Fluorinated acrylates as additives for Li-ion battery electrolytes

Abstract

A compound of formula (I) ##STR00001## for use in electrolyte compositions for electrochemical cells, wherein R.sup.1 and R.sup.2 are selected independently from each other from H, F, CN, R′, OR′, OC(O)R′, and OP(O)R″.sub.2, R.sup.3 is selected from H, C.sub.1 to C.sub.12 alkyl, C.sub.3 to C.sub.6 (hetero)cycloalkyl, C.sub.2 to C.sub.12 alkenyl, C.sub.2 to C.sub.12 alkynyl, C.sub.5 to C.sub.12 (hetero)aryl, and C.sub.6 to C.sub.24 (hetero)aralkyl, R.sup.4 is selected from C.sub.1 to C.sub.12 alkyl, C.sub.3 to C.sub.6 (hetero)cycloalkyl, C.sub.2 to C.sub.12 alkenyl, C.sub.2 to C.sub.12 alkynyl, C.sub.5 to C.sub.12 (hetero)aryl, and C.sub.6 to C.sub.24 (hetero)aralkyl, or R.sup.3 and R.sup.4 are bound together and form together with the group —C—C(O)—O— a 5- to 6-membered heterocycle which may be substituted by one or more substituents selected from F and optionally fluorinated C.sub.1 to C.sub.12 alkyl.

Claims

1. An electrolyte composition comprising: at least one additive comprising the compound of formula (I) ##STR00011## wherein R.sup.1 and R.sup.2 are selected from the group consisting of F, CN, R′, OR′, and OP(O)R″.sub.2, wherein R′ is selected from the group consisting of C1 to C12 alkyl, C3 to C6 (hetero)cycloalkyl, C2 to C12 alkenyl, C2 to C12 alkynyl, C5 to C12 (hetero)aryl, and C6 to C24 (hetero)aralkyl, wherein the alkyl, the (hetero)cycloalkyl, the alkenyl, the alkynyl, the (hetero)aryl, and the (hetero)aralkyl may be substituted by one or more substituents selected from the group consisting of F and CN, wherein R″ is selected from the group consisting of OR′ and R′ and wherein the two R″ may form a 5-membered heterocycle or a 6-membered heterocycle together with the P-atom, and wherein at least one of R.sup.1 and R.sup.2 is F or is selected from the group consisting of R′, OR′, and OP(O)R″.sub.2, in which R′ or R″ is substituted by one or more F; R.sup.3 is selected from the group consisting of H, C1 to C12 alkyl, C3 to C6 (hetero)cycloalkyl, C2 to C12 alkenyl, C2 to C12 alkynyl, C5 to C12 (hetero)aryl, and C6 to C24 (hetero)aralkyl, wherein the alkyl, the (hetero)cycloalkyl, the alkenyl, the alkynyl, the (hetero)aryl, and the (hetero)aralkyl may be substituted by one or more substituents selected from the group consisting of F and CN; R.sup.4 is selected from the group consisting of C1 to C12 alkyl, C3 to C6 (hetero)cycloalkyl, C2 to C12 alkenyl, C2 to C12 alkynyl, C5 to C12 (hetero)aryl, and C6 to C24 (hetero)aralkyl, wherein the alkyl, the (hetero)cycloalkyl, the alkenyl, the alkynyl, the (hetero)aryl, and the (hetero)aralkyl may be substituted by one or more substituents selected from the group consisting of F and CN; or R.sup.3 and R.sup.4 are bound together with the group —C—C(O)—O— and form a 5-membered heterocycle or a 6-membered heterocycle which may be substituted by one or more substituents selected from the group consisting of F and C1 to C12 alkyl, wherein the C1 to C12 alkyl may be substituted with one or more F; wherein the electrolyte composition contains in total 0.1 to 10 wt. % of the at least one additive compound of formula (I), based on the total weight of the electrolyte composition; (ii) at least one aprotic organic solvent; and (iii) at least one lithium conducting salt.

2. The electrolyte composition according to claim 1, wherein at least one of R.sup.1 and R.sup.2 is selected from the group consisting of F and C1 to C12 alkyl, wherein the C1 to C12 alkyl may be substituted with one or more F.

3. The electrolyte composition according to claim 1, wherein both R.sup.1 and R.sup.2 are selected from the group consisting of F and C1 to C12 alkyl, wherein the C1 to C12 alkyl may be substituted with one or more F.

4. The electrolyte composition according to claim 1, wherein at least one of R.sup.1 and R.sup.2 is a perfluorinated C1 to C12 alkyl.

