Method for manufacturing a polymer electrolytic capacitor

The present invention concerns a polymer electrolytic capacitor and a method to manufacture the same .

Polymer electrolytic capacitors are used in electronic circuits where high capacitance , low ESR and enhanced ripple current capability are needed .

However, such capacitors are limited to a working voltage of not more than 400 V at room temperature ( see e . g . : Next Generation of Polymer Capacitors : Boosting Performance of Electrolytic Capacitors with Conductive Polymer Dispersions , U . Merker, K . Wussow, K . Astemann - Proceedings of 1 st PCNS Conf . , 2017 ) .

In cases where working voltages of more than 400 V and enhanced temperatures are required, polymer electrolytic capacitors are sensitive to short circuits (breakdown) between the anode and the cathode electrodes . Therefore , other capacitor types like liquid electrolytic capacitors are used in electronic circuits at working voltages higher than 400 V .

Up to now, capacitor manufacturers have not been able to develop and produce high voltage polymer electrolytic capacitors since the reliability of these products was not adequate to use in commercial electronic circuits . However, e . g . for electrical vehicle chargers and inverters , capacitors voltage classes of more than 400 V become relevant . The present invention refers to a method for manufacturing of a polymer electrolytic capacitor comprising several steps .

In a first step, a metal foil is provided . The metal foil comprises aluminium with a layer on the surface comprising aluminium oxide . Preferably, the metal foil is an aluminium foil consisting of aluminium . The surface of the metal foil is preferably fully covered by a layer . The layer comprises or consists of aluminium oxide .

In a further step, an anode foil is punched out of the metal foil . The anode foil may have a rectangular shape . The anode foil may also have another appropriate shape . A rectangular shape allows to build a compact , cuboid capacitor . A punched out rectangular foil may have a length and a width of preferably at least 0 . 1x0 . 1 cm, more preferably 3x3 cm, and preferably not more than 100x100 cm, more preferably 10x10 cm . The foil may be but does not have to be quadratic . Furthermore the punched out foil may comprise a connection snip protruding from the rectangular shape .

During punching, during further 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 in the layer may occur . Further, punched out edges of the foil lie free after punching since they are not covered by the oxide layer, so far .

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 punched out edges to cover and protect the edges. Preferably, the oxide layer fully covers the foil and the edges after this step.

By reforming, defects in the oxide layer are healed. Possible defects may be e.g. structural defects in structure of the oxide or non-oxidized spots at the surface of the anode foil.

Forming and reforming of the oxide layer is performed by an oxidizing process. Preferred conditions of the oxidizing process are specified below.

The oxide layer forms a dielectrical layer of the capacitor.

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

Preferably, the impregnation step is conducted by vacuum impregnation and followed by drying in a ventilated oven. Drying is preferably conducted at a temperature higher than 100 °C, more preferably higher than 120 °C.

In further steps, the capacitor is built from the anode foil.

Preferably, the defined steps are conducted in the stated order. Optional further steps may conducted before, after or between the stated steps. In an embodiment, no further steps may be conducted between two following of the stated steps. In particular, the step of reforming and forming the oxide layer is preferably conducted directly before impregnation with the polymer electrolyte solution. This means, no further steps are conducted between reforming and forming the oxide layer and impregnating the oxidi zed anode foil with a polymer electrolyte solution .

By reforming and forming the oxide layer directly before impregnation, a formation of electrical conductive short-cuts through the oxide layer is suppressed . Thus , a breakdown voltage of the capacitor can be optimi zed . The breakdown voltage of a capacitor is defined as a voltage at which the dielectric layer loses desired dielectrical properties and a signi ficant high current flow appears between anode and cathode of the capacitor .

Furthermore , by the disclosed method a capacitor with desired low ESR (Equivalent Series Resistance ) values at high frequencies of alternating current can be provided .

In an embodiment , the manufacturing method comprises a further step of stacking the anode foil , a separator paper sheet and a cathode foil to build a capacitor stack . Preferably, the separator sheet is placed directly on the oxide layer of the anode foil . Also preferably, the cathode foil is placed directly on the separator sheet . The cathode foil may also comprise aluminium or consist of aluminium . The cathode foil may also be covered by an oxide layer, preferably by an aluminum oxide layer .

The step of stacking is conducted after impregnation with the polymer electrolyte solution . Thus , impregnation can be conducted directly after reforming of the oxide layer .

