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

Todevelop 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 elementalsubstitutions. Herein, we report the novel selenophosphate-based lithiumargyrodites Li6-x PSe5-x Br1+x (0 <= x <= 0.2) exhibiting ionic conductivities up to 8.5mS center dot cm(-1) and uncover the origin of their fastLi(+) transport. Rietveld refinement of neutron powder diffractiondata reveals a better interconnection of the Li+ cagescompared to the thiophosphate analogue Li6PS5Br, by the occupation of two additional Li+ sites, facilitatingfast Li+ transport. Additionally, a larger unit cell volume,lattice softening, and higher structural disorder between halide andchalcogenide are unveiled. The application of Li5.85PSe4.85Br1.15 as the catholyte in In/LiIn|Li6PS5Br|LiNi0.83Co0.11Mn0.06O2:Li5.85PSe4.85Br1.15 solid-state batteries leads to severe degradation upon chargingof the cell, revealing that selenophosphate-based lithium argyroditesare not suitable for applications in lithium nickel cobalt manganeseoxide-based solid-state batteries from a performance perspective.This work further expands on the understanding of the structure-transportrelationship in Li+ conducting argyrodites and re-emphasizesthe necessity to consider chemical and electrochemical stability ofsolid electrolytes against the active materials when developing fastLi(+) conductors.