GB2087131A - Electrode assembly for e.g. secondary batteries - Google Patents

Abstract

An electrode assembly comprising: a porous electrode having a first and second exterior face (122a,122b), and having a cavity (114) formed in the interior thereof between first and second interior faces (120a,120b) positioned opposite the first and second exterior faces (122a,122b); a counter electrode (not shown) positioned facing each of the first and second exterior faces (122a,122b) of the porous electrode; means (not shown) for passing an oxidant through said porous electrode; and screening means (126) for blocking the interior faces (120a,120b) of the porous electrode a greater amount than it blocks the respective exterior faces (122a,122b) of the porous electrode, thereby maintaining a differential of oxidant electrode surface between the interior faces (120a,120b) and the exterior faces (122a,122b). The electrode assembly is useful in a metal-halogen-halogen hydrate electrical energy storage device. <IMAGE>

Description

SPECIFICATION Electrode assembly This invention relates to electrode assemblies and, although the invention is not so restricted, it relates more particularly to electrical energy storage devices (EESD), especially a rechargeable EESD.

An EESD has utility in electric vehicle markets or in stationary power systems. Both of these markets may have a requirement to electrodepositthe reducible metal in a smooth dense manner and to remove it uniformly during discharge. In the electric vehicle market, there may be multiple shallow depth discharges occurring prior to a complete discharge. During discharge, difficulty has arisen when an oxidant is passed through a porous electrode. It may be significantly more electrochemically active than the counter electrode due to its high surface area. Due to the increase in current density, the metal of the counter electrode is removed quickly during discharge. Additionally, chemical corrosion of the reducible metal of the EESD by the presence of the oxidant in the electrolyte has a tendency to decrease the effectiveness of any EESD.

These problems are collectively referred to as the edge activity of an oxidant electrode. The control of the edge effects of a porous oxidant electrode is the object of the present invention.

According to the present invention there is provided an electrode assembly comprising: a porous electrode having a first and a second exterior face, and having a cavity formed in the interior thereof between first and second interior faces which are positioned opposite the first and second exterior faces; a counter electrode positioned facing each of the first and second exterior faces of the porous electrode; means for passing an oxidant through said porous electrode; and screening means for blocking the interior faces of the porous electrode by a greater amount than it blocks the respective exterior faces of the porous electrode, thereby maintaining a differential of oxidant electrode surface between the interior faces and the exterior faces.

There is also provided according to the present invention a method of discharging an electrical energy storage device comprising the steps: providing a porous electrode having a first and a second exterior face, and having a cavity formed in the interior thereof between first and second interior faces which are positioned opposite the first and second exterior faces; providing a counter electrode, comprised of an electrochemically reducible substance, positioned facing each of the first and second exterior faces of the porous electrode; providing a current carrying electrolyte between said electrodes; passing an oxidant through said porous electrode; screening the porous electrode thereby decreasing its electrochemical activity by blocking the interior faces by a greater amount than the respective exterior faces of the porous electrode, and maintaining a differential of electrode surface area between the interior faces and the exterior faces; and closing the circuit between the counter electrodes and the porous electrode, thereby oxidizing the substance of the counter electrodes and reducing the oxidant at the porous electrode.

In either case, the screening means may be W-shaped and be formed of an inert plastics material; the porous electrode may be comprise of a carbonaceous material or a film forming metal; and the screening means may extend along the interior faces 0.05 to 0.30 inches (1.27 to 7.62 mm) further than it extends along the exterior faces.

The electrode assembly preferably further comprises means for passing electrolyte into said cavity.

In said method the counter electrodes may be comprised of a metal and the oxidant may be a halogen. For example, the counter electrodes may be comprised of zinc and the oxidant may be chlorine.

In said method the electrical energy storing device may be a metal-halogen hydrate device and the electrolyte may be on aqueous electrolyte.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view of an electrical energy storage device (EESD) according to the present invention; Figure 2 is a cross-sectional view of part of an EESD such as that shown in Figure 1; Figure 3 is a case for supporting a submodule stack of electrolytic cells of an EESD according to the present invention; Figure 4 is a sectional view taken along line 4-4 of Figure 2; Figure 5 is a sectional view taken along line 5-5 of Figure 4; Figure 6 is an exploded view of electrodes of an EESD according to the present invention; Figure 7 is a sectional view of a cell distribution manifold of an EESD according to the present invention; and Figure 8 is a cross-sectional view of a portion of an electrode assembly used in an EESD according to the present invention, showing the internal/external masking or screening effect.

