Ionic liquids expected to shine in next-generation solid-state lithium metal batteries
Researchers from Tokyo?University have developed a new quasi-solid cathode for solid-state lithium metal batteries that greatly reduces the interfacial 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 cell also showed good capacity retention, and while finding the optimal ionic liquid remains challenging, the idea promises to provide a new direction for the development of solid-state lithium batteries for practical applications.
Lithium-ion batteries have become ubiquitous and are found everywhere in our smartphones, laptops, power tools and electric cars. But as we look for a better solution with higher energy density, scientists have turned to solid-state lithium-metal batteries. Lithium metal batteries have a higher energy density than lithium ion batteries. They are considered to be the future of batteries that can power large-scale vehicles and power grids.
The addition of ionic liquids to the cathode material fills structural voids and provides a better interface with the solid electrolyte for a better interface
However, technical problems have prevented solid-state lithium metal batteries from entering applications with more demanding environmental conditions. One of the major issues is the design of the interface between the electrodes and the solid electrolyte. The electrolyte in lithium-ion batteries is usually liquid and highly flammable, posing a safety hazard. This is why there have been attempts to use solid electrolytes instead. However, good contact between the electrode and the solid electrolyte is difficult to achieve. A rough surface on either side can lead to high interfacial resistance, which plagues the performance of the cell. There has been some work on the design of solid electrolytes, but cathode design is still an open problem.
A team led by Professor Kiyoshi Kanamura at the University of Tokyo has been developing new ways to improve the contact between the cathode and the solid electrolyte in solid-state lithium metal batteries. Now, they have succeeded in creating a quasi-solid lithium cobalt oxide (LiCoO2) cathode that contains a room-temperature ionic liquid. The ionic liquids consist of positive and negative ions, and they can also transport ions. Importantly, they fill the tiny voids at the cathode/solid electrolyte interface. As the voids are filled, the interfacial resistance decreases significantly.
The team's approach offers other benefits as well. Not only are the ionic liquids ionically conductive, but they are also virtually non-volatile and generally non-flammable. They also present little obstruction to cathode formation, making the fabrication process virtually unobtrusive. The team demonstrated a prototype battery made with their quasi-solid cathode and a solid "garnet" electrolyte (referring to its structure) that showed good rechargeability, with 80% capacity retention after 100 charge/discharge cycles at 60°C. Further studies also found that the optimal ionic liquid content was 11 wt%.
But problems remain, such as the urgent need to find a better, non-degradable ionic liquid now. However, the team's new paradigm offers exciting new directions for research on solid-state lithium metal batteries and has the potential to bring them out of the lab and into our lives.
This work was supported by the Japan Science and Technology Agency's (JST) Advanced Low Carbon Technology Research and Development Program (ALCA) - Special Promotion Research for Innovative Next Generation Batteries (SPRING).
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Professor Kiyoshi Kanamura, Tokyo University
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