Binder containing polyvinylidene fluoride capable of fixing to a metal and associated lithium-ion battery electrode

Abstract

The present invention relates to a binder which can be used in a lithium-ion battery, comprising at least one polyvinylidene fluoride and at least one acrylic copolymer including monomers having functional groups which have an affinity for metals or are capable of fixing to the metals. According to the invention, in a characteristic manner, said polyvinylidene fluoride is such that a solution of N-methyl-2-pyrrolidone containing 5 wt % of said polyvinylidene fluoride has a viscosity, measured at 23 C. with an imposed shear rate of 30 rpm, of 125 mPa.Math.s to 1500 mPa.Math.s.

Claims

1. A binder for a lithium-ion battery comprising at least one vinylidene fluoride polymer and at least one acrylic copolymer, said acrylic copolymer comprising monomers comprising functional groups exhibiting an affinity for metals or which are capable of becoming fixed to metals, characterized in that said acrylic copolymer comprises monomers including at least one type of functional group chosen from among the following groups: carboxyl, carboxylic acid anhydride, epoxy, mercapto, sulfide and phenolic and wherein a 5% by weight solution of said vinylidene fluoride polymer in N-methyl-2-pyrrolidone exhibits a viscosity, measured at 23 C. with a controlled shear rate of 30 revolutions/min, equal to or greater than 125 millipascal-seconds, and less than 1500 millipascal-seconds.

2. The binder as claimed in claim 1, wherein said vinylidene fluoride polymer is such that said viscosity of said solution of N-methyl-2-pyrrolidone containing 5% by weight of said vinylidene fluoride polymer is about 150 millipascal-seconds or about 700 millipascal-seconds.

3. The binder as claimed in claim 1, wherein said acrylic copolymer comprises monomers comprising at least one type of functional group chosen from the following groups: carboxyl, carboxylic anhydride, epoxy, mercapto, sulfide, phenolic and ester.

4. The binder as claimed in claim 1, wherein said acrylic copolymer contains 10 mol % of monomers bearing functional groups exhibiting an affinity for metals or capable of becoming fixed to metals and in particular 10 mol % of methacrylic acid groups.

5. The binder as claimed in claim 1, wherein said binder contains, by weight, a content of acrylic copolymer equal to or greater than 0.1% and equal to or less than 25%.

6. An electrode for a lithium-ion battery of the type comprising a metal collector, at least one face of which is covered with a layer of substrate containing an active substance and a binder, characterized in that wherein said binder is the binder of claim 1.

7. The electrode as claimed in claim 6, wherein said substrate contains, by weight, a content of said binder of equal to or greater than 1% and of equal to or less than 5% and in particular of equal to 3%.

8. The electrode as claimed in claim 6, wherein said substrate contains, as active substance, a lithium metal oxide and optionally carbon black.

9. The electrode as claimed in claim 6, wherein said substrate contains, as active substance, at least one ingredient chosen from coke, carbon black, graphite, activated carbon and carbon fibers.

10. The electrode as claimed in claim 1, wherein said solution of N-methyl-2-pyrrolidone containing 5% by weight of said vinylidene fluoride polymer exhibits a viscosity, measured at 23 C. with a controlled shear rate of 30 revolutions/min, is equal to or greater than 300 millipascal-seconds.

11. The electrode as claimed in claim 1, wherein said solution of N-methyl-2-pyrrolidone containing 5% by weight of said vinylidene fluoride polymer exhibits a viscosity, measured at 23 C. with a controlled shear rate of 30 revolutions/min, is equal to or greater than 700 millipascal-seconds, and less than 1200 millipascal-seconds.

12. The binder as claimed in claim 5 wherein said binder contains, by weight, a content of acrylic copolymer equal to or less than 20%.

13. The binder as claimed in claim 12 wherein said binder contains, by weight, a content of acrylic copolymer equal to or less than 10%.

14. The electrode as claimed in claim 7, wherein said substrate contains, by weight, a content of said binder equal to or greater than 1% and of equal to or less than 3%.

15. The binder as claimed in claim 1, wherein the 5% by weight solution of said vinylidene fluoride polymer in N-methyl-2-pyrrolidone exhibits a viscosity, measured at 23 C. with a controlled shear rate of 30 revolutions/min, equal to or greater than 300 millipascal-seconds, and less than 1200 millipascal-seconds.

16. The binder as claimed in claim 1, wherein said acrylic copolymer is a copolymer of methyl methacrylate and methacrylic acid.

