Lithium-air batteries break through technical bottlenecks and may become the next generation of battery technology standards

Last Thursday, researchers at the University of Cambridge in the UK published a document showing that they have developed a lithium-air battery that successfully solves some of the practical problems in this technology - especially the problem of chemical instability. Before this, due to this chemical instability, lithium-air batteries would show a rapid decline in performance. The lithium-air battery they developed has a high energy density and can be recharged more than 2,000 times. The battery has a theoretical energy efficiency of more than 90%.

Scientists are very hopeful that lithium-air batteries will one day replace the lithium-ion batteries we currently use. "Lithium-ion rechargeable batteries have been used for nearly 25 years," said Professor Clare P. Grey of the Department of Chemistry at the University of Cambridge by phone. "Twenty-five years ago, the more compact lithium-ion battery paved the way for the emergence of portable electronics, making the electronic devices we carry with us lighter and more portable. Lithium-ion battery technology was more suitable for consumers at the time, and now it is time for lithium-air batteries to replace it."

No chemist or engineer would say that lithium-ion batteries are perfect. As electric cars become more popular, researchers have begun to focus their efforts on lithium-air batteries. Because lithium-air batteries are much lighter than lithium-ion batteries, lighter cars mean longer driving range. To be sure, lithium-air batteries ideally have a higher energy density. In theory, only this type of battery can allow electric cars to have a driving range comparable to gasoline and diesel cars without having to carry a huge and bulky battery pack.

It will take another 10 years to be put into commercial use

In a press release, scientists at the University of Cambridge said that although their research has successfully overcome the biggest obstacle in lithium-air battery technology, it will take at least 10 years to put lithium-air batteries for commercial use.

The basic chemical principle of lithium-air batteries is very simple. During discharge, lithium ions from the negative electrode react with oxygen in the air at the positive electrode to produce a solid product called lithium peroxide, which fills the pores of the carbon electrode. During charging, the chemical process is reversed and the lithium peroxide is decomposed to release oxygen.

The prototype of lithium-air battery has been successfully manufactured a long time ago. Theoretically, the battery has a storage capacity 10 times that of lithium-ion batteries currently on the market. However, due to the extreme chemical instability of lithium metal, there are many major defects in practical application. How to reliably make the above reaction happen repeatedly over many cycles is the biggest challenge facing this technology.

The battery's reaction product, lithium peroxide, and the intermediate product, lithium superoxide, are both highly reactive and will decompose the electrolyte. Therefore, the battery capacity will drop sharply after several charge and discharge cycles, and the battery life is short. Since lithium peroxide has poor conductivity, it is difficult to decompose during charging and requires a very high charging voltage, which will also lead to side effects such as decomposition of the electrolyte and carbon electrode.

During discharge, lithium peroxide will block the porous carbon electrode, causing the discharge to end prematurely; during charging, the surface of the lithium metal negative electrode will grow toward the positive electrode in a dendrite-like manner, which may eventually cause a short circuit and pose a safety hazard; lithium metal will react with water vapor, nitrogen, and carbon dioxide in the air, resulting in the consumption of the negative electrode material and ultimately causing the battery to fail.

Improved chemical stability

Researchers at the University of Cambridge switched to using multi-layered macroporous graphene as the positive electrode material, and used water and lithium iodide as electrolyte additives. What was ultimately produced and decomposed was lithium hydroxide, rather than the lithium peroxide in previous batteries. Lithium hydroxide is more stable than lithium peroxide, which greatly reduces side reactions in the battery and improves battery performance. In addition to helping decompose lithium hydroxide, lithium iodide also seems to play a role in protecting the lithium metal negative electrode, making the battery immune to excess water. Without it, the same amount of water would directly cause the battery to fail, making it completely impossible to charge and discharge. Because graphene oxide is porous, researchers estimate that this battery can be cycled more than 2,000 times.

The researchers said at a press conference that they have reduced the voltage gap in lithium-air batteries to 0.2V, successfully improving battery performance and efficiency. The lithium-air battery model they developed has a storage capacity of about 3,000 watt-hours per kilogram, which is about 8 times that of existing lithium-ion batteries. It can be charged and discharged thousands of times, and the first cycle charge and discharge efficiency is as high as 93%, that is, 93% of the energy charged into the battery can be used during discharge.

There are still technical difficulties to be overcome

However, there are still some problems with current lithium-air batteries. The reduction in voltage gap and the large capacity of graphene oxide electrodes result in them only being able to accommodate a smaller rate of charge and discharge, and the metallic lithium at the negative electrode of the battery sometimes still forms dendrites that affect battery performance. Moreover, as we mentioned earlier, there is more than just oxygen in the air. Other compounds in the air may also cause lithium-air batteries to be unstable.

The fact that these problems have not yet been solved also means that lithium-air batteries cannot be truly put into commercial use. It is easy to develop new battery technology, but it still requires overcoming many technical difficulties to put it into use. The researchers said they are currently working with several companies to strive to advance this technology as soon as possible.

Via arstechnica