Meaning
Concentration gradients in the electrolyte solution reduce the availability of lithium ions at the electrode surface during heavy discharge. High current draws cause salt depletion when the rate of ion consumption exceeds the rate of diffusion through the solvent. The reaction stops when the local concentration reaches zero.
Ion Gradient
Lithium salts must move through the porous separator to maintain the flow of current. Salt depletion occurs first in the center of the electrode where the path to the bulk electrolyte is longest. This creates an uneven current distribution that can damage the active materials.
Current Threshold
Discharge rates are limited by the speed at which ions can move through the liquid. When a battery reaches the salt depletion limit, the voltage drops sharply even if there is still energy stored in the electrodes. This effect is more pronounced in thick electrodes designed for high energy density.
Cold weather worsens the problem because the viscosity of the electrolyte increases and slows down the diffusion process. Engineers must design the electrolyte chemistry to provide enough ion mobility for the intended power application.
Electrolyte Performance
Optimizing the salt concentration involves a balance between conductivity and viscosity. Avoiding salt depletion requires a high enough salt density to handle peak loads without increasing the resistance of the cell. This optimization is a fundamental part of developing batteries for high power electric vehicles.
