Recent Advancements in the Conversion-Type Pnictide-Based

Feature Article pubs.acs.org/JPCC

Recent Advancements in the Conversion-Type Pnictide-Based Electrodes for Li-Ion Batteries L. Monconduit*,†,‡ †

Institut Charles Gerhardt Montpellier-UMR 5253 CNRS, ALISTORE European Research Institute (3104 CNRS), Université Montpellier 2, 34095 Montpellier, France ‡ Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, France ABSTRACT: Nowadays conversion-type electrode materials definitively lie as the core of any research programs related to Liion batteries. Requirements are high capacity, good rate capability, and a long cycle life. Indeed, the goal of much lithium battery research is to achieve the highest energy density battery as possible. In the case of pnictide materials, such performances are the results of the following conversion reaction: MxXy + 2yLi ↔ xM0 + yLi3X (X = P, Sb; M = Fe, Ni, Co, ...). However, these materials are still suffering from serious issues such as (i) low Coulombic efficiency, (ii) high polarization, (iii) poor cycle life (volume expansion), and (iv) limited rate capability that unfortunately still prevent them for any close commercial viability. In this article, the most recent research developments of our group and through collaborations in this specific field will be reported. In the interest of overcoming the limitations listed above, a cautious and rigorous scrutinizing of the electrochemical behavior of any studied materials is necessary. In our research group, we have extensive experience in the use of sophisticated in and ex situ characterization tools, in the aim to probe bulk pnictide in the Li batteries and the electrolyte/electrode surface as well. Indeed, thanks to these methods, we could unambiguously show that electrochemical conversion reactions are leading to some unstable phases, which cannot be synthesized via common chemical reaction paths. One can observe the key role of the solid/solid Li3X/ M0 interfaces in the reversibility of the conversion mechanism. Contrarily, during the process, the solid/liquid electrode/ electrolyte interfaces are subject to continuous parasitic reactions which drastically limit the cycle life of the battery. Fortunately, both nanostructuration of the pnictide electrodes as well as the confinement of pnictide into a porous carbon matrix play a great role in improving the performance of the cell mainly due (i) to the shortening of the distance over which Li+ diffuses or (ii) to the buffer effect of the carbon matrix against the local volume change during the charge and discharge process. could also be described with sulfides, nitrides, fluorides, and pnictides such as phosphides and antimonides as well.2−13 As compared to some classical insertion reactions that govern the energy stocked in the actual Li-ion batteries, which are limited to 1e− or even 0.5e− per 3d metal atom (LiCoO2). These new conversion reactions able to involve 2e−, 3e−, or more were shown as an original way to create a new class of electrodes with staggering capacity gains over various voltage ranges, depending on the nature of the X anion (O, S, P, F, ...). The reversibility of a conversion reaction resides in the formation of very energetic nanoparticles upon a complete reduction of the metal due to a large amount of the interfacial surface. The fingerprint of the reduction process is a characteristic voltage plateau with typical length equal to the amount of electrons required for a full reduction of the compound. In the case of the pnictides, in particular phosphides, it is worth noting that the redox centers

1. INTRODUCTION The most predictable decline of the fossil fuels combined with global warming are two of the biggest challenges that humanity will have to face in the future. Overcoming them will require development and implementation of some important and sophisticated technology for energy storage. For example, the design of new batteries showing high energy density, high power, and high cyclability will definitively meet the Li ion’s share in the global automotive world market. Nowadays, Li-ion batteries are already a kernel component of the world mass production electric vehicles. One can see a huge market potential and possibility to strongly reduce atmospheric pollutants and greenhouse gas emissions and meet consumers’ expectations in terms of gas consumption as well. In the past, Tarascon et al. have showed that simple oxides can electrochemically react toward Li, leading to some sustainable reversible capacities as high as 900 Ah/kg.1 These results were undoubtedly demonstrated by the following conversion reaction: MxOy + 2ye− + 2yLi+ → xM0 + yLi2O (M: transition metal). Lately, it was demonstrated that this new Li reactivity mechanism was not only devoted to oxides but © 2014 American Chemical Society

Received: December 5, 2013 Revised: March 26, 2014 Published: March 27, 2014 10531

dx.doi.org/10.1021/jp411918x | J. Phys. Chem. C 2014, 118, 10531−10544

The Journal of Physical Chemistry C

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Figure 1. (a) Potential-composition profile of a FeP electrode versus lithium and derivative −dx/dV vs potential in inset. F, first discharge; solid line, first charge and second discharge. (b) First two cycles of FeP, from top to bottom, the ratio LixFeP/FeP derived from the Mössbauer spectra, the amount of Li3P detected by in situ XRD, the relative amounts of metallic iron determined by both magnetic measurements and Mössbauer spectroscopy (77 K), the number of lithium atoms per FeP, and the evolution of the cell potential, all drawn versus time.

affecting the cycling life of the conversion electrode materials. The last part of this article will be committed to the recent improvements achieved in the conversion-type electrode material field, with respect to their positive redox properties. We will see the critical role of the electrode formulation or the nanostructuration in view of technological challenges.

are not exclusively located on the transition metal but electron transfer also occurs into bands that have a strong anion contribution.14,15 Indubitably, this phenomenon has to be directly correlated with the covalence of the M−X bond. Finally, materials such as pnictides show high gravimetric and volumetric capacities (up to 1000 Ah/kg and 7000 Ah/L) as compared to some graphites (800 Ah/L), moderate polarization, limited loss of capacity in the first cycle, and very promising capacity retention during the first cycles, showing evidence of a reversibility of the redox process. Nevertheless, several issues still remain for the pnictide (X = P, Sb) materials to be considered as a commercial reality. Among those, the most critical are (i) the strong structural reorganization induced by the chemical changes, producing large volume changes that lead to a particle decohesion and a limited cycling life, (ii) a voltage hysteresis observed between the discharge and charge steps which still remains high, (iii) a large Coulombic inefficiency observed in the first cycle and further partly due to a electrolyte degradation, and (iv) a low rate capability. Like we wrote earlier, the ambition of this article is to describe the most recent developments realized in our research team and in collaboration with the mission of reducing most of the inhibition factors. These advancements were performed with the assistance of a combination of powerful in and ex situ characterization techniques. Electrochemistry, X-ray diffraction, SEM, 119Sn and 121Sb Mössbauer spectrometry, 7Li and 31P NMR analysis, X-ray photoemission spectroscopy (XPS), magnetization, and impedance spectrometry measurements were extensively used to cautiously probe the bulk material and the electrode/electrolyte interfaces as well. Indeed, we clearly attested that, before conversion, intermediate phases can be stabilized; furthermore, phases formed on cycling can be peculiarly metastable. The role of the solid/solid LixX/M0 interfaces in the reversibility of the conversion process is emphasized while the solid/liquid electrode/electrolyte interfaces are subjected to important parasitic reactions, strongly

2. MECHANISM CHARACTERIZATION The conversion mechanism is often described as a direct transformation of the pristine compound into LixX and metallic particles, but intermediate ternary phase formation can also happen before the conversion process. Both CuO and Cu2O have been known to be easily changed into Cu nanoparticles and Li2O when they are reduced by lithium; in addition, Cu2O is generally observed as an intermediate species both upon discharge and charge of a battery containing CuO.16,17 In the case of sulfides, CuS also displays a reversible conversion reaction to Cu and Li2S, which proceeds through an intermediate step that results in the formation of Cu2−xS and Li2S.18,19 Regarding FeS2, the conversion process is preceded by an intermediate step of insertion forming Li2+xFe1−xS2 phases.20,21 Practically, the formation of a variety of lithium polysulfide intermediates has been observed. These particular species are partially soluble in the electrolyte, reducing its conductivity, and can eventually go back and forth to the negative electrode, leading to severe inefficiencies.22,23 For phosphides and antimonides, due to their bond covalent nature, the redox activity may result in extensive lithium insertion before conversion to Li3X (Pn = P, Sb)/M0. The existence of an intermediate process suggests that a direct conversion to metallic particles M0 from binary compounds MxXy is not energetically preferred. This is generally observed when the transition metal oxidation state in the starting process is high enough to allow some intermediate oxidation states to be reached, prior to the complete metal reduction. The mechanism of conversion systems can be relatively complex. For example, FePx (x = 1, 2, 10532



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1), doubtlessly due to some morphological changes, a two-step insertion/conversion (eq 2 and eq 3) occurs in further cycles. 2.1.2. NiP2. NiP2 is also an interesting example of transition metal phosphides for which an indirect conversion happens. We recently reported the electrochemical study of NiP2 toward Li, and demonstrated that the monoclinic NiP2 phase can reversibly catch up to 5 lithiums per formula unit, leading to reversible capacities of 1000 Ah/kg at an average potential of 0.9 V.28 Combined investigations using electrochemical tests, in situ XRD, 31P NMR, and first-principles DFT calculations, have also shown that a two-step insertion/conversion reaction transforms the NiP2 starting electrode into an intermediate ternary Li2NiP2 single phase, and that before the conversion into Li3P/Ni0 nanocomposite. The proposed mechanism is shown in Figure 2.29

