Properties and Lithium Battery Applications of Solvate Ionic Liquid/CO2 Binary Mixtures

Abstract

Introduction We have previously reported the intriguing physico-chemical properties of a series of solvate ionic liquids (SILs), which share many of the features of conventional ionic liquids, including high thermal stabilities, wide electrochemical windows and high ionic conductivities, making them attractive as electrolytes for future energy storage applications.1,2SILs may be exemplified by the 1:1 combination of Li[TFSA] and tetraglyme (G4), forming [Li(G4)][TFSA], shown in Fig.1. Recently, we extended our study of the properties of these SILs upon dilution with several molecular solvents.3 An important criterion4 of a SIL is the stability of the complex cation i.e. [Li(G4)]+. This can be conveniently determined by pulsed field gradient NMR (ratio of self-diffusion coefficients Dglyme/DLi is approximately unity), and furthermore by assessment of the residual (“free”) glyme in solution by Raman spectroscopy (deconvolution of the relevant modes gives typically <5% “free” glyme). Of particular note are the low polarity solvents such as toluene and HFE (1,1,2,2-tetrafluoroethyl,2,2,3,3-tetrafluoropropyl ether), which can effectively dilute the SILs, decreasing viscosity and increasing ionic conductivity, without disruption of the complex cation, [Li(G4)]+. Indeed, we have previously demonstrated the use of HFE in particular as a diluting solvent for a SIL applied to the lithium-sulfur battery.5Dilution of the SIL with HFE enables increased coulombic efficiency and rate capability, and also further reduces the solubility of the polysulfides. Moreover, as HFE is a non-flammable diluting solvent, the inherent safety advantage of the low-volatility SIL is not compromised. However, the use of a perfluorinated diluting solvent may be problematic environmentally. The formation of binary mixtures of ILs and high pressure carbon dioxide (CO2) has been studied extensively.6 As a low polarity solvent, it is reasonable to assume that CO2 would also not disrupt the complex cation structure of the SIL. Inspired by these analogous systems, we report here our findings for the dilution of a SIL with environmentally benign, high pressure CO2. Experimental Conductivity was measured by the complex impedance method with two platinum electrodes. The cell constant was determined by calibration with a dilute KCl solution, before transfer of the electrode assembly to the high pressure vessel. The determination of solubility of CO2 in the SIL was performed as described in detail elsewhere.7 Briefly, (i) the pressure change upon sorption of a known quantity of CO2 in the SIL, and (ii) the volume expansion of the SIL upon CO2sorption were determined, from which the solubility could be calculated. The Li deposition/dissolution and battery measurements were performed in a hand-made screw-plate cell that was assembled and then placed inside the high pressure vessel in an Ar-filled glovebox. Results and Discussion The large solubility of CO2 in the SIL promotes a dramatic enhancement in conductivity, as shown in Fig.2 below. This increase is conductivity is not unprecedented. Notable earlier reports in this area include that of Tominaga et al., who, after studying the effect of high pressure CO2 processing on polymer electrolytes, observed in situ a significant enhancement in conductivity for a series of solid polymer electrolytes, such as PMEO10LiTFSA (PMEO: poly-(oligo-oxyethylene glycol methacrylate)), under CO2 pressure.8 The reversible deposition and dissolution of lithium metal was studied in this SIL/CO2 binary mixture, as shown in Fig.3. The result obtained under Ar pressure is also shown for comparison. CO2, with high solubility, promotes efficient deposition and dissolution of Li. This can in part be attributed to higher conductivity (the use of high pressure Ar does not dramatically change the conductivity of the SIL). As shown in Fig.4, the binary SIL/CO2 mixture can also be applied to a typical Li-ion battery cathode such as LiCoO2(LCO), showing improved energy efficiency. In our presentation of this work, we will further discuss the solution properties of the SIL/CO2binary system, further electrochemical characterisation of the Li deposition/dissolution process, and comment on the application of this novel electrolyte for other conventional battery electrodes, including graphite. Acknowledgement This study was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST).