High-compaction-density lithium iron phosphate positive plate and preparation method thereof

Abstract

The invention discloses a high-compaction-density lithium iron phosphate positive plate and a preparation method thereof, and relates to the technical field of lithium battery positive electrode preparation, the high-compaction-density lithium iron phosphate positive plate comprises a positive electrode current collector and a positive electrode active layer on the surface of the positive electrode current collector, the active layer contains a lithium iron phosphate active material system, a conductive network system, a cross-linking curing type bonding system and a channel lining type inorganic nanometer component. During preparation, channel lining type inorganic nano components are deposited on the surfaces of removable core material particles to form core-shell pore-forming units, the core-shell pore-forming units are mixed with the systems to prepare slurry, the slurry is coated, dried and rolled, and then low-temperature vacuum heat treatment is performed to remove the removable core material and enable the cross-linking curing type bonding system to be cross-linked and cured. A macro-hole channel is formed, and the hole wall is provided with a hole wall lining layer formed by a channel lining type inorganic nano component. The positive plate is suitable for a thick electrode scene, the electrolyte infiltration uniformity can be remarkably improved, the dry core and stagnant gas are effectively reduced, the standing time is shortened, and the compaction density and the peel strength are kept.

Description

Technical Field

[0001] This invention relates to the field of lithium battery cathode preparation technology, specifically to a high-density lithium iron phosphate cathode sheet and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) batteries are widely used in electric vehicles and energy storage systems due to their safety and lifespan advantages. As requirements for vehicle range, system integration efficiency, and the cost per kilowatt-hour (kWh) of energy storage increase, large-capacity square or pouch cells (e.g., from 100Ah to over 300Ah) are increasingly employing cathodes with higher areal density to improve volumetric energy density. In engineering, this is typically achieved by increasing the compaction density of the LFP cathode and combining it with a thicker coating (greater coating thickness and areal capacity) to increase the amount of active material loaded per unit volume. Simultaneously, in roll-to-roll manufacturing, the cycle time and consistency of processes such as coating drying, roll forming for thickness determination, slitting, and stacking / winding must be considered. In this application scenario, electrolyte injection and wetting are critical post-assembly processes, and their timing and uniformity directly affect formation efficiency, internal resistance distribution, and batch consistency.

[0003] Existing technologies improve compaction density through particle size distribution, secondary particle densification, carbon coating, and rolling, and promote wetting through processes such as vacuum injection, heated settling, or multiple injections. Other technologies attempt to adjust the pore structure at the electrode level to improve ion transport. However, when the positive electrode simultaneously meets the requirements of high compaction density and thick electrode, the electrode porosity decreases, the pore size distribution shifts towards smaller pores, and the tortuosity increases. Rolling may also create a relatively dense surface region, leading to a decrease in the electrolyte penetration rate in the thickness direction, accompanied by gas retention. The wetting front is hindered from advancing inside the electrode, easily resulting in insufficient wetting near the current collector side or in localized areas. Insufficient wetting can affect the formation and uniformity of the interfacial film during the formation stage, resulting in higher local ion / electron transport impedance. This leads to uneven current distribution, increased polarization, and temperature rise differences, manifesting as decreased capacity utilization, increased DC internal resistance dispersion, and the risk of early cycle decay. In large-area electrodes and large cells, this non-uniformity is even more difficult to completely cover with process margins, often requiring extended resting time or increased process control, which in turn brings manufacturing cycle time, work-in-process inventory, and cost pressure.

[0004] Furthermore, in practical products, the electrolyte ratio is often controlled to increase energy density, and the electrolyte capacity available to the electrode is limited. When the capillary driving force is insufficient or the pore connectivity decreases, even if the outer layer has absorbed liquid, the interior may still remain in a low-saturation state for a long time, leading to an amplification of impedance and heat generation differences over time. Therefore, the technical problem that the existing technology urgently needs to solve is that in high-density and thick-coated lithium iron phosphate cathode sheets, it is difficult to simultaneously achieve the wetting rate and uniformity of the electrolyte in the thickness direction, which easily leads to local dry cores and gas retention. Summary of the Invention

[0005] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a high-density lithium iron phosphate cathode sheet and its preparation method. During preparation, channel-lined inorganic nanomaterials are deposited on the surface of removable core material particles to form a core-shell pore-forming unit. This unit is then mixed with the aforementioned systems to form a slurry, coated, dried, and rolled. A low-temperature vacuum heat treatment is then performed to remove the removable core material and allow the cross-linked curing adhesive system to cross-link and cure, forming macroporous channels with a pore wall liner composed of channel-lined inorganic nanomaterials. This significantly improves electrolyte wetting uniformity, effectively reduces dry core and stagnant gas, shortens settling time, and maintains compaction density and peel strength, thus solving the technical problems described in the background art.

