Functional organic salt for lithium-ion batteries

Abstract

An electrolyte for a lithium-ion electrochemical cell comprises a non-aqueous solution of a lithium salt and a redox shuttle salt compound in a non-aqueous solvent, wherein the redox shuttle compound comprises an amino-substituted cyclopropenium salt of Formula (I) as described herein.

Claims

1. An electrolyte for a lithium-ion electrochemical cell comprising a non-aqueous solution of a lithium salt and a redox shuttle salt in an organic solvent; wherein the redox shuttle salt comprises a compound of Formula (I): ##STR00010## wherein X.sup. is an anion selected from the group consisting of a halide, and bis(trifluoromethanesulfonyl)imidate, tetrafluoroborate, hexafluorophosphate, and an anion of Formula (II): ##STR00011## Z is selected from the group consisting of alkylsulfonyl, alkylcarbonyl, fluorinated-alkylsulfonyl, and fluorinated-alkylcarbonyl; R and R are independently selected from the group consisting of C.sub.1 to C.sub.8 alkyl, aryl, alkylaryl, and poly(alkylene glycol) groups; and the alkyl, aryl, arylalkyl and alkylaryl groups optionally are substituted by one of more substituent selected from halogen, hydroxyl, alkoxy, alkylsulfonyl, and arylsulfonyl; and wherein the compound of Formula (I) is present in the organic solvent at a concentration in the range of about 0.005 M to about 0.5 M.

2. The electrolyte of claim 1, wherein R and R are ethyl.

3. The electrolyte of claim 1, wherein X.sup. is a halide.

4. The electrolyte of claim 1, wherein X.sup. is chloride.

5. The electrolyte of claim 1, wherein X.sup. is a sulfonimidate anion.

6. The electrolyte of claim 1, wherein X.sup. is an anion of Formula (II): ##STR00012## wherein Z is selected from the group consisting of alkylsulfonyl, alkylcarbonyl, fluorinated-alkyl sulfonyl, and fluorinated-alkylcarbonyl.

7. The electrolyte of claim 1, wherein X.sup. is bis(trifluoromethanesulfonyl)imidate anion.

8. The electrolyte of claim 1, wherein the lithium salt comprises one or more salt selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imidate (LiTFSI), lithium 2-trifluoromethyl-4,5-dicyanoimidazolate (LiTDI), lithium 4,5-dicyano-1,2,3-triazolate (LiTDI), lithium trifluoromethanesulfonate (LiTf), lithium perchlorate (LiClO.sub.4), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium tetrafluoroborate (LiBF.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium thiocyanate (LiSCN), lithium bis(fluorosulfonyl)imidate (LIFSI), lithium bis(pentafluoroethylsulfonyl)imidate (LBETI), lithium tetracyanoborate (LiB(CN).sub.4), and lithium nitrate.

9. The electrolyte of claim 1, wherein the lithium salt is present in the electrolyte at a concentration in the range of about 0.1 M to about 3 M.

10. The electrolyte of claim 1, wherein the non-aqueous solvent comprises one or more solvent selected from the group consisting of an ether, a carbonate ester, a nitrile, a sulfoxide, a sulfone, a fluoro-substituted linear dialkyl carbonate, a fluoro-substituted cyclic alkylene carbonate, a fluoro-substituted sulfolane, and a fluoro-substituted sulfone.

11. The electrolyte of claim 1, wherein the non-aqueous solvent comprises one or more carbonate esters selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate.

12. The electrolyte of claim 1, wherein the non-aqueous solvent comprises ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in an EC:EMC ratio of about 3:7 (v:v).

13. The electrolyte of claim 12, wherein the lithium salt comprises about 1 to 1.5 M LiPF.sub.6 and the compound of Formula (I) comprises about 0.3 to 0.5 M CP-Cl.

14. The electrolyte of claim 12, wherein the lithium salt comprises about 1 to 1.5 M LiPF.sub.6 and the compound of Formula (I) comprises about 0.3 to 0.5 M CP-TFSI.

15. An electrochemical cell comprising an anode, a cathode, a lithium ion-porous membrane between the anode and the cathode, and the electrolyte of claim 1 contacting the anode, the cathode, and the membrane.

