Scientists have developed a lithium-ion battery that, unlike conventional ones, consists entirely of solid compounds and is non-flammable, even at very high temperatures. 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. 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 such a battery is improperly charged (overcharging) or left 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," said Jennifer Rupp, Professor at ETH Zurich. 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. Researchers developed an improved electrode-electrolyte interface. 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. "During production, we made sure that the solid electrolyte layer obtained a porous surface," said Jan van den Broek, a student in Rupp's group. The researchers then applied the material of the negative pole in a viscous form, allowing it to seep into the pores. Finally, the scientists tempered the battery at 100 degrees Celsius. "With a liquid or gel electrolyte, it would never be possible to heat a battery to such high temperatures," said van den Broek. Researchers were able to significantly enlarge the contact area between the negative pole and the solid electrolyte, which ultimately means that the battery can be charged faster. Batteries produced like this could theoretically operate at a normal ambient temperature, said Semih Afyon, a former research scientist in Rupp's group, now a professor at the Izmir Institute of Technology in Turkey.
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