Surface-science modalities shed new light on lithium diffusion in battery materials
OCI Vacuum Microengineering, a Canadian manufacturer of specialist instrumentation for the surface analysis of thin films, is applying its collective domain knowledge and expertise to the study of localized lithium diffusion within a range of energy-storage materials. The hope is that the in-house R&D initiative, if translated into wide-scale commercial adoption across the battery supply chain, will yield game-changing analytical capabilities to fast-track the evaluation and optimization of next-generation electrode materials, interlayers and stabilizing compounds for lithium-based battery technologies.
In terms of project specifics, the OCI team is tracking solid-state lithium diffusion from a gas phase source into thin-film battery materials using two “analytical workhorses” of the surface-science world: Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). Deployed in tandem, the two modalities provide complementary insights on the sample under study, with AES interrogating the elemental composition of the near-surface environment (typically to a depth of 3–10 nm), while LEED determines the surface structure of single-crystalline materials via bombardment with a collimated beam of low-energy electrons (and subsequent observation of diffracted electrons on a fluorescent screen).
Established in 1990, OCI already has an international R&D customer base that employs its LEED and AES spectrometers to characterize all manner of nanomaterials. Key applications include 2D materials, organic thin films for electronic devices, advanced photovoltaics and magnetic thin films (for spintronic and superconducting applications) – in each case ensuring compatibility with almost any vacuum thin-film deposition system (including molecular beam epitaxy and chemical vapour deposition).
“Right now, the use of surface-science tools to evaluate lithium diffusion in energy-storage materials is a proof-of-principle endeavour on our part,” explains Jozef Ociepa, president and chief scientist at OCI. The goal, he adds, is to use real-world experimental data to educate prospective and existing customers about the utility of AES/LEED for their battery R&D programmes – and, in the process, open up new commercial opportunities for OCI. “We want to show battery manufacturers and advanced materials companies how LEED and AES can help them to look with ‘new eyes’ at battery performance – evaluating the fundamental physics of new anode and cathode materials, for example, in the early stages of the product development cycle.”
All of which is important given the battery industry’s relentless search for innovative electrode materials capable of accumulating more lithium ions in their crystalline structures, while also ensuring high lithium-ion mobility, stable charge cycling and extended operational lifetimes. “For sure, lithium-ion-based battery technology is a proven success, but there are still fundamental performance issues to address,” notes Ociepa. Those issues include low energy density, capacity degradation and dendrite growth (tree-like lithium structures that can lead to catastrophic battery failure). “The use of LEED and AES will open up a wider spectrum of analytical capabilities to better characterize the next generation of battery materials,” he adds.
Ociepa and colleagues have been developing their Lithium Diffusion Tester, which requires an ultrahigh-vacuum (UHV) operating environment, for the past 18 months and presented initial research findings for a range of materials at the Electrochemical Society (ECS) annual meeting in Atlanta, GA, in October last year (see “How fundamental physics drives battery performance”, below). Given that AES and LEED instruments are tried-and-tested OCI product lines, the technology breakthrough lies in the integration of multiple core building blocks into the diffusion test system – specifically, the AES/LEED configuration, the lithium evaporation source, sample stage cooling and heating, as well as the load-lock and glove box.
“The Lithium Diffusion Tester is now a turnkey system that’s ready to ship to customers with a six-month lead-time from order,” notes Ociepa. “We’re currently at the stage of implementing and validating the platform on a range of battery materials, including nanostructured silicon, silicon carbide and highly oriented pyrolytic graphite.”
Localization is key
Technology innovation is also ongoing, with the OCI team recently integrating a UHV-compatible electrochemical test cell alongside the Lithium Diffusion Tester. This extended configuration opens the way to in-situ surface characterization of individual battery electrodes using LEED and AES, with those components transferable from the electrochemical test cell to the diffusion test chamber without breaking vacuum conditions.
The big win here is the use of surface-science modalities to measure lithium diffusion within individual electrodes separately from the battery cell – a significant advance for battery makers, whose traditional electrochemical test methods track lithium diffusion across anode, cathode and electrolyte combined together in the cell. “Our AES/LEED approach offers unprecedented localization and a more granular picture to inform performance testing, degradation and failure analysis, and lifetime prediction measurements on candidate materials for next-generation batteries,” notes Ociepa.
Ultimately, concludes Ociepa, the combined modalities have the potential to generate unique data sets on lithium diffusion that industry can’t get any other way. “We think this capability will yield an alternative view on battery performance, fast-tracking uptake of new candidate materials while pinpointing critical failure points early in the product development cycle.”
How fundamental physics drives battery performance
Lithium transport in battery materials and subcomponents is among the key factors governing device performance, reliability and lifetime. To inform the product innovation cycle, it’s therefore instructive for scientists to study the fundamentals of solid-state lithium diffusion (defined as the process of lithium atom/ion migration under a concentration gradient and activated by thermal energy from atomic vibrations of the host structure at room temperature).
Understanding the passive lithium diffusion process also yields a better understanding of the active diffusion processes at the heart of lithium-based batteries (in the presence of an applied electrical potential). Fundamentally, it is expected that materials exhibiting good passive lithium diffusion properties will also exhibit attractive diffusion behaviour under the influence of an external potential.
In this context, OCI’s dual-modality Lithium Diffusion Tester offers a unique opportunity to observe the free movement of lithium atoms/ions into a solid sample and, in turn, to simplify the understanding of diffusion processes. That’s especially the case for single-crystal structures, in which the lithium diffusion process is promoted by interstitials, vacancies and dislocations within a lattice that is free from grain boundaries.
“Our AES/LEED approach enables us to categorize materials that are attractive for lithium diffusion based on the pure lattice component,” explains Ociepa. “The conditions which limit lithium diffusion – such as lithium oxidation and the presence of grain boundaries – can also be investigated selectively and independent of other factors.”
In their studies to date, OCI scientists have identified three categories of material versus the capacity for “natural” lithium diffusion: materials exhibiting rapid lattice diffusion and no effect on long-range structural order (e.g. pyrolytic graphite); moderate lithium diffusion and some effect on long-range order (e.g. silicon carbide, synthetic diamond, lithium niobate and titanium dioxide); and no lattice diffusion and a strong effect on long-range structural order (e.g. silicon, which requires a nanoengineering process to create a lithium diffusion path).
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