The invention relates to a method for manufacturing a perforated electrode active material film, to a method for manufacturing an electrode for an energy storage device, to a perfo- rated electrode active material film, to an electrode comprising the perforated film, and to an energy storage device comprising the electrode.

Electrical energy storage cells including batteries and various species of capacitors and ultracapacitors are widely used to provide power to different kinds of devices. In particular, lithium-ion batteries (LiBs) have attracted considerable attention for its application in elec- trie vehicles (EVs). One of the most important performance parameters of the energy storage cells is their internal resistance, also known as equivalent series resistance, which limits both charge and discharge rates of the electrodes and loses energy which dissi pates as heat. Maximizing the charge and discharge rates is critical in many applications. For instance, cells with high discharge rates used in EVs can provide high instantaneous power during acceleration and climbing. In addition to high power density, the fast charging ability will decrease the charging time greatly, which is beneficial for reducing the range anxiety for EVs.

In general, the C-rates performance of an electrode may be increased by decreasing the thickness of the electrode active material or lowering its compacted density. Both strate- gies will sacrifice the energy density of the energy storage device. In addition, the cost per energy unit will be increased since more current collector and package materials are re quired. Another feasible method to increase the C-rates performance is to make holes on the electrode to decrease the lithium-ion transmission path, which can alleviate the polari zation of the electrode to great extent under high current density. A common way for mak- ing holes in electrodes is to use a laser to etch the electrodes. However, this requires ex pensive and complicated instrumentation, and the etching speed is quite slow, which does not match with commercial roll-to-roll production lines. Mechanical punching is also used to make holes in electrodes which comprise current collectors coated with the active ma- terial. However, metal particles from current collectors may be introduced when punching, which may result in internal short circuit in the energy storage device. This can cause thermal runaway of the cells.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features, which are specified in the description below and in the claims that follow. The invention is defined by the independent patent claims, while the dependent claims define advantageous embodiments of the invention.

In a first aspect, the invention relates more specifically to a method for manufacturing a perforated electrode active material film, wherein the method comprises the steps of: manufacturing a self-supporting electrode active material film; and making holes in the electrode active material film to obtain a self-supporting perforated electrode active mate rial film.

By making holes in the self-supporting electrode active material film before the film is lam- inated to a current collector, the risk of introducing particles from the current collector, typically metal particles, into a final energy storage device is removed. The holes in the perforated electrode active material film may penetrate the film and be spread out over the surface, typically relatively evenly over the surface of the film. The self-supporting film should be able to support itself and be handled, e.g. rolled onto and out from a storage roll, without requiring to be attached to a further support material and without breaking. In one embodiment the tensile strength of the film may be 100 N/m or more to enable to the self-supporting function. By “self-supporting” is also meant the film may be formed with sufficient tensile strength and/or ductility so that the film may be free-standing.

The self-supporting electrode active material film may for example be produced by blend- ing a mixture of active material, conductive additive, fibrillizable polymer, and non- fibrillizable polymer, and then fibrillizing the mixture using high shear. As alternatives to blending, the mixture may also be mixed using acoustic resonance or tumbling. The mix ture may then be compressed using a laminator to form a self-supporting electrode film with the desired thickness. The thickness of the electrode film may for example be 5-500 pm. The laminator may be a calender machine. The rolls of the laminator may be heated when the film passes through the gap between the two rolls, for example to a temperature of 80-200 ^° C. With this heating, the electrodes will become more flexible and have more mechanical strength, which is beneficial to the further production process. The step of making holes in the electrode active material film may comprise punching holes in the film using mechanical force, for example punching, drilling, or cutting. This is a simple, cost-effective, and commercially compatible method for making holes, compared to e.g. using a laser. The normal drawbacks of this simple method are avoided by making holes in the electrode active material film before it is laminated to a current collector. The holes may for example be made using a roll with microneedles, wherein the shape of the needles is tapered or columnar. Using a roll with microneedles is particularly well suited for production of energy storage devices, since rolls are often involved in the process, e.g. for calendering, and the hole punching step may therefore be performed as a continuous process. Alternatively, microneedles may be positioned on a surface of a ram that can be punched repetitively towards the film to make a plurality of holes in each punch, in which case for each punch the film should be translated a distance corresponding to the size of the ram surface. The diameter of columnar needles may be e.g. 20-100 pm, and tapered needles may have a bottom diameter of e.g. 20-100 pm. This may produce through holes with a diameter of about 20-100 pm at the top of the electrode. The distance of neighbour ing needles may be about 20-500 pm.

