Manganese rich cathode compositions for sodium-ion batteries and methods of making the same

Abstract

A cathode composition designed for use in a sodium-ion battery is provided. In some embodiments, the cathode composition may have the formula NaxMn3-yMyO7, where M is one or more metal elements and wherein x is greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than 0 and less than or equal to 0.5. The compositions of the present embodiments of the cathode have the capacity comparable to current lithium (Li)-ion batteries, yet at a reduced cost and with a focus on sustainable elements. By doping several transition metals into Na2Mn3O7, new compositions with exceptionally high energy density and low voltage hysteresis can be created.

Description

Attorney Docket No.: MIT 25470 PCT | 88212-414294MANGANESE RICH CATHODE COMPOSITIONS FOR SODIUM-ION BATTERIES AND METHODS OF MAKING THE SAME CROSS REFERENCE TO RELATED APPLICATION

[0001] The present disclosure claims priority to and the benefit of U.S. Provisional Application No.63 / 595,286, entitled “Manganese Rich Cathode Compositions for Sodium-on Batteries and Methods of Making Same,” filed on November 1, 2023, the content of which is incorporated by reference herein in its entirety. FIELD

[0002] The present disclosure relates to cathode compositions and methods for preparing and using the same, and more particularly relates to cathode compositions for a sodium-ion battery and electrochemical cell capable of engaging in high-valent redox reactions. BACKGROUND

[0003] The recent global awareness of the deleterious effects of gasoline and other fossil fuel-based energy sources on our environment has greatly increased the demand for alternate energy sources. Batteries, and lithium-ion (Li-ion) batteries in particular, have seen a dramatic rise in demand due, at least in part, to their inclusion in various domestic and commercial applications. Moreover, the recent advancements in electric vehicles have further increased the demand for lithium-ion batteries. To date, lithium mining has become a controversial practice due, at least in part, to its expense and environmental applications. As a result, various cathode compositions have been explored as potential alternatives to Li-ion batteries. Sodium-ion batteries (Na-ion batteries) have particularly garnered attention due, at least in part, to their perceived cost-effectiveness and high-energy density. Specifically, Na2Mn3O7has demonstrated electrochemical activity in Na-ion cells, capable of engaging in high-valent redox reactions, (i.e., the over-oxidation of oxygen or transition metals). This property allows for a higher specific capacity and operating voltages that may be able to compete with widely used lithium-ion (Li-ion) battery technologies. Moreover, Na2Mn3O7 is composed of inexpensive and abundant elements, making it an excellent candidate for low- cost and environmentally friendly batteries while avoiding issues related to the critical mineral supply chain.Attorney Docket No.: MIT 25470 PCT | 88212-414294

[0004] The foregoing notwithstanding, use of Na2Mn3O7has several shortcomings. For example, Na2Mn3O7may experience a decline in performance due, at least in part, to electrolyte decomposition at high voltages and / or structural distortions during cycling. Additionally, Na2Mn3O7may experience reduced cycle life, lower voltage tolerance, and / or diminished specific capacity with respect to commercially available batteries.

[0005] Accordingly, there is a need for improving the cathode of sodium-ion batteries before the batteries can become a cost-effective substitute for Li-ion counterparts in commercial Na-ion cells. SUMMARY

[0006] The present disclosure is directed to cathode compositions for a sodium-ion battery and electrochemical cell capable of engaging in high-valent redox reactions. The cathode compositions can include Na2Mn3O7 (NMO) and one or more dopants, which can result in improvements in performance and / or reversibility over conventional sodium-ion batteries. These new cathode materials can be developed with stable high-valent redox through substitution of redox-active transition metals by employing cationic ordered transition metal vacancies in the crystal lattice and partial transition metal substitution as design principles in tandem. The dopants can include metal elements, and notably, transition metals that can be incorporated into the structure of the NMO. Incorporation can occur by virtue of substitution of the metal element for at least a portion of the manganese in NMO. Alternatively, the dopant can be added to the NMO without incorporation into its structure, and instead modifying a grain morphology of the NMO. By doping several transition metals into Na2Mn3O7, new compositions with exceptionally high energy density and low voltage hysteresis can be created.

[0007] One exemplary cathode composition for a sodium-ion battery includes a compound having the formula NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.5.

[0008] In the composition, y can be less than or equal to about 0.25. M can include one or more metal elements or can include a metal. M can include one or more of iron (Fe),Attorney Docket No.: MIT 25470 PCT | 88212-414294 aluminum (Al), nickel (Ni), copper (Cu), or combinations thereof. In some embodiments, M can include one or more of an oxide, hydroxide, carbonate, or nitrate.

[0009] The composition can further include one or more of polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), carbon blacks, carbon nanotubes, graphitic carbon, adhesion promoters, or carboxymethylcellulose. An electrically conductive diluent can facilitate electron transfer from the cathode composition to a current collector. A specific capacity of the cathode composition can be greater than about 50 mAh / g after at least 10 charging and discharging cycles.

[0010] The compound having the formula NaxMn3-yMyO7can have an improved cycle discharge capacity as compared to a cycle discharge capacity of Na2Mn3O7. In some embodiments, the compound having the formula NaxMn3-yMyO7can have an improved anionic voltage hysteresis as compared to an anionic voltage hysteresis of Na2Mn3O7.

[0011] One example embodiment of a sodium-ion electrochemical cell includes an anode, an electrolyte, and a cathode. The cathode includes a compound having the formula: NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.5.

[0012] In some embodiments, an example electronic device can include the sodium-ion electrochemical cell. M can include one or more metal elements. In some embodiments, M can include a transition metal or combinations thereof.

[0013] One example method of making a cathode composition includes combining a precursor having the formula: Na2Mn3O7with a compound M, and heating the mixture to form a compound having the formula: NaxMn3-yMyO7. M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.5.

[0014] The compound M can be incorporated into the structure of the precursor Na2Mn3O7by virtue of substitution of the compound M for a portion of the Mn in the precursor Na2Mn3O7. The method can further include optimizing an amount of the compound M that is substituted into the structure of the precursor Na2Mn3O7. Optimizing can include adjustingAttorney Docket No.: MIT 25470 PCT | 88212-414294 the amount of the compound M based on improving one or more of structure, ability, or electrochemical activity of the cathode composition.

[0015] The compound M can include one or more metal elements. In some embodiments, the compound M can include a transition metal or combinations thereof. The compound M can modify a grain morphology of the precursor Na2Mn3O7without incorporating itself into its structure.

[0016] The precursor with the compound M can include mechanically milling the precursor and the compound with a mortar and pestle for 30 minutes. Heating the mixture can include placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace with open ends. In some embodiments, heating the mixture can include placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, sealing it from the atmosphere, and flowing 100% oxygen gas. Heating the mixture can also include placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, sealing it from the atmosphere, and flowing a mixture of oxygen and argon gas. In some embodiments, heating the mixture can include placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, heating the furnace approximately in a range of about 400 °C to about 600 °C at about 5 °C per minute and then holding constant temperature approximately in a range of about 400 °C to about 600 °C for approximately in a range of about 4 hours to about 24 hours. Heating the mixture can also include placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, heating the furnace to about 550 °C at about 5 °C per minute and then holding constant temperature at about 550°C for about 12 hours.

