Lithium-ion batteries are commonly used for electrical energy storage in portable devices and are promising systems for
large-scale energy storage. However, their application is still limited due to electrode degradation and stability issues. To
enhance the fundamental understanding of electrode degradation this work reports on the Raman spectroscopic characterization
of LiCoO2 and LiNi1/3Mn1/3Co1/3O2 (NMC) cathode materials of working Lithium-ion batteries.
The aim of the work is to gain new insights into the dynamics of cathode materials for lithium-ion batteries through
spatially and time resolved experiments under working conditions to get new knowledge about the cause of fatigue and
difference in the state of charge of individual oxide particles.
As a result of the cycling, the composition of the composite cathode changes locally, e.g. by diffusion of the carbon additive.
This reduces the electronic contact of individual oxide particles to the matrix causing a decrease in the integrated
electrochemical performance. An analysis of individual oxide particles, under working conditions, shows how the state of
charge or the dynamics of individual particles differ from each other. In addition to the heterogeneous composition of the
composite cathodes a heterogeneous behaviour of the oxide particles can be observed . The electrochemical processes do
not proceed homogeneously at different points of the electrode surface. Mapping electrode heterogeneity under working
conditions provides information on the diffusion behavior of lithium ions in the solid and of their de-/intercalation. The
analysis of dynamics of the electrolyte improve the understanding of the diffusion behaviour of individual oxide particles.
The typical interplay of intercalation and solvatation of the active material and the electrolyte is analyzed. In addition to
temporarily inactive or inactive particles of the active material, solid-state diffusion in the active materials is observed.
On the one hand, this shows that the exchange of lithium ions does not take place on the entire electrode surface. On
the other hand, the active materials show heterogeneous intercalation behavior during electrochemical cycling. Active
and inactive behaviour of different oxide particles makes it difficult to identify and apply a spectroscopic indicator for
determining the state of charge of cathode materials.
For further analysis, strategies to increase the sensitivity of in situ/operando spectroscopy have been developed on the
methodological side, through the use of material properties as well as by additives.
The Raman microscope setup was extended. The possibility of spatially resolved multi-channel measurement was developed
and evaluated by metal oxide materials for Lithium-ion batteries (CeO2 und Li2MnO3).
Furthermore, the potential of Raman spectroscopy for spatially and time resolved analysis under working conditions was
explored. The basis for this is provided by resonance-enhanced in situ Raman microscopy, which increases the sensitivity
by exploiting resonance effects in the material. For this purpose, a multi-wavelength approach was used in order
to analyse the materials with different wavelengths and to compare the information content. By selecting the excitation
wavelength, resonance effects in the electrode materials, LiCoO2, LiNiyCo1-yO2 and LiNi1/3Mn1/3Co1/3O2 (NMC), can
be shown. A resonance Raman effect of LiCoO2 for the wavelengths 385 and 514 nm, a pre-resonance effect for the wavelength
632 nm and no resonance effect for the wavelength 256,7 nm was observed. Previously unidentified overtone
signals at 980, 1070 and 1320 cm-1 could be assigned. By Raman signals of the overtone assignment, misorders of the
cobalt ions in LiCoO2 can be detected. The Raman shift of LiNiyCo1-yO2 shows a linear change with the nickel content
that is independent of the excitation wavelength. Thus, the nickel content of the oxides can be determined. The promising
electrode material NMC shows a Raman spectrum depending on the used excitation wavelength. By selecting the
excitation wavelength, due to resonance and absorption effects, different phases of the material are probed. The use of the excitation wavelength 632 nm has the least impact of resonance and absorption effects. There is an indication that
different phases of the single components are present in the NMC. The analysis of the different phases under working
conditions is possible by selecting the excitation wavelength. Last, the excitation wavelength 256,7 nm shows a high
sensitivity to the electrode surface and can be used to detect synthesis residues (LiNO3) and surface species (Li2CO3).
To facilitate the spectroscopic analysis of the SEI (solid electrolyte interface) SHINERS (shell-isolated nanoparticleenhanced
Raman spectroscopy) is applied by using Au nanoparticles coated with a thin SiO2 layer (Au@SiO2). A
surface-enhanced Raman signal of Li2CO3 at 1090 cm-1 was observed during electrochemical cycling as an intermediate.
Its formation/decomposition highlights the role of Li2CO3 as a component of the SEI on LiCoO2 composite cathodes.
The results demonstrate the potential of Raman spectroscopy to monitor electrode/electrolyte interfaces of lithium-ion
batteries under working conditions thus allowing relations between electrochemical performance and structural changes
to be established. | English |