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Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries Christian Masquelier*,†,§,∥ and Laurence Croguennec‡,§,∥ †

Laboratoire de Réactivité et de Chimie des Solides, UMR CNRS 7314, Université de Picardie Jules Vernes, 80039 Amiens Cedex 1, France ‡ CNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France § ALISTORE-ERI, FR CNRS 3104 33 Rue Saint-Leu, 80039 Amiens Cedex 1, France ∥ Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, France 3.2.2. Solution-Based Syntheses 3.2.3. Carbon Coating and Purity Control of LiFePO4 3.3. The Highly Insulating LiMnPO4 3.4. Substitutions of Mn and/or Co for Fe in LiFePO4 3.5. Intrinsic Physicochemical Properties of LiFePO4 and Mechanism of Li+ Extraction 3.5.1. Crystal Structure, Defects, e− Transport, Li+ Diffusion 3.5.2. Reactivity with Moisture and/or Air 3.5.3. Mechanism 4. Alternative Polyanionic Structures and Compositions: Hydrated Phosphates, Diphosphates, Alluaudites, Silicates, and Borates 4.1. Fe3+/Fe2+ Couple in Amorphous or Crystalline Iron Phosphates: FePO4·nH2O 4.2. V 4+ /V3+ , Ti 4+ /Ti 3+ , and Fe 3+ /Fe 2+ Redox Couples in Diphosphates and Diarsenates LixMX2O7 (M = Fe, V, Ti; X = P, As) and Fe4(P2O7)3·nH2O 4.3. Transition Metal Silicates Li2MSiO4 (M = Fe, Mn, Co) 4.4. Na3Fe3(PO4)4 and Alluaudite Phases 4.5. Lithiated Transition Metal Borates 5. Oxy-, Hydroxy-, Fluoro-phosphates and -sulfates: New Promising Electrode Materials 5.1. V5+/V4+ and V4+/V3+ Couples in LixVOXO4 (X = P, As) 5.2. The Nb5+/Nb4+ and Ti4+/Ti3+ Couples in LixMOXO4 (X = S, P, Si) 5.3. V4+/V3+ and V3+/V2+ Couples in NaxVPO4F and LixVPO4F 5.4. Fe3+/Fe2+ and Ti4+/Ti3+ Couples in LiFeIIIPO4F and LiTiIIIPO4F 5.5. Fe3+/Fe2+ Couple in A2MPO4F (A = Li, Na; M = Fe, Mn) 5.6. V3+/V4+ Redox Couple in A3V2(PO4)2F3 and A5V(PO4)2F2 5.7. Fe3+/Fe2+ in Tavorite-like LiFeIISO4F and LiFeIIIPO4OH. Extension to the Triplite Family 6. Conclusions and Future Outlook

CONTENTS 1. Introduction: From Oxides to Ionically Conducting Polyanionic Frameworks as Positive Electrodes in Li Batteries 2. The Early Days: The NASICON and Anti-NASICON Structures Used as Model Frameworks 2.1. Structural Considerations 2.2. The Inductive Effect: Tuning the Mn+/M(n−1)+ Redox Couple in the NASICON Structure by Changing the Chemical Nature of the XO4n− Groups 2.3. Relative Positions of Various Mn+/M(n−1)+ Redox Couples (M = Fe, Ti, V, Nb) in NASICON-type Phosphates 2.3.1. Position of the Ti4+/Ti3+ Couple versus Li+/Li or Na+/Na 2.3.2. Position of the Fe3+/Fe2+ Couple versus Li+/Li or Na+/Na 2.3.3. Position of the Vn+/V(n−1)+ Couples versus Li+/Li or Na+/Na 2.4. Complex Redox Phenomena in Anti-NASICON Compositions LixM2(PO4)3 (0 < x < 5 ; M = Fe, V) 2.4.1. Li+ Insertion into Monoclinic Fe2(SO4)3, Li3Fe2(PO4)3, and Li3Fe2(AsO4)3 2.4.2. Li+ Insertion/Extraction into Monoclinic Li3V2(PO4)3 3. LiMPO4 Compositions Based on the Olivine Structure: Fifteen Years of Great Achievements 3.1. The Early Days: From an Academic Curiosity to First Industrial Realizations 3.2. A Myriad of Synthesis Routes Developed for Optimal Electrochemical Response of LiMPO4 Powders, Industrial Process Ability, and Cost Reduction 3.2.1. Solid-State Syntheses © XXXX American Chemical Society

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Received: May 8, 2012

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dx.doi.org/10.1021/cr3001862 | Chem. Rev. XXXX, XXX, XXX−XXX

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Review

ceeds while operating on the M4+/M3+ redox couple, located between 4 and 5 V versus Li+/Li. LiCoO2 had been first proposed by J. B. Goodenough’s group1 and remains the main material used as positive electrode in commercial lithium-ion batteries. LiNi1−y−zCoyAlzO22 and more recently LiCo1/3Ni1/3Mn1/3O23 were considered as being competitive to LiCoO2 with lower cost, higher energy and power, as well as highly improved safety for the latter. LiNi0.80Co0.15Al0.05O2 and LiCo1/3Ni1/3Mn1/3O2 in particular reached commercial success. Materials such as xLi2MnO3·(1− x)LiMO2 (the “LiMO2” component can be LiMn2O4 (spinel) or more generally layered oxides such as LiMn1/2Ni1/2O2 and LiCo1/3Ni1/3Mn1/3O2) exhibit high reversible capacities (>200 mA·h/g) after an “activation” process at high voltage (>4.5 V vs Li).4,5 The peculiar crystallographic structure of the two-dimensional oxides Li1−xMO2 (M = Co, Ni, Fe, Mn) leads to stacking modifications with slabs gliding or to irreversible structural instabilities when the number x of extracted lithium is high (end of charge). Irreversible migration of transition metals within the lithium layers may occur and lead to important capacity loss on cycling. For this reason, J. B. Goodenough6 and M. M. Thackeray7−9 had envisioned in the early 1980s the possible use of three-dimensional oxides such as the spinel Li[Mn2]O4. In such spinel structure, MnO6 octahedra are connected to each other through edge-sharing and define a three-dimensional network of conduction paths for lithium motion (Figure 1). Spinel LiMn2O4 exhibits an operating voltage of 4.1 V versus Li+/Li, and its high potential analogue, Li[Ni1/2Mn3/2]O4, lies at about 4.7 V versus Li+/Li. Coupled with an elevated-potential negative electrode such as the spinel Li4Ti5O12, that latter system provides a route to develop a promising new generation of 12 V batteries.10 Besides those “simple” oxides (which may lead actually to quite complicated mixed cation arrangements and properties), three-dimensional frameworks built on transition metals and polyanions (XO4)n− have become in the last 15 years the subject of very intensive research worldwide since the discovery of the electrochemically active LiFePO4.11,12 Despite the “weight penalty” (smaller theoretical gravimetric capacity) arising from the presence of polyanion groups such as (PO4)3−, (SiO4)4−, and (SO4)2−, the positive attributes of such materials are as follows: • Very stable frameworks provide long-term structural stability, essential for extensive cycling and safety issues.

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1. INTRODUCTION: FROM OXIDES TO IONICALLY CONDUCTING POLYANIONIC FRAMEWORKS AS POSITIVE ELECTRODES IN LI BATTERIES For more than 20 years, most of the technological achievements for the realization of positive electrodes for practical rechargeable Li battery systems have been devoted to transition metal oxides such as LixMO2 (M = Co, Ni, Mn), LixMn2O4, LixV2O5, or LixV3O8. The first two classes of materials built on close-packed oxygen stacking adopt bidimensional and tridimensional crystal structures, respectively (Figure 1), from which lithium ions may be easily intercalated or extracted in a reversible manner. These oxides are reasonably good ionic and electronic conductors, and lithium insertion/extraction pro-

• The chemical nature of the polyanion allows the monitoring of a given Mn+/M(n−1)+ redox couple, through the inductive effect introduced by Goodenough,13,14 and gives rise to higher values versus Li+/ Li than in oxides. • An immense variety of atomic arrangements and crystal structures were adopted with an extreme versatility toward cation and anion substitutions for a given structural type. In this contribution, we will attempt to establish an overall review, as complete as possible, on two decades of discoveries and technological developments of these polyanion-based materials as positive electrodes for Li batteries. The overall structure of this Review is as follows: • the early days: the AxMM′(XO4)3 NASICON structure used as model compounds

Figure 1. Representations of the main structural families of transition metal oxides used as positive electrodes in Li batteries. B


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• LiMPO4 compositions based on the olivine structure: 15 years of great achievements • more “exotic” structures: hydrated phosphates, diphosphates, alluaudites, silicates, borates • new promising electrode materials based on/affiliated with the tavorite structure: AxM(XO4)Y Other complementary review contributions of interest can be found in the following references: 15−33.

dral (R3̅ for instance) or monoclinic (C2/c for instance) space groups.

2. THE EARLY DAYS: THE NASICON AND ANTI-NASICON STRUCTURES USED AS MODEL FRAMEWORKS The NASICON (na-super-ionic-conductors) structure has been the object of immense research effort since the discovery in 197634,35 of Na+ ion transport properties approaching those of β-Al2O3. At that time, when energy storage and generation issues started to really matter, the focus in solid-state ionics was on the development of superionic ceramic conductors to be used as solid electrolytes in Na/S-type batteries for instance. The seminal range of compositions of this structural family is the Na1+xZr2P3−xSixO12 (0 ≤ x ≤ 3) solid solution, which shows a pronounced maximum of ionic conductivity for x = 2, while the two end members NaZr2(PO4)336 and Na4Zr2(SiO4)337 are poor ionic conductors. The chemical and structural characteristics responsible for the fast ion transport in the NASICON structure were clearly addressed by Goodenough35 and are summarized as follows: • highly covalent three-dimensional framework generating a large interstitial space • elasticity of the framework for better accommodation of local compositional changes • weak framework−alkali cation interactions, and mainly electrostatic interactions between neighboring alkali cations • three-dimensional network of interconnected conduction pathways • nonreducible transition element when in contact with metallic alkali cation The concept of investigating three-dimensional frameworks based on the NASICON structure as hosts for reversible insertion/extraction of alkali cations (electrodes) arose in the mid 1980s mostly from concerns about possible stability/ reactivity versus metallic Na (or Li) when used as solid electrolytes. While NASICON compositions containing Zr, Sc, or Hf, for instance, showed no reactivity versus Li or Na, Torardi38,39 and Delmas40−43 were the first to demonstrate reversible alkali electrochemical insertion into Fe2(MoO4)3 and (Li,Na)Ti2(PO4)3, respectively. Goodenough immediately envisaged the possibility of tuning and monitoring the properties of these electrodes through polyanion substitutions in Fe-based 3-D structures: the inductive effect engendered by the polyanion has a direct impact on the position of the Fe3+/ Fe2+ redox couple in a given structural-type, shifting from 3.0 V versus Li+/Li for Fe2(MoO4)3 or Fe2(WO4)313,38,44 to 3.6 V versus Li+/Li for Fe2(SO4)314,45 and to ∼2.8 V versus Li+/Li for Li3Fe2(PO4)3.12,46−48

Figure 2. NASICON (generally rhombohedral) and anti-NASICON (generally monoclinic) frameworks of general formula AxMM′(XO4)3.

The basic MM′(XO4)3 repeating unit, usually named “lantern”, is made of three tetrahedra connected to two octahedra. Each lantern is connected to six other lanterns, hence generating overall a large interstitial space that may accommodate between 0 (Fe2(SO4)3) and 5 (Li5Fe2(PO4)3) alkali cations per structural formula. These lantern units are stacked parallel to the [001] direction of the hexagonal unitcell. The exceptional adaptability of this structural arrangement to numerous chemical substitutions onto the An+, Mn′+, or Xn″+ sites has been widely demonstrated. Importantly, the number x of alkali cations per structural formula AxMM′(XO4)3 may be adjusted depending on the stable oxidation states of transition metals and on the X element chosen. For instance, niobium, titanium, and iron, mostly stable in air at their oxidation states +5, +4, and +3, respectively, lead, when combined with the PO43− anion, to compositions such as NbTi(PO4)3,50,51 LiTi2(PO4)3,43 Na2FeTi(PO4)3,52 and Na3Fe2(PO4)353−55 that all adopt the NASICON structure and are easy to prepare as pure powders. Other examples include Fe2(SO4)349 and Na4Zr2(SiO4)3.37 Depending on the preparation procedure, AxMM′(XO4)3 may adopt a distinct crystal structure (usually monoclinic P21/n or orthorhombic Pcan, referred to as anti-NASICON) into which the lantern units are stacked differently in three dimensions: antiparallel in this case, alternately along the directions ∼(2b ± c) of the monoclinic unit-cell.56,57 This arrangement results in a less open structure than in the NASICON case, hence less favorable for alkali ion transport (Figure 2). The interconnection between MO6 octahedra and XO4 tetrahedra generates fast ion conduction, but quite poor intrinsic electronic conductivity (major advantage for a solid electrolyte): no direct ··−M−O−M−·· electronic delocalization is possible as the MO6 octahedra are isolated from each other and separated by the XO4 groups. Tillement et al.51,58,59 reported first the existence of mixed-valence NASICON structures with unusual oxidation states for transition metals, such as NbIV, TiIII, and FeII.

2.1. Structural Considerations

The NASICON structure, of general formula AxMM′(XO4)3, is built on a three-dimensional framework of MO6 and M′O6 octahedra sharing all their corners with XO4 tetrahedra and vice versa (Figure 2)34,49 and commonly described with rhomboheC


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2.2. The Inductive Effect: Tuning the Mn+/M(n−1)+ Redox Couple in the NASICON Structure by Changing the Chemical Nature of the XO4n− Groups

provided as well an elegant measure of the Fe−O bond strengths modified by the XO4n− polyanions. 2.3. Relative Positions of Various Mn+/M(n−1)+ Redox Couples (M = Fe, Ti, V, Nb) in NASICON-type Phosphates

The NASICON framework was used by Goodenough in the late 1980s as a very demonstrative example of the possibility for the chemist to elaborate electrode materials functioning at controlled operating voltages. For the NASICON-type ironbased compositions of general formula AxFe2(XO4)3, the inductive effect concept was successfully used to fully explain qualitatively the respective positions of the Fe3+/Fe2+ redox couple where X varies from P (2.8 V vs Li+/Li48) to Mo (3.0 V vs Li+/Li13,38) to W (3.0 V vs Li+/Li13) and to S (3.6 V vs Li+/ Li14) (Figure 3). Hence, isostructural Li3Fe2(PO4)348,60−62 and

A second important input from the early fundamental studies on Li+ (or Na+) insertion into the NASICON framework deals with the determination of the respective positions of redox couples of a series of transition metal elements such as Fe, Ti, V, and Nb. This structure indeed offers a very rich ability for chemical substitutions. The number x of alkali cations A+ in AxMM′(XO4)3 is “tuned” by the oxidation state of M and M′ and by the nature of X (SiIV, PV, SVI in particular). The phosphate group (PO4)3− offers the widest variety of M,M′ chemical substitutions, given that x may vary from 0 in NbTi(PO 4 ) 3 , Nb 2 (PO 4 ) 3 , 64 or V 2 (PO 4 ) 3 65 to 5 in Li5Fe2(PO4)348,60−62 in the NASICON framework. Straightforward synthesis routes (in air or Ar) easily yield compositions with transition metals at the oxidation states V3+, Fe3+, Ti4+, and Nb5+. A short illustration of accessible compositions, chemically and electrochemically, is given in Figure 4.

Figure 3. Respective positions of the Fe3+/Fe2+ redox couple vs Li+/Li in NASICON AxFe2(XO4)3 compositions (X = P, Mo, W, S).

Figure 4. Accessible compositions and respective Mn+/M(n−1)+ redox couples (vs Li) in NASICON AxMM′(XO4)3 structures.

