Parri Adeli: Could you briefly take us through your journey in the battery field?
Stefano Passerini: I entered the Li-battery field in 1986 in the laboratory of Professor Bruno Scrosati. I got a PhD in electrochemistry in Italy and then I moved to Minnesota, USA, for a research position at the University of Minnesota. Afterwards, I went back to Italy to join ENEA and now I am in Germany, so it is a long journey. More than 35 years! Now, I am the director of Helmholtz Institute Ulm (HIU). I am also a professor at Karlsruhe Institute of Technology. I have about 50 students and postdocs in my group working on various topics.
PA: Could you give an overview of the different approaches you have implemented for engineering cathodes?
SP: There is no doubt that most cars will be powered by batteries in a decade. We need to investigate sustainable materials. That is why we started to investigate high-voltage nickel manganese oxide spinel. These materials are cobalt-free and could tackle the cobalt shortage issue. We are also working quite a lot on Ni-rich and Li-rich materials. The high voltage spinel had some problems with degradation of the material. We then started to work on the crystalline structure and tried to optimize the nanoparticles, especially on the coatings of these particles to protect them against degradation. This is what we are mostly involved in. For our work with regard to high-Ni cathodes, we do not make the NMC-811 ourselves but we get it from commercial sources. What we are trying to do is to develop the old cell chemistry in order to get the best performance out of it.
Another point we are stressing very much, is to scale up the Li-ion battery production to the level we seek. We also need to work on the processes to make the batteries. For example, the positive electrode production still employs less desirable solvents. These are the two subjects we are addressing mostly, trying to make cathode production less polluting, more environmentally friendly, moving towards aqueous binders rather than fluorinated binders and engineering the materials in order to avoid their degradation when we actually apply these aqueous binders.
There is no doubt that most cars will be powered by batteries in a decade. We need to investigate sustainable materials. That is why we started to investigate high-voltage nickel manganese oxide spinel.
PA: Could you describe the effect of coating for LiNi0.5Mn1.5O4 (LNMO) cathodes on the battery performance? Particularly NiPOx coating that is the subject of your 2020 paper in Materials Today.
SP: This is sort of the outcome of the work on binders. A few years ago, the general opinion in the field was that you cannot use aqueous binders with nickel cobalt manganese oxide because they will degrade. We have optimized the recipe which at that time was adding a little bit of phosphoric acid into the slurry. We noticed that the cathode material was not degrading anymore so we investigated this to understand the reason. We realized that as soon as the transition metal oxides dissolved into the water phase then the phosphate anions reacted with them and precipitated as nickel cobalt manganese phosphate. This is what was protecting the material. We have a patent on this process and then there was a follow-up patent from a friend of mine, Dr. Ilias Belharouak. He also did some additional modifications. He actually showed very good cycle performance for NCM 811. This was something that we developed to enable the aqueous process for the cathode.
We told ourselves if the formation of phosphates can protect in a nasty environment (because for these oxides water is a nasty environment), why don’t we try putting the coatings directly on the particles before exposing them to the aqueous environment? We are still working on both ways. One is forming the phosphates in situ during the slurry preparation and then coating of the electrode. The other way is coating the particles before in a separate environment. It seems to be working pretty nicely. Some phosphates are better than others. This was also one of the reasons why we wanted to do it in a separate environment so that we could decide which phosphate we have - nickel phosphate, cobalt phosphate, or manganese phosphate. Because when we do it in the slurry, we have no choice. Whatever comes out of the particles is going to precipitate as phosphates and we do not control the chemistry. We are seeing that Ni phosphate is pretty good.
PA: One of the main challenges for high voltage cathodes is the electrolyte. You are carrying out great work in tackling that issue with various electrolyte additives such as the ones in your 2021 paper published in the Journal of Power Sources.
SP: At a certain point, we synthesized one Li-rich nickel manganese cobalt oxide compound with no matched property. However, there was the issue of reaction with electrolyte, so we started to look at electrolyte additives. Neither of those two additives implemented in that paper tris(trimethylsilyl) phosphite (TTSPi) and bis(2,2,2-trifluoroethyl) carbonate (TFEC) are brand new but the combination of these two additives (TTSPi and TFEC) is particularly effective for protecting at the interphase. We have a patent on this as the synergy of the effects for enhancing the performance was not known.
