Modifications of the charge-discharge behaviour of Fe₂(MoO₄)₃ in all-solid-state lithium-ion batteries

The solidification of lithium-ion batteries (LIBs) by replacing liquid electrolytes with solid electrolytes enables the development of a new class of LIBs, namely all-solid-state lithium-ion batteries (ASSLIBs), with improved safety and energy density. Such battery solidification can greatly influence the properties of battery components, as exemplified by a recent report suggesting that the (dis)charge behaviour of Fe₂(MoO₄)₃ (FMO), a promising two-phase electrode material, differs on solid electrolytes compared to liquid electrolytes. However, its underlying mechanism remains unclear. Here we examined the (de)lithiation behaviour of FMO thin films on solid electrolytes using operando synchrotron X-ray diffraction (XRD) to gain insights into the influence of the solidification on the (dis)charge mechanism of electrode materials. The XRD results revealed that FMO on solid electrolytes exhibits a monotonic peak shift over a wide capacity range, accompanied by a temporary peak broadening. This suggests that FMO possesses an expanded solid-solution reaction region and a narrower two-phase reaction region in solidified batteries compared to liquid-based LIBs. The altered (dis)charge behavior was suggested to be thermodynamically driven, as it remained largely unchanged with varying rates and under open circuit conditions. Qualitative analysis considering stress-induced variations in Gibbs free energy curves demonstrated that external stress, potentially caused by the constraint of chemo-mechanical expansion, can thermodynamically narrow the two-phase region when the chemical expansion coefficients of the two phases of FMO differ. These findings highlight the significant impact of the battery solidification on electrode material properties, emphasizing the importance of considering these unique issues in the design of ASSLIBs.