Method for synthesizing coated modified lithium iron phosphate

Abstract

The invention relates to the technical field of batteries, in particular to a synthesis method of coated modified lithium iron phosphate. Comprising the following steps: mixing iron phosphate, a lithium source, a reducing short-chain low-molecular-weight carbon source and a doped metal source, and sintering in an inert atmosphere to obtain lithium iron phosphate, the molar ratio of the lithium element to the carbon element is 1: (0.30-0.35); the molecular weight of the reducing short-chain low-molecular-weight carbon source is 150-200g / mol; mixing the lithium iron phosphate with a mixed carbon source, and sintering in an inert atmosphere to obtain carbon-coated lithium iron phosphate; the mixed carbon source comprises dextrose monohydrate, a cross-linking reaction type carbon source and a modified dispersant; the main chain in the molecule of the modified dispersant comprises aromatic hydrocarbon, and the side chain comprises ether group. The lithium iron phosphate material obtained by the method has high compaction density and excellent electrochemical performance, and the volume capacity density is improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a method for synthesizing coated and modified lithium iron phosphate. Background Technology

[0002] In recent years, lithium-ion batteries have been widely used in electric vehicles, portable electronic devices, and large-scale energy storage systems due to their advantages such as high energy density, long cycle life, and safety. Among them, lithium iron phosphate (LiFePO4), as a cathode material with an olivine structure, is considered one of the most promising cathode materials for lithium-ion batteries due to its high theoretical capacity (170 mAh / g), low toxicity, low cost, and environmental friendliness. However, lithium iron phosphate materials have some inherent defects, such as a relatively low lithium-ion diffusion coefficient (approximately 10–14 cm⁻¹). 2 The electrical conductivity (approximately 10⁻⁹ S / cm) and compaction density (typically <2.5 g / cm³) are difficult to improve using traditional processes. 3 These issues severely limit further improvements in the energy density of lithium iron phosphate, especially at high charge-discharge rates, where the specific capacity decreases significantly.

[0003] To address these issues, researchers have explored various methods to improve the performance of lithium iron phosphate (LFP). Carbon coating is a simple and effective method that can improve the material's conductivity and stability to some extent. However, existing carbon coating methods still suffer from problems such as uneven coating, complex processes, and high costs. Furthermore, limitations exist in the selection and use of carbon sources, making it difficult to completely reduce the ferric iron in LFP, resulting in high carbon residue and affecting the material's electrochemical performance. Glucose, an existing carbon source, tends to crystallize independently during spray drying, forming crystalline particles that accumulate and, after sintering, easily form particulate carbon, failing to achieve uniform coating and leading to poor electrical performance in LFP materials. In addition, improving compaction density by controlling the material's microstructure and preparation process is also an important research direction. However, how to maintain high compaction density while simultaneously ensuring good electrochemical performance remains a pressing technical challenge.

[0004] Patent document CN114014291A discloses a method for preparing high-density lithium iron phosphate (LFP) material, LFP, and lithium-ion battery cathode materials including the same. The preparation method includes: preparing LFP material using a lithium source, a phosphorus source, and an iron source; mixing the LFP material with a carbon source and performing a secondary sintering to prepare carbon-added LFP material; and subjecting the carbon-added LFP material to airflow milling to prepare high-density LFP material. The volumetric capacity density of the LFP material obtained by this prior art needs further improvement. Summary of the Invention

[0005] The present invention aims to solve the above problems by providing a method for synthesizing coated and modified lithium iron phosphate. The lithium iron phosphate material obtained by this method has both high compaction density and excellent electrochemical performance, and the volumetric capacity density is improved.

