Lithium-ion batteries store a lot of energy in a small space, making them the energy source of choice for mobile electronic devices. Today, mobile phones, laptops, e-bikes and electric cars are all powered by such batteries. Researchers at ETH Zurich have now developed a type of battery that, unlike conventional ones, consists entirely of solid chemical compounds and is non-flammable.

Conventional lithium-ion batteries are not without their dangers: mobile phone batteries have exploded several times in the past, resulting in injuries, and only six months ago an entire row of houses burned down in the old town of Steckborn on Lake Constance. The blaze was caused by a model-making battery that allegedly caught fire due to being charged improperly.

Solid-state batteries can be heated

In conventional lithium-ion batteries as well in most other batteries, the positive and negative poles – the two electrodes – are made of solid conductive compounds; charges move between these electrodes in a liquid or gel electrolyte. If you charge such a battery improperly (overcharging) or leave it sitting out in the sun, the liquid can ignite or the gel can swell up.

This is not the case with solid-state batteries, which are currently in development in research laboratories worldwide: in these types of batteries, both the electrodes and the intermediary electrolyte are made of solid material. "Solid electrolytes do not catch fire even when heated to high temperatures or exposed to the air," explains Jennifer Rupp, who, as Professor of Electrochemical Materials at ETH Zurich, is leading the development of this new type of battery.

Research at the interface

One of the challenges in developing solid-state batteries is to connect the electrodes and electrolyte in such a way that the charges can circulate between them with as little resistance as possible. The ETH researchers have now developed an improved electrode-electrolyte interface.

In the laboratory, they constructed a sandwich-like battery featuring a layer of lithium-containing compound (lithium garnet), which acts as a solid electrolyte between the two electrodes. Lithium garnet is one of the materials with the highest known conductivity for lithium ions.