Patent Publication Number: US-2009239144-A1

Title: Nickel-metal hydride rechargeable cell

Description:
TECHNICAL FIELD 
     The present invention relates to a nickel-metal hydride rechargeable cell. 
     BACKGROUND ART 
     Nickel-metal hydride rechargeable cells are used for various purposes because of their high capacity. On the other hand, the nickel-metal hydride rechargeable cells are easy to decrease in remaining capacity due to self-discharge, and occasionally, they have to be freshly recharged when used some time after being charged. 
     In order to remedy the problem associating with self-discharge, for example, the nickel-metal hydride rechargeable cell disclosed in Unexamined Japanese Patent Publication No. 62-115657 uses as a separator a nonwoven fabric that is made from a sulfonated polyolefin-based resin. 
     The nickel-metal hydride rechargeable cell with the separator described in the publication, however, does not sufficiently suppress the self-discharge. 
     DISCLOSURE OF THE INVENTION 
     It is an object of the present invention to provide a nickel-metal hydride rechargeable cell in which self-discharge is suppressed for a long period. 
     The inventors have arrived at the invention after conducting various studies over and over to achieve the above object. 
     The invention provides a nickel-metal hydride rechargeable cell having an electrode assembly accommodated in a container together with an alkaline electrolyte. The electrode assembly is formed by winding a positive plate, a negative plate and a separator. The positive plate includes nickel hydroxide particles, a coating layer that coats at least a part of a surface of each of the nickel hydroxide particles and includes a cobalt compound as a base, in which an average valence of cobalt is more than bivalent, and an additive that is distributed between the nickel hydroxide particles and contains niobium and yttrium. The negative plate includes hydrogen storage alloy with a cobalt content of 2.0 percent by mass or less. The separator includes fiber having a sulfo group. The alkaline electrolyte contains sodium hydroxide as a base for a solute. 
     The nickel-metal hydride rechargeable cell of the invention is capable of suppressing self-discharge for a long period by synergistic effect of the constituents mentioned above without reducing a cycle life property, as compared to conventional nickel-metal hydride rechargeable cells. Once recharged, the cell of the invention does not need to be recharged before being put to use even some time after charge. Accordingly, the cell is convenient for the user and yet environmentally friendly as the unnecessary power consumption caused by self-discharge is reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The FIGURE is a perspective view of a nickel-metal hydride rechargeable cell according to one aspect of the invention, partially broken away, and illustrates in a circle a cross-section schematically showing a part of a positive electrode in an enlarged scale. 
     
