CONTINUOUS COATED IRON ELECTRODE

Abstract

Provided is a high quality and high performance nickel-iron battery comprising an iron electrode which is prepared by a continuous process. The process comprises preparing a formulation comprising an iron active material and a binder, and continuously coating a continuous substrate material on at least one side with the formulation. The coated continuous substrate material is dried, compacted and blanked. In one embodiment, the iron electrode comprises a PVA binder.

Claims

1. A nickel-iron battery prepared by the method of: (i) creating an iron anode by preparing an active material coating mix comprising an iron active material, from 2.5 to 1.5 wt % elemental sulfur, from 8 to 20 wt % nickel, and a polyvinyl alcohol binder with the amount of polyvinyl alcohol binder in the range of from 2.5 to 4 wt % of the mix; continuously coating a continuous substrate material on at least one side with the active material coating mix to prepare an active material coating on the substrate; drying, compacting and cutting the iron electrode to size; and recovering an iron anode which comprises elemental sulfur, nickel, and polyvinyl alcohol in the range of from 2.5 to 4 wt %; and (ii) combining the iron anode with a nickel cathode to prepare the nickel-iron battery.

2. The battery of claim 1, wherein the amount of polyvinyl alcohol binder used in the mix is about 3.5 wt %.

3. The batter of claim 1, wherein the amount of nickel used in the mix ranges from 16 to 20 wt %.

4. The iron electrode of claim 1, wherein the iron active material comprises Fe metal.

5. The iron electrode of claim 1, wherein the iron active material comprises Fe.sub.3O.sub.4.

6. The battery of claim 1, wherein the substrate material comprises a conductive material.

7. The battery of claim 6, wherein the conductive material comprises a perforated metal foil or sheet, metal mesh or screen, woven metal, or expanded metal.

8. The battery of claim 7, wherein the conductive material comprises a nickel plated perforated foil.

9. The battery of claim 1, wherein the substrate material comprises a three dimensional material.

10. The battery of claim 9, wherein the three dimensional material comprises a metal foam or metal felt.

11. The battery of claim 1, wherein the coating of the continuous substrate comprises layering of materials having different properties.

12. The battery of claim 11, wherein the layers have different porosities and/or densities.

13. The battery of claim 12, wherein the layers have different concentrations of additives.

14. The battery of claim 1, wherein the drying step is conducted with a combination of IR, microwave or UV drying in a first step, and convection drying in a second step.

15. The battery of claim 14, wherein the convection drying occurs once the amount of water or solvent remaining is in the 10-20% weight range.

16. The battery of claim 1, wherein the drying step is conducted in an inert atmosphere.

17. The battery of claim 1, wherein the compaction is accomplished by rolling mill, vertical pressing or magnetic compaction.

18. The battery of claim 17, wherein the compaction is sufficient to achieve an iron electrode having a thickness of 0.005 to 0.50 inches.

19. The battery of claim 18, wherein the porosity of the electrode after compaction is in the range of from 10 to 50%.

20. The battery of claim 1, further comprising a polymer coating.

Description

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

[0015] FIG. 1 is a perspective view of a coated iron electrode of the present invention.

[0016] FIG. 2 is a side view and cross-section view of an iron electrode coated on both sides of the substrate in accordance with the present invention.

[0017] FIG. 3 is a perspective view of a current pocket iron electrode.

[0018] FIG. 4 is a side view and a cross-section view of a current pocket iron electrode.

[0019] FIG. 5 is discharge capacities for Ni—Fe cells with iron electrodes having varied binder compositions.

[0020] FIG. 6 is discharge capacities for Ni—Fe cells with iron electrodes having varied nickel and iron content.

[0021] FIG. 7 is discharge capacities for Ni—Fe cells with iron electrodes having varied sulfur content.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention provides an iron electrode comprised of a single, coated conductive substrate which is coated with an active iron material, preferably with a binder, e.g., polyvinyl alcohol (PVA). In one embodiment, the electrode is made using a continuous process.

