WO2015114257A1 - Electrode material, preparation process and use in a lithium secondary battery - Google Patents

Abstract

This crosslinked electrode material, of three-dimensional structure, consists of particles of inorganic electrode material connected together by an organic compound, characterized in that it comprises covalent bonds between the inorganic particles and the organic compound.

Description

 ELECTRODE MATERIAL, PROCESS FOR PREPARATION AND USE IN A

LITHIUM ACCUMULATOR

FIELD OF THE INVENTION

The invention relates to a crosslinked electrode material and to a process for its preparation. The electrode containing this material is intended to be mainly used in a lithium battery. The field of use of the present invention relates particularly to the conversion of chemical energy to electrical energy and in the form of heat.

PRIOR ART The use of lithium (Li-ion) accumulators as autonomous energy sources is becoming more common. This is particularly the case in the field of portable equipment, where they gradually replace nickel-cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) batteries. In this respect, and in recent years, sales of Li-ion batteries have outpaced those of Ni-MH and Ni-Cd batteries. This market reversal is a direct result of the continuous improvement in the performance of lithium batteries, which have made it possible to reach densities of mass and volume energy which are much higher than those of the Ni-Cd and Ni-MH accumulators.

Indeed, while the first Li-ion accumulators had an energy density of about 85 Wh / kg, nearly 200 Wh / kg can now be obtained, the energy density being related to the mass of the Li cell. -ion complete. For comparison, the performances of the Ni-MH and Ni-Cd accumulators are respectively capped at 100 Wh / kg and 50 Wh / kg.

Advances in lithium batteries have made it possible to diversify their use (hybrid or all-electric cars, energy storage of photovoltaic cells, etc.). Increasing energy demands (per unit mass and / or volume) have led to the development of new Li-ion battery electrode materials.

For example, the main active electrode materials used in commercial accumulators are:

for the positive electrode, (1) lamellar compounds such as LiCoO ₂ (theoretical and experimental specific capacities respectively of 275 mAh / g and 140 mAh / g), Li i0 ₂ (theoretical and experimental specific capacities respectively of 148 mAh / g and 120 mAh / g), and mixed lithiated oxides (Ni, Co, Mn, Mg, Al), or (2) spinel structure compounds of composition close to LiMn ₂ 0 ₄ . These materials make it possible to obtain an operating potential relative to the lithium metal close to 4 volts;

for the negative electrode, carbon (graphite, coke, ...) or possibly Li ₄ spinel TisOi ₂ or a metal forming an alloy with lithium (Sn, Si, Sb ...).

In general, the electrode materials need to be formulated with other components (polymers, electronic conductors, etc.) in order to ensure the solidity of the electrode and its good electrochemical behavior.

Be that as it may, even though the prior art electrode inks make it possible to prepare electrodes having satisfactory properties, they are not necessarily adapted to high surface area electrode materials for the same reasons. following:

the high specific surface of these materials requires working with compositions having a very low solids content in order to obtain inks whose viscosity allows good dispersion of the components during mixing, as well as obtaining satisfactory rheological properties for the deposition process; the surface developed by the active material is very important, it is necessary to use high levels of binders to ensure the continuity of the particle network and thus obtain a good adhesion and satisfactory mechanical properties for the electrode. However, this solution is unacceptable from the point of view of the electronic conductivity (formation of an insulating layer due to the polymer) and from the point of view of the densities of mass energy (high proportion of inactive mass). There is therefore a need to modify these high surface area electrode materials to facilitate their implementation. This is the main technical problem solved by the present invention. SUMMARY OF THE INVENTION

The present invention relates to an electrode material and its use in an aqueous composition that can be used as an electrode ink. This electrode material comprises an active material but also a compound forming a three-dimensional network between the particles of this active material, creating chemical bridges. This structural modification of the electrode active material makes it possible to reduce the quantities of binder and solvent required, compared with the prior art inks of electrodes.

More specifically, the present invention relates to a cross-linked electrode material, of three-dimensional structure, consisting of particles of inorganic electrode material connected to each other by an organic compound. This organic compound is a crosslinking agent.

This crosslinked material comprises covalent bonds between the inorganic particles and the organic compound.

