Manganese oxide compositions and their use as electrodes for aqueous phase energy storage devices

Abstract

A composition and method of preparation of mixed valence manganese oxide, nickel-doped mixed valence manganese oxide and cobalt-doped mixed valence manganese oxide nanoparticles as well as tri-manganese tetroxide, nickel-doped tri-manganese tetroxide and cobalt-doped tri-manganese tetroxide nanoparticles for use as electrodes for aqueous energy storage devices.

Claims

1. A composition comprising nanoparticles of mixed valence oxides of the formula MnOx wherein Mn consists of a mixture of valence states of 2+, 3+ and 4+ where x has a value between 1 and 2 and having an average particle size of 10 nm to 50 nm, wherein Mn 4+ is present at a level of 40 mole percent to 60 mole percent, Mn 3+ is present at a level of 10 mole percent to 20 mole percent and Mn 2+ is present at a level of 20 mole percent to 40 mole percent and wherein said nanoparticles of mixed valence oxides is combined with carbon black and is in the form of an electrode that indicates a potential window of 2.5V in an aqueous electrochemical half-cell.

2. The composition of claim 1 wherein said mixed valence oxide MnOx is doped with Ni to provide the doped mixed valence oxide NiMnOx wherein Mn has a mixture of valence states of 2+, 3+ and 4+ and Ni has a valence of 2+.

3. The mixed valence oxide of claim 2 wherein said Ni doped mixed valence oxide has the formula Ni.sub.yMn.sub.3-yO where y has the value 0.1, 0.5 and 0.75 and 3<x<6.

4. The composition of claim 1 wherein said composition is a mixture of 10% to 40% by weight of carbon black and 60% to 90% by weight of said MnOx particles.

5. The composition of claim 2 wherein said composition is combined with carbon black to provide a mixture of 10% to 40% by weight of carbon black and 60% to 90% by weight of said doped mixed valence oxide NiMnOx.

6. The composition of claim 5 wherein said composition is in the form of an electrode.

7. An electrochemical cell comprising: two current collectors having deposited thereon a composition comprising 10-40% by weight of carbon black and 60-90% by weight of MnO, nanoparticles having particle size of 10 to 50 nm wherein Mn consists of a mixture of valence states 2+, 3+ and 4+ and l<x<2; a separator; and an electrolyte comprising an aqueous solution of an alkali salt, wherein Mn 4+ is present at a level of 40 mole percent to 60 mole percent, Mn 3+ is present at a level of 10 mole percent to 20 mole percent and Mn 2+ is present at a level of 20 mole percent to 40 mole percent and wherein said nanoparticles of mixed valence oxides is combined with carbon black and is in the form of an electrode that indicates a potential window of 2.5V in an aqueous electrochemical half-cell.

8. The electrochemical cell of claim 7, wherein said MnOx nanoparticles are doped with Ni and comprise the doped mixed valence oxide NiMnOx wherein Ni is present at a valence of 2+.

9. A method of preparing MnOx nanoparticles comprising: (a) supplying a manganese (II) salt of the formula Mn(A).sub.2 where A is any halide or a NO.sub.3.sup.; (b) reacting said manganese (II) salt with an alkali base in air and forming said MnOx nanoparticles; and (c) heating said nanoparticles at a temperature of 200 C.-400 C. and recovering MnOx nanoparticles at a particle size of 10 nm to 50 nm wherein Mn has a mixture of valence states of 2+, 3+ and 4+ and where x has a value between 1 and 2.

10. A composition comprising nanoparticles of mixed valence oxide CoMnOx wherein Mn and Co consist of a mixture of valence states of 2+, 3+ and 4+ where x has a value between 1 and 2 and having an average particle size of 10 nm to 50 nm wherein said nanoparticles of mixed valence oxides is combined with carbon black and is in the form of an electrode that indicates a potential window of 2.5V in an aqueous electrochemical half-cell.

