Production and fusion of zinc particles in porous electrodes

Abstract

To provide a method for producing and fusing zinc particles in a porous electrode.SOLUTION: A method of immersing a monolith in a liquid is provided in which a mixture of zinc particles, water, a water-soluble thickener, and water-insoluble inorganic porogen particles is placed in a mold, the water is evaporated to form a monolith, and the monolith is heated to fuse the zinc particles and remove the porogen particles. Also provided is a method of placing a mixture of zinc particles, aqueous acetic acid, and porogen particles in a mold, evaporating the water to form a monolith, and immersing the monolith in a liquid that removes the porogen particles.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This application claims the benefit of U.S. Provisional Patent No. 63 / 299,071, filed January 13, 2022. This provisional application, and all other publications and patent documents referenced throughout this non-provisional application, are hereby incorporated by reference.

[0002] The present disclosure relates generally to zinc electrodes for batteries. [Background technology]

[0003] Traditionally, zinc electrodes have limited cycle life due to the perforated formation of dendrites, which short-circuit the battery if they extend long enough to penetrate the cell's separator and make electrical contact with the counter electrode. A solution to this dendrite formation problem is to fabricate the Zn anode as an aperiodic pore-solid structure, in which 3D interconnected void volume is co-continuous with a 3D interconnected solid zinc network, i.e., a "sponge" form factor (Hopkins et al. (2020), Sustain, Energy and Fuels 4, 3363-3369; Hopkins et al. (2020), Energy Storage Mater. 27, 370-376; Hopkins et al. (2020), J. Vis. Exp. (163); Ko et al. (2018), ACS Appl. Energy Mater. 2, 212-216; Parker et al. (2017), Science 356, 415-418; Parker et al. (2016), J. Electrochem. Soc. 163, A351-A355; Parker et al. (2014), Energy Environ. Sci. 7, 1117-1124). The metallic, conductive 3D pathways improve current distribution throughout the electrode structure and avoid uneven reaction sites prone to dendrite formation during charge-discharge cycles. Additionally, the entangled void network around the 3D zinc scaffold allows for confined volume elements with a high surface (zinc)-to-volume (electrolyte) ratio. This confinement induces zincate saturation at a lower concentration than occurs in open solution, resulting in minimal shape change due to dehydration to zinc oxide (ZnO) early in the discharge process.

[0004] These conventional Zn sponge fabrication protocols used emulsion-based compositions that required organic pore formers (carboxymethylcellulose (CMC) or cornstarch). After drying and compacting the zinc-porogen bodies and subsequent heat treatment under an inert gas flow (nitrogen or argon), the organic porogen must be removed by heating in an oxygen-containing environment, typically air, which would result in excessive oxidation of the zinc to zinc oxide. The entire process is carried out in a furnace and requires approximately 10 hours for tube purging, heating, and cooling. Summary of the Invention

[0005] Disclosed herein is a method comprising providing a mixture comprising zinc-containing particles, water, a water-soluble thickener, and water-insoluble inorganic porogen particles, placing the mixture in a mold, evaporating the water from the mixture to form a monolith, heating the monolith to fuse the zinc particles, and immersing the monolith in a liquid that removes the porogen particles.

[0006] Also provided herein is a method comprising providing a mixture comprising particles comprising zinc, an aqueous acetic acid solution, and porogen particles, placing the mixture in a mold, evaporating water from the mixture to form a monolith, and immersing the monolith in a liquid that removes the porogen particles.

