Tracking Electrolyte Motion in Cylindrical Li-ion Cells Using Moment of Inertia Measurements

High energy density cylindrical Li-ion cells are densely packed with active materials, inactive materials and electrolyte. Injected electrolyte generally fills all pore spaces in the electrodes and separators of manufactured cells with possibly some excess. When such cells are charged, the overall volume of the electrode materials increases and therefore some electrolyte is pushed under hydraulic pressure to the spaces outside the electrode winding at the ends of the cylindrical can and also possibly into the hollow core of the cylindrical electrode winding. During discharge this electrolyte reenters the pore spaces of the electrodes as electrode particles contract. Therefore, the moment of inertia of the cell about an axis perpendicular to the axis of the cylindrical can change as the cell is charged and discharged. We have built a torsional oscillator that can measure the resonant frequency, and hence the moment of inertia, of a cylindrical Li-ion cell as it is charged and discharged. Because the moment of inertia of the cell depends on the electrolyte distribution, we can "watch" the electrolyte move within the cell. The design and operation of the instrument is described here as well as experiments that demonstrate the electrolyte motion that occurs in cylindrical cells. Consequences of this electrolyte motion are discussed.