As the name suggests these are a prismatic block, normally with the outer case made from aluminium. Again these come in many different formats and there is little standardisation apart from the VDA PHEV2 size of cell. These cells need a pressure applied to the main faces of the cell in order to maintain their performance over time.
The extension to the prismatic cell is the “blade” cell as originally termed by BYD. This is an elongated prismatic cell with the terminals at each end, designed to be assembled directly into a battery enclosure. Hence cell to pack.
Active Material Package
The active material within a prismatic cell is layered and these layers are arranged in a roll or as individual sheets stacked together. The roll is wound on a simple jig and then quashed to form it into a rectangular shape. The stacked layers are all cut to size and then stacked together before all of the anodes are joined electrically and all of the cathodes are joined electrically.
Cell shown has a jelly roll with the axis aligned to the longest side of the cell.
Pressure on Cell Surface
The cell electrode pressure is required to keep the cell operating at it’s peak performance over it’s lifetime. As the cell is charged lithium ions move into the graphite anode and the cell will increase in thickness. Silicon in the anode will increase this swelling significantly. The layers of the cell are likely to fatigue and fracture over a lifetime of charging and discharging. The external pressure can help to maintain the contact of the layers over time. Also, gas generation can cause the active layers to delaminate, hence reducing the active working area of the cell and reducing capacity and power capability. Applying a pressure normal to the active planes will keep the layers working together.
When assembling prismatic cells into a module there will be an initial pressure requirement and at end of life there will be a final pressure.
For a typical 12 cell module made using PHEV2 format prismatic cells (148mm x 91mm x 26.5mm) the initial force applied to the end plates is ~3kN.
148mm x 91mm = 13468mm2 = 0.013468m2
Pressure = 3000N / 0.013468m2 = 222750Nm-2 = 2.23 bar
At end of life this force can increase to ~30kN, a pressure of 22.3bar.
VDA Prismatic Cells
The closest you will find to a standard for a prismatic cell format. Based on a standard written by the German automotive industry. VDA stands for Verband der Automobilindustrie (German Association of the Automotive Industry).