Solid additives for iron negative electrodes

Abstract

According to one aspect, an additive for an iron negative electrode of an alkaline electrochemical cell may comprise a powder of discrete particles comprising agglomerated particles comprising at least one metal sulfide.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of priority of International Patent Application No. PCT / US23 / 30484, filed on August 17, 2023, and U.S. Provisional Patent Application 63 / 378,132, filed on October 3, 2022, the entire contents of all of which are incorporated herein by reference. Background Art

[0003] Energy storage technologies are playing an increasingly important role in the power grid. These energy storage assets provide smoothing to better match power generation and demand on the grid. The services performed by energy storage devices are beneficial to the power grid on multiple time scales, from milliseconds to years. Today, there are energy storage technologies that can support time scales from milliseconds to hours, but there is still a need for long-duration and very long-duration (generally, > 8 h) energy storage systems. The benefit is potentially low-cost rechargeable battery chemistries that enable long-duration large-scale energy storage. Summary of the Invention

[0004] According to one aspect, an additive for an iron negative electrode of an alkaline electrochemical cell may comprise a powder of discrete particles including agglomerated particles, the agglomerated particles comprising at least one metal sulfide.

[0005] In some embodiments, the at least one metal sulfide of the agglomerated particles is greater than 50 wt% of the discrete particles. The at least one metal sulfide of the agglomerated particles may be greater than 80 wt% of the discrete particles.

[0006] In certain embodiments, based on weight percentage, the average particle size of the discrete particles may be greater than about 30 microns and less than about 800 microns.

[0007] In some embodiments, as determined by mercury intrusion porosimetry, the median pore diameter of the discrete particles of the at least one metal sulfide of the agglomerated particles is greater than about 75 nm and less than about 15 microns.

[0008] In certain embodiments, the discrete particles may have a first average apparent density, and the particles including the at least one metal sulfide have a second average apparent density, the first average apparent density being less than the second average apparent density. The first average apparent density may be greater than 1.0 g / cm³ and less than 2.1 g / cm³.

[0009] In some embodiments, according to European Pharmacopoeia 2.9.41.-2 (Method B), the discrete particles may have a friability with a weight loss of less than about 15%.

[0010] In some embodiments, solid-state bonding can hold together at least some of the agglomerated particles in the discrete particles such that there is solid-state bonding between the agglomerated particles.

[0011] In some embodiments, at least some of the agglomerated particles of the discrete particles can be metal matrix composites bonded by infiltration.

[0012] In some embodiments, the discrete particles can include an adhesive, and at least some of the agglomerated particles of the discrete particles are bonded by the adhesive. As an example, the adhesive can be soluble in an alkaline electrolyte and / or the adhesive can be reactive to form a substance that is soluble in an alkaline electrolyte.

[0013] In some embodiments, the at least one metal sulfide can include zinc sulfide (ZnS). The powder of the discrete particles can be greater than or equal to 90 wt% zinc sulfide (ZnS). As an example, greater than 60 wt% zinc sulfide (ZnS) can be in a sphalerite structure as determined by x-ray diffraction.

[0014] In some embodiments, the agglomerated particles of the discrete particles can have surface-connected pores. As an example, as determined by mercury intrusion porosimetry, the surface-connected pores of the agglomerated particles can be greater than or equal to 7 vol% and less than or equal to 40 vol% of the agglomerated particles.

[0015] In some embodiments, the at least one metal sulfide can include iron sulfide (FeS), tin sulfide (SnS), bismuth sulfide (Bi2S3), aluminum sulfide (Al2S3), antimony(III) sulfide (Sb2S3), antimony(V) sulfide (Sb2S5), manganese sulfide (MnS), molybdenum(IV) sulfide (MoS2), or a combination thereof.

[0016] In some embodiments, the discrete particles can further include tin oxide (SnO2), tin (Sn), bismuth (Bi), zinc selenide (ZnSe), potassium hydroxide (KOH), sodium hydroxide (NaOH), or a combination thereof.

[0017] In some embodiments, the discrete particles can further include a conductive material. As an example, the conductive material can include tin, graphite, carbon black, or a combination thereof.

[0018] In some embodiments, the discrete particles may further include particles of at least one pore-forming agent. Based on weight percentage, the average particle size of the particles of the at least one pore-forming agent may be greater than about 5 microns and less than about 20 microns. In some cases, the particles of the at least one metal sulfide may have a first average particle size based on weight percentage, and the particles of the at least one pore-forming agent have a second average particle size based on weight percentage, and the second average particle size is greater than or equal to the first average particle size. In certain cases, the at least one pore-forming agent may be soluble in an alkaline electrolyte. As an example, the at least one pore-forming agent may include potassium oxide (K2O), lithium oxide (LiO), sodium oxide (Na2O), potassium hydroxide (KOH), lithium hydroxide (LiOH), sodium hydroxide (NaOH), sodium sulfate (Na2S), potassium sulfate (K2S), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), sodium stannate (Na2[Sn(OH)6]), potassium stannate (K2[Sn(OH)6]), or a combination thereof. Additionally or alternatively, the at least one pore-forming agent may include poly(methyl methacrylate), starch, sodium chloride (NaCl), potassium chloride (KCl), ammonium bicarbonate (NH4HCO3), or a combination thereof.

[0019] According to another aspect, an iron negative electrode for an alkaline electrochemical cell may comprise a first powder including an iron active material and a second powder including any one or more of the above powders as additives, the first powder and the second powder forming a powder blend, wherein the second powder is dispersed relative to the first powder.

[0020] In certain embodiments, the first powder may include any one or more of iron oxide, atomized iron powder, sponge iron powder, or ground iron powder. As an example, the ratio of the average particle size of the discrete particles of the second powder to the average particle size of the particles of the iron active material may be from 0.5 to 2.

[0021] In some embodiments, the average particle size of the discrete particles may be greater than or equal to the average pore size of the powder blend.

[0022] In certain embodiments, the apparent density of the powder blend may be less than the apparent density of the first powder alone.

[0023] In some embodiments, the iron active material may be greater than about 70% of the total weight of the first powder and the second powder.

[0024] In certain embodiments, the thickness of the active layer of the electrode may be greater than about 8 mm and less than about 50 mm.

[0025] In some embodiments, the concentration of the second powder in the powder blend may have a predetermined gradient in the thickness dimension of the active layer of the powder blend.

[0026] Still according to another aspect, a method for preparing an additive for an iron negative electrode of an alkaline electrochemical cell may include: forming a raw material including granular material having a predetermined composition, and processing the raw material including the granular material into a powder of discrete particles including agglomerated particles of the granular material, the agglomerated particles including at least one metal sulfide.

[0027] In some embodiments, processing the raw material of the granular material may include solid-state bonding of the granular material. As an example, solid-state bonding of the granular material may include heating the granular material to a temperature of about 500 °C to about 1400 °C. Additionally or alternatively, solid-state bonding of the granular material may include sintering the granular material into the agglomerated particles. As an example, the at least one metal sulfide includes zinc sulfide (ZnS), and sintering is performed at a temperature greater than 900 °C and less than 1300 °C. In certain cases, solid-state bonding of the granular material may include hot pressing the granular material. As an example, the at least one metal sulfide may include zinc sulfide (ZnS), and hot pressing is performed at 900 °C with a uniaxial pressure of about 410 kPa for about 10 minutes.

[0028] In certain embodiments, forming the raw material may include introducing at least one polymer binder into the granular material. In some cases, processing the raw material into the discrete particles may include pyrolyzing the at least one polymer binder to form a graphitized film on the granular material. The at least one polymer binder may be pyrolyzable in an inert atmosphere and has a yield of residual solids of > 7 wt% after pyrolysis in a non-oxidizing atmosphere up to 600 °C. Additionally or alternatively, the at least one polymer binder may include pitch, zinc stearate, stearic acid, polyalcohol, poly(methyl methacrylate), polyolefin, polymer having an aromatic ring, poly(ethylene glycol), poly(tetrafluoroethylene), polyvinylidene fluoride, carboxymethyl cellulose, poly(acrylic acid), or a copolymer of any one or more of the foregoing. Still additionally or alternatively, in introducing the at least one polymer binder into the granular material, the granular material may be subjected to thermomechanical bonding in a processing atmosphere including hydrogen.

[0029] In some embodiments, forming the raw material may include introducing at least one inorganic binder into the granular material. The at least one inorganic binder may include an oxide-based binder, a silicate-based binder, an alumina-containing binder, or a combination thereof.

[0030] In certain embodiments, the raw material for processing the granular material further includes a raw material for compacting the granular material. As an example, the raw material for compacting the granular material includes roll-compacting the raw material of the granular material.

[0031] In some embodiments, the granular material may include particles of the at least one metal sulfide. The raw material for forming the granular material may include mixing at least two metal sulfides in a predetermined weight ratio relative to each other. As an example, the at least two metal sulfides may include zinc sulfide (ZnS) and iron sulfide (FeS). In some cases, the at least two metal sulfides may include zinc sulfide (ZnS) and iron sulfide (FeS) in the discrete particles in the predetermined weight ratio relative to each other. In certain cases, forming the raw material including the granular material may include mixing a conductive material with the at least one metal sulfide. As an example, the conductive material may include tin, graphite, carbon black, or a combination thereof. In certain cases, the conductive material may be solid at room temperature, and the raw material for processing the granular material includes heating the raw material of the granular material in an inert environment to melt the at least one metal sulfide and the conductive material such that the conductive material wets the at least one metal sulfide, and in the case where the conductive material wets the at least one metal sulfide, cooling the raw material of the granular material such that the at least one metal sulfide and the conductive material in the raw material of the granular material form a metal matrix composite bonded by infiltration. In some embodiments, the particles of the at least one metal sulfide may include particles of zinc sulfide (ZnS). In some cases, processing the raw material may include exposing the granular material to a chemical reduction environment including one or more reducing agents at 700 °C to 1000 °C such that zinc oxide (ZnO) in the granular material is chemically reduced to zinc sulfide (ZnS). The one or more reducing agents may include solid carbon, gaseous carbon monoxide, or gaseous hydrogen. In some cases, processing the raw material may include exposing the granular material to a sulfur-containing gas to convert zinc oxide (ZnO) to zinc sulfide (ZnS). As an example, the sulfur-containing gas may include hydrogen sulfide (H2S), carbonyl sulfide (OCS), methanethiol (CH3SH), sulfur dioxide (SO2), or a combination thereof. In some cases, the raw material for forming the granular material may include blending particles of a pore-forming agent with the particles of the at least one metal sulfide, the pore-forming agent being soluble in an alkaline electrolyte. In some cases, the ratio of the average particle size of the particles of the pore-forming agent to the average particle size of the particles of the at least one metal sulfide may be greater than or equal to 1:1 and less than about 5:1.

