WO2010053257A2

WO2010053257A2 - Method of fabricating negative electrode for nickel/zinc secondary battery

Info

Publication number: WO2010053257A2
Application number: PCT/KR2009/005219
Authority: WO
Inventors: Yun Jung Kwak; Tae Hyuk Kang; Dong Pil Park
Original assignee: Energreen Co Ltd
Current assignee: Energreen Co Ltd
Priority date: 2008-11-04
Filing date: 2009-09-14
Publication date: 2010-05-14
Anticipated expiration: 2011-05-04
Also published as: WO2010053257A3

Abstract

The present disclosure relates to a method of fabricating a negative electrode for nickel/zinc secondary batteries. The method can prevent paste agglomeration during formation of paste containing active materials and prevent deformation of the negative electrode or formation of dendrites caused by dissolution of the negative electrode in an alkaline electrolyte solution, thereby improving life span and charge/discharge efficiency of the batteries.

Description

METHOD OF FABRICATING NEGATIVE ELECTRODE FOR NICKEL/ZINC SECONDARY BATTERY

The present invention relates to a method of fabricating a negative electrode for nickel/zinc secondary batteries and, more particularly, to a method of fabricating a negative electrode for nickel/zinc secondary batteries, which can enhance conductivity of the negative electrode, prevent paste agglomeration during formation of paste containing an active material, and prevent deformation of the negative electrode or formation of dendrites caused by dissolution of the negative electrode in an alkaline electrolyte solution, thereby improving life span and charge/discharge efficiency of the batteries.

Currently, most countries have introduced a variety of environmental regulations for preservation of the environment. As part of this movement, lead acid batteries and nickel/cadmium batteries are replaced by nickel/hydrogen batteries, lithium ion batteries and the like in the field of small batteries. In the field of large industrial batteries, however, the lead acid batteries and nickel/cadmium batteries are still used due to failure to develop substitute batteries. Accordingly, development of environmentally friendly batteries having high capacity is attracting increasing attention and is intensively carried out in the art.

As one of batteries developed to replace the lead acid battery, a nickel (Ni)/zinc (Zn) secondary battery has an operating voltage of 1.6 V/cell or more, high energy density per unit weight and volume, and a specific power density of 875 W/kg, which is superior to the specific power density, 535 W/kg, of the lead acid battery. Further, advantageously, the Ni/Zn secondary battery has a charge/discharge cycle life of 500 cycles or more to a capacity equivalent to 80% of the maximum capacity of the secondary battery, which is more stable that the charge/discharge cycle life of 200~700 cycles of the lead acid battery. Furthermore, zinc alkaline secondary batteries employ zinc for an anode active material and are widely applied to secondary batteries not only for driving but also for stationary power storage due to its inexpensive price. In the zinc alkaline secondary batteries, however, since the zinc negative electrode is dissolved in an alkaline solution and undergoes repetitious elution and precipitation of zinc by charge/discharge reaction, electrode plates are deformed by the charge/discharge reaction. Further, the eluted zinc is not uniformly precipitated but grows in a dendritic phase during charging of the battery, so that the zinc dendritic phase penetrates separators of the secondary battery and causes short circuit of the battery, thereby decreasing life span of the secondary battery.

Various techniques have been developed to solve such problems.

Japanese Laid-open Publication No. 1985-185372 discloses a nickel/zinc secondary battery that includes zinc electrodes containing an oxide and a hydride of In and Ti to maintain high energy density in order to increase the charge/discharge cycle life of the nickel/zinc secondary battery.

Japanese Laid-open Publication No. 1987-108467 discloses a zinc alkaline battery that includes zinc electrodes containing indium metal, indium oxide and thallium, and an electrolyte containing germanium ions to increase the charge/discharge cycle life of the zinc alkaline battery.

Japanese Laid-open Publication No. 1986-016366 discloses a zinc alkaline battery that includes zinc electrodes containing inactive and non-conductive organic compounds to increase the charge/discharge cycle life of the zinc alkaline battery while preventing deformation of zinc electrode plates thereof.

