Lithium iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a lithium iron phosphate positive electrode material as well as a preparation method and application thereof. The lithium iron phosphate positive electrode material comprises a lithium iron phosphate matrix (LFP) and a coating layer coated on the surface of the lithium iron phosphate matrix, wherein the coating layer contains aromatic polyamide and carbon. The lithium iron phosphate positive electrode material can improve the electrochemical performance of LFP at low temperature, and improve the low-temperature cycling stability and capacity retention ratio of the battery.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a lithium iron phosphate cathode material, its preparation and application. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) has become one of the most important cathode materials in the fields of power batteries and energy storage batteries due to its advantages such as high safety, long cycle life, and low cost. However, its intrinsic low electronic conductivity (10⁻⁶ ppm) is a significant drawback. -9 S / cm) and slow lithium-ion diffusion rate (10 - ¹ 4 ~10 - ¹ 6 Its low-temperature performance is extremely poor (cm² / s). Below 0°C, its discharge capacity decays sharply, and the capacity retention rate is usually less than 60% at -20°C, which severely limits its application in cold regions.

[0003] Currently, conventional methods to improve the low-temperature performance of LFP include: (1) Carbon coating: This is the most common modification method, which improves electronic conductivity by coating the surface of LFP particles with an amorphous carbon layer. However, the carbon layer produced by the pyrolysis of traditional carbon sources (such as glucose and sucrose) has high disorder and many lattice defects, which has limited improvement on ion conduction ability, and the interfacial impedance between the carbon layer and the electrolyte is still large at low temperatures. (2) Particle nano-sizing: This shortens the lithium-ion diffusion path. However, nanoparticles are prone to agglomeration, have low tap density, and increase side reactions with the electrolyte, leading to processing difficulties and decreased cycle stability. (3) Ion doping: This is achieved by using Mg²⁺ to ionize the LFP particles. + Ti 4+ Doping with elements such as [specific element name] expands the lithium-ion diffusion channels. However, the process control is complex and may introduce impurity phases, sacrificing some specific capacity. Summary of the Invention The purpose of this invention is to provide a lithium iron phosphate cathode material, its preparation method and application. This lithium iron phosphate cathode material can improve the electrochemical performance of LFP at low temperatures and enhance the low-temperature cycle stability and capacity retention of the battery.

[0004] To achieve the objectives of this invention, the following technical solution is adopted: On one hand, the present invention provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix and a coating layer covering the surface of the lithium iron phosphate matrix, wherein the coating layer contains polyarylamide and carbon.

[0005] In this embodiment of the invention, the carbon is carbon quantum dots.

[0006] In embodiments of the present invention, the polyarylamide is poly(p-phenylene terephthalamide) and / or poly(m-phenylene isophthalamide).

[0007] Preferably, the polyarylamide is poly(p-phenylene terephthalamide).

[0008] In this embodiment of the invention, the particle size range of the lithium iron phosphate matrix is ​​0.15~6μm.

[0009] In an embodiment of the present invention, the thickness of the composite coating layer is 50~200nm.

[0010] Preferably, the polyarylamide accounts for 0.5wt% to 2.5wt% of the weight of the lithium iron phosphate matrix.

[0011] Preferably, the molecular weight of the polyarylamide is 4000-6000.

[0012] Preferably, the carbon quantum dots account for 0.5wt% to 2.5wt% of the weight of the lithium iron phosphate matrix.

[0013] Preferably, the particle size of the carbon quantum dots is in the range of 5~20 nm.

[0014] On the other hand, the present invention also provides a method for preparing the lithium iron phosphate cathode material as described above, comprising the following steps: Preparation of lithium iron phosphate matrix; Carbon quantum dots are uniformly dispersed to form a dispersion. Raw material A for synthesizing polyarylamide is added to the dispersion and dissolved, and then the lithium iron phosphate matrix is ​​added to form an LFP dispersion; under low temperature and rapid stirring, raw material B for synthesizing polyarylamide is added to the LFP dispersion, and after in-situ polymerization reaction, lithium iron phosphate cathode material is obtained.

[0015] In an embodiment of the present invention, the preparation of the lithium iron phosphate matrix is ​​carried out through the following steps: mixing an iron source, a lithium source and a phosphorus source, ball milling, drying and then heat treating to obtain the lithium iron phosphate matrix.

[0016] Preferably, the iron source includes at least one of ferric phosphate, ferrous oxalate, ferric nitrate, iron oxide red, iron tetroxide, and ultrafine iron powder.

[0017] Preferably, the lithium source includes at least one of lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium chloride, and lithium nitrate.

[0018] Preferably, the phosphorus source includes at least one of ferric phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ferrous ammonium phosphate.

