Orthorhombic sodium closo-hydroborate preparation method

Abstract

Methods of synthesizing orthorhombic-structure closo-hydroborate include mixing orthorhombic-structure closo-hydroborate precursors with low shear mixing to form a precursor mixture, placing the precursor mixture in a reaction vessel with an inert environment, heating the precursor mixture to a reaction temperature for the precursor mixture, and quenching the reaction vessel from the reaction temperature. The resulting orthorhombic-structure closo-hydroborate has ionic conductive stability at 25°C. Cathode composites that include the orthorhombic- structure closo-hydroborate therein are disclosed as well as solid-state batteries or all-solid-state batteries that have the orthorhombic-structure closo-hydroborate made according to the methods as a solid electrolyte and / or as part of the cathode composite.

Description

Attorney Docket 24636.772WO1 (SD2025-065-2PCT) ORTHORHOMBIC SODIUM CLOSO-HYDROBORATE PREPARATION METHOD RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No.63 / 697,350, filed September 20, 2024, which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to methods of synthesizing orthorhombic-structure closo-hydroborate that includes heating and then quenching to form an orthorhombic-structure closo-hydroborate that has ionic conductive stability at 25°C, cathode composite comprising the same, and solid state batteries having one or both therein. BACKGROUND

[0003] Superionic conductivity in sodium and / or lithium closo-hydroborates is generally observed for its cubic-phase structure due to large defects, such as vacancies, and anions reorientation, in the crystal structure. The crystal structure of the sodium or lithium closo- hydroborate at room temperature (generally 25 °C) can be realized by mixing different anions using high energy ball milling followed by thermal treatment with a heating rate of 5 °C / min. While the orthorhombic-phase sodium closo-hydroborate (o-NBH), with a chemical composition of Na3(BH4)(B12H12) shown in FIG.1 possesses high ionic conductivity at room temperature, the density function theory (DFT) calculations of the Na-B-H provided in FIG.2 suggest that the o- NBH is not thermodynamically stable at room temperature and has an energy over Hull of 16 meV / atom. The reported “high” ionic conductivity is 4 mS / cm is at about 365K and is 0.5 mS / cm at room temperature.

[0004] The lack of thermodynamic stability renders these substances less than useful in practical applications. There is a need to produce sodium and / or lithium closo-hydroborates that are thermodynamically stable at room temperature to harness the high ionic conductivity thereof. SUMMARY

[0005] The subject matter disclosed herein utilizes a low-energy mixing and low temperature treatment to realize a highly ionic conductivity solid electrolyte which reduce the production cost.

[0006] A first aspect of the disclosed embodiments provides a method of synthesizing orthorhombic-structure closo-hydroborate. The method comprises mixing orthorhombic-structureAttorney Docket 24636.772WO1 (SD2025-065-2PCT) closo-hydroborate precursors with low shear mixing to form a precursor mixture. The method comprises exposing the precursor mixture to heat in a reaction vessel having an inert environment. The mixing and the exposing may occur simultaneously. In another embodiment, mixing occurs before placing the precursor mixture int the reaction vessel. The method includes heating the precursor mixture to a reaction temperature and then quenching the reaction vessel from the reaction temperature. An orthorhombic-structure closo-hydroborate is formed that has ionic conductive stability at room temperature.

[0007] The low shear mixing may comprise hand mixing, mortar mixing, low shear rotary blade mixing, or low shear magnetic impeller mixing. The reaction temperature may be between about 300°C to about 450°C. The heating may occur for at least an hour or at least four hours. The quenching may comprise introducing the reaction vessel into a water bath or liquid nitrogen, or introducing the reaction vessel to an inert air temperature having a temperature sufficient to cool the reaction vessel faster than 5°C / min.

[0008] The orthorhombic-structure closo-hydroborate precursors may comprise sodium cations or lithium cations. The orthorhombic-structure closo-hydroborate formed may be selected from the group consisting of M2+x(BH4)x(B10H10)1-x, M2+x(BH4)x(B12H12)1-x, M2-x(BH4)x(B11H14)1-x, x, 1-x,

[0009] A second aspect provides cathode composites. The cathode composites comprise a cathode material, an orthorhombic-structure closo-hydroborate made by the process described above, and optionally, a carbon additive. The cathode composite may further comprise a coating configured to reduce decomposition of a solid electrolyte. The cathode composite may comprise about 10% wt / wt to about 60% wt / wt of the orthorhombic-structure closo-hydroborate, or even about 10% wt / wt to about 40% wt / wt of the orthorhombic-structure closo-hydroborate. The cathode composite may comprise about 1% wt / wt to about 10%wt / wt of carbon additives.Attorney Docket 24636.772WO1 (SD2025-065-2PCT)

[0010] The cathode composite may comprise about 5% wt / wt to about 90% wt / wt of the cathode, or about 60% wt / wt to about 90% wt / wt of the cathode. The cathode material may be selected from a Na-ion or Li-ion layered oxide selected from the group consisting of MCrO2, MFeO2, MMnO2, MCoO2, MNi0.5Mn0.5O2, MNi1-x-yCoxMnyO2, wherein 0≤x≤1 / 3 and 0≤y≤1 / 3, Na2 / 3MnO2, Na2 / 3CoO2, Na2 / 3Ni1-x-yFexMnyO2, wherein 0≤x≤0.5 and 0≤y≤0.5, and combination thereof, or a Na-ion or Li-ion polyanion material selected from the group consisting of MFePO4, M3V2(PO4)3, M3V2(PO4)2F3, M2FeP2O7, and combination thereof, or a Na-ion or Li-ion nPrussian blue analogues material selected from MxFe[Fe(CN)6], MxMn[Fe(CN)6], MxNi[Fe(CN)6], and combination thereof, wherein x is in a range of 1 to 2 and M is a sodium cation or a lithium cation, or a conversion-type material selected from the group consisting of S, S-C, FeySz, NiySz, MoySz, wherein the y, z pair is selected from 1, 1; 1, 2; 2, 3; 3, 4; and 7, 8, and combination thereof.

