MASTERBATCH OF CARBON-BASED CONDUCTIVE FILLERS FOR LIQUID FORMULATIONS, ESPECIALLY IN LI-ON BATTERIES

Abstract

A masterbatch in agglomerated solid form including carbon nanofibers and/or nanotubes and/or carbon black, the content of which is between 15 wt % and 40 wt %, preferably between 20 wt % and 35 wt %, relative to the total weight of the masterbatch; at least one solvent; at least one polymer binder, which represents from 1 wt % to 40 wt %, preferably from 2 wt % to 30 wt % relative to the total weight of the masterbatch. Also, a concentrated masterbatch, characterized in that it is obtained by eliminating all or part of the solvent from the masterbatch described previously. Also, a process for preparing said masterbatches and to the uses of the latter, especially in the manufacture of an electrode or of a composite material for an electrode.

Claims

1. A masterbatch in agglomerated solid form comprising: a) carbon nanofibers and/or nanotubes and/or carbon black, the content of which is between 15 wt % and 40 wt %, preferably between 20 wt % and 35 wt %, relative to the total weight of the masterbatch; b) at least one solvent; c) at least one polymer binder, which represents from 1 wt % to 40 wt %, preferably from 2 wt % to 30 wt % relative to the total weight of the masterbatch.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0117] The invention will now be illustrated by the following examples, the purpose of which is not to limit the scope of the invention, defined by the appended claims. In these examples, reference is made to the appended figures, in which:

[0118] FIGS. 1A and 1B illustrate, using SEM and at two different working distances, the dispersion of the CNTs within the masterbatch obtained in example 1;

[0119] FIGS. 2A and 2B illustrate, using SEM and at two different working distances, the dispersion of the CNTs within the masterbatch obtained in example 2;

[0120] FIG. 3 illustrates the discharge capacity of a battery containing a cathode obtained from the masterbatch according to the invention, as a function of the number of cycles; and

[0121] FIG. 4 illustrates the electrochemical performances of an electrode manufactured from the masterbatch according to the invention.

DETAILED DESCRIPTION AND EXAMPLES

Example 1

Preparation of a CNT/PVDF/NMP Masterbatch

[0122] A 5 wt % solution of PVDF (Kynar HSV 900 from ARKEMA) was produced previously by dissolving the powder of the polymer in N-methyl pyrrolidone (NMP); the solution was stirred at 50 C. for 60 min.

[0123] The CNTs (Graphistrength C100 from ARKEMA) were introduced into the first feed hopper of a BUSS MDK 46 (L/D=11) co-kneader, equipped with a discharge extrusion screw and a granulation device. The 5% solution of PVDF (Kynar HSV 900) in N-methyl pyrrolidone (NMP) was injected in liquid form at 80 C. into the first zone of the co-kneader. The temperature settings and the throughput within the co-kneader were the following: zone 1: 80 C., zone 2: 80 C., screw: 60 C., throughput: 15 kg/h.

[0124] At the outlet of the die, the masterbatch was cut into granules under dry conditions. The granules were packaged in an airtight container to avoid loss of NMP during storage. The composition of the final masterbatch was the following: 30 wt % of carbon nanotubes, 3.5 wt % of PVDF resin and 66.5 wt % of NMP.

[0125] Observations of the dried masterbatch using a scanning electron microscope (SEM) showed that the carbon nanotubes were well dispersed (FIGS. 1A and 1B).

Example 2

Use of the CNT/PVDF/NMP Masterbatch for Manufacturing an Electrode

[0126] Step a) 20 g of masterbatch granules from example 1 were wetted with 160 g of NMP solvent. After 2 h of static impregnation under ambient conditions, the masterbatch granules were dispersed in the solvent using a Silverson L4RT mixer at 6000 rpm for 15 minutes. A significant temperature rise was observed during the dispersion operation: the mixture containing the CNTs reached a temperature of 67 C. The solution obtained was denoted by CNT premix.

[0127] Step b) 14.3 g of Kynar HSV 900 were dissolved in 276 g of NMP solvent using a flocculator-type agitator for 4 hours.

[0128] Step c) 279 g of LiFePO.sub.4/C (LFP) (grade P1 from Phostech) powder were dispersed in the Kynar solution; during this step, the LiFePO.sub.4 powder was added gradually while stirring (600 rpm). The suspension obtained was denoted by LFP premix.