[0006] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: A high-compact-density lithium iron phosphate cathode sheet includes: a cathode current collector and a cathode active layer on its surface, wherein the compaction density after removing the removable core material is not less than 2.65 g / cm³, and the active layer comprises lithium iron phosphate active material system component A, conductive network system component B, cross-linked curing adhesive system component C, and channel-lined electrophilic inorganic nano-component E, wherein component E comprises boehmite nanoparticles or boehmite nanofibers, and component C comprises sodium carboxymethyl cellulose, styrene-butadiene rubber latex, polyacrylic acid, and a water-dispersible polycarbodiimide crosslinking agent or a water-dispersible polyepoxy crosslinking agent; The active layer forms a through macropore channel, and the macropore channel wall is provided with a pore wall liner composed of component E. The macropore channel is formed after the removable core material in the core-shell pore-forming unit D@E is removed, and the shell of the core-shell pore-forming unit D@E contains component E.

[0007] Furthermore, based on the positive electrode active layer after removing the removable core material, the content of component B is 1.2 to 2.8% by mass, the content of component C is 1.0 to 2.2% by mass, the content of component E is 0.05 to 0.60% by mass, and the balance is component A, and the pore size of the macropore channel is greater than 5 micrometers.

[0008] Furthermore, component A includes secondary particulate lithium iron phosphate A1 and fine-particle lithium iron phosphate A2; The median particle size D50 of A1 is 6 to 12 micrometers, the tap density is not less than 1.3 g per cubic centimeter, and the carbon coating content is 0.8 to 2.0% by mass. The median particle size D50 of A2 is 0.6 to 1.8 micrometers, and the content of A2 in the total amount of A1 and A2 is 5 to 25% by mass.

[0009] Furthermore, component B includes conductive carbon black B1 and conductive carbon nanotubes B2, with a mass ratio of B1 to B2 of 6 to 12 to 1. The conductive carbon nanotubes have a diameter of 6 to 15 nanometers and a length of 5 to 20 micrometers. The conductive carbon black is a medium specific surface area conductive carbon black with a nitrogen adsorption specific surface area of ​​40 to 90 square meters per gram and a DBP oil absorption value of 120 to 220 milliliters per 100 grams, or a high specific surface area high structure conductive carbon black with a nitrogen adsorption specific surface area of ​​600 to 1600 square meters per gram and a DBP oil absorption value of 350 to 520 milliliters per 100 grams.

[0010] Furthermore, the total content of component C is 1.0 to 2.2% by mass, the content of crosslinking agent in the final dry film is 0.05 to 0.25% by mass, and component E further includes nano-silica with a particle size of 10 to 40 nanometers.

[0011] A method for preparing a high-density lithium iron phosphate cathode sheet, comprising: An inorganic dispersion containing boehmite and polyacrylic acid was prepared and deposited on the surface of removable core material particles to obtain a core-shell porous unit D@E; A positive electrode slurry was prepared by mixing lithium iron phosphate active material system component A, conductive network system component B, cross-linked curing adhesive system component C with core-shell pore-forming unit D@E; the positive electrode slurry was coated on the positive electrode current collector and dried and rolled; the rolled precursor electrode was subjected to vacuum heat treatment to remove the removable core material and cross-link the adhesive system to obtain a positive electrode with an inorganic nano-liner layer on the pore wall of the macropore channel.

[0012] Furthermore, the inorganic dispersion has a pH of 7.0 to 9.0, a boehmite solid content of 5 to 12% by mass, a polyacrylic acid concentration of 0.2 to 0.6% by mass, and is subjected to high-speed shear dispersion for 20 to 40 minutes and ultrasonic dispersion for 10 to 20 minutes.

[0013] Furthermore, the removable core material is adamantane or a high-melting-point adamantane derivative of the same group. The removable core material includes a first core material particle D1 with a median particle size D50 of 12 to 20 micrometers and a second core material particle D2 with a median particle size D50 of 2 to 5 micrometers. The mass ratio of D1 to D2 is 4:1 to 8:1. The amount of removable core material added is 0.8 to 3.5% by mass relative to the total solids of components A, B, C and E in the positive electrode active layer after removing the removable core material. The shell mass fraction of the core-shell pore-forming unit D@E is 6 to 20% by mass relative to the removable core material and the shell thickness is 80 to 350 nanometers.

[0014] Furthermore, the solid content of the positive electrode slurry is 58 to 68% by mass, the core-shell pore-forming unit D@E is added after the slurry is homogenized by low-shear mixing for 8 to 15 minutes and vacuum degassing for 10 to 20 minutes, and the pre-electrode containing the removable core material is rolled to achieve a compaction density of 2.75 to 2.90 g / cm³.

[0015] Furthermore, the vacuum heat treatment is carried out under conditions of vacuum degree of 20 to 500 Pa, temperature of 105 to 150 degrees Celsius, and holding time of 0.5 to 6 hours.

[0016] (III) Beneficial Effects This invention provides a high-density lithium iron phosphate cathode sheet and its preparation method, which has the following beneficial effects: Under the conditions of maintaining high compaction density and thick coating, a core-shell pore-forming unit composed of removable core material and inorganic nano-components is introduced into the slurry. After roll pressing, a low-temperature vacuum heat treatment is performed to remove the removable core material without destroying the already formed dense skeleton, thereby forming a through macropore channel. The inorganic nano-components remain in situ on the channel wall along with the core-shell structure, which significantly reduces the wetting resistance of the channel wall to the electrolyte and inhibits gas retention. This accelerates the penetration of the electrolyte along the thickness direction and improves the wetting uniformity, reducing local dry core and the resulting internal resistance dispersion and polarization amplification.

