An Electrochemical Study on NH4VOPO4: Can Ion-Exchange Improve Side Reactions?

Abstract

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+.