Neutron depth profiling technique nurtured at NIST can improve battery technology
The newest generation of lithium-ion batteries now being developed uses thin-film, solid-state technology and could soon safely power cell phones, electric vehicles, laptops, and other devices. However, like all batteries, solid-state lithium-ion batteries have a drawback: Impedance—electrical resistance—can build up as batteries are discharged and recharged, limiting the flow of electric current.
Researchers at the National Institute of Standards and Technology (NIST), together with collaborators at Sandia National Laboratories, the Naval Research Laboratory, and several universities, recently used two complementary techniques—contact potential difference measurements and neutron depth profiling (NDP)—to determine which parts of the battery contribute most to impedance. Their work was published in the journal ACS Energy Letters and was described in a November 16 news release from NIST.
Depiction of the three-step neutron depth profiling reaction in lithium-6 (not to scale). (Image: J. Weaver/NIST)
A little-known technique: NDP uses a beam of cold neutrons generated by the NIST Center for Neutron Research (NCNR) to probe the nanoscale distribution and concentration of certain isotopes, including lithium-6. When low-energy neutrons are absorbed by lithium-6, they produce energetic charged particles—an alpha particle (helium-4) and tritium (hydrogen-3). The charged particles generated and the energy they retain after passing through surrounding material—in this case through layers of the battery—produce a snapshot of the isotope’s dispersion in the sample. The nondestructive technique can be used while a battery is running.
A lithium-ion battery has two sheetlike terminals, the anode (negative terminal) and the cathode (positive terminal), separated by an ion-conducting medium called the electrolyte. NDP measurements revealed that lithium ions were piling up and impeding the electric current at the boundary between the electrolyte and the anode. The researchers concluded that the impedance could be significantly reduced if layers of other material were added in between the anode and the electrolyte.
“This work demonstrates that neutron depth profiling, combined with Kelvin probe force microscopy and theoretical modeling, continues to advance our understanding of the inner workings of lithium-ion batteries,” said Jamie Weaver, a radioanalytical chemist at NIST and one of the coauthors of the paper.
NIST’s NDP expert explains the science: Weaver joined NIST as a National Research Council postdoctoral associate in 2017 and was appointed the NDP instrument custodian in 2018. NIST has been developing and fine-tuning NDP for decades, but the technique is unfamiliar to many, even within the nuclear science community. In a November 17 blog post, “Detecting the Flavors of Important Elements With Neutron Depth Profiling,” Weaver explained that NDP is a radiochemistry technique that to date works with only a handful of isotopes: lithium-6, helium-3, boron-10, and nitrogen-14.
“The definition I most often give is that it is a technique that uses neutrons to measure both the amount and physical distribution of select atomic elements in a material, without destroying the material,” Weaver said.
“While it is true that the pool of elements detectable by NDP is small, the elements it can measure are incredibly important to sustaining our modern lifestyles,” Weaver explained. “And while there are other techniques that can measure these elements, none can do so across the dynamic concentration range achievable by NDP and do it in a way that doesn’t change or destroy the sample.”