11. The composition of claim 10, wherein said composition is a mixture of 10% to 40% by weight of carbon black and 60% to 90% by weight of said doped mixed valence oxide CoMnOx.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features and advantages will be better understood by reading the following detailed description, taken together with the drawings wherein:

(2) FIG. 1 is an image of Mn.sub.3O.sub.4 nanoparticles by transmission electron microscopy (TEM).

(3) FIG. 2 is an image of Mn.sub.3O.sub.4 nanoparticles by high resolution TEM.

(4) FIGS. 3A and 3B are images of Co.sub.0.1Mn.sub.2.9O.sub.4 (3A) and Ni.sub.0.1Mn.sub.2.9O.sub.4 (3B) nanoparticles by TEM.

(5) FIG. 4 is a plot of X-ray diffraction (XRD) data of Mn.sub.3O.sub.4, Co.sub.0.1Mn.sub.2.9O.sub.4 and Ni.sub.0.1Mn.sub.2.9O.sub.4.

(6) FIGS. 5A and 5B are plots of capacitance vs. scan rates of Mn.sub.3O.sub.4 and Co.sub.xMn.sub.3-xO.sub.4 (x: 0.1, 0.5) (5A) Mn.sub.3O.sub.4 and Ni.sub.xMn.sub.3-xO.sub.4 (x: 0.1, 0.5, 0.75) (5B) from half-cell cyclic voltammetry measurements.

(7) FIG. 6 is a plot of capacitive retention vs. number of cycles of various button-cells using Mn.sub.3O.sub.4 nanoparticles as electrode materials.

(8) FIG. 7 is a plot of in situ XRD data of Mn.sub.3O.sub.4 during the charge-discharge measurement, showing a stable structure of Mn.sub.3O.sub.4 spinel during the cycling.

(9) FIG. 8 is a plot of cyclic voltammetry measurements of Mn.sub.3O.sub.4 nanoparticles with a stable potential window of 2.5 V (from 1.7 V to 0.8 V vs mercury sulfate reference electrode [MSE]) in an aqueous Na.sub.2SO.sub.4 electrolyte at scan rates of 0.1 V/second and 1 V/second.

(10) FIGS. 9A and 9B are plots of chronopotentiometric measurements of Mn.sub.3O.sub.4 nanoparticles with a stable potential window of 2.7 V (from 1.7 V to 1.0 V vs. MSE) in an aqueous Na.sub.2SO.sub.4 electrolyte with charge-discharge current density of 10 A/gram in a half-cell. FIG. 9A presents the 1st to 10th charge-discharge cycles, and FIG. 9B presents the 91st to 100th charge-discharge cycles.

(11) FIG. 10 is a TEM image of the mixed valence oxide herein MnOx.

(12) FIG. 11 is a relatively higher resolution TEM image of the mixed valence oxide MnOx.

(13) FIG. 12A is a TEM image of the mixed valence oxide with cobalt doping comprising Co.sub.0.1Mn.sub.2.9Ox.

(14) FIG. 12B is a TEM image of the mixed valence oxide with nickel doping comprising Ni.sub.0.1Mn.sub.2.9O.sub.x.

(15) FIG. 13 identifies graph (a) a complete and graph (b) pre-edge feature of X-ray absorption near edge structures (XANES) spectra of mixed valence manganese oxide (MnOx) and commercial Mn.sub.3O.sub.4 (with a size large than 200 nm) and MnO.sub.2 bulk at Mn K edge.

(16) FIG. 14 identifies X-ray diffraction diagrams (XRD) of the identified mixed valence oxides MnOx, Co.sub.0.1Mn.sub.2.9Ox and Ni.sub.0.1Mn.sub.2.9O.sub.x.

(17) FIG. 15 illustrates cyclic voltammetries (CVs) of graph (a) MnO.sub.X and graph (b) commercial Mn.sub.3O.sub.4 carried on a three-electrode half-cell in 0.1 M Na.sub.2SO.sub.4 electrolyte at various scan rates using a mercury sulfate reference electrode (MSE). As illustrated, the MnO.sub.X nanoparticles exhibit a stable potential window of 2.5 V, while commercial Mn.sub.3O.sub.4 bulk materials (with a size large than 200 nm) can only be operated under 1.2 V.

