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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 applications.
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 atomic 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 instrument.
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 μm.
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 characteristic peaks:
- 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, approximately 65%.
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 μm.
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 Co3O4.
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 μm.
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 imaging.
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