Sodium-ion batteries have been extensively pursued as economic alternatives to Lithium-ion batteries. Investigating the polyanion chemistry, alluaudite structured Na 2 Fe II 2 (SO 4 ) 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 Fe III /Fe II 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 Na 2+2x Mn II 2-x (SO 4 ) 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, Na 2+2x Mn II 2-x (SO 4 ) 3 has been found to be a potential high-voltage (ca. 4.4 V) cathode material for sodium batteries.

Exploring future cathode materials for sodium-ion batteries, alluaudite class of Na2Fe(II)2(SO4)3 has been recently unveiled as a 3.8 V positive insertion candidate (Barpanda et al. Nat. Commun. 2014, 5, 4358). It forms an Fe-based polyanionic compound delivering the highest Fe-redox potential along with excellent rate kinetics and reversibility. However, like all known SO4-based insertion materials, its synthesis is cumbersome that warrants careful processing avoiding any aqueous exposure. Here, an alternate low temperature ionothermal synthesis has been described to produce the alluaudite Na2+2xFe(II)2-x(SO4)3. It marks the first demonstration of solvothermal synthesis of alluaudite Na2+2xM(II)2-x(SO4)3 (M = 3d metals) family of cathodes. Unlike classical solid-state route, this solvothermal route favors sustainable synthesis of homogeneous nanostructured alluaudite products at only 300 °C, the lowest temperature value until date. The current work reports the synthetic aspects of pristine and modified ionothermal synthesis of Na2+2xFe(II)2-x(SO4)3 having tunable size (300 nm ∼5 μm) and morphology. It shows antiferromagnetic ordering below 12 K. A reversible capacity in excess of 80 mAh/g was obtained with good rate kinetics and cycling stability over 50 cycles. Using a synergistic approach combining experimental and ab initio DFT analysis, the structural, magnetic, electronic, and electrochemical properties and the structural limitation to extract full capacity have been described.

A novel alluaudite, Na2+2xCo2−x(SO4)3, has been unveiled as a potential high-voltage (ca. 4.8–5.8 V) sodium battery cathode demonstrating antiferromagnetic ordering.

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Rechargeable lithium-ion battery remains the leading electrochemical energy-storage device, albeit demanding steady effort of design and development of superior cathode materials. Polyanionic framework compounds are widely explored in search for such cathode contenders. Here, lithium metal borate (LiM-BO 3 ) forms a unique class of insertion materials having the lowest weight polyanion (i.e., BO 3 3− ), thus offering the highest possible theoretical capacity (ca. 220 mAh/ g). Since the first report in 2001, LiMBO 3 has rather slow progress in comparison to other polyanionic cathode systems based on PO 4 , SO 4 , and SiO 4 . The current review gives a sneak peak to the progress on LiMBO 3 cathode systems in the last 15 years highlighting their salient features and impediments in cathode implementation. The synthesis and structural aspects of borate family are described along with the critical analysis of the electrochemical performance of borate family of insertion materials.

We demonstrate a machine-learning analysis of large-capacity/high-voltage battery cathodes, which quantitatively evaluates the importance of ever-attempted technical solutions.

Magnesium aluminate spinel (MgAl 2 O 4 ) forms an interesting system having tetrahedral and octahedral voids filled with near similar sized divalent Mg 2+ and trivalent Al 3+ cations. Structural disorder (e.g., Mg-Al antisite defects) can be tuned by synthetic conditions. This study reports the evolution of Mg/Al disorder in MgAl 2 O 4 prepared by combustion synthesis using different types of fuels. The effect of nature of fuel and the final calcination temperature (600°C-900°C for 9 h) on degree of cation ordering has been investigated combining powder Xray (XRD) and neutron (NPD) diffraction. The results indicate very high degree of inversion in the samples crystallized at low annealing temperature, which on further annealing reduces toward the thermodynamically stable values. Raman spectroscopy, probing MgO 4 , and AlO 4 tetrahedral bonds, confirmed the results at a local level.

Rechargeable batteries have emerged as ubiquitous and indispensable technologies of the 21st century, propelling myriads of consumer electronics and ushering a new era of electric vehicles and stationary grid storage. Since the commercialization of Li‐ion batteries by SONY (approximately in the year 1991), the secondary battery sector has seen unprecedented growth with diverse applications touching global populations across socioeconomic strata. There is a steady quest to develop a wide range of batteries to cater diverse global demands from milliwatt‐scale electronics to megawatt‐scale grid‐storage applications. In this journey, Li‐ion batteries are complemented by various post‐Li‐ion chemistry (e.g., Na+, K+, Mg2+, Ca2+, and Al3+‐ion batteries), conversion mechanism‐based systems (e.g., Li–S, Na–S, and Li–O2) as well as renewed development of pre‐Li‐ion era technologies like aqueous batteries. Cathodes sit at the core with command over the net cost and performance of batteries. Various polyanionic cathodes have been developed to date, often guided by structure of naturally occurring minerals. One such mineral system is alluaudite, named after French geologist François Alluaud. The current article portrays the discovery and development of the alluaudite class of polyanionic cathode materials for rechargeable batteries. The structure and electrochemical properties of various alluaudite insertion materials are gauged along with possible future perspectives.