PCT/US2004/026859 discloses a nickel/zinc secondary battery which employs surfactants as a dispersion agent to prevent performance deterioration of the battery caused by agglomeration of zinc oxide particles occurring in preparation of a slurry or paste for a negative electrode for nickel/zinc secondary batteries. However, since the dispersion agent can provide negative results in battery reaction if it is not decomposed, it is necessary to heat-treat the zinc negative electrode at 300 or more in a vacuum and an inactive gas atmosphere when the zinc negative electrode is fabricated using the dispersion agent. However, such an increase in the number of processes leads to an excessive increase in process costs and inefficiency of the process, and the heat-treated electrode is likely to undergo pore closing, thereby making it difficult to ensure performance of the battery. Further, PCT/US2004/026859 discloses a method of fabricating a nickel/zinc secondary battery which includes a separator layer to prevent formation of dendrites on a negative electrode and contains a borate and a fluoride in a potassium hydroxide electrolyte solution.

Further, US Patent No. 6,649,305 B1 notes the very low conductivity of zinc oxide and suggests use of conductive ceramics. However, when using the conductive ceramics, there is a problem in that agglomeration of active materials cannot be prevented during fabrication of the slurry or paste.

As such, various attempts have been made to solve the problems of the nickel/zinc secondary battery in the art, and resulted not only in an increase of manufacturing costs, but also in unsatisfactory effects.

It is an aspect of the present invention to provide a method of fabricating a negative electrode for nickel/zinc secondary batteries, which can enhance conductivity of the negative electrode, prevent paste agglomeration during formation of paste containing an active material, and prevent deformation of the negative electrode or formation of dendrites caused by dissolution of the negative electrode in an alkaline electrolyte solution to improve life span and charge/discharge efficiency of the battery.

In accordance with an aspect of the present invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (a1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (a2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (a3) adding a binder to the slurry to prepare a composition for negative electrodes; (a4) coating the composition on a current collector; and (a5) drying the composition coated on the current collector.

In accordance with another aspect of the present invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (b1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (b2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (b3) adding a binder to the slurry to prepare a composition for negative electrodes; (b4) coating a release agent on base films, followed by coating the composition thereon; (b5) disposing two base films having the composition coated thereon at opposite sides of a current collector to face each other, followed by compressing the base films by application of heat and pressure; and (b6) removing the base films from the current collector.

In accordance with a further aspect of the present invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (c1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (c2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (c3) adding a binder to the slurry to prepare a composition for negative electrodes; (c4) coating a surface of a copper or copper alloy current collector with a dispersion liquid prepared by dispersing activated carbon in a binder; (c5) coating the composition on the current collector; and (c6) drying the composition coated on the current collector.

In accordance with yet another aspect of the present invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (d1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (d2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (d3) adding a binder to the slurry to prepare a composition for negative electrodes; (d4) coating the composition on a current collector; (d5) drying the composition coated on the current collector to form a negative electrode; (d6) coating and drying a separator with a dispersion liquid prepared by dispersing porous ceramic or silica in a binder or a thickening agent; and (d7) compressing the separator to the negative electrode by application of heat and pressure.

According to embodiments of the invention, graphite powder or activated carbon such as acetylene black is physically mixed with an anode active material using a ball-mill process, thereby significantly improving conductivity of the negative electrode of the nickel/zinc secondary battery. In addition, the method prevents agglomeration of active materials in a slurry or paste while maintaining a tap density as high as possible during fabrication of the negative electrode of the nickel/zinc secondary batteries, thereby improving performance of the negative electrode of the secondary battery. Further, the method prevents deformation of the negative electrode or formation of dendrites caused by dissolution of the negative electrode in an alkaline electrolyte solution, thereby improving life span and charge/discharge efficiency of the battery.

The above and other aspects, features and advantages of the invention will be more clearly understood from the detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagram of a nickel/zinc secondary battery in accordance with one embodiment of the present invention;

Fig. 2 is a diagram of the nickel/zinc secondary battery in accordance with the embodiment of the present invention, in which positive electrodes and negative electrodes are stacked on each other;

Fig. 3 is a flowchart of a method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with a first embodiment of the present invention;

Fig. 4 is a sectional view of a negative electrode for nickel/zinc secondary batteries before and after roll pressing in the method according to the first embodiment of the present invention;

Fig. 5 is a flowchart of a method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with a second embodiment of the present invention;

Fig. 6 shows a base film on which a release agent and a composition for negative electrodes are coated in operation (b4) of the method in accordance with the second embodiment of the present invention;

Fig. 7 shows two base films before being compressed to a current collector in operation (b5) of the method in accordance with the second embodiment of the present invention;

Fig. 8 shows a fabricated negative electrode after removing the compressed base films from the current collector in operation (b6) of the method in accordance with the second embodiment of the present invention;

Fig. 9 is a flowchart of a method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with a third embodiment of the present invention;

Fig. 10 shows a copper-foam current collector having an activated carbon layer formed on a surface thereof by coating a dispersion liquid containing activated carbon in operation (c4) of the method in accordance with the third embodiment of the present invention;

Fig. 11 is a flowchart of a method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with a fourth embodiment of the present invention;

Fig. 12 shows a process of compressing separators to the negative electrode in operation (d6) of the method in accordance with the fourth embodiment of the present invention;

Fig. 13 is a graph depicting performance of a nickel/zinc secondary battery fabricated by Example 1; and

Fig. 14 is a graph depicting performance of a nickel/zinc secondary battery fabricated by Example 2.

