A battery welding process based on screw fixation and solder conductive

Abstract

The application relates to the technical field of battery manufacturing, and discloses a battery welding process based on screw fixing and tin soldering conduction, which comprises the following steps: applying a solid-state dormant flux composition to a region to be connected and solidifying the composition into a film; fixing a connecting piece through cold riveting; breaking the flux film by mechanical force to expose the internal active agent in situ; prepositioning low-temperature solder at the base of the connecting piece; and performing local low-temperature heating to make the solder melt and infiltrate under the action of the active agent, so that a conductive path is formed. The composition comprises a breakable polymer matrix and an active agent coated by the matrix. The application establishes a force and chemical synergistic mechanism, activates the flux by mechanical force, and solves the technical problem that low-temperature solder is difficult to wet the surface of an aluminum alloy; meanwhile, local low-temperature heating avoids thermal damage of high temperature to a battery cell, and realizes a battery connection with high mechanical strength, low interface resistance and safety and reliability.

Description

Technical Field

[0001] This invention relates to the field of battery manufacturing technology, specifically to a battery welding process based on screw fixing and solder conductivity. Background Technology

[0002] In the manufacturing process of power battery packs, individual battery cells (especially large cylindrical cells such as the 4680 type) need to be electrically connected using connectors (such as threaded studs). This connection not only requires extremely low interfacial contact resistance to carry large currents, but also extremely high mechanical strength and long-term reliability.

[0003] Currently, achieving this connection mainly relies on high-temperature welding technologies, such as laser welding. While laser welding can form a high-strength metallurgical bond, its high energy input and instantaneous high temperature inevitably transfer a large amount of heat to the battery cap and interior. This heat input poses a serious threat to the heat-sensitive components inside the battery (such as the separator and electrolyte), leading to irreversible degradation of the battery's electrochemical performance and even causing safety hazards.

[0004] To avoid high-temperature thermal damage, the industry has explored other connection methods. While purely mechanical connections, such as cold riveting, avoid heat input, the conductivity of the connection interface relies entirely on physical contact. Because the aluminum alloy cap always has a hard passivating oxide film (Al2O3) on its surface, this purely mechanical connection results in high and unstable interface resistance, failing to meet the low resistance requirements of power batteries, and is prone to electrical contact failure under vibration and thermal cycling.

[0005] Another approach is to use low-temperature solders (such as tin-bismuth alloys) for welding. However, low-temperature welding faces key technical obstacles when applied to aluminum alloy substrates. The passivation oxide film on the aluminum surface is chemically extremely stable, and conventional low-temperature fluxes are insufficiently active to effectively remove it within the 150°C to 200°C temperature window. On the other hand, traditional liquid fluxes with excessive activity (such as acidic fluxes) can cause severe corrosion to the connection points, and their residues are difficult to clean, seriously affecting the long-term reliability of the product. Therefore, how to achieve effective metallurgical bonding and high-strength mechanical fixation of the aluminum substrate while ensuring battery safety (i.e., low temperature) is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a battery welding process based on screw fixing and solder conductivity, which solves the problem of thermal damage to batteries caused by traditional welding methods, as well as the problem of difficulty in achieving high-strength metallurgical bonding of materials such as aluminum alloys at low temperatures.

[0007] To achieve the above objectives, the present invention provides the following technical solution: Firstly, the present invention provides a battery welding process based on screw fixing and solder conductivity, employing the following technical solution:

[0008] A battery welding process based on screw fixing and solder conductivity includes the following steps: S1: Applying a solid dormant flux composition containing an activator and a fragile polymer matrix to the area to be connected of the battery electrode and curing it to form a solid flux film; S2: Fixing a threaded connector with a flange to the area to be connected by cold pressing and riveting, exposing the activator inside the solid flux film; S3: Pre-placing low-temperature solder at the base of the threaded connector; S4: Locally heating the base of the threaded connector at low temperature to melt the low-temperature solder, which then impregnates under the action of the exposed activator, and forms a conductive path after cooling.

[0009] By adopting the above technical solution, this invention constructs a synergistic connection mechanism of mechanical activation-low-temperature chemical welding. Specifically, in step S1, the solid flux film, under normal conditions, encapsulates the highly active activator component in a dormant state through the physical coating of the polymer matrix, achieving pre-positioning and precise positioning of the flux and avoiding premature corrosion or failure of the activator on the substrate. Step S2 is the core activation step of this process. Utilizing the huge instantaneous mechanical impact force generated during the cold-press riveting process, the solid flux film, which has brittle characteristics, is forced to undergo in-situ physical breakage. This mechanical breakage breaks the coating shell of the polymer matrix, thereby exposing the internally encapsulated activator at specific points within the microscopic area of ​​the riveting interface. In step S4, since the activator has been awakened by mechanical force and directly contacts the metal surface to be joined, when a local low-temperature heat source is applied, the exposed activator rapidly reacts with the oxide layer on the substrate surface, removing the oxide film that hinders welding. The molten low-temperature solder then spreads and wets the fresh metal surface, ultimately forming a reliable metallurgical bond. This process achieves decoupling and optimization of the connection function by using mechanical fixation to bear the main structural load and low-temperature soldering to bear the main current transmission.