4) and NiP2 can be particularly appealing to illustrate the formation of intermediate phases during the battery cycling process. 2.1. Intermediate Phases. 2.1.1. FePx. FePx (x = 1, 2, 4) are conversion-type materials exhibiting high gravimetric and volumetric capacities. Among them, FeP has been recently investigated.24,25 While a direct conversion was easily demonstrated in first discharge by both X-ray diffraction (XRD) and high resolution transmission electronic microscopy (HRTEM), during the cycling process, an amorphization of the electrode under charge came to complicate the understanding of the mechanism, setting up the need to research this process in more detail. In this specific matter, the combination of 57Fe Mössbauer spectroscopy with measurements of magnetic susceptibilities has been particularly efficient to study the redox mechanism more thoroughly.26 Mössbauer spectroscopy provides data about the local environment of the probing nucleus, independently of the presence of long-range order. Magnetic measurements give details of the magnetic behavior of all phases (mainly ferromagnetic and antiferromagnetic), bringing up data on the nature, morphology, and surface state of nanoparticles. These two techniques are complementary in their ability of measuring magnetic susceptibilities and detecting very small amounts of a ferromagnetic phase such as metallic iron. Additionally, firstprinciples T = 0 K phase diagram calculations were attentively investigated to determine the most energetically stable phases for the Li/Fe/P system.25 Results of this study combined with a XRD analysis are summarized in Figure 1. According to the reaction mechanism FeP + 3Li → Li3P + Fe,27 the simultaneous formation of Li3P and metallic iron during the first discharge (F inset Figure 1a) was clearly demonstrated by magnetic and Mössbauer measurements. The increase of the potential of the battery to 1 V vs Li+/ Li (process 2′, Figure 1) leads to (i) an almost complete disappearance of Li3P and metallic iron and (ii) the formation of an intermediate which has been clearly identified by Mössbauer spectroscopy as a cubic LiFeP phase. Extraction of a part of the lithium atoms from Li3P destabilizes the matrix, favoring the reaction with metallic iron to form LiFeP according to the following reaction: Fe + Li3P-2Li → LiFeP. Finally, it is noticeable that the intermediate phase corresponds well to the ternary phase deduced from the first-principles T = 0 K phase diagram calculations.25 Additional charge up to 2 V (process 1′) causes the partial disappearance of LiFeP and the growth of FeP. The second discharge to 0.5 V (process 1) leads to the transformation of some FeP to LiFeP, and further discharge to 0 V (process 2) forms again Li3P and metallic iron. To summarize the electrochemical mechanism involved in the FeP/Li cell, three equations were proposed: First discharge: FeP + 3Li → Li3P + Fe

Figure 2. Mechanism for the lithiation of the NiP2 electrode.

The intermediate ternary Li2NiP2 phase has been fully characterized and was described to be structurally very close to the starting material NiP2, regarding the Ni ion environment. However, its nucleation upon delithiation (charge) coming from the fully converted Li3P/Ni composite is shown to be kinetically limited, which strongly suggests that a restricted lithiation is required for the best cyclability of the NiP2/Li cell. One can notice that most of these intermediate phases, i.e., LiFeP for FeP or Li2NiP2 for NiP2, are difficult to identify due to their X-ray amorphous features. The growth of these intermediate phases is ruled by very specific and local thermodynamic conditions inside the electrode and during the cycling of the battery. All of these phases are unknown for the most part in the literature. As far as our knowledge goes, all attempts to synthesize them using various synthetics routes failed, presumably due to the difficulty to mimic the electrochemical thermodynamic conditions. An existing competition between lithium insertion and partial conversion occurs. In the case of the FeP mechanism described above, the intermediate process is kinetically controlled in the same way as for Co3O4.30 In most cases, this competition can be thermodynamically controlled (e.g., in Fe2O3),30 with particle size either having an indirect effect on the current density for lithium insertion or a direct energetic effect for a partial conversion. The existence of an intermediate process indicates that a direct conversion to metallic particles from some binary compounds is not of energetic advantage. DFT calculations on fluorides FeF331 have suggested that the formation of very small metallic nanoparticles creates a handicap in cohesive energy that makes the formation of the intermediate LiFeF3 more favorable than the complete reduction of iron into metallic Fe0. Such a hypothesis can also be applied to similar compounds that undergo insertion prior to conversion. The next part illustrates through two electrode examples (FeP and NiSb2) the following: (i) the calendar instability of the electrochemically formed phases especially at low voltage and (ii) the need to use in situ powerful tools to fully characterize them.

(1)

Further cycles: Li3P + Fe ↔ LiFeP + 2Li LiFeP ↔ FeP + Li

(Process 2′ /2)

(Process 1′/1)

(2) (3)

In conclusion, definitively, this work demonstrates that, while a direct conversion happens during the first discharge process (eq 10533



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2.2. Stability of Electrochemically Formed Phases. 2.2.1. FeP/LiFeP. To evaluate the calendar stability of an electrode at low potential, we have followed the evolution of a FeP electrode after its removal from the electrochemical cell at the end of the second discharge (eqs 2 and 3). To this end, several Mössbauer spectra were recorded over a period of 4 weeks.26 The spectra are shown in Figure 3. The spectrum was

Generally speaking, the spontaneous reformation of LiFeP proves the high reactivity of the LinX/M nanocomposite at the end of discharge, which leads to large charge capacities for most conversion-type materials.32−34 These observations draw even more attention to evaluation of the shelf life of electrodes based on conversion reactions, since their instability can be a serious handicap for industrial applications. Attempts to mimic the high reactivity of the LixX/M0 (X = F; M = Cu, Fe) have been performed for fluorides: equivalent nanocomposites have been prepared in various ways, and have been charged in a lithium battery.35,36 Although partial formation of MxXy has been experimentally proven in a few cases, the corresponding electrochemical performances are much lower and the electrochemical signatures quite different. This indicates that the chemically prepared LinX/M0 composite is less efficiently mixed and less active than the composite electrochemically formed in the battery. 2.2.2. Calendar Evolution in the NiSb2 Electrode. NiSb2 was recently proposed as a new conversion electrode material delivering reversible capacity of 520 Ah/kg (4150 Ah/L)37,38 near the theoretical capacity of 558 Ah/kg. A first electrochemical study has been coupled to operando XRD and has shown the following: (i) During the first discharge, NiSb2 undergoes a conversion process, leading to (Ni0 + 2Li3Sb). (ii) During the first charge, an original conversion reaction happens, leading to a modified NiSb2 phase, which is back converted during the second (and further) discharges into Li3Sb and Ni nanoparticles. Note that the presence of Ni at the end of the discharge was first only supposed because any XRD analysis could not clearly identify the presence of this metal. The NiSb2 polymorph that is stabilized at the end of the charge will be further discussed in section 2.3. We have explored NiSb2 mechanism vs Li by both in situ operando and ex situ XAS analyses to clarify the oxidation state of Ni during the discharge and more specifically at the end of the discharge. Ex situ XAS and Ni K-edge analysis have been carried out, and no metallic Ni0 was found at the end of the discharge. On the contrary, in situ measurements have shown that nickel metallic state (Ni0) particles are present in the electrode at the end of discharge, confirming the conversion process of NiSb2 into Li3Sb/Ni0. It is important to keep in mind that ex situ measurements are performed on electrodes which have already relaxed, while in situ measurements give a real time signature. Such results demonstrate the metastability of the Li3Sb/Ni0 discharged electrode that evolves with time under OCV conditions. Relaxation phenomena in the electrode at low potential are here clearly confirmed, in agreement with the potential increase of the NiSb2/Li battery under OCV conditions. This potential increase and stabilization around 0.8 V is the chemical mark of a new equilibrium related to the stabilization of a new phase coming from a spontaneous reaction between Li3Sb and highly reactive Ni nanoparticles.39 2.3. Thermodynamic Intriguing Properties of the Electrochemically Formed Phases. As we discussed earlier, intermediate phases can be formed before the conversion of pnictide electrodes into LixX/M0 at the end of the discharge. Most of them are not described in the literature due to the specific growth conditions inside the battery. Phases formed during the charge process can also show intriguing thermodynamic features, as we will see hereafter with NiSb2. NiSb2. As described above, the conversion mechanism of NiSb2 vs Li into Li3Sb/Ni0 was demonstrated by combining XRD, Sb Mössbauer spectrometry, and XAS analyses.

Figure 3. Evolution of a FeP electrode with time after two complete discharges. Mössbauer spectra recorded (a) immediately after the second discharge at 77 K and 2 (b), 5 (c), 7 (d), and 9 days (e) and 4 weeks (f) after the end of the electrochemical test at 295 K.

recorded at 77 K immediately after the second discharge. It shows the sextet and singlet of magnetically blocked and superparamagnetic iron, respectively, and a sextet of magnetically ordered FeP. Spectrum b was recorded at room temperature 2 days after the discharge was stopped. We can still detect the presence of metallic iron, although the lines of the sextet collapse to an extremely broad singlet due to the higher fluctuation rate of the magnetization at room temperature as compared to 77 K. Finally, FeP gives rise to a doublet. Spectrum c was recorded 5 days later. No metallic iron was found, neither magnetically blocked nor superparamagnetic species. Instead, the doublet of the ternary phase LiFeP comes into view. During the following 3 weeks, its intensity keeps increasing (spectra d−f). Spectrum f can be considered as the new equilibrium state, reflected by an increase of the open circuit voltage (OCV) from zero to 0.22 V, which is the characteristic potential of LiFeP formation. Note that LiFeP is predicted to be stable by theoretical calculations.25 Field-cooled/zero-fieldcooled magnetic measurements have also been performed on a fully discharged electrode and have shown the presence of the metallic iron grains, nanometer size (4−5 nm).26 The massive interface formed between nanoscale solid phases Fe and Li3P provides a pathway for ionic transport which favors the spontaneously formation of the intermediate LiFeP phase from Li3P/Fe0 even at room temperature in OCV mode. 10534



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Figure 4. Mechanism for the lithiation of the NiSb2 electrode with the formation of a modified high pressure NiSb2 at the end of charge.

