A thin SEI is always present on the surface of the anode at all times. This layer is formed as a result of the reaction between the lithium and the thionyl chloride electrolyte, and it starts forming as soon as a cell is filled with electrolyte. It is only when this layer becomes excessively thick that passivation becomes a problem. In fact, some anode passivation is necessary because it is responsible for the extraordinary shelf life of these cells (>10 years). Without passivation, cells would self discharge in a matter of weeks.
Anode passivation is responsible for the condition known as voltage delay, which is the drop in potential observed when a load is placed on a cell. In a freshly manufactures cell, the drop in running potential may last for less than a second but in a heavily passivated cell, the voltage may drop below 3.0 volts for an extended period.
If given enough time, the voltage of a passivated cell will eventually return to a stable running potential that is equivalent t o the running potential of a fresh cell. The goal, when manufacturing and using Li/SOCl2 cells is not to eliminate the passivation but to manage the growth of the passivating (SEI) layer.
Figure 2 represents a typical start up (loading) of a passivated and fresh cell. The running voltage of the passivated cell in this figure eventually returned to a running voltage that was similar to the running voltage of the fresh cell.
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