A High-Entropy Multicationic Substituted Lithium Argyrodite Superionic Solid Electrolyte

Bulk-type solid-state batteries (SSBs) constitute a promising next-generation technology for electrochemical energy storage. However, in order for SSBs to become competitive with mature battery technologies, (electro)chemically stable, superionic solid electrolytes are much needed. Multicomponent or high-entropy lithium argyrodites have recently attracted attention for their favorable material characteristics. In the present work, we report on increasing the configurational entropy of the Li6+aP1–xMxS5I solid electrolyte system and examine how this affects the structure-conductivity/stability relationships. Using electrochemical impedance spectroscopy and 7Li pulsed field gradient nuclear magnetic resonance (NMR) spectroscopy, multicationic substitution is demonstrated to result in a very low activation energy for Li diffusion of ∼0.2 eV and a high room-temperature ionic conductivity of ∼13 mS cm–1 (for Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I). The transport properties are rationalized from a structural perspective by means of complementary neutron powder diffraction and magic-angle spinning NMR spectroscopy measurements. The Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I solid electrolyte was also tested in high-loading SSB cells with a Ni-rich layered oxide cathode and found by X-ray photoelectron spectroscopy to suffer from interfacial side reactions during cycling. Overall, the results of this study indicate that optimization of conductivity is equally important to optimization of stability, and compositionally complex lithium argyrodites represent a new playground for a rational design of (potentially advanced) superionic solid electrolytes.