Applications Note

QuantumLeap XAS

Analyses of Observed
Speciation of Cobalt in
NMC Cathodes Using
Laboratory XAS
We report the X-ray Absorption Spectroscopy (XAS) investigation of foursamples of NMC electrodes using QuantumLeapTM H2000. Co XANES show
three isosbestic points. Principal component analysis (PCA) and Iterative
Target Transform Factor Analysis (ITTFA) reconstruction of the experimental X-ray Absorption Near Edge Structure (XANES) show that two of thesamples contain two unique species of Co. In the remaining samples, thespeciation of Co can be described by the linear combinations of the two
distinct species, validating QuantumLeap’s synchrotron-like performance.
This applications note discusses battery science enabled by Sigray QuantumLeap XAS.

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Applications Note

Analyses of Observed Speciation of Cobalt in NMC cathodes Using Laboratory
X-ray Absorption Spectroscopy (XAS)
Authors: Dr. Aniruddha Deb, Dr. Srivatsan Seshadri | Sigray, Inc.

Background: Continuing to meet the pace of global energy
demand necessitates higher performance, lower cost, andenvironmentally-friendly

energystoragetechnologies,

including stationary energy storage solutions for power grids
and mobile energy storage solutions for electric vehicles.
Lithium-ion batteries with a high energy density are promising
candidates, not only for electric vehicles but also for grid
applications to efficiently store energy harvested from wind,solar, and other renewable sources. To achieve these goals,

Novel Approach: Sigray QuantumLeap XAS
Sigray’s QuantumLeapTM H200 is a laboratory XAS instrumentthat provides synchrotron-like capabilities and features:•A patented ultrahigh brightness X-ray source,•Wide energy range coverage from 4.5 to 25 keV,•

Cylindrically curved Johansson crystals,•A highly efficient data acquisition approach,•Bothdifferent chemistries are being explored to develop newmaterials for electrodes and electrolytes that can deliver
higher capacity, energy density, operating voltages, structuralstability, electron-transfer rates, safety, and lower cost.
X-ray absorption spectroscopy (XAS) provides a powerfulelement-specific probe to investigate the oxidation state,
short-range structural, and coordination environments,
allowing scientists to understand the complex chemistries at
the atomic level of these newly developed electrode materials
as prepared and under real operating conditions. XAS is
predominantly performed at synchrotron facilities because
they provide tunable energy X-ray beams of high brightness[1,2]. However, scarce beamtime availability limits the scopeof their use as a routine technique for most research groups,
particularly for researchers in industry. Laboratory-based XAS
instruments such as Sigray QuantumLeap enable academicand industrial researchers to perform routine measurements
and analyses to increase their research productivity andshorten the time scales involved to develop novel highefficiency electrode systems.

transmission

andfluorescencemodemeasurement options which allow analyzing samples
with concentrations as low as 0.1% wt, and•Customizability to enable operando experiments undervarious environments.

MethodFour NMC samples were acquired on QuantumLeap
H2000 using a Ge(331) crystal. Co K-edge measurements
were performed in the fluorescence mode due to Co’s
low concentration (~3-5 wt.%) in the NMC electrodes. For
all samples, a quick XANES scan ~300 eV from the Co-K
edge was chosen. The results obtained were processed in
the QuantumLeap GUI and then imported to the Athena
program suite of the IFEFFIT package [3] for further analysis.
The data from Athena were exported and then analyzed forthe principal component analysis (PCA), least squares fitting,and iterative target transformation factor analysis (ITTFA).

Applications NoteExperiments and Results
Co-K edge XANES spectra for all four samples (numbered2A

S1S1, S1S2, S1S3, and S1S4) were collected at 8 hours and areshown in Fig. 1. We can clearly see that the XANES for thefour samples show three isosbestic points (Fig. 1A). The three
isosbestic points qualitatively indicate the presence of two
distinct species of cobalt. Furthermore, since S1S1 and S1S2
samples shown in blue and red represent the most reducedand oxidized species, it allowed us to explore the possibility
that the remaining two Co species in S1S3 and S1S4 are alinear combination of the existing two species.

2B1AFigure 2: Least squares linear combination fitting for S1S3 (2A) and S1S4(2B). For the fits, S1S1 and S1S2 XANES were used as principal components
for the linear combination.

Principal component analysis (PCA) [4] of Co XANES was

1B

performed over the XANES energy range of 7673-7810 eV.
The PCA cumulative variance, shown in Fig. 3A, shows thatwith only 2 components, 99.98% (dashed red line) of thespectra from 4 samples can be accounted for, while the PCA
scree plot (Fig. 3B) also shows that there are only two uniquecomponents of cobalt (red line) present in the samples further
validating the LCF analysis that there are mainly 2 principal
cobalt components for all of the observed electrode XANES.

