Electrolyte compositions for sodium-based batteries and methods of making the same

Abstract

Electrolyte compositions for sodium-based batteries and methods of forming the same are described. The electrolyte compositions include at least one sodium borate ester salt and a solvent. In some embodiments, the sodium borate ester is tetra(phenol)borate (NaB[OC6H5]4) or sodium tetra(isopropyl)borate (NaB[OC3H7]4), and the solvent is an ether-based solvent.

Description

ELECTROLYTE COMPOSITIONS FOR SODIUM-BASED BATTERIESAND METHODS OF MAKING THE SAMEBACKGROUND

[0001] A range of sodium electrolytes have previously been studied for their application in sodium batteries, predominantly based on the salts NaPFe, NaTFSI, NaFSI and NaCIO4. Carbonate electrolytes are commonly considered, with some works investigating ether- based solvents, particularly for sodium metal batteries. The performance of the electrolyte in a sodium battery is tied to the chemistry of both the salt and the solvent. Generally, NaPFe is treated as the current state-of-the-art due to the relative stability of the solid electrolyte interface (SEI) it forms at the sodium metal and sodium-carbon anode, which enables high Coulombic efficiency (CE). NaCIC , NaFSI and NaTFSI suffer from more substantial reactions (i.e. , lower CE) at the anode and a less stable SEI. Carbonate electrolytes tend to form stable SEI products to achieve high CE in later cycles, at the expense of consuming more sodium from the cell initially to form these side products, which reduces cell capacity and initial CE during early cycles. Ether electrolytes are less reactive at the anode, improving initial CE and consuming less sodium, however they tend to form SEI products that are less stable, inhibiting the battery from reaching high CE at longer cycles. Accordingly, a need exists for improved electrolytes for sodium batteries.SUMMARY

[0002] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary, and the foregoing Background, is not intended to identify key aspects or essential aspects of the claimed subject matter. Moreover, this Summary is not intended for use as an aid in determining the scope of the claimed subject matter.

[0003] In some embodiments, an electrolyte composition for use in sodium -based batteries is described. The electrolyte composition includes at least one sodium borate ester salt having the formula NaB[OR]4, NaBFy[OR]4-y (where y = 1 , 2, or 3), NaB[X][OR]2, NaB[X]F2, or NaB[X]2, wherein R in any of the preceding formula is selected from the groupconsisting of an alkyl group, a fluoroalkyl group, a resonant group, a fluoro-resonant group, an acetyl group, and a fluoro-acetyl group, and X in any of the preceding formula is selected from the group consisting of a diol group, a fluoro-diol group, a dicarboxyl group, and a fluoro-dicarboxyl group; and at least one ether-based solvent, fluoroether-based solvent, carbonate-based solvent, or ionic liquid solvent. In some embodiments, the sodium borate ester salt may have different R or X groups, i.e., having the formula NaBFx[ORa]y[ORb]z (where x+y+z = 4 and where x, y, z = 0 to 3), NaB[ORa][ORb][X], or NaB[Xa][Xb],

[0004] In some embodiments, an electrolyte composition for use in sodium -based batteries is described. The electrolyte composition includes at least one sodium borate ester salt selected from the group consisting of sodium tetra(phenol)borate and sodium tetra(isopropyl)borate, and at least one ether-based solvent.

[0005] In some embodiments, a method of forming an electrolyte composition for use in a sodium-based battery is described. The method includes a step of combining sodium borohydride, a solvent, and an alcohol having the formula ROH, where R is selected from the group consisting of an alkyl group, a fluoroalkyl group, a resonant group, and a fluoro- resonant group. Combining the sodium borohydride, solvent, and alcohol forms a solution comprising the solvent and a sodium borate ester salt having the formula NaB(OR)4.

[0006] In some embodiments, a method of forming an electrolyte composition for use in a sodium-based battery is described. The method includes a step of combining sodium borohydride, a solvent, and a ketone having the formula RO, where R is selected from the group consisting of an alkyl group and a fluoroalkyl group. Combining the sodium borohydride, solvent, and ketone forms a solution comprising the solvent and a sodium borate ester salt having the formula NaB(OR)4.

[0007] In some embodiments, a method of forming an electrolyte composition for use in a sodium-based battery is described. The method includes a step of combining sodium borohydride, a solvent, and a carboxylic acid having the formula ROH, where R is selected from the group consisting of an acetyl group and a fluoro-acetyl group. Combining the sodium borohydride, solvent, and carboxylic acid forms a solution comprising the solvent and a sodium borate ester salt having the formula NaB(OR)4.

[0008] In some embodiments, a method of forming an electrolyte composition for use in a sodium-based battery is described. The method includes a step of combining sodium borohydride, a solvent, and phenol. Combining the sodium borohydride, solvent, and phenol forms a solution comprising the solvent and sodium tetra(phenoxy)borate.

