Safer lithium-ion battery design prevents thermal runaway that can cause fires

Traditional lithium-ion batteries are known to be a fire hazard and, in some cases, can cause an explosion. Reducing the fire risk of lithium-ion batteries has become a top priority, as they are widely used in everything from electric cars to electric toothbrushes. There is a strong need for lithium-ion battery designs that balance long cycle life, high voltage, and safety.

A fire hazard also occurs if a lithium-ion battery suffers any physical damage, is overcharged, or has a manufacturing defect. This causes thermal runaway when the anions (negatively charged ions) break their bond with the lithium and release heat. For traditional lithium-ion batteries, this can result in temperature changes of more than 500°C.

However, Chinese researchers have discovered a way to significantly reduce the heat released when lithium-ion batteries are damaged. Their study, published in Nature Energy, details the new design and experimental results from nail penetration tests, where the temperature rise was only about 3.5°C.

This new design was made possible by the team’s discovery that ionic association in the electrolyte within the battery lowers the temperature at which thermal runaway occurs by about 94 degrees Celsius. They found that replacing some of the solvent in the battery with a different material should increase the temperature at which thermal runaway begins and reduce the risk of thermal runaway. This replacement also allows the formation of a solid electrolyte interface (SEI) at low temperatures, a normal feature of lithium-ion batteries.

So the researchers developed a “solvent relay strategy” that promotes ion association for SEI formation at room temperature, but induces dissociation at higher temperatures for safety. The new design includes a solvent called lithium bis(fluorosulfonyl)imide, which binds to the lithium in existing solvents only at high temperatures, suppressing anionic bonding that ultimately causes thermal runaway.

The research team tested the new design by driving a nail into a 1.1 Ah pouch cell. This is a common safety evaluation test for lithium-ion batteries.

“This approach enabled 4.5 V graphite NCM811 pouch cells (1.1 Ah) that demonstrated an excellent cycle life of 4,100 hours with approximately 81.9% capacity retention (1,000 cycles at 0.45 C). These ampere-hour scale cells also demonstrated improved thermal stability, with a temperature rise of 3.5 °C during nail penetration testing. (compared to 555.2 °C for the NCM811 pouch cell),” the study authors wrote.

This new design clearly advances the safety of lithium-ion batteries. Although some more testing is needed, these changes could be incorporated into lithium-ion batteries in the near future.

“This study elucidates the critical impact of ionic bonding on thermal runaway and establishes an effective strategy to achieve extended cycle life, higher cutoff voltage, and enhanced safety in ampere-hour-level lithium-ion batteries,” the study authors wrote.

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