Positive and negative differentiated integrated electrode for static film-free zinc-bromine battery and battery stack

Abstract

The invention belongs to the technical field of electrochemical energy storage, and relates to a positive and negative differential integrated electrode for a static filmless zinc-bromine battery and a battery stack. The electrode comprises a flexible graphite composite bipolar plate, one side of the flexible graphite composite bipolar plate is compounded with a non-conductive negative electrode thin pre-oxidized felt subjected to oleophobic modification, the other side of the flexible graphite composite bipolar plate is compounded with a positive electrode thick activated graphite felt which has a gradient pore structure and is loaded with a bromine oxidation reduction catalyst, the contact side of the bipolar plate and the negative electrode felt is activated, and the three layers are compounded into an integrated structure. The stack is horizontally placed, positive and negative electrode felts of adjacent electrodes are directly attached and compressed, no diaphragm exists, and the compression ratio is controlled by a limiting column. Three-dimensional uniform deposition of zinc is guided through the negative electrode non-conductive felt, the bromine oil phase is locked through cooperation of positive electrode gradient pores and gravity settling, the contact resistance is reduced through the integrated structure, non-film static operation is achieved, the cycle life exceeds 500 times, the self-discharge rate is lower than 0.45%, the energy efficiency is higher than 80%, the structure is simplified, and the lithium ion battery is suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to an integrated positive and negative differentiated electrode and battery stack for a static membrane-free zinc-bromine battery. Background Technology

[0002] Zinc-bromine batteries possess unique advantages in large-scale electrochemical energy storage due to their combination of high energy density, low cost, and intrinsic safety. In recent years, to simplify system structure and reduce manufacturing costs, researchers have focused on developing pump-free, statically circulating zinc-bromine batteries. These batteries eliminate the complex circulation pipelines and pump / valve components of traditional flow batteries, achieving mass transport through static electrolyte storage and natural diffusion, significantly improving system compactness and reliability. However, in further advancing membrane-free, horizontal structural designs, existing technologies still face a series of technical bottlenecks that are difficult to resolve collaboratively.

[0003] Uncontrollable zinc deposition on the negative electrode side is one of the core issues limiting battery cycle life. Static zinc-bromine batteries typically use conductive plastic or metal-based bipolar plates as the negative electrode current collector, which have relatively smooth surfaces. During charging and reduction, zinc tends to preferentially grow along the direction of current concentration, easily forming sharp dendritic crystals. Because the electrolyte is stationary in a static system, there is a lack of physical inhibition of dendrite formation by flow scouring. These dendrites continue to extend during repeated charging and discharging, eventually penetrating the narrow gap between the positive and negative electrodes, causing internal short circuits and battery failure. Even with a horizontal placement structure to utilize gravity to stabilize the interface, the zinc dendrite problem remains unresolved, making it difficult to improve the battery's safe areal capacity, and the cycle life is far from meeting the requirements for commercial applications.

[0004] The self-discharge problem caused by bromine shuttle at the positive electrode also severely restricts battery performance. During charging, elemental bromine or bromine complexes are generated at the positive electrode. If the positive electrode material's adsorption and locking capacity for the bromine oil phase is insufficient, bromine species will diffuse through the electrolyte to the negative electrode region, reacting chemically with zinc and causing self-discharge. Currently, the positive electrode of most static zinc-bromine batteries uses ordinary carbon felt or carbon cloth. These materials have limited porosity and insufficient surface active sites, resulting in weak storage capacity for the high-density bromine oil phase. After charging, the bromine oil phase cannot be effectively anchored in the positive electrode region, leading to a large amount of bromine shuttles to the negative electrode. This not only reduces coulombic efficiency but also accelerates the corrosion and passivation of the zinc at the negative electrode.

[0005] Furthermore, the contact method between the bipolar plates and electrodes also affects battery performance. In traditional structures, the bipolar plates and positive and negative electrode felts are usually assembled separately, relying on mechanical pressure to achieve bonding. This interface contact method has a large contact resistance, increasing the battery's internal losses, and it is difficult to ensure the consistency of the contact state of each cell during assembly, affecting the uniformity and reliability of the stack. To achieve ion conduction between the positive and negative electrodes while preventing electronic short circuits, most existing static zinc-bromine batteries still rely on separators for physical isolation. However, separator materials are expensive, have high ion conduction resistance, and are prone to contamination and mechanical damage during long-term cycling, further increasing system costs and failure risks.

