WO2000077875A1

WO2000077875A1 - Method for making a multilayer structure for lithium polymer generators

Info

Publication number: WO2000077875A1
Application number: PCT/FR1999/001385
Authority: WO
Inventors: Michel Coulon; Pierre-Yves Silvert
Original assignee: Le Carbone Lorraine
Priority date: 1999-06-11
Filing date: 1999-06-11
Publication date: 2000-12-21

Abstract

The invention concerns a method for making electrode films for a Li-ion polymer generator comprising at least a strip casting of an electrolyte-separator (13) on a support (3) so as to form a double layer of electrolyte-separator/support (40), and at least an operation for casting an electrode (12, 14) on said electrolyte-separator layer (13) of said double layer (40). The inventive method is advantageous in that it does not require a particular support layer for each electrode layer and eliminates the problems of delamination between the electrode and its support. Moreover, it eliminates the calendering operations, thereby considerably reducing the risks of defects inherent in each calendering operation.

Description

PROCESS FOR MANUFACTURING MULTILAYERS FOR POLYMER-BASED LITHIUM GENERATORS

Field of the invention

The present invention relates to batteries, or rechargeable cells, based on alkali metals, and more precisely rechargeable electrochemical cells based on lithium comprising layers of active polymers. The invention relates very particularly to a process for manufacturing multilayer composites based on polymers intended for the manufacture of said electrochemical generators

State of the art

Rechargeable electrochemical cells based on alkali metals have been the subject of numerous developments aimed in particular at increasing their technical performance, such as longevity and energy density, and at limiting the sensitivity to air of the active components during their manufacture

The most recent developments have led to the introduction of generators called "lithium-ion polymers" (or "Li-ion polymers"), such as those described in particular in American patent US Pat. No. 5,296,318 These generators include series of layers, of which at least one positive collector, one positive electrode, one polymer electrolyte separator, one negative electrode and one negative collector Typically, the collectors are made of a metal such as copper for the anode or aluminum for the cathode, electrodes consist of a composite comprising a polymer and a compound capable of interposing the alkaline salt, and the polymer electrolyte separator consists of a polymer impregnated with an alkaline salt and acting as an electrolyte of high ionic conductivity. The polymer electrolyte separator may itself comprise two layers It is also known to form stacks comprising a greater number of layers, such as symmetrical stacks which comprise a positive collector, a positive electrode, a polymer electrolyte separator, a negative electrode, negative collector, negative electrode, polymer electrolyte separator, electrode positive and a positive collector These elementary cells can be associated in series and / or parallel assemblies to constitute a battery having the desired capacity and electrical voltage

These stacks make it possible to produce light and very thin batteries which are sought after for the power supply of portable devices, such as mobile telephones, portable computers, electronic cameras,

The various layers can be assembled by a simple winding under pressure, in order to ensure the intimacy of the contact between the layers, this pressure being maintained over time by confining the packaging which serves to isolate it from the air. ambient and manipulate it

It is also known to weld the layers to one another by successive operations of hot compression or calendering. This method offers the advantage of allowing the use of flexible and light plastic envelopes since the packaging no longer has to be ensured. the mechanical pressure maintenance function on the assembly

For large-volume industrial production of this type of batteπes, the stacks are produced by calendering of continuous films, at speeds as high as possible, so as to form long multilayer strips made up of the desired stack Several calendering operations are generally required to obtain stacks The Li-ion polymer generators are then obtained from these strips, for example, by cutting or winding such a strip

Problem

The successive calendering operations are extremely critical for the quality of the final multilayer strip, which can comprise more than 100 layers distributed in several cells, since a single fault in connection between two layers can lead to the scrapping of the generator set II is therefore essential to implement means to facilitate the handling of films, reduce the risk of defects adhesion between the different layers and, as far as possible, reduce the number of calenders.

The requirements of productivity lead to the widening of the width and, on the other hand, the search for energy density in the final generator means that the electrode films have to be made thicker and thinner and thinner and thinner. loaded with binder and increasingly thin separator-electrolyte films. These films, which by design are becoming more and more fragile, are handled at high speed continuously and automatically during the film processing operations which follow the casting and drying, that is to say typically:

• unwinding and rewinding for control;

• unwinding and rewinding for the slitting which adjusts the film strips to their final width;

• first calendering operations between the electrodes, the collectors and the separator;

• cutting of elementary multilayers to their final dimension in the generator.

