context: As indicated by the above results, the Li/Al-ion electrolyte exhibits better cycle stability than the Al-ion electrolyte. To further investigate the electrolytes, the voltammograms of these electrolytes measured at different scan rates were analysed using the Randles–Sevick equation (Fig. S7†). The calculated Li-ion diffusion coefficients are 2.84 × 10−12 cm2 s−1 and 7.96 × 10−13 cm2 s−1 in the WO3 and the Ti-V2O5 films, and the Al-ion diffusion coefficients are 2.72 × 10−14 cm2 s−1 and 7.99 × 10−15 cm2 s−1 in the WO3 and the Ti-V2O5 films, respectively. The obtained diffusion coefficients are all two magnitudes lower for Al3+ than Li+, owing to the strong electrostatic interaction between trivalent Al3+ and the host lattice, which slows the ion diffusion in the films. For the mixed Li/Al-ion electrolyte, the obtained diffusion curves are between those of the pure Li-ion electrolyte and the pure Al-ion electrolyte (Fig. S7d, h†). Such a performance is much better compared with the coefficient of pure Al-ions in the films, indicating better ion diffusion in the lattices and promoting the cycle stabilities. Previous reports also indicated that the mixed Li/Al-ion electrolyte can promote the cycling performance in Al-anode batteries. The utilization of mixed ion electrolytes can be a promising direction and deserves further investigation.
answer: Al-ion electrolyte.