[0032] In some embodiments, processing the raw materials into the discrete particles can include introducing at least one binder into the discrete particles, the at least one binder being soluble in an alkaline electrolyte.

[0033] In some embodiments, processing the raw materials into the discrete particles can include forming the granular material into one or more intermediates and changing the size of the one or more intermediates to form the discrete particles. Forming the granular material into the one or more intermediates can include cold pressing the granular material. Cold pressing the granular material can include forming agglomerates of the granular material. In some cases, changing the size of the one or more intermediates to form the discrete particles can include sintering the one or more intermediates together to form the discrete particles. As an example, forming the granular material into the one or more intermediates can include sintering the granular material into a sintered body having at least one dimension of approximately 1 mm or greater, and changing the size of the one or more intermediates includes reducing the size of the one or more intermediates to form the discrete particles. In some cases, the one or more intermediates can include a monolith of a ceramic material, the monolith having at least one dimension of approximately 1 mm or greater, and changing the size of the one or more intermediates includes reducing the size of the one or more intermediates to form the discrete particles. In certain cases, changing the size of the one or more intermediates to form the discrete particles can include any one or more of the following: grinding, drum granulation, fluidized bed granulation, spray drying, high shear mixing granulation, twin screw granulation, extrusion granulation, open pan granulation / disc granulation, wet granulation, or dry granulation.

[0034] The various aspects may include methods for preparing electrodes for a battery pack. Each aspect may include a method for preparing an electrode for a battery pack, which may include: forming an electrode structure comprising an iron active material and a zinc source, and converting the zinc source into zinc sulfide on or in the electrode structure through a sulfidation reaction. In some embodiments, the zinc source may include zinc oxide powder. Each aspect may include a method for preparing an electrode for a battery pack, which includes: contacting an electrode structure comprising an iron active material with a solution containing dissolved zinc, and exposing the structure in contact with the solution to a sulfide source to convert at least a portion of the dissolved zinc into zinc sulfide. In some embodiments, the sulfide source may include a solution containing Na2S. In some embodiments, the sulfide source may include a gas containing H2S. In some embodiments, the solution containing dissolved zinc may include a solvent and a gelling agent. Some embodiments may further include drying the exposed solution-contact structure, wherein the drying occurs at one or more temperatures of 800 degrees Celsius or below 800 degrees Celsius. In some embodiments, the drying occurs in a furnace. Each aspect may include a method for preparing an electrode for a battery pack, which may include: forming a sulfidation solution composed of a solvent and a sulfide source, forming an electrode structure comprising an iron active material and a solid zinc source, contacting the electrode structure with the sulfidation solution, and maintaining the contact of the sulfidation solution with the electrode structure for a period of time such that zinc sulfide is incorporated on or in the electrode structure. In some embodiments, the sulfidation solution may be a component of an electrolyte solution, and contacting the electrode structure with the sulfidation solution may include adding the sulfidation solution to a battery cell of a battery pack including the electrode structure such that the sulfidation solution contacts the electrode structure. In some embodiments, the solvent includes KOH and the sulfide source includes Na2S. In some embodiments, the solid zinc source is formed of solid zinc oxide particles or agglomerated particles of zinc oxide. In some embodiments, the molar ratio of Na2S in the sulfidation solution to zinc oxide in the electrode structure is from 1 to 2. In some embodiments, the period of time may be less than 168 hours. Each aspect may include a method for preparing an electrode for a battery pack, which may include: forming a structure comprising an iron active material and a solid zinc source through one or more processes carried out at a first processing temperature or above the first processing temperature, and exposing the formed structure to a sulfide source gas while maintaining a selected concentration of the sulfide source gas. The one or more processes carried out at the first processing temperature or above the first processing temperature may include one or more of sintering and hot pressing.In some embodiments, exposing the formed structure to the sulfide source gas while maintaining the selected concentration of the sulfide source gas includes exposing the formed structure to the sulfide source gas while maintaining the selected concentration of the sulfide source gas and a second processing temperature below the first processing temperature such that a sulfidation reaction of zinc oxide occurs to form zinc sulfide. In some embodiments, the first processing temperature can be equal to or higher than 800 degrees Celsius, and the second processing temperature can be equal to or lower than 600 degrees Celsius. In some embodiments, the second processing temperature can be equal to or lower than 400 degrees Celsius. In some embodiments, the sulfide source gas is H2S, and the selected concentration of the sulfide source gas is 0.25% to 1%. In some embodiments, exposing the formed structure to the sulfide source gas occurs as part of a cooling stage of the last one of the one or more processes for forming the formed structure. In some embodiments, zinc oxide is formed from solid zinc oxide particles or agglomerated particles of zinc oxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1A is a schematic representation of an electrochemical cell.

[0036] Figure 1B is a schematic representation of a rechargeable battery pack.

[0037] Figure 2A is a schematic representation of an iron negative electrode including a bed of a powder blend, the powder blend including a first powder of an iron-containing active material and a second powder of an additive material.

[0038] Figure 2B is Figure 2A a close-up schematic representation of the second powder of, the second powder including discrete particles containing agglomerated particles, the agglomerated particles including at least one metal sulfide.

[0039] Figure 2C is a diagram showing a powder of discrete particles of agglomerated particles mixed with a powder of an iron active material.

[0040] Figure 3 is Figure 2A the measured capacity (normalized) of the iron negative electrode of varying with Figure 2A a series of graphs of the average particle size of the discrete particles of the iron negative electrode of, where the type of metal sulfide remains constant, and different graphs correspond to experiments conducted using different electrolytes, electrolyte concentrations, amounts (weight %) of the discrete particles used, and cycling conditions such as temperature and C-rate.

[0041] Figure 4 is a flowchart of an exemplary method 400 for preparing an additive for an iron negative electrode for an alkaline electrochemical cell.

[0042] Figure 5A and Figure 5B is the phase diagram of zinc sulfide (ZnS).

[0043] Like reference symbols in different figures represent like elements. DETAILED DESCRIPTION

[0044] The embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. The reference to specific examples and embodiments is for illustrative purposes and is not intended to limit the scope of the claims. The following description of the embodiments of the present disclosure is not intended to limit the present disclosure to these embodiments, but rather to enable those skilled in the art to make and use the present disclosure. Unless otherwise noted, the drawings are not drawn to scale.

[0045] As used herein, unless otherwise noted, the recitation of ranges of values herein is merely intended to be a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each separate value within the range is incorporated into the specification as if it were individually recited herein.

[0046] The following examples are provided to illustrate various embodiments of the systems and methods of the present disclosure. These examples are for illustrative purposes and may be predictive and should not be considered limiting and do not otherwise limit the scope of the present disclosure.

[0047] The various embodiments of the systems, devices, techniques, methods, activities, and operations set forth in this specification may be used in a variety of other activities and other fields beyond those set forth herein. Additionally, for example, these embodiments may be used with other devices or activities that may be developed in the future; and, with existing devices or activities that may be modified in part based on the teachings of this specification. Additionally, the various embodiments and examples set forth in this specification may be used in whole or in part with one another and may be used with one another in different combinations and multiple combinations. Thus, for example, the configurations provided in the various embodiments of this specification may be used with one another. Thus, the configurations provided in the various embodiments of this specification may be used with one another. For example, according to the teachings of this specification, the components of an embodiment having A, A', and B and the components of an embodiment having A", C, and D may be used with one another in a variety of combinations, such as A, C, D, and A, A", C, and D, etc. The scope of protection provided by the present disclosure should not be limited to the specific embodiments, configurations, or arrangements set forth in the specific embodiments, examples, or embodiments of the specific figures.

[0048] As used herein, unless otherwise specified, room temperature is 25 °C. Also, standard temperature and pressure are 25 °C and 1 atmosphere. Unless otherwise explicitly stated, all tests, test results, physical properties, and values related to temperature, pressure, or both are provided at standard ambient temperature and pressure.

[0049] Unless the context otherwise indicates or explicitly provides, all references in this disclosure to average particle size shall be understood to refer to the average particle size on a weight percentage basis. Accordingly, some references in this disclosure to average particle size may occasionally omit specific mention of "weight percentage basis" for purposes of clarity and readability.

[0050] Embodiments of the present disclosure include devices, systems, and methods for long-duration and ultra-long-duration energy storage. As used herein, "long (duration)" and / or "ultra-long (duration)" may refer to an energy storage cycle of 8 hours or longer, such as an 8-hour energy storage cycle, an 8-hour to 20-hour energy storage cycle, a 20-hour energy storage cycle, a 20-hour to 24-hour energy storage cycle, a 24-hour energy storage cycle, a 24-hour to one-week energy storage cycle, a one-week to one-year (e.g., from a few days to a few weeks to a few months) energy storage cycle, and the like. In other words, a "long (duration)" and / or "ultra-long (duration)" energy storage device or system may refer to an energy storage device or system that can be configured to store energy over a time span of days, weeks, or seasons. For example, an energy storage device or system may be configured to store energy generated by a solar cell during summer months when sunlight is abundant and solar power generation exceeds grid demand, and release the stored energy during winter months when sunlight may be insufficient to meet grid demand.

[0051] According to other embodiments, the present invention includes devices, systems, and methods for shorter (duration) energy storage of less than about 8 hours. For example, an electrochemical cell may be configured to store energy generated by a solar cell during a day-night cycle, where solar power generation during midday may exceed grid demand, and release the stored energy during the evening when sunlight may be insufficient to meet grid demand. As another example, the invention may include energy storage for use as a backup power source during periods of insufficient grid power for facilities including homes, commercial buildings, factories, hospitals, or data centers, where the required discharge duration may range from a few minutes to several days.

[0052] An electrochemical cell, such as a battery pack, stores electrochemical energy by taking advantage of the difference in electrochemical potential that creates a voltage difference between a positive electrode and a negative electrode. If the electrodes are connected by a conductive element, this voltage difference generates an electric current. In a battery pack, the negative and positive electrodes are connected in series through an internal resistance element and an external resistance element. Typically, the external element conducts electrons and the internal element (the electrolyte) conducts ions. Since a charge imbalance cannot be maintained between the negative and positive electrodes, these two flows must supply ions and electrons at the same rate. In operation, the electron flow can be used to drive an external device. A rechargeable battery pack can be charged by applying a reverse voltage difference that drives the current and ion flow in a direction opposite to that of the discharging battery pack in service.

