Safer sodium battery eliminates thermal runaway with a heat-triggered polymer barrier

Some batteries have been known to catch fire or explode at high temperatures or when under stress. This safety concern has pushed researchers to experiment with different ways to design safer batteries that can ideally still perform reliably and efficiently. Sodium-ion batteries (NIBs) are considered a promising alternative to lithium-ion batteries, but still face safety risks, especially at high capacities. But now, a team of researchers in China has designed a new type of electrolyte for NIBs that may eliminate these risks, allowing for stable performance across a wide temperature range.

The thermal runaway problem

The main risk associated with batteries involves a process called thermal runaway. Thermal runaway is a rapid and uncontrolled increase in temperature that occurs when heat generation exceeds heat dissipation. This can lead to intense, self-sustaining fires or explosions that are exceptionally difficult to extinguish, release toxic gases, and can even reignite after being extinguished.

Some electrolytes are designed to be "nonflammable," often by using phosphate esters or fluorinated compounds. However, most nonflammable electrolytes only prevent fire, and do not fully eliminate thermal runaway in large batteries. The team involved in the new study notes that the thermal stability of the electrolyte, the stability of the electrode–electrolyte interfaces and the interactions between the anode and cathode at high temperatures must be considered comprehensively when creating a truly safe battery that can resist thermal runaway.

A polymerizable, nonflammable electrolyte

In the new study, published in Nature Energy, researchers take a different approach to stopping thermal runaway. Instead of relying on reactions between decomposition products and free radicals in an electrolyte to stop thermal runaway, the team developed a new polymerizable, nonflammable electrolyte (PNE) for sodium-ion batteries. The electrolyte works by forming a protective polymer barrier when temperatures increase, blocking dangerous reactions between the electrodes. This also impedes side reactions and the generation of reductive gases.

"This design not only achieves the non-flammability but also enables thermal self-protection through in situ polymerization of phosphoric acid, a decomposition product of triethyl phosphate (TEP), forming an insulating polymer network to block the mechanical and chemical crosstalk between cathode and anode at high temperature," the study authors explain.

Impressive results in safety and performance tests

The team tested the new electrolyte in commercial-sized 1.45 Ah and 3.5 Ah cylindrical sodium-ion cells in a series of safety tests, including nail-penetration tests, accelerating rate calorimeter (ARC) testing for thermal stability and thermal abuse testing. The electrolyte experienced no thermal runaway, even at 300°C (572°F) or after nail-penetration tests.

Electrochemical performance was measured over hundreds of charge/discharge cycles at various temperatures and showed that the batteries had a high energy density and stable performance across a wide temperature range. When compared to other electrolytes, the PNE electrolyte outperformed them in safety and durability under stress.

"Using CNFM as cathode and HC as anode with the designed PNE electrolyte, the cell possesses a capacity of 3.5 Ah and can endure up to 700 cycles at room temperature with 85.7% capacity retention. Moreover, it demonstrates exceptional durability even under high-temperature conditions of 60 °C, allowing for stable cycling up to 700 cycles with 88.1% capacity retention. [Additionally], excellent discharge capacity retention can be [retained] at low temperature environments of −20 °C (92.6%), −30 °C (84.5%) and −40 °C (64.1%), respectively," the study authors write.

The new electrolyte design could inspire safer battery designs for grid storage, electric vehicles and other high-capacity applications. Although the study focused on specific cell chemistries, further research may adapt this approach to other battery chemistries and formats, and under real-world conditions.

Written for you by our author Krystal Kasal, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.