[0009] In some embodiments, a method of forming an electrolyte composition for use in a sodium-based battery is described. The method includes a step of combining sodium borohydride, a solvent, and isopropanol. Combining the sodium borohydride, solvent, and isopropanol forms a solution comprising the solvent and tetra(isopropyl)borate.

[0010] In some embodiments, a method of forming an electrolyte composition for use in a sodium-based battery is described. The method includes a step of combining borane (BH3), a solvent, a sodium salt having the formula NaORa, and an alcohol or carboxylic acid having the formula RaOH or RbOH, where Ra / bis selected from the group consisting of an alkyl group, a fluoroalkyl group, a resonant group, a fluoro-resonant group, an acetyl group, and a fluoro-acetyl group. Combining the borane, solvent, sodium salt and alcohol or carboxylic acid forms a solution comprising the solvent and a sodium borate ester salt having the formula NaB[ORa]y[ORb]4-y (where y = 1 , 2, 3, or 4). In some embodiments, the borane, solvent and alcohol or carboxylic acid are first reacted to form a borane ester with the formula B(ORa / b)3, which is then reacted with the sodium salt having the formula NaORato yield a sodium borate ester salt having the formula NaB[ORa]y[ORb]4-y. In some embodiments, the borane is instead boron trifluoride, which may be combined with a sodium salt having the formula NaOR and the alcohol or carboxylic acid to yield a sodium borate ester salt with the formula NaBFy[OR]4-y (y = 1 , 2, or 3). In some embodiments, the sodium salt may have the formula Na2[X] where X is selected from the group consisting of a diol group, a fluoro-diol group, a dicarboxyl group, a fluoro-dicarboxyl group, that, when combined with the borane, solvent, and alcohol or carboxylic acid, yields a sodium borate ester salt having the formula NaB[X][OR]2, NaB[X]F2, or NaB[X]2.

[0011] The electrolyte platforms detailed herein overcome the challenges set forth in the Background by providing high initial CE to achieve high initial capacity, while achieving high CE in later cycles to retain this capacity and enable long term battery cycling. Thetechnology described herein offers a range of compelling advantages, including, but not necessarily limited to:

[0012] 1 ) cost reduction: The adoption of borate-based electrolytes promises low material costs, potentially lowering the overall production cost of sodium batteries.

[0013] 2) versatility: borate-based electrolytes open the door to diverse combinations of salts and electrolyte solutions, adaptable to various battery applications. This versatility enables tailored solutions for specific energy storage requirements.

[0014] 3) Fluorine-free electrolytes: a notable advantage of borate-based electrolytes lies in the ability to formulate fluorine-free salts without sacrificing critical ionic conductivities, thus addressing environmental concerns associated with fluorinated compounds.

[0015] These and other aspects of the technology described herein will be apparent after consideration of the Detailed Description and Figures herein. It is to be understood, however, that the scope of the claimed subject matter shall be determined by the claims as issued and not by whether given subject matter addresses any or all issues noted in the Background or includes any features or aspects recited in the Summary.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Non-limiting and non-exhaustive embodiments of the disclosed technology, including the preferred embodiment, are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

[0017] Figure 1A is a flow diagram illustrating a method for forming an electrolyte composition in accordance with various embodiments described herein.

[0018] Figure 1 B is a flow diagram illustrating a method for forming an electrolyte composition in accordance with various embodiments described herein.

[0019] Figure 1 C is a flow diagram illustrating a method for forming an electrolyte composition in accordance with various embodiments described herein.

[0020] Figure 1 D is a flow diagram illustrating a method for forming an electrolyte composition in accordance with various embodiments described herein.

[0021] Figure 2 is an illustration of various sodium borate ester salts suitable for use in electrolyte compositions configured in accordance with various embodiments described herein.

[0022] Figure 3 illustrates boron NMR data for synthesized sodium borate ester salts as disclosed herein, demonstrating purity of synthesized salts and lack of other boron peaks associated with other salts or precursors.

[0023] Figure 4 illustrates galvanostatic cycling at increasing current (up to 6 mA) of Na metal vs Al foil coin cells with various electrolyte compositions configured in accordance with various embodiments described herein.

[0024] Figure 5 illustrates performance characteristics of a full battery cell cycling with a sodium tetra(phenoxy)borate in ether electrolyte configured in accordance with various embodiments described herein.DETAILED DESCRIPTION

[0025] Embodiments are described more fully below with reference to the accompanying Figures, which form a part hereof and show, by way of illustration, specific exemplary embodiments. These embodiments are disclosed in sufficient detail to enable those skilled in the art to practice the invention. However, embodiments may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein. The following detailed description is, therefore, not to be taken in a limiting sense.

