Vanadyl Phosphates A_xVOPO₄ (A = Li, Na, K) as Multielectron Cathodes for Alkali-Ion Batteries

Vanadyl phosphates comprise a class of multielectron cathode materials capable of cycling two Li⁺, about 1.66 Na⁺, and some K⁺ ions per redox center. In this review, structures, thermodynamic stabilities, and ion diffusion kinetics of various A_xVOPO₄ (A = Li, Na, K, NH₄) polymorphs are discussed. Both the experimental data and first-principle calculations indicate kinetic limitations for alkali metal ions cycling, especially between for 0 ≤ x ≤ 1, and metastability of phases with x > 1. This creates challenges for multiple-ion cycling, as the slow kinetics call for nanosized particles, which being metastable and reactive with organic electrolytes are prone to side reactions. Thus, various synthesis approaches, surface coating, and transition metal ion substitution strategies are discussed here as possible ways to stabilize A_xVOPO₄ structures and improve alkali metal ion diffusion. The role of advanced characterization techniques, such as X-ray absorption spectroscopy, diffraction, pair distribution function analysis and ⁷Li and ³¹P NMR, in understanding the reaction mechanism from both structural and electronic points of view is emphasized.