[0023] The substrate is used as a current conducting and collecting material that houses the active material (iron) of the electrode in a mechanically stable design. In current pocket electrode designs, the substrate encompasses the active material and holds the material between two layers of conductor, therefore requiring two substrates per electrode. In this process, pockets are formed by interlocking two perforated Ni-coated strips into which the active material is compressed. While such a design offers long life, the energy density is poor.

[0024] An alternative electrode utilizes a porous sintered structure of iron powder, which is filled with iron hydroxide by either an electrochemical process or by impregnation of the pores with an appropriate iron salt, followed by immersion in alkaline solution. Such electrodes suffer from poor active material loading and corrosion of the iron porous plaque during impregnation, leading to limited life.

[0025] To address these short-comings, U.S. Pat. No. 4,236,927 describes a process whereby iron powder and a reducible iron compound are mixed together and sintered into a stable body. This mixture is then sintered at high temperature to form a plate of desired shape. While this eliminates the need for a sintered plaque substrate or pockets of Ni-coated steel, it requires high temperature sintering under hydrogen atmosphere. Such processes add considerable complexity and cost in volume manufacturing.

[0026] In contrast, the present invention uses a single layer of substrate for the iron electrode. This single layer acts as a carrier with coated material bonded to at least one side. The substrate may be a thin conductive material such as perforated metal foil or sheet, metal mesh or screen, woven metal, or expanded metal. The substrate may also be a three-dimensional material such as a metal foam or metal felt. In one embodiment, a nickel plated perforated foil has been used.

[0027] The coating mixture applied to the substrate is a combination of binder and active materials in an aqueous or organic solution. The mixture can also contain other additives such as pore formers or conductive additives. Conductive additives include but are not limited to carbon, graphite, or Ni powder. Pore formers can be incorporated to enhance electrode porosity and include but are not limited to ammonium carbonate and ammonium bicarbonate. Other additive that may be included in the coating mixture are bismuth, tin, sulfur, and metal sulfides. The binder materials have properties that provide adhesion and bonding between the active material particles, both to themselves and to the substrate current collector. The binder is generally resistant to degradation due to aging, temperature, and caustic environment. The binder can comprise polymers, alcohols, rubbers, and other materials, such as an advanced latex formulation that has been proven effective. A polyvinyl alcohol (PVA) binder is used in one embodiment. Use of a binder to mechanically adhere the active material to the supporting single substrate eliminates the need for expensive sintering or electrochemical post-treatment. Aqueous based solutions have the advantage of lower toxicity and removal of water during the drying process is environmentally friendly and does not require further treatment or capture of the solvent.

[0028] There are several advantages to employing PVA as a binder versus conventional binders. PVA is readily water soluble, simplifying the manufacturing process by allowing for direct addition of a PVA solution to the active material mix and eliminating issues associated with shelf life common with PTFE binders. PVA does not impart a hydrophobic nature to the electrode surface, insuring good contact between the active material and the alkaline electrolyte. PVA can be added to the active material paste in the form of a concentrated solution or in powder form. PVA that is hydrolyzed between 98.5 and 100% is preferred in one embodiment. A most preferred embodiment uses PVA that is hydrolyzed between 99.2 and 100%. Furthermore, the PVA has a 4% water solution viscosity between 3-70 cP at 20° C. In a preferred embodiment, the viscosity of a 4% water solution of the PVA is between 20-40 cP at 20° C. In a most preferred embodiment, the viscosity of a 4% water solution of the PVA is between 27-33 cP at 20° C. Concentrations of PVA in the final paste are 1 to 10% by total weight. Preferred concentrations of PVA are in the range of 1 to 5% and a most preferred concentration of PVA in the paste is between 2.5 to 4%. Lower concentrations of PVA do not provide sufficient binding of the active material, while higher concentrations result in an increase in electrode electrical resistance, degrading the performance of the battery under high current loads.

[0029] The active material for the mix formulation is selected from iron species that can be reversibly oxidized and reduced. Such materials include metal Fe, iron oxide materials or mixtures thereof. The iron oxide material will convert to iron metal when a charge is applied. A suitable iron oxide material includes Fe.sub.3O.sub.4. A preferred form of iron is hydrogen reduced with a purity of about 96% or greater and having a 325 mesh size. In addition, other additives may be added to the mix formulation. These additives include but are not limited to sulfur, antimony, selenium, tellurium, bismuth, tin, and metal sulfides. Conductive additives that my be added include but are not limited to nickel powder, carbon, and graphite.

