Synergistic Ion Diffusion in Lithium Titanium Phosphate Conductors: A Tale from Solo to Ensemble

The rational design of solid electrolytes for the next-generation batteries entails an accurate understanding of ionic transport mechanisms. To elucidate the detailed ion hopping processes in different coordinate environments, two solid electrolytes, LiTi2(PO4)3 and Li3Ti2(PO4)3, with the same NASICON-type framework but different sites for accommodating mobile ions, were synthesized and investigated by in situ neutron diffraction and theoretical calculations. The temperature-dependent anisotropic thermal vibrational ellipsoids and migration paths from the maximum entropy method (MEM) indicated that Li ions move faster at higher coordinate architectures, exhibiting three-dimensional (3D) diffusion pathways. In this rhombohedral structure, "one" node (M1 site) out of "three" interconnected transition sites was found to be the lithium configuration of NASICON. Li ions located at the nodes along the 3D pathway in LiTi2(PO4)3 can only drive out another Li-ion species at the node site, while Li ions located at transition sites between two nodes in Li3Ti2(PO4)3 have repulsive force from their five surrounding Li ions. These different configurations lead to distinct overall transport modes. In LiTi2(PO4)3, concerted Li ions transport along a separated chain, while in Li3Ti2(PO4)3, concerted motion occurs along multiple cooperating chains in the 3D channels. Theoretical calculations further indicated that a larger diffusion bottleneck size of Li3Ti2(PO4)3 enables lower hopping energy compared to LiTi2(PO4)3. This study clarifies the detailed ionic hopping processes and the underlying structure–conductivity relationships. Overall, these results elucidate the synergistic events in Li-ion hopping from thermodynamic and kinetic points of view, which will greatly benefit the rational design of solid electrolytes for next-generation batteries.