High-energy-density lithium iron phosphate cylindrical battery and preparation method thereof

Abstract

The invention discloses a high-energy-density lithium iron phosphate cylindrical battery and a preparation method thereof, and relates to the technical field of lithium iron phosphate batteries, the battery is of a full-tab or tab-free winding structure, the end face of a micro tab at the positive electrode end and an aluminum or aluminum alloy collector ring are pressed tightly and are subjected to laser welding at a welding window; an annular two-layer gradient micropore composite interface layer is arranged on the end face of the collector ring and comprises a bonding passivation seed layer and a heat-conducting and electricity-conducting micropore network layer, the two layers both contain aluminum fluoride particles and lithium phosphate particles, and the outer layer contains hexagonal boron nitride, aluminum nitride and a conductive carbon skeleton. The diaphragm is provided with an anode end function band, and the electrolyte contains lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, lithium difluorophosphate and lithium difluoro (oxalato) borate; and pre-charging and pulse directional activation formation are carried out after liquid injection infiltration. The battery inhibits end hot spots and corrosion and reduces end temperature difference under fast charging and high power, connection resistance drift and gas production are reduced, a fast charging window is improved, the cycle life is prolonged, and the consistency is improved.

Description

Technical Field

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

[0002] In the power systems of electric passenger vehicles and light commercial vehicles, in order to balance safety, cost and cycle life, lithium iron phosphate systems are gradually evolving towards large-size cylindrical cell forms, and are combined with designs such as full-tab / tabless winding structures, thin current collectors and high-voltage solid-thickness electrodes to improve volume utilization and adapt to high-rate charging and discharging.

[0003] Especially in large cylindrical sizes such as the 46 series, cooling is mostly implemented from the cylindrical sidewalls, resulting in relatively unfavorable heat dissipation boundary conditions at the ends and end caps. Simultaneously, the cells are often arranged axially side-by-side within the battery pack, limiting the contact between the ends and structural components, increasing the difficulty of end temperature monitoring and uniform temperature control. These cells require maintaining low DC internal resistance and stable end electrical connections under high current density conditions in scenarios such as rapid charging at charging stations, long downhill energy recovery, and frequent acceleration. Existing technologies typically achieve current collection through welding, crimping, or conductive connections between the end current collectors and electrode edges, combined with conventional separator and electrolyte systems to complete formation and long-term cycling. However, under high energy density constraints, the amount of electrolyte used inside the cell tends to converge, the electrode pore structure becomes denser, the end space is compact, and the heat diffusion path is limited. Furthermore, differences in materials and contact states exist between the end caps, current collectors, and welding areas, leading to more sensitive electrical, thermal, and interface reactions at the ends, resulting in different aging characteristics and consistency fluctuations compared to the central region under high-rate operating conditions.

[0004] In the above application scenarios, the single core technical problem that existing high-energy-density lithium iron phosphate cylindrical batteries generally face is that the current collection connection area at the end of the all-tab / tablet is prone to forming hot spots that are coupled with contact resistance drift, local heating and interface corrosion, and generate positive feedback evolution during cycling.

[0005] The mechanism typically manifests as follows: the short current convergence path and concentrated current density at the end point, along with minute contact resistance differences around the weld nugget or at the micro-contact points, cause localized Joule heating and temperature gradients. This temperature rise accelerates side reactions of the electrolyte under high potential / high temperature conditions at the end point, generating acidic or fluorine-containing intermediates and promoting corrosion, film thickening, or gas generation on the current collector metal surface. The accumulation of corrosion products and gas can also cause the formation of interfacial micropores and attenuation of contact pressure, thereby reducing the effective contact area and further concentrating the current, while simultaneously exacerbating the end impedance distribution gradient. If this problem cannot be effectively suppressed, it may lead to increased fast-charging polarization and a narrower usable charging rate window, increased DC internal resistance dispersion, premature capacity knee in cycle life, and increased risk of bulging and safety valve activation under high-temperature storage or high-power pulse conditions, thus affecting the reliability and consistency of the battery cell in the vehicle and energy storage system. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] To address the shortcomings of existing technologies, this invention provides a high-energy-density lithium iron phosphate cylindrical battery and its preparation method. The battery comprises a bonding and passivation seed layer and a thermally and electrically conductive microporous network layer, both containing aluminum fluoride particles and lithium phosphate particles. The outer layer contains hexagonal boron nitride, aluminum nitride, and a conductive carbon framework. The separator has a positive terminal functional band, and the electrolyte contains lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium difluorophosphate, and lithium difluorooxalate borate. After electrolyte injection and wetting, pre-charging and pulse-directed activation formation are performed. This battery suppresses end hot spots and corrosion and reduces end temperature differences under high-power fast charging, reducing connection resistance drift and gas generation, and improving cycle life and consistency. It solves the technical problems described in the background art.

[0008] (II) Technical Solution

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A high-energy-density lithium iron phosphate cylindrical battery includes a housing, an end cap, a fully tabbed or tabless spiral-wound electrode assembly placed inside the housing, and an electrolyte; the electrode assembly includes a lithium iron phosphate positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode each have bare foil edges and form micro-tab teeth.

[0011] At the positive terminal, the end face of the micro-tab is pressed tightly against the positive terminal current collector ring of aluminum or aluminum alloy. The annular region of the end face of the positive terminal current collector ring is provided with a double-layer gradient microporous composite interface layer. The double-layer gradient microporous composite interface layer is formed by sequentially stacking a bonding-passivation seed layer near the positive terminal current collector ring and a thermally conductive-conductive microporous network layer on its outer side, with a gap left at the welding window. The thickness of the bonding-passivation seed layer is 2 to 8 micrometers, and the thickness of the thermally conductive-conductive microporous network layer is 10 to 30 micrometers with a porosity of 25% to 50%. Both the bonding-passivation seed layer and the thermally conductive-conductive microporous network layer contain aluminum fluoride nanoparticles and lithium phosphate nanoparticles. The thermally conductive-conductive microporous network layer also contains hexagonal boron nitride flake particles, aluminum nitride particles, vapor-grown carbon fibers, and conductive carbon black.

[0012] Furthermore, the dimensions of the welding window are 0.2×0.6 mm to 0.6×1.5 mm, and the circumferential spacing is 0.8 to 2.0 mm; the thickness of the positive end collector ring is 0.30 to 0.80 mm, and the end face is provided with surface microgrooves with a groove depth of 0.05 to 0.25 mm, a groove width of 0.15 to 0.50 mm, and a groove spacing of 0.30 to 1.00 mm.

[0013] Furthermore, the diaphragm is a three-layer microporous membrane of polypropylene / polyethylene / polypropylene with a double-sided alumina ceramic coating. One long side of the diaphragm has a positive end functional band with a width of 0.6 to 1.6 mm, a dry film thickness of 3 to 8 micrometers, and a porosity of 35 to 55%. The positive end functional band includes hexagonal boron nitride flake particles, alumina nanoparticles, lithium phosphate nanoparticles, aluminum fluoride nanoparticles, and polyvinylidene fluoride-hexafluoropropylene copolymer binder. A microporous window pattern is formed on the surface of the functional band, with a 0.3 × 0.6 mm low coating area set every 2 to 4 mm.

