Waterproof and impermeable energy storage cable and preparation method thereof

Abstract

The invention relates to the technical field of energy storage cables, in particular to a waterproof and impermeable energy storage cable and a preparation method thereof, and the energy storage cable sequentially comprises a conductor layer which is formed by twisting and compressing a plurality of annealed soft copper wires from inside to outside; the inner-layer waterproof water-blocking structure comprises water-blocking yarns uniformly and sparsely wound around the conductor layer and an inorganic semi-conductive water-blocking tape overlapped and wrapped on the outer layer of the water-blocking yarns; the insulating layer is a composite flame-retardant insulating layer, and the composite flame-retardant insulating layer comprises the following raw materials in parts by weight: 100 parts of matrix resin, 10-15 parts of expansible graphite, 3-6 parts of polyethylene grafted maleic anhydride and 5-8 parts of a synergistic flame retardant; and the outer sheath is a nano modified polyethylene sheath layer. The cable has the advantages that the expandable graphite and the polyethylene grafted maleic anhydride are specifically compounded and applied to a cable insulating layer, and the nano-modified outer sheath is cooperated, so that the energy storage cable which is active in water resistance, efficient in fire resistance, excellent in flexibility, long-acting, reliable and excellent in comprehensive performance is successfully prepared; the cable solves the harsh requirement of the current energy storage system on the cable performance, and has great industrial application value.

Description

Technical Field

[0001] This invention relates to the field of energy storage cables, and in particular to a waterproof and impermeable energy storage cable and its preparation method. Background Technology

[0002] As the global energy structure shifts towards renewable energy, energy storage technology has become crucial for balancing the power grid and improving energy efficiency. Energy storage systems (ESS) are often deployed in complex and harsh environments such as outdoors, in containers, underground, and even coastal areas. Their internal cables, especially the DC-side battery connection cables, face challenges such as high humidity, condensation, and even direct water immersion. Moisture infiltration can lead to a decrease in cable insulation resistance, causing electrochemical corrosion and dendrite growth, ultimately resulting in discharge breakdown or system short circuits, seriously threatening the safety and lifespan of the energy storage system.

[0003] In existing technologies, improving the waterproof performance of cables mainly relies on structural water-blocking, such as filling the conductor gaps with water-blocking paste or wrapping with water-blocking tape. For example, Chinese patent application CN202321762662.3 discloses a connecting cable for a highly waterproof energy storage system. However, such methods are passive protection; once the sheath is damaged or ages over a long period, moisture can still penetrate. On the other hand, the fire resistance of cables is equally important. Traditional fire-resistant solutions often use mica tape wrapping or add inorganic flame retardants such as aluminum hydroxide / magnesium hydroxide, but this often leads to increased cable hardness, decreased flexibility, and limited flame-retardant efficiency.

[0004] Therefore, there is an urgent need to develop an energy storage cable that integrates active waterproofing, high-efficiency flame retardancy, excellent flexibility and long-term reliability to cope with increasingly demanding application scenarios. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a waterproof and seepage-proof energy storage cable and its preparation method. This cable achieves multi-level and synergistic high-efficiency waterproofing, seepage prevention and safety protection by constructing an active expansion water-blocking and flame-retardant mechanism in the insulation layer and strengthening the passive protection barrier in the outer sheath.

[0006] This invention is achieved through the following technical solution: On one hand, a waterproof and seepage-proof energy storage cable is provided, which comprises, from the inside out: The conductor layer is made of multiple annealed soft copper wires twisted and pressed together; The inner waterproof and water-blocking structure includes water-blocking yarn that is uniformly and loosely wound around the conductor layer, and an inorganic semi-conductive water-blocking tape that is overlapped and wrapped around the outer layer of the water-blocking yarn. The insulating layer is a composite flame-retardant insulating layer. By weight, the raw materials of the composite flame-retardant insulating layer include: 100 parts of matrix resin, 10-15 parts of expandable graphite, 3-6 parts of polyethylene grafted maleic anhydride, and 5-8 parts of synergistic flame retardant. The outer sheath is a nano-modified polyethylene sheath layer.

