IRON ELECTRODE EMPLOYING A POLYVINYL ALCOHOL BINDER

Abstract

The present invention provides one with an iron electrode employing a binder comprised of polyvinyl alcohol (PVA) binder. In one embodiment, the invention comprises an iron based electrode comprising a single layer of a conductive substrate coated on at least one side with a coating comprising an iron active material and a binder, wherein the binder is PVA. This iron based electrode is useful in alkaline rechargeable batteries, particularly as a negative electrode in a Ni—Fe battery.

Claims

1. A method of preparing an iron electrode comprising the steps of i) preparing a paste formulation which comprises an iron active material, sulfur and from 2 to 5 wt % of a polyvinyl alcohol binder; ii) providing a single layer substrate; and iii) coating the paste formulation on at least one side of the single layer substrate.

2. The method of claim 1, wherein the paste formulation further comprises a pore former, carbon, graphite or Ni powder.

3. The method of claim 1, wherein the single layer substrate comprises a thin conductive material.

4. The method of claim 3, wherein the thin conductive material comprises a perforated metal foil or sheet, metal mesh or screen, woven metal, or expanded metal.

5. The method of claim 4, wherein the thin conductive material comprises a nickel plated perforated foil.

6. The method of claim 1, wherein the single layer substrate comprises a three dimensional material.

7. The method of claim 6, wherein the three dimensional material comprises a metal foam or metal felt.

8. The method of claim 1, wherein the sulfur is present in the paste formulation in the amount of from 0.25 to 1.5% by weight.

9. The method of claim 1, wherein the paste formulation is coated on both sides of the substrate.

10. The method of claim 1, wherein the sulfur comprises elemental sulfur.

11. The method of claim 1, wherein the active iron material comprises iron metal, an iron oxide material, or a mixture thereof.

12. The method of claim 11, wherein the iron oxide material comprises Fe.sub.3O.sub.4.

13. The method of claim 1, wherein the polyvinyl alcohol binder comprises polyvinyl alcohol that is hydrolyzed between 98.5 and 100%.

14. The method of claim 1, wherein the polyvinyl alcohol binder comprises polyvinyl alcohol that is hydrolyzed between 99 and 100%.

15. The method of claim 1, wherein the polyvinyl alcohol binder comprises from 2.5 to 4 wt % of the iron electrode.

Description

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

[0021] FIG. 1 is a perspective view of a coated iron electrode of the present invention comprising a PVA binder;

[0022] 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;

[0023] FIG. 3 is a perspective view of a current pocket iron electrode; and

[0024] FIG. 4 is a side view and a cross-section view of a current pocket iron electrode.

[0025] FIG. 5 shows cycling data for cells with different concentrations of PVA in the iron electrode.

[0026] FIG. 6 is discharge capacities for Ni—Fe cells with iron electrodes having varied nickel and iron content.

[0027] FIG. 7 is discharge capacities for Ni—Fe cells with iron electrodes having varied sulfur content.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The invention comprises an iron electrode comprised of a single, coated conductive substrate employing a PVA binder to affix the active material to the substrate.

[0029] In the present invention, a single layer of substrate is used. 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.

[0030] The coating mixture is a combination of PVA binder and active materials in an aqueous or organic solution. The mixture can also contain other additives such as pore formers, conductive additives such as carbon, graphite, or Ni powder, and reaction promoting additives such as sulfur and sulfur bearing materials such as FeS, Mgs and BiS. Pore formers can be incorporated to enhance electrode porosity. The PVA binder provides adhesion and bonding between the active material particles, both to themselves and to the substrate current collector. 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.

[0031] It has been discovered that there are several advantages to employing PVA as a binder in an iron electrode of the present invention 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. This property permits ready use in a continuous coating process. PVA does not impart a hydrophobic nature to the electrode surface, insuring good contact between the active material and the alkaline electrolyte. It has also been found that PVA minimizes any increase in cell resistance and offers the highest mAh/g capacity when used in an iron electrode.

[0032] 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 99.0 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 formulation 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.

[0033] While PVA is not generally considered an acceptable binder for electrodes employing a single substrate, the unique properties of the pasted iron electrode of this invention enable its use as a binder. During electrochemical cycling of the iron electrode, iron is converted to iron oxides and iron hydroxides which are only very sparingly soluble in the electrolyte. Therefore, these reactions occur at the surface of the iron particles. During charge, as the iron oxides and iron hydroxides are reduced back to iron metal, the small iron particles effectively fuse together, providing strong mechanical binding between active material particles. Thus, unlike conventional battery electrodes that undergo mechanical swelling and shrinking which result in physical degradation of the electrode over time, the iron electrode physical strength improves with charge/discharge cycling. It is this distinction that enables the use of PVA as a binder for an iron electrode, and allows one to successfully take advantage of PVA and its desirable properties, as discussed above.

[0034] The active material for the mix formulation is selected from iron species that can be reversibly oxidized and reduced. Such materials include iron metal, iron oxide materials and 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 and conductivity improvers such as nickel.

[0035] 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.

[0036] 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. 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.

[0037] FIGS. 3 and 4 of the drawing show a conventional pocket iron electrode. In FIG. 3, the two substrates 21 and 22 are shown to form the pocket which holds the iron active material. In FIG. 4, the iron active material 23 is held between the two substrates 21 and 22.

ILLUSTRATIVE EXAMPLES

Paste Preparation

[0038] 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 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.

Example 1

[0039] 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

[0040] 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 2 C capacities of the Ni—Fe batteries, the best results at 2 C discharge rate were obtained in cells employing PVA as a binder.

Example 2

[0041] Water based pastes (Table 2) 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-00002 TABLE 2 PVA concentration Iron in electrode Capacity Sample (%) (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

[0042] 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.

[0043] 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 3

[0044] 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.

[0045] 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%.

[0046] Electrodes from Example 3 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 0.059 Ah/g 0.076 Ah/g 0.075 Ah/g 0.084 Ah/g 0.034 Ah/g 0.105 Ah/g (total electrode) Ah/cm.sup.3 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 (total electrode) Type of Pocket Pocket Pocket Pocket Pocket Continuous iron plate plate plate plate plate coated electrode (Pasted)

Example 4

Paste Preparation

[0047] 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 3, the sulfur content was 0.5%, and the binder content was 3.5%.

[0048] Water based pastes with varying nickel and iron content (Table 4) 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 80 4 20 76

[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] 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.

Examples 5

Paste Preparation

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 2 C discharge rates.

[0055] In the foregoing examples, the invention Ni—Fe battery used an electrolyte comprised of sodium hydroxide (NaOH), lithium hydroxide (LiOH), and sodium sulfide (Na.sub.2S). A sintered nickel electrode impregnated with nickel hydroxide was used as the positive electrode in the foregoing examples using the iron electrode of the present invention. The separator used in the inventive Ni—Fe battery was a 0.010 inch thick polyolefin non-woven mesh. The electrolyte used in the conventional Ni—Fe battery was potassium hydroxide (KOH), and the anode and cathode was kept electrically isolated using a spacer. The results show a vast improvement in performance characteristics for the inventive Ni—Fe battery.

[0056] 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.