Classifications

• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/06—Electrodes for primary cells
  • H01M4/08—Processes of manufacture
  • H01M4/12—Processes of manufacture of consumable metal or alloy electrodes
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M12/00—Hybrid cells; Manufacture thereof
  • H01M12/04—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type
  • H01M12/06—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type with one metallic and one gaseous electrode
• • H—ELECTRICITY
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  • H01M12/00—Hybrid cells; Manufacture thereof
  • H01M12/08—Hybrid cells; Manufacture thereof composed of a half-cell of a fuel-cell type and a half-cell of the secondary-cell type
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/24—Electrodes for alkaline accumulators
  • H01M4/244—Zinc electrodes
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
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  • H01M4/24—Electrodes for alkaline accumulators
  • H01M4/26—Processes of manufacture
  • H01M4/30—Pressing
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  • H01M4/00—Electrodes
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  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/362—Composites
  • H01M4/364—Composites as mixtures
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
  • H01M4/42—Alloys based on zinc
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
  • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
  • H01M4/621—Binders
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
  • H01M4/621—Binders
  • H01M4/622—Binders being polymers
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M2004/021—Physical characteristics, e.g. porosity, surface area
• • H—ELECTRICITY
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  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M2300/00—Electrolytes
  • H01M2300/0085—Immobilising or gelification of electrolyte
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• This invention relates to electrodes for batteries, and in particular, to zinc electrodes and to a method of manufacturing such electrodes.
• Zinc (Zn) has been used in many battery applications as the anode material. This is due to the high energy capacity of zinc and to its chemical stability in the electrolyte. Zinc electrodes provide high current densities and a flat discharge curve. Battery systems such as nickel-zinc, silver-zinc, zinc-chloride, zinc-bromide, zinc-manganese etc. are well known. A zinc electrode can be made from a solid plate, pellets or powdered zinc materials.
• an organic gelling agent may be added to allow sufficient electrolyte penetration and to maintain Zn - Zn particle contact area. This is important in order to obtain high electronic conductivity.
• gelling agents including their tendency to break up due to repeated swelling and contraction of the zinc electrode, destruction of the uniformity of distribution of zinc powder by impact and the formation of passive layers under high discharge current densities. Accordingly an object of the present invention is to provide an electrode with improved Zn - Zn particle contact area.
• Reaction rate at a zinc electrode made from zinc powder is dependent on active surface area and so theoretically shape and available surface area of the zinc particles should be optimal to give best performance.
• Various additives have also been used in zinc electrodes in an attempt to increase activity and stability. For example, graphite has been added to electrodes to increase particle-particle contact area and calcium zincate has been used to improve the stability.
• a further object of the present invention is to provide a zinc electrode with increased activity and stability.
• Shape change is defined as the migration of zinc. Most common is the migration from the top and sides of the electrode to the bottom. This is due in part to the dissolution of zinc during the discharge cycle. Zinc, in oxide form, and partially dissolved in potassium hydroxide, is moved to other areas of the electrode. Movement occurs down the concentration gradient of the dissolved zinc hydroxides and so zinc is moved to other areas of the electrode, to a great extent in the lower portion, where the zinc is redeposited on charge. During charge / discharge cycles, the general tendency is for zinc to leave certain areas of the electrode and concentrate in other areas. Usually zinc is concentrated in the lower part of the electrode due to gravitational forces, causing the lower portion of the electrode to become denser than the upper part. Potential differences over the electrode surface may also result in local shape changes.
• Zinc is extremely soluble in strong alkaline environments. The high solubility allows for rapid current spikes typically unattainable with other battery systems.
• zinc diffusion leads to the well-known phenomena of electrode shape change and the presence of zinc dendrites within the battery. Attempts to control shape change and dendrite formation include:
• French Patent No. 2264401 describes a secondary zinc electrode which is produced by applying to a collecting grid a non-hardened mixture containing particulate zinc oxide, a binder such as PTFE, and other substances, such as rayon fibres or metal powders.
• this electrode still suffers from problems of dendritic growth and, in particular, shape changes or deformations, probably as a result of movement of the electrolyte parallel to the surface of the electrode during the charge and discharge cycles.
• a further possible approach to controlling shape change and dendrite formation in rechargeable zinc electrodes is by containing the zinc in a matrix by encapsulation, typically using a gelling agent.
• Organic gelling agents with high ionic conductivity such as acrylate polymers (e.g. CARBOPOL), can be used to encapsulate zinc powder.