16. A battery comprising a plurality of electrochemical cells of claim 15 electrically connected together in series, in parallel, or in both series and parallel.

17. An electrolyte for a lithium-ion electrochemical cell comprising a non-aqueous solution of a lithium salt and a redox shuttle salt in an organic solvent wherein the redox shuttle salt comprises tris(diethylamino)cyclopropenium chloride (CP-Cl), which has the formula: ##STR00013## and wherein the concentration of the redox shuttle salt is about 0.3 to 0.5 M; the non-aqueous solvent comprises one or more carbonate esters selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate; and the lithium salt comprises about 1 to 1.5 M LiPF.sub.6.

18. The electrolyte of claim 17, wherein the non-aqueous solvent comprises ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in an EC:EMC ratio of about 3:7 (v:v).

19. The electrolyte of claim 9, wherein the non-aqueous solvent comprises ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in an EC:EMC ratio of about 3:7 (v:v).

20. An electrolyte for a lithium-ion electrochemical cell comprising a non-aqueous solution of a lithium salt and a redox shuttle salt in an organic solvent; wherein the redox shuttle salt comprises tris(diethylamino)cyclopropenium bis(trifluoromethanesulfonyl)imidate (CP-TSFI), which has the formula: ##STR00014## and wherein the concentration of the redox shuttle salt is about 0.3 to 0.5 M; the non-aqueous solvent comprises one or more carbonate esters selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate; and the lithium salt comprises about 1 to 1.5 M LiPF.sub.6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

(2) FIG. 1 illustrates compounds of tris(diethylamino)cyclopropenium-type additives of Formula (I) including tris(diethylamino)cyclopropenium chloride (CP-Cl) and tris(diethylamino)cyclopropenium bis(trifluoromethanesulfonyl)imidate (CP-TFSI).

(3) FIG. 2 shows cyclic voltammograms of electrochemical cells utilizing an electrolyte containing CP-Cl (10 mM) at various scan rates.

(4) FIG. 3 shows cyclic voltammograms of electrochemical cells utilizing an electrolyte containing CP-TFSI (10 mM) at various scan rates.

(5) FIG. 4 shows a voltage profile plot of overcharge data (top) and a capacity versus cycling number plot (bottom) from coin cells using an electrolyte containing CP-Cl (0.4 M); charging rate of C/10 and overcharge rate of 100%. The plot shows data for the first 225 hours of cycling.

(6) FIG. 5 shows a voltage profile plot of overcharge data (top) and a capacity versus cycling number plot (bottom) from coin cells using an electrolyte containing CP-TFSI (0.4 M); charging rate of C/10 and overcharge rate of 100%. The plot shows data for approximately the first 130 hours of cycling.

(7) FIG. 6 schematically illustrates a lithium-ion electrochemical cell.

(8) FIG. 7 schematically illustrates a lithium-ion battery.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

(9) Described herein are non-aqueous electrolytes for a lithium-ion batteries comprising a lithium salt and a redox shuttle salt additive in a non-aqueous solvent. The redox shuttle salt additives are tris(amino)-substituted cyclopropenium salts of Formula (I).

(10) ##STR00006##
wherein X.sup. is an anion. In some embodiments X.sup. is an anion selected from a halide (e.g., F.sup., Cl.sup., Br.sup.), bis(trifluoromethanesulfonyl)imidate, tetrafluoroborate, hexafluorophosphate and an anion of Formula (II):

(11) ##STR00007##
wherein Z is selected from alkylsulfonyl, alkylcarbonyl, fluorinated-alkylsulfonyl, and fluorinated-alkylcarbonyl; (e.g., bis(methylsulfonyl)imidate, bis(trifluoromethanesulfonyl)imidate, bis(trifluoroacetyl)imidate, and the like). In some preferred embodiments X.sup. is chloride or bis(trifluoromethanesulfonyl)imidate.