(18) FIG. 16 illustrates capacitance vs. scan rates of MnO.sub.X, Ni.sub.0.1Mn.sub.2.9O.sub.x and Co.sub.0.1Mn.sub.2.9O.sub.x from half-cell cyclic voltammetry measurements.

(19) FIG. 17 illustrates in situ (a) XANES and (b) XRD data of MnO.sub.x during the charge-discharge measurement, showing a stable crystalline and electronic structure during the identified cycling.

(20) FIG. 18 illustrates a configuration of a symmetric button-cell. Loading of each electrode includes 4 mg MnOx and 1 mg carbon, and a total of 300 uL of 1M Na.sub.2SO.sub.4 is added. Loading density is around 3 mg/cm.sup.2 for each electrode.

(21) FIG. 19 illustrates galvanostatic cycles (from 4996.sup.th cycle to 5000.sup.th cycle) of the MnO.sub.X button-cell at a current density of 10 A/g with a stable potential window of 3.0 V (from 1.5 V to 1.5 V). Symmetric button-cell with a configuration of MnO.sub.X/Na.sub.2SO.sub.4/MnO.sub.X was applied. Loading includes 4 mg MnO.sub.X, 1 mg carbon, and 300 uL of 1 M Na.sub.2SO.sub.4. The loading density is around 3 mg/cm.sup.2.

(22) FIG. 20 illustrates in graph (a) energy density and (b) power density of the MnO.sub.X button-cell at current density of 10, 20 and 50 A/g for 5,000 charge-discharge cycles.

DETAILED DESCRIPTION

(23) There are now provided details for the preparation of manganese nanoparticles, such as nanoparticles of tri-manganese tetroxide (Mn.sub.3O.sub.4), cobalt-doped tri-manganese tetroxide Co.sub.xMn.sub.3-xO.sub.4 and nickel-doped tri-manganese tetroxide Ni.sub.xMn.sub.3-xO.sub.4 (x: 0.1, 0.5, 0.75).

(24) Compounds of manganese in the +2, +3, +4, +5, +6 and +7 oxidation states are known, but many are unstable in the solid state. The Mn.sup.4+ (Mn.sup.IV) is known to be stable in the solid state, therefore, manganese dioxide (MnO.sub.2) has been used as a main material for energy storage devices for many years. In addition to MnO.sub.2, manganese oxide compounds (Mn.sub.3O.sub.4) are also widely used as electrode materials, especially lithium manganese oxide compounds such as Li.sub.1+XMn.sub.2-XO.sub.4+Y have been used as positive electrode material (cathode) for rechargeable lithium-ion batteries. Typically, these compounds are formed by calcination (thermally heating) of a mixture of a manganese source compound (e.g., manganese carbonate [MnCO.sub.3] and manganese dioxide [MnO.sub.2]) and a lithium source compound at elevated temperatures (between 700 and 900 C.), resulting in the particles size with micrometer size and wide size distribution.

(25) According to a preferred embodiment of the invention, the tri-manganese tetroxide nanoparticles, cobalt-doped manganese oxide nanoparticles, and nickel-doped manganese oxide nanoparticles may be prepared by the following general reaction scheme:

(26) ##STR00001##

(27) In the above equation, A may be any halide such as Cl.sup. or (NO.sub.3).sup., and B or C can be Cl.sup., (NO.sub.3).sup., (SO.sub.4).sup.2 or (C.sub.10H.sub.14O.sub.4).sup.2 (acetylacetonate). The value of x is from 0.1 to 0.75.

