Preparation Of Carbon/Manganese Oxide Composites And Their Capacitive Performance

Supercapacitors are widely used in automobiles,aviation,power grids,and other fields due to the advantages of high power density and long cycle life.However,the low specific capacitance of porous carbon greatly limits the energy density of the device.According to the energy density formula,the capacity of material and operating voltage window are the keys to determine the energy density.Therefore,the introduction of pseudocapacitive materials with high specific capacitance and constructing lithium ion capacitors using organic lithium ion electrolytes with a wide voltage window are expected to improve the energy density of supercapacitors.As a kind of ideal pseudocapacitive material,manganese-based oxides have the advantages of low cost,non-toxic nature,high natural abundance,and superb theoretical specific capacitance.Unfortunately,the electrochemical performance is limited by the poor intrinsic conductivity and the structure degradation caused by the volumetric expansion during the cycle.Based on the design of nanostructures,the synthesis of carbon/manganese oxide composites with efficient conductive network and structural stability is a feasible solution to the above problems.In this paper,a sandwich carbon/MnO₂ pseudocapacitive composite was synthesized based on the constant current electrodeposition method,which effectively improved the intrinsic defects of manganese oxide while achieving high capacity.Then,a self-standing MnO@hollow carbon pseudocapacitive composite was assembled from bottom to top to achieve the nano-scale mixing of carbon and manganese oxide.The pseudocapacitive composite is further assembled into a lithium ion capacitor device with a wide voltage window due to the low lithium insertion potential of MnO and the outstanding performance of composite.Three-electrode,half-cell and full-cell performance of the above pseudocapacitive materials were investigated respectively.The main conclusions are as follows:（1）In order to obtain high-capacity electrode materials,a sandwich-shaped MnO2/CNTs/MnO2 pseudocapacitive composite was prepared by layer-by-layer loading of MnO2 and carbon nanotubes（CNTs）on the surface of activated carbon paper by constant current electrodeposition,and its performance in aqueous electrolyte was evaluated.The influence of different layer structures on the pseudocapacitive performance of the composite was investigated,and it was found that the sandwich material had the better structure and electrochemical performance.The composite has abundant mesopores,and the synthesizedα-MnO₂ presents a fluffy and porous sponge-like structure,which could effectively reduce the expansion stress of the composite.The intermediate layer CNTs of composite serve as conductive media relay between the inner and outer layers of MnO2 to greatly improve the conductivity of composite.Three-electrode tests show that the reversible specific capacitance is 428.8 F g^-1 at a current density of 0.1 A g^-1.The capacity retention rate is 80%at 0.5 A g^-1.Moreover,the Sandwich/CP still has an excellent cycle stability（95%retention rate）at a current density of 1 A g^-11 after 6000 cycles.The multi-layer structure scheme provided in this chapter can provide a new idea for the design of manganese oxide composite electrodes,and it is expected to solve the common problems of some metal oxide materials used in hybrid capacitors.（2）In order to obtain high energy density energy storage device,a self-standing MnO@hollow carbon nanofibers film（HCF）prepared from bottom to top was used as the anode to construct an organic lithium ion capacitor with a wide voltage window.The influence of the etching time on the composite structure and electrochemical performance of anode was investigated,and it was found that the size of the internal cavity was positively related to the etching time.As the acid etching time increased,the electrode specific capacity increased first and then decreased.When the etching time was at 8 h,the electrode showed the best electrochemical performance.The LIC with the MnO@HCF anode and activated carbon cathode can deliver a high power density up to 10750 W kg^-1 at 48.7 Wh kg-1 and a high capacity retention of 98.6%after 3000 cycles under the mass loading of 7.1 mg cm^-2.The research of self-standing film anode in this chapter opens up a new avenue for designing the high-loading electrode with excellent performance,which may have a great prospect in the practical application.