In a second aspect, the invention relates more particularly to a method for manufacturing an electrode for an energy storage device, where the method comprises the steps of: manufacturing a perforated self-supporting electrode active material film according to the first aspect of the invention; and stacking the perforated film onto a coated current collec tor, wherein the coated current collector is coated with a conductive primer layer, to obtain a laminated electrode. The current collector may typically be a metal foil, and the thick ness of the primer layer may typically be 1-5 pm. This method is cost-effective, and the risk of including particles from the current collector in a hole-punching step is avoided. The perforated film may typically be stacked on both sides of the current collector.

In one embodiment, the step of stacking the perforated film onto a coated current collector coated with a conductive primer layer may comprise casting a conductive adhesive slurry onto a current collector. The conductive adhesive slurry may for example contain a mix ture of 49% polyacrylic acid (PAA), 50% carbon black, and 1% carbon nanotubes (CNT) in water, or 49% poly(1,1-difluoroethylene) (PVDF), 50% carbon black, and 1% CNT in N- methyl-2-pyrrolidone (NMP). When the coated current collector is coated with an adhesive wet slurry, stacking of the perforated film onto the current collector may be carried out with low pressure, for example less than 50 pounds per linear inch (8.9 kg per linear cm), to avoid the risk of blocking the holes due to a high pressure. This may for example be per formed using a calender machine. The laminated electrode may thereafter be rolled onto a storage or handling roll for further processing. The adhesion between the perforated film and the coated current collector will typically be strong even at room temperature and un der low pressure lamination.

The method may additionally comprise the step of drying the laminated electrode after its manufacture, for example using a blast oven to dry the final electrode roll. Removing all, or at least almost all, traces of water will remove or decrease the risk of gassing within the final energy storage device, and thereby increase the safety of the device. Other sub stances which potentially may be volatile in the assembled cell may also be removed in this way. In one embodiment, the method may additionally comprise drying the coated current col lector to obtain a dry conductive primer layer before the step of stacking the perforated film onto the coated current collector to obtain a laminated current collector. In this way it is not necessary to dry the laminated electrode after its manufacture since only dry com ponents are used in the assembly. An advantage of this is that water and other potentially volatile substances are more easily removed from the current collector before they are entrapped between the current collector and the perforated film.

The step of stacking the perforated film onto the coated current collector to obtain a lami nated electrode may comprise compressing the perforated film and the coated current collector between two rolls of a laminator, for example with a pressure of around 200-3000 pounds per linear inch (36-536 kg per linear cm). Additionally, the step of stacking the perforated film onto the coated current collector to obtain a laminated electrode may com prise heating of at least one of the rolls, for example to a temperature of 80-200 °C. These two embodiments, i.e. including compressing the perforated film and the coated current collector between two rolls with a high pressure and/or heating at least one of the rolls, are particularly advantageous if the coated current collector is dried before the perforated film is stacked onto it, or if a dry, non-adhesive primer layer is used in the manufacture of the electrode, since a high pressure and/or a high temperature may increase the adhesion between the perforated film and the coated current collector. The best results are typically obtained when using both high pressure and high temperature. In a third aspect, the invention relates more particularly to a perforated electrode active material film for an energy storage device, wherein the film is self-supporting. The film may be manufactured using the method according to the first aspect of the invention. The film may have a thickness of e.g. 30-200 pm for optimum rate capability. The holes may typically penetrate the film, be distributed relatively uniformly in the film, and have a diam eter of 5-200 pm, for example 20-100 pm.

