Graphene thick electrode with gradient channel structure and preparation method and application thereof

Abstract

The invention discloses a graphene thick electrode with a gradient channel structure and a preparation method and application thereof, and relates to the technical field of electrochemical energy storage device manufacturing, and the internal average pore size of a large channel layer, a transition channel layer and a small channel layer designed in the thickness direction meets D1gt; d < 2gt >; by means of the structural design, the effective diffusion coefficient and the peller modulus of the porous medium are physically optimized, the reaction rate distribution of the whole electrode in the thickness direction is more uniform, and the problem that the bottom utilization rate of a traditional thick electrode is low is solved. Therefore, the electrode can give consideration to both rapid ion transmission and high-activity substance loading capacity within the limited thickness.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage device manufacturing technology, specifically to a graphene thick electrode with a gradient channel structure, its preparation method, and its application. Background Technology

[0002] In the field of electrochemical energy storage, electrode design has always faced the contradiction between energy density and power density. On the one hand, in order to improve energy density, the industry has gradually developed thick electrodes with high areal loading of active materials; on the other hand, as the electrode thickness increases, especially when the thickness exceeds 100 μm, the transport path of ions inside the electrode is significantly lengthened, and the transport resistance increases sharply, resulting in a significant decrease in the thickness utilization rate and rate performance of thick electrodes.

[0003] In existing technologies, thick electrodes typically have a pore structure that is essentially uniform along the thickness direction. This single pore structure makes it difficult to simultaneously meet the requirements of ion transport and active material loading: when the overall pore size is large, although it is beneficial for electrolyte penetration and rapid ion diffusion, the mass of active material that can be loaded per unit volume is small, the volumetric energy density is low, and the electrode skeleton is loose, making it easy to peel off from the current collector, resulting in poor mechanical stability; when the overall pore size is small, although the electrode structure is dense and the volumetric capacity is high, the electrolyte cannot penetrate to the bottom of the electrode in time under high-rate conditions, and the active material near the current collector is prone to being in a "dormant" state due to concentration polarization, resulting in low utilization rate and material waste in the thickness direction.

[0004] Hierarchical transport systems in nature, such as leaf veins and alveoli, demonstrate that by introducing non-uniform, hierarchical, or gradient channel structures in space, it is possible to balance high-throughput transport and effective load within a limited volume. However, current thick electrode fabrication processes generally struggle to precisely construct similar gradient channel structures along the electrode thickness direction. The pore size distribution and channel layout within the electrode still lack designability and programmability, making it difficult to fundamentally alleviate the contradiction between ion transport and active material loading in thick electrodes. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a graphene thick electrode and its preparation method. By designing a continuously transitioning and integrally connected micron-level channel layer in the thickness direction, a porous structure with gradient or hierarchical channel characteristics is formed inside the electrode. While ensuring a high areal loading, the ion transport performance in the thickness direction is significantly improved, and the mass transfer efficiency and structural stability are optimized.

[0006] The technical problem to be solved by this invention is achieved by the following technical solution: One objective of this invention is to provide a graphene thick electrode with a gradient channel structure, comprising a current collector and a porous electrode layer bonded to the surface of the current collector; the porous electrode layer has a non-uniform pore structure in the thickness direction, and is sequentially divided from the side away from the current collector (i.e., the electrolyte side) to the side closer to the current collector (i.e., the current collector side) into the following sections: The large channel layer contains micron-sized channels with an average pore size of D1. The transition channel layer contains micron-sized channels with an average pore size of D2. The small channel layer contains micron-sized channels with an average pore size of D3. Among them, D1>D2>D3.

[0007] Furthermore, the total thickness of the porous electrode layer is 50~2000 μm, and the areal loading is 3~50 mg / cm². 2 The thickness ratio of the large channel layer, transition channel layer, and small channel layer is adjusted according to the total thickness of the electrode and the target rate performance.

[0008] Furthermore, D1 is 50~200 μm; D2 is 10~80 μm; D3 is 1~20 μm; and the channels between adjacent layers are interconnected.

[0009] Furthermore, the channel volume fraction (porosity) of the large channel layer is higher than that of the transition channel layer and the small channel layer, with the small channel layer having the lowest channel volume fraction. This results in a gradient increase in the overall density of the porous electrode layer from the electrolyte side to the current collector side, thereby forming a dense conductive and bonding network at the current collector interface to provide the largest specific surface area and the strongest mechanical anchoring capability on the side closer to the current collector.

[0010] In this invention, the large channel layer serves as a "high-speed entrance" for ions, with a maximum pore size of 50-200 μm. This design aims to sacrifice some volumetric density in exchange for extremely high liquid phase permeability, thereby significantly reducing entrance resistance. The transition channel layer is configured with medium-sized pores of 10-80 μm, serving as a "diversion hub" for transport, used to balance ion transport rate and active material loading. The small channel layer is configured with minimum-sized pores of 1-20 μm, serving as an "electron collection station" and a "mechanical anchoring point." Here, since the ion flux requirement has decreased, the high-density carbon framework can provide the maximum specific surface area to store charge and provide the strongest mechanical adhesion.

[0011] Furthermore, the porous electrode layer is composed of one or more of graphene or graphene-type porous carbon materials, such as laser-induced graphene, chemical vapor deposition graphene, reduced graphene oxide, and graphitized carbon.

[0012] Furthermore, the geometry of the micron-scale channel includes one or more of the following: spherical pores, tubular channels, slit-type channels, and irregular interconnected networks; the inner wall of the micron-scale channel is composed of few-layer graphene, multi-layer graphene, or amorphous carbon.

[0013] Furthermore, the construction methods of the micron-level channels include, but are not limited to, directly forming a gradient channel structure in the thickness direction by changing the channel generation conditions, or preparing graphene porous membranes with different micron-level channels and then stacking them by lamination, bonding or solvent-assisted interface fusion to form a gradient channel structure in the thickness direction.

[0014] Furthermore, the methods for regulating the channel generation conditions include, but are not limited to, changing the amount of foaming agent and foaming conditions, changing the concentration of etchant and etching time, changing the laser scanning parameters, and using one or more of spatially selective freeze drying or phase separation.

[0015] In some specific embodiments, the micron-scale channels are prepared using a sacrificial template pore-forming process. This involves introducing template agents of different particle sizes and amounts into a carbon precursor, followed by film formation, drying, and carbonization, after which the template agents are removed, resulting in a micron-scale channel network with a gradient distribution of average channel size and channel volume fraction along the thickness direction. The carbon precursor is selected from one or more aromatic polymers or carbonizable resins such as phenolic resin, polyethersulfone, polyimide, polyamic acid, polyacrylonitrile, epoxy resin, lignin, and asphalt. The template agent is selected from one or more materials that can generate gas through thermal decomposition or be removed by solvent cleaning, such as polyethylene terephthalate, polymethyl methacrylate, polystyrene, urea-formaldehyde resin, ammonium chloride, ammonium bicarbonate, and calcium carbonate. The carbonization process includes, but is not limited to, one or more of laser-induced carbonization, flash Joule heat treatment, and high-temperature carbonization in a tube furnace. Carbonization is used to pyrolyze carbon precursors into conductive carbon skeletons. At the same time, thermal energy is used to decompose, vaporize or sublimate template agents in situ, thereby leaving pore structures in the conductive carbon skeletons that correspond to the geometric characteristics of the template agents.

[0016] Furthermore, the current collector includes, but is not limited to, one or more of the following: copper foil, aluminum foil, nickel foil, titanium foil, stainless steel foil, metal mesh, nickel foam, and carbon paper.

[0017] In this invention, the graphene thick electrode can be directly fabricated in situ on the surface of the current collector to form an electrode layer; alternatively, the graphene thick electrode layer can be first fabricated on a supporting substrate, and then peeled off from the supporting substrate to obtain a self-supporting electrode. Because the graphene thick electrode layer has good conductivity and structural integrity, the peeled self-supporting electrode can be used in some applications without an additional metal current collector, or in combination with a current collector as needed.

[0018] A second objective of this invention is to provide the application of the graphene thick electrode with the gradient channel structure in electrochemical energy storage devices. The graphene thick electrode exhibits a significant micron-scale channel gradient structure in the thickness direction, enabling it to provide high porosity and good channel connectivity under high areal loading conditions, significantly reducing thickness-direction ion transport impedance, and improving the thickness utilization rate of the active material inside the thick electrode.

[0019] Furthermore, the electrochemical energy storage device includes supercapacitors, lithium-ion capacitors, lithium-ion batteries, sodium-ion batteries, and hybrid-ion capacitors. This electrochemical energy storage device can fully utilize the rapid ion transport channels and effective working thickness provided by the gradient micron-level channel structure, while also possessing high specific capacitance / specific capacity, excellent rate performance, and good cycle stability.