(28) As noted, the reaction is preferably carried out in a semi-batch reactor containing Mn and Ni aqueous salt solutions with a controlled addition of an aqueous base solution such as KOH or NaOH. In a semi-batch reactor, the manganese (II) salt, and optionally a cobalt (II) salt or nickel(II) salt, is dissolved in water inside the reactor under vigorous stirring, while an aqueous alkali base solution is slowly injected into the reactor via a syringe pump. The design of semi-batch reactor allows the fine control of the nucleation and growth of the Mn.sub.3O.sub.4, Co.sub.xMn.sub.3-xO.sub.4 or Ni.sub.xMn.sub.3-xO.sub.4 nanoparticles. The resulting precipitates are collected and thermally treated (heating to 200 C. to 400 C.). As noted, the value of x in Co.sub.xMn.sub.3-xO.sub.4 or Ni.sub.xMn.sub.3-xO.sub.4 can range from 0.1 to 0.75 and all or any increments in between. Accordingly, one may form any of the following compounds: Co.sub.0.1Mn.sub.2.9O.sub.4, Co.sub.0.5Mn.sub.2.5O.sub.4, Co.sub.0.75Mn.sub.2.25O.sub.4, Ni.sub.0.1Mn.sub.2.9O.sub.4, Ni.sub.0.5Mn.sub.2.5O.sub.4, and Ni.sub.0.75Mn.sub.2.25O.sub.4.

(29) Preferably, the above is achieved by first mixing a manganese (II) precursor salt as a 50 mM solution, such as MnCl.sub.2, and a cobalt (II) precursor salt or nickel (II) precursor salt in a 10 mM solution, such as Co(NO.sub.3).sub.2 or Ni(NO.sub.3).sub.2, in water at room temperature under an open air environment and relatively vigorous stirring. The reaction volume can be easily scaled from 50 mL to 1000 mL. An alkali base, such as a solution of 250 mM KOH or NaOH (pH=13.4), is added to the metal precursor solution within 30 to 60 minutes at a constant flow rate. The resulting precipitate was permitted to stand for an additional 30 minutes at room temperature with stirring. The resulting product was then centrifuged and washed thoroughly with H.sub.2O, air-dried, then calcination takes place under air by thermally treating at 100 C. for 2 hours. Optionally, other suitable temperatures may be selected between 50 C. and 200 C. The final products are Mn.sub.3O.sub.4, or optionally Co.sub.xMn.sub.3-xO.sub.4, or Ni.sub.xMn.sub.3-xO.sub.4 (where x may be 0.1, 0.5, or 0.75) spinel-type nanoparticles, such as Co.sub.0.3Mn.sub.2.5O.sub.4 or Ni.sub.0.5Mn.sub.2.5O.sub.4, with a purity of 95.0-100%, at a particle size of 10 nm to 30 nm. The particles were preferably found to have an average particle size of 17 nm with a variation of +/7 nm.

(30) The electrode materials are prepared by mixing 60% to 90% weight percent (preferably 80%) of Mn.sub.3O.sub.4, Co.sub.xMn.sub.3-xO.sub.4 or Ni.sub.xMn.sub.3-xO.sub.4 nanoparticles with 10% to 40% weight percent (preferably 20%) of carbon black (commercial carbon black with sizes ranging from 30 nm to 100 nm and a surface area of 75 m.sup.2/gram) and then copious amount of water to form final ink slurry. In a typical preparation, 0.8 gram of Mn.sub.3O.sub.4, Co.sub.xMn.sub.3-xO.sub.4 or Ni.sub.xMn.sub.3-xO.sub.4 nanoparticles and 0.2 gram of acetylene carbon black are mixed with about 3 gram of water, and sonicated for 15 minutes to form the ink slurry.

(31) Suitable manganese (II) precursor salt compounds may include MnCl.sub.2 or Mn(NO.sub.3).sub.2. Suitable cobalt (II) precursor salt compounds may include Co(NO.sub.3).sub.2, CoSO.sub.4, or CoCl.sub.2. Suitable nickel (II) precursor salt compounds may include Ni(NO.sub.3).sub.2, NiSO.sub.4, C.sub.10H.sub.14NiO.sub.4, or NiCl.sub.2. Suitable manganese (II) precursor salt solution concentrations may be selected from the range of 25 mM to 100 mM, and all or any increments in between, such as 50 mM. Suitable cobalt (II) precursor or nickel (II) precursor salt solution concentrations may be selected from the range of 2 mM to 10 mM, and all or any increments in between, such as 5 mM.