PA: Which types of electrolytes are you exploring for your solid-state batteries research?
SP: I have been working on polymer electrolytes since 1986, right after Michel Armand came up with polyethylene oxide (PEO), Scrosati jumped into the field and I was the guy doing things. A few years ago, I started a collaboration with Samsung (Japanese branch). They wanted us to explore new cathode materials for solid-state batteries using sulfidic electrolytes. We got quite good knowledge in the field and now we are very active. Pure solid-state can be very difficult. We published a paper in Small in 2020 on using ionic liquid interlayer to reduce interfacial resistance. We demonstrated that putting a few layers of ionic liquid at the interface of Lithium lanthanum zirconate and Li metal as well as on the cathode side cuts the interfacial resistance substantially. In the first paper, we just reported the better performance.
There is a follow-up paper on this subject together with Prof. Jurgen Janek where we collaborated within one of the German-funded projects. In the collaboration, we are explaining why we get better performance. When you have solid-solid interface, the two solids might behave differently and then you have the problem of matching their interface. The solid electrolyte has its own interface, it stays there. The electrodes expand and contract upon cycling. Ionic liquid allows filling these gaps that are formed during the operation and reduces the interfacial resistance by one or two orders of magnitude. This paper that we published in 2020 inspired us to keep going on this project. We are about to submit a manuscript that goes farther in this direction. We are using a high-Ni cathode (NMC811) on the positive electrode side and Li metal on the negative electrode side. We also developed this hybrid solid-state electrolyte, which is flexible. We can even demonstrate cells that cycle between 8 and 13 volts.
We are using a high-Ni cathode (NMC811) on the positive electrode side and Li metal on the negative electrode side. We also developed this hybrid solid-state electrolyte, which is flexible. We can even demonstrate cells that cycle between 8 and 13 volts.
PA: So you have explored sulfides, oxides, and hybrid electrolytes with ionic liquid incorporated. How did they compare to each other?
SP: Yes. Sulfides are kind of easier because they are very ductile. Less need of matching the interface of the two solid components. If you apply a little bit of pressure the sulfidic electrolyte will deform and match the electrode surface. When you go to oxidic electrolytes they are like rocks and really hard. If you have a consumption of Li metal electrode, for example, you get a void and there is no contact anymore. Therefore, we decided to introduce this ionic liquid interlayer.
PA: Single ion-conducting polymer electrolytes are excellent against dendrites, but they suffer from compatibility issues with high-energy cathodes. Could you explain the work you are doing on solving this matter in your recent papers employing LiNi6Mn0.2Co0.2O2 and Li[Ni0.8Co0.1Mn0.1]O2 cathodes?
SP: Yes, we also have a manuscript under review with NMC811 with the same single ion conducting polymer electrolyte. As I mentioned earlier, for 35 years, I have been working on PEO-based electrolytes which is a follow-up of Michel Armand's work. Especially in the 90s we really tried everything. So very frequently now, I see papers regarding polymers that have already been investigated 20 or 30 years ago but they kind of ignore it. And still, I did not see any pure polymer electrolyte working better than PEO so far. A few years ago, one of my colleagues in Grenoble, France, came up with this polyanion. In the polyanion, the counter ion is Li+ but the mobility was really low, so, Li was not moving. Then we came up with the idea of adding a molecular solvent that could in a sense support transfer of Li from one site to another. This seems to be working. Still, there is some liquid inside so in a sense, it is a quasi-dry polymer electrolyte. But first, the liquid is not really free, but bound to the polymer, so it is not evaporating easily or leaking out. And secondly, there is no PF6-. This is a typical anion which is a pretty nasty chemical when it gets to composition, so this is kind of a combined approach. It is not a pure polymer electrolyte, but it is much less dangerous from many points of view than the conventional liquid electrolyte or the conventional gel electrolyte and it only transports Li-ion. This is again another field that we are addressing together with my colleague Dominic Bresser. We collaborate intensively and are addressing these questions with German-funded projects.
PA: Great talking to you!
SP: Thank you!