Susanne Wilken’s research paves the way for safer large lithium-ion batteries
Thursday, December 11, 2014
The expansion of consumer oriented lithium-ion batteries towards large-scale applications like vehicles is well under way. However, problems with safety, cycle life and capacity grow with the size of the batteries. In her doctoral thesis, Susanne Wilken emphasizes the need for a deeper understanding of the shortcomings of the lithium-ion batteries used today.
The electrolytes that are common in lithium-ion batteries for portable devices suffer from chemical and thermal instability, which may cause a number of failures. These could possibly be tolerated for small consumer batteries, but become unacceptable when the batteries are up-scaled for large applications such as vehicles. The overall aim of Susanne Wilken’s research has been to understand more about why the electrolyte fails, and to find ways to mitigate the failures.
Water or heat common causes for failure
The safety of lithium-ion batteries is related to the electrolyte and its interaction with other battery components. Most failures start due to impurities like water or because of thermal conditions such as heat.
– There are two options for dealing with these problems, says Susanne Wilken. Either we stay with the standard electrolytes from the consumer supply chain and find good additives to deal with water or heat issues on the large-scale level, or we completely change the chemistry. In my research I have gone with the first of these options.
Surprising test results for common flame retardant
Susanne Wilken has focused on flame retardants, in particular phosphate based compounds and ionic liquids. A number of new approaches and experimental techniques have been employed in the search for further understanding of the failure mechanisms
Part of the research was done in collaboration with fellow doctoral students from KTH and Uppsala University, all members of the Swedish Hybrid Vehicle Centre (SHC). The researchers for example examined triphenylphosphate (TPP) as a flame retardant additive in high power applications.
– We built a battery with graphite and lithium iron phosphate electrodes and different concentrations of TPP in the carbonate based electrolyte, Susanne Wilken explains. To our surprise, we found that this additive is not at all suitable for high power applications, though it is very common for example in the space industry.
The flame retardant was found to participate in film formation on the electrode and increased the viscosity of the electrolyte, leading to decreased performance. Susanne Wilken concludes that the search for useful additives continues:
– We need to find an additive either less degrading or with higher flame-retarding performance.
Valuable collaboration paints the whole picture
Collaborating with other doctoral students has given the work additional strength, Susanne Wilken says
– The collaboration made it possible for us to paint the whole picture and present it in research papers, not only saying what happens but also why it happens.
The research has been financed by SHC and Susanne Wilken has been an active participant in the centre’s activities.
– The mingling and networking within SHC have been very valuable for my research, Susanne Wilken says. I have had unique opportunities to learn what others are doing and to understand the whole chain from battery to vehicle application.
As her work at Chalmers draws to a close, Susanne Wilken plans to work with industrial research.
Susanne Wilken will defend her thesis “Failure Mechanisms of Lithium-ion Battery Electrolytes: Detection and Mitigation” on 12 December, 10.00 at Lecture Hall PJ, 4th floor, Origo, Chalmers University of Technology, Göteborg
Prof. Patrik Johansson (project leader), Susanne Wilken (PhD student), Dr. Johan Scheers (co-supervisor), the Department of Applied Physics, Chalmers University of Technology
The project “Battery properties: designed, controlled and safer lithium ion cells – Electrolyte additives” has been part of the Swedish Hybrid Vehicle Centre.
Text and photo: Emilia Lundgren