Iron-based mixed polyanion positive electrode material and preparation method and application thereof

Abstract

This invention provides an iron-based hybrid multi-anion cathode material, its preparation method, and its applications. NLFPP-GQDs uses sodium-lithium composite polyanions as the core, modified with graphene quantum dots to form a mesoporous spherical structure. It inherits the structural stability and excellent thermal safety of polyanion materials, while the continuous conductive network constructed by GQDs solves the problem of poor conductivity in traditional polyanion materials, significantly improving the Li⁺ diffusion coefficient and rate performance, and simultaneously optimizing cycle stability. Li⁺ intercalation and GQDs loading are completed simultaneously through hydrothermal pre-intercalation. Combined with two-stage high-temperature solid-phase reconstruction and dilute acid purification, the cumbersome electrochemical ion exchange process is eliminated, shortening the synthesis cycle by 60% and reducing production costs by 22.7%. Furthermore, the process parameters are precisely controllable, adapting to the needs of industrial mass production. In applications, this material can be matched with hard carbon and other anodes to assemble high-energy-density full batteries, balancing fast-charging performance and cycle life.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, and more specifically, to an iron-based hybrid multi-anion cathode material, its preparation method, and its application. Background Technology

[0002] With the rapid development of the new energy industry, lithium-ion batteries are placing higher demands on the energy density, rate performance, and cost-effectiveness of cathode materials. Iron-based polyanionic materials, due to their abundant iron reserves, environmental friendliness, and stable polyanionic framework (strong covalent bonds suppress oxygen release), have become an important direction for replacing traditional transition metal oxide cathodes. However, existing iron-based materials generally suffer from bottlenecks: single polyanionic materials (such as LiFePO4 and Li2FeP2O7) have low voltage platforms (<3.5V) and limited capacity utilization; while fluorophosphates (Li2FePO4F) offer improved voltage, they rely on expensive fluorine sources and are difficult to synthesize; silicates (Li2FeSiO4) have poor chemical stability and are prone to structural collapse during cycling. Hybrid multi-anionic materials introduce two or more polyanions (such as PO4³⁻ and P2O7) to address these challenges. 4 (⁻), utilizing the "polyanion synergistic effect" to regulate voltage and stability, a typical example being Na4Fe3(PO4)2(P2O7) (NFPP): However, the preparation of existing NFPP-derived lithium-based materials (such as NaLi3Fe3(PO4)2(P2O7)) has the following problems: Existing technologies rely on an electrochemical ion exchange method involving "sodium half-cell pre-sodium removal-lithium half-cell lithium insertion," which requires multiple battery assembly and disassembly processes. This process is complex (involving at least four glove box operations), time-consuming (total cycle exceeding 120 hours), and difficult to scale up. Electrochemical ion exchange is prone to insufficient Li⁺ / Na⁺ exchange (exchange rate is usually <95%), leaving unreacted Na⁺ sites in the products, causing lattice distortion (lattice parameter fluctuation ±2%) and increased ion migration impedance (charge transfer resistance >100Ω). Without targeted optimization of the conductive network and ion diffusion channels, capacity decay is severe at high rates (>10C) (30C capacity is usually <82mAh / g), making it difficult to meet the fast charging requirements of power batteries.

[0003] This invention combines graphene quantum dot interface modification to solve the problems of complex processes, uneven products, and poor kinetics in existing processes, thereby achieving efficient and low-cost preparation of iron-based mixed multi-anion cathode materials. Summary of the Invention

[0004] The present invention aims to solve the technical problems mentioned in the background art and provide an iron-based hybrid multi-anion cathode material, its preparation method and application.

[0005] To achieve the above objectives, the present invention provides the following technical solution: an iron-based hybrid multi-anion cathode material, comprising a graphene quantum dot modified sodium-lithium composite polyanion cathode composite material, namely NLFPP-GQDs, having a mesoporous spherical structure, wherein the surface of the graphene quantum dots is rich in hydroxyl functional groups, which are uniformly distributed on the surface and grain boundaries of the NLFPP particles to form a continuous conductive network.

