Carboxylate ester-based electrolytes for sodium batteries

Abstract

An electrolyte for sodium ion batteries can comprise a salt comprising one or more of sodium bis (fluoro sulfonyl) imide (NaFSI) salt, NaOTf, NaTFSI, NaBF4, NaPFe, and NaCICh, and an electrolyte solvent comprising at least one carboxylate ester as a dominant solvent. A sodium ion battery cell (100) can comprise the electrolyte (140) having a salt comprising one or more of NaFSI salt, NaOTf, NaTFSI, NaBF4, NaPFe, and NaC104, and an electrolyte solvent comprising at least one carboxylate ester as a dominant solvent. The battery cell (100) can also include an anode (130) in electrical contact with the electrolyte (140) and a cathode (150) in electrical contact with the electrolyte (140). A casing (120) can enclose the electrolyte (140), the anode (130), and the cathode (150) sufficient to prevent oxygen and moisture from entering the casing (120).

Description

[0001] CARBOXYLATE ESTER-BASED ELECTROLYTES FOR SODIUM BATTERIES

[0002] CROSS REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Application No. 63 / 643,526, filed May 7, 2024, which is hereby incorporated herein by reference.

[0004] STATEMENT REGARDING FEDERALLY SPONSORED

[0005] RESEARCH OR DEVELOPMENT

[0006] Not applicable.

[0007] NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

[0008] Not applicable.

[0009] INCORPORATION BY REFERENCE STATEMENT

[0010] Not applicable.

[0011] BACKGROUND

[0012] Sodium-ion batteries (SIBs) are a promising technology for next-generation energy storage. However, their performance is limited at low temperatures due to the inferior bulk and interfacial resistance of current electrolytes.

[0013] Among various economic sectors, transportation and electricity generation are the two primary sources of CO2 emissions. Decarbonization of these sectors requires the electrification of vehicle fleets as well as scaling up renewable electricity generation from solar and wind. Both require high-performance and cost-effective energy storage technology. Rechargeable batteries are a promising solution for this need, but the mainstream Li-ion batteries (LIBs) is expensive due to the usage of costly metals such as Li, Co and Ni. Due to their potential lower cost and better sustainability, Na-ion batteries (SIBs) are attracting increasing attention as an alternative to LIBs.

[0014] The development of SIBs is facing several challenges. First, most anode materials have a low initial Coulombic efficiency (ICE) of 30%-70%. Such low ICE means that a large portion of Na+extracted from the cathode during first charge is consumed by side reactions, which significantly compromises the energy density and cycling stability. Second, their rate capability and cycling stability is inferior to those of LIBs. The sluggish diffusion of the large Na+leads to a poor rate capability. The huge volume changes upon Na insertion and the unsatisfactory surface passivation at the anode / cathode surface compromises cycling stability. Thirdly, they have poor low-temperature performance. The state-of-art (SOA) SZBs electrolyte, typically made of 1 M NaC104 or NaPE, dissolved into a blend of ethylene carbonate (EC) and linear carbonate, generally has a minimum operating temperature of ca. -20 °C. This cannot satisfy the needs for stationary energy storage applications, especially for regions at high altitude and / or latitude where outdoor temperatures can go below -20 °C or even -30 °C during the winter. At such low temperatures, SIB electrolytes can crystallize or even freeze, thus failing battery operation. Despite work that has been done to address the first two challenges, less attention has been paid to enhance the low-temperature performance.

[0015] Among various battery design factors, electrolyte design plays a pivotal role in governing the low-temperature performance due to three reasons: (1) it determines the concentration of free ions and their mobility, thereby regulating electrolyte conductivity; (2) its decomposition dictates the composition of the cathode / solid electrolyte interphase (CEESEI), thereby regulating the interphase resistance; (3) it determines the solvent-cation interaction, thereby regulating the de-solvation energy during the interfacial charge transfer reaction.

[0016] The poor low-temperature performance of current SIB electrolytes can be attributed to inferior bulk and interfacial properties. The main solvent, EC, has a high melting point (37 °C) and it becomes very viscous or even solidifies at low temperatures, which significantly reduces electrolyte conductivity and increases interfacial resistance. For example, at -20 °C, the conductivity of 1.0 M NaPFe-EC / EMC (EMC: ethyl methyl carbonate) reduces to 1.4 mS / cm, and the charge transfer resistance increases by two orders of magnitude from 322 Q to ~31,000 . Such a dramatic loss of performance can also be seen in other SIB electrolytes.

[0017] To address this challenge, many strategies have been explored. Adding co-solvent with low melting point and low viscosity can prevent the electrolyte from freezing and increase its conductivity at low temperatures. For example, adding 25% methyl acetate (MA) into 1 M LiPFe-EC / DEC / DMC (EEl) (DEC: diethyl carbonate; DMC: dimethyl carbonate) can increase its conductivity at -60 °C by one order of magnitude from < 0.1 mS / cm to ~1 mS / cm. This design is effective, but it compromises the compatibility of electrolytes with electrodes at high operating temperatures (such as 45 °C). Another approach involves suppressing the solvent-cation interaction by using weakly polar solvents to decrease charge transfer resistance. For example, Li+-solvent binding energy can be reduced by -25% when the polarity of ethyl acetate is weakened by grafting two F atoms to the acetyl group. However, this strategy leads to poor salt dissociation and low conductivity due to the reduced solvent polarity.

[0018] To further expand the application of SIBs, energy density can be increased. One effective strategy to achieve this is by increasing the cutoff charging voltage of the battery. However, when operating at high voltages, the cathode material of SIBs encounters several problems, including oxygen precipitation, phase transitions and fragmentation of cathode particles, dissolution and migration of transition metals (TMs), and oxidation of the electrolyte on the high-voltage cathode. Improving electrolyte stability by using solvents with high oxidation resistance can help enhance the performance of high-voltage SIBs. High voltage solvents can contribute to the formation of a stable cathode electrolyte interphase (CEI), thereby improving the electrochemical performance of the battery by forming a protective layer through self-decomposition or interaction with the electrode surface. The most common electrolyte in sodium-ion batteries (SIBs) is 1 M NaPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a 1 :1 volume ratio. EC primarily contributes to interphase formation, while DEC facilitates ion transport within the electrolyte. However, this electrolyte exhibits poor oxidative stability above 4.3 V vs. Li / Li+, which corresponds to approximately 4.0 V vs. Na / Na+.

[0019] SUMMARY

[0020] An electrolyte for sodium ion batteries can comprise one or more of a sodium bis (fluoro sulfonyl) imide (NaFSI) salt, sodium trifluoromethane sulfonate (NaOTf), sodium bis (trifluoromethyl sulfonyl) imide (NaTFSI), NaBF4, NaPFe, andNaCICh, as a sodium salt, and an electrolyte solvent comprising at least one carboxylate ester as a dominant solvent. Typically, the electrolyte can be provided as a liquid at room temperatures. Consistent with these principles, a sodium ion battery cell can comprise an electrolyte, the electrolyte can be any of the variations described herein. The battery cell can also include an anode which is in electrical contact with the electrolyte and a cathode which is in electrical contact with the electrolyte. A casing can enclose the electrolyte, the anode, and the cathode sufficient to prevent oxygen and moisture from entering the casing.

[0021] There has thus been outlined, rather broadly, the more important features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken with the accompanying drawings and claims, or may be learned by the practice of the invention.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1A is a schematic illustration of a coin cell sodium ion battery in accordance with one example.

[0024] FIG. IB is an exploded view of the battery shown in FIG. 1A.

[0025] FIG. 2A is an illustration of an SEI layer during solvent reduction and anion reduction when sodium salt concentration is low.

[0026] FIG. 2B is an illustration of an SEI layer during solvent reduction and anion reduction when sodium salt concentration is high.

[0027] FIGs. 3A-3D show physical properties of carboxylate versus carbonates. FIG. 3A is a graph of melting point vs. number of carbon atoms, FIG. 3B is a graph of viscosity vs. number of carbon atoms, FIG. 3C is a graph of dielectric constant vs number of carbon atoms, and FIG. 3D is a graph of donor number vs. number of carbon atoms. The dielectric constant of MIB and IP A, as well as the donor number of MB and MIB, are not available in the literature.

[0028] FIG. 4A shows salt effect with Conductivity of MA-based electrolytes with different Na salts.

[0029] FIG. 4B and 4C are curves of HC / Na cell using 1 m and 4 m MA-based electrolyte with different Na salts.

[0030] FIG. 5 shows structures and solubility in methyl acetate of several sodium salts. FIG. 6A-6J show various concentration effects of NaFSI in methyl acetate, with salt concentrations in molality (x = 0.5, 1, 2, 3, 4, 8, 11.4) and discharge and charge rates of 0.1 C. FIG. 6A shows charging and discharging profiles of the 2ndcycle; FIG. 6B shows capacity vs concentration; FIG. 6C shows EIS after the 5thcycle; FIG. 6D shows Rohm and Rint vs concentration; FIG. 6E shows CE (top) and capacity (bottom) vs cycle number; FIG. 6F shows ICE (left) and average CE (21-30th) (right) vs concentration. FIGs. 6G and 6H show FT-IR profiles of NaFSI-MA with different salt concentration; FIG. 61 shows atomic percentage of C in the SEI on the cycled HC anode surface characterized by XPS; and FIG. 6J shows atomic percentage of F in the SEI on the cycled HC anode surface characterized by XPS.

[0031] FIGs. 7A and 7B show conductivity and solvation structure of NaFSI-DMC and NaFSI-MA electrolytes.

