GB2068630A

GB2068630A - Electrochemical cell having internal short inhibitor

Info

Publication number: GB2068630A
Application number: GB8033811A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1980-01-29
Filing date: 1980-10-20
Publication date: 1981-08-12
Also published as: CA1166687A; GB2068630B; JPS56128571A; DE3101099A1; FR2474767A1

Abstract

Electrochemical cells, particularly sealed lead acid cells of cylindrical spiral-wound plate design, are susceptible to shorting due to severe discharge and charge reversal. To prevent this the cell includes, besides the positive (25) and negative (23) electrodes and (multilayer) separator (26) between them, inert porous short-inhibitor members (22) between the negative electrode (23) and separator (26). <IMAGE>

Description

SPECIFICATION Electrochemical cell having internal short inhibitor The present invention relates to electrochemical cells, and seeks inter alia to provide a sealed lead acid cell having improved performance and less susceptibility to shorting due to severe discharge and charge reversal.

A significant improvement in the well-known lead acid cell is the fully sealed lead acid cell which makes use of a cylindrical spiral-wound plate design for high energy density and low internal impedance, and can be used, i.e., charged and discharged, in any position. Such cells typically include spaced-apart positive and negative lead plates having a grid-like construction. The grid structure is filled with the active materials to form either positive (lead dioxide) or negative (sponge lead) electrodes. Sandwiched between the plates is a thin porous separator, the plate-separator assembly being wound into a compact rugged cylindrical form. The separator electrically isolates the plates, and also functions as an effective wick to retain the cell's electrolyte (an aqueous solution of sulfuric acid) and keep it evenly distributed in the working area.The thin, highly porous separator keeps the ionic path between the plates short and permits rapid diffusion of electrolyte, these factors all contributing to the cell's ability to be discharged at high rates. The typical cell being described also generally includes means such as excess negative plate material, for minimizing the formation of gases in the cell, and a resealable vent for releasing internal pressure in the cell should unwanted gases be generated.

While the above-described cell represents a significant improvement over prior lead acid cells, it is desirable to further improve its performance and lengthen its life cycle, especially when the cell is subjected to extreme conditions. For example, when a lead acid cell is allowed to stand on open circuit, a slow electrochemical discharge occurs, the rate of self-discharge depending on the cell temperature and its state of charge. If a cell is allowed to self-discharge completely, i.e., until substantially all of the sulfate ion in the electrolyte has reacted with the plate materials, the lead sulfate becomes slightly soluble in the very dilute electrolyte and is free to diffuse into the separator between the plates.

Attempting to recharge the cell in this condition may result in the formation of lead dendrites in the separator between the plates, eventually shorting the cell and ending its useful life. Similarly, where a discharged lead-acid cell is for one reason or another connected to a charge in reverse, the cell will accept a charge (the positive plate becomes a negative plate and vice versa), but is vulnerable to being shorted out by deposits of metallic lead and lead sulfate in the separator.

In addition, it has been found that there are certain problems associated with winding the plateseparator assembly of known cells. More particularly, in the manufacture of electrolytic cells or batteries, the positive and the negative plate materials are formed as strips which are then cut into cell strip lengths and wound, with separator material therebetween, into a coil. The coil is then inserted into a preformed cylindrical container. Electrolyte is added to the container and the container is closed and sealed. The sealed cell is then charged. The coiled positive plate is connected to the positive terminal and the coiled negative plate is connected to the negative terminal.Typically, the plate strips and separators are wound into the composite coiled assembly about a mandrel with a kiss roller or the inner surface of a cylindrical winding nest engaging one side of the coil as it is being wound. One of the difficulties with this arrangement, however, is that the forces required for winding are applied through the mandrel directly to the strips being wound. Thus, tensile forces are applied to the separators as the separators and plates are pulled by the arbor into the composite, coiled assembly. Because the separators have a low tensile strength, these winding forces can cause damage to and/or breakage of the separators, and can result in a shorted coil, which, of course, adversely affects overall cell quality.

The present invention provides an electrochemical cell comprising a housing containing a positive electrode, a negative electrode, a porous electrically non-conductive separator disposed between said positive and negative electrodes, an inert porous short-inhibitor member disposed adjacent to and intermediate said negative electrode and said separator, and an electrolyte disposed within the housing in contact with said electrodes, said separator, and said short-inhibitor member.

The electrochemical cell may be a sealed lead-acid cell which includes an inner assembly comprising spaced-apart positive and negative plate members, a thin porous separator member disposed between the positive and negative plate members, and a short-inhibitor member sandwiched between the negative plate member and the separator member.