[0011] A third aspect herein is solid-state batteries. The solid-state battery has a cathode, an orthorhombic-structure closo-hydroborate solid electrolyte made by the process described above. The solid-state battery typically has an anode, but an anode free version is also possible. In some embodiments, the cathode may be a cathode composite as described above. The anode may comprise one or more of a sodium metal, a tin alloy, an antimony alloy, a hard carbon material, and a phosphorous material.

[0012] The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Other features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in and constitute a part of this specification, show certain aspects of the subject matter disclosed herein and, together with the description, help explain some of the principles associated with the disclosed implementations. In the drawings,

[0014] FIG.1 is a ball and stick representation of the crystal structure of orthorhombic-phase sodium closo-hydroborate with a chemical composition of Na3(BH4)(B12H12) having partial sodium occupancy.

[0015] FIG.2 has two Na-B-H phase diagrams for orthorhombic sodium closo-hydroborate.Attorney Docket 24636.772WO1 (SD2025-065-2PCT)

[0016] FIG.3 is a graph of ionic conductivity at 30 °C and activation energy of o-NBH prepared with different cooling rates of 0.5 °C / min, 2 °C / min, and quenching.

[0017] FIG.4 is a graph of current leakage due to different bias.

[0018] FIG.5 is a graph showing a linear relationship between an applied bias and current leakage, which can be correlated to Ohm’s law (V=I*R).

[0019] FIG.6 is a graph of XRD patterns of Na3(BH4)(B12H12) at different cooling rates as compared to quenching.

[0020] FIG.7 is a solid-state nuclear magnetic resonance (NMR) spectra probed by11B solid- state NMR for the resultant crystal structures of the different cooling shown in FIG.6.

[0021] FIG.8 is a solid-state nuclear magnetic resonance (NMR) spectra probed by23Na solid- state NMR for the resultant crystal structures of the different cooling shown in FIG.6.

[0022] FIG.9 is a representation of an electrode composite architecture to realize high areal.

[0023] FIG.10 is a 3-dimensional reconstructed model of sodium cobalt oxide (NCO) / sodium borohydrate (NBH), NCO / sodium zirconium chloride (NZC,) and NCO / NZC / NBH.

[0024] FIG.11 is a Linear sweep voltammetry graph of o-NBH and the electrode redox potential.

[0025] FIG.12 is a 3- dimensional composite density and specific areal resistance increase graph with increasing NZC content due to the differences in the gravimetric density.

[0026] FIG.13 is a graph of initial potential profiles of Na9Sn4 / o-NBH / NCO.

[0027] FIG.14 is a reversible capacity retention graph of the cathode composite.

[0028] FIG.15 are actual enlarged images of cathode thickness with various areal loading in full-cell configurations.

[0029] FIG.16 is a graph of initial potential profiles for the cathodes in a full-cell configuration.

[0030] FIG.17 is a graph of the specific capacity retention and efficiency with various loading of the cathodes in a full-cell configuration.

[0031] FIG.18 is a graph of the potential profiles of 45 mgNCO / cm2at low temperatures of an Sn anode.

[0032] FIG. 19 is a Nyquist plot of Li3(BH4)(B12H12) prepared according to the Na3(BH4)(B12H12) protocol.Attorney Docket 24636.772WO1 (SD2025-065-2PCT)

[0033] FIG.20 is a graph of XRD patterns of Na3(BF4)(B12H12) at different cooling rates as compared to quenching.

[0034] FIG.21 is a representation of a simple solid state battery.

[0035] FIG.22 is a flow chart for a synthesis method for forming an orthorhombic-structure closo-hydroborate that is ionically stable at room temperature. DETAILED DESCRIPTION

[0036] The following detailed description will illustrate the general principles of the invention, examples of which are additionally provided in the accompanying drawings.

[0037] As used herein, percent or the percent symbol, is understood to mean a percent by weight of any total composition, unless expressly stated otherwise. It should also be noted that in specifying any range of concentration or amount, any particular upper concentration or amount can be associated with any particular lower concentration or amount disclosed herein.

[0038] Unless otherwise indicated, all numbers expressing quantities or ingredients, measurement of properties and so forth used in the specification and embodiments are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the present specification and claims can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claimed embodiments, each numerical parameter and the word “about” as applied thereto should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0039] As used herein, “room temperature” means 25 °C + / -3 °C, more preferably + / -2°C. Methods of making an orthorhombic-structure closo-hydroborate solid electrolyte