[0026] Described herein are electrolyte compositions having suitability for use in sodium-based batteries. As used herein, the term sodium-based battery is intended to encompass all types of batteries incorporating sodium material in the cell architecture, such as the use of sodium as a material in the electrodes. Sodium-based batteries may include, but are not necessarily limited to, sodium batteries, sodium-ion batteries, sodium metal batteries, and anode-free sodium batteries. The electrolyte compositions described herein include at least one sodium borate ester salt, and are therefore well suited for use in sodium - based batteries.

[0027] In some embodiments, the electrolyte compositions described herein also beneficially incorporate glyme-based solvents, though not all embodiments disclosed herein require glyme-based solvents. In those embodiments incorporating glyme-based solvents, the electrolyte compositions may allow for battery operation and a wider range of temperatures by providing a generally non-flammable non-aqueous electrolyte.

[0028] In some embodiments, the electrolyte composition generally includes at least one sodium borate ester salt. The sodium borate ester salt may have the formula NaB(OR)4, NaBFy[OR]4-y (where y = 1 , 2, or 3), NaB[X][OR]2, NaB[X]F2, or NaB[X]2, wherein R in any of the preceding formula is selected from the group consisting of an alkyl group, a fluoroalkyl group, a resonant group, a fluoro-resonant group, an acetyl group, and a fluoro-acetyl group, and X in any of the preceding formula is selected from the group consisting of a diol group, a fluoro-diol group, a dicarboxyl group, and a fluoro-dicarboxyl group. The electrolyte composition further includes at least one ether-based solvent, fluoroether-based solvent, carbonate-based solvent, or ionic liquid solvent. While the composition may, in some embodiments, include one sodium borate ester salt and one solvent, other embodiments of the electrolyte composition may include multiple different types of sodium borate ester salts and / or solvents. For example, the electrolyte composition described herein may include two different sodium borate ester salts and one ether-based solvent, one sodium borate ester salt and two different ether-based solvents, two different sodium borate ester salts and two different ether-based solvents, one sodium borate ester salt, one ether-based solvent and one carbonate-based solvent, etc.

[0029] With respect to the one or more sodium borate ester salts included in the electrolyte composition, the sodium borate ester salt may have the formula NaB(OR)4, NaBFy[OR]4-y (where y = 1 , 2, or 3), NaB[X][OR]2, NaB[X]F2, or NaB[X]2, wherein R in any of the preceding formula is selected from the group consisting of an alkyl group, a fluoroalkyl group, a resonant group, a fluoro-resonant group, an acetyl group, and a fluoro-acetyl group, and X in any of the preceding formula is selected from the group consisting of a diol group, a fluoro-diol group, a dicarboxyl group, and a fluoro-dicarboxyl group. Any species within each of the R groups and X groups can be used in the sodium borate ester salt.

[0030] With respect to the formula NaB(OR)4, exemplary, though non-limiting, sodium borate ester salts having the formula NaB(OR)4 that are suitable for use in the electrolyte compositions described herein include sodium tetra(phenol)borate (NaBph), sodium tetra(isopropyl)borate, sodium tetra(trifluoro ethyl)borate (NaBtfe), sodium tetra(pentafluoro propyl)borate (NaBpfp), sodium tetra(nonafluoro tertbutyl)borate, and sodium tetra(hexafluoro isopropyl)borate (NaBhfip).

[0031] When a sodium borate ester salt having the formula NaBFy[OR]4-y, NaB[X][OR]2, or NaB[X]2 is used in the electrolyte composition, it should be noted that the sodium borate ester salt may have two different R or X groups with the sodium borate salt. For example, in the case of NaBFy[OR]4-ywhere the sodium borate ester salt includes two different R groups, the formula for the sodium borate ester salt can be presented as NaBFx[ORa]y[ORb]z, where Raand Rbare each a different R group, though each of Raand Rbare still selected from within the classes of R groups previously described. In this formula, x+y+z = 4 and x, y, z = 0 to 3.

[0032] Similarly, when the sodium borate ester salt has the formula NaB[X][OR]2, but includes two different R groups, the formula for the sodium borate ester salt can be presented as NaB[ORa][ORb][X], with Raand Rbeach being a different R group selected from within the classes of R groups previously described.

[0033] In the case of NaB[X]2, the sodium borate ester salt can include two different X groups, and as such, the formula for this sodium borate salt can be reflected as NaB[Xa][Xb], with Xaand Xbeach being a different X group selected from within the classes of X groups previously described.

[0034] As described previously, the electrolyte composition may include more than one sodium borate ester salt. For example, an electrolyte composition may include both sodium tetra(phenol)borate and sodium tetra(isopropyl)borate.

