Understanding Lithium-Ion Transport in Selenophosphate-Based Lithium Argyrodites and Their Limitations in Solid-State Batteries

To develop solid-state batteries with high power and energy densities, solid electrolytes with fast Li⁺ transport are required. Superionic lithium argyrodites have proven to be a versatile system, in which superior ionic conductivities can be achieved by elemental substitutions. Herein, we report the novel selenophosphate-based lithium argyrodites Li_6-xPSe_5-xBr_1+x (0 ≤ x ≤ 0.2) exhibiting ionic conductivities up to 8.5 mS·cm^-1 and uncover the origin of their fast Li⁺ transport. Rietveld refinement of neutron powder diffraction data reveals a better interconnection of the Li⁺ cages compared to the thiophosphate analogue Li₆PS₅Br, by the occupation of two additional Li⁺ sites, facilitating fast Li⁺ transport. Additionally, a larger unit cell volume, lattice softening, and higher structural disorder between halide and chalcogenide are unveiled. The application of Li_5.85PSe_4.85Br_1.15 as the catholyte in In/LiIn|Li₆PS₅Br|LiNi_0.83Co_0.11Mn_0.06O₂:Li_5.85PSe_4.85Br_1.15 solid-state batteries leads to severe degradation upon charging of the cell, revealing that selenophosphate-based lithium argyrodites are not suitable for applications in lithium nickel cobalt manganese oxide-based solid-state batteries from a performance perspective. This work further expands on the understanding of the structure-transport relationship in Li⁺ conducting argyrodites and re-emphasizes the necessity to consider chemical and electrochemical stability of solid electrolytes against the active materials when developing fast Li⁺ conductors.