Production of metallic sodium by a two-temperature electrolysis method

Abstract

A novel two-temperature electrochemical method and system for producing metallic sodium from sodium polysulfide have been discovered. The technology of the present invention provides high electrical conductivity for sodium ions and extends the service life of the electrochemical cell.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to U.S. Provisional Application No. 62 / 985,287, filed Mar. 4, 2020, which is incorporated herein by reference in its entirety. BACKGROUND OF THE INVENTION

[0003] Liquid hydrocarbons containing many petroleum feedstocks typically contain sulfur in the form of organic sulfur compounds that are difficult to remove, as well as metals and other heteroatom-containing compounds that impede the use of the hydrocarbons. Sulfur can cause air pollution and can poison catalysts used in petroleum processing or catalysts designed to remove hydrocarbons and nitrogen oxides from motor vehicle exhaust. Restricting the sulfur content in hydrocarbon fuels such as gasoline, diesel, and fuel oil, including marine fuels, has become a global trend. Metals contained in hydrocarbon streams can also poison hydrotreating catalysts commonly used to remove sulfur, whereby hydrogen reacts under extreme conditions to break down sulfur-containing organic sulfur molecules.

[0004] Sodium metal has been considered potentially effective for treating high-sulfur hydrocarbons, including petroleum distillates, crude oil, heavy oil, bitumen, and shale oil. Sodium is capable of reacting with the oil and its contaminants to significantly reduce the sulfur, nitrogen, oxygen, and metal content in the oil by forming sodium sulfide compounds (sulfides, polysulfides, and hydrosulfides) and other sodium-containing by-products. For such desulfurization technologies to be economically viable, the sodium metal must be regenerated from the sodium sulfide / polysulfide and other sodium-containing by-products. Although sodium metal can be electrochemically regenerated from such sulfides / polysulfides and by-products, the short service life of the electrochemical cells used in these methods has hindered their commercial use. Thus, until now, there has been no cost-effective and robust electrochemical system and method for generating sodium metal from sulfides / polysulfides. SUMMARY OF THE INVENTION

[0005] A novel electrochemical method for generating sodium metal from polysodium sulfide has been discovered. The technology of the present invention provides high electrical conductivity for sodium ions and extends the service life of the electrochemical cell. The inventors developed the technology of the present invention after their extensive research revealed that the polysodium sulfide / glycol anolyte is chemically unstable under the electrolysis conditions for generating sodium metal from polysodium sulfide salts. Surprisingly, it was found that the side reactions leading to the degradation of the anolyte exhibit an extreme and hitherto unrecognized temperature dependence. The technology of the present invention provides a novel two-temperature electrolysis method for the electrochemical cell with high throughput and long service life.

[0006] On the one hand, the technology of the present invention provides a method for preparing metallic sodium from sodium salts, such as sodium sulfide / polysulfide or sodium halide / polyhalide. The method includes introducing anolyte at a first temperature into the anolyte chamber of an electrochemical flow battery (or a hybrid flow battery). The anolyte contains an effective amount of a sodium salt (e.g., sulfur-containing sodium salt) dissolved in a temperature-sensitive solvent. The anolyte chamber contains an anode including the sodium salt. The electrochemical flow battery further includes a catholyte chamber and a ceramic sodium ion-conducting membrane separating the anolyte chamber from the catholyte chamber. The catholyte chamber contains a molten sodium cathode. The method further includes: enabling sodium ions from the sodium salt to pass from the anolyte chamber through the ceramic sodium ion-conducting membrane to the catholyte chamber; reducing the sodium ions to metallic sodium at the molten sodium cathode; and cooling the anolyte outside the anolyte chamber to a second temperature. In any embodiment, when the anolyte is in the anolyte chamber (i.e., at the battery operating temperature), the electrical conductivity of the ceramic sodium ion-conducting membrane is at least 10 mS cm -1 or at least 50 mS cm -1 . The second temperature causes the temperature-sensitive solvent to degrade at a rate less than 20% of the rate that occurs at the first temperature. Thus, by heating only a small portion of the anolyte for a short period when it enters the electrochemical cell and rapidly cooling it when it leaves the cell, anolyte side reactions are minimized and the battery life is greatly extended.

[0007] In any embodiment, the method further includes oxidizing the anion of the sodium salt at the anode. In any embodiment, the sodium salt contains sodium sulfide, polysulfide, a mixture of sodium sulfide and one or more polysulfides. In some such embodiments, the method further includes oxidizing sulfide and / or polysulfide to higher polysulfide and / or sulfur at the anode. In any embodiment of the method of the present invention, the anolyte contains sulfur, which can optionally be recovered from the anolyte. In any embodiment, at least a portion of the metallic sodium formed in the catholyte chamber can be removed and / or recovered.

[0008] On the other hand, the technology of the present invention provides a system for implementing the method of the present invention. Accordingly, a system for producing metallic sodium by dual-temperature electrolysis is provided, the system comprising: an anolyte source including an anolyte; at least one pump for pumping the anolyte to the system; a heater for heating the anolyte to a first temperature; a refrigerator for cooling the anolyte to a second temperature lower than the first temperature; and an electrolyte flow battery. The electrolyte flow battery comprises: an anolyte chamber and a catholyte chamber separated by a sodium ion-conducting ceramic membrane; an anode including a sodium salt disposed in the anolyte chamber; a molten sodium cathode disposed in the catholyte chamber; and a power source electrically connected to the anode and the cathode. In the electrolyte flow battery, the anolyte chamber further includes an anolyte inlet and an anolyte outlet; and the catholyte chamber includes an opening through which the molten sodium can flow. The anolyte source is in fluid connection with the pump and the anolyte inlet of the anolyte chamber; the heater is adapted to heat the anolyte before or when the anolyte from the anolyte source enters the anolyte chamber; and the refrigerator is adapted to cool the anolyte when the anolyte leaves the anolyte chamber. It should be understood that the system may optionally use any of the elements described herein, including but not limited to anolyte, anode, catholyte, cathode, current collector, and other equipment, such as in Figure 8 and the accompanying text, including equipment for sulfur and / or metallic sodium recovery. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1A and 1B is a schematic description of illustrative embodiments of an electrochemical cell that can be used in the method and system of the present invention. Figure 1C Shows an exemplary embodiment of a series electrochemical cell that is arranged to oxidize sodium sulfide to polysulfide and ultimately to elemental sulfur, while also removing metallic sodium from the anolyte.

[0010] Figure 2 is a graph showing the conductivity of a Na2S5 anolyte as a function of wt.% Na and temperature.

[0011] Figure 3 Shows the FT-IR spectrum of Na2S5 in which 7% sodium is dissolved in EG and incubated at different temperatures for 195 hours.

[0012] Figure 4 Shows the normalized FT-IR peak height of the peak at 1725 cm -1 wavenumber against the baseline, which is a characteristic feature of 2-hydroxyethyl acetate.

[0013] Figure 5 Shows the peak at 3250 cm -1The normalized FT-IR peak height of the peak at the wavenumber against the baseline, which is the O-H stretch of ethylene glycol.

[0014] Figure 6 Is a graph showing the mass loss observed in the Na2S5 anolyte at 3% and 7% sodium at 110 °C, 125 °C and 150 °C during thermal incubation.