Preferably, the capacitor stack comprises at least two separator sheets and two cathode foils , arranged in the stated order on both sides of the anode foil to achieve a desired electrical capacity .

Preferably, the foils and sheets are stacked one after each other while each new stacked sheet or foil is impregnated by the polymer electrolyte solution before stacking the next sheet or foil . Preferably, each impregnation step is conducted by vacuum impregnation and followed by drying in a ventilated oven .

In a preferred embodiment , the step of reforming the oxide layer and forming the oxide layer at the punched out edges comprises several steps .

In one of such steps , the surface of the anode foil or already existing oxide layer may be treated with boric acid, wherein an oxidation reaction is performed by using a boric acid solution . In particular, aluminium at the surface of the anode foil may be oxidi zed by the boric acid solution and heal defects or void spots in the oxide layer .

Preferably, the surface treatment with boric acid is performed at an oxidi zing temperature of at least 70 ° C, more preferably at least 90 ° C

In particular, a direct current of at least 8 mA, preferably at least 10 mA may be applied to the anode foil to stimulate the oxidation reaction and heal the defects in the oxide layer until a forming voltage is reached .

In particular, when applying a direct current to the anode , an applied voltage increases until the defects in the oxide layer are healed and the voltage stabili zes at the forming voltage .

In a preferred embodiment , a direct current of at least 8 mA, preferably 10 mA, is applied to the anode foil for at least additional 1 to 2 minutes after the forming voltage is reached to heal even more defects and stabili ze the oxide layer . The direct current may also be applied for a longer period .

In a further step, the anode foil may be depolari zed by heating the anode foil to more than 100 ° C, preferably more than 140 ° C, more preferably more than 150 ° C . Depolari zation may be conducted in an appropriate oven for several minutes , preferably for 3 to 7 minutes , more preferably for at least 5 minutes .

After depolari zation, the surface treatment by boric acid comprising the application of the oxidi zing boric acid solution and the application of the direct current to stimulate the oxidation reaction may be repeated .

Thereafter, a step of chemical depolari zation in an ammonium phosphate solution may be conducted in order to remove any excess charge in the oxide that could lead to increased leakage and hence decreased breakdown voltage . The chemical depolari zation in an ammonium phosphate solution is preferably conducted between 60 ° C to 80 ° C for 7 to 13 minutes , more preferably to at least 70 ° C for more than 10 minutes .

Preferably, thereafter the surface treatment by boric acid comprising the application of the oxidi zing boric acid solution and the application of the direct current to stimulate the oxidation reaction may be repeated again.

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

Preferably, the polymer electrolyte solution for impregnation may comprise a PEDOT : PSS-solution or consist of a PEDOT :PSS- solution. PEDOT:PSS stands for poly ( 3 , 4-ethylenedioxy- thiophene) polystyrene sulfonate. PEDOT :PSS is a polymer mixture of two ionomers. One component in this mixture is made up of sodium polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component poly (3,4- ethylenedioxythiophene ) (PEDOT) is a conjugated polymer and carries positive charges and is based on polythiophene. Together the charged macromolecules form a macromolecular salt .

More preferably, as a PEDOT : PSS-solution for impregnation Clevios™ K, a PEDOT :PSS polymer dispersions from Heraeus® is used, which has a conductivity of up to 1000 S/cm and good chemical, thermal and UV stability.

Using the specified polymer electrolyte solution allows a further improvement of electrical characteristics of the capacitor. In particular, the breakdown voltage of the capacitor may be further increased. According to another aspect , the present invention refers to a polymer electrolytic capacitor as manufactured by the method as speci fied before .

According to another aspect , the present invention refers to any polymer electrolytic capacitor comprising a stack of level , flat layers , wherein the layers comprise an anode layer, a separator paper sheet and an anode layer, and wherein the anode layer is impregnated with a polymer electrolyte solution, preferably a PEDOT : PSS-solution .

Each of the level layers is formed in a homogenous flat plane .

The polymer electrolytic capacitor may have any feature as speci fied before regarding the manufacturing methods and vice versa .

In particular, the surface of the anode layer may be fully covered by an oxide layer comprising aluminium oxide with a minimum density of defects .

Thus , more preferably, operating voltages 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 may be achieved, wherein the breakdown voltage of the capacitor is higher than the operating voltage .

