Pursuit of Sustainable Iron-Based Sodium Battery Cathodes: Two

Jan 21, 2016 - Case Studies. Prabeer Barpanda*,‡. ‡. Faraday Materials Laboratory (FaMaL), Materials Research Center, Indian Institute of Science,...
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Pursuit of Sustainable Iron-Based Sodium Battery Cathodes: Two Case Studies Prabeer Barpanda*,‡ ‡

Faraday Materials Laboratory (FaMaL), Materials Research Center, Indian Institute of Science, C. V. Raman Avenue, Bangalore 560012, India ABSTRACT: Rechargeable batteries have been the torchbearer electrochemical energy storage devices empowering small-scale electronic gadgets to large-scale grid storage. Complementing the lithium-ion technology, sodium-ion batteries have emerged as viable economic alternatives in applications unrestricted by volume/weight. What is the best performance limit for new-age Na-ion batteries? This mission has unravelled suites of oxides and polyanionic positive insertion (cathode) compounds in the quest to realize high energy density. Economically and ecologically, iron-based cathodes are ideal for mass-scale dissemination of sodium batteries. This Perspective captures the progress of Fe-containing earth-abundant sodium battery cathodes with two best examples: (i) an oxide system delivering the highest capacity (∼200 mA h/g) and (ii) a polyanionic system showing the highest redox potential (3.8 V). Both develop very high energy density with commercial promise for large-scale applications. Here, the structural and electrochemical properties of these two cathodes are compared and contrasted to describe two alternate strategies to achieve the same goal, i.e., improved energy density in Fe-based sodium battery cathodes.

1. INTRODUCTION With the ever-growing global energy consumption, the 21st century acutely needs solutions for large-scale and eco-efficient energy generation−storage−delivery trilogy.1 Innovative technologies (portable electronics to automobiles) and their rapid global dissemination have led to significant increase in global energy consumption. The resulting rapid consumption of fossil fuels and growing CO2 emission have triggered imperative searches of new materials and technologies. It is ideal to have clean energy generation with zero emission, which demands novel materials development for electricity generation along with reliable energy storage. In the energy storage sector, secondary lithium-ion batteries offer the most efficient option for portable electronics, smart grid, and transportation usages. The paucity of Li minerals and their uneven geographic distribution has led to the question: Is lithium the new gold?2 Here, sodium-ion batteries can work wonders as an alternate charge carrier option especially for stationary applications such as grid storage, which are not restricted by volume/weight of the battery. Sodium is a worthy contender in the battery field due to its abundance, uniform global distribution, easy reclamation, material economy, and competitive electrochemical properties. It has led to worldwide research on suites of oxides and polyanionic insertion materials for development of robust sodium batteries.3 Positive insertion (cathode) materials can be generally written as AxMy(XOn)z, where A = alkali metals (Li/Na/Mg/ Zn/Al), M = 3d/4d transition metals, and X = auxiliary anions (P, S, and Si, etc.). While (XOn)z imparts overall thermal/ © 2016 American Chemical Society

structural stability, 3d metals undergo reversible change in redox state parallel to the insertion/extraction of alkali metals. From an elemental composition point of view, it is important to employ abundant alkali and 3d metals to have low-cost batteries. While Na is the most abundant alkali metal, Fe is the ideal 3d metal for cathode design owing to its abundance, economy, and low toxicity. So, the ideal cathode composition can be NaxFey(XOn)z. But there is a problem. The combination of Na−Fe drastically reduces the net redox potential as (i) the Na/Na+ redox couple (−2.71 V vs SHE) is lower than Li/Li+ redox couple (−3.04 V vs SHE) and (ii) the Fe redox couple is over 1 V lower than the Co redox couple. The decrease in redox potential limits the overall energy density. To overcome this issue, it will be ideal to have NaxFey(XOn)z chemistry with either very high reversible capacity (close to 200 mAh/g) and/ or high redox potential (near 4 V). While the former goal can be accomplished with low molecular weight oxides, the latter can be realized by polyanionic chemistry with tunable crystal structure (and local M−O bond coordination). Here, two such champion materials are summarized, one each for oxide and polyanion systems, which demonstrate formidable energy density in earth-abundant “Na−Fe”-based cathodes for secondary sodium batteries. Received: October 7, 2015 Revised: January 21, 2016 Published: January 21, 2016 1006

