Many might be familiar with their smartphones or other electronic devices losing the charge over time, but the lithium-ion batteries that power them actually forego a sizable chunk of their capacity before they even get started. This is caused by impurities that form during their very first cycle, but a team led by the Nobel-Prize-winning inventor of rechargeable lithium batteries may have found a solution to this problem, in the form of a novel coating that protects against these losses at the outset.
The impurities that underly these issues with lithium batteries can be found in the nickel-rich cathodes on the positive side of the device, which serve as one of its two electrodes. The nickel in this component is key to the impressive energy density of lithium-ion batteries, but it is also unstable.
This causes impurities to form on the cathode surface during the very first charge and discharge cycle, which in turn immediately reduce the battery's storage capacity by 10 to 18 percent. Additionally, the nickel creates instabilities beneath the surface within the cathode structure, which over time, also start to degrade the battery's storage capacity.
In 2019, Stanley Whittingham won the Nobel Prize in Chemistry along with two other scientists for the development of the lithium-ion battery in the 1970s. The technology has come a long way since, but researchers, including Whittingham, are still working to improve them by experimenting with different materials, and a promising one for use as the cathode is a nickel-manganese-cobalt material called NMC 811.
Whittingham led a team of researchers from the State University of New York at Binghamton, the Department of Energy and Oak Ridge National Laboratories in testing out an experimental chemical treatment for NMC 811, involving lithium-free niobium oxide. The hope was that this would prevent the instabilities in the cathode, something the researchers investigated through X-ray and neutron diffraction studies.
“Neutrons easily penetrated the cathode material to reveal where the niobium and lithium atoms were located, which provided a better understanding of how the niobium modification process works,” says study author Hui Zhou. “The neutron scattering data suggests the niobium atoms stabilize the surface to reduce first-cycle loss, while at higher temperatures the niobium atoms displace some of the manganese atoms deeper inside the cathode material to improve long-term capacity retention.”
The chemical treatment stabilized the structure to reduce the capacity loss that normally takes place during the first charging cycle. Ultimately, it provided better performance over the longer term, too, leading to a capacity retention of 93.2 percent over 250 charging cycles. The scientists see plenty of potential in the new battery design, especially where high-density storage is a priority, such as in the world of electric transportation.
“The improvements seen in electrochemical performance and structural stability make niobium-modified NMC 811 a candidate as a cathode material for use in higher energy density applications, such as electric vehicles,” says Whittingham. “Combining a niobium coating with the substitution of niobium atoms for manganese atoms may be a better way to increase both initial capacity and long-term capacity retention. These modifications can be easily scaled-up using the present multi-step manufacturing processes for NMC materials.”
The research was published in the journal ACS Energy Letters.
Source: Oak Ridge National Laboratory