Methods for measuring electrolytes, capacitors, and OCC

Abstract

The present invention provides an electrolyte for electrolytic capacitors, a capacitor having the electrolyte, and a method for measuring the oxide formation capacity (OCC) value. [Solution] The capacitor 1 has a case 2 sealed by a cover 3. Together the cover and case constitute a housing. A winding element 4, which is an example of a capacitive element, is arranged and configured within the housing and includes a cathode which can be any suitable cathode, and an anode which can be a sintered anode having an aluminum substrate and an aluminum particle-based sintered portion arranged on each main surface of the substrate. The anode and cathode are separated by separator paper within the winding element, and the separator paper is immersed in an electrolyte. The electrolyte has an oxide formation capacity (OCC) value of at least 1.3 V / sec or a refractive index of at least 1.42 at 20°C.

Description

Detailed Description of the Invention

[0001] There is a high demand for providing capacitors with good volume efficiency. This means there is a high demand regarding increasing the capacitance of a capacitor while maintaining its size, or regarding reducing the size of a capacitor while maintaining its capacitance.

[0002] In this context, the inventors of the present invention have found that at least some capacitors with high volume efficiency tend to have a shortened lifespan under the same operating conditions as capacitors with low volume efficiency.

[0003] Therefore, the object of the present application is to provide a capacitor that can have an improved lifespan. According to another object, a capacitor that can have improved volume efficiency may be provided.

[0004] The electrolytes of claims 1 and 5 and the capacitor of claim 11 help to at least partially overcome some of the above problems, or at least partially help to achieve at least one of the above objects. Advantageous embodiments are provided in the dependent claims.

[0005] The inventors of the present invention have found that a sintered anode can help increase the volume efficiency of a capacitor. A so-called sintered anode has a sintered portion. The sintered portion contains fused metal particles that can be formed, for example, by sintering from a powder or a powder slurry. Any anode having a sintered portion can be treated as a sintered anode.

[0006] According to the embodiment, the sintered anode may include one or more valve metals. The sintered portion preferably includes the one or more valve metals. "Valve metal" can be understood in a general technical sense and is not limited thereto. For example, valve metals include at least aluminum, titanium, tantalum, niobium, tungsten, chromium, zirconium, hafnium, zinc, vanadium, bismuth, and antimony. Of these, aluminum, tantalum, and vanadium are particularly preferred. Aluminum is most preferred.

[0007] According to the embodiment, the sintered anode can be a so-called bulk anode, which is a sintered portion as the main body. For example, a lead tab can be used to contact the sintered anode.

[0008] According to an embodiment that can be preferably used for a capacitor having a winding element, the sintered anode has a substrate containing a first valve metal and a sintered portion on at least one main surface of the substrate. The valve metal in the substrate and the valve metal in the sintered portion may be the same or different. It is preferable that both the substrate and the sintered portion contain aluminum or have aluminum as a main component.

[0009] High-surface-area anodes, including those that are partially etched, particularly sintered, can generally lead to increased gas formation and increased leakage current. In the case of sintered anodes, this can occur during processing, for example, due to slit formation, winding, or delamination or cracking of the cold weld layer in the sintered portion. Chemical reactions on these exposed metal surfaces can create gas. Due to the mechanical properties of sintered anodes, moderate formation parameters are usually applied, such as low-temperature heat treatment or low concentrations and temperatures during chemical depolarization. Thus, the number of defects in the oxide layer is not sufficiently reduced, which can lead to insufficient re-healing of the oxide. This can cause high leakage current during storage and high operating leakage current in the capacitor. Furthermore, there are hydration challenges when using multipolarization, and the oxide layer cannot be stabilized against them.

[0010] The inventors have found that it is advantageous if the electrolyte is capable of oxidizing the aforementioned portion. In particular, the inventors have found that an electrolyte with good oxide creation capability (OCC) is advantageous and can help extend the lifespan of the capacitor.

[0011] According to the embodiment, an electrolyte having an oxide formation capacity of at least 1.3 V / sec is provided. This value enables rapid oxide formation in the working electrolyte of the capacitor with respect to exposed metal parts such as cracked areas or delamination areas. When the OCC is 1.3 V / sec or higher, the electrolyte can be advantageously used for the sintered anode of the capacitor.

[0012] Furthermore, according to the embodiments, the inventors of the present invention have found that the maximum voltage according to the embodiments can be increased to 400V or higher.

[0013] Measuring OCC is not limited to general methods. In particular, it can be recorded by any means of plotting the generation of oxidation voltage as a function of time when applied to a standardized aluminum anode with a defect site.

