A long-life cyanide-free silver plating solution, its electroplating method, applications, and products.

Abstract

This invention discloses a long-life cyanide-free silver plating solution, its electroplating method, applications, and products, belonging to the field of electroplating technology. The cyanide-free silver plating solution comprises: a silver ion source; methanesulfonic acid; and a silver ion complexing agent, wherein the molar ratio of the silver ion complexing agent to silver ions is 1 or more, and the silver ion complexing agent has the following general formula: HO-CH2CH2-[SCH2CH2] n -OH, where n is an integer from 1 to 3; a composite stabilizer, which consists of a first stabilizer and a second stabilizer; and water. Through the synergistic effect of the composite stabilizer, the plating bath life is significantly increased to over 10 MTO, supporting continuous industrial mass production. The coating has an ultra-low coefficient of friction of 0.20–0.40 without the use of lubricant, a contact resistance of less than 1 mΩ, stable performance after heat treatment at 200℃, and a insertion / removal life exceeding 50,000 cycles.

Description

Technical Field

[0001] This invention belongs to the field of electroplating technology, and relates to a cyanide-free silver plating solution, its electroplating method, application, and products. More specifically, this invention relates to a cyanide-free silver plating solution with ultra-long plating solution life, excellent wear resistance, and stable contact resistance, which is particularly suitable for the preparation of functional coatings under frequent plugging and unplugging and high current density conditions, such as fast charging connectors for electric vehicles, electrical connectors for rail transit traction systems, and high-frequency pluggable power interfaces for data centers. Background Technology

[0002] Electric vehicles need to address two key issues in energy replenishment: First, charging time should be short enough, for example, charging to over 80% in just a few minutes, comparable to refueling time; second, battery capacity should be large enough to support a driving range of over 1000 kilometers on a single charge. This presents a technical challenge: the connector needs to handle a considerable current during charging, generating significant heat. Among all connector metals, silver has the best electrical and thermal conductivity, making it an ideal material for charging gun connectors. However, pure silver has poor wear resistance and cannot meet the requirements of repeated insertion and removal.

[0003] To address the insufficient wear resistance of pure silver, existing technologies have proposed various improvement schemes. Patents CN112680756A, TWI905996B, CN112941578B, CN116601338A, CN118302919A, and CN110997985A disclose technical solutions to improve the wear resistance of silver plating by doping with bismuth atoms, where the bismuth atom content ranges from 0.01% to 10%. However, under heating conditions, the oxidation or segregation of bismuth in these silver-bismuth alloy platings leads to increased contact resistance and a higher coefficient of friction, failing to meet the long-term stable operation requirements of fast-charging connectors under thermal cycling conditions.

[0004] EP4317536B1 discloses a method for manufacturing silver-plated products, which uses an aqueous solution containing potassium silver cyanide, potassium cyanide, and mercaptothiazole to improve the wear resistance of silver through the doping of mercaptothiazole. However, this system contains highly toxic cyanide, which does not meet environmental protection requirements. In addition, mercaptothiazole has extremely strong adsorption on nickel or silver surfaces, which may lead to poor adhesion and high internal stress problems caused by intercalation.

[0005] CN121250485A and CN117677733A proposed a "copper-nickel-graphite silver" gradient composite coating structure. By optimizing the material ratio and preparation process of each coating layer, they achieved a reduction in temperature rise and an improvement in insertion / removal life under high current conditions. However, this system also contains cyanide, which does not meet environmental protection requirements; at the same time, the dispersion of graphite and other particles in the plating solution is poor, resulting in uneven carbon distribution in the coating layer.

[0006] CN115125591B discloses a cyanide-free silver plating composition comprising a silver ion source, thiodiethylene glycol (HO(CH2)2-S-(CH2)2OH), and a sulfonated anionic polymer (such as naphthalenesulfonic acid formaldehyde condensate) for depositing a silver layer with a low coefficient of friction. Studies have shown that this plating solution has extremely poor stability; the plating layer becomes rough after less than one MTO (Metal Turn Over, where one MTO represents the total amount of metal added during the use of the plating solution, exactly equal to the total amount of metal used in the initial preparation of the plating solution), failing to meet the requirements of industrial mass production.

[0007] Besides the issue of plating bath life, another key drawback of existing cyanide-free silver plating systems is that the wear resistance coefficient of the plating layer easily rises above 0.50 under thermal cycling conditions, failing to meet the lifespan requirement of ≥10,000 mating cycles for fast-charging connectors. The root cause of this problem is that existing systems have failed to effectively address the stability of the friction coefficient and contact resistance of the silver plating layer after heat treatment.

