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Aqueous zinc-ion batteries form a key post-Li-ion batteries to cater the rising demand for grid storage. Fe-based compounds can be used as economical cathodes for zinc-ion batteries. Herein, we explored iron-based flourophosphate as a potential polyanionic cathode. Involving the Fe3+/2+ redox process, it can reversibly intercalate Zn2+ yielding a capacity of ∼80 mA h g−1, involving a solid-solution mechanism. Polyanionic Fe-based phosphate frameworks can be harnessed as potential low-cost cathodes for secondary zinc-ion batteries. [ABSTRACT FROM AUTHOR]

In search of high energy density cathode materials, the eldfellite mineral-type NaVIII(SO4)2 compound has been theoretically predicted to be a promising cathode insertion host for sodium-ion batteries. Synergizing computational and experimental investigations, the current work introduces NaVIII(SO4)2 as a novel versatile cathode for Li-ion and Na-ion batteries. Prepared by a low temperature sol–gel synthesis route, the eldfellite NaV(SO4)2 cathode exhibited an initial capacity approaching ∼79% (vs. Li+/Li) and ∼69% (vs. Na+/Na) of the theoretical capacity (1e− ≅ 101 mA h g−1) involving the V3+/V2+ redox potential centered at 2.57 V and 2.28 V, respectively. The bond valence site energy (BVSE) approach and DFT-based calculations were used to gain mechanistic insight into alkali ion migration and probe the redox center during (de)insertion of Li+/Na+ ions. Post-mortem and electrochemical titration tools revealed the occurrence of a single-phase (solid-solution) redox mechanism during reversible Li+/Na+ (de)insertion into NaVIII(SO4)2. With the multivalent vanadium redox center, eldfellite NaVIII(SO4)2 forms a new cathode insertion host for Li/Na-ion batteries with potential two-electron uptake. [ABSTRACT FROM AUTHOR]

The search for an alternative high-voltage polyanionic cathode material for Li-ion batteries is vital to improve the energy densities beyond the state-of-the-art, where sulfate frameworks form an important class of high-voltage cathode materials due to the strong inductive effect of the S6+ ion. Here, we have investigated the mechanism of cationic and/or anionic redox in LixM(SO4)2 frameworks (M = Mn, Fe, Co, and Ni and 0 ≤ x ≤ 2) using density functional calculations. Specifically, we have used a combination of Hubbard U corrected strongly constrained and appropriately normed (SCAN+U) and generalized gradient approximation (GGA+U) functionals to explore the thermodynamic (polymorph stability), electrochemical (intercalation voltage), geometric (bond lengths), and electronic (band gaps, magnetic moments, charge populations, etc.) properties of the bisulfate frameworks considered. Importantly, we find that the anionic (cationic) redox process is dominant throughout delithiation in the Ni (Mn) bisulfate, as verified using our calculated projected density of states, bond lengths, and on-site magnetic moments. On the other hand, in Fe and Co bisulfates, cationic redox dominates the initial delithiation (1 ≤ x ≤ 2), while anionic redox dominates subsequent delithiation (0 ≤ x ≤ 2). In addition, evaluation of the crystal overlap Hamilton population reveals insignificant bonding between oxidized O atoms throughout the delithiation process in the Ni bisulfate, indicating robust battery performance that is resistant to irreversible oxygen evolution. Finally, we observe that both GGA+U and SCAN+U predictions are in qualitative agreement for the various properties predicted. Our work should open new avenues for exploring lattice oxygen redox in novel high voltage polyanionic cathodes, especially using the SCAN+U functional. [ABSTRACT FROM AUTHOR]

Mineral exploration forms a key approach for unveiling functional battery electrode materials. The synthetic preparation of naturally found minerals and their derivatives can aid in designing of new electrodes. Herein, saranchinaite Na2Cu(SO4)2 and its hydrated derivative kröhnkite Na2Cu(SO4)2·2H2O bisulfate minerals have been prepared using a facile spray drying route for the first time. The phase stability relation during the (de)hydration process was examined synergising in situ X-ray diffraction and thermochemical studies. Kröhnkite forms the thermodynamically stable phase as the hydration of saranchinaite to kröhnkite is highly exothermic (−51.51 ± 0.63 kJ mol−1). Structurally, kröhnkite offers a facile 2D pathway for Na+ ion migration resulting in 20 times higher total conductivity than saranchinaite at 60 °C. Both compounds exhibited a conversion redox mechanism for Li-ion storage with the first discharge capacity exceeding 650 mA h g−1 (at 2 mA g−1vs. Li+/Li) upon discharge up to 0.05 V. Post-mortem analysis revealed that the presence of metallic Cu in the discharged state is responsible for high irreversibility during galvanostatic cycling. This study reaffirms the exploration of Cu-based polyanionic sulfates, which while having limited (de)insertion properties, can be harnessed for conversion-based electrode materials for batteries. [ABSTRACT FROM AUTHOR]