3A

3B

Figure 1: Co-K edge XANES of NMC electrodes prepared under different
conditions of thermal treatment and coating conditions (1A). A zoomed
view of the XANES region is bottom (1B).

To answer this, we performed a least squares linear
combination fit (LCF) with S1S1 and S1S2 to S1S3 and S1S4
respectively. As shown in Fig. 2, S1S3 (2A) can be representedas a linear combination of 23.5% S1S1 and 76.4% S1S2, while
S1S4 (2B) shows the linear combination of 56.5% of S1S1 and43.5% of S1S2.

Figure 3: Cumulative variance of the PCA analysis (3A). Scree plot for thePCA analysis (3B). Both the Cumulative variance and the scree plot showsthat there are 2 principal components of cobalt (red dash line) for all theNMC electrode samples studied here.

Applications Note

Iterative target transform factor analysis (ITTFA) [5] was thenutilized to align the components that were obtained from5BPCA analysis to the real experimental XANES. The ITTFA
reconstructed experimental cobalt unique species XANES
spectra are shown in Fig. 4.

45C

Figure 4: ITTFA reconstructed experimental two principal XANES
components (component 1 in blue and component 2 in orange) of cobalt
investigated in the series of NMC electrode samples.

The PCA components (components 1 and 2) that were obtained
from ITTFA (shown in Fig. 4) were utilized to perform a leastsquares linear combination fit to the experimental XANES for5D

each of the NMC electrode samples, from which the fractional
composition of each component was determined. The resultsof the fit for each of the NMC electrodes as shown in Fig. 5.
The fit results with the PCA components are shown in Table 1.

5A

Figure 5: Least squares linear combination fit using the ITTFA PCA
components 1 and 2 shown previously in Figure 4. Fit results of thesample are as follows: S1S1 in 5A, S1S2 in 5B, S1S3 in 5C, and S1S4 in 5D.
Colors: The data is shown in blue, fit in red, residual (difference between
the data and the fit) in gray. PCA component 1 is shown in yellow and PCA
component 2 is shown in purple.

Applications Note

SummarySample

PCA Component 1(in %)PCA Component 2(in %)S1S1

099.95

S1S2

96.36

3.73

of the experimental XANES show that there are two unique

S1S374.60

25.16

species of cobalt in the series of NMC electrodes that wereS1S4

41.92

58.94

Investigation of the series of NMC electrodes using Sigray
QuantumLeap H2000 shows the isosbestic points in thecobalt XANES. PCA analysis and the ITTFA reconstruction

studied. The ITTFA PCA component 1 shows that it resembles
the XANES spectra of S1S2 while the PCA component 2resembles the XANES spectra of S1S1. This is also evident

Table 1: Least squares linear combination fits using the PCA components

from the quantitative analysis shown in Table 1. Both linear
combination fits of samples S1S1 and S1S2 and the PCA

From the PCA analysis and the ITTFA component fit resultsin Table 1, we see that S1S1 resembles the PCA component
2 (with 99.95%) as one of the unique Co species and S1S2
resembles PCA component 1 (with 96.36%), while S1S3 andS1S4 are linear combinations of S1S1 and S1S2. These PCA
linear combinations for S1S3 and S1S4 (Table 1) are also ingood agreement with the results that were obtained earlier
by the linear combination least squares fit shown in Fig. 2.

analysis are in agreement and show that there are two uniquespecies of cobalt present in this series of NMC electrodes.
The

above

results

demonstrate

thesynchrotron-like

capabilities of laboratory-based QuantumLeap to produce
the quality of data required for detailed and accurate analyses
using PCA and ITTFA analysis (in this case, to understand thecobalt speciation in NMC electrodes).

1 mm

1. A. Deb, et al. X-ray absorption spectroscopy study of the LixFePO4 cathode during cycling using a novel electrochemical
in situ reaction cell, J. Synchrotron Rad. 11 (2004) 497-504.
2. W.-S Yoon, et al. In Situ X-ray Absorption Spectroscopic Study on LiNi0.5Mn0.5O2 Cathode Material during Electrochemical Cycling, Chem. Mater. 15 (16) (2003) 3161-3169.
3. B. Ravel and M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using
IFEFFIT, J. Synchrotron Rad. 12 (2005) 537–541.
4. I.T. Jolliffe and J. Cadima, Principal component analysis: a review and recent developments. Phil. Trans. R. Soc. A. 374(2016) 20150202.
5. A. Roßberg, T. Reich and G. Bernhard, Complexation of uranium(VI) with protocatechuic acid—application of iterativetransformation factor analysis to EXAFS spectroscopy, Anal. Bioanal. Chem. 376 (5) (2003) 631–638.

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info@sigray.com