WO2007059687A1

WO2007059687A1 - A composition for zinc negative electrode material, and zinc negative electrode and secondary battery comprising the same

Info

Publication number: WO2007059687A1
Application number: PCT/CN2006/002945
Authority: WO
Inventors: Zhijian Zhu
Original assignee: Byd Company Limited
Priority date: 2005-11-23
Filing date: 2006-11-02
Publication date: 2007-05-31
Also published as: CN1971978A; CN100449826C

Abstract

A zinc negative electrode material composition comprises elementary zinc and/or zinc oxide, wherein said composition further comprises a zinc salt insoluble in alkaline electrolyte solution. When used as a negative electrode material of battery, the composition for a zinc negative electrode material provided by the present invention can effectively suppress the shape change of the zinc electrode without reducing the energy density of the battery. After multiple cycles, the rate of the residual area of zinc negative electrode is high and the capacity attenuation of the battery is slow.

Description

A Composition for Zinc Negative Electrode Material, and Zinc Negative

Electrode and Secondary Battery Comprising the Same

Field of the Invention The present invention relates to a composition for negative electrode material, and negative electrode and secondary battery comprising the same, in particular, it relates to a composition for zinc negative electrode material, and zinc negative electrode and secondary battery comprising the same.

Background of Invention

The capacity of a traditional secondary battery comprising zinc negative electrode, such as a zinc-silver battery or a zinc-nickel battery, attenuates rapidly along with the increase of the cycle number. Such capacity attenuation is ascribed to the morphological variation of the zinc electrode, which is called "shape change of zinc electrode" by those skilled in the art. Zinc comprised in an electrode of a secondary battery with elementary zinc and/or zinc oxide as negative electrode active substance converts into zinc oxide during the discharging process of the battery, while zinc oxide converts into metal zinc again during the charging process. During the discharging process of the battery, elementary zinc comprised in the zinc electrode is oxidized to form zinc oxide (ZnO) or zinc hydroxide (Zn(OH)₂), which can be dissolved in strong alkaline electrolyte solution in a large amount. During the charging process, because these products have very high solubility in the alkaline electrolyte solution and convert into zincates such as potassium zincate or sodium zincate after dissolving in the alkaline electrolyte solution, most of zinc in the oxidation state cannot deposit on its original position where it dissolves. Therefore, the amount of zinc dissolved on the brim of the electrode is greater than that depositing on the brim of the electrode, while the situation is just the opposite for the central part of the electrode, leading to the redistribution of zinc on the electrode and concentration of the active substance from the brim of the electrode to the central part. Along with the increase of the cycle number, the active substance on the brim of the electrode continuously decreases, while the active substance on the central part of the electrode continuously increases, leading to the shape change of the electrode, whereby results in the decrease of the surface area capable of conducting electrochemical reaction. Therefore, the discharging speed and battery capacity are lowered and the cycle life of the battery is shortened. Besides, a part of the zincate depositing round the electrode and in the membrane makes the mass transfer of the zinc electrode difficult and forms dendritic deposits on some spots of the outer surface of the electrode, which may easily pierce the diaphragm.

To solve the above problems, US 3873367 discloses a negative electrode for an alkaline storage cell comprising active substance which is selected from the group consisting of metallic zinc and zinc compounds, mixed with a further compound of an alkaline-earth reactant capable of forming low-solubility zincates upon the passage of zinc into solution on discharge of the electrode, the proportion of said active substance decreasing outwardly from the interior of said mass to zero at the exterior thereof and the proportion of said reactant increasing outwardly from the interior of said mass to 100% at the exterior thereof. In regard to this, the , reactant is calcium. This process converts the discharging products of zinc into insoluble calcium zincate or magnesium zincate by adding calcium hydroxide or magnesium hydroxide to the electrode so that the solubility of the discharging products of zinc in the electrolyte solution is lowered because calcium zincate or magnesium zincate does not dissolve in the electrolyte solution but deposits on the surface of the zinc electrode or inside the zinc electrode. US 5302475 discloses a rechargeable cell containing an electrode comprising an electrochemically active zinc material, and an aqueous alkaline electrolyte which inhibits shape change in said zinc electrode, said electrolyte comprising: a) KOH, in a concentration of about 3.2M; b) KF, in a concentration of about 1.8M; and c) K₂CO₃, in a concentration of about 1.8M. US 4224391 discloses a battery having a positive electrode, a negative electrode and an electrolyte with the positive electrode having zinc or zinc alloy as an active anodic material, the negative electrode having a metal oxide or hydroxide as an active cathodic material, and the electrolyte comprising a solution of a salt formed by the reaction of one or more acids selected from the group consisting of boric acid, phosphoric acid and arsenic acid with an alkali or earth alkali hydroxide present in an amount to produce a stoichiometric excess of said hydroxide to said acid in the range of 0.02 to 3.0 equivalents per liter..