5. The electrolyte composition according to claim 1, wherein R.sup.3 is selected from the group consisting of H and C1 to C12 alkyl.

6. The electrolyte composition according to claim 1, wherein R.sup.4 is selected from the group consisting of H and C1 to C12 alkyl, wherein C1 to C12 alkyl may be substituted by one or more substituents selected from F and CN.

7. The electrolyte composition according to claim 1, wherein the at least one compound of formula (I) is selected from the compounds of formulae (1.1) to (1.4) ##STR00012##

8. An electrochemical cell comprising the electrolyte composition according to claim 1.

9. The electrochemical cell according to claim 8 wherein the electrochemical cell is a lithium battery.

10. The electrolyte composition according to claim 1, wherein the electrolyte composition further comprises at least one further additive different from the compounds of formula (I), wherein the at least one further additive is selected from the group consisting of polymers, SEI forming additives, flame retardants, overcharge protection additives, wetting agents, HF and/or H.sub.2O scavengers, stabilizer for LiPF.sub.6 salt, ionic salvation enhancer, corrosion inhibitors, and gelling agents.

Description

PREPARATION OF COMPOUNDS OF FORMULA (I)

Compound I.1: Methyl-(4-trifluoro-3-(trifluoromethyl))-but-2-enoate

(1) Two reaction vessels, each equipped with a stirrer, were connected to a cold trap. Vessel A was charged with 245 g (2.4 moles) of acetic anhydride and 6 g of concentrated sulfuric acid. Vessel B was charged with 67 g (200 mmol) of methyl(triphenylphosphoranyliden)acetate and 200 ml of pentane and connected to a dry ice reflux condenser. Vessel A was warmed to 50° C., the cold trap was cooled with acetone/dry ice. Into vessel A was introduced 90 g (400 mmol) of hexafluoroacetone trihydrate. The evolving gaseous hexafluoroacetone was condensed in the cold trap. Subsequently, vessel B was cooled with acetone/dry ice and the cold trap was allowed to warm to room temperature. When all of the hexafluoroacetone had evaporated from the cold trap, the temperature in vessel B was raised to −30° C. and the mixture was stirred for three hours. The reaction mixture was allowed to warm to room temperature overnight. The precipitate was filtered off and the pentane was distilled off using a 20 cm distillation column filled with Raschig rings. The liquid residue was distilled at normal pressure on a rotary band column. There was obtained 22.9 g of product with a purity of 98.9% (GC). The product was characterized via its mass spectrum.

Compound I.2: 3-(2,2,2-Trifluoro-1-methyl-ethenyl)-tetrahydrofuran-2-one

(2) In an argon atmosphere, 6.3 g (157 mmol) of sodium hydride (60% in paraffin) were suspended 150 mL of dry tetrahydrofuran (THF). While stirring and cooling with ice water, 33.3 g (150 mmol) 3-(diethylphosphonio)tetrahydrofuran-2-one were added at 0 5° C. After the addition, the temperature was allowed to rise to room temperature and stirring was continued until the evolution of hydrogen had ceased (ca. 1 hour). The reaction mixture was again cooled to 0-5° C. and a solution of 22.4 g (200 mmol) trifluoroacetone in 50 ml of dry THF was added. The mixture was stirred at 0-5° C. for another hour, then warmed to room temperature. A yellowish jelly formed at the bottom of the flask. The THF was decanted from the jelly and 150 mL of 20% hydrochloric acid were added to the flask. This mixture was extracted with 3×10 ml of diethyl ether. The combined organic phases were washed with water and brine, dried over sodium sulfate and the solvent was evaporated. There was obtained 17.6 grams of colorless liquid.

(3) The previously decanted THF was evaporated under vacuum at 50° C. and a biphasic residue was obtained. The phases were separated and gave 1.7 g of upper phase and 23.5 g of lower phase.

(4) The lower phase was combined with the liquid gained from the acidic work-up of the gelatinous residue and distilled at 0.3-0.4 mbar on a rotary band column. The fraction boiling at 50° C. was collected (8.7 g) and characterized. 1H-NMR (400 MHz): 2.45 ppm (s, 3H), 3.15 ppm (m, 2H), 4.4 ppm (dd, 2H); FT-IR: 1672 cm-1 (C═C), 1788 cm-1, 1808 cm-1 (C═O); FI-HMS: C7H7O2F3.