Sodium-ion batteries can be designed as a low-cost alternative to lithium-ion batteries, where various layered transition metal oxides are frontrunner positive electrode materials. Owing to the inexpensive and abundant Mn resources and a large reversible capacity approaching 200 mA h g−1, α type (O′3 type) NaMnO2 is considered as a competent and economical candidate for sodium-ion batteries. However, O′3 NaMnO2 suffers from rapid capacity fading during charge–discharge cycling, for which the reasons remain elusive. The current work probes the underlying mechanisms behind this capacity degradation based on the correlation between the crystal structure and electrochemical properties. O′3 type NaMnO2, having a monoclinic O3-type structure, undergoes (de)intercalation of sodium ions through numerous potential plateaus and jumps corresponding to a number of intermediate phases. In situ and ex situ X-ray diffraction analyses reveal that the structure changes with different degrees of (de)sodiation and that eight different crystalline phases (co)exist. Furthermore, we have optimized the appropriate voltage window to achieve excellent cycling stability. [ABSTRACT FROM AUTHOR]

Rechargeable batteries based on Li-ion and post Li-ion chemistry have come a long way since their inception in the early 1980s. The last four decades have witnessed steady development and discovery of myriads of cathode materials taking into account their processing, economy, and performance along with ecological sustainability. Though oxides rule the battery sector with their high energy and power density, polyanionic insertion compounds work as gold mines for designing insertion compounds with rich structural diversity leading to tuneable redox potential coupled with high structural/chemical/thermal stability. The scope of polyanionic compounds can be taken a step further by combining two or more different types of polyanions to get suites of mixed polyanionic materials. While most cathodes are built with metal polyhedra constituted by oxygen (MOm‖XOm, M = 3d metals, X = P, S, Si, B, W, etc., m = 3–6), in some cases, selected oxygen sites can form bonding with hydrogen to form OH/H2O ligands. It can lead to the family of hydroxy-based mixed-polyanionic cathode materials. The presence of hydroxy components can affect the crystal structure, local chemical bonding, and electronic, magnetic, diffusivity and electrochemical properties. Employing a mineralogical survey, the current review renders a sneak peek on various hydroxy-based polyanionic cathode materials for Li-ion and post Li-ion batteries. Their crystal structure, and electrochemical properties have been overviewed to outline future research focus and scope for real-life application. [ABSTRACT FROM AUTHOR]

Metal–air batteries with high energy density have emerged as key players in the energy storage sector. They operate on two underlying processes, namely, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). It provides the impetus to design efficient, earth-abundant and economic bifunctional electrocatalysts vis-à-vis precious metal-based catalysts. In this perspective, a few polyanionic battery insertion materials have been reported as potential electrocatalysts. In the current work, metal fluorophosphate (Na2MPO4F, M = Fe/Co/Mn) family of sodium insertion materials have been shown as a new class of bifunctional electrocatalysts with robust structural stability. In particular, Na2CoPO4F was found to exhibit superior catalytic performance with an onset potential of 0.903 V (vs. RHE) for the ORR and an overpotential of 380 mV (vs. RHE) for the OER. The underlying mechanism and kinetics were explored using ab initio computational studies. Overall, polyanionic transition metal fluorophosphates were explored for the first time as bifunctional electrocatalysts capable of working as potential cathode materials in hybrid metal–air batteries. [ABSTRACT FROM AUTHOR]

In an effort to rationally design economic electrocatalysts, zinc-substituted cobalt phosphate and pyrophosphate were prepared using facile template-free combustion synthesis. They act as efficient stable bifunctional electrocatalysts due to the tuning of oxygen affinity by zinc substitution and catalytically active cobalt sites. Exploiting their bifunctional activity, these cobalt (pyro)phosphates were incorporated into a zinc–air battery in an alkaline electrolyte. [ABSTRACT FROM AUTHOR]