[0019] Preferably, the ball milling specifically includes: using grinding beads of 1~4mm, and ball milling for 3~6 hours at a rotation speed of 300~500rpm.

[0020] Preferably, the heat treatment specifically includes: heating to 300℃~400℃ in an inert gas at a rate of 2~5℃ / min, holding at that temperature for 2h~5h, and then heating to 500~650℃ at a rate of 2~5℃ / min, holding at that temperature for 8~16h.

[0021] In this embodiment of the invention, the raw material A of the polyarylamide is p-phenylenediamine and / or m-phenylenediamine; the raw material B of the polyarylamide is at least one of terephthaloyl chloride, isophthaloyl chloride, trimesoyl chloride and oxaloyl chloride.

[0022] In this embodiment of the invention, the temperature of the low-temperature rapid stirring is -20℃ to 0℃, and the stirring speed is 100~300 rap / min; the time of the in-situ polymerization reaction is 10~18h.

[0023] In an embodiment of the present invention, the preparation method further includes: heating the obtained lithium iron phosphate cathode material to 300°C to 500°C at a rate of 2 to 5°C / min under the protection of an inert gas, and holding at that temperature for 2 to 4 hours.

[0024] In another aspect, the present invention also provides a positive electrode sheet, which includes the lithium iron phosphate positive electrode material as described above, or the lithium iron phosphate positive electrode material prepared by the preparation method described above. In another aspect, the present invention also provides a lithium-ion battery comprising the positive electrode sheet as described above. Compared with the prior art, the beneficial effects of the present invention are as follows: The lithium iron phosphate cathode material of the present invention comprises a lithium iron phosphate matrix and a coating layer covering the surface of the lithium iron phosphate matrix, wherein the coating layer contains polyarylamide and carbon. The amide bonds (-NH-CO-) on the polyarylamide molecular chain have strong intermolecular forces with electrolyte solvent molecules (ethylene carbonate or dimethyl carbonate), which can efficiently adsorb and lock in the electrolyte, ensuring sufficient ion transport medium at low temperatures. Furthermore, the porous network structure formed by the polyarylamide provides a buffer space for ion migration and a uniformly distributed scaffold for the carbon, preventing its aggregation. The carbon solves the electrochemical inertness problem of the coating layer and improves the conductivity of the material. The synergistic effect of both ensures high capacity output and long-term cycling stability of the material at low temperatures. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the lithium iron phosphate cathode material of the present invention; The attached figures are labeled as follows: 1: Lithium iron phosphate matrix; 2: Coating layer containing polyarylamide and carbon. Detailed Implementation

[0026] To better understand and implement this application, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of this application, and not all of them.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0028] Unless otherwise stated, all numerical values ​​for the amounts of expressed components, reaction conditions, etc., used in the specification and claims are to be understood as being modified by the term "about". Therefore, unless otherwise indicated, the numerical parameters set forth herein are approximate values ​​that can be varied to obtain the desired performance.

[0029] For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range or individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0030] The word “and / or” as used in this article refers to one or all of the elements mentioned.

[0031] The terms "include" and "contain" as used in this article cover both cases where only the mentioned elements exist and cases where other unmentioned elements exist in addition to the mentioned elements.

[0032] Currently, existing technologies have failed to fundamentally solve the problems of high ion migration barriers and difficulties in interfacial desolvation at low temperatures on the surface of LFP particles. Therefore, developing an innovative modification technology that can simultaneously enhance electron transport, ion conduction, and stabilize the interface has significant scientific and commercial value.

[0033] The purpose of this invention is to provide a lithium iron phosphate (LFP) cathode material, which is a high-performance, low-temperature cathode material with a composite coating layer. The composite coating layer strategy is used to modify the LFP, improving its electrochemical performance at low temperatures and enhancing the battery's low-temperature cycle stability and capacity retention. The following is a detailed description of this application.

[0034] Lithium iron phosphate cathode material like Figure 1 As shown, the present invention provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate substrate 1 and a coating layer 2 covering the surface of the lithium iron phosphate substrate 1, wherein the coating layer contains polyarylamide and carbon.

[0035] The material of "lithium iron phosphate matrix (LFP)" is not limited to unmodified lithium iron phosphate, but also includes modified lithium iron phosphate, such as doped modified lithium iron phosphate; preferably, the material of "lithium iron phosphate matrix" is unmodified lithium iron phosphate.

[0036] "Coating" is not limited to direct coating, but also includes indirect coating; that is, the coating layer can be directly coated on the lithium iron phosphate substrate, and there can be one or more other structures between the coating layer and the lithium iron phosphate substrate; preferably, the coating layer is directly coated on the lithium iron phosphate substrate, that is, there are no other structures between the coating layer and the lithium iron phosphate substrate.