(32) Crystalline nanoparticles of Mn.sub.3O.sub.4, Co.sub.xMn.sub.3-xO.sub.4 and Ni.sub.xMn.sub.3-xO.sub.4 (x at a value of 0.1 to 0.75) can now be prepared with relatively high purity (99%-100%) and substantially free of other oxidation states, such as Mn.sub.2O.sub.3 or MnO.sub.2. Substantially free means that other manganese oxidation state impurities (manganese oxides other than Mn.sub.3O.sub.4) may be detected by X-ray diffraction from 1% (weight) and up to a maximum of 5% (weight), and any increments in between at 0.1% difference, such as 1.1% (weight), 1.2% (weight) etc., up to 5.0% (weight).

(33) For the preparation of cobalt-doped, tri-manganese tetroxide nanoparticles the molar ratio of the precursors in the preferred procedure (e.g. MnCl.sub.2:Co(NO.sub.3).sub.2) above may vary from about 29:1 to 3:1; For the preparation of nickel-doped, tri-manganese tetroxide nanoparticles the molar ratio of the precursors in the preferred procedure (e.g. MnCl.sub.2:Ni(NO.sub.3).sub.2) above may vary from about 29:1 to 3:1.

(34) According to an embodiment of the invention, aqueous energy storage devices may include two current collectors (such as carbon paper, copper foil or stainless steel foil), a separator (commercial Whatman cellulose filter paper), and a carbon black/metal oxides composition prepared as described above deposited on one side of each current collector, wherein the metal oxide is a Mn.sub.3O.sub.4, Co.sub.xMn.sub.3-xO.sub.4 or Ni.sub.xMn.sub.3-xO.sub.4 (where x may be 0.1, 0.5, or 0.75) nanoparticle. For a button cell, 3 to 7 mg of the Mn.sub.3O.sub.4/Carbon black, Co.sub.xMn.sub.3-xO.sub.4/Carbon black or Ni.sub.xMn.sub.3-xO.sub.4/Carbon black may be applied as electrode on each current collector. The electrolyte may include the aqueous KCl (from 0.1 M to 4 M), Na.sub.2SO.sub.4 (from 0.1 M to 3 M), and K.sub.2SO.sub.4 (from 0.1 M to 0.5 M). Other suitable electrolytes may be selected form KNO.sub.3, NaCl, and NaNO.sub.3, and more commonly a mixture of KCl and Na.sub.2SO.sub.4.

(35) FIGS. 1 and 3 are TEM images of Mn.sub.3O.sub.4, Co.sub.0.1Mn.sub.2.9O.sub.4 and Ni.sub.0.1Mn.sub.2.9O.sub.4 nanoparticles prepared according to the procedures described above. All the materials show similar morphologies of nanoparticles with diameters ranging from 10 nm to 30 nm. High resolution TEM image shown in FIG. 2 demonstrates that the Mn.sub.3O.sub.4 nanoparticles show a highly crystalline structure with distinct lattice fringes.

(36) FIG. 4 shows X-ray diffraction (XRD) data of Mn.sub.3O.sub.4, Co.sub.0.1Mn.sub.2.9O.sub.4 and Ni.sub.0.1Mn.sub.2.9O.sub.4 nanoparticles. All three materials show very similar XRD patterns. All the diffraction peaks can be perfectly indexed to the tetragonal cell of the Mn.sub.3O.sub.4 spinel-type structure, corresponding to the well-defined data shown in Joint Committee on Powder Diffraction Standards (JCPDS number: 24-0734).

(37) FIG. 5 shows the plots of gravimetric capacitance as a function of scan rates of Mn.sub.3O.sub.4, Co.sub.xMn.sub.3-xO.sub.4 (x: 0.1, 0.5) and Ni.sub.xMn.sub.3-xO.sub.4 (x: 0.1, 0.5, 0.75) nanoparticles. The measurements are conducted using a three-electrode half-cell between 0 V and 0.9 V (vs. Ag/AgCl) in a 0.1M Na.sub.2SO.sub.4 electrolyte. Data show that addition of Co or Ni components into Mn.sub.3O.sub.4 spinel structure increase its capacitance considerably.

