Analysis of Lithium Battery Injection Process

The role of lithium battery electrolyte is to conduct ions between the positive and negative electrodes and act as a medium for charging and discharging, just like blood in the human body. How to make the electrolyte fully and evenly infiltrate the interior of the lithium battery has become an important issue. Therefore, the injection process is a very important process that directly affects the performance of the battery.

The injection process is to inject the battery electrolyte into the battery cell in a quantitative manner after the battery cell is assembled. The process can be divided into two steps. The first step is to inject the electrolyte into the battery cell, and the second step is to fully infiltrate the injected electrolyte with the electrode and diaphragm in the battery cell. The infiltration time will affect the production cost of lithium-ion batteries. In this process, too much injection can easily cause the battery to bulge and cause uneven battery thickness. Too little injection will lead to a reduction in battery capacity and cycle times, and uneven injection will lead to consistency differences in battery capacity and cycle performance.

during the commercial battery assembly process, the electrolyte is injected into the sealed chamber through a metering pump, the battery is placed in the injection chamber, and then the vacuum pump evacuates the injection chamber, and a vacuum environment is formed inside the battery. Then the injection nozzle is inserted into the battery injection port, the electrolyte injection valve is opened, and the electrolyte chamber is pressurized to 0.2-1.0Mpa with nitrogen. The pressure is maintained for a certain period of time, and the injection chamber is vented to normal pressure. Finally, it is left to stand for a long time (12-36h) to allow the electrolyte to fully infiltrate the positive and negative materials and diaphragm of the battery. When the injection is completed, the battery is sealed. In theory, the electrolyte will penetrate into the diaphragm and electrode from the top of the battery, but in fact a large amount of electrolyte flows downward and gathers at the bottom of the battery, and then penetrates into the pores of the diaphragm and electrode through capillary pressure, as shown in Fig.1

Usually, the diaphragm is composed of porous hydrophilic materials with relatively large porosity, while the electrode is composed of porous media composed of various particles. It is generally believed that the electrolyte penetrates faster in the diaphragm than in the electrode. Therefore, the flow process of the electrolyte should be to first penetrate into the diaphragm and then penetrate into the electrode through the diaphragm, as shown in Fig.1

Fig.1 Schematic diagram of electrolyte soaking battery cell

In the electrode, a larger cavity is formed between three or four large active material particles, and the cavities are connected by narrow channels between two parallel particles. The electrolyte first converges in the cavity and then diffuses to the nearby throat. Therefore, the wetting rate of the electrolyte is mainly controlled by the throat between the connected cavities and the cavity volume. As shown in Fig.2, the α cavity is composed of four particles and is connected to the surrounding cavities through four throats, and the β cavity is composed of three particles and is connected to the surrounding cavities through three throats.

Fig.2 Schematic diagram of the electrode cavity structure

As shown in Figure 3, the mechanism of electrolyte diffusion in the electrode pores can be seen as the interaction between three forces: the pressure Fl from the electrolyte flow, the capillary force due to the surface tension Fs, and the resistance Fg caused by the air in the pores. When injecting liquid, evacuating the battery can reduce the resistance caused by the air, while pressurizing the electrolyte injection can increase the driving force for liquid flow. Therefore, vacuum-pressurized injection is conducive to the infiltration of the electrolyte.

Fig.3 Schematic diagram of electrolyte diffusion dynamics in pores

The capillary motion of the electrolyte can be described by the Washburn equation:

h is the liquid penetration height at time t, r is the capillary radius, γlv is the liquid-gas surface tension, ϑ is the contact angle, Δρ is the density difference, and η is the viscosity. It can be seen that the viscosity of the electrolyte, the wetting contact angle with the electrode, the surface tension and other characteristics will affect the wetting process.

Electrolyte infiltration is the process of driving air out of the electrode pores. Due to the random distribution of the size and shape of the pore structure, the electrolyte infiltration rate is often different, which causes air to gather near the current collector, surrounded by electrolyte, and trapped in the electrode. The electrolyte infiltration saturation is always less than 1. Almost all large voids are filled with electrolyte, but there are small voids in many places. Small voids represent residual air surrounded by solid particles. Therefore, how to minimize this residual air is the key to improving the degree of infiltration.

In summary, liquid injection will directly affect the performance of lithium-ion batteries. By injecting a certain amount of electrolyte into the battery cell through liquid injection equipment, the technical problem of uneven liquid injection in the liquid injection process can be well solved. Therefore, the liquid injection equipment can be said to be a key factor in achieving a good liquid injection effect in the liquid injection process. If the liquid injection equipment in the lithium battery production line has poor stability and maintainability, the liquid injection method is backward, and it cannot meet the liquid injection requirements with high quality, it will greatly limit the liquid injection production efficiency. Simple and crude liquid injection equipment may also cause safety hazards, increase production costs, and even bring serious quality risks to the company.

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