[0014] Furthermore, the lithium salt in the electrolyte comprises 0.80 to 1.00 mol / L lithium hexafluorophosphate and 0.10 to 0.25 mol / L lithium difluorosulfonylimide; the solvent comprises ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and methyl acetate in a volume ratio of 10 to 25: 25 to 45: 25 to 45: 0 to 20; and contains 0.3 to 1.5% by mass lithium difluorophosphate, 0.2 to 1.2% by mass lithium difluorooxalate borate, 0.5 to 4.0% by mass fluoroethylene carbonate, 0.1 to 0.8% by mass tris(trimethylsilyl)phosphate and 0.1 to 1.0% by mass ethylene sulfate.

[0015] Furthermore, the bonding-passivation seed layer comprises, on a dry powder basis, 30 to 55% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 10 to 30% by mass of conductive carbon black, 1 to 5% by mass of carbon nanotubes, 10 to 35% by mass of aluminum fluoride nanoparticles, and 5 to 20% by mass of lithium phosphate nanoparticles.

[0016] The thermally and electrically conductive microporous network layer, on a dry powder basis, comprises 25 to 50% by mass of hexagonal boron nitride flake particles, 8 to 20% by mass of aluminum nitride particles, 5 to 15% by mass of vapor-grown carbon fibers, 2 to 12% by mass of graphite flakes, 0.5 to 4% by mass of carbon nanotubes, 2 to 8% by mass of conductive carbon black, 10 to 25% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 1 to 8% by mass of aluminum fluoride nanoparticles, and 1 to 8% by mass of lithium phosphate nanoparticles; wherein the D50 of the aluminum fluoride nanoparticles is 20 to 80 nanometers, and the D50 of the lithium phosphate nanoparticles is 50 to 200 nanometers.

[0017] A method for preparing a high-energy-density lithium iron phosphate cylindrical battery includes: preparing a lithium iron phosphate positive electrode sheet and a negative electrode sheet with bare foil edges and forming micro-ear teeth; sequentially forming a bonding-passivation seed layer with a thickness of 2 to 8 micrometers and a thermally conductive-conductive microporous network layer with a thickness of 10 to 30 micrometers in the annular region of the current collector ring end face of the aluminum or aluminum alloy positive electrode, the two layers constituting a two-layer gradient microporous composite interface layer, and reserving a welding window;

[0018] The positive electrode, separator and negative electrode are wound and the functional band of the positive end of the separator is positioned at the positive end. A pressure of 1 to 3 MPa is applied to the end face of the micro electrode to press the end face of the micro electrode tab tightly onto the two layers of gradient microporous composite interface layer and laser welding is performed at the welding window.

[0019] After vacuum drying, electrolyte is injected and allowed to stand for 8 to 24 hours. After pre-charging, 2 to 10 sets of pulse charge and discharge are performed, followed by routine formation and aging capacity testing.

[0020] Furthermore, the slurry for the bonding-passivation seed layer comprises, on a dry powder basis, 30 to 55% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 10 to 30% by mass of conductive carbon black, 1 to 5% by mass of carbon nanotubes, 10 to 35% by mass of aluminum fluoride nanoparticles, and 5 to 20% by mass of lithium phosphate nanoparticles. The slurry is prepared using a mixed solvent of acetone and N-methylpyrrolidone and applied by screen printing or precision dispensing.

[0021] Furthermore, the slurry of the thermally conductive-conductive microporous network layer, on a dry powder basis, comprises 25 to 50% by mass of hexagonal boron nitride, 8 to 20% by mass of aluminum nitride, 5 to 15% by mass of vapor-grown carbon fibers, 2 to 12% by mass of graphite sheets, 0.5 to 4% by mass of carbon nanotubes, 2 to 8% by mass of conductive carbon black, 10 to 25% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 1 to 8% by mass of aluminum fluoride, and 1 to 8% by mass of lithium phosphate.

[0022] The mass ratio of acetone to N-methylpyrrolidone in the mixed solvent of the slurry is 70:30 to 95:5, and the solid content is 45 to 60% by mass. Further, cell assembly is completed in a dry environment with a dew point not exceeding -40 degrees Celsius, and vacuum drying includes vacuum treatment at 110 degrees Celsius for 12 to 24 hours. After liquid injection, the cells are allowed to stand at 25°C for 12 hours; pre-charged at 0.10C to 3.45V and allowed to stand for 30 minutes; six sets of pulses are performed, each set consisting of 1.0C charging for 30 seconds, standing for 60 seconds, 1.0C discharging for 30 seconds, and standing for 60 seconds; then conventional formation is performed: charged at 0.33C to 3.65V, constant voltage to 0.05C, and allowed to stand for 30 minutes; discharged at 0.33C to 2.50V; aged for 24 to 72 hours and then capacity testing is performed.

[0023] (III) Beneficial Effects

[0024] This invention provides a high-energy-density lithium iron phosphate cylindrical battery and its preparation method, which has the following beneficial effects:

[0025] Two layers of gradient microporous composite interface layer are set at the positive end current collector connection of the all-tab cylindrical battery cell and a welding window is reserved, so that the microtab and the current collector ring can simultaneously obtain metallurgical connection and stable contact, thus suppressing current congestion from the source.

[0026] In the composite interface layer, hexagonal boron nitride and aluminum nitride construct end-effector thermal diffusion channels, while the conductive carbon framework maintains a low and constant interfacial resistance. Aluminum fluoride nanoparticles and lithium phosphate nanoparticles form reaction seeds within the micropores, cooperating with lithium difluorophosphate and lithium difluorooxalate borate in the electrolyte, and undergoing directional activation to induce pulse-induced preferential film formation at the ends, thereby inhibiting corrosion and gas generation and blocking the positive feedback of increased resistance and accelerated temperature rise.

[0027] The positive end functional band of the diaphragm further provides channels for heat conduction and acid capture, reducing end temperature difference and aging dispersion. Since the key structure is only locally arranged and does not increase the proportion of the main body inactive components, it can significantly improve the fast charging window, power retention and long-term consistency while maintaining high energy density, and further significantly improve formation cycle time and mass production consistency. Attached Figure Description

[0028] Figure 1This is a schematic diagram of the overall structure of the large cylindrical omnipolar battery cell in this design.

[0029] Figure 2 This is a magnified cross-sectional view of the positive extreme part of the structure of this scheme;

[0030] Figure 3 This is a schematic diagram of the two-layer gradient microporous composite interface layer structure of this scheme;

[0031] Figure 4 This is a flowchart of the cell manufacturing and assembly process for this solution;

[0032] Figure 5 This is a schematic diagram of the end-failure suppression mechanism of this scheme;

[0033] Figure 6 This is a comparison chart of the hot spots at the fast charging end of this solution. Detailed Implementation

[0034] 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.

[0035] Please see Figures 1-6 This invention provides a high-energy-density lithium iron phosphate cylindrical battery and its preparation method, comprising:

[0036] Component A: High energy density lithium iron phosphate cathode sheet

[0037] A1 Positive Electrode Active Material

[0038] Material: Carbon-coated lithium iron phosphate (LiFePO4) secondary particles;

[0039] Parameter range: D50: 3–8 micrometers; D90: ≤15 micrometers; Specific surface area (BET): 6–15 m² / g; Carbon coating: 1.0–3.0% by mass (based on active material); Tap density: ≥1.1 g / cm³; Moisture: ≤300 ppm (Kal Fischer method);

[0040] Example values: D50 approximately 5.0 micrometers; D90 approximately 12 micrometers; carbon coating approximately 2.0%; moisture ≤ 200 ppm.

