Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
XAS Studies of Nanocatalysis and Chemical Transformation
XAS Studies of Battery Materials
Faisal M. AlamgirNational Institute of Standards and Technology
XAS Studies of Nanocatalysis and Chemical Transformation
Acknowledgements• Ela Strauss• Marten denBoer• Chi-Chang Kao - In-situ of LiCoO2 – based battery• Jay F. Whitacre
• Dan Fischer• Zugen Fu• Jason VanSlutman - Local strain of LiCoO2• Lydia Nemirovskaya
• Esther Takeuchi • Amy Colon• Krista Martocci - Role of V and Ag in SVO batteries• Tom Reddy• Steve Geenbaum
XAS Studies of Nanocatalysis and Chemical Transformation
LiCoO2-based Cathodes Set the Benchmark in Performance in a Variety of UsesThese ultra-compact power sources are being
developed for:
• A new generation of spacecraft• Automotive power• Portable electronics • Implantable medical devices, such as drug
delivery systems
XAS Studies of Nanocatalysis and Chemical Transformation
Uses of LixAg2V4O11-based Batteries
• Particularly useful in medical applications• High power delivery• Non-toxic
XAS Studies of Nanocatalysis and Chemical Transformation
LiCoO2-based Microbattery design
2 µm
A microbattery is comprised of the following components:
• An Electrolyte - Lithium phospho-oxynitride glass (LIPON)
• Anode - Metallic Lithium • Cathode - LiCoO2• The cathode is deposited
directly onto Ti current collectors by sputtering, annealed at low temperature (~500oC compared to ~600oC for commercial batteries).
The total cell thickness is less than 20 µm.
XAS Studies of Nanocatalysis and Chemical Transformation
In-situ XAS of LiCoO2-based Battery
• We expect the following things to change as Li is de-intercalated:– Cell parameter (coupled changes in atomic positions)– Oxidation state of Co and/or O for charge compensation– Local ordering (decoupled changes in atomic positions– Scattering amplitude (fewer Li ions)
XAS Studies of Nanocatalysis and Chemical Transformation
Electrochemical behavior
3.85 3.90 3.95 4.00 4.05 4.10 4.15 4.20 4.25 4.30-0.00005
0.00000
0.00005
0.00010
0.00015
0.00020
0.00025
0.00030
0.00035
Capacity(Ah)
Voltage (V)
Charge_Capacity(Ah)
3.85 3.90 3.95 4.00 4.05 4.10 4.15 4.20 4.25 4.30-0.0005
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
Derivative
(dQ/dV)
Voltage (V)
0 2 4 6 8 10 122.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
discharging
rest
charg ing
Volta
ge (V
)
t im e (hours)
minimum
Electrochemically, the charge/discharge behavior corresponds to that of a bulk LiCoO2 cell.
For example, we find the same features on the dQ/dV vs. V plot that has been reported by Reimers et al.
a
bc
XAS Studies of Nanocatalysis and Chemical Transformation
X-ray absorption near-edge structure of Co
0.0
0.5
1.0
1.5
2.0
2.5
0.0
7.7 7.72 7.74
Nor
m. a
bsor
ptio
n [a
.u.]
Photon energy [keV]
1.3 eV
•The white line of the near-edge undergoes a shift of ~1.3 eV
• similar shift for a LixNi0.8Co0.2O2 cell has been reported [Johnson and Kropf].
XAS Studies of Nanocatalysis and Chemical Transformation
Real-space manifestation of cycling
2.0
4.0
0 2 4 6
FT
( χ(k
)*k
2)
R [Å]
0.5
1.0
1.5
2.0
2.5
0 2 4 65 10 15 20 25 30 35
FT
( χ(k
)*k
2)
R [Å]
scan number
Charging
DischargingRest
•The amplitude of the Fourier transform peaks change as a function of charge/discharge•The two ovals in the inset show the regions of the layered structure that correspond to the peaks. •The inter-/intra-layer atoms distances contract as a function of charging.
XAS Studies of Nanocatalysis and Chemical Transformation
PCA Formulation
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
•
⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢
⎣
⎡
•
⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
=
⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
nnnn
n
n
nn
ij
mnmm
n
n
n
mnmm
n
n
n
www
wwwwww
vv
vv
mmm
cccbbbaaa
.....
.
.
.00...0.00.0
............
..
..
..
............
..
..
..
