Key Points in Lithium Battery Injection Design

The role of lithium battery electrolyte is to facilitate the conduction of ions between the positive and negative electrodes. It serves as a medium for lithium ion transportation and charge transfer. The common electrolyte for lithium-ion batteries consists of inorganic lithium salt electrolyte, organic carbonate solvents, and additives. As a medium for lithium ion migration and charge transfer, it is an essential component of lithium-ion batteries and forms the foundation for achieving high voltage, high energy density, and high cycling performance advantages.

The most important parameters for assessing battery injection are the injection volume, wetting effect (thorough and uniform), and injection accuracy. These three aspects are achieved through the performance of the injection machine. Therefore, the injection machine is also crucial in the lithium battery production process as it directly impacts battery performance.

The principle of the injection machine for electrolyte injection is to inject the electrolyte into the limited internal cavity of the battery (which includes the cell and the unfilled space) using a specific process such as vacuum, pressure, and time. A portion of the electrolyte infiltrates into the interior of the cell (composed of positive and negative electrode sheets and separator), while another portion occupies the unfilled space. The total amount of electrolyte injected is the injection volume. The more electrolyte infiltrates into the interior of the cell, the better the wetting effect relatively. The shorter the time required to infiltrate the electrolyte into the cell, the better the process capability of the filling machine. The deviation between the actual injection volume and the set required injection volume for a specific battery is the injection accuracy. For the same batch of batteries, the better the consistency of the injection volume and the more concentrated the injection volume, indicated by a higher CPK value of the injection weight, the better the overall performance of the injection machine.

A complete set of injection equipment consists of key components such as a vacuum pump, injection pump, electrolyte transfer tank, code reading system, weighing system, MES system, leak detection system, supply system, and settling mechanism.

Vacuum Pump: Screw pumps are commonly used as vacuum pumps in the injection machine. If placed near the injection machine, the vacuum utilization efficiency is high, resulting in energy savings. However, if the vacuum pump is located far away, it requires pipelines to transport the vacuum, and the loss of vacuum due to the pipelines needs to be considered. The longer and narrower the pipelines, the greater the loss of vacuum flow rate and vacuum level.

Injection Pump: Electric pumps are commonly used, with ceramic pump heads. The accuracy of the electric pump for the electrolyte is generally around 0.25%. It is important to avoid pump clogging during actual production.

Electrolyte Transfer Tank: The main purpose of the electrolyte transfer tank is to supply the electrolyte (to the injection cup) at atmospheric pressure and maintain it within a small and constant range. The electrolyte in the electrolyte tank is pressurized to around 0.2 MPa due to the presence of nitrogen gas as a protective gas. The pressure decreases during use. When necessary, a two-layer or double-tank structure can be used for the electrolyte transfer tank. This allows for degassing of the upper tank to improve the consistency and accuracy of the injection volume. Filtering of the electrolyte can also be done if necessary.

MES System: The MES system includes battery barcode scanning, pre-weighing, post-weighing, and checking the injection volume deviation for qualification. It enables the integration of the first injection and second injection processes, as well as the interconnection of the entire factory.

Leak Detection System: Sometimes it is necessary to test the sealing of the sealing rubber plug and the battery. Batteries with inadequate sealing are not filled with electrolyte. The leak detection is performed using either a vacuum or pressure retention method.

Supply System: The supply system includes the injection pump, valve, and pipelines. Some systems are equipped with temporary storage cups to improve efficiency. In some cases, a mobile injection needle method is used, which allows for injection multiple cups with fewer pumps.

Settling Mechanism: The settling mechanism includes the injection cup, sealing rubber plug, battery tray, pressing mechanism, and the pressure-vacuum valve and pipeline system. For pouch batteries（related equipment）, a vacuum-atmosphere cycling settling method is generally used. For cylindrical batteries, a vacuum-atmosphere-positive pressure-atmosphere cycling settling method is commonly employed.