CA1149867A - Process for producing cupric sulfide electrodes - Google Patents

Abstract

Disclosed in a wet process for producing a CuS cathode having a high capacity and reduced tendency to induce swelling when incorporated into a cell having a non-aqueous electrolyte and a lithium anode. Elemental copper and sulfur are dispersed in an aqueous HCl solution free of ionic contaminants. After reaction, the resulting cupric sulfide is washed with an aqueous solution of a mixture of, e.g., ammonium chloride and hyrochloric acid, to remove cuprous ions. The product is then washed with water until the chloride ion content of the wash water is less than about 5 ppm. The washed solid product is then dried to subline residual NH4Cl, pressed to form a shape retaining cathode, subjected to a final heat treatment and incorporated into a cell or battery having a lithium metal anode and a nonaqueous electrolyte.

Description

BACKGROUND OF THE INVENTION
This invention relates to a method of producing cupric sulfide electrodes having improved electrical characteristics and to an improved lithium-cupric sulfide cell~
One type of cell current.Ly used to power cardiac pace-makers comprises a cupric sulfide cathode, a lithium metal anode, and a nonaqueous electrolyte. The processes currently employed to synthesize cupric sulfide for use as cathodes in such cells involve the so-called dry method wherein copper metal particles and sulfur are mixed together, heated to form cupric sulfide, and su~sequently extracted with aqueous acid solutions to remove impurities. Another prior art dry process involves the formation of CuS ~y direct reaction between copper and sulfur at lo~ tem-pera~ures over a num~er of hours or days under controlled con-ditions of low humidity designed to minimize the production of contaminants. The cupric sulfide product is then throughly dried, optionally mixed with a ~inder, and pressed in a mold to form a shape-retaining cathode.
The stringent quality control requirements imposed on ~O all components used in cardiac pacemakers re~uire that cells exhibiting a capacity ~elow 1~6 ampere-hours ~for a 7.0 g cathode~
~e rejected. The cathodes made in accordance with the foregoing processes, when incorporated into cells of the type described a~ove, exhi~it a capacity which varies hetween a~out 1.4 and 1~8 ampere-hours, experience with the dry manufacturing methods has shown that this variation in capacity cannot ~e avoided ~y pro-ducing cupric sulfide with a standard protocol. It thus has not ~een possible to develop a dry cupric sulfide production tech-nique which consistently results in cells having a least a 1.6 ampere-hour capacity.

~ .

;i7 1 Another pro~Dlem with dry processed cupric sulfide cathodes is encountered when the cathodes are assem~led into cells with a lithium anode and a nonaqueous electrolyte, Spec-ifically, such cells give off hydrogen gas when initially sub-jected to a current drain. Because of this, the cells tend to swell and cannot be hermetically sealed~ Prior to heing incorp-oratPd into a pacemaker, the cells are su~jected to a current drain and then stored for 20 to 40 days until the swelling and gas li~eration diminishes. `Some cells swell to such an extent that they must ~e discarded.
While the precise reactions responsi~le for the li~er-ation of the hydrogen gas and the reasons why cells having a standard sized anode and cathode vary in capacity remain o~scure, it has been hypothesized that the presence of impurities in the cathodes contributes to these pro~lems. However~ attempts to remove such impurities from the dry processed copper sulfide or to control the reaction so as to avoid impurity formation have not heen successful in satisfactorily eliminating these prohlems, As a result, the cell manufacturing process must ~e essentially cQmpleted ~efore it can be determined whether the cathodes ex-hi~it the required properties.
SUM~R~ OF THE INVENT~ON
The instant invention provides a process for producing cupric sulfide cathodes which, ~hen incorporated in electrochem-ical cells including a lithium anode and an nonaqueous electrolyte, consistently result in cells which do not swell and have a cap-acit~ in excess 1,6 ampere-hours. The invention also provides lithium-cuprice sulfide cells having improved electrical charac-teristics.
In accordance with the process of the invention, copper 1 metal is com~ined with sulfur in an aqueous HCl solu-tion su~-stantially free of extraneous metallic cations to produce cupric sulfide. Prefera~ly, the aqueous reaction medium contains at least 1% HCl, and more prefera~ly at least 5~ to 10%. After completion of the reaction, the cupric sulfide product is washed with an aqueous solution to remove cuprous chloride. It has ~een found that a solution of two compounds is necessary for t~is step. Mixtures of alkali metal chlorides and HCl or NE40H
and NH4Cl work well, but a mixture of HCl and NH4Cl is preferred ~ because of its low cost and because ions introduced into the cupric sulfide product may be easily removed by s~bsequent heat-ing and drying steps. Thereafter, the cupric sulfide product is washed with water until the chloride ion content is helow at least a~out 5 ppm, prefera~ly 1.5 ppm. The cupric sulfide is then thoroughly dried under conditions to su~lime NH3, H20, and HCl mixed therewith, and then pressed into a shape retaining cathode~ Preferably, the wash solution for removing cuprous ion contains 10~ HCl and 10% NH4Cl. A slight stoichiometric e~cess of copper over that required to com~ine with the sulfur may be used so that any cuprous ions producad do not disrupt the desired stoichiometric ratio of reactants necessary to produce pure CuS.
The reasons why cathodes produced in this manner ef~ectively deal ~ith the pro~lems of swelling and varying capa-city is not understood. However, by following the teachings disclosed herein those skilled in the art can consistently pro-duce cupric sulfide-lithium cells which uniformly have a capacity in excess of 1.6 (typically in excess of 1,8~ ampere hours (for a 7.0 g CuS cathode) and do not produce significant quantities of hydrogen. Furthermore, the process of the inve~tion is safer than the prior art dry process, allows CuS to be produced in less .