Embodiments of the invention will hereinafter be described in detail with reference to the accompanying drawings.

In accordance with a first embodiment of the invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (a1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (a2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (a3) adding a binder to the slurry to prepare a composition for negative electrodes; (a4) coating the composition on a current collector; and (a5) drying the composition coated on the current collector.

Referring to Figs. 1 and 2, a method of fabricating a negative electrode for nickel/zinc secondary batteries will be briefly described. Fig. 1 schematically shows a nickel/zinc secondary battery in accordance with one embodiment of the invention. Fig. 2 schematically shows the nickel/zinc secondary battery in accordance with the embodiment, in which positive electrodes and negative electrodes are stacked on each other.

First, electrodes are prepared. Compositions for the electrodes are prepared using anode or cathode active materials, a thickening agent and a binder, and the compositions are coated and dried on current collectors, thereby providing the electrodes. After forming the electrodes, separators are covered on each of the electrodes and terminals are exposed outside the separators. Then, the electrodes are disposed in a case, which is sealed after injecting an electrolyte solution thereinto, so that a nickel/zinc secondary battery can be produced. Here, the negative electrodes and the positive electrodes are alternately stacked on each other to improve capacity of the secondary battery, as shown in Fig. 2.

Next, the method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with the first embodiment will be described with reference to Figs. 3 and 4. Fig. 3 is a flowchart of the method in accordance with a first embodiment of the invention. Fig. 4 is a sectional view of a negative electrode for nickel/zinc secondary batteries before and after roll pressing in the method according to the first embodiment of the invention.

Referring to Fig. 3, an anode active material is mixed with activated carbon or graphite powder using a ball mill in operation a1.

The anode active material may include, but is not limited to, at least one selected from the group consisting of zinc oxide, calcium zincate, and zinc powder.

The activated carbon may include, but is not limited to, at least one selected from the group consisting of acetylene black and Ketjen black. The activated carbon and graphite may be provided in the form of powders having an average particle size less than 10.

In the method according to the first embodiment, the activated carbon or graphite is mixed with the active material using the ball-mill process, thereby improving electrical conductivity of the electrode while preventing agglomeration of the active material in the slurry or paste during fabrication of the electrode. The activated carbon such as acetylene black and Ketjen black has a large volume to cause a decrease in tap density when used as a conductive agent. Since the decrease in tap density leads to a decrease in energy density of the battery, it is necessary to keep the tap density as high as possible. When using the ball-mill process, the active material and the activated carbon can be strongly bonded to each other while keeping the tap density as high as possible, thereby preventing agglomeration of the active material.

In the mixture prepared by operation a1, the anode active material and the activated carbon are mixed in a ratio of 95~99 wt% and 1~5 wt% by weight, respectively. If the content of activated carbon exceeds 5 wt%, the electrode is lowered in activity, and, if the content of activated carbon is less than 1 wt%, the electrode is lowered in electrical conductivity.

In this operation, additives may be added to the anode active material and the activated carbon. The additives may include various metal oxides such as Ca(OH)₂, Bi₂O₃, Tl₂O₃, In₂O₃, and SnO. Ca(OH)₂ may be added to prevent zincate ions from being dissolved in the electrolyte solution, and the metal oxides such as Bi₂O₃ and the like may be added to reduce gas generation by increasing hydrogen overvoltage.

Next, after wetting the mixture prepared in operation a1, a thickening agent is added to the mixture to prepare a slurry in operation a2.

The thickening agent may include, but is not limited to, at least one selected from the group consisting of CMC, HEC and acrylic ester.

The thickening agent may be used after being substituted Na+ with K+.

For Ca(OH)₂, which is an additive used to prevent the zincate ions from being dissolved after charging the battery, there is rapid agglomeration resulting from reaction with a water mixed binder. However, the method according to the first embodiment of the invention can prevent such a phenomenon. Here, since agglomeration of a small amount of Ca(OH)₂ can affect performance of the battery, the thickening agent may be used after being substituted Na+ with K+ to solve this phenomenon. This operation may be achieved by adding potassium hydroxide, sodium hydroxide, and the like.

