Non-equilibrium molecular dynamics study on atomistic origin of grain boundary resistivity in NASICON-type Li-ion conductor

Grain boundary (GB) resistance to ion conduction in solid-state electrolytes is one of the main issues on next-generation, high-performance rechargeable batteries. Thus, it is required to understand the origin of the GB resistance from the atomistic point of view. In this paper, a method to investigate the local ion flux using the non-equilibrium molecular dynamics (NEMD) is proposed, and it is demonstrated that the atomistic origin of the GB resistance in NASICON-type LiZr2(PO4)3 is clarified by the local ion-flux analysis of poly-crystalline system containing over half-million atoms in combination with Li-ion site potential energy analysis. The local ion-flux analysis enables us to visualize where Li ions migrate fast or slow in poly-crystalline structures, and it is observed, for the first time, that Li ions move toward lower reaches within grains and pass through specific regions of GBs. The Li-ion site potential energy analysis provides atomic-level details of the differences between high-flux and low-flux regions. It is confirmed from the analyses that the GB resistance comes from deep Li-ion traps and high-energy Li-ion migration paths made of rings of PO4 and ZrO6 polyhedra that do not exist in the crystalline structure.