Temperature effect on hybrid capacitors containing protic acetate-based ionic liquids as electrolytes

This work reports an investigation about the use of the electrolyte containing 1 M of lithium acetate (LiOAc) in the fluorine-free and low-cost protic ionic liquid 1-methylimidazolium acetate (1-MimOAc) in hybrid capacitors containing lithium iron phosphate (LFP) and activated carbon (AC) electrodes. The physicochemical properties of the electrolyte were first evaluated as a function of temperature, and its electrochemical stability window was determined by linear sweep voltammetry. We show that devices containing this electrolyte display high cycling stability, retaining 75% of their initial capacity after 4000 cycles at 25°C. In addition, we showed that their performance is affected by the operating temperature. Specifically, at 50°C, the activated carbon electrode undergoes oxidative surface activation and reduced stability, while at 10°C, deposition occurs on the surface of the LFP electrode, likely associated with limited ion mobility and altered interfacial changes. These phenomena were examined by SEM, EDX and XPS analyses. These results indicate that different temperature-dependent side reactions dominate at low and high temperatures. Overall, the findings demonstrate that temperature control plays a crucial role in interfacial phase evolution and cycle performance and highlights the potential of acetate-based ionic liquids as a sustainable, fluorine-free hybrid energy storage system.