P3-type layered K0.48Mn0.4Co0.6O2 was synthesized using a solid-state method. By stabilising into a rhombohedral structure [s.g. R3m (#160)], it delivers a reversible capacity of 64 mA h g−1 with a nominal voltage of ∼3.0 V (vs. K/K+) and it has good cycling stability. It involves a solid-solution redox mechanism, and forms an economical and stable oxide insertion material for potassium-ion batteries. [ABSTRACT FROM AUTHOR]

Aqueous rechargeable zinc-ion batteries have immense commercial potential in the primary battery market, which is largely occupied by alkaline zinc/manganese oxide batteries. These storage systems are desirable for large-scale applications due to their superior safety, high capacity, fast ionic conductivity and low cost. Pursuing new Mn-based cathodes for Zn-ion batteries, cryptomelane with a specific chemical composition of K1.33Mn8O16 was synthesized using a simple sonochemical technique. The formation of phase pure tetragonal K1.33Mn8O16 with I4/m symmetry was confirmed by X-ray diffraction. When tested as a cathode for Zn-ion batteries, cryptomelane K1.33Mn8O16 demonstrated a high reversible capacity of 312 mA h g−1 at a rate of C/10. Robust cycling stability was observed retaining more than 80% of initial capacity after 650 cycles with ∼99% coulombic efficiency. The intercalation chemistry of Zn–MnO2 systems is very rich involving polymorphism and phase transformations during charge/discharge cycles. Synergizing ex situ X-ray diffraction and spectroscopy with the electrochemical titration technique, we have examined the detailed mechanism of (de)intercalation of Zn2+ into cryptomelane K1.33Mn8O16, which is proposed as a potential cathode for Zn-ion batteries. [ABSTRACT FROM AUTHOR]

In order to design earth-abundant low cost electrocatalysts, this communication exploits polymorphism in Na2MnP2O7 pyrophosphate sodium insertion materials. Two polymorphs of Na2MnP2O7 have been prepared with a short annealing duration of 30 minutes. These scalable materials exhibit efficient bifunctional electrocatalytic activity stemming from the Mn redox centre and robust structural framework. [ABSTRACT FROM AUTHOR]

Kröhnkite-type Na2Fe(SO4)2·2H2O mineral is a sustainable and promising polyanionic cathode that has been experimentally found to offer a high redox potential (3.25 V vs. Na/Na+) along with fast-ion diffusion and high reversibility. Owing to the structural complexity, Na+ diffusion was assumed to occur along a convoluted channel along the b-axis. However, theoretical work related to this material still appears missing to support that statement. In this work, DFT+U calculations have been performed with the primary aim to unveil the Na+ diffusion mechanism in this material. The electronic structure and charge transfer are also envisaged to reveal evidence of Fe2+/3+ redox reaction and a vital role of structural H2O. Based on formation energies of this material with varied Na concentration, a calculated voltage profile is determined to show two voltage plateaus at 4.81 and 3.51 V, corresponding to experimental results. Nudged elastic band calculation reveals that Na+ diffusion is primarily occuring in the [011̅] direction with a moderate ionic mobility due to the structural distortion induced during migration, suggesting the possibility of defect-assisted diffusion. Intriguingly, the formation of small hole polarons is first observed, and could play a key role in the electronic conduction of this material. [ABSTRACT FROM AUTHOR]

Reversible electrochemical potassium-ion intercalation in P2-type NaxCoO2 was examined for the first time. Hexagonal Na0.84CoO2 platelets prepared by a solution combustion synthesis technique were found to work as an efficient host for K+ intercalation. They deliver a high reversible capacity of 82 mA h g−1, good rate capability and excellent cycling performance up to 50 cycles. [ABSTRACT FROM AUTHOR]

Twisted-melilite dipotassium cobalt pyrophosphate (K2CoP2O7, P42/mnm, #136), originally reported by Gabelica-Robert (1981), was synthesized in powder form by a standard solid-state reaction route. The magnetic properties of the material were studied by magnetometry and its magnetic structure determined using neutron powder diffraction for the first time. Below TN = 11 K, the material adopts a G-type antiferromagnetic structure with moments aligned in the ab-plane (magnetic space group Pn′nm, #58.3.473). Ab initio calculations were performed to examine the isotropic magnetic spin exchange parameters as well as the preferred direction of magnetic moments due to spin–orbit coupling. The relationship between crystal structure geometry and the strength of the magnetic interactions was examined and compared to those of melilite-type Sr2CoGe2O7. [ABSTRACT FROM AUTHOR]

