Large-ripple-current-resistant sintered anode foil, preparation method thereof and electrolytic capacitor

Abstract

The invention relates to the technical field of anode foil processing, in particular to a large-ripple-current-resistant sintered anode foil, a preparation method thereof and an electrolytic capacitor. Comprising the following steps: carrying out dry mixing on aluminum powder and a dry powder binder to form composite powder; after a clean aluminum base band subjected to surface roughening treatment is preheated, the composite powder is deposited on the surface of the aluminum base band under the action of a pulse electrostatic field to form a gradient pore structure coating; and carrying out in-situ heating and pre-curing on the aluminum base band with the gradient pore structure coating, and then sintering to form metallurgical bonding of the gradient pore structure, so as to obtain the sintered anode foil resistant to large ripple current. The sintered anode aluminum foil with the gradient pore structure is prepared by adopting a near-dry pulse electrostatic powder coating process, the structure has multiple effects of rapid ion transmission and efficient heat conduction, and when the anode foil prepared by the method disclosed by the invention is applied to an aluminum electrolytic capacitor, the capacitor can have a relatively good effect of resisting large ripple current.

Description

Technical Field

[0001] This invention relates to the field of anode foil processing technology, specifically to a sintered anode foil resistant to large ripple current and its preparation method, as well as an electrolytic capacitor. Background Technology

[0002] Aluminum electrolytic capacitors are indispensable passive components in power electronics, new energy vehicles, industrial control, and consumer electronics. The performance of their core component, the anode foil, directly determines the capacitor's specific capacitance, equivalent series resistance (ESR), ripple current withstand capability, and lifespan. As electronic devices continue to evolve towards higher power density and miniaturization, the requirements for aluminum electrolytic capacitors' ability to withstand large ripple currents are becoming increasingly stringent. If the Joule heat generated by the ripple current within the capacitor cannot be effectively dissipated, it will accelerate electrolyte evaporation, oxide film degradation, significantly shorten device lifespan, and may even lead to thermal runaway and safety accidents.

[0003] Traditional aluminum electrolytic capacitor anode foil is mainly prepared through electrochemical corrosion process (corrosion foil), which uses strong acids and alkalis to form corrosion tunnels on the aluminum foil surface to increase the specific surface area. However, this process has the following problems: high cost of treating corrosion waste liquid and serious environmental pollution; corrosion pore size and pore depth are limited by the thickness of the aluminum foil substrate, and the improvement of specific capacitance tends to be a bottleneck; deep corrosion causes a significant decrease in the mechanical strength of aluminum foil, increasing the risk of breakage in subsequent processing.

[0004] To overcome the shortcomings of corrosion foil, aluminum powder sintered anode foil technology has gradually gained attention. Currently, most mainstream sintered foils adopt a wet coating process: aluminum powder, organic binder, and a large amount of organic solvent (such as NMP, benzene solvents, etc.) are mixed to form a slurry and then coated onto an aluminum substrate. The wet process has the following drawbacks: the volatilization of organic solvents causes serious pollution, the cost of waste gas treatment is high, and solvent residue affects the reliability of the device; the drying shrinkage of the slurry generates internal stress, leading to microcracks, delamination, or uneven porosity in the coating, which weakens the electrolyte wetting efficiency; if the organic binder is not fully decomposed during the high-temperature sintering stage, the remaining carbon residue increases the leakage current; the slurry preparation, drying, and solvent recovery systems are complex, resulting in high production costs.

[0005] In the existing technology, dry rolling is used for the preparation of electrode sheets in the fields of lithium-ion batteries and supercapacitors. However, the above-mentioned existing technologies are aimed at electrochemical energy storage electrodes that operate on the mechanism of lithium insertion / extraction or double-layer charge-discharge. Their core performance indicators are energy density and rate performance, and the requirements for electrode thermal management are relatively relaxed, with the operating frequency generally not exceeding several hundred hertz.

[0006] However, the working mechanism of aluminum electrolytic capacitors differs fundamentally from that of the aforementioned energy storage devices. The anode foil of aluminum electrolytic capacitors uses aluminum oxide film as the medium and operates under high frequency (up to hundreds of kHz) and high ripple current conditions, resulting in extremely high internal heat power density. Under high ripple current conditions, the porous layer of the anode foil must simultaneously meet two mutually restrictive and stringent requirements: (1) the electrolyte must rapidly and deeply penetrate the porous layer to ensure unobstructed ion transport channels and reduce ion transport impedance; (2) the porous layer itself must possess efficient axial (i.e., from the aluminum substrate outward) thermal conductivity to quickly dissipate the Joule heat generated by the ripple current and avoid local overheating. This dual requirement imposes far more stringent performance constraints on the pore structure, pore size distribution, and pore connectivity of the porous layer than on lithium battery or supercapacitor electrodes, and existing technologies cannot directly meet these special requirements.