[0030] Sulfur as an additive has been found to be useful in concentrations ranging from 0.25 to 1.5% and higher concentrations may improve performance even more. Nickel has been used as a conductivity improver and concentrations ranging from 8 to 20% have been found to improve performance and higher concentrations may improve performance even more.

[0031] A further advantage of the electrode of the present invention is that additives can be combined into the paste formulation since electrode processing is done at relatively low temperatures where the additives would otherwise be lost at high temperatures. Use of a sintered construction as described in the prior literature precludes addition of additives such as sulfur to the active mass since they would be lost during the sintering process.

[0032] In one embodiment, the coating method is a continuous process. The process comprises mixing an iron active material, generally with a binder in suitable solvent, coating a continuous substrate material on at least one side with the active material mix, drying the coating to the desired thickness, blanking or cutting the coated substrate to the desired size, and attaching a tab to the electrode. The coating step applies the active material mixture to the substrate, such as spraying, dip and wipe, extrusion, low pressure coating die, or surface transfer. A low pressure coating die is used in one embodiment. The coating method must maintain a high consistency for weight, thickness, and coating uniformity. This insures that finished electrodes will have similar loadings of active material to provide uniform capacity in the finished battery product.

[0033] The coating method of the invention is conducive to layering of various materials and providing layers of different properties, such as porosities, densities, and thicknesses. For example, the substrate can be coated with three layers; the first layer being of high density, second layer of medium density, and final layer of a lower density to create a density gradient. This gradient improves the flow of gases from the active material to the electrolyte and provides better electrolyte contact and ionic diffusion with the active material throughout the structure of the electrode.

[0034] After coating, the electrode is dried to remove any residual liquid, i.e., aqueous or organic solvent. The drying methods will generally provide a continuous method for liquid removal from the coated active material which will enhance the adhesion and binding effects of the dry constituents without iron ignition. This drying method provides a uniform and stable active material coating with the substrate material. Two stages of drying can be used. For example, the first stage can be radiation for bulk drying, for cost and quality control, followed by convection drying to remove the remaining liquid. The radiation used can be any radiation, such as infrared, microwave, or UV, and is very fast. In one embodiment, IR radiation is used. However, the radiation creates a high temperature at the surface of the coated electrode. This high temperature is acceptable as long as there is sufficient water present to act as a heat sink. Therefore, the water is generally removed to about 10-20% by weight water. This can generally be determined using a control chart. Going below 10% water is dangerous, as the electrode becomes too dry and the high surface temperature can ignite the iron active material. Thus, using the convection drying to complete the removal of water or solvent is a preferred embodiment, once the amount of water or solvent remaining is in the 10-20% by weight range. In another embodiment, radiation can be used to complete the drying if the process is conducted in an inert atmosphere.

[0035] The compaction methods can be accomplished by rolling mill, vertical pressing, or magnetic compaction of the active material to achieve the desired thickness from 0.005 to 0.50 inches and porosities from 10% to 50%, for high quality and low cost continuous processing. In one embodiment, the porosity of the electrode is from 34-43% porosity. These compaction methods can be used in conjunction with the layering method described above for providing material properties of density, thickness, porosity, and mechanical adhesion.

[0036] In addition, continuous in-line surface treatments can be applied continuously throughout any of the steps, including coating, layering, and drying processes. The treatments can apply sulfur, polymer, metal spray, surface laminate, etc. In one embodiment, a polymer post-coat is applied.

[0037] Blanks of the electrode are cut to the desired size from the continuous substrate material. The lengthwise size of the blanks will depend on the battery into which the electrode is to be used. The blanks can be cut before the drying step, with each of the separate blanks then dried. The blanks can also be cut to the desired size after drying but before compaction. In this embodiment, each blank is then compacted to the desired thickness. In one embodiment, the blanks are cut as noted in FIG. 1, after the drying and compaction steps.

