context: Among ceramic electrolytes, the garnet-structured oxide Li7La3Zr2O12 (LLZO) has attracted attention as a core material of next-generation batteries (post-LIBs) because it exhibits excellent chemical/electrochemical stability against Li metals and has large electrochemical stability window. In particular, LLZO offers significant advantages in inhibiting Li dendrite formation because it is a single-ion conductor (transference number ∼1), has a high shear modulus (∼60 GPa) and stable interface with Li metals. Despite these intrinsic advantages, LLZO still has the problem of short circuit formation caused by Li propagation through LLZO particularly under high current densities. The penetration of Li through the polycrystalline LLZO electrolyte has been recently interpreted in terms of wettability of Li metals on LLZO, mechanical failure induced by stresses during Li plating that caused flaws on the LLZO surface, high electronic conductivity and a high interfacial resistance as well as non-uniform current distribution by lithium carbonate (Li2CO3) formed on the LLZO surface by the reaction between water vapor, carbon dioxide and LLZO in ambient air. Li propagation is known to occur via preferential plating at the grain boundaries (GBs), and the interface property of Li metal and LLZO plays a critical role in Li penetration when a high current density is applied to the interfaces. Because of the impurities or defects that predominantly exist on the GBs, different energy states for reduction at GBs might lead to non-uniform distribution of Li plating and preferential propagation of Li ions through the GBs. However, the mechanism of Li dendrite formation is unclear and remains controversial. Here, we investigated the mechanism of Li dendrite formation for the crystalline Ta-doped LLZO (Li6.5La3Zr1.5Ta0.5O12, LLZTO) by examining their energy band structures and defect states using various analytical techniques including reflection electron energy loss spectroscopy (REELS), scanning photoelectron microscopy (SPEM), and nanoscale charge-based deep level transient spectroscopy (Nano Q-DLTS). The measurement results revealed that the metallic Li plating along the GBs originates from the plentiful defect states in the GBs, implying that the formation of Li dendrites can be avoided if a thin layer with a wide bandgap is coated on LLZTO grains. Based on these analytical measurements, we adopted the laser annealing of LLZTO as a bandgap engineering method to suppress the Li dendrite formation by forming a surface structure of amorphous LLZTO and Li2O2 with wide bandgaps, which can block the electron injection into the grain boundaries. In addition, our electrochemical measurement results for the 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, defect states in developing ceramic solid electrolytes for Li metal batteries and the practicality of surface modification by laser treatment.
answer:  Li7La3Zr2O12 (LLZO)