[0015] Figure 7 Is a graph showing the mass loss observed in the anolyte with Na2S5 in the ethylene glycol anolyte at 5 wt% at temperatures ranging from 80 °C to 150 °C.

[0016] Figure 8 Shows a process flow diagram (PFD) of an illustrative embodiment of a system for performing the two-stage anolyte temperature electrolysis process of the present invention.

[0017] Figure 9 Shows the applied voltage (right y-axis) required to reach 57 mA / cm over time for an illustrative embodiment of an electrochemical cell and a two-temperature electrolysis method using the technology of the present invention. 2 Required.

[0018] Figure 10 Shows the voltage and current density over time for high-temperature operation (125 - 150 °C) of an illustrative embodiment of the technology of the present invention as discussed in Example 1.

[0019] Figure 11 Shows at 1580 cm -1 The normalized FT-IR peak height of the peak at the wavenumber against the baseline.

[0020] Figure 12 Shows at 1725 cm -1 The normalized FT-IR peak height of the peak at the wavenumber against the baseline. Detailed Description

[0021] As defined below, the following terms are used throughout.

[0022] As used in this specification and the appended claims, unless otherwise indicated herein or clearly contradicted by context, the singular articles "a", "an", and "the" and similar references in the context of describing an element (especially in the context of the following claims) shall be understood to cover both the singular and the plural. Unless otherwise indicated, the recitation of a range of values herein is merely intended to be a shorthand method of referring individually to each separate value falling within the range, and each separate value is incorporated into the specification as if recited individually herein. Unless otherwise specified herein or clearly contradicted by context, all methods described herein can be performed in any suitable order. Unless otherwise noted, the use of any and all examples or exemplary language (e.g., "such as") provided herein is merely intended to better illustrate the embodiments and does not pose a limitation on the scope of the claims. No language in this specification should be construed as indicating any non-claimed element as essential.

[0023] As used herein, "about" will be understood by persons of ordinary skill in the art and will vary to some extent depending on the context in which it is used. If there is ambiguity in the use of the term by persons of ordinary skill in the art, "about" will mean up to plus or minus 10% of the particular term, taking into account the context in which the term is used.

[0024] "Ceramic sodium ion conductive ceramic membrane" means any suitable ceramic membrane that prevents contact of the molten sodium cathode with the anode and the anolyte but allows sodium ions to selectively pass through the membrane from the anode / anolyte to the cathode.

[0025] "Alkylene diol" or "alkane diol" refers to an alkane having 2 to 6 carbon atoms (i.e., 2, 3, 4, 5 or 6 carbon atoms) or an alkylene ether having 4 to 8 carbon atoms (i.e., 4, 5, 6, 7 or 8 carbon atoms) with two hydroxyl groups attached to different carbons. It should be understood that an alkane is a saturated straight-chain or branched-chain hydrocarbon. Similarly, "alkylene triol" or "alkane triol" refers to an alkane having 3 to 6 carbon atoms (i.e., 2, 3, 4, 5 or 6 carbon atoms) or an alkylene ether having 4 to 8 carbon atoms (i.e., 4, 5, 6, 7 or 8 carbon atoms) with three hydroxyl groups attached to three different carbons. Examples of alkane diols and triols include, but are not limited to, ethylene glycol (i.e., ethane-1,2-diol), propylene glycol (e.g., propane-1,2-diol, propane-1,3-diol), glycerol, butylene glycol (e.g., butane-1,2-diol, butane-1,3-diol, butane-2,3-diol, butane-1,4-diol, 2-methylpropane-1,3-diol, etc.), butanetriol (e.g., butane-1,2,3-triol, butane-1,2,4-triol, 2-hydroxymethylpropane-1,3-diol, etc.), pentylene glycol (e.g., pentane-1,2-diol, pentane-1,3-diol, pentane-2,3-diol, pentane-1,4-diol, pentane-1,5-diol, 2-methylbutane-1,4-diol, etc.), pentanetriol (e.g., pentane-1,2,5-triol, pentane-1,3,5-triol, 2-hydroxymethylbutane-1,4-diol, etc.), hexylene glycol (e.g., hexane-1,2-diol, hexane-1,6-diol, hexane-1,3-diol, hexane-2,3-diol, hexane-3,4-diol, etc.), hexanetriol (e.g., hexane-1,2,3-triol, hexane-1,2,6-triol, hexane-1,3,6-triol, etc.). Examples of alkylene diols and triols.

[0026] "Cycloalkylene diol" or "cycloalkane diol" refers to a cycloalkane having 3, 4, 5 or 6 carbon atoms with two hydroxyl groups attached to different carbons. "Cycloalkylene triol" or "cycloalkane triol" refers to a cycloalkane having 3, 4, 5 or 6 carbon atoms with three hydroxyl groups attached to different carbons. Examples include cyclopropane-1,2-diol, cyclobutane-1,3-diol, cyclopentane-1,2-diol, cyclohexane-1,2-diol, cyclohexane-1,3-diol, cyclohexane-1,4-diol, etc.

[0027] As used herein, "temperature-sensitive solvent" refers to a solvent used in the electrolyte of an electrochemical cell to dissolve or partially dissolve one or more substances (e.g., salts or neutral, uncharged compounds), and which becomes increasingly unstable as the temperature increases, undergoing decomposition and / or other side reactions that have an adverse effect on the performance of the electrolyte.

[0028] High sodium ion conductivity is desired to ensure high fluxes of metallic sodium produced by electrolysis of sodium salts such as sodium polysulfide. To achieve the desired conductivity for a particular combination of sodium sulfide salts and sodium polysulfide salts, a balance must be achieved among the selection of the anolyte, the anode, and the ceramic ion-conducting membrane that are part of the electrochemical cell. The inventors have found that low molecular weight diols and triols (having a molecular weight of, for example, less than 400 Da, such as from about 62 Da to less than about 200 Da) provide good solubility for sodium sulfide / polysulfide and lower solubility for elemental sulfur while maintaining maximum sodium ion conductivity of the cell. However, it has been found that the performance of such cells deteriorates within a few days, rendering the cells unsuitable for long-term commercial use.

[0029] The following side reactions of sodium polysulfide (Na2S x ) and / or diol / triol (such as ethylene glycol (EG)) have been identified:

[0030] 1. Hydrogen sulfide gas is produced due to EG acting as an acid and Na2S x acting as a base.

[0031] 2. Sodium sulfuroxygenates (such as sodium thiosulfate, sodium sulfate, etc.) are formed by Na2S x extracting oxygen from ethylene glycol.

[0032] 3. S in Na2S x extracts methylene from EG to form sulfided or desulfurized oligomers of sulfur and methylene.

[0033] 4. Carboxylate salts (such as sodium acetate, sodium glycolate, sodium formate, sodium oxalate, etc.) are formed by electrochemical oxidation of EG in an alkaline medium.

[0034] 5. Gases such as dimethyl disulfide, carbon disulfide, carbon dioxide, etc. are formed.

