Electrochemical cells were used for study of the processes occurring in lithium-air batteries. (CREDIT - Daniil Itkis)
A team of researchers from the Faculties of Materials Science and Chemistry, Lomonosov Moscow State University, are involved in improving lithium-air batteries, which can greatly surpass the main parameters of lithium-ion systems. The research findings have been published in the Journal of Physical Chemistry C.
Scientists and manufacturers typically have to deal with the issue of elaborating batteries of new types: lighter and simultaneously more powerful and with higher energy storage capacity. One of the possible approaches to achieve that is to replace latest lithium-ion batteries with professed lithium-air batteries. Such batteries can accumulate five times more power compared to lithium-ion ones. One area that can be revolutionized is electric cars, which currently use lithium-ion batteries.
The researchers at the Lomonosov Moscow State University are focused on studying the methods of electrochemical oxygen reduction in a lithium-air battery. The functions of the lithium-air battery are explained briefly: when battery discharges, the negative electrode, signified by lithium foil, dissolves creating lithium ions, which travel through the electrolyte layer to the positive electrode.
The positive electrode is a porous carbon sponge that is wetted by the electrolyte. Atmospheric oxygen entering a cell from the environment dissolves in electrolyte and reaches the carbon positive electrode. Right at the interface of electrolyte and carbon, one of the main processes - electrochemical oxygen reduction - occurs.
Oxygen molecules accept electrons from the carbon material and subsequently connect with lithium ions. Consequently, a battery discharge product, namely, solid lithium peroxide is obtained, which settles within the pores in carbon. However, peroxide is not formed instantly. Initially, very active particles - superoxide anions - are created. Then after some time these species are transformed into the final product.
Lithium-ion battery operation is quite different. In contrast, lithium metal is not present in it: both in positive and negative electrodes, lithium is present in the form of ions. The specific energy of the lithium-ion battery realized for the study is approximately 220-240 W?h/kg (calculated per cell mass with battery case as well).
Active materials that accomodate lithium ions, add over half of the cell weight. The rest refers to current collectors, electrolyte, various additives, and casing materials. As active materials are not required for lithium ions found in a lithium-air battery, the mass of the battery is lower. Therefore, much higher specific energy can be realized in such batteries.
Elaboration of novel metal-air batteries with nonaqueous electrolytes and namely lithium-air power sources, after creating a lot of noise several years ago, has now reached a deadlock. It has turned out that oxygen reduction in such lithium-air batteries proceeds with many difficulties, through many stages and is accomplished by lots of side reactions. The desire, typical for many researchers and innovators, to provide the soonest commercialization of such batteries, which could exhibit much higher performance in comparison to lithium-ion ones, has failed due to the lack of deep understanding of the processes, taking place inside a battery.
Daniil Itkis, Ph.D. Senior Researcher, Lomonosov Moscow State University
Currently, it is not possible to recharge a lithium-air battery a number of times. After a couple of recharge cycles, the carbon positive electrode, where oxygen reduction and additional reaction with lithium ions occurs, becomes electrically passivated. It occurs because of side reactions with intermediate species - superoxide anions.
These particles are so active that they trigger reactions of electrolyte and carbon electrode oxidation. But in this process, materials are damaged and an electrolyte is wasted for these side reactions. Scientists will benefit if sore spots in the carbon material that suffer from such side processes are found, leading to development of usable lithium-air batteries, and as a result transferring them from laboratory to large-scale fabrication.
Previously, researchers from the Lomonosov Moscow State University partnered with American colleagues and discovered that oxygen reduction could be done differently, based on the peculiarities of the electrolyte used. Now Daniil Itkis' research team has demonstrated that the mechanism of the reaction could also differ based on the degree of the carbon electrode material imperfection.
In their study, the team compared how the process happens on various model graphitic electrodes. In their previous research, they guessed that an attack of superoxide anions on carbon materials began in places where there were defect spots in carbon. In the present study, the researchers proved this hypothesis in the electrolytes frequently used in lithium-air battery research.
In general, the result is disappointing as there are no carbon materials without imperfections in reality. It means that we should look for ways which can help to shift the region where the reaction proceeds, farther from the carbon material. Now we are actively thinking this matter over. I t's difficult to say now whether lithium-air batteries will be cheaper or more expensive than lithium-ion ones. We could assume that they will be cheaper. But the problem is often hidden in details. It can turn out that in order to reach rechargeability we'd have to add some very expensive additives. I could say that we won't get any prototypes till 2020-2025.
Itkis , Ph.D. S enior Researcher, Lomonosov Moscow State University