[0006] A method for preparing an iron-based hybrid multi-anion cathode material includes the following steps: S1. Preparation of Li-NFPP-GQDs precursor by hydrothermal pre-intercalation: FeC2O4·2H2O, NaH2PO4·2H2O, (NH4)2HPO4, Li2CO3, and polyvinylpyrrolidone (PVP) were dissolved in deionized water at a molar ratio of 5:3:1:3:0.1 and stirred to form a suspension; GQDs were added and ultrasonically dispersed for 20-30 min; the mixture was transferred to a hydrothermal reactor and kept at 170-190℃ for 10-14 h; after centrifugation and washing, it was vacuum dried at 60-80℃ for 6-10 h to obtain the Li-NFPP-GQDs precursor; S2. High-temperature solid-phase reconstruction: The precursor is placed in a tube furnace and heated to 320-380℃ at 1-3℃ / min under an Ar atmosphere, and held for 3-5 hours; then the temperature is switched to an Ar-H2 mixture and heated to 680-720℃ at 4-6℃ / min, and held for 8-12 hours; the furnace is cooled to room temperature and ground through a 200-300 mesh sieve. S3. Purification treatment: Disperse the calcined product in a 0.08-0.12 mol / L dilute H2SO4 solution, stir at room temperature for 25-35 min, centrifuge and wash until pH=6.8-7.2, and vacuum dry at 80-100℃ for 10-14 h to obtain NLFPP-GQDs cathode material.

[0007] Further preferred embodiment: In step S1, the mass ratio of GQDs to Fe source is 2-4%, the ultrasonic dispersion power is 250-350w, the frequency is 40kHz, the centrifugation speed is 8000-10000rpm, and the centrifugation time is 8-12min.

[0008] Further preferred embodiment: In step S2, the volume ratio of Ar-H2 mixed gas is 90:10-98:2, the Ar gas flow rate is 40-60 mL / min, and the Ar-H2 mixed gas flow rate is 70-90 mL / min; during the high-temperature insulation stage, the H2 atmosphere can inhibit Fe²⁺ oxidation and drive the directional exchange between Li⁺ and residual Na⁺.

[0009] A further preferred embodiment: In step S3, the liquid-to-solid ratio of dilute H2SO4 to the calcined product is 10-15:1 (mL / g), the centrifugation speed is 10000-12000 rpm, and the centrifugation time is 12-18 min.

[0010] A lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the above-described positive electrode material or a positive electrode material prepared by any of the above-described preparation methods. Beneficial effects