[0007] Furthermore, the matrix resin is selected from one or two of ethylene-vinyl acetate copolymer and low-density polyethylene.

[0008] Furthermore, the synergistic flame retardant is selected from at least one of nano-aluminum hydroxide, nano-magnesium hydroxide, and ammonium polyphosphate.

[0009] Furthermore, the expandable graphite has an initial expansion temperature of 180°C to 200°C and a particle size of 200 mesh to 300 mesh.

[0010] Furthermore, the grafting rate of the polyethylene grafted with maleic anhydride is 0.5wt%-1.2wt%.

[0011] Furthermore, the nano-modified polyethylene sheath layer comprises a polyethylene matrix and nano-inorganic fillers dispersed therein; the nano-inorganic fillers are at least one of nano-silica, nano-montmorillonite, and nano-silicon carbide; and the amount of nano-inorganic fillers added is 2% to 8% based on the mass of the polyethylene matrix.

[0012] Another method for preparing a waterproof and seepage-proof energy storage cable as described above is provided, comprising the following steps: Step S1. Conductor preparation; Oxygen-free copper rods are drawn and annealed to obtain annealed soft copper wires, which are then stranded and pressed to form a conductor layer. Step S2. Construct a waterproof and water-blocking structure; Water-blocking yarn is uniformly and loosely wound around the conductor layer, and then inorganic semiconducting water-blocking tape is wrapped around the outer layer of the water-blocking yarn. Step S3. Prepare composite flame-retardant insulating masterbatch; Expandable graphite, polyethylene grafted maleic anhydride, and synergistic flame retardant are premixed and then melt-blended and granulated at a temperature 20-30°C lower than the initial expansion temperature of expandable graphite to obtain composite flame-retardant insulating masterbatch. Step S4. Extruding the insulation layer; The composite flame-retardant insulating masterbatch is mixed with the matrix resin and extruded onto the wire core treated with S2 at a temperature of 110℃-175℃ using an extruder to form an insulating layer. Step S5. Extrude the outer sheath; The nano-modified polyethylene sheath material is extruded onto the outside of the insulation layer using an extruder at a temperature of 120℃-200℃ to form an outer sheath. Step S6. Post-processing; The extruded cable is cooled, sized, and then wound up.

[0013] Furthermore, in step S3, the melt blending granulation is carried out using a twin-screw extruder.

[0014] Further, in step S4, the temperatures of the extruder from the feeding section to the die head are set sequentially as follows: feeding section 110℃-130℃, compression section 130℃-145℃, homogenization section 145℃-160℃, and die head / die opening 160℃-175℃.

[0015] Further, in step S5, the temperatures of the extruder from the feeding section to the die head are set sequentially as follows: feeding section 140℃-160℃, compression section 160℃-180℃, homogenization section 180℃-190℃, and die head / die opening 190℃-200℃.

[0016] Beneficial effects The insulation material design of this invention utilizes expandable graphite, which rapidly expands physically upon heating, resulting in a significant increase in volume. This effectively blocks potential water seepage channels within and on the surface of the insulation layer, forming an "active" barrier. Polyethylene grafted with maleic anhydride serves as a highly efficient compatibilizer; its polyolefin segments are compatible with the matrix resin, while the maleic anhydride functional groups can form strong hydrogen bonds and even chemical bonds with the oxygen-containing groups at the edges of the EG sheets. This solves the problem of poor dispersion and easy detachment of EG in non-polar polyolefins. The synergy of these two components not only ensures the stable performance of EG, but the resulting dense, expanded carbon layer also possesses excellent heat and oxygen insulation properties, giving the cable superior flame retardancy.

[0017] The inner structure of the cable structure of this invention provides the first physical barrier; the middle insulation layer, as the core functional layer, provides active protection; the outer nano-modified polyethylene sheath, utilizing the "maze effect" of nanoparticles, greatly enhances the sheath's density, mechanical strength, and resistance to environmental stress cracking, forming a robust passive protective shell. These three elements work synergistically to comprehensively resist the intrusion of moisture and harmful media.

