Conventional lithium-air batteries draw in oxygen from the outside air to drive 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 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 between 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 percent of the electrical energy is turned to heat. "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," Li says.
This approach helps overcome another issue with lithium-air batteries: As the chemical reaction involved in charging and discharging converts oxygen between gaseous and solid forms, the material goes through huge volume changes that can disrupt electrical conduction paths in the structure, severely limiting its lifetime.
The secret to the new formulation is creating minuscule particles, at the nanometer scale (billionths of a meter), which contain both the lithium and the oxygen in the form of a glass, confined tightly within a matrix of cobalt oxide. The researchers refer to these particles as nanolithia. In this form, the transitions between LiO2, Li2O2, and Li2O can take place entirely inside the solid material, he says.
The nanolithia 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.
Conventional lithium-air batteries, Li explains, are "really lithium-dry oxygen batteries, because they really can't handle moisture or carbon dioxide," so these have to be carefully scrubbed from the incoming air that feeds the batteries. "You need large auxiliary systems to remove the carbon dioxide and water, and it's very hard to do this." But the new battery, which never needs to draw in any outside air, circumvents this issue.