[0011] 1. By setting up a hydrothermal reaction, Li⁺ pre-intercalation and GQDs loading are completed simultaneously in the hydrothermal reaction stage, with a Li⁺ intercalation rate exceeding 90%. This pre-occupies the active Na sites in the NFPP precursor, laying a solid foundation for the directional and efficient Li⁺ / Na⁺ exchange in the subsequent solid-phase reaction, completely eliminating the complex process of traditional electrochemical ion exchange. GQDs are uniformly coated on the particle surface and grain boundaries through ultrasonic dispersion and coordination, eliminating the need for additional modification steps and improving the overall process efficiency by 60%. 2. By setting up a two-stage calcination process of low-temperature degreasing and high-temperature reconstruction, combined with H2 atmosphere control, the Fe²⁺ content is guaranteed to be ≥95%, and the Na⁺ removal rate is over 98%. With the addition of dilute acid purification process, the purity and structural stability of the product are significantly improved. Compared with traditional methods, the synthesis cycle is shortened by 60% and the production cost is reduced by 22.7%. At the same time, the rate performance, cycle stability and safety of the material are comprehensively optimized, making it more suitable for industrial mass production. 3. By modifying with GQDs, GQDs are uniformly loaded onto the surface and grain boundaries of NLFPP particles through ultrasonic dispersion during the hydrothermal stage. The abundant hydroxyl functional groups in GQDs coordinate with Fe²⁺, significantly enhancing interfacial bonding and constructing a continuous three-dimensional conductive network. This not only solves the problem of poor intrinsic conductivity in polyanionic materials, reducing the charge transfer resistance (Rct) to 65Ω, far lower than the 133Ω of traditional materials, but also shortens the Li⁺ diffusion path, increasing the Li⁺ diffusion coefficient to 4.8 × 10⁻⁻⁻⁴ ... 9 The GQDs modulator achieves a 284% increase in capacity compared to the unmodified material, resulting in superior rate performance. At a high rate of 30C, the discharge capacity remains at 86.7 mAh / g, with no significant capacity decay after rate recovery. Furthermore, the GQDs coating inhibits the aggregation and growth of NLFPP particles during charge and discharge, enhancing structural stability. This results in a capacity retention of 88.3% after 500 cycles at 0.5C and 86.8% after 200 cycles at 2C in the full battery. GQDs also improve the material's thermal stability, with a thermal decomposition initiation temperature of 320℃, 40℃ higher than LiFePO4, significantly enhancing battery safety. Moreover, the GQDs modification and hydrothermal process are completed simultaneously, requiring no additional steps, balancing performance improvement with process simplification, making it more suitable for industrial production. 4. In summary, this iron-based hybrid multi-anion cathode material, its preparation method, and its application, NLFPP-GQDs, uses sodium-lithium composite polyanions as the core and is modified with graphene quantum dots to form a mesoporous spherical structure. It inherits the structural stability and excellent thermal safety of polyanion materials, while also solving the problem of poor conductivity in traditional polyanion materials through the continuous conductive network constructed by GQDs. This results in a significant improvement in the Li⁺ diffusion coefficient and rate performance, with simultaneous optimization of cycle stability. The Li⁺ diffusion coefficient is simultaneously completed through hydrothermal pre-intercalation. By embedding with GQDs loads and combining two-stage high-temperature solid-phase reconstruction and dilute acid purification, the cumbersome electrochemical ion exchange process is eliminated, shortening the synthesis cycle by 60% and reducing production costs by 22.7%. Moreover, the process parameters are precisely controllable, making it suitable for industrial mass production. In application scenarios, this material can be matched with anodes such as hard carbon to assemble high-energy-density full batteries, balancing fast-charging performance and cycle life. It is suitable for energy storage fields with strict requirements for safety, rate performance, and cost, providing a new direction for the research and industrialization of high-performance lithium-ion battery cathode materials. Detailed Implementation

[0012] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention.

[0013] In this embodiment of the invention, an iron-based hybrid multi-anion cathode material, its preparation method and application are disclosed. The iron-based hybrid multi-anion cathode material includes a graphene quantum dot modified sodium-lithium composite polyanion cathode composite material, namely NLFPP-GQDs, which has a mesoporous spherical structure. The graphene quantum dot surface is rich in hydroxyl functional groups, which are uniformly distributed on the surface and grain boundaries of NLFPP particles to form a continuous conductive network. A method for preparing an iron-based hybrid multi-anion cathode material includes the following steps: S1. Preparation of Li-NFPP-GQDs precursor by hydrothermal pre-intercalation: FeC2O4·2H2O, NaH2PO4·2H2O, (NH4)2HPO4, Li2CO3, and polyvinylpyrrolidone (PVP) were dissolved in deionized water at a molar ratio of 5:3:1:3:0.1 and stirred to form a suspension; GQDs were added and ultrasonically dispersed for 20-30 min; the mixture was transferred to a hydrothermal reactor and kept at 170-190℃ for 10-14 h; after centrifugation and washing, it was vacuum dried at 60-80℃ for 6-10 h to obtain the Li-NFPP-GQDs precursor; S2. High-temperature solid-phase reconstruction: The precursor is placed in a tube furnace and heated to 320-380℃ at 1-3℃ / min under an Ar atmosphere, and held for 3-5 hours; then the temperature is switched to an Ar-H2 mixture and heated to 680-720℃ at 4-6℃ / min, and held for 8-12 hours; the furnace is cooled to room temperature and ground through a 200-300 mesh sieve. S3. Purification treatment: Disperse the calcined product in a 0.08-0.12 mol / L dilute H2SO4 solution, stir at room temperature for 25-35 min, centrifuge and wash until pH=6.8-7.2, and vacuum dry at 80-100℃ for 10-14 h to obtain NLFPP-GQDs cathode material.

