Method for manufacturing polymer electrolytic capacitor

Abstract

The present invention relates to a method for manufacturing a polymer electrolytic capacitor, comprising the steps of: providing a metal foil comprising aluminum having a layer comprising aluminum oxide on a surface; punching an anode foil (1) from the metal foil; re-forming an oxide layer and forming an oxide layer at the stamped edge; and impregnating the oxidized anode foil (1) with a polymer electrolyte solution.

Description

Technical Field

[0001] The present invention relates to a polymer electrolytic capacitor and a method of manufacturing the same. Background Art

[0002] Polymer electrolytic capacitors are used in electronic circuits where high capacitance, low ESR, and enhanced ripple current capabilities are required.

[0003] However, such capacitors are limited to operating voltages not exceeding 400V at room temperature (see, for example: Next Generation Polymer Capacitors: Improving the Performance of Electrolytic Capacitors with Conductive Polymer Dispersions, U. Merker, K. Wussow, K. Astemann - Proceedings of the 1st PCNS Conference, 2017).

[0004] In cases where operating voltages above 400V and enhanced temperature are required, polymer electrolytic capacitors are sensitive to short - circuits (breakdown) between the anode and cathode electrodes. Therefore, other capacitor types such as liquid electrolytic capacitors are used in electronic circuits operating at voltages above 400V.

[0005] So far, capacitor manufacturers have not been able to develop and produce high - voltage polymer electrolytic capacitors because the reliability of these products is not sufficient for use in commercial electronic circuits. However, for example, for electric vehicle chargers and inverters, capacitor voltage ratings above 400V become relevant. Summary of the Invention

[0006] The present invention relates to a method for manufacturing a polymer electrolytic capacitor, which includes several steps.

[0007] In a first step, a metal foil is provided. The metal foil includes aluminum, which has a layer including aluminum oxide on its surface. Preferably, the metal foil is an aluminum foil composed of aluminum. The surface of the metal foil is preferably completely covered by the layer. The layer includes or consists of aluminum oxide.

[0008] In a further step, an anode foil is stamped from the metal foil. The anode foil may have a rectangular shape. The anode foil may also have another suitable shape. A rectangular shape allows for the construction of a compact cube capacitor. The stamped rectangular foil may have a length and width of preferably at least 0.1×0.1 cm, more preferably 3×3 cm, and preferably not exceeding 100×100 cm, more preferably 10×10 cm. The foil may or may not be square. Additionally, the stamped foil may include connection snips protruding from the rectangular shape.

[0009] During stamping, during additional optional or non-optional manufacturing steps, or during transportation, the oxide layer on the surface of the metal foil may be damaged. For example, cracks may occur in the layer. Further, the stamped edges of the foil are free after stamping, as they have not been covered by the oxide layer so far.

[0010] In a further step, the oxide layer on the surface is reformed. Preferably, in the same step, the oxide layer is also formed at the stamped edges to cover and protect the edges. Preferably, after this step, the oxide layer completely covers the foil and the edges.

[0011] By reforming, the defects in the oxide layer are repaired. Possible defects can be, for example, structural defects in the oxide structure or non-oxidized spots at the surface of the anode foil.

[0012] The formation and reformation of the oxide layer are carried out by an oxidation process. The preferred conditions of the oxidation process are detailed below.

[0013] The oxide layer forms the dielectric layer of the capacitor.

[0014] In a further step, the oxidized anode foil is impregnated with a polymer electrolyte solution (preferably a PEDOT:PSS-solution). The polymer electrolyte solution can at least partially diffuse in the oxide layer and enable a defined desired current to flow through the dielectric layer.

[0015] Preferably, the impregnation step is carried out by vacuum impregnation and then dried in a ventilated oven. The drying is preferably carried out at a temperature above 100 °C, more preferably above 120 °C.

[0016] In a further step, the capacitor is constructed from the anode foil.

