Graphite Polarization

Overpotential Cause

Three distinct contributions drive the deviation of the anode potential from equilibrium. Activation polarization arises from the energy barrier of the charge-transfer reaction at the electrode interface. Ohmic polarization relates to the electrical resistance of the active material and the ionic resistance of the electrolyte.

Concentration polarization occurs when the diffusion of lithium ions through the electrode pores cannot keep pace with the applied current, creating a depletion zone near the current collector. These three mechanisms act in parallel, with concentration polarization dominating during the final stages of a high-current charging step when the salt concentration gradients are steepest.

Plating Hazard

Excessive anode polarization drives the local potential of graphite below zero volts against lithium. This triggers metallic lithium plating on the electrode surface. Plating leads to rapid capacity loss and dendritic growth.

Consequently, cell manufacturers must set conservative charge cutoff voltages to prevent internal short circuits under cold conditions.

Control Strategy

Battery management systems utilize sophisticated charging algorithms to mitigate graphite polarization. These algorithms adjust the charging current dynamically based on real-world voltage and temperature inputs. Pack designers specify high-conductivity electrolytes and optimized electrode thicknesses to minimize polarization during high-rate charging.

Sourcing contracts for electric vehicle cells always include polarization limits under standard test conditions.