COMPOSITION FOR BATTERY ELECTRODES

Abstract

The present invention relates to a composition for battery electrodes in which at least one solvent is a composition comprising between 80% and 95% by mass of N-methylpyrrolidone (NMP).

Claims

1. A composition comprising: at least one solvent comprising N-methylpyrrolidone in mass contents of between 80% and 98.99%, limits inclusive, water in mass contents of less than 1% and at least one compound bearing at least one γ-butyrolactam or γ-butyrolactone ring, NMP excluded, each of these compounds being present in mass proportions ranging from 0.01% to 19%, limits inclusive, alone or in combination, the total of these compounds not exceeding 19%, the solvent being present in mass proportions of greater than or equal to 50%, at least one electron conductor in solid form in proportions of less than or equal to 50%, 0 excluded.

2. The composition as claimed in claim 1, also comprising an active filler and at least one polymer.

3. The composition as claimed in claim 1, also comprising a free-radical generator.

4. A process for obtaining an electrode, comprising the following steps: depositing the composition as claimed in claim 2 onto an electrical current collector; evaporating off the solvent; optionally calendering.

5. The process as claimed in claim 4, wherein the solvent recovered by evaporation comprises the solvent.

6. An electrode obtained according to the process of claim 4.

7. A battery obtained with one or two electrodes as claimed in claim 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0020] The compositions of the invention comprise less than 50% by mass of electron conductor in solid form, preferably less than 35%, preferentially less than 25%, more preferably less than 10% and more preferably less than 5% by mass.

[0021] The term “electron conductor in solid form” means carbon blacks, graphenes, single-walled or multi-walled carbon nanotubes, graphites and carbon fibers, alone or in combination. Preferably, the electron conductor comprises carbon nanotubes.

[0022] The compositions of the invention also comprise at least one compound bearing at least one pyrrolidone ring (γ-butyrolactam), NMP excluded, and/or γ-butyrolactone, each of these compounds being present in proportions ranging from 0.01% to 19%, alone or in combination, the total of these compounds not exceeding 19%.

[0023] The term “compound bearing at least one pyrrolidone ring (γ-butyrolactam) or γ-butyrolactone” may refer to any compound with a molecular weight of less than 15 000 g/mol, preferably less than 10 000 g/mol, and in which the presence of a γ-butyrolactone or γ-butyrolactam ring is characterized (mass spectroscopy, NMR or infrared spectroscopy).

[0024] The γ-butyrolactone and γ-butyrolactam structures may bear substituents comprising C, H, N or O.

[0025] Among these compounds are γ-butyrolactone, succinimides such as N-methylsuccinimide and N-hydroxysuccinimide, formylpyrrolidone, 5-hydroxy-N-methylpyrrolidone, 2-pyrrolidone, or N-hydroxymethylpyrrolidone, among the main ones. They may also be oligomers bearing a γ-butyrolactam and/or γ-butyrolactone ring of a form that is more or less complex or difficult to characterize as regards either their nature or their concentration.

[0026] It is considered that all the impurities derived from the γ-butyrolactam or γ-butyrolactone structure present in the solvent derived from evaporation during the electrode manufacturing process may be present in the compositions of the invention. Lighter compounds whose structure does not comprise a γ-butyrolactam or γ-butyrolactone structure may also be found.

[0027] Preferably, these compounds are present in contents of between 0.01% to 19% by mass, more preferably between 0.1% and 10% and more preferably between 0.5% and 5% by mass, limits inclusive.

[0028] This solvent composition is obtained by recovering NMP vapors in the presence of air for temperatures above 120°.

[0029] This solvent composition may also be obtained by purposely adding γ-butyrolactam and/or γ-butyrolactone derivatives to NMP whose purity is greater than 99%, although this is not the prime objective of the invention, which uses NMP without the need for purification, thus containing these γ-butyrolactone and/or γ-butyrolactam compounds.

[0030] The compositions of the invention may also comprise an active filler. The term “active filler” means lithiated transition metal oxides such as LiMO.sub.2, of the LiMPO.sub.4 type, of the Li.sub.2MPO.sub.3F type or of the Li.sub.2MSiO.sub.4 type in which M is Co, Ni, Mn, Fe or a combination thereof, of the LiMn.sub.2O.sub.4 type or of the S.sub.8 type, artificial or natural graphites or silicon or silicon modified with carbide, nitrides or oxides.

[0031] The compositions of the invention may also comprise one or more polymers, chosen from poly(vinylidene fluoride) (PVDF) polymers, poly(vinylpyrrolidone), poly(phenylacetylene), poly(meta-phenylenevinylidene), polypyrrole, poly(para-phenylenebenzobisoxazole), poly(vinyl alcohol), carboxymethylcellulose and mixtures thereof, and polyacrylonitrile and copolymers thereof. Preferably, it is poly(vinylidene fluoride) (PVDF) and poly(N-vinylpyrrolidone).

