Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy

Summary

Cryogenic Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) techniques can provide key insights into the chemistry and morphology of intact solid-liquid interfaces. Methods for preparing high quality Energy Dispersive X-ray (EDX) spectroscopic maps of such interfaces are detailed, with a focus on energy storage devices.

Abstract

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar energy and fuel generation, and electrochemical energy storage. Nanoscale characterization of such interfaces has recently been achieved using cryogenic electron microscopy, thereby providing a new path to advancing our fundamental understanding of interface processes.

This contribution provides a practical guide to mapping the structure and chemistry of solid-liquid interfaces in materials and devices using an integrated cryogenic electron microscopy approach. In this approach, we pair cryogenic sample preparation which allows stabilization of solid-liquid interfaces with cryogenic focused ion beam (cryo-FIB) milling to create cross-sections through these complex buried structures. Cryogenic scanning electron microscopy (cryo-SEM) techniques performed in a dual-beam FIB/SEM enable direct imaging as well as chemical mapping at the nanoscale. We discuss practical challenges, strategies to overcome them, as well as protocols for obtaining optimal results. While we focus in our discussion on interfaces in energy storage devices, the methods outlined are broadly applicable to a range of fields where solid-liquid interface play a key role.

Introduction

Interfaces between solids and liquids play a vital role in the function of energy materials such as batteries, fuel cells, and supercapacitors^1,2,3. While characterizing the chemistry and morphology of these interfaces could play a central role in improving functional devices, doing so has presented a substantial challenge^1,3,4. Liquids are incompatible with the high vacuum environments needed for many common characterization techniques, such as x-ray photoemission spectroscopy, scan....

Protocol

1. Prepare the sample and transfer into the SEM chamber

1. Set up the microscope
   1. For systems that convert between room temperature and cryogenic equipment, install the cryo-SEM stage and anticontaminator according to the equipment manufacturer’s instructions and evacuate the SEM chamber.
   2. Adjust the gas injection system (GIS) platinum source so that when inserted it sits approximately 5 mm further away from the sample surface compared to typical room temperature experiments. This position needs to be optimized for each system to ensure even coating of the sample surface. On the FIB used here, this is done by loosening a set screw o....

Results

This method has been developed on a dual FIB/SEM system equipped with a commercially available cryogenic stage, anticontaminator, and preparation chamber. For details, see the table of materials. We have primarily tested this method on lithium metal batteries with a number of different electrolytes, but the method is applicable to any solid-liquid interface that will endure the amount of dose applied during EDX mapping.

Figure 1 illustrates the various components .......

Discussion

The cryogenic preparation method described here is important and must be done correctly for the chemistry and morphology to be preserved⁸. The foremost concern is freezing the sample quickly since this is what allows the liquid to be vitrified⁸. If the sample cools too slowly, liquids may crystalize resulting in a change in morphology⁶. To prevent crystallization, slush nitrogen is used in this procedure, as it reduces the Leidenfrost effect and acce.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
INCA EDS	Oxford instruments		Control software for X-max 80
PP3010T Cryo-preparation system	Quorum Technologies, Inc.		FIB/SEM cryogenic preparation system. Includes pumping station, transfer rod system, preparation (prep) chamber, cryogenic stages, sample shuttles
Strata 400 DualBeam System	FEI Co. (now Thermo Fisher Scientific)		Dual beam FIB/SEM
X-Max 80	Oxford Instruments		80mm2 EDX detector
xT Microscope Control	FEI Co. (now Thermo Fisher Scientific)		Software for controlling FEI Strata