[0129] Step d) In order to obtain a good dispersion of the CNTs around the active LFP material, the two CNT and LFP premixes respectively obtained during steps a) and c) were mixed for 10 minutes using a flocculator agitator at 600 rpm then using a Silverson L4RT mixer for 15 minutes at 6000 rpm and finally using a Retsch Minicer ball mill for 30 minutes at 2000 rpm using 0.7 to 0.9 mm ceramic balls. The composition of the ink, as dry matter, was the following: 2% of CNTs; 5% of Kynar HSV 900 and 93% of LiFePO.sub.4/C with a solids content of 40% in the NMP solvent.

[0130] Step e) Using a Sheen film applicator and an adjustable BYK-Gardner applicator, un film with a thickness of 100 m was produced on a 25 m aluminum foil.

[0131] Step f) The film produced during step e) was dried at 70 C. for 4 h in a ventilated oven then compressed under 200 bar.

[0132] SEM observations showed that the CNTs are well dispersed around the micron-sized particles of LiFePO.sub.4/C (FIGS. 2A and 2B).

Example 3

Evaluation of the Electrochemical Performances of an Electrode According to the Invention

[0133] The CEA/LITEN laboratories in Grenoble evaluated the electrochemical performances of the positive electrode (cathode) from example 2 by combining it with a graphite anode.

[0134] The formulation of the cathode containing 2 wt % of CNTs and 5 wt % of PVDF binder was compared to a standard formulation containing, as conductive additive, 2.5 wt % of Super P carbon black from Timcal (CB) and 2.5 wt % of VGCF carbon fibers from Showa Denko (CF) with 5 wt % of PVDF binder. This standard formulation is obtained by mixing of powders, without going through the preparation, then the dilution, of a masterbatch according to the invention.

[0135] On an Li-ion battery having a capacity of 500 mAh, the results obtained under various charge/discharge regimes 1C, 2C, 3C, 5C and 10C (cf. FIG. 4) show that greater capacities are obtained with the battery containing 2 wt % of CNTs in the cathode compared to the standard battery (CB+CF), even more so when the charge/discharge regime is faster (10C). This example therefore illustrates the better electrochemical performances of the electrode obtained from a masterbatch according to the invention.

Example 4

Preparation of a CNT/PVDF/NMP Masterbatch

[0136] A 5 wt % solution of PVDF (Kynar HSV 900 from ARKEMA) was produced previously by dissolving the powder of the polymer in N-methyl pyrrolidone (NMP); the solution was stirred at 50 C. for 60 min.

[0137] The CNTs (Graphistrength C100 from ARKEMA) were introduced into the first feed hopper of a BUSS MDK 46 (L/D=11) co-kneader, equipped with a discharge extrusion screw and a granulation device. The 5% solution of PVDF (Kynar HSV 900) in N-methyl pyrrolidone (NMP) was injected in liquid form at 80 C. into the first zone of the co-kneader. The temperature settings and the throughput within the co-kneader were the following: zone 1: 80 C., zone 2: 80 C., screw: 60 C., throughput: 15 kg/h.

[0138] At the outlet of the die, the masterbatch was cut into granules under dry conditions. The granules were packaged in an airtight container to avoid loss of NMP during storage. The composition of the final masterbatch was the following: 25 wt % of carbon nanotubes, 4 wt % of PVDF resin and 71 wt % of NMP.

[0139] Observations of the dried masterbatch using a scanning electron microscope (SEM) showed that the carbon nanotubes were well dispersed.

Example 5

Study of the Stability of the Batteries Obtained from the Masterbatch According to the Invention

[0140] The stability of the batteries was studied using, as cathode conductive additive, raw CNTs that contain between 2 and 3% of Fe. In order to do this, aging tests at 55 C. were carried out by the CEA/LITEN laboratories on 25 mAh Pouch cell batteries comprising a cathode with 93 wt % of the LiNi1/3Co1/3Al1/3O2(NCA) active material with no iron and 2 wt % of raw CNTs and 5 wt % of PVDF binder combined with a graphite anode. After 100 cycles at 55 C. with a charge/discharge rate of C/5, the discharge capacity drops by 20% but ICP chemical analysis of the anode does not show an increase in the iron content which remains equal to 3 ppm. There is not therefore any migration of the iron contained in the CNTs of the cathode to the anode (cf. FIG. 3).