[0020] A third objective of this invention is to provide an electrochemical energy storage device, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode and / or the negative electrode employs the graphene thick electrode with a gradient channel structure.

[0021] The design principle of the graphene thick electrode described in this invention is as follows: (1) The present invention constructs a porous graphene electrode layer on the current collector and divides the electrode layer into structural partitions in the thickness direction, so that a large channel layer with a larger average pore size is formed on the side away from the current collector, a transition channel layer with a medium average pore size is formed in the middle, and a small channel layer with a smaller average pore size is formed on the side close to the current collector. (2) This invention achieves a gradient channel structure in the thickness direction of the large channel layer, transition channel layer and small channel layer by controlling the pore size, porosity and layer thickness, thereby obtaining a graphene thick electrode with a gradient micron-level channel distribution in the thickness direction. (3) The present invention can perform post-processing on the graphene thick electrode as needed to optimize conductivity, pore structure and mechanical stability; the post-processing includes, but is not limited to, annealing or secondary carbonization in an inert atmosphere, secondary laser scanning of laser-induced graphene, chemical reduction or doping modification of reduced graphene oxide, hot pressing or rolling of porous electrode layer under a certain temperature and / or pressure, and one or more of the interface reinforcement treatment of adhesive impregnation-drying / curing, so that each layer of graphene skeleton forms a continuous conductive network in the thickness direction and optimizes its mechanical stability and interface bonding strength.

[0022] The beneficial effects of this invention are as follows: The internal average pore size of the large channel layer, transition channel layer and small channel layer designed in the thickness direction of this invention satisfies the mathematical relationship D1>D2>D3. This structural design physically optimizes the effective diffusion coefficient and Thiele modulus of the porous medium, making the reaction rate distribution of the entire electrode more uniform in the thickness direction. It solves the problem of low bottom utilization of traditional thick electrodes, and enables the electrode to simultaneously achieve rapid ion transport and high active material loading within a limited thickness. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the graphene thick electrode prepared in Example 1; Figure 2 The image shows a surface scanning electron microscope (SEM) image of the graphene thick electrode prepared in Example 1. Figure 3 A cross-sectional SEM image of the graphene thick electrode prepared in Example 1; Figure 4 The galvanostatic charge-discharge (GCD) curves of the three-electrode system using the graphene thick electrode prepared in Example 1 as the working electrode at different current densities are shown. Figure 5 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Example 2 as the working electrode at different current densities are shown. Figure 6 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Example 3 as the working electrode at different current densities are shown. Figure 7 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Comparative Example 1 as the working electrode at different current densities are shown. Figure 8 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Comparative Example 2 as the working electrode are shown at different current densities. Figure 9 The Nyquist plots of electrochemical impedance spectroscopy for three-electrode systems using graphene thick electrodes prepared in Example 1, Comparative Example 1, and Comparative Example 2 as working electrodes are shown. Figure 10 The GCD curves of the two-electrode energy storage device using the graphene thick electrode prepared in Example 1 as the positive electrode are shown at different current densities. Figure 11 The GCD curves of the two-electrode energy storage device with the graphene thick electrode as the positive electrode prepared in Comparative Example 1 are shown at different current densities. Figure 12 The Nyquist plots are electrochemical impedance spectroscopy spectra of two-electrode energy storage devices using graphene thick electrodes prepared in Example 1 and Comparative Example 1 as positive electrodes, respectively. Detailed Implementation

[0024] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments and illustrations.

[0025] Example 1 In this embodiment, a graphene thick electrode with a total thickness of 450 μm was constructed. The micron-level channel structure design in the thickness direction follows the positive gradient principle (D1>D2>D3).

[0026] First, a small channel layer with a thickness of 150 μm and an average internal pore size of 5 μm is constructed on the surface of the copper foil current collector. Then, a transition channel layer with a thickness of 150 μm and an average internal pore size of 50 μm is constructed on the small channel layer. Finally, a large channel layer with a thickness of 150 μm and an average internal pore size of 100 μm is constructed on the transition channel layer. The specific preparation steps are as follows: (1) Weigh 4 g of polyethersulfone (PES) powder and dissolve it in 16 mL of N-methylpyrrolidone (NMP). Stir at 800 r / min for 5 h to obtain a uniform and transparent PES film-forming solution.

[0027] (2) Polyethylene terephthalate (PET) particles with average particle sizes of 5 μm, 50 μm and 100 μm were added to the above PES film-forming solution respectively. The amount of PET was 5 wt%, 5 wt% and 10 wt% of the mass of PES respectively. The PET particles were stirred to disperse the PET particles evenly, and slurry C, slurry B and slurry A were obtained respectively for constructing small channel layer, transition channel layer and large channel layer.

[0028] (3) Preparation of the first small channel layer Slurry C was spin-coated onto a 50 μm thick copper foil current collector at a spin speed of 400 r / min for 40 s. The resulting wet film was dried at 80℃ for 2 h, then at 200℃ for 2 h, resulting in a dry film thickness of 50 μm. The dried film was then exposed to a CO2 laser beam for laser-induced carbonization treatment at a laser power of 8 W, a scanning speed of 100 mm / s, and a line spacing of 0.1 mm. During the laser scanning process, the PES precursor was transformed into a laser-induced graphene (LIG) porous carbon framework under localized high temperatures. Simultaneously, the template agent PET underwent pyrolysis and vaporization, resulting in template agent removal and the formation of micron-sized channels with an average pore size of 5 μm, thus obtaining the first small-channel layer.

[0029] (4) Preparation of the second and third small channel layers Step (3) is repeated twice on the surface of the first small channel layer prepared in step (3) to obtain a three-layer superimposed small channel layer with a total thickness of 150 μm, which serves as the support framework for the thick electrode.

[0030] (5) Preparation of the fourth to sixth transition channel layers Following the "spin-coating-drying-laser carbonization" process in step (3), a micron-scale transition channel layer with a thickness of 50 μm and an average pore size of 50 μm is constructed on the surface of the three-layer small channel layer prepared in step (4) using slurry B. Three layers are then spin-coated sequentially to form a transition channel layer with a thickness of 150 μm, providing a smooth transition for ion transport from the small channel layer to the large channel layer.

[0031] (6) Preparation of the seventh to ninth large channel layers Following the "spin-coating-drying-laser carbonization" process in step (3), a micron-sized large channel layer with a thickness of 50 μm and an average pore size of 100 μm is constructed on the surface of the three-layer transition channel layer prepared in step (5) using slurry A. Three layers are then spin-coated sequentially to form a large channel layer with a thickness of 150 μm, which facilitates rapid electrolyte wetting and provides a wide entry point for ions to enter the thick electrode.

[0032] Figure 1 This is a schematic diagram of the structure of the graphene thick electrode prepared in Example 1. Figure 1 It can be seen that the thick electrode is a porous graphene bulk material, containing three types of channel structures: macropores, mesopores, and micropores. Several large-diameter circular pores are arranged near the upper surface, representing the macrochannel layer; medium-diameter pores are embedded in the middle of the electrode, corresponding to the transition channel layer; and the lower part of the electrode is mainly composed of a microchannel layer consisting of densely packed fine pores. The overall pore size gradually decreases from top to bottom along the thickness direction, while the framework density gradually increases, forming a typical gradient micron-scale channel network.

[0033] Figure 2 This is a surface SEM image of the graphene thick electrode prepared in Example 1. From... Figure 2 As can be seen, the electrode surface exhibits a typical three-dimensional porous undulating structure. In addition to nano / micron-sized fine pores, macropores / channels on the order of 100 μm formed after the removal of the template agent PET can also be observed. This facilitates rapid electrolyte wetting and ion transport on the electrode surface, providing a rich interface for subsequent electrochemical reactions.

[0034] Figure 3 This is a cross-sectional SEM image of the graphene thick electrode prepared in Example 1. From... Figure 3 It can be seen that the total thickness of the electrode is approximately 450 μm, indicating a dense connection.

[0035] The total areal loading of the graphene thick electrode prepared in Example 1 was determined using the mass difference method. The method was as follows: The active layer of the graphene thick electrode, along with a copper foil current collector measuring 2 cm × 2 cm × 50 μm, was weighed; the total mass was 0.217 g. Then, the graphene was completely scraped off from the copper foil surface using a scraper, and the copper foil was weighed; the mass was 0.179 g. The difference between the two weighings was 0.038 g, which is the mass of the active layer of the graphene thick electrode. The calculated total areal loading of the graphene thick electrode was 9.55 mg / cm². 2 .

