Chemical Science

Research Frontiers 2022

Intact sea-island nanostructures of solid-state electrolytes
snapshotted with a femtosecond X-ray laser
All-solid-state batteries are promising as nextgeneration batteries for electric vehicles, so applications
and fundamental research have been intensivelyactive in both industry and academia in recent years.
Solid electrolytes can replace the conventional liquidelectrolytes, which are highly reactive and flammable;thus, solid electrolytes can improve the safety, lifetime,and energy density of batteries. Glass-ceramics, whichhave an amorphous phase and one or more crystalline
phases, have attracted significant interest as a solid
electrolyte material. In particular, sulfide glass-ceramics
have achieved ionic conductivity at room temperature
equivalent to or higher than that of liquid electrolytes [1].
Knowledge of the relationship between the structural
information of sulfide glass-ceramics and its battery
performances will accelerate further improvementof solid electrolytes towards widely practical use. So
far, various methods such as Raman spectroscopy,
nuclear magnetic resonance, transmission electronmicroscopy (TEM), and molecular dynamics simulation
have provided new insights into the structure of sulfide
glass-ceramics. Among them, high-resolution TEM isa powerful tool that enables direct observation of thesea-island (amorphous-crystal) structure of sulfide
glass-ceramics with nanometer spatial resolution andin situ measurement [2]. However, imaging intact
nanostructure is still challenging because the crystal
grains become amorphous during the observation
owing to electron beam damage.
In the current study [3], we visualized the structureof an intact sulfide glass-ceramic electrolyte particleusing pulsed coherent X-ray solution scattering(PCXSS). Additionally, we suggested an image
processing method that makes use of morphologicaloperations to obtain crystal images and the volume

ratio of the amorphous to crystalline phases from thetwo-dimensional projection image produced via PCXSS(Fig. 1). PCXSS is a coherent diffractive imaging
technique based on the X-ray free-electron laser (XFEL)and micro-liquid enclosure array (MLEA). Since XFEL
pulses are extremely short (in the femtosecond range),
it is possible to collect diffraction data from samplesbefore radiation damage sets in. MLEA integrates manysmall solution wells into a single device and makes it
possible to hold specimens in a vacuum environment
with control over parameters such as temperature andsolvent.
PCXSS measurements of (Li2S)70–(P2S5)30 glassceramic particles were carried out at SACLA BL2 EH3.
The incident XFELs with a photon energy of 4 keVwere focused to ~1 mm on the sample using totalreflection mirrors. Immediately prior to the PCXSS
measurement, the particles dispersed in a heptane
solvent were enclosed in the MLEA (Fig. 2) using asemiautomated assembly system. Heptane solvent
prevented the deliquescence of sulfide glass-ceramics
and enhanced the slight image contrast between theamorphous and crystalline phases, as previously
described [3]. Figure 3(a) shows a single-shot X-ray
laser diffraction pattern from a glass-ceramic particle.
The diffraction pattern reached the edge of the detector
with a good signal-to-noise ratio, though the presenceof the solvent generally reduces diffracted intensities
from the sample. The maximum spatial frequency
was 0.105 nm–1. Figure 3(b) is a projection image
reconstructed by applying the phasing method to thediffraction pattern (Fig. 3(a)). The spatial resolution was
14.5 nm, as evaluated by the phase retrieval transferfunction. The particle size was approximately 300 nm,
and there were many bright spots all throughout theSingle-shot damage-freemeasurement

Solid-state electrolyte
Femtosecond
X-ray laser

Phase retrieval &MorphoCIEP
Sample in solution
for image-contrast
enhancement
Orange: Crystalline phase
Green: Outer shape

100 nmFig. 1. Schematic of a femtosecond X-ray laser and cutting-edge image-processing techniquethat reveal intact sea-island nanostructures of fragile solid-state electrolytes for batteries. [3]86Research Frontiers 2022

(a)

(b)