[0010] Preferably, in step S2, cold pressing is applied using a multi-point annular indenter, which simultaneously breaks up the solid flux film and creates a residual stress field around the area to be joined. By employing this technique, the multi-point annular indenter induces a controlled distribution of high-energy residual stress around the riveting point. According to the principles of physical metallurgy, high-stress areas have higher surface Gibbs free energy and stronger adsorption to liquid metal. Preferably, in step S4, the molten low-temperature solder preferentially wets the residual stress field and releases the residual stress during the wetting process. By employing this technique and utilizing the stress-inducing mechanism, the liquid solder is directionally guided to fill the area with the tightest riveting bond, achieving precise self-leveling of the solder. Simultaneously, the alloying process between the solder and the substrate releases the destructive stress originally concentrated at the riveting point, converting it into stable chemical bonding energy, thereby improving the fatigue resistance of the joint under long-term vibration conditions.

[0011] Preferably, in step S4, the localized low-temperature heating employs high-frequency induction heating, with the peak heating temperature controlled between 150°C and 200°C. By adopting the above technical solution, utilizing the skin effect of the high-frequency electromagnetic field, energy is confined within the extremely thin surface layer at the bottom of the threaded connector, achieving millisecond-level rapid heating and cooling. The process window of 150°C to 200°C ensures sufficient melting and leveling of the low-temperature solder, while remaining strictly below the pore-closing temperature of the polymer separator inside the battery and the decomposition temperature of the electrolyte, fundamentally eliminating the damage to the electrochemical performance of the battery cell caused by welding heat input.

[0012] Preferably, in step S3, the low-temperature solder is a porous pre-fabricated solder pad, and the alloy composition of the solder pad is a tin-bismuth alloy. By adopting the above technical solution, the tin-bismuth alloy (such as Sn42Bi58) has a low eutectic melting point of 138°C, which is suitable for low-temperature processes. The porous pre-fabricated pad provides capillary channels during the melting process, promoting solder climbing and wetting, and at the same time providing an escape path for volatile gases generated by flux reaction, effectively reducing the porosity inside the weld and improving the density of the conductive path.

[0013] Preferably, before step S1, a pretreatment step of argon plasma cleaning of the area to be joined is included.

[0014] By employing the above technical solution, this pretreatment step aims to obtain a clean interface with high surface energy to be joined. This step is performed in a vacuum plasma device. The mechanism involves high-purity argon gas being excited into a plasma state under a radio frequency (RF) or direct current (DC) electric field, generating high-kinetic-energy argon ions (Ar). + A mixture of electrons and free radicals.

[0015] High-energy argon ions can effectively remove trace organic contaminants (such as grease, mold release agent residue) and physically adsorbed water molecules from the surface of the area to be joined (such as the surface of an aluminum pole) through physical bombardment, and then remove them by the vacuum system.

[0016] Simultaneously, this ion bombardment selectively etches the surface at the microscale, introducing micro-roughness and breaking chemical bonds on the substrate surface, generating a large number of dangling bonds. This highly activates the treated surface, increasing its surface wetting tension (surface energy).

[0017] This clean and high-energy surface state ensures that the solid flux film in step S1 can be uniformly coated and obtain a strong interfacial adhesion, preventing the film from falling off in subsequent operations; and it provides extremely favorable substrate conditions for the spread of molten solder in step S4.

[0018] Specifically, this process can employ a radio frequency (RF) plasma source, controlling the process gas pressure within the vacuum chamber to be between 10 Pa and 100 Pa; the applied RF power to be between 50 W and 300 W; and the processing time to be between 30 seconds and 300 seconds. By controlling these parameters, optimal surface cleaning and activation effects can be achieved without causing macroscopic damage to the substrate.

[0019] In a second aspect, the present invention provides a solid dormant flux composition, applied to a battery welding process based on screw fixing and solder conductivity as described in the first aspect, employing the following technical solution:

[0020] A solid dormant flux composition comprising the following components in parts by weight: fragile polymer matrix: 10-20 parts by weight; activator: 5-15 parts by weight; rheology modifier: 1-5 parts by weight; organic solvent: 60-80 parts by weight.

[0021] By employing the above-described technical solution, this composition is specifically designed for a synergistic mechanical and chemical process. The fragile polymer matrix is ​​the key carrier for achieving the dormancy-activation mechanism. After solvent evaporation and curing, the matrix forms a continuous brittle phase that physically encapsulates the surfactant particles, preventing contact between the surfactant and ambient moisture or the substrate. When subjected to riveting impact, the matrix undergoes brittle fracture rather than ductile deformation, thus ensuring the instantaneous release of the surfactant. Rheology modifiers impart thixotropic properties to the paste, making it suitable for screen printing or dispensing processes and ensuring uniform coating thickness.

[0022] Preferably, the fragile polymer matrix is ​​a cross-linked copolymer, the raw materials of which include styrene monomer, divinylbenzene monomer as a cross-linking agent, and glycidyl methacrylate monomer as an adhesion promoter. To achieve the aforementioned fragile properties, the preparation method of the polymer matrix includes: dissolving styrene monomer, divinylbenzene monomer, glycidyl methacrylate monomer, and a free radical initiator (such as azobisisobutyronitrile) in a good solvent (such as toluene), and performing free radical solution polymerization under inert gas protection; after polymerization, the product is dropped into a poor solvent (such as methanol) for precipitation, filtration, drying, and pulverization to obtain a brittle copolymer powder with high cross-linking degree and high glass transition temperature. By adopting the above technical solution, the high proportion of divinylbenzene introduces a dense three-dimensional cross-linked network, giving the material extremely high hardness and brittleness, ensuring that it is easily broken under mechanical impact. The epoxy groups introduced by glycidyl methacrylate provide an anchoring effect to the metal substrate, preventing the film from accidentally detaching during operation.

