GB1604137A

GB1604137A - Electrochemical storage cell or battery

Info

Publication number: GB1604137A
Application number: GB18286/78A
Authority: GB
Original assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1977-05-07
Filing date: 1978-05-08
Publication date: 1981-12-02
Also published as: DE2720726A1; DE2720726C3; DE2720726B2

Description

(54) ELECTROCHEMICAL STORAGE CELL OR BATTERY (71) We, BROWN, BOVERI & CIE AG.D 6800 Mannheim-Kafertal, Kallstadter Strasse 1, Germany, a German Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described, in and by the following statement: The invention relates to an electrochemical storage cell or battery with at least one anode chamber for alkali metal functioning as anolyte and at least one cathode chamber for sulphur-bearing catholyte substance, separated from each other by an alkali ion-conducting solid electrolyte and defined by a light-metal cell wall, which is provided with a protective coating and also functions as current collector.

The use of the cell wall as current collector results in a substantial simplification of the construction of storage cells of the kind described above. Another advantage is obtained by the use of light-metal or light-metal alloys for the cell walls, a feature which confers a high power density on the storage cells or batteries.

However, corrosion of the cell wall material by the catholyte substance, more particularly by the sulphur or sodium polysulphide melt, is a substantial disadvantage.

To overcome this problem, it has already been proposed that a screening electrode of a material having a greater corrosion resistance, more particularly graphite, be arranged in front of the cell wall since it has been found that, in an electric field, lightmetal alloys corrode much more rapidly, more particularly in the form of the dangerous pitting corrosion (see German Offenlegungsschrift 2 457 418).

It has also been attempted, by means of a coating of MoS2, to protect light-metal cell walls of such storage cells against corrosion (see US Patent Specificaton 3 749 603). An attempt has also been made to coat the light-metal cell walls with cobalt sulphides or chromium sulphides.

The above-mentioned facilities were able to briefly improve the corrosion characteristics, but in the long term new disadvantages occurred. More particularly arnong these is the rise of internal resistance of the cell.

Other detrimental influences occur due to insufficient knowledge of the material parameter combinations required for such a complex stress condition.

The object of the present invention was therefore in first place to define the material parameters for a coating material which will provide long-term advantages in cells of the construction described above, both in respect of the electrical properties of the cell (in particular a uniform low electrical resistance) and in terms of service life, i.e. to make possible maximum inhibition of the ageing and corrosion processes. Special attention was to be given to find a solution which was advantageous in terms of manufacture and costs.

According to the invention the problem is solved in that the light-metal, at least in the region in contact with the catholyte substance, is coated with an undercoat and with a coating of a metallic alloy which adheres well in the operating state and is a good conductor of electric current and in the corroded state still has a minimum conductivity of approximately 0.03 Q 'cm' and prevents the formation of a contiguous stratum of aluminium sulphide or other corrosion products with a resistance in excess of approximately 2Qcm2 and whose corrosion products in the catholyte substance have a solubility less than approximately 10 mg/g.

The following requires explanation with regard to the individual features: Light-metal in first place refers to the metals aluminium, magnesium, titanium and their alloys. Aluminium containing up to 5% of magnesium is of special significance. The first-mentioned feature, i.e. the criterion of good adhesion and electrical conductivity, is not surprising in the present context but it is even more surprising for the following feature which presupposes corrosion of the protective coating. In this respect the invention differs materially from earlier tests which, using molybdenum disulphide or graphite, proceeded from the assumption that contact with the sulphur-bearing catholyte substance with the protective coating should not result in the corrosion thereof.

Within the scope of the present invention it is assumed that the protective coating is to corrode in the course of operation while the other criteria mentioned in the characterizing part are maintained.

It has been found in numerous tests that the minimum conductivity of 0.03 Q-lcm~l is sufficient to ensure that the cell has good electrical properties. On the other hand it is of great importance to ensure that the corroded stratum thus produced offers adequate resistance to the diffusion of aluminium so as to reliably prevent the formation of A12S3 and similar products with a resistance exceeding 2cm2, recognized within the scope of the invention as being particularly harmful because of its high resistance value. Finally, the alloy intended for the protective coating had to be selected with a view to avoiding the coating, produced by corrosion, dissolving in the course of operation in the catholyte substance by virtue of a specific minimum solubility. Failure to pay attention to the last-mentioned critenon resulted in the failure of some known solutions to the problem.

Within the context of the abovementioned criteria it is possible to select alloys of different kind. Generally, they will be alloys with suitable contents of chromium, cobalt, nickel, molybdenum or tungsten, based on nickel or cobalt. This includes various alloy steels and so-called superalloys. Some of these alloys have become known as casings for sodium sulphur cells but not in connection with their use as protective coatings on light-metals or their alloys and not under the specific aspects according to the present invention.

An alloy based on at least one metal of the group iron, cobalt or nickel, provided this contains 10 to 50% by weight, more particularly 20 to 30% by weight of chromium, was found to be particularly suitable as the result of investigations carried out for the present invention. It has been found that the chromium content plays a special part in certain alloys of the above-mentioned kind.

