Deep Insight into Reactivity of Li3PS4 Electrolyte for Solid Li-Ion Batteries Towards Humidity Using Large Instruments
Authors: Profatilova Irina , Leteyi Mfiban Ivan , Vanpeene Victor , Tarnopolskiy Vasily , Thompson Oskar , Villevieille Claire , DiMichiel Marco , Daniel Lise , Lyonnard Sandrine , Reytier Magali 
Published: December 2024

Wide commercialization of Li-ion batteries (LIBs) for electric transport stimulates a demand for safer batteries with higher energy density. Thiophosphate solid electrolytes (TSEs) for LIBs are one of the most promising candidates to replace a classical liquid electrolyte with volatile components owing to their high ionic conductivity values and appropriate mechanical properties for processing [1]. However, one of the main drawbacks of TSEs is their high sensitivity towards humidity even in trace amounts. The reactivity with water and moisture impacts the electrochemical properties of the electrolytes and leads to toxic H2S emissions. This aspect plays a crucial role for the choice of LIB manufacturing environment (dry room characteristics) and requires fine understanding of underlying mechanisms to optimize a performance / cost ratio for solid-state batteries (SSBs).
In the present study, a unique combination of characterisation methods was applied to investigate the reaction between densified Li3PS4 pellet and different levels of humidity (dew point (DP) -40 °C and DP=12 °C for 15 min and 2 h) from chemical and morphological/microstructural points of view. H2S gas evolution was accurately quantified upon a TSE exposure to humid atmosphere using a home-made flow-through setup with differentiation between surface and bulk pellet reactivity [2]. Further, top-view propagation of Li3PS4 pellet degradation was investigated using SEM and EDX techniques showing fractures generated through gas released leading to some increase in the pellet porosity at DP= -40 °C; apparition of cracks and oxygen content raise in case of DP = 12 °C exposure. The bulk of the exposed pellets was probed using non-destructive X-ray nanotomography (ESRF, ID16b). Different scenarios were observed at low and high levels of humidity during 15 min and 2 h of the pellet exposure (see Fig.). Degradation front containing large pores and cracks propagated up to 50 µm to the depth of the pellet (~400 µm total thickness) after 2 h of exposure to DP= 12 °C. X-ray diffraction-computed tomography (XRD-CT, ESRF, ID15a) was employed to observe structural changes in 3D for Li3PS4 SE upon its reaction with humidity at different depths. Minor changes were find in crystalline structure of Li3PS4 after DP= -40 °C exposure while a reaction at DP=12 °C provoked formation of new phases propagating up to 90 µm depth of the pellet. Some of newly formed compounds were identified, i.e. P2S2O3, Li2HPO3, P2O5 and Li2S2O6.2H2O. Structural strains for Li3PS4 crystalline phase after its exposure to DP=12 °C were revealed thanks to advanced XRD-CT analysis. Finally, the impact of the pellet exposure to humidity at different conditions on its ionic conductivity was investigated.
As a result of this study, a solid link was found between the morphological, microstructural, chemical degradation of the TSE and its ionic conductivity upon reaction with water. It was evidenced that the crack formation and propagation to the bulk of TSE pellet is a result of a complex cyclic mechanism, which includes chemical attack of H2O molecules on Li3PS4 pellet surface, creation of mechanical strain for the TSE lattice, formation of solid degradation products and gaseous H2S, crack propagation due to the strain release and the gas evolution, further penetration of water to the bulk of the pellet, etc. To the best of our knowledge, this is a first comprehensive study of reaction between SSE and humidity taking into account all the aspects above. This work contributes to understanding of key triggers for TSE deterioration in contact with different concentrations of water and to definition of proper environment for solid-state batteries production and handling.
Beamtime at the ESRF was granted within the Battery Pilot Hub MA4929 “Multi-scale Multi-techniques investigations of Li-ion batteries: towards a European Battery Hub.

References:

[1] Y. Zheng et al., A review of composite solid-state electrolytes for lithium batteries: fundamentals, key materials and advanced structures, Chem. Soc. Rev., 2020, 49, 8790.

[2] X. Randrema, I. Leteyi Mfiban, M. Soler, I. Profatilova et al, Towards a Practical Use of Sulfide Solid Electrolytes in Solid-State Batteries: Impact of Dry Room Exposure on H2S Release and Material Properties, Batteries& Supercaps 2024, 7, e20230038.

Figure 1