[0041] A2 conductive system

[0042] Objective: To maintain electronic pathways and reduce end-micro-contact resistance sensitivity under high-pressure solid-thickness coating.

[0043] Parameter ranges (based on total solids in the cathode coating): Acetylene black: 0.6–1.5%; Multi-walled carbon nanotubes: 0.15–0.60%; Vapor-grown carbon fibers: 0.15–0.80%;

[0044] Example (by weight):

[0045] Acetylene black 1.0%; Multi-walled carbon nanotubes 0.30%; Vapor-grown carbon fibers 0.40%;

[0046] A3 adhesive system

[0047] Polyvinylidene fluoride (PVDF) is used as the main binder (solvent method, which facilitates high-pressure compaction and edge cleanliness); parameter range: PVDF: 2.0–3.5% (based on positive electrode coating solids);

[0048] Example: Polyvinylidene fluoride 2.50%;

[0049] A4 Positive Electrode Formulation Summary

[0050] Example (mass percentage, solids): Carbon-coated lithium iron phosphate: 95.80%; Acetylene black: 1.00%; Multi-walled carbon nanotubes: 0.30%; Vapor-grown carbon fibers: 0.40%; Polyvinylidene fluoride: 2.50%;

[0051] A5 cathode structure and process parameters

[0052] Current collector: Aluminum foil thickness 10–14 μm (12 μm in example); Single-sided dry film thickness: 160–185 μm; preferably 165–175 μm (170 μm in example); Single-sided areal density (including binder and conductive agent): 32–45 mg / cm² (41 mg / cm² in example); Porosity after compaction: 28–31% (30% in example); Bare foil edge required for end tab design; Positive electrode bare foil edge width: 1.2–2.2 mm (1.6 mm in example); Coating boundary burr height of bare foil edge area: ≤10 μm (≤8 μm in example); Permissible coating residue in bare foil edge area: ≤0.5 mg / dm²;

[0053] Component B high energy density negative electrode

[0054] B1 negative electrode active material system: To balance energy density and fast charging availability window: mainly graphite, with a small amount of silicon-oxygen / carbon composite to improve capacity but avoid excessive expansion.

[0055] Parameter range (based on total solids in the negative electrode coating): Artificial graphite: 88–95%; Silicon-oxygen / carbon composite: 2–8%; Hard carbon or soft carbon: 0–3% (optional, for improving low temperature / rate).

[0056] Example: Artificial graphite 91.50% (D50 14–20 μm, moisture ≤200 ppm); silicon oxide / carbon composite 5.00% (D50 2–6 μm, surface pre-carbonized); hard carbon 1.00%;

[0057] B2 conductive system

[0058] Parameter range: Conductive carbon black: 0.5–1.5%; Carbon nanotubes: 0.05–0.30%; Example: Conductive carbon black 0.80%; Multi-walled carbon nanotubes 0.20%;

[0059] B3 binder system: The aqueous system improves electrode toughness, reduces edge pulverization, and facilitates end micro-contact stability.

[0060] Parameter range: Sodium carboxymethyl cellulose: 0.4–1.2%; Styrene-butadiene rubber: 0.4–1.5%; Lithium polyacrylate or polyacrylic acid: 0.1–0.6% (for enhanced silicone bonding and elastic recovery);

[0061] Example: Sodium carboxymethyl cellulose 0.70%; styrene-butadiene rubber 0.60%; lithium polyacrylate 0.20%;

[0062] Summary of B4 negative electrode formulations

[0063] Examples (by weight, solids): Artificial graphite: 91.50%; Silicon-oxygen / carbon composite: 5.00%; Hard carbon: 1.00%; Conductive carbon black: 0.80%; Multi-walled carbon nanotubes: 0.20%; Sodium carboxymethyl cellulose: 0.70%; Styrene-butadiene rubber: 0.60%; Lithium polyacrylate: 0.20%;

[0064] B5 negative electrode structure and process parameters:

[0065] Current collector: copper foil 6–10 micrometers (8 micrometers in this example); single-sided dry film thickness: 110–130 micrometers; preferably 115–125 micrometers (120 micrometers in this example); single-sided areal density: 14–24 mg / cm² (18 mg / cm² in this example); porosity after compaction: 28–36% (34% in this example); edge width of the bare negative electrode foil: 1.4–2.4 mm (1.8 mm in this example); burr height of the bare foil edge area: ≤10 micrometers (≤8 micrometers in this example).

[0066] Component C electrolyte system:

[0067] C1 lithium salt system: Parameter range: Lithium hexafluorophosphate: 0.80–1.00 M mol / L; Lithium difluorosulfonyl imide: 0.10–0.25 mol / L; Example: Lithium hexafluorophosphate 0.90 mol / L; Lithium difluorosulfonyl imide 0.20 mol / L;

[0068] C2 solvent system: Parameter range (volume ratio): Ethylene carbonate: 10–25; Ethyl methyl carbonate: 25–45; Dimethyl carbonate: 25–45; Methyl acetate: 0–20 (used for viscosity reduction, improved wetting and fast-charge polarization);

[0069] Example volume ratio: ethylene carbonate: ethyl methyl carbonate: dimethyl carbonate: methyl acetate = 20:35:35:10;

[0070] C3 Additive Package: Key Additive 1: Lithium difluorophosphate (LiPO2F2); Mass percentage range: 0.3–1.5% (Example 1.0%); Provides a phosphorus-fluorine film-forming precursor in hot / high potential regions, enabling localized rapid passivation in conjunction with end phosphate / fluoride seeding.

[0071] Key Additive 2: Lithium difluorooxalate borate (LiDFOB); mass percentage range: 0.2–1.2% (0.8% in examples); inhibits aluminum side corrosion, improves high-temperature interface stability; and forms a more heat-resistant composite inorganic / organic film-forming component together with the phosphate system. Anode Film Enhancement: Fluorinated ethylene carbonate (FEC); mass percentage range: 0.5–4.0% (2.0% in examples); used for film formation on silicon-oxygen anodes and for expanding the fast-charging lithium suppression deposition window.

[0072] End-cap HF management / flame retardant aid: Tris(trimethylsilyl)phosphate; mass percentage range: 0.1–0.8% (0.4% in examples); works with end-cap fluoride / phosphate seeds to reduce HF-driven corrosion chain reactions. Film densification: Ethylene sulfate; mass percentage range: 0.1–1.0% (0.3% in examples); promotes the formation of denser, more heat-resistant interfacial films.

