Manganese dioxide electrodes, process for producing the same...

Details Manganese dioxide electrodes, process for producing the same... Manganese dioxide electrodes, process for producing the same...

1999-10-05

2002-01-08

Gupta, Yogendra N. (Department: 1751)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C252S500000, C252S503000, C252S506000, C252S518100, C252S519200, C252S520100, C429S209000, C429S218200, C429S222000, C429S224000, C429S229000, C429S232000

Reexamination Certificate

active

06337160

ABSTRACT:

The invention relates to novel manganese dioxide electrodes comprising modified, electrochemically active manganese dioxide, to a method for fabricating these novel manganese dioxide electrodes and to their use in rechargeable cells.
Typical components of an alkaline primary cell are a cathode consisting of manganese dioxide, an anode preferably made of zinc, an alkaline electrolyte and an electrolyte-permeable separator material. The manganese dioxide which is customarily used for fabricating the cathode, being an electrolyte manganese black having a &ggr;-structure, exhibits very high activity. The discharge mechanism of manganese dioxides has been studied very thoroughly in the past. An overview of such studies is provided, for example, by A. Kozowa in: Batteries, Vol. 1, Manganese Dioxide, Ed.: K. V: Kordesch, Chapter 3, Marcel Dekker 1974. The use of manganese black in alkaline zinc-MnO
2
cells from a commercial point of view has been set forth by R. F. Scarr and J. C. Hunter in: Handbook of Batteries, Ed. D. Linden, Chapter 10, McGraw-Hill 1995.
To increase the electrical conductivity, such manganese dioxide electrodes are customarily admixed with particles of coal, carbon black or graphite. Organic or inorganic additives are used as binders.
The zinc electrode as a rule consists of zinc powder having a large surface area and of a gelling agent, e.g. carboxymethylcellulose, as the stabilizer. Also known, however, are cold-pressed or hot-pressed elctrodes, with or without a binder, or sintered zinc powder electrodes. In addition to amalgamated zinc anodes, mercury-free zinc anodes are increasingly being used.
The alkaline electrolyte commonly consists of an aqueous potassium hydroxide solution. Alternative options, however, are solutions of other hydroxides such as sodium hydroxide solutions or lithium hydroxide solutions and mixtures thereof.
The separator material present between the electrodes has the purpose of electronically insulating the two electrodes.
On economic and ecological grounds, attempts have been made for some time to fabricate a rechargeable alkali-manganese cell.
The greatest problem in this context has so far been posed by the manganese dioxide electrode, since it does not have an adequate discharge/charging cycle stability. During discharge, protons are inserted into the MnO
2
lattice, as a result of which &ggr;-MnO
2
is reduced to the isostructural &agr;-MnOOH. This takes place in a homogeneous solid-phase reaction and entails expansion of the manganese black structure. If a further discharge occurs, the stability limit is exceeded, i.e. the expanded structure collapses and phases are formed which have a different crystal structure (e.g. Mn(OH)
2
, Mn
3
O
4
). This is an irreversible collapse of the MnO
2
crystal lattice. After this, recharging is no longer possible. Changes in the lattice structure of the &ggr;-MnO
2
commence at a degree of intercalation of about 0.8 protons per manganese atom. In the case of known rechargeable MnO
2
—Zn cells this is taken into account by the discharge being limited either by the final discharge voltage of about 0.9 V or by the cyclable amount of zinc (K. Kordesch; K. Tomantschger and J. Daniel-Ivan in: Handbook of Batteries, Ed.: D. Linden, Chapter 34, McGraw-Hill 1995).
DE 33 37 568 A1 describes the preparation of &ggr;-MnO
2
by direct-current electrolysis from manganese(II) salt solutions, which had been admixed with titanyl ions. Using the MnO
2
thus produced, it is possible, according to the description, for the number of discharge/charging cycles for a discharge level of 33% in alkaline aqueous electrolytes to be more than 100 cycles, more than doubled that of the reference manganese black (International Common Sample (ICS) No. 2) at 42 cycles. The publication points out that it is not possible simply by admixing the manganese black with titanium dioxide powder, to effect an improvement in the cycling capability.
