Electrochemical behavior of $\mathrm{LiV_{3}O_{8}}$ positive electrode in hybrid Li,Na–ion batteries

Vanadium(V)-containing oxides show superior intercalation properties for alkaline ions, although the performance of the material strongly depends on its surface morphology. In this work, intercalation activity of LiV$_{3}$O$_{8}$, prepared by a conventional solid state synthesis, is demonstrated for the first time in non-aqueous Li,Na-ion hybrid batteries with Na as negative electrode, and different Na/Li ratios in the electrolyte. In the pure Na-ion cell, one Na per formula unit of LiV$_{3}$O$_{8}$ can be reversibly inserted at room temperature via a two-step process, while further intercalation leads to gradual amorphisation of the material, with a specific capacity of 190 mAhg$^{−1}$ after 10 cycles in the potential window of 0.8–3.4 V. Hybrid Li,Na-ion batteries feature simultaneous intercalation of Li$^+$ and Na$^+$ cations into LiV$_{3}$O$_{8}$, resulting in the formation of a second phase. Depending on the electrolyte composition, this second phase bears structural similarities either to Li$_{0.7}$Na$_{0.7}$V$_{3}$O$_{8}$ in Na-rich electrolytes, or to LiV$_{3}$O$_{8}$ in Li-rich electrolytes. The chemical diffusion coefficients of Na+ and Li+ in crystalline LiV$_{3}$O$_{8}$ are very close, hence explaining the co-intercalation of these cations. As DFT calculations show, once formed, the Li$_{0.7}$Na$_{0.7}$V$_{3}$O$_{8}$-type structure favors intercalation of Na$^+$, whereas the LiV$_{3}$O$_{8}$-type prefers to accommodate Li$^+$ cations