This electrode material therefore comprises particles of the same inorganic material, which are connected together.

In this crosslinked electrode material, the inorganic electrode material particles advantageously represent 50 to 99% by weight based on the weight of the crosslinked electrode material. On the other hand, the organic compound advantageously represents 1 to 50%; by weight relative to the weight of the crosslinked electrode material. It preferably contains from 80 to 99% by weight of particles of inorganic electrode material relative to the weight of the crosslinked electrode material, and even more preferably from 85 to 95% by weight of particles of inorganic electrode material. The process for preparing this crosslinked electrode material comprises the following steps:

a) preparing an aqueous composition comprising, by weight relative to the weight of said composition:

 - 10 to 90% of at least one inorganic electrode material whose surface has hydroxyl functions and / or metal hydrides;

 0.1 to 45% of at least one organic crosslinking agent comprising at least two functions capable of reacting with the hydroxyl functions and / or metal hydrides of the electrode material;

b) crosslinking the inorganic electrode material, by drying the aqueous composition thus obtained, preferably at a temperature between 25 and 160 ° C, and even more preferably at a temperature between 25 and 120 ° C;

c) obtaining a powder by grinding the crosslinked electrode inorganic material.

The aqueous composition prepared during step a) may be in the form of a paste. It can be milled, advantageously manually in a mortar, and this prior to step b) drying / crosslinking. It can also be left standing between 5 minutes and 48 hours in a closed container to promote the crosslinking reaction.

Thus, this crosslinked electrode material results from the reaction between:

 50 to 99 parts by weight of at least one inorganic electrode material, the surface of which has hydroxyl functions and / or metal hydrides; and 1 to 5 parts by weight of at least one organic crosslinking agent comprising at least two functions capable of reacting with the hydroxyl functions and / or metal hydrides of the electrode material.

Advantageously, the crosslinking temperature of the electrode active material is between 25 and 100 ° C, more preferably between 25 and 70 ° C.

The duration of step b) is advantageously between 1 and 24 hours, more preferably between 4 and 12 hours.

At the end of the crosslinking step b), the crosslinked electrode material may be in the form of an agglomerate of particles. It is milled during step c), according to the techniques forming part of the knowledge of those skilled in the art and in particular, by manual grinding mortar, mill lease, attritor or air jet. In general, and after the grinding step c), the particles of electrode active material have a size advantageously less than 60 micrometers, and more advantageously between 1 and 60 micrometers. The size corresponds to the largest dimension of these particles of material. This is the diameter for spherical particles.

This grinding step makes it possible in particular to ensure that the particles of crosslinked electrode material have a size suitable for use in an electrode ink.

The present invention also relates to the method of preparing an electrode containing this crosslinked electrode material, comprising the following steps:

a) preparing an aqueous composition comprising, by weight relative to the weight of said composition:

 10 to 90% of at least one inorganic electrode material whose surface has hydroxyl functions and / or metal hydrides;

 0.1 to 45% of at least one organic crosslinking agent comprising at least two functions capable of reacting with the hydroxyl functions and / or metal hydrides of the electrode material;

b) crosslinking the inorganic electrode material, by drying the aqueous composition thus obtained, preferably at a temperature between 25 and 160 ° C, and even more preferably at a temperature between 25 and 120 ° C;

c) obtaining a powder by grinding the crosslinked electrode inorganic material; d) forming an aqueous dispersion containing the powder thus obtained, at least one electronic conductor, and at least one binder;

e) depositing the aqueous dispersion on a substrate;

f) drying the deposit;

g) obtaining an electrode in the form of a film.

