Polarizable cesium cations for energy storage from electrolyte characterization to-EDLC application

Abstract

In the quest for new electrolytes on electrochemical energy storage devices, we present, characterize and test a simple organic electrolyte based on cesium bis (trifluoromethanesulfonyl) imide (CsTFSI) for supercapacitor (SC) application. The physicochemical properties of CsTFSI-based electrolytes in acetonitrile (ACN) and propylene carbonate (PC), alone and mixed with LiTFSI as a co-salt, were characterized under various concentrations and temperatures and revealed a different solution organization system, depending on the solvent and nature of the salt. The solvation radii based on the Jones-Dole-Kaminsky model indicated that ACN forms a single sphere of solvation around the cation Cs+, unlike PC, which does not solvate the large and polarizable Cs cation. The CsTFSI -ACN (1 mol L−1) electrolyte presents a suitable viscosity (<1 mPa•s) and density (36 mS cm−1) for SC application, ionic mobility within an extended temperature range (−40–60 °C), and its molecular level is compatible with the porosity of activated carbon (AC) electrodes. The nanopores of the ACs display good electronic conductivity and ion transport capability, characteristic of electrochemical double-layer capacitors EDLC within an electrochemical window of 3 V and a wide temperature span, i.e., −20–40 °C. In-situ measurements of accumulated gas pressure during typical galvanostatic charge-discharge protocols exposed a minimum pressure at 40 °C (2.6 × 108 Pa for CsTFSI-ACN, 1 mol L−1) compared to standard electrolytes (Et4BF4-ACN, 1 mol L−1) and yielded specific capacitances (Csp) of 115 F g−1 at a normalized current density of 2 A g−1. Standard accelerated aging floating tests confirmed the stability of the SCs (>100 h → 12% decrease on Csp). Finally, the suitability of CsTFSI as a co-salt (with lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) was demonstrated for SC devices, aiming to boost Csp (i.e.,164 F g−1 for equimolar amounts of LiTFSI and CsTFSI). The above results attest to the functionality of CsTFSI as a promising salt for energy storage applications.