Description

EXAMPLES

(1) Products Used for the Composition of the Binders

(2) PVDF 1: Vinylidene fluoride homopolymer characterized by a melt viscosity, measured according to ASTM D3835, of between 4450 Pa.Math.s and 5450 Pa.Math.s at 230 C. and 100 s.sup.1. PVDF 2: Vinylidene fluoride homopolymer characterized by a melt viscosity, measured according to ASTM D3835, of between 3400 Pa.Math.s and 4000 Pa.Math.s at 230 C. and 100 s.sup.1. PVDF 3: Vinylidene fluoride homopolymer characterized by a melt viscosity, measured according to ASTM D3835, of between 3000 Pa.Math.s and 3300 Pa.Math.s at 230 C. and 100 s.sup.1. KynarHSV900/Kynar720 PVDF mixture in proportions by weight of 1:1 characterized by a melt viscosity, measured according to ASTM D3835 at 232 C. and 100 s.sup.1, of 3800 Pa.Math.s. CMM: Copolymer of methyl methacrylate and methacrylic acid. This copolymer contains 10 mol % of methacrylic acid. It has an intrinsic viscosity of 27 cm.sup.3/g. nonfunctionalized PMMA: Acrylic polymer based on methyl methacrylate which has an intrinsic viscosity of 46 cm.sup.3/g. This polymer does not contain functionalized comonomer bearing carboxyl functional groups.
Compositions of the Binders

(3) The different binder compositions studied are collated in table I below.

(4) TABLE-US-00001 TABLE I % by % by weight of Type of acrylic weight acrylic PVDF polymer of PVDF polymer Example 1 PVDF 1 CMM 80 20 Example 2 PVDF 1 CMM 90 10 Example 3 PVDF 1 CMM 97.5 2.5 Comparative PVDF 1 100 0 example 1 Comparative PVDF 2 100 0 example 2 Comparative PVDF 3 100 0 example 3 Comparative PVDF 1 nonfunctionalized 90 10 example 4 PMMA
Measurement of the Viscosity of Solutions of Binders in N-methyl-2-pyrrolidone

(5) The viscosity was measured of solutions of N-methyl-2-pyrrolidone (NMP) respectively containing: 5% by weight of PVDF 1 alone (comparative example 1 of table I) 5% by weight of PVDF 2 alone (comparative example 2 of table I) 5% by weight of PVDF 3 alone (comparative example 3 of table I) 5% by weight of a Kynar HSV900/Kynar 720 PVDF mixture; 5% by weight of a mixture of PVDF 1 and CMM, this consisting of 90% by weight of PVDF 1 and 10% by weight of CMM (example 2 of table I); and 5% by weight of a mixture of PVDF 1 and CMM, this consisting of 97.5% by weight of PVDF 1 and 2.5% by weight of CMM (example 3 of table I).

(6) The viscosity of each of the abovementioned solutions was measured at 23 C. using a Brookfield DV-II Pro viscometer equipped with a rotor of SC4-34 type with a rotational speed of the spindle controlled at 30 revolutions per minute.

(7) The results obtained appear in table II below, which also collates the different binder compositions. Table II shows that the addition of an acrylic copolymer bearing methacrylic acid functional groups to a PVDF polymer makes it possible to reduce the viscosity of the PVDF/acrylic polymer mixture in comparison with the PVDF alone. This reduced viscosity makes it possible to more easily spread the mixture which forms the substrate containing the active substance of the electrode. Moreover, the viscosity of a 5% solution in N-methyl-2-pyrrolidone of a Kynar HSV900/Kynar 720 PVDF mixture as disclosed in the document EP 2 953 193 is 80 mPa.Math.s.

(8) TABLE-US-00002 TABLE II Solution viscosity (mPa .Math. s) Example 2 450 Example 3 620 Comparative 700 example 1 Comparative 150 example 2 Comparative 75 example 3 Kynar HSV900/ 80 Kynar HS 720 PVDF mixture
Preparation of Cathodes

(9) In addition to the polymeric binders described above, the following products were used for the preparation of cathodes: lithium metal oxide LiNMC 1:1:1, sold by Umicore under the name of MX10, carbon black Super P Li, sold by Timcal.

(10) As regards the polymeric binder, the same compositions as those appearing in table I are used.

