WO2002047188A1

WO2002047188A1 - Method for the preparation of a cathode for lithium ion batteries

Info

Publication number: WO2002047188A1
Application number: PCT/SI2001/000030
Authority: WO
Inventors: Marjan Bele; Miran Gaberscek; Robert Dominko; Jernej Drofenik; Stane Pejovnik
Original assignee: Kemijski, Institut
Priority date: 2000-11-28
Filing date: 2001-11-12
Publication date: 2002-06-13
Also published as: SI20777A; AU2002214523A1

Abstract

A new process for the preparation of a cathode is suggested allowing a more equal distribution of conductible particles (for instance carbon black) and a more equal distribution of a binder. Instead of a classic binder (poly(tetrafluorethylene), poly(vinyliden)) a water-soluble macromolecule is used as a binder, for instance gelatin, water solube cellulose derivatives, gum arabic, pectines and similar natural polyelectrolytes and vinyl polymeres (for instance: polyvinyl alcohol).

Description

METHOD FOR THE PREPARATION OF A CATHODE FOR LITHIUM ION BATTERIES

1. Field of the invention

The invention belongs to the field of chemical technology, more specifically of chemical sources of electric power. It refers to a new method for the preparation of a cathode made of lithiated oxide and to a cathode prepared in this way meant for lithium ionic batteries.

2. Prior art

A subgroup of lithium ionic batteries reaching current density from 1 μAcm^"2 to 1 mAcrn^"2 comprises a cathode usually made of lithiated oxide powder (active cathodic material), the particles of which (agglomerates) have a typical dimension of 1-50 μm. Lithiated oxides with the following chemical composition are most commonly used- iCo0₂, LiN.O₂, mixtures of both said oxides, LiMn₂0 , LiMnO₂. Active cathodic material is a good ionic and a poor electronic conductor. A good operation of a cathode is possible only if the active cathodic material is also a good conductor of electrons. To serve this purpose, to an active cathodic material is added an electronically conductive material, most commonly carbon black, the particles of which have typical dimensions smaller than 1 μm. The final composite has carbon black and active particles mixed at random. Mechanical stability of the composite is achieved by an addition of 5-10 mass % of a binder (polytetrafluoroethylen or polyvinylidendyfluor.de). A thick suspension of composite is then applied on a current collector (most often aluminium foil). Before use the application is dried for several hours at a temperature of 100-150°C.

2.1 Classic preparation of a cathode for lithium ionic batteries

An active cathodic material (LiCoO₂, LiNiO₂, LiMn₂O , LiMnO₂ or an adequate mixture thereof) is prepared by using classic ceramic or salt-gel procedures. The agglomerates of an active cathodic material of typical dimensions 1-50 μm are the most adequate for the preparation of a cathode. Agglomerates consist of small particles of a 1 μm magnitude order. Agglomerate is dispersed in 1- methyl-2-pyrolydone, ethanol or acetone. To the dispersion is added 5-40 mass % of high conductive carbon black and 5-10 mass % of a binder (polytetrafluoroethylene or polyvinylidendifluoride). The obtained mixture is well mixed and applied on aluminium foil. The electrode prepared in this way is pressed under a pressure of 500-2000 kPa. Final thickness of the application amounts to 50-200 μm.

2.2. Adsorption of polyelectrolytes on particles

Adsorption of polyelectrolytes (for instance gelatin) is studied and exploited in food industry (A.G. Ward, A. Courts in »The Science and Technology of Gelatin«, Academic Press, London, 1977), photography (C.E. Mees, C.E. Kenneth in »The Theory of Photographic Process«, Macmillan, New York, 1966), electrochemistry (G.M. Brown, G.A. Hope, J. Electroanal. Chem., 397 (1995) 293), biology, medicine, etc. Polyelectrolytes are used as stabilisers both in suspensions ((1) TJ. Maternaghan, O.B. Banghan, R.H. Ottewill, J. Photogr. Sci. 28 (1980); (2) V.V. Rodin, V. Izmailova, Polym. Sci., 272 (1994) 433) and emulsions (H.J. Muller, H. Hermel, Colloid Polym. Sci. 272 (1994) 433). In the first case stabilisation is achieved by a steric barrier, which is - after the completion of adsorption on a substrate - represented by molecules of a polyelectrolyte (hairy structure), in the later case emulsions become stable due to the changed characteristics of the interfacial layer.

