The batteries that power our cell phones, laptops, and electric vehicles rely on cobalt. Cobalt mining is problematic for the environment and miners. In order to find other solutions for lithium-ion (Li-ion) batteries that move away from a dependency on cobalt, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have participated in a collaborative study to identify new potential materials for the positive terminal of a battery, called a cathode.
In a battery, Li-ion is inserted into a cathode during charging and released during discharging, providing electricity. The new cathodes offer two advantages: They are both cobalt-free and stable, which means that they do not undergo a structural failure, such as cracking, as they are repeatedly charged and discharged.
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 Li-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. These metals include molybdenum, niobium, and titanium.
“You can think of building a cathode like building a house out of different kinds of bricks,” said Argonne Physicist Wenqian Xu. “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 percent 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.
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 Li-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.”
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