[0007] Given the above background, there is an urgent need to develop a solvent-free, low-pollution anode foil preparation process specifically designed for high-frequency, high-ripple conditions in aluminum electrolytic capacitors. Summary of the Invention

[0008] To address the technical problems of severe organic solvent contamination, easy cracking and uneven pore structure in coatings, residual carbonization of binders, and inability to specifically control the pore structure in existing wet aluminum powder sintering foil processes, this invention provides a sintered anode foil resistant to high ripple currents, its preparation method, and an electrolytic capacitor. This invention employs a quasi-dry pulsed electrostatic powder coating process to prepare a sintered anode aluminum foil with a gradient pore structure. This structure exhibits multiple benefits, including rapid ion transport and efficient thermal conduction. Applying the anode foil prepared by this method to aluminum electrolytic capacitors enables the capacitors to exhibit better resistance to high ripple currents.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0010] A method for preparing a sintered anode foil resistant to high ripple current includes the following steps:

[0011] S1. Aluminum powder and dry powder binder are dry-mixed to form a composite powder;

[0012] S2. After preheating, cleaning and surface roughening treatment of the aluminum substrate, the composite powder is electrostatically deposited on the surface of the aluminum substrate under the action of a pulsed electrostatic field to form a gradient pore structure coating.

[0013] S3. The aluminum substrate with the gradient pore structure coating is pre-cured in situ by heating, and then sintered to form a metallurgical bond with a gradient pore structure, thereby obtaining a sintered anode foil resistant to large ripple current.

[0014] Furthermore, aluminum powder with a single-peaked average particle size distribution is selected to form a composite powder with a dry powder binder, or aluminum powder with a multi-peaked average particle size distribution is selected to form a multi-level composite powder with a progressively increasing particle size gradient with a dry powder binder. Then, the aluminum substrate is subjected to multi-level pulsed electrostatic spraying to deposit the composite powder or the multi-level composite powder. By controlling the following parameters for multi-level deposition, a gradient pore structure coating with progressively increasing porosity can be formed from the substrate surface outwards: the pulsed electrostatic field negative voltage increases sequentially from 10 kV to 100 kV, the pulse frequency decreases sequentially within the range of 1500 Hz to 50 Hz, and the duty cycle increases sequentially within the range of 5% to 95%. The pulsed electrostatic field is used to suppress the "reverse ionization" phenomenon during the dry powder accumulation process.

[0015] Furthermore, the purity of the aluminum powder is at least 4N, and the median particle size of the aluminum powder is in the range of 0.5 micrometers to 8 micrometers;

[0016] The dry powder binder is selected from polyvinylidene fluoride (PVDF) powder and / or thermoplastic acrylic resin powder, and the average particle size of the dry powder binder is less than 1 micrometer; the mass percentage of the dry powder binder in the composite powder is 0.5%-5%;

[0017] The total thickness of the gradient porous structure coating is 30 micrometers to 80 micrometers.

[0018] Furthermore, the formation of the gradient porosity coating is controlled by the following parameters: aluminum powders with various median particle size distributions ranging from small to large are selected and mixed with dry powder binders to form multi-level composite powders with increasing particle size gradients; then, under the action of a pulsed electrostatic field, the aluminum substrate is sequentially passed through at least two pulsed electrostatic spraying chambers for multi-level pulsed electrostatic spraying deposition. The pulsed electrostatic spraying chambers can spray the multi-level composite powders onto the surface of the aluminum substrate. The number of pulsed electrostatic spraying chambers is set according to the number of particle size grades of the multi-level composite powders, and the spraying parameters of the multiple pulsed electrostatic spraying chambers are adjusted so that the multi-level composite powders are sequentially coated onto the surface of the aluminum substrate in a gradient manner with increasing particle size grades, thereby forming a gradient porosity coating on the surface of the aluminum substrate with increasing porosity from the substrate outwards.

[0019] Furthermore, the multi-stage composite powder specifically includes primary composite powder and secondary composite powder;

[0020] The primary composite powder is composed of aluminum powder with an average particle size of 1μm-3μm and a dry powder binder;

[0021] The secondary composite powder is composed of aluminum powder with an average particle size in the range of 3μm-8μm and a dry powder binder.

[0022] Furthermore, the first-level composite powder is first applied to the surface of the aluminum substrate by a first-level pulse electrostatic spraying in the first pulse electrostatic spraying chamber to form a base layer, and then the second-level composite powder is applied to the surface of the base layer by a second-level pulse electrostatic spraying in the second pulse electrostatic spraying chamber to form a surface layer.

[0023] The parameters for the first-stage pulse electrostatic spraying are as follows: the negative voltage of the pulse electrostatic field is adjusted from 10 kV to 50 kV, the pulse frequency is 600 Hz to 1000 Hz, and the duty cycle is 20% to 40%, forming a bottom layer with a porosity of 35% to 45%.

[0024] The secondary pulse electrostatic spraying parameters are as follows: the pulse electrostatic field is adjusted within the range of negative voltage 50kV to negative voltage 100kV, the pulse frequency is 100Hz-500Hz, and the duty cycle is 50%-80%, which can form a surface layer with a porosity of 50%-60%.

[0025] Furthermore, the multi-stage composite powder specifically includes a first composite powder, a second composite powder, and a third composite powder;

[0026] The first composite powder is composed of aluminum powder with an average particle size in the range of 0.5μm-2μm and a dry powder binder;

[0027] The second composite powder is composed of aluminum powder with an average particle size of 2μm-4μm and a dry powder binder;

[0028] The third composite powder is composed of aluminum powder with an average particle size in the range of 4μm-8μm and a dry powder binder.