US 4273841 discloses a secondary electrochemical battery, wherein one of the active substances in the electrode is zinc and the electrolyte solution is an aqueous solution comprising 5-10% by weight of potassium hydroxide, 5-15% by weight of potassium fluoride, and 10-20% by weight of potassium phosphate.

All of US Patent Nos. 5302475, 4224391 and 4273841 reduce the solubility of the zinc discharging products in the electrolyte solution and thus suppress the shape change of the electrode by introducing some ions such as fluorine ion, carbonate radical ion, phosphate radical ion, borate radical ion, arsenate radical ion, silicate radical ion, etc. into the electrolyte solution.

Although the above methods can have an effect of suppressing the shape change of the zinc electrode, the area residual rate of the zinc electrode is not high enough and the capacity attenuation of the battery is rapid after many times of cycle. Besides, the addition of the above ions lowers the concentration of the hydroxide radical ion (OH^"), which strongly affects the electric conductivity of the electrolyte solution, so that the electric conductivity of the electrolyte solution is reduced and the internal resistance of the battery is increased.- Therefore, high current discharge of the battery is impossible. That is, the performance of the high current discharge is poor. Besides, although the method disclosed in US 3873367, wherein calcium hydroxide is added to the electrode, can suppress the shape change of the zinc electrode to a certain extent, the more materials which do not participate in the discharge are added, the lower the content of zinc and the less the released energy, leading to a reduction of the energy density of the battery.

Summary of the Invention

An object of the present invention is to overcome the above-mentioned shortcomings in the prior art, and to provide a composition for zinc negative electrode material able to effectively suppress the shape change of the zinc electrode, raise the rate of the residual area of zinc negative electrode comprised in a secondary battery after multiple cycles, and slower the capacity attenuation of the battery, a zinc negative electrode and a secondary battery comprising the same.

The composition for zinc negative electrode material provided by the present invention comprises elementary zinc and/or zinc oxide, wherein said composition further comprises a zinc salt insoluble in alkaline electrolyte solution.

The zinc negative electrode of a secondary battery provided by the present invention comprises a current collector and a negative electrode material coated on and/or filled in the current collector, wherein said negative electrode material is the composition for zinc negative electrode material provided by the present invention. The secondary battery provided by the present invention comprises an electrode core, an alkaline electrolyte solution, and a battery shell, said electrode core and alkaline electrolyte solution are sealed in the battery shell, said electrode core comprises a positive electrode, a negative electrode, and a membrane, wherein said negative electrode is the zinc negative electrode of secondary battery provided by the present invention. Because the zinc salt insoluble in alkaline electrolyte solution is added to the composition for zinc negative electrode material provided by the present invention, during the- discharging process of a battery, the zinc in oxidation state in zinc salts comprised in the above composition used as the battery negative electrode material forms crystal nuclei on the electrode after reduction for the deposition of the zincates dissolved in the electrolyte solution, allowing the discharging products of the zinc active substance, which enters the electrolyte solution and dissolves in it, to deposit on the original region, where it leaves from due to dissolution, during the charging process. The formed crystal nuclei are uniformly distributed in the electrode because the zinc salt added to the electrode is uniformly distributed in the electrode. Therefore, the discharging products of zinc dissolved in the electrolyte solution uniformly redeposit on the electrode during the charging process through these uniformly distributed crystal nuclei formed from the zinc salts added to the electrode and thus avoid the tendency of depositing on the central part of the electrode, thereby effectively suppressing the shape change of the zinc electrode. Addition of said salt insoluble in electrolyte solution can not only well suppress the shape change of the zinc electrode, but also maintain the energy density of the battery with such an electrode as the negative electrode because said zinc salt comprises a large amount of zinc having electrochemical activity. Besides, the concentration of the hydroxide radical ion in the alkaline electrolyte solution is not lowered because the present invention does not change the electrolyte solution and the zinc salt added to the electrode does not dissolve in the alkaline electrolyte solution, thus, the problem of affecting the high current discharge performance of the battery does not exist.

