How battery electrolytes behave when heated

Lea Westphal of the Technical University of Munich (TUM) is the lead author of a new study that provides the most detailed, temperature-dependent picture of ethylene carbonate (EC) to date. EC is a solvent used in the electrolytes of lithium-ion batteries.

EC is found in almost all lithium-ion batteries, such as those in smartphones or electric cars. The substance helps form a protective layer on graphite anodes. “Thanks to this layer, the battery lasts longer because the electrodes and electrolyte are protected from degradation,” explains Lea Westphal, a doctoral student at the Heinz Maier-Leibnitz Zentrum.

Better predictions for industry

Even though EC is solid at room temperature and must be mixed with solvents that have a lower melting point to remain liquid, a precise understanding of how the molecules behave as temperatures change provides scientists and battery manufacturers with significantly better guidance. The team investigated how EC molecules arrange themselves and move from temperatures near absolute zero up to their melting point.
To track tiny changes in the EC crystal during heating, Lea Westphal’s team used neutron powder diffraction at two neutron sources: the SPODI instrument at TUM’s FRM II in Garching and the ECHIDNA instrument at ANSTO in Sydney. The measurements showed that EC maintains the same ordered crystal structure consisting of a fixed repeating unit from near absolute zero (-270°C) up to 36°C.

Expansion with rising temperature

The researchers also observed that the repeating unit expands noticeably as the temperature rises, as the molecules vibrate more strongly.
To understand the structural changes that occur during melting, the researchers analyzed the samples using high-energy X-ray powder diffraction at DESY (PETRA III) in Hamburg. They discovered that when EC melts, the regular arrangement of the molecules is lost, but each individual molecule retains its shape. “This means that the molecules survive the transition to the liquid state and merely lose their long-range alignment,” says Lea Westphal.

Useful for longer-lasting electrolytes

Lea Westphal explains why the study’s findings are important: “In practice, these improved predictions can accelerate the development of safer, longer-lasting electrolytes and help us select solvent mixtures or additives that perform reliably over a wider temperature range.”