Post on 12-Aug-2020
Operando X-Ray Scattering and
Spectroscopic Analysis of
Germanium Nanowire Anodes in
Lithium-Ion Batteries
Katharine Silberstein CFES 2015 Conference
February 26, 2015
Outline
Motivation for battery research
Operando cell design and methods
• X-ray Diffraction (XRD)
• X-ray Absorption Spectroscopy (XAS)
Results on Ge NW anodes
Conclusions
2
Motivation
3
Why Batteries?
• Store energy from
alternative, intermittent
sources via chemical
reactions for later use in
electronics, transportation,
grid load leveling.
4
Batteries Today
5
Abruña, H.; Kiya, Y.; Henderson, J. Physics Today 61, 2008, 43-47.
Battery Research Goals • Synthesize new materials with enhanced performance
• Inexpensive – organic, sulfur cathodes
• Higher capacity – germanium, silicon anodes
• Correlate molecular structure to bulk electrochemical
properties • Structural
• Computation
• Analytical methods
• NMR
• Raman/Infrared
• X-ray scattering/spectroscopy
• Electrochemical
• Cyclic voltammetry in solution and film
• Coin cell testing
6
Operando Applications
• To better understand mechanism of charge storage, need
to look inside functioning cell
7
Analytical Methods
8
Coin Cell Design
9
Pros and Cons of Cell Design
BENEFITS
• Facile assembly and alignment in beam path
• At synchrotron source, experimental timescale dictated by
sample, not instrumentation
LIMITATIONS
• Cannot be reassembled or reused (dictated by coin cells)
• Still some interference from electrolyte and separator
10
Powder X-Ray Diffraction (XRD)
11
• High energy (20-30
keV) x-rays impinge
upon polycrystalline
sample
• Bragg’s Law:
nλ = 2d sin(θ)
• Interference of
diffracted waves gives
powder pattern
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Potential Limitations of XRD • Only probes crystalline domains
• Patterns change in tandem with electrochemistry
• Possible radiation damage
• Shifting to new position gives same pattern
• High background
• Keeping a constant cell design allows for rational subtraction
Azimuthal
integration
Plot scans vs.
elapsed time
12
Contour plot
of peaks
Plot scans vs.
voltage profile
X-Ray Absorption Spectroscopy (XAS)
13
Courtesy of S. DeBeer
Unlike diffraction, XAS requires tunable source to sweep through energies of interest.
X-Ray Absorption Spectroscopy (XAS)
14
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Χ 𝐸 =μ 𝐸 − μ0(𝐸)
∆μ(𝐸0)
http://ssrl.slac.stanford.edu/nilssongroup/corelevel.html
Obtaining EXAFS Fits
• Beyond edge, periodic
modulation of
absorption coefficient
• Transform to k-space
• Fourier transform to
radial distribution from
photoabsorber
• Fit to known structures
15
Potential Limitations of XAS
• Material of interest must have absorption edge that is
accessible at synchrotron source
• Bulk-sensitive measurement
16
Germanium Anodes XRD and XAS
17
Moving Beyond Carbon
18
Courtesy of B. Richards
• State-of-the-art cells use graphite anodes: (372 mAh/g)
• Germanium, silicon have much higher theoretical capacity (1600, 4200 mAh/g)
• But… 300-400% volume expansion from intercalation – nanowires!
Ge-Li Phase Diagram Known
19
Sangster, J.; Pelton, A. J. Phase Equilib. 1997, 18, 289–294.
5 10 15
250
200
150
100
50
High
2(degrees)
Low 0 1 2 3Potential vs Li/Li+(V)
Diffra
ctio
n S
ca
n N
um
ber
Operando XRD
20
50 100 150 200 250
0.0
0.2
0.4
0.6
0.8
1.0N
orm
aliz
ed Inte
nsity
Diffraction Scan Number
Ge
GeLi
Ge4Li9
Ge2Li7
Ge4Li15
Phase Analysis
21
EXAFS
Can Crystallinity Be Preserved?
• Large volume change between Ge and Ge4Li15 believed
to be main culprit in capacity loss
• By reversing the cell polarity above the point of
amorphization, can we preserve some crystallinity?
• Select 0.3V vs Li/Li+ as voltage cutoff
23
4 6 8 10 12 14
80
70
60
50
40
30
20
10
2 (degrees)
Diffra
ctio
n S
can N
um
ber
Low High Potential vs Li/Li+(V)0 1 2 3
Operando XRD
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20 40 60 80
0.00
0.02
0.04
0.06
0.6
0.8
1.0
No
rma
lize
d I
nte
nsi
ty
Diffraction Scan Number
Ge
GeLi
Ge4Li9
Ge2Li7
Ge4Li15
Phase Analysis
25
EXAFS
26
Conclusions
• Crystalline and amorphous phases able to be probed by
combining XRD and XAS studies
• Nanostructuring not enough to prevent amorphization
brought about by full discharge
• By limiting depth of discharge, crystallinity can be
preserved and restored for at least the first few cycles
27
Acknowledgements
• Ben Richards
• Dr. Michael Lowe
• Dr. Jie Gao
• Prof. Tobias Hanrath
• Prof. Héctor Abruña
Beamline support:
• Dr. Jacob Ruff
• Dr. Darren Dale
• Dr. Ken Finkelstein
• James Pastore
28