Key Factor for Attaining Long Cycle Life of Graphite Coated Anode of Li-ion Secondary Batteries

Abstract

Coated graphite electrodes are widely used as the anode in Li-ion batteries, in which an electrically conductive additive is mixed with the active material for keeping good electric contact between the active material and current collector. We examined the state of mixing of the conductive additive in the coated electrode in detail. The electrode coated with a homogeneous distribution of conductive additive over the electrode prolonged the cycle performance with high discharge capacity, while the electrode with inhomogeneously distributed conductive additive exhibited remarkable degradation of cycleability even though the same materials were used in the same ratio. The SEM images of the well-mixed electrode showed no appreciable change after many cycles, while the poorly mixed electrode showed a number of solid deposits over the surface after cycling. The deposit was identified to be electrically nonconductive Li2CO3, which is presumed to be the cause of the suppressed cycleability. Such an inhomogeneous electrical contact also enlarged the initial charging capacity loss. The results of the analysis of impedance spectra for both electrode types during cycling agreed with the results obtained through electrochemical evaluation.