Cell Electrode Pressure and Expansion

The cell electrode pressure and expansion feel like two conflicting areas that have to be managed to enable performance over lifetime.

Expansion

As you charge a cell it expands, when you discharge a cell it contracts and as the cell ages over its lifetime we see a continuing cell expansion. Thus the cell expansion can be divided into:

  • Reversible cell expansion
  • Irreversible cell expansion

Pressure

Diagram showing pressure on cell active layers

The cell electrode pressure is required to keep the cell operating at it’s peak performance over it’s lifetime. However, is there an optimum pressure and why exactly does the cell need it?

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.

Managing Cell Expansion

Over the lifetime of a cell the stack thickness and hence cell thickness will increase by 10 to 20%.

One option for managing this with pouch and prismatic cells is to use gap pads, an elastic medium. These allow the cell to expand whilst maintaining a pressure on the surface of the cell. However, these pads take up volume, add weight and cost.

Allowing for 20% increase in volume is a significant mechanical and packaging challenge. Therefore, we need to balance the expansion with an increase in pressure.

sunwoda module

A module that is made up of 12 PHEV2 prismatic cells. The face area of these cells is 91mm x 148mm ~ 0.0135m2

The initial assembly force ~3kN and hence ~2.2bar

NMC cells with graphite anodes will see an end of life force ~30kN ~22bar

NMC cells with graphite-silicon anodes will see an end of life force ~50kN ~37bar

This is a new core page and the intention is to add articles from experts, latest materials to manage this and a look at modelling the electrochemical-mechanical system.