Lithium-ion batteries will continue powering e-mobility for the foreseeable future, and having explored the six different battery chemistry types; we now focus on the battery cells housing these chemistries. Between cylindrical, prismatic, and pouch-shaped forms, cylindrical are the most common, although battery manufacturers will leverage each type’s distinct features that suit the application. Cost is certainly one determining factor, but equally important are the contents within the cells structures.
Here, we’ll look at each cell’s profile, advantages, disadvantages, and applications they might be suited for. It’s worth noting that while the cylindrical shape is the most technologically mature of the three types, prismatic and pouch cells continue to grow steadily.
Battery cell profiles
Cylindrical batteries have achieved the highest market penetration, powering everything from household gadgets such as TV remotes via the infamous AA or AAA batteries to being specifically engineered to power 40-ton trucks. This is possible due to the vast size options available, though one of the most common is the 18650 model (18mm diameter, 65mm height, 0 representing the circular shape).
This cell’s anodes, cathodes, and separators are compressed in a sheet-like form, rolled up, and packaged into a cylinder case. It’s a shape that makes automated manufacturing very easy, paving the way for mass production and rapid market dominance.
The main standard characteristics of this battery include high capacity, output voltage, and current discharge. Further, they perform well across a wide temperature range. This makes the shape ideal for electric vehicles, particularly off-highway (OHEV).
Prismatic cells are fast becoming favorites in the automotive industry. There aren’t many standard sizes to choose from, which could mean that automakers will need to design a battery case from scratch, as the standard sizes available might not suit their needs. However, since the shape makes for increased efficiency, design can be flexible for this structure. The negative trade-off, in this case, is the lack of a unified production process, which drives up costs.
Prismatic cells first entered the market to power gadgets that followed a similar profile to their flat rectangular shape, such as mobile phones, tablets, and medical devices. However, as testing in different applications continued, the cell technology developed and was scaled to begin powering larger devices. The flat, wide surface is ideal for packing density and is fast becoming a favorite for road-going vehicles.
As for the internal structure, the anode, cathode, and separator sheets are pressed together and rolled before fitting them into a rectangular metallic (aluminum or steel) or hard plastic casing. This hard-shell casing reduces the risk of bulging should pressure build-up internally.
Also known as polymer cells, pouch cells use a foil laminate bag-like structure instead of a hard casing like prismatic cells. The pouch’s outer protective layers are usually made from nylon BOPA (Biaxially Oriented Polyamide) or PET (Polyethylene terephthalate), while the middle batteries are made of aluminum foil.
By using a soft aluminum coating, the size can be adapted to the use and intended battery mission, making it easier to manufacture different shapes, cuts, and sizes depending on what they’ll be powering. This adaptability makes pouch cells ideal for applications that are tight on space, and since it’s a younger technology than cylindrical or prismatic cells, research and development is still at a relatively nascent stage.
In fact, OEMs and vehicle manufacturers have only started using this cell structure in vehicles and Non-Road Mobile Machinery (NRMM) recently. This means that as more tests are done and data collected, expect this technology to find use in more applications.
Cell advantages and disadvantages
The high market maturity means that buyers have plenty of suppliers to choose from or switch between, as the latest technology is readily available to all, leading to minor differences in production costs and performance ratings. While this certainly counts as an advantage, since choice strengthens the buyer’s hand, it also means that cylindrical cells have almost peaked in terms of technological innovation. This is by no means an outright negative, as prismatic and pouch cells have plenty of catching up to do.
A critical advantage cylindrical cells offer that prismatic and pouch don’t is how the circular shape enhances heat dissipation and mechanical stability. This is one of the reasons these are used at Xerotech, where we also individually fuse cells and encase each in fire retardant foam. This protects the entire module in short circuit or thermal events, as the threat is dealt with on a cellular level.
It must be said, however, that this shape also prevents the space from being used to its maximum potential, as the same gaps that help heat dissipation prevent more cells from being added to the module. Therefore, more cells would be needed to reach similar power levels as the prismatic type, and since the cells also need a mounting bracket to be kept in place, more weight is added to the pack.
Ideal for packing density, the more straightforward structure requires fewer electrical connections to be welded than cylindrical cells. Furthermore, given the size differences, in certain circumstances, one prismatic cell could contain the energy equivalent of 20-100 cylindrical cells. The shape also makes it easier to stack the cells, while the use of screw poles makes battery assembly and element replacement easier.
However, the downside to the shape is that more stress is placed on the electrode and separator sheets closer to the container corners. This could lead to electrode coating damage and an unequal electrolyte distribution, and heat dispersion also suffers with this shape, as there’s no space between the cells. And while the lack of standard sizes means flexibility, the flip side is that the lack of standardization between models makes prismatic cells more expensive to produce.
The most recent addition to the market and the most flexible cell option, pouch cells offer high energy density and can be up to 40% lighter than steel or aluminum-cased batteries of equal capacity. The low-cost casing helps bring down the initial cost of production; however, since these cells have low-to-medium capacity, many would need to be welded together to function in industrial battery packs. This means that should a fault develop, the whole module would need to be replaced.
Furthermore, extra protection and design planning are needed to protect pouch cells since the casing is relatively fragile, too weak to prevent thermal events, and can swell up to 10% of the original size after 500 charge cycles. Sharp edges pose a severe threat, and the pouch sizes create distance between cooling mechanisms and the cell centre, making it harder to stop the creation of hot spots.
Despite the current safety concerns, future developments could see pouch cells become the cell structure for next-generation batteries. They offer up to 95% better packaging efficiency and better energy density, which makes it worth the extra testing and design planning needed to ensure the safety and functionality of this cell type to unlock its full potential. However, if safety and pouch structure integrity remain challenges, it’s unlikely pouch cells will find extensive use in diverse markets.
Why cylindrical cells are still the way to go
Cylindrical cells remain the best option for the OHEV market by offering increased safety and better mechanical stability while operating better across a broad spectrum of temperatures. While prismatic cells might offer better packing density, scalable and customizable platforms such as Xerotech’s Hibernium® platform mitigate that difference.
Battery cell technology will continue developing, undoubtedly making for a more interesting lithium-ion battery market. Not only do end users get a plethora of choices, but battery manufacturers will be pushing each other to reach new innovative heights, developing better systems that will further power the change to a zero-emissions world.
Xerotech is intent on empowering this change, so if you want to find out just how we can power your application, reach out to a member of our team, and we’ll be thrilled to electrify your operations.
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