CA2304395C - Coating for lithium cell battery current collector - Google Patents

Abstract

The present invention relates a protective coating of current applied to the metal collector of an electrode. The protective coating comprises a mineral binder glassy or partially and optionally a additive of electronic conduction. This coating is applied form of a solution or dispersion on the collector of the electrode, and dried, so as to cover and protect at least one part of the surface of the metal of the collector to prevent the formation of insulating films generated by the active species from other components of the generator.

Description

TITLE

Coating of current collectors for lithium accumulator FIELD OF THE INVENTION

The present invention relates to an applied current protective coating on the metal collector of an electrode. The protective coating includes a binder partially or totally vitreous mineral and optionally an additive of conduction electronic. This coating is applied as a solution or dispersion on the collector of the electrode, and dried, so as to cover and protect at least a part of the metal surface of the collector to avoid the formation of insulating films generated by active species from other components of the generator.

PRIOR ART

In principle, lithium batteries offer the unique advantage of achieving unsurpassed specific and specific energies that make them particularly interesting for a wide range of stationary or mobile applications that go from the microelectronics, from portable electronics to large installations for the electric or hybrid electric vehicles.

These systems typically use liquid aprotic electrolytes mainly lithium-ion type and more recently electrolytes polymers, these the last being either of the dry solvating polymer type, operating between 40 and 100 C with lithium anodes, either of gelled type, using a solvating polymer or no, and operating at room temperature thanks to the addition of liquid solvents aprotic

-2-in combination with lithium-ion electrodes of which the cathodes operate at high voltages (- 4V). The extreme conditions of temperature or high operating voltage specific to these systems tend to make to age different components of the generator to cycling and / or as a function of time.

The aging of the components is particularly reflected in the current collectors and results in the formation of passivation films or degradation on the surface of the latter by reaction with the generator components, the occurrence the active materials of the electrodes and the chemical components of the organic electrolyte. The formation of such films more or less insulating interfaces greatly alters the quality of electronic exchanges between collectors and the active electrode materials, which are generally present in the form of composites.

In a polymer electrolyte medium, the effect of the passivation phenomena is sometimes magnified because in the solid state, the products formed by the solvent reactions organic, lithium salt, electrode materials or from more Generator components tend to accumulate at the interface due to lack of convection solvent or solubilization of the formed films, or for lack of reaction of corrosion / dissolution of the metal and renewal of the exchange surface.
The attack collectors or the formation of surface passivation films by oxidation / dissolution of the metallic conductor is usually caused by reactions electrochemical, in particular oxidation or reduction, initiated by radicals, acid-base reactions or chemical reactions of oxidation-reduction more or less catalyzed by the materials involved. Figures la) and lb) illustrate a

-3-collector / composite electrode assembly and the film location of passivation to the collector / electrode interface after cycling.

Passivation phenomena are particularly evident in the case of aluminum collectors frequently used for their low cost and for their properties of thermal and electrical conduction with cathodes associated with end voltages of charge frequently above 3 and even 4 volts.

In lithium-ion type systems using liquid electrolytes or of the gelled polymers with liquids, corrosion of aluminum from collector of the cathode is generally avoided by the use of a salt or a fluorinated additive type LiBF4 and LiPF6 which easily form a fluorinated film on the surface of aluminum, or again by the oxidizing anion C104, thus avoiding deep corrosion or dissolution of the manifold aluminum. With other particularly stable fluorinated salts, such as TFSI's formula (CF3SO2) 2NLi corrosion of aluminum above 4 volts can lead to the complete disintegration of the collector.