[0035] Observations 1, 2, and 3 are consistent with the following side reactions, as shown

[0036] Na2S.Sx + 2(CH2OH-CH2OH) ---------> 2H2S + Na2SO4 + (CH2)2Sy

[0037] Observations 4 and 5 are consistent with the following side reactions, as shown:

[0038] Na2S.S x + 2(CH2OH-CH2OH) ---------> (COONa)2 + (CH3)2S x

[0039] Thus, it has surprisingly been found that the use of ethylene glycol results in an undesired increase in the area specific resistance (ASR) of the electrolytic cell, leading to a decrease in current over several days.

[0040] Thermal degradation and electrolysis tests as a function of the anolyte temperature show that as the process temperature is decreased from above 125 °C to lower temperatures, the rate of the degradation reaction drops sharply and the equilibrium shifts to the left (favoring the reactants in the above degradation reaction). In fact, when the anolyte temperature does not exceed 110 °C, and particularly when the anolyte temperature does not exceed 100 °C, the degradation rate is significantly reduced. See, for example Figures 3 - 7 each plot in the figures shows electrolyte degradation at the high end of this spectrogram, i.e., at the preferred temperatures for excellent conductivity and current density.

[0041] The above reactions become apparent at temperatures above 120 °C, especially above 125 °C. Given that such temperatures are still desirable for providing high sodium ion conductivity of the membrane during electrolysis, the technology of the present invention provides methods and systems for minimizing anolyte degradation while maintaining high sodium ion conductivity at operating temperatures, for example, from 125 °C to 150 °C.

[0042] Thus, on the one hand, the technology of the present invention provides a method that includes:

[0043] introducing an anolyte at a first temperature into an anolyte chamber of an electrochemical flow battery, where

[0044] the anolyte includes an effective amount of a sodium salt dissolved in a temperature-sensitive solvent;

[0045] the anolyte chamber includes an anode containing a sodium salt;

[0046] the electrochemical flow battery further includes a catholyte chamber and a ceramic sodium ion-conducting membrane separating the anolyte chamber from the catholyte chamber; and

[0047] the catholyte chamber includes a molten sodium cathode;

[0048] allowing sodium ions from the sodium salt to pass from the anolyte chamber through the ceramic sodium ion-conducting membrane to the catholyte chamber;

[0049] reducing the sodium ions to metallic sodium at the molten sodium cathode; and

[0050] cooling the anolyte outside the anolyte chamber to a second temperature;

[0051] where the second temperature causes the temperature-sensitive solvent to degrade at a rate less than 20% of the rate that occurs at the first temperature.

[0052] On the other hand, the technology of the present invention provides a method, which includes:

[0053] introducing anolyte at a second temperature into the anolyte chamber of an electrochemical flow battery, where

[0054] the anolyte includes an effective amount of a sodium salt dissolved in a temperature-sensitive solvent;

[0055] the anolyte chamber includes an anode containing a sodium salt;

[0056] the electrochemical flow battery further includes a catholyte chamber and a ceramic sodium ion conductive membrane separating the anolyte chamber from the catholyte chamber; and

[0057] the catholyte chamber includes a molten sodium cathode;

[0058] causing sodium ions from the sodium salt to pass through the ceramic sodium ion conductive membrane from the anolyte chamber to the catholyte chamber; and

[0059] reducing the sodium ions to metallic sodium at the molten sodium cathode;

[0060] wherein the second temperature causes the temperature-sensitive solvent to degrade at a rate less than 20% of the rate that would occur at the first temperature.

[0061] In any embodiment, the sodium salt can include sodium sulfide, polysodium sulfide, a mixture of sodium sulfide and one or more polysodium sulfides, sodium halide, polyhalide sodium, or a mixture of sodium halide and one or more polyhalide sodiums. In any embodiment, the sodium salt can include sodium sulfide, polysodium sulfide, a mixture of sodium sulfide and one or more polysodium sulfides. In any embodiment, the polysulfide can have the formula Na2S x, where x is an integer from 1 to 32, i.e., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32 or a range between any two of the above values and including any two values (e.g., 1 to 7 or 1 to 5). However, those skilled in the art should understand that in some embodiments, the measured value of x will reflect the amount of sulfur in the mixture. It should also be understood that the sodium salt in the anolyte includes anions, and the method further includes oxidizing the anions at the anode. In cases where, for example, the anolyte contains sodium sulfide and / or polysulfide, the method may include oxidizing sulfide and polysulfide anions to higher polysulfides and / or sulfur such that the anolyte may include sulfur. In any embodiment of the method, sulfur can be recovered from the anolyte by, for example, separating sulfur as a solid and / or immiscible liquid phase and removing it from the anolyte or otherwise recovering it. Phase separation and recovery can be accomplished by standard methods such as cooling and precipitation or crystallization of sulfur or cooling and sedimentation of liquid sulfur, whereby it can be drawn off from the remaining anolyte.

[0062] In any embodiment herein, the amount of sodium in the anolyte can range from about 1 wt% to about 10 wt%, such as about 1 wt%, about 2 wt%, about 3 wt%, about 4 wt%, about 5 wt%, about 6 wt%, about 7 wt%, about 9 wt%, about 10 wt% or a range between any two of the above values and including any two values. For example, the amount of sodium in the anolyte can range from about 3 wt% to about 7 wt%.

[0063] In any embodiment herein, the temperature-sensitive solvent of the technology of the present invention can include alkyl diols, alkyl triols, cycloalkyl diols, and / or cycloalkyl triols. For example, the temperature-sensitive solvent can be selected from the group consisting of: ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, and glycerol. In any embodiment, the temperature-sensitive solvent can include lactams (e.g., N-methyl-2-pyrrolidone, (NMP)), cyclic ureas (e.g., N,N′-dimethylpropyleneurea, DMPU), C 1-6Alkyl amides (e.g., formamide, N,N-dimethylformamide, acetamide), carbonates (e.g., dimethyl carbonate, diethyl carbonate), ethers (e.g., dimethyl ether, dioxane), or mixtures of any of the foregoing with water. In any embodiment, the temperature-sensitive solvent can comprise a mixture of two or more of the foregoing solvents (or solvent classes). Thus, for example, the anolyte can comprise a mixture of two solvents in a weight ratio of 1:99 to 99:1, 5:95 to 95:5, or more typically 50:50 to 95:5, such as 50:50, 55:45, 60:40, 70:30, 80:20, 90:10, 95:5, or a range between any two of the foregoing ratios and including any two ratios. In any embodiment, while any compatible pair of temperature-sensitive solvents can be used, examples include ethylene glycol and NMP, ethylene glycol and water, tetraethylene glycol and dimethyl ether, and ethylene glycol and glycerol.