1 time, produces cathodes ~ith more uni~orm properties among batches, and produces cathodes with improved mechanical strength.
Accordingly, the process of the invention represents a signifi-cant improvement over the prior art dry processing methods.
The invention also provides a lithium-cupric sulfide cell which remains hydrogen gas-free when initially su~jected to a current drain and has a capacity of at least 0.25 ampere-hours per gram of chatode. These improved properties may be traced to the purity of the CuS cathode which contaîns less than 0.35 water, less than 0.35~ sulfates, less than 0.60% free sulfur, and less than 0.30% of substances extractable with concentrated NH40~ (chiefly hydrated cuprous sulfites~. By following the teachings disclosed herein, it is routinely possible to produce CuS cathodes containing less than 0.05~ water, less than 0.20%
sulfates, and less than 0.20% free sulfur. It is not uncommon for cells containing a 7 gram cathode to have a capacity in excess of a~out l.q ampere-hours ~0.27 ampere-hour per gram~.
U,S, Patent No. 2,332,145 to ~ohn O, Hay discloses a method of producing cupric sulfide from copper metal and sulfur in an aqueous acidic solution of a metallic salt, According to the Hay patent, metal salts such as sodium or cupric chloride~
when acidified, are capa~le of catalyzing the reaction between copper and sulfur in aqueous solution, In his examples, Ha~ dis-closes that copper metal and sulfur, present in approximately a

2 to 1 weight ratio, can ~e reacted to form cupric sulfide in an aqueous solution comprising Q.5~ to 2% HCl or H2SO4 and 20~ to 4~% cupric sulfate or sodium chloride.
O~jects of the invention are to provide improved cuprlc sulfide~lithium cells, to furnish a protocol for making cupric sulfide cathodes which results in a product of consistent elec-trochemical properties, and to produce cupric sulfide cathodes ~ ~ ~4~67 1 which~ ~hen incorporated in an electrochemical system toge~her with lithium metal and a nonaqueous electrolyte, exhibit a sign-ificantly reduced tendency or no tendency to evole gas. Another o~ject is to produce a CuS cathode which, when incorporated in a cell having a lithium anode, consistently exhibits a capacity of at least a~out 0.25 ampere-hours per gram of CuS.
These and other o~jects and features of the invention will ~e apparent froM the following de~cription of a preferred embodiment and from the drawing wherein the sole figure depicts an exploded view of a lithium-cupric sulfide cell.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
At the heart of the invention is the discovery tha-t cathodes formed from cupric sulride produced in an acidified aqueous system and su~sequently freed of impurities in the manner disclosed herein are characterized by certain useful properties, the presence of which could not ~e predicted ~rom the processing scheme. I~hen the cathodes are assem~led into cells comprising a lithium anode and a nonaqueous electrolyte, hydrogen gas lib-eration iseither completely eliminated or reduced to relatively lo~Y levels Furthermore~ the cathodes uni~ormly result in cells having an accepta~le capacity. Accordingly, the need to de-gas the cells prior to their incorporation into pacemakers is sub-stan~ially eliminated and the frequency of rejections greatly decreased Sulfur~ prefera~ly in finely divided form~ is placed in an aqueous HCl solution preferably comprising at least 1~
and more preferably greater than 5~ HCl in distilled wat~r at room temperature A stoichiometric equivalent or a slight stoichiometric excess of particulate copper is then incrementally added to the sulfur in the aqueous hydrochloric acid solution.

