Optimization of Organic/Water Hybrid Electrolytes for High‐Rate Carbon‐Based Supercapacitor

Abstract

Water-in-salt" (WIS) electrolytes with wide electrochemical stability windows (ESWs) have made a breakthrough in energy density of aqueous batteries and supercapacitors (SCs), but the sluggish ion diffusion limits their widespread application. Although the ion diffusion of WIS electrolytes can be improved by the addition of organic co-solvents, the effects of types and amounts of added organic solvents on the physicochemical properties of hybrid electrolytes are not clear. Here, the conductivity, ESW, and flammability of a series of hybrid electrolytes prepared by adding different organic solvents to a typical lithium bis(trifluoromethane sulfonyl) imide (LiTFSI)-based WIS electrolyte are systematically studied. The results show that acetonitrile (ACN) is the best one to improve ion diffusion while maintaining high-level safety and wide ESW. Furthermore, a ternary phase diagram of LiTFSI/H 2 O/ACN is drawn to comprehensively show the relationship among the conductivity, flammability, and solubility of the hybrid electrolytes. According to the guidance of this phase diagram, an optimal hybrid electrolyte (LiTFSI/H 2 O/(ACN) 3.5 ) is obtained, and the carbon-based symmetric SC using such hybrid electrolyte is able to fully work at 2.4 V with superior rate capability and excellent cycling stability over 40 000 cycles.

Perspective on High‐Energy Carbon‐Based Supercapacitors

“…10 Wh kg −1 @ 7.6 kW kg −1 81%, 10 000 cycles @ 5 A g −1 AC YP-50F 1533 m 2 g −1 [197] 17 mol kg − 85%, 20 000 cycles @ 5 A g −1 AC YP-50F 1533 m 2 g −1 [199] 6 mol kg − 18.9 Wh kg −1 @ 21.7 kW kg −1 82%, 15 000 cycles @ 5 A g −1 AC [219] KOH (K 2 SO 4 )//H 2 SO 4 (K 2 SO 4 )/H 2 O c)) 2.0 V 36.9 Wh kg −1 @ 248 W kg −1 8.8 Wh kg −1 @ 4 kW kg −1~1 00%, 10 000 cycles @ 5 A g −1 AC [220] KOH//H 2 SO 4 /H 2 O c)) 2. Energy and power densities were based on the total device.…”

Hybrid Aqueous/Nonaqueous Water‐in‐Bisalt Electrolyte Enables Safe Dual Ion Batteries

“…At present, WiS electrolytes have already been successfully applied to supercapacitors and metal ion batteries to improve their safety. [ 28–32 ] However, the reported WiS and its derivatives with extended working voltage window still cannot meet the requirements for high working voltage DIBs due to the unique anion insertion mechanism. Therefore, matching appropriate electrolytes and electrodes that can fully support the operation of DIBs in a safe manner is fundamentally important to facilitate the practical application of the related batteries.…”

Recent advances in carbon-based supercapacitors

“…169 The low ionic conductivity is generally considered to be the main disadvantage of WIS electrolytes due to the strong coordination between Li + and water molecules, leading to unsatisfactory rate/power capabilities of CSs. [174][175][176][177] More recently, introducing extra ions with weak/ inert interactions into the WIS electrolytes has further boosted the salt/water ratio to an unprecedented level, and these waterin-bisalt electrolytes with upgraded operational potential still maintain competitive conductivities and viscosities in contrast to typical organic electrolytes (Fig. 12).…”