New battery concept promises more compact energy storage

Lithium-ion batteries are used everywhere – in mobile phones, in laptops, in electric cars. It is a common experience that the capacity of your battery is never quite large enough: You usually have to charge your phone every day, and electric cars are limited in their range. A key challenge for battery research is therefore to increase the amount of energy stored per weight or volume (the so-called energy density), allowing more energy to be stored without increasing the size of the battery.

Lithium-ion-batteries store energy through a process involving two electrochemical reactions: in the first, an externally supplied electrical current drive out lithium ions from a lithium-containing compound (generally referred to as the cathode); the lithium ions then travel through the electrolyte and, in another electrochemical reaction, into the anode which is usually made of graphite. During discharge the opposite process occurs, accompanied by a flow of electricity in the external circuit. At present, the main limiting component for increasing the energy density is the cathode.

Now, a new European Horizon 2020 research project LiRichFCC sets out to change the way lithium ions are stored in the cathode. In traditional cathodes (based on transition metal oxides) the lithium ions go into the empty spaces between the atoms of the existing crystal lattice of the host material, by a process called intercalation. At the same time, the host has to accommodate the accompanying electrons without changing structure. This severely limits the amount of lithium that can go into the cathode. The new oxyfluoride materials to be studied in the project have a completely different mechanism for lithium storage. The materials, which have a crystal structure known as face-centered cubic (FCC),  can incorporate lithium directly into their crystal lattice. This not only allows a much higher energy density (since more lithium can be stored in the cathode) but it also leads to fast transport of lithium in and out of the structure, which in turn means that batteries will be able to deliver high amounts of energy in a short time. Similarly, the time to recharge the battery will be much shorter.

The so-called Li-rich FCC materials were only discovered recently and have not been investigated systematically yet. This will be done in LiRichFCC which will focus on the fundamental insights needed to understand and improve the new materials. The project aims to demonstrate a doubling of the energy density compared to conventional Li-ion batteries using traditional transition metal oxide cathodes. This will make a significant impact on applications such as electric vehicles and portable electronics. DTU Energy will contribute with computational predictions of new materials compositions, identification of the transport mechanisms in the materials and at their interfaces, and will also take part in the experimental characterization of the materials developed in the project.