Origin of Intergranular Li Metal Propagation in Garnet-Based Solid Electrolyte By Direct Electronic Structure Analysis and Performance Improvement By Bandgap Engineering

摘要

Garnet-structured oxide electrolyte (Li_7La_3Zr_2O_(12), LLZO) has the significant advantage of being chemically and electrochemically stable against Li metal, which allows the implementation of Li metal batteries. However, a short-circuit failure by Li penetration through the LLZO electrolyte has remained a crucial issue for safety and is a major hurdle for Li-based batteries to overcome. Here, we investigate the mechanism of Li dendrite formation for the crystalline Ta-doped LLZO (LLZTO) by examining their energy band structures and defect states using reflection electron energy loss spectroscopy (REELS), scanning photoelectron microscopy (SPEM), and nanoscale charge-based deep level transient spectroscopy (Nano Q-DLTS) techniques. The experimental results reveal that the Schottky barrier height (SBH) was lowered by 0.5 eV due to the defect states localized in grain boundaries and the metallic Li propagation along the grain boundaries is caused by the SBH reduction. Based on the analytical results, the laser annealing of LLZTO was performed as a bandgap engineering method to suppress the Li dendrite formation by forming a mixed surface layer of amorphous LLZTO and Li_2O, which has a wide bandgap to block the electron injection into the grain boundaries. The electrochemical measurements for the laser treated LLZTO demonstrate that the stability and cycling performance are significantly improved. This study sheds light on the importance of electronic structure, in particular, defect states to develop high performance oxide solid electrolytes for Li metal batteries and the practicality of surface modification by laser treatment.