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2. OXIDE SYSTEM P2−NAX[FE0.5MN0.5]O2 Broadly speaking, transition metal oxides were the “holy grail” in the field of “solid-state chemistry”, starting from magnetic memory, superconductors to primary/secondary batteries. The conception of reversible Li+ insertion into TiS2 ushered the story of rechargeable battery with exploration of various oxidebased cathodes for both Li-ion and Na-ion batteries. Solid-state chemists explored a suite of oxide cathodes starting in the 1980s, some principal materials being Na x CoO 2 and NaxFeO2.3,4 These 3d-metal-containing oxides have a layered structure built from stacks of edge-sharing MO6 octahedra accommodating Na atoms in the interlayer spaces. Based on the stacking along the c-axis, the NaFeO2 oxides broadly adopt either P2- or O3-type structure (as proposed by Delmas et al.),5 where the Na+ site has the prismatic or octahedral coordination, respectively (Figure 1). In the case of P2-structure, edge-

Figure 2. Galvanostatic charge−discharge profile of O3−NaFeO2 vs Na. Reprinted with permission from ref 6. Copyright 2013 Electrochemical Society.

stability up to 300 °C, enough for safe practical application. But, care should be taken to avoid any moisture contamination of O3−NaFeO2, which readily reacts with H2O to form NaOH and FeOOH.6 The net discharge capacity is greatly affected by the voltage window, with higher cutoff voltage leading to further Na+ extraction but with large polarization and low capacity.7,8 Though the degree of Na+ extraction increases with larger cutoff potential, beyond 3.5 V, it leads to structural degradation and irreversible phase transition. Synchrotron study suggests at high-voltage Fe3+ species partially occupy tetrahedral coordination. During desodiation of O3−NaFeO2 cathode, some tetrahedral vacancies are formed adjacent to FeO6 octahedra. The Fe-ions from FeO6 can migrate to adjacent tetrahedal voids as it is energetically favorable. The Fe in tetrahedral voids blocks Na+ migration so as to deterioriate the net electrochemical activity. Although the net electrochemical activity of O3−NaFeO2 is restricted to 35% of its theoretical capacity, it registers Fe4+/Fe3+ electrochemical redox activity involving low polarization and fast Na+-migration. While its commercial implementation is not viable due to poor capacity, it has triggered widespread research into various 3d metal homolgoue oxides (e.g., NaMnO2, NaCoO2, NaNiO2, and NaVO2, etc.) and their solid solutions (e.g., NaFe1/3Co1/3Ni1/3O2, and NaFe1/2Co1/2O2, etc.).9 In general, while the O3-type NaMO2 cathodes deliver limited capacity of 100−120 mA h/g unable to reach even 50% of their theoretical values, the P2-type NaxMO2 (e.g., P2−Na0.6MnO2 and P2−Na0.7CoO2) materials are known to deliver high capacity. In this case, it will be ideal to implement P2−NaxFeO2 as an earth-abundant cathode displaying high capacity. Unfortunately, the synthesis of P2−NaxFeO2 is difficult owing to the instability of Fe4+ species in oxide framework. Because its Mnhomologue (i.e., P2−NaxMnO2) is thermodynamically stable, Yabuuchi et al. cleverly prepared a mixed-metal oxide, P2− Na2/3[Fe1/2Mn1/2]O2, by partial subsitution of Fe by Mn.10 This composition can be made by one-step annealing of oxide precursors at 700−900 °C (in air). This as-synthesized P2-type mixed-metal oxide, when tested in a sodium half-cell system without any further cathode optimization, showed a multistep charge−discharge profile leading to an impressive discharge capacity of 190 mA h/g (Figure 3), which far exceeds the capacity of earlier-discussed O3−NaFeO2. It also delivered decent cycling stability and rate kinetics retaining 70% of

Figure 1. Structural illustration of P2-type and O3-type NaFeO2 polymorphs showing edge-sharing FeO6 octahedra (green) and Na atoms (yellow).

