A Surprising Failure Mechanism in Symmetric Supercapacitors at High Voltages
Ionic liquids (ILs) are attractive candidates for high-voltage electrochemical energy storage systems, owing to their high electrochemical stability. Recently, a unique eutectic mixture of ILs was reported to demonstrate outstanding performance in supercapacitor systems at low temperatures. Yet, many publications using this or similar IL mixtures reported only a limited voltage or cyclability when utilizing them with practical activated carbon electrodes. With supercapacitors consisting of symmetric electrodes, in which voltages higher than 3V are applied, fast capacity fading and activity termination are observed. In order to exceed the limit of 3V for supercapacitors that use electrolyte solutions possessing wide electrochemical windows, we thoroughly investigated the (unexpected) failure mechanism, using several analytical methods. This is the most important aspect of the paper. By this, we discovered a pronounced difference in the electrochemical behavior of the negative and the positive electrodes, which has significant implications on the operation of full symmetric cells at high voltages. Finally, we propose a solution that enables stable operation of cells up to 3.4V. By balancing the mass of the electrodes, we prevent high-voltage failure and control the voltage split to use the full electrochemical window of each electrode and obtain a higher cell voltage of 3.4V and an energy density higher than 40Wh/kg (of the electrode materials). The most important aspect of this work was a rigorous study of the failure mechanism.