[0017] The cross-linked curing bonding system, combined with low-temperature vacuum heat treatment, locks in the structural stress during pore formation and enhances the bonding strength between the coating and the current collector, preventing macropore collapse and powder shedding. Simultaneously, it inhibits swelling and migration after electrolyte immersion, maintaining long-term channel connectivity. Inorganic nano-components function by positioning within the pore walls, achieving significant wetting enhancement at low addition levels and reducing adverse effects on compaction, deposition, and slurry rheology. Furthermore, core removal and shaping are completed in the same process, reducing additional wet processing steps and mitigating the risk of water introduction. This makes it suitable for continuous roll-to-roll manufacturing and consistent control of high-capacity cells. This positive electrode significantly shortens the electrolyte injection settling time. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall process of the thick electrode preparation method of the present invention; Figure 2 This is a schematic diagram of the core-shell perforation unit D@E of the present invention; Figure 3 This is a schematic diagram of the cross-sectional distribution of the precursor electrode of the core-shell porous unit D@E of the present invention; Figure 4 This is a schematic diagram illustrating the mechanism of the integrated low-temperature vacuum core removal and cross-linking shaping process of the present invention; Figure 5 This is a schematic diagram of the composite structure of macropore channels and pore wall liner in the final positive electrode active layer of the present invention; Figure 6 This is a schematic diagram comparing the electrolyte wetting path of the thick electrode sheet of the present invention with that of a conventional high-voltage thick electrode sheet; Figure 7 This is a comparison chart of key performance indicators between the embodiments and comparative examples of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Please see Figures 1-7 This invention provides a high-density lithium iron phosphate cathode sheet and its preparation method: The following are all based on the final positive electrode active layer after removing the D core material; the D core material will be added in the preparation method and removed in subsequent processes.

[0021] Component A: Lithium iron phosphate active material system (A1+A2) A1: High tap density secondary particle type lithium iron phosphate Median particle size D50: 6–12 micrometers; D90: ≤18–22 micrometers; Tap density: ≥1.3 g / cm³ (preferably ≥1.5 g / cm³); Carbon coating content: 0.8–2.0% by mass% Specific surface area (nitrogen adsorption): 8–18 m² / g; Moisture content: ≤300ppm (ideally ≤200ppm); A2: Fine-particle-size lithium iron phosphate interstitial component (used to improve compaction and interparticle contact) D50: 0.6–1.8 micrometers; Content: A2 accounts for 5–25% of the total mass of A1 + A2; Function: To fill the gaps between secondary A1 particles, improve packing and compaction, without excessively increasing the specific surface area.

[0022] Component B: Conductive network system (B1+B2) Objective: To form a continuous electronic network even in high-compaction, thick coatings, and to avoid localized increases in resistance due to compaction.

[0023] B1a: Medium specific conductivity carbon black (SuperP type): BET: 40–90 m² / g (preferably 55–75); DBP oil absorption value: 120–220 mL / 100g; Alternatively: B1b: High specific surface area, high structural conductivity carbon black (Ketjenblack type): BET: 600–1600 m² / g; DBP oil absorption value: 350–520 mL / 100g.

[0024] B2: High aspect ratio conductive carbon nanotubes (multi-walled or few-walled) Diameter: 6–15 nanometers; Length: 5–20 micrometers; Purity: ≥95%; Ash content: ≤1.0%; Total B (B1 + B2): 1.2–2.8% by mass; range: 1.6–2.3% by mass; B1:B2 mass ratio: (6–12):(1) (i.e., low nanotube content, which plays a bridging role) Component C: Cross-linked curing adhesive system (C1+C2+C3+C4); Objective: To simultaneously complete cross-linking and curing during the heat treatment for removing D, so that: The structure does not collapse after drilling, the inorganic liner E is anchored to the channel wall, and the swelling of the binder is reduced under electrolyte immersion (reducing subsequent dry core and powder shedding).

[0025] The process employs an aqueous coating method (matching the mainstream aqueous coating method for lithium iron phosphate), and the bonding system is as follows: C1: Sodium carboxymethyl cellulose Degree of substitution: 0.7–1.2; Viscosity of 2% aqueous solution (25°C): 800–2500 mPa·s; C2: Styrene-butadiene rubber latex; Solid content: 45–55%; Glass transition temperature: -30°C to -5°C; C3: Polyacrylic acid (or a portion thereof of its lithium / sodium salt): Number average molecular weight: 1×10 5 –6×10 5 ; Function: Provides carboxyl sites, which interact strongly with the hydroxyl / aluminum centers on the inorganic E surface and participate in cross-linking.

[0026] C4: Scheme C4-1: Water-dispersible polycarbodiimide crosslinking agent; C4 is a water-dispersible polycarbodiimide crosslinking agent, added at a rate of 0.05–0.30% of the total dry film mass; Crosslinking and shaping were completed under vacuum heat treatment at 110–140°C for 0.5–4 h.

[0027] Total C (based on the final electrode dry film): 1.0–2.2% by mass; Preferred: 1.2–1.6% by mass; C4 (crosslinking agent) accounts for 0.05–0.25% of the final dry film mass.