When porous electrodes are used in an EESD, their electrochemical activity must be taken into consideration during the discharge reaction because the oxidant will be reduced not only at the exterior electrode surface (generally longitudinal face) of the electrode, but also in the interior portion of the porous electrode.

In a preferred embodiment the electrochemical reactions of discharge are: Zn (metal) < Zn++ 2e Cl2#2Cl# -2e Zn (metal) + Cl2 < Zn++ + 2 Cl It has been found that to control the edge effects of a porous electrode there should be some means for decreasing, screening or masking the electrochemical activity of the porous electrode by having a differential mechanical mask on the exterior faces and interior faces of the electrode.

The positive electrodes of the present invention are porous electrodes and may be carbonaceous electrodes, that is, comprised of carbon, activated carbon, graphite, activated graphite, or mixtures thereof, with or without other fillers that may be present in a carbonaceous electrode. The porous electrode may also be comprised of a film forming metal, such as titanium, titanium alloys, tantalum, tantalum alloys, zirconium, zirconium alloys, niobium, niobium alloys, tungsten, tungsten alloys, or mixtures thereof.Any of the electrodes may further comprise catalytic materials well known in the art, i.e. noble metals such as gold and silver and the like, or Group VIII elements of the Periodic Table of Elements (HANDBOOK OF CHEMISTRY AND PHYSICS, 55th Ed., 1974-1975, published by CRC Press) such as ruthenium, rhodium, palladium, osmium, nickel, iridium, platinum, the oxides thereof, mixtures thereof, or the like. Generally, when film forming metals are used, a catalyst is also used, e.g., ruthenized titanium.

The electrode assembly according to the present invention may be used in any EESD or any electrochemical reaction where a porous electrode is used, e.g. those using hydrogen, or oxygen; halogens, such as chlorates, bromates; or primary or secondary fuel cells, such as the metal hydride type, metal halogen systems, and the like. Most preferred is the EESD of the metal-halogen-hydrate type such as the metal-halogen device described in U.S. Patent Specifications Nos. 3713888 or 4049 880.

Operations of a zinc chloride battery system are described in Electric Power Research Institute (EPRI) EM-249 Report for Project 226-1 Interim Report, September 1976; EM-1051, Parts 1-4, Project 226-3 Interim Report, April 1979; Cost Analysis of 50 KWH Zinc-Chlorine Batteries for Mobile Applications, U.S. Dept. of Energy Report COO-2966-1, January 1978; and Safety and Environmental Aspects of Zinc-Chlorine Hydrate Batteries for Electric Applications, U.S. Dept. of Energy Report CO0-2966-2, March 1978.

It has been found that an electrode assembly according to the present invention is particularly useful in an EESD where a current carrying electrolyte is employed such as an aqueous electrolyte. Any of the electrolytes well known in the art for the EESDs described above may be employed. Electrolytes may be acidic or alkaline. The most preferred electrolyte is that used in the metal-halogen hydrate device described in the aforementioned patents such as a zinc chlorine EESD.

It is preferred that when carbonaceous electrodes are employed in the electrode assembly in the present invention that the electrodes be activated in accordance with the disclosure in the Journal of the Electrochemical Society, August, 1978, Vol.125, No.8, "Abstract of the Electrochemical Society Meeting", and Extended Abstracts of the Electrochemical Society, Vol. 78-2 for Fall Meeting, October 15-20, 1978, in particular Abstract No. 73. An electrode assembly according to the present invention is also preferably used as a bipolar electrode in accordance with U.S. Patent Specification No. 4 100 332.

