Localized high-salt concentration electrolyte containing a solvent based on a long-chain ethylene glycol ether and a fluorinated diluent and its use

Abstract

Localized high-salt concentration electrolytes, each containing a salt, an ethylene glycol ether as a solvent, and a fluorinated diluent. In certain embodiments, the ethylene glycol ether has the chemical formula R1-(O-CH2-CH2) n -O-R2, where n = 1 to 4 and at least one of R1 and R2 is a hydrocarbon side chain having at least 2 carbon atoms and where the salt is soluble in the ethylene glycol ether. In certain embodiments, the fluorinated diluent is selected from the group consisting of fluorinated ethylene glycol ethers and fluorinated ethers. In certain embodiments, the salt includes an alkali metal salt. In certain embodiments, the salt includes an alkaline earth metal salt. The salt may include a perfluorosulfonimide salt. Also disclosed is an electrochemical device including the localized high-salt concentration electrolyte of the present disclosure.

Description

[0001] Related application information

[0002] This application claims the benefit of priority of U.S. Provisional Patent Application No. 62 / 832,676, filed on Apr. 11, 2019, entitled “HIGH SALT CONCENTRATION ELECTROLYTES WITH DILUENTS FOR SECONDARY LITHIUM BATTERIES”, which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0003] The present invention generally relates to the field of electrolytes for electrochemical devices. Specifically, the present invention relates to localized high salt concentration electrolytes containing solvents based on longer side chain ethylene glycol ethers and fluorinated diluents and their uses. BACKGROUND OF THE INVENTION

[0004] High salt concentration electrolytes are known to enhance the cycle life in rechargeable or secondary batteries with lithium metal anodes. Specifically, electrolytes with LiFSI or LiTFSI salts dissolved in ethylene glycol ether (glyme)-based solvents at greater than 2 moles (M) are known to enhance the charge-discharge cycle life performance in batteries with lithium metal anodes. In conventional electrolytes with a lithium salt concentration of ~1 M, solvent molecules undergo reduction at the surface of the lithium metal anode to form a solid-electrolyte interphase (SEI) (passivation) layer. In high salt concentration lithium-based electrolytes, most of the solvent molecules will remain associated with the solvated Li + ions and are not available to form the SEI layer. In the absence of free solvent molecules, a more compact and stable SEI is formed by the fluorinated anions of the lithium salt on the surface of the lithium anode, which results in a higher lithium plating / stripping Coulombic efficiency and enhanced cycle life performance of the battery.

[0005] However, high salt concentration electrolytes have various disadvantages, such as low conductivity, high viscosity, and consequent poor wetting of the electrodes and separators, resulting in lower charge-discharge rates (C-rates) than those typically used in conventional secondary batteries. High salt concentration electrolytes also result in higher production costs due to their high salt content, where the salt is typically the most expensive component of the electrolyte. Diluting high salt concentration electrolytes with an excess of solvent creates more free solvent molecules that react with the lithium metal anode and consume the lithium metal anode, thereby reducing the Coulombic efficiency and cycle life of the battery.

[0006] In addition, to counteract the high viscosity due to the high salt concentration, these electrolytes often utilize low-boiling DME (1,2-dimethoxyethane or monoglyme or ethylene glycol dimethyl ether) as a solvent. The use of a low-boiling solvent such as DME typically results in significant gas generation in battery cells that are simultaneously exposed to high ambient temperatures in the charged state. SUMMARY OF THE INVENTION

[0007] In one embodiment, the present disclosure relates to an electrolyte comprising a salt; a solvent comprising an ethylene glycol ether of the formula R1-(O-CH2-CH2) n -O-R2, where n = 1 to 4 and at least one of R1 and R2 is a hydrocarbon side chain having at least 2 carbon atoms, where the salt is soluble in the solvent; and a diluent selected from the group consisting of fluorinated ethylene glycol ethers and fluorinated ethers. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For purposes of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the invention is not limited to the exact methods and apparatuses shown in the drawings, in which:

