High‐Performance and Ultra‐Stable Aqueous Supercapacitors Based on a Green and Low‐Cost Water‐In‐Salt Electrolyte

Abstract

The emergence of "water-in-salt" carbon-based supercapacitors presents great potential for further improving the energy density, owing to the extended electrochemical stability window. However, not only low conductivity and high viscosity hinder the superior rate performance, but also the high cost of salts limits their commercialization. Here, we report a low-cost concentrated sodium nitrate electrolyte for high-performance and ultra-stable supercapacitors, based on good electrochemical stability, high conductivity, and low viscosity. In the wide operation voltage range of 0-2.1 V, the constructed symmetric supercapacitor based on commercial active carbon exhibits a high specific capacitance of 32.68 F g À 1 at 1 A g À 1 . Moreover, superior rate capability (83 % capacitance retention from 1 to 20 A g À 1 ) and excellent cycle stability (90 % capacitance retention after 9000 cycles) are also demonstrated. Consequently, this electrolyte system could be the most attractive candidate for promising carbon-based supercapacitors to further improve the energy density.[a] Prof.

The Applications of Water‐in‐Salt Electrolytes in Electrochemical Energy Storage Devices

“…The asymmetry of widened windows of reported WISEs. These data are summarized from the following references: 21 m LiTFSI, [ 2 ] 30 m ZnCl 2 , [ 41 ] 17 m NaClO 4 , [ 25 ] 22 m LiNO 3 , [ 35 ] 12 m NaNO 3 , [ 26 ] 25 m NH 4 CH 3 COO, [ 15 ] 22 m KCF 3 SO 3 , [ 79 ] 21 m LiTFSI + 7 m LiOTF, [ 16 ] 32 m KOAc + 8 m LiOAc, [ 19 ] 9.26 m NaCF 3 SO 3 + 1 m Na 2 SO 4 , [ 68 ] 9 m NaOTF + 22 m TEAOTF, [ 42 ] 42 m LiTFSI + 21 m Me 3 EtN•TFSI, [ 43 ] LiTFSI/(H 2 O) 2.6 (ACN) 3.7 , [ 30 ] 21 m LiTFSI‐H 2 O/9.25 m LiTFSI‐DMC (1:1), [ 32 ] 8 m NaClO 4 /(H 2 O) 1.5 (AN) 2.4 , [ 29 ] LiTFSI + 94%PEG + 6%H 2 O, [ 44 ] 2 M NaNO 3 + 66.7 wt% sucrose. [ 122 ] …”

“…Specifically, the ESWs of 30 m ZnCl 2 , 25 m NH 4 CH 3 COO, 17 m NaClO 4 , 22 m LiNO 3 , and 12 m NaNO 3 could reach 2.3, 2.95, 2.8, 2.55, and 2.56 V, respectively. [ 15,25,26,35,41 ] Among them, the cheap NaClO 4 and NaNO 3 could effectively solve the issue of high cost of TFSI‐based WISE, and they are regarded as promising alternatives for the commercialization of WISEs.…”

“…[ 2,15,19–24 ] Obviously, there are some disadvantages of WISEs hindering their practical applications compared to traditional aqueous electrolytes and organic electrolytes: 1) The high viscosity and low conductivity compared with the traditional dilute electrolytes will deteriorate the rate performance of the devices employing WISEs; 2) the high cost of WISEs caused by high concentration and ionic liquids commonly used is a key factor limiting their industrial application; 3) the strong corrosivity of some WISEs resulting from the high concentration puts forward higher requirements for the current collectors, diaphragms, and packaging materials; 4) WISEs have poor wettability to the electrodes, which will increase the impedance of the devices, deteriorate the cycling performance and lead to the longer time for the electrolyte injection process; 5) the low temperature application of WISEs is limited by the salting‐out because of the decrease of solubility at lower temperatures. [ 25–30 ] Therefore, the dedicated discussion on some studies focusing on solving these shortcomings of WISEs will be conducted in Section 3.…”

“Water-in-salt” electrolyte enhanced high voltage aqueous supercapacitor with carbon electrodes derived from biomass waste-ground grain hulls

“…. 3 But the ammable, volatile and toxic for OE and the high viscosity and low conductivity for IL have rendered their further application for SCs. Compared with IL and OE, aqueous electrolytes have many merits, such as high ionic conductivity, low cost, and environmentally friendly.…”

Aqueous Supercapacitor with Ultrahigh Voltage Window Beyond 2.0 Volt

“…Correspondingly, the price of 17 m NaClO 4 electrolyte (0.33 $ g −1 ) is not only cheaper than 21 m LiTFSI electrolyte (6.16 $ g −1 ) but also lower than those of the current commercial organic electrolytes (Figure 7g). Except 17 m NaClO 4 electrolytes, [11b,48] there were other WIS electrolytes with similar cheap price aiding the corresponding aqueous SCs achieving the voltage window over 2.0 V, such as 12 m NaNO 3 [ 49 ] and 8.96 m LiCl WIS electrolyte. [ 50 ] Unfortunately, there were a significant gap in voltage window between the aqueous SCs with these WIS electrolytes and those with typical LiTFSI WIS eletrolytes.…”