Sulfur-Doped CoNi Layered Double Hydroxide/Carbon Nanofiber Composite Films for Flexible Supercapacitors

Carbon nanofiber (CNF) films prepared by traditional methods have limited capacity; thus, they cannot meet the requirements of high-performance flexible supercapacitors. Compounding CoNi layered double hydroxides (CoNi-LDHs) with flexible CNF films should be a promising strategy to increase energy density and maintain high power density and durable stability. Here, the flexible CNF films were obtained by electrospinning polyacrylonitrile films treated by an acetate-assisted alcohol thermal method and then carbonized at high temperatures. Subsequently, sulfur-doped CoNi-LDH/CNF composite films are prepared by a hydrothermal method. The CNF films provide a three-dimensional (3D) conductive network structure, ensuring that the redox-active center of CoNi-LDH nanosheets has excellent electron and ion transport properties and sufficient active sites. Sulfur doping can not only provide more anchoring sites for loading CoNi-LDH nanosheets but also further improve its electronic conductivity, thereby reducing the electrochemical impedance. The synergistic effect of the doping effect and structural advantage is beneficial to improve electrochemical performance and cycle stability. The S-CoNi-LDH/CNF composite film has an ultrahigh specific capacitance of 1618 F g–1 at 1 A g–1. Besides, the S-CoNi-LDH/CNF composite film and activated carbon (AC) are assembled into S-CoNi-LDH/CNF//AC asymmetric supercapacitors with a maximum energy density of 61.7 Wh kg–1 at 400 W kg–1. It also has excellent cycle stability (90.3% capacitance retention) over 10,000 charge–discharge cycles. This work provides an effective strategy for designing flexible supercapacitors with a high energy density and excellent cycle stability.