In dry polymer medium, the formation of passivation films on the collector of aluminum of a cathode based on vanadium oxide such as V205, does not not to dissolution of the collector, but rather to the formation of passivation films more or less insulators that increase the electrical resistance between the collector and the composite cathode.
We then observe the formation of oxidation films of aluminum based on of oxygen and of fluorine, visible in electron microscopy, reaching thicknesses higher to the alumina films initially present on the surface of aluminum. Such movies are

-4-more or less electrical insulators and therefore harm the passage of electrons between the collector and active materials and electronic conduction present in the cathode.
A known way for a long time to protect metal collectors from current of electrochemical accumulators of the phenomena of passivation / dissolution is to cover them with an electronically conductive carbon coating little oxidizable. Generally, dispersions of carbon blacks in binders organic or minerals are used in the form of a more or less water-resistant basecoat at the generator electrolyte so as to avoid corrosion phenomena Electrochemical. In addition, these coatings avoid direct contact with collector with the active materials of the electrode (see for example US Pat. No. 5,262,264).
These Solutions are used successfully in several commercial applications.
However, none of them is perfectly satisfactory, particularly when the Electrochemical generators are used in extreme conditions previously described over a long period of time, in particular because of the lack of impermeability and chemical or electrochemical stability of binders organic, or further metal conduction additives or conjugated polymers.

U.S. Patent 5,580,686 (Fauteux et al.) Discloses a protective coating ("primer") based on carbon dispersed in a metal polysilicate used in lithium-ion type electrolytic cell comprising a cathode oxide cobalt and a graphite anode. Polysilicates have several limitations because of of their strong basicity. Thus, they are reactive vis-à-vis active electrode materials acids such as vanadium oxide. In addition, they are chemically reactive with type materials

-5-iron phosphate. Their basic character also makes them incompatible with additives of conduction consisting of conjugated polymers of polyaniline type, doped polypyrole etc.

In most of these applications carbon is the generally preferred additive at because of its high chemical inertness and its resistance to corrosion electrochemical.
SUMMARY OF THE INVENTION

The present invention relates to an electrochemical generator comprising a separating electrolyte between two electrodes including at least one of the electrodes uses a metal current collector partially or completely covered with a coating protective conductor, chemically compatible with the electrode material adjacent, said coating comprising a vitreous or partially vitreous mineral binder in which is optionally dispersed an electronic conduction additive ensuring exchanges between the electrode and the collector, said coating being in touch sealed with said collector to protect the covered metal surface of the formation of passivation films generated by the reactive species from generator components.

In a second aspect of the invention, there is a coating method partial or total of a current protector on the metal collector of a electrode, the process comprising:

a) the preparation of an aqueous solution dispersion of a mineral binder partially or totally glassy that is neutralized so as to ensure the compatibility of the binding with the

-6-active material of the electrode and in which it is optionally dispersed minus one electronic conduction additive;

b) coating the mixture obtained in step a) on the metal of the collector and drying of way to cover and protect at least a part of the surface of the metal so to avoid the formation of passivation films generated by the active species from others generator components.

In a third aspect of the invention, there is an electrode comprising a metal current collector partially or completely covered by a coating protective conductor, chemically compatible with the electrode material adjacent, said coating comprising a vitreous or partially vitreous mineral binder in which is optionally dispersed an electronic conduction additive ensuring exchanges between the electrode and the collector, said coating being in touch sealed with said collector to protect the covered metal surface of the formation of passivation films generated by the reactive species from generator components.

IN THE DRAWINGS

Figures la) and 1 b) illustrate a collector / composite electrode assembly and the location of the passivation film at the collector / electrode interface after cycling.

Figures 2a) and 2b) illustrate an electrochemical generator comprising a collectionizr whose surface has been coated with a coating in accordance with this invention.

Figures 3 and 4 illustrate the cycling results of the generator electrochemical described in Example 3 in the presence of a protective coating of

-7-collector based on (LiPO3) r, and carbon black (Figure 3) and in the absence of such coating (Figure 4).

