Stone salts can potentially double the capacity of Li batteries

Lithium batteries have steadily growing impact on our everyday lives, from batteries in smartphones and computers over electric cars and kitchen utensils, where low weight and long service life are of great importance, as well as large scale storage of surplus energy from renewable energy sources like wind turbines and solar cells. A high capacity electrode material based on vanadium oxyfluoride, Li₂VO₂F, has recently attracted considerable interest as it can potentially double the capacity of lithium battery cathode materials, thereby having a potentially great impact on modern society.

Batteries release power when discharged as positively charged ions move inside the battery from anode (-) to the cathode (+). The new material can release twice as many electrons as the existing cathode types for lithium batteries used in electric cars, cell phones, etc.

Twice as many movable electrons

Existing types of batteries usually use cathode materials of lithium containing manganese, nickel and/or cobalt oxides, or lithium iron phosphates, and normally, when a lithium battery is charged, a transition material occurs.  When charging lithium ion batteries, positively charged electrons are added to the system, as ex. iron atoms in the positive electrode change from Fe2+ to Fe3+, and lithium is bound in the negative electrode. The electrical energy supplied by the battery is thus stored as chemical energy, explains Professor Poul Norby from DTU Energy. He headed the recently finished EU-funded research project Hi-C (www.hi-c.eu) that studied interfaces in batteries and supercapacitors.

The interfaces of batteries are important for the stability, storage and, in particular, transport of ions and electrons, and the Hi-C project had several breakthroughs, one of which was the discovery by researchers from Karlsruher Institut für Technologie (KIT) of the materials based on a stone salt structure, the Li₂VO₂F.

Uses of battery materials are typically limited by the fact that each metal atom can only deliver or receive one additional electron, such as iron atoms changing from Fe²⁺ to Fe³⁺ without the structure of the material changing. However, vanadium in synthesized Li₂VO₂F can be oxidized from V³⁺ -> V⁴⁺ and again from V⁴⁺ -> V⁵⁺, enabling extraction of twice as many electrons from Li₂VO₂F material than from iron-based materials and thereby potentially doubling the capacity of Lithium battery cathodes.

Another major positive thing about the new material is the ability to move the Li-ions without altering the bindings or volume in the material. Unlike more conventional materials, ions are extracted from a dense material, and it is not easy to explain why it works as well as it does, explains Professor Poul Norby.

Ion-movement without alterations

Professor Poul Norby compares the classic Li-based battery materials with a sponge or a game card, where acceptance and delivery of ions is done in an intercalation process in 3-dimensional or layered materials.

“Imagine a sponge with water. You squeeze the sponge hard and drain it for water, but what you have left is still a sponge. Or imagine a stacked game card where you stick a knife between two of the cards and take it out again. The cards have not changed form and the layered structure is the same. It's incredibly smart because it means charging and unloading a battery without the materials changing structure or shape. In the new materials, the process of extracting lithium is a little different, but the structure is still surprisingly stable even if you pull 2/3 of the cations out of the packaged material. This is one of the reasons why we do both experimental and theoretical research in the studies of these materials," says Professor Poul Norby.

The discovery of the stone salt vanadium oxyfluoride, Li₂VO₂F, has led KIT, DTU Energy and three other partners from Hi-C to continue their cooperation and research in how lithium ions are stored in the cathode, This is done in the EU-funded Horizon 2020 research project LiRichFCC (LiRichFCC, “A new class of powerful materials for electrochemical energy storage: Lithium-rich oxyfluorides with cubic dense packing”).

“We are quite confident and optimistic regarding the new materials", says Professor Poul Norby.

The finished project Hi-C had participation of eight European universities and companies from Sweden, the UK, Denmark, Germany and France. The budget was 6.3 mio. Euro, of which 4.6 mio. Euro came from EU.