Figure 5. Potential profiles (left) and their derivatives (right) for TiSnSb (a) and (Ti+Sn+Sb) intimate grounded powdered electrode (b).

the normal marcasite type to the new polymorph is reversible and reproducible. As described in Figure 4, the main difference between HP- and m-NiSb2 is found in the linkage of NiSb6 octahedra (Figure 4), presumably caused by a change in the ligand field. In the m-NiSb2, NiSb6 octahedra are arranged by sharing two edges with adjacent octahedra parallel to the c-axis, when they are linked to the adjacent octahedral by sharing one edge and four corners in HP-NiSb2. This leads to an effective packing of NiSb6 octahedra, as compared to the ambient pressure form. The stabilization of this high pressure polymorph at ambient temperature and pressure is intriguing and shows the

It is worth noting that the Li3Sb formed during the discharge of NiSb2 is the high temperature Li3Sb polymorph and not the hexagonal low temperature Li3Sb phase. This odd thermodynamic behavior has been previously already reported for antimonides.40,41 In addition, we have demonstrated that, upon charge, the lithium extraction occurs through an original conversion process, leading to the formation of the high pressure NiSb2 polymorph (HP). This NiSb2 polymorph was described in the literature by Takizawa et al.,42 as obtained by high pressure/ temperature treatment of normal marcasite-type NiSb2 (mNiSb2 hereafter) at 6 GPa and 600 °C. The phase transition from 10535



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difficulties in assessing the thermodynamic features in the battery, especially for conversion materials. This last part has proved the difficulty in identifying the phases which are formed upon cycling of the battery likely due to (i) loss of an atomic long-range order in these phases which requires the use of atomic-scale investigations, (ii) an instability for which it is necessary to use in situ characterization techniques, and finally (iii) the unexpected nature of these phases often unknown in the literature. 2.4. Conversion Mechanism: A Story of Interfaces. Conversion reactions are systematically associated with a high voltage hysteresis between the charge and the discharge. Despite the fact that the voltage hysteresis decreases with the covalent M−Pn bonding and is smaller for pnictides than for oxides or fluorides,2 it still remains important. The origin of the hysteresis is partly linked to a thermodynamic origin that is correlated to the inherent nature of the starting material and to its modification during the cycling process. The asymmetric voltage response upon charge and discharge is due to the growth of different interfaces, which induces different electrochemical equilibriums. This hysteresis voltage has been computed for conversion and is in very good agreement with experiments, as was recently demonstrated by Doublet et al.43 A new methodology based on first-principles DFT calculations has recently been proposed by the same group for rationalizing this phenomenon taking into account the interface electrochemistry in the conversion materials.44 It is noticeable that a large polarization is often consistent with a poor reversibility. The next part of our discussion will be dedicated to the impact of the interfaces, between the phases produced during the discharge, and the reversibility of the conversion mechanism. One can observe that another part of the hysteresis of conversion reactions is associated with a kinetic contribution and can be avoided or seriously weakened by appropriate engineering. Some examples will be discussed in the section 3. 2.4.1. Solid/solid Interfaces in the Hearth of the Conversion Mechanism. The lithium-ion-battery electrode material TiSnSb shows excellent electrochemical performance in terms of capacity (550 Ah/kg) and rate capability (over hundreds of cycles).45 To illustrate the key role of the solid/solid Li3X/M interfaces (in the discharged electrode) in the setting up the reversible conversion reaction in TiSnSb, a full study comparing the electrochemical mechanisms of TiSnSb and a Ti/Sn/Sb composite vs Li has been undertaken by combining XRD, 119Sn Mössbauer, and 7Li NMR spectroscopic in situ measurements.46 During the first discharge, TiSnSb experienced a direct conversion reaction (eq 4) at the same time as Ti/Sn/Sb composites proceed by a stepwise alloying process (eq 5), with TiSnSb showing a larger polarization than Ti/Sn/Sb composites (Figure 5), both leading to a mixture of Li3Sb, Li7Sn2, and Ti (eqs 4 and 5). More amazingly, the charge occurs differently regenerating a similar “TiSnSb” phase in the first case (eq 4′) and the formation of Sn and Sb in the second case (eq 5′).

Discharge: Ti/Sn/Sb + 6.5Li → Li3Sb + 0.5Li 7Sn2 + Ti

Charge: Li3Sb + 0.5Li 7Sn2 + Ti → Sn + Sb + Ti + 6.5Li

Figure 6. Schematic view of the mechanism for the lithiation of the TiSnSb electrode showing the beneficial effect of starting from crystalline TiSnSb for the setting up of reversible conversion in cycling.

atoms, provides a structural fingerprint or a memory for the [Li3Sb/Li7Sn2/Ti] mixture in the discharged electrode. Due to this memory effect, the Li3Sb, Li7Sn2, and Ti very small particles are homogeneously distributed and in very close contact through nanosized interfaces. This electrochemical formation of Ti metallic nanoparticles embedded in the homogeneous Li3Sb/ Li7Sn2 matrix allows a rapid mass transport, likely due to a short diffusion distance between the redox centers producing a good reversibility and life cycling. In contrast, an agglomerate of the composite Ti/Sn/Sb leading to Li3Sb, Li7Sn2, and Ti species is

(4)

Charge: Li3Sb + 0.5Li 7Sn2 + Ti → “TiSnb” + 6.5Li

(5′)

Remembering (i) that the morphology, the grain size, the chemical nature of the active metals, the electrode formulation process, and the cycling conditions were identical for both pristine electrodes and (ii) that the chemical nature of the discharged electrodes had been shown to be similar in all cases, i.e., a mixture of Li3Sb, Li7Sn2, and Ti, we concluded that the better performance of TiSnSb compared to Ti/Sn/Sb electrode is related to intrinsic parameters. In the reversibility of the conversion process, the crucial role of the solid/solid interfaces between the LixX (X = Sn, Sb) matrix and the Ti metal nanoparticles was beyond a doubt demonstrated. Further, we showed that the nature of these interfaces between lithiated phases (Li3Sb, Li7Sn2) and Ti in the discharged electrode is connected to the long-range order of Sb, Sn, and Ti atoms in the starting crystallized material. As presented by the schematic view in Figure 6, the starting crystalline TiSnSb, with a long-range order of Ti, Sn, and Sb

Discharge: TiSnSb + 6.5Li → Li3Sb + 0.5Li 7Sn2 + Ti

(5)

(4′) 10536



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Figure 7. Combination of characterization techniques to investigate the TiSnSb electrode/electrolyte surface reactions.

is a sloping curve that follows the conversion plateau due to the formation of a SEI. Besides the charge consumed during the reduction of the solvent, this polymeric layer is also known to be additional lithium storage on its surface in a capacitive way,48 able to contribute to some extra capacities. An enhancement of the electrolyte decomposition can be induced by using nanoparticulate electrodes.49 Although numerous studies have already been dedicated to the degradation of electrolyte in the carbon systems, as far as we know, just relatively few studies have been focused on the SEI mechanism for conversion electrodes. In the case of pnictide materials, opposite to the benefit from nanosized solid/solid interfaces (see section 2.4.1) concerning the reversibility and associated cyclability, reactions occurring at the solid/liquid interfaces between the active material (AM) and the organic liquid electrolyte represent the main issue for the conversion reaction electrode materials.50 The nature of the electrolyte decomposition has been analyzed extensively for carbon-based negative electrodes but much less in conversiontype material.51,52 These side reactions with the electrolyte are commonly concomitant to the electrochemical reduction process and have been studied on TiSnSb conversion-type electrode material.53 In spite of promising electrochemical performance in terms of capacity and capacity retention, the Coulombic efficiency of TiSnSb/Li remains far away from the expected 100%. At low potential, the high surface area caused by the phase changes upon cycling emphasizes electrolyte degradation, which still consists of the main factor limiting the Coulombic efficiency. To fully understand the formation of SEI in the presence of alkylcarbonate-based electrolytes and its evolution upon cycling, surface characterization techniques were combined. Electrochemical impedance spectroscopy (EIS) was used for monitoring

produced during the discharge accompanied by a memory effect of microsized grains of Sb, Sn, and Ti initially present in the starting electrode. The microsized interfaces between Li3Sb, Li7Sn2, and Ti do not allow the formation of a ternary phase and instead result in separate Sn and Sb particles. In conclusion, starting from a crystalline phase and minimizing the sizes of the interfaces between the metallic nanoparticles M0 and the lithiated LixM matrix favors the reversibility of the conversion process. Indeed, starting from a crystalline TiSnSb, we could reach an impressive electrochemical performance, with the electrode maintaining a 550 Ah/kg capacity after 250 cycles at a rapid 4C rate (4Li/h). These achievements were significantly improved in the course of our latest studies. The memory effect discussed above is quite different from those so far reported in the literature. In fact, the memory effect has been directly related to the morphology for the cobalt oxides, where regardless of the disintegration into metallic particles of 1−10 nm in size, dispersed in the matrix of LinX, conservation of the initial particle shape was reported.1,16 More recently, a “memory effect” has also been reported for a charge of the positive electrode LiFePO4.47 This peculiar phenomenon happens after only one cycle of partial charge and discharge and was explained by a particle-by-particle connection charge/discharge model. This effect commonly present in the battery use is especially enhanced in the case of a slight voltage modification; it can lead to substantial miscalculations in estimating the state of charge of batteries. 2.4.2. Solid/Liquid Interfaces. For conversion compounds, the analysis of the capacity values, reported upon the first discharge at low potential, readily reveals an inclination toward higher capacity values with respect to those predicted by the conversion reaction. The characteristic of the additional capacity 10537