[0040] With reference to FIG.22, methods of synthesizing orthorhombic-structure closo- hydroborate 200 are represented by a flow chart. The method 200 includes mixing orthorhombic-structure closo-hydroborate precursors 202 with low shear mixing to form a precursor mixture, heating the precursor mixture 203, 208 in a reaction vessel having an inert environment to a reaction temperature, and then quenching the reaction vessel from the reaction temperature 210 to form an orthorhombic-structure closo-hydroborate that has ionic conductive stability at 25°C. The method can be conducted in various orders. In one embodiment, the method starts by placing two or more precursors in a reaction vessel 201, then introducing anAttorney Docket 24636.772WO1 (SD2025-065-2PCT) inert environment 206b, which includes sealing the reaction vessel. Once the vessel is prepared, the vessel can be rotated, tilted back and forth, or moved in any other fashion suitable to perform low shear mixing of the two or more precursors 202. As represented in the upper and right flow path of FIG.22, this mixing and heating can occur simultaneously. Alternately, the mixing can occur before the heating. After a reaction temperature has been achieved and maintained for a preselected time period, the method includes quenching the reaction vessel from the reaction temperature 210 and then removing the orthorhombic-structure closo-hydroborate from the reaction vessel 212. In another embodiment, the method starts by mixing two or more precursors with low shear mixing 202 and once mixed, introducing the mixture into a reaction vessel 204, introducing an inert environment 206a, which can include sealing the reaction vessel, heating the reaction vessel 208, and quenching the reaction vessel 210 once a reaction temperature is reached and maintained for a preselected time period. Thereafter, the method can include removing the orthorhombic-structure closo-hydroborate from the reaction vessel 212.

[0041] Low energy mixing, also known as low shear mixing, refers to the gentle blending of materials at controlled speeds and intensities to minimize the force exerted on the particles. This method is ideal for mixing delicate or heat-sensitive substances without altering their properties or causing damage. Low shear mixers are commercially available under the brands such as Lancaster mixers, Milton Roy mixers, MGT Industries Ltd., and Ebara Scott mixers. Impeller type and rotating speed are parameters that affect whether the mixing is low shear or high shear. Unlike high shear mixing, which applies significant energy to rapidly and thoroughly mix powder materials, low shear mixing prioritizes controlled agitation to maintain the integrity of materials. Some example low shear mixers, often described by the type of impeller, include but are not limited to, screw-type agitators, UZ impeller-type agitators, gate-type agitators, marine- type agitators, hydrofoil turbine agitators, pitched blade agitators, helical blade agitators, folding impeller agitators, magnetic agitators. Ball mill mixing is high shear mixing.

[0042] The inert environment, as non-limiting examples, can be an argon environment or a nitrogen environment. The inert environment can be introduced under vacuum. Under vacuum is understood by those skilled in the art to be less than atmospheric pressure, typically significantly less than 1 atm and nearing 1 x 10-7atm.

[0043] The reaction temperature may be between about 300°C to about 450°C. The heating may occur for at least an hour or at least four hours. The quenching may comprise introducingAttorney Docket 24636.772WO1 (SD2025-065-2PCT) the reaction vessel into a water bath or liquid nitrogen, or introducing the reaction vessel to an inert air temperature having a temperature sufficient to cool the reaction vessel faster than 5 °C / min, preferably faster than 2 °C / min, more preferably faster than 1°C / min, and still preferably faster than 0.5 °C / min. In most embodiments, quenching provides instantaneous or nearly instantaneous cooling.

[0044] The precursors can be one or more of NaBH4, NaBF4, Na2B10H10, Na2B12H12, NaCB9H10, NaCB11H12, NaB3H8, and NaB11H14, or one or more of LiBH4, LiBF4, Li2B10H10, Li2B12H12, LiCB9H10, LiCB11H12, LiB3H8, and LiB11H14. Using various combinations of these precursors, numerous closo-hydroborate solid electrolytes can be made by the methods disclosed herein, including but not limited to: Na2+x(BH4)x(B10H10)1-x, Li2+x(BH4)x(B10H10)1-x, Na2+x(BH4)x(B12H12)1-x, Li2+x(BH4)x(B12H12)1-x, Na2-x(BH4)x(B11H14)1-x, Li2-x(BH4)x(B11H14)1-x, Na2-x(BH4)x(CB9H10)1-x, Li2-x(BH4)x(CB9H10)1-x, Na2-x(BH4)x(CB11H12)1-x, Li2-x(BH4)x(CB11H12)1-x, Na4(B12H12)x(B10H10)1-x, Li4(B12H12)x(B10H10)1-x, Na4(B12H12)x(B11H14)1-x, Li4(B12H12)x(B11H14)1-x, Na2-x(B12H12)x(CB9H10)1-x, Li2-x(B12H12)x(CB9H10)1-x, Na2-x(B12H12)x(CB11H12)1-x, Li2-x(B12H12)x(CB11H12)1-x, Na2+x(BF4)x(B12H12)1-x, Li2+x(BF4)x(B12H12)1-x, Na2+x(BF4)x(B10H10)1-x, Li2+x(BF4)x(B10H10)1-x, Na2-x(BF4)x(B11H14)1-x, Li2-x(BF4)x(B11H14)1-x, Na2-x(BF4)x(CB9H10)1-x, Li2-x(BF4)x(CB9H10)1-x, Na2-x(BF4)x(CB11H12)1-x, Li2-x(BF4)x(CB11H12)1-x, Na2+x(B3H8)x(B12H12)1-x, Li2+x(B3H8)x(B12H12)1-x, Na2+x(B3H8)x(B10H10)1-x, Li2+x(B3H8)x(B10H10)1-x, Na2-x(B3H8)x(B11H14)1-x, Li2-x(B3H8)x(B11H14)1-x, Na2-x(B3H8)x(CB9H10)1-x, Li2-x(B3H8)x(CB9H10)1-x, Na2-x(B3H8)x(CB11H12)1-x, Li2-x(B3H8)x(CB11H12)1-x, Na2+x(B11H14)x(B12H12)1-x, Li2+x(B11H14)x(B12H12)1-x, Na2+x(B11H14)x(B10H10)1-x, Li2+x(B11H14)x(B10H10)1-x,Attorney Docket 24636.772WO1 (SD2025-065-2PCT) Na2-x(B11H14)x(B11H14)1-x, Li2-x(B11H14)x(B11H14)1-x, Na2-x(B11H14)x(CB9H10)1-x, Li2-x(B11H14)x(CB9H10)1-x, Na2-x(B11H14)x(CB11H12)1-x, Li2-x(B11H14)x(CB11H12)1-x. wherein x is 1. In all embodiments, the orthorhombic-structure closo-hydroborate formed has ionic conductive stability at 25°C. In one embodiment, the closo-hydroborate solid electrolyte formed is Na3(BH4)(B12H12). Cathode Composites comprising orthorhombic-structure closo-hydroborate