As indicated so far by tests carried out within the scope of the invention a strongly adhering and particularly dense chromiumrich substrate, which substantially blocks diffusion of the base metals towards the melt, is formed in alloys of the abovementioned kind. If the chromium content drops below 10% by weight there is evidently no contiguous barrier layer while chromium sulphides with less favourable chracteristics are evidently formed by chromium contents exceeding 50% by weight, where the protective section of the stratum also diminishes. Chromium cosulphides, which lead to the formation of very dense substrates with a good barrier effect, are formed in the preferred range between 20 and 30%.

The metallic alloys applied as coating in accordance with the invention are therefore partially sulphided by the melts of the cathode chamber. By contrast to sulphiding of the light-metal, the corrosion products formed by the process, for example heavy metal sulphides and thiospinells, are not harmful to the cell characteristics because they are less readily soluble in the melt and have a better conductivity for electric current than corresponding products of the unprotected light-metal. As shown by tests it is not necessary for the coating to form an absolutely sealing-tight covering layer because the light-metal itself is also substantially corrosion-resistant.

Two exemplified embodiments will be explained hereinbelow by reference to the accompanying drawning: The drawing shows a cup-shaped storage cell as a vertical section.

The storage cell or battery has an outer casing in the form of a vessel of aluminium 1. The vessel was produced as follows: the individual part members are first provided with an undercoat consisting of 96% nickel and 4% aluminium, designated with the numeral 2 in the illustration, before the actual stratum according to the invention is applied. This is followed by coating with the protective stratum, referenced with the numeral 3 in the illustration, namely a superalloy comprising 56.71% by weight of nickel, 18.4% of cobalt, 12.3% of chromium, 3.25% of molybdenum, 5% of aluminium, 4.33% of titanium. Both coatings were applied by plasma spraying using Plasmadyne apparatus rated at 36 kW.

Argon was used as plasma gas at a current of 500 A with a voltage supply of 35 V. The part members of the cell casing thus treated were then joined by electron beam welding.

The bottom, designated with the numeral 4 in the illustration, was manufactured in the same manner.

The above-described laboratory production can of course be replaced by more elegant methods. The cathode chamber, filled in the usual manner with a graphite felt 6, is situated between the cell wall or the casing 3 and the solid electrolyte designated with the numeral 5. The socket, designated 7 in the illustration, is provided for evacuating purposes after the catholyte substance is filled in. The seal member 8 of aluminium, connected to the casing 1, is pressed by means of a union flange 9 on the Al203 ring 10. In this way the cathode chamber is sealed to atmosphere.

The following is a comparison between a sodium-sulphur cell having a casing of steel and one according to the invention. Both cells are constructed as shown in principle in the illustration. The electrolyte tube has a length of 220 mm, a diameter of 25 mm and a wall thickness of approximately 1 mm.

The distance between the electrolyte tube and the casing wall is 7 mm. Due to corresponding coating of the aluminium the same material in both cells of the example has a composition as stated above and is in contact with the melt.

The thickness of the casing wall is 1 mm.

In the steel cell and by contrast to the illustration the seal is constructed in accordance with the German Offenlegungsschrift 24 59 530.

Given a discharge time of 2 hours such cells can deliver 100 Wh.

The weight of the steel cell is approximately 1 kg and that of the coated aluminium cell is approximately 650 g. The advantage of the use of aluminium as regards weight or energy density is thus clearly shown.

It was shown that both cell types reached a capacity of 78 + 2%, remaining constant over 100 cycles within the error limits, if 0.8 mole % of tetracyanoethylene (see also German Offenlegungsschrift 2 633 456) was added to the sulphur at a charging current density of 75 mA/(cm2 AI203). The two cells under comparison are therefore equivalent as regards capacity and ageing but this does not apply to the aluminium casing which is coated in accordance with the prior art.

In a cell whose casing consisted of aluminium coated by plasma spraying with molybdenum, subsequently converted by contact with sulphur into MoS2, a cell voltage of only 1.52 V was obtained with a discharge current of 75 mA/cm2 (referred to the electrolyte surface area) while the voltage of a cell constructed according to the invention is much higher at the same discharge current density, namely at 1.88 V.

This voltage proved to be constant over more than 100 cycles.

In a cell with an aluminium casing which was vacuum-coated with graphite the initial cell voltage at 75 mA/cm was 1.7 V but a cell voltage of only 1.4 V was obtained after several cycles at a current rate of 35 mAl cm2.

A comparison with a cell wall of uncoated aluminium is not possible since uncoated aluminium corrodes so rapidly when the cell wall is used as current collector and the internal resistance of the cell is increased by the strongly insulating aluminium sulphides so that in practice the cell cannot be operated.

It will be understood that the invention is not confined to the kind of cell described in the preceding example but can also be applied to other conventional shapes.

