Investigation of Initial Capacity Loss in Li-Ion Full Cells with Blended Siox/Graphite Anodes

Abstract

SiOx based materials are top anode candidates for next generation high energy cells. Incremental gains in specific capacity, enabled by blending small percentages (<10%) of SiOx with graphite, may be sufficient to meet next gen performance and cost targets. To date, significant progress in cycle life performance of SiOx materials has been demonstrated. In some cases, the cycle life performance exceeds 80% retained capacity to 500 cycles using aerial anode capacities of 4.0mAh/cm2 or greater. However, a common concern related to SiOx based anode materials is the initial capacity loss (Figure 1). Here, the capacity loss behavior occurs with two distinct slopes. Initially, a rapid loss of 10% occurs before 75 cycles. The rapid initial loss is followed by a stable, linear capacity loss slope that mirrors loss in cells without SiOx. At low percentages of SiOx, it is accepted that the dominant failure mechanisms in full cells are SEI growth at the anode and consumption of Li-ions from the system. Consequently, depletion of Li-ions also causes further degradation at the cathode. In this work, we investigate the impact of SEI growth and Li-ion consumption on initial capacity loss. Using a highly robust 3-electrode cell, we monitor the cycling and impedance behavior of the anode and cathode during initial and extended full cell cycling. Three-electrode cell data is coupled with data from half-cell testing, X-ray diffraction, and SEM/EDS to fully understand “cross-talk” between anode and cathode, degradation, and the shift in cell balancing. Additionally, to aid in cell design, we discuss variables such as the percentage of silicon in the anode (5-20%) and the initial anode to cathode capacity ratio (AC ratio). Ultimately, we aim to mitigate this initial capacity loss and further improve cycle life performance. Figure 1