Renata SA Passivation Phenomena - Manual

Passivation Phenomena
Lithium is among the most reactive elements. It
easily reacts with a number of substances,
including water and air. Because of this high
reactivity, the commercial exploitation of lithium-
based electrochemical systems has been for long
time hindered by the reaction between lithium and
several electrolytes. Only in the 80s suitable
electrolytes were developed, based on aprotic
organic solvents which are stable when in contact
with lithium metal.
The reason for the stability of electrolytes based
on organic solvents lies in the passivation layer
that is built at the lithium surface.
This protective layer (also called SEI, Solid-
Electrolyte-Interphase) stops the reaction between
electrolyte and lithium and due to its mechanical
characteristics also ensures good stability for long
times. Therefore the formation of a layer of right
properties is a key element for the achievement of
long-term storage properties.
The formation of the SEI layer is influenced by a
number of factors, including the formulation of the
electrolyte and the production conditions. In
addition, a particular step of the manufacturing
process plays a decisive role in the formation of
the right SEI layer: the pre-discharge step (i.e. a
discharge limited to some % of the theoretical
capacity of the cell) of 100% of the produced
cells. By carefully controlling the pre-discharge
parameters, a passivation layer of optimized
physical-chemical characteristics is created at the
interphase lithium-electrolyte.
Unlike other lithium-based battery technologies,
the CR (Li/MnO
2
) system is not characterised by a
passivation layer of growing thickness after long-
term ageing of the cells or after short exposures
at high temperature. The SEI layer of CR cells built
at the beginning does not change significantly
even after years of storage at controlled
temperature (see related section in this chapter –
FAQ about recommended storage conditions). In
other lithium systems, instead, a growth of the
layer with ageing time, is observed, turning out in
a reduced pulse capability (the well-known
"voltage delay effect, especially observed for
liquid cathode systems when trying to request
high pulses after long time storage at room
temperature, or after short periods at high
temperature). For these other lithium systems it is
necessary to apply a continuous load of low
current to minimize passivation phenomena; on
the contrary, for CR systems this precaution is not
necessary.
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