Highly compressible garnet thin films for high-energy-density Li metal batteries

Oxide-based electrolytes are promising with high ion conductivity and a wide electrochemical window (0–5 V). However, the brittleness and poor compressibility severely inhibit their application. Here, inspired by the “flexible eggshell membrane”, we fabricated a 20 µm, flexible and compressible Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 thin film by in-situ healing the LLZTO crystal chips with poly(di(ethylene glycol) ethyl ether acrylate) electrolytes. Fracture mechanics analysis demonstrates the superior mechanical properties of the flexible LLZTO films. With the package pressure increasing from 0.6 to 1.4 t, the Li metal symmetric cell displays a decreasing internal resistance from ∼281 to ∼156 Ω cm 2 , corresponding to the low limit of ion conductivity from ∼0.6 × 10 −4 to ∼2.1 × 10 −4 S/cm. The symmetric cell can be a stable cycle for ∼220 h at 0.4 mA/cm 2 and ∼300 h at 0.1 mA/cm 2 with a bending angle of 30°. The full cell LiFePO 4 /LLZTO/Li shows a superior rate performance of 150 mA/g and a long cycle life of ∼200 cycles. Pairing with the LiNi 0.8 Co 0.1 Mn 0.1 O 2 , the cell displays a Coulombic efficiency of ∼99.7 % for ∼150 cycles. These results indicate that we have successfully fabricated flexible and compressible LLZTO films, and this will open an avenue for the application of high-energy-density thin film batteries.