[0008] In summary, current electric vehicle fast-charging connectors commonly employ silver-based plating to meet the requirements of high conductivity and high thermal conductivity. The mainstream processes include cyanide-containing silver plating and cyanide-free silver plating, which improve the wear resistance of the plating by doping with inorganic elements, organic substances, or graphite. While cyanide-containing systems offer stable plating performance, the highly toxic cyanide leads to high wastewater treatment costs and stringent environmental approvals. Cyanide-free systems are mostly based on coordination systems such as thiodiglycol and hydantoin, but cyanide-free silver plating has an extremely short lifespan, failing to meet the demands of industrial mass production. Furthermore, none of the above systems can completely solve the problem of coating friction coefficient and contact resistance stability under thermal cycling conditions. Therefore, there is an urgent need in this field to develop a cyanide-free silver electroplating composition with a long plating bath life, excellent wear resistance, and good thermal stability. Summary of the Invention

[0009] The present invention aims to address the aforementioned deficiencies in the prior art by providing a long-life cyanide-free silver plating solution, its electroplating method, applications, and products. Specifically, the technical problems to be solved by the present invention include: (1) The existing cyanide-free silver plating system has a short lifespan (<3 MTO), which cannot support continuous industrial mass production. (2) When the temperature rises, the contact resistance of the existing silver-bismuth alloy coating doped with bismuth atoms increases due to the oxidation or segregation of bismuth, and the coefficient of friction easily rises to above 0.50, which cannot meet the lifespan requirement of ≥10,000 insertion and removal cycles for fast charging connectors. To achieve the above objectives, the present invention provides the following technical solutions.

[0010] In a first aspect, the present invention provides a cyanide-free silver plating solution, comprising: a) Silver ion source; b) Methylsulfonic acid; c) A silver ion complexing agent, wherein the molar ratio of the silver ion complexing agent to silver ions is 1 or more, and the silver ion complexing agent has the following general formula: HO-CH2CH2-[SCH2CH2] n -OH, where n is an integer from 1 to 3; d) A composite stabilizer, comprising a first stabilizer and a second stabilizer; the first stabilizer is selected from one or more of alkyl sulfonates, lignin sulfonates, polystyrene sulfonates, naphthalene sulfonate formaldehyde condensates, methyl naphthalene sulfonate formaldehyde condensates, and benzyl naphthalene sulfonate formaldehyde condensates; the second stabilizer is selected from one or more of linear alcohol polyoxyethylene ethers, ethylene oxide-propylene oxide block polymers, alkylphenyl polyoxyethylene ethers, and alkylnaphthol polyoxyethylene ethers; and e) Water.

[0011] Methylsulfonic acid (CH3SO3H) plays a role in maintaining the pH value of the plating solution, keeping it at a strongly acidic environment below pH 2. This strongly acidic environment helps maintain the stability of silver ions in the solution, preventing hydrolysis and precipitation, and also facilitates anodic dissolution. Methylsulfonic acid also has good electrical conductivity, effectively improving the dispersion and coverage of the plating solution.

[0012] This invention employs a compound with the general formula HO-CH2CH2-[SCH2CH2] n Compounds with -OH groups act as complexing agents, where n is an integer from 1 to 3. Experiments show that when n is greater than 3, the solubility of the complexing agent decreases, and it cannot form a stable complex with silver ions. Therefore, the upper limit of n is limited to 3.

[0013] The inventors discovered that using a compound with the specific structure described above as a silver ion complexing agent, when the molar ratio of the complexing agent to silver ions is not less than 1, can form a sufficiently stable complex with the silver ions, which is beneficial to the long-term stability of the plating solution. During the electroplating process, the complexing agent, by complexing with silver ions, adjusts the discharge potential of the silver ions, making the silver deposition process more gradual, which is conducive to obtaining a fine and dense silver plating layer.

[0014] The main functions of the first stabilizer are: (i) to associate with silver ions through its own sulfonate groups, affecting the mass transfer and discharge process of silver ions; and (ii) to partially embed into the coating during electrodeposition, introducing carbon and sulfur elements to improve the wear resistance of the coating. The second stabilizer plays a synergistic polarization role in this invention: with the presence of the first stabilizer, the addition of the second stabilizer can further increase the polarization of silver deposition, making the current density of silver deposition at the same potential smaller, i.e., making silver deposition more difficult, thereby greatly extending the life of the plating solution. Experimental results show that when the first stabilizer is added alone, although silver can still be deposited, the life of the plating solution is shorter, and the coating quickly becomes rough; however, after adding the second stabilizer to the first stabilizer, a bright coating can still be obtained even at 10 MTO or higher.

[0015] This invention significantly improves the stability of the plating bath by controlling the molar ratio of the complexing agent to silver ions to be above 1 and through the synergistic effect of the first and second stabilizers. In the silver electroplating composition of this invention, the synergistic effect of the first and second stabilizers is not only key to achieving an ultra-long plating bath life (>10 MTO), but also an important factor in obtaining a coating with a low coefficient of friction (0.2–0.4). Figure 6 As shown, under the combined action of the first and second stabilizers, the polarization curve of silver shifts significantly negatively, indicating that silver deposition becomes more difficult. This is beneficial for suppressing the spontaneous reduction reaction of silver ions in the plating bath, thereby extending the life of the plating bath. Simultaneously, the synergistic adsorption of the two stabilizers regulates the silver electrocrystallization process, refining the coating grains and increasing density, thus improving the tribological properties of the coating. SEM-EDX analysis of the electroplated coating shows that the coating contains small amounts of carbon and sulfur elements (such as...). Figure 5 As shown in the figure, this atomic-scale doping is more uniformly dispersed than the doping of traditional graphite or graphene particles, and can significantly reduce the coefficient of friction and improve wear resistance without the need for lubricants.