[0009] Figure 1A is a graph of the cycle life versus lithium salt concentration for a plurality of exemplary localized high-salt concentration (LHSC) electrolytes, each composed of lithium bis(fluorosulfonyl)imide salt (LiFSI), 1,2-diethoxyethane (DEE), and 1,2-(1,1,2,2-tetrafluoroethoxy)-ethane (TFE) and having different DEE:TFE volume percent ratios;

[0010] Figure 1B is for Figure 1A the cycle life of different salt (LiFSI) concentrations of the LHSC electrolyte versus the DEE volume percent;

[0011] Figure 2 is Figure 1A and 1B the cycle life in various LHSC electrolytes versus the DEE / LiFSI (solvent / salt) molar ratio;

[0012] Figure 3A is a graph showing the cycle life for a plurality of different electrolytes, including the 2M LiFSI + DEE + TFE LHSC electrolyte of the present disclosure having a 70:30 DEE:TFE volume percent ratio;

[0013] Figure 3B is a graph showing the normalized gas generation for Figure 3A the electrolyte; and

[0014] Figure 4 is a high-level diagram of an electrochemical device fabricated in accordance with aspects of the present disclosure. DETAILED DESCRIPTION

[0015] In certain aspects, the present disclosure relates to localized high salt concentration (LHSC) electrolytes prepared using long side chain ethylene glycol ethers and fluorinated diluents. The LHSC electrolytes prepared in accordance with the present disclosure can have a high concentration of lithium salts such as LiFSI or LiTFSI, for example, dissolved in a long side chain ethylene glycol ether-based solvent such as DEE (1,2-diethoxyethane or ethylene glycol diethyl ether), where the long side chain ethylene glycol ether-based solvent has one or more side chains that extend the length of the molecular structure of the base ethylene glycol ether, whether it is a monoethylene glycol ether or a polyethylene glycol ether. In the case of a monoethylene glycol ether, the base ethylene glycol ether is DME (1,2-dimethoxyethane). As used herein and in the appended claims, the term "long side chain ethylene glycol ether-based solvent" refers to an ethylene glycol ether-based solvent having an additional hydrocarbon molecular structure attached to at least one end of the ethylene glycol ether molecular structure. Examples of long side chain ethylene glycol ether-based solvents are described in detail below. The LHSC electrolytes prepared in accordance with the present disclosure can also include a fluorinated hydrocarbon, such as a fluorinated ethylene glycol ether or a fluorinated ether, added as a diluent to overcome the disadvantages typically associated with high salt concentration electrolytes.

[0016] The fluorinated diluent is typically miscible with the long side chain ethylene glycol ether-based solvent used in the LHSC electrolyte; however, due to its low polarity it is designed or selected to have a much lower lithium salt solubility relative to the ethylene glycol ether-based solvent. As a result, even after the addition of the diluent, the solvent molecules of the long side chain ethylene glycol ether-based solvent and the lithium ions from the lithium-based salt will remain associated with each other and thus retain the advantages of the high salt concentration electrolyte. In addition, the addition of the fluorinated diluent reduces the overall viscosity and helps improve the conductivity and wetting properties of the LHSC electrolyte without increasing the amount of free solvent molecules in the electrolyte. Further, this allows the use of higher charge-discharge rates, such as the charge-discharge rates typically used in conventional secondary batteries such as lithium ion batteries, without sacrificing the high Coulombic efficiency and cycle life of the battery. In addition, because the diluent is fluorinated, it can also help form a tighter and more stable solid-electrolyte interphase (SEI) on the lithium metal surface of, for example, the negative electrode in a lithium metal secondary battery, in a manner similar to the fluorinated anions of the lithium salt.

