There is significant effort to enable lithium metal anodes for rechargeable batteries due to its low electrode potential (-3.04 V vs. standard hydrogen electrode) and high theoretical specific capacity (3860 mAhg^-1). However, despite nearly a half-century of research efforts, several challenges still exist that prevent widespread adoption, such as the high reactivity with the electrolyte preventing the formation of a stable solid electrolyte interphase (SEI) and lithium metal dendrite formation. While various techniques have been used to characterize lithium-ion batteries, there are limitations to characterizing lithium metal because of its intrinsic high chemical reactivity, low thermal stability, and low atomic number making it prone to contamination and melting, while exhibiting weak scattering characteristics for electrons and x-rays [1].

Recently, cryogenic transition electron microscopy (cryo-TEM) techniques have proven to be powerful for observing the fundamental structure and SEI composition of nanoscale electrochemically deposited lithium metal [2-3]. However, this technique alone is limited to very thin specimens, so cannot be applied to traditional bulk battery materials. Therefore, we explore the ability of cryogenic focused ion beam (cryo-FIB) to mill bulk lithium metal structures while minimizing local heating damage and gallium ion reactions. Using cryo-FIB, we observe notably different features from those of room temperature FIB milled cross-sections of electrochemically deposited lithium metal and solid-state thin-film lithium metal batteries. With this technique we demonstrate the ability to investigate cross-sections of plated lithium metal to observe nucleation, morphology, and density. Further, we demonstrate the ability to cryo-FIB prepare TEM lamellae of solid-state thin-film lithium metal batteries for more comprehensive chemical and structural analyses.

[1] D. Lin et al. “Reviving the lithium metal anode for high-energy batteries,” Nature Nanotechnology. 13 (2017) 194-206.

[2] X. Wang et al. “New insights on the structure of electrochemically deposited lithium metal and its solid electrolyte interphases via cryogenic TEM,” Nano Letters. 10.1021/acs.nanolett.7b03606.

[3] Y. Li et al. “Atomic structure of sensitive battery materials and interfaces revealed by cryo–electron microscopy,” Science. 358 (2017) 506-510.