[0064] In any embodiment of the present disclosure, the anolyte optionally contains one or more sodium salts as sodium ion conductivity enhancers. The presence of these sodium salts in the anolyte, along with sodium polysulfide, increases the conductivity and current density of the electrolyte flow battery. The sodium ion conductivity enhancer of the technology of the present invention comprises a sodium salt that is selected to be substantially (preferably completely) soluble, ionizable at the concentrations used, and thermally stable in a temperature-sensitive solvent at the first and second temperatures used in the method of the present invention. Non-limiting examples of the sodium ion conductivity enhancer include sodium halides (e.g., NaCl, NaBr, and NaI), sodium hydroxide, sodium carbonate, sodium sulfur-containing oxygen compounds (Na2SO4, Na2SO3, Na2S2O3), sodium hydrosulfide (NaSH), and mixtures of any two or more thereof. In any embodiment, the anolyte may contain a sodium halide, sodium hydroxide, or a mixture of any two or more thereof. In any embodiment, 0.01 wt% to 20 wt% of the sodium conductivity enhancer may be present in the anolyte. For example, the anolyte may contain 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 16 wt%, 18 wt%, and 20 wt% of the sodium ion conductivity enhancer or a range between any two of the above values and including any two values. Thus, for example, the anolyte may optionally contain 0.1 wt% to 20 wt%, 1 wt% to 18 wt%, or 5 wt% to 15 wt% of the sodium ion conductivity enhancer. A conductivity enhancement of at least 10% - 100% can be achieved using an anolyte containing such an enhancer compared to the same anolyte without such an enhancer. In some embodiments, the enhancement is at least 10%, at least 20%, at least 40%, at least 60%, at least 80%, at least 100%, or a range between any two of the above values and including any two values.

[0065] In the technology of the present invention, the molten sodium cathode comprises sodium and optionally a sodium alloy. In any embodiment, the sodium of the molten sodium cathode may contain a small amount of foreign impurities but may still be substantially pure (e.g., at least 99% sodium, at least 99.5% sodium or at least 99.9% sodium). Those skilled in the art should understand that suitable sodium alloys mainly consist of metallic sodium. In any embodiment, the sodium alloy is at least 80 wt% metallic sodium, such as at least 80 wt%, at least 85 wt%, at least 90 wt%, at least 95 wt%, at least 96 wt%, at least 97 wt%, at least 98 wt%, at least 99 wt% or between any two of the above values and including the range of any two values. For example, in any embodiment, the sodium alloy may be 80 wt% to 99 wt% metallic sodium. The alkali metal alloy may contain, for example, an alloy having one or more of the following: Si, Ge, Sn, Pb, Hg, Cs, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, and Cd. In any embodiment, the liquid alkali metal may be a sodium alloy containing Cs. The amount of non-metallic sodium in the alloy may account for, for example, 1-20 wt% of the total sodium alloy by weight. Thus, the non-metallic sodium may be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt%, 10 wt%, 15 wt%, 20 wt% or between any two of the above values and including the amount of any two values.

[0066] In any embodiment of the methods and systems herein, the anode current collector in electrical contact with the anode is disposed in the anolyte chamber of the electrochemical cell. In any such embodiment, the anode current collector may be or comprise one or more of nickel (e.g., foam or mesh), various carbon types (e.g., carbon foam, carbon felt), steel, Kovar, or cobalt. In any embodiment, the anode current collector may comprise nickel.

[0067] In any embodiment, the electrochemical flow battery may further comprise a cathode current collector. The cathode current collector may comprise nickel or other suitable materials known in the art.

[0068] In any embodiment, the ceramic sodium ion conducting membrane may be a sodium superionic conductor, a sodium ion conducting garnet-like ceramic, sodium β″-alumina, or a sodium conducting glass ceramic. For example, in the case where the negative electrode active material comprises sodium, the alkali ion conducting membrane may include any suitable type of NaSICON membrane, a sodium ion conducting garnet-like ceramic, Na-β″-alumina, or a sodium ion conducting glass ceramic membrane. The NaSICON composition may include, but is not limited to, Na3Zr2Si2PO 12 、Na 1+x Si x Zr2P 3-x O 12(where x = 1.6 to 2.4), yttrium-doped NaSICON (e.g., Na 1+x+y Zr 2-y Y y Si x P 3-x O 12 、Na 1+x Zr 2-y Y y Si x P 3-x O 12-y , where x = 1.6 - 2.4, y = 0 - 0.25), Na 1+x Zr2X y (PO4)3, where x ranges from 0 to 3, y ranges from 0 - 1.5 and X is a dopant (e.g., Fe, Al, Ti, Hf, Co, Ni, Nb) and Fe-doped NaSICON (Na3Zr 2 / 3 Fe 4 / 3 P3O 12 ). Non-limiting examples of Na-β″-alumina membranes are Na (1.53-1.73) Li (0.28-0.32) Al (10.66-10.72) O 17 . In any embodiment, the sodium ion conducting ceramic membrane can be a sodium ion conducting garnet-like ceramic with the general formula A x B2C3O 12 , where A is an alkali metal ion, where x = 3 - 9 (B = Te 6+ , Ta 5+ , Nb 5+ , Zr 4+ ; C = La 3+ ; Y 3+ ; Nd 3+ ). Non-limiting examples of Na-conducting ceramic glasses include sodium phosphates such as xNa2O.yP2O5, sodium silicates such as xNa2O.ySiO2, sodium borates such as xNa2O.yB2O3, sodium aluminates such as xNa2O.yAl2O3, and mixtures of any two or more thereof; in any of the above, the molar ratio of x:y can range from 1:3 to 3:1, 1:2 to 3:1, 1:2 to 2:1, 1:2 to 1:1, 1:3 to 2:1, or 1:3 to 1:1.

[0069] In any embodiment, the ceramic sodium ion conducting membrane is a NaSiCON or a Na-β″-alumina membrane. In any embodiment, the conductivity of the ceramic sodium ion conducting membrane can be at least 10 mS cm -1 or at least 50 mS cm -1 , for example, the conductivity is 10 mScm -1 to 100 mS cm-1 For example, the conductivity of the ceramic sodium ion conductive membrane can be 10 cm -1 , 20 cm -1 , 30 cm -1 , 40 cm -1 , 50 mS cm -1 , 60 mS cm -1 , 70 mS cm -1 , 80 mS cm -1 , 90 mS cm -1 , 100 mS cm -1 or a range between any two of the above values and including any two values.

[0070] As disclosed herein, the dual-temperature method of the technology of the present invention reduces or avoids the degradation of temperature-sensitive solvents used as electrolytes, such as the anolyte in an electrolyte flow battery, to maintain high throughput and extend service life. The temperature control in the method of the present invention can be carried out in several ways. In any embodiment, the anolyte can be heated to a first temperature shortly before or at the time of entering the anolyte chamber, and can be cooled to a second temperature immediately or shortly after leaving the anolyte chamber. In any embodiment, the ceramic sodium ion conductive membrane or the ceramic sodium ion conductive membrane and the anolyte chamber are heated to the first temperature, for example, by a thermal shroud around the anolyte chamber or by a heat exchange element within the anolyte chamber. The anolyte can thus be heated to the first temperature by the anolyte chamber, or can also be heated in whole or in part before entering the heated anolyte chamber.

[0071] In any embodiment, the range of the first temperature can be from about 115 °C to about 150 °C, such as about 115 °C, about 120 °C, about 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, 150 °C or a range between any two of the above values and including any two values. In any embodiment, thus the range of the first temperature can also be from about 120 °C to about 150 °C or from about 125 °C to about 150 °C or about 145 °C.

[0072] In any embodiment, the range of the second temperature can be from about 80 °C to less than 115 °C, such as about 80 °C, about 85 °C, about 90 °C, about 95 °C, about 110 °C, less than 115 °C or a range between any two of the above values and including any two values. In any embodiment, thus the range of the second temperature can also be about 80 °C or from about 85 °C to about 110 °C.