[0017] Preferably, the defined steps are carried out in a prescribed order. Optional additional steps can be carried out before, after, or between the prescribed steps. In one embodiment, no additional steps may be carried out between two subsequent steps of the prescribed steps. In particular, the steps of reforming and forming the oxide layer are preferably carried out directly before impregnation with the polymer electrolyte solution. This means that no additional steps are carried out between reforming and forming the oxide layer and impregnating the oxidized anode foil with the polymer electrolyte solution.

[0018] By directly reforming and forming an oxide layer before impregnation, the formation of conductive short circuits through the oxide layer is inhibited. Thus, the breakdown voltage of the capacitor can be optimized. The breakdown voltage of a capacitor is defined as the voltage at which the dielectric layer loses its desired dielectric properties and a significantly high current appears between the anode and cathode of the capacitor.

[0019] In addition, by the disclosed method, a capacitor having a desired low ESR (equivalent series resistance) value at high frequencies of alternating current can be provided.

[0020] In one embodiment, the manufacturing method includes an additional step of stacking the anode foil, separator paper, and cathode foil to construct a capacitor stack. Preferably, the separator is directly placed on the oxide layer of the anode foil. Also preferably, the cathode foil is directly placed on the separator. The cathode foil may also include or consist of aluminum. The cathode foil may also be covered with an oxide layer, preferably an alumina layer.

[0021] The step of stacking is performed after impregnation with the polymer electrolyte solution. Thus, impregnation can be performed directly after the reformation of the oxide layer.

[0022] Preferably, the capacitor stack includes at least two separators and two cathode foils arranged on both sides of the anode foil in a prescribed order to achieve a desired capacitance.

[0023] Preferably, the foils and papers are stacked one by one, and each newly stacked sheet or foil is impregnated with the polymer electrolyte solution before the next sheet or foil is stacked. Preferably, each impregnation step is performed by vacuum impregnation and then dried in a ventilated oven.

[0024] In a preferred embodiment, the step of reforming the oxide layer and forming the oxide layer at the punched-out edges includes several steps.

[0025] In one of such steps, the surface of the anode foil or the existing oxide layer can be treated with boric acid, where the oxidation reaction is carried out by using a boric acid solution. In particular, the aluminum at the surface of the anode foil can be oxidized by the boric acid solution, and the defects or void points in the oxide layer can be repaired.

[0026] Preferably, the surface treatment with boric acid is performed at an oxidation temperature of at least 70 °C, more preferably at least 90 °C.

[0027] In particular, a direct current of at least 8 mA, preferably at least 10 mA, can be applied to the anode foil to stimulate the oxidation reaction and repair the defects in the oxide layer until the formation voltage is reached.

[0028] In particular, when a direct current is applied to the anode, the applied voltage is increased until the defects in the oxide layer are repaired and the voltage stabilizes at the formation voltage.

[0029] In a preferred embodiment, after reaching the formation voltage, a direct current of at least 8 mA, preferably 10 mA, is applied to the anode foil for at least an additional 1 to 2 minutes to repair even more defects and stabilize the oxide layer. The direct current can also be applied for a longer period of time.

[0030] In a further step, the anode foil can be depolarized by heating the anode foil to above 100 °C, preferably above 140 °C, more preferably above 150 °C. The depolarization can be carried out in a suitable oven for a few minutes, preferably for 3 to 7 minutes, more preferably for at least 5 minutes.

[0031] After depolarization, the surface treatment with boric acid can be repeated, which includes applying an oxidizing boric acid solution and applying a direct current to stimulate the oxidation reaction.

[0032] Thereafter, a step of chemical depolarization in an ammonium phosphate solution can be carried out in order to remove any excess charge in the oxide that may cause increased leakage and thus a reduced breakdown voltage. The chemical depolarization in the ammonium phosphate solution is preferably carried out at between 60 °C and 80 °C for 7 to 13 minutes, more preferably at above 70 °C for more than 10 minutes.

[0033] Preferably, thereafter the surface treatment with boric acid can be repeated again, which includes applying an oxidizing boric acid solution and applying a direct current to stimulate the oxidation reaction.