[0037] "Coating the surface of the lithium iron phosphate substrate" can mean coating the entire surface of the lithium iron phosphate substrate or coating a portion of the surface of the lithium iron phosphate substrate; preferably, "coating the surface of the lithium iron phosphate substrate" means coating the entire surface of the lithium iron phosphate substrate, that is, the coating layer completely covers the surface of the lithium iron phosphate substrate.

[0038] This invention employs a dual-strategy modification approach using a composite interface to improve the slow kinetics of low-temperature lithium-ion polymers (LFPs) and increase conductivity, thereby addressing issues such as low capacity retention and poor rate performance at low temperatures. A high-performance low-temperature cathode material with a composite coating layer is synthesized. Through innovative material combination, this invention, for the first time, combines polyarylamide, an insulating polymer with excellent electrolyte affinity, with carbon materials exhibiting superior conductivity for LFP modification. Specifically, the amide bonds (-NH-CO-) on the polyarylamide molecular chain have strong intermolecular forces with electrolyte solvent molecules (ethylene carbonate or dimethyl carbonate), efficiently adsorbing and locking in the electrolyte to ensure sufficient ion transport medium at low temperatures. Furthermore, the porous network structure formed by the polyarylamide provides a buffer space for ion migration and a uniformly distributed scaffold for the carbon, preventing its aggregation. The carbon solves the electrochemical inertness problem of the coating layer and improves the material's conductivity. The synergistic effect of both ensures high capacity output and long-term cycling stability at low temperatures.

[0039] In this embodiment of the invention, carbon is carbon quantum dots (CQDs). Carbon quantum dots possess a huge specific surface area and abundant edge defects, making them excellent electronic conductors capable of forming a continuous, high-speed ion / electron transport interface layer. Their surface functional groups, such as carbonyl and carboxyl groups, can serve as "activation sites" for lithium ions, effectively reducing the lithium content of lithium. + The "desolvation energy barrier" before the electrolyte enters the cathode material lattice significantly reduces the interfacial impedance at low temperatures, playing a very important role in overcoming the bottleneck of low-temperature performance.

[0040] In this embodiment of the invention, the polyarylamide is poly(p-phenylene terephthalamide) (PPTA) and / or poly(m-phenylene isophthalamide) (PMIA). PPTA and PMIA have excellent electrolyte affinity and good thermal stability, and can efficiently adsorb and lock in the electrolyte, ensuring sufficient ion transport medium at low temperatures.

[0041] Preferably, the polyarylamide is poly(p-phenylene terephthalamide) (PPTA). PPTA has the advantages of high mechanical strength and excellent electrolyte affinity; the high thermal stability and high mechanical strength of PPTA can also provide top-level protection for LFP particles, suppressing the risk of structural breakage and thermal runaway during cycling. In this embodiment of the invention, carbon is carbon quantum dots (CQDs), and polyarylamide is poly(p-phenylene terephthalamide) (PPTA). PPTA and CQDs can play a synergistic role in ion conduction, electronic conduction, mechanical and thermal stability, specifically manifested as: (1) Synergistic ion conduction: The negatively charged functional groups (-COOH, -OH) on the surface of CQDs can effectively promote lithium ion (Li) + (1) The adsorption and desolvation process of PPTA significantly reduces the interfacial impedance at low temperatures; the good liquid retention of PPTA ensures that there is always sufficient electrolyte at the interface; (2) Electron conduction synergy: CQDs construct an electron channel through the PPTA insulation layer, which makes up for the defect of PPTA non-conductivity and realizes the efficient collection and transmission of electrons on the particle surface. (3) Mechanical and thermal stability synergy: The high thermal stability and high mechanical strength of PPTA provide top protection for LFP particles, suppressing the risk of structural breakage and thermal runaway during cycling. The PPTA network provides a uniformly distributed support for CQDs to prevent their aggregation; CQDs solve the insulation problem of PPTA; the combination of the two forms a "ion-electron" dual continuous high-speed transport interface layer on the surface of LFP particles.

[0042] Preferably, the weight of carbon quantum dots (CQDs) accounts for 0.5 wt% to 2.5 wt% of the weight of the lithium iron phosphate (LFP) matrix; for example, the weight of CQDs accounts for 0.5 wt%, 0.7 wt%, 0.9 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.5 wt% of the weight of the LFP matrix, etc. A suitable range of CQD content not only ensures a suitable coating thickness but also guarantees a certain level of conductivity; when the CQD content is low, the conductivity is insufficient; when the CQD content is high, it not only affects the coating thickness but also the ion diffusion distance, increasing the interfacial impedance.

