Cell Electrode 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.

Gas generation is a byproduct of electrochemical and chemical reactions inside the battery, which can occur when the battery is operational or in storage. The gas generation rate is dependent on chemistry, manufacturing quality, and battery management. Gas generation can be accelerated by increasing the ambient temperature, the discharge current, and by overcharging and over-discharging. Common gas species generated from typical lithium-ion battery composition are often toxic, e.g. CO, HF, SO₂, NO₂, NO and HCl.

Aalund R, Endreddy B and Pecht M, How Gas Generates in Pouch Cells and Affects Consumer Products, Front. Chem. Eng, 4:828375

Conclusions from experimental work conducted by Mussa et al [1] and Müller et al [2]:

• Mechanical pressure improves the electrical contact in Li-ion batteries.
• Reduced ionic pore resistance gets dominant in compressed cells at high C-rates.
• Compressibility is strongly dependent on the number of layers.
• Uncompressed Li-ion batteries tend to Li deposition.
• An optimum compressive pressure exists that extend the battery life.
• Cyclable lithium loss is reduced at the optimum pressure.
• Pressure-induced current distribution does not explain ageing in parallel connection.
• High compressive pressure impedes the electrochemical kinetics and mass transport.

Overall the results show a strong coupling between electrochemistry and mechanics.

Barai et al [3] show the opposing capacity fade and power fade result behaviour is related to the wettability increase and separator creep within the cell after compressive load is applied. 

1. When pressure is applied, the separator pores creep, thus the porosity and tortuosity of the separator changes. With applied pressure, the change of porosity and tortuosity is not linear [24]. The ion transport resistance of the separator is directly related to the porosity and tortuosity.
2. In contrast, when pressure is applied, the wettability of the electrodes increases; the penetration of the electrolyte into the porous electrodes increases. Increased wettability leads to better contact between electrode materials and electrolyte, i.e., increased double layer which increases the double layer capacitance and reduces ion-transport resistance.

These two mechanisms along with SEI growth due to cycling mainly dominate the degradation of cell performance reported in this study.

Their study also showed:

• An evolution of compressive pressures over multiple cycles, showing that pressure increases with cycling.

Typical Pressure Requirements

The pouch cell has a soft case design and so needs the module structure to supply all of the mechanical requirements for the cell.

Typically an NMC/Graphite pouch cell will require 20kPa to 40kPa pressure at the beginning of it’s life.

As the cell is charged and discharged the cell will expand and contract.

Typically a pouch cell thickness will increase by ~10% over it’s lifetime.

Cell Format and Module Design

The mechanical requirements around applying this pressure are dependent on the cell format and hence impact the module/pack design.

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.

This pressure used in prismatic module design feels very high compared to what we see generally within the literature. Some of this could be a result of the cell case design with the very stiff corner structure of the prismatic case and a requirement to maintain the geometry of the case. However, this is just a view of the author and it would be great to have more data to understand this design.

Generating and maintaining this pressure over the lifetime of the cell with the cell increasing in thickness is a difficult design challenge. Often foam interlayers are used between the cells to allow the cells to expand.

There are several novel approaches to managing the force/pressure.

Freudenberg [5] have a cell barrier and expansion material that can manage the pressure with the cell expanding.

References

1. Abdilbari Shifa Mussa, Matilda Klett, Göran Lindbergh, Rakel Wreland Lindström, Effects of external pressure on the performance and ageing of single-layer lithium-ion pouch cells, Journal of Power Sources, Volume 385, 2018
2. Verena Müller, Rares-George Scurtu, Michaela Memm, Michael A. Danzer, Margret Wohlfahrt-Mehrens, Study of the influence of mechanical pressure on the performance and aging of Lithium-ion battery cells, Journal of Power Sources, Volume 440, 2019
3. Anup Barai, Ravichandra Tangirala, Kotub Uddin, Julie Chevalier, Yue Guo, Andrew McGordon, Paul Jennings, The effect of external compressive loads on the cycle lifetime of lithium-ion pouch cells, Journal of Energy Storage, Volume 13, 2017
4. Aalund R, Endreddy B and Pecht M, How Gas Generates in Pouch Cells and Affects Consumer Products, Front. Chem. Eng, 4:828375
5. Ensuring that the innovative spark is ignited, Fruedenberg

Power versus Energy Cells

Comparing power versus energy cells we see there are some fundamental differences. A high energy cell will have better volumetric and gravimetric energy density at the expense of the ability to deliver a high current. The power cell will have a low internal resistance and will be optimised to deliver current over energy density.