Next-generation battery chemistries promise higher energy densities, enabling electric cars to drive further and mobile devices to last longer during usage while retaining efficient charging speeds and long lifetimes. Currently, the biggest player in rechargeable battery technologies is lithium-ion batteries (LIBs). These are used in many applications, from electric vehicles to handheld vacuum cleaners. While improvements to this technology are a continuing topic of study, increasing demand for batteries with high energy densities also necessitates the exploration of new chemistries.

One promising new chemistry is the lithium-air battery, where lithium ions react with oxygen in the air to produce electrical energy and form lithium peroxide. On charging, electrical energy is applied to the battery to remove the lithium peroxide and recover the lithium ions and oxygen gas. This is opposed to LIBs, which shuttle and store lithium ions in transition metal oxides to produce or store energy. Since lithium-air batteries do not require heavy transition metal oxide electrodes, the overall energy per weight is significantly increased. In an electric vehicle, this energy density is particularly attractive as it enables lighter loads and means users can drive further per charge.

For lithium-air batteries, one important factor affecting the overall efficiency, rate performance, and energy provided is how efficiently oxygen is transported to and from the electrode surface. In this process, oxygen gas from the air must diffuse into the battery and to the electrode where it will react with lithium ions and electrons. If this transport is slow the power output of the battery will be very low.

Additives can improve battery performance and some researchers have drawn inspiration from haemoglobin in the blood. Hemes are a class of biomolecules that contain an iron atom at the center. This iron center can bind to (and release) oxygen. In the blood, heme transports oxygen from the air in our lungs to other parts of the body. Similarly, by adding these molecules into a Li-air battery, oxygen transport from the air to the electrodes can be greatly improved. This allows the battery to be cycled at higher rates, resulting in faster charging.

“Bio-inspired additives such as hemoglobin are a promising performance enhancer for lithium-air batteries”

Heme groups are also known to undergo redox reactions with peroxides. Redox reactions, short for reduction-oxidation reactions, correspond to reactions in which electrons are transferred from the reductant to the oxidant. These electron-transfer reactions between hemes and peroxides are typically very fast and are toxic to red blood cells. On the other hand, in a Li-air battery, the redox reactions of heme are a major performance improving property. Deleterious side reactions often take place in a Li-air battery where electrons are transferred to other species in the battery, instead of to the oxygen or peroxides. By aiding in electron transfer directly between the electrode, oxygen, and the peroxide, additives help avoid undesirable side reactions.

Several research groups have investigated the effects of heme biomolecules as redox mediators and oxygen transport enhancers in lithium-air batteries. One study showed stable battery cycling when using heme additives. Their results demonstrated that the reversibility of the lithium and oxygen reactions is greatly improved with the heme biomolecules. This means that a battery with heme additives can be charged and discharged for many more cycles than a battery without.

Another detailed study into the mechanisms of heme in the lithium-air battery used ultra-violet absorption measurements to determine the oxygen-bound and oxygen-free states of the heme molecule. They then used these measurements while charging or discharging a lithium-air battery to understand the ongoing chemical reactions. From their results, they hypothesized that a complicated, multi-step reaction takes place. The heme molecules first engage in redox reactions by accepting (or donating) electrons from the electrode, and then subsequently bind to oxygen. Both the oxygen-binding and electron transfer aided in improving the lithium-air battery performance. However, the authors also demonstrate further optimisation of this system is required, as the beneficiary reactions of the heme molecules are sensitive to battery composition.

In a related study, researchers synthesized catalysts for lithium-air batteries using a hemoglobin precursor extracted from blood waste. They presented various synthetic methods and conditions for the catalysts and later tested their electrochemical performance. When they used the catalysts in lithium-air batteries, charging became more efficient and there were fewer undesired side reactions. Their work suggests an interesting method of recycling bio-waste as well as promoting efficient energy storage from bio-inspired molecules.

There are several major challenges in adapting these biomolecules to lithium-air batteries. First, the detailed reaction steps need to be fully characterised and understood in order to better tailor the molecules. Next, the concentrations of these additives need to be optimised. In addition, the molecule must not degrade during charge and discharge, nor over time.

As demand for energy storage continues to increase, research into high energy-density battery chemistries are becoming increasingly relevant. Bio-inspired additives such as hemoglobin are a promising performance enhancer for lithium-air batteries. Hemes are able to improve both the oxygen transport and electron transfer at the electrode, thus enabling efficient reactions, with a high cycle life, high rate, and high energy density. Further studies are necessary to understand the mechanisms, full benefits, and drawbacks of using these additives in lithium-air batteries.