[0018] The capacity-enhancing and toughening effects of PE-g-MAH compensate for the potential material embrittlement issues that may arise from adding EG, maintaining the cable's necessary flexibility and bending performance to meet the requirements of compact wiring within energy storage systems. The nano-modified outer sheath further enhances the cable's abrasion resistance, weather resistance, and chemical corrosion resistance. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the cross-sectional structure of a waterproof and seepage-proof energy storage cable according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the process flow according to an embodiment of the present invention.

[0020] In the diagram: 1-Conductor layer; 2-Water-blocking yarn; 3-Inorganic semi-conductive water-blocking tape; 4-Composite flame-retardant insulation layer; 5-Nano-modified polyethylene outer sheath. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0022] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise stated, all percentages, ratios, proportions, or parts are by weight.

[0023] Unless otherwise specified, the reagents and raw materials used in the embodiments and comparative examples of this invention are commercially available.

[0024] Example 1 A waterproof and seepage-proof energy storage cable, such as Figure 1 and Figure 2 As shown, from the inside out, the layers are: conductor layer 1, water-blocking yarn 2, inorganic semi-conductive water-blocking tape 3, composite flame-retardant insulation layer 4, and nano-modified polyethylene outer sheath 5.

[0025] Its preparation method is as follows: Step 1) Preparation of the conductor layer; Using Φ8.0mm oxygen-free copper rod, annealed soft copper wire with a diameter of 0.3mm was produced through continuous drawing and annealing; 19 of these copper wires were then twisted and compressed to produce a nominal cross-section of 2.5mm². 2 The conductor layer.

[0026] Step 2) Construction of the inner waterproof and water-blocking structure; Water-blocking yarn is evenly and loosely wound around the conductor layer with an overlap rate of ≥30%; then an inorganic semiconducting water-blocking tape is wrapped around it.

[0027] Step 3) Preparation of composite flame-retardant insulating masterbatch; One part of 300-mesh expandable graphite EG1 (initial expansion temperature 200℃), four parts of polyethylene grafted with maleic anhydride PE-g-MAH (grafting rate 1.2%), and five parts of nano-aluminum hydroxide ATH were premixed for 5 minutes in a high-speed mixer. The mixture was then melt-blended, underwater pelletized, and dried using a twin-screw extruder within a temperature range of 155-165℃ to obtain a composite flame-retardant insulating masterbatch.

[0028] Step 4) Extruding the insulation layer; The above masterbatch was mixed with 100 parts of ethylene-vinyl acetate copolymer (EVA, VA content 28%) and extruded onto the wire core treated in step (2) through an extruder to form an insulation layer. The temperatures of each section of the extruder were set as follows: feeding section 110℃-130℃, compression section 130℃-145℃, homogenization section 145℃-160℃, and die head / die 160℃-175℃. The nominal thickness of the insulation layer was 1.2mm.

[0029] Step 5) Extrude the outer sheath; Nano-silica modified polyethylene sheath material (nano-SiO2 content 5wt%) is extruded onto the outside of the insulation layer using an extruder to form the outer sheath. The extruder temperatures are set as follows: feeding section 140℃-160℃, compression section 160℃-180℃, homogenization section 180℃-190℃, and die head / die opening 190℃-200℃. The nominal thickness of the sheath is 1.5mm.

[0030] Step 6) Post-processing: cooling, sizing, coding, and winding to obtain the finished cable.

[0031] Example 2 The difference from Example 1 is that in the preparation of the composite flame-retardant insulating masterbatch, 3 parts of 260-mesh expandable graphite EG1 with an initial expansion temperature of 190℃, 4.5 parts of polyethylene grafted with maleic anhydride PE-g-MAH with a grafting rate of 1.0%, and 6 parts of ammonium polyphosphate APP were premixed for 5 minutes in a high-speed mixer. The composite flame-retardant insulating masterbatch was then obtained by melt blending, underwater pelletizing, and drying using a twin-screw extruder within a temperature range of 155-165℃.

[0032] In the preparation of the extrusion outer sheath, the outer sheath uses nano-montmorillonite modified polyethylene sheath material (nano-montmorillonite content 3wt%). The remaining preparation steps are the same as in Example 1.