The specific surface of the electrode material used is advantageously, before crosslinking, between 10 and 1200 m ² / g, more advantageously between 10 and 250 m ² / g, and still more advantageously between 50 and 150 m ² / boy Wut. Typically, the inorganic electrode material having hydroxyl functions and / or metal hydrides is a material selected from the group consisting of titanium oxides; alloys based on titanium oxide (for example Li ₄ Ti ₅ O ₁₂ ); activated carbon (activated carbon); graphite carbon (G); silicon (Si); tin (Sn); antimony (Sb); phosphates (for example LiMP0 ₄ with M = Fe, Co, Mn, Ni); silicates (for example LiMSiO ₄ with M = Mn, Fe, Co); lamellar oxides (for example LiM0 ₂ (M = Mg, Mn, Co, Ni, Al), spinel oxides (for example LiM0 ₄ with M = Mg, Mn, Ni) and mixtures thereof These materials comprise hydroxyl functional groups and / or metal hydrides In the case of LTO, surface hydroxyls naturally exist by reaction with moisture.

Where appropriate, the aforementioned electrode materials may be pretreated for the purpose of possessing said hydroxyl functions and / or metal hydrides. For example, the carbon-based material may be treated by acid-base treatment, or hydrogenation.

It may be a material called "polyanionic framework" such as phosphates, or silicates, for example.

The organic crosslinking agent comprises functions capable of reacting with the hydroxyl functions and / or metal hydrides of the electrode material. These reactive functions can in particular be chosen from the group comprising the aldehyde functions; isocyanate; silane; carboxylic acid; alkene; alkyne; and their mixtures.

The organic crosslinking agent can thus comprise identical or distinct reactive functions. Advantageously, it comprises at least two reactive functions. By way of example, it may comprise at least one alkyne function and at least one carboxylic acid function.

As already indicated, the organic crosslinking agent crosslinks the particles of inorganic electrode material together. In other words, this crosslinking allows the connection between inorganic particles by means of an organic molecule. According to an advantageous embodiment, the organic crosslinking agent may be chosen from the group comprising:

 aliphatic or aromatic dialdehydes of formula HC (= O) -A-CH (= O); diisocyanates of formula O = C = N-A-N = C = O;

disilanes of formula (R-O) ₃ Si-A-Si (RO-R) ₃ ;

 the dicarboxylic acids of formula HO-C (= O) -A-C (= O) -OH;

dialkenes of formula H ₂ C = CAC = CH ₂ ;

 dialynes of formula HC≡C-A-C≡CH;

with R = H, or a linear or branched alkyl group comprising 1 to 6 carbon atoms;

and A = (CH ₂ ) _n , n being an integer between 0 and 20, or an aromatic group optionally comprising at least one heteroatom (O, N, Si, P ...).

An aromatic group is a divalent group corresponding to a hydrocarbon to which two hydrogen atoms have been removed, and whose free valence is not engaged in a double bond. For example, it may be 1,4-phenylene. In this case, the crosslinking agent may in particular be 1,4-phenylene diisocyanate.

Advantageously, the organic crosslinking agent may be chosen from the group of dialdehydes. It may especially be glyoxal (dialdehyde with A = (CH ₂ ) _n and n = 0); succinaldehyde (dialdehyde with A = (CH ₂ ) _n and n = 2); or glutaraldehyde (dialdehyde with A = (CH ₂ ) _n and n = 3).

According to a particular embodiment, step d) of the method for preparing an electrode may also comprise the addition of at least one additive that can be chosen especially from the group comprising binders and electronic conductors.

By way of example, the binder may be chosen from the group comprising carboxymethylcellulose (CMC); polyacrylic acid (PAA); alginates (sodium alginate E401); styrene butadiene latexes (SBR); acrylonitrile butadiene latex (NBR); and their mixtures.

The electronic conductor, advantageously in the form of particles, can be chosen from the group comprising carbon black, carbon nanotubes, carbon fibers and their mixture. By "aqueous dispersion" (step d)) is meant a liquid composition whose liquid phase is predominantly water, or a water / alcohol mixture and preferably only water. Advantageously, the electrode ink has a solids content greater than 20% by weight, based on the weight of the electrode ink, more preferably between 20 and 50%. In other words, the electrode ink comprises at least 20% by weight of solid after removal of the volatile compounds. Advantageously, the aqueous dispersion implemented in step d) to prepare the electrode according to the invention may comprise, by weight relative to the weight of the aqueous dispersion:

 0.1 to 10% of at least one electronic conductor, preferably 0.2 to 7.5%; 0.1 to 5% of at least one binder, advantageously 0.2 to 2.5%.

