CA1123898A

CA1123898A - Lithium-lead sulfate primary electrochemical cell

Info

Publication number: CA1123898A
Application number: CA341,849A
Authority: CA
Inventors: Keith A. Klinedinst; Carl R. Schlaikjer
Original assignee: GTE Laboratories Inc
Current assignee: Verizon Laboratories Inc
Priority date: 1978-12-26
Filing date: 1979-12-13
Publication date: 1982-05-18
Also published as: FR2445628B3; GB2038537B; US4176214A; DK552379A; FR2445628A1; DE2951520A1; GB2038537A

Abstract

LITHIUM-LEAD SULFATE PRIMARY ELECTROCHEMICAL CELL

ABSTRACT OF THE DISCLOSURE
Disclosed is a primary electrochemical cell which employs a lithium metal anode, a lead sulfate cathode, and an electrolyte solution comprising a lithium salt in an organic solvent. The cell has an operating voltage and energy density sufficiently close to conventional 1.5V cells to permit their direct replacement.

Description

~123~98 The present invention rela-tes to the ~ield of electrochemical cells; and more particularly -to the ~ield o~
primary electrochemical cells having a discharge voltage of I around 1.5V.

Primary electrochemical cells are a class of voltaic cells. Voltaic cells are those electrochemical cells in which chemical changes produce electrical energyl in distinction to electrolysis cells in which electrical energy from an outside source produces chemical changes within the cell. Primary cells are those voltaic cells which cannot be convenien~ly recharged, which usually are discarded after a single exhaustion of their component elements, or which require replacement of their - 15j exhausted chemical constituents to bring them back to their original condition. These cells are distinguished from another class of voltaic cells, namely, secondary cells, in which the exhausted cell is charged by passing elec-trical current from an outside source through it in the reverse direction of the 20, discharge current.
In a primary cell, chemical energy is converted to electrical energy with a reduction in the free energy of the system. In the course of the cell reaction, negative charge leaves the anode, travels through an external driven circuit, and re-enters the cell at the cathode. Thus, the cathode is the positive electrode and the anode is the negative electrode.
By virtue of the established electromotive series, it is possible to select suitable cathodes and anodes -to obtain a ~2--~' ~Z~B98 desired theoretical voltage. Tle ideal cell would give the theoretical voltage under con-tinued, constant load and the loss in free energy would manifest itself entirely as electrical energy outside the cell. However, this ideal is never attained in practice, because the internal resistance of a cell is not zero and the reactions within the cell are never completely reversible. Moreover, problems of incompatibility of the cathode and anode with each other or with the electrolyte, polari~ation, and other well known problems preventperformance at theoretical values.
The demand for existing aqueous electrolyte primary electrochemical cells with actual discharge voltages o a~out 1.5V is increasing rapidly with the popularity of cordless electrical entertainment, communications, and technical - lS equipment. Currently, the major part of this demand is satisfied by two types of cells often referred to as the Leclanché or alkaline manganese dioxide~zinc cell. These cells use zinc anodes and manganese dioxide cathodes, wherein the cathode material is mi~ed with carbon to provide electronic conductivity. The electrolyte is in an a~ueous solution of either ammonium chloride and zinc chloride or potassium hydroxide with potassium zincate. These aqueous cells suffer ~rom the possibility of gas formation by reaction of the anode with the electrolyte and may not, therefore, be hermetically sealed. Further, the working voltage at constant load for these cells decreases steadily with the extent of discharge.
There have recently appeared in the literature a number of different electrochemical cells which employ lithium as the active anode manterial and which discharge at about 1.5V.

Lithium has several inherent advantages associated with its ^ -~23~

use, amOn~J which are hi~h capaeit:Les ~ncl hicJh specifie energics coupl~d with a hi~h degree of stability and lon~
storage life -times. Amon~ these are the Li/CuO s~stem (SAFT;
G. Lehmann et al, 5th :[n-ternational Power Sourees Sympesium, Brighton, 1974), the Li/PbCrO4 and LijPbO2 systems tG Pistoia et al, Electroehemiea Acta, 22l pp. 1141-1145, 1977), the Li/Bi2o3 system (Varta; U.S. Patent 4,085,259), the Li/antimony oxide systems (Sb2O3 , SbO2 and Sb2Os) (Var~a; Ger. Of~en.

