Solvent-free preparation method and application of gradient porosity electrode

Abstract

The invention belongs to the technical field of chemical power sources, and relates to a solvent-free preparation method and application of a gradient porosity electrode, and the preparation method comprises the following steps: firstly, mixing an active material, a conductive agent, a binder and a pore-forming additive to obtain a raw material mixture with different pore-forming additive contents, and pressing to obtain electrode plates with different porosities; and then, according to a porosity stepped change sequence, stacking and thermally rolling a plurality of electrode plates, and combining with a current collector to obtain the gradient porosity electrode. Compared with the prior art, the porosity of the electrode can be efficiently and accurately controlled, and gradient distribution is formed, so that the performance of the electrode in energy storage equipment such as a lithium ion battery and a supercapacitor is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical power sources, and relates to a solvent-free preparation method and application of a gradient porosity electrode, in particular to a preparation method and application of a lithium-ion battery electrode, and specifically to a solvent-free preparation method and application of a gradient porosity electrode of a lithium-ion battery through multi-layer composite. Background Art

[0002] Lithium-ion batteries have become an indispensable mainstream technology in the energy storage field due to their high energy density, excellent charge and discharge efficiency, and long cycle life. However, with the rapid expansion of the new energy vehicle market and the rapid growth of the demand for renewable energy storage, the existing lithium-ion battery technology is facing a series of severe challenges, including the increasing cost pressure, increasingly stringent environmental protection regulations, and the urgent need for high-performance indicators. These issues have put forward higher requirements for the manufacturing process and design method of lithium batteries. The design of gradient porosity electrodes is a key technology to promote the performance improvement of lithium batteries. Traditional electrodes with uniform porosity show significant bottlenecks in ion and electron transport in the application of thick electrodes, while gradient porosity can effectively solve this problem by optimizing the porosity distribution. Its design concept is to design a low porosity near the current collector to improve conductivity according to the transport requirements, and a high porosity near the electrolyte to promote electrolyte infiltration and ion transport. This hierarchical structure design of gradient pores has the following advantages: 1. Optimize the transport path: By adjusting the length and efficiency of the electron and ion transport paths, the gradient design can significantly reduce the internal resistance and concentration polarization, and improve the rate performance and cycle stability. 2. Improve the energy density: The gradient porosity structure can make the reaction inside the electrode more uniform by efficiently using the active material, thus improving the overall energy density of the battery. 3. Adapt to various material systems: This design method is not limited to a specific material system, has strong universality, and can be widely applied to positive and negative electrode materials with different chemical compositions and structures.

[0003] Currently, the wet coating and lamination preparation method is usually adopted to prepare the gradient porosity electrode. For example, Chinese Patent Application CN116632164A discloses a preparation method for preparing a gradient porosity electrode by means of lamination coating. Among them, the first layer of the electrode needs to be coated on the current collector. After drying and curing, the second coating is carried out. By controlling the porosity of the electrodes coated twice, the gradient porosity electrode is prepared. However, the method adopted in the patent has a complex preparation process for the electrode and high energy consumption, which is not conducive to actual commercial production because the next coating operation can only be carried out after layer-by-layer cooling and curing. In addition, Chinese Patent CN111725479B discloses a lithium-ion battery electrode sheet with gradient porosity and its preparation method, which includes: (1) according to the designed number of electrode layers, mixing different contents of powder pore-forming agents with active materials, conductive agents, and polymer binders to obtain multiple portions of electrode slurries with different pore-forming agent contents; (2) coating the electrode slurry with the lowest pore-forming agent content onto the current collector, drying and then rolling and compacting it for the first time with a roller to obtain the first coating; (3) sequentially coating the electrode slurries on the surface of the first coating in the order of increasing pore-forming agent content from low to high; (4) heating and drying the electrode sheet to decompose and escape the pore-forming agent, forming gradient pores in the coating; (5) carrying out the second rolling of the dried multi-layer structured electrode sheet with a patterned roller with a continuous hemispherical bump structure. Although the related process can realize the preparation of the gradient porosity electrode, the wet coating process adopted requires the use of a large amount of organic solvents such as N-methylpyrrolidone (NMP), etc. This not only increases the complexity of the production process but also has a non-negligible impact on the environment. In addition, the energy consumption of the wet process is mainly concentrated in the electrode drying and solvent recovery links, resulting in high production costs. Especially when the market demand for low-cost and high-efficiency energy storage is gradually emerging, the limitations of the traditional wet coating process become more prominent. In terms of performance, with the important trend of thick electrodes to improve the energy density of batteries, the wet coating technology is also facing severe challenges. The thick electrode design can increase the mass fraction of the active material, thereby improving the energy density per unit volume. However, the thickness that can be prepared for the electrode by wet coating is greatly limited. This is because during the drying process, due to the different shrinkage degrees of the solid phase and the liquid phase in the mixed slurry, too high a loading amount will cause phenomena such as powdering, cracking, and even peeling of the electrode. This bottleneck is particularly prominent in high-power and long-life applications and has become the core problem restricting the development of lithium battery technology.

[0004] Therefore, there is an urgent need to propose a new preparation method for electrodes that does not rely on solvents and has high charge and discharge capabilities. Summary of the Invention

[0005] The object of the present invention is to provide a solvent-free preparation method and application of a gradient porosity electrode for a lithium-ion battery by multi-layer composite. By combining the solvent-free preparation technology with the gradient porosity design, the ion transport resistance and concentration polarization problems in thick electrodes are effectively alleviated. By controlling the content of the pore-forming additive and the solvent-free preparation process parameters, the accurate control of the electrode porosity and the construction of the gradient porosity structure are realized, effectively improving the battery performance such as the discharge capacity of the half-cell, laying a solid foundation for the production of high-performance lithium-ion batteries. At the same time, based on the solvent-free technology, the present invention is environmentally friendly and efficient, and is suitable for the industrial production of lithium-ion batteries.

[0006] The object of the present invention can be achieved by the following technical solutions:

[0007] The first aspect of the present invention provides a solvent-free preparation method of a gradient porosity electrode, comprising the following steps:

[0008] S1: Mix the active material, conductive agent, binder and pore-forming additive to obtain a raw material mixture with different contents of pore-forming additive, and press to obtain electrode sheets with different porosities;

[0009] S2: Stack and press multiple electrode sheets in the order of stepwise change of porosity, and then combine with the current collector to obtain a gradient porosity electrode.

[0010] Based on the solvent-free preparation technology, the present invention proposes a preparation method of a multi-layer composite gradient porosity electrode, which gets rid of the dependence on toxic solvents and reduces the energy consumption in production preparation. The present invention not only meets the dual requirements of environmental protection and economy, but also provides a greater space for the flexibility and adaptability of the manufacturing process, laying a solid foundation for the production of high-performance lithium-ion batteries.

[0011] In some specific embodiments, in step S1, the active material includes a positive electrode active material and a negative electrode active material,

[0012] The positive electrode active material is selected from at least one of carbon fluoride (CF), lithium iron phosphate (LFP), lithium cobalt oxide (LCO) or lithium nickel cobalt manganese ternary material (NCM);

[0013] The negative electrode active material is selected from at least one of graphite, silicon, hard carbon or silicon carbide.

[0014] In some specific embodiments, the conductive agent is selected from carbon black or carbon nanotubes; preferably, the carbon nanotubes are selected from single-walled carbon nanotubes or chemical vapor deposition carbon nanotubes.

[0015] In some specific embodiments, the binder is selected from polytetrafluoroethylene (PTFE), polyimide (PI), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), or polyvinylidene fluoride (PVDF).

[0016] In some specific embodiments, the pore-forming additive includes a sacrificial-phase additive or a non-sacrificial-phase additive.

[0017] The sacrificial-phase additive is selected from one of ammonium bicarbonate (NH4HCO3), ammonium carbonate ((NH4)2CO3), or ammonium nitrate (NH4NO3).

[0018] The non-sacrificial-phase additive is selected from active carbon or graphene oxide; generally, different pore-forming additives are selected according to different active materials.

[0019] In some specific embodiments, in the raw material mixture, the mass ratio of the active material, the conductive agent, and the binder is 80-96:2-6:2-5; the content of the pore-forming additive is 2-15 wt%, and generally, the higher the target porosity of the electrode, the higher the content of the pore-forming additive.

[0020] In some specific embodiments, in the mixture composed of the active material, the conductive agent, and the binder, the active material accounts for 90-96% wt, the binder accounts for 2-6% wt, and the conductive agent accounts for 2-5% wt. According to the different target porosities of the electrode, the proportions of each component vary slightly. The higher the target porosity, the generally larger the proportions of the binder and the conductive agent.

[0021] Similarly, the content of the pore-forming additive is adjusted according to the target porosity of the electrode sheet. Generally, the higher the target porosity of the electrode, the higher the content of the pore-forming additive.

[0022] In some specific embodiments, in step S1, the mixing method used includes at least one of mechanical stirring, gas stirring, grinding mixing, or ball milling mixing.

[0023] In some specific embodiments, mechanical shear stirring is used to mix the active material, the conductive agent, the binder, and the pore-forming additive. The binder fibrillates under the shear force generated by high-speed stirring during the mixing process, forming a fibrous structure and adhering and aggregating the powder materials. The fibrillated structure is a fibrous structure generated by a super-high molecular polymer binder when receiving high-speed shear force. Preferably, polytetrafluoroethylene is used as the binder to form polytetrafluoroethylene fibers.

[0024] In some specific embodiments, planetary ball milling is used to mix the active material, the conductive agent, the binder, and the pore-forming additive.

[0025] In some specific embodiments, in step S1, during the pressing, the pressing method includes one of hot pressing, cold pressing, hot roll pressing, cold roll pressing or calendaring; the pressing is carried out by a solvent-free processing method, and during the pressing process, a specific pressure is applied to the powder material by a press to form electrode layers with different target porosities. The magnitude of the pressure, the roll pressing speed, and the temperature are controlled by process parameters, and the specific porosity can be calculated by the ratio of the compaction density to the theoretical density;

[0026] In some preferred embodiments, when hot roll pressing is used, the roll pressing temperature is 150 - 250 °C.

[0027] In some specific embodiments, in step S2, the porosity of the electrode sheet is 10 - 55%, during the stacking process, the difference in porosity between adjacent electrode sheets is 5 - 15%, and the number of stacking layers is 3 - 5 layers. The formed gradient porosity electrode has a pore gradient that, starting from the current collector side, includes an increasing porosity and a decreasing porosity.

[0028] In some specific embodiments, in step S2, during the stacking process, the thicknesses of multiple electrode sheets are all 180 - 220 μm, and the preferred thickness is 200 μm; starting from the current collector side, the porosities of the multiple electrode sheets are 18 - 24%, 28 - 34%, 38 - 42%, 48 - 52% in sequence.

[0029] Correspondingly, in step S1, when multiple electrode sheets are prepared by hot roll pressing, the thicknesses of the roll spacings are 120 - 135 μm, 140 - 155 μm, 160 - 180 μm, 190 - 210 μm respectively; preferably, the thickness ratios of the roll spacings when multiple electrode sheets are prepared are 127, 148, 170, 197 μm respectively.

[0030] In some specific embodiments, in step S2, during the stacking process, the thicknesses of multiple electrode sheets are all 80 - 120 μm, and the preferred thickness is 100 μm; in the obtained gradient porosity electrode, starting from the current collector side, the porosities are 18 - 22%, 33 - 37%, 48 - 52% in sequence.

