WO2019094907A2

WO2019094907A2 - PREPARATION OF: I. INTERCALATIVE METAL OXIDE/CONDUCTIVE POLYMER COMPOSITES AS ELECTRODE MATERIALS FOR RECHARGEABLE BATTERS; II. SODIUM RICH MANGANESE OXIDE HYDRATE WITH CAPACITY FOR AQUEOUS Na-ion ELECTROCHEMICAL ENERGY STORAGE

Info

Publication number: WO2019094907A2
Application number: PCT/US2018/060677
Authority: WO
Inventors: Teng XIAOWEI
Original assignee: The University Of New Hampshire
Priority date: 2017-11-13
Filing date: 2018-11-13
Publication date: 2019-05-16
Also published as: WO2019094907A3

Abstract

The present invention is directed at intercalative metal oxide/conductive polymer composites suitable for use as electrode materials for rechargeable batteries. The composites can be prepared by agitation of the metal oxide and the conductive polymer in aqueous media. The present invention is also directed at a sodium rich layered manganese oxide hydrate prepared by annealing manganese (II, III) oxide and sodium hydroxide. The sodium rich manganese (III, IV) oxide so formed indicates an enhanced capacity for Na-ion storage suitable for the use of electrode materials for aqueous energy storage.

Description

Preparation of: I. Intercalative Metal Oxide/Conductive Polymer Composites as Electrode Materials for Rechargeable Batteries; II. Sodium Rich Manganese Oxide Hydrate With Capacity For Aqueous Na-ion Electrochemical Energy Storage

Government Rights Clause

This invention was made with government support under Prime Contract No. DE- SC0010286 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

Field

Background

The preparation of effective electrode materials especially cathode materials for rechargeable energy storage devices including supercapacitors and batteries have attracted increased attention. Many metal oxide, for example vanadium pentoxide, manganese oxide, cobalt oxide are the most promising layered electrode materials for rechargeable lithium/sodium/potassium batteries because of its high-energy storage capacities. However, the capacity retention of these electrode materials upon cycling and power performance (time required to charge and discharge the devices) are unsatisfactory, partially due to their poor electrical conductivity. Several approaches have been developed on the preparation of new electrode materials to ameliorate these limitations, including simply mixing electrode material with carbon black or coating the electrode material with more conductive layers. In particular, the preparation of metal oxide/conductive polymer composite is a very rapidly developing area of electrode materials development. Compared with simple mixtures of metal oxide and conducting polymer, preparation of intercalative metal oxide/conducting polymers is a very promising approach to making electrode materials, where conductive polymer is inserted between metal oxide layers, so that semiconductive metal oxide materials are in close proximity to conductive polymer. However, typical preparation routines for intercalative composite materials include in situ polymerization, or require assistance of heat or even microwave radiation, and therefore involve relatively long processing time, arduous preparation procedures and have difficulty in scale-up production. It is, therefore, necessary to develop simpler and more energy-efficient methods of synthesis method in this regard.

In related context, developing electrochemical energy storage (EES) technologies using safe and earth-abundant materials becomes increasingly attractive for economically storing the electric power generated by solar and wind. Aqueous EES devices using Na-ions as charge carriers have been promising alternatives to non-aqueous lithium-ion batteries (LIBs) for its low cost, high safety and availability of Na sources in terrestrial reserves. However, the storage of Na-ions is challenging because of its relatively large ionic radius, so that LIB host materials (especially cathode) usually with a close-packed array of oxide ion may not be able to accommodate the Na-ion for reversible insertion and extraction.

Two design principles have been used to tackle the intercalation of Na-ion. One is the replacement of oxygen anions (0^2~) with anions that have weaker bonding with metal cations, so that cations are sufficiently mobile in the electrode. Recent studies show promise of hexacyano ions (C≡N)₆ ^6~ based electrode materials for Na- and K-ions storage due to their weakened bonding between cyanide (C≡N)^~ and cations. Some reports indicate that potassium copper hexacyanoferrate and its analogues can function as stable electrode materials for aqueous K- and Na-ion storage. Sodium manganese hexacyanoferrate has been reported by to show relatively good energy performance and cycling life in a non-aqueous electrolyte.

Another approach to design a Na-ion electrode is to introduce a relatively large interstitial host framework. These materials with planar or zigzag layers show different polymorphs (P2, P3 or 03 symmetry) with respect to the sites of the intercalated alkali ions by altering the stacking of transition metal-oxygen octahedra ([ΜΟβ]). However, the mechanistic understanding of storage of Na-ion inside various host materials is still far from settled. This is largely due to the different intercalation chemistry of Na-ions from that of Li- ions so that the fundamental understanding obtained from Li-ion storage may not be directly applied to Na-ion.

Recently, efforts have been devoted to the studies of the storage mechanisms of various alkali ions. For example, 03-type layered LiMnC (ABC oxygen stacking) suffered from degradation to spinel structure and thus the impaired capacity for Li-storage due to the migration of Mn ions during the cycling. In contrast, the NaMnC counterpart had a high energy barrier for Mn ion migration, which prevented cation mixing and thus sustained the layered structure during the Na-ion intercalation/deintercalation. Compared with 03-type NaMnC , birnessite δ-Μηθ2 also has a layered structure containing two-dimensional sheets of edge-shared Μηθβ octahedra with a general formula of AxMnC -EbO (A: H⁺, Li⁺, Na⁺, or K⁺; X: usually less than 0.2). ^{1 5} The studies of birnessite electrode in aqueous electrolyte have been reported. However, even though the birnessite has a rather large interlayer distance (~ 7 A), the storage capacity was typically low for aqueous Na-ion storage (< 60 mA h g^"1) due to the limited potential window (~ 1.2 V) and ineffective redox process. Much less work on improving the aqueous Na-ion storage capacity in birnessite has been reported to date.

Brief Description of the Drawings

FIG. 1 shows photos of bulk V2O5 in water, simple mixture of V2O5, PEDOT:PSS and water, as well as intercalative V205/PEDOT:PSS composite. The color change reflects structural differences of intercalative V20₅/PEDOT:PSS composite compared with V2O5 or simple mixture of V2O5 and PEDOT:PSS.

FIG. 2 shows XRD patterns of commercial V2O5 (the precursor for making intercalative V20₅/PEDOT:PSS composite) and intercalative V20₅/PEDOT:PSS composite.

FIG. 3 shows SEM and TEM images of commercial V2O5 (the precursor for making intercalative V₂05/PEDOT:PSS composite) and intercalative V₂05/PEDOT:PSS composite. The latter shows distinct morphological changes from bulk particles to layered structure.

FIG. 4 shows neutron pair distribution function (PDF) analysis of commercial V2O5 (the precursor for making intercalative V20₅/PEDOT:PSS composite), and intercalative V20₅/PEDOT:PSS composite. The latter shows distinctly decreased coherent length, corresponding well to the formation of layered structure.