[0017] In some embodiments, the method can include combining the cathode composition with an anode and an electrolyte to form a sodium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] This disclosure will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:

[0019] FIG.1 illustrates a schematic of a lattice structure with ordered vacancies of an example embodiment cathode composition having the formula NaxMn3-yMyO7;Attorney Docket No.: MIT 25470 PCT | 88212-414294

[0020] FIG.2 is a graph illustrating X-ray diffraction (XRD) patterns of Comparative Example 1 and of Examples 1 to 7 of the present embodiments;

[0021] FIG.3A is a graph illustrating the voltage curves of the cells of Comparative Example 1 and of Examples 1 to 3 and 7;

[0022] FIG.3B is a graph illustrating the voltage curves of the cells of Comparative Example 1 and of Examples 4 to 6;

[0023] FIG.4A is a graph illustrating the differential capacity curves of the cells of Comparative Example 1 and of Example 1;

[0024] FIG.4B is a graph illustrating the differential capacity curves of the cells of Comparative Example 1 and of Example 2;

[0025] FIG.4C is a graph illustrating the differential capacity curves of the cells of Comparative Example 1 and of Example 3;

[0026] FIG.4D is a graph illustrating the differential capacity curves of the cells of Comparative Example 1 and of Example 4;

[0027] FIG.4E is a graph illustrating the differential capacity curves of the cells of Comparative Example 1 and of Example 5;

[0028] FIG.4F is a graph illustrating the differential capacity curves of the cells of Comparative Example 1 and of Example 6;

[0029] FIG.4G is a graph illustrating the differential capacity curves of the cells of Comparative Example 1 and of Example 7;

[0030] FIG.5A is a graph illustrating the discharge capacity versus the cycle number of the cell of Comparative Example 1 and of Examples 1 to 3 and 7;

[0031] FIG.5B is a graph illustrating the discharge capacity versus the cycle number of the cell of Comparative Example 1 and of Examples 4 to 6;

[0032] FIG.6A is a graph illustrating powder X-ray diffraction of Na2Mn3O7(NMO) and Na2.5Mn2.5Fe0.5O7 (NMFO) with major peaks labeled;Attorney Docket No.: MIT 25470 PCT | 88212-414294

[0033] FIG.6B is a schematic illustration of a lattice with ordered vacancies having three unique Na+ sites in the lattice of NMFO;

[0034] FIG.7A is an image of a particle on which electron energy loss spectroscopy (EELS) is performed; with labeled areas on which measurements are taken;

[0035] FIG.7B is a graph illustrating associated EELS spectra with the Mn L-edges labeled; and

[0036] FIG.7C is a graph illustrating associated EELS spectra with the Fe L-edges labeled. DETAILED DESCRIPTION

[0037] Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the systems and methods disclosed herein. The description is inclusive of all descriptions, including any accompanying figures, whether in the present document and / or any materials or other information incorporated by reference herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the compositions and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present disclosure is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present disclosure. To the extent the present disclosure includes illustrations and descriptions that include schematic illustrations of compositions, results, quantitative measurements, and the like, a person skilled in the art will recognize how to rely upon the present disclosure to integrate the compositions, methods, and the like into a composition., production, method, etc. of forming cathode compositions for batteries. Additionally, to the extent the present disclosure includes various terms for components and / or processes of the disclosed compoinents, methodologies, and the like, one skilled in the art, in view of the claims, present disclosure, and knowledge of the skilled person, will understand such terms are merely examples of such components and / or processes, and other components, designs, processes, and / or actions are possible.Attorney Docket No.: MIT 25470 PCT | 88212-414294

[0038] Further, to the extent features, layers, sides, objects, steps, or the like are described as being “first,” “second,” third,” etc., and / or “lower,” “upper,” “middle,” etc., such numerical and / or location ordering / identification is generally arbitrary, and thus such numbering can be interchangeable unless indicated or otherwise understood by those skilled in the art to not be interchangeable. To the extent that the instant disclosure includes various terms for components and / or processes of the disclosed systems, compositions, designs, and methods, and the like, one skilled in the art, in view of the claims, present disclosure, and knowledge of the skilled person, will understand such terms are merely examples of such components and / or processes, and other components, designs, processes, and / or actions are possible.

[0039] Definitions

[0040] For the purposes of this disclosure, the following terms are used as follows:

[0041] The terms “sodiate” and “sodiation” refer to a process for adding sodium to an electrode material;

[0042] The terms “desodiate” and "desodiation” refer to a process for removing sodium from an electrode material;

[0043] The terms “charge and “charging” refer to a process for providing electrochemical energy to a cell;

[0044] The terms “discharge” and “discharging refer to a process for removing electrochemical energy from a cell, e.g., when using the cell to perform desired work;

[0045] The phrase “positive electrode” refers to an electrode (often called a cathode) where electrochemical reduction and sodiation occurs during a discharging process; and

[0046] The phrase “negative electrode” refers to an electrode (often called an anode) where electrochemical oxidation and desodiation occurs during a discharging process.

[0047] The term “dopants” can refer to additives, e.g., Na2Mn3O7, as provided for herein, which can be incorporated in the structure (i.e., substituted for at least a portion of the manganese), and to additives that cannot be incorporated in the structure.

[0048] COMPOSITIONSAttorney Docket No.: MIT 25470 PCT | 88212-414294

[0049] At least one novel aspect of the present disclosure lies in the compositions for cathodes of sodium-ion batteries and methods for making such compositions. Sodium-ion batteries have the potential to meet the growing demand for energy storage due, at least in part, to their low costs stemming from natural resource abundances. To improve the cathode energy densities of the sodium-ion batteries, high voltage capacity through reversible high- valent redox reactions can be accessed. Such reactions usually cause instability in cathode materials, but Na2Mn3O7 (NMO) can achieve excellent performance and reversibility in the high-valent regime due, at least in part, to its unique lattice structure with ordered Mn vacancies. New cathode materials can be developed with stable, reversible high-valent redox through substitution of redox-active transition metals by employing cationic ordered vacancies and / or partial transition metal substitution as design principles, and / or both cationic ordered vacancies and partial transition metal substitution in tandem. In some embodiments, cationic ordered vacancies can be synergized with tunable metal-ligand hybridization through partial metal substitution to access the high voltage capacity. For example, Fe3+for Mn4+can be incorporated in NMO to make Na2.5Mn2.5Fe0.5O7 and achieve improved high-valent redox behavior. This Fe substitution can lead to larger specific capacities (171 mAh / g to 159 mAh / g first cycle), enhanced cycle stability (97 mAh / g to 60 mAh / g after 50 cycles), and / or superior rate performance.

[0050] In some embodiments, the compositions of the present disclosure may include Na2Mn3O7 and one or more dopants. In some embodiments, the dopants can include one or more metals and / or combinations thereof. The metals can be, for example, transition metals and / or combinations thereof, though, in at least some embodiments, the dopant can be any cation (metal) substitution. A lattice structure of the resultant compound is shown in FIG.1, with the sodium, manganese, and oxygen atoms labeled, and with the metal structures of the lattice stemming from the darkened portions of the accompanying periodic table. As shown, some non-limiting examples of metals with which the Na2Mn3O7 structure can be doped can include iron (Fe), aluminum (Al), nickel (Ni), and / or copper (Cu).

[0051] In some embodiments, manganese in Na2Mn3O7 can be substituted with one or more dopants M by adding one or more of an oxide, hydroxide, carbonate, nitrate, and / or other compound to the precursor material before a heating step. In some embodiments, the dopant that is used to substitute the manganese can be a metal element to modify the formula of the cathode to NaxMn3-yMyO7 where M is one or more metal elements and where y isAttorney Docket No.: MIT 25470 PCT | 88212-414294 approximately in the range of greater than about 0 to less than or equal to about 0.5. The provided cathode compositions can exhibit improved electrochemical cycling performance and / or higher capacities compared to Na2Mn3O7, when incorporated into Na-ion electrochemical cells.

[0052] In some embodiments, the cathode compositions may be single phase, having a Na2Mn3O7 type crystal structure. That is, in some embodiments, when the Na2Mn3O7 cathode materials of the present disclosure are single phase (as evidenced by their respective XRD patterns) (e.g., Na2Mn3O7), a dopant may be incorporated into the Na2Mn3O7 structure. In at least some such embodiments, non-limiting examples of the metal element(s) can modify the formula of Na2Mn3O7to one or more of: Na2.1Mn2.9Fe0.1O7, or Na2.25Mn2.75Fe0.25O7,

[0053] In other embodiments the cathode compositions may predominantly include a single phase with a Na2Mn3O7type crystal structure, but also can include a secondary phase that includes a metal oxide and / or a sodium transition metal oxide. That is, in various embodiments, the Na2Mn3O7 cathode materials of the present disclosure include two or more phases (as evidenced by their respective XRD patterns) (e.g., Na2.5Mn2.5Fe0.5O7, Na2.25Mn2.75Al0.25O7, Na2.5Mn2.75Ni0.25O7, Na2.5Mn2.75Cu0.25O7, Na2.75Mn2.5Fe0.25Ni0.25O7), indicating that some or all of the dopants are present in the second phase and that a dopant may or may not be incorporated into the Na2Mn3O7 structure. That is, in some embodiments, the compound may modify a grain morphology of the precursor Na2Mn3O7without incorporating itself into its structure. Some non-limiting examples of such secondary phases can include MnO2or Fe2O3, among others. Moreover, in such embodiments, the dopants may modify the grain morphology of the Na2Mn3O7 phase (i.e., cause a change in particle shape compared to undoped Na2Mn3O7) and / or significantly improve its electrochemical performance.

[0054] An amount of sodium in the composition can vary depending, at least in part, upon the charged and discharged states of the cathode when incorporated into a sodium-ion battery. For example, sodium can move from and to the cathode to the anode during charging and discharging. After sodium has moved from the cathode to the anode for the first time, some of the sodium originally in the cathode material can remain in the anode. This sodium, which can be measured as irreversible capacity, is usually not returned to the cathode and is usually not useful for further charging and discharging of the battery. It will be appreciated thatAttorney Docket No.: MIT 25470 PCT | 88212-414294 during subsequent charging and discharging cycles it is possible that more sodium becomes unavailable for cycling.

[0055] In some embodiments, the doped Na2Mn3O7cathode materials of the present disclosure can react reversibly with Na when incorporated in Na half-cells. In some embodiments, electrochemical cells incorporating the doped Na2Mn3O7cathode materials of the present disclosure can cycle better than Na2Mn3O7, under similar cycling conditions. Further, in some embodiments, electrochemical cells can have low voltage hysteresis. These improvements in cycle life can be accompanied by differences in the differential capacity of sodium half cells of doped materials compared to undoped Na2Mn3O7, which may indicate that at least some of the dopants are being incorporated into the Na2Mn3O7, structure.

[0056] When forming the cathode electrode from the instantly provided cathode active material compositions (material that is electrochemically active and storing Na+ions), one or more additives can be added. Some non-limiting examples of such additives can include binders (e.g., polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR)), conductive diluents (e.g., carbon blacks, carbon nanotubes), fillers (e.g., graphitic carbon, adhesion promoters, thickening agents for coating viscosity modification, such as carboxymethylcellulose) and other additives known by those skilled in the art can be mixed in a suitable coating solvent (e.g., water, N-methylpyrrolidinone (NMP)) to form a coating dispersion and / or coating mixture. The coating dispersion and / or coating mixture can be mixed thoroughly and then applied to a current collector by any appropriate coating technique, including but not limited to reverse roll coating, knife coating, notched bar coating, dip coating, spray coating, electrospray coating, and / or gravure coating. The current collectors can be foils of conductive metals, such as, for example, copper, aluminum, stainless steel, and / or nickel foil. The slurry (i.e., mixture of active material and inactive binders dispersed in a coating solvent) can be coated onto the current collector foil and then allowed to dry in air, followed usually by drying in a heated oven, typically at about 80 °C to about 300 °C for at least an hour to remove the solvent. The resulting cathodes may then be compressed to increase their density.

[0057] The cathode compositions can include an electrically conductive diluent to facilitate electron transfer from the powdered cathode composition to a current collector. Electrically conductive diluents can include, but are not limited to, carbon (e.g., carbon black), metal, metal nitrides, metal carbides, metal silicides, and metal borides. Representative electricallyAttorney Docket No.: MIT 25470 PCT | 88212-414294 conductive carbon diluents can include carbon blacks such as SUPER P carbon black (both from Timcal Ltd., Bodio Switzerland), SHAWANIGAN BLACK (Chevron Chemical Co., Houston, Tex.), Ketjen Black (AkzoNobel Functional Chemicals, Chicago, U.S.A.) acetylene black, furnace black, lamp black, graphite, carbon fibers and / or combinations thereof.

[0058] A person skilled in the art will recognize that cathodes usually have high electrochemical potential while anodes have low potential, which forms a driving force for sodium ions to move from the anode to the cathode during discharge, allowing us to take advantage of this chemical driving force by having a discharging battery do electrical work to power devices. It will be appreciated that while the composition of the present embodiments is discussed with respect to the cathode, in some embodiments these composition can be used as an anode in a battery, e.g., a sodium-ion battery. In such embodiments, the compositions of the present disclosure can be paired with another material with even higher electrochemical potential, as known by persons skilled in the art, resulting in a lower driving force, and a battery that stores energy.

[0059] METHODS

[0060] The cathode compositions of the present embodiments can be synthesized by any suitable method, e.g., by ball milling or jet milling to combine precursors of the metal elements (e.g., oxides, hydroxides, nitrates, carbonates, and the like), followed by heating to generate the cathode composition. Heating may be conducted in an ambient, pure oxygen, partial oxygen (balanced by inert gas), or other environment at a maximum temperature of at least 400 °C or at least 500 °C or at least 600 °C, so approximately in a range of about 400 °C to about 600 °C. Some non-limiting examples of inert gases that can be used include argon and / or nitrogen. In some embodiments, the method of making the provided cathode compositions can include co-precipitation of soluble precursors of the desired composition by taking stoichiometric amounts of water-soluble salts of the metals desired in the final composition (excepting sodium and oxygen) and dissolving them in an aqueous mixture. As examples, sulfate, nitrates, and / or halide salts can be utilized. Exemplary sulfate salts may be useful as precursors to the provide compositions include sulfates, such as transition metal sulfates, including iron sulfate, nickel sulfate, and / or manganese sulfate. The aqueous mixture can then be made basic (i.e., to a pH greater than about 9) by the addition of ammonium hydroxide and / or another suitable base, as will be known by those of ordinary skill in the art. The metal hydroxides, which are not soluble at high pH, can precipitate outAttorney Docket No.: MIT 25470 PCT | 88212-414294 and be filtered, washed, and dried thoroughly to form a blend. One or more of sodium carbonate, sodium hydroxide, sodium peroxide, and / or a combination can be added to the blend to form a mixture. In some embodiments, the mixture can be sintered by heating it in an environment with oxygen to a temperature above about 400 °C for a period approximately in a range from about 1 hour to about 10 hours. The mixture can then be heated above about 500 °C for an additional period of time until a stable composition is formed.

[0061] An amount of metal M being substituted into the NMO can vary. For example, in some embodiments, an amount of metal M that is used to substitute for manganese can be optimized based on one and up to several parameters. Some non-limiting examples of such parameters can include presence of ionic sites, charge balancing state, boundary conditions, temperature, amount of heat and / or gas flow, and / or phase stability. Optimization that maintains one or more of a structure, its stability, and / or activity can be influenced, for example, by adjusting an amount of dopant added thereto. Specifically, an amount of dopant can affect performance of the resulting cathode composition, with excess dopant causing the cathode composition to form a different phase without the desired electrochemical properties, while insufficient amounts of dopant can result in no meaningful impact on electrochemical performance. Dopant amounts and their impact on performance of the cathode are discussed in greater detail in the examples below.

