Meaning
Diffusion processes govern the rate at which moisture from the surrounding environment penetrates the protective seals of a closed electrochemical cell. The calculation of water vapor ingress kinetics allows battery engineers to predict the gradual degradation of lithium-ion cells over several years of storage or operation. This transport rate depends on the concentration gradient of water vapor, the temperature, the permeability of the sealing materials, and the geometry of the leak paths.
Limiting this ingress is essential because moisture reacts with electrolyte salts to form corrosive acids that destroy the cell’s active materials.
Transport Mechanism
Molecular diffusion across the polymer seals or weld micro-cracks is the primary driving force for moisture penetration. When analyzing water vapor ingress kinetics, the transport rate is modeled using Fick’s laws of diffusion, where the flow of water molecules is proportional to the concentration gradient across the seal. In rubber or plastic seals, moisture dissolves into the polymer matrix before diffusing through to the interior of the cell.
This process is much slower than flow through open physical cracks but remains a major long-term degradation pathway.
Degradation Consequence
Internal chemical reactions between the entered moisture and the cell chemistry lead to irreversible capacity loss and cell swelling. The study of water vapor ingress kinetics shows that even parts-per-million levels of water can trigger the formation of hydrofluoric acid. This acid decomposes the solid electrolyte interphase and dissolves transition metals from the positive electrode.
This cycle of degradation results in a rise in cell impedance and a decrease in safety.
Sourcing Analysis
Procurement teams use lifetime models to select cell designs with superior environmental barriers. Evaluation of water vapor ingress kinetics helps buyers compare polymer and glass-to-metal sealing technologies to minimize the risk of moisture-induced capacity fade.
