Passivation is a phenomenon of liquid
cathode lithium cells related to the interaction of the metallic lithium
anode and the oxyhalide 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 milliamp/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 below.
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 periodsand/or high storage temperatures 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.
Eventually 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. 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 higher 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 useable 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.
Electrochem utilizes additives in
many of its cell chemistries (including BCX, CSC, PMX and MWD
cells) to minimize passivation formation and enhance restart performance.
Under most operating conditions, depassivation of an Electrochem
cell is unnecessary. However, under some more severe conditions
(such as high temperature storage) it may be beneficial to depassivate
a cell. For the most effective depassivation, Electrochem generally
recommends discharging a cell at or near the specified maximum continuous
discharge rate. The table below shows typical depassivation loads
for several common Electrochem cell chemistries and sizes. Note that
the load given in the table will yield close to the rated maximum
continuous current for an individual cell. The load should be adjusted
accordingly for multi-cell battery packs.
A depassivation load should be applied until the cell voltage recovers
to a normal level (> 3.0 volts). The recovery duration will depend
on the severity of the passivation. Any questions regarding the performance
of Electrochem cells should be directed to an Electrochem Sales
or Customer Service representative, or to an authorized Electrochem
dealer.