[0043] Preferably, the particle size range of carbon quantum dots is 5~20 nm. For example, the particle size range of carbon quantum dots is 5 nm, 7 nm, 10 nm, 13 nm, 16 nm, 18 nm, 20 nm, etc.

[0044] Preferably, the polyarylamide accounts for 0.5 wt% to 2.5 wt% of the weight of the lithium iron phosphate matrix. For example, the polyarylamide content is 0.5 wt%, 0.7 wt%, 0.9 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.5 wt%, etc., of the lithium iron phosphate matrix. A suitable polyarylamide coating amount ensures a continuous coating layer, thereby guaranteeing sufficient mechanical strength and suppressing volume expansion during cycling. When the coating amount is too low, the effect of suppressing volume expansion during cycling is weakened, affecting cycle life; when the coating amount is too high, the ion transport distance becomes longer, affecting the ion diffusion rate.

[0045] Preferably, the molecular weight of the polyarylamide is 4000-6000. Exemplary examples include polyarylamides with molecular weights of 4000, 4300, 4500, 4900, 5200, 5500, and 6000. Selecting polyarylamides with higher molecular weights ensures a certain level of mechanical strength and reduces volume expansion during cycling.

[0046] In this embodiment of the invention, the particle size range of the lithium iron phosphate matrix is ​​0.15~6 μm. Exemplarily, the particle size range of the lithium iron phosphate matrix is ​​0.15 μm, 0.2 μm, 0.8 μm, 2 μm, 3.5 μm, 4.8 μm, 5.6 μm, 6 μm, etc. Selecting the particle size of the lithium iron phosphate matrix within this range can reduce the resistance to lithium-ion solid-phase transport and improve kinetics.

[0047] In this embodiment of the invention, the thickness of the composite coating layer is 50-200 nm. Exemplarily, the thickness of the composite coating layer is 50 nm, 60 nm, 80 nm, 95 nm, 100 nm, 130 nm, 180 nm, 200 nm, etc. A suitable coating layer not only ensures a certain level of conductivity but also isolates electrolyte side reactions, ensuring the absorption of a large amount of electrolyte. When the coating layer is too thin, uneven coating may occur, and after a certain number of cycles, the coating layer may crack, easily leading to side reactions with the electrolyte. When the coating layer is too thick, the interfacial impedance increases, affecting ion transport kinetics.

[0048] Preparation method of lithium iron phosphate cathode material On the other hand, the present invention also provides a method for preparing the lithium iron phosphate cathode material as described above, comprising the following steps: Preparation of lithium iron phosphate matrix; Carbon quantum dots are uniformly dispersed to form a dispersion. Raw material A for synthesizing polyarylamide is added to the dispersion and dissolved, and then the lithium iron phosphate matrix is ​​added to form an LFP dispersion; under low temperature and rapid stirring, raw material B for synthesizing polyarylamide is added to the LFP dispersion, and after in-situ polymerization reaction, lithium iron phosphate cathode material is obtained.

[0049] In the above preparation method, polyarylamide can form a three-dimensional nanofiber network through in-situ polymerization, which can provide excellent flexible support, electrolyte lock-in and physical isolation; CQDs can be uniformly embedded in the PPTA network, thus better playing the role of "miniature conductive station" and "ion activation center", providing an ultra-fast transport path for electrons and lithium ions.

[0050] In an embodiment of the present invention, the preparation of the lithium iron phosphate matrix is ​​carried out through the following steps: mixing an iron source, a lithium source and a phosphorus source, ball milling, drying and then heat treating to obtain the lithium iron phosphate matrix.

[0051] Preferably, the iron source includes at least one of ferric phosphate, ferrous oxalate, ferric nitrate, iron oxide red, iron tetroxide, and ultrafine iron powder.

[0052] Preferably, the lithium source includes at least one of lithium carbonate, lithium dihydrogen phosphate, lithium hydroxide, lithium chloride, and lithium nitrate.

[0053] Preferably, the phosphorus source includes at least one of ferric phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ferrous ammonium phosphate.

[0054] In some embodiments, the iron source and phosphorus source are both iron phosphate, the lithium source is lithium carbonate, and the molar ratio of iron phosphate to lithium carbonate is 1:(1.05~1.1).

[0055] Preferably, the ball milling specifically includes: using grinding beads of 1-4 mm, and milling at a rotation speed of 300-500 rpm for 3-6 hours. More preferably, the grinding beads are zirconia beads, and the ball-to-material ratio is (9-11):1.

[0056] Preferably, the drying process specifically involves drying in a drying oven at 80-100℃ for 12-16 hours.

