Revealing the Pnma crystal structure and ion-transport mechanism of the Li3YCl6 solid electrolyte

Chloride solid electrolytes represented by Li3YCl6 excel simultaneously in ionic conductivity, deformability, and oxidative stability; their structure-property relationship would provide guiding principles for designing high-performance solid electrolytes. Here, we report that the prototype system Li3YCl6 does not exhibit the P3¯m1 symmetry as commonly believed. This structure occurs only when the material partially decomposes at an overly high annealing temperature of 550°C. With the decomposition being suppressed at 450°C, the material shows a Pnma symmetry instead. Based on this orthorhombic structure, the ion-transport mechanism is clarified through neutron diffraction and first-principles computation. Guided by the established structure-property relationship, the efficient ion transport previously achievable only in the low-crystallinity state is realized in highly crystalline materials. The all-solid-state cells formed by this high-crystallinity material and LiNi0.8Mn0.1Co0.1O2 deliver performance exceeding most reported Li3YCl6-based cells; under 3 C at 25°C, the capacity retention is above 80% for 780 cycles.