[0032] FIGs. 8A-8H show solvent effects on anode performance, (solvent = carboxylate (MA, DMC, EA) or carbonate (EMC, NPA, DEC) Discharge / charge rate: 0.1C. FIG. 8A shows charging and discharging profiles of the 2ndcycle; FIG. 8B shows capacity; FIG. 8C shows an EIS curve of each cell after the 5thcycle; FIG. 8D shows Rohm and Rint after the 5thcycle; FIG. 8E shows CE (top) and capacity (bottom) with cycling; FIG. 8F shows ICE and average CE (21 -30th); FIGs. 8G and 8H show SEI component analyzed by XPS.

[0033] FIG. 9A-9J show solvent effect on cathode performance, (solvent = MA, DMC). FIG. 9A shows LSV of NaFSI-MA and NaFSI-DMC in Na / Al cell; FIG. 9B shows voltage-time profile of NVP / Na cell using 3 m NaFSI-MA electrolyte; FIGs. 9C to 9H compare electrochemical performance and internal resistance between 4 m NaFSI-MA and 4 m NaFSI-DMC for NVP / Na cell, discharged and charged at 1C rate. FIG. 9C shows charging and discharging profiles of the 2ndcycle; FIG. 9D shows capacity; FIG. 9E shows an EIS curve of each cell after the 5thcycle; FIG. 9F shows Rohm and Rim after the 30thcycle; FIG. 9G shows CE (top) and capacity (bottom) with cycling; FIG. 9H shows ICE and average CE (21-30th); FIGs. 91 and 9J show cathode electrolyte interphase (CEI) components analyzed by XPS.

[0034] FIGs. 10A-10B are graphs showing low temperature performance. FIG. 10A shows a capacity comparison between 3 m NaFSI-MA, 3 m NaFSI-DMC and SOA electrolyte for HC / Na cell both at 25 °C and -20 °C, discharged and charged at 0.1 C rate. FIG. 10B shows an EIS curve of a cell cycled at -20 °C after the 30 cycles.

[0035] FIGs. 11A-11C show consequences and mechanism of solvent decomposition and correlation between interfacial resistance with total lost capacity after cycling. FIG. 11A shows the concentration effect and FIG. 1 IB shows the chain length effect. FIG. 11C shows a reductive decomposition mechanism of MA, DMC and EC.

[0036] FIG. 12A and 12B are graphs showing a VH analysis of the tested cyclic carboxylates as electrolytes.

[0037] FIG. 12C is an FT-IR analysis of the Al current collector surface, disassembled from the Na / Al cell during the VH test in accordance with one example.

[0038] FIG. 12D-12E are graphs showing first (1st) and second (2nd) scans of the LSV profiles for different cyclic carboxylates with varying ring sizes.

[0039] FIG. 12F-12G are graphs showing first and second scans of the LSV profiles for the DVL derivatives.

[0040] FIG. 13A-13B are graphs showing HC / NFM performance with various diluents.

[0041] FIG. 13C is a graph showing HC / NFM cycle profile for cyclic carboxylate electrolytes with different ring sizes.

[0042] FIG. 13D-13E are graphs of capacity -voltage profiles and cycling performance of DVL / TTE and ECL / TTE.

[0043] FIG. 14A-14B are graphs of HC / NFM performance with various co-solvent.

[0044] FIG. 14C-14D are graphs of 1.5-4.2V and 1.5-4.5V cycling performance of NPA-4 and BE.

[0045] These drawings are provided to illustrate various aspects of the invention and are not intended to be limiting of the scope in terms of dimensions, materials, configurations, arrangements or proportions unless otherwise limited by the claims.

[0046] DETAILED DESCRIPTION

[0047] While these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. Thus, the following more detailed description of the embodiments of the present invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the present invention, to set forth the best mode of operation of the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the present invention is to be defined solely by the appended claims.

[0048] Definitions

[0049] In describing and claiming the present invention, the following terminology will be used.

[0050] The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a cell” includes reference to one or more of such features and reference to “the electrode” refers to one or more of such electrodes.

[0051] As used herein with respect to an identified property or circumstance, “substantially” refers to a degree of deviation that is sufficiently small so as to not measurably detract from the identified property or circumstance. The exact degree of deviation allowable may in some cases depend on the specific context.

[0052] As used herein, “adjacent” refers to the proximity of two structures or elements. Particularly, elements that are identified as being “adjacent” may be either abutting or connected. Such elements may also be near or close to each other without necessarily contacting each other. The exact degree of proximity may in some cases depend on the specific context.

[0053] As used herein, the term “about” is used to provide flexibility and imprecision associated with a given term, metric or value. The degree of flexibility for a particular variable can be readily determined by one skilled in the art. However, unless otherwise enunciated, the term “about” generally connotes flexibility of less than 2%, and most often less than 1%, and in some cases less than 0.01%.

[0054] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

[0055] As used herein, the term “at least one of’ is intended to be synonymous with “one or more of.” For example, “at least one of A, B and C” explicitly includes only A, only B, only C, or combinations of each.

[0056] Numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly recited limits of 1 to about 4.5, but also to include individual numerals such as 2, 3, 4, and subranges such as 1 to 3, 2 to 4, etc. The same principle applies to ranges reciting only one numerical value, such as “less than about 4.5,” which should be interpreted to include all of the above-recited values and ranges. Further, such an interpretation should apply regardless of the breadth of the range or the characteristic being described.

[0057] Any steps recited in any method or process claims may be executed in any order and are not limited to the order presented in the claims. Means-plus-function or step-plus- function limitations will only be employed where for a specific claim limitation all of the following conditions are present in that limitation: a) “means for” or “step for” is expressly recited; and b) a corresponding function is expressly recited. The structure, material or acts that support the means-plus function are expressly recited in the description herein. Accordingly, the scope of the invention should be determined solely by the appended claims and their legal equivalents, rather than by the descriptions and examples given herein. Example Embodiments

[0058] As explained above, it has been difficult to make electrolytes for sodium ion batteries that can provide good performance across a range of temperatures and which can provide high voltage performance. The solvents used in the electrolytes can have a great impact on the battery performance. From the perspective of solvent selection, the ideal solvent for low- temperature electrolytes should have: (1) a low melting point so it does not freeze at low temperatures; (2) a high dielectric constant and a low viscosity to ensure good conductivity; (3) a low donor number (DN) that leads to weak solvent-cation binding and fast charge transfer kinetics. The technology described herein involves electrolytes for sodium ion batteries that include a salt comprising one or more of a sodium bis(fluoro sulfonyl) imide (NFSI) salt, NaOTf, NaTFSI, NaBF i, NaPFe, and NaCICU, as a sodium salt in an electrolyte solvent that includes at least one carboxylate ester as a dominant solvent. As used herein, “dominant solvent” refers to a solvent that is present in the electrolyte at a greater concentration than any other solvent. In some cases, the electrolyte can include a single carboxylate ester as the only solvent, without any other solvents. In other examples, the electrolyte can include a combination of multiple carboxylate esters as the only solvents. In still other examples, a majority of the total solvents present in the electrolyte can be a single carboxylate ester or a combination of multiple carboxylate esters. In still further examples, the carboxylate ester or combination of multiple carboxylate esters may not make up the majority of the electrolyte solvents, but the one or more carboxylate esters can be present in a concentration that is greater than any other individual solvent in the electrolyte.

[0059] In some examples, the carboxylate ester can be selected from the group consisting of methyl acetate, ethyl acetate, methyl propionate, n-propyl acetate, i-propyl acetate, ethyl propionate, methyl butyrate, methyl isobutyrate, and combinations thereof. In these cases, the carboxylate ester can be non-cyclic carboxylate ester (e.g. straight chain or branched). In one example, the carboxylate ester is methyl acetate. As a general guideline, the carboxylate ester can have a low carbon count. For example, the carboxylate ester can have from 3 to 10 carbons, or from 3 to 8 carbons, or in some cases from 3 to 5 carbons. These carboxylate esters can be particularly useful for low-temperature performance.

[0060] In other examples, high-voltage sodium batteries can benefit from using cyclic carboxylate esters or lactones. In one example, suitable cyclic carboxylate esters can include unsubstituted cyclic carboxylate esters having from three to six carbons (e.g. four to sevenmember rings). Specific non-limiting examples of such unsubstituted cyclic carboxylate esters can include P-propiolactone (BPL), y-butyrolactone (GBL), 5-valerolactone (DVL), and s-caprolactone (ECL). In some examples, the cyclic carboxylate esters can be substituted having the general structure: where A is a ring of three to sixteen carbons, and in some cases four to seven carbons, and R can be substituted at one or more of a, P, y, and 8 positions. R can be one or more of an alkyl group having one to ten carbons, halogen (e.g. F, Cl, etc), and phenyl group. In some cases, multiple R groups can be substituted in ring A and in some cases can include two R groups. In such cases, the R groups can be the same or different from one another. Nonlimiting examples of such substituted cyclic carboxylate esters can include where A includes five carbons: and where R is a Cl to C9 alkyl, halogen, or phenyl. In some examples, the at least one carboxylate ester can include one or more non-cyclic carboxylate ester and one or more cyclic carboxylate esters. As an example, depending on the cathode material, a charge cutoff voltage can be stable above 4.2V, such as from 4.2V to 5V.