[0035] The total amount of sodium borate ester salt in the electrolyte composition, whether based on one type of sodium borate ester salt or multiple types of sodium borate ester salts that may be used in the composition, is generally in the range of from about 0.01 mol / L to 6 mol / L of the electrolyte composition.

[0036] With respect to the one or more solvents included in the electrolyte composition, the solvent may be selected from the group consisting of ether-based solvents, fluoroether- based solvents, carbonate-based solvent, or ionic liquid solvents. In some embodiments, ether-based solvents may be preferred. Exemplary, though non-limiting, ether-based solvents that can be used in the electrolyte compositions described herein include 1 ,2- dimethoxyethane (DME), Diethylene glycol dimethyl ether (DEGDME / diglyme), tetraethylene glycol dimethyl ether (TEGDME / tetraglyme), and 1 ,3-dioxolane. Exemplary, though non-limiting, fluoroether-based solvents that can be used in the electrolyte compositions described herein include 2, 2-dimethoxy-4-(trifluoromethyl)-1 ,3-dioxolane ether, and 1 ,1 ,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. As described previously, the electrolyte composition may include more than one solvent. For example, an electrolyte composition may include both 1 ,2-dimethoxyethane and diglyme.

[0037] The total amount of solvent in the electrolyte composition, whether based on one type or species of solvent or multiple types or species of solvents that may be used in the composition, is generally in the range of from about 1 wt% to about 99 wt% of the electrolyte composition. That is to say, when an electrolyte composition includes more than one type or species of solvent, the sum of the content of the multiple solvents present in the electrolyte composition is within the range of from 1 wt% to 99 wt%.

[0038] While the core components of the electrolyte composition are one or more sodium borate ester salts and one or more solvents, the electrolyte composition may include other materials as well. In some embodiments, the electrolyte composition includes one or more ether based solvents as the primary solvent, along with one or more non-ether based solvents. The electrolyte composition may also include one or more supplemental salts and / or one or more additives. These components may be present in the electrolyte composition in any suitable amount, though in some embodiments, these materials may be present at a wt% that is less than the amount of sodium borate ester salts and primary solvents used in the electrolyte composition.

[0039] With respect to non-ether based solvents, any suitable non-ether based solvents can be used provided that the non-ether based solvents do not prevent the electrolyte composition from functioning in the sodium-based battery. Exemplary, thoughnon-lim iting, non-ether based solvents that can be used in the electrolyte composition include ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and tetrahydrofuran (THF). Both aqueous and non-aqueous non-ether based solvents can be used. Secondary solvents may be used in the electrolyte to improve wettability, increase oxidative stability, modify viscosity, modify flammability, or vary the composition and properties of the anode-electrolyte and cathode-electrolyte interface.

[0040] With respect to supplemental salts, the electrolyte composition can include as supplemental salts any salt that are non-sodium borate ester salts and which do not prevent the electrolyte composition from functioning in the sodium-based battery. Exemplary, though non-limiting, supplemental salts that can be used in the electrolyte composition include sodium hexafluorophosphate (NaPFe), sodium perchlorate (NaCICk), sodium tetrafluoroborate (NaBF4), sodium tetraphenylborate, sodium nitrate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium (trifluoromethanesulfonyl)imide, sodium iodide, sodium bis(fluorosulfonyl)imide, iron trifluoromethanesulfonate, iron tetrafluoroborate, and iron acetylacetonate. Supplemental salts may be used in the electrolyte composition to provide various beneficial effects, including, but not limited to increasing ionic conductivity of the electrolyte, improving thermal stability, increasing wettability, modifying viscosity, improving the stability and transport through the anodeelectrolyte interphase and cathode-electrolyte interphase, improving electrode conductivity, and reducing solvation of electrode materials.

[0041] With respect to additives, any suitable additive can be used provided that the additives do not prevent the electrolyte composition from functioning in the sodium -based battery. Exemplary, though non-limiting, additives that can be used in the electrolyte composition include fluoroethylene carbonate (FEC), trans-difluoroethylene carbonate (DFEC), vinyl carbonate (VC), vinyl ethylene carbonate (VEC), trimethyl phosphite (TMP), tris(trimethylsilyl)phosphite (TMSP), tris(trimethylsilyl)borate, dimethyl acetamide, 2,2- dimethoxy-propane, vinyl acetate, and gamma-butyrolactone (GBL), biphenyl, thiophene, triphenylamine, and 1 ,4-benzodiozane-6,7-diol. Additives may be used in the electrolyte composition to provide various beneficial effects, including, but not limited to, reducingflammability, suppressing anode dendrite growth, improving the stability and transport through the anode-electrolyte interphase and cathode-electrolyte interphase, improving oxidative stability, improving electrode conductivity, and reducing solvation of electrode materials.