2,516,703), and the Li/PbSiO3 and Li~CuA12O4 systems (Varta;

Ger. Offen. 2,521,769) The Pistoia reference further identifies some of the inherent advantages în the use of lead eompounds whieh makes them worthy o~ further investigation as eathodes for lithium eells; however; there is no mention in thi~ referenee of a sel~
employing a lead sulfate eathode.

Accordingly, the present invention provides a primary eleetrochemical cell comprising: (a) an anode eomprising lithium metal; (b) a eathode comprising lead suLate; said anode and cathode being spaced rom each other in contaet with (c) an electrolytic solution which eomprises a dissociable lithium salt dissolved in a liquid vrganie solvent.

~ ' ~
:~238~1~

In a pre.Eerred embodiment of the invention, the cathode will contain a minor amount of a material less electro-positive than the lead sulate, whereby a first constant dis~
charge voltage will be obtained by virtue of the lead sulfate until exhaustion of the lead sulfate, and then a second, reduced discharge voltage will be effected to signal near exhaustion of the cell but permit continued use.

Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:-FIGURE 1 is a cross-sectional schematic diagram of one embodiment o an electrochemical cell according to the present invention;
FIGURE 2 is a graph showing t.he variation betwePn discharge voltage and discharge time for a cell as shown in Figure 1 and as described in Example l;
FIGURE 3 is a cross-sectional schema~ic representa-tion o a button cell made in accordance with the present invention;
FIGURE 4 is a graph showing the discharge voltage as a function of time for a cell as shown in Figure 3 and as described in Example 9;
FIGURE 5 is a cross-sectional schematic represen-tation of an AA cell constructed in accordance with the prese~t

3~39~

invention; and FIG~RE 6 is a graph showing the variation ofdischarge voltage with discharge time for a cell as shown in Figure 5 and as described in Example 10.

The cells described herein have lithium anodes and lead sulfate cathodes. They have operating voltages at a constant load, with a current density of about 1.0 mA/cm2, o~
about 1.5V. Thus, these cells may be used as direct replace-ments for all applicàtions for which either conventionalLeclanché or alkaline manganese cells are used. The theoretical energy density in watt hours per pound or per cubic inch of these lithiumjlead sulfate cells is about the same as that of the - standard Leclanché cell. However, because a non-a~ueous _ 15 electrolyte is employed, there is no danger of the gas formation associated with the Leclanché cell, and the cells may be hermetically sealed. Si~niicantly, the ~:~rking voltage does not decline substantially with time ~mtil close to the end o~
the discharge. Thus, the cell discharges more e~iciently and reliably than the standard Leclanché cell. And, in a pre~erred embodiment of the invention, near exhaustion o~
the cell is indicated by a second plateau or stepped down voltage near the end of the useful life of the cell.
Referring to Figure 1, there is seen a cross-sectional view o~ a cell in accordance with the presentinvention. The cell has a lead sulfate ~PbSO4~ cathode 2 and a lithium metal anode 4. In this particular embodiment, an excess of electrolytic solution 6 is provided within the sealed container 8 which can be of suitable material such as ~23~398 glass. The con~ainer top 10 can be sealed to the main body portion oE the con~ainer 8 by a ~round gl~ss joint 12 or other suitable sealing arrangement. Current leads 14 and 16, which can be sealed to the container top by sui-table ylass to 5 metal seals 18, are connected to current collectors 20 at the cathode and 22 at the anode. Mechanical separators 24, between the electrodes are preferably used.
A button cell embodiment of this invention is shown in cross-section in Figure 3. The cathode 102 and the anode 104 are both shown as flat discs. The cathode 102 is in direct contact with the bottom 108 of the button cell can and the anode 104 is in direct contact with the top 110 of the cell.
An insulating ring 126 is shown between the bottom 108 and top 110 of the can. The cathode 102 and anode 104 are spaced from each other by spacer124 which is saturated with electrolyticsolution.
A conventional AA size cell, but made in accordance with the present invention, is shown in cross-sectîon in Figu~e 5. In this embodiment,the cathode 202 is shown in the center of the cell, spaced from the anode 20~ by porous separator 224. The cathode is also spaced from the bottom o~
cell can 208 by an insulating disc 226. The anode 204 is in direct contact with the can 208, the can thereby f:unctioning as the current collector. The cathode 202 is provided with current collector 220 which passes to the exterior of the cell through seal 2180 In this embodiment, sufficient space is provided in the head of the cell for excess electrolyte 206.
The anodes employed according to this invention comprise lithium metal which is preferably attached to an ` --:1~23~

