Polymer binders - towards more practical solid-state batteries
Polymer chemists at the University of Oxford, working as part of the interdisciplinary SOLBAT project team, have filed two patent families for novel cathode binder materials for solid-state batteries (SSBs). By avoiding the use of fluorinated materials and reducing the external pressure needed to be applied to the battery, the research points a way to a more practical SSB for commercial use in EVs. The novel materials help to mitigate challenges surrounding the expansion and contraction of the particles of cathode active materials on charge and discharge.
Challenges of solid-state battery volume expansion
All-solid-state batteries stand out for their lack of a flammable liquid electrolyte, offering improved safety characteristics and superior stability. They potentially open the way for use of more energy-dense anodes, such as lithium metal (rather than graphite). This would deliver a transformative boost in volumetric and gravimetric energy densities (the energy stored in a given volume or weight). A step change in energy density could extend EV driving ranges significantly, accelerating adoption.
| By the numbers | |
|---|---|
| 2 | Patent families filed for the use of polymers as SSB cathode binders |
| 50-70 | Megapascals stack pressure typically used in the lab to mitigate SSB volume expansion |
| 1 | Megapascal stack pressure deployed when using polymeric cathode binders |
| 300 | Test cycles where the SSB cathode incorporating a polymer binder performed well |
The market opportunity for SSBs is huge. Fortune Business Insights forecasts growth from US$99mln in 2024 to more than US$1.3bln in 2032. SSBs are increasingly being deployed in demonstration / pilot fleets, or as semi-solid or “hybrid” solid-state batteries. Multiple automotive companies including Toyota and Nissan, as well as many battery companies, are hoping to begin commercial or mass production between 2026 and 2030. Multiple technical challenges still remain to meet these milestones, including the ability to deliver a stable, composite cathode compatible with a solid-state system.
As SSBs charge and discharge, the active material in the composite cathode changes volume, leading to loss of contact between particles of active material and the solid-state electrolyte, reducing the ability of lithium ions to move between anode and cathode, reducing battery performance, and eventually leading to cell failure.
Therefore, in solid-state systems pressure has to be applied to hold all the components together – which isn’t needed in a liquid system because the liquid electrolyte can flow into voids to maintain ion transport paths.
Stack pressures of 50-70 megapascals (MPa) – or around 50 times atmospheric pressure – are not unheard of to achieve desired performance from an SSB. This simply isn’t practical outside the lab – it’s not a solution for auto manufacturers. Finding other ways to mitigate volumetric expansion of SSBs to eliminate or reduce the need of applying external pressure to the cells is an ongoing research and commercialisation challenge.
Additionally, the binders used in SSBs – the materials that hold the particles in a composite electrode together – are typically based on fluorinated materials. However, they are environmentally hazardous, and don’t conduct lithium ions particularly well.
A new approach was needed.
Getting into a bind
A potential solution came from the field of polymer chemistry, and a research group that previously hadn’t worked on batteries.
Professor Charlotte Williams’ group at the Chemistry Department at the University of Oxford, focuses on new sustainable technologies for polymer production and carbon dioxide usage. Their research is developing highly active catalysts that transform abundant renewable resources and wastes into polymers. Charlotte comments:
We’d been working on polymer chemistries that are oxygen-rich, which is one of the most important factors in improving battery performance. So in collaboration with Professor Sir Peter Bruce at the Department of Materials we started looking at whether our materials could be translated across to the battery field.”
Working as part of the multi-disciplinary Faraday Institution SOLBAT project, researchers tested a range of flexible polymers that could potentially be used as cathode binders, holding the particles in a composite electrode together without the need for a high pressures. As components change volume, polymers can fill the voids and spring back to shape during charge and discharge.
Dr Victor Riesgo-Gonzalez, a Postdoctoral Research Fellow in Charlotte’s group, comments:
We had to find a polymer that would allow good ion transport, be adhesive, display the right mechanical properties, and be stable in the harsh battery environment. It took a lot of optimisation of the materials, but the team settled on block and terpolymers with the right combination of properties.”
By using controlled polymerisation strategies, including reactions that directly use carbon dioxide as a monomer, the team make block polymers that allow precise functional group spacing, architecture and chain length – all features that are important for making lithium ion conducting binders for solid-state batteries. This has led to both fine-tuning of stability, conductivity and adhesion, and to a considerable reduction in the stack pressure that needs to be applied to achieve good battery performance.
Dr James Runge, another Postdoctoral Research Associate in the group, continues:
We’ve been able to demonstrate that our oxygenated polymers used as a binder could outperform fluorinated polymers both in terms of battery capacity and cycle life – and remove the need to use hazardous materials.”
Powering up
Working with Oxford University Innovation, the University’s technology transfer office, the researchers successfully filed two patent families based on the technology. The team is now in active discussions with several companies who may wish to license the technology for commercialisation in battery applications and other potential areas.
Charlotte comments:
The polymers are made from commercial monomers and CO2. The methods to make them are compatible with large-scale manufacturing and the products can be processed using conventional equipment; all these factors are significant benefits in scaling-up the polymer technologies. We’re now working with various commercial partners to test their use in battery formulations.”
Future research directions
Professor Williams’ team are now researching how to optimise the binders to improve battery performance, by varying, for example, the composition of the polymers and the processing conditions.
Charlotte continues:
You can optimise the polymer, but how that translates to what happens in the composite is less clear. You might make a polymer that is extremely ionically conductive and assume that will make a battery with a high capacity, but that’s not necessarily the case. So it’s important we get a better understanding of the microstructure and the relative importance of the components. The polymer only represents about 5% of the cathode, but it’s a very important component in larger-scale cathode production and use.”
Initial tests have also been conducted to assess the viability of use of polymer binders in alternate cell chemistries and formats.
Dr Sylwia Walus, Research Programme Manager at the Faraday Institution concludes:
There’s clearly much research, development and scale up required in this area, but this workstream demonstrates the gains that can be made in battery performance by drawing on the expertise of researchers from fields outside traditional battery science. The team at the Chemistry Department has brought truly new ideas for novel designs of battery components to the table.”