[0057] Preferably, the heat treatment specifically includes: heating to 300℃~400℃ in an inert gas at a rate of 2~5℃ / min, holding at that temperature for 2h~5h, and then heating to 500~650℃ at a rate of 2~5℃ / min, holding at that temperature for 8~16h. A segmented heat treatment / calcination method is adopted. The first stage of heat treatment (300℃~400℃, holding for 2h~5h) is mainly to remove water of crystallization, and the second stage of heat treatment (500~650℃, holding for 8~16h) is mainly to promote crystal phase formation. Further, the inert gas includes at least one of argon, nitrogen, helium, and neon, preferably argon.

[0058] In embodiments of the present invention, raw material A of the polyarylamide is p-phenylenediamine (PPDA) and / or m-phenylenediamine (MPD); raw material B of the polyarylamide is terephthaloyl chloride (TPC) and / or isophthaloyl chloride (IPC). In some embodiments, the polyarylamide is PPTA, raw material A is p-phenylenediamine, and raw material B is terephthaloyl chloride; in other embodiments, the polyarylamide is PMIA, raw material A is m-phenylenediamine, and raw material B isophthaloyl chloride.

[0059] Preferably, the mass ratio of raw material A to raw material B of polyarylamide is (0.5~3): (1.2~4.8).

[0060] In this embodiment of the invention, the dispersion also contains an acid absorbent. Preferably, the acid absorbent is sodium hydroxide.

[0061] In this embodiment of the invention, the temperature of the low-temperature rapid stirring is -20℃ to 0℃, and the stirring speed is 100~300 rap / min; the time of the in-situ polymerization reaction is 10~18h.

[0062] In an embodiment of the present invention, the preparation method further includes: after the in-situ polymerization reaction is completed, the solid product is collected by vacuum filtration, washed several times with deionized water and ethanol alternately until the filtrate is neutral, and the filter cake is dried at 80-120°C for 12-24 hours.

[0063] In an embodiment of the present invention, the preparation method further includes: heating the obtained lithium iron phosphate cathode material to 300°C-500°C at a rate of 2-5°C / min under the protection of an inert gas, and holding at that temperature for 2-4 hours. Further, the inert gas includes at least one of argon, nitrogen, helium, and neon, preferably argon.

[0064] Positive electrode sheet In another aspect, the present invention also provides a positive electrode sheet, which includes the lithium iron phosphate positive electrode material as described above, or the lithium iron phosphate positive electrode material prepared by the preparation method described above.

[0065] In embodiments of the present invention, the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode material. Further, the positive electrode film layer also includes a conductive agent and a binder. The present invention does not impose any particular limitations on the material of the positive current collector or the types of conductive agents and binders in the positive electrode film layer, as long as the purpose of this application can be achieved.

[0066] Lithium-ion batteries In another aspect, the present invention also provides a lithium-ion battery comprising the positive electrode sheet as described above. In embodiments of the present invention, the lithium-ion battery further includes a negative electrode, an electrolyte, and a separator. The separator is disposed between the positive and negative electrodes, and the electrolyte fills the pores of the separator and wets both the positive and negative electrodes. During battery charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes; the electrolyte acts as a conductor between the positive and negative electrodes; and the separator, disposed between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through. The present invention does not impose any particular limitations on the negative electrode, electrolyte, or separator, as long as the objective of this application is achieved.

[0067] The present invention will be further illustrated below with reference to the embodiments: Example 1: The preparation method of the lithium iron phosphate cathode material in this embodiment includes the following steps: 1. Preparation of LiFePO4 matrix Ferric phosphate dihydrate (FePO4·2H2O) and lithium carbonate (Li2CO3) with a molar ratio of 1:1.05 were mixed in a planetary ball mill under the following conditions: anhydrous ethanol (purity ≥99.5%) as solvent, zirconium oxide balls (3 mm in diameter, ball-to-powder ratio 10:1) as milling media, a rotation speed of 400 rpm, a vacuum environment, and a time of 5 h (alternating forward and reverse rotation, switching directions every 30 min). The mass ratio of ethanol to powder (i.e., the mixture of ferric phosphate dihydrate and lithium carbonate) was 3:1. After ball milling, the mixture was dried in a drying oven at 80 °C for 12 h. After drying, the mixture was calcined in a tube furnace under an argon atmosphere in stages: first, the temperature was increased from room temperature to 350 °C at a rate of 2 °C / min and held for 4 h, then increased to 650 °C at a rate of 5 °C / min and held for 12 h. After natural cooling to room temperature, a lithium iron phosphate matrix with a particle size of 0.35 μm was obtained.

