Intercalation Potential Shifts in Large Format Lithium Cells
Local intercalation overpotentials in large format cells force anode potentials below 0V vs Li/Li+, causing lithium plating long before terminal voltage limits.
Ohmic Polarization
Meaning
Instantaneous drop in battery cell voltage caused by the internal resistance of the electrodes, electrolyte, and physical connections occurs when current begins to flow. This voltage drop is referred to as ohmic polarization, and it represents the resistive losses that obey Ohm’s law within the electrochemical system. It is the first and fastest component of the total polarization that occurs during battery operation, appearing and disappearing immediately when the circuit is closed or opened.
Minimizing this resistance is crucial for reducing heat generation and maximizing the energy efficiency of the battery pack.
Resistive Components
Internal resistance is composed of electronic and ionic resistance working in series throughout the cell. Electronic resistance arises from the metallic collectors, active materials, and physical weld joints between cells. Ionic resistance comes from the movement of lithium ions through the liquid electrolyte and the porous separator.
Each of these components contributes to the total voltage drop and must be minimized through careful material selection and assembly.
Heat Generation
The voltage drop caused by this resistance leads directly to heat generation within the cell during high current charging or discharging. This heat must be removed by the thermal management system to prevent the cell from reaching unsafe temperatures. Since heat generation is proportional to the square of the current, even a small reduction in resistance can significantly lower the heat load.
This relationship makes resistance control a primary goal of cell design.
Diagnostics
Measuring this voltage drop allows the battery management system to estimate the health and state of charge of the cell. By applying a short current pulse and measuring the instant voltage change, the system calculates the internal resistance. An increase in this resistance over time indicates cell degradation, such as electrolyte dry out or collector corrosion.
This diagnostic capability is vital for predicting battery failure before it occurs.