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

24

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