[0073] In view of the guidance provided herein, the following is within the skill in the art: Select a first temperature to provide a desired conductivity and flux and select a second temperature that limits the degradation of temperature-sensitive solvents in the anolyte to provide a long service life at the desired flux while minimizing the energy consumption for heating / cooling the anolyte. Thus, while in some embodiments the second temperature will be selected such that the temperature-sensitive solvent degrades at a rate less than 20% of the rate that occurs at the first temperature, a second temperature that limits degradation to other rates can be readily selected by one of ordinary skill in the art. Thus, in some embodiments, the second temperature is selected to cause less than 25%, less than 20%, less than 15%, less than 10%, less than 5%, less than 4%, 3%, 2%, or 1% of the degradation rate that occurs at the first temperature. In any embodiment, the

[0074] Thus, in any embodiment, the method may comprise:

[0075] Introducing an anolyte at a first temperature into an anolyte chamber of an electrochemical flow battery, wherein

[0076] the anolyte chamber includes an anode comprising a sodium salt;

[0077] the anolyte includes an effective amount of a sodium salt dissolved in a temperature-sensitive solvent;

[0078] the electrochemical flow battery further includes a catholyte chamber and a ceramic sodium ion-conducting membrane separating the anolyte chamber from the catholyte chamber; and

[0079] the catholyte chamber includes a molten sodium cathode;

[0080] causing sodium ions from the sodium salt to pass from the anolyte chamber through the sodium ion-conducting membrane to the catholyte chamber;

[0081] reducing the sodium ions to metallic sodium at the molten sodium cathode; and

[0082] cooling the anolyte outside the anolyte chamber to a second temperature;

[0083] wherein

[0084] the conductivity of the ceramic sodium ion-conducting membrane is at least 50 mS cm -1 ; and

[0085] the second temperature causes the temperature-sensitive solvent to degrade at a rate less than 20% of the rate that occurs at the first temperature;

[0086] The sodium salts are selected from the group consisting of sodium sulfide, polysodium sulfide, a combination of sodium sulfide and one or more polysodium sulfides, sodium halide, polyhalide sodium, and a combination of sodium halide and one or more polyhalide sodiums; and

[0087] The temperature-sensitive solvent includes an alkyl diol and / or a cycloalkyl diol.

[0088] In some such embodiments, the sodium salt is selected from the group consisting of sodium sulfide, polysodium sulfide, or a combination of sodium sulfide and one or more polysodium sulfides.

[0089] The method of the present invention can use a variety of electrochemical cell configurations. Figure 1A An illustrative embodiment of one such cell configuration is schematically shown. Cell 100 includes a molten sodium cathode 110 (which can be sodium or a sodium alloy) disposed in a catholyte chamber 115. The cell also includes an anode 120 (i.e., a sodium salt) disposed in an anolyte chamber 125. A ceramic sodium ion-conducting membrane 130 (e.g., NaSICON, Na-β″-alumina, a sodium ion-conducting garnet-like ceramic, a sodium-conducting glass ceramic, etc.) separates the catholyte chamber and the anolyte chamber and their contents. Membrane 130 can be fixed to the cell housing with O-rings 140A and 140B. The cell can include a cathode and an anode current collector 150A and 150B that are in electrical contact with the cathode and the anode, respectively. During cell operation, the anolyte can be circulated in and out of the anolyte chamber. An optional external chamber that is fluidly connected to the catholyte chamber is not shown, and excess molten sodium can flow into the optional external chamber during the regeneration of metallic sodium. It should be understood that the electrochemical cells of the technology of the present invention can be configured with sensors, controllers, monitors, regulators, flow meters, access ports, and alarm mechanisms to allow the concentration and ratios of components such as sodium, elemental sulfur, temperature-sensitive solvent, oxidation state, open-circuit cell voltage, etc. to be monitored, measured, and maintained.

[0090] Alternatively, Figure 1BA schematic cross-sectional view showing another illustrative embodiment of an electrolytic cell of the technology of the present invention is presented. The cell includes a housing 310, which is generally an electrical insulator and chemically resistant to solvents and sodium sulfide. A ceramic sodium ion conductive membrane 312, in this case in the form of a tube, separates the catholyte chamber 314 from the anolyte chamber 316. Inside the catholyte chamber are a cathode (molten sodium or sodium alloy) and a cathode current collector. The cathode current collector 324 can be configured to penetrate the housing 310 or have a lead 325 that penetrates the housing 310, such that a connection can be established with the negative terminal (not shown) of a DC power supply. Inside the anolyte chamber 316 is an anode current collector 326, which in this case is shown as a cylindrical porous mesh electrode that surrounds the membrane tube 312. A lead 328 penetrates the housing such that a connection can be established with the positive terminal of a DC power supply. Anolyte solution flows through the anolyte inlet 330. The anolyte includes an anode (i.e., a sodium salt) as described herein. As the anolyte flows through the inlet 330, the anolyte also exits through the outlet 332. In some cases, a second liquid phase of molten sulfur can also leave with the anolyte. An optional second outlet can be provided from the anolyte chamber at a location below the anolyte outlet 332. The second lower outlet can be more for removing molten sulfur that has settled and accumulated at the bottom of the cell. The space between the cathode 324 and the membrane 312 is generally filled with molten alkali metal. When the cell operates, alkali metal ions pass through the membrane 312 and are reduced at the cathode 324 to form an alkali metal in the catholyte chamber 314, causing the alkali metal to flow through the catholyte outlet 334.

[0091] The cell can have multiple anodes, anode current collectors, cathodes, cathode current collectors, and membranes. Inside the cell, the anodes / current collectors are all in parallel, and the cathodes / current collectors are all in parallel.

[0092] Referring to Figure 1B , the electrolytic cell housing 310 can be an electrical insulating material such as most polymers. The material is also preferably chemically resistant to solvents. Polytetrafluoroethylene (PTFE) as well as polyvinylidene fluoride or high-density polyethylene (HDPE) are particularly suitable. The cell housing 310 can also be made of a non-insulating and non-chemically resistant material, provided that the interior of the housing 310 is lined with such an insulating and chemically resistant material. Other suitable materials would be inorganic materials such as alumina, silica, aluminosilicates, and other insulating refractory materials or ceramic materials.

[0093] The ceramic sodium ion conductive membrane 312 is preferably substantially permeable only to sodium and substantially impermeable to anions, polyanions, and dissolved sulfur. The membrane 312 can be partially made of an alkali metal ion conductive material. If the metal to be recovered by the battery is sodium, a useful material for the separator is NaSICON, which has a relatively high ionic conductivity. Typical NaSICON compositions can be as disclosed herein. The membrane 312 can have a portion of its thickness that has negligible porosity such that the liquids in the anolyte chamber 316 and the catholyte chamber 314 cannot transfer from one chamber to the other, but substantially only sodium ions can transfer from the anolyte chamber 316 to the catholyte chamber 314. The membrane can also be partially comprised of an alkali metal (i.e., sodium) ion conductive glass ceramic such as a material produced by Ohara Glass of Japan.

[0094] The anode current collector 326 is positioned within the anolyte chamber 316. The anode current collector can be made of a conductive material such as stainless steel, nickel, iron, iron alloy, nickel alloy, and other anode materials known in the art. The anode 326 is connected to the positive pole of a DC power source. The anode 326 can be a mesh, foam, monolithic structure, or can be a monolithic material having features that permit passage of the anolyte (including the sodium salt anode) to the anode structure. The anolyte solution is fed into the anolyte chamber through an inlet 330 and exits the chamber through an outlet 332. The electrolytic cell 300 can also be operated in a semi - continuous manner where the anolyte chamber is fed and partially drained through the same passage.

[0095] The conductive cathode current collector 324 can be in the form of a strip, ribbon, rod, or mesh. The cathode current collector 324 can comprise mostly electronic conductors such as nickel, steel, iron, copper, or graphite. A portion of the cathode current collector is disposed within the catholyte chamber 314 and a portion is outside the catholyte chamber 314 and the cell housing 310 for electrical contact. Alternatively, a lead 325 can extend from the cathode current collector outside the cell housing 310 for electrical contact. Within the catholyte chamber 314 is a molten sodium bath.

[0096] A non - limiting example of the operation of the electrolytic cell 300 is described as follows: The anolyte solution is fed into the anolyte chamber 316. The electrode current collectors 324, 326 are connected to a voltage source such that there is a voltage greater than Na2S between the anode current collector 326 and the cathode current collector 324. xThe potential of the dissociation voltage, depending on the composition, ranges between approximately 1.8 V and approximately 2.5 V. For example, the dissociation voltage can be 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, or 2.5 V or a range between any two of the above values and including any two values. At the same time, sodium ions pass through the membrane 312 into the cathode compartment 314, where the sodium ions are reduced to the metallic state within the cathode compartment 314, with electrons being provided through the cathode current collector 324, and sulfides and polysulfides are oxidized at the anode current collector 326 such that lower polysulfide anions are converted into higher polysulfide anions and / or elemental sulfur forms. When sulfur is formed, the sulfur dissolves wholly or partially in the anode liquid solvent. At sulfur saturation or upon cooling, the sulfur can form a second liquid phase that settles to the bottom of the anode compartment 316 of the electrolytic cell. The sulfur can be removed together with the anode liquid to settle in a container outside the cell, or the sulfur can be removed directly from the settling zone 336 through an optional sulfur outlet 338, as shown. Alternatively, the cell 300 can be part of an electrochemical system that otherwise removes sulfur from the anode liquid, and the sulfur outlet 338 is absent.

[0097] The cells of the technology of the present invention can be arranged in series. That is, the anode liquid of one electrolytic cell can flow into a second cell, where in each successive cell, the ratio of sodium to sulfide decreases as the polysulfide form becomes higher order. Such embodiments are illustrated in Figure 1C .

[0098] The methods disclosed herein can be carried out in various types of electrochemical systems. By way of illustrative example only, Figure 8 the process flow diagram (PFD) shown in Figures 1A - 1C) wherein sodium metal 245 is regenerated from the sodium salt and removed from the cell. As it exits the electrochemical cell, spent anolyte 242 is cooled in a second heat exchanger 250 to a temperature below 110 °C as described herein, e.g., from about 80 to about 100 °C. The cooled anolyte 255, which may contain some dissolved elemental sulfur, is recycled to the anolyte tank 210.

[0099] At the splitter 220, a portion of the anolyte 260 exiting the anolyte tank is directed to a crystallizer 265 where the anolyte is cooled to a temperature between about 15 °C and 80 °C by the circulating incoming (266A) and outgoing (266B) cooling fluid. Other suitable temperatures within this range may be used, including, for example, 15 °C to 60 °C, 30 °C to 80 °C, or 40 °C to 80 °C. Sulfur 277 precipitates out, including as crystals, and is then filtered out as the anolyte 270 passes through a sulfur filter 275. The lower temperature in this part of the system and process not only reduces the solubility of sulfur in the anolyte, causing precipitation / crystallization of sulfur (S8), but also renders Na2S x unstable, promoting S8 formation and precipitation / crystallization. The desulfurized anolyte 280 is then recycled back to the anolyte tank 210. Those skilled in the art will appreciate that other methods of removing dissolved sulfur from the anolyte 260 may be employed, such as gravimetric methods (e.g., centrifugation). Alternatively, a different anolyte solvent system with lower sulfur solubility may be used at a temperature above the sulfur melting point such that elemental sulfur can be removed as a liquid. Other sulfur removal techniques may also be used, such as extraction with a non-polar solvent immiscible with the anolyte. The following are within the skill in the art: modifying the systems and processes of the present invention to use any suitable sulfur removal technique and making other minor modifications, such as including additional fluid drives (e.g., pumps), filters, heat exchangers, etc. as needed and arranging such components to meet the needs at hand.

[0100] Examples

[0101] Materials. Ethylene glycol was obtained from Univar.

[0102] Example 1 - Electrochemical Cell for Producing Sodium Metal from Sodium Polysulfide Using a NaSICON Ceramic Membrane

[0103] A hybrid electrochemical flow battery was constructed for use in the disclosed method. The flow battery has an anolyte chamber, a catholyte chamber, and a 4.5 mm thick NaSICON ceramic membrane that separates the anolyte and catholyte chambers. The anolyte chamber contains a nickel anode current collector disposed therein, as well as an anolyte inlet and an anolyte outlet. The catholyte chamber contains a molten sodium cathode and a nickel cathode current collector. A power supply (programmable Ametek Sorensen XHR series: 7.5 V, 130 A) is electrically connected to the current collectors in each chamber. During battery operation, an anolyte containing dissolved Na2S and sulfur in ethylene glycol flows into the anolyte chamber through the inlet. The anolyte enters the anolyte chamber at a temperature between 115 °C (or 125 °C) and 150 °C and is heated to this temperature shortly before entering the battery (e.g., in a heat exchanger as shown in Figure 8 ). The sulfide / polysulfide is partially or completely oxidized to form higher polysulfides and elemental sulfur, which are at least partially dissolved in the ethylene glycol of the anolyte. Sodium ions from the oxidized sulfide / polysulfide are transported across the NaSICON membrane to the catholyte chamber, where a molten sodium cathode and a nickel current collector are disposed. The sodium ions are reduced to metallic sodium at the cathode, and the excess sodium flows out through the catholyte chamber outlet. The "spent" anolyte containing higher polysulfides and elemental sulfur flows out of the chamber through the anolyte outlet. A portion of the anolyte is sent to a crystallizer, where a portion of the anolyte is further cooled to precipitate / crystallize elemental sulfur, which is filtered out of the anolyte and then returned to the storage tank. A portion of the spent anolyte is returned to the storage tank, where additional sodium sulfide / polysulfide is dissolved into the anolyte, and then it is recycled back to the electrochemical cell to produce more sodium, higher polysulfides, and elemental sulfur. Figure 9 Shows the applied voltage required for this battery to reach 57 mA / cm over time in a dual-temperature system of the technology of the present invention operating at low temperature (110 °C), dual temperatures (100 °C and 125 °C), and 125 °C. 2 Figure 10 Shows the applied voltage and current density at a battery inlet temperature of 125 °C to 150 °C. After being exposed to high temperature (>125 °C) for more than 350 hours, the battery performance returns to the baseline conditions of 57 mA / cm 2 and 3.08 V, indicating that the dual-temperature system prevents anolyte degradation over the entire temperature range.

