Packing plenty of energy in a small and light package, lithium-air batteries are a promising candidate for the battery of tomorrow: however, in their current state, these cells are still too complex, inefficient, and short-lived to be practical. Now, researchers at MIT have developed a new cell design that surmounts those obstacles and could lead to cheap high-performance batteries that power anything from personal electronics to fast-charging electric cars.
Commercial batteries are usually self-contained, but this is not the case for lithium-air batteries, which adopt a so-called "open-cell" design requiring oxygen to be constantly transferred to and from the cell. In the process, the oxygen also changes states – from gaseous to solid as it binds with lithium to store charge, and then from solid back to gaseous as charge is drained.
This setup, however, is inconvenient for at least three reasons. Firstly, humidity and carbon dioxide need to be filtered out of the incoming air to avoid damaging the cell. Secondly, the mismatch between the 3.7 V charging voltage and the 2.5 V discharge voltage means that 32 percent of the energy applied during charging is lost to heat (which can cause explosions if the cell is charged too quickly). Lastly, the constant switching between gaseous and solid forms of oxygen puts a great deal of mechanical stress on the cell, causing it to fail prematurely.
Professor Ju Li and team have now managed to circumvent these limitations by creating a lithium-air cell cathode that works in the much more practical self-contained design.
In the new cell, oxygen remains inside the cell and in a solid state at all times. The oxygen is bound to lithium to form a glass-like material. These molecules are in turn encased in a matrix of cobalt oxide, forming what the researchers call "nanolithia" – nanostructures that act as a catalyst for the chemical reactions that take place in the cell.
This setup reduces the charging loss from 32 to just eight percent, making the battery not only more efficient, but also able to be safely charged quickly. The scientists also saw a stable performance, with a less than 2 percent capacity loss after 130 charge/discharge cycles. What’s more, the cathode is extremely light, and the researchers believe that, with some work, these cells could soon end up storing much more energy per unit weight and volume than current commercial lithium-ion cells.
"Right now, the gravimetric [per unit weight] energy capacity of our dried cathode paste is about 30 percent more than dried cathode paste of mature Li-ion cathodes," Li told us. "The volumetric energy capacity of our dried cathode paste is about the same as that of mature Li-ion cathode. But drastic improvement in both can be expected in the next 12 months."
The researchers are hoping to go from proof of concept to a prototype within a year. Such a battery could find use for a wide range of applications including personal electronics, electric cars, and large-scale power grid storage – competing with lithium-ions not only in performance, but in cost as well.
"Since the raw material uses much less heavy transition metal elements like cobalt or nickel than traditional Li-ion cathodes, the eventual cost per kWh could be much cheaper," Li told us.
A study describing the advance appears in the journal Nature Energy.
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