sharing FeO6 octahedra is stacked with two distinct [FeO2]∞ layers (following ABBA pattern) housing the Na-ions in large trigonal prismatic sites. However, the O3-structure (rock-salt structure) is built from edge-sharing FeO6 octahedra stacked having three crystallographically distinct [FeO2]∞ layers (following ABCABC pattern) locating the large Na-ions between these layers in a distinct octahedral site.5 These NaFeO2 layered compounds can be easily synthesized by classical solid-state route involving ambient high-temperature annealing of readily available oxide precursors. In the pursuit of Fe-based sodium insertion materials, NaFeO2 (O3 and P2) polymorphs can be an ideal cathode combining excellent theoretical capacity (QTh = 242 mA h/g), ease of synthesis, and materials economy. Okada’s group reported the first demonstration of electrochemical activity of O3-type NaFeO2 cathode.6 In half-cell sodium architecture, it delivered reversible Na-insertion with a clear plateau at 3.3 V (Figure 2). Considering an Na−Fe-based oxide system, it is a substantially high redox potential. During the initial charge, up to 0.42 mol of Na+ can be extracted leading to a reversible discharge capacity of 85 mAh/g. Mössbauer analysis of O3− NaFeO2 revealed the partial oxidation of (up to 18%) Fe3+ → Fe4+, marking the first such reversible Fe4+/Fe3+ redox activity in any sodium-based cathodes. This cathode offers thermal 1007

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Figure 3. Electrochemical analysis of P2-type Na2/3[Fe1/2Mn1/2]O2 cathode in Na half-cell architecture: (left) Galvanostatic voltage-capacity curves and (right) discharge capacity as a function of rate. Reprinted with permission from ref 10. Copyright 2012 Nature Publishing Group.

capacity even at 1C rate. The initial phase of charging up to 3 V (first step) can be assigned to Mn4+/Mn3+ redox activity whereas the final phase of charging (3−4.3 V) is ascribed to Fe4+/Fe3+ redox reaction as proved by synchrotron XRD and Mössbauer spectroscopy. Throughout the Na-(de)insertion process, a single-phase (solid-solution) mechanism is observed involving reversible gliding of [Fe1/2Mn1/2]O2 layers. With an average redox potential of 2.75 V (vs Na), P2− Na2/3[Fe1/2Mn1/2]O2 is the second such example of sodium cathode involving Fe4+/Fe3+ redox reaction delivering an energy density exceeding 520 W h/kg. This high energy density coupled with low cost and easy and scalable synthesis makes P2−Na2/3[Fe1/2Mn1/2]O2 the champion oxide system with commercial promise to rival Li-based batteries in largescale applications.

3. POLYANIONIC SYSTEM NA2+2XFE2−X(SO4)3 Oxide cathodes do command a long-standing pedigree of fundamental research and scientific understanding as well as product commercialization. However, their limited (layered) crystal structure and strong M−O bonding restricts the net redox potential. Further, possible structural instability limits the degree of desodiation in oxide cathodes. In this scenario, Manthiram and Goodenough suggested “inductive effect principle” to realize tunability of the M redox center.11,12 It suggests the addition of a spectator anion X (X = B, P, S, Si, V, Mo, and W, etc.) can create strong X−O bonding thereby inducing ionicity in M−O bonding. The (weaker) ionic bonding in M−O increases the distance between its antibonding orbitals vis-à-vis Na/Na+ redox couple, leading to higher redox potential. Thus, moving from oxides (NaxFeyOz) to polyanionic composition [NaxFey(XOn)z] can offer materials with high redox potential. Unlike the layered oxides, the presence of polyanion develops a robust three-dimensional framework structure with chemical/thermal stability with the possibility of complete desodiation in principle. Over the past decade, various polyanionic insertion materials have been unveiled such as Na2FePO4F, Na2FeP2O7, NaFeSO4F, and Na4Fe3(PO4)2(P2O7), etc.3 Nevertheless, these spectator polyanions augment the molecular weight and thereby decrease the theoretical capacity. The only way polyanionic compounds can compete with oxide cathodes is by having high Fe redox potential, the higher the better. As per the inductive effect, higher electronegativity in X can draw O closer to itself thereby enhancing the ionicity of local Fe−O bonds and the formal redox potential of Fe center. In this case, looking at the trend of Pauling electronegativity values of elements (Figure 4), it is ideal to choose X = S having the highest electronegativity. Sulfur chemistry is known to have high-voltage operation in

Figure 4. Pauling electronegativity values of various anions with sulfur (highlighted by circle) showing the highest value.

