A greener glue for solid-state batteries

Solid-state batteries offer safer, higher-energy storage because they replace flammable liquid electrolytes with solid materials. But building all-solid battery requires combining several solid components - cathode particles, solid electrolyte, conductive additives – and these need a binder to hold them together. This binder must not only “glue” the materials in place but also allow ions to pass through it, stay stable at high voltages and ideally be recyclable.

Up to now, many binders have been made from fluorinated polymers, which raise environmental and regulatory concerns. The European Chemicals Agency is even considering banning their use. As a result, finding new, safer binder materials has become an important challenge for advancing solid‑state battery technology.

Fabrication of composite cathode and solid-state cell configuration.

A team of researchers from the University of Oxford and the Faraday Institution published a paper in Chemical Science detailing their designed block copolymer binders made of polyester/polycarbonate segments that are Li-ion conducting, high-voltage stable, and recyclable. They incorporated these into composite cathodes and demonstrated performance rivalling fluorinated binders, they performed Small-Angle X-ray Scattering (SAXS) experiments at Diamond’s labSAXS instrument.

Composite cathodes in solid-state batteries typically mix the active cathode, a solid electrolyte, carbon for conductivity, and a polymer binder. This binder also has to deal with mechanical stress, let lithium ions move through it, and stay stable at high voltages.

Because fluorinated binders are often non-conductive and environmentally problematic, the team asked: can a non-fluorinated, recyclable binder be made that still meets the performance requirements?

They designed polymers as a mix of two components. A-block: a stiff mechanical modifier - poly(4-vinyl cyclohexene carbonate) and B-block: a soft, ion-conductive polyester/carbonate (a random copolymer of caprolactone (CL) and trimethylene carbonate (TMC)).

By adjusting how these blocks were arranged and how much of each they used, they could fine‑tune the binder’s strength and ion‑transport ability.

They mixed the polymers with a lithium salt to make thin films and tested how they handled heat, how well ions moved through them, how flexible or sticky they were, and how stable they remained. They then used the binders in real composite cathodes and built full solid‑state battery cells to see how well they performed during charging and discharging at elevated temperature.

To find out more about Diamond’s laboratory SAXS instrument please contact principal beamline scientist Nick Terrill: nick.terrill@diamond.ac.uk