Can We Utilise Phonons to Enhance Li-Ion Diffusion?

Solid state electrolytes offer the potential to drastically increase the overall stability of rechargeable lithium batteries as well as provide the means to realise the use of Li-metal anodes maximising the charge capacity of a device. At present, batteries use liquid electrolytes such as [LiPF₆]^-which although they possess high ionic conductivities of 1x10^-2S cm^-1 limit the safe temperature ranges a battery can be operated at as well as forbidding the use of Li-metal anodes due to dendrite formation leading to short circuiting and “thermal runaway”. At present, however, there is a dearth of materials suitable for the role of a highly conductive, stable, solid electrolyte. Current highly conductive materials such as the agyrodites (Li₆PS₅X; X=Cl,Br ) or LGPS (Li₁₀GeP₂S₁₂) typically possess stability issues, whilst more stable materials show low conductivities, e.g. tetragonal LLZO (Li₇La₃Zr₂O₁₂). Understanding the fundamental processes that govern ionic conductivity in such materials has long been a focus of research, however limited information is available on how the intrinsic lattice dynamics of a material influence this, and whether it is a governing factor. In this work we look at understanding the phonon processes between the bulk crystal lattice and a vacanncy migration event in the Li-rich antiperovskites (Li₃OX ; X=Cl,Br,I). In studying the phonons in this way, we aim to question how phonon-based descriptors can be applied to materials discovery and whether diffusion processes can be enhanced via external stimuli.