[0062] Additional examples of optimization can include optimizing for phase stability and / or the interfacial condition for stabilizing structure of the cathode composition. For example, heating of the mixture at certain temperatures, e.g., about 650 °C or about 700 °C, can cause the structure to be destroyed. Moreover, use of some dopants over others can improve one or more of structure, ability, and / or electrochemical activity while simultaneously worsening another of these features. For example, in some embodiments, substitution of aluminum, which is not redox reactive, can serve as a structural stabilizer while having little to no impact on electrochemical activity. Alternatively, substitution of nickel can enhance electrochemical activity while having little to no impact on stability.

[0063] In some embodiments, the provided cathode compositions can have high specific capacity (mAh / g) retention when incorporated into a sodium-ion battery and cycled through multiple charge / discharge cycles. For example, the provided cathode compositions can have a specific capacity of greater than about 40 mAh / g, greater than about 50 mAh / g, or even greater than 60 mAh / g after 10 or more charging and discharging cycles at rates of C / 10Attorney Docket No.: MIT 25470 PCT | 88212-414294 when the battery is cycled between about 2.0 V and about 4.3 V versus Na and the temperature is maintained at about room temperature (about 25°C).

[0064] The cathode compositions of the present embodiments can be combined with an anode and an electrolyte to form an Na-ion battery. Examples of suitable anodes include carbonaceous materials such as soft carbons (carbonaceous materials that can graphitize when heated under atmospheric pressure), hard carbons (carbonaceous materials that cannot graphitize when heated under atmospheric pressure) and / or hydrogen containing carbons (carbonaceous materials made from organic precursors heated at temperatures near about 700 °C). Some non-limiting examples of suitable anodes can include metals that can alloy with Na (active metals, such as tin and lead) and alloys of active metals with metals that cannot alloy with Na (inactive metals, such as iron).

[0065] In some embodiments, electrochemical cells can contain an electrolyte in it. Representative electrolytes can be in the form of a solid, liquid, or gel. Exemplary solid electrolytes can include polymeric media such as polyethylene oxide, polytetrafluoroethylene, polyvinylidene fluoride, fluorine-containing copolymers, polyacrylonitrile, combinations thereof, and / or other solid media that will be familiar to those skilled in the art. Examples of liquid electrolytes can include organic carbonates, such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, and / or fluoroethylene carbonate. The electrolyte can be provided with a sodium electrolyte salt. Exemplary sodium salts can include NaPF6, NaClO4, NaN(CF3SO2), NaN(C2F5SO2)2, NaAsF6, and / or combinations thereof. In some embodiments, sodium-ion electrochemical cells that include the provided cathode compositions can be made by taking a positive electrode and a negative electrode, as described above and placing them in an electrolyte.

[0066] A microporous separator, such as microporous material available from Celgard LLC, Charlotte, N.C., can be used to prevent the contact of the negative electrode directly with the positive electrode.

[0067] The electrochemical cells can be used in a variety of devices including, without limitation, electric cars, grid-storage for long duration, portable computers, tablet displays, mobile telephones, other motorized devices (e.g., personal or household appliances),Attorney Docket No.: MIT 25470 PCT | 88212-414294 instruments, illumination devices (e.g., flashlights), and / or heating devices. One or more electrochemical cells can be combined to provide a battery pack.

[0068] The embodiments of the present disclosure will be further described with regard to the following detailed examples. These examples are offered to further illustrate various specific embodiments and techniques. It should be understood, however, that many variations and modifications may be made while remaining within the scope of the present disclosure.

[0069] EXAMPLES

[0070] Preparation of NaxMn3-yMyO7

[0071] Stoichiometric amounts of NaNO3(J. T. Baker, 99%), MnCO3(Sigma Aldrich, 99.9%), and an oxide or hydroxide of the dopant element (e.g., Fe2O3 (Sigma Aldrich, 99.995%) or Ni(OH)2(Sigma Aldrich)) were weighed and mixed by hand in a mortar and pestle for about 30 minutes. The sample was removed and placed in an alumina boat, which was subsequently placed in a tube furnace with open ends. The furnace was heated to about 550 °C at about 5 °C per minute and then held at about 550 °C for about 12 hours. The furnace was then allowed to cool to about 150 °C. The sample was then immediately placed in an Ar filled glovebox where it was ground in a mortar and pestle by hand. Some of the sample was removed for X-ray diffraction (XRD) measurement.

[0072] It will be appreciated that in some embodiments, the furnace can be heated to approximately a range of about 400 °C to about 600 °C at about 5 °C per minute and then holding its temperature constant approximately in a range of about 400 °C to about 600 °C for approximately in a range of about 4 hours to about 24 hours. In some embodiments, heating the mixture can also include placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, heating the furnace to about 550 °C at about 5 °C per minute and then holding constant temperature at about 550°C for about 12 hours.

[0073] NaxMn3-yMyO7Electrode Preparation

[0074] About 0.5 g of active material, about 0.0625 g of carbon black (Thermo Scientific, 99%) and about 0.0625 g of Polyvinylidine Fluoride (Sigma Aldrich, PVDF) were weighed into a 35 mL THINKY cup with about 2 mL of N-methylpyrrolidone (Sigma Aldrich, 99.5%Attorney Docket No.: MIT 25470 PCT | 88212-414294 anhydrous) in an Ar filled glovebox. The cup of slurry was then sealed, removed from the glovebox, and was mixed for about 20 minutes at speeds ramping from about 800 rpm to about 1200 rpm to about 2000 rpm in a THINKY ARE-310 planetary mill. The slurry was then coated onto carbon coated aluminum foil using a doctor blade with an approximately 100 μm gap in an Ar filled glovebox. The coating was dried at about 120 °C for several hours on a hotplate in an Ar filled glovebox prior to use. Electrode disks about 16 mm in diameter were punched from the electrode for use in 2032 coin cells. Each disk had approximately 6 mg to about 11 mg of NaxMn3-yMyO7active material.

[0075] Electrolyte and Coin Cell Preparation

[0076] All electrolyte and coin cell preparation was performed in an Ar filled glovebox with less than about 0.1 ppm moisture and oxygen. Electrolytes were prepared by dissolving NaPF6 (Thermo Scientific, 98%) salt in propylene carbonate (Sigma Aldrich, 99.7%) with about 4 v% fluoroethylene carbonate (Sigma Aldrich 99%) additive. Coin cells were constructed from 2032 coin cell hardware. Na metal electrodes were prepared by punching about 16 mm disks from Na foil that had been rolled from Na chunks (Fisher Scientific). Each cell contained an Na foil electrode, Whatman Glass microfiber separator, electrolyte, a NaxMn3-yMyO7 disk electrode, and a stainless steel spring and spacer.

[0077] Electrochemical Measurements

[0078] All coin cells were electrochemically cycled at a rate of C / 10, based on about 155 mAh / g for Na2Mn3O7 using a Bio-Logic VMP-300 Potentiostat. The fade rate of cells was measured as the discharge capacity loss between cycle 3 and cycle 10. Based on the differential capacity, the low voltage redox plateau corresponding to the Mn3+ / Mn4+redox reaction happens below about 2.4 V, and there is concern with respect to the hysteresis in the voltage window corresponding to predominantly anionic redox (> about 2.4 V) and solely anionic redox (O2- / O- redox plateau). Therefore, the voltage hysteresis was determined as the average voltage during the second discharge half cycle above about 2.4 V subtracted from the average voltage during the second charge half cycle above about 2.4 V. The anionic voltage hysteresis was calculated as the average voltage during the second discharge half cycle in the anionic redox plateau, which was determined to be the region over about 4.1 V corresponding to where the differential capacity was greater than about 1 mAh / V, subtracted from the average voltage during the second charge half cycle in the anionic redox plateau.Attorney Docket No.: MIT 25470 PCT | 88212-414294

[0079] XRD Analysis

[0080] X-ray powder diffraction can be done using, for example, either a Panalytical Empyrean or Bruker D8 Venture diffractometer. Both are equipped with an Mo source.

