Meaning
Concentration gradients drive the passive movement of chemical species through a host medium from regions of higher density to lower density. In electrochemical systems, fickian diffusion describes the transport of lithium ions within both the liquid electrolyte and the solid active material particles. It dictates the rate at which a battery can be charged and discharged without inducing mechanical or chemical degradation.
Ionic Transport
Solute movement through the porous electrode matrix depends heavily on the tortuosity and porosity of the separator. For lithium ions, fickian diffusion is the primary mechanism of replenishment at the electrode surface during deep discharge cycles. If the electrolyte formulation lacks sufficient ionic conductivity, concentration gradients become severe.
This leads to early voltage cutoffs and underutilization of the cathode material.
Sourcing Variable
Purchasing teams utilize the diffusion coefficient as a performance indicator when comparing active material chemistries. A higher solid-state fickian diffusion coefficient allows for the selection of larger primary particles, which generally have lower surface area and experience fewer parasitic side reactions with the electrolyte. Sourcing managers use this trade-off to optimize the cell design for long cycle life.
In addition, it reduces the dependence on complex and expensive nano-structuring techniques during cathode synthesis.
Rate Limitation
High-power applications suffer when ionic flux cannot match the electrical current demanded by the load. At high charge rates, slow fickian diffusion in the graphite anode leads to localized lithium accumulation on the particle surfaces. This results in lithium plating, which reduces the cell capacity permanently and can cause internal short circuits.
Sourcing contracts for electric vehicle cells must specify the diffusion-limited current density to prevent unsafe operation.