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between the AM particles and the C network, or between the electrode and the current collector, is a source of capacity fade, especially for electrodes that present volume changes upon charge and discharge processes.59,60 Distribution of connectivity/resistance between AM particles and the carbon matrix exists and may affect the electrode degradation mechanism.61−63 The conversion processes entail massive structural reorganization and volumetric changes, which lead to particle isolation and cracking as a result of electrode grinding, and to a subsequent fading of the capacity after a few cycles. Attempts to solve this issue using the most suitable polymer binder and conductive additive have been done. For FeSn2, NiSb2,64 and TiSnSb,65 three conversion-type AMs, this approach has resulted in remarkable improvements, indicating that a considerable amount of carbon coupled with an adequate choice of binder might be a suitable baseline strategy for conversion-based electrodes. These kinds of formulated electrodes show very good homogeneity, adhesion to the current collector, and relatively high porosities (75%). Both binders and carbon additives allowed capacity retention to be enhanced as well as the rate capability of the conversion intermetallic-based negative electrodes vs Li. As illustrated in Figure 8 for TiSnSb, carboxymethyl

in situ the resistivity of the SEI, while XPS and solid state NMR spectroscopy provided useful information on the solid electrolyte interphase chemical composition and its evolution during cycling (Figure 7). For the first discharge in alkylcarbonates containing LiPF6, we demonstrated that a solid electrolyte interphase is intensively formed at the TiSnSb electrode surface. The SEI grows at low voltage until the end of the conversion reaction through the creation of new interfaces. The capacity loss systematically observed during the first cycle is strongly related to that SEI growth wherein a part of Li ions are trapped. EIS study indicates that this SEI is relatively thick and resistive, and XPS and NMR analyses both prove it is mainly constituted of lithiated carbonate species such as ROCO2Li, Li2CO3, and polycarbonates but also likely of LiF and fluorophosphates. Li2CO3 is the main compound of the SEI present upon discharge. A LiOH species has been identified as well and related to a side reaction with the water contained in the carboxymethyl cellulose (CMC) used as a binder. During the charge process, a decrease in the thickness of the SEI and a reorganization of the layer caused by the contraction of the electrode occur simultaneously with partial LiF dissolution. It has been observed that the SEI formation is depending on the rate of cycling, leading to a better Coulombic efficiency when rapid cycling rates are applied. This could be related to the formation of a thinner and less resistive SEI layer at high cycling rates. The aging also has an impact on the SEI formation. After 20 cycles, a growth of the SEI is still observed, but the composition has changed in the course of the cycling. The major compound in SEI formed at the electrode surface is no longer Li2CO3 but LiOH. This preliminary study showed that the species due to alkylcarbonate-based electrolyte degradation are the same as those observed with a graphite electrode. The use of more efficient SEI former additives is required (and discussed in the next part), to limit the amount of Li trapped in the SEI which leads to a poor Coulombic efficiency and prevents their use in future Li-ion applications. It is noteworthy that, although the SEI has typically been studied for materials that react at low voltages, side reactions involving the electrolyte have also been observed following the conversion of metal fluorides, hinting at the possible catalytic effect of the generated metal nanoparticles.54

3. TOWARD IMPROVEMENT OF CONVERSION ELECTRODE MATERIALS To date and despite their very attractive specific capacities, conversion pnictide materials have suffered from limited cycle life mainly due to agglomeration of active elements, reducing reactivity toward the electrolyte, and strong volume expansion which leads to electrode disintegration upon cycling. Different initiatives to solve those problems by preparing composites either with carbon or a second metal have shown limited success. Consequently, we have decided to use various strategies of electrode structuration and formulation to improve the performance of conversion pnictide electrodes in terms of cycle life and rate capability. This part described the main strategies we used: (i) the electrode formulation through the selection of the binder and of the conductive additive, as well as of the processing conditions, and (ii) the nanostructuration of the electrode (on metallic foam, foil, or rods). 3.1. Electrode and Electrolyte Formulation. 3.1.1. Electrode Formulation. Recent results55,56 suggest that contributions of the nonelectroactive components to the electrode polarization can be very significant, and the binder chemistry could play a critical role in the AM/electrolyte interface behavior (Li+ charge transfer and SEI formation).57,58 The loss of contacts

Figure 8. (top) SEM images before and after electrode formulation (CMC/VGCF) of TiSnSb and (bottom) capacity as a function of the number of cycles of formulated TiSnSb films at various rates in comparison with the powdered TiSnSb electrode (red) (C/n rate means insertion/deinsertion of 1 Li in n hours).

cellulose/vapor grown carbon fiber (CMC/VGCF) used as binder/conductive additive allow retaining 100% of the specific capacity during hundreds of cycles at a rapid rate, while a dramatic fading occurs after just a few cycles at a low rate in the case of classical powered electrodes. Due to the formation of a flexible conductible network in the electrode in which the 10538



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Figure 9. (a and b) SEM images of NiP2 deposited on Ni foam at two magnifications and in the inset the corresponding galvanostatic curve; (c) SEM images of NiP2 on Ni nanorods with corresponding EDX (d).

precipitating and accumulating inside the electrode at each cycle. Indeed, TiSnSb electrode can display a capacity of approximately 600 Ah/kg, a cycle life of more than 1000 cycles, limited irreversible losses, and satisfactory rate capability with a capacity of 250 Ah/kg even at high rate (384 Ah/kg). Under such cycling conditions, TiSnSb could favorably compete with silicon if the practical gravimetric capacity is considered.65 3.2. Electrode Structuration. Recently, multiple efforts have been directed toward addressing the cycle life problem by designing the nanoscale of the particles including sol−gel, ballmilling, and electrodeposition.72−74 The use of engineered nanoparticles or electrodes may not only offer the advantage of shortening diffusion paths for lithium, resulting in enhanced capacity and rate capability in the first few cycles, but it may also bypass or at least limit the first cycle textural formatting. On the other hand, reducing the metal particles to nanodimensions does not reduce the extent of volume change but does render the phase transitions that accompany conversion reaction easier, and reduces cracking within the electrode.75 Many strategies have been used to enhance the kinetics of reaction by increasing specific surface area and shortening Li diffusion lengths, like the electrochemically assisted template growth of metallic nanorods onto a metallic foil current collector followed by electrochemical plating of Fe3O4 or Ni3Sn4 onto the rods.76,77 Using such electrodes demonstrated a great improvement in power density as well as in capacity retention.

microparticles of active material (TiSnSb) get entangled, a material volume change occurs during lithium alloying/dealloying without detrimental effect. Furthermore, the high porosity could buffer the expansion of the particles.66 We have also demonstrated that improved cycle life is likely due to better ability of CMC to cover the particles compared to PVDF (polyvinylidene fluoride), resulting not only in stronger interparticles bonding but also in a better SEI layer. If the electrode formulation is crucial for guaranteeing good cycle life, the electrolyte formulation is just as important as developed in the next part. 3.1.2. Role of Additive in Electrolyte. Fluoroethylene carbonate (FEC) and vinyl carbonate (VC) have been shown to be promising electrolyte additives which improve the cycle life of Si thin films67−69 as well as the Si-based composite electrodes70 and Si-nanowire (SiNW) electrodes.71 For TiSnSb electrode, we recently reported significant electrochemical performance improvement when formulated as described above (CMC/carbon nanofibers) and cycled in fluoroethylene carbonate containing electrolyte. On discharge FEC is reduced earlier than ethylene carbonate (EC) (from the standard used electrolyte: LiPF6 in EC:PC:3DMC) and forms a more stable SEI form constituted by long chain flexible polycarbonates, which allows a better stability and ability to accommodate the volume variations of the conversion-type materials and limit the contact between the electrode and the liquid electrolyte. This would reduce the amount of SEI products 10539



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Figure 10. (a) SEM images of Cu nanorods, (b) Cu nanorods covered with Cu3P after phosphorus vaporization, and (c) Cu3P/Cu nanorods after lithiation with corresponding capacity retention (d) (comparison with the Cu3P powder electrode).

