Lithium battery - winding machine principle

The principle of winding

(1) Principle of winding machine

The winding machine has positive and negative feeding units, and the mechanism that winds the positive, negative, and separator together is called a winding needle.

According to the shape of the battery cell, it is mainly divided into square and cylindrical battery cell winding machines. 

Generally, the winding equipment adopts a structure of two or more winding needles and single-sided needle pulling. 

The main mechanisms of the winding machine include: active unwinding of positive and negative pole pieces and diaphragms, 

pole piece and diaphragm rewinding, automatic correction, and automatic tension detection and control. 

The polarizer is introduced into the needle winding part by the clamping roller drive mechanism 

and wound together with the diaphragm according to the process requirements. 

After the winding is completed, the workstation will be automatically changed, the diaphragm will be cut off, and the termination tape will be applied. 

After the finished bare cells are automatically cut off, they will be pre pressed and scanned. 

The good finished bare cells will be automatically transferred to the tray and then transferred to the subsequent process. 

Defective bare cells are automatically unloaded to the collection area for defective bare cells. 

The process flow of winding is shown in the following figure:

Process flow diagram of winding equipment

(2) Explanation of winding mechanism

① Pre winding: The initial process of feeding the positive and negative electrode plates, 

during which the positive and negative electrode plates are clamped by the electrode plate feeding mechanism and fed into the winding needle at a certain speed. 

It is necessary to control the rotation angle and speed of the winding needle to match the electrode plate feeding mechanism. 

This process involves two synchronizations: the unwinding speed of the diaphragm is synchronized with the needle winding speed, 

and the feeding speed of the polarizer is synchronized with the needle winding speed.

② Winding process: After completing the initial feeding process of the positive and negative electrode pieces, 

the positive and negative electrode pieces are tightly wrapped by the diaphragm and wound around the winding needle. 

Subsequently, the winding needle can be rotated to achieve continuous winding. 

During this process, the tension of the material roll is detected to adjust the discharge speed of the pole piece discharge motor, 

ensuring a constant tension of the material roll during the winding process.

Schematic diagram of winding process

The control problem in pre winding belongs to open-loop control problem, 

and it is impossible to accurately measure whether the winding needle, diaphragm, 

and pole piece are truly synchronized between each other. 

This requires us to establish an accurate winding control model, especially for the winding requirements of larger batteries. 

The tension of the winding material belt can be controlled using closed-loop feedback control technology.

In addition, during the winding process, we actually control the speed of the winding motor rotation, 

and the actual speed is a function of the actual radius of each material roll and the winding needle winding, 

which is dynamically changing. 

At present, without actual sensor measurement, we assume that the core of the coil gradually increases after one feeding, 

and the variation law of the middle radius fully conforms to Archimedes' spiral law. The initial roll radius is pre-set through a program.

③ Dynamic control model for winding process: As the pre winding process belongs to open-loop control, 

an accurate mathematical model is the key to the success or failure of the winding control system. 

Especially when the linear speed of the polarizer is greater than 1m/s, 

an accurate winding model is the key to controlling the stability of winding tension and the quality control of winding.

④ High quality winding of battery cells: 

The core issue of high-quality winding of battery cells is that the diaphragm 

and pole pieces of the wound battery cells are uniformly adhered without gaps, 

and the diaphragm and pole pieces maintain uniform contact stress in all directions during battery use.

This poses two requirements for the winding machine. 

Firstly, the core must maintain consistent adhesion stress even after the needle is drawn, which is crucial for designing the contour shape of the needle. 

Especially for square wound batteries, it is necessary to ensure uniform adhesion stress between the pole piece and the diaphragm after the needle is drawn. 

The winding contour curve must be a closed curve with a continuous first-order derivative, 

and the judgment principle is that the curve is continuous, smooth, and free of sharp corners. 

Secondly, when the diaphragm and pole piece enter the winding needle, 

the tension is consistent in the direction of the tangent line of the winding needle. 

