Cell 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

The reversible cell expansion comes from changes in graphite layer spacing associated with different graphite-lithium intercalation compounds induce reversible macroscopic expansion and contraction of composite graphite electrodes during galvanostatic cycling.

The irreversible electrode deformation is correlated with irreversible electrode capacity loss [2]. Potentiostatic voltage hold experiments show that the formation and growth of the solid electrolyte interphase (SEI) directly induce macroscopic expansion of graphite composite electrodes.

Jones et al [2] explain this reversible and irreversible cell expansion:

1. A pristine graphite particle has an initial size of φ_o determined by the characteristic layer spacing of d_o between graphite layers.
2. Lithium intercalation causes the graphite layer spacing to increase by d at the atomic scale, which is translated to the microscale as an overall size increase of the graphite particle by φ_Li. Deposition of electrolyte reduction products on the surface of the graphite particle during the formation of the solid-electrolyte interphase (SEI) causes a further increase in the graphite particle size of φ_SEI.
3. Upon delithiation, deformation due to lithium intercalation is recovered, but the increase in the particle size due to SEI formation is irreversible.

In addition to this there is a gas build up in the cell over the lifetime [4]. This is a common phenomenon of the degradation of battery performance, a result of the electrolyte decomposition and happening whether the battery is in use or not. This gassing can be exacerbated under abuse conditions such as overcharging and overheating. This can result in cell failure.

• Overcharging – the gassing occurs mainly through the electrochemical oxidation of electrolyte solvents on the cathode with the Li⁺ ions from the electrolyte being reduced into metallic Li on the anode.
• Overheating – the gassing takes place through not only the redox decomposition but also the chemical decomposition of the electrolyte solvents on both the anode and cathode besides the vapor expansion of volatile electrolyte solvents.

Wenjia Du et al [5] used lab based x-ray to understand the swelling of a cell versus cycling, in this instance an unrestrained pouch cell. Gas agglomeration led to cell deformation in different areas, observed in 4D (3D + time), the subsequent quantification including the volume fraction, surface area and thickness showed a heterogeneous gas distribution, revealing the degradation mechanism involving the coalescence of gas.

If we use the ‘L’ to denote the thickness and ‘ΔL’ for the thickness change. Having these in mind, it would be safe to say:

• The anode thickness (L_anode) was 136 to 177 um at the middle region of the cell after 200 cycles.
• For individual electrode (local measure), the gas formation dominates the cell expansion, which can be quantified via thickness change (ΔL_gas/anode = 20-40 um). While the cathode (ΔL_cathode = 5-10 um) plays a less important role in the expansion.
• If we count one anode and one cathode, the total thickness can be calculated by L_total = L_anode + L_cathode = ∼ 300-310 um. However, L is less helpful than ΔL because thickness change varies against different locations.
• Overall cell thickness increase from 4.0 mm to 6.9 mm after the 200^th cycle, resulting in a total expansion (also termed as deformation ratio) reached to 72.5 % (ΔL_cell-level= (L_200th – L_pristine)/L_pristine = (6.9-4)/4 = 0.725).

Mohtat et al [1] show the magnitude of reversible and irreversible expansion for an NMC111-Graphite pouch cell, including a variation in the constraining pressure.

This paper is looking at cell expansion as an indicator for the control system: “The focus of this paper is to systematically verify the capability of aging diagnostics using cell expansion under variety of aging conditions.” However, it does show some interesting experimental data.

The new cell was 4mm thick when new and as you can see from the middle graph the irreversible cell expansion at the lowest 6.9kPa (1psi) pressure was 400µm = 0.4mm or 10% irreversible expansion over the lifetime of the cell.

The reversible expansion is ~100µm at the beginning of life which is roughly 2.5% of the cell thickness.

Note that the reversible expansion decreases as the cell ages.

You do need to look carefully at these plots and their axes.

Spingler et al [3] show that the expansion of the cell is rate dependent and they also show that the expansion is not uniform across the cell at higher charge rates.

This post has been built based on the support and sponsorship of: Quarto Technical Services, TAE Power Solutions, h.e.l group and The Limiting Factor. 

a) shows average expansion across the cell surface with severe expansion overshoots at 1.5C and 2.0C. At the end of the CC-CV charging, cells charged with higher C-rates exhibit greater average cell thickness, i.e. not all of the expansion overshoot is reversible. Possibly, the overshoot is caused by lithium plating and subsequent stripping of most but not all of the previously plated lithium

b-d) show local overshoots of up to 50 µm at different C-rates.

This post is an introduction to the subject of cell expansion and hopefully an overview of the mechanisms. The different cells and test regimes make it difficult to paint an overall conclusion.

However, the work by Wenjia Du et al [5] shows that the gas generation is a significant mechanism for cell expansion.

It is important to read this post in conjunction with the post on cell electrode pressure. Hopefully you can see that these articles approach this problem from slightly different viewpoints.

References

1. Peyman Mohtat, Suhak Lee, Jason B. Siegel and Anna G. Stefanopoulou, Reversible and Irreversible Expansion of Lithium-Ion Batteries Under a Wide Range of Stress Factors, 2021 J. Electrochem. Soc. 168 100520
2. E. M. C. Jones, O.O. Capraz, S. R. White and N. R. Sottos, Reversible and Irreversible Deformation Mechanisms of Composite Graphite Electrodes in Lithium-Ion Batteries, Journal of The Electrochemical Society, 163 (9) A1965-A1974 (2016)
3. Franz B. Spingler, Wilhelm Wittmann, Johannes Sturm, Bernhard Rieger, Andreas Jossen, Optimum fast charging of lithium-ion pouch cells based on local volume expansion criteria, Journal of Power Sources, Volume 393, 2018
4. Sheng S. Zhang, Insight into the gassing problem of Li-ion battery, Front. Energy Res., 05 December 2014, Sec. Electrochemical Energy Conversion and Storage
5. Wenjia Du, Rhodri E. Owen, Anmol Jnawali, Tobias P. Neville, Francesco Iacoviello, Zhenyu Zhang, Sebastien Liatard, Daniel J.L. Brett, Paul R. Shearing, In-situ X-ray tomographic imaging study of gas and structural evolution in a commercial Li-ion pouch cell, Journal of Power Sources, Volume 520, 2022

If you have written a paper or article on this subject and think we should have included it please do drop us a line.

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.