Novel 'firewall' boosts battery safety

Chinese scientists have developed a liquid electrolyte that solidifies the instant a battery overheats, creating a "firewall" that effectively extinguishes potential fires before they start.

The research marks the world's first demonstration of sodium-ion batteries at a commercial scale — specifically the ampere-hour level found in smartphones and power tools — that can completely prevent thermal runaway. Thermal runaway is a dangerous chain reaction where a battery's temperature rises uncontrollably, often leading to explosions. The findings were recently published in the journal Nature Energy.

For decades, the industry operated on the belief that battery fires were caused primarily by the liquid electrolyte catching fire. To combat this, researchers focused on developing non-flammable liquids.

The research team challenged this convention, when they discovered that even with flame-retardant liquids, batteries could still undergo severe thermal runaway. They identified the true culprit as the melting of the separator, a thin plastic membrane that keeps the positive and negative sides of a battery apart. When that membrane melts, it causes a short circuit, triggering a series of heat-releasing side reactions.

Using this discovery, the team created a "polymerizable" electrolyte. In simple terms, this is a liquid that can turn into a solid plastic-like substance when triggered. The electrolyte uses two specific sodium salts, sodium tetrafluoroborate and sodium hexafluorophosphate. These salts form protective films on the battery's electrodes, which are the components that allow electricity to enter and leave the battery. This setup improves the battery's lifespan while ensuring safety.

If a battery reaches an abnormal temperature of 150 C, the liquid electrolyte rapidly hardens into a solid. This physical barrier blocks all interaction between the positive and negative electrodes, effectively shutting the battery down safely.

To prove the technology's reliability, researchers conducted two rigorous tests. In the first, they drove a steel nail directly through the battery to simulate an internal short circuit. In the second, they placed the battery in an oven heated to 300 C. In both scenarios, the sodium-ion battery produced no smoke, fire or explosions.

Hu Yongsheng, a professor at the Chinese Academy of Sciences' Institute of Physics, noted that these tests were performed on batteries sized for consumer electronics rather than small laboratory "coin cells". This indicates the technology is nearly ready for mass production.

Hu said the safety breakthrough does not compromise performance. The battery remains stable at high voltages and operates in extreme weather, ranging from minus 40 C to 60 C. If installed in an electric vehicle, Hu said there would be no range anxiety even in northern China's winter, where temperatures often drop to minus 20 C.

While lithium-ion batteries currently dominate the market, sodium-ion versions offer several advantages. Lithium is a limited resource that is often expensive and imported. Sodium, the main element in common table salt, is abundant and cheap.

While sodium-ion batteries currently trail lithium-ion models in energy density — the amount of energy a battery can hold compared to its weight — Hu believes they are highly promising for large-scale power grids and commercial vehicles. Ongoing research aims to close the performance gap and expand production capacity.

limenghan@chinadaily.com.cn