The effects of pristine and carboxylated multi-walled carbon nanotubes as conductive additives on the performance of LiNi0.33Co0.33Mn0.33O2 and LiFePO4 positive electrodes

Abstract

The influence of two types of multi-walled carbon nanotubes, pristine (MWCNTs) and carboxylated (MWCNTs–COOH), on the electrochemical performance of LiNi0.33Co0.33Mn0.33O2 (NCM)- and LiFePO4 (LFP)-based positive electrodes is investigated. Several characterization techniques, e.g., electronic conductivity measurements, galvanostatic cycling, cyclic voltammetry, electrochemical impedance spectroscopy, and SEM/EDX, were employed. The results suggest that improved electronic conductivity is not necessarily synonymous with enhanced performance. A clear dependence of both rate capability and electrode morphology on the combination of conductive additive and active material can be observed. In particular, while LFP performs better with MWCNTs–COOH, pristine MWCNTs appear to be better suited for NCM. Although the use of MWCNTs–COOH improves the connective network between the active particles for both NCM and LFP, the presence of COOH moieties appears to have a strong effect on the surface properties of the NCM-based electrodes. MWCNTs–COOH formed a shell around the active particles, which may function as a physical barrier and hinder Li+ insertion/extraction.