Transition Metal Oxalate-Based Materials : An Emerging Material Family for Alkali-Ion Battery Cathodes

Alkali-ion rechargeable batteries, including Li⁺/Na⁺/K⁺/Mg²⁺-ion batteries, etc., are promising technologies for energy storage applications. Polyanion compounds (PCs) represent an important cathode material category by virtue of their robust framework structures with facile alkali-ion migration paths and high-voltage properties derived from the inductive effect of anion groups. Many PCs have been well-studied, but the exploration of PCs is still fundamentally important for high-performance batteries. Recently, transition metal oxalate-based materials (TMOBMs) have emerged as a competitive cathode material family for diverse alkali-ion batteries. Oxalate anions (C₂O₄^2-) demonstrate strong and flexible coordinating ability with both redox-active transition metals and various anion groups (e.g., PO₄^3-, SO₄^2-, F^-), leading to the exploration of many TMOBMs with diverse framework structures (including 1D chained, 2D layered, and 3D tunneled) and intriguing electrochemical properties. This review first focuses on the development of TMOBMs as cathode materials. Their classification, crystal structures, synthesis methods, physiochemical properties, and applications in Li⁺/Na⁺/K⁺/Mg²⁺-ion batteries are comprehensively introduced. Investigation of TMOBMs cathodes is at the early stage. We believe that this review is timely and of great significance to further enrich the family of PCs and promote their development and applications.