[0017] In addition, the fluorinated diluent also allows the use of more stable solvents based on longer side-chain ethylene glycol ethers that are themselves relatively more viscous than DME in the preparation of high-salt-concentration electrolytes (e.g., DEE (1,2-diethoxyethane or ethylene glycol diethyl ether), DPE (1,2-dipropoxyethane or ethylene glycol dipropyl ether), DBE (1,2-dibutoxyethane or ethylene glycol dibutyl ether), diethylene glycol diethyl ether, diethylene glycol dipropyl ether, diethylene glycol dibutyl ether, triethylene glycol diethyl ether, triethylene glycol dipropyl ether, triethylene glycol dibutyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol dipropyl ether, tetraethylene glycol dibutyl ether, etc.). Further, the use of the more stable solvents based on longer side-chain ethylene glycol ethers of the present disclosure prevents or reduces the gas generated in a charged electrochemical cell unit exposed to a high ambient temperature. The decomposition products of the solvents based on longer side-chain ethylene glycol ethers will typically be relatively larger molecules with higher boiling points (e.g., relative to DME), which may tend to dissolve more into the LHSC electrolyte rather than generating gas inside the cell unit. In addition, having one or more longer side chains increases the molar volume of the ethylene glycol ether-based solvent, which in turn reduces the amount of salt (molar concentration or molality) required per unit volume of the LHSC electrolyte to maintain the association between Li + ions and solvent molecules, and thus reduces the volumetric cost of producing the LHSC electrolyte.

[0018] In another aspect, the present disclosure relates to the use of lithium-based LHSC electrolytes prepared according to the present disclosure. For example, these LHSC electrolytes can be used in any suitable lithium-based electrochemical device, such as a battery or a supercapacitor. The lithium-based LHSC electrolytes prepared according to the present disclosure can provide, for example, a greater charge-discharge cycle life and reduced gas generation to the electrochemical device, while providing desired wetting and SEI formation characteristics and allowing a good C-rate, etc.

[0019] Details of the above and other aspects of the present disclosure are described below. Throughout the present disclosure, the term "about", when used in conjunction with a corresponding numerical value, means ±20% of that numerical value, typically ±10% of that numerical value, often ±5% of that numerical value, and most often ±2% of that numerical value. In certain embodiments, the term "about" may be considered to precisely indicate the actual numerical value.

[0020] Localized high-salt-concentration electrolyte

[0021] Exemplary solvents based on longer side-chain ethylene glycol ethers

[0022] Structure 1 below shows the longer side-chain ethylene glycol ether R1-(O-CH2-CH2) that can be used in the LHSC electrolyte prepared according to the present disclosure n-O-R2 general molecular structure. The longer side-chain ethylene glycol ethers of the present disclosure will have at least one side-chain structure (R1 or R2) having 2 to 6 carbon atoms. The side-chain can be linear, branched, cyclic, partially or fully saturated. The side-chains can have the same or different molecular structures (i.e., R1 = R2 or R1 ≠ R2). A methyl (-CH2) side-chain having one carbon atom on both sides (i.e., for each of R1 and R2) will yield DME, which is conventionally used as a solvent in certain high-concentration electrolytes as mentioned above. Also as mentioned above, DME has certain unique drawbacks in certain electrochemical cell unit applications such as secondary lithium metal batteries.

[0023] n = 1 to 4 (1)

[0024] n = 1 and R1 = R2 = -CH2-CH3 (2)

[0025] For illustrative purposes, Structure 2 above is the molecular structure of DEE (1,2-diethoxyethane or ethylene glycol diethyl ether or CH3-CH2-O-CH2-CH2-O-CH2-CH3), where both side-chains R1 and R2 are ethyl (-CH2-CH3) groups. DEE is utilized below as an exemplary longer side-chain ethylene glycol ether-based solvent of the present disclosure. However, those skilled in the art will readily appreciate that DEE is merely one example and is not intended to limit the selection of another longer side-chain ethylene glycol ether-based solvent for the LHSC electrolytes prepared according to the present disclosure, such as a solvent having the molecular structure of Structure 1 above as summarized above.

[0026] Exemplary fluorinated diluents

[0027] In certain embodiments, the high-salt concentration electrolytes of the present disclosure can be prepared using fluorinated hydrocarbons, such as fluorinated ethylene glycol ethers or fluorinated ethers. Structure 3 below illustrates a fluorinated ethylene glycol ether in the form of 1,2-(1,1,2,2-tetrafluoroethoxy)-ethane (TFE).