[0028] Component D: Removable core material Option D: Adamantane or a high-melting-point adamantane derivative from the same family (as a sublimable solid core material), and specify: Particle size: D1D50 = 12–20 μm, D2D50 = 2–5 μm; D1:D2 = 4:1–8:1; Drying section limits: not exceeding 65°C (or using a drying section with an extended time of 45–60°C) to ensure no significant loss of D; Core removal section: vacuum 20–200Pa, temperature 120–150°C, 2–6h (the temperature is increased to ensure thorough core removal, and it is not necessary to adhere to 105–120°C).

[0029] D core material usage (relative to the total solids of A+B+C+E): 0.8–3.5% by mass; Preferred: 1.5–2.8% by mass; Granularity design: D1 (main channel core): D50 = 12–28 micrometers, D90 ≤ 40 micrometers; D2 (connecting bridge core, optional): D50 = 2–6 micrometers; D1:D2 mass ratio: (3–8):1.

[0030] Component E: Channel-lined electrophilic electrolyte inorganic nanocomponent (E1, may contain E2) E material uses battery grade, with feed moisture (Karl Fischer) ≤0.3%; E is vacuum dried at 120°C for 6–12 hours before being added to the slurry; After core removal / shaping, the finished electrode sheet moisture (Karl Fischer) is ≤300ppm (preferably ≤200ppm).

[0031] E1: Boehmite (alumina hydrate) nanoparticles / nanofibers; Primary particle size: 20–80 nanometers; Or nanofiber morphology: diameter 10–30 nm, length 80–300 nm; Specific surface area: 60–160 square meters / gram; Loss on ignition (or structural water related indicators): Control moisture adsorption according to battery materials.

[0032] Particle size: 10–40 nanometers; Specific surface area: 100–250 square meters / gram; Total E (final dry film): 0.05–0.60% by mass; Preferred: 0.15–0.40% by mass. Example

[0033] The final dry film (after removing the D core material) is designed to be 100 parts by weight; the D core material is added during preparation and removed subsequently.

[0034] 1. Example formulation (final dry film basis: 100 parts by weight); A: 96.3 parts of lithium iron phosphate active material system; A1 (secondary granular lithium iron phosphate): 78.0 parts; A2 (fine-particle-size interstitial lithium iron phosphate): 18.3 parts; B: Conductive network system 2.0 parts; B1 (High-structure conductive carbon black): 1.7 parts; B2 (carbon nanotubes): 0.3 parts; C: 1.4 parts of cross-linking curing adhesive system; C1 (sodium carboxymethyl cellulose): 0.55 parts; C2 (styrene-butadiene rubber, by solids content): 0.55 parts; C3 (polyacrylic acid): 0.20 parts; C4 (water-dispersible polyepoxy crosslinking agent): 0.10 parts; E: 0.3 parts of inorganic lining nano-components; E1 (boehmite nanofibers or nanoparticles): 0.25 parts; E2 (nano silica, optional): 0.05 parts; D (removable core material) addition amount: 2.2 parts (relative to the above 100 parts solids); Where D1:D2=6:1 (mass ratio), that is, D1=1.89 parts and D2=0.31 parts.

[0035] D will be removed in the subsequent vacuum heat treatment, leaving behind a macroporous channel network + wall lining E.

[0036] 2. Preparation of core-shell pore-forming units (D@E) Objective: To ensure that E is not randomly distributed, but grows on the wall of the future channel, and retains its shape / does not collapse after rolling.

[0037] Step D@E-1: Preparation of E nano-dispersion Polyacrylic acid (as a dispersing and anchoring precursor) is added to deionized water to a concentration of 0.2–0.6 by mass.

[0038] Boehmite E1 (and optionally silica E2) is added to make the inorganic solid content 5–12% by mass.

[0039] High-speed shear dispersion for 20–40 minutes, followed by ultrasonic dispersion for 10–20 minutes (controlling slurry temperature ≤30°C) to obtain a uniform dispersion.

[0040] Adjust the pH of the dispersion to 7.0–9.0 (this can be achieved with a small amount of acetic acid / citric acid) to improve dispersion stability and the uniformity of subsequent adsorption and shell formation. If pH adjustment is necessary, it is preferable to use regulators that do not introduce high corrosiveness, such as ammonia / triethanolamine / bicarbonate; polyacrylic acid is preferably added in its sodium / lithium salt partially neutralized form, which is beneficial for both dispersion and subsequent crosslinking.

[0041] Step D@E-2: Forming a shell on the surface of the camphor core material Premix camphor particles D1 and D2 at 20–25°C to ensure they are fully homogeneous.

[0042] Slowly spray / drop the above E dispersion into the camphor particles while gently tumbling and mixing them, so that the inorganic particles gradually deposit on the camphor surface.

[0043] Add a small amount of sodium carboxymethyl cellulose solution (0.1–0.3% by mass as a temporary bridging agent) and continue mixing for 10–20 minutes.

[0044] The wet particles were dried at 25–35°C and under vacuum of 10–30 kPa until their fluidity was restored and their surfaces were no longer sticky, thus obtaining the core-shell porous unit D@E.