• a further problem with such gelling agents is the swelling nature of the agents when in contact with the alkaline electrolyte (e.g. KOH) due to incorporation of the electrolyte into the organic polymer.
• Such swelling of the polymer may result in both positive and negative effects on the electrode. Severe swelling results in an increased particle contact resistance within the electrode as the zinc particles are removed from each other under repeated charge / discharge cycling. This reduces the capacity of the system and limits the number of charge / discharge cycles.
• the current collector pressed into the electrode or implemented in the battery can also suffer from these effects as part of it loses electronic contact with the zinc matrix. In addition dry spots can occur in the electrode, reducing the utilisation of zinc.
• An object of the present invention is to provide a zinc electrode which is stable after repeated charge / discharge cycling.
• the present invention provides an electrode for a secondary battery comprising zinc powder as active electrode material, the electrode further comprising an organic gelling agent, an organic binding agent, calcium zincate and an electro- active element for increasing the electrochemical activity of the electrode under charge and discharge.
• the resulting electrode is able to deliver high current densities with good stability at low overpotentials and both high ionic and electronic conductivity are maintained within the electrode after repeated charge / discharge cycling.
• the term "gelling agent” relates to a polymeric material with high OH ⁇ conductivity.
• the function of the gelling agent within the electrode is to form a three dimensional ion conductive structure in the electrode covering the zinc particles reducing zincate mobility and thereby reducing the tendency to dendrite formation. Ionic conductivity of the gelling agent is obtained as the electrolyte (for example aqueous potassium hydroxide) is absorbed into the polymer structure which in turn has a tendency to swell the polymer structure.
• the term "binding agent” relates to a polymeric material which does not absorb OH " and maintains the mechanical stability of the electrode. The function of the binding agent within the electrode is to counteract the tendency of the gelling agent to swell on absorption of the electrolyte thereby reducing shape changes and dendrite formation with repeated charge / discharge cycles.
• the zinc powder has a particle size distribution by weight such that: (i) at least 90 % of the particles have a size within the range 5 to 400 ⁇ m; (ii) at least 25 % of the particles are large particles with a size of 100 ⁇ m or greater; and
• At least 25 % of the particles are small particles with a size of less than 100 ⁇ m; large and small particles being intimately distributed to facilitate particle to particle contact area.
• the use of such a zinc powder gives a good pore size distribution within the electrode and high particle-particle contact.
• An almost bimodal particle size distribution is provided with large particles which would leave gaps in the packing of the particles and small particles which will fill in those gaps.
• Such a particle size distribution allows low ohmic losses to be obtained and no passivation is observed even at high reaction rates.
• the zinc powder has at least 25 % of large particles with a size greater that 200 ⁇ m. More preferably the zinc powder has at least 30 % of large particles with a size greater than 200 ⁇ m and at least 30 % small particles with a size less than 100 ⁇ m.
• the zinc powder is formed as a mixture of first and second zinc powders, the first zinc powder comprising predominantly large particles with a size of 100 ⁇ m or greater and the second zinc powder comprising predominantly small particles with a size of 100 ⁇ m or less.
• the first zinc powder has at least 90 % by weight of particles in the range 75 to 400 ⁇ m and the second zinc powder has at least 80 % by weight particles less than 100 ⁇ m.
• the first and second zinc powders are in a weight ratio of 1:4 to 4:1, most preferably about 1:1. The use of two grades of zinc powder gives improved charge / discharge stability and allows high current densities without passive film formation.
• the gelling agent is a polymer with high OET conductivity which effectively controls the fluidity of the electrode.
• the polymer may be in powder form, such as carboxymethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose, polyvinyl alcohol, polyethylene oxide, polyacrylate or salts thereof, or the polymer may be a granular gelling agent selected from cross-linked and branched type (meth) acrylates and salts thereof (such as CARBOPOL), carboxymethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose, polyvinyl alcohol, starch or a starch derivative.
• the gelling agent is preferably present in an amount of less than 5 wt %, most preferably 0.5 wt % to 2 wt %.
• the binding agent is a polymer or plastics material that reduces swelling of the electrode.
• the polymer may preferably be selected for example from polytetrafluoroethylene (PTFE), polyethylene, polypropylene, and rubber polymers.
• PTFE polytetrafluoroethylene
• the binding agent is present in an amount less than 4 wt %, most preferably less than 3 wt % or less than 2 wt %.