[0007] A more complete understanding may be readily obtained by reference to the following description of example embodiments and the accompanying drawings, in which: [Brief explanation of the drawings]

[0008] [Figure 1] Figure 1 shows the X-ray diffraction (XRD) pattern of a zinc monolith (Method 1) after heat treatment (580°C, 1 hour) and washing with 1M HCl for 10 minutes. [Figure 2] Photographs of Zn sponge monoliths fabricated with inorganic porogen (Method 1) molded into different form factors are shown. [Figure 3]1 shows a scanning electron micrograph of a zinc monolith synthesized by Method 1. The micrograph shows the interior surface exposed by removing a slice from a rod-shaped zinc monolith. [Figure 4] Primary discharge of a zinc electrode in a Zn-air cell. Cathode: acetylene black carbon: cryptomelane MnOx aerogel catalyst: PTEE binder in a 4:1:1 ratio; anode: zinc sponge electrode prepared according to Method 1; electrolyte: 9M KOH; separator: Freudenberg nonwoven, Celgard 3501. Discharged at 10 mA cm-2. [Figure 5] Primary discharge of a zinc electrode in an Ag-Zn cell. Cathode: silver oxide (primarily AgO), anode: zinc sponge electrode prepared according to Method 1, electrolyte: 9M KOH, separator: 2x cellophane, Freudenberg nonwoven, Celgard 3501. Discharged at 10 mA cm-2. [Figure 6] Discharge capacity retention as a function of cycles for a Ni-Zn cell. Cathode: NiOOH taken from a commercial Ni-Zn battery (PK cell). Anode: zinc sponge electrode prepared according to Method 1. Electrolyte: 6M KOH, 1M LiOH. Separator: Freudenberg nonwoven, Celgard 3501. Discharged at 10 mA cm-2. The areal current density is defined by the geometric area of ​​the zinc sponge electrode. [Figure 7] 1 shows a scanning electron micrograph of a chemically sintered zinc electrode (prepared according to Method 2). The micrograph shows the internal surface exposed by removing a slice from a rod-shaped zinc monolith. Magnification: 250x. [Figure 8] 1 shows a scanning electron micrograph of a chemically sintered zinc electrode (prepared according to Method 2). The micrograph shows the internal surface exposed by removing a slice from a rod-shaped zinc monolith. Magnification: 500x. [Figure 9] 1 shows a scanning electron micrograph of a chemically sintered zinc electrode (prepared according to Method 2). The micrograph shows the internal surface exposed by removing a slice from a rod-shaped zinc monolith. Magnification: 1000x. [Figure 10] 1 shows a scanning electron micrograph of a chemically sintered zinc electrode (prepared according to Method 2). The micrograph shows the internal surface exposed by removing a slice from a rod-shaped zinc monolith. Magnification: 1500x. [Figure 11] Primary discharge of a zinc electrode in a Zn-air cell. Cathode: acetylene black carbon: cryptomelane MnOx aerogel catalyst: PTEE binder in a 4:1:1 ratio; anode: zinc sponge electrode prepared according to Method 2; electrolyte: 9M KOH; separator: Freudenberg nonwoven, Celgard 3501. Discharged at 10 mA cm-2. [Figure 12] Discharge capacity retention as a function of cycles for a Ni-Zn cell. Cathode: NiOOH taken from a commercial Ni-Zn battery (PK cell). Anode: zinc sponge electrode prepared according to Method 2 before zinc acetate removal. Electrolyte: 6M KOH, 1M LiOH. Separator: Freudenberg nonwoven, Celgard 3501. Discharged at 10 mA cm-2. The areal current density is defined by the geometric area of ​​the zinc sponge electrode. [Figure 13] Discharge capacity retention as a function of cycles for a Ni-Zn cell. Cathode: NiOOH taken from a commercial Ni-Zn battery (PK cell). Anode: zinc sponge electrode prepared according to Method 2 after removing the zinc acetate. Electrolyte: 6M KOH, 1M LiOH. Separator: Freudenberg nonwoven, Celgard 3501. Discharged at 10 mA cm-2. The areal current density is defined by the geometric area of ​​the zinc sponge electrode. [Figure 14] A photograph of a silver sponge electrode is shown. [Figure 15] 1 shows a scanning electron micrograph of the inner surface of a silver sponge. DETAILED DESCRIPTION OF THE INVENTION

[0009] In the following description, for purposes of explanation and not limitation, specific details are set forth to provide a thorough understanding of the present disclosure. However, it will be apparent to those skilled in the art that the present subject matter may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as not to obscure the present disclosure with unnecessary detail.

