Highly conductive gel polymer electrolytes for sodium-ion batteries with hard carbon anodes

•The study presents gel electrolytes with optimized polymer matrix and plasticizer.•Sulfolane/ionic liquid binary plasticizer improves the properties of electrolytes.•10-50% PMMA in polymer matrix widens the electrochemical window of the electrolyte.•The activation energy of charge transfer is comparable with Li-ion batteries.•Samples 9:1_NTf2 and 7:3_NTf2 have the best compatibility with a hard carbon anode.