Synthesis of a free-standing and edge-site-free GMS-sheet with hierarchically porous structure. Image: Wei Yu, Hirotomo Nishihara et al.
Synthesis of a free-standing and edge-site-free GMS-sheet with hierarchically porous structure. Image: Wei Yu, Hirotomo Nishihara et al.

Lithium-air batteries, sometimes known as lithium-oxygen batteries (Li-O2), comprise a lithium metal anode, an organic electrolyte and a porous carbon cathode. During discharge, oxygen in the surrounding air reacts with lithium at the cathode, releasing energy in the process. Given their extremely high energy density, Li-O2 batteries could potentially lead the way in generating greener sources for energy security.

Yet advances in the technology have stalled because the specially designed carbon cathodes lack certain characteristics. Namely, abundant active sites where chemical reactions can take place and space large enough to accommodate the nucleation and growth of discharge products, something necessary for achieving a high energy density.

Now, researchers from Tohoku University in Japan and their partners have developed a special type of porous carbon sheet called a graphene mesosponge sheet (GMS-sheet) for use as a cathode. This novel sheet, reported in a paper in Advanced Energy Materials, could significantly improve the energy density and cycle stability of Li-O2 batteries.

"The rational design of the porous structure for the carbon cathode is crucial for achieving a high-performance, but it is also a major challenge," says Hirotomo Nishihara, professor at Tohoku University's Advanced Institute for Materials Research (WPI-AIMR) and co-corresponding author of the paper. "We creatively developed an angstrom-to-millimeter controllable synthesis of free-standing cathodes with minimally stacked graphene free from edge sites."

To do this, Nishihara and his colleagues utilized a chemical vapor deposition (CVD) process and rationally controlled three synthesis parameters: the pelletization force, the amount of aluminum oxide (Al2O3) template and the CVD's duration. Doing so resulted in a series of GMS-sheets with different porosity, thickness and number of carbon layers.

"It is interesting to see that the specific mass/areal capacities of Li-O2 batteries using GMS-sheets cathodes can be controlled by these three synthesis parameters," says Wei Yu, assistant professor at Tohoku University's WPI-AIMR and co-corresponding author of the paper. "By optimizing these parameters, we're excited to achieve impressive energy-storage capacities, surpassing the performance of the best carbon cathodes, with more than 6300 milliampere-hours per gram and more than 30.0 milliampere-hours per square centimeter when normalized to the mass and area of GMS-sheets, respectively.

"With the help of our collaborators from the National Institute for Materials Science, Ochanomizu University, Hokkaido University, Osaka University, and 3DC Inc., we characterized the discharge-charge mechanism using comprehensive in situ techniques and unlocked the key to superior battery performance: the hierarchical porous structure of GMS-sheet."

Nishihara and his team believe that the GMS-sheet represents a milestone carbon cathode for Li-O2 batteries. "We will continue to promote the practical use of Li-O2 batteries based on our GMS-sheet, and our landscape also covers other metal-gas batteries such as Na-O2, Li-CO2 and Zn-O2 batteries, for which a high-performance carbon cathode is also needed," says Nishihara.

This story is adapted from material from Tohoku University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.