Insights into solid solution-assisted polyanion cathodes: Present status, environmental focus and advancement in phosphate and borate chemistry for Li- and Na-ion battery technologies

The development of high energy-density and low-cost alkali-ion batteries is critically important for future portable electronics, heavy electric vehicles, and grid storage. Polyanion solid solution cathodes that are chemically functionalized through molecular or atomic nano-engineering and possess unique microstructures would offer significantly enhanced electro-chemical properties for commercial prospects due to their high energy density, specific capacity, facile ion-diffusion, high safety, and superior structural stability under low or high temperature environments. The low molecular weight borates have a great potential to form the desirable polyanion solid solution than the other mature polyanion families, i.e., PO ₄ ³⁻, SiO ₄ ^4-, CO ₃ ^2-, SO ₄ ^2- etc. The beauty of borate chemistries have been revealed owing to its smaller formula mass (58.8), low reaction energy, anti-oxidation, and robust polyanionic structure which make them suitable for composite interaction. In addition, these composites are superior hosts for Li ⁺ or Na ⁺ per formula unit, multi-electron redox chemistry and stoichiometric composition, thereby preventing rapid capacity and voltage decay via the coupling effects of cathode active components. The present review explores the recent progress of solid solution based polyanion nanomaterials containing BO ₃ ^3- and PO ₄ ³⁻ polyanions which also includes physico-chemical characteristics of Zn, Cu, Co, Mn, Fe and V-based polyanionic materials. Moreover, there are no literature reported so far on the Zn, Cu, and Co-based borate polyanions for Li- and Na-ion batteries (LIBs and NIBs). However, unlike phosphate based electrodes, there are very few review articles published on Li-ion batteries using borate materials. Therefore, this review provides the extensive literature on the solid composites containing BO ₃ ^3- or mixed-borate polyanions, and their comparison with phosphate cathodes which would inculcate deeper outlook for the safer design and development of high energy-density LIBs and NIBs.