[0061] In some cases, the electrolyte solvent consists essentially of the at least one carboxylate ester, especially as an anolyte solvent. In other examples, the electrolyte solvent further comprises a co-solvent which is not a carboxylate ester. Non-limiting examples of suitable co-solvents can include linear carbonate molecules (diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate, etc.), cyclic carbonate molecules (such as ethylene carbonate, propylene carbonate, etc.), cyclic carboxylate molecules (such as y -butyrolactone, y-valerolactone, etc ), nitrile molecules (such as acetonitrile, butyl nitrile, etc.), ether molecules (such as diethyl ether and dimethoxyethane), pyrrolidone molecules (such as N- methyl-2-pyrrolidone), phosphate molecules (such as trimethyl phosphate and triethyl phosphate) and sulfone molecules (such as sulfolane), and combinations thereof. Although specific concentrations can vary, in one example, the at least one carboxylate ester comprises 70% to 99% by volume of the electrolyte solvent, in some cases 70% to 80% by volume of the electrolyte solvent, and in other cases 90% to 99% by volume of the electrolyte solvent.

[0062] Any of the electrolytes described herein can be used in sodium ion batteries. In some examples, a sodium ion battery cell can include the electrolyte, an anode that is in electrical contact with the electrolyte and cathode that is in electrical contact with the electrolyte. A casing can enclose the electrolyte, the anode, and the cathode. In some examples, the casing can be sufficient to prevent oxygen and moisture from entering the casing. FIGs. 1 A and IB show one example of a sodium ion battery 100 having a coin cell form factor. The coin cell has a stainless-steel anode casing 110 and a stainless-steel cathode casing 120 containing the other battery components. FIG. IB shows an exploded view, with a sodium anode 130, electrolyte 140, and a cathode 150 inside the casing. The casing can substantially enclose the battery cell sufficient to exclude oxygen from the battery cell. This enclosure can maintain oxygen exclusion for at least an expected product life (i.e. two years, three years, five years, or more depending on the design, environment and expected application).

[0063] Carboxylate ester-based electrolytes are described herein for SIB applications, due to their favorable properties (i.e., low melting point, low viscosity and high dielectric constant). The effect of salt, concentration and solvent molecular structure are systematically examined and compared with carbonate-based electrolytes. By combining electrochemical tests with spectroscopic characterizations, the performance of selective carboxylate ester-based electrolytes in hard carbon / Na and Na3V2(PO4)s / Na half-cells were evaluated. The carboxylates enabled high electrolyte conductivities, especially at low temperatures, due to their favorable properties (i.e., low melting point, low viscosity and high dielectric constant). However, carboxylates alone may be inadequate to form a stable interphase due to their high reactivity, which can be addressed by forming anion-rich Na+solvation via increasing salt concentration. For example, FIG. 2A-2B are illustrations of an SEI layer during solvent reduction and anion reduction. As shown in FIG. 2A, when the sodium salt concentration is low, the electrolyte can decompose continuously because of solvent reduction. However, as shown in FIG. 2B, when a higher salt concentration is used, stable SEI formation occurs and solvent reduction is lowered. Fundamental knowledge on the chemistry-property- performance correlation of this new family of electrolytes was obtained, and their benefits and pitfalls were examined. The technology described herein can include electrolytes that include linear carboxylate esters as the main solvent for use in SIBs having enhanced low-temperature performance. A systematic study including salt screening, concentration optimization and comparison with carbonate was performed to examine the multiple design freedoms of carboxylate ester-based electrolytes. By combining electrochemical tests with spectroscopic characterizations, the performance of selective carboxylate ester-based electrolytes was evaluated in hard carbon (HC) / Na and NaaV2(PO4)3 (NVP) / Na half-cells and their performance was correlated with their structure and properties (both transport and interfacial). Fundamental knowledge on the chemistry -property -performance correlation of this new family of electrolytes was obtained, and their benefits and pitfalls are discussed herein.

[0064] Several linear carboxylate esters (abbreviated as carboxylate hereafter) that can be used in the electrolytes include the following: methyl acetate (MA); ethyl acetate (EA); methyl propionate (MP); n-propyl acetate (NPA); i-propyl acetate (IP A); ethyl propionate (EP); methyl butyrate (MB); methyl isobutyrate (MIB); and combinations thereof. For comparison, some common carbonate esters (abbreviated as carbonate hereafter) include: dimethyl carbonate (DMC); ethyl methyl carbonate (EMC); and diethyl carbonate (DEC). Structurally, carboxylate (formula: R(C=O)OR', where R and R' are alkyl groups) contains a carbonyl group in which the C atom is singly bonded to a second oxygen atom, while carbonate (formula: RO(C=O)OR') consists of a carbonyl group flanked by two alkoxy groups. Specifically, methyl acetate (MA), ethyl acetate (EA) and n-propyl acetate (NPA) have the same CH3C(=0)0RI structure with different Ri groups. Methyl propionate (MP) and ethyl propionate (EP) have the same CH CH2C(=O)OR2 structure with different R2 groups. EA and MP are isomers, and NPA, isopropyl acetate (IP A), EP, methyl butyrate (MB) and methyl isobutyrate (MIB) are isomers.

[0065] FIGs. 3A-3D show several physical properties of carboxylates compared with carbonates. FIG. 3A shows a graph of melting points versus number of carbon atoms. FIG. 3B shows a graph of viscosity at 25 °C versus number of carbon atoms. FIG. 3C shows a graph of dielectric constant at 25 °C versus number of carbon atoms. FIG. 3D shows a graph of donor numbers versus number of carbon atoms. These graphs show that many carboxylates have lower melting points, lower viscosity, higher dielectric constant, and comparable donor numbers when compared with carbonates.

[0066] Among different properties, the melting point, the dielectric constant and viscosity, and the donor number (DN) respectively determine the liquid range, the conductivity, and the de-solvation energy of the electrolyte, thereby influencing the low-temperature performance. Generally, all carboxylates have melting points below -70 °C, lower than those of carbonates and far below those of SOA electrolytes (-30 °C). Their viscosities are in the range of 0.36-0.69 cP, lower than those of carbonates. Due to the asymmetry of the molecular structure, they are much more polar than carbonates, with dielectric constants above 5.6. Their donor numbers, around 16 kcal / mol, are comparable to those of carbonates except for EMC and MP. As the chain length increases, the melting point and viscosity increases due to the larger intermolecular interaction, while the dielectric constant decreases due to the larger molecular size and, consequently, higher molecular polarizabilities. Overall, compared with carbonates, carboxylates have lower melting points and viscosities, higher dielectric constants, and comparable donor numbers. These features make them ideal candidates for making low-temperature electrolytes. Among all carboxylates, MA stands out because it has the lowest melting point, the lowest viscosity, and the highest dielectric constant. Therefore, in the below discussion, MA is used as the example carboxylate for a systematic study. Carbonates with the same chain length as MA, i.e., DMC, are compared whenever possible.

[0067] As mentioned above, the electrolytes described herein can include one or more of NaFSI, NaOTf, NaTFSI, NaBF4, NaPFe, and NaCICh, as a sodium salt in the carboxylateester based solvent described above. In some examples, one or more of NaFSI, NaOTf, and NaTFSI can be particularly suited for use with low-temperature electrolyte operation. In some cases, NaFSI can be the only sodium salt present in the electrolyte. In other examples, the sodium salt can further comprise a co-salt and the NaFSI can be a dominant salt. Similarly, NaBF4, NaPFe, and NaC104 can be particularly suitable for use as a dominant salt when configuring the electrolyte for high-voltage operation. In these cases, as an example, NaPFe can be the dominant salt or can be the only salt.

[0068] As used herein, “dominant salt” refers to a salt that is present at a greater concentration than any other salt. In some cases, this may be a majority concentration but this is not required. Therefore, other sodium salts can be present and the dominant salt can be present in the highest concentration out of all sodium salts in the electrolyte.

[0069] Various co-salts can be used in addition to the dominant salt. As non-limiting examples, the co-salt can be one or more of NaFSI, NaPFe, NaC104, NaOTf, NaTFSI, NaBETI, NaSCN, NaNO3, NaBOB, NaDFOB, Na2C2O4, NaBF4, LiTFSI and Mg(TFSI)2. It is understood that these co-salts are also listed as primary salts such that a co-salt would be a second salt which is different from the primary salt. For example, if NaFSI is the primary salt, then NaFSI would not also be chosen as a co-salt. In one specific example, the sodium salt consists of or consists essentially of the NaFSI. In some examples, the salts present in the electrolyte can be from 50% to 100% NaFSI, or from 70% to 100% NaFSI, or from 80% to 100% NaFSI, or from 90% to 100% NaFSI, based on the total weight of salt present in the electrolyte. In other examples, the salts present in the electrolyte can include from 50% to 99% NaFSI, or from 50% to 90% NaFSI, or from 50% to 80% NaFSI, or from 50% to 70% NaFSI, based on the total weight of salt present in the electrolyte. As an example, when designing low-temperature electrolytes, using a co-salt (e.g. NaPFe) can be desirable. In contrast, when designing a high-voltage electrolyte, lower amounts of co-salt or no co-salt can be desirable. Specifically, in the design of low-temperature cells, a relatively high concentration of NaFSI can be used to tailor the SEI (solid electrolyte interphase) for stable cycling. However, in the design of high-voltage cells, the FSI anion is incompatible with the cathode current collector (Al), so one or more of NaPFe, NaBF4or NaC104may be suitable in that case.

[0070] Various additives can also be used. In some examples, an additive can be present in an amount less than 5% of the total electrolyte weight. As non-limiting examples, the additives can be one or more of vinylene carbonate, fluoroethylene carbonate, sodium (oxalate) difluoro borate, tris (trimethylsilyl) phosphite, RbPFe, CsPFe, LiPFe, SbF3, biphenyl, succinonitrile, ethylene sulfite, 1,3-propane sultone, prop-l-ene-l,3-sultone, sulfolane, and sodium bis(oxalato)borate.

