Preparation And Modification Of Carbon Nanotubes As Conductive Additives For Lithium-Ion Batteries

The conductive agent is an essential component of the cathode of lithium-ion batteries,and the conductive network constructed by it plays a decisive role in the electron transfer rate between the active materials of the cathode,which is a crucial factor affecting the performance of the battery multiplier.Among various materials for conductive agents,carbon nanotubes have significant physical and chemical properties and application value due to their excellent physicochemical properties,and the catalytic chemical vapor deposition（CCVD）method is currently used for the large-scale preparation of carbon nanotubes.However,the products produced by the CCVD method can break theσandπbonds and graft oxygen-containing functional groups of carbon nanotubes during the purification process of acid,which leads to the reduction of carbon nanotube conductivity and side reactions,thus limiting their application in the field of conductive agents for lithium-ion batteries.To address the above problems,the following three experiments are designed for the overall process of"catalyst improvement-carbon nanotube preparation-nitrogen doping modification-conductive agent application"1.The catalyst is a crucial variable affecting the CCVD method,and a catalyst with less sintering and uniform distribution of active components is beneficial to improve the yield and quality of carbon nanotubes.In this paper,molybdenum was used as the co-catalyst,iron as the active component,and alumina as the carrier to prepare different Fe-to-molybdenum ratios of catalysts by the impregnation method,and propane was used as the carbon source to prepare multi-walled carbon nanotubes by CCVD method.The experimental results showed that the yield of carbon nanotubes prepared under the same CCVD conditions increased to 1623%.The specific surface area increased to 275 m²/g when the Fe-to-molybdenum ratio in the impregnation solution was adjusted to 1:0.2.This experiment demonstrated the positive effect of molybdenum in improving the yield and quality of carbon nanotubes under the above reaction conditions.2.In order to eliminate the adverse effects of carbon nanotube defects and oxygen-containing functional groups after purification by strong acid,the nitrogen-doped carbon nanotubes were designed to be prepared by post-synthesis heat treatment using melamine as the nitrogen source.The experimental results showed that the nitrogen content in the prepared nitrogen-doped carbon nanotubes was 5.55 atom%,and the percentage of graphitic nitrogen groups was 88.16%at a heat treatment temperature of 800°C.When the nitrogen-doped carbon nanotubes were used as the conductive agent for lithium manganate batteries at 2wt%,the battery discharge capacity could reach 106.8 m Ah/g with 92.45%capacity retention in 200cycles at the current density of 5 C.The battery discharge capacity was still 90 m Ah/g at the current density of 10 C.This experiment proved the feasibility of nitrogen-doped carbon nanotubes as the conductive agent for lithium-ion batteries.3.In order to study the elimination effect of ammonia on oxygen-containing functional groups and the advantages of the composite conductive agent,the nitrogen-doped carbon nanotubes were prepared by post-synthesis heat treatment using ammonia as the nitrogen source and mixed with conductive carbon black as the hybrid conductive agent.The experimental results showed that after the ammonia heat treatment,the carboxyl and hydroxyl groups on the surface of the prepared nitrogen-doped carbon nanotubes were significantly reduced,while the nitrogen content was 1.51 atom%and the percentage of each nitrogen group was relatively balanced.When the nitrogen-doped carbon nanotubes and conductive carbon black were used as the conductive agent of LiNi_0.5Mn_0.3Co_0.2O₂ battery at 2+2wt%,the discharge capacity of the battery could reach 135.5m Ah/g with 99.04%capacity retention in 200 cycles at the current density of 3C;the discharge capacity of the battery still had 122.7m Ah/g at the current density of 10C;the lithium-ion diffusion rate of the composite conductive agent battery is significantly improved compared with the battery using SP alone;this experiment proves the reduction effect of ammonia on carbon nanotubes,as well as the positive effect of the composite conductive agent on the dispersion of components,ion,and electron transport capacity in lithium-ion batteries.