Dual influence of protonation on Li-ion transport in garnet solid electrolytes: A first-principles study

Abstract

Garnet-type cubic Al-doped Li7La3Zr2O12 (LLZO) is a promising solid electrolyte for all-solid-state Li-ion batteries because of its excellent ionic conductivity and stability against Li metal. However, its sensitivity against moisture poses a significant challenge, as LLZO undergoes protonation upon exposure to water, which has been experimentally observed to reduce Li-ion conductivity. Despite this, the exact impact of protonation-induced structural changes and the role of protons in Li-ion transport within LLZO remain unclear. A thorough understanding of these mechanisms is crucial for enhancing the performance of LLZO as a solid electrolyte. In this study, we employed density functional theory (DFT) and DFT-based molecular dynamics (DFT-MD) simulations to investigate the effect of protonation on the structure, Li-ion transport, and proton dynamics in LLZO. Our results demonstrate that protonation induces a phase transition from the centrosymmetric Ia3‾d to a distorted non-centrosymmetric I4‾3d symmetry, concomitantly reducing the Li-ion conductivity, consistent with experimental observations. Furthermore, we identify two competing effects of protonation on Li-ion transport: protons tend to occupy octahedral sites, which trap Li-ions in more stable tetrahedral sites, thereby impeding their mobility. Conversely, proton insertion enlarges the bottlenecks along Li-ion diffusion pathways from a range of 1.78–1.90 Å to 1.80–1.92 Å, potentially lowering the diffusion barrier for Li transport. Therefore, protonation introduces both inhibitory and facilitating factors for Li-ion transport. Additionally, the reduction in Li-ion concentration due to protonation further diminishes the overall ionic conductivity of LLZO.