In this operation, the content of the thickening agent may be 0.5~5 wt% with reference to the mixture of the anode active material and the activated carbon. If the content of the thickening agent exceeds 5 wt%, the slurry increases in viscosity and the electrode resistance increases, thereby making it difficult to coat the compound on the current collector while deteriorating performance of the battery. If the content of the thickening agent is less than 0.5 wt%, the slurry becomes thin, thereby making it difficult to form the electrode.

Next, a binder is added to the slurry to prepare a composition for the negative electrode in operation a3.

The binder may include PTFE, PE, SBR, and the like.

In this operation, the binder may be added in an amount of 0.5~5 wt% with reference to the mixture of the anode active material and the activated carbon. If the content of the binder exceeds 5 wt%, the slurry increases in viscosity and the electrode resistance increases, thereby deteriorating performance of the battery. If the content is less than 0.5 wt%, a binding force with respect to the current collector and other anode active material is lowered, thereby deteriorating performance of the battery, and the binder is liable to be dissolved in the electrolyte solution.

Next, the composition is coated on the current collector in operation a4, and is then dried thereon in operation a5.

After coating and drying the composition, the prepared electrode may be subjected to roll pressing. Roll pressing more densely couples the active material, activated carbon and the like to each other. The current collector may include a metal sheet, an expanded metal, a punching metal, a metal foam, and the like.

In accordance with a second embodiment of the invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (b1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (b2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (b3) adding a binder to the slurry to prepare a composition for negative electrodes; (b4) coating a release agent on base films, followed by coating the composition thereon; (b5) disposing two base films having the composition coated thereon at opposite sides of a current collector to face each other, followed by compressing the base films by application of heat and pressure; and (b6) removing the base films from the current collector.

Next, referring to Figs. 5 to 8, the method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with the second embodiment will be described in detail. Fig. 5 is a flowchart of the method of fabricating the negative electrode for nickel/zinc secondary batteries in accordance with the second embodiment. Fig. 6 shows a base film on which a release agent and a composition for negative electrodes are coated in operation (b4) of the method in accordance with the second embodiment. Fig. 7 shows two base films before being compressed to a current collector in operation (b5) of the method in accordance with the second embodiment. Fig. 8 shows a fabricated negative electrode after removing the compressed base films from the current collector in operation (b6) of the method in accordance with the second embodiment.

In the method in accordance with the second embodiment, operations b1 to b3 correspond to the operations a1 to a3 of the method according to the first embodiment.

Referring to Fig. 5, operations b1 to b3 of the method according to the second embodiment are performed as in operations a1 to a3 of the method according to the first embodiment. In operation b3, PTFE, PE, SBR, and the like may be used as the binder. Further, when heat is applied, low density polyethylene (LDPE) may be used as the binder since it becomes highly flexible by application of heat, improving binding capabilities.

Then, after a release agent is coated on the base film, the composition for negative electrodes is coated thereon in operation b4. Referring to Fig. 6, after coating a release agent layer 212 on a base film 214, the composition 213 prepared in operation b3 is coated thereon. The release agent layer 212 allows the base film 214 to be easily removed from the negative electrode and may include a silicone-based release agent, but is not limited thereto. The base film 214 may contain PP, PE or the like.

Next, two base films having the composition coated thereon are disposed at opposite sides of the current collector to face each other, followed by compressing the base films by application of heat and pressure to the base films in operation b5. Referring to Fig. 7, after disposing the two base films 214 having the release layer 212 and the composition 213 continuously stacked thereon to face each other, the current collector 220 is located between the base films 214, which in turn are compressed by application of heat and pressure. The compression may be performed using a heating roll press, a heating plate press or the like. The compression may be performed at 70~120 for a thermal bonding duration of 10 seconds to 3 minutes. Here, when using the heating roll press, a pressure of 300~1000 kg/ may be applied, and when using the heating plate press, a pressure of 100~1000 kg/ may be applied. The current collector may include a metal sheet, an expanded metal, a punching metal, a metal foam, and the like. As such, when the electrode plates are formed by lamination, the electrode plates are strongly coupled to the current collector to reduce solubility in an alkaline solution during charge/discharge reaction as compared with the conventional negative electrode, thereby significantly improving charge/discharge efficiency.

Last, after forming the electrode plates on the current collector through compression in operation b5, the base film is removed therefrom in operation b6. By removing the base films, a negative electrode can be obtained as shown in Fig. 8.