secondary batteries.13−15 However, it was barely pursued for sodium-ion batteries with the exception of Fe2(SO4)3 (3.3 V vs Na)16 and NaFeSO4F (3.5 V vs Na)17 with limited electrochemical performance. Recently (2014), putting SO4 chemistry on the anvil, a novel alluaudite-structured Na2+2xFe2−x(SO4)3 insertion compound has been reported having a capacity of 100 mA h/g with the highest Fe3+/Fe2+ redox potential (ca. 3.8 V vs Na), thereby delivering robust energy density.18 The target compound, Na2+2xFe2−x(SO4)3, was initially synthesized by classical solid-state synthesis using an intimate 2:3 molar precursor mixture of Na2SO4 and anhydrous FeSO4. Since SO4 materials are prone to thermal decomposition over 450 °C involving SO2 gas evolution, the annealing was conducted at moderate temperature of 350 °C for 24 h under steady Ar-flow. It yields the desired alluaudite product albeit with minor impurities such as thermodynamically stable FeSO4, Na2Fe(SO4)2·4H2O, and Na6Fe(SO4)4. Following, this compound can also be obtained by using FeSO4·H2O monohydrate precursor. These monohydrate and anhydrous precursors can be prepared by controlled annealing of commercial FeSO4·7H2O at 100−250 °C for 2−5 h (under Ar-flow). Pursuing alternate synthesis, various solvothermal methods involving two steps such as ionothermal, polyolassisted synthesis, and spray drying as well as chimie-douce method have been successfully developed.19 They all lead to the same product with different size/morphology involving the final annealing temperature range of 300−350 °C for 6−24 h. For example, ionic liquid or diol can act as a templating agent and/ or chelating media to develop nanoscale and platelet morphology, whereas spray drying using water can develop 1008

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Figure 5. Formation of various products in Na2SO4−FeSO4 binary phase diagram. The off-stoichiometric Na2+2xFe2−x(SO4)3 is a thermodynamically stable product. All other compounds can appear as impurity/unreacted species.

slightly lower energy (i.e., stable state) in the case of C2/c, albeit with energy near similar to that of the P21/c case. The Na2+2xFe2−x(SO4)3 structure is strikingly different from the earlier reported NASICON-type AxFe2(SO4)3 consisting of lantern units [Fe2(SO4)3]. Instead, it belongs to a broad alluaudite family with generic formula AA′BM2(XO4)3, where B = Na-1 site, A and A′ = partially occupied Na-2 site and Na-3 site, respectively, M = Fe site, and X = S. Here, all Fe occupy a crystallographically distinct site. The structure is built from isolated edge-sharing FeO6−FeO6 [Fe2O10] octahedral dimers, abridged together by SO4 tetrahedra strictly in a corner-sharing fashion (Figure 3b), delimiting tunnels along the c-axis. The Na occupies three crystallographically distinct sites: Na-1, Na-2, and Na-3 (Figure 3). While Na-1 is fully occupied albeit with no open channels for migration, the Na-2 and Na-3 are partially occupied with large tunnels for efficient one-dimensional migration (along the c-axis). Theoretical study as well as bond valence sum (BVS) calculation point at very fast Na+ dynamics particularly for Na-3 sites, which can be beneficial for cathode application. Although the alluaudite class of cathodes is well-known [e.g., mixed-valence NaMnFe 2 (PO 4 ) 3 ], 22 Na2+2xFe2−x(SO4)3 marks the first sulfate-based alluaudite with solely Fe2+ without any Na−Fe site mixing. It paves the way for discovery of other 3d alluaudite homologues such as Na2+2xMn2−x(SO4)3.23 The open alluaudite framework is suitable for Na+-insertion dynamics. While the Na+ in Na-2 and Na-3 sites can be easily extracted along the c-axis, the Na+ in the Na-1 site can take part in electrochemical activity by jumping to Na-3 sites. Thus, theoretically complete (de)sodiation reaction can be realized leading to its theoretical capacity. When this newfound compound was tested in standard half-cell architecture, it delivered reversible Na-(de)insertion with capacity approaching 100 mA h/g (Figure 7), which is over 80% of the theoretical capacity. As per DFT calcualtions, complete desodiation seems improbable as it leads to structural instability in Fe2(SO4)3 and irreversible desodiation reaction. There is a drop between first and subsequent charge profiles, which hints at irreversible structural rearrangement during the first desodiation process, similar to the case of Li2FeSiO4 and Li2FeP2O7 insertion materials.24 The presence of a gradual sloping voltage-capacity profile involving minimal volume strain (∼2%) is a signature of underlying single-phase (solid-solution) redox behavior. This low strain is a plus point for long-term cycling involving less aggressive electrochemical grinding during battery operation. A keen look reveals the presence of a multistep voltage profile undergoing a series of structural ordering during the Na+ (de)intercalation process with the average Fe3+/Fe2+ redox potential centered at 3.8 V (vs Na). It benchmarks the highest Fe redox potential ever reported among all known polyanionic materials as well as oxide cathodes even including Fe4+/Fe3+ redox value in the high-capacity oxide cathodes as described in

