The serious drawbacks of lithium-air batteries include wasting much of the injected energy as heat and degrading relatively quickly. They also require extra components for oxygen gas pumping, in an open-cell configuration that is very different from conventional sealed batteries.
Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering at MIT, explained the shortcomings of lithium-air batteries as the mismatch between the voltages involved in charging and discharging the batteries. The batteries’ output voltage is more than 1.2 volts lower than the voltage used to charge them, which represents a significant power loss incurred in each charging cycle. “You waste 30% of the electrical energy as heat in charging. It can actually burn if you charge it too fast,” said Li.
In a conventional lithium-air battery, oxygen from the outside air drives a chemical reaction with the battery’s lithium during the discharging cycle, and this oxygen is then released again to the atmosphere during the reverse reaction in the charging cycle.
In the new nano-lithia cathode battery variant, the same kind of electrochemical reactions take place between lithium and oxygen during charging and discharging, but they take place without ever letting the oxygen revert to a gaseous form.
Instead, the oxygen stays inside the solid and transforms directly among its three redox states, while bound in the form of three different solid chemical compounds—Li2O, Li2O2 and LiO2—which are mixed together in the form of a glass.
This reduces the voltage loss by a factor of five, from 1.2 volts to 0.24 volts, so only 8% of the electrical energy is turned to heat, according to Li: “This means faster charging for cars, as heat removal from the battery pack is less of a safety concern, as well as energy efficiency benefits.”
The secret to the new formulation is creating nanometer particles called nano-lithia, which contain both the lithium and the oxygen in the form of a glass, confined tightly within a matrix of cobalt oxide. Transitions among LiO2, Li2O2 and Li2O can take place entirely inside the solid material, according to Li.
The nano-lithia particles would normally be very unstable, so the researchers embedded them within the cobalt oxide matrix, a sponge-like material with pores just a few nanometers across. The matrix stabilizes the particles and also acts as a catalyst for their transformations.
The new battery never needs to draw in any outside air.
The new battery is also inherently protected from overcharging because the chemical reaction in this case is naturally self-limiting—when overcharged, the reaction shifts to a different form that prevents further activity. “We have overcharged the battery for 15 days, to a hundred times its capacity, with no damage at all,” said Li.
In cycling tests, a lab version of the new battery was put through 120 charging-discharging cycles, and showed less than 2% loss of capacity, indicating that such batteries could have a long, useful lifetime. Such batteries could be installed and operated just like conventional, solid, lithium-ion batteries, and could be easily adapted to existing installations or conventional battery pack designs for cars, electronics or even grid-scale power storage, according to Li.
The much lighter “solid oxygen” cathode design could store as much as double the amount of energy for a given cathode weight, and with further refinement of the design, the new batteries could ultimately double that capacity again, according to Li.
The carbonate used as the liquid electrolyte “is the cheapest kind,” and the cobalt oxide component weighs less than 50% of the nano-lithia component, making the new battery system “very scalable, cheap and much safer” than lithium-air batteries, according to Li.
The team expects to move from this lab-scale proof of concept to a practical prototype within about a year.
The nano-lithia cathode battery is described in the journal Nature Energy in a paper by Ju Li; postdoctoral scholar Zhi Zhu; and five others at MIT, Argonne National Laboratory and Peking University in China. The work was supported by the National Science Foundation and the U.S. Department of Energy.