[0016] Furthermore, the silver ion source is selected from silver nitrate (AgNO3) or silver methanesulfonate (CH3SO3Ag). Both of these silver salts have good water solubility and are easy to prepare into stable electroplating solutions.

[0017] Furthermore, the concentration of silver ions is controlled within the range of 10–70 g / L, the concentration of methanesulfonic acid within the range of 20–60 g / L, the concentration of the first stabilizer within the range of 1–50 g / L, and the concentration of the second stabilizer within the range of 0.1–10 g / L. If the silver ion concentration is too low, the electroplating efficiency will be insufficient and the deposition rate will be slow; if the silver ion concentration is too high, the plating solution will carry away silver ions, leading to higher costs. The amount of methanesulfonic acid used should be sufficient to adjust the pH value to below 2. If the concentration of the first stabilizer is too low, the above effects will be insignificant; if the concentration is too high, it may cause excessive entrainment of organic matter in the coating, affecting the purity and conductivity of the coating. If the concentration of the second stabilizer is too low, the synergistic effect will be insignificant; if the concentration is too high, it may affect the conductivity and deposition rate of the plating solution.

[0018] Furthermore, the cyanide-free silver plating solution of the present invention may also contain carbon particles, wherein the carbon particles are selected from one or more of graphite, graphite oxide, and carbon nanotubes, and the concentration is 0.01–5 g / L. The addition of carbon particles can further improve the wear resistance of the coating. As a solid lubricant dispersed in the silver plating layer, the carbon particles can reduce the direct contact area and lower the coefficient of friction during friction. The concentration of carbon particles should not be too high, otherwise it will affect the uniformity and conductivity of the coating.

[0019] Furthermore, the lifespan of the cyanide-free silver plating solution of the present invention is over 10 MTO.

[0020] In a second aspect, the present invention provides a method for electroplating silver on a substrate, comprising: S1 providing a substrate; S2 contacting the substrate with the cyanide-free silver plating solution of the present invention; and S3 applying an electric current to the cyanide-free silver plating solution and the substrate to electroplat a silver layer on the substrate.

[0021] Furthermore, the electroplating temperature of the cyanide-free silver plating solution is 30–60°C, and the current density is 0.1–10 ASD. If the electroplating temperature is too low, the ion migration rate is slow, the deposition rate is low, and the coating stress increases; if the electroplating temperature is too high, the evaporation of the plating solution accelerates, the complexing agent may decompose, and the coating crystals may become rough. If the current density is too low, the deposition rate is too slow, resulting in low production efficiency; if the current density is too high, it may cause the coating to burn, become rough, and have reduced adhesion.

[0022] Furthermore, the substrate is selected from nickel, copper, or copper alloys.

[0023] Thirdly, the present invention provides the use of the above-mentioned cyanide-free silver plating solution in the preparation of plating for contacts of fast charging connectors for electric vehicles, electrical connectors for rail transit traction systems, or high-frequency pluggable power interfaces for data centers.

[0024] Fourthly, the present invention provides an article comprising a substrate and a silver layer located on the substrate, the silver layer being formed by electrodeposition using the cyanide-free silver plating solution of the present invention.

[0025] Furthermore, the silver layer has a coefficient of friction of 0.2 to 0.4 without the use of a lubricant.

[0026] Furthermore, the contact resistance of the silver layer is less than 1 mΩ. This resistance value refers to the resistance value of the product obtained by electroplating without heat treatment.

[0027] Furthermore, after heat treatment at 200°C for 6 hours, the coefficient of friction of the silver layer is still in the range of 0.2 to 0.4, and the contact resistance is still below 1.2 mΩ.

[0028] Compared with the prior art, the technical solution of the present invention has the following significant technical effects: (1) The lifespan of the plating solution is significantly increased to more than 10 MTO, which can support continuous industrial mass production. Through the synergistic optimization of the first and second stabilizers and the precise control of the amount of complexing agent, the silver electroplating composition of the present invention maintains the appearance, friction coefficient and contact resistance of the plating layer basically unchanged after the silver ions are consumed by 10 MTO, which completely solves the technical problem that the existing cyanide-free silver plating system has a short plating solution lifespan and cannot meet the needs of industrial mass production.

[0029] (2) The wear resistance coefficient of the coating is stable at 0.20-0.40, and the contact resistance is below 1 mΩ. Even after heat treatment at 200℃ for 6 hours, the friction coefficient and contact resistance remain stable, and the insertion and extraction life exceeds 50,000 cycles. The coating of this invention achieves an ultra-low friction coefficient without the need for external lubricant, which is far superior to the existing cyanide-free silver plating system (typically with a friction coefficient > 0.5). At the same time, the contact resistance of the coating of this invention is extremely low (below 1 mΩ), which can meet the conductivity requirements under high current and high thermal conductivity conditions. More importantly, after heat treatment at 200℃ for 6 hours, the friction coefficient and contact resistance of the coating remain basically unchanged, indicating that the coating of this invention has excellent thermal stability and can meet the long-term use requirements of fast charging connectors under thermal cycling conditions.