[0028]

[0029] TFE is used herein as an exemplary fluorinated diluent, particularly in combination with the solvents based on longer-chain ethylene glycol ethers of DEE as presented in the previous section. It is noted that the backbone structure of TFE is similar to that of DEE. Thus, TFE and DEE are more likely to be miscible with each other. The presence of eight fluorine atoms in the TFE molecule makes it less polar, such that lithium salts are less soluble in TFE, for example, about 10 times or more less soluble. Thus, when selecting diluents and solvents according to the present disclosure, it is desirable to select diluents and solvents having similar backbone structures.

[0030] Two additional embodiments of the fluorinated diluent are illustrated in Structures 4 and 5 below.

[0031]

[0032]

[0033] The fluorinated diluent of Structure 4 is the fluorinated ether 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and the fluorinated diluent of Structure 5 is the fluorinated ether bis(2,2,2-trifluoroethyl) ether (BTFE). The foregoing are merely several embodiments of fluorinated ethylene glycol ethers and fluorinated ethers suitable for use in high-salt-concentration electrolytes according to the present disclosure. In certain embodiments, the diluent of the present disclosure comprises suitable hydrocarbon molecules having at least one oxygen (-O-) bond and at least one fluorine (-F) substitution.

[0034] Exemplary Li-based salts

[0035] A variety of lithium-based salts can be used to prepare the high-salt-concentration electrolytes of the present disclosure. For example, in certain embodiments, it is desirable to use lithium bis(fluorosulfonyl)imide (LiFSI), the molecular structure of which is illustrated in Structure 6 below.

[0036]

[0037] In certain embodiments, it is desirable to use a lithium-based salt having the general formula (Li + )(CF3-(CF2) n -SO2-N - -SO2-(CF2) n -CF3), where n ≥ 0. The molecular structure of this general formula is illustrated in Structure 7 below. Further, Structure 8 below illustrates the case where n = 0, which is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0038]

[0039]

[0040] It is noted that while lithium perfluorosulfonimide salts are exemplified herein, the teachings of the present disclosure can be practiced using other salts, such as alkali metal-based salts and alkaline earth metal-based salts, such as perfluorosulfonimide salts based on sodium or magnesium, but not limited thereto. In addition, salts commonly used in lithium ion batteries, such as LiPF6, LiAsF6, LiBF4, LiBOB, Li trifluoromethanesulfonate, etc., can be used in the high salt concentration electrolytes prepared according to the present disclosure.

[0041] Broadly, the electrolytes prepared according to the present disclosure can have a salt concentration in the range of about 0.1 M to about 10 M, while in some embodiments a salt concentration in the range of about 1 M to about 5 M is desirable, and in other embodiments a salt concentration in the range of about 2 M to about 3 M is desirable. In some embodiments, the solvent:diluent ratio of the electrolytes prepared according to the present disclosure can be in the range of about 10:90 to 100:0. In some embodiments, a solvent:diluent ratio in the range of about 40:60 to about 90:10 is desirable, and in other embodiments a solvent:diluent ratio in the range of about 60:40 to about 80:20 is desirable.

[0042] Exemplary High Salt Concentration Electrolytes and Test Data

[0043] The following exemplary LHSC electrolytes utilize LiFSI as the salt, DEE as the solvent based on longer side chain ethylene glycol ethers, and TFE as the fluorinated diluent. Some LHSC electrolyte compositions are prepared using various concentrations (1 M to 5 M) of the LiFSI salt and different volume ratios (30% to 100%) of DEE:TFE, and these compositions are tested for cycle life and gas formation.

