The strong correlations between the performance of a KB supercapacitor and the properties of NaClO 4 and LiFSI electrolytes over wide concentration ranges

Aqueous supercapacitors have received significant attention for their high safety, environmental friendliness, and high ionic conductivity. In particular, the supercapacitors with highly concentrated water-in-salt electrolytes (WiSEs) exhibit outstanding characteristics such as wide electrochemical stability windows (2-3 V) and fast ion transport behaviors. However, previous studies on the WiSE supercapacitors have mostly focused on the effects of high concentrations, and few have examined the influence of the electrolytic structure upon the performance of supercapacitors with various concentrations. Hence, the present study examines how the concentration affects the performance of supercapacitors with various concentrations of NaClO4 (1-17 mol kg−1) and LiFSI (1-35 mol kg−1) electrolytes. The study reveals a strong correlation between the electrochemical performance and the concentration-dependent properties of the electrolyte. Sudden increases in the cell voltages, along with the maximum response speeds of the supercapacitors (−Φ120 Hz = 80.5° and 77.9° for NaClO4 and LiFSI, respectively) are observed in the middle of the concentration range (i.e., at 5-6 mol kg−1), and are strongly associated with a critical change in the ion and water aggregates in the unique biphasic aqueous electrolyte. The present work is of great importance in gaining a fundamental understanding of the aqueous electrolytes with chaotropic ion salts and is expected to contribute greatly to the development of ultrafast supercapacitors based on the highly conductive electrolytes.