An electrode assembly and method according to the present invention will now be described with reference to the accompanying drawings. Figure 1 is a schematic diagram of an electrode compartment as used in the present invention, in an EESD such as the zinc-chlorine-chlorine hydrate system. Sealed in place in a container 10 is an electrolyte reservoir 12 with a plastics reservoir 14. The electrolyte reservoir 12 functions as a sump from which electrolyte is pumped via a line 16 by means of a pump Pinto each of the stacks or submodules 18 via a plurality of conduits 20. A valve V is placed in line 16 so that the electrolyte may be changed or dumped as desired. While the apparatus 10 is shown as having a hood 22, it is to be appreciated that the design of such equipment may be modified to fit into an electric vehicle or be used in the standing power market.It is further to be appreciated that the electrolyte that is flowing from the sump 12 via line 16 into submodules 18 can be heated or cooled as desired by auxiliary apparatus (not shown).

Figure 2 is a cross-section of an EESD such as shown in Figure 1 showing the electrolyte sump 12 and a tray 22 in which a series of electrical cells are arranged in bipolar fashion having current terminals 19 and 21.

Current is passed through the current terminals to conventional bus bars, which in turn are connected to connector studs (not shown), thereby passing the current to each of the individual cells in each submodule.

Stacks of electrodes are retained in submodule tray 22, a sectional view of which is shown in Figure 3. The submodule tray 22 has an electrolyte drain cup 24 to which is attached a conduit 26 which in turn is connected to a passageway for passing the electrolyte away from the submodule to the sump via an exit line 28. In order to prevent parasitic losses during the charging of the stack, and to decrease the short circuiting that could possibly occur, the electrolyte passes down the conduit 26 and through a pair of opposed serpentine like channels best shown in Figure 3 as channels 30 and 32, the flow being in the direction shown by the arrows.

In a preferred embodiment an electrolyte flows through and past the electrodes during the electrolytic reaction. In order to supply the flowing electrolyte, an electrolyte distribution manifold 34 is provided for each submodule. The electrolyte flows from the sump 12 out of an exit port 36 and is then pumped back to the submodules. The submodules 18 which are stacks of ten cells are inserted into the interior 35 (see Figure 3) of the submodule tray 22, and the electrolyte distribution manifold 34 is joined with the submodule tray by positioning the manifold into channels 38.

Figure 4 shows a sectional view of a portion of a stack of electrodes with a porous carbonaceous electrode, as is preferably used in a zinc chloride EESD.

The porous chlorine electrode 40 consists of a pair of porous carbon plates 40a and 40b which are joined together with a cavity 41 between them to allow passage of electrolyte therethrough as shown by arrows 42 (see Figure 5). Gas venting holes (not shown) may be provided at the top of the porous chlorine electrode.

The tops of three chlorine electrodes are shown in the right side of Figure 4 while the remaining portion of Figure 4 is a sectional view. To prevent distortion of the porous chlorine electrodes, a stub 44 is provided in the middle of the chlorine electrode to give strength thereto. The porous chlorine electrodes are manufactured to have an indented portion 46 (see Figure 6), in which an electrolyte feed tube 48 may be inserted. The electrolyte feed tube 48 is in turn connected to the internal electrolyte distribution manifold at a point 50 (see Figure 5). The electrolyte distribution manifold is comprised of a pair of complementary members 52 and 54 which are fastened together by nuts 56 and bolts 58.

As shown in Figures 4 and 5 a bipolar intermediate bus 60 is machined to receive the chlorine electrodes at points 62 and 64, while adjacent thereto is a metal or zinc electrode 68 which fits into the intermediate bipolar bus at a point 70. To prevent short circuiting, to ensure a tight fit, to control the discharge rates of the chlorine electrode, and to control the edge effects thereof, spacers 72 and 74 are provided to joint the chlorine and zinc electrodes together. The chlorine and zinc electrodes are arranged in bipolar fashion. The masking or screening effect is performed by spacers 72 and 74.

In operation, the electrolyte flows from the sump 12 through an external conduit 80 (see Figure 7) into an interior conduit 82 which is connected to the electrolyte distribution manifold 34 at a point 84. From the electrolyte distribution manifold 34, the electrolyte is passed through tubes 48, it leaves tubes 48 at the bottom of the halogen electrode at a point 83 (see Figure 5), and flows through the porous electrodes, up the intercell spacing 84, into drain cup 24, down the exit conduit 26, into channels 30 and 32 (as described above), and out the exit 28 back to the sump.