Fe2(SO4)313,45 display a difference of 0.8 V upon Li+ insertion, further confirmed by Padhi63 for the mixed anionic framework LiFe2(SO4)2(PO4): the Fe3+/Fe2+ redox couple is placed at ∼3.3 V versus Li+/Li, that is, at an intermediate value between those of the sulfate and the phosphate. The inductive effect of XO4 groups that, depending on the electronegativity of X, weakens or strengthens the covalency of the Fe−O bonds is at the origin of these significant operating voltage differences: each transition metal is involved in six M− O−X bond sequences, and, as a result, the substitution of S for Mo or W diminishes the covalency of the Fe−O bonds, lowers the energy of the antibonding states, and hence increases the difference between the Fe3+/Fe2+ and the Li+/Li couples14 (Figure 3). Additionally, Bruce’s work44 on the insertion of Na+ in Fe2(XO4)3 compositions (X = Mo, W) showed an operating voltage of 2.7 V versus Na+/Na (0.3 V smaller than vs Li+/Li), which pointed out very weak interactions between the alkali cations and the oxygen atoms of the NASICON framework, a favorable feature for high ionic transport. These early considerations have prompted the identification of new positive electrode materials with attractive operating voltages for the Fe3+/Fe2+ couple in lithium batteries and theoretical capacities in the range of ∼120 mA·h/g. They

2.3.1. Position of the Ti4+/Ti3+ Couple versus Li+/Li or Na+/Na. Delmas41−43 showed the possible insertion of lithium or sodium into LiTi2(PO4)3 and NaTi2(PO4)3, respectively, toward the end-members A3Ti2(PO4)3 (A = Li, Na) into which titanium is at the oxidation state +III. Lithium insertion into LiTi2(PO4)3 occurs at 2.48 V versus Li+/Li for the Ti4+/Ti3+ redox couple (Figure 5) according to a two-phase mechanism between LiTi2(PO4)3 and Li3Ti2(PO4)3, further confirmed.66,67 On going from LiTi2(PO4)3 to Li3Ti2(PO4)3, a remarkable increase in the c/a ratio of the hexagonal unit-cell occurs, due to stronger electrostatic repulsions between the [Ti2(PO4)3] lanterns along [001] when the octahedral Li site (M1), initially occupied and located halfway between two MO6 octahedra along [001]hex, is emptied. As a result, the crystal structure of Li3Ti2(PO4)3 can no longer be described by the space group R3c̅ , and a new set of tetrahedral sites (labeled M3′ and M3″) was found to be occupied by the Li+ ions through neutron diffraction studies,66 as for the isostructural Li3Fe2(PO4)3.60 The isotypic composition NaTi2(PO4)3 is also active toward the electrochemical insertion of sodium according once again to a two-phase mechanism. The reversible redox process occurs at 2.15 V versus Na+/Na for the Ti4+/Ti3+ couple. This value is once again a clear demonstration of very weak interactions D


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from the sodium counterpart Na3Fe2(PO4)3.53−55 As for Li3Ti2(PO4)3 obtained by electrochemical insertion of Li+ into LiTi2(PO4)3, the ion exchange between Na+ ions (in the octahedral M1 site) and Li+ (in tetrahedral M3 sites) results in a strong increase in the c/a ratio of the rhombohedral unit-cell, described in the space group R3̅.60,61,71 The electrochemical insertion of Li+ into rhombohedral Li3Fe2(PO4)3 (theoretical capacity = 128 mA·h g−1) occurs following a continuous decrease in the cell voltage as a function of composition, at an average voltage of 2.85 V versus Li+/Li, between 3.1 and 2.6 V.62 Excellent reversibility has been demonstrated over long-range cycling, and the fully discharged composition, Li5Fe2(PO4)3, constitutes the first example, to our knowledge, of a NASICON framework hosting five alkali cations. A crystal structure (apparently wrong from interatomic distances considerations) of this phase has been proposed by Eyob et al. 72 Interestingly, the insertion of Na+ into Na3Fe2(PO4)3 is limited to Na4Fe2(PO4)3 for which all of the octahedral M1 and strongly distorted tetrahedral M2 sites are filled.73 The crystal structure of Na4Fe2(PO4)3 has been refined recently74 in the space group R3̅c. The cases of the mixed NASICON compositions Li2+xFeTi(PO4)3 and Na2+xFeTi(PO4)3 are instructive as Fe3+ → Fe2+ reduction occurs first at ∼2.5 V versus Na+/Na (∼2.8 V vs Li+/ Li) before the reduction of Ti4+ into Ti3+ at 2.2 V versus Na+/ Na (∼2.5 V vs Li+/Li). For all of these mixed compositions, Patoux et al. reported full solid solution behaviors (Figure 6) and excellent reversibility.52 2.3.3. Position of the Vn+/V(n−1)+ Couples versus Li+/Li or Na+/Na. Gopalakrishnan was the first to report65 on the chemical oxidation of NASICON Na3V2(PO4)3 to yield the new mixed-valence (V5+ and V4+) composition V2(PO4)3. Vanadium can be oxidized from V3+ to V4+ and then V5+, thus providing the possibility of exchanging more than one electron per transition metal. Additionally, V3+ may be reduced to V2+ through sodium insertion at 1.6 V versus Na+/Na. The electrochemical extraction of Na+ from Na3V2(PO4)3 to NaV2(PO4)3 occurs following a two-phase reaction at 3.4 V versus Na+/Na, the latter phase being isotypical with NaTi2(PO4)3. NASICON Li3V2(PO4)3 (isostructural with Li3Fe2(PO4)3 and Li3Ti2(PO4)3) was prepared by ion exchange from the sodium analogue, and the first report of Li+ extraction from this phase was published by Gaubicher.75 Oxidation of V3+ to V4+ occurs at 3.77 V versus Li+/Li in a two-phase process, as evidenced by in situ X-ray diffraction.76 The charged phase, LiV2(PO4)3, is isostructural with LiTi2(PO4)3 (space group R3)̅ as supported as well by NMR.75 Cushing reported on only partial ion exchange from Na 3 V 2 (PO 4 ) 3 to yield Li2NaV2(PO4)3.77

Figure 5. Top: GITT electrochemical data of Li+ insertion into LiTi2(PO4)3. Bottom: Electrochemical behavior, vs Li, of chemically prepared Li3Ti2(PO4)3. Adapted with permission from ref 41. Copyright 1987 Elsevier Press.

between the alkali cations and the framework in the NASICON structure as the ∼0.3 V difference between the operating voltages of Li/Li1+xTi2(PO4)3 and Na/Na1+xTi2(PO4)3 reflects the difference between the Li+/Li and Na+/Na couples. Sodium insertion results in the formation of Na3Ti2(PO4)3 isostructural with Na3Fe2(PO4)3.53−55,68 Mixed (Li,Na)x(M,M′)(PO4)3 compositions were investigated by Tillement,50,51,58 Padhi,47 and Patoux52 who revealed that the position of a given Mn+/M(n−1)+ redox couple was mostly independent of the chemical nature of the second transition metal element M′, due to the absence of direct connectivity between two (M,M′)O6 octahedra. For instance, the Ti4+/Ti3+ couple was found to lie at the same value of ∼2.2 V versus Na+/Na in NaxTiNb(PO4)3, Na1+xTi2(PO4)3, and Na2+xFeTi(PO4)3. The investigation of Cr-containing compositions such as Li2CrTi(PO4)3, for instance, supports these findings and allows one to locate precisely the Ti4+/Ti3+ couple.69 2.3.2. Position of the Fe3+/Fe2+ Couple versus Li+/Li or Na + /Na. The rhombohedral form (NASICON) of Li3Fe2(PO4)3 is not the stable form that one may prepare by standard ceramic routes: it can instead be obtained48,54,60,70 by ion exchange in aqueous LiNO3 solution or molten LiNO3

2.4. Complex Redox Phenomena in Anti-NASICON Compositions LixM2(PO4)3 (0 < x < 5 ; M = Fe, V)

As mentioned previously, the stable Li3M2(PO4)3 (M = Fe, V) crystalline form obtained from ceramic synthesis adopts an antiNASICON framework into which the “lantern” units are oriented antiparallel, thus delimiting smaller interstitial cavities that can accommodate preferentially Li+ ions.78 Depending on the ordering, or not, of Li+ ions into this framework, they crystallize in monoclinic or orthorhombic systems, as extensively described by the group of Maksimov, and others, in the mid-1980s.53,56,57,71,79−87 At that time, the issue was to E


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inserted as well into the monoclinic arsenate Li3Fe2(AsO4)389 toward Li5Fe2(AsO4 )3 .48 The shape of the potential− composition curve is very similar to that of Li3Fe2(PO4)3, with the existence of an intermediate composition Li4Fe2(AsO4)3 between two two-phase reactions. These reactions take place at 2.91 and 2.62 V versus Li+/Li, that is, are better separated than for Li3Fe2(PO4)3. Substitution of AsO43− for PO43− has a very minor influence on the global inductive effect on iron due to the similar electronegativities of As and P. This was confirmed by Gaubicher90,91 who found the same redox potential values (4.0 V vs Li+/Li) for the V5+/V4+ couple in isostructural LixVOPO4 and LixVOAsO4. Noticeably, these structures have been recently investigated by three independent groups92−94 as model compounds for the understanding of complex Li NMR signals in paramagnetic compounds, and useful insights into the activation energies for hopping between the lithium sites were provided. Shirakawa investigated the electronic structures of such compounds and their associated modifications upon Li+ insertion.95,96 2.4.2. Li + Insertion/Extraction into Monoclinic Li3V2(PO4)3. With no doubt, the most investigated antiNASICON composition is Li3V2(PO4)3 that can be oxidized through lithium extraction at potentials ranging from 3.4 to 4.6 V versus Li+/Li.76,88,97−109 Li3V2(PO4)3 shows a complicated series of four successive two-phase transitions upon Li extraction toward Li 0.1V 2 (PO 4 ) 3, that is, by operating successively on both V3+/V4+ and V4+/V5+ redox couples for a theoretical capacity of 197 mA·h g−1 (Figure 7).

Figure 6. Reversible Li+ insertion/extraction into NASICON Li2TiFe(PO4)3 vs Li. The reaction proceeds through a single phase Li2+xTiFe(PO4)3, as witnessed by in situ X-ray diffraction (bottom). Adapted with permission from ref 52. Copyright 2003 American Chemical Society.

investigate the structural modifications induced by alkali order− disorder transitions within these superionic conductors. 2.4.1. Li + Insertion into Monoclinic Fe 2 (SO 4 ) 3 , Li3Fe2(PO4)3, and Li3Fe2(AsO4)3. Manthiram14 was the first to report on the insertion of lithium into monoclinic Fe2(SO4)3 at an attractive potential of 3.6 V versus Li+/Li. Okada46 showed subsequently that NASICON and monoclinic Fe2(SO4)3 had very similar electrochemical signatures and that insertion of lithium into monoclinic Fe2(SO4)3 occurs through a two-phase reaction, the end-member Li2Fe2(SO4)3 being of orthorhombic symmetry (Pcan). In the same study, the electrochemical insertion of Li+ into Li3Fe2(PO4)3 located the Fe3+/Fe2+ couple at 2.8 V versus Li+/Li, and this material was envisaged as an interesting buffer material to be mixed with Fe2(SO4)3 for protection against overdischarge. Further investigations48,62 on Li3+xFe2(PO4)3 revealed though that two biphasic reactions take place at 2.88 and 2.73 V versus Li+/Li with the existence of a definite composition Li4Fe2(PO4)3 in between. In situ X-ray diffraction88 revealed that the framework is very well-maintained and that the intermediate phases involved are extremely similar to the pristine one. Reversible capacities of ∼115 mA·h/g at C/10 regime could be attained with excellent capacity retention once particle sizes and surface were optimized, in particular through low-temperature (solution) synthesis and mechanical grinding of the phosphate with conductive carbon. Lithium may be

Figure 7. Galvanostatic Li+ insertion/extraction and respective redox couples in anti-NASICON LixM2(PO4)3 (M = Fe, V) compositions. Adapted with permission from ref 88. Copyright 2003 Elsevier.

The crystal structure of the pristine Li3V2(PO4)3 has been determined from single-crystal X-ray diffraction by Huang101 and from combined X-ray synchrotron/neutron diffraction data by Patoux.88 As for Li3Fe2(PO4)3, lithium ions fully occupy three crystallographic sites in Li3V2(PO4)3,92,93 and anisotropic thermal motion factors suggest a favored two-dimensional ion transport. Huang101 and Saı̈di100 demonstrated early excellent reversibility at reasonable charging/discharging rates for nanostructured electrodes embedded in a conductive carbon matrix F


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of formula Li3V1.5Al0.5(PO4)3 from which vanadium V3+ can still be fully oxidized to V5+, thus leading to an even greater theoretical capacity of 203 mA·h g−1 thanks to the lighter mass of Al versus V. Electrochemical extraction of Li from Li3V1.5Al0.5(PO4)3 was achieved for more than 100 cycles with excellent cycle efficiency.108

(Figure 8). The complexity of the phase transformations sequence and the reversibility of the overall reaction could be

3. LiMPO4 COMPOSITIONS BASED ON THE OLIVINE STRUCTURE: FIFTEEN YEARS OF GREAT ACHIEVEMENTS 3.1. The Early Days: From an Academic Curiosity to First Industrial Realizations

Since the demonstration by Padhi et al.11,12 that lithium can be extracted reversibly from the triphylite LiFePO4 at ∼3.5 V versus Li+/Li, it quickly became the material of choice for Li insertion/extraction among the so-called polyanionic structures (Figure 9).

Figure 8. Galvanostatic Li+ insertion/extraction, cyclability, and response to high currents of anti-NASICON Li3V2(PO4)3. Adapted with permission from ref 101. Copyright 2002 Wiley.

spotted by Morcrette76 through in situ X-ray diffraction (Figure 7). A unit-cell contraction of ΔV/V = −6.8% occurs between Li3V2(PO4)3 and LiV2(PO4)3 as well as a slight increase on further oxidation to Li0.1V2(PO4)3. Despite these numerous phase transitions and non-negligible volume changes, the monoclinic Li3V2(PO4)3 system is structurally highly reversible, and once possible vanadium dissolution in the electrolyte and reactivity with the electrolyte at potentials as high as 4.6 V versus Li+/Li have been circumvented, Li3V2(PO4)3 will stand as a promising positive electrode for Li-rechargeable batteries. Excellent power capability data and cycling efficiency on optimized electrodes were recently reported by Barker et al.,109 and many groups, especially in China, have embarked on the development of alternative synthesis routes for large-scale production.110−113 Yin et al.106 succeeded in isolating the partially deintercalated composition Li2.5V2(PO4)3 (stable phase at 3.65 V vs Li) and demonstrated that lithium ions are partly disordered, as in the orthorhombic high-temperature (∼300 °C) form, γ, of Li3V2(PO4)3. They could hence demonstrate superior ion mobility within this intermediate composition and, noticeably, V3+/V4+ charge ordering. Another interesting approach aimed at enhancing Li+ transport within the structure was proposed by Morgan99 and further consolidated in 2007 by Barker’s group.108 The concept was to substitute part of aluminum for vanadium according to the structural formula Li3V2−xAlx(PO4)3. The substitution of 0.5 V by Al leads to a quite interesting positive electrode material

Figure 9. Representation of the crystal structure of LiFePO4 in Pnma space group description.

The triphylite LiFePO4 adopts an olivine-type crystal structure, built on distorted oxygen hexagonal packing into which Li+ and Fe2+ occupy one-half of the octahedral sites and P occupies 1/8 of the tetrahedral sites. This structural type had been first investigated for its magnetic properties.114−117 The unit-cell adopts an orthorhombic symmetry described either in the Pmnb or Pnma (equivalent) space groups (a = 10.338(1) Å, b = 6.011(1) Å, c = 4.695(1) Å in Pnma description). The peculiar distribution of Li+ and Fe2+ within the octahedral sites G


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promising results of Li134 happened to be extremely difficult to reproduce for several years. Yamada reviewed indeed extensively in a seminal paper137 the strategy, achievements, and optimal LiFe1−xMnxPO4 compositions developed by SONY. While LiFePO4 is a semiconductor with a 0.3 eV band gap, LiMnPO4 is an insulator with ca. 2 eV band gap, a major intrinsic obstacle to a smooth redox reaction at 4 V in the Mn-rich phase, besides additional lattice frustration effects induced by the strong electron−lattice interaction (Jahn−Teller effect on Mn3+) in charged Li1−yFe1−xMnxPO4 electrodes. Besides LiMnPO4, Amine,138 Okada,139 and Lloris140 envisaged the utilization of the isostructural LiCoPO4, an extremely challenging material in terms of theoretical energy density to be delivered (oxidation of Co3+ → Co4+ at 4.8 V vs Li+/Li). With no doubt, the paper published in 2002 by the group of Y. M. Chiang141 constitutes the milestone for the development of olivine-type materials as positive electrodes for lithium ion batteries. Besides the important academic controversy142−144 that followed on the effective role of 1% “dopants” such as M = Zr or Nb, on the 8 orders of magnitude increase in electrical conductivity of Li1−xMxFePO4 reported, this study undoubtedly caught the attention of a vast community of people, as it identified for the first time LiFePO4-based materials as materials of choice for power applications. The creation, in parallel, of the A123 Co., which quickly produced and smartly marketed real commercialized batteries using LiFePO4-type electrodes for the first time, paved the way for much industrial interest.

generates layers that have a direct impact on both electronic and ionic conductivities. FeO6 octahedra share corners between each other, not edges (Figure 9), and electronic delocalization is hence difficult. Lithium conductivity is not three-dimensional and proceeds mostly along [010]Pnma.118 Additionally, and very noticeably, the average FeII−O distance is much longer than what is expected for Fe2+ in octahedral coordination, due to the fact that each FeO6 octahedron shares one edge with a PO4 tetrahedron and corners with four other PO4 tetrahedra. As a consequence, electrostatic repulsions between Fe and P ions weaken the Fe− O bond strength, which is at the origin of the unusually high operating voltage of this electrode material when lithium ions are extracted from the framework (3.45 V vs Li+/Li).119 The very first breakthrough concerning the triphylite LiFePO4 was presented by Ravet and Armand in 1999120,121 who showed that full electrochemical utilization was made possible through a thin conductive carbon coating at the surface to compensate for its low electrical conductivity, as illustrated in Figure 10. An inspiring approach for carbon coating on

3.2. A Myriad of Synthesis Routes Developed for Optimal Electrochemical Response of LiMPO4 Powders, Industrial Process Ability, and Cost Reduction

Hundreds of research papers and patents have been filed over the last 10 years on many synthesis procedures developed to prepare “optimized” LiMPO4 powders for battery applications. We try to draw in this Review a comprehensive review of what has been achieved in the last 10 years. We will divide this overview into two main parts: solid-state synthesis and solution-based synthesis. Solid-state synthesis is the most “robust” and conventional method used industrially to synthesize powder materials for lithium batteries because of its (apparent) simplicity, ideal for continuous large-scale production. Solution-based syntheses are more versatile, with the possible use of a large panel of solvents: aqueous, classical organic, polyols, or, from quite recent reports, ionic liquids. For further details, a comprehensive review on the synthesis of nanostructured materials for Li batteries was recently published by Cho and Nazar.21 3.2.1. Solid-State Syntheses. Solid-state synthesis of powders of LiFePO4 usually involves (i) grinding or ball milling of the precursors, (ii) a first thermal treatment at low temperature (∼350 °C), (iii) a new grinding with preparation of pellets, and (iv) a second thermal treatment at temperatures higher than 500 °C. Annealing is generally performed under Ar/H2 atmosphere to reduce Fe3+ into Fe2+ if the precursors used are Fe3+-rich and/or to prevent oxidation of Fe2+. Sintered micrometer sized particles are usually obtained through “classical” solid-state reactions. To tackle this important issue, LiFePO4 being intrinsically a poor ionic and electronic conductor, Ravet and Armand first proposed to fabricate and optimize carbon/ LiFePO4 composites.120 Thin carbon coatings can be obtained

