Numerous incidents occurred in the past decades involving lithium batteries, from the explosion of car battery pack to the smartphones burning, highlighting the security issues of this kind of battery. The main danger comes from the liquid electrolyte which is thermally unstable and represents a weak barrier against lithium dendrite growth upon cycling. Polymer electrolytes are an interesting alternative to current liquid ones. Solid polymer electrolytes (SPE) are solvent free electrolyte and are composed of a salt and a polymer which gives mechanical properties. These SPE present an important safety increase but the electrochemical performance, especially at room temperature, remains very low. Another material family is under development: the Gel Polymer electrolyte (GPE). GPE are composed of a lithium salt, a polymer and a solvent which improves the electrochemical performance^1,2.

In this work, we want to develop an original ‘’wet’’ chemical process to realize the electrolyte thin film. The chosen gel polymer electrolyte (GPE) is composed of an acrylate polymer matrix which gives the mechanical behavior and a liquid phase which gives the electrochemical properties. The liquid phase is a mixture of an ionic liquid and a lithium salt. The electrolyte is polymerized by UV-curing (Fig.1). In a first step, the GPE was characterized with an original in-situ impedance spectroscopy to monitor the polymerization reaction (Fig.2). Conductivity measurements reveal very high ionic conductivity (> 1 mS/cm), whatever the electrolyte composition, which was expected considering the high ionic content in the liquid phase. However, the lithium transference number is very low (< 0.1) leading to low lithium conductivity (~10^-12S/cm). Raman spectroscopy was used to determine Li-TFSI coordination (Fig.3) which, could partially explain the low transference number. Electrolyte composition was improved to increase the transference number (up to 0.29). The as-obtained electrolyte was integrated in an all solid state battery with LiCoO2 cathode. Galvanostatic cycles of batteries integrating a GPE or a standard liquid electrolyte are presented in Fig.4. Cycling was done at ambient temperature with a high 1C C-rate and electrochemical results are very similar despite a slightly higher polarization with the GPE.

[1] M. Armand, Solid State lonics 69 (1994) 309-319.

[2] A. Arya, Ionics (2017).