Machine learning molecular dynamics simulation identifying weakly negative effect of polyanion rotation on Li-ion migration

ZM Xu and HY Duan and Z Dou and MB Zheng and YX Lin and YH Xia and HT Zhao and YY Xia, NPJ COMPUTATIONAL MATERIALS, 9, 105 (2023).

DOI: 10.1038/s41524-023-01049-w

Understanding the physical picture of Li ion transport in the current ionic conductors is quite essential to further develop lithium superionic conductors for solid-state batteries. The traditional practice of directly extrapolating room temperature ion diffusion properties from the high-temperature (>600 K) ab initio molecular dynamics simulations (AIMD) simulations by the Arrhenius assumption unavoidably cause some deviations. Fortunately, the ultralong-time molecular dynamics simulation based on the machine-learning interatomic potentials (MLMD) is a more suitable tool to probe into ion diffusion events at low temperatures and simultaneously keeps the accuracy at the density functional theory level. Herein, by the low-temperature MLMD simulations, the non-linear Arrhenius behavior of Li ion was found for Li3ErCl6, which is the main reason for the traditional AIMD simulation overestimating its ionic conductivity. The 1 & mu;s MLMD simulations capture polyanion rotation events in Li7P3S11 at room temperature, in which four PS4(3-) tetrahedra belonging to a part of the longer-chain P2S7(4-) group are noticed with remarkable rotational motions, while the isolated group PS4(3-) does not rotate. However, no polyanion rotation is observed in Li10GeP2S12, & beta;-Li3PS4, Li3ErCl6, and Li3YBr6 at 300 K during 1 & mu;s simulation time. Additionally, the ultralong-time MLMD simulations demonstrate that not only there is no paddle-wheel effect in the crystalline Li7P3S11 at room temperature, but also the rotational PS4(3-) polyanion groups have weakly negative impacts on the overall Li ion diffusion. The ultralong-time MLMD simulations deepen our understanding of the relationship between the polyanion rotation and cation diffusion in ionic conductors at room environments.

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