The smallest working unit in a battery is the layered ‘Electrode Assembly’, consisting of two current collectors, a separator sheet which is sandwiched between a cathode and an anode. The whole assembly is soaked in a liquid electrolyte. This basic arrangement or ‘Electrode Assembly’ is common to all Li-ion cells irrespective of the chemistry. This layered assembly is extremely thin (~250 µm) or approximately 4 times the thickness of a human hair. When fully formed, the electrode assembly looks somewhat like a thick and long sheet of paper.Packaging of Li-ion cells
During cell manufacturing, this electrode assembly is packaged into a cell and there are multiple ways to do it. It may be folded over many times to form a pouch or prismatic cell or it may be rolled up into a cylinder to form a cylindrical cell. The choice of shape is dictated primarily by the final application of the battery. For example, when designing the battery for a phone or a tablet, the battery needs to be flat and as thin as possible so that it can easily fit into the device. As a result, the pouch cell is a natural choice. The packaging of cell affects almost all the performance metrics to some extent including the energy density, c-rate capability, cycle life, efficiency and safety.Cylindrical Cell
The cylindrical cell continues to be one of the most widely used packaging styles for Li-ion batteries. One of the main reasons for this is the relative simplicity of the manufacturing equipment which rolls up the electrode assembly into a cylinder. This translates to lower cost of the equipment and a higher throughput rate during production. Also, traditionally a lot of the devices using batteries have been designed to accept cylindrical cells and to some extent this legacy continues. An additional characteristic of the cylindrical cells is that they have a solid and sturdy casing which helps the battery to withstand internal pressures without deforming. Some internal pressures in cells is commonly observed and the situation in rare cases may be exacerbated when operating in adverse conditions such as sustained high c-rate or elevated ambient temperatures. Due to the sturdiness of the casing, the cycle life of cylindrical cells often benefits in addition to the safety. One of the key drawbacks however, relates to the packing density. Due to its cylindrical shape, the battery packs contain a lot of empty spaces in between which lowers the volumetric energy density of the system (Wh/L). The most popular type of cylindrical cell is often called 18650. The name arises from the dimensions of the cell; 18 mm is the diameter and 65 mm is the length of the cell. A larger variation of the cell (21700) is also now becoming popular. These cells have a diameter of 21 mm with a length of 70 mm. The larger cell helps reduce the weight of inactive cell components resulting in improved energy density. Typical applications for the cylindrical cell are power tools and medical instruments. Currently, they are also covering a major market in the transportation sector (partly due to high production rates and lower cost) although this may change in the future. The current Tesla Model S uses an 85 kWh Li-ion battery consisting of approximately 7000 cylindrical (18650) cells. Although, the battery pack designers could potentially save a lot of space by shifting to prismatic cells, the decision is not that simple as we will see.Prismatic Cell
The construction of a prismatic cell is very similar to a pouch cell with two important differences. This cell has a hard casing which is sturdy like the cylindrical cell. It also is much thicker than a pouch cell making it look somewhat like a fat book. Prismatic cells offer the advantage of excellent packing density. They can be packed together with little space in between due to the rectangular shape. Additionally, they also offer flexibility in cell size. For example, the BMW i3 which has a 33 kWh battery pack consists of only 96 cells in total. Thus, each prismatic cell is approximately 350 Wh which is approximately 35 times larger than a standard 18650. Using large batteries significantly reduces the complexity and cost of the assembly of the battery pack. For stationary storage systems which are multiple MWh sized, this difference would be even more pronounced. There is a challenge though that is encountered in case of prismatic cells and it relates to thermal management. The empty spaces between cylindrical cells leaves enough gaps to allow easy dissipation of the heat that cells generate during operation though natural or forced air convection. Prismatic cell battery packs need a little more infrastructure to ensure the same level of cooling. Therefore, the decision to choose a certain cell type is often not so straightforward. Nevertheless, once the companies do make a choice, more often than not they are quite wary of changing it. This is because it also entails a complete redesign of the pack and repetition of a lot of battery testing.Pouch Cell
In the case of EVs the market may be divided but in case of portable electronics such as cell phones, tablets and laptops, its completely owned by the pouch cells. Ergonomics and portability are critical for these devices and the pouch cell fits the requirements perfectly. Older laptops would often use the cylindrical cells but for obvious reasons that is now in the past. The main difference which separates pouch cell from other types, is the lack of hard casing material. The covering for a pouch cells is made of a flexible plastic/aluminium foil composite material. Although, it is very tightly sealed, their ability to handle internal pressure is quite limited. If you look inside the plastic case of a cell phone battery you would find a pouch cell packaged in an aluminium foil. Depending on the age of the battery in rare cases you may find that the battery is a little swollen. The is because the flexible packaging material for a pouch cell is less sturdy compared to the hard casing. In addition to safety, deformation of the pouch cells can also reduce the cycle life of the cell. The compromise on the robustness of packaging however allows the pouch cell to maximize the energy density. So what is your application and which cell format would you choose?