21
22221
11211
22
11
21
33231
22221
11211
21
33231
22221
11211
µµµ
χχχβββααα
[A] = [B] • [V] • [w]t
XAS Studies of Nanocatalysis and Chemical Transformation
PCA: Components and Reconstruction
7.68 7.69 7.70 7.71 7.72 7.73 7.74-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
a
principal component 1 principal component 2 principal component 3 Co XANES at x = 0.12N
orm
aliz
ed a
bsor
ptio
n
Energy (KeV)
0.0
0.4
0.8
1.2
1.6
7.69 7.70 7.71 7.72 7.73 7.74
0.0
0.4
0.8
1.2
1.6
0.0
0.4
0.8
1.2
1.6
Energy (KeV)
x = 0.37
******** experimental___ reconstructed using principal components
Nor
mal
ized
abs
orpt
ion
x = 0.7
b
x = 0.12
XAS Studies of Nanocatalysis and Chemical Transformation
PCA: Possible Reaction Pathways
P
Increasing x
P
Q
R
QR
XAS Studies of Nanocatalysis and Chemical Transformation
7.685 7.690 7.695 7.700 7.705 7.710 7.715 7.7200.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6 C
B
A
x in Li(1-x)CoO2 =
s to d
Nor
mal
ized
Abs
orpt
ion
Energy (keV)
0.12 0.18 0.25 0.37 0.40 0.47 0.55 0.63 0.70
Co K-edge in Li(1-x)CoO2
The Co K-edge XANES in Li(1-x)CoO2 as a function of x.
The s to d transition and s to p transition (A, B and C) are labeled.
Co does appears to be involved in charge recompensation
…because
7.688 7.6900.00
0.01
0.02
0.03
0.04
0.05
0.06
if the d-band occupancy of Co changes with increased delithiation, then this peak amplitude should increase
XAS Studies of Nanocatalysis and Chemical Transformation
Theoretical fit to real-space data
0 1 2 3 4 5 60.0
0.5
1.0
1.5
2.0
2.5x in Li(1-x)CoO2 =
Second clusterCo-Co&Co-Li
Co-O
First cluster
|F
T [χ
'(k)]|
R (Å)
0.12 0.18 0.25 0.37 0.40 0.47 0.55 0.63 0.70
XAS Studies of Nanocatalysis and Chemical Transformation
0.1 0.2 0.3 0.4 0.5 0.6 0.7
1.4
1.6
1.8
2.0
2.2
2.4
2.6
Co neighbor Oxy neighbor 1 Oxy neigbor 2
Nea
rest
nei
ghbo
r dis
tanc
es o
f Co
(Å)
x in Li(1-x)CoO2
Theoretical fit to real-space data
The atomic structure around Co shows charge compensation occurs first by formation of O holes (for x<0.25).
XANES shows us that for x>0.25, charge compensation occurs through Co d-hole formation.
XAS Studies of Nanocatalysis and Chemical Transformation
Spectro-electrochemical Cell
Spectro-electrochemical cell for in-situ XAS at the O K-edge being developed.
Half-cell built and tested.
appropriate
XAS Studies of Nanocatalysis and Chemical Transformation
How Optimal is the Structure in Thin-film LiMCoO2 Cathode?
• The process of thin-film deposition may render the structure to be far from equilibrium.
• Non-equilibrium structure will affect the performance of the cell.
XAS Studies of Nanocatalysis and Chemical Transformation
LiNiO2 cathodes: thin-film and bulk
0.0
5.0
10.0
0.0
-5.0
-10.0
-15.0
5 10 15
χ(k
)*k
3
k [Å -1 ]
thin-film
bulk powdera
0.05
0.1
0.15
0.2
0.25
0.3
0 2 4 6 8
FT
( χ(k
)*k
2)
R [Å]
thin-film
bulk powder
b
•The a) oscillating function χ(k) and its Fourier transform, b) FT[ χ(k)*k2 ] function for LiNiO2, as we go from bulk (blue) to thin-film (red).
•The amplitude of the Ni-Ni peak of the thin-film (in the plane of the substrate) is reduced and a shoulder is observed. This total effect cannot be explained by Jahn-Teller distortions.
XAS Studies of Nanocatalysis and Chemical Transformation
Theoretical simulation of metal environment
0.05
0.1
0.15
0.2
0 2 4 6
FT
( χ(k
)*k
3)
R [Å]
Individual scattering paths with 0.14 Å splitting
Experimental
Model calculated by FEFF
0 2 4 6 8
R [Å]
Ni Environment in LiNiO2 thin filmNi Environment in LiNiO2 bulk powder
Experimental
Model calculated by FEFF
• Our simulation shows that the set of metal-metal neighbors break away from a six-fold symmetry to a two-fold symmetry; i.e., the six atoms are split into a set of two (at a slightly shorter distance) and four (at slightly larger distance).
