CH718616A2 - Process for the production of a current collector for a thin battery. - Google Patents

Abstract

According to the method of the invention, a current collector (5a, 5b) for a thin battery is produced by a technique of printing or sputtering onto a substrate formed from a battery conditioning material. The layer obtained by printing or sputtering comprises particles of an electrically conductive material. The printing or spraying step is followed by curing using a light source, thereby obtaining the current collector. The printed or sputtered layer is produced having the required shape of the current collector, so that no punching or other shaping operations are required. The current collectors of a battery according to the invention have mechanical and electrical properties that are comparable or improved with respect to conventional current collectors based on sheet or mesh.

Description

Field of the invention

The present invention relates to thin batteries such as those used in electronic watches or other portable devices.

State of the art.

[0002] Thin batteries for the electronic device market comprise at least one pair of current collectors in physical contact respectively with the cathode and the anode of the battery. Cathode and anode materials are based on a given chemistry, often featuring lithium for the anode and magnesium oxide for the cathode for making Li-ion based batteries. The electrodes are separated by a separator sheet which contains a liquid or solid electrolyte and this assembly of layers is packaged in a suitable packaging material.

[0003] The standard way of producing such batteries involves fabricating a current collector sheet or mesh, formed from an electrically conductive material, e.g. copper or aluminum, followed by coating cathode and anode materials on the current collector and subjecting the collector foil or mesh to a punching procedure to achieve the required size and geometry of the collectors as determined by the geometry and the dimensions of the battery packaging material.

[0004] This punching procedure is known to result in defects such as coating cracking or edge defects in the current collector foil or mesh, which can be potentially detrimental to battery performance. Moreover, any change in the form factor of the current collector requires the development of new tools increasing the cost and the production time.

Summary of the invention

The invention aims to provide a solution to the problems described above. This object is achieved by the method as disclosed in the appended claims, and by a battery capable of being obtained by the application of said method. According to the method of the invention, a current collector for a thin battery is produced by forming a current collector layer on a substrate formed from a battery conditioning material, using a technique printing or spray deposition. The layer includes particles of an electrically conductive material. The printing or sputtering step is followed by curing the layer using a light source, thereby obtaining the current collector. According to preferred embodiments, the curing technique applied is a low temperature sintering process, such as photon sintering or laser sintering. The printed or sputtered layer is produced having the required shape of the current collector, so that no punching operation or other shaping operations are required, which solves the problems described above relating to such operations. The inventors have established that current collectors produced by printing or spraying and curing in accordance with the invention have mechanical and electrical properties comparable or improved compared to conventional current collectors based on sheet or mesh. A battery according to the invention is characterized in that the resistivity of the current collectors of the battery is greater than the volume resistivity of the material of the current collectors.

Brief Description of Figures

[0006] Figure 1 shows a packaging substrate for a thin battery, cut to a geometry that is compatible with the method of the invention. Figure 2 illustrates the geometry of the current collectors obtained by screen printing or spray deposition and curing, on the substrate of Figure 1. Figure 3 illustrates the production of the electrodes on the current collectors. Figure 4 shows the assembled thin battery obtained by carrying out additional assembly steps after the steps illustrated in Figures 1 to 3. Figures 5 to 7 illustrate alternative conditioning substrates and the application thereto of a method in accordance with the invention. Figure 8 compares the electrical performance of three batteries, by comparing the performance of batteries comprising current collectors obtained by the method of the invention with a battery comprising conventional current collectors.

Detailed Description of Embodiments of the Invention

Referring to Figure 1, the first step of the method of the invention is to provide a substrate 1 in the form of a planar packaging material for a thin battery. In the embodiment shown, the substrate 1 is a pre-cut piece of packaging material, which is bendable along a centerline 2. In thin-film battery technology, this type of substrate is also called a pocket. The material may be any type of known battery packaging material, for example a multi-layered material comprising a metal layer sandwiched between layers of synthetic material. The substrate 1 is placed on a planar surface with the layer which is intended to be in contact with the current collectors of the battery facing upwards. This upwardly facing layer is formed from a heat sealable material, for example a polypropylene material or a polyethylene material. In the embodiment shown, the packaging material is pre-cut in the form of a rectangle extending on both sides of the fold line 2, said fold line dividing the substrate into a first and a second rectangular part 3a and 3b. A first and a second tongue 4a and 4b extend outward from the upper edge of the respective rectangular parts 3a and 3b, at locations which are eccentric with respect to the center lines of said rectangular parts 3a and 3b. The off-center positions of tabs 4a and 4b are chosen so that when substrate 1 is folded along fold line 2, tabs 4a and 4b do not overlap (see below).

