WO2000011739A1

WO2000011739A1 - Heat treated fine carbon for alkaline manganese cathodes

Info

Publication number: WO2000011739A1
Application number: PCT/US1998/008825
Authority: WO
Inventors: Lewis F. Urry
Original assignee: Eveready Battery Company, Inc.
Priority date: 1998-05-01
Filing date: 1998-05-01
Publication date: 2000-03-02
Also published as: EP1080507A1; AU7172998A

Abstract

Heat treated fine carbon has small particle size and can be used as the electroconductive element in cathodes of electrochemical cells to reduce the volume taken up by non-active materials by increasing contact between the active material and the electroconductive element.

Description

HEAT TREATED FINE CARBON FOR ALKALINE MANGANESE CATHODES

BACKGROUND OF THE INVENTION

The present invention generally relates to a heat treated fine carbon as a

conductor in a cathode mixture of electrochemical cells.

Conventionally, the positive electrode of alkaline manganese batteries

comprises mixtures of electrolytic manganese dioxide (EMD) as the positive electrode

active material, and carbon as the electroconductive material. The electroconductive

material is necessary because the specific conductivity of manganese dioxide alone is

extremely low. When electroconductive carbon materials are used in large quantities, the quantity of manganese dioxide that can be used in a battery's fixed internal volume is decreased. Consequently, the discharge capacity density of the battery is decreased

to a very great extent. On the other hand, when an insufficient amount of the

electroconductive carbon is used, there is decreased contact between the manganese

dioxide and the carbon. This results in a decreased electron conduction network, and

the overall utilization rate of the manganese dioxide in the electrode is thereby

decreased. By using a finer conductor material, especially compared to the size of the manganese dioxide, a lesser amount of conductor material is needed to get an

adequate electron matrix. The finer particle-size particles reduce the volume percent

solids without reducing the input per volume of active materials, or increases the input

of active materials per unit volume of the solids packing. The advantages of using a

very fine conductor material are well known, but difficult to achieve. Graphite has been widely used as a conductor and has the advantage of being highly oxidation resistant. However, due to graphite's particle size (the smallest

particle size commercially available is about 2.5 microns), a large volume percent of the graphite conductor is necessary to provide electron distribution to the active

material throughout discharge. One approach to reducing the minimum volume

percent of the graphite conductor is to grind it finer; however, this is difficult to do and costly. Moreover, the finest ground material commercially available is still not

less than 0.5 micron.

Acetylene black, the finest of the carbon blacks, has also been used as a

conductor, and has the advantage of finer particle size. However, conventional

acetylene black is oxidized much faster by Mnθ2 than graphite, and thus,

storageability suffers as Mnθ2 capacity decreases and carbonate is produced.

Optimization of the mixing ratios cells using conventional acetylene black and

manganese dioxide for high efficiency is taught by U.S. Patent No. 5,017,445. This

reference teaches using an acetylene black of a limited specific surface area, with the

weight ratio of manganese dioxide:acetylene black ranging from 7: 1 to 12:1.

It is desirable to have a fine carbon conductor that has the optimum properties

good oxidation resistance and permits good electrochemical performance at a very

low volume percent (<6%) of the positive electrode.

SUMMARY OF THE INVENTION

This invention is an electrochemical cell having an anode, a cathode, and an

electrolyte, wherein the cathode comprises a heat-treated fine carbon as an electronic conductor.

In yet another aspect, this invention is an alkaline cell having a cathode

comprising a conductor at less than 6 volume percent of the positive electrode.

In still another aspect, this invention is an alkaline cell having a cathode

mixture comprising fine carbon having a high oxidation resistance of less than 30

milliliters ^C^O /gram, as determined by a potassium dichromate digestion test

described herein.

The alkaline cell of this invention has a cathode that has good oxidation

resistance, and good electrochemical performance at a low volume percent of the

positive electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph of effect of particle size on oxidation resistance of heat treated fine

carbon vs. graphites.

Figure 2 is a graph comparing three D-size cells containing different types of

conductor materials.

DESCRIPTION OF THE INVENTION

According to the present invention, a mixture comprising a heat treated fine

carbon with manganese dioxide provides an improved positive electrode for alkaline

cells. The heat treated fine carbon of this invention can be produced using a fine

carbon material such as acetylene black, that is treated, for example, at a temperature of about 3000 C in an inert atmosphere for about one hour. The resulting heat treated fine carbon particles have an average particle size diameter in the range of about 50

Angstroms to about one micron, as measured by scanning electron microscopy. The

fine carbon particles facilitate formation of an electronically conductive network, with a lesser volume percent of the conductor as compared to graphite, thereby resulting in

enhanced utilization of the manganese dioxide particles on the overall mixed positive

electrode active material.

The fine carbon, as used in this invention, can be obtained from various

commercial sources such as Chevron Corporation, Cabot Corporation, Denka

Corporation, Sedema Corporation, to name a few. After subjecting the fine carbon to

heat treatment, the carbon particles have improved oxidation resistance over

conventional fine carbon, as measured by the following potassium dichromate test.