[0042] Methods of making the electrolyte compositions described herein are also part of the present disclosure. Figures 1A, 1 B, 1 C and 1 D generally outline four similar approaches to making the electrolyte compositions described herein.

[0043] With respect to Figure 1 A, a method 100A for making an electrolyte composition generally includes the step 110A of combining sodium borohydride, a solvent, and an alcohol having the formula ROH. When combined, a reaction between the sodium borohydride and the alcohol leads to the formation of a sodium borate ester salt carried by the solvent.

[0044] The specific order in which the sodium borohydride, the solvent, and the alcohol having the formula ROH are combined is generally not limited. In some embodiments, the sodium borohydride is combined with the solvent, after which the alcohol is added. However, the alcohol could be added to the solvent first, followed by addition of the sodium borohydride, or the sodium borohydride and the alcohol could be added to the solvent at the same time, as well as combining the sodium borohydride and the alcohol, following by adding the solvent.

[0045] The solvent used in step 110A is similar or identical to the solvent described previously with respect to the electrolyte composition. For example, the solvent may include one or more of 1 ,2-dimethoxyethane, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1 ,3-dioxolane. In embodiments where an ether-based solvent is used, the method may also include the use of a non-ether based solvent depending on the specific electrolyte composition being formed. For example, the method may use one or more ether based solvents and one or more non-ether based solvents.

[0046] In one embodiment of method 100A, the sodium borohydride and alcohol are combined with a non-ether based solvent and no ether based solvent is used in the method. This variation is suitable for use when preparing the embodiment of the electrolyte composition described herein that does not use an ether based solvent.

[0047] The alcohol used in step 110A is an alcohol having the formula ROH, where R is selected from the group consisting of an alkyl group, a fluoroalkyl group, and a resonant group. The R group used in the alcohol generally dictates that type of sodium borate ester salt formed by the reaction between the sodium borohydride and the alcohol. That is to say, when sodium borate ester salt is formed via step 110A, the sodium borate ester salt will have the formula NaB(OR)4, where R will be the same R as was present in the alcohol having the formular ROH.

[0048] Method 100A may optionally include an additional step 120A, wherein some or all of the solvent is removed. Any suitable method or methods for removing the solvent can be used as part of step 120A. In some embodiments, solvent is removed by heating the solution to evaporate off some or all of the solvent. The solution can also be subjected to vacuum in order to remove some or all of the solvent. The removal of some or all of the solvent can be useful to, e.g., change the concentration of the sodium borate ester salt in the solution that can be used as an electrolyte composition. Removing solvent generally increases the concentration of the sodium borate ester salt in the solution, which may be desirable depending on the specific sodium-based battery in which the solution will be used as an electrolyte composition.

[0049] In cases where all of the solvent is removed, this generally results in providing a dry form of the sodium borate ester salt. Having a dry form of the sodium borate ester salt may be beneficial for, for example, transport of the material, as well as allowing for the sodium borate ester salt to be reconstituted as needed, and with whatever solvent is desired by the end user.

[0050] In one specific example, the alcohol used in the method 100A is isopropanol. When the alcohol used in method 100A is isopropanol, the method may result in the formation of a solution comprising the solvent and tetra(isopropyl)borate.

[0051] With respect to Figure 1 B, a method 100B for making an electrolyte composition generally includes the step 110B of combining sodium borohydride, a solvent, and a ketone having the formula RO. When combined, a reaction between the sodium borohydride and the ketone leads to the formation of a sodium borate ester salt carried by the solvent.

[0052] The specific order in which the sodium borohydride, the solvent, and the ketone having the formula RO are combined is generally not limited. In some embodiments, the sodium borohydride is combined with the solvent, after which the ketone is added. However, the ketone could be added to the solvent first, followed by addition of the sodium borohydride, or the sodium borohydride and the ketone could be added to the solvent at the same time, as well as combining the sodium borohydride and the ketone, following by adding the solvent.

[0053] The solvent used in step 11 OB is similar or identical to the solvent described previously with respect to the electrolyte compositions. For example, the glyme solvent may include one or more of dimethoxyethane, diglyme, tetraglyme, and 1 ,3-dioxolane. In embodiments where an ether-based solvent is used, the method may also include the use of a non-ether based solvent depending on the specific electrolyte composition being formed. For example, the method may use one or more ether based solvents and one or more non-ether based solvents

[0054] In one embodiment of method 100B, the sodium borohydride and ketone are combined with a non-ether based solvent and no ether based solvent is used in the method. This variation is suitable for use when preparing the embodiment of the electrolyte composition described herein that does not use an ether based solvent.

