Ageing of NCA/Graphite-Silicon Cells : Investigating the feasibility of accelerated ageing

Sammanfattning: Reliable test data on cell ageing is important to have when deciding on which cell to use in a vehicle. Accelerated ageing could, if performed in a good way, increase the amount of information about the cell ageing in an early stage of a vehicle project, reducing the risk of costly battery replacements. Since cells with NCA chemistry, in most cases, does not show accelerated ageing at elevated temperatures, cycling at higher C-rates and different SOC windows may be more effective in order to accelerate the ageing. However, in order to reflect the ageing in a reference cell, the accelerated aged cell must not be charged with a C-rate causing an excess of lithium plating. Thus, to perform accelerated ageing, it is important to have a good understanding of the ageing phenomena and to be able to detect the different types of ageing in the cell. A cycle life study is therefore performed in order to investigate the impact of the C-rate, SOC window placement and ∆SOC on the ageing and the possibility to use any of these cycling parameters to perform accelerated ageing. Furthermore, in order to find a suitable charging current, where lithium plating is kept at low levels while at the same time, the charging time is kept at a minimum, two lithium plating detection methods is investigated, Impedance Tracking (IT) and the Voltage Relaxation Profile (VRP) method. The differential voltage method together with half-cell measurements has in recent years proven to be a powerful way to understand the ageing in Li-ion cells and in this work, it is used to get a better understanding of the degradation of the silicon content in the negative electrode. From the cycle ageing study, it was seen that the cells cycled with higher charging current experienced a rapid capacity fade, indicating that lithium plating may be the predominant cause of ageing. From SEM images performed in a parallel project, major surface film formation was also seen at the negative electrode of these cells. Accelerated ageing was also seen when utilizing the whole cell capacity by cycling with a low cut-off current. This led to a very high increase in the cell impedance and here the predominant cause of ageing was instead expected to be due to volumetric stress in the positive electrode active material. Lastly, accelerated ageing was also seen when cycling at low SOC. This due to the utilization of the silicon content of the negative electrode causing volumetric expansion and contraction of the silicon particles. The ageing of the silicon in the negative electrode was further confirmed by studying the differential voltage. Both lithium plating detection methods investigated was effective at detecting lithium plating at lower temperatures, however at elevated temperatures, the impedance tracking method showed a better ability to indicate lithium plating. However, it appears that both methods require a large amount of lithium plating to occur in order to show clear indications of lithium plating.