[0023] Preferably, the activator comprises zinc chloride and stannous chloride. More preferably, the weight ratio of zinc chloride to stannous chloride in the activator is 4:1 to 2:1.

[0024] By adopting the above technical solution, a dual reaction mechanism of film breaking and displacement was designed to address the difficulty in pre-treating aluminum alloy surfaces:

[0025] Film breaking reaction: The exposed zinc chloride, in a molten state under heat, strongly erodes and dissolves the dense alumina (Al2O3) film on the aluminum surface through the Lewis acid-base reaction mechanism, exposing the fresh aluminum substrate;

[0026] Displacement reaction: Stannous chloride undergoes a displacement reaction with the fresh aluminum substrate, depositing a layer of metallic tin in situ on the aluminum surface. This pre-deposited tin layer serves as a transition layer, exhibiting excellent miscibility with the subsequently molten tin-bismuth solder, thus achieving wetting and metallurgical bonding on the originally unsolderable aluminum surface. A ratio of 4:1 to 2:1 optimizes the balance between the film removal rate and the tin deposition rate, preventing excessive corrosion of the substrate due to over-reaction.

[0027] This invention provides a battery welding process based on screw fixing and solder conductivity. It has the following beneficial effects:

[0028] 1. This invention achieves functional decoupling between mechanical fixation and electrical connection through cold-press riveting and localized low-temperature heating welding for conductivity. The peak temperature of this process is much lower than that of traditional laser welding, effectively avoiding thermal damage to heat-sensitive components such as the separator and electrolyte inside the battery, thereby ensuring the original electrochemical performance and cycle life of the battery and improving process safety.

[0029] 2. This invention provides a robust mechanical anchoring foundation through cold-press riveting. Simultaneously, the mechanical force of the riveting activates the dormant solid flux, releasing activators that effectively break down the passivation oxide film on the aluminum alloy surface, promoting reliable metallurgical bonding of the low-temperature solder at the interface. This synergistic effect of mechanical anchoring and metallurgical bonding ensures that the connection point possesses both high mechanical pull-out strength and long-term stable low interfacial contact resistance.

[0030] 3. This invention utilizes a solid dormant flux and mechanical activation to establish a synergistic process of force and chemical action. The flux is inert upon application and is activated only in the target area where riveting stress is concentrated, thus achieving in-situ, targeted release of the activator. This mechanism solves the problems of insufficient activity or easy corrosion residue caused by traditional liquid fluxes at low temperatures, successfully overcoming the technical challenge of achieving reliable metallurgical bonding on aluminum alloy surfaces at low temperatures. Furthermore, the process is precisely controlled and achieves high cleanliness. Attached Figure Description

[0031] Figure 1 This is a process flow diagram of an embodiment of the present invention;

[0032] Figure 2 This is a schematic diagram of the structure of a dormant solid flux film covering the surface of a battery electrode according to an embodiment of the present invention.

[0033] Figure 3 This is a schematic diagram illustrating the principle of how a threaded connector in an embodiment of the present invention breaks down a solid flux film and exposes the activator through cold-press riveting.

[0034] Figure 4 This is a schematic diagram of the final conductive path connection structure formed after welding in an embodiment of the present invention. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0036] Preparation Examples 1-4:

[0037] Preparation Example 1: Preparation of Solid Dormant Flux (F-1)

[0038] Synthesis of the fragile polymer matrix (P-1): Toluene (500 mL) was added to a 1000 mL three-necked flask under N2 atmosphere, followed by styrene, divinylbenzene (DVB, 80% mixture, glycidyl methacrylate, molar ratio 75:20:5). The mixture was heated to 80 °C and stirred. Azobisisobutyronitrile (AIB) initiator was added, accounting for 1.0 wt% of the total monomer mass. The reaction was carried out at 80 °C for 10 hours. After the reaction was completed, the polymer solution was cooled and precipitated dropwise into 2000 mL of methanol. The precipitate was collected by filtration, washed three times with methanol, and dried in a vacuum oven at 60 °C for 24 hours to obtain a white powdery fragile polymer matrix (P-1).

[0039] Preparation of flux paste (F-1): 15 parts by weight of fragile polymer matrix (P-1) powder, 8 parts by weight of zinc chloride, 2 parts by weight of stannous chloride, and 2 parts by weight of fumed silica are added to 73 parts by weight of methyl isobutyl ketone and mixed evenly in a planetary mixer to obtain solid dormant flux paste (F-1).

[0040] Preparation Example 2: Preparation of Solid Dormant Flux (F-2, Low Brittleness / High Toughness)

[0041] Synthesis of polymer matrix (P-2): Except for the monomer molar ratio being changed to styrene:divinylbenzene:glycidyl methacrylate = 90:5:5, the rest of the synthesis steps and conditions were the same as those described in Preparation Example 1 (1), and a white powdery low-brittle polymer matrix (P-2) was obtained.