Figure 10. Thin carbon coating at the surface of highly crystalline LiFePO4 yields to high benefits for electrochemical response at high rate.

spinel LiMn2O4 had been published by Dominko in that period.122 Huang123 and Yamada124 were the first to publish comprehensive studies on small LiFePO4 particle growth and composite electrode optimization, achieving excellent (already) cyclability for ∼90% of the theoretical capacity at C/2. Excellent properties were early reported as well on polymer-type batteries, which benefited from the higher temperatures of operation (∼90 °C) needed with this technology.125,126 Anderson addressed the issue of the thermal stability of this new type of positive electrode material,127−129 and Tucker published early studies of Li NMR in LiMPO4 (M = Fe, Mn, Co, Ni) allotropes.130,131 The early work of Padhi11 highlighted that lithium extraction from isostructural compositions LiFe1−xMnxPO4 was also possible, locating the Mn3+/Mn2+ redox couple at 4.1 V versus Li+/Li. Yamada132,133 investigated the Li1−yFe1−xMnxPO4 phase diagram in detail and demonstrated, in parallel with Li,134−136 a significant increase in energy density when Mn is substituted for Fe. However, the presence of manganese brings in even more severe kinetic limitations than with iron, and the H


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either from the decomposition of additives such as resorcinolformaldehyde,123,145 polysaccharides,145−147 or cellulose148,149 added on purpose from the very beginning of the synthesis, from the decomposition of oxalates (acetates) used as precursors for iron and lithium,124,148,147 or from intimate mixtures (mechanical grinding, annealing at high temperature) of LiFePO 4 (after synthesis) with a carbon precursor.11,121,122,146 From a general point of view, the addition of conductive additives prior to the synthesis improved significantly the overall transport properties of the carbon-coated LiFePO4 material, through decrease in the Li+ diffusion lengths in “nanoparticles” and improvement of electronic wiring between particles. As pointed out by Chen,146 the overall amount of coated carbon should be optimized (not too low, not too high) so as to promote better reversible capacity and kinetic response without scarifying the tap density and therefore the volumetric energy density. Interesting reports (in terms of chemistry used and performances reached) include the formation of a core−shell structure for a carbon/LiFePO4 nanocomposite150 and the use of an Fe3+-containing Li4P2O7 glass/LiFePO4 nanocomposite.151 In situ polymerization of aniline was achieved by introducing Fe 3+ ions (FeCl 3 ) into a solution containing PO 4 3− (NH4H2PO4) and aniline to produce an FePO4/polyaniline core−shell, subsequently annealed at 700 °C with lithium acetate and sucrose. The resulting 20−40 nm LiFePO4 particles are homogeneously covered with a 1−2 nm carbon-coating152 and sustain rates as high as 60C with still 80 mA·h g−1 of delivered capacity (Figure 11). Ceder’s group reported151 that an inorganic Fe3+-containing Li4P2O7 glass (resulting from the annealing of a global “LiFe1−2yP1−yO4−δ” (y = 0.05) composition) covering the surface of LiFePO4 can significantly improve its transport properties. The 50 nm particles thus obtained exhibit apparent extraordinary performance: 130 mA·h/g at 50C rate (for a 15 wt % carbon-containing electrode (Figure 12)). 3.2.2. Solution-Based Syntheses. In a first approach, mixing Li, Fe, and PO4 precursors in aqueous solution generates the precipitation of the two insoluble salts Li3PO4 and Fe3(PO4)2·nH2O whose progressive annealing at high temperature under inert slightly reducing atmosphere (N2/H2: 90/10) results in 100−200 nm particles of LiFePO4.144,153 Whittingham’s group145,154,155 early studied the parameters of importance for optimizing the preparation of fine LiFePO4 particles through hydrothermal procedures under mild conditions and autogenous pressure. Depending on the precipitation sequence step, (i) coprecipitation of Li3PO4 and Fe3(PO4)2 or (ii) successive precipitations of Li3PO4 first and then of Fe3(PO4)4, isolated primary nanoparticles or 300 nm aggregates of primary particles are formed.21 Poor rate capability is obtained at high rates as soon as large-sized particles are formed. Alternatively, carbon-containing surfactants can be introduced during the coprecipitation reaction to form carbon-coated nanoparticles. Conventional sol−gel and precipitation methods most often require an additional heating step at high temperature (>500 °C) to obtain crystalline phases,153 thus producing relatively large particles. Delacourt has developed a low-temperature direct precipitation method144,156−159 to yield LiFePO4 or LiMnPO4 directly in boiling water under ambient pressure. Aqueous solution mixtures of FeII sulfate FeSO4·7H2O and of

Figure 11. Polymer-assisted synthesis and performances of LiFePO4 electrodes, as reported by Wan et al. in ref 150. Reproduced with permission from ref 150. Copyright 2008 Wiley.

H3PO4 are, for instance, brought to a pH value close to neutrality by slowly adding LiOH. This low-temperature synthesis leads actually to the formation of defective LixFeyPO4 compositions showing off-stoichiometric amounts of Fe3+ and Li+ ions, Fe vacancies, and also antisite defects (in particular, Fe onto the lithium site). This resulted in a complete modification of the electrochemical properties, from a classical two-phase reaction for stoichiometric LiFePO4 to a full solid solution at room temperature for these defective powders.159 This has been confirmed and even strengthened recently by Hamelet160 who showed extensive cation-mixing, complete redistribution within the Li and Fe sites, and complex new electrochemical phenomena between 3.4 and 2.7 V versus Li. Hydrothermal syntheses are usually performed in water and at low temperature (∼100−200 °C) within an autoclave.145 The surface of LiFePO4 being prone to easy oxidation in aqueous media, reducing agents such as ascorbic acid, sucrose, and citric acid are generally used. The size of such particles I


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depends on the pH used, but thin platelets are mostly always formed with (010)Pnma as the basal plane (perpendicular to Li+ privileged diffusion direction).161,162 It has been found, too, that LiMnPO4 or LiCoPO4 particles have these diffusion channels within the platelets. Solvothermal reactions offer an interesting alternative, especially when the target compound or the precursors are sensitive to water. Organic solvents such as dimethoxyethane, ethanol, tetraethylene glycol, etc., were used:145 the obtained LiFePO4 particles show significant amounts of antisite defects, which can be eliminated by annealing at higher temperatures. Microwave-assisted solvothermal synthesis was recently proposed by Murugan:163 nanocrystallites (nanorods) of LiMPO4 (M = Fe, Mn, Co, Ni) were successfully obtained within much shorter reaction times (5−15 min) to complete the reaction at 300 °C. Syntheses in polyol media are performed at intermediate temperatures, that is, between 200 and 400 °C depending on the boiling point of the solvent (314 °C for tetraethylene glycol, 285 °C for triethylene glycol, and 245 °C for diethylene glycol).164,165 The reaction is controlled under reflux at atmospheric pressure and usually yields nanoparticles thanks to the reaction medium, which acts both as the solvent to dissolve the precursors and as a particle growth inhibitor. Higher reaction temperatures promote higher crystallinity and, depending on the nature of the polyol itself (its boiling point in particular), particles ranging from 20 to 50 nm are obtained. Recent interesting approaches included the use of templating techniques to produce hierarchically porous LiFePO4 electrodes,166 monolithic LiFePO4/C composites with well-defined macropores.167,168 Significant efforts have been recently devoted to original chemical routes aimed at producing LiFePO4 powders within a large spectrum of particle sizes and morphologies: egg-like shapes, hollow structures and barrel-like particles,169 nanospheres or nanoplates,170 monodisperse LiFePO4 microspheres,171,172 or single-crystalline nanowires173 (Figure 13). Armand, Tarascon, and Recham174−176 developed a new extensive approach to prepare materials unstable at high temperature. The use of ionic liquids in a so-called ionothermal route resulted in the formation of highly crystalline particles with all sorts of morphologies (Figure 13) depending on the anions used (TFSI−, BF4−, CF3SO3−, C(CN)3−, Cl−) and on the length of cationic chains: imidazolium, pyrollidinium, pyridinium. The method resembles that of inorganic molten

Figure 12. X-ray diffraction patterns (top) and galvanostatic discharge data at high rate (bottom) of LiFePO4 coated with an inorganic Li− P−O glass. Adapted with permission from ref 151. Copyright 2009 Nature.

Figure 13. SEM images illustrating various shapes of LiFePO4 crystals, as reported in refs 175 (left) and 169 (c−f on the right). Reproduced with permission from refs 169 and 175. Copyright 2009 American Chemical Society. J


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LiMnPO4 arise most likely more from its extremely low ionic and electrical conductivities and from structural distortions induced by the Jahn−Teller Mn3+ ions upon oxidation to MnPO4.156,195−199 As a result, many initial electrochemical reports showed poor capacity and reversibility156,200 despite promising data published by SONY in 2002.134,135 Drezen and Exnar demonstrated the benefits of reducing particle size to very small values together with the positive attributes, once again, of a thin carbon coating for improving the electrochemical performances of sol−gel prepared LiMnPO4201,202 (Figure 14). Using a polyol-type synthesis

salt synthesis, but it uses ionic liquids as a synthetic medium. Most ionic liquids are stable up to 250−300 °C, and the hydrophobicity (polarity) with the inorganic reactants, the viscosity, and the melting point can be adjusted. 3.2.3. Carbon Coating and Purity Control of LiFePO4. As mentioned previously, improvement of the electrochemical performance of LiFePO4 often arises from surface modifications. Numerous carbon-coated LiFePO4 materials were prepared using different synthesis methods,146,177−182 and Raman spectroscopy proved to be a quite sensitive technique to determine the nature and the quality of this carbon coating.183−185 In fact, visible Raman spectra give information on the carbon in-plane correlation length, whereas a UV Raman study is required to determine the sp3/sp2 ratio of the carbon.185 Electronic conductivity measurements are of course very sensitive to the carbon nature and especially to its order− disorder state. For lithium-deficient Li1−xFePO4 compositions, Fe2P2O7 was found to be often formed as an intermediate impurity before being reduced at higher temperature (>800 °C) into iron-rich phosphide materials (Fe2P, Fe75P15C10) whose conductivity is larger than 10−2 S cm−1 (vs less than 10−9 S cm−1 for LiFePO4).143,156 The presence of these iron phosphide impurities was spotted using a combination of Mössbauer, transmission electron microscopy, and X-ray photoelectron spectroscopies.186 The impurities form, in fact, a percolating nanonetwork within the grain boundaries, responsible for the enhanced conductivity. Fe and P deficient LiFe1−2xP1−xO4−δ materials were shown to deliver exceptional high rate electrochemical performances.151 Surface analyses performed with XPS revealed the formation of an ionically conductive glass similar to Li4P2O7 containing a second phase, probably of Fe3+-containing Li4P2O7-type. Note that the exact nature of this conductive surface is still under debate187 and will be at the origin of further research. Careful control of the purity of obtained LiFePO4 powders is of major importance, particularly at the industrial level, as the presence of impurities (Fe2P, Fe2O3, etc.) strongly affects the reversible capacity retention (especially vs graphite carbon negative electrodes), in particular at “high” temperature.188 Magnetic measurements were shown to be highly sensitive to low impurity concentrations.189−192 The intrinsic magnetic properties of LiFePO4 are well-known:114,119 it undergoes a transition to an ordered antiferromagnetic state at a Néel temperature of ∼50 °C. The isothermal plots of the magnetic moment versus applied magnetic field are then very sensitive to the presence of nanosized ferro- or ferrimagnetic impurities. Mössbauer spectroscopy analyses are also extremely powerful to detect the presence, the nature, and the quantity of Fecontaining impurities in LiFePO4.124,182 Delacourt identified,158 through high-resolution transmission electron microscopy and EDX analyses, the presence of crystalline NbOPO4 and/or an amorphous (Nb, Fe, C, O, P)-containing particles surrounding LiFePO4 for Li1−xNbxFePO4 samples that had been unsuccessfully doped. Noticeably, an elegant and original approach was proposed recently by Lepage to coat LiFePO4 with PEDOT conducting polymer at ambient temperature.193

Figure 14. Galvanostatic charge−discharge characteristics of LiMnPO4 vs Li for various particle sizes. Adapted with permission from ref 201. Copyright 2006 The Electrochemical Society.

method, they were able to prepare at 200 °C thin crystalline nanoparticles of LiMnPO4 ( 2, M = Zr, Nb) type materials sintered at 800 °C resulted in exceptional electronic conductivity, increased by 8 orders of magnitude, as compared to standard stoichiometric LiFePO4.212−214 A large debate began (Figure 16) at that time on the interpretation of these results: was the substitution effective? If substitution occurred, was it at the origin of this drastic increase in electronic conductivity? Indeed, such a Li1−xMz+xFePO4 (z > 2) stoichiometry would imply for charge compensation that iron valence would be less than Fe2+ or that vacancies would be formed. On the basis of the drastic conductivity increase, the hypothesis of a mixed valence (Fe2+/Fe3+) compound was considered by the authors, with the presence of dopants and lithium vacancies on the lithium site but without crystallographic evidence to support this hypothesis. Armand’s group reported immediately on experiments performed so as to discriminate between the effect of doping and that of carbon coming from the decomposition at high temperature of carbon-containing precursors and revealed that the conductivity of “doped LiFePO4” increased only when additional carbon is present.142 Nevertheless, other studies of high-temperature doped and undoped LiFePO 4 revealed high electronic conductivities143,144,158,162,215 due to metallic and or conductive carbon impurities (iron phosphides, iron phosphocarbides, niobium oxyphosphate, etc.).216 Calculations by Islam’s group showed that the accommodation of high valence cations on the Li and Fe sites within the olivine structure would in fact be highly unfavorable.217,218 Wagemaker et al., through the combined analysis of X-ray and neutron diffraction data, revealed that for Li1−xMz+xFePO4 compositions similar to those reported by Chiang’s group, the dopant (M = Zr, Nb, Cr) preferentially occupies the lithium site, although the quantity of dopants in the lattice was much smaller than expected.219 In contrast, compounds with nominal compositions corresponding to charge-compensated stoichiometries (i.e., Li1−xMz+xFePO4, x ≤ 0.03) are found to incorporate roughly all of the dopants preferentially in the Li site. In all cases, the incorporation of the aliovalent dopant was shown to be balanced by lithium vacancies and as a result by the presence of only Fe2+ ions in the lattice.

Figure 15. Investigation of thermal stability of Li x MnPO 4 compositions. Adapted with permission from ref 209. Copyright 2009 The Electrochemical Society.

Figure 16. Left: Schematic of elemental maps generated from TEM analysis showing the location of Fe2P (gray), LiFePO4 (yellow), and C/ Fe75P15C10 (blue). Reproduced with permission from ref 143. Copyright 2004 Nature Publishing Group. Right: Traces of NbOPO4 observed at the surface of LiFePO4.158 Adapted with permission from ref 158. Copyright 2006 Elsevier. L


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Figure 17. Left: In situ X-ray diffraction investigation of Li+ extraction mechanism from LiCoPO4. Reproduced with permission from ref 231. Copyright 2007 American Chemical Society. Right: Mix cation effect on the electrochemical properties of LiMnPO4. Reproduced with permission from refs 203 and 238. Copyright 2011 The Electrochemical Society and 2010 Elsevier.

