Lithium ion batteries (LIBs) have been extensively studied as energy sources for devices ranging from portable electronics to electric vehicles (EVs). LIBs safety remains one of their biggest concerns due to use of flammable organic liquid as electrolytes¹. In recent years, the emergence of gel polymer electrolytes (GPE) based on ionic liquid solvent opened up an avenue for safer solid-state battery technology due to the nonflammability nature of the liquid. These type of GPEs consist of a liquid electrolyte solvent with lithium salt and a polymer². Addition of fillers in GPEs have been shown to improve both ionic conductivity and mechanical properties of the composite electrolyte³. In this work, we are reporting a gel polymer electrolyte (GPE) which was synthesized by blending of a polymerized ionic liquid (PIL) based on pyrrolidinium cation, an imidazolium cation based ionic liquid and lithium salt. Later on, the effect of addition of certain percentages of inorganic silica fillers in GPE were observed by performing both electrochemical and thermal analysis of the polymer electrolyte and their applicability in Li batteries. The prepared GPEs and the composite GPEs (CGPE) were free standing and mechanically stable. Electrochemical Impedance Spectroscopy (EIS) was performed to measure and compare the room temperature ionic conductivity of the GPEs and CGPEs. Chronoamperometry tests were also performed to observe the lithium ion transference number of both type of electrolytes. Almost a two-fold increase in ionic conductivity and ~10% increase in lithium ion transference number was observed for CGPE compared to GPE. This may be the cause of improved ion conduction pathways for lithium ions due to the addition of inorganic silica fillers. Thermogravimetric analysis was performed to observe the thermal stability of the electrolytes and it has been observed that both the electrolytes were thermally stable upto 390^oC. The electrolytes also possess high electrochemical stability window (~ 5V vs Li/Li⁺) which indicates their good affinity of the PIL, Ionic Liquid, Lithium salt and the fillers. The chemical stability of the electrolytes against lithium anode were evaluated and compared with the ionic liquid electrolyte (ILE) by performing EIS of Li/GPE or CGPE or ILE/Li symmetric cells at different storage time under open circuit condition at room temperature. The growth rate of interfacial resistances in terms of storage time is significantly lower for both the GPE and CGPE compared to ILE system. To observe the electrochemical stability of the electrolytes, galvanostatic cycling tests were performed on Li/Li symmetric cells at high current densities. It has been observed that both the electrolytes have lower and stable overpotential compared to ILE which is indicative of uniform lithium deposition on both GPE and CGPE. The electrochemical results obtained in this study, indicate that the CGPE are expected to perform as solid-state composite electrolyte in new generation lithium batteries.

References:

1. J. Kalhoff, G. G. Eshetu, D. Bresser, and S. Passerini, ChemSusChem, 8, 2154–2175 (2015).

2. M. Safa, A. Chamaani, N. Chawla, and B. El-Zahab, Electrochim. Acta, 213, 587–593 (2016).

3. C. Ma et al., J. Power Sources, 317, 103–111 (2016) http://dx.doi.org/10.1016/j.jpowsour.2016.03.097.