Manganese Oxides/graphene Composite Electrodes For Electrochemical Performance Of Asymmetrical Capacitor

As a kind of energy storage devices with high power density and long cycle life,supercapacitors have attracted widespread attention.However,the traditional symmetric carbon-based electric double-layer capacitors have low energy density and cannot satisfy the increasing demands for energy storage.Therefore,researchers have designed an asymmetric capacitor composed of an electrode material based on electric double layer energy storage mechanism and an electrode material based on Faradic energy storage mechanism.Because of the different energy storage mechanisms of the positive and negative electrodes,the new energy storage device can have the characteristics of high power density,high energy density and long cycle life.According to different Faraday reaction principles,asymmetric capacitors can be divided into asymmetric supercapacitors with pseudocapacitive reactions and asymmetric hybrid ion capacitors with battery reactions.In this paper,manganese oxides with good pseudocapacitive and lithium storage characteristics were selected as electrode materials.However,manganese oxide has poor electrical conductivity and large volume changes during reaction.Therefore,manganese oxides are combined with graphene and have morphological structure design and modification.Such-obtained manganese oxides are performed to assemble two types of asymmetric capacitors to study their electrochemical performance.(1)Based on the excellent lithium storage performance of manganese oxides,oxygen defective manganese dioxide/graphene composite electrode materials are designed.Through in-situ precipitation and annealing,manganese dioxide/graphene composite electrode material with abundant oxygen vacancies were synthesized.The performance in lithium half-cell was optimized by adjusting the manganese dioxide/graphene ratio and annealing conditions.In lithium half-cells 1M LiPF6 with EC:DMC:EMC(v:v:v=1:1:1)was used as electrolyte.Electrodes delivered excellent specific capacity of 1052 mAh g-1 at 0.1 A g-1 and lifespan(91.4%after 400 cycles).At the same time,asymmetric lithium ion capacitors were assembled using manganese dioxide containing abundant oxygen vacancies/graphene composites as negative electrodes and nitrogen-doped carbon nanosheet electrodes as positive electrodes.Through the mass matching of the positive and negative electrodes and the pre-lithiation control of the negative electrode,the electrochemical performance of the asymmetric lithium ion capacitor was further optimized.Asymmetric lithium ion capacitors exhibit a maximum energy density of 206.2 Wh kg-1 and a maximum power density of 25000 W kg-1.(2)Based on the good pseudocapacitive characteristics of manganese oxides,trimanganese tetroxide/graphene composite electrode materials were designed.By coprecipitation and hydrogenation,trimanganese tetraoxide/graphene composite electrode materials with C-O-Mn bonding was synthesized,and the pore size distribution of samples was adjusted by controlling the temperature of the hydrogenation treatment to optimize the electrode performance.In three-electrode system,the electrochemical performance of the electrodes was tested in a 1M sodium sulfate electrolyte.Mn3O4/rGO composites with the optimal pore size of 5-10nm deliver high specific capacitance(351 F g-1 at 0.5 A g-1)and excellent cycling life(100%after 10,000 cycles)At the same time,asymmetric supercapacitors were assembled using trimanganese tetroxide/graphene composite electrode materials as positive electrodes and commercial activated carbon as negative electrodes.The asymmetrical supercapacitor presents excellent energy density(36.76 Wh kg-1)and cycling performance(93.5%after 5000 cycles).