[0044] The cycle life was tested using the same pouch-type secondary battery cell constructed with a nickel-manganese-cobalt (NMC) positive electrode, a lithium metal negative electrode, and a microporous polyolefin-based separator. During the test, the cells containing the respective compositions in the LHSC electrolyte composition were cycled at a C / 3 - C / 2 charge-discharge rate between 3V and 4.3V. The table below shows the charge-discharge cycle life (at 80% capacity retention) obtained in lithium battery cells fabricated with various LiFSI+DEE+TFE LHSC electrolyte compositions of the present disclosure. In the table below, the cycle life for the DEE:TFE ratio is indicated by "-" as the LiFSI salt at that DEE:TFE ratio is not fully soluble. Generally, if a higher amount of diluent (here, TFE) is used, then less LiFSI salt can be dissolved in the LHSC electrolyte because TFE is a fluorinated ethylene glycol ether with low polarity and salt solubility.

[0045]

[0046] Turning now to the drawings, Figure 1A shows the variation of the cycle life with respect to the lithium salt concentration of the various LiFSI+DEE+TFE LHSC electrolyte compositions mentioned above, and Figure 1B shows the variation of the cycle life with respect to the volume percentage of DEE used in these electrolyte compositions. As seen in Figure 1A and 1B and generally, the cycle life of the cells tends to be high at lithium salt concentrations approaching 2M or 3M and at a DEE:TFE volume ratio approaching 70:30.

[0047] Figure 2 shows the variation of the cycle life with respect to the solvent / salt (DEE / LiFSI) molar ratio in the LHSC electrolyte, independent of the amount of diluent (TFE). It is known that a 2:1 solvent / salt molar ratio is optimal for the cycle life performance of the cells in conventional high-salt-concentration lithium-based electrolytes because each lithium ion associates with an average of two DME molecules in the electrolyte. In this regard, Figure 2It also indicates enhanced cycle life for battery cells using the LHSC electrolytes of the present disclosure with a solvent:salt (DEE / LiFSI) molar ratio of about 2:1. This confirms the fact that even after the addition of the TFE diluent, the solvent (DEE) molecules and the solvated lithium ions remain associated, thus retaining the advantages of a high-salt concentration electrolyte while improving the charge-discharge rate performance of the battery cell. In certain embodiments, the solvent:salt molar ratio ranges from about 1:1 to about 4:1. In certain embodiments, the solvent:salt ratio ranges from about 1.5:1 to about 3:1. A 2:1 solvent:salt molar ratio for DEE / LiFSI roughly corresponds to about 3.6 moles of LiFSI salt per liter of DEE solvent in the electrolyte.

[0048] Figure 3A Compare the cycle life obtained using various electrolytes in battery cells cycled at a charge-discharge rate of C / 3 - C / 2 between 3V and 4.3V. The LHSC electrolytes of the present disclosure (LHSC electrolytes are labeled as "2M DEE:TFE(70:30)" in Figure 3A ), having a solvent based on a longer side-chain ethylene glycol ether and a fluorinated ethylene glycol ether diluent, show higher cycle life performance than "conventional" carbonate-based electrolytes and high-salt concentration electrolytes without a diluent (labeled as "5MDME", "5M DEE", and "2M DEE" in Figure 3A ). In this regard, "conventional" carbonate-based electrolytes include electrolytes using LiFSI, a cyclic carbonate solvent (e.g., EC (ethylene carbonate) or FEC (fluoroethylene carbonate)), and an optional linear carbonate diluent (e.g., EMC (ethyl methyl carbonate)). In one embodiment, the "conventional" electrolyte in this regard can be a 1M to 3M LiFSI solution in EC, and in another embodiment, the "conventional" electrolyte in this regard can be a 1M to 3M LiFSI solution in FEC:EMC in a 2:8 volume ratio.