[0006] In summary, existing static zinc-bromine batteries face multiple technical contradictions that mutually restrict each other in terms of negative electrode dendrite suppression, positive electrode bromine locking, interface contact optimization, and film-free structure design. This makes it difficult to improve battery cycle life, energy efficiency, and areal capacity in a coordinated manner, and the manufacturing cost and assembly complexity remain high. There is an urgent need for an innovative electrode structure solution that can systematically solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to provide a static membrane-free zinc-bromine battery with a differentiated integrated positive and negative electrode and a battery stack. By designing the positive and negative electrodes with different functions and combining them into an integrated structure, the invention aims to suppress the growth of zinc dendrites in the negative electrode and the bromine shuttle in the positive electrode from the source, reduce the contact resistance between the electrode and the bipolar plate, and eliminate the need for a separator and pumping system. This results in a synergistic improvement in battery cycle life, energy efficiency and areal capacity, while simplifying the stack structure and reducing manufacturing costs.

[0008] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides an integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery, characterized in that it comprises a flexible graphite composite bipolar plate, wherein a thin pre-oxidized negative electrode felt is laminated on one surface of the bipolar plate, and a thick activated graphite felt for the positive electrode is laminated on the other surface; the surface of the bipolar plate in contact with the thin pre-oxidized negative electrode felt is an activation layer; the thin pre-oxidized negative electrode felt is an oleophobic modified pre-oxidized polyacrylonitrile fiber felt with a resistivity ≥10. 6 Ω·cm; the positive electrode thick activated graphite felt has a gradient pore structure along the thickness direction, and the carbon fiber surface of the activated graphite felt is loaded with a bromine oxidation-reduction catalyst; the bipolar plate, the negative electrode thin pre-oxidized felt and the positive electrode thick activated graphite felt are integrated into a single structure through a composite process.

[0009] Furthermore, the activation layer is formed by treating the surface of the bipolar plate with at least one of roughening, partial oxidation, or surface loading of a tin-based catalyst.

[0010] Furthermore, the roughening is performed by mechanical polishing or plasma etching, and the surface roughness Ra after roughening is 0.5–2.0 μm; the partial oxidation is performed by electrochemical oxidation, with an oxidation voltage of 1.5–2.0 V and a processing time of 10–60 s; the tin-based catalyst supported in the activation layer is tin or SnO2 nanoparticles, wherein the particle size of the tin nanoparticles is 10–100 nm.

[0011] The activation treatment on the contact side between the bipolar plate and the negative electrode pre-oxidized felt is mainly to increase surface roughness and active sites. Roughening or plasma etching can create a micro-uneven structure on the surface of the bipolar plate, increasing the actual contact area with the pre-oxidized felt. Simultaneously, these protrusions generate a local electric field enhancement effect during charging, inducing zinc ions to preferentially nucleate at these locations. Electrochemical oxidation can introduce oxygen-containing functional groups onto the graphite surface. These functional groups have a certain affinity for zinc ions, helping to reduce the overpotential of zinc deposition. Supported tin-based catalysts utilize the alloying effect between tin and zinc, causing zinc to tend to spread uniformly around tin particles during deposition rather than forming sharp dendrites. The core objective of all three methods is to make the initial nucleation of zinc more uniform and dense, avoiding dendrite growth caused by localized current concentration.

[0012] Furthermore, the thickness of the positive electrode thick activated graphite felt is 5.0–7.0 mm, and the areal density is 400–1000 g / m³. 2 Graphitization degree ≥95%.

[0013] Furthermore, the gradient pore structure is formed by integrally hot-pressing carbon fiber layers with different porosities after layering; wherein the porosity on the side closer to the bipolar plate is 90% to 95%, and the porosity on the side farther from the bipolar plate is 75% to 85%.

[0014] Further, the bromine oxidation-reduction catalyst is selected from at least one of nitrogen-doped carbon materials, metal oxides, or metal / metal oxide composites, with a loading of 0.1–5 wt%. Specifically, the nitrogen-doped carbon material is selected from at least one of nitrogen-doped graphene, nitrogen-doped carbon nanotubes, or nitrogen-doped mesoporous carbon; the metal oxide is selected from at least one of NiO, Co3O4, or MnO2; and the metal / metal oxide composite is Co / CoO2. x (1≤x≤1.5).

