A new study appears to identify potential alternative materials to use in the positive terminals of consumer electronics batteries, replacing cobalt.
The batteries used in cell phones, laptops and electric vehicles rely on cobalt, a hard-to-find metal that is largely drawn from poorly regulated African mines, and associated with considerable environmental destruction, exploitative labour practices and other problems in places like the Democratic Republic of the Congo.
The new cathodes are said to offer the advantages of being both cobalt-free and stable, and so less afflicted with a propensity towards structural failure, such as cracking, as they are repeatedly charged and discharged – a tradeoff that has limited progress in this direction to date.
As a battery constituent, cobalt offers thermal stability, which means it functions even as it is heated to higher temperatures, as well as structural stability. Researchers have been looking for different materials that could offer these same advantages without cobalt’s flaws.
In the new study, a research team led by the University of California, Irvine created and analyzed a material for a lithium-ion cathode that uses no cobalt and is instead rich in nickel. This cathode chemistry is compositionally complex, meaning that it contains small amounts of a wide range of other metals, including molybdenum, niobium and titanium.
The research team used the resources of the Advanced Photon Source (APS), a DOE Office of Science user facility at the US Department of Energy’s (DOE) Argonne National Laboratory. A paper based on the study appeared in a September issue of Nature.
“You can think of building a cathode like building a house out of different kinds of bricks,” said Argonne physicist Wenqian Xu, a co-author on the paper. “By having a variety of different shapes and sizes of bricks, we can enhance the stability of the house. Multiple elements help to ensure the integrity of the cathode particles.”
The researchers wanted to investigate the structural and thermal stability of the new cathode. Other nickel-rich cathodes typically have poor heat tolerance, which can lead to oxidization of battery materials and thermal runaway, which could in some cases lead to explosions. Additionally, even though high-nickel cathodes can accommodate larger capacities, large changes in volume from repeated expansion and contraction can result in poor stability and safety concerns.
To test the new battery, the researchers cycled it more than a thousand times. They discovered that in the process, the cathode material underwent less than 0.5% of volume expansion. This is roughly a tenth of the volume expansion experienced by previous nickel-rich cathodes, which all had stability problems to varying degrees.
“Keeping the volume of the cathode consistent is essential for ensuring its stability,” said Argonne physicist Tianyi Li, a co-author on the paper.
To characterize the heat tolerance of the new cathode material, called HE-LMNO, the UC Irvine team used beamline 11-ID-C at the APS, with the support of Xu and Li, to examine what would happen to the material at high temperatures. As opposed to previous high-nickel cathodes, which showed severe nanocracking at high temperatures, the HE-LMNO undergoes a phase change that allows it to continue to perform and retain capacity. The HE in HE-LMNO stands for high-entropy, a characteristic that refers to the large number of different elements included in the alloy.
“The APS significantly advanced our understanding of the high-entropy doped material we studied,” said UC Irvine’s Huolin Xin, the lead author of the study. “Our results suggest the high-entropy effect is transferable to a broader class of compounds that could form the basis of new battery materials.”
According to Xu, the research could provide design rules for a host of new battery cathodes that could help reduce next-generation lithium-ion batteries’ reliance on cobalt. “We haven’t just found one new battery,” he said. “Really, by mixing different transition metals in the structure, we could potentially see many more interesting cathode candidates. There will potentially be some even better than we have already found.”
To create electrodes for the experiment, the researchers used Argonne’s Cell Analysis, Modeling and Prototyping (CAMP) Facility. “We collaborate with researchers spanning national labs, industry and academia by fabricating electrodes using commercial and novel materials. Our approach enables us to provide baseline electrodes and to validate promising chemistries using pilot scale equipment, which is critical for assessing advanced materials,” said Argonne battery scientist Steve Trask.