Katharina Quade

In my doctorate at RWTH Aachen University, one of the largest and most renowned technical universities in Germany with over 45,000 students, I am working on the experimental and data-based ageing analysis of batteries. This area of research is highly interdisciplinary: it combines knowledge in the construction and use of test equipment (electrical, mechanical and optical) with a deep understanding of electro-chemical processes and well-founded data analysis. This combination is the only way to meaningfully evaluate the ageing data from a large number of cells under different operating conditions.

Even during my studies at RWTH Aachen University, I was already working on the topic of batteries: my bachelor's and master's degree theses were written in this field, and I also worked for several years as an assistant at institutes that are involved in developing batteries. For my doctorate, I ultimately decided to focus on the lifetime analysis of new cell technologies, with the aim of gaining a deeper understanding of ageing mechanisms, and thus enabling their rapid and meaningful use in applications.

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UC Berkeley Campus

The focus of my research trip to UC Berkeley, which was also the final step of my doctorate, was on the analysis of the degradation of commercial sodium-ion round cells. Sodium-ion batteries are considered a promising alternative to the lithium-ion batteries that are currently widely in use. They offer advantages in terms of sustainability, raw material availability and potentially also cost.

However, there are important questions remaining to be answered regarding the longevity of this technology. In preparation for the research trip, I had already been working with colleagues the previous year at the CARL of RWTH Aachen (editor's note: Acronym for Center for Ageing, Reliability, and Lifetime Prediction of Electrochemical and Power Electronic Systems) and started extensive ageing tests on sodium-ion batteries in order to investigate them in more depth with the students from UC Berkeley. The analysis started with an evaluation of the ageing data, from which relevant features were derived, for example from regularly recorded discharge curves and their differential profiles. These features were correlated with battery ageing.

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Golden Gate Bridge

Surprisingly, it turned out that our batteries age particularly quickly in the lower state of charge range, something that is not common with lithium-ion batteries. To investigate the causes in more detail, we spontaneously carried out CT measurements in Aachen via a colleague, which showed deformations in the electrode structure. This was a particular highlight for us, as these ageing mechanisms and deformations have not yet been observed in sodium-ion batteries.

University of California, Berkeley

UC Berkeley is one of the world's most renowned universities and is distinguished by its interdisciplinary research into future-oriented, highly socially relevant topics. The laboratory of Prof. Moura, energy, controls, & application lab (eCAL), combines expertise in algorithm development, control technology and data-based methods.

Even prior to my arrival, in preparation for my visit Prof. Moura had made a PhD student available to me to work together with me on my topic. This student was originally from China and had been at UC Berkeley for about a year. We got along well and our profiles complemented each other perfectly: the PhD student specialised in artificial intelligence methods for batteries, and I brought experience in test data and electrochemical analysis to the project. Thanks to a detailed timetable worked out in advance together with Prof. Moura, I was able to start research immediately on arrival.

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Ice cream for lunch

On my first day at UC Berkeley, the "battery guys" took me from the lab through the building and showed me my workstation. Compared to the large laboratories at RWTH Aachen, eCAL seemed almost like a little family: there are about ten PhD students working there under Prof. Moura, as well as some bachelor's and master's degree students. The way of working in the laboratory at UC Berkeley differed significantly from that in Germany. While PhD students at RWTH Aachen regularly supervise theses and work closely with research assistants, the focus at Berkeley was more on one's own projects and doctoral theses.

In this respect, the PhD students had fewer organisational obligations and were able to concentrate on their research. In Berkeley, the PhD students were also closely supervised and there were fortnightly meetings with the professor. For example, we regularly exchanged information with Prof. Moura on the current status of our work and were supported in our project beyond the period of my visit. The eCAL covers a wide range of topics: from battery technologies to charging infrastructure for electric vehicles to autonomous driving. I found the discussions on topics that were outside my previous focus, such as autonomous driving, especially exciting.

The manageable size of the laboratory made it really easy for me to get going: conversations started up easily, and despite the fact that the students were working across different buildings on campus, our interactions were easy-going and open. The laboratory was highly international, with the majority of international students coming from Asia and Latin America, and the students' very different cultural backgrounds meant that our discussions often represented a mix of specialist topics, cultural perspectives and an exchange of experiences from PhD programmes that differed worldwide.

During my visit, we worked intensively on a joint paper. There were many open and stimulating discussions. Our exchanges of ideas were characterised by different perspectives. These conversations often made me think, questioning my own approach and showing me how valuable different perspectives are in research. I learnt a lot from these continuous and stimulating exchanges, and I am convinced that it has made me a better researcher.

As well as work, there were also social events, such as lunches together, ice cream trips and excursions. This also made collaboration in the laboratory really easy-going. Even in moments when we reached an impasse during conversation, the mood always remained relaxed and often even amusing. This was an important part of the trip for me because it brought together work and life on-site.

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View on Golden Gate Bridge from campus

The campus itself was impressive, especially the location in the east of the "Bay Area". At some points you could see San Francisco directly, and if it wasn't foggy, you could even see the Golden Gate Bridge from campus. But what surprised me the most was the strong sense of community: the students there identify very much with the university, and this energy was constantly felt on campus and in Berkeley.

Personal life in the USA

Finding a place to live in Berkeley wasn't easy. The university does offer some housing options, but as I was only looking around for apartments just before my visit, I opted for an AirBnB. If you want to live more cheaply, you will often find shared rooms, and many students also live in nearby cities such as Oakland. I deliberately chose Berkeley because I didn't have a car and wanted to be able to go to the university on foot.

Berkeley itself is quite a small town in the East Bay. As my stay was over the summer semester, there was less activity on campus and in the city overall than usual. Going to San Francisco at the weekends was easy thanks to the BART train.

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Sea lions at Pier 39

In San Francisco itself, you usually use the bus and metro or of course, the famous "cable cars". I really liked the city overall, and it offers a lot to discover both in terms of the culinary and the cultural. For example, I took a trip to Alcatraz, visited exhibitions and ate very well in Chinatown and Little Italy. However, you need to pay attention to the microclimates: it can quickly get cold, windy or foggy in the "Bay Area".

California offers an enormous variety of landscapes and has the most national parks of any US state. Many of these are accessible from Berkeley, but this usually requires a car. During my visit, I had the opportunity to visit Yosemite, Sequoia, Redwood, and the Death Valley National Parks.

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Trip to Death Valley

I found it particularly fascinating how much the natural world can vary within a single state: from the impressive forests of Redwoods and Sequoias, to the dry and hot desert landscapes in Death Valley. I often went on these trips with other "visiting students" I met at the pizza or bar evenings organised by UC Berkeley.

My conclusion

My time in California has given me a lot, both professionally and personally, and will stay with me for a long time to come. The interactions with the other students in the lab, who came from many different countries and brought different perspectives to my work and everyday life, was particularly enriching. Thanks to these conversations, I learned a lot about different approaches in research and further developed my ability to approach other people with greater openness. I am still in contact with some of the PhD students and look forward to our next meeting in Aachen.

I am also very happy that I was able to work intensively and in a really focused way on my research project at UC Berkeley. This has made a major contribution to the fact that I can now successfully complete my doctorate. Overall, the stay has strengthened my passion for research, which also motivates me further to complete our joint publication in the coming weeks and months.

I am particularly grateful to Prof. Moura from UC Berkeley, Prof. Sauer from RWTH Aachen University and my colleagues based in Germany who were also involved in the analysis work. Many thanks to the DAAD-Stiftung's "KSB Foundation Scholarship" who made this special stay abroad possible for me in the first place.