[0053] Now referring Figure 1A , an electrochemical cell 100 (e.g., a battery pack) can include a negative electrode 102 separated from a positive electrode 103 by a separator 104. The separator 104 can be supported, for example, by a mesh 105 (e.g., a polypropylene mesh) and a frame 108 (e.g., polyethylene or polypropylene) of the electrochemical cell 100. Current collectors 107 can be associated with the respective electrodes of the negative electrode 102 and the positive electrode 103 and are supported by a backplane 106 (e.g., a polyethylene backplane or a polypropylene backplane). In some embodiments, the temperature of the electrochemical cell 100 can be controlled, for example, by insulation around the electrochemical cell 100 and / or by a heating element 150. For example, the heating element 150 can increase the temperature of the electrochemical cell 100 and / or the specific composition of the battery, such as the electrolyte permeating the negative electrode 102 and the positive electrode 103. The electrolyte can be an aqueous solution. In certain embodiments, the electrolyte can be an alkaline solution (pH > 10). In certain embodiments, the electrolyte can be a near-neutral solution (10 > pH > 4).

[0054] The electrochemical cell 100 is merely an example of one electrochemical cell configuration according to multiple embodiments and is not intended to be limiting. Other configurations, such as electrochemical cells with different types of meshes and / or without a mesh 105, electrochemical cells with different types of frames and / or without a frame 108, electrochemical cells with different types of current collectors and / or without a current collector 107, electrochemical cells with a reservoir structure, electrochemical cells with different types of backplanes and / or without a backplane 106, electrochemical cells with different types of insulation and / or without insulation, and / or electrochemical cells with different types of heating elements and / or without a heating element 150, can replace Figure 1A the exemplary configuration of the electrochemical cell 100 shown, and other configurations meet the requirements of multiple embodiments.

[0055] In some embodiments, a plurality of Figure 1AThe electrochemical cells 100 therein can be electrically connected in series to form a stack. In some other embodiments, multiple electrochemical cells 100 can be electrically connected in parallel. In some other embodiments, the electrochemical cells 100 are connected in a hybrid series-parallel electrical configuration to achieve a favorable combination of the current and voltage transmitted.

[0056] Now referring Figure 1B , the rechargeable battery pack 10 can include a positive electrode 12, a negative electrode 14, and a separator 16 within a container 18. The container 18 is filled with an electrolyte 20 up to a level 22 that is at least as high as the respective tops 32, 34 of the electrodes 12, 14. The space above the level 22 of the electrolyte 20 can be referred to as the headspace 24. The positive electrode 12 can be electrically connected to the positive terminal 42 of the rechargeable battery pack 10 and can include an active material that can undergo a reduction reaction during discharge and an oxidation reaction during charging. The negative electrode 14 can be electrically connected to the negative terminal 44 of the rechargeable battery pack 10 and can include an active material that can undergo an oxidation reaction during discharge of the rechargeable battery pack 10 and a reduction reaction during charging of the rechargeable battery pack 10. Figure 1B The rechargeable battery pack 10 in

[0057] In multiple embodiments, the electrolyte 20 can be an aqueous or non-aqueous solution that is alkaline, neutral, or acidic. For example, the electrolyte solution can include potassium hydroxide (KOH), sodium hydroxide (NaOH), lithium hydroxide (LiOH), or a combination thereof.

[0058] In some embodiments, the battery pack 10 can include a separator 16 that allows ions to transfer between the electrodes 12, 14 through the electrolyte. In some embodiments, the separator can be selected based on its ability to allow the selective transfer of desired molecules or materials while substantially restricting or preventing the transfer of undesired molecules or materials. For example, some separator membranes are ion-selective, allowing the transfer of negative ions (or positive ions) while substantially preventing the transfer of positive ions (or negative ions). In other instances, the separator material can be selected based on its ability to allow or prevent bubbles from crossing from one side (associated with one electrode) to the opposite side (associated with the counter electrode).

[0059] In multiple embodiments, the container 18 can be made of any suitable material and structure capable of containing the electrolyte, electrodes, and at least a minimal amount of gas pressure. For example, the container 18 can be made of metal, plastic, composite materials, or other materials. In some embodiments, the battery pack container 18 can be sealed to prevent any gas generated during the operation of the battery pack from escaping.

[0060] In some embodiments, the battery pack container 18 may include a pressure relief valve to allow the release of gas when the gas pressure within the battery pack container 18 exceeds a predetermined threshold.

[0061] Although electrodes 12, 14 are shown in the figures as being substantially spaced apart, in some embodiments, the electrodes may be very close to each other or even pressed against each other, with the separator 16 therebetween. Further, although the figures may show a single positive electrode 12 and a single negative electrode 14, battery pack systems within the scope of the present disclosure may also include two or more positive electrodes 12 and / or two or more negative electrodes 14.

[0062] Now referring to Figure 1A and Figure 1B , it is generally to be understood that, unless the context otherwise requires or clearly dictates, the negative electrode 102 of the electrochemical cell 100 and / or the negative electrode 14 of the rechargeable battery pack 10 may include a metal or metal oxide, such as iron, zinc, cadmium, or other metals and / or oxides or hydroxides of these or other metals. Further, it is generally to be understood that, unless the context otherwise requires or clearly dictates, the negative electrode 102 of the electrochemical cell 100 and the negative electrode 14 of the rechargeable battery pack 10 have similar or identical characteristics, and for the sake of efficient description, these characteristics are not described separately for each negative electrode. Accordingly, in light of the foregoing, the embodiments in the following specification are described in the context of the negative electrode 14, and more specifically, in the context of the negative electrode 14 being an iron negative electrode. Thus, the negative electrode 14 shall hereinafter be referred to as the "iron negative electrode 14", and all such references shall be understood to be intended to include references to the other types of active metals described herein and the other negative electrodes described herein (e.g., negative electrode 102), unless the context otherwise requires or clearly dictates.

[0063] Now referring to Figure 1B , Figure 2A and Figure 2B , the iron negative electrode 14 may include a bed 201 of a powder blend 202 having pores, an electrolyte (e.g., Figure 1BThe electrolyte 20) in [the electrochemical cell] can permeate through the bed 201 of the iron negative electrode 14 via pores to support the flow of ions when the iron active material of the negative electrode 14 undergoes oxidation and reduction reactions during the operation of an electrochemical cell (e.g., a rechargeable battery pack 10). The powder blend 202 can include a first powder 204 and a second powder 206. As described in more detail below, the first powder 204 can include an iron active material. Also as described in more detail below, the second powder 206 can include discrete particles 208, and the discrete particles 208 include agglomerated particles 210, and the agglomerated particles 210 include at least one metal sulfide. As an example, the iron active material can be greater than about 70% of the total weight of the first powder 204 and the second powder 206, which may be useful for balancing competing considerations related to the performance (e.g., capacity utilization of the iron active material), cost, and size of an electrochemical cell (e.g., a rechargeable battery pack 10) including the iron negative electrode 14.

[0064] Generally, as described in the following paragraphs, the discrete particles 208 of the agglomerated particles 210 overcome significant challenges in particle size matching, dry powder processing, and floating / shedding risks compared to powders of loose particles of solid-state additives used in iron negative electrodes.

[0065] As an example, the second powder 206 including the discrete particles 208 of the agglomerated particles 210 containing an additive material may help overcome design challenges typically associated with particle size mismatches between the active material and the additive material in an iron negative electrode. Specifically, for large-scale cost-effective production, many solid-state additive powders are formed using preparation techniques that produce small particle sizes. While small particle sizes (e.g., << 100 microns) may be useful for achieving the dissolution rate of certain types of solid-state additives (e.g., manganese sulfide, tin-containing additives, and zinc sulfide) in the iron negative electrode electrolyte, these small particle sizes are generally incompatible with the large particle sizes of the iron active material required to achieve the packing and flow properties necessary for efficient performance of the iron negative electrode. In other words, there is typically a particle size mismatch between the powder of loose particles of the additive (generally < 10 microns) and the powder of the iron active material (generally 100 microns).

[0066] Due to this size contrast between the solid additive and the iron powder used, the powder mixture used in the iron negative electrode can achieve a packing density higher than expected, and solely due to geometric effects, the smaller solid additive particles fill the interstitial spaces of the larger iron active material particles. Additionally or conversely, if the particles of the solid additive are small enough relative to the larger iron active material particles, the particles of the solid additive can act as a "flow enhancer" or "packing enhancer" by reducing interparticle friction. Such a higher packing density may result in a low porosity of the iron negative electrode, leading to poor performance of the resulting iron negative electrode. In this context, such poor performance may be attributed to at least one or more of rate performance, achievable areal loading, specific capacity, and voltage efficiency. However, using agglomeration of particles to form the particles 208 of the second powder 206 helps to overcome the particle size mismatch between the loose particles of the solid additive and the iron powder used in the powder mixture of the iron negative electrode. Additionally or instead, using agglomeration of particles to form the discrete particles 208 of the additive renders the geometry of the particles of the solid additive independent of the performance of the solid additive in the iron negative electrode 14 - helping to independently control the specific surface area of the additive material, the additive chemistry, the size of the particles of the additive, or any combination thereof in the powder blend 202 in the bed 201.

[0067] As another example, the second powder 206 including the discrete particles 208 of the agglomerated particles 210 facilitates the use of dry powder processing to form the iron negative electrode 14. In the context of aqueous long - term energy storage, the electrode areal loading and thickness ratio are much higher than in the context of other battery pack electrodes. As a comparison, lithium - ion electrodes (commonly used in shorter - term energy storage technologies) typically have an active layer strata with a thickness usually less than ∼1 mm, while the iron negative electrode 14 may have an active layer with a thickness greater than about 8 mm to less than about 50 mm. As used in this context, it should be understood that the thickness of the active layer refers to the thickness of the layer of the iron negative electrode 14 that includes the first powder 204 comprising the iron active material (and thus also includes the second powder 206, as part of the powder blend 202). Wet processes based on slurry processing and slurry rheology are commonly used in the production of lithium - ion electrodes to maintain a uniform mixing of the electrode active material, additives, and binders throughout the preparation of the active layer. However, the drying time of the electrodes processed by the wet method increases super - linearly with the increase in the thickness of the active layer material, making it impractical to use the wet method when the iron negative electrode 14 is much thicker than the lithium - ion electrode. Although dry powder processing is superior to wet processing in terms of preparation time, the effects of powder rheology and the resulting tendency for segregation in dry powder processing can be much more severe than in wet electrode processing.

[0068] Therefore, for the preparation of the iron negative electrode 14 using dry powder processing of the powder blend 202, special attention must be paid to the considerations surrounding particle size design to meet the desired powder packing (for high performance) and consistency criteria, while taking into account different constraints. Thus, while the second powder 206 including discrete particles 208 comprising agglomerated particles 210 of the additive material may be relevant to any processing route, the second powder 206 may be particularly useful in the context of dry powder processing to form the powder blend 202 of the iron negative electrode 14. That is, dry powder processing of the second powder 206 to form the powder blend 202 may be beneficial for simultaneously meeting the performance-based requirements for the surface area and dispersion of the additive material within the iron negative electrode 14, while also meeting the processing-based requirements for particle size matching and minimization of segregation.