[0006] The technical solution to the problem of this invention is to provide a method for synthesizing coated and modified lithium iron phosphate, comprising the following steps:

[0007] S1. Mix iron phosphate, lithium source, reducing short-chain low molecular weight carbon source and doped metal source, and sinter under an inert atmosphere to obtain lithium iron phosphate;

[0008] The molar ratio of lithium to carbon is 1:(0.30~0.35);

[0009] The molecular weight of the reducing short-chain low molecular weight carbon source is 150-200 g / mol;

[0010] S2. The lithium iron phosphate is mixed with a mixed carbon source and sintered under an inert atmosphere to obtain carbon-coated lithium iron phosphate;

[0011] The mixed carbon source includes glucose monohydrate, a cross-linking reactive carbon source, and a modified dispersant;

[0012] The main chain of the modified dispersant molecule includes aromatic hydrocarbons, and the side chains include ether groups.

[0013] In this invention, firstly, an appropriate amount of reducing short-chain low-molecular-weight carbon source is added during the preparation of lithium iron phosphate to reduce all the trivalent iron in the iron phosphate to divalent iron, achieving a carbon residue of no more than 0.1%. Secondly, based on this, a self-assembling cross-linking reactive carbon source and an aromatic hydrocarbon ether dispersant are used in conjunction with glucose monohydrate for carbon coating. This can inhibit glucose crystallization, ensure uniform distribution of the carbon source, avoid the formation of individual carbon powder particles, and form a uniform thin-layer coated carbon layer. Finally, the compaction density and specific capacity of the carbon-coated lithium iron phosphate are improved.

[0014] In step S1 By using an appropriate amount of reducing short-chain low-molecular-weight carbon source, the carbon residue can be controlled to achieve reduction to divalent iron while keeping the carbon residue extremely low. This trace amount of amorphous carbon exists in the form of carbon dots or ultrathin layers at the grain boundaries or surface of lithium iron phosphate particles, providing a good substrate for subsequent carbon coating.

[0015] If the carbon residue is too high, it can easily fill the interparticle spaces as isolated islands or coat the particle surface in a thick layer, affecting lithium-ion diffusion. Therefore, the amount of reducing short-chain low-molecular-weight carbon source needs to be limited and controlled so that the molar ratio of lithium to carbon is 1:(0.30 to 0.35), for example, 1:0.30, 1:0.31, 1:0.32, 1:0.33, 1:0.34, or 1:0.35.

[0016] The molecular weight of the reducing short-chain low-molecular-weight carbon source also needs to be limited. An appropriate molecular weight is beneficial for controlling the carbon residue rate. If the molecular weight is too high, there will be too much carbon residue, and if the molecular weight is too low, it may completely volatilize before it can exert its reducing effect. The molecular weight of the reducing short-chain low-molecular-weight carbon source is preferably 150-200 g / mol. For example, the reducing short-chain low-molecular-weight carbon source can be glucose, fructose, etc.

[0017] The reducing power of the reducing short-chain low-molecular-weight carbon source is preferably limited. If the reducing power is too high, it may continue to reduce ferrous iron to elemental iron; if the reducing power is too low, it obviously cannot reduce ferric iron to ferrous iron completely. Therefore, as a preferred embodiment of the present invention, the reducing short-chain low-molecular-weight carbon source is selected from anhydrous glucose.

[0018] Furthermore, the amounts of iron phosphate, lithium source, and doped metal source are not limited. As a preferred embodiment of the present invention, the molar ratio of iron, phosphorus, lithium, carbon, and doped metal is (0.96~0.99):1:1:(0.30~0.35):(0.002~0.004).

[0019] The lithium source is not limited, but as a preferred embodiment of the present invention, the lithium source is selected from inorganic acid salts of lithium.

[0020] The choice of doping metal is generally unrestricted, but a suitable doping metal is beneficial for further improving the material's properties. Preferably, the doping metal is selected from Ti. Titanium preferentially occupies Li sites, which helps suppress FeLi defects, increases specific capacity, and simultaneously enhances the particle skeleton strength, further improving compaction density. Preferably, the doping metal source can be selected from at least one of the following: alkoxides, inorganic acid salts, hydroxides, oxides, and organic complexes of the doping metal.