• the calcium zincate component provides Ca-ions in the matrix of the zinc electrode which reduces dendrite growth from the electrode by causing rapid deposition of zincate within the electrode matrix, thereby preventing diffusion of Zn-ions.
• An additional effect of calcium zincate is increased stability of the current density under discharge and prevention of passive film formation especially at high current densities.
• Calcium zincate may be present in an amount of 5 wt % or less which helps to maintain reasonable reaction rates. Higher amounts of calcium zincate are preferable for extended cycle life of the electrode, but at high current densities an activator material that increases the reaction rate should also be added.
• the electrode further comprises an electro-active element. The effect of adding such an electro-active element is that electrochemical activity is increased under both charge and discharge and remains high even with the addition of zincate.
• the electro-active element is preferably a metal oxide in which the metal has at least as high electronic conductivity as Zn.
• Preferred electro-active elements include silver oxide, Ag 2 O; aluminium oxide, Al 2 O 3 ; zinc oxide, ZnO and cobalt oxide, CoO.
• the electrode comprises silver oxide or cobalt oxide in an amount of 2 wt % or less.
• Such electro-active elements may be usefully used in zinc electrodes for secondary batteries or for high power primary batteries.
• the present invention provides a method of forming a zinc electrode for a secondary battery comprising the following steps:
• the present invention relates to the use of the zinc electrode described above in a zinc-air battery, a zinc-silver battery or a zinc/nickel battery for portable electronics applications.
• the invention provides a secondary battery containing a zinc electrode as described above or made according to the method described above, wherein the battery is a zinc-air battery, a zinc-silver battery or a zinc/nickel battery.
• Figure 1 shows the results from potentiodynamic sweeps on electrodes comprising (a) zinc alloy GNlO (powder of small particle fraction), (b) zinc alloy GN40 (powder of large particle fraction) and (c) 1:1 zinc alloy GNlO and GN40;
• Figure 2 shows a potentiodynamic plot of electrodes prepared by (c) adding the gelling agent to the zinc powder, then adding the binding agent, and (d) adding the binding agent to the zinc powder, then adding the gelling agent;
• Figure 3 shows a potentiodynamic plot of electrodes containing (e) 0.1 g PTFE, (f) 0.2 g PTFE and (g) 0.4 g PTFE;
• Figure 4 shows a potentiodynamic plot of a sample prepared with 1.8 wt % PTFE compared with a sample without PTFE;
• Figure 5 shows a cyclic voltammogram (200 cycles) plot of sample (h) prepared in accordance with the method of the invention
• Figure 6 shows potentiodynamic sweeps of zinc electrodes with (1) and without (m) Ag 2 O;
• Figure 7 shows potentiodynamic sweeps of zinc electrodes containing Ag 2 O, ZnO or Al 2 O 3 ;
• Figure 8 shows a cyclic voltammogram plot of a zinc electrode containing silver oxide (n). The cycles were performed between -200 mV vs. Zn to +200 mV vs. Zn;
• Figure 9 shows discharge/charge performance of a zinc electrode sample with no additives
• Figure 10 shows discharge/charge performance of a zinc electrode sample containing 1 weight % CoO
• Figure 11 shows a cyclic voltammogram plot of a zinc electrode containing 14.5 wt % calcium zincate (i);
• Figure 12 shows potentiodynamic sweeps of zinc electrodes containing 4.9 wt % calcium zincate (j) and 1.9 wt % calcium zincate (Ic);
• Figure 13 shows the potentiodynamic sweep of a zinc electrode containing 8.2 wt % calcium zincate and 1.6 wt % Ag 2 O.
• samples with calcium zincate concentrations of 3.2 wt %, 5.1 wt % and 8.2 wt % without Ag 2 O are included.
• AU samples were made in accordance with the method of the invention.
• the invention provides new zinc electrodes.
• the electrode according to a first aspect of the invention comprises zinc powder as the active electrode material and the electrode also includes an organic gelling agent, an organic binding agent, calcium zincate and an electro-active element.
• the electrode reduces and preferably prevents dendrite formation and unwanted shape changes of the electrode and is able to deliver high current densities with good stability at low overpotentials.
• the amount of gelling agent is preferably ⁇ 5 wt %, more preferably 0.5 wt % to 2 wt %.
• the use of a larger amount of gelling agent increases the contact resistance between zinc particles and gives a reduction in the volume available for the addition of zinc. If the amount of gelling agent is too low this will affect the utilisation of zinc in the electrode and the dendrite inhibiting properties.