[0030] (3) Through the synergistic effect of the first stabilizer and the second stabilizer, the present invention can achieve atomic-scale carbon and sulfur doping in the coating. Compared with traditional graphite or graphene particle doping, the dispersion is more uniform and there is no problem of uneven dispersion and internal stress caused by the incompatibility between graphite and metallic silver, which further reduces the friction coefficient of the coating. Attached Figure Description

[0031] Figure 1 This is an optical photograph of the friction surface of the silver-plated ball in Example 1.

[0032] Figure 2 This is an optical photograph of the friction surface of the silver-plated layer in Example 1.

[0033] Figure 3 The results show the contour measurement of the surface scratches on the silver plating layer in Example 1.

[0034] Figure 4 The friction coefficient measurement results are for the silver plating layer in Example 1.

[0035] Figure 5 The images show SEM images and EDX analysis results of the silver plating layer in Example 1.

[0036] Figure 6 The effect of the stabilizer provided by this invention on the silver deposition potential. Detailed Implementation

[0037] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention.

[0038] The test methods used in the following embodiments are described below: Friction coefficient testing: Tribological tests were performed using an MXW-002 pin-disc tribometer (Jinan Yihua) equipped with a linear reciprocating stage in linear mode. The moving friction partner, silver-plated, consisted of a 6 cm diameter copper ball plated with the wear-resistant silver described in the examples. All tests were performed in a "like-on-like" manner. A vertical force of 2 N was applied, with a stroke of 1 cm, and friction was repeated twice per second, with the coefficient of friction recorded. The coefficient of friction (COF) was calculated as the ratio between the required frictional force and pressure.

[0039] Contact resistance test: A DC double-arm bridge (QJ44, Shanghai Zhengyang) is used to measure contact resistance to eliminate the influence of the test lead resistance on the measurement results. Two current leads and two voltage leads are used for current supply and voltage measurement respectively, thereby improving measurement accuracy. The probe pressure is controlled above 10 N until the contact resistance is relatively stable.

[0040] Plating bath life test: Continuous electroplating was performed using copper sheets, with silver ions added continuously during the process. The changes in coating appearance, coefficient of friction, and contact resistance with silver ion consumption (in MTO) were observed. For every increase of 1 MTO in silver ion consumption (i.e., the total amount of silver ions consumed equals the total amount of silver ions in the initial plating bath), a new sample was taken for testing.

[0041] Sample preparation: On a 3 cm × 3 cm copper sample, the sample was first degreased and cleaned, then a 2 μm nickel underlayer was electroplated (using the AURIFAB Ni-5600 nickel plating system). A silver underlayer was then deposited using a base bath consisting of 1 g / L silver metal from silver methanesulfonate, 10 g / L thiodiethylene glycol, and 20 g / L methanesulfonic acid solution, with a 2 ASD (A / dm²) coating applied. 2 The DC current density was maintained at room temperature for 15 seconds. Finally, silver was electroplated to a thickness of 8 micrometers using the silver plating solutions of each embodiment and comparative example at 40–60°C.

[0042] Heat treatment conditions: 200℃ for 6 hours, in an air atmosphere.

[0043] Example 1 In a 2 L beaker, add silver nitrate (providing 10 g / L of silver ions), 40 g / L of methanesulfonic acid, 20 g / L of thiodiethylene glycol (HOCH2CH2SCH2CH2OH, i.e., n=1, the molar ratio of complexing agent to silver ions is 1.77), 50 g / L of first stabilizer 1 (sodium dodecylbenzenesulfonate, CAS No.: 25155-30-0), 0.1 g / L of second stabilizer 1 (polymer of methyl ethylene oxide with 1,2-ethylenediamine and ethylene oxide, CAS No.: 26316-40-5), and the remainder is water. The electroplating temperature is 30℃, and the current density is 2 ASD.

[0044] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is 0.26, and the contact resistance is 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is 0.28, and the contact resistance is 1.1 mΩ. The silver deposit in the coating does not penetrate the substrate through the wear marks and is essentially flat relative to the background baseline. Figures 1-4 As shown, continuous electroplating was performed using copper sheets with silver ions added continuously during the process. It was found that after 10 MTO of silver ions were consumed, the appearance, coefficient of friction, and contact resistance of the plating layer remained basically unchanged, and the plating solution life was >10 MTO.

[0045] Example 2 In a 2 L beaker, add silver methanesulfonate (providing 20 g / L of silver ions), 50 g / L of methanesulfonic acid, 40 g / L of HO(CH2CH2S)2CH2CH2OH (i.e., n=2, molar ratio of complexing agent to silver ions is 1.19), 30 g / L of first stabilizer 2 (sodium polystyrene sulfonate, CAS No.: 25704-18-1), 1 g / L of second stabilizer 2 (nonylphenol polyoxyethylene ether, CAS No.: 127087-87-0), and the remainder is water. The electroplating temperature is 40℃, and the current density is 4 ASD.

