Study On The Controlled Synthesis And Electrochemical Performances Of Magnesium Cobaltite Electrode Materials

As a new emerging energy storage device,supercapacitors?SCs?have the merits of high power density,fast charge-discharge rate,long-term cycling stability,and environmental friendliness,which have been applied to hybrid electric vehicles,emergency lighting system,and backup sources.However,the relatively low energy density limits the further application of SCs,and the development of high-performance electrode materials has become a key strategy to solve this problem.As a type of binary transition metal oxide,MgCo₂O₄ has attracted increasing attention due to its low cost,environmental friendliness,and high theoretical specific capacitance of about³122 F/g.This thesis aims to synthesize MgCo₂O₄electrode materials with controlled morphologies and excellent electrochemical performances.The main research contents are summarized as following:?1?Flower-like MgCo₂O₄ microstructures were successfully synthesized through a hydrothermal reaction at 150^oC for 15 h combined with a subsequent calcination treatment at400^oC for 3 h,and the influence of reaction temperature,reaction time,and amount of urea on the morphology of the samples were investigated in details.The MgCo₂O₄ flowers?S1?exhibited a specific capacitance of 749.2 F/g at a current density of 1 A/g with a rate capability of about 73.3%at 15 A/g.67.8%of the original specific capacitance was maintained after 5000 charge-discharge cycles at 5 A/g.An asymmetric supercapacitor?ASC?was fabricated by using S1 and activated carbon?AC?as positive and negative electrodes,respectively.The ASC delivered an energy density of 35.2 Wh/kg at a power density of 859.2W/kg,and the specific capacitance was maintained 100.9%after 5000 cycles at 5 A/g.The MgCo₂O₄ samples synthesized with hydrothermal reaction of 3,5,9,and 12 h delivered the specific capacitances of 462.6,553.1,625.4,and 692.8 F/g,respectively.?2?Flower-like MgCo₂O₄ hieratical structure was prepared via a solvothermal method using absolute ethanol as solvent,and combined with an annealing treatment of the precursor at 350^oC for 2 h in air.The influence of reaction time,volume ratio of ethanol to water,and the dosage of urea on the morphology and size of the MgCo₂O₄ products were investigated in details.Such MgCo₂O₄ flowers delivered a specific capacitance of 703.4 F/g at a current density of 1 A/g with a rate capability of 69.1%at 16 A/g.After 5000 continuous charge-discharge cycles at 5 A/g,75.0%of the original capacitance was remained.?3?Using ethylene glycol as solvent,the MgCo₂O₄ twinned-hemispheres were successfully obtained at 150 ^oC for 15 h with annealing treatment at 500 ^oC for 2 h.The influences of reaction time,reaction temperature,and dosage of urea on the morphology of MgCo₂O₄products were investigated.In 6 M of KOH aqueous electrolyte,the specific capacitance of MgCo₂O₄ twinned-hemispheres electrode?S1?could reach 626.5 F/g at 1 A/g,and rate capability was 71.8%as the current density increased to 16 A/g.After 5000 cycles at 5 A/g,about 68.9%of the original capacitance could be preserved.The fabricated ASC with AC as negative electrode delivered an energy density of 30.6 Wh/kg at a power density of 861 W/kg.In addition,two MgCo₂O₄ samples obtained with 5 and 40 mmol of urea and another two obtained with reaction time of 6 and 9 h delivered the specific capacitance of 469.2,354.2,345.7,and 533.8 F/g,respectively.