-5~

9B6~7 1 The mixture is then allo~ed to react for a sufficient amount of time to produce a fine black powder comprising su~stantially pure cupric sulfide, As a result of the reaction, trace quant-ities of cuprous ions and other impurities are produced, Rather than attempting to minimize or eliminate the production of such impurities, it is preferred to include in the reaction mixture a slight stoichiometric excess of copper, and then to wash out the impurities, Thus, the particulate product is washed with an aqueous solution o~ a mixture of HCl and NH4Cl, a mixture of alkali metal chloride and HCl, or a mixture of NH40H and NH4Cl, It has been discovered that a mixed solution is capable of dis-solving cuprous chloride, whereas, individually, water t HCl so-lution, NH4Cl solution, NH40H solution, or alkali metal chloride solution are ineffective.
Next~ the solid produc~ is ~ashed repeatedly with dis-tilled water until the chloride content of the wash water is less than about 5parts per million, preferably less than a~out 1.5 parts per million~ At this point the cupric sulfide is substant-ially free of extraneous cuprous, cupric and chloride ion, Sub-se~uently~ the powder is heated to approximately 100C under Yacuum to remove waterr ammonia, and any residual HCl. If an alkali metal salt is used in the first washing step, sufficient ~ashing should be done to lower the alkali metal ion content -to acceptable levels, e,g.~ less than about 2 ppm, The resulting product is then molded in a press to form a shape retaining cathode having a density on the order of 52-56 grams per cubic inch, and baked at a temperature greater than 2Q0C~ e,g,r 25~ C~ The introduction of a binder material can increase the impurity level and is not recommended~ However, certain binders may ~e used without seriously adversely affecting I the cathode's behavior.
To produce a cellr a lithium wafer la is pressed against the bottom of a cleaned stainless steel cell case 12 under an inert atmosphere, a pair of inert, e.~g., polypropylene, separators 14 are placed over the lithium wafer r and the cathode 16 is fitted atop the separators. A polymeric ~e.g., polypropy-lene~ gasket 18 is then fitted ~ithin the rim of the case and a stainless steel cap 2a is fitted thereon. This unsealed assem~ly is then soaked in a water-free electrolyte comprising 1~ 2q% 1,2-dimethoxy ethane, 61% 1,3 dioxolane, 10% lithium per-chlorate, and 0.5~ 3,5 dimethylisoxazole. After the cell is permeated with the electrolyte, the cap is crimped in place, the cell is tested, and is then ready for use.
~ hen employing cupric sulfide cathodes made with the dry process discussed a~ove, ~efore the cells can ~e used they must be subjected to a current drain and stored for a period of typically 20 to 4Q days until the liberation of hydrogen gas and swelling~ if any, ceases. ~he tendency of the cells to liberate ~drogen can ~e diminished in some cases by firing the dry pro-cessed cathodes for long periods of time, e~g., 24 hours, undervacuum. Thereafter, the cells are tested for capacity, and if found to be capable of delivering at least L 6 ampere-hours of - current, are suitable for incorporation into a cardiac pacemaker.
A su~stantial quantity of cells produced ~y this prior art - method ~ere found wanting in capacity. Essentially all such swell, and many swell to such an extent that they cannot be u~ed.
In contrast~ cells made ~ith the cupric sulfide cathodes produced in accordance with this invention either do not generate gas or in some instances generate a small quantity of gas, and exhibit a capacity in the Yicinity of 1.8 to 2.~ ampere-hours 1 The invention will be further understood from the following non-limiting examples.
EXP~LE
1.344 kilograms of sublimed sulfur are emulsified in a production si2e blender in 4 liters of distilled ~ater con-taining 0.2~ nonionic surfactant ~Pluronic 68F* Wyandott Chem-ical Company)~ The sulfur emulsion is transferred to a reaction vessel together with 4 liters of distilled water containing sufficient hydrochloric acid (37%~ to produce a 10~ concentration of HCl in the total solution. ~hile stirring, 2.789 kilograms of electrolytic copper powder ~1.05 times the stoichiometric weight required to react with the weight of sulfur~ is added to the solution, The reaction vessel is e~uipped with a reflux condenser and boiled for one hour to produce a fine black powder.
The solid product is then allowed to settle~ the supernatant i5 removed, and 8 liters of distilled water containing 10% HCl and 10% ammonium chloride are addea to the reaction flask~ After ~oiling for five minutes, the supernatant is again removed and 8 liters of distilled water are added with stirring for a five minute rinse. This washing procedure is repeated three additional times. A fourth 8 liter aliquot of water is then mixed with the solid product, and the two-phase mixture is passed through a Buchner funnel ~ith the ald of a vacuum. The cupric sulfide product is then exposed to 380 to 400 liters of deionized water until the chloride content is less than 1~5 ppm, as measured with a chloride ion electrode. The wet product is then trans-ferred to a stainless steel rotary dryer and dried under vacuum at lOQ C to remove water and su~lime residual NH4Cl, NH40H, NH3, or HCl. Ammonium chloride and hydroxide are removed by virtue of the follo~ing reactions:

