Kinetic analysis of redox processes in Salen-type polymers at sub-zero temperatures

Batteries and supercapacitors are widely used in various types of portable electronic devices and electric vehicles. Typically, they retain energy and power density at room temperature and at temperatures up to +60° C. However, at sub-zero temperatures, the energy as well as the power density of the power sources dramatically decline due to the sluggish kinetics, mostly connected with solvation/desolvation processes. One of the approaches to solving the problem is using organic electrode materials, among which NiSalen-type polymers may be considered as one of the prospective. Here, we investigate the kinetic restrictions of the redox processes in such polymers at sub-zero temperatures. Temperature-dependent relationships of the kinetic parameters of charge/discharge of poly[Ni(Salen)], poly[Ni(CH₃Salen)] and poly [Ni(CH₃Saltmen)] were determined in electrolyte solutions based on acetonitrile and ethyl acetate with Et₄NBF₄, LiClO₄ and LiTFSI salts. It was shown that injection of counter ions into the polymer film limits the redox processes, and the activation energy of the process depends on the film structure as well as electrolyte composition. Proper combination of the electrolyte and polymer ensures low-activation process of counter ion injection, which corresponds to the absence of desolvation stage. As a result, fast charge and discharge of such materials is possible at the temperatures as low as –40 °C.