[0014] In particular, OCC is most preferably measured by the following method. First, two mainly identical aluminum foils are prepared. These aluminum foils have an aluminum purity of at least 99.96% by weight in their metal portions. Each of these aluminum foils has two main surfaces insulated by an oxide having a thickness of at least 620 nm. This oxide is preferably gamma aluminum oxide. However, the edges of the foils are covered only by a thin natural oxide formed on the aluminum exposed to air. Such an oxide may be several nanometers thick, for example, 2-3 nm. The thickness of the aluminum foil before forming the 620 nm thick oxide is 150 μm. The width of both foils is 5 mm, and the height of each foil is 70 mm. One foil is used as the anode and the other as the cathode. Thus, the foils are immersed in the electrolyte to a depth of 20 mm and the oxide formation ability is measured. The measurement is carried out at a electrolyte temperature of 85 ± 1 °C. For example, the measurement can be carried out in a double-walled glass beaker. The electrolyte temperature can be controlled, for example, by a thermostat with a circulating water bath. The anode and cathode are then connected to a power supply. This power supply must be technically capable of being set to an infinite voltage. The upper voltage limit for a 700V power supply should be sufficient for the experiment. The power supply can be, for example, an LTRONIX B606DPM-L 700V. The current is set to 1mA. Thus, the power supply forces a constant 1mA current to flow between the anode and cathode. At the point when power is supplied and current begins to flow, the voltage applied by the power supply is recorded over time. As oxide grows due to oxidation of the anode, the voltage applied by the power supply to maintain 1mA is continuously increased. In the so-called linear range, which is between 300 and 400V, the slope of the curve is measured, for example, by applying a linear fit. This slope is the OCC value.

[0015] At voltages higher than 400V, more preferably higher than 450V, or even more preferably higher than 475V, an inflection point can be reached where the charge transport mechanism changes and sparks occur. From this point onward, no further oxide growth occurs. As mentioned above, this maximum voltage is preferably not too low (preferably above 400V) so that the electrolyte can grow a sufficiently thick oxide.

[0016] The inventors have found that electrolytes satisfying the above-defined OCC are advantageous for the reasons described. By defining OCC, the inventors have found a value that can determine the properties of an electrolyte that are particularly well-suited for sintered foil, independently of the specific composition of the electrolyte.

[0017] According to another embodiment that can be combined with the previous embodiment but does not necessarily have to be combined with the previous embodiment, the inventors have found that an advantageous electrolyte used in capacitors, particularly capacitors having a sintered anode, has a refractive index of 1.42 as measured at a temperature of 20°C.

[0018] In particular, the inventors have found that electrolytes having a refractive index of 1.42 or higher at 20°C have compositions that are advantageous overall for reducing gas generation. Electrolytes having a refractive index of at least 1.42 at 20°C have often reduced water content, which helps reduce gas generation.

[0019] According to the embodiment, the electrolyte contains a boron compound. In particular, it is most preferable that the electrolyte contains a boron oxide compound. An alternative boron compound can be boric acid. In general, boron compounds, as well as boron oxide and boric acid, together with other components of the electrolyte, can be a source of water that does not exist as free water. The inventors have found that by using boron oxide or boric acid, the oxidizing capacity of water can be provided to the electrolyte without the drawbacks of oxide hydration.

[0020] According to the embodiment, the content of boron compounds such as boron oxide or boric acid can be 2 to 4% by weight. Preferably, it can be 2.75 to 3.25% by weight, and for example, 3 ± 0.1% by weight.

[0021] According to another embodiment, the electrolyte may include polyethylene glycol or a derivative of polyethylene glycol. The content of polyethylene glycol or a derivative of polyethylene glycol may be between 4 and 7% by weight.

[0022] In yet another embodiment, the pH of the electrolyte can be between 4.7 and 6.6. Preferably, the pH is in the range of 4.9 to 6.4. The inventors have found that several examples satisfying the above OCC can be found within this pH range.

[0023] According to further embodiments, the conductivity of the electrolyte is 3000 μS / cm or less at a temperature of 30°C. More preferably, the conductivity is 2700 μS / cm or less at 30°C. Having this low conductivity helps to reduce leakage current.

[0024] Furthermore, the inventors have found that water can be unfavorable in an electrolyte for the reasons mentioned above. According to the embodiments, the water content of the components added to form the electrolyte is preferably less than 1%. It is even more preferably less than 0.5% by weight, and even more preferably no water is added at all. Note that other components of the electrolyte may generate small amounts of water when mixed.