[0042] Preparation of flux paste (F-2): 15 parts by weight of low brittle polymer matrix (P-2) powder, 8 parts by weight of zinc chloride, 2 parts by weight of stannous chloride, and 2 parts by weight of fumed silica are added to 73 parts by weight of methyl isobutyl ketone and mixed evenly in a planetary mixer to obtain low brittle solid dormant flux paste (F-2).

[0043] Preparation Example 3: Preparation of Solid Dormant Flux (F-3, High Activator Concentration)

[0044] Synthesis of polymer matrix (P-1): As described in Preparation Example 1 (1), a fragile polymer matrix (P-1) was obtained.

[0045] Preparation of flux paste (F-3): 15 parts by weight of fragile polymer matrix (P-1) powder, 12 parts by weight of zinc chloride, 3 parts by weight of stannous chloride, and 2 parts by weight of fumed silica are added to 68 parts by weight of methyl isobutyl ketone and mixed evenly in a planetary mixer to obtain a solid dormant flux paste (F-3) with a high activator concentration.

[0046] Preparation Example 4: Preparation of Solid Dormant Flux (F-4, Low Active Agent Concentration)

[0047] Synthesis of polymer matrix (P-1): As described in Preparation Example 1 (1), a fragile polymer matrix (P-1) was obtained.

[0048] Preparation of flux paste (F-4): 15 parts by weight of fragile polymer matrix (P-1) powder, 4 parts by weight of zinc chloride, 1 part by weight of stannous chloride, and 2 parts by weight of fumed silica are added to 78 parts by weight of methyl isobutyl ketone and mixed evenly in a planetary mixer to obtain a solid dormant flux paste (F-4) with low activator concentration.

[0049] Examples 1-4:

[0050] Example 1: Battery connection under preferred process conditions

[0051] This embodiment provides a method for connecting a threaded connector to a battery cap under preferred process parameters. Detailed process flow can be found in [reference needed]. Figure 1 As shown, the specific steps include:

[0052] Interface pretreatment and dormant flux coating:

[0053] The positive electrode cap (3003 series aluminum alloy) of the 4680 cylindrical lithium-ion battery was sent into an argon plasma cleaner and treated for 60 seconds under a pressure of 80Pa and a radio frequency power of 200W.

[0054] The battery was removed from the cleaning machine, and the solid dormant flux paste (F-1) obtained in Example 1 was screen-printed onto the center area of ​​the cap, forming a ring pattern with a diameter of 6.5 mm. After printing, it was placed in an N2 atmosphere oven and baked at 70°C for 15 minutes to form a solid flux film with a thickness of approximately 20 μm. At this time, the connection interface is in a flux dormant state, and its structure can be found in [reference needed]. Figure 2 As shown.

[0055] Stress field preset and mechanically activated riveting:

[0056] Precisely align the tin-plated C1100 oxygen-free copper stud (with dovetail groove) with the center of the flux-coated cap.

[0057] A servo press and a multi-point annular indenter were used to cold-press and rivet the studs. The total stroke for riveting was set to 0.7 mm, the peak pressure to 7.5 kN, and the holding time to 200 ms. For the mechanical activation mechanism of the solid flux film during this cold-press riveting process, please refer to [link to relevant documentation]. Figure 3 As shown.

[0058] Pre-installation of porous pre-fabricated solder pads:

[0059] An annular porous solder pad made of Sn42 / Bi58 alloy with a porosity of 25% is fitted onto the base of the riveted stud.

[0060] Stress relief and selective reflow soldering:

[0061] The battery was placed in an N2 atmosphere local reflow soldering workstation, and the base of the stud was heated using a high-frequency induction heating coil (operating frequency 1.0MHz).

[0062] The heating parameters were set as follows: heating rate 120℃ / second, peak temperature 170℃, and holding time 1.5 seconds. The N2 purging flow rate was 15L / min.

[0063] Cooling and bonding complete:

[0064] Heating is stopped, and rapid cooling is achieved through N2 purging, completing the connection between the stud and the battery cap. In this final connection structure, a conductive path is formed between the threaded connector and the battery cap through a metallurgically bonded weld; its structure can be found in [reference needed]. Figure 4 As shown.

[0065] Example 2: Battery connection in low temperature / low pressure / energy saving mode

[0066] This embodiment provides a method for connecting a threaded connector to a battery cap under relatively low temperature and pressure conditions, including the following steps:

[0067] Interface pretreatment and dormant flux coating:

[0068] The positive electrode cap of a 4680 cylindrical lithium-ion battery was sent into an argon plasma cleaner and treated for 50 seconds under a pressure of 60 Pa and a radio frequency power of 150 W.

[0069] The solid dormant flux paste (F-1) obtained in Example 1 was prepared by screen printing on the central area of ​​the cap. After printing, it was placed in an N2 atmosphere oven and baked at 60°C for 20 minutes to form a solid flux film with a thickness of about 15 μm.

[0070] Stress field preset and mechanically activated riveting:

[0071] Precisely align the tin-plated C1100 oxygen-free copper stud (with dovetail groove) with the center of the flux-coated cap.

[0072] A servo press and a multi-point annular press head are used to cold-press and rivet the studs. The total stroke of the riveting press is set to 0.5mm, the peak pressure is 5.5kN, and the holding time is 250ms.

[0073] Pre-installation of porous pre-fabricated solder pads:

[0074] An annular porous solder pad made of Sn42 / Bi58 alloy with a porosity of 20% is fitted onto the base of the riveted stud.