[0068] 2. Preprocessing of CQDs 1g of CQDs (particle size 5nm) was dispersed in 100ml of anhydrous NMP and sonicated for 4 hours to form a uniform dispersion. 3. Preparation of PPTA / CQDs@LFP (1) Preparation of amine monomer solution: In a glove box, 0.68g of PPDA was dissolved in the above dispersion, and 0.5g of NaOH was added as an acid absorbent. The mixture was magnetically stirred until completely dissolved to obtain solution A. (2) Dispersion of LFP: 100g of dry lithium iron phosphate matrix powder was added to solution A, mechanically stirred and ultrasonically applied to fully wet and encapsulate the lithium iron phosphate matrix particles to form solution B. (3) Preparation of acyl chloride monomer: Dissolve 1.28g TPC in 300mL of anhydrous n-hexane to obtain solution C. (4) Transfer slurry B to a reactor equipped with a condenser and a constant pressure dropping funnel. Under ice-water bath (-20°C) and vigorous stirring (200 rap / mins), slowly add solution C dropwise (about 10 s / drop per hour) to slurry B. After the addition is complete, continue the reaction for 14 hours. (5) Post-processing: After the reaction is complete, the solid product is collected by vacuum filtration and washed several times with deionized water and ethanol alternately until the filtrate is neutral. The filter cake is dried in an 80°C vacuum drying oven for 16 hours. (6) Heat treatment: The dried powder is heated to 300℃-500℃ in a tube furnace under argon protection at a rate of 2℃ / min, held for 4 hours, and then naturally cooled to room temperature to obtain the cathode material PPTA / CQDs@LFP; wherein the thickness of the PPTA / CQDs composite coating layer is 100nm.

[0069] Example 2: The only difference between this embodiment and Example 1 is that in the preparation of the amine monomer solution in step (1), the amount of PPDA used is 0.22g; In step (3) preparation of acyl chloride monomer, the amount of TPC used is 0.42g; The thickness of the obtained PPTA / CQDs composite coating layer is 60 nm.

[0070] Example 3: The only difference between this embodiment and Example 1 is that in the preparation of the amine monomer solution in step (1), the amount of PPDA used is 1.1g; In step (3) preparation of acyl chloride monomer, the amount of TPC used is 2.1g; The thickness of the obtained PPTA / CQDs composite coating layer is 180 nm.

[0071] Example 4: The only difference between this embodiment and Embodiment 1 is that in the pretreatment of CQDs in step 2, the amount of CQDs used is 0.5g and the particle size of CQDs is 10nm.

[0072] Example 5: The only difference between this embodiment and Embodiment 1 is that in the pretreatment of CQDs in step 2, the amount of CQDs used is 2g and the particle size of CQDs is 20nm.

[0073] Comparative Example 1: According to step 1 of Example 1 (i.e., preparation of LiFePO4 matrix), a lithium iron phosphate matrix was obtained; this lithium iron phosphate matrix was not modified.

[0074] Comparative Example 2: According to step 1 in Example 1 (i.e., preparation of LiFePO4 matrix), lithium iron phosphate matrix is ​​obtained; 100g of dried lithium iron phosphate matrix powder was mixed with 1g of sucrose in a tube furnace. Under argon protection, the temperature was increased to 300℃-500℃ at 2℃ / min, held for 4 hours, and then naturally cooled to room temperature to obtain the final product C@LFP; wherein the thickness of the C coating layer is about 60nm.

[0075] Comparative Example 3: A method for preparing PPTA-coated LFP includes the following steps: 1. Preparation of LiFePO4 matrix According to step 1 in Example 1 (i.e., preparation of LiFePO4 matrix), lithium iron phosphate matrix is ​​obtained; 2. Preparation of PPTA@LFP (1) Preparation of amine monomer solution: In a glove box, 1.5g PPDA was dissolved in 100ml anhydrous NMP, and 0.5g NaOH was added as an acid absorbent. The mixture was magnetically stirred until completely dissolved to obtain solution A. (2) Dispersion of LFP: 100g of dry lithium iron phosphate matrix was added to solution A, mechanically stirred and ultrasonically applied to fully wet and encapsulate the lithium iron phosphate matrix to form solution B. (3) Preparation of acyl chloride monomer: Dissolve 3g TPC in 300mL of anhydrous n-hexane to obtain solution C. (4) Transfer slurry B to a reactor equipped with a condenser and a constant pressure dropping funnel. Under ice-water bath (-20°C) and vigorous stirring (200 rap / mins), slowly add solution C dropwise (about 10 s / drop per hour) to slurry B. After the addition is complete, continue the reaction for 14 hours. (5) Post-processing: After the reaction is complete, the solid product is collected by vacuum filtration and washed several times with deionized water and ethanol alternately until the filtrate is neutral. The filter cake is dried in an 80°C vacuum drying oven for 16 hours. (6) Heat treatment: The dried powder is heated to 300℃-500℃ in a tube furnace under argon protection at a rate of 2℃ / min, held for 4 hours, and then naturally cooled to room temperature to obtain the final product PPTA@LFP.