• The same effect is observed in LiCoO2 thin films.
XAS Studies of Nanocatalysis and Chemical Transformation
EXAFS Experiments Designed to Study the Local Strain in LiCoO2 Thin Films
Samples:
• Multiple 700nm of LiCoO2 was deposited on 2” Si wafer• Samples annealed to different temperatures• Fluorescence EXAFS data collected at X23B and X11B
XAS Studies of Nanocatalysis and Chemical Transformation
Bulk Stress Relaxation With Annealing on Isostructural LiCoO2
100 150 200 250 300 350 400 450 500
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
1 hr.1 hr.
1 hr.
5 hrs.Intri
nsic
Stre
ss (G
Pa)
Anneal Temp ( 0C)
Wafer curvature measurement of stress, shows bulk stress being annealed out with heat treatment
XAS Studies of Nanocatalysis and Chemical Transformation
Co Atomic Neighborhood with Annealing
0 1 2 3 4 5 6
475 oC, 10 hrs.
0 1 2 3 4 5 6
350 oC
0 1 2 3 4 5 6
As deposited
0 1 2 3 4 5 6
100 oC
0 1 2 3 4 5 6
200 oC
0 1 2 3 4 5 6
150 oC
0 1 2 3 4 5 6
700 oC, 2 hrs.
0 1 2 3 4 5 6
bulk powder
R (Å)
Co-O Co-Co
Relaxation of stress takes place in a complex set of steps.
Stress on the Co-O bonds relaxed by 10 hrs. anneal at 475 0C
Stress remains on the Co-Co plane after annealing for 10 hrs at 475 0C.
Stress released by annealing at 700 0C for 2 hrs. However a c-axis expansion remains.
XAS Studies of Nanocatalysis and Chemical Transformation
Angle-resolved XAS
0.0
0.5
1.0
1.5
7.68 7.7 7.72 7.74 7.76
0 -10 -20010
Nor
m. a
bsor
ptio
n [a
.u.]
Photon energy [keV]
Angle/ deg
• No texturing effect picked up by the plane-polarized X-rays.
• Orientation of the thin film with respect to the polarized X-ray beam is not the cause of this effect.
XAS Studies of Nanocatalysis and Chemical Transformation
The same effect is observed in LiCoO2
• The effect of the amplitude change at the Co-Co bond can be modeled by changing the level of strain on the Co-Co bond.
0 2 4 6
Thin-film LiCoO2annealed at 100 deg C for 1 hr.
R (Å)
|FT
{k2 χ
(k)}
|
Thin-film LiCoO2annealed at 200 deg C for 1 hr.
Bulk LiCoO2 theoreticalexperimental
XAS Studies of Nanocatalysis and Chemical Transformation
Experiments at the O K-edge
U7 endstation at the NSLS:
•1200 line grating mono;
•Focused beam (~1.5mm x1.5mm);
•Fluor. and P.E.Y. detector;
•motorized sample manipulator (4 degrees of motional freedom)
XAS Studies of Nanocatalysis and Chemical Transformation
O K Fluorescence Yield from LiCoO2 on Si wafer
0.5
1.0
1.5
2.0
5.4 5.6 5.8
abso
rptio
n [a
.u.]
photon energy [keV]Photon energy (10-1 KeV)
σ*
150, 1 hr250, 5 hrs400, 5 hrs
475, 10 hrsSurface oxide state
The degree of covalent σ*
bonding increases with annealing
at the expense of
surface oxide states.
XAS Studies of Nanocatalysis and Chemical Transformation
O K Electron Yield from LiCoO2 on Si wafer
0.5
1.0
1.5
5.4 5.6 5.8
abso
rptio
n [a
.u.]
photon energy [keV]Photon energy (10-1 KeV)
150, 1 hr250, 5 hrs400, 5 hrs
475, 10 hrs
σ*
Surface oxide state
Surface oxide states dominate the electron yield signal. They diminish with annealing
while
the degree of covalent σ*
bonding increases with annealing.
XAS Studies of Nanocatalysis and Chemical Transformation
Comparison of O K FY data with FEFF8 Theory
XAS Studies of Nanocatalysis and Chemical Transformation
Ex-situ Study of Li Intercalation in Ag2V4O11
Ag and V K-edges are well separated in energy and this cathode is not optimized for in-situ studies.