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     The FIGURE shows a nickel-metal hydride rechargeable cell according to one aspect of the present invention. 
     The cell has an exterior can  1  in a shape of a circular cylinder with a bottom. An electrode assembly  2  is accommodated in the exterior can  1  together with an alkaline electrolyte, not shown. The electrode assembly  2  is produced by winding a positive plate  3 , a negative plate  4 , and a separator  5  into a spiral-wound configuration so that the separator  5  is sandwiched between the plates  3  and  4 . An outer end of the negative plate  4  is positioned in an outermost circumference of the electrode assembly  2  as viewed into a spiral direction. The negative plate  4  is electrically connected to an inner circumferential wall of the exterior can  1 . 
     A circular cover plate  8  having a vent hole  7  in the center thereof is set inside an open end of the exterior can  1  through a ring-like insulating gasket  6 . The insulating gasket  6  and the cover plate  8  are fastened by caulking an open end edge of the exterior can  1 . Between the positive plate  3  of the electrode assembly  2  and an inner surface of the cover plate  8 , there is disposed a positive lead  9  that electrically connects the positive plate  3  and the cover plate  8 . A rubber valve element  10  is positioned to an outer surface of the cover plate  8  so as to close the vent hole  7 . A cylindrical positive terminal  11  with a flange is attached onto the outer surface of the cover plate  8 , encircling the valve element  10 . 
     An annular insulating plate  12  is placed on the open end edge of the exterior can  1 . The positive terminal  11  extends through and protrudes from the insulating plate  12 . Reference numeral  13  represents an exterior tube. The exterior tube  13  covers an outer circumferential edge of the insulating plate  12 , an outer circumferential surface of the exterior can  1 , and an outer circumferential edge of a bottom wall of the exterior can  1 . 
     The positive plate  3 , the negative plate  4 , the separator  5 , and the alkaline electrolyte will be described below in detail. 
     The positive plate  3  is constituted by a conductive positive substrate and a positive mix that is carried by the positive substrate. 
     The positive substrate is a porous substrate having a three-dimensional network structure. For example, a nickel porous body may be applied as the positive substrate. The nickel porous body is obtained by nickel-coating a sponge used as a three-dimensional network material and then roasting this nickel-coated sponge. 
     The positive mix contains composite particles  14  containing nickel hydroxide as a base (major component) which serves as a positive active material, Nb-based particles  15  containing niobium as a base, Y-based particles  16  containing yttrium as a base, and a binding agent  17 . 
     More specifically, each of the composite particles  14  has a center (core) made of a nickel hydroxide particle  18  in a substantially globular shape. The nickel hydroxide particle  18  may be a solid solution containing either one or both of cobalt and zinc. The nickel hydroxide of the nickel hydroxide particle  18  may be high-order nickel hydroxide in which an average valence of nickel is more than bivalent. 
     The nickel hydroxide particle  18  has a surface that is entirely or at least partially coated with a coating layer  19  containing a cobalt compound as a base. The cobalt compound included in the coating layer  19  is a high-order cobalt compound consisting of cobalt oxide or cobalt hydroxide in which an average valence of cobalt is more than bivalent. The cobalt compound of the coating layer  19  may contain an alkali cation, such as Na, K, and Li. The cobalt compound may also have a distorted crystal structure. 
     As the Nb-based particle  15 , for example, a particle of metallic niobium or niobium compound may be applied. Applicable niobium compounds include Nb 2 O 5 , NbF 5 , etc. 
     As the Y-based particle  16 , for example, metal or an yttrium compound may be applied. The yttrium compound may be Y 2 O 3 , YF 3  or the like. 
     Instead of using the Nb-based particle  15  and the Y-based particle  16 , it is possible to use a particle of a compound containing both niobium and yttrium as a base. In short, what is important is that an additive containing niobium and yttrium is distributed between the composite particles  14 . 
     Materials that may be applied as the binding agent  17  include, for example, carboxymethyl cellulose, methyl cellulose, PTFE dispersion, HPC dispersion, etc. 
     The positive plate  3  may be fabricated, for example, in the following manner. First, a positive slurry is prepared by mixing and kneading the composite particles  14 , the Nb-based particles  15 , the Y-based particles  16 , the binding agent  17  and water. Subsequently, the positive slurry is applied to and filled into the positive substrate. The positive substrate is pressed and cut after the positive slurry is dried, to thereby obtain the positive plate  3 . 