[0045] Turning now to FIGS. 9 and 10, an electrode composite architecture 100, more specifically a solid-state cathode composite, is represented. The architecture has a highly conductive solid electrolyte 102 in which electrochemical stable solid electrolyte 104 is dispersed. Each electrochemical stable solid electrolyte 104 comprises an active material 106. Additionally, the composite architecture includes an electronic conductor 108, which may be present in the form a fibers. The electrochemical stable solid electrolyte 104 includes the high ionic conductive closo- hydroborate solid electrolytes discussed above, which can include any of the examples in the immediately preceding list. The closo-hydroborate solid electrolytes 104 is incorporated into the halide solid electrolyte 102 to improve the ionic kinetics.

[0046] With reference to FIGS.13 and 14, the experimental testing evidences a high areal capacity of about 4.99 mAh / cm2(~92.5 % of the active material) at room temperature with one proposed cathode composite architecture, and a higher capacity retention than the single solid electrolyte architecture.

[0047] In all embodiments, the concentration of the orthorhombic-structure closo-hydroborate solid electrolyte in the cathode composite can be in a range of about 10% wt / wt to about 99% weight, preferably about 15% weight to about 99% weight, more preferably about 15% wt / wt to about 80% wt / wt. In other embodiments, the orthorhombic-structure closo-hydroborate solid electrolyte in the cathode composite can be in a range of about 15% wt / wt to about 70% weight, about 15% to about 60% wt / wt, or about 15% to about 40% wt / wt. Solid State Batteries having an orthorhombic-structure closo-hydroborate solid electrolyte and / or a cathode composite comprising the same

[0048] Referring to FIG.21, in another aspect, a solid-state battery, generally referenced as 100, has a housing 102 enclosing an anode 104 and a cathode or a cathode composite 106. The battery 100 can include a negative terminal 108 and a positive terminal 110 or the exteriorAttorney Docket 24636.772WO1 (SD2025-065-2PCT) surface of the housing at the anode 104 and the cathode 106, respectively, can function as the terminals. Here, an electrolyte separator 105 is present between the anode 104 and the cathode 106. The electrolyte separator 105 can be one of the closo-hydroborate solid electrolytes disclosed herein. In one embodiment, the electrolyte separator 105 and the cathode composite 106 each include one of the closo-hydroborate solid electrolytes disclosed herein. The closo- hydroborate solid electrolytes for the separator 105 and in the cathode composite 106 can be the same or they can be different. In one embodiment, either or both of the separator 105 and the cathode composite 106 include a plurality of closo-hydroborate solid electrolytes. In an alternate embodiment, only the electrolyte separator 105 includes one or more closo-hydroborate solid electrolytes. In yet another embodiment, only the cathode composite 106 one or more closo- hydroborate solid electrolytes.

[0045] Cathode composites are disclosed herein that have a cathode material and an orthorhombic closo-hydroborate made by methods herein, i.e., one that has ionic conductive stability at 25°C. The cathode composition can comprise 10% wt / wt to about 60% wt / wt of the orthorhombic-structure closo-hydroborate, preferably 10% wt / wt to about 50% wt / wt of the orthorhombic-structure closo-hydroborate.

[0046] The cathode material is selected from the group consisting of a conversion-type material, a layered oxide material, a polyanion-type cathode material, Prussian blue analogs, and combinations thereof. The layered oxide can be a Na-ion O3-type, Na-ion P2-types, or a Li-ion O3-type material, which may include one or more of MCrO2, MFeO2, MMnO2, MCoO2, MNi0.5Mn0.5O2, MNi1-x-yCoxMnyO2, wherein M is a sodium cation or a lithium cation, and 0≤x≤1 / 3 and 0≤y≤1 / 3, or Na2 / 3MnO2, Na2 / 3CoO2, Na2 / 3Ni1-x-yFexMnyO2, wherein 0≤x≤0.5, and 0≤y≤0.5. The polyanion material may be a sodium polyanion or a lithium polyanion, which may include one or more of MFePO4, M3V2(PO4)3, M3V2(PO4)2F3, M2FeP2O7, wherein M is a sodium cation or a lithium cation. When a polyanion-type material is present, it can be selected from the group consisting of sodium nickel iron manganese oxide, iron-based mixed phosphate- pyrophosphate Na2Fe3(PO4)2(P2O7), sodium titanium phosphate (NaTi2(PO4)3), sodium iron sulfide (Na2FeS2), lithium iron phosphate (LiFePO4), and lithium iron manganese phosphate (LiFe1-xMnxPO4wherein 0≤x≤1), and combinations thereof. The Prussian blue analogues may include one or more of NaxFe[Fe(CN)6], NaxMn[Fe(CN)6], NaxNi[Fe(CN)6], wherein x is in a range of 1 to 2 and M is a sodium cation or a lithium cation. The conversion-type material mayAttorney Docket 24636.772WO1 (SD2025-065-2PCT) include one or more of S, S-C, FeySz, NiySz, MoySz, wherein the y, z pair is selected from 1, 1; 1, 2; 2, 3; 3, 4; and 7, 8.