[0008] According to a preferred embodiment, the upwardly facing surface of the conditioning substrate is pretreated with a surface activating treatment, such as a corona plasma treatment. Surface activation increases surface energy and decreases the risk of defects in the subsequent printing step or subsequent sputter deposition step applied to produce a current collector layer. The term "current collector layer" is defined in the present context as a layer capable of being obtained as the direct result of printing or sputtering onto the packaging substrate. The current collector layer includes electrically conductive particles that do not form a homogeneous conductive sheet. When screen printing is used, this last step is carried out using a metallic ink such as silver ink or copper ink, or any other metallic ink suitable for screen printing. a metal which is suitable to serve as a current collector for a thin battery. The screen printing step is performed in areas which are offset from the boundaries of the rectangular portions 3a and 3b and the tabs 4a and 4b, resulting in thin current collector layers 5a and 5b of the printed metallic ink, in rectangular zones which are offset with respect to the limits of the rectangular parts 3a and 3b. The current collector layers 5a and 5b include tongue zones 6a and 6b offset with respect to the limits of the tongues 4a and 4b. The offset distance is preferably the same everywhere along the circumference of the printed areas, creating a margin 10 of constant width.

Screen printing is known as such and the details of this process therefore do not need to be described. A commercially available screen printing tool can be used, for example from the supplier Aurel®. The screen printing of the layers 5a and 5b (comprising the tabs 6a and 6b) is carried out under conditions which are compatible with the packaging material on which the layers are printed. According to preferred embodiments, the thickness of the printed current collector layers is between 2 μm and 50 μm. The inks used in the method of the invention are inks comprising nanoparticles of electrically conductive material, preferably metal. Silver or copper inks of this type are known and available from various suppliers, such as Dupont®.

[0010] Other printing processes can be used to obtain current collector layers having characteristics similar to the layers obtained by screen printing, such as flexographic gravure and 3D printing. As an alternative to printing, the current collector layers 5a and 5b having the characteristics described above can be obtained by sputter deposition.

[0011] The layer obtained by printing or sputtering is a layer comprising particles, preferably nanoparticles, of the electrically conductive material. Whether or not such a particle-based layer is obtainable by printing or spray deposition depends primarily on the material used. Ag and Cu are prime examples of materials suitable for this purpose, but the invention is not limited thereto. Ni can also be used. Stainless steel, however, is not a suitable material for this.

[0012] After the step of printing or deposition by sputtering, the current collector layers 5a and 5b (comprising the tabs 6a and 6b) are subjected to hardening in order to produce the current collectors strictly speaking under the form of a homogeneous electrically conductive layer. Curing is effected by illuminating the printed or sprayed layers with a light source so that the homogeneous sheet is obtained without excessive heating of the packaging material. According to preferred embodiments, low temperature sintering techniques are used for this purpose, such as photonic sintering or laser sintering. Low temperature sintering is defined as sintering which is applicable at room temperature, more particularly at any temperature between 15°C and 30°C, or between 20°C and 25°C.

[0013] Photonic sintering is a process known as such, involving the application of short pulses of high energy light on a layer comprising nanoparticles in order to heat the layer and obtain a homogeneous layer of the particulate material, in very short time, and with minimal thermal impact on the substrate on which the layer was deposited. Tools known for the application of this process can be used in the method of the invention, such as those available for example from the supplier Novacentrix®. It has been found that a typical thin battery packaging material is compatible with a photonic sintering process applied at room temperature. According to preferred embodiments, the parameters of the photonic sintering process applied in the method of the invention lie within the following ranges: Pulse length between 1 and 20 ms Pulse frequency (number of pulses per second) between 200 and 800 Sintering impulse voltage: between 1 kV and 4 kV Duration of the photo-sintering process: between 0.1 seconds and 5 seconds According to additional preferred embodiments, the energy input by the photonic sintering process in the current collector layers is between 0.5 and 4 J /cm<2>.

The sintering process results in the current collectors 5a and 5b as such, in which the nanoparticles of the printed ink layers are homogenized into uniform layers. The sintering process thus amplifies the electrical conductivity of the current collectors 5a and 5b. This completes the process steps of the method of the invention for the production of the current collectors. The invention however relates just as much to a process for the production of a battery comprising the current collectors thus produced. The process steps for the production of the battery as such are described below.

[0015] As a result of the curing process, electrodes 7a and 7b are produced on current collectors 5a and 5b, as shown in Figure 3. particular, for example Li for the anode and MgO2 for the cathode. The layers can comprise said active materials, mixed with a binder, for example a copolymer of vinylidene fluoride (PVdF), an agent conferring electron conductivity (for example carbon black) and a solvent. The electrode layers 7a and 7b can be applied by any method known in the state of the art, including screen printing. The layers of electrodes 7a and 7b are preferably offset with respect to the limits of the main rectangular part of the current collectors 5a and 5b, that is to say that the electrodes are not produced on the tabs 6a and 6b of said current collectors.

