Three-Dimensional Imaging of Large-Volume Lithium Metal Microstructure by Xenon Plasma Focused Ion Beam
Authors: Peng Yueying , Nishikawa Kei , Kanamura Kiyoshi 
Published: December 2024

Advanced energy storage systems are increasingly in demand due to the rising needs of portable electronics, electric vehicles, and grid-scale energy storage. Lithium (Li) metal stands out as an exceptional battery anode with its high theoretical specific capacity (3860 mA g-1 compared to graphite's 372 mA g-1) and low redox potential (-3.04 V vs. the standard hydrogen electrode). However, practical Li metal anodes often face challenges such as low Coulombic efficiency, short cycle life, and safety concerns, mainly due to the reactive nature of Li metal and the unpredictable deposition with diverse morphologies.

Developing advanced Li metal anodes requires a deep understanding of their electrode structure. It is indispensable to find three-dimensional (3D) images. However, obtaining accurate 3D imaging of Li anodes poses significant challenges. On one hand, the non-uniform electrodeposition of Li metal necessitates a large sample to accurately represent the entire electrode. On the other hand, achieving high resolution is essential for distinguishing the complex phases within the Li electrode, including metallic Li, solid electrolyte interphase (SEI), and pores. While popular 3D imaging technologies like X-ray computed tomography and nuclear magnetic resonance imaging offer comprehensive size information, they often struggle with limitations in resolution or imaging contrast. Traditional focused ion beam-scanning electron microscopy (FIB/SEM) can provide high-resolution images at nanoscale, but its use of gallium ion (Ga+) leads to issues such as ion implantation and limited size (≤ 10 μm). An emerging alternative, plasma FIB/SEM (PFIB/SEM), coupled with Xenon (Xe) gas, featured by both high resolution and large volume size, presents a promising solution, making it appealing for characterization of Li electrode.

In this presentation, Xe PFIB is employed for 3D imaging of electrodeposited Li with a large volume (200 μm in dimension) and high resolution (50 nm per pixel). This approach facilitates the clear identification of two phases: deposited Li metal and SEI/pores. It enables the quantification of their spatial distribution, packing density, and specific surface area, crucial parameters determining the Coulombic efficiency of the deposited Li anode. Furthermore, the ability to separate individual Li particles provides quantitative information on volume size, particle number, and shape. These quantitative results establish a robust mathematical relationship with the overpotential of Li deposition, significantly enhancing our understanding of the Li electrodeposition process.

Leveraging the large volume and high-resolution 3D information obtained through Xe PFIB-SEM, this work demonstrates its significance in assessing and understanding electrodeposited Li, offering valuable insights for designing advanced Li metal anodes. Xe PFIB-SEM methodology is poised to become a pivotal technique for 3D characterization in energy storage materials and related fields.