ABSTRACT
A major drawback of α-MnO2-based zinc-ion batteries (ZIBs) is the poor rate performance and short cycle life. Herein, an oxygen-deficient α-MnO2 nanotube (VO-α-MnO2)-integrated graphene (G) and N, P codoped cross-linked porous carbon nanosheet (CNPK) composite (VO-α-MnO2/CNPK/G) has been prepared for advanced ZIBs. The introduction of VO in MnO2 can decrease the value of the Gibbs free energy of Zn2+ adsorption near VO (ca. -0.73 eV) to the thermal neutral value. The thermal neutral value demonstrates that the Zn2+ adsorption/desorption process on VO-α-MnO2 is more reversible than that on α-MnO2. The as-made Zn/VO-α-MnO2 battery is able to deliver a large capacity of 305.0 mAh g-1 and high energy density up to 408.5 Wh kg-1. The good energy storage properties can be attributed to VO. Additionally, the VO-α-MnO2/CNPK/G composite possesses the structure of nanotube arrays, which results from the vertical growth of α-MnO2 nanotubes on CNPK. This unique array structure helps to realize fast ion/electron transfer and stable microstructure. The electrochemical performance of VO-α-MnO2 has been comprehensively improved by compositing with G and CNPK. The VO-α-MnO2/CNPK/G can achieve capacity up to 405.2 mAh g-1, energy density of 542.2 Wh kg-1, and long cycle life (80% capacity retention after 2000 cycles).
ABSTRACT
Although nickel-cobalt bimetallic sulfides have been widely studied for supercapacitor electrodes, how to obtain high specific capacity and cycle stability is still an important challenge. Here, an efficient chemical redox method is used to adjust the crystal and electronic structure of cobalt-nickel sulfide (NCS) via B doping, combined with electrospinning technology and conductive polymer polypyrrole (PPy) coating to facilitate faraday redox reactions and obtain high energy density electrode materials. The resulting composite with boron-doped nickel-cobalt sulfide on electrospinned carbon nanofibers with polypyrrole-coating (PPy@B-NCS/CNF) has a high specific capacity (751.61C/g at 1 A/g) and good cycle stability (82.49 % retention after 4000 cycles at 5 A/g). With PPy@B-NCS/CNF as the positive electrode and activated carbon as the negative electrode, an asymmetric supercapacitor (ASC) is prepared. It has excellent electrochemical properties with a power density of 65.58 Wh kg-1 and an energy density of 819.72 W kg-1. The low-temperature performance test shows high reversibility, which provides the possibility for the development of low-temperature electrolytes. Finally, density functional theory (DFT) explains that B-doped NCS has better electrochemical properties from the energy band and state density.
ABSTRACT
As a pseudocapacitive electrode material, nickel-cobalt bimetallic phosphide has attracted wide attention with its advantage in capacitance and chemical activity. While, like Ni-Co oxides or sulfides, the application of nickel-cobalt bimetallic phosphide is generally hampered by its confined conductivity, low chemical stability and unsatisfactory cycle durability. Herein, this work demonstrates a NiCoP@CNT@PPy (NCP@CNT@PPy) composite that is obtained by polymerizing pyrrole monomer on the surface of NiCoP@CNT complex. According to density functional theory (DFT), it is theoretically demonstrated that the bimetallic Ni-Co phosphide (NiCoP) can exhibit more electrons near the Fermi level than single Ni or Co phosphide. Under the combined effects of carboxylic carbon nanotubes (c-CNTs) and polypyrrole (PPy), the NCP@CNT@PPy electrode exhibits excellent electrochemical performance. In addition, a flexible asymmetric supercapacitor (ASC) is prepared, which demonstrated high energy density and admirable heat-resistance and flexibility performance, showing huge potential in the application of heat-resistant storage energy systems and portable wearable devices.