A systematically comparative study on LiNO3 and Li2SO4 aqueous electrolytes for electrochemical double-layer capacitors

In this work, highly soluble LiNO 3 was adopted as the neutrally aqueous electrolyte for active carbon (AC)-based electrochemical double-layer capacitors (EDLCs), of which the electrochemical performances were evaluated. Simultaneously, the physicochemical properties such as the ionic conductivity and viscosity of the LiNO 3 aqueous solution were investigated. As compared with the most studied Li 2 SO 4 aqueous solution, the LiNO 3 aqueous solution displays more favorable physicochemical properties and electrochemical performances as the neutral electrolytes for EDLCs. To be specific, the conductivity of the 5.0 M LiNO 3 aqueous solution can reach up to 154.8 mS cm −1  at 25 °C, which is nearly two times of the maximum conductivity of 77.6 mS cm −1 for the 2.0 M Li 2 SO 4 aqueous solution under the identical testing conditions. Even at a concentration as high as 9.0 M, the absolute viscosity of the LiNO 3 aqueous solution is only 2.4, while that of the Li 2 SO 4 aqueous solution achieves 3.0 at the maximum concentration of 2.5 M. Additionally, 5.0 M LiNO 3 aqueous solution exhibits a wide electrochemical potential stability window from −0.9 to 0.9 V (vs. SCE) at the AC electrode, giving rise to an operating cell voltage of 1.8 V, which is comparable to that of 2.0 M Li 2 SO 4 aqueous solution. Further, with the 5.0 M LiNO 3 aqueous solution as the electrolyte, the as-fabricated EDLC delivers an energy density up to 21.16 Wh Kg −1  at a power density of 100.09 W kg −1 , which is higher than 18.43 Wh Kg −1 for the EDLC with the 2.0 M Li 2 SO 4 aqueous electrolyte at the identical power density. Even though the power density reaches as high as 5970 W kg −1 , the energy density of the EDLC with the 5.0 M LiNO 3 aqueous electrolyte can still remain at 13.1 Wh Kg −1 , substantially higher than 8.71 Wh Kg −1 of the EDLC with the 2.0 M Li 2 SO 4 aqueous electrolyte at the same power density. Moreover, the EDLC with the 5.0 M LiNO 3 aqueous electrolyte also holds good cyclic stability. After 10000 charge–discharge cycles at a current density of 1 A g −1 and a cut-off voltage of 1.8 V, the capacity retention of this EDLC retains more than 90%. These results can render an insight to explore safe, eco-friendly, inexpensive and neutrally aqueous electrolytes for supercapacitors .