Nanoscale Interface Characterization in Battery Materials with Vibrational Spectroscopy in a Scanning Transmission Electron Microscope

The growing incorporation of electric vehicles and portable smart devices in our lives has placed higher requirements on the safety, performance, and longevity of batteries used in such technology (1). Facile ionic transport is a primary criterion to design future battery materials and configurations. However, interfacial ion transport, including both across interfaces between different battery components, e.g., electrode-electrolyte interfaces, and across grain boundaries within single components, e.g., in polycrystalline cathodes or solid electrolytes, is often sluggish. The relevant origin is still unknown. This is largely because ion transport behavior in solids is correlated to local atomic arrangements while a technique that can probe both atomic structure and ion transport behavior is still missing. Apart from properties of the diffusing species that affect their migration, the low energy optical phonon frequency is an example of a property that correlates with the activation energy for ion transport (2). Aberration-corrected scanning transmission electron microscopy (STEM) has been widely used to probe atomic structure in all classes of materials. Recently, advances in electron monochromation have enabled nanoscale vibrational spectroscopy to be performed in a STEM via electron energy loss spectroscopy (EELS). Thus, in monochromated STEM EELS, we have a technique that can probe atomic structure and ion transport behavior simultaneously with better than nanometer spatial resolution. The technique has already demonstrated its efficacy towards investigating compositional variation in the core-loss regime and bonding arrangements in the vibrational-loss regime along with atomic structure across individual nanoscale interfaces (3,4). However, it has not yet been used to characterize interfaces in battery materials. In this study, we use monochromated STEM EELS to investigate composition and bonding arrangements along with local atomic structure across individual grain and phase boundaries in polycrystalline LiCoO₂, a common battery cathode material.<br/><br/>Polycrystalline LiCoO₂ specimens were prepared for STEM EELS analysis using either focused ion beam machining (FIB) or mechanical polishing and ion milling. Monochromated core-loss and vibrational-loss STEM EELS was performed on a Nion UltraSTEM100 microscope. Preliminary core-loss measurements show that the Li concentration drops at both grain and phase boundaries. Correlative vibrational-loss measurements show that the signal associated with Li-O motion drops in intensity at both boundaries. Further experimental results and corresponding simulations based on density functional theory from all three specimens will be presented.<br/><br/>References:<br/>1. Li et al. An advance review of solid-state battery: Challenges, progress, and prospects. Sustain Mater Technol. 2021 e00297.<br/>2. Bachman et al. Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction. Chem Rev. 2016 116(1):140–62.<br/>3. Venkatraman et al. The influence of surfaces and interfaces on high spatial resolution vibrational EELS from SiO₂. Microscopy. 2018 67(suppl_1):i14–23.<br/>4. Collins et al. Functional Group Mapping by Electron Beam Vibrational Spectroscopy from Nanoscale Volumes. Nano Lett. 2020 20(2):1272–9.<br/>5. This research was supported by the U.S. DOE Office of Science Early Career project ERKCZ55 and was conducted, in part, using instrumentation within ORNL’s Materials Characterization Core provided by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. DOE and sponsored by the Laboratory Directed Research and Development Program of ORNL, managed by UT-Battelle, LLC, for the U.S. DOE. All experiments were performed at the Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility.