Aqueous rechargeable cadmium metal batteries for energy storage

Abstract

The present technology relates generally to a secondary electrochemical cell including a cathode comprising a metal oxide, a current collector, an anode comprising cadmium metal disposed on the current collector, and an aqueous electrolyte comprising a cadmium salt and having a pH of about 4 to about 7.

Description

Atty. Dkt. No. 118347-0134 (24-011-UPR)AQUEOUS RECHARGEABLE CADMIUM METAL BATTERIES FOR ENERGY STORAGECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Appl. No. 63 / 647,963, filed May 15, 2024, the contents of which are incorporated herein by reference in its entirety for any and all purposes.GOVERNMENT RIGHTS

[0002] This invention was made with government support under OIA- 1849243 awarded by The National Science Foundation, and 80NSSC19M0236 awarded by The National Aeronautics and Space Administration. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present technology relates generally to secondary aqueous batteries with cadmium anodes, and methods of making the same.SUMMARY

[0004] In an aspect, a secondary electrochemical cell is presented. The electrochemical cell includes a cathode including a metal oxide, a current collector, an anode including cadmium metal disposed on the current collector, and an aqueous electrolyte including a cadmium salt and having a pH of about 4 to about 7.

[0005] In any embodiment, the cadmium salt may include CdX2, CdSCh, Cd(NOs)2, Cd(COOH)2, CdCl2Hi20i4, C2CdF6O6S2, C4CdFi2N2O8S4, or CdF4N2O8S4, where X is Cl, F, Br, or I. The cadmium salt may be CdCh present in the aqueous electrolyte at a concentration of about 0.1 M to about 1.2 M. The current collector may include copper or titanium. The current collector may be copper foil. The metal oxide may include MnCh, V2O5, elemental sulfur, FeFe(CN)e, or a combination of two or more thereof. The metal oxide may include MnCh. The aqueous electrolyte may further include a manganese salt. The aqueous electrolyte may include 0.01 M to about 0.5 M MnCh. The secondary electrochemical cell may further include a separator disposed between the cathode and the anode, the separator including cellulose, glass fiber, or polypropylene. The cathode may -1- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)further include a binder including poly(vinylidene fluoride), carboxymethyl cellulose, polyacrylic acid, polytetrafluoroethylene), styrene-butadiene rubber, or a combination of two or more thereof. The aqueous electrolyte may further include a lithium salt, a sodium salt, or a potassium salt. The cadmium metal may be a foil, the cadmium metal may be electrochemically plated onto the current collector, or the cadmium metal may be sputtered onto the current collector. The cadmium metal may be the foil.

[0006] In another aspect, a method of forming the secondary electrochemical cell disclosed herein is presented. The method includes disposing the anode, the current collector, and the cathode in the aqueous electrolyte to form the secondary electrochemical cell.

[0007] In another aspect, another method of forming the secondary electrochemical cell disclosed herein is presented. The method includes disposing the current collector and the cathode in the aqueous electrolyte; and forming the anode by electrochemically plating the cadmium metal from the cadmium salt dissolved in the aqueous electrolyte onto the current collector to form the secondary electrochemical cell.

[0008] In another aspect, a method of operating the secondary electrochemical cell disclosed herein is presented. The method includes cycling the secondary electrochemical cell at 300 mAh g'1for at least 300 cycles with a capacity retention of at least 60% and an average Coulombic efficiency of at least 99%.

[0009] In another aspect, a secondary electrochemical cell is presented. The secondary electrochemical cell includes a cathode comprising P-phase MnCh, carbon black, and poly vinylidene fluoride in a mass ratio of 8: 1 : 1; a titanium current collector; an anode comprising a cadmium metal foil disposed on the titanium current collector; and an electrolyte comprising 1 M CdCh and 0.1 M MnCh dissolved in water, and having a pH of about 4 to about 7.

[0010] Further aspects and embodiments of the present technology are described herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is an illustration of a cadmium metal battery with an aqueous electrolyte having a pH of about 4 to about 7, according to the working examples.-2- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0012] FIG. 2A is a graph of galvanostatic charge / discharge (GCD) curves of symmetrical Cd||Cd batteries at 0.5 mA cm’2and 0.25 mAh cm’2, according to the working examples.

[0013] FIG. 2B is a graph of rate performance of symmetrical Cd||Cd batteries, according to the working examples.

[0014] FIG. 2C is a graph of GCD curves of symmetrical Cd||Cd batteries at 1.0 mA cm’2and 0.25 mAh cm’2, according to the working examples.

[0015] FIG. 2D is a graph of GCD curves of symmetrical Cd||Cd batteries at 2.0 mA cm’2and 1.0 mAh cm’2, according to the working examples.

[0016] FIG. 2E is a graph of GCD curves of the Cd||Cu battery in FIG. 2C at 1.0 mA cm’2and 1.0 mAh cm’2, according to the working examples.

[0017] FIG. 2F is a graph of Coulombic efficiency of the Cd||Cu battery in FIG. 2C at 1.0 mA cm’2and 1.0 mAh cm’2over more than 1000 cycles, according to the working examples.

[0018] FIG. 3A is a graph of the voltage profile of GCD of a Cd||Cu battery at 1.0 mA cm’2current and 5.0 mAh cm’2capacity over 50 cycles, according to the working examples.

[0019] FIG. 3B is a graph of Coulombic efficiency of the battery in FIG. 3A over 200 cycles, according to the working examples.

[0020] FIG. 3C is a graph of the voltage profile of GCD of a Cd||Cu battery at 1.0 mA cm’2current and 10.0 mAh cm’2capacity over 50 cycles, according to the working examples.

[0021] FIG. 3D is a graph of Coulombic efficiency of the battery in FIG. 3C over 50 cycles, according to the working examples.

[0022] FIG. 3E is a graph of the voltage profile of GCD of a Cd||Cu battery at 0.5 mA cm’2current and 1.0 mAh cm’2capacity over 50 cycles, according to the working examples.

[0023] FIG. 3F is a graph of Coulombic efficiency of the battery in FIG. 3E over more than 500 cycles, according to the working examples.

[0024] FIG. 3G is a graph of the voltage profile of GCD of a Cd||Cu battery at 0.25 mA cm’2current and 1.0 mAh cm’2capacity over 50 cycles, according to the working examples.-3- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0025] FIG. 3H is a graph of Coulombic efficiency of the battery in FIG. 3G over 250 cycles, according to the working examples.

[0026] FIG. 31 is a graph of the voltage profile of GCD of a Cd||Cu battery at 0.1 mA cm’2current and 1.0 mAh cm’2capacity over 20 cycles, according to the working examples.

[0027] FIG. 3J is a graph of Coulombic efficiency of the battery in FIG. 31 over 42 cycles, according to the working examples.

[0028] FIG. 4A is a GCD curve of cadmium metal plating at 1.0 mA cm’2and1.0 mAh cm’2with an illustration of the morphological evolution of the Cd metal during the plating process, according to the working examples.

[0029] FIG. 4B is an SEM image of Cd metal plated at a plating capacity of 0.5 mAh cm’2, according to the working examples.

[0030] FIG. 4C is another SEM image of Cd metal plated at a plating capacity of 0.5 mAh cm’2, according to the working examples.

[0031] FIG. 4D is an SEM image of Cd metal plated at a plating capacity of 1.0 mAh cm’2, according to the working examples.

[0032] FIG. 4E is another SEM image of Cd metal plated at a plating capacity of 1.0 mAh cm’2, according to the working examples.

[0033] FIG. 4F is an SEM image of Cd metal plated at a stripping capacity of0.5 mAh cm’2, according to the working examples.

[0034] FIG. 4G is another SEM image of Cd metal plated at a stripping capacity of 0.5 mAh cm’2, according to the working examples.

[0035] FIG. 5A is a graph of GCD curves of a Cd||MnO2 battery at 100 mA g'1with an electrolyte of 1.0 M CdCh and 0.1 M MnCh, according to the working examples.

[0036] FIG. 5B is a graph of the rate performance of the battery in FIG. 5A, according to the working examples.-4- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0037] FIG. 5C is a graph of cyclic voltammetry (CV) curves of a Cd||MnO2 electrochemical cell at different scanning rates with an electrolyte of 1.0 M CdCh and 0.1 M MnCh, according to the working examples.

[0038] FIG. 5D is a graph of cycling performance of a Cd||MnO2 battery at 300 mA g'1with an electrolyte of 1.0 M CdCh and 0.1 M MnCh, according to the working examples.

[0039] FIG. 6 is a graph of GCD curves of a Cd||V2Os battery at 100 mA g’1, according to the working examples.

[0040] FIG. 7 is a graph of GCD curves of the Cd||FeFe(CN)e battery at 100 mA g’1, according to the working examples.

[0041] FIG. 8 is a graph of GCD curves of the Cd||sulfur battery at 100 mA g’1, according to the working examples.DETAILED DESCRIPTION

[0042] It is to be appreciated that certain aspects, modes, embodiments, variations, and features of the present methods are described below in various levels of detail in order to provide a substantial understanding of the present technology.Definitions

[0043] Unless defined otherwise, all technical and scientific terms used herein generally have the same meaning as commonly understood by one of ordinary skill in the art to which this technology belongs.

[0044] As used herein and in the appended claims, singular articles such as “a” and “an” and “the” and similar referents in the context of describing the elements (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all-5- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the embodiments and does not pose a limitation on the scope of the claims unless otherwise stated. No language in the specification should be construed as indicating any non-claimed element as essential. For example, reference to “a cell” includes a combination of two or more cells, and the like. Generally, the nomenclature used herein and the laboratory procedures in cell culture, molecular genetics, organic chemistry, analytical chemistry and nucleic acid chemistry and hybridization described below are those well-known and commonly employed in the art.

[0045] As used herein, “about” will be understood by persons of ordinary skill in the art and will vary to some extent depending upon the context in which it is used. If there are uses of the term which are not clear to persons of ordinary skill in the art, given the context in which it is used, “about” will mean up to plus or minus 10% of the particular term - for example, “about 10 wt.%” would be understood to mean “9 wt.% to 11 wt.%.” It is to be understood that when “about” precedes a term, the term is to be construed as disclosing “about” the term as well as the term without modification by “about” — for example, “about 10 wt.%” discloses “9 wt.% to 11 wt.%” as well as disclosing “10 wt.%.”

[0046] The phrase “and / or” as used in the present disclosure will be understood to mean any one of the recited members individually or a combination of any two or more thereof -for example, “A, B, and / or C” would mean “A, B, C, A and B, A and C, B and C, or the combination of A, B, and C.”

[0047] As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 atoms refers to groups having 1, 2, or 3 atoms.-6- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)Similarly, a group having 1-5 atoms refers to groups having 1, 2, 3, 4, or 5 atoms, and so forth.

[0048] Throughout this disclosure, various publications, patents, and published patent specifications are referenced by an identifying citation. Also within this disclosure are Arabic numerals referring to referenced citations, the full bibliographic details of which are provided immediately preceding the claims. The disclosures of these publications, patents, and published patent specifications are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.The Present Technology

[0049] Disclosed herein are cadmium metal electrochemical cells with aqueous electrolyte at a pH of about 4 to about 7, and methods of forming and using the same. The cadmium metal electrochemical cell may be used as a primary or secondary battery. The cadmium metal electrochemical cell includes a cathode, an anode, and an aqueous electrolyte, as disclosed herein.