[0029] Furthermore, the first composite powder is electrostatically sprayed onto the surface of the aluminum substrate in the first pulse electrostatic spraying chamber to form a base layer. Then, the second composite powder is electrostatically sprayed onto the surface of the base layer in the second pulse electrostatic spraying chamber to form an intermediate layer. Finally, the third composite powder is electrostatically sprayed onto the surface of the intermediate layer in the third pulse electrostatic spraying chamber to form a surface layer.

[0030] The first pulse electrostatic spraying parameters are as follows: the negative voltage of the pulse electrostatic field is adjusted from 10 kV to 50 kV, the pulse frequency is 600 Hz to 1000 Hz, and the duty cycle is 20% to 35%.

[0031] The second pulse electrostatic spraying parameters are as follows: the pulse electrostatic field is adjusted within the range of negative voltage 50kV to negative voltage 65kV, the pulse frequency is within the range of 400Hz-600Hz, and the duty cycle is within the range of 35%-50%.

[0032] The third pulse electrostatic spraying parameters are as follows: the negative voltage of the pulse electrostatic field is adjusted from 65kV to 100kV, the pulse frequency is 100Hz-400Hz, and the duty cycle is 50%-80%.

[0033] Furthermore, the aluminum base strip is preheated after undergoing alkaline degreasing treatment, water washing, drying, surface roughening treatment, and then in sequence.

[0034] The surface roughening treatment is to make the surface roughness of the aluminum substrate reach at least Ra≥0.8 μm;

[0035] The preheating temperature of the aluminum base strip is 50℃-80℃.

[0036] Furthermore, the in-situ heating pre-curing temperature is 20℃-50℃ higher than the softening point temperature of the dry powder adhesive, and the pre-curing holding time is 3 to 10 seconds, which allows the adhesive to melt slightly and initially fix the aluminum powder onto the aluminum base strip.

[0037] Furthermore, the sintering process employs vacuum sintering, with the following parameter settings: a vacuum degree of at least 10... -3 Sintering at 550℃-630℃ for 1 to 3 hours allows the aluminum powder particles to form a metallurgical bond, creating a three-dimensional porous framework structure. At the same time, the dry powder binder is completely decomposed and volatilized, leaving no carbon residue.

[0038] The second aspect of the present invention provides a sintered anode foil resistant to large ripple current prepared by the above preparation method, the structure of which includes an aluminum substrate and a gradient porosity structure coating deposited and sintered on the surface of the aluminum substrate; the microstructure of the gradient porosity structure coating is a porosity structure with a porosity that increases sequentially from the substrate outward in the range of 20%-80%.

[0039] A third aspect of the present invention provides an electrolytic capacitor comprising a sintered anode foil resistant to large ripple current, prepared by the above-described preparation method.

[0040] Beneficial technical effects: This invention achieves a gradient pore structure coating with progressively increasing porosity on the surface of an aluminum substrate by controlling the particle size of aluminum powder and the parameters of pulse electrostatic spraying. The invention precisely controls the powder packing density through pulse electrostatic field parameters (voltage, frequency, duty cycle), resulting in adjustable porosity, good process repeatability, and precise controllable porosity. Furthermore, the amount of nano-level dry powder binder used is small (≤5wt%), and it completely decomposes and volatilizes during the vacuum high-temperature sintering stage, leaving no carbon residue in the anode foil. The aluminum electrolytic capacitor made from the specially structured sintered aluminum foil produced by this invention achieves a specific capacitance of over 0.9μF / cm², a relatively smaller equivalent series resistance (ESR), can withstand a large ripple current of over 7.5A, and has a lower leakage current of below 190μA, resulting in high capacitor reliability.

[0041] This invention employs a quasi-dry pulsed electrostatic powder coating process to prepare sintered anode aluminum foil with a gradient porosity structure. Compared to traditional wet coating, the solvent-free system of this invention completely eliminates the use of organic solvents, thus eliminating solvent evaporation pollution and waste liquid treatment issues, significantly reducing environmental compliance costs. Furthermore, it eliminates the need for drying and recycling equipment, and the closed-loop circulating spraying device recovers undeposited powder for reuse, achieving an aluminum powder utilization rate of over 95%. This invention is more environmentally friendly. Additionally, since the process involves no liquid phase, it fundamentally eliminates the microcracks and delamination defects caused by drying shrinkage stress resulting from wet coating. The sintered foil prepared by this method is crack-free and has uniform pores, exhibiting a pore structure with a gradient increasing porosity from the aluminum substrate surface outwards. The dense bottom layer ensures heat conduction, while the porous surface layer ensures ion transport, providing multiple benefits of rapid ion transport and efficient heat conduction. When applied to aluminum electrolytic capacitors, this results in capacitors with lower internal resistance and better resistance to large ripple currents. Detailed Implementation

[0042] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Unless otherwise specifically stated, the numerical values ​​set forth in these embodiments do not limit the scope of the invention. Techniques and methods known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques and methods should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that values ​​expressed, for example, as "within the range of ab" or "between the range of ab," do not include the endpoint values ​​a and b; values ​​expressed as "for ab," "is ab," or "ab" include the endpoint values ​​a and b.

