Suppressing early capacitance fade of electrochemical capacitors with water-in-salt electrolytes

The use of water-in-salt electrolytes (WiSEs) in electrochemical capacitors is an interesting alternative to state-of-the-art electrolytes based on organic solvents. However, making use of their wide electrochemical stability in electrochemical capacitors has been challenging, as shown in battery materials. In this work, we used three concentrations of LiTf₂N in water (7, 15 and 21 mol kg⁻¹) with activated carbon, and the determination of the maximum operating voltage showed stability up to 2.2, 2.4 and 2.6 V, respectively. This determination suggested positive/negative-electrode mass balances of 2.1, 1.5 and 1.2, but the experiments showed that the cycle life of devices using WiSEs of 15 and 21 mol kg⁻¹ can be improved. Thus, we find that increasing the amount of active material in the positive electrode gave higher capacitance retention over cycling. With the optimization for a longer cycle life, the observed capacitance retentions for the devices with WiSEs of 7, 15 and 21 mol kg⁻¹ were 97%, 70% and near 100%, respectively, after 10000 cycles. The higher amount of active material in the positive electrode penalized the energy-power performance, but the higher electrochemical stability compensated for it, and WiSEs of 7, 15 and 21 mol kg⁻¹ stored 15, 16 and 21 W h kg⁻¹ when they operated at 0.1 A g⁻¹.