Nonaqueous electrolyte compositions comprising cyclic carbonate and non-fluorinated acyclic carbonate

Abstract

Disclosed herein are electrolyte compositions comprising: a) a first solvent comprising a cyclic carbonate; b) a second solvent comprising a non-fluorinated acyclic carbonate; c) at least one electrolyte component selected from: i) a fluorinated acyclic carboxylic acid ester; ii) a fluorinated acyclic carbonate; iii) a fluorinated acyclic ether; or iv) a mixture thereof; and d) an electrolyte salt; wherein the electrolyte component is present in the electrolyte composition in the range of from about 0.05 weight percent to about 10 weight percent, based on the total weight of the first and second solvents.

Claims

1. An electrolyte composition comprising: a) a first solvent comprising a cyclic carbonate, wherein the cyclic carbonate comprises 4-fluoroethylene carbonate; b) a second solvent comprising a non-fluorinated acyclic carbonate; c) at least one electrolyte component selected from i) a fluorinated acyclic carboxylic acid ester; ii) a fluorinated acyclic carbonate; iii) a fluorinated acyclic ether; or iv) a mixture thereof; and d) an electrolyte salt; wherein the first solvent comprising the cyclic carbonate is present in the electrolyte composition in the range of from 20 to 35 weight percent, based on the total weight of the first and second solvents; wherein the second solvent comprising the non-fluorinated acyclic carbonate is present in the electrolyte composition in the range of from 65 to 80 weight percent, based on the total weight of the first and second solvents; and wherein the electrolyte component is present in the electrolyte composition in the range of from about 0.05 weight percent to about 10 weight percent, based on the total weight of the first and second solvents.

2. The electrolyte composition of claim 1, wherein the cyclic carbonate further comprises ethylene carbonate; propylene carbonate; vinylene carbonate; vinyl ethylene carbonate; dimethylvinylene carbonate; ethyl propyl vinylene carbonate; 4,5-difluoro -1,3-dioxolan-2-one; 4,5-difluoro-4-methyl-1,3-dioxolan-2-one; 4,5-difluoro-4,5-dimethyl-1,3-dioxolan-2-one; 4,4-difluoro-1,3-dioxolan-2-one; or 4,4,5-trifluoro-1,3-dioxolan-2-one.

3. The electrolyte composition of claim 1, wherein the non-fluorinated acyclic carbonate comprises dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, or ethyl methyl carbonate.

4. The electrolyte composition of claim 1, wherein the electrolyte component comprises a fluorinated acyclic carboxylic acid ester represented by the formula:
R.sup.1—COO—R.sup.2, wherein i) R.sup.1 is H, an alkyl group, or a fluoroalkyl group; ii) R.sup.2 is an alkyl group or a fluoroalkyl group; iii) either or both of R.sup.1 and R.sup.2 comprises fluorine; and iv) R.sup.1 and R.sup.2, taken as a pair, comprise at least two carbon atoms but not more than seven carbon atoms.

5. The electrolyte composition of claim 4, wherein R.sup.1 and R.sup.2, taken as a pair, further comprise at least two fluorine atoms, with the proviso that neither R.sup.1 nor R.sup.2 contains a FCH.sub.2—group or a —FCH— group.

6. The electrolyte composition of claim 4, wherein the fluorinated acyclic carboxylic acid ester comprises CH.sub.3—COO—CH.sub.2CF.sub.2H, CH.sub.3CH.sub.2—COO—CH.sub.2CF.sub.2H, F.sub.2CHCH.sub.2—COO—CH.sub.3, F.sub.2CHCH.sub.2—COO—CH.sub.2CH.sub.3, CH.sub.3—COO—CH.sub.2CH.sub.2CF.sub.2H, CH.sub.3CH.sub.2—COO—CH.sub.2CH.sub.2CF.sub.2H, F.sub.2CHCH.sub.2CH.sub.2—COO—CH.sub.2CH.sub.3, CH.sub.3—COO—CH.sub.2CF.sub.3, CH.sub.3CH.sub.2—COO—CH.sub.2CF.sub.2H, CH.sub.3—COO— CH.sub.2CF.sub.3, H—COO—CH.sub.2CF.sub.2H, H—COO—CH.sub.2CF.sub.3, or mixtures thereof.

7. The electrolyte composition of claim 6, wherein the fluorinated acyclic carboxylic acid ester comprises CH.sub.3—COO—CH.sub.2CF.sub.2H.

8. The electrolyte composition of claim 7, further comprising lithium bis(oxalato)borate, ethylene sulfate, and maleic anhydride.

9. The electrolyte composition of claim 1, wherein the electrolyte component comprises a fluorinated acyclic carbonate represented by the formula:
R.sup.3—OCOO—R.sup.4, wherein i) R.sup.3 is a fluoroalkyl group; ii) R.sup.4 is an alkyl group or a fluoroalkyl group; iii) R.sup.3 and R.sup.4, taken as a pair, comprise at least two carbon atoms but not more than seven carbon atoms.

10. The electrolyte composition of claim 8, wherein R.sup.3 and R.sup.4, taken as a pair, further comprise at least two fluorine atoms, with the proviso that neither R.sup.3 nor R.sup.4 contains a FCH.sub.2— group or a —FCH— group.

