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Utilizing Ultraviolet Photoemission Spectroscopy (UPS) and Low Energy Inverse Photoemission Spectroscopy (LEIPS) to Gain Thermodynamic Insight into Real World Battery Materials

Surface Analysis Spotlight: XPS

by James Johns Staff Scientist

Consumer demand for lightweight portable electronics, electric vehicles, and the global imperative to store energy from renewable sources is driving researchers to develop batteries that are lighter, longer lasting, and have higher capacity and efficiency.  Unfortunately, these demands are often in tension with each other at the chemical and material level. For example, chemical tweaks aiming to increase a battery’s efficiency might also reduce its lifetime by speeding up unwanted side reactions, or changing a material hoping to increase its capacity to store charges might also increase its self-leakage rate. As a consequence, next generation batteries comprise a milieu of chemically varied materials in different functional roles (electrodes, electron/hole blockers, anion/cation conductors, separators, protecting or capping layers, etc.).  These distinct materials then work together as a system to facilitate the electrochemical reactions needed to power our phones, our cars, and our electrical grid. In an ideal world, directly comparing the electrical performance of different batteries containing chemically modified materials would provide mechanistic insight and sensibly guide new directions to explore.  Unfortunately, the interplay between materials inside working battery cells adds complexity beyond properties of the individual components alone, making it very challenging to determine causal relationships between chemical changes and electrical performance, and the number of chemical variations that could be tested is nigh infinite.  How, then, can researchers gain true chemical insight to guide the design of new battery materials, maximizing their gains while minimizing materials and human time wasted with inefficient trial and error experiments?

Motivated by these concerns, recent work by Terashima et al.¹ demonstrates the utility of analytical surface science for elucidating the relationship between the chemical/interfacial composition of a material system and the thermodynamic forces driving the movement of electrical charges within it. Specifically, they study the key ingredients of an all-solid-state battery. They examine a family of nickel-containing lithium-based cathodes (NCM’s) being developed for electric vehicles, a sulfide-based solid electrolyte, and a lithium niobate capping layer which is used to protect the NCM cathodes against deleterious side reactions with the sulfide solid electrolyte.  The key physical and chemical insight underlying this work is that most of the processes (excluding ion current) in a battery are electrochemical charge transfers occurring at the interfaces between dissimilar materials. When current is flowing inside a battery, the Fermi levels (E_F) of each of the materials are equilibrated; therefore, if the ionization potential (IP, i.e. the energy required to remove an electron), the electron affinity (EA, i.e. the energy released when an electron is added to a material), and the energetic positioning of the Fermi level are known for both materials at an interface, one has all of the information to understand the thermodynamics of electron/hole transport across that interface. 

Figure 1: Thermodynamic information obtained from a combined LEIPS / UPS Experiment

The researchers extracted that exact information on the battery materials described above by applying two surface science techniques: Ultraviolet Photoemission Spectroscopy (UPS) and Low Energy Inverse Photoemission Spectroscopy (LEIPS).  In their UPS experiments, a UV photon (the 21.22 eV photon emitted from a helium 2pà1s transition in this case) ejects electrons from a material.  The highest energy electrons inside the material, those at the top of the valence band (VBM in Figure 1), leave with the highest kinetic energy and enable the measurement of the valence band onset relative to the Fermi level of the instrument. The researchers then apply a negative bias to the sample to accelerate electrons away from the sample, allowing for the measurement of electrons which can just barely manage to escape the material with the addition of the 21.22 eV energy. By definition, this is the vacuum level (E_VAC), and it determines the exact positioning of the Fermi level in a gapped material when combined with the data from a LEIPS measurement.

A LEIPS experiment is effectively a UPS measurement run in reverse, in which an electron beam with known kinetic energy impinges onto the sample and those electrons enter the conduction band. A small fraction of those electrons then relaxes deeper toward the bottom of the conduction band, emitting a UV photon. By varying the energy of the incoming electron beam, one can map out the energy of the conduction band relative to the instrument's Fermi level and the material’s vacuum level. Finally, by combining the results of these two experiments, one arrives at the full thermodynamic picture of low energy occupied and unoccupied states available for charge transfer, the band gap, and the location of the material’s Fermi level within that gap, as illustrated by the energy diagram in Figure 1.

Traditional inverse photoemission has been around since the 1970s; however, it was only the invention of its low-energy cousin, LEIPS, in 2012 by Hiroyuki Yoshida² that advanced the technique to tackling a much wider array of applications involving sensitive materials including organic semiconductors and lithium-ion batteries.  LEIPS prevents sensitive materials from being damaged by keeping the landing energy of the electron beam under 5 eV, below the threshold for electron-induced damage for most materials.

 

Figure 2: Band diagrams from UPS/LEIPS measurements of three NCM battery materials.  Left: Combined UPS (blue) and LEIPS (red) spectra near the Fermi Level for three NCMS with Ni content increasing from top to bottom.  Dotted lines mark the onset of the VBM and CBM. Right: Band Diagrams derived from the spectroscopic data shown on the left. Reproduced from Terashima et al.²

Terashima et al. utilize combined UPS and LEIPS spectroscopies to measure the effect of nickel concentration on the band gap and Fermi Level location in a series of NCMs as shown in Figure 2 with differing nickel concentrations.  They conclude that increasing the nickel concentration in this family leads to a closing of the band gap, a migration of the Fermi level from near the conduction band edge to near the valence band edge, and a transition from being a majority electron carrier to a majority hole carrier.  This transition facilitates the transfer of holes from oxygen and explains the loss of molecular oxygen and instability in the nickel-rich NCM’s observed by others. Similarly, they use these same tools to measure the alignment of Li₁₀GeP₂S₁₂ (the sulfur solid-state electrolyte) and NCM811, which allows them to explain oxygen diffusion across this interface.  Finally, they utilize UPS/LEIPS to measure the band diagram of two potential capping layers to prevent this oxygen hole diffusion, Li₃PO₄ and LiPON, and based on their measurements, they can successfully explain that the deeper Fermi level from the LiPON layer creates a more chemically robust battery.

The research by Terashima et al. successfully demonstrates that LEIPS and UPS are excellent techniques for the direct measurement of important material properties that drive the performance of specific electrochemical interfaces in lithium-ion batteries.   They conclude that increasing nickel content lowers both the band gap and the Fermi level in NCM-based cathodes and explain why LiPON is a better protective material than Li₂PO₄ for NCMs in an LGPS electrolyte.  They can reach these conclusions because UPS and LEIPS are uniquely suited to measuring the energy required for single electron transfer processes.  These conclusions could not have been made from other techniques such as reflection electron energy loss spectroscopy (REELS) or UPS + optical absorption spectroscopy which rely on excited states of the neutral system (i.e. no electron transfer) to get information about the conduction band. 

To learn more about how these techniques work and how they compare to other analytical techniques in the field, join our upcoming where we explain how to use surface science techniques to measure the band diagram of materials on August 22, 2024, at 10:00 AM CST. Register for webinar.

1. Terashima, M. “Selection of Surface Coating Materials for Ni-Rich NCM in All-Solid-State Batteries through Electronic Band Structure Analysis Using UPS/LEIPS” J. Phys. Chemi. C. 128, 22 (2023                                                                 

2. Yoshida, H. “Near-ultraviolet inverse photoemission spectroscopy using ultra-low energy electrons” Chem. Phys. Lett., 539–540 (2012), pp. 180-185