[0050] In one aspect, the cadmium metal electrochemical cell includes an aqueous electrolyte including a cadmium salt and having an acidic or neutral pH. The cadmium salt may include CdX2, CdSC , Cd(NO3)2, Cd(COOH)2, CdCl2Hi20i4, C2CdF6O6S2, C4CdFi2N2O8S4, CdF4N20sS4, or a combination of any two or more thereof, where X is Cl, F, Br, or I. The cadmium salt may be dissolved in water at a concentration of about 0.1 M to about 1.2 M (e.g., about 0.2 M to about 1.2 M, about 0.5 M to about 1.2 M, 0.8 M to about 1.1 M, or about 0.9 M to about 1.1 M). The pH may be about 3 to about 8 (e.g., 4 to 7, 4 to 6, 4 to 5, 4.5 to 5, about 5, or about 7). For example, the electrolyte may be CdCl2at a concentration of about 1.0 M at a pH of about 4.5 to about 5.

[0051] In any embodiment, the aqueous electrolyte may further include an additive. The additive may be a metal salt, where the metal of the metal salt corresponds to the metal oxide of the cathode, used to substantially reduce or prevent dissolution of the metal oxide in the cathode. The additive may also include lithium salt, a sodium salt, or a potassium salt. The additive may be dissolved in the electrolyte at a concentration of about 0.001 M to about 1.0 M (e.g., 0.01 M to about 0.5 M, 0.05 to about 0.2 M, or about 0.1 M). For example, the cathode may include MnO2, and the additive may be a 0.05 M to 0.2 M manganese salt (e.g., MnCh, MnBr2, Mnl2, or MnSCh).-7- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0052] The anode of the cadmium metal electrochemical cell includes cadmium metal. The metallic cadmium may be cadmium foil, metallic cadmium electrochemically plated (e.g., via galvanostatic charge / discharge cycling) onto a current collector, metallic cadmium sputtered onto a current collector, metallic cadmium mixed with a conductive carbon, and / or metallic cadmium intercalated in a conductive carbon.

[0053] The anode may further include a conductive carbon. The conductive carbon may include carbon black (e.g., Ketjen black, acetylene carbon black, or Super P), graphite, hard carbon, amorphous carbon, soft carbon, mesocarbon microbeads (MCMB), mesoporous carbon, carbon nanotubes, carbon nanofibers, graphene, or a mixture of any two or more thereof.

[0054] The cadmium metal electrochemical cell may further include current collectors for the anode and / or the cathode. The current collectors may include copper, stainless steel, titanium, platinum, gold, aluminum, nickel, or a mixture or alloy of two or more thereof. The current collectors may be in the form of a foil, mesh, or screen.

[0055] The cadmium metal electrochemical cell may include a composite cathode. The composite cathode may include a cathode active material, and may optionally include a binder and / or a conductive carbon.

[0056] The cathode active material may include MnCh, V2O5, elemental sulfur, FeFe(CN)e, or a combination of two or more thereof. For example, the cathode active material may include P-MnCh. The active material may be evenly dispersed in the composite cathode. The active material may be present in an amount of about 25 wt.% to about 85 wt.% (e.g., about 25 wt.% to about 75 wt.% sulfur, about 25 wt.% to about 50 wt.%, about 15 wt.% to about 30 wt.%, about 40 wt.% to about 60 wt.%, about 25 wt.%, or about 50 wt.%).

[0057] The composite cathode material may further include a binder. The binder may be poly(vinylidene fluoride) (PVDF), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), poly(tetrafluoroethylene) (PTFE), styrene-butadiene rubber (SBR), or a mixture of two or more thereof. For example, the binder may be PVDF. The binder may be present in an amount of about 0 wt.% to about 20 wt.% (e.g., about 5 wt.% to about 20 wt.%, about 5 wt.% to about 15 wt.%, or about 10 wt.%).-8- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0058] The composite cathode may further include a conductive carbon. The conductive carbon may include carbon black (e.g., Ketjen black, acetylene carbon black, or Super P), graphite, hard carbon, amorphous carbon, soft carbon, mesocarbon microbeads (MCMB), mesoporous carbon, carbon nanotubes, carbon nanofibers, graphene, or a mixture of any two or more thereof. The conductive carbon may be present in an amount of about 0 wt.% to about 50 wt.% (e.g., about 5 wt.%, about 40 wt.%, about 10 wt.% to about 30 wt.%, about 15 wt.% to about 25 wt.%, or about 20 wt.%).

[0059] The cadmium metal electrochemical cell may include a separator disposed between the anode and the cathode to prevent an electrical short in the cell. The separator may be a microporous polymer film, glass fiber, paper fiber, and ceramic material. Illustrative microporous polymer films include, but are not limited, nylon, cellulose, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, polybutene, or a blend or copolymer thereof. The separator may be a commercially available separator. Commercially available separators include those such as, but not limited to, Celgard® 2400, 2025, 3501, and 2325. Separators of a wide range of thicknesses may be used. For example, the separator may be from about 5 pm to about 50 pm thick.

[0060] In another aspect, a method of forming the cadmium metal electrochemical cell is provided. The method may include disposing the anode, the current collector, and the cathode, respectively as disclosed herein, in the aqueous electrolyte, as disclosed herein, to form the cadmium metal electrochemical cell.

[0061] The method may include forming the anode by depositing cadmium metal onto a current collector via electroplating (e.g., via galvanostatic charge / discharge cycling). The deposited cadmium metal may have a smooth, flat or rounded morphology and lack any sharp dendritic structures. The method may include using a cadmium metal foil or mesh as the anode.

[0062] The cathode may be prepared by mixing the cathode active material with a conductive carbon material and a binder in the presence of a solvent to form a slurry. The solvent may be N-methylpyrrolidone (NMP), acetone, water, or the like. The cathode may be prepared by coating and drying the mixture of the cathode active material, conductive carbon material, and binder directly on a current collector, or by casting the mixture on a separate support to form a film and then laminating the film on a current collector.-9- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0063] When the cathode include includes a S-C composite, the S-C composite may be prepared by a melt-infusion method. For example, a mixture of sulfur and carbon black may be milled to create a homogenous mixture. The mixture may be pressed into a pellet, and then melt-infused in an autoclave at a temperature of about 100 °C to about 200 °C. After melt-infusion, the pellet may be ground into a fine S-C powder before being used to form the cathode.

