# Cell Gas Pressure

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.

#### 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.

### Gases

There are a number of papers [3, 4] that have looked at the gases inside cells, how they are produced and consumed during different phases/use cases for the cell. Typically the gas includes: CO2, CO, H2, and C2H2.

### Ideal Gas Law

The approximation of the ideal gas is usually permissible when the pressure of the gas is small compared to the critical pressure. Below we list some of the gases observed inside an NMC/Graphite cell.

For all these gases, the critical pressure is significantly higher than the pressure inside the battery cells, so in this case the gas can be assumed to be ideal for temperature independent processes.

This means that a reduction of the gas volume due to an expansion of the electrode would result in the gas pressure increasing. Thus following the ideal gas law PV=nRT

Gulsoy et al [1] measured the gas pressure versus surface temperature, allowing the cell temperature to stabilise between measurements.

The relationship between temperature and pressure looks almost linear, but there is a 2nd order term.

### Gas Pressure Post Formation

Gulsoy et al [1] developed a technique to measure the accumulated pressure within the cylindrical cell post formation, the cell in question was the LG INR21700 M50. The method involved opening the cell and inserting a sensor without incurring a loss of pressure. A bespoke test rig was designed to achieve this. Three cells were tested and the average gas pressure was 260 mbar.

### Gas Pressure vs SoC

Gulsoy et al [1] show the gas pressure versus SoC for an LG INR21700 M50 cell and Hemmerling et al [2] show the gas pressure versus SoC for an LG INR18650 MJ1 cell.

Internal gas pressure of instrumented cells with respect to 0% SOC during the processes of (a) charging with C/3 rate and (b) discharging with 1C rate at different ageing stages

Hemmerling et al [2] show the gas pressure versus SoC for an LG INR18650 MJ1 cell during a stepped C/3 charge cycle with a relaxation time.

The Δp for both cells in this case is very similar.

### Gas Pressure vs Charge Rate

Hemmerling et al [2] show the gas pressure versus SoC for an LG INR18650 MJ1 cell at different charge rates.

Higher charge rates => higher rate of pressure rise.

### Gas Pressure vs Cycling

Gulsoy et al [1] show the gas pressure increasing as the cell is cycled 100 times. The step after every 20 cycles is for the reference performance test. They noticed from the data, that the pressure rise in the cell appeared to include both reversible and non-reversible components. An observation supported by previous studies.

The increase in pressure for the 3 cells tested was 105 mbar, 114 mbar and 52 mbar. Approximately 1mbar/cycle for this 21700 cell.

This testing is extremely difficult to do as it is an intrusive measurement. Hence, a significant amount of research is being applied to the instrumentation of cells.

#### References

1. B. Gulsoy, T.A. Vincent, C. Briggs, J.E.H. Sansom, J. Marco, In-situ measurement of internal gas pressure within cylindrical lithium-ion cells, Journal of Power Sources, Volume 570, 2023
2. Jessica Hemmerling, Johannes Schäfer, Tobias Jung, Tina Kreher, Marco Ströbel, Carola Gassmann, Jonas Günther, Alexander Fill, Kai Peter Birke, Investigation of internal gas pressure and internal temperature of cylindrical Li-ion cells to study thermodynamical and mechanical properties of hard case battery cells, Journal of Energy Storage, Volume 59, 2023
3. Chengyu Mao, Rose E. Ruther, Linxiao Geng, Zhenglong Li, Donovan N. Leonard, Harry M. Meyer, Robert L. Sacci and David L. Wood, Evaluation of Gas Formation and Consumption Driven by Crossover Effect in High Voltage Lithium-Ion Batteries with Ni-Rich NMC Cathodes, ACS Applied Materials & Interfaces
4. N. Е. Galushkin, N. N. Yazvinskaya and D. N. Galushkin, Mechanism of Gases Generation during Lithium-Ion Batteries Cycling, Journal of The Electrochemical Society, Volume 166, Number 6