In a new concept for battery cathodes, nanometer-scale particles made of lithium and oxygen compounds (depicted in red and white) are embedded in a sponge-like lattice (yellow) of cobalt oxide, which keeps them stable. The researchers propose that the material could be packaged in batteries that are very similar to conventional sealed batteries yet provide much more energy for their weight. Courtesy of the researchers. Click to enlarge.

It also displays stable cycling performance (only 1.8% loss after 130 cycles in lithium-matched full-cell tests against Li₄Ti₅O₁₂ anode), as well as a round-trip overpotential of only 0.24 V. Further, the cathode is automatically protected from O₂ gas release and overcharging through the shuttling of self-generated radical species soluble in the carbonate electrolyte.

Because gas evolution and phase change between O₂ (gas) and O^x− (condensed phase) are required at the cathode in Li–air batteries, the nucleation and growth of such phase changes with a 10⁴-fold difference in specific volume entail a huge overpotential (η, the difference between practical and theoretical potential values) with η_discharging > 0.1 V in O₂ (gas) → O^x− (condensed phase), and η_charging > 1.1 V in O^x− (condensed phase) → O₂ (gas). The alarmingly large η charging indicates severe kinetic bottlenecks in gas-evolving solid products (for example, Li₂O and Li₂O₂) being dynamically dismantled during charging . The serious overpotential loss of charge and discharge (>1.2 V) causes severe energy efficiency and thermal management problems. Repeated phase changes with large overpotential also cause chemo-mechanical damage that limits cyclability.

Here we develop an oxygen anion-redox cathode that does not release/take O₂ (gas).

—Zhu et al.

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% 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.

—Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering at MIT

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 key to the new formulation is creating particles at the nanometer scale 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.

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.

The new battery is also inherently protected from overcharging, the team says, because the chemical reaction in this case is naturally self-limiting. When overcharged, the reaction shifts to a different form that prevents further activity.

With a typical battery, if you overcharge it, it can cause irreversible structural damage or even explode. [With the nanolithia battery] we have overcharged the battery for 15 days, to a hundred times its capacity, but there was no damage at all.

—Ju Li

In cycling tests, a lab version of the new battery was put through 120 charging-discharging cycles, and showed less than a 2% loss of capacity, suggesting that such batteries could have a long useful lifetime. And because such batteries could be installed and operated just like conventional solid lithium-ion batteries, without any of the auxiliary components needed for a lithium-air battery, they could be easily adapted to existing installations or conventional battery pack designs for cars, electronics, or grid-scale power storage.

Because these “solid oxygen” cathodes are much lighter than conventional lithium-ion battery cathodes, the new design could store as much as double the amount of energy for a given cathode weight, the team says. And with further refinement of the design, Li says, the new batteries could ultimately double that capacity again.

All of this is accomplished without adding any expensive components or materials, according to Li. The carbonate they use as the liquid electrolyte in this battery “is the cheapest kind” of electrolyte, he says. And the cobalt oxide component weighs less than 50% of the nanolithia component. Overall, the new battery system is “very scalable, cheap, and much safer” than lithium-air batteries, Li says. The team expects to move from this lab-scale proof of concept to a practical prototype within about a year.

This is a foundational breakthrough, which may shift the paradigm of oxygen-based batteries. In this system, commercial carbonate-based electrolyte works very well with solvated superoxide shuttles, which is quite impressive and may have to do with the lack of any gaseous O₂ in this sealed system. All active masses of the cathode throughout cycling are solid, which presents not only large energy density but compatibility with the current battery manufacturing infrastructure.

—Xiulei Ji, assistant professor of chemistry at Oregon State University, who was not involved in this work

The research team included MIT research scientists Akihiro Kushima and Zongyou Yin; Lu Qi of Peking University; and Khalil Amine and Jun Lu of Argonne National Laboratory in Illinois. The work was supported by the National Science Foundation and the US Department of Energy.

Resources

• Zhi Zhu, Akihiro Kushima, Zongyou Yin, Lu Qi, Khalil Amine, Jun Lu & Ju Li (2016) “Anion-redox nanolithia cathodes for Li-ion batteries” Nature Energy 1, Article number: 16111 doi: 10.1038/nenergy.2016.111