Partition
Diffraction

XFEL pulse

5 mmSilicon nitride
membrane

Solvent containing
sample particles

Fig. 2. (a) Photograph of an MLEA. The
MLEA comprises 576 micro-liquid enclosures.
(b) Schematic of the cross-section of an MLEA
chip. The micro-liquid enclosures are separated
from each other by epoxy grid partitions. [3]particle. Because the outer shape of the particle issmooth, these bright spots can be attributed to crystal
grains with higher electron densities rather than to theroughness of the particle surfaces.
We applied a newly developed image processing
technique, morphological crystal-image-extraction
processing (MorphoCIEP), to the reconstructed image
in Fig. 3(b) to quantitatively extract the outer shape andcrystal images. Figure 3(c) shows the superposition of
the crystal (orange) and outer shape (green) images.
The extracted crystal image clarified the sea-island
nanostructures. MorphoCIEP consists of several
steps, including high-pass filtering (HPF), rotational
morphological processing (RMF) [4], and interpolation
processing. HPF and RMF act complementarily in thereciprocal and real spaces to effectively emphasize
crystal grains. Parameters in each image processing
step can be optimized by repeatedly performing

Spatial Frequency (nm–1)

Photons/(11×11 pixels)

(a)

10–1

106

0.10

the process while feeding back the output results.
The sizes of the crystal grains were approximately
10–50 nm, which is consistent with observations of theTEM images. There were 23 crystal grains in total, andthey covered the entire particle, from the edge to thecenter. Furthermore, the volume ratio of amorphous tocrystalline phases in the particles was determined
to be approximately 2.5:1 using the intensity values ofthe extracted images and known electron densities ofamorphous and crystalline phases of (Li2S)70 –(P2S5)30.
The proportion and distribution of crystalline grains in
the particles are related to the lithium-ion conductivity,
which is essential to battery performance.
The quantitative morphological information obtained
via PCXSS with MorphoCIEP will guide the optimizationof various properties of solid electrolytes, including
thermal stability, moisture resistance, potential window,
interface resistance, and ionic conductivity. In future
work, we aim to perform in situ temperature-controlled
PCXSS measurements to observe the dynamics ofstructural changes during the annealing process in thesynthesis of sulfide solid-state electrolytes. The studied
sulfide solid-state electrolyte is a kind of metastablematerial. With the progress of materials informatics [5],metastable materials have been found to be innovative
materials with unprecedented functions. However,as the name implies, the low thermodynamic stabilityof metastable materials makes it difficult to fully
analyze their structures by conventional measurement
methods. PCXSS with MorphoCIEP has the potential
to contribute to the development of reliable, safer, andhigher-performance batteries and advanced metastablematerials for realizing a sustainable society.

Image Intensity (arb. units)
01(b)

(c)

0.05
0.00

–0.05
–0.10
–0.10 –0.05 0.00 0.05 0.10

100 nm

100 nmSpatial Frequency (nm–1)

Fig. 3. (a) Measured diffraction pattern from a glass-ceramic particle. (b) Reconstructed image
obtained via phase retrieval. (c) Superposition of crystal (orange) and outer-shape (green)
images after applying MorphoCIEP to the reconstructed image in Fig. 3(b). [3]Akihiro Suzuki* and Yoshinori Nishino
Research Institute for Electronic Science,
Hokkaido University*Email: akihiro.suzuki@es.hokudai.ac.jp

References[1] A. Hayashi et al.: Nat. Commun. 10 (2019) 5266.
[2] H. Tsukasaki et al.: Sci. Rep. 7 (2017) 4142.
[3] A. Suzuki, H. Tanaka, H. Yamashige, Y. Orikasa, Y. Niida,
T. Kimura, K. Tono, M. Yabashi, T. Ishikawa, Y. Bessho,Y. Joti, Y. Nishino: Nano Lett. 22 (2022) 4603.
[4] Y. Kimori: J. Synchrotron Rad. 20 (2013) 848.
[5] D. P. Tabor et al.: Nat. Rev. 3 (2018) 5.
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