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Characterization of sulfur and nanostructured sulfur battery cathodes in electron microscopy without sublimation artefacts

Barnaby D.A. Levin1, Michael J. Zachman1, Jörg G. Werner2, Ritu Sahore2, Kayla X. Nguyen1, Yimo Han1, Baoquan Xie2, Lin Ma2, Lynden Archer3, Emmanuel P. Giannelis2, Ulrich Wiesner2, Lena F. Kourkoutis1,4, David A. Muller1,4

1. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA, 14853.

2. Department of Materials Science and Engineering, Cornell University, Ithaca, NY, USA, 14853.

3. School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY, USA, 14853.

4. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY, USA, 14853.

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Figure S1. Schematic diagram of airSEM. Sample can be transferred between optical and electron microscopes on a moveable stage. Under electron microscope, sample in air is separated from electron optics in vacuum by a thin (~ 10 nm) electron transparent silicon nitride membrane. Working distance ~ 50 μm. Sample is mounted on STEM detector prior to imaging.

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Figure S2. Cryo-STEM images of carbon-sulfur composites with corresponding sum spectra from XEDS mapping for a) activated gyroidal mesoporous carbon aGDMC-15-10h. Scale bar 1 μm. b) Non-activated gyroidal mesoporous carbon GDMC-15-1600°C. Scale bar 1 μm. c) Carbon nanotubes. Scale bar 2 μm.

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Optimizing cryo-TEM and XEDS imaging conditions for carbon-sulfur compositesDue to the low temperature of the cryo-holder, ice can build up on the sample during loading into the holder, transfer of the holder into the TEM, and as the sample sits in the specimen chamber. Typically, crystalline ice particles form during loading, and an amorphous ice layer gradually forms over the sample in the specimen chamber. The formation of ice over regions of interest on the sample has the effect of reducing image contrast, and increasing the thickness of material through which x-rays and inelastically scattered electrons must travel to be detected spectroscopically. Ice formation over the sample during loading can be mitigated by loading under dry nitrogen gas. Ice formation during transfer is mitigated by protecting the sample with a cryo-shield on the tip of the cryo-holder. In the specimen chamber, ice formation on the sample is mitigated by the use of cryo-blades. When imaging sulfur in cryo-TEM, if ice build-up becomes problematic, one can take advantage of the difference between the sublimation point of sulfur and ice by heating the sample to ~ -80oC to remove some ice by sublimation.

When imaging carbon-sulfur composites in particular, ultra-thin, holey, or lacey carbon support films for TEM grids are recommended in order to reduce the carbon background signal for XEDS and EELS. An alternative film material such as silicon nitride may be used to eliminate a carbon background in spectroscopic signals, but the ADF image contrast of a carbon particle on a silicon nitride film will be weaker than on a carbon film of the same thickness.

As carbon is a low-Z element that emits relatively few x-rays, relatively long exposure times are required in order to achieve good signal to noise ratio in carbon XEDS maps. On an electron microscope with only a single XEDS detector with a small window for signal collection, carbon signal attenuation will limit the maximum size of objects that can be mapped reliably to ~ 1 or 2 μm before carbon x-ray absorption within the object prevents full mapping of the carbon in the particle. On a TEM grid with an ultrathin carbon film, there will be a small carbon background signal in addition to the signal from carbon-sulfur composite samples.

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Imaging modes and XEDS mapping in airSEM

The default imaging mode in airSEM is backscattered electron (BSE) imaging. BSE imaging of carbon is challenging because carbon is a light element (low-Z) that scatters electrons relatively weakly. When imaging carbon-sulfur composite samples in airSEM, we were able to obtain stronger contrast, and greater signal to noise using a bright field “airSTEM” detector, operating in bright-field STEM mode in air with a detector below the sample (Nguyen et. al., 2014; Han et. al. 2015). Figure S3 shows BSE and bright field airSTEM images, and XEDS maps of sample 20-2-1.5-50S HPC, a highly porous activated carbon material derived from ice-templation and infiltrated with sulfur by melt infusion with a 1:1 carbon to sulfur ratio by weight (Sahore et. al. 2015). The contrast and signal to noise of the composite particles are visibly stronger in the airSTEM image than in the BSE image.

XEDS mapping suggests these have weaker sulfur infiltration than the larger particle in the center. In addition to sulfur and carbon signals from the sample, the XEDS sum spectrum from the field of view in airSEM shows a range of background fluorescence signals, some of which would not typically appear in a vacuum SEM or TEM. The silicon signal has two main sources, firstly a background signal from the silicon detector on which the sample rests, and secondly a fluorescence signal from the silicon chip that holds the window in place. There will be some contribution to the silicon signal from the SiN window itself, although this will be relatively small because the window used was only ~ 10 nm thick. Oxygen and nitrogen background signals can be attributed mainly to air, with some oxygen background signal from a uniform passivating oxide layer on the detector, and a small nitrogen signal from the SiN window. The copper Lα peak is due to fluorescence from a copper TEM grid bar from the grid the sample was dispersed on. This copper signal appears when doing XEDS in vacuum TEM. There may also be some small background carbon signal from the ultra-thin carbon support film on the TEM grid. Again, this carbon background also appears in vacuum TEM.

Synthesis notes for sample in Figure S3. Disordered, activated mesoporous carbon structures 20-2-1.5-50S HPC were synthesized using an ice-templation method described by Sahore et. al. Activation was achieved by heating the HPCs in CO2 at 950oC for 1.5 hours. Sulfur infiltration was attempted by melt infusion at a sulfur:carbon ratio of 1:1 by weight (Sahore et. al. 2015).

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Figure S3. AirSEM BSE and STEM images of carbon-sulfur composite particles. BF STEM imaging is more sensitive to the low-Z carbon, so signal to noise and contrast are stronger. Corresponding XEDS maps of carbon and sulfur show sulfur has infiltrated into the particle in the center of the image (yellow on overlay). Scale bars 2 μm. XEDS sum spectrum from field of view contains fluorescence signals from air, the copper grid, and the silicon detector in addition to the carbon and sulfur signals.

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Figure S4. Bright field airSTEM images of a) sulfur particle, and b) carbon-sulfur spheres with corresponding X-ray sum spectra from XEDS mapping. Scale bars: a) 5 μm, b) 10 μm. Silicon XEDS signal due primarily to x-ray excitation from STEM detector. Copper XEDS signal due to fluorescence from copper TEM grid.