[0104] Example 2 - XRD Analysis of Spent Anolyte

[0105] ​The electrochemical cell of Example 1 is operated at an anolyte temperature of about 130 °C. The spent anolyte is collected, ethylene glycol is vacuum distilled and the remaining solid is dried and examined by X-ray powder diffraction (XRD). Sodium sulfur oxygen compounds (such as sodium thiosulfate, sodium sulfate, etc.) are identified as due to the passage of Na2S x extracting oxygen from ethylene glycol. Further, sodium carboxylates (such as sodium acetate, sodium glycolate, sodium formate, sodium oxalate, etc.) are identified as due to the electrochemical oxidation of EG in an alkaline medium.

[0106] Example 3 - Effect of temperature on anolyte containing Na2S5

[0107] The effect of temperature on an anolyte containing the model polysulfide Na2S5 in the model anolyte solvent ethylene glycol was studied by FT-IR using a Bruker Tensor 37. Na2S5 was prepared by mixing a 1:4 molar ratio of Na2S and S8 mixture in EG until it dissolved to nominally provide Na2S5. Figure 3 FT-IR spectra of Na2S5 dissolved in EG and incubated at 110 °C, 125 °C, and 150 °C for 195 h are shown compared to the baseline (i.e., prepared without any thermal incubation). The data show a minor change in the peak height of the 110 °C data, but large changes at 125 °C and 150 °C, including the appearance of new peaks, indicating the formation of new compounds.

[0108] At Figure 4 and 5 the change in peak height in the FT-IR spectra at 1725 cm -1 and 3250 cm -1 wavenumbers with respect to the baseline (i.e., normalized) is plotted. The absorption band at about 1725 cm -1 is associated with the byproduct 2-hydroxyethyl acetate, and the absorption band at 3250 cm -1 is associated with EG. The data show that during the first 65 h of incubation, the anolytes are similarly affected at the three test temperatures (110 °C, 125 °C, and 150 °C). However, further incubation shows no further deterioration at 110 °C, while further deterioration is observed at 125 °C and 150 °C. Specifically, the increase in the absorption rate at 1725 cm -1 with increasing temperature indicates an increase in the side reaction producing 2-hydroxyethyl acetate. The decrease in the absorption rate at 3250 cm -1 with increasing temperature indicates that EG disappears with increasing temperature as it may be converted to byproducts such as 2-hydroxyethyl acetate. Figure 6Shows the mass loss as a percentage of the baseline amount after 195 hours for two sets of anolytes with 3 wt% and 7 wt% sodium content at three temperatures. The data shows that the mass loss is minimal at 110 °C compared to 125 °C and 150 °C.

[0109] In a separate test, anolyte of Na2S5 in EG at 5 wt% sodium was prepared and incubated at various temperatures between 80 °C and 150 °C. The results are shown in Figure 7 as a plot of mass loss % versus incubation time. The data shows a similar trend to the 7% and 3% Na samples, where samples at 110 °C and below show negligible weight loss.

[0110] The anolytes of the cells (in Example 1) operated at low temperature (110 °C), dual temperature (100 °C and 125 °C), and 125 °C (as Figure 9 shown) were analyzed by FT-IR. The results of the three samples are shown in Figure 11 and 12 as the normalized peak heights at 1580 cm -1 and 1725 cm -1 respectively, indicating the presence of decomposition products in the anolyte. The data shows that greater degradation occurs at a constant temperature of 125 °C (no temperature fluctuation test) compared to 110 °C and the dual temperature tests.

[0111] Example 4 - Electrochemical Cell for Producing Metallic Sodium from Sodium Polysulfide Using a β″-Alumina Ceramic Membrane

[0112] A hybrid electrochemical flow battery was constructed using a β″-alumina ceramic membrane of the same thickness as the NaSICON ceramic membrane as in Example 1 to separate the anolyte chamber and the catholyte chamber. Due to the lower conductivity of the β″-alumina membrane, the cell operates at approximately one-third of the current density of the corresponding cell using NaSICON in Example 1.

[0113] Alternatively, a hybrid electrochemical flow battery was constructed using a B′-alumina membrane with approximately one-quarter to one-half the thickness of the NaSICON ceramic membrane as in Example 1 to separate the anolyte chamber and the catholyte chamber. The current density of this electrochemical flow battery is expected to be comparable to that of the corresponding cell using NaSICON in Example 1.

[0114] Example 5 - Electrochemical Cell for Producing Metallic Sodium from Sodium Polysulfide Using a Mixture of NaSICON Ceramic Membrane and a Temperature-Sensitive Solvent

[0115] Hybrid electrochemical flow batteries are constructed using different temperature-sensitive solvents in the anolyte as in Example 1. In this battery, an 80:20 w / w mixture of ethylene glycol and NMP is used as the temperature-sensitive solvent. The battery is operated under the same conditions as in Example 1. It is expected that adding NMP to ethylene glycol will increase the sodium ion conductivity of the anolyte.

[0116] Equivalents

[0117] Although certain embodiments have been illustrated and described, those of ordinary skill in the art can make changes, equivalent substitutions, and other types of changes to the methods, electrolytic cells, electrolytes, electrodes, systems, and operating conditions of the present invention as set forth herein after reading the foregoing specification. Each of the foregoing aspects and embodiments may also have incorporated or included therein variations or aspects as disclosed with respect to any or all of the other aspects and embodiments.

[0118] The technology of the present invention is also not limited to the specific aspects described herein, which are intended to be illustrative of individual aspects of the technology of the present invention only. Many modifications and variations of the present invention are possible and will be apparent to those skilled in the art without departing from the spirit and scope of the present invention. In addition to the methods recited herein, functionally equivalent methods within the scope of the technology of the present invention will be apparent to those skilled in the art from the foregoing description. Such modifications and variations are also intended to fall within the scope of the appended claims. It is to be understood that the technology of the present invention is not limited to the specific methods, solvents, electrolyte reagents, compounds, compositions, cells, and conditions, and of course, the specific methods, solvents, electrolyte reagents, compounds, compositions, cells, and conditions may vary. Unless otherwise indicated herein or otherwise clearly contradicted by the context, all methods described herein may be performed in any suitable order. It is also to be understood that the terms used herein are for the purpose of describing particular aspects only and are not intended to be limiting. Accordingly, it is intended that the present specification be considered exemplary only, where the breadth, scope, and spirit of the technology of the present invention are indicated only by the appended claims, the definitions therein, and any equivalents thereof. No language in the present specification should be construed as indicating any non-claimed element as essential.

[0119] Embodiments of the illustrative description herein can be practiced appropriately without any one or more elements, one or more limitations not specifically disclosed herein. Thus, for example, terms such as "comprising", "including", "containing", etc. should be understood expansively and without limitation. Additionally, the terms and expressions employed herein are used in a descriptive rather than limiting sense, and in using such terms and expressions, there is no intention to exclude any equivalents or portions thereof of the features shown and described, but it should be recognized that various modifications can be made within the scope of the claimed technology. Additionally, the phrase "consisting essentially of" will be understood to include those elements specifically recited and additional elements that do not materially affect the basic and novel characteristics of the claimed technology (e.g., the conductivity or current density of the claimed embodiment). The phrase "consisting of" excludes any element not specified. Further, the use of any of the above terms in the description of a particular element or embodiment also contemplates the use of any other of the terms. For example, the use of "including" with respect to an element or embodiment will also be understood to disclose the use of "consisting essentially of" or "consisting of" with respect to the same element or embodiment, and vice versa.