* Trade ~ark 8 " .

.. ... :
' , , ' .

NH4Cl ---> NH3 ~ ~ICl NH40H ---> NH3 ~ H 0 Seven gram samples of the purified cupric sulfide are then molded under pressure ~e.g 45 tons of force exerted over an area of approximately 1.6 in.2~ to a density o~ 56.6 grams per cu~ic inch to form cathodes ~approximately 0 Q8 in~ thick) which are then baked or "fired" at 250 + 15C for four minutes.
The cathodes are then incorporated into cells. In a glove ~ox with an inert argon atmosphere, a lithium disk is 1~ pressed into the bottom of a cleaned stainless steel case. A
pair of porous, polypropylene separators (0. oas in. thick and 0.022 in. thick) and a cupric sulfide cathode are placed atop the lithium wafer. A polypropylene gasket i5 next fitted into the case, and a stainless steel cap placed over the gasket.
These unsealed cells are then soaked in a mixed, water-free electrolyte comprising 2~% 1,2 dimethox~ ethane~ 61% 1,3 dioxolane, 10% lithium perchlorate, and 0.5% 3,5 dimethylisoxazole. The cell caps are subsequently crimped in place to seal the assem~ly.
Advantageously, the cells may ~e hermetically sealed without - ~ danger of swelling.
When cells constructed in accordance with the akove~
procedure were subject to a current drain, no hydrogen gas was produced and no swelling occurred. The average capacity of - these cells is ~etween a~out 1.85 and 1.90 ampere-hours and the range of capacity is found to ~e ~etween ahout 1.8 and 2.0 ampere~
hours.
Twenty-five cathodes produced as descri~ed a~ove were - su~jected to analysis and ~ere found to contain~ on average, 0,02% water, O.Q6% sulfate, O.Q8% free sulfur~ and 0.22% of sub-stances extracta~le with concentrated NH40H comprising water- ~

insolu~le cuprous salts ~elieved to predominantly consisting of ::

hydrated sulfites. The Cu~ content of these cathodes reported as 66.56%.
Test cathodes produced ~y the process set forth above ~ere incorporated into cells comprising a lithium anode, various dif~erent separators, and the non-aqueous electrolyte described a~ove. The reaction time for the copper and sulfur in the aqueous HCl solution was varied ~etween 1 and 5 hours, and the density o~ the cathodes was varied between 52 and about 56 grams per cu~ic inch. Thirty-one groups of 7 or 8 cells were tested ~or capacity~ The results are set forth ~elow, Capacity da-ta is given in ampere hours, measured not to the end of cell life but rather until the loaded voltage drops to 1,0 volt.