FIG. 5A shows electrochemical tests of intercalative V20₅/PEDOT:PSS composite at the scan rate ranging from 10 mV/sec to 1000 mV/sec in an aqueous KC1 electrolyte. FIG. 5B shows electrochemical tests of bulk V2O5, simple mixture of V2O5 and PEDOT:PSS, as well as intercalative V₂0₅/PEDOT:PSS composite. The latter showed enhanced capacity (more than three times).

FIG. 6A shows TEM images of LiMnC bulk material.

FIG. 6B shows intercalative LiMn0₂/PEDOT:PSS nanosheet.

FIG. 7 shows cyclic voltammetry measurements of bulk LiMnC materials.

FIG. 8 shows cyclic voltammetry measurements of intercalative LiMn0₂/PEDOT:PSS nanosheet.

FIG. 9 shows a comparison of specific capacity toward K-ion electrochemical storage of intercalative LiMnCVPEDOTiPSS nanosheets and bulk LiMnC materials where intercalative nanosheets show at least three times higher capacity.

FIG. 10 shows EDS characterizations of NasMnOx materials with (a) MnsOs obtained by directly heating Mn30₄ nanoparticles, (b)Nao.i3MnOi.74-H20, (c) Nao.25MnOi.84-H20, and (d) Nao.29Mn02-H20 obtained by the thermal solid-state reactions of NaOH and M¾0₄ as the molar ratios of 0.5:1, 1:1, 2:1, respectively.

FIG. 11 shows (a) EDS characterization of Na₍8)MnO_x-H₂0 (Nao.i3MnOi.74-H₂0; Nao.25MnOi.84-H20 and Nao^MnC -EhO) materials obtained by the thermal solid-state reaction of NaOH and M¾0₄ as a molar ratio of 4:1; (b) the summaried atomic ratios of sodium to manganese for various Na5MnO_x-H20 materials; and (c) XRD patterns of Nao.29Mn02-H20 materials syntheized via solid state reaction of NaOH:Mn30₄ as molar ratios of 2: 1 and 4:1, indicating the forming of a stable Na₀.29MnO2 material even with an increased amount of NaOH precursor.

FIG. 12 shows EDS characterization of Na5Mn02-¾0 birnessite synthesized via the wet chemistry method showing the Na/Mn atomic is 0.17:1 with the comparision to that of other works in Table 1.

FIG. 13 shows (a) TEM image of of Nao.29MnO2.H2O materials; (b) Experimental neutron PDFs of Na5MnOx-H20 materials, where coherent lengths of the materials decreased as the Na concentration increased;(c) Phase percentage of Nao.29Mn02-H20 in Na5MnOx.H20 materials obtained from neutron PDF analysis. FIG. 14 shows TEM characterizations of

materials with (a) MnsOs,

(b)Nao.i3MnOi.74-H₂0, (c) Nao.25MnOi.84-H₂0. FIG. 15 shows neutron PDF analysis of (a) Mn₅Os, (b) Nao.13MnO1.74.H2O, (c) Nao.25MnO1.84.H2O and (d) Nao.29Mn02-H20 materials.

FIG. 16 shows XRD patterns of NasMnOx materials comapred with MnsOs and Mn02 standards indicating the structural evolution from MnsOs to Mn02 as sodium concentrations increased, and the Rietveld refinement analyese of XRD pattern of

FIG. 17 shows neutron PDFs of NasMnOx materials normalized by the intensity of the peak at 1.9 A associated with Mn-0 pair. The atom pair associated with each peak (PI to P7) can be attributed to (a) water, (b, d, e, g, i) MnsOs polyhedra in black and (c, f, h) Mn02 polyhedra in blue.

FIG. 18 shows the proposed formation mechanism for Nao.2₉Mn02-H20 driven by solid-state Na-ion intercalation in (Mn²⁺: green; Mn³⁺: orange; Mn⁴⁺: purple; Na⁺: brown; O: red).

FIG. 19 shows XRD characterizations of Nao.2₉MnO_x materials with the thermal solid-state reacted Nao.29Mn02 and washed Nao.29Mn02 (the diffraction peaks from M¾04 are indexed in black and the black solid dot from NaOH, the resulting heated anhydrous Nao.29Mn02 and washed Nao.29Mn02-H20 are labelled as well).

FIG. 20 shows electrochemcial CV meausrements in half-cells. Cyclic Voltammetry (CV) scans of (a) Mn₅0₈, (b) Nao.i3MnOi,7₄-H₂0, (c) Na₀.25MnOi.84-H₂O, and (d)

Nao.29Mn02-H₂0 between -1.25 V to 1.25 V (vs Ag/AgCl) in 0.1 M Na₂S04 electrolyte at the scan rates from 5 to 1000 mV s ¹.

FIG. 21 shows electrochemical half-cell measurements with (a) Cyclic Voltammetry (CV) scans of Nao.29MnO2.H2O material between -1.25 V to 1.25 V (vs Ag/AgCl) at various scan rates in 0.1 M Na2S04 electrolyte; (b) calculated specific charge storage capacities of sodium-manganese oxides as a function of scan rates; Symmetric full-cell measurements with (c) discharge electrode capacities of Nao.29MnO2.H2O material at the various current densities of 1 A g^_1, 2 A g^_1, 5 A g^_1 and 10 A g^_1 (after 5000 galvonstatic charge and discharge process unless specified otherwise); (d) electrode capacites of Nao.2₉Mn02-H20 as a function of cycle number up to 5000 at the current densities from 1 A g^_1 to 10 A g^_1; (e) coulombic and energy efficiencies of Nao.2₉Mn02-H20 at various current densities as a function of current density (1, 2, 5 and 10 A g^_1); (f) Ragone plot with gravimetric specific energy and power of the symmetric Nao.29MnO2.H2O full-cell after 5000 galvanostatic cycles. The aqueous (empty symbols) and non-aqueous (solid symbols) devices are reported, and the gravimetric specific energy and power are calculated by the mass of electrode materials except the Panasonic (17500) Li-ion batteries.

FIG. 22 shows CVs of sodium-manganese oxides at the scan rate of 50 mV s^_1, showing the anodic peak of Nao.29Mn02-H20 shifted to a lower potential and the cathodic peak shifed to a higher potential compared with those of other materials as the Na concentration increased. FIG. 23 shows diffusivity measurements of NasMnOx materials with (a) MnsOs, (b)

Nao.i3MnOi.74-H₂0, (c) Na₀.25MnOi.84-H₂O, (d) Na₀.2₉MnO2-H₂O, (e) the relaxation steps of NasMnOx materials and (f) (Eo-E) as a function of (l/t^1/2) curves for the slope calculations, where Eo is the open circuit voltage.

FIG. 24 shows electrode capacities of Nao.29MnO2.H2O as a function of voltage at the current denties of (a) 1 A g ¹, (b) 2 A g ¹, (c) 5 A g ¹ and (d)10 A g ¹ during galvanostatic charge and dischrage process.

