Watching Batteries Work — Electron by Electron

I am trying to understand what happens in batteries – or the cathode of a battery to be precise – when a charge moves into it. Most cathode materials like Li-rich transition metal oxides are actually semiconductors when we look at the band structure. But they do conduct quite well. The reason for this is the development of polarons in the crystal lattice of the material. A polaron just describes the coupling between a polarizable lattice and a charge, which mostly leads to a distortion – so a change in the bond lengths surrounding the charge. The charge is then localized in that lattice, instead of delocalized like in a metal. The charge then moves through the material with this distortion together – as a polaron.

How exactly that happens and what pathways are more probable than others, that is what I am trying to figure out. I am using X-ray spectroscopy and scattering for that, at X-ray free electron lasers, as we are looking here at dynamics in the femto- to picosecond time range. So we are using very short X-ray pulses to probe that. Scattering can give us information about structural changes in materials, and spectroscopy can give us information about changes in the oxidation state and some limited information on the geometry as well. So with those techniques we are able to take snapshots of the material while the charge moves and put those then together to get a movie – just instead of having a frame per second, we have a frame per pico-/femtosecond.

I would like to give the students some sort of introduction to X-ray physics and batteries if they didn’t have that already. I also thought of a prop to kind of visualise the polaron hopping in the lattice. It is a little preliminary as we don’t know yet how they actually move, but it should be good for an introduction – especially as this is not an easy topic to understand – and would engage the students.

That is what my research is about. I really hope we can find helpful information on how the charge transfer in cathode materials might be enhanced and give some guidelines for material properties. As research on new cathode materials right now is mainly done by trial-and-error approaches, I hope my research could facilitate that process and allow us to move faster towards new battery materials, helping with our current energy crisis.

So how did I end up here? I grew up in Berlin, Germany. I went to the 'Gymnasium' there – I am not sure if it is the exact same as in Denmark, but it might be comparable. We all have to choose 2 subjects we want to take more seriously, and my first choice was Physics and Maths. Unfortunately, at my school only two students wanted to do Physics, and therefore the course could not be provided. So I did Chemistry instead and did quite well in there.

I then went to a study program for high school students at a university in the far west of Germany, during my last year of school, where they wanted us to get an introduction to engineering and sciences. My plan was to study Physics there after I finished school and I wanted to get a head start. But I went there and got so overwhelmed with the physics and maths they taught (it was the second semester for the students), that I thought that I was just not smart enough for it and decided to go back to Berlin and start studying Chemistry, as I at least had a good education there from school.

I didn’t understand at that time that you can’t expect to understand things from higher semesters by just being thrown in there. You grow with each semester and with each task, and things that seem super hard in the beginning just become possible by the time you get to it. I still thought a lot about my dream of studying Physics during my Bachelor's. I finished those and moved to a different university in the south of Germany to do a study program in 'physical and theoretical chemistry'. It was a compromise. I would not do a second Bachelor in Physics, but rather move closer to Physics with my Master's.

I traveled a lot during my Master's, as I had fewer lectures and was able to work remotely. By doing so, I realised that I love traveling and meeting new people. After finishing a Master's in Chemistry, it is very common to do a PhD. In fact, our professors told us from the very beginning that if we stopped after our Master’s, we might as well just do a different degree. So I spent little thought on what other options I might have, and once I finished my degree, I was not prepared to leave academia.

I was pretty certain I would want to move to a different country, as the PhD is limited in time and maybe this would be my last chance of actually living somewhere else. I then pretty spontaneously chose Copenhagen and applied for positions. In the end I chose my current job because I wanted to 'save the world' – and because, as we are using quite special X-ray pulses, we have to travel to particle accelerators. The thought of telling my kids one day that I actually saw a particle accelerator from the inside was too exciting. A bonus is also that we have to travel to those facilities, and I am able to see the whole world while working. And funny enough, I am now at DTU Physics – so I get my Physics degree in the end after all.