[0049] Furthermore, the use of a more stable solvent based on a longer side-chain ethylene glycol ether such as DEE will reduce the amount of gas generated in a charged battery cell exposed to a high ambient temperature. Figure 3B Shows the relative amount of gas generated by various electrolytes in pouch cells (described above) that are fully charged (state of charge (SOC) 100) and exposed to a 45°C ambient temperature for one week. The carbonate-solvent-based electrolytes conventionally used in lithium-ion batteries generate the largest amount of gas in the battery cell. Inhibiting gas additives in carbonate electrolytes do not result in any significant reduction in gas volume. High-salt concentration electrolytes prepared with a monoethylene glycol ether solvent (in Figure 3BThe one labeled as "5MDME" also shows gas generation similar to that of the carbonate-based electrolyte. The use of solvents based on longer side-chain ethylene glycol ethers such as DEE in high-salt-concentration electrolytes results in a significant reduction in the gas volume (labeled as "5M DEE" in Figure 3B ). In addition, the LHSC electrolyte composition of the present disclosure in this example with enhanced cycle life performance (labeled as "2M LiFSI in DEE:TFE (70:30)" in Figure 3B ) also shows significantly lower gas generation.

[0050] The lower gas generation, along with enhanced cycle life performance at higher charge-discharge rates, makes the LHSC electrolyte of the present disclosure containing solvents based on longer side-chain ethylene glycol ethers and fluorinated ethylene glycol ethers or fluorinated ether diluents a target electrolyte for lithium metal electrochemical devices such as secondary batteries and supercapacitors.

[0051] Exemplary uses of the LHSC electrolyte of the present disclosure

[0052] As mentioned above, the LHSC electrolyte of the present disclosure can be used as an electrolyte for electrochemical devices and the like. Figure 4 FIG. 400 illustrates an electrochemical device manufactured according to an aspect of the present disclosure. Those skilled in the art will readily appreciate that the electrochemical device 400 can be, for example, a battery or a supercapacitor. In addition, those skilled in the art will readily understand that Figure 4 FIG. 400 illustrates only some of the basic functional components of the electrochemical device and a realistic example of an electrochemical device such as a secondary battery or a supercapacitor will typically be embodied using a wound configuration or a stacked configuration. In addition, those skilled in the art will understand that the electrochemical device 400 will include other components not shown in Figure 4 for the sake of simplicity of illustration, such as electrical terminals, seals, thermal breakdown layers, and / or vent holes, etc.

[0053] In this example, the electrochemical device 400 includes a positive electrode and a negative electrode 404, 408 spaced apart from each other respectively, and a pair of corresponding current collectors 404A, 408A. A porous dielectric separator 412 is located between the positive electrode and the negative electrode 404, 408 to electrically isolate the positive electrode and the negative electrode but allow the ionic flow of the LHSC electrolyte 416 prepared according to the present disclosure to penetrate therethrough. The porous dielectric separator 412 and / or one, the other, or both of the positive electrode and the negative electrode 404, 408, if porous, are impregnated with the LHSC electrolyte 416. In Figure 4In [the figure], the positive electrode and the negative electrodes 404 and 408 are illustrated as porous in the manner in which the LHSC electrolyte 416 is illustrated as extending into them. In certain embodiments, for example, certain embodiments of a lithium metal secondary battery having a solid-type lithium metal negative electrode, one, the other, or both of the positive electrode and the negative electrodes 404 and 408 do not need to be porous. As described above, the benefits of using the LHSC electrolyte of the present disclosure for the LHSC electrolyte 416 can include the fact that, because of the larger molecules of the solvent based on longer side-chain ethylene glycol ethers and the fluorinated diluent, less salt is required than in DME-based electrolytes, less gas is generated at higher temperatures, the cycle life performance is enhanced, a higher Coulombic efficiency is possible, and a higher charge-discharge rate is possible. Examples of LHSC electrolytes suitable for use as the LHSC electrolyte 416 are described above. In certain embodiments, the LHSC electrolytes of the present disclosure are particularly suitable for use in rechargeable lithium metal-based electrochemical devices, such as secondary lithium metal batteries. As disclosed herein, various compositions of the LHSC electrolytes of the present disclosure can achieve cycle life performance superior to other types of electrolytes. The electrochemical device 400 includes a container 420 that houses current collectors 404A, 408A, a positive electrode and a negative electrode 404, 408, a porous dielectric separator 412, and the LHSC electrolyte 416.