Test Batch Reaction Ca-thode Separ- Average Range of Num~er Time Density ators* Capacity Capacity .

1 5 Hrs~ 52 PF 1.90 1.83-1.94 2 54 PF 1.82 1.78-1.87

3 - 56 PF 1.86 1.80-1.93

4 52 PFR 1.81 1.75-1.86 3 Hrs. 54 PFR 1.87 1.83-1.91 6 56 PFR 1.87 1.80-1.93 7 , 52 PFR 1.86 1.79-1.94 8 1 Hr. 54 PFR 1.84 1.76-1.89 Q 56 PFR 1,87 1.78-1.91 52 PF 1.87 1~81-1.89 11 3 Hrs. 54 PF 1.88 1.84-1.91 12 56 PF 1.82 1.79-1.84 13 52 PF 1.85 1.78-1.89 14 1 Hr. 54 PF 1~89 1.84-1.94 56 PF 1.88 1~87-1,90 ~6 52 PF 1.87 1.84-1.90 17 54 PF 1~93 1.87-1.95 18 2 Hrs. 56 PF 1.91 1~87~1.23 19 54 PP 1.91 1.87-1~95 56 PP 1.92 1.85-1~5 21 52 PF 1.73 1.69-1.77 22 54 PF 1.84 1.83-1.85 23 2 Hrs. 56 PF 1.88 1.84-1.90 24 54 PP 1.90 1085-1.94 30 ` ~5 56 PP l.9a 1~85 1.94 -lQ-1 Test Batch Reaction Cathode Separ- Average Range of Num~er Time Density ators* Capacit~ Capaci-t~
.. ..

26 52 PF 1.81 1.77 - 1.84 27 3 Hrs. 54 PF 1,84 1.81-1~87 28 56 PF 1.92 1.87 - 1098 2~ ' 52 P~ 1.78 1,71-1.82 1 Hr. 54 PF 1.84 1.83-1.86 31 56 PF 1.88 1.83-1.94 *PFR 0.~08 in. polypropylene + 0.010 in. fiberglass PF 0, 022 in. polypropylene + 0.010 in. fi~erglass PP 0.022 in. polypropylene ~ 0.008 in. polypropylene None of these test cells generated any significant quantity of hydrogen gas when subjected to a current drain, Comparative Example Stoichiometric proportions of electrolytic copper powder and su~limed sulfur are mixed and spread in shallow stain-less steel trays in a uniform layer 4 mm thick. The trays are placed inside aging cham~ers which are maintained at a temperature ~etween 70F and 75F and a relative humidity between 40% and 55%. The powder mix is spread in thin layers using a depth guage so as to minimiæe the development of "hot spots" which can result in nonuniform composition of the CuS product and increae the ~ -risk of an uncontrolled exothermic reaction. The reaction be-tween Cu and S occurs spontaneously over a four to five day period. Every day the trays containing the partially reacted powder, which tends to form into a crust, are removed, and the crust is ~roken up, sifted, respread evenly on the trays, and returned to the aging cham~er. At the end of the final day the powder is sifted again, and pressed in a mold as disclosed a~ove to produce 7.0 gram cathodes. These are fired at 250C for 4 minutes.
Thirty-one dry processed cathodes produced as disclosed a~ove were su~jected to analysis and found to contain, on average, '11--. . . . . . . .

