GB2109153A

GB2109153A - Electrochemical cells including CuS electrodes

Info

Publication number: GB2109153A
Application number: GB8228165A
Authority: GB
Inventors: Alexis E Ushakoff
Original assignee: Cordis Corp
Current assignee: Cordis Corp
Priority date: 1979-11-28
Filing date: 1982-10-01
Publication date: 1983-05-25
Also published as: CA1149867A; DE3044177A1; JPS56123671A; GB2063842B; GB2109153B; GB2063842A

Abstract

An electrochemical cell has a compacted, particulate CuS electrode as the cathode (16) and a lithium metal anode (10) contained in a crimped stainless steel case (12, 20) with a non-aqueous electrolyte. The cathode contains less than 0.35 weight % of each of water and sulfates, less than 0.6 weight % free sulfur and less than 0.30 weight % of substances extractable with concentrated NH4OH. The cell has a capacity of 0.27 ampere hours or more per gram of cathode, and is suitable, inter alia, for use as a power source for cardiac pacemakers. <IMAGE>

Description

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GB2 109 153 A

1

SPECIFICATION

Electrochemical cells including CuS electrodes

5 This invention relates to an improved cell embodying a CuS electrode, e.g. a lithium-cupric sulfide cell.

One cell currently used to power cardiac pacemakers comprises a cupric sulfide cathode, a lithium metal anode, and a nonaqueous electrolyte. The processes currently employed to synthesize cupric sulfide for cathodes in such cells involve the so-called dry method wherein copper particles and sulfur are mixed together, heated to form cupric sulfide, and subsequently extracted with aqueous acid solutions to remove 10 impurities. Another prior art dry process involves the formation of CuS by direct reaction between copper and sulfur at low temperatures over a number of hours or days under controlled conditions of low humidity designed to minimize the production of contaminants. The cupric sulfide product is then thoroughly dried, optionally mixed with a binder, and pressed in a mold to form a shape-retaining cathode.

The stringent quality control requirements imposed on all components used in cardiac pacemakers 15 require rejection of cells exhibiting a capacity below 1.6 ampere-hours (for a 7.0 g cathode). Cathodes made in accordance with the foregoing processes, when incorporated into cells of the type described above, exhibit a capacity which varies between about 1.4 and 1.8 ampere-hours. Experience with the dry manufacturing methods has shown that this variation in capacity cannot be avoided by producing cupric sulfide under standardised conditions. It thus has not been possible to develop a dry cupric sulfide 20 production technique which consistently results in cells having at least a 1.6 ampere-hour capacity.

Another problem with dry processed cupric sulfide cathodes is encountered when the cathodes are assembled into cells within a lithium anode and a nonaqueous electrolyte. Specifically, such cells give off hydrogen gas when initially subjected to a current drain. Because of this, the cells tend to swell and cannot be hermetically sealed. Priorto being incorporated into a pacemaker, the cells are subjected to a current 25 drain and then stored for 20 to 40 days until the swelling and gas liberation diminishes. Some cells swell to such an extent that they must be discarded, however.

While the precise reactions responsible for the liberation of the hydrogen gas and the reasons why cells having a standard sized anode and cathode vary in capacity remain obscure, it has been hypothesized that the presence of impurities in the cathodes contributes to these problems. However, attempts to remove such 30 impurities from the dry processed copper sulfide or to control the reaction so as to avoid impurity formation have not been successful in satisfactorily eliminating these problems. As a result, the cell manufacturing process must be essentially completed before it can be determined whether the cathodes exhibit the required properties.

The invention the subject of our G.B. Patent Application No. 8036598 (Serial No. G.B. 2,063,842A) provides 35 a process for producing cupric sulfide cathodes for electrochemical cells including a lithium anode and a non-aqueous electrolyte which consistently do not swell and have capacities in excess of 1.6 ampere-hours. The present application is divided from the aforesaid G.B. application.

