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Distribution Of Lithium Concentration In A Lithium Battery
- Jul 06, 2018 -

The cathode material is removed or embedded in the charge and discharge process, and the lithium concentration distribution is directly related to the charge state of the material, which is closely related to the stress and strain of the volume expansion or contraction of the electrode material. If we know the lithium distribution in the lithium ion battery pole, we can get a lot of electrode response information, understand the charge and discharge process, and explain the failure mechanism of the battery.

The working principle of lithium ion batteries:

(1) when charging: Li is inlaid from the cathode material (such as LiCoO2 material) and embedded in the anode material (such as Graphite material) through electrolytes, while an equal number of electrons enter the anode material along the opposite path with the discharge.

(2) discharge: Li+ is inlaid from anode material (negative electrode) and embedded in cathode material (Zheng Ji) through electrolytes. At the same time, an equal number of electrons flow out from anode material, through negative electrode fluid, external circuit and Zheng Ji's collector into cathode material, so that positive negative electrode is oxidized and reduced respectively.

The difference between charging and discharging is that when charging, the electrons can not move spontaneously outside the circuit, so the external power is needed to do the work.

Electrochemical simulation prediction of lithium concentration distribution

The electrochemical pseudo two-dimensional (P2D) model of lithium ion battery is based on the theory of porous electrode and the theory of concentrated solution. As shown in Figure 1, the actual chemical reaction process in the battery is considered, including the solid phase diffusion process, the liquid phase diffusion and migration process, the load transfer process and the equilibrium process of the solid-liquid phase potential. The Butler-Volmer equation is used to describe the electrochemical reaction on each electrode and the process of embedding and releasing the lithium on the surface. The diffusion process of lithium ion in the particles is described by the Fick second diffusion law. Some partial differential equations describing the reaction process and the corresponding boundary condition constitute the model. The charge discharge curves of the external characteristics of the reactive batteries can be obtained at a very short time. At the same time, the solid state concentration distribution and the solid state potential distribution of the positive and negative materials and the liquid concentration of the electrolyte can be obtained. The details of cloth and solid potential distribution are accurate, comprehensive and based on mechanism.


Figure 1 electrochemical pseudo two-dimensional (P2D) model for lithium ion batteries

The distribution of lithium in the detailed electrode materials can be calculated when the pseudo two-dimensional model is extended. The distribution of lithium in the detailed electrode materials can be calculated. As shown in Figure 2, the lithium cobalt acid electrode is distributed in different SOC charge states, and the local inhomogeneous distribution of lithium can be seen.


Figure 2 simulation results of lithium concentration distribution in lithium cobalt electrode

On line detection of lithium concentration distribution by neutron diffraction

The distribution of lithium concentration predicted by electrochemical simulation can explain many problems, but this is not a real result after all. It is an ideal assumption for the electrode process of lithium ion batteries. Neutron diffraction technology is a technology for analyzing materials by using different materials for different shielding rates of neutron radiation. The penetration force of neutron radiation is strong, the scattering length is independent of the atomic number Z and is sensitive to the light atom. Therefore, the neutron is very sensitive to the lithium atom and the nickel manganese cobalt transition metal atom in the lithium ion battery material. We can carry out the distribution of the internal Li in the lithium ion battery without destroying the lithium ion battery structure. The analysis of the position.

Owejan et al. Used the device shown in Figure 3 to assemble graphite negative electrode and lithium plate into a semi battery. The transmission process and distribution of lithium in graphite electrode were detected by neutron photography. The neutron beam penetrates the PTFE package material, imaging the cross section of the battery electrode, directly detecting the distribution of lithium on the cross section of the electrode, the single side coating of the electrode, the width of 5, and the length of 15 of the detection surface, as shown in Figure 4a. Then, through theoretical analysis, they establish direct relation between the intensity of the neutron spectrum and the concentration of lithium, so that the distribution of lithium concentration on the section of the electrode can be measured directly.


Fig. 3 lithium battery construction device for high resolution neutron on-line detection

Fig. 4 is the distribution of lithium in a graphite electrode during the first discharge process. Figure 4a is a diagram of the electrode sample and its detection surface. The figure 4b is the distribution map of the lithium concentration corresponding to the different discharge times. The diagram 4C is the evolution process of the potential of the battery at the corresponding time. The lithium concentration and distribution of the electrodes correspond well to the electric potential of the electrodes. Similarly, FIG. 5 is the lithium concentration distribution and the corresponding potential at the time when the graphite electrode is detached from lithium during the first charge.


Figure 4 the distribution of electrode cross section lithium concentration in the first discharge of graphite in the process of graphite discharge, (a) photographic schematic diagram, (b) the lithium distribution at different discharge times, (c) the voltage evolution of (c) battery. (rate C/9)


Fig. 5 the distribution of lithium concentration during the first lithium charging process, (a) the lithium concentration distribution at different charging time and (b) the voltage evolution of the battery (C/9).

The neutron beam patterns in Figures 4 and 5 can quantitatively analyze the concentration of lithium ions. In the discharge / charging process, although the ratio is very small (C/9), it is still possible to observe the uneven distribution of lithium distribution near the collector and the two sides of the diaphragm. This difference quantitative analysis, as shown in Figure 6, is higher near the side of the diaphragm side than the collector side, and the difference increases with the increase of the lithium intercalation.


Fig. 6 difference of lithium concentration between electrode plate and collector side during discharge process

In addition, the author pays attention to the lithium ion concentration in the electrode after the graphite electrode is embedded and then degenerated in the electrode. As shown in Figure 7, this part of lithium leads to the capacity loss and irreversible capacity. In the fourth discharge / charging cycle of the graphite electrode, the amount of lithium remaining in the graphite electrode was shown in Figure 8. The irreversible lithium loss occurred mainly in the first cycle, and the residual lithium amount was almost no longer changed after several cycles.


Fig. 7 lithium distribution in the first cycle full charge state pole piece.


Fig. 8 the first 4 cycles of discharge capacity and the remaining lithium capacity

With the development of experimental technology, researchers continue to develop on-line detection technology to study the mechanism of lithium ion battery. Besides neutron beam on-line detection, there are many technologies such as on-line Raman spectrum detection, X ray online detection and so on.