Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
'rhis invention r~lates to -the rechar~Jeability of
secondary electrochemical cells and in part:icular l~.o -the irnproved
cyclinq efficiency of a lithi.um aluminum al].oy elec-trode opera-ting
in an organic electrolyte.
It i.s well known -that there is considerable di.fficulty
in obtaining high cycli.nq e:Eficiencies for lithium anodes in
secondary li-thi.um batteri~s b~cause of the ~ndri-tic nature o:E
the e]ectrodeposi-ted metal. For this reason, considerable interest
is evident in the recen-t literature in the lithium-aluminum elec~
trode as an alternative -to pure lithium in room -tempera-ture cells.
Most fundamental work involving -the Li-Al systerrl has
been carried out at higher temperatures in connec-tion wi-th the
use of the alloy electrode in molten sal-t systems. Phase diayrams
and structural and thermodynamic data have been reported in the
literature~ Lithi~lm ~orms a solid solution with aluminum with up
to ~7 atomic percent (a/o) Li (the ~-phase). For higher lithium
content, the ~ -phase, LiAl is formed; however, this phase is
nonstoichiometric, having compositions in the rancJe 47 to at
leas-t 56 a/o Li. At higher lithium concentra-tions, compounds
suc~ as Li3A12, Li2Al, and LigA14 are formed.
In the case of l,iAl/~l electrode operating in an
organic elec-trolyte, four steps can be distinguished in the
reduction process: (a) migration of Li ions in -the passivation
layer, (b) charge transfer, (c) dif-fusion of Li in ~-LiAl, and
(d) reaction of Li and Al. Because the mobility of Al in ~-LiA]
is negligible in compari.son w.:ith the mob:i.lity of I.:i, the LiAl
deposi-t i.s never dend:ri-tic, and thereore, rnuch .less vulnerable -to
corrosion. I-lowever, e~en i~ the ~olution reacts eEficient~.y
with -the qrain boundaries of -the LiA:L alloy, one of -the products,
~ \
3;3~
namel.y ~1, may p:rov1cl~! goocl elc~ctr:Lcal contac-t between th~
grains. Moreove:r, formation of l.iA.l. occurs ~t a poten-tial rnore
positive than the potentia] necessary for deposition o:E metallic
lithium. These dif.Eerences explain why the cyclincJ efficiency o-E
the Li~l/Al electrode is relatively hiyh and practically indepen-
dent of the natuxe of -the organlc elect:ro]yte. In addition,
LiAl is more thermodynamically s-ta~le than I,i.
I-t is thus reasonable to assume tha-t the cycling
efficiency of the Li-Al al.loy elec~rode is much more determined
by -the properties of the Li-Al alloy i-tself.
It has been found that the cycling efficiency of the
lithium-aluminum alloy electrode is limited by cracking of the
lithium-aluminum layer formed on an a].uminum substrate duri.ny
cycling of the electrode. The cracking occurs because of the fact
that -the molar volume of the alloy is greater than that of alumi-
num alone. The cracking is accompanied by isolation of active
grains of alloy ma-terial which even-tually fall off -the electrode
thereby significantly reducing cycling efficiency.
It is thus an o~ject of -the invention to improve the
cycliny efficiency of a. lithium-aluminum alloy anode operating in
an organic electrolyte at room tempera-ture.