WHAT WE CLAIM IS: 1. An electrochemical storage cell or battery with at least one anode chamber for alkali metal functioning as anolyte and at least one cathode chamber for sulphurbearing catholyte substance, separated from each other by an alkali ion-conducting solid electrolyte and defined by a light-metal cell wall, which is provided with a protective coating and also functions as current collector, characterized in that the light-metal , at least in the region in contact with the catholyte substance, is coated with an undercoat and with a coating of a metallic alloy which adheres well in the operating state and is a good conductor of electric current and in the corroded state still has a minimum conductivity of approximately 0.03 Q-'cm-' and prevents the formation of a contiguous stratum of aluminium sulphide or other corrosion products with a resistance in excess of approximately 2Qcm2 and whose corrosion products in the catholyte substance have a solubility less than approximately 10 mg/g.

An An electrochemical storage cell or battery as claimed in claim 1, wherein the alloy is based on at least one metal of the group iron, cobalt, nickel, containing 10 to 50% by weight of chromium.

3. An electrochemical storage cell or battery as claimed in claim 2, wherein the alloy contains chromium at the rate of 20 to 30% by weight.

4. An electrochemical storage cell or battery as claimed in claim 4, wherein the undercoat is based on aluminium-nickel.

5. An electrochemical storage cell or battery as claimed in any one of the claims 2 to 4, wherein coating is performed by plasma or flame spraying.

6. An electrochemical storage cell or battery substantially as herein described with reference to and as shown in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

7 in the illustration, is provided for evacuating purposes after the catholyte substance is filled in. The seal member 8 of aluminium, connected to the casing 1, is pressed by means of a union flange 9 on the Al203 ring 10. In this way the cathode chamber is sealed to atmosphere.

The following is a comparison between a sodium-sulphur cell having a casing of steel and one according to the invention. Both cells are constructed as shown in principle in the illustration. The electrolyte tube has a length of 220 mm, a diameter of 25 mm and a wall thickness of approximately 1 mm.

The distance between the electrolyte tube and the casing wall is 7 mm. Due to corresponding coating of the aluminium the same material in both cells of the example has a composition as stated above and is in contact with the melt.

The thickness of the casing wall is 1 mm.

In the steel cell and by contrast to the illustration the seal is constructed in accordance with the German Offenlegungsschrift 24 59 530.

Given a discharge time of
2 hours such cells can deliver 100 Wh.

The weight of the steel cell is approximately 1 kg and that of the coated aluminium cell is approximately 650 g. The advantage of the use of aluminium as regards weight or energy density is thus clearly shown.

It was shown that both cell types reached a capacity of 78 + 2%, remaining constant over 100 cycles within the error limits, if 0.8 mole % of tetracyanoethylene (see also German Offenlegungsschrift 2 633 456) was added to the sulphur at a charging current density of 75 mA/(cm2 ssAI203). The two cells under comparison are therefore equivalent as regards capacity and ageing but this does not apply to the aluminium casing which is coated in accordance with the prior art.

In a cell whose casing consisted of aluminium coated by plasma spraying with molybdenum, subsequently converted by contact with sulphur into MoS2, a cell voltage of only 1.52 V was obtained with a discharge current of 75 mA/cm2 (referred to the electrolyte surface area) while the voltage of a cell constructed according to the invention is much higher at the same discharge current density, namely at 1.88 V.

This voltage proved to be constant over more than 100 cycles.

In a cell with an aluminium casing which was vacuum-coated with graphite the initial cell voltage at 75 mA/cm was 1.7 V but a cell voltage of only 1.4 V was obtained after several cycles at a current rate of 35 mAl cm2.

A comparison with a cell wall of uncoated aluminium is not possible since uncoated aluminium corrodes so rapidly when the cell wall is used as current collector and the internal resistance of the cell is increased by the strongly insulating aluminium sulphides so that in practice the cell cannot be operated.

It will be understood that the invention is not confined to the kind of cell described in the preceding example but can also be applied to other conventional shapes.

WHAT WE CLAIM IS: 1. An electrochemical storage cell or battery with at least one anode chamber for alkali metal functioning as anolyte and at least one cathode chamber for sulphurbearing catholyte substance, separated from each other by an alkali ion-conducting solid electrolyte and defined by a light-metal cell wall, which is provided with a protective coating and also functions as current collector, characterized in that the light-metal , at least in the region in contact with the catholyte substance, is coated with an undercoat and with a coating of a metallic alloy which adheres well in the operating state and is a good conductor of electric current and in the corroded state still has a minimum conductivity of approximately 0.03 Q-'cm-' and prevents the formation of a contiguous stratum of aluminium sulphide or other corrosion products with a resistance in excess of approximately 2Qcm2 and whose corrosion products in the catholyte substance have a solubility less than approximately 10 mg/g.

An An electrochemical storage cell or battery as claimed in claim 1, wherein the alloy is based on at least one metal of the group iron, cobalt, nickel, containing 10 to 50% by weight of chromium.
3. An electrochemical storage cell or battery as claimed in claim 2, wherein the alloy contains chromium at the rate of 20 to 30% by weight.
4. An electrochemical storage cell or battery as claimed in claim 4, wherein the undercoat is based on aluminium-nickel.
5. An electrochemical storage cell or battery as claimed in any one of the claims 2 to 4, wherein coating is performed by plasma or flame spraying.
6. An electrochemical storage cell or battery substantially as herein described with reference to and as shown in the accompanying drawing.