uniform homogeneous spherical nanoparticles. Detailed reports on these solvothermal syntheses will be reported in the future. Though initially, the alluaudite product was thought to be stoichiometric Na2Fe2(SO4)3, the need to use an offstoichiometric sodium-rich precursor mixture hinted at possible formation of off-stoichiometric composition. As shown in Figure 5, the reaction between Na2SO4 and FeSO4 can form metastable Na2Fe(SO4)2, stable vanthoffite Na6Fe(SO4)4, and a range of off-stoichiometric Na2+2xFe2−x(SO4)3 products. Attempts to produce stoichiometric Na2Fe2(SO4)3 invariably led to alluaudite products with a significant presence of impurities such as α-/β-polymorphs of FeSO4 and Fe2O3. To consume the (unreacted) FeSO4 impurities, an excess amount of Na 2 SO 4 is required forming off-stoichiometric Na2+2xFe2−x(SO4)3 (x = 0.2−0.25).20,21 Excess usage of Na2SO4 leads to sodium-rich impurities such as vanthoffite Na6Fe(SO4)4. At this point, it seems the stoichiometric Na2Fe2(SO4)3 is metastable in nature. Because Fe-deficiency leads to lower theoretical capacity in off-stoichiometric alluaudites, it is important to find a soft-chemistry approach to stabilize stoichiometric Na2Fe2(SO4)3 having the highest capacity (ca. 120 mA h/g) among all alluaudites. It remains a challenge for chemists. The Na−Fe−S−O alluaudite assumes a monoclinic framework (Figure 6). Initially, it was a puzzle as to whether it has P21/c (No. 14) or C2/c (No. 15) symmetry. Detailed investigation using synchrotron radiation points at C2/c symmetry. Further, recently first-principles calculations confirm

Figure 6. (a) Illustration of the alluaudite Na2+2xFe2−x(SO4)3 crystal structure consisting of interconnected FeO6 octahedra (green) and SO4 tetrahedra (yellow) having Na atoms (blue) in three distinct sites. (b) Bridging of neighboring edge-sharing Fe2O10 dimers by SO4 units and local coordination of (c) Na-site 2 and (d) Na-site 3. While the Na-1 site is fully filled, the Na-2 and Na-3 sites are partially occupied. 1009

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Figure 7. Electrochemical measurement of alluaudite Na2+2xFe2−x(SO4)3 insertion material: (left) galvanostatic potential-capacity curve depicting a reversible 3.8 V activity resulting from multistep redox behavior (inset); (right) discharge capacity as a function of cycling rate showing fast (de)sodiation kinetics. The discharge voltage profiles at different rates are compared in the inset. Reprinted with permission from ref 18. Copyright 2014 Nature Publishing Group.

polyanionic insertion compounds have been unravelled, practical Fe-based high-performance cathodes have been recently realized in (i) P2-type Na2/3[Fe1/2Mn1/2]O2 oxide (energy density = 520 W h/kg) and (ii) Na2+2xFe2−x(SO4)3 polyanionic material (energy density = 380−455 W h/kg) discovered in 2012 and 2014, respectively. While the former has the highest reversible capacity, the latter show the highest Fe redox potential among all known Fe-based insertion materials, as illustrated in Figure 8. These materials can be easily

the earlier section. It can also rival Li-based commercial cathode (e.g., LixCoO2) in terms of energy density. Operating well within the practical safe limit of current generation organic electrolytes, it works as a high-voltage insertion host both for Na+ and Li+ carriers.18,21 This unusually high Fe3+/Fe2+ redox potential can be rooted to its crystal structure. The presence of edge-sharing FeO6 octahedra brings the central Fe atoms closer to each other, which develops more pronounced repulsion in the charged state (i.e., Fe3+−Fe3+ repulsion).25 This leads to high Gibbs free energy in the charged state [E(NaFeIII2(SO4)3)], thereby developing higher (net) redox potential (V) that can be expressed as V = E(Na) + E(NaFe III 2(SO4 )3 ) − E(Na 2Fe II 2(SO4 )3 )

