Reducing Surface Roughness to Achieve Li₂CO₃‐Existent Lithiophilic Interface in Garnet‐Type Solid‐State Batteries

Abstract The presence of Li₂CO₃ has been identified as the cause of poor lithophilicity in garnet‐type Li₇La₃Zr₂O₁₂ (LLZO) solid‐state batteries. A Li₂CO₃‐free garnet is expected to enhance the Li/LLZO interface contact. However, permanently eradicating regenerative Li₂CO₃ from the LLZO surface is extremely challenging and the influence of regenerated Li₂CO₃ is often ignored. Herein, it is found that glossy Li₂CO₃ pellets can also be perfectly wetted by molten Li, contradicting the common belief that Li₂CO₃ is lithiophobic. Therefore, reducing the surface roughness of LLZO allows it to be directly wetted by lithium metal, regardless of the presence of Li₂CO₃. Additionally, smooth LLZO exhibits better air stability due to its reduced active area. The symmetric cell with a smooth LLZO pellet shows a low interfacial impedance of 2 Ω cm² and a high critical current density of 1.4 mA cm⁻² at 25 °C. This work highlights the surface physics of garnet which significantly influences its interface properties, apart from surface chemistry.