[0015] The thick activated graphite felt for the positive electrode employs a gradient pore structure. The large pores near the bipolar plate ensure smooth electrolyte penetration into the felt, preventing mass transfer limitations. The smaller pores away from the bipolar plate physically trap the bromine oil phase. Because the bromine oil phase is denser than the electrolyte, it settles to the bottom of the felt under gravity. The high specific surface area and tortuous channels of the small pore layer effectively adsorb and lock the bromine oil, preventing it from diffusing back to the positive electrode surface or crossing to the negative electrode side. A supported bromine redox catalyst can reduce the Br2 / Br2 ratio. - The activation energy of the redox couple accelerates the rate at which bromine is reduced during discharge, reducing the accumulation time of bromine in the positive electrode region, thereby further reducing the probability of bromine shuttle.

[0016] Furthermore, the thickness of the negative electrode thin pre-oxidized felt is 0.3–1.0 mm, and the areal density is 150–250 g / m³. 2 The oleophobic modification treatment is achieved by immersing the pre-oxidized felt in a 0.5-5 wt% fluorosilane ethanol solution for 10-60 minutes, and then drying and curing it at 80-120°C.

[0017] The negative electrode thin pre-oxidized felt uses non-conductive pre-oxidized polyacrylonitrile fiber, which remains insulated without carbonization. This prevents electrons from conducting between the positive and negative electrodes when they are directly bonded, fundamentally avoiding physical short circuits. Simultaneously, the non-conductivity means that zinc ions must pass through the pores of the pre-oxidized felt to reach the bipolar plate surface for deposition. The deposited zinc grows inward along the felt fiber direction, forming a three-dimensional structure embedded in the felt body. This structure is denser and less prone to detachment than planar deposition. Oleophobic modification involves grafting a fluorosilane molecular layer onto the pre-oxidized felt surface. The fluorocarbon segments in the fluorosilane molecules have extremely low surface energy, making the felt body repellent to the bromine oil phase (contact angle > 90°). Simultaneously, the silane ends of the fluorosilane form chemical bonds with the hydroxyl groups on the pre-oxidized felt surface, ensuring the stability of the oleophobic layer. Even if a small amount of bromine oil diffuses from the positive electrode side, it is difficult to wet and penetrate the pre-oxidized felt, effectively adding a chemical barrier on the negative electrode side, forming a two-way protection with the adsorption and locking on the positive electrode side.

[0018] Furthermore, the thickness of the flexible graphite composite bipolar plate is 0.6–1.2 mm.

[0019] Furthermore, the area of ​​the flexible graphite composite bipolar plate is larger than the area of ​​the attached thick activated graphite felt for the positive electrode and the thin pre-oxidized felt for the negative electrode, with a width margin of 2 to 10 mm on all four sides, and the four sides are treated with insulation and hydrophobicity.

[0020] A second aspect of the present invention provides a static membrane-free zinc-bromine battery stack, the stack comprising at least two integrated electrodes as described above; the stack is placed horizontally, and in two adjacent integrated electrodes, the positive electrode thick activated graphite felt of one integrated electrode and the negative electrode thin pre-oxidized felt of the other integrated electrode are oppositely arranged and directly bonded and compressed, without a separator in between; the stack is provided with limiting posts or bosses to control the assembly thickness, so that the assembly compression ratio of the positive electrode thick activated graphite felt and the negative electrode thin pre-oxidized felt is independently 12% to 20%.

[0021] Furthermore, the stack is filled with a dissolved oil-phase bromine complexing agent aqueous electrolyte, which contains ZnBr2 1-3 mol / L, KBr 0.5-2 mol / L, and quaternary ammonium salt or pyridine salt oil-phase bromine complexing agents.

[0022] The stack is placed horizontally, allowing the positive and negative electrode felts to be directly bonded and compressed, eliminating the need for a diaphragm and pump system, thus significantly simplifying the structure. Horizontal placement allows the bromine oil phase generated during charging to naturally settle to the bottom of the thick positive electrode felt under gravity, rather than suspending in the electrolyte and diffusing. Combined with the gradient pore structure of the positive electrode, the bromine oil is stably locked in a region far from the negative electrode. Limiting posts or bosses control the assembly compression ratio to 12%-20%. This range ensures good electrical contact between the positive and negative electrode felts and the bipolar plates without clogging the pore channels due to excessive compression, thus preventing interference with ion conduction. The positive and negative electrode felts are in direct contact between adjacent electrodes. Since the negative electrode felt is non-conductive, even with tight bonding, electronic short circuits will not occur, and ions can freely migrate through the electrolyte in the pores of the felt, enabling normal charging and discharging without a membrane.