[0071] Six commercially available Na salts were compared in terms of solubility and conductivity (see FIG. 5). In addition, their compatibility with the SOA SIB anode material, hard carbon (HC), was also compared. The solubility of these Na salts follows the order of NaBF4« NaOTf « NaC104< NaTFSI < NaPFe < NaFSI. Such difference in salt solubility is related to their lattice energy: salts with higher lattice energy have lower solubility. Owing to their extremely low solubilities, NaBF4 and NaOTf were excluded in this study. At a salt concentration of 1 m, the conductivity of MA electrolytes made with different Na salts follows the order of NaC104 < NaTFSI < NaPFg < NaFSI (FIG. 4A shows a graph of the conductivity at 25 °C vs. concentration of the salt in MA). For all salts, the conductivity shows a non-monotonical dependence on salt concentration and peaks at an intermediate concentration of 1-3 m due to the competing effect of charge carrier concentration and viscosity. NaFSI enables the highest conductivity among different salts, showing a maximum conductivity of 14.1 mS / cm at a concentration of 3 m. In certain examples, the salt in the electrolyte can be present at a concentration from about 1 mol / kg to about 3 mol / kg.

[0072] In certain examples, different electrolyte compositions can be used for the anode side and the cathode side of a sodium battery. The electrolyte used in the anode side (the anolyte) can include a carboxylate ester solvent and NaFSI salt as described above, along with any of the options described above. The cathode side electrolyte (the catholyte) can include a second salt and a catholyte solvent, where the catholyte solvent does not comprise carboxylate ester. In certain examples, the second salt can be some other salt that is not NaFSI. For example, the second salt can be NaPFe, NaPF4, NaCICh, NaOTf, NaTFSI, NaBF4, or a combination thereof. The catholyte solvent can include ethyl carbonate, diethyl carbonate, dimethyl carbonate, or a combination thereof. In some examples, the sodium battery can include a membrane separating the catholyte from the anolyte, such as an ion exchange membrane.

[0073] Example 1

[0074] The compatibility of different salts with HC was examined by conducting cyclic voltammetry (CV) in HC / Na cells. FIG. 4B shows a graph of current vs. voltage for electrolytes with 1 m salt concentration in MA, and FIG. 4C shows a similar graph for 4 m salt concentration. During the cathodic scan, both Na+intercalation into HC and electrolyte decomposition can happen. If there is a reversible reaction, an anodic peak is expected during the subsequent anodic scan. For NaFSI-MA electrolyte, there are both reduction and oxidation peaks at a normal salt concentration (1 m) and a high concentration (4 m). In contrast, no oxidation peaks are observed for NaC104-MA, NaTFSI-MA and NaPFe-MA at either 1 m or 4 m, suggesting the reduction reactions in these electrolytes are likely only irreversible electrolyte decomposition. Such difference can be explained by the well-known nature of FST to form stable ani on-dominated SEI upon reduction that can prevent continuous electrolyte decomposition.

[0075] Summarizing the salt effect study, NaFSI stands out among all six sodium salts due to its highest solubility, highest conductivity, and compatibility with HC. Therefore, NaFSI was selected as the salt for further study as discussed below. The structure and solubility in MA of the salts is shown in FIG. 5.

[0076] Salt concentration plays an important role in determining the bulk and interfacial properties of the electrolyte by regulating the cation solvation shell and ultralow concentration electrolyte. Salt concentration of the NaFSI-MA electrolyte affects the performance of HC / Na half cells. Specifically, the capacity, CE and cycling stability were characterized and correlated with interfacial properties and electrolyte structures.

[0077] The charge-discharge profiles of the HC / Na half cells (FIG. 6A) show a typical slopeplateau curve, where the slope corresponds to Na+insertion into parallel graphene layers of HC, while the plateau corresponds to the Na1insertion into HC nanopores. The capacity is non-monotonically dependent on salt concentration (FIG. 6B), and a maximum capacity of 176 mAh / g is observed at a concentration of 3 m. The electrochemical impedance spectra (EIS) were collected (FIG. 6C), and the ohmic resistance (Rohm) (FIG. 6D, left) and interfacial resistance (Rint) (FIG. 6D, right) were obtained by fitting EIS with an equivalent circuit. Here the interfacial resistance includes both the charge transfer resistance (Rct) and the SEI resistance (RSEI), but they cannot be deconvoluted from the only one semi-circle in the EIS. Both Rohm and Rint show a non-monotonic dependence on salt concentration and reach the minimum at 3 m, consistent with the capacity-concentration correlation. This suggests the capacity's dependence on concentration can be explained by the internal resistance, i.e., cells with higher internal resistance are more prone to reach cut-off voltage under constant-current operation, therefore delivering less capacity. Another key observation is that Rint is 2-3 orders higher than Rohm, indicating the internal resistance is dominated by the interfacial resistance. The coulombic efficiency (CE) is compared in FIGs. 6E-6F. The ICE shows a nonmonotonic dependence on salt concentration and 4 m electrolyte shows the maximum ICE (55.00%). After 30 cycles, high concentration electrolytes (> 3 m) all demonstrate average CE of > 99.4%, suggesting the formation of a stable SEI. In contrast, low concentration electrolytes (< 1 m) cannot form stable SEI as indicated by the fast capacity decay, the low ICE (< 25%) and low average CE. Correspondingly, they show much higher interfacial resistance than other electrolytes.

[0078] The CE’s dependence on salt concentration can be explained by considering the solvation structure of Na+ions in these electrolytes. The Na+solvation structure was examined by Fourier-transform infrared spectroscopy (FT-IR). Peak at 1710-1770 cm (FIG. 6G) is assigned to C=O stretch in MA, in which the feature at 1740 cm ' corresponds to uncoordinated (free) MA. Its intensity decreases gradually, while another band attributed to solvated MA emerges at — 1730 cm1and increases with salt concentration. This observation indicates that the fraction of free MA molecules decreases with salt concentration. Peak at —574 cm1(FIG. 6H) is assigned to the SaSCE of FSF, which is contributed to free FST and / or FSF within a solvent-separated ion pair (SSIP) without direct coordination to Na+. As NaFSI concentration increases, this peak shifts from 574 to 567 cm indicating the coordination of FST with Na+. When FST anion coordinates with Na+, different local structures include contact ion pair (CIP) and aggregate (AGG) form. The results show there are less free anions and more Na-anion coordination as salt concentration increases. Such coordination of FST with Na+in concentrated electrolyte decreases the lowest unoccupied molecular orbital (LUMO) of FST, rendering them more prone to reduction compared to free FSI. Given the high reactivity of MA, free MA molecules are susceptible to reduction at HC anode, but their decomposition does not form stable SEI. The change of Na+solvation structure in high-concentration electrolytes suppress the reduction of MA while promoting the reduction of FST, thus allowing the formation of a stable SEI. This hypothesis is supported by X-ray photoelectron spectroscopy (XPS) results (FIGs. 61- 6J). In the SEI on cycled HC, the amount of organic components, an indicator of MA decomposition, decreases with salt concentration (FIG. 61). On the contrary, inorganic components, indicative of anion decomposition, increase with the salt concentration (FIG. 6J). In summary, increasing salt concentration leads to anion-rich solvation shell, which favors anion decomposition that can form stable SEI on HC anode. In addition, 3 m NaFSI- MA electrolyte shows the highest capacity due to its lowest internal resistance.

[0079] To understand the uniqueness of carboxylate chemistry, a systematic comparison of carboxylate and carbonate is provided. Of particular interest is how the minor difference in molecular structure between carboxylate and carbonate affects their physical properties and solvation structure, thus governing their electrochemical performance. As a representative linear carbonate, DMC is widely used as a co-solvent to reduce electrolyte melting point and viscosity. Structurally, it has a symmetric molecular structure compared to the asymmetric structure of MA, making it less polar but more resistant to reduction. Here, the conductivity, solvation structure and electrochemical performance of MA and DMC are compared in both HC / Na and NVP / Na cells, then the influence of chain length and molecule configuration is examined.

[0080] NaFSI-MA electrolytes show much higher conductivity than NaFSI-DMC electrolytes at both 25 °C and -20 °C (FIG. 7 A), which can be attributed to MA’s lower melting point, lower viscosity and higher dielectric constant. Specifically, Im NaFSI-MA shows a room temperature conductivity of 10.72 mS / cm and it decreases to 8.06 mS / cm at - 20 °C, whereas Im NaFSI-DMC has a room temperature conductivity of 7.82 mS / cm and it decreases dramatically to 0.61 mS / cm at -20 °C. The FT-IR (FIGs 6G-6H) of both electrolytes were measured and the solvation number, i.e. the average number of solvent molecules coordinated to Na+, was calculated and compared in FIG. 7B. At all measured concentrations, NaFSI-MA electrolytes have a lower solvation number than NaFSI-DMC electrolytes. At 3 m, the solvation number of NaFSI-MA is 1.92, while that of NaFSI-DMC is 3.07. This result indicates there are less solvents and more anions in Na+solvation shell of NaFSI-MA compared to NaFSI-DMC.