Electrochemical energy storage has recently seen tremendous emphasis being placed on the large-scale (power) grid storage. Sodium-ion batteries are capable of achieving this goal with economic viability. In a recent breakthrough in sodium-ion battery research, the alluaudite framework (Na2Fe2(SO4)3) has been reported, with the highest Fe3+/Fe2+ redox potential (ca. 3.8 V, Barpanda, et al., Nat. Commun., 2014, 5, 4358). Exploring this high-voltage sodium insertion system, we report the discovery of Na2+2xCo2−x(SO4)3 (x = 0.16) as a new member of the alluaudite class of cathode. Stabilized by low-temperature solid-state synthesis (T≤ 350 °C), this novel Co-based compound assumes a monoclinic structure with C2/c symmetry, which undergoes antiferromagnetic ordering below 10.2 K. Isotypical to the Fe-homologue, it forms a complete family of solid-solution Na2+2x(Fe1−yCoy)2−x(SO4)3 [y = 0–1]. Ab initio DFT analysis hints at potential high voltage operation at 4.76–5.76 V (vs. Na), depending on the degree of desodiation involving a strong participation of the oxygen sub-lattice. With the development of safe organic electrolytes, Na2+2xCo2−x(SO4)3 can work as a cathode material (∼5 V) for sodium-ion batteries. [ABSTRACT FROM AUTHOR]

Sodium-ion-based batteries have evolved as excellent alternatives to their lithium-ion-based counterparts due to the abundance, uniform geographical distribution and low price of Na resources. In the pursuit of sodium chemistry, recently the alluaudite framework Na2M2(SO4)3 has been unveiled as a high-voltage sodium insertion system. In this context, the framework of density functional theory has been applied to systematically investigate the crystal structure evolution, density of states and charge transfer with sodium ions insertion, and the corresponding average redox potential, for Na2M2(SO4)3 (M = Fe, Mn, Co and Ni). It is shown that full removal of sodium atoms from the Fe-based device is not a favorable process due to the 8% volume shrinkage. The imaginary frequencies obtained in the phonon dispersion also reflect this instability and the possible phase transition. This high volume change has not been observed in the cases of the Co- and Ni-based compounds. This is because the redox reaction assumes a different mechanism for each of the compounds investigated. For the polyanion with Fe, the removal of sodium ions induces a charge reorganization at the Fe centers. For the Mn case, the redox process induces a charge reorganization of the Mn centers with a small participation of the oxygen atoms. The Co and Ni compounds present a distinct trend with the redox reaction occurring with a strong participation of the oxygen sublattice, resulting in a very small volume change upon desodiation. Moreover, the average deintercalation potential for each of the compounds has been computed. The implications of our findings have been discussed both from the scientific perspective and in terms of technological aspects. [ABSTRACT FROM AUTHOR]

Sodium-ion batteries have been extensively pursued as economic alternatives to lithium-ion batteries. Investigating the polyanion chemistry, alluaudite structured Na2FeII2(SO4)3 has been recently discovered as a 3.8 V positive electrode material (Barpanda et al., Nature Commun., 5:4358, 2014). Registering the highest ever FeIII/FeII redox potential (vs. Na/Na+) and formidable energy density, it has opened up a new polyanion family for sodium batteries. Exploring the alluaudite family, here we report isotypical Na2+2xMnII2−x(SO4)3 (x = 0.22) as a novel high-voltage cathode material for the first time. Following low-temperature (ca. 350 °C) solid-state synthesis, the structure of this new alluaudite compound has been solved adopting a monoclinic framework (s.g. C2/c) showing antiferromagnetic ordering at 3.4 K. Synergising experimental and ab initio DFT investigation, Na2+2xMnII2−x(SO4)3 has been found to be a potential high-voltage (ca. 4.4 V) cathode material for sodium batteries. [ABSTRACT FROM AUTHOR]