[0073] Component D membrane system

[0074] D1 base membrane: Structure: Polypropylene / polyethylene / polypropylene three-layer microporous membrane;

[0075] Parameter range: Thickness: 10–16 micrometers (12 micrometers in example); Porosity: 35–55% (40% in example); Average pore size D50: 30–120 nanometers (80 nanometers in example); Air permeability (Gurley): 60–160 seconds / 100 cubic centimeters (90 seconds / 100 cubic centimeters in example);

[0076] D2 Conventional ceramic coating: A porous coating layer on each side, with a thickness of 1.0–2.5 micrometers (1.5 micrometers in the example);

[0077] Dry powder composition range: Alumina: 90–97%; Binder (such as styrene-butadiene rubber or acrylic emulsion system): 3–10%;

[0078] D3 Positive Terminal Functional Strip: Arrangement: The functional strip is set only on the long side of the diaphragm; this long side is positioned at the positive terminal (the side close to the positive current collector end cap) during winding.

[0079] Geometric parameters: Functional band width: 0.6–1.6 mm (1.0 mm in example); Functional band dry film thickness: 3–8 μm (4 μm in example); Functional band porosity: 35–55% (approximately 45% in example); Functional band dry powder formulation (example, mass percentage);

[0080] Hexagonal boron nitride flake particles: 45% (D50 approximately 2 μm, flake thickness 0.2–0.4 μm); alumina nanoparticles: 25% (D50 approximately 300 nm); lithium phosphate nanoparticles: 15% (D50 approximately 100 nm).

[0081] Aluminum fluoride nanoparticles: 5% (D50 approximately 40 nm); Polyvinylidene fluoride-hexafluoropropylene copolymer binder: 10%;

[0082] Microporous window patterns are made on the surface of the functional strip (e.g., a 0.3×0.6 mm low-coating area is set every 2–4 mm) to: 1) avoid excessive obstruction of end electrolyte wetting; 2) provide a stable liquid replenishment channel for the end composite interface layer; 3) reduce the risk of functional strip particle migration.

[0083] Component E end current collection and three-functional composite interface layer system:

[0084] E1 collector component: E1-1 positive end collector ring (or end cap collector plate).

[0085] Materials: High-purity aluminum or aluminum alloys (e.g., 1050, 1070, 3003); Parameter range: Thickness: 0.30–0.80 mm (0.50 mm in this example); Surface roughness Ra: 1.5–4.0 μm (2.5 μm in this example); Surface microgrooves: used to improve clamping contact and retention of the composite interface layer; Groove depth: 0.05–0.25 mm (0.15 mm in this example); Groove width: 0.15–0.50 mm (0.30 mm in this example); Groove spacing: 0.30–1.00 mm (0.60 mm in this example);

[0086] E1-2 Negative End Current Collector Component: Material: Copper (C1020, etc.) or nickel-plated copper; Thickness: 0.25–0.60 mm (0.40 mm in the example); E2 Positive End Trifunctional Composite Interface Layer: E2-0 Structural Definition: This interface layer is a two-layer gradient structure, and covers the contact zone between the positive electrode tabless micro-tab end face and the positive electrode current collector ring (e.g., a continuous ring around the end) with an annular region, but leaves a partial gap at the welding window.

[0087] The purpose of the two-layer gradient is: the inner layer is more passivating and bonding, controlling corrosion to start from the metal surface; the outer layer is more thermally and electrically conductive, spreading out hot spots and maintaining electron pathways.

[0088] Both layers contain fluoride / phosphate seeds, which work synergistically with electrolyte additives to achieve adaptive passivation at hot spots.

[0089] E2-1 Inner Layer: Bonding-Passivation Seed Layer: Thickness: 2–8 μm (4 μm in example); Porosity: 5–25% (10–20% in example); Dry Powder Formulation Range (mass percentage); Polyvinylidene Fluoride-Hexafluoropropylene Copolymer: 30–55%; Conductive Carbon Black: 10–30%; Carbon Nanotubes: 1–5%; Aluminum Fluoride Nanoparticles: 10–35%; D50 20–80 nm; Lithium Phosphate Nanoparticles: 5–20%; D50 50–200 nm; Example Formulation (mass percentage)

[0090] Polyvinylidene fluoride-hexafluoropropylene copolymer: 40%; Conductive carbon black: 20%; Multi-walled carbon nanotubes: 3%; Aluminum fluoride nanoparticles: 25%; Lithium phosphate nanoparticles: 12%.

[0091] E2-2 Outer Layer: Thermally and Electrically Conductive Microporous Network Layer: Thickness: 10–30 μm (18 μm in examples); Porosity: 25–50% (25–45% in examples); Dry Powder Formulation Range (mass percentage); Hexagonal boron nitride flake particles: 25–50%; Aluminum nitride particles: 8–25%; Vapor-grown carbon fibers: 5–15%; Graphite sheets (or conductive graphite): 2–12%; Carbon nanotubes: 0.5–4%; Conductive carbon black: 2–8%; Polyvinylidene fluoride-hexafluoropropylene copolymer: 10–25%; Aluminum fluoride nanoparticles: 1–8%; Lithium phosphate nanoparticles: 1–8%

[0092] Example formulation (percentage by mass);

[0093] Hexagonal boron nitride: 38%; aluminum nitride: 15% (for surface stabilization treatment to reduce water absorption and hydrolysis tendency); preferably: surface passivated aluminum nitride particles are used, the surface of which has an aluminum oxide or aluminum fluoride passivation layer: passivation layer thickness: 2–30 nm (preferably 5–15 nm); or expressed as a mass fraction: 0.2–2.0 wt% surface aluminum oxide; and the aluminum nitride content range is slightly narrowed to: 8–20% (15% in example);

[0094] Cell assembly is completed in a dry environment with a dew point ≤−40℃ to reduce hydrolysis triggering.

[0095] Vapor-grown carbon fibers: 10%; Graphite sheets: 6%; Multi-walled carbon nanotubes: 2%; Conductive carbon black: 4%; Polyvinylidene fluoride-hexafluoropropylene copolymer: 18%; Aluminum fluoride: 4%; Lithium phosphate: 3%;

[0096] For PVDF-HFP systems using a binder framework, repeatable micropores are formed using differential evaporation rates and solvent-induced phase separation (SIPS) without introducing pore-forming agents that require washing or leave uncontrollable residues.

[0097] Outer layer (thermally conductive-conductive microporous layer) slurry solvent system (mass ratio): acetone: N-methylpyrrolidone = 85:15 (range 70:30 to 95:5); solid content: 45–60% (52% in example);

[0098] Post-coating drying procedure: Pre-dry at 35–45℃ for 5–15 min (allowing acetone to evaporate rapidly, triggering local precipitation of PVDF-HFP and forming primary pores); Main drying at 70–90℃ for 20–40 min (allowing NMP to migrate and evaporate slowly, stabilizing the pore structure); Vacuum at 110℃ for 4–8 h (residual solution <500ppm); Light pressure shaping: Light pressure at 0.1–0.5MPa to orient hexagonal boron nitride without collapsing the pore structure.

[0099] Porosity determination methods: mass / volume method (estimated based on coating thickness, area, mass and solid density) or mercury intrusion method (if available).

[0100] E2-3 Morphology and Orientation Control Parameters: During outer layer coating, a shear orientation process is employed (e.g., slot coating / blade coating direction is consistent with the circumferential direction), and light pressing is performed after drying to ensure that the hexagonal boron nitride flake particles are preferably arranged parallel to the end faces. Inner thermal conductivity of outer layer: 3–12 W / (m·Kelvin) (target ≥5 for examples); Thick thermal conductivity of outer layer: 0.6–3 W / (m·Kelvin) (target ≥1 for examples); Area resistivity of outer layer after compression: 0.05–0.20 mΩ·cm² (under compression pressure of 1–3 MPa).