[0036] Example 2 The graphene thick electrode was prepared according to the method of Example 1, except that the average particle size of the PET particles added when preparing slurry C, slurry B and slurry A was adjusted to 20 μm, 80 μm and 200 μm, respectively.

[0037] Example 3 In this embodiment, a graphene thick electrode with a total thickness of 450 μm was constructed. The micron-level channel structure design in the thickness direction follows the positive gradient principle (D1>D2>D3).

[0038] First, a small channel layer with a thickness of 150 μm and an average internal pore size of 5 μm is constructed. Then, a transition channel layer with a thickness of 150 μm and an average internal pore size of 50 μm is constructed on the small channel layer. Finally, a large channel layer with a thickness of 150 μm and an average internal pore size of 100 μm is constructed on the transition channel layer. The specific preparation steps are as follows: (1) Weigh 4 g of polyethersulfone (PES) powder and dissolve it in 16 mL of NMP. Stir at 800 r / min for 5 h to obtain a uniform and transparent PES film-forming solution.

[0039] (2) PET particles with average particle sizes of 5 μm, 50 μm and 100 μm were added to the above PES film-forming solution respectively. The amount of PET was 5 wt%, 5 wt% and 10 wt% of the mass of PES respectively. The PET particles were stirred to disperse the PET particles evenly, and slurry C, slurry B and slurry A were obtained respectively for constructing small channel layer, transition channel layer and large channel layer.

[0040] (3) Preparation of the first small channel layer precursor membrane A flat glass plate was selected as the film-forming substrate, and a release layer was applied to the surface of the glass plate (e.g., a PTFE release film was laid, or a water-soluble PVA release layer was coated and dried on the surface of the glass plate for subsequent overall peeling). Slurry C was applied to the released glass substrate using a spin coating method at a speed of 400 r / min for 40 s. The resulting wet film was dried at 80 ℃ for 2 h, and then dried at 200 ℃ for 2 h to obtain the first small channel layer precursor film with a dry film thickness of 50 μm.

[0041] (4) Preparation of the second and third small channel layer precursor membranes Step (3) is repeated twice on the surface of the first small channel layer precursor film obtained in step (3) to obtain a three-layer superimposed small channel layer precursor film with a total thickness of 150 μm, which serves as the carrier skeleton precursor of the thick electrode.

[0042] (5) Preparation of precursor membranes for the fourth to sixth transition channel layers Following the spin-coating-drying process in step (3), a transition channel layer precursor film with a thickness of 50 μm is constructed on the surface of the three-layer small channel layer precursor film prepared in step (4) using slurry B. Three layers are then spin-coated sequentially to form a transition channel layer precursor film with a thickness of 150 μm, providing a smooth transition for ion transport from the small channel layer to the large channel layer.

[0043] (6) Preparation and overall peeling of precursor membranes for the seventh to ninth large channel layers Following the spin-coating-drying process in step (3), a large channel layer precursor film with a thickness of 50 μm is constructed on the surface of the three-layer transition channel layer precursor film prepared in step (5) using slurry A. Three layers are then spin-coated sequentially to form a large channel layer precursor film with a thickness of 150 μm. This yields a PES@PET monolithic precursor film with a total thickness of 450 μm and a gradient PET template distribution of 5 / 50 / 100 μm. Subsequently, this monolithic precursor film is completely peeled off from the release substrate to obtain a self-supporting PES@PET gradient channel precursor film.

[0044] (7) Flash joule thermal carbonization treatment The self-supporting precursor film obtained in step (6) was placed in a nitrogen-protected sealed space, and flash Joule heat treatment was performed on it using carbon paper as the resistance heating and thermal coupling medium: the precursor film was sandwiched between two layers of carbon paper to form a "carbon paper / precursor film / carbon paper" sandwich structure, and an insulating high-temperature resistant pressure plate was used to hold it on the outside of the sandwich to ensure uniform heat transfer and suppress warping; the carbon paper was energized to generate Joule heat in a short time, and the heat was quickly transferred to the precursor film to achieve rapid carbonization. The flash time was 0.5 s, and the number of flashes was 1. During the flash process, the PES precursor rapidly carbonized and formed a graphene-type porous carbon skeleton, and the template agent PET decomposed and volatilized, thereby forming a micron-level gradient channel structure (D1>D2>D3) with a thickness decreasing from large to small along the thickness direction, resulting in a gradient channel graphene thick electrode with a total thickness of 450 μm.

