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Lithium batteries, a power source for many portable devices such as
cellular phones, laptops, camcorders, etc. are also used in electrical
vehicles and aerospace and military applications. The development of
lithium batteries is advancing rapidly.
Due to longer life, lower self-discharge rate, and higher energy
density compared to other rechargeable batteries, along with higher
cost-effectiveness and smaller toxicity, layered lithium cobalt oxide
LiCoO2 (Fig. 1) is used as a cathode for commercial
Figure 1. Layered LiCoO2 structure.
The ordinary reversible lithium intercalation of the non-defective
lithium battery is key to rechargeable Li battery. However, effects of
protracted cycling or prolonged storing pose a threat to performance in
course of time. Hence, to improve the life-time of batteries, the
distribution of degraded areas on the surface of positive electrodes
must be understood.
The most valuable methods for structural characterization of the
electrodes in rechargeable lithium batteries are Raman spectroscopy and
force microscopy (AFM). Results of an LiCoO2 cathode
characterization using AFM
and Raman techniques on the NTEGRA
Spectra (NT-MDT) instrument integrated with Renishaw inVia Raman
spectrometer is presented. Simultaneous recording of AFM
and Raman images from the same sample area are allowed by the
Figure 2. Structure of Lithium-ion battery
Figure 2 shows the typical structure of a lithium-ion rechargeable
cylindrical cell. The anode, cathode, and electrolyte are the three
primary functional components of a lithium-ion battery. Carbonaceous
material (graphite is the most popular) is used for the anode, the
cathode is made from LiCoO2, and lithium salt in an organic
solvent is used as the electrolyte.
The individual lithium-ion cells from different laptop lithium
battery packs were examined. The first battery pack, used for about 3
years (about 1200 charging-recharging cycles), had only about 25% of
its nominal capacity, and was ~30% charged. The second battery was new
and ~65% charged.
Figure 3. AFM topography (a),(b), phase
(c),(d), magnitude (e),(f) and optical images (g),(h) of the surface of
LiCoO2 cathodes from the used battery and from the new one.
Pictures (a), (c) exhibit high roughness and grain structure which is
typical for used batteries. Size of all images: 50x50 mm.
Figure 3 shows the surface morphology of the LiCoO2
cathode, as determined by AFM.
Information about the size, shape and orientation is provided by AFM
topography. The lattice constants of cathodes are known to be a
function of lithium concentration in the material (Li1-xCoO2).
Because of this, the cathode from the new battery is much smoother than
the one from the used battery - Li extraction has caused the crystal
cell of the used cathode material to expand, so that the individual
microcrystals can be observed on the surface.
Phase and cantilever oscillation magnitude are also recorded during
the AFM experiment. These can provide additional data to the topography
scan - for example, phase measurements allow more sharp definition of
grain edges, as they are not affected by height differences. Optical
images provide complementary data, although it is not possible to
correlate the optical and AFM images completely.
Raman spectra of the LiCoO2 cathode are dependent on the
electrochemical history. The different spectra observed in Fig. 4 are
from points on the cathode in different states of degradation.
Figure 4. Raman spectra of cathode: LiCoO2
(red), delithiated LiCoO2 (green), and Co3O4
(blue). The resolved bands may be related with the structural
distortion or the surface change during the extraction of Li.
The total irrducible representation for the vibrational modes of
LiCoO2 can be found by factor group analysis to be A1g+ 2 A2u
+ Eg + 2 Eu. The gerade modes are Raman
active, and the ungerade modes are IR active. In the Raman
spectra, only the A1g and Eg bands should be
observed. In this experiment, two strong bands are observed at 472 and
579 cm- 1 with an intensity ratio of 1:3, which correspond
to oxygen vibrations involving v2 (Eg), O-Co-O bending, and
v1 (A1g), Co-O stretching modes, respectively.
Figure 5. Atomic displacements of the
Raman-active modes of LiCoO2
Some changes are seen in the Raman spectra when lithium is extracted
during the charging process (green line in Fig. 4). The main peaks from
the LiCoO2 at 472cm-1 and 579 cm-1
undergo a small shift. The 515cm-1 and 674cm-1 bands increase in
intensity - these can be assigned to vibrational modes of Li2O
and Co3O4 respectively.
We can identify the state of different regions of the lithium ion
battery cathode from their Raman spectra, using the following
- Two intense peaks, 472 and 579 cm-1 characterize the
areas of the cathode in the intercalated state (LiCoO2).
- Comparable intensities of Raman bands, at 579 and 674 cm-1,
characterizes the delithiated cathode, (Li1-xCoO2).
- Strong peaks at 674 cm-1 and disappearance of peaks at
579 cm-1 characterize the degraded areas of cathode.
Figure 6 shows the two-dimensional Raman and AFM
maps from the same place of the cathode removed from the new battery
pack. The higher areas on the AFM
topography correspond to the more intercalated state of the cathode.
Approximately 60% of the cathode area is relevant to the delithiated
material. This is in accordance with the charge level of battery,
Figure 6. AFM and Raman images from the same
place of the new cathode: (a) AFM topography height, (b) phase, (c)
magnitude; (d) Raman intensity map with 579 cm-1 peak, (e)
Raman intensity map using 674 cm-1 peak; (f) chemical map
(red color corresponds to the LiCoO2, green color is
delithiated LiCoO2 (Co3O4 are absent
on this cathode, because the intensity of peak at 674 cm-1
is never rank over the intensity of peak 579 cm-1); (g)
ratio of peak intensities at 579 and 674 cm-1. Black color
corresponds to the delithiated cathode, yellow color corresponds to the
intercalated state of cathode. Size of all images: 50x50 mm.
In the cathode removed from the used battery pack, the Raman images
are more complicated (Fig. 7 a-g). In addition, they also have degraded
Figure 7. AFM and Raman images from the same
place of cathode removed from the old battery: (a) AFM topography, (b)
phase, (c) magnitude; (d) Raman intensity map using 579 cm-1
peak, (e) Raman intensity map using 674 cm-1 peak; (f) ratio
of peak intensities at 579 and 674 cm-1. Black color
corresponds to the Co3O4, because such areas are
characterized by a strong peak at 674 cm-1 and very weak
peak at 579 cm-1;(g) chemical map (blue color corresponds to
the Co3O4 , red color corresponds to the non
degraded LiCoO2 Size of all images: 50x50 mm.
Comparing AFM topography (Fig. 7a) with the Raman map (Fig.
7g) reveals that flatter topography with larger grains and fewer grain
boundaries is seen for degraded areas of the cathode material (outside
of oval areas) and non-degraded parts of the cathode correspond to
areas with a larger proportion of smaller grains and more grain
boundaries (shown by ovals in the Fig. 7).
The importance of correlated AFM-Raman
imaging for Li-ion battery studies is shown in this study. Raman
imaging allows the lithium intercalation processes and degradation of
the cathode to be characterized in detail. The chemical properties of
the cathode can be correlated with its topography by simultaneous AFM
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