High‐Temperature‐Endurable, Flexible Supercapacitors: Performance and Degradation Mechanism

Current state-of-the-art supercapacitors all have a limited operational temperature, and thus, extension of the temperature range is in high demand. In this work, high-temperature-endurable, flexible supercapacitors were fabricated by a very simple method and by using commercially available, low-cost materials. The device could be operated efficiently at 120 °C, and even after 10 000 cycles at an operation voltage of 2.5 V, approximately 75 % of its capacitance was retained. Furthermore, its performance remained essentially unchanged even under high bending conditions. Cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy revealed that there were three factors causing capacitance fading and an increase in the internal resistance. The first one was fusion of the separator during high-temperature electrochemical charging/discharging, which led to an increase in the internal resistance. The second factor was decomposition of the separator, which resulted in the accumulation of deposits on the surfaces of the positive and negative electrodes. The third factor was that possible physical separation of the active materials on the positive electrode from the current collector led to a drastic increase in internal resistance.