Detailed Description of the Preferred Embodiments

According to the present invention, various zinc salts insoluble in alkaline electrolyte solution can be used to achieve the object of the present invention, preferably, the zinc salt insoluble in alkaline electrolyte solution comprised in said composition for zinc negative electrode material is one or more selected from the group consisting of zinc carbonate, zinc silicate, zinc phosphate, and zinc borate.

According to the present invention, the content of said zinc salt insoluble in electrolyte solution is 0.5-50% by weight, preferably 1-40% by weight based on the total weight of said composition for zinc negative electrode material.

Said negative electrode active substance comprises elementary zinc and/or zinc oxide.

Said elementary zinc such as zinc powder and/or zinc oxide is a conventional negative electrode active substance used in the zinc negative electrode secondary battery. Besides, said negative electrode active substance can further comprise other conventional negative electrode active substance known to those skilled in the art.

Generally, the composition for negative electrode material further comprises an adhesive. The species and content of said adhesive are known to those skilled in the art. For example, said adhesive can be one or more selected from the group consisting of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, sodium polyacrylate, and polytetrafluoroethylene, and the content of the adhesive is known to those skilled in the art, generally being 0.01-10% by weight, preferably 0.02-8% by weight based on the total weight of the negative electrode active substance.

Said composition for zinc negative electrode material can further comprise a conductive agent. The conductive agent is not specially limited and can be a conventional conductive agent used for negative electrode in the art such as one or more selected from the group consisting of conductive acetylene black, conductive graphite, conductive carbon black, conductive metal power such as copper powder, tin powder. Generally, the content of said conductive agent is 0.1-15% by weight, preferably 0.5-10% by weight based on the total weight of the composition for zinc negative electrode material.

According to the zinc negative electrode of secondary battery provided by the present invention, said current collector can be a conventional negative electrode current collector in the secondary battery comprising zinc negative electrode such as punched copper belt, woven copper wire and porous drawn copper net. In the embodiments of the present invention, punched copper belt is used as the current collector.

The zinc negative electrode of secondary battery provided by the present invention can be used in various secondary batteries such as secondary zinc-silver battery, secondary zinc-manganese battery, and zinc negative electrode accumulator. According to the present invention, said zinc negative electrode of secondary battery can be made by various processes in the prior art. For example, said zinc negative electrode of secondary battery can be made by the process comprising coating a slurry comprising a negative electrode active substance, an adhesive, and a solvent on a current collector, drying, optionally rolling, wherein said negative electrode active substance is the composition for zinc negative electrode material provided by the present invention and said solvent can be selected from any solvents which enable said negative electrode active substance, adhesive, and solvent to form a paste, preferably is water. The amount of said solvent is able to provide the paste with viscosity and flowability, and coat said paste on the current collector. Generally, the content of said solvent is 10-60% by weight, preferably 15-55% by weight based on the total weight of the negative electrode active substance.

Said slurry can be prepared by first uniformly mixing the zinc salt, elementary zinc, and/or zinc oxide, and optional conductive agent dry powder, and then mixing the resultant mixture with the adhesive and solvent to obtain the slurry, or by directly uniformly mixing the zinc salt, elementary zinc, and/or zinc oxide, optional conductive agent together with adhesive and water to obtain the slurry. Said adhesive can be used in powder form or solution form. The process for coating said slurry on said current collector and the process and conditions for drying and rolling are known to those skilled in the art.

