The positive and negative materials are desorbed or embedded in lithium ions during charge and discharge. The lithium concentration distribution is directly related to the state of charge of the material, and is closely related to the stress and strain when the volume of the electrode material expands or contracts.In the lithium ion battery pole piece, if you know the lithium distribution, you can get a lot of electrode reaction information, understand the charge and discharge process, and explain the battery failure mechanism.
How lithium-ion batteries work:
(1) At the time of charging: Li is deintercalated from a cathode material (for example, a LiCoO 2 material), and an electrolyte is intercalated into an anode material (for example, a Graphite material), while an equal amount of electrons enter the anode material in a path opposite to that at the time of discharge.
(2) During discharge: Li+ is deintercalated from the anode material (negative electrode), and the electrolyte is embedded in the cathode material (positive electrode), while an equal amount of electrons flow out from the anode material, through the current collector of the negative electrode current collector, the external circuit, and the positive electrode. The cathode material is introduced to cause oxidation and reduction reactions of the positive and negative electrodes, respectively.
The difference between the charging and discharging processes is that when charging, the electrons cannot move spontaneously on the external circuit, and the power supply must be applied.
Electrochemical simulation for predicting lithium concentration distribution
The electrochemical pseudo-two-dimensional (P2D) model of lithium-ion battery is based on the theory of porous electrode and concentrated solution theory. As shown in Figure 1, the actual chemical reaction process inside the battery is considered, including solid phase diffusion process, liquid phase diffusion andMigration process, transfer process, solid-liquid phase potential balance process. The Butler-Volmer equation is used to describe the electrochemical reaction on each electrode and the surface embedding and deintercalation process. Fick's second diffusion law is used to describe the diffusion process of lithium ions inside the particle. Several partial differential equations describing the reaction process and corresponding boundary conditions constitute a model. The charge and discharge curves of the external characteristics of the reaction cell can be obtained in a short calculation time, and the solid phase of the positive and negative materials in the internal process can also be obtained.Details such as concentration distribution and solid-phase potential distribution, as well as liquid phase concentration distribution and solid-phase potential distribution of the electrolyte, have the advantages of accuracy, comprehensiveness, and mechanism-based.
Fig.1 Electrochemical pseudo two-dimensional (P2D) model of lithium ion battery
The pseudo two-dimensional model is extended. When the geometric model adopts a three-dimensional structure, the lithium distribution in the electrode material can be calculated in detail. As shown in Fig. 2, the lithium cobaltate electrode has a lithium concentration distribution under different SOC state of charge. See local unevenness in the distribution of lithium.
Figure 2 Simulation results of lithium concentration distribution of lithium cobaltate electrode
Neutron diffraction on-line detection of lithium concentration distribution
The lithium concentration distribution predicted by electrochemical simulation can explain many problems, but this is not a true measurement result, and is an ideal assumption for the electrode process of lithium ion batteries. The neutron diffraction technique is a technique for analyzing materials by using different materials for different occlusion rates of neutron radiation.Neutron radiation has strong penetrating power, the scattering length is independent of atomic number Z, and is sensitive to light atoms. Therefore, neutrons are very sensitive to lithium atoms and nickel manganese cobalt transition metal atoms in lithium ion battery materials. The in-situ analysis of the distribution of Li inside the lithium ion battery was carried out without destroying the structure of the lithium ion battery.
Owejan et al. used the device shown in Figure 3 to assemble a graphite negative electrode and a lithium plate into a half-cell. The transmission process and distribution of lithium in the graphite pole piece were detected online by neutron photography. The neutron beam penetrates the PTFE encapsulating material, and the cross-section of the battery pole piece is imaged, and the distribution of lithium in the cross section of the electrode is directly detected. The one-sided coating of the pole piece has a width of 5 mm and a length of the detecting surface of 15 mm, as shown in Fig. 4a. Then, through theoretical analysis, they establish a direct relationship between the intensity of the neutron spectrum and the lithium concentration, so that the distribution of lithium concentration on the pole piece can be directly quantitatively measured.
Figure 3 is a lithium battery construction device for high-resolution neutron online detection
Figure 4 is a diagram showing the distribution of lithium embedded in the electrode sheets during the first discharge of the graphite electrode sheets. 4a is a schematic view of a pole piece sample and its detecting surface, FIG. 4b is a lithium concentration distribution map corresponding to different discharge timings, and FIG. 4c is a potential evolution process of the battery at a corresponding time. The lithium concentration of the electrode and its distribution correspond well to the potential of the electrode. Similarly, FIG. 5 is a lithium concentration distribution of the graphite electrode sheet during the first charge-off of lithium and a potential at a corresponding time.
Figure 4 shows the lithium concentration distribution of the electrode section during the first lithium insertion process of graphite, (a) a schematic diagram of the photo, (b) the distribution of lithium at different discharge times, and (c) the voltage evolution of the battery. (magnification C/9)
Fig. 5 Distribution of lithium concentration during the first delithation of graphite, (a) distribution of lithium concentration at different charging times and (b) voltage evolution of the battery (magnification C/9)
The neutron beam patterns of Figures 4 and 5 allow quantitative analysis of lithium ion concentrations. During the discharge/charge process, although the magnification is small (C/9), it is still possible to observe the uneven distribution of lithium near the current collector and the two sides of the separator. The quantitative analysis of this difference is shown in Fig. 6. The lithium concentration near the separator side is higher than the collector side, and as the amount of lithium intercalation increases, the difference increases.
Figure 6. Difference in lithium concentration embedded in the diaphragm and collector side of the pole piece during discharge
In addition, the authors pay attention to the lithium ion concentration remaining in the pole piece after the lithium electrode is intercalated with lithium, as shown in Fig. 7, this part of lithium causes capacity loss and is irreversible capacity. In the first four discharge/charge cycles of the graphite electrode, the amount of lithium remaining in the graphite electrode is as shown in Fig. 8. The irreversible lithium loss mainly occurs in the first cycle, and in the subsequent cycles, the amount of residual lithium hardly changes.
Figure 7 The first 4 cycles of discharge capacity and the residual lithium capacity
With the development of experimental technology, researchers continue to develop online detection technology to study the mechanism of lithium-ion batteries. In addition to online detection of neutron beams, there are many technologies such as Raman spectrum online detection and x-ray online detection.
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