[0014] In step S1, the mass ratio of GQDs to Fe source is 2-4%, the ultrasonic dispersion power is 250-350W, the frequency is 40kHz, the centrifugation speed is 8000-10000rpm, and the centrifugation time is 8-12min. In step S2, the volume ratio of Ar-H2 mixed gas is 90:10-98:2, the Ar gas flow rate is 40-60mL / min, and the Ar-H2 mixed gas flow rate is 70-90mL / min. During the high-temperature holding stage, the H2 atmosphere can inhibit Fe²⁺ oxidation and drive the directional exchange of Li⁺ and residual Na⁺. In step S3, the liquid-solid ratio of dilute H2SO4 to calcined product is 10-15:1 (mL / g), the centrifugation speed is 10000-12000rpm, and the centrifugation time is 12-18min.

[0015] A lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes the positive electrode material described above or the positive electrode material prepared by any of the above methods. The active material of the negative electrode is a composite material of hard carbon, graphite, and silicon carbon, with a negative electrode active layer mass ratio of 95-97:2-3:1-2; the electrolyte is 1 mol / L LiPF6 dissolved in EC / DMC / EMC at a volume ratio of 1:1:1, containing 4-6 wt% fluoroethylene carbonate (FEC); the separator is a Celgard 2400 polypropylene separator; the positive electrode includes an aluminum foil current collector and a positive electrode active layer coated on the surface of the current collector; the positive electrode active layer includes NLFPP-GQDs positive electrode material, conductive agent, and binder in a mass ratio of 92-96: The composition is 2-4:2-4; the positive electrode slurry viscosity is 4800-5200 mPa·s, and the solid content is 68-72 wt%; the single-sided coating density of the electrode is 127-133 g / m², and the thickness is 80-100 μm; the conductive agent is a mixture of SuperP and carbon nanotubes at a mass ratio of 3-5:1; the binder is polyvinylidene fluoride (PVDF); the electrode is dried in stages at 80-120℃, with the drying process being 80℃ / 3-5h and 110℃ / 10-14h, and the rolling pressure is 4-6 MPa; the battery operates at 25℃ with a voltage range of 1.5-4.2V. Under the specified conditions, the discharge capacity at 0.2C rate is ≥105mAh / g, and the discharge capacity at 30C rate is ≥85mAh / g; the capacity retention rate after 500 cycles at 0.5C is ≥85%; the capacity retention rate of the full cell (NLFPP-GQDs / / hard carbon) after 200 cycles at 2C is ≥85%, and the energy density is ≥240Wh / kg. The graphene quantum dot modified sodium-lithium composite polyanionic cathode material, namely NLFPP-GQDs, has a mesoporous spherical structure with a particle size of 1-2μm, a mesopore pore size of 8-15nm, and a GQDs coating thickness of 1-3... nm, GQDs loading 1.5-2.5wt%; material is orthorhombic Pn21a space group, lattice parameters a=10.40-10.45Å, b=13.60-13.70Å, c=8.75-8.80Å, volume 1225-1230ų; Na / Li atomic ratio 1:(2.95-3.00), Fe²⁺ content ≥95%; in step S1, Fe²⁺ content of FeC₂O₄・₂H₂O ≥31%, Li₂CO₃ particle size less than 5μm; in step S2, Na⁺ removal rate ≥98%. In preparing the cathode material, FeC2O4・2H2O was selected as the iron source. It was analytically pure and had an Fe²⁺ content ≥31%. This avoided lattice defects caused by the introduction of Fe³⁺ sources such as Fe(NO3)3, as Fe³⁺ would occupy Li⁺ sites and reduce ion migration rate. The sodium and phosphorus sources were NaH2PO4・2H2O and (NH4)2HPO4 dual phosphorus sources. The molar ratio was adjusted (3:1) to ensure the balance between PO4³⁻ and P2O7. 4 The ratio meets the NLFPP lattice requirements (PO4³⁻:P2O7). 4(⁻=2:1); Lithium source: Battery-grade Li₂CO₃, particle size <5μm, small particle size ensures uniform diffusion of Li⁺ into the NFPP precursor lattice during the hydrothermal stage; Modifier: Graphene quantum dots (GQDs), particle size 5-10nm, oxygen content 15-20%, hydroxyl functional groups can form coordination with Fe²⁺, improving interfacial bonding; Dispersant: Polyvinylpyrrolidone (PVP, K30), assists in raw material dispersion and precursor molding, can be completely removed by low-temperature calcination, weight loss ≥98% at 350℃, no Residual impurities; FeC2O4・2H2O (5mmol, 1.07g), NaH2PO4・2H2O (3mmol, 0.54g), (NH4)2HPO4 (1mmol, 0.13g), Li2CO3 (3mmol, 0.26g), and PVP (0.5g) were added to 50mL of deionized water in the specified proportions, and the mixture was magnetically stirred for 30min at 600rpm until a homogeneous suspension was formed, ensuring no significant particle sedimentation; GQDs were added to the suspension. 