Editorial：Mike
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Issue Date：2019-09-30
Views：10924
The battery cell used stacking technology has the advantages of small internal resistance, long life, high space utilization, and high energy density after group. In terms of battery performance, compared with the winding technology, the lamination stacking technology can increase the energy density of the battery by 5%, increase the cycle life by 10% and reduce the cost by 5% under the same conditions.
The lamination & stacking process is a lithium polymer battery manufacturing process in which a positive electrode, a negative electrode is cut into small pieces and a separator is laminated to form a small cell, and a single cell is stacked in parallel to form a large cell. However, there are different ways to stacking process. Let’s watch a video from Scienceviz: [embed]https://youtu.be/8WEGVxliKJ8[/embed] As the video shows, there are four stacking processes, and Grepow uses the Z-Folding with single electrodes method, which is less complicated and increased the pass rate of the battery.
As I mentioned before, the lamination stacking technology can increase the energy density, cycle life, and reduce the cost. Then I will compare the advantages and disadvantages of the stacking and winding process from different aspects.
1. The internal resistance of the battery is different
The cell produced by the stacking process has a lower internal resistance and a higher winding internal resistance. Because the winding cell is usually a single pole, the laminated cell can be seen as a multi-pole type, which greatly reduces its internal resistance. The difference in internal resistance causes the difference in heat generation between the finished battery during the charge and discharge cycle and the decay of the battery capacity. It is obvious that the battery capacity of the stacked battery is attenuated more slowly.
2. Different battery life
As the charge and discharge cycle continues, heat is generated inside the battery, which in turn affects the temperature of the battery. For the laminated battery, the internal temperature distribution is relatively uniform, and the winding battery has a single-direction heat transfer mode between the pole piece and the diaphragm, which causes the temperature gradient distribution phenomenon to be serious, and the internal high temperature and external appearance occur the low-temperature phenomenon. The uneven temperature distribution leads to the inactivation of the live material at the high-temperature position during the charging and discharging process, and the function of deintercalation lithium ions cannot be performed, thereby affecting the rapid decay of other locations and affecting the performance of the battery.
The temperature distribution of stacking and winding cell (The picture from Internet. All rights reserved)
3. The stress inside the cell is different
Two ways of making the battery have different mechanical characteristics. The laminated area of the laminated core electrode diaphragm has the same force area, no specific point of stress concentration, and the battery does not have sharp damage in a certain part during use. At the edge of the winding cell, stress is concentrated. According to the previous experience of battery disassembly analysis, the pole piece bend is more prone to micro-short circuit, electric breakdown and lithium deposition. The stress concentration point is the primary location for battery deactivation, which also results in reduced cycle life of the winding battery.
4. Battery rate performance is different
The stacking process is equivalent to the parallel connection of multi-pole pieces, which makes it easier to discharge large currents in a short time, which is beneficial to the rate performance of the battery. The winding process is just the opposite, with a single tab causing a slightly lower rate performance.
5. Different battery capacity density
The laminated battery has a higher capacity density because its internal space is more fully utilized. In contrast, the winding battery has a circular shape on both sides of the battery and the last two layers of the coil occupy a certain thickness, so the capacity density is low.
The safety is also different, from the aspects of internal resistance, stress, temperature distribution, etc., the winding battery with high internal resistance and inconsistence temperature are less safe.
1. The film production complexity is different
The winding process is simpler and has been operated, making it easy for automated manufacturing. Most companies currently on the market use a form of winding. The stacking process is very cumbersome and the pass rate of the pole piece is very low. For a winding core, only two knives at the beginning and the end are required, and each piece of the laminated piece requires four knives. The quality of the pole piece (section, burr, etc.) is difficult to maintain a high degree of consistency, which affects the ultimate performance of the battery.
2. The complexity of battery manufacturing is different
Winding cells are easy to operate and can be completed quickly, either semi-automatic or fully automatic. The stacking process is highly complex, manual operation is time-consuming and laborious, and automation is difficult to industrialize due to equipment problems. In addition, in the quality control of the battery core, the winding type is easier to control, and the stacking type is difficult to achieve good consistency due to the cumbersome process steps. In short, it is necessary to select a suitable process according to its own conditions and battery requirements. After the process change, the pole piece cutting and the subsequent welding and assembly need to be greatly changed.
Film Cutting: The coiled lithium battery is convenient for cutting and has a high pass rate. Each cell only needs to perform one slitting of the positive and negative electrodes, which is difficult and has a low probability of producing defective products. The laminated lithium battery is cumbersome and has a low pass rate. Each battery has dozens of small pieces, each of which has four cut faces, and the slicing process is easy to produce bad punching, so the probability of producing a pole piece and burr is greatly increased for a single battery.
Grepow has the 0.6-second high-speed stacking technology production equipment, which can realize mass production of stacking square batteries, and the stacking efficiency of single-chip is about 1.0 seconds/chip.
Keep an eye on Grepow's official blog, and we'll regularly update industry-related articles to keep you up-to-date on the battery industry.
Grepow: https://www.grepow.com/
Grepow Blog: https://www.grepow.com/blog.html
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The Important of Separator of Battery Cell
Lithium-ion batteries can be classified into pouch, prismatic and cylindrical batteries according to the packaging method and shape. From the perspective of the internal molding process, pouch and prismatic batteries can be wound or stacked, and cylindrical batteries such as 18650 battery, 21700 battery, 4680 battery, etc. have curvatures everywhere, so they can only be wound.
