Page 363 - Lindens Handbook of Batteries

14.28 PriMAry BATTerieS

14.6.1 Chemistry
The Li/SOCl cell consists of a lithium anode, a porous carbon cathode, and a nonaqueous
2
SOCl :LiAlCl electrolyte. Other electrolyte salts, such as LiGaCl have been employed for special-
4
4
2
ized applications. Thionyl chloride is both the electrolyte solvent and the active cathode material.
There are considerable differences in electrolyte formulations and electrode characteristics. The
proportions of anode, cathode, and thionyl chloride will vary depending on the manufacturer and the
desired performance characteristics. Significant controversy exists as to the relative safety of anode-
limited vs. cathode-limited designs. Some cells have one or more electrolyte additives. Catalysts,
23
metallic powders, or other substances have been used in the carbon cathode or in the electrolyte to
enhance performance.
The generally accepted overall reaction mechanism is
4Li + 2SOCl → 2 4LiCl ↓+ SSO 2
+
The sulfur and sulfur dioxide are initially soluble in the excess thionyl chloride electrolyte,
and there is a moderate buildup of pressure due to the generation of sulfur dioxide during the
discharge. The lithium chloride, however, is not soluble and precipitates within the porous
carbon cathode as it is formed. Sulfur may precipitate in the cathode at the end of discharge.
in most cell designs and discharge conditions, this blocking of the cathode is the factor that
limits the cell’s service or capacity. Formation of sulfur as a discharge product can also present
a problem because of a possible reaction with lithium, which may result in a thermal runaway
condition.
The lithium anode is protected by reacting with the thionyl chloride electrolyte during stand,
forming a protective LiCl film on the anode as soon as it contacts the electrolyte. This passivating
film, while contributing to the excellent shelf life of the cell, can cause a voltage delay at the start of a
discharge, particularly on low-temperature discharges after long stands at elevated temperatures. The
presence of trace qualities of moisture leads to the formation of HCl, which increases passivation, as
does the presence of ppm levels of iron. Some products have special anode treatments or electrolyte
additives to overcome or lower this voltage delay.
The low freezing point of thionyl chloride (below -110°C) and its relatively high boiling point
(78.8°C) enable the cell to operate over a wide range of temperature. The electrical conductivity of
the electrolyte decreases only slightly with decreasing temperature. Some of the components of the
Li/SOCl systems are toxic and flammable; thus exposure to open or vented cells or cell components
2
should be avoided.

14.6.2 Bobbin-Type Cylindrical Batteries
Li/SOCl bobbin batteries are manufactured in a cylindrical configuration, most in sizes conforming
2
to ANSi standards. These batteries are designed for low- to moderate-rate discharge and are not typi-
cally subjected to continuous discharge at rates higher than the C/50 rate. They have a high energy
density. For example, the D-size cell delivers 19.0 Ah at 3.4 V, compared with 15 Ah at 1.5 V for the
conventional zinc-alkaline cells (see Tables 8.5 and 14.11).

Construction. Figure 14.16 shows the constructional features of the cylindrical Li/SOCl cell,
2
which is built as a bobbin-type construction. The anode is made of lithium foil which is swaged
against the inner wall of a stainless or nickel-plated steel can; the separator is made of nonwoven
glass fibers. The cylindrical, highly porous cathode, which takes up most of the cell volume, is
made of Teflon-bonded acetylene black. The cathode also incorporates a current collector, which is
a metal cylinder in the case of the larger cells and a pin in the case of smaller cells that do not have
an annular cavity.