Type of prismatic cells – winding and Z stacking
4 min read
There are three types of form factors when it comes to lithium-ion cells: cylindrical, pouch, and prismatic cells. A little-known fact is that there are two types of cell internal structure designs in a prismatic cell. Let us talk about them in detail and talk about their advantages below:
Winding type
Winding is the traditional way of building a prismatic cell, in which the electrodes and separators are wounded in an elliptical fashion and placed in a prismatic square.
Winding type cells are slightly cheaper to produce. Smaller capacity cells, e.g. 100Ah, use a single winding roll. Larger capacity cells use double winding rolls. The space utilisation in the above image is not optimal in the winding-type cell, and there can be additional tension on the edges of the roll.
There is a change in the structure, and the cell structure is no longer uniform after 1000 cycles because of the non-uniform distribution of the tension in the electrode design. This change keeps happening with 1000s of more cycles, and it is difficult to meet the claims of 6000 cycles and above because the cell will reach the knee point earlier because of distortion in the cell’s internal structure. Due to the lack of uniform electrode design, cell bulging in winding-type prismatic cells begins much faster.
Knee point highlighted on the cycle life graph
A knee point is defined as a point on the cycle life graph where capacity degradation begins to occur at an unusually high pace.
Z stacking type
Z stacking, also known as the Z folding or laminating method, is a type of building the prismatic cell. It is like building a very thick pouch cell stack and then inserting it inside the prismatic can. Z stacking type cells have alternate stacking of positive and negative electrodes, between which there is a separator for insulation. There is a healthy gap between the left and right edges of the electrodes and the folded part of the separator, and the design has no tension/stress on the sides.
Internal structure of a new 280Ah Prismatic cell with Z stacking design
Cell internal structure of Z stacking type Prismatic cell after 1000 cycles
The cell’s internal structure mostly remains the same after 1000 cycles. Constantly expanding, the stress generated can also be released to the surroundings at any time, and the electrodes will not break after cycling. This type of design ensures high safety and ultra-long-life performance. It makes the cell work for longer cycles, and it is easy to reach 15,000 cycles (at lower SoH) without reaching a knee point in the cell degradation pattern due to a lack of distorted internal structure.
Each electrode has an individual tab, which are welded together. This ensures a stable and uniform connection of each electrode to the positive and negative terminals, thereby reducing the overall internal resistance of the cells and less heat generation during cell charging and discharging, which in turn leads to better cell efficiency. Some companies that manufacture cells with the Z stacking method promise cell functionality up to 60% SoH (state-of-health). This allows for second-life use of the cell after 10-12 years.
The Z stacking type cells can truly work for 20 years at a BESS level at 0.3C charge and 0.3C discharge rate (a popular rate of operation for various BESS projects) at 90% DoD at 35°C (air cooling and liquid cooling system ensures the cell temperature comes down to 35°C in the most extreme hot operating temperature zones). The Z stacking type cells can truly work for 2 cycles per day projects for 12 years where 8760 cycles are definitely expected and back it with extended warranty for such project requirements.
The above images are from a CT scan. Prismatic cell bulging is a known fact, and to control the bulge, a counter pressure needs to be applied to keep the cell from bulging. Hence, all prismatic cell manufacturers mention to apply a specific pressure. For example, a 280Ah LFP prismatic cell is recommended to use a clamping pressure of 300±30Kgf on the side of the cell with a higher surface while building a battery pack.
More details about winding vs. Z stacking) type prismatic cell to follow in Part 2.
About the author:
Rahul Bollini is an R&D expert in Lithium-ion cells with 9 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.
Also read: Technical analysis of unified cell by Powerco (VOLKSWAGEN)
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