A quasi-solid-state polymer gel binder (PGB) has been prepared and investigated for application in high-temperature lithium-ion batteries. The effects of binder on the aging behavior of LiFePO₄ (LFP) cathodes were investigated by electrochemical methods and post mortem analysis. Conventional binder polyvinylidene difluoride (PVDF) was used as a benchmark. At 60 °C, LFP/PGB half cells delivered a capacity of 98 mAh g⁻¹, which corresponds to 92% of the initial value 140 mA g⁻¹ over 1000 cycles at 10C. These results were far superior to the ones obtained for LFP with PVDF which only exhibited a capacity of 58 mAh g⁻¹. We showed that a significant degradation of LFP in the LFP/PVDF electrode accompanied by iron dissolution in the electrolyte and deposition on the anodic surface together with a strong swelling of PVDF in the electrolyte as well as some contact loss between the electrode and the current collector represent the main reasons for the capacity fading at 60 °C. In contrast, these effects were much less pronounced in the LFP/PGB electrode. Furthermore, thick LFP/PGB electrodes with 450 μm thickness, corresponding to a 4.5 mg cm⁻² mass loading, showed a better cycling performance and more favorable electrochemical kinetics than the electrode incorporating PVDF.