The secondary battery provided by the present invention comprises an electrode core, an alkaline electrolyte solution, and a battery shell, said electrode core and alkaline electrolyte solution are sealed in the battery shell, said electrode core comprises a positive electrode, a negative electrode and a membrane, wherein said negative electrode is the zinc negative electrode of secondary battery provided by the present invention.

Because the present invention only relates to the improvement of the battery negative electrode, the positive electrode and membrane constituting the electrode core as well as the , alkaline electrolyte solution and battery shell are not specifically limited and can be the ones conventionally used in the art.

For example, said positive electrode can be a nickel positive electrode obtained by stirring a positive electrode active substance (such as one of nickel hydroxide, silver oxide, and manganese dioxide), an adhesive, and an additive corresponding to the positive electrode active substance, and deionized water to form a slurry, coating the slurry on foam nickel to which;, a current conducting belt is welded, then drying, rolling, and cutting the resultant foam nickel to yield the nickel positive electrode. Said additive corresponding to the positive electrode active substance is determined by the used positive electrode active substance. For example, when nickel hydroxide is used as the positive electrode active substance, the corresponding additive is cobalt protoxide, conductive carbon black, or nickel powder. Said membrane is located between the positive electrode and negative electrode and possesses electric insulating performance and liquid retaining performance and can obstruct the penetration of dendrites. The membrane is contained together in the battery body with said electrode core and alkaline electrolyte solution. Said membrane can be selected from various composite membrane used in the alkaline secondary battery with zinc as the negative electrode, such as various composite membrane prepared by combining microporous membrane with liquid storage membrane able to obstruct the penetration of zinc dendrites. Said microporous membrane can be selected from various microporous membrane wettable by alkaline electrolyte solution known to those skilled in the art such as one or more of polypropylene microporous membrane treated with nonionic surfactant or one or more of the series of polyethylene radiation graft membrane. It can also be the hydrated cellulose membrane or glass paper membrane or polyvinyl alcohol membrane. The liquid-storing membrane constituting another part of the composite membrane can be selected from modified polypropylene felt, vinylon felt, or nylon felt used in the alkaline secondary battery. The composite membrane is a sheet element. The location, character, and species of said membrane are known to those skilled in the art. Said alkaline electrolyte solution can be at least one selected from the solutions of KOH, NaOH, and LiOH. The amount of the injected electrolyte solution is generally 0.9-4.5 g/Ah and the concentration of the electrolyte solution is generally 6-8 mol/1.

Said battery shell can be various shells usable to various types of batteries and those skilled in the art can easily select the suitable battery shell according to the requirement. The following examples will further illustrate the present invention.

Example 1

(1) Preparation of the negative electrode

70 g zinc oxide, 20 g zinc carbonate, and 3 g conductive carbon black were uniformly mixed in a stirrer, and then mixed with 20 g 3wt% aqueous solution of polyvinyl alcohol, 25 g 2wt% aqueous solution of hydroxypropyl methyl cellulose, 4.8 g polytetrafluoroethylene, and 5 g deionized water to form a sticky slurry. The above slurry was coated on a punched copper belt with a thickness of 0.07 mm and a width of 42 mm plated with Pb-Sn alloy layer. Then the coated foam nickel was dried at 105°C, rolled, cut, and welded to a current conducting belt to yield a negative electrode sheet with a dimension of 95 mmx42 mmxθ.6 mm. Said negative electrode comprised 5 g negative electrode material. (2) Preparation of the positive electrode

92 g spherical nickel hydroxide, 7 g cobalt protoxide, H g conductive carbon black, 3 g polytetrafluoroethylene, 0.2 g carboxymethyl cellulose, and 52 g deionized water were fully stirred and mixed to form a slurry, and the resulted slurry was coated on a foam nickel to which a current conducting belt was welded. Then the coated foam nickel was dried at 105°C, rolled, cut, to yield a positive electrode sheet with a dimension of 65 mmx40 mmxθ.6 mm. Said positive electrode comprised 4.5 g positive electrode material.

