Studies of Local Degradation Phenomena in Composite Cathodes forLithium-Ion Batteries Page: 4 of 5
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Interestingly, almost all micro-Raman spectra collected
from the anode and separator reveal contributions from
inorganic and organic components of the SEI layer. The peak
at 1091 cm-1 corresponds to Li2CO3 and the broad band at 1450
cm-1 as well as peaks that overlap with carbon D and G bands
could be consistent with the symmetric vibrations of -C-O and
-C=O groups of organic products of the electrolyte
decomposition. Micro-Raman spectra of the anode and
separator were often hampered by very strong fluorescence
signal which comes form fluorophosphate compounds the
decomposition of LiPF6 to [24,25,26].
Carbon
u2Co0
400 600 800 1000 1200 1400 1600 1800
Wavenumber, cm 1
Figure 5. Micro-Raman spectra of carbon particles found at the surface
of the Li anode and trapped in the separator of the cycled cell.
To extract carbon particles from the anode and eliminate
contributions from the SEI layer organic and inorganic
contaminants, the Li anode was dissolved in distilled water.
The remaining black residue was filtered out, dried and
analyzed. Figure 6 shows micro-Raman spectra of the black
residue with reference to the spectra of 13C carbon black and1000 1200 1400 1600
Wavenumber (cm-)10
Figure 6. Micro-Raman spectra of the carbon residue removed from the
Li-anode of the cycled cell. The spectra of "3C carbon black and C
graphite additives are provided for reference.12C graphite. Interestingly, all carbon particles were highly
amorphous unlike the original graphite and 13C carbon black
On the other hand, contributions from the corresponding 13C
and 12C carbon D and G bands can be clearly identified.
These amorphous carbons could be natural contaminants
of the graphite and 13C carbon black additives. They also could
result from mechanical processing during electrode
manufacturing. Another possible mechanism of carbon additive
degradation in the cathode is gradual carbon particle
delamination due to PF- anion intercalation-deintercalation
during electrochemical cycling.
There have been numerous studies of PF- anion
intercalation into graphite for potential applications in "dual
graphite" cells [27,28,29]. Discharge capacities of about 100
mA/g were reported the potentials above 4.2 V. The
irreversible losses of capacity observed during charge-
discharge cycles prevent any practical application of the
graphite cathode in a non-aqueous battery cell. It was
suggested that electrolyte decomposition and/or graphite
structure destruction by cointercalation of PF- ions and solvent
molecules is responsible for the graphite electrode degradation
[29]. Unfortunately, no similar studies were carried out on the
effects of anion intercalation into amorphous carbons.
However, one can assume that carbon blacks could exhibit a
similar mode of degradation. Experimental evidence to support
this hypothesis will be the subject of a separate study.
Although the amount of carbon additives that could
undergo structural damage during mechanical processing and
charge-discharge cycling is very small, the consequences for
the electrochemical performance of the composite cathode
could be severe. Even slight loss and/or rearrangement of
carbon additives on the surface of the active material
agglomerates can lead to an increased resistance within the
agglomerate where small crystallites were originally in poor
electronic contact with their neighbors. The loss of direct
electronic contact between primary particles and the receding
carbon matrix will lead to partial and, eventually, total isolation
of some oxide particles, which responsible for the observed cell
power and capacity fade.
The experimental evidence presented in this work
suggests that carbon particle decrepitation upon anion
intercalation/deintercalation followed by removal from the
cathode is the likely mode of the composite cathode
degradation. However, other detrimental processes such as loss
electrode mechanical integrity, gas evolution, surface film
formation, and electrophoretic transport of fine carbon particles
across the separator toward the anode can also affect the
electrode electrochemical performance. Further fundamental
studies of the specific phenomena will be conducted to
determine the critical issues of the materials science and Li-ion
battery engineering.
AcknowledgmentsThis work was supported by the Assistant Secretary for
Energy Efficiency and Renewable Energy, Office of
0 FreedomCAR and Vehicle Technologies of the U.S.
Department of Energy under Contract No. DE-AC03-
76SF00098.Carbon residue
13C carbon
black
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Kerlau, M.; Marcinek, M.; Srinivasan, V. & Kostecki, R.M. Studies of Local Degradation Phenomena in Composite Cathodes forLithium-Ion Batteries, article, November 1, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc902351/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.