Power Electronics / Power Management

Self-extinguishing batteries

29 February 2024 Power Electronics / Power Management

In a recently published study, a design for a self-extinguishing rechargeable battery is described. It replaces the most commonly used electrolyte, which is highly combustible – a medium composed of a lithium salt and an organic solvent – with materials found in a commercial fire extinguisher.

An electrolyte allows lithium ions that carry an electric charge to move across a separator between the positive and negative terminals of a lithium-ion battery. By modifying affordable commercial coolants to function as battery electrolytes, a battery that puts out its own fire could be produced.

The designed electrolyte worked well across a wide temperature range, from -70 to 80°C. Batteries that were produced in the lab with this electrolyte transferred heat away from the battery very well, and extinguished internal fires effectively.

These batteries were subjected to the nail penetration test, a common method for assessing lithium-ion battery safety. Driving a stainless-steel nail through a charged battery simulates an internal short circuit; if the battery catches fire, it fails the test. When a nail was driven through these modified charged batteries, they withstood the impact without catching fire.

Why it matters

By nature, a battery’s temperature changes as it charges and discharges, due to internal resistance (opposition within the battery to the flow of lithium ions). High outdoor temperatures or uneven temperatures within a battery pack seriously threaten batteries’ safety and durability.

Energy-dense batteries, such as the lithium-ion versions that are widely used in electronics and electric vehicles, contain an electrolyte formulation dominated by organic molecules that are highly flammable. This worsens the risk of thermal runaway which is an uncontrollable process in which excess heat inside a battery speeds up unwanted chemical reactions that release more heat, triggering further reactions. Temperatures inside the battery can rise by hundreds of degrees in a second, causing a fire or explosion.

Another safety concern arises when lithium-ion batteries are charged too quickly. This can cause chemical reactions that produce very sharp lithium needles called dendrites on the battery’s anode, the electrode with a negative charge. Eventually, the needles penetrate the separator and reach the other electrode, short-circuiting the battery internally and leading to overheating.

As scientists studying energy generation, storage and conversion, a strong interest in developing energy-dense and safe batteries is paramount. Replacing flammable electrolytes with a flame-retardant electrolyte has the potential to make lithium-ion batteries safer and can buy time for longer-term improvements that reduce inherent risks of overheating and thermal runaway.

How the project was done

An electrolyte that was non-flammable, would readily transfer heat away from the battery pack, could function over a wide temperature range, was very durable, and would be compatible with any battery chemistry needed to be developed. However, most known non-flammable organic solvents contain fluorine and phosphorus, which are expensive and can have harmful effects on the environment.

Instead, focus was shifted onto adapting affordable commercial coolants that were already widely used in fire extinguishers, electronic testing and cleaning applications, so that they could function as battery electrolytes.

Focus was directed towards a mature, safe and affordable commercial fluid called Novec 7300, which has low toxicity, is non-flammable, and does not contribute to global warming. By combining this fluid with several other chemicals that added durability, an electrolyte that had the features we sought and would enable a battery to charge and discharge over a full year without losing significant capacity, could be produced.

What is still unknown

Because lithium, an alkali metal, is scarce in our Earth’s crust, it is important to investigate how well batteries that use other, more abundant alkali metal ions, such as potassium or sodium, fare in comparison. For this reason, the study focused predominantly on self-extinguishing potassium-ion batteries, although it also showed that the developed electrolyte works well for making self-extinguishing lithium-ion batteries.

It remains to be seen whether this electrolyte can work equally well for other types of batteries that are in development, such as sodium-ion, aluminium-ion and zinc-ion batteries. The goal is to develop practical, environmentally friendly, sustainable batteries regardless of their ion type.

For now, however, since this alternative electrolyte has similar physical properties to currently used electrolytes, it can be readily integrated with current battery production lines. If the industry embraces it, it can be expected that companies will be able to manufacture non-flammable batteries using their existing lithium-ion battery facilities.

This article edited from The Conversation, www.theconversation.com

Original research paper can be sourced at https://bitly.ws/3d4Rq

For more information contact www.nature.com

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