[0031] Correspondingly, in step S1, when multiple electrode sheets are prepared by hot roll pressing, the roll spacings are 50 - 65 μm, 60 - 75 μm, 75 - 90 μm respectively; further preferably, the roll spacings when multiple electrode sheets are prepared are 57, 65, 80 μm respectively.

[0032] In some specific embodiments, in step S2, during the stacking process, the thicknesses of multiple electrode sheets are all 120 - 170 μm, and the preferred thickness is 150 μm; in the obtained gradient porosity electrode, starting from the current collector side, the porosities are 8 - 12%, 13 - 17%, 18 - 22% in sequence.

[0033] Correspondingly, in step S1, the hot pressing pressures during the hot pressing preparation of multiple electrode sheets are 6.8 - 7.0 MPa, 6.3 - 6.5 MPa, and 4.9 - 5.1 MPa respectively; preferably, the hot pressing pressures during the hot pressing preparation of multiple electrode sheets are 6.9 MPa, 6.4 MPa, and 5 MPa respectively.

[0034] In some specific embodiments, in step S2, during the pressing and forming, the thickness of the pressed product is 1 - 1.5 times that of the electrode sheet.

[0035] In some specific embodiments, in step S2, the pressing and forming adopts hot roll pressing, the roll pressing temperature is 80 - 220 °C, and the roll gap is 0.5 - 1 times the thickness of the electrode sheet.

[0036] In some specific embodiments, in step S2, the pressing and forming adopts hot pressing, the hot pressing temperature is 160 - 200 °C, and the hot pressing pressure is 4 - 6 MPa.

[0037] According to the thickness of the electrode sheet and the target electrode thickness, adjust the roll pressing spacing or the hot pressing pressure and the number of pressing times, and perform one pressing. After pressing, the laminated electrode is completely bonded and there is no obvious delamination.

[0038] In some specific embodiments, the speed of the hot roll is generally 0.2 - 18 revolutions per minute.

[0039] Select different pressing methods and process parameters according to the types of the active material and the binder. Generally, the higher the melting point of the binder, the greater the press pressure, the slower the roll speed, and the higher the roll pressing temperature.

[0040] In some specific embodiments, in step S2, the laminated electrode obtained by hot roll pressing is hot - pressed on the current collector. The equipment used is a roll press, the temperature is 180 - 220 °C, and the roll gap is generally 0.6 times the total thickness of the electrode and the current collector.

[0041] In some specific embodiments, in step S2, the laminated electrode obtained by hot roll pressing is hot - pressed on the current collector. The equipment used is a hot press, the temperature is 180 - 220 °C, and the pressure is 8 - 9 MPa.

[0042] In some specific embodiments, the current collector is selected from copper foil or aluminum foil.

[0043] The second aspect of the present invention provides an application of a gradient porosity electrode, including using the gradient porosity electrode to prepare an electric energy storage device.

[0044] In some specific embodiments, the gradient porosity electrode is used to prepare capacitors such as lithium - ion batteries and supercapacitors.

[0045] Compared with the existing technology, the present invention can efficiently and accurately control the porosity of the electrode and form a gradient distribution, thereby improving the performance of the electrode in energy storage devices such as lithium-ion batteries and supercapacitors. Specifically, it has the following features:

[0046] 1. Gradient Porosity Design Improves Performance: This invention optimizes the overall electrode structure by designing an electrode layer with a gradient porosity. This gradient porosity electrode provides a better ion transport path and higher energy density during battery charge and discharge, significantly improving the battery's cycle performance, power density, and energy efficiency.

[0047] 2. Multi-layer composite structure enhances electrode performance: Using a laminated electrode structure with different porosities, a multi-layered structure is formed by stacking and compounding to improve the mechanical strength and stability of the electrode. During use, this structure helps reduce the stress caused by battery expansion and contraction, extending the battery life.

[0048] 3. Precise porosity control: The present invention precisely controls the porosity of different electrode layers and rationally adjusts the pore structure of the electrode without relying on solvents through the rational formulation of pore-forming additives. This allows the electrode to be designed with the most suitable porosity distribution according to specific needs, thereby optimizing battery performance. 4. Solvent-free and environmentally friendly process: Currently, most commercial electrode preparations are carried out by wet coating, which dissolves the binder in an organic solvent and adds a certain proportion of active materials, conductive agents, etc. to form a uniformly mixed slurry. The slurry is then applied to the current collector metal foil by doctor blade coating, and the corresponding electrode is prepared after drying. In order to form a stable and uniform slurry, NMP is usually used as the organic solvent. Compared with such wet techniques, the solvent-free preparation technology used in the present invention is based on the fiberization effect of PTFE powder binder. During the high-speed stirring and mixing of the powder material, PTFE fiberizes and adheres to the particles of active materials, conductive agents, etc., forming a self-supporting structure of the electrode. This not only avoids the use of solvents and related pollution problems, but also reduces the cost of solvent recovery and treatment. This process significantly improves production efficiency while meeting environmental protection requirements and reducing the impact on the environment.

[0049] 5. Improved conductivity and battery performance: The high-shear stirring process causes the binder to undergo fiberization, forming a fibrous structure that effectively adheres and aggregates the powder material, thereby promoting the conductivity and stability of the electrode. This better connects the electrode active material and the conductive agent, improving the battery's charge and discharge efficiency and cycle life.