[0055] The ketone used in step 110B is a ketone having the formula RO, where R is selected from the group consisting of an alkyl group and a fluoroalkyl group. The R group used in the ketone generally dictates that type of sodium borate ester salt formed by the reaction between the sodium borohydride and the ketone. That is to say, when sodium borate ester salt is formed via step 110B, the sodium borate ester salt will have the formula NaB(OR)4, where R will be the same R as was present in the ketone having the formular RO.

[0056] Method 100B may optionally include an additional step 120B, wherein some or all of the solvent is removed. Any suitable method or methods for removing the solvent can be used as part of step 120B. In some embodiments, solvent is removed by heating the solution to evaporate off some or all of the solvent. The solution can also be subjected to vacuum in order to remove some or all of the solvent. The removal of some or all of thesolvent can be useful to, e.g., change the concentration of the sodium borate ester salt in the solution that can be used as an electrolyte composition. Removing solvent generally increases the concentration of the sodium borate ester salt in the solution, which may be desirable depending on the specific sodium-based battery in which the solution will be used as an electrolyte composition.

[0057] In cases where all of the solvent is removed, this generally results in providing a dry form of the sodium borate ester salt. Having a dry form of the sodium borate ester salt may be beneficial for, for example, transport of the material, as well as allowing for the sodium borate ester salt to be reconstituted as needed, and with whatever solvent is desired by the end user.

[0058] With respect to Figure 1 C, a method 100C for making an electrolyte composition generally includes the step 110C of combining sodium borohydride, a solvent, and a carboxylic acid having the formula ROH. When combined, a reaction between the sodium borohydride and the carboxylic acid leads to the formation of a sodium borate ester salt carried by the solvent.

[0059] The specific order in which the sodium borohydride, the solvent, and the carboxylic acid having the formula ROH are combined is generally not limited. In some embodiments, the sodium borohydride is combined with the solvent, after which the carboxylic acid is added. However, the carboxylic acid could be added to the solvent first, followed by addition of the sodium borohydride, or the sodium borohydride and the carboxylic acid could be added to the solvent at the same time, as well as combining the sodium borohydride and the carboxylic acid, following by adding the solvent.

[0060] The solvent used in step 110C is similar or identical to the solvent described previously with respect to the electrolyte composition. For example, the solvent may include one or more of 1 ,2-dimethoxyethane, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1 ,3-dioxolane. In embodiments where an ether-based solvent is used, the method may also include the use of a non-ether based solvent depending on the specific electrolyte composition being formed. For example, the method may use one or more ether based solvents and one or more non-ether based solvents.

[0061] In one embodiment of method 100C, the sodium borohydride and carboxylic acid are combined with a non-ether based solvent and no ether based solvent is used in the method. This variation is suitable for use when preparing the embodiment of the electrolyte composition described herein that does not use an ether based solvent.

[0062] The carboxylic acid used in step 110C is a carboxylic acid having the formula ROH, where R is selected from the group consisting of an acetyl group and a fluoro-acetyl group. The R group used in the carboxylic acid generally dictates that type of sodium borate ester salt formed by the reaction between the sodium borohydride and the carboxylic acid. That is to say, when sodium borate ester salt is formed via step 110C, the sodium borate ester salt will have the formula NaB(OR)4, where R will be the same R as was present in the carboxylic acid having the formular ROH.

[0063] Method 100C may optionally include an additional step 120C, wherein some or all of the solvent is removed. Any suitable method or methods for removing the solvent can be used as part of step 120C. In some embodiments, solvent is removed by heating the solution to evaporate off some or all of the solvent. The solution can also be subjected to vacuum in order to remove some or all of the solvent. The removal of some or all of the solvent can be useful to, e.g., change the concentration of the sodium borate ester salt in the solution that can be used as an electrolyte composition. Removing solvent generally increases the concentration of the sodium borate ester salt in the solution, which may be desirable depending on the specific sodium-based battery in which the solution will be used as an electrolyte composition.

[0064] In cases where all of the solvent is removed, this generally results in providing a dry form of the sodium borate ester salt. Having a dry form of the sodium borate ester salt may be beneficial for, for example, transport of the material, as well as allowing for the sodium borate ester salt to be reconstituted as needed, and with whatever solvent is desired by the end user.

[0065] With respect to Figure 1 D, a method 100D for making an electrolyte composition generally includes the step 110D of combining sodium borohydride, a solvent, and phenol. When combined, a reaction between the sodium borohydride and the phenol leads to the formation of sodium tetra(phenoxy)borate carried by the solvent.