[0076] Comparative Example 4: A method for preparing CQDs-coated LFP includes the following steps: According to step 1 in Example 1 (i.e., preparation of LiFePO4 matrix), lithium iron phosphate matrix is ​​obtained; 100g of dried lithium iron phosphate matrix powder was mixed with 1g of CQDs in NMP and then directly dried and sintered to obtain CQDs@LFP.

[0077] Performance testing To test the electrochemical performance of the lithium-ion cathode materials obtained in the examples and comparative examples, the cathode materials obtained in Examples 1-5 or Comparative Examples 1-4, the binder polyvinylidene fluoride (PVDF), and the conductive agent SP were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 80:10:10 and stirred to form a uniform and stable slurry. The slurry was then coated onto aluminum foil and dried to obtain the cathode sheet. The cathode sheets were cut, weighed, and placed in a glove box. In the glove box, using a sodium metal sheet as the negative electrode, LiPF6 dissolved in diethylene glycol dimethyl ether as the electrolyte, and PP as the separator, CR2032 button cells were assembled. The button cells were then tested sequentially using a Land battery testing system at a current density of 0.1V within a voltage window of 2.5~3.65V. The test conditions and results are shown in Table 1. (1) Low temperature test and cycle test Activation: Charge and discharge at 0.1C constant current for 3 cycles at 25℃ (2.5-3.65 V); Low temperature test: The battery was placed in a high and low temperature test chamber (-20℃±0.5℃) and kept at a constant temperature for 2 hours; First charge and discharge: The nominal specific capacity was set to 170mAh / g. The battery was charged to 3.65V at 0.2C and then discharged to 2.5V at a constant current. The specific capacity of the first discharge cycle was recorded. Specific capacity = capacity / mass of active material. Cyclic testing: At -20℃, the battery is charged to 3.65V at a constant current and constant voltage of 1C, left to stand for 30 minutes, and then discharged to 2.5V at a constant current of 1C, left to stand for 30 minutes. The discharge capacity of the 1st and 500th cycles is recorded. Capacity retention rate = (Nth discharge capacity / 1st discharge capacity) × 100%.

[0078] (2) Electrochemical impedance spectroscopy (EIS) test Test conditions: Frequency range: 100 kHz ~ 10 mHz; Amplitude: 5 mV; Test temperature: -20℃ (tested after battery is kept at constant temperature for 2 hours).

[0079] (3) The method for testing the low-temperature capacity retention rate is as follows: ① Charge the battery to 3.65V at a constant current of 0.33C at room temperature; ② Discharge the battery to 2.5V at 0.33C and record the discharge capacity C1 at room temperature; ③ Let the battery stand at -20℃ for 4 hours; charge the battery to 3.65V at a constant current of 0.33C, and then charge it to 3.65V at a constant voltage of 3.65V until the current is ≤0.05C and the battery is cut off; ④ Discharge the battery to 2.5V at -20℃ at 0.33C and record the discharge capacity C2 at this time; Repeat the above steps 3 times. Low-temperature capacity retention rate = C2 (average) / (C1 (average), where C1 is the battery capacity charged during constant current discharge at room temperature, and C2 is the battery capacity during constant current discharge at -20℃.

[0080] (4) Liquid absorption capacity: After drying the coated electrode, cut it into a 2*10cm shape, fix one end, and place the other end in a box containing electrolyte. Observe the highest liquid height that the liquid on the electrode can reach after 18 hours. This represents the liquid absorption capacity of the electrode.

[0081] Table 1. Electrochemical performance test results of each example and comparative example.

[0082] Referring to Table 1 and comparing Examples 1-3, it was found that a coating layer of appropriate thickness is needed to obtain better capacity. A thicker coating layer leads to a partial loss of capacity. The same conclusion can be drawn for Examples 4-5. Comparing Comparative Examples 1-4 with Examples 1-5, it was found that Comparative Example 1, without modification, has insufficient discharge capacity. The main reason is the insufficient kinetics of lithium ions at low temperatures, resulting in slow diffusion. Therefore, modification is necessary.

[0083] Table 1 shows that Examples 1-5 have better capacity retention, mainly because: on the one hand, PPTA not only inhibits volume expansion but also ensures liquid absorption capacity, making PPTA the main factor in maintaining capacity retention; on the other hand, CQDs can also avoid crack side reactions. Comparative Examples 1-4, in particular, have relatively poor capacity retention, especially the unmodified Comparative Example 1.

