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

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 V 4+/5+ and V 3+/4+ redox couples, resulting in a gravimetric capacity of 305 mAh g −1 . In order to employ the VOPO 4 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 VOPO 4 framework with large cations. KVOPO 4 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. NH 4 VOPO 4 is similar to KVOPO 4 where the KTP-type VOPO 4 framework is enlarged and distorted due to the presence of large cations. In this study, we investigate the electrochemical performance of NH 4 VOPO 4. While the pristine material suffers greatly from parasitic reactions, we show that electrochemistry and thermal stability can be improved by exchanging some of the NH 4 + with Na + .