In addition, it advantageously comprises 16 to 47.5% of crosslinked electrode material, by weight relative to the weight of the aqueous dispersion.

According to a particular embodiment, the substrate on which the aqueous composition is deposited, is a current collector made of metal (for example copper, aluminum, nickel and their alloys or non-woven carbon).

The drying of this deposit during step f) is advantageously carried out in an oven or in an oven in line.

The drying time is advantageously between 1 minutes and 24 hours, more preferably between 1 and 60 minutes.

The drying temperature is advantageously between 30 and 80 ° C, more preferably between 30 and 70 ° C, and still more preferably of the order of 50 ° C.

Advantageously, the film (electrode) obtained after drying adheres to the substrate, and has cohesion properties facilitating the production of a battery, including a lithium battery. The present invention also relates to the electrode obtainable by the method described above, but also the accumulator containing this electrode, preferably a lithium battery. Due to the crosslinking of the electrode material, the aqueous dispersion (electrode ink) implemented in step d) above, makes it possible to obtain a homogeneous dispersion of the particles of electrode active material, and without requiring excessive dilution of the latter. In addition, the viscosity of the aqueous dispersion (having a solids content advantageously greater than 20% by weight) is compatible with the various deposition methods forming part of the general knowledge of those skilled in the art.

At the end of the deposition (for example by coating) and the drying of the composition, it is thus possible to obtain grammages greater than 1.5 mg / cm ² of active material.

The invention and the advantages thereof will appear more clearly from the following figures and examples given to illustrate the invention and not in a limiting manner. DESCRIPTION OF THE FIGURES

Figure 1 illustrates the curve representing the load capacity (in mAh / g) as a function of the load current regime.

 FIG. 2 illustrates the curve representing the discharge capacity (in mAh / g) as a function of the discharge current regime.

EXAMPLES OF CARRYING OUT THE INVENTION

In all of the following examples, the electrode material used is a titanate (Li ₄ Ti ₅ O ₁₂ , or LTO) having a surface area of 150 m ² / g, and supplied by Jonhson Matthey. Example 1: Production of an electrode (prior art)

An aqueous dispersion is prepared from the mixture containing:

3.4 g of a 3% by weight aqueous solution of CMC (carboxymethylcellulose, binder),

 0.3 g of carbon black (electronically conductive), and

 16.86 g of demineralized water. This mixture is then mechanically dispersed in order to deagglomerate the carbon black and TiO 2 particles, the maximum particle size being 40 micrometers.

A second binder is then introduced into the aqueous dispersion in order to adjust the viscosity of the final aqueous ink before the coating step, in order to optimize the quality of the coating. 0.4 g of SBR (copolymer of butadiene and styrene (LD417 BASF) are thus added to the aqueous dispersion.

The aqueous ink thus formed is then coated (by spreading the ink) on all or part of an aluminum current collector, so as to form a uniform and uniform aqueous ink layer.

The water is then removed from the aqueous ink layer by drying in an oven at a temperature of 12 hours at 55 ° C.

This electrode is thus prepared according to the following main steps:

 preparing an aqueous composition comprising an inorganic electrode material, a binder (CMC), and electronically conductive particles; formation of an aqueous dispersion;

adding a second binder (SBR);

 depositing the aqueous dispersion on a substrate;

drying the deposit. Example 2: Production of an electrode (prior art)

An aqueous dispersion is prepared from the mixture containing:

9.5 g of an aqueous solution at 3% by mass of CMC,

 0.1 g of carbon black (electronic conductor).

This mixture is mechanically dispersed (without the addition of additional water, water from the CMC solution) in order to deagglomerate the carbon black particles and the TisO 4 particles, the maximum particle size being 40 microns.

A second binder is then introduced into the aqueous dispersion in order to adjust the viscosity of the final ink before the coating step, in order to optimize the quality of the coating. 0.8 g of SBR are added to the aqueous dispersion.

The aqueous ink thus formed is then coated (by spreading the ink) on all or part of an aluminum current collector, so as to form a uniform and uniform aqueous ink layer. The water is then removed from the aqueous ink layer by drying in an oven for 12 hours at 55 ° C.