[0045] Comparative Example 1 Graphene thick electrodes were prepared according to the method of Example 1, except that no template agent was added when preparing the slurry, resulting in dense graphene thick electrodes.

[0046] Comparative Example 2 The graphene thick electrode was prepared according to the method of Example 1, except that the design of the micron-level channel structure in the thickness direction of the electrode follows the reverse gradient principle (D1 < D2 < D3). That is, the top layer of the electrode (the side in contact with the electrolyte) is a small channel layer with an average pore size of 5 μm, the middle layer is a transition channel layer with an average pore size of 50 μm, and the bottom layer (the side in contact with the current collector) is a large channel layer with an average pore size of 100 μm.

[0047] Electrochemical performance testing of a three-electrode system was performed: Using the graphene thick electrodes prepared in Examples 1, 1, and 2 as working electrodes, platinum sheets as counter electrodes, Ag / AgCl electrodes as reference electrodes, and zinc sulfate solution (1 mol / L) as electrolytes, three-electrode systems were assembled. Constant current charge-discharge and electrochemical impedance spectroscopy tests were performed on the three-electrode systems on an electrochemical workstation.

[0048] Figure 4 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Example 1 as the working electrode are shown at different current densities. Figure 4 It can be seen that in the current density range of 0.5~10 mA / cm 2 Within the specified range, the GCD curve exhibits a nearly symmetrical isosceles triangle shape with a flat voltage plateau, indicating that the thick electrode possesses low ohmic polarization and good capacitance characteristics. Based on discharge times at different current densities, the areal capacitance of the electrode at a current density of 0.5 mA / cm² is calculated to be 149.1 mF / cm². 2 When the current density increases to 10 mA / cm2 At that time, the area capacitance of the electrode can still maintain more than 71.7% of the initial value, which shows that under high areal load conditions, the porous thick electrode still has excellent rate performance and thickness utilization.

[0049] Figure 5 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Example 2 as the working electrode are shown at different current densities. Figure 5 It can be seen that in the current density range of 0.5~10 mA / cm 2 Within the specified range, the GCD curve exhibits a nearly symmetrical isosceles triangle shape with a flat voltage plateau, indicating that the thick electrode possesses low ohmic polarization and good capacitance characteristics. Based on the discharge time calculations at different current densities, at a current density of 0.5 mA / cm²... 2 At that time, the area capacitance of the electrode was 129.2 mF / cm². 2 When the current density increases to 10 mA / cm 2 At that time, the area capacitance of the electrode can still maintain more than 78% of the initial value, indicating that the thick electrode prepared in Example 2 still has good rate performance and thickness utilization under high areal loading conditions.

[0050] Figure 6 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Example 3 as the working electrode are shown at different current densities. Figure 6 It can be seen that in the current density range of 0.5~10 mA / cm 2 Within the specified range, the GCD curve exhibits a nearly symmetrical isosceles triangle shape, indicating good reversibility of the charge-discharge process. This suggests that the thick electrode possesses low polarization and good capacitive behavior. Based on the discharge time calculations at different current densities, the discharge time at a current density of 0.5 mA / cm² was [data missing]. 2 At that time, the area capacitance of the electrode was 135.4 mF / cm². 2 When the current density increases to 10 mA / cm 2 At that time, the area capacitance of the electrode could still maintain more than 74.3% of the initial value, indicating that the thick electrode prepared in Example 3 could also maintain a high usable capacity / specific capacitance under high rate conditions.

[0051] Figure 7 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Comparative Example 1 as the working electrode are shown at different current densities. Figure 7 It can be seen that in the current density range of 0.5~10 mA / cm 2Within this range, the GCD curve generally maintains a nearly symmetrical isosceles triangle shape, but the discharge time shortens significantly with increasing current density. Based on the discharge time calculations at different current densities, the discharge time at a current density of 0.5 mA / cm² is [data missing]. 2 At that time, the area capacitance of the electrode was 201.6 mF / cm². 2 When the current density increases to 10 mA / cm 2 At that time, the areal capacitance of the electrode dropped to 70.0 mF / cm², and the capacitance retention rate was only 34.7%, significantly lower than that of the graphene thick electrode prepared in Example 1. This indicates that without the introduction of a gradient channel structure, the thickness utilization and rate performance of the battery are significantly limited.

