Power batteries are generally divided into three forms: prismatic, pouch, and cylindrical. They are mostly produced using two processes: winding and stacking, each with its own advantages and disadvantages.
A battery composed of cells assembled by winding is called a wound battery; a laminated battery is a lithium battery for vehicles that uses a laminated process.
In terms of battery discharge platform, wound lithium batteries have a slightly lower discharge platform due to their high internal resistance and large polarization, with some voltage being consumed by internal polarization. Stacked lithium batteries have lower internal resistance and smaller polarization, so their discharge platform is higher than that of wound batteries and closer to the material's own discharge platform.
For many electrical devices with high discharge cutoff voltages, stacked batteries with higher discharge platforms are undoubtedly the preferred choice.
In terms of battery capacity, wound lithium batteries have a slightly lower volumetric capacity because the internal space is not fully utilized due to factors such as the thickness of the tabs, the circular shape of the cell sides, and the two layers of separators at the end taking up unnecessary thickness. Stacked lithium batteries, on the other hand, make full use of the internal space, resulting in a higher volumetric capacity compared to wound batteries.
The difference in battery capacity is only noticeable in thicker batteries (where insufficient space is used on the winding side) and thinner batteries (where the thickness of the winding tabs occupies unnecessary space). For batteries of standard size, the difference exists, but it is not particularly significant.
In terms of battery energy density, wound lithium batteries have lower energy density than stacked lithium batteries due to their lower volumetric capacity and lower discharge platform. Stacked lithium batteries have a higher discharge platform and higher volumetric capacity than wound batteries, resulting in a correspondingly higher energy density.
In terms of applicable battery thickness, wound lithium batteries have a narrower range of applications. For ultra-thin batteries, excessive electrode thickness can negatively impact battery capacity. For ultra-thick batteries, not only are the electrodes too long and difficult to control during winding, but the space on both sides of the battery cannot be fully utilized, which also reduces battery capacity.
Stacked lithium batteries have a wide range of applications. Whether it's making ultra-thin or ultra-thick batteries, the stacking process can handle it.
Wound batteries have no advantage in terms of ultra-thin and ultra-thick batteries. However, it should be noted that ultra-thin batteries are not widely used at present. Ultra-thick batteries can be achieved by stacking two thinner batteries in parallel (but at the cost of reducing capacity).
From the perspective of battery thickness control, wound lithium batteries are prone to excessive thickness at the tabs, separator ends, and both sides of the cell due to the uneven internal structure of the cells. Stacked lithium batteries, on the other hand, have easier thickness control. The uniform internal structure of the cells results in consistent thickness across all parts of the battery, making thickness control easier.
Because the thickness of wound lithium batteries is difficult to control, extra margin must be left in the thickness during the design process, which reduces the design capacity of the battery.
In terms of battery thickness deformation, wound lithium batteries have uneven internal structures, resulting in uneven internal reactions and rates during charging and discharging. Therefore, thicker wound batteries may deform after high-rate charge-discharge cycles or multiple cycles.
Stacked lithium batteries are not easily deformed. Their internal structure is uniform, and their reaction rates are relatively consistent, making even thicker cells less prone to deformation.
In terms of battery shape, wound lithium batteries have a single shape; they can only be made into rectangular batteries. Stacked lithium batteries, on the other hand, offer greater flexibility in size. The size of each electrode can be designed according to the battery dimensions, allowing the battery to be made into any shape.
Flexible dimensions are a clear advantage of stacking technology, but currently, the market demand for irregularly shaped batteries doesn't seem to be very high. This is also one reason why wound batteries are not suitable for very large thicknesses.
From a slitting perspective, wound lithium batteries are easier to slit and have a higher yield rate. Each cell only needs to be slitted once for the positive and once for the negative electrodes, which is less difficult and has a lower probability of producing defective products. Stacked lithium batteries, on the other hand, are cumbersome to slit and have a lower yield rate. Each battery has dozens of small pieces, and each small piece has four cut surfaces. The slitting process is also prone to producing defects through punching, so for a single battery, the probability of producing electrode breaks and burrs is greatly increased.
Although the small electrodes of stacked batteries can be strictly controlled through screening after slitting, the cost of fully controlling hundreds of thousands of small electrodes would be very high.
From the perspective of battery production control, the production control of wound lithium batteries is relatively simple. Each battery has two electrodes, making control easier. The production control of stacked lithium batteries is more complex. Each battery has dozens of electrodes, and testing, transportation, and statistics are all challenging aspects.
For a factory of any moderate size, a daily output of tens of thousands means a million stacked electrodes every day! If the output is in the hundreds of thousands, it could even be close to ten million small electrodes! The difficulty of turnover and monitoring during the production process is unimaginable.
From an operator's perspective, winding lithium batteries has low requirements. Becoming proficient in winding is difficult, but completing it to a acceptable standard is not. Once you understand the process and control the electrode alignment, you can get started.
Stacked lithium batteries require highly skilled operators. The stacking process is difficult, and the design of the negative electrode being too long or too wide relative to the positive electrode is generally not too significant, so operators need to have a certain level of operational expertise.
From a barrier-to-entry perspective, winding lithium batteries has a low entry barrier. Manual winding is easy to operate, and when funds are insufficient, manual operation can be considered, saving the cost of purchasing large automated equipment and lowering the entry barrier.
The entry barrier for stacked lithium-ion batteries is high. Automated equipment is not yet mature, and the cumbersome manual stacking process leads to increased labor costs, thus raising the entry barrier for stacked lithium-ion batteries.
In summary, the lamination process still needs further technological maturation before it can be widely adopted. The winding process, due to its higher cost-effectiveness, is currently still widely used.
But clearly, stacking technology is the mainstream direction for the future!