[0032] The compositions of the invention may also comprise one or more free-radical generators, such as peroxides, redox couples, azo compounds or alkoxyamines, alone or in combination in contents of less than 5% and preferably less than 1% by mass relative to the NMP.

[0033] The invention also relates to a process for obtaining an electrode, comprising the following steps: [0034] deposition of the composition of the invention comprising at least one active filler, at least one polymer on an electrical current collector, [0035] evaporating off the solvent, [0036] optionally calendering.

[0037] The invention also relates to a process for obtaining an electrode, in which the solvent recovered by evaporation comprises the solvent of the invention.

[0038] The metallic electrical current collector is chosen from the following metals: Al, Cu, Ni in a nonlimiting manner, and has a thickness of between 8 and 35 μm. The metallic collector may also be coated with a primer deposited on the electron conductor in a thickness of between 0.5 and 5 μm.

[0039] The calendering consists in compressing the electrode between two counter-rotating rolls, in which the gap between the rolls is less than the thickness of the electrode.

[0040] The electrode may be a cathode or an anode. Preferably, it is a cathode.

[0041] For the cathodes, the active material is chosen from lithiated transition metal oxides such as LiMO.sub.2, of the LiMPO.sub.4 type, of the Li.sub.2MPO.sub.3F type, of the Li.sub.2MSiO.sub.4 type, where M is Co, Ni, Mn, Fe or a combination thereof, of the LiMn.sub.2O.sub.4 type or of the S.sub.8 type.

[0042] For the anodes, the active material is chosen from artificial or natural graphites or silicon or silicon modified with carbide, nitrides or oxides.

[0043] The invention also relates to a battery using a cathode and/or an anode obtained according to the process of the invention.

[0044] The invention also relates to the use of the compositions of the invention in fields such as inks and paints, or in petroleum extraction.

[0045] The formulation of the electrode (cathode or anode) may be as follows, in a nonlimiting manner: [0046] a solids content of between 20% and 90% by mass, the remainder being the solvent (composition comprising N-methylpyrrolidone in mass contents of between 80% and 98.99%, limits inclusive, water in mass contents of less than 1% and at least one compound bearing at least one γ-butyrolactam or γ-butyrolactone ring, NMP excluded, each of these compounds being present in mass proportions ranging from 0.01% to 19%, limits inclusive, alone or in combination, the total of these compounds not exceeding 19%), [0047] one or more electron conductors in proportions of between 0.1% and 5% of the total formulation for manufacturing the electrode, [0048] polymeric or nonpolymeric additives (binder, dispersant) in proportions of between 0.3-5% of the total formulation for manufacturing the electrode.

Example 1

[0049] Preparation of the dispersion of Graphistrength® C100 HP carbon nanotubes in electronic-grade NMP solvent (reference).

[0050] Graphistrength® C100 HP CNT is the commercial grade from Arkema containing <20 ppm of metallic impurities. This grade of purified CNTs is recommended for use in the cathodes of Li-ion batteries. These carbon nanotubes are multi-walled (between 10 and 15 walls) and their specific surface area is between 180 and 240 cm.sup.2/g.

[0051] Preparation of the Dispersion:

[0052] 100 g of Graphistrength® C100 HP in powder form are premixed with 400 g of NMP (electronic grade, purity >99.8% by mass), using a deflocculator equipped with a 55 mm paddle.

[0053] 25 g of PVP K 30 (BASF) are added gradually over one hour with stirring at 1000-1600 rpm.

[0054] A further 70 g of NMP are added after 30 minutes, and a further 65 g of NMP are added 30 minutes later.

[0055] The predispersion in an amount of 660 g, containing 15% of CNT and 3.8% of PVP, is ready for the step of milling in a Brandt horizontal ball mill (HBM), with a mill chamber volume of 250 ml.

[0056] The mill is charged with 180 ml of ceramic beads with a diameter of 0.4-0.7 mm and the gap size is 0.1-0.15 mm.

[0057] The milling circuit is primed with 230 g of NMP alone for 5 minutes.

[0058] The predispersion is gradually added over 15-20 minutes, while increasing the rotor speed to 3000 rpm and the pump to 35% of the capacity.

[0059] Solids content and absorbance measurements were taken to monitor the evolution of the dispersion (table 1).