To summarize, “dopants” can be incorporated in LiFePO4 if the target stoichiometry is properly charge balanced. Nevertheless, to our knowledge, these “doped” compounds do not exhibit increased conductivity as it was shown to come from metallic impurities and/or carbon. Compositions within the solid solution domain Li(Mny Fe 1−y )PO 4 were investigated early by Padhi and Yamada.11,132,133,206 The respective widths of the two Li+ extraction plateaus are directly proportional to the amounts of Fe and Mn to be oxidized, at 3.45 and 4.1 V versus Li, respectively. The increase in Mn content generates therefore higher global energy density, at the expense however of rate capability, and Yamada established a quite interesting phase diagram for the Li x (Mn y Fe 1−y )PO 4 (0 ≥ x, y ≤ 1) compositions.137,206 Whatever the concentration of manganese, the Mn3+/Mn2+ redox reaction occurs as a first-order phase transition between the Jahn−Teller-active Mn3+-rich phase and the Mn2+-rich phase. Nevertheless, as just mentioned for LiMnPO4, for compositions rich in manganese (y > 0.8) the materials suffer from strains. The Fe3+/Fe2+ redox reaction occurs as a first-order phase transition reaction only for y = 0, whereas two types of reactions (solid solution and first-order phase transition) are observed for manganese-substituted materials. The composition Li(Mn0.6Fe0.4)PO4 was early spotted by Yamada as the most promising one with a reversible capacity of 160 mA·h/g132 and a delivered energy density both larger than

that of LiMn2O4 in W·h/L and larger than that of LiCoO2 in W·h/kg (for a charge cutoff at 4.2 V). It was investigated in detail through in situ synchrotron X-ray diffraction by Bramnik,220,221 who proposed an alternative global reversible mechanism based on two successive two-phase reactions with only a small solid solution range for 0.55 ≤ x ≤ 0.67. Significant activity focused recently on LiFe1−xMnxPO4 compositions with higher Mn content so as to increase the energy density.222 Carbon-coated LiMn0.85Fe0.15PO4 particles show less severe dissolution problems over prolongated cycling.223 LiMn0.75Fe0.25PO4 nanorods grown on graphene show very satisfactory response at high discharge rates with excellent reversibility.224 Recently, Mauger’s group in collaboration with Yamada225 showed from magnetic analyses that for y ≤ 0.6, all of the Mn3+ ions in MnyFe1−yPO4 are in the high-spin state (S = 2), whereas for larger manganese concentration, Mn3+ ions undergo a transition to the low-spin state (S = 1). This partial spintransition for Mn3+ would be at the origin of the strains observed for deintercalated (MnxFe1−x)PO 4 (y > 0.8) compositions, and thus of the loss in electrochemical capacity. Given what has been found and achieved globally for the complex chemistry of LiFePO4, much still remains to be fully understood for Li(MnyFe1−y)PO4 compositions. The possible presence of intrinsic nonstoichiometry and structural defects226 and their influence on the electrochemical properties should be M


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traction along [100] and [010] and, more intriguing, slight elongation along [001] in Pnma description. The FeO6 octahedron undergoes quite severe distortion.119 The magnetic structures of both LiFePO4 and FePO4 show antiferromagnetic ordering below 52 and 125 K, respectively, as a result of complex superexchange and super superexchange interactions.114,119,244,245 The possible occurrence of structural defects within LiFePO4 was questioned and much debated following Chung’s report141 on possible doping by tiny amounts of metals supervalent to Li+ (Mg2+, Al3+, Ti4+, Zr4+, Nb5+), which were reported to increase the electronic conductivity by a factor of ∼108. The overall electronic conductivity of LiFePO4 was reported by Chiang and co-workers141,212,246 and Delacourt196 to be of ∼10−9 S cm−1 at 298 K. Zhou et al.214 were among the first to draw a rather precise description of the electronic structure of LiFePO4 through computation (GGA + U method) and optical (UV− vis−IR diffuse reflectance spectroscopy) band gap measurements. A significant band gap of 3.8−4.0 eV is induced by strong electron correlation at the transition metal, and it was argued that olivines were likely to be polaronic conductors, later demonstrated by Maxish using first-principles pseudopotential calculations247 and Ellis through T-controlled Mossbauer spectroscopy for the (1−x)LiFePO4−(x)FePO4 system248 who showed strong evidence of highly coupled ion-electron transport. First-principle methods were successfully used by Morgan et al.118 to envisage a strong anisotropy in the transport mechanism of Li+ in the LiFePO4 framework, soon after confirmed by Islam217 through atomistic simulation techniques. Dominant 1D diffusion along [010]Pnma with little crossover between the 1D channels occurs, although transport measurements on single crystals are in apparent contradiction.249 Importantly, Morgan pointed out too that this 1-D diffusion (D ≈ 10−9 cm2/s) was indeed rapid enough for use of LiFePO4 as a positive electrode with, however, possible rate problems due to defects blocking the 1-D channels,118 as discussed in more detail recently by Malik250 and Yang.251 The possible occurrence of Fe on Li sites, and hence reduced Li mobility within hydrothermally prepared LiFePO4, was early mentioned by Yang.154 The successful development of low temperature synthesis techniques aimed at producing nanoscale particles of LiFePO4 resulted in significant improvements in terms of kinetics at the expense of the tap density of the electrodes. Nuspl et al.279 and Delacourt et al. proposed that under a certain particle-size threshold (∼150 nm), carbon coating of LiFePO4 particles was no longer mandatory to obtain extremely good performances (147 mAh g−1 at 5C rate).157 This was later supported by Gaberscek280 through a comparative study of extensive data published by independent groups who concluded that the effect of carbon coating on LiFePO4 was marginal due to the smaller ionic conductivity (∼10−11 S cm−1 at room temperature) as compared to the electronic conductivity (∼10−9 S cm−1 at room temperature). The discovery by Delacourt et al.252 (Figure 18) and Dodd et 253 al. of a complete LixFePO4 solid solution (0 ≤ x ≤ 1) upon heating xLiFePO4 + (1−x)FePO4 mixtures at ∼350 °C under N2 allowed the determination of the crystal structures of triphylite type compositions with fractional occupation of Li sites254 (Figure 19). Nishimura et al.255 were hence able to fully confirm the zigzag overall 1-D mechanism of Li+ transport envisaged by Morgan and Islam and reported on an

addressed carefully as well as their thermal stability in the delithiated state. Other olivine-type compositions have been/are investigated. LiCoPO4, first identified by Amine138 as a high voltage positive electrode material (∼4.8 V vs Li), caught the attention of several other research groups,220,221,227−232 with only limited success until recently. However, Bramnik pointed out, from in situ synchrotron X-ray diffraction, several interesting phenomena231 (Figure 17). The first charge occurs as two successive Li+ extraction plateaus, revealing the existence of an intermediate phase Li0.7CoPO4. Recently, Moreau reported such an intermediate composition (Na0.7FePO4) upon sodium extraction from the triphylite-type NaFePO4 (isolated after Na+ insertion into heterosite FePO4).233 The completely delithiated phase CoPO4 appeared to be unstable in air and underwent amorphization. Moreover, the extraction of Li+ (at 4.8 V vs Li) is accompanied by significant electrolyte degradation. As reported by Jow and co-workers,234 the cyclability of LiCoPO4-based electrodes can be significantly improved through Fe3+ substitution on the Li and Co sites, which appears to stabilize the charged CoPO4 structure. Multicomponent olivines LiFe1−x−yMnxCoyPO4235−237 and Li1+0.5xCo1−xVx(PO4)1+0.5x (x = 0, 0.05, 0.10)238 have recently caught attention. Wang et al. reported drastic improvements of cyclability of LiCoPO4-type positive electrodes, with 115 mA·h/g maintained after 25 cycles at C/10 for Li1.025Co0.95V0.05(PO4)1.025/C (Figure 17). The group of K. Kang investigated in particular the compositions LiFe1/3Mn1/3Co1/3PO4, prepared via a coprecipitation of mixed transition metal oxalates,239 and provided experimental evidence of reversible Li+ extraction through a single phase LixFe1/3Mn1/3Co1/3PO4 solid solution.240 Whittingham241 and Kang242 focused on substituting vanadium for iron according to LiFe1−3y/2VyPO4 and suggest enhancement of electrochemical performances. 3.5. Intrinsic Physicochemical Properties of LiFePO4 and Mechanism of Li+ Extraction

The industrial importance of olivine-type positive electrode materials and their puzzling exceptional power capability despite rather poor intrinsic transport properties have prompted an immense research activity in academia on the physicochemical characteristics of LiFePO4. We will give below an overview of the literature addressing the main properties of this fascinating material, in terms of crystal structure, electron and/or Li+ transport, reactivity with air, and mechanism of electrochemical Li+ extraction/insertion. 3.5.1. Crystal Structure, Defects, e− Transport, Li+ Diffusion. The crystal structure of LiFePO 4 is quite straightforward, described with an orthorhombic unit-cell in Pmnb or Pnma space groups that generate four formula units per cell. As recognized early by Yamada,137 all of the oxygen ions form strong covalent bonds with phosphorus to form the phosphate groups and generate a stable three-dimensional framework that favors stable operation at relatively high temperatures and safety under abusive conditions. A comprehensive description of the structural variations in the lithiophylite-triphylite series is given in ref 243. In LiFePO4, the average Fe−O distance (2.157 Å) is significantly higher than what would be expected for Fe2+ in octahedral coordination as a result of one edge sharing between FeO6 and PO4. The delithiated phase FePO4 possesses the same framework as LiFePO4 with anisotropic lattice parameter variations: conN


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substantial dependence upon crystal size and morphology, hence sometimes apparent contradictory results from different groups.254,256−262 3.5.2. Reactivity with Moisture and/or Air. The synthesis of the FeII-containing LiFePO4 requires careful control of the surrounding atmosphere, which can lead to either formation of conductive metal phosphides (FeP, Fe2P, etc.) under reducing conditions (in the presence of carbon or reducing gases)143,144,186 or FeIII-containing phases (LiFePO4OH, Li3Fe2(PO4)3, FePO4, LiFeP2O7, Fe2O3, etc.) under slightly oxidizing environment. Synthesis techniques developed so as to yield submicrometer particles of LiFePO4 with higher kinetics have generated powders with enhanced surface reactivity under ambient atmosphere (moisture and/or air). As a consequence, undesired surface species such as Li2CO3 and Li3PO4 may form, in particular if the surface of LiFePO4 is not covered by carbon-coating.263 A comprehensive study on the reactivity of LiFePO4 nanoparticles with air and/or moisture or water was reported by the groups of Guyomard, Kanno, and Yamada264−267 (Figure 20). It was proposed that storage of LiFePO4−C nanocomposites at 120 °C in hot air (for ∼30 days) led to partial delithiation and to the formation of an amorphous ferric phosphate side-phase LixFePO4(OH)x through a water-driven aging mechanism. The resulting powder presents degraded electrochemical performances; consequently, much care has to

Figure 18. Phase distribution diagram on cooling for LixFePO4 (0 ≤ x ≤ 1) established from temperature-controlled X-ray diffraction data. Reproduced with permission from ref 252. Copyright 2005 Nature Publishing Group.

Figure 19. Experimental visualization of zigzag Li+ transport in Li0.6FePO4. Reproduced with permission from ref 255. Copyright 2008 Nature Publishing Group. Neutron diffraction pattern refinement of single-phase Li0.5FePO4 at 350 °C. Reproduced with permission from ref 254. Copyright 2005 Elsevier.

“experimental visualization of lithium diffusion in LixFePO4” by means of refinements of anisotropic displacement parameters in the crystal structure of Li0.6FePO4 extracted from neutron diffraction data (Figure 19). This paved the way to numerous studies on the thermodynamics and kinetics of LixFePO4 phase formation/stabilization upon heating/cooling, which reveal

Figure 20. Lattice constant changes for Li1−xFePO4 after various air contact experiments (top) and overview of the consequences of atmosphere exposure of olivine LiFePO4 at 120 °C (bottom). Adapted with permission from refs 264 and 267. Copyright 2008 The Electrochemical Society and 2011 Elsevier. O


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many research groups worldwide has generated an immense activity and passionate debates on the understanding of the mechanism(s) involved. In a seminal series of papers,61,127−129 Andersson and Thomas were the first to explore these issues through in situ X-ray diffraction, Mö ssbauer spectroscopy, and neutron diffraction. Electrodes at various states of charge were carefully analyzed and described as two-phase mixtures of LiFePO4 and FePO4 with respective ratios in good accordance with the nominal Li+ extracted electrochemically. Andersson mentioned early (in 2000) that higher efficiency of LiFePO4 electrodes at higher temperatures would make it an “almost ideal positive electrode materials in electric vehicle applications”. In these early days, only 80% of the theoretical capacity could be reached for micrometric-size particles: Andersson and Thomas tried to provide a reasonable explanation to the “source of firstcycle capacity loss”129 by proposing, at the particle level, two models labeled as “mosaic” (which considers the feasibility of the extraction/reinsertion to occur at many sites within a given particle) and “radial”, similar to the core−shell model proposed by Padhi11 later modeled mathematically by Srinivasan and Newman.273,274 Importantly, Srinivasan was the first to envisage concentration ranges of single-phase regions LiαFePO4 (0 ≤ α ≤ 0.02) and Li1−βFePO4 (0 ≤ β ≤ 0.047) needed to define the phase boundaries in the shrinking core model and to account for the changes of potential with Li concentration within these regions.273 Soon after, Yamada,189 Delacourt,252,254 and Dodd253,275 brought experimental evidence of LixFePO4 solid solution domains. Yamada demonstrated, from X-ray diffraction data analysis on partially charged LiFePO4/FePO4 electrodes, slight deviation of the orthorhombic unit cell constants from those of the stoichiometric end members of LiFePO4 and FePO4.189 Neutron diffraction data of a two-phase mixture of a nominal Li0.5FePO4 electrode confirmed the coexistence of nonstoichiometric Li0.05FePO4 and Li0.89FePO4 compositions.276 The properties of nanoscale LiFePO4 particles were closely investigated by Yamada276,281 and Chiang282 who demonstrated a size-dependent miscibility gap, that is, maximum values of α and β in the solid solution domains LiαFePO4 and Li1‑βFePO4 when the particle size is downsized to ∼30−40 nm. Meanwhile, Gibot et al.159 reported that nanosized particles with high crystalline disorder and nonstoichiometry lead to a full solidsolution Li+ extraction/insertion mechanism at RT with a sloping voltage variation and continuous lattice parameters change for the whole LixFeyPO4 composition range. As pointed out by Kobayashi et al.,281 anisotropic surface redox effects in nanosized particles should not be neglected as they can lead to a wide dispersion of voltage values.283−285 Moreover, as was recently precisely argued by Wagemaker et al.,286 the thermodynamics of nanosized LiFePO4 is distinctly different from that of the bulk. For particle sizes below 35 nm, LixFePO4 solubility limits depend strongly on the overall composition. Note that LixFePO4 compositions isolated at ∼350 °C were found to transform back upon cooling to complex mixtures of LiαFePO4 and LiβFePO4 metastable phases,254 among which Li0.6FePO4 and Li0.33FePO4 appeared to be the most stable.257 Chen et al.161 proposed a first experimental visualization of coexisting LiFePO4 and FePO4 domains within micrometer platelet-like crystals. They spotted the occurrence of disordered transition zones at the phase boundary and clearly suggested that the well-adopted core shell model did not apply to

be taken to implement carefully optimum storage conditions of LiFePO4−C nanopowders.267 Xia et al.268 found significant traces of Fe2O3 in hot-air-exposed (200 °C) LiFePO4 and puzzling additional redox phenomena at ∼2.7 V versus Li. In parallel, Hamelet et al. found that gentle annealing of LiFePO4 in air (100−400 °C) would generate major cation distribution rearrangements within the LiFePO4 structure and completely new mechanisms of Li+ insertion/extraction.269 Combined investigations with Mössbauer and NMR spectroscopies besides T-controlled X-ray and neutron diffraction demonstrated that the oxidation of LiFePO4 in dry air up to ∼350 °C (accompanied by significant mass gain) leads to the extrusion of Fe toward the particle surface and hence to Fe vacancies (Figure 21). A pure FeIII-containing olivine-type

Figure 21. TG variation and lattice volume expansion upon exposure of LiFePO4 in air as a function of temperature (top) and galvanostatic charge discharge characteristics of the resulting highly defective LixFeyPO4 powder (bottom). Adapted with permission from refs 269 and 270. Copyright 2009 The Royal Society of Chemistry and 2011 American Chemical Society.

phase was obtained at ∼350 °C, where 1/3 of the iron initially contained in the LiFePO4 pristine particles had been extruded to form nanoclusters of Fe2O3. Complex and new redox phenomena are then associated with the Li+ insertion/ extraction into/from these new highly defective “LiFe1−yPO4” compositions.269 It was very recently demonstrated270 that, in such air oxidized LiFe 1−y PO4 powders, Fe atoms are redistributed onto the Li and Fe crystallographic sites of the triphylite framework and lead, when significant amounts of Fe vacancies have been created, to complex superstructures (Figure 21) similar to what had been previously described for the natural transformation of fayalite to laihunite.271,272 3.5.3. Mechanism. The apparent contradiction between the mostly insulating nature of both LiFePO4 and FePO4 compositions involved in the process of Li+ extraction/insertion and the extremely fast electrochemical response obtained by P


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Figure 22. Three main types of Li+ extraction mechanisms illustrated. Adapted with permission from refs 287, 161, and 278. Copyright 2008 Nature Publishing Group, 2006 The Electrochemical Society, and 2006 American Chemical Society.

extraction of Li+ occurs by proposing the so-called “dominocascade” model,287 seriously questioned by Ramana288 and Badi289 but recently confirmed by Brunetti.290 From the analysis of the FWHM (full width at half-maximum) of X-ray diffraction peaks on electrochemically deintercalated samples and in coherence with HRTEM studies, the domino-cascade model proposes that during electrochemical cycling, single particles are composed either of LiFePO4 or of FePO4 but do not consist of both coexisting phases as a result of structural constraints and of high concentration of charge carriers occurring at the reaction interface (Figure 22). It appears then that the models proposed may strongly depend on the morphology of the particles, on their size distributions, and on the type of Li+ extraction performed (chemically, electrochemically). We argue then that there might not be major contradictions between the results found by Chen (big crystals, twinning, and effects produced so as to compensate for the unit-cell volume change at the interface), Laffont (chemical

individual crystallites (Figure 22). Prosini considered meanwhile277 a model within which the delithiated phase grows from the center of the particle so as to reduce the stress originating from volume contraction associated with the lattice mismatch between the two end-member phases. Individual particles (prepared by chemical extraction of Li+ from LiFePO4) composed of both LiFePO4 and FePO4 phases were observed by Laffont et al.278 by means of STEM and HREELS techniques: in submicrometer platelet-shaped crystals, anisotropic delithiation occurs along the [010]Pnma direction giving rise to a core of FePO4 surrounded by a shell of LiFePO4 (Figure 22) in accordance with Prosini. The nanointerface between single-phase areas of LiFePO4 or FePO4 was described as a juxtaposition of the two end-members rather than as a LixFePO4 solid solution278 in contrast to the highly disordered zone observed by Chen et al.161 Delmas et al. brought an important original contribution to the passionate debates about how the electrochemical Q


