GB2118762A

GB2118762A - Electrochemical cell with compacted cathode containing polyolefin powder additive

Info

Publication number: GB2118762A
Application number: GB08305996A
Authority: GB
Inventors: John Richard Fehling; Pao-Soong Lou
Original assignee: Duracell International Inc
Current assignee: Duracell Inc USA
Priority date: 1982-03-29
Filing date: 1983-03-04
Publication date: 1983-11-02
Also published as: FR2524208A1; IT8320352A0; CA1186373A; ZA831265B; ES8403668A1; DE3309833A1; BR8301564A; GB2118762B; AU1192783A; JPS58178959A; BE896052A; AU554320B2; GB8305996D0; ES520710A0

Abstract

An electrochemical cell having a compacted e.g. extruded cathode wherein small amounts of a powdered polyolefin such as polypropylene are added to the active cathode material prior to compaction. The cathode mixture may comprise graphite, MnO2, polypropylene and KOH electrolyte and may be used with a zinc anode.

Description

SPECIFICATION Electrochemical cell with compacted cathode containing polyolefin powder additive This invention relates to electrochemical cells having compacted cathodes and more particularly to alkaline electrolyte cells having extruded cathodes.

Cathodes for cells such as alkaline electrolyte cells have been constructed by various compaction procedures whereby active materials such as mercuric oxide (HgO), silver oxide (Ag2O) and manganese dioxide (MnO2) are pelletized or extruded into desired configurations. Cathode pellets are generally constructed under relatively high pressures (1 0,000 psi or more) to form independent structures. In the extrusion procedure a cathode active material together with conductive additives such as graphite or carbon and the cell electrolyte are admixed and placed in a cell container. The mixture is then extruded against the cell container walls by a punch at pressures of about 1200 psi. Since the extruded cathode is not structurally independent lower compacting pressures are adequate.

In both the pelletizing and extrusion procedures the cathode active materials are admixed with graphite or carbon for both conductivity and for lubrication during the compacting. However, despite the presence of the graphite or carbon as a lubricant in relatively large amounts of generally about 6% or more of the cathode weight several problems have been encountered in the compacting procedures particularly with "wet" extrusions (compaction of a cathode mixture which includes fluid electrolyte).

Additionally, cells made in accordance with such procedures have suffered from reduced delivered capacity because of such problems. The extruded cathodes tend to have large residual bottom thicknesses with such material being generally unavailable for discharge therefore resulting in reduced cell capacity. Furthermore, despite the presence of the lubricating graphite or carbon, the cathodes do not have the entirely desired uniformity of density and as a result cell efficiency is detrimentally affected.

It is an object of the present invention to provide a means whereby the discharge characteristics of cells having compacted cathodes are enhanced.

It is a further object of the present invention to provide such means whereby the procedure of cathode compaction for said cells is also facilitated.

These and other objects, features and advantages of the present invention will become more evident from the following discussion.

Generally, the present invention comprises an electrochemical cell having a compacted cathode wherein, prior to the compaction of the cathode such as by extrusion, a small amount of a finely powdered polyolefin such as polypropylene or polyethylene is added to the cathode materials. It has been discovered that small amounts of the powdered polyolefin, on the order of less than about 3% by weight of the cathode, unexpectedly markedly improve the structure of the compacted cathode and the capacity of the cell containing such cathode. Additionally, particularly in extrusion procedure, substantially reduced extrusion pressure (15% or more reduction) can be employed to form the requisite extruded cathode e.g. an extrusion pressure of about 1000 psi or less is sufficient.

Concomitantly, wear of the extrusion punch is also minimized.

The polyolefin powder additives, which are useful in the practice of the present invention have particle sizes in the finished cell ranging from about 30 microns to about 300 microns. Larger particle sized may, however, be initially added to the cathode mix prior to compaction provided that precompaction procedures (such as mixing with an intensifier bar) comminutes the particles to the requisite dimensions prior to the actual compaction (e.g. extrusion). The amount of the polyolefin powder added to the cathode should be minimal since it is non conductive and, unlike the conductive graphite or carbon, its presence in larger amounts would impair conductivity and capacity of the cathode. The amount of polyolefin powder added to the cathode materials prior to compaction is between about 0.1% to a maximum of about 3% by weight.It is preferred that the amount of polyolefin powder be present in an amount ranging from about 0.25% to about 1% The polyolefin additives of the present invention are chemically stable and may be utilized with generally any powdered and compacted cathode material such as the aforementioned HgO, Ag2O and MnO2 active cathode materials. Because of the stability of the polyolefin additives they may be utilized in various cell environments such as alkaline cells having zinc anodes and sodium or potassium hydroxide electrolyte solutions or non-aqueous electrolyte cells such as those having lithium or other alkali or alkaline earth metal anodes.