• problems with gelling agents include swelling when in contact with the electrolyte, destruction of the uniformity of the zinc powder by impact and the formation of passive layers under high discharge current densities.
• the present invention relates to an electrode in which the mobility of zincate ions resulting from the dissolution of zinc under discharge is reduced by encapsulating zinc particles with a gelling agent. Further, the negative effects due to swelling of the zinc electrode as described above are prevented by adding one or more binding agents.
• the binding agent is made from an inert material that does not take part in any reactions during charge / discharge cycles.
• the object of adding the binding agent is to maintain high ionic and electronic conductivity within the zinc electrode as the electrode undergoes swelling due to the gelling agent and due to variations in density caused by zinc oxide formation during discharge.
• the binding agent may be a polymer / plastics material that increases the binding properties of the zinc electrode thereby preventing dendrite / shape change properties of the zinc agglomerate or clay or slurry.
• binding agents are polytetraflouroethylene (PTFE), polyethylene, polypropylene, and rubber polymers (for example the latex of styrene-butadiene rubber (SBR), the latex of acrylonitril-butadiene rubber (NBR) and the latex of ethylene- propylene-diene monomer (EPDM)).
• PTFE polytetraflouroethylene
• SBR styrene-butadiene rubber
• NBR acrylonitril-butadiene rubber
• EPDM ethylene- propylene-diene monomer
• the effect of the binding agent is the formation of a mechanically strong zinc powder agglomerate with good zinc particle-particle contact, and a high exposed surface area.
• An additional benefit observed on addition of the binding agent to the zinc powder is that at high dissolution rates the formation of a passive layer is reduced, thus giving better stability at high loads.
• the amount of the binding agent should be ⁇ 4 wt %, preferably ⁇ 3 wt %, to maintain high electronic conductivity of the electrode.
• the content of PTFE lower than 2 wt % the reaction rate of the electrode is in the same range as electrodes without PTFE. However, such electrodes with PTFE show enhanced charge / discharge stability.
• a mixed polymer comprising a binding agent polymer and a gelling agent polymer.
• the employment of the binding agent polymer preferably PTFE, improves the strength of the electrode.
• the employment of the gelling agent polymer effectively controls the fluidity of the electrode.
• the expansion due to ZnO formation is preferably prevented by the addition of ZnO inside the electrode matrix under preparation.
• a void is then created within the electrode. This void is then filled under discharge without expanding the electrode.
• the active electrode material comprises zinc powder which preferably has a particle size distribution by weight such that:
• At least 25 % of the particles are small particles with a size of less than 100 ⁇ m; large and small particles being intimately distributed to facilitate particle to particle contact area.
• the zinc powder has at least 25 % of large particles with a size greater that 200 ⁇ m. More preferably the zinc powder has at least 30 % of large particles with a size greater than 200 ⁇ m and at least 30 % small particles with a size less than 100 ⁇ m.
• the zinc powder is formed as a mixture of first and second zinc powders, the first zinc powder comprising predominantly large particles with a size of 100 ⁇ m or greater and the second zinc powder comprising predominantly small particles with a size of 100 ⁇ m or less.
• the first zinc powder has at least 90 % by weight of particles in the range 75 to 400 ⁇ m and the second zinc powder has at least 80 % by weight particles less than 100 ⁇ m.
• the first and second zinc powders are in a weight ratio of 1:4 to 4:1, most preferably about 1:1. The use of two grades of zinc powder gives improved charge / discharge stability and allows high current densities without passive film formation.
• the active electrode material also comprises calcium zincate (Ca(OH) 2 . nZn(OH) 2 .nH 2 O) and an electro-active element.
• Ca-ions in the matrix of the zinc electrode reduces dendrite growth from the electrode.
• the effect of the Ca-ions is a rapid deposition of zincate within the electrode matrix, thereby preventing the diffusion of Zn-ions.
• Incorporation of Ca-ions into the electrode can be achieved by adding calcium zincate to the electrode to obtain Ca-ion rich areas close to the zinc particles and within the encapsulated agglomerates of zinc powder and additives.
• Electro-active elements such as metal oxides
• One explanation for this increased electrochemical activity is that the addition of metal oxides increases the available surface area of zinc by activating the reaction sites for reduction and oxidation of zinc, hi addition, the particles of metal oxides distributed inside the electrode increase the particle-particle contact area. Thereby the ohmic resistance of the electrode is reduced.