As described above, the negative electrode for nickel/zinc secondary batteries is prepared using the lamination process according to the second embodiment, so that the electrode plates can be more uniformly coated, allowing easy coating of new layers, and can be more strongly coupled to the current collector.

In accordance with a third embodiment of the invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (c1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (c2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (c3) adding a binder to the slurry to prepare a composition for negative electrodes; (c4) coating a surface of a copper or copper alloy current collector with a dispersion liquid prepared by dispersing activated carbon in a binder; (c5) coating the composition on the current collector; and (c6) drying the composition coated on the current collector.

Next, referring to Figs. 9 to 10, the method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with the third embodiment will be described in detail. Fig. 9 is a flowchart of the method in accordance with the third embodiment. Fig. 10 shows a copper-foam current collector having an activated carbon layer formed on a surface thereof by coating a dispersion liquid containing activated carbon in operation c4 of the method in accordance with the third embodiment.

In the method in accordance with the third embodiment, operations c1 to c3 correspond to operations a1 to a3 of the method according to the first embodiment.

Referring to Fig. 10, operations c1 to c3 of the method according to the third embodiment are performed as in operations a1 to a3 of the method according to the first embodiment.

Next, the surface of the copper or copper alloy current collector is coated with the dispersion liquid containing the activated carbon in operation c4. In this embodiment, to improve efficiency of the nickel/zinc secondary battery by enhancing electrical conductivity of the negative electrode of the secondary battery, a copper or copper alloy-based metal sheet, expanded metal, punching metal, metal foam, or the like is used. Further, to prevent the copper-based current collector from being corroded by the alkaline electrolyte solution, the surface of the current collector is coated with the dispersion liquid, which contains 80~97 wt% activated carbon and 3~20 wt% binder. Here, the binder may be PE, PTFE, PVA, PVdF, and the like. In this case, the solvent may be distilled water, alcohol, and an organic solvent, such as THF (tetra-hydofuran), NMP (NMethyl-pyrrolidone), or the like.

If the content of activated carbon exceeds 97 wt%, the binding force of the composition to the current collector is decreased due to the thick activated carbon layer on the surface of the current collector, and if the content of activated carbon is less than 80 wt%, it is difficult to effectively suppress corrosion of the current collector.

Fig. 10 shows a copper-foam current collector 220 having an activated carbon layer 230 formed on the surface thereof by coating the dispersion liquid containing the activated carbon.

The current collector 220 coated with the dispersion liquid has 40~120 PPI (pore per inch).

Next, the composition for negative electrodes is coated on the current collector in operation c5, and is dried thereon in operation c6, thereby providing a negative electrode.

After coating and drying the composition on the current collector, the negative electrode may be subjected to roll pressing. The roll pressing may be carried out as described above.

In accordance with a fourth embodiment of the invention, a method of fabricating a negative electrode for nickel/zinc secondary batteries includes: (d1) mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture; (d2) wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry; (d3) adding a binder to the slurry to prepare a composition for negative electrodes; (d4) coating the composition on a current collector; (d5) drying the composition coated on the current collector to form a negative electrode; (d6) coating and drying a separator with a dispersion liquid prepared by dispersing porous ceramic or silica in a binder or a thickening agent; and (d7) compressing the separator to the negative electrode by application of heat and pressure.

Next, referring to Figs. 11 to 12, the method of fabricating a negative electrode for nickel/zinc secondary batteries in accordance with the fourth embodiment will be described in detail. Fig. 11 is a flowchart of the method in accordance with the fourth embodiment. Fig. 12 shows a process of compressing separators to the negative electrode in operation d6 of the method in accordance with the fourth embodiment.

In the method in accordance with the fourth embodiment, operations d1 to d3 correspond to operations a1 to a3 of the method according to the first embodiment.

Referring to Fig. 11, operations d1 to d3 of the method according to the fourth embodiment are performed as in operations a1 to a3 of the method according to the first embodiment.

Next, the composition for negative electrodes is coated on the current collector in operation d4, and is dried thereon in operation d5, thereby providing a negative electrode.