[0023] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: The negative electrode side uses uncarbonized pre-oxidized felt with oleophobic modification. Its non-conductive properties allow the positive and negative electrode felts to be directly bonded without short-circuiting, eliminating the need for a separator. Simultaneously, the oleophobic layer effectively repels bromine oil diffusion to the negative electrode. The contact side between the bipolar plate and the negative electrode felt undergoes activation treatment, providing dense nucleation sites for zinc deposition. Combined with the three-dimensional framework structure of the pre-oxidized felt, zinc grows uniformly within the felt, preventing the formation of sharp dendrites. The battery can still operate normally after more than 500 cycles. The positive electrode side features a gradient structure with gradually decreasing porosity along the thickness direction. Large pores ensure electrolyte penetration, while small pores utilize gravity to settle and lock the bromine oil phase at the bottom of the felt. Combined with a catalyst loaded on the carbon fiber surface, the bromine reduction reaction rate is accelerated, and the self-discharge rate is controlled below 0.5%. The bipolar plate and the positive and negative electrode felts are composited into a single unit through hot pressing or conductive adhesive, reducing the interfacial contact resistance caused by separate assembly. This improves voltage efficiency by more than 4 percentage points compared to traditional separate structures. The entire fuel cell stack is placed horizontally, without pumps or membranes, resulting in a simple and reliable structure, significantly reduced manufacturing costs, and good practical value. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the integrated electrode structure prepared in Example 1. In the figure, 1-positive electrode thick activation felt; 2-flexible graphite composite bipolar plate; 3-activation layer; 4-negative electrode thin pre-oxidized felt.

[0025] Figure 2 This is a schematic diagram of the cross-sectional assembly of the battery prepared in Example 1. In the figure, 1-integrated electrode; 2-limiting post; 3-battery casing.

[0026] Figure 3 This is a schematic diagram of the three-dimensional deposition of zinc inside the thin pre-oxidized felt of the negative electrode.

[0027] Figure 4 This is a schematic diagram of the bromine oil phase settling under the thick activated graphite felt at the positive electrode. Detailed Implementation

[0028] 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. Unless otherwise specified, the raw materials used in the embodiments are all commercially available products.

[0029] Example 1 This embodiment provides an integrated positive and negative differentiated electrode and battery stack for a static membrane-free zinc-bromine battery, the preparation method of which includes the following steps: (1) Bipolar plate pretreatment A reinforced flexible graphite composite bipolar plate with a thickness of 0.6 mm was selected. Tin nanoparticles were loaded onto the side of the bipolar plate in contact with the negative electrode pre-oxidized felt for activation using an electrodeposition method. The bipolar plate was immersed in an electrolyte containing 0.05 mol / L SnCl2 and 0.1 mol / L sodium citrate, with the bipolar plate as the cathode and a platinum sheet as the anode, at a current density of 5 mA / cm². 2 Electrodeposition for 60 seconds forms an activation layer of tin nanoparticles with a particle size of 50–100 nm (average particle size of about 75 nm) on the surface of the bipolar plate.

[0030] (2) Treatment of negative electrode thin pre-oxidized felt The material used is uncarbonized pre-oxidized polyacrylonitrile fiber felt, 0.8 mm thick, with a surface density of 200 g / m². 2 The material was impregnated in a 2.0 wt% perfluorodecyltrimethoxysilane ethanol solution for 30 min, and then dried and cured at 100℃ for 45 min to obtain an oleophobic modified pre-oxidized felt. The measured resistivity of the pre-oxidized felt after treatment was 2.0 × 10⁻⁶. 6Ω·cm, with a contact angle of 130° for the bromine oil phase.

[0031] (3) Treatment of thick activated graphite felt with positive electrode PAN-based graphite felt was selected, with an initial thickness of 5.0 mm and a surface density of 600 g / m³. 2 A gradient pore structure was formed by layering carbon fiber layers with different porosities: a macroporous layer (2.0 mm thick) with a porosity of 92% was placed near the bipolar plate, while a microporous layer (3.0 mm thick) with a porosity of 75% was placed away from the bipolar plate. The two layers were then hot-pressed (120℃, 1 MPa) to form a single unit, followed by activation in air at 600℃ for 2 hours, resulting in a measured graphitization degree of 96%. A bromine oxidation-reduction catalyst was then loaded using an impregnation method: the gradient felt was impregnated with a solution containing 0.5 wt% Co(NO3)2. After being removed from the aqueous solution of 6H2O, the sample was pyrolyzed at 300℃ in air for 30 min, followed by heat treatment at 800℃ in nitrogen for 2 h to obtain supported Co / CoO. x (1 ≤ x ≤ 1.5) The positive electrode of the composite catalyst is a thick activated graphite felt with a cobalt loading of 0.3 wt%.

