Rational design of bilayer heterogeneous poly(ionic liquid)s electrolytes for high-performance flexible supercapacitors with ultraslow self-discharge rate

Supercapacitors have high power density and long cyclic life, but always face a great challenge of rapid self-discharge. Herein, we design a series of bilayer heterogeneous poly(ionic liquid)s electrolytes (BHPE) with anionic and cationic poly(ionic liquid)s (PolyILs), to construct high-performance supercapacitors with ultraslow self-discharge rate. Zeta potential and ionic conductivity of cationic PolyILs are adjusted by changing the volume of counter anions (BF4-, PF6-, TFSI-) and the length of alkyl side chain (ethyl, butyl and hexyl). Both experimental and simulated results reveal that the interaction energy between the cationic PolyILs and counter anion greatly depends on the sizes of anions and the lengths of alkyl side chains of cationic PolyILs, which has intrinsic effect on the zeta potential difference between anionic and cationic PolyILs of the BHPEs, and further can efficiently regulate the self-discharge rate of supercapacitors. The developed supercapacitor based on BHPE with using cationic PolyILs with small anion of BF4- and long alkyl side chain of hexyl exhibits an ultraslow self-discharge rate of −0.04 V h−1 and a high energy density of 34.8 Wh kg−1, which shows great promise for further practical applications in the future.