11. The electrolyte composition of claim 9, wherein the fluorinated acyclic carbonate comprises CH.sub.3—OC(O)O—CH.sub.2CF.sub.2H, CH.sub.3—OC(O)O—CH.sub.2CF.sub.3, CH.sub.3—OC(O)O—CH.sub.2CF.sub.2CF.sub.2H, HCF.sub.2CH.sub.2—OCOO—CH.sub.2CH.sub.3, CF.sub.3CH.sub.2—OCOO—CH.sub.2CH.sub.3, or mixtures thereof.

12. The electrolyte composition of claim 11, wherein the fluorinated acyclic carbonate comprises CH.sub.3—OC(O)O—CH.sub.2CF.sub.2H.

13. The electrolyte composition of claim 1, wherein the electrolyte component comprises a fluorinated acyclic ether represented by the formula:
R.sub.5—O—R.sub.6, wherein i) R.sup.5 is a fluoroalkyl group; ii) R.sup.6 is an alkyl group or a fluoroalkyl group; iii) R.sup.5 and R.sup.6, taken as a pair, comprise at least two carbon atoms but not more than seven carbon atoms.

14. The electrolyte composition of claim 1, wherein the electrolyte component is present in the electrolyte composition in the range of from about 0.05 weight percent to about 5 weight percent, based on the total weight of the first and second solvents.

15. The electrolyte composition of claim 1, further comprising an additive selected from a lithium boron compound, a cyclic sultone, a cyclic sulfate, a cyclic carboxylic acid anhydride, or a combination thereof.

16. The electrolyte composition of claim 15, wherein the electrolyte component comprises a fluorinated acyclic carboxylic acid ester represented by the formula:
R.sup.1—COO—R.sup.2, wherein i) R.sup.1 is H, an alkyl group, or a fluoroalkyl group; ii) R.sup.2 is an alkyl group or a fluoroalkyl group; iii) either or both of R.sup.1 and R.sup.2 comprises fluorine; and iv) R.sup.1 and R.sup.2, taken as a pair, comprise at least two carbon atoms but not more than seven carbon atoms.

17. An electrochemical cell comprising: (a) a housing; (b) an anode and a cathode disposed in the housing and in ionically conductive contact with one another; (c) the electrolyte composition of claim 1 disposed in the housing and providing an ionically conductive pathway between the anode and the cathode; and (d) a porous separator between the anode and the cathode.

18. The electrochemical cell of claim 17, wherein the electrochemical cell is a lithium ion battery.

19. The electrochemical cell of claim 18, wherein the cathode comprises a cathode active material exhibiting greater than 30 mAh/g capacity in the potential range greater than 4.6 V versus a Li/Li.sup.+ reference electrode, or a cathode active material which is charged to a potential greater than or equal to 4.35 V versus a Li/Li.sup.+ reference electrode.

20. The electrochemical cell of claim 18, wherein the cathode comprises: a) a lithium-containing manganese composite oxide having a spinel structure as active material, the lithium-containing manganese composite oxide being represented by the formula:
Li.sub.xNi.sub.yM.sub.zMn.sub.2−y−zO.sub.4−d, wherein x is 0.03 to 1.0; x changes in accordance with release and uptake of lithium ions and electrons during charge and discharge; y is 0.3 to 0.6; M comprises one or more of Cr, Fe, Co, Li, Al, Ga, Nb, Mo, Ti, Zr, Mg, Zn, V, and Cu; z is 0.01 to 0.18, and d is 0 to 0.3; or b) a composite material represented by the structure of Formula:
x(Li.sub.2−wA.sub.1−vQ.sub.w+vO.sub.3−e).(1−x)(Li.sub.yMn.sub.2−zM.sub.zO.sub.4−d) wherein: x is about 0.005 to about 0.1; A comprises one or more of Mn or Ti; Q comprises one or more of Al, Ca, Co, Cr, Cu, Fe, Ga, Mg, Nb, Ni, Ti, V, Zn, Zr or Y; e is 0 to about 0.3; v is 0 to about 0.5 w is 0 to about 0.6; M comprises one or more of Al, Ca, Co, Cr, Cu, Fe, Ga, Li, Mg, Mn, Nb, Ni, Si, Ti, V, Zn, Zr or Y; d is 0 to about 0.5; y is about 0 to about 1; and z is about 0.3 to about 1; and wherein the Li.sub.yMn.sub.2−zM.sub.zO.sub.4−d component has a spinel structure and the Li.sub.2−wQ.sub.w+vA.sub.1−vO.sub.3−e component has a layered structure; or c) Li.sub.aMn.sub.bJ.sub.cO.sub.4Z.sub.d wherein J is Ni, Co, Mn, Cr, Fe, Cu, V, Ti, Zr, Mo, B, Al, Ga, Si, Li, Mg, Ca, Sr, Zn, Sn, a rare earth element, or a combination thereof; Z is F, S, P, or a combination thereof; and 0.9<a<1.2, 1.3<b<2.2, 0<c<0.7, 0<d<0.4, or d) Li.sub.aNi.sub.bMn.sub.cCo.sub.dR.sub.eO.sub.2−fZ.sub.f, wherein: R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, Zr, Ti, a rare earth element, or a combination thereof; Z is F, S, P, or a combination thereof; and 0.8<a<1.2, 0.1<b<0.9, 0.0<c<0.7, 0.05<d<0.4, 0<e<0.2; wherein the sum of b+c+d+e is about 1; and 0<f<0.08; or e) Li.sub.aA.sub.1−b,R.sub.bD.sub.2, wherein: A is Ni, Co, Mn, or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, Zr, Ti, a rare earth element, or a combination thereof; D is 0, F, S, P, or a combination thereof; and 0.90<a<1.8 and 0<b<0.5.