A gram of the material to be evaluated is accurately weighed in a 100 milliliter volumetric flask. A mixture of 75 ml 0. 1 N ^C^Oγ

solution and 15 ml of 1 : 1 H2SO4 is added to the flask. The flask is

sealed with a stopper and transferred to a water bath, maintained at

95 ° C for 4 hours. The flask is removed from the water bath and cooled

on ice. A 1 :1 solution of H2SO4 is made, and 10 milliliters of the

H2SO4 solution are added to the flask, and mixed thoroughly. A

portion of the solution is centrifuged. An aliquot of the centrifuged

clear solution is titrated with 0.1 N ferrous ammonium sulfate using

sodium diphenylamine sulfonate as

indicator. A solution blank is run along with the samples. The

normality factor is calculated from the blank which converts milliliters Fe++ to milliliters Cr O₇ ^"2. The normality factor is used to calculate

the number of milliliters of potassium dichromate solution consumed

per gram of sample in a given time.

The oxidation resistance is expressed as the number of milliliters of K Cr O

solution consumed per gram of sample. The lower the value, the greater its resistance

to oxidation. The heat treated fine carbon according to this invention is 4.5 times more

resistant to oxidation compared to the starting material.

RESISTANCE TO OXIDATION

Oxidation resistance (milliliters of K₂Cr O

Carbon Material consumed per gram of sample)

Chevron Acetylene Black - conventional 118.3 mL/gram

Chevron Acetylene Black - heat treated 25.83 mL/gram

DETAILED DESCRIPTION OF DRAWINGS

Figure 1 is a graph of effect of particle size on oxidation resistance of heat

treated fine carbon vs. graphites. As can be seen by this graph, the carbon after

graphitization had the same oxidation resistance as graphite per unit surface area or

particle size.

Figure 2 is a graph comparing three D-size cells containing different types of

conductor materials. Cell 1 has heat treated acetylene black as a conductor at 5.8

volume percent; cell 2 has graphite as a conductor at 13.7 volume percent; and cell 3

has combined graphite and conventional acetylene black. EXAMPLES

Three D-size alkaline cells are constructed for comparison. A first cell is constructed using an acetylene black in the cathode mixture. The

acetylene black, obtained from Chevron Corporation, is subjected to heating at

3000 ° C for one hour in an inert atmosphere. The resulting heat treated acetylene

black particles, having average diameter of about 75 Angstroms, is mixed with electrolytic manganese dioxide active material and teflon as a binder, in the relative

volume ratios of 7.7 volume percent carbon, 90.9 volume percent of electrolytic

manganese dioxide, 1.4 volume percent teflon. The mixture is then packed to a solids

packing of 75 volume percent, so that in the finished cathode, carbon is present at 5.8

volume percent, electrolytic manganese dioxide is 68.2 volume percent, teflon is 1.0

volume percent, with the remainder being non-solids, mainly as electrolyte and void

volume.

A second comparative cell is constructed using a cathode comprising 12

volume percent graphite, 62 volume percent electrolytic manganese dioxide, and 1

volume percent teflon as binder.

A third comparative cell is constructed using a cathode comprising 14 volume

percent combined graphite and conventional acetylene black, 54 volume percent

electrolytic manganese dioxide, 7 volume percent inorganic binder.

Thus, all three cells were constructed with the cathode having a total 75

volume percent solids. The remaining 25 volume percent represents non-solids

electrolyte filling at cell assembly. The three cells were subjected to a 2.3 ohm

continuous discharge. The results, as can also be seen in Figure 3, show that the cell

comprising the heat treated acetylene black exhibited a much higher discharge voltage

profile than cells with conventional acetylene black cathode mixture. Although the present invention has been fully described by way of example

with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the ait. Therefore,

unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.

Claims

CLAIMS:

1. An electrochemical cell having an anode, a cathode, and an electrolyte, said

cathode comprising a heat treated fine carbon.

2. An electrochemical cell having an anode, a cathode, and an electrolyte, said cathode comprising less than 6 volume percent of a conductor present in the cathode.

3. The electrochemical cell according to claim 1, wherein the fine carbon is

acetylene black.

4. The electrochemical cell according to claim 1 , wherein the fine carbon has an

average particle diameter from about 50 Angstroms to about 1 micron.

5. The electrochemical cell of claim 1, wherein the fine carbon has an average

particle diameter between about 50 Angstroms to about 200 Angstroms.

6. The electrochemical cell according to claim 1 , wherein the fine carbon has an

oxidation resistance of less than 30 milliliters 0.1 N potassium dichromate digested

per gram of carbon, as measured by a potassium dichromate test.

7. The electrochemical cell according to claim 1, wherein the electrolyte is

potassium hydroxide.

8. An electrochemical cell having an anode, a cathode, and an electrolyte, said

cathode comprising a heat treated fine carbon having an oxidation resistance of less

than 30 milliliters 0.1 N potassium dichromate digested per gram of carbon, as

measured by a potossium dichromate test.

9. The electrochemical cell according to claim 2, wherein the fine carbon is

acetylene black.

10. The electrochemical cell according to claim 2, wherein the average particle

diameter of the heat treated carbon is from about 50 Angstroms to about 1 micron.

11. The electrochemical cell of claim 2, wherein the conductor has an average particle size between about 50 Angstroms to about 200 Angstroms.

12. The electrochemical cell according to claim 2, wherein the conductor has an oxidation resistance of less than 30 milliliters 0.1 N potassium dichromate digested

per gram of carbon, as measured by a potassium dichromate test.

13. An electrochemical cell having an anode, a cathode, and an electrolyte, said

cathode comprising a conductor having an oxidation resistance of less than 30

milliliters 0.1 N potassium dichromate digested per gram of carbon, as measured by a

potassium dichromate test.