Sulfate Chemistry for High-Voltage Insertion Materials: Synthetic,Structural and Electrochemical Insights

Rechargeable batteries based on Li and Na ions have been growing leaps and bounds since their inception in the 1970s. They enjoy significant attention from both the fundamental science point of view and practical applications ranging from portable electronics to hybrid vehicles and grid storage. The steady demand for building better batteries calls for discovery, optimisation and implementation of novel positive insertion (cathode) materials. In this quest, chemists have tried to unravel many future cathode materials by taking into consideration their eco-friendly synthesis, material/process economy, high energy density, safety, easy handling and sustainability. Interestingly, sulfate-based cathodes offer a good combination of sustainable syntheses and high energy density owing to their high-voltage operation, stemming from electronegative SO₄²⁻ units. This review delivers a sneak peak at the recent advances in the discovery and development of sulfate-containing cathode materials by focusing on their synthesis, crystal structure and electrochemical performance. Several family of cathodes are independently discussed. They are 1) fluorosulfates AMSO₄F], 2) bihydrated fluorosulfates AMSO₄F ⋅ 2H₂O], 3) hydroxysulfate AMSO₄OH], 4) bisulfates A₂M(SO₄)₂], 5) hydrated bisulfates A₂M(SO₄)₂ ⋅ nH₂O], 6) oxysulfates Fe₂(SO₄)₂O] and 7) polysulfates A₂M₂(SO₄)₃]. A comparative study of these sulfate-based cathodes has been provided to offer an outlook on the future development of high-voltage polyanionic cathode materials for next-generation batteries.