[0010] This paper presents a synthesis method that uses a thermally stable inorganic pore former to form interconnected zinc structures. After a simplified heat treatment at low temperature, the inorganic porogen is chemically removed. By incorporating a thermally stable inorganic porogen, the heat treatment time and synthesis cost are significantly reduced. In a second variant, the need for additional heat treatment is minimized or eliminated by chemically sintering zinc particles into a monolithic body, thereby reducing synthesis cost and time.

[0011] In the first method, a mixture of zinc, water, a water-soluble thickener, and inorganic porogen particles is made. The zinc particles may be any of those described in, for example, U.S. Patent Nos. 9,802,254, 10,008,711, 10,720,635, 10,763,500, 10,804,535, 11,069,889, and 11,296,373.

[0012] The water-soluble thickener may be, for example, carboxymethyl cellulose. The porogen particles are water-insoluble and may comprise, for example, calcium carbonate, magnesium carbonate, and magnesium oxide. The porogen may be thermally stable in air up to 600°C and easily removable after heat treatment.

[0013] The mixture is then placed in a mold, and the water is evaporated to form a monolith. The monolith is then heated to fuse the zinc particles. The heating step can be performed, for example, at up to 600°C for up to two hours.

[0014] After heating, the monolith is immersed in a liquid that removes the porogen particles, which may be, for example, an aqueous solution of hydrochloric acid to remove the calcium carbonate particles.

[0015] In a second method, zinc particles are mixed with an aqueous acetic acid solution and porogen particles. The acetic acid treatment of the zinc powder can occur before, during, and / or after mixing with a suitable porogen for chemical sintering. The porogen particles may comprise, for example, zinc acetate dihydrate, halide salts (such as NaCl, KCl), urea, calcium carbonate, or other highly water-soluble compounds.

[0016] As described above, the mixture is placed in a mold and the water is evaporated to form a monolith. With acetic acid and zinc acetate dihydrate, no heating step is required to fuse the zinc particles. The monolith is then immersed in a liquid that removes the porogen particles. The liquid can be, for example, water, ethanol, or an aqueous hydrochloric acid solution. Optionally, the monolith is then heated below the melting point of zinc.

[0017] Other variations and parameters of these methods may be incorporated from the disclosures in U.S. Patent Nos. 9,802,254, 10,008,711, 10,720,635, 10,763,500, 10,804,535, 11,069,889, and 11,296,373.

[0018] An aspect of this synthesis protocol is the use of CaCO3 as an inorganic porogen to fabricate porous zinc electrodes. Calcium carbonate is used for its thermal stability and high solubility in acidic aqueous solutions; the former allows zinc to exceed its melting point, while the thermally resistant CaCO3 maintains its form factor, allowing for easy removal after heat treatment. Previous zinc sponge fabrication methods in the art use organic porogens that must be removed by high-temperature oxidation. In these previous examples, removal of the porogen leaves only air to diffuse heat within the zinc structure, a situation that can lead to uneven internal heating of the zinc body. Because the thermal conductivity of CaCO3 (2.25 W / mK) is approximately 100 times greater than that of air (0.025 W / mK), infusing CaCO3 into the voids of a zinc monolith through heat treatment distributes heat more uniformly within the zinc body and promotes the fusion of individual zinc particles into a mechanically cohesive network. The result is a zinc sponge that is at least as strong, complete, and easy to handle as those obtained with organic porogen-based methods, while utilizing a simpler and shorter heat treatment step. The use of hydrochloric acid (HCl) to remove CaCO3 after heat treatment also dissolves some of the surface ZnO that forms during heating. For some rechargeable zinc battery configurations (Ag-Zn, Zn-air, Zn-MnO2), it is desirable for the sponge to have a relatively low ZnO content when initially assembled into the battery.

