Page 168 - Battery Reference Book

9/8 Lithium batteries

and lithium-silver chromate cells. Matsushita supply rated load of 2.7V compared with 1.5V for most
lithium-carbon fluoride cells. Venture Technology conventional types of battery. The specific voltage on
(formerly Ever Ready (Berec)), Ray-o-Vac and Say0 discharge is dependent on the discharge rate, discharge
(Japan) supply lithium-manganese dioxide cells. A temperature and state of charge. The end or cut-
further type of cell which has reached commercial off voltage is 2V. The lithium system is capable of
realization is the Venture Technology lithium-ferrous maintaining more stable voltage plateaux at higher
sulphide cell (1.5 V/cell nominal). It has a volumetric currents than any other cell of comparable size.
energy density of 500-900Wh/dm3 and has an It is also claimed that the lithium-sulphur dioxide
excellent shelf and working life and very good low- system operates very efficiently over a wide range of
temperature performance, with similar applications to temperatures (typically from -40 to +70"C), achiev-
the lithium-manganese dioxide cell. ing higher discharge rates at lower temperatures than
are possible with other types of cell, which provide
9.2 Lithium-sulphur dioxide primary little service below -18°C. The cells can be operated
batteries with success at elevated temperatures. When operated
at very high currents outside the recommended lim-
Of all the lithium battery systems developed over its, the cells may produce heat and high internal gas
the last decade, the system based on sulphur dioxide pressures, and these conditions should be avoided. The
and an organic solvent is now acknowledged to have cells are, however, an excellent source for high pulse
emerged as the most successful both commercially currents. Multicell batteries contain additional insula-
and technically. Other lithium batteries are capable tion as well as a fuse for protection from mishandling
of delivering as great or greater energy densities, in such as short-circuits.
particular the lithium-thionyl chloride systems. How- A typical energy density of a lithium-sulphur diox-
ever, the latter system may give rise to spontaneous ide cell is 420Wh/dm3 or 260Whkg according to
decomposition of explosive violence in high-rate bat- one manufacturer and 330 W hkg and 525 W h/dm3
teries, while the other lithium systems do not have the according to another. These are nearly three times
high-rate capabilities of the lithium-sulphur dioxide the values expected for mercury-zinc cells, four times
battery. that of an alkaline manganese dioxide cell and two to
The reactivity of lithium necessitates controlled- four times higher than that of conventional zinc and
atmosphere assembly during manufacture, and in some magnesium type batteries.
cases the use of expensive materials in the cell con- The high volumetric energy densities reflect the
struction to avoid corrosion and the provision of a high voltages of the lithium-based systems. One reason
sophisticated seal design. for some lack of acceptance in miniature applications
Although lithium batteries have high-rate discharge
capability, their use at very high rates or accidental is that although one lithium cell could be specified
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 incorporate vents approximately one-half that of the equivalent mercury
and/or fuses to minimize these risks. cell, and the frequency of battery charging would in
Honeywell Inc. and the Mallory Battery Company in
the USA have introduced lithium batteries based on the extreme cases be correspondingly increased.
lithium-sulphur dioxide electrochemical couple. The The volumetric ampere hour capacity of mercuric
positive active material in these batteries, liquid sul- oxide-zinc cells is higher than that of lithium-based
phur dioxide, is dissolved in an electrolyte of lithium systems. However, in many cases, using two lithium
bromide, acetonitrile and propylene carbonate, and is cells in parallel or one larger lithium cell will give
reduced at a porous carbon electrode. the same ampere hour capacity as can be achieved
This type of battery has a spiral-wound electrode in an equal or even smaller volume by an equiv-
pack, made from rectangular foil electrodes. Lithium alent two-cell series mercury -zinc battery of sim-
foil is rolled on to an expanded metal mesh current ilar voltage. This is illustrated in Table 9.7. One
collector as the negative electrode, and is separated lithium-sulphur dioxide cell (voltage 2.7.5 V) occu-
from the similarly supported cathode by a polypropy- pies about 30% more space than two series mer-
lene separator. Two types of cell construction are used: curic oxide-zinc cells (voltage 2.5 V). Admittedly,
jelly-roll electrodes in crimp-sealed or hermetically this compares the worst cited case for lithium against
sealed cylindrical cells, and large 20-100Ah 12V the best for mercuric oxide-zinc. Higher energy den-
flat-plate electrodes in large reserve batteries. It is a sity systems such as lithium-vanadium pentoxide and
relatively high-pressure system and cells must have lithium-sulphur dioxide would show significant vol-
safety vents to avoid explosion in the event of acci- ume savings over an equivalent ampere hour mercuric
dental incineration (see Part 2 for further details of oxide-zinc system. In fact lithium-sulphur dioxide
construction). systems are being increasingly considered for high-rate
The lithium-sulphur dioxide cell has an open circuit miniature power source applications including military
voltage of 2.92V at 20°C and a typical voltage under applications where it is found that a two-cell mercuric