Structure comprising a fluoro primer and electrode based on this structure

Especially for electrodes, there is provided a structure successively comprising a layer of a metal L1, a fluoro primer L2 and a layer of a fluoro polymer L3 in which the flouro primer L2 is derived from a fluoro polymer chemically modified by a partial dehydrofluorination with a base followed by an oxidation step, especially with H 2 O 2 . According to one specific form, the structure is an electrode of a lithium-ion battery in which the metal L1 is the collector and the fluoro polymer L3, which has a high content of carbon and/or oxides, is the electroactive layer thereof.

EXAMPLE 1 
 Preparation of a Chemically Modified Polyvinylidene Fluoride In this example, the fluoro polymer used as starting material is a polyvinylidene fluoride (PVDF) latex prepared according to the emulsion process as described in U.S. Pat. No. 4,025,709. After drying at 105° C. for 24 hours, this latex gives a dry powder. This powder, when melted, has a flow index of 0.6 to 1 g/10 min at 230° C. under 10 kg. This latex, referred to as Latex 1 hereinbelow, contains 40% by weight of PVDF. The process according to the present invention can, however, be applied in particular to any PVDF latex or VF 2 copolymer obtained by an emulsion process or to any suspension of PVDF or VF 2 copolymer obtained by a suspension process. Dehydrofluorination Step To begin with, 7.2 kg of an aqueous sodium hydroxide solution containing 15% by weight of NaOH in water is prepared in a stirred 20 liter reactor. This solution is brought to 70° C. and 7.2 kg of Latex 1, optionally diluted in deionized water so as to have a given solids content, are then added thereto at a rate of 0.72 kg/min with stirring at 180 rpm. A brown-coloured coagulated emulsion is thus obtained, which turns even darker the further the degradation proceeds. Depending on the duration of the dehydrofluorination reaction, a fine black powder is obtained which gradually becomes insoluble in the usual organic solvents, in particular dimethylformamide (DMF) or N-methylpyrrolidone (NMP). Step of Reacting With an Oxidizing Agent The reaction medium, still stirred and maintained at a temperature of 70° C., is acidified to pH&equals;5 by adding about 2.5 kg of hydrochloric acid at a concentration of 36% by weight. 1.68 kg of hydrogen peroxide at a concentration of 35% by weight are then added at a rate of 0.4 kg/min, and the pH is then increased to a value of between 6.6 and 7.6 by adding a sodium hydroxide solution containing 15% by weight of NaOH. The mixture is left to react while maintaining the pH between 6.6 and 7.6 by addition of the same sodium hydroxide solution. A gradual decolorization of the coagulated emulsion is observed, which turns pale yellow to ochre. Finishing The solid coagulate in suspension is filtered off to give a pale yellow powder which is washed with three dispersions in 20 liters of water with stirring and successive filtrations. A powder is thus obtained which is dried in an oven at 105° C. to constant weight. Characterization The characterization of the product of this powder is carried out by measuring the absorbance at 300 mm which is obtained by analysis with a Perkin-Elmer LC-75 spectrophotometer using a concentration of 0.1% by weight of product in NMP. The dissolution time before carrying out the measurements is 24 hours. 
 EXAMPLE 2 
 Preparation of a Polyvinylidene Fluoride Homopolymer and of Two Copolymers of Polyvinylidene Fluoride and of Hexafluoropropene (HFP) that are Chemically Modified These tests are carried out in a similar manner to that of Example 1 under the following experimental conditions and with the following analytical results: 1 Absorbence Solids Oxidation at 300 nm in % by mass content of Degradation treatment solution in Sample Starting latex of HFP Melt index the latex time (min) time (mm) NMP A Kynar ® 50 0 0.6-1( 1 ) 42% 30 150 0.19 B Kynar ® 50 0 0.6-1( 1 ) 42% 60 150 0.206 C Kynar ® 50 0 0.6-1( 1 ) 42% 90 200 0.262 D Kynarflex ® 2800 11 0.8-1( 2 ) 11% 230 75 0.154 E Kynarflex ® 2750 15 3-4( 2 ) 20% 250 75 0.174 ( 1 )at 230° C. under 10 Kg ( 2 )at 230° C. under 5 Kg 
 EXAMPLE 3 
 Preparation of a PVDF Coating 1 to 2 &mgr;m Thick on a Metal Foil 3 g of polyvinylidene fluoride are dissolved in 97 g of N-methyl-2-pyrrolidone (abbreviated to NMP hereinbelow, from Merck, purity>99%) with magnetic stirring at 55° C. for at least 30 minutes (up to 4 h for grades which are difficult to dissolve). Once cooled, this solution is applied with a brush or a cloth to the metal foil (copper for the negative electrode or aluminium for the positive electrode) and the solvent is then evaporated off at 120° C. for 10 minutes. The thickness of the layer of PVDF thus formed on the metal ranges between 1 and 2 &mgr;m. 
