Excellent Stability of a Li-Ion-Conducting Solid Electrolyte upon Reversible Li+/H+ Exchange in Aqueous Solutions

Batteries with aqueous catholyte and Li metal anode have attracted intensive research interests due to their exceptional energy density and high charge/discharge rate. The long-term operation of such batteries requires particular conditions for the solid electrolyte separator between the anode and aqueous solutions. In addition to the compatibility with Li, the separator must be stable over a wide pH range. Unfortunately, no such compound has yet been reported. In this study, an excellent stability in neutral and strong basic solutions was observed in a Li-stable solid electrolyte, the cubic Li₇La₃Zr₂O₁₂ garnet. The material experienced 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 2M 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.