1 O.Q~ water, 0.4Q% sulfate, 0~77% free sulfur~ and ~,77% of substances extractable with concentrated NH40H comprising water-in~olu~le cuprous salts believed to predominantly consist of hydrated sulfites. The Cu++ content of these cathodes reported as 66.05~. Ten batches of dry processed cathodes ~ere incor-porated into cells comprising a lithium anode, a separa-tor, and the non-aqueous electrolyte descri~ed above. The cells were tested for capacity. Using the same test conditions as those employed for the wet processed cathodes, the following results were o~tained ~capacities in ampere-hours~, Lowest Measured Highest Measured Batch Average Capacity Capacity Capacity 1 1.71 - 1.60 1.78 2 1.53 1.44 1.66 ' 3 1.54 1.42 1.74 4 1.62 1.53 1.6~
1.43 1.23 1,66 6 1.66 1.62 1.72 7 1.6~ 1.61 1.77 8 1.66 1.60 1.71 67 1,61 ' 1,74 1.74 1.67 1.80 Most of the test cells spontaneously generated hydrogen gas when subjected to a current drain.
In general, dry processed cathodes exhibit rather low capacities at high current drains and higher capacities at lower current drains. In contrast, cathodes made in accordance with the inventionexhibit a capacity which is substantially unaffected ~y the magnitude of the current drain within the 330 ohm to 3000 ohm range tested.
' In view of the foregoing, those skilled in the art will appreciate that various modifications can be made in the process of the invention without departing from the spirit and scope thereof~ Accordingly,, other em~odiments are within the following claims.

.
: ~ ' '' . ' , . .

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a cupric sulfide cathode for use in an electrochemical system which includes a lithium anode and a nonaqueous electrolyte, said process comprising the step of:
A. reacting copper metal with sulfur in an aqueous HCl solution substantially free of extraneous metallic cations to produce solid particulate cupric sulfide;
B. removing cuprous chloride from the solid cupric sulfide of step A by washing with a mixed aqueous solution com-prising a pair of components selected from the group consisting of alkali metal chloride and HCl, ammonium chloride and HCl, and ammonium hydroxide and ammonium chloride;
C. washing the solid product of step B until the chloride content of the wash water is less than about 5 ppm;
D. during the solid product of step C under conditions to remove HCl, NH4OH, NH3, and NH4Cl, if present in the solid product; and E. pressing the dried cupric sulfide into a shape-retaining cathode.

2. The process of claim 1 wherein the wash solution of step B consists of a mixture of HCl and NH4Cl.

3. The process of claim 1 wherein the wash of step C is repeated until the chloride content of the wash water is less than about 1.5 ppm.

4. The process of claim 1 wherein the wash solution of step B includes alkali metal cations and the wash of step C is repeated-until the alkali metal ion content of the wash water is less than about 2 ppm.

5. The process of claim 1 wherein the solution of step A is a 10% HCl solution.

6. The process of claim 1 wherein the solution of step contains at least 1% HCl.

7. The process of claim 1 wherein the wash solution of step B comprises a solution containing 10% HCl and 10% NH4Cl.

8. The process of claim 1 wherein a small excess of copper metal over than necessary to react with the sulfur is used in step A.

9. The process of claim 1 including the additional step of subjecting said shape-retaining cathode to a heat treatment in excess of 200°C

10. In an electrochemical cell comprising a lithium anode, a cupric sulfide cathode, and a nonaqueous electrolyte, the im-provement wherein said cathode contains less than 0 35% water, less than 0.35% sulfates, less than 0.60% free sulfur, and less than 0.30% of substances extractable with concentrated NH4OH, said cell being free of the tendency to generate hydrogen gas when initially subjected to a current drain and having a capacity of at least about 0.25 ampere-hour per gram of cathode.

11. The cell of claim 10 wherein said cathode is produced from cupric sulfide powder formed by reaction of copper and sulfur in an aqueous HCl solution.

12. The cell of claim 10 wherein said cathode has a density no less than 52g/in3.

13. The cell of claim 10 further comprising a hermetically sealed cell case.

14. The cell of claim 10 wherein said cathode has a den-sity no less than 54g/in3 and contains less than 0.05% water, less than 0.20% sulfates, and less than 0.20% free sulfur, said cell having a capacity of at least 0.27 ampere-hours per gram of cathode.