2.3 Deposition of carbon black on a substrate treated with a solution of polyelectrolyte

Deposition of carbon black on a substrate is performed in two different steps. The first step includes adsorption of a water soluble polyelectrolyte from a solution onto a substrate (a substrate can also be in the form of particles), the next step is the formation of layers of carbon black particles on the substrate by immersing into carbon black dispersion ((1) J.O: Besenhard, H. Meyer, H.P. Gausmann, GER Patent 1991 , 41 13 407; (2) J.O: Besenhard, O. Clauβen, H.P. Gausmann, H. Meyer, GER Patent 1991 , 41 41 416 (3) J.O: Besenhard,

0. Clau^βen, H.P. Gausmann, H. Meyer, H. Mahlkov, United States Patent 1998, 5705219; (4) J.O. Besenhard, O. Clauβen, H. Meyer, M. Kϋhlkamp United States Patent 1999, 5,916,485; (5) M. Bele, S. Pejovnik, J.O. Besenhard, V. Ribitsch, Colloid. Surface, A 1998, 143, 17; (6) M: Bele, K. Kocevar, I. Musevic, J.O. Besenhard and S. Pejovnik, Colloid. Surface, A 2000, 168, 231).

2.4Techniques similar to the present invention

2.4.1 In the patent (A. Kozawa, S. Mase, A. Sato, United States Patent 1999, 5,958,623), the authors showed that electric chemical cells can be prepared in which a dispersion of an organic fine carbon material is used. An organic fine carbon material can be used for the formation of a conductive layer on the surface of a cathodic active material. An organic fine carbon material is a composite material with a chemical composition similar to that of a cathode prepared by the new method, described in item 4. Yet, the method of preparation of an organic fine carbon material totally differs from the method of cathode preparation, described in item 4.

2.4.2 In the patent (J.O. Besenhard, H. Meyer, O. Clau^βen, M. Kϋhlkamp, Europaische Patentschrift 1992, EP 0 616 720 B1) the authors showed that composite materials with high conductivity can be prepared by the procedure (method) consisting of 5 steps:

1. powder or fibres are brought into contact with a water solution containing a macromolecular substance that is well soluble in water

II. rinse with water if needed

III. the obtained material is brought into contact with the dispersion of fine conductible particles, superficially active substances and salt IV. rinse with water if needed

V. shape into a final product.

3. Technical problem

Cathodes prepared according to the classic procedure described in item 2.1 are not optimised with respect to: a) contents and distribution of carbon black. The distribution of carbon black in a classic cathodic composite is coincidental. Therefore a much bigger quantity of carbon black than necessary should be added in order to obtain the required electronic conductivity of the final composite. b) contents and distribution of a binder. A binder used in the classic method is an electronic insulator, it is therefore required that its contents be at the lowest level possible. The binder in the composite is distributed coincidentally, so a much higher quantity of a binder than needed should be added in order to satisfy mechanical characteristics of the final cathodic composite.

Cathodes may also be prepared according to the method 2.4.2, however this type of cathode preparation proved to be very rare in practice. If cathodes are prepared according to the method 2.4.2 they have the following disadvantages:

A) Owing to rinsing with water in step II), described in item 2.4.2, the adsorption of macromolecular substance on the cathodic material is not necessarily optimal, the procedure however is technically demanding due to the separation of particles from the solution of macromolecules.