[0075] Stress relief and selective reflow soldering:

[0076] The battery was placed in an N2 atmosphere local reflow soldering workstation, and the base of the stud was heated using a high-frequency induction heating coil (operating frequency 0.8MHz).

[0077] The heating parameters were set as follows: heating rate 100℃ / second, peak temperature 160℃, and holding time 2.0 seconds. The N2 purging flow rate was 10L / min.

[0078] Cooling and bonding complete:

[0079] Stop heating and use N2 purging for rapid cooling to complete the connection between the stud and the battery cap.

[0080] Example 3: Battery connection in high temperature / high pressure / fast mode

[0081] This embodiment provides a method for connecting threaded connectors to battery caps under conditions of higher temperature and pressure, and shorter heating time, to meet high production cycle requirements, including the following steps:

[0082] Interface pretreatment and dormant flux coating:

[0083] The positive electrode cap of a 4680 cylindrical lithium-ion battery was sent into an argon plasma cleaner and treated for 75 seconds under a pressure of 100Pa and a radio frequency power of 300W.

[0084] The solid dormant flux paste (F-1) obtained in Example 1 was prepared by screen printing on the central area of ​​the cap. After printing, it was placed in an N2 atmosphere oven and baked at 80°C for 10 minutes to form a solid flux film with a thickness of about 25 μm.

[0085] Stress field preset and mechanically activated riveting:

[0086] Precisely align the tin-plated C1100 oxygen-free copper stud (with dovetail groove) with the center of the flux-coated cap.

[0087] A servo press and a multi-point annular press head were used to cold-press and rivet the studs. The total stroke of the riveting press was set to 0.9 mm, the peak pressure was 9.0 kN, and the holding time was 150 ms.

[0088] Pre-installation of porous pre-fabricated solder pads:

[0089] A ring-shaped porous solder pad made of Sn42 / Bi58 alloy with a porosity of 30% is fitted onto the base of the riveted stud.

[0090] Stress relief and selective reflow soldering:

[0091] The battery was placed in an N2 atmosphere local reflow soldering workstation, and the base of the stud was heated using a high-frequency induction heating coil (operating frequency 1.8MHz).

[0092] The heating parameters were set as follows: heating rate 150℃ / second, peak temperature 180℃, and holding time 1.0 second. The N2 purging flow rate was 20L / min.

[0093] Cooling and bonding complete:

[0094] Stop heating and use N2 purging for rapid cooling to complete the connection between the stud and the battery cap.

[0095] Example 4: Battery connection under high activator concentration flux

[0096] This embodiment provides a flux with a high activator concentration to support a wide range of flux components, including the following steps:

[0097] Interface pretreatment and dormant flux coating:

[0098] The positive electrode cap of a 4680 cylindrical lithium-ion battery was sent into an argon plasma cleaner and treated for 60 seconds under a pressure of 80 Pa and a radio frequency power of 200 W.

[0099] The solid dormant flux paste (F-3) obtained in Example 3 was prepared by screen printing in the central area of ​​the cap. After printing, it was placed in an N2 atmosphere oven and baked at 70°C for 15 minutes to form a solid flux film with a thickness of about 20 μm.

[0100] Stress field preset and mechanically activated riveting:

[0101] Precisely align the tin-plated C1100 oxygen-free copper stud (with dovetail groove) with the center of the flux-coated cap.

[0102] A servo press and a multi-point annular pressure head were used to cold-press and rivet the studs. The total stroke of the riveting press was set to 0.7 mm, the peak pressure was 7.5 kN, and the holding time was 200 ms.

[0103] Pre-installation of porous pre-fabricated solder pads:

[0104] An annular porous solder pad made of Sn42 / Bi58 alloy with a porosity of 25% is fitted onto the base of the riveted stud.

[0105] Stress relief and selective reflow soldering:

[0106] The battery was placed in an N2 atmosphere local reflow soldering workstation, and the base of the stud was heated using a high-frequency induction heating coil (operating frequency 1.0MHz).

[0107] The heating parameters were set as follows: heating rate 120℃ / second, peak temperature 170℃, and holding time 1.5 seconds. The N2 purging flow rate was 15L / min.

[0108] Cooling and bonding complete:

[0109] Stop heating and use N2 purging for rapid cooling to complete the connection between the stud and the battery cap.

[0110] Comparative Examples 1-5:

[0111] Comparative Example 1: Traditional Laser Welding Connection

[0112] Compared to Example 1, the difference lies in that this connection method does not use dormant flux, mechanical activation riveting, or porous pre-fabricated solder pads. Instead, it employs a conventional process, directly using a 2kW fiber laser (wavelength 1070nm) under argon protection to weld C1100 oxygen-free copper studs (without tin plating or dovetail grooves) directly to the 3003 series aluminum alloy cap of the 4680 battery via circumferential welding. The remaining steps (such as cleaning) are identical to the corresponding pretreatment steps in Example 1.

[0113] Comparative Example 2: Cold riveting connection only

[0114] Compared to Example 1, the difference lies in that this connection method does not use dormant flux or porous pre-fabricated solder pads. The studs are secured to the battery cap solely through mechanical activation riveting in step 2 of Example 1 (using the studs and riveting parameters of Example 1). No subsequent heat reflow soldering step is performed. The remaining steps are identical to the corresponding pretreatment steps in Example 1.

