Passivation

 

Passivation Information

Cell passivation is a lithium battery characteristic that can be very difficult to understand for many users of batteries. In most cases, passivation is not a factor in cell performance, and in those cases that passivation becomes an issue, simple steps can be undertaken to overcome the effects. This section will help to explain steps to overcome passivation.

Passivation is a very thin, high resistant layer formed in a cell between the lithium anode and the carbon based cathode of a lithium cell. It is formed as a result of a chemical reaction between the battery electrolyte and the lithium anode. Without the passivation layer, this type of lithium battery would not exist because the lithium would discharge and degrade quite rapidly. An advantage of the passivation layer is it allows the battery to have a very low self discharge rate and extremely long shelf life.

For an excellent discussion on the chemistry that causes the passivation layer, as well as other tips on working with the passivation layer, please go to the Electrochem Battery website.

The most obvious affect of the passivation layer is voltage delay. Voltage delay is the voltage drop that occurs when a load is placed on the cell. Depending on the extent of passivation, the voltage may drop rapidly after the load is applied but it usually rises after a short period of time.

The extent of the passivation layer and its effects on cells and batteries is dependant on the following factors:

Storage Time
The longer cells are stored, the more the passivation layer will grow. This may result in increased voltage delay once the load is applied.

Storage temperature
High temperature storage increases the amount of passivation. Storage of batteries in non-air-conditioned areas during the summer months in hot climates can cause adverse passivation problems. It is recommended that batteries be stored in climate controlled areas.

Discharge Temperature
Just as high temperature storage may cause passivation effects, discharging batteries at very low temperatures may cause similar characteristics.

Prior use of cell or battery pack
Prior discharge of cells and batteries may cause the passivation layer to form thicker after prior use. Thus, voltage delay on a second run may be more pronounced after the first use of the cell or battery.

Resistive load of circuit
High resistive loads may cause the passivation layer be more pronounced on a particular cell or battery than if the load was small. Thus, a low rate cell with a high load will be more susceptible to the effects of passivation than a high rate cell with the same load. For passivation purposes, it is important to select the proper cell type for the expected resistive load.

Working with Passivation

With most Electrochem cells and applications, passivation is not a factor, and users do not notice voltage delay when hooking up to the circuit. On the other hand, if passivation is noticeable, fortunately, there are simple recommendations to reduce or eliminate the effects. Rather than repeat information here, it is recommended that the Electrochem Battery website be consulted for the proper procedures to remove or eliminate passivation.