Meaning
Electrical resistance to ion and electron transport occurs across the physical boundary separating the active electrode material and the electrolyte. Interface impedance dictates the speed at which lithium ions can cross from the solid-state electrolyte into the active material of the electrode during rapid charge and discharge cycles. High impedance at this junction leads to large overpotentials and reduces the usable capacity of the cell.
This reduction in capacity is particularly pronounced at low operating temperatures where ionic diffusion through the solid interface is naturally sluggish.
Electrochemical Effect
Spheroidizing the active material particles improves the contact conformity and reduces the interfacial resistance. During cycling, the expansion and contraction of the electrode can create gaps that increase the interface impedance. This mechanical disconnection limits high-rate capability.
Diagnostic Testing
Electrochemical impedance spectroscopy measures this property by applying a small sinusoidal voltage across the cell over a wide frequency range. The resulting Nyquist plot reveals the distinct charge-transfer resistance of the boundary. This measurement helps locate degradation in the cell.
Material Adjustment
Protective coatings such as lithium niobate are applied to the active material to suppress unwanted side reactions. These coatings lower the charge-transfer barrier and stabilize the boundary against chemical oxidation. A stable boundary maintains low impedance over long term operation.