[0047] In all embodiments, the cathode composite can include a binder. Typically, a binder is added to improve mechanical integrity of the cathode during large scale processing / manufacturing. Any binder suitable for this purpose can be included. The binder may be present as less than 5% wt / wt site, less than 4% wt / wt, less than 3% wt / wt, less than 2% wt / wt, and less than 1% wt / wt of the cathode composite. Some non-limiting examples of binders includes polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), hydrogenated nitrile butadiene rubber (HNBR), polyacrylic acid (PAA), polyvinyl alcohol, styrene butadiene rubber (SBS), and combinations thereof.

[0048] In all embodiments, the cathode composite can include a coating configured to reduce decomposition of a solid electrolyte. The coating can be present on all exterior surfaces of the cathode composite or only on surfaces that contact the solid electrolyte. Examples coating materials / compositions include Al2O3, TiO2, ZrO2, LiNbO3, Li2ZrO3, Li1.3Al0.3Ti1.7(PO4)3, Li2ZrCl6, Li3InCl6, Na2SiO3, Na2ZrO3, NaZr2Cl9, Na2ZrCl6, and combinations thereof. Example 1: Synthesis of sodium orthorhombic-structure closo-hydroborate

[0049] A bulk sample was prepared by facile hand-mixing of two precursors, Na2B12H12and NaBH4, using a mortar and pestle, thereby eliminating the energy intensive ball milling method. The bulk sample weighed less than one gram. The sample was placed in a glass tube, e.g., a glass ampoule, and an argon inert atmosphere under vacuum was introduced and the glass tube was sealed. The glass tube with the sample therein was heated to 400 °C for four hours. Then, the heated sample was rapidly cooled by dipping the same into a water bath, which stabilized the metastable-nature orthorhombic-structured closo-hydroborate to form a solid electrolyte.

[0050] The resulting crystal structure had a high ionic conductivity of 4.6 mS / cm at 303 K (about 30 °C) as supported by FIG.3. The ionic conductivity is one order of magnitude higher than those reported in prior literature.

[0051] With reference to FIG.3, the ionic conductivity and activation energy of the o-NBH increases and decreases, respectively, with increasing cooling rate. The ionic conductivity (30 °C) peaks at 4.6 mS / cm, which is higher than many sodium solid-state electrolyte reported, when the material was rapidly cooled, e.g., by quenching the heated reaction vessel. A direct-current polarization experiment suggests that the electronic conductivity is around 10−5mS / cm, fiveAttorney Docket 24636.772WO1 (SD2025-065-2PCT) orders of magnitude lower than the ionic conductivity. This is good agreement of the metastable- nature of o-NBH when rapid cooling was utilized, which evidences a stabilized structure at room temperature.

[0052] The crystal structure and local environment of o-NBH cooled at different rates were investigated by synchrotron X-ray diffraction (s-XRD) and solid-state nuclear magnetic resonance spectra (ss-NMR). With reference to FIG.6, the material cooled with near instantaneous quenching as compared to 0.5 °C / min showed different s-XRD patterns. The rapid cooling of the material by quenching the capillary (sealed reaction vessel) in water showed only the o-NBH phase with little or no precursors patterns observed at room temperature, which evidences a room stable thermodynamically stable structure. Conversely, the 0.5 °C / min cooling rate resulted in room-temperature s-XRD patterns matched well with the Na2B12H12 and weak NaBH4 signals. This implies that o-NBH exhibits a reversible phase transition to its enthalpy- preferred phase(s) precursors. Table 1. CASTEP calculation of the chemical shift NaBH4 16 17 47, , cooling rates were further probed by1H,11B, and23Na solid-state nuclear magnetic resonance (ssNMR) spectroscopy, respectively. The11B ssNMR spectrum in FIG.7 indicates the Na2B12H12and NaBH4have a chemical shift at −16 ppm and −42 ppm (6), respectively, while o- NBH has two chemical shifts (−16 ppm and −45 ppm) attributing to the B environment of B12H122−and BH4−. The more negative chemical shift of the BH4−in o-NBH (from –42 to –45 ppm) stemmed from the extra shielding from the higher electron density donated by Na ions from Na2B12H12 in the o-NBH structure which is verified by CASTEP simulation (see Table 1 above). The11B spectra also indicates that the B-H in Na3(B12H12)(BH4) maintain the same polymorph after thermal treatment. The content of the o-NBH structure in the material by the integral of the spectra is estimated to be 97 wt.% and 61.2 wt.% when it is quenched and cooled at 2 °C / min, respectively. This further supports the metastable-nature of the o-NBH that needs rapid cooling to stabilize the highly ionic conductive phase at room temperature. Besides, the fullAttorney Docket 24636.772WO1 (SD2025-065-2PCT) width at half maximum (FWHM) resonance of B12H122−in o-NBH is much narrower than that in the Na2B12H12 and the relaxation time dropped from 0.5 / 15 s to 5 / 150 ms suggesting more rapid dynamics.

