Meaning
Chemical degradation occurs when moisture reacts with lithium hexafluorophosphate salts to create an acidic byproduct that attacks the internal components of a battery cell. This hydrofluoric acid corrosion is a leading cause of premature failure in lithium ion batteries that have been exposed to humidity or have poor seal integrity. The acid is highly reactive and can dissolve the current collectors, the active materials and the separator.
It is a persistent threat that manufacturers must mitigate through strict moisture control during the assembly process. This reaction also leads to gas generation which can cause the cell to swell or vent.
Electrolyte Breakdown
Moisture contamination triggers a series of reactions that decompose the electrolyte salt into phosphorus oxyfluoride and hydrogen fluoride. These byproducts then combine with any remaining water to form the hydrofluoric acid corrosion that damages the cell. This process is self sustaining because the reaction can release more water or acidic components as it proceeds.
It is especially problematic in cells that use high voltage cathodes, which can further catalyze the decomposition of the electrolyte. Maintaining a dry environment in the battery plant is the primary defense against this chemical attack.
Component Damage
Metal surfaces inside the cell are particularly vulnerable to the acidic environment created by the decomposition of the salt. The aluminum current collector on the cathode side can be etched and weakened, leading to a loss of electrical contact and increased resistance. This hydrofluoric acid corrosion also attacks the binder that holds the active material to the foil, causing the electrode to peel or flake.
In severe cases, the acid can penetrate the separator and create a path for an internal short circuit. The resulting debris from this corrosion further contaminates the electrolyte and reduces the efficiency of the lithium ion transport.
Longevity Impact
Cycle life is significantly shortened when acidic byproducts are allowed to build up inside the battery housing. The constant removal of material from the electrodes leads to a steady decline in capacity and an increase in internal heating. If hydrofluoric acid corrosion is present, the cell may show a high rate of self discharge and a rapid increase in impedance.
This makes the battery less capable of handling high power loads and reduces the overall safety of the system. Purchasers often look for low moisture specifications and high quality sealing to ensure their systems are protected from this degradation. Monitoring the pressure and temperature of a battery pack can provide early warning signs of this internal chemical damage.