appropria-te metallic current collector. The purity o ~he lithium should be 95~ or better. The preferred anode will thus consist cssentially of pure lithium metal. Lithium prepared by fused salt electrolysis is presently preferred. The shape of the anode is not importantl but can be of any suitable configuration for the type of cell desired.
The cathodes according to this invention should contain as high a percentage-of lead sulfate as possible;
however, while they preferably consist essentially of PbSO4, they preferably contain amounts of a conductivity-improving material and a binder material as are necessary to provide effective cathodes under the desired conditions of use. Also, as will be described in more detail below, they may contain a minor amount of a material, such as lead sulfide, ~PbS), which is less electropositive than the lead sulfate, to act as an indicator that the cell is nearly exhausted but yet permits continued use at a lower, constant discharge voltage.
A suita~le polymeric material such as polytetrafluoro-ethylene (PTFE) ean be employed to effectively bond the cathodes together. While any level of this or other binding agent which is effective to provide sufficient dimensional stability to hold thecathode together under conditions of intended use may be employed, it is presently believed that inclusion of from about 3 to 50% of a binder is preferred.
The cathodes also preferably contain sufficient conductive material to give the cathode sufficient electronic conductivity. Preferably, the conductive material will be a material selected from the group consisting of carbon hlack, 1'1238~8 graphite, lead powder, and mixtures thereof. ~o obtain a cathode capable of e*ficiently operating in an electrochemical cell, its specific conductivity should be greater than about 10 3 ohm 1 cm 1, and preferably should be about 5 x 10 3 ohm~lcm~
or above. Typically, from 5 io 20% by weight of the graphite, carbon ~lack or other inert conductor or mixtures of conductors will be employed. Preferably, about 10 wt.% total of a mixture of graphite and carbon black is preferred. Best cell perormances have been obtained using 85% lead sulfate, 5~ graphite, 5 carbon ~lack and 5% PTFE, all percentages being by weight.
Discharge curves for cells prepared according to the present invention show that the final degree o porosity of the cathode affects discharge characteristics of the cell~ The porosity of the cathode can be increased from 8% to 18~
resulting in a large increase in cell capacity~ Similarly, a cell having a cathode constrained within a standard battery exhibits a reduced cell capacity as opposed to a cell having the catilode in an excess of electrolyte. Thèse results suggest that it is the volume available for cathode expansion during eell discharge whieh determines the eompleteness ~f the discharge process. In the presence of excess electrolyte and with the anode and cathode separated from each other by at least several millimeters, twice as much electrical energy is produced during cell discharge as when the two electrodes are constrained within the walls of a standard battery can and are separated by only a thickness of a sheet of porous separator paper. Thus, it appears that of the two following cell discharge reactions:

~l23~3~8 ~1 ) 2 Li ~ PbSO~ls ~ Pb ~ Li 2SO~
~ 2) 4 Li -~ PbSO,~ --~ Pb ~ Li2S03 -~ Li2~
reaction (1) is believed to predominate during discharge of the cell where the cell components are confined wi-thin a standara battery case or the cathode is o~herwise confined, ànd produces two equivalents of electric charge per mole of lead sulfa-te; however, if the cathode is not so confined but is able to expand freely as it discharges, the reaction of equation (2) is believea to predominate and twice as much elec-trical energy will be produced per mole of lead sulfate~ Theoperating characteristics o a cell constructed to perform according to equation ( 2 ) would thus be preferred; howevex, the weight and volume charge densities of a cell construct~d to perform according to equation (1) compare favorably to prior art cells, including the Li/Bi2O3 cell described by Varta vide supra. While not wishing to be bound to any particular theory of operation, the foregoing explanation is provided for the purpose of setting forth the best understanding of the operation of the invention at this time.
According to a preferred method of preparing the cathode from its various components, the components are pxovided in powdered form and are than well mixed and, if`desired, co-ground to further homogenize the mixture. The mixture is then preferably pressed in an appropriately shaped die so as to cause the particles of lead sulfate, binder material and inert conductor to come into intimate contact. A metallic or non-metallic current collector, such as an expanded metal grid, may be embedded with the pressed mat~rial during this operation or the consolidated cathode may be later pre~sed onto an 3~

