Large Cylindrical Lithium-ion Battery Process Difficulties And Solutions

The current production efficiency and yield of large cylindrical batteries are still relatively low, and there are still the following process difficulties in achieving high-efficiency mass production:

1) Full-tab forming: The difficulty lies in controlling the flattening accuracy and strength to avoid damage to the current collector or the generation of debris, dust, etc.

2) Collector plate and post terminal: The difficulty lies in the high requirements for welding accuracy control, penetration control, and pressure control, and both false welding and welding perforation must be avoided.

3) Sealing welding: The difficulty lies in the deviation of the reference plane under high-speed conditions, which affects the welding accuracy. The main pain point is that the nickel-plating layer falls off during welding, causing the shell to rust.

4) Winding: The main pain point is the uncontrollable risk of tab shape changes during cutting, winding, transportation and winding. The difficulty lies in the integrated control of laser control and precision automation after the die-cutting and winding processes are combined, as well as improving the lug cutting quality and winding alignment accuracy through real-time closed-loop control.

5) Electrolyte filling: Since the internal space utilization rate of the large cylinder is higher, its internal stress is stronger, which can easily lead to problems such as difficulty in electrolyte infiltration and low electrolyte filling efficiency.

1. Difficulties and solutions of full-tab forming process

During the manufacturing process of large cylindrical batteries, in order to avoid scratches on the inner wall of the battery cans by the tab when the battery cell is put into the cans, and to ensure the welding effect of the battery tab and the collector plate, full-tab forming process is required. However, since large cylindrical batteries mostly use full-tab process, the number of battery tabs is large, and the full-tab forming process has high requirements, mainly:

1) The speed of full-tab forming is too fast, and the electrode is easy to turn outward.

2) If the full-tab forming process is not well controlled, dust is easy to be generated;

3) The critical stress value of the collector structure is low, resulting in damage to the collector during the shaping process.

2. Difficulties and solutions for collector plate welding process

Laser welding technology is the bottleneck of the yield and production efficiency of large cylindrical batteries with full-tabs. It is mainly in collector plate welding, post terminal welding, and sealing welding. The difficulties in welding collector plates and battery tabs are:

1) The non-coated "blank" part of the edge is very short, and the control requirements for welding accuracy and temperature are high. Technically, it is necessary to avoid both cold welding and welding perforation, and to avoid battery short circuit caused by thermal shrinkage of the separator or splash breakdown due to increased welding temperature.

2) The problem of narrow process window for copper collector plate welding.

The main solutions are:

1) Improve the weldability of collector plate materials, such as reasonable thickness design, surface treatment, etc.

2) Improvement of laser welding technology.

3) Improvement of online detection technology for laser welding quality.

3. Difficulties and solutions for post terminal welding process

The difficulty of post terminal laser welding process is mainly that the post terminal is thick and requires a large amount of energy to penetrate the post terminal. The collector plate is thin, which makes it difficult to control the energy during the welding process and it is easy to weld through the collector plate.

The main solutions are:

1) Specific design of the pole, such as thickness control and surface material treatment.

2) Improvement of laser energy control and welding accuracy.

3) Improvement of online detection technology for laser welding quality.

4. Difficulties and solutions for sealing welding

The difficulty of sealing welding lies in: controlling the welding accuracy and quality under high-speed rotation conditions.

At the same time, laser welding can easily damage the nickel-plating layer of the shell, causing the shell to rust.

For thin-walled shells, it is necessary to ensure both welding strength and the sealing performance of the battery cell, and the control of welding accuracy is higher.

Different from small cylindrical batteries, large cylindrical batteries have lower cans strength, more electrolyte in the battery, and higher internal pressure, which puts higher requirements on the pressure resistance and stability of the port.

In addition to optimizing the welding process, sealing welding can also optimize battery materials such as shells and electrolytes to reduce the internal pressure of the battery or improve the rust resistance of the shell, so as to reduce poor welding and improve the stability of the welding effect.

5. Difficulties and solutions for winding alignment

Compared with small cylindrical batteries, the diameter of large cylindrical batteries is almost doubled. In terms of electrode winding, the main pain point is the uncontrollable risk of changes in the shape of the battery tab during cutting, winding, transportation. The difficulty lies in the integrated control of laser control and precision automation after the die-cutting and winding processes are merged, and the improvement of the pole ear cutting quality and winding alignment accuracy through real-time closed-loop control.

The main solutions are:

1) Applying an integrated production method of laser cutting and winding to reduce the risk of battery tab shape changes in the transit transportation link.

2) Optimize the materials to reduce the error of raw materials such as electrode and reduce winding production defects.

3) Strengthen the detection system, monitor the thickness of each electrode incoming material, the spacing between battery tabs, etc. in real time, and feedback to the back-end winding machine, so that it can make corresponding precision adjustments, thereby improving the alignment accuracy of the winding machine.

6. Difficulties and solutions for electrolyte infiltration

The difficulty of electrolyte infiltration is also one of the difficulties in the production process of large cylindrical batteries. Compared with small cylindrical batteries, the internal space utilization rate of large cylindrical batteries is higher, and the content of active materials (positive and negative electrodes) inside the battery increases, thereby improving the energy density. However, the more compact internal space of the shell leads to higher viscosity of the electrolyte, which greatly reduces the wettability of the electrolyte. In addition, the internal expansion of the cylindrical battery itself squeezes the inner wall of the structural part, which will lead to a reduction in the actual effective contact between the pole piece and the electrolyte, thereby affecting the subsequent cycle and capacity of the battery.

The solution can be achieved by:

1) optimizing the electrolyte formula;

2) using bell-shaped cavity injection;

3) appropriately increasing the vacuum pressure and alternating cycles, etc. to improve the electrolyte infiltration effect and shorten the infiltration time.