CA2137965A1

CA2137965A1 - Ni/metal hydride secondary cell

Info

Publication number: CA2137965A1
Application number: CA002137965A
Authority: CA
Inventors: Frank Lichtenberg; Klaus Kleinsorgen; Gunter Hofmann
Original assignee: Individual
Current assignee: VARTA Batterie AG
Priority date: 1993-12-18
Filing date: 1994-12-13
Publication date: 1995-06-19
Also published as: CN1107259A; DE4343322A1; JPH07211317A; KR950021843A; EP0658947A1

Abstract

Abstract of the Disclosure The massive irreversible loss of capacity of conventional Ni/metal hydride button cells with positive compacted-powder electrodes produced from a mixture of Ni(OH)2, Ni powder and oxide compounds of cobalt, when subjected to a relatively severe high temperature short circuit test (at 65°C and after 6 cycles), is made smaller and only temporary if the positive mass is additionally admixed with from 0.1 to 15% of manganese and/or a manganese oxide. The addition of manganese is made to prevent reductive destruction of the previously formed conductive CoOOH matrix under the conditions of the high temperature short circuit test, by stabilizing the matrix or promoting its ability to regenerate.

Description

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: ' "' ' Ni/METAL HYDRIDE SECONDARY. CELL

Backqround of the Invention The present invention relates to secondary cells of the Ni/metal hydride type, which generally employ a negative electrode capable of electrochemically storing hydrogen, and a corresponding positive electrode of a type used in conventional secondary batteries 6uch as the Ni/Cd accumulator. The positive and negative electrodes are arranged in an alkaline electrolyte, and are appropriately separated from one another to develop an operating cell.
When an electric current is applied to the negative electrode of a Ni/metal hydride cell, the active material of the electrode (i.e., a metal N capable of absorbing hydrogen) is charg~d by the absorption of hydrogen:

:
M + H20 + e ~ M-H + OH (1) -, During discharging, the stored hydrogen is released so that an electric current is generated:
' M-H + OH~ M + H20 + e ( 2 ?

Both reactions are reversible.
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~u A similar situation applies to the reactions taking -:
~ place at the positive nickel hydroxide electrode: ~
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~137~5 Ch~rging: Ni(oN~7 + OH ~ NiOOH ~ HzO + e~ (3) Discharging: NiOoH + H2O + e~ I Ni~OH)2 + OH (4) Ni/metal hydride batteries, even today, offer distinct advantages compared to conventional secondary batteries. This is because an environmentally friendly energy source, hydrogen, is used in conjunction with the well-established, in principle identical positive electrode of a Ni/Cd accumulator.
Ni/metal hydride batteries are gas-tight and maintenance-free. Due to rather advanced electrode manufacturing technologies, such cells have already attained energy densities , :
of 50 Wh/k~. Here, the quality of the positive electrode plays a particularly important part, since it limits the capacity.
According to present standards, for example, in accordance with U.S. Patent No. 4,935,318, the positive active ~15 mass of a Ni/metal hydride cell includes nickel hydroxide, nickel ~etal powder, cobalt metal powder, certain foreign hydroxides, in particular Co(OH)2, and a binder, which are mixed in a dry state.
The dry mixture is then mixed with water, to form a paste, which is spread into a highly porous three-dimensional nickel matrix.
An addition of cobalt hydroxide to the positive mass serves to generate an excess capacity, which is required by the negative hydride electrode to develop a discharge reserve. When the as yet unsealed cell is charged for the first time, the fact that Co(OH)2 has a lower oxidation potential than Ni(oH)2 results in the formation of higher cobalt oxides (Co2O3) prior to the ~. ~, ~nc~ -, ~UO~O~ ~ .

~137965 conversion of Ni(OH) 2 to NiOOH. The negative electrode is thus given a pre-charge which corresponds to the total charge supplied. As a result, if, after subsequent discharge the useful capacity of the positive electrode is exhausted, the cobalt oxides remain stable so that the negative electrode still has a discharge reserve (due to its pre-charge). What is more, cobalt oxides are electron conductive, which has a beneficial effect on the capacity behavior of the mass.
However, a serious problem is presented by most commercial nickel/hydride cells, whether they are round cells, prismatic cells or of the button cell type. Specifically, such cells do not, as a rule, withstand the high temperature short circuit (HTSC) test which is conventionally performed by battery customers in the industry. In this test, cells conditioned by a ~
few ~tart-up cycles are short-circuited (in the discharged state) -with a 2n re~istor and ~tored ~or 3 days at 65C. Cycling is ; ;
then continued at room temperature, and the residual capacity is .
determined. Ordinarily, massive irreversible losses in capacity are observed in this process.