app~opriate current collector. If desil-ed, the pellet of pressed cathode material can be partially broken up for use in particulate form or for later reEormation into ano-ther desired shape.
The solvent in which the lithium salt is dissol~ed to form the electrolyte solution pre~erably comprises one or more organic liquids which singly or mixed together are capable of dissolving the dissociable lithium salt. ~ wide variety of organic compounds can be employed singly or mixed, as long as they are capable of dissolving the lithium salt to produce a conductive solution, and do not foster or take part in undesirable side reactions involving the anode, cathode, separator, or cell hardware. It is preferred that compounds have a liquid range between -40 and +50C, a viscosity not exceeding 1 centipoise at 25C, and a dielectric constant of at least 10 at 25C. However, mixtures may be employed to take advantage of desirable features and offset less desirable ones.
Thus, a compound with a high melting point and high viscosity but high dielectric constant (ethylene carbonate) or a compound with a low boiling point and a low dielectric constant, but a low ~reezing point and low viscosity (diethyl ether), would still be useful when present in a mixture.
Among the various organic liquids suitable as solvents will be: (1) aliphatic ethers, acetals, and ketals; ~2) alicyclic ethers, acetals, and ketals; (3) esters, orthoesters and cyclic esters; (4) aliphatic and aromatic nitro compounds;
(5) inorganic esters; (6) aliphatic, alicyclic and aromatic tertiary amines; (7) substituted and cyclic amides; (8~ nitrides, (9) aldehydes and (10) ketones.

3~g~

It is preEerred to use a mi~ture of solvents selected ~rom suitable aliphatic ethers, esters and cyclic e-thers.
Preferred solvents are those selected from the group consisting of propylene carbonate ~PC), dimethoxyethane (DME), dimethyl carbonate (DMC), tetrahydrofuran (THF), 1,4-dioxane and mixtures of these. Binary mixtures are preferred, especially - where propylene carbonate is selected as one solvent. The preferred solvent mixture contains from about 20 wt.% to about 60 wt.% of PC. Preferred systems can comprise about 40 wt.~
PC and about 60 wt.~ of an aliphatic ether like DME, an ester like DMC or a cyclic ether like THF or 1,4-dioxane. A
particularly preferred solvent system will comprise 4Q wt.
PC and about 60 wt.% DME.
The lithium salt which is necessary to form the electrolyte solution is preferably one which is substantially - soluble and dissociates to ionic species such that the specific conductivity is yreater than 10-4 ohm~lcm~l. The reactivity of the salt with tne solvent, the anode, the cathode, the separator, or cell hardware, is preferably ne~ligible or does not allo~ a loss oE electrical capacity on standiny in excess of 5~ per year for hermetically sealed cells. Most preferably, the salt is a member selected from the ~roup of lithium sal-ts which consists essentially of lithium perchlorate (LiC104), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorophosphate (LiPF6), lithium tetrachloroaluminate (LiAlC14) and lithium tetrafluoroborate (LiBF4), and mixtures of these.
As referred to briefly above, these cells can be made with a built-in indicator system which allows discharye at a first nearly constant potential of around 1.5V for the major ~3~