(38) FIG. 6 shows the capacitive stability of Mn.sub.3O.sub.4 nanoparticles prepared according to the procedure above at charged voltages of 1.4 and 1.6 V measured via two-electrode button cell. After 10,000 charge-discharge cycles, Mn.sub.3O.sub.4 electrode shows a superior cycle life, with nearly 80% and 72% capacitive retention at 1.4 V and 1.6 V, respectively.

(39) FIG. 7 shows a series of XRD patterns of Mn.sub.3O.sub.4 nanoparticles measured during the cyclic voltammetry measurements from 0 V to 1.2 V at a scan rate of 1 mV/second. This in situ measurement can reflect the changes occurring during the fast charge/discharge processes. The data show that overall crystalline structure of Mn.sub.3O.sub.4 remains stable between potential windows from 0 V to 1.2 V during the charge-discharge processes.

(40) FIG. 8 shows that cyclic voltammetry measurements of Mn.sub.3O.sub.4 nanoparticles, prepared according to the procedure above in paragraph 23, in a 0.1M Na.sub.2SO.sub.4 aqueous electrolyte using a three-electrode half-cell with scan rates of 0.1 V/second and 1 V/second. The data clearly show a stable potential window of 2.5 V (from 1.7 V to 0.8 V vs MSE) without a distinct oxygen evolution reaction at higher potential ranges and hydrogen evolution reaction at lower potential ranges, which has not been reported by far.

(41) Similarly, FIG. 9 shows chronopotentiometric measurements of Mn.sub.3O.sub.4 nanoparticles measured using a three-electrode half-cell in a 0.1M Na.sub.2SO.sub.4 aqueous electrolyte. Data show that when a charge-discharge current density of 10 A/gram was applied, a stable potential window can be reached up to 2.7 V (from 1.7 V to 1.0 V). The data show that the 1st to 10th charge-discharge cycles are very similar to the 91st to 100th charge-discharge cycles, indicating a negligible degradation of Mn.sub.3O.sub.4 nanoparticles electrode during cycling between a 2.7 V potential-window.

(42) Thus, the compositions prepared according to the methods described herein exhibit one or more of the following advantageous properties that makes them good candidates for electrode components: (1) the particle size of the compositions is relatively homogeneous within the range of about 10 nm-30 nm; (2) the compositions are prepared from relatively inexpensive and non-toxic raw materials that are relatively abundant and easy to procure; (3) the nanoparticles are preferably of high purity (95% to 100%) of Mn.sub.3O.sub.4 or Co.sub.xMn.sub.3-xO.sub.4 (x is preferably 0.1, 0.5) or Ni.sub.xMn.sub.3-xO.sub.4 (x is preferably 0.1, 0.5 or 0.75) that makes them particularly suitable for electrodes; (4) the manufacturing process is relatively fast (within few hours), amenable to scale up, conducted under ambient atmosphere and pressure, and relatively low temperature (thermal treatment at 50 C. to 200 C.), and do not require organic solvents to form the nanoparticle crystals; (5) the manufacturing process may be easily converted from batch to continuous production set-up; and (6) the Mn.sub.3O.sub.4, Co.sub.xMn.sub.3-xO.sub.4 (x is preferably 0.1, 0.5) and Ni.sub.xMn.sub.3-xO.sub.4 (x is preferably 0.1, 0.5, 0.75) nanoparticle prepared according to the procedures herein can be used as effective electrode materials for aqueous phase energy storage devices including batteries and electrochemical capacitors. Energy storage devices using such electrode materials are safe and inexpensive, can work at relatively wide voltage window (2.5 V to 2.7 V) with very long cycle life, such as above 10000 cycles.

