Atomistic insight into the dopant impacts at the garnet Li7La3Zr2O12 solid electrolyte grain boundaries

Abstract

The garnet-type Li7La3Zr2O12 (LLZO) as one of the most promising solid electrolytes (SEs) has attracted great research attention owing to its high compatibility with Li metal anodes. Doping with a supervalent cation is an effective strategy to stabilize cubic LLZO with desired high ion conductivity. The behaviour of dopants at the grain boundary (GB) (e.g. segregation) is expected to have a great influence on the properties of LLZO but is far from understood. Here we have performed first-principles calculations to reveal the atomic-scale impact of dopants at the GB of the LLZO SE. The results show that Al and Ga dopants at the GB are preferentially segregated at the 24d site of Li with three neighbouring Li-ions, and Nb and Ta dopants prefer to locate at the 5-coordinated and partially distorted 6-coordinated Zr sites at the GB. The segregation of a Nb-like dopant at the GB will improve Li-ion conductivity, while the GB with an Al-like dopant shows conductivity comparable to that of the undoped one and fragmentation of the Li-ion diffusion network. Moreover, the electronic state calculations indicate electron accumulation at the doped GBs, in contrast to the mitigation effect of the dopants on dendrite formation along LLZO GBs revealed by the calculation of Li interstitial formation energy. We also explored the potentially existing phases at the doped coarse GBs, and a series of products have been proposed. These comprehensive calculations provide valuable atomistic insights into the dopants at the GB in the LLZO SE and substantial knowledge of optimization of this material.