Enhancing the stability of lithium-air batteries

Lithium-air batteries, also known as lithium-oxygen batteries, could be the next generation of high-energy electricity storage devices. Their theoretical energy storage capacity is higher than that of conventional lithium-ion batteries of the same weight, but they are not yet chemically stable enough to provide a reliable solution. Now, a collaborative research project in which a team from the University of Oldenburg, led by chemist Professor Dr Gunther Wittstock, is testing a new concept to extend the life of these battery cells.

The project is led by IOLITEC Ionic Liquids Technologies, a company based in Heilbronn, Germany. The MEET (Münster Electrochemical Energy Technology) Battery Research Centre at the University of Münster and the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in Bremen are also taking part. Lithium-air batteries work in the same way as conventional battery types, but in this type of battery the reaction of lithium ions with oxygen from the air at the positive electrode is used to generate electricity. The advantage is that lithium-air batteries can store almost as much energy per kilogram as fossil fuels. This means that they have a similar specific energy to today’s batteries but weigh less, which makes them attractive for use in electric cars as well as in stationary energy storage. Wittstock said that there are a number of technical problems to be solved before the batteries can be used in electric cars.

One of these challenges is the lack of electrolytes that are chemically stable at both the positive and the negative electrode. These conductive fluids or solids are located in the area between the two electrodes. In lithium-air batteries, one of the electrodes is made of metallic lithium while the other — called the gas diffusion electrode — consists of a porous network and conducting material where oxygen (O₂) from the air is reduced in an oxidation-reduction reaction. When the battery is discharging, positively charged lithium ions move across the electrolyte from one electrode to the gas diffusion electrode, where they combine with oxygen and electrons from an external electrical circuit to form lithium oxide. This generates an electric current which can be used to provide energy for electrical devices. During charging, lithium and oxygen separate once more and the ions and electrons travel in the opposite direction.

To increase the stability of the lithium-air battery, the project team aims to design a membrane that separates the positive electrode from the negative electrode, thus allowing different electrolytes to be used on either side — this could significantly expand the options for electrolytes. The scientists plan to test a separator with a special coating on each side that protects the lithium electrode and the gas diffusion electrode. The Oldenburg team led by Wittstock is using various methods, including surface spectroscopy and scanning electrochemical microscopy (SECM) to investigate the processes on the surfaces of the separator and electrodes. IOLITEC is developing the separating layer with a team from MEET Battery Research Centre at the University of Münster, led by Verena Küpers. “We are testing different coatings that are specifically adapted to the challenges posed by each type of electrode,” Küpers said.

Image caption: A lithium-air battery cell in front of a test bench. Image credit: Fraunhofer IFAM