[0045] D@E's structure / parameters: Shell mass fraction (relative to core D): 6–20% (approximately 12% in this example); Shell thickness: approximately 80–350 nanometers (formed as a porous, granular, stacked shell); Shell pores: nanoscale open pores to ensure that sublimation gases can escape; 3. Preparation of positive electrode slurry Step S-1: Adhesive masterbatch Dissolve sodium carboxymethyl cellulose in deionized water to prepare a 1.0–2.0% by mass solution and stir until completely swollen.

[0046] Add polyacrylic acid solution to make the total concentration of carboxyl polymer meet the above formula.

[0047] Add water-dispersible polyepoxy crosslinking agent (C4) and stir for 20 minutes.

[0048] Finally, add styrene-butadiene rubber latex (by solids content) and mix under low shear for 15–30 minutes to obtain the binder masterbatch.

[0049] Step S-2: Pre-dispersion of conductive agent Conductive carbon black and carbon nanotubes are added to part of the binder mother liquor.

[0050] High-speed shearing for 20–30 minutes, with short-term ultrasonic assistance if necessary, is used to prevent carbon nanotubes from forming bundles and carbon black from agglomerating significantly.

[0051] Step S-3: Add active material and prepare slurry Lithium iron phosphate A1 and A2 were added to the mixing tank in batches while stirring.

[0052] Control the solid content (non-water volatiles) to 58–68% by mass (for thick coatings, it is more likely to be in the range of 60–65% to balance coating and drying uniformity).

[0053] Use planetary agitation or double planetary mixing for 60–120 minutes to make the slurry uniform.

[0054] Step S-4: Finally, add D@E After the slurry is basically homogeneous, the core-shell pore-forming unit D@E is slowly added.

[0055] Use low-shear mixing for 8–15 minutes to avoid excessive shell breakage while ensuring uniform dispersion.

[0056] Vacuum degassing for 10–20 minutes reduces the false dry core caused by air bubbles after coating.

[0057] 4. Integrated steps of coating, drying, rolling, and low-temperature vacuum core removal + cross-linking and shaping. Step C-1: Coating Current collector: Aluminum foil or carbon-coated aluminum foil (thickness commonly 12–16 micrometers).

[0058] Single-sided wet film thickness: set according to the target single-sided dry film thickness of 90–140 micrometers.

[0059] Target surface density: can be designed in the range of 18–25 mg / cm² (depending on the target energy density of the cell).

[0060] Step C-2: Segmented low-temperature drying (to avoid premature loss and migration of D) First stage: 50–65°C, 2–5 minutes (rapid setting, inhibiting component migration) Second stage: 70–90°C, 3–8 minutes (to remove most of the moisture). Moisture content after drying: controlled within the process allowable window (followed by vacuum baking and further dehydration). Step C-3: Roller pressing (compact first, then create holes) Roller temperature: 25–45°C; Objective: To achieve a compaction density of 2.75–2.90 g / cm³ for the precursor electrode containing D (to reserve for maintaining ≥2.65 g / cm³ after subsequent removal of D); Step C-4: Low-temperature vacuum heat treatment Performed in a vacuum oven or continuous vacuum heat treatment equipment: Temperature increase: from 40°C to 105–120°C at a rate of 0.5–2°C / min; Vacuum level: 50–500 Pa; Insulation: 1.5–4 hours; This occurs simultaneously during the process: D-core material sublimates / evaporates and escapes → forming a high-speed macroporous channel network; C4 crosslinking agent triggers the formation of crosslinking and curing with carboxyl / hydroxyl network → structural fixation and enhanced resistance to electrolytes; The shell E of D@E is left on the channel wall → forming an electrophilic liner (capillary wetting + degassing and anti-bubble).

[0061] II. Formulation and processing schemes for the 6 groups of samples Unified geometry and datum conditions Current collector: 16μm aluminum foil.

[0062] Single-sided coating: target areal density 33.2±0.3mg / cm² (including binder / conductive agent / functional components, excluding the removed D core material).

[0063] Drying: Segmented drying at 50–65℃ (pre-curing) + 70–90℃ (dehydration), followed by uniform vacuum drying (dehydration).

[0064] The target final compaction density was controlled at 2.70–2.72 g / cm³, except for the low compaction contrast.

