The ageing of automotive lithium-ion batteries has been studied in many collaborative projects between industry and academia, resulting in new improved battery design and usage. Even so, there are still a considerable extent of unevenness, or “heterogeneity”, in degradation phenomena on several levels observed in the batteries. This heterogeneous ageing is still not well understood, and there is a need to to find the reasons behind it to mitigate these problems in the future.
The goal of the research project HALIBatt, Heterogenic Aging in Large Intercalation Batteries, is to quantify, explain, and describe the uneven distribution of cell ageing observed in various large cells. An improved understanding of the root causes of this heterogeneity, as well as methods to diagnose and ultimately predict it, would help improve model development and lifetime prediction, provide insight to OEMs in developing battery systems, and ultimately contribute to a more effective use of battery materials.
Cell teardown analyses from previous research have revealed that spatial heterogeneity presents numerous scientific and technological development challenges. Additionally, phenomena such as ageing between the two sides of an electrode coating have not been fully captured or predicted by current battery models, at least not with spatial discretization. Furthermore, heterogeneity along the length of a wound electrode has not been well studied. In general nowadays manufacturers are producing bigger batteries than in the past, which can be the cause of, or at least enhance, the heterogeneity of degradation
The scope of HALLIBatt is a joint effort and a broad collaboration between industry and academy with different complementing competences involving four PhD students.
Battery cycling
The main battery cycling is done by PhD student Daniel Poposki at Chalmers University of Technology and Volvo Group Trucks Technology.
“My work is mainly revolved around experimental studies on how batteries age and perform in various conditions, as well as gather data that can be utilized for creating useful models of the battery. Observation of heterogenic degradation within large format battery cells can in some cases correlate with temperature gradients that occur within the battery. Focus is therefore on deepening the understanding of how thermal aspects affect the performance and lifetime of battery technologies that are newly emerging in the market, in collaboration with all HALIBatt partners,” says Daniel Poposki.
Post-mortem analysis
The cycled batteries will then be handled for post-cycling characterization, which is mainly done by PhD students Aamer Siddiqui and Agnes Matilda Mattson.
“My work mainly involves conducting teardowns and post mortem analysis. Teardown enables me to separate/segment each material and analyse them with various post mortem tools. Such as electrochemistry, BIB SEM, image analysis, OLSA, and various other techniques. This helps to answer questions regarding ageing mechanism and understand the degradation. A large part of my contribution also goes towards developing these methods,” says Aamer Siddiqui, PhD student at Scania.
Agnes-Matilda Mattsson’s work with the cycled batteries is with focus on X-ray diffraction. Agnes is a PhD student at Uppsala University, an organisation with a strong history and culture within this area.
“X-ray diffraction is an efficient method when characterizing heterogenous ageing in battery cells. We will analyse the influence of temperature, pressure, and cycling protocols from cells provided by the industry partners Volvo and Scania. One project that will take place this autumn is the investigation of ageing using X-ray diffraction to create a map of the electrodes. With this method we can study the heterogeneity both on the electrodes, but also throughout the cells ,” says Agnes-Matilda Mattsson.
Developing mathematical models for prediction
The insights from the post-mortem analyses will be utilized to develop mathematical models for prediction of hetereogenous aging. These models can then be used in the industry to handle batteries in the most optimal way which will avoid deterioration and in the end extend battery lifetime. Gian Marco Trippetta is a PhD student at KTH Royal Institute of Technology in Stockholm, and his main role is within this area.
“Agnes and my work are overlapping but I’m mainly in charge of developing the models. So far the results are only on an experimental stage, but I hope to present more in the beginning of next year. It’s an exciting and important project and I enjoy the collaboration with other academics and the industry. If we can understand, interpret, and predict the mechanism that cause heterogeneity, it has a lot of potential to improve battery lifetime,” says Gian Marco Trippetta.