• any metal oxide may be used to achieve an increase in the reaction rate of zinc in an electrode, but the electro-active element is preferably a metal oxide in which the metal has at least as high electronic conductivity as Zn. Both silver oxide powder (Ag 2 O) and cobalt oxide powder (CoO) have shown to give good results. Other preferred electro- active elements include aluminium oxide, Al 2 O 3 and zinc oxide, ZnO. Small amounts OfAg 2 O ( ⁇ 8 wt %) increase the current density of the electrode under both charge and discharge. This is probably partly due to the redox reactions Of Ag 2 O during charge / ' discharge of the battery. For battery applications AgO is used as the cathode material in zinc-silver batteries. AgO is reduced to Ag 2 O under discharge and Ag 2 O is oxidised to AgO under charge. AgO will oxidise Zn resulting in ZnO and Ag 2 O formation.
• a zinc electrode comprising calcium zincate and an electro-active element as described above may be used advantageously in high power primary batteries or secondary batteries, including battery systems such as nickel-zinc, silver-zinc, zinc-chloride, zinc- bromide, zinc-manganese etc.
• Elements that can reduce the hydrogen formation from the zinc electrode may also preferably be used in the electrodes of the invention.
• Alloying elements, additives and inhibitors are used to reduce the rate of hydrogen formation.
• Alloying elements which may be used include materials with a high overpotential for the hydrogen reaction such as antimony, bismuth, gallium, indium, lead, mercury, thallium, cadmium and tin or oxides or hydroxides of such materials. Ions of these elements can be added into the electrolyte or salts can be added into the zinc powder mixture. The elements will then deposit onto the zinc surface and prevent hydrogen formation.
• Inhibitors that adsorb on the reaction sites for hydrogen formation can be added into the electrolyte.
• Inhibitors for the hydrogen reaction are well known to those skilled in the art.
• the term "zinc powder" also includes metal powders where the metal is zinc together with one or more alloying elements such as those described above.
• Electrodes according to the present invention may be used in any secondary or primary battery application known to those skilled in the art. In particular they may be used in battery systems such as nickel-zinc, silver-zinc, zinc-chloride, zinc-bromide, zinc- manganese etc.
• the invention provides a method of producing a zinc electrode. It has been found that the method in which the zinc electrode is prepared influences dendrite formation and shape changes and the reaction rate of the electrode.
• a zinc powder is provided.
• This preferably consists of a powder having a particle size distribution as defined above in order to give a good pore size distribution within the electrode, and high particle-particle contact between the particles. With the use of such a particle size distribution, low ohmic losses are obtained and no passivation is observed even at high reaction rates.
• Calcium zincate an electro-active element and any other optional materials (e.g. powder additives such as activation agents, hydrogen inhibiting materials and passivation preventing materials) are added.
• powder additives such as activation agents, hydrogen inhibiting materials and passivation preventing materials
• an organic binding agent as described above is added to the powder mixture.
• the zinc powder and additives are partly pressed into the organic binder forming agglomerates.
• an organic gelling agent as described above is added to the zinc powder and binding agent agglomerate and mixed so that the gelling agent powder surrounds the agglomerates. Mechanical force may be added to make a homogeneous mixture.
• electrolyte e.g. aqueous KOH
• the zinc powder mixture is encapsulated by the organic gelling agent.
• the conducting current collector is pressed into the paste.
• the conducting current collector may be formed from a mesh-like, sponge-like, fibrous or felt-like metallic porous body made of nickel, stainless steel, copper or metals coated by such metals.
• EXAMPLE l This example shows that a distribution of the zinc particle size fraction gives high reaction rates and increased stability of the zinc electrode.
• Zinc electrodes containing zinc powder and an organic gelling agent were tested.
• the electrodes were prepared by mixing 6 g of zinc powder with 0.18 g of Carbopol 940 (Carbopol 940 is a gelling agent which consists of a cross linked polyacrylic acid polymer). Different particle size fractions of the zinc powder were used.
• the zinc powders (GNlO and GN40) are products of Grillo, Germany and the Carbopol 940 is a product of BF Goodrich.
• the samples were prepared by adding the Carbopol 940 to the zinc powder. The mixture was then mixed with the application of mechanical energy before 0.6 ml of aqueous KOH electrolyte (6.6 M) was added to the powders in order to form a paste. The paste was then coated on a nickel mesh used as a current collector. The mesh was bent and sandwiched over the anode layer to prepare the electrode.
