Meaning
Concentration of basic chemical species such as lithium hydroxide or lithium carbonate on the exterior of active particles defines this parameter. Measuring residual surface alkali provides an indicator of the potential for parasitic reactions within a battery cell. These compounds form when the cathode material reacts with moisture or CO2 in the air.
High levels are particularly common in nickel rich layered oxides.
Chemical Stability
Presence of these basic salts can lead to the gelation of the electrode slurry during the coating process. When residual surface alkali levels are too high, the basicity of the mixture triggers a reaction with the polyvinylidene fluoride binder. This reaction increases the viscosity and makes the application of a uniform layer impossible.
Managing the storage environment is the primary way to limit this growth. Manufacturers often use titration methods to quantify the specific amount of lithium carbonate present on the surface.
Slurry Interaction
Interaction between the cathode surface and the liquid solvent depends on the pH of the interface. High residual surface alkali can dissolve into the NMP solvent and alter the polymerization state of the additives. This change affects the adhesion of the active material to the current collector.
Careful washing or coating steps are often implemented to neutralize these surface species.
Electrochemical Impact
Decomposition of carbonate species at high voltages leads to gas evolution inside the battery pouch. If the residual surface alkali is not removed, the resulting CO2 can cause the cell to swell and fail. These basic layers also increase the charge transfer resistance at the interface.
Removing them improves both the safety and the cycle life of the device.
