Lithium-Ion Battery Degradation: Measuring Rapid Loss of Active Silicon in Silicon-Graphite Composite Electrodes

In an effort to increase the specific energy of lithium-ion batteries, silicon additives are often blended with graphite (Gr) in the negative electrode of commercial cells. However, due to the large volumetric expansion of silicon upon lithiation, these Si-Gr composites are prone to faster rates of degradation than conventional graphite electrodes. Understanding the effect of this difference is key to controlling degradation and improving cell lifetimes. Here, we investigate the effects of state of charge and temperature on the ageing of a commercial cylindrical cell with a Si-Gr electrode (LG M50T). Using degradation mode analysis, we were able to quantify the rates of degradation for Si and Gr separately. Loss of active Si is shown to be worse than Gr under all operating conditions, but especially at low state of charge and high temperature, with up to 80% loss in Si capacity after 4 kA h of charge throughput (~400 equivalent full cycles). The results indicate cell lifetimes can be improved by limiting the depth of discharge of cells containing Si-Gr, which suggests Si is not beneficial for all applications. The degradation mode analysis methods developed here provide valuable new insight into the causes of cell ageing by separating the effects of the two active materials in the composite electrode. These methods provide a suitable framework for data analysis of any experimental investigations on cells involving composite electrodes.