Thus, it has become essential to handle the electrode and separator films on support films which provide the structural role with respect to the mechanical stresses exerted during the operations described above, until the multilayer produced by calendering successive has enough layers to be able to support itself. The film which serves as a support for strip casting typically performs this structural mechanical function and is chosen to be able to perform this function. The delamination of the bilayer caused by the mechanical stresses exerted by the process has thus become the serious production incident which requires the entire line to be stopped and the entire delaminated part to be scrapped.

If the adhesion between the film and its support must be without failure until the number and the mechanical resistance of the calendered layers make it possible to overcome the mechanical role of the support, it must on the other hand not be too strong in order to do not damage the film during peeling which eliminates the support film. Indeed, the electrode films are more and more fragile because, as indicated above, the amount of binder and plasticizer is increasingly reduced in order to increase the energy density of the generator

Consequently, the choice of a satisfactory support for the electrodes is subject to several requirements. In particular

- the support must be as thin and as light as possible in order to reduce its cost and handling difficulties,

- it must have the closest possible thickness and grammage tolerance,

- It must be strong enough to withstand the mechanical stresses linked to the manufacturing process,

- it must resist the solvent of the slip when it has a casting operation,

- it must withstand the temperature of the calendering (s),

- It must have enough adhesion with the electrode to ensure bonding without failure during the manufacturing process, but also weak enough to

15 do not exceed the mechanical strength of the film during the final peeling, ^~

- it must have the lowest possible cost since it is not directly recyclable due, among other things, to the cutting operation

Due to all of these constraints, the choice is very limited. A film of super-calendered paper or polyester is generally used, the surface treatment of which is tried to adjust so as to obtain an adhesion comprised within the narrow range. range of acceptable values There is currently no known surface treatment which gives complete satisfaction

The Applicant has sought an industrial manufacturing process for a multilayer electrode film for a polymer Li-ion generator which makes it possible to avoid the drawbacks of the prior art

Description of the invention

D 0

According to the invention, the method of manufacturing electrode films for polymeric polymer generator comprises at least one operation of casting in a strip a separator-electrolyte film on a support, so as to form at least a bilayer. separator-electrolyte / support, and at least one operation of casting an electrode on said separator-electrolyte layer of said bilayer

Thus, instead of looking for a specific support for the electrode, the Applicant has had the idea of using as a casting support for the electrodes the separator-electrolyte film on its support, that is to say a separator bilayer -electrolyte / support The Applicant has in fact realized that it was possible to use the fact that the separator-electrolyte film does not present at all the same difficulties as the electrodes with regard to adhesion since it is very plastic and has a higher content of binder than the electrodes The method according to the invention therefore makes it possible to benefit for all the layers of the generator from the good adhesion and the ease of peeling specific to the separator-electrolyte bilayer /support

By exploring this path, the Applicant has also found that when the electrode is poured onto the electrolyte separator

- the solvent of the slip of the electrode induces a surface fusion of the separator-electrolyte which ensures a welding of very good quality of the two layers Welding is moreover undetectable with the metallurgical optical microscope on polished sections (see figure 5) No diffusion of conductive particles from the electrode through the separator has been observed which could create a short circuit,

the electrolyte separator has sufficient compliance to withstand the stresses due to the drying shrinkage of the electrode, even when the latter is as thin as 15 μm,

- the three-layer does not delaminate during the process

The method according to the invention makes it possible to eliminate the waste in line due to delamination between the electrodes and their supports, and the waste of generators for lack of calendering between the electrodes and the electrolyte separator

Figures

Figure 1 shows, schematically, a typical multilayer strip from which a polymer Li-ion generator can be made FIG. 2 schematically represents a method of manufacturing a multilayer electrode film for a polymer Li-ion generator of the prior art

FIG. 3 schematically represents a method of manufacturing a multilayer electrode film for a polymer Li-ion generator of the prior art

FIG. 4 schematically represents a method for manufacturing a multilayer electrode film for a polymer Li-ion generator of the invention

FIG. 5 is a micrograph which shows the connection between the separator (13) and an electrode (12) manufactured according to the invention

Detailed description of the invention

As illustrated in Figure la), a typical multilayer strip (1) from which a polymer Li-ion generator can be made includes a positive collector (11), a positive electrode (12), a polymer electrolyte separator (13), a negative electrode (14) and a negative collector (15) Figure lb) shows another multilayer strip (1) characterized by a symmetrical stacking of the layers and comprising a positive collector (111), a positive electrode (121 ), a polymer electrolyte separator (131), a negative electrode (141), a negative collector (15), a negative electrode (142), a polymer electrolyte separator (132), a positive electrode (122) and a collector positive (112)

