Two naphthalene-based polyimide derivatives are fabricated through a facile condensation polymerization by 1,4,5,8-naphthalenetetracarboxylic dianhydride (NT) with different two diamines, e.g., urea and ethylenediamine, respectively. Their structures with morphologies are studied and the electrochemical properties are investigated as cathode materials for rechargeable lithium-ion batteries (LIBs). By comparison to NT, the polymers with carbonyl functionalities as electroactive groups show improved stability in electrolyte. The obtained derivative (NOP) from NT and urea with more redox active sites exhibits a higher discharge capacity and better rate performance, which might be attributed to its relatively lower molecule weight of the framework and the introduction of additional carbonyl group inherited from urea unit. It displays an average reversible capacity of 174.5 mAh g−1 at 20 mA g−1 and a good rate performance with a high value of 133.5 mAh g−1 at 500 mA g−1. With an optimized polymerization conditions, its capacity could remain 153 mAh g−1 after 60 cycles of charge-discharge processes at 50 mA g−1, making it a potential material for greener and sustainable electrode for electrochemical storage devices.

Flexible solid-state fiber supercapacitors are fabricated by directly electrodepositing ultrathin manganese dioxide (MnO2) nanosheets on commercial carbon fiber yarns. The deposition process is well controlled and the composition of MnO2 in fiber electrodes is optimized to enable fiber SCs to possess high specific capacitance. Conductive carbon fibers concurrently serve as current collectors in fiber SCs and as flexible substrates for the deposition of MnO2. A single MnO2/CFs fiber electrode exhibits a specific volumetric capacitance of 58.7 F cm−3 with a specific gravimetric capacitance of 428 F g−1 based on the MnO2 mass. Two hybrid carbon fiber electrodes are assembled together in parallel with polyvinyl pyrrolidone/Na2SO4 gel, which is used as both an electrolyte and a separator. The assembled flexible device exhibits a high volumetric energy density of 3.8 mW h cm−3 at a power density of 89 mW cm−3 with a good flexibility (CV curves almost unchanged after 2000 bending times) and a superior long cycle stability (an 85.8% capacitance retention after 10000 cycles). Moreover, the integrated SCs could power a commercial light-emitting-diode (LED), demonstrating its strong potential for the practical applications of flexible energy storage devices.