If the influence of kine-c factors is negligible, the changes in the lithia-on/delithia-on behaviour of FMO on the solid electrolyte can be a5ributed to thermodynamic factors. A primary thermodynamic factor poten-ally influencing the lithia-on/delithia-on behaviour of FMO on a solid electrolyte is the stress arising from the constraint of the volume change of the FMO thin film by the solid electrolyte substrate during lithia-on/delithia-on. FMO is known to exhibit a large laoce constant change of up to several percent upon lithia-on/delithia-on . Assuming an elas-c modulus of approximately 100 GPa for FMO for simplicity, the constraint of this laoce deforma-on is roughly es-mated to generate a stress ranging from several hundred MPa to a few GPa within the FMO thin film. Such large stresses may alter the lithia-on/delithia-on mechanism of FMO. Indeed, stress is reported to alter the Li chemical poten-al of each phase in the two-phase cathode material LiFePO4 32 . Furthermore, it has been reported that interfacial strain between the substrate and thin film can modify the lithia-on mechanism of Fe3O4 thin films, a conversion-type anode material . In the following, therefore, we discussed whether stress can thermodynamically alter the lithia-on/delithia-on behaviour of FMO. More specifically, we discussed if stress can thermodynamically expand the solid solu-on region and contract the two-phase region of FMO as suggested by our XRD measurements. Furthermore, we discussed what proper-es of FMO rendered its lithia-on/delithia-on mechanism stress-dependent.