According to the present invention, there is provided an electrochemical cell comprising a lithium anode, a cupric sulfide cathode, and a non-aqueous electrolyte, the cathode containing less than 0.35% water, less 40 than 0.35% sulfates, less than 0.60% free sulfur and less than 0.30% of substances extractable with concentrated NH4OH, the cell being substantially free of the tendency to generate hydrogen gas when initially subjected to a current drain, and having a capacity of at least 0.25 ampere-hour per gram of cathode.

The invention comprehends a heart pacemaker powered by a cell in accordance with the last paragraph.

The explanation which follows is given by way of example only.

45 In producing a cell embodying the invention, (by the method disclosed and claimed in G.B. Application No. 8036598), copper metal is combined with sulfur in an aqueous HCI solution substantially free of extraneous metallic cations to produce cupric sulfide. Preferably, the aqueous reaction medium contains at least 1% HCI, and more preferably at least 50% to 10%. After completion of the reaction, the cupric sulfide product is washed with an aqueous solution to remove cuprous chloride. It has been found that a solution of two 50 compounds is necessary. Mixtures of alkali metal chlorides and HCI or NH4OH and NH4CI work well, but a mixture of HCI and NH4Cl is preferred because of its low cost and because ions in the mixture introduced into the cupric sulfide product may be easily removed by subsequent heating and drying steps. Thereafter, the cupric sulfide product is washed with water until the chloride ion content is below about 5 ppm, preferably 1.5 ppm. The cupric sulfide is then thoroughly dried under conditions which ensure removal of NH3, H20, 55 and HCI mixed therewith, and is at last pressed into a shape retaining cathode. Preferably, the wash solution for removing cuprous chloride contains 10% HCI and 10% NH4CI. A slight stoichiometric excess of copper over that required to combine with the sulfur may be used so that any cuprous products incidentally produced do not disrupt the desired stoichiometric ratio of reactants necessary to produce pure CuS.

The reasons why cathodes produced in this manner effectively overcome the problems of swelling and go varying capacity is not understood. However, by following the teachings disclosed herein those skilled in the art can consistently produce 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 presently-described process is a significant improvement over the prior art dry process, is safer, allows CuS to be produced in less time, produces cathodes with more uniform g5 properties among batches, and produces cathodes with improved mechanical strength.

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GB2 109 153 A

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A lithium-cupric sulfide cell according to the invention remains hydrogen gas-free when initially subjected to a current drain and has a capacity of at least 0.25 ampere hours per gram of cathode. These improved properties may be traced to the purity of the CuS cathode which contains less than 0.35% water, less than 0.35% sulfates, less than 0.60% free sulfur, and less than 0.30% substances extractable with concentrated 5 NH40H (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 about 1.9 ampere hours (0.27 ampere hour per gram).

U.S. Patent Specification No. 2,332,145 discloses a method of producing cupric sulfide from copper metal 10 and sulfur in an aqueous acidic solution of a metallic salt. According to this patent, metal salts such as sodium or cupric chloride, when acidified, are capable of catalyzing the reaction between copper and sulfur in aqueous solution. In examples given in the Specification, it is said that copper metal and sulfur, present in approximately a 2 to 1 weight ratio, can be reacted to form cupric sulfide in an aqueous solution comprising 0.5% to 2% HCI or H2S04 and 20% to 40% cupric sulfate or sodium chloride.

15 The invention will now be described in more detail with reference to the sole accompanying drawing which depicts an exploded view of a lithium-cupric sulfide cell according to the invention.

At the heart of the invention is the discovery that cathodes formed from cupric sulfide produced in an acidified aqueous system and subsequently freed of impurities in the manner disclosed herein are characterized by certain useful properties, the presence of which could not be predicted from the processing 20 scheme. When the cathodes are assembled into cells comprising a lithium anode and a nonaqueous electrolyte, hydrogen gas liberation is either completely eliminated or reduced to low levels. Furthermore, the cathodes result in cells consistently having an acceptable capacity. Accordingly, the need to de-gas the cells prior to their incorporation into pacemakers is substantially eliminated and the frequency of rejections greatly decreased.

