Origin of intergranular Li metal propagation in garnet-based solid electrolyte by direct electronic structure analysis and performance improvement by bandgap engineering

Abstract

Garnet-structured oxide electrolytes (Li₇La₃Zr₂O₁₂, LLZO) have significant advantage of being chemically and electrochemically stable against Li metals and allow implementation in Li metal batteries. However, a short-circuit failure due to Li penetration through the LLZO electrolyte has remained a crucial issue for safety and is a major hurdle for Li-based batteries to overcome. In, we investigated a mechanism of Li dendrite formation for the crystalline Ta-doped LLZO (LLZTO) electrolyte 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 revealed that the Schottky barrier height (SBH) was lowered by 0.5 eV due to defect states localized in grain boundaries and that the metallic Li propagation along the grain boundaries is caused by the SBH reduction. Based on analytical results, the laser annealing of LLZTO was performed via bandgap engineering method to suppress the Li dendrite formation by forming a mixed surface layer of amorphous LLZTO and Li₂O₂, which has a wide bandgap to block the electron injection into the grain boundaries. The electrochemical measurements of laser-treated LLZTO demonstrated that the stability and cycling performance were significantly improved. This study sheds light on the importance of electronic structure, in particular, the defect states to develop high-performance oxide solid electrolytes for Li metal batteries and the practicality of surface modification by laser treatment.