Dense fluoropolymer film

Abstract

The present invention pertains to a process for the manufacture of a dense film. The process includes processing, in a molten phase, a solid composition [composition (C)] comprising; at least one vinylidene fluoride (VDF) fluoropolymer comprising one or more carboxylic acid functional end groups [polymer (F)], at least one poly(alkylene oxide) (PAO) of formula (I):
HO—(CH.sub.2CHR.sub.AO).sub.n—R.sub.B  (I) wherein R.sub.A is a hydrogen atom or a C.sub.1-C.sub.5 alkyl group, R.sub.B is a hydrogen atom or a —CH.sub.3 alkyl group and n is an integer comprised between 2000 and 40000, preferably between 4000 and 35000, more preferably between 11500 and 30000, and -optionally, at least one inorganic filler [filler (I)]; thereby providing a dense film having a thickness of from 5 μm to 30 μm. The present invention also pertains to the dense film provided by this process and to the use of the dense film as dense separator in electrochemical devices.

Claims

1. A process for the manufacture of a dense film, said process comprising: processing, in a molten phase, a solid composition (C), said composition (C) comprising: at least one polymer (F), wherein polymer (F) is a fluoropolymer comprising recurring units derived from vinylidene fluoride (VDF), recurring units derived from at least one (meth)acrylic monomer (MA) of formula (II): ##STR00004## wherein: R.sub.1, R.sub.2 and R.sub.3, equal to or different from each other, are independently selected from a hydrogen atom and a C.sub.1-C.sub.3 hydrocarbon group, and R.sub.x is a hydrogen atom or a C.sub.1-C.sub.5 hydrocarbon group and, optionally, from one or more fluorinated monomers different from VDF; and one or more carboxylic acid functional end groups, at least one poly(alkylene oxide) (PAO) of formula (I):
HO—(CH.sub.2CHR.sub.AO).sub.n—R.sub.B  (I) wherein R.sub.A is a hydrogen atom or a C.sub.1-C.sub.5 alkyl group, R.sub.B is a hydrogen atom or a —CH.sub.3 alkyl group and n is an integer comprised between 2000 and 40000, and optionally, at least one inorganic filler [filler (I)]; thereby providing a dense film having a thickness of from 5 μm to 30 μm formed of at least one grafted fluoropolymer [polymer (Fg)] comprising: at least one fluorinated backbone comprising recurring units derived from vinylidene fluoride (VDF) and from one or more hydrogenated monomers, and at least one pendant side chain linked to one or more fluorinated backbones of the polymer (Fg) through one or more ester functional groups, said pendant side chain comprising alkylene oxide recurring units of formula:
—(CH.sub.2CHR.sub.AO).sub.n—.

2. The process according to claim 1, wherein monomer (MA) is acrylic acid (AA).

3. The process according to claim 1, wherein the PAO of formula (I) has an average molecular weight comprised between 100000 and 1800000.

4. The process according to claim 1, wherein the PAO of formula (I) is a poly(ethylene oxide) (PEO) complying with formula (I-A):
HO—(CH.sub.2CH.sub.2O).sub.n—CH.sub.3  (I-A) wherein n is an integer comprised between 2000 and 40000.

5. The process according to claim 4, wherein n is an integer comprised between 11500 and 30000.

6. The process according to claim 1, wherein the composition (C) comprises: from 20% to 95% by volume, relative to the total volume of composition (C), of at least one polymer (F), and from 5% to 80% by volume, relative to the total volume of composition (C), of at least one PAO of formula (I).

7. The process according to claim 6, wherein the composition (C) comprises: from 45% to 90% by volume, relative to the total volume of composition (C), of at least one polymer (F), and from 10% to 55% by volume, relative to the total volume of composition (C), of at least one PAO of formula (I-A).

8. The process according to claim 1, wherein composition (C) is processed in molten phase using melt-processing techniques.

9. A dense film obtained by the process according to claim 1.

10. A process for the manufacture of an electrochemical device, said process comprising: interposing a dense separator between a negative electrode and a positive electrode thereby assembling an electrochemical device, and injecting an electrolyte into the electrochemical device, wherein the dense separator is obtained by the process according to claim 1.

11. The process according to claim 10, wherein the electrochemical device is a secondary battery such as an alkaline or alkaline-earth secondary battery.

12. The process according to claim 10, wherein the electrochemical device is a Lithium-ion battery.

Description

EXAMPLE 1

Blend of Polymer (F-1) and PEO-1 (50:50 Volume Ratio)

(1) A film having a thickness of 24 μm was prepared by hot blown film extrusion from a 50:50 by volume blend of polymer (F-1) and PEO-1. FT-IR spectroscopic analyses on the dense film so obtained have shown the appearance of an ester band at about 1730-1740 cm.sup.−1.