This electrode is thus prepared according to the following main steps:

 preparing an aqueous composition comprising an inorganic electrode material, a binder (CMC), and electronically conductive particles; formation of an aqueous dispersion;

 adding a second binder (SBR);

 depositing the aqueous dispersion on a substrate;

 drying the deposit.

Example 3: Production of an electrode (counterexample)

An aqueous dispersion is prepared from the mixture containing:

- 7.16 g of water,

0.2 g of a solution of Glyoxal at 20% by mass. The mixture is mechanically dispersed, in order to deagglomerate the particles of

1.1 g of CMC, 0.2 g of carbon black and 2.76 g of water are then introduced into this mixture. These compounds are immediately mechanically dispersed, the maximum particle size being 50 microns.

The aqueous ink thus formed is then coated (by spreading the ink) on all or part of an aluminum current collector, so as to form a uniform and uniform aqueous ink layer.

The water is then removed from the aqueous ink layer by drying in an oven for 12 hours at 55 ° C. This electrode is thus prepared according to the following main steps:

 preparing an aqueous composition comprising an inorganic electrode material, a crosslinking agent (glyoxal), and electronically conductive particles;

 formation of an aqueous dispersion;

addition of a binder (CMC);

 depositing the aqueous dispersion on a substrate;

 drying the deposit.

Example 4: Production of an electrode according to the invention

The active material is prepared by mixing with a mortar 3 g of Li ₄ Ti ₅ O ₁₂ , 1 g of glyoxal at 40% by weight in water, and 10 g of water. The mixture is then dried for at least 12 hours in an oven at a controlled temperature of 55 ° C. This step makes it possible to crosslink the inorganic particles of Li ₄ Ti ₅ 0i _{2 with} each other.

The material obtained is then milled mechanically milling bolt with 9 balls agath of 1cm diameter to reduce the particle size to at least 40 micrometers. An aqueous dispersion is then produced by mixing 2.82 g of the material melt, 0.9 g of a 3% by weight aqueous solution of CMC, 0.24 g of carbon black (electronic conductor), and 5.6 g of demineralized water.

The mixture is mechanically dispersed to deagglomerate the carbon black and Li ₄ TiO ₂ particles, the maximum particle size being 40 micrometers.

The aqueous ink thus formed is then coated (by spreading the ink) on all or part of a copper current collector, so as to form a uniform and uniform aqueous ink layer.

The water is then removed from the aqueous ink layer by drying in an oven for 12 hours at 55 ° C.

This electrode is thus prepared according to the following main steps:

 preparing an aqueous composition comprising an inorganic electrode material, a crosslinking agent (glyoxal), and electronically conductive particles;

 crosslinking the particles of inorganic electrode material by drying the composition;

 obtaining a powder;

 forming an aqueous dispersion comprising this powder and a binder (CMC); depositing the aqueous dispersion on a substrate;

 drying the deposit.

Comparison of Examples 1-4

The appearance of each of the electrodes is visually controlled once the ink has dried. The resulting electrode must form a continuous film for use in a lithium battery.

 The electrode is then electrochemically tested as a button cell.

The "button cell" type lithium battery consists of a stack consisting of a negative lithium electrode, a positive electrode based on the silicon / carbon composite material to be tested and a Celgard type polymer separator. The face of the electrode comprising the inorganic electrode material to be tested is opposite the negative lithium metal. The negative electrode is formed by a circular film 16 millimeters in diameter and 100 microns thick, deposited on a stainless steel disc. The separator is impregnated with a liquid electrolyte based on LiPF ₆ at a concentration of 1 mol / l in a mixture of carbonates.

Table 1: Composition of the inks according to Examples 1-4 (in grams).

These results show the importance of the crosslinking step before the addition of binder and therefore before the deposition of the ink to form the electrode.

Table 2: Observations relating to the electrodes according to Examples 1-4. LTO reacted with the

 Initial LTO

 Glyoxal (Example 4)

 156 72.6

Table 3: Specific Surface Measurement of Different Materials Li.iTi ₅ 0 ₁₂ (m ² / g). In view of the cross-linking of the electrode material particles, the present invention makes it possible to reduce the specific surface area of the electrode material. Consequently, the electrode material according to the invention does not require a large quantity of binder, and makes it possible to prepare electrode inks comprising a conventional solids content, a higher level of active matter, even if the material initial electrode (that is to say before crosslinking) did not allow these conditions in view of its large surface area.