     The composite particle  14  is made, for example, by subjecting the nickel hydroxide particle  18  coated with a cobalt compound to alkali/heat treatment. 
     To be more concrete, the alkali/heat treatment sprays an alkaline solution onto the nickel hydroxide particle  18  coated with the cobalt compound while stirring in a heating atmosphere. This makes the cobalt compound coating the nickel hydroxide particle  18  into a high-order cobalt compound. 
     As a result of the alkali/heat treatment, the crystal structure of the cobalt compound forming the coating layer  19  becomes distorted, and the cobalt compound comes to contain an alkali cation, such as Li, Na and K, in accordance with the kind of the alkaline solution. 
     The negative plate  4  is constituted by a conductive negative substrate and a negative mix that is carried by the negative substrate. For example, a punching metal may be applied as the negative substrate. 
     The negative mix contains hydrogen storage alloy particles, a binding agent, and if necessary, a conductive agent. As to the binding agent, it is possible to use the same binding agent as the one used in the positive mix. The binding agent may also be used in combination with, for example, sodium polyacrylate. For example, carbon powder may be applied as the conductive agent. 
     The hydrogen storage alloy particles of the negative plate  4  are made of hydrogen storage alloy with a cobalt content of 2.0 percent by mass or less. Applicable alloys include a rear earth-Mg—Ni-based hydrogen storage alloy having a Ce 2 Ni 7 -type structure or a structure similar thereto and a LaNi-based hydrogen storage alloy having an AB 5 -type structure. 
     The negative plate  4  may be fabricated, for example, in the following manner. A first step is to prepare a negative slurry that is made of hydrogen storage alloy particles, a binding agent, water, and if necessary, a conductive agent. Secondly, the negative slurry is applied onto the negative substrate. The negative substrate is pressed and cut after the negative slurry is dried, to thereby obtain the negative plate  4 . 
     The separator  5  is made of a nonwoven fabric including chiefly of polyolefin-based synthetic resin fibers that are provided with a sulfo group (—SO 3 H). Synthetic resins applicable as the polyolefin-based synthetic resin include polyethylene, polypropylene, etc. The sulfo group is provided by treating the nonwoven fabric with acid containing a sulfate group such as sulfuric acid, fuming sulfuric acid or the like. 
     The alkaline electrolyte is a Na-rich caustic alkali solution containing sodium hydroxide (NaOH) as a base of a solute. One liter of the alkaline electrolyte preferably contains potassium hydroxide ranging from 0 gram equivalent to 1 gram equivalent, NaOH ranging from 5 gram equivalent to 7 gram equivalent, and lithium hydroxide (LiOH) ranging from 0.3 gram equivalent to 1.3 gram equivalent. 
     In the above-described nickel-metal hydride rechargeable cell, the average valence of the cobalt contained in the cobalt compound of the coating layer  19  is more than bivalent, and the Nb-based particles  15  and the Y-based particles  16  are distributed between the composite particles  14 . The hydrogen storage alloy of the negative plate  4  contains cobalt of 2.0 percent by mass or less, and the separator  5  includes the fibers having the sulfo group. Moreover, the alkaline electrolyte contains NaOH as a base of the solute. Consequently, albeit for unknown reasons, the nickel-metal hydride rechargeable cell has a very excellent self-discharge property. In short, the cell makes it possible to effectively suppress the self-discharge for a long period when left charged. 
     EMBODIMENT 
     Embodiment 1 
     1. Fabrication of the Negative Plate 
     Metal materials were weighed and mixed so that composition is La 0.10 Ce 0.05 Pr 0.35 Nd 0.50 Mg 0.10 Ni 3.70 Al 0.22 . This mixture was melted in a high-frequency melting furnace and cast into an ingot. This ingot was heated for 10 hours in an argon atmosphere having a temperature of 1000 degrees centigrade, to thereby make a crystal structure of the ingot into a Ce 2 Ni 7 -type structure or a structure similar thereto. Subsequently, the ingot was mechanically pulverized and sieved in an inert atmosphere, to thereby obtain rare-earth-Mg—Ni-based hydrogen storage alloy particles having the above composition. The rare-earth-Mg—Ni-based hydrogen storage alloy particles that had been obtained had an average particle diameter of 50 μm. The average particle diameter was defined as a particle diameter corresponding to 50 percent in weight integration in the particle diameter distribution of the rare-earth-Mg—Ni-based hydrogen storage alloy particles that was measured with a laser diffraction/scattering particle size distribution analyzer. 