[0064] In another aspect, a method of using the cadmium metal electrochemical cell is disclosed. The method may include conducting galvanostatic charging and discharging of the electrochemical cell over about 100 cycles to about 500 cycles (e.g., at least 100 cycles, at least 200 cycles, at least 300 cycles, at least 400 cycles, or at least 500 cycles) with a capacity retention of about 40% to about 90% (e.g., at least 50%, at least 60%, at least 70%, or at least 80%) and an average Coulombic efficiency of about 90% to about 100% (e.g., at least 90%, at least 95%, at least 98%, at least 99%, or at least 99.5%).EXAMPLES

[0065] The present technology is further illustrated by the following Examples, which should not be construed as limiting in any way. The examples herein are provided to illustrate advantages of the present technology and to further assist a person of ordinary skill in the art with preparing or using the compositions and systems of the present technology. The examples should in no way be construed as limiting the scope of the present technology, as defined by the appended claims. The examples can include or incorporate any of the variations, aspects, or embodiments of the present technology described above. The variations, aspects, or embodiments described above may also further each include or incorporate the variations of any or all other variations, aspects, or embodiments of the present technology. The following Examples demonstrate the preparation, characterization, and use of illustrative cadmium metal batteries. In these examples, the cadmium metal plating was investigated in cadmium metal batteries with an aqueous electrolyte of CdCh at near-neutral pH (e.g., about 4 to about 7).Example 1: CdllCd, CdllCu, and CdllMnCh Electrochemical CellsMaterials and Methods-10- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0066] Materials. The cadmium foil (99.95% purity, 0.25 mm thickness) and cadmium chloride (99.99% purity) were purchased from Sigma Aldrich. Copper foil (9 pm thickness) was used as current collectors. The P-phase MnCh material was synthesized by a hydrothermal method as follows. 0.25 g of polyvinyl pyrrolidone (PVP, K-30, Mw=40,000) was dissolved in 20 mL H2O. Then 0.13 g KMnCh was dissolved in 10 mL H2O, which was added dropwise to the PVP solution under continuous stirring; in a separate flask, 0.05 g (NHThSCh was dissolved in 10 mL H2O, which was added dropwise to the above solution. After vigorous stirring for 15 minutes, the resulting solution was transferred to a 100 mL PTFE lined stainless steel autoclave. The hydrothermal reaction was maintained at 130 °C for 10 hours. After the reaction cooled down to room temperature, the obtained precipitate was centrifuged and washed with H2O three times. The material was dried at 70 °C for 2 hours to 24 hours, and then was further calcinated in a muffle furnace for 3 hours (target temperature: 450 °C, ramp rate: 5 °C min'1). The final P-MnCh material was black in color.

[0067] Electrode Preparations. The MnCh electrode was prepared by mixing the P-phase MnCh, Ketjen black carbon, and poly vinylidene fluoride (PVDF) binder into a homogenous slurry solution, which was coated on carbon fiber current collectors (Fuel Cell Store, brand: AvCarb MGL370, thickness: 0.37 mm; diameter: 1.0 cm). The MnCh electrode was dried in an air-forced oven, and the mass ratio between active mass, carbon, and binder was 8:1:1. The active mass loading was in the range of 1.5-2.0 mg cm'2. To carry out ex-situ XRD analysis, a MnCh self-standing film electrode was prepared, where the film included 70 wt.% MnCh, 20 wt.% Ketjen carbon, and 10 wt.% polytetrafluoroethylene (PTFE) binder. The cadmium foil was cut into round-shaped electrodes for use in symmetrical Cd||Cd and asymmetrical Cd||Cu batteries, with an area of 0.71 cm2(diameter: 3 / 8 inches). The area for Cd foil was 1.27 cm2(diameter: 1 / 2 inches) in Cd||MnO2 batteries.

[0068] Battery Assembly and Testing. Symmetrical Cd||Cd batteries (2032 type coin cells) were made by disposing two Cd foils on either side of a glass fiber separator. The asymmetrical Cd||Cu batteries (2032 type coin cells) were made by using the Cd foil as the counter / reference electrodes and copper foil as the working electrode. The electrolyte was about 100 pL a 1.0 M CdCh aqueous solution. The Cd||MnO2 batteries were made in two-electrode Swagelok cell configuration, where the working electrode was MnCh, and the counter / reference electrode was the Cd foil. The electrolyte for Cd||MnO2 batteries was about 100 pL a 1.0 M CdCh and 0.1 M MnCh aqueous solution. To observe the ex-situ SEM-11- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)images or the ex-situ XRD patterns, the Cd or MnCh electrodes were retrieved carefully from the cycled batteries, and these electrodes were washed with water and isopropanol multiple times. To analyze the cyclic voltammetry curve of the electrolyte, three-electrode Swagelok cells were assembled, where the working, counter, and reference electrode were a titanium foil, a Cd foil, and silver / silver chloride electrode (Ag / AgCl, saturated, +0.20 V vs. SHE), respectively. The linear scanning voltammetry of the 0.1 M NH4CI solution was made in a three-electrode breaker cell configuration, with the Cd foil, a large piece of activated carbon, and Ag / AgCl served as the working, counter, and reference electrode, respectively.Galvanostatic charge / discharge (GCD) curves were obtained.

[0069] Physical Characterization. X-ray diffraction (XRD) patterns of the MnCh powders and self-standing film electrodes were collected with CuKa radiation source (X = 1.5406 A). Scanning electron microscopy (SEM) images and energy dispersive X-ray spectra (EDS) mapping of MnCh and cadmium foils were recorded with a field emission scanning electron microscope (SEM, JEOL, JSM-6480LV).

[0070] Calculation of the Cd-MnCh Battery Energy Density. With the N / P ratio of 1 : 1 , the average capacity was calculated as Cl x C2 / (C1+C2) = 310 * 477 / (310+477) = 188 mAh g'1. The battery voltage was 1.0 V. Therefore, the energy density is calculated as 188*1 = 188 Wh kg'1(based on the cathode and anode active mass).