[0050] 6. High controllability of electrode performance: The performance of the electrode is affected by the active material loading, and the differences in the active material loading of the electrode mainly come from two aspects: the electrode thickness and the porosity. The solvent-free preparation technology adopted in the present invention can, on the one hand, achieve the preparation of ultra-thick electrodes, and on the other hand, at the same thickness, due to the controllable porosity of the electrode and the self-supporting characteristics of the electrode structure, the areal density is often greater than that of the electrodes prepared by wet coating. Therefore, even at the same thickness, the present invention can achieve a higher discharge capacity than the wet coating process by regulating the porosity. That is, high controllability of the electrode performance is achieved through the free regulation of the electrode thickness and porosity.

[0051] 7. Flexibility in process control: The pressing process and the operating parameters of the laminated composite electrode (such as rolling speed, temperature, pressing pressure, etc.) in the present invention can be finely adjusted according to the requirements of electrode performance. This flexible process control ability makes the method highly adaptable and operable in the manufacture of different types of batteries.

[0052] 8. Enhanced mechanical strength and structural integrity: By adjusting the pressing parameters and the design of multi-layer composite, the mechanical strength of the electrode material after forming is improved. The electrode layers are tightly bonded and there is no obvious delamination, reducing the risk of electrode rupture or damage caused by internal stress, and further enhancing the overall reliability and service life of the battery.

[0053] 9. Applicable to multiple battery types: The method of the present invention is applicable to the electrode materials of multiple lithium-ion batteries, including common positive electrode materials (such as lithium iron phosphate, lithium cobaltate, ternary materials, etc.) and negative electrode materials (such as graphite, silicon-based materials, hard carbon, etc.), and has broad application prospects.

[0054] 10. The preparation method proposed in the present invention can meet the requirement of simultaneously preparing electrode sheets with different porosities. Only one preparation time is required to prepare all the required electrodes, greatly shortening the electrode preparation process and significantly improving the electrode preparation efficiency. Description of the Drawings

[0055] Figure 1 It is the thermogravimetric (TG) analysis curve of the electrode material with a set porosity of 50% obtained in step 3 of Example 1 in the temperature range from room temperature to 150 °C.

[0056] Figure 2 It is the SEM images of the cross-sections of the electrode sheets with different porosities in Example 1. Among them, Figure A is the electrode sheet with a porosity of 20%, Figure B is the electrode sheet with a porosity of 30%, Figure C is the electrode sheet with a porosity of 40%, and Figure D is the electrode sheet with a porosity of 50%.

[0057] Figure 3Discharge performance curves of the multi-layer composite lithium-ion battery gradient porosity electrode prepared in Example 1 and the lithium-ion battery electrode prepared in Comparative Example 1 in a half-cell.

[0058] Figure 4 Comparison of the rate performance of the multi-layer composite lithium-ion battery gradient porosity electrode prepared in Example 2 and the lithium-ion battery electrode prepared in Comparative Example 2 in a half-cell.

[0059] Figure 5 Comparison diagram of the preparation process flows of the multi-layer composite lithium-ion battery gradient porosity electrodes of Example 1 and Comparative Example 3.

[0060] Figure 6 Comparison of the performance data of the multi-layer composite lithium-ion battery gradient porosity electrodes prepared in Example 1 and Comparative Example 3. Detailed implementation mode

[0061] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation mode and specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0062] In the following embodiments, unless otherwise specified for raw material reagents or processing technologies, it means that they are all conventional commercially available products or conventional processing technologies in the art.

[0063] Example 1

[0064] A solvent-free preparation method of a multi-layer composite lithium-ion battery gradient porosity electrode includes the following steps:

[0065] Step 1: Select carbon fluoride (CF) as the active material of the lithium-ion battery positive electrode, carbon black as the conductive agent, and polytetrafluoroethylene (PTFE) as the binder. Weigh and mix them according to the ratio of active material: conductive agent: binder = 90 wt%: 5 wt%: 5 wt% to obtain a mixed active material;

[0066] Step 2: Select ammonium bicarbonate (NH4HCO3) as the pore-forming additive for the electrode, and set the target porosities of the electrode to be 20%, 30%, 40%, and 50% respectively, and adjust the content of the pore-forming additive. Among them, for the electrode with a porosity of 20%, the content of ammonium bicarbonate additive in the raw material is 4 wt%; for the electrode with a porosity of 30%, the content of ammonium bicarbonate additive in the raw material is 7 wt%; for the electrode with a porosity of 40%, the content of ammonium bicarbonate additive in the raw material is 12 wt%; for the electrode with a porosity of 50%, the content of ammonium bicarbonate additive in the raw material is 15 wt%;

[0067] Step 3: Add ammonium bicarbonate to the mixed active material obtained in Step 1. Select high-speed mechanical stirring as the mixing method, set the rotation speed to 18,000 rpm, and the stirring time to 8 min. Conduct sufficient stirring to ensure uniform mixing of the materials. At the same time, the binder fibrillates under shear stress to produce a fibrous structure;

[0068] Step 4: Preparation of the electrode sheet. Set the target thickness of the electrode to 200 μm. Select hot roll pressing as the preparation method of the electrode. Press the uniformly mixed material obtained in Step 3 under solvent-free processing conditions. During the pressing process, the roll gap of the 20% porosity electrode is set to 127 μm; the roll gap of the 30% porosity electrode is set to 148 μm; the roll gap of the 40% porosity electrode is set to 170 μm; the roll gap of the 50% porosity electrode is set to 197 μm. The roll pressing temperature is 150 °C for all, and the roll speed is 3.4 revolutions per minute for all. After roll pressing, electrode sheets with four porosities are prepared.