The development time of the winding process is relatively longer, the process is mature, the cost is low, and the yield rate is high. However, comparing stacking battery vs winding, with the promotion and development of electric vehicles, the stacking process has become a rising star due to its advantages such as high volume utilization, stable structure, small internal resistance, and long cycle life.
Winding and stacking are the core links in the assembly process of lithium battery middle cells. Winding is a production process that rolls pole pieces, diaphragms, termination tapes, etc. that match in size after slitting into pole cores.
Different from stacking battery vs winding, stacking battery is a production process in which pole pieces and separators are alternately stacked together to finally complete the multi-layer laminated pole core.
From the perspective of battery shape, pouch batteries can only use stacking technology. The prismatic battery can use either the stacking process or the winding process, and the current mainstream is the winding process. Cylindrical batteries can only use winding technology.
Stacking
Winding
Energy density
Higher. Higher space utilization.
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Lower. There is a C angle, and the larger the capacity, the lower the utilization rate.
Structural stability
Higher. The internal structure is uniform and the reaction rate is relatively low.
Lower. There is a C angle, which leads to uneven rate of internal reaction of charging and discharging.
Fast charging adaptation
Better. The multi-pole plates are connected in parallel, the internal resistance is low, and the charge and discharge of large current can be completed in a short time, and the rate performance of the battery is high.
Poor. During the charge and discharge process, the degradation rate of the active material at the high temperature position is accelerated, and the other positions are rapidly attenuated.
Safety
The risk is low. Stress distribution is more consistent, which keeps the interface flat and more stable.
Lower. Potential problems such as powder shedding, burrs, pole piece expansion, and separator stretching are easy to occur at the bend.
Cycle life
Longer. Low internal resistance, relieve battery heating during fast charging, improve battery chemical system stability and prolong service life.
Shorter. It is easy to deform in the later stage, which in turn affects the cycle life of the battery.
Productivity
Large-capacity batteries are generally low, mainly 6-8PPM.
Higher, generally at 12-13PPM.
Yield
Low, the glitch problem is prominent.
Higher automation, higher yield rate, higher number of pole pieces.
Process maturity
Low, the number of pole pieces is large, and the investment in equipment is large.
Higher, fewer pole pieces, mature equipment and low investment cost.
Higher battery energy density
There is a curvature at the corner of the winding, and the space utilization rate is lower than that of the stacking. The stacking battery can make full use of the corner space of the battery, so the energy density of the cell formed by the stacking is higher under the same volume of the cell design. Compared stacking battery vs winding battery, the energy density of stacking structures can be increased by about 5%.
More stable internal structure
Comparing stacking battery vs winding battery, the stacking battery does not have the problem of uneven internal stress at the corners. During the repeated use of the battery, the expansion force of each layer is similar, so the stacking battery can maintain a flat interface.
During the use of the wound battery, with the insertion of lithium ions, both the cathode and anode will expand, and due to the inconsistent internal stress of the inner and outer layers at the corner of the winding, wavy deformation will occur. This deformation will lead to the deterioration of the interface of the battery, uneven current distribution, and accelerate the instability of the internal structure of the battery.
Higher safety
The stacking battery is evenly stressed, there is no bending problem at both ends, and the battery safety is higher. However, compared stacking battery vs winding, after the pole pieces at both ends of the winding are bent, the coating material will be greatly bent and deformed, and the "C angle" bends are prone to powder drop and burr problems. In severe cases, it will cause an internal short circuit of the battery and cause thermal runaway.
Longer cycle life
The number of lugs of stacking batteries is twice that of winding, and the more the number of tabs, the shorter the electron transmission distance and the smaller the resistance.
It is well known that when the voltage and time are constant, the larger the resistance, the less heat generated, and the smaller the resistance, the smaller the heat generated, so the service life of stacking batteries is relatively longer than winding batteries to compare stacking battery vs winding battery.
The efficiency of the equipment is low, and the floor space of the stacking machine is larger than that of the winding machine.
High equipment investment
From the perspective of a single production line, a production line needs 10 winding machines. According to the price of 3-3.5 million RMB/set, the cost of a winding machine under a production line is about 30-35 RMB. The number of stacking machines required for a production line is related to the number of cells.
Low yield rate
The winding battery is easy to cut and has a high pass rate. Each battery cell only needs to cut the positive and negative electrodes once, which is less difficult. However, compared stacking battery vs winding, each battery has dozens of small pieces in stacking cutting, and each small piece has four cut surfaces, which is prone to defective products.
Difficult to control
There are only two pole pieces for winding a battery, and the winding spot welding is easy, and each battery only needs two spot welding, which is easy to control. However, different from stacking battery vs winding, the number of stacking pole pieces is large, and it is easy to have false welding. All the pole pieces must be spot welded to one solder joint, which is difficult to operate.
The winding process is to roll the slitted cathode and anode and the separator together by controlling the speed, tension, relative position, etc. of the pole pieces. The characteristics of the process make it only possible to prepare lithium batteries with regular shapes.
Compared stacking battery vs winding, the stacking process is to alternately stack the cathode and anode and the separator through the sheet feeding mechanism to form stacked cores, which can produce regular-shaped or special-shaped lithium batteries with higher flexibility.
The internal resistance of the winding battery is large, and the internal resistance can be greatly reduced through structural improvement. For example, the full tab structure can achieve a level of internal resistance similar to that of the stacking structure, but it requires higher equipment capabilities and quality control.
Different from stacking battery vs winding, the stacking battery has a flat structure, low internal resistance, and high space utilization. It shines in the field of power batteries. Some of top 10 lithium battery companies in the world represented by LG and BYD stick to the stacking route.
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