In the preferred embodiment of the present invention, the plate-separator assembly is spiral-wound into a coil such that a short-inhibitor member is adjacent each of the major surfaces of the negative plate member; a separator member is adjacent each of the short-inhibitor members; and the positive plate member is adjacent one of the separator members. Each short-inhibitor member may be formed from a fine mesh fabric, either woven or non-woven, comprising an acid and oxidation resistant polymer fiber such as polyester, polypropylene, etc., which may or may not be combined with a sintered polymer filler.The short-inhibitor member may have a thickness in the range of .0009 - .010 inches, preferably about .0012 inches; a basic weight (gms/yd2) in the range of 18 - 30, preferably, about 20; a bulk density (gm/cm3) in the range of .64 - .78, preferably about .78; and a porosity in the range of 40 - 50%. It will be noted that while the preferred location of the short-inhibitor member, as described above, is on the negative plate member of the cell, said short-inhibitor member may be located anywhere in the plate-separator sandwich. Each separator member, which may include one or more layers, may be typically formed from a non-woven glass micro fiber mat. It will be understood, however, that other types of separators may also be used.

The subject plate/separator/short-inhibitor assembly is contained in a housing which also contains the electrolyte, typically an aqueous solution of sulfuric acid. The housing of course includes a pair of external terminals each of which is connected to a positive or negative plate member.

It will be noted that while the subject assembly has been described above with respect to a lead acid cell, the assembly may also be used in cells having zinc electrodes.

The invention further provides a method for making an electrochemical cell, comprising the steps of: providing negative and positive electrodes, a separator disposed therebetween for insulating the electrodes from one another, and a porous shortinhibitor member adjacent to and intermediate said negative electrode and said separator, said shortinhibitor member being a fine mesh fabric formed from an acid and oxidation resistant polymer fiber; placing said electrodes, separator, and shortinhibitor member into a housing, said housing including external terminals each of which is connected to one of said electrodes; adding an electrolyte into the housing such that such electrolyte comes into contact with said electrodes, separator and short-inhibitor member; and sealing said housing.

The method may include the step of winding said electrodes, separator and short-inhibitor member into a coiled assembly before the step of placing said electrodes, separator and short-inhibitor member into the housing. This will involve a plurality of short-inhibitor members, each of which may have a length greater than that of said electrodes and separator member so as to define an extension portion on each short-inhibitor member, said step of winding said electrodes, separator and shortinhibitor members into a coiled assembly including the step of applying a winding means to said short-inhibitor extension portions such that the brunt of the tensile forces effected during said winding step are borne by said short-inhibitor members.

In order that the invention may be clearly understood, a preferred embodiment thereof will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a perspective, cross-sectional view of a battery cell formed in accordance with the embodi ment of the invention; Figure 2 is a plan view, broken away in part, of the battery cell of Figure 1; and Figure 3 is a schematic diagram illustrating the plate-separator-short-inhibitor assembly prior to its being wound into a cylindrical coil.

Referring to Figures 1 and 2, the electrochemical cell 10 includes an inner assembly 20 which compris es a negative plate electrode 23; first and second short-inhibitor members 22, each of which being disposed adjacent one of the major surfaces of negative plate electrode 23; first and second separator members 26, each of which being disposed adjacent one of the short-inhibitor members 22; and a positive plate electrode 25 disposed adjacent one of the separator members 26. As illustrated in the drawings, the preferred embodiment comprises a spirally wound assembly 20, but it will be understood that a flat plate-separator-short-inhibitor assembly may also be employed in cells which incorporate a starved electrolyte system.Where the cell is of the lead acid type, negative plate electrode 23 and positive plate electrode 25 are each con structedfrom lead metal grids which are cut into strips. These grids are filled with the active materials to form a negative (sponge lead) electrode 23 and a positive (lead dioxide) electrode 25. As indicated above, the present invention may also be employed with zinc electrode cells, and thus, in such an embodiment, the specific construction of the negative and positive electrode plates of the cell will be changed pursuant to known practice.

Each short-inhibitor member 22 is formed from an inert, fine mesh fabric, either woven or non-woven, comprising an acid and oxidation resistant polymer fiber such as polyester, polypropylene, etc..., which may or may not be combined with a sintered polymer filler. In the preferred embodiment of the invention, short-inhibitor members 22 are formed from polyester, and have a thickness in the range of .0009 - .010 inches, preferably about .0012 inches, a basic weight (gms/yd2) in the range of 18 to 30, preferably, about 20; a bulk density (gm/cm3) in the range of.64 to .78, preferably about .78; and a porosity in the range of 40 to 50%, preferably about 45%.In addition, it is preferable that short-inhibitor members 22 have a length greater than plates 23 and 25 and separator members 26 such that extension portions 27 on members 22 are provided (See Figure 3), the function of these extension portions being described below.