[0046] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.22, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.26, and the contact resistance is 0.9 mΩ. Using continuous electroplating with copper sheets and constant replenishment of silver ions, it was found that after 10 MTO of silver ions were consumed, the appearance, coefficient of friction, and contact resistance of the coating remained essentially unchanged, indicating a plating bath life >10 MTO.

[0047] Example 3 In a 2 L beaker, add silver methanesulfonate (providing 20 g / L of silver ions), 20 g / L of methanesulfonic acid, 50 g / L of HO(CH2CH2S)3CH2CH2OH (i.e., n=3, molar ratio of complexing agent to silver ions is 1.12), 1 g / L of first stabilizer 3 (methylnaphthalenesulfonate formaldehyde condensate, CAS No.: 9047-27-2), 10 g / L of second stabilizer 3 (2-naphthol polyoxyethylene ether, CAS No.: 35545-57-4), and the remainder is water. The electroplating temperature is 40℃, and the current density is 4 ASD.

[0048] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.31, and the contact resistance is 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.34, and the contact resistance is 1.2 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0049] Example 4 In a 2 L beaker, silver methanesulfonate (providing 40 g / L of silver ions), 40 g / L of methanesulfonic acid, 50 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 1.11), 10 g / L of first stabilizer 1 (sodium dodecylbenzenesulfonate), 10 g / L of second stabilizer 2 (nonylphenol polyoxyethylene ether), and 0.01 g / L of carbon nanotubes were added, with the remainder being water. The electroplating temperature was 40℃, and the current density was 4 ASD.

[0050] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.28, and the contact resistance is 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.32, and the contact resistance is 1.1 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0051] Example 5 In a 2 L beaker, add silver methanesulfonate (providing 40 g / L of silver ions), 60 g / L of methanesulfonic acid, 50 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 1.11), 20 g / L of first stabilizer 2 (sodium polystyrene sulfonate), 10 g / L of second stabilizer 3 (2-naphthol polyoxyethylene ether), 5 g / L of graphite oxide, and the remainder is water. The electroplating temperature is 50℃, and the current density is 4 ASD.

[0052] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.29, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.33, and the contact resistance is 1.0 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0053] Example 6 In a 2 L beaker, silver nitrate (providing 70 g / L of silver ions), 40 g / L of methanesulfonic acid, 80 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 1.01), 5 g / L of first stabilizer 3 (methylnaphthalenesulfonate formaldehyde condensate), 10 g / L of second stabilizer 1 (polymer of methyl ethylene oxide with 1,2-ethylenediamine and ethylene oxide), and the remainder is water. The electroplating temperature is 60 °C, and the current density is 6 ASD.

[0054] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.35, and the contact resistance is 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.38, and the contact resistance is 1.1 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0055] Example 7 In a 2 L beaker, silver methanesulfonate (providing 30 g / L of silver ions), 50 g / L of methanesulfonic acid, 60 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions 1.77), 20 g / L of first stabilizer 4 (naphthalenesulfonate formaldehyde condensate, CAS No.: 9084-06-4), and 5 g / L of second stabilizer 1 (polymer of methyl ethylene oxide with 1,2-ethylenediamine and ethylene oxide) were added, with the remainder being water. The electroplating temperature was 45℃, and the current density was 0.5 ASD.

[0056] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.30, and the contact resistance is 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.33, and the contact resistance is 1.0 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0057] Example 8 In a 2 L beaker, add silver nitrate (providing 30 g / L of silver ions), 60 g / L of methanesulfonic acid, 70 g / L of HO(CH2CH2S)2CH2CH2OH (n=2, molar ratio of complexing agent to silver ions is 1.38), 15 g / L of first stabilizer 5 (benzylnaphthalene sulfonate formaldehyde condensate), 8 g / L of second stabilizer 2 (nonylphenol polyoxyethylene ether), and the remainder is water. The electroplating temperature is 50℃, and the current density is 10 ASD.

[0058] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.32, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.35, and the contact resistance is 1.1 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0059] Example 9 In a 2 L beaker, add silver nitrate (providing 10 g / L of silver ions), 40 g / L of methanesulfonic acid, 20 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 1.77), 50 g / L of first stabilizer 1 (sodium dodecylbenzenesulfonate), 0.1 g / L of second stabilizer 1 (polymer of methyl ethylene oxide with 1,2-ethylenediamine and ethylene oxide), and the remainder is water. The electroplating temperature is 30℃, and the current density is 0.1 ASD.

[0060] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.27, and the contact resistance is 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.29, and the contact resistance is 1.1 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0061] Example 10 In a 2 L beaker, add silver methanesulfonate (providing 25 g / L of silver ions), 45 g / L of methanesulfonic acid, 45 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 1.59), 20 g / L of first stabilizer 6 (sodium lignosulfonate, CAS No. 8061-51-6), 3 g / L of second stabilizer 4 (linear alcohol polyoxyethylene ether, specifically lauryl alcohol polyoxyethylene ether, CAS No. 9002-92-0), with the remainder being water. The electroplating temperature is 45℃, and the current density is 3 ASD.

