Photo-cured Cationic Polyoxazoline Macromonomers as Gel Polymer Electrolytes for Lithium-Ion Batteries

In this work, we present a cationic vinylimidazolium-terminated poly(2-ethyl-2-oxazoline) (PEtOx) macromonomer as a key component of gel polymer electrolytes (GPE) for lithium-ion batteries. GPE production followed a scalable process based on UV curing of the cationic PEtOx macromonomer with polyfunctional acrylic comonomers dissolved in an organic electrolyte (LP30), affording electrolyte-swollen polymeric ionic liquid (PIL) networks with PEtOx side chains. Thus, cathodes coated with a GPE layer of less than 200 mm thickness were readily manufactured. The PIL brush-type GPE is highly insoluble but swellable in LP30 and exhibits pronounced electrolyte retaining ability against evaporation. At 160 °C, the weight loss of the GPE amounted to around 5%. This is 12% less compared to an LP30-soaked commercial Celgard separator. At room temperature, the ionic conductivity was 3.6 × 10-4 S/cm, surpassing that of a comparable Celgard/LP30 system. Contrary to LP30 in Celgard, conductivity measurements for the PIL brush GPE did not indicate any crystallization of the liquid electrolyte at subambient temperatures. This was confirmed by differential scanning calorimetry, suggesting improved ionic mobility in the GPE over a wide temperature range. The electrochemical stability window of the PIL brush GPE is wide enough and fits all common lithium-ion cathode materials. In fact, the GPE exhibited exceptional oxidative stability of 5.2 V vs Li/Li+. Half-cell cycling experiments using a lithium iron phosphate cathode revealed high capacity values of 150 mAh/g at a current rate of C/10. When the current was increased to C/2, the capacity decreased to 120 mAh/g and the cell reached 80% of its initial capacity (referred to C/2) after 180 cycles. Thus, according to the first physicochemical and electrochemical investigations, the PEtOx-based PIL brush GPE represents a promising candidate with respect to lithium-ion battery operation.