[0115] Comparative Example 3: Welding joints using conventional liquid flux

[0116] Compared to Example 1, the difference lies in that this connection method does not use dormant flux or mechanically activated riveting. Instead, a commercially available weakly acidic liquid flux (for aluminum welding) is applied to the connection area between the stud flange and the cap. The stud (untinned, without dovetail grooves) is secured by a simple clamping device without applying a stress field. Subsequently, localized heating is performed using a hot air gun, with the temperature and time closely mimicking the reflow soldering process of Example 1, to melt the solder (using the same Sn42 / Bi58 solder) and fill the connection area. The remaining steps are identical to the corresponding pretreatment steps in Example 1.

[0117] Comparative Example 4: Connection using low-brittle flux

[0118] The difference from Example 1 is that this comparative example uses the low-brittle solid dormant flux (F-2) paste obtained in Preparation Example 2 for coating, and all other materials and process parameters are exactly the same as in Example 1.

[0119] Comparative Example 5: Riveting connection without pre-set stress field

[0120] Compared to Example 1, the difference lies in the riveting step: a flat pressure head is used instead of the multi-point annular pressure head in Example 1 to ensure that a predetermined residual stress field is not formed on the cap surface around the stud during the riveting process. All other materials and process parameters are exactly the same as in Example 1.

[0121] Test Example 1-2:

[0122] Test Example 1: Verification of Process Feasibility and Safety

[0123] This test case aims to verify the feasibility of the connection process of the present invention in practical operation and to confirm that the process does not have a negative impact on the internal structure and sealing of the battery, thereby ensuring production safety. All test samples were connected using the process parameters of Example 1.

[0124] Process highest internal temperature monitoring

[0125] Experimental objective: To measure the highest temperature inside the battery cap during connection in order to assess the potential impact of heat on the battery cell.

[0126] Experimental steps:

[0127] Prepare 5 4680 cylindrical lithium-ion battery samples to be connected.

[0128] Drill a 0.5mm diameter hole at the center of the bottom of the negative terminal of each battery. Carefully insert a type K thermocouple (probe diameter 0.2mm) into the battery, ensuring the thermocouple tip is in close contact with the inner surface of the positive terminal cap, and guaranteeing that the thermocouple has no electrical contact with the battery's internal current collector or other electrode components. Seal the hole with epoxy resin to ensure the battery's seal is not compromised.

[0129] Perform the stud connection according to the steps in Example 1.

[0130] During the induction heating process of stress relief and selective reflow soldering, the temperature readings of the K-type thermocouples were continuously recorded using a data acquisition system, and the peak temperature of each sample was recorded.

[0131] Sealing test:

[0132] Experimental objective: To evaluate the impact of cold riveting and localized induction heating processes on the battery sealing integrity.

[0133] Experimental steps:

[0134] Prepare 5 4680 cylindrical lithium-ion battery samples to be connected.

[0135] Use a helium gas spectrometer leak detector (leak detection accuracy better than 5×10⁻⁶). -8 (atm·cc / s), the initial helium leakage rate of each cell was measured before the connection process.

[0136] Perform the stud connection according to the steps in Example 1.

[0137] After connection, clean the battery surface and use the same helium gas spectrometer leak detector again to measure the final helium leakage rate of each battery.

[0138] Compare the helium leakage rates before and after the connection.

[0139] Visual inspection

[0140] Experimental objective: To check for visible defects, flux residue, or cap deformation at the connection interface.

[0141] Experimental steps:

[0142] Visual inspection was performed on the five battery samples that had been connected.

[0143] Observe the solder joint morphology at the connection between the stud and the battery cap, including the wetting and spreading of the solder, whether there is solder splatter, flux sintering residue, etc.

[0144] Check the surface of the battery cap for obvious macroscopic deformation or discoloration.

[0145] Record the inspection results:

[0146] Table 1. Summary of Feasibility and Safety Test Results for Sample Process in Example 1

[0147] S1-A 73.1 <![CDATA[2.3×10 -7 ]]> <![CDATA[2.6×10 -7 ]]> Good, no splashing S1-B 76.5 <![CDATA[3.0×10 -7 ]]> <![CDATA[3.2×10 -7 ]]> Good, no residue S1-C 69.4 <![CDATA[2.7×10 -7 ]]> <![CDATA[2.9×10 -7 ]]> Good, no deformation S1-D 75.2 <![CDATA[3.1×10 -7 ]]> <![CDATA[3.4×10 -7 ]]> Good, solder joints are rounded. S1-E 71.8 <![CDATA[2.8×10 -7 ]]> <![CDATA[3.0×10 -7 ]]> Good quality, non-corrosive

[0148] in conclusion:

[0149] The results of this test demonstrate that the battery connection process of the present invention has good feasibility and safety.

[0150] During the monitoring of the highest internal temperature during the process, the peak internal temperature of all samples remained between 69.4°C and 76.5°C during induction heating. This temperature range is significantly lower than the pore-closing temperature of lithium-ion battery separators (typically above 120°C) and the thermal decomposition temperature of the electrolyte, verifying that the localized low-temperature induction heating strategy employed in this invention effectively avoids thermal damage to the battery cell. This is achieved by decoupling the mechanical fixation of the studs from the conductive function of the solder, and by precisely controlling the melting point of the low-temperature solder and the localized heating range.

