A rational design of garnet-type Li7La3Zr2O12 with ultrahigh moisture stability

Lithium-stuffed garnets, one of the most promising solid electrolytes for all-solid-state lithium batteries (ASSLBs), are typically vulnerable to water or moisture. In this work, Li6.5La3Zr1.5Ta0.5O12 (LLZT) with ionic conductivity of 7.36 × 10−4 S cm−1 at room temperature and ultrahigh moisture stability is designed and synthesized by a solvent-free route with no excess lithium source. After storing in ambient atmosphere for four months, the LLZT maintains an ionic conductivity of 6.41 × 10−4 S cm−1. With optimized lithium contents, the LLZT is stored in an atmosphere rich in H2O and CO2 at 65°C for 24 hours and the ionic conductivity only decreases by 6.9% to 6.6 × 10−4 S cm−1. Even undergone water-immersion, the LLZT pellet shows good electrochemical stability, which allows an interfacial resistance of 14.6 Ω cm2 with Li and stable cycling performance of Li|LLZT|LFP cell, exhibiting a high capacity retention of 93% after 100 cycles. Several important features including high relative density, few grain boundaries, water-stable secondary phase of La2Zr2O7, and the Li+-deficient garnet lattice are combined to contribute to the moisture stability as suggested by morphology and surface chemistry analysis and first-principles calculations. This study provides valuable insights into synthesizing fast and moisture-stable lithium garnets in a time-efficient way, which is vital for developing garnet-based ASSLBs.