[0069] As yet another example, the second powder 206 including discrete particles 208 comprising agglomerated particles 210 of the additive material may help reduce the risk of solid additives floating / shedding in the iron negative electrode. That is, a particular risk associated with the use of solid additives in the iron negative electrode is that particles of these such solid additives may attach to bubbles generated during the electrochemical cycling (e.g., by hydrogen evolution or oxygen evolution) or otherwise be entrapped or dragged by bubbles generated during the electrochemical cycling (e.g., by hydrogen evolution or oxygen evolution), resulting in the loss of loose particles of the solid additives from the iron negative electrode. This can be particularly problematic in cases where the solid additives are intended to remain in the iron negative electrode during the electrode lifetime, as any amount of solid additives lost from the iron negative electrode may result in performance degradation during the lifetime of a given iron negative electrode and / or lead to increased costs to compensate for the loss of the solid additives.

[0070] Several factors influence the floating / shedding risk of the solid additives. The first factor influencing the floating / shedding risk is particle size. While obtaining solid additives of small size (e.g., 5 microns) is generally the least costly, at these length scales, the interaction of the solid additives with bubbles can be severe. Additionally or alternatively, if the particle size of the discrete particles 208 of at least one metal sulfide is approximately the same as or larger than the pore size of the iron negative electrode 14, the pores of the iron negative electrode itself can act as a "filter" for the solid additives. Although capillary interactions can be modulated by surface chemistry, viscous drag forces are fundamental to the passage of bubbles through the iron negative electrode and may result in the loss of solid additive particles, regardless of the surface chemistry of the solid additive particles. The second factor influencing the floating / shedding risk is surface chemistry. Additives with non-polar surfaces or low surface energy will tend to adsorb more strongly to the bubble surface and will thus be subject to enhanced drag by the bubbles.

[0071] Accordingly, compared to a powder of loose particles using a solid additive, a second powder 206 of discrete particles 208 of agglomerated particles 210 including a solid additive can reduce the risk of loss of solid additive material particles. That is, the discrete particles 208 of the agglomerated particles 210 are significantly larger than each individual particle and can thus be more resistant to the drag force of the gas bubbles or other forces used to remove the solid additive from the iron negative electrode 14. Additionally or alternatively, since the size of the discrete particles 208 of the agglomerated particles 210 is independent of the size of the individual particles, the size of the discrete particles 208 can be selected such that the discrete particles 208 are not "suitable" to pass through the pores of the iron negative electrode 14. As an example, the iron negative electrode 12 can be formed as a packed bed filter through which hydrogen gas bubbles and electrolyte move / flow, exerting an accompanying viscous drag force and capillary drag force. Discrete particles 208 having a size approximately the same as the pore diameter of the iron negative electrode 14 can geometrically prevent migration out of the iron negative electrode, thus keeping the solid additive in the desired position within the iron negative electrode 14. Accordingly, in some embodiments, the average particle size of the discrete particles 208 is greater than or equal to the average pore diameter of the powder blend 202, which may help reduce the risk of the solid additive floating / shedding from the iron negative electrode 14. Additionally or alternatively, in a packed bed filter, good filtration typically occurs if the filter pore diameter is within 4 times the particle size to be filtered. Thus, in some cases, the average particle size of the discrete particles 208 can be less than the average pore diameter of the powder blend 202 of the iron negative electrode, in which case the discrete particles 208 can be sufficiently retained within the iron negative electrode 14.

[0072] Now referring to Figure 1B 、 Figure 2A 、 Figure 2B and Figure 3 , while the larger average size of the discrete particles 208 is generally useful for addressing the floating / shedding risk, experiments have determined that the average size of the discrete particles 208 can affect the capacity of the iron negative electrode 14. Specifically, as shown by the experimental results of Figure 3 , the capacity performance of the iron negative electrode 14 increases with an increase in the average particle size of the discrete particles 208 up to a certain optimal capacity and then decreases as the average particle size of the discrete particles 208 further increases. In other words, the range of reduction in the floating / shedding risk based on the size of the discrete particles 208 may be limited by the effect of the size of the discrete particles 208 on the capacity of the iron negative electrode 14. Specifically, based on these experimental results, on a weight percentage basis, as determined by laser diffraction particle size analysis, the average particle size of the discrete particles 208 is greater than about 30 microns and less than about 800 microns. This range may be useful for balancing the competing size considerations of reducing the floating / shedding risk versus achieving the peak capacity of the iron negative electrode 14.

[0073] In some embodiments, the risk of floating / shedding can be additionally or alternatively reduced by generating gas bubbles in the iron negative electrode 14 of the second powder 206 away from the discrete particles 208 including the agglomerated particles 210. For example, the concentration of the second powder 206 in the powder blend 202 can have a predetermined gradient in the thickness dimension of the active layer of the iron negative electrode 14 (corresponding Figure 2A to the thickness of the powder blend 202 therein). Thus, with such a gradient in the active layer of the iron negative electrode, hydrogen evolution in the iron negative electrode 14 can occur at a position away from the second powder 206, minimizing its risk of floating / shedding.

[0074] Generally, the first powder 204 of the powder blend 202 in the iron negative electrode 14 can include any one or more types of iron active materials suitable for use in any one or more of the various different electrochemical cells described herein. For example, the iron active material of the first powder 204 can include one or more iron-containing compounds, which can be any one or more of a variety of different shapes in the first powder 204 (e.g., spherical, agglomerated, flattened, or sintered). The range of one or more iron-containing compounds can be from highly reduced forms of iron (more metallic) to highly oxidized forms of iron (more ionic). Thus, for example, one or more iron-containing compounds can include iron alloys or iron-containing compounds such as iron oxides, mixed iron oxides, iron hydroxides, iron sulfates, iron carbonates, iron sulfides, or any combination thereof. Additionally or alternatively, one or more iron-containing compounds can include purified or refined iron materials such as carbonyl iron or electrolytic iron, or iron ores such as magnetite, maghemite, siderite, hematite, goethite, limonite, or other iron materials. In certain embodiments, the iron active material of the first powder 204 can include one or more of iron oxides, atomized iron powder, sponge iron powder, ground iron powder, other additives including conductive carbon additives, or any combination thereof.

[0075] Generally, in the powder blend 202 having the first powder 204, the second powder 206 may contribute to the formation of the iron negative electrode 14 by dry material filling. As an example, the powder blend 202 can be added to a mold, and the powder blend 202 can be pressed to form the iron negative electrode 14. According to this technique and other preparation techniques, the apparent density of the powder blend 202 can be less than the apparent density of the separate first powder blend 202 under other similar compactions. That is, compared to the iron negative electrode formed without the second powder 206, the powder blend 202 can facilitate the formation of the iron negative electrode 14 as a low-density / high-porosity electrode. For example, such low density / high porosity can facilitate exposing more of the iron active material of the first powder 204 to the electrolyte, thus improving the performance of the iron negative electrode 14.

[0076] The size of the iron active material of the first powder 204 can be adjusted relative to the size of the discrete particles 208 to facilitate dry powder processing to form the iron active electrode 14 and / or achieve the target porosity of the iron active electrode 14. To reduce the likelihood of accidental separation of the second powder 206 from the first powder 204 in the powder blend 202, the discrete particles 208 of the second powder 206 and the iron active material of the second powder can have similar particle sizes such that the first powder 204 and the second powder 206 have similar flow characteristics. The average particle size of the discrete particles 208 that minimizes separation from the first powder 204 can be close to but not necessarily equal to the average particle size of the first powder 204. Matching or nearly matching the average particle size of the discrete particles 208 with the average particle size of the iron active material of the first powder 204 can remove solid particles of at least one metal sulfide from the interstitial space between the particles of the iron active material of the first powder 204. As an example, the ratio of the average particle size of the discrete particles 208 of the second powder 206 to the average particle size of the particles of the iron active material of the first powder 204 can be from 0.5 to 2. At this relative size, the first powder 204 and the second powder 206 can generally flow together during the dry material filling process while also increasing the porosity of the iron negative electrode to facilitate better ion transport / rate performance of the iron negative electrode 14.

[0077] In some embodiments, at least one metal sulfide of the agglomerated particles 210 can be greater than 50 wt% (e.g., greater than 80 wt%) of the discrete particles 208. That is, the agglomerated particles 210 can be formed primarily of at least one metal sulfide such that the volume occupied by the agglomerated particles 210 is not significantly greater than when it is useful as an additive in the iron negative electrode 14. As described in more detail below, the technique for agglomerating particles of at least one metal sulfide to form the agglomerated particles 210 can be carried out without using a binder or by efficiently using a binder.

[0078] Although the powder blend 202 in the iron negative electrode 14 has a porosity defined by the first powder 204 and the second powder 206, it should be understood that the discrete particles 208 can contribute to the overall porosity of the powder blend 202 through the porosity of the discrete particles 208 themselves. For example, as determined by mercury intrusion porosimetry, the median pore diameter of the discrete particles 208 of the agglomerated particles 210 of at least one metal sulfide can be greater than about 75 nanometers and less than about 15 micrometers. Thus, more generally, the discrete particles 208 can have a first average apparent density, the particles forming the agglomerated particles 210 include a second average apparent density, and the first average apparent density is less than the second average apparent density. As an example, the first average apparent density can be greater than 1.0 grams per cubic centimeter and less than 2.1 grams per cubic centimeter. The lower apparent density of the discrete particles 208 relative to the apparent density of the particles forming the agglomerated particles 210 can be beneficial for exposing a large surface area of at least one metal sulfide to the electrolyte in the electrochemical cell. In some cases, the agglomerated particles 210 of the discrete particles 208 can have surface-connected pores to facilitate the cost-effective preparation of the discrete particles 208 while also facilitating the exposure of a large surface area of at least one metal sulfide to the electrolyte in the electrochemical cell. As an example, as determined by mercury intrusion porosimetry, the surface-connected pores of the discrete particles 208 can be greater than or equal to 7 volume % of the agglomerated particles 210 and less than or equal to 40 volume % of the agglomerated particles 210.

