Making order out of disorder with new lithium cathodes

Californian scientists have reported major progress in the development of cathodes made with so-called ‘disordered’ materials, which could lead to a promising new type of lithium-ion battery. Their research has been published in the journals Physical Review Letters and Nature Communications.

The cathode material in lithium batteries is typically ‘ordered’, meaning the lithium and transition metal atoms are arranged in neat layers, allowing lithium to move in and out of the layers. A few years ago, scientists led by Gerbrand Ceder at the Lawrence Berkeley National Laboratory discovered that certain types of disordered material could store even more lithium, giving batteries higher capacity.

Yet despite their attractive properties, “discovering new disordered materials has been mostly driven by trial and error and by relying on human intuition”, revealed Alexander Urban, lead author of the PRL paper. “Now we have for the first time identified a simple design criterion to predict novel disordered compositions. The new understanding establishes a relationship between the chemical species, local distortions of the crystal structure and the tendency to form disordered phases.”

One advantage of using disordered materials is the ability to avoid the use of cobalt, a limited resource, with more than half the world’s supply existing in politically unstable countries. By moving to disordered rock salts, battery designers could be free to use a wider range of chemistries. For example, disordered materials have been made using chromium, titanium and molybdenum.

“We want the ability to have more compositional freedom, so we can tune other parameters,” said Ceder. “There are so many properties to optimise — the voltage, the long-term stability, whether it’s easy to synthesise — there’s so much that goes into taking a battery material to a commercial stage. Now we have a recipe for how to make these materials.”

Another major advance in lithium-ion batteries is reported in the Nature Communications paper, which shows that disordered materials can be fluorinated, unlike other battery materials. Fluorination confers two advantages: it allows more capacity and makes the material more stable. In a battery, the increased stability would translate into a device with long cycle life and that is less likely to catch fire.

The lead author of the paper, former Berkeley Lab researcher Jinhyuk Lee, worked with scientists at Berkeley Lab’s Advanced Light Source (ALS), a source of X-ray beams for scientific research, to conduct in situ experiments. As noted by Ceder, “The ALS was really important to understand the mechanism by which we get higher capacity.

“What’s really cool is you can look at the battery while it’s operating, and look at the electronic structure of the cathodes,” Ceder continued. “So you learn how it charges and discharges, where the electrons go, which is a crucial aspect of charge storage.”

According to Ceder, the ALS research was crucial in showing that the disordered materials “are more stable and don’t lose oxygen”. Now that his team has demonstrated the concept, Ceder plans to follow up by trying to add even more fluorine to the materials.

“New cathode materials is the hottest direction in Li-ion batteries,” Ceder said. “The field is a bit stuck. To get more improvements in energy storage there are only a few directions to go. One is solid-state batteries and the other is to keep improving the energy density of electrode materials. The two are not mutually exclusive. This research line is definitely not exhausted yet.”