This requires that the correction amplitude of the diaphragm pole piece should not be too large, 

and should be ensured within a limit within the elastic range of the diaphragm and pole piece.

⑤ GAP problem of square wound battery cells

Even if the needle winding curve of a square battery cell is a closed curve with a continuous first-order derivative, 

and there is no gap between the electrode pieces and the separator after flattening, during the charging and discharging process of the battery, 

the inconsistent degree of expansion and contraction of the electrode pieces will cause the gap between the electrode pieces to gradually increase with the charging cycle. 

When the electrolyte here is not abundant, lithium ions cannot be transferred, which will affect the capacity utilization. 

Long term use will bring about lithium deposition safety issues. 

Moreover, with the increasing demand for energy density, the negative electrode is gradually introduced into the silicon negative electrode system. 

Due to the large expansion of silicon negative electrode plates, the winding method of the electrode group is prone to inner ring plate breakage, 

which affects the service life of the battery and limits the amount of silicon material added. 

The recommended approach is to increase the amount of abundant electrolyte appropriately during injection.

Characteristics of winding

According to the degree of automation of the winding machine, it can be divided into manual, semi-automatic, 

fully automatic, and integrated machine types. 

According to the size of the produced core package, it can be divided into small, medium, large, super large, etc. 

The following are schematic diagrams of several winding machines.

The characteristics of winding core are as follows:

♦ Continuous integration of polarizer and diaphragm, high manufacturing efficiency;

♦ The winding has only two edges, with few edges and complete polarizer, making it easy to control burrs;

♦ Simple production control, easy operation, and low control difficulty;

♦ It is not advisable to roll it too thick or affect each other between layers, as it is prone to deformation;

♦ High flexibility requirements for polarizer;

♦ The lateral tension of the polarizer is inconsistent, and there may be gaps inside, making it difficult to achieve uniform bonding stress;

♦ The expansion of the polarizer causes GAP problems and makes it difficult to achieve high quality.

Key structure of winding machine

The main modules of the equipment include: automatic unwinding module for polarizer/diaphragm, 

polarizer/diaphragm rewinding module, automatic correction module, guide roller module, polarizer ear guide smoothing module, 

main drive module, tension control module, tension measurement/display and storage module, polarizer feeding module, diaphragm anti-static device, 

polarizer ear folding/folding and polarizer damage detection module, CCD online detection module, polarizer cutting module, 

dust removal system, polarizer and diaphragm defective product single roll and removal module, winding head assembly, 

diaphragm cutting module, diaphragm adsorption module, adhesive tape termination module, automatic unloading module, 

bare cell pre pressing module, unloading module, equipment frame and large board module.

Layout diagram of winding machine

The development trend of winding machines

① High speed and high precision: The linear speed of winding pole pieces has been developed from the existing 2-3m/s to 4-5m/s, 

and the alignment accuracy of winding pole pieces has been improved from the existing ± 0.3mm to ± (0.1-0.2) mm.

② High pass rate: CPK increased from 1.33 to 1.67, and eventually developed to 2.0 or above, reaching the level of exemption from inspection.

③ Stability: Improve the average time between failures from tens or hundreds of hours to thousands or tens of thousands of hours.

④ Digitization and intelligent control of equipment: online monitoring of winding tension, alignment of pole pieces and separators, 

closed-loop optimization of winding parameters and final battery performance parameters, and improvement of winding qualification rate.

⑤ Laser die-cutting and winding integration: The combination of laser die-cutting and winding processes achieves equipment integration and integration.

⑥ High speed winding machine: Through the breakthrough of diaphragm continuous winding technology, the winding efficiency is doubled.

Based on years of technological accumulation and accumulation, the winding process has obvious advantages in production equipment, technical processes, efficiency, cost, and other aspects. 

However, under the trend of standardization, large capacity, and large size demand for automotive grade power batteries, the winding process has begun to be "powerless". 

How to improve the quality of winding is the core issue facing the industry.