In this post, I will be focusing on the work I did on cathode materials over the course of my internship. This part of my internship really honed my intuitive understanding of materials and how to mix solutions, emulsions, and suspensions with very different properties. I also think it advanced my bread-making skills by several years. My experience trying to research this cathode and my experience trying to bake bread were unbelievably similar.
Both my personal bread baking project and my professional cathode project started with some literature research. I had to look through papers (and recipes) to find mixtures that made sense and were feasible to make. After conducting a quick literature review and getting an idea of what my first mixing procedure would be, I had to collect my materials and prepare for a first run. In both cases I had much too high expectations; they ended in a shaggy dough too unwieldy to work with. Below is a picture of my first cathode mixture. Unfortunately, I didn’t take a picture of my awful first bread dough (I’m really kicking myself now, but at the time my hands were way too sticky for me to use my phone). Trust me though, it looked very similar to this cathode.
Fig 1. Shaggy cathode mixture
In both cases, I underestimated the stickiness of the binder (polymer for the cathode, gluten for the bread), and how that would affect my ability to mix my ingredients in.
I’m going to bench the bread analogy for now, because I think it’s getting a little tedious, but I will bring it back at the end. For making an experimental cathode, an intuitive understanding of these battery materials is really important to the mixing process. Many of the chemicals one uses on a daily basis are proprietary or don’t have a lot of published information, and without knowing exactly how they’re made, it’s difficult to know how they should be mixed.
After more literature research, advice from my team members, and mixture iterations, I ended up with several different procedures to make a smooth cathode as pictured below. Each of them had slightly different compositions, wet viscosity curves, grain size, and dry characteristics.
Fig 2. This is what one of the final wet cathode mixtures looked like. It was smooth and viscous with small grains in its wet state.
Fig. 3 This is a cast of one of the cathodes. It has large grains and little adhesion.
Fig 4. This cathode cast had much smaller grains but had small holes in the application after drying.
There was an additional question about side reactions in the cathode mixtures. We wanted to answer this through XRD analysis of different mixtures of cathode components. This process involved making many different mixtures, mixing and heating them for several weeks, checking for discharge, and finally packaging them and sending them to another lab. In the end, we decided that another analytical techique was a better fit than XRD, but unfortunately my internship ended before the solutions were actually analyzed.
Also, unfortunately, I didn’t have enough time to characterize the effects of each additive and each component's concentration on the rheology of the cathode mixtures. My final mixtures had viscosity curves promising for printing, but in the end they weren’t fully characterized. Here is where the bread analogy comes back in: I am getting closer but unfortunately have never fully succeeded in baking edible bread. So all in all for this summer, in bread-making and cathodes, results are inconclusive but promising!