Furthermore , by the enclosed method a capacitor with desired low ESR (Equivalent Series Resistance ) values at high frequencies of alternating current can be provided . Further, the operating temperature of the provided capacitor may be elevated up to 125 ° C, more preferably up to 150 ° C without serious losses in the electrical properties .

Further, a level layer stack allows the design of a cuboid capacitor which can be arranged in a more compact and more flexible way than existing wound electrolytic capacitors . A more compact design also means a reduction of used space and weight of the capacitor, which is in particular useful for automotive applications .

Due to compact arrangement , short and thus low-inductive electrical connections between the capacitor and connected electrical components such as e . g . IGBT-modules may be reali zed . Parasitic impedances can be minimi zed .

Furthermore , several cuboid capacitors may be stacked together or may be arranged in parallel to provide bigger capacitors having a higher total capacity in a very simple way .

Further, the capacitors may be connected in series to provide a capacitor with higher total breakdown voltage .

Further, the electrical connections of the anode and cathode foils 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 .

According to another aspect , the present invention refers to the use of a polymer electrolytic capacitor at operating voltages up to 650 V . The polymer electrolytic capacitor may have any feature as speci fied before , in particular regarding the manufacturing methods , and vice versa .

According to another aspect , the present invention refers to an assembly comprising an electronic module with a planar base area and a capacitor as speci fied before , wherein an outer plane surface of the capacitor and the planar base area are arranged in the same plane .

This allows a compact arrangement of the capacitor and the electronic module in a device , e . g . in a motor . Further this allows e . g . a compact arrangement of both, the electronic module and the capacitor on a common cooling body having a simple plane cooling surface .

In a preferred embodiment , the whole capacitor stack may be housed by a casing made of aluminium . The casing may be electrically connected to the cathode foils or may replace the cathode foils of the capacitor . An aluminium casing allows a high cooling performance and an improved temperature regulation of the capacitor .

As a result of a desired improved cooling, the capacitor stacks may arranged even closer to connected electrical components in order to further minimi ze electrical losses .

Further, as a result of improved cooling the capacitors may be used at higher temperatures , for examples in motors of automotive vehicles . In the following, examples of the capacitor are described in detail by using figures. However, the invention is not restricted to the following examples.

The figures show:

Figure 1: Photograph of a stack of capacitor foils and separating paper sheets.

Figure 2: Schematical drawing of an anode foil.

Figure 3: Schematical drawing of an exemplarily setup used for reforming of the anode foils.

Figure 4: Diagram showing average ESR curves of electrolytic capacitors at different AC frequencies normalized to the ESR of the capacitors at an AC frequency of 100 Hz.

Figure 5: Diagram showing I-U-curves and breakdown voltages of electrolytic capacitors.

Figure 6: Diagram showing average ESR curves of electrolytic capacitors at different AC frequencies and at different temperatures .

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

Figure 8: Side view of a second example of a capacitor stack.

Figure 1 shows different foils and sheets which are to be stacked to build a capacitor according to an embodiment of the invention. Anode foils 1 and cathode foils 2 are punched out of metal foils , e . g . in a rectangular shape with a protruding rectangular snip 4 for connection, as shown in Figure 2 .

The anode foil is preferably an aluminium foil providing a layer on the surface comprising aluminium oxide . The cathode foil may be an aluminium foil as well .

Separator paper sheets 3 are cut slightly larger to ensure a complete coverage between the electrodes .

The paper sheets 3 may comprise cellulose paper, with a thickness of e . g . 90 pm and a density of e . g . 60 g/cm ² . Such paper provides good protection to mechanical shorts from burs on the foils .

Figure 3 shows an exemplarily setup used for reforming of the anode foils 1 .

After punching the oxide layer of the anode foil 1 is reformed by using a boric acid solution preferably at 90 ° C . A constant direct current of e . g . 10 mA may be applied running from the anode foil 1 to a cylindrical counter electrode 5 as seen in figure 3 .

This results in a slowly increasing voltage while defects in the oxide layer are healed, until a forming voltage is reached .

The forming voltage is reached, when a voltage curve has stabili zed indicating that the reformation of the oxide is completed . When the forming voltage is reached, the current is left on for several additional minutes, e.g. 2 minutes, after which the current is turned off.

Hereafter, a thermal depolarization step is performed, wherein the foil is heated up to 150 °C, e.g. for 5 min in an oven .