[0062] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is 0.27, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is 0.30, and the contact resistance is 1.0 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0063] Example 11 In a 2 L beaker, add silver nitrate (providing 30 g / L of silver ions), 50 g / L of methanesulfonic acid, 55 g / L of HO(CH2CH2S)2CH2CH2OH (n=2, molar ratio of complexing agent to silver ions is 1.09), 15 g / L of first stabilizer 2 (sodium polystyrene sulfonate, CAS No. 25704-18-1), 2 g / L of second stabilizer 2 (nonylphenol polyoxyethylene ether, CAS No. 127087-87-0), 1.5 g / L of graphite particles (average particle size 1-5 μm), and the remainder is water. The electroplating temperature is 50℃, and the current density is 2.5 ASD.

[0064] After electroplating, the coating exhibits a uniform grayish-black metallic luster. The measured coefficient of friction for the silver deposit is 0.24, and the contact resistance is 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color slightly darkened but did not peel off, with a coefficient of friction of 0.28 and a contact resistance of 1.1 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the appearance, coefficient of friction, and contact resistance of the coating remained essentially unchanged, indicating a plating bath life >10 MTO.

[0065] Example 12 In a 2 L beaker, add silver methanesulfonate (providing 20 g / L of silver ions), 40 g / L of methanesulfonic acid, 30 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 1.33), 10 g / L of first stabilizer 7 (calcium lignosulfonate, CAS No. 8061-52-7), 5 g / L of second stabilizer 5 (ethylene oxide-propylene oxide block polymer, Poloxamer 188, CAS No. 9003-11-6), 0.5 g / L of graphite (average particle size 1-5 μm), and the remainder is water. The electroplating temperature is 40℃, and the current density is 1.5 ASD.

[0066] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is 0.26, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is 0.29, and the contact resistance is 1.0 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the coating appearance, coefficient of friction, and contact resistance remained essentially unchanged, indicating a plating bath life >10 MTO.

[0067] Comparative Example 1 In a 2 L beaker, add silver nitrate (providing 10 g / L of silver ions), 40 g / L of methanesulfonic acid, 10 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 0.89), 10 g / L of first stabilizer 3 (methylnaphthalenesulfonate formaldehyde condensate), and the remainder is water. The electroplating temperature is 40℃, and the current density is 2 ASD.

[0068] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.28, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains largely unchanged, the coefficient of friction is approximately 0.31, and the contact resistance is 1.1 mΩ. Continuous electroplating with copper sheets, with continuous replenishment of silver ions, revealed that the coating became rough and unusable after 0.4 MTO. This indicates that when the molar ratio of thiodiethylene glycol to silver ions is less than 1, the long-term stability of the plating solution is insufficient, possibly due to insufficient complexation leading to a rough coating.

[0069] Comparative Example 2 In a 2 L beaker, add silver nitrate (providing 10 g / L of silver ions), 40 g / L of methanesulfonic acid, 30 g / L of HO(CH2CH2S)4CH2CH2OH (i.e., n=4, the molar ratio of complexing agent to silver ions is 1.07), 10 g / L of first stabilizer 1 (sodium dodecylbenzenesulfonate), 5 g / L of second stabilizer 2 (nonylphenol polyoxyethylene ether), and the remainder is water. The electroplating temperature is 40℃, and the current density is 2 ASD.

[0070] The plating solution was inherently turbid during preparation. After electroplating, the coating exhibited a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit was approximately 0.35, and the contact resistance was 0.9 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remained largely unchanged, with a coefficient of friction of approximately 0.37 and a contact resistance of 1.1 mΩ. Continuous electroplating with copper sheets, with continuous replenishment of silver ions, revealed that the coating became rough and unusable after 0.5 MTO. This indicates that when the complexing agent n>3, the solubility of the complexing agent is insufficient, the plating solution itself is turbid, and the rough coating may result from the decomposition of the complexing agent in later stages.

[0071] Comparative Example 3 In a 2 L beaker, add silver nitrate (providing 10 g / L of silver ions), 40 g / L of methanesulfonic acid, 20 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 1.77), 10 g / L of the second stabilizer 2 (nonylphenol polyoxyethylene ether), and the remainder is water (without adding the first stabilizer). The electroplating temperature is 40℃, and the current density is 2 ASD.

[0072] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 1.05, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 1.02, and the contact resistance is 1.1 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that after 10 MTO of silver ions were consumed, the appearance, coefficient of friction, and contact resistance of the coating remained essentially unchanged. This indicates that although the plating bath can have a long lifespan without the addition of the first stabilizer, the coefficient of friction of the coating is significantly higher (>1.0) due to the lack of atomic-level carbon / sulfur doping provided by the first stabilizer.

[0073] Comparative Example 4 In a 2 L beaker, add silver nitrate (providing 10 g / L of silver ions), 40 g / L of methanesulfonic acid, 40 g / L of thiodiethylene glycol (n=1, molar ratio of complexing agent to silver ions is 3.54), 20 g / L of the first stabilizer 3 (methylnaphthalenesulfonate formaldehyde condensate), and the remainder is water (no second stabilizer added). The electroplating temperature is 40℃, and the current density is 2 ASD.