The separation between the porous halogen electrode and the metal electrode ranges from about 40 to about 250 mils (1 mm to 6.35 mm), preferably 80 mils (2 mm).

The differential masking of the present invention is graphically shown in Figure 8. The porous electrode is comprised of two elements 100a and 1 00b which are normally joined together at top (not shown) and bottom. Figure 8 shows a "W" shaped element, made of an inert plastics material, e.g. Kynar (Trade Mark), which has grooves 102a and 1 02b into which the elements 100a and 1 00b are respectively fitted. The porous electrode of Figure 8 is similar to the porous electrode of Figure 4. An electrolyte distribution inlet 106 functions as the electrolyte feed tube 48 of Figure 6. For ease of distribution of electrolyte, an inlet channel 108 is formed between members 110 and 112.The electrolyte flows from the sump 12 down distribution inlet 106, nearly to the base of the porous electrode, out of channel 108, fills cavity 114, and then passes through porous electrodes 100a and 100b, through internal faces 120a and 120b and the external faces 122a and 122b.

During operation (charge and discharge) of an EESD, the longitudinal faces 122a and 122b are blocked by an external mask 124a and 124b which physically covers the longitudinal (external) electrode face opposite the counter electrode 68. An internal mask 126 also physically blocks the interior faces of the porous electrode. A differential in physical screening or masking of the longitudinal (External) face versus the internal faces is maintained such that the height of the external mask (measured from the base of the porous electrode 128a or 1 28b to the top of external mask 103a or 130b, respectively) is much less than the height of the internal mask (measured from the base of the porous electrode 128a or 1 28b to the top of the internal mask 132).The differential between the interior screen or mask and the exterior mask ranges from about 0.05 in. (1.27 mm) to about 0.3 in. (7.62 mm), and is preferably 0.18 in. (1.27 mm).

The spacers 72 and 74, shown in Figures 4 and 6, perform the same function on the sides of the electrodes as the internal and external screen or mask at the base of the porous electrode of Figure 8.

It is to be appreciated that the cells and submodules described herein can be combined in series or parallel relationship as is well-known in the art.

Any means for storing and/or charging the oxidant can be used. The storage compartment 25 is connected to line 16 for operation during charging or during discharge of a primary or secondary (electrically rechargeable) EESD via line 23.

In a preferred embodiment, chlorine formed during charging of a zinc chlorine battery with an aqueous zinc chloride electrolyte is converted to chlorine hydrate. The hydrate is then stored and is available for discharge by decomposing the chiride hydrate to chlorine and water.

The halogen hydrate formation apparatus necessary for forming and storing the halogen hydrate during the charging and discharging of the electrical energy storage device is assembled to the remaining apparatus of Figure 1. Any conventional equipment may be used such as that described in U.S. Patent Specification Nos. 3713888 and 3 823 036; or the Electric Power Research Institute and Department of Energy reports mentioned above.

A description will now be given of an investigation which was carried out to evaluate the performance of an electrode assembly according to the present invention.

A Kynar (Trade Mark) electrode assembly was machined to the configuration of Figure 8 incorporating various degrees of differential masking in order to evaluate their effectiveness in controlling the discharge edge activity in a zinc chlorine chlorine hydrate EESD. The evaluation was performed in a test cell consisting of two pairs of mechanically framed chlorine electrodes measuring 4 in x 2.65 in. x 0.080 in. (101.6 mm x 67.31 mm x 2.03 mm) and three zinc electrodes measuring 4 in. x 2.745 in. x 0.390 in. (101.6 mm x 69.72 mm x 2.03 mm). The exposed apparent area for each chlorine electrode after framing (longitudinal face) is calculated to be 61.3 cm2 (245.2 cm2 per cell). The exposed apparent area for each zinc electrode is calculated to be 65.9 cm2 per face. Two porous graphite electrodes (Union Carbide PG-60) were inserted into the Kynar frame.The cavity between the longitudinal (exterior) faces of the chlorine electrode is 0.08 in. The temperature of the electrolyte was controlled by circulating the the electrolyte through a titanium coil immersed in a constant temperature water bath and held at a temperature of 300C + 0.5 C. The volume of electrolyte used was approximately 800 millilitres. In the charge mode, chlorine gas produced electrochemically was vented from the sump. In the discharge mode, the required chlorine gas was fed to the sump via a gas dispersion tube from a chlorine gas cylinder.

