Doping strategy and mechanism for oxide and sulfide solid electrolytes with high ionic conductivity

Ionic conductivity is a critical parameter required for superionic conductors to be successfully applied as solid electrolytes in all-solid-state batteries. Various methods have been developed to improve the ionic conductivity of solid electrolytes by researchers worldwide. Herein, the research progress achieved by Kilner's group in improving the ionic conductivity of garnet-type solid electrolytes is summarized, focusing on the effects and the underlying mechanism of the doping strategies. Moreover, the characterization methodologies for ion diffusion are discussed in detail, where a 6Li:7Li isotope couple is employed for inter-diffusion and the corresponding isotopic profiles are tested, followed by tracer experiments to directly measure the diffusion coefficient. Inspired by this work, we extend similar strategies to argyrodite sulfide SE (Li6PS5I) to greatly improve its ionic conductivity. This work can therefore serve as a handy tool for improving ionic conductivity in both oxide and sulfide solid electrolytes, providing an in-depth understanding of the underlying lithium diffusion mechanism and improving the methodology.