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Notes on electron radiation damage to sulfur.With mass loss by sublimation suppressed, one of the major factors that will limit our ability to characterize sulfur in cryo-TEM is electron radiation damage. There are two major radiation damage mechanisms that sulfur will be vulnerable to in the electron microscope. The first is ionization damage, or radiolysis, which occurs when an incident electron ionizes atoms in the specimen by inelastic scattering (Egerton, 2011). In an insulating specimen, such as sulfur, the electronic bonds broken by ionization damage will not be repaired by free electrons, resulting in rapid structural degradation, and potentially leading to mass loss (Egerton et. al., 2004).The second is displacement damage, which occurs due to elastic scattering of electrons in the electron beam from the nuclei of atoms in the specimen. When the energy transferred to an atomic nucleus is greater than the binding energy of that atom in its site in the material, then it is ejected from that site. Surface sputtering is the form of displacement damage of most relevance to this discussion as it causes mass loss by removing material from the exit surface (Egerton et. al., 2004). Sulfur is a molecular solid, consisting of S8 molecules, which are held together by relatively weak Van der Waals forces (~ 1.1 eV per molecule). The bond energy of sulfur atoms within each S8 ring is ~ 2.72 eV per atom. Energy transfer from high energy radiation to sulfur nuclei can cause S8 rings to dissociate, resulting in the sputtering of a range of species of sulfur molecule (Chisney et. al. 1988). Our observations of radiation damage on sulfur in cryo-TEM suggest a combination of radiation damage processes affect the sample. Figure S5a shows a thin region of a sulfur particle in cryo-STEM (sample temperature ~ -173oC) before exposure to a stationary electron probe, with the probe position indicated by the red arrow. Figure S5b shows the same particle after exposure to the electron beam, with ~ 10 pA beam current and a ~ 2 Å probe diameter, for a total of 30 seconds. From comparing the two images, it can be seen that material surrounding the position of the electron probe has been lost. The initial thickness of the sulfur at the position indicated in Figure S5a was estimated to be ~150 nm based on the low loss EELS spectrum, acquired using a very low beam current and low exposure time (Figure S5), and using the method of Iakoubovskii to calculate the mean free path of electrons in sulfur (Iakoubovskii et. al., 2008).Core loss EELS spectra featuring the sulfur L-edge were recorded at 5 second intervals during exposure to the electron beam. Figure S5c shows the background subtracted sulfur L-edge EELS spectra over an energy loss range of 130-270 eV. The sulfur L-edge is observed to consist of a small pre-peak at ~ 163 eV energy loss, followed by is a broad edge, with an onset at ~165 eV energy loss. The decrease in signal intensity with time is due to loss of material from the region surrounding the beam. Initially, mass loss is relatively slow, consistent with a loss of ~11% of the initial mass of material over the first 20 seconds of exposure to the beam (Figure S5d). Over this period, the decrease in sulfur mass that we observe in Figure S5 appears to be primarily due to sputtering damage, as we do not observe any reduction in the size of the pre-peak at ~163 eV, which would be expected to accompany bond breaking caused by ionization damage. Mass loss rapidly accelerates after 20 seconds, with a further 16% of the initial mass of material lost between 20 and 25 seconds of exposure, and a further 65% of the initial mass of material lost between 25 and 30 seconds of exposure (Figure S5d). The relatively slow initial rate of mass loss, followed by a period of much more rapid mass loss is consistent with the presence of a

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protective surface layer on the sulfur, probably a layer of ice that formed due to the cryogenic temperature of the sample. The rate of mass loss increases once the surface layer has been degraded by radiation damage, allowing sulfur particles to escape into vacuum more easily. Overall, these observations highlight the need for caution when attempting to characterize sulfur at high resolution in the electron microscope, in order to avoid excessive radiation damage to the specimen. More study will be required in future in order to quantify the limits that radiation damage will place on the resolution that can be attained before radiation damage obscures the inherent structure of specimens containing sulfur.

Figure S5. Cryo-STEM images of edge of sulfur particle a) before and b) after exposure to stationary electron probe. Scale bar 200 nm. c) Sequential EELS spectra acquired from point indicated in a) and b). Beam current ~ 10 pA, acquisition time 5s. This suggests a decrease in thickness of material in path of beam over time due to radiation damage. Sample temperature ~ -173oC. d) % Mass lost from the material with time, as measured from the spectra in c).

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Figure S6. Low loss spectrum recorded from edge of sulfur particle. Thickness calculated to be ~ 1 inelastic mean free path from spectrum, ~ 150 nm for sulfur (Iakoubovskii et. al., 2008).

References

Chisney, D.B., Boring, J.W., Johnson, R.E. & Phipps, J.A. (1988), Molecular ejection from low temperature sulfur by keV ions. Surface Science, 195, 594-618.

Egerton, R.F., Li, P. & Malac, M., (2004) Radiation damage in the TEM and SEM, Micron 35, 399–409.

Egerton, R. F. (2011) Electron Energy Loss Spectroscopy in the Electron Microscope. 3rd ed. New York, USA: Springer.

Iakoubovskii, K., Mitsishi, K., Nakayama, Y. & Furuya, K. (2008) Thickness measurements with electron energy loss spectroscopy. Microscopy Research and Technique 71, 626–631.

Sahore, R., Estevez, L.P., Ramanujapuram, A., DiSalvo, F.J. & Giannelis, E.P. (2015) High-rate lithium–sulfur batteries enabled by hierarchical porous carbons synthesized via ice templation. Journal of Power Sources, 297, 188-194.

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