[0101] E2-4 Welding Window Pattern: Welding Window: The interface layer leaves a bare metal window in the annular area for laser spot welding / short seam welding to achieve metal-to-metal metallurgical connection.

[0102] Parameter range: Window size: 0.2×0.6 mm to 0.6×1.5 mm (0.4×1.0 mm in the example); Circumferential spacing: 0.8–2.0 mm (1.2 mm in the example); Number and distribution of windows: Ensure electrical connection redundancy and avoid hot spots caused by single-point failure.

[0103] Preparation method

[0104] Key drying / assembly is completed in a dry room environment (dew point ≤ -40℃).

[0105] Step 1: Raw Material Pre-drying: Carbon-coated lithium iron phosphate: Vacuum drying at 120℃ for 12–16 hours, moisture content ≤200ppm. Conductive carbon black, carbon nanotubes, vapor-grown carbon fibers, hexagonal boron nitride, aluminum nitride, aluminum fluoride, lithium phosphate: Vacuum drying at 80–120℃ for 6–12 hours. Polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer: Vacuum drying at 60–80℃ for 6–10 hours. Copper foil / aluminum foil: The rolls are allowed to stand in the drying room to equilibrate, avoiding condensation and moisture absorption.

[0106] Step 2: Preparation and Coating of Positive Electrode Slurry

[0107] Preparation of adhesive solution: Add polyvinylidene fluoride to N-methylpyrrolidone and stir at 50–60°C until completely dissolved to obtain an 8–12% (mass fraction) adhesive stock solution.

[0108] Conductive agent dispersion: Take a portion of the binder mother liquor and add acetylene black, carbon nanotubes, and vapor-grown carbon fibers in sequence. Disperse under high shear for 30–60 minutes to ensure that the slurry does not have obvious agglomeration.

[0109] Adding lithium iron phosphate: Gradually add pre-dried carbon-coated lithium iron phosphate, planetarily stir for 30–60 minutes, control the solid content to 62–68%, and the viscosity at 25°C to 1500–3500 mPa·s.

[0110] Degassing and filtration: Vacuum degassing for 15–30 minutes, then pass through a 200–400 mesh sieve.

[0111] Coating: Coating is applied to both sides of a 12-micron aluminum foil with narrow slits, leaving a 1.6 mm margin for the bare positive electrode foil.

[0112] Drying: Segmented drying at 80–120℃, residual solvent ≤500ppm.

[0113] Roll pressing: Roll press to the target porosity of about 30% to ensure that no powder falls off the edges.

[0114] Slitting and end micro-tab forming: Micro-tab teeth are formed on the edge of the bare foil using laser or precision punching, with a tooth pitch of about 0.6 mm, a tooth depth of about 1.3 mm, and burrs ≤ 8 micrometers.

[0115] Step 3: Preparation and Coating of Negative Electrode Slurry

[0116] Prepare an aqueous solution of sodium carboxymethyl cellulose (1–2%); add artificial graphite, silicon-oxygen / carbon composite, hard carbon and conductive agent to the aqueous solution, and disperse for 30–60 minutes; add styrene-butadiene rubber latex and lithium polyacrylate, and mix under low shear for 20–40 minutes, with a solid content of 45–52%; double-sided coating on 8-micron copper foil, leaving a 1.8 mm margin for the bare negative electrode foil; dry in sections at 80–110℃, with residual water ≤500 ppm; roll-press to a porosity of approximately 34%; slit and form end micro-electrode teeth (tooth pitch 0.6 mm, tooth depth 1.4 mm, burrs ≤8 microns).

[0117] Step 4: Diaphragm preparation or selection and end functional strip coating

[0118] A three-layer microporous base membrane is selected and a double-sided ceramic coating is completed; an end functional strip (1.0 mm wide, 4 μm dry thickness) is coated on the long side of one side of the diaphragm: hexagonal boron nitride, alumina, lithium phosphate, and aluminum fluoride are added to a polyvinylidene fluoride-hexafluoropropylene copolymer solution (solvent can be acetone / a small amount of N-methylpyrrolidone), stirred and ultrasonically / high sheared dispersed; the slurry solid content is controlled at 30–45%, and dried at 60–90℃ after coating; a patterned roller / mask is used to form a microporous window pattern.

[0119] Before coating the end functional tape, the surface of the diaphragm base film is corona treated to make the surface tension ≥38mN / m (preferably ≥42mN / m); the adhesion of the functional tape (mass loss after tape peeling) ≤0.5mg / dm².

[0120] Step 5: Preparation of the composite interface layer between the current collector ring and the end cap

[0121] Positive electrode current collector surface treatment: degreasing (anhydrous solvent), micro-blasting or micro-etching to achieve Ra of approximately 2.5 micrometers; Inner layer coating (bonding-passivation seed layer): preparation of inner layer slurry: dissolving polyvinylidene fluoride-hexafluoropropylene copolymer in acetone / a small amount of N-methylpyrrolidone; adding conductive carbon black, carbon nanotubes, aluminum fluoride, and lithium phosphate for dispersion; coating the annular area on the end face of the current collector using screen printing or precision dispensing, and drying to obtain a 4-micrometer inner layer.

[0122] Outer layer coating (thermally conductive-conductive microporous network layer): Prepare the outer layer slurry and control the rheology of the high filler system (a small amount of rheology modifier may be added if necessary, but avoid water / strong polarity residue); apply by scraping / slit coating along the circumference to form an 18-micron outer layer; reserve welding windows (0.4 × 1.0 mm, circumferential spacing 1.2 mm) through a mask.

[0123] Drying and vacuum treatment: staged drying at 60–120℃, followed by vacuum drying at 110℃ for 4–8 hours to ensure residual solvent ≤500ppm.

[0124] Step 6: Winding and End Connection

[0125] The cells are wound into a core in the following order: positive electrode / separator / negative electrode / separator, ensuring that: the functional band at the end of the separator is positioned at the positive terminal; the toothed end faces of the positive and negative electrode micro-tabs are neat, and the end face runout is controlled within the allowable range of the process. End clamping: Press the end face of the positive terminal micro-tab against the positive electrode current collector ring coated with a composite interface layer, applying an end face pressure of 1–3 MPa to ensure full contact between the interface layer micropores and the micro-tab end face. Welding: Perform laser spot welding / short-seam welding in the reserved window area to form a metallurgical connection; the interface layer covered area is not directly welded through, thus avoiding voids and corrosion channels caused by polymer ablation. The negative terminal can use a similar structure or a simplified structure.

[0126] Step 7: Shell insertion, drying, electrolyte injection and formation

[0127] The core is inserted into the casing and the insulation is installed; vacuum drying: 110℃ vacuum for 12–24 hours; after liquid injection, it is left to stand at 25℃ for 8–24 hours, for example, 12 hours; pre-charge to 3.45V at 0.10C and let stand for 30 minutes; perform 2–10 short pulse charge-discharge cycles, for example, 6 sets of pulses: pulse current 0.5–1.5C (for example, 1.0C); pulse duration 10–60 s (for example, 30 s);

[0128] For example: let stand for 60 seconds → discharge at 1.0C for 30 seconds → let stand for 60 seconds; then process according to standard procedure: charge at 0.33C to 3.65V and maintain constant voltage to 0.05C, let stand for 30 minutes, discharge at 0.33C to 2.50V; age for 24–72 hours and then divide the capacity.