Also, the rich presence of electronegative SO4 units favor high redox potential as per inductive effect principle. The alluaudite cathode has decent cycling stability, retaining over 90% of initial discharge (first cycle) capacity even after 50 cycles. In addition, it demonstrated very fast rate kinetics retaining over 85% and 70% of initial capacity (at C/20 rate) even at fast rates of 1 and 10 C, respectively (Figure 7). It makes alluaudite Na2+2xFe2−x(SO4)3 an ideal cathode having a high-voltage-propelled greater energy density along with fast rate kinetics and cycling stability. It is crucial to note this material can be obtained from economic earth-abundant precursors and can be prepared by low-temperature routes. Further, the as-synthesized compound can be utilized in battery fabrication without any further cathode optimization such as particle downsizing and/or carbon coating. One major concern is its stability. Like all SO4-based compounds, the alluaudite sulfate is prone to moisture attack, which can be taken care of by careful handling in inert ambience. Also, it tends to decompose above 450 °C, but it provides enough stability for practical battery applications. Overall, the newfound alluaudite forms a commercially promising cathode for large-scale sodium batteries.

Figure 8. Summary of observed redox potential (vs Na) and gravimetric capacity of major Fe-based cathode insertion materials for sodium batteries. Selected Fe-based polyanionic compounds (blue), oxide cathodes (green), and alluaudite cathodes (yellow) are illustrated. The best oxide and alluaudite cathodes (marked with star symbol) deliver very high energy density.

synthesized and offer desirable electrochemical performance without demanding any cathode optimization such as carbon coating, which is beneficial to obtain practical cathode with good tap density. They can be integrated into a practical battery using Na-containing anodes (such as Na2Ti6O13) to make a Nametal-free safe operational battery. Because Na does not undergo alloying with Al, a cheap battery can be assembled using Al (instead of Cu) current collectors. Based on elemental abundant Na−Fe−Mn−O and Na−Fe−S−O systems, these two champion Fe-based cathodes have comparable or better

4. CONCLUSIONS Sodium-ion batteries have emerged as a viable economic alternative to Li-ion batteries, particularly where weight/volume restriction does not apply. In the quest to take sodium batteries from laboratory to commercial market, we must steer away from rare metals (Co/Ni) to earth-abundant metals (Fe/Mn) based cathode materials. While numerous oxides and 1010