[0032] (4) Integrated composite A silver-based conductive adhesive is uniformly coated on both sides of the bipolar plate. The pre-oxidized negative electrode felt is then bonded to one side of the activated layer of the bipolar plate, and the thick activated graphite positive electrode felt is bonded to the other side. The plate is placed in a hot press and held at 120°C and 1 MPa for 10 minutes to bond the three layers into an inseparable whole. After cooling, it is cut into 10 cm × 10 cm electrode sheets. The four sides of the bipolar plate are 5 mm wider than the felt, and an epoxy resin insulating layer is applied to the four sides for insulation and hydrophobic treatment. A schematic diagram of the resulting integrated electrode structure is shown below. Figure 1 As shown.

[0033] (5) Stack assembly Six of the aforementioned integrated electrodes are horizontally stacked inside the battery casing, ensuring direct contact between the thick activated graphite felt of the lower electrode and the thin pre-oxidized felt of the upper electrode, without any separator in between. End plates are installed at both ends of the stack, and the total assembly thickness is controlled by limiting posts, ensuring that the assembly compression ratio of both the thick activated graphite felt of the positive electrode and the thin pre-oxidized felt of the negative electrode is 15%. An electrolyte is injected into the stack, consisting of: 2 mol / L ZnBr, 1 mol / L KBr, 5 vol% tetrabutylammonium bromide, and the remainder being deionized water. A schematic diagram of the battery cross-section assembly is shown below. Figure 2 As shown. The zinc deposition inside the negative electrode pre-oxidation felt is uniform and dense, without sharp dendrites, and its deposition morphology is as follows. Figure 3 Schematic diagram; a large amount of bromine oil phase is adsorbed at the bottom of the thick felt of the positive electrode, and no trace of bromine shuttle is observed. Its sedimentation morphology is as follows. Figure 4Indication.

[0034] Example 2 This embodiment provides an integrated positive and negative differentiated electrode and battery stack for a static membrane-free zinc-bromine battery, the preparation method of which includes the following steps: (1) Bipolar plate pretreatment A reinforced flexible graphite composite bipolar plate with a thickness of 1.0 mm was selected. The side of the bipolar plate in contact with the negative electrode pre-oxidation felt was activated by plasma etching roughening. The etching power was 200 W and the treatment time was 5 min, so that the surface roughness Ra reached 1.0 μm.

[0035] (2) Treatment of negative electrode thin pre-oxidized felt The material used is uncarbonized pre-oxidized polyacrylonitrile fiber felt, 0.5 mm thick, with a surface density of 150 g / m². 2 The material was impregnated in a 2.5 wt% perfluorodecyltrimethoxysilane ethanol solution for 25 min, and then dried and cured at 105℃ for 40 min to obtain an oleophobic modified pre-oxidized felt. The measured resistivity of the pre-oxidized felt after treatment was 2.3 × 10⁻⁶. 6 Ω·cm, with a contact angle of 132° for the bromine oil phase.

[0036] (3) Treatment of thick activated graphite felt with positive electrode PAN-based graphite felt was selected, with an initial thickness of 6.0 mm and a surface density of 800 g / m³. 2 A gradient pore structure was formed by layering carbon fiber layers with different porosities: a macroporous layer (2.5 mm thick) with a porosity of 93% was placed near the bipolar plate, while a microporous layer (3.5 mm thick) with a porosity of 77% was placed away from the bipolar plate. The two layers were then hot-pressed (120℃, 1 MPa) to form a single composite, followed by activation in air at 620℃ for 1.8 hours, resulting in a measured graphitization degree of 96.2%. A bromine redox catalyst was then loaded using an impregnation method: the gradient felt was immersed in a mixed dispersion containing 0.2 wt% nitrogen-doped graphene and 0.2 wt% NiO nanoparticles, then pyrolyzed in nitrogen at 330℃ for 25 min to obtain a thick activated graphite felt with a total loading of 0.4 wt% for the composite catalyst.