[0081] Comparative Example 1

[0082] Na2Mn3O7 powder was prepared using MnCO3 and NaNO3 precursors. The XRD pattern of this powder is shown in FIG.2, which shows that a single-phase crystalline oxide was produced. The small peak at about 6.5° indicates that ordered vacancies are present. From this powder Na2Mn3O7versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 V to about 4.3 V versus Na / Na+. The voltage curve for the first cycle of this cell is shown in FIGS.3A-3B, with FIG.3A illustrating the electrochemical profiles for Na2Mn3O7 that contains iron (Fe) and FIG.3B illustrating the electrochemical profiles for Na2Mn3O7 that does not contain Fe. Corresponding differential capacity curves are shown in FIGS.4A-4G. The capacity of the cell remained constant with cycle number, as shown in FIGS.5A-5B. Specifically, FIG.5A illustrates capacity versus cycle for Na2Mn3O7 that contains Fe, and FIG.5B illustrates capacity versus cycle for Na2Mn3O7that does not contain Fe, while having a fade rate of about -3.23 mAh / g, which means that the capacity increased between cycle 3 and cycle 10 for this voltage range.

[0083] Example 1

[0084] Na2.1Mn2.9Fe0.1O7 powder was prepared using MnCO3, Fe2O3, and NaNO3 precursors. The XRD pattern of this powder is shown in the correspondingly labeled trend line of FIG.2 and shows that two phases are present. The small peak at about 6.5° indicates that ordered vacancies are present. The other peaks in the diffraction pattern are indicative of a crystalline oxide phase. This phase may be Na2Mn3O7 or Na2Mn3O7 with some incorporated iron (e.g., Na2+xMn3-xFexO7). From this powder Na2.1Mn2.9Fe0.1O7 versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 V to about 4.3 V versus Na / Na+. The voltage curve for this cell is shown in the correspondingly labeled trend line of FIG.3A, and the differential capacity is shown in the correspondingly labeled trend line of FIG.4A. It will be appreciated that there are significant differences between the differential capacity plots of this material and the cell from Comparative Example 1. For example, the peaks in the differential capacity occur at slightly different voltages and have different peak widths. This may indicate that the material in Example 1 isAttorney Docket No.: MIT 25470 PCT | 88212-414294 structurally different than the material in Comparative Example 1 including, for instance, the inclusion of iron in the Mn layer of Na2Mn3O7. It may also indicate that the material in Example 1 has different redox mechanisms than the material in Comparative Example 1. The capacity versus cycle number of the Na2.1Mn2.9Fe0.1O7versus Na coin cell is shown in the correspondingly labeled trend line of FIG.5A. The cell had higher capacity and similar capacity retention compared to that of Comparative Example 1. The cell also had a lower voltage hysteresis of about 41 mV and lower anionic voltage hysteresis of about 38 mV compared to Comparative Example 1.

[0085] Example 2

[0086] Na2.25Mn2.75Fe0.25O7 powder was prepared using MnCO3, Fe2O3, and NaNO3 precursors. The XRD pattern of this powder is shown in the correspondingly labeled trend line of FIG.2 and shows that two phases are present. The small peak at about 6.5° indicates that ordered vacancies are present. The minor splitting of the peaks at about 14.5°, about 17.5°, and about 29.5° along with a peak at about 19° in the diffraction pattern is indicative that a minor impurity phase is present in this sample, but the Na2Mn3O7 structure is present. The other peaks in the diffraction pattern are indicative of a crystalline oxide phase. This phase may be Na2Mn3O7 or Na2Mn3O7 with some incorporated iron (e.g., Na2+xMn3-xFexO7). From this powder Na2.25Mn2.75Fe0.25O7versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 V to about 4.3 V versus Na / Na+. The voltage curve for this cell is shown in the correspondingly labeled trend line of FIG.3A, and the differential capacity is shown in the correspondingly labeled trend line of FIG.4A. It will be appreciated that there are significant differences between the differential capacity plots of this material and the cell from Comparative Example 1. For example, the peaks in the differential capacity occur at slightly different voltages and have different peak widths. This may indicate that the material in Example 2 is structurally different than the material in Comparative Example 1 including, for instance, the inclusion of iron in the Mn layer of Na2Mn3O7. It may also indicate that the material in Example 2 has different redox mechanisms than the material in Comparative Example 1. The capacity versus cycle number of the Na2.25Mn2.75Fe0.25O7 versus Na coin cell is shown in FIG.5A. The cell had lower capacity and similar capacity retention compared to that of Comparative Example 1. The cell also had a higher voltage hysteresis of about 147 mV and higher anionic voltage hysteresis of about 51 mV compared to Comparative Example 1.Attorney Docket No.: MIT 25470 PCT | 88212-414294

[0087] Example 3

[0088] Na2.5Mn2.5Fe0.5O7 powder was prepared using MnCO3, Fe2O3, and NaNO3 precursors. The XRD pattern of this powder is shown in the correspondingly labeled trend line of FIG.2 and shows that two phases are present. The small peak at about 6.5° indicates that ordered vacancies are present. The minor splitting of the peaks at about 14.5°, about 17.5°, and about 29.5° along with a peak at about 19° in the diffraction pattern is indicative that a minor impurity phase is present in this sample, but the Na2Mn3O7structure is present. The other peaks in the diffraction pattern are indicative of a crystalline oxide phase. This phase may be Na2Mn3O7 or Na2Mn3O7 with some incorporated iron (e.g., Na2+xMn3-xFexO7). From this powder Na2.5Mn2.5Fe0.5O7versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 V to about 4.3 V versus Na / Na+. The voltage curve for this cell is shown in the correspondingly labeled trend line of FIG.3A, and the differential capacity is shown in the correspondingly labeled trend line of FIG.4A. It will be appreciated that there are significant differences between the differential capacity plots of this material and the cell from Comparative Example 1. For example, the peaks in the differential capacity occur at slightly different voltages and have different peak widths. This may indicate that the material in Example 3 is structurally different than the material in Comparative Example 1 including, for instance, the inclusion of iron in the Mn layer of Na2Mn3O7. It may also indicate that the material in Example 3 has different redox mechanisms than the material in Comparative Example 1. The capacity versus cycle number of the Na2.5Mn2.5Fe0.5O7versus Na coin cell is shown in the correspondingly labeled trend line of FIG.5A. The cell had higher capacity and worse capacity retention compared to that of Comparative Example 1. The cell also had a lower voltage hysteresis of about 121 mV and lower anionic voltage hysteresis of about 1 mV compared to Comparative Example 1.

[0089] Example 4

[0090] Na2.25Mn2.75Al0.25O7powder was prepared using MnCO3, Al(OH)3, and NaNO3precursors. The XRD pattern of this powder is shown in the correspondingly labeled trend line of FIG.2 and shows that two phases are present. The small peak at about 6.5° indicates that ordered vacancies are present. The minor splitting of the peaks at about 17.5° and about 25° in the diffraction pattern is indicative that a minor impurity phase is present in this sample, but the Na2Mn3O7 structure is present. The other peaks in the diffraction pattern are indicative of a crystalline oxide phase. This phase may be Na2Mn3O7 or Na2Mn3O7 withAttorney Docket No.: MIT 25470 PCT | 88212-414294 some incorporated aluminum (e.g., Na2+xMn3-xAlxO7). From this powder Na2.25Mn2.75Al0.25O7versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 to about 4.3 V versus Na / Na+. The voltage curve for this cell is shown in the correspondingly labeled trend line of FIG.3B, and the differential capacity is shown in the correspondingly labeled trend line of FIG.4B. It will be appreciated that there are significant differences between the differential capacity plots of this material and the cell from Comparative Example 1. The peaks in the differential capacity occur at slightly different voltages and have different peak widths. This may indicate that the material in Example 4 is structurally different than the material in Comparative Example 1 including, for instance, the inclusion of aluminum in the Mn layer of Na2Mn3O7. It may also indicate that the material in Example 4 has different redox mechanisms than the material in Comparative Example 1. The capacity versus cycle number of the Na2.25Mn2.75Al0.25O7 versus Na coin cell is shown in the correspondingly labeled trend line of FIG.5B. The cell had similar capacity and lower capacity retention compared to that of Comparative Example 1. The cell also had a higher voltage hysteresis of about 288 mV and lower anionic voltage hysteresis of about 38 mV compared to Comparative Example 1.

[0091] Example 5

[0092] Na2.5Mn2.75Ni0.25O7powder was prepared using MnCO3, Ni(OH)2, and NaNO3precursors. The XRD pattern of this powder is shown in the correspondingly labeled trend line of FIG.2 and shows that two phases are present. The small peak at about 6.5° indicates that ordered vacancies are present. The minor splitting of the peaks at about 17.5°, about 25°, and about 29.5° in the diffraction pattern is indicative that a minor impurity phase is present in this sample, but the Na2Mn3O7 structure is present. The other peaks in the diffraction pattern are indicative of a crystalline oxide phase. This phase may be Na2Mn3O7or Na2Mn3O7 with some incorporated nickel (e.g., Na2+xMn3-xNixO7). From this powder Na2.5Mn2.75Ni0.25O7 versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 V to about 4.3 V versus Na / Na+. The voltage curve for this cell is shown in the correspondingly labeled trend line of FIG.3B, and the differential capacity is shown in the correspondingly labeled trend line of FIG.4B. It will be appreciated that there are significant differences between the differential capacity plots of this material and the cell from Comparative Example 1. For example, the peaks in the differential capacity occur at slightly different voltages and have different peak widths. This may indicate that the materialAttorney Docket No.: MIT 25470 PCT | 88212-414294 in Example 5 is structurally different than the material in Comparative Example 1 including, for instance, the inclusion of nickel in the Mn layer of Na2Mn3O7. It may also indicate that the material in Example 5 has different redox mechanisms than the material in Comparative Example 1. The capacity versus cycle number of the Na2.5Mn2.75Ni0.25O7versus Na coin cell is shown in the correspondingly labeled trend line of FIG.5B. The cell had higher capacity and lower capacity retention compared to that of Comparative Example 1. The cell also had a higher voltage hysteresis of about 158 mV and higher anionic voltage hysteresis of about 65 mV compared to Comparative Example 1.

[0093] Example 6

[0094] Na2.5Mn2.75Cu0.25O7 powder was prepared using MnCO3, CuO, and NaNO3 precursors. The XRD pattern of this powder is shown in the correspondingly labeled trend line of FIG.2 and shows that two phases are present. The small peak at about 6.5° indicates that ordered vacancies are present. The minor splitting of the peaks at about 17.5°, about 25°, and about 29.5° in the diffraction pattern is indicative that a minor impurity phase is present in this sample, but the Na2Mn3O7 structure is present. The other peaks in the diffraction pattern are indicative of a crystalline oxide phase. This phase may be Na2Mn3O7or Na2Mn3O7 with some incorporated copper (e.g., Na2+xMn3-xCuxO7). From this powder Na2.5Mn2.75Cu0.25O7versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 V to about 4.3 V versus Na / Na+. The voltage curve for this cell is shown in the correspondingly labeled trend line of FIG.3B, and the differential capacity is shown in the correspondingly labeled trend line of FIG.4B. It will be appreciated that there are significant differences between the differential capacity plots of this material and the cell from Comparative Example 1. For example, the peaks in the differential capacity occur at slightly different voltages and have different peak widths. This may indicate that the material in Example 6 is structurally different than the material in Comparative Example 1 including, for instance, the inclusion of copper in the Mn layer of Na2Mn3O7. It may also indicate that the material in Example 6 has different redox mechanisms than the material in Comparative Example 1. The capacity versus cycle number of the Na2.5Mn2.75Cu0.25O7 versus Na coin cell is shown in the correspondingly labeled trend line of FIG.5B. The cell had higher capacity and similar capacity retention compared to that of Comparative Example 1. The cell also had a lower voltage hysteresis of about -9 mV and lower anionic voltage hysteresis of about 21 mV compared to Comparative Example 1. The negative voltage hysteresis is an artifact ofAttorney Docket No.: MIT 25470 PCT | 88212-414294 the way such calculation was defined because the difference between the low voltage charge and discharge capacities are significantly larger than in the other examples.

[0095] Example 7

[0096] Na2.75Mn2.75Fe0.25Ni0.25O7 powder was prepared using MnCO3, Fe2O3, Ni(OH)2, and NaNO3precursors. The XRD pattern of this powder is shown in the correspondingly labeled trend line of FIG.2 and shows that two phases are present. The small peak at about 6.5° indicates that ordered vacancies are present. The minor splitting of the peaks at about 14°, about 17°, about 17.5°, about 20°, about 24.5°, and about 29.5° along with a peak at about 19° in the diffraction pattern is indicative that a minor impurity phase is present in this sample, but the Na2Mn3O7 structure is present. The other peaks in the diffraction pattern are indicative of a crystalline oxide phase. This phase may be Na2Mn3O7or Na2Mn3O7with some incorporated iron and nickel (e.g., Na2+x+2yMn3-x-yFexNiyO7). From this powder Na2.75Mn2.75Fe0.25Ni0.25O7versus Na coin cells were constructed. One cell was cycled at a rate of C / 10 between about 2.0 V to about 4.3 V versus Na / Na+. The voltage curve for this cell is shown in the correspondingly labeled trend line of FIG.3A, and the differential capacity is shown in the correspondingly labeled trend line of FIG.4A. It will be appreciated that there are significant differences between the differential capacity plots of this material and the cell from Comparative Example 1. For example, the peaks in the differential capacity occur at slightly different voltages and have different peak widths. This may indicate that the material in Example 7 is structurally different than the material in Comparative Example 1 including, for instance, the inclusion of iron and nickel in the Mn layer of Na2Mn3O7. It may also indicate that the material in Example 7 has different redox mechanisms than the material in Comparative Example 1. The capacity versus cycle number of the Na2.75Mn2.75Fe0.25Ni0.25O7versus Na coin cell is shown in the correspondingly labeled trend line of FIG.5A. The cell had higher capacity and similar capacity retention compared to that of Comparative Example 1. The cell also had a higher voltage hysteresis of about 271 mV and an about equal anionic voltage hysteresis of about 76 mV compared to Comparative Example 1.

[0097] Table 1 below summarizes the results of the Examples and Comparative Examples. A person skilled in the art, in view of the present disclosure, will appreciate other embodiments of the present disclosure are within the scope of the appended claims.Attorney Docket No.: MIT 25470 PCT | 88212-414294 TABLE 1

[0099] Na2+xMn3-xFexO7 (NMFO) can be investigated by substituting a fraction of Mn in Na2Mn3O7(NMO) with Fe (x = 0.5). Compared to other top performing cathode materials in the literature, NMFO is a competitive candidate. NMFO has comparable specific capacity, but a significant portion of the capacity comes from high voltage anionic redox, leading to superior average voltage and therefore more competitive energy density. Further, many competitive candidates rely on elements like Ni and Co, which are relatively scarce and expensive compared to Mn and Fe. Assuming Fe3+can be substituted for Mn4+NMO, the charge difference is compensated with extra Na+.

[0100] FIG.6A is a graph illustrating that as-synthesized Na2.5Mn2.5Fe0.5O7 (NMFO) exhibits a similar XRD pattern to Na2Mn3O7 (NMO) with triclinic structure and ^1^space group except the small peak at 2θ ≈ 19°, which likely corresponds to a minor α-NaFeO2impurity phase. Additionally, the interlayer spacing in NMFO is about 5.562 Å, corresponding to the ^11^0^ planes, compared to about 5.555 Å for NMO. The small

[0001] peak at 2θ ≈ 6.5° is formed from the ordering of the vacancies and TM ions in the TMO slab, so its presence confirms the existence of ordered vacancies globally.