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a vitreous mineral binder or partially vitreous, based on phosphate, polyphosphate, borate, or polyborate metals alkalis such as lithium or potassium alone or in the form of mixtures, in which optionally dispersing at least one electronic conduction additive.
The Phosphate compounds represent a preferred embodiment for wet and thus protect metal collectors used in generators Electrochemical. In addition, these compounds in solution in water allow control the pH and avoid acid-base reactions between the mineral binder and the additives or materials of electrodes during the implementation of the films and the use of the generator electrochemical. Glass-forming additives such as silica hydrolysed, the Siloxanes, aluminates, organometallic titanates, partially or totally hydrolyzed are included in the present invention as long as they remain chemically compatible with conduction additives and with the active materials of the electrode, that is, as long as one can control their acid-base properties so as to avoid chemical reactions harmful to the operation of the generator. The additive former of glass is preferably added as an aqueous solution or alcoholic in the solution of the mineral binder before neutralization, which is preferably performed at a pH
varying between 4 and 9.

-8-Various electronic conduction additives are possible with binders of the present invention. Among these are carbon blacks, graphites, metals such as copper and silver, conductive mineral compounds Metallic type carbides, nitrides, silicides or even metal chalcogenides or still the doped conjugated polymers such as polyanilines, polypyrols etc., and their mixtures. Carbon blacks and graphite are the conduction additives electronic and may be added in varying concentrations preferably between 2 and 20% by volume. These additives are preferably added in the form dispersed in the pure mineral binder or mixed in order to induce a conductivity essential electronics for the maintenance of electron exchanges between the substratum metal and the active materials of the electrode. Non-carbon additives are particularly sought after in high power devices or the level of Required conductivity is well above 1 S / cm.

The important property sought for the mineral binder is its ability to wet and protect the metal surface of the current collector, so to prevent access of the generator components to the surface of the generator, in case the electrolyte, the active materials of the electrode etc. The protected area not being subject to corrosion / passivation reactions, and optionally having a additive conduction, then it is sufficient to maintain a permanent electronic contact between the metal contacted and the conductive components of the adjacent electrode.

In particular in a dry polymer medium, where the complete dissolution of the collector is not observed, it is not necessary to cover the whole of the metal surface of

-9 collection, as long as only the non-covered surface will eventually be passivated without preventing electronic exchanges at protected areas.
This characteristic of the invention thus makes it possible to optimize the level of binder mineral required from to leave sufficient residual porosity to allow at least one anchorage surface of the composite electrode on the conductive coating of protection.

The desired thicknesses for the coatings of the present invention will on the order of a few microns to minimize the weight or dead volume of the recovery relative to the active materials of the generator.
Preferably, the thickness of the generator is less than 10 micrometers, and more preferably less than 4 micrometers.

An optional way to realize the invention to optimize the content is to use all or part of a conductive additive electronic which is also an active material of the electrode, in which case we will use a material finely ground, ie less than a micrometer, to reduce the stress generated by the volume variations of the electrode material, and select materials of which the variations in volume during cycling are less than 10%, and preferably less than 5%. Even if the preferential rate of mineral binder must be between 15 and 95%, in this embodiment of the invention, the level of inorganic binder will be as high as possible, preferably above 30%, in order to preserve mechanical integrity and the role of glass binder despite the discharge / charge cycles and variations of volume.

-10-A particularly preferred embodiment of the invention consists of use a lithium polyphosphate of general formula (LiPO3) ,,, which can to be prepared in aqueous solution from a neutralized solution of the acid (HPO3), preferentially between pH 4 and 9, with a lithium salt such as Li 2 O, LiOH or LiZCO3. The electronic conduction additive is then dispersed in the solution and coating on the current collector.

The collector coatings of the present invention can be used various types of current collectors, in particular aluminum and metallizations of aluminum a few hundred angstrom. It is interesting to note that these The latter are normally incompatible with a basic mineral binder such as the lithium polysilicates.

The following examples are provided to illustrate the present invention, and born must at no time be considered as limiting the scope.