 EXAMPLE 4 
 Preparation of a PVDF Coating on an Aluminium Foil Coated or not Coated With a Primer 10 g of polyvinylidene fluoride are dissolved in 90 g of NMP with magnetic stirring at 55° C. for at least 30 minutes (up to 4 h for grades which are difficult to dissolve). Once cooled, this solution is spread onto an aluminium foil 20 &mgr;m thick, coated or not coated with a primer, and a film is then formed by means of a manual Doctor Blade scraper. The film is dried at 120° C. in a ventilated oven for 30 minutes. The layer of PVDF thus formed on the metal is about 40 &mgr;m thick. 
 EXAMPLE 5 
 Preparation of a Solution for Forming a Negative Electrode for an Li-ion Battery 5 g of polyvinylidene fluoride are dissolved in 85 g of NMP with magnetic stirring at 55° C. for at least 30 minutes (up to 4 h for grades which are difficult to dissolve). 45 g MCMB 6-28 graphite powder with an average particle size of 6 &mgr;m obtained from Osaka Gaz, are added to this solution. These powders are dispersed in the solution by magnetic stirring at room temperature for 30 minutes, and then for 3 minutes in a Dispermat brand multi-paddle turbomixer at high stirring speed (2000 rpm). 
 EXAMPLE 6 
 Preparation of a Negative Electrode for an Li-ion Battery The solution of Example 5 is spread on a copper foil 20 &mgr;m thick and a film is then formed by means of a manual Doctor Blade scraper set at 400 &mgr;m. The film is dried at 90° C. in a ventilated oven for 15 minutes, and then at 140° C. under vacuum overnight. The conductive layer thus formed on the copper foil is thus composed of 10% by weight of polyvinylidene fluoride and 90% of graphite. Its density, or “weight per unit area”, is about 12 g/cm 2 and its average thickness is 120 &mgr;m. 
 Example 7 
 Preparation of a Solution to Form a Positive Electrode for an Li-ion Battery 3 g of polyvinylidene fluoride are dissolved in 62 g of NMP with magnetic stirring at 55° C. for at least 30 minutes (up to 4 h for grades which are difficult to dissolve). 1.5 g of conductive carbon black powder of acetylene type (Denka Black) and 45.5 g of LiCoO 2 powder with an average particle size of 5 &mgr;m, obtained from Union Minière, are added to this solution. These powders are dispersed in the solution by magnetic stirring at room temperature for 30 minutes, and then for 3 minutes in a Dispermat brand multi-paddle turbomixer at high stirring speed (2000 rpm). 
 Example 8 
 Preparation of a Positive Electrode for an Li-ion Battery The solution of Example 7 is spread on an aluminium foil 20 &mgr;m thick and a film is then formed by means of a manual Doctor Blade scraper set at 350 &mgr;m. The film is dried at 90° C. in a ventilated oven for 15 minutes, and then at 140° C. under vacuum overnight. The conductive layer thus formed on the aluminium foil is thus composed of 6% by weight of polyvinylidene fluoride, 3% of conductive carbon black and 91% of LiCoO 2 . Its density, or “weight per unit area”, is about 1.9 g/cm 2 and its average thickness is 120 &mgr;m. 