[0019] The following examples are provided to illustrate specific applications. These specific examples are not intended to limit the scope of the disclosure in this application. Method 1: Inorganic template + one-step heat treatment in air

[0020] In one embodiment, 8 mL of 18 MΩ·cm water is mixed with 80 mg of high-viscosity CMC thickener (Sigma Aldrich, C5013). Separately, 3.0 g of calcium carbonate (CaCO3) is dry-mixed with 65 g of 50 μm zinc (Zn) powder. After achieving uniform mixing, the dry powder is mixed into the CMC thickener solution until homogeneous. The viscous paste is then molded into the desired shape and dried overnight at 50–70 °C. To fuse the zinc particles, the dried monolith is placed in an alumina crucible and placed in a box furnace preheated to 580 °C for 1 hour. After this period, the sample is removed from the furnace and allowed to cool to room temperature. To remove the CaCO3 porogen, the heat-treated Zn monolith is immersed in 1 M HCl for 5–10 minutes, then washed with copious amounts of water and then ethanol. After heat treatment and acid washing, the zinc structure comprises about 85 wt. % metallic zinc and about 15 wt. % zinc oxide (ZnO) according to reference intensity ratio (RIR) fitting of the XRD pattern (Figure 1).

[0021] The final shape factor of the finished monolith is derived primarily from the dimensions of the initial mold used to contain the viscous paste derived from the Zn sponge precursor, and various examples fabricated in our laboratory are shown in Figure 2. Scanning electron micrographs of the cross section (Figure 3) show well-fused and interconnected zinc particles.

[0022] Electrochemical testing of the disclosed Zn monoliths demonstrates high discharge capacity (in Zn-air and silver-zinc (Ag-Zn) cells) and rechargeability (in nickel-zinc (Ni-Zn) cells). In first-discharge tests (Figures 4 and 5), achievable capacities are 500-550 mAhg. -1The discharge capacity was determined to be 60-68% of the theoretical zinc discharge capacity. These results suggest that, while promising compared to conventional zinc electrodes, some ZnO remains, limiting the zinc metal available for extremely high-energy battery discharges. A prototype Ni-Zn cell using a Zn anode obtained with the disclosed method and a NiOOH cathode taken from a commercial cell demonstrates rechargeability and capacity retention at 20% depth of discharge (DOD) for 25 cycles with 100% coulombic efficiency (Figure 6). Model 2: Inorganic templating using chemical fusion of zinc particles

[0023] A sintering strategy is disclosed that chemically, rather than thermally, fuses zinc particles into a 3D network. This process involves introducing acetic acid into the zinc production protocol, either before or after mixing with an appropriate porogen. Upon exposure to acetic acid, the inherent adventitious surface oxide of the zinc powder dissolves, initiating a self-exchange mechanism with the zinc metal surface (Yan et al. (2015), RSC Adv. 5, 83781-83797; Stock et al. (2018), ACS Appl. Energy Mater. 1, 5579-5588). This process converts ZnO into Zn 2+ and often these species are redeposited as Zn metal at the joints where the Zn particles meet, bonding the particles together.

[0024] In one embodiment, 28 g of Zn powder is mixed with 5 g of zinc acetate dihydrate. After homogenization, 3 mL of 5% acetic acid is added. After achieving uniform mixing, the viscous paste is cast into the desired shape and dried overnight at 50-70°C. To remove the zinc acetate, the zinc monolith is immersed in water or ethanol at room temperature to 60°C for 2 hours. A vacuum can be applied to remove the zinc acetate from the large monolith. The acetic acid treatment is sufficient to fuse the zinc particles (Figures 7-10) and form a monolith that can be electrochemically discharged in a Zn-air cell (Figure 11).

[0025] In another embodiment, 27 g of Zn powder is mixed with 2 mL of 5% acetic acid and thoroughly stirred. The zinc powder is then filtered through a vacuum flask and washed with ethanol. 5 g of finely ground zinc acetate dihydrate is then mixed with the zinc powder. In a separate beaker, 4 mL of water and high-viscosity CMC (Sigma Aldrich, C5013) are mixed. After achieving uniform mixing, the dry powder is mixed into a CMC-thickened aqueous solution until homogeneous. The viscous paste is then molded into the desired shape and dried overnight at 50-70°C. To remove the zinc acetate, the zinc monolith is immersed in water or ethanol at room temperature to 60°C for 2 hours. Vacuum wetting can be used to dissolve the zinc acetate from large monoliths.