[0034] In a preferred embodiment, the surface treatment with boric acid is repeated at least three times. Thus, a stable oxide layer with a minimum number of defects can be provided, and a desired high breakdown voltage can be achieved. Preferably, the breakdown voltage can be higher than 450 V, preferably higher than 550 V or higher than 600 V and up to 650 V.

[0035] Preferably, the polymer electrolyte solution for impregnation can comprise or consist of a PEDOT:PSS-solution. PEDOT:PSS stands for poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. PEDOT:PSS is a polymer mixture of two ionomers. One component in this mixture consists of sodium polystyrene sulfonate, which is a sulfonated polystyrene. A part of the sulfonyl groups is deprotonated and carries a negative charge. The other component poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer and carries a positive charge and is based on polythiophene. The charged macromolecules together form a macromolecular salt.

[0036] More preferably, as the PEDOT:PSS solution for impregnating Clevios TM K, a PEDOT:PSS polymer dispersion from is used, which has a conductivity of up to 1000 S / cm and good chemical, thermal, and UV stability.

[0037] Using the specified polymer electrolyte solution allows for further improvement of the electrical characteristics of the capacitor. In particular, the breakdown voltage of the capacitor can be further increased.

[0038] According to another aspect, the present invention relates to a polymer electrolytic capacitor manufactured by the method as specified previously.

[0039] According to another aspect, the present invention relates to any polymer electrolytic capacitor including a stack of horizontal flat layers, where the layers include an anode layer, separator paper, and an anode layer, and where the anode layer is impregnated with a polymer electrolyte solution (preferably a PEDOT:PSS solution).

[0040] Each of the horizontal layers is formed in a homogeneous flat plane.

[0041] The polymer electrolytic capacitor can have any of the features specified previously regarding the manufacturing method, and vice versa.

[0042] In particular, the surface of the anode layer can be completely covered by an oxide layer including alumina with a minimum defect density.

[0043] Thus, more preferably, an operating voltage of the capacitor of more than 450 V, more than 500 V, more than 550 V, more than 600 V, and most preferably up to 650 V can be achieved, where the breakdown voltage of the capacitor is higher than the operating voltage.

[0044] Furthermore, by a sealing method, a capacitor having a desired low ESR (equivalent series resistance) value at high frequencies of alternating current can be provided.

[0045] In addition, the operating temperature of the provided capacitor can be raised up to 125 °C, more preferably up to 150 °C, without significant loss of electrical performance.

[0046] In addition, the horizontal layer stack allows for the design of a cube capacitor that can be arranged in a more compact and flexible manner than existing wound electrolytic capacitors. The more compact design also means a reduction in the use space and weight of the capacitor, which is particularly useful for mobile applications.

[0047] Due to the compact arrangement, a short and thus low-inductive electrical connection between the capacitor and the connected electrical components (such as an IGBT module for example) can be achieved. The parasitic impedance can be minimized.

[0048] Furthermore, several cubic capacitors can be stacked together or can be arranged in parallel, thus providing a larger capacitor with a higher total capacitance in a very simple manner.

[0049] In addition, capacitors can be connected in series to provide a capacitor with a higher total breakdown voltage.

[0050] Moreover, the electrical connection of the anode foil and the cathode foil can be configured in various shapes and can be arranged at any side of the capacitor stack in order to contact various electrical components in a compact and preferably low-inductive manner.

[0051] According to another aspect, the invention relates to the use of a polymer electrolytic capacitor at an operating voltage of up to 650V.

[0052] The polymer electrolytic capacitor can have any of the features specified previously, particularly with regard to the manufacturing method, and vice versa.

[0053] According to another aspect, the invention relates to a component comprising an electronic module having a planar substrate area and a capacitor as specified previously, wherein the outer planar surface of the capacitor and the planar substrate area are arranged in the same plane.

