Impact of Silicon/Graphite Composite Electrode Porosity on the Cycle Life of 18650 Lithium-Ion Cell

The impact of porosity of a negative electrode containing a silicon/carbon/graphite composite on the cycling performance of 18650 cells with a NMC-based cathode was investigated. Anode porosity variation between 30 and 40% led to a significant difference in the cycle life of 18650 cells: 225 and 347 cycles were achieved, respectively, before the drop of a discharge capacity to 75% of the initial value. A detailed postmortem study of 18650 cells with 30 and 40% anodes after formation and cycling steps was conducted including measurements of average electrode mass and thickness changes, electrochemical cycling in half-cells, impedance measurements in symmetrical cells, study of the electrode surface compositions via X-ray photoelectron spectroscopy (XPS), and acquisition of cross-sectional scanning electron microscopy images. It was found that the degradation processes taking place in 18650 cells were different as a function of the negative electrode porosity at the beginning of tests. Thus, Li+ ion loss in the solid electrolyte interphase layer was the main reason for the performance loss in the case of 40% anode porosity. Compression of the anode to 30% of porosity resulted in Li metal deposition on the negative electrode surface and within the separator during cycling of the 18650 cell, which was evidenced by visual observations and XPS analysis. Li+ ion transport was largely impeded by this deposit leading to poor C-rate performance. The present work contributes to a better understanding of important parameters to take into account for the ongoing practical implementation of prospective Si-containing electrodes for lithium-ion batteries.