[0021] The specific preparation process of lithium iron phosphate is not limited in principle, but a suitable treatment method is beneficial to reducing the particle size and further improving the compaction density. As a preferred embodiment of the present invention, iron phosphate, a lithium source, a reducing short-chain low-molecular-weight carbon source, and a doped metal source are mixed and then sequentially ball-milled, spray-dried, sintered under an inert atmosphere, and then air-jet pulverized to obtain the lithium iron phosphate. Ball milling initially reduces the particle size, spray drying obtains uniformly sized spherical particles, and air-jet pulverization reduces hard agglomeration.

[0022] Preferably, iron phosphate, lithium source, reducing short-chain low molecular weight carbon source, and doped metal source are added to water with a solid content of 30% to 50%, and then ball milled; for example, the solid content can be 30%, 35%, 40%, 45%, or 50%.

[0023] Preferably, the slurry is ball-milled to a particle size of 200–400 nm; for example, the particle size of the slurry can be 200 nm, 250 nm, 300 nm, 350 nm, or 400 nm.

[0024] Preferably, the inert gas used in sintering is selected from argon.

[0025] Preferably, during sintering, the heating rate is 2–5 °C / min, the holding temperature is 550–650 °C, and the holding time is 2–4 h. For example, the heating rate can be 2 °C / min, 3 °C / min, 4 °C / min, or 5 °C / min; the holding temperature can be 550 °C, 560 °C, 570 °C, 580 °C, 590 °C, 600 °C, 610 °C, 620 °C, 630 °C, 640 °C, or 650 °C; and the holding time can be 2 h, 2.5 h, 3 h, 3.5 h, or 4 h.

[0026] In step S2 A dense and thin carbon-coated shell was constructed by uniformly and continuously growing glucose monohydrate, a cross-linking reactive carbon source, and a modified dispersant whose main chain includes aromatic hydrocarbons and whose side chains include ether groups on a trace amount of amorphous carbon on the surface of lithium iron phosphate. Glucose monohydrate provides the sp... 3 The carbon skeleton, with its water of crystallization escaping first to form venting pores for pyrolysis gases and prevent the carbon shell from cracking, is used. The cross-linked reactive carbon source first cross-links to form a three-dimensional network during heat treatment, and then this three-dimensional network carbonizes, transforming into a carbon network on the carbon skeleton and inhibiting glucose crystallization. The modified dispersant is adsorbed onto the surface of lithium iron phosphate modified with trace amounts of carbon via the aromatic hydrocarbon backbone. The ether side chains separate the particles through steric hindrance, achieving uniform dispersion and ensuring that the lithium iron phosphate particles are fully and uniformly surrounded by carbon source molecules, avoiding self-aggregation or local enrichment of carbon. Ultimately, a uniform, complete, and high-hardness carbon shell is formed, which is not easily damaged during electrode compaction, effectively protecting the lithium iron phosphate particles and maintaining the integrity of the conductive network, thus improving compaction density and electrical performance.

[0027] As a preferred embodiment of the present invention, the reactive crosslinking carbon source is selected from at least one of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and polyacrylic acid (PAA).

[0028] PVP, PVA, and PAA can be used individually, in pairs, or all three simultaneously. Different combinations can further improve the material's microstructure, compaction density, and electrical properties.

[0029] Preferably, the reactive crosslinking carbon source comprises at least polyvinyl alcohol (PVA). For example, in some embodiments, the reactive crosslinking carbon source is composed of polyacrylic acid (PAA) and polyvinyl alcohol (PVA). PVA and PAA form a uniform and stable complex adsorbed onto the particle surface via hydrogen bonding. In other embodiments, the reactive crosslinking carbon source is composed of polyvinyl alcohol (PVA) and polyvinylpyrrolidone (PVP). Still in some embodiments, the reactive crosslinking carbon source is composed of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and polyacrylic acid (PAA).

[0030] Preferably, in this invention, the main chain of the modified dispersant molecule is selected from a styrene-maleic anhydride copolymer, and the side chains are selected from at least one of polyetheramine and polyether alcohol. Specifically, the maleic anhydride ring-opening in the styrene-maleic anhydride copolymer is linked to the terminal primary amine-modified polyethylene glycol chain via an amide bond, or the maleic anhydride ring-opening in the styrene-maleic anhydride copolymer is linked to the terminal hydroxyl-containing polyethylene glycol chain via an ester bond. For example, it could be SMA-g-MPEG.