[0065] Example (E1): The scheme of this application (core-shell pore creation D@E + cross-linking and shaping + low temperature vacuum core removal) (1) Final positive electrode dry membrane formulation (based on 100 parts by weight after removing D) Component A (lithium iron phosphate system) 96.3 parts: A1: 78.0 parts of secondary particulate lithium iron phosphate (D50=6–12μm, tap density ≥1.3g / cm³, carbon coating 0.8–2.0%). A2: 18.3 parts of fine-grained interstitial lithium iron phosphate (D50 = 0.6–1.8 μm); Component B (conductive network) 2.0 parts; 1.7 parts of high-structure conductive carbon black (BET 50–80 m² / g, DBP 250–380 mL / 100 g). 0.3 parts of carbon nanotubes (diameter 6–15 nm, length 5–20 μm); Component C (crosslinking and curing adhesive system) 1.4 parts; Sodium carboxymethyl cellulose 0.55 parts (DS 0.7–1.2); Styrene-butadiene rubber (solid content) 0.55 parts (Tg -30 to -5℃); 0.20 parts polyacrylic acid (number average molecular weight 1×10⁻⁶) 5 –6×10 5 ); 0.10 parts of water-dispersible polyepoxy crosslinking agent (number average molecular weight 300–800; epoxy equivalent 150–350 g / eq); Component E (inorganic nanomaterials lining the pore walls) 0.30 parts; 0.25 parts of boehmite nanocomposition (20–80 nm or nanofiber morphology, specific surface area 60–160 m² / g); 0.05 parts of nano-silica (10–40 nm, optional, for shell structure stabilization); (2) The additional D core material added during preparation (will be removed) Total addition of D: 2.2 parts (relative to the above 100 parts of solids); D-particle size: D1 (main channel core) D50=12–28μm, D2 (bridging core) D50=2–6μm; D1:D2=6:1; D morphology: Core-shell porous unit D@E (i.e., D surface preloaded with E shell; shell mass fraction 6–20%, shell thickness approximately 80–350 nm, forming a porous shell to facilitate sublimation and escape). (3) Process Rolling: Roll the D@E-containing precursor electrode to a high precursor density (e.g., on the order of 2.85–2.90 g / cm³, as a density reserve after core removal).

[0066] Vacuum heat treatment (core removal + cross-linking and shaping in one step): 105–120℃, 50–500Pa, 1.5–4h; Simultaneously: D sublimation removal → forming macropore channels; cross-linking and curing → pore structure shaping and anti-swelling; E remaining on the pore wall → forming an electrophilic liner.

[0067] Comparative Example (C1): Conventional high-voltage solid-thickness electrode sheet (without D, without E, and without cross-linking) Formulation: A96.3; B2.0; the bonding system consists of only CMC+SBR totaling 1.7 (or totaling 1.4), free of polyacrylic acid, crosslinking agent, and inorganic E.

[0068] Process: Conventional drying + roller pressing to 2.72g / cm³ + vacuum drying to remove water.

[0069] Comparative Example (C2): Conventional optimization to increase porosity by reducing roll pressure; Formula: Same as C1.

[0070] Process: Roller pressing to 2.55 g / cm³ (significantly low compaction), the rest are the same.

[0071] Comparative Example (C3): Using only sublimable porous core material D (without shell E, without crosslinking) Formulation: Add D (2.2 parts, D1:D2=6:1) to C1, but D does not form a core-shell (no E shell); the bonding system is still CMC+SBR (no crosslinking).

[0072] Process: Roll pressing (with high-density pre-reserved for the precursor) + vacuum sublimation at 105–120℃ to remove D.

[0073] Comparative Example (C4): Core-shell D@E, but without cross-linking and shaping. Formulation: Same as Example E1 (containing D@E), but the bonding system is reduced to CMC+SBR (no polyacrylic acid, no crosslinking agent).

[0074] Process: Same as E1 (vacuum core removal), but without cross-linking reaction.

[0075] Comparative Example (C5): D+E but uniformly blended (without a positioning liner) and cross-linked (demonstrating the non-obviousness of the positioning distribution). Formula: The overall composition is equivalent to E1 (A, B, C, E, and D are used in the same amounts), the only difference is: D represents bare core material (without core-shell). E (boehmite / silica) is uniformly incorporated into the slurry as a free nanoparticle, rather than being preloaded onto the D surface.

[0076] Process: Same as E1 (vacuum core removal + cross-linking).

[0077] III. Test Items and Test Standards 1) Coating thickness Standard: IS4593 (Mechanical scanning method for thickness of plastic films and sheets) or equivalent GB / T6672-2001.

[0078] Diameter: Mechanical scanning thickness gauge / dead weight thickness gauge is used; ≥10 points are taken along the width and length of each electrode sheet; Coating thickness = (total electrode thickness - aluminum foil thickness); aluminum foil thickness is measured separately or verified using the nominal value of aluminum foil from the same batch.

[0079] Thickness change after immersion: The thickness was measured again after immersion in electrolyte for 24 hours, and the difference ΔT was taken.

[0080] 2) Compacted density (coating bulk density) Industry-standard calculation method: Take the coating mass of a known area (e.g., a 20mm diameter stamping sheet) (excluding the aluminum foil mass). Volume = Area × Coating Thickness; Compacted Density = Mass / Volume.

[0081] Thickness measurement shall refer to GB / T6672 or IS4593.

[0082] 3) Pore size distribution, open porosity, and macropore volume fraction Standard: IS15901-1 (Method for determining pore size distribution and porosity by mercury indentation).

[0083] Aperture: Sample: Coating after aluminum foil removal (or directly measure coating peeling sample).

[0084] Output indicators: open porosity (%); pore size distribution; and the cumulative pore volume in the >5μm pore size range is converted to obtain the macropore volume fraction (vl%).

[0085] 4) Electrolyte absorption rate, t95 (time to reach 95% of the theoretical absorption capacity) IS62 or ASTM D570 (plastic liquid absorption / water absorption mass increment method) is used for mass increment-time determination framework; in the battery field, the medium is often replaced with electrolyte while maintaining the same weighing diameter.