Example 1 A binder solution of the type (LiPO3) n is neutralized at a pH of about 7, a second solution is brought to a pH of 11. The two solutions are then put in contact with a V205 powder. In the first case, that is when the pH is close to 7, the solution retains the orange color of the V205, while in the second case, the formation of a green solution resulting from a reaction chemical between the binder and solid oxide. A similar observation is made when the same oxide of vanadium is brought into contact with an aqueous solution of polysilicate lithium. These observations are made to illustrate the importance of ensuring chemical compatibility of mineral binder protection of the collector with the active materials of the electrode so avoid a gradual deterioration of the performance of a generator at cycling.

Example 2 An aqueous solution of acid (HPO 3) 2 is neutralized to a pH of about 7 with lithium hydroxide. Carbon black is dispersed (KetjenblackTM EC-600) to rate of 8% by volume, relative to the glass of formula (LiPO 3) 2. Suspension is then coated on a 13 micron aluminum collector. The thickness obtained after drying at 150 C is about 3 micrometers. We notice the aspect shiny and adhesive protective coating and its electronic surface conductivity.
A try identical using potassium hydroxide to neutralize the acid (HPO3) ,, give a sample whose coating is even more flexible and adhesive.

Example 3 The collector based on (LiPO31, from Example 2 for the production of an electrochemical generator according to the following steps. Cathode composite is coated on the protected collector based on (LiPO 3) n of Example 2 to to form a film thin from a dispersion in acetonitrile of the following elements, be a) a ethylene oxide-based copolymer (55% by volume) containing LiTFSI salt a concentration corresponding to an O / Li molar ratio of 30; b) V2O5 powder (40%), and c) carbon black (KetjenblackTM, 5% volume). After coating, the film is dried to 80 C under vacuum for 12 hours. We realize a complete cell by assembly sequence, by thermal transfer of the cathode films, the separator 20 micrometers also consisting of a copolymer of ethylene oxide and LiTFSI, and a anode of metallic lithium. The reversible capacity of the battery is 5.03 C / cm2. The Figure 3 illustrates the behavior of this cell according to the cycling at 60 C.
notes the good capacity maintenance in successive cycles, and mainly the maintaining the the value of the Area Specific Impedance (ASI) which reflects the sum of phenomena of ohmic resistance, charge transfer and diffusion, and thus allows judge the maintaining the quality of the electrical contacts. In this example, the UPS
is about 100 S2 after about thirty cycles.

In contrast, Figure 4 illustrates an identical cell using however an aluminum collector not covered with the protective coating according to present invention. Both cells are recharged at constant current in 6 hours in using a higher voltage limit of 3.1 V.
in this second case, a significant initial loss of capacity and a value of the UPS high as soon the start of cycling. This loss is nearly four times greater than of the test preceding, thus confirming the protective role of the coating forming the object of this request.

Example 4 A binder based on potassium polymetaphosphate (KPO3) õ obtained according to the procedure of Example 2 is used with a carbon black dispersion (KetjenblackTM) and iron phosphate whose particles are submicron.
The mixed protective coating is about 4 microns thick.
Appearance this coating is semi-glossy, qualitatively confirming the little character porous protective coating. The glass content used is 70%
volume iron phosphate (20% by volume) and carbon black (10%
in volume), in order to obtain a substantially dense and slightly porous coating. A
test of the electrochemical activity of this coating is carried out using this coating directly with a polymer electrolyte consisting of an oxide copolymer of ethylene and of LiTFSI and a metal lithium anode in a montage identical to that of Example 3. There is then an electrochemical activity specific to the additive of iron phosphate with a 3.3 V discharge tray and capacity proportional to the amount of additive. A low and stable ASI value according to cycles of discharge / charge is also observed.

Although the present invention has been described using implementations specific, it is understood that several variations and modifications may add to implemented, and the present application is intended to cover such changes, uses or adaptations of the present invention according to, in general, the principles of invention and including any variation of the present description which will become known or Convention in the field of activity in which this invention, and which can be applied to the essential elements mentioned above, in agreement with the scope of the following claims.