[0025] According to further embodiments, the electrolyte has the following components: The amount of ethylene glycol in the electrolyte can be between 74% and 86% by weight. The amount of polypropylene can be between 0% and 10% by weight. The amount of diethylene glycol can be between 0% and 10% by weight. The amount of dicarboxylic acid, such as sebacic acid or azelaic acid, can be between 3% and 6% by weight, for example, between 4% and 5% by weight. The amount of ammonia can be between 0.7% and 0.9% by weight, for example, between 0.75% and 0.8% by weight. The amount of polyvinyl alcohol can be between 0% and 1% by weight, for example, 0.75% or 1% by weight. The polyethylene glycol content can be between 4% and 7% by weight. For example, the amount of shorter polyethylene glycol with an average of 300 to 500 repeating units can be present in amounts between 0% and 5%, and the amount of polyethylene glycol with an average of 1500 to 2500 repeating units can be present in amounts between 0% and 4% by weight. The amount of p-nitroacetophenone can be 0.5 ± 0.2% by weight, and more preferably 0.5 ± 0.1% by weight. The amount of boron oxide or boric acid can be the amount defined above.

[0026] The inventors have found that these compositions help reduce gas generation or achieve the above-mentioned OCC value or the above-defined refractive index. The content values ​​presented herein individually help to achieve this.

[0027] According to a further embodiment, a capacitor is described. The capacitor has an anode and a cathode. The anode and the cathode can be arranged and configured as parts of a capacitive element. The capacitive element can be configured to be charged and discharged during the operation of the capacitor. The capacitive element, or the cathode and anode inside it, can be contacted by a conductive element such as a wire, a lead, or a lead tab. The capacitive element is preferably formed as a wound element in which the anode and the cathode are wound. Further, the capacitor is preferably an electrolytic capacitor in which a separator is arranged between the anode and the cathode. The separator can be wetted or immersed with an electrolyte. The electrolyte is preferably the electrolyte defined above. Further, as described above, the anode can be a sintered anode as described above.

[0028] Further advantageous and further embodiments of the electrolyte or the capacitor will become apparent from the exemplary embodiments described below in conjunction with the figures. It should be noted that the present invention is not limited to the exemplary embodiments shown in the figures and described in their context. Further, note that the exemplary embodiments at least partially depict figures showing schematic views. These schematic views are not to scale and may be drawn in a way that absolute and relative dimensions are distorted. Individual elements may be drawn exaggeratedly large for better representability or better understanding. Therefore, absolute or relative dimensions cannot be interpreted from the schematic views unless otherwise indicated.

Brief Description of the Drawings

[0029] [Figure 1] It is a figure showing an embodiment of a capacitor. [Figure 2] It is a figure showing an OCC curve regarding an exemplary embodiment of an electrolyte. [Figure 3] It is a figure showing the result of a storage test comparison between an electrolyte of a comparative example and an exemplary embodiment of an electrolyte according to the present invention.

[0030] In Figure 1, capacitor 1 is shown in a schematic cross-sectional view. Capacitor 1 has a case 2 sealed by a cover 3. Together the cover and case constitute a housing. In general, any other housing can also be used for capacitors. A wound element 4, which is an example of a capacitive element, is arranged and configured within this housing. Although not explicitly shown, the wound element includes a cathode, which can be any suitable cathode. Furthermore, it includes an anode, which can be a sintered anode having an aluminum substrate and sintered portions based on aluminum particles arranged on each main surface of the substrate. Furthermore, the anode and cathode are separated by separator paper within the wound element. The separator paper is immersed in an electrolyte. The electrolyte has an OCC value of 1.3 V / sec or higher and a refractive index of at least 1.42 at 20°C. The anode and cathode are in contact by lead tabs 5.

[0031] For example, the following electrolyte compositions, as shown in Table 1, can be used.

[0032] [Table 1]

[0033] The inventors have found that the electrolyte compositions shown in Table 1 or the modifications described in the introduction are advantageous for use in the capacitor shown in Figure 1.

[0034] The analysis of these electrolytes is shown in Figure 2. Graphs represented by dashed lines with longer dashes alternating with dots are associated with electrolyte E600. Graphs represented by dashed lines with short dashes are associated with electrolyte E601. Graphs represented by dashed lines with medium-length dashes are associated with electrolyte E602. Graphs represented by continuous lines are associated with electrolyte E603. The electrolytes shown in Table 1 were each prepared in double-walled glass beakers maintained at a temperature of 85±1°C by a thermostat. The thermostat used a circulating water bath. Anodes and cathodes were prepared using the same method. Both had a thickness of 150 μm before oxide formation. The foil width was 5 mm. The foil height was 70 mm. The main planes of the anode and cathode foils were oxidized and had at least 620 nm of gamma alumina oxide. However, the edges of the foils were not covered with a thick oxide and only had natural oxide with a thickness in the range of, for example, 2-3 nm. The purity of the foil prepared before oxidation is 99.95% by weight or higher. Both the anode and cathode are immersed in the working electrolyte, which is maintained at a temperature of 85±1°C. The immersion depth is 20 mm. Both the anode and cathode are in contact with a power supply LTRONIX B606DPM-L 700V. The power supply is set to a maximum voltage of 700V, and the fixed current is set to 1 mA. As shown in Figure 2, the voltage applied by the power supply to maintain a current of 1 mA is recorded as a function of time. The relevant value obtained from the plot here is the average gradient between 300V and 400V. This value can be read by any means. For example, a fitted curve could be applied to this region. All electrolytes were shown to have an average gradient above 1.3 V / sec. Therefore, they have favorable properties. As can be seen from the curves, electrolytes E602 and E603 have even better OCC and are more favorable. Note that inflection points in the voltage curves can be observed for each curve. Above the inflection point, sparks may be induced through the oxide, but further oxide growth will not occur. This is the maximum voltage that can be applied to the electrolyte.The aforementioned maximum voltage for all electrolytes is clearly above 400V. Note that the straight lines drawn in the graph are not fitting functions, but merely highlight the formation of inflection points and two different regimes of charge transport.

