Vacancy-Suppressed Garnet Electrolytes for Durable Solid-State Batteries

Cubic-garnet-type solid electrolytes (SEs) are promising candidates for all-solid-state batteries (ASSBs) due to their high ionic conductivity and stability against lithium metal. However, intrinsic lithium and oxygen vacancies formed during high-temperature sintering can lead to interfacial metal reduction and phase transitions, ultimately causing short circuits. In this study, we quantified these intrinsic vacancies in Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) and proposed a strategy to achieve vacancy-suppressed, lithium-stuffed garnet SEs by simply tuning the lithium content in both the green pellet and bedding powder during sintering. The optimized Li-stuffed LLZTO exhibited lithium occupancy exceeding 6.5 per formula unit (pfu) and oxygen-vacancy concentrations below 0.02 pfu, resulting in improved chemical stability at the electrolyte-electrode interface and enhanced air stability. The garnet electrolyte demonstrated a critical current density of 1.00 mA cm^-2 at 30°C and 1.75 mA cm^-2 at 60°C, along with stable cycling performance over 3000 h in lithium symmetric cells and 2000 cycles in hybrid full-cells, demonstrating 90% capacity retention. These findings highlight the pivotal role of intrinsic vacancy control in enhancing the structural and electrochemical integrity of garnet electrolytes, thereby promoting their practical application in ASSBs.