In a fourth aspect, the invention relates to an electrode comprising the perforated elec trode active material film according to the third aspect of the invention. In a fifth aspect, the invention relates more particularly to an energy storage device com prising the electrode according to the fourth aspect of the invention. The energy storage device may for example be a battery, a supercapacitor, or a lithium-ion capacitor.

In the following is described examples of preferred embodiments illustrated in the accom panying drawings, wherein: Fig. 1 shows selected steps of a process of manufacturing an electrode of an en ergy storage device in accordance with an embodiment of the invention;

Fig. 2 illustrates the principle of the hole-punching step of the process of Fig. 1; Fig. 3 shows a setup for the hole-punching step of Fig. 2 performed using calen der rolls; and Fig. 4 shows a setup for the step of laminating the film and the current collector.

In the following is described examples of embodiments of the invention. In the drawings, the reference numeral 1 indicates a self-supporting electrode active material film. The drawings are illustrated in a schematic manner, and the features therein are not neces sarily drawn to scale. Figure 1 shows a flow diagram for a method according to the invention for manufacturing a perforated electrode active material film and an electrode comprising the film. The start ing point for the perforated electrode active material film is to provide active materials, conductive additives, and binders, which are mixed by e.g. blending, acoustic resonance, or tumbling. This mixture is then combined with a fibrillizable binder and mixed by high shear force to obtain a bulk material and binder mixture, which is calendered to obtain a self-supporting electrode active material film. Holes are then punched in the film by means of mechanical force to obtain the perforated self-supporting electrode active material film. To obtain the electrode comprising the film, a binder, conductive additive, and solvent are mixed to obtain a conductive primer slurry. This slurry is then cast coated onto a current collector, e.g. a copper foil, to obtain a coated current collector which is coated with the conductive primer. Finally, the coated current collector with conductive primer and the perforated self-supporting electrode active material film are laminated by calendaring to obtain the electrode comprising the perforated film.

Figure 2 illustrates the principle of using a roll 3 with microneedles 5 to punch holes 9 in a self-supporting electrode active material film 1 to obtain a self-supporting perforated elec trode active material film 7.

Figure 3 shows how the step of figure 2 can be performed practically in large scale. A long sheet of the self-supporting electrode active material film 1 is unrolled from a first storage roll 11 and suspended between a first 13 and second 15 suspension rolls to provide a flat region without any bends for the hole-punching step. A rotating roll 3 with microneedles 5 will punch holes in the film 1 to provide a perforated film 7. A plate 17 with holes which are complementary to the microneedles 5 is positioned on the other side of the film 1 than the roll 3 to provide a firm background to press the film 1 against. In this way it is assured that the microneedles 5 will punch holes 9 in the film 1, and not push the film 1 slightly away or break it. A dust collector 19 is positioned below the plate 17 to collect any dust which may be produced in the hole-punching step. The perforated film 7 is then run onto a storage roll 23 for further processing. Tension control rolls 21, 25 are employed to control the ten sion of film 1 , 7 at different positions.

Figure 4 shows how the perforated self-supporting film 7 and the coated current collector 35 are stacked together to obtain a laminated electrode 29 for an energy storage device. A current collector 27 is run out from a current collector roll 31, and an adhesive conduc tive primer slurry 33 is cast onto the current collector 27 to obtain the coated current col lector 35 which is coated with a conductive primer layer. The perforated film 7 and the coated current collector 35 are then stacked together between two rolls 37, 39 to provide the laminated electrode 29. Finally, the laminated electrode 29 is rolled onto a laminated electrode storage roll 41 for further processing. Tension control rolls 43, 45, 47 are em ployed to control the tension of the current collector 27, the laminated electrode 29, and the perforated self-supporting film 7, respectively, at different positions. For most practical uses, a perforated self-supporting film 7 may be stacked on each side of the current col lector 27, either in one step with two perforated self-supporting films 7 or by repeating the stacking process shown in figure 4.