Next, the separator is coated with the dispersion liquid prepared by dispersing the porous ceramic or silica in the binder or the thickening agent, followed by drying the separator in operation d6. More specifically, in the method according to the fourth embodiment, the separator is compressed to the negative electrode by application of heat and pressure to the separator to prevent formation of dendrites caused by dissolution of the negative electrode. At least one separator may be used for the negative electrode. Here, the separator is composed of a fine porous film, and the fine pores of the separator may be closed by heat during compression of the separator. Accordingly, in the method according to the fourth embodiment, the porous ceramic or silica is uniformly dispersed in the binder including PTFE, PE, SBR and the like, and the thickening agent such as CMC, HEC and acrylic ester to prepare the dispersion liquid, which in turn is coated and dried on the surface of the separator, in order to maintain porosity of the separator. As such, when the dispersion liquid is coated on the surface of the separator, the number of pores closed by the application of heat and pressure can be significantly reduced to 5~20% of the number of pores of the separator before the application of heat. The dispersion liquid may contain 1~10 parts by weight of the porous ceramic or silica with reference to an average active material for each electrode plate.

Further, to allow the separator to be more easily bonded to the negative electrode, PE powders having enhanced binding capability upon application of heat, calcium hydroxide (Ca(OH)₂) chemically coupled to charge products of zinc to prevent dissolution of the electrode in the alkaline solution, and the like may be further added to the dispersion liquid, which is coated on the separator. The dispersion liquid may contain 1~10 parts by weight of calcium hydroxide with reference to an average active material for each electrode plate.

Last, the separator is compressed to the negative electrode prepared in operation d5 by applying heat and pressure in operation d7. Referring to Fig. 12, two separators 107 are disposed at opposite sides of a negative electrode 103 to face each other, followed by compressing the separators by application of heat and pressure, in operation d5. The compression may be performed using a heating roll press, a heating plate press or the like. The compression may be performed at a temperature of 70~120 and a pressure of 50~500 kg/ for a thermal bonding duration of 5~180 seconds.

As such, in the method according to the fourth embodiment, the lamination process is used to form a dense interface by compressing the separators to the negative electrode, so that the solubility of the negative electrode is reduced in the alkaline solution during charge/discharge reaction, as compared with the negative electrode employing conventional bag-shaped separators, thereby significantly improving charge/discharge efficiency.

Next, preferred examples will be descried for thorough understanding of the present invention. Here, it should be understood that the following examples are given by way of illustration only and do not limit the scope of the invention. Further, it will be apparent that various modifications and changes can be made without departing from the spirit and scope of the invention. The scope of the invention should be limited only by the accompanying claims.

Example 1

ZnO 1764 g, AB 40g, Ca(OH)₂ 44g, and Bi₂O₃ 72g were mixed at 180 rpm in a container for 3 hours using a ball mill process, and passed through a sieve to separate anode materials from balls. After wetting the prepared anode materials using distilled water, substituted CMC was added to the anode materials to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE, PE and SBR, and stirred to prepare a composition for negative electrodes, which was coated on foam-type current collectors. The coated electrodes were dried at 90 for 2 hours in an oven, roll-pressed to 30% of an initial thickness thereof, and covered with box-shaped separators, thereby providing negative electrodes. Here, two Celgard 3407 films were used as the separators for each electrode. A compositional ratio of the materials for fabrication of the negative electrodes is shown in Table 1.

Table 1

	ZnO	AB	Ca(OH)₂	Bi₂O₃	PTFE	PE	SBR	CMC
Composition (wt%)	88.2	2.0	2.2	3.6	0.6	2.0	0.4	1.0

On the other hand, after loading Ni(OH)₂ 3657 g, Ni powder 116 g, and CoO 116 g in a mixer, CMC was added to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE and PE, and stirred to prepare a composition for positive electrodes, which was coated on current collectors. The coated electrodes were dried at 90 for 2 hours in an oven, roll-pressed to 30% of an initial thickness thereof, and covered with box-shaped separators, thereby providing positive electrodes. Here, two NKK non-woven films were used as the separators for each electrode. A compositional ratio of the materials for fabrication of the positive electrodes is shown in Table 2.

Table 2

	Ni(OH)₂	Ni powder	CoO	PTFE	PE	CMC
Composition (wt%)	91.43	2.9	2.9	0.6	1.97	0.2

A battery having a desired capacity was fabricated by alternately stacking the prepared positive and negative electrodes. 11 positive electrode plates and 12 negative electrode plates were alternately stacked on each other to produce a battery having a capacity of 50 Ah. The stacked electrode plates were received in a battery case and capped by a cover. Then, an electrolyte solution was supplied into the battery case, which in turn was subjected to aging in a vacuum at room temperature for 6 hours to activate the battery, thereby completing a nickel/zinc secondary battery.

