Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-03-20
2002-04-09
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S206000, C429S232000
Reexamination Certificate
active
06368748
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the invention
The present invention relates to a nickel-metal hydride storage cell, and more particularly to a nickel-metal hydride storage cell which employs a positive electrode containing a nickel hydroxide active material whose particles are coated with a cobalt compound and a negative electrode containing a hydrogen-absorbing alloy, and a manufacturing method of the cell.
(2) Description of the Related Art
A nickel positive electrode for use in a nickel-metal hydride storage cell can be either a sintered type or a paste type (non-sintered type). The sintered type is produced by filling a nickel powder-sintered substrate with an active material, whereas the paste type is produced by filling a highly porous nickel substrate such as a nickel fiber-sintered porous member or a foam nickel porous member with an active material paste.
The sintered type has drawbacks that the filing operation of an active material is complicated and that it is hard to enhance the energy density of the electrode because there are limits to an increase in the porosity of the substrate. In contrast, the paste type is easy to handle, which allows a high-density filling. For this reason, the paste type nickel electrode has become the mainstream, as the demand for higher energy density and lower cost of cells grows.
In spite of these advantages, however, the paste type nickel electrode has the following disadvantages: the electric contact between the active material and the substrate is insufficient because the pores in the substrate have large diameters. Consequently, the electrode has poor efficiency in electricity collection, and the generating ability of the high-density active material cannot be fully brought out.
In order to overcome these drawbacks of the paste type nickel positive electrode, various techniques have been suggested as follows: (1) Japanese Laid-open Patent Application No. 62-222566 discloses a technique of forming cobalt hydroxide layers over the surfaces of solid solution active material powder particles containing nickel hydroxide and either cadmium hydroxide or cobalt hydroxide. (2) Japanese Laid-open Patent Application No. 3-62457 discloses a technique of forming a solid solution of nickel hydroxide and cobalt hydroxide onto the surfaces of nickel hydroxide particles. (3) Japanese Laid-open Patent Application No. 5-151962 discloses a technique of forming hydrophilic organic layers onto the cobalt compound-coated layers which are formed onto the nickel hydroxide power particles, as an improved technique of the above-mentioned Japanese Laid-open Patent Application No. 62-222566.
These techniques have successfully improved the electric conductivity among active material particles and increased the active material utilization rate of the nickel positive electrode, thereby expanding the capacity of the nickel positive electrode. However, the expansion of the nickel positive electrode capacity does not directly lead to the improvement of the performance of an alkali-nickel storage cell. The reason for this is as follows.
As the active material utilization rate grows, the actual capacity of the positive electrode expands. However, when this positive electrode is used with a conventional negative electrode, the excess capacity (charge reserve) of the negative electrode reduces in proportion to the expansion of the actual capacity of the positive electrode. Consequently, more hydrogen dissociates from the negative electrode during a charging operation, which causes the internal pressure of the cell to be raised. Furthermore, the negative electrode performance deteriorates with the progress of the charge/discharge cycle, and the domination of the positive electrode is easily collapsed. The increase in dissociated hydrogen and the collapse of the positive electrode control cause the safety valve to operate to release the electrolyte to the outside the storage cell, thereby deteriorating the cycle life of the cell.
Therefore, in order to lead the expansion of the capacity of a nickel positive electrode to the improvement of the performance of a alkaline nickel storage cell, it is necessary to employ a negative electrode which is suitable for the performance of the nickel positive electrode, and to balance the capacities of these electrodes.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a nickel-metal hydride storage cell which has a large actual capacity and excellent cycle characteristics, and restricts a rise in the cell internal pressure during a charging operation, by using a nickel positive electrode having a high active material utilization rate and a hydrogen-absorbing alloy electrode which has an excellent low-temperature discharge characteristic, and by balancing these electrodes.
In order to achieve the object, the nickel-metal hydride storage cell of the present invention comprises a non-sintered nickel positive electrode which is filled with a cobalt-coated nickel active material including mother particles exclusively or mainly composed of nickel hydroxide, and cobalt compound layers partly or entirely coating the surfaces of the mother particles; a metal hydride negative electrode which is filled with a hydrogen-absorbing alloy which absorbs and desorbs hydrogen; and an electrolyte which includes an alkali aqueous solution. In the nickel-metal hydride storage cell, a positive electrode non-reactive capacity rate is 16% or lower and a negative electrode charge depth is 80% or lower after an initial charge/discharge operation.
The positive electrode non-reactive capacity rate is determined by a following equation 1:
positive electrode non-reactive capacity rate %=(positive electrode theoretical capacity−actual cell capacity)/positive electrode theoretical capacity×100 Eq. 1.
The negative electrode charge depth is determined by a following equation 2:
negative electrode charge depth %=(negative electrode remaining capacity+actual cell capacity)
egative electrode whole capacity×100 Eq. 2.
The nickel-metal hydride storage cell of the present invention can be manufactured by a method comprising the following four steps: a first step of producing a cobalt-coated nickel active material by dispersing mother particles exclusively or mainly consisting of nickel hydroxide into a cobalt compound-contained solution, and by precipitating a cobalt compound by adding an alkali solution to the cobalt compound-contained solution with a pH value being adjusted; a second step of applying a heat treatment to the cobalt-coated nickel active material by adding an alkali metal solution to the cobalt-coated nickel active material and by heating the cobalt-coated nickel active material in a presence of oxygen; a third step of producing a non-sintered nickel positive electrode whose non-reactive capacity rate expressed by the Equation 1 is 16% or lower, by using the cobalt-coated nickel active material containing the cobalt compound which has been heat-treated in the second step; a fourth step of assembling a nickel-metal hydride storage cell whose negative electrode charge depth expressed by Equation 2 after an initial charge/discharge operation is restricted to 80% or lower, by using the non-sintered nickel electrode and a metal hydride negative electrode which is filled with a hydrogen-absorbing alloy.
The present invention will be detailed hereinafter, based on the above-explained method of manufacturing a nickel-metal hydride storage cell.
In the first step, nickel mother particles are dispersed into a solution which contains a cobalt compound, and the pH of this solution is adjusted to a predetermined value. As a result, the cobalt compound is precipitated in the manner that it coats the surfaces of the nickel mother particles.
In the second step, the nickel mother particles which have been coated with the cobalt compound precipitate is soaked in an alkali metal solution, and heat-treated in the presence of oxygen. As a result, the oxidation number of the cobalt conta
Ise Tadashi
Tadokoro Motoo
Takee Masao
Yamawaki Akifumi
Chaney Carol
Sanyo Electric Co,. Ltd.
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