[0066] The specific order in which the sodium borohydride, the solvent, and the phenol are combined is generally not limited. In some embodiments, the sodium borohydride is combined with the solvent, after which the phenol is added. However, the phenol could be added to the solvent first, followed by addition of the sodium borohydride, or the sodium borohydride and the phenol could be added to the solvent at the same time, as well as combining the sodium borohydride and the phenol, followed by adding the solvent.

[0067] The solvent used in step 110D is similar or identical to the solvent described previously with respect to the electrolyte composition. For example, the solvent may include one or more of 1 ,2-dimethoxyethane, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1 ,3-dioxolane.

[0068] Method 100D may optionally include an additional step 120D, wherein some or all of the solvent is removed. Any suitable method or methods for removing the solvent can be used as part of step 120D. In some embodiments, solvent is removed by heating the solution to evaporate off some or all of the solvent. The solution can also be subjected to vacuum in order to remove some or all of the solvent. The removal of some or all of the solvent can be useful to, e.g., change the concentration of the sodium tetra(phenoxy)borate in the solution that can be used as an electrolyte composition. Removing solvent generally increases the concentration of the sodium tetra(phenoxy)borate in the solution, which may be desirable depending on the specific sodium -based battery in which the solution will be used as an electrolyte composition.

[0069] In cases where all of the solvent is removed, this generally results in providing a dry form of the sodium tetra(phenoxy)borate. Having a dry form of the sodium tetra(phenoxy)borate may be beneficial for, for example, transport of the material, as well as allowing for the sodium tetra(phenoxy)borate to be reconstituted as needed, and with whatever solvent is desired by the end user.

[0070] While not illustrated in the Figures, another method of forming the electrolyte composition described herein generally includes a step of combining borane (BH3), a solvent, a sodium salt having the formula NaORa, and an alcohol or carboxylic acid having the formula RaOH or RbOH, where Ra / bis selected from the group consisting of an alkyl group, a fluoroalkyl group, a resonant group, a fluoro-resonant group, an acetyl group, anda fluoro-acetyl group. Combining the borane, solvent, sodium salt and alcohol or carboxylic acid forms a solution comprising the solvent and a sodium borate ester salt having the formula NaB[ORa]y[ORb]4-y, where y = 1 , 2, 3, or 4. In some embodiments of this method, the borane, solvent, and alcohol or carboxylic acid are first reacted to form a borane ester with the formula B(ORa / b)3, which is then reacted with the sodium salt having the formula NaORato yield a sodium borate ester salt having the formula NaB[ORa]y[ORb]4-y.

[0071] In another embodiment of making an electrolyte composition as described herein, boron trifluoride is combined with a sodium salt having the formula NaOR, solvent, and an alcohol or carboxylic acid to yield a sodium borate ester salt with the formula NaBFy[ORi]4-y, where y = 1 , 2, or 3.

[0072] In still another embodiment of making an electrolyte composition as described herein, a sodium salt having the formula Na2[X] where X is selected from the group consisting of a diol group, a fluoro-diol group, a dicarboxyl group, a fluoro-dicarboxyl group, that, is combined with borane, solvent, and an alcohol or carboxylic acid. This process yields a sodium borate ester salt having the formula NaB[X][OR]2, NaB[X]F2, or NaB[X]2.

[0073] In another embodiment of making an electrolyte composition as described herein, a sodium salt having the formula Na2[X] where X is selected from the group consisting of a diol group, a fluoro-diol group, a dicarboxyl group, a fluoro-dicarboxyl group, that, is combined with boron trifluoride, solvent, and an alcohol or carboxylic acid to yield a sodium borate ester salt with the formula NaB[X]Fy[OR]2-y, where y = 0, 1 , or 2.

[0074] Experimental Synthesis and Characterization

[0075] Scalable Synthesis of sodium borate ester salt- based electrolytes

[0076] Several of the sodium borate ester salts described herein have been synthesized directly as electrolytes and dried under heat and vacuum to yield powders. These salts include those highlighted in Figure 2. Electrolytes may be obtained by synthesizing the sodium borate ester salt directly in glyme solvent or by dissolving the dried sodium borate ester salt in glyme salt, both yielding a liquid electrolyte.

[0077] Synthesized salts have been characterized using liquid NMR as shown in Figure 3 and have been tested in sodium battery half cells using CV (to characterize oxidative andreductive stability), and battery cycling (to determine coulombic efficiency, maximum current density, overpotential and cycle life).

[0078] Figure 4 shows galvanostatic cycling at increasing current and CV data for two additional sodium borate ester salt electrolytes in glyme solvent: sodium tetra(phenol)borate and sodium tetra(trifluoro ethyl)borate (shown in Figure 2).

[0079] Figure 5 shows capacity retention and Coulombic efficiency of a full battery cell cycling with an electrolyte comprising the sodium tetra(phenol)borate salt in ether solvent. The graph shows excellent capacity retention over 50 cycles of the cell.

