A Study Of Sn-modified MnO₂Used For The Positive Electrode Of Asymmetric Supercapacitor

Supercapacitors are practical and efficient energy storage devices, which combine the advantage of traditional capacitors and rechargeable batteries. MnO2is a potential electrode material for supercapacitors. It is resources widely, environment friendly and it has a variety of oxidation state and wide electrochemical window.In this thesis, we successfully synthesized Sn-modified MnO2positive electrode material for asymmetric supercapacitor. X-ray diffraction, scanning electron microscopy and electrochemical tests were used to characterize the prepared MnO2. Results showed that the pH value of reaction solution significantly influenced the structure, morphology and electrochemical performance of the MnO2. y-MnO2and δ-MnO2were, respectively, obtained when the reaction solutions are acid and alkaline. The layer structure of8-MnO2which was advantageous to the ion transfer and diffusion played a basic role for high capacitance. The electrochemical tests indicated that good high rate performance was observed on the Sn-modified MnO2material when the molar ratio of Sn to Mn was1:50, the pH of reaction solutions was9and refluxing time was2h. A specific capacitance of178F/g was obtained at the current density of0.3A/g, which was68%higher than that of the undoped material. A specific capacitance of166F/g was remained after the current density increased to2.0A/g. During the processes of charging and discharging at a current density of1.OA/g, a specific capacitance of165F/g was remained even after1000cycles, which was94%of the initial capacitance. Because Sn was involved in MnO2crystal structure and participated in redox reaction, Sn-modified MnO2exhibited favorable capacitance retention ability and high-rate performance.Asymmetric supercapacitor generally consists of two types of electrodes. One is electric double layer electrode; the other is Faradaic pseudocapacitor electrode. The integration of the two types of electrodes can better meet the requirements of electric vehicles for high energy density and high power density. In this thesis, asymmetric supercapacitors were successfully assembled. positive electrode was Sn-modified MnO2material which was synthesized when the molar ratio of Sn to Mn was1:50, the pH of reaction solutions was9and refluxing time was2h. negative electrode was commercial activated carbon (AC). CV tests showed that the best voltage range of asymmetric supercapacitor was0-1.8V. Constant current charge and discharge test, AC impedance test and life performance test indicated that asymmetric capacitor exhibited the best electrochemical performance when m(Sn-MnO2):m(AC)=1.5:1. A specific capacitance of52.1F/g was obtained at the current density of1.0A/g. More than85%of the initial capacitance was remained when the current density increased to5.0A/g.93%of the initial capacitance was remained after1000cycles. A energy density of23.4Wh/kg was obtained when the power density was1800W/kg. After the power density increased5times to9000W/kg, a energy density of20.0Wh/kg was still remained, which was86%of the initial energy density. The asymmetric supercapacitor is promising practical applications for electric vehicles with high power density and high energy density.