Ageing in Commercial Li-ion Batteries: Lifetime Testing and Modelling for Electrified Vehicle Applications
Results from lifetime tests are used in the development of an empirical ageing model, to study the ageing as a result of different user cases in a vehicle application. The results showed that the battery lifetime in a vehicle application could be prolonged, without interfering with the driving itself, by better planning of the charging.
Lifetime test results and ageing analysis of the tested cells are used as guidance for the development of a physics-based ageing model. The model includes capacity loss and resistance increase due to resistive film formation and loss of active material.
The lifetime test results showed that for the studied cell, staying below 40% SOC level will improve the lifetime considerably. This was seen in both the calendar ageing tests and the cycling ageing tests. The lifetime tests performed in small SOC intervals at different SOC levels showed that the ageing is separated into two groups, with more pronounced ageing at high SOC and less ageing at low SOC.
The main ageing contribution is the growth of Solid Electrolyte Interphase (SEI), which is also the case seen in the model when using parameters for the SEI growth extracted from calendar ageing data. The more pronounced ageing at higher SOC levels can partly be explained by the SEI growth that is increased at higher SOC level and partly from the contribution of manganese dissolution. This was confirmed by Post Mortem (PM) analysis and successfully captured by the model.
Battery lifetime tests
Plug-in Hybrid Electric Vehicle (PHEV)
Chalmers, Elektroteknik, Elkraftteknik
In the experimental testing we could observe the anticipated ageing behaviour, where the LiB is ageing faster at higher temperatures, at higher State of Charge (SOC) levels, and when charged/discharged with larger currents. For some type of LiBs low SOC can also accelerate ageing, however, that is not true for the LiB studied here. In particular, when only a small part of the battery energy was used (10% SOC intervals) the lifetime was substantially improved when keeping SOC below 50%.
The experimental results were used to develop a model where different user perspectives of electrified vehicle were taken into account. For a personal vehicle application, the car is parked a substantial amount of its lifetime. The model showed that by planning the charging, and maintaining a low charge level during parking, the battery lifetime could be significantly prolonged. By also charging the battery only as much as needed for the drive, the lifetime could be further improved. These changes are something the driver of an electrified vehicle easily can adapt to improve the battery lifetime.
The project also took a deep dive into the material level ageing of the LiB by opening and analysing the materials of tested cells at Uppsala University. Based on the results, I studied more in detail what triggers the different ageing mechanisms observed. This was done through a physics-based model describing the electrochemical processes. Here we could see that the resistance increase came from the negative electrode material, while the capacity loss came from trapping of Li-ions, and loss of active material in the positive electrode.
Elektroteknik och elektronik
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4633
KC, Kemigården 4
Opponent: Dr. Daniel Abraham, Argonne National Laboratory, USA