Photoinitiated Polymerization of Methacrylate/Ionic Liquid Based Polymer Electrolytes: Effect of the Curing Sequence on the Electrochemical Properties
Authors: Cesbron Marius , Oukassi Sami , Bazin Arnaud , Piana Giulia , Armel Vanessa , Porthault Hélène , Poncet Séverine , Geffraye Françoise , Salot Raphaël 
Published: February 2020

Polymer electrolytes have been widely investigated as promising candidates to prepare lithium ion batteries because of their excellent properties such as safety, mechanical stability and flexibility [1]. In this context, several polymerization techniques have been carried out and recently photo-initiated polymerization has become a well-established technology in this field [2,3]. Photo-initiated polymerization is a fast, low thermal impact and high spatial resolution method [4]. Furthermore, photo-initiated polymerization allow tailoring of the properties of the polymer since it can be carried out under a wide range of conditions, including variations in chemical composition (monomer structures, reaction mixture) and irradiation conditions (light intensity, fluence, temperature) [5].

In the present work, emphasis was devoted to the effect of the curing sequence on the electrochemical properties of methacrylate/ionic liquid based polymer electrolyte. The reaction mixture consisted of bisphenol A ethoxylate dimethacrylate (BisEMA, 29 wt.%), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 16 wt.%), 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI, 52 wt.%), and 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur, 3 wt.%). The quaternary system exhibited a total miscibility behavior and consisted of a homogeneous mixture of all components, proven by differential scanning calorimetry (DSC) measurements before polymerization. Ultra violet (UV) photoinitiated polymerization was carried out with light intensities ranging between 0.3 and 50mW/cm². Photo-DSC measurements showed that the light intensity had no effect on the final conversion rate (100% in all cases), whereas it considerably influenced the polymerization rate and therefore the structural properties of the polymer. The resulting polymer electrolytes were investigated regarding their ionic conductivity. It was found that for the same reaction mixture, polymers obtained using very low light intensity presented at room temperature higher ionic conductivities (up to +50%) compared to those obtained at high light intensity. The temperature dependence of the ionic conductivity (figure 1) exhibited a Vogel–Tamman–Fulcher (VTF) type behavior in all considered photoinitiated polymerization conditions; furthermore it showed that light intensity affected the interactions between the ions and the polymer network. Particular attention was put on the investigation of the influence of the resulted polymer structures on the transport properties of the electrolytes using dynamic mechanical analysis (DMA) and solid state nuclear magnetic resonance (NMR).

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Figure 1