(43) From the above it may be readily appreciated that the present invention also applies to the preparation of manganese nanoparticles for electrodes comprising mixed-valence manganese oxide of the formula MnO.sub.x, wherein Mn has an oxidation state of 2+, 3+ and 4+ and the value of x is within the range between 1 and 2 (1<x<2). In terms of molar percent, preferably 40% to 60% of Mn has the valence of 4+, preferably 10% to 20% of Mn has the valence 3+ and preferably 20% to 40% of Mn has the valence of 2+. In addition, the present invention also applies to mixed valence cobalt doped MnOx where the manganese and cobalt are present at a valence state of 2+, 3+ and 4+ or to mixed valence nickel doped MnOx where the manganese is present at a valence state of 2+, 3+ and 4+ and nickel is present at a valence state of 2+.

(44) More specifically, one may preferably prepare a Co doped MnOx of the formula Co.sub.yMn.sub.3-yO.sub.x where y has the value of 0.1 or 0.5 and 3<x<6. One may also preferably prepare a Ni doped MnO.sub.x of the formula Ni.sub.yMn.sub.3O.sub.x where y has the value 0.1, 0.5 and 0.75 and 3<x<6. In the Co doped MnOx both Co and Mn are present in a mixed valence state of 2+, 3+ and 4+. In the Ni doped MnOx the Mn is present at a mixed valence state of 2+, 3+ and 4+ and the Ni is present at a valence state of 2+.

(45) Preferably, the mixed valence oxide MnOx nanoparticles, the Co doped MnOx nanoparticles (CoMnOx) and/or the Ni doped MnOx nanoparticles (NiMnOx) are prepared according the following general reaction scheme in a semi-batch reactor:

(46) ##STR00002##

(47) In the above equation, A may be any halide such as Cl.sup. or (NO.sub.3).sup.. The alkali base may preferably be NaOH but may include any other suitable alkali, such as KOH. The resulting product is collected via centrifuge, air-dried, followed by a mild temperature calcination (thermal treatment in air or oxygen) at temperatures of 200 C. to 400 C. (225-275 C. is preferred). In the above x is within the range between 1 and 2 (1<x<2), y is within the range between 2 and 4 (2<y<4), and z is within the range between 2 and 3 (2<z<3). In terms of molar percent, of Co-doped MnOx preferably 40% to 60% of Co has the valence of 4+, preferably 10% to 20% of Co has the valence 3+ and preferably 20% to 40% of Co has the valence of 2+; of Ni-doped MnOx, Ni always has the valence of 2+.

(48) In the semi-batch reactor, which is reference to a reactor that allows for reactant addition or product removal over time, the alkali base solution such as NaOH is preferably injected into the reactor containing the identified Mn (and/or Ni and Co) precursors in a relatively slow and controlled injection rate. More specifically, the alkali base solution is preferably injected at an injection rate from 0.1 mL/min to 100 mL/min with respect to a preferred reactor size of 50 ml to 50 liters. Thus, nucleation of Mn(OH).sub.2 is preferably controlled by the injection rate of the alkali base solution and as a consequence. It is noted that under the preferred conditions of normal atmospheric conditions and room temperature (e.g. 20 C. to 30 C.), dissolved oxygen gas in the water present (with a preferred saturation concentration of 0.27 mM) will oxide the Mn(OH).sub.2 and or Ni(OH).sub.2 and Ni(OH).sub.2 into mixed valence oxides with valences of 2+, 3+ and 4+. The oxide product is collected via centrifuge, air-dried, followed by a mild temperature calcination (thermal treatment in air or oxygen) at temperatures of 200 C. to 400 C. (225-275 C. is preferred). The final products can be readily prepared with an average nanoparticle size of 10 nm to 50 nm. The particles herein, which may therefore include the mixed valence oxide MnOx or the corresponding doped mixed valence oxide CoMnOx and/or NiMnOx, preferably have an average particle size of 20 nm with a variation of +/7 nm. Such size recitation is reference to particle diameter.

(49) In connection with the above, reference is now made to FIG. 10 which shows a TEM image of the mixed valence oxide MnOx, FIG. 11 providing a relatively higher resolution view of such oxide. FIG. 12A is a TEM image of the mixed valence oxide with cobalt doping comprising Co.sub.0.1Mn.sub.2.9Ox. FIG. 12B is a TEM image of the mixed valence oxide with nickel doping comprising Ni.sub.0.1Mn.sub.2.9O.sub.x.