[0044] Furthermore, it should be noted that the use of terms such as "first-level," "second-level," "first," and "second" to define composite powders or pulse electrostatic spraying is merely for the purpose of distinguishing between different materials and steps. Unless otherwise stated, these terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0045] Experimental methods not specified in the following examples are generally performed according to national standards; if there is no corresponding national standard, they are performed according to general standard requirements or general methods.

[0046] Example 1

[0047] A method for preparing a sintered anode foil resistant to high ripple current includes the following steps:

[0048] S1. Preparation of composite powder: Select aluminum powder with a single-peaked average particle size distribution and dry powder binder to form composite powder. Specifically, select high-purity aluminum powder with a purity of 99.99wt% (i.e., purity 4N) (average particle size D50 = 2μm) and nano PVDF powder (average particle size D50 = 200nm) and dry mix them in a V-type mixer for 30 minutes to obtain composite powder. The mass percentage of nano PVDF powder in the composite powder is 0.5%.

[0049] S2. Pretreatment of aluminum substrate: The aluminum substrate (thickness 20μm, purity 99.5wt%) is degreased by 3wt% NaOH solution (55℃, 2min), rinsed with deionized water, and dried with hot air. Then, the surface roughening treatment is carried out until the surface roughness of the aluminum substrate reaches at least Ra≥0.8 μm. Then, it is uniformly preheated to 60℃.

[0050] Two-stage series pulse electrostatic powder coating: Under the action of a pulse electrostatic field, the aluminum substrate is sequentially passed through two series pulse electrostatic spraying chambers for two-stage pulse electrostatic spraying. The pulse electrostatic spraying chambers can spray the composite powder to be deposited on the surface of the aluminum substrate. First, the composite powder is sprayed onto the surface of the aluminum substrate in the first pulse electrostatic spraying chamber to form a bottom layer. Then, the composite powder is sprayed onto the surface of the bottom layer in the second pulse electrostatic spraying chamber to form a top layer.

[0051] The parameters of the first pulse electrostatic spraying are as follows: S1 step composite powder, -50kV (negative voltage), pulse frequency 600Hz, duty cycle 40%, deposition thickness of about 20μm, forming a bottom layer with a porosity of about 40% (relatively dense), small particle size + high frequency and low duty cycle, forming a relatively dense interface layer, providing high thermal conductivity and good adhesion.

[0052] The parameters of the second pulse electrostatic spraying are as follows: S1 step composite powder, -70kV (negative voltage), pulse frequency 400Hz, duty cycle 80%, deposition thickness of about 20μm, forming a surface layer with a porosity of about 55% (relatively loose). Large particle size + low frequency and high duty cycle form a loose surface layer with wide pores, which is conducive to rapid wetting of electrolyte.

[0053] After two stages of pulse electrostatic spraying, the total powder coating thickness is about 40μm. The porosity of the base layer gradually changes from 40% at the bottom layer to 55% at the top layer, forming a gradient pore structure coating with progressively increasing porosity from the base layer surface outward.

[0054] S3. In-situ heating pre-curing: The aluminum substrate with the gradient pore structure coating is brought into the infrared heating zone, and the temperature of the substrate surface reaches 185°C (about 25°C higher than the softening point of PVDF) and is held for 5 seconds.

[0055] Vacuum sintering: at a vacuum degree of 2×10 -4 At Pa, the temperature is increased to 610℃ at a heating rate of 10℃ / min and held for sintering for 2 hours. Then, the furnace is cooled to room temperature to form a metallurgical bond with a gradient pore structure between aluminum powder particles, thus constructing a three-dimensional porous skeleton structure. At the same time, the dry powder binder is completely decomposed and volatilized without carbon residue, resulting in a sintered anode foil resistant to large ripple current.

[0056] Example 2

[0057] A method for preparing a sintered anode foil resistant to high ripple current includes the following steps:

[0058] S1. Preparation of composite powder: Select aluminum powder with a multi-peaked average particle size distribution and form multi-level composite powder with dry powder binder, with increasing particle size gradient. That is, select aluminum powder with various median particle size distributions from small to large particle size and form multi-level composite powder with increasing particle size gradient with dry powder binder, as detailed below:

[0059] Primary composite powder: High-purity aluminum powder with a purity of 99.99wt% (i.e., purity 4N) (average particle size D50 = 1.5μm) and nano-PVDF powder (average particle size D50 = 200nm) are dry-mixed in a V-type mixer for 30 minutes to obtain primary composite powder. The mass percentage of nano-PVDF powder in the composite powder is 0.5%.

[0060] Secondary composite powder: High-purity aluminum powder with a purity of 99.99wt% (i.e., purity 4N) (average particle size D50 = 4.0μm) and nano-PVDF powder (average particle size D50 = 200nm) are dry-mixed in a V-type mixer for 30 minutes to obtain secondary composite powder. The mass percentage of the nano-PVDF powder in the composite powder is 0.5%.