[0069] Step 5: Laminating the composite electrode. Select hot roll pressing as the preparation method of the composite electrode. Adjust the roll gap to 300 μm, the roll pressing temperature to 220 °C, and the roll speed to 1.2 revolutions per minute. Stack the four electrode sheets with different porosities in a four-in-one stacking manner and conduct one pressing. After pressing, the laminated electrode is completely bonded and there is no obvious delamination. The electrode thickness is 303 μm. Subsequently, send the laminated electrode and the carbon-coated aluminum foil (MA-EN-CU-0003, Canrd) current collector into the roll press to roll-coat the current collector. The roll pressing temperature is 180 °C and the roll speed is 0.8 revolutions per minute. Finally, a laminated electrode with a current collector-20%-30%-40%-50% gradient porosity is prepared.

[0070] Example 2

[0071] A solvent-free preparation method for a multi-layer composite lithium-ion battery gradient porosity electrode, comprising the following steps:

[0072] Step 1: Select lithium iron phosphate (LFP) as the active material of the lithium-ion battery cathode, carbon black as the conductive agent, and polytetrafluoroethylene (PTFE) as the binder. Weigh and mix them according to the ratio of active material: conductive agent: binder = 90 wt%: 5 wt%: 5 wt% to obtain a mixed active material;

[0073] Step 2: Select activated carbon as the pore-forming additive for the electrode. Set the target porosities of the electrode to 20%, 30%, 40%, and 50% respectively, and adjust the content of the pore-forming additive. Among them, for the electrode with a porosity of 20%, the content of activated carbon additive in the raw material is 4 wt%; for the electrode with a porosity of 30%, the content of activated carbon additive in the raw material is 7 wt%; for the electrode with a porosity of 40%, the content of activated carbon additive in the raw material is 12 wt%; for the electrode with a porosity of 50%, the content of activated carbon additive in the raw material is 15 wt%.

[0074] Step 3: Add the activated carbon to the mixed active material obtained in Step 1. Select high-speed mechanical stirring as the mixing method, set the rotation speed to 18,000 revolutions per minute, and the stirring time to 8 minutes. Conduct sufficient stirring to ensure uniform mixing of the materials. At the same time, the binder fibrillates under shear stress to produce a fibrous structure;

[0075] Step 4: Electrode preparation. Set the target thickness of the electrode to 200 μm. Select hot roll pressing as the electrode preparation method, and press the uniformly mixed material obtained in Step 3 under solvent-free processing conditions. During the pressing process, the roll gap of the 20% porosity electrode is set to 127 μm; the roll gap of the 30% porosity electrode is set to 148 μm; the roll gap of the 40% porosity electrode is set to 170 μm; the roll gap of the 50% porosity electrode is set to 197 μm. The roll pressing temperature is 180 °C, and the roll speed is 3.4 revolutions per minute. After roll pressing, electrode sheets with four porosities are prepared.

[0076] Step 5: Laminated composite electrode. Select hot roll pressing as the composite electrode preparation method, adjust the roll gap to 300 μm, the roll pressing temperature to 220 °C, and the roll speed to 1.2 revolutions per minute. Stack the four electrode sheets with different porosities in a four-in-one stacking manner and conduct one pressing. After pressing, the laminated electrode is completely bonded and there is no obvious delamination. The electrode thickness is 303 μm. Subsequently, send the laminated electrode and the carbon-coated aluminum foil current collector into the roll press for roll-coating the current collector. The roll pressing temperature is 180 °C, and the roll speed is 0.8 revolutions per minute. Finally, a laminated electrode with a current collector-20%-30%-40%-50% gradient porosity is prepared.

[0077] Example 3

[0078] A solvent-free preparation method for a multi-layer composite lithium-ion battery gradient porosity electrode, comprising the following steps:

[0079] Step 1: Select graphite as the active material for the negative electrode of the lithium-ion battery, carbon nanotubes as the conductive agent, and polyvinylidene fluoride (PVDF) as the binder. Weigh and mix them according to the ratio of active material: conductive agent: binder = 96 wt%: 2 wt%: 2 wt% to obtain a mixed active material;

[0080] Step 2: Select ammonium bicarbonate (NH4HCO3) as the pore-forming additive for the electrode, set the target porosity of the electrode to 20%, 35% and 50% respectively, and adjust the content of the pore-forming additive. Among them, for the electrode with a porosity of 20%, the content of ammonium bicarbonate additive in the raw material is 3wt%; for the electrode with a porosity of 35%, the content of ammonium bicarbonate additive in the raw material is 6wt%; for the electrode with a porosity of 50%, the content of ammonium bicarbonate additive in the raw material is 10wt%.

[0081] Step 3: Add ammonium bicarbonate to the mixed active material obtained in Step 1, select planetary ball milling as the mixing method, set the rotation speed to 3000 revolutions per minute, and the stirring time to 5 minutes, and perform sufficient stirring to ensure that the materials are evenly mixed;

[0082] Step 4: Electrode preparation. Set the target thickness of the electrode to 100μm, select hot roll pressing as the electrode preparation method, and press the uniformly mixed powder material obtained in Step 3 under solvent-free processing conditions. During the pressing process, the roll gap of the 20% porosity electrode is set to 57μm; the roll gap of the 35% porosity electrode is set to 65μm; the roll gap of the 50% porosity electrode is set to 80μm. The roll pressing temperature is 250°C, the roll speed is 4 revolutions per minute, and electrode sheets with three porosities are prepared after roll pressing;

[0083] Step 5: Laminated composite electrode. Select hot roll pressing as the composite electrode preparation method, adjust the roll gap to 50μm, the roll pressing temperature to 180°C, and the roll speed to 1.2 revolutions per minute; stack the electrode sheets with three different porosities in a three-in-one stacking manner, and perform one pressing. After pressing, the laminated electrode is completely bonded and there is no obvious delamination, and the electrode thickness is 105μm. Subsequently, send the laminated electrode and the carbon-coated copper foil current collector into the roll press to roll-coat the current collector. The roll pressing temperature is 180°C, and the roll speed is 0.8 revolutions per minute. Finally, a laminated electrode with a current collector - 20% - 35% - 50% gradient porosity is prepared.