Fig. 13 is a graph depicting performance of the nickel/zinc secondary battery fabricated by Example 1. It can be corroborated from Fig. 13 that the nickel/zinc secondary battery fabricated in Example 1 has superior discharge efficiency and cycle life.

Example 2

ZnO 1764 g, AB 40g, Ca(OH)₂ 44g, and Bi₂O₃ 72g were mixed at 180 rpm in a container for 3 hours using a ball mill process, and passed through a sieve to separate anode materials from balls. After wetting the prepared anode materials using distilled water, substituted CMC was added to the anode materials to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE, PE and SBR, and stirred to prepare a composition for negative electrodes. Then, a release agent layer was coated on each of PP base films using a silicone-based release agent, followed by coating the composition for negative electrodes on the release agent layer. With the PP base films disposed to face each other, a copper expanded metal current collector was located between the PP base films, which in turn were compressed at a temperature of 90 and a pressure of 500 kg/ for 20 seconds using a heating roll press. Then, the base films were removed from the current collector, which in turn was covered with box-shaped separators, thereby providing negative electrodes. Here, two Celgard 3407 films were used as the separators for each electrode. A compositional ratio of the materials for fabrication of the negative electrodes is shown in Table 3.

Table 3

On the other hand, after loading Ni(OH)₂ 3657 g, Ni powder 116 g, and CoO 116 g in a mixer, CMC was added to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE and PE, and stirred to prepare a composition for positive electrodes, which was coated on current collectors. The coated electrodes were dried at 90 for 2 hours in an oven, roll-pressed to 30% of an initial thickness thereof, and covered with box-shaped separators, thereby providing positive electrodes. Here, two NKK non-woven films were used as the separators for each electrode. A compositional ratio of the materials for fabrication of the positive electrodes is shown in Table 4.

Table 4

Fig. 14 is a graph depicting performance of the nickel/zinc secondary battery fabricated by Example 2. It can be corroborated from Fig. 14 that the nickel/zinc secondary battery fabricated by Example 2 has superior discharge efficiency and cycle life.

Example 3

ZnO 1764 g, AB 40g, Ca(OH)₂ 44g, and Bi₂O₃ 72g were mixed at 180 rpm in a container for 3 hours using a ball mill process, and passed through a sieve to separate anode materials from balls. After wetting the prepared anode materials using distilled water, substituted CMC was added to the anode materials to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE, PE and SBR, and stirred to prepare a composition for negative electrodes. Then, surfaces of copper-foam shaped current collectors were coated by spraying a dispersion liquid containing 95 wt% activated carbon and 5 wt% PTFE. After drying the current collectors, the composition for negative electrodes was coated on the surfaces of the current collectors. Then, the electrodes were dried at 90 for 2 hours in an oven, roll-pressed to 30% of an initial thickness thereof, and covered with box-shaped separators, thereby providing negative electrodes. Here, two Celgard 3407 films were used as the separators for each electrode. A compositional ratio of the materials for fabrication of the negative electrodes is shown in Table 5.

Table 5

On the other hand, after loading Ni(OH)₂ 3657 g, Ni powder 116 g, and CoO 116 g in a mixer, CMC was added to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE and PE, and the slurry was stirred to prepare a composition for positive electrodes, which was coated on current collectors. The coated electrodes were dried at 90 for 2 hours in an oven, roll-pressed to 30% of an initial thickness thereof, and covered with box-shaped separators, thereby providing positive electrodes. Here, two NKK non-woven films were used as the separators for each electrode. A compositional ratio of the materials for fabrication of the positive electrodes is shown in Table 6.

Table 6

Example 4

ZnO 1764 g, AB 40g, Ca(OH)₂ 44g, and Bi₂O₃ 72g were mixed at 180 rpm in a container for 3 hours by a ball mill process, and passed through a sieve to separate anode materials from balls. After wetting the prepared anode materials using distilled water, substituted CMC was added to the anode materials to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE, PE and SBR, and stirred to prepare a composition for negative electrodes, which was coated on foam-type current collectors. The coated electrodes were dried at 90 for 2 hours in an oven, roll-pressed to 30% of an initial thickness thereof, and covered with box-shaped separators, thereby providing negative electrodes. Then, a dispersion liquid having porous ceramics, calcium hydroxide, PTFE and CMC uniformly dispersed therein was coated on surfaces of Celgard 3407 films provided as separators. After disposing the prepared negative electrode between the separators, the separators were compressed with respect to the negative electrode at a temperature of 90 and a pressure of 100 kg/ for 10 seconds using a heating roll press, thereby fabricating final negative electrodes. A compositional ratio of the materials for fabrication of the negative electrodes is shown in Table 7.

