Capacity loss and faradaic efficiency of lithium thionyl chloride cells Page: 2 of 4
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0)
.
.0
Q.
W-
00
"60C ambient100
Storage Days200
Figure 2 Carbon capacity for cathode limited C cells.
6 ................-.--....... .. -...................... --.
0 4
3 - -- - --
2a -
1
7 12 17 22 27
Lithium Thickness
Figure 3 Capacity discharged at constant resistance load
of 100 ohm at ambient after aging at ambient or 60*C up to
225 days. Lines are statistical fits for cells aged at 225
days at 60*C (solid) and at ambient (dotted).
Figure 3 presents the discharged cell capacity as a function
of lithium thickness, discharge load, storage temperature
and time. A statistical analysis of the result is shown in
Table I. The adjusted R2 was 86.4% and the p-value of the
store-day was 0.083. The misfit was mainly caused by the
data from the truly cathode limited cells stored longer than
200 days. However, dependent variables such as lithium
thickness, 1/T, and discharge load were consistent as shown
by the very small p-values. No other work we could find
had evaluated truly cathode limited cells. In some work,
cells would start as cathode limited, but after storage, the
lithium was corroded away and the cells became lithium
limited. " Figure 3 shows that the cells are cathode limited
when the initial lithium thickness is greater is than 21-mil.
At 235 days and 60*C, the loss of lithium is about one Ali
per cell. Cathode capacity is conventionally estimated
assuming 2.0 to 3.5 Ali per gram of carbon. Considering
storage time (235 days) and temperature (60*C), an
additional one Ah of lithium is needed for this cell design.
We plan to continue the experiment for another 12 months
to observe the behavior of these cells.Effects of Direct and Pulsed Current
on the Self-Discharge
Self-discharge of Li/TC cells comes about because of the
direct reaction of lithium with the solvent, according to6 -
5.5
5-
4.5
4-
3.5.
3 _ _ _ _ _ *-(1)
Useful energy can only be extracted by means of the two
separate reactionsLi -+ Li++ e at the anode
and
4Li++4e~+2SOC12- 44LiC1+SO2+S
at the cathode.(2)
(3)Accumulating LiCl at the lithium-electrolyte interface slows
reaction (1) and has been referred to as a passivation layer
or solid electrolyte interface, SEI.3 The SEI presumably is a
pure ion conductor that blocks all charge transfer reactions,
but allows Li ions to freely migrate from the lithium metal
surface into the liquid electrolyte.
Passage of current enhances the self-discharge of Li/TC
cells, apparently because the passivation layer breaks down
and allows thionyl chloride to directly react with the lithium
anode according to reaction 1. If reactions 1 and 2 proceed
completely independent of each other, then any current-
induced exposure of lithium metal will bring about a
proportional increase of the self-discharge rate and lead to a
faradaic efficiency of less than 1. In the absence of such a
surface-cleaning current, the cell will still self-discharge in
proportion to the area of lithium that is not protected by the
lithium chloride layer. Averaged over time, this translates
into reduced faradaic efficiency.
Some applications call for uninterrupted use of Li/TC cells,
while others require pulse conditions where a comparatively
long off period follows a short period of high current. The
latter conditions were expected to present the worst possible
situation, but as Figure 4 shows, the lowest faradaic
efficiency is associated with the lowest average current
density. The data in Figure 4 are all based on
microcalorimetric determinations of the heat flow from
Li/TC cells during constant discharge and pulsed discharge
loads. Because of the slow rate at which cells approach
steady state conditions, each test was run for a minimum of
3 hours. D cells with surface area from 45 to 345 cm2 were
used in a Hart Model 1701 Microcalorimeter. Apart from
controlling the on-off duty cycle and the pulse height of the
discharge current, two temperatures, 5 and 30*C, were
selected for evaluation, as were cells with different anode
areas and different electrolyte concentrations. Using
current densities, in lieu of current, eliminates cell size as4 Li+2 SOC12 -4 LiC1+ SO2+ S
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Hoier, S.; Schlaikjer, C.; Johnson, A. & Riley, S. Capacity loss and faradaic efficiency of lithium thionyl chloride cells, article, May 1, 1996; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc670262/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.