For the anode fabrication, the active material may be for example graphite, hard carbon, soft carbon, Si, SiO, lithium titanate (LTO), or a mixture thereof. For the cathode fabrica- tion, the active material may be lithium-ion phosphate (LFP), lithium nickel manganese cobalt (NCM), lithium manganese oxide (LCO), lithium nickel cobalt aluminum (NCA) or a mixture thereof. The dry blend could comprise about 70% to 98% active materials, about 0.5% to 10% fibrillizable polymer PTFE, about 0.5% to 5% non-fibrillizable binder, about 3% to 10% conductive additives. The non-fibrillizable binder may be e.g. PAA, PVDF,

CMC, SBR, polyethylene (PE), polypropylene (PP), or a mixture thereof.

Examples

Example 1: Particles of graphite, carbon black and binders are dry mixed to provide the active material film. A V-blender is used for the dry mixing to get the mixture of dry pow- ders. The powders and proportions are as fellow: 92 percent by weight of graphite, 2% fibrillizable PTFE, 3% non-fibrillizable polymer poiyvinylidene fluoride (PVDF), about 3% carbon black. High shear is used to fibrillize the dry powder mixtures. The high shear equipment is a jet mill, wherein the high pressure gas is applied to the dry mixed powders which physically stretch the polymer particles to form fiber net to bind the graphite and carbon black. The high pressure gas here is compressed air with the pressure between 20 PSI to 110 PSI. After fibrillizing the dry powder mixtures, the mixture is fed into a high pressure nip to form electrode film, in this case the equipment is a calender machine. The mixed dry powder is then fed into the gap between two rolls. The thickness of the film is adjusted according to the gap of the two rolls. The thickness of the film is 80 pm. The rolls of calender are heated to approximately 160 °C to increase the flexibility and mechanical strength of the film.

Example 2: Particles of graphite, carbon black and binders are dry mixed. A V-blender is used for the dry mixing to get the mixture of dry powders. The powders and proportions are as fellow: 92 percent by weight of graphite, 2% fibrillizable PTFE, 3% non-fibrillizable polymer polyacrylic acid (PAA), about 3% carbon black. The jet mill with high pressure of air is used to fibrillize the dry powder mixtures. After fibrillizing the dry powder mixtures, the mixure is fed into the gap of two rolls of calender machine. The thickness of the film is 60 pm. The rolls of the calender are heated to approximately 160 °C to increase the flexi bility and mechanical strength of the film. Example 3: Particles of activated carbon, carbon black and PTFE are dry mixed using a V-blender to get the mixture of dry powders. The powders and proportions are as fellow: 95 percent by weight of activated carbon, 2% fibrillizable PTFE, about 3% carbon black. A jet mill with high pressure of air is used to fibrillize the dry powder mixtures. After fibrillizing the dry powder mixtures, the mixture is fed into the gap of two rolls of calender machine to get the activated carbon electrode film. The thickness of the film is 60 pm. The rolls of calender are heated to approximately 160 °C to increase the flexibility and mechanical strength of the film. Example 4: Particles of LFP, carbon black and PTFE are dry mixed using the V-blender to get the mixture of dry powders. The powders and proportions are as follows: 95 percent by weight of LFP, 2% fibrillizable PTFE, about 3% carbon black. The jet mill with high pressure of air is used to fibrillize the dry powder mixtures. After fibrillizing the dry powder mixtures, the mixture is fed into the gap of two rolls of a calender machine to get the LFP electrode film. The thickness of the film is 80 pm. The rolls of calender are heated to ap proximately 160 °Cto increase the flexibility and mechanical strength of the film.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodi ments without departing from the scope of the appended claims. In the claims, any refer- ence signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.