Cathodes with disordered structures enhance stability in Li-ion batteries

SOPHIA CHEN Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Sophia Chen.  

As more people drive electric cars, the demand for lithium ion batteries is going up. However, these batteries have many design limitations. Bin Ouyang of Florida State University and his colleagues are trying to make a better lithium-ion battery.

Ouyang’s team is focusing on making a better negative terminal, or cathode, for lithium ion batteries. Commercial batteries use cobalt and nickel in their cathodes, where these metals give up electrons as the battery charges. 

However, nickel and cobalt are rare and expensive materials. For example, 60 percent of the world’s cobalt comes from one country, the Democratic Republic of the Congo, says Ouyang. This poses supply chain issues for lithium-ion batteries.  

BIN OUYANG This raises up a material science problem and whether we can find a new material that is much less expensive to replace the state-of-the-art cathode material.

CHEN In addition, when charging and discharging the batteries over many cycles, the cathode material will expand and contract in volume and degrade. Current cathode materials change in volume by about 5 percent, leading to cracks in the cathode and potentially battery failure.

In their new research, Ouyang and his colleagues simulated and then fabricated new cathode materials that don’t use cobalt or nickel and also degrade less after being charged and discharged. To achieve this, they found that they needed to design a material with disorder in its crystal structure.

OUYANG Statistically, it’s possible to find the two species or more than two species in a single crystalline symmetric site of a lattice.

CHEN You can think of their material’s structure in analogy with a Jengo tower. As you remove blocks from the tower, the tower keeps its shape. In a similar way, when you remove lithium from the tower, the disorder allows the cathode mostly to keep its shape.

OUYANG The structure can still hold and it does not really need to change the volume or the shape to compensate.

CHEN These disordered structures contrast with current cathode materials, which occur in alternating layers of lithium, oxygen, and then a transition metal such as cobalt or nickel. In these layered structures, as the lithium ions leave the cathode, the transition metal atoms tend to migrate to the holes left by the lithium. In this way, the cathode’s volume changes.

This doesn’t happen with their new cathode materials, consisting of vanadium and niobium, instead of cobalt and nickel, which they synthesized and tested for their volume change.

OUYANG Those disordered materials can really have a small change of volume. Some of the examples that were demonstrated here is like 0.7%. And we have the best case, which is only 0.2%, and which is verified to experiment.

CHEN To design these materials, Ouyang’s team turned to a first-principles simulation. He says they chose these particular compounds for the cathode because they were easy to synthesize. Ultimately, the materials in their paper may not be the best for commercial batteries. 

But that’s not the point. Ouyang says other researchers can use their model to look for other material candidates to further improve cathode material design. 

OUYANG We actually develop a model, which is as a function of composition and the species and charge states of different atoms in the materials and then you can calculate the volume change of any composition.

CHEN In future work, Ouyang plans to use these models to help design lithium ion batteries that are entirely made of solids. These solid-state batteries should be less flammable, and thus safer, than the liquid electrolyte found in lithium ion batteries today.

This work was published in a recent issue of Joule. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.