portion o~ the cell liEe, followed b~ discharge at a second, reduced, but llearly constallt potential for an additional period of time. This indica-tor efEect is obtained by employin~ with the lead sulfate cathode a minor amount of lead sulfide, a material less electropositive than the lead sulfate. Thus, when the lead sulfate is near exhaustion, the potential drops from its first, constant level of about 1.5V to a second level.
Lead sulfide can typically be employed at a level o~ fro~ 5 to 50 wt.~ with about 15-20 wt.~ being most preferred. As stated above, the Li/PbSO4 cell can discharge at about 1.5V (at a current density of aboutl mA/cm2). A similar cell in which the cathode contains a mixture of PbSO4 and PbS also discharges at about 1.5V. However, after the 1.5V discharge, the potential drops to about 1.25V and remains steady at this voltage until the end of the discharge. The fractîon of the discharge which occurs at 1.5V can be controlled by varying the relative amounts of PbSO4 and PbS contained within the cathode. By this sudden lowering of its output potential by about 15% close to the end of its discharge, the cell can be made to `inform' the electronic circuitry into which it is incorporated that the end of its useful life is approaching. This automatic signaling feature can be used to notify the operator of the device that the cell should be replaced, or alternatively the device itself could be pro~rammed to automatically switch to a standby (backup) cell when this signal has been received. This new signaling feature ~akes the Li/PbSO4 cell preferable to the standard 1.5V Leclanche-type or alkaline manganese cell for which it can be exchanged. These standard "dry cell" batteries experience a gradual decline in potential throughout their ~2~

useful life, and thus do not possess a similar aefiniteability to warn of impending failure.
The following examples are presented for the purpose of further illustration and explaining the present invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.

Example l This example illustrates the construction and discharge of a cell of the type shown in Figure l, in accordance with the present invention.
Cathode material was prepared containing 70~ PbSO4, 20% graphite and 10% PTFE by thoroughl~ mixing and grinding 0.450 gm PbSO4, 0.128 gm graphite, and O.Q64 gm PTFE with a mor-tar and pestle. A pellet die was then used to press a cathcde wafer about 2.5 cm2 in area. An expanded nickel metal current collector was embedded within the wafer during the pressing operation. This cathode wafer was then sandwiched between two 0.030 inch thick lithium anodes as shown in Figure l.
A single piece of 0.005 inch thick porous glass separator paper was placed between each face of the cathode wafer and the lithium anode. Nickel metal current collectors and lead wires were used and the cell was contained within a Pyrex glass container. The cell was assembled and the glass ~l23~3t~

container was then evacuated. The evacua-ted cell was filled with a l.OM solution of LiC104 as the electrolyte salt in a solvent mixture of 40 wt.~ propylene carbonate and 60 wt.%
dimethoxyethane.
The cell was discharged across a constant 1250 ohm resistor, and the resulting discharge curve is shown in Figure 2. The average aischarge voltage was 1.44V and the average current density was O.24 mA/cm2.
Another identical cell was assembled and dischar~ed across a constant 300 ohm resistor. The resulting discharge curve is also shown in Figure 2. In this case, the average cell voltage was 1.37V, and the average current density was 0.86 mA/cm .

Example 2 Another Li/PbS04 cell was prepared, identical to that described in Example 1, except that the elec-trolyte salt was l.OM LiAsF6. The cell was discharged across a 300 ohm resistor.
The avera~e cell voltage was 1.43V, and the ave~age current density was O.92 mA/cm2.

Example 3 Another Li/PbS04 cell was prepared according to Example 1, again changing only the electrolyte salt, this time employing a l.OM solution of LiPF6. The cell was discharged across a 300 ohm resistor. The average cell voltage was 1.47V, and the average current density was O.89 mA/cm .

1~2;3~

Example 4 _ A further Li/PbS04 cell was prepared as in Example 1, but this time employing a l.OM solution of LiAl~l~ as the electrolyte salt. The cell was discharged across a 300 ohm resistor. The average cell voltage was 1.53V, and the average current density was 0.99 mA/cm2.

Example 5 Another Li/PbS04 cell was prepared as described in Example 1, but this time employing a l.OM solution of LiBF4 as the electrolyte salt. The cell was discharged across a 300 ohm resistor. The average cell voltage was 1.70V, and the average current density was O.98 m~/cm .

Example 6 Another Li/PbS04 cell was prepa~ed as described in Example 1, but this time employing a l.OM solution of LiC104 dissolved in 40 wt.~ propylene carbonate and 60 wt.% dioxane.
The cell was discharged across a 300 ohm resistor. The average cell voltage was 1.42V, and the average current density was 0.88 mA/cm2.

