Currently, Li-ion batteries are based on inorganic oxides as cathodes, although they fail in terms of safety and sustainability. Recently redox-active polymers have attracted considerable attention as an alternative to inorganic cathode materials due to their low solubility in common electrolytes, film forming ability, excellent thermal stability and tunable redox properties. Among redox polymers, polyimides (PIs) are known for their ability to undergo reversible electrochemical reduction/oxidation as well as for offering high capacities.

To extensively explore the influence of chemical structure on the electrochemical properties of polymers a series of PIs based on three aromatic dianhydrides and various diamines incorporating ether segments, ionic fragments and additional lactone cycle were synthesized (Fig. 1). The expected improvement in the performance of the cells based on ionic PIs through the additional solvation of Li⁺ by ionic fragments was not reached feasibly because of the electrostatic repulsion effect. In contrast, the insertion of short ether moieties in PI’s backbone allows for the desired Li⁺ solvation, increase of their mobility and rise in the batteries capacity. Among synthesized series, PIs derived from 1,4,5,8-naphthalene tetracarboxylic acid dianhydride and 4,7,10-trioxa-1,13-tridecanediamine showed best battery performance: excellent charge/discharge efficiency, high specific capacity (up to 140 mAh g^-1) and the capability to reversibly operate at relatively high rates (up to C/5).

This work was supported by the Russian Foundation for Basic Research (projects no. 14-29-04039_ofi_m and 16-03-00768_a) and by European Commission (project no. 318873 «IONRUN»). Electrochemical experiments were run at CIC Energigune through a collaboration with Polymat.