(11) A cathode was manufactured by following the following stages: a 5% by weight solution of polymeric binder in N-methyl-2-pyrrolidone is prepared until the polymeric binder has completely dissolved. Super P Li carbon black is then added to this solution. The solution is mixed using a mechanical stirrer. The lithium metal oxide LiNMC MX10 is then added. A paste is obtained which contains, by weight, 94 parts of LiNMC, 3 parts of carbon black and 3 parts of binder per 100 parts of the LiNMC/carbon black/binder mixture. The paste obtained is deposited on an aluminum sheet so as to have a wet thickness of 350 m. The NMP is then evaporated by heating the coated sheet at 90 C. for 15 minutes and then at 150 C. for 30 minutes. A coating having a thickness of 11015 m is thus obtained. It is found that the paste formed with the binders of examples 1 to 3 cited in table I is easier to spread over the aluminum sheet.
Measurement of the Adhesion

(12) The adhesion between the layer formed of the LiNMC/carbon black/binder mixture and the aluminum sheet is subsequently measured. To do this, a strip with a width of 25 mm is cut out. This strip is subsequently adhesively bonded to a stiff aluminum plate using a double-sided adhesive, the adhesive being deposited on the side of the LiNMC/carbon black/binder coating. The peel test is carried out using a dynamometer from MTS Systems of Synergie 200H type by fixing, in one jaw, the stiff aluminum plate and, in the other jaw, the flexible aluminum sheet on which the deposition was carried out. In this configuration, the peel angle is 180 C. The rate of displacement of the jaws is set at 100 mm/min. The results of the adhesion tests for the binder examples appearing in table I are collated in table III which follows. Table III shows that an improved adhesion is obtained with the addition to a vinylidene fluoride polymer of an acrylic copolymer containing functional groups exhibiting an affinity for metals or capable of becoming fixed to metals. It also shows that the improvement in the adhesion is due, at least in part, to the abovementioned functional groups. Finally, the results of Table III show that the proportion of acrylic copolymer as mentioned above can range at least up to 20% by weight of the binder and that the higher this proportion, the better the adhesion. Nevertheless, a reduced improvement in the adhesion, with respect to the increase in the amount of acrylic copolymer as mentioned above, is found when the proportion by weight of this acrylic polymer reaches 10%.

(13) TABLE-US-00003 TABLE III Peel force (N/25 mm) Example 1 0.85 0.03 Example 2 0.80 0.07 Example 3 0.65 0.05 Comparative 0.4 0.05 example 1 Comparative 0.25 0.03 example 2 Comparative 0.1 0.03 example 3 Comparative 0.45 0.03 example 4

(14) The adhesive properties of a Kynarx HSV900/Kynarx 720 PVDF mixture as disclosed in the document EP 2 953 193 were also measured, as shown in table IV below:

(15) TABLE-US-00004 TABLE IV Peel force (N/25 mm) PVDF 2 0.25 0.03 Kynar HSV900/Kynar 0.15 0.02 720 1:1 PVDF 2 + 10% CMM 0.4 0.02 Kynar HSV900/Kynar 0.2 0.05 720 1:1 + 10% CMM

(16) The results obtained show that the Kynarx HSV900/Kynarx 720 PVDF mixture does not make it possible to obtain adhesive properties sufficient for the application as binder for a lithium-ion battery.

(17) Measurement of the Swelling in the Electrolyte and Content of Compounds Passing into the Electrolyte (Extractables)

(18) The first stage for the measurement of the uptake in weight of the binder in the electrolyte consists in preparing a binder-free film. For this, a 5% by weight solution of polymeric binder in N-methyl-2-pyrrolidone is prepared until the polymeric binder has completely dissolved. An amount of solution is poured into a glass petri dish so as to have approximately 20 milligrams of solution per square centimeter. The solvent is subsequently evaporated by placing the petri dish in an oven at 120 C. overnight. A dry film of binder is thus obtained. Ten square centimeters are cut out. The sample is subsequently weighed. Its weight is denoted W.sub.0.

(19) Subsequently, the sample is immersed at 60 C. for 24 hours in a test tube containing the electrolyte consisting of an ethyl carbonate/dimethyl carbonate/diethyl carbonate mixture having a 1/1/1 composition by weight. On conclusion of the 24 hours, the film is removed from the test tube and given a cursory wipe in order to remove the surface electrolyte drops. It is subsequently weighed. This weight is denoted W.sub.1.

(20) Subsequently, the sample is immersed in dimethyl carbonate at ambient temperature for 2 hours. It is then dried in an oven at 120 C. for 2 hours. On conclusion of this stage, the sample is weighed. Its weight is denoted W.sub.2.

(21) The degree of swelling or uptake in weight is calculated according to the following formula:

(22) $\frac{W_1-W_0}{W_0}$

(23) The content of extractables (or content of compounds passing from the binder into the electrolyte) is calculated according to the following formula:

(24) $\frac{W_0-W_2}{W_0}$

(25) This protocol was applied to the composition of example 2. We obtained an increase in weight of 23% and a content of extractables of 2%.