We expect the following things to change as Li is intercalated:- atomic positions (coupled/de-coupled)- oxidation state of V and Ag with charge compensation
XAS Studies of Nanocatalysis and Chemical Transformation
Change in the Oxidation State of V
5.464 5.466 5.468 5.470 5.472
0.1
0.2
0.3
0.4
0.5
Nor
mal
Abs
orbt
ion
(au)
Energy (keV)
Ag2V4O11 0.72LiAg2V4O11 2.13LiAg2V4O11 5.59LiAg2V4O11 V2O5 VO2 V2O3
XAS Studies of Nanocatalysis and Chemical Transformation
Changes in V Atomic Neighborhood
0 2 40.000
0.001
0.002
0.003
0.004
0.005
FT (k
2 *χ(k
))
Real Space (Å)
Ag2V4O11 0.72LiAg2V4O11 2.13LiAg2V4O11 5.59LiAg2V4O11
XAS Studies of Nanocatalysis and Chemical Transformation
Ionic Neighbor Distances Around Vanadium
Equilibrium ionic distances will be a function of the local Coulumbic interactions
We observe that V-V ionic distance decreases with even 0.79 mol Li/ mol SVO
XAS Studies of Nanocatalysis and Chemical Transformation
0 1 2 3 4 5 60.000
0.001
0.002
0.003
0.004
0.005
FT (k
2 *χ(k
))
Real Space (Å)
LC of 67% Ag2V4O11 & 33% 5.59LiAg2V4O11
2.13LiAg2V4O11
0 2 4 6 80.000
0.001
0.002
0.003
0.004
0.005
FT (k
2 *χ(k
))
Real Space (Å)
LC of 98% Ag2V4O11 and 2% 5.59LiAg2V4O11
0.72LiAg2V4O11
LC of V Ionic Neighborhood
Phase segregation between SVO and the phase at 5.59 mol Li/ mol SVO
However, this phase segregation not observed at lower lithiation
XAS Studies of Nanocatalysis and Chemical Transformation
Ag K-edge analysis of LixAg2V4O11
0 1 2 3 4 5 60.000
0.005
0.010
0.015
0.020
FT(k
2 *χ(k
))
R (Å)
Ag foilAg2V4O11Li0.72Ag2V4O11Li2.13Ag2V4O11Li5.59Ag2V4O11
• FT {χ′} at the silver k-edge of the samples. Features of SVO are well resolved from those of metallic Ag.
XAS Studies of Nanocatalysis and Chemical Transformation
S02 Determination in LixAg2V4O11
S02 (fcc Ag) = 0.79
S02 (Li5.59Ag2V4O11) = 0.77
S02 (Li2.13Ag2V4O11) = 0.76
0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.000.006
0.007
0.008
0.009
0.010
0.011
0.012
0.013
0.014
0.015
0.016
σ2
So2
Li2.13Ag2V4O11 k-weighting 0 k-weighting 1 k-weighting 2 k-weighting 3
N (fcc Ag) = 12N (Li5.59Ag2V4O11) = 11.3
N (Li2.13Ag2V4O11) = 11.25
XAS Studies of Nanocatalysis and Chemical Transformation
Disorder: Ag K-edge analysis of LixAg2V4O11
0 1 2 3 4 5 60.000
0.005
0.010
0.015
0.020
FT(k
2 *χ(k
))
R (Å)
Ag foilAg2V4O11Li0.72Ag2V4O11Li2.13Ag2V4O11Li5.59Ag2V4O11
The remaining difference in amplitude at the Ag-Ag bond is from disorder: σ2 (fcc Ag) = 89% σ2 (Li2.13Ag2V4O11)
XAS Studies of Nanocatalysis and Chemical Transformation
LC XANES Using SVO and Ag as Endpoints
• A different story from Ag EXAFS
0 1 2 3 4 5 60.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Per
cent
age
Amount of Lithium (mol)
Ag Ag2V4O11
XAS Studies of Nanocatalysis and Chemical Transformation
Concluding Remarks
• XANES and EXAFS tell complementary stories– Interpretation of XANES and EXAFS data should be internally
consistent • PCA is very useful in providing objective global picture prior to more
subjective analysis of XANES and EXAFS• Attempt should be made to look at all the relevant atomic species in your
system– Low energy excitations can provide some very useful information, e.g.
3d transition metal L-edge data.• Linear Combination approaches can simplify an otherwise complicated
picture– Especially useful for systems where the correct structural model is not
known.• In EXAFS analysis, de-coupling steps should be taken to extract information
about correlated properties.