In the processes of the prior art, such as that illustrated in FIG. 2, the multilayer strip (1) for a polymer Li-ion generator is produced by a series of parallel and / or successive calendering operations of the elementary layers, which are self-supporting, that is to say they must have sufficient mechanical strength to allow their manipulation. The base films, namely the electrode film (12) and the separator-electrolyte film (13) , are produced separately by a shaping process which is generally casting on a support (2, 3) (Figure 2a) The support (2, 3) is then peeled (Figure 2b) so as to release the self-adhesive film. carrier (12, 13) (Figure 2c) The two films are then joined together by calendering so as to form a bilayer (Figure 2d). It is also possible to combine the calendering of the elementary layers differently (for example, an electrode film can be coated with the collector layer before being joined to a separator-electrolyte film), but, in all cases, the calendering of the layers is carried out from layers without support.

In an evolution of the prior art imposed by the increasing fragility of the films, which are no longer self-supporting, as illustrated in FIG. 3, the electrode film (12) is cast on its support (2) so as to forming an electrode / support bilayer (41) and the separator-electrolyte film (13) is cast on its support (3) so as to form a separator-electrolyte / support layer (42) (Figure 3a). The bilayer electrode / support (41) is calendered on the collector (11) (FIG. 3b), forming the multilayer (43), then the support (2) is peeled (FIG. 3c), the metal collector (11) therefore playing the structural role which allows the manipulation without damage of the electrode film (12). The collector / electrode bilayer (44) is calendered on the electrolyte / support separator (42) (FIG. 3d). It is then possible to get rid of the support (3) by peeling and thus obtain the collector / electrode / separator-electrolyte multilayer (46) (FIG. 3e). The principle of this manufacturing method is to use the casting supports to manipulate the films until they are transferred to the multilayer and to peel them before the following calendering on the surface which they mask and protect. It is therefore just as well, after the step described in 3b, calendering another electrode / support bilayer (41) on the free face of the collector (11) before proceeding to the step described in 3c.

In the method according to the invention, as illustrated in FIG. 4, a support (3) is firstly coated by casting with a separator-electrolyte film (13) so as to form a first two-layer intermediate strip ( 40) (Figure 4a). This strip has mechanical characteristics capable of allowing relatively easy handling thereof. The intermediate strip (40) is then coated with a positive (12) or negative (14) electrode film by casting (FIG. 4b). The three-layer intermediate strip (51, 52) thus formed benefits from the mechanical characteristics provided by the support (3) and eliminates the need for a particular support for the electrode film. The film the electrode then no longer needs to be self-supporting and can thus be very loaded with active materials at the expense of the binder, and therefore very fragile.

The three-layer (51, 52) can then be coated with a positive (11) or negative (15) collector by a calendering operation, so as to form a third intermediate strip (61, 62) (FIG. 3c). The peeling of this intermediate strip (61, 62) then releases the elementary multilayer (71, 72) which has become self-supporting thanks to the structural role of the metal collector, corresponding either to the positive pole (71), or to the negative pole (72 ) of the final generator (figure 3d).

The elementary multilayers (71, 72) can be stacked so as to form the different combinations of layers desired. FIG. 3d illustrates the case where two elementary multilayers are joined together by calendering so as to form an elementary cell (8).

The support (3) is chosen from a material which simultaneously provides sufficient adhesion between the two layers during the manufacturing operations of the multilayer and a peelability (or "peelability") of said support when the multilayer produced by calendering is self carrier and that we need to free the face of the multilayer masked by the support for the following calendering. Said support may be a plastic film, such as a polyester film or a polypropylene film, or a nonwoven film, such as a super-calendered paper or a nonwoven of polyaramide fibers.

Said support is advantageously a raw plastic film of manufacture, that is to say one which has not undergone any surface treatment, which makes it possible to simplify the process and reduce its cost. Said support can also be a plastic film having undergone a matting surface treatment such as a chemical attack, a corona treatment or a flame treatment intended to reinforce if necessary the adhesion.

Said support can also be a plastic film provided with a primary coating of the silicone, polyvinyl alcohol or acrylic type, this coating having been carried out on a raw film or having undergone a surface treatment. Primary coating further increases the adhesion of the separator-electrolyte film if necessary, to the extent that it does not degrade said peelability

The method of manufacturing electrode films for a Li-ion polymer generator therefore comprises, according to the invention, at least one operation of casting in a strip a separator-electrolyte film (13) on a peelable support (3), so as to form a first intermediate multilayer (40), which comprises at least the separator-electrolyte / support bilayer, and at least one operation of casting an electrode (12, 14) on said multilayer (40), on the side of the separator-electrolyte layer (13), said intermediate multilayer (40) then being used as a support

It may be necessary to provide a specific drying operation for the layers obtained by casting, so as to accelerate the evaporation of the solvents used for casting.

