Ion‐Exchange‐Induced Phase Transition Enables an Intrinsically Air Stable Hydrogarnet Electrolyte for Solid‐State Lithium Batteries

Abstract Inferior air stability is a primary barrier for large‐scale applications of garnet electrolytes in energy storage systems. Herein, a deeply hydrated hydrogarnet electrolyte generated by a simple ion‐exchange‐induced phase transition from conventional garnet, realizing a record‐long air stability of more than two years when exposed to ambient air is proposed. Benefited from the elimination of air‐sensitive lithium ions at 96 h/48e sites and unobstructed lithium conduction path along tetragonal sites (12a) and vacancies (12b), the hydrogarnet electrolyte exhibits intrinsic air stability and comparable ion conductivity to that of traditional garnet. Moreover, the unique properties of hydrogarnet pave the way for a brand‐new aqueous route to prepare lithium metal stable composite electrolyte on a large‐scale, with high ionic conductivity (8.04 × 10⁻⁴ S cm⁻¹), wide electrochemical windows (4.95 V), and a high lithium transference number (0.43). When applied in solid‐state lithium batteries (SSLBs), the batteries present impressive capacity and cycle life (164 mAh g⁻¹ with capacity retention of 89.6% after 180 cycles at 1.0C under 50 °C). This work not only designs a new sort of hydrogarnet electrolyte, which is stable to both air and lithium metal but also provides an eco‐friendly and large‐scale fabrication route for SSLBs.