     The obtained alloy particles of 100 parts by mass were kneaded after being added with sodium polyacrylate of 0.5 parts by mass, carboxymethyl cellulose of 0.12 parts by mass, PTFE dispersion (dispersion media: water; specific gravity 1.5; solid content of 60 percent by mass) of 0.5 parts by mass (in terms of solid content), carbon black of 1.0 parts by mass, and water of 30 parts by mass, to thereby prepare a negative slurry. The negative slurry was then applied onto a punching sheet made of nickel. After the negative slurry was dried, the punching sheet was pressed and cut. In this manner, a negative plate for an AA-size cell was fabricated. 
     2. Fabrication of the Positive Plate 
     A mixed solution of nickel sulfate, zinc sulfate, and cobalt sulfate was prepared so that a mass ratio of Ni, Zn and Co is 96:3:1 in reduced quantity. Sodium hydroxide was gradually added into the mixed solution, stirring constantly, to induce a reaction. At the same time, the mixed solution in the process of the reaction was maintained in a range of from 13 to 14 in pH. Nickel hydroxide particles of a substantially spherical shape were consequently precipitated in the mixed solution. 
     In the next place, cobalt sulfate solution was added into the mixed solution in which the nickel hydroxide particles were precipitated, to thereby cause a reaction. At the same time, the mixed solution in the process of the reaction was maintained in a range of from 9 to 10 in pH. In this way, cobalt hydroxide was precipitated in surfaces of the substantially spherical nickel hydroxide particles that had been previously precipitated. The substantially spherical nickel hydroxide particles whose surfaces were coated with the cobalt hydroxide were rinsed three times with ten parts purified water. The particles were then dehydrated and dried. Consequently, nickel hydroxide particles whose surfaces were coated with cobalt hydroxide were obtained. 
     Thereafter, the obtained particles were subjected to alkali/heat treatment. More specifically, the nickel hydroxide particles whose surfaces were coated with cobalt hydroxide were constantly stirred in a heating atmosphere having a temperature of 100 degrees centigrade, and were simultaneously sprayed with sodium hydroxide solution having a concentration of 25 percent by mass for 0.5 hours. As a result, the cobalt hydroxide coating the nickel hydroxide particles was oxidized and turned into a high-order cobalt compound. 
     The particles that had subjected to the alkali/heat treatment were rinsed three times with ten parts purified water. The particles were subsequently dehydrated and dried. This provided composite particles that were nickel hydroxide particles whose surfaces were coated with respective coating layers made of a high-order cobalt compound having a distorted crystal structure and containing an alkali cation. 
     The next step was to mix the composite particles of 100 parts by mass, diniobium pentoxide (Nb 2 O 5 ) powder of 0.3 parts by mass, diyttrium trioxide (Y 2 O 3 ) powder of 0.9 parts by mass, and HPC (hydroxypropylcellulose) dispersion liquid (dispersion media: water of 40 parts by mass and solid content of 60 parts by mass) of 0.3 parts by mass so that the composite particles, the Nb 2 O 5  powder, and the Y 2 O 3  powder were evenly dispersed. Accordingly, a positive slurry was obtained. The positive slurry was filled into a nickel porous body. After the positive slurry was dried, the nickel porous body was pressed and cut, to thereby fabricate a non-sintered positive plate for an AA-size cell. 
     3. Fabrication of the Separator 
     A nonwoven fabric was prepared, which was made of polypropylene resin fibers, and was 45 g/m 2  in basis weight (areal weight) and 0.2 mm in thickness. The nonwoven fabric was subjected to sulfonation treatment using fuming sulfuric acid. This provided a separator having a sulfo group. 
     4. Alkaline Electrolyte Preparation 
     A Na-rich alkaline electrolyte was prepared by mixing potassium hydroxide (KOH) solution, sodium hydroxide (NaOH) solution, and lithium hydroxide (LiOH) solution. The solutions were mixed together in such a ratio that one liter of the alkaline electrolyte contained 0.5 gram equivalent of potassium hydroxide, 6.0 gram equivalent of sodium hydroxide, and 1.0 gram equivalent of lithium hydroxide. 
     5. Fabrication of the Nickel-Metal Hydride Rechargeable Cell 
     The obtained positive and negative plates and separator were wound into a spiral-wound configuration so that the separator was sandwiched between the plates, to thereby fabricate an electrode assembly. The obtained electrode assembly was accommodated into an exterior can, and a predetermined affixing process was carried out. The Na-rich alkaline electrolyte was then injected into the exterior can. The open end of the exterior can was sealed with a cover plate, etc. In this manner, a sealed cylindrical nickel-metal hydride rechargeable cell of AA-size according to Embodiment 1 was fabricated. 
     Comparative Examples 1 to 5 
     As shown in TABLE 1, a nickel-metal hydride rechargeable cell was fabricated in the same manner as in Embodiment 1, except for one or more modifications described under the following items (i) to (vi). 
     (i) The nickel hydroxide particles coated with the cobalt compound were not subjected to the alkali/heat treatment, and the average valence of cobalt contained in the cobalt compound of the coating layer was set to bivalent.
 