[0033] Example 3 The difference from Example 1 lies in the preparation of the composite flame-retardant insulating masterbatch. Ten parts of 280-mesh expandable graphite (EG) with an initial expansion temperature of 200°C, 3.5 parts of polyethylene grafted with maleic anhydride (PE-g-MAH) with a grafting rate of 1.0%, and 7 parts of nano-magnesium hydroxide (MH) were premixed for 5 minutes in a high-speed mixer. The mixture was then melt-blended, underwater pelletized, and dried using a twin-screw extruder within a temperature range of 155-165°C to obtain the composite flame-retardant insulating masterbatch. The remaining preparation steps were the same as in Example 1.

[0034] Example 4 The difference from Example 1 lies in the preparation of the composite flame-retardant insulating masterbatch. In this example, 5 parts of 200-mesh expandable graphite EG (with an initial expansion temperature of 180°C), 5 parts of polyethylene grafted with maleic anhydride (PE-g-MAH) at a grafting rate of 1.0%, and 3 parts of nano-aluminum hydroxide (ATH) and 3 parts of ammonium polyphosphate (APP) as synergistic flame retardants are premixed for 5 minutes in a high-speed mixer. The mixture is then melt-blended, underwater pelletized, and dried using a twin-screw extruder within a temperature range of 155-165°C to obtain the composite flame-retardant insulating masterbatch.

[0035] In the preparation of the extruded insulation layer, the matrix resin is low-density polyethylene (LDPE); the remaining preparation steps are the same as in Example 1.

[0036] Example 5 The difference from Example 1 lies in the preparation of the composite flame-retardant insulating masterbatch. In this example, 2 parts of 260-mesh expandable graphite EG1 (initial expansion temperature 200℃), 6 parts of polyethylene grafted with maleic anhydride PE-g-MAH (grafting rate 0.5%), and 6 parts of nano-aluminum hydroxide ATH as the synergistic flame retardant are premixed for 5 minutes in a high-speed mixer. The mixture is then melt-blended, underwater pelletized, and dried using a twin-screw extruder within a temperature range of 155-165℃ to obtain the composite flame-retardant insulating masterbatch.

[0037] In the preparation of the extruded insulation layer, the matrix resin is a mixture of EVA and LDPE (mass ratio 1:1). In the preparation of the extrusion outer sheath, the outer sheath uses nano-silicon carbide modified polyethylene sheath material (nano-silicon carbide content 4wt%); the remaining preparation steps are the same as in Example 1.

[0038] Example 6 The difference from Example 1 is that in the preparation of the composite flame-retardant insulating masterbatch, 14 parts of 260-mesh expandable graphite EG1 with an initial expansion temperature of 200℃, 3 parts of polyethylene grafted with maleic anhydride PE-g-MAH with a grafting rate of 1.0%, and 8 parts of ammonium polyphosphate APP as a synergistic flame retardant are premixed for 5 minutes in a high-speed mixer. The composite flame-retardant insulating masterbatch is then melt-blended, underwater pelletized, and dried using a twin-screw extruder at a temperature of 165℃-170℃; the remaining preparation steps are the same as in Example 1.

[0039] Comparative Example 1 The difference from Example 1 is that the insulating layer does not use the composite flame-retardant insulating layer of the present invention. Specifically, steps 3) and 4) are omitted, and commercially available ordinary halogen-free low-smoke flame-retardant polyolefin material is used directly (its formulation contains a large amount of traditional flame retardants such as aluminum hydroxide, but does not contain expandable graphite and PE-g-MAH extruded insulating layer). The remaining preparation steps are the same as in Example 1.

[0040] Comparative Example 2 The difference from Example 1 is that the composite flame-retardant insulation layer formulation does not contain the compatibilizer polyethylene grafted maleic anhydride PE-g-MAH. Specifically, in step 3), only 11 parts of expandable graphite EG and 5 parts of nano-aluminum hydroxide ATH are directly mixed with 100 parts of ethylene-vinyl acetate copolymer EVA, without masterbatch preparation, and the insulation layer is directly extruded. The remaining preparation steps are the same as in Example 1.