[0071] With a N / P ratio of 2: 1, the theoretical capacity of the Cd metal was 477 / 2 = 238.5 mAh g'1. The average capacity was 310 * 238.5 / (310+238.5) = 135 mAh g'1. The theoretical energy density was 135 x 1 = 135 wh kg'1(based on the cathode and anode active mass). Assuming 40-50 wt.% mass burden from non-active components, including the current collectors, electrolytes, carbon, binders, and packages, the energy density was 135 x (50-60%) = 67-80 Wh kg'1.Results and Discussion

[0072] FIG. 1 is an illustration of a cadmium metal battery 100 with an aqueous electrolyte 150 having a pH of about 4 to about 7, illustrating cadmium plating / stripping on the cadmium anode 110. The cadmium metal battery 100 include the cadmium metal anode 110 disposed on a current collector 130, the aqueous electrolyte 150 including a cadmium salt, and a MnCh cathode 120 disposed on a current collector 140.-12- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0073] To explore the Cd plating behavior, a 1.0 M CdCh solution was used as the electrolyte, which had a near-neutral pH value of 4.91 and high ionic conductivity of 20.8 mS cm’1. The cyclic voltammetry (CV) curve of the electrolyte was conducted in a three-electrode cell configuration, where the working, counter, and reference electrode was a titanium foil, a Cd foil, and silver / silver chloride electrode (Ag / AgCl, saturated, +0.20 V vs. SHE), respectively. When the scanning potential reached -0.64 V vs. Ag / AgCl, the cathodic current proliferated, which resulted from the Cd plating. During the positive scanning, the anodic peak occurred at -0.58 V vs. Ag / AgCl, which was attributed to Cd stripping. The average reaction potential was calculated as -0.61 V vs. Ag / AgCl, which corresponded to -0.41 V vs. SHE, in agreement with the standard electrode potential (-0.40 V).

[0074] To evaluate the reaction kinetics and stability of Cd metal plating, symmetrical Cd||Cd batteries were fabricated and tested at 0.5 mA cm’2current and 0.25 mAh cm’2capacity. A cycle included a charge and discharge step, and was 1 hour for each cycle. FIG.2A displays the corresponding Galvanostatic charge / discharge (GCD) curves, where a low polarization value of about 5 mV was observed. After a long cycling period of about 2600 hours, there was no noticeable polarization increase or battery short-circuit, indicating a stable Cd plating process.

[0075] The polarization value was 1-2 orders of magnitude lower than most divalent metal electrodes, such as Mn (1100 mV), Fe (200 mV), Ni (300 mV), Cu (25 mV), and Zn (50 mV). This property indicated faster reaction kinetics for Cd as compared to other divalent metal electrodes. The rate capability of symmetrical Cd||Cd batteries was tested. As shown in FIG. 2B, at higher current densities of 1, 2, 4, 8, and 10 mA cm’2, the polarization value was about 6, 8, 12, 18, and 21 mV, respectively. Electrochemical impedance spectroscopy (EIS) analysis of the symmetrical battery was performed, which exhibited a marginal chargetransfer impedance of about 0.29 ohm, which may account for the smaller polarization values.

[0076] To further examine the Cd metal stability, symmetrical Cd||Cd batteries were tested at larger currents and capacities. At a plating / stripping condition of 1.0 mA cm’2and 0.25 mAh cm’2, the symmetrical battery retained stable GCD curves for about 2400 hours (FIG.2C). At 2.0 mA cm’2current and 1.0 mAh cm’2capacity, the symmetrical battery showed smooth and well-defined GCD curves for about 1600 hours (FIG. 2D). These results collectively indicated the stability of the Cd metal anode.-13- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0077] Cd||Cu batteries were also assessed. The plating current and capacity was 1.0 mA cm'2and 1.0 mAh cm'2, respectively. As shown in FIG. 2E, the Coulombic efficiency (CE) was 93.72% in the 1stcycle, which increased to 98.50, 99.79, 99.83, and 99.90% at the 2nd, 10th, 20th, and 50thcycle, respectively. FIG. 2F displays the long-term cycling of the Cd||Cu battery, where the average CE was 99.92% over 1096 cycles (cycling time: about 2190 hours).

[0078] Cd metal plating performance was tested at a current of 1.0 mA cm'2and a plating capacity of 5.0 mAh cm'2. The average CE was as high as 99.82% over 200 cycles (FIGS.3A-3B) The cycling time was extended to about 1997 hours, and there was no sudden battery failure or short-circuit, which further indicated the stability of the plating / stripping processes. The plating capacity was increased to 10 mA cm'2, where the Cd metal still maintained a high CE of 99.75% after 50 cycles and about 1000 hours (FIGS. 3C-3D).

[0079] In another example, the plating capacity was 1.0 mAh cm'2and the current density was 0.5-0.1 mA cm'2. At 0.5 mA cm'2current (FIGS. 3E-3F), the average CE was 99.81% over 548 cycles, and the cycling time was prolonged to about 2190 hours. When the current was lowered to 0.25 mA cm'2, the Cd metal maintained a high CE value of 99.60% (256 cycles, about 2042 hours, FIG. 3G-3H). The Cd plating efficiency at a small current of 0.1 mA cm'2was tested, where each plating process was 10 hours per cycle. The average CE was about 98.6% (84 cycles, about 1670 hours), which further indicated Cd plating reversibility.

[0080] FIG. 31 is a graph of the voltage profile of GCD of a Cd||Cu battery at 0.10 mA cm'2current and 1.0 mAh cm'2capacity over 20 cycles. At this smaller current density of 0.10 mA cm'2the HER may play a significant role, because the discharge rate is slow enough (discharge time: 10 hours). Conventional aqueous metal batteries can suffer from parasitic hydrogen evolution reactions (HER), which can lead to inferior plating CE values. FIG. 31 shows the GCD curves, where the CE value at the 1st, 3rd, 5th, 10th, and 20thcycle was 90.2, 97.4, 98.8, 99.03, and 98.46%, respectively. FIG. 3J is a graph of Coulombic efficiency of the battery in FIG. 31 over 42 cycles. The average CE was 98.7% over 42 cycles, and the total cycling time was over 840 hours.

