There are three common battery cell types: prismatic, polymer or pouch, and cylindrical. Each of these cell formats are available in different sizes and chemistries, And our battery packs mostly use the cylindrical Lithium-ion (Li-ion) rechargeable types(18650 & 32650) and prismatic cells.
- Prismatic Cells
Prismatic cells came out in the early 1990s. These cells are packaged in a hard, welded aluminum or steel casing "metal can". Its exterior is robust, yet satisfies the need for a thin cell. There are only a few standard sizes available on the market and are named based on their thickness, width, and length. For example, a standard 103450 is 10mm thick, 34mm wide, and 50mm long. The low-profile design of the prismatic cell improves volume utilization inside the battery pack, but this cell type can be expensive to manufacture. These cells are also less efficient in thermal management, have shorter cycle life than cylindrical cells and may exhibit swelling. Today, Li-prismatic cells are commonly found in thin profile, handheld devices like phones, tablets, and laptops but are also available in larger formats for applications such as electric vehicles/buses and hybrids.
2. Polymer Cells
Polymer, or pouch cells, Introduced in the early 1990s, the modern prismatic cell satisfies the demand for thinner sizes. They are like the prismatic cell, but do not have a hard, exterior casing. These cells are sealed in a foil pouch (laminate) and their electrolyte flexibility makes it easy to manufacture thin profile sizes in different custom shapes, cuts, and curves. Pouch cells have the highest packaging efficiency and great energy density, but tend to swell and therefore are more vulnerable to penetration. Normal size Li-polymer cells are commonly found in thin profile handheld devices like mobile phones, modern tablets, and thin laptops. They are also popular in portable applications requiring high load currents, such as drones and hobby gadgets. Curved polymer cells tend to be used in wearable consumer and medical devices, whereas the large format polymer cells are used in Energy Storage System (ESS) applications and electric and hybrid vehicles.
Prismatic cells are also available in large formats. Packaged in welded aluminum housings, the cells deliver capacities of 20–50Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Figure 5 shows the prismatic cell.
3. Cylindrical Cells
The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder can withstand high internal pressures without deforming.
These cells are enclosed in a metal can and are named based on their diameter and length. For Li-ion rechargeable batteries, the most common sizes are the 18650 (18mm diameter, 65mm length), the 26650 (26mm diameter, 65mm length), and the 21700 (21mm diameter, 70mm length).
Many lithium and nickel-based cylindrical cells include a positive thermal coefficient (PTC) switch. When exposed to excessive current, the normally conductive polymer heats up and becomes resistive, stopping current flow and acting as short circuit protection. Once the short is removed, the PTC cools down and returns to the conductive state. Cylinders are symmetrical, have a robust mechanical form, and can be efficiently packed. This shape helps minimize stress and internal pressure concentration. When assembled in a battery pack, Li-ion cylindrical cells have a higher energy density than flat Li-ion cells. The cylindrical cell is extremely versatile and the preferred choice for multi-cell configurations commonly used in applications such as medical, military, consumer, industrial, EV and more.