[0129] The microporous network of the end composite interface layer retains aluminum fluoride and lithium phosphate at the end, forming a local reaction field.

[0130] When hot spots appear, lithium difluorophosphate and lithium difluorooxalate borate preferentially form a fluorine-containing / phosphorus-containing composite passivation film in this reaction field, cutting off the positive feedback of corrosion-resistance increase; at the same time, the high in-plane thermal conductivity of hexagonal boron nitride / aluminum nitride flattens the thermal peak, further inhibiting the runaway decomposition of additives; since the passivation effect occurs more precisely, the global additive requirement can be lower, which is more conducive to the design of high energy density low electrolyte.

[0131] I. Samples and Common Conditions

[0132] 1) Common cell and electrode system (consistent across all groups)

[0133] Battery cell: A large cylindrical cell (4680 specification) with a diameter of 46mm and a length of 80mm, featuring a full-tab / tabless winding structure; Positive electrode: Carbon-coated lithium iron phosphate system (containing acetylene black, multi-walled carbon nanotubes, vapor-grown carbon fibers, and polyvinylidene fluoride binder); Negative electrode: Artificial graphite + silicon-oxygen / carbon composite + a small amount of hard carbon, with an aqueous binder system (sodium carboxymethyl cellulose / styrene-butadiene rubber / lithium polyacrylate).

[0134] Separator: Polypropylene / polyethylene / polypropylene three-layer microporous membrane + double-sided alumina ceramic coating; Welding: The end current collector ring and micro tabs are laser welded (weld point / weld density is consistent); Sample quantity: n=8 cells per group, used to take the average value and dispersion (the average value is expressed in the table; the dispersion is usually in the range of capacity ±0.05Ah and internal resistance ±0.05mΩ).

[0135] II. Six sets of experiments (1 set of example experiments + 5 sets of comparative experiments)

[0136] Design Principles: Comparative Example 1: Represents a typical existing tabless cylinder (without an end chemical-thermal synergistic interface layer); Comparative Example 2: Represents a thermal management-only improvement (only a thermally and electrically conductive interface layer is created, without forming a film seed / reaction field); Comparative Example 3: Represents an electrolyte system improvement only (using common film-forming / corrosion-inhibiting salt additives, but the ends are still bare metal / directly welded); Comparative Example 4: Represents a simple combination of Comparative Example 2 and Comparative Example 3 (each exerts its original effect); Comparative Example 5: Represents a scheme closer to this one, but lacks key synergistic elements (lacking end functional bands and directional activation formation, or lacking a key point in the two-layer gradient microporous structure).

[0137] Implementation Example (IE)

[0138] Electrolyte: Advanced electrolyte C1 (see below); End caps: Two-layer gradient microporous composite interface layer (containing aluminum fluoride nanoparticles + lithium phosphate nanoparticles as film-forming seeds), with welding windows; Separator: Positive end functional band (narrow band, containing hexagonal boron nitride / lithium phosphate / aluminum fluoride, etc.); Formation: End cap directional activation formation pulse (first low-rate pre-charge, then short pulse to promote preferential film formation of end cap seeds + additives).

[0139] B. Comparative Example 1 (CE1)

[0140] Electrolyte: Baseline electrolyte C0 (see below); End caps: No composite interface layer, micro tabs directly welded to aluminum current collector ring;

[0141] Diaphragm: No end functional band; Formation: Conventional formation (no directional activation pulse).

[0142] C. Comparative Example 2 (CE2: Thermal / Electrical Interface Layer Only)

[0143] Electrolyte: Baseline electrolyte C0; End caps: Single-layer thermally and electrically conductive interface layer (hexagonal boron nitride + aluminum nitride + conductive carbon + polyvinylidene fluoride-hexafluoropropylene copolymer), without aluminum fluoride and lithium phosphate film-forming seeds; Welding window provided; Separator: No end functional band; Formation: Conventional formation.

[0144] D. Comparative Example 3 (CE3: Advanced Electrolyte Only)

[0145] Electrolyte: Advanced electrolyte C1; End caps: No composite interface layer, direct welding; Diaphragm: No end functional band; Formation: Conventional formation.

[0146] E. Comparative Example 4 (CE4: a simple combination of Comparative Example 2 and Comparative Example 3)

[0147] Electrolyte: Advanced electrolyte C1; End caps: Single-layer thermally and electrically conductive interface layer (without film-forming seed), with welding windows; Separator: No end functional band; Formation: Conventional formation.

[0148] F. Comparative Example 5 (CE5)

[0149] Electrolyte: Advanced electrolyte C1; End caps: Two-layer gradient microporous composite interface layer (containing film-forming seeds) with welding windows, similar to the one used in this scheme; Separator: No end functional band; Formation: No end directional activation pulse (only conventional formation).

[0150] III. Clarification of Electrolyte Formulation (C0 / C1)

[0151] 1) Baseline electrolyte CO

[0152] Lithium salt: Lithium hexafluorophosphate 1.0 mol / L; Solvent (volume ratio): Ethylene carbonate: Ethyl methyl carbonate: Dimethyl carbonate = 30:40:30; Additives (mass percentage): Vinyl carbonate 2.0%, Fluorinated ethylene carbonate 1.0%.

[0153] 2) Advanced electrolyte C1

[0154] Lithium salt: Lithium hexafluorophosphate 0.90 mol / L + lithium difluorosulfonyl imide 0.20 mol / L; Solvent (volume ratio): Ethyl carbonate: Ethyl methyl carbonate: Dimethyl carbonate: Methyl acetate = 20:35:35:10;

[0155] Additives (by weight): Lithium difluorophosphate 1.0%; Lithium difluorooxalate borate 0.8%; Fluorinated ethylene carbonate 2.0%; Tris(trimethylsilyl)phosphate 0.4%; Ethylene sulfate 0.3%.

[0156] IV. Testing Standards and Methods

[0157] 1) Electrical performance and lifespan: IEC / National Standard System

[0158] IEC 62660-1:2018: Battery cells for electric road vehicles – Performance testing (test procedures for capacity, power, internal resistance, etc.). IEC 62660-2:2018: Battery cells – Reliability and abuse testing (including a framework for cycle and storage testing procedures). GB / T 31486-2024: Electrical performance requirements and test methods for power batteries for electric vehicles; GB / T 31484-2015: Cycle life requirements and test methods for power batteries for electric vehicles (current).

[0159] The specific test procedures described below are organized according to the general caliber of IEC 62660-1 / 2; they can be applied to similar items in GB / T 31486-2024 and GB / T 31484-2015.

[0160] 2) End static connection resistance (end contact resistance): ASTM

[0161] ASTM B539-20: Methods for measuring the resistance of static electrical connections (terminals / solder joints / crimp joints / friction joints, etc.) (including dry circuit and rated current load levels).