(2) The amount of recurring units of formula —CH.sub.2CH.sub.2O— of PEO-1 grafted to the polymer (F-1) was 34% by weight, relative to the total weight of PEO-1 in the blend.

(3) The dense film so obtained had an ionic conductivity of 4.58×10.sup.−4 S/cm. The mechanical properties of the dense film so obtained in machine direction (MD) and transversal direction (TD) at 23° C., according to ASTM D638 standard procedure (Type V) (grip distance: 25.4 mm, Lo: 21.5 mm, speed rate: 1-50 mm/min) are set forth in Table 2 here below.

(4) TABLE-US-00002 TABLE 2 Stress Strain Modulus Yield Stress Yield Strain at break at break [MPa] [MPa] [%] [MPa] [%] MD 449 15.2 7.6 18.1 194.6 TD 725 13.0 3.6 12.0 141.1

(5) It has been thus found that the dense films provided by the process according to the invention are advantageously endowed with superior ionic conductivity values as compared with those of commercially available dense films made of the following fluoropolymers: polymer (F-1): 2.13×10.sup.−5 S/cm polymer (F-2): 4.47×10.sup.−5 S/cm SOLEF® 21508 VDF/HFP: 5.1×10.sup.−5 S/cm

EXAMPLE 2

Blend of Polymer (F-1) and PEO-1 (90:10 Volume Ratio)

(6) A film having a thickness of 13 μm was prepared by flat cast film extrusion from a 90:10 by volume blend of polymer (F-1) and PEO-1.

(7) FT-IR spectroscopic analyses on the dense film so obtained have shown the appearance of an ester band at about 1730-1740 cm.sup.−1.

(8) The amount of recurring units of formula —CH.sub.2CH.sub.2O— of PEO-1 grafted to the polymer (F-1) was 88% by weight, relative to the total weight of PEO-1 in the blend.

(9) The mechanical properties of the dense film so obtained in transversal direction (TD) at 23° C. and −30° C., according to ASTM D638 standard procedure (Type V) (grip distance: 25.4 mm, Lo: 21.5 mm, speed rate: 1-50 mm/min) are set forth in Table 3 here below.

(10) TABLE-US-00003 TABLE 3 SOLEF ® 6008 SOLEF ® 6008 Ex. 2 Ex. 2 PVDF PVDF Modulus [MPa]  699 (23° C.) 2248 (−30° C.)  1842 (23° C.)  3224 (−30° C.)  Yield Stress [MPa] 34.3 (23° C.) 76.7 (−30° C.) 60.0 (23° C.) 104.3 (−30° C.)  Yield Strain [MPa] 11.3 (23° C.) 11.5 (−30° C.)  6.3 (23° C.)  6.6 (−30° C.) Stress at break [MPa] 82.1 (23° C.) 75.3 (−30° C.) 91.4 (23° C.) 88.3 (−30° C.) Strain at break [%]  452 (23° C.)  157 (−30° C.)  460 (23° C.) .sup. 14 (−30° C.) Energy [mJ/mm3] —  102 (−30° C.) — 11.5 (−30° C.)

(11) The results set forth in Tables 2 and 3 here above have shown that the dense films provided by the process according to the invention are advantageously endowed with superior mechanical properties in a wide range of temperatures of from −30° C. to 100° C. as compared with those of commercially available dense films.

(12) In view of the above, it has been found that the dense films provided by the process according to the invention, advantageously combining both outstanding ionic conductivity and outstanding mechanical properties, are particularly suitable for use as dense separators in electrochemical devices.

(13) Manufacture of a Lithium-Ion Battery

(14) A coin cell was prepared by placing the dense film as prepared according to Example 1 between Lithium metal negative electrode and a positive electrode containing LiFePO.sub.4 as active material, SOLEF® 5130 PVDF as binder and Super P® Li conductive carbon black.

(15) The coin cell was filled with 200 μl of Selectilyte® LP30 electrolyte consisting of a 1 M solution of LiPF.sub.6 in ethylene carbonate/dimethyl carbonate (1:1 weight ratio).

(16) The discharge capacity values of the coin cell so obtained at different discharge rates are set forth in Table 4 here below.

(17) TABLE-US-00004 TABLE 4 Average Discharge Rate [mAh/g] [%] 5 Discharge 5D 58.2 37.1 2 Discharge 2D 116.5 74.2 1 Discharge D 133.6 85.0 0.33 Discharge D/3 149.0 94.9 0.2 Discharge D/5 151.3 96.4 0.1 Discharge D/10 155.0 98.7 0.05 Discharge D/20 154.8 98.6