[0081] The Cd plating morphology may contribute to its high efficiency. Four representative states were selected for morphology observation in a plating / stripping cycle (FIG. 4A), namely plating for 0.5 mAh cm'2(point A), plating for 1.0 mAh cm'2(point B),-14- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)stripping for 0.5 mAh cm'2(point C), and a fully stripped state (point D). The scanning electron microscopy (SEM) images are shown in FIGS. 4B-4G.

[0082] At a moderate plating capacity (point A, 0.5 mAh cm'2), the plated Cd metal appeared as irregular-shaped but surface-flattened chunks or lumps, whose size distribution was about 30 pm to about 100 pm (FIG. 4B). Taking a closer look, the Cd lumps included some smaller particles residing on a top surface, as shown in FIG. 4C.

[0083] When the plating capacity reached 1.0 mAh cm'2(point B), the overall Cd materials were better connected, and the particle size was further enlarged to 50 pm to about 100 pm (FIG. 4D) Close observation revealed that the morphology was different from that at 0.5 mAh cm'2capacity. As presented in FIG. 4E, many plate-like Cd chips were observed underneath the previously observed small particles (marked by dashed circles), and these plates were closely and densely packed together. Without being bound by any theory, the initially plated Cd particles may facilitate the nucleation process, which may promote the subsequent Cd metals to grow into well-defined plates. Energy dispersive spectra (EDS) and mapping analysis was performed on the deposits, which indicated plating of high-purity Cd metals.

[0084] During the stripping process (0.5 mAh cm'2, point C), the Cd metal did not experience a noticeable particle size decrease, as compared in FIGS. 4D and 4F. However, at a higher magnification, it was observed that the smaller particles on the top were diminished, and the plate-stacking texture was more evident (FIG. 4G). This observation may suggests that the stripping process starts with the top of the Cd metal instead of its base, which may be beneficial for uniform plating / stripping. At the fully stripped stage (point D), all the plate-like metals disappeared from the Cu substrate.

[0085] There was no dendrite growth during the plating / stripping cycle, and the Cd deposits were micron-sized plates or particle aggregates, which were closely packed to decrease their surface areas. This morphology may be conducive to reducing electrodeelectrolyte side reactions, and increasing the Coulombic efficiency.

[0086] Cd plating morphology was tested at 5.0 mAh cm'2and 10.0 mAh cm'2capacity, where Cd retained a relatively flat and smooth surface without dendrites. After cycling in symmetrical batteries for 100 hours, the Cd metal surface was still dense and dendrite-free.-15- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0087] A manganese dioxide (MnCh) material was selected as a cathode active material. MnCh has a capacity of about 200-500 mAh g'1and is earth-abundant. The MnCh material was prepared by a hydrothermal method, which produced a tetragonal structure with a space group of P42 / mnm (JCPSD # 24-0735, ?-phase). The MnCh had a well-defined nanorod morphology, where the rod length was about 5 pm to about 10 pm, and the rod diameter was a few 100 nm to 500 nm. EDS results of this i-MnCh indicated Mn and O elements were present in the sample, indicating high material purity.

[0088] Cd||MnO2 batteries were assembled in a two-electrode cell configuration. The electrolyte was 1.0 M CdCh with 0.1 M MnCh dissolved in DI water, which may prevent or reduce MnCh dissolution and increase cycling stability. As shown in FIG. 5A, the Cd||MnO2 battery delivered a discharge capacity of about 330 mAh g'1and a flat discharge plateau at 0.84 V in the first cycle. Subsequently, the discharge capacity stabilized at about 310 mAh g' and the discharge plateau increased to about 1.0 V. Without being bound by any theory, the voltage elevation may be attributed to the material morphology change and electrode activation.

[0089] The theoretical energy density for this Cd||MnO2 battery was calculated as 188 Wh kg'1, based on the cathode capacity (310 mAh g'1), anode capacity (477 mAh g'1), cell voltage (1.0 V), and a negative / positive (N / P) electrode capacity ratio of 1 : 1. If an N / P = 2:1 ratio is used for the full cell, the theoretical energy density is 135 Wh kg'1. With a 40-50 wt.% mass burden from non-active components (current collectors, electrolytes, carbon, binders, and packages), the final energy density may be 67-80 Wh kg'1, which exceeds that of commercial Ni-Cd batteries (40-60 Wh kg'1).

[0090] Besides the promising energy density, the Cd||MnO2 battery demonstrated fast reaction kinetics and reasonable cycling stability. As displayed in FIG. 5B, the discharge capacity was about 310, 270, 240, 210, and 188 mAh g'1at 0.1, 0.2, 0.3, 0.5, and 0.8 A g'1, respectively. Even at a higher current of 1.0 A g'1, the capacity retained 171 mAh g'1, which corresponded to about 55 % capacity utilization. To further explore the reaction kinetics, CV tests were carried out at different scanning rates of 1-5 mV s'1(FIG. 5C). The log(v) and log(z) relationship was analyzed and the b values for cathodic and anodic peaks were close to 0.5, suggesting that the insertion reaction was diffusion-controlled.-16- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0091] FIG. 5D is a graph of the cycling performance of a Cd||MnO2 battery at 300 mA g'1(about 1C rate). After about 1000 cycles, the discharge capacity degraded from 222 mAh g'1to 97.5 mAh g’1, which rendered a capacity retention of 44% and average Coulombic efficiency of 99.6%.Example 2: CdllViQs Electrochemical Cells

[0092] Unless otherwise noted, the same materials and methods as described with respect to Example 1 were also used for Example 2. The V2O5 electrode was prepared by forming a V2O5 / C composite. Mortar and pestle were used to grind V2O5 with Ketjen black carbon in a mass ratio of 7:2 for about 20 minutes to about 30 minutes, and then the material was transferred to the high-energy ball-mill and milled for about 4 hours.