` 20 Summarv of the Invention ~''"',',, It is therefore the primary object of the present ~-invention to provide an alkaline secondary cell of the Ni/metal hydride type which can stand up to HTSC testing.

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- This and other objectives are achieved with a Ni/metal hydride secondary cell having a positive nickel hydroxide electrode, a negative electrode comprised of a hydroger. storage alloy and an alkaline electrolyte. In accordance with the present invention, the positive electrode is a three-dimensional metallic conductive matrix formed predominantly of nickel hydroxide, and the active mass of the positive electrode contains additions of manganese, oxide compounds of manganese, ternary alkali metal/manganese oxides, and combinations thereof, in addition to the more conventional additions of cobalt, oxide compounds of cobalt, and combinations thereof.
For further detail regarding the Ni/metal hydride secondary cell of the present invention, reference is made to the following detailed description, and a single figure illustrating lS a graph showing comparative performance characteristias.
' Detailed De~cription of Preferred Embodiments In accordance with the present invention, the loss in capacity of conventional Ni/metal hydride cells is overcome with a modified composition for the positive active mass. To this end, the mass mixture, which is predominantly (i.e., to more than 50%) comprised of Ni(OH) 2 and conventional additions of cobalt metal powder and oxidic compounds of cobalt (e.g., CoO), is further provided with additions of metallic manganese or oxide w~u~compounds of manganese, both individually and mixed together.
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Possible oxide compounds include any of the manganese oxides MnOx (e.g., manganese dioxide, MnO2, for x=2), as well as the aqueous derivatives of manganese oxide6, MnO~. This may (where y = z) include any of the manganese hydroxides, and (where y $ z) any of the oxide hydrates of manganese (e.g., ~-MnOOH, manganite).
Ternary alkali metal/manganese oxides likewise form suitable additions. This group includes, for example, lithium manganese spinels of the type Li~Mn204.
Also usually present in the mass mixture of the positive electrode is nickel powder, as a conductive material, and a binder (e.g., in the form of PTFE).
Apart from the addition, in accordance with the present invention, of manganese and/or its oxide compounds to the cobalt-containing positive nickel hydroxide mass, additions of further ~-. -., foreign metals such as Zn, Cd and Sn (preferably in the form of ~ :
;~ oxidic compounds) are also beneficial. ~
The proportion by weight of all of the cobalt- ;-containing constituents in the mass mixture (as a whole) is - ;
preferably from 0.1 to lS%. The proportion by weight of all of the manganese-containing constituents in the mas6 mixture should ~-likewise be from 0.1 to 15%. The latter proportion includes any ~;~
oxide compounds of Zn, Cd and Sn which are present, and the metals themselves.
The oxides of manganese, and of the other metals mentioned, are (like their hydroxides and oxyhydroxides) ~u electroconductive and/or are converted by electrochemical ;~
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. :,' , treatment to their higher-valent or their low~r-valent oxidation species. This is accomplished without a significant deterioration of the properties of the positive electrode as a result.
The success of the foregoing improvements suggests that the loss in capacity triggered by high temperature short circuit testing is caused by the positive electrode. Under the test conditions previously mentioned, the potential of the positive electrode can decline toward more negative values, and the electrically conductive matrix (which is composed of a compound of the type, CoOOH, built up electrochemically) is reductively de~troyed. This destruction is counteracted by the manganese- -containing additions made to the mass mixture, evidently by stabllization or enhancement of the ability to regenerate the co uctive ~atrix, CoOOH. Evidence of this possible "manganese ; ef~ect" is presented by the following experimentation, performed ; on button cells (which, because they are readily fitted, were selected a~ representative of all Ni/metal hydride cells).
To produce cells in accordance with the present ~
~`20 invention, positive electrodes were produced from a mixture of ;~;
- ~ 60~ by weight of Ni(OH)2, 9% by weight of CoO, 1% by weight of -Co, 27% by weight of Ni and 3% by weight of MnO2. The test electrodes weré produced in~the usual way, as compacted-powder `-pellets with a nickel fabric casing. Prior art comparison electrodes were produced in the same manner, but did not contain .~ MnO2 and instead contained an additional 3% by weight of Ni.
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2137~fi5 ~, Prior to assembly, all the positive electrodes were f stored in a Co-containing alkali (KOH) for approximately 20 hours at 80C. Thereafter, the positive electrodes were assembled together with negative electrodes made from a hydrogen-storing 5alloy powder, which were likewise pressed to form pellets. After assembly, the "manganese cells~' of the present invention and the "standard cells" were put into service and, after the 6th cycle, were sub~ected to an HTSC test. After the HTSC test, cycling of the cells was continued for an additional 5 cycles under the 10initial conditions. The charging and discharging currents were .-, ...... ....
S0 mA, at all time~, and the end-of-discharge voltage was 0.7 V. -The results of such testing are illustrated in the single figure of the drawings, which shows the capacity development in both cell groups (capacity C[Ah] represented as a function of tn] cycles). In each case, the illustrated curves ~-are averaged from data measured for the 6 cells from one or the other groups of cells under test. ~
AB i8 apparent from curve 1, representing the HTSC test ;
for the standard cells, there is a serious drop in capacity from which the cells do not recover in the further course of their --cyclic treatment. In contra6t, and referring now to curve 2, there i8 a much smaller decline in capacity of the "manganese cells" of the presenb invention, and within 2 to 3 subseguent cycles, a recovery takes place in which the original capacity is regained.

