Air-stable and lithium-compatible garnet pellet enabled by surface doping for high-performance solid-state batteries†

Garnet-type solid-state electrolytes (SSEs), typically Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT), hold great promise for next-generation lithium metal batteries (LMBs). However, LLZT, with a high content of reactive Li⁺, is air-sensitive, which forms insulating and lithiophobic impurities, jeopardizing its practical applications. Here, we demonstrate that crust-localized Fe³⁺ doping of the LLZT pellet (CF-LLZT) ensures high air stability and lithium compatibility without hurting its ionic conductivity. Briefly, Fe₂O₃ nanofilms are first deposited onto the LLZT substrate, and subsequent high-temperature sintering drives Fe³⁺ into the underneath LLZT, forming a Li⁺ deficient crust with the bulk structure unchanged. This surface-renovated LLZT can extend air-exposure time up to 1 month without forming Li₂CO₃ containments. The symmetric cell of Li/CF-LLZT/Li shows a low interfacial resistance of 6 Ω cm² (1580 Ω cm² for Li/LLZT/Li) and stable electrochemical performance (>5000 h). The assembled LMBs using different cathode materials, particularly LiFePO₄ and LiNi_0.83Co_0.07Mn_0.1O₂, demonstrate high reversible capacity and promising cycling capability. Unlike bulk Fe³⁺ doping, which results in a significant decline in Li⁺ conductivity and renders it unsuitable for use in SSEs, our study highlighted the importance of surface structure modulation of SSEs as an effective research avenue to circumvent the interfacial challenge to facilitate their future commercialization.