Investigation of Battery Cell Expansion using Dilatometry

Measurements of the expansion behaviour of battery cells are a critical aspect during the development of new cell technologies.

Knowledge of the maximum cell expansion and its dependence on state of charge and cell ageing is essential for the safe and efficient integration into modules and battery packs.

Using the measurement setups developed and patented by VIRTUAL VEHICLE, these parameters for pouch-format battery cells can be measured precisely and reliably even at an early stage of development. Contactless capacitive displacement sensors with sub-µm resolution are used to measure expansion. This allows for the analysis of details within individual cycles, subcycles, and current pulses.

A defined pressure can be applied to the cells, which is measured via load cells and can be analyzed separately. The setups are housed in a climate chamber—meaning that the ambient temperature is also controlled. Additionally, the temperature at the cell surface can be measured using thermocouples.

The cells are connected to a cycler, and an arbitrary current or voltage profile can be specified, such as Quasi-OCV, GITT, C-rate tests, pulse tests, ageing tests, or measurements to determine voltage hysteresis. The measurement method is not limited to a single cell chemistry but can be applied to various anode and cathode formulations. In addition to established lithium-ion systems, alternative systems, such as sodium-ion cells, can also be investigated.

The measurements serve as a valuable supplement to the electrical characterization of new cell chemistries and provide important information for further development.

Key Features

The following overview summarizes the key features of the measurement system:

A total of 6 test rigs are currently available for measurements.

Functionality – Setup

The following diagram shows the structure of the measurement system and the functions of the individual components.

Features

Measurement of thickness change and force in a test setup
Cell under a defined contact pressure
Controlled and monitored temperature
High-resolution capacitive displacement sensors
High mechanical stability of the test setup
Any electrical current profile can be applied

Functionality – Cell Holders

The two available cell holder models offer flexible options for pressurizing, making contact with, and measuring the temperature of cells.

Version 1

Positioning holes for cell alignment
Open contact area
Temperature measurement on the underside

Version 2

No positioning holes
Connection via contact pins
Temperature measurement on the top side

Example: Analysis of a Full Cycle

Dilatometry measurements enable the simultaneous analysis of cell voltage, cell expansion, and force, thereby providing detailed insights into the behaviour of battery cells during the charging and discharging process.

Example: Analysis of a Hysteresis Measurement

Special measurement profiles enable a detailed analysis of voltage and expansion hysteresis in battery cells.

Certain SoC points of the cell (e.g., every 10%) are reached once after charging and once after discharging. The difference between the voltage and the cell thickness at these points corresponds to the hysteresis.

Based on these measurements, the hysteresis in cell voltage and thickness change can be analyzed for various currents, SoC changes per step, applied pressure, temperature, etc.

Schematic representation of a discharge cycle during hysteresis measurement—circles of the same colour indicate points at which the cell has the same SoC.

Cell voltage and change in thickness during the discharge cycle, with SoC on the x-axis. The cell voltage plot shows the so-called hysteresis loops, which are characteristic of this cell chemistry with a silicon-containing anode.

C-Rate Testing and Detection of Lithium Deposits

C-rate tests allows researchers to investigate the effects of current rate at different temperatures and pressures on the behaviour of battery cells, as well as to identify critical effects such as the deposition of metallic lithium on the surface of the anode (known as lithium plating).

Deposited lithium causes greater expansion compared to lithium stored within the anode.

A distinction can be made between reversible and irreversible plating, as shown in the graph in the middle row, particularly on the far left and in the center (2C current rate at 5 °C and 25 °C, respectively).

Mechanical Finite Element Simulations

The combination of experimental characterization and finite-element simulation enables the investigation of stresses, deformations, and layer changes in the cell stack.

The experiments can be combined with finite element simulations of the cell to investigate changes in the layer structure and mechanical stresses within the cell stack in response to the following influences:

Pressure application / compression
Expansion / contraction of the anode during the charging or discharging process

Approach: meso-mechanical (taking into account the individual layers of the cell components in the cell stack).

Prerequisites: Knowledge of the layered structure and the mechanical properties of the cell components (which can be determined through tensile and compression tests).

References

Kargl, P., Drews, V., Daubinger, P., Schweighofer, O., Marinaro, M., Giffin, G.A., Wohlfahrt-Mehrens, M. and Thaler, A., 2022. Investigation of voltage and expansion hysteresis of Si-alloy-C/NMC622 pouch cells using dilatometry. Journal of Power Sources, 548, p.232042.

Kargl, P., Rasch, B., Pichler, F., Stangl, C., Marinaro, M., Wohlfahrt-Mehrens, M. and Thaler, A., 2025. Dilatometry study on current rate, temperature, and pressure influence on the expansion behaviour of Si-alloy/graphite-NMC622 pouch cells. Journal of Power Sources, 654, p.237752.

Kargl, P., Investigation of the expansion behaviour of silicon-based lithium-ion battery cells with a combined approach of dilatometry and finite-element simulations, Dissertation, Institut für Materialphysik, TU Graz, DOI: https://doi.org/10.3217/04wx2-kx578