3.2.1. Electrode Nanoarchitecturation. Following this way, we have exploited the promise provided by phosphides and antimonides by creating new electrode configurations. Through a vapor-phase procedure, pnictides were directly grown on metallic foam,78 foil,79,80 or nanorods. 81,82 Using such procedures, we were able to obtain carbon-free self-supported pnictide electrodes capable of sustaining high capacities over many cycles while having enhanced rate capabilities as demonstrated for (i) NiP2 deposited on Ni foam78 (Figures 9a, 9b) or (ii) Cu3P grown on Cu nanopillars (Figures 10a, 10b, 9c, 9d).81 The enhanced performances of supported nanostructured metal-pnictide (Figures 9b, 10d)were attributed to (i) the optimized interface between the active material and the current collector, improving the electron transport; (ii) the larger surface area developed by the supported active material toward the liquid electrolyte, improving the ionic transport; and (iii) the mechanical buffering of the volume changes upon cycling thanks to the free intrinsic volume of the nanostructured composite electrode (Figure 10c). Despite the issues associated with the cost of these synthetic methods and with the lower volumetric energy density due to the thinness of these films, the benefit of such electrode structuration is unquestionable. 3.2.2. Mesoporous Carbon/Pnictide Composites. Phosphorus in its red allotropic form can also be very attractive with a theoretical specific capacity of 2596 Ah/kg. However, owing to a poor electronic conductivity, the experimental capacity is far from the theory and dramatically fades after a few cycles. Via the simple vaporization−condensation of red phosphorus onto mesoporous carbon, we have recently successfully prepared efficient P/mesoporous C composite electrodes. 83 Such composites were tested vs Li and have shown enhanced

electrochemical properties in terms of capacity retention upon cycling and rate capability. In contrast to what was observed for pure unsupported phosphorus, Li storage in P/C composite occurs through the reversible formation of Li3P during the discharge process, as clearly evidenced by in situ XRD, leading to capacities higher than 900 Ah/kg after 20 cycles.84 This concept has successfully inspired other groups who have demonstrated improved electrochemical performance of amorphous red phosphorus as an anode material for Na ion batteries85 or for lithium storage.86 The benefit of preparing such composites of pnictides (or more simply P) with mesoporous carbon comes from the various characteristics of the carbon including (i) a high specific area, (ii) a low electrical resistivity, and (iii) a porous mesostructure, allowing good electrolyte accessibility through the electrode and buffering the volume expansion. In the same spirit, the Ni2P/graphene sheet composite has recently been accomplished via a one-pot solvothermal method. The as-obtained Ni2P nanospheres effectively prevent the agglomeration of graphene sheets. The cyclic stability and rate capability of Ni2 P are significantly improved after the incorporation of graphene sheets and can (i) deliver a capacity of 450 mA h g−1 at a current density of 54 mA g−1 and (ii) partly decrease the voltage polarization.87

4. CONCLUSION Our recent research has contributed to undeniably demonstrate that pnictides are exciting electrode materials which deliver high capacities through conversion reactions with lithium. The issues to surmount in order to make these materials available in terms of industrial and commercial reality were clearly identified in previous reviews.3 Concerning pnictides, few issues have been 10540



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performances, as illustrated by recent results provided for mesoporous Fe2O3.88 In this case, the porous structure exhibits extra space to accommodate the volume change during lithium ion insertion/extraction, that can help in maintaining the integrity of the electrode and thus improving the cycling stability and rate performance. Such a benefit is also achievable by using a macroporous architecture, as recently shown for NiO conversion material.89 In the course of advancement of the performance of conversion electrode materials, the electrolyte formulation has not been forgotten. As demonstrated above for the TiSnSb and in other recent studies, additives in electrolytes like FEC can significantly improve the cycling performance of P-, Sn-, and Sbconversion materials, by enhancing the cyclability by minimizing electrolyte decomposition.90 If additives in electrolytes are able to improve the performances of the conversion-type materials, new types of electrolytes have to also be considered as serious options, as recently demonstrated for copper sulfide-based batteries. While rapid capacity fading was found in cells containing carbonate-based electrolytes, copper sulfide cells with ether-based electrolytes (i.e., 1 M LiTFSI in DOL/DME) have shown much better electrochemical performance in terms of capacity retention and Coulombic efficiencies as well.91 The final word has to be given in the direction of the rebirth of the Na systems. Conversion-type electrodes may also be very efficient for sodium batteries. Very encouraging results on Na batteries have recently demonstrated the viability of the use of conversion material as a possible electrode.92−95 Phosphides96 and antimonides97,98 have shown excellent performances in Na batteries through the conversion reaction, which reserves also thermodynamical surprises. As recently published by Adelhalm et al., we propose to investigate deeper into the more comprehensive analysis of sodium-based conversion reactions.99 The promise of enhancing the storage capacity of current electrode materials fully justifies the pursuit of study on the fascinating electrochemical reactivity of conversion materials.

addressed in our group and through collaborations and are reported in this Feature Article. The use of an arsenal of in and ex situ characterization tools allowed getting a deeper insight of the complex conversion mechanism. These in situ tools alloy “catching” signatures of phases which are metastable and not accessible by classical ex situ tools. It has been evidenced for pnictides that (i) an intermediate ternary phase is mostly preferred to the direct conversion to metallic particles and (ii) the composite LixX/M0 electrodes are calendar instable at low potential. We have also pointed out that fundamental work is still needed to understand the thermodynamic oddness in the battery which is reflected by the electrochemical formation in the battery of phases which are unknown in the literature or synthesized under very specific temperature and pressure conditions. On the other hand, from the study of pnictide conversion materials, we assumed that the origin of the voltage hysteresis comes mainly from inherently different paths of reaction and also interfacial thermodynamics during phase transformation, which leads to asymmetric responses upon charge and discharge. The interfaces between the phases which are formed during the conversion reaction have been shown to keep a fingerprint of the starting crystalline phase. This structural memory provides (i) an homogeneous distribution of LixX and M0 in very close contact via nanosized interfaces in the discharged electrode and (ii) rapid mass transport, since the diffusion distances remain short between the redox centers, leading to good reversibility and cyclability. Unfortunately, these multiple interfaces which are in continuous evolution as a function of the discharge and charge accelerate the electrolyte degradation and remain one of the main factors limiting the cycling life, the reversibility, and the Coulombic efficiency of conversion electrodes. To beat the electrode pulverization and the loss of electrical contact that can cause massive particle reorganization (inherent to the complex conversion reaction), we have set out strategies based on electrode engineering. We have demonstrated that playing on the electrode and electrolyte formulation led to unexpected performance in terms of capacity, kinetics, and cyclability. However, many challenges still remain in developing P and Sb conversion-based electrode materials for lithium batteries, because they undergo complex conversion reactions toward lithium, which can deteriorate the electrode and block the kinetics of the electrochemical response. One can see that enhancement of the electrochemical performance can be achieved, either by downsizing to the nanoscale, depositing on nanorods, or embedding with conductive carbon materials. Future works have to be dedicated to developing new methods for synthesizing efficient carbon-pnictide, antimonide composites with the following properties: (i) easy to prepare, (ii) nontoxic, (iii) low cost, (iv) environmentally friendly, and (v) eliminating the need from expensive P or Sb precursors, especially since the natural abundance of Sb will be critical in the near future. A recent study has shown an elegant synthesis of Ni3P nanoparticles embedded in carbon nanosheets, based on using a surfactant/layered-metal hydroxide precursor that has allowed simultaneous carbon coating and phosphorization in a one-step procedure. The nanocomposite exhibited high capacity and good cycling stability. This green synthesis route is able to be readily extended to the preparation of transition metal sulfide by facilely altering the intercalated surfactant. Indeed, a study dedicated to the conversion-type material designing its own porous structure can also be used to enhance

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AUTHOR INFORMATION

Corresponding Author

*Phone: 33 (0) 4 67 14 33 35. E-mail: Laure.monconduit@um2. fr. Notes

The authors declare no competing financial interest. Biography

Laure Monconduit is a Full Senior Research Scientist of the French National Scientific Research Council (CNRS) at the Laboratory for Aggregates, Interfaces and Materials for Energy, Charles Gerhardt 10541