[0162] 3) Corrosion mass loss of end manifolds (post-disassembly mass method): ASTM

[0163] ASTM G1 (G0001-03R17e1 series): Methods for the preparation, cleaning and evaluation of mass loss of corrosion specimens.

[0164] V. Specific Test Calibration

[0165] 1) Capacity

[0166] Environment: 25℃±2℃; Charging: 1C constant current to 3.65V, then switch to constant voltage until the current drops to 0.05C; let stand for 30min; Discharging: 0.33C constant current to 2.50V; Basis: Capacity test standard of IEC62660-1 (or refer to GB / T31486-2024).

[0167] 2) DC internal resistance DCIR (50% SOC pulse method)

[0168] Pre-processing: Calibrate the SOC according to the usable capacity measured by the capacity test; adjust the cell to 50% SOC and let it stand for 60 minutes; apply a 10s discharge pulse (3C current), record the voltage difference 0.1s and 10s after the pulse starts, and calculate DCIR=ΔV / ΔI; based on: power / internal resistance pulse test of IEC62660-1.

[0169] 3) Fast charging hotspot test (end temperature rise and temperature difference)

[0170] Conditions: 25℃±2℃, wind speed <0.5m / s (natural convection), initial SOC=10%; fast charging: 3C constant current charging to 3.65V, constant voltage charging to 0.2C current;

[0171] Temperature measurement points: 1 point on the positive end surface (near the center of the end cap and the collector); 1 point on the middle surface of the cylinder; Record: the highest end temperature rise ΔT_end,max, and the maximum end-to-middle temperature difference ΔT_end-mid; Note: Temperature monitoring and environmental control are standard requirements of IEC62660 testing;

[0172] 4) End connection resistor (ASTM B539)

[0173] Sampling: Disassemble 3 cells in each group and take the positive current collector-micro tab end face connection area to make a static connection sample; Testing: Apply the specified clamping force (e.g., 20N) according to ASTM B539-20 dry circuit method (Kelvin four-wire method), test the current 100mA, and measure the connection resistance; Test points: initial after formation and after 500 fast charge cycles.

[0174] 5) Fast charging cycle life (10–80% SOC)

[0175] Conditions: 25℃±2℃; Pre-calibrate rated capacity Q using 0.33C; Each cycle: Charge from 10% SOC to 80% SOC using a constant current of 3C (controlled by 0.70Q of charge input, or constant voltage to 0.2C after reaching 3.65V, but cut off at the upper limit of the charge); rest for 10 minutes; Discharge back to 10% SOC using a constant current of 3C (controlled by 0.70Q of discharge, or cut off before reaching 2.50V); rest for 10 minutes; Perform a 0.33C capacity retest and a DCIR pulse test at 50% SOC every 100 cycles. Criterion: Cycle until the capacity decays to 80% of the initial capacity is considered the end of the lifespan; Basis: Cycle life test framework of IEC62660-1 / 2 (or refer to GB / T31484-2015).

[0176] 6) High-temperature storage (60℃, fully charged)

[0177] Charge to 100% SOC; let stand at 60℃ for 7 days; restore to 25℃ and let stand for 24 hours; measure capacity and DCIR; based on: IEC62660-2 storage / reliability test framework.

[0178] 7) Mass loss of end manifold (ASTM G1 cleaning + weighing)

[0179] Disassemble 3 cells in each group and take the positive current collector ring; clean and remove corrosion products according to ASTM G1 and weigh them (compare with the reference mass of the same batch before assembly) and calculate the mass loss.

[0180] VI. Performance Test Data

[0181]

[0182]

[0183] VII. Conclusion

[0184] The CE1 (the existing baseline) exhibits significantly higher end-point temperature rise and end-mid temperature difference during 3C fast charging (ΔT_end,max = 32℃, ΔT_end-mid = 9℃). Furthermore, after 500 fast charging cycles, the end-point connection resistance deteriorates from 210µΩ to 520µΩ (significant instability), corresponding to a 52% increase in DCIR and a lifespan of only 900 cycles. This aligns with the positive feedback loop of end-point contact deterioration → localized current concentration → temperature rise → side reactions / corrosion → further contact deterioration (hot spots become hotter, more corroded, and have increased resistance).

[0185] Compared to CE1, CE2 (thermal interface only) shows a decrease in end temperature rise (32→28℃), but the corrosion mass loss remains high (8.2mg), and the end connection resistance still increases significantly (205→430µΩ). Ultimately, the DCIR still increases by 40%, and the lifespan is 1100 cycles. → This indicates that simply spreading the heat cannot stop the corrosion-resistance increase chain.

[0186] Compared to CE1, CE3 (advanced electrolyte only) showed some improvement in high-temperature storage and corrosion (6.0 mg mass loss), but hot spots at the ends were still obvious (temperature difference of 30℃ and 8℃), and the connection resistance still increased significantly (230→380µΩ). This indicates that improving the electrolyte alone is insufficient to specifically address localized electro-thermal coupling failure points at the ends.

[0187] The improvement of CE4 (advanced electrolyte + single-layer thermal interface) compared to CE2 / CE3 is significant, but it is still within the expected range of improvement.

[0188] The end temperature rise is 26°C, the connection resistance is 220Ω → 300µΩ, the DCIR is increased by 28%, and the lifespan is 1400 cycles. This is in line with the improvement of each factor doing its own thing: the thermal interface reduces the temperature rise, and the advanced electrolyte reduces side reactions, but it still lacks end adaptive passivation to cut off the positive feedback.

[0189] CE5 (two-layer gradient microporous seed interface layer + advanced electrolyte) shows a significant leap compared to CE4 (simple combination):

[0190] The end temperature rise further decreased (26→23℃), and more importantly, the growth of end connection resistance was significantly suppressed (300→220µΩ), corrosion mass loss was significantly reduced (4.2→2.5mg), DCIR increase decreased from 28% to 18%, and lifespan increased from 1400 to 1700 cycles.

[0191] This indicates that the combination of film-forming seeds, microporous reaction field, and advanced electrolyte film-forming salt creates a local preferential film-forming / self-passivation mechanism at the end, which weakens the positive feedback loop of corrosion → increased resistance → heat generation → more corrosion, rather than simply the superposition of heat and electrolyte.

[0192] Based on CE5, IE (this embodiment) further enhances the end passivation by using end functional bands and directional activation formation, making the end passivation more controllable and front-mounted, further stabilizing the end connection resistance (220→195µΩ), reducing DCIR to 12%, and increasing the lifespan to 1900 cycles.

[0193] Ultimately, by using an end-gradient microporous seed interface layer (spatial fixation reaction sites) + film-forming salt additives (which can form a corrosion-resistant composite film) + process-oriented activation (to enable preferential film formation and self-limitation at the end), the positive feedback failure chain at the end is suppressed, resulting in significantly lower end temperature difference, smaller connection resistance drift, lower corrosion quality loss, and longer fast-charging life.