[0079] In certain embodiments, the at least one metal sulfide of the agglomerated particles 210 can include zinc sulfide (ZnS) relative to a similar electrode without zinc sulfide, which can be useful for improving the capacity of the iron negative electrode 14. To achieve such an improvement in the performance of the iron negative electrode 14 using the volume efficiency of the size of the discrete particles 208, the second powder 206 of the discrete particles 208 can be greater than or equal to 90 weight % zinc sulfide. In some embodiments, as determined by x-ray diffraction, the zinc sulfide can be in a low-temperature sphalerite structure (e.g., greater than 60 weight % of the zinc sulfide can be in a low-temperature sphalerite structure). As described in more detail below, in the case where the at least one metal sulfide includes zinc sulfide, the low-temperature sphalerite structure of the zinc sulfide can be useful for certain types of agglomeration (e.g., sintering) that can be used to form the agglomerated particles 210.

[0080] Although at least one metal sulfide of the agglomerated particles 210 has been described as including zinc sulfide, it should be understood that any one or more other types of metal sulfides may additionally or alternatively be included in the agglomerated particles 210. For example, one or more metal sulfides of the discrete particles 208 may additionally or alternatively include iron sulfides (e.g., FeS, Fe3S4, Fe2S3, and / or other forms), stannous sulfide (SnS), bismuth sulfide (Bi2S3), aluminum sulfide (Al2S3), antimony(III) sulfide (Sb2S3), antimony(V) sulfide (Sb2S5), manganese sulfide (MnS), molybdenum(IV) sulfide (MoS2), iron disulfide, iron copper sulfide, tin sulfide, copper sulfide, cadmium sulfide, silver sulfide, titanium disulfide, lead sulfide, nickel sulfide, antimony sulfide, any one or more of the polymorphs including these. In some cases, the discrete particles 208 may also include lower oxides of metal sulfides, or solid solutions of metal sulfides with oxides or hydroxides. In certain cases, the discrete particles 208 may also include minerals such as sulfosalt minerals, which are salts of metals (e.g., Cu, Pb, Ag, Fe, Hg, Zn, V), metalloids (e.g., As, Sb, Bi, Ge), and sulfur. Exemplary sulfosalts include proustite Ag3SbS3 and tetrahedrite Cu 12 As4S 13 . In some cases (e.g., depending on the electrolyte composition or other factors), the discrete particles 208 may additionally include non-metal sulfide compounds. Additionally or alternatively, the discrete particles 208 may also include tin oxide (SnO2), tin (Sn), bismuth (Bi), iron selenide (FeSe), stannous selenide (SnSe), zinc selenide (ZnSe), potassium hydroxide (KOH), sodium hydroxide (NaOH), or combinations thereof.

[0081] In some embodiments, the discrete particles 208 may further include particles of a pore former 212 to impart strength to the discrete particles 208 even if the discrete particles 208 are porous. The average particle size of the pore former 212 may be selected based on geometric considerations related to the initial size of the particles of at least one metal sulfide forming the agglomerated particles 210 and geometric considerations related to the final expected size of the discrete particles 208 after the particles of at least one metal sulfide form the agglomerated particles 210. Generally, effective pores can be formed when the average particle size of the pore former 212 can be significantly smaller than the average particle size of the discrete particles 208 and equal to or greater than the average particle size of the particles of at least one metal sulfide forming the agglomerated particles 210. For example, if the average particle size of the particles agglomerating to form the agglomerated particles 210 is ∼5 μm and the average particle size of the discrete particles 208 is 100 μm, the average particle size of the pore former 212 can be greater than about 5 μm and less than about 20 μm on a weight percentage basis. Additionally or alternatively, the particles forming the agglomerated particles 210 of at least one metal sulfide may have a first average particle size on a weight percentage basis, the particles of the pore former 212 may have a second average particle size on a weight percentage basis, and the second average particle size may be greater than or equal to the first average particle size. That is, the size of the pore former 212 can be designed to impart a greater porosity to the discrete particles 208 of the second powder 206 than can be achieved using only the particles forming the agglomerated particles 210. Additionally or alternatively, the particles of the pore former 212 may be soluble in an alkaline electrolyte such that the particles of the pore former 212 dissolve (e.g., rapidly or over time) and leave pores in the powder blend 202 of the iron negative electrode 14.

[0082] The particles of the pore former 212 may include any one or more different types of materials that serve as low-cost and easily removable spaceholders. As an example, the particles of the pore former 212 may include potassium oxide (K2O), lithium oxide (LiO), sodium oxide (Na2O), potassium hydroxide (KOH), lithium hydroxide (LiOH), sodium hydroxide (NaOH), sodium sulfate (Na2S), potassium sulfate (K2S), sodium carbonate (Na2CO3), potassium carbonate (K2CO3), sodium stannate (Na2[Sn(OH)6]), potassium stannate (K2[Sn(OH)6]), or combinations thereof. A subset of these materials including KOH, NaOH, Na2CO3, K2CO3 can effectively thermally dissociate into constituent salts and gaseous by-products (e.g., 2KOH + heat → K2O + H2O or K2CO3 + heat → K2O + CO2). The production of gaseous by-products is useful because the spaceholder is large in volume but low in cost. Additionally or alternatively, the particles of the pore former 212 may include poly(methyl methacrylate), starch, sodium chloride (NaCl), potassium chloride (KCl), ammonium bicarbonate (NH4HCO3), or combinations thereof.

[0083] Now referring to Figures 2A to 2C , typically, the second powder 206 of the discrete particles 208 should generally be able to be blended and processed with the first powder 204 without disintegrating or otherwise losing its intended size. Disintegration or partial disintegration can be defined as a significant reduction in the size of the secondary particles of the solid additive after blending and processing. A significant reduction in size can be defined as a measurable shift towards smaller particle sizes in the particle size distribution, including the formation of a measurable "second mode" in the particle size distribution (due to the formation of smaller particles through disintegration), which mode is the approximate original particle size.

[0084] Disintegration can also be identified microscopically. For example, the other powders in the blend can be imaged before and after blending, and the surfaces of the other powders can be examined to see if portions of the solid additive material are deposited on the surface. Additionally or alternatively, by monitoring the particle shape of the secondary particles throughout the blending process, the solid additive can be shown not to disintegrate during blending. If the particles are shown to become rounded or otherwise change shape throughout the blending, then the particles have at least partially disintegrated during the blending process.

[0085] According to the European Pharmacopoeia 2.9.41.-2 (Method B) (the entire content of which is hereby incorporated by reference), the anti-disintegration property can be quantified as the friability of the second powder 206 of the discrete particles 208. The test procedure can be adjusted for different materials to distinguish particles that are not suitable for use as battery pack additives from particles that are suitable for use as battery pack additives. In one embodiment, the friability based on ZnS particles is defined. The material is shaken in a friability tester at a frequency of 400 vibrations per minute for 240 seconds, with 10 ± 1 grams of material in the tester bottle. It has been shown that materials with high friability produce a large amount of fine powder, the amount of which is considered not usable for the second powder 206. Specifically, a friability of 20% was observed according to the procedure outlined below. A friability of less than about 15% (e.g., less than 10%, less than 5%) can be used for the second powder 206, with small variations allowed for test tolerances. The starting material is a powder with a particle size of 150 to 500 microns. Due to the relatively fine starting material particle size, a dry particle size analyzer (Camsizer X2, dispersion pressure of 30 kPa) was used to obtain the particle size distribution of the material and to quantify the difference in particle size distribution of the material before and after shaking in the friability tester. After shaking in the friability tester, a second mode was found at finer particle sizes in the particle size distribution; this was interpreted as being due to particle breakage.

[0086] The amount of material attributed to this mode is quantified by:

[0087] 1) Plot the change in q3 (the derivative of the cumulative particle size distribution) against the particle size of the powder before shaking in the friability tester and the particle size of the powder after shaking in the friability tester.

[0088] 2) If the friability of the material is low enough, the particle size distribution before and after shaking in the friability tester should have a substantially similar particle size distribution, with the mode in the particle size distribution being approximately the same as before shaking, and possibly a new mode added at the finer particle sizes - this new mode at the finer particle sizes is called the fines mode. In the case where there is no mode at approximately the same particle sizes before and after shaking in the particle size distribution in the friability tester, the material is too friable as it has disintegrated in the tester. These materials are defined as having a higher friability than the desired level, even if an exact value cannot be calculated.

[0089] 3) By comparing the particle size distributions before and after shaking in the friability tester, the particle size that separates the fines mode from the original mode in the particle size distribution can be selected. This is typically chosen as the local minimum of q3 as a function of particle size. This particle size is called the cut-off size.

[0090] As a quantitative measure, a weight loss of less than about 10% of the original sample as fines according to European Pharmacopoeia 2.9.41.-2 (Method B) can indicate suitable anti-disintegrability, which is useful for reliably forming the iron negative electrode 14 by processing using the second powder 206 and for use in the iron negative electrode 14.

[0091] Figure 2C is a diagram showing the mixing of a powder of discrete particles (e.g., Figure 2A and Figure 2B the second powder 206 of discrete particles 208 therein) with a powder of an iron active material (e.g., Figure 2A and Figure 2B the first powder 204 therein). In the diagram, the powder of discrete particles is light-colored and the powder of the iron active material is dark-colored. In this example, the solid-state additive (ZnS) is granulated with poly(vinyl alcohol) and carboxymethyl cellulose. Both binders can produce agglomerates. In some cases, the particle sizes produced by this process are similar to those of the iron active material. Incorporating these materials into the Fe electrode powder mixture successfully reduces the apparent density of the powder mixture, demonstrating that electrode porosity can be retained. Verification of the sintered ZnS concept was also carried out, and materials with suitable strength and properties were also produced. Incorporating these additives into the electrochemical tests shows enhanced performance.

[0092] Figure 4 is a flowchart of an exemplary method 400 for preparing an additive for an iron negative electrode for an alkaline electrochemical cell. Unless otherwise indicated or explicitly specified in the context, any one or more aspects of the exemplary method 400 can be used to prepare the second powder 206 ( Figure 1B ) for the iron negative electrode 14 ( Figure 1B and Figure 2A ) for a rechargeable battery pack 10 ( Figure 2Aand Figure 2B )。

[0093] As shown in step 410, the exemplary method 400 can include forming a feedstock that includes a granular material having a predetermined composition. The granular material should be understood to refer to loose particles of a material that agglomerate together to form any one or more agglomerated particles 210( Figure 2B ). Thus, the granular material can include particles of at least one metal sulfide prior to agglomeration into agglomerated particles. By way of example and not limitation, the particles of at least one metal sulfide can include particles of zinc sulfide (ZnS).