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(15) Gillot, F.; Bichat, M.-P.; Favier, F.; Morcrette, M.; Doublet, M.-L.; Monconduit, L. The Lixmpn4 Phases (M/Pn=Ti/P, V/As): New Negative Electrode Materials for Lithium-Ion Rechargeable Batteries. Electrochim. Acta 2004, 49 (14), 2325−2332. (16) C, L.; Poizot, P.; Leriche, J. B.; Tarascon, J. M. A Transmission Electron Microscopy Study of the Reactivity Mechanism of Tailor-Made CuO Particles Toward Lithium. J. Electrochem. Soc. 2001, 148, A1266− A1274. (17) Xiang, J. Y.; Tu, J. P.; Yuan, Y. F.; Wang, X. L.; Huang, X. H.; Zeng, Z. Y. Electrochemical Investigation on Nanoflower-Like Cuo/Ni Composite Film as Anode for Lithium Ion Batteries. Electrochim. Acta 2009, 54, 1160−1165. (18) Bonino, F.; Lazzari, M.; Rivolta, B.; Scrosati, B. ElectrochemicalBehavior of Solid Cathode Materials in Organic Electrolyte Lithium Batteries - Copper Sulfides. J. Electrochem. Soc. 1984, 131, 1498−1502. (19) Timmons, A.; Dahn, J. R. In Situ Optical Observations of Particle Motion in Alloy Negative Electrodes for Li-ion Batteries. J. Electrochem. Soc. 2006, 153, A1206−A1210. (20) Golodnitsky, D.; Peled, E. Pyrite as Cathode Insertion Material in Rechargeable Lithium/Composite Polymer Electrolyte Batteries. Electrochim. Acta 1999, 45, 335−350. (21) Strauss, E.; Calvin, S.; Mehta, H.; Golodnitsky, D.; Greenbaum, S. G.; denBoer, M. L.; Dusheiko, V.; Peled, E. X-ray Absorption Spectroscopy of Highly Cycled Li/Composite Polymer Electrolyte/ FeS2 Cells. Solid State Ionics 2003, 164, 51−63. (22) Aurbach, D.; Pollak, E.; Elazari, R.; Salitra, G.; Kelley, C. S.; Affi nito, J. On the Surface Chemical Aspects of Very High Energy Density, Rechargeable Li-Sulfur Batteries. J. Electrochem. Soc. 2009, 156, A694− A702. (23) Yamin, H.; Peled, E. Electrochemistry of a Non-Aqueous Lithium Sulfur Cell. J. Power Sources 1983, 9, 281−287. (24) Hall, J. W.; Membreno, N.; Wu, J.; Celio, H.; Jones, R. A.; Stevenson, K. J. Low-Temperature Synthesis of Amorphous FeP2 and Its Use as Anodes for Li Ion Batteries. J. Am. Chem. Soc. 2012, 134, 5532−5535. (25) Boyanov, S.; Bernardi, J.; Gillot, F.; Dupont, L.; Womes, M.; Tarascon, J. M.; Monconduit, L.; Doublet, M. L. FeP: Another Attractive Anode for the Li-Ion Battery Enlisting a Reversible Two-Step Insertion/ Conversion Process. Chem. Mater. 2006, 18, 3531−3538. (26) Boyanov, S.; Womes, M.; Monconduit, L.; Zitoun, D. Mössbauer Spectroscopy and Magnetic Measurements as Complementary Techniques for the Phase Analysis of FeP Electrodes Cycling in LiIon Batteries. Chem. Mater. 2009, 21 (15), 3684−3692. (27) Boyanov, S.; Zitoun, D.; Menetrier, M.; Jumas, J. C.; Womes, M.; Monconduit, L. Comparison of the Electrochemical Lithiation/ Delitiation Mechanisms of FePx (x = 1, 2, 4) Based Electrodes in LiIon Batteries. J. Phys. Chem. C 2009, 113, 21441−21452. (28) Gillot, F.; Boyanov, S.; Dupont, L.; Doublet, M. L.; Morcrette, M.; Monconduit, L.; Tarascon, J. M. Electrochemical Reactivity and Design of NiP2 Negative Electrodes for Secondary Li-Ion Batteries. Chem. Mater. 2005, 17, 6327−6337. (29) Boyanov, S.; Bernardi, J.; Bekaert, E.; Menetrier, M.; Doublet, M. L.; Monconduit, L. P-Redox Mechanism at the Origin of the High Lithium Storage in NiP2-Based Batteries. Chem. Mater. 2008, 21, 298− 308. (30) Larcher, D.; Sudant, G.; Leriche, J. B.; Chabre, Y.; Tarascon, J. M. The Electrochemical Reduction of Co3O4 in a Lithium Cell. J. Electrochem. Soc. 2002, 149, A234−A241. (31) Doe, R. E.; Persson, K. A.; Meng, Y. S.; Ceder, G. First-Principles Investigation of the Li-Fe-F Phase Diagram and Equilibrium and Nonequilibrium Conversion Reactions of Iron Fluorides with Lithium. Chem. Mater. 2008, 20, 5274−5283. (32) Dupont, L.; Grugeon, S.; Laruelle, S.; Tarascon, J. M. Structure, Texture and Reactivity Versus Lithium of Chromium-Based Oxides Films as Revealed by TEM Investigations. J. Power Sources 2007, 164, 839−848. (33) Morales, J.; Sanchez, L.; Martin, F.; Berry, F.; Ren, X. L. Synthesis and Characterization of Nanometric Iron and Iron-Titanium Oxides by

Institute for Molecular Chemistry and Materials, in Montpellier, France. Current research directions include the synthesis and characterization of new electrode materials for Li-ion and post-Li systems: Na-, Mg-, Ca-ion batteries, improvement of performance by playing (i) on the nanostructuration and the formulation of electrode and (ii) on the confinement of active element in carbon matrix and through the study of the mechanisms that govern their Performance by ex and in situ characterizations. She is strongly involved in European and French partnership: active member of the European network ALISTORE-ERI and of the French network RSE2. She also leads or is involved in numerous scientific projects: ANR (ICARES, DESCARTES, ...), FP7 (EUROLIS), bilateral (Israel, U.K., Germany, ...).

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ACKNOWLEDGMENTS L.M. thanks C. Villevieille, M. Womes, D. Zitoun, C. Marino, A. Darwiche, S. Boyanov, F. Gillot, M. L. Doublet, B. Lestriez, H. Wilhelm, M. Sougrati, B. Fraisse, J. Fullenwarth, L. Stievano, W. Zhang, H. Martinez, R. Dedryvère, P. L. Taberna, P. Simon, J. M. Tarascon, and M. Morcrette for participating on the work presented in the present paper, B. Donnadieu and N. Louvain for helping in the writing of this paper, and the academic members of the ALISTORE-ERI for sharing interesting discussions.

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REFERENCES

(1) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J.-M. Nano-Sized Transition-Metal Oxides as Negative-Electrode Materials for Lithium-ion Batteries. Nature 2000, 407, 496−499. (2) Palacin, M. R. Recent Advances in Rechargeable Battery Materials: A Chemist’s Perspective. Chem. Soc. Rev. 2009, 38, 2565−2575. (3) Cabana, J.; Monconduit, L.; Larcher, D.; Palacin, M. R. Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions. Adv. Mater. 2010, 22, E170−E192. (4) Godshall, N. A.; Raistrick, I. D.; Huggins, R. A. Thermodynamic Investigations of Ternary Lithium-Transition Metal-Oxygen Cathode Materials. Mater. Res. Bull. 1980, 15, 561−570. (5) Thackeray, M. M.; Coetzer, J. A Preliminary Investigation of the Electrochemical Performance of Alpha-Fe2O3 And Fe3O4 Cathodes in High-Temperature Cells. Mater. Res. Bull. 1981, 16, 591−597. (6) Gabano, J.-P. Lithium Batteries; Academic Press: New York, 1983. (7) Kaun, T. D.; Nelson, P. A.; Redey, L.; Vissers, D. R.; Henriksen, G. L. High-Temperature Lithium Sulfide Batteries. Electrochim. Acta 1993, 38, 1269−1287. (8) Souza, D.; Pralong, V.; J. Jacobson, A.; Nazar, L. F. A Reversible Solid-State Crystalline Transformation in a Metal Phosphide Induced by Redox Chemistry. Science 2002, 296, 2012−2015. (9) Park, C. M.; Sohn, H. J. Black Phosphorus and its Composite for Lithium Rechargeable Batteries. Adv. Mater. 2007, 19, 2465−2468. (10) Kim, Y. U.; Cho, B. W.; Sohn, H. J. The Reaction Mechanism of Lithium Insertion in Vanadium Tetraphosphide. J. Electrochem. Soc. 2005, 152, A1475−1478. (11) Woo, S. G.; Jung, J. H.; Kim, H.; Kim, M. G.; Lee, C. K.; Sohn, H. J.; Cho, B. W. Electrochemical Characteristics of Ti-P Composites Prepared by Mechanochemical Synthesis. J. Electrochem. Soc. 2006, 153, A1979−1983. (12) Pralong, V.; Souza, D. C. S.; Leung, K. T.; Nazar, L. F. Reversible Lithium Uptake by Cop3 at Low Potential: Role of the Anion. Electrochem. Commun. 2002, 4, 516−520. (13) Doe, R. E.; Persson, K. A.; Meng, Y. S.; Ceder, G. First-Principles Investigation of the Li-Fe-F Phase Diagram and Equilibrium and Non Equilibrium Conversion Reactions of Iron Fluorides with Lithium. Chem. Mater. 2008, 20, 5274−5283. (14) Doublet, M. L.; Lemoigno, F.; Gillot, F.; Monconduit, L. The LixVPn4 Ternary Phases (Pn = P, As): Rigid Networks for Lithium Intercalation/Deintercalation. Chem. Mater. 2002, 14 (10), 4126−4133. 10542



