Page 184 - Lindens Handbook of Batteries

BATTERY ELECTROLYTES 7.9

7.3.2 Inorganic-Solvent Electrolytes
A class of liquid cathode primary batteries has been developed that uses purely inorganic solvents.
These cells have very high energy density, in part because the electrolyte carries out the dual role
of electrolyte solution and cathode active material. The main representatives are thionyl chloride,
SOCl , and sulfuryl chloride, SO Cl , although a number of other solvents have appeared in the
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patent literature. Both types have been modified by the inclusion of additives in the electrolyte,
BrCl in the thionyl chloride electrolyte and Cl in the sulfuryl chloride electrolyte. The effect on
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the energy output of the cells due to the additives is small, but there are some advantages in resis-
tance of the cells to abuse conditions. Generally, the shelf life of the cells is adversely affected
by the additives. It is somewhat surprising that the conductivities of the electrolytes are relatively
high (1M LiAlCl solution in thionyl chloride = 14.6 mS/cm; in sulfuryl chloride = 7.4 mS/cm),
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since the dielectric constants of these solvents are low (permittivity of thionyl chloride = 9.25; of
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sulfuryl chloride = 9.15). The preferred salt in all of these solutions is LiAlCl although some
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work has been carried out with LiGaCl to show improved conductivity and reduced passivation.
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These systems are surprisingly stable even though the oxidizing liquids are in direct contact with
lithium metal and one would expect at least a strong reaction, if not an explosive one under these
circumstances. Extensive study of the lithium surface has shown, however, that a tight, compact
layer of lithium chloride is formed which impedes further reaction. The shelf life is accordingly
very long (better than many organic solvent systems), although a delayed response in current is
often shown after a long period of storage as the layer must be at least somewhat disrupted in order
for current to pass. Chapter 14 of this work discusses these and other aspects of the systems in
detail. Further insight into these systems can be derived from Refs. 20 and 21.
The lithium-sulfur dioxide liquid cathode system is very important in military and industrial
applications. The electrolyte phase is a mixture of an organic solvent with condensed-phase sulfur
dioxide. Acetonitrile is usually used as the organic solvent because of its high solubility for and
stability with sulfur dioxide. Acetonitrile has a moderately high dielectric constant (35.95) and very
low viscosity (0.341 cP). This combination (30/70 by volume) with a1M LiBr salt gives a conductiv-
ity of about 52 mS/cm at ambient temperature, approaching the conductivity of aqueous solutions
(see Chap. 14 for the temperature dependence of conductivity). As with oxyhalide liquid cathode
cells, a compact protective film on the lithium metal is the enabling feature of the electrolyte, only
in this case the material formed is lithium dithionite (Li S O ), which is also the reaction product of
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the cell. This material has very low electronic conductivity and forms a very compact layer, which
on cell discharge must be somewhat disrupted to allow the lithium ions to enter the electrolyte. The
cell has lower energy content than oxyhalide cells because of a lower voltage and the dilution of the
electrolyte with acetonitrile. It also has to contain pressurized sulfur dioxide, which has a boiling
point of -10°C. This electrolyte allows LiSO cell to provide excellent low temperature performance
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to -40°C.
7.4 IONIC lIqUIDs

Ionic liquids are defined as liquids that are primarily dissociated into ions even though they have
complex polyatomic structures of each of the ions. Many of these are actually liquid at room tem-
perature and below and also support the dissolution of lithium salts at reasonable concentrations, such
as 1 M. Because they have very low vapor pressures in general, they offer flame-retardant properties
that few other electrolytes accomplish. Also, because of their high concentration of ions, the conduc-
tivities are comparable to many organic solvent systems, even though the viscosities tend to be much
higher. The high viscosity can cause problems in filling cells in short time periods as well as creating
wetting problems of electrodes and separators. Typical structures are shown in Fig. 7.3 from a recent
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paper on this topic. An electrochemical difficulty with most ionic liquids occurs due to the fact that
the onium cations are reduced at more positive potentials than lithium deposition or intercalation in
graphite, and the SEI formed with these materials is frequently unstable due to dissolution in these