Thereafter, the oxide layer is reformed by treatment with boric acid solution, again.

Thereafter, the foil is chemically depolarized in an ammonium phosphate solution at e.g. 70 °C for 10 min.

After this, the oxide layer may be reformed by treatment with boric acid solution, again.

The mentioned steps remove defects in the oxide layer and remove any excess charge in the oxide layer that could lead to an increased leakage and hence a decreased breakdown voltage of the oxide layer and thus of the capacitor.

After the described initial steps, the connection snips 4 may be covered, too, using polystyrene, to prevent leakage to the non-treated parts of the snip.

Hereafter, the anode foil 1 is vacuum impregnated with an PEDOT : PSS-based polymer electrolyte solution, ensuring good penetration of the oxide layer structure with the electrolyte. Preferably, the used electrolyte solution is the commercial PEDOT : PSS-based electrolyte solution CLEVIOS™ K SD 1 by Heraeus®. The polymer electrolyte solution contains PEDOT :PSS in aqueous dispersion along with a mixture of di f ferent additives . Some of these additives may be polyglycols which increase breakdown voltage for polymer based capacitors . Glycerin may be a further additive , as it can help tuning the viscosity of the solution .

After impregnation of the anode foil 1 , a capacitor stack is build up by cathode - paper - anode - paper - cathode , as seen in Figure 1 . Preferably, the stack is built up layer by layer on a sample holder . Between every layer the sample is vacuum impregnated with the polymer electrolyte solution and dried in a ventilated oven, e . g . at 120 ° C, ensuring good connection between anode and cathode foils . After a final cathode foil is placed, a glass plate may be placed on top and clamped in place on the sample holder . Hereafter the stack may be dried at for several days , to ensure the evaporation of as much water as possible .

ESR measurements were performed at room temperature on the finished capacitor stacks , at 200 frequencies logarithmically distributed between 20 Hz and 100 kHz using a Keysight E4980AL LCR Meter . A logarithmic frequency distribution of data points was chosen as the ESR shows very flat tendencies at higher frequencies , why a lower density of data can be used . ESR measurements of eight di f ferent stacks according to the above described example were compared to a reference stack where the polymer electrolyte solution was exchanged for a commercial liquid high voltage electrolyte .

Figure 4 shows an average ESR curve of the polymer electrolytic capacitor of the first embodiment at di f ferent AC frequencies normali zed to the ESR of the capacitor at an alternating current frequency of 100 Hz . Further, the ESR curve of a liquid electrolyte reference sample is shown, also normali zed to an ESR at 100 Hz . At 100 Hz the ESR of the two capacitors is very similar at around 10 Ohm .

For liquid electrolytes the seen behavior is that the ESR drops to around 2 / 3 of the ESR at 100 Hz before flattening out and become close to constant at high frequencies around 100 kHz .

For the polymer electrolytic capacitor, the ESR at 100 kHz drops to 11 % compared to the ESR at 100 Hz . This signi ficant larger decrease in ESR at high frequencies , causes the polymer aluminium electrolytic capacitor, to outperform traditional liquid electrolyte electrolytic capacitors , when it comes to ESR at high frequencies , where modern wide bandgap semiconductors normally operate .

Further, for breakdown testing a Vitrek 951 i Electrical Safety Compliance Analyzer was used . A ramping voltage of 3 V/ s was applied across the sample while voltage U and current I were measured . Peaks in the I-U-curve indicate breakdown phenomena .

In Figure 5 , breakdown voltages of the identical eight capacitor stacks which were also used for 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 .

Consistent breakdowns above 550 V, approximately between 560 V and 595 V can be seen . This is signi ficantly higher than any previous results for breakdown of polymer electrolyte capacitors known from literature . Further measurements show that ESR hardly di f fers at elevated temperatures up to 150 ° C . For example , figure 6 shows average ESR curves of electrolytic capacitors at room temperature and for comparison at 150 ° C .

Figures 7 and 8 show another example of a capacitor stack . The stack of rectangular planar layers allows a compact capacitor design and a compact arrangement of the connection snips 4 next to each other . For example , the connection snips 4 of the anode foil 1 and the cathode foils 2 can be shi fted 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 .

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

List of reference signs 1 anode foil

2 cathode foil

3 separator paper sheet

4 connection snip

5 cylindrical counter electrode