Cell Electrode Pressure and Expansion

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

Compression Pads

Compression pads are used to manage the dimensional change of the cell during the breathing process and for the gradual permanent swelling that occurs throughout the battery lifetime.

Gases are generated in the cathode, anode and electrolyte during the formation and cycling of a battery cell. As this is a closed system the result is a cell gas pressure. What this gas is composed of, how it behaves and how it changes with State of Charge (SoC) and the cycling of the cell are interesting relationships we can build on.

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

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.

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

EVE LF50F prismatic cell clamping arrangement for lifetime testing

LFP Cell Clamping Pressure

Here we want to look at the requirements for Lithium Iron Phosphate chemistry. In the specifications for the EVE LF50F prismatic LFP cell they state that for lifetime testing the cell should be held in a jig.

LFP Expansion During Cycling and Lifetime – during the charge and discharge processes, Lithium Iron Phosphate (LFP) cells experience highly restricted cell volume variation. This structural stability is fundamentally attributed to the robust PO₄ tetrahedral structure and strong covalent bonds within the crystal lattice, which prevent significant physical expansion but also contribute to the material’s slow ion diffusivity.

Operational Pressure Measurements

Schematic experimental setup: The pouch cell (orange) and pressure mapping sensor (blue) are situated between the compression pads (black) and CompreCell Pouch cell fixture (grey).

Mapping Pouch Cell Pressure Distribution in Operando

During cycling in constant volume mode, an increase in pressure in the charged state was apparent both in the total force recorded by the CompreDrive, as well as in the in operando TekScan pressure mapping sensor. On the other hand, this pressure increase was absent (in both sensors) while cycling the cell in the actively regulated constant pressure mode of the CompreDrive.

Pressure

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.