[0151] In the sealing test, the difference in helium leakage rate between all samples before and after the connection process was minimal, remaining at 3.4 × 10⁻⁶. -7 Below atm·cc / s, far below the industry-accepted seal failure threshold (typically 10). -6 (atm·cc / s). This indicates that the cold riveting process of the stud and the subsequent local induction heating did not damage the sealing structure of the battery cap, verifying the non-invasiveness of the riveting and soldering process of this invention.

[0152] Visual inspection revealed that the solder at the connection points was evenly spread, with no solder splatter, obvious flux residue, or macroscopic deformation of the battery cap observed. This is thanks to the mechanical activation properties of the solid dormant flux of this invention, ensuring that the flux is activated only in the riveting area, achieving selective wetting. At the same time, the porous solder pads facilitate the uniform flow of molten solder, avoiding splatter and unnecessary surface contamination.

[0153] In summary, the connection process of this invention, through its unique functional decoupling, force and chemical synergistic activation, and stress-induced mechanism, can effectively control heat transfer and avoid mechanical damage while ensuring connection strength, thus providing a safe and reliable connection solution for batteries.

[0154] Test Example 2: Comparison Test of Connection Performance and Reliability

[0155] This test case aims to quantitatively evaluate the differences in electrical performance, mechanical strength, and long-term reliability between the embodiments of the present invention and various comparative connection schemes, thereby fully demonstrating the technical advantages of the present invention.

[0156] Initial contact resistance (ACIR) test:

[0157] Experimental objective: To measure the initial DC contact resistance at the connection between the stud and the battery cap, and to evaluate the conductivity of the connection interface.

[0158] Experimental steps:

[0159] Five fully connected battery samples were selected from each embodiment and comparative example.

[0160] Using a four-wire microohmmeter (with a measurement accuracy better than 0.1 μΩ), measurements were taken at the top of the stud and 5 mm from the edge of the battery cap at the connection point. A 10 A DC current was applied, and the voltage drop after stabilization was recorded. The contact resistance was then calculated.

[0161] Each sample was measured three times, and the average value was taken.

[0162] Pull-out force test:

[0163] Experimental objective: To measure the force required to pull the stud off the battery cap and to evaluate the mechanical strength of the connection.

[0164] Experimental steps:

[0165] Five fully connected battery samples were selected from each embodiment and comparative example.

[0166] Secure the battery to the fixture of the tensile testing machine with the stud pointing vertically upwards.

[0167] Apply a tensile force at a constant tensile speed of 5 mm / min until the stud detaches from the cap, and record the peak pull-out force.

[0168] Each sample was measured three times, and the average value was taken.

[0169] Aging resistance (ACIR change rate):

[0170] Experimental objective: To evaluate the long-term electrical stability of the connection under extreme temperature cycling.

[0171] Experimental steps:

[0172] Five fully connected battery samples were selected from each embodiment and comparative example.

[0173] First, the initial ACIR of each sample is measured.

[0174] The sample was placed in a high and low temperature cycling chamber and subjected to 100 thermal shock cycles. Each cycle consisted of: holding at -40°C for 30 minutes, raising the temperature to 85°C and holding for 30 minutes, and then cooling to -40°C.

[0175] After the cycle was completed, the sample was brought back to room temperature and its ACIR was measured again.

[0176] Calculate the ACIR change rate for each sample = (Aging ACIR - Initial ACIR) / Initial ACIR × 100%.

[0177] Impact of battery electrochemical performance (capacity retention):

[0178] Experimental objective: To evaluate the impact of the connection process on the electrochemical performance of the battery itself.

[0179] Experimental steps:

[0180] Five fully connected battery samples were selected from each embodiment and comparative example.

[0181] Activate all batteries according to the initial activation procedure recommended by the battery manufacturer.

[0182] Using battery cycle testing equipment, the battery was charged at 25°C with constant current and constant voltage at a rate of 1C to 4.2V, and then discharged at a rate of 1C with constant current to 2.5V.

[0183] The battery discharge capacity was measured every 100 cycles.

[0184] The discharge capacity retention rate after 500 cycles is calculated as (capacity after 500 cycles / initial capacity) × 100%.

[0185] Table 2. Summary of test results for connection performance and reliability

[0186] Example 1 0.165 138.2 8.7 94.3 Example 2 0.191 115.9 12.1 93.8 Example 3 0.158 145.7 7.9 94.5 Example 4 0.173 135.0 9.5 94.1 Comparative Example 1 0.152 149.1 78.3 83.5 Comparative Example 2 18.3 125.6 112.5 N / A Comparative Example 3 0.354 72.8 38.6 91.2 Comparative Example 4 0.871 55.4 69.1 89.9 Comparative Example 5 0.170 118.0 28.3 93.0

[0187] Note: N / A indicates that normal charging and discharging is not possible.

[0188] in conclusion:

[0189] The test results demonstrate that the connection process of this invention has advantages in terms of initial performance, mechanical strength, and long-term reliability.