Separator members 26 are each formed from a thin, highly porous insulating material which, along with short-inhibitor members 22, completely electrically isolate positive and negative electrode plates 25 and 23. As mentioned above, separators 26 function as an effective wick to retain the cell's electrolyte and keep it evenly distributed in the working area. The porous separator 26 and inhibitor members 22 provide a short ionic path between plates 23 and 25 and permit rapid diffusion of the electrolyte. Preferably, separator members 26 are formed from a non-woven glass micro fiber mat, but other types of separators in common use may also be employed. In addition, as illustrated in Figures 1 and 2, each separator 26 may include one or more layers, such as, for example, four layers.

The electrode assembly 20 may be spirally wound using automatic belt winders, manual belt winders or an arbor winding technique such as that schematically illustrated in Figure 3. More particularly, as shown in Figure 3 (and referred to above), it is preferable that short-inhibitor members 22 have a length greater than that of electrode plates 23 and 25 and separator members 26 such that extension portions 27 are provided on short-inhibitors 22. In winding assembly 20, the arbor 24 pulls on inhibitor extensions 27, which in-turn, also effects the winding of the electrode plates and separators.Because inhibitor members 22 have sufficient tensile strength to withstand the pull forces exerted by the arbor in winding the electrode/separator/short-inhibitor strips, the above-described problems associated with the tensile forces being directly applied to the fragile separator members, which generally have a very low tensile strength, are obviated.

Referring again to Figures 1 and 2, the coiled electrode assembly 20 is enclosed in a cylindrical housing 15, which includes an inner case 12 formed from an electrically non-conductive material, e.g., a chemically stable polypropylene. Housing 15 also includes an outer metallic case 11, encasing inner case 12, which provides for mechanical rigidity and strength. Typically, outer case 11 may be formed from aluminium or steel. By providing housing 15 with inner and outer cases 12 and 11, respectively, the rate of gas diffusion through the housing wall is minimized, and loss of water from the electrolyte is virtually eliminated.

Housing 15 further includes an outer cover member 13 and a pair of terminals 14 and 16. Terminals 14 and 16 protrude through outer cover 13, each terminal being electrically connected to either positive plate 25 or negative plate 23 within housing 15.

Housing 15 also includes a resealable safety vent mechanism 17 which provides for the harmless venting of gases that can be generated under extreme operating conditions such as an excessive overcharge rate. The electrolyte, typically an aqueous solution of sulfuric acid, is preferably added to the housing under pressure. The acid concentration within a cell varies with the state of charge, the concentration being highest when the cell is fully charged and lowest when the cell is discharged. The amount of electrolyte used in the cell is selected to permit efficient utilization of the active plate materials while still preventing the accumulation of any free electrolyte, i.e., unretained by the electrode plates 23 and 25, separators 26 and short-inhibitor layers 22.As indicated above, under normal use conditions, virtually no water is lost from the elect#lyte. However, a small portion of the water may be temporarily involved in generating gas during overcharging.

The following examples are presented to compare results obtained in testing conventional cells against cells containing short-inhibiting layers in accordance with the invention.

Example 1 Test cells were built in accordance with the preferred embodiment of the present invention, i.e., incorporating polyester short-inhibitor layers. Some of the test cells were filled with electrolyte in the conventional manner, the other test cells having the electrolyte forced into the cell under pressure to increase its rate of entry. After filling the cell, the pressure was relieved. Each short-inhibitor member had a thickness in the range of .0009 - .010 inches, a basic weight in the range of 18 - 30 grams per square yard, a density in the range of .64 - .78 grams per cubic centimeter, and a porosity in the range of about 40 to 50%.In addition, control cells, i.e., cells not incorporating the subject polyester shortinhibitor members were prepared, some of said control cells being filled with electrolyte in the conventional manner, the other control cells having the electrolyte forced into the cell as described above with respect to some of the test cells.

All of the test cells and control cells were then subjected to a reversal test which comprises discharging the freshly formed cells at a rate of 250 mA for 24 hours. This completely discharged the cells, tending to drive them into polarity reversal. The control cells, regardless of how the electrolyte was added, tended to develop shorts by deposits of metallic lead and lead sulfate in theseparatorwith ninety percent (90%) of the control cells developing shorts between the electrodes. None of the test cells developed shorts. Moreover, the test cells in which the electrolyte was added under pressure revealed no traces of lead sulfate in the separators or short-inhibitor members.

Example 2 Test cells incorporating the short-inhibitor mem- bers of the present invention were prepared having the electrolyte forced into the cells under pressure, the pressure being relieved after the cells were filled.