[0054] Turning now to FIG.8, additionally, the existence of at least two crystal structures in o- NBH-2 is further supported by23Na and cross-peak intensities in the spin diffusion-mediated 2D11B ss-NMR spectra. The o-NBH-2 exhibits two well defined Na environments, assigned to NaBH4and Na2B12H12, while o-NBH-Q has one broad peak. The broader FWHM Na environment in o-NBH-Q indicates that there is rapid exchange between the two environments which is expected in single crystal structure. Example 2: Synthesis of lithium orthorhombic-structure closo-hydroborate

[0055] Further demonstration of rapid cooling preparation of solid electrolyte is demonstrated using Li3(BH4)(B12H12) and Na3(BF4)(B12H12). Li3(BH4)(B12H12) was prepared according to the Na3(BH4)(B12H12) protocol of Example 1 using the LiBH4and Li2B12H12precursors, except that the reaction temperature was increased to 500 °C to 525 °C. Room temperature ionic conductivity of Li3(BH4)(B12H12) is around 8 × 10−5S / cm, which is three orders of magnitude higher than Li2B12H12. Example 3: Solid-State Battery

[0056] Cross-section images of some example solid-state batteries comprising one of the orthorhombic-structure closo-hydroborate solid electrolytes disclosed herein and a cathode composite comprising the same are provided in FIG.15. The cathode composite can have different thicknesses. The range illustrated in FIG.15 is from about 110 µm to about 310 µm, which is not meant to be limiting.

[0057] The solid-state batteries disclosed herein can have an anode comprising one or more of a sodium metal, a tin alloy, an antimony allow, tin (Sn), antimony, a hard carbon material, and phosphorus material. Some non-limiting examples for the anode include Na-Sn, Na-Sb, NaxSnO, wherein x is whatever number of sodium cations are necessary for a zero charge. Turning now to FIGS.16 and 17, the solid state batteries having the Na-Sn anode and an anode free (AF) battery show high reversible areal capacity at room temperature (25 °C) with a closo- hydroborate solid electrolyte present as the electrolyte separator. Further, as shown in FIG.18, the solid-state batteries have high reversible areal capacity at low temperature, such as 0 °C, -5Attorney Docket 24636.772WO1 (SD2025-065-2PCT) °C, -10 °C, and -15 °C. Room temperature and these low temperatures are just examples picked for the experimental data and are in no manner meant to be limiting.

[0058] The electrochemical stability of the highly conductive o-NBH is studied using the grand potential phase diagram approach and linear sweep voltammetry. The linear sweep voltammetry of FIG.11 shows a high reduction stability against sodium but limited oxidation stability. The symmetrical Na9Sn4 / o-NBH / Na9Sn4 cell showed a stable reversible charge transfer with a small increase in polarization after the 50 cycles of 1 mAh cm−2of (de)sodiation which can be attributed to the increase in impedance due to the minor reduction of electrolyte. According to the grand potential calculation, the oxidations at 2.6 and 3.8 V vs. Na / Na+can be attributed to the oxidation of the BH4−and B12H122−anions, respectively.

[0059] In comparison, the oxidation at around 4.2 V vs. Na9Sn4can be attributed to the oxidation of Na2B12H12 to B9H11 and B (see the equation below). The electrochemical oxidation is further probed by galvanostatic desodiation with different cut-off voltage (Vcut-off) was conducted to study the implication of the oxidation products. A high initial desodiation capacity of 38 and 90 mAh g−1 when the Vcut-off increases to 3.4 and 4.9 V respectively, before the negligible capacity in the subsequent cycle. A large interfacial impedance was observed after the oxidizing of the material to 4.9 V vs. Na9Sn4as the oxidation products of Na2B12H12are gaseous and this potentially creates the void between the materials. 3.0 V vs Na / Na+: 4 Na3(B12H12)(BH4) → 7.33 Na2B12H12 + 4 B2H5 + 4.67 Na 4.0 V vs Na / Na+: 11 Na2B12H12→ 12 B9H11+ 24 B + 22 Na

[0060] The possible passivating oxidation product of o-NBH was further investigated by assembling an all-solid-state battery using bare NaCrO2 (NCO) and oxidation-resistant sodium- zirconium-chloride coated NaCrO2 (NCO@NZC) as the cathode. NCO was chosen due to its relatively low redox potential of 3.2 V vs Na9Sn4which is very close to the first oxidation of Na3(BH4)(B12H12) which is hypothesized to form the Na2B12H12 and B2H5 interphase.