• Electrochemical measurement was carried out using the 1286 electrochemical interface Solartron. A Zn reference electrode and a Ni counter electrode were used. The electrolyte was a 6.6 M KOH solution. Several potentiodynamic and potentiostatic experiments were performed on the electrodes. The figures illustrate the benefit of the distribution of particle size fractions in the Zn electrode.
• Figure 1 shows the results from potentiodynamic sweeps on the electrodes. As can be seen from the figure with samples (a) (powder of small particle fraction) and (b) (powder of large particle fraction) passivation occurs at high reaction rates. On the other hand, with sample (c) (1:1 mixture of large and small particle fraction) high reaction rates are obtained.
• the experiment shows that to obtain a zinc electrode with high reaction rates a combination of small particles, giving good particle-particle contact and high active surface area, and large particles, giving porosity for electrolyte penetration is essential.
• This example shows that the method of production of the electrode containing a binding agent affects the reaction rate.
• the zinc electrode was prepared by mixing 5.6 g of zinc powder (consisting of 2 different particle fractions 50 wt % GNlO and 50 wt % GN40) with 0.1 g of PTFE (as the binding agent) and 0.1 g of Carbopol 940 (as the gelling agent).
• the zinc powder is the product of Grillo, Germany
• the PTFE is the product of Dupont in USA
• the Carbopol 940 is a product of BF Goodrich.
• the samples were prepared by two different procedures. In the first method, the Carbopol 940 was added to the zinc powder and mixed with the application of mechanical energy. Then the PTFE was added. This is shown in Figure 2 as sample (c). In the second method, the PTFE was first added to the zinc powder and mixed with the application of mechanical energy before the Carbopol 940 was finally added. This is shown in Figure 2 as sample (d).
• the paste was then coated on a nickel mesh used as a current collector.
• the mesh was bent and sandwiched over the anode layer to prepare the electrode.
• Electrochemical measurement was carried out using the 1286 electrochemical interface Solartron. A Zn reference electrode and a Ni counter electrode were used. The electrolyte was an aqueous 6.6 M KOH solution. Several potentiodynamic and potentiostatic experiments were performed on the electrodes. The figures illustrate the benefit of the production method of the invention for a zinc electrode containing a binding agent.
• Figure 2 shows the result of electrochemical measurements on the electrodes.
• Sample (d) exhibits higher activity than the sample (c).
• twice the current density at equal potential is reached for (d) as compared to (c). This shows that the method of the invention may be used to produce electrodes containing a binding agent which have high activity.
• zinc electrodes were synthesized with increasing amount of PTFE.
• the first sample was prepared by mixing 5.6 g of zinc powder (consisting of 2 different particle fractions 50 wt % GNlO and 50 wt % GN40) with 0.1 g of PTFE (1.7 wt %) and 0.1 g of Carbopol 940. This is shown in Figure 3 as sample (e).
• the second sample was prepared by mixing 5.6 g of zinc powder (consisting of 2 different particle sizes 50 wt % GNlO and 50 wt % GN40) with 0.2 g of PTFE (3.5 wt %) and 0.1 g of Carbopol 940. This is shown in Figure 3 as sample (f).
• the last sample was prepared by mixing 5.6 g of zinc powder (consisting of 2 different particle sizes 50 wt % GNlO and 50 wt % GN40) with 0.4 g of PTFE (6.9 wt %) and 0.1 g of Carbopol 940. This is shown in Figure 3 as sample (g).
• Figure 3 shows the results of the electrochemical experiments on the zinc electrodes. With increasing amount of PTFE the activity of the zinc electrode decreases.
• Figure 4 is a comparative experiment. It shows a sample prepared with 1.8 wt % PTFE compared with a sample without PTFE. In order to prevent losses related to particle- particle contact due to the swelling of the electrode without PTFE, the electrode was pressed into a holding frame. The figure shows that when the amount of PTFE is below 2 wt % no negative effect of the PTFE content is observed.
• the example shows that, if the binding agent is PTFE 5 the amount of binding agent should be ⁇ 4 wt % to maintain high reaction rates without passive film formation.
• EXAMPLE 4 This example shows that the electrodes can undergo charge / discharge cycling without dendrite formation or unwanted shape changes due to the combined effect of adding a gelling agent and a binding agent.
• Dendrite formation under repeated charge / discharge cycling of the zinc electrode is prevented by encapsulation of the zinc particles. To maintain the mechanical integrity of the electrode binding agents are added.