The casting operations, as well as any drying operations and subsequent calendering operations, can be carried out online, which increases productivity

EXAMPLES

Example 1

To make an electrolyte separator, a first homogeneous slip was prepared by mixing a part of a mixture comprising

- 30% by weight of PNDF / HFP copolymer (Kynarflex 2801® from ELF- ATOCHEM),

- 20% Pyrogenic silica,

- 50% dibutyl phthalate (DBP), with two parts of acetone Then, with this first slip, a film 15 μm thick was produced after drying on a coating line with a doctor blade using a raw polyester film of manufacture 36 μm thick as support. The bilayer thus obtained could be wound and unwound several times on a cylinder 10 mm in diameter, it could be split into strips as narrow as 10 mm without any delamination being observed

An anodic film was produced by preparing a second homogeneous slip comprising 58 parts of mixture - 71% by weight of graphite MCMB 25-28 ® from OSAKA Gas,

- 15.5% by weight of DBP,

- 10.5% PNDF / HFP,

- 3% carbon black; and 42 parts of acetone.

This second slip was used to manufacture on the same line as above a film of 200 g / m ² after drying on various supports listed in Table I. The results concerning the adhesion are also indicated in Table I, second and third columns The supports tested could not be used on the film packaging line because either the adhesion was too weak and premature delamination occurred (second column), or, on the contrary, the adhesion was too strong to allow peeling without damage to the film, especially after calendering (third column)

A cathode film was produced by preparing a third homogeneous slip comprising 55 parts of mixture:

- 72% by weight of commercial LiCoO ₂ from OMG,

- 13% by weight of DBP,

- 10% PVDF / HFP, - 5% carbon black, and 45 parts of acetone This third slip was used to manufacture, on the same line as above, a film of 200 g / m ² after drying on various supports listed in Table I. The results concerning the adhesion were equivalent to those obtained with the film anodic and recorded in Table I, second and third columns. None of the films tested was suitable for the process.

Example 2

A separating film was produced in the manner described in Example 1. An anodic film was produced using the second slip as prepared for Example 1, but using the bilayer formed by the film on the coating line. separator film and its support as a coating support. No delamination was observed on the production line and the polyester support film peeled very well after having calendered the three-layer on the collector.

A cathode film was produced using the third slip as prepared for Example 1, but using on the coating line the bilayer formed by the separating film and its support as a coating support. No delamination was observed on the production line and the polyester support film peeled very well after having calendered the three-layer on the collector. Examination of a polished section under an optical microscope shows that there is a perfect weld between the electrode and the separator (see Figure 5).

TABLE I

Note: the references designate Dupont, Sopal or Coveme grades. Advantages of the invention

The method according to the invention provides a solution to the very important problem of supporting the electrodes in the increasingly frequent case at present where the latter are no longer self-supporting. The search for the adhesion / "peelability" compromise is thus limited to the separator which is a much easier case than that of the electrodes and for which there are economic solutions available on the market which make it possible to radically eliminate the rejects and stops of linked lines. delamination of the supports. The invention also has the advantage of no longer requiring a particular support layer for each electrode layer, which leads to a substantial reduction in the manufacturing cost. In addition, it eliminates calendering operations, and more precisely an calendering operation by electrode (that which according to the prior art welds the electrode to the separator). The limited number of calendering operations required by the method according to the invention considerably reduces the risks of defects inherent in each calendering operation.

Claims

A method of manufacturing electrode films for a polymer Li-ion generator comprising at least one operation of tape-forming a separator-electrolyte film on a support, so as to form at least one separator-electrolyte / support layer, and at less an operation of casting an electrode on said separator-electrolyte layer of said bilayer

The method of claim 1, wherein said support is a plastic film, such as a polypropylene film or a polyester film

The method of claim 1, wherein said support is a nonwoven film, such as super-calendered paper or a nonwoven of polyaramide fibers.

The method of claim 2, wherein said plastic film is raw of manufacture

The method of claim 2, wherein said plastic film has undergone a matte surface treatment, such as chemical attack, corona treatment or flame treatment

Method according to claim 4 or 5, in which said plastic film is provided with a primary coating of the silicone, polyvinyl alcohol or acrylic type

Method according to one of claims 1 to 6, in which the two casting operations are carried out in line