EP 0 146 201 A1 describes the addition of bismuth ions and lead ions to the manganese black, as an activator. This publication claims the intensive mechanical admixing of ICS manganese black and of Na birnessite (Na, Mn
x
O
4
) with Bi
2
O
3
(or Bi
2
S
3
) and PbO. Given a discharge level of 50% (based on the theoretical 2 e
−
capacity) the material obtained exhibits good cyclability. The electrodes fabricated using the material described do have a very high excess of graphite, and consequently their use in practice is likely to be problematic. The publications EP 0 136 172 A2 and EP 0 136 173 A2 describe the use of corresponding modified manganese dioxides in MnO
2
—Zn cells.
PCT/WO92/14273 A1 describes a method for preparing manganese black which is rechargeable in aqueous solution, said method involving the impregnation or mixing of conventional &ggr;- or &bgr;-manganese black with an aqueous solution of bismuth ions, lead ions or copper ions.
PCT/WO93/12551A1 discloses barium additives, especially barium oxide, barium hydroxide and barium sulphate as an additive to cathode pastes for alkaline zinc-manganese dioxide cells. As a result of the addition of these barium compounds, higher cumulative capacities are achieved compared with standard cells. The method of operation is described by stating that the barium ions located in the vicinity of active cathode material impede access of the zinc ions to the manganese dioxide and thus prevent or retard the formation of electrochemically entirely inactive hetaerolite (ZnO×Mn
2
O
3
). Also discussed is a positive effect on the porosity of the manganese black electrode. Corresponding barium additives are also described in U.S. Pat. No. 5,424,145 A. This publication likewise discloses the addition of oxides, spinels and perovskites, especially compounds of nickel, cobalt, aluminium, zinc, iron, manganese, chromium, vanadium, titanium and silver. The purpose of these additives is to protect the manganese dioxide against overcharging and to prevent further oxidation to higher-valence manganese compounds such as manganates.
It is therefore an object of the invention to provide cyclically stable MnO
2
electrodes which are suitable for use in rechargeable alkaline cells. Another object of the invention is to provide, for the fabrication of manganese dioxide electrodes, inexpensive, suitably modified manganese dioxide which, given an effective discharge level has adequate cycle stability, ideally without any change in the lattice structure, exhibits improved reversibility characteristics compared with conventional material while discharge times are extended and cell voltages during the discharge are increased, this applying both to high and to low discharge rates. A further object of the invention was to provide an inexpensive and simple method for fabricating these modified manganese dioxide electrodes.
This object is achieved by manganese dioxide electrodes fabricated from a conventional manganese dioxide whose characteristics have been modified beforehand by the addition of coated inorganic particles.
These coated inorganic particles may be of a type where the support particles consist of a material selected from the group consisting of mica, SiO
2
, Al
2
O
3
, ZrO
2
and ZnO. Single or multiple coatings of these particles may be composed of dielectric substances and in particular of ferroelectric, piezoelectric or pyroelectric substances. Such coatings may consist of titanates, stannates, tungstates, niobates or zirconates; also possible, however, are silicate coatings, depending on the type of the base particles selected. Particles with coatings from mixtures of these substances are likewise suitable. Suitable inorganic particles may also have coatings consisting of metal oxides from the group consisting of Fe
2
O
3
, NiO, CoO, ZrO
2
, SnO
2
, TiO
2
, Sb
2
O
3
, PbO, Pb
3
O
4
, Bi
2
O
3
, or mixtures thereof. The individual coatings which, per se, consist of one substance may be doped with foreign ions, an example being SnO
2
coatings doped with foreign ions. The manganese dioxide used as the base material may be present in a structu

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