[0094] In some embodiments, the granular material of the feedstock can include a combination of multiple types of solid additive particles having different chemical properties and / or particle sizes. As an example, in order to facilitate achieving optimal performance in an electrochemical cell, it may be useful to substantially uniformly mix multiple types of particles with the iron active particles of any one or more of the powder blends described herein. Mixing multiple types of solid additive particles in the feedstock in a predetermined weight ratio relative to each other prior to processing can facilitate achieving such approximate uniformity in the powder blend that will subsequently be formed by processing with the iron active particles. That is, for any two additives A and B, the granular materials of only A and only B will necessarily result in A-rich pockets and B-rich pockets, while the granular material of a suitable A + B mixture will generally result in better distribution uniformity of both additives A and B. As an example, the feedstock of the granular material can include a mixture of at least two metal sulfides (e.g., zinc sulfide and iron sulfide). In some embodiments, the granular materials of at least two sulfides can be mixed together in a predetermined weight ratio relative to each other. By processing the feedstock to form discrete particles, this weight ratio can advantageously persist such that the discrete particles can have at least two metal sulfides in a predetermined weight ratio.

[0095] In some embodiments, forming a feedstock that includes a granular material having a predetermined composition can include blending particles of a pore former (e.g., any one or more of the various different pore formers described herein) with particles of at least one metal sulfide. In some cases, the pore former can be soluble in an alkaline electrolyte. The ratio of the average particle size of the particles of the pore former in the feedstock to the average particle size of the particles of at least one metal sulfide can be greater than or equal to 1:1 and less than about 5:1.

[0096] In some embodiments, forming the feedstock can include mixing a conductive material with at least one metal sulfide. The conductive material can include tin, graphite, carbon black, or a combination thereof. In cases where the feedstock includes a conductive material, it should be understood that the discrete particles produced by any one or more of the processing techniques described below can include the conductive material.

[0097] In the specific case of incorporating a solid additive into the iron negative electrode, the particle size of the iron active material particles can be from about 50 micrometers to about 900 micrometers, and the particle size of the granular material of at least one metal sulfide can generally be about 10 micrometers or less. There are several reasons for this: 1) Many solid additives are produced by chemical synthesis and related techniques for battery pack applications, and these techniques generally have the highest yield and lowest cost when producing particles with a size of ~500 nm to 10 μm; 2) The function of the additive may rely on uniform and fine dispersion throughout the iron negative electrode to enhance electrode performance; or 3) The additive may require a high surface area to enhance electrode performance. The particle size of the granular material of one or more additives may be affected by one or more of these reasons. Some examples of the most effective additives with a granular material having a particle size finer than that of iron in the iron negative electrode include MnS, Al2S3, Sb2S3, Sb2S5, FeS, Bi2S3, MoS2, conductive additives such as graphite or carbon black, tin-containing solid compounds, and ZnS.

[0098] In some embodiments, the forming feedstock may include introducing at least one polymer binder to the granular material. As described in more detail below, the at least one polymer binder can be used as part of a sintering process such that the agglomerated particles formed from the granular material of the feedstock are solid-state bonded together.

[0099] As shown in step 420, the exemplary method 400 may include processing a feedstock including granular material into a powder of discrete particles including agglomerated particles of the granular material, the agglomerated particles including at least one metal sulfide. As described in more detail below, processing the feedstock including granular material into a powder of discrete particles may include any one or more of a variety of different granulation techniques that can be implemented economically and effectively to produce a powder of discrete particles with a target friability to achieve the performance goals of the iron negative electrode formed using the powder of discrete particles.

[0100] In some embodiments, the particulate material of the raw material can be bound together in the solid state to form discrete particles of agglomerated particles. For example, such solid state binding can include binding the material in the solid state using bonds of the same material. The solid state binding can be carried out according to any one or more of a variety of different techniques. However, the solid state binding formed according to such techniques can be defined by the presence of a neck or solid state contact between the agglomerated particles (e.g., which can be observed by microscopic examination of the surface and / or cross-section of the agglomerated particles), including cases where the neck or bond size is > 5% of the particle diameter. Additionally or alternatively, the solid state binding is characterized in that processing the material by any one or more processes results in a significant increase in the strength of the material that has undergone the process, provided that the increase in strength is attributable to the solid state binding. Examples of processes that can be used to form solid state bonds include sintering (including sintering at a temperature above half of the melting temperature of the material), hot pressing (including hot pressing at a temperature of 30% of the melting temperature of the material or at a temperature above 30% of the melting temperature of the material), cold sintering, infiltration, and reaction bonding. The equipment for carrying out the solid state binding can include a calcining furnace, a continuous straight-line furnace (e.g., a belt furnace, a pusher furnace, and a walking beam furnace), and / or an intermittent furnace.

[0101] As an example, the particulate material of the raw material can be solid state bound at a temperature of about 500 degrees Celsius to about 1400 degrees Celsius. This solid state binding can occur in a non-oxidizing atmosphere such as a reducing atmosphere or an inert atmosphere. In the case of at least one metal sulfide including zinc sulfide, an oxidizing atmosphere may be less desirable due to the possible formation of harmful sulfur dioxide and sulfur oxide gases and the reduction of the ZnS content by the formation of ZnO from ZnS. The inert atmosphere can include nitrogen or argon. The reducing atmosphere can include hydrogen or carbon monoxide. Mixtures of these atmospheres (e.g., a combination of nitrogen and hydrogen) can be used. For fine ZnS particulate material, satisfactory sintering can be experimentally observed at a temperature above 900 degrees Celsius and below 1300 degrees Celsius (e.g., ~1100 °C), and for fine ZnS powder, hot pressing at 900 °C, a uniaxial pressure of about 60 psi (about 410 kPa), and a time of about 10 minutes is shown to be satisfactory. The sintering and / or hot pressing temperature, pressure, and other processing conditions will vary with the material used and the desired density.

[0102] In certain embodiments, processing the raw material into discrete particles can include introducing at least one binder into the discrete particles of the agglomerated particles. Continuing with this example, at least one binder introduced into the discrete particles can be soluble and / or reactive to form a soluble substance in an alkaline electrolyte, such that the binder can bind the particulate material to the discrete particles of the agglomerated particles to facilitate handling the powder of the discrete particles to form an iron negative electrode, and the binder can dissolve in the iron negative electrode when the rechargeable battery pack is cycled.

[0103] Returning to the example where the feedstock includes introducing at least one polymeric binder to the particulate material, processing the feedstock into discrete particles can include pyrolyzing at least one polymeric binder to form a graphitic film on the particulate material. The type of binder used can vary with the material being bound and how the electrode is processed. In the case of ZnS agglomeration, at least one binder can include CMC, poly(vinyl alcohol), and poly(ethylene glycol). At least one binder and subsequent processing can be selected such that during the electrochemical cycling, the binder holds the solid additives within the powder bed, and in particular such that small particles are bound in place during the electrochemical cycling. A binder having a large amount of char upon pyrolysis during high temperature processing (such as sintering, hot pressing) can produce discrete particles of the agglomerated particles, which keeps the particles within the discrete particles / reduces the risk of particle floating. A high char binder can be defined as a binder that pyrolyzes in an inert atmosphere and has a residual solid yield > 7 wt% after pyrolysis in a non-oxidizing atmosphere up to 600 °C. Carboxymethyl cellulose (CMC) or poly(acrylic acid) can be used as high char binders. Polyacrylonitrile (PAN) or other polymers having aromatic rings can be used as high char binder systems. In such cases, the pyrolysis behavior can be used simultaneously to graphitize the char, thereby producing a strong coherent graphitic film on the solid additives while maintaining the additives in the solid state. Pitch can be used as a binder or tackifier. In many cases, a mixture of binders can be used in the granulation process. These binders can be used for different purposes. For example, poly(vinyl alcohol) can be used in combination with CMC and / or PAN to customize the rheology, particle strength, and char level of the particles. In some embodiments, the binder can remain within the discrete particles throughout the life of the electrochemical cell. In such cases, the binder can be compatible with the alkaline electrolyte and can thus include poly(tetrafluoroethylene), carboxymethyl cellulose, poly(ethylene), poly(propylene), and polyvinylidene fluoride. In some cases, binder pyrolysis and residual char can be detrimental to the solid additives, and a low char binder may be required. In such cases, poly(ethylene), poly(propylene), poly(vinyl alcohol), or other binders that produce low residual carbon can be usefully used to form discrete particles of the agglomerated particles. In some embodiments, inorganic binders can be used, such as clay or silicate-based binders, alumina-containing binders, or any other oxide-based binder common in the art for binding powdered materials together.

[0104] In certain embodiments, the techniques for processing the feedstock to form discrete particles of the agglomerated particles can include granulation processes that do not involve binders. Such processes can include compacting (e.g., roll pressing) or sintering the powder body and reducing the sintered body to a suitable size by, for example, pulverization.

[0105] Processing the feedstock to form discrete particles may additionally or alternatively include any one or any other suitable granulation technique that may be useful for binding smaller powder particles together into coarser powder particles: fluidized bed granulation, spray drying, high shear mixing granulation, twin screw granulation, roll compaction, intensive mixing, wet granulation, and extrusion granulation, and open pan granulation / disc granulation.

[0106] In some cases, a combination of any one or more of the granulation techniques described herein may be used. For example, the strength sometimes required may be greater than that which can be reasonably achieved by granulation using a binder, but a tight particle size and high sphericity may also be required. In such cases, spray drying techniques may be used to agglomerate the granular material, and the resulting material may then be sintered in a process called the agglomeration and sintering process.

[0107] In some cases, the attraction between particles may be sufficient such that no binder is required to form discrete particles of agglomerated particles. For example, nano-sized powders can agglomerate without the need for a binder.

[0108] In some embodiments, processing the feedstock to form discrete particles of agglomerated particles may include binder-based granulation, including drum granulation, spray drying, wet granulation, or dry granulation.

[0109] In some cases, an additive may be used as a binder for other additives. For example, a first additive may be heated in the presence of a second low melting point additive that solidifies upon cooling to room temperature. For example, the first additive may be FeS or ZnS, and the second additive may be Sn. The heating may be carried out in an inert atmosphere to prevent oxidation of the molten phase. The crystallographic and chemical properties of both the first additive and the second additive can be adjusted to ensure that the first additive is wetted by the second additive, and thus to ensure uniform binding and high strength upon cooling. The resulting material can be considered a metal matrix composite bonded by infiltration. Incorporating the bonded / infiltrated second additive in powder form during the infiltration process results in a higher degree of uniformity of the material properties of the resulting discrete particles and can create more pores in the discrete particles, thus usefully increasing the rate performance and / or reducing the impedance of the resulting iron negative electrode formed from the powder of the discrete particles.

[0110] In some embodiments, the particulate material of the raw material can be solid-state bonded with a shape factor close to the desired size and shape. For example, the already agglomerated powder can be solid-state bonded by many techniques known in the art for powder agglomeration and / or sintering. As a specific example, ZnS powder can be pelletized and, in some cases, ground to the desired size distribution, and then the resulting agglomerates can be sintered (e.g., in a continuous sintering furnace). In certain cases, the ZnS powder can be spray-dried to form agglomerates in a suitable size range, and then the resulting agglomerates can be sintered. The resulting sintered material can be lightly ground in a rod mill or similar equipment to break any slight bonding between the secondary particles that occurs during the sintering step without further disrupting the discrete particles of the agglomerated particles.

