Chemists from the University of Waterloo have successfully resolved two of the most challenging issues surrounding lithium-oxygen batteries, and in the process created a working battery with near 100% coulombic efficiency.

The new work, published in Science, shows that four-electron conversion for lithium-oxygen electrochemistry is highly reversible. The Waterloo team is the first to achieve four-electron conversion, which doubles the electron storage of lithium-oxygen, also known as lithium-air, batteries.

Thermodynamics and configuration of the Li-O₂ cell. (A) Gibbs reaction energy for formation of Li₂O and Li₂O₂ as a function of temperature. The thermodynamic data were calculated according to the database of HSC chemistry version 5. (B) Configuration of the inorganic electrolyte Li-O₂ cell and schematic illustration of Li₂O formation during discharge. Xia et al.

There are limitations based on thermodynamics. Nevertheless, our work has addressed fundamental issues that people have been trying to resolve for a long time.

—Linda Nazar, Canada Research Chair of Solid State Energy Materials and senior author

The high theoretical-energy density of lithium-oxygen (Li-O₂) batteries and their relatively light weight have made them a key focus for R&D for next-generation battery systems, especially for EVs. However, long-standing issues with the battery’s chemistry and stability have kept them in the realm of academia.

Two of the more serious issues involve the intermediate of the cell chemistry (superoxide, LiO₂) and the peroxide product (Li₂O₂) reacting with the porous carbon cathode, degrading the cell from within. In addition, the superoxide consumes the organic electrolyte in the process, which greatly limits the cycle life.

Nazar and her colleagues switched the organic electrolyte to a more stable inorganic molten salt and the porous carbon cathode to a bifunctional metal oxide catalyst.

We demonstrate that by increasing the operating temperature and exploiting stable inorganic electrolytes and ORR catalysts, the reversible formation of Li₂O leads to a highly rechargeable Li-O₂ cell with high capacity, low overpotential with transfer of 4 e^–/O₂, and excellent cycling performance.

—Xia et al.

By operating the battery at 150 ˚C (302 ˚F), they found that the more stable product Li₂O is formed instead of Li₂O₂. This results in a highly reversible Li-oxygen battery with coulombic efficiency approaching 100 per cent.

By storing O₂ as lithium oxide (Li₂O) instead of lithium peroxide (Li₂O₂), the battery not only maintained excellent charging characteristics, it achieved the maximum four-electron transfer in the system, thereby increasing the theoretical energy storage by 50%.

By swapping out the electrolyte and the electrode host and raising the temperature, we show the system performs remarkably well.

—Linda Nazar

Resources

• C. Xia, C. Y. Kwok, L. F. Nazar (2018) “A high-energy-density lithium-oxygen battery based on a reversible four-electron conversion to lithium oxide” Science Vol. 361, Issue 6404, pp. 777-781 doi: 10.1126/science.aas9343