[0054] This allows for a compact arrangement of the capacitor and the electronic module in a device (such as in an electric motor). Moreover, this allows for a compact arrangement of, for example, both the electronic module and the capacitor on a common cooling body having a simple planar cooling surface.

[0055] In a preferred embodiment, the entire capacitor stack can be accommodated in a housing made of aluminum. The housing can be electrically connected to the cathode foil or can replace the cathode foil of the capacitor. The aluminum housing allows for high cooling performance of the capacitor and improved temperature regulation.

[0056] As a result of the desired improved cooling, the capacitor stack can be arranged even closer to the connected electrical components in order to further minimize the electrical losses.

[0057] In addition, as a result of the improved cooling, the capacitor can be used at higher temperatures, for example in an electric motor of a motor vehicle. Description of the Drawings

[0058] Hereinafter, examples of the capacitor are described in detail by using figures. However, the invention is not limited to the following examples.

[0059] The figures show:

[0060] Figure 1 : Photograph of a stack of capacitor foils and separator papers.

[0061] Figure 2 : Schematic diagram of an anode foil.

[0062] Figure 3 : Schematic diagram of an exemplary setup for reforming the anode foil.

[0063] Figure 4 : Chart showing the average ESR curves of electrolytic capacitors normalized to the ESR of a capacitor at an AC frequency of 100 Hz at different AC frequencies.

[0064] Figure 5 : Chart showing the I-U curve and breakdown voltage of an electrolytic capacitor.

[0065] Figure 6 : Chart showing the average ESR curves of electrolytic capacitors at different AC frequencies and different temperatures.

[0066] Figure 7 : Top view of a second example of a capacitor stack.

[0067] Figure 8 : Side view of a second example of a capacitor stack. Detailed Description

[0068] Figure 1 Shows different foils and sheets according to an embodiment of the present invention, which are stacked to construct a capacitor.

[0069] As Figure 2 shown, the anode foil 1 and the cathode foil 2 are stamped from a metal foil, for example in a rectangular shape with a protruding rectangular segment 4 for connection.

[0070] The anode foil is preferably an aluminum foil, which provides a layer including aluminum oxide on its surface. The cathode foil can also be an aluminum foil.

[0071] The separator paper 3 is cut slightly larger to ensure complete coverage between the electrodes.

[0072] The paper 3 can include cellulose paper, which has a thickness of, for example, 90 μm and a density of, for example, 60 g / cm 2 . Such paper provides good protection against mechanical short circuits from burrs on the foil.

[0073] Figure 3 Shows an exemplary setup for reforming the anode foil 1.

[0074] After stamping, the oxide layer of the anode foil 1 is preferably reformed at 90 °C by using a boric acid solution. A constant direct current of, for example, 10 mA can be applied, which runs from the anode foil 1 to the cylindrical counter electrode 5, as Figure 3 seen in

[0075] This results in a slowly increasing voltage while the defects in the oxide layer are repaired until the formation voltage is reached.

[0076] When the voltage curve has stabilized (indicating that the reformation of the oxide is complete), the formation voltage is reached.

[0077] When the formation voltage is reached, the current is kept on for an additional few minutes, for example 2 minutes, after which the current is switched off.

[0078] Thereafter, a thermal depolarization step is performed, in which the foil is heated up to 150 °C, for example for 5 minutes in an oven.

[0079] Thereafter, the oxide layer is reformed again by treatment with a boric acid solution.

[0080] Thereafter, the foil is chemically depolarized in an ammonium phosphate solution at, for example, 70 °C for 10 minutes.

[0081] After this, the oxide layer can be reformed again by treatment with a boric acid solution.

[0082] The steps mentioned remove the defects in the oxide layer and any excess charge in the oxide layer, which could lead to increased leakage and thus a reduced breakdown voltage of the oxide layer and the capacitor thereby.

[0083] After the initial steps described, polystyrene can also be used to cover the connecting segment 4 to prevent leakage to the untreated part of the segment.