Feature Article

Mechanical Milling: Electrochemical Properties as Anodic Materials in Lithium Cells. J. Electrochem. Soc. 2005, 152, A1748−A1754. (34) Choi, H. C.; Lee, S. Y.; Kim, S. B.; Kim, M. G.; Lee, M. K.; Shin, H. J.; Lee, J. S. Local Structural Characterization for Electrochemical Insertion-Extraction of Lithium into CoO with X-ray Absorption Spectroscopy. J. Phys. Chem. B 2002, 106, 9252−9260. (35) Wang, F.; Robert, R.; Chernova, N. A.; Pereira, N.; Omenya, F.; Badway, F.; Hua, X.; Ruotolo, M.; Zhang, R.; Wu, L.; et al. Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes. J. Am. Chem. Soc. 2011, 133, 18828−18836. (36) Zhou, Y.-N.; Zhang, H.; Wu, X.-J.; Fu, Z.-W. Li3N-Co Nanocomposite: A New Promising Lithium-Ion Storage Material. Electrochem. Solid-State Lett. 2008, 11, A51−A54. (37) Villevieille, C.; Ionica-Bousquet, C. M.; Ducourant, B.; Jumas, J. C.; Monconduit, L. NiSb2 as Negative Electrode for Li-ion Batteries: An Original Conversion Reaction. J. Power Sources 2007, 172, 388−394. (38) Xie, J.; Zhao, X. B.; Cao, G. S.; Zhao, M. J.; Su, S. F. Electrochemical Li-Uptake Properties of Nanosized NiSb2 Prepared by Solvothermal Route. J. Alloys Compd. 2005, 393, 283−286. (39) Marino, C. T. U. M. Ph.D. Thesis, Université Montpellier 2, 2012. (40) Morcrette, M.; Larcher, D.; Tarascon, J. M.; Edstrom, K.; Vaughey, J. T.; Thackeray, M. M. Influence of Electrode Microstructure on the Reactivity of Cu2Sb with Lithium. Electrochim. Acta 2007, 52, 5339−5345. (41) Villevieille, C.; Fraisse, B.; Womes, M.; Jumas, J. C.; Monconduit, L. A New Ternary Li4FeSb2 Structure Formed Upon Discharge of the FeSb2/Li Cell. J. Power Sources 2009, 189, 324−330. (42) Takizawa, H.; Uheda, K.; Endo, T. A New High-Pressure Polymorph of NiSb2. Intermetallics 2000, 8, 1399−1403. (43) Khatib, R.; Dalverny, A. L.; Saubanère, M.; Gaberscek, M.; Doublet, M. L. Origin of the Voltage Hysteresis in the CoP Conversion Material for Li-Ion Batteries. J. Phys. Chem. C 2013, 117, 837−849. (44) Dalverny, A. L.; Filhol, J. S.; Doublet, M. L. Interface Electrochemistry in Conversion Materials for Li-ion Batteries. J. Mater. Chem. 2011, 21, 10134−10142. (45) Sougrati, M. T.; Fullenwarth, J.; Debenedetti, A.; Fraisse, B.; Jumas, J. C.; Monconduit, L. TiSnSb, a New Efficient Negative Electrode for Li-Ion Batteries: Mechanism Investigations by OperandoXRD and Mossbauer Techniques. J. Mater. Chem. 2011, 21, 10069− 10076. (46) Marino, C.; Sougrati, M. T.; Gerke, B.; Pottgen, R.; Huo, H.; Menetrier, M.; Grey, C. P.; Monconduit, L. Role of Structure and Interfaces in the Performance of TiSnSb as an Electrode for Li-Ion Batteries. J. Chem. Mater. 2012, 24, 4735−4743. (47) Sasaki, T.; Ukyo, Y.; Novak, P. Memory Effect in a Lithium-ion Battery. Nat. Mater. 2013, 12 (6), 569−575. (48) Gachot, G.; Grugeon, S.; Armand, M.; Pilard, S.; Guenot, P.; Tarascon, J. M.; Laruelle, S. Deciphering the Multi-Step Degradation Mechanisms of Carbonate-Based Electrolyte in Li Batteries. J. Power Sources 2008, 178, 409−421. (49) Larcher, D.; Bonnin, D.; Cortes, R.; Rivals, I.; Personnaz, L.; Tarascon, J. M. Combined XRD, EXAFS, and Mossbauer Studies of the Reduction by Lithium of Alpha-Fe2O3 with Various Particle Sizes. J. Electrochem. Soc. 2003, 150, A1643−A1650. (50) Qian, C.; Kim, F.; Ma, L.; Tsui, F.; Yang, P.; Liu, J. Solution-Phase Synthesis of Single-Crystalline Iron Phosphide Nanorods/Nanowires. J. Am. Chem. Soc. 2004, 126, 1195−1198. (51) Grugeon, S.; Laruelle, S.; Dupont, L.; Tarascon, J. M. An Update on the Reactivity of Nanoparticles Co-Based Compounds Towards Li. Solid State Sci. 2003, 5, 895−904. (52) Laruelle, S.; Grugeon, S.; Poizot, P.; Dollet, M.; Dupont, L.; Tarascon, J. M. On the Origin of the Extra Electrochemical Capacity Displayed by MO/Li Cells at Low Potential. J. Electrochem. Soc. 2002, 149, A627−A634. (53) Marino, C.; Darwiche, A.; Dupré, N.; Wilhelm, H. A.; Lestriez, B.; Martinez, H.; Dedryvère, R.; Zhang, W.; Ghamouss, F.; Lemordant, D.; et al. Study of the Electrode/Electrolyte Interface on Cycling of a Conversion Type Electrode Material in Li Batteries. J. Phys. Chem. C 2013, 117, 19302−19313.

(54) Gmitter, A.; Badway, F.; Rangan, S.; Bartynski, R. A.; Halajko, A.; Pereira, N.; Amatucci, G. G. Formation, Dynamics, and Implication of Solid Electrolyte Interphase in High Voltage Reversible Conversion Fluoride Nanocomposites. J. Mater. Chem. 2010, 20, 4149−4161. (55) Guy, D.; Lestriez, B.; Bouchet, R.; Gaudefroy, V.; Guyomard, D. Tailoring the Binder of Composite Electrode for Battery Performance Optimization. Electrochem. Solid-State Lett. 2005, 8, A17−A21. (56) Dominko, R.; Gaberscek, M.; Drofenik, J.; Bele, M.; Pejovnik, S.; Jamnik, J. The Role of Carbon Black Distribution in Cathodes for Li Ion Batteries. J. Power Sources 2003, 119−121, 770−773. (57) Sisbandini, C.; Brandell, D.; Gustafsson, T.; Thomas, J. O. Effect of Short-Chain Amine Coatings on the Performance of LiFePO4 Li-Ion Battery Cathodes. Electrochem. Solid-State Lett. 2009, 12, A99−A101. (58) El Ouatani, L.; Dedryvere, R.; Ledeuil, J.-B.; Siret, C.; Biensan, P.; Desbrieres, J.; Gombeau, D. Surface Film Formation on a Carbonaceous Electrode: Influence of the Binder Chemistry. J. Power Sources 2009, 189, 72−80. (59) Kostecki, R.; Lei, J.; McLarnon, F.; Shim, J.; Striebel, K. Diagnostic Evaluation of Detrimental Phenomena in High-Power Lithium-Ion Batteries. J. Electrochem. Soc. 2006, 153, A669−A672. (60) Shim, J.; Striebel, K. A. Characterization of high-Power LithiumIon Cells During Constant Current Cycling - Part I. Cycle Performance and Electrochemical Diagnostics. J. Power Sources 2003, 122, 188−194. (61) Stephenson, D. E.; Hartman, E. M.; Harb, J. N.; Wheeler, D. R. Modeling of Particle-Particle Interactions in Porous Cathodes for Lithium-Ion Batteries. J. Electrochem. Soc. 2007, 154, A1146−A1155. (62) Guy, D.; Lestriez, B.; Guyomard, D. New Composite Electrode Architecture and Improved Battery Performance from the Smart Use of Polymers and Their Properties. Adv. Mater. 2004, 16, 553−557. (63) Zhang, S. S.; Xu, K.; Jow, T. R. Evaluation on a Water-Based Binder for the Graphite Anode of Li-ion Batteries. J. Power Sources 2004, 138, 226−231. (64) Sivasankaran, V.; Marino, C.; Chamas, M.; Soudan, P.; Guyomard, D.; Jumas, J. C.; Lippens, P. E.; Monconduit, L.; Lestriez, B. Improvement of Intermetallics Electrochemical Behavior by Playing with the Composite Electrode Formulation. J. Mater. Chem. 2011, 21, 5076−5082. (65) Wilhelm, H. A.; Marino, C.; Darwiche, A.; Monconduit, L.; Lestriez, B. Significant Electrochemical Performance Improvement of TiSnSb as Anode Material for Li-Ion Batteries with Composite Electrode Formulation and the Use of VC and FEC Electrolyte Additives. Electrochem. Commun. 2012, 24, 89−92. (66) Lestriez, B.; Bahri, S.; Sandu, I.; Roue, L.; Guyomard, D. On the Binding Mechanism of CMC in Si Negative Electrodes for Li-ion Batteries. Electrochem. Commun. 2007, 9, 2801−2806. (67) Ulldemolins, M.; Le Cras, F.; Pecquenard, B.; Phan, V. P.; Martin, L.; Martinez, H. Investigation on the Part Played by the Solid Electrolyte Interphase on the Electrochemical Performances of the Silicon Electrode for Lithium-Ion Batteries. J. Power Sources 2012, 206, 245− 252. (68) Choi, N.-S.; Lee, Y.; Kim, S.; Shin, S.-C.; Kang, Y.-M. Improving the Electrochemical Properties of Graphite/LiCoO2 Cells in Ionic Liquid-Containing Electrolytes. J. Power Sources 2010, 195, 2368−2371. (69) Chen, L.; Wang, K.; Xie, X.; Xie, J. Effect of Vinylene Carbonate (VC) as Electrolyte Additive on Electrochemical Performance of Si Film Anode for Lithium Ion Batteries. J. Power Sources 2007, 174, 538−543. (70) Mazouzi, D.; Delpuech, N.; Oumellal, Y.; Gauthier, M.; Cerbelaud, M.; Gaubicher, J.; Dupré, N.; Moreau, P.; Guyomard, D.; Roué, L.; et al. New Insights Into the Silicon-Based Electrode’s Irreversibility Along Cycle Life Through Simple Gravimetric Method. J. Power Sources 2012, 220, 180−184. (71) Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D. Effect of Fluoroethylene Carbonate (FEC) on the Performance and Surface Chemistry of Si-Nanowire Li-Ion Battery Anodes. Langmuir 2012, 28, 965−976. (72) Mukaibo, H.; Osaka, T.; Reale, P.; Panero, S.; Scrosati, B.; Wacthler, M. Optimized Sn/SnSb Lithium Storage Materials. J. Power Sources 2004, 132, 225−228. 10543