[0035] In Figure 3 and Table 2, a comparative electrolyte, referred to as "E521 - Conventional Electrolyte" (steeper curve; dotted line; triangle), which does not have an OCC of 1.3 V / sec or higher, or a refractive index of at least 1.42 at 20°C, is compared with electrolyte E603 (less steep curve: continuous line: square) in a storage test. The values ​​measured in this test are summarized in Table 2. This test is conducted as follows: A capacitor with electrolyte E603 is stored for the indicated time. Leakage current is continuously measured for 5 minutes after the operating voltage is applied to the capacitor. As can be seen from Figure 3 and Table 2 below, over storage times of 500 hours and longer, the capacitor with the electrolyte of the present invention is far superior to the conventional electrolyte. The inventors believe this is due to the oxide generation capability.

[0036] [Table 2] [Explanation of Symbols]

[0037] 1 Capacitor 2 Casing 3 Cover 4. Winding element 5 Lead Tabs

Claims

1. An electrolyte for electrolytic capacitors having an oxide formation capacity (OCC) value of at least 1.3 V / sec.

2. The electrolyte according to claim 1, having a maximum voltage of at least 400V.

3. The aforementioned oxide formation capacity (OCC) value is, A step of preparing two aluminum foils having a purity of 99.96% by weight or higher, wherein the main surface of the aluminum foil has a growth oxide with a thickness of at least 620 nm and the edges are covered only with natural oxide, the thickness of the planar aluminum foil before the growth oxide is attached is 150 μm, and the width of each aluminum foil is 5 mm, The steps include immersing both foils, as anode and cathode, in the electrolyte to a depth of 20 mm at a temperature of 85 ± 1 °C, The steps include connecting the anode and cathode to a power supply configured to technically provide an infinite voltage and limit the current to 1 mA, The steps include recording the voltage as a function of time, The steps include calculating the average gradient of the curve between 300 and 400V, and The electrolyte according to claim 1 or 2, measured by [method].

4. The electrolyte according to any one of claims 1 to 3, wherein the electrolyte has a refractive index of at least 1.42 at 20°C.

5. An electrolyte for electrolytic capacitors having a refractive index of at least 1.42 at 20°C.

6. The electrolyte according to any one of claims 1 to 5, comprising boron and / or boron oxide.

7. The electrolyte according to any one of claims 1 to 6, comprising polyethylene glycol or a derivative of polyethylene glycol.

8. 4. An electrolyte according to any one of claims 1 to 7, having a pH of 4.7 to 6.

6.

9. The electrolyte according to any one of claims 1 to 8, having a conductivity of 3000 μS / cm or less at a temperature of 30°C.

10. The electrolyte according to any one of claims 1 to 9, wherein the original water content of the electrolyte before mixing all of its components is less than 1% by weight.

11. A capacitor having an anode, a cathode, and an electrolyte according to any one of claims 1 to 10.

12. The capacitor according to claim 11, wherein the anode has a sintered portion.

13. The capacitor according to claim 11 or 12, wherein the anode contains aluminum or has aluminum as a main component.

14. A method for determining the oxide-forming ability of an electrolyte, A step of preparing two aluminum foils having a purity of 99.96% by weight or higher, wherein the main surface of the aluminum foil has a growth oxide with a thickness of at least 620 nm and the edges are covered only with natural oxide, the thickness of the planar aluminum foil before the growth oxide is attached is 150 μm, and the width of each aluminum foil is 5 mm, The steps include immersing both foils in the electrolyte to a depth of 20 mm as the anode and cathode, The steps include connecting the anode and cathode to a power supply set to a technically infinite voltage and limiting the current to 1 mA, The steps include recording the voltage as a function of time, The steps include calculating the average gradient of the curve between 300 and 400V, A method that includes [a certain feature].

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