21. The electrochemical cell of claim 18, wherein the cathode comprises Li.sub.aA.sub.1−xR.sub.xDO.sub.4−fZ.sub.f, wherein: A is Fe, Mn, Ni, Co, V, or a combination thereof; R is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, Zr, Ti, a rare earth element, or a combination thereof; D is P, S, Si, or a combination thereof; Z is F, Cl, S, or a combination thereof; 0.8<a<2.2; 0<x<0.3; and 0<f<0.1.

22. The electrochemical cell of claim 18, wherein the electrolyte component comprises a fluorinated acyclic carboxylic acid ester, and the fluorinated acyclic carboxylic acid ester comprises CH.sub.3—COO—CH.sub.2CF.sub.2H, CH.sub.3CH.sub.2—COO—CH.sub.2CF.sub.2H, F.sub.2CHCH.sub.2—COO—CH.sub.3, F.sub.2CHCH.sub.2—COO—CH.sub.2CH.sub.3, CH.sub.3—COO—CH.sub.2CH.sub.2CF.sub.2H, CH.sub.3CH.sub.2—COO—CH.sub.2CH.sub.2CF.sub.2H, F.sub.2CHCH.sub.2CH.sub.2—COO—CH.sub.2CH.sub.3, CH.sub.3—COO—CH.sub.2CF.sub.3, CH.sub.3CH.sub.2—COO—CH.sub.2CF.sub.2H, CH.sub.3—COO—CH.sub.2CF.sub.3, H—COO—CH.sub.2CF.sub.2H, H—COO—CH.sub.2CF.sub.3, or mixtures thereof.

23. The electrochemical cell of claim 18, wherein the electrolyte component comprises a fluorinated acyclic carbonate, and the fluorinated acyclic carbonate comprises CH.sub.3— OC(O)O—CH.sub.2CF.sub.2H, CH.sub.3—OC(O)O—CH.sub.2CF.sub.3, CH.sub.3—OC(O)O—CH.sub.2CF.sub.2CF.sub.2H, HCF.sub.2CH.sub.2—OCOO—CH.sub.2CH.sub.3, CF.sub.3CH.sub.2—OCOO—CH.sub.2CH.sub.3, or mixtures thereof.

24. An electronic device, transportation device, or telecommunications device, comprising an electrochemical cell according to claim 17.

Description

EXAMPLES

(1) The concepts disclosed herein are illustrated in the following Examples, which are not intended to be used or interpreted as a limitation of the scope of the claims unless this intention is expressly stated. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of the concepts disclosed herein, and without departing from the spirit and scope thereof, can make various changes and modifications to adapt to various uses and conditions.

(2) The meaning of abbreviations used is as follows: “° C.” means degrees Celsius; “g” means gram(s), “mg” means milligram(s), “μg” means microgram(s), “L” means liter(s), “mL” means milliliter(s), “μL” means microliter(s), “mol” means mole(s), “mmol” means millimole(s), “M” means molar concentration, “wt %” means percent by weight, “mm” means millimeter(s), “μm” means micrometer(s), “ppm” means parts per million, “h” means hour(s), “min” means minute(s), “psig” means pounds per square inch gauge, “kPa” means kiloPascal(s), “A” means amperes, “mA” mean milliampere(s), “mAh/g” mean milliamperes hour(s) per gram, “V” means volt(s), “xC” refers to a constant current which is the product of x and a current in A which is numerically equal to the nominal capacity of the battery expressed in Ah, “rpm” means revolutions per minute, “NMR” means nuclear magnetic resonance spectroscopy, “GC/MS” means gas chromatography/mass spectrometry, “Ex” means Example and “Comp. Ex” means Comparative Example.

(3) Materials and Methods

Representative preparation of 2,2-difluoroethyl acetate (DFEA)

(4) The 2,2-difluoroethyl acetate used in the Examples and Comparative Examples was prepared by reacting potassium acetate with HCF.sub.2CH.sub.2Br. The following is a typical procedure used for the preparation.

(5) Potassium acetate (Aldrich, Milwaukee, Wis., 99%) was dried at 100° C. under a vacuum of 0.5-1 mm of Hg (66.7-133 Pa) for 4 to 5 h. The dried material had a water content of less than 5 ppm, as determined by Karl Fischer titration. In a dry box, 212 g (2.16 mol, 8 mol % excess) of the dried potassium acetate was placed into a 1.0-L, 3 neck round bottom flask containing a heavy magnetic stir bar. The flask was removed from the dry box, transferred into a fume hood, and equipped with a thermocouple well, a dry-ice condenser, and an additional funnel.