It is another object of the invention -to provide an
improved method for making a lithium-aluminum alloy anode for use
in an electrochemical cell which employs a suitable organic liquid
elec-trolyte and a cathode~
Accordiny to -the invention, a method of improving the
cycling efficiency of a lithium alum:inum alloy anode opera-ting in
a suita~le oryanic liquid elec-trolyte in an e]ectrochemical cell
is provided, which rnethod compri.ses compress:iny -the anode in situ
at a pressure of 1.0 to 2.5 kg~cm~
L,~
The li-thium-aluminum alloy anodes are typica:LI.y formed
by electroplat.inq lithium on an al.umin~ml anode substra-te from a
suitable organic electroly-te solutlon con-tai.ning a suitabl.e
lithium salt. More spe~i:Eically, lit~ m aluminum a].loy is most
effectively formed by electropla-tlng l:i.thium on aluminum from a
non-aqueous solution con-ta:i.ning a lithium sal-t; for exampl.e, from
a propylene carbonate sol.ution containi.ng 1 M lithi~lm bromide~
On the basis of our s-tudi.es, aluminum containing a small amount
of i.ron~ magnesium, or silicon is superior with respect to ul-tra-
pure aluminum. For ins-tance, aluminum foil and sheeting available
for household use normally contains iron and other impurities.
Ideally, only about 50% of the original aluminum should be conver-
-ted to the alloy so that -the resul-ting electrode remains mechani-
cally stable. If the fraction converted -to alloy is significantly
less, the energy density of the battery sys-tern is reduced, and
the capacity on storage can drop due to conversion of the almost
stoichiometric ~-LiAl to -the ~-alloy in which the lithium concen-
tration and mobility are much less. The optimum form of the
aluminum is probably foil with a thickness of 0.075 mm (mass =
20 mg cm 2) con-taining lithlum deposited with total charge of
18C cm 2 on each side. Such an electrode can be charged or dis-
charged with a maximum curren-t densi-ty of about 10mA cm 2.
Thus~ an improved method of makinq a lithium-aluminum
alloy anode for use in an electrochem.ical cell employing a suitable
organic liquid electrolyte and a cathode is also con-templa-ted,
said method comprising electroplating lithiwn on an alumi.num anode
substrate from said organic ].iquid electrolyte containing a Sl1it-
able lithium salt, the improvement comprlsing compressiny the anode
in situ a-t a pressure of 1.0 to 2~5 kg/cm2.
3~ 5
Compression oE the anode provides good e]ectrical
contac~ be-tween lithium~aluminum grains forrned on the ano~e
substrate, and minimizes -their tendency to separate frorn the
aluminum substrate to mechanically s-tabilize the anode struc-tureO
Compression of the anode is eEfected in situ in an
electrochemical cell by means of a suitable separator which is
inert with respect to the lithium-a:LIlminum alloy anode, the
oxganic electrolyte and the cathode. Further, the separator is
of a suitable material which is sufficierltly porous -to provide
for access of electrolyte to the anode to permit ionic
conduc-tion while pxeven-ting migration of the anode material.
The separator material should also be wet-table by -the electrolyte.
Suitable separator ma-terials include porous polypropy-
lene materials and glass microfibre materials.
Referring ayain -to the pressure which is applled to the
anode in situ via a separator, if -the pressure is too low, -the
separator is not effective in retaining granules of the lithium-
aluminum alloy formed on the aluminum substrate during cycling
which may otherwise separa-te from the substrate. If -the pressure
is too high, the porosi-ty of the separator could be reduced,
resulting in higher resistance in the electrolyte. A pressure
range of 1.0 to 2.5 Kg/cm2 is thus contemplated.
Suitable organic electrolytes for use in electrochemical
systems described herein include propylene carbonate, acetonitrile,
tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxye-thane, dimethyl
sulphoxide and combinations thereof.
The electrolyte further contains a suitable lithium
salt such as lithiurn bromide, lithium iodide, lithium hexafluoro~
arsenate, li-thium perc~llora-te and combinations -thereof to provlde
-
a 0.5 molar to saturated solution in t:he electroly-te. In the
case of LiASF6, saturated ls about 3.5 molar.
The cathode may ~e an intercallatior, materla] such as
vanadium oxide (V6O13), molybdenum disulphide or titanium disul-
phide.
In the drawiny which scrves to illustrate the embodi-
ments of the invention,
Figures 1 and 2 are graphs which illus~rate -the effect
of pressure applied to the anode surface to improve -the cycling
1~ efficiency of a li-thium-aluminum alloy anode, and
Figure 3 is a side eleva-tion in section which illustrates
a typical battery sys-tem which employs a compressed lithium-
aluminum alloy anode.