In order to more fully illustrate the efficacy of the present invention the following examples are presented. It is understood, however, that such examples are for illustrative purposes only and the invention is not limited to specifics contained therein. Unless otherwise indicated all parts are parts by weight.

Example 1 (prior art) A cathode mix comprised of 76% MnO2, 13.5% graphite (particle size of less than 10 microns), and 10.5% 7.2N KOH electrolyte solution was prepared and a 7.8 gram portion thereof was placed in a cell container (0.55" (1.4 cm) Lox 1.96" 5.0 cm) HT) and extruded therein by a punch at 1200 psi against the cell container wall to form a cathode with an ID of 0.366" (0.93 cm). The cell was completed with the insertion of a closed tubular separator into said cathode, the further insertion of about 3 grams of a gelled amalgamated zinc anode (93% Zn, 7% Hg) into the separator and the addition of about 2 cc of 40% KOH electrolyte. After sealing of the cell it was discharged at a continuous discharge rate of 4 ohms with the results given in Table I.

Example 2 A cell was made in accordance with the procedure and with the materials in Example 1 but with 75.5% MnO2 and 0.5% powdered polypropylene (about 150 micron particle size). The cell was discharged as above with the results given in Table Table I Hours Hours Voltage example 1 cell example 2 cell %Improvement 1.10 1.58 1.77 12 1.00 2.82 2.98 5.7 0.90 3.45 3.73 8.1 0.80 3.89 4.38 12.6 0.75 4.10 4.60 12.2 Example 3 (prior art) A cell was made as in Example 1 but from a cathode mix 74.5% MnO2, 14.5% graphite (about 35 microns particle size) and 10.5% 7.2N KOH and discharged at a continuous rate of 4 ohms. The results are given in Table II.

Example 4 A cell was made as in Example 3 but with 74.5% MnO2 and 0.5% powdered polypropylene (about 150 micron particle size in the cathode mix). The cell was discharged as above and the results are given in Table II.

Table II Hours Hours Voltage example 3 cell example 4 cell % Improvement 1.10 1.52 2.05 34.9 1.00 2.60 3.26 25.4 0.90 3.42 4.05 18.4 0.80 3.92 4.63 18.1 0.75 3.92 4.78 21.9 It is readily apparent from the above examples that despite the reduction of active cathode materials in the cathodes, the cells of Examples 2 and 4 (having the additive of the present invention) are markedly superior at all cutoff voltages. It is further evident from Examples 3 and 4 that despite changes in amount and type of graphite and the reduction of active cathode material which causes reduction in capacity of the cell in Example 3, the capacity of the cell in Example 4 actually exhibits improved capacity.

It is understood that the above examples are for illustrative purposes only and that further changes in cell and cathode compositions and constructions may be made without departing from the scope of the present invention as defined in the following claims.

Claims

1. A method for improving the compacting of cathode for an electrochemical cell, comprising adding a particulate polyolefin to a cathode mixture comprising a particulate cathode active material and a particulate electronically conductive material, prior to compacting of the mixture, in an amount ranging between 0.1% of 3% of said cathode by weight.

2. The method of claim 1 wherein said compacting comprises extrusion of said cathode within a cell container.

3. The method of claim 2 wherein said extrusion is a wet extrusion.

4. The method of claim 1, 2 or 3 wherein said polyolefin is selected from polyethylene and polypropylene.

5. The method of any preceding claim wherein said conductive material is selected from graphite and carbon.

6. The method of any preceding claim wherein said polyolefine has a particle size ranging between 30 and 300 microns.

7. A method of making a cathode for an electrochemical cell, substantially as set forth in the foregoing Example 2 or 4.

8. A cathode for an electrochemical cell made in accordance with the method of any of claims 1 to 7.

9. The cathode of claim 8 wherein said cathode active material is selected from HgO, Ag2O and MnO2.

10. An electrochemical cell containing the cathode of claim 8 or 9.

11. An electrochemical cell containing a zinc anode, an alkaline electrolyte comprising KOH, and an extruded cathode comprising MnO2, graphite and from 0.1% to 3% of a polypropylene powder having a particle size ranging between 30 and 300 microns.