Na-ion batteries are currently the focus of significant research activity due to the relative abundance of sodium and its consequent cost advantages. Recently, the pyrophosphate family of cathodes has attracted considerable attention, particularly Li2FeP2O7 related to its high operating voltage and enhanced safety properties; in addition the sodium-based pyrophosphates Na2FeP2O7 and Na2MnP2O7 are also generating interest. Herein, we present defect chemistry and ion migration results, determined via atomistic simulation techniques, for Na2MP2O7 (where M = Fe, Mn) as well as findings for Li2FeP2O7 for direct comparison. Within the pyrophosphate framework the most favourable intrinsic defect type is found to be the antisite defect, in which alkali-cations (Na/Li) and M ions exchange positions. Low activation energies are found for long-range diffusion in all crystallographic directions in Na2MP2O7 suggesting three-dimensional (3D) Na-ion diffusion. In contrast Li2FeP2O7 supports 2D Li-ion diffusion. The 2D or 3D nature of the alkali-ion migration pathways within these pyrophosphate materials means that antisite defects are much less likely to impede their transport properties, and hence important for high rate performance. [ABSTRACT FROM AUTHOR]

Twisted-melilite dipotassium cobalt pyrophosphate (K2CoP2O7, P42/mnm, #136), originally reported by Gabelica-Robert (1981), was synthesized in powder form by a standard solid-state reaction route. The magnetic properties of the material were studied by magnetometry and its magnetic structure determined using neutron powder diffraction for the first time. Below TN = 11 K, the material adopts a G-type antiferromagnetic structure with moments aligned in the ab-plane (magnetic space group Pn′nm, #58.3.473). Ab initio calculations were performed to examine the isotropic magnetic spin exchange parameters as well as the preferred direction of magnetic moments due to spin–orbit coupling. The relationship between crystal structure geometry and the strength of the magnetic interactions was examined and compared to those of melilite-type Sr2CoGe2O7. [ABSTRACT FROM AUTHOR]

Aqueous zinc-ion batteries form a key post-Li-ion batteries to cater the rising demand for grid storage. Fe-based compounds can be used as economical cathodes for zinc-ion batteries. Herein, we explored iron-based flourophosphate as a potential polyanionic cathode. Involving the Fe 3+/2+ redox process, it can reversibly intercalate Zn 2+ yielding a capacity of ∼80 mA h g -1 , involving a solid-solution mechanism. Polyanionic Fe-based phosphate frameworks can be harnessed as potential low-cost cathodes for secondary zinc-ion batteries.

Mineral exploration forms a key approach for unveiling functional battery electrode materials. The synthetic preparation of naturally found minerals and their derivatives can aid in designing of new electrodes. Herein, saranchinaite Na 2 Cu(SO 4 ) 2 and its hydrated derivative kröhnkite Na 2 Cu(SO 4 ) 2 ·2H 2 O bisulfate minerals have been prepared using a facile spray drying route for the first time. The phase stability relation during the (de)hydration process was examined synergising in situ X-ray diffraction and thermochemical studies. Kröhnkite forms the thermodynamically stable phase as the hydration of saranchinaite to kröhnkite is highly exothermic (-51.51 ± 0.63 kJ mol -1 ). Structurally, kröhnkite offers a facile 2D pathway for Na + ion migration resulting in 20 times higher total conductivity than saranchinaite at 60 °C. Both compounds exhibited a conversion redox mechanism for Li-ion storage with the first discharge capacity exceeding 650 mA h g -1 (at 2 mA g -1 vs. Li + /Li) upon discharge up to 0.05 V. Post-mortem analysis revealed that the presence of metallic Cu in the discharged state is responsible for high irreversibility during galvanostatic cycling. This study reaffirms the exploration of Cu-based polyanionic sulfates, which while having limited (de)insertion properties, can be harnessed for conversion-based electrode materials for batteries.

P3-type layered K 0.48 Mn 0.4 Co 0.6 O 2 was synthesized using a solid-state method. By stabilising into a rhombohedral structure [s.g. R3m (#160)], it delivers a reversible capacity of 64 mA h g -1 with a nominal voltage of ∼3.0 V (vs. K/K + ) and it has good cycling stability. It involves a solid-solution redox mechanism, and forms an economical and stable oxide insertion material for potassium-ion batteries.

In order to design earth-abundant low cost electrocatalysts, this communication exploits polymorphism in Na2MnP2O7 pyrophosphate sodium insertion materials. Two polymorphs of Na2MnP2O7 have been prepared with a short annealing duration of 30 minutes. These scalable materials exhibit efficient bifunctional electrocatalytic activity stemming from the Mn redox centre and robust structural framework.

