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 AxVOPO4 (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 AxVOPO4 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.

Several advanced electrolytes (mainly ether-based) have shown promising electrochemical performance in high-energy-density lithium-metal batteries. This work evaluates their thermal stability under abuse conditions to elucidate their safety limits compared to carbonate electrolytes typically used in Li-ion batteries. Electrolyte stability was assessed in conjunction with a LiNi0.8Mn0.1Co0.1O2 cathode and a Li-metal anode at ultra-high voltages (≤4.8 V) and temperatures (≤300 °C). The onset and extent of heat release were monitored via isothermal microcalorimetry and differential scanning calorimetry. Most ether-based electrolytes show improved thermal resilience over carbonate electrolytes. While extreme voltages severely destabilize the ether-based electrolytes, a phosphate-based localized high-concentration electrolyte exhibits improved stability over carbonate electrolytes, even at 60 °C. Although thermal analysis during the first charge process may be insufficient to conclude the long-term advantages of these electrolytes, a more stable electrolyte identified under extreme voltage and temperature conditions provides valuable guidance for the safety of future electrolyte designs.

Increasing demands for higher energy density batteries have inspired multi-electron cathodes which can double the energy density per transition metal cation. In Li-ion batteries, vanadyl phosphates have been shown to intercalate 2 Li+ per V center by activating V4+/5+ and V3+/4+ redox couples, resulting in a gravimetric capacity of 305 mAh g−1. In order to employ the VOPO4 structure in earth abundant and cheaper alkali-ion batteries, channels must be expanded to allow the diffusion of larger cations. This can be achieved by pillaring the VOPO4 framework with large cations. KVOPO4 has been shown to be a high capacity cathode in Na-ion batteries, however not all of the K could be removed from the structure, limiting the accessible capacity. NH4VOPO4 is similar to KVOPO4 where the KTP-type VOPO4 framework is enlarged and distorted due to the presence of large cations. In this study, we investigate the electrochemical performance of NH4VOPO4. While the pristine material suffers greatly from parasitic reactions, we show that electrochemistry and thermal stability can be improved by exchanging some of the NH4 + with Na+.

LiVOPO4 is a promising next-generation multi-electron cathode material, boasting a theoretical capacity of 305 mAh/g, significantly higher than any commercially used Li-ion battery cathode material. However, the material still faces...

Multi-electron cathodes are an exciting class of energy storage materials that can intercalate more than one alkali-ion per transition metal. One such case, nano-sized ε-VOPO4 can intercalate two Li-ions to obtain the theoretical capacity of 305 mAh g−1, despite its inherently poor ionic and electronic conductivity. While carbon additives can compensate for cathode material’s poor conductivity, the type of carbon additive can play a key role in achieving full theoretical capacity of ε-VOPO4. Here, we explore the electrochemical behavior of two sourced carbons while systematically tracking V valence through operando X-ray absorption spectroscopy. The degree of V redox largely depends on the carbon additive’s electrical conductivity and surface coverage, with graphene enabling full 2 li-ion (de)intercalation whereas the use of acetylene black leads to trapped Li-ion. In both cases however, side reactions are promoted when the limits of facile Li (de)intercalation are reached resulting in excess capacities inconsistent with V valence. This excess capacity is more strongly correlated to carbon loading and surface area of the carbon additive rather than any exotic redox mechanism of ε-VOPO4 such as oxygen redox.