Article
Challenges in Characterizing Battery Materials
Surface Analysis Spotlight: XPS
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by Sarah Zaccarine XPS Scientist |
Lithium metal is generally considered the most promising battery electrode material due to its high capacity, negative redox potential, and low electrochemical valence. Lithium-ion batteries (LIBs) have experienced great success including rechargeability and long lifetimes, but their limited energy density restricts applications moving forward. Lithium metal batteries (LMBs) offer similar benefits but with much higher achievable energy densities, making them a promising future battery technology. Both LIBs and LMBs are multi-layered, complex systems with many materials and interfaces that each play a critical role in performance and stability. Tuning the composition and morphology of these materials is necessary to create stable, high-performing devices, but these materials are very difficult to handle and characterize.
Li metal is highly reactive with air and forms dendrites that can pierce the separator and cause short circuits, and the Li metal anode can contract significantly during charging and discharging, damaging the protective solid-electrolyte interphase (SEI) film and reducing efficiency. Developing a chemically and mechanically stable SEI has become a major area of research since it can isolate metallic Li from the electrolyte and mitigate Li dendrite formation to improve cell durability, but the SEI layer is notoriously difficult to characterize because of its mosaic multi-layer composition of organic and inorganic layers, dynamic nature during charging/discharging, and nuanced synergistic effects that are difficult to correlate to performance. In order to optimize materials properties and improve battery lifetimes, there is a pressing need for physicochemical characterization approaches with high spatial resolution, chemical and morphological analysis, and correlation of properties to performance.
Secondary X-ray induced Electron (SXI) imaging using 10 µm X-ray spot in XPS is used to locate lithium/electrolyte interface.
J. Phys. Chem. A, 2021, 125, 1069
X-ray photoelectron spectroscopy (XPS) is commonly used to characterize the chemical composition of battery materials, and the depth resolution (~ 10nm) is ideal for analyzing thin layers and interfaces. PHI XPS instruments provide a variety of operating modes and analytical accessories to enable thorough characterization of surfaces and interfaces in battery materials and how these properties evolve. With PHI’s scanning microprobe allowing for spatial resolution down to a few microns, tunable beams with small and large spot sizes can be used to track changes in composition and local features. X-ray induced secondary electron imaging (SXI), including high-resolution mosaic imaging over a wide sample area, can be used for easy identification of important features and morphological imaging of various battery component materials to complement chemical information. Chemical mapping can visualize distribution of individual elements or chemical species to understand how materials are dispersed and change with testing. Additional options address stability concerns associated with Li materials, including a cooling stage to preserve battery chemistry during extended experiment times, where time-resolved profiles are used to compare chemical degradation under controlled temperatures. Combining Al Kα and Cr Kα X-rays can be used for non-destructive XPS and hard X-ray photoelectron spectroscopy (HAXPES) analysis of surface (up to ~10nm, Al) and subsurface (up to ~30nm, Cr) composition. Further capabilities can provide electronic characterization from the same sample location, including reflection electron energy loss spectroscopy (REELS), ultraviolet photoelectron spectroscopy (UPS), and low-energy inverse photoelectron spectroscopy (LEIPS), which can be used to calculate work function and electronic properties for small- and large-band-gap materials.
The full energy diagram can be obtained using UPS/LEIPS. LEIPS uses a low-energy electron beam, which ensures damage-free energy diagram evaluation! Performing XPS and UPS/LEIPS in the same area provides a direct link between chemical properties and electronic structure of the electrolyte/electrode interface.
These experiments require safe sample handling solutions. PHI provides the ability to transfer battery materials from the glovebox to the instrument via controlled-environment transfer vessels to minimize environmental exposure. Beyond the standard sample holders, which can accommodate sample sizes up to 80 mm x 80 mm in lateral dimensions and 20 mm in height, PHI’s 4-contact stage holder and electrochemical cell enable in-situ heating, cooling, and polarization studies to measure how properties change under various operation conditions. Overall, PHI provides an excellent suite of analytical capabilities to research all components of batteries at all stages of production.
Operando XPS can probe the evolution of the chemical structure and the surface potential at the electrode/electrolythe interface of lithium-ion batteries under electrochemical conditions.
J. Phys. Chem. A, 2021, 125, 1069
For more information about solutions that PHI has for batteries, including analysis by Auger Electron Spectroscopy and TOF-SIMS, please read our flyer and come to my talk "XPS Analysis of Battery Materials" at AVS 68 in Pittsburgh in November.