Ceramic-based flexible electrolyte sheets for lithium metal batteries

Researchers at Tokyo Metropolitan University have developed a new method for making flexible ceramic-based electrolyte sheets for lithium metal batteries. They combined a garnet-like ceramic, a polymer binder, and an ionic liquid, producing a nearly solid sheet electrolyte. The synthesis is carried out at room temperature, requiring significantly less energy than existing processes at high temperature (> 1000 ° C). It works over a wide temperature range, which makes it a promising electrolyte for electric vehicle batteries, for example. For more information, see the IDTechEx Electric Vehicle Lithium-Ion Batteries 2020-2030 report.
There is a global demand to switch to cleaner renewable energy sources. Advanced energy storage systems are needed to use intermittent renewables more efficiently. Lithium-ion batteries have had a profound impact on modern society, powering a wide range of electronic devices and portable devices such as cordless vacuum cleaners since their introduction by Sony in 1991. But the use of these batteries in Electric vehicles (EVs) still require a substantial improvement in the capacity and safety of advanced Li-ion technology.
This has led to a revival of research interest in lithium metal batteries: lithium metal anodes have a much higher theoretical capacity than graphite anodes currently used in commerce. There are still technological hurdles associated with lithium metal anodes. In liquid-based batteries, for example, lithium dendrites (or arms) can develop, which could short circuit the battery and even cause fires and explosions. This is where solid-state inorganic electrolytes come in: they are significantly safer, and a garnet-like ceramic (structure type) Li7La3Zr2O12, better known as LLZO, is now widely regarded as a material. Electrolytic solid state electrolytic promising for its high content of ionic ions. conductivity and compatibility with Li metal. However, the production of high density LLZO electrolytes requires very high sintering temperatures, which can reach 1200 ° C. This is both energy inefficient and time consuming, making it difficult to produce LLZO electrolytes on a large scale. In addition, poor physical contact between fragile LLZO electrolytes and electrode materials generally results in high interfacial resistance, greatly limiting their application in Li-metal solid-state batteries.
So a team led by Professor Kiyoshi Kanamura of Tokyo Metropolitan University set out to develop a flexible composite LLZO sheet electrolyte that can be manufactured at room temperature. They poured a suspension of LLZO ceramic onto a thin polymeric substrate, like spreading butter on toast. After drying in a vacuum oven, the 75 micron thick sheet electrolyte was soaked in ionic liquid (IL) to improve its ionic conductivity. ILs are salts that are liquid at room temperature, known to be highly conductive while being virtually non-flammable and non-volatile. Inside the sheets, IL succeeded in filling microscopic voids in the structure and filling in LLZO particles, forming an efficient pathway for Li ions. They also effectively reduced interfacial resistance at the cathode. Upon further investigation, they found that Li ions diffuse through both IL and LLZO particles in the structure, highlighting the role played by both. The synthesis is simple and suitable for industrial production: the entire process is carried out at room temperature without requiring high temperature sintering.

Although challenges remain, the team says the mechanical robustness and operability of the flexible composite sheet at a wide temperature range make it a promising electrolyte for Li-metal batteries. The simplicity of this new synthesis method may mean that we will see high capacity lithium metal batteries on the market sooner than we think.

Source: Tokyo Metropolitan University

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