This is followed by the application of a separator sheet (not shown), as is well known in the art. The separator sheet can comprise a solid electrolyte or it can be supplied with a liquid electrolyte dispensed onto the separator sheet after the positioning of the sheet.

[0017] Next, the substrate 1 is folded along the fold line 2 and sealed along the margin 10, as shown in Figure 4. The tabs 4a and 4b do not overlap, and the margins of the material of packaging along the edges of the tabs are not sealed, leaving the current collector tabs 6a and 6b available as external contacts for the battery. This type of sealing process is known as such, and can be carried out according to any known method in use today for the production of thin batteries.

According to an alternative embodiment illustrated in Figures 5 to 7, two separate conditioning substrates 1a and 1b are provided, having the same geometry as the parts 3a + 4a and 3b + 4b of the substrate 1 of the first mode of achievement. On these conditioning substrates, the current collectors 5a and 5b are obtained by the process of the invention, for example by screen printing and photonic sintering, and the electrodes 7a and 7b are obtained by a known process. Joining these separate substrates 1a and 1b results in a sealed battery having the same appearance as the battery shown in Figure 4.

The method of the invention is just as applicable to a conditioning substrate which is not provided with tabs 4a and 4b, but in which the conditioning material comprises a metal layer sandwiched between synthetic layers, each provided with an opening that exposes the sandwiched metal layer of the package. The current collector produced by the method of the invention is then in direct contact with the exposed metallic layer of the packaging material. The opening on the other side of the packaging then serves as a contact strip for the battery. This type of packaging material is described for example in the patent publication document WO2020007584A1, included here by way of reference.

The current collectors obtained according to the invention have a resistivity which falls within the line of stainless steel current collectors produced in a conventional manner, or represents an improvement with respect to these, and they allow the production batteries having excellent performance, while not suffering from the main problems of the state of the prior art set out in the introduction. The current collectors are produced directly in the correct geometry, so that procedures for punching the current collectors are no longer necessary, thereby overcoming the problems relating to such procedures. Additionally, developing and producing different battery geometries and sizes requires less time and equipment, reducing production costs and complexity.

[0021] While the resistivity of the current collectors obtained by the method of the invention is comparable to or lower than that of conventional stainless steel current collectors, a characteristic of the current collectors (and consequently of the batteries) produced by the method of the invention resides in the fact that the resistivity of the current collectors is greater than the volume resistivity of the electrically conductive material of which the collectors are formed. This means that the resistivity of a current collector in a battery according to the invention is greater than the resistivity of a current collector based on a sheet or a mesh formed from the same material. At the same time, the thickness of the current collectors in a battery according to the invention is comparable to the thickness of conventional current collectors based on sheet or mesh. According to preferred embodiments, the thickness of the current collectors in a battery according to the invention is between 2 μm and 50 μm. According to preferred embodiments, the resistivity of a current collector of a battery according to the invention is between 2 and 100 times the volume resistivity of the electrically conductive material applied in the current collectors, the thickness of the current collectors being between 2 µm and 50 µm. For Ag and Cu, this means that the resistivity of the current collector of a battery produced according to the method of the invention is respectively within the following ranges: For Ag: between 3.18E-08 Ωm and 1.59E-06 Ωm For Cu: between 3.36E-08 Ωm and 1.68E-06 Ωm

A certain number of tests have been carried out by the inventors, which are proof of the excellent performance of the current collectors obtained by the method of the invention. The tests and results are described below.

Preparation of silver-based current collector

[0023] A silver ink for screen printing applications was purchased from Dupont®. The ink was screen printed using a screen printing process machine purchased from Aurel®. Before printing, the pocket was pretreated with a corona treatment at 200 W. The stencil used in this example was of the 325-326 steel stencil type allowing a printed thickness varying between 10 and 20 μm. After printing, a sintering process was applied using a photonic sintering machine from the company Novacentrix®. A 2 ms long, 2 kV pulse envelope was used to deliver a total energy of 2 J/cm<2>. Finally, the electrical conductivity and thickness were measured and compared with a reference current collector sheet produced by the conventional production technique.

[0024] Table 1 shows the results. Reference 25 ± 1 4.4E-02 ± 1.7E-03 62 ± 2 Current collector printed in Ag 13 ± 1 3.6E-02 ± 5.0E-03 28 ± 9

Table 1

*expressed as the number of times the bulk Ag resistivity must be multiplied to arrive at the sample resistivity

As shown in Table 1, the printed/sintered current collector has acceptable thickness, comparable sheet resistance, and improved resistivity compared to a conventional stainless steel current collector sheet used in this example as a reference. In test batteries produced based on these current collectors (see later), it has been observed that the higher resistivity of the stainless steel current collector sheet measured in Table 1 results in a lower capacity compared to printed/sintered current collectors.

