Electrified vehicles, both pure electric and hybrid, are becoming increasingly popular. Most of the electrified vehicles are using a lithium ion battery (LiB) as energy source. Although the battery price is decreasing it is still a very expensive component, which makes it important that the lifetime of the battery is sufficiently long. In my thesis I’m trying to answer what causes the battery to age and how we can use the battery smarter and prolong the lifetime by taking into account all aspects from the usage in a vehicle down to what happens in the battery and its materials. The project was carried out in collaboration with Uppsala University, Volvo Car Corporation, and ABB, and was funded by the Swedish Energy Agency with some support from the Swedish Electromobility Centre.
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.