Batteries supply electrons by undergoing reversible chemical reactions. That has meant that all the reactants have to be inside the battery, which adds to its weight and volume. Lithium-air batteries could potentially change that situation. At one electrode, they have pure lithium metal rather than a lithium-containing chemical. At the other, the lithium reacts with oxygen in the air. When the battery is charged, this reaction is reversed, and the oxygen is returned to our atmosphere.
With far fewer chemicals permanently inside the battery, it's possible to achieve a much higher energy density—there have been demonstrations of lithium-air batteries with an energy density five times that of current lithium-ion tech. The only drawback? They have a lifespan of about a month, in part because both oxygen and metallic lithium are pretty reactive and in part because air offers a lot of things other than oxygen that can react.
Now, a team of researchers has figured out a way to protect against many of these reactions and showed that the resulting battery can survive hundreds of charge/discharge cycles in an air-like atmosphere. Which probably means the researchers are ready to figure out what goes wrong when this material meets actual air. The hope is that will be an easier issue to solve.
Side reactions
The work was done by a US-based collaboration between academics and Argonne National Lab. The focus was on blocking what are termed side reactions, or chemical reactions that don't contribute to the functioning of the battery and/or destroy some of its components. These can affect the electrodes, the lithium that carries charge, and the electrolyte that allows the lithium to transit between the electrodes. Complicating matters further, air contains a variety of chemicals—nitrogen, oxygen, water, and more—that can potentially participate in these side reactions.
To limit them, the researchers used a combination of computer simulations (using density functional theory) and building actual hardware. To simplify matters, they tested the hardware under an air-like atmosphere, with realistic percentages of oxygen, nitrogen, carbon dioxide, and water, but without any further potential complications.
The battery design they used had a lithium-metal anode. To protect that, the researchers coated it with lithium carbonate, which was extremely easy to do: they simply ran a few charge/discharge cycles under a pure carbon dioxide atmosphere. This formed a dense mesh of crystals on the surface of the lithium, and the simulations showed that these crystals were sufficient to block the transit of oxygen, carbon dioxide, and nitrogen. Getting these molecules across that barrier is energy expensive, which ensures that no reactions take place at the anode. Reactions with water are also energetically unfavorable. As a result, cycling the battery through a charge/discharge cycle returned 99.97 percent of the lithium to the anode, where it started.
For the electrolyte, the researchers used a mixture of organic chemicals that had been demonstrated in earlier work (1-ethyl-3-methylimidazolium tetrafluoroborate mixed with dimethyl sulfoxide). This turned out to provide a big contribution to the battery's stability by dispersing any gas or water molecules that ended up dissolved in it. Most of the reactions with the battery components required two or more molecules of water or carbon dioxide, but the simulations indicated that the electrolyte ensured that only single molecules were typically available at any surfaces.
Waiting for air
Finally, the researchers made their cathode out of molybdenum disulfide. This material forms a surface that's compatible with the product of the lithium-oxygen reaction, Li2O2, allowing it to form a thin film over the cathode. This film is inert in the presence of water and carbon dioxide, preventing unwanted reactions. Even when this film is dissociated into the electrolyte, it doesn't react efficiently with the water dispersed there.
The resulting battery showed stable behavior over hundreds of cycles—the authors tested one out to 700 cycles without a failure, although the potential gap gradually increased from 1.3V to 1.6V. They conclude that "the protected lithium anode, electrolyte blend, and high-performance air cathode all work in synergy to provide a lithium–oxygen battery with a long cycle life under simulated air conditions."
That last bit—the "simulated air conditions"—is kind of critical. There's no reason that the researchers wouldn't also have tested this battery using actual air and no reason they wouldn't have reported the results if it had done well. The lack of these results in the paper suggests it might not have performed as well when presented with actual air. But this work has clearly solved some of the biggest and most obvious problems with lithium-air batteries. There's a good chance that any remaining issues are comparatively minor.
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