Stepwise reaction and degradation in solution synthesis of Li6PS5Br from P4S10

Abstract

Transport is the only sector to have increased its CO2 emissions since 1990, and regular passenger cars make up the largest part of these transport emissions. Thus, a growing need for sustainable mobility is clear, leading to an expanding electric vehicle market. Lithium-ion batteries are considered most suitable for these vehicles owing to their large volumetric and gravimetric capacities. The need for increased battery performance as well as improved safety makes solid-state batteries the main contender for next-generation Lithium-ion batteries in all battery development roadmaps. Among solid-state electrolytes, sulfide-based materials such as LGPS (Li10GeP2S12), lithium thiophosphates (Li3PS4 and related compounds), and lithium argyrodite-type (generally Li6PS5X, X being Cl, Br, or I) materials are attracting much attention because of their high ionic conductivity, which is comparable in magnitude to that of known liquid electrolytes, potential for improved safety, and potential for more sustainable production. Synthesis of both lithium thiophosphates and their derived products, lithium argyrodites, can be performed in several ways: solid-state methods offer proven ways to achieve these materials, although they come at significant cost in both reaction time as well as energy consumption. Solution-based methods have been shown to produce these materials with lower reaction times, requiring a smaller energy investment, and offering access to metastable phases. A solution-based route starting from the precursor P4S10 towards the final Li6PS5Br argyrodite-type product has been described in literature by Yubuchi et al., whereby P4S10 and Li2S are first reacted in a tetrahydrofuran (THF) solvent to form Li3PS4, followed by addition of Li2S and LiBr in ethanol to form the Li6PS5Br product in a mixed-solvent solution. This work examines the presence of intermediate compounds in the first step of this synthesis (performed in THF), and the formation of ethanolic degradation products over time in the second step of the synthesis (performed in the THF-ethanol mixed solvent system) by a combination of analytical techniques including MAS-31P-NMR, liquid 31P-NMR and ICP-OES.