[0041] Comparative Example 3 The difference from Example 1 is that the inner waterproof and water-blocking structure is incomplete. Specifically, in step 2), only the inorganic semi-conductive water-blocking tape is wrapped around the inner layer, omitting the step of uniformly and loosely winding the water-blocking yarn. The remaining preparation steps are the same as in Example 1.

[0042] Performance testing ① Permeability test Place a 3m long cable in a water pipe at an ambient temperature of 20±10℃. The water level should be 1m above the cable's central axis. The sample should remain for 24 hours, then undergo 10 thermal cycles using an electric heating method to raise the conductor temperature to 5K-10K above the cable's normal operating temperature, but not exceeding 100℃. Each thermal cycle should last 8 hours, during which the conductor temperature should be maintained within the specified range for at least 2 hours, followed by 3 hours of natural cooling. The water level should be maintained at 1m. No water should seep from either end of the cable during the test. The equipment is commercially available.

[0043] ②Fire resistance test When the cable outer diameter is less than or equal to 20mm, the tests shall be conducted according to C, W, and Z as specified in BS6387:2013, and the line shall remain intact.

[0044] Refractory only With 300V applied to both ends of the cable, each circuit is connected to an indicator light and a 2A circuit breaker. The circuit remained intact for 180 minutes under a 950℃ flame: the indicator lights did not go out, and the circuit breaker operated normally.

[0045] Refractory water treatment A 300V voltage was applied to both ends of the cable. Each circuit was connected to an indicator light and a 2A circuit breaker. After being exposed to a 650℃ flame for 15 minutes, the cable was sprayed with water at a rate of 0.25~0.30L / m / s for 5 seconds. The spraying was repeated every 60 seconds, and the circuit remained intact after 5 sprays. The indicator lights did not go out, and the circuit breaker operated normally.

[0046] Refractory and mechanical impact A 300V voltage was applied to both ends of the cable, and each circuit was connected to an indicator light and a 2A circuit breaker. The cable was tested under a 950℃ flame. After maintaining the flame for 10 minutes (±10 seconds), the cable was subjected to an impact test. Following the initial impact, a further impact was performed after 10 minutes (±10 seconds). Subsequent impacts were performed at intervals of 10 minutes (±10 seconds): 3 times at 30 minutes, 6 times at 60 minutes, and 12 times at 120 minutes. The circuit remained intact: the indicator lights did not extinguish, and the circuit breaker operated normally.

[0047] When the cable outer diameter is greater than 20mm, the test shall be conducted in accordance with BS8491:2008, and the line shall remain intact.

[0048] A 1000V voltage was applied to both ends of the cable. Each circuit was connected to an indicator light and a 2A circuit breaker. Under a flame temperature of 830℃~870℃, the sample was impacted by an impact device after 10 minutes (±10 seconds) of flame exposure. After the initial impact, the sample was impacted again after 10 minutes (±10 seconds). Subsequent impact intervals were 10 minutes (±10 seconds): 3 times at 30 minutes, 6 times at 60 minutes, and 12 times at 120 minutes. Five minutes before the end of the flame and impact tests, a water spray device was activated and sprayed water for 5 seconds. Sixty seconds after the initial water spray, another 5 seconds of water spray was applied. This process was repeated until five water sprays were completed. At the end of the entire test, the circuit remained intact: the indicator lights did not extinguish, and the circuit breaker operated normally.

[0049] ③ Battery acid resistance test Cable samples were pre-soaked in a mixture of 25% sulfuric acid and 75% water with a relative density of 1.28 for 10 seconds, air-dried for 3 minutes, and then placed in an aging chamber for 240 hours of thermal aging at a conductor maximum temperature of 125℃. After aging, the samples were removed and placed at room temperature of 23±5℃ for 30 minutes. Then, according to the specifications in Tables 3-1 and 3-2, cables with different conductor cross-sections were subjected to a winding test at a certain winding speed after applying weight. After the winding test, AC voltages of 50Hz or 60Hz were applied to both ends of the cable. For cables used in DC 900V power supply systems, the withstand voltage was 1.5 kV; for cables used in DC 1500V power supply systems, the withstand voltage was 2.5 kV. Each withstand voltage was maintained for 5 minutes. No cracks were found on the cable surface, and the core was not broken down. The winding device was commercially available.