[0061] S2, Aluminum substrate pretreatment is the same as in Example 1;

[0062] Two-stage series pulsed electrostatic powder coating: Under the action of a pulsed electrostatic field, the aluminum substrate is sequentially passed through two series-connected pulsed electrostatic spraying chambers for two-stage pulsed electrostatic spraying. The pulsed electrostatic spraying chambers can spray the composite powder to be deposited on the surface of the aluminum substrate. First, the first-stage pulsed electrostatic spraying of the first-stage composite powder is performed on the surface of the aluminum substrate in the first pulsed electrostatic spraying chamber to form a bottom layer. Then, the second-stage pulsed electrostatic spraying of the second-stage composite powder is performed on the surface of the bottom layer in the second pulsed electrostatic spraying chamber to form a surface layer.

[0063] The parameters of the first-stage pulse electrostatic spraying are as follows: first-stage composite powder, -50kV (negative voltage), pulse frequency 600Hz, duty cycle 30%, deposition thickness of about 20μm, forming a bottom layer with a porosity of about 35% (relatively dense), small particle size + high frequency and low duty cycle, forming a relatively dense interface layer, providing high thermal conductivity and good adhesion.

[0064] The parameters of the secondary pulse electrostatic spraying are as follows: secondary composite powder, -70kV (negative voltage), pulse frequency 400Hz, duty cycle 50%, deposition thickness of about 20μm, forming a surface layer with a porosity of about 55% (relatively loose). The large particle size + low frequency and high duty cycle form a loose surface layer with wide channels, which is conducive to rapid wetting of electrolyte.

[0065] After two stages of pulse electrostatic spraying, the total powder coating thickness is about 40μm. The porosity of the base layer gradually changes from 35% at the bottom layer to 55% at the top layer, forming a gradient pore structure coating with progressively increasing porosity from the base layer surface outward.

[0066] S3. In-situ heating pre-curing: The aluminum substrate with the gradient pore structure coating is brought into the infrared heating zone, and the temperature of the substrate surface reaches 185°C (about 25°C higher than the softening point of PVDF) and is held for 5 seconds.

[0067] Vacuum sintering: at a vacuum degree of 2×10 -4 At Pa, the temperature is increased to 600℃ at a heating rate of 10℃ / min and held for sintering for 3 hours. Then, the furnace is cooled to room temperature to form a metallurgical bond with a gradient pore structure between aluminum powder particles, thus constructing a three-dimensional porous skeleton structure. At the same time, the dry powder binder is completely decomposed and volatilized without carbon residue, resulting in a sintered anode foil resistant to large ripple current.

[0068] Example 3

[0069] A method for preparing a sintered anode foil resistant to high ripple current includes the following steps:

[0070] S1. Preparation of composite powder: Select aluminum powder with a multi-peaked average particle size distribution and form multi-level composite powder with dry powder binder, with increasing particle size gradient. That is, select aluminum powder with various median particle size distributions from small to large particle size and form multi-level composite powder with increasing particle size gradient with dry powder binder, as detailed below:

[0071] First composite powder: High-purity aluminum powder with a purity of 99.99wt% (i.e., purity 4N) (average particle size D50 = 1.0μm) and nano-PVDF powder (average particle size D50 = 200nm) are dry-mixed in a V-type mixer for 30 minutes to obtain the first composite powder. The mass percentage of the nano-PVDF powder in the composite powder is 0.5%.

[0072] Second composite powder: High-purity aluminum powder with a purity of 99.99wt% (i.e., purity 4N) (average particle size D50 = 2.5μm) and nano-PVDF powder (average particle size D50 = 200nm) are dry-mixed in a V-type mixer for 30 minutes to obtain the second composite powder. The mass percentage of the nano-PVDF powder in the composite powder is 0.5%.

[0073] The third composite powder: High-purity aluminum powder with a purity of 99.99 wt% (i.e., purity 4N) (average particle size D50 = 4.5 μm) and nano-PVDF powder (average particle size D50 = 200 nm) are dry-mixed in a V-type mixer for 30 minutes to obtain the third composite powder. The mass percentage of the nano-PVDF powder in the composite powder is 0.5%.

[0074] S2, Aluminum substrate pretreatment is the same as in Example 1;

[0075] Three-stage series pulsed electrostatic powder coating: Under the action of a pulsed electrostatic field, the aluminum substrate is sequentially passed through three series-connected pulsed electrostatic spraying chambers for three-stage pulsed electrostatic spraying. The pulsed electrostatic spraying chambers can spray the composite powder to be deposited on the surface of the aluminum substrate. First, the first pulsed electrostatic spraying chamber performs a first pulsed electrostatic spraying of the first composite powder to the surface of the aluminum substrate to form a bottom layer. Then, the second pulsed electrostatic spraying chamber performs a second pulsed electrostatic spraying of the second composite powder to the surface of the bottom layer to form an intermediate layer. Finally, the third pulsed electrostatic spraying chamber performs a third pulsed electrostatic spraying of the third composite powder to the surface of the intermediate layer to form a top layer.

[0076] The parameters of the first pulse electrostatic spraying are: first composite powder, -45kV (negative voltage), pulse frequency 800Hz, duty cycle 25%, deposition thickness of about 12μm, forming a bottom layer with a porosity of about 30%.