[0037] (4) Integrated composite A silver-based conductive adhesive is uniformly coated on both sides of the bipolar plate. The pre-oxidized negative electrode felt is then bonded to one side of the activated layer of the bipolar plate, and the thick activated graphite positive electrode felt is bonded to the other side. The plates are allowed to cure at room temperature for 24 hours, allowing the three layers to bond into an inseparable whole. After cooling, the plates are cut into 10 cm × 10 cm electrode sheets, with each side of the bipolar plate extending 5 mm beyond the felt. An epoxy resin insulating layer is then applied to the four sides for insulation and hydrophobic treatment.

[0038] (5) Stack assembly Six of the aforementioned integrated electrodes are horizontally stacked, with the thick activated graphite felt of the lower electrode directly bonded to the thin pre-oxidized felt of the upper electrode, without any diaphragm in between. End plates are installed at both ends of the stack, and the total assembly thickness is controlled by limiting posts, ensuring that the assembly compression ratio of both the thick activated graphite felt of the positive electrode and the thin pre-oxidized felt of the negative electrode is 12%. An electrolyte is injected into the stack, with the following composition: 2 mol / L ZnBr2, 1 mol / L KBr, 5 vol% tetrabutylammonium bromide, and the remainder being deionized water.

[0039] Example 3 This embodiment provides an integrated positive and negative differentiated electrode and battery stack for a static membrane-free zinc-bromine battery, the preparation method of which includes the following steps: (1) Bipolar plate pretreatment A reinforced flexible graphite composite bipolar plate with a thickness of 1.2 mm was selected. The side of the bipolar plate in contact with the negative electrode pre-oxidation felt was activated by electrochemical oxidation at a voltage of 1.8 V for 30 s.

[0040] (2) Treatment of negative electrode thin pre-oxidized felt The material used is uncarbonized pre-oxidized polyacrylonitrile fiber felt, 1.0 mm thick, with a surface density of 250 g / m². 2 The material was impregnated in a 3.0 wt% perfluorodecyltrimethoxysilane ethanol solution for 20 min, and then dried and cured at 115℃ for 30 min to obtain an oleophobic modified pre-oxidized felt. The measured resistivity of the pre-oxidized felt after treatment was 1.9 × 10⁻⁶. 6 Ω·cm, with a contact angle of 131° for the bromine oil phase.

[0041] (3) Treatment of thick activated graphite felt with positive electrode PAN-based graphite felt was selected, with an initial thickness of 7.0 mm and a surface density of 1000 g / m³. 2 A gradient pore structure was formed by layering carbon fiber layers with different porosities: a macroporous layer (3.0 mm thick) with 95% porosity was placed near the bipolar plate, while a microporous layer (4.0 mm thick) with 80% porosity was placed away from the bipolar plate. The two layers were then hot-pressed (120℃, 1 MPa) to form a single unit, followed by activation in air at 580℃ for 2.5 hours, resulting in a measured graphitization degree of 95.8%. A bromine oxidation-reduction catalyst was then loaded using an impregnation method: the gradient felt was immersed in a suspension containing 0.3 wt% Co3O4 nanoparticles, then pyrolyzed at 320℃ in a nitrogen atmosphere for 30 min to obtain a thick activated graphite felt with a Co3O4 catalyst loading of 0.3 wt%.

[0042] (4) Integrated composite A silver-based conductive adhesive is uniformly coated on both sides of the bipolar plate. The pre-oxidized negative electrode felt is then bonded to one side of the activated layer of the bipolar plate, and the thick activated graphite positive electrode felt is bonded to the other side. The plate is placed in a hot press and held at 180°C and 3 MPa for 5 minutes to bond the three layers into an inseparable whole. After cooling, the plate is cut into 10 cm × 10 cm electrode sheets, with each side of the bipolar plate extending 5 mm beyond the felt body. An epoxy resin insulating layer is then applied to the four sides for insulation and hydrophobic treatment.

[0043] (5) Stack assembly Six of the aforementioned integrated electrodes are horizontally stacked, with the thick activated graphite felt of the lower electrode directly bonded to the thin pre-oxidized felt of the upper electrode, without any diaphragm in between. End plates are installed at both ends of the stack, and the total assembly thickness is controlled by a boss limiting structure, ensuring that the assembly compression ratio of both the thick activated graphite felt of the positive electrode and the thin pre-oxidized felt of the negative electrode is 20%. An electrolyte is injected into the stack, with the following composition: 2 mol / L ZnBr2, 1 mol / L KBr, 5 vol% tetrabutylammonium bromide, and the remainder being deionized water.

