CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a national phase filing under section 371of PCT/EP2018/070166, filed Jul. 25, 2018, which claims the priority of German patent application 10 2017 117 160.6, filed Jul. 28, 2017, each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a polymer capacitor and to a polymer capacitor. In particular, the capacitor may be a hybrid polymer capacitor comprising a solid polymer electrolyte and a liquid electrolyte.

SUMMARY

Such a capacitor usually comprises an anode foil, a cathode foil and separator foils in between, wound to a winding. The anode foil is covered by a dielectric layer. The polymer covers the anode foil, in particular the dielectric layer of the anode foil, and acts as a cathode. The polymer is applied by impregnating the winding with a dispersion containing a solvent, a polymer precursor and optionally additives.

In standard methods as described in U.S. Pat. No. 7,497,879 B2, the dispersion is applied by impregnation and, thereafter, the solvent is removed by evaporation in vacuum and/or at high temperatures. These conditions may lead to a degradation of the foils and may lead to unwanted reduction of the amount of additives.

Embodiments provide an improved method of manufacturing a polymer capacitor. Further embodiments provide a polymer capacitor having improved properties.

Various embodiments provide a method of manufacturing a polymer capacitor. The polymer capacitor may be a hybrid polymer capacitor comprising a polymer forming a solid electrolyte and further comprising a liquid electrolyte. The separator foils may be impregnated with the liquid electrolyte.

The method comprises winding an anode foil, a cathode foil and separator foils to form a winding, impregnating the winding with a dispersion comprising a solvent and a polymer precursor and extracting the solvent from the winding in a supercritical fluid extraction process.

In particular, the polymer precursors are particles forming a polymerized polymer after removal of the solvent. The polymer is conductive. The dispersion may comprise one or more additives. The additives may be used for increasing the conductivity of the polymer and, thereby, for reducing the ESR (equivalent series resistance) of the capacitor. Additives may alternatively or additionally added to the winding element in other ways than by the dispersion. As an example, one or more additives may be added to the foils before winding the foils into the winding element.

In the supercritical fluid extraction process, a fluid in its supercritical phase is used for extracting the solvent. As examples, carbon dioxide or nitrous oxide may be used as the fluid. The winding is stored in an extraction cell, in which the fluid is introduced. The temperature and pressure in the extraction cell are set such that the supercritical phase of the fluid is achieved.

Within the temperature-pressure region of the supercritical phase of the liquid, the temperature and pressure can be adjusted such that the solvent is selectively extracted from the winding. In particular, other components of the dispersion may be not or only to a small amount extracted from the winding in the supercritical fluid extraction process. This enables retaining additives, which may be contained in the initial dispersion or may be added by other methods to the winding element, at a high amount or even at the initial amount during the extraction process. In particular, additives having a low boiling point can be retained in the winding.

The supercritical fluid extraction process may be carried out at a low process temperature, e.g., below 60° C. Thereby, damage of the winding due to high temperature can be prevented. In a conventional evaporation process the solvent evaporates as steam, which may lead to damage of the winding, for example by oxide and hydroxide generation on the foils. This may lead to a reduction of life time and reduced electrical performance.

The supercritical fluid extraction process may be carried out at a pressure above 1 atm. In contrast to that, conventional evaporation processes are often carried out at low pressure or vacuum conditions which may lead to unwanted removal of additives. By carrying out the process at a pressure above 1 atm additives can be retained in the winding and damage of the winding is prevented.

In an embodiment, one or more additives in the dispersion may have a low boiling point. In particular, the boiling point may be below 130° C. The additive may comprise a single component, for example. In some embodiments, the additive may comprise an azeotropic mixture of at least two components. An azeotropic mixture has a characteristic boiling point, which may be different from the boiling points of its individual components. In this case, the azeotropic mixture may have a low boiling point, in particular below 130° C.

The supercritical fluid extraction process enables retaining these low boiling point additives in the winding during the extraction process. Accordingly, low boiling point additives can be used for improving the characteristics of the winding and, in particular, the characteristics of the polymer. In a standard evaporation process, such low boiling point additives will be removed during the process.

Using the supercritical fluid extraction process generally enables retaining the additives, which may be low boiling point additives and/or higher boiling point additives, at a high amount in the winding. In particular, the additive may remain in the winding at least in an amount of 80% of the initial amount, i.e. in the amount before extracting the solvent. This may be the initial amount in the dispersion or in the winding element before the extraction step. In other embodiments, the additive may remain in a lower amount in the winding, for example in an amount of 50% to 75%. Depending on the process parameters, the amount may be between 1% and 30%, for example.

In one embodiment, at least one of the additives may have a higher boiling point, e.g., a boiling point above 130° C.

Additives having a low boiling point may consist or comprise a short or middle chain alky alcohol, a low molecular weight ester, 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. Additives having a higher boiling point may consist or comprise specific salts and polymers, for example. In particular, such additives may consist or comprise polyols, polyalkylene glycol(s), for example mannitol, sorbitol, xylitol and polyethylene glycol(s) such as PEG300-PEG2000.

Embodiments further provide a polymer capacitor manufactured by the method described above. In particular, the polymer capacitor may be a hybrid polymer capacitor comprising a polymer and a liquid electrolyte. Due to the supercritical fluid extraction process, the polymer capacitor may comprise additives which are not present in comparable capacitors manufactured by a standard evaporation process.

According to further embodiments a polymer capacitor comprises an anode foil, a cathode foil and separator foils wound to a winding. The polymer covers at least the anode foil. The polymer may be located also on the cathode foil or the separator foils. The capacitor may be a hybrid polymer capacitor.

One or more additives are distributed in the polymer. The additives may be initially contained in a dispersion comprising a solvent and a polymer precursor. At least one of the additives may have a boiling point below 130° C. The capacitor may be manufactured by the above disclosed method. The capacitor and, in particular, the additives may have the characteristics as described in connection with the disclosed method.

In particular, at least one of the additives may consist of or comprise a short or middle chain alkyl alcohol, a low molecular weight ester, ethyl acetate or propyl acetate.

Additionally or alternatively to low boiling point additives, the capacitor may comprise one or more additives having a higher boiling point. The additives may be present in a high amount.

As an example, the weight of at least one of the additives having a low boiling point is at least 0.2% of the dry weight of the winding. In some embodiments, the weight may be at least 30% of the dry weight of the winding. The dry weight of the winding is the weight of the winding before addition of any kind of dispersion, additives and liquid electrolyte.

The weight of at least one of the additives having a low boiling point may be at least the same as the weight of the polymer. In some embodiments, the weight may be up to or more than 15 times the weight of the polymer.

Additionally or alternatively, the surface area of at least one of the additives having a low boiling point may be at least 25% of the surface area of the anode foil. In some embodiments, the relative surface area may be at least 50%.

High boiling point additives may be present at a weight of at least 2% of the dry weight of the winding. In some embodiments, the weight may be up to or more than 30% of the dry weight of the winding.

Every feature described with respect to the method and the capacitors is also disclosed herein with respect to the other aspect, even if the respective feature is not explicitly mentioned in the context of the specific aspect.

