PASSIVATION IN LITHIUM BATTERIES
What Is Passivation in Lithium Batteries?
Passivation is a chemical term referring to the inherent phenomena observed within all Lithium Thionyl Chloride Primary cells whereby a high resistant film (Solid Electrolyte Interface -SEI) of Lithium Chloride (LiCL) is formed internally on the surface of the lithium anode within cells themselves. A LiCl film is formed as a result of a chemical reaction between the interaction of the battery electrolyte and the fixed lithium anode.
Long Shelf Life
Due to the formation of a high resistance LiCl film on the surface of the anode within dormant Passivated Lithium cells , self-discharge rates are very low. Without a passivation layer cells of this chemistry could not be stored and would not exist, the Lithium would discharge and degrade very rapidly. The typical rated shelf life of a Lithium Thionyl Chloride Primary battery is over 10 years, this is primarily due to the capability of the Passivation layer to protect the cell from high self-discharge rates and allow a ‘dormant’ state to exist.
Drawback – Voltage Delay
After placing an electrical load on a Passivated cell the measured output voltage drops in proportion to the requested current, this is due to the internal voltage drop over the resistive passivation layer. Current flow through the passivation layer will quickly break down the resistive LiCl film and reduce the internal voltage drop, hence quickly allowing the cells output voltage to recover to steady level. If the load is increased the output voltage may periodically dip once more until again recovering to a steady state.
In many applications where low discharge rates are required, the passivation layer does not degrade cell performance and the voltage drop at load start-up is insignificant. If Lithium Thionyl Chloride Primary cells are used in high load application after long periods of storage the voltage drop caused by the passivation layer does cause a problem.
Working with passivation in lithium batteries
If a voltage delay at system start-up cannot be tolerated in the application equipment, there are several possible techniques that can be employed to reduce the severity of the passivation effect.
The passivation layer can be prevented from growing too thick by either applying continuous low current or periodic high current discharge during battery storage, this will however shorten the expected life of the cell during storage. Alternatively the battery can be placed under a high load just prior to usage, this method will not increase the self discharge rate of the cell during storage, but will still waste capacity during the depassivation procedure.