[0111] In certain embodiments, the raw materials can be cold-pressed together to form discrete particles that can retain sufficient adhesion for handling, such as in the case of cold-pressing of tin powder or various pelletizing techniques for ceramic powders.

[0112] Using any one or more of the various different techniques described herein, the binding / granulation process can result in bound material particles larger than the desired average particle size of the powder of discrete particles (e.g., at least one dimension is approximately 1 mm or greater). In such cases, the resulting bound / sintered mass can be an intermediate that can be machined (e.g., by suitable crushing, grinding, milling, and sizing / sieving techniques) to achieve the target average particle size of the powder of discrete particles. Such processing usefully separates the binding process from the sizing process and thus facilitates a wider process window and material combinations.

[0113] In some embodiments, the raw materials can be processed into a monolithic block of a larger ceramic material (by, for example, pressing and sintering a geometry with at least one dimension approximately 1 mm or greater). Then, the resulting larger-scale solid-state bonded material can be crushed, ground, or otherwise reduced in size to produce discrete particles of the desired size.

[0114] In some embodiments, alloying elements can be included that extend the stability of the cubic phase of ZnS to higher temperatures, thereby maintaining the cubic phase during hot working operations or facilitating the transformation of the hexagonal phase to the cubic phase.

[0115] Now refer to Figure 4 、 Figure 5A and Figure 5B, which is a ZnS phase diagram showing the phase behavior as a function of temperature and composition. The phase diagram shows the solubility of FeS in ZnS and that doping with FeS can usefully control the phase transition temperature. It can be noted that the solubility of Fe in ZnS is up to ~40 mol%. In some embodiments, it may be useful to have ZnS in the low-temperature wurtzite structure. Iron lowers the phase transition temperature from wurtzite to sphalerite, making it easier for ZnS to transform into the wurtzite crystal structure. Correlatively, iron contamination inhibits the re-transformation of the wurtzite structure back into the sphalerite structure. This transformation may be slow relative to the time scales associated with industrial processing (tens (10’s) of minutes to several hours). The combination of the lowering of the phase transition temperature, chemical partitioning, and the generation of a drag effect upon introduction of the Fe alloying element, along with the slow diffusion kinetics at lower temperatures, makes the phase transition slow.

[0116] In the case where at least one metal sulfide includes ZnS, the cubic ZnS phase can be retained during a hot working process. For example, in such cases, the starting material can be sintered at a temperature above the sphalerite ZnS / wurtzite ZnS phase transition temperature (1020 °C for pure ZnS) and annealed at a temperature below the sphalerite ZnS / wurtzite ZnS phase transition temperature during cooling to promote the transformation back to sphalerite ZnS. Annealing to transform to sphalerite ZnS may require careful balancing of the kinetics and driving force of the transformation to occur on a useful time scale (tens of minutes to several hours). Generally, at temperatures below the phase transition temperature, as the supercooling increases, the driving force for transformation to the cubic phase increases, but the kinetics of the transformation are somewhat slow. The annealing conditions to achieve the transformation from the wurtzite structure to the sphalerite structure can be from about 700 °C to about 1000 °C (e.g., from about 800 °C to about 950 °C) with a time scale of from about 15 minutes to about 4 hours. In some cases, the thermal exposure can include an isothermal hold within the listed temperature range. The material can be deliberately cooled slowly enough within the listed temperature range where the crystal structure transformation occurs. In some embodiments, a combination of slow cooling and isothermal hold can be used to achieve the transformation of the crystal structure. In certain cases, the material can be reheated to achieve the transformation to the cubic phase of the ZnS material (as compared to performing solid-state bonding and crystal structure transformation in one step).

[0117] Continuing with this example, sintering can usefully be performed at a temperature below the sphalerite ZnS / wurtzite ZnS phase transition temperature to produce sintered particles having a sphalerite ZnS crystal structure without further processing to transform the ZnS into the sphalerite structure, thus saving expensive processing time and energy.

[0118] In some embodiments, processing the starting material to form discrete particles can include managing C and O impurities.

[0119] As an example, ZnO and C-based impurities in a ZnS additive may be undesirable. If ZnO remains in discrete particles, it may be reduced during subsequent thermal processing or dissolve to form zincate ions when discrete particles of the agglomerated particles enter the electrolyte. Reduction may cause evaporation of metallic Zn and subsequent deposition in the thermal processing equipment. Alternatively, if ZnO enters an electrochemical cell, it may dissolve in the alkaline electrolyte of the electrochemical cell, potentially affecting the electrochemical performance. Thus, both reduction and dissolution may be undesirable.

[0120] Thus, continuing with this example, a reducing and / or decarburizing atmosphere can be used to remove ZnO in a controlled manner. ZnO reduction typically occurs at 700 °C to 1000 °C, producing gaseous Zn metal and a redox species. In various embodiments, the reduction can be carried out with a reducing agent such as solid carbon, gaseous carbon monoxide, or gaseous hydrogen.

[0121] In some embodiments, an S-containing gas can be used to reduce the likelihood of ZnO evaporation. ZnS typically contains trace amounts of ZnO, and in a processing atmosphere, ZnS can react with O to produce ZnO. ZnO can be reduced to form Zn-based vapors, especially during high-temperature electrode processing. Zn evaporation can be detrimental to the thermal processing equipment, and thus it is desirable to avoid or at least reduce this possible likelihood. Accordingly, in some embodiments, an S-containing gas can be added to increase the sulfur potential in the processing atmosphere. ZnO can be converted to ZnS, preventing Zn evaporation. The gas can be selected as a reducing gas. As an example, the S-containing gas can be any one or a combination of the following: 1) hydrogen sulfide, H2S; 2) carbonyl sulfide, OCS; 3) methanethiol, CH3SH; and / or 4) sulfur dioxide, SO2.

[0122] In some embodiments, the effect of the formation of gaseous zinc on electrode formation and / or furnace operation can be reduced. In the presence of a carbon source, oxides co-existing in a zinc sulfide source (e.g., ZnO) or zinc oxide formed by oxidation on other oxide-free surfaces may cause the carbothermal reduction of ZnO to Zn(g) when heated to temperatures above 800 °C. The formation of this gaseous zinc can be controlled to prevent or at least reduce the likelihood of interfering with furnace operation or electrode preparation. The carbothermal reduction can include each of the following: 1) both carbon and zinc oxide are present in the heated electrode; and 2) a high enough temperature. Thus, preventing or controlling the formation of gaseous zinc from a zinc source containing any amount of zinc oxide may require removing one of the above factors. In one embodiment, the granular material can include zinc sulfide, an iron source, and some carbon source, which can be compacted at a temperature of about 750 °C, about 750 °C to 800 °C, about 800 °C, about 800 °C to 850 °C, or about 850 °C to reduce the likelihood of excessive zinc oxide being reduced.

[0123] As an additional or alternative example, C-based contamination in ZnS may be undesirable. It has been found that when processing ZnS with Fe at high temperatures, C contamination can increase undesirable side reactions. In various embodiments, managing C can be carried out through three routes: 1) minimizing or eliminating the use of binders or other carbon-containing materials during processing; 2) using low-coke binders when in use; 3) using a suitable atmosphere for heating and sintering ZnS. In some embodiments, ZnS can be pressed and sintered in the absence of a polymer binder, thus eliminating the risk of C contamination. In some embodiments, a C-containing polymer can be added as a pressure aid, binder, or other functional agent. Polymers with acceptable coke levels can include any low-coke binders used for low-residue C pyrolysis in an inert or reducing atmosphere, such as: zinc stearate, stearic acid, polymeric alcohols such as poly(vinyl alcohol), poly(methyl methacrylate), polyolefins (e.g., poly(propylene), poly(ethylene), low-molecular-weight analogs such as paraffin wax and microcrystalline wax), and copolymers of these materials. Additionally or alternatively, inorganic binders can be used, such as soluble silicates, soluble phosphates, or soluble aluminates. Silicates can reduce the battery pack performance of the iron negative electrode, so using aluminate as an inorganic binder may be useful. Additionally or alternatively, gaseous hydrogen can be included in the processing atmosphere to promote the removal of residual C in ZnS through the reaction Cs + 2H 2,g →CH 4,g (or other related reactions).

[0124] Various embodiments can include methods for preparing electrodes for a battery pack (e.g., the iron negative electrode 14 of battery pack 10, the negative electrode 102 of electrochemical cell 100, etc.), including forming a structure comprising an iron active material and a zinc source, and converting the zinc source to zinc sulfide on or in the structure through a sulfidation reaction. In various embodiments, the zinc source can be in various forms, including zinc oxide powder, zinc oxide solution, etc., and the zinc oxide can be in various forms, such as solid zinc oxide particles, agglomerated particles of zinc oxide, etc. In various embodiments, forming the structure and / or converting the zinc source to zinc sulfide on or in the structure can include one or more different operations, including but not limited to exposure to one or more liquid and / or powder and / or gas solutions, drying, heating, cooling, pressing, sintering, etc.

[0125] In some embodiments, an electrode structure (e.g., a structure for forming the iron negative electrode 14, the iron negative electrode 14 itself, etc.) can be placed on, immersed in, submerged in, or have poured thereon a gel, gel-like, or viscous mixture of ZnS, a gelling agent (e.g., agar, lignosulfonate, polyethylene glycol, etc.), and a solvent (e.g., water, isopropyl alcohol, dichloromethane, dimethyl sulfoxide, dimethylformamide, toluene, etc.), where in the presence of the gelling agent (0.1 wt% to 10 wt% gelling agent), a minimal amount of solvent can be used to make the system flowable. In some mixtures, zinc sulfide can be the major component. In other mixtures, the selected gelling agent can be the major component. The penetration of the mixture into the electrode pores can occur naturally until sufficient mixture has been absorbed into the electrode (the increased mass being equal to the desired ZnS wt%, including the weight of the solvent and organics), or can be accelerated by using a vacuum until the same result is achieved. The mixture can then be dried at a temperature not exceeding 800 degrees Celsius. In some embodiments, the gelling agent is removed by washing with a compatible solvent in which the agent is sufficiently soluble (e.g., water, isopropyl alcohol, dichloromethane, dimethyl sulfoxide, dimethylformamide, toluene, etc.). In some embodiments, the gelling agent is removed by heating in an oven until complete combustion or evaporation, as measured by a change in mass. In some embodiments, the gelling agent can be partially removed until the desired mass remains in the electrode or until sufficient coke has formed.