The short-inhibitor members in each cell had the same thickness, weight, density and porosity as the test cells of Example 1. In addition, control cells, i.e., cells which did not incorporate the short-inhibitor members of the present invention, were also prepared. The cells were cycled at 480C with each cycle comprising a charge period of 18 hours on a constant voltage of 2.45 volts, and a discharge of 1.8 amperes to a cell cutoff voltage of 1.4 volts. With "failure" being defined as a drop in capacity to one-half the rated capacity of 1.8 ampere-hours at the above discharge rate, the test cells lived, i.e., did not undergo "failure" for a period about twice as long as the control cells.

In summary, the electrochemical cell described above employs short-inhibitor members within the electrode plate/separator sandwich. This cell extends the time for the incidence of permanent cell failure as a result of shorting caused by deposits of lead dentrites or tracks in the plate insulation (separators) when the cell is subjected to abusive conditions such as overdischarge and cell reversal.

In addition, the cell has a significantly improved cycle life over known cells. Further, the cell is very resistant to damage to the cell separator as a result of winding the plate-separator assembly, the relatively strong polyester short-inhibitor members being the portion of the assembly that carries the largest portion of the tensile forces during winding.

Claims

1. An electrochemical cell comprising a housing containing a positive electrode, a negative electrode, a porous electrically non-conductive separator disposed between said positive and negative electrodes, an inert porous short-inhibitor member disposed adjacent to and intermediate said negative electrode and said separator, and an electrolyte disposed within the housing in contact with said electrodes, said separator, and said short-inhibitor member.

2. An electrochemical cell as claimed in Claim 1, wherein the short-inhibitor member comprises a mesh fabric formed from an acid and oxidation resistant polymer fiber.

3. An electrochemical cell as claimed in Claim 2, wherein the mesh fabric is woven.

4. An electrochemical cell as claimed in Claim 2, wherein the mesh fabric is non-woven.

5. An electrochemical cell as claimed in any one of the preceding claims, wherein the short-inhibitor member includes a sintered polymer filler.

6. An electrochemical cell as claimed in any of the preceding claims, wherein the short-inhibitor member is formed from polyester.

7. An electrochemical cell as claimed in any one of Claims 1 to 5, wherein the short-inhibitor member is formed from polypropylene.

8. An electrochemical cell as claimed in any one of the preceding claims, wherein the short-inhibitor member has a density in the range of .64 - .78 grams per cubic centimeter, and a porosity in the range of 40-50%.

9. An electrochemical cell as claimed in any one of the preceding claims, wherein the separator is a fibrous glass member.

10. An electrochemical cell as claimed in Claim 9, wherein the separator member is a non-woven glass micro fiber mat.

11. An electrochemical cell-as claimed in any one of the preceding claims, wherein the electrolyte is an aqueous solution of sulfuric acid.

12. An electrochemical cell as claimed in any one of the preceding claims, wherein the electrodes, separator and short-inhibitor member are spirally wound into a coiled assembly.

13. An electrochemical cell as claimed in Claim 12, wherein there are a plurality of short-inhibitor members and each of said short-inhibitor members has a length greater than that of said electrodes and separator members so as to define an extension on each short-inhibitor memberfor facilitating the winding of said electrode assembly.

14. An electrochemical cell as claimed in any one of the preceding claims, wherein the housing includes an inner case formed from an electrically non-conductive material and an outer case formed from a metallic material, said outer case enclosing said inner case.

15. A method for making an electrochemical cell, comprising the steps of: providing negative and positive electrodes, a separator disposed therebetween for insulating the electrodes from one another, and a porous shortinhibitor member adjacent to and intermediate said negative electrode and said separator, said shortinhibitor member being a fine mesh fabric formed from an acid and oxidation resistant polymer fiber; placing said electrodes, separator, and shortinhibitor member into a housing, said housing including external terminals each of which is connected to one of said electrodes; adding an electrolyte into the housing such that said electrolyte comes into contact with said electrodes, separator and short-inhibitor member; and sealing said housing.

16. A method as claimed in Claim 15, wherein the short-inhibitor member is polyester.

17. A method as claimed in Claim 15 or Claim 16, wherein the electrolyte is added into the housing under pressure.

18. A method as claimed in any one of Claims 15 to 17, including the step of winding said electrodes, separator and short-inhibitor member into a coiled assembly before the step of placing said electrodes, separator and short-inhibitor member into the housing.

19. A method as claimed in Claim 18, wherein there are a plurality of short-inhibitor members each of which has a length greater than that of said electrodes and separator member so as to define an extension portion on each short-inhibitor member, said step of winding said electrodes, separator and short-inhibitor members into a coiled assembly including the step of applying a winding means to said short-inhibitor extension portions such that the brunt of the tensile forces effected during said winding step are borne by said short-inhibitor members.

20. An electrochemical cell as claimed in Claim 1, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

21. A method for making an electrochemical cell as claimed in Claim 15, substantially as hereinbefore described.

22. An electrochemical cell made by the method claimed in any one of Claims 15 to 19.