[0084] Thereafter, the anode foil 1 is vacuum impregnated with a PEDOT:PSS-based polymer electrolyte solution, thus ensuring good penetration of the oxide layer structure by the electrolyte. Preferably, the electrolyte solution used is the commercially available PEDOT:PSS-based electrolyte solution CLEVIOS TM K SD 1. The polymer electrolyte solution contains a mixture of PEDOT:PSS in an aqueous dispersion together with different additives. Some of these additives can be polyethylene glycol, which increases the breakdown voltage of the polymer-based capacitor. Glycerol can be an additional additive as it can help to adjust the viscosity of the solution.

[0085] After impregnation of the anode foil 1, the capacitor stack is constructed of cathode - paper - anode - paper - cathode, as Figure 1As seen in. Preferably, the stack is built up layer by layer on a sample holder. Between each layer, the samples are vacuum impregnated with the polymer electrolyte solution and dried in a ventilated oven, for example at 120 °C, thus ensuring a good connection between the anode and cathode foils. After placing the final cathode foil, a glass plate can be placed on top and clamped in place on the sample holder. Thereafter, the stack can be dried for several days to ensure the evaporation of as much moisture as possible.

[0086] An Agilent E4980AL LCR meter was used to perform ESR measurements on the completed capacitor stacks at room temperature at 200 frequencies logarithmically distributed between 20 Hz and 100 kHz. A logarithmic frequency distribution of the data points was chosen because the ESR shows a very flat trend at higher frequencies, which is why a lower density of data can be used. The ESR measurements of eight different stacks according to the example described above were compared to a reference stack, where the polymer electrolyte solution was replaced by a commercially available liquid high-voltage electrolyte.

[0087] Figure 4 Shows the average ESR curves of polymer electrolytic capacitors of a first embodiment normalized to the ESR of the capacitor at an AC frequency of 100 Hz at different AC frequencies. In addition, the ESR curve of a liquid electrolyte reference sample is shown, which is also normalized to the ESR at 100 Hz. At 100 Hz, the ESR of the two capacitors is very similar at around 10 Ohm.

[0088] For the liquid electrolyte, the behavior seen is that the ESR drops to around 2 / 3 of the ESR at 100 Hz before flattening out and becomes nearly constant at high frequencies around 100 kHz.

[0089] For the polymer electrolytic capacitor, the ESR at 100 kHz drops to 11% compared to the ESR at 100 Hz. This significantly larger reduction in ESR at high frequencies makes the polymer aluminum electrolytic capacitor superior to traditional liquid electrolyte electrolytic capacitors when it comes to the ESR at high frequencies, at which modern wide-bandgap semiconductors typically operate.

[0090] In addition, for the breakdown test, a Vitrek 951i Electrical Safety Compliance Analyzer was used. A ramp voltage of 3 V / s was applied across the sample while measuring the voltage U and the current I. The peak in the I-U curve indicates the breakdown phenomenon.

[0091] In Figure 5 the breakdown voltages of the same eight capacitor stacks also used for the ESR measurements were measured at room temperature and compared to a reference stack using the same liquid electrolyte as the one used for the ESR measurements.

[0092] Persistent breakdowns can be seen above 550 V, between approximately 560 V and 595 V. This is significantly higher than any previous results known from the literature for the breakdown of polymer electrolyte capacitors.

[0093] Additional measurements show that the ESR hardly differs at elevated temperatures up to 150 °C. For example, Figure 6 shows the average ESR curve of the electrolytic capacitor at room temperature and compares it with that at 150 °C.

[0094] Figure 7 and 8 shows another example of a capacitor stack. The stacking of rectangular planar layers allows for a compact capacitor design and a compact arrangement of connection segments 4 next to each other. For example, the connection segments 4 of the anode foil 1 and the cathode foil 2 can be shifted side by side as shown in Figure 7 so that multiple electrical components can be electrically connected to the capacitor stack in a compact arrangement.

[0095] Furthermore, the compact arrangement makes it easy to connect several capacitors in series to obtain a capacitor with an even higher operating voltage. In addition, several stacks can be arranged parallel to each other or stacked on top of each other to obtain a capacitor with a higher total capacitance.