[0126] In one embodiment, a sol-gel mixture consisting of a zinc oxide compound and a sulfide source (e.g., Na2S, H2S) can be used to introduce an additive into an electrode structure (e.g., a structure for forming the iron negative electrode 14, the iron negative electrode 14 itself, etc.). The electrode structure can be immersed in whole or in part in a zinc oxide solution consisting of a mixture of zinc oxide with a solvent (e.g., water, isopropyl alcohol, dichloromethane, dimethyl sulfoxide, dimethylformamide, toluene, etc.) and a gelling agent (e.g., agar, lignosulfonate, polyethylene glycol, etc.), and then exposed to a solution of Na2S or gaseous H2S. The electrode structure can then be dried in air or in an oven at a temperature not exceeding 800 degrees Celsius to form ZnS. The formation can be confirmed by performing XRD on a representative sample from the electrode, as well as by ICP-OES to confirm that the mass ratio of Zn:S is approximately 1:1 (the theoretically acceptable range = 0.5:1 to 2:1).

[0127] Compared with soluble sulfide sources (e.g., Na2S, K2S), ZnS has a lower solubility, enabling the preferential formation of ZnS in solution, where the relative solubility is determined by comparing the solubility products (Ksp) of the respective compounds in water. Facilitated by a solvent, this exchange reaction of ZnO with soluble sulfide can be summarized as ZnO + Na2S + H2O → ZnS + 2NaOH or equivalently Zn(OH)4 2- + Na2S → ZnS + 2NaOH + 2OH - , where the solvent shown here is water within a certain pH range. This exchange with a more soluble sulfide for forming ZnS is known to occur readily in alkaline electrolytes. A similar substitution reaction can be carried out using gaseous hydrogen sulfide (H2S) or other sulfur-containing gases, e.g., ZnO + H2S(g) → ZnS + H2(g). Thus, any embodiment that allows the addition of ZnO in the form of particles or aggregates can subsequently be exposed to a sulfur ion source (e.g., S 2- 、HS -etc.) to form ZnS particles or aggregates similar in size to the initial aggregates. The surface area and porosity of the particulate material can be designed to allow the conversion of ZnO to ZnS on the time scale of interest, where the time scale may depend on the chosen conversion method (e.g., gaseous H2S vs. Na2S). In an illustrative embodiment, ZnO can be placed in a KOH-based solvent (6M to 8M KOH) and 10 mM to 750 mM of dissolved sodium sulfide is added. The molar ratio of sodium sulfide to ZnO can be from 1 to 2. In this illustrative embodiment, as verified by XRD, ZnO can react at room temperature within 1 week to form ZnS. If the KOH-based solvent is the electrolyte of the battery, this conversion process can effectively occur within the battery cells of the battery pack, thus combining the ZnS synthesis step and the step of filling the battery with the electrolyte. In another illustrative embodiment, ZnO can be converted to ZnS by exposure to H2S gas at elevated temperatures. In some cases, ZnO can be embedded in a matrix of iron-containing compounds (such as metallic iron and iron oxides, iron hydroxides, and iron oxyhydroxides). In some cases, the sulfur activity can be controlled such that the ZnO undergoes a sulfidation reaction, but other components of the blend (such as iron) do not undergo sulfidation. By, for example, controlling the partial pressure ratio of H2S and H2, selective sulfidation of ZnO can be achieved without sulfidation of the Fe-containing material. Such a controlled sulfidation reaction can limit the total amount of sulfur added to the powder mixture, which is beneficial for electrochemical performance and durability. An overly high starting sulfur concentration can lead to the accumulation of sulfates in the electrolyte and thus to sulfate precipitation. The reaction of ZnO with H2S can be carried out at relatively low temperatures (including as low as 250 °C), and the conversion time scale is approximately several (4 to 9) hours. By increasing the temperature to increase the reaction kinetics to, for example, 300 °C or 400 °C, faster kinetics can be obtained. At an H2S concentration of 0.25% to 1% at the reactor inlet, complete sulfidation can be achieved at 400 °C to 600 °C in a time of 10 to 60 minutes. This high-temperature sulfidation process can be carried out during the cooling stage of a sintering or hot pressing operation, thus utilizing the heat already transferred to the electrode during the electrode formation process.

[0128] The sulfidation reactions discussed above can be applied to solid ZnO particles or aggregates of ZnO particles (sometimes referred to as ZnO granules). The specific surface area of the ZnO particles can be adjusted to allow sufficient sulfidation kinetics to achieve low processing costs and rapid processing times.

[0129] In some embodiments, the sulfidation reaction of H2S with ZnO can be combined with the high-temperature processing steps of iron materials. As an example, a sintered or hot-pressed iron negative electrode (such as iron negative electrode 14, etc.) containing ZnO can be formed. During the cooling process, H2S can be added to the cooling gas stream so that ZnO is in-situ converted to ZnS in the powder bed. Such in-situ sulfidation steps can effectively utilize the heated part and can further effectively avoid the direct reduction of ZnS by metallic Fe. The direct reduction of ZnS by metallic Fe is not desirable because it causes ZnS loss, thereby increasing the cost of the resulting Fe electrode. The direct reduction of ZnS by Fe is kinetically / thermodynamically facile only above 800 °C, while the sulfidation reaction of ZnO to ZnS can occur at much lower temperatures. Therefore, during the hot processing of the iron negative electrode, there is an important wide temperature window in which in-situ sulfidation can be effectively carried out.

[0130] One or more of the foregoing embodiments discussed above (including the use of binders or pore formers) can be combined with one or more of the methods discussed above for preparing cells for a battery pack (such as the iron negative electrode 14 of battery pack 10, the negative electrode 102 of electrochemical cell 100, etc.). For example, the ZnO aggregates discussed herein can be formed using one or more binders in a manner similar to that for forming ZnS aggregates discussed above. In another non-limiting example, a Na2S pore former of a suitable particle size, such as produced by chemical synthesis or by granulation, can be introduced into the iron powder mixture for the electrode before pressing to form an electrode with sufficient porosity. After the electrode is formed, Na2S can be exposed to a suitable solvent and dissolved, which can then facilitate the reaction of the sulfide with ZnO to form ZnS in the electrode. In another embodiment, ZnO aggregates can be introduced into the electrode before processing. After processing, the electrode can then be exposed to an electrolyte containing sulfide (such as dissolved Na2S, K2S, soluble sulfides, or mixtures thereof). It is then known that sulfide ion exchange forms ZnS in place of ZnO.

[0131] The above method descriptions are provided only as exemplary examples and are not intended to require or imply that the steps of the various embodiments must be carried out in the order presented. As those skilled in the art will understand, the order of the steps in the above embodiments can be carried out in any order. For example, words such as "thereafter", "then", "next", etc. are not necessarily intended to limit the order of the steps; these words can be used to guide the reader in understanding the description of the method. In addition, any reference to a claim element in the singular form, such as the use of the articles "a / an" or "the", should not be construed as limiting the element to the singular.

[0132] The foregoing description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the embodiments. In addition, any step of any of the embodiments described herein can be used in any other embodiment. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein can be applied to other embodiments without departing from the scope of the disclosure. Thus, the disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein.

Claims

1. A method for preparing an electrode for a battery pack, comprising: Form an electrode structure comprising an iron active material and a zinc source; and Convert the zinc source into zinc sulfide on or in the electrode structure by a sulfidation reaction.

2. The method according to claim 1, wherein the zinc source comprises zinc oxide powder.

3. A method for preparing an electrode for a battery pack, comprising: Contact an electrode structure comprising an iron active material with a solution containing dissolved zinc; and Expose the structure in contact with the solution to a sulfide source to convert at least a portion of the dissolved zinc into zinc sulfide.

4. The method according to claim 3, wherein the sulfide source is a solution containing Na2S.

5. The method according to claim 3, wherein the sulfide source is a gas containing H2S.

6. The method according to claim 3, wherein the solution containing dissolved zinc comprises a solvent and a gelling agent.

7. The method according to claim 3, further comprising drying the structure in contact with the exposed solution, wherein the drying occurs at one or more temperatures of 800 degrees Celsius or below 800 degrees Celsius.

8. The method according to claim 7, wherein the drying occurs in a furnace.

9. A method for preparing an electrode for a battery pack, comprising: Form a sulfidation solution composed of a solvent and a sulfide source; Form an electrode structure comprising an iron active material and a solid zinc source; Contact the electrode structure with the sulfidation solution; and Maintain contact of the sulfidation solution with the electrode structure for a period of time such that zinc sulfide is incorporated on or in the electrode structure.

10. The method according to claim 9, wherein: The sulfidation solution is a component of an electrolyte solution; and Contacting the electrode structure with the sulfidation solution includes adding the sulfidation solution to a battery cell of a battery pack including the electrode structure such that the sulfidation solution contacts the electrode structure.

11. The method according to claim 9, wherein the solvent comprises KOH and the sulfide source comprises Na2S.

12. The method according to claim 9, wherein the solid zinc source is formed of solid zinc oxide particles or agglomerated particles of zinc oxide.

13. The method according to claim 12, wherein, The molar ratio of Na2S in the sulfidation solution to zinc oxide in the electrode structure is from 1 to 2.

14. The method according to claim 13, wherein the period of time is less than 168 hours.

15. A method for preparing an electrode for a battery pack, comprising: Form a structure comprising an iron active material and a solid zinc source by one or more processes carried out at a first processing temperature or above the first processing temperature; and Expose the formed structure to a sulfide source gas while maintaining a selected concentration of the sulfide source gas.

16. The method according to claim 15, wherein the one or more processes carried out at the first processing temperature or above the first processing temperature include one or more of sintering and hot pressing.

17. The method according to claim 15, wherein exposing the formed structure to the sulfide source gas while maintaining the selected concentration of the sulfide source gas includes exposing the formed structure to the sulfide source gas while maintaining the selected concentration of the sulfide source gas and a second processing temperature below the first processing temperature, such that a sulfidation reaction of zinc oxide occurs to form zinc sulfide.

18. The method according to claim 17, wherein the first processing temperature is equal to or higher than 800 degrees Celsius, and the second processing temperature is equal to or lower than 600 degrees Celsius.

19. The method according to claim 17, wherein the second processing temperature is equal to or lower than 400 degrees Celsius.

20. The method according to claim 15, wherein the sulfide source gas is H2S, and the selected concentration of the sulfide source gas is from 0.25% to 1%.

21. The method according to claim 15, wherein exposing the formed structure to the sulfide source gas occurs as part of a cooling stage of the last one of the one or more processes for forming the formed structure.

22. The method according to claim 15, wherein the zinc oxide is formed from solid zinc oxide particles or agglomerated particles of zinc oxide.

Images