Effects Of Nanocrystallization And Graphene Doping On Charge-discharge Performance Of MoS₂

With the continuous development of cathode materials and electrolytes in battery systems,higher charge and discharge specific capacity and cycle stability are required for anode materials.Molybdenum disulfide?MoS₂?has a theoretical specific capacity as high as 669 m Ah·g-1,which is twice that of the commercial graphite negative electrode.As a result,the MoS₂ has received extensive attention from scientific research workers.However,the presence of problems,including structural distortion,poor conductivity and dissolution of persulfur groups,severely restrict the development of the MoS₂ negative electrode material.In this paper,MoS₂ and MoS₂/graphene negative electrode materials were prepared by hydrothermal method and ultrasonic assisted method.The structure of negative electrode material,optimization of preparation process,nanostructure,three-dimensional framework and other structural properties were systematically studied,and the transfer resistance and lithium ion transfer resistance,charge and discharge cycle performance stability and high rate charge and discharge performance and other electrochemical properties were optimized.In order to optimize the crystal structure of MoS₂,we changed the temperature of hydrothermal synthesis.It was found that the interplanar spacing of the?002?MoS₂ surface is closely related to the synthesis temperature.When the synthesis temperature is lower,the synthesized two Molybdenum sulphide has a smaller?002?crystal plane spacing,and low-temperature synthesis of MoS₂ has a low crystallinity,which is conducive to lithium ion diffusion.However,low-temperature synthesis will generate molybdenum trioxide?Mo O3?impurity phase,which needs to be purified by ammonia treatment.The final MoS₂ anode material has a nanocrystal structure with a degree of freedom of 3 and a diameter of about 2 nm.After analysis,the boundary exists unsaturated sulfur atoms,so the MoS₂ material has good cycle stability?the capacity loss is less than 15% after 40 cycles?and higher specific charge-discharge capacity.?At current density of 0.1 A·g-1,the specific discharge capacity was 826 m Ah·g-1 after 40 cycles.?In order to further improve the conductivity of the MoS₂ negative electrode materials,we doped graphene with MoS₂,which was realized by mixing the hydrothermally synthesized MoS₂ nanocrystalline material and the reduced graphene ethanol solution in an ultrasonic assist method.The resistivity decreases with increasing graphene incorporation,but the rate of decline gradually slows down.Thus,there is an optimal mass content?20%?.On the other hand,the ultrasonic time also has a certain degree of influence on the conductivity of the negative electrode material.The short ultrasonic time is not conducive to the dispersion of the MoS₂ particles,however,the too long ultrasonic time will lead to the rupture of the graphene sheet which is not conducive to the formation of the conductive network.The negative electrode material synthesized by this method can stably circulate 150 turns,and its first charge-discharge capacity loss decreases.However,the two-dimensional composite material of MoS₂/graphene can not prevent the dissolution of persulfur groups during charge and discharge.Therefore,a three-dimensional anode material of MoS₂/graphene was prepared by a hydrothermal method,because graphene oxide can be self-assembled in high-pressure waterheating kettles.MoS₂ nanoparticles were formed at the interface of graphene and water or high-temperature water droplets.Afterwards,the chemical bonds on the surface of graphene oxide formed a three-dimensional skeleton,and a threedimensional anode material was finally obtained.When the synthesis temperature is 160 °C,the concentration of graphene oxide is 1 g/L,and after freeze-drying,the resulting anode material shows a good charge-discharge performance?at a current density of 1 A·g-1,the discharge specific capacity is 838 m Ah·g-1 after 500 cycles?and stability under high rate conditions?at a current density of 10 A·g-1,the discharge specific capacity is 112 m Ah·g-1 after 1000 cycles?.When the synthesis temperature is 200 °C,the concentration of graphene oxide is 1.5 g/L,and after freeze-drying,the obtained anode material has a better charge-discharge specific capacity?at a current density of 1 A·g-1,the discharge specific capacity was 953 m Ah·g-1 after 200 cycles?,but its cycle stability was poor,which can only maintain stable cycling of 200 cycles.