Preparation Of Manganese-Cobalt Metal Oxide Electrode Materials For High-performance Supercapacitors

Supercapacitors(SCs),a highly potential energy storage device,possess the advantages of high power density,long cycle life,fast charge/discharge rate,and so on.Nevertheless,the practical applications of SCs are limited due to their lower energy density.It is well known that the electrochemical performance of SCs is highly depended on electrode material.Hence,designing electrode materials with high electrochemical performance is of great significance.Co3O4 stands out among various transition metal oxide electrode materials due to its low cost,environmental-friendliness and ultra-high theoretical specific capacitance(CS).Nevertheless,it is still less than ideal when it comes to the cycling stability and rate capability,which is difficult to meet the needs of practical applications.The purpose of this thesis is to design and synthesize Co3O4-based electrode materials with high specific capacitance and superior cycling stability.The main research contents are summarized as follows:(1)Partially substitute Co in Co3O4 structure with a relatively inexpensive Mn to obtain the MnCo2O4.5 spindle-like electrode material,which was successfully prepared for the first time by solvothermal-calcination methods in a mixed solution containing water and ethanol.By adjusting the reaction time,urea dosage,the volume ratio of ethanol to water to regulate and control the size and morphology of the products.The MnCo2O4.5 spindles(S1)and layered quasi-cubes structure(S2)were obtained with R(defined as volume ratio of ethanol to water)equal to 1/1 and 0/1,respectively,and the differences of the electrochemical performance were discussed in detail.Considering the differences in specific surface area and morphology,there inevitablely existed differences in pseudocapacitor properties.The spindle-like electrode possessed higher specific capacitance and better rate capability,at a current density of 0.5 A/g,the specific capacitance of S1 and S2 was 343 F/g and 290 F/g,respectively.After 5000 charge-discharge cycles at 4 A/g,the final CS could reach 81.3%and 75.1%of the initial values.(2)The Mn-doped Co3O4 oblique prisms were solvothermally synthesized in a mixed solvent of ethanol and water with a subsequent calcination treatment process.It was confirmed by XRD,TEM and XPS analysis that the obtained product was Mn2+doped Co3O4.The effects of solvothermal time,temperature,and urea dosage on the size and morphology of the final products were discussed in detail.Under the same synthesis condition as that of Mn@Co3O4,pure Co3O4 powder was prepared without manganese salt.Electrochemical test showed that Mn@Co3O4 exhibited better performance than that of pure Co3O4.The porous Mn@Co3O4 oblique prism provided a specific capacitance of 909 F/g at 1 A/g,which was higher than that of pure Co3O4 quasi-cubes(640 F/g).When the current density increased to 16 A/g,the rate capabilities of the two electrode materials were 78.2%and 61.7%,respectively.After 5000 charge-discharge cycles,the cycle stability of Mn@Co3O4 was as high as 71.2%.(3)The MnCo2O4/Co3O4 composite was synthesized by a hydrothermal method with a subsequent calcination treatment process at 550?.Additionally,the influences of reaction time and temperature on the dimension and morphology of the final products were also investigated.The specific capacitance of the MnCo2O4/Co3O4 ellipsoid-like microstructures at 1 A/g was 614 F/g,which was close to 1.8 times(343 F/g)of the pure Co3O4 nanosheets.When the current density increased to 10 A/g,the Cs value of the two kinds of materials maintained 76.9%and 74.6%of the initial values,respectively.The MnCo2O4/Co3O4 ellipsoids could achieve 5000 continuous cycles at 5 A/g with a cycling stability of 77.5%.