FIG. 25 shows (a) CV scans of disorder Nao.29Mn02-H20, high temperature treated Nao.2₉Mn02-H20 and commercial Mn02 bulk materials at the scan rate of 5 mV s ¹ in a 2.5 V potential window in half-cell; (b) calculated Tafel slopes at the scan rate of 5 mV s ¹ ; (c) Specific capacities at the scan rate of 5, 10 and 50 mV s ¹; (d) Oxygen K-edge sXAS of electrochemically cycled Nao.29Mn02, MnsOs and anhydrous commercial Mn02 bulk materials.

FIG. 26 shows CV scans of (a) disordered Nao.29MnO2.H2O, (b) high temperature treated Nao.2₉Mn02-H20 and (c) commercial Mn02 bulk materials between -1.25 V to 1.25 V (vs Ag/AgCl) in 0.1 M Na₂S0₄ electrolyte at the scan rates of 5, 10 and 50 mV s ¹ ; Tafel plots of (d, e, f) HER and (g, h, i) OER for disordered Nao.2₉Mn02, high temperature treated Nao.2₉Mn02-H20 and commercial Mn02 bulk at scan rates of 5, 10 and 50 mV s ¹, and the summaried Tafel slopes of (j) HER and (k) OER. Detailed Description The present invention is directed at preparation of intercalative (layered) metal oxide/conductive polymer composites as electrode materials for rechargeable batteries. Preferred metal oxides include those oxides that can form a two-dimensional plane with relatively strong in-plane chemical bonding with a dissociation energy of 4 to 7 electron volts (eV) (the average vanadium-oxygen ionic bonding is around 6.7 eV and manganese-oxygen ionic bonding is around 4.2 eV) and relatively weak Van der Waals bonding between planes with a dissociation energy of about 0.01 eV. Preferred examples include V2O5 and LiMnC . It is contemplated that other suitable oxides may include T1O2, M0O2, M0O3, Nb₂0₅ and L1C0O2. The conductive polymer herein is preferably a positively charged polymeric ionomer in combination with a negatively charged polymeric ionomer. Reference to ionomer herein is to be understood as a charged polymer stabilized by ionic cross-links.

One particularly preferred conductive polymer includes as the positive charged ionomer poly(3,4-ethylene dioxythiophene) (PEDOT) in combination with the negatively charged ionomer poly(styrenesulfonate) (PSS). The conductive polymer may therefore be conveniently identified as PEDOT:PSS. In PEDOT:PSS, part of the sulfonyl groups are deprotonated and carry a negative charge. The PEDOT is a conjugated polymer and carries positive charges based upon polythiophene.

The intercalative structure (layering of the oxide and conductive polymer) is preferably achieved herein by agitation (e.g. stirring) of the metal oxide and the conductive polymer (i.e. positively charged polymeric ionomer in combination with negatively charged polymeric ionomer) in aqueous media. For example, the intercalative structure herein was observed to form when agitating the metal oxide with the PEDOT:PSS in water for an extended period of time, such as for 100 to 200 hours. Preferably, the metal oxide to conductive polymer weight ratio is in the range of 1 : 1 to 8 : 1 , more preferably 3 : 1 to 5 : 1. One particularly preferred weight ratio of metal oxide to conductive polymer is 4: 1.

The above procedure is a relatively scalable synthetic routine and is preferably carried on at room temperature without heat or radiation. The resulting nanocomposites have been characterized by powder X-ray diffraction, Raman spectroscopy and transmission electronic microscope analyses. The thickness of the layered structure is contemplated to fall in the range of 1-5 nm up to 30 nm.

Furthermore the application potential of the nanocomposites herein have been tested in an aqueous sodium batteries test, which display some synergistic effects between the metal oxides (V2O5, NaMnC ) and the intercalative conductive polymer (PEDOT:PSS). The results showed that intercalative metal oxide/conductive polymer composites show 100% to 400% enhanced capacity, as well as much improved power performance.

The preparation of intercalative V205/PEDOT:PSS nanocomposite was as noted preferably conducted in aqueous solution at room temperature using the conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) [PEDOT:PSS]. A mixture of 100 mg of commercial V205 bulk material and 25 mg of PEDOT:PSS were submerged in 6 mL of deionized (DI) water in a scintillation vial. The mixture was vigorously stirred at approximately 500 rpm for one week, accompanied with a noticeable color change (FIG. 1).

The dispersion of the commercially available V2O5 powder in DI water is light yellow in color. The conductive polymer PEDOT:PSS, a light blue color, is added to the V2O5 giving the mixture a slightly green tinge. Over the one week mixing period, a color change from the green-yellow color of the mixture to a dark green color occurs. The oxidation state of the V⁵⁺ likely reduced to a V^4+/5+ mixture due to the interaction with PEDOT:PSS.

Through XRD a significant change can be clearly seen that the crystal structure and morphology is affected by the interaction with the PEDOT:PSS (FIG. 2). The V₂0₅ is initially highly crystalline having relatively strong Bragg features, orthorhombic cell with Pmmn symmetry. However after mixing in DI water with the PEDOT:PSS for two weeks a layered structure is formed. The PEDOT:PSS appears to insert into the interlayer gap of the bulk V2O5 materials having a weak Van der Waals force between layers, which finally facilitates the formation of the intercalative V20₅/PEDOT:PSS nanosheets through a exfoliation process. The diffraction pattern of the exfoliated layered structure produces only a few small Bragg peaks in the XRD pattern. The d-spacing of the layered peak dooi=13.67 A (3.05° 2Θ).

SEM and TEM images show that the V2O5 before exfoliation appears as particles and after exfoliation the V205-PEDOT:PSS appear as ribbon like strands appearing to pull away from each other and the bulk particles (FIG. 3). Neutron pair distribution function (PDF) analysis was chosen to elucidate the interaction between the V2O5 and PEDOT:PSS due to the lack of Bragg features in the diffraction pattern (FIG. 4). The exfoliated layer phase was identified as a bi-layered V2O5 structure, having a coherence length of 15 A, a significant decrease as compare to the bulk phase. There was still a detectable bulk phase in the PDF, 8% by mass. A simplified model currently has atoms with the correct ratio of elements for the PEDOT:PSS in the interlayer spacing but not in the correct conformation. The low r-space data is dominated by water and the PEDOT:PSS. The first strong negative peak at ~ 1.01 A is due to both the O-H interaction water (0.96 A) and the interaction of the polymer C-H (1.1 A). The strong positive peak at -1.45 A is the C-C interaction from the conductive polymer and a small portion is due to the H-H correlation from water. The second negative peak at 2.09 A is C-H interaction but a carbon atom interacting with a hydrogen atom bonded to a nearest neighbor carbon atom.