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Figure 23. Highly divided hydrated FePO4·nH2O powders and their corresponding electrochemical response vs Li. Reproduced with permission from ref 319. Copyright 2009 The Electrochemical Society.

analysis of the experimental aging data of available commercial cells is underway in several groups and companies.

extraction resulting in numerous nucleation centers and LFPFP interfaces), and Delmas (small particles, electrochemical extraction). All of these experimental observations, thermodynamic considerations, and proposed models called for in situ studies (synchrotron X-ray diffraction, X-ray absorption, etc.) of electrodes during battery operation, which were realized by several groups. Chang et al.291 reported on a delayed structural transformation as the result of slow nucleation kinetics of the resulting FePO4 phase. Chiang et al.292,293 envisaged that the delay in the appearance of crystallized FePO4 was due to the existence of a transient intermediate amorphous “FePO4” component, the amount of which being dependent on (i) the particle size, (ii) the applied electrical overpotential, and (iii) the amplitude of the misfit strain between the lithiated and delithiated crystalline phases. These interpretations were seriously questioned by Leriche et al.,294 who indeed observed an apparent delay in the crystallization of FePO4 upon charging but no formation of amorphous FePO4. It is most likely that theorists and experimentalists will continue to debate and argue vividly around the intriguing processes associated with the reversible Li+ extraction/insertion from LiFePO4, given the fact that it is extremely sampledependent (size, shape, nonstoichiometry, crystallinity, etc.). Most recent contributions point out that (i) antisite defects in nanoparticles obstruct Li+ diffusion and thus are detrimental to electrochemical performance,250,289 (ii) coating LiFePO4 with a fast Li+ conducting inorganic matrix resulted in extremely fast electrochemical response,295 and (iii) phase separation is dynamically suppressed during normal battery operation, that is, above a critical current density for which the particles fill homogeneously.296 In terms of real utilization of LiFePO4 in commercial cells for long-life applications (i.e., as a positive electrode material in future electric and hybrid electric vehicles), there is a growing need to implement reliable global models so as to improve battery-pack design and reach final integration to a battery management system on board.297−299 To this end, in-depth

4. ALTERNATIVE POLYANIONIC STRUCTURES AND COMPOSITIONS: HYDRATED PHOSPHATES, DIPHOSPHATES, ALLUAUDITES, SILICATES, AND BORATES Besides the huge amount of work and great hopes placed on the olivine-type LiFePO4 to stand as a material of choice for electric vehicle applications, alternative compositions and structures with similar theoretical capacities have been investigated: other crystalline varieties of FePO4, “hydrated” phosphates, silicates, alluaudites and garnets, and borates. Interesting properties could be obtained for some of them, mostly using Fe as the active transition metal element. 4.1. Fe3+/Fe2+ Couple in Amorphous or Crystalline Iron Phosphates: FePO4·nH2O

Several studies focused on evaluating the electrochemical properties of other iron phosphates, among which the allotropic forms of heterosite FePO4 (H-FePO4)127,119 obtained upon lithium removal from LiFePO4. Recently, successful sodium insertion into heterosite FePO4 has been demonstrated.233 Besides amorphous FePO4, numerous crystalline allotropes of FePO4, metastable at ambient temperature, are known: αquartz FePO 4 (Q-FePO 4 30 0 ), tridymite FePO 4 (TFePO 4301,302), a high-pressure form, isostructural with CrVO4,303,304 as well as FePO4-II and FePO4-III, obtained upon dehydration of P-FePO4·2H2O (phosphosiderite) and SFePO4·2H2O (strengite), respectively.301,302,305 For most of these anhydrous allotropes, the electrochemical response upon Li+ insertion/extraction is worse than that of heterosite FePO4,304−309 the best performance being reported by Croce310 for a α-quartz FePO4/5% RuO2 composite exhibiting a capacity of 110 mA·h g−1 at a rate of C/3 and by Shi311 for amorphous nanoporous FePO4 particles obtained through a template synthesis method (120 mA·h g−1 at C/10 and 65 mA·h g−1 at 3C). Recent contributions include virus-assisted R


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synthesis of amorphous FePO4 nanowires312 or carbon nanotubes-amorphous FePO4 core−shell nanowires.313 Besides the anhydrous FePO4 allotropes, hydrated forms of formula FePO4·nH2O were also envisaged.305,314 Because the inductive effect of the phosphate groups, the average potential associated with the Fe3+/Fe2+ redox couple is located at ∼3 V versus Li+/Li. Several crystalline allotropes of formula FePO4·∼2H2O are reported in the literature: P-FePO4·2H2O (phosphosiderite) and S-FePO4·2H2O (strengite), which crystallize in the space groups P21/n and Pbca, respectively,305,315,316 and M-FePO4·2.3H2O (metastrengite I), for which the structure has not, to our knowledge, been solved so far.316 Such compounds are easily prepared from precipitation in aqueous media, which leads to powders of submicrometer particle size with high surface area.144,302 Despite their slightly lower theoretical specific capacity of ca. 140 mA·h g−1 as compared to that of the anhydrous FePO4, the electrochemical responses for both crystalline and amorphous FePO4·nH2O are definitely worth mentioning.306314,317,318 For instance, 0.9 Li could be inserted into amorphous FePO4·1.6H2O at C/20 while 0.6 Li only for crystalline PFePO4·2H2O at the same charge/discharge rate without knowing, at this point, whether these differences are due to different kinetics (difference in particle size for instance), or if lithium insertion mechanisms into amorphous and crystalline forms differ from a thermodynamic viewpoint (Figure 23). The beneficial role of constitutional “water”, which may act as a promoter for faster ionic diffusion, has been proposed.314 Within the operating voltage window used (2−4 V vs Li+/Li), these water molecules look unaffected as very good electrochemical stability was demonstrated for extensive cycling.314 This stability suggests that, as for the crystalline form of FePO4·2H2O, where the 6-fold coordination of Fe is ensured by 4 oxygen belonging to PO4 groups and 2 oxygen belonging to H2O groups, a similar local arrangement occurs for the amorphous FePO4·nH2O powders. Delacourt et al.319 investigated in detail the mechanisms of electrochemical reaction within amorphous and crystalline forms of FePO4·nH2O through combined techniques such as in situ X-ray diffraction, Mö ssbauer, and X-ray absorption spectroscopies. The crystalline forms undergo two-phase lithium-insertion reactions giving rise to isosbestic points on the superposition of normalized XAS spectra. Amorphous FePO4·nH2O compositions are described as disordered solid mixtures of [Fe(PO4)4H2O2] clusters: such a description accounts for both the presence of isosbestic points on the superposition of XAS spectra recorded at different stages of Li insertion and the sloping open-circuit potential. All of those compounds exhibit decent electrochemical properties with capacities ranging between 120 and 140 mA·h/g, and pretty good capacity retention. These compounds are interesting candidates for lithium−metal polymer batteries because of their moderate potential, and the sloping potential variation upon discharge/charge offers advantages for state of charge monitoring. The obtained values are 3.0 and 3.2 V for amorphous FePO4·2H2O and Fe4(P2O7)3·4H2O, respectively. This difference may arise from different local environments around Fe atoms.

4.2. V4+/V3+, Ti4+/Ti3+, and Fe3+/Fe2+ Redox Couples in Diphosphates and Diarsenates LixMX2O7 (M = Fe, V, Ti; X = P, As) and Fe4(P2O7)3·nH2O

As for the “hydrated” FePO4·nH2O compositions, a comparative study of Fe4(P2O7)3 and Fe4(P2O7)3·4H2O was undertaken.314 Possible electrochemical insertion of lithium into Fe4(P2O7)3 had already been mentioned and discussed in terms of the relatively high position of the Fe3+/Fe2+ redox couple versus Li.12 The theoretical capacity (∼144 mA·h g−1) and the operating voltage (∼3.1 V vs Li+/Li) make it an interesting positive electrode material if the 4 FeIII atoms per formula unit can be reduced to FeII. Only partial reduction was reached (3 Li+ inserted for 4 FeIII reducible cations) for amorphous Fe4(P2O7)3 with, noticeably, much better response than for crystalline Fe4(P2O7)3. An amorphous powder of composition Fe4(P2O7)3·4H2O showed superior electrochemical capacities close to the theoretical one and, interestingly, an average voltage (3.2 V vs Li+/Li) significantly higher than that encountered for FePO4·nH2O (3.0 V vs Li+/Li).314 This higher voltage was interpreted, in terms of the inductive effect provided by the PO4 groups, as the consequence of higher P/Fe ratio in Fe4(P2O7)3·nH2O as compared to FePO4·nH2O.47 LixMX2O7 (M = Ti, Fe, V, Mn) compositions320,321 are clearly, at first sight, non competitive positive electrode materials in terms of theoretical capacities to be delivered because of the weight penalty of two phosphate groups per transition metal. They provide, however, an interesting panel of crystal structure arrangements and electrochemical insertion mechanisms that deserve to be mentioned here. The recent discovery by Nishimura322 of the new lithiated diphosphates Li2FeIIP2O7 (isostructural with Li2MnP2O7323) that operate at 3.5 V versus Li+/Li (Figure 24) has prompted renewed interest, in particular for substituted Li 2 Fe 1−x Mn x P 2 O 7 324 and Li2CoP2O7325 compositions into which the investigation of the oxidation of M2+ to M4+ for a theoretical capacity of 220 mA·h g−1 deserves special attention. The electrochemical performance of Li2Fe1−xMnxP2O7 increases with the iron concentration: the highest capacity obtained being ∼80% of the theoretical capacity expected for one lithium deintercalated, that of Li2MnP2O7 being negligible. From experimental and ab initio computational results, the extraction of the second lithium per redox center, associated with the M3+/M4+ redox couple, would occur at high voltage (>5 V vs Li+/Li) and thus require development of electrolytes stable in larger potential windows.324 Li2CoP2O7 is electrochemically active with an average voltage around 4.9 V versus Li+/Li, but also a very limited capacity in discharge. In an early comparative study, Wurm326 described the preparation of LiFeP2O7, LiFeAs2O7, and LiVP2O7 as pure microcrystalline powders via an aqueous solution route that allowed the formation of finely dispersed small particles. The accessible redox couples are Fe3+/Fe2+ for Li1+xFeX2O7 and, interestingly, V4+/V3+ and V3+/V2+ for Li1±xVP2O7. LiFeP2O7 and LiVP2O7 are isostructural327−331 and crystallize in the space group P21. Each FeO6 (or VO6) octahedron is linked to five different P2O7 groups, one of them acting as a “chelating” sequence around Fe (or V). LiFeAs2O7 adopts its own crystal structure,394 described in the space group C2. There is in this case no bidentate corner sharing, and thus each FeO6 octahedron is linked to six different As2O7 groups. Padhi et al. showed first12 that the electrochemical insertion of lithium into LiFeP2O7 occurred at 2.9 V versus Li+/Li, that S


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processes, at 2.63 and 2.57 V versus Li+/Li. This constitutes the highest operating voltage reported so far for the Ti4+/Ti3+ redox couple in Ti-phosphates. The formation of Li0.96TiP2O7 at the end of discharge corresponds to a global unit cell volume expansion of ΔV/V = +4, 4% as compared to the pristine material TiP2O7. As for many polyanionic frameworks, excellent reversibility was demonstrated as they easily accommodate relatively large volume expansions/contractions over extensive electrochemical cycling. Alternative interesting diphosphates compositions have been investigated recently, such as LiFe1.5P2O7,337 Li2VOP2O7,338 LiCrP 2 O 7 , 339 FeH 2 P 2 O 7 , 340 Li 4+x Co 2−x (P 2 O 7 ) 2 , 341 and Li9V3(P2O7)3(PO4)2.342 4.3. Transition Metal Silicates Li2MSiO4 (M = Fe, Mn, Co)

The raw cost of materials (Fe, Si, O) and the quest (still to be achieved/demonstrated) for the exchange of two electrons per transition metal have prompted significant research efforts on “tetrahedral” silicates Li2MSiO4 (M = Fe, Mn, Co) since the first reports of reversible lithium extraction from Li2FeSiO4 by Armand343 and collaborators344,345 (Figure 25). The low

Figure 24. Galvanostatic response of Li+ extraction/insertion from Li2FeP2O7 vs Li and schematic representation of the crystal structure of Li2FeP2O7. Adapted with permission from refs 322 and 324. Copyright 2010 American Chemical Society and 2011 American Chemical Society.

is, at a slightly higher operating voltage than for Li3Fe2(PO4)3 but with limited experimental capacity attained. Wurm showed that the electrochemical activity of these materials is drastically enhanced by an intimate mixing of finely divided particles of pristine materials with conductive carbon, through ball milling.326 Crystalline LiMX2O7 (M = Fe, V; X = P, As) particles react electrochemically with lithium through twophase mechanisms, characterized by insertion plateaus located at 2.90 V for LiFeP2O7 (Fe3+/Fe2+ couple), 1.99 V for LiVP2O7 (V3+/V2+ couple), and 2.55 V for LiFeAs2O7 (Fe3+/Fe2+ couple) on first discharge. The Li-driven second phase is crystalline, with a larger unit-cell in the case of isostructural Li1+xMP2O7 (M = Fe, V). As for Li3V2(PO4)3, LiVP2O7 offers as well the possibility of operating on the V4+/V3+ redox couple if lithium is extracted. The electrochemical extraction of one lithium from LiVP2O7 occurs as a two-phase process at a relatively high equilibrium potential of 4.26 V versus Li+/Li. The compound obtained at the end of the oxidation (either electrochemically or chemically) is lithium-free, and the transformation between LiVP2O7 and VP2O7 is displacive, that is, without breaking of V−O and P−O bonds. The crystal structure of VP2O7 was determined from synchrotron X-ray powder diffraction.332 As compared to LiVP2O7, a large cell distortion occurs in VP2O7 due to a modification of the shape of the tunnels that contained lithium ions along with an oxidation of the vanadium VIII to VIV that induces shorter V−O bond lengths. Barker showed later333 that carefully optimized carbothermal synthesis procedures could yield LiVP2O7 powders with excellent capacity retention at moderate cycling regimes. Patoux334 showed the reversible insertion of lithium into LixTiP2O7335,336 that proceeds through two reversible redox

Figure 25. Crystal structure representations of two polymorphs of Li2FeSiO4360 and first I = f(E) plot of Li+ extraction/insertion between Li2FeSiO4 and LiFeSiO4.345 Reproduced with permission from refs 360 and 345. Copyright 2009 American Chemical Society and 2005 Elsevier.

intrinsic conductivity of Li2FeSiO4 has been overcome not only by carbon coating but also by lowering the particle size, resulting in satisfactory performance in terms of capacity and cycle life.346−350 These results have highlighted the potential of Li 2 FeSiO 4 or even higher-operating voltage materials, Li2Fe1−xMnxSiO4,351 as new positive electrode materials for Li-ion batteries. Until only recently, the crystal chemistry of Li2FeSiO4 remained quite ambiguous due to its rich polymorphism and hence to the difficulties encountered in obtaining single phase T


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samples. The seminal study of Tarte352 and the early publication of Nyten345 proposed that Li2FeSiO4 is isostructural with β-Li3PO4, that is, crystallizing in the orthorhombic space group Pmn21 with lattice parameters a = 6.27 Å, b = 5.33 Å, c = 5.01 Å. As first revealed by Quoirin,353−355 the indexation given in ref 345 was highly questionable, and it was revealed that Li2FeSiO4 underwent a complex series of phase transformations: the Cmma space group with a = 10.66 Å, b = 12.54 Å, c = 5.02 Å was thus proposed for Li2FeSiO4 annealed at 800 °C. Nishimura356 determined the crystal structure of Li2FeSiO4 (synthesized from a ceramic-type route at 800 °C) in monoclinic symmetry with a = 8.23 Å, b = 5.02 Å, c = 8.23 Å, β = 99.20°, which was confirmed by Boulineau 357 and Sirisopanaporn358 who found the existence and solved the crystal structure of a new metastable polymorph, obtained by rapid quenching at ambient temperature from 900 °C. The structural models describing Li2FeSiO4 are derived from Li3PO4-based structures, in which one-half of the tetrahedral sites, generated by a distorted hexagonal close packing of oxygen atoms, are occupied by cations. Li3PO4 itself crystallizes in two main groups of polymorphs (β and γ), which differ in their respective orientations of filled tetrahedra: all T+ (oriented upward) in the low-temperature β form, T+ and T − (oriented downward) for the high-temperature γ form360 (Figures 25 and 26). In more complicated chemical systems (three types of

Figure 27. Polymorphism in Li2MSiO4 tetrahedral structures, computed by DFT calculations, in ref 372 Reproduced with permission from ref 372. Copyright 2011 American Chemical Society.

