Rotational Inertia Measurements of Cylindrical Li-Ion Cells: Watching Electrolyte Move and the Impact of Cell Orientation
Authors: Butler Emily Joan , Kowalski Nicholas , Fitzner R. Christian , Aiken Connor P , Harlow Jessie , Johnson Michel B. , Bond Toby Mishkin , Jager Julianne , Metzger Michael , Dahn Jeff R. 
Published: December 2024

The novel Rotational Inertia Measuring (RIM) system provides unique insights into electrolyte motion in cylindrical Li-ion cells1. As electrode particles expand during charging, a portion of liquid electrolyte is forced out of the electrodes and into the hollow ends of the can, and also into the cavity at the core of the electrode winding. Then, as electrode particles contract while discharging, electrolyte is pulled back into the electrode winding. This movement of electrolyte gives rise to changes in the moment of inertia of a cell that is oscillated about its central diameter, which in turn cause changes in the frequency at which that oscillating cell resonates. RIM is a low-cost and non-destructive torsional oscillator that is used to track small changes in the resonant frequency of a cell as it cycles, as shown in Figure 1. RIM thereby improves our understanding of how electrolyte moves and how electrolyte movement impacts cell performance and lifetime. Animations of high resolution operando synchrotron X-ray CT images verify that conclusions drawn from RIM measurements are accurate.
The rate at which electrodes are “re-wetted” as electrolyte exits and re-enters them can be measured with RIM. Insights from RIM could be used to assist in efforts to extend the lifetime of cylindrical cells with electrode materials that exhibit large volume changes, such as silicon, and of cells that undergo fast-charging. RIM can also be used to examine the usefulness of resting cells between cycles in certain scenarios.
In addition to the first RIM instrument for horizontal 18650 cells, three new RIM instruments are presented: one for tracking electrolyte movement in horizontal 21700 cells, one for vertical 21700 cells, and one for vertical 46800 cells. The rate at which measurements of resonant frequency are collected by RIM has been doubled to improve the fidelity of measurements taken with high cell cycling rates. New examinations of the correlation between changes in the amplitude of resonant frequency peaks and state of charge have yielded insights into jellyroll expansion in relation to cell age. Using RIM, we will present information on the impact of cell orientation on lifetime, the effects of aging on electrode windings and electrolyte motion, and the effects of increasing the size of high-performance cylindrical cells.
Figure 1: RIM instruments have been constructed in four different configurations, including instruments used to examine electrolyte motion in horizontally cycled 18650 cells (left) and vertically cycled 21700 cells (bottom right). Changes in the resonant frequency and in the oscillation amplitude observed at resonance are tracked and associated to a corresponding cell voltage (top right) to obtain insights about electrolyte distributions at all states of charge.
References:
[1] C. P. Aiken et al 2023 J. Electrochem. Soc.
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Figure 1