Energizing hybrid supercapacitors by using Mn2+-based active electrolyte

Abstract

Supercapacitors have high power density and a long lifespan but poor energy density in contrast with rechargeable batteries, restricting their widespread applications. Adding soluble redox-active ingredients into electrolyte is an effective strategy to increase specific energy. However, an ion-selective membrane is generally needed in such supercapacitors to avoid the mixing of anolyte and catholyte, which significantly increases the cost. Here we report a supercapacitor that consists of a modified solid Ti₃C₂T_x anode and an active catholyte containing Mn²⁺, where the conversion between soluble Mn²⁺ and solid MnO₂ occurs at the cathode, and the redox of Ti–O with the bonding/de-bonding of H₃O⁺ occurs at the anode. Impressively, this hybrid supercapacitor displays a gratifying specific energy of 43.4 W h kg⁻¹, without using any ion-selective membrane, and excellent cycling stability over 20 000 cycles. Moreover, we also demonstrate its superior low-temperature performance even though the electrolyte has been frozen at −70 °C.