Polyanionic Cathode Materials: A Comparison Between Na-Ion and K-Ion Batteries

Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are considered the next-generation candidates for future energy storage systems to partially substitute commercial lithium-ion batteries because of their abundant sodium/potassium reserves, cost-effectiveness, and high safety. Polyanionic cathode materials are widely used in alkali ion batteries due to their stable structural framework, high thermal stability, excellent alkali ion diffusion kinetics, and adjustable working voltage. Generally, the polyanionic cathodes used for SIBs surpass PIBs in the aspect of energy density and cycle life in most cases, however, the PIBs also have their unique advantages that are seldom reported. To this end, the polyanionic materials are classified by the valence states of active metal sites, the storage mechanism of Na⁺ and K⁺ in different crystal structures is summarized, and the electrochemical performance between SIBs and PIBs is compared. Particularly, some unique advantages of polyanionic cathodes in PIBs, such as high working voltage, superior rate capability, and excellent capacity retention are revealed, and the possible reasons are discussed in detail. Finally, various viable solutions are proposed to improve the battery performance of polyanionic compounds for future development.