25 Sulfur, preferably in finely divided form, is placed in an aqueous HCI solution preferably comprising at least 1% and more preferably greater than 5% HCI in distilled water 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. The mixture is then allowed to react for a sufficient time to produce a fine black powder comprising substantially pure cupric sulfide. As a result of the reaction, trace quantities of 30 cuprous salts and other impurities are produced. Rather than attempt 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 of a mixture of HCI and NH4CI, a mixture of alkali metal chloride and HCI, or a mixture of NH4OH and NH4CI. It has been discovered that such a mixed wash solution is capable of dissolving cuprous chloride, 35 whereas, individually, water, HCI solution, NH4CI solution, NH4OH solution, or alkali metal chloride solution are ineffective.

Next, the solid product is washed repeatedly with distilled water until the chloride content of the wash water is less than 5 parts per million, preferably less than 1.5 parts per million. At this point the cupric sulfide is substantially free of extraneous cuprous, cupric and chloride ions. Subsequently, the powder is heated to 40 approximately 100°C under vacuum to remove water, ammonia, and any residual HCI. If the mixed wash solution contains an alkali metal salt sufficient washing with water should be done to lower the alkali metal ion content to acceptable levels, e.g., less than 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 (3.17 to 3.42 grams per cubic centimeter), and baked at a temperature 45 greater than 200°C, e.g. 250°C. The introduction of a binder material can increase the impurity level and is not recommended. However, certain binders may be used without seriously adversely affecting the cathode's behavior.

To produce a cell, a lithium wafer 10 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 overthe lithium 50 wafer, and the CuS cathode 16 is fitted atop the separators. A polymeric (e.g. polypropylene) gasket 18 is then fitted within the rim of the case and a stainless steel cap 20 is fitted thereon. This unsealed assembly is then soaked in a water-free electrolyte comprising 29% 1,2-dimethoxy ethane, 61% 1,3 dioxolane. 10% lithium perchlorate, 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.

55 When employing cupric sulfide cathodes made with by dry processes discussed above, the cells must be subjected to a current drain and stored for a period of typically 20 to 40 days until the liberation of hydrogen gas and swelling, if any, ceases, before they can be used. The tendency of the cells to liberate hydrogen can be diminished in some cases by firing the dry processed cathodes for long periods of time, e.g., 24 hours, under vacuum. Thereafter, the cells are tested for capacity, and if found to be capable of delivering at least 00 1.6 ampere-hours of current, are suitable for incorporation into a cardiac pacemaker. A substantial quantity of cells produced by the prior art dry methods have been found wanting in capacity. Essentially all swell, and many swell to such an extent that they cannot be used. In contrast, cells made with the cupric sulfide cathodes produced in accordance with this invention either do not generate gas or in some instances generate only a small quantity of gas, and exhibit a capacity in the vicinity of 1.8 to 2.0 ampere-hours.

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GB2 109 153 A

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Example

1.344 kilograms of sublimed sulfur are emulsified in a production size blender in 4 liters of distilled water containing 0.2% nonionic surfactant (Pluronic 68F, Wyandott Chemical Company). The sulfur emulsion is transferred to a reaction vessel together with 4 liters of distilled water containing sufficient hydrochloric acid 5 (37%) to produce a 10% concentration of HCI in the total solution. While stirring, 2.789 kilograms of 5

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 equipped 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 is removed, and 8 liters of distilled water containing 10% HCI and 10% ammonium chloride are added to the reaction flask.

10 After boiling for five minutes, the supernatant is again removed and 8 liters of distilled water were added iq 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 with the aid of suction. 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.