[0077] The parameters of the second pulse electrostatic spraying are as follows: second composite powder, -60kV (negative voltage), pulse frequency 550Hz, duty cycle 40%, deposition thickness of about 13μm, forming an intermediate layer with a porosity of about 45%.

[0078] The parameters of the third pulse electrostatic spraying are as follows: third composite powder, -75kV (negative voltage), pulse frequency 350Hz, duty cycle 55%, deposition thickness of about 15μm, forming a surface layer with a porosity of about 58%.

[0079] After three stages of pulse electrostatic spraying, the total powder coating thickness is about 40μm. The porosity of the substrate cross section gradually changes from 30% at the bottom layer to 58% at the surface layer, forming a gradient pore structure coating with progressively increasing porosity from the substrate surface outward.

[0080] S3. In-situ heating pre-curing: The aluminum substrate with the gradient pore structure coating is brought into the infrared heating zone, and the temperature of the substrate surface reaches 185°C (about 25°C higher than the softening point of PVDF) and is held for 5 seconds.

[0081] Vacuum sintering: at a vacuum degree of 2×10 -4 At Pa, the temperature is increased to 630℃ at a heating rate of 10℃ / min and held for sintering for 1 hour. Then, the furnace is cooled to room temperature to form a metallurgical bond with a gradient pore structure between aluminum powder particles, thus constructing a three-dimensional porous skeleton structure. At the same time, the dry powder binder is completely decomposed and volatilized without carbon residue, resulting in a sintered anode foil resistant to large ripple current.

[0082] Comparative Example 1

[0083] This case study describes the preparation of wet-coated sintered foil, which includes the following steps:

[0084] Slurry preparation: High-purity aluminum powder with D50=2.5μm, PVDF and NMP are mixed at a mass ratio of 80:5:15 and ball-milled for 2 hours to prepare a slurry. The slurry is then coated onto a roughened aluminum substrate (same as the corresponding operation in Example 1) with a doctor blade, dried at 120°C for 2 hours, and NMP is recovered to obtain a dry film with a thickness of about 40μm. Then, vacuum sintering is performed (same as the corresponding operation in Example 2).

[0085] Comparative Example 2

[0086] In this case, the sintered foil is formed by using a single-peak distributed aluminum powder and a dry powder binder to form a composite powder, which is then coated with a single-stage pulse electrostatic spray to form a single-layer uniform (gradient-free porous structure) coating. Specifically, the preparation process of the sintered foil in this case is the same as in Example 2, except that: the average particle size D50 of the aluminum powder in the composite powder of S1 is 2.5μm, and S2 is coated with powder by single-stage pulse electrostatic spray with the following parameters: -60kV (negative voltage), pulse frequency 550Hz, duty cycle 40%, and deposition thickness of about 40μm, forming a single-layer coating with a porosity of about 45% and a gradient-free porous structure.

[0087] Comparative Example 3

[0088] This case uses DC electrostatic spraying for powder coating without pulse modulation. The specific operations for composite powder and aluminum substrate pretreatment are the same as those in Comparative Example 2. Single-stage electrostatic spraying is used for powder coating with the following parameters: spraying voltage -60kV, continuous DC electric field (no pulse, duty cycle equivalent to 100%), and the rest are the same as in Comparative Example 2.

[0089] Comparative Example 4

[0090] In this case, the sintered foil is formed by multi-peak distributed aluminum powder and dry powder binder to form a composite powder, and then a single-stage pulse electrostatic spraying is performed to form a single-layer uniform (gradient-free porous structure) coating. Specifically, the preparation process of the sintered foil in this case is the same as in Example 2, except that: in S1, high-purity aluminum powder with D50=1.5μm and high-purity aluminum powder with D50=4.0μm are mixed evenly in equal mass ratio and then mixed with nano PVDF powder to form a composite powder (where PVDF accounts for 0.5wt%). In S2, single-stage pulse electrostatic spraying is used for powder coating with the following parameters: -60kV (negative voltage), pulse frequency 550Hz, duty cycle 40%, and deposition thickness of about 40μm, forming a single-layer coating with a gradient-free porous structure.

[0091] Comparative Example 5

[0092] The sintered foil preparation process in this case is the same as in Example 2, except that this case uses a reverse gradient, that is, a gradient pore structure coating with decreasing porosity is formed from the substrate surface outwards. The specific parameters are as follows:

[0093] First, perform pulse electrostatic spraying to form a base layer with a porosity of approximately 55%. Spraying parameters: secondary composite powder (aluminum powder D50=4.0μm), -70kV (negative voltage), pulse frequency 400Hz, duty cycle 50%, deposition thickness approximately 20μm.

[0094] Then, pulse electrostatic spraying is performed to form a surface layer with a porosity of about 35%. Spraying parameters: primary composite powder (aluminum powder D50=1.5μm), -50kV (negative voltage), pulse frequency 600Hz, duty cycle 30%, deposition thickness about 20μm.

[0095] Test case

[0096] The anode foils from each case were applied to aluminum electrolytic capacitors (400V 3300μF, Φ50×100), and the performance of each capacitor was tested. The results are shown in Table 1 below.