(12) R and R are independently selected from alkyl (e.g., C.sub.1 to C.sub.8 alkyl, such as methyl, ethyl, propyl, isopropyl, butyl, pentyl, and the like), aryl (e.g., phenyl), arylalkyl (e.g., benzyl, 2-phenylethyl, and the like), alkylaryl (e.g., C.sub.1 to C.sub.8 alkyl-substituted phenyl, such as 4-ethylphenyl, 2,4,6-trimethylphenyl, and the like), and a poly(alkylene glycol) group. As used herein the term poly(alkylene glycol) refers to an oligomer or polymer of alkylene glycol monomer units (e.g., an ethylene glycol polymer or oligomer, a propylene glycol polymer or oligomer, an ethylene glycol-propylene glycol copolymer, and the like). For example, a poly(alkylene glycol) group for use as a substituent R or R can have the formula: C(R).sub.2C(R).sub.2(OC(R).sub.2C(R).sub.2).sub.nOC(R).sub.2C(R).sub.2OR where R can be H, methyl, or ethyl, and n is 3 to 100. In some preferred embodiments, R and R are independently C.sub.1 to C.sub.4 alkyl (e.g., methyl, ethyl, propyl, isopropyl and t-butyl).

(13) The redox shuttle salt additive can be present in the electrolyte at any concentration, but preferably is present at a concentration in the range of about 0.005 M to about 0.5 M. In some embodiments, the additive is present in the electrolyte at a concentration in the range of about 0.01 M to about 0.4 M, or about 0.03 M to about 0.3 M.

(14) The electrolyte can include any lithium salt that is suitable for use as a lithium ion source in electrolytes for lithium-ion batteries, which salts are well known in the secondary battery art. Non-limiting examples of lithium salts useful in the electrolyte compositions described herein include, e.g., lithium bis(trifluoromethanesulfonyl)imidate (LiTFSI), lithium 2-trifluoromethyl-4,5-dicyanoimidazolate (LiTDI), lithium 4,5-dicyano-1,2,3-triazolate (LiTDI), lithium trifluoromethanesulfonate (LiTf), lithium perchlorate (LiClO.sub.4), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium tetrafluoroborate (LiBF.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium thiocyanate (LiSCN), lithium bis(fluorosulfonyl)imidate (LIFSI), lithium bis(pentafluoroethylsulfonyl)imidate (LBETI), lithium tetracyanoborate (LiB(CN).sub.4), lithium nitrate, combinations of two or more thereof, and the like. In some preferred embodiment, the lithium salt comprises lithium nitrate in combination with at least one other salt, e.g., LiTFSI. Preferably, the lithium salt is selected from one or more of LiF.sub.2BC.sub.2O.sub.4, LiPF.sub.6, LiBF.sub.4, LiB(C.sub.2O.sub.4).sub.2, LiClO.sub.4, lithium bis(fluorosulfonyl)imidate (LiFSI), lithium bis(trifluoromethanesulfonyl)imidate (LiTFSi), and LiAsF.sub.6. The lithium salt can be present in the electrolyte at any concentration suitable for lithium-ion battery applications, which concentrations are well known in the secondary battery art. In some embodiments, the lithium salt is present in the electrolyte at a concentration in the range of about 0.1 M to about 3 M, e.g., about 0.5 M to 2 M, or 1 M to 1.5M.

(15) The electrolyte comprises a non-aqueous solvent, wherein the solvent comprises one or more solvent compound selected from an ether, a carbonate ester, a nitrile, a sulfoxide, a sulfone, a fluoro-substituted linear dialkyl carbonate, a fluoro-substituted cyclic alkylene carbonate, a fluoro-substituted sulfolane, and a fluoro-substituted sulfone. For example, the solvent can comprise an ether (e.g., glyme or diglyme), a linear dialkyl carbonate (e.g., dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and the like), a cyclic alkylene carbonate (ethylene carbonate (EC), propylene carbonate (PC) and the like), a sulfolane (e.g., sulfolane or an alkyl-substituted sulfolane), a sulfone (e.g., a dialkyl sulfone such as a methyl ethyl sulfone), a fluoro-substituted linear dialkyl carbonate, a fluoro-substituted cyclic alkylene carbonate, a fluoro-substituted sulfolane, and a fluoro-substituted sulfone. The solvent can comprise a single solvent compound or a mixture of two or more solvent compounds. in some embodiments, the solvent comprises a mixture of a cyclic alkylene carbonate and a linear dialkyl carbonate, for example, a mixture of ethylene carbonate and ethyl methyl carbonate (EC/EMC), e.g., in a weight to weight ratio of about 3:7 EC:EMC.

