The Application Of Multi-Walled Carbon Nanotubes As Conductive Additives In Lithium-ion Power Battery

Since its discovery, Carbon nanotube (CNT) is always a research hotspot, and considered to be one of the most promising nanomaterials. CNT is a one-dimensional tubular nanomaterial formed by curled monolayer or multilayer graphite, and possesses many excellent physical and chemical properties. Due to its unique electronic conductivity, CNT shows an attractive application prospect on the conductive additive for lithium ion power battery. As a novel fibrous conductive additive, CNT can form a three-dimensional conductive network. Compared with traditional conductive additives, such as conductive carbon black, CNT possesses higher conductivity, and the addition amount is relatively lower, which is favorable to improve the capacity, the cycle life, especially high-rate charge-discharge performance of battery. Today, CNT has become one of the main developing directions of conductive additives. However, to use as lithium ion power battery conductive additive, CNT has two main defaults:the residual metal catalyst will affect the battery performance; the strong van der Waals interactions between CNTs makes it difficult to uniformly disperse CNT in the electrode active materials. Therefore, there are still many works to be done for utilizing CNT as conductive additive. In this dissertation, the effect of the residual Fe catalyst in CNT on the battery performance was studied. Through high temperature graphitization treatment, the residual metal catalyst could be effectively removed. Subsequently, acid ultrasonic treatment and sand grinding processing were conducted to prepare CNT conductive slurry. The prepared CNT conductive slurry was used as conductive additive for lithium iron phosphate battery, which achieved good results. The main research contents and results are shown as follow:1. Through analyzing the batteries with internal short circuit, it was found that Fe impurity is the main reason to cause internal short circuit. CNT prepared by chemical vapor deposition (CVD) method contains many residual metal catalysts, such as Fe and Ni, which will cause the internal short circuit. Through high temperature graphitization purification, the content of Fe impurity could be greatly reduced. The battery self-discharge experiment showed that after purification, the battery performance was significantly improved.2. CNT conductive slurry has been prepared by strong acid ultrasonic treatment and sand grinding process. The acid treated CNTs are still not stable in N-methyl pyrrolidone (NMP), furthermore, the strong acid ultrasonic process will destroy the structure of CNTs. On the contrary, CNT conductive slurry prepared by sand grinding method showed good dispersibility in NMP, After6000rpm centrifugation, the stability ratio is>90%, and after12000rpm centrifugation, the stability ratio>65%. While mixing with lithium iron phosphate, the SEM images showed that CNTs were tightly contacted with lithium iron phosphate particles.3. CNT conductive slurry prepared by sand grinding process was used as conductive additive in lithium ion power battery. The performances of battery using CNT or Super-p as conductive additives were tested. The compacted density of CNT contained positive plate was2.071g/cm3, and the mean electrical resistivity is9.73Ω·cm. Compared with Super-p, CNT showed better performances to the specific capacity, internal resistance, high-rate charge-discharge, low temperature discharge and cycle life of battery.