Meaning
Dynamic electrochemical flow restrictions in battery cells describe the overall opposition to the transfer of lithium ions and electrons across the electrode-electrolyte interfaces during operation. This resistance arises from the activation energy required to drive the electrochemical reactions and the concentration gradients that develop within the cell. Polarization resistance directly reduces the operating voltage of the cell during discharge and increases it during charge, leading to energy losses as heat.
Minimizing this resistance is a key objective for improving the efficiency and power density of batteries.
Ohmic Effects
Internal voltage drops in the cell are caused by both immediate ohmic resistance and time dependent polarization effects. While the ohmic resistance is instantaneous, the polarization resistance develops more slowly as the current flows and the concentration of lithium ions at the electrode surfaces changes. This rise in resistance is influenced by the thickness of the electrodes and the porosity of the separator, which affect the transport of ions through the electrolyte.
Thinner electrodes generally exhibit lower polarization effects, enabling higher power output.
Rate Capability
High current operation of the battery is limited by the magnitude of these dynamic resistance effects. When the polarization resistance is high, the cell reaches its cut off voltage prematurely, reducing the delivered capacity of the battery during high rate discharge. This limitation is particularly pronounced at low temperatures, where the ionic conductivity of the electrolyte is reduced and the reaction kinetics are slower.
To mitigate these effects, battery chemistries are optimized with high surface area materials and high conductivity electrolytes.
Thermal Management
Heat generation within the battery pack is directly proportional to the square of the current and the internal resistance, including these polarization effects. During rapid charging or high power discharge, the energy lost due to polarization resistance is converted into thermal energy, which must be removed by the cooling system of the pack. Effective thermal management and low polarization cells are required to prevent the battery from reaching unsafe temperatures during demanding drive cycles.
Selecting cells with low polarization characteristics is essential for high performance electric vehicles.