Both the charge and discharge processes were operated under constant current. Cell voltage was measured using two voltage probes, separate from the current carrying terminal located at the top of the chlorine and zinc bus bars. The operating conditions are as follows: TABLE 1 Charge: 5 hrs at 27 mA/cm2 (i.e. 6.62 amp) Discharge: to 0 volt at 40 mA/cm2 (i.e. 9.8 amp) Chlorine Electrode 245.2 cm2 Area Electrolyte: Before charge: 25% ZnCl2 (2.3M) pH: 0.18 Flow rate: 2 ml/cm2/min Cl2 concentration: approximately 2 9/1 The external shoulder (mask) size was held constant at 0.05 in. (1.27 mm) (mechanical masking on longitudinal face of the chlorine electrode) while the size of the internal shoulder (mask) was varied to obtain the various differential mask sizes (interior face).To determine the effectiveness of varying the internal and external mechanical screening or mask, the internal mask had an increase in size over the external mask of 0.05 in. (1.27 mm), 0.09 in. (2.29 mm), 0.20 in. (5.08 mm), and 0.45 in. (11.43 mm). All tests were conducted with the same electrodes under the same operating conditions. The effect of differential masking on the charge profile was negligible except to the extent that a good uniform smooth deposit of zinc was obtained.

Most significant were the losses in zinc area coverage at the various discharge steps as is shown below in Table 2.

TABLE 2 Effect of Differential Masking On The Area Loss of Zinc Coverage Area Loss of Zinc Coverae (%) Mask (inches) (mm. in brackets) at Discharge Depth of Differential Internal External 50% 75% 90% 0.05(1.27) 0.10(2.54) 0.05(1.27) 5 12 46 0.09 (2.30) 0.14 (3.56) 0.05 (1.27) 3 8.25 - 0.20(5.08) 0.25(6.35) 0.05(1.27) 3 4 13 Observation of the zinc metal during various stages of discharge is quite significant. At 50% depth of discharge, a patch-type zinc plate had already developed. The size and shape of the zinc path was similar for all differential mask sizes evaluated. At this stage of discharge, the top edge plate started baring of zinc, averaging 3% loss of zinc area.

A 75% depth of discharge, the size and shape change of the zinc deposits had becomes more significant.

The decrease in area coverage of zinc was 12% for the 0.05 in. (1.27 mm) differential mask, 8.25% for the 0.09 in. (2.30 mm) differential mask, and 4% forthe 0.20 in. (5.08 mm) differential mask. It is seen that the difference in decrease in area coverage between the 50% and 75% depth of discharge was relatively small for the 0.20 in. (5.08 mm) differential mask, but significantly largerforthe 0.05 in. (1.27 mm) differential mask.

At 90% depth of discharge, a very well defined zinc patch had developed, the decrease in area coverage of zinc being 46% for the 0.05 in. (1.27 mm) differential mask as compared to 13% for the 0.20 in. (5.08 mm) differential mask. At this stage of discharge, the area coverage of zinc for the 0.20 in. (5.08 mm) differential mask is still considered to be satisfactory.

In the case of the 0.45 in. (11.43 mm) differential mask, the graphite substrate at about 90% depth of discharge showed a reverse shaped patch. The centre portion was bare of zinc, implying an over-mask effect.

It is not known exactly what causes the observations described above but one theory is that with a porous electrode, i.e. a flow-through modeof operation, a portion of the chlorine electrode surface, behind the physical external mask, is participating in chlorine reduction resulting in localized increased current along the external mask edges which causes an increase in the rate of anodic dissolution at the edges of the zinc electrode.