15 The wet product is then transferred to a stainless steel rotary dryer and dried under vacuum at 100°Cto 15

remove water and residual NH4CI, NH40H, NH3, or HCI. Ammonium chloride and hydroxide are removed by virtue of the following reactions:

NH4CI->NH3 + HCI

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NH40H-*NH3 + H20

Seven gram samples of the purified cupric sulfide are then molded under pressure (e.g. 45 tons or 45720 kg of force exerted over an area of approximately 1.6 in.2 or 16.5 cm2) to a density of 56.6 grams per cubic inch 25 (3.45 g/cc) to form cathodes (approximately 0.08 in. or 0.2 cm. thick) which are then baked or "fired" at 250 ± 25 15°C for four minutes.

The cathodes are incorporated into cells. In a glove box with an inert argon atmosphere, a lithium disk is pressed into the bottom of a cleaned stainless steel case. A pair of porous, polypropylene separators (0.008 in. or 0.02 cm. thick and 0.022 in. or 0.06 cm. thick) and a cupric sulfide cathode are placed atop the lithium 30 wafer. A polypropylene gasket is next fitted into the case, and a stainless steel cap placed over the gasket. 30 These unsealed cells are then soaked in a mixed, water-free electrolyte comprising by weight 29% 1,2 dimethoxy 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 assembly. Advantageously, the cells may be hermetically sealed without danger of swelling.

35 When cells constructed in accordance with the above procedure subject to a current drain, no hydrogen 35 gas was produced and no swelling occurred. The average capacity of these cells is between about 1.85 and 1.90 ampere-hours and the range of capacity is found to be between about 1.8 and 2.0 ampere-hours.

Twenty-five cathodes produced as described above were subjected to analysis and were found to contain, on average, by weight 0.02% water, 0.06% sulfate, 0.08% free sulfur, and 0.22% of substances extractable 40 with concentrated NH4OH comprising water-insoluble cuprous salts believed to consist predominantly of 40 hydrated sulfites. The Cu++ content of these cathodes reported as 66.56%.

Test cathodes produced by the process set forth above were incorporated into cells comprising a lithium anode, various different separators, and the non-aqueous electrolyte described above. The reaction time for the copper and sulfur in the aqueous HCI solution was varied between 1 and 5 hours, and the density of the 45 cathodes was varied between 52 and about 56 grams per cubic inch. Thirty-one groups of 7 or 8 cells were 45 tested for capacity. The results are set forth below. Capacity data is given in ampere hours, measured not to the end of cell life but rather until the loaded voltage drops to 1.0 volt.

4 GB2 109 153 A

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Test

Batch Reaction

Cathode

Separ

Average

Range of

Number

Time

Density ators*

Capacity

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52

PF

1.90

1.83-1.94

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5 Hrs.

54

PF

1.82

1.78-1.87

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56

PF

1.86

1.80-1.93

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52

PFR

1.81

1.75-1.86

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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

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1 Hr.

54

PFR

1.84

1.76-1.89

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56

PFR

1.87

1.78-1.91

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52

PF

1.87

1.81-1.89

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3 Hrs.

54

PF

1.88

1.84-1.91

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56

PF

1.82

1.79-1.84

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52

PF

1.85

1.78-1.89

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1 Hr.

54

PF

1.89

1.84-

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56

PF

1.88

1.87-1.90

16

52

PF

1.87

1.84-1.90

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54

PF

1.93

1.87-1.95

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2 Hrs.

56

PF

1.87-1.93

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54

PP

1.91

1.87-1.95

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56

PP

1.92

1.85-1.95

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52

PF

1.73

1.69-1.77

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54

PF

1.84

1.83-1.85

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2 Hrs.

56

PF

1.88

1.84-1.90

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54

PP

1.90

1.85-1.94

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56

PP

1.90

1.85-1.94

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52

PF

1.81

1.77-1.84

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3 Hrs.

54

PF

1.84

1.81-1.87

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56

PF

1.92

1.87-1.98

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52

PF

1.78

1.71-1.82

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1 Hr.

