Method of Manufacturing a Polymer Capacitor and Polymer Capacitor

Abstract

A method for manufacturing a polymer capacitor and a polymer capacitor are disclosed. In an embodiment a polymer capacitor includes an anode foil, a cathode foil and separator foils wound to a winding, wherein the anode foil is covered by a polymer, wherein one or more additives are distributed in the polymer, and wherein at least one of the additives has a boiling point below 130° C.

Claims

1. A polymer capacitor comprising: an anode foil; a cathode foil; and separator foils wound to a winding, wherein the anode foil is covered by a polymer, wherein one or more additives are distributed in the polymer, and wherein at least one of the additives has a boiling point below 130° C.

2. The polymer capacitor of claim 1, wherein at least one of the additives comprises a short or middle chain alkyl alcohol, a low molecular weight ester, an ethyl acetate or propyl acetate.

3. The polymer capacitor according to claim 2, wherein the at least one of the additives comprises the short or middle chain alkyl alcohol in form of a butyl alcohol, an isopropyl alcohol or an unsaturated alcohol.

4. The polymer capacitor according to claim 2, wherein the at least one of the additives comprises the low molecular weight ester having a molecular weight smaller than 200 g/mol.

5. The polymer capacitor of claim 1, wherein a weight of at least one of the additives is at least the same as a weight of the polymer.

6. The polymer capacitor of claim 1, wherein a surface area of at least one of the additives is at least 25% of a surface area of the anode foil.

7. The polymer capacitor of claim 1, further comprising a liquid electrolyte.

8. The polymer capacitor of claim 1, wherein the polymer is a solid polymer.

9. The polymer capacitor according to claim 1, wherein the polymer capacitor is a hybrid polymer capacitor comprising the polymer and a liquid electrolyte.

10. The polymer capacitor according to claim 1, wherein the at least one of the additives increases a conductivity of the polymer.

11. The polymer capacitor according to claim 1, wherein the polymer capacitor is free of a solvent.

12. The polymer capacitor according to claim 11, wherein the solvent is water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Further features, refinements and expediencies become apparent from the following description of the exemplary embodiments in connection with the figures.

[0029] FIG. 1 schematically shows the structure of a polymer capacitor;

[0030] FIG. 2 shows steps in a method of fabricating a polymer capacitor;

[0031] FIG. 3 shows a pressure-temperature phase diagram; and

[0032] FIG. 4 schematically shows a device for carrying out a supercritical fluid extraction process.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0033] FIG. 1 shows a polymer capacitor 1 in a partially unwound view. The capacitor 1 comprises an anode foil 2, a cathode foil 3 and separator foils 4 in between. In this figure, only one separator foil 4 is shown. A further separator foil is present between the cathode foil 3 and the anode foil 2 at the other side of the foils 2, 3. The foils 2, 3, 4 are wound around a common axis into a winding 5.

[0034] The winding 5 may be located in a can (not shown in the figure). The can may comprise a metal such as aluminum.

[0035] The anode foil 2 is electrically contacted by a first contact structure 6. The cathode foil 3 is electrically contacted by a second contact structure 7. Each of the contact structures 6, 7 may comprise multiple stacked contact elements 8, 9. The contact structures 6, 7 and the multiple contact elements 8, 9 may have the shapes of tabs. The contact elements 8, 9 extend to opposite sides of the winding 5. In particular, the first contact structure 6 may have a lead-out 10 extending upwards and the second contact structure 7 may have a second lead-out 11 extending downwards. Furthermore, the contact structures 6, 7 are located at opposite lateral sides of the winding 5. By using multiple first contact elements 8 forming a first contact structure 6 and multiple second contact elements 9 forming a second contact structure 7 the metal resistance of the winding 5 can be reduced. Thereby, the ESR of the capacitor 1 is reduced.

[0036] Alternatively, each of the contact structures 6, 7 may be formed by a single contact element 8, 9.

[0037] The cathode foil 3 may have an extension 12 in an axial direction of the winding 5. In particular, the cathode foil 3 may extend axially beyond the anode foil 2. The extension 12 may be electrically and also mechanically connected to a bottom of a can. Additionally or alternatively, the anode foil 2 may have an extension 12 in an axial direction, e.g., in the opposite axial direction.

[0038] The anode foil 2 may comprise an aluminum foil covered by a dielectric layer, in particular an oxide layer. Also the cathode foil 3 may comprise an aluminum foil covered by an oxide layer. The separator foil 4 may be formed by a paper material.

[0039] The anode foil 2, in particular the dielectric layer formed on the anode foil 2, is covered by a polymer 13. The polymer 13 is conductive and serves as a solid electrolyte. The polymer 13 may also be disposed on the cathode foil 3 and the separator foil 4. The polymer 13 may be applied by impregnating the winding 5 with a dispersion containing a solvent, a polymer precursor and one or more additives 20. After that, the solvent is extracted and the polymer precursor polymerizes to form the polymer 13. The additives 20 are distributed in the polymerized polymer 13.

[0040] The function of the additives 20 may be to improve the inter-connection of the polymer 13, e.g., between polymer particles. As an example, the additives 20 may strengthen secondary chemical bonds and intermolecular connections. Thereby, the conductivity of the polymer 13 can be reduced, whereby the ESR of the capacitor 1 is reduced.