[0054] As will be understood by those skilled in the art, depending on the type and design of the electrochemical device, each of the positive electrode and the negative electrodes 404 and 408 includes a suitable material that is compatible with the alkali metal ions and other components in the LHSC electrolyte 416. Each of the current collectors 404A and 408A can be made of any suitable conductive material, such as copper or aluminum, or any combination thereof. The porous dielectric separator 412 can be made of any suitable porous dielectric material, such as a porous polymer, etc. Various battery and supercapacitor configurations capable of being used to construct Figure 4 the electrochemical device 400 are known in the art. If any of such known configurations are used, then the novelty of the electrochemical device 400 lies in the composition of the LHSC electrolyte 416.

[0055] In certain aspects, the present disclosure relates to an electrolyte that includes: a salt; a solvent that includes an ethylene glycol ether of the formula R1-(O-CH2-CH2) n -O-R2, where n = 1 to 4 and at least one of R1 and R2 is a hydrocarbon side chain having at least 2 carbon atoms, where the salt is soluble in the solvent; and a diluent selected from the group consisting of fluorinated ethylene glycol ethers and fluorinated ethers.

[0056] In one or more embodiments of the electrolyte, the salt includes an alkali metal salt.

[0057] In one or more embodiments of the electrolyte, the salt includes a perfluorosulfonylimide salt.

[0058] In one or more embodiments of the electrolyte, the perfluorosulfonylimide salt includes a lithium-based perfluorosulfonylimide salt.

[0059] In one or more embodiments of the electrolyte, the lithium-based perfluorosulfonylimide salt includes lithium bis(fluorosulfonyl)imide.

[0060] In one or more embodiments of the electrolyte, the salt includes a lithium salt.

[0061] In one or more embodiments of the electrolyte, the concentration of the salt in the electrolyte ranges from about 0.1 M to about 10 M.

[0062] In one or more embodiments of the electrolyte, the concentration of the salt in the electrolyte ranges from about 1 M to about 5 M.

[0063] In one or more embodiments of the electrolyte, the concentration of the salt in the electrolyte ranges from about 2 M to about 3 M.

[0064] In one or more embodiments of the electrolyte, the electrolyte has a volume percentage ratio of solvent: diluent from about 10:90 to about 100:0.

[0065] In one or more embodiments of the electrolyte, the electrolyte has a volume percentage ratio of solvent: diluent from about 40:60 to about 90:10.

[0066] In one or more embodiments of the electrolyte, the electrolyte has a volume percentage ratio of solvent: diluent from about 60:40 to about 80:20.

[0067] In one or more embodiments of the electrolyte, the electrolyte has a solvent: salt molar ratio in the range of about 1:1 to about 4:1.

[0068] In one or more embodiments of the electrolyte, the electrolyte has a solvent: salt molar ratio in the range of about 1.5:1 to about 3:1.

[0069] In one or more embodiments of the electrolyte, the electrolyte has a solvent: salt molar ratio in the range of about 2:1.

[0070] In one or more embodiments of the electrolyte, R1 = R2.

[0071] In one or more embodiments of the electrolyte, the hydrocarbon side chain has 2 to 6 carbon atoms.

[0072] In one or more embodiments of the electrolyte, n = 1 and each of R1 and R2 is an ethyl group, such as in 1,2 - diethoxyethane (DEE).

[0073] In one or more embodiments of the electrolyte, the diluent includes a fluorinated ether.

[0074] In one or more embodiments of the electrolyte, the diluent includes a fluorinated ethylene glycol ether.

[0075] In one or more embodiments of the electrolyte, the fluorinated ethylene glycol ether is 1,2 - (1,1,2,2 - tetrafluoroethoxy) - ethane (TFE).

[0076] In one or more embodiments of the electrolyte, the salt includes lithium bis(fluorosulfonyl)imide.

[0077] In one or more embodiments of the electrolyte, the concentration of the salt is at least 2M.

[0078] In one or more embodiments of the electrolyte, the solvent and the diluent are present in a solvent:diluent volume percentage ratio in which the solvent is at least 40% and the diluent is at most 60%.