The intercalative V20₅-PEDOT:PSS nanocomposite showed an increase in capacitance as compared to a mixture of the V2O5 and PEDOT:PSS (FIG. 5B). The capacity of 160 niAh/g at a scan rate of 10 mV/s is among the best reported K-ion electrochemical storage using aqueous electrolytes. As shown in FIG. 5B, over the scan rate of 10 mV/s the intercalative V20₅-PEDOT:PSS nanocomposite indicates a capacity of greater than 75 mAh/g, or in the range of 75 MAh/g to 160 mAh/g. The redox features remain visible though all scans rates tested from 10 - 1000 mV/s, some shoulders start to appear at higher scan rates. FIGS. 6-9 show the corresponding evaluations and comparisons of bulk LiMnC and intercalative LiMnCVPEDOTiPSS nanosheets material prepared by the procedure herein of agitation of LiMnC in water in the present of PEDOT:PSS. Once again, as shown in FIG. 9, the intercalative LiMnCVPEDOTiPSS nanosheets indicate at least three times higher electrochemical storage capacity as compared to bulk and non-layered LiMnC . More specifically, as show in FIG. 9, the intercalative LiMnC /PEDOTiPSS indicates the following: (1) at a scan rate of 10-20 mV/sec, a specific capacity of greater than or equal to 60 mAh/g or in the range of 60 mAh/g to 70 mAh/g; (2) at a scan rate of 10-500 mV/sec, a specific capacity in the range of 20 mAh/g to 70 mAh/g.

Turning next to the sodium-rich manganese oxide hydrates with capacity for aqueous sodium ion electrochemical energy storage, the following is noted. Preferably, Na-rich Μηθ2-Η2θ suitable for use for aqueous Na-ion storage can be made in the solid state, preferably by annealing the mixture of Mn30₄ and NaOH, involving conversion from Mn30₄ spinel to an ordered MnsOs layered structure and finally to Na-rich MnC -EhO driven by Na-ion insertion. The Na-rich manganese oxide hydrate herein is represented by the formula Na(8)MnOx-H20 wherein δ has a value greater than 0.17, or more preferably, in the range of > 0.17 to 0.29; and x has a value in the range of 1.74 to 2.0. The reaction was confirmed by neutron total scattering measurements and pair distribution function (PDF) analysis. Storage capacity up to about 150 mA h g ¹ is observed through increase of the potential window and promotion of the redox charge transfer process towards the aqueous Na-ion storage. It should be noted that the Mn30₄ precursor is a manganese (II, III) oxide, where the valences of the Mn element are 2+ and 3+. In addition, the

is a manganese (III, IV) oxide which includes Mn⁴⁺ and Mn³⁺.

The resulting Nao^MnC -EhO material exhibits a relatively high overpotential (~ 0.6 V) towards oxygen and hydrogen evolution reactions and therefor enables a kinetically stable potential window of 2.5 V in the half-cell in an aqueous electrolyte without gas evolution. Moreover, the Na-rich structure improves diffusion- limited redox charge storage encouraging up to a 0.41 electron transfer reaction. Overall, the resulting Nao^MnC -EhO demonstrates a reversible capacity of about 130 to 160 mA h g^_1 (a scan rate of 5 mV s^_1 in the half -cell) in aqueous Na-ion storage, a high energy density of 20 to 30 Wh kg^-1 (a rate of 23 °C in a full-cell), and a relatively good cycling life (70 to 100 mAh g^_1 after 5000 cycles at an electric current rate of 1 A g ^_1 in a full-cell). Different from wet chemistry methods involving the oxidation of Mn²⁺ or reduction of permanganate at room temperature,

materials (Nao.i3MnOi.74-H20; Nao.25MnOi.84-H20 and Nao^MnC -EhO) were preferably prepared at 270 °C in the air via a solid-state reaction between NaOH and M¾0₄ nanoparticles, followed by water rinsing. The temperature range for the solid state reaction may fall in the range of 200 °C to 400 °C, more preferably 250 °C to 300 °C. The Mm0₄ nanoparticles may have a size range of 10 to 30 nm. By altering the molar ratios between NaOH and Mm0₄ from 0 to 2, various sodium manganese oxides (Nao.i3MnOi.74-H20; Nao.25MnOi.84-H20 and Nao.29Mn02-H20) were prepared, verified by energy dispersive X-ray spectroscopy (EDS) measurement (FIG. 10). Atomic ratios of Na/Mn remained the maximum value of 0.29 when the ratio of NaOH: M¾0₄ increased to 4: 1 (FIG. 11). Such a high Na/Mn ratio of 0.29 can only be achieved via the solid-state annealing, while birnessite made via a wet chemistry approach has a Na/Mn ratio of 0.17 (FIG. 12, Table 1). The morphology of Na(8)MnO_x-H20 materials evolved from faceted nanoparticles, to a mixture of layers and particles, and finally to a complete layered structure with a planar dimension up to 200 nm, when the Na concentration (δ) increased from 0 to 0.29 (FIG. 22a & 23).

FIG. 13b shows the PDF data for various Na(8)MnOx-H20 materials (Nao.13MnO1.74·- H2O; Nao.25MnOi.84-H20 and Nao^MnC -EhO) obtained from neutron total scattering measurements. The lattice parameters obtained after refinement were shown in FIG. 15 and Tables 2-5. Unlike Rietveld refinement that only analyzes the Bragg scattering (FIG. 16), the PDF analyzes both Bragg and diffusive scattering and provided details on crystalline structure and the structural deviations from perfect crystallinity such as defects, mismatch or disorder of the materials in atomic scale. The peak of atomic pair in PDF vanished at a distance longer than the longest interatomic distance of the materials (coherent length), which decreased from > 50 A to ~ 30 A as δ increases from 0 to 0.29 (FIG. 13b). The results demonstrated that crystalline order in NasMnOx becomes more confined as δ increased. Namely, NasMnOx- H2O with high concentration of sodium cannot sustain long-range crystallinity and became disordered. Reference to long-range crystallinity is reference to order of greater than 5.0 nm (i.e. the position of the atoms repeat in lattice space in a regular array). Accordingly, in the context of the present disclosure, disordered

may be understood as having crystalline order only up to 3.0 nm and ordered is to be understood as having crystalline order greater than 5 nm. See FIG. 13b. FIG. 13c shows that as Na concentration (δ) changed,

(Nao.i3MnOi.74-H20; Nao.25MnOi.84-H20 and Nao^MnC -EhO) showed pure phase MnsOs (δ = 0), mixture of Mn₅0₈ and layered Μη0₂ (δ = 0.13 and 0.25). When δ reached 0.29, a pure triclinic birnessite structure formed with a chemical formula of Nao.29(8)Mn02-H20, where the Na cations and water molecules occupied the interlay er regions of edge-sharing [Μηθβ] octahedra with an interplanar distance of 7.14 A.