[0101] FIG.6B illustrates a lattice with ordered vacancies having three unique Na+sites in the lattice: Na1 is directly above the ordered vacancy and is prismatically coordinated while Na2 is in an octahedral site not directly adjacent to an ordered vacancy. NMFO can have one more Na+ion per unit cell than NMO (two formula units per unit cell), and this extra Na+ion sits in the TM vacancy site. The morphology of NMFO can be dominated by plateletAttorney Docket No.: MIT 25470 PCT | 88212-414294 particles that are about 1 μm to about 2 μm in diameter and about 100 nm in thickness. Scanning transmission electron microscopy (STEM) imaging can be used to confirm the layered structure and the presence of ordered vacancies along certain zones axes, e.g, [01^2]). The remaining compounds and their positions in the lattice structure are illustrated in FIG. 6B.

[0102] The homogeneous distribution and mixing of Mn and Fe on this particle suggest their integration into the NMO lattice. EDX chemical mapping on a larger set of particles supports the homogeneity of Fe incorporation into the lattice. Electron energy loss spectroscopy (EELS) analysis on the same particle can corroborate the EDX results, as shown in FIGS.7A-7C. Specifically, FIGS.7A-7C show EELS measurements for Na2.5Mn2.5Fe0.5O7. The particle on which these measurements are taken is shown in FIG.7A, with the white boxes indicating the areas of the particle on which the measurements were taken moving from the edge of the particle (A) to the bulk (D). The associated EELS spectra are shown in FIGS.7B-7C, and the Mn- and Fe-L edges are labeled, with the arrow indicating the shift in Mn L3-edge.

[0103] As shown in FIG.7C, the Fe L-edge peak (both L3and L2) is present at multiple locations moving from the edge to the bulk, confirming the presence of Fe throughout the particle. Moreover, the Mn L3-peak shifts to higher energies moving from the particle edge (A in FIG.7A) towards the bulk (D in FIG.7A), which is indicative of oxidation from edge to bulk. This suggests that the surface has lower valent Mn species than the bulk, which could be from surface reduction. The surface is also coated with a few unit cells without the ordered vacancy present, requiring the Mn to reduce to preserve charge neutrality. Further, the presence of α-NaFeO2impurity can also be observed.

[0104] The electrochemical performance of NMFO compared to NMO was evaluated over the voltage window of about 1.5 V to about 4.3 V versus Na / Na+where the electrolyte is stable. Above about 4.3 V, the electrolyte decomposes, and a significant amount of O2-ions are oxidized, which could lead to a very strong driving force to form covalent bonds coupled with in-plane Mn migration, leading to material instability. The electrochemical cycling was carried out with a current density corresponding to C / 20 (based on 1C = 155 mA / g then recalculated after the first cycle based on measured discharge capacity). During the first discharge, both NMO and NMFO show clear plateaus at about 4.2 V, which corresponds to O2- / O- redox, and at about 2.0 V, which corresponds to Mn3+ / Mn4+redox. During the firstAttorney Docket No.: MIT 25470 PCT | 88212-414294 discharge, in the high-valent regime (> about 3.0 V), the capacity contribution in NMO is dominated by the O2- / O- redox plateau, which contributes a discharge capacity of about 30 mAh / g. Further discharge leads to a steep drop in voltage then an Mn3+ / Mn4+redox plateau, which contributes about 120 mAh / g. For NMFO, the O2- / O- redox plateau contributes about 20 mAh / g and is followed by a sloping profile. This could be characteristic of O2- / O- redox shifting to TM redox as was predicted by computation and has been observed in Li-ion chemistries. Similar observations can be made for cycling in a larger voltage window (approximately in the range of about 1.5 V to about 4.7 V versus Na / Na+) and at higher C- rate (i.e., C / 10) cycling in the same voltage range.

[0105] Over the first three cycles, the discharge capacity of NMO can drop significantly (from about 159 mAh / g to about 88 mAh / g). A closer look at the cycling profiles hints that the capacity loss is severe in the lower voltage region (i.e., during Mn3+ / Mn4+redox). This can possibly be due to Mn-dissolution, which is common in lithium- and sodium-containing TMOs that derive capacity from Mn redox reactions. The rapid capacity loss in the lower voltage regime coupled with relative stability in the high voltage regime of NMO can cause the average discharge voltage to increase from about 2.4 V to a maximum of about 3.0 V. A similar phenomenon may occur when cycling at C / 10. With progressive cycling (>10 cycles), the low voltage regime of NMO remains relatively stable while the high voltage O2- / O- redox plateau fades, causing the average discharge voltage to decrease.

[0106] The initial discharge capacity is higher for NMFO (about 171 mAh / g), likely stemming from the fact that NMFO can accommodate more Na in its structure. Most of the initial capacity fade from cycling comes from the low voltage cycling (again possibly due to Mn-dissolution) but degrades at a much slower rate than in NMO, causing the overall specific capacity to degrade much slower. In NMFO, both the high and low voltage regimes degrade slowly, leaving the average discharge voltage relatively constant. At C / 10, the low-valent capacity degrades quickly, which is similar to NMO, but the high-valent capacity degrades much slower than in NMO. This suggests that Fe introduces an increased energetic penalty for in-plane TM migration, leading to less structural deformation. NMFO maintains superior performance over NMO in terms of specific capacity and specific energy at higher cycles (about 97 mAh / g to about 60 mAh / g after 50 cycles, about 49.5 mAh / g to about 19.6 mAh / g after 100 cycles) and outperforms even as the rate is increased. It will be appreciated thatAttorney Docket No.: MIT 25470 PCT | 88212-414294 improvements to cycle stability can be based, at least in part, on Mn-dissolution and include coating with carbon, alumina, and / or other materials as possible solutions.

[0107] NMFO can outperform NMO in terms of rate capability. To study the kinetics further, electrochemical impedance spectroscopy (EIS) can be performed on pristine cells of NMO and NMFO. NMFO had a superior Na diffusivity (DNa+) of about 3.78 x 10-16cm2 / s compared to about 2.56 x 10-16cm2 / s for NMO. Galvanostatic intermittent titration technique (GITT) can be also used to study the kinetics throughout charge and discharge of NMO and NMFO. The DNa+ of NMO varies between about 9.3 x 10-15and about 3.9 x 10-10cm2 / s during cycling while that of NMFO varies between about 2.7 x 10-12and about 1.3 x 10-9cm2 / s. For a given voltage and half cycle (i.e., charge or discharge), NMFO can have a superior DNa+. The primary limiting factor for Na+kinetics in NMFO can be the fact that the Na+ion must transition through different coordination environments as it moves through the material.

[0108] As a result of the above, Fe substitution for Mn in NMO can enhance TM-O hybridization, enabling reversible and stable high-valent cationic and anionic capacity while maintaining the robust ordered vacancy structure motif. Through combined computational and experimental investigation, although Na2.5Mn2.5Fe0.5O7 (NMFO) exhibits a similar high- valent redox mechanism as NMO, its ability to accommodate more Na in its structure and its increased interlayer spacing (due, at least in part, to the extra electrons on Fe3+versus Mn4+) results in superior electrochemical performance during cycling in terms of specific capacity, energy density, cycle stability, and / or rate capability. Ordered cationic vacancies can thus be achieved in TMs beyond Mn, and by combining this with other design rules, material candidates with exceptional high-valent redox behavior can be expanded. Additionally, the partial substitution of TMs may not only tune TM-O hybridization strength and govern the redox mechanism / redox-active species, but can also tune structures (such as interlayer distance or phases), having a significant impact on material stability, rate capability, and overall electrochemical performance in the high-valent regime.