[0044] Comparative Example 1 The difference between this comparative example and Example 1 is that the negative electrode thin pre-oxidized felt in Example 1 is not modified to be oleophobic (uncarbonized pre-oxidized polyacrylonitrile fiber felt, thickness 0.8 mm, areal density 200 g / m²). 2 The rest is the same as in Example 1.

[0045] Comparative Example 2 The difference between this comparative example and Example 1 is that the gradient pore structure of the thick activated graphite felt in Example 1 is replaced with a uniformly porous activated graphite felt with a porosity of 90%, a thickness of 5.0 mm, and a surface density of 600 g / m³. 2 No gradient, same load Co / CoO x (1 ≤ x ≤ 1.5) composite catalyst, otherwise the same as in Example 1.

[0046] Comparative Example 3 The difference between this comparative example and Example 1 is that the electrodes are not integrally composited, but rather assembled separately. That is, the bipolar plate, the thin pre-oxidized felt for the negative electrode, and the thick activated graphite felt for the positive electrode are each independent and are bonded together by mechanical pressure. The compression ratio is still 15%, and the rest is the same as in Example 1.

[0047] Comparative Example 4 The difference between this comparative example and Example 1 is that the side of the bipolar plate that contacts the negative electrode pre-oxidation felt is not subjected to any activation treatment, maintaining the original flat and smooth surface; otherwise, it is the same as Example 1.

[0048] Comparative Example 5 The difference between this comparative example and Example 1 is that the thick activated graphite felt for the positive electrode does not support a bromine redox catalyst; otherwise, it is the same as Example 1.

[0049] Performance testing Coulombic efficiency, voltage efficiency, energy efficiency: current density 20 mA / cm² 2 Charged to a surface capacity of 60 mAh / cm² 2 Then discharge to the discharge cutoff voltage, record the charge and discharge capacity and energy, and calculate the coulombic efficiency (discharge capacity / charge capacity × 100%), voltage efficiency (average discharge voltage / average charge voltage × 100%) and energy efficiency (discharge energy / charge energy × 100%).

[0050] Self-discharge rate: After fully charging the battery and letting it rest for 72 hours, test the discharge capacity before and after the rest period, and calculate it using the following formula: Self-discharge rate = (Discharge capacity before rest) / (Discharge capacity before rest) Discharge capacity after settling) / Discharge capacity before settling × 100%.

[0051] Cycle life: at 20 mA / cm 2 Current density, 60 mAh / cm 2 Under the condition of battery capacity, charge and discharge cycles were performed, and the number of cycles when the battery experienced an internal short circuit or its capacity decayed to 80% of its initial capacity was recorded.

[0052] The test results are shown in Table 1.

[0053] Table 1 Performance Test Results

[0054] The performance test results above show that the integrated electrode and stack of the present invention exhibit excellent performance under different parameter combinations. The coulombic efficiency of all three embodiments reaches over 95.5%, indicating that the non-conductive pre-oxidized felt of the negative electrode effectively suppresses bromine shuttle, and the gradient pores of the positive electrode, in synergy with the catalyst, control the self-discharge rate below 0.44%. The voltage efficiency is between 84.2% and 85.8%, and the energy efficiency is between 80.4% and 82.6%, and all can stably cycle more than 500 times without short circuits, proving that the present invention has robustness and a wide process window.

[0055] In Comparative Example 1, after removing the oleophobic modification of the negative electrode pre-oxidized felt, the self-discharge rate increased to approximately 1.8%, and the cycle life decreased to approximately 350 cycles, indicating that the oleophobic modification plays a crucial role in preventing the diffusion of bromine oil to the negative electrode and reducing self-discharge. In Comparative Example 2, after replacing the gradient pores with uniform pores, the self-discharge rate increased to 1.2%, indicating that the matching design of the gradient pore structure with the direction of gravity sedimentation significantly improved the locking ability of the positive electrode to the bromine oil phase. In Comparative Example 3, after adopting a split assembly, the voltage efficiency decreased to 81.5%, and the energy efficiency decreased to 78.2%, proving that the integrated composite structure effectively reduced the interfacial contact resistance. In Comparative Example 4, after removing the bipolar plate activation treatment, a micro-short circuit occurred in less than 300 cycles, indicating that the bipolar plate activation treatment plays an important role in inducing uniform zinc nucleation and extending cycle life. In Comparative Example 5, after removing the positive electrode catalyst loading, the coulombic efficiency slightly decreased to 95.2%, the voltage efficiency decreased to 82.5%, and the self-discharge rate increased to 0.56%, indicating that catalyst loading has an auxiliary enhancing effect on improving the kinetics of bromine oxidation-reduction reaction and reducing self-discharge.