Reversible electrochemical potassium-ion intercalation in P2-type Na x CoO 2 was examined for the first time. Hexagonal Na 0.84 CoO 2 platelets prepared by a solution combustion synthesis technique were found to work as an efficient host for K + intercalation. They deliver a high reversible capacity of 82 mA h g -1 , good rate capability and excellent cycling performance up to 50 cycles.

Twisted-melilite dipotassium cobalt pyrophosphate (K 2 CoP 2 O 7 , P4 2 /mnm, #136), originally reported by Gabelica-Robert (1981), was synthesized in powder form by a standard solid-state reaction route. The magnetic properties of the material were studied by magnetometry and its magnetic structure determined using neutron powder diffraction for the first time. Below T N = 11 K, the material adopts a G-type antiferromagnetic structure with moments aligned in the ab-plane (magnetic space group Pn'nm, #58.3.473). Ab initio calculations were performed to examine the isotropic magnetic spin exchange parameters as well as the preferred direction of magnetic moments due to spin-orbit coupling. The relationship between crystal structure geometry and the strength of the magnetic interactions was examined and compared to those of melilite-type Sr 2 CoGe 2 O 7 .

Electrochemical energy storage has recently seen tremendous emphasis being placed on the large-scale (power) grid storage. Sodium-ion batteries are capable of achieving this goal with economic viability. In a recent breakthrough in sodium-ion battery research, the alluaudite framework (Na 2 Fe 2 (SO 4 ) 3 ) has been reported, with the highest Fe 3+ /Fe 2+ redox potential (ca. 3.8 V, Barpanda, et al., Nat. Commun., 2014, 5, 4358). Exploring this high-voltage sodium insertion system, we report the discovery of Na 2+2x Co 2-x (SO 4 ) 3 (x = 0.16) as a new member of the alluaudite class of cathode. Stabilized by low-temperature solid-state synthesis (T ≤ 350 °C), this novel Co-based compound assumes a monoclinic structure with C2/c symmetry, which undergoes antiferromagnetic ordering below 10.2 K. Isotypical to the Fe-homologue, it forms a complete family of solid-solution Na 2+2x (Fe 1-y Co y ) 2-x (SO 4 ) 3 [y = 0-1]. Ab initio DFT analysis hints at potential high voltage operation at 4.76-5.76 V (vs. Na), depending on the degree of desodiation involving a strong participation of the oxygen sub-lattice. With the development of safe organic electrolytes, Na 2+2x Co 2-x (SO 4 ) 3 can work as a cathode material (∼5 V) for sodium-ion batteries.

Sodium-ion-based batteries have evolved as excellent alternatives to their lithium-ion-based counterparts due to the abundance, uniform geographical distribution and low price of Na resources. In the pursuit of sodium chemistry, recently the alluaudite framework Na2M2(SO4)3 has been unveiled as a high-voltage sodium insertion system. In this context, the framework of density functional theory has been applied to systematically investigate the crystal structure evolution, density of states and charge transfer with sodium ions insertion, and the corresponding average redox potential, for Na2M2(SO4)3 (M = Fe, Mn, Co and Ni). It is shown that full removal of sodium atoms from the Fe-based device is not a favorable process due to the 8% volume shrinkage. The imaginary frequencies obtained in the phonon dispersion also reflect this instability and the possible phase transition. This high volume change has not been observed in the cases of the Co- and Ni-based compounds. This is because the redox reaction assumes a different mechanism for each of the compounds investigated. For the polyanion with Fe, the removal of sodium ions induces a charge reorganization at the Fe centers. For the Mn case, the redox process induces a charge reorganization of the Mn centers with a small participation of the oxygen atoms. The Co and Ni compounds present a distinct trend with the redox reaction occurring with a strong participation of the oxygen sublattice, resulting in a very small volume change upon desodiation. Moreover, the average deintercalation potential for each of the compounds has been computed. The implications of our findings have been discussed both from the scientific perspective and in terms of technological aspects.

We have investigated polymorphs of LiFeSO4F, tavorite and triplite, which have been reported as cathode materials for lithium ion batteries. The predicted voltages are 3.64 and 3.90 V for tavorite and triplite, respectively, which agreed excellently with experimental data. It is found that the lithiated states (LiFeSO4F) of the polymorphs are almost degenerate in energy. The difference in voltage is mainly due to the difference in the stabilities of the delithiated states (FeSO4F). This is rationalized by the Fe(3+)-Fe(3+) repulsion in the edge sharing geometry of the triplite structure.