Excellent stability of a lithium-ion-conducting solid electrolyte upon reversible LI⁺/H⁺ exchange in aqueous solutions

Batteries with an aqueous catholyte and a Li metal anode have attracted interest owing to their exceptional energy density and high charge/discharge rate. The long-term operation of such batteries requires that the solid electrolyte separator between the anode and aqueous solutions must be compatible with Li and stable over a wide pH range. Unfortunately, no such compound has yet been reported. In this study, an excellent stability in neutral and strongly basic solutions was observed when using the cubic Li₇La₃Zr₂O₁₂ garnet as a Li-stable solid electrolyte. The material underwent a Li⁺/H⁺ exchange in aqueous solutions. Nevertheless, its structure remained unchanged even under a high exchange rate of 63.6%. When treated with a 2 m LiOH solution, the Li⁺/H⁺ exchange was reversed without any structural change. These observations suggest that cubic Li₇La₃Zr₂O₁₂ is a promising candidate for the separator in aqueous lithium batteries.