• the zinc electrode (h) was obtained by mixing 5.6 g of zinc powder (consisting of 2 different particle fractions 50 wt % GNlO and 50 wt % GN40) with 0.1 g of PTFE (as the binding agent) and 0.1 g of Carbopol 940 (as the gelling agent).
• the zinc powder is a product of Grillo in Germany
• the PTFE is a product of Dupont in USA
• the Carbopol 940 is a product of BF Goodrich.
• the samples were prepared by agglomerating PTFE with zinc powder before adding Carbopol 940.
• the whole mixture was mixed with the addition of mechanical energy, before 0.6 ml of aqueous KOH electrolyte (6.6 M) was added in order to form a paste.
• the paste was coated on a nickel mesh acting as the current collector.
• the mesh was bent and sandwiched over the anode layer to prepare the electrode.
• Electrochemical measurement was carried out using the 1286 electrochemical interface Solartron. A Zn reference electrode and a Ni counter electrode were used. The electrolyte was an aqueous 6.6 M KOH solution. Several potentiodynamic and potentiostatic experiments have been performed on the electrodes. The figures illustrate that dendrite formation can be reduced by the addition of a binding agent to the zinc electrode.
• Figure 5 shows the charge / discharge curves for the electrode. The figure shows that both the discharge and charge behavior of the electrode is reasonably stable and high current densities are obtained. The slow increase in the current can be explained by an increase of the active surface area after repeated cycling. After 200 cycles the experiment was stopped. From visual inspection of the electrode no dendrite formation could be observed and the electrode did not suffer from shape changes.
• This example shows that adding small amounts of metal oxides will increase the reaction rate of the zinc electrode.
• a first zinc electrode (1) was prepared by mixing up 5.4 g of zinc powder (50 wt % GN40 and 50 wt % GNlO particle size powder) with 0.1 g of calcium zincate, 0.1 g of Ag 2 O and 0.1 g of PTFE. The mixture was mixed with the addition of mechanical energy, and then 0.1 g of Carbopol 940 was added. After mixing all the powders together, 0.6 ml of aqueous 6.6 M KOH was slowly added to form a paste.
• a second zinc electrode (m) was prepared by mixing up 5.4 g of zinc powder (50 wt % GN40 and 50 wt % GNlO particle size powder) with 0.1 g of calcium zincate and 0.1 g of PTFE. The mixture was mixed with the application of mechanical energy, and then 0.1 g of Carbopol 940 was added. After mixing all the powders together, 0.6 ml of aqueous 6.6 M KOH was slowly added to form a paste.
• the paste was coated on a nickel mesh acting as the current collector.
• the mesh was bent and sandwiched over the anode layer to prepare the electrode.
• Electrochemical measurement was carried out using the 1286 electrochemical interface Solartron. A, Zn reference electrode and a Ni counter electrode were used. The electrolyte was an aqueous 6.6 M KOH solution. Several potentiodynamic and potentiostatic experiments have been performed on the electrodes. The figures illustrate the additional benefit of the components in the zinc electrode.
• Figure 6 shows the potentiodynamic curve for the sample containing Ag 2 O (1) and the sample without silver oxide (m). As can be seen from the figure, when silver oxide is added to the zinc electrode an increase in the reaction rates is obtained.
• Figure 7 shows the potentiodynamic curve for samples containing various metal oxides. These electrodes were prepared in the same way as those described above and in addition to zinc powder, PTFE and Carbopol 940, these electrodes also contained 0.2 wt % calcium zincate and: (i) 1.7 wt % Al 2 O 3 ; (ii) 1.7 wt % ZnO;
• Figure 8 shows cyclic sweeps of a zinc electrode containing silver oxide (n). The figure shows stable charge / discharge behavior of the electrode. No dendrite formation was observed even after several hundred cycles.
• EXAMPLE 6 This example shows that adding small amounts of metal oxides will increase the reaction rate of the zinc electrode.
• the effect of CoO on the electrochemical behavior of zinc electrodes has been investigated by performing cycling tests.
• a first zinc electrode was prepared by mixing up 5.4 g zinc powder (particle size 70 - 100 ⁇ m) and 0.1 g of PTFE. The mixture was mixed with the addition of mechanical energy, and then 0.1 g of Carbopol 940 was added. After mixing all the powders together, 0.6 ml of aqueous 6.6 M KOH was slowly added to form a paste.
