An international team of researchers has demonstrated a new way to increase the robustness and energy storage capability of a particular class of lithium-rich cathode materials by using a carbon dioxide-based gas mixture to create oxygen vacancies at the material’s surface. The treatment improved the energy density—the amount of energy stored per unit of mass—of the cathode material by up to 30% to 40%, according to the researchers.

The discovery sheds light on how changing the oxygen composition of lithium-rich cathode materials could improve battery performance, particularly in high-energy applications such as electric vehicles.

“We’ve uncovered a new mechanism at play in this class of lithium-rich cathode materials," says Shirley Meng, nanoengineering professor at the University of California San Diego (UCSD) and one of the principal research investigators.

Meng’s group collaborated with researchers from the Chinese Academy of Sciences to develop a way to introduce oxygen vacancies in a class of cathode materials known as lithium-rich layered oxides. These materials have been gaining popularity among battery researchers because they can potentially house more energy than other cathode materials. But lithium-rich cathode materials also have their drawbacks, including slow discharge rates and an issue called voltage fade, which is characterized by a drop in cell voltage with each charge-discharge cycle.

The team found that treating the lithium-rich cathode particles with a carbon dioxide-based gas mixture created oxygen vacancies uniformly throughout the surface of the particles. The treatment only left oxygen vacancies within the first 10 to 20 nanometers without altering the rest of the material’s atomic structure.

SEM image of lithium-rich cathode particles treated with a carbon dioxide-based gas mixture to introduce oxygen vacancies on the surface. Image credit: UCSD Laboratory for Energy Storage and Conversion. In electrochemical tests, the treated material exhibited a relatively high discharge capacity (300 milliamp-hours per gram) with minimal voltage loss after 100 charge-discharge cycles.

Through characterization studies in collaboration with groups from Brookhaven National Laboratory and Oak Ridge National Laboratory, the researchers postulated several reasons for oxygen vacancies improving the cathode material’s performance—among them that they allow lithium ions to move around more easily throughout the cathode, leading to high discharge capacity and faster discharge rates. The vacancies also increase the material’s stability by inhibiting the formation of highly reactive oxygen radicals at the cathode material’s surface, which are typically responsible for degrading the electrolyte while the battery is operating. This could mean longer battery lifetime, the researchers say.

As a next step, the research team will work on scaling up the treatment reported in this study. They will also conduct further studies on the oxygen activity in other materials to determine how it could be leveraged to improve battery performance.