Feature Article

(73) Amadei, I.; Panero, S.; Scrosati, B.; Cocco, G.; Schiffini, L. The Ni3Sn4 Intermetallic as a Novel Electrode in Lithium Cells. J. Power Sources 2005, 143, 227−230. (74) Mukaibo, H.; Sumi, T.; Yokoshima, T.; Momma, T.; Osaka, T. Electrodeposited Sn-Ni Alloy Film as a High Capacity Anode Material for Lithium-Ion Secondary Batteries. Electrochem. Solid-State Lett. 2003, 6, A218−A220. (75) Bruce, P.; Scrosati, B.; Tarascon, J. M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed. 2008, 47, 2930− 2946. (76) Taberna, P.-L.; Mitra, S.; Poizot, P.; Simon; Tarascon, J.-M. High Rate Capabilities Fe3O4-Based Cu Nano-Architectured Electrodes for Lithium-Ion Battery Applications. Nat. Mater. 2006, 5 (7), 567−573. (77) Hassoun, J.; Panero, S.; Simon, P.; Taberna, P. L.; Scrosati, B. High-Rate, Long-Life Ni-Sn Nanostructured Electrodes for Lithium-Ion Batteries. Adv. Mater. 2007, 19 (12), 1632−1635. (78) Gillot, F.; Boyanov, S.; Dupont, L.; Doublet, M. L.; Morcrette, M.; Monconduit, L.; Tarascon, J. M. Electrochemical Reactivity and Design of NiP2 Negative Electrodes for Secondary Li-Ion Batteries. Chem. Mater. 2005, 17, 6327−6337. (79) Boyanov, S.; Annou, K.; Villevieille, C.; Pelosi, M.; Zitoun, D.; Monconduit, L. Nanostructured Transition Metal Phosphide as Negative Electrode for Lithium-Ion Batteries. Ionics 2008, 14 (3), 183−190. (80) Pfeiffer, H.; Tancret, F.; Bichat, M.-P.; Monconduit, L.; Favier, F. d. r.; Brousse, T. Air Stable Copper Phosphide (Cu3P): a Possible Negative Electrode Material for Lithium Batteries. Electrochem. Commun. 2004, 6, 263−267. (81) Villevieille, C.; Robert, F.; Taberna, P. L.; Bazin, L.; Simon, P.; Monconduit, L. The Good Reactivity of Lithium with Nanostructured Copper Phosphide. J. Mater. Chem. 2008, 18, 5956−5960. (82) Villevieille, C.; Ionica-Bousquet, C. M.; De Benedetti, A.; Morato, F.; Pierson, J. F.; Simon, P.; Monconduit, L. Self Supported Nickel Antimonides Based Electrodes for Li Ion Battery. Solid State Ionics 2011, 192, 298−303. (83) Marino, C.; Debenedetti, A.; Fraisse, B.; Favier, F.; Monconduit, L. Activated-Phosphorus as New Electrode Material for Li-Ion Batteries. Electrochem. Commun. 2011, 13, 346−349. (84) Marino, C.; Boulet, L.; Gaveau, P.; Fraisse, B.; Monconduit, L. Nanoconfined Phosphorus in Mesoporous Carbon as Electrode for LiIon Batteries: Performance and Mechanism. J. Mater. Chem. 2012, 22, 22713−22720. (85) Kim, Y.; Park, Y.; Choi, A.; Choi, N.-S.; Kim, J.; Lee, J.; Ryu, J. H.; Oh, S. M.; Lee, K. T. An Amorphous Red Phosphorus/Carbon Composite as a Promising Anode Material for Sodium Ion Batteries. Adv. Mater. 2013, 25, 3045−3049. (86) Wang, L.; He, X.; Li, J.; Sun, W.; Gao, J.; Guo, J.; Jiang, C. NanoStructured Phosphorus Composite as High-Capacity Anode Materials for Lithium Batteries. Angew. Chem., Int. Ed. 2012, 51, 9034−9037. (87) Lu, Y.; Wang, X.; Mai, Y.; Xiang, J.; Zhang, H.; Li, L.; Gu, C.; Tu, J.; Mao, S. X. Ni2P/Graphene Sheets as Anode Materials with Enhanced Electrochemical Properties Versus Lithium. J. Phys. Chem. C 2012, 116, 22217−22225. (88) Xu, Y.; Jian, G.; Liu, Y.; Zhu, Y.; Zachriah, M.-R.; Wang, C. Superior Electrochemical Performance and Structure Evolution of Mesoporous Fe2O3 Anodes for Lithium-Ion Batteries. Nano Energy 2014, 3, 26−35. (89) Wen, W.; Wu, J.-M.; Cao, M.-H. Rapid One-Step Synthesis and Electrochemical Performance of NiO/Ni with Tunable Macroporous Architectures. Energy 2013, 2, 1383−139. (90) Jang, J. Y.; Park, G.; Lee, S.-M.; Choi, N.-S. Functional Electrolytes Enhancing Electrochemical Performance of Sn-Fe-P Alloy as Anode for Lithium-Ion Batteries. Electrochem. Commun. 2013, 35, 72−75. (91) Jache, B.; Mogwitz, B.; Klein, F.; Adelhelm, P. Copper Sulfides for Rechargeable Lithium Batteries: Linking Cycling Stability to Electrolyte Composition. J. Power Sources 2014, 247 (1), 703−711.

(92) Alcántara, R.; Jaraba, M.; Lavela, P.; Tirado, J. L. NiCo2O4 Spinel: First Report on a Transition Metal Oxide for the Negative Electrode of Sodium-Ion Batteries. Chem. Mater. 2002, 14, 2847−2848. (93) Kim, J.-S.; Ahn, H.-J.; Ryu, H.-S.; Kim, D.-J.; Cho, G.-B.; Kim, K.W.; Nam, T.-H.; Ahn, J. H. The Discharge Properties of Na/Ni3S2 Cell at Ambient Temperature. J. Power Sources 2008, 178, 852−856. (94) Sun, Q.; Ren, Q.-Q.; Li, H.; Fu, Z.-W. High Capacity Sb2O4 Thin Film Electrodes for Rechargeable Sodium Battery. Electrochem. Commun. 2011, 13, 1462−1464. (95) Valvo, M.; Lindgren, F.; Lafont, U.; Bjorefors, F.; Edstrom, K. Towards More Sustainable Negative Electrodes in Na-ion Batteries via Nanostructured Iron Oxide. J. Power Sources 2014, 245, 967−978. (96) Fullenwarth, J.; Darwiche, A.; Soares, A.; Donnadieu, B.; Monconduit, L. NiP3: A Promising Negative Electrode for Li- and Na-ion Batteries. J. Mater. Chem. A 2014, 2 (7), 2050−2059. (97) Qian, J.; Chen, Y.; Wu, L.; Cao, Y.; Ai, X.; Yang, H. High Capacity Na-Storage and Superior Cyclability of Nanocomposite Sb/C Anode for Na-ion Batteries. Chem. Commun. 2012, 48 (56), 7070−7072. (98) Darwiche, A.; Marino, C.; Sougrati, M. T.; Fraisse, B.; Stievano, L.; Monconduit, L. Better Cycling Performances of Bulk Sb in Na-Ion Batteries Compared to Li-Ion Systems: An Unexpected Electrochemical Mechanism. J. Am. Chem. Soc. 2012, 134, 20805−20811. (99) Klein, F.; Jache, B.; Bhide, A.; Adelhelm, P. Conversion Reactions for Sodium-Ion Batteries. Phys. Chem. Chem. Phys. 2013, 15, 15876− 15887.

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