[0074] After electroplating, the coating exhibits a metallic texture and a semi-bright appearance. The measured coefficient of friction for the silver deposit is approximately 0.35, and the contact resistance is 0.8 mΩ. After heat treatment at 200℃ for 6 hours, the coating color remains essentially unchanged, the coefficient of friction is approximately 0.42, and the contact resistance is 1.1 mΩ. Continuous electroplating using copper sheets, with continuous replenishment of silver ions, revealed that the coating became rough and unusable after 0.5 MTO of silver ions were consumed. This indicates that even with a molar ratio of thiodiethylene glycol to silver ions greater than 1 and an increased amount of the first stabilizer, the plating bath life remains short due to the lack of the second stabilizer provided by this invention, making it difficult to meet the requirements of continuous industrial production.

[0075] The components and test results of each embodiment and comparative example are summarized in Table 1.

[0076] Table 1. Components and test results of each embodiment and comparative example.

[0077] First stabilizer 1: Sodium dodecylbenzenesulfonate (CAS No.: 25155-30-0); First stabilizer 2: Sodium polystyrene sulfonate (CAS No.: 25704-18-1); First stabilizer 3: Methylnaphthalenesulfonate formaldehyde condensate (CAS No.: 9047-27-2); First stabilizer 4: Naphthalenesulfonate formaldehyde condensate (CAS No.: 9084-06-4); First stabilizer 5: Benzylnaphthalenesulfonate formaldehyde condensate; First stabilizer 6: Sodium lignosulfonate CAS No. 8061-51-6; First stabilizer 7: Calcium lignosulfonate CAS No. 8061-52-7; Second stabilizer 1: Polymer of methyl ethylene oxide with 1,2-ethylenediamine and ethylene oxide (CAS No.: 26316-40-5); Second stabilizer 2: Nonylphenol polyoxyethylene ether (CAS No.: 127087-87-0); Second stabilizer 3: 2-Naphthol polyoxyethylene ether (CAS No.: 35545-57-4); Second stabilizer 4: Lauryl alcohol polyoxyethylene ether, CAS No. 9002-92-0; Second stabilizer 5: Ethylene oxide-propylene oxide block polymer, Poloxamer 188, CAS No. 9003-11-6).

[0078] By comparing Examples 1-12 with Comparative Examples 1-4, the following conclusions can be drawn: Regarding the lifespan of the plating solution: In Comparative Example 1, when the molar ratio of thiodiethylene glycol (n=1) to silver ions was less than 1, the long-term stability of the plating solution was insufficient, and the coating became rough after only 0.4 MTO. In Comparative Example 2, when a complexing agent with n=4 was used, the plating solution was turbid due to insufficient solubility of the complexing agent, and the coating became rough after 0.5 MTO. Comparative Example 4, based on Comparative Example 1, increased the amount of thiodiethylene glycol (molar ratio to silver greater than 1) and the first stabilizer. Due to the lack of a second stabilizer, the stability of the plating solution was still insufficient, and the coating became rough after 0.5 MTO. In contrast, in Examples 1 to 12 of this invention, the molar ratio of the complexing agent to silver ions was greater than 1, and the n value of the complexing agent was limited to the range of 1 to 3, and the plating solution lifespan all reached more than 10 MTO.

[0079] Regarding the coefficient of friction: In Comparative Example 3, no first stabilizer was added. Although the plating bath life was long (>10 MTO), the coefficient of friction of the coating was significantly high (1.05), which could not meet the requirements for connector insertion and removal. In contrast, in Examples 1-12 of this invention, through the synergistic effect of the first and second stabilizers, the coefficient of friction of the coating was stabilized in the range of 0.20-0.40, which was far superior to that of Comparative Example 3. The mechanism of this result is that the first stabilizer is adsorbed on the cathode surface during silver electrodeposition and partially embedded in the coating during the electrodeposition process, thereby introducing atomic-scale carbon and sulfur doping into the coating, resulting in more uniform dispersion and effectively reducing the coefficient of friction of the coating.

[0080] Regarding the current density range: Examples 7 (0.5 ASD), 9 (0.1 ASD), and 8 (10 ASD) demonstrate that the compositions of the present invention function properly and produce high-performance coatings over a wide current density range from 0.1 ASD to 10 ASD.

[0081] Regarding thermal stability: After heat treatment at 200°C for 6 hours, the coefficient of friction and contact resistance of the coatings in each embodiment remained stable with minimal variation. This indicates that the coating of the present invention possesses excellent thermal stability, meeting the requirements for long-term use of fast-charging connectors under thermal cycling conditions.

[0082] Regarding the synergistic effect of the first and second stabilizers: Figure 6 The effects of these two stabilizers on silver deposition are explained. When the first stabilizer is added alone, although silver can still be deposited, the plating bath life is short, and the coating quickly becomes rough. Adding the second stabilizer to the first stabilizer further increases the polarization of silver—the current density for silver deposition at the same potential decreases, meaning silver deposition becomes more difficult—thus greatly extending the plating bath life, achieving a bright coating even at 10 MTO or higher. Meanwhile, as... Figure 5 As shown, SEM-EDX analysis of the coating indicates that the coating is doped with a small amount of carbon and sulfur. This atomic-scale doping is more uniformly dispersed than nano- or micro-scale graphite / graphene particles, and there is no internal stress problem caused by uneven dispersion. Thus, an ultra-low coefficient of friction can be obtained without the need for external lubricant.