[0050] Table 1 Cable outer diameter, spindle outer diameter, weight, and number of turns (unsheathed cable)

[0051] Table 2 Cable outer diameter, mandrel outer diameter, weight, and number of turns (sheathed cable)

[0052] ④ Thermal life performance test The thermal life of the cable was evaluated according to the methods of GB / T11026.1 and GB / T11026.2. The performance selected for life determination was the elongation at break retention rate, with 50% as the endpoint.

[0053] The temperature index T1 at 20000h is extrapolated from the Arrhenius regression curve, and it is required that T1 should not be lower than 120℃.

[0054] ⑤ Softness performance test Cable cross-section is 70mm 2 Cables of 100 and below should undergo a flexibility test.

[0055] One end of the cable sample is fixed, and the other end is subjected to a certain load pressure. It is placed in an aging chamber at 80±2℃ for pretreatment. After 24 hours, it is taken out and placed in an environment at 20±2℃ and 50±5% relative humidity for 72 hours. The test is carried out according to the commercially available device. The free rebound angle of the sample should not be greater than the specified in Table 3-3.

[0056] Table 3 Requirements for Softness Test

[0057] ⑤ Bending performance test Take a 1m cable sample. Bend the sample cable 180° around the bending wheel (first bend), then bend it 180° in the opposite direction (second bend). For cables with an outer diameter of 14mm or less, perform two repeated bends; for cables with an outer diameter of more than 14mm, perform one repeated bend. After the bending test, visually inspect the cable sample; the metal sheath of the sample should be free of cracks and broken wires.

[0058] ⑥ Combustion performance test Single-strand flame retardant performance The cable shall pass the single-strand flame retardant test in accordance with the requirements of GB / T18380.12 standard.

[0059] Bundle Combustion Test Cables shall pass the bundled flame retardant Class C and Class D combustion tests in accordance with the requirements of GB / T18380.35 and GB / T18380.36 standards.

[0060] ⑦ Salt spray resistance test The cable shall be aged for 336 hours in accordance with GB / T2423.17 and placed at room temperature for at least 16 hours. Then, the tensile strength and elongation at break shall be tested in accordance with the relevant provisions of GB / T2951.11. The change rate of tensile strength and elongation at break before and after the salt spray test shall not exceed -30%.

[0061] The above tests were performed on Examples 1-6 and Comparative Examples 1-3, and the test data are as follows:

[0062] As shown in the table above, Examples 1-6 and Comparative Example 2 all passed the water permeability test. This is based on the capillary effect of the water-blocking yarn effectively blocking the conductor gaps. The water-blocking tape expands upon encountering trace amounts of water vapor in the tiny gaps caused by thermal cycling, achieving a "self-healing seal." This further proves that the nano-modified sheath significantly slows down the radial permeation rate of water molecules. Comparative Example 1 showed trace amounts of water vapor, and its insulation layer was made of traditional halogen-free materials, without expandable graphite (EG) or polyethylene grafted maleic anhydride (PE-g-MAH). Although the outer waterproof structure was intact, the interface between the traditional insulation material and the semiconducting water-blocking tape was prone to micro-gaps under thermal cycling due to the difference in thermal expansion coefficients. Moisture slowly permeates through this interface and condenses into trace amounts of water vapor at the cold end; this further demonstrates the importance of the insulating layer material of the present invention in maintaining the long-term stability of the overall waterproof structure; water seepage occurred in Comparative Example 3 because it lacked a water-blocking yarn layer, causing water to rapidly intrude along the continuous longitudinal capillary channels formed by the conductor twisting, forming a "wick effect"; the water quickly reached the end and seeped out, causing the waterproof function to completely fail, fully demonstrating that the physical barrier layer in the inner waterproof structure is indispensable.