0.1g, concentration 1mg / mL, solvent is deionized water, ultrasonically dispersed for 20min, power 300W, frequency 40kHz, to uniformly coat the surface of the raw material particles with GQDs through ultrasonic cavitation effect, with a coating rate ≥90%; the mixture is transferred to a 100mL PTFE-lined hydrothermal reactor, tightened and placed in an oven, kept at 180℃ for 12h, heating rate 5℃ / min, and then naturally cooled to room temperature, cooling rate ≤2℃ / min to avoid precursor cracking; the reactor is removed and the product is centrifuged. The mixture was separated at 8000 rpm for 10 min, and washed alternately with 50 mL of deionized water three times and 50 mL of anhydrous ethanol once to remove residual PVP and unreacted salts. It was then vacuum dried at 60 °C for 8 h to obtain a light green powdery Li-NFPP-GQDs precursor with a particle size distribution of 0.8-1.5 μm. The Li-NFPP-GQDs precursor (5 g) was evenly spread on a corundum boat (thickness ≤ 5 mm to avoid particle sintering) and placed in the central constant temperature zone of a tube furnace. Ar gas with a purity of 99% was introduced.999%, flow rate 50 mL / min, replace furnace air for 30 min; the first stage of low-temperature degreasing is heated to 350℃ at 2℃ / min and held for 4 h to remove residual PVP and water of crystallization in the precursor and avoid organic carbonization contamination products during the high-temperature stage; the second stage of high-temperature reconstruction is heated to 700℃ at 5℃ / min and held for 10 h, then switched to Ar-H2 mixed gas with a volume ratio of 95:5 and a flow rate of 80 mL / min; the H2 atmosphere can reduce any Fe³⁺ that may be generated (Fe³⁺ + H2 → Fe²⁺ + H2O), ensuring that the Fe²⁺ content is ≥95%; at the same time, the high temperature drives the directional exchange of Li⁺ and residual Na⁺ in the lattice (Na⁺ volatilizes in the form of Na2O and is carried out by the Ar-H2 gas flow), forming NaLi3Fe3(PO4)2(P2O7)-GQDs composite material; after calcination, maintain Ar gas flow rate of 50 mL / min. The product was cooled to room temperature in the furnace for ≥8 hours. The product was then removed and ground through a 200-mesh sieve using an agate mortar to obtain a dark green crude NLFPP-GQDs product. The calcined crude product (4g) was dispersed in a 40mL 0.1mol / L dilute H2SO4 solution and magnetically stirred at 400rpm for 30 minutes at room temperature to dissolve residual Li2CO3 (Li2CO3+H2SO4→Li2SO4+CO2↑+H2O) and Na3PO4 (Na3PO4+H2SO4→Na2SO4+NaH2PO4) impurities on the surface. The mixture was centrifuged at 10000rpm for 15 minutes and washed with deionized water until the supernatant pH reached 7, approximately three times, 50mL each time. The mixture was then vacuum dried at 80℃ for 12 hours at a vacuum degree ≤10Pa to obtain high-purity NLFPP-GQDs cathode material with a yield ≥85%. Li⁺ is pre-intercalated in the NFPP precursor lattice via hydrothermal reaction with an intercalation rate ≥90%, pre-occupying active Na sites and laying the foundation for directional Li⁺ / Na⁺ exchange in subsequent solid-state reactions, avoiding the cumbersome process of electrochemical ion exchange. Simultaneously, GQDs are loaded during the hydrothermal stage, eliminating the need for additional modification steps and improving process efficiency by 60%. A two-stage calcination process of low-temperature degreasing and high-temperature reconstruction avoids particle sintering caused by high-temperature calcination alone, while utilizing an H₂ atmosphere to precisely control the Fe valence state, ensuring the electrochemical activity of the product. Na⁺ is removed through high-temperature volatilization, eliminating the need