“Water-In-Salt” Electrolyte-Based High-Voltage (2.7 V) Sustainable Symmetric Supercapacitor with Superb Electrochemical Performance—An Analysis of the Role of Electrolytic Ions in Extending the Cell Voltage

Abstract

The present research on an aqueous electrolyte(s)-based supercapacitor, considered to be the next-generation storage device, is mainly focused on to achieve a wide potential window with high energy density. However, the water decomposition in the aqueous electrolyte has posed a major challenge to be tackled to perceive its future prospect. In this context, the present work has undertaken a systematic investigation to analyze the role of ions in the recently explored “water-in-salt” electrolyte(s) attaining the high cell voltage for the symmetric supercapacitor cell (SSC), constructed using N-doped reduced graphene oxide as the electrode material. The nature of electrolytic ions toward affecting the H-bonding network of the water structure and thereby influencing the cell voltage stability has been investigated by employing different electrolytes: 7 m CH3COONa; 2, 7, 12, and 17 m NaClO4; 2, 5, 7, and 11 m NaNO3; and 7, 17, 22, and 27 m CH3COOK. It exhibited the highest cell voltage of 2.7 V in 17 m NaClO4 with an energy density of 140 W h/kg at 640 W/kg, whereas for 11 m NaNO3, a cell voltage of 2.3 V is observed with an energy density of 72 W h/kg at 545 W/kg. In contrast, for 7 m CH3COONa and 27 m CH3COOK, cell voltages of 1.9 and 2.0 V have been obtained with energy densities of 40 W h/kg at 474 W/kg and 41 W h/kg at 500 W/kg, respectively. Among these electrolytes, the ClO4 – anion and NO3 – anion are found to be the water structure breakers (chaotrope), but the ClO4 – anion exhibited much better chaotropicity as compared to the NO3 – anion. In contrast, the CH3COO– anion is found to act as a water structure maker (kosmotrope) anion. To the best of our knowledge, this is among the highest cell voltages (2.7 V) obtained in the aqueous electrolyte-based symmetric supercapacitor with superb energy density and power density. Its potential for energy storage application has been illustrated by lighting 54 white light-emitting diodes (>3 V) for more than 6 min upon charging a single SSC for about 15 s.

Small‐Molecular Crowding Electrolyte Enables High‐Voltage and High‐Rate Supercapacitors

“…Operating potential limits of SC using 95EG‐H 2 O electrolyte were further evaluated according to the procedure described by Weingarth et al (Figure S4, Supporting Information). [ 33 ] Figure 4c shows S‐value of the SC using 95EG‐H 2 O electrolyte in the potential range from −1.7 to 1.7 V. The positive operating potential limit of SC is 1.2 V and the negative operating potential limit is −1.6 V, which is consist with the result of chronoamperometry tests (Figure S5, Supporting Information). Galvanostatic charge–discharge (GCD) curves of SCs with the 95EG‐H 2 O electrolyte are shown in Figure 4d.…”

Physicochemical and Electrochemical Properties of Water‐in‐Salt Electrolytes

“…For concentration values >20 m , existence of full hydrogen bond network is generally obscured and the local electric field induced by ions in the solution will affect translational and rotational dynamics of water molecules [31] . The type of anions is of significant importance on water properties [35] . For example, NO 3 − anion could not disrupt hydrogen bond network as much as TFSI − (as seen in proton NMR spectra) [31] .…”

Role of FTFSI Anion Asymmetry on Physical Properties of AFTFSI (A=Li, Na and K) Based Electrolytes and Consequences on Supercapacitor Application

“…[29] The desolvation energies of ions at polarized interfaces is indicative of faster partial charge transfer kinetics during adsorption desorption in the microporosity. [17,30] Thus, interactions between the electrolyte and AC can then affect the lifetime and self-discharge of ESs. [31,32] Among organic solvents, those based on alkylcarbonates i. e., ethylene carbonate (EC), propylene carbonate (PC), dimethylcarbonate (DMC) and ethyl methyl carbonate (EMC) in mixture with alkali salts AX (A = Li, Na, K) [33,34] have the advantage of offering a wide operating voltage (up to 3 V), [35,36] especially those based on imide salts such as LiFSI, [37,38] LiTFSI [37,38] KTFSI [39] or NaTFSI.…”

“…In organic media, the electrolytes are composed of salts dissolved in a pure solvent or a mixture of solvents such as nitriles, [25,26] alkylcarbonates [27,28] or ethers [29] . The desolvation energies of ions at polarized interfaces is indicative of faster partial charge transfer kinetics during adsorption desorption in the microporosity [17,30] . Thus, interactions between the electrolyte and AC can then affect the lifetime and self‐discharge of ESs [31,32] .…”