The influence of battery case on the consistency of lithium battery

The consistency of lithium battery cells is an important indicator of power batteries. The performance of single cells determines the overall performance level of the battery pack. If there is a consistency problem, the single cells are in multiple series and parallel connections. Later, it will directly affect the life and safety of the battery pack, as well as the manufacturing cost and maintenance cost of the power battery. Based on this, battery manufacturers and electric vehicle manufacturers all regard the consistency of battery cells as an important consideration.

Part Ⅰ Factors affecting the consistency of lithium battery cells

The consistency of battery cells has caused considerable trouble to battery manufacturers and electric vehicle manufacturers. The inconsistency of battery cell capacity, voltage, and internal resistance not only affects battery performance, but also poses safety risks for battery applications. There are many factors that affect the consistency of the battery, and raw materials, production processes, BMS systems and even factory inspections may all be the source of the problem.

Battery material: the ratio and purity of the electrolyte, the purity of the solvent, the particle and distribution of active material, the stability of the material composition, the stability and uniformity of the diaphragm parameters, including the gap of the diaphragm, thickness error, static electricity, etc.

In terms of equipment: poor stability of battery cell production equipment, low automation, low equipment processing accuracy, and poor equipment and process matching will affect the consistency of power cells.

Personnel operation: In enterprises with a low level of automation, personnel failing to carry out standardized operations, inconsistent job proficiency, fluctuations in employee morale caused by management out of control, and employee mobility will all affect the consistency of power cells.

Production methods: operating instructions are unclear or defective, incoming inspection documents are defective, process changes frequently, process standards are not perfect, etc.

Production environment: factory buildings do not meet standards, equipment failures cause temperature and humidity to do not meet standards, weather effects, etc.

In addition, the design level of the BMS system also directly affects the consistency of the power cell.

Part Ⅱ High-quality battery case materials provide a new guarantee for consistency

For a long time, the industry's attention to solve the consistency problem of power cells has mostly focused on the inside of the battery, mostly from the aspects of diaphragm and electrolyte, but to solve the consistency problem of power cells, the battery case material is indispensable.

Especially in terms of power batteries, the number of power cells for new energy vehicles is relatively large. It is not easy to ensure that hundreds of cells have the same life and the same safety quality. In addition to the chemical system inside the battery, BMS, PACK In addition to seeking solutions, the influence of the case material of the power cell cannot be ignored.

In the full life cycle of the power cell, various application environments will cause the power cell to be vibrated, squeezed or even deformed. Therefore, the quality of the cell case is very important. Good case materials and good technology can be used in To a large extent, reduce security risks.

Lithium battery case materials are generally divided into aluminum case and steel case. Square lithium batteries mainly use aluminum materials that are easy to process and shape, while cylindrical lithium batteries mostly use steel materials with strong physical stability as the case material. In order to prevent the positive electrode active material of the battery from oxidizing the steel case, manufacturers usually use nickel plating to protect the iron matrix of the steel case.

It is worth noting that a link that has a greater impact on the consistency of the power cell is the nickel plating of the case material. After nickel plating and pre-nickel plating, the results are quite different.

The pre-nickel-plated battery case material adopts a special heat treatment diffusion process during the production process, so that a deeper nickel-iron diffusion layer is formed between the nickel layer and the base band, which ensures the compactness, adhesion and synchronous extensibility of the nickel plating. In this way, the nickel-plated steel plate still has excellent coating adhesion after cylindrical processing, and at the same time has stronger corrosion resistance.

The steel strip is first plated with nickel and then stamped into a case, so that the plating layer of the steel case is uniform, and the problem of basically no nickel at the bottom of the plated steel case and excessive nickel layer at the mouth is solved. In the electroplating process of the post-nickel plating process, the inner wall coating is uneven and the coating is easy to fall off. The metal powder is a fatal risk in the lithium battery, and the pre-nickel plating can reduce this risk.

For the consistency of the power cell, the stamping equipment used for the case material is also very important. The more stamping equipment used, the more difficult it is to ensure the consistency of the case material produced, and the greater the accuracy tolerance of its dimensions.

Of course, the high requirements for the consistency of the battery case are things that manufacturers of lithium battery cells must do well. For lithium battery pack assembly manufacturers, what they can do is to choose battery cells with good consistency. Before assembling the battery pack, use a battery cycle tester to do a charge-discharge test to test the actual capacity and self-discharge rate of the battery cell, and to do a grade sorting. After the battery cell capacity sorting is completed, use a battery sorter to test the open circuit voltage and internal resistance of the cells, and do the sorting. Doing the above two points can ensure the consistency of the battery to the greatest extent, thereby improving the quality and cycle life of the battery pack.