The Lithium Measurement Challenge
Global shifts in policy and increased demand for clean energy are driving unprecedented growth in the lithium-ion battery industry. From researching novel chemistries to recycling end-of-life batteries, scientists and engineers need to characterize lithium in battery materials. Despite its importance, reliably measuring lithium is challenging for traditional analytical techniques, especially in air- and moisture-sensitive battery materials.
A new analytical technique combining Laser Ablation Laser Ionization (LALI) with Time-of-Flight Mass Spectrometry (TOF-MS), developed by Exum Instruments, addresses this gap by quantitatively measuring lithium, spatially and in depth, under vacuum conditions. This article explains LALI-TOF-MS and explores its applications for battery materials characterization.
Mapping Lithium’s Journey Through Silicon Anodes
Measuring the distribution of lithium in battery materials is critical for understanding lithium ion behavior during charge and discharge cycles, characterizing the solid electrolyte interface (SEI) layer, and identifying areas of accumulation that may lead to plating or other degradation mechanisms. While lithium’s role in the cathode is well understood, its interactions with the electrolyte and formation of the SEI significantly impact a battery’s lifespan. This is especially true for silicon anodes. Silicon offers several advantages for the battery industry – it is relatively inexpensive, abundant, eco-friendly, and has a high capacity. However, before silicon can revolutionize electric vehicle batteries or grid storage, scientists must decode the complex chemistry it undergoes during cycling.
The process of cycling battery cells introduces challenges such as inert metallic lithium deposition, dendrite formation, and the development of secondary compounds, all of which can compromise the integrity of the anode. Traditional techniques such as X-ray analysis, electron microscopy, and electrochemical methods provide valuable insights into structural and morphological changes, yet they often lack the sensitivity needed to “see” (i.e., directly measure) lithium.
LALI-TOF-MS: Unveiling Lithium’s Hidden Pathways
Laser Ablation Laser Ionization Time-of-Flight Mass Spectrometry (LALI-TOF-MS) combines traditional TOF-MS with an innovative dual-laser ionization system.
- Laser Ablation: The first laser ablates, or removes, 10s-100s of nanometers (nm) of material from the sample’s surface. This enables direct analysis of solid or powdered materials without extensive sample preparation.
- Laser Ionization: The ionization laser targets neutral particles created by the ablation process, creating ions that are more representative of the sample’s constituents than plasma-generated ions in traditional mass spectrometry techniques
- Time-of-Flight (TOF): The TOF mass analyzer creates a full mass spectrum at each laser spot, detecting low-mass elements like lithium and carbon to high-mass metallic elements. This capability supports comprehensive multi-element quantification and detailed elemental mapping.
LALI-TOF-MS addresses limitations in traditional characterization techniques by providing:
- Elemental Mapping & Depth Profiling: Investigating the spatial distributions of elements helps characterize complex battery materials, track lithium, and identify failure mechanisms.
- Quantification from Lithium to Uranium: Full quantification of elements from trace to bulk concentrations enables identification of contaminants, characterization of degradation processes, and verification of material purity.
- Air-free Characterization: Analysis performed under vacuum allows accurate characterization of air- and moisture-sensitive materials.
Stanford’s Insights: Visualizing Lithium in Action
Researchers at Stanford’s Synchrotron Radiation Lightsource (SSRL) at SLAC National Accelerator Laboratory use LALI-TOF-MS as a complement to their x-ray-based techniques. For these researchers, the true power of LALI-TOF-MS lies in its capability to effectively “photograph” the behavior of lithium in electrodes.
By separating charged particles by their mass-to-charge ratios, the technique enables detection of elemental lithium, 7Li, along with compounds formed by lithium in combination with oxygen, silicon, and carbon as seen in signals like 23LiO+ and 33Li3C. The researchers have determined that lithium’s combination with oxygen (e.g., 23LiO+) is indicative of the inorganic part of the SEI layer and lithium’s combination with carbon (e.g., 33Li3C) represents the SEI’s organic portion.
Figure 2 shows the elemental maps acquired by LALI-TOF-MS on a set of pristine and cycled silicon anode samples. Comparing three distinct ablation layers reveals the distribution of key species both on the surface and beneath it. This comparison highlights essential differences between the pristine and cycled states of the anode.
By analyzing the layered structure, researchers gain millimeter-scale insights into how compounds evolve as the battery cycles. This approach allows tracking of lithium throughout microenvironments within the system, a breakthrough that deepens understanding of degradation mechanisms and informs strategies to optimize and prolong battery life.
3D Reconstruction of Top-of-Charge Anode
Commercializing silicon anode products involves determining optimal cell geometry and formation processes. This involves characterizing lithium distribution at different states of charge.
In this study, LALI-TOF-MS was used to create 3D depth profiles of a set of silicon anodes at different charge states. Each anode was 40 microns thick and deposited on a copper current collector. The analysis examined areas of 1 mm by 1 mm with a 50-micron lateral resolution, continuing until the system ablated through the anode and into the copper current collector. The video in Figure 3 displays a 3D reconstruction of the top-of-charge anode. Results showed that lithium was uniformly distributed throughout the anode’s cross section, providing data that can inform electrode and cell geometry optimization based on available lithium ions.
In addition to lithium, the analysis detected fluorine and trace amounts of cobalt. Since fluorine is a major constituent of this battery’s electrolyte, its presence in the anode confirms SEI layer formation. The trace amounts of cobalt reveal some impurities in the anode material. Understanding SEI formation and trace impurities in electrodes is critical to optimizing the cell’s performance, enhancing cycle life, and improving safety.
Because each voxel contains a full mass spectrum, this dataset allows cell manufacturers to understand lithium-ion distribution at different charge states, characterize SEI layer formation, and identify trace contaminants.
Advancing Battery Science Through Lithium Visibility
Traditionally, lithium is a challenging element to reliably measure and quantify with conventional techniques. Addressing these limitations, LALI-TOF-MS emerges as a valuable solution for depth profiling, mapping, and quantifying lithium in battery materials. The studies included in this article illustrate how this technique helps scientists and engineers understand lithium ion behavior at different states of charge, characterize SEI layer formation, and identify trace contaminants that may affect cell performance.
Pinpointing the location of lithium goes far beyond a technical inquiry—it is key to deciphering and controlling the chemical processes determining next-generation batteries’ efficiency and safety. Integrating advanced analytical techniques like LALI-TOF-MS not only enriches researchers’ scientific understanding but also propels innovation in sustainable energy solutions.
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Acknowledgements
Article contributed by Ellen Williams, EVP of Business Development for Exum Instruments.
Special thanks to Erick Espinosa-Villatoro, Postdoctoral Fellow and member of the Weker Group at SLAC National Accelerator Laboratory, for sharing his work characterizing silicon anodes.
