In energy storage, electromagnetic radiation can be divided into different types according to energy and frequency, and infrared spectroscopy (FTIR) and Raman spectroscopy (RAMAN) are commonly used electromagnetic radiation detection technologies.
The compounds with covalent bonds in the infrared spectrum absorb electromagnetic waves in the infrared region of the electromagnetic spectrum, and the covalent bonds will stretch or bend under the influence of infrared. When the molecular vibration frequency is consistent with the infrared frequency, infrared absorption occurs [15,16]. The absorbed infrared energy causes the transition between energy levels, and the process is described in Figure 4.58.
When the molecule in the ground state (n=0) is excited (n=1), a basic transition will occur and the amplitude will increase. The wavelength of the basic transition is shown as follows:
V (0 → 1) represents the transition from the ground state to the first excited state, Xa is the probability of transition, v0 is the wavelength of the ground state, and c is the speed of light. Due to weak absorption, the harmonic transition to the higher excited state (n=2,3,4) is not obvious in the spectrum. The vibrational transition probability is proportional to the second power of the transition dipole distance.
Where, (v) and (v ‘) are the vibration wave functions of the initial and final states; μ Is the dipole moment.IR absorption will only occur due to changes in vibration mode or dipole moment. The strength of absorption band (or peak) is defined as follows:
Where, μ Is the dipole moment; Q is the normal coordinate.
Different absorption spectra of different organic functional groups can be observed in the mid infrared region of 4000~400 cm-1 (see Figure 4.59). The change of the wave number or intensity of the absorption band represents the change of the chemical structure of the compound or the surrounding chemical environment. Infrared spectra can be obtained by different methods, including transmission, reflection, diffuse reflection and internal reflection .
1) The transmission mode is only applicable to transparent plates or transparent samples where the human radiation is not absorbed. Mix the powder sample with KBr, and press it into a small transparent disc for testing. The absorption of infrared radiation can be expressed by transmissivity and absorptivity.
2) The reflection mode is also called reflection absorption infrared spectrum (RAIRS) or infrared reflection absorption spectrum (IRAS). It is used for metal single crystal samples and adopts small angle incidence.
3) In the diffuse reflection mode, the human radiation angle scatters on the rough surface of the powder, and the scattered light is collected to obtain the absorption spectrum. This method is suitable for samples with low transmittance.
4) In internal reflection, continuous reflection occurs when light enters internal reflection elements (IREs) (such as diamonds, faults and ZnSe). The sample is coated on IRE, and the light is absorbed by the sample due to its low reflection coefficient in the infrared region. At attenuated total reflection (ATR) is caused by the reduction of IRE reflectivity.
Because each internal reflected infrared light is mainly absorbed on the sample surface, surface analysis can be carried out.
The following information can be obtained by FTIR analysis of lithium ion battery:
1) Composition and local structure of liquid and polymer electrolyte;
2) Local structure of inorganic electrode materials;
3) The composition of SEI film on the electrode surface was determined by non in-situ and in-situ analysis.
Figure 4.60 is an example of the non in-situ internal reflection spectrum of FTIR analysis, which gives the composition analysis results of the SEI film formed by the reduction of 1M LiPF6/EC: EMC (3:7) electrolyte on the electrode surface. By comparing the spectra of the reference sample and the electrode surface, it can be found that the SEI film contains vinyl lithium bicarbonate (CH2OCOLi) 2 . The existence of lithium alkyl carbonate (LiOCO2R) in SEI membranes and whether it is converted into crisps (ROCOR ‘) or carboxylates (R-CO2 -) are controversial. However, the peak of lithium vinyl carbonate can be clearly observed in the infrared spectrum, and the calculated frequency is consistent with the test frequency. The corresponding molecular structure is shown in Figure 4.61.
The electrolyte decomposes with the redox reaction on the electrode surface to form SEI film. In situ FTIR is the most effective method to obtain information about membranes. Figure 4.62 shows the infrared spectrum (SNIF-TIRS) obtained by differential normalized interface Fourier transform on an in-situ FTIR cell. More information about the formation mechanism of SEI film can be obtained by analyzing the IR spectra obtained from PC decomposition under different voltages .
In Figure 4.62b, the peak of 1731 cm-1 observed at 4.2 V corresponds to monkey base, while the peak of 1413 cm-1 and
The 1221cm-1 peak corresponds to C-O stretching vibration of carboxyl group. From this we can see that the oxidation of PC to carboxyl group starts at 4.2 V.