for washing and simplifying the process. The above-mentioned cathode material was then subjected to performance testing under the conditions of 25℃ and a voltage range of 1.5-4.2V. The test results are as follows: initial discharge capacity at 0.2C rate was 108.5mAh / g, 103.2mAh / g at 0.5C, 99.8mAh / g at 1C, 94.4mAh / g at 5C, 91.9mAh / g at 10C, 85.8mAh / g at 20C, and still reached 86.7mAh / g at 30C; when the rate recovered from 30C to 0.2C, the capacity rebounded to 109.2mAh / g, indicating that... The material exhibits excellent structural stability; after 500 cycles at 0.5C, the discharge capacity is 95.8 mAh / g, with a capacity retention of 88.3% and a coulombic efficiency consistently ≥99.5%; after 200 cycles at 2C, the capacity retention is 87.1%; cyclic voltammetry (CV, CHI660E) tests at scan rates of 0.1–1.0 mV / s show a good linear relationship between the oxidation and reduction peak currents and the square root of the scan rate (R²>0.99), and the calculated Li⁺ diffusion coefficient is 4.8 × 10⁻. 9 cm² / s, compared to NLFPP prepared by electrochemical ion exchange (1.25 × 10⁻⁻⁶ cm² / s). 9 The efficiency (cm² / s) is increased by 3.8 times; electrochemical impedance spectroscopy (EIS) tests show that the charge transfer resistance (Rct) is only 65Ω, far lower than the 133Ω of the traditional method; differential scanning calorimetry (DSC, NETZSCHTG209) analysis shows that the thermal decomposition onset temperature of the material is 320℃, which is 40℃ higher than that of LiFePO4 (280℃), and the heat release is only 280J / g, with no obvious exothermic peak (<500℃), and the safety is significantly improved; Then, the above-mentioned positive electrode material is used to prepare electrode sheets and batteries, adapting them to industrial coating processes: NLFPP-GQDs (94wt%), conductive agent (SuperP and carbon nanotubes mixed at a mass ratio of 3:1, 3wt%), and PVDF binder (3wt%) were added to N-methylpyrrolidone (NMP) solvent. The mixture was planetarily stirred at 2000 rpm for 2 hours, with ultrasonic dispersion every 30 minutes for 5 minutes at 200W. The slurry viscosity was adjusted to 5000±200 mPa·s using a rotational viscometer (NDJ-8S), with a solid content of 70±2wt%. A doctor blade coater was used to uniformly coat the slurry onto a 12μm thick aluminum foil with a purity of 99.9% and a single-sided coating density of 130±3 g / m². The foil was first dried in a forced-air dryer at 80℃ for 4 hours, followed by vacuum drying at 110℃ for 12 hours. The dried electrode sheets were then rolled using a roller press at a pressure of 5 MPa to a thickness of 80-100 μm and cut into 14mm diameter discs. An active area of ​​1.54 cm² was prepared for use. In an argon glove box, with H₂O < 1 ppm and O₂ < 1 ppm, NLFPP-GQDs electrodes were used as the working electrode, lithium foil (500 μm thick, 15 mm diameter) as the counter electrode, and a Celgard 2400 polypropylene membrane (25 μm thick) as the separator. A 1 mol / L LiPF₆-EC / DMC / EMC (1:1:1, v / v / v, containing 5 wt% FEC) electrolyte was used, with a filling volume of 50 μL. A CR2032 coin cell was assembled. The negative electrode used hard carbon (HC), with an areal capacity 1.2 times that of the positive electrode. The electrode preparation method was the same as for the positive electrode. The ratio of active material: conductive agent: binder was 97:2:1, and the viscosity was 3000 mPa·s. A soft-pack full cell was assembled, measuring 50 mm × 50 mm, with a capacity of 1 Ah, an N / P ratio of 1.2, and an electrolyte filling volume of 1.5 g / Ah. The charge-discharge performance of the half-cell was tested using the NewareBTS2300 battery testing system, and the full cell was cycled 200 times at a 2C rate. The results showed that the initial discharge capacity of the full cell was 94.3 mAh / g, and the capacity after 200 cycles was 81.7 mAh / g, with a retention rate of 86.8% and an energy density of 245 Wh / kg, calculated based on the mass of the positive electrode. It only takes 18 minutes to charge to 80% capacity at 1C, meeting the requirements for fast charging.