[0052] Figure 8 The GCD curves of the three-electrode system using the graphene thick electrode prepared in Comparative Example 2 as the working electrode are shown at different current densities. Figure 8 It can be seen that in the current density range of 0.5~10 mA / cm 2 Within the specified range, the GCD curve exhibits a nearly symmetrical isosceles triangle shape with a flat voltage plateau, indicating that the thick electrode possesses low ohmic polarization and good capacitance characteristics. Based on the discharge time calculations at different current densities, at a current density of 0.5 mA / cm²... 2 At that time, the area capacitance of the electrode was 147.5 mF / cm². 2 When the current density increases to 10 mA / cm 2 At that time, the area specific capacitance of the electrode was 52.2% of the initial value. This indicates that the graphene thick electrode with the anti-gradient channel structure prepared in Comparative Example 2 exhibits significantly inferior rate performance compared to Example 1 under the same areal loading.

[0053] Figure 9 The Nyquist plots of the electrochemical impedance spectroscopy (EIS) of the three-electrode systems using the graphene thick electrodes prepared in Example 1, Comparative Example 1, and Comparative Example 2 as working electrodes are shown. Figure 9 It can be seen that the high-frequency intercepts (Rs) of these three electrodes are similar, indicating that the electrolyte resistance and current collector contact resistance are not significantly different. The mid-to-high frequency semicircle diameters show that the charge transfer resistance (Rct) of these three electrodes is relatively small, with Example 1 and Comparative Example 2 being slightly lower than Comparative Example 1. In the low-frequency region, the curve of Example 1 is closer to the imaginary axis, with a larger slope and shorter extension length, indicating that the gradient channel structure significantly reduces the thickness-to-thickness ion diffusion impedance. The low-frequency curve morphology and slope of Comparative Example 2 are between those of Example 1 and Comparative Example 1. The curve of Comparative Example 1 deviates significantly from the imaginary axis in the low-frequency region and exhibits a long tailing characteristic, indicating that ion transport inside the dense, thick electrode without micron-level channels is severely restricted. The above trends are consistent with the GCD test results of the specific capacitance decay of the three electrodes at high current densities.

[0054] Assemble a two-electrode energy storage device and conduct electrochemical performance testing: Two-electrode energy storage devices were constructed using graphene thick electrodes prepared in Example 1 and Comparative Example 1 as positive electrodes, commercial zinc foil as negative electrodes, and zinc sulfate solution (1 mol / L) as electrolyte. Constant current charge-discharge and electrochemical impedance spectroscopy tests were performed on the two-electrode energy storage devices on an electrochemical workstation.

[0055] Figure 10 The GCD curves of a two-electrode energy storage device using the graphene thick electrode prepared in Example 1 as the positive electrode are shown at different current densities. Figure 10 It can be seen that at 0.5 mA / cm 2 At low current densities, the device's charge-discharge curves approximate symmetrical isosceles triangles. Based on the discharge time calculations at different current densities, it was found that at a current density of 0.5 mA / cm²... 2 At that time, the areal capacitance of the device was 252.2 mF / cm². 2 When the current density increases to 10 mA / cm 2 At that time, the areal capacitance of the device was 178.8 mF / cm². 2 The specific capacitance retention rate is 70.9%. This shows that, under high areal loading conditions, the two-electrode energy storage device using the graphene thick electrode prepared in Example 1 as the positive electrode still has excellent rate performance and high thickness utilization.

[0056] Figure 11 The GCD curves of the two-electrode energy storage device using a thick graphene electrode as the positive electrode, prepared in Comparative Example 1, are shown at different current densities. Figure 11 It can be seen that at 0.5 mA / cm 2 At low current densities, the device's charge-discharge curves generally maintain an isosceles triangular shape; when the current density increases to 10 mA / cm², the curves change. 2 At this time, the discharge time is significantly shortened, the curve is compressed to a greater extent, and a more obvious rate performance degradation is observed. Based on the discharge time calculations at different current densities, at 0.5 mA / cm²... 2 At that time, the areal capacitance of the device was 337 mF / cm². 2 When the current density increases to 10 mA / cm 2 At that time, the area specific capacitance of the device dropped to 115 mF / cm², and the specific capacitance retention rate was only 34.1%, which was significantly lower than that of the two-electrode energy storage device with graphene thick electrode as positive electrode prepared in Example 1. This further illustrates that under the same areal load conditions, the thickness utilization rate and rate performance of the two-electrode energy storage device with dense graphene thick electrode as positive electrode are significantly limited.