[0056] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A positive and negative differentiated integrated electrode for a static membrane-free zinc-bromine battery, characterized in that, The invention includes a flexible graphite composite bipolar plate, wherein a thin pre-oxidized negative electrode felt is laminated on one surface of the bipolar plate, and a thick activated graphite felt for the positive electrode is laminated on the other surface; the surface of the bipolar plate in contact with the thin pre-oxidized negative electrode felt is an activated layer; the thin pre-oxidized negative electrode felt is a pre-oxidized polyacrylonitrile fiber felt that has undergone oleophobic modification and has a resistivity ≥10. 6 Ω·cm; The positive electrode thick activated graphite felt has a gradient pore structure along the thickness direction, and the carbon fiber surface of the activated graphite felt is loaded with a bromine oxidation-reduction catalyst. The bipolar plate, the thin pre-oxidized negative electrode felt, and the thick activated graphite felt positive electrode are integrated into a single structure through a composite process.

2. The integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery according to claim 1, characterized in that, The activation layer is formed by treating the surface of the bipolar plate with at least one of roughening, partial oxidation, or surface loading of a tin-based catalyst.

3. The integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery according to claim 2, characterized in that, The roughening is performed by mechanical polishing or plasma etching, and the surface roughness Ra after roughening is 0.5–2.0 μm; the partial oxidation is performed by electrochemical oxidation, with an oxidation voltage of 1.5–2.0 V and a processing time of 10–60 s; the tin-based catalyst supported in the activation layer is tin or SnO2 nanoparticles, wherein the particle size of the tin nanoparticles is 10–100 nm.

4. The integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery according to claim 1, characterized in that, The thickness of the positive electrode thick activated graphite felt is 5.0–7.0 mm, and the areal density is 400–1000 g / m³. 2 Graphitization degree ≥95%.

5. The integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery according to claim 1, characterized in that, The gradient pore structure is formed by integrally hot-pressing carbon fiber layers with different porosities after layering; wherein the porosity on the side closer to the bipolar plate is 90% to 95%, and the porosity on the side farther from the bipolar plate is 75% to 85%.

6. The integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery according to claim 1, characterized in that, The bromine oxidation-reduction catalyst is selected from at least one of nitrogen-doped carbon materials, metal oxides, or metal / metal oxide composites, and the loading is 0.1 to 5 wt%.

7. The integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery according to claim 1, characterized in that, The thickness of the negative electrode thin pre-oxidized felt is 0.3–1.0 mm, and the areal density is 150–250 g / m³. 2 The oleophobic modification treatment is achieved by immersing the pre-oxidized felt in a 0.5-5 wt% fluorosilane ethanol solution for 10-60 minutes, and then drying and curing it at 80-120°C.

8. The integrated positive and negative differential electrode for a static membrane-free zinc-bromine battery according to claim 1, characterized in that, The thickness of the flexible graphite composite bipolar plate is 0.6–1.2 mm.

9. A static membrane-free zinc-bromine battery stack, characterized in that, The fuel cell stack includes at least two integrated electrodes as described in any one of claims 1-8; the fuel cell stack is placed horizontally, and in two adjacent integrated electrodes, the positive electrode thick activated graphite felt of one integrated electrode and the negative electrode thin pre-oxidized felt of the other integrated electrode are arranged opposite to each other and directly bonded and compressed, without a diaphragm in between; the fuel cell stack is provided with limiting posts or bosses to control the assembly thickness, so that the assembly compression ratio of the positive electrode thick activated graphite felt and the negative electrode thin pre-oxidized felt is independently 12% to 20%.

10. The static membrane-free zinc-bromine battery stack according to claim 9, characterized in that, The stack is filled with a dissolved oil-phase bromine complexing agent aqueous electrolyte, which contains ZnBr2 1-3 mol / L, KBr 0.5-2 mol / L, and quaternary ammonium salt or pyridine salt oil-phase bromine complexing agents.

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