FIG. 17 shows the PDF in the range from 0.8 to 4.3 A, revealing a phase transition from MnsOs, to a mixture of MnsOs and MnC birnessite and finally to MnC birnessite as δ increased. The peaks of PDF can be indexed as O-H pair at 0.95 A (PI) from water (a), Mn- O pairs around 1.9 A (P2) from the [Μηθβ] octahedral unit and 2.2 A (P3) from Mn atoms in prismatic sites relative to O, Mn-Mn or 0-0 pair around 2.8 A (P4), and Mn-0 pair around 3.5 A (P5) from the nearest neighbors of [Μηθβ] octahedral units. It is notable that O-H pair (PI) and Mn-0 pairs (P2, P3, P5 and P7) showed negative peaks due to negative coherent neutron scattering lengths of H and Mn atoms (-3.74 femtometer and -3.73 femtometer, respectively). The Mn-0 pair around 1.9 A (P2) is attributed to MnsOs (b) and layered Nao.29Mn02 (c), respectively. The Mn-0 pair around 2.2 A (P3) is attributed to Mn(II)-0 from MnsOs phase (d), which decreased relatively to Mn(IV)-0 pair at P2 as δ increased, congruent with the decreasing phase fractions of MnsOs. The positive peaks at 2.8 A (P4) is attributed to Mn-Mn or 0-0 bonding from adjacent [Μηθβ] octahedral units in MnsOs (<?) and Nao.29Mn02 if) phases. Therefore, as δ increased the intensity of the PDF peaks at 2.8 A of each material did not change significantly relatively to Mn(IV)-0 pair. Similar trends can be found in Mn-0 pair at 3.5 A (P5) from adjacent [Μηθβ] in MnsOs (g) and Na₀.29MnO2 phases (h). The peaks at ~ 4.0 A (P6 and P7) showed a rather interesting transition from positive to negative direction as δ increased. The positive peak at 3.96 A (P6) related to O- O pair (?) in MnsOs either within the same [Mn(IV)0₆] octahedral unit or [Mn(II)-0] units where Mn²⁺ located in the trigonal prismatic site. In contrast, negative peak at 4.0 A (P7) is attributed to Mn-Na pair at 4.11A (h) from the interaction between Na-ion at the interlayer and Mn⁴⁺ from [Μηθβ] octahedral unit or Mn-0 pair at 3.73 A (h) from the interaction between ¾0 at the interlayer and Mn⁴⁺, both from Nao.29Mn02 layered phase. The interplay, between negative peaks of Mn-Na and Mn-Ow pairs (Ow from interlayer H2O) in Nao.29Mn02 phase and positive peak of 0-0 pair from MnsOs phase at around 4.0 A, explained the overall peak changed from positive to negative direction when δ increased, again reflecting the phase transition from MnsOs to Nao.29Mn02 birnessite driven by the Na-ion insertion during the solid-state annealing.

Based on above analysis, a formation mechanism of Nao.29Mn02-H20 birnessite is proposed in FIG. 18, where Mn30₄ was converted into MnsOs through oxidation of [Mn(III)06] octahedra of Μ¾0₄ into [Mn(IV)0₆] units, followed by Na-ion driven conversion from MnsOs to Nao.29Mn02-H20 birnessite during thermal annealing in the air. Although MnsOs and Nao.29Mn02-H20 have different crystalline structures, where the former is crystalline monoclinic and the latter is disordered triclinic, both compounds share similar structural characteristics. MnsOs has a layered structure and consists of sheets of [Mn ⁺ 0₈]^4~ in the be plane. Each Mn⁴⁺ atom is coordinated by six oxygen atoms and form edge-sharing octahedral unit ([Μηθβ])· Half of the Mn⁴⁺ sites in the main octahedral sheets are not fully occupied, above and below these vacant sites are Mn²⁺ sites. Therefore, the negatively charged octahedral sheets are further neutralized and held together by Mn²⁺ atoms located between layers, giving a compositional formula of Μη²⁺2Μη⁴⁺3θ8. Unlike Mn⁴⁺ ions, Mn²⁺ ions have larger radius and thus show trigonal prismatic coordination with oxygen atoms. It is apparent that the [Mn ⁺ 0₈]^4~ sheets resemble the structure of Nao^MnC -EhO birnessite comprised of infinite [Μηθβ] octahedral layer with intercalated Na cations in between. The transition from MnsOs to Nao^MnC -EhO birnessite is an equivalent process of ion-exchange of Mn²⁺ ions in the Mn2⁺Mn ⁺0₈ with Na⁺ ions in solid state.

Without being limited, it is believed that the Mn²⁺ ions with trigonal prismatic coordination located between the interlayer of MnsOs had higher mobility than the Mn⁴⁺ ions within octahedral coordination. Accordingly, the insertion of Na-ions into the Mn²⁺ site was kinetically favored, accompanied with the migration of Mn²⁺ ions into the vacant sites in [Μη⁴⁺3θ8]^4" layers, and finally drove the formation of Nao.29Mn02. XRD showed that anhydrous Nao.29Mn02 had interlayer distance of 5.58 A (FIG. 19), very similar to that of MnsOs (5.2 A). Upon water intercalation, the resulting Nao.₂9Mn0₂- H₂0 showed an increased interlayer distance of 7.14 A. Note that the Na-ion driven conversion from MnsOs to Nao.29Mn02 disclosed herein contrasts the formation of Li-MnC via the ion-exchange between Ca2Mn30s (Ca²⁺2Mn⁴⁺30s), isomorphic structure of MnsOs (Mn²⁺2Mn⁴⁺30s), and molten lithium nitrate. In the formation of Li-MnC , Li-ions occupied all the available octahedral sites between the [Mn ⁺0₈]^4~ layers rather than the trigonal prismatic sites occupied by Ca²⁺ in the parent Ca2Mn30s compound due to much smaller size of Li⁺ compared with Ca²⁺, resulting in the complete conversion to layered LiMnC with R3m or 03 symmetry.

Electrochemical performance of

were tested in a 0.1 M Na2S0₄ electrolyte in a three-electrode half-cell using cyclic voltametry (CV) measurements between -1.25 V to 1.25 V (vs Ag/AgCl) at scan rates ranging from 5 to 1000 mV s^"1 (FIG. 20). FIG. 21a showed the CVs of Nao^MnC -EhO, where distinct redox peaks can be observed at all the tested scan rates. As the scan rate increased, the anodic peaks shifts to higher potenital from 0.78 V to 1.00 V, while the cathodic peaks shifted to lower potential from 0.12 V to - 0.17 V. Compared with other NasMnOx-FhO materials (Nao.i3MnOi.7₄-H20 and Nao.25MnOi.8₄-H20), Nao^MnC -EhO showed the least peak-shifting, indicating it has a more faciliated redox processes requiring lower overpotential for Na-ion transport (FIG. 22). In FIG. 21b, Nao^MnC -EhO material showed higher specific capacities compared with other materials at all scan rates with a maxmium specific capacity 147 mAh g^_1 at a scan rate of 5 mV s^"1. To further evaluate the Na-ion transport in NasMnOx-fhO materials, the diffusion coefficient was measured using a current-pulse relaxation technique.⁷ As shown in FIG. 23, the relative diffusion coefficients (regarding to MnsOs) of Nao.i3MnOi.74-H20, Nao.25MnOi.84-H20, and Nao^MnC -EhO were 2.4, 6.6 and 38.7, demonstrating the Na-ion intercalation has less energy barrier in the Nao^MnC -EhO electrode, congruent with CV data showing less overpotential for ionic trasport for Nao^MnC -EbO.

