Researchers at Tokyo State University have developed a new method for preparing flexible ceramic electrolyte sheets for lithium-metal batteries. They mixed garnet ceramics, polymer binders and ionic liquids to produce a quasi-solid, flake electrolyte. The synthesis is performed at room temperature, requiring much less energy than existing high-temperature (>1000°C) processes. It can operate over a wide temperature range and is a promising battery electrolyte for, for example, electric cars.
Fossil fuels account for most of the world’s energy needs, including electricity. But fossil fuels are running out, and burning them also releases pollutants like carbon dioxide and other toxic nitrogen oxides directly into the atmosphere. There is a global need to shift to cleaner, renewable energy sources. But the main sources of renewable energy like wind and solar are intermittent — the wind doesn’t blow all the time and there’s no sunlight at night. As a result, advanced energy storage systems need to make more efficient use of renewable, intermittent energy. Since SONY commercialized lithium-ion batteries in 1991, they have had a profound impact on modern society, powering many portable electronic devices and household appliances such as cordless vacuum cleaners. But using the battery in electric cars still requires substantial improvements in the capacity and safety of state-of-the-art lithium-ion technology.
This has led to a surge of interest in lithium-metal batteries: the lithium-metal anode has a higher theoretical capacity than the graphite anode currently in commercial use. There are still technical obstacles to the lithium anode. In liquid batteries, for example, lithium dendrites can grow, causing short circuits and even fires and explosions. This is the place where solid inorganic electrolyte function: they are significantly safer, a garnet structure type ceramic Li7La3Zr2O12, better known name is lithium LLZO lanthanum zirconium oxygen, and now is widely regarded as a kind of promising solid electrolyte material, because it has a high ionic conductivity and the compatibility with lithium metal. However, the production of high-density LLZO electrolytes requires very high sintering temperatures, up to 1200℃. This wastes energy and takes time, making mass production of LLZO electrolytes difficult. In addition, poor physical contact between brittle LLZO electrolyte and electrode material results in high interfacial resistance, which greatly limits its application in all-solid lithium metal batteries.
Therefore, a team led by Professor Kiyoshi Kanamura of Tokyo State University set out to develop a flexible LLZO electrolyte sheet that can be prepared at room temperature. They poured LLZO ceramic mud on thin polymer substrates, like butter on toast. After drying in a vacuum oven, the 75 micron thick sheet electrolyte was immersed in ionic liquid (IL) to improve its ionic conductivity. ILs is a salt that is liquid at room temperature and is known to be highly conductive, but hardly flammable or volatile. Within the thin sheet, IL successfully filled the microscopic gap in the structure and bridged the LLZO particles, forming an effective channel for lithium ions. They also effectively reduce the interfacial resistance at the positive end. In further studies, they found that lithium ions diffuse through IL and LLZO particles in the structure, highlighting the role of both. The synthesis method is simple and suitable for industrial production: the whole process is carried out at room temperature without high temperature sintering.
While challenges remain, the team says the mechanical strength and maneuverability of the flexible composite sheet over a wide temperature range make it an ideal electrolyte for lithium-metal batteries. This simple new synthesis method means we will see high-capacity lithium-metal batteries on the market sooner than previously thought.