[0097] Table 1. Anode foil parameters and corresponding aluminum electrolytic capacitor performance of the examples and comparative examples

[0098]

[0099] As shown in Table 1, this invention achieves a gradient pore structure coating with progressively increasing porosity on the surface of an aluminum substrate by controlling the particle size of aluminum powder and adjusting the pulse electrostatic spraying parameters. The aluminum foil with the special structure prepared by this invention can replace the traditional etched foil to prepare aluminum electrolytic capacitors of the corresponding models. The specific capacitance reaches more than 0.9 μF / cm², the equivalent series resistance (ESR) is relatively smaller, it can withstand large ripple current, and the leakage current is smaller, below 190 μA.

[0100] Comparative Example 1 uses a conventional wet-coated sintered foil, while Comparative Example 3 uses a composite powder formed by a single-peak distributed aluminum powder and a dry powder binder, followed by single-stage electrostatic spraying (without pulse modulation). Both examples result in a single-layer powder-coated sintered foil structure without a gradient. The resulting aluminum electrolytic capacitors exhibit a specific capacitance less than 0.85 μF / cm², an equivalent series resistance greater than 25 mΩ, a ripple current withstand of no more than 7.0 A, and a leakage current of at least 220 μA. In Comparative Example 1, the wet-coated sintered foil develops noticeable microcracks after drying, negatively impacting the performance of subsequent capacitors. In Comparative Example 3, the continuous DC field causes charge accumulation in the powder deposition layer, resulting in significant "reverse ionization," mutual repulsion between powder particles, excessively thick or sparse local deposition, and poor porosity uniformity, all of which negatively affect the performance of subsequent capacitors.

[0101] Comparative Example 2 uses a composite powder formed by a single-peak distributed aluminum powder and a dry powder binder, followed by single-stage pulse electrostatic spraying. Compared to traditional etched foil, the performance of the capacitor produced is improved. However, compared to Example 1 of this invention, the capacitor produced in Comparative Example 2 has slightly worse performance than that of Example 1 because the single-layer powder coating does not form a gradient pore structure. Comparative Example 4 uses aluminum powders with D50=1.5μm and D50=4.0μm of equal mass uniformly mixed and then subjected to single-stage pulse electrostatic spraying. There is no pore gradient distribution in the cross-sectional direction. However, because the porosity of the sintered layer on the surface of the aluminum substrate is uniform, a dense high thermal conductivity channel cannot be formed. Therefore, simply widening the particle size distribution cannot bring about the synergistic benefits of thermal conduction and ion transport. Only by adopting the spatial gradient distribution of porosity of this invention can the performance of high ripple current resistance be effectively improved. Compared to Example 1, Comparative Example 5 uses a reverse gradient pore structure. Due to the low porosity of the surface layer, it is a relatively dense surface layer, which hinders the rapid wetting of the electrolyte in the capacitor, resulting in an increase in ion transport impedance. Furthermore, the bottom layer has a high porosity and is a relatively loose surface layer, resulting in poor thermal conductivity. The heat inside the capacitor cannot be effectively conducted to the aluminum substrate, and ripple heat accumulation is obvious. Therefore, the capacitor made with the reverse gradient pore structure is not resistant to large ripple current.

[0102] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a sintered anode foil resistant to high ripple current, characterized in that, Includes the following steps: S1. Aluminum powder and dry powder binder are dry-mixed to form a composite powder; S2. After preheating, cleaning and surface roughening treatment of the aluminum substrate, the composite powder is electrostatically deposited on the surface of the aluminum substrate under the action of a pulsed electrostatic field to form a gradient pore structure coating. S3. The aluminum substrate with the gradient pore structure coating is pre-cured in situ by heating, and then sintered to form a metallurgical bond with a gradient pore structure, thereby obtaining a sintered anode foil resistant to large ripple current.

2. The method for preparing a sintered anode foil resistant to large ripple current according to claim 1, characterized in that, Aluminum powder with a single-peaked average particle size distribution is selected to form a composite powder with a dry powder binder, or aluminum powder with a multi-peaked average particle size distribution is selected to form a multi-level composite powder with a particle size gradient increasing with dry powder binder. Then, the aluminum substrate is subjected to multi-level pulse electrostatic spraying to deposit the composite powder or the multi-level composite powder. By adjusting the following parameters to perform multi-level deposition, a gradient pore structure coating with progressively increasing porosity can be formed from the surface of the substrate outward: the negative voltage of the pulse electrostatic field increases sequentially from 10 kV to 100 kV, the pulse frequency decreases sequentially within the range of 1500 Hz to 50 Hz, and the duty cycle increases sequentially within the range of 5% to 95%. The aluminum powder has a purity of at least 4N and a median particle size in the range of 0.5 micrometers to 8 micrometers; the dry powder binder is selected from polyvinylidene fluoride powder and / or thermoplastic acrylic resin powder, and the average particle size of the dry powder binder is less than 1 micrometer. The dry powder binder accounts for 0.5%-5% of the mass of the composite powder; The total thickness of the gradient porosity coating is 30 micrometers to 80 micrometers.

