Li₅AlO₄-Assisted Low-Temperature Sintering of Dense Li₇La₃Zr₂O₁₂ Solid Electrolyte with High Critical Current Density

Abstract

In recent years, solid electrolytes (SEs) have been developed a lot due to the superior safety of solid-state batteries (SSBs) upon liquid electrolyte-based commercial batteries. Among them, garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is one of the few SEs that is stable to lithium anode with high Li⁺ conductivity and the feasibility of preparation under ambient air, which makes it a promising candidate for fabricating SSBs. However, high sintering temperature (>1200 °C) prevents its large-scale production, further hindering its application. In this work, the Li₅AlO₄ sintering aid is proposed to decrease the sintering temperature and modify the grain boundaries of LLZO ceramics. Li₅AlO₄ generates in situ Li₂O atmosphere and molten Li-Al-O compounds at relatively low temperatures to facilitate the gas-liquid-solid material transportation among raw LLZO grains, which decreases the densification temperature over 150 °C and strengthens the grain boundaries against lithium dendrites. As an example, Ta-doped LLZO ceramics without excessive Li sintered with 2 wt % Li₅AlO₄ at 1050 °C delivered high relative density > 94%, an ionic conductivity of 6.7 × 10^-4 S cm^-1, and an excellent critical current density (CCD) of 1.5 mA cm^-2 at room temperature. In comparison, Ta-doped LLZO with 15% excessive Li sintered at 1200 °C delivered low relative density < 89%, a low ionic conductivity of ∼2 × 10^-4 S cm^-1, and a poor CCD of 0.5 mA cm^-2. Li symmetric cells and Li-LFP full cells fabricated with Li₅AlO₄-assised ceramics were stably cycled at 0.2 mA cm^-2 over 2000 h and at 0.8C over 100 cycles, respectively.