[0016] The following specific examples further illustrate the effectiveness of the technical solution in this application. Example 1: 5 mmol FeC2O4·2H2O, 3 mmol NaH2PO4·2H2O, 1 mmol (NH4)2HPO4, 3 mmol Li2CO3, and 0.5 g PVP were added to 50 mL of deionized water. After stirring for 30 min, 0.1 g GQDs were added, followed by sonication for 20 min, hydrothermal treatment at 180℃ for 12 h, centrifugation and washing, and vacuum drying at 60℃ for 8 h to obtain the Li-NFPP-GQDs precursor. The precursor was then held in an Ar atmosphere at 350℃ for 4 h in a tube furnace, followed by Ar... The material was heated at 700℃ for 10 hours in a -H2 (95:5) atmosphere, cooled in the furnace, and then ground through a 200-mesh sieve. It was washed with 0.1 mol / L H2SO4 for 30 minutes, centrifuged and dried to obtain NLFPP-GQDs material. The discharge capacity at 0.2C was 108.5 mAh / g, the capacity at 30C was 86.7 mAh / g, and the retention rate after 500 cycles at 0.5C was 88.3%. The NLFPP-GQDs / / HC full cell had a retention rate of 86.8% after 200 cycles at 2C, and it took 18 minutes to fast charge to 80% capacity at 1C.

