Page 94 - Battery Reference Book

2/12 Guidelines to battery selection
temperature range and be stable towards lithium and Table 2.7
the cathode material.
In order to exploit the value of a lithium-based sys- Lithium-sulphur Mercuric
tem to the maximum, the positive electrode (cathode) dioxide oxide - zinc
material should also be of high energy density. The Volumetric energy
search for the ideal combination of cathode mater- density (W h/dm3) 420 500 (best)
ial and electrolyte has attracted a great deal of effort
in the 1970s. The lithium-polycarbon-monofluoride Normal working 2.75 1.25
voltage (V)
system is one of two lithium systems that have been
commercially promoted. Developed by the Matsushita Volumetric capacity
Electric Industrial Co. in Japan, the cells are avail- (A Wdm) 153 400
able in several cylindrical sizes. The patented cathode Relative cell volume
material is of the form (CF,), where x has a value per Ah 2.6 1.0
between 0.5 and 1.0 and is formed by reacting carbon
with fluorine under various conditions of temperature
and pressure, depending on the type of carbon used comparison of lithium-sulphur dioxide and mercuric
as the starting material. Except where batteries are oxide-zinc cells.
intended for low-rate applications, acetylene black or Thus one lithium- sulphur dioxide cell (voltage
graphite is added to the electrode to improve con- 2.75V) occupies about 30% more space than
ductivity. The electrolyte is lithium tetrafluoroborate two series mercuric oxide-zinc cells (voltage
dissolved in a-butyrolactone. Honeywell Inc. and the 2.5 V). Admittedly, this compares the worst cited
Mallory Battery Co. in the United States have intro- case for lithium against the best for mercuric
duced lithium batteries based on the lithium-sulphur oxide-zinc. Higher energy density systems such
dioxide electrochemical couple. The positive active as lithium-vanadium pentoxide and lithium-sulphur
material in these batteries, liquid sulphur dioxide, is dioxide would show significant volume savings over
dissolved in an electrolyte of lithium bromide, ace- an equivalent ampere hour mercuric oxide-zinc
tonitrile and propylene carbonate, and is reduced at a system. In fact lithium-sulphur dioxide systems
porous carbon electrode. are being increasingly considered for high-rate
Both types of lithium battery have a spiral-wound miniature power source applications including military
electrode pack, made up from rectangular foil elec- applications where it is found that a two-cell mercuric
trodes. Lithium foil is rolled on to an expanded metal oxide-zinc battery can occupy considerably more
mesh current collector as the negative electrode, and space than the equivalent lithium-sulphur dioxide
is separated from the similarly supported cathode by a cell; there are the added advantages inherent in the
polypropylene separator. lithium-sulphur dioxide system of excellent storage
Practical open-circuit voltages of the lithium- life (5-10 years, that is, very low self-discharge),
polycarbon-monofluoride and lithium- sulphur dioxide wide operating temperature range (-50 to +60"C) and
systems are approximately 2.8 V and 2.9 V respectively stable voltage characteristics on load.
at 20°C. The high voltage means that these batteries are The reactivity of lithium necessitates controlled-
not interchangeable with other electrochemical systems atmosphere assembly - in some cases the use of expen-
in existing equipment, unless a 'dummy' cell is also sive materials in the cell construction to avoid cor-
included. rosion, and in some cases (for example, lithium-
The high volumetric energy densities reflect the sulphur dioxide) the provision of a sophisticated seal
high voltages of the lithium-based systems. One reason design.
for some lack of acceptance in miniature applications Although lithium batteries have high-rate discharge
is that although one lithium cell could be specified capability, their use at very high rates or accidental
where it is necessary to use two mercury cells in shorting could result in temperatures leading to seal
series, a lithium button cell would have a capacity failure or explosion. Manufacturers have incorporated
approximately one-half that of the equivalent mercury vents and/or fuses to minimize these risks.
cell, and the frequency of battery changing would in The zinc-air system, which attracted a great deal
extreme cases be correspondingly increased. of investment in the late 1960s and early 1970s to
The volumetric ampere hour capacity of mercuric make a consumer product in standard cylindrical sizes,
oxide-zinc cells is higher than that of lithium-based suffered initially from four problems. It was difficult
systems. However, in many cases using two lithium to produce air-breathing cathodes of consistent qual-
cells in parallel or one larger lithium cell will give ity; the need to allow air into the cell led to elec-
the same ampere hour capacity that can be achieved trolyte leakage; carbonation of the electrolyte occurred
in an equal or even smaller volume than an equiv- on long-term discharge; during intermittent discharge
alent two-cell series mercury -zinc battery of similar oxygen ingress products caused wasteful corrosion of
voltage. This is illustrated in Table 2.7, which gives a the active material.