Author: Zhu Yulong

Today, let’s talk about the technology of assembling battery cells inside a battery.

In the discussion of the technical route for the power battery pack, the controversy about choosing cylindrical, pouch, or prismatic cells continues. Similar controversies also appear on the feasibility of cell manufacturing. There are two competing techniques for assembling mid-section cells in lithium batteries: stacking process and winding process.

These two methods compete in terms of utilization rate of cell space, cell life, manufacturing efficiency, and scale of manufacturing investment.

Differences between Stacking and Winding Techniques

The two methods have different approaches:

• Winding process

By controlling the factors such as speed, tension, size, and deviation of the pole piece, the matched pole piece, diaphragm, and tape are divided into strips and rolled into pole cores according to size.

• Stacking process

The stacking process is a production technology in which pole pieces and diaphragms are alternately stacked together to form a multilayer stacked pole core.

As shown in the above figure, from the perspective of battery shape, both pouch and prismatic cells are designed and produced around the stacking process, while prismatic cells can use both the stacking and winding process.

Currently, China’s battery companies’ main technical direction is still mainly oriented around the winding process. As a mature product form, cylindrical batteries have always used winding process. However, in the longterm, with the advancement of the stacking process technology, a large number of battery companies are beginning to transition from the original winding process to the stacking process era.

Advantages of Stacking Technology

From the perspective of the final battery product, batteries produced using the stacking process have higher energy density, more stable internal structure, higher safety, and longer life compared to products produced using the winding process.

• Higher Energy Density

The wounding process used in cylindrical cells has a curvature at the winding corner, resulting in lower spatial utilization efficiency. In contrast, the stacking process can fully utilize the space of the battery. Therefore, under the same volume design of the battery, the energy density is correspondingly improved.

• More Stable Structure

In the use of batteries, the positive and negative electrodes will both expand due to the insertion of lithium ions. The wound battery will wave-like deform at the corner where the internal stress of the outer and inner layers is uneven, causing the battery’s interface to deteriorate, uneven current distribution, and the internal structure of the battery will become increasingly unstable. This process will accelerate. In contrast, due to the equal expansion force of each layer, the stacking process’s battery can maintain a flat interface during the cyclic use.

• Higher Safety

The expansion and contraction of electrode plates and the stretching of diaphragms will cause battery deformation. Minimizing cell deformation is a guarantee of battery safety. During the wounding process, the bending of both ends of the electrode plate will cause large bending deformation of the coating material, which is prone to problems such as dusting and burrs. The pulling force on the electrode plate and diaphragm is prone to wrinkling and uneven problems. In contrast, the battery of the stacking process has uniform force and small deformation, and higher safety.

• Longer Cycle Life

Cycle life is one of the key performance parameters of batteries. The stacking process has more electrode tabs, shorter electronic transmission distance, smaller resistance, and less heat generation. The winding process is prone to problems such as deformation and expansion, affecting battery decay performance.

Disadvantages of the stacking process

The stacking process also has its problems: low production efficiency, high equipment investment, low yield rate, etc.

Low Equipment EfficiencyThe efficiency of general power battery winding is 12PPM, and the efficiency of square winding machine can reach this efficiency when the pole piece length is 6000mm. However, the traditional Z-folded stacking has only 4PPM efficiency, which is three times lower.

In addition, the footprint of the stacking equipment is larger than that of the winding machine, which further reduces efficiency.

High equipment investment

From the perspective of a single production line, the number of winding machines required is related to the length of each battery. Typically, one production line requires 10 winding machines with an investment of around 30 million yuan.

Using the stacking process, the number of stacking machines required for a production line is related to the number of battery sheets. The investment in stacking equipment for a production line is 60 million yuan or even more.

Low yield rate

Winding the battery is convenient for cutting and has a high pass rate. Each cell only needs to be cut once for the positive and negative poles, with low difficulty. On the other hand, the cutting of the stacked battery is tedious. Each cell has dozens of small pieces, which is prone to producing defective products, such as section breakage.

Let’s mention the popular blade-shaped battery. This shape is more suitable for stacking. This is because:

The blade-shaped battery is 960mm in length and 90mm in height. When produced by the stacking process, the alignment accuracy can be controlled within 0.3mm, and the stacking efficiency is 0.3s/pcs. For example, Bee Power recently launched a short-bladed battery product with a stacking efficiency of 0.125s/pcs, covering full-size short-bladed battery products from L300-L600.

How to choose the technical route?

The development direction of cylindrical, square shell, and soft pack batteries varies, and different companies make different choices based on their own situation. Each has its own advantages and disadvantages.

From the perspective of current technology scene

◎ Cylinder: With the standardization of 4680 cells and the use of winding technology, the manufacturing speed of individual cells is increasing, and the usage scenarios are expanding.

◎ Soft pack: Transitioning to short and long knives, the use of stacking has greater advantages.

◎ Square shell: With the current size remaining the same, continuing to use winding technology is possible, but transitioning to a square shell stacking size will naturally adopt stacking technology after becoming a short or long knife.

From the perspective of manufacturing efficiency and yield rate

◎ The largest potential is the cylindrical battery produced through winding. This is because we have a lot of imagination space in the process of importing dry electrodes around the mature process, and Tesla and BMW are actively promoting it.

◎ The fastest development is stacking: With the participation of companies such as BYD, EVE Energy, Heter, and CATL, Chinese battery companies are all designing towards the superposition of film + cutting battery solutions. Many overseas companies are also trying, such as Samsung SDI, LG Chemical, SK, and even Panasonic. There is a lot of potential for technological development in this area.

◎ Currently, the development route with the most stable development trend is winding: Shell winding has become a mature process, and we have also seen the strongest trend in the construction of original production capacity along this route.

Summary: Battery companies need to make trade-offs in their choice of process, but the trend is becoming increasingly clear. With the development of technological innovation, the era of TWh-scale manufacturing of power batteries is an inevitable point for each battery company. Whoever can do well in this area can find their own fundamental position in the stage where power batteries become standardized products on a large scale.

This article is a translation by ChatGPT of a Chinese report from 42HOW. If you have any questions about it, please email bd@42how.com.