Long-term energy and power perfromance of Nao^MnC -EhO material were tested in symmetric full-cells for 5,000 galvanostatic cycles at a potential window of 2.5 V. Nearly linear votalge-capacity profiles at all the tested current densities pointed out a single-phase solid solution redox reaction (FIGS. 21c, 24). FIG. 21d shows that the electrode capacities of Nao.2₉Mn02-H20 material varied from 83 mAh g^_1 to 24 mAh g^_1 as the current density increased from 1 to 10 A g^_1, corresponded with the discharge time from 160 s (a C-rate of 23) to 4.5 s (a C-rate of 800). Nao^MnC -EhO material exhibited an excellent cycle stability up to 5000 cycles without obvious capacity loss, as well as nearly 100% coulombic efficiency and high energy efficiency at different current densities (FIG. 21e). At the low current densities Nao^MnC -EhO showed a continuous increase capacity upon cycling. Such behaviour has been attributed to the slow building-up of ionic interface during the initial cycling before the electrode reached its best electrochemical condition. FIG. 21f shows that Nao.2₉Mn02-H20 exhibited the specific energy from 26 to 7.5 Wh kg^-1 and the specific power from 625 to 6250 W kg^-1. These values are higher or comparable with several aqueous or non-aqueous EES devices, including Panasonic (17500) Li-ion battery (data reported in less than 5 cycles), a-MnC , δ-Μηθ2 or amorphous birnessites, and tunnel-structured Nao.44Mn02 and 03 type NaMnC . The limited capacity for aqueous Na-ion found in typical birnessite is attributed to the limited potential window (~ 1.2 V) and ineffective redox process. In order to elucidate the origin of high capacity found in Nao^MnC -EhO birnessite (147 mAh g^_1), the roles of disordered nature on increasing the voltage window and therefore inhibiting the gas evolution reaction is considered, as noted below.

To determine whether the structure found in Nao^MnC -EhO affected the voltage window for aqueous Na-ion storage, CV measurement and Tafel analysis for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were conducted using disordered Nao^MnC -EhO, high-temperature treated Nao^MnC -EhO made via thermally treating disordered Nao^MnC -EhO at 500 °C, and commercial anhydrous MnC bulk materials. Compared with high- temperature treated Nao^MnC -EbO and MnC , disordered Nao.2₉Mn02-H20 showed much weaker HER current at a potential of up to -1.25 V (equivalent to overpotential of 0.63 V towards HER) and higher Tafel slopes at various scan rates (FIGS. 25a, 25b, 26), suggesting a sluggish kinetics of HER. FIG. 25b showed that although high-temperature treated and disordered Nao.29Mn02-H20 were both inactive towards oxygen evolution reaction (OER) even at a potential of 1.25 V (equivalent to overpotential of 0.63 V towards OER), only disordered Nao.2₉Mn02-H20 showed high overpotential towards HER and OER, suggesting that the disordered nature lead to high resistance to gas evolution reactions and therefore a kinetically stable potential window of 2.5 V in an aqueous electrolyte. In contrast, high-temperature treated Nao.2₉Mn02-H20 and commercial Mn02 showed much inferior capacities compared with disordered Na₀.29MnO2- H2O (FIG. 25c), which could also be explained by the parasitic gas evolution reactions especially HER that could deteriorate the electrode and cause capacity loss at prolonged cycles.

Working Examples

Material synthesis. Mn30₄ nanoparticles were first synthesized via a solution phase method. In a typical synthesis, MnCi2-4H20 (0.7 g, Alfa Aesar, 99% metals basis) was fully dissolved by deionized water (140 mL, 18.2 ΜΩ; Millipore, Inc.) in a 500 mL flask under vigorous stirring at room temperature. The aqueous solution of NaOH (Alfa Aesar, 99.98% metals basis) with a concentration of 0.123 g mL ¹ was injected at a rate of 0.167 mL min ¹ for 50 min using an automatic syringe (HSW Inc.). After injection, the mixture continuously reacted for another 30 min till dark brown precipitate was formed. The resulting product was separated by centrifuging and then washed by deionized water and ethanol consecutively. The obtained products (M¾0₄ nanoparticles) were finally vacuum-dried.

In the synthesis of sodium-manganese oxides, NaOH (Alfa Aesar, 99.99% metals basis) and 100 mg M¾0₄ nanoparticles were ground in mortar with the molar ratios of 0.5, 1, 1.5, 2 and 4, respectively. The resulting mixture of NaOH and M¾0₄ was heated in tube furnace (Thermal Scientific, Inc.) in the open air at 270 °C for 6 hours. The obtained solids were thoroughly washed with deionized water to remove the possible NaOH residual and vacuum-dried for overnight. The high-temperature treated Nao^MnC -EhO material was obtained by thermal treatment of the as-synthesized disordered Nao.2₉Mn02 at 500 °C for 2 hours in the open air. The MnC birnessite with low sodium concentration was synthesize via a wet chemistry method. Aqueous MnC (5 mg mL ¹) precusor was injected into 20 mL NaOH solution with a concetration of 5.7 mg mL ¹ at the rate of 0.167 mL min ¹ for 1 hour, and the obtained product was vacuum-dried after washed by deionized water and ethanol. And then the solids was annealed in the open at 270 °C for 6 hours. Electrochemical Measurements.

Half-cell test: Cyclic voltammetry (CV) measurements of sodium-manganese oxide were conducted using a three-electrode half-cell powered by CHI 660d single channel electrochemical workstation. The three-electrode system contained a glassy carbon rotating disc electrode (Pine Instrument) as the working electrode, platinum wire and silver-silver chloride (Ag/AgCl) electrode as counter and reference electrodes, respectively. The ink material was prepared by grinding mixture of 7 mg sodium- manganese oxide and 3 mg carbon black (Alfa Aesar, > 99.9%). The resulting mixture was mixed with deionized water to make an ink solution of 0.5 mg mL ¹. The resulting solution was subsequently sonicated until the materials were homogeneously dispersed. In a typical half-cell measurement, 10 suspension containing 3.5 μg sodium-manganese oxide and 1.5 μg carbon black was drop- cast onto the glassy carbon disc electrode (0.5 cm in diameter) and vacuum-dried. The CV measurements of electrodes were conducted in a 250 mL flat-bottom flask containing 100 mL argon-purged Na2S0₄ aqueous electrolyte (0.1 M) at a rotating rate of 500 rpm. The CV data were obtained within an applied potential range from - 1.25 V to 1.25 V (vs Ag/AgCl) for 3 cycles, and the third CV cycle was used for the calculation of storage capacity.