[0080] This data indicates that sodium borate ester salt in glyme electrolytes described herein can support highly reversible cycling for very long cycle life, provide high oxidative stability and chemical stability with sodium battery cathodes for use in full battery cells, and enable rapid Na ion transport with minimal overpotential, all of which are optimal metrics for a sodium battery electrolyte.

[0081] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

[0082] Although the technology has been described in language that is specific to certain structures and materials, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific structures and materials described. Rather, the specific aspects are described as forms of implementing the claimed invention. Because many embodiments of the invention can be practiced without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

[0083] Unless otherwise indicated, all number or expressions, such as those expressing dimensions, physical characteristics, etc., used in the specification (other than the claims) are understood as modified in all instances by the term "approximately". At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the claims, each numerical parameter recited in the specification or claims which is modified by the term "approximately" should at least be construed in light of the number of recitedsignificant digits and by applying rounding techniques. Moreover, all ranges disclosed herein are to be understood to encompass and provide support for claims that recite any and all sub-ranges or any and all individual values subsumed therein. For example, a stated range of 1 to 10 should be considered to include and provide support for claims that recite any and all sub-ranges or individual values that are between and / or inclusive of the minimum value of 1 and the maximum value of 10; that is, all sub-ranges beginning with a minimum value of 1 or more and ending with a maximum value of 10 or less (e.g., 5.5 to 10, 2.34 to 3.56, and so forth) or any values from 1 to 10 (e.g., 3, 5.8, 9.9994, and so forth).

Claims

CLAIMSI / We claim:1 . An electrolyte composition comprising: a sodium borate ester salt selected from the group consisting of sodium tetra(phenol)borate (NaB[OCeH5]4) and sodium tetra(isopropyl)borate (NaB[OC3H?]4); and at least one ether-based solvent.

2. The electrolyte composition of claim 1 , wherein the at least one ether-based solvent is selected from the group consisting of dimethoxyethane, diglyme, tetraglyme, and 1 ,3- dioxolane.

3. The electrolyte composition of claim 1 , further comprising: at least one supplemental sodium salt.

4. The electrolyte composition of claim 3, wherein the at least one supplemental salt is selected from the group consisting of sodium hexafluorophosphate (NaPFe), sodium perchlorate (NaCIC ), sodium tetrafluoroborate (NaBF4), sodium tetraphenylborate, sodium nitrate, sodium bis(oxalato)borate, sodium difluoro(oxalato)borate, sodium (trifluoromethanesulfonyl)imide, sodium iodide, and sodium bis(fluorosulfonyl)imide.

5. The electrolyte composition of claim 1 , further comprising: at least one additive.

6. The electrolyte composition of claim 5, wherein the at least one additive is selected from the group consisting of fluoroethylene carbonate (FEC), trans-difluoroethylene carbonate (DFEC), vinyl carbonate (VC), vinyl ethylene carbonate (VEC), trimethyl phosphite (TMP), tris(trimethylsilyl)phosphite (TMSP), tris(trimethylsilyl)borate, dimethyl acetamide, 2,2-dimethoxy-propane, vinyl acetate, and gamma-butyrolactone (GBL), biphenyl, thiophene, triphenylamine, and 1 ,4-benzodiozane-6,7-diol.

7. The electrolyte composition of claim 1 , wherein the electrolyte composition further comprises at least one non-ether based solvent.

8. A method of making an electrolyte composition comprising: combining sodium borohydride, a solvent, and phenol to thereby form a solution comprising the solvent and a sodium tetra(phenoxy)borate.

9. The method of claim 8, further comprising: removing some or all the solvent from the solution.

10. The method of claim 9, wherein all of the solvent is removed from the solution to thereby provide dried sodium tetra(phenoxy)borate, the method further comprising: combining a second solvent with the dried sodium tetra(phenoxy)borate to thereby provide an electrolyte composition.11 . The method of claim 9, wherein removing some or all of the solvent from the solution comprises heating the solution to evaporate some or all of the solvent, subjecting the solution to vacuum, or both.

12. A method of making an electrolyte composition comprising: combining sodium borohydride, a solvent, and isopropanol to thereby form a solution comprising the solvent and a sodium tetra(isopropyl)borate.

13. The method of claim 12, further comprising: removing some or all the solvent from the solution.

14. The method of claim 13, wherein all of the solvent is removed from the solution to thereby provide dried sodium tetra(isopropyl)borate, the method further comprising: combining a second solvent with the dried sodium tetra(isopropyl)borate to thereby provide an electrolyte composition.

15. The method of claim 13, wherein removing some or all of the solvent from the solution comprises heating the solution to evaporate some or all of the solvent, subjecting the solution to vacuum, or both.

Images