Calendar aging and post-mortem analysis of NCA/Si-graphite 18650 lithium-ion batteries

Summary:

Lithium-ion batteries have emerged as a key enabler for a sustainable energy system. Their wide applications in electrification of traffic and the storage of fluctuating renewable energy require more and more performance in terms of safety, lifetime, energy density, costs and so on. Recently, silicon-based materials have been widely introduced in the anode of commercial cells for electrical vehicles in order to increase their energy density. Surprisingly, while a substantial amount of research works focuses on cycle performance of half-cell with Si-based electrode, very few studies have investigated in details the calendar aging of commercial cells with Si-graphite anode. It is, however, critical for automotive applications since vehicles are often charged for a long period during parking.
In the present work, an extensive set of calendar aging tests and post-mortem analysis are carried out with NCA/Si-graphite 18650 cells manufactured by Sony. The impacts of state of charge (SoC), check-up frequency and charging strategy during the storage on the aging of cells are investigated. The resulted degradations are analysed by differential voltage, post-mortem analysis and a continuum aging model in order to assess the mechanisms of degradation and sudden death of cells.
Our study shows a strong dependency of the cell degradation on storage SoC and check-up frequency. It also reveals a plateau region in which the capacity fade is independent from the SoC. Furthermore, cells stored at high temperature and high SoC failed very quickly due to the interruption by the CID (Current Interrupt Device) caused by an excessive pressure inside the cell. Post-mortem analysis of the most aged cells indicates prominent degradation of the anode and structural change of cathode active materials. These results are mainly explained by side reactions between electrolyte/additives and anode. The possible mechanisms and the influence of different factors are interprated and discussed under the light of modelling results.
The present study highlights that, cells with Si-graphite anode show different calendar aging behaviours from cells with graphite anode; they therefore require different design and operating strategies. Our findings provide hints to manufacturers of cells and BMS to improve the lifetime and safety of next-generation high-energy batteries.