Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-12-11
2002-09-24
Nuzzolillo, Maria (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S232000, C429S236000
Reexamination Certificate
active
06455196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non-sintered positive electrode for alkaline storage batteries and an alkaline storage battery using the same.
2. Description of Related Art
Recently, with the spread of portable apparatuses, alkaline storage batteries are strongly demanded to be high in capacity. Particularly, nickel-metal hydride storage batteries are secondary batteries comprising positive electrodes mainly composed of nickel hydroxide and negative electrodes mainly composed of a hydrogen-storing alloy, and these batteries have rapidly spread as secondary batteries of high capacity and reliability.
The positive electrodes for alkaline storage batteries will be explained.
The positive electrodes for alkaline storage batteries are roughly classified into sintered type and non-sintered type. The former are made by impregnating a nickel sintered substrate of about 80% in porosity obtained by sintering a core material such as punching metal and a nickel powder with a nickel salt solution such as an aqueous nickel nitrate solution and subsequently with an aqueous alkaline solution, thereby to produce nickel hydroxide in the porous nickel sintered substrate. In these positive electrodes, the porosity of the substrate is difficult to further increase, and amount of nickel hydroxide cannot be increased. Thus, there is a limit in enhancement of capacity.
As the latter non-sintered type positive electrodes, JP-A-50-36935 proposes those which comprise a foamed nickel substrate of about 95% in porosity comprising three-dimensionally communicating pores in which nickel hydroxide particles are held and which are being widely used as positive electrodes for alkaline storage batteries of high capacity. In these non-sintered positive electrodes, spherical nickel hydroxide particles having a high bulk density are used from the point of increase in capacity. As disclosed in JP-A-62-136761, particle size of the nickel hydroxide particles and size of the pores of the foamed nickel substrate are adjusted to proper values. Moreover, metallic ions such as cobalt, cadmium and zinc are usually dissolved in the nickel hydroxide particles in the state of solid solution for the improvement of discharge characteristics, charge acceptance and life characteristics.
Here, since the size of the pores of the foamed nickel substrate is set sufficiently larger than the particle size of nickel hydroxide, the charge and discharge reaction smoothly proceeds in the nickel hydroxide particles present near the substrate skeleton which maintains current collection, but the reaction does not sufficiently proceed in the nickel hydroxide particles apart from the skeleton. Therefore, in non-sintered positive electrodes, the nickel hydroxide particles are electrically connected to each other using a conductive agent to improve utilization ratio of the packed nickel hydroxide particles. In many cases, divalent cobalt oxides such as cobalt hydroxide and cobalt monoxide are used as the conductive agent. These divalent cobalt oxides per se have no electrical conductivity, but are electrochemically oxidized to &bgr;-cobalt oxyhydroxide having conductivity in the initial charging in the battery, which functions as an electrically conductive network. Thanks to the presence of the conductive network, the utilization ratio of active material packed at high density can be greatly increased in the non-sintered positive electrodes and thus increase of capacity can be attained as compared with in the sintered positive electrodes.
However, even the non-sintered positive electrodes having the above construction are not complete in current collecting performance of the conductive network and have their upper limits in utilization ratio of nickel hydroxide particles. Furthermore, the above positive electrodes suffer from the problems that when the battery is overcharged or kept with being short-circuited or stored for a long period or at high temperatures, the capacity of the positive electrodes lowers by the subsequent charging and discharging. This is because the electrochemical oxidation reaction in the battery as mentioned above cannot completely change a bivalent cobalt oxide to &bgr;-cobalt oxyhydroxide and besides the function of the conductive network is apt to deteriorate.
Recently, as a means for improving the incompleteness of the conductive network, JP-A-8-148145 and U.S. Pat. No. 5,629,111 disclose a method which comprises heat treating (oxidizing) cobalt hydroxide in the active material of the positive electrode in the presence of an aqueous alkaline solution and oxygen (air) outside the battery to modify the cobalt hydroxide to a cobalt oxide having a disordered crystal structure and an oxidation number higher than 2. Similarly, JP-A-9-147905 discloses improvement of cobalt oxides having a cobalt valence of 2.5-2.93, and JP-A-9-259888 discloses characteristics of a battery made using &bgr;-cobalt oxyhydroxide prepared in the similar manner.
Moreover, the above-mentioned JP-A-8-148145 additionally mentions application of the similar heat treatment to nickel hydroxide solid solution particles having a coating layer of cobalt hydroxide (hereinafter referred to as “Co(OH)
2
-coated Ni particles”). This process has an advantage that amount of cobalt used can be reduced because of improvement in dispersion of cobalt by previously preparing the Co(OH)
2
-coated Ni particles. On the other hand, as to the method of preparation in this case, JP-A-9-73900 discloses a method which comprises heating Co(OH)
2
-coated Ni particles containing an aqueous alkaline solution in a fluidized granulator under fluidization or dispersion. This treatment has the advantage that troubles such as formation of particle lumps due to agglomeration can be diminished.
The main object of the above-mentioned techniques published recently is basically that cobalt oxidation reaction which takes place at the initial charging of batteries is sufficiently performed outside the battery, since the reaction does not satisfactorily proceed under normal conditions. Accordingly, the defect caused by the incompleteness of the conductive network referred to above can be improved.
However, the above-mentioned cobalt oxide cannot be said to be complete in oxidation state and further improvement is required.
BRIEF SUMMARY OF THE INVENTION
The inventors have perceived the above points and conducted detailed experiments and analyses, and, as a result, have found that characteristics of an active material for positive electrodes which comprises &ggr;-cobalt oxyhydroxide having a cobalt valence higher than 3.0 can be further improved as compared with other active materials, but the positive electrode using the above active material shows a greater reduction in capacity upon repetition of charging and discharging cycles at high temperatures than conventional positive electrodes.
The reduction in capacity of batteries upon repetition of charging and discharging cycles at high temperatures is a phenomenon recognized also in conventional positive electrodes. In the case of conventional positive electrodes in which a bivalent cobalt oxide is added to a foamed nickel substrate as a conductive agent and the conductive network is formed by charging (oxidation) in the battery, the bivalent cobalt oxide is dissolved in an electrolyte and re-precipitated (production of cobalt complex ion and re-precipitation as cobalt hydroxide) during the period of from filling the electrolyte to the initial charging. Therefore, the nickel hydroxide solid solution particles are bonded to the foamed nickel substrate through cobalt hydroxide, and when oxidation of cobalt hydroxide takes place by the initial charging and the cobalt hydroxide changes to &bgr;-cobalt oxyhydroxide or the like which does not dissolve in the electrolyte, the active material particles (which mean here the sum of the nickel hydroxide solid solution particles and the cobalt oxide conductive agent) are strongly bound to the foamed nickel substrate. However, when the charging and
Dansui Yoshitaka
Kato Fumio
Suzuki Tatsuhiko
Tanigawa Futoshi
Yuasa Kohji
Matsushita Electric - Industrial Co., Ltd.
Nuzzolillo Maria
Ruthkosky Mark
Stevens Davis Miller & Mosher LLP
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