[0096] List of reference symbols

[0097] 1 Anode foil

[0098] 2 Cathode foil

[0099] 3 Separator paper

[0100] 4 Connection segment

[0101] 5 Cylindrical counter electrode

Claims

1. A method for manufacturing a polymer electrolytic capacitor, comprising the following steps: providing a metal foil comprising aluminum having a layer comprising alumina on the surface; stamping an anode foil (1) from the metal foil; re-forming an oxide layer and forming the oxide layer at the stamped edges; impregnating the oxidized anode foil (1) with a polymer electrolyte solution and constructing the capacitor from the anode foil.

2. The method for manufacturing a polymer electrolytic capacitor according to claim 1, wherein, no additional steps are performed between re-forming and forming the oxide layer and impregnating the oxidized anode foil.

3. The method for manufacturing a polymer electrolytic capacitor according to claim 1 or 2, comprising the step of stacking the anode foil (1), separator paper (3) and cathode foil (2) to construct a capacitor stack.

4. The method for manufacturing a polymer electrolytic capacitor according to any one of claims 1 to 3, wherein, the step of re-forming the oxide layer and forming the oxide layer at the stamped edges comprises the following steps: surface treatment with boric acid, which comprises oxidizing the surface of the anode foil (1) by using a boric acid solution and applying a direct current of at least 8 mA, preferably at least 10 mA, to the anode foil (1) until the formation voltage is reached to repair defects in the oxide layer; depolarization of the anode foil (1) by heating the anode foil (1) to above 100 °C; repeating the surface treatment with boric acid; and chemical depolarization in an ammonium phosphate solution.

5. The method for manufacturing a polymer electrolytic capacitor according to claim 4, wherein, the surface treatment with boric acid is performed at an oxidation temperature of at least 70 °C, preferably at least 90 °C.

6. The method for manufacturing a polymer electrolytic capacitor according to claim 4 or 5, wherein, after reaching the formation voltage, a direct current of at least 8 mA, preferably 10 mA, is applied to the anode foil (1) for at least an additional 1 to 2 minutes.

7. The method for manufacturing a polymer electrolytic capacitor according to any one of claims 4 to 6, wherein, the depolarization of the anode foil (1) by heating the anode foil (1) is performed for 3 to 7 minutes.

8. The method for manufacturing a polymer electrolytic capacitor according to any one of claims 4 to 7, wherein, the depolarization of the anode foil (1) by heating the anode foil (1) is performed by heating the anode foil (1) to above 150 °C.

9. The method for manufacturing a polymer electrolytic capacitor according to any one of claims 4 to 8, wherein, the chemical depolarization in an ammonium phosphate solution is performed at 60 °C to 80 °C for 7 to 13 minutes.

10. The method for manufacturing a polymer electrolytic capacitor according to any one of claims 4 to 9, wherein, the surface treatment with boric acid is repeated at least three times.

11. The method for manufacturing a polymer electrolytic capacitor according to any one of claims 1 to 10, wherein, The polymer electrolyte solution includes a PEDOT:PSS solution.

12. A polymer electrolytic capacitor manufactured by the method according to any one of claims 1 to 11.

13. A polymer electrolytic capacitor comprising a stack of horizontal layers, the layers including an anode layer (1), a separator paper sheet (3), and a cathode layer (2), wherein, at least the anode layer (1) is impregnated with a polymer electrolyte solution, preferably a PEDOT:PSS solution.

14. The polymer electrolytic capacitor according to claim 13, wherein, the surface of the anode layer (1) is completely covered by an oxide layer including alumina having a minimum defect density.

15. Use of the polymer electrolytic capacitor according to any one of claims 12 to 14 at an operating voltage of up to 650V.

16. An assembly comprising an electronic module having a planar substrate region and a capacitor according to any one of claims 12 to 14, wherein, the outer planar surface of the capacitor and the planar substrate region are arranged in the same plane.

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