B) The connection between carbon black, covered active particles obtained by the step IV of the method 2.4.2, is not good; attractive forces between the particles are too weak to obtain a mechanically stable cathode, adequate to be used in a lithium ionic battery. 4. Description of the solution with embodiments

We suggest a new procedure for the preparation of a cathode allowing a more equal distribution of conductible particles (for instance carbon black) and a more equal distribution of a binder. Instead of a classic binder (poly(tetrafluoroethylene), poly(vinyliden difluorid)) a water soluble macromolecule is used, for instance gelatin, water-soluble derivatives of cellulose, gum arabic, pectines and similar natural polyelectrolytes and vinyl polymers (example: poly vinyl alcohol). The suggested procedure is similar to that described in the patent (J.O. Besenhard, H. Meyer, O. Clauβen, M. Kϋhlkamp, Europaische Patentschrift 1992, EP 0 616 720 B1) which is cited in item 2.4.2. Step (II) is changed and a new step is added following step (III). In the changed step (II) rinsing is performed not with water but with an acid or a base with a known pH, or drying. In the new step (following step III) dispersed particles are flocculated by an addition of a polyelectrolyte into the dispersion. The scheme of the new procedure is thus:

i. active particles are brought into contact with a water solution containing a macromolecular substance, which is well soluble in water ii. rinse with a base or an acid having a similar pH as the solution under item i), or dry iii. the obtained material is brought into contact with a dispersion of fine conductible particles, superficially active substances and salt iv. an additional quantity of water solution is added which contains a macromolecular substance thus causing flocculation of all present hard particles v. rinse with water or dry vi. form into a final product.

In step iv) the dispersed particles are not only flocculated but also additionally bound to each other. The suggested step „iv" essentially contributes to better mechanical and electrochemical characteristics of a cathode compared to the characteristics obtained by the procedure described in the patent cited under item 2.4.2. The final composite contains approximately 97 mass % of an active cathodic material, which is much more than in the classic method (80-90 mass %)•

The invention is explained by means of enclosed figures 1-3.

Figure 1 shows the first discharging (deintercalation) and charging of a cathode made from active particles LiMn₂O₄, treated in a solution of gelatin and then in a carbon black solution.

Figure 2 shows a dependence of the reversible capacity of the cathode made from active particles of LiMn₂O , treated in a solution of gelatin and then in a carbon black solution, from the number of cycles discharging - charging.

Figure 3 shows a dependence of a logarithm of specific conductibility of the cathode made from active particles of LiMn₂O , treated in a solution of gelatin and then in a carbon black solution, from mass contents (%) of carbon black in the final cathodic material.

4.1 Preparation of a solution of a modified polyelectrolyte

The solution was prepared by dissolving a water-soluble polyelectrolyte in water. Polyelectrolytes were used which form a hairy structure on the boundary between a particle and an electrolyte (for instance proteins, cellulose derivatives, gums, etc.). Hairy structure means that after the completion of adsorption, tails or loops of a polyelectrolyte protrude from the surface of the particle into the interior of the solution.

Prior to the adsorption the pre-prepared solution of polyelectrolyte was modified by changing pH values of the solution or by adding a suitable ionic-superficially active substance. This changed the density of the charge on the polyelectrolyte, which has an influence on its properties, like its shape and adsorption capacity, in order to obtain optimal adsorption.

4.2 Treatment of active particles in a solution of a polyelectrolyte (item ii)

A polyelectrolyte, modified in compliance with item 4.1 , was used to superficially treat the particles of an active cathodic material, which were later used as an electrochemically active cathodic material. While stirring a known quantity of an active cathodic material was added into an adequate quantity of a solution of a modified polyelectrolyte. After 10 - 30 s the treated particles were separated from the solution with decantation or better filtration. An alternative possibility is to dry the mixture. The treated particles were used for the carbon black deposition.

4.3 Deposition of carbon black on particles treated with a solution of a polyelectrolyte (item iii)

The particles treated with a polyelectrolyte were added a dispersion of carbon black of known composition and concentration. The dispersion should be adequately homogenised and adequately stable, which is achieved by an adequate content of superficially active substances and salts. After the addition of carbon black dispersion, when smoothly stirred, part of dispersed carbon black deposits on the surface of treated particles of the active material. A part of dispersed carbon black remains in the dispersion. The obtained mixture of active cathodic particles treated with a polyelectrolyte, covered with a layer and dispersed particles of carbon black was used in the process of flocculation, described in 4.3.

4.4 Flocculation (item iv)

The obtained mixture, described under 4.2, is added an adequate quantity of a polyelectrolyte of known composition and concentration. While smoothly stirring flocculation is carried out where floccules emerge and get precipitated. The rest of the clear solution is decanted. An alternative option is to dry the mixture.