TABLE-US-00001 TABLE 1 Time in Solids Mass % Absorbance minutes content % CNT at 50 ppm CNT 0 8.55 6.84 0.931 10 8.54 6.83 1.498 40 8.7 6.96 2.617 70 8.67 6.94 2.895 120 8.78 7.02 3.378 200 9.37 7.50 3.442 260 8.22 6.58 3.434 Addition of NMP to target 4% of CNT 320 4.93 3.94 3.346

[0060] For each absorbance measurement, the dispersion taken from the mill was diluted to 50 ppm of CNT. It was measured using a DR/2000 spectrometer (Hach) at a wavelength of 355 nm.

Example 2: Recovery NMP and CNT Dispersion Based on this Solvent (Invention)

[0061] The NMP solvent of example 1, of “electronic” grade, was used to prepare the typical “cathode” formulation for the Li battery. For 960 g of NMC 622 produced by Umicore (LiNi.sub.0.6Mn.sub.0.2Co.sub.0.2O.sub.2), 20 g of Kynar® HSV 1810 PVDF produced by Arkema were added. The dry premix was dispersed in 1950 ml of NMP using a disc mixer for 30 minutes. 50 g of the CNT dispersion of example 1 were then added. The solvent was overdosed relative to the formulations used in the production of the cathodes since the purpose of this example is to model the recovery of the solvent in the presence of the typical ingredients of a cathode.

[0062] The dispersion was placed in a 5 l Lab Rotovap brand rotary evaporator. The evaporator was heated to 145° C. and the condensate vessel was maintained at 110° C. The condenser vacuum valve was kept open to maintain the contact of the NMP condensate with the air.

[0063] Over 24 hours, 900 g of solvent with a pronounced yellow color were recovered in the vessel.

[0064] The recovered NMP was analyzed by the Karl Fischer method for the presence of water, the value obtained being 650 ppm. The NMP purity amount of 94.6% was obtained by mass spectroscopy. Thus, about 5% of recovered NMP may be attributed to the nonvolatile NMP oxidation products.

[0065] This recovery NMP sample was used to prepare the CNT dispersion according to the method described in example 1, adhering to the same conditions.

[0066] Solids content and absorbance measurements were taken to monitor the evolution of the dispersion (table 2).

TABLE-US-00002 TABLE 2 Times in % Solids Mass % Absorbance minutes content CNT at 50 ppm CNT 0 8.51 6.81 1.022 15 8.64 6.91 1.634 40 8.67 6.94 2.801 60 8.81 7.05 3.244 90 8.85 7.08 3.645 120 9.51 7.61 3.845 Addition of NMP to target 4% of CNT 320 5.06 3.05 3.742

[0067] For the absorbance control, the recovery NMP was used for the reference cell. It is observed that the evolution of the dispersion is faster in the case of the recovery NMP. After 90 minutes of milling, the absorbance approaches saturation, which is observed after 120 minutes of milling in example 1. The absorbance value is higher in example 1, which may reflect a better performance of the dispersion of the invention.

Example 3: Electrical Performance of the CNT Dispersion of Examples 1 and 2 in the Cathode Formulation

[0068] Preparation of the Cathode Formulation

[0069] The solution of Kynar® HSV 1810 PVDF at 8% in “electronic” grade NMP in an amount of 12.5 g was mixed with the same amount of the CNT dispersion of example 1 using a disc mixer, at 400 rpm. After 15 minutes of mixing, 98.5 g of NMC 622 and a further 10 g of NMP are gradually added to maintain good fluidity of the dispersion. The objective for the viscosity is between 3500 and 5000 cPs. Mixing was complete after 30 minutes at 1500 rpm.

[0070] The dispersion was then applied to a polyethylene terephthalate (PET) support to a thickness of 100 μm using a doctor blade, targeting a coating thickness of 120 μm. The coating was dried in a ventilated oven for 30 minutes at 130° C.

[0071] The solids content of this “model” cathode formulation is as follows:

[0072] NMC: 98.5%; PVDF 1%; CNT 0.5%

[0073] The coated PET sheet is cut to obtain 3×4 cm samples. The ends of each sample are covered with a conductive ink containing silver. The electrical resistivity was measured using a Keithlley brand electrometer.

[0074] The same protocol was applied to obtain a cathode model with the CNT dispersion of example 2. The results of the electrical measurements are summarized in Table 3.

TABLE-US-00003 TABLE 3 Dispersion Electrode NMP purity in quality (cathode) the CNT measured by resistivity dispersion absorbance Ohm .Math. cm example .sup. >99% 3.346 62 comparative NMP Example >94.6% 3.742 39 composition invention, recovery

[0075] Under similar conditions, the CNT dispersion prepared based on recovery NMP demonstrates better electrical performance in the NMC622 cathode type formulation, the resistivity of the electrode is lower, which is favorable to the correct functioning of a battery.