Increasing the size of the differential mask decreases the usable area behind the masks and compensates for the otherwise higher edge activity on discharge. This is reflected in all three of the experimental criteria selected for evaluating the differential masking approach to controlling edge activity on discharge. As can be seen from the above example, although the 0.45 in. (11.43 mm) differential mask size displayed a satisfactorily flat discharge profile, its average discharge voltage and coulombic efficiency were low. An over-mask effect was confirmed by visual inspection of the zinc deposit near the end of the discharge. The differential mask size of 0.20 in. (5.08 mm) was the most effective for retaining the shape of the zinc deposit near the end of the discharge and at the same time giving a satisfactory discharge profile.

It is to be appreciated that the physical mask can be manufactured by a wide variety of processes, for example by injection moulding fluoroplastic Kynar (Trade Mark) or similar inert materials such as polyvinyl chloride or polyester resins.

Figure 8 shows the masking to have been located at the base of the porous electrode, but it should be appreciated that the physical masking may be used on the side of the oxidant electrode, as in Figure 8, or at the top of the oxidant electrode, depending upon how one wishes to insert the oxidant into the porous electrode. Alternatively, the internal screening or mask may be used on all sides of the porous oxidant electrode depending on the oxidant employed and the flowing electrolyte. The masking may also take the form of a coating of an inert substance onto the internal and longitudinal (external) faces of the porous electrode.

Claims (17)

1. An electrode assembly comprising: a porous electrode having a first and a second exterior face, and having a cavity formed in the interior thereof between first and second interior faces which are positioned opposite the first and second exterior faces; a counter electrode positioned facing each of the first and second exterior faces of the porous electrode; means for passing an oxidant through said porous electrode; and screening means for blocking the interior faces of the porous electrode by a greater amount than it blocks the respective exterior faces of the porous electrode, thereby maintaining a differential of oxidant electrode surface between the interior faces and the exterior faces.

2. An electrode assembly as claimed in claim 1 in which the screening means is W-shaped and is formed of an inert plastics material.

3. An electrode assembly as claimed in claim 1 or 2 in which the porous electrode is comprised of a carbonaceous material.

4. An electrode assembly as claimed in claim 1 or 2 in which the porous electrode is comprised of a film forming metal.

5. An electrode assembly as claimed in any preceding claim in which the screening means extends along the interior faces 0.05 to 0.30 inches (1.27 mm to 7.62 mm) further than it extends along the exterior faces.

6. An electrode assembly as claimed in any preceding claim further comprising means for passing electrolyte into said cavity.

7. An electrode assembly substantially as hereinbefore described with reference to the accompanying drawings.

8. A method of discharging an electrical energy storage device comprising the steps: providing a porous electrode having a first and a second exterior face, and having a cavity formed in the interior thereof between first and second interior faces which are positioned opposite the first and second exterior faces; providing a counter electrode, comprised of an electrochemically reducible substance, positioned facing each of the first and second exterior faces of the porous electrode; providing a current carrying electrolyte between said electrodes; passing an oxidant through said porous electrode; screening the porous electrode thereby decreasing its electrochemicalactivity by blocking the interior faces by a greater amount that the respective exterior faces of the porous electrode, and maintaining a differential of electrode surface area between the interior faces and the exterior faces; and closing the circuit between the counter electrodes and the porous electrode, thereby oxidizing the substance of the counter electrodes and reducing the oxidant at the porous electrode.

9. A method as claimed in claim 8 in which the screening means is W-shaped and is formed of a plastics material.

10. A method as claimed in claim 8 or 9 in which the porous electrode is comprised of a carbonaceous material.

11. A method as claimed in claim 8 or 9 in which the porous electrode is comprised of a film forming material.

12. A method as claimed in any of claims 8 to 11 in which said screening means extends along the interior faces 0.05 to 0.30 inches (1.27 mm to 7.62 mm) further than it extends along the exterior faces.

13. A method as claimed in any of claims 8 to 12 in which the counter electrodes are comprised of a metal; and the oxidant is a halogen.

14. A method as claimed in claim 13 in which the counter electrodes are comprised of zinc and the oxidant is chlorine.

15. A method as claimed in claim 13 or 14 in which the electrical energy storage device is a metalhalogen-halogen hydrate device.

16. A method as claimed in any of claims 13 to 16 in which the electrolyte is an aqueous electrolyte.

17. A method substantially as hereinbefore described with reference to the accompanying drawings.