• a second zinc electrode was prepared by mixing up 5.4 g of zinc powder with 0.1 g of PTFE and 1 weight % CoO. The mixture was mixed with the application of mechanical energy, and then 0.1 g of Carbopol 940 was added. After mixing all the powders together, 0.6 ml of aqueous 6.6 M KOH was slowly added to form a paste. The paste was coated on a nickel mesh acting as the current collector. The mesh was bent and sandwiched over the anode layer to prepare the electrode.
• Electrochemical measurement was carried out on half-cells, using Zn as reference electrode and a Ni as counter electrode.
• the electrolyte was an aqueous 6.6 M KOH solution.
• Figure 9 shows the discharge and charge performance at C/4 rate for the sample without CoO.
• Figure 10 shows the discharge and charge performance at C/4 rate for the sample with 1 % CoO.
• an average voltage of about -30 mV in discharge and 30 mV in charge was obtained.
• FIG. 9 shows a loss in capacity of about 34 % from the 5 th to the 20 th cycle for the sample with no CoO. Although the addition of CoO causes a small decrease of the cell capacity, as seen in Figure 10, the overall cycling behavior is improved by exhibiting only a 3 % loss in capacity form the 5 th to the 20 th cycle.
• the experiment shows that by adding small amounts of cobalt oxide to the zinc electrode, the reaction rate was improved.
• the experiment also showed that adding cobalt oxide improved the cycle stability of the cell.
• calcium zincate can be added to the zinc electrode.
• the example shows that by adding an increasing amount of calcium zincate to the zinc electrode increased stability is obtained.
• the example also shows that high reaction rates with calcium zincate are obtained by the addition of metal oxides.
• the solid product containing mainly calcium zincate Ca(OH) 2 .nZn(OH) 2 .nH 2 O was rinsed with distilled water 3 times. The deposited product was then dried in an oven (70 °C, for 48 hours).
• a first zinc electrode (i) was prepared by mixing up 4.76 g of zinc powder (50 wt % GN40 and 50 wt % GNlO) with 0.84 g of Ca(OH) 2 . nZn(OH) 2 .nH 2 O (14.5 wt %) and 0.1 g of PTFE. The powders were then mixed with the application of mechanical energy, and 0.1 g of Carbopol 940 was added. After mixing all powders together, 0.6 ml of aqueous 6.6 M KOH was slowly added to form a paste.
• a second zinc electrode (j) was prepared by mixing up 5.5 g of zinc powder (50 wt % GN40 and 50 wt % GNlO particle size powder) with 0.11 g of Ca(OH) 2 .nZn(OH) 2 .nH 2 O (1.9 wt %) and 0.1 g of PTFE. The powders were then mixed with the application of mechanical energy, and then 0.1 g of Carbopol 940 was added. After mixing all powders together, 0.6 ml of aqueous 6.6 M KOH was slowly added to form a paste.
• a third zinc electrode (k) was prepared by mixing up 5.22 g of zinc powder (50 wt % GN40 and 50 wt % GNlO particle size powder) with 0.28 g of Ca(OH) 2 .nZn(OH) 2 .nH 2 O (4.9 wt %) and 0.1 g of PTFE. The powders were then mixed with the application of mechanical energy, and then 0.1 g of Carbopol 940 was added. After mixing all powders together, 0.6 ml of aqueous 6.6 M KOH was slowly added to form a paste.
• the paste was coated on a nickel mesh acting as the current collector.
• the mesh was bent and sandwiched over the anode layer to prepare the electrode.
• Electrochemical measurement was carried out using the 1286 electrochemical interface Solartron. A Zn reference electrode and a Ni counter electrode were used. The electrolyte was an aqueous 6.6 M KOH solution. Several potentiodynamic and potentiostatic experiments have been performed on the electrodes. Figure 11 shows cyclic sweeps of the zinc electrode (i) containing 14.5 wt % calcium zincate. The addition of calcium zincate improves the charge / discharge stability of the electrode. A decrease in the reaction rate is observed. No dendrite formation was observed.
• Figure 12 potentiodynamic sweeps of the zinc electrodes containing 1.9 wt % calcium zincate Q) and 4.9 wt % calcium zincate (Ic). As the amount of calcium zincate is reduced the reaction rate of the zinc electrode increases.
• calcium zincate can be added. Undesirably, as shown, this reduces the reaction rate of the zinc electrode.
• Figure 13 shows the potentiodynamic sweep for a zinc electrode containing 8.2 wt % calcium zincate

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