Example 6

Preparation of a CNT/CMC/Water Masterbatch

[0141] A 10 wt % solution of low-weight carboxymethyl cellulose (CMC) (Finnfix 2 grade) was produced previously by dissolving the powder of the CMC polymer in demineralized water. The solution was stirred at ambient temperature for 60 min.

[0142] The CNTs (Graphistrength C100 from ARKEMA) were introduced into the first feed hopper of a BUSS MDK 46 (L/D=11) co-kneader, equipped with a discharge extrusion screw and a granulation device. The 10% solution of CMC in demineralized water was injected in liquid form at 30 C. into the first zone of the co-kneader. The balance of the CMC (22 wt %) was introduced in powder form into the first feed hopper. The temperature settings and the throughput within the co-kneader were the following: zone 1: 30 C., zone 2: 30 C., screw: 30 C., throughput: 15 kg/h.

[0143] At the outlet of the die, the masterbatch was cut into granules under dry conditions. The granules were dried in an oven at 80 C. for 6 hours to remove the water. The composition of the final masterbatch was the following: 40 wt % of carbon nanotubes, 60 wt % of CMC.

[0144] The granules were packaged in an airtight container to avoid uptake of water during storage.

Example 7

Preparation of a CNT/CMC/Water Masterbatch

[0145] A 10 wt % solution of low-weight carboxymethyl cellulose (CMC) (Finnfix 2 grade) was produced previously by dissolving the powder of the CMC polymer in demineralized water. The solution was stirred at ambient temperature for 60 min.

[0146] 20 kg of CNTs (Graphistrength C100 from ARKEMA) were introduced into the first feed hopper of a BUSS MDK 46 (L/D=11) co-kneader, equipped with a discharge extrusion screw and a granulation device. 61.1 kg of 10% solution of CMC in demineralized water were injected in liquid form at 30 C. into the first zone of the co-kneader. The balance of the CMC (18.9 kg) was introduced in the form of powder into the first feed hopper. The temperature settings and the throughput within the co-kneader were the following: zone 1: 30 C., zone 2: 30 C., screw: 30 C., throughput: 15 kg/h.

[0147] The composition of the mixture exiting the die was the following: 20% CNTs/25% CMC and 55% water.

[0148] At the outlet of the die, the masterbatch was cut into granules under dry conditions. The granules were dried in an oven at 80 C. for 6 hours to remove the water. The composition of the final masterbatch was the following: 45 wt % of carbon nanotubes, 55 wt % of CMC. The granules were packaged in an airtight container to avoid uptake of water during storage.

Example 8

Dispersion of a CNT/CMC Masterbatch in Water

[0149] The dried masterbatch obtained in example 7 is introduced into hot water at 90 C. with gentle stirring so as to obtain a nanotube concentration of 2 wt %. The stirring is continued for 1 hour, which results in a gradual cooling of the dispersion.

[0150] Under these conditions, an effective dispersion of the nanotubes in water is obtained. Such a dispersion may be used for example as an aqueous formulation base for the manufacture of an electrode or of paints.

Example 9

Preparation of a Masterbatch Based on Carbon Nanofibers

[0151] A 5 wt % solution of PVDF (Kynar HSV 900 from ARKEMA) was produced by dissolving the powder of the polymer in N-methyl pyrrolidone (NMP); the solution was stirred at 50 C. for 60 min.

[0152] Carbon nanofibers (VGCF from SHOWA DENKO) were introduced into the first feed hopper of a BUSS MDK 46 (L/D=11) co-kneader, equipped with a discharge extrusion screw and a granulation device. The 5% solution of PVDF (Kynar HSV 900) in N-methyl pyrrolidone (NMP) was injected in liquid form at 80 C. into the first zone of the co-kneader. The temperature settings and the throughput within the co-kneader were the following: zone 1: 80 C., zone 2: 80 C., screw: 60 C., throughput: 15 kg/h.

[0153] At the outlet of the die, the masterbatch was cut into granules under dry conditions. The granules were packaged in an airtight container to avoid loss of NMP during storage. The composition of the final masterbatch was the following: 25 wt % of nanofibers, 3.75 wt % of PVDF resin and 71.25 wt % of NMP.

