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

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Battery cell manufacturers strive to increase the energy density and specific energy of lithium-ion batteries to suit market needs. Using silicon in the negative electrode is one route towards this goal due to the high specific capacity of silicon (around 10 times higher than graphite). However, pure silicon degrades too quickly to be of use in commercial cells, in part due to the volume increasing by around 300% upon charging. To combat this, cell manufacturers use small amounts of silicon additives alongside graphite in composite negative electrodes with the aim of increasing energy density without compromising on cell lifetime. However, it is currently unclear how these composite electrodes age; understanding this behaviour will be key to optimising capacity and lifetime of these next generation materials.
In the work presented here, we investigate the effects of state of charge and temperature on the aging 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 aging by separating the effects of the two active materials in the composite electrode. These methods also provide a suitable framework for data analysis of any future experimental investigations on cells involving composite electrodes.

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