(6) Sulfolane (500 mL, Aldrich, 99%, 600 ppm of water as determined by Karl Fischer titration) was melted and added to the 3 neck round bottom flask as a liquid under a flow of nitrogen. Agitation was started and the temperature of the reaction medium was brought to about 100° C. HCF.sub.2CH.sub.2Br (290 g, 2 mol, E.I. du Pont de Nemours and Co., 99%) was placed in the addition funnel and was slowly added to the reaction medium. The addition was mildly exothermic and the temperature of the reaction medium rose to 120-130° C. in 15-20 min after the start of the addition. The addition of HCF.sub.2CH.sub.2Br was kept at a rate which maintained the internal temperature at 125-135° C. The addition took about 2-3 h. The reaction medium was agitated at 120-130° C. for an additional 6 h (typically the conversion of bromide at this point was about 90-95%). Then, the reaction medium was cooled down to room temperature and was agitated overnight. Next morning, heating was resumed for another 8 h.

(7) At this point the starting bromide was not detectable by NMR and the crude reaction medium contained 0.2-0.5% of 1,1-difluoroethanol. The dry-ice condenser on the reaction flask was replaced by a hose adapter with a Teflon® valve and the flask was connected to a mechanical vacuum pump through a cold trap (−78° C., dry-ice/acetone). The reaction product was transferred into the cold trap at 40-50° C. under a vacuum of 1-2 mm Hg (133 to 266 Pa). The transfer took about 4-5 h and resulted in 220-240 g of crude HCF.sub.2CH.sub.2OC(O)CH.sub.3 of about 98-98.5% purity, which was contaminated by a small amount of HCF.sub.2CH.sub.2Br (about 0.1-0.2%), HCF.sub.2CH.sub.2OH (0.2-0.8%), sulfolane (about 0.3-0.5%) and water (600-800 ppm). Further purification of the crude product was carried out using spinning band distillation at atmospheric pressure. The fraction having a boiling point between 106.5-106.7° C. was collected and the impurity profile was monitored using GC/MS (capillary column HP5MS, phenyl-methyl siloxane, Agilent 19091S-433, 30 m, 250 μm, 0.25 μm; carrier gas—He, flow rate 1 mL/min; temperature program: 40° C., 4 min, temp. ramp 30° C./min, 230° C., 20 min). Typically, the distillation of 240 g of crude product gave about 120 g of HCF.sub.2CH.sub.2OC(O)CH.sub.3 of 99.89% purity, (250-300 ppm H.sub.2O) and 80 g of material of 99.91% purity (containing about 280 ppm of water). Water was removed from the distilled product by treatment with 3A molecular sieves, until water was not detectable by Karl Fischer titration (i.e., <1 ppm).

Synthesis of 2,2-Difluoroethyl Methyl Carbonate (DFEMC)

(8) A solution of 404 mL 2,2-difluoroethanol (DFE; 525 g; 6.40 mol; mw=82.05; D=1.30; bp=95° C.; Synquest 2101-3-02) and 11.6 g 4-(dimethylamino)pyridine (DMAP; 94.9 mmol; 1.5 mol %; mw=122.17; Aldrich 107700) in 4644 mL dichloromethane (DCM) was cooled via a circulating chiller as it stirred under nitrogen in a 20-L jacketed flask with bottom let-down valve, a condenser, overhead stirrer and a dropping funnel. Aqueous NaOH (441 mL; 50 wt % NaOH; 8.3 mol; 30% excess; 0.75 g NaOH/mL; 18.8 M; D=1.52; Aldrich 415413) was added all at once and the mixture was stirred and chilled to 1° C. The mixture was stirred rapidly as 584 mL cold methyl chloroformate (MCF, 712 g; 7.54 mol; 18% excess; mw=94.50; D=1.22; bp=70° C., Aldrich M35304) was added at 5-10 mL/min. The chiller was set at −20° C. to maintain the reaction temperature at 2-3° C. After about half the MCF had been added, the salts in the aqueous phase crystallized and, in the absence of liquid aqueous NaOH, the reaction essentially stopped. Water (300 mL) was added to liquify the salts and the reaction proceeded again. When the MCF had all been added (1.5 hr total addition time), the dichloromethane solution was sampled and analyzed by gas chromatography (30-m DB-5; 30° C./5 min, then 10° C./min; He: 13.8 cc/min): 0.97 min (0.006%, DFE); 1.10 min (61.019%, DCM); 1.92 min (0.408%, dimethyl carbonate, DMC); 4.38 min (38.464%, 2,2-difluoroethyl methyl carbonate, DFEMC). DFEMC:DFE=6410; DFEMC:DMC=94. The dichloromethane product solution was drawn off via the bottom valve and the flask was washed out with water; the dichloromethane solution was then returned to the flask and was stirred sequentially with 2×750 mL 5% hydrochloric acid followed by 1.5 L sat sodium bicarbonate and finally dried with magnesium sulfate.

(9) The dichloromethane was distilled off at ˜40° C./500 torr from a 5-L flask through a 12″ empty column topped with a simple still head. Then the residual pot material was distilled at 100°/250 torr to yield 866 g crude 2,2-difluoroethyl methyl carbonate; GC analysis showed DFE 0.011%; DCM 4.733%; DMC 0.646%; DFEMC 94.568%; bis(2,2-difluoroethyl) carbonate (BDFEC) 0.043%. This is a 91% yield of 2,2-difluoroethyl methyl carbonate. The crude DFEMC was redistilled from a 95-113° bath at 285 torr through an 18″ glass column packed with 0.16-in SS316 mesh saddles. Fractions 7-10 distilled at about 90° C./285 torr from a 105-113° C. bath. GC-FID analysis of these fractions is provided in Table 1. The pot (25 g) was mostly BDFEC.