Example 7 .
Another Li/PbS04 cell was prepared as described in Example 1, except that the electrolyte was a l.OM solution of LiC104 dissolved in 40 wt.% propylene carbonate and 60 wt.% ~
dimethyl carbonate. The cell was discharged across a 300 ohm resistor. The average cell voltage was 1.45V, and the average current density was 0.90 mA/cm2.

3~2~8~1~

Example 8 __ .
Another Li/PbS04 cell was prepared according -to the description in Example 1, except that this -time the electrolyte was a l.OM solu-tion of LiC104 dissolved in 40 wt.% propylene carbonate and 60 wt.% tetrahydrofuran. The cell was discharged across a 300 ohm resistor. The average cell voltage was 1.46V, and the average current density was 0.91 mA/cm2.

Example 9 This example illustrates the preparation of a button cell as generally shown in Figure 3, according to the present invention.
A cathode mixture containing 85% PbS04, 5% graphite, 5% carbon black, and 5% PTFE was prepared by finely grinding and mixing 0.43~ gm PbS04, 0.025 gm graphite, 0.025 gm carbon black, and 0.025 gm PTFE with a mortar and pestle. The mixture was placed in a pellet die and pressed to form a cathode wafer about 0.65 cm2 in area. This cathode wafer was placed in the bottom of a button cell can, a piece of porous glass separator paper was placed on top of the cathode, and the top o the b~ltton cell can containing a 0~030 inch thick circular anode was pressed onto the separator paper. The button cell was vacuum-filled with an electrolyte solution containing a l.OM solution of LiC104 dissolved in 40 wt.%
propylene carbonate and 60 wt.% dimethoxyethane and was then crimped shut.
The cell was then discharged across a constant 4300 ohm load. The resulting discharge curve is shown in Figure 4.
The average discharge voltage was about 1.5V, and the average ~Z3~

current densi~y was about 0.5 m~/cm . The discharge capacity was about 61 m~hr (to a 1.OV cutoff).

Example 10 This example illustrates the preparation of an AA
size cell, generally as shown in Figure 5, according -to the present invention.
A cathode mixture containing 85% PbS04, 5% graphite, 5% carbon black, and 5% PTFE was prepared by mixing and finely grinding 7.00 gm PbS04, 0.42 gm graphite, 0.42 gm carbon black, and 0.42 gm PTFE, with a mortar and pestle. A large pellet die was used to pelletize the mixed cathode materials, after which the resulting cathode pellets were partially bro~en apart to form smalle; pieces of pelle-tized material.
A cylindrical piece of 0~030 inch thick lithium metal was pressed onto the inside wall o~ a AA can, the inside bottom of which was lined with a sheet of PTFE. A cylindrical piece of porous glass separator paper was then placed against the exposed lithium surface, and the center of the A~ can was filled with the pelletized cathode material. A nickel metal current collector was pressed into the center of the resulting column of cathode material and was connected to the electrically insulated center pin which passed through the top of the AA
can. The celL was vacuum-filled with a l.OM LiC10~ electrolyte solution containing 40 wt.% propylene carbonate and 60 wt.~
dimethoxyethane as the solvent. The top of the cell was welded to the can bottom.
The cell was then discharged across a constant 133 ohm resistor. The resulting discharge curve is shown in ~313~8 ~ic~ure 6. The avera~e cell volta~e was about l.~V, and the average current density was about 0.~ mA/cm2. The discharge capacity was about 1.23 Ahr (to a l.OV cutoff).