the synthesis of single phase samples of Li2FeSiO4 polymorphs, but also very similar electrochemical properties (voltage, volume variation, and electronic structure). Minimization of the electrostatic repulsions between Fe3+ and Si4+ induces the 2D to 3D transformation upon lithium deintercalation from Li2FeSiO4 polymorphs, whereas the deintercalation of the second lithium (Fe3+/Fe4+ redox couple) is predicted to occur at a very high voltage (4.7 V vs Li+/Li) with severe structural distortions, both being detrimental to reversible cycling of the second lithium ion. Islam has shown that lithium diffusion involves different pathways in zigzag between the two lithium sites of this 3D framework stabilized upon cycling, with octahedral sites sharing faces with the LiO4 tetrahedra as intermediate sites.373 One of the peculiarities of this structural family is its quite poor intrinsic conductivity, due to isolated FeO4 tetrahedra and tightly bound Li+ cations in tetrahedral sites. As a consequence, most of the electrochemical data reported so far were recorded at 60 °C374,375 so as to favor easier ion and electron transport. Recent work has demonstrated also that ionic liquid electrolytes are particularly suitable for the cycling of these materials at elevated temperature.376 Various synthesis procedures have been developed, either by direct precipitation in H2O under ambient pressure,355 by hydrothermal reaction at ∼200 °C,368,376 by a Peccini sol−gel process,368 by hydrothermal-assisted sol−gel process,347 by ceramic synthesis in Ar-filled sealed tubes,358 under microwaveassisted solvothermal conditions,377 by solution-polymerization approach,348 or by combination of spray pyrolysis and wet ballmilling followed by annealing.378 The crystallization of phase pure Li2FeSiO4 is relatively tricky as one has to avoid the presence of undesired lithium-silicate phases such as Li2SiO3 or the partial oxidation of FeII into FeIII. Moreover, Li2FeSiO4 is highly sensitive to air (O2 and moisture)379 and should be handled carefully in inert atmosphere for optimal storage. The mechanism of lithium extraction/insertion in Li2FeSiO4 is rather complicated and not fully understood yet. However, a first important input was provided by Nyten380 from in situ X-

Figure 26. 6Li MAS NMR spectra of Pmn21 (LFS@400 sample), Pmnb (LFS@900 sample), and P21/n polymorphs (LFS@700 sample).359 Reproduced with permission from ref 359. Copyright 2011 American Chemical Society.

cations, for instance), the structures of Li2MSiO4 analogues (M = Zn, Mn, Mg, and Co) have been reported to adopt “simple” β-type or γ-type structures or their distorted derivatives.361−366 The Li2MSiO4 family offers then a very rich structural chemistry with many subtle variations between the connectivity of tetrahedral sites occupied by Li+, Si4+, and M2+ (Figure 27). Such complex polymorphism is encountered as well for the Mn and Co analogues Li2MnSiO4 and Li2CoSiO4.367−371 The relative stability and electrochemical properties of various Li2FeSiO4 polymorphs were very recently investigated from first-principles calculations.372 All of them were shown to have very similar energies, which explains the difficulties to control U


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composite, obtained using a solution-polymerization approach, is performed up to 4.8 V versus Li+/Li. Nevertheless, as was also predicted by calculations,372 slow capacity decay is observed versus a smaller potential window due to irreversible structural rearrangements. Armand et al.387,388 very recently proposed that nitrogen and fluorine substitution for oxygen in Li2FeSiO4 could be interesting to investigate in detail as both would induce a decrease in the second lithium deintercalation voltage associated with the Fe3+/Fe4+ redox couple (for instance, from 4.86 V in Li2FeSiO4 to 4.1 V in Li2FeSiO4N). Nevertheless, fluorine substitution worsens the reversible capacity in contrast to nitrogen that would participate in the redox processes.

ray diffraction and ex situ Mössbauer spectroscopy of charged and discharged compositions (Figure 28). One of the peculiar

4.4. Na3Fe3(PO4)4 and Alluaudite Phases

Na3Fe3(PO4)4 is a layered material that was recently shown to accommodate not only sodium ions but also lithium ions.389,390 Around 1.5 Li (Na) could be reversibly intercalated and deintercalated, with, interestingly, better diffusion evidenced for sodium ions. (Na1−xLix)MnFe2(PO4)3 alluaudite phases, prepared by sol−gel reaction and characterized by a threedimensional structure with two sets of tunnels, were tested as a positive electrode in lithium and sodium batteries; reasonable performances could be obtained for NaMnFe2(PO4)3 in lithium batteries with the exchange of 1.2 Li+ ions (about 80 mA·h/g), whereas in sodium cells the reversible capacity was lower (about 50 mA·h/g) (Figure 29).391 Lithium-rich phases exhibit smaller electrochemical reversible capacity as compared to the nonlithiated NaMnFe2(PO4)3.392

Figure 28. Charge−discharge characteristics of Li2FeSiO4 and Li2MnSiO4 up to 4.5 V vs Li. Adapted with permission from ref 377. Copyright 2010 American Chemical Society.

properties of Li2FeSiO4 is the discrepancy observed between the first charge and discharge plateau, located at 3.1 and 2.8 V versus Li+/Li, respectively. This latter operating voltage is the one experimentally observed by several authors for subsequent cycles. Nyten proposed,380 and it was recently supported by calculations,372 that the observed lowering of the potential plateau from 3.10 to 2.80 V during the first cycle could be explained by a structural rearrangement in which some of the Li+ ions and Fe atoms become interchanged within their respective crystallographic sites. Particle size (preferably small) and carbon coating play an important role for the reobtention of electrochemically active powders, and recent achievements347,374,381 have demonstrated close to full capacity utilization at reasonable rates. Unlike Li2FeSiO4, other members of the Li2MSiO4 family such as Li2MnSiO4 have not shown promising electrochemical characteristics so far.358,382,383,367 Li2MnSiO4 exhibits also a rich polymorphism.368,384 The crystal structure of its high-temperature form has been published by Politaev,259 and its poor ion and electron transport characteristics were recently addressed.385 Additionally, an important loss of crystallinity occurs during the first oxidation.368 Soon after the inherent electrochemical nonstability of Li2MnSiO4 positive electrode material had been confirmed, the use of mixed Fe/Mn orthosilicates was proposed367,382,383,386 so as to stabilize the unstable local environment of Mn3+ in tetrahedral coordination by Fe. Yang’s group reported on an optimal composition of Li2Fe0.5Mn0.5SiO4 able to deliver quite high capacity (214 mA·h g−1) within a wide voltage window but with serious capacity fading during cycling.383 Dominko showed, through in situ XANES and Mössbauer spectroscopies, that capacity retention and structural stability of Li2FezMn(1−z)SiO4 depend heavily on the upper cutoff voltage used, preferably below 4.2 V versus Li+/Li.386 The overall changes of oxidation states did not exceed more than 0.8 electron per both transition metals even though the electrochemical experiment suggested that more than 1 electron per compound formula had been exchanged. More recently, Yang’s group348 reported using ex situ Mössbauer significant oxidation of Fe3+ to Fe4+ (Fe4+/FeTotal ≈ 0.6) when lithium deintercalation from a novel Li2FeSiO4/C

4.5. Lithiated Transition Metal Borates

LiMBO3 borate phases were considered for the first time in 2001 as possible alternatives for positive electrode materials. The goal was to increase the specific capacity and energy versus those of LiFePO4 due to their lower mass.393 Different polymorphs exist,393−396 described either in the monoclinic space group C2/c (M = Fe, Mn, Co) with chains made of edgesharing trigonal bipyramids M2O8 and linked through planar BO3 groups (Figure 30), or in the hexagonal space group P6̅ (M = Mn) with chains made of edge-sharing square pyramids and connected through borate groups. In these structures, lithium ions occupy tetrahedral sites. Legagneur et al. reported only very limited reversible capacities for LiMBO3, with a potential around 3.0 V versus Li+/Li for the Fe2+/Fe3+ redox couple. Solid-state syntheses are generally used to prepare these borate phases under argon with LiBO2 or H3BO3 as boron precursors. As for the olivine phase LiFePO4, the formation of LiMBO3/C composites (M = Fe, Mn) allowed one to demonstrate the ability of these materials to deliver encouraging performances, despite potential windows too large for applications.397−399 More recently, Yamada interestingly reported attractive reversible capacity obtained for LiFeBO3 corresponding to the exchange of almost 1Li per Fe (>190 mA·h/g at C/20, room temperature, and in the large potential range [1.5−4.5 V vs Li+/Li])400 (Figure 30). Yamada associated these performances with the formation of nanosized particles and with the special attention paid to surface poisoning due to contact with ambient atmosphere. Ceder compared for the first time the electrochemical performance obtained for the monoclinic and hexagonal LiMnBO3 polymorphs,401 the first being obtained pure at 500 °C and the second at 800 °C. From this study, monoclinic LiMnBO3 was revealed as the most interesting Mnrich polymorph, with a lower potential for lithium deinV


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Figure 30. Crystal structure of monoclinic LiFeBO3 (top),400 and galvanostatic response vs Li upon Li+ extraction/insertion from LiFeBO3 (bottom403). Reproduced with permission from refs 400 and 403. Copyright 2010 Wiley and 2011 American Physical Society.

5. OXY-, HYDROXY-, FLUORO-PHOSPHATES AND -SULFATES: NEW PROMISING ELECTRODE MATERIALS From the recent achievements on tavorite-type LiVPO4F109 and LiFeSO4F,404 in particular, we have found it particularly appropriate to establish an overview on the great variety of AxMXO4Y chemical compositions that can be prepared and investigated as positive electrode materials in Li and/or Na batteries. Basically, the structural types described in this section are built on: • XO4 tetrahedra (X = P, As, S) • MOnYy (M = Fe, V, Nb, Ti, Co, Mn: Y = O, F, OH) octahedra (sometimes very distorted) generating a 3-D framework • A+ cations located in the interstitial space (A+ = Li+, Na+, H+) Fluoro-phosphate and fluoro-sulfate materials benefit in particular from the inductive effects of both sulfate-phosphate and F anions, which lower the energy of a given M(n+1)+/Mn+ redox couple. Most of these compounds have hydroxylcontaining mineral analogues, such as the amblygonite LiAlPO4(OH1−xFx)405 and the tavorite LiFePO4(OH). These tavorite-type structures are capable of very high rates with lithium diffusion expected to be one-dimensional.406 Few (only

Figure 29. Crystal structure representation of the alluaudite-type phase (Li,Na)MnFe2(PO4)3. Adapted with permission from refs 391 and 392. Copyright 2010 American Chemical Society.

tercalation (3.7 V vs Li+/Li in comparison to 4.1 V for the hexagonal form) and with the formation of the stable MnBO3 phase that promotes better cyclability. Yamada’s group also recently synthesized the complete solid solution Li(MnxFe1−x)BO3 in the monoclinic form, in inert atmosphere and at moderate temperature (600 °C).402 As expected, the potentials associated with the redox couples Mn2+/Mn3+ and Fe2+/Fe3+ were found to be at 3.7 and 3.0 V versus Li+/Li, respectively. Experimental and theoretical studies have shown that small volume variation is expected upon lithium deintercalation from these LiMBO3 phases, which could promote long-range cyclability.400,403 Nevertheless, Li//LiMBO3 cells still show high polarization that may be due to lithium diffusion intrinsically kinetically limited; as lithium diffusion is quasi1D in these phases, the presence of antisite defects would be highly detrimental and has thus to be carefully controlled.401,403 W


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adopts only the α form. The β-VOPO 4 structure is orthorhombic (Pnma space group) into which the VO6 octahedra are strongly distorted, forming chains along.100 While only VOPO4 may be prepared directly in the β form, Gaubicher succeeded in stabilizing it for X = As412 by preparing β-LiVOAsO4 from a mixture of VOAsO4·2H2O and Li2CO3 heated under Ar to 600 °C. As a consequence, the position of the V5+/V4+ redox couple in this β structure could be determined by either the electrochemical insertion of Li+ into VOPO4 or the electrochemical extraction of lithium from LiVOAsO4. The experimental values are extremely close, equal to 3.98 V and 4.02 V versus Li+/Li for X = P and As, respectively, consistent with the similar electronegativities of P and As. Moreover, this places the V5+/V4+ redox couple at an intermediate value (∼4 V vs Li+/Li) between those found for LiV2(PO4)3 → V2(PO4)3 (>4.5 V) and for V2O5 (∼3.4 V). Gaubicher90 interpreted this feature as a result of the peculiar structural arrangement in β-VOPO4, itself intermediate between a NASICON structure (only ··−V−O−X−·· sequences) and an infinite oxide (only ··−V−O−V−··· sequences). Subsequently, Barker419 succeeded in preparing directly the βLiVOPO4 phase by carbothermal reduction of VOPO4·2H2O and Li2CO3. Azmia420 demonstrated good response at high rate for carefully ball-milled electrode composites. Kerr416 used the small particles of the ε form of VOPO4 (monoclinic P21/n, obtained from thermal oxidation of VPO4·H2O421) to intercalate lithium at a reasonable regime due to the superior ionic transport within this phase. The electrochemical reduction of ε-VOPO4 with lithium occurs at 3.83 V versus Li to yield an α-LiVOPO4 phase (triclinic P-1422) (Figure 31). Although only 2/3 of the theoretical capacity (163 mA·h g−1) was achieved, excellent cyclability was demonstrated.416 Song,423 Ren,424 and Xiong425 confirmed later these observations. The electrochemical activity of triclinic αLiVPO4O was reported to be poor in the high voltage range426 and to decrease drastically in the low voltage range:427 it was tentatively attributed to an intrinsic very low electronic conductivity and to extreme change in the VO6 environment upon lithium intercalation and deintercalation.

4 to our knowledge) structural families of lithium and sodium transition metal fluorophosphates have been reported before in the literature: AMPO4F, A3M2(PO4)2F3,407 A2MPO4F,408,409 and A5M(PO4)2F2.410 5.1. V5+/V4+ and V4+/V3+ Couples in LixVOXO4 (X = P, As)

A quite early series of studies on this family of compounds was initiated by the group of M. Quarton with the works of Gaubicher,90,91,411,412 Dupré,413−415 and Kerr416 on various LixVOXO4 hosts for the reversible insertion of lithium. An interesting overview had been given at that time by Whittingham.17 As for the NASICON structure, the inductive effect of XO4n− polyanion generates attractive operating voltages for the V5+/V4+ and V4+/V3+ redox couples in lithium batteries. However, each MO6 octahedron being connected to 4 XO4 tetrahedra only (instead of 6 for the NASICON structure), a weaker global inductive effect is generated in AxVOXO4-type structures than in AxM2(XO4)3. From a structural point of view, two main classes of VOXO4 (X = P, As, S) compositions, noted as α and β, should be distinguished (Figure 31). The α-VOXO4 structure is tetragonal, space group P4/n, and is built of [VO5]∞ chains of VO6 octahedra sharing corners along the quaternary axis [001].417,418 The four remaining oxygen corners of the VO6 octahedra each belong to a different XO4 tetrahedron. VOAsO4

5.2. The Nb5+/Nb4+ and Ti4+/Ti3+ Couples in LixMOXO4 (X = S, P, Si)

The chemical and electrochemical insertions of lithium into two structural forms of NbPO5, α-NbOPO4 (P4/n) and βNbPO5 (P21/c), were investigated by Patoux,427 the polyhedral connectivity being essentially the same as that encountered in α and β VOPO4. The high-temperature β form is described as well as the member m = 2 of the “monophosphate tungsten bronze with pentagonal tunnels” (MPTBp) family of general formula Ax(NbO3)2m(PO2)4 (A= Li, Na, Ag).428 Both structures present distinct behaviors with regard to lithium insertion. β-LixNbPO5 rapidly and reversibly uptakes lithium leading to a sustained capacity of 90−120 g (i.e., operating to the Nb5+/Nb4+ couple) for more than 100 cycles at ∼2 V versus Li+/Li (Figure 32). On the other hand, αLixNbOPO4 displays an insertion plateau at ∼1.7 V versus Li+/ Li indicative of a two-phase process that was found to be poorly reversible. Both materials, when discharged at voltages lower than 1 V, present irreversible reactions with complete amorphization and reduction to metallic niobium. Oxotitanium materials of general formula LixTiXO5 (X = S, P, Si) were investigated in detail by Patoux334 so as to provide comparative data on how the Ti4+/Ti3+ couple would be shifted

Figure 31. Polymorphism of VOPO4 and galvanostatic signature vs Li upon Li + insertion/extraction into VOPO 4 (from ref 416). Reproduced with permission from ref 416. Copyright 2000 The Electrochemical Society. X


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nected by corner-sharing PO4 tetrahedra: the resulting 3-D framework delimitates different types of tunnels larger than 3 Å in diameter, with two possible crystallographic sites for the lithium ions (Figure 33). Recently, Ellis reported the partial

Figure 32. GITT curves of the first cycle of Li+ insertion/extraction into β-NbOPO4 (top427) and into TiOSO4 (bottom334). Reproduced with permission from refs 427 and 334. Copyright 2011 The Electrochemical Society and 2002 American Chemical Society.

by the inductive effect of S, P, and Si on the Ti−O bond. The frameworks of Li2TiSiO5 and LiTiPO5 are similar to those of αVOPO4417,418 and α-NbPO5,427 respectively, whereas TiSO5 exhibits the bronzoı̈d structure of β-NbPO5.429 This latter phase can be viewed as the m = 2 member of the MTBPT family (monophosphate tungsten bronzes with pentagonal tunnels) of general formula Ax(NbO3)2m(PO2)4. TiSO5 adopts then an ReO3-type arrangement. Whereas the already Li-filled Li2TiSiO5 presents no electrochemical activity, 0.6 Li could be inserted reversibly into LiTiPO5 at an equilibrium cell voltage of 1.5 V versus Li+/Li, that is, similar to titanium oxides. More interesting is the behavior of the sulfate TiOSO4, which operates through a solid solution process between 3.0 and 1.8 V versus Li+/Li (Figure 32). The Li/LixTiSO5 is reversible on subsequent cycles as the XRD pattern of the pristine phase is mostly recovered after a full cycle, although the global intensity is slightly reduced. At low voltage, electrode degradation presumably results from departure of sulfate groups within the electrolyte. At this point, it is important to stress that, although the inductive effect of the SO4 groups is beneficial to higher operating voltages as compared to phosphates and oxides, it also induces weaker Ti− O bonds and thus weakens the structural stability of the electrode over cycling.

