There are two main packaging methods in the production of Lithium batteries: Rolling and Stacking. We will explore both methods in this article.
Rolled cells are also known as winding cells or jelly rolls. Rolling was the first method introduced in producing Lithium batteries. This method has been used for a long time in the production of NiMH (Nickel-Metal Hydride) and LiPo (Lithium-ion Polymer) batteries. In this method, long layers of material are stacked and rolled.
This method allows for a high level of efficiency and consistency in the turnout of the product.
Some disadvantages include a low space utilization caused by the fixed cylindrical shape and temperature distribution caused by poor radial thermal conductivity. The number of cycles also cannot be too large; generally, one 1650 battery will have 20 cycles. Due to this, the capacity is small, and a large number of cells must be packed into the battery modules for electric vehicles.
Although the Cylindrical Rolling method appears simple, the internal design is anything but. Its complex design requires symmetry between the contact between the positive and negative electrodes, heat distribution, and the production machinery.
Let us explore two examples of batteries that are produced with this method:
The image above includes the internal dimensions in mm of a 18650 battery. The length of the electrodes between the cathode and anode nearly comes out to 1.5 meters when laid out while the width of the tabs comes out to only 4mm.
In order to reduce the uneven current and temperature distribution, engineers must carefully consider how they will arrange the tabs and electrode coatings.
The image above reflects warping, which occurs due to the inconsistent volume changes of the electrode layers. This is one of the reasons that cause the aging failure of batteries produced under a winding method.
The images below show the magnified electrode shapes of 18650 batteries under CT after storage, low-rate cycling, and high-rate cycling. Warping can be seen in the first image, labeled a). The second image, labeled b), shows how a pin is added in the middle to suppress the deformation of the core.
Although beneficial, the pins increase the weight of the battery and production costs.
2. Rectangular Stacking (Pouch Cells)
Pouch cells are created by stacking multiple electrode sheets on top of each other in what is known as the Stacking method.
There are many advantages to these pouch-shaped cells:
- Due to the contact of the anode and cathode and its ratios allow for better heat dissipation.
- The shape allows for higher efficiency and utilization in space, and they make modules and battery packs easy to form. Our batteries are more customizable, and we have ultra-thin batteries that can be around 0.4mm in thickness.
- The distribution of current density is uniform as each electrode layer has a flange and tabs welded together.
There is one disadvantage to this method:
- The pouch layer is so thin that the pouch itself is semi-conductive. This thus results in positive pouch corrosion when utilized inside devices. A simple solution to this is to wrap the cell in Kapton tape.
In summary, in order to compare the advantages and disadvantages of various types, we need to analyze from many angles such as thermal, mechanical, and reliability, not just the traditional electrochemical characteristics. That’s the packaging methods in the production of lithium batteries.
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