Preparation of copper-based current collector

[0027] The process steps described in the previous paragraph have also been carried out to produce copper-based printed current collectors. Copper-based ink was purchased from Novacentrix®. The ink was printed using a screen printing machine already detailed in the example of the silver current collector. The stencil used in this example was a PET 140-34 allowing a printed thickness varying between 10 and 20 µm. After printing, a photonic sintering process was applied. A pulse envelope of 2 ms in length and 1 kV was used to provide a total energy of 1 J/cm2. Table 2 compares the results of the screen-printed copper current collector with the foil-based current collector used for reference. Reference 25 ± 1 4.4E-02 ± 1.7E-03 66 ± 2 Current collector printed in Cu 17 ± 2 4.6E-02 ± 7.5E-03 50 ± 2

Table 2

[0028] In this case, the resistivity value of the printed/sintered copper current collector is also slightly improved compared to the conventional stainless steel current collector sheet.

[0029] After characterizing the current collector, primary batteries based on the Li/MnO2 chemical system were assembled and tested electrochemically. In order to identify the performance of each printed current collector, one of the conventional current collector foils was replaced with a printed/sintered foil. This approach allows the individual characterization of each printed current collector. In order to compare the results, reference batteries based on conventional current collector foils were also assembled and tested. The batteries were discharged at a constant current rate of C/10 corresponding to 10 hours to discharge the cells down to 2 V. Three types of batteries were assembled and tested: a. The reference named CP042350 (the two current collectors are sheet collectors, using stainless steel sheets having a thickness of 25 µm) b. A printed copper current collector named CP042350GS (a foil collector and a printed Cu collector) c. A printed silver current collector named CP042350GS+ (one foil collector and one printed Ag collector) d. The results in Figure 8 represent how the printed/sintered current collector can be a valid alternative to conventional foil without impacting discharge performance.

While the invention has been illustrated and described in detail in the foregoing drawings and description, such illustration and description should be considered illustrative or exemplary and not restrictive. Other variations to the disclosed embodiments may be understood and implemented by those skilled in the art in practicing the claimed invention from a study of the accompanying drawings, disclosure and claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "a" does not exclude a plurality. The mere fact that certain measures are cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. No reference sign in the claims should be construed as limiting the scope.

Claims (13)

1. Method for the production of a current collector (5a, 5b) for a battery, the method comprising the steps of: providing a substrate (1) formed from a planar battery conditioning material, – production, on a predefined area of the substrate, of a current collector layer by a technique of printing or sputtering, the current collector layer comprising particles of an electrically conductive material, – hardening of the current collector layer by illuminating the layer with a light source, which makes it possible to obtain the current collector (5a, 5b). 2. The method of claim 1, wherein the current collector layer is produced by a printing technique selected from the group consisting of screen printing, flexographic gravure and 3D printing. 3. Method according to claim 1 or 2, in which the step of hardening is carried out by sintering at low temperature. 4. Method according to claim 3, in which the hardening step is carried out by photonic sintering or laser sintering. 5. Method according to claim 4, in which the curing step is carried out by a pulse photonic sintering process, by applying the following parameters: – Pulse length between 1 and 20 ms – Pulse frequency (number of pulses per second) between 200 and 800 – Impulse sintering voltage: between 1 kV and 4 kV – Duration of the photo-sintering process: between 0.1 seconds and 5 seconds. 6. Process according to claim 5, in which the energy input applied during the photonic sintering step is between 0.5 J/cm2 and 4 J/cm2. 7. Method according to any one of the preceding claims, further comprising the step of pretreating the substrate (1) by a surface activation treatment. 8. A method according to any preceding claim, wherein the curing step is carried out at room temperature. 9. Method according to any one of the preceding claims, in which the current collector layer obtained by printing or sputtering has a thickness between 10 µm and 20 µm. 10. A method for producing a battery, comprising the steps of producing a first and a second current collector (5a, 5b) by applying a method according to any preceding claim, followed by the steps of: – production of a first electrode (6a) on the first current collector (5a), – production of a second electrode (6b) on the second current collector (5b), – production of a separator material comprising an electrolyte on the first and/or the second electrode, – assembly of the first and second electrodes and the current collectors and the sealing of the packaging materials used in the production of the current collectors, thus obtaining the battery. 11. Battery comprising a planar conditioning substrate comprising a first and a second current collector (5a, 5b), in which the current collectors are adapted to be obtained by printing or sputter deposition of current collector layers on the substrate, and by curing the current collector layers. 12. Battery comprising a planar conditioning substrate comprising a first and a second current collector (5a, 5b), characterized in that the current collectors are formed of an electrically conductive material and in which the resistivity of the current collectors is greater than the volume resistivity of said material. 13. Battery according to claim 12, in which the resistivity of the current collectors (5a, 5b) is between 2 and 100 times said bulk resistivity and in which the thickness of the current collectors is between 2 μm and 50 μm.