(50) FIG. 13 shows that mixed valence manganese oxides prepared herein with normalized absorbance as indicated in which Mn valence positions between commercial Mn.sub.3O.sub.4 bulk materials (with a diameter larger than 200 nm) and MnO.sub.2 bulk materials (with a diameter larger than 1000 nm). This is considered to therefore confirm that Mn in the recited MnOx herein has a valence of 2+, 3+ and 4+. FIG. 15 provides evidence that the mixed valence oxide MnOx herein can be operated within a potential window of 2.5 V in a half-cell, while commercial Mn.sub.3O.sub.4 bulk materials can only be prepared with a potential window of 1.2 V in the half-cell.

(51) Electrode materials of the mixed valence oxides of MnOx [Mn: 2+, 3+, 4+], CoMnOx [Mn/Co: 2+, 3+, 4+] and/or NiMnOx [Mn: 2+, 3+, 4+; Ni: 2+] can be conveniently prepared by mixing 60% to 90% weight percent (preferably 80%) of MnOx, CoMnOx or NiMnOx nanoparticles with 10% to 40% weight percent (preferably 15%-25% by weight) of carbon black (commercial carbon black with sizes ranging from 30 nm to 100 nm and a surface area of 75 m.sup.2/gram) and then copious amount of water to form final ink slurry. In a typical preparation, 0.8 gram of MnOx, CoMnOx or NiMnOx nanoparticles and 0.2 gram of acetylene carbon black are mixed with about 3 gram of water, and sonicated for 15 minutes to form the ink slurry.

(52) Similar to the above, an aqueous energy storage device can be prepared with the mixed valence oxides of MnOx, CoMnOx and/or NiMnOx, which again involves the aforementioned two current collectors and a separator. A carbon black/metal oxide composition is prepared as described above and deposited on one side of each current collector, where the metal oxide is now, as noted, mixed valence oxides of MnOx, CoMnOx and/or NiMnOx. For a symmetric button cell (in which the anode and cathode are identical), as shown in FIG. 18, such may preferable include 4 mg of MnOx and 1 mg carbon and 300 uL of Na.sub.2SO.sub.4 is added as the electrolyte solution. FIG. 19 identifies the galvanostatic cycles showing that a stable potential window of 3.0 V in which the potential of the button-cell changes from 1.5V to 1.5V with respective to time while keeping a constant output current density of 10 A/g.

(53) FIG. 20, identifies in graph (a) the energy density and in graph (b) the power density of the mixed valence oxide herein MnOx button cell at current density of 10, 20 and 50 A/g for 5,000 charge/discharge cycles. Accordingly, a relatively long cycle life is observed for the mixed valence oxides herein and it is contemplated that the mixed valence oxides herein of MnOx and/or CoMnOx and/or NiMnOx will be on the order of up to and including 10,000 charge/discharge cycles, and higher, such as up to 20,000 cycles.

(54) As can be appreciated from the above, the present disclosure reveals an improvement to the potential window of electrochemical energy storage systems in the present of an aqueous electrolyte, which is an important feature to designing new electrochemical energy storage devices to provide high energy and power densities, good reliability and relatively low cost compared to existing lithium ion battery use. In the present invention, among other things, a new synthesis is identified for a mixed valence manganese oxide (and cobalt- and nickel-doped mixed valence manganese oxide) nanoparticles that indicated relatively high overpotential (larger than 0.6 V) to hydrogen and oxygen evolution reactions in the presence of an electrolyte. Beyond the thermodynamic potential window of water (1.23V), the mixed valence manganese oxide nanoparticles demonstrated potential windows of 2.5V in the half-cell and 3.0V in a symmetric button cell. This demonstrates excellent energy density, power density and columbic efficiency after upwards of about 20,000 charge-discharge cycles.

(55) While the principles of the invention have been described herein, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation as to the scope of the invention. Other embodiments are contemplated within the scope of the present invention in addition to the exemplary embodiments shown and described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.