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

Abstract The presence of Li 2 CO 3 has been identified as the cause of poor lithophilicity in garnet‐type Li 7 La 3 Zr 2 O 12 (LLZO) solid‐state batteries. A Li 2 CO 3 ‐free garnet is expected to enhance the Li/LLZO interface contact. However, permanently eradicating regenerative Li 2 CO 3 from the LLZO surface is extremely challenging and the influence of regenerated Li 2 CO 3 is often ignored. Herein, it is found that glossy Li 2 CO 3 pellets can also be perfectly wetted by molten Li, contradicting the common belief that Li 2 CO 3 is lithiophobic. Therefore, reducing the surface roughness of LLZO allows it to be directly wetted by lithium metal, regardless of the presence of Li 2 CO 3 . 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 cm 2 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.