[0093] FIG. 6 is a graph of GCD curves of a Cd||V2Os battery at 100 mA g'1. A discharge capacity of about 200 mAh g'1and an average discharge voltage of about 0.45 V was attained.Example 3: Cd||FeFe 6 Electrochemical Cells

[0094] Unless otherwise noted, the same materials and methods as described with respect to Example 1 were also used for Example 3. The Prussian blue material FeFe(CN)e was prepared by a solution-based precipitation method. KsFe(CN)6 aqueous solution (50 mL, 0.1 M) was added to an aqueous solution of FeCh (100 mL, 0.1 M) under magnetic stirring, and then the mixed solution was heated at 60°C for 6 hours for reaction, forming dark green precipitates of FeFe(CN)e. The precipitates were centrifuged and washed with water multiple times, and then dried in an oven at 60°C for about 12 hours. The FeFe(CN)e electrode was prepared using the FeFe(CN)e material.

[0095] FIG. 7 is a graph of GCD curves of the Cd||FeFe(CN)e battery at 100 mA g'1. This battery delivered an initial discharge capacity of about 64 mAh g'1with multiple discharge plateaus, which may relate to a phase transformation process. During charging, there was a longer charge slope at about 1.2 V and a plateau at about 1.3 V. The charge capacity was over 200 mAh g'1. Following the first cycle, the charge / discharge curves became more stable, and the discharge capacity was about 30 mAh g'1to about 40 mAh g'1.Example 4: CdllS / C Electrochemical Cells-17- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0096] Unless otherwise noted, the same materials and methods as described with respect to Example 1 were also used for Example 4. The sulfur / carbon (S / C) cathode was prepared by forming a S / C composite with about 60 wt.% sulfur. The S / C composite was prepared by a melt-diffusion method as follows. 0.3 g sulfur and 0.2 g Ketjen black carbon was ground with a mortar and pestle for 30 minutes, and then the resulting mixture was transferred to a planetary ball-milling jar and subjected to ball-milling at about 400 rpm for about 5 hours. Then the S / C mixture was formed into a pellet using a hydraulic press at 3 tons. The pellet was transferred to an autoclave for melt-infusion reaction at a temperature of 155°C for 6 hours. Following melt-infusion, the pellet was ground into fine S / C powders for use in the S / C electrode.

[0097] FIG. 8 is a graph of GCD curves of the Cd|| S / C battery at 100 mA g'1. The Cd||S / C battery exhibited a discharge capacity of about 2300 mAh g'1in the first cycle, and the discharge voltage was 0.1 V. The discharge voltage was 0.3 V in the following cycles.EQUIVALENTS

[0098] While certain embodiments have been illustrated and described, a person with ordinary skill in the art, after reading the foregoing specification, can effect changes, substitutions of equivalents and other types of alterations to the compounds of the present technology or salts, compositions, derivatives, and mixtures as set forth herein. Each aspect and embodiment described above can also have included or incorporated therewith such variations or aspects as disclosed in regard to any or all of the other aspects and embodiments.

[0099] The present technology is also not to be limited in terms of the particular aspects described herein, which are intended as single illustrations of individual aspects of the present technology. Many modifications and variations of this present technology can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods within the scope of the present technology, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. It is to be understood that this present technology is not limited to particular methods, reagents, compounds, compositions, labeled compounds or biological systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to be -18- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)limiting. Thus, it is intended that the specification be considered as exemplary only with the breadth, scope and spirit of the present technology indicated only by the appended claims, definitions therein and any equivalents thereof.

[0100] The embodiments, illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising,” “including,” “containing,” etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claimed technology. Additionally, the phrase “consisting essentially of’ will be understood to include those elements specifically recited and those additional elements that do not materially affect the basic and novel characteristics of the claimed technology. The phrase “consisting of’ excludes any element not specified.

[0101] In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0102] As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like, include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.-19- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)

[0103] All publications, patent applications, issued patents, and other documents (for examplejournals, articles and / or textbooks) referred to in this specification are herein incorporated by reference as if each individual publication, patent application, issued patent, or other document was specifically and individually indicated to be incorporated by reference in its entirety. Definitions that are contained in text incorporated by reference are excluded to the extent that they contradict definitions in this disclosure.