Diffusivity Measurements

The diffusivity measurements was tested in a typical half-cell setting as described above, except 40 ug active materials sodium-manganese oxides was loaded on working electrode and 0.25 M Na2S0₄ was used as electrolyte. A constant negative current pulse of 1 uA was first applied to working electrode and was held for 15 seconds to discharge the electrode from the open circuit potential. After that, the working electrode was relaxed and potential changes were collected for another 1000 seconds.

Full-Cell Test

Symmetric two-electrode full-cells with Nao^MnC -EhO electrodes were assembled and measured to characterize the energy/power performance and the long cycle stability as well. Electrodes were made by drop casting the slurry containing ~ 5 mg Nao^MnC -EhO and 1.25 mg carbon black as a mass ratio of 4: 1 on Toray carbon paper (E-Tek, Inc., 1.5 cm in diameter). The resulting electrodes were weighed with an accurate mass loading of active material after vacuum-dried over-night. Two symmetric electrodes were separated by cellulose-based filter paper (Whatman), and 150 Na2S0₄ aqueous solution (1 M) was used as the electrolyte. The cell stack of electrodes and separator was tightened by stainless plate and compression spring to ensure good electrical contact, and then assembled in the split button-cells (model: EQ-STC, MTI Corp.). Galvanostatic charge and discharge measurements of symmetric full-cells were conducted on the battery analyzer (model: B-TG, Arbin Instruments) within 2.5 V potential window for 5000 cycles at the constant current densities of 1, 2, 5 and 10 A g ¹. All the electrochemical calculations are provided in the supporting information.

X-ray and Neutron Scattering Characterization

X-ray diffraction measurements were conducted at 17-BM-B at the Advanced Photon Source at the Argonne National Laboratory with a wavelength of λ = 0.72768 A. In-situ XRD of electrochemical half-cell measurements were conducted in a home-made cell consisted of thin carbon paper (E-Tek, Inc.) as working electrode, platinum wire and micro Ag/AgCl electrode as counter and reference electrodes, respectively. The Na2S0₄ aqueous electrolyte (1 M) was used as the electrolyte. The suspension of a mixture of Nao.2₉Mn02 and carbon black was drop cast on the thin carbon paper, and then dried naturally in air. The cellulosed based filter paper was used as separator. The cell was then assembled for X-ray measurements. In-situ XRD tests were performed during CV scans from -1.25 V to 1.25 V (vs Ag/AgCl) at the scan rates of 5 mV s ¹. GSAS-II software was used to analyze the structural changes during the charge and discharge processes. The total neutron scattering experiment was conducted at the Nanoscale-Ordered Materials Diffractometer (NOMAD) beamline at Spallation Neutron Source at Oak Ridge National Laboratory. The pair distribution function (PDF) analysis was conducted using PDFgui software.

EDS And TEM Characterizations

Energy dispersive X-ray spectroscopy (EDS) was conducted for elemental analysis by an Amray 3300FE field emission SEM with a PGT Imix-PC microanalysis system at University of New Hampshire. Regular transmission electron microscopy (TEM) images were collected on Zeiss/LEO 922 Omega TEM at University of New Hampshire.

The present invention therefore describes the synthesis of Na-rich Na₀.29MnO2- H₂0 via a solid-state reaction between Mn30₄ and NaOH. The conversion from Mn30₄ spinel to monoclinic MnsOs, and to triclinic Nao.2₉Mn02-H20 birnessite driven by the Na intercalation was confirmed by neutron total scattering experiments and PDF analysis. The O-K edge soft X-ray absorption measurements and Tafel analysis for gas evolution reactions suggested that interplay between Na-ion, structural water and Mn valences found in high-temperature treated Nao.29Mn02 might account for its high overpotential toward gas evolution reactions and thereby the kinetically stable potential window of 2.5 V in an aqueous electrolyte. Moreover, electro-kinetic analysis and in-situ XRD measurements both pointed to a high electron transfer reaction (0.36 and 0.41 electrons) during charging/discharging processes, benefited from the Na-rich structure. The reported promotional effects of the disordered and Na-rich structure on storage capacity of layered birnessite open up a new strategy to design high capacity electrode materials for aqueous energy storage.

Table 1

Summaried atomic ratio A/Mn (A is the cation including Na⁺, K⁺) of MnC birnessite synthesized via a solid-state reaction compared with that of a wet chemistry method and those of other works.

*Note: the averaged A/Mn ratio of Mn02 made via wet chemistry methods is about 0.14.

References

1 Yeager, M. et al. Highly Efficient K0.15MnO2 Birnessite Nanosheets for Stable Pseudocapacitive Cathodes. The Journal of Physical Chemistry C 116, 20173- 20181, doi: 10.1021/jp304809r (2012). Qu, Q. et al. Electrochemical Performance of Mn02 Nanorods in Neutral Aqueous Electrolytes as a Cathode for Asymmetric Supercapacitors. The Journal of Physical Chemistry C 113, 14020-14027, doi: 10.1021/jp8113094 (2009).

Wang, J., Zhang, G. & Zhang, P. Layered birnessite-type Mn02 with surface pits for enhanced catalytic formaldehyde oxidation activity. /. Mater. Chem. A 5, 5719- 5725, doi: 10.1039/C6TA09793F (2017).

Zhu, H. T. et al. Birnessite-type Mn02 Nanowalls and Their Magnetic Properties. The Journal of Physical Chemistry C 112, 17089-17094, doi: 10.1021/jp804673n (2008).

Cai, J., Liu, J. & Suib, S. L. Preparative Parameters and Framework Dopant Effects in the Synthesis of Layer-Structure Birnessite by Air Oxidation. Chem. Mat. 14, 2071-2077, doi: 10.1021/cm010771h (2002).

Table 2

Refined crystal structural parameters of MnsOs obtained by using the fitting of neutron scattering data with R_wp = 5.93%. The x, y, z and mult indicated the atom positions and atom numbers in the unit cell, respectively. Frac and Uiso represents the occupation and isotropic thermal parameters, respectively. The mult shows the atom numbers in the unit cell. All the corresponding values are provided in the table below.

Refined crystal structural parameters of MnsOs

Atom Type X y z frac mult Uiso

Mnl Mn+4 0.000 0.000 0.500 1.000 2 0.014

Mn2 Mn+4 0.000 0.258 0.000 1.000 4 0.010

Mn3 Mn+2 0.277 0.000 0.347 1.000 4 0.021

01 0-2 0.890 0.227 0.598 1.000 8 0.015

02 0-2 0.899 0.000 0.088 1.000 4 0.015

03 0-2 0.395 0.000 0.069 1.000 4 0.015

Space group: C 2/m

b=5.725 c=4.882 (A) cc=90 β=109.816 ^90 size: 0.008 μιη V=273.392 (A³)

Table 3

Refined crystal structural parameters of Nao.i3MnOi.74-H20 (MnsOs and MnC ) with R.

