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) During charging: Li is deintercalated from a cathode material (such as LiCoO2 material) and embedded in an anode material (such as a Graphite material) through an electrolyte. At the same time, an equal number of electrons enter the anode material along a path opposite to that during discharge.
(2) During discharge: Li+ is deintercalated from the anode material (negative electrode), and the electrolyte is embedded in the cathode material (positive electrode). At the same time, an equal amount of electrons flow out from the anode material, through the anode current collector, the external circuit and the current collector of the positive electrode. Entering the cathode material, so that the positive and negative electrodes respectively undergo oxidation and reduction reactions.
The difference between the charging and discharging process is: When charging, the electrons cannot move spontaneously on the external circuit, and the power must be applied to do the work.
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 electrodes and the theory of concentrated solution. As shown in Figure 1, the actual chemical reaction process inside the battery is considered, including solid-phase diffusion process, liquid phase diffusion and Migration process, transfer process, solid-liquid phase equilibration 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 lithium ion inside the particle. Diffusion process. 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 positive and negative electrodes of the internal process of the reaction can also be obtained. The solid phase concentration distribution and solid phase potential distribution of the material, as well as the liquid phase concentration distribution and solid phase potential distribution of the electrolyte, have the advantages of accuracy, comprehensiveness, and mechanism.
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 concentration of the lithium cobaltate electrode under different SOC state of charge can be obtained. See the local unevenness of the lithium distribution.
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. It is an ideal hypothesis for the electrode process of lithium ion batteries. The neutron diffraction technique is a kind of neutron radiation using different materials. The occlusion rate is different, the technique of analyzing the material. The neutron radiation has strong penetrating power, the scattering length is independent of the atomic number Z, and is also sensitive to light atoms. Therefore, the neutron is responsible for the lithium atom in the lithium ion battery material. Nickel-manganese-cobalt transition metal atoms are very sensitive. We can analyze the distribution of Li in lithium-ion battery in situ without destroying the structure of 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 neutron beam was used to detect the transport and distribution of lithium in the graphite pole piece. The neutron beam penetrates the PTFE packaging material. 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 will The intensity of the sub-spectrum is directly related to the lithium concentration, which can directly quantitatively measure the distribution of lithium concentration on the cross-section of the pole piece.
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 sheet during the first discharge of the graphite electrode sheet. Figure 4a is a schematic view of the pole piece sample and its detection surface, and Figure 4b is a lithium concentration distribution map corresponding to different discharge times. 4c is the potential evolution of the battery at the corresponding time. The lithium concentration of the electrode and its distribution are well correlated with the potential of the electrode. Similarly, Figure 5 shows the lithium concentration distribution of the graphite electrode sheet during the first charge and lithium removal. And the potential at the corresponding moment.
Fig. 4 Electrode cross-section lithium concentration distribution during the first lithium discharge of graphite, (a) photograph, (b) lithium distribution at different discharge times, (c) voltage evolution of the battery. (magnification C/9)
Figure 5: Lithium concentration distribution during the first de-lithium removal of graphite, (a) Lithium concentration distribution at different charging times and (b) Battery voltage evolution (magnification C/9)
The neutron beam patterns in Figures 4 and 5 can be used to quantify the lithium ion concentration. During the discharge/charge process, although the magnification is small (C/9), it is still possible to observe that the pole piece is close to the current collector and close to the sides of the diaphragm. The lithium distribution unevenness, quantitative analysis of this difference is shown in Figure 6, the lithium concentration near the diaphragm side is higher than the collector side, and as the amount of lithium insertion 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 lithium removal from the graphite electrode. As shown in Fig. 7, this part of lithium causes capacity loss and is irreversible capacity. The first four discharges/charge of the graphite electrode In the cycle, the amount of lithium remaining in the graphite electrode is shown in Fig. 8. The irreversible lithium loss mainly occurs in the first cycle, and in the subsequent cycles, the residual lithium amount 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 techniques such as online detection of Raman spectra and on-line detection of x-rays.