[0081] HC anode performance was shown where a cell with NaFSI-MA delivers less capacity than that with NaFSI-DMC (176 mAh / g vs 207 mAh / g), and both are lower than SOA electrolyte (218 mAh / g) (IM NaPFe-EC / DEC 1 : 1 by volume). FIG. 8A shows a graph of voltage vs. capacity for several different electrolytes that included 3 m NaFSI in different solvents. FIG. 8B shows a graph of capacity vs. carbon atoms in the solvent. This is consistent with the impedance study (FIG. 8C-8D), which shows NaFSI-MA has higher ohmic (4.9 Q. vs 2.7 Q) and interfacial resistance (2000 Q vs 416 Q) than its carbonate counterpart. Notably, the interfacial resistance of NaFSI-MA is almost 10 times higher than that of SOA electrolytes (182 Q), suggesting the highly resistive SEI. Cells with NaFSI-MA and NaFSI-DMC show comparable ICE (50.03% vs 53.16%) and average CE (99.52% vs 99.57%) (FIG. 8E-8F), both lower than that of SOA electrolyte (75.99% and 99.69%). EC, an indispensable component in SOA electrolyte, can form an alkyl carbonate (sodium double alkyl carbonate, NEDC) in SEI. Such alkyl carbonate is believed to play a key role in forming a stable SEI in SOA electrolyte. The inferior CE of NaFSI-MA and NaFSI-DMC can be attributed to their solvent reactivity at low potential and inability to form stable SEI. The lower ICE of NaFSI-MA than NaFSI-DMC can be explained by the different Na+solvation structure. As previously discussed, the anion-rich solvation shell of NaFSI-MA leads to more anion decomposition, as confirmed by XPS (FIG. 8G). In general, the SEI formed in both electrolytes have similar organic and inorganic species. However, NaFSI-MA shows stronger peak density of Na Is, S 2p, F Is, N Is and C Is in, indicating more electrolyte decomposition, consistent with its low CE and higher interfacial resistance. Moreover, the SEI in NaFSI-MA has more NaF (FIG. 8H), a decomposition product of FST, than its carbonate counterpart. These findings demonstrate the anion-domain solvation shell in NaFSI-MA gives rise to an inorganic-rich SEI on HC surface, and the low ICE and high interfacial resistance suggests stable SEI formation needs more electrolyte decomposition.

[0082] Building on the understanding of MA and DMC, the chain length and structure of the carboxylate affect their electrochemical performance. For both carboxylate and carbonate, cell capacity decreases as the solvent chain length increases, and cells with carboxylate show less capacity than their carbonate counterpart (FIG. 8A-8B) due to their larger internal resistance (FIG. 8C-8D). Additionally, the internal resistance is dominated by the interfacial resistance, which increases as solvent chain length probably due to the decreasing cathodic stability. Of particular interest is that the interfacial resistances of carboxylate electrolyte are one order of magnitude higher than those of the carbonate-based electrolyte, suggesting the decomposition products from carboxylate is much more resistive to Na+conduction. Cells with carboxylate-based electrolytes show less ICE, but the average CEs become comparable with cycling (FIG. 8E-8F). This suggests carboxylate can form a stable SEI on HC anode, albeit at the expense of more electrolyte decomposition. In addition, the difference in ICE becomes larger as solvent chain length increases, suggesting the carboxylate molecules become much more reactive as they are longer. Their compatibility with hard carbon was further assessed by CV, which shows molecules with shorter chain length are more compatible with HC. After 30 cycles, cells using carboxylate-based electrolytes lose more capacity than their carbonate counterpart (FIG. 8E). For example, the cell with NaFSI-MA exhibits a capacity retention of 55.39%, whereas the cell with NaFSI-DMC show a capacity retention of 89.90%. This confirms the conj ecture that carboxylate decomposes more to form a stable SEI. Interestingly, the capacity retention becomes worse as solvent chain length increases both for carboxylate and carbonate-based electrolytes, indicating smaller molecules exhibit greater stability in performance.

[0083] Various isomers of carboxylate were compared. Their consistent electrolyte conductivity means the solvent configuration poses negligible effect on electrolyte conductivity. For example, 5-carbon carboxylates with different configurations all exhibit a conductivity of approximately 7 mS / cm. Electrolytes using acetate show higher capacity and CE than that of propionate and butyrate, which means acetates are most stable in the HC / Na cell. For example, electrolyte using EA delivers higher capacity (63 mAh / g vs 44 mAh / g), ICE (32.70% vs 25.28%) and CE (99.10% vs 98.75%) than electrolyte using its isomer, MP. For carboxylates with branched chains (IPA and MIB), they are incapable of completing 30 charge-discharge cycles, highlighting the adverse effect of branched chains on forming stable SEI.

[0084] In summary, NaFSI-carboxylate electrolytes show less capacity, lower CE and less capacity retention than NaFSI-carbonate electrolytes in HC / Na half-cells. While carboxylate-based electrolytes do possess better bulk properties (conductivity), their electrochemical performance is greatly limited by their inferior interfacial properties, including higher internal resistance and more electrolyte decomposition. Furthermore, both the chain length and configuration of the carboxylate affect its performance. The capacity and ICE worsen as solvent chain length increases. Molecules with the same formula but different configurations display similar conductivity, but acetates show best performance.

[0085] NaFSI-MA stands out among carboxylate-based electrolytes. Anodic stability and compatibility with battery cathode can also be provided by examining NVP / Na half cells.

[0086] The anodic stability of the electrolyte, i.e., the resistance against oxidation under high voltage, was examined by linear sweep voltammetry (LSV) in Na / Al half-cells (FIG. 9A). 3 m NaFSI-MA and 3 m NaFSI-DMC start to oxidize at 3.2 V and 3.6 V, respectively, suggesting that MA-based electrolyte is less resistant to oxidation than DMC-based electrolyte. Consequently, 3 m NaFSI-MA electrolyte failed in NVP / Na cells (FIG. 9B) and cannot be charged beyond 3.5 V. A long plateau at ~3.3 V is observed that indicates continuous electrolyte oxidation. Apparently, the electrolyte decomposition fails to form a stable CEI on the cathode surface. By increasing the concentration to 4 m the anodic stability ofNaFSI-MA can be improved to 3.5 V (FIG. 9A) and it enables successful charge to 3.6 V (FIG. 9C). However, a lower capacity (93 mAh / g vs 113 mAh / g) is observed (FIG. 9D), consistent with its higher internal resistance (FIG. 9E-9F). It also exhibits a lower capacity retention (79.46%@ 30thcycle) than that of NaFSI-DMC (97.23%@ 30thcycle) (FIG. 9G). Meanwhile, 4 m NaFSI-MA electrolyte shows lower ICE (77.41%) and average CE (59.59%) than its DMC counterpart (95.98% and 84.81%) (FIG. 9G-9H), suggesting more electrolyte consumption during charging, consistent with its inferior anodic stability. As evidenced by the XPS results (FIG. 91-9 J), the CEI of NaFSI-MA electrolyte shows more anion decomposition product, NaF, than that of NaFSI-DMC electrolyte. Overall, although carboxylate-based electrolytes possess better bulk properties than their carbonate counterparts, their electrochemical performance can be compromised due to their inferior interfacial properties, including poor ability to form interphase, more electrolyte decomposition, and the resulting high interfacial resistance.

[0087] Finally, the low temperature performance ofNaFSI-MA was compared with NaFSI- DMC and SOA electrolyte in HC / Na half cells (FIG. 10A). Respectively, at -20 °C they keep 6.2%, 44.3% and 16.5% of their room-temperature capacity. The unsatisfying low temperatures capacity of SOA electrolyte can be explained by its low conductivity, which decreases from 10.42 mS / cm at room temperature to 0.79 mS / cm at -20 °C (FIG. 7A). In contrast, NaFSI-MA shows the worst capacity retention at low temperatures because of its highest interfacial resistance (FIG. 10B). Meanwhile, NaFSI-DMC shows a moderate conductivity (2.43 mS / cm) at low temperatures and interfacial resistance, which leads to the highest low-temperatures performance.

[0088] Carboxylate-based electrolytes can be used for SIBs and the effect of salt chemistry, concentration, and molecular structure can be seen on the electrochemical performance of both HC / Na and NVP / Na half-cells. Carboxylate-based electrolytes demonstrate better transport properties (conductivity) at both room and low temperatures than the carbonate counterpart due to their lower melting point, lower viscosity and stronger polarity, as well as comparable CE after SEI formation. However, they are more reactive, which leads to low ICE, larger interfacial resistance, and lower capacity. The interfacial resistance of the HC / Na half-cells in the carboxylate-based electrolytes is 5-8 times higher than that in the carbonate- based electrolyte, and 1-2 orders of magnitude higher than that in the SOA STBs electrolyte. The large interfacial resistance in the carboxylate-based electrolytes is likely a result of thick and resistive SEI due to large amount of irreversible electrolyte decomposition. This hypothesis is confirmed by the low ICE and a linear correlation between the accumulated lost capacity and the total interfacial resistance (FIG. 11A-11B). This study also shows that carboxylate-based electrolytes can form stable SEI after cycling with comparable average CE to carbonate-based electrolytes and SOA electrolytes. However, this can only be achieved when anion participates in SEI formation at a salt concentration > 2 m.

[0089] To understand the difference between carboxylate and carbonate, the reductive decomposition mechanism of MA (representative of carboxylate), DMC (representative of carbonate) and EC (component in SOA electrolytes) are reviewed and compared. As shown in FIG. 11C, they undergo different reaction routes and generate different products. Alkyl carbonate formed by EC decomposition, such as NEDC in SIBs and LEDC in LIBs, is believed to be the ingredient to form stable SEI, because of their propensity to form a multimer conformation network. In contrast, extensive studies have demonstrated that carboxylate, including linear carbonate, are incapable of forming stable interphase. This deficiency implies that their decomposition products are less effective in inhibiting continuous electrolyte reduction, evidenced by the lower ICE and higher interfacial resistance in MA- and DMC- based electrolyte than that of SOA electrolyte. In addition, considering that carboxylates exhibit higher dielectric constants than their carbonate counterparts, it can be inferred that their SEI is more prone to dissolve. Consequently, SEI formed in carboxylate may experience continuous dissolution-regeneration, which results in larger interfacial resistance and continuous capacity loss. This provides a plausible explanation for the lower CE of carboxylate electrolyte than their carbonate counterpart.

