Improving the safety of lithium-ion batteries with salt

Researchers at the SLAC National Accelerator Laboratory and Stanford University have shown how to improve the safety of lithium-ion batteries using a new non-flammable electrolyte. Batteries containing this polymer-based electrolyte were shown to be safer, provide excellent performance and continue to function at high temperatures without starting a fire due to the introduction of a lot more salt.

With rechargeable lithium-ion batteries in so many electronic devices, electric cars, and storing the energy from solar panels, solving the problem of the batteries catching fire when their temperature gets too high is paramount, especially as there is currently no safe alternative electrolyte.

Standard lithium-ion electrolytes have a lithium salt dissolved in a liquid organic solvent, but although the solvent improves the battery’s performance by moving lithium ions around, it can also catch fire. As batteries produce heat as they work, punctures or defects can mean they quickly heat up, with the small molecules of solvent in the electrolyte evaporating and changing from liquid to gas until going on fire.

There are two kinds of safe electrolyte, ceramic solid-state electrolytes, which have good ionic conductivity at room temperature but are brittle, and polymer electrolytes, which are easier to manufacture but limited by room temperature ionic conductivity. Polymer electrolytes use a polymer matrix instead of the usual salt–solvent solution to move ions around, but are less efficient. This study assessed the effect of more salt being added to polymer electrolytes, increasing it from less than 50% of the electrolyte’s weight to 63%, making it one of the saltiest ever polymer-based electrolytes.

As reported in Matter [Huang et al. Matter (2022) DOI: 10.1016/j.matt.2022.11.003], this Solvent-Anchored non-Flammable Electrolyte (SAFE) addresses the ionic conductivity issue of polymer electrolytes and the manufacturing difficulties of ceramic ones. The extra salt acts as an anchor for the solvent molecules, stopping them from evaporating and catching fire, and as the solvents are anchored by salts and polymers, they increase the ionic conductivity.

The new electrolyte could find applications in consumer electronics and in electric cars, where lithium-ion batteries filled with an electrolyte such as SAFE means they can be packed closer together without overheating, allowing more room and increasing the overall energy density so that cars can travel farther between charges. As first author Rachel Huang told Materials Today, “A safe electrolyte that is ready to replace the existing flammable electrolyte in lithium-ion batteries is highly desirable”.

The electrolyte being a liquid with higher viscosity than existing electrolytes was a problem during manufacturing, particularly when on injection into a battery, so it will be critical to fine-tune its mechanical properties. The researchers also hope to investigate different salt, polymer and solvent combinations to further reduce cost.

“A safe electrolyte that is ready to replace the existing flammable electrolyte in lithium-ion batteries is highly desirable”Rachel Huang