[0084] Example 4

[0085] A solvent-free preparation method for a multi-layer composite lithium-ion battery gradient porosity electrode, comprising the following steps:

[0086] Step 1: Select silicon carbide (SiC) as the active material for the negative electrode of the lithium-ion battery, carbon nanotubes as the conductive agent, and polytetrafluoroethylene (PTFE) as the binder, and weigh and mix them according to the ratio of active material: conductive agent: binder = 92wt%: 3wt%: 5wt% to obtain a mixed active material;

[0087] Step 2: Select activated carbon (AC) as the pore-forming additive for the electrode. Set the target porosity of the electrode to 10%, 15%, and 20% respectively, and adjust the content of the pore-forming additive. Among them, for the electrode with a porosity of 10%, the content of the activated carbon additive in the raw material is 5 wt%; for the electrode with a porosity of 15%, the content of the activated carbon additive in the raw material is 7 wt%; for the electrode with a porosity of 20%, the content of the activated carbon additive in the raw material is 10 wt%.

[0088] Step 3: Add the activated carbon to the mixed active material obtained in Step 1. Select high-speed gas flow stirring as the mixing method, with a stirring time of 3 min, and carry out sufficient stirring to ensure uniform mixing of the materials;

[0089] Step 4: Electrode preparation. Set the target thickness of the electrode to 150 μm, select hot pressing as the electrode preparation method, and press the uniformly mixed powder material obtained in Step 3 under solvent-free processing conditions. During the pressing process, the pressure of the 10% porosity electrode is set to 6.9 MPa; the pressure of the 15% porosity electrode is set to 6.4 MPa; the pressure of the 20% porosity electrode is set to 5 MPa. The hot pressing temperature is 170 °C, and the hot pressing time is 7 min. After hot pressing, electrode sheets with three porosities are prepared, and the electrode thicknesses are 152 μm, 155 μm, and 154 μm respectively;

[0090] Step 5: Laminated composite electrode. Select hot pressing as the composite electrode preparation method, adjust the pressure to 5 MPa, the hot pressing temperature to 180 °C, and the hot pressing time to 200 s. Stack the three electrode sheets with different porosities in a three-in-one stacking manner and perform one pressing. After pressing, the stacked electrode is completely bonded and there is no obvious delamination, and the electrode thickness is 147 μm. Subsequently, send the laminated electrode and the carbon-coated copper foil current collector into the hot press for pressure lamination of the current collector. The hot pressing temperature is 180 °C, the hot pressing pressure is 8 MPa, and the hot pressing time is 1 minute. Finally, a laminated electrode with a current collector - 10% - 15% - 20% gradient porosity is prepared.

[0091] Comparative Example 1

[0092] A lithium-ion battery electrode prepared by wet coating includes the following steps:

[0093] Step 1: Weigh and mix the active material carbon fluoride (CF), carbon nanotube conductive agent, and polyvinylidene fluoride (PVDF) binder of the lithium-ion battery cathode according to the ratio of 93 wt%: 3 wt%: 4 wt%. Subsequently, add N-methylpyrrolidone to the mixed material until the solid content is about 50 wt%;

[0094] Step 2: Place the mixed materials in a planetary ball mill to fully mix and homogenize the above raw materials, ensuring uniform distribution of the slurry mixture and good fluidity;

[0095] Step 3: Use a vacuum adsorber to flatly adsorb the current collector on the workbench, and use a coater and a 200-μm doctor blade to coat the slurry;

[0096] Step 4: After coating, send the electrode into a vacuum drying oven to dry at 80 °C for 10 h. After drying, perform cold rolling to further improve the mechanical strength of the electrode, and the roll gap is 0.4 times the electrode thickness;

[0097] Step 5: Send the electrode and the carbon-coated aluminum foil current collector into a rolling press to roll and cover the current collector. The rolling temperature is 180 °C, the rolling speed is 0.8 revolutions per minute, and finally a wet-coated single-layer electrode is prepared. The measured porosity of the electrode is 22.7%.

[0098] Comparative Example 2

[0099] A lithium-ion battery electrode prepared by a solvent-free method includes the following steps:

[0100] Step 1: Weigh and mix the active material lithium iron phosphate (LFP) for the negative electrode of the lithium-ion battery, carbon nanotube conductive agent, and polytetrafluoroethylene (PTFE) binder in a ratio of 85 wt%:10 wt%:5 wt%;

[0101] Step 2: Place the mixed materials in a high-speed mixer, use high-speed shear stress to fibrillate the binder, and at the same time fully mix and homogenize the above raw materials to ensure uniform particle distribution of the mixture and good fluidity of the powder;

[0102] Step 3: Set the electrode thickness to 200 μm, and use a hot rolling press to press the uniformly mixed powder material into a sheet electrode at 150 °C. The roll gap is 124 μm, and the thickness of the pressed electrode is 203 μm;

[0103] Step 4: Send the electrode and the carbon-coated aluminum foil current collector into a rolling press to roll and cover the current collector. The rolling temperature is 180 °C, the roll gap is 0.4 times the electrode thickness, and finally a single-layer electrode is prepared. The measured porosity of the electrode is 13.8%.