4.4. Preparation of a cathode (item vi)

Active cathodic particles, treated with a polyelectrolyte, covered with a layer of carbon black and subsequently flocculated in carbon black dispersion with the addition of a polyelectrolyte are applied onto aluminium foil. The application is pressed under pressure 100-5000 kPa and dried in vacuum or inert atmosphere. The final thickness of the application amounts to 50-200 μm. The dried electrode is transported into a dry chamber to undergo electrochemical tests.

A method of preparation of a cathode for lithium batteries by the present invention is performed as follows: a) prepare a solution of a polyelectrolyte adequate for the formation of a hairy structure on the surface of carbon particles by solving 0.1 to 10 g of a polyelectrolyte selected among proteins, derivatives of cellulose, gums or mixtures thereof in 1 I of deionised water while smoothly stirring at a temperature of 30 to 100°C. b) into 1 I of the obtained solution, heated to room temperature and modified to pH 3 to 12, mix 1 to 10 g of particles of an active cathodic material with a dimension of 0.1 to 10 μm and a specific surface of 2 to 50 m²/g, leave for 10 s to 30 minutes, decant c) suspension of active particles under b) is rinsed with a base or an acid having a similar pH to that of the solution under item a) or dried d) mix 1-100 g of well conductible carbon black, 0.001-1 mol of a superficially active substance and 0.005-0.1 mol of salt if needed. The mixture should be added an adequate quantity of water. After 1-15 minute stirring with approx. 200 rpm at room or higher temperature, the particles are additionally homogenised, for instance with a turbo stirrer, with ultrasonic bath or with a mill (for instance dispermat) to obtain a stabilised dispersion. e) active particles obtained according to item c) are added to the dispersion under item d) f) rinse with water and dry or only dry, without rinsing g) form a final product (cathode) adequate for electrochemical measurements

Embodiment A: Preparation of a solution of a modified polyelectrolyte

In experimental work 0,01-1 % of solution of gelatin No. 48722, Fluk producer, was used. The solution was prepared by solving 0.1-10 g of gelatin in 1 litre of water at 30-100°C while constantly stirring with a magnetic stirrer with approx.

200 rpm. Prior to use it was always tempered to room temperature.

Before starting adsorption, the pre-prepared solution of gelatin was modified with an adequate quantity of 0.1 M NaOH to obtain a pH value between 7 and

9.

Embodiment B: Preparation of a polyelectrolyte treated particles of an active cathodic material

Gelatin or gum arabic Gum-arab spraygum irx 28830 Colloides Naturels International, France, modified according to item 4.1 , was used to superficially treat cathodic active particles that were later used as an electrochemically active cathodic material. Into 1-100 ml of modified polyelectrolyte was added 1- 10 g LiMn₂O₄ while simultaneously stirring. After 2-30 minutes the treated particles of the active cathodic material were decanted. The obtained mixture was used in carbon black deposition.

Embodiment C: Preparation of carbon black dispersion

Dispersion of carbon black was prepared by mixing 1-100 g of Printex XE2, Degussa producer, 0.001-1 mol CTAB (cetyltrimethylamonium bromide), producer Aldrich Nr. 85-582-0 and 0.01-0.1 mol of Na acetate, of the producer Merck Nr. 6268. Water was added to the mixture to obtain the final mass of 1 kg. After 1-15 min stirring with approx. 200 rpmin at approx. 40°C, the particles were additionally homogenised by a turbo stirrer, where approx. 20000 rpm were used in 0.5 to 10 minutes. The obtained dispersion was additionally homogenised for 30 minutes in ultrasonic bath at approx. 50°C.

Embodiment D: Deposition of carbon black onto polyelectrolyte treated particles of an active cathodic material

To 10 g of polyelectrolyte treated particles of an active cathodic material were added 10-200 g of prepared dispersion. Moderate stirring was applied for 1-30 minutes.

Embodiment E: Flocculation

5-200 ml of the solution of a polyelectrolyte prepared in compliance with the embodiment A were added into the mixture obtained by the embodiment D. After 1-30 minutes of moderate stirring the flocculated particles were left to entirely precipitate and the solution was then decanted.

