Preparation And Electrochemical Energy Storage Properties Of C-coated CoMoO₄ Composites

Transition metal oxides（MnO2,NiO,Co3O₄ and RuO）were very popular in the electrode materials of lithium-ion battery（LIB）and supercapacitor（SC）owing to high theoretical capacity and good electrochemical activity.However,their bad conductivity leads to the low capacity.The volume change makes the structural instability during charge/discharge process,which causes the poor cycle and rate performance.Therefore,the binary transition metal oxides have attracted much attention.Compared with single metal oxides,the binary transition metal oxides have smaller band gap,higher electronic conductivity and larger theoretical capacity,so the performances of lithium batteries and supercapacitors have been improved obviously.However,the improvement still far from enough.Compared with other electrode materials,binary transition metal oxides still behave low conductivity.Combination with other materials（metal oxides,carbon materials and conductive polymers,etc.）could significantly improve the conductivity of the electrode material.This effective modification method can accelerate the transfer of ions/electrons and improve the electrochemical properties of electrode materials.My researchis focus on the carbon coating modification of CoMoO₄ composites with special nanostructure by a facile hydrothermal-calcination method.The X-ray diffraction（XRD）,energy dispersive spectrometer（EDS）and field emission scanning electron microscopy（FE-SEM）were used to study the structure characteristics of C-coated CoMoO₄ composites.Cyclic voltammetry（CV）,galvanostatic charge/discharge（GCD）and electrochemical impedance（EIS）were used to analyse the lithium battery performance and pseudocapacitive performance.The effect of carbon coating on the structure and electrochemical energy storage of CoMoO₄ was investigated.The main contents of this paper are as follows: 1.Synthesis of C-coated CoMoO₄ nanosheet arrays grown on Ni foam by two hydrothermal reactions and annealing process.CoMoO₄ nanosheet arrays grown on foam Ni substrates were synthesized by IV hydrothermal method and CoMoO₄ were carbon-coated by hydrothermal-calcination method.After the first hydrothermal reaction,XRD and SEM results show that the ordered,interconnected three-dimensional reticular CoMoO₄ porous nanosheet arrays have grown on the Ni foam substrate,but the surface of nanosheet is rough,there are many mesopores on the surface of nanosheets.After hydrothermal-calcination,the sample was tested by XRD,EDS and SEM.XRD shows that the sample was a complex of C and β-CoMoO₄.EDS shows that the carbon material have been coated on the surface of the CoMoO₄ nanosheets,the mass ratio of C was 5.76%.SEM shows the structure of CoMoO₄ nanosheets was not changed,but the surface of CoMoO₄ nanosheets becomes finer and smoother.Especially,these nanoholes in the nanosheet surface are filled and disappear,which further indicated that carbon had been successfully coated on the surface of the CoMoO₄ nanosheets.The loadings of pure CoMoO₄ and C-coated CoMoO₄ active materials were 1.4 and 2.1 mg cm-2,respectively.And the thermogravimetry（TG）shows that the glucose was completely carbonized at 400 °C,which is the appropriate calcination temperature.2.The electrochemical performance of CoMoO₄ and C-coated CoMoO₄ nanosheet arrays were evaluated as the anode materials for LIBs.After carbon coating,CoMoO₄ nanosheets show higher specific capacity,better cycling performance and higher rate performance,lithium battery performance has been improved significantly.At the current density of 100 m A g^-1,the intial impulse / discharge capacity of the C-coated CoMoO₄ nanosheet array was 768/769 mAh g^-1,which was higher than that of the pure CoMoO₄ nanosheet array(662/711 mAh g^-1).The average charge/discharge specific capacity of the C-coated CoMoO₄ composite electrode was 630/651 mAh g^-1 during 50 cycles and the capacity retention rate（relative to the maximum discharge specific capacity）is 82%,larger than 599/618 mAh g^-1 and 77% of pure CoMoO₄.After carbon coating,the cycling performance and specific capacity have been remarkably improved.At high current of 0.8,1 and 2C,the discharge specific capacities of the C-coated CoMoO₄ were 344,275 and 240 mAh g^-1,which were far greater than the average discharge capacity of CoMoO₄(229,165 and 80 mAh g^-1),respectively.This result shows the excellent rate performance of C-coated CoMoO₄.The CV,GCD and EIS curves show that the carbon coating improves the conductivity of CoMoO₄,accelerates the transfer of electrons and ions in the electrochemical reaction,and improves the electrochemiacal performance for LIB.3.The pseudocapacitance performances of pure CoMoO₄ CoMoO₄ and C-coated CoMoO₄ nanosheet arrays were evaluated as the active materials for SC.During 1000 cycles,the maximum specific capacitance of the CoMoO₄ and C-coated CoMoO₄ nanosheet arrays were 1492 and 1865 F·g^-1,and the capacity retention were 74% and 80%,respectively.After carbon coating,the specific capacitance and cycle performance of the CoMoO₄ were improved obviously.At current densities of 1,2,4,6,8 and 1A·g^-1,the specific capacitance of C-coated CoMoO₄ were 1265,1251,1137,1128,1105,1317 F g^-1,the specific capacity still increased with the increase of current and the specific capacitance also increased slightly when the current density was restored to 1A g^-1,when the current density back to 1A g^-1,the specific capacitance was also slightly increased,indicating the good rate performance of C-coated CoMoO₄.Compared with CoMoO₄,the charging platform voltage of C-coated CoMoO₄ decreases and the discharge platform rises,which suggests that carbon coating improves the conductivity of CoMoO₄,which was the main reason for the improvement of electrochemical performance.