[0031] The amounts of glucose monohydrate, cross-linking reactive carbon source, and modified dispersant are preferably limited. For example, excessive amounts of glucose monohydrate may lead to an excessively thick carbon layer, affecting ion diffusion; excessive amounts of cross-linking reactive carbon source may lead to high cross-linking, resulting in a brittle carbon shell; and excessive amounts of modified dispersant may hinder the contact between the carbon source and lithium iron phosphate. Preferably, the glucose monohydrate accounts for 50wt% to 70wt% of the mass of the mixed carbon source, for example, 50wt%, 55wt%, 60wt%, 65wt%, or 70wt%; the cross-linking reactive carbon source accounts for 20wt% to 30wt% of the mass of the mixed carbon source, for example, 20wt%, 22wt%, 25wt%, 27wt%, or 30wt%; and the modified dispersant accounts for 10wt% to 20wt% of the mass of the mixed carbon source, for example, 10wt%, 12wt%, 15wt%, 17wt%, or 20wt%.

[0032] The optimal ratio of lithium iron phosphate to the mixed carbon source is limited. Too much mixed carbon source reduces the proportion of active material, while too little mixed carbon source results in an incomplete carbon shell and affects compaction density. Preferably, the mass ratio of the mixed carbon source to the lithium iron phosphate is 1:(12-20), for example, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, or 1:20.

[0033] The specific preparation process of carbon-coated lithium iron phosphate is not limited in principle, but a suitable treatment method is beneficial to reducing the particle size and further improving the compaction density. As a preferred embodiment of the present invention, lithium iron phosphate is mixed with a mixed carbon source and then sequentially ball-milled, spray-dried, sintered under an inert atmosphere, and then air-jet pulverized to obtain the carbon-coated lithium iron phosphate. Ball milling initially reduces the particle size, spray drying obtains uniformly sized spherical particles, and air-jet pulverization reduces hard agglomeration.

[0034] Preferably, lithium iron phosphate and a mixed carbon source are added to water with a solid content of 50% to 70%, and then ball milled; for example, the solid content can be 50%, 55%, 60%, 65%, or 70%.

[0035] Preferably, the slurry is ball-milled to a particle size of 200–400 nm; for example, the particle size of the slurry can be 200 nm, 250 nm, 300 nm, 350 nm, or 400 nm.

[0036] Preferably, the inert gas used in sintering is selected from argon.

[0037] Gradient sintering is preferred to precisely control the chemical reaction sequence of the mixed carbon source at different temperature stages. Preferably, during sintering, the initial holding temperature is 250–350°C for 0.5–1.5 h; the subsequent holding temperature is 550–650°C for 0.5–1.5 h; and the final holding temperature is 780–790°C for 5–7 h; the heating rate is 2–5°C / min. The first holding establishes a stable framework, the second holding involves the interweaving of glucose and cross-linking reactive carbon sources to form a carbon layer, and the third holding enhances the graphitization degree of the carbon layer. For example, the initial insulation temperature can be 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, or 350℃, and the initial insulation time can be 0.5h, 1h, or 1.5h; the secondary insulation temperature can be 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, or 650℃. At 0℃, the secondary heat preservation time can be 0.5h, 1h, or 1.5h; the tertiary heat preservation temperature can be 780℃, 781℃, 782℃, 783℃, 784℃, 785℃, 786℃, 787℃, 788℃, 789℃, or 790℃, and the secondary heat preservation time can be 5h, 5.5h, 6h, 6.5h, or 7h; the heating rate can be 2℃ / min, 3℃ / min, 4℃ / min, or 5℃ / min.

