Cylindrical Design

 

Advanced cylindrical design uses rolled up sheets comprised of anode, cathode, and separators. The roll is encapsulated inside a “can” and the electrodes are tab-welded to the current collectors. The number (and quality) of welds, current collector width (and thickness), and cell length will influence the impedance and power capability. For example, more welds, thicker current collectors and shorter length will lower impedance. Another advantage of the cylindrical cell is its inherent ability to contain high gas pressures. This becomes important for recombinant batteries that produce high gas pressures during charge or discharge.

The disadvantages of a cylindrical cell include high overhead volume requirements for current collectors and packing two or more cells into an array. In addition to volume losses, the act of rolling the electrode pair put high demands on the separator, thus limiting the types of separators that can be used. This may result in high cell impedance and lower manufacturing yield. Last, as cell diameter increases so does the temperature gradient of the cell from the center to the surface. This can cause stability issues and uneven cell aging. The volumetric overhead of the current collectors is between 5% and 15% of the total volume for this type of packaging. The cell to cell packing efficiency has a 15% to 25% loss as well, thus the packaging volume to active material is between 20% and 40% of the total volume.

The majority of cylindrical cells have become very popular for portable electronics because of their versatile shape, compact size, and high production levels. This makes them ideal for low capacity applications.

 

Prismatic Design

In this type of construction, there are multiples of the same electrode connected in parallel. Because prismatic cells take advantage of paralleling many electrodes, they are the ideal choice for high capacity cells. There are two common methods to introduce the separator. One method is to ‘bag’ either the anodes or the cathodes, and the other method is to ‘zig-zag’ the separator between the electrodes. Bagging the electrode retains electrode materials that may cause soft shorts, however it is more complicated and time consuming to implement. In most prismatic constructions, to limit the loss of volume that the current collectors occupy, both anode and cathode current collectors are fused together on one end of the cell. This is good from a volumetric point of view, however in high rate applications, this becomes problematic due to uneven current paths. This can cause stability issues and uneven cell aging as well. Last, prismatic cells require more mechanical support for battery chemistry types that generate high internal gas pressures. For this type of chemistry, single cell applications are not common. One of the big advantages of the prismatic cell is the packing utilization. In applications where thermal management is required, an engineered minimum space between cells can be achieved. Last, there is between 10% and 20% volume overhead that must be allocated to packing the current collectors.

 

Bi-Polar Design

The main advantage this construction type offers is the common/shared current collector. This important feature reduces the volumetric overhead of the current collector and inherently results in uniform current flow across the cell, thus making it ideal for high rate discharge applications. The volumetric overhead is approximately zero as compared to the cylindrical and prismatic technologies. This gives an immediate 10% or greater volumetric density advantage. Uniform current and resistance paths also promotes uniform I2R heat generation. Uniform temperature generation also helps with uniform electrochemical aging translating into longer cell life. The size of the surface area and thickness of the cell are directly related to the capacity of the cell. This limits the capacity to less than 40Ah for practical purposes.