[0190] In the initial ACIR test, samples from Examples 1-4 all exhibited excellent contact resistance as low as 0.158 mΩ to 0.191 mΩ, comparable to Comparative Example 1 (laser welding) and significantly lower than Comparative Example 2 (cold riveting only) and Comparative Example 4 (low-brittle flux). This indicates that the present invention successfully establishes a low-resistance electrical path through mechanical activation of the flux and low-temperature solder, while avoiding the thermal damage to the battery caused by traditional laser welding. The extremely high ACIR value of Comparative Example 2 confirms the shortcomings of simple mechanical riveting in terms of conductivity. The higher ACIR of Comparative Example 4 illustrates the importance of the fragility of the flux matrix for activator release and effective wetting.

[0191] In the pull-out force test, the pull-out forces of the samples in Examples 1-4 ranged from 115.9 N to 145.7 N, demonstrating a robust mechanical connection. This strength level is close to or better than Comparative Example 1 (laser welding) and significantly higher than Comparative Example 3 (conventional liquid flux welding). The lower pull-out force of Comparative Example 3 reflects the limitations of conventional welding processes in terms of mechanical strength for purely metallurgical bonds in the absence of mechanical anchoring.

[0192] The aging resistance test results showed that the ACIR change rate of samples 1-4 of this invention remained below 15%, demonstrating excellent long-term stability. In contrast, the ACIR change rate of Comparative Example 1 (laser welding) was as high as 78.3%, indicating that its rigid connection was prone to fatigue failure under thermal cycling stress. The ACIR change rate of Comparative Example 2 (cold riveting only) exceeded 100%, confirming that its contact interface deteriorated rapidly after aging. The ACIR change rates of Comparative Example 3 (conventional liquid flux welding) and Comparative Example 4 (low-brittle flux) were also higher than those of the examples, indicating that the synergistic effect of mechanical activation and stress induction of this invention is crucial for maintaining the long-term stability of the connection interface. The ACIR change rate of Comparative Example 5 (stress-free pre-set riveting) was also higher than that of the examples, which confirms the contribution of the stress induction mechanism to improving connection reliability.

[0193] In the electrochemical performance test, the capacity retention rates of samples in Examples 1-4 after 500 cycles were all above 93%, comparable to the performance of unconnected batteries and far higher than the 83.5% of Comparative Example 1 (laser welding). This strongly demonstrates the non-thermal damage characteristics of the process of this invention, which can effectively protect the internal structure and electrochemical performance of the battery from being affected. The capacity decay of Comparative Example 1 directly reflects the irreversible damage to the battery electrochemical system caused by the thermal damage introduced during its connection process. Comparative Example 2 could not be effectively evaluated for electrochemical performance due to poor initial electrical connection.

[0194] In summary, this invention achieves highly conductive, mechanically activated solid-state dormant flux, and stress-induced metallurgical bonding mechanisms to achieve a battery connection that is highly conductive, mechanically strong, and reliable over the long term. This solution avoids the thermal damage associated with traditional high-temperature welding while synergistically enhancing the overall performance of the connection interface, outperforming existing technologies and comparative solutions lacking key features.

Claims

1. A battery soldering process based on screw fixation and solder conductive, characterized by, Includes the following steps: S1: A solid dormant flux composition comprising an activator and a fragile polymer matrix is ​​applied to the area to be connected of a battery electrode and cured to form a solid flux film. The fragile polymer matrix is ​​a crosslinked copolymer whose raw materials include styrene monomer, divinylbenzene monomer as a crosslinking agent, and glycidyl methacrylate monomer as an adhesion promoter. S2: The threaded connector with the flange is fixed to the area to be connected by cold pressing and riveting, and the activator inside the solid flux film is exposed. S3: Low-temperature solder is pre-placed at the base of the threaded connector; S4: Locally heat the base of the threaded connector at low temperature to melt the low-temperature solder, and then wet it under the action of the exposed activator. After cooling, a conductive path is formed.

2. The battery welding process based on screw fixing and solder conductivity according to claim 1, characterized in that, In step S2, the cold pressing riveting is applied by a multi-point annular pressure head, which, while breaking the solid flux film, creates a residual stress field around the area to be joined.

3. The battery welding process based on screw fixing and solder conductivity according to claim 2, characterized in that, In step S4, the molten low-temperature solder preferentially wets the residual stress field and releases the residual stress during the wetting process.

4. The battery welding process based on screw fixing and solder conductivity according to claim 1, characterized in that, In step S4, the local low-temperature heating adopts high-frequency induction heating, and the peak heating temperature is controlled between 150°C and 200°C.

5. The battery welding process based on screw fixing and solder conductivity according to claim 1, characterized in that, In step S3, the low-temperature solder is a porous pre-fabricated solder pad, and the alloy composition of the solder pad is a tin-bismuth alloy.

6. The battery welding process based on screw fixing and solder conductivity according to claim 1, characterized in that, Prior to step S1, a pretreatment step of argon plasma cleaning of the area to be joined is also included.

7. The battery welding process based on screw fixing and solder conductivity according to claim 1, characterized in that, The solid dormant flux composition comprises the following components in parts by weight: Fragile polymer matrix: 10-20 parts by weight; Surfactant: 5-15 parts by weight; Rheology modifier: 1-5 parts by weight; Organic solvent: 60-80 parts by weight.

8. A battery welding process based on screw fixing and solder conductivity according to claim 7, characterized in that, The activator comprises zinc chloride and stannous chloride.

9. A battery welding process based on screw fixing and solder conductivity according to claim 8, characterized in that, In the activator, the weight ratio of zinc chloride to stannous chloride is 4:1 to 2:1.

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