In the Raman spectrum, the light intensity is weaker than the infrared spectrum, but the single wavelength laser can easily focus on the specific surface of the sample, so the interference from water or carbon dioxide is less. Raman scattering is the inelastic scattering of light, and its energy is lower than that of human light. The scattering occurs because part of the energy is used for the vibration of molecules in the sample. A human photon induces an electric dipole, which interacts with molecular vibration or vibrational energy levels to form a new energy level. As the electric dipole finally resonates with the human light in the frequency and absorbs or loses vibration or rotational energy, the Stokes spectral line or anti Strokes spectral line can be observed.
As shown in Figure 4.63, electrons are excited to the virtual energy state during the interaction between photons and molecules. When the electron transitions from the virtual energy state to the ground state, it releases light. In energy storage, when the energy of human photon is the same as that of emitted photon, this phenomenon is called Rayleigh scattering; When the energy loss photons have higher vibrational energy, it is called Stokes scattering; If the vibrational energy of the energy photon is lower, it is called anti Stokes scattering. Assumed electric field strength, electric dipole moment M= α E（ α： Polarization. The dipole distance of molecules in the electric field can be expressed as formula (4.72), which consists of Rayleigh scattering and Raman scattering.
Based on Rayleigh scattering, the longer (lower frequency) than it is called Stokes line
The long and short lines are called anti Stokes lines. Since most compounds exist in the form of ground state at room temperature, they usually interact with the light of Stokes line. The Raman shift is the difference between Rayleigh line and Stokes line. Raman spectrum is represented by Raman shift frequency and Raman line intensity.
Raman spectroscopy is used to observe the molecular vibration in the near infrared, mid infrared and far infrared regions and provide information on the structure and composition of samples. This method can complement the infrared spectroscopy. The infrared spectrum measures the vibration energy through the transition dipole moment, while the Raman spectrum is based on different selection rules, that is, the change of polarizability.
The sample is irradiated with a laser beam to maximize the molecular absorption, so that the strong Raman spectrum of chromophore molecular vibration caused by resonant Raman scattering can be obtained. Strong resonance Raman spectroscopy is helpful for the analysis of trace and low concentration samples.
Laser microscope Raman spectrometer is equipped with optical microscope, laser generator, monochromator and high sensitivity CCD (charge coupled detector). By irradiating a laser beam to a part of the sample, we can obtain a high-resolution spectrum in a short time and observe it in detail.
The other form of Raman spectrum is hyperspectral image, which can extract information from thousands of Raman spectra. Because different components in a specific area are represented by different colors, the heterogeneity of the sample can be identified with the naked eye. Figure 4.64 shows LiNio8Coo in different colors 15 Alo. 0502. Hyperspectral images of graphite and ethylene fast black. From the diagram, it is easy to find the composition change caused by electrode deterioration over time . Graphite and ethylene fast black exist inside the electrode, and the surface is LiNio8Coo 15 Alo. The battery of osO2 shows the largest capacity attenuation. This indicates that the cycle performance is affected by the connectivity between particles and whether they are uniformly mixed with carbon.
The confocal microscope irradiates the point along the z-axis of the sample, and the detection can be carried out in the depth direction of the sample with only a small sample area. In addition to the side and depth resolution of 250 nm, the method can also provide high resolution in several micrometer space. Based on the auto XYZ platform in confocal mode, maps of different depth profiles can be obtained. We can also analyze the local structure, composition distribution and different phase structure of the sample.
For lithium secondary battery, the structure of lithium metal oxide and carbon of positive electrode can be analyzed. Recently, confocal microscopy has been used to detect the structural changes with voltage changes before and after non in-situ electrode cycling. The positive electrode shown in Figure 4.65 is composed of LiNi0.8Co0.15Al0.05O2, acetylene black and graphite carbon. According to the different Raman spectra obtained from different parts, we can know that the material distribution at different positions inside the cathode is uneven.
As shown in Figure 4.66, similar to FTIR, in-situ Raman spectroscopy can be used to effectively detect the reaction at the electrode surface . Raman spectra of LiCoO2 electrodes at different voltages can be obtained by using in-situ Raman cells. As shown in Figure 4.67, high voltage will weaken the intensity of Ag mode in LiCoO2. Since the Alg mode corresponds to the vibration of the c-axis , the results show that the orientation of Li1-xCoO2 particles changes in the lattice plane.