[0063] Regarding fire resistance and flame retardancy, a comparison between Examples 1-6 and Comparative Examples 1-3 shows that the insulation layer of Comparative Example 1, under 950°C flame and mechanical impact, lacks the high-strength expanded carbon layer skeleton formed by EG, making its carbon layer fragile and prone to collapse. It cannot maintain insulation integrity under impact, resulting in a short circuit. In Examples 1-6, when the expandable graphite is heated, the intercalated acidic substances rapidly vaporize, generating a huge thrust that causes the graphite sheets to expand tens to hundreds of times in a direction perpendicular to the substrate, forming a "worm-like" expanded carbon layer. Ammonium polyphosphate (APP) decomposes upon heating to generate strong dehydrating agents such as polyphosphoric acid, which can catalyze the dehydration and cross-linking of polymers containing hydroxyl and ester groups, forming a dense, carbon-rich aromatic carbon layer. Polyethylene grafted with maleic anhydride (PE-g-MAH) forms strong interfacial interactions with the oxygen-containing groups on the surface of EG, the phosphate groups of APP, and the hydroxyl groups of fillers such as ATH / MH through hydrogen bonding and esterification. Through steric hindrance and interfacial wetting, it prevents the agglomeration of inorganic fillers such as EG sheets and APP particles, allowing them to achieve uniform dispersion at the nanoscale in the matrix, thereby forming a dense and tough carbon layer. In other words, the "expansion-char formation" of expandable graphite (EG) and ammonium polyphosphate (APP) synergistically expands EG to form a heat-insulating and oxygen-barrier layer, while APP catalyzes the dehydration and cross-linking of the EVA matrix to generate a dense char layer. This dual char layer greatly inhibits the volatilization and combustion of combustibles, reducing the heat release rate and smoke production. In contrast, Comparative Example 2, although containing EG, lacks PE-g-MAH, resulting in uneven filler dispersion and extremely weak interfacial bonding. Due to the lack of effective interfacial conduction and synergy, the flame-retardant efficiency of EG and APP is low, and the formed char layer is loose and incomplete, thus achieving a flame-retardant rating of only Class D and a smoke density as low as 58%. In the stringent "fire resistance plus mechanical impact" (Class Z) test, its fragile interface cannot withstand the dual damage of high temperature and mechanical stress. The expanded char layer rapidly disintegrates under impact due to debonding from the matrix, leading to line interruption. Therefore, PE-g-MAH is not simply a processing aid, but an indispensable key component for achieving efficient synergy between EG / APP, forming a high-temperature, high-strength char layer, and ensuring cable safety under extreme conditions such as fires.

[0064] Regarding environmental resistance and mechanical durability, compared to Comparative Example 2, Comparative Example 2 exhibited surface cracks in the winding test after acid etching and thermal aging; the lack of PE-g-MAH resulted in extremely weak interfacial bonding between inorganic fillers such as EG and ATH and the EVA matrix. Under acid erosion and thermal stress, the interface debonded first, forming stress concentration points that propagated into cracks during bending. In contrast, Examples 1-6, due to their strong interfacial bonding, did not exhibit this problem.

[0065] Example 6 had the highest T1, but the worst flexibility; that is, the high EG content and low PE-g-MAH content led to excessively high material stiffness and decreased toughness. This reflects the trade-off between high flame retardancy and high flexibility. Conversely, Examples 1, 3, and 5 maintained excellent flexibility while having a T1 > 130°C, achieving a balance in comprehensive performance of flame retardancy, heat resistance, and flexibility. The tensile and elongation rate of Comparative Example 1 was close to the failure boundary. The large amount of exposed inorganic flame retardant in its traditional material is prone to interfacial hydrolysis and ion erosion in salt spray environment, accelerating material aging; while the embodiments of the present invention significantly improved weather resistance through the barrier effect of PE-g-MAH-coated modified inorganic filler and nano-modified sheath.

[0066] In summary, this invention, by specifically compounding expandable graphite with maleic anhydride grafted onto polyethylene and applying it to the cable insulation layer, in conjunction with a nano-modified outer sheath, successfully prepared an energy storage cable with excellent comprehensive performance, including active waterproofing, high-efficiency fire resistance, excellent flexibility, and long-term reliability. This solves the stringent requirements of current energy storage systems for cable performance and has significant industrial application value.