[0041] As an example, the weight of at least one of the low boiling point additives 20 may be at least the same as the weight of the polymer 13.

[0042] The additives 20 may comprise or consist of a material having a low boiling point, e.g., short or middle chain alkyl alcohols, low molecular weight ester (<200 g/mol), ethyl acetate or propyl acetate. Short or middle chain alkyl alcohols include butyl alcohol, isopropyl alcohol, unsaturated alcohols like allyl alcohol and brand chain alcohols (t-butyl alcohols), for example.

[0043] The additive 20 may comprise an azeotropic mixture of at least two components. An azeotropic mixture cannot be separated into its components by distillation. An azeotropic mixture has a characteristic boiling point, which may be different from the boiling points of its individual components.

[0044] In addition to that, at least one of the additives 20 may comprise or consist of a material having a high boiling point, such as specific salts or polymers.

[0045] Additionally, the winding 5 may be impregnated with a liquid electrolyte. The separator foil 4, in particular, may be impregnated with the liquid electrolyte. In this case, the polymer capacitor 1 is a hybrid polymer capacitor.

[0046] The capacitor 1 can be manufactured by the process described in the following.

[0047] FIG. 2 shows a flow diagram of a method of manufacturing a polymer capacitor, such as the capacitor 1 shown in FIG. 1.

[0048] In step A, an anode foil 2, a cathode foil 3 and separator foils 4 are prepared and wound into a winding 5. Prior to forming the winding 5, one or more first contact elements 8 may be attached to the anode foil 2 and one or more second contact elements 9 may be attached to the cathode foil 3.

[0049] Then, in step B, the winding 5 is impregnated with a dispersion comprising a solvent and a polymer precursor. The dispersion may also comprise one or more additives 20. The solvent may comprise water or may mainly consist of water.

[0050] After that, in step C, the solvent is extracted and, thereby, partially or completely removed. The removal of the solvent enables the polymerization of the polymer precursor and the inter-connection of the polymer 13. The extraction of the solvent is accomplished by using a supercritical fluid extraction process, in which a supercritical fluid is used for extracting the solvent.

[0051] As an example, a fluid like carbon dioxide (CO.sub.2) or nitrous oxide (N.sub.2O) is brought in its supercritical state.

[0052] FIG. 3 shows a pressure-temperature phase diagram depicting a solid phase S, a gas phase G, a liquid phase L and a supercritical fluid phase SCF. By setting the temperature T and pressure P above a critical temperature T.sub.C and critical pressure P.sub.C, a critical point C is exceeded and the fluid is brought in its supercritical state. As an example, the critical temperature T.sub.C for CO.sub.2 is 31° C. and the critical pressure P.sub.C is 74 bar.

[0053] The supercritical fluid extraction process enables removing the solvent at low temperatures, e.g., temperatures below 60° C. Additionally, the supercritical fluid extraction process enables removing the solvent without setting vacuum conditions (<1 atm). Furthermore, adjusting the temperature T and pressure P within the supercritical region to specific values enables selectively extracting the solvent without extracting the additives 20 at the same time.

[0054] This process enables using additives 20 in the polymer dispersion which have low boiling points, whereby the additives 20 remain in the winding 5 during the extraction process. As an example, a low boiling point additive 20 may have a boiling point below 130° C.

[0055] Furthermore, the low process temperature prevents damage of the anode and cathode foils 2, 3 by high temperature water, in particular steam, which may be produced in conventional evaporation methods and may lead to a degradation of the foils 2, 3. Such a damage may lead to a change of capacitance and stability of the anode and cathode foils 2, 3 and to a reduction of life time and electrical performance of the capacitor 1. The supercritical fluid extraction enables preserving the anode and cathode foils 2, 3 in their states, which ensures a high life time and good electrical performance of the capacitor 1.

[0056] Furthermore, the supercritical fluid extraction enables maintaining the amount of additives 20 at the initial level or at a only slightly reduced level. Accordingly, the resulting capacitor 1 may comprise a high amount of additives 20, thereby maximizing the conductivity of the polymer 13 and reducing the ESR of the capacitor 1. In contrast to that, conventional evaporation methods will result in huge losses of the amount of additives due to high temperature and/or vacuum.

[0057] Returning to FIG. 2, after conducting the supercritical fluid extraction process, step D may be carried out, wherein step D comprises impregnating the winding 5 with a liquid electrolyte and final assembly of the capacitor 1.

[0058] FIG. 4 shows an exemplary device 14 for carrying out the supercritical fluid extraction process in the method described in FIG. 2.

[0059] The device 14 comprises a storage tank 15, in which a fluid is stored. The fluid may be in its gas phase. The fluid may be CO.sub.2 or N.sub.2O, for example. The fluid is compressed by a compressor 16 and pumped into an extraction cell 17, in which the winding 5 impregnated with the dispersion is stored.

[0060] By a T-P control unit 18, the temperature T in the extraction cell 17 is set to a temperature larger than the critical temperature T.sub.C of the fluid. The pressure P in the extraction cell 17 is set to a pressure larger than the critical pressure P.sub.C of the fluid. The pressure P may be regulated by a backpressure regulator 19 to ensure that the optimum pressure is set. In particular, the pressure and temperature is set such that the fluid is in its supercritical state and that water is selectively extracted from the impregnated winding 5. The outflow of the extracted solvent and the fluid is controlled by a flow meter 21.