(10) TABLE-US-00001 TABLE 1 Distillation Fraction Composition by GC-FID Analysis Fraction DFE % DMC % DFEMC % BDFEC % Yield, g 7 0.0089 0.8403 99.0496 0.0500 501 8 0.0019 0.0023 99.9283 0.0522 128 9 0.0094 0.0300 99.3358 0.5787 61 10 0.0110 — 99.0150 0.9240 11

(11) Fractions 7-9 were combined and distilled under partial vacuum (70 torr) from a 100° C. oil bath through a 20-cm x 2.2 cm column packed with 0.16-in SS316 mesh saddles (Ace Glass 6624-04) in four fractions: #1 (23 g), #2 (20 g), #3 (16 g) and #4 (13 g), to remove DFE. The DFE content of the distillates was analyzed by GC: #1 (0.100%), #2 (0.059%), #3 (0.035%) and #4 (0.026%). The pot material (602 g) was analyzed by GC-FID: DFE 0.0016%; DMC 0.1806%; DFEMC 99.6868%; BDFEC 0.1132%. The sum of DMC, DFEMC and BDFEC accounted for 99.9808% of the product, which contained 16 ppm DFE. The product also contained 18 ppm water by Karl-Fischer titration.

Lithium Bis(Oxalato)Borate (LiBOB) Purification

(12) In a nitrogen purged dry box, lithium bis(oxalato)borate (LiBOB, Sigma Aldrich, 757136-25G) was purified using the following procedure. 25 grams of LiBOB were added to a 500 mL Erlenmeyer flask equipped with a Teflon-coated stir bar. To this, 125 mL of anhydrous acetonitrile (Sigma Aldrich, Fluka, molecular sieves) was added. The flask was heated at 45° C. for 10 minutes using an oil bath. The mixture was filtered through a fine-pore glass frit (Chemglass, F, 60 mL) into a 500 mL filter flask with the use of vacuum. The solution was allowed to cool to room temperature, forming a clear solution, and 125 mL of cold toluene (Freezer @ −25° C., Sigma Aldrich CHROMASOLV®) was added. Immediate precipitation was observed and this mixture was allowed to sit for 20 minutes to allow additional solid formation. The solution was filtered through a fine-pore glass frit (Chemglass, F, 60 mL) into a 500 mL round bottom. The filter cake was washed with cold anhydrous toluene (2× 20 mL) and using a glass funnel, transferred to a cylindrical long neck flask. This flask was capped tightly, removed from the glove box, and attached to a Kugelrohr, which was subsequently attached to a high vacuum. This flask was dried under high vacuum (60-100 millitorr) at room temperature overnight, and then at 140° C. under high vacuum (60-80 millitorr) for an additional three days. At this time, the flask was capped and returned to the dry box for further purification. Propylene carbonate was used to further purify the LiBOB, as described below.

(13) Propylene Carbonate Purification (Used to Purify LiBOB Further)

(14) Propylene carbonate (PC, Aldrich, CHROMASOLV for HPLC, 99.7%) was transferred to the dry box and placed on activated molecular sieves to dry the solvent. 300 mL PC was added to a round bottom flask with a stir bar. This was attached to a single piece distillation apparatus with a Vigreux column. The apparatus was then put under high vacuum (˜500 millitorr), and the solution was degassed with stirring. The temperature was then increased to 50° C., and then 90° C. Eventually, the vacuum increased to ˜250 millitorr, and the PC fraction began to distill over. Seven fractions were collected. The last two fractions (totaling ˜290 mL) were used in subsequent steps.

(15) A sacrificial portion of LiBOB was used to trap any remaining impurities in the fractionally distilled propylene carbonate. 10.2 g of LiBOB (Rockwood Lithium, Frankfurt, Germany) was combined with 200 mL of the distilled PC. This was stirred overnight in the dry box, at 100° C. The mixture was then attached to a simple distillation apparatus, and the PC was distilled off and collected into a round bottom flask. A heat gun was used multiple times to help the PC distill over. This propylene carbonate which was collected was then transferred to the dry box and used to purify LiBOB.

(16) In the glove box, to a 250 mL round bottom equipped with a Teflon coated stir bar, 17 g of LiBOB (previously purified using acetonitrile and toluene, as described above) and 75 mL of purified propylene carbonate were added. This was stirred at room temperature in the glove box for ˜2 hours. If the solution was not clear, the temperature was increased to 60° C. and stirred for ˜15 minutes.

(17) This tubing was then attached to a simple distillation apparatus. The distillation apparatus was sealed using a receiver round bottom and clamped rubber tubing. The apparatus was then removed from the dry box. The rubber tubing was attached to the schlenk line/high vacuum, and the apparatus was put under vacuum (˜150 millitorr). The receiver flask was surrounded by a dry ice/acetone trap, and the LiBOB/PC flask was heated in an oil bath (55-70° C.). The temperature was adjusted based on the efficiency of the high vacuum. If the temperature is too high, the LiBOB will start to collect in the top of the distillation head. After most of the PC was removed, a heat gun was used to help droplets move from the distillation head. This was repeated until no droplets appeared. The dry ice/acetone receiver trap was then replaced by a liquid nitrogen trap, and the oil bath temperature was slowly increased to 115-130° C. (again depending on the vacuum). A heat gun was again used to remove droplets of PC in the distillation head. This was repeated until no more PC was being removed.

