Dr. John Uhlrich, Associate Editor for Energy Technology, talks to Professor Jim Yang Lee, National University of Singapore, about his recently published article on dual carbon networks in lithium-ion battery cathodes.
Professor Lee and colleagues have developed a new scheme for increasing the conductivity and performance of lithium-ion battery cathodes. To achieve this, they use graphene in combination with carbon nanopainting to achieve greater connectivity of the conductive carbon network throughout the LiFePO4 nanoparticles. Such a construction supports fast electron transport between the aggregated LiFePO4 nanocrystals as well as within them. Consequently, the LiFePO4/C composite delivers very good rate performance, even at very high discharge rates.
Could you briefly explain the focus and findings of your article to a non-specialist and why they are of current interest?
High energy density batteries are desired for the development of large energy systems, and this requires a new generation of battery materials. Some of these new materials have yet to demonstrate their full potential because they could not be integrated electrically into the battery electrodes in an efficient manner. Their poor extrinsic conductivity is the main reason for this. While conducting carbon additives have traditionally been used to integrate conventional battery materials in the electrodes, this approach had not been optimized for the new generation of high energy density materials. In this work we are suggesting a new scheme for improving electrical integration based on the use of conducting carbon at different length scales.
Could you please explain the motivation behind the study?
The motivation came from the lackluster performance of some new materials, e.g., phosphoolivine cathodes such as LiFePO4, which are supposed to be theoretically superior. Research over the years has shown that electron transport between the particles of active material can be an issue. The common mitigation method is to deposit a conducting carbon coating on the active particles (“carbon nanopainting”).
We feel that this may not be adequate as the interparticle contacts are still primarily point contacts. Hence, we propose to “interject” a 2D conducting material (graphene) between the active particles to increase the number of contact points. At the same time we can also make use of the 2D conducting material as a high-capacity conduit to “shuttle” the electrons collected from the particles to and from the current collector. We find this to be the same in concept as the planning of a city transportation system where both arterial roads and smaller roads are needed.
Why was your attention focused on LiFePO4 for the cathode material?
We chose the phosphoolivine LiFePO4 because it has been used in some commercial systems. We wanted to see if we could improve its current performance – and we did.
How long did this investigation take?
The practical work was not too long, about six months, but the conceptualization and tweaking of the experimental conditions took longer. All in all it took a PhD student about a year to complete the project.
How will you follow up on this discovery?
The natural next order of business is quantification – that is, determining the best balance between the “arterial roads” (graphene) and “small roads” (nanopainted carbon) for a particular application. By optimizing the ratio and arrangement, the most efficient electron transport can be achieved, which results in optimal charging and discharging of the battery. However, this can be a massive optimization exercise that is best left for the industry to explore.
What challenges still need to be overcome before this composite material impacts commercial batteries?
The only concern we have now is the stability and cost of the 2D material, graphene. Unlike graphite or other forms of carbon, graphene is a relatively new material for which the long-term performance has yet to be confirmed, even though it has a strong suite of functional properties initially.
- Dual-Carbon Network for the Effective Transport of Charged Species in a LiFePO4 Cathode for Lithium-Ion Batteries,
Bo Ding, Ge Ji, Zhou Sha, Jishan Wu, Li Lu, Jim Yang Lee,
Energy Technol. 2014.
Professor Jim Yang Lee is a Member of the Editorial Board of Energy Technology. He was also Guest Editor of a special issue of Energy Technology focusing on Energy Storage Materials in April of last year.