Comparison of the polymorphs of VOPO4 as multi-electron cathodes for rechargeable alkali-ion batteries

Multi-electron polyanion cathodes offer the potential for achieving both high voltage and high capacity in rechargeable alkali-ion batteries. Among the few materials known to exhibit multi-electron cycling, the polymorphs of VOPO4, which operate on the V3+-V4+-V5+ redox couples, are particularly promising due to the high gravimetric capacities that have been achieved and the high voltage of the V4+/5+ couple. In this work, we performed a systematic first principles investigation, supported by careful electrochemical characterization and published experimental data, of the relative thermodynamic stability, voltage, band gap, and diffusion kinetics for alkali intercalation into the beta, epsilon and alpha(I) polymorphs of VOPO4. We find that all VOPO4 polymorphs remain reasonably stable with the insertion of one alkali ion per V, but are significantly destabilized with the insertion of two alkali ions per V. The voltages for Na insertion are similar to 0.33-0.69 V lower than those for Li insertion. We find that the alpha(I) polymorph is predicted to have higher Li+ migration barriers and larger band gaps than the beta and epsilon polymorphs, which account for the relatively worse electrochemical cycling performance observed. On the other hand, only the alpha(I) polymorph exhibits reasonably low barriers for Na+ migration compared to the beta and epsilon polymorphs, which are consistent with observed electrochemical performances reported thus far in the literature. We also show that differences in the voltage, kinetics and rate capability of these different polymorphs for Li and Na insertion can be traced back to their fundamentally different VO6/VO5-PO4 frameworks.