6.04%, showing the phase fraction of MnsOs and MnC (by mass) is 56% to 44%.

Refined crystal structural parameters of MnsOs

Atom Type X y z frac mult Uiso

Mnl Mn+4 0.000 0.000 0.500 1.000 2 0.005

Mn2 Mn+4 0.000 0.269 0.000 1.000 4 0.018

Mn3 Mn+2 0.262 0.000 0.349 1.000 4 0.016

01 0-2 0.888 0.229 0.582 1.000 8 0.026

02 0-2 0.901 0.000 0.095 1.000 4 0.015

03 0-2 0.405 0.000 0.094 1.000 4 0.031

Space group: C 2/m

b=5.723 c=4.876 (A) cc=90 β=110.012 ^90 size: 0.009 μιη V=273.539 (A³)

Refined crystal structural parameters of Μηθ2

Atom Type X y z frac mult Uiso

Mnl Mn+4 0.000 0.000 0.000 1.000 2 0.029

01 0-2 0.386 -0.055 0.131 1.000 4 0.050

Nal Na+1 0.544 0.365 0.450 0.147 4 0.006

02 0-2 0.604 0.333 0.513 0.602 4 0.106

Space g ;roup: C-1

a=5.058 b= =2.731 c= =7.387 (A) ot= =87.511 β= =104.993 γ=91.302 size: 0.004 μιη ν=98.471 Table 4

Refined crystal structural parameters of Nao.25MnOi.84-H20 (MnsOs and MnC ) with R.

9.40%, showing the phase fraction of MnsOs and MnC (by mass) is 36% to 65%.

Refined crystal structural parameters of MnsOs

Atom Type X y z frac mult Uiso

Mnl Mn+4 0.000 0.000 0.500 1.000 2 0.011

Mn2 Mn+4 0.000 0.281 0.000 1.000 4 0.020

Mn3 Mn+2 0.248 0.000 0.327 1.000 4 0.006

01 0-2 0.888 0.230 0.595 1.000 8 0.016

02 0-2 0.903 0.000 0.094 1.000 4 0.037

03 0-2 0.428 0.000 0.080 1.000 4 0.038

Space group: C 2/m

b=5.740 c=4.876 (A) cc=90 β=109.807 ^90 size: 0.005 μιη V=273.954 (A³)

Refined crystal structural parameters of Μηθ2

Atom Type X y z frac mult Uiso

Mnl Mn+4 0.000 0.000 0.000 1.000 2 0.052

01 0-2 0.391 -0.044 0.138 1.000 4 0.048

Nal Na+1 0.515 0.333 0.450 0.137 4 0.007

02 0-2 0.611 0.342 0.516 0.448 4 0.112

Space £ ;roup: C-1

a=5.070 b= =2.739 c= =7.363 (A) ot= =86.894 β: =104.782 γ==90.886 size: 0.011 μιη ν=98.730 Table 5

Refined crystal structural parameters of Nao^MnC -EhO (no MnsOs was observed) with

Refined crystal structural parameters of Nao.29MnC>2

Atom Type X y z frac mult Uiso

Mnl Mn+4 0.000 0.000 0.000 1.000 2 0.018

01 0-2 0.384 -0.038 0.135 1.000 4 0.052

Nal Na+1 0.565 0.161 0.450 0.145 4 0.006

02 0-2 0.590 0.330 0.514 0.500 4 0.107

Space group: C-l

5.048 b=2.755 c=7.381 (A) cc=86.479 β=104.175 ^90.402 size: 0.007 μιη V=99

(A³)

Claims

Claims:

1. An electrode comprising:

a layered metal oxide/conductive polymer wherein said metal oxide has in-plane chemical bonding with a dissociation energy of 4 to 7 electron volts and said conductive polymer comprises a positively charged polymeric ionomer in combination with a negatively charge polymeric ionomer.

2. The electrode of claim 1 wherein said metal oxide is selected from V2O5, LiMnC , T1O2, M0O2, M0O3, Nb₂0₅ and L1C0O2.

3. The electrode of claim 1 wherein said positively charged polymeric ionomer comprises poly(3,4-ethylene dioxythiophene).

4. The electrode of claim 1 wherein said negatively charged polymeric ionomer comprises poly(styrenesulfonate).

5. The electrode of claim 1 wherein said metal oxide and conductive polymer are present at a weight ratio of 1: 1 to 8:1.

6. The electrode of claim 1 wherein said layered metal oxide conductive polymer is present at a thickness in the range of 1 nm to 30 nm.

7. The electrode of claim 1 wherein metal oxide comprises V2O5 and said conductive polymer comprises poly(3,4-ethylene dioxythiophene) in combination with poly(styrenesulfonate) and indicates a capacity of greater than 75 MAh/g at a scan rate of 10 mV/s.

8. The electrode of claim 7 wherein said layered metal oxide/conductive polymer indicates a capacity in the range of 75 MAh/g to 160 MAh/g at a scan rate of 10 mV/s.

9. The electrode of claim 1 wherein said metal oxide comprises LiMnC and said conductive polymer comprises poly(3,4-ethylene dioxythiophene) in combination with poly(styrenesulfonate) and indicates a capacity of greater than or equal to 60 mAh/g at a scan rate of 10-20 mV/sec.

10. The electrode of claim 9 wherein said layered metal oxide/conductive polymer indicates a capacity in the range of 20 mAh/g to 70 mAh/g at a scan rate of 10-20 mV/sec.

11. A layered manganese oxide hydrate comprising

wherein δ has a value greater than 0.17 and x has a value greater than 1.74.

12. The layered manganese oxide hydrate of claim 11 wherein δ has a value in the range of greater than 0.17 to 0.29 and x has a value greater than 1.74 to 2.0.

13. The layered manganese oxide hydrate of claim 11 which indicates a reversible capacity of 130 mA h g ¹ to 160 mA h g ¹ at a scan rate of 5 mV s ¹ in a half cell in aqueous Na-ion storage.

14. The layered manganese oxide hydrate of claim 11 which indicates a cycling life of 70 mAh g ¹ to 100 mAh g ¹ after 5000 cycles at a rate of 1 A ¹ C in a full cell.

15. A method of preparing layered manganese oxide hydrate comprising combining NaOH with Mn30₄ nanoparticles at a size of 10 to 30 nm, in the solid state, at a temperature range of 200 °C to 400 °C and recovering said manganese oxide hydrate having the formula

wherein δ has a value greater than 0.17 and x has a value of 1.74 to 2.0

16. The method of claim 15 wherein δ has a value in the range of greater than 0.17 to 0.29.

17. The method of claim 15 wherein said temperature is in the range of 250 °C.