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

Therational design of solid electrolytes for the next-generationbatteries entails an accurate understanding of ionic transport mechanisms.To elucidate the detailed ion hopping processes in different coordinateenvironments, two solid electrolytes, LiTi2(PO4)(3) and Li3Ti2(PO4)(3), with the same NASICON-type framework but different sitesfor accommodating mobile ions, were synthesized and investigated by in situ neutron diffraction and theoretical calculations.The temperature-dependent anisotropic thermal vibrational ellipsoidsand migration paths from the maximum entropy method (MEM) indicatedthat Li ions move faster at higher coordinate architectures, exhibitingthree-dimensional (3D) diffusion pathways. In this rhombohedral structure,"one" node (M1 site) out of "three" interconnectedtransition 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 speciesat the node site, while Li ions located at transition sites betweentwo nodes in Li3Ti2(PO4)(3) have repulsive force from their five surrounding Li ions. Thesedifferent configurations lead to distinct overall transport modes.In LiTi2(PO4)(3), concerted Li ionstransport along a separated chain, while in Li3Ti2(PO4)(3), concerted motion occurs along multiplecooperating chains in the 3D channels. Theoretical calculations furtherindicated that a larger diffusion bottleneck size of Li3Ti2(PO4)(3) enables lower hoppingenergy compared to LiTi2(PO4)(3). Thisstudy clarifies the detailed ionic hopping processes and the underlyingstructure-conductivity relationships. Overall, these resultselucidate the synergistic events in Li-ion hopping from thermodynamicand kinetic points of view, which will greatly benefit the rationaldesign of solid electrolytes for next-generation batteries.