[0067] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A waterproof and seepage-proof energy storage cable, characterized in that, The energy storage cable comprises, from the inside out: The conductor layer is made of multiple annealed soft copper wires twisted and pressed together; The inner waterproof and water-blocking structure includes water-blocking yarn that is uniformly and loosely wound around the conductor layer, and an inorganic semi-conductive water-blocking tape that is overlapped and wrapped around the outer layer of the water-blocking yarn. The insulating layer is a composite flame-retardant insulating layer. By weight, the raw materials of the composite flame-retardant insulating layer include: 100 parts of matrix resin, 10-15 parts of expandable graphite, 3-6 parts of polyethylene grafted maleic anhydride, and 5-8 parts of synergistic flame retardant. The outer sheath is a nano-modified polyethylene sheath layer.

2. The waterproof and seepage-proof energy storage cable according to claim 1, characterized in that, The matrix resin is selected from one or two of ethylene-vinyl acetate copolymer and low-density polyethylene.

3. The waterproof and seepage-proof energy storage cable according to claim 1, characterized in that, The synergistic flame retardant is selected from at least one of nano aluminum hydroxide, nano magnesium hydroxide, and ammonium polyphosphate.

4. The waterproof and seepage-proof energy storage cable according to claim 1, characterized in that, The expandable graphite has an initial expansion temperature of 180°C to 200°C and a particle size of 200 mesh to 300 mesh.

5. The waterproof and seepage-proof energy storage cable according to claim 1, characterized in that, The grafting rate of the polyethylene grafted with maleic anhydride is 0.5wt%-1.2wt%.

6. The waterproof and seepage-proof energy storage cable according to claim 1, characterized in that, The nano-modified polyethylene sheath layer comprises a polyethylene matrix and nano-inorganic fillers dispersed therein; the nano-inorganic fillers are at least one of nano-silica, nano-montmorillonite, and nano-silicon carbide; and the amount of nano-inorganic fillers added is 2% to 8% based on the mass of the polyethylene matrix.

7. A method for preparing a waterproof and seepage-proof energy storage cable as described in any one of claims 1-6, characterized in that, Includes the following steps: Step S1. Conductor preparation; Oxygen-free copper rods are drawn and annealed to obtain annealed soft copper wires, which are then stranded and pressed to form a conductor layer. Step S2. Construct a waterproof and water-blocking structure; Water-blocking yarn is uniformly and loosely wound around the conductor layer, and then inorganic semiconducting water-blocking tape is wrapped around the outer layer of the water-blocking yarn. Step S3. Prepare composite flame-retardant insulating masterbatch; Expandable graphite, polyethylene grafted maleic anhydride, and synergistic flame retardant are premixed and then melt-blended and granulated at a temperature 20-30°C lower than the initial expansion temperature of expandable graphite to obtain composite flame-retardant insulating masterbatch. Step S4. Extruding the insulation layer; The composite flame-retardant insulating masterbatch is mixed with the matrix resin and extruded onto the wire core treated with S2 at a temperature of 110℃-175℃ using an extruder to form an insulating layer. Step S5. Extrude the outer sheath; The nano-modified polyethylene sheath material is extruded onto the outside of the insulation layer using an extruder at a temperature of 120℃-200℃ to form an outer sheath. Step S6. Post-processing; The extruded cable is cooled, sized, and then wound up.

8. The preparation method according to claim 7, characterized in that, In step S3, the melt blending granulation is carried out using a twin-screw extruder.

9. The preparation method according to claim 7, characterized in that, In step S4, the temperatures of the extruder from the feeding section to the die head are set sequentially as follows: feeding section 110℃-130℃, compression section 130℃-145℃, homogenization section 145℃-160℃, and die head / die opening 160℃-175℃.

10. The preparation method according to claim 7, characterized in that, In step S5, the temperatures of the extruder from the feeding section to the die head are set sequentially as follows: feeding section 140℃-160℃, compression section 160℃-180℃, homogenization section 180℃-190℃, and die head / die opening 190℃-200℃.

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