[0084] Comparing Examples 1-5, it can be observed that the impedance is lower as the CQDs content increases, which is due to the superconducting ability of the CQDs. However, when the CQD content exceeds a reasonable range, the coating layer thickness increases significantly, leading to an increase in impedance. In Example 3, when PPTA was increased, the thicker coating layer resulted in an increase in impedance. Combining Examples 1-5 with Comparative Examples 1-4, it is found that the main factors determining the conductivity and interfacial impedance are the amount of CQDs coated and the coating layer thickness. The low-temperature performance of the battery also shows the same trend. Among them, Comparative Example 2 also involves carbon coating, and although the improvement effect is higher than that of single-layer PPTA coating, the conductivity is far inferior to that of CQDs.

[0085] Comparing Examples 1-5, it was found that the liquid absorption capacity increased continuously with increasing PPTA content, which is due to the inherent properties of the material. However, once the PPTA content reached saturation, the liquid absorption capacity was saturated. Further increasing the PPTA content would lead to an increase in the coating layer thickness, resulting in increased membrane impedance. Simultaneously, a suitable content of CQDs, with a large specific surface area and appropriate coating layer thickness, could also increase the liquid absorption capacity. The comparative examples demonstrate that PPTA is a key material for ensuring liquid absorption capacity, a conclusion consistent with Comparative Examples 1-4.

[0086] The technical means disclosed in this application are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A lithium iron phosphate cathode material, characterized in that: It includes a lithium iron phosphate matrix and a coating layer covering the surface of the lithium iron phosphate matrix, the coating layer containing polyaramid and carbon.

2. The lithium iron phosphate cathode material as described in claim 1, characterized in that: The carbon is carbon quantum dots; and / or, the polyarylamide is poly(p-phenylene terephthalamide) and / or poly(m-phenylene isophthalamide).

3. The lithium iron phosphate cathode material as described in claim 1, characterized in that: The particle size range of the lithium iron phosphate matrix is ​​0.15~6μm; and / or the thickness of the composite coating layer is 50~200nm.

4. The lithium iron phosphate cathode material as described in claim 2, characterized in that: The polyarylamide accounts for 0.5wt% to 2.5wt% of the weight of the lithium iron phosphate matrix; and / or, the molecular weight of the polyarylamide is 4000 to 6000.

5. The lithium iron phosphate cathode material as described in claim 2, characterized in that: The carbon quantum dots account for 0.5wt% to 2.5wt% of the weight of the lithium iron phosphate matrix; and / or, the particle size of the carbon quantum dots ranges from 5 to 20 nm.

6. A method for preparing a lithium iron phosphate cathode material as described in any one of claims 1 to 5, characterized in that: Includes the following steps: Preparation of lithium iron phosphate matrix; Carbon quantum dots are uniformly dispersed to form a dispersion. Raw material A for synthesizing polyarylamide is added to the dispersion and dissolved, and then the lithium iron phosphate matrix is ​​added to form an LFP dispersion; under low temperature and rapid stirring, raw material B for synthesizing polyarylamide is added to the LFP dispersion, and after in-situ polymerization reaction, lithium iron phosphate cathode material is obtained.

7. The method for preparing the lithium iron phosphate cathode material as described in claim 6, characterized in that: The preparation of the lithium iron phosphate matrix is ​​carried out through the following steps: mixing iron source, lithium source and phosphorus source, ball milling, drying and heat treatment to obtain the lithium iron phosphate matrix.

8. The method for preparing the lithium iron phosphate cathode material as described in claim 6, characterized in that: At least one of the following conditions must be met: (1) The raw material A of the polyarylamide is p-phenylenediamine and / or m-phenylenediamine; the raw material B of the polyarylamide is at least one of terephthaloyl chloride, isophthaloyl chloride, trimesoyl chloride and oxaloyl chloride; (2) The temperature of the low-temperature rapid stirring is -20℃ to 0℃, and the stirring speed is 100~300rap / min; the time of the in-situ polymerization reaction is 10~18h; (3) The preparation method further includes: under the protection of an inert gas, heating the obtained lithium iron phosphate cathode material to 300℃~500℃ at a rate of 2~5℃ / min and holding it at that temperature for 2~4 hours.

9. A positive electrode sheet, characterized in that: Including the lithium iron phosphate cathode material according to any one of claims 1 to 5, and / or The lithium iron phosphate cathode material prepared by any one of claims 6 to 8.

10. A lithium-ion battery, characterized in that: Includes the positive electrode sheet as described in claim 9.

Images