The characterization of the composition of electrode materials for lithium-ion batteries mainly includes inductively coupled plasma (ICP), X-ray fluorescence spectrometer (XRF), energy dispersive ray spectroscopy (EDX), secondary ion mass spectrometry (SIMS) and so on. Among them, SIMS can analyze the depth distribution of elements and has high sensitivity. The characterization of element valence states mainly include scanning transmission ray imaging (STXM), electron energy loss spectroscopy (EELS), X-ray near-edge structure spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), etc. Since the change in the valence state leads to the change in the magnetic properties of the material, the information about the change in the valence state of the elements in the material can also be obtained indirectly by measuring the magnetic susceptibility, paramagnetic resonance (ESP), and nuclear magnetic resonance (NMR). If it contains Fe and Sn elements, it can also be studied by Mossbauer spectroscopy. In addition, for the measurement of the carbon content in the carbon-coated electrode material, a carbon-sulfur analyzer can be used.
The morphology of electrode materials is generally characterized by scanning electron microscope (SEM), transmission electron (TEM), STXM, and scanning probe microscope (SPM). The atomic force microscope (AFM) in SPM is widely used in the observation of the surface morphology of thin film materials and metallic Li, and is mainly used in the observation of nano-level flat surfaces. Characterization of the crystal structure of materials mainly includes X-Ray diffraction (XRD), extended X-Ray absorption fine spectroscopy (EX AFS), neutron diffraction (neutron diffraction), and nuclear magnetic resonance (nuclear). magnetic resonate, NMR) and spherical aberration correction scanning transmission electron microscope, etc. Vibrational spectroscopy (infrared spectroscopy and Raman spectroscopy) is very sensitive to the symmetry properties and local bonding of materials, and can quickly provide structural information of materials. Therefore, it has been widely used in solid-state chemistry and other fields. Vibrational spectroscopy can perform qualitative analysis of materials, and can detect amorphous and semi-amorphous compounds that are difficult to analyze by X-ray diffraction methods. If there are certain groups of atoms, complex ions, etc. which can be regarded as isolated kinetics in the crystal, that is, when the frequency of some of their internal vibrations or all internal vibrations is significantly higher than the external vibrations, then identify the vibrations of certain crystals It’s greatly simplified. The spectra of a series of compounds containing such atomic groups or complex ions have common characteristics, and these characteristics are related to their internal vibrations. In addition, Raman scattering can also judge the crystal structure and its symmetry through the characteristic peaks and peak widths related to lattice vibration.
In addition to the conventional charging and discharging tests, electrochemical characterization mainly includes cyclic vol-tanmogram (CV) and electrochemical impedance spectrosco-py (EIS). Cyclic voltammetry is the most popular in electrochemical research. One of the commonly used test methods, based on the spike potential and peak current in the CV diagram, can analyze and study the electrochemical reaction of the electrode within the potential range, identify the reaction type, reaction step or reaction mechanism, and judge the reversibility of the reaction. And to study chemical reactions such as adsorption, passivation, deposition, diffusion, and coupling on the electrode surface. Electrochemical impedance test is also one of the most commonly used test methods in electrochemical research, and can obtain information about ohmic resistance, adsorption/desorption, electrochemical reaction, surface film and kinetic parameters of electrode process.
Since the extraction/intercalation of lithium ions in intercalation compounds is a key step to realize energy storage and output, the intercalation and deintercalation kinetics of ions in these materials has become a very important parameter to characterize their electrochemical performance. For lithium ion batteries, the commonly used electrochemical test methods to characterize the kinetics of lithium ion insertion and desorption mainly include cyclic voltammetry, electrochemical impedance spectroscopy, constant current intermittent titration (GITT) and potential step method (PSCA) Wait.
Cyclic voltammetry can be used to obtain the linear relationship between the peak current (Ip,) and the square root of the scan rate (V^1/2) at different scan rates. Figure 1-2 shows the LiNi0.5Mn1.5O4 with Fd-3m space group structure synthesized at 700℃, the CV curve of the material at different scanning speeds and the linear relationship between the peak current and the square root of the scanning speed.
The electrode reaction is controlled by lithium ion diffusion, which conforms to the semi-infinite solid phase diffusion mechanism. For semi-infinite diffusion-controlled electrode reactions, the diffusion coefficient of lithium ions can be calculated using the Randles-Sevcik formula:
In the formula, Ip is the peak current, A; A is the electrode surface area, cm²; n is the number of reaction electrons (for lithium ions, n=1); DLi is the diffusion coefficient, cm²·s-1; CLi is the concentration of lithium ions , Mol·cm-3. The calculated lithium ion diffusion coefficient is between 4.7×10-9~8.27×10-9cm²·s-1, and the average lithium ion diffusion coefficient is 6.33×10-9cm²·s-1.