Meaning
Electrochemical resistance shifts away from equilibrium potential during charge and discharge operations, creating an overpotential that reduces cell voltage efficiency. Polarization stems from activation barriers at solid electrolyte interfaces, concentration gradients within liquid electrolytes and ohmic drops across current collectors. Operating currents drive these deviations higher, converting electrical input into thermal output rather than stored chemical energy.
The phenomenon ceases the instant current interruption allows the system to relax back to open circuit voltage.
Electrode Kinetics
Charge transfer resistance governs activation polarization at the boundary where solid active material meets liquid electrolyte. Surface area limitations and slow electron exchange force a kinetic penalty before Faradaic reactions proceed at the desired rate. High-power cycling demands specific conductive additives and optimized porosity to shrink this activation barrier.
Decreasing reaction temperatures aggravate the penalty, stiffening the interface and multiplying the voltage drop during initial pulse application.
Concentration Gradients
Mass transport limitations trigger concentration polarization when ionic species fail to replenish depleted reaction sites fast enough. Lithium-ion depletion near cathode surfaces and accumulation at anode interfaces generate internal concentration gradients that restrict high-rate discharge. Electrolyte salt conductivity and separator tortuosity dictate how swiftly ions diffuse through the pore network to restore equilibrium.
Extended high-current demands eventually exhaust local reactants, causing abrupt voltage collapse long before theoretical capacity runs out.
Thermal Feedback
Internal resistance heating accelerates polarization by altering reaction kinetics and ionic mobility inside sealed enclosures. Elevated cell temperatures lower solution resistance temporarily, yet simultaneous degradation reactions build resistive solid electrolyte interphase layers over time. Long-term pack performance degrades permanently as repeated polarization cycles thicken these parasitic films and permanently trap active lithium ions.
Cell aging correlates directly with the irreversible growth of polarization over extended cycling histories.
