Page 208 - Battery Reference Book

16/4 Other fast-ion conducting solid systems
reactions in lead-acid and alkaline batteries), and the 7. Safety. Neither short-circuit nor voltage reversal
ability of the electrode to absorb a sufficient quantity of causes pressure build-up or chemical reaction.
the active ion to provide systems with very attractive
energy densities. Table 16.1 summarizes the major physical and
A third, more traditional, concept being applied in electrical characteristics of these batteries.
advance battery design is the application of fused-salt
electrolytes. The use of fused salts, of course, implied
elevated temperatures, but fused-salt electrolytes can Table 16.1 Duracell solid electrolyte batteries
allow the drawing of higher current densities than
are currently possible with solid-electrolyte-based bat- Duracell type no.
teries. The most notable current development using 305127 305159
a fused-salt electrolyte is the lithium-iron sulphide
battery, developed at the Argonne National Labora- Nominal voltage (V) 2.0 4.0
tory (USA). This battery operates at around 400°C No. of cells 1 2
and uses a fused-salt eutectic electrolyte mixture of Rated capacity* (mA h) 350 350
lithium chloride and potassium chloride. Using a Dimensions
28.9 * 0.13
lithium-aluminium anode, the theoretical energy den- Diameter (mm) 2.54 f 0.25 29.7 + 0.13
5.8 k 1.8
Height (mm)
sity is 650 W Nkg, similar to that of sodium-sulphur, Volume (cm3) 1.44 4.04
and the expected actual energy densities of the two Weight (g) 7.25 15.85
systems are comparable.
The alumina PbIz PbS fast ion solid state bat- *Rated at 1 pA discharge at 21°C
tery relies on ionic conduction in the solid state.
This is a low-rate process where temperature is an
important variable. Discharge efficiency on maximum The solid electrolyte cell manufactured by Duracell is
load is particularly affected and a 350 mA h cell is typ- made of the following materials:
ically rated at I uA at room temperature - well below
its maximum capability. This reduces its dependence Anode: high-purity lithium sheet.
on temperature and guarantees a high discharge effi- Cathode: mixture of lead iodide, lead sulphide
ciency over a wide range .of temperatures. The closely and lead.
matched temperature characteristics of solid electrolyte Electrolyte: blend of lithium iodide and lithium-
batteries and CMOS logic circuits is another property hydroxide and activated alumina.
which makes the solid electrolyte battery ideally suited
for memory-retention applications. The major advan- At the anode, the lithium loses electrons forming
tages of the solid electrolyte battery are as follows: lithium ions (Li+). The ions travel through the solid
electrolyte layer and the electrons travel through the
1. Virtually unlimited shelf life. The Duracell solid external load to reach the cathode. At the cathode, the
electrolyte battery has a projected shelf life in lithium ions react with the composite cathode material
excess of 20 years under normal storage conditions and the incoming electrons to form the discharge
and is capable of extended storage at temperatures products. The discharge reactions can be expressed by
as high as 120°C. the following equations:
2. Wide operating temperature range. Solid electrolyte
cells can operate from 40 to over 120°C. Opera- 2Li + PbIz + 2LiI + Pb (16.2)
tion at higher temperatures is possible with mod- 2Li + PbS + LizS + Pb (16.3)
ified designs. The current capability is a function
of temperature. At 95°C the current capability is Lithium iodide is virtually a pure ionic conductor. The
10-20 times the room temperature performance; ionic conductivity is 10-7/(Gkm) at room temperature.
however, at -40°C it is only 2-3% of that at room The conductivity can be enhanced by incorporating
temperature. high-surface-area alumina in the solid lithium iodide.
3. High energy density. A volumetric energy density The solid electrolyte used in these cells has an ionic
of 300-500 W h/dr~-~ is superior to most conven- conductivity of about 10-5/Skm) at room temperature,
tional battery systems. which enables the cell to deliver currents of 10 yA/cm2
4. High voltage density. The thin cell structure and at 20°C with high utilization of the active materials.
high cell voltage (1.9V) gives a high voltage-to- These batteries are available in button and cylin-
height ratio. drical designs with capacities up to 0.35Ah.
5. No gassing, corrosion or leakage. The use of solid Solid electrolyte batteries are designed primarily
cell components and the absence of chemical reac- for low-power, long-service life applications (15-20
tions eliminates gassing and leakage. years), and should be used in accordance with the
6. Hermetic, leakproof design. Only one ‘seal’ is manufacturers’ specifications. Although such con-
required per battery. ditions should be avoided, these cells can withstand