One of the new frontiers in battery technology is creating safer versions of the ubiquitous lithium-ion battery, like those that power electric cars and the computers or phones you read these words on. These little suckers are great at packing large amounts of energy into tight spaces, but can run into trouble at high temperatures. Versions that replace combustible, liquid electrolytes with solid parts is one way this problem might be overcome and researchers have just thrown up one possible answer, building a solid-state lithium-ion battery that can be heated all the way up to 100° Celsius without bursting into flames.

If you've ever left your phone out in the sun on a summer's day, you may recall an on-screen temperature warning, advising you to let the phone cool down before using it again. This is because the liquid electrolyte within the battery can ignite or swell up under high temperatures. Improper charging, or overcharging, can be another cause for this type of malfunction.

Such instances of lithium battery failure are very rare, but a busted iPhone is one thing and an electric vehicle bursting into flames is another. The sheer amount of lithium batteries in use around the world every day means that there is plenty of opportunity for something, somewhere to go wrong at some point.

This has led researchers to explore how the safety of these batteries might be improved. Smart chips that can be embedded inside to monitor the battery's health is one possibility, and replacing the liquid electrolyte with solid components is another that is gaining some attention in laboratories around the world.

The electrolyte is the solution tasked with carrying the charge between the battery's positive and negative electrodes. The idea behind solid-state batteries is to replace this solution with something that can endure high temperatures. But this concept brings on another set of problems, among which is how to connect the solid electrolyte with the solid electrodes in a way that allows the charge to circulate with as little resistance as possible, maximizing its run time on a full charge.

Researchers at Switzerland's ETH Zurich have come up with a battery design they say addresses this problem. They liken the battery to a sandwich, with two electrodes as the bread and a layer of solid lithium garnet electrolyte as the meat inside. Garnet is a mineral that forms gemstones, is used as an abrasive material in waterjet cutting and also happens to be a material with one of the highest known conductivities for lithium ions.

The team crafted the solid garnet electrolyte in a way that gave it a porous surface. The negative electrode was then applied in a viscous form which allowed it to seep into these pores. This creates a larger contact area between the electrode and electrolytes, and means that the battery can be charged faster. The design also meant that it could withstand temperatures of up to 100° Celsius (212° F) when the team put it to the test.

"With a liquid or gel electrolyte, it would never be possible to heat a battery to such high temperatures," says Jan van den Broek, one of the authors of the study.

In its current form, the battery works best at 95° Celsius (203° F) and above, temperatures that better facilitate the movement of the lithium ions. This could see it put to use in battery storage power plants that save excess energy for a later date.

"Today, the waste heat that results from many industrial processes vanishes unused," says Semih Afyon, a former research scientist at ETH Zurich and now with the Izmir Institute of Technology in Turkey. "By coupling battery power plants with industrial facilities, you could use the waste heat to operate the storage power plant at optimal temperatures."

But with further development, the team says that the sandwich-like solid form of the device could be adapted to thin-film batteries. These could change the way things like phones and laptops are powered, and even be placed directly onto silicon chips.

The research was published in the journal Advanced Energy Materials.

Source: ETH Zurich

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