[0079] In one or more embodiments of the electrolyte, the solvent and the diluent are present in a solvent:diluent volume percentage ratio in which the solvent is at least 60% and the diluent is at most 40%.

[0080] In one or more embodiments of the electrolyte, the concentration of the salt is at least 3M.

[0081] In one or more embodiments of the electrolyte, the solvent and the diluent are present in a solvent:diluent volume percentage ratio in which the solvent is at least 40% and the diluent is at most 60%.

[0082] In one or more embodiments of the electrolyte, the solvent and the diluent are present in a solvent:diluent volume percentage ratio in which the solvent is at least 60% and the diluent is at most 40%.

[0083] In one or more embodiments of the electrolyte, the salt includes lithium bis(fluorosulfonyl)imide.

[0084] In one or more embodiments of the electrolyte, the salt is selected from the group consisting of lithium bis(fluorosulfonyl)imide and salts of the following molecular structures:

[0085]

[0086] where n is equal to or greater than 0.

[0087] In some aspects, the present disclosure relates to an electrochemical device including: a positive electrode; a negative electrode spaced apart from the positive electrode; a porous dielectric separator located between the positive electrode and the negative electrode; and an electrolyte according to any one of the electrolyte embodiments described above and accommodated within at least the porous dielectric separator.

[0088] In one or more embodiments of the electrochemical device, the negative electrode comprises lithium metal.

[0089] In one or more embodiments of the electrochemical device, the electrochemical device is a secondary battery and the negative electrode is a lithium metal electrode.

[0090] The foregoing has been a detailed description of illustrative embodiments of the invention. Note that in the specification and the appended claims, unless specifically stated or indicated otherwise, the conjunctive language used, such as in the phrases “at least one of X, Y, and Z” and “one or more of X, Y, and Z,” should be construed to mean that each item in the list of conjuncts may exist in any number excluding every other item in the list or in any number in combination with any or all of the other items in the list, each of which may also exist in any number. Applying this general rule, the conjunctive phrases in the foregoing examples, where the list of conjuncts consists of X, Y, and Z, should respectively cover: one or more of X; one or more of Y; one or more of Z; one or more of X and one or more of Y; one or more of Y and one or more of Z; one or more of X and one or more of Z; and one or more of X, one or more of Y, and one or more of Z.

[0091] Various modifications and additions can be made without departing from the spirit and scope of the invention. The features of each of the various embodiments described above can be combined with the features of the other described embodiments as appropriate to provide various combinations of features in associated new embodiments. Further, while many separate embodiments have been described above, what has been described herein is merely illustrative of the application of the principles of the invention. Additionally, while the specific methods herein may be illustrated and / or described as being performed in a particular order, the order is highly variable within the common general knowledge to achieve aspects of the present disclosure. Accordingly, this description is merely exemplary and does not otherwise limit the scope of the invention.

[0092] Exemplary embodiments have been disclosed above and illustrated in the drawings. Those skilled in the art will understand that various changes, omissions, and additions can be made to what is specifically disclosed herein without departing from the spirit and scope of the invention.

Claims

1. An electrolyte, the electrolyte comprising: Lithium bis(fluorosulfonyl)imide as its salt; 1,2 - Diethoxyethane as its solvent; and 1,2-(1,1,2,2 - tetrafluoroethoxy)-ethane (TFE) as its diluent; wherein the concentration of the salt in the electrolyte is from 2 M to 3 M; and the electrolyte has a volume percentage ratio of solvent:diluent of 60:40 to 80:

20.

2. The electrolyte according to claim 1, the electrolyte having a solvent:salt molar ratio in the range of 2:1 to 4:

1.

3. The electrolyte according to claim 1, the electrolyte having a solvent:salt molar ratio in the range of 2:1 to 3:

1.

4. The electrolyte according to claim 1, the electrolyte having a solvent:salt molar ratio of 2:

1.

5. The electrolyte according to claim 1, wherein the concentration of the salt is less than 3 M.

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