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(10) Yabuuchi, N.; Kajiyama, M.; Iwatate, J.; Nishikawa, H.; Hitomi, S.; Okuyama, R.; Usui, R.; Yamada, Y.; Komaba, S. P2-type Nax(Fe1/2Mn1/2)O2 made from earth-abundant elements for rechargeable Na batteries. Nat. Mater. 2012, 11, 512−517. (11) Manthiram, A.; Goodenough, J. B. Lithium insertion into Fe2(SO4)3 frameworks. J. Power Sources 1989, 26, 403−408. (12) Padhi, A. K.; Manivannan, V.; Goodenough, J. B. Tuning the position of the redox couples in materials with NASICON structure by anionic substitution. J. Electrochem. Soc. 1998, 145, 1518−1520. (13) Barpanda, P. Sulfate chemistry for high-voltage insertion materials: Synthetic, structural and electrochemical insights. Isr. J. Chem. 2015, 55, 537−557. (14) Barpanda, P.; Ati, M.; Melot, B. C.; Rousse, G.; Chotard, J. N.; Doublet, M. L.; Sougrati, M. T.; Corr, S. A.; Jumas, J. C.; Tarascon, J. M. A 3.90 V iron-based fluorosulphate material for lithium-ion batteries crystallizing in the triplite structure. Nat. Mater. 2011, 10, 772−779. (15) Ati, M.; Melot, B. C.; Rousse, G.; Chotard, J. N.; Barpanda, P.; Tarascon, J. M. Structural and electrochemical diversity in LiFe1‑dZndSO4F solid solution: A Fe-based 3.9 V positive-electrode material. Angew. Chem., Int. Ed. 2011, 50, 10574−10577. (16) Okada, S.; Arai, S.; Yamaki, J. Denki Kagaku oyobi Kogyo Butsuri Kagaku 1997, 65, 802−808. (17) Barpanda, P.; Chotard, J. N.; Recham, N.; Delacourt, C.; Ati, M.; Dupont, L.; Armand, M.; Tarascon, J. M. Structural, transport, and electrochemical investigation of novel AMSO4F (A= Na, Li; M = Fe, Co, Ni, Mn) metal fluorosulphates prepared using low temperature synthesis routes. Inorg. Chem. 2010, 49, 7401−7413. (18) Barpanda, P.; Oyama, G.; Nishimura, S.; Chung, S. C.; Yamada, A. A 3.8 V earth-abundant sodium battery electrode. Nat. Commun. 2014, 5, 4358. (19) Dwibedi, D.; Okada, S.; Barpanda, P. Patent in preparation. (20) Oyama, G.; Nishimura, S.; Suzuki, Y.; Okubo, M.; Yamada, A. Off-stoichiometry in alluaudite-type sodium iron sulfate Na2+2xFe2‑x(SO4)3 as an advanced sodium battery cathode material. ChemElectroChem 2015, 2, 1019−1023. (21) Ming, J.; Barpanda, P.; Nishimura, S.; Okubo, M.; Yamada, A. An alluaudite Na2+2xFe2‑x(SO4)3 (x = 0.2) derivative phase as insertion host for lithium battery. Electrochem. Commun. 2015, 51, 19−22. (22) Trad, K.; Carlier, D.; Croguennec, L.; Wattiaux, A.; Ben Amara, M.; Delmas, C. NaMnFe2(PO4)3 alluaudite phase: synthesis, structure and electrochemical propertie as positive electrode in lithium and sodium batteries. Chem. Mater. 2010, 22, 5554−5562. (23) Dwibedi, D.; Araujo, R. B.; Chakraborty, S.; Shanbogh, P. P.; Sundaram, N. G.; Ahuja, R.; Barpanda, P. Na2.44Mn1.79(SO4)3: a new member of the alluaudite family of insertion compounds for sodium ion batteries. J. Mater. Chem. A 2015, 3, 18564−18571. (24) Shimizu, D.; Nishimura, S.; Barpanda, P.; Yamada, A. Electrochemical redox mechanism in 3.5 V Li2‑xFeP2O7 (0 < x < 1) pyrophosphate cathode. Chem. Mater. 2012, 24, 2598−2603. (25) Chung, S. C.; Barpanda, P.; Nishimura, S.; Yamada, Y.; Yamada, A. Polymorphs of LiFeSO4F as cathode materials for lithium ion batteries - a first principle computational study. Phys. Chem. Chem. Phys. 2012, 14, 8678−8682.

energy density even exceeding that of many Li-ion cathodes offering practical opportunity to employ them instead of Li-ion batteries in specific applications. Their reversible capacity can be tailored by (i) morphology engineering (e.g., nanosizing), (ii) intimate and suitable carbon coating, (iii) substitution of Mn/Co into Fe sites [e.g., Na2(Fe1−xMnx)2(SO4)3], and (iv) tuning the Na−Fe stoichiometry (e.g., using Na-rich composition). It attests to the richness of Fe chemistry for cathode discovery and further inspires the battery community to explore Na−Fe systems for real-life sodium-ion batteries.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-80-2293-2873. Fax: +91-80-2360-7316. Notes

The authors declare no competing financial interest. Biography Prabeer Barpanda is currently an Assistant Professor at the Indian Institute of Science (IISc) (Bangalore, India). He received his B.Eng. degree with distinction from National Institute of Technology Rourkela (NITR, India), M.Phil. from the University of Cambridge (U.K.) and Ph.D. from RutgersThe State University of New Jersey (USA). Following 5 years postdoctoral work in UPJV-CNRS (France) and the University of Tokyo (Japan), he joined the faculty at IISc in 2013. He directs the Faraday Materials Laboratory (FaMaL) at IISc broadly exploring inorganic materials chemistry for electrochemical energy storage (http://prabeer.weebly.com).

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ACKNOWLEDGMENTS I acknowledge the Department of Science and Technology (DST), Government of India for research funding under the Solar Energy Research Initiative (SERI) programme (DST/ TMC/SERI/FR/169) and the Department of Atomic Energy (DAE) for a DAE-BRNS Young Scientists Research Award (YSRA). I am grateful to Prof. Shigeto Okada for technical discussions and for hosting me at Kyushu University (Fukuoka, Japan) during the summer of 2015.

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REFERENCES

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DOI: 10.1021/acs.chemmater.5b03926 Chem. Mater. 2016, 28, 1006−1011