Ionic liquids expected to shine in next-generation solid-state lithium metal batteries

Researchers from Tokyo Metropolitan University have developed a new quasi-solid cathode for solid-state lithium metal batteries that greatly reduces the interface resistance between the cathode and the solid electrolyte. By adding an ionic liquid, their modified cathode can maintain good contact with the electrolyte. The newly developed prototype battery also showed good capacity retention. Although finding the optimal ionic liquid is still challenging, this idea is expected to provide a new direction for the development of solid lithium batteries for practical applications.

Lithium-ion batteries have become ubiquitous, found in our smartphones, laptops, power tools and electric cars. But as we look for better solutions with higher energy density, scientists have turned to solid-state lithium metal batteries. Lithium metal batteries have higher energy density than lithium-ion batteries. They are considered the future of batteries, capable of powering large-scale vehicles and grids.

However, technical problems prevent solid-state lithium metal batteries from entering applications with harsh environmental conditions. One of the major issues is the design of the interface between the electrode and the solid electrolyte. The electrolyte in lithium-ion batteries is usually a liquid, which is highly flammable and poses a safety hazard. That's why people have been trying to use solid electrolytes instead. However, it is difficult to achieve good contact between the electrode and the solid electrolyte. Roughness on either surface can lead to high interfacial resistance, which hampers battery performance. There has been some work investigating the design of solid electrolytes, but cathode design remains an open problem. A team led by Professor Kiyoshi Kanamura of Tokyo Metropolitan University has been developing new methods to improve the contact between the cathode and solid electrolyte in solid-state lithium metal batteries. Now, they have succeeded in creating a quasi-solid lithium cobalt oxide (LiCoO2) cathode containing a room-temperature ionic liquid. Ionic liquids are composed of positive and negative ions, and they can also transport ions. Importantly, they can fill tiny voids at the cathode/solid electrolyte interface. As the voids are filled, the interfacial resistance decreases significantly. The team's approach brings other benefits as well. Not only are ionic liquids ionically conductive, they are also virtually non-volatile and generally non-flammable. They also create little hindrance to the formation of the cathode, leaving the manufacturing process virtually unaffected. The team demonstrated a prototype battery made with their quasi-solid cathode and solid "garnet" electrolyte (referring to its structure), which showed good rechargeability, performing 100 charge/discharge cycles at a high temperature of 60°C After cycling, the capacity retention rate reaches 80%. Further research also found that the optimal ionic liquid content is 11wt%. But problems still exist, such as the urgent need to find a better ionic liquid that is not easily degraded. However, the team's new paradigm provides an exciting new direction for solid-state metal lithium battery research and has the potential to bring them out of the laboratory and into our lives.