Claims (26)

1. Electrochemical generator comprising an electrolyte separator between two electrodes of which at least one of the electrodes comprises a current collector metallic partially or totally covered with a protective conductive coating, said coating comprising a vitreous or partially vitreous mineral binder including a phosphate, a polyphosphate, borate or polyborate of an alkali metal, or their mixtures, said coating being brought into sealing contact with said collector in order to protect the surface metal covered with the formation of passivation films. 2. A generator according to claim 1, in which is dispersed at least an additive of electronic conduction ensuring the electronic exchanges between the electrode and the collector. 3. A generator according to claim 1 wherein the generator is a generator at lithium, and the alkali metal includes lithium, potassium or their mixtures. 4. A generator according to claim 1 wherein the coating comprises a glass former. 5. A generator according to claim 2 wherein the conduction additive electronic includes carbon blacks, graphites, metals, compounds conductive minerals metals, doped conjugated polymers, and mixtures thereof. 6. A generator according to claim 2 wherein the conduction additive electronic comprises an electrochemically active material functioning as a material of electrode. 7. A generator according to claim 6 wherein the material electrochemically active is the same as that of the adjacent active electrode, 8. A generator according to claim 1 wherein the adjacent electrode is a cathode whose collector metal is aluminum, or whose collector is a metallization to aluminum base. 9. A generator according to claim 1 wherein the adjacent electrode is a cathode comprising a vanadium oxide or a phosphate of a metal of structure transition nasicon. 10. A generator according to claim 9 wherein the second electrode is an anode of lithium. 11. A generator according to claim 9 wherein the electrolyte separator is a polymer electrolyte. 12. A generator according to claim 1 wherein the adjacent electrode is a anode. 13. Process for partial or total coating of a current protector on the surface of a metal collector of an electrode, the method comprising:

a) the preparation of an aqueous solution dispersion of a vitreous mineral binder or partially vitreous, including a phosphate, polyphosphate, borate or polyborate of an alkali metal, or a mixture thereof;

b) coating the mixture obtained in step a) on the surface of the metal of the collector, followed by drying so as to cover and protect at least part of the surface of the metal to avoid formation of passivation films. 14. Process according to claim 13, in which, in step a), at least one electronic conduction additive. 15. Process according to claim 14, in which step a) is added a former of glass in the dispersion. 16. Process according to claim 14, in which the mixture of step a) includes at least one electronic conduction additive. 17. A method according to claim 13 wherein the alkali metal comprises lithium, the potassium or mixtures thereof. 18. Process according to claim 14, in which the conduction additive electronic includes carbon blacks, graphites, metals, compounds conductive minerals metals, doped conjugated polymers, and mixtures thereof. 19. Electrode comprising a metallic current collector partially or totally covered with a protective conductive coating, said coating comprising a mineral binder glassy or partially glassy, including a phosphate, a polyphosphate, a borate or a polyborate of an alkali metal, said coating being brought into sealing contact with said collector in order to protect the covered metal surface from the formation of films of passivation. 20. Electrode according to claim 19 in which an additive is dispersed conductive electronics ensuring the electronic exchanges between the electrode and the collector. 21. Electrode according to claim 19 in which the alkali metal includes the lithium, potassium or mixtures thereof. 22. An electrode according to claim 19 wherein the coating comprises a glass former. 23. Electrode according to claim 20, in which the conduction additive electronic includes carbon blacks, graphites, metals, compounds conductive minerals metals, doped conjugated polymers, and mixtures thereof. 24. Electrode according to claim 20, in which the conduction additive electronic comprises an electrochemically active material functioning as a material of electrode. 25. An electrode according to claim 24 wherein the material electrochemically active is the same as that of the adjacent active electrode. 26. Electrode according to claim 23, in which the conduction additive electronic includes carbon blacks or graphites whose content is between 2 and 20% volume.