[0104] Comparative Example 3

[0105] A preparation method of a multi-layer composite lithium-ion battery gradient porosity electrode includes the following steps:

[0106] Step 1: Select carbon fluoride (CF) as the active material for the positive electrode of the lithium-ion battery, carbon black as the conductive agent, and polytetrafluoroethylene (PTFE) as the binder. Weigh and mix them according to the ratio of active material: conductive agent: binder = 90 wt%: 5 wt%: 5 wt% to obtain the mixed active material;

[0107] Step 2: Select ammonium bicarbonate (NH4HCO3) as the pore-forming additive for the electrode. Set the target porosity of the electrode to be 20%, 30%, 40%, and 50% respectively, and adjust the content of the pore-forming additive. Among them, for the electrode with a porosity of 20%, the content of ammonium bicarbonate additive in the raw material is 4 wt%; for the electrode with a porosity of 30%, the content of ammonium bicarbonate additive in the raw material is 7 wt%; for the electrode with a porosity of 40%, the content of ammonium bicarbonate additive in the raw material is 12 wt%; for the electrode with a porosity of 50%, the content of ammonium bicarbonate additive in the raw material is 15 wt%;

[0108] Step 3: Add ammonium bicarbonate to the mixed active material obtained in Step 1, and then mix the obtained raw material mixture with N-methylpyrrolidone in a mass ratio of 1:1. Select high-speed mechanical stirring as the mixing method, set the rotation speed to 18000 rpm, and the stirring time to 8 min, and carry out sufficient stirring to ensure uniform mixing of the materials to prepare the electrode slurry;

[0109] Step 4: Electrode preparation. Coat the electrode slurry with a target porosity of 20% on the carbon-coated aluminum foil current collector, heat and dry it at 150 °C to decompose and escape ammonium bicarbonate, and roll it at a pressure of 0.2 MPa and a roll gap of 30 μm to obtain the first coating;

[0110] Coat the electrode slurry with a target porosity of 30% on the first coating, heat and dry it at 150 °C to decompose and escape ammonium bicarbonate, and roll it at a pressure of 0.18 MPa and a roll gap of 55 μm to obtain the second coating;

[0111] Coat the electrode slurry with a target porosity of 40% on the second coating, heat and dry it at 150 °C to decompose and escape ammonium bicarbonate, and roll it at a pressure of 0.16 MPa and a roll gap of 80 μm to obtain the third coating;

[0112] Coat the electrode slurry with a target porosity of 50% on the third coating, heat and dry it at 150 °C to decompose and escape ammonium bicarbonate, and roll it at a pressure of 0.16 MPa and a roll gap of 105 μm to obtain the fourth coating;

[0113] Step 5: Adjust the roll gap to 100 μm, the rolling temperature to 220 °C, and the roll speed to 1.2 revolutions per minute. Perform one pressing to obtain a laminated electrode sheet with a gradient porosity of collector - 20% - 30% - 40% - 50%. The multi-layer electrodes after pressing are completely bonded and show no obvious delamination. The electrode thickness is 124 μm. The rest is the same as in Example 1.

[0114] Application Example:

[0115] This example is used to comparatively characterize the thermal stability, porosity, and electrode performance of the electrodes prepared in Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, and Comparative Example 3;

[0116] Among them, the electrode performance test methods include:

[0117] 1. Under the condition of a temperature higher than the decomposition temperature of ammonium bicarbonate, perform thermogravimetric analysis on the mixed material in Example 1 to characterize the thermal stability and weight change of the electrode material at this temperature;

[0118] 2. Perform liquid nitrogen brittle fracture on the electrode sheets with different porosities prepared in Example 1. Subsequently, use the secondary electron mode to observe the cross-sectional morphology of the brittle-fractured electrode, and observe the change in electrode porosity through the morphological change of the electrode cross-section;

[0119] 3. Take the gradient porosity electrodes prepared in Example 1 and Comparative Example 1, and assemble them with a lithium sheet negative electrode, gasket, spring piece, battery case, and separator into a CR2032 coin cell. Subsequently, let the assembled coin cell stand at 25 °C for 24 hours, and then use a multi-channel electrochemical workstation to perform a constant current discharge test on the battery, and record data such as voltage and discharge specific capacity.

[0120] 4. Take the gradient porosity electrodes prepared in Example 2 and Comparative Example 2, and assemble them with a lithium sheet negative electrode, gasket, spring piece, battery case, and separator into a CR2032 coin cell. Subsequently, let the assembled coin cell stand at 25 °C for 24 hours, and then use a multi-channel electrochemical workstation to perform a constant current discharge test on the battery at different current densities, and record data such as voltage and discharge specific capacity of the battery at different rates.

[0121] The characterization results are as follows:

[0122] Figure 1 It is the thermogravimetric (TG) analysis curve of the 50% porosity electrode sheet prepared in Step 4 of Example 1. It can be analyzed that after heating at 150 °C, 15 wt% of the ammonium bicarbonate pore-forming agent is completely decomposed by heat, and the remaining mass ratio is 85 wt% of the mixed material of active substances, conductive agents, and binders. The TG curve proves the thermal stability of the active substances, conductive agents, and binders at the processing temperature and the complete thermal decomposition of the pore-forming additive. This ensures the reliability of the subsequent electrode preparation.

[0123] Figure 2 SEM images of the cross-sectional morphologies of the electrode sheets with different porosities in Example 1, where Figures A, B, C, and D are the cross-sectional images of the electrode sheets with porosities of 20%, 30%, 40%, and 50%, respectively. It can be seen that as the porosity in the electrode sheet increases, the degree of structural fragmentation of the electrode sheet cross-section gradually increases, showing more internal pores. After calculation, the porosities of the electrode sheets with activated carbon contents of 4wt%, 7wt%, 12wt%, and 25wt% are 22.3%, 33.1%, 41.8%, and 50.7%, respectively.

[0124] Figure 3 Figure for comparing the discharge curves of Example 1 and Comparative Example 1 in a half-cell at a 0.1C rate. From the curves, it can be observed that the discharge voltage plateau of the half-cell of Example 1 assembled with the gradient porosity electrode is significantly higher than that of the half-cell of Comparative Example 1, which indicates the improvement effect of the gradient porosity structure on the electrochemical kinetics of the electrode. At the same time, due to the improvement of the electrolyte wettability and the increase of the solid-liquid contact interface brought about by the higher porosity, the discharge specific capacity of the half-cell of Example 1 is also significantly higher than that of the half-cell of Comparative Example 1. These data significantly prove that the present invention can effectively construct a gradient porosity electrode structure and thus greatly improve the electrode performance.