(18) The apparatus was then removed from heat/liquid nitrogen and placed under nitrogen. After the LiBOB had cooled, and the PC had warmed to room temperature, the apparatus tubing was clamped using forceps to keep it under nitrogen. It was then transferred to the dry box by purging the antechamber with a continuous flow of nitrogen for ˜20 minutes.

(19) Ethylene Sulfate (ES) Purification

(20) In a glove box, 12 g of ethylene sulfate (Chemlmpex, Wood Dale, Ill.) was added to a sublimator equipped with an insert for dry ice/acetone. This was sealed in the dry box, removed, and attached to the high vacuum (˜100 millitorr). The tubing was first put under vacuum before the valve was opened to put the sublimator under vacuum. The insert was filled with dry ice and acetone and the bottom of the sublimator was submerged in an oil bath, preheated to 60° C. This was heated for ˜4 hours, or until all of the white solid adhered to the cold finger. At this time, the valve was sealed to keep the contents under vacuum. The tubing was put under nitrogen and removed. The dry ice/acetone trapped was then emptied, and the outside of the sublimator was cleaned off. This was transferred into the glove box, and the white solid was collected into a dried glass container using a plastic funnel. An NMR of the sublimed ES in CDCl.sub.3 was then obtained to confirm its purity. The purified ES was stored in a glass container in the freezer until needed.

(21) Maleic Anhydride (MA) Purification

(22) In the glove box, 27 g of maleic anhydride (Aldrich, Milwaukee, Wis.)) was added to a large sublimator equipped with an insert for dry ice/acetone. This was sealed in the dry box, removed, and attached to the high vacuum (˜100 millitorr). The tubing was first put under vacuum before the valve was opened to evacuate the sublimator. The insert was filled with dry ice and acetone and the bottom of the sublimator was submerged in an oil bath, preheated to 60° C. This was heated for one hour, and then the temperature was increased to 85° C. for another 7 hours, or until all of the sublimed white solid adhered to the cold finger. At this time, the valve was sealed to keep the contents under vacuum. The tubing was put under nitrogen and removed. The material in the dry ice/acetone trap was then emptied, and the outside of the sublimator was cleaned off. This was transferred into the glove box, and the white solid was collected into a dried glass container using a plastic funnel. An NMR of the MA in CDCl.sub.3 was then obtained to verify its purity. The purified MA was stored in a glass container in the dry box until needed.

(23) Electrolyte Preparation

(24) For Example 1, the electrolyte composition was prepared as follows. Three stock solutions of diethyl carbonate (DEC), (BASF, Independence, Ohio) and ethyl methyl carbonate (EMC), (BASF, Independence, Ohio) with the following weights were prepared in a nitrogen purged dry box: 1) 13.848 g DEC and 36.158 g EMC, 2) 13.847 g DEC and 36.155 g EMC, and 3) 13.848 g DEC and 36.162 g EMC. Molecular sieves (3 A) were added and the mixtures was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter.

(25) Next, 113.200 g of the DEC/EMC mixtures described above were combined with 15.097 g of fluoroethylene carbonate (FEC), (BASF, Independence, Ohio) and 22.638 g of propylene carbonate (PC), (as received, BASF, Independence, Ohio) in a nitrogen-purged dry box. Molecular sieves (3 A) were added and the mixture was dried to less than 1 ppm water as determined by Karl Fischer titration and filtered through a 0.25 micron PTFE syringe filter.

(26) Then, 9.0136 g of the DEC/EMC/FEC/PC mixture was combined with 0.1126 g of LiBOB and gently agitated overnight. 0.0568 g of MA, 0.5615 g of DFEA, and 1.3540 g of LiPF.sub.6 (BASF, Independence, Ohio) were then added. The mixture was gently agitated to dissolve the components. A separate vial of 0.1688 g of ethylene sulfate (ES) was prepared, and the two were combined immediately before use to provide the electrolyte composition of Example 1.

(27) For Comparative Example A, the electrolyte composition was prepared by combining 113.200 g of the EMC/DEC mixture described in Example 1 with 15.097 g of FEC and 22.638 g of propylene carbonate in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixture was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter. 9.0096 g of the mixture was combined with 0.1073 g of LiBOB and gently agitated overnight. 0.0543 g of MA and 1.3600 g of LiPF.sub.6 were then added. The material was gently agitated to dissolve the components. A separate vial of 0.1602 g of ethylene sulfate was prepared, and the two components were combined immediately before use to provide the electrolyte composition of Comparative Example A.

