The discovery of triphylite LiFePO₄ cathode for Li-ion battery pushed the polyanionic materials to the forefront of battery cathodes. The polyanionic material offers rich crystal chemistry, robust framework, voltage tunability, and high redox potential based on the inductive effect due to the polyanionic unit [(XO₄)_m^n-, X = S, P, Si, W, Mo, etc.]. Among them, SO₄-based polyanionic systems can offer higher redox potential and ease/versatility of synthesis at lower temperatures. Following, various polyanionic sulfate compounds have been reported as promising candidates for alkali ion batteries.

We have investigated the polyanionic sulfate materials having bisulfate [A_2-xM(SO₄)₂: A= Li, Na, K; x= 0,1] and hydroxy(fluoro)sulfate [AMSO₄(OH^-/F^-): A = Li, Na, K] framework. We reaffirm the change in the performance of polyanionic sulfates by the introduction of secondary anions such as OH^- and F^- in the framework, triggering polymorphism and/or varying the size of (de)intercalating ion, in the following case studies.

(i) Spray drying route was used to discover a metastable monoclinic polymorph of Li₂Ni^II(SO₄)₂ belonging to space group P2₁/c. As per first-principal calculations, it can work as 5.5 V (vs. Li⁺/Li) cathode for Li-ion battery. We further examined the crystal chemistry, phase stability landscape and the ground state magnetic structure.

(ii) The eldfellite NaV^III(SO₄)₂ (C2/m) is demonstrated as a versatile cathode material for both Li-ion (2.57 V, 80 mAh/g) and Na-ion (2.28 V, 70 mAh/g) battery at current rate of C/20 and based on solid solution reaction mechanism.

(iii) The electrochemical performance of orthorhombic polymorphs of Fe_IIISO₄OH, synthesized using hydrothermal route, as 3.2 V (110 mAh/g, C/20) Li-ion battery cathode working on the monophasic reaction is investigated. We further demonstrated the first reversible Na-ion (de)insertion in monoclinic Fe_IIISO₄OH at ~2.9 V based on solid solution reaction mechanism with a discharge capacity of 85 mAh/g (C/100).

(iv) The electrochemical activity and phase evolution of ionothermally prepared (Li_x)FeSO₄F fluorosulfate as a function of temperature have been investigated. The effect of temperature on monophasic/biphasic redox mechanism will be discussed.