Preparation And Electrochemical Properties Of Copper Sulfide

As a new energy storage device,supercapacitors have attracted wide attention due to their advantages of high power density,fast charging and discharging rate,environmental friendliness and good cycle stability.However,low energy density restricts its further development.As is known to all,electrode materials are the key to determine the energy density and other major performance indexes of supercapacitors.Therefore,the design and preparation of electrode materials with excellent performance has become the core topic of supercapacitor research.Among metal sulfides,copper sulfides（Cu_xS,x=1-2）not only are low in price and abundant in material sources,but also have unique non-stoichiometric ratio,self-doping and high theoretical specific capacity,which make them an ideal material for pseudcapacitance electrode.However,the specific capacity of the actual prepared Cu_xS as an electrode material is low,and other electrochemical properties,such as the capacity rate and cycle stability,are not ideal.The regulation of structure,morphology and composition as well as the introduction of defect states are effective ways to improve the electrochemical performance of electrode materials.Therefore,this paper designed and controllably prepared three-dimensional copper sulfides of CuS,Cu₂S,Cu₉S₅and Cu_1.96S and regulated their defect states.The influences of structure,morphology,composition and defect state on electrochemical performance were studied.The work is mainly divided into the following three parts:1.Preparation of CuS and Cu₉S₅and study on their electrochemical properties.CuS and Cu₉S₅with different morphologies and non-stoichiometric ratios were prepared using one-step hydrothermal method by adjusting solvent type and reaction time.The internal relationship between solvent type and reaction time and the formation process and morphologies of CuS and Cu₉S₅,and their influences on electrochemical properties were investigated.The results showed that non-stoichiometric rose-like Cu₉S₅was more easily obtained in methanol solution,while other flower-like CuS were more easily obtained in ethanol,ethylene glycol and isopropanol.At the same time,with the extension of reaction time,CuS and Cu₉S₅morphologies were prone to collapse,resulting in poor crystallinity,which leaded to the degradation of electrochemical performance.Therefore,24 h was the best reaction time in this experiment.In addition,electrochemical analysis showed that Cu₉S₅（92 C/g）showed better electrochemical performance than those of CuS（84 C/g,75 C/g,49 C/g）obtained from the other three solvents at the current density of 1 A/g,which was related to the rose-like morphology of Cu₉S₅and its abundant valence changes.2.Regulation of morphology and defect state of Cu₉S₅and its effect on the electrochemical performance.Scanning electron microscopy（SEM）and X-ray photoelectron spectroscopy（XPS）analysis showed that the annealing atmosphere and annealing time played an important role in the control of the morphology and defect state of Cu₉S₅.Especially when annealing under argon（Ar）for 2 h,a large number of holes and folds appeared on the surface of the rose-shaped Cu₉S₅,and meanwhile appropriate sulfur vacancies were generated in Cu₉S₅,which effectively increased the specific surface area of Cu₉S₅,improved its ion transport capacity and enriched REDOX reaction,thus leading to a significant improvement in the electrochemical performance of Cu₉S₅.The results showed that the specific capacity of the sample annealed for 2 h in Ar can reach 337 C/g,which is nearly four times that of the original Cu₉S₅.Meanwhile,the assembled asymmetric supercapacitor Cu₉S₅//AC also showed good energy storage characteristics.For examples,when the power density was 789.5 W/kg,the energy density can reach 13.2 Wh/kg,and the capacitance retention can still be close to100%after 2000 cycles.3.Regulation of phase and morphology of CuS and its effect on the electrochemical performance.By adjusting the annealing time and temperature in Ar atmosphere,the phase transition of CuS from CuS/Cu₉S₅to Cu_1.96S and the corresponding morphology regulation were realized.The experimental results showed that prolonging the annealing time at 250℃is beneficial to the formation of new Cu₉S₅phase in CuS.However,when the annealing temperature was increased to 450℃（annealing time was 2 h）,the transformation of CuS from CuS/Cu₉S₅to Cu_1.96S was realized.Due to the generation of S vacancy and the appearance of complex flower/spherical morphology with rich holes and folds after annealing for CuS,the annealed samples,especially Cu_1.96S,showed better electrochemical performance than that of CuS before annealing,such as the specific capacitance of 238 C/g（at 1 A/g current density）and the capacity rate of 92%.