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 Li 7 La 3 Zr 2 O 12 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 Li 7 La 3 Zr 2 O 12 is a promising candidate for the separator in aqueous lithium batteries.