[0090] Since carboxylate alone may not form a stable interphase, the role of salt becomes meaningful. By using NaFSI salt, it is possible to make carboxylate compatible with HC and achieve high CE. In contrast, electrolytes with other salts may not be compatible with HC and exhibit irreversible electrolyte decomposition. Previous studies have shown that the organic content (mainly formed by solvent decomposition) of the SEI on sodiated hard carbon decreases in the order: NaPFe > NaC104 > NaTFSI > NaFSI, which suggests that first three anions are less effective in contributing to SEI formation compared to NaFSI. This further explains the capability of FSf to form stable SEI compared to other salts. A notable observation in the present results is the improvement of ICE and capacity with NaFSI concentration. This enhancement can be ascribed to the unique feature of high concentration electrolyte. As the salt concentration increases, more anions enter the solvation shell, driving the LUMO shift from the solvent to the anion and making anions more prone to decompose. As a result, the SEI formed in high concentration electrolytes exhibits an inorganic-rich, anion-derived composition.

[0091] So far there has been no single solvent that can simultaneously possess low melting point (to prevent freezing at low temperatures), high dielectric permittivity and low viscosity (to facilitate ion transport), while still enable both stable SEI and CEI. Carboxylates have shown the potential to provide superior mass transport properties in the bulk at both room and low temperatures. Their poor ability to form stable interface can potentially be addressed by mixing co-solvent and / or additives.

[0092] In conclusion, to evaluate the potential of carboxylate-based electrolytes in enhancing low-temperature performance of SIBs, the effect of salt, concentration and solvent molecular structure are described and the results are compared with carbonate-based electrolytes. Carboxylates enable high electrolyte conductivities, especially at low temperatures, due to their favorable properties (i.e., low melting point, low viscosity and high dielectric constant). However, carboxylates alone may be inadequate to form a stable interphase due to their high reactivity, leading to low CE and fast capacity degradation. By forming anion-rich Na+solvation via increasing salt concentration, it is possible leverage anion decomposition to form inorganic-rich SEI that can stabilize the interface, thereby permitting stable cycling of SIBs. However, the large extent of electrolyte decomposition leads to very large interfacial resistance, which significantly compromises the low-temperature performance of the carboxylate electrolytes.

[0093] At a fundamental level, the comprehensive results presented establish a clear chemistry-property-performance correlation of carboxylate-based electrolytes for SIB application. Interphase engineering can improve stable interphase and utilize their advantageous bulk transport property.

[0094] Methods The electrolytes were prepared by dissolving sodium fluoroborate (NaBF4, 99.9%, Aladdin), sodium trifluoromethanesulfonate (NaOTf, 99.5%, Solvionic), sodium perchlorate (NaCICh, 98%, Thermo Scientific Chemicals), sodium hexafluorophosphate (NaPFe, 99+%, Thermo Scientific Chemicals), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI, 99.5%, Solvionic), and sodium bis(fluorosulfonyl)imide (NaFSI, 99.9%, Solvionic) respectively as per the required concentrations in various carboxylate and carbonate solvents at argon-filled glove box. All the salts were dried in vacuum at 100 °C for 24 h, and the solvents were dried over fresh 3 A molecular sieves for three days.

[0095] The hard carbon anode was prepared as follows: a slurry made up of 90 wt% hard carbon powder (Kureha Co.), 5 wt% carbon black, and 5 wt% poly (vinylidene fluoride) in N-methyl-2-pyrrolidone solution was pasted onto Al foil. This electrode was dried overnight at room temperature, and then punched into circles with 5 / 16-inch diameter. The HC loading amount was 1.2-1.8 mg / cm2. The NVP powder was purchased from Kejing Star Technology Co., Ltd. (Shenzhen). Then, it was well mixed with poly (vinylidene fluoride) and carbon black (8: 1 : 1 wt.%), and further coated on aluminum foil. This electrode was dried overnight at room temperature, and then punched into circles with 5 / 16-inch diameter. The NVP loading amount was 1.2- 1.4 mg / cm2. Both HC anode and NVP cathode were dried in vacuum at 100 °C overnight before use.

[0096] Electrochemical tests.

[0097] All experiments were conducted using 2023-type coin cells assembled in an argon- filled glove box, where both the moisture and oxygen contents were maintained at below 1 ppm. The coin cells used a glassy fiber membrane as the separator and thick Na foil as a counter electrode, and 40 pl of electrolyte. The electrochemical performances were carried out on the Landt CT3001A battery test system. For the HC / Na half-cell cycling, 0.1C rate charging and discharging protocol was used within the voltage range of 0.001-2 V. 1C rate corresponds to 300 mAh / g on the weight basis of the HC active material. Each sample was tested twice in parallel to ensure the accuracy of the results. For low temperature testing of HC / Na, cells were discharged and charged 5 cycles with 0.1C rate at room temperature first, and then soaked in a low-temperature thermostatic bath to maintain at -20 °C for 2 hours. Subsequently, as-made cells were cycled with 0.1C rate. For NVP / Na half cells, cycling was performed at 2.0-3.6 V at 1C rate. The conductivity of the electrolyte was assessed using electrochemical impedance spectroscopy (EIS) using a Gamry Interface 1010T. Symmetrical cells (SS | electrolyte | SS, SS denote stainless steels) were assembled with one piece of glass fiber membrane soaked with electrolyte (40 pl) and two stainless steels. Such cells were tested by a potentiostatic EIS test in the frequency range of 1 to 200,000 Hz. The ionic conductivity of the electrolyte was estimated using the following equation: o = (1 / R) (1 / S), where R is the resistance, S is the area of the membrane, and G is the ionic conductivity. For the low-temperature conductivity test, the cell was hold in an oil bath capable of cooling down to -20 °C.

[0098] Characterization.

[0099] Fourier-transform infrared spectroscopy (FT-IR) was collected by the Nicolet iS50 FTIR spectrometer with diamond ATR crystal, on which the electrolyte was placed directly on the windows testing holders during the test. All FTIR spectra were deconvoluted with the Gauss Amp function by PeakFit software.

[0100] X-ray photoelectron spectroscopy (XPS) and argon sputtering were performed by Kratos Axis Ultra DLD. Samples were transferred through the antechamber to avoid any air contact. An area of 300 pm x 700 pm was irradiated using a filament voltage of 15 kV, an emission current of 8 mA, and a pass energy of 40 eV for high-resolution scans and 160 eV for the low-resolution survey scans. For the XPS sputter depth profiling measurements, a sputter crater of 3 mm x 3 mm area was produced by the Ar+ion beam using an emission current of 20 mA and a filament voltage of 4 kV. The XPS spectra were calibrated by referencing sp2carbon to 284.0 eV.

[0101] Example 2

[0102] Four unsubstituted cyclic carboxylates BPL, GBL, DVL, and ECL with ring sizes of 4, 5, 6, and 7, respectively were tested having the following structures:

[0103]

[0104] GBL shares the same five-membered ring size as EC but contains one fewer C-0 bond. Each of these lactones was dissolved in Im NaPF6 to prepare electrolytes, which were then tested in Na / Al cells using the voltage hold (VH) method. In this method, the cells were rested for 5 minutes, charged to 4.5 V, and held at that voltage until the current dropped below 0.005 mA. A baseline electrolyte (BE) underwent the same procedure.

[0105] FIG. 12A compares the current response over time for each tested electrolyte. The peak currents for BE, BPL, and GBL were around 0.5 mA, while those for DVL and ECL remained below 0.2 mA, indicating that DVL and ECL exhibit higher oxidative stability at 4.5 V. FIG. 12B compares the total capacity and average current during the hold period, revealing the following trend: GBL > BPL > BE > DVL > ECL. GBL and BPL show significant oxidative activity at high voltage, consistent with literature reports that GBL is not oxidation resistant. Although DVL exhibited a lower initial current, its total oxidation capacity and average current were comparable to those of BE. In contrast, ECL demonstrated the lowest oxidation capacity and average current, highlighting its superior anti -oxidation capability.

[0106] After the tests, the Al current collectors were disassembled, washed with dimethoxyethane (DME), and dried overnight in a vacuum oven at 80 °C. The treated Al disks were then analyzed via FT-IR spectroscopy as shown in FIG. 12C. The pristine Al showed characteristic Al-0 peaks at 946 cm1and 1067 cm These peaks nearly disappeared in the sample cycled with ECL, suggesting the formation of a passivating film that suppressed the Al signal. Other samples exhibited weak Al-0 peaks and prominent new peaks corresponding to C=O stretching, indicating decomposition products. These findings confirm that ECL rapidly forms a protective passivation layer on the cathode, effectively suppressing further electrolyte oxidation. DVL, while slower to react, still undergoes decomposition. BE, BPL, and GBL, on the other hand, exhibit significant oxidative degradation at 4.5 V.

[0107] Linear sweep voltammetry (LSV) was also performed in Na / Al cells, as shown in FIG. 12D-12E. During the first scan, the oxidation stability followed the order: BPL > GBL > BE > DVL > ECL. After passivation layer formation, the second scan showed reduced oxidative activity, maintaining the same order. Although some differences exist between the VH and LSV results — particularly regarding DVL and ECL — both methods consistently identified DVL and ECL as the most oxidation-resistant solvents.