The process for the preparation of a cathode for lithium ionic batteries is thus characterised in that a) a solution of a polyelectrolyte adequate for the formation of a hairy structure on the surface of carbon particles is prepared by preparing 0.01 to 10 mass % of the solution of a polyelectrolyte in water, whereby a polyelectrolyte may be gelatin or other proteins, derivatives of cellulose, gum or mixtures thereof, solved under constant stirring at a temperature of 30 to 100°C, b) a solution, heated to approximately room temperature and modified to pH 3 to 12, is used for the preparation of a suspension with 0.1 to 50 mass % of particles of an active cathodic material with dimension 0.1 to 10 μm and specific surface of 2 to 50 m²/g is left for 10 s to 30 minutes, then decant c) the suspension of active particles under b) is then rinsed with a base or an acid having a similar pH to that in the solution under A) or only dry, without rinsing, d) mix 0.00001 to 1 mol of a superficially active substance and if needed 0.0005-0.1 mol of salt to 1 g well conductible carbon black and to this mixture an adequate quantity of water is added and mixed for 1 to 15 minutes with approximately 200 rpm at room or higher temperature and then these particles are additionally homogenised, for instance with a turbo stirrer, with ultrasonic bath or a mill, preferably a dispermat to obtain a stabilised dispersion, e) active particles obtained according to item c) should be added to the dispersion under item d), f) rinse with water and dry or only dry without rinsing g) form into a final product, preferably a cathode, suitable for electrochemical measurements.

Gelatin, gum arabic or cellulose derivative may be used as a polyelectrolyte.

The active material may be LiMn₂O₄ with spinel structure, LiCo0₂, LiNi0₂ or any mixture thereof.

Carbon black with specific surface above 10 m²g^{"1 |S use}d ^{as we}" conductible carbon black.

The cathode for lithium ionic batteries is manufactured by the described process and described substances.

Claims

1. A method for the preparation of lithium ionic batteries characterised in that a) a solution of a polyelectrolyte adequate for the formation of a hairy structure on the surface of carbon particles is prepared by preparing 0.01 to 10 mass % of the solution of a polyelectrolyte in water, whereby a polyelectrolyte may be gelatin or other proteins, derivatives of cellulose, gum or mixtures thereof, solved under constant stirring at a temperature of 30 to 100°C, b) a solution, heated to approximately room temperature and modified to pH 3 to 12, is used for the preparation of a suspension with 0.1 to 50 mass % of particles of an active cathodic material with dimension 0.1 to 10 μm and specific surface of 2 to 50 m²/g is left for 10 s to 30 minutes, then decant c) the suspension of active particles under b) is then rinsed with a base or an acid having a similar pH to that in the solution under A) or only dry, without rinsing, d) mix 0.00001 to 1 mol of a superficially active substance and if needed 0.0005-0.1 mol of salt to 1 g well conductible carbon black and to this mixture an adequate quantity of water is added and mixed for 1 to 15 minutes with approximately 200 rpm at room or higher temperature and then these particles are additionally homogenised, for instance with a turbo stirrer, with ultrasonic bath or a mill, preferably a dispermat to obtain a stabilised dispersion, e) active particles obtained according to item c) should be added to the dispersion under item d), f) rinse with water and dry or only dry without rinsing g) form into a final product, preferably a cathode, suitable for electrochemical measurements.

2. A process according to claim 1, characterised in that gelatin is used as a polyelectrolyte.

3. A process according to claim 1 , characterised in that gum arabic is used as a polyelectrolyte.

4. A process according to claim 1 , characterised in that cellulose derivative is used as a polyelectrolyte.

5. A process according to claim 1 , characterised in that LiMn₂O with spinel structure is used as an active material.

6. A process according to claim 1 , characterised in that LiCo0₂ is used as an active material.

7. A process according to claim 1 , characterised in that LiNi0₂ is used as an active material.

8. A process according to claim 1 , characterised in that any mixture of compounds cited in claims 5 to 7 is used as an active material.

9. A process according to claim 1 , characterised in that carbon black with specific surface above 10 m g^"1 is used as well conductible carbon black.

10. A cathode for lithium ionic batteries characterised in that it is manufactured according to claims 1 to 9.