(16) The electrolytes can be incorporated in a lithium ion electrochemical cell comprising a positive electrode (cathode), a negative electrode (anode), and a porous separator between the cathode and anode, with the electrolyte in contact with both the anode and cathode, as is well known in the battery art. A battery can be formed by electrically connecting two or more such electrochemical cells in series, parallel or a combination of series and parallel. The electrolyte can be utilized with any anode or cathode compositions useful in lithium-ion batteries. Electrochemical cell and battery designs and configurations, anode and cathode materials, as well as electrolyte salts, solvents and other battery or electrode components (e.g., separator membranes, current collectors), which can be used in the electrolytes, cells and batteries described herein, are well known in the lithium battery art, e.g., as described in Lithium Batteries Science and Technology Gholam-Abbas Nazri and Gianfranco Pistoia, Eds., Springer Science+Business Media, LLC; New York, N.Y. (2009), which is incorporated herein by reference in its entirety.

(17) The following non-limiting examples illustrate various features of the electrolytes and materials described herein, as well as methods of synthesizing such compounds.

EXAMPLE 1

Synthesis of Redox Shuttle Additives

A. Synthesis of tris(diethylamino)cyclopropenium chloride (CP-Cl).

(18) ##STR00008##

(19) To 10 mL of dry dichloromethane was added pentachlorocyclopropane (3.3 g, 15.4 mmol) at 0 C. Diethylamine (7.9 g, 108 mmol) was slowly added to the solution over 1 hour. The reaction mixture was vigorously stirred for 4 hours at 0 C. and then at room temperature for 18 hours before heating to 65 C. The reaction mixture was cooled to room temperature and precipitated with 30 mL of acetone and the diethylammonium chloride salt byproduct was removed by filtration as a white solid. This purification process was repeated for three times. The supernatant was concentrated in vacuo to yield tris(diethylamino)cyclopropenium chloride as an orange oil (2.5 g, 56.5%). .sup.1H NMR (CDCl.sub.3, 300 MHz): (ppm) 3.42-3.49 (q, J=7.2 Hz, 12H), 1.28-1.33 (t, J=7.3 Hz, 18H); .sup.13C NMR (CDCl.sub.3, 75 MHz): (ppm) 116.1, 47.2, 14.4.

B. Synthesis of tris(diethylamino)cyclopropenium bis(trifluoromethanesulfonyl)imidate (CP-TFSI).

(20) ##STR00009##

(21) To the solution of CP-Cl (1.2 g, 4.17 mmol) in 40 mL of deionized water was added bis(trifluoromethanesulfonyl)imidate lithium salt (3.6 g, 12.5 mmol) at room temperature and the reaction mixture was vigorously stirred overnight. Then the reaction mixture then was extracted with chloroform and washed with water. The organic layer was dried over Na.sub.2SO.sub.4 and concentrated in vacuo to yield tris(diethylamino)cyclopropenium bis(trifluoro-methanesulfonyl)imidate as an orange-brownish oil (1.2 g, 53.9%). .sup.1H NMR (CDCl.sub.3, 300 MHz): (ppm) 3.36-3.43 (q, J=7.2 Hz, 12H), 1.26-1.64 (t, J=7.3 Hz, 18H); .sup.13C NMR (CDCl.sub.3, 75 MHz): (ppm) 116.3, 46.9, 14.1; .sup.19F NMR (CDCl.sub.3, 282 MHz): (ppm) 78.8.

EXAMPLE 2

Electrochemical Evaluation of CP-Cl and CP-TFSI in Lithium Ion Cells

(22) All electrodes utilized herein are from the Argonne National Laboratory (ANL) Cell Analysis, Modeling and Prototyping (CAMP) facility.

