Page 435 - Handbook of Battery Materials

14.3 Lithium Alloys as an Alternative 407

disconnected from the underlying metal, so that they cannot participate in the
electrochemical reaction, and the rechargeable capacity of the electrode is reduced.
This is a common problem when using elemental lithium negative electrodes in
contact with electrolytes containing organic cationic groups, regardless of whether
the electrolyte is an organic liquid or a polymer [4].
In order to achieve good rechargeability, one has to maintain a consistent
geometry on both the macro and micro scales and avoid electrical disconnection of
the electroactive species.

14.3
Lithium Alloys as an Alternative

Attention has been given for some time to the use of lithium alloys as an alternative
to elemental lithium. Groups working on batteries with molten salt electrolytes that
◦
operate at temperatures of 400–450 C, well above the melting point of lithium,
were especially interested in this possibility. Two major directions evolved. One
involved the use of lithium–aluminum alloys [5, 6], while the other was concerned
with lithium–silicon alloys [7–9].
Whereas this approach can avoid the problems related to lithium melting, as well
as the others mentioned above, there are always at least two disadvantages related
to the use of alloys. Because they reduce the activity of the lithium, they necessarily
reduce the cell voltage. In addition, the presence of additional species that are not
directly involved in the electrochemical reaction always brings additional weight,
and generally, volume. Thus the maximum theoretical values of the specific energy
and the energy density are always reduced in comparison with what might be
attained with pure lithium.
In practical cases, however, the excess weight and volume due to the use of alloys
may not be very far from those required with pure lithium electrodes, for one
generally has to operate with a large amount of excess lithium in rechargeable cells
in order to make up for the capacity loss related to the filament growth problem
upon cycling.
Lithium alloys have been used for a number of years in the high-temperature
‘thermal batteries’ that are produced commercially for military purposes. These
devices are designed to be stored for long periods at ambient temperatures, where
their self-discharge kinetic behavior is very slow, before they are used. They must
be heated to high temperatures when their energy output is desired. An example
is the Li alloy/FeS 2 battery system that employs a molten chloride electrolyte. In
order to operate, the temperature must be raised to above the melting point of
the electrolyte. This type of cell typically uses either Li–Si or Li–Al alloys in the
negative electrode.
The first use of lithium alloys as negative electrodes in commercial batteries to
operate at ambient temperatures was the employment of Wood’s metal alloys in
lithium-conducting button-type cells by Matsushita in Japan. Development work