[0061] Referring to FIG.10, the ability to form an intimate contact between the electrode and the distribution of the materials is studied by reconstructing the cross-section scanning electron microscopy images prepared by focus-ion beam milling. The different Z-values of the NCO, NZC, and o-NBH allow a different contrast in the backscattered electron beam microscopy. Given the same cathode:solid electrolyte weight composition ratio, the NCO / o-NBH, NZC / NZC and NCO / NZC / o-NBH have a 64.3, 43.9, and 55.7 vol.% occupied by the solid electrolyte,Attorney Docket 24636.772WO1 (SD2025-065-2PCT) respectively. Additionally, FIG.12 shows the trend of cathode composite volumetric density with NCO / o-NBH (1.82 gcomposite / cm3) has the lowest density than NCO@NZC / o-NBH an NCO@NZC (2.60 and 3.26 gcomposite / cm3, respectively). A different volume fraction of NCO particles in the cathode composite stemmed from NZC has a higher gravimetric density resulting in the smaller volume fraction of the solid electrolyte in the structure. The ionic tortuosity based on the solid electrolyte in the cathode composite was estimated to be around 1.53, 4.91, and 3.46, for NCO / o-NBH, NCO / NZC, and NCO / NZC / o-NBH. The ionic tortuosity of the cathode composite is also estimated experimentally by EIS with NZC as the electron blocking electrodes. As expected, NCO / o-NBH (765.7 Ω cm2) shows higher ionic tortuosity followed by NCO@NZC / o-NBH (966.1 Ω cm2) and NCO@NZC (2070.9 Ω cm2). This is attributed to the higher ionic conductivity of o-NBH than NZC and good volumetric content of o-NBH in the cathode composite. It is noted that while the o-NBH-containing composite shows a larger volume, resulting in a long ion diffusion path for the same areal loading, the inclusion of o-NBH in the composite improves the ion diffusion through the composite.

[0062] Sodium inventory excess all-solid-state batteries with a cathode mass loading of 15 mgNCO / cm2(or, a theoretical capacity of 1.8mAh / cm2) were assembled using the NCO / o-NBH, NCO@NZC, or NCO@NZC / o-NBH as cathode and Na9Sn4as the counter electrode to investigate the oxidation stability of o-NBH. An “all” solid-state battery is one that has no liquid present, in particular, no liquid electrolyte, not even at the cathode. Due to the higher ionic percolation, FIG.13 shows the NCO exhibits a smaller overpotential than that of NCO / NZC and NCO / NZC / o-NBH which agrees with the ionic tortuosity in the cathode composite. The NCO cell exhibited good material utilization and a reversible discharge capacity of 1.67 mAh / cm2(72.7% of the theoretical capacity) due to the high initial Coulombic efficiency (ICE) of 92.7% (FIG.13). Although the NCO / NZC and NCO / NZC / NBH cells exhibited a lower initial charge capacity (~110 mAh / g1), which can be attributed to the slight increase of the cathode composite impedance and / or amorphization of the NCO due to ball milling, it exhibited a higher ICE of 98.2% which can be beneficial in the full-cell configuration. It is also noticeable that both NCO / NZC and NCO / NZC / o-NBH exhibited an average Coulombic efficiency of 99.3% during the first four low current density cycles while the average Coulombic efficiency of NCO cell is merely 99.6% Furthermore, the NCO cell suffered from capacity fading the cell retained less than 75% of its initial discharge capacity after 90 cycles with an average Coulombic efficiency ofAttorney Docket 24636.772WO1 (SD2025-065-2PCT) 99.6% (FIG.14). Credit to the stable electrochemical interface, both NCO@NZC and NCO@NCO / o-NBH cells exhibited a more stable cycling retaining more than 75 and 66% of its initial capacity after 400 cycles, respectively.

[0063] Building on the cell configuration, sodium inventory limited all-solid-state batteries with various cathode mass loading (15 and 45 mgNCO / cm2, or a thickness of around 110 µm and 308.9 µm, respectively, in FIG.15) and tin as anodes or anode-free configuration were assembled. The differences in the ICE (anode-free 95% vs. Sn 70-80%) in FIG.16 can be attributed to the irreversibility of Sn anode. Nevertheless, a high reversible capacity of 1.56 and 4.86, and 1.34 and 3.52 mAh / cm2for anode-free and Sn at various loading were evidenced, respectively. Given so, it is noteworthy that current study is the highest reported reversible areal capacity in literature. After the initial loss of sodium inventory, the cells show a capacity retention of >90% after 50 cycles (FIG.17).

[0064] To further demonstrate capability of the cell configuration, the high cathode areal loading (45 mgNCO / cm2) cell is further investigated at low temperatures (e.g., 10, 5, 0, −5, −10, and −15 °C) showing its capability in different climates. Moreover, the cell in FIG.18 exhibited a high discharge capacity at low temperatures. This highlights the all-solid-state battery can operate in freezing temperatures which liquid electrolyte battery is unable to.

[0065] Referring now to FIGS.19 and 20, FIG.19 is a Nyquist plot of Li3(BH4)(B12H12), which was prepared according to Example 1 herein. FIG 20 is a graph of the in-situ XRD patterns of Na3(BF4)(B12H12), formed from a mixture of NaBF4and Na2B12H12, with increasing temperature. A new phase is formed at 500 °C and this phase is preserved after quenching in water.

[0066] The subject matter described herein can be embodied in systems, apparatus, methods, and / or articles depending on the desired configuration. The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and / or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and subcombinations of the disclosed features and / or combinations and subcombinations of several further features disclosed above. In addition, theAttorney Docket 24636.772WO1 (SD2025-065-2PCT) logic flows depicted in the accompanying figures and / or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. For example, the logic flows may include different and / or additional operations than shown without departing from the scope of the present disclosure. One or more operations of the logic flows may be repeated and / or omitted without departing from the scope of the present disclosure. Other implementations may be within the scope of the following claims.