Example 10

Preparation of a Masterbatch Based on Carbon Black

[0154] A 5 wt % solution of PVDF (Kynar HSV 900 from ARKEMA) was produced by dissolving the powder of the polymer in N-methyl pyrrolidone (NMP); the solution was stirred at 50 C. for 60 min.

[0155] Carbon black (Super P from TIMCAL) was introduced into the first feed hopper of a BUSS MDK 46 (L/D=11) co-kneader, equipped with a discharge extrusion screw and a granulation device. The 5% solution of PVDF (Kynar HSV 900) in N-methyl pyrrolidone (NMP) was injected in liquid form at 80 C. into the first zone of the co-kneader. The temperature settings and the throughput within the co-kneader were the following: zone 1: 80 C., zone 2: 80 C., screw: 60 C., throughput: 15 kg/h.

[0156] At the outlet of the die, the masterbatch was cut into granules under dry conditions. The granules were packaged in an airtight container to avoid loss of NMP during storage. The composition of the final masterbatch was the following: 25 wt % of carbon black, 3.75 wt % of PVDF resin and 71.25 wt % of NMP.

Example 11

Use of the CNT/CMC Masterbatch for Manufacturing a Conductive Material for Electrodes

[0157] Preliminary step) The electrode active material LiFePO.sub.4 was synthesized according to the procedure described in the example 1 of patent FR 2 848 549. 5 g of the iron (III) nitrilotriacetic complex were introduced into an autoclave reactor in 800 ml of a 0.0256 mol/l solution of lithium hydrogen phosphate, Li.sub.2HPO.sub.4. The reaction was carried out at 200 C. under an autogenous pressure of 20 bar for 2 hours. The mixture was cooled slowly, without stirring, via inertia of the reactor (over around 12 hours). When the reactor had returned to ambient temperature and to atmospheric pressure, the autoclave was opened and the powder recovered was filtered over a Bchner flask. The cake obtained was washed with deionized water, then suction-filtered.

[0158] Step a) The suction-filtered cake comprising the electrode active material LiFePO.sub.4 preliminarily prepared was put into suspension in 100 ml of water in the filter using a flocculator-type agitator.

[0159] Step b) 134 mg of the CNT/CMC concentrated masterbatch obtained according to example 7 (consisting of 45 wt % of carbon nanotubes and of 55 wt % of CMC) were dispersed in the suspension prepared in step a).

[0160] Step c) After suction-filtering the cake, the LiFePO.sub.4/CNT composite active material is dried at 60 C. under vacuum.

[0161] An LiFePO.sub.4/CNT conductive material for an electrode containing 3 wt % of CNTs was obtained. It was observed that the CNTs were advantageously well distributed at the surface of the LiFePO.sub.4 particles. CMC is compatible with applications in the batteries.

Embodiments

[0162] 1. A masterbatch in agglomerated solid form comprising: [0163] a) carbon nanofibers and/or nanotubes and/or carbon black, the content of which is between 15 wt % and 40 wt %, preferably between 20 wt % and 35 wt %, relative to the total weight of the masterbatch; [0164] b) at least one solvent; [0165] c) at least one polymer binder, which represents from 1 wt % to 40 wt %, preferably from 2 wt % to 30 wt % relative to the total weight of the masterbatch.

[0166] 2. The masterbatch as in embodiment 1, characterized in that said solvent is an organic solvent, water or mixtures thereof in any proportions.

[0167] 3. The masterbatch as in embodiment 2, characterized in that said organic solvent is chosen from N-methyl pyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), ketones, acetates, furans, alkyl carbonates, alcohols and mixtures thereof.

[0168] 4. The masterbatch as in one of embodiments 1 to 3, characterized in that said polymer binder is chosen from the group consisting of polysaccharides, modified polysaccharides, polyethers, polyesters, acrylic polymers, polycarbonates, polyimines, poly-amides, polyacrylamides, polyurethanes, polyepoxides, polyphosphazenes, polysulfones, halogenated polymers, natural rubbers, functionalized or unfunctionalized elastomers, especially elastomers based on styrene, butadiene and/or isoprene, and mixtures thereof.

[0169] 5. The masterbatch as in embodiment 4, characterized in that the polymer binder is chosen from the group consisting of halogenated polymers and preferably from fluoropolymers.