[0038] After the drying, compaction and blanking steps, a tab is generally attached to the electrode for connection purposes. The tab is constructed of a conductive material and can be attached using conventional methods, such as welding.

[0039] The iron electrode can be used with a suitable positive electrode (cathode) to make a battery, e.g., a Ni—Fe battery with a nickel cathode and the iron electrode of this invention. The battery can be made as is conventional, with a standard electrolyte and battery separator. The electrolyte, for example, can be a potassium hydroxide based electrolyte.

[0040] The present batteries including the iron electrode can be used, for example, in a cellphone, thereby requiring an electrode with only a single side coated. However, both sides are preferably coated, allowing the battery to be used in many applications as is known in the art.

[0041] Turning to the figures of the drawing, FIG. 1 is a prospective view of a coated iron electrode. The substrate 1 is coated on each side with the coating 2 comprising the iron active material and binder. This is further shown in FIG. 2. The substrate 1 is coated on each side with the coating 2 of the iron active material and binder. The substrate may be coated continuously across the surface of the substrate, or preferably, as shown in FIGS. 1 and 2, cleared lanes of substrate may be uncoated to simplify subsequent operations such as welding of current collector tabs.

[0042] FIGS. 3 and 4 show a conventional pocket iron electrode. In FIG. 3, the two substrates 30 are shown to form the pocket which holds the iron active material. In FIG. 4, the iron active material 40 is held between the two substrates 41 and 42.

ILLUSTRATIVE EXAMPLES

Paste Preparation

[0043] A water based paste comprised of hydrogen reduced iron powder (325 mesh size), 16% nickel powder #255, 0.5% elemental sulfur powder (precipitated, purified) and 3.5 wt % of PVA binder was prepared using a digital stirring device and 3-wing stirring blade operating at 1300 RPM for 10-15 minutes. Deionized water was added to the mixture to create a paste with a viscosity between 120,000-130,000 cP.

Electrode Preparation

Example 1

[0044] The water based paste was applied to a 1.63″ wide nickel-plated continuous perforated strip with 2-mm perforations by feeding the strip fed through the top of an open-bottomed pot attached to a doctor-blade fixture with a gap width set to 0.068″. The paste mixture is poured into the pot and the perforated strip is pulled down at a rate of 2.7 ft/min coating the perforated strip with the paste mixture. Segments ranging 4-5″ are cut from the coated strip and placed into a drying oven at 150° F. for 20 minutes.

[0045] After drying the coated strips were cut to a standard length of 3″ and then compressed to thickness to achieve a porosity of approximately 40%. Dried paste mixture was removed from the top 0.25″ of the strip in order to provide a clean space for a stainless steel tab to be spot-welded.

Example 2

[0046] A series of iron electrodes were prepared by impregnating nickel foam with various pastes comprising several different binder compositions described in Table 1. The discharge capacities of the individual cells prepared from these electrodes were measured and plotted against the amount of iron in the anode in FIG. 5. The effect of rate on capacity was evaluated by discharging the cells at multiple rates of C/10, C/5, C/2, and 2C where C represents the current required to discharge the cell in one hour.

TABLE-US-00001 TABLE 1 Cell # Binder Binder g of iron 1 1% CMC 1% PTFE 6.4 2 1% PVA 1% PTFE 8.5 3 1% CMC 1% AL-2002 latex 7.9 4 1% CMC 1% AL-3001 latex 7.4 5 1% PVA 1% AL-1002 latex 8.3

[0047] Since the binder can contribute to electrode resistance, it is desirable to employ a binder that minimizes an increase in cell resistance and offers the highest mAh/g capacity. Comparing the 2C capacities of the Ni—Fe batteries, the best results at 2C discharge rate were obtained in cells employing PVA as a binder.

Example 3

[0048] Water based pastes (Table 2) were applied to a 1.63″ wide nickel-plated perforated strip with 2-mm perforations by continuously feeding the strip fed through the top of an open-bottomed pot attached to a doctor-blade fixture with a gap width set to 0.068″. The paste mixture is poured into the pot and the perforated strip is pulled down at a rate of 2.7 ft/min coating the perforated strip with the paste mixture. Segments ranging 4-5″ are cut from the coated strip and placed into a drying oven at 150° C. for 20 minutes.