Claims

Attorney Docket 24636.772WO1 (SD2025-065-2PCT) What is claimed:

1. A method of synthesizing orthorhombic-structure closo-hydroborate, the method comprising: mixing orthorhombic-structure closo-hydroborate precursors with low shear mixing to form a precursor mixture; exposing the precursor mixture to heat in a reaction vessel having an inert environment; heating the precursor mixture to a reaction temperature; and quenching the reaction vessel from the reaction temperature; wherein an orthorhombic-structure closo-hydroborate is formed that has ionic conductive stability at 25°C.

2. The method of claim 1, wherein low shear mixing comprises hand mixing, mortar mixing, low shear rotary blade mixing, or low shear magnetic impeller mixing.

3. The method of claim 1, wherein the reaction temperature is between about 300 °C to about 450 °C.

4. The method of claim 1, wherein the heating occurs for at least an hour.

5. The method of claim 1, wherein the heating occurs for at least four hours.

6. The method of claim 1, wherein quenching comprises introducing the reaction vessel into a water bath or liquid nitrogen, or introducing the material in the reaction vessel to an inert air temperature having a temperature sufficient to cool the material faster than 5 °C / min.

7. The method of claim 1, wherein the orthorhombic-structure closo-hydroborate precursors comprise sodium cations or lithium cations.

8. The method of claim 1, wherein the orthorhombic-structure closo-hydroborate formed is selected from the group consisting of M2+x(BH4)x(B10H10)1-x, M2+x(BH4)x(B12H12)1-x, M2-x(BH4)x(B11H14)1-x, M2-x(BH4)x(CB9H10)1-x, M2-x(BH4)x(CB11H12)1-x, M4(B12H12)x(B10H10)1-x, M4(B12H12)x(B11H14)1-x, M2-x(B12H12)x(CB9H10)1-x, M2-x(B12H12)x(CB11H12)1-x,Attorney Docket 24636.772WO1 (SD2025-065-2PCT) M2+x(BF4)x(B12H12)1-x, M2+x(BF4)x(B10H10)1-x, M2-x(BF4)x(B11H14)1-x, M2-x(BF4)x(CB9H10)1-x, M2- x(BF4)x(CB11H12)1-x, M2+x(B3H8)x(B12H12)1-x, M2+x(B3H8)x(B10H10)1-x, M2-x(B3H8)x(B11H14)1-x, M2-x(B3H8)x(CB9H10)1-x, M2-x(B3H8)x(CB11H12)1-x, M2+x(B11H14)x(B12H12)1-x, M2+x(B11H14)x(B10H10)1-x, M2-x(B11H14)x(B11H14)1-x, M2-x(B11H14)x(CB9H10)1-x, M2-x(B11H14)x(CB11H12)1-x, and combinations thereof; wherein M is a sodium cation or a lithium cation.

9. A cathode composite comprising: a cathode material and an orthorhombic-structure closo-hydroborate made by the process of any of claims 1 to 10.

10. The cathode composite of claim 9, further comprising a coating configured to reduce decomposition of a closo-hydroborate solid electrolyte.

11. The cathode composite of claim 9, wherein the orthorhombic-structure closo-hydroborate comprises about 10% wt / wt to about 60% wt / wt of the cathode composite.

12. The cathode composite of claim 9, wherein the orthorhombic-structure closo-hydroborate comprises about 10% wt / wt to about 40% wt / wt of the cathode composite.

13. The cathode composite of claim 9, wherein the cathode material comprises about 5% wt / wt to about 90% wt / wt of the cathode composite.

14. The cathode composite of claim 9, wherein the cathode material comprises about 5% wt / wt to about 90% wt / wt of the cathode composite.

15. The cathode composite of claim 9, wherein the cathode material is selected from the group consisting of a conversion-type material, a layered oxide material, a polyanion-type cathode material, Prussian blue analogs, and combinations thereof.

16. The cathode composite of claim 9, wherein the cathode material comprises a Na-ion or Li- ion layered oxide selected from the group consisting of MCrO2, MFeO2, MMnO2, MCoO2,Attorney Docket 24636.772WO1 (SD2025-065-2PCT) MNi0.5Mn0.5O2, MNi1-x-yCoxMnyO2, wherein 0≤x≤1 / 3 and 0≤y≤1 / 3, Na2 / 3MnO2, Na2 / 3CoO2, Na2 / 3Ni1-x-yFexMnyO2, wherein 0≤x≤0.5 and 0≤y≤0.5, and combination thereof, or a Na-ion or Li-ion polyanion material selected from the group consisting of MFePO4, M3V2(PO4)3, M3V2(PO4)2F3, M2FeP2O7, and combination thereof, or a Na-ion or Li-ion nPrussian blue analogues material selected from MxFe[Fe(CN)6], MxMn[Fe(CN)6], MxNi[Fe(CN)6], and combination thereof, wherein x is in a range of 1 to 2 and M is a sodium cation or a lithium cation, or a conversion-type material selected from the group consisting of S, S-C, FeySz, NiySz, MoySz, wherein the y, z pair is selected from 1, 1; 1, 2; 2, 3; 3, 4; and 7, 8, and combination thereof.

17. A solid-state battery comprising: a cathode; an orthorhombic-structure closo-hydroborate solid electrolyte made by the process of any of claims 1 to 10; and an anode; wherein the cathode, the orthorhombic-structure closo-hydroborate solid electrolyte, and the anode are arranged to form a solid-state battery.

18. The solid-state battery of claim 17, wherein the cathode is a cathode composite according to any of claims 11 to 16.

19. The solid-state battery of claim 17, wherein the anode comprises one or more of a sodium metal, a tin alloy, an antimony allow, a hard carbon material, and a phosphorous material.

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