[0083] The cyanide-free silver plating solution, electroplating method, and application of this invention have significant industrial practical value. This plating solution completely eliminates cyanide, belonging to an environmentally friendly green manufacturing technology, and meets the latest limit requirements of GB21900-2008. The plating solution life can reach over 10 MTO, supporting continuous industrial mass production, significantly reducing maintenance costs and downtime risks of electroplating production lines. The coating wear resistance coefficient is stable at 0.20–0.40, and the contact resistance is approximately below 1 mΩ. Even after heat treatment, the friction coefficient and contact resistance remain stable, and the insertion / removal life exceeds 50,000 cycles.

[0084] The products and methods of this invention are particularly suitable for preparing functional coatings on the contact surfaces of fast-charging connectors for electric vehicles. They can also be used for coatings on electrical connectors in rail transit traction systems under high current density conditions, and for wear-resistant conductive coatings on high-frequency pluggable power interfaces in data centers. Compared to existing technologies, this invention achieves breakthrough improvements in environmental friendliness, plating solution stability, coating wear resistance, and thermal stability, and has broad market application prospects.

[0085] Although embodiments of the present invention have been shown and described above, it is understood that these embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions, and alterations to the above embodiments within the scope of the present invention without departing from its principles and spirit. The scope of protection of the present invention is defined by the claims and their equivalents.

Claims

1. A long-life, cyanide-free silver plating solution, characterized in that, The cyanide-free silver plating solution comprises: a) Silver ion source; b) Methylsulfonic acid; c) A silver ion complexing agent, wherein the molar ratio of the silver ion complexing agent to silver ions is 1 or more, and the silver ion complexing agent has the following general formula: HO-CH2CH2-[SCH2CH2] n -OH, where n is an integer from 1 to 3; d) A composite stabilizer, comprising a first stabilizer and a second stabilizer; the first stabilizer is selected from one or more of alkyl sulfonates, lignin sulfonates, polystyrene sulfonates, naphthalene sulfonate formaldehyde condensates, methyl naphthalene sulfonate formaldehyde condensates, and benzyl naphthalene sulfonate formaldehyde condensates; the second stabilizer is selected from one or more of linear alcohol polyoxyethylene ethers, ethylene oxide-propylene oxide block polymers, alkylphenyl polyoxyethylene ethers, and alkylnaphthol polyoxyethylene ethers; and e) Water.

2. The cyanide-free silver plating solution according to claim 1, characterized in that, The silver ion source is selected from silver nitrate or silver methanesulfonate.

3. The cyanide-free silver plating solution according to claim 1, characterized in that, The concentration of silver ions is 10–70 g / L, the concentration of methanesulfonic acid is 20–60 g / L, the concentration of the first stabilizer is 1–50 g / L, and the concentration of the second stabilizer is 0.1–10 g / L.

4. The cyanide-free silver plating solution according to claim 1, characterized in that, The cyanide-free silver plating solution also contains carbon particles, which are selected from one or more of graphite, graphite oxide, and carbon nanotubes, and their concentration is 0.01 to 5 g / L.

5. The cyanide-free silver plating solution according to any one of claims 1 to 4, characterized in that, The lifespan of the cyanide-free silver plating solution is over 10 MTO.

6. A method for electroplating silver on a substrate, characterized in that, Includes the following steps: S1 provides the substrate; S2 brings the substrate into contact with the cyanide-free silver plating solution according to any one of claims 1 to 5; as well as S3 applies an electric current to the cyanide-free silver plating solution and the substrate to electroplate a silver layer on the substrate.

7. The method according to claim 6, characterized in that, The electroplating temperature in step S3 is 30–60°C, and the current density is 0.1–10 ASD.

8. The method according to claim 6, characterized in that, The substrate is selected from nickel, copper, or copper alloys.

9. The use of the cyanide-free silver plating solution according to any one of claims 1 to 5, characterized in that, The cyanide-free silver plating solution is used to prepare the plating layer for fast charging connector contacts for electric vehicles, electrical connectors for rail transit traction systems, or high-frequency pluggable power interfaces for data centers.

10. An article comprising a substrate and a silver layer thereon, characterized in that, The silver layer is formed by electrodeposition using the cyanide-free silver plating solution described in any one of claims 1 to 5.

11. The article of claim 10, characterized in that, The silver layer has a coefficient of friction of 0.2 to 0.4 without the use of lubricant.

12. The article of claim 10, characterized in that, The contact resistance of the silver layer is less than 1 mΩ.

13. The article of claim 10, characterized in that, After being heat-treated at 200°C for 6 hours, the silver layer still has a friction coefficient in the range of 0.2 to 0.4 and a contact resistance below 1.2 mΩ.

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