Figure 33. Representation of the tavorite structural arrangement (a) and overview of corresponding LixMPO4Y compositions and redox voltages (b).

occupancy (0.82:0.18) of two LiO4F subsites by Li+ ions in a distorted LiO5F cavity, whereas from analysis of combined Xray and neutron diffraction data Ateba Mba proposed the occupancy of a single site, that corresponding to the most occupied one reported by Ellis.437−439 LiVPO4F can be prepared through a two-step carbothermal solid-state reaction involving carbon, VPO4, and LiF431 at 700 °C, which results in the formation of large agglomerates of 10− 40 μm with residual conductive carbon. The synthesis of smaller (70 nm) particles of LiVPO4F embedded in a carbon network has also been reported, using a V2O5·nH2O hydrogel as the vanadium precursor, which allowed the formation of LiVPO4F at significantly lower temperature (550 °C).440,441 The preparation of such compositions is not that trivial however, as mentioned by Sauvage.442 NaVPO4F adopts a different crystal structure, apparently related to Na3Al2(PO4)2F3.407 It can be obtained through a simple solid-state reaction between VPO4 and NaF at 750 °C under argon.430 Sodium-ion cells using hard carbon at the negative electrode and NaVPO4F as the positive electrode were successfully fabricated: they provided an average operating

5.3. V4+/V3+ and V3+/V2+ Couples in NaxVPO4F and LixVPO4F

AVPO4F materials belong to the tavorite-type family and were first proposed as positive electrodes by Barker et al.109,430−436 The crystal structure of LiVPO4F is based on the tavorite-type arrangement, characterized by corner-sharing one-dimensional chains of FeO4F2 octahedra, two octahedra being bridged by fluorine atoms along the chains. These chains are interconY


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voltage of 3.7 V.430,443 Additionally, it was mentioned that the extraction of Na+ from NaVPO4F occurs via two distinct reversible oxidation steps, located at 3.8 and 4.3 V versus Li+/ Li. LiVPO4F shows highly reversible lithium extraction/insertion reactions in two potential ranges, around 1.8 and 4.2 V versus Li+/Li. This difference in potential between these two redox reactions (2.4 V) allowed Barker433 to propose the realization of a symmetrical cell with LiVPO4F at both negative and positive electrodes (Figure 34). Nevertheless, Yamaki’s group

Figure 35. Various redox reactions in homeotypic LiVPO4F and LiVPO4O compositions. Adapted with permission from ref 439. Copyright 2012 American Chemical Society.

vs Li+/Li), while, surprisingly, a single one is observed during the next discharge439,445 (Figure 35). Noticeably, long-range cyclability of graphite/LiVPO4F cells has been demonstrated at a C/2 cycling rate, with more than 120 mA·h g−1 after 200 cycles.447 In addition, it was recently proposed, from accelerating rate calorimetry tests on delithiated LiVPO4F in 1 M LiPF6 EC/DEC electrolyte, that the thermal stability of the VPO4F−LiVPO4F is comparable to that of LiFePO4−FePO4.446 Partial aluminum substitution for vanadium in LiVPO4F was shown to induce a decrease in the overall capacity, but to result in lower polarization, higher average potential, smaller irreversibility, and long-range cycling.447,448

Figure 34. V3+/2+ and V3+/4+ redox couples in the LiVPO4F electrode vs Li and cyclability of a symmetrical LiVPO4F// LiVPO4F cell.433 Reproduced with permission from ref 433. Copyright 2005 The Electrochemical Society.

has recently reported that the symmetric cell using LiVPO4F at the two electrodes shows reduced cycle life due to LiVPO4F dissolution at the anodic side in the highly acidic LiPF6-based organic electrolyte; much more stable cyclability, even at high temperature, was achieved in ionic-liquid electrolytes.444 • In the lower potential range, lithium is inserted reversibly into LiVPO4F via a voltage plateau associated with the V3+/V2+ redox couple at ∼1.8 V versus Li+/Li in a twophase reaction mechanism.437,445 This reaction is characterized by a very low polarization and a high reversible capacity of 140 mA·h/g (i.e., 90% of the theoretical one). The inserted phase thus formed, Li2VPO4F, crystallizes in the higher symmetry monoclinic C2/c space group, with an exchange between the two lithium sites on the millisecond time scale over an energy barrier of 0.44 eV437,438 (Figure 35). • In the upper potential range, lithium is extracted reversibly from LiVPO4F toward VPO4F, thus involving the V3+/V4+ redox couple at ∼4.2 V versus Li+/Li with a reversible capacity of 155 mA·h/g (97% of the theoretical one) and, again, a small polarization.446 Lithium deintercalation from LiVPO4F proceeds through two voltage plateaus, very close in potential (4.24 and 4.28 V

5.4. Fe3+/Fe2+ and Ti4+/Ti3+ Couples in LiFeIIIPO4F and LiTiIIIPO4F

Tavorite-type LiFePO4F was recently prepared by the groups of Tarascon and Nazar through a rather straightforward solid-state reaction449 or by ionothermal or solvothermal reactions.450−452 LiTiPO4F was also obtained by Recham by solid-state and ionothermal reactions.450 • Microsized particles were obtained from solid-state reaction by either a mixture of ball-milled MF3 (M = Fe, Ti) and Li3PO4 heated at 700 °C in a stainless steel or platinum tube filled with argon or a mixture of FePO4 and LiF heated in N2 at 575 °C. Nanosized particles were obtained through ionothermal reactions at 260 °C; MF3 (M = Fe, Ti) and Li3PO4 were used as precursors, and 1butyl-3-methylimidazolium triflate (for Fe) or 1,2dimethyl-3-(3-hydroxypropyl)imidazolium bis(trifluoromethane sulfonyl)imide (for Ti) as the ionic liquid. Z


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• Solvothermal reaction allows formation of matchsticklike particles (below 100 nm in diameter and up to 1 μm in length) from anhydrous FeF3 and LiH2PO4 in ethyl alcohol at 230 °C during 3 days. The crystal structure of LiFePO4F has been determined from Rietveld refinement of powder X-ray diffraction data449 in the triclinic space group P1̅, that is, isostructural with the tavorite LiFePO4(OH). Lithium ions have been located on two independent crystallographic sites, partly occupied (0.75 and 0.25).449,453 Contrary to LiVPO4F, lithium extraction from LiFePO4F is not presently feasible as this would involve oxidation of Fe3+ to Fe4+ at much too high a voltage. Highly reversible Li intercalation (0.96 Li) was reported to occur close to 3 V versus Li+/Li, providing a specific capacity very close to the theoretical one (152 mA·h g−1). The operating voltage associated with the Fe3+/Fe2+ couple (∼3.0 V vs Li+/Li) is higher than for LiFePO4(OH)454 thanks to the inductive effect of the fluorine anion (Figure 36). With no surprise, it is smaller than for LiFePO4 because of the smaller connection of Fe to phosphate groups, as was previously discussed by Gaubicher90 and Patoux427 for MOPO4 structures. The tavorite-type structure is maintained for Li2FePO4F, despite an overall unit-cell volume expansion of 8%.452 Interesting modeling and magnetic considerations have been published recently.455,456 Also recently, a mixed hydroxy-fluorophosphate LiFePO4(OH)0.4F0.6 of tavorite-type structure was reported to insert lithium through a solid solution reaction at an average voltage intermediate between those of LiFePO4(OH) and LiFePO4F.452 It was prepared at 200 °C through a hydrothermal reaction in water with FePO4·2H2O and LiF as precursors. Surprisingly, two distinct redox steps were reported by Recham450 for LiTiPO4F, at 2.9 V (extraction of Li+) and 1.7 V versus Li+/Li (insertion of Li+), that the authors attributed to the Ti3+ → Ti4+ oxidation and Ti3+ → Ti2+ reduction, respectively. A fully reversible process was reported for a sustained reversible capacity of 150 mA·h g−1. To our opinion, this system should be reinvestigated to fully characterize the pristine material and to verify, for instance, whether the pristine material is really of LiTiPO4F composition. 5.5. Fe3+/Fe2+ Couple in A2MPO4F (A = Li, Na; M = Fe, Mn)

Figure 36. Comparison of discharge−charge characteristics of AFePO4X compositions vs Li. Adapted with permission from refs 453, 454, 450, and 449. Copyright 2010 The Royal Society of Chemistry, 2010 American Chemical Society, 2010 American Chemical Society, 2010 The Electrochemical Society.

Fluorophosphates of general formula A2MPO4F (A = Li or Na; M II = Fe, Mn, Co, or Ni) adopt three structural types that differ in the connectivity of the MO4F2 octahedra. They were considered as very promising materials to be investigated because their theoretical capacity (if two alkali cations can be extracted) is twice as large as that of olivine-type LiMPO4 compounds, especially if Co and Mn are concerned. • The crystal structure of Na2MnPO4F is characterized by two types of Mn2O8F2 chains running along the b axis of the monoclinic unit cell (P2/n space group), the cornersharing MnO4F2 octahedra being connected through fluorine atoms.408 These chains are linked by PO4 tetrahedra to form a three-dimensional framework where the Na+ cations are located in channels.

isolated PO4 tetrahedra. The lithium ions are sitting in channels along the [010] direction. • The crystal structures of Na 2 CoPO 4 F, 458 (Li,Na)2FePO4F, Na2NiPO4F, and Na2(Fe1−xMx)PO4F (M = Co, Mg)459,460 consist of a layered framework described in the orthorhombic space group Pbcn (Figure 37). Infinite chains of M2O7F2 units, made of two facesharing MO4F2 octahedra and connected via bridging fluorine atoms, are interconnected through PO 4 tetrahedra forming layers between which sodium cations are located. Okada457 was the first, to our knowledge, to envisage the use of such materials (Li2CoPO4F) as high voltage positive electrode materials, but he did not succeed in proving reversible Li+ extraction/insertion besides electrolyte oxidation. Recent

• The crystal structures of Li 2 NiPO 4 F 4 0 9 and Li2CoPO4F457 are also three-dimensional (Figure 37): they adopt orthorhombic symmetry (space group Pnma) and are built of infinite chains of edge-sharing NiO4F2 octahedra joined together by sharing corners with AA


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Figure 37. Crystal structures and electrochemical properties of Li2CoPO4F (left481) and Na2FePO4F (right459,460). Reproduced with permission from refs 481, 459, and 460. Copyright 2010 Elsevier, 2007 Nature Publishing Group, and 2010 American Chemical Society.

• Sol−gel synthesis, using stoichiometric amounts of Fe(CH3COO)2, NaCH3COO, NaF, and H3PO4, stirred in a solution of dimethoxyethane. After solvent evaporation, the homogeneous gel obtained was fired under conditions similar to those already described for the solid-state synthesis. • Hydrothermal synthesis, with reagents (H 3 PO 4 , (NH4)2Fe(SO4)2, CoCl2, or MgCl2) sealed inside a Teflon-lined Parr reactor in alkaline aqueous media (NaF, NaOH) and heated to 170−220 °C under autogenous pressure. The powders as prepared were then heated at 500−625 °C for 4−6 h under argon. The sol−gel and solid-state synthesis routes enable preparation of isotropic crystallites (50−200 nm in diameter), whereas the hydrothermal reaction leads to the formation of rod-like particles, 75−100 nm thick and 300−700 nm long. Carbon-coated synthesized Na2FePO4F exhibits promising electrochemical properties with an average voltage of 3.3 V

reports461,462 suggest that an irreversible phase transformation occurs during the first charge, involving rotations of CoO4F2 octahedra and PO4 tetrahedra with 5% unit-cell expansion. The new framework demonstrates reversible Li-intercalation/ deintercalation with a discharge capacity of only 60 mA·h g−1 when the potential window is limited to 5 V versus Li+/Li, whereas it can be increased to 110 mA·h g−1 when the upper potential limit is shifted to 5.5 V versus Li+/Li463 (Figure 37). Nazar’s group459,460 successfully demonstrated the reversible activity of (Li,Na)2FePO4F and of magnesium-substituted Na2(Fe1−xMgx)PO4F. These compositions were synthesized by various synthesis routes: • A ceramic route involving stoichiometric amounts of Na(CH3COO), NaF, and NH4H2PO4, ball-milled for 4− 6 h with Fe(C2O4)3·2H2O prior to firing for 6 h at 525− 625 °C. AB


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versus Li+/Li for the oxidation of Fe2+ into Fe3+, and a reversible capacity of 115 mA·h/g at C/10 and of 80 mA·h/g at 5C459 (Figure 37). This indicates, as previously reported for Nasicon-type materials41,43,52 and for Na3V2(PO4)2F3,464,465 that sodium-containing materials are able to cycle reversibly even when cycled in lithium-based electrolytes. The electrochemical extraction/insertion occurs according to a quasi solidsolution reaction associated with minor structural changes, with less than 4% volume contraction upon charge, that is, significantly smaller than what occurs for the triphylite LiFePO4. A small distortion of the lattice to a monoclinic unit cell is observed for the intermediate compositions Na1.5FePO4F, possibly due to a charge (Fe2+, Fe3+) or ion (Na, vacancy) ordering. Na/Li ion-exchange was also performed and allowed the preparation of isostructural Li2FePO4F, revealing the high mobility of the alkali ions in this two-dimensional framework. Preliminary results460 suggest small improvements of electrochemical properties when partially substituting Mg for Fe in Na2(Fe1−xMgx)PO4F. On the other hand, replacement of Fe by Co, Ni, and Mn in Na2MPO4F did not lead to any noticeable property so far. 5.6. V3+/V4+ Redox Couple in A3V2(PO4)2F3 and A5V(PO4)2F2

Na3V2(PO4)2F3 belongs to another family of fluorophosphates (Na3M2(PO4)2F3; M = Al, V, Cr, Fe, Ga) first reported by Le Meins.407 It is characterized by a three-dimensional framework built up from V2O8F3 bioctahedra and PO4 tetrahedra, two vanadium octahedra being bridged by fluorine atoms. Sodium ions occupy two crystallographic positions in two channels (Figure 38). Na3V2(PO4)2F3 can be prepared via a carbothermal synthesis route involving VPO4 and NaF as precursors. When used in lithium batteries, it shows a highly reversible alkali Li + extraction/insertion specific capacity of 120 mA·h/g (oxidation/reduction V3+ ⇔ V4+) with two distinct redox processes centered at 3.8 and 4.3 V versus Li+/Li464 (Figure 38). Low capacity fading has been observed for 220 cycles, and possible extraction of the remaining Na+ cation (V4+/V5+ couple at ∼4.9 V vs Li+/Li) was demonstrated. Graphite/Li+ electrolyte/ Na3V2(PO4)2F3 cells have demonstrated quite interesting electrochemical performances, and highly lithiated graphite was shown to be electrochemically stable in the presence of Na+ ions in the Li+ electrolyte.465 During the first charge, Na+ ions are extracted from the fluorophosphate positive electrode while concurrent lithium ion intercalation occurs at the graphite negative electrode. During the next discharge, the reaction at the positive electrode is likely based on a mixed Li/Na insertion mechanism as found for (Li,Na)2FePO4F.459,460 Li5V(PO4)2F2 and Li5Cr(PO4)2F2 are characterized by a twodimensional structure, with sheets made of VO4F2 octahedra and PO4 tetrahedra. Lithium ions are located within six crystallographic sites between these layers, but also within tunnels. Interestingly, this phase was prepared by Nazar’s group by a so-called dimensional reduction technique from the pristine Li3V2(PO4)3.299,466 The electrochemical extraction of two Li+ ions from Li5V(PO4)2F2 (V3+ to V5+) would lead to a theoretical capacity of 170 mA·h g−1, but only one Li+ could be extracted (with high reversibility and very small polarization) at an average voltage of 4.15 V versus Li+/Li. Variable-temperature NMR studies and 2D exchange spectroscopy (EXSY) were used to probe lithium ion dynamics in the reduced phase Li4V(PO4)2F2 and thus to identify the more mobile lithium ions

Figure 38. Crystal structure, sodium extraction, and lithium insertion vs Li from Na3V2(PO4)2F3.464 Reproduced with permission from ref 464. Copyright 2006 Elsevier.