[0109] Examples of the above-described embodiments can include the following: 1. A cathode composition for a sodium-ion battery, comprising: a compound having the formula NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2Attorney Docket No.: MIT 25470 PCT | 88212-414294 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.5. 2. The cathode composition of claim 1, wherein y is less than or equal to about 0.25. 3. The cathode composition of claim 1 or claim 2, wherein M comprises one or more metal elements. 4. The cathode composition of claim 3, wherein M comprises a metal. 5. The cathode composition of any of claims 1 to 4, wherein M includes one or more of iron (Fe), aluminum (Al), nickel (Ni), copper (Cu), or combinations thereof. 6. The cathode composition of claim 1 or claim 2, wherein M comprises one or more of an oxide, hydroxide, carbonate, or nitrate. 7. The cathode composition of any of claims 1 to 6, further comprising one or more of polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), carbon blacks, carbon nanotubes, graphitic carbon, adhesion promoters, or carboxymethylcellulose. 8. The cathode composition of any of claims 1 to 7, further comprising an electrically conductive diluent to facilitate electron transfer from the cathode composition to a current collector. 9. The cathode composition of any of claims 1 to 8, wherein a specific capacity of the cathode composition is greater than about 50 mAh / g after at least 10 charging and discharging cycles. 10. The cathode composition of any of claims 1 to 9, wherein the compound having the formula NaxMn3-yMyO7 has an improved cycle discharge capacity as compared to a cycle discharge capacity of Na2Mn3O7. 11. The cathode composition of any of claims 1 to 10, wherein the compound having the formula NaxMn3-yMyO7has an improved anionic voltage hysteresis as compared to an anionic voltage hysteresis of Na2Mn3O7. 12. A sodium-ion electrochemical cell, comprising: an anode;Attorney Docket No.: MIT 25470 PCT | 88212-414294 an electrolyte; and a cathode that comprises a compound having the formula: NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.5. 13. The cell of claim 12, wherein M comprises one or more metal elements. 14. The cell of claim 13, wherein M comprises a transition metal or combinations thereof. 15. A method of making a cathode composition comprising: combining a precursor having the formula: Na2Mn3O7 with a compound M; and heating the mixture to form a compound having the formula: NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.5. 16. The method of claim 15, wherein the compound M is incorporated into the structure of the precursor Na2Mn3O7 by virtue of substitution of the compound M for a portion of the Mn in the precursor Na2Mn3O7. 17. The method of claim 14 or claim 15, further comprising optimizing an amount of the compound M that is substituted into the structure of the precursor Na2Mn3O7. 18. The method of claim 17, wherein optimizing further comprises adjusting the amount of the compound M based on improving one or more of structure, ability, or electrochemical activity of the cathode composition. 19. The method of any of claims 15 to 18, wherein the compound M comprises one or more metal elements. 20. The method of claim 20, wherein the compound M comprises a transition metal or combinations thereof. 21. The method of claim 15 or claim 16, wherein the compound M modifies a grain morphology of the precursor Na2Mn3O7without incorporating itself into its structure.Attorney Docket No.: MIT 25470 PCT | 88212-414294 22. The method of any of claims 15 to 21, wherein the precursor with the compound M comprises mechanically milling the precursor and the compound with a mortar and pestle for about 30 minutes. 23. The method of any of claims 15 to 22, wherein heating the mixture comprises placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace with open ends. 24. The method of any of claims 15 to 23, wherein heating the mixture comprises placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, sealing it from the atmosphere, and flowing 100% oxygen gas. 25. The method of any of claims 15 to 24, wherein heating the mixture comprises placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, sealing it from the atmosphere, and flowing a mixture of oxygen and argon gas. 26. The method of any of claims 15 to 25, wherein heating the mixture comprises placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, heating the furnace approximately in a range of about 400 °C to about 600 °C at about 5 °C per minute and then holding constant temperature approximately in a range of about 400 °C to about 600 °C for approximately in a range of about 4 hours to about 24 hours. 27. The method of any of claims 15 to 26, wherein heating the mixture comprises placing the precursors in an alumina boat and subsequently placing the boat in a tube furnace, heating the furnace to about 550 °C at about 5 °C per minute and then holding constant temperature at about 550°C for about 12 hours. 28. The method of any of claims 15 to 27, further comprising combining the cathode composition with an anode and an electrolyte to form a sodium-ion battery. 29. An electronic device comprising the sodium-ion electrochemical cell of claim 12.

[0110] One skilled in the art will appreciate further features and advantages of the disclosures based on the provided for descriptions and embodiments. Accordingly, the inventions are not to be limited by what has been particularly shown and described. To the extent the present disclosure includes illustrations and descriptions that include prototypes, bench models, or schematic illustrations of set-ups, a person skilled in the art will recognizeAttorney Docket No.: MIT 25470 PCT | 88212-414294 how to rely upon the present disclosure to integrate the techniques, systems, composition, designs, and methods provided for into a product and / or production method. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0111] Some non-limiting claims that are supported by the contents of the present disclosure are provided below.

Claims

Attorney Docket No.: MIT 25470 PCT | 88212-414294 What is claimed is:

1. A cathode composition for a sodium-ion battery, comprising: a compound having the formula NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.

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2. The cathode composition of claim 1, wherein y is less than or equal to about 0.

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3. The cathode composition of claim 1, wherein M comprises one or more metal elements.

4. The cathode composition of claim 3, wherein M comprises a metal.

5. The cathode composition of claim 1, wherein M includes one or more of iron (Fe), aluminum (Al), nickel (Ni), copper (Cu), or combinations thereof.

6. The cathode composition of claim 1, further comprising one or more of polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), carbon blacks, carbon nanotubes, graphitic carbon, adhesion promoters, or carboxymethylcellulose.

7. The cathode composition of claim 1, further comprising an electrically conductive diluent to facilitate electron transfer from the cathode composition to a current collector.

8. The cathode composition of claim 1, wherein a specific capacity of the cathode composition is greater than about 50 mAh / g after at least 10 charging and discharging cycles.

9. The cathode composition of claim 1, wherein the compound having the formula NaxMn3-yMyO7 has an improved cycle discharge capacity as compared to a cycle discharge capacity of Na2Mn3O7.

10. The cathode composition of claim 1, wherein the compound having the formula NaxMn3-yMyO7has an improved anionic voltage hysteresis as compared to an anionic voltage hysteresis of Na2Mn3O7.

11. A sodium-ion electrochemical cell, comprising: an anode,Attorney Docket No.: MIT 25470 PCT | 88212-414294 an electrolyte, and a cathode that comprises a compound having the formula: NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.

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12. The cell of claim 11, wherein M comprises one or more metal elements.

13. The cell of claim 12, wherein M comprises a transition metal or combinations thereof.

14. A method of making a cathode composition comprising: combining a precursor having the formula: Na2Mn3O7 with a compound M; and heating the mixture to form a compound having the formula: NaxMn3-yMyO7, where M includes one or more compounds and wherein x is approximately in a range from about greater than or equal to 2 and less than or equal to 3.5 and wherein y is greater than about 0 and less than or equal to about 0.

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15. The method of claim 14, wherein the compound M is incorporated into the structure of the precursor Na2Mn3O7 by virtue of substitution of the compound M for a portion of the Mn in the precursor Na2Mn3O7.

16. The method of claim 13, further comprising optimizing an amount of the compound M that is substituted into the structure of the precursor Na2Mn3O7.

17. The method of claim 16, wherein optimizing further comprises adjusting the amount of the compound M based on improving one or more of structure, ability, or electrochemical activity of the cathode composition.

18. The method of claim 14, wherein the compound M modifies a grain morphology of the precursor Na2Mn3O7without incorporating itself into its structure.

19. The method of claim 14, further comprising combining the cathode composition with an anode and an electrolyte to form a sodium-ion battery.

20. An electronic device comprising the sodium-ion electrochemical cell of claim 11.

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