The modified material according to the invention gives much more capacity than the material formulated according to the state of the art, whether in charge (Figure 1) or discharge (Figure 2).

The comparison of the capacities restored during the different current regimes (FIGS. 1 and 2) shows that the performances of the system according to the invention are improved, and this by implementing a lower amount of polymer.

Claims

 Cross-linked electrode material, of three-dimensional structure, consisting of particles of inorganic electrode material interconnected by an organic compound, characterized in that it comprises covalent bonds between the inorganic particles and the organic compound. Crosslinked electrode material according to Claim 1, characterized in that the particles of inorganic electrode material represent 50 to 99% by weight, and in that the organic compound represents 1 to 50% by weight relative to the weight of the material crosslinked electrode. Crosslinked electrode material according to claim 1 or 2, characterized in that the inorganic electrode material particles represent 80 to 99% by weight based on the weight of the crosslinked electrode material, preferably 85 to 95% by weight. A process for preparing the crosslinked electrode material according to one of claims 1 to 3, comprising the steps of: a) preparing an aqueous composition comprising, by weight relative to the weight of said composition:  10 to 90% of at least one inorganic electrode material whose surface has hydroxyl functions and / or metal hydrides;  0.1 to 45% of at least one organic crosslinking agent comprising at least two functions capable of reacting with the hydroxyl functions and / or metal hydrides of the electrode material; b) crosslinking the inorganic electrode material, by drying the aqueous composition thus obtained; c) obtaining a powder by grinding the crosslinked electrode inorganic material. A method of preparing an electrode, comprising the steps of: a) preparing an aqueous composition comprising, by weight relative to the weight of said composition: 10 to 90% of at least one inorganic electrode material whose surface has hydroxyl functions and / or metal hydrides; 0.1 to 45% of at least one organic crosslinking agent comprising at least two functions capable of reacting with the hydroxyl functions and / or metal hydrides of the electrode material; b) crosslinking the inorganic electrode material, by drying the aqueous composition thus obtained; c) obtaining a powder by grinding the crosslinked electrode inorganic material; d) forming an aqueous dispersion containing the powder thus obtained, at least one electronic conductor and at least one binder; e) depositing the aqueous dispersion on a substrate; f) drying the deposit; g) obtaining an electrode in the form of a film. Process according to claim 4 or 5, characterized in that the inorganic electrode material having hydroxyl functions and / or metal hydrides is a material selected from the group consisting of titanium oxides; alloys based on titanium oxide; activated carbon; graphite carbon; silicon; tin; antimony; phosphates; silicates; lamellar oxides; spinel oxides; and their mixtures. Process according to Claim 4 or 5, characterized in that the organic crosslinking agent is chosen from the group comprising:  aliphatic or aromatic dialdehydes of formula HC (= O) -A- CH (= 0);  diisocyanates of formula O = C = N-A-N = C = O;  disilanes of formula (R-O) 3 Si-A-Si- (O-R) 3; the diacids of formula HO-C (= O) -A-C (= O) -OH; dialkenes of formula H ₂ C = CAC = CH ₂ ;  dialynes of formula HC≡C-A-C≡CH; with R = H, or a linear or branched alkyl group comprising 1 to 6 carbon atoms; and A = (CH ₂ ) _n , where n is an integer from 0 to 20, or an aromatic group. 8. Process according to claim 4 or 5, characterized in that the organic crosslinking agent is chosen from the group of dialdehyde comprising glyoxal; succinaldehyde; and glutaraldehyde. 9. The method of claim 5, characterized in that the f) drying step is carried out at a temperature between 30 and 80 ° C for a period of between 1 minute and 24 hours. 10. The method of claim 5, characterized in that the aqueous dispersion is deposited by coating, and in that the substrate is a metal current collector. 11. Electrode obtainable according to the method according to one of claims 5 to 10.