Compound I.3: Methyl-(4-trifluoro-3-methyl)-but-2-enoate

(5) Under an atmosphere of dry argon, 67.0 g (200 mmol) methyl(triphenylphosphoranylidene)-acetate were suspended in 300 mL of n-pentane and the mixture was cooled to −78° C. 33.6 g of trifluoroacetone were added rapidly and stirring at −78° C. was continued for another three hours. The reaction mixture was allowed to warm to room temperature overnight. The precipitate was filtered by frit and the pentane was distilled off using a 20 cm distillation column filled with Raschig rings. The residue was distilled at normal pressure on a rotary band column. There were obtained 87.3 g of pentane and four fractions boiling from 106-111° C. containing the target product. The first two of these fractions had assays of 67% and 85.6% (GC) and weighed 0.8 and 1.1 g respectively. The last two fractions contained 97.4% and 97.9% of target product (GC) and weighed 8.9 and 11.6 g respectively. The product was characterized via its mass spectrum.

(6) Electrolyte Compositions

(7) Electrolyte compositions were prepared containing 1 M LiPF.sub.6 in a mixture of propylene carbonate (PC) or ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a ratio of 3:7 by mass and 2 or 1 wt.-% of different comparative and inventive compounds as shown in Table 1.

(8) TABLE-US-00001 TABLE 1 Reduction potential peak Example Structure vs. Li.sup.+/Li [V] Compound I.1 (inventive) 2.07 Compound I.2 (inventive) 1.68 Compound I.3 (inventive) 1.57 3.3-dimethylacrylacid methyl ester (comparative) 0 0.97

(9) Electrochemical Tests

(10) Reduction potential peak values were obtained from differential capacity plots of 2032 coin-type cells comprising a CMC-bonded graphite working electrode on a Cu current collector and a PVDF bonded lithium iron phosphate (LFP, BASF) counter electrode (cell voltages were converted into working electrode potential vs. Li.sup.+/Li considering an average counter electrode potential of 3.45 V.sub.Li). Cells were galvanostatically charged at C/100 rate from open circuit voltage to 3.6 V.sub.Li using a composition of EC/EMC 3/7 by weight containing 1M LiPF.sub.6 and 2 wt.-% of the respective additive. The results are shown in Table 1.

(11) Capacity retention of Li-ion cells using the inventive electrolyte additives reported below was investigated in a full cell configuration with coin-type cells (2032) with the same anode as described above for the determination of the reduction potential. The cathode used was PVdF (polyvinylidenefluoride)-bonded Li(Ni.sub.0.5Co.sub.0.2Mn.sub.0.3)O.sub.2 (also referred to as NCM523, BASF) on an Al current collector. A glass-fiber separator (Whatman GF/D) was used as the separator, which was soaked with 95 μl of a mixture of PC/EMC 3/7 by weight containing 1M LiPF.sub.6 and 1 wt.-% of the respective additive. All cells were assembled in an argon-filled glove box (Unilab, MBraun) having oxygen and water levels below 0.1 ppm. Afterwards the test cells were transferred to a battery test station comprising a Maccor battery test system and a climatic chamber tempered at 25° C. Cells were cycled at 0.5 C rate after 2 formation cycles at 0.1 C at 25° C. between 3.0-4.3 V. Capacity retention is reported in Table 2 as percentage of 1.sup.st cycle discharge capacity.

(12) The results are shown in Table 2.

(13) TABLE-US-00002 TABLE 2 10.sup.th cycle 20.sup.th cycle 50.sup.th cycle retained 0.5 C retained 0.5 C retained 0.5 C 1.sup.st cycle capacity capacity capacity capacity (% of 1.sup.st cycle) (% of 1.sup.st cycle) (% of 1.sup.st cycle) Comparative example 1 0 0 0 0 (3.3-dimethylacrylacid methyl ester) Inventive example 1 149.4 97.0 97.6 97.0 (Compound I.1) Inventive example 2 160.2 98.9 98.0 95.5 (Compound I.2) Inventive example 3 155.1 97.2 96.0 93.2 (Compound I.3)

(14) The cell of comparative example 1 with an electrolyte containing a non-fluorinated acrylic acid ester cannot be charged. It is assumed that the failure is caused by exfoliation of the graphite contained in the anode by co-intercalation of propylene carbonate and Li ions present in the electrolyte composition. The acrylic acid ester does not seem to have any effect in respect to the protection of the graphite. The inventive fluorinated acrylic acid esters clearly show good capacity retention and protect the graphitic anode against exfoliation by propylene carbonate.