Lithium-ion batteries have become commonplace, with applications in smartphones, computers, power equipment, and electric vehicles. However, scientists have turned to solid-state lithium metal batteries as they seek better alternatives with higher energy density. Li metal batteries have the potential to have a substantially better energy density than Li-ion batteries. They are viewed as the future batteries, capable of massively powering cars and grids.
A team has recently created a new quasi-solid-state cathode for solid-state lithium metal batteries. It has a much lower interfacial resistance between the cathode and a solid electrolyte. They were able to retain great contact with the electrolyte by adding an ionic liquid to their modified cathode. Their prototype battery also demonstrated good capacity retention. Though locating the ideal ionic liquid remains challenging, the concept is still relevant for the Solid-State Battery Market as it opens up new avenues for solid lithium battery development for practical applications.
Technical difficulties with lithium-ion batteries prevent solid-state lithium metal batteries from entering demanding applications. One of the most important is the interface design between electrodes and solid electrolytes. Li-ion battery electrolytes are often liquid and highly flammable, providing a safety threat. That's why people have been experimenting with solid-state electrolytes instead. However, good contact between electrodes and solid electrolytes is difficult to accomplish. Any surface roughness on either side causes significant interfacial resistance, wreaking havoc on battery performance. There has been some research on the design of the solid electrolyte, but the cathode design remains a mystery.
Researchers have been working on novel techniques to improve the contact between the cathode and the solid-state electrolyte in solid-state lithium metal batteries. They have created a quasi-solid-state lithium cobalt oxide (LiCoO2) cathode containing a room-temperature ionic liquid. Ionic liquids are made up of positive & negative ions and can transport ions. They can also fill any microscopic holes at the cathode/solid electrolyte interface. The interfacial resistance was dramatically reduced once the spaces were filled.
The team's method has other advantages. Ionic liquids are ionically conductive and also almost non-volatile, and rarely flammable. They also have little impact on the slurry from which the cathode is generated, leaving the production process unaffected. The team displayed a prototype battery produced with their quasi-solid-state cathode and a solid "garnet" electrolyte (its structure). This proved good rechargeability, with 80 per cent capacity retention after 100 charge/discharge cycles at 60°C. Further research found an appropriate ionic liquid level of 11wt%.
There are still issues to be solved, such as developing a better ionic liquid that does not degrade as quickly. The team's new paradigm has the potential to progress research into solid-state lithium metal batteries with the possibility of commercialisation.