Aqueous-Eutectic-in-Salt Electrolytes for High-Energy-Density Supercapacitors with an Operational Temperature Window of 100 °C, from −35 to +65 °C

Abstract

Water-in-salt (WIS) electrolytes are gaining increased interest as an alternative to conventional aqueous or organic ones. WIS electrolytes offer an interesting combination of safety, thanks to their aqueous character, and extended electrochemical stability window, thanks to the strong coordination between water molecules and ion salt. Nonetheless, cost, the tendency of salt precipitation, and sluggish ionic transfer leading to poor rate performance of devices are some intrinsic drawbacks of WIS electrolytes that yet need to be addressed for their technological implementation. It is worth noting that the absence of "free'' water molecules could also be achieved via the addition of a certain cosolvent capable of coordinating with water. This is the case of the eutectic mixture formed between DMSO and H 2 O with a molar ratio of 1:2 and a melting point as low as −140 °C. Interestingly, addition of salts at near-saturation conditions also resulted in an increase of the boiling point of the resulting solution. Herein, we used a eutectic mixture of DMSO and H 2 O for dissolution of LiTFSI in the 1.1−8.8 molality range. The resulting electrolyte (e.g., the so-called aqueous-eutectic-in-salt) exhibited excellent energy and power densities when operating in a supercapacitor cell over a wide range of extreme ambient temperatures, from as low as −35 °C to as high as +65 °C.

Aqueous Co‐Solvent in Zwitterionic‐based Protic Ionic Liquids as Electrolytes in 2.0 V Supercapacitors

“…Thus, [C][Z][A]‐type ZPILs were mixed with water and mixtures of water with ACN and/or DMSO to obtain highly concentrated electrolyte solutions (Figures S16 and S17 and Tables S6 and S7 in the Supporting Information), and we investigated their performance as electrolytes in an SC cell. The SC cell was built up using two mesoporous carbons [52,53] (Figure S18 and Table S8 in the Supporting Information) as electrodes. The mass of each electrode per surface area of current collector was 5 mg cm −2 .…”

“…The latter is actually an additional advantage of WcS-in-ZPIL electrolytes as the formation of a SEI layer in LiTFSI-based electrolytes typically results in fading of both capacity and coulombic efficiency during cycling. [53,58]…”

“…Moreover, taking into consideration differential scanning calorimetry scans and electrochemical impedance spectroscopy measurements, ESW widening in our “water‐in‐ZPIL” and “WcS‐in‐ZPIL” should be most likely ascribed to water participation in the H‐bond complexes forming the eutectic (and, hence, no longer as “free”) water molecules rather than to the formation of a solid‐electrolyte interphase (SEI) layer. The latter is actually an additional advantage of WcS‐in‐ZPIL electrolytes as the formation of a SEI layer in LiTFSI‐based electrolytes typically results in fading of both capacity and coulombic efficiency during cycling [53,58] …”

“…Electrodes preparation : Porous carbon were prepared using the procedure described elsewhere [52,53] . Briefly, 4 g sodium citrate and 4 g urea were dissolved in 30mL of deionized water by stirring over 2 h. The resulting homogeneous aqueous solution was first frozen at −80°C and then freeze‐dried using a Thermo Savant (Micromodulyo‐230).…”

“…ZPILs in the form of [C][Z] [A] were prepared using the same experimental conditions as for the above-mentioned PILs; BET was mixed with PILs (e.g., 0.2, 0.5, 1, or 1.1 mols of BET per mol of PIL) previously obtained, followed by a thermal treatment at approximately 90°C in a N 2 atmosphere over 2-3 h. DES-like mixtures between BET and either MSAH or PTSAH were prepared by mixing the individual components in a 1 : 3 molar ratio and submitting the resulting mixture to approximately 90°C over 2-3 h. Water and aqueous co-solvents ZPILs were prepared by adding Electrodes preparation: Porous carbon were prepared using the procedure described elsewhere. [52,53] Briefly, 4 g sodium citrate and 4 g urea were dissolved in 30mL of deionized water by stirring over 2 h. The resulting homogeneous aqueous solution was first frozen at À 80°C and then freeze-dried using a Thermo Savant (Micromodulyo-230). The resulting white powder was thermally treated at 800°C under N 2 atmosphere with a heating rate of 3°C min À 1 and held at this temperature for 2 h. After returning to room temperature, the resulting carbons were thoroughly washed several times, first with diluted HCl (1 m) and then with deionized H 2 O, to remove any rest of inorganic salt.…”