Meaning
Moving phase boundaries represent the localized structural transition between lithiated and unlithiated regions in insertion electrodes. The two phase reaction front arises when the chemical potential difference forces a sharp separation between two distinct phases during lithiation. This behavior is characteristic of materials like lithium iron phosphate and silicon anodes.
Structural Boundary
Charge transport becomes concentrated along the narrow region of the two phase reaction front. This boundary moves through the particle as the lithiation progresses, converting the starting material into the fully lithiated phase. The velocity of this movement is determined by the ionic and electronic conductivity of the material.
Mechanical Strain
Tremendous mechanical stress accumulates at the interface between the distinct phases. Because each phase has a different volume, the presence of the two phase reaction front generates shear stress that can cause microcracking within the particle. These cracks expose fresh surfaces to the electrolyte, causing further degradation.
Over many cycles, this cracking can isolate regions of the active material, preventing them from participating in the electrochemical reaction and reducing overall cell capacity.
Diffusion Rate
Lower charging currents mitigate the localized mechanical strain by slowing boundary propagation. This control allows the two phase reaction front to move uniformly, giving the material time to relax and distribute the strain. The optimization of this rate is a key component in the design of fast-charging profiles.
