In this paper, sulfur-doped carbon nanotubes were synthesized and modi ed at 600, 700 and 800°C. The results showed that the amount of sulfur doped in carbon nanotubes increased with the increase of temperature, which were 0.78%, 0.98%, and 1.07%, respectively, but the carbon/sulfur binding mode did not change. At the same time, sulfur doping signi cantly increased the speci c surface area, which was conducive to improving the in ltration of the electrolyte into the electrode piece. Sulfur-doped carbon nanotubes are used as conductive agents for the cathode NCM523 of lithium-ion batteries, and compared with untreated carbon nanotubes, they effectively improve the battery polarization, reduce the internal resistance, and greatly improve the ratio performance, and in terms of cycling performance, the capacity retention rate of the battery is increased from 71.3% to 81 ~ 85%.
IntorductionLithium ion batteries have the characteristics of high energy density, high rate, and long cycle, which makes their market share in elds such as digital 3C and power vehicles increasingly widespread [1] . With the increasing demand for applications, some problems of lithium-ion batteries also urgently need to be solved. Most active materials used in cathode materials for lithium-ion batteries have poor conductivity, resulting in relatively large electrode internal resistance and low utilization of active material, which seriously affects the performance of the battery in terms of rate capability, cycling stability, and safety [2,3] , the contraction and expansion of materials in the process of charging and discharging also greatly affect the safety and life of the battery [4,5] . Conductive additives can increase the conductive contact between active materials, improve the electronic conductivity, generate micro-current between active materials and collector surfaces, reduce electrode contact resistance, accelerate electron mobility [5][6][7] .Therefore, it is often necessary to add additional conductive materials to improve battery performance.Due to the requirements for stability and resistance to acid and alkaline corrosion, conductive additives in lithium-ion batteries are mostly carbon-based materials [8][9][10][11] , including conductive carbon black, conductive graphite, carbon bers, graphene, and carbon nanotubes. In addition to their excellent electrical conductivity, carbon nanotubes have a ber-like structure that provides good exibility and mechanical stability [12][13][14] , It is conducive to improving the processability of the battery plate and the cycle life of the battery. Therefore, they are favored by researchers and manufacturers.Common preparation methods for carbon nanotubes include arc discharge, laser evaporation and chemical vapor deposition [15][16][17] . Due to the need for batch preparation, the chemical vapor deposition method is currently the most widely used. The resulting carbon nanotubes contain residual catalysts such as iron, cobalt, nickel, and aluminum. To remove these impurities, puri cation usi...
