Operando XAS to Illustrate the Importance of Electronic Conductivity in Vanadyl Phosphate Systems

Abstract

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.