[0086] step: Stamping: 20mm in diameter; vacuum dried at 105℃ for 12h; cooled in a desiccator for 30min; weighed initial mass m0.

[0087] Electrolyte: 1 ml / L lithium hexafluorophosphate / carbonate system (e.g., ethylene carbonate / methyl ethyl carbonate / diethyl carbonate volume ratio 3 / 5 / 2), 25±1℃.

[0088] The lamination was placed in the electrolyte, and a vacuum of 200 Pa was drawn and maintained for 5 minutes (simulating liquid injection and degassing). Then, the pressure was restored to normal and the timing was started.

[0089] Samples were taken at t=1, 3, 5, 10, 20, 40, 60, 90, and 120 min: the film was removed and the surface free liquid was gently absorbed with lint-free paper (fixed for 5 s), weighed mt, and immediately put back.

[0090] Calculate the absorption rate W(t) = (mt-m0) / m0 × 100%.

[0091] Based on the theoretical calculation of the volume conversion of the opening obtained from IS15901-1, the absorbable liquid mass m_thery is defined as R(t)=(mt-m0) / m_thery×100%.

[0092] t95: The time when R(t) reaches 95% (obtained by interpolation).

[0093] 5) Hole-making core material residue (sufficiency of core removal) Standard: IS11358-1 (General principles for thermogravimetric analysis TG).

[0094] Diameter: Take the coated sample after core removal and perform thermogravimetric analysis (nitrogen atmosphere). Evaluate the residual mass fraction (wt%) based on the decomposition / volatilization temperature range of the target core material.

[0095] 6) Coating adhesion to aluminum foil (peel strength) Standard: GB / T2792-2014 (180° peel strength test method).

[0096] Diameter: With 180° peeling and a fixed peeling rate (e.g., 300 mm / min), record the average peeling force in the stable range; the unit is expressed in N / cm.

[0097] The sample width and bonding method can be referenced in GB / T2792 for general fixture and speed settings.

[0098] IV. Performance test data (n=5, mean ± standard deviation)

[0099] All samples ultimately (after core removal) had the same density of positive electrode active layer; For samples containing D and cored, the solid areal density during coating needs to be increased: if the amount of D increases by 2.2 parts relative to 100 parts of the final dry film, then the target areal density during coating = 33.2 × (1 + 0.022) ≈ 33.93 mg / cm²; after core removal, it returns to 33.2 mg / cm². Tests such as compaction density and liquid absorption are all performed in the cored state.

[0100] Table 2. Immersion / Absorption Dynamics and Mechanical Integrity Indicators (Liquid absorption test: Refer to the mass increment method framework of IS62 / ASTM D570, with the medium replaced by electrolyte; t95 is defined as 95% of the theoretical absorbable liquid mass; adhesion is in accordance with GB / T2792-2014.) The saturated aspirate volume W∞ is defined as the increase in aspirated mass over 24 hours (or the plateau value reached in 120 minutes); t95 is defined as the time to reach 0.95 × W∞.

[0101] V. Conclusion Comparison between E1 and C1: Both have a compaction density of 2.72 g / cm³ and similar open porosity (19.1% vs 18.3%), but the t95 decreases from 78 min to 18 min, and the liquid absorption rate at 10 min increases from 28% to 84%.

[0102] Note: The mechanism by which this solution addresses the issue of dry cores / slow wetting in thick electrodes is not based on overall pore enlargement (otherwise, it should manifest as a significant increase in porosity), but rather on the reconstruction of the mass transfer path brought about by the high-speed macropore channels, hydrophilic linings on the pore walls, and instantaneous shaping after core removal.

[0103] With the same formulation, different distribution patterns yield significantly different results (C5 vs E1). C5 and E1 have equivalent raw material usage (both contain D, E, and crosslinking), the only difference is: C5:E represents the uniform blending of free nanoparticles; E1: E is preloaded on the surface of D to form D@E, which automatically becomes the hole wall liner after core removal.

[0104] Results: The t95 (18 min) of E1 was still significantly better than that of C5 (29 min).

[0105] C4 (with D@E but no crosslinking) showed good wetting (t95=24min), but its 24h immersion thickness increase and peel strength deteriorated significantly (ΔT=6.0μm, peel strength 0.90N / cm), indicating that the pore structure / interface was more unstable after immersion.

[0106] While maintaining faster wetting, E1 significantly reduced the thickness increment after soaking and maintained high peel strength (ΔT=1.8μm, peel strength 1.28N / cm). This indicates that the instantaneous cross-linking and shaping during core removal and pore formation is not a dispensable conventional addition, but rather provides structural stability for high-pressure compaction and macropore channels, avoiding engineering problems such as pore collapse, pulverization, and lining migration.

[0107] Reducing compaction can indeed improve wetting, but at the cost of volumetric energy density: C2 reduced the compaction time to 32 minutes, but the compaction density decreased from 2.72 to 2.55 g / cm³.

[0108] E1 achieves faster wetting while maintaining high pressure density, which is closer to the real motivation of needing both energy density and yield / tick rate.