E~ample 11 This example illustrates the construction and discharge of a cell in accordance with the present invention employing a second active cathode material to provide an indicator.
The cathode contained 49~ PbSO~, 21% PbS, 20% graphite, and 10% PTFE. Finely divided powders of 0.317 gm PbS04, 0.136 gm PbS, 0.129 gm graphite, and 0.065 gm PTFE were thorouyhly mixed with a mortar and pestle. A pellet die was then used to press a cathode wafer about 2.5 cm2 in area. An expanded nickel metal current collector was embedded within the wafer during the pressing operation. This cathode wafer was then i5 sandwiched between two 0.030 inch thick lithium anodes as shown in the schematic diagram of the electrochemical cell in Fisure 1. A single piece of 0.005 inch thick porous glass separator paper was placed between each face of the cathode wafer and the lithium anode. Nickel metal current collectors and lead wires were used, and the cell was contained within a Pyrex glass container.
~ fter the cell was assembled, the glass container was evacuated, and the evacuated cell was filled with an electrolyte solution containing l.OM LiC104 in 40% PC-60% DME.
The cell was then discharged across a constant 1250 ohm resistor. The average voltage for the first portion of the discharge ~about 30 hours) was about 1.67V with an average current density about O.26 mA/cm2, while for the second portion of the discharge (about 30 to 50 hours) the average ~2~ 8 voltage was about 1.3~V with an average current density about 0.22 mA/cm2.

Example 12 This example illustrates a variation of -the indicator-type cell described in Example 11.
The cathode wafer contained three layers of material.
The central layer contained 70~ PbSO4,20% graphite, and 10%
PTFE, while each surface layer contained 70~ PbS, 20~ graphite, and 10% PTFE. All four constituents were in the *orm of finely divided powders. Thus, 0.317 gm PbSO4, 0.090 gm graphite, and 0.045 gm PTFE were thoroughly mixed with a mortar and pestle.
A pellet die was then used to press a cathode wafer about 2.5 cm2 in area. An expanded nickel metal current collector was embedded within the wafer during the pressing operation.
Then, two PbS-containing mixtures were prepared by thoroughly mixing 0.068 gm PbS, 0.019 gm graphite, and 0.010 gm PTFE with a mortar in pestle. The pellet die was then used to press one of these PbS-containing mixtures onto each side of the previously formed PbSO4-containing cathode wafer, thus forming the three layered cathode structure, and as in Example 11, the PbSO4 to PbS weigh~ ratio was 7 to 3.
An electrochemical cell containing this cathode was then assembled and tested as in Example 11. The discharge characteristics of this cell were very similar to those for Example 11.
The above disclosure is for the purpose of explaining the present invention to those skilled in the art, and is not intended to describe all those obvious moaifications -2~-. ~
~3~98 and variations of the invention which will become apparent upon reading the disclosure. Applicants do intend, however, to include all those obvious modifications and variations within the scope of -the invention which is defined by the followiny claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. A primary electrochemical cell comprising:
(a) an anode comprising lithium metal;
(b) a cathode comprising lead sulfate;
said anode and-cathode being spaced from each other and in contact with (c) an electrolytic solution which comprises a dissociable lithium salt dissolved in a liquid organic solvent.

2. A primary electrochemical cell according to Claim 1 wherein the anode consists essentially of lithium metal.

3. A primary electrochemical cell according to Claim 1 wherein the cathode consists essentially of lead sulfate.

4. A primary electrochemical cell according to Claim 3 wherein the cathode contains an amount of a binder material effective to provide dimensional stability to the cathode.

5. A primary electrochemical cell according to Claim 3 wherein the cathode contains an amount of a material selected from the group consisting of carbon black, graphite, lead powder, and mixtures thereof, which are effective to provide electronic conductivity to the cathode.

6. A primary electrochemical cell according to Claim 1 wherein the organic solvent comprises propylene carbonate.

7. A primary electrochemical cell according to Claim 6 wherein the organic solvent comprises a mixture of propylene carbonate with at least one other organic liquid selected from the group consisting of dimethoxyethane, dimethyl carbonate, tetrahydrofuran and 1,4-dioxane.

8. A primary electrochemical cell according to Claim 7 wherein the solvent comprises from about 20 wt.% to about 60 wt.% of propylene carbonate.

9. A primary electrochemical cell according to Claim 1 wherein the dissociable lithium salt is a member selected from the group consisting of LiClO4, LiAsF6, LiPF6, LiAlCl4, LiBF4 and mixtures of these.

10. A primary electrochemical cell according to Claim 9 wherein the dissociable lithium salt comprises LiAlCl4.

11. A primary electrochemical cell according to Claim 1 wherein the cathode further comprises a minor amount of a material less electropositive than the lead sulfate.

12. A primary electrochemical cell according to Claim 11 wherein the material less electropositive than the lead sulfate, comprises lead sulfide.