TABLE-US-00002 TABLE 2 PVA concen- Iron in Capacity Sample tration (%) electrode (g) (mAh/g Fe) 1 3.5 8.3 117 2 3.5 8.45 116 3 3.5 11.4 112 4 5 8.25 89 5 7 10.1 69 6 9 8.55 8

[0049] After drying the coated strips were cut to a standard length of 3″ and then compressed to thickness to achieve a porosity of approximately 40%. Dried paste mixture was removed from the top 0.25″ of the strip in order to provide a clean space for a stainless steel tab to be spot-welded onto.

[0050] A series of continuously coated iron electrodes were prepared by coating perforated NPS with an aqueous mixture of iron powder, nickel powder as a conductivity aid, elemental sulfur and employing PVA as a binder. Multiple levels of PVA were employed in the mixes to evaluate the effect of binder concentration on mechanical stability of the electrode and rate capability of the electrode. At concentrations below 2 weight percent PVA, the physical integrity of the electrodes was unacceptable. Concentrations of binder above about 5 weight percent showed a sharp drop in discharge capacity, most likely due to increased electrode resistance and possibly masking of the active material from the electrolyte interface. Data for cells with varying levels of PVA is summarized in Table 2.

Example 4

[0051] A 10 wt % solution of PVA (Elvanol 7130) preheated to between 120-125° F. was added to a jacketed container with iron powder (325 mesh), nickel powder #255, and sulfur preheated to 120° F. This mixture was stirred for 30 minutes at 120° F. The solid component mixture of this paste was 80% iron, 16% nickel, 0.4% sulfur, and 3.5% PVA. Viscosity measurements of the paste had a range of 25000 to 39000 cP immediately after removal from the container and after a further 90 seconds, the viscosity ranged from 22000 to 31000 cP.

[0052] The paste mixture was then transferred to a jacketed holding tank preheated to 110° F. where it was stirred. The paste was pumped to a paste hopper where a perforated nickel plated steel strip was coated. The coated strip was then passed through a doctor blade to achieve a coating thickness between 0.040-0.050″ and introduced to a vertical drying oven. The first stage of drying consisted of IR heating at 240° F. for 1.67 minutes followed by heating in a conventional oven at 240° F. for 3.35 minutes. The second drying stage with a residence time of 1.7 minutes consisted of forced hot air with a set drying temperature of 260° F. The paste temperature exiting the ovens did not exceed 210° F. After cooling, the finished coating was calendared to a thickness of 0.025″. Pieces of the coating were cut to size and weighed to obtain coating porosity. The porosity ranged from 34-43% with a targeted porosity of 38%.

[0053] Electrodes from Example 4 were used to construct a Ni—Fe battery. Table 3 shows the performance of the iron electrode in comparison to other commercial Ni—Fe batteries employing pocket plate electrodes.

TABLE-US-00003 TABLE 3 Electrode Chinese Chinese of present Cell Seiden Taihang Ukrainian Russian Zappworks invention Ah/g 0.095 Ah/g 0.130 Ah/g 0.117 Ah/g 0.116 Ah/g — 0.126 Ah/g (powder) Ah/g (total 0.059 Ah/g 0.076 Ah/g 0.075 Ah/g 0.084 Ah/g 0.034 Ah/g 0.105 Ah/g electrode) Ah/cm.sup.3 (total 0.199 Ah/cm.sup.3 0.203 Ah/cm.sup.3 0.216 Ah/cm.sup.3 0.238 Ah/cm.sup.3 0.099 Ah/cm.sup.3 0.430 Ah/cm.sup.3 electrode) Type of Pocket Pocket Pocket Pocket Pocket Continuous iron plate plate plate plate plate coated electrode (Pasted)

Example 5

Paste Preparation

[0054] A water based paste comprised of hydrogen reduced iron powder (325 mesh size), nickel powder #255, elemental sulfur powder (precipitated, purified) and the appropriate amount of binder was prepared using a digital stirring device and 3-wing stirring blade operating at 1300 RPM for 10-15 minutes. Deionized water was added to the mixture to create a paste with a viscosity between 120,000-130,000 cP. The nickel and iron content was varied according to Table 4, the sulfur content was 0.5%, and the binder content was 3.5%.