Table 7

On the other hand, after loading Ni(OH)₂ 3657 g, Ni powder 116 g, and CoO 116 g in a mixer, CMC was added to prepare a viscous slurry. Then, binders were added to the slurry in the sequence of PTFE and PE, and the slurry was stirred to prepare a composition for positive electrodes, which was coated on current collectors. The coated electrodes were dried at 90 for 2 hours in an oven, roll-pressed to 30% of an initial thickness thereof, and covered with box-shaped separators, thereby providing positive electrodes. Here, two NKK non-woven films were used as the separators for each electrode. A compositional ratio of the materials for fabrication of the positive electrodes is shown in Table 8.

Table 8

Claims

A method of fabricating a negative electrode for nickel/zinc secondary batteries comprising:

mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture;

wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry;

adding a binder to the slurry to prepare a composition for negative electrodes;

coating the composition on a current collector; and

drying the composition coated on the current collector.
A method of fabricating a negative electrode for nickel/zinc secondary batteries comprising:

mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture;

wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry;

adding a binder to the slurry to prepare a composition for negative electrodes;

coating a release agent on base films, followed by coating the composition thereon;

disposing two base films having the composition coated thereon at opposite sides of a current collector to face each other, followed by compressing the base films by application of heat and pressure; and

removing the base films from the current collector.
A method of fabricating a negative electrode for nickel/zinc secondary batteries comprising:

mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture;

wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry;

adding a binder to the slurry to prepare a composition for negative electrodes;

coating a surface of a copper or copper alloy current collector with a dispersion liquid prepared by dispersing activated carbon in a binder;

coating the composition on the current collector; and

drying the composition coated on the current collector.
A method of fabricating a negative electrode for nickel/zinc secondary batteries comprising:

mixing an anode active material and activated carbon or graphite powder using a ball-mill process to prepare a mixture;

wetting the mixture, followed by adding a thickening agent to the mixture to prepare a slurry;

adding a binder to the slurry to prepare a composition for negative electrodes;

coating the composition on a current collector;

drying the composition coated on the current collector to form a negative electrode;

coating and drying a separator with a dispersion liquid prepared by dispersing porous ceramic or silica in a binder or a thickening agent; and

compressing the separator to the negative electrode by application of heat and pressure.
The method according to any one of claims 1 to 4, wherein the anode active material comprises at least one selected from the group consisting of zinc oxide, calcium zincate, and zinc powder.
The method according to any one of claims 1 to 4, wherein the activated carbon comprises at least one selected from the group consisting of acetylene black and Ketjen black.
The method according to any one of claims 1 to 4, wherein the thickening agent comprises at least one selected from the group consisting of CMC, HEC and acrylic ester.
The method according to claim 7, wherein the thickening agent is substituted Na+ with K+.
The method according to any one of claims 1 to 4, wherein the binder comprises at least one selected from the group consisting of PTFE, PE, and SBR.
The method according to any one of claims 1 to 4, wherein the anode active material and the activated carbon are mixed in a ratio of 95~99 wt% and 1~5 wt% by weight, respectively.
The method according to any one of claims 1 to 4, wherein a metal oxide selected from the group consisting of Ca(OH)₂, Bi₂O₃, Tl₂O₃, In₂O₃ and SnO is added to the mixture when mixing the anode active material and the activated carbon.
The method according to claim 1, further comprising:

roll-pressing a nickel/zinc negative electrode fabricated after drying the composition coated on the current collector.
The method according to claim 3, wherein the dispersion liquid comprises 80~97 wt% of activated carbon and 3~20 wt% of a binder.
The method according to claim 3, wherein a solvent used for preparation of the dispersion liquid is selected from distilled water, alcohol, tetra-hydrofuran (THF), and N-methyl-pyrrolidone.
The method according to claim 3, wherein the binder for the dispersion liquid comprises at least one selected from the group consisting of PE, PTFE, PVA and PVdF.
The method according to claim 3, wherein the current collector coated with the dispersion liquid has 40~120 PPI.
The method according to claim 4, wherein the dispersion liquid contains 1~10 parts by weight of the porous ceramics or silica with reference to an average active material for each electrode plate.
The method according to claim 4, wherein the dispersion liquid further comprises PE powder and calcium hydroxide.
The method according to claim 18, wherein the dispersion liquid contains 1~10 parts by weight of calcium hydroxide with reference to an average active material for each electrode plate.