[0026] In another embodiment, CaCO porogen is used in conjunction with both chemical and thermal sintering. This process involves pretreating zinc powder with acetic acid, mixing it with calcium carbonate porogen, and then heat-treating it for a short period (e.g., 0.5-2 hours) at a temperature below the melting point of zinc (e.g., 400°C) in an inert gas (e.g., nitrogen or argon) atmosphere. It has been found that acetic acid treatment of zinc allows it to be sufficiently softened below its melting point (419°C). In contrast, unacetic acid-treated zinc must be heated to around 600°C to form a structurally sound monolith.

[0027] Prototype cells using zinc anodes (Method 2) and NiOOH cathodes taken from commercial cells demonstrate remarkable rechargeability and a capacity retention of 20% DOD. If the zinc acetate porogen is not removed before assembling the cells, 50 cycles at 100% efficiency are obtained before performance declines (Figure 12). In contrast, if the zinc acetate is removed by washing with water before assembling the cells, significantly extended and improved performance is observed (Figure 13). The cells used in Figure 13 operated for over 150 cycles before the coulombic efficiency fell below 100% and lasted for over 200 cycles and 2 months in the cell before falling below 85%.

[0028] The methods described herein and in U.S. Patent Nos. 9,802,254, 10,008,711, 10,720,635, 10,763,500, 10,804,535, 11,069,889, and 11,296,373 can also be applied to metals other than zinc for the production of porous interconnected monoliths for aqueous batteries. Silver, commonly used as a high-rate cathode in combination with a zinc anode, can be produced using these methods. For example, 200 mg of silver oxide (AgO)(I) is thoroughly mixed with 100 mg of NaCl and pressed into a 15 mm diameter pellet using a 3-ton hydraulic press. After pelletization, the mass is placed in an atmospheric furnace at 700°C for 30 minutes to convert the silver oxide to silver metal and simultaneously fuse the silver particles. After cooling, the pellet is thoroughly washed with hot water at 70°C to remove NaCl, resulting in a completely metallic silver sponge electrode (Figs. 14, 15).

[0029] Many modifications and variations are possible in light of the above teachings. Accordingly, claimed subject matter may be practiced other than as specifically described. For example, reference to a claim element in the singular, using the articles "a," "an," "the," or "said," is not to be construed as limiting the element to the singular.

Claims

1. providing a mixture comprising zinc-containing particles, water, a water-soluble thickener, and water-insoluble inorganic porogen particles; placing the mixture in a mold; evaporating the water from the mixture to form a monolith; heating the monolith to fuse the zinc particles; immersing the monolith in a liquid that removes the porogen particles; A method for providing the above.

2. The method of claim 1 , wherein the water-soluble thickener is carboxymethyl cellulose.

3. The method of claim 1 , wherein the porogen particles comprise calcium carbonate.

4. The method of claim 3 wherein the liquid is an aqueous hydrochloric acid solution.

5. 10. The method of claim 1, wherein heating the monolith is performed at 600°C or less for 2 hours or less.

6. providing a mixture comprising particles comprising zinc, an aqueous acetic acid solution, and porogen particles; placing the mixture in a mold; evaporating water from the mixture to form a monolith; immersing the monolith in a liquid that removes the porogen particles; A method for providing the above.

7. The method of claim 6 , wherein the porogen particles comprise zinc acetate dihydrate.

8. The method of claim 6 , wherein the porogen particles comprise a halide salt or urea.

9. The method of claim 6 , wherein the liquid is water or ethanol.

10. The method of claim 6 , wherein the porogen particles comprise calcium carbonate.

11. The method of claim 10, wherein the liquid is an aqueous hydrochloric acid solution.

12. heating the monolith at a temperature below the melting point of zinc; The method of claim 10 further comprising:

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