[0017] Example 2: To verify the influence of key process parameters, a single-factor variable experiment was designed, and the results are as follows: variable Level 1 Level 2 (Optimal) Level 3 0.2C capacity change hydrothermal temperature 160℃ (98.2mAh / g) 180℃ (108.5mAh / g) 200℃ (102.3mAh / g) +10.5% High temperature insulation temperature 650℃ (100.1mAh / g) 700℃ (108.5mAh / g) 750℃ (103.7mAh / g) +8.4% GQDs addition amount 0.05g (95.7mAh / g) 0.1g (108.5mAh / g) 0.2g (101.2mAh / g) +13.4% The results show that the optimal parameters are 180℃ hydrothermal temperature, 700℃ high-temperature insulation, and 0.1g GQDs addition, which can achieve a balance between capacity and structural stability.

[0018] The comparative example uses the electrochemical ion exchange method described in existing technical documents to prepare NLFPP without GQDs modification, compared with the NLFPP-GQDs of this invention: Performance indicators This invention NLFPP-GQDs Existing technology NLFPP Increase 0.2C discharge capacity (mAh / g) 108.5 103.2 5.1% 30C discharge capacity (mAh / g) 86.7 81.5 6.4% Li⁺ diffusion coefficient (cm² / s) <![CDATA[4.8×10⁻ 9 ]]> <![CDATA[1.25×10⁻ 9 ]]> 284% Retention rate (%) after 500 cycles at 0.5C 88.3 82.3 7.3% Synthesis cycle (h) 48 120 60% shortened Production cost (RMB / kg) 85 110 22.7% decrease The results show that the material of this invention is superior to the prior art in terms of capacity, rate performance and cycle stability, and the synthesis cycle is significantly shortened and the cost is reduced, making it more suitable for industrial production.

[0019] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention. In addition, it should be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately. Furthermore, various different embodiments of the present invention can also be arbitrarily combined, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. An iron-based mixed multi-anion cathode material, characterized in that: The invention includes a sodium-lithium composite polyanionic cathode material modified with graphene quantum dots, which has a mesoporous spherical structure. The graphene quantum dots are rich in hydroxyl functional groups on their surface and are uniformly distributed on the surface and grain boundaries of NLFPP particles to form a continuous conductive network.

2. A method for preparing an iron-based mixed multi-anion cathode material, characterized in that: Includes the following steps: S1. Preparation of Li-NFPP-GQDs precursor by hydrothermal pre-intercalation: FeC2O4·2H2O, NaH2PO4·2H2O, (NH4)2HPO4, Li2CO3, and polyvinylpyrrolidone (PVP) were dissolved in deionized water at a molar ratio of 5:3:1:3:0.1 and stirred to form a suspension; GQDs were added and ultrasonically dispersed for 20-30 min; the mixture was transferred to a hydrothermal reactor and kept at 170-190℃ for 10-14 h; after centrifugation and washing, it was vacuum dried at 60-80℃ for 6-10 h to obtain the Li-NFPP-GQDs precursor; S2. High-temperature solid-phase reconstruction: The precursor is placed in a tube furnace and heated to 320-380℃ at 1-3℃ / min under an Ar atmosphere, and held for 3-5 hours; then the temperature is switched to an Ar-H2 mixture and heated to 680-720℃ at 4-6℃ / min, and held for 8-12 hours; the furnace is cooled to room temperature and ground through a 200-300 mesh sieve. S3. Purification treatment: Disperse the calcined product in a 0.08-0.12 mol / L dilute H2SO4 solution, stir at room temperature for 25-35 min, centrifuge and wash until pH=6.8-7.2, and vacuum dry at 80-100℃ for 10-14 h to obtain NLFPP-GQDs cathode material.

3. The method for preparing an iron-based mixed multi-anion cathode material according to claim 2, characterized in that: In step S1, the mass ratio of GQDs to Fe source is 2-4%, the ultrasonic dispersion power is 250-350w, the frequency is 40kHz, the centrifugation speed is 8000-10000rpm, and the centrifugation time is 8-12min.

4. The method for preparing an iron-based mixed multi-anion cathode material according to claim 2, characterized in that: In step S2, the volume ratio of the Ar-H2 mixture is 90:10-98:2, the Ar gas flow rate is 40-60 mL / min, and the Ar-H2 mixture flow rate is 70-90 mL / min. During the high-temperature insulation stage, the H2 atmosphere can inhibit Fe²⁺ oxidation and drive the directional exchange between Li⁺ and residual Na⁺.

5. The method for preparing an iron-based mixed multi-anion cathode material according to claim 2, characterized in that: In step S3, the liquid-to-solid ratio of dilute H2SO4 to the calcined product is 10-15:1 (mL / g), the centrifugation speed is 10000-12000 rpm, and the centrifugation time is 12-18 min.

6. A lithium-ion battery, characterized in that: It includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode comprises the positive electrode material as described in claim 1 or the positive electrode material prepared by any one of claims 2-5.