[0057] Figure 12The Nyquist plots of the electrochemical impedance spectroscopy (EIS) of two-electrode energy storage devices using graphene thick electrodes prepared in Example 1 and Comparative Example 1 as positive electrodes are shown. Figure 12 It can be seen that the two-electrode energy storage device with graphene thick electrode as positive electrode prepared in Example 1 has a curve closer to the imaginary axis and a shorter diffusion tail in the low frequency region, which indicates that the gradient channel structure significantly reduces the thickness-to-thickness ion diffusion impedance; while the two-electrode energy storage device with graphene thick electrode as positive electrode prepared in Comparative Example 1 has a longer curve tail, indicating that the internal ion transport is more severely restricted.

[0058] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A graphene thick electrode with a gradient channel structure, comprising a current collector and a porous electrode layer bonded to the surface of the current collector, characterized in that: The porous electrode layer has a non-uniform pore structure in the thickness direction, and is divided into the following sections from the side away from the current collector to the side closer to the current collector: The large channel layer contains micron-sized channels with an average pore size of D1. The transition channel layer contains micron-sized channels with an average pore size of D2. The small channel layer contains micron-sized channels with an average pore size of D3. Among them, D1>D2>D3.

2. The graphene thick electrode with a gradient channel structure according to claim 1, characterized in that: The D1 is 50~200 μm; the D2 is 10~80 μm; the D3 is 1~20 μm; Preferably, the channel volume fraction of the large channel layer is higher than that of the transition channel layer and the small channel layer, and the channel volume fraction of the small channel layer is the lowest. Preferably, the total thickness of the porous electrode layer is 50~2000 μm, and the areal loading is 3~50 mg / cm². 2 .

3. The graphene thick electrode with a gradient channel structure according to claim 1, characterized in that: The porous electrode layer is composed of one or more of laser-induced graphene, chemical vapor deposition graphene, reduced graphene oxide, and graphitized carbon. Preferably, the current collector is one or more of copper foil, aluminum foil, nickel foil, titanium foil, stainless steel foil, metal mesh, nickel foam, and carbon paper.

4. The graphene thick electrode with a gradient channel structure according to claim 1, characterized in that: The geometry of the micron-scale channel includes one or more of the following: spherical pores, tubular channels, slit-type channels, and irregular interconnected networks; the inner wall of the micron-scale channel is composed of few-layer graphene, multi-layer graphene, or amorphous carbon.

5. The graphene thick electrode with a gradient channel structure according to claim 1, characterized in that: The micron-level channels are constructed by either directly forming a gradient channel structure in the thickness direction by changing the channel generation conditions, or by preparing graphene porous membranes with different micron-level channels and then stacking and assembling them through lamination, bonding, or solvent-assisted interface fusion to form a gradient channel structure in the thickness direction.

6. The graphene thick electrode with a gradient channel structure according to claim 5, characterized in that: The channel generation conditions are controlled by changing the amount of foaming agent and foaming conditions, changing the concentration of etchant and etching time, changing the laser scanning parameters, or using one or more of the following: spatial selective freeze drying or phase separation.

7. The graphene thick electrode with a gradient channel structure according to claim 1, characterized in that: The micron-scale channels are prepared by a sacrificial template pore-forming process, in which template agents with different particle sizes and amounts are introduced into the carbon precursor, and the template agents are removed after film formation, drying and carbonization, forming a micron-scale channel network with a gradient distribution of average channel size and channel volume fraction in the thickness direction. Preferably, the carbon precursor is selected from one or more of phenolic resin, polyethersulfone, polyimide, polyamic acid, polyacrylonitrile, epoxy resin, lignin, and pitch. Preferably, the template agent is selected from one or more of polyethylene terephthalate, polymethyl methacrylate, polystyrene, urea-formaldehyde resin, ammonium chloride, ammonium bicarbonate, and calcium carbonate.

8. The application of the graphene thick electrode with gradient channel structure according to any one of claims 1 to 7 in electrochemical energy storage devices.

9. The application according to claim 8, characterized in that: The electrochemical energy storage devices include supercapacitors, lithium-ion capacitors, lithium-ion batteries, sodium-ion batteries, and hybrid-ion capacitors.

10. An electrochemical energy storage device, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode and / or the negative electrode is a graphene thick electrode with a gradient channel structure as described in any one of claims 1 to 7.

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