[0055] Water based pastes with varying nickel and iron content (Table 3) were applied to a 1.63″ wide nickel-plated perforated strip with 2-mm perforations by feeding the strip fed through the top of an open-bottomed pot attached to a doctor-blade fixture with a gap width set to 0.068″. The paste mixture is poured into the pot and the perforated strip is pulled down at a rate of 2.7 ft/min coating the perforated strip with the paste mixture. Segments ranging 4-5″ are cut from the coated strip and placed into a drying oven at 150° C. for 20 minutes.

TABLE-US-00004 TABLE 4 Sample Nickel (%) Iron % 1 8 88 2 12 84 3 16 60 4 20 76

[0056] After drying the coated strips were cut to a standard length of 3″ and then compressed to thickness to achieve a porosity of approximately 40%. Dried paste mixture was removed from the top 0.25″ of the strip in order to provide a clean space for a stainless steel tab to be spot-welded onto.

[0057] Ni—Fe cells were constructed using electrodes fabricated from the pastes with varying nickel and iron content. The data is shown in FIG. 6. The cell performance does not appear to be very dependent upon nickel concentration in the concentration range between 8-16% but improved capacity at high (1 C) and low rates (C/10) is observed for electrodes with 20% nickel.

Example 6

Paste Preparation

[0058] A water based paste comprised of hydrogen reduced iron powder (325 mesh size), nickel powder #255, elemental sulfur powder (precipitated, purified) and the appropriate amount of binder was prepared using a digital stirring device and 3-wing stirring blade operating at 1300 RPM for 10-15 minutes. Deionized water was added to the mixture to create a paste with a viscosity between 120,000-130,000 cP. The nickel content was 16%, polyvinyl alcohol 3.5%, and the sulfur content was varied between 0 and 1.5% with the remainder of the electrode composition being iron powder.

[0059] Water based pastes with varying sulfur content were applied to a 1.63″ wide nickel-plated perforated strip with 2-mm perforations by feeding the strip fed through the top of an open-bottomed pot attached to a doctor-blade fixture with a gap width set to 0.068″. The paste mixture is poured into the pot and the perforated strip is pulled down at a rate of 2.7 ft/min coating the perforated strip with the paste mixture. Segments ranging 4-5″ are cut from the coated strip and placed into a drying oven at 150° C. for 20 minutes.

[0060] After drying the coated strips were cut to a standard length of 3″ and then compressed to thickness to achieve a porosity of approximately 40%. Dried paste mixture was removed from the top 0.25″ of the strip in order to provide a clean space for a stainless steel tab to be spot-welded onto.

[0061] Ni—Fe cells were constructed using electrodes fabricated from the pastes with varying sulfur content. The data is shown in FIG. 7. Increasing the sulfur content of the electrode increases the capacity at the C/10 discharge rate until the sulfur content reaches about 1.5% where there is no further increase in capacity. Increasing the sulfur content increased the capacity of the iron electrode even at sulfur contents up to 1.5% at the 1 C and 2C discharge rates.

[0062] In NI—Fe cells constructed with iron electrodes of the foregoing examples, an electrolyte comprising sodium hydroxide (NaOH), lithium hydroxide (LiOH), and sodium sulfide (Na.sub.2S) was used. A sintered nickel electrode impregnated with nickel hydroxide was used as the positive electrode and a 0.010 inch thick polyolefin non-woven mesh was used as the separator in these examples of Ni—Fe cells with the iron electrode of the present invention. The electrolyte used in the conventional Ni—Fe battery was potassium hydroxide (KOH) and the anode and cathode were kept electrically isolated using a spacer. The results show a vast improvement in performance characteristics for the inventive Ni—Fe battery.

[0063] While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention.