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

Sodium-ion batteries are emerging as an alternative to lithium-based chemistries, with several manufacturers anticipating commercial production in the short perspective. However, the use of sodium is not yet as well established as lithium and thus, requires further development to overcome the safety issues, incompatibility of sodium with many electrode materials, and limited specific energy. The presented study aims to introduce new sodium conductive gel polymer electrolytes with binary polymer matrix and binary plasticizer and discuss their compatibility with a hard carbon anode. In this study, the polymer matrix consists of polyacrylonitrile and poly(methyl methacrylate) in three mass ratios, 9:1, 7:3, and 5:5. The plasticizer is a mixture of sulfolane and imidazolium-based ionic liquid with BF4– or NTf2– anion. The measurements revealed that a small amount of PMMA improves the properties of gel electrolytes, including size stability at high temperatures, thermal properties, and transfer number of cations in the gel electrolytes. The electrochemical properties strongly depend on the anion of the ionic liquid. However, the kinetics of Na+ conduction in a hard carbon anode is favorable in the presence of an ionic liquid with NTf2– anion. Furthermore, systems with hard carbon anode and gel electrolyte with sulfolane/[EMIm][NTf2] plasticizer exhibit activation energy of charge transfer process similar to the lithium-based chemistries. This fact makes these electrolytes good candidates for use in a sodium-ion battery.