[0194] 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-energy-density lithium iron phosphate cylindrical battery, characterized in that: include, It has a housing, end caps, and a fully tabbed or tabless wound electrode assembly and an electrolyte placed inside the housing; the electrode assembly includes a lithium iron phosphate positive electrode, a negative electrode, and a separator, wherein the positive electrode and the negative electrode each have bare foil edges and form micro-tab teeth; At the positive end, the end face of the micro electrode is pressed tightly against the positive end current collector ring of aluminum or aluminum alloy. The annular area of ​​the end face of the positive end current collector ring is provided with a double-layer gradient microporous composite interface layer. The double-layer gradient microporous composite interface layer is formed by sequentially stacking a bonding-passivation seed layer near the positive end current collector ring and a thermally conductive-conductive microporous network layer on its outer side, and leaving a gap at the welding window. The thickness of the bonding-passivation seed layer is 2 to 8 micrometers, and the thickness of the thermally conductive-conductive microporous network layer is 10 to 30 micrometers with a porosity of 25% to 50%. Both the bonding-passivation seed layer and the thermally conductive-conductive microporous network layer contain aluminum fluoride nanoparticles and lithium phosphate nanoparticles. The thermally conductive-conductive microporous network layer also contains hexagonal boron nitride sheet particles, aluminum nitride particles, vapor-grown carbon fibers, and conductive carbon black.

2. A high-energy-density lithium iron phosphate cylindrical battery according to claim 1, characterized in that: The welding window has dimensions of 0.2×0.6 mm to 0.6×1.5 mm and a circumferential spacing of 0.8 to 2.0 mm; the positive end collector ring has a thickness of 0.30 to 0.80 mm and a surface microgroove on its end face with a groove depth of 0.05 to 0.25 mm, a groove width of 0.15 to 0.50 mm, and a groove spacing of 0.30 to 1.00 mm.

3. A high-energy-density lithium iron phosphate cylindrical battery according to claim 2, characterized in that: The diaphragm is a three-layer microporous membrane of polypropylene / polyethylene / polypropylene with a double-sided alumina ceramic coating. One long side of the diaphragm has a positive end functional band with a width of 0.6 to 1.6 mm, a dry film thickness of 3 to 8 micrometers, and a porosity of 35 to 55%. The positive end functional band includes hexagonal boron nitride flake particles, alumina nanoparticles, lithium phosphate nanoparticles, aluminum fluoride nanoparticles, and polyvinylidene fluoride-hexafluoropropylene copolymer binder. A microporous window pattern is formed on the surface of the functional band, with a 0.3 × 0.6 mm low coating area set every 2 to 4 mm.

4. A high-energy-density lithium iron phosphate cylindrical battery according to claim 3, characterized in that: The electrolyte comprises lithium salts of 0.80 to 1.00 mol / L lithium hexafluorophosphate and 0.10 to 0.25 mol / L lithium difluorosulfonylimide; solvents comprise ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and methyl acetate in a volume ratio of 10 to 25: 25 to 45: 25 to 45: 0 to 20; and contains 0.3 to 1.5% by mass lithium difluorophosphate, 0.2 to 1.2% by mass lithium difluorooxalate borate, 0.5 to 4.0% by mass fluoroethylene carbonate, 0.1 to 0.8% by mass tris(trimethylsilyl)phosphate and 0.1 to 1.0% by mass ethylene sulfate.

5. A high-energy-density lithium iron phosphate cylindrical battery according to claim 4, characterized in that: The binder-passivation seed layer comprises, on a dry powder basis, 30 to 55% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 10 to 30% by mass of conductive carbon black, 1 to 5% by mass of carbon nanotubes, 10 to 35% by mass of aluminum fluoride nanoparticles, and 5 to 20% by mass of lithium phosphate nanoparticles. The thermally and electrically conductive microporous network layer, on a dry powder basis, comprises 25 to 50% by mass of hexagonal boron nitride flake particles, 8 to 20% by mass of aluminum nitride particles, 5 to 15% by mass of vapor-grown carbon fibers, 2 to 12% by mass of graphite flakes, 0.5 to 4% by mass of carbon nanotubes, 2 to 8% by mass of conductive carbon black, 10 to 25% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 1 to 8% by mass of aluminum fluoride nanoparticles, and 1 to 8% by mass of lithium phosphate nanoparticles; wherein the D50 of the aluminum fluoride nanoparticles is 20 to 80 nanometers, and the D50 of the lithium phosphate nanoparticles is 50 to 200 nanometers.

6. A method for preparing a high-energy-density lithium iron phosphate cylindrical battery, characterized in that: include: Prepare lithium iron phosphate positive and negative electrode sheets with bare foil edges and micro-ear-shaped teeth; A bonding-passivation seed layer with a thickness of 2 to 8 micrometers and a thermally conductive-conductive microporous network layer with a thickness of 10 to 30 micrometers are sequentially formed in the annular region of the positive end current collector ring of aluminum or aluminum alloy. The two layers constitute a two-layer gradient microporous composite interface layer, and a welding window is reserved. The positive electrode, separator and negative electrode are wound and the functional band of the positive end of the separator is positioned at the positive end. A pressure of 1 to 3 MPa is applied to the end face of the micro electrode to press the end face of the micro electrode tab tightly onto the two layers of gradient microporous composite interface layer and laser welding is performed at the welding window. After vacuum drying, electrolyte is injected and allowed to stand for 8 to 24 hours. After pre-charging, 2 to 10 sets of pulse charge and discharge are performed, followed by routine formation and aging capacity testing.

7. The preparation method according to claim 6, characterized in that: The slurry for bonding and passivating the seed layer comprises, on a dry powder basis, 30 to 55% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 10 to 30% by mass of conductive carbon black, 1 to 5% by mass of carbon nanotubes, 10 to 35% by mass of aluminum fluoride nanoparticles, and 5 to 20% by mass of lithium phosphate nanoparticles. The slurry is prepared using a mixed solvent of acetone and N-methylpyrrolidone and applied by screen printing or precision dispensing.

8. The preparation method according to claim 7, characterized in that: The slurry of the thermally conductive and electrically conductive microporous network layer, on a dry powder basis, comprises 25 to 50% by mass of hexagonal boron nitride, 8 to 20% by mass of aluminum nitride, 5 to 15% by mass of vapor-grown carbon fibers, 2 to 12% by mass of graphite sheets, 0.5 to 4% by mass of carbon nanotubes, 2 to 8% by mass of conductive carbon black, 10 to 25% by mass of polyvinylidene fluoride-hexafluoropropylene copolymer, 1 to 8% by mass of aluminum fluoride, and 1 to 8% by mass of lithium phosphate. The mass ratio of acetone to N-methylpyrrolidone in the mixed solvent of the slurry is 70:30 to 95:5, and the solid content is 45 to 60% by mass.

9. The preparation method according to claim 8, characterized in that: The battery cell assembly is completed in a dry environment with a dew point no higher than -40 degrees Celsius, and vacuum drying includes vacuum treatment at 110 degrees Celsius for 12 to 24 hours.

10. The preparation method according to claim 9, characterized in that: After injection, allow to stand and soak at 25°C for 12 hours; pre-charge to 3.45V at 0.10C, and let stand for 30 minutes; perform 6 sets of pulses, each set consisting of 1.0C charging for 30 seconds, standing for 60 seconds, 1.0C discharging for 30 seconds, and standing for 60 seconds; then perform conventional formation: charge to 3.65V at 0.33C, maintain constant voltage to 0.05C, let stand for 30 minutes, and discharge to 2.50V at 0.33C; after aging for 24 to 72 hours, divide into volumes.

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