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An explanation of this "manganese effect" is not presently available~ However, it is believed that in the absence of manganese in conventional Ni/metal hydride secondary cells, the higher cobalt oxides which were originally formed and which have a capacity-increasing effect, do not remain stable under the special conditions of an HTSC test, but are in part reduced to metallic Co. Due to its residual oxidation potential, the mangane~e possibly prevents such reduction by promoting the restoration of CoOOH.
It will be understood that various changes in the details, material~; and arrangement of parts which have been herein described and illustrated in order to explain the nature of this invention may be made by those skilled in the art within the principle and scope of the invention as exprqssed in the fo110wing c1aims.

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Claims

What is claimed is:

1. A Ni/metal hydride secondary cell having a positive nickel hydroxide electrode, a negative electrode comprised of a hydrogen storage alloy, and an alkaline electrolyte, wherein the positive electrode is a three-dimensional metallic conductive matrix formed predominantly of nickel hydroxide, having an active mass which includes a manganese-containing addition selected from the group consisting of manganese, oxide compounds of manganese, ternary alkali metal/manganese oxides, and combinations thereof.

2. The secondary cell of claim 1 wherein the oxide compounds of manganese are manganese oxides having the composition MnOx.

3. The secondary cell of claim 1 wherein the oxide compounds of manganese are manganese hydroxides having the composition MnOyHz, where y = z.

4. The secondary cell of claim 1 wherein the oxide compounds of manganese are manganese oxyhydroxides or manganese oxide hydrates having the composition MnOyHz, where y ? z.

5. The secondary cell of claim 1 wherein the oxide compounds are ternary alkali metal/manganese oxides.

6. The secondary cell of claim 1 wherein the active mass of the positive electrode additionally includes an oxide compound of an element selected from the group consisting of Cd, Zn and Sn.

7. The secondary cell of claim 1 wherein the metallic conductive matrix of the positive electrode is formed from nickel powder or a nickeled substrate.

8. The secondary cell of claim 1 wherein the active mass is admixed with a binder.

9. The secondary cell of claim 1 wherein the active mass of the positive electrode further includes a cobalt-containing addition selected from the group consisting of cobalt, oxide compounds of cobalt, and combinations thereof.

10. The secondary cell of claim 9 wherein the cobalt-containing additions have a proportion by weight of the total mass of the positive electrode of from 0.1 to 15%, and the manganese-containing additions, including any additional oxide compounds, have a proportion by weight of from 0.1 to 15%.