This work was performed by Barnaby D.A. Levin and Kayla Nguyen, under supervision of Prof. David A. Muller, Cornell University Applied Physics. Samples were provided by Weidong Zhou, supervised by Prof. Hector D. Abruña, Cornell University Chemistry.
The lithium sulfur (Li-S) battery is a promising technology with the potential to provide a greater energy density at lower cost than current lithium ion batteries. Analyzing the distribution of sulfur in these electrodes is critical for creating durable Li-S batteries with high energy density. Reliable characterization of sulfur electrodes by TEM is presently problematic due to Sulfur sublimation. Unless it can be encapsulated, Sulfur will sublimate in the sub 10-7 Torr vacuum of a TEM at typical room temperature (~18oC). The vapor pressure of sulfur is ~6×10-7 Torr at room temperature.
The value of airSEM™ technology in material science is in the ability to image samples, such as those containing Sulfur, under ambient conditions. This avoids unwanted phase changes or mass loss that may occur as a result of the exposure of samples to the vacuum of a typical SEM or TEM.
Challenges - Suppressing sulfur sublimation
There are 2 ways to suppress sulfur sublimation in the electron microscope:
- Use a Cryo-electron microscope
- Use environmental or air electron microscope
Cryo-TEM does effectively suppress Sulfur sublimation but the technique is slow in terms of sample throughput – roughly a few hours per sample.
The advantage of using the airSEM for imaging Sulfur samples, relative to other techniques, is that it is very high in terms of throughput; many samples can be imaged in a relatively short space of time – sometimes just a few minutes per sample.
Fig1: a) Bright-Field 200keV STEM image of sulfur particles in PAN shell. b) The sample was transferred to airSEM and imaged again with the airSTEM detector. c) and d) Fresh sample prepared and imaged using an airSTEM detector now showing partially filled rather than empty shells. Samples were prepared for TEM and airSEM by dispersing the material onto a TEM grid using distilled water.
One potential cathode material for a lithium sulfur battery ispolyacrylonitrile (PAN) encapsulated spheres of sulfur. The goal is to partially fill the spheres with sulfur, to allow space for expansion when the electrode is charged with Li. When imaging this material in the TEM (Figure 1 a), it is evident that some PAN shells are completely filled with sulfur. They appear dark because of the thickness of the sulfur inside them. It is also evident that there are completely empty PAN shells, which appear faint in the images. It was not clear whether these empty shells were the result of sulfur sublimation in the TEM through pores in some of the PAN shells during imaging, or whether some of the shells were damaged and lost their sulfur during synthesis.
The PAN-sulfur sample was transferred from the TEM to the airSEM and imaged using the prototype airSTEM detector for the airSEM. This indicated that all of the essential features seen in the 200keV STEM image, such as empty PAN shells ~10nm thick, could also be resolved with 30keV airSTEM (Figure 1 b).
Fresh samples were then prepared for imaging in airSTEM. In many instances, we were able to see PAN shells that were partially filled with sulfur (Figure 1 c and d). These partially-filled shells were not observed in the vacuum-exposed TEM sample suggesting sublimation in the TEM empties any partially-filled shells.
Using the airSEM allowed us to see the inherent distribution of sulfur in the PAN-sulfur sample after synthesis without sublimation artifacts.
Link to PDF
Link to paper:
Characterizing Sulfur in TEM and STEM, with applications to Lithium Sulfur Batteries. Barnaby D.A. Levin, Michael J. Zachman, Jörg G. Werner, Ulrich Wiesner, Lena F. Kourkoutis, David A. Muller. Microscopy and Microanalysis, 2014
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- Characterizing Sulfur in TEM and STEM, with applications to Lithium Sulfur Batteries. Barnaby D.A. Levin, Michael J. Zachman, Jörg G. Werner, Ulrich Wiesner, Lena F. Kourkoutis, David A. Muller. Microscopy and Microanalysis, 2014.
Work supported by the Energy Materials Center at Cornell, DOE EFRC BES (DE-SC0001086). EM Facility support from NSF MRSEC program (DMR 1120296).