[0108] Seven derivatives of DVL with P-substitution by alkyl chains of varying lengths (e.g. Compound II where R was 1, 3, 4, 5, 6, 7, and 9 were studied using LSV. These molecules were named according to the length of their alkyl chains. For example, the methyl -substituted compound was named DVL-C1, while the butyl-substituted one was named DVL-C4. The electrolytes were prepared by dissolving Im NaPFe in the corresponding solvents. The results showed that P-substitution negatively impacted the oxidative stability of the electrolyte (FIG. 12F-12G). For instance, DVL-C1 exhibited a significantly higher oxidation current compared to DVL. However, as the alkyl chain length increased, the oxidation current decreased. Notably, the oxidation profile of DVL-C9 closely resembled that of DVL. These findings suggest that longer alkyl chains can mitigate the adverse effects of P-substitution, likely due to steric hindrance that shields the P-site from oxidation.

[0109] Due to the high viscosity of the cyclic molecule, an additional solvent can be used in the electrolyte formulation to reduce viscosity and improve ionic conductivity, thereby ensuring favorable bulk properties. Solvents were tested as diluents, including dichloromethane (DCM), fluorobenzene (FB), methyl trifluoroacetate (MTFA), ethyl difluoroacetate (EDFA), ethyl trifluoroacetate (ETFA), methyl acetate (MA), methyl propionate (MP), and 1,1,2,2-tetrafluoroethylene 2, 2, 3, 3 -tetrafluoropropyl ether (TTE). The electrolytes were prepared by dissolving Im NaPFe in a mixture of DVL and one of the diluent solvents at a 1:9 weight ratio. Additionally, 2 wt% fluoroethylene carbonate (FEC) was added as an additive to enhance performance on the HC anode. Each electrolyte was named according to its diluent solvent.

[0110] FIG. 13 A presents the initial coulombic efficiency (ICE) and specific discharge capacity at 0.1C, while FIG. 13B displays the average coulombic efficiency (CE, from the 50th to 100th cycle) and capacity retention at the 100th cycle for the hard carbon / sodium nickel iron manganese oxide (HC / NFM) full cell (with an N:P ratio of 1.5) operated between 1.5-4.2V. Although TTE did not exhibit the highest cycling performance, showing an average CE of 98.30% and capacity retention of 74.55%, it delivered the best initial performance, with an ICE of 65.70% and a specific discharge capacity of 137 mAh / g. MP, EDFA, and ETFA demonstrated higher average CE, and MA showed better capacity retention. These four solvents are suitable for use as co-solvents.

[0111] FIG. 13C presents the performance of HC / NFM cells using different cyclic carboxylate-based electrolytes. Each electrolyte was prepared by dissolving Im NaPF6 in a mixture of a cyclic carboxylate and TTE at a 1 :9 weight ratio, with 2 wt% FEC added as an additive. A control electrolyte containing only TTE (without any cyclic carboxylate), labeled "TTE," was tested under identical conditions to isolate the effect of the cyclic carboxylates. The results show that both the TTE and BPL / TTE electrolytes failed during the first cycle. The GBL / TTE electrolyte survived the first cycle but began decomposing around 3.8 V during the charging process in the second cycle. In contrast, DVL / TTE and ECL / TTE electrolytes enabled stable charge-discharge cycling for three cycles between 1.5-4.2 V at 0.1C and exhibited similar capacity-voltage profiles (FIG. 13D). ECL showed the highest initial coulombic efficiency (ICE), but a lower average CE compared to DVL, suggesting that the greater initial decomposition of DVL may contribute to the formation of a more stable electrode-electrolyte interphase. FIG. 13E shows the cycling performance of ECL / TTE and DVL / TTE electrolytes at 1C, where both demonstrated comparable specific discharge capacities and coulombic efficiencies over 100 cycles.

[0112] TTE was added as a diluent to reduce the high viscosity introduced by the cyclic carboxylates. However, TTE is a low-polarity solvent with poor salt dissociation capability, which limits the overall performance of the full cell. Therefore, the electrolyte can be adjusted by introducing co-solvents to improve the bulk properties further. Two classes of co-solvents were tested: linear carboxylates and linear carbonates.

[0113] The ECL / TTE electrolyte was selected as the control because it exhibited a higher initial coulombic efficiency (ICE) than DVL / TTE. Electrolytes containing 40 wt% cosolvent were named according to the co-solvent used. For example, an electrolyte composed of Im NaPF6 in ECL / MA / TTE with a weight ratio of 1 :4:5 and 2 wt% FEC was labeled "MA-4". The labels MA-1, MA-4 correspond to electrolytes containing 10%, respectively. The ECL ratio is fixed at 10% for all electrolyte.

[0114] FIG. 14A compares the ICE and specific discharge capacity at 0.1C, while FIG. 14B shows the average CE (over cycles 21-30) and capacity retention at the 30th cycle for HC / NFM full cells cycled between 1.5-4.2 V. The results indicate that, compared to the ECL / TTE, most co-solvents, except for DEC, enhanced the ICE, suggesting reduced side reactions during the first cycle. However, most co-solvents resulted in a reduction in specific discharge capacity. Shorter-chain co-solvents such as MA, EA, EMC, and DMC showed a decrease of more than 38 mAh / g compared to the electrolyte without any co-solvent. Notably, although MA exhibited the highest capacity retention in the earlier diluent screening, it delivered significantly lower capacity in this example (e.g. 62 mAh / g less) likely due to increased internal resistance. In contrast, longer-chain carboxylates and carbonates, such as NPA and DEC, had minimal impact on capacity, with NPA-1 and NPA-4 even showing improved specific discharge capacities. For cycling, all cosolvent can increase the average CE significantly and most cosolvent can increase the capacity retention.

[0115] FIG. 14C-14D compare the 1C cycling performance of NPA-4 and the baseline electrolyte (BE) over voltage ranges of 1.5-4.2 V and 1.5-4.5 V. NPA-4 exhibited better capacity retention than BE at 4.2 V, and its advantage became more pronounced at the higher cutoff voltage of 4.5 V. After 200 cycles, NPA-4 retained 65.36% of its discharge capacity, whereas BE dropped to 50.67% after just 100 cycles.

[0116] This example demonstrates that the oxidation resistance of cyclic carboxylate can be tuned by modifying the ring size and the alkyl chain length of beta-alkyl substitutions. Among them, electrolytes based on DVL and ECL enable stable cycling in HC / NFM full cells within a voltage range of 1.5-4.2 V. By introducing diluents and co-solvents, this example electrolyte can be adapted for 4.5 V sodium-ion batteries, achieving a capacity retention of 65.36% after 200 cycles at 1C rates, significantly higher than that of commercial EC / DEC-based electrolytes.

[0117] Reference was made to the examples illustrated in the drawings and specific language was used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the technology is thereby intended. Alterations and further modifications of the features illustrated herein and additional applications of the examples as illustrated herein are to be considered within the scope of the description.

[0118] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more examples. In the preceding description, numerous specific details were provided, such as examples of various configurations to provide a thorough understanding of examples of the described technology. It will be recognized, however, that the technology may be practiced without one or more of the specific details, or with other methods, components, devices, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the technology.

[0119] For example, various features and elements of the above-described invention can be used alone or in various combinations where elements descried in one example can be utilized in other described examples. More specifically, the invention can be generally described by the following clauses:

[0120] Clause 1. An electrolyte for sodium batteries comprising: a salt comprising one or more of sodium bis (fluoro sulfonyl) imide (NaFSI) salt, NaOTf, NaTFSI, NaBF4, NaPF6, and NaCIC ; and an electrolyte solvent comprising at least one carboxylate ester as a dominant solvent.

[0121] Clause 2. The electrolyte of any of Clauses 1 and 3-23, wherein the electrolyte is designed for low-temperature operation and the salt comprises a dominant salt selected from the group consisting of NaFSI, NaOTf, and NaTFSI

[0122] Clause 3. The electrolyte of any of Clauses 2, wherein the NaFSI salt is the dominant salt, and optionally the only salt.

[0123] Clause 4. The electrolyte of any of Clauses 1 and 5-23, wherein the electrolyte is designed for high-voltage operation and the salt comprises a dominant salt selected from the group consisting of NaBF4, NaPFe, and NaC104.

[0124] Clause 5. The electrolyte of Clause 4, wherein the NaPFe salt is the dominant salt, and optionally wherein the NaPFe salt is the only salt. Clause 6. The electrolyte of any of Clauses 1 -5 and 6-23, wherein the salt further comprises one or more of NaPF6, NaCIC , NaOTf, NaTFSI, NaBETI, NaSCN, NaNO3, NaBOB, NaDFOB, Na2C2O4, NaBF4, LiTFSI and Mg(TFSI)2.

[0125] Clause 7. The electrolyte of any of Clauses 1-6 and 8-23, further comprising at least one additive in an amount less than 5 weight percent based on total electrolyte weight.

[0126] Clause 8. The electrolyte of Clause 7, wherein the additive is one or more of vinylene carbonate, fluoroethylene carbonate, sodium (oxalate) difluoro borate, tris (trimethyl silyl) phosphite, RbPFe, CsPFe, LiPFe, SbF3, biphenyl, succinonitrile, ethylene sulfite, 1,3-propane sultone, prop-l-ene-l,3-sultone, sulfolane, and sodium bis(oxalato)borate.

[0127] Clause 9. The electrolyte of any of Clauses 1-8 and 10-23, wherein the salt consists essentially of the NaFSI salt.

[0128] Clause 10. The electrolyte of any of Clauses 1-9 and 11-23, wherein the salt is present at a concentration from about 1 mol / kg to about 3 mol / kg.

[0129] Clause 11. The electrolyte of any of Clauses 1-10 and 12-23, wherein the carboxylate ester is selected from the group consisting of methyl acetate, ethyl acetate, methyl propionate, n-propyl acetate, i-propyl acetate, ethyl propionate, methyl butyrate, methyl isobutyrate, and combinations thereof.

[0130] Clause 12. The electrolyte of any of Clauses 1-11 and 13-23, wherein the carboxylate ester has from 3 to 10 carbons.