(ii) The Nb 2 O 5  powder and the Y 2 O 3  powder were not added into the positive slurry.
 
(iii) The CoO powder of 1.9 parts by mass was added into the positive slurry.
 
(iv) The negative plate was made using hydrogen storage alloy having a composition of Mm 1.0 Ni 3.65 Co 0.75 Mn 0.35 Al 0.30 .
 
(v) The nonwoven fabric was subjected to fluorine gas treatment instead of the sulfonation treatment. The fluorine gas treatment is to process a nonwoven fabric with a mixed gas obtained by adding oxygen gas, CO 2  gas, sulfur gas, and the like, into fluorine gas that is diluted with inert gas.
 
(vi) As the alkaline electrolyte, K-rich alkaline electrolyte was used instead of the Na-rich alkaline electrolyte. To be concrete, KOH solution, NaOH solution and LiOH solution were mixed together at such a ratio that one liter of the alkaline electrolyte contains 6.0 gram equivalent of potassium hydroxide, 1.0 gram equivalent of sodium hydroxide, and 0.2 gram equivalent of lithium hydroxide.
 
     6. Evaluation of Capacity Retention Rates of the Cells 
     The cells of Embodiment 1 and Comparative Examples 1 to 5, which had been subjected to initial activation treatment were charged at a charging current of 1 ItA in a differential voltage control mode in an environment having a temperature of 25 degrees centigrade. After a 60-minute quiescent period following the charging, each of the cells was discharged at a discharging current of 740 mA, and then a discharging capacity was measured as a full charge capacity. 
     (1) Capacity Retention Rate (40° C. for 3 Months) 
     Each of the cells, which had been discharged to measure a full charge capacity, was charged at a charging current of 1 ItA in a differential voltage control mode in an environment having a temperature of 25 degrees centigrade. After the charged cells were left to stand at a temperature of 40 degrees centigrade for three months, each of the cells was discharged at a discharging current of 740 mA in an environment having a temperature of 25 degrees centigrade. Then a discharging capacity was measured as a remaining capacity. A rate of the remaining capacity in relation to the full charge capacity is shown in TABLE 1 under Capacity retention rate (40° C. for 3 months). 
     (2) Capacity Retention Rate (25° C. for 1 Year) 
     Each of the cells, which had been discharged to measure a full charge capacity, was charged at a charging current of 1 ItA in a differential voltage control mode in an environment having a temperature of 25 degrees centigrade. After the charged cells were left to stand at a temperature of 25 degrees centigrade for one year (360 days), each of the cells was discharged at a discharging current of 740 mA in an environment having a temperature of 25 degrees centigrade. Then a discharging capacity was measured as a remaining capacity. A rate of the remaining capacity in relation to the full charge capacity is shown in TABLE 1 as a capacity retention rate (25° C. for 1 year). 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Positive plate 
                   
                 Cell evaluation 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Average 
                   
                   
                 Negative plate 
                   
                   
                 Capacity 
                 Capacity 
               
               
                   
                 valence of Co 
                 Nb 2 O 5   
                   
                 Presence of Co 
                 Separator 
                   
                 retention 
                 retention 
               
               
                   
                 in the 
                 powder and 
                   
                 in hydrogen 
                 Pre- 
                 Electro- 
                 rate (40° C. 
                 rate (25° C. 
               
               
                   
                 coating layer 
                 Y 2 O 3  powder 
                 CoO powder 
                 storage alloy 
                 processing 
                 lyte 
                 for 3 months) (%) 
                 for 1 year) (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Embodiment 
                 More than 
                 Added 
                 Not added 
                 Not present 
                 Sulfonation 
                 Na-rich 
                 81.5 
                 84.1 
               
               
                 1 
                 bivalent 
               
               
                 Comparative 
                 Bivalent 
                 Not added 
                 Added 
                 Present 
                 Fluorine 
                 K-rich 
                 0.0 
                 0.0 
               
               
                 example 1 
                   
                   
                   
                   
                 gas 
               
               
                 Comparative 
                 More than 
                 Added 
                 Added 
                 Present 
                 Fluorine 
                 K-rich 
                 21.8 
                 23.0 
               
               
                 example 2 
                 bivalent 
                   
                   
                   
                 gas 
               
               
                 Comparative 
                 Bivalent 
                 Not added 
                 Not added 
                 Not present 
                 Fluorine 
                 K-rich 
                 26.4 
                 28.0 
               
               
                 example 3 
                   
                   
                   
                   
                 gas 
               
               
                 Comparative 
                 Bivalent 
                 Not added 
                 Added 
                 Present 
                 Sulfonation 
                 K-rich 
                 53.8 
                 55.7 
               
               
                 example 4 
               
               
                 Comparative 
                 Bivalent 
                 Not added 
                 Added 
                 Present 
                 Fluorine 
                 Na-rich 
                 59.9 
                 62.2 
               
               
                 example 5 
                   
                   
                   
                   
                 gas 
               
               
                   
               
            
           
         
       
     
     TABLE 1 clearly shows the following facts. 
     The cell of Embodiment 1 is excellent in terms of capacity retention rate, as compared to the cells of Comparative Examples 1 to 5. This is because, in the cell of Embodiment 1, the average valence of the cobalt contained in the cobalt compound of the coating layer is more than bivalent; the positive slurry is not added with the CoO powder but with the Nb 2 O 5  powder and the Y 2 O 3  powder; the hydrogen storage alloy of the negative plate does not contain Co of more than 2.0 percent by mass; the separator is subjected to the sulfonation treatment; and the alkaline electrolyte is Na-rich. 
     The invention is not limited to the one aspect and the embodiment thereof, and may be modified in various ways. The cell may be square in shape. There is no particular restriction to a mechanical structure of the cell. 
     According to the one aspect, the positive plate  3  includes the Nb-based particles  15  and the Y-based particles  16  as additives distributed between the composite particles  14 . However, the positive plate  3  may include additives of other kinds. If the positive plate  3  includes particles containing Co as additive, it is preferable that the Co contained in the additive should have a mass of 0.1 percent or less of the mass of the nickel hydroxide contained in the positive plate  3 . This is because, if the mass of the Co contained in the additive is more than 0.1 percent of the mass of the nickel hydroxide contained in the positive plate  3 , the cell is deteriorated in self-discharge property.