54

PF

1.84

1.83-1.86

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56

PF

1.88

1.83-1.94

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15

20

30

35

40

* PFR 0.008 in. polypropylene + 0.010 in. fiberglass 45 PF 0.022 in. polypropylene + 0.010 in. fiberglass 45

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.

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Comparative Example

Stoichiometric proportions of electrolytic copper powder and sublimed sulfur are mixed and spread in shallow stainless steel trays in a uniform layer 4 mm thick. The trays are placed inside aging chambers which are maintained at a temperature between 70°F and 75°F and a relative humidity between 40% and 55%. The 55 powder mix is spread in thin layers using a depth gauge so as to minimize the development of "hot spots" 55 which can result in nonuniform composition of the CuS product and increase the risk of an uncontrolled exothermic reaction. The reaction between 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 broken up, sifted, respread evenly on the trays, and returned to the aging chamber. At the go end of the final day the powder is sifted again, and pressed in a mold as disclosed above to produce 7.0 gram go cathodes. These are fired at 250°C for 4 minutes.

Thirty-one dry processed cathodes produced as disclosed above were subjected to analysis and found to contain, on average by weight 0.09% water, 0.49% sulfate, 0.77% free sulfur, and 0.77% of substances extractable with concentrated NH40H comprising water-insoluble cuprous salts believed predominantly 05 consist of hydrated sulfites. The Cu++ content of these cathodes reported as 66.05%. Ten batches of these 55

GB2 109 153 A

dry processed cathodes were incorporated into cells comprising a lithium anode, a separator, and the non-aqueous electrolyte described above. The cells were tested for capacity. Using the same test conditions as those employed for the wet processed cathodes, the following results were obtained (capacities in ampere hours).

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15 4 l.bz i.oo i.do 15

Batch

Average Capacity

Lowest Measured Capacity

Highest Measured Capacity

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1.71

1.60

1.78

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1.53

1.44

1.66

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1.54

1.42

1.74

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1.62

1.53

1.68

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1.43

1.23

1.66

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1.66

1.62

1.72

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1.65

1.61

1.77

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1.66

1.60

1.71

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1.67

1.61

1.74

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1.74

1.67

1.80

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25 y l.o/ l.bl I./4 25

Most of the test cells spontaneously generated hydrogen gas when subjected to a current drain.

30 In general, dry processed cathodes exhibit rather low capacities at high current drains and higher 30

capacities at lower current drains. In contrast, cathodes made in accordance with the invention exhibit a capacity which is substantially unaffected by the magnitude of the current drain within the 330 ohm to 3000 ohm range tested.

Unless stated to the contrary, percentages given herein are by weight.

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Claims

1. An electrochemical cell comprising a lithium anode, a cupric sulfide cathode, and a non-aqueous electrolyte, the cathode containing less than 0.35% water, less than 0.35% sulfates, less than 0.60% free

40 sulfur, and less than 0.30% of substances extractable with concentrated NH4OH,the cell being substantially 40 free of the tendency to generate hydrogen gas when initially subjected to a current drain, and having a capacity of at least 0.25 ampere-hour per gram of cathode.

2. The cell according to claim 1, wherein the cathode is produced from cupric sulfide powder formed by reaction of copper and sulfur in an aqueous HCI solution.

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3. The cell according to claim 1 or claim 2, wherein the cathode has a density no less than 3.17 g/cc. 45

4. The cell according to claim 1 or claim 2, wherein the cathode has a density no less than 3.29 g/cc and contains less than 0.05% water, less than 0.20% sulfates, and less than 0.20% free sulfur, the cell having a capacity of at least 0.27 ampere hours per gram of cathode.

5. The cell according to any of claims 1 to 4, further comprising a hermetically sealed cell case.

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6. An electrochemical cell according to claim 1, and substantially as herein described with reference to 50 and as shown in the accompanying drawings.

7. A cardiac pacemaker powered by a lithium/cupric sulfide cell as claimed in any of claims 1 to 6.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1983. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.