Example 1
Figure 1 shows the charge capacity Q of a LiAl electrode
(plotted as the logarithm) as a function of -the number of cyc],es
N. In the experiment Li was repeatedly transEerred from one Al
electrode to a second at a curren-t density of 1 mAcm ~. Both
electrodes are made of substantially pure aluminum i.e. 99O999% Al.
The results desicJnated (b) were obtained for free s-tanding elec-
trodes in propylene carbonate containing lM LiAsF6. The da-ta
designated (a) were obtained for electrodes prc3cnt agains-t a
porous polypropylene separator soaked with the same electrolyte~
The electrodes were squeezed together and -tightly wound together
wi-th Parafilm, a trademark for a polymeric hydrocarbon film made
by Marathon Corporation of ~enasha, Wisconsin. The force used to
compress the two electrodes together was about 2.5 ky cm 2. The
average cycling efficiency under compression was 97% whereas that
for the free standing electrode was only 9~%.
-- 5
39;~ti5
Example 2
Figure 2 shows -the logaxithm of the charge capacity
Q against the number of cycle~ ~or Li~l electro~es ma~le from
commercial aluminum suppliecl by Homeshield Industries Itd.,
Bramalea, Ontario. This material is oE a -thickness of about 0.4 mm
and includes about 0.8 %/w Fe, 0.]gO/w Cu, 0.15~/wZn and 2.4%/w Mn
as impuri-ties. The electroly-te solution was the same as usecl
above. Experiments were carried out with free s~anding electrodes
(~), e]ectrodes pressed to the separator with a pressure of about
0.2 kg cm 2(~) and electrodes pressed to the separator with a
pressure of 2.5 kg cm 2(~. Compression affects the cycling
efficiency only after ~35 cycles. For N ~35, -the average cycling
efficiencies are 94% (P = 0), 95gO (P - 0.25 ]cg cm 2) and 98%
(P ~ 2 5 kg cm ).
Similar experiments were conducted using a glass micro-
fibre separator and the results were practically identical.
E~ample 3
A simple battery constructed with such a separator
pressed against the lithium-aluminum electrode is shown schemati-
cally in Figure 3.
Referring to Figure 3, the battery 10 is seen tocomprise an ou-ter casing 12, a lithium-aluminum alloy anode 16,
a cathode 14 and a porous separator 18 between the anode and
cathode. Biasing means 20 in the form of coil springs 20 and
biasing pad 22 serve to compress -the cathode ayainst -the anode.
The springs are selected to provide the appropriate compression
i.e. abou-t 2O5 Kg cm . The casing acting to contain e~cess
electrolyte which is also absorbed within the porous separa-tor
material. The anode surface is thus physically covered by the
separator.
.~ L~
The cathode was Inclcle .(':1-0111 an int.ercallatlon material,
namely vanadium ox:ide (V~013) obtained by the thermal decomposition
~`, of NH4V03, The battery was comp1ete(l wit:h a ~e~ndall E-]452 separ~
ator soaked with IM LiAsF6 in propyLerle carbona-te. Two cells wi-th
capacity limited either by the cathocle or by -the anode were tes-ted.
The result of the cycling experiment conducted with the cell limi-ted
by the anode resulted in a cycling e~ficiency for 15 cycles of
greater than 99%0 Char~e and discharge cycles for the Li~l/V6013
cell were conducted. A large change in potential was observed
during the discharge process. This is typical for an intercalla-
tion cathode.
A1-though the arrangement illustrated is a flat plate
arrangement, it will be appreciated by those skilled in -the ~rt
that a coil arrangement in which the anode, separator and cathode
are coiled for use in a standard commercial cylindrical cell is
also contemplated. In such an arrangement r it is expected that
the elasticity of the separa-tor would provide the appropriate
pressure, thus eliminating the need for any addi-tional biaslng
means.