[0125] Figure 4 Figure for comparing the rate performance of Example 2 and Comparative Example 2 in a half-cell. Due to the performance improvement brought about by the gradient porosity structure of the electrode in Example 2, although the thickness is much greater than that of the electrode in Comparative Example 2, the half-cell assembled with Example 2 is significantly superior to Comparative Example 2 in terms of rate performance. The gradient pore structure ensures the ion transport near the electrolyte and separator sides and the electron transfer near the current collector side, thereby improving the charge and discharge ability of the electrode at a higher rate.

[0126] Figure 5 Schematic diagram for comparing the preparation processes and procedures of Example 1 and Comparative Example 3. Due to the solvent-free preparation technology adopted in Example 1, multiple electrode layers with different porosities in Example 1 can be prepared simultaneously, and then the preparation of the gradient porosity electrode can be completed only by one-time lamination and rolling. However, due to the limitation of the wet coating process in Comparative Example 3, the coating process of the next layer of the electrode must be carried out after the coating of the previous layer of the electrode is completed and fully dried. This preparation process not only makes the preparation process more cumbersome but also leads to additional energy consumption and the increase of production and time costs. The comparison of the process routes strongly illustrates that the present invention can effectively improve the technical drawbacks of the existing technology for preparing gradient porosity electrodes based on wet coating and greatly improve the preparation efficiency of gradient porosity electrodes.

[0127] Figure 6Performance data comparison of the gradient porosity electrodes prepared in Example 1 and Comparative Example 3. Due to the improvement in the thickness of the electrode that can be prepared by the solvent-free method, the active material loading of the electrode in Example 1 is significantly higher than that in Comparative Example 3, and the discharge capacity of the half-cell assembled is also significantly higher than that of the half-cell assembled with the electrode in Comparative Example 3.

[0128] The above description of the embodiments is to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A solvent-free preparation method of a gradient porosity electrode, characterized in that, It includes the following steps: S1: Mix the active material, conductive agent, binder and pore-forming additive to obtain a raw material mixture with different contents of pore-forming additive, and press to obtain electrode sheets with different porosities; S2: Stack and press multiple electrode sheets in the order of stepwise change of porosity, and then combine with the current collector to obtain a gradient porosity electrode.

2. The solvent-free preparation method of the gradient porosity electrode according to claim 1, characterized in that In step S1, the active material includes a positive electrode active material and a negative electrode active material. The positive electrode active material is selected from at least one of carbon fluoride, lithium iron phosphate, lithium cobaltate or lithium nickel cobalt manganese ternary material; The negative electrode active material is selected from at least one of graphite, silicon, hard carbon or silicon carbide; The conductive agent is selected from carbon black or carbon nanotubes; The binder is selected from polytetrafluoroethylene, polyimide, carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid or polyvinylidene fluoride; The pore-forming additive includes a sacrificial phase additive or a non-sacrificial phase additive; The sacrificial phase additive is selected from one of ammonium bicarbonate, ammonium carbonate or ammonium nitrate; The non-sacrificial phase additive is selected from activated carbon or graphene oxide; In the raw material mixture, the mass ratio of the active material, conductive agent and binder is 80-96:2-6:2-5; the content of the pore-forming additive is 2-15 wt%.

3. The solvent-free preparation method of the gradient porosity electrode according to claim 1, wherein In step S1, the mixing method used includes at least one of mechanical stirring, gas stirring, grinding mixing or ball milling mixing.

4. The solvent-free preparation method of the gradient porosity electrode according to claim 1, wherein In step S1, in the pressing, the pressing method includes one of hot pressing, cold pressing, hot roll pressing, cold roll pressing or rolling; Preferably, when hot roll pressing is adopted, the roll pressing temperature is 150-250 °C.

5. The solvent-free preparation method of the gradient porosity electrode according to claim 1, wherein, In step S2, the porosity of the electrode sheet is 10-55%, and during the stacking process, the difference in porosity between adjacent electrode sheets is 5-15%, and the number of stacking layers is 3-5 layers.

6. The solvent-free preparation method of the gradient porosity electrode according to claim 5, characterized in that, In step S2, during the stacking process, the thicknesses of multiple electrode sheets are all 180-220 μm, and the porosities are 18-24%, 28-34%, 38-42%, 48-52% in sequence.

7. The solvent-free preparation method of the gradient porosity electrode according to claim 5, characterized in that, In step S2, during the stacking process, the thicknesses of multiple electrode sheets are all 80-120 μm, and the porosities are 18-22%, 33-37%, 48-52% in sequence.

8. The solvent-free preparation method of the gradient porosity electrode according to claim 5, wherein In step S2, during the stacking process, the thicknesses of multiple electrode sheets are all 120-170 μm, and the porosities are 8-12%, 13-17%, 18-22% in sequence.

9. The solvent-free preparation method of the gradient porosity electrode according to claim 4, characterized in that, In step S2, in the press forming, the thickness of the pressed product is 1-1.5 times that of the electrode sheet; The press forming adopts hot roll forming, the roll pressing temperature is 80-220 °C, and the roll gap is 0.5-1 times the thickness of the electrode sheet; The press forming adopts hot press forming, the hot press temperature is 160-200 °C, and the hot press pressure is 4-6 MPa.

10. Use of a gradient porosity electrode prepared by the method according to any one of claims 1 to 9, characterized in that The gradient porosity electrode is used for preparing an electric energy storage device.

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