[0120] In addition, in the case where the features or aspects of the present disclosure are described in terms of a Markush group, those skilled in the art will recognize that the present disclosure is also thereby described in terms of any single member or subgroup of members of the Markush group. Each of the narrower types and subgeneric groups belonging to the general disclosure also forms part of the technology. This includes the general description of the technology, the preconditions or negative limitations of which remove any subject matter from the genus, whether or not the material so removed is specifically recited herein.

[0121] As those skilled in the art will understand, for any and all purposes, particularly with respect to providing a written description, all ranges disclosed herein also cover any and all possible subranges and combinations of subranges thereof. Any listed range can be readily considered to be fully described and enabled to be decomposed into at least equal halves, thirds, fourths, fifths, tenths, etc. of the same range. As a non-limiting example, each range discussed herein can be readily decomposed into lower third, middle third, upper third, etc. As those skilled in the art will also understand, all language such as "at most", "at least", "greater than", "less than", etc. includes the recited numbers and refers to ranges that can be subsequently decomposed into subranges as described above. Finally, as those skilled in the art will understand, ranges include each individual member, and each individual value is incorporated into the specification as if recited individually herein.

[0122] All publications, patent applications, issued patents, and other documents (e.g., journals, articles, and / or textbooks) cited in this specification are hereby incorporated by reference as if each individual publication, patent application, issued patent, or other document was specifically and individually indicated to be incorporated by reference in its entirety. To the extent that definitions contained in the text incorporated by reference conflict with the definitions in the present disclosure, the said definitions are not included.

[0123] Other embodiments are set forth in the following claims, along with the full scope of equivalents to such claims.

Claims

1. A method for preparing metallic sodium from sodium salts, the method comprising: introducing anolyte at a first temperature of 115 °C to 150 °C into the anolyte chamber of an electrochemical flow battery, wherein the anolyte is heated to the first temperature shortly before or upon entering the anolyte chamber; and wherein the anolyte comprises sulfur and an effective amount of sodium salt dissolved in a temperature-sensitive solvent; the anolyte chamber comprises an anode, wherein the anode comprises the sodium salt; the electrochemical flow battery further comprises a catholyte chamber and a ceramic sodium ion-conducting membrane separating the anolyte chamber from the catholyte chamber; and the catholyte chamber comprises a molten sodium cathode; allowing sodium ions from the sodium salt to pass from the anolyte chamber through the ceramic sodium ion-conducting membrane to the catholyte chamber; reducing the sodium ions to metallic sodium at the molten sodium cathode; and cooling the anolyte outside the anolyte chamber to a second temperature of 80 °C to less than 115 °C immediately upon or shortly after leaving the anolyte chamber, without recovering sulfur; wherein the second temperature causes the temperature-sensitive solvent to degrade at a rate less than 20% of the rate occurring at the first temperature; Wherein the temperature-sensitive solvent is selected from the group consisting of: alkyl diols, alkyl triols, cycloalkyl diols, cycloalkyl triols, lactams, cyclic ureas, C 1-6 alkylamides, carbonates, ethers, mixtures of any two or more thereof, or mixtures of any one of the foregoing with water.

2. The method according to claim 1, wherein the sodium salt comprises sodium sulfide, polysodium sulfide, or a mixture of sodium sulfide and one or more polysodium sulfides.

3. The method according to claim 2, wherein the polysulfide has the formula Na2S x , where x is an integer from 1 to 32.

4. The method according to claim 1, wherein the amount of sodium in the anolyte ranges from 1 wt% to 10 wt%.

5. The method according to claim 1, wherein the amount of sodium in the anolyte ranges from 3 wt% to 7 wt%.

6. The method according to claim 1, further comprising recovering the sulfur from the anolyte at a temperature of 15 °C to 60 °C.

7. The method according to claim 1, wherein the temperature-sensitive solvent is selected from the group consisting of ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, glycerol, and mixtures of two or more thereof.

8. The method according to claim 1, wherein the temperature-sensitive solvent is ethylene glycol.

9. The method according to claim 1, wherein an anode current collector in electrical contact with the anode is disposed in the anolyte chamber of the electrochemical cell.

10. The method according to claim 9, wherein the anode current collector comprises one or more of nickel, various carbon types, steel, Kovar, or cobalt.

11. The method according to claim 10, wherein the anode current collector comprises nickel.

12. The method according to claim 1, wherein the electrochemical flow battery further comprises a cathode current collector.

13. The method according to claim 12, wherein the cathode current collector comprises nickel.

14. The method according to claim 1, wherein the ceramic sodium ion-conducting membrane comprises a sodium ion-conducting garnet-like ceramic, a Na-conducting ceramic glass, a NaSiCON, or a Na-β”-alumina.

15. The method according to claim 1, wherein the ceramic sodium ion-conducting membrane or the ceramic sodium ion-conducting membrane and the anolyte chamber are heated to the first temperature.

16. The method according to claim 1, wherein the ceramic sodium ion conductive membrane or the ceramic sodium ion conductive membrane and the anolyte chamber are heated to the first temperature by a thermal hood.

17. The method according to claim 1, wherein when the anolyte is in the anolyte chamber, the conductivity of the ceramic sodium ion conducting membrane is at least 10 mS cm -1 .

18. The method according to claim 1, wherein when the anolyte is located in the anolyte chamber, the conductivity of the ceramic sodium ion conductive membrane is at least 50 mS cm -1 .

19. The method according to claim 1, wherein the conductivity of the ceramic sodium ion conductive membrane is 10 mS cm -1 to 100 mS cm -1 .

20. A method for preparing metallic sodium from a sodium salt, the method comprising: introducing an anolyte at a first temperature of 115°C to 150°C into an anolyte chamber of an electrochemical flow battery, wherein the anolyte is heated to the first temperature shortly before or upon entering the anolyte chamber; and wherein the anolyte comprises an effective amount of a sulfur-containing sodium salt dissolved in a temperature-sensitive solvent comprising an alkyl diol; the anolyte chamber comprises an anode, wherein the anode comprises the sulfur-containing sodium salt; the electrochemical flow battery further comprises a catholyte chamber and a ceramic sodium ion conductive membrane separating the anolyte chamber from the catholyte chamber; and the catholyte chamber comprises a molten sodium cathode; allowing sodium ions from the sodium salt to pass from the anolyte chamber through the ceramic sodium ion conductive membrane to the catholyte chamber; reducing the sodium ions to metallic sodium at the molten sodium cathode; cooling the anolyte outside the anolyte chamber to a second temperature of 80°C to less than 115°C immediately upon or shortly after leaving the anolyte chamber without recovering sulfur; wherein the second temperature causes the temperature-sensitive solvent to degrade at a rate less than 20% of the rate that occurs at the first temperature.

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