Effect of particle size on the slurry-based processability and conductivity of t-Li7SiPS8

Abstract

All-solid-state batteries (ASSBs) promise higher energy and power densities and improved safety over lithium-ion batteries (LIBs) by using non-flammable solid electrolytes (SEs), with thiophosphate-based SEs having the highest ionic conductivities. In this study, we present the slurry-based processing of tetragonal Li₇SiPS₈ (t-Li₇SiPS₈) into freestanding SE-sheets using six different slurry formulations based on anisole, p-xylene and toluene as solvents and polyisobutene (PIB) and two hydrogenated nitrile butadiene rubbers (HNBR-17 and HNBR-34) as binders, respectively. We systematically investigate their chemical compatibility and morphology and show the effect of different particle size distributions on the slurry-based processability. Depending on the particle size, significant differences are observed in sheet homogeneity and relative density. Specifically, higher ionic conductivities are observed for sheets based on larger particles, likely stemming from smaller inter-particle grain boundary effects as demonstrated by electrochemical impedance spectroscopy (EIS). We confirm the positive correlation between Li diffusivity and particle size through pulsed field gradient nuclear magnetic resonance (PFG NMR) experiments, with larger particle sizes resulting in higher diffusivities. Our study suggests a beneficial effect of larger particles for SEs in terms of transport properties and can be considered as a strategy to maximize the performance of future ASSBs.