Investigating the Influence of Transition Metal Substitution in Lithium Argyrodites on Structure, Transport, and Solid-State Battery Performance

Lithium argyrodites have gained significant attention as candidates for solid electrolytes in solid-state batteries due to their superior ionic conductivities and favorable mechanical properties. However, during charging, oxidative decomposition reactions occur at the interface between the solid electrolyte and cathode active material, which impede cell performance. In this study, transition metal substitution of the solid electrolyte is investigated with the intention of tuning the composition of the cathode electrolyte interphase (CEI) and thereby improving the cycling performance. Hence, the Li_5.5–2xZn_xPS_4.5Cl_1.5 (0 ≤ x ≤ 0.15) and Li_6–2xZn_xPS₅Br (0 ≤ x ≤ 0.15) substitution series are investigated to elucidate how substitution affects structure, Li⁺ transport, and the performance of the materials as catholytes in solid-state batteries. Corefinement of the neutron and powder X-ray diffraction data unveils the occupation of Li⁺ positions by Zn²⁺. This leads to blocking of Li⁺ transport pathways within the Li⁺ cages causing a decrease of ionic conductivities along with increasing activation energies for Li⁺ transport. By using a combination of cycling experiments, impedance spectroscopy and X-ray photoelectron spectroscopy, the composition of the CEI and the state-of-charge dependence of the CEI growth when using Li_5.5–2xZn_xPS_4.5Cl_1.5|NCM-83 composites was investigated in half-cells, revealing that Zn²⁺ substitution leads to faster decomposition kinetics and affects the CEI composition. Overall, this work explores the influence of Li⁺ substitution by Zn²⁺ on structure and transport in lithium argyrodites and the potential of transition metal substitutions as means to tune the kinetics of CEI growth, the CEI composition, and thereby cell performance.