(28) For Example 2, the electrolyte composition was prepared using the same procedures as described in Example 1, but with the following differences. The electrolyte composition was prepared by combining 113.200 g of the EMC/DEC mixture described in Example 1 with 15.097 g of FEC and 22.638 g of propylene carbonate in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixture was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter. 9.0046 g of the mixture was combined with 0.1094 g of lithium bis(fluorosulfonyl)imide (LiFSI, Nippon Shokubai Co., LTD, Japan), 0.5501 g of DFEA, and 1.3602 g of LiPF.sub.6. The material was gently agitated to dissolve the components and prepare the final electrolyte composition of Example 2.

(29) For Comparative Example B, the electrolyte composition was prepared using the same procedures as described in Example 1, but with the following differences. The electrolyte was prepared by combining 113.200 g of the EMC/DEC mixture described in Example 1 with 15.097 g of FEC and 22.638 g of propylene carbonate in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixture was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter.

(30) 9.0033 g of the mixture was combined with 0.1059 g of lithium bis(fluorosulfonyl)imide (LiFSI, Nippon Shokubai Co., LTD, Japan) and 1.3600 g of LiPF.sub.6. The material was gently agitated to dissolve the components and prepare the final electrolyte composition of Comparative Example B.

(31) For Example 3, the electrolyte composition was prepared using the same procedures as described in Example 1, with the following differences. The electrolyte was prepared by combining 113.200 g of the EMC/DEC mixture described in Example 1 with 15.097 g of FEC and 22.638 g of propylene carbonate in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixture was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter. 9.0072 g of the mixture was combined with 0.5402 g of DFEA and 1.3606 g of LiPF.sub.6 (BASF, Independence, Ohio). The material was gently agitated to dissolve the components and prepare the final electrolyte composition of Example 3.

(32) For Example 4, the electrolyte composition was prepared using the same procedures as described in Example 1, with the following differences. 27.698 g of diethyl carbonate and 72.302 g of ethyl methyl carbonate were combined in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixtures was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter. The electrolyte was prepared by combining 56.566 g of the mixture described above with 7.553 g of FEC and 11.317 g of propylene carbonate in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixture was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter. 9.0030 g of the mixture was combined with 0.5489 g of 2,2-difluoroethyl methyl carbonate (DFEMC) and 1.3600 g of LiPF.sub.6. The material was gently agitated to dissolve the components and prepare the final electrolyte composition of Example 4.

(33) For Comparative Example C, the electrolyte composition was prepared using the same procedures as described in Example 4, except that 2,2-difluoroethyl methyl carbonate (DFEMC) was not added. Hence, 9.0525 g of the diethyl carbonate/ethyl methyl carbonate mixture, described in Example 4, was combined with 1.2077 g of FEC and 1.8134 g of propylene carbonate. 5.9985 g of this mixture was combined with 0.9062 g of LiPF.sub.6.

(34) For Example 5, the same procedures as described in Example 1 were used, with the following differences. 27.698 g of diethyl carbonate and 72.302 g of ethyl methyl carbonate were combined in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixtures was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter. The electrolyte was prepared by combining 56.566 g of the mixture described above with 7.553 g of fluoroethylene carbonate (FEC, BASF, Independence, Ohio) and 11.317 g of propylene carbonate (BASF, Independence, Ohio) in a nitrogen purged dry box. Molecular sieves (3 A) were added and the mixture was dried to less than 1 ppm water and filtered through a 0.25 micron PTFE syringe filter. 9.0003 g of the mixture was combined with 0.5444 g of DFEA and 1.3602 g of LiPF.sub.6 (BASF, Independence, Ohio). The material was gently agitated to dissolve the components and prepare the final electrolyte composition of Example 5.

(35) Pouch Cells

(36) Pouch cells were purchased from Pred Materials (New York, N.Y.) and were 200 mAh cells containing an NMC 532 cathode and a graphitic anode.

(37) Before use, the pouch cells were dried in the antechamber of a dry box under vacuum overnight at 80° C. Approximately 900 microliters of the desired electrolyte composition was injected through the bottom, and the bottom edge sealed in a vacuum sealer. For each Example and Comparative Example, two pouch cells were prepared using the same electrolyte composition.

(38) Two different lots of pouch cells were used. Although nominally the same, the pouch cells of the two different lots appear to provide different electrochemical performance under the same conditions. For comparison purposes, the electrochemical results are compared for pouch cells of the same lot. Below, Table 2 presents results for one lot of pouch cells, and Table 3 for a second lot.

(39) Pouch Cell Evaluation Procedure

(40) The cells were placed in fixtures which applied a pressure of 66 kPa to the electrodes through an aluminum plate fitted with a foam pad. The cells were held in an environmental chamber (model BTU-433, Espec North America, Hudsonville, Mich.) and evaluated using a battery tester (Series 4000, Maccor, Tulsa, Okla.) for the formation procedures (at 25° C.) and the high temperature cycling (at 45° C.). In the following procedures, the currents for the C-rates were determined assuming the cell would have a capacity of 170 mAh per g of NMC. Thus currents of 0.05 C, 0.25 C, and 1.0 C were implemented in the tester using, respectively, currents of 8.5, 42.5, and 170 mA per gram of NMC in the cell.

(41) The pouch cells were conditioned using the following cycling procedure. In a first cycle, the cell was charged for 36 min at 0.25 C, corresponding to approximately 15% state of charge; this was followed by a four hour rest at open circuit voltage. The first charge was continued using constant current (CC) of 0.25 C to 4.35 V. The cell was held at a constant voltage (CV) at 4.35 V until the current dropped below (or tapered off to) 0.05 C. This was followed by CC discharge at 0.5 C to 3.0 V.

