Flexible polytriphenylamine-based cathodes with reinforced energy-storage capacity for high-performance sodium-ion batteries

Owing to the excellent redox reversibility and structural diversity, polytriphenylamine (PTPAn) has been regarded as one of the promising cathode candidates for sodium-ion batteries. However, it still suffers from the bulk aggregation and low operating capacity in practical applications. Assisted by the in-situ polymerization, leaf-like PTPAn nanosheets are uniformly introduced on the surface of carbon nanofibers (CNFs) to form the free-standing CNF@PTPAn composite electrodes. Interestingly, the formation mechanism of the leaf-on-branch structure of CNF@PTPAn composites is systematically explored, confirming that the controlling of oxidation rate and growth degree plays crucial roles in tuning the morphology and active material content of the composite electrodes. Supported by the capacity-cutting analysis, the effective coupling between the active PTPAn and conductive CNFs can provide fast electron/ion-shuttling channels and deepen the electrochemical reaction process. At 50 mA g−1, the capacity of the optimized CNF@PTPAn composite can reach 105 mA h g−1, with a stable rate capability of 78 mA h g−1 even at 400 mA g−1 after 500 cycles in a half cell. The detailed kinetic analysis confirms that the ion-storage behaviors in the low-voltage region can be tailored for the improved capacitive contribution and diffusion coefficients. Meanwhile, the flexible CNF-based full cell with CNF@PTPAn as the cathode and CNFs as the anode exhibits a high energy density of 60 W h kg−1 at 938 W kg−1. Based on this, the rational design is expected to provide more possibilities to obtain advanced freestanding electrode systems.