(3) Assembly of the battery

The negative electrode obtained in (1), a wettable polyolefin microporous membrane with which vinylon felt was combined, and the positive electrode obtained in (2) were sequentially laminated and reeled to form a volute electrode core. The obtained electrode core was put into an AA-type steel battery shell with one end being open, and then 1.1 g/Ah electrolyte solution (a mixed aqueous solution of KOH and LiOH comprising 30% by weight KOH and 1.5% by weight LiOH) was added. AA-type cylindrical zinc-nickel battery Al was obtained after the battery shell was sealed.

Examples 2-8

The compositions of zinc negative electrode material, zinc negative electrodes of secondary batteries, and secondary batteries A2-A8 comprising said negative electrode compositions were prepared according to the process in Example 1, except that the amounts of zinc oxide, elementary zinc, and zinc salt were shown in the following Table 1.

Table 1

Comparative Example 1

The positive electrode and negative electrode were prepared and assembled into reference battery ACl according to the process in Example 1, except that no zinc salt insoluble in the electrolyte solution was added to the zinc electrode.

Comparative Example 2

The positive electrode and negative electrode were prepared and assembled into reference battery AC2, except that 20 g calcium hydroxide was added to the zinc electrode.

Test Example 1

Secondary batteries A1-A8 prepared in Examples 1-8 and reference secondary batteries ACl and AC2 prepared in Comparative Examples 1-2 were tested respectively for their performance.

I. Activation of the batteries Secondary batteries A1-A8 prepared in Examples 1-8 and reference secondary batteries ACl and AC2 prepared in Comparative Examples 1-2 were respectively activated by charging at a current of 70 mA for 16 hours, and then discharging at 350 mA till the battery voltage was 1.3 V.

II. Test of the performance of the batteries The above activated batteries A1-A8 and AC1-AC2 were respectively charged at 140 mA, laid-aside for 10 min, and then discharged at 700 mA till the voltage was 1.3 V, and laid-aside for 5 min. The above procedures were repeated for 250 times and the discharge capacities of the first cycle and the 250^th cycle were recorded. The rates of retaining capacity of the batteries were calculated by the following formula: Rate of retaining capacity=(discharge capacity of the 250^th cycle/discharge capacity of the first cycle)xlOO%.

The results are shown in Table 2.

The batteries were dissected and determined for the area of the zinc electrode on which the current collector was not exposed. Said area is denoted as Sl . The area residual rates after 250 times of cycles were calculated according to the formula: area residual rate=(Sl/S) x 100%, wherein S represents the original area of the electrode (95 mmx42 mm). The results are shown in Table 2. Table 2

It can be seen from the results shown in Table 2 that compared with the reference secondary batteries, the secondary batteries provided by the present invention have obviously higher rate of residual area of zinc negative electrode, slower capacity attenuation of the battery, and better cycle performance.

Claims

What is claimed is:

L A composition for zinc negative electrode material comprising elementary zinc and/or zinc oxide, wherein said composition further comprises a zinc salt insoluble in alkaline electrolyte solution.

2. The composition according to claim 1, wherein said zinc salt insoluble in alkaline electrolyte solution is one or more selected from the group consisting of zinc carbonate, zinc silicate, zinc phosphate and zinc borate.

3. The composition according to claim 1, wherein the content of said zinc salt insoluble in alkaline electrolyte solution is 0.5-50% by weight based on the total weight of said composition.

4. The composition according to claim 3, wherein the content of said zinc salt insoluble in alkaline electrolyte solution is 1-40% by weight based on the total weight of said composition.

5. A zinc negative electrode of a secondary battery comprising a current collector and a negative electrode material coated on the current collector, wherein said negative electrode material is the composition for zinc negative electrode material in any of claims 1-4.

6. A secondary battery comprising an electrode core, an alkaline electrolyte solution, and a battery shell, said electrode core and alkaline electrolyte solution being sealed in the battery shell and said electrode core comprising a positive electrode, a negative electrode, and a membrane of the battery, wherein the negative electrode is the zinc negative electrode of a secondary battery in claim 5.