Less flammable batteries?

14 August 2019

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Scientists have found a material which could lead us to having less flammable batteries...

Rechargeable lithium ion batteries have been known to catch fire, and this is because of the highly flammable liquid electrolyte within. Replacing this with a less flammable material would make the batteries much safer. Now scientists at the University of Louvain, in Belgium, have found a new alloy, which might just do the trick.

The electrolyte is the part of the battery which separates the negative terminal - the cathode - from the positive anode. When a battery discharges, negatively-charged electrons flow from the cathode, around the circuit, and back into the anode. To recharge the battery, the electrons are pushed back from the anode into cathode, travelling through the electrolyte inside the battery as they go.

“A defect in manufacturing, or an external shock, can put the anode and the cathode in contact. Then a lot of heat can be released,” explains Geoffroy Hautier, author of the study. Heat, combined with oxygen in the air and a potential fuel means a fire can start. Typically, the electrolyte is made up of an organic liquid, which is very flammable.

Liquids are more flammable than solids, as the molecules have more surface area to react with oxygen in the air, which is needed for something to burn. “One thing battery researchers have been trying for some time now, is to replace this liquid electrolyte with a solid; this would be much safer,” says Hautier.

Electrolytes are made of organic liquids, so that the lithium ions to power the battery can be pushed through quickly. But finding a solid material, which will let lithium ions flow through quickly enough to be practical has proved to be a challenge. Solids are made of rigid lattice structures with all the atoms held firmly in place, making any movement difficult. Now, Hautier and his team have developed a new alloy, which is a solid but lets lithium flow through easily.

“This material we’ve found is basically around 10 times higher in this diffusion coefficient - this mobility of lithium - then the best, previously-known solid,” says Hautier. The diffusion coefficient is a measure of how quickly something can flow through a material - like how fast a drop of ink can diffuse into a glass of water.

The new alloy is made up of a combination of lithium, titanium, phosphorus and sulphur and works by having a “frustrated” lattice structure. The lithium ions in this structure are not fixed to lattice sites and are constantly moving around. This means that, with some external “push” from a battery charger, the ions can flow through the solid.

One of the shortcomings of this new alloy is that it contains titanium, which is not very stable. Hautier hopes to be able to replace this in due course with zirconium, which will result in a more stable alloy. But the most important finding for him is the discovery of this new type of crystal structure in the first place.

“We understood why this material is so good. We understood what in the crystal structure makes the lithium so mobile. Now, with this understanding, we can go back to other materials and maybe try to find materials with the same types of properties which haven’t been looked at yet.”

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