Key improvement to lithium-oxygen batteries revealed

Lithium-air batteries, sometimes referred to as lithium-oxygen batteries (Li-O₂), comprise a lithium metal anode, an organic electrolyte and a porous carbon cathode. During discharge, the oxygen in the surrounding air reacts with lithium at the cathode, releasing energy. Due to their high density (>3500 Wh kg-1), Li-O₂ batteries could help generate greener sources for energy security. However, advances in the technology have stalled because specially designed carbon cathodes lack certain characteristics, such as abundant active sites where chemical reactions can take place, and space large enough to accommodate the nucleation and growth of discharge products, something necessary to achieve high energy density.

Researchers from Tohoku University have developed a porous carbon sheet called a graphene mesosponge sheet (GMS-sheet) that improves the energy density and cycle stability in Li-O₂ batteries. Hirotomo Nishihara, co-corresponding author of the paper, said that although the rational design of the porous structure for the carbon cathode is crucial for achieving a high performance, it is also a challenge. “We creatively developed an angstrom-to-millimetre controllable synthesis of freestanding cathodes with minimally stacked graphene free from edge sites,” Nishihara said.

To do this, the researchers rationally controlled three synthesis parameters during a chemical vapour deposition (CVD) process: the pelletisation force, the amount of Al₂O₃ template and the CVD’s duration. Doing so resulted in a series of GMS-sheets with different porosity, amounts of carbon layers and sheet thickness.

Wei Yu, co-corresponding author of the paper, said that the specific mass/areal capacities of Li-O₂ batteries using GMS-sheets could be controlled by these three synthesis parameters. “By optimising 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 centimetre when normalised to the mass and area of GMS-sheets, respectively,” Yu said.

The researchers characterised the discharge-charge mechanism using comprehensive in-situ techniques and, in doing so, determined that the hierarchical porous structure of GMS-sheets is vital for enhancing battery performance. Nishihara believes that the GHS-sheet will be 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,” Nishihara said.

The research findings were published in the journal Advanced Energy Materials.

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