[0104] The present technology may include, but is not limited to, the features and combinations of features recited in the following lettered paragraphs, it being understood that the following paragraphs should not be interpreted as limiting the scope of the claims as appended hereto or mandating that all such features must necessarily be included in such claims:A. A secondary electrochemical cell comprising:a cathode comprising a metal oxide;a current collector;an anode comprising cadmium metal disposed on the current collector; and an aqueous electrolyte comprising a cadmium salt and having a pH of about 4 to about 7.B. The secondary electrochemical cell of Paragraph A, wherein the cadmium salt comprises CdX2, CdSO4, Cd(NO3)2, Cd(COOH)2, CdChHnOu, C2CdF6O6S2, C4CdFi2N2O8S4, or CdF4N2O8S4, where X is Cl, F, Br, or I.C. The secondary electrochemical cell of Paragraphs A or B, wherein the cadmium salt is CdCl2and present in the aqueous electrolyte at a concentration of about 0.1 M to about 1.2 M.D. The secondary electrochemical cell of any one of Paragraphs A-C, wherein the current collector comprises copper or titanium.E. The secondary electrochemical cell of any one of Paragraphs A-D, wherein the current collector is copper foil.-20- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)F. The secondary electrochemical cell of any one of Paragraphs A-E, wherein the metal oxide comprises MnCh, V2O5, elemental sulfur, FeFe(CN)e, or a combination of two or more thereof.G. The secondary electrochemical cell of any one of Paragraphs A-F, wherein the metal oxide comprises MnCh.H. The secondary electrochemical cell of Paragraph G, wherein the aqueous electrolyte further comprises a manganese salt.I. The secondary electrochemical cell of Paragraph H, wherein the aqueous electrolyte comprises 0.01 M to about 0.5 M MnCh.J. The secondary electrochemical cell of any one of Paragraphs A-I, wherein the secondary electrochemical cell further comprises a separator disposed between the cathode and the anode, the separator comprising cellulose, glass fiber, or polypropylene.K. The secondary electrochemical cell of any one of Paragraphs A- J, wherein the cathode further comprises a binder including poly(vinylidene fluoride), carboxymethyl cellulose, polyacrylic acid, poly(tetrafluoroethylene), styrene-butadiene rubber, or a combination of two or more thereof.L. The secondary electrochemical cell of any one of Paragraphs A-K, wherein the aqueous electrolyte further comprises a lithium salt, a sodium salt, or a potassium salt.M. The secondary electrochemical cell of any one of Paragraphs A-L, wherein the cadmium metal is a foil, the cadmium metal is electrochemically plated onto the current collector, or the cadmium metal is sputtered onto the current collector.N. The secondary electrochemical cell of Paragraph M, wherein the cadmium metal is the foil.O. A method of forming the secondary electrochemical cell of any one of Paragraphs A-N, the method comprising disposing the anode, the current collector, and the cathode in the aqueous electrolyte to form the secondary electrochemical cell.P. A method of forming the secondary electrochemical cell of any one of Paragraphs A-N, the method comprising disposing the current collector and the cathode in the aqueous -21- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)electrolyte; and forming the anode by electrochemically plating the cadmium metal from the cadmium salt dissolved in the aqueous electrolyte onto the current collector to form the secondary electrochemical cell.Q. A method of operating the secondary electrochemical cell of any one of Paragraphs A-N, the method comprising cycling the secondary electrochemical cell at 300 mAh g'1for at least 300 cycles with a capacity retention of at least 60% and an average Coulombic efficiency of at least 99%.R. A secondary electrochemical cell comprising:a cathode comprising P-phase MnCh, carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1;a titanium current collector;an anode comprising a cadmium metal foil disposed on the titanium current collector;andan electrolyte comprising 1 M CdCh and 0.1 M MnCh dissolved in water, and having a pH of about 4 to about 7.

[0105] Other embodiments are set forth in the following claims, along with the full scope of equivalents to which such claims are entitled.-22- 4908-0612-8188.2

Claims

Atty. Dkt. No. 118347-0134 (24-011-UPR)CLAIMS1. A secondary electrochemical cell comprising:a cathode comprising a metal oxide;a current collector;an anode comprising cadmium metal disposed on the current collector; and an aqueous electrolyte comprising a cadmium salt and having a pH of about 4 to about 7.

2. The secondary electrochemical cell of claim 1, wherein the cadmium salt comprises CdX2, CdSO4, Cd(NO3)2, Cd(COOH)2, CdChHnOu, C2CdF6O6S2, C4CdFi2N2O8S4, or CdF4N2O8S4, where X is Cl, F, Br, or I.

3. The secondary electrochemical cell of claim 1, wherein the cadmium salt is CdCh and present in the aqueous electrolyte at a concentration of about 0.1 M to about 1.2 M.

4. The secondary electrochemical cell of claim 1, wherein the current collector comprises copper or titanium.

5. The secondary electrochemical cell of claim 1, wherein the current collector is copper foil.

6. The secondary electrochemical cell of claim 1, wherein the metal oxide comprises MnCh, V2O5, elemental sulfur, FeFe(CN)e, or a combination of two or more thereof.

7. The secondary electrochemical cell of claim 1, wherein the metal oxide comprises MnCh.

8. The secondary electrochemical cell of claim 7, wherein the aqueous electrolyte further comprises a manganese salt.

9. The secondary electrochemical cell of claim 8, wherein the aqueous electrolyte comprises 0.01 M to about 0.5 M MnCh.

10. The secondary electrochemical cell of claim 1, wherein the secondary electrochemical cell further comprises a separator disposed between the cathode and the anode, the separator comprising cellulose, glass fiber, or polypropylene.

11. The secondary electrochemical cell of claim 1, wherein the cathode further comprises a binder including poly(vinylidene fluoride), carboxymethyl cellulose, polyacrylic acid,-23- 4908-0612-8188.2Atty. Dkt. No. 118347-0134 (24-011-UPR)poly(tetrafluoroethylene), styrene-butadiene rubber, or a combination of two or more thereof.

12. The secondary electrochemical cell of claim 1, wherein the aqueous electrolyte further comprises a lithium salt, a sodium salt, or a potassium salt.

13. The secondary electrochemical cell of claim 1, wherein the cadmium metal is a foil, the cadmium metal is electrochemically plated onto the current collector, or the cadmium metal is sputtered onto the current collector.

14. The secondary electrochemical cell of claim 13, wherein the cadmium metal is the foil.

15. A method of forming the secondary electrochemical cell of any one of claims 1-14, the method comprising disposing the anode, the current collector, and the cathode in the aqueous electrolyte to form the secondary electrochemical cell.

16. A method of forming the secondary electrochemical cell of any one of claims 1-14, the method comprising disposing the current collector and the cathode in the aqueous electrolyte; and forming the anode by electrochemically plating the cadmium metal from the cadmium salt dissolved in the aqueous electrolyte onto the current collector to form the secondary electrochemical cell.

17. A method of operating the secondary electrochemical cell of any one of claims 1-14, the method comprising cycling the secondary electrochemical cell at 300 mAh g'1for at least 300 cycles with a capacity retention of at least 60% and an average Coulombic efficiency of at least 99%.

18. A secondary electrochemical cell comprising:a cathode comprising P-phase MnCh, carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1;a titanium current collector;an anode comprising a cadmium metal foil disposed on the titanium current collector;andan electrolyte comprising 1 M CdCh and 0.1 M MnCh dissolved in water, and having a pH of about 4 to about 7.-24- 4908-0612-8188.2

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