[0109] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A high-density lithium iron phosphate cathode, characterized in that: include: The positive current collector and its surface positive active layer have a compaction density of not less than 2.65 g / cm³ after removing the removable core material. The active layer contains lithium iron phosphate active material system component A, conductive network system component B, cross-linked curing adhesive system component C, and channel liner type electrolyte-loving inorganic nano component E. Component E includes boehmite nanoparticles or boehmite nanofibers, and component C includes sodium carboxymethyl cellulose, styrene-butadiene rubber latex, polyacrylic acid, and water-dispersible polycarbodiimide crosslinking agent or water-dispersible polyepoxy crosslinking agent. The active layer forms a through macropore channel, and the macropore channel wall is provided with a pore wall liner composed of component E. The macropore channel is formed after the removable core material in the core-shell pore-forming unit D@E is removed, and the shell of the core-shell pore-forming unit D@E contains component E.

2. The high-density lithium iron phosphate cathode sheet according to claim 1, characterized in that: Based on the positive electrode active layer after removing the removable core material, the content of component B is 1.2 to 2.8% by mass, the content of component C is 1.0 to 2.2% by mass, the content of component E is 0.05 to 0.60% by mass, and the balance is component A, and the pore size of the macropore channel is greater than 5 micrometers.

3. The high-density lithium iron phosphate cathode sheet according to claim 2, characterized in that: Component A includes secondary particulate lithium iron phosphate A1 and fine-particle lithium iron phosphate A2; The median particle size D50 of A1 is 6 to 12 micrometers, the tap density is not less than 1.3 g per cubic centimeter, and the carbon coating content is 0.8 to 2.0% by mass. The median particle size D50 of A2 is 0.6 to 1.8 micrometers, and the content of A2 in the total amount of A1 and A2 is 5 to 25% by mass.

4. The high-density lithium iron phosphate cathode sheet according to claim 3, characterized in that: Component B includes conductive carbon black B1 and conductive carbon nanotubes B2, with a mass ratio of B1 to B2 of 6 to 12 to 1. The conductive carbon nanotubes have a diameter of 6 to 15 nanometers and a length of 5 to 20 micrometers. The conductive carbon black is a medium specific surface area conductive carbon black with a nitrogen adsorption specific surface area of ​​40 to 90 square meters per gram and a DBP oil absorption value of 120 to 220 milliliters per 100 grams, or a high specific surface area high structure conductive carbon black with a nitrogen adsorption specific surface area of ​​600 to 1600 square meters per gram and a DBP oil absorption value of 350 to 520 milliliters per 100 grams.

5. A high-density lithium iron phosphate cathode according to claim 4, characterized in that: The total content of component C is 1.0 to 2.2% by mass, the content of crosslinking agent in the final dry film is 0.05 to 0.25% by mass, and component E further includes nano-silica with a particle size of 10 to 40 nanometers.

6. A method for preparing a high-density lithium iron phosphate cathode sheet, characterized in that: include: An inorganic dispersion containing boehmite and polyacrylic acid was prepared and deposited on the surface of removable core material particles to obtain a core-shell porous unit D@E; A positive electrode slurry was prepared by mixing lithium iron phosphate active material system component A, conductive network system component B, cross-linked curing adhesive system component C with core-shell pore-forming unit D@E; the positive electrode slurry was coated on the positive electrode current collector and dried and rolled; the rolled precursor electrode was subjected to vacuum heat treatment to remove the removable core material and cross-link the adhesive system to obtain a positive electrode with an inorganic nano-liner layer on the pore wall of the macropore channel.

7. The preparation method according to claim 6, characterized in that: The inorganic dispersion has a pH of 7.0 to 9.0, a boehmite solid content of 5 to 12% by mass, and a polyacrylic acid concentration of 0.2 to 0.6% by mass. The inorganic dispersion is subjected to high-speed shear dispersion for 20 to 40 minutes and ultrasonic dispersion for 10 to 20 minutes.

8. The preparation method according to claim 7, characterized in that: The removable core material is adamantane or a high-melting-point adamantane derivative of the same group. The removable core material includes a first core material particle D1 with a median particle size D50 of 12 to 20 micrometers and a second core material particle D2 with a median particle size D50 of 2 to 5 micrometers. The mass ratio of D1 to D2 is 4:1 to 8:

1. The amount of removable core material added is 0.8 to 3.5% by mass relative to the total solids of components A, B, C and E in the positive electrode active layer after removing the removable core material. The shell mass fraction of the core-shell pore-forming unit D@E is 6 to 20% by mass relative to the removable core material and the shell thickness is 80 to 350 nanometers.

9. The preparation method according to claim 8, characterized in that: The positive electrode slurry has a solid content of 58 to 68% by mass. The core-shell pore-forming unit D@E is added after the slurry is homogenized by low-shear mixing for 8 to 15 minutes and vacuum degassing for 10 to 20 minutes. Roll pressing is then applied to achieve a compaction density of 2.75 to 2.90 g / cm³ for the precursor electrode containing the removable core material.

10. The preparation method according to claim 9, characterized in that: Vacuum heat treatment is carried out under conditions of vacuum degree of 20 to 500 Pa, temperature of 105 to 150 degrees Celsius, and holding time of 0.5 to 6 hours.

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