(those extracted more easily from the 3D framework).467 For the first time to our knowledge, oxidation of Cr3+ into Cr6+ into a polyanion structure has been reported at 4.6 V versus Li+/ Li.410 5.7. Fe3+/Fe2+ in Tavorite-like LiFeIISO4F and LiFeIIIPO4OH. Extension to the Triplite Family

The discovery by Reddy et al.348 of promising electrochemical properties for the newly synthesized monoclinic phase FeIIISO4(OH) paved the way very recently for intense research on sulfate-based polyanion structures (particularly LiFeSO4F) as hosts for reversible Li+ extraction/insertion. Lithium insertion into Fe(OH)SO4 was shown to occur in two successive redox steps located at 3.26 and 3.13 V versus Li+/ Li, for a total reversible capacity of 110 mA·h/g at a C/20 rate. In the meantime, Marx showed a reversible Li+ intercalation into the tavorite LiFeIIIPO4(OH) at 2.6 V versus Li+/Li for a reversible capacity of 100 mA·h/g.453 Once again, a clear illustration of the shift of the position of a given redox couple (Fe3+/Fe2+ here) when (PO4)3− polyanions are replaced by (SO4)2− ones is given, as illustrated in Figure 39. Since then, tremendous research effort has been put on the lithiated phase LiFeIISO4F, in particular by the groups of Tarascon and Nazar.404,451,469−476 Various synthetic routes have been developed, including ionothermal, solvothermal, and AC


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Figure 39. Respective positions of Fe3+/Fe2+ and V4+/V3+ redox couples vs Li in tavorite and triplite-type LiMXO4Y (M = Fe, V; X = P, S; Y = OH, F) compositions upon Li+ insertion/extraction.

ionic transport properties (quasi 1D) of this material,476 which might stand as a promising positive electrode material despite its strong reactivity with moisture. The oxidized product FeSO4F was proposed initially404 to adopt an isostructural triclinic (P1)̅ symmetry, but this model has been challenged recently,474 and the structure could be fully determined in the monoclinic space group C2/c, as is the case, interestingly, for FePO4·H2O.454 Lithium diffusion is of the same order of magnitude in LiFeSO4F as in LiFePO4, with an easier mobility in the lithium-rich composition than in the lithium-poor.477 The fluorosulfate family was recently extended, through cation substitutions, to NaMSO4F, Na(Fe1−xMx)SO4F, and (Na1−xLix)MSO4F (M = Fe, Co, Ni) phases.472,473 NaFeSO4F seems to be the only one to display some electrochemical activity. Even more recently, a new polyanionic framework of triplitetype structure was reported by Liu et al. for LiFeSO4F478 and by Barpanda et al. for LiFe1−δMnδSO4F, with the highest voltage ever reported for the redox couple Fe3+/Fe2+ (3.9 V vs Li+/Li)479 (Figure 40). The transition metal ions are coordinated by four SO4 anionic groups and two fluorine atoms, but the triplite structure consists of edge-sharing chains with MO4F2 octahedra instead of corner-sharing chains in the tavorite structure. Fluorine atoms are in cis configuration rather than in trans in tavorites, whereas the lithium ions are almost equally distributed with the transition-metal ions between the two metal sites. Manganese substitution was shown to promote tavorite to triplite structural transformation. Nearly no volume change is observed upon cycling (0.6% only, vs 7−10% for LiFePO4 and tavorite-LiFeSO4F). More recently, Ellis et al. revisited the tavorite−triplite system LiFe1−xMxSO4F (M = Mn, Zn) and suggested that the tavorite structure could be an intermediate metastable and cation-ordered phase before the formation of the cation-disordered triplite phase.480 A lot of questions still remain concerning this new and very attractive system: what are the criteria for triplite versus tavorite stabilization? What is the lithium diffusion pathway in this cation-disordered phase (Figure 41)?

solid-state dry reactions. The use of ionic liquids (EMI-TFSI, for instance) to promote the crystallization of LiFeSO4F at temperatures under which it would be decomposed (∼350 °C) from a structurally related precursor such as FeSO4·H2O proved to be a very elegant synthesis route404,451 at first stage. However, as pointed out by Tripathi and Nazar,474 the prohibitive cost of ionic liquids and the difficult recovery of excess LiF used in the procedure necessitate the development of alternative synthesis routes. A low-temperature solid-state synthesis route was recently proposed by Ati: it consists of reacting FeSO4·H2O with excess of LiF in a dry and closed environment at T = 290 °C.471 A solvothermal reaction was also recently proposed by Nazar’s group, which consists of heating at 220 °C a mixture of FeSO4·H2O and LiF (NaF) in tetraethylene glycol (TEG) for 60 h for LiFeSO4F (48 h for NaFeSO4F).474 These lithiated fluorosulfates adopt the tavorite-like structure, described in the triclinic space group P1̅ for LiFeSO4F and in the monoclinic space group P2/c for NaFeSO4F.474 The extraction of Li+ from LiFeSO4F proceeds at ∼3.6 V versus Li+/Li, that is, at slightly higher voltage than that of LiFePO4 (Figure 40). Good response to increased current density was attained, without any specific carbon coating or downsizing of particles, thanks to the satisfactory

6. CONCLUSIONS AND FUTURE OUTLOOK Considered first as an academic curiosity, a fundamental research in which very few people were interested 15 years ago, polyanion-based structures have been, since the discovery of reversible lithium extraction from LiFePO4, the object of very intense research and development activities, at both academic and industrial levels. These materials suffer intrinsically, undoubtedly, from their lower theoretical gravimetric capacity due to the presence of “heavy” XO4 (X = P, S, Si, Mo, W) groups besides transition metals. However, for most of them, they offer a particularly stable open 3-D framework (especially for X = P) ideal for long-term cycling and fast ion motion. This gives rise to a fascinating and rich structural chemistry, heavily investigated by many groups. This Review demonstrates that an immense variety of structural types can accommodate reversibly Li+ and/or Na+ insertion/extraction. The operating voltages versus Li (or Na) are generally high, thanks to the inductive effect of the polyanion groups (in particular for SO4 and PO4) and also of fluorine in fluoro-phosphates and fluoro-sulfates. Importantly, as gathered in Tables 1, 2, and 3, while Co, Mn, and Ni are the most frequently used metal elements in layered or spinel oxide positive electrodes for Li and Na batteries, iron and vanadium AD


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Figure 40. Tavorite and triplite forms of LiFeSO4F and their respective electrochemical properties vs Li. Reproduced with permission from ref 479. Copyright 2011 Nature Publishing Group.

Figure 41. Lithium diffusion paths in tavorite LixFeSO4F, LixVPO4F, and LixVPO4O.482 Reproduced with permission from ref 482. Copyright 2011 American Chemical Society.

AE


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Table 1. Overview of Iron-Containing Phosphates, Silicates, Borates, Fluorophosphates, and Sulfates That Have Been Investigated as Positive Electrodes in Li Batteries

NASICON

anti-NASICON

hydrated phosphates

diphosphates and diarsenates

triphylite, olivine-type silicate borate tavorite

triplite marinite silimanite KTP layered

initial/final active material

redox couple

average potential (V vs Li)

theoretical capacity (mA·h/g)

energy density (W·h/kg)

Fe2(MoO4)3/Li2Fe2(MoO4)3 Fe2(WO4)3/Li2Fe2(WO4)3 Fe2(SO4)3/Li2Fe2(SO4)3 Li3Fe2(PO4)3/Li5Fe2(PO4)3 LiFe2(SO4)2PO4/Li3Fe2(SO4)2PO4 Li3Fe2(PO4)3/Li5Fe2(PO4)3 Li3Fe2(AsO4)3/Li5Fe2(AsO4)3 Fe2(SO4)3/Li2Fe2(SO4)3 P-FePO4·2H2O/LiFePO4·2H2O S-FePO4·2H2O/LiFePO4·2H2O M-FePO4·2H2O/LiFePO4·2H2O Fe4(P2O7)3·4H2O/Li4Fe4(P2O7)3·4H2O Fe4(P2O7)3/Li4Fe4(P2O7)3 LiFeP2O7/Li2FeP2O7 Li2FeP2O7/LiFeP2O7 LiFeAs2O7/Li2FeAs2O7 LiFePO4/FePO4 LiFe2/3PO4/Li1.6Fe2/3PO4 Li2FeSiO4/LiFeSiO4 LiFeBO3/FeBO3 LiFePO4F/Li2FePO4F FeSO4(OH)/LiFeSO4(OH) LiFePO4(OH)/Li2FePO4(OH) LiFeSO4F/FeSO4F LiFeSO4F/FeSO4F Li2Fe(SO4)2/LiFe(SO4)2 LiZn0.1Fe0.9SO4F KFeSO4F/FeSO4F LiFeSO4(OH)/FeSO4(OH)

Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+

3.0 3.0 3.6 2.8 3.3 2.88 and 2.73 2.91 and 2.62 3.6 3.0 3.0 3.0 3.2 3.1 2.9 3.5 2.55 3.45 3.3 and 2.8 2.7 2.8 2.9 3.25 2.6 3.6 3.9 3.83 3.60 3,.25 and 3.5 and 4.1 3.6

91 63 134 128 130 128 98 134 157 157 157 131 144 113 113 107 170 128 165 219 151 159 153 151 151 102 150 128 151

273 189 482 358 429 358 269 482 471 471 471 419 446 328 395 273 586 390 445 613 438 517 398 544 589 391 540 461 544

Table 2. Overview of Iron-Containing Phosphates, Silicates, Borates, Fluorophosphates, and Sulfates That Have Been Investigated as Positive Electrodes in Na Batteries initial/final active material NASICON

hexagonal olivine-type layered alluaudite layered

Fe2(MoO4)3/NaFe2(MoO4)3 Fe2(WO4)3/Na2Fe2(WO4)3 Fe2(SO4)3/Na2Fe2(SO4)3 Na3Fe2(PO4)3/Na4Fe2(PO4)3 Na3Fe2(AsO4)3/Na4Fe2(AsO4)3 NaFePO4/FePO4 Na2FePO4F/NaFePO4F Na3Fe3(PO4)4/Na5Fe3(PO4)4 NaFe(SO4)2/Na2Fe(SO4)2

redox couple 3+

average potential (V vs Na)

theoretical capacity (mA·h/g)

energy density (W ·/kg)

2.7 2.7 3.3 2.5 2.4 3.0 3.0 2.6 3.5

91 63 134 115 90 154 124 130 99

246 170 442 287 216 462 372 338 346

2+

Fe /Fe Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+ Fe3+/Fe2+

• Vanadium-containing positive electrodes (Table 3) offer an even wider range of possible Li+ (or Na+) electrochemical insertion/extraction reactions, thanks to the possible operation on three redox couples: V5+/V4+, V4+/ V3+, V3+/V2+. The most commonly used redox couple is V4+/V3+, usually located at around 4 V versus Li. These materials offer, therefore, theoretical gravimetric energy densities significantly higher than for their iron-counterparts, as high as 655 W·h/kg for LiVPO4F versus Li and 525 W·h/kg for NaVPO4F versus Na. Recent demonstration of exceptional high rate capability of Na3V2(PO4)2F3 versus Na offers potential hope for a viable positive electrode for Na batteries. For a longer, riskier, approach, such polyanionic materials should/could be considered as interesting host structures for the development of Na-based batteries, both as solid electro-

offer the most promising characteristics for polyanionic-type positive electrodes. • Among Fe-containing positive electrodes (Tables 1 and 2), LiFePO4, LiFeBO3, and LiFeSO4F offer the highest theoretical gravimetric energy densities for Li batteries. While the industrial use of LiFePO 4 has been demonstrated, LiFeBO3 and LiFeSO4F still suffer from serious kinetic and air/moisture stability limitations. For sodium batteries, the choice is much narrower, but NaFePO4-based compositions/structures deserve peculiar attention. Iron-based materials offer very promising opportunities for the development of large scale applications for which the cost of materials plays a major role. AF


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Table 3. Overview of Vanadium-Containing Polyanionic Structures That Have Been Investigated as Positive Electrodes in Li and Na Batteries initial/final active material lithium batteries

sodium batteries

Li3V2(PO4)3/Li5V2(PO4)3 Li3V2(PO4)3/LiV2(PO4)3 LiV2(PO4)3/V2(PO4)3 ε-VPO4O/LiVPO4O VPO4O·H2O/LiVPO4O·H2O VPO4O·2H2O/LiVPO4O·2H2O LiVPO4O/VPO4O LiVPO4O/Li2VPO4O Li4VO(PO4)2/Li5VO(PO4)2 NaVPO4F/VPO4F LiVPO4F/VPO4F LiVPO4F/Li2VPO4F Li5V(PO4)2F2/Li4V(PO4)2F2 Li2VO(HPO4)2/LiVO(HPO4)2 VO(H2PO4)2/LiVO(H2PO4)2 LiVP2O7/VP2O7 LiVP2O7/Li2VP2O7 Li2VOP2O7/LiVOP2O7 Li9V3(P2O7)3(PO4)2/ Li6V3(P2O7)3(PO4)2 V2(SO4)3/Li2V2(SO4)3 β-LiVOAsO4/α-VOAsO4 Na3V2(PO4)3/NaV2(PO4)3 NaV2(PO4)3/V2(PO4)3 Na3V2(PO4)3/Na5V2(PO4)3 NaVPO4F/VPO4F Li1.1Na0.4VPO4.8F0.7/VPO4.8F0.7 Na3V2(PO4)2F3/NaV2(PO4)2F3

redox couple involved

average potential (V vs Li or vs Na)

theor. capacity (mA·h/g)

theor. energy density (W·h/kg)

VII/VIII VIII/VIV VIV/VV VIV/VV VIV/VV VIV/VV VIV/VV VIII/VIV VIII/VIV VIII/VIV VIII/VIV VII/VIII VIII/VIV VIV/VV VIII/VIV VIII/VIV VII/VIII VIV/VV VIII/VIV

1.7 3.7 4.1 3.8 3.6 3.7 3.9 2.4 4.1 4.05 4.2 1.8 4.0 4.2 4.2 4.3 2.0 4.1 4.1

131 131 68 165 149 135 159 159 94 143 156 156 85 98 103 116 116 105 87

223 485 279 627 536 500 620 382 385 579 655 281 340 412 433 499 232 431 357

VII/VIII VIV/VV VIII/VIV VIV/VV VII/VIII VIII/VIV VIII/VIV, VIV/VV VIII/VIV

2.6 4.0 3.4 3.85 1.6 3.7 4.0 3.9

138 126 118 59 118 143 142 128

359 504 401 227 189 529 568 499

lytes and electrodes. Therefore, one may envisage, for hightemperature applications, for instance, the development of all solid-state ceramic batteries built on successions of polyanionic frameworks at all stages of the positive/electrolyte/negative sequence.

for 25 years on the crystal chemistry of sodium ion conductors and positive electrode materials for Li-ion batteries, in particular phosphate-based positive electrodes. He graduated (Ph.D.) from Paris-XI Orsay University in 1991, spent 2 years as a Post-Doc at the Osaka National Research Institute, Japan, and 2 additional years as a Post-Doc at the University of Texas at Austin, TX. He became Associate Professor in Chemistry in 1996 and joined the Université de Picardie Jules Verne in Amiens in 2000. He is the coauthor of ∼100 publications in this field.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Laurence Croguennec is currently a CNRS researcher at the Institut de Chimie de la Matière Condensée in Bordeaux (ICMCB), France. She has been working for 18 years on the crystal chemistry of electrode materials developed for Li-ion batteries and especially in the characterization of mechanisms involved upon cycling: layered oxide and phosphate-type positive electrode materials. She graduated

Christian Masquelier is currently a Professor in Chemistry at Université Picardie Jules Verne Amiens, France, and has been working AG


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ACKNOWLEDGMENTS C.M. is grateful to many colleagues, ex-Post-Doc, and Ph.D. students he shared research efforts with: J. B. Goodenough, J. Gaubicher, J. Rodriguez-Carvajal, M. Morcrette, J. M. Tarascon, J. B. Leriche, L. Dupont, S. Levasseur, P. Gibot, M. CasasCabanas, D. Bonnin, P. Poizot, C. Grey, R. Dominko, G. Rousse, C. Wurm, S. Patoux, C. Delacourt, G. Quoirin, S. Hamelet, C. Sirisopanaporn, J. M. Ateba Mba, L. Tao, and R. Amisse. L.C. is grateful to C. Delmas, D. Carlier, M. Ménétrier, F. Weill, M. Maccario, N. Marx, J. M. Ateba Mba, K. Trad, A. Castets, F. Le Cras, A. Wattiaux, L. Bourgeois, and E. Suard for their cooperative research and fruitful discussions on different polyanionic systems as positive electrode materials for lithiumion batteries. REFERENCES (1) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (2) Guilmard, M.; Pouillerie, C.; Croguennec, L.; Delmas, C. Solid State Ionics 2003, 160, 39. (3) Ohzuku, T.; Makimura, Y. Chem. Lett. 2001, 642. (4) Lu, Z. H.; MacNeil, D. D.; Dahn, J. R. Electrochem. Solid-State Lett. 2001, 4, A191. (5) Thackeray, M. M.; Johnson, C. S.; Vaughey, J. T.; Li, N.; Hackney, S. A. J. Mater. Chem. 2005, 15, 2257. (6) Goodenough, J. B.; Thackeray, M. M.; David, W. I. F.; Bruce, P. G. Rev. Chim. Miner. 1984, 21, 435. (7) Thackeray, M. M.; David, W. I. F.; Goodenough, J. B. Mater. Res. Bull. 1983, 17, 785. (8) Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461. (9) Thackeray, M. M.; Johnson, P. J.; de Picciotto, L. A.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1984, 19, 79. (10) Imazaki, M.; Ariyoshi, K.; Ohzuku, T. J. Electrochem. Soc. 2009, 156, A780. (11) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188. (12) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1609. (13) Manthiram, A.; Goodenough, J. B. J. Solid State Chem. 1987, 71, 349. (14) Manthiram, A.; Goodenough, J. B. J. Power Sources 1989, 26, 403. (15) Delmas, C.; Ménétrier, M.; Croguennec, L.; Levasseur, S.; Pérès, J. P.; Pouillerie, C.; Fournès, L.; Weill, F. Int. J. Inorg. Mater. 1999, 1, 11. (16) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (17) Whittingham, M. S. Chem. Rev. 2004, 104, 4271. (18) Grey, C. G.; Dupré, N. Chem. Rev. 2004, 104, 4493. (19) Fergus, J. W. J. Power Sources 2010, 195, 939. (20) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587. (21) Song, H. K.; Lee, K. T.; Kim, M. G.; Nazar, L. F.; Cho, J. Adv. Funct. Mater. 2010, 1. (22) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem. Mater. 2010, 22, 691. (23) Tarascon, J. M.; Recham, N.; Armand, M.; Chotard, J. N.; Barpanda, P.; Walker, W.; Dupont, L. Chem. Mater. 2010, 22, 724. (24) Adams, S. Cosmos 2011, 7, 11. (25) Goodenough, J. B.; Kim, Y. J. Power Sources 2011, 196, 6688. (26) Yuan, L. X.; Wang, Z. H.; Zhang, W. X.; Hu, X. L.; Chen, J. T.; Huang, Y. H.; Goodenough, J. B. Energy Environ. Sci. 2011, 4, 269. AH


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