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Lithium Cell Differences

What you should know

Pouch Type

Swelling Pouch Cells

 

Swelling or bulging as a result of gas generation during charge and discharge is a concern with poly pack type coverings. The pouch cell uses laminated architecture in a bag. It is light and cost-effective but exposure to humidity and high temperature can shorten life. Adding a light stack pressure prolongs longevity by preventing delamination. Swelling of 8–10 percent over 500 cycles must be considered with some cell designs. The pouch cell offers a simple, flexible, and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life.

Pouch cells are a somewhat minimalistic approach to packaging because they do not have a rigid container.   In fact, they are only sealed by flexible foil, that is why they are also called “coffee bag cells”. Furthermore, cathodes, separators, and anodes are stacked instead of wound. This approach increases packaging density to the maximum and saves weight, thus increasing energy density of the cell. This type of packaging is used only for Li-ion chemistries. Instead of a liquid electrolyte, a polymer electrolyte also acting as separator is sometimes used in this configuration. Pouch cells usually have more than one electrochemical cell inside. These very flat cells perfectly fit e.g. into tablet computers (capacities around 4 Ah). They are also employed for high-power and for high-energy (EV) applications (capacity example: 20 Ah A5-size with 20 cathode, 20 anode, and 40 separator sheets, respectively [8]). High currents either in charging or discharging mode result in internal pressure. Serious swelling of the package occurs when the cell is overheated or shortened. Swelling occurs as a normal process during the initial charging step (formation), though. To cope with that, manufacturers usually oversize the package, resulting in a separate “bag” to which excess gases can escape. After formation, the package is resealed to its final form, and the gas-bag is cut off. Nevertheless – more than prismatic cells, pouch cells need careful temperature management and support structures when placed into a module.

Review on Electrochemical Storage Materials and TechnologyAIP Conf. Proc. 1597, 204-218 (2014); doi: 10.1063/1.4878489© 2014 Karl Young, EVs, Li-ion (LTO), supercaps, fast-charging, hybridsM.E. energy storage, electric motors, electric aviation, Founder Precision Technology inc.

Advantages

Before Tesla appeared on the scene, many electric bicycles and scooters have power packs built-up using Li-ion 18650 cells.

Compared to the pouch or prismatic cells, cylindrical cells like the 18650 / 26650  can be produced more quickly; so more kWh of cell produced per hour due to its configuration - another reason for lower $/kWh.

Another advantage of cylindrical cells over the flat cells (e.g. pouch, prismatic) is that its electrodes are wound evenly and tightly and encased in a metal casing. This minimizes electrode material from breaking up from the mechanical vibrations, thermal cycling from charging and discharging, and mechanical expansion of the current conductors inside the cell from thermal cycling.

Expansion of Lithium Ion Pouch Cell Batteries: Observations from Neutron Imaging

Jason B. Siegel4,5,1, Anna G. Stefanopoulou1, Patrick Hagans2, Yi Ding4,3 and David Gorsich3

Published 27 April 2013 • © 2013 The Electrochemical Society
Journal of The Electrochemical Society, Volume 160, Number 8 Citation Jason B. Siegel et al 2013 J. Electrochem. Soc. 160 A103

Aerolithium batteries, using the cylindrical type form of LiFePO4, have a VERY high energy density which produces greater cranking power than comparable batteries with pouch-type cells.


The cell's special lithium processing is different from other cheaper LiFePO4 cells other battery companies use. This results in a more robust cell that provides greater safety improvements over lighter weight pouch type cells and offers greater abuse tolerance to events like overcharging or short extreme discharge pulses which are common in aircraft engine starting. 


Other advantages include very low self-discharge, < 1% a month, giving these cells a higher cycle life, more reliability, and of course no chance of swelling up like pouch-type cells.

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high power aircraft Lithium cells
Lithium Iron Phosphate cells

Let's Compare

Cylindrical Cells with Prismatic Batteries vs. Pouch Cells

Let's compare Cylindrical Cells with Prismatic Batteries and Pouch Cells

Now that you have some idea about the technology behind these different types of battery packaging, let’s dive a little deeper into a comparison between cylindrical cells, prismatic batteries, and pouch cells.

Here are some of the main pros and cons you should look for when thinking about cylindrical vs. pouch cells – but of course, the type of cell you ultimately choose will depend on the type of application you need it for:

  • Cylindrical cells, due to their widespread and standardized use, offer a more robust automation process and better manufacturing techniques (providing a more consistent product) than pouch cells.

  • Prismatic pouch cells are sensitive to deformation in high-pressure environments, whereas cylindrical cells handle high internal pressures much better and without deforming.

  • Cylindrical lithium battery electrodes are easier to weld when compared with lithium polymer pouch cells or prismatic cells.

  • Pouch cells, while lightweight, are vulnerable to damage stemming from humidity and high temperatures. This can reduce their lifespans and increasing their overall cost because they’ll need to be replaced more often.

  • If a single cylindrical battery goes bad, the impact on the entire pack is relatively low. But with prismatic cells, if one battery goes bad, it can then compromise the entire pack.

  • Cylindrical cells are stable, inexpensive, standardized across industries with high cycle life. (due to its cylindrical design). Prismatic cells and pouch cells are not standardized across industries, which makes them more expensive to produce. That said, their rectangular shapes utilize limited space better, and those with hard shells can be stacked. In most cases, softer pouch cells should not be stacked.

  • Cylindrical cells radiate heat and control temperature more effectively when compared to pouch or prismatic cells. These batteries die faster due to less effective thermal management.

  • The chemical activity at the edges and corners of prismatic batteries is rather poor. The less-than-optimal chemical activity of prismatic batteries means their performance decreases substantially after long-term use. Cylindrical cell batteries have a much longer lifespan compared to prismatic or pouch-type batteries.