(42) For the second cycle, the cell was charged at constant current (CC charge) of 0.2 C to 4.35 V followed by a CV voltage-hold step at 4.35 V until current dropped below 0.05 C. This was followed by a CC discharge at 0.2 C to 3.0 V. This cycle was used as a check of the capacity of the cell.

(43) Ten additional cycles were performed using 1 C—CCCV protocols which consisted of CC charges at 1 C to 4.35V, a CV constant voltage step where the current was allowed to taper to 0.05 C, followed by a discharge cycle at 1.0 C to 3.0 V.

(44) For the 25° C. formation cycles and the 45° C. cycling described below, the cells also had a 10 min rest following each charge and each discharge step. Pressure (66 kPa) was applied to the cells during formation and cycling, and the cells were evacuated and resealed after the final discharge cycle.

(45) Cycling Protocol

(46) The cells were placed in an environmental chamber at 45° C. and cycled: CC charge 1 C to 4.35 V+CV charge to 0.05 C, and CC discharge at 1 C to 3.0 V.

(47) Results for Examples 1-3 and Comparative Examples A and B are presented in Table 2, and results for Example 4 and Comparative Example C are presented in Table 3. In the Tables, the cycle life to 80% capacity retention is the number of cycles needed to reach 80% of the maximum capacity achieved during cycling at 45° C.

(48) TABLE-US-00002 TABLE 2 Electrolyte Composition .sup.1 Cycle Discharge Additives, Life to 80% Capacity at Electrolyte Amount .sup.4 Pouch Capacity Cycle 10 Example Solvent Mixture .sup.2 Component .sup.3 (wt %) Cell Retention (mAh/g) 1 10/15/54/21 5 wt % DFEA 1 wt % LiBOB, 1-1 449 156 FEC/PC/EMC/DEC 1.5 wt % ES, 0.5 wt % MA 1-2 439 157 Comp. 10/15/54/21 — 1 wt % LiBOB, A-1 326 158 Ex. A FEC/PC/EMC/DEC 1.5 wt % ES, 0.5 wt % MA A-2 330 163 2 10/15/54/21 5 wt % DFEA 1 wt % LiFSi 2-1 797 162 FEC/PC/EMC/DEC 2-2 712 157 Comp. 10/15/54/21 — 1 wt % LiFSi B-1 700 161 Ex. B FEC/PC/EMC/DEC B-2 952 165 3 10/15/54/21 5 wt % DFEA — 3-1 754 162 FEC/PC/EMC/DEC 3-2 968 161 Notes: .sup.1 All electrolyte compositions 1M in LiPF.sub.6 .sup.2 Solvent mixture given in weight ratios, based on solvents only .sup.3 Electrolyte component wt % based on total weight of solvent mixture .sup.4 Additive wt % based on total weight of solvent mixture

(49) The results in Table 2 show the beneficial effect of including an electrolyte component as disclosed herein in the electrolyte composition. Example 1, containing 5 wt % DFEA, shows improved cycle life to 80% capacity compared to Comparative Example A, which contains the same solvent mixture (in the same solvent ratios) and the same additives, except no DFEA. Hence, the effect of DFEA is to significantly improve the cycle life performance of the electrochemical cell.

(50) Example 2 and Comparative Example B show the impact of LiFSI on cell performance, with and without DFEA. In either case, these two examples show very significantly superior cycle life capability compared with Comparative Example A, which does not contain DFEA and/or LiFSI.

(51) TABLE-US-00003 TABLE 3 Cycle Discharge Discharge Coulombic Electrolyte Composition .sup.1 Life to 80% Capacity at Capacity at Efficiency at Electrolyte Capacity Cycle 150 Cycle 10 Cycle 10 Example Solvent Mixture .sup.2 Component .sup.3 Retention (mAh/g) (mAh/g) (%) 4 10/15/54/21 5 wt % DFEMC 643 173 187 99.84 FEC/PC/EMC/DEC 823 172 183 99.66 Comp. 10/15/54/21 —  .sup. N/A .sup.4 152 164 99.83 Ex. C FEC/PC/EMC/DEC N/A 151 162 99.84 5 10/15/54/21 5 wt % DFEMC N/A 152 162 99.96 FEC/PC/EMC/DEC N/A 152 162 99.80 Notes: .sup.1 All electrolyte compositions 1M in LiPF.sub.6 .sup.2 Solvent mixture given in weight ratios, based on solvents only .sup.3 Electrolyte component wt % based on total weight of solvent mixture .sup.4 N/A means not available

(52) Example 4 shows that electrolytes containing difluoroethyl methyl carbonate (DFEMC) show improvement in both the cycle life and the capacity retention (at ten cycles) compared to Comparative Example C, which does not contain the DFEMC component. Example 5 shows that electrolyte compositions containing difluoroethyl acetate (DFEA) show an improvement in coulombic efficiency (the ratio of the discharge capacity to charge capacity, which is an indication, at least in part, of the loss of cyclable Li and electrolyte degradation) at cycle 10 compared with that for Comparative Example C, which contained the same solvent mixture but no DFEA.