(23) Coin Cells:

(24) The positive electrode material was composed of about 80 wt % LiFePO.sub.4 (LFP), about 8 wt % polyvinylidene fluoride (PVDF) binder, and about 12 wt % carbon black coated on an aluminum current collector. The loading density of the positive electrode active material was about 12.1 mg/cm.sup.2. The negative electrode was composed of 87 wt % lithium titanate (Li.sub.4Ti.sub.5O.sub.12) (LTO), 5 wt % carbon black, and about 8 wt % PVDF binder coated on a copper current collector. The loading density of the negative electrode active material was about 8 mg/cm.sup.2. The separator used in the coin cell assembly was a polypropylene/polyethylene/polypropylene separator (PP/PE/PP; CELGARD 2325). The base electrolyte used was composed of 1.2 M LiPF.sub.6 in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in an EC/EMC volume:volume (v:v) ratio of about 3/7), referred to as Electrolyte A herein, and containing the redox shuttle additive at specified concentrations. All electrodes used were dried at 100 C. prior to use and all coin cells were assembled in an argon-atmosphere under constant-temperature (30 C.).

(25) Electrochemical Data:

(26) All coin cell-derived electrochemical data were collected on MACCOR cyclers. Cyclic voltammetry (CV) was performed in Electrolyte A containing the shuttle salt at specified concentrations using a three electrode apparatus with a Pt electrode, a Li counter electrode and a Li reference electrode (Pt|Li|Li) at various scan rates.

(27) FIG. 2 shows cyclic voltammograms of CP-Cl (10 mM) in Electrolyte A using the three electrode apparatus (Pt|Li|Li) at various scan rates. One pair of electrochemically reversible peaks was observed between 3.9 and 4.1 V vs Li/Li.sup.+, which is well below the stability threshold voltage (about 4.8 V vs. Li/Li.sup.+) of common electrolyte components such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and LiPF.sub.6. The exceptional electrochemical reversibility of CP-Cl makes this compound useful as a redox shuttle additive for overcharge protection of lithium-ion batteries.

(28) FIG. 3 provides cyclic voltammograms of CP-TFSI (10 mM) in Electrolyte A using the three electrode apparatus (Pt|Li|Li) at various scan rates. One pair of electrochemically reversible peaks was observed between 3.9 and 4.1 V vs Li/Li.sup.+, which is well below the stability threshold voltage (about 4.8 V vs. Li/Li.sup.+) of common electrolyte components such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and LiPF.sub.6. The exceptional electrochemical reversibility of CP-TFSI makes this compound useful as a redox shuttle additive for overcharge protection of lithium-ion batteries.

(29) FIG. 4 shows voltage profiles of overcharge tests of coin cells using an LTO anode and an LFP cathode, and comprising 0.4 M CP-Cl in Electrolyte A, during the course of 225 hours. The charging rate was C/10 and the overcharge rate was 100%. The tris(diethylamino)cyclopropenium chloride additive demonstrated excellent overcharge protection performance in Electrolyte A.

(30) FIG. 5. shows voltage profiles of overcharge tests of coin cells using an LTO anode and an LFP cathode, and comprising 0.4 M CP-TFSI in Electrolyte A, during the course of approximately 130 hours. The charging rate was C/10 and the overcharge rate was 100%. The tris(diethylamino)cyclopropenium bis(trifluoromethanesulfonyl) imidate additive demonstrated excellent overcharge protection performance in Electrolyte A.

EXAMPLE 3

Electrochemical Cells

(31) FIG. 6 schematically illustrates a cross-sectional view of lithium-ion electrochemical cell 10 comprising cathode 12, and anode 14, with porous separator membrane 16 therebetween. Electrolyte 18, comprising a solution of a lithium salt in a non-aqueous solvent containing a redox shuttle salt additive, contacts electrodes 12 and 14 and separator 16. The electrodes, separator and electrolyte are sealed within housing 19. FIG. 7 schematically illustrates a lithium-ion battery comprising a first array 20 consisting of three series-connected electrochemical cells 10, and a second array 22 consisting of three series-connected electrochemical cells 10, in which first array 20 is electrically connected to second array 22 in parallel.

(32) The use of the terms a and an and the and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms comprising, having, including, and containing are to be construed as open-ended terms (i.e., meaning including, but not limited to,) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(33) Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.