Battery Chat with Parri #2: Prof. Feng Lin

Dr. Feng Lin is an assistant professor in the chemistry department of Virginia Tech. In this Battery Chat, he talks to Parri Adeli about his scientific journey and his research into cathodes and catalysts.

Professor Feng Lin
Professor Feng Lin

Dr. Feng Lin completed his PhD in Materials Science at Colorado School of Mines, with graduate research at the National Renewable Energy Laboratory. Afterwards, he joined the Lawrence Berkeley National Lab as a postdoctoral fellow and later QuantumScape Corporation as a Senior Member of Technical Staff.

Currently, he is an assistant professor at Virginia Tech’s department of chemistry. He also holds courtesy appointments in the Department of Materials Science and Engineering, and Macromolecules Innovation Institute at Virginia Tech. His research activities focus on electrochemical energy systems, including rechargeable batteries, smart windows, and catalysts for chemicals and renewable fuels.

Parri Adeli: Could you please share with us your scientific journey?

Professor Feng Lin: My scientific discipline has always been materials science from undergrad all the way to PhD. When I was in grad school, I worked on energy-saving smart windows for buildings that can change their optical properties based on the ion intercalation chemistry. After finishing my grad studies, I became a postdoc at Berkeley National Lab and started to work on Li-ion batteries under Dr. Marca Doeff. Nickel has been in my research focus for more than ten years. After my postdoc, I went to work at QuantumScape (QS) which was a startup company at the time. I left QS in 2016 and joined Virginia Tech as an assistant professor. My lab has been focusing on various aspects of battery technology, especially on the cathode side.

PA: What are the challenges your group strives to address regarding cathodes?

FL: We are one of the junior research groups in the country still researching conventional layered oxide cathodes. If you look at the well-known names in the field who work on these traditional cathodes, they are usually people who have been in this field for a very long time and have made significant contributions. I see the opportunity of further improving energy density in conventional layered cathodes.

Professor Feng Lin and his team
Professor Feng Lin and his team

One approach is designing new materials like high-Ni containing cathodes. Our approach is different and considers capacity utilization percentage. Improving the charging homogeneity and chemomechanical properties has been our research focus in the past few years. Today, our Li-ion batteries cannot deliver stable cathode capacity greater than 220 mAh/g due to side reactions with the electrolyte etc. We recently performed a study where we looked at electrodes and their thousands of particles. It turns out most of the particles undergo incomplete charging. A significant portion of the particles is not even charged at all. If we can utilize that part of the capacity, it would be great. To achieve that we need to homogenize the charging reactions across the entire electrode. That will entail the engineering of different cell components. On top of that, most of these high-Ni containing layered oxides are polycrystalline materials. They have a lot of smaller grains, so how can you ensure each grain has the same charging pattern? If we can engineer each small particle to charge and discharge at the same rate, at the same time, then we can better utilize the capacity.

This is our strategy for solving the issues encountered in high-Ni content, however, the fundamental aspect is widely applicable to any battery electrode materials.

Another interesting project is to design battery materials that are resistant to extreme conditions, such as high-energy irradiation. This is really looking into the future, such as outer space exploration and nuclear power industries.

If we can engineer each small particle to charge and discharge at the same rate, at the same time, then we can better utilize the capacity. This is our strategy for solving the issues encountered in high-Ni cathodes.

PA: Is nickel pivotal in other areas in your current research?

FL: Aside from Ni-containing cathodes, we are working on Ni catalysts in the group as well. More than 100 years ago, Thomas Edison tried to invent a battery based on Ni hydroxide. The challenge he had was that the counter electrode contains iron and the iron can dissolve in the electrolyte. The dissolved iron can incorporate into the Ni hydroxide lattice, at the atomic scale. When iron gets incorporated into Ni hydroxide, it activates it, making Ni hydroxide an excellent catalyst for oxygen evolution. Oxygen evolution in this battery system is not good and will lower Coulombic efficiency. 