[0131] Clause 13. The electrolyte of any of Clauses 1-12 and 13-23, wherein the carboxylate ester is a substituted or unsubstituted cyclic carboxylate ester selected from the group consisting of P-propiolactone (BPL), y-butyrol acton e (GBL), 8-valerolactone (DVL), and s-caprol acton e (ECL).

[0132] Clause 14. The electrolyte of any of Clauses 1-13 and 14-23, wherein the electrolyte solvent consists essentially of the at least one carboxylate ester.

[0133] Clause 15. The electrolyte of any of Clauses 1-14 and 16-23, wherein the electrolyte solvent further comprises a co-solvent which is not a carboxylate ester.

[0134] Clause 16. The electrolyte of Clause 15, wherein the co-solvent is selected from linear carbonate molecules (diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate, etc.) , cyclic carbonate molecules (such as ethylene carbonate, propylene carbonate, etc ), cyclic carboxylate molecules (such as y-butyrol acton e, y-valerolactone, etc.), nitrile molecules (such as acetonitrile, butyl nitrile, etc.), ether molecules (such as diethyl ether and dimethoxyethane), pyrrolidone molecules (such as N-methyl-2- pyrrolidone), phosphate molecules (such as trimethyl phosphate and triethyl phosphate) and sulfone molecules (such as sulfolane) ,and combinations thereof.

[0135] Clause 17. The electrolyte of Clause 15, wherein the at least one carboxylate ester comprises 70% to 99% by volume of the electrolyte solvent.

[0136] Clause 18. The electrolyte of Clause 15, wherein the at least one carboxylate ester comprises 70% to 80% by volume of the electrolyte solvent.

[0137] Clause 19. The electrolyte of Clause 15, wherein the at least one carboxylate ester comprises 90% to 99% by volume of the electrolyte solvent.

[0138] Clause 20. The electrolyte of any of Clauses 1-19 and 23, further comprising a catholyte adjacent the electrolyte, the catholyte comprising a second salt and a catholyte solvent, wherein the catholyte solvent does not comprise carboxylate ester.

[0139] Clause 21. The electrolyte of Clause 20, wherein the second salt is not NaFSI.

[0140] Clause 22. The electrolyte of Clause 17, wherein the second salt is one or more of NaPF6, NaPF4, NaC104, NaOTf, NaTFSI, and NaBF4.

[0141] Clause 23. The electrolyte of Clause 20, wherein the catholyte solvent is selected from the group consisting of ethyl carbonate, diethyl carbonate, dimethyl carbonate, and combinations thereof.

[0142] Clause 24. A sodium ion battery cell, comprising: an electrolyte comprising a salt comprising one or more of sodium bis (fluorosulfonyl) imide (NaFSI) salt, NaOTf, NaTFSI, NaBF4, NaPFe, and NaC104, and an electrolyte solvent comprising at least one carboxylate ester as a dominant solvent; an anode which is in electrical contact with the electrolyte; a cathode which is in electrical contact with the electrolyte; and a casing which encloses the electrolyte, the anode, and the cathode sufficient to prevent oxygen and moisture from entering the casing.

[0143] Clause 25. The sodium ion battery cell of any one of Clauses 24 and 26-28, wherein the electrolyte is the electrolyte of any one or more of claims 1 to 20. Clause 26. The sodium ion battery cell of any one of Clauses 24-25 and 27-28, further comprising an anion-dominated solid electrolyte interphase (SEI) layer on the anode.

[0144] Clause 27. The sodium ion battery cell of any one of Clauses 24-26 and 28, wherein the anode comprises hard carbon.

[0145] Clause 28. The sodium ion battery cell of any one of Clauses 24-27, wherein the cathode comprises sodium, Na2V2(PO4)3, or a combination thereof.

[0146] Although the subject matter has been described in language specific to structural features and / or operations, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features and operations described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. Numerous modifications and alternative arrangements may be devised without departing from the spirit and scope of the described technology.

Claims

CL IMSWhat is claimed is:

1. An electrolyte for sodium batteries comprising: a salt comprising one or more of sodium bis (fluoro sulfonyl) imide (NaFSI) salt, sodium trifluoromethane sulfonate (NaOTf), sodium bis (trifluoromethyl sulfonyl) imide (NaTFSI), NaBF4, NaPFe, and NaC104; and an electrolyte solvent comprising at least one carboxylate ester as a dominant solvent.

2. The electrolyte of claim 1, wherein the electrolyte is designed for low- temperature operation and the salt comprises a dominant salt selected from the group consisting of NaFSI, NaOTf, and NaTFSI.

3. The electrolyte of claim 1, wherein the NaFSI salt is the dominant salt.

4. The electrolyte of claim 3, wherein the salt consists essentially of the NaFSI salt.

5. The electrolyte of claim 1, wherein the electrolyte is designed for high-voltage operation and the salt comprises a dominant salt selected from the group consisting of NaBF4, NaPF6, and NaC104.

6. The electrolyte of claim 5, wherein the NaPFe salt is the dominant salt.

7. The electrolyte of claim 6, wherein the salt consists essentially of the NaPFe salt.

8. The electrolyte of claim 4, wherein the salt further comprises one or more of NaPF6, NaC104, NaOTf, NaTFSI, NaBETI, NaSCN, NaNO3, NaBOB, NaDFOB, Na2C2O4, NaBF4, LiTFSI and Mg(TFSI)2.

9. The electrolyte of claim 1, further comprising at least one additive in an amount less than 5 weight percent based on total electrolyte weight.

10. The electrolyte of claim 9, wherein the additive is one or more of vinylene carbonate, fluoroethylene carbonate, sodium (oxalate) difluoro borate, tris (trimethylsilyl) phosphite, RbPFe, CsPFe, LiPFe, SbF.3, biphenyl, succinonitrile, ethylene sulfite, 1,3- propane sultone, prop-l-ene-l,3-sultone, sulfolane, and sodium bis(oxalato)borate.11 . The electrolyte of claim 1 , wherein the salt is present at a concentration from about 1 mol / kg to about 3 mol / kg.

12. The electrolyte of claim 1, wherein the carboxylate ester is selected from the group consisting of methyl acetate, ethyl acetate, methyl propionate, n-propyl acetate, i- propyl acetate, ethyl propionate, methyl butyrate, methyl isobutyrate, and combinations thereof.

13. The electrolyte of claim 1, wherein the carboxylate ester has from 3 to 10 carbons.

14. The electrolyte of claim 1, wherein the carboxylate ester is a substituted or unsubstituted cyclic carboxylate ester selected from the group consisting of -propiolactone (BPL), y-butyrolactone (GBL), 8-valerolactone (DVL), and 8-caprolactone (ECL).

15. The electrolyte of claim 1, wherein the electrolyte solvent consists essentially of the at least one carboxylate ester.

16. The electrolyte of claim 1, wherein the electrolyte solvent further comprises a co-solvent which is not a carboxylate ester.

17. The electrolyte of claim 16, wherein the co-solvent is selected from linear carbonate molecules (diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate, etc.) , cyclic carbonate molecules (such as ethylene carbonate, propylene carbonate, etc.), cyclic carboxylate molecules (such as y-butyrolactone, y-valerolactone, etc.), nitrile molecules (such as acetonitrile, butyl nitrile, etc.), ether molecules (such as diethyl ether and dimethoxyethane), pyrrolidone molecules (such as N-methyl-2-pyrrolidone), phosphatemolecules (such as trimethyl phosphate and triethyl phosphate) and sulfone molecules (such as sulfolane) ,and combinations thereof.

18. The electrolyte of claim 16, wherein the at least one carboxylate ester comprises 70% to 99% by volume of the electrolyte solvent.

19. The electrolyte of claim 16, wherein the at least one carboxylate ester comprises 70% to 80% by volume of the electrolyte solvent.

20. The electrolyte of claim 16, wherein the at least one carboxylate ester comprises 90% to 99% by volume of the electrolyte solvent.

21. The electrolyte of claim 1, further comprising a catholyte adjacent the electrolyte, the catholyte comprising a second salt and a catholyte solvent, wherein the catholyte solvent does not comprise carboxylate ester.

22. The electrolyte of claim 21, wherein the second salt is not NaFSI.

23. The electrolyte of claim 21, wherein the second salt is one or more of NaPF6, NaPF4, NaCIC , NaOTf, NaTFSI, and NaBF4.

24. The electrolyte of claim 21, wherein the catholyte solvent is selected from the group consisting of ethyl carbonate, diethyl carbonate, dimethyl carbonate, and combinations thereof.

25. A sodium ion battery cell, comprising: an electrolyte comprising a salt comprising one or more of sodium bis (fluorosulfonyl) imide (NaFSI) salt, sodium trifluoromethane sulfonate (NaOTf), sodium bis (trifluoromethyl sulfonyl) imide (NaTFSI), NaBF4, NaPFe, and NaC104, and an electrolyte solvent comprising at least one carboxylate ester as a dominant solvent; an anode which is in electrical contact with the electrolyte; a cathode which is in electrical contact with the electrolyte; anda casing which encloses the electrolyte, the anode, and the cathode sufficient to prevent oxygen and moisture from entering the casing.

26. The sodium ion battery cell of claim 25, wherein the electrolyte is the electrolyte of any one or more of claims 1 to 20.

27. The sodium ion battery cell of claim 25, further comprising an anion-dominated solid electrolyte interphase (SEI) layer on the anode.

28. The sodium ion battery cell of claim 25, wherein the anode comprises hard carbon.

29. The sodium ion battery cell of claim 25, wherein the cathode comprises sodium, Na2V2(PO4)3, or a combination thereof.

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