Preparation And Electrochemical Properties Of Cobalt Sulfide/Carbon Composites

Due to the high power density,long cycle life and short charging time,supercapacitors have become a research hotspot for energy storage devices.However,the lower energy density compared with batteries limits their commercial application.As a key factor affecting the performance of supercapacitors,the improvement in electrochemical performance of electrode materials has attracted researchers’ wide attention.Among the supercapacitor electrode materials,cobalt sulfide has become a promising material due to its high theoretical specific capacitance,better conductivity than oxide and low cost.However,its particle agglomeration problem,inadepuate redox reaction and worse conductivity than electrical double layer materials lead to lower specific capacitance,poor rate performance and less cycle stability,which limits its largescale application in the field of high-performance supercapacitors.This article mainly starts from three aspects to prepare high-performance electrode materials,including inhibiting the agglomeration of cobalt sulfide particles to increase the active area,introducing mixed metal sulfides to increase the redox reactions and compositing carbon materials to optimize the conductive path between cobalt sulfide particles.We have designed cobalt sulfide/carbon tube,nickel-cobalt sulfide/carbon tube and cobalt sulfide/graphene-like carbon three different composite materials.The influences of different process parameters on the microstructures of the composite materials were explored,the relation between different microstructures and energy storage properties of the composite materials was analyzed,and the best performance samples were selected to assembled with activated carbon to form an asymmetric supercapacitor device.In order to suppress the agglomeration of cobalt sulfide particles and increase the active area,we designed a pea pod-like cobalt sulfide/carbon tube composite electrode material.By adjusting the preparation process,it was found that when the annealing temperature is 400 oC and the mass ratio of carbon-coated nanowires to sulfur powder is 1:1,the resulting carbon tube is thinner and transparent,and the internal hollow cobalt sulfide particles are fine and well dispersed.Electrochemical tests show that the electrode material has the best performance,the specific capacitance of the composite material can reach 1195 F g^-1 at a current density of 1 A g^-1.In order to increase redox reactions and improve the specific capacitance of the electrode material,we further expanded from single metal sulfide to mixed metal sulfide,that is,preparing nickel-cobalt sulfide/carbon tube composite electrode material.By adjusting the content of the nickel-cobalt sulfide multilayer balls added in the electrospinning process,it can be found that when the mass of nickel-cobalt sulfide is 0.6 g,the number of the particles in the transparent carbon network is large and the dispersion is good.The electrochemical test results show that the sample with the multi-layer sphere content of 0.6 g has the best electrochemical performance.When the current density is 1 A g^-1,the corresponding specific capacitance value is 1327 F g^-1.In order to optimize the conductive path of the composite electrode material,we introduced graphene-like carbon with high specific surface area and high in-plane conductivity,that is,preparing cobalt sulfide/graphene-like carbon composite electrode material.After adjusting the parameters,we can find that when adding sulfur in the onepot method and the mass ratio of PVP to cobalt nitrate is 2/6,the carbon layer of the obtained composite material is thin and transparent,the cobalt sulfide particles on the carbon layer are clear and the particle size is small.Electrochemical tests show that this material has the best performance,the specific capacitance value at 1 A g^-1 current density is 1443.2 F g^-1,and can retain 64% of the initial capacitance at 20 A g^-1.When the material is assembled into a water-based asymmetric supercapacitor,the working voltage range is expanded to 1.6 V,and the maximum energy density can reach 60.9 Wh kg^-1.