Meaning
Physical constants representing the rate at which active species migrate through an electrode material determine the charge and discharge speed of an electrochemical cell. Sourcing managers evaluate the diffusion coefficient of lithium ions within the active materials to predict the high-rate performance of a cell during rapid charging. This value governs the flux of ions through the intercalation host under a concentration gradient, establishing the fundamental limit of current delivery.
The measurement holds valid only under constant temperature conditions as thermal energy alters the rate of migration.
Mass Transport
Analysis of mass transport through the active particles reveals how the active material behaves during high power demands. A high diffusion coefficient minimizes the concentration gradients that form within the cathode particles during rapid discharge, preventing premature voltage cutoff. When these values are low, the cell suffers from high internal resistance and reduced usable capacity at high C-rates.
It is therefore an essential material parameter for power-optimized cell designs.
Rate Calculation
Determination of this rate parameter involves electrochemical techniques like galvanostatic intermittent titration or potentiometric titration. The resulting diffusion coefficient is calculated from the transient voltage response following a current pulse. This analysis provides the necessary data to model the rate capability of a cell during the design phase.
Technical Limitation
Phase transitions within the electrode during cycling can cause the measured value to change by orders of magnitude. For this reason, a single value cannot represent the entire state of charge of the battery.