 EXAMPLE 9 
 Determination of the Adhesion Properties Between the Conductive Layer and the Metal Foil Strips 25 mm wide and at least 10 cm long are cut out in the assembly described in Example 4, Example 6 or Example 8 and then attached to a rigid metal support by means of a double-sided adhesive tape (of brand TESA, reference &num;4970) of the same width onto the conductive layer side. The assembly is pressed against the support by simply pressing by hand. The rigid metal support&plus;double-sided adhesive tape&plus;conductive layer&plus;metal foil is referred to as the “peeling test piece”. The peeling test pieces are then installed on a DY30 dynamometer distributed by Adamel Lhomargy. The metal support is kept fixed. Rupture between the conductive layer and the aluminium foil is initiated either by hand or using a razor blade. The free part of the aluminium foil is fixed to a mobile jaw and is then pulled at 180° at a pulling speed of 100 mm/min. The instantaneous tensile force is determined by means of a force cell used of 10 N. The average value of this force during the peeling between the metal foil and the conductive layer is referred to as the “peeling force”. 
 EXAMPLE 10 
 Comparison of a PVDF Coating With or Without Primer Kynar® 761 sold by Elf Atochem is used to form a coating according to Example 4 on an aluminium foil, which is uncoated or coated with a primer of the chemically modified PVDF homopolymer “A” of Example 2 according to Example 4. If the aluminium foil is not coated with a primer, the Kynar® 761 does not adhere to the aluminium. If the aluminium foil is coated with a primer of the chemically modified PVDF homopolymer “A” of Example 2, the Kynar® 761 adheres to the aluminium, and using the process described in Example 9, the peeling force between the Kynar® 761 coating and the aluminium foil can be measured as 0.22 N/25 mm with a standard deviation of 0.06 N/25 mm. 
 Example 11 
 Comparison of a Negative Electrode With or Without Primer Kynar®761 sold by Elf Atochem is used to form a negative electrode according to Example 5 and Example 6, which is uncoated or coated with a primer of chemically modified PVDF homopolymers “A”, “B” and “C” of Example 2 according to Example 4, or with a primer of the chemically modified PVDF/HPF copolymers “D” and “E” of Example 2 according to Example 4. The chemically modified PVDF homopolymer “A” of Example 2 or the PVDF homopolymer KF-1300 from Kureha (market “standard”) is also used to form a negative electrode according to Example 5 and Example 6 and to compare them with the previous ones. Using the process described in Example 9, the peeling force between the conductive layer and the aluminium foil can be measured, and the results are collated in the table below: 2 Peeling force Binder used Primer (g/25 mm) Kynar ® 761 no 55 Kynar ® 761 A 110 Kynar ® 761 B 200 Kynar ® 761 C 200 Kynar ® 761 D 170 Kynar ® 761 E 200 A no 220 KF-1300 no 140 Thus, a PVDF homopolymer which is a binder of medium quality, such as Kynar® 761, can be used in the manufacture of a negative electrode together with a primer for a chemically modified PVDF homopolymer or for a chemically modified PVDF/HFP copolymer to form an electrode of good quality, as is evidenced by the comparison with the electrodes manufactured with KF-1300 or the chemically modified PVDF homopolymer “A”. 
 EXAMPLE 12 
 Comparison of a Positive Electrode With or Without Primer Kynar® 761 sold by Elf Atochem is used to form a positive electrode according to Example 7 and Example 8, which is uncoated or coated with a primer for the chemically modified PVDF homopolymers “A”, “B” and “C” of Example 2 according to Example 4. The chemically modified PVDF homopolymer “A” of Example 2 or the PVDF homopolymer KF-1300 from Kureha (market “standard”) is also used to form a negative electrode according to Example 7 and Example 8 and to compare them with the previous ones. Using the process described in Example 9, the peeling force between the conductive layer and the aluminium foil can be measured, and the results are collated in the table below: 3 Peeling force Binder used Primer (g/25 mm) Kynar ® 761 no 25 Kynar ® 761 A 430 Kynar ® 761 B 450 Kynar ® 761 C 400 A no 340 KF-1300 no 60 Thus, a PVDF homopolymer which is a binder of medium quality, such as Kynar® 761, can be used in the manufacture of a positive electrode together with a primer for a chemically modified PVDF homopolymer to form an electrode of good quality, as is evidenced by the comparison with the electrodes manufactured with KF-1300 or the chemically modified PVDF homopolymer “A”. The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. Also, the preceding specific embodiments are to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding French application 00/04.201, are hereby incorporated by reference. From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.