Passivation of Primary Lithium Cells

Passivation is a phenomenon of all lithium primary cells related to the interaction of the metallic lithium anode and the electrolyte. A thin passivation layer forms on the surface of the anode at the instant the electrolyte is introduced into the cell. This layer is important because it protects the anode from reaction while the cell is dormant – resulting in a long shelf-life.

During low rate discharge (5-10 microamps/cm2 ), the lithium ions that allow the cell to operate can migrate through the passivation layer. As the rate of discharge increases (0.1-1.0 milli-amp/cm2 ), so does the porosity of the passivation layer, allowing greater ion flow and higher power output. This change in the structure of the passivation layer is illustrated in the diagram.

Under normal conditions, the thin passivation layer does not degrade cell performance. When the layer grows too thick, however, discharge performance may be affected. The growth of the passivation layer is influenced greatly by storage conditions. Long storage periods of months or years and/or storage of the cells above room temperature (23o C) will cause the passivation layer to grow thicker. A passivated cell may exhibit voltage delay, which is the time lag that occurs between the application of a load on the cell and the voltage response. As the passivation layer thickens, the voltage delay becomes more severe. On continued discharge though, the voltage of a passivated cell will rise to a level equivalent to the load voltage of an unpassivated cell.

Adjusting storage conditions to reduce the likelihood of passivation is the best way to reduce voltage delay problems. However, there are several effective methods for dealing with excessive passivation when storage conditions cannot be controlled. The layer can be kept from growing too thick by maintaining a light load on the cell during storage. Alternatively, a high load, placed on the cell at regular intervals during storage, or just prior to the anticipated start-up of the cell, can be used to disrupt the passivation layer and restore normal performance.

Both of these methods will have an impact on the capacity of the cell. In particular, a low rate discharge tends to increase the normal self-discharge reaction of the cell and reduce the available capacity.