3. The method for preparing a sintered anode foil resistant to large ripple current according to claim 2, characterized in that, Aluminum powders with various median particle size distributions, ranging from small to large, are selected and mixed with dry powder binders to form multi-level composite powders with increasing particle size gradients. Then, under the action of a pulsed electrostatic field, the aluminum substrate is sequentially passed through at least two pulsed electrostatic spraying chambers for multi-level pulsed electrostatic spraying deposition. Each pulsed electrostatic spraying chamber can spray the multi-level composite powder onto the surface of the aluminum substrate. The number of pulsed electrostatic spraying chambers is set according to the number of particle size grades of the multi-level composite powder, and the spraying parameters of the multiple pulsed electrostatic spraying chambers are adjusted so that the multi-level composite powder is sequentially coated onto the surface of the aluminum substrate according to the increasing particle size gradient, thereby forming a gradient pore structure coating on the surface of the aluminum substrate with progressively increasing porosity from the substrate outwards.

4. The method for preparing a sintered anode foil resistant to large ripple current according to claim 3, characterized in that, The multi-stage composite powder specifically includes primary composite powder and secondary composite powder; The primary composite powder is composed of aluminum powder with an average particle size of 1μm-3μm and a dry powder binder; The secondary composite powder is composed of aluminum powder with an average particle size in the range of 3μm-8μm and a dry powder binder.

5. The method for preparing a sintered anode foil resistant to large ripple current according to claim 4, characterized in that, First, the primary composite powder is applied to the surface of the aluminum substrate by primary pulse electrostatic spraying in the first pulse electrostatic spraying chamber to form a base layer. Then, the secondary composite powder is applied to the surface of the base layer by secondary pulse electrostatic spraying in the second pulse electrostatic spraying chamber to form a surface layer. The parameters for the first-stage pulse electrostatic spraying are as follows: the negative voltage of the pulse electrostatic field is adjusted from 10 kV to 50 kV, the pulse frequency is 600 Hz to 1000 Hz, and the duty cycle is 20% to 40%. The secondary pulse electrostatic spraying parameters are as follows: the pulse electrostatic field is adjusted within the range of negative voltage 50kV to negative voltage 100kV, the pulse frequency is 100Hz-500Hz, and the duty cycle is 50%-80%.

6. The method for preparing a sintered anode foil resistant to large ripple current according to claim 3, characterized in that, The multi-stage composite powder specifically includes a first composite powder, a second composite powder, and a third composite powder; The first composite powder is composed of aluminum powder with an average particle size in the range of 0.5μm-2μm and a dry powder binder; The second composite powder is composed of aluminum powder with an average particle size of 2μm-4μm and a dry powder binder; The third composite powder is composed of aluminum powder with an average particle size in the range of 4μm-8μm and a dry powder binder.

7. The method for preparing a sintered anode foil resistant to large ripple current according to claim 6, characterized in that, First, the first composite powder is electrostatically sprayed onto the surface of the aluminum substrate in the first pulse electrostatic spraying chamber to form a bottom layer. Then, the second composite powder is electrostatically sprayed onto the surface of the bottom layer in the second pulse electrostatic spraying chamber to form an intermediate layer. Finally, the third composite powder is electrostatically sprayed onto the surface of the intermediate layer in the third pulse electrostatic spraying chamber to form a top layer. The first pulse electrostatic spraying parameters are as follows: the negative voltage of the pulse electrostatic field is adjusted from 10 kV to 50 kV, the pulse frequency is 600 Hz to 1000 Hz, and the duty cycle is 20% to 35%. The second pulse electrostatic spraying parameters are as follows: the pulse electrostatic field is adjusted within the range of negative voltage 50kV to negative voltage 65kV, the pulse frequency is within the range of 400Hz-600Hz, and the duty cycle is within the range of 35%-50%. The third pulse electrostatic spraying parameters are as follows: the negative voltage of the pulse electrostatic field is adjusted from 65kV to 100kV, the pulse frequency is 100Hz-400Hz, and the duty cycle is 50%-80%.

8. A method for preparing a sintered anode foil resistant to high ripple current according to any one of claims 1-7, characterized in that, The aluminum base strip is subjected to alkaline degreasing treatment, water washing, drying, surface roughening treatment and then preheating. The surface roughening treatment is to make the surface roughness of the aluminum substrate reach at least Ra≥0.8 μm; The preheating temperature of the aluminum base strip is 50℃-80℃; The in-situ heating pre-curing temperature is 20℃-50℃ higher than the softening point temperature of the dry powder adhesive, and the pre-curing holding time is 3 seconds to 10 seconds. The sintering process employs vacuum sintering, with the following parameter settings: a vacuum degree of at least 10... -3 Sintering is carried out for 1 to 3 hours at a temperature of 550℃-630℃.

9. A sintered anode foil resistant to high ripple current, characterized in that, The material is prepared by any one of claims 1-8, and its structure includes an aluminum substrate and a gradient pore structure coating deposited and sintered on the surface of the aluminum substrate. The microstructure of the gradient porosity coating is a porosity structure with progressively increasing porosity from the base zone outwards.

10. An electrolytic capacitor, characterized in that, This includes sintered anode foil resistant to high ripple current, prepared by the preparation method according to any one of claims 1-8.

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