On the OER (oxygen evolution reaction), we do want to have high activity so now we are working on a Ni hydroxide-based oxygen evolution catalyst and we are incorporating iron into the lattice. This incorporation is either done in-situ in the cell or ex-situ in the synthesis. We recently published a paper in Nature Catalysis, which describes how we can in-situ engineer the chemical composition of the catalyst. There are other aspects of our research utilizing Ni, for example, sodium ion batteries.

PA: Your research has led to more than 100 publications including several impactful publications on Ni-containing cathodes. Which entity is financially supporting this research?

FL: My research on the Ni side is funded through the US Department of Energy (DOE) and the National Science Foundation (NSF). For the project with the DOE, we are looking at how we can significantly reduce the cobalt and move towards Co-free cathodes. Our papers on lithium nickel oxide cathode materials are part of DOE’s effort. Another DOE project we are funded for is to look at solid-state polymer electrolytes. We are incorporating promising new solid-state polymer electrolytes with Ni-based cathodes.

On the NSF project, we are working on the mechanical properties of cathodes. Mechanical properties of cathode materials have not received intensive attention until recently. We started working on this five years ago and recognized the problem with cracking. We are working on the fundamental mechanism for mechanical failure. How can we modify the dislocation and grain boundary inside the particle? We recently published a paper in Advance Materials where we noted our discovery that some level of dislocation inside the particle can facilitate initial charging. Different grain orientations in NMC will give different grain boundaries. If you engineer grain orientation to a specific orientation, you should be able to facilitate charging and discharging leading to less internal stress building up, hence, mechanical stability can be potentially enhanced. We recently published a paper in Nature Communications to highlight how the grain crystallographic orientation governs the charging behavior and mechanical properties of Ni-rich layered cathodes.

Some dopants prefer to stay in bulk and others prefer to segregate to the surface. Having this multidimensional dopant distribution, you will be able to stabilize not just the surface but also the bulk.

PA: Could you explain your observation on the surface chemistry of Co-free, Ni-rich layered oxides? Have you tried dopants other than the ones you recently reported (Ti and Mg) in Advanced Energy Materials?

FL: We noticed that surface of the cathodes is very sensitive to how you handle the sample. One of the reaction mechanisms is that lithium can come out of the lattice and react with water and CO2 to form carbonate or bicarbonate species. We wanted researchers to be very careful when making claims about ‘less carbonate so better material’. Concerning dopants in the cathodes, magnesium can distribute very homogenously in the particles whereas titanium is more segregated to the surface. Ti is able to lock oxygen into the lattice very nicely. The basic principle is that Ti and oxygen have a stronger bonding energy. Other groups have found tungsten can do similar work. We have developed many other doping chemistries, most of which have not been published. We tried replacing Ti with manganese, but Mn is not as strong in terms of oxygen retention. Some dopants prefer to stay in bulk and may be able to stabilize the structure through mitigating phase transitions. For dopants that are segregated to the surface, they can protect the surface against side reactions with the electrolyte.

Having this multidimensional dopant distribution, you will be able to stabilize not just the surface but also the bulk. A good example can be seen in our recent paper about Mg/Ti co-doped LiNiO2.

PA: Where do you see the direction of future cathode chemistries?

FL: In the foreseeable future (10 years), from the commercialization standpoint, the focus will probably be still on Ni-based cathodes. New cathode chemistries are interesting fundamentally, but the problems and challenges are clear e.g. the voltage profile.

With regard to Ni-containing cathodes, if we can demonstrate NMC811 or even higher Ni NMCs in electric vehicle batteries safely and cost-effectively applied at a large scale, that would be great. If we can get the safety to a good position, I see the market growing for high-Ni. Research needs to progress on multiple aspects in battery, for example electrolytes, anodes, and communication between different components. Moving forward, the future of cathode chemistries also relies on progress in other sections of the battery.

PA: Many thanks for joining us for this informative and interesting chat!

FL: My pleasure, thank you for chatting with me.

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