[0170] 6. The masterbatch as in embodiment 5, characterized in that the fluoropolymer binder is chosen from: [0171] (i) those comprising at least 50 mol % of at least one monomer of formula (I):

CFX.sub.1CX.sub.2X.sub.3 (I) [0172] where X.sub.1, X.sub.2 and X.sub.3 independently denote a hydrogen atom or halogen atom (in particular a fluorine or chlorine atom), such as polyvinylidene fluoride (PVDF), preferably in a form, polytrifluoroethylene (PVF3), polytetrafluoroethylene (PTFE), copolymers of vinylidene fluoride with either hexafluoropropylene (HFP), or trifluoroethylene (VF3), or tetrafluoroethylene (TFE), or chlorotrifluoroethylene (CTFE), fluoroethylene/propylene (FEP) copolymers, copolymers of ethylene with either fluoroethylene/propylene (FEP), or tetrafluoroethylene (TFE), or chlorotrifluoroethylene (CTFE); [0173] (ii) those comprising at least 50 mol % of at least one monomer of formula (II):

ROCHCH.sub.2 (II) [0174] where R denotes a perhalogenated (in particular perfluorinated) alkyl radical, such as perfluoropropyl vinyl ether (PPVE), perfluoroethyl vinyl ether (PEVE) and copolymers of ethylene with perfluoromethyl vinyl ether (PMVE).

[0175] 7. The masterbatch as in embodiment 4, characterized in that the polymer binder is chosen from the group consisting of modified polysaccharides and more preferably from modified celluloses such as carboxymethyl cellulose.

[0176] 8. The masterbatch as in any one of embodiments 1 to 6, characterized in that it contains: from 20 to 30 wt % of carbon nanotubes, from 2 to 5 wt % of PVDF resin and from 65 to 75 wt % of NMP.

[0177] 9. The use of the masterbatch as in one of embodiments 1 to 8 for manufacturing an electrode.

[0178] 10. A process for preparing a masterbatch as in one of embodiments 1 to 8 comprising: [0179] (a) dissolving a powder of the polymer binder in the solvent to form a solution; [0180] (b) mixing said solution with the carbon nanofibers and/or nanotubes and/or the carbon black, in a compounding device; [0181] (c) kneading said mixture.

[0182] 11. The process as in embodiment 10, characterized in that the kneading is carried out via a compounding route using a co-rotating or counter-rotating twin-screw extruder or using a co-kneader.

[0183] 12. A concentrated masterbatch, characterized in that it is obtained by removing all or part of the solvent from the masterbatch as in any one of embodiments 1 to 8.

[0184] 13. The concentrated masterbatch as in embodiment 12, characterized in that it contains from 20 to 98%, preferably from 25 to 60%, or even from 40 to 60% in the case of an aqueous solvent or from 60 to 95% in the case of an organic solvent, by weight of carbon-based fillers, and advantageously a binder/carbon-based filler weight ratio of less than 2, or even of less than 1.6.

[0185] 14. A process for preparing an electrode, comprising the following steps: [0186] a) the preparation of a mixture by dispersion in a dispersion solvent of the masterbatch as in one of embodiments 1 to 8 or 12 to 13, or obtained according to the process as in either of embodiments 10 and 11, containing at least a first binder and optionally at least a first solvent; [0187] b) the preparation of a solution by dissolving at least a second polymer binder in at least a second solvent; [0188] c) the addition of an electrode active material to said solution; [0189] d) the mixing of the products resulting from steps a) and c); [0190] e) the deposition of the composition thus obtained on a substrate in order to form a film; [0191] f) the drying of said film.

[0192] 15. An electrode capable of being obtained according to the process as in embodiment 14.

[0193] 16. A process for preparing a composite active material for an electrode, comprising the following steps: [0194] a) the provision of an electrode active material in the form of an aqueous solution or dispersion; [0195] b) the addition and mixing of the masterbatch as in one of embodiments 1 to 8 or 12 to 13, or obtained according to the process as in either of embodiments 10 and 11, said masterbatch containing a water-soluble or water-dispersible binder, to the aqueous solution or dispersion obtained in step (a); [0196] c) the suction-filtering and drying of the mixture obtained in step (b).

[0197] 17. A composite active material for an electrode capable of being obtained according to the process as in embodiment 16.

[0198] 18. The use of the masterbatch as in any one of embodiments 1 to 8, 12 and 13 for the preparation of liquid formulations containing carbon nanofibers and/or nanotubes and/or carbon black.