[0038] The beneficial effects of this invention are:

[0039] In this invention, firstly, an appropriate amount of reducing short-chain low-molecular-weight carbon source is added during the preparation of lithium iron phosphate to reduce all the trivalent iron in the iron phosphate to divalent iron, achieving a carbon residue of no more than 0.1%. Secondly, based on this, a self-assembling cross-linking reactive carbon source and an aromatic hydrocarbon ether dispersant are used in conjunction with glucose monohydrate for carbon coating. This can inhibit glucose crystallization, ensure uniform distribution of the carbon source, avoid the formation of individual carbon powder particles, and form a uniform thin-layer coated carbon layer. Finally, the compaction density and specific capacity of the carbon-coated lithium iron phosphate are improved. Attached Figure Description

[0040] Figure 1 This is a TEM image of carbon-coated lithium iron phosphate particles obtained in Example 1;

[0041] Figure 2 This is a TEM image of carbon-coated lithium iron phosphate particles obtained in Comparative Example 1. Detailed Implementation

[0042] The following are specific embodiments of the present invention, and the technical solution of the present invention will be further described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0043] Example 1

[0044] A method for synthesizing coated and modified lithium iron phosphate includes the following steps:

[0045] S1. Mix 1000g (6.6mol) of iron phosphate (iron-to-phosphorus ratio 0.990), 242g (6.6mol) of battery-grade lithium carbonate, 65g (2.2mol) of anhydrous glucose, 6.8g (0.02mol) of tetrabutyl titanate and water until the solid content is 40% to obtain a mixture.

[0046] The mixture was ball-milled to obtain a nano-slurry with a particle size of 300 nm. The nano-slurry was spray-dried to obtain dry powder particles with a particle size of 25 μm. The dry powder particles were then subjected to gradient temperature sintering under an inert atmosphere, with a heating rate controlled at 3 °C / min, a holding temperature of 600 °C, a holding time of 3 h, and natural cooling to room temperature. After air jet milling, lithium iron phosphate with a particle size of 0.5–1 μm was obtained.

[0047] S2. Mix 750g of prepared lithium iron phosphate, 45g of mixed carbon source (in which the mass ratio of glucose monohydrate, PVP, and SMA-g-MPEG is 7:2:1) and water until the solid content is 60% to obtain a mixture.

[0048] The mixture was ball-milled to obtain a nano-slurry with a particle size of 350 nm. The nano-slurry was then spray-dried to obtain dry powder particles with a particle size of 25 μm. The dry powder particles were then subjected to gradient temperature sintering under an inert atmosphere: a heating rate of 3 °C / min, a holding temperature of 300 °C for 1 h; a heating rate of 3 °C / min, a holding temperature of 600 °C for 1 h; and a heating rate of 3 °C / min, a holding temperature of 785 °C for 6 h. Finally, the mixture was naturally cooled to room temperature and then air-jet pulverized to obtain carbon-coated lithium iron phosphate with a particle size of 0.7–1 μm.

[0049] TEM images of the obtained carbon-coated lithium iron phosphate particles are shown below. Figure 1 As shown.

[0050] Example 2

[0051] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PAA, and SMA-g-MPEG is 7:2:1.

[0052] Example 3

[0053] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PVA, and SMA-g-MPEG is 7:2:1.

[0054] Example 4

[0055] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PAA, PVP, and SMA-g-MPEG is 7:1.3:0.7:1.

[0056] Example 5

[0057] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PVP, PAA, and SMA-g-MPEG is 7:1.3:0.7:1.

[0058] Example 6

[0059] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PAA, PVA, and SMA-g-MPEG is 7:1.3:0.7:1.

[0060] Example 7

[0061] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PVP, PVA, and SMA-g-MPEG is 7:1:1:1.

[0062] Example 8

[0063] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PAA, PVP, PVA, and SMA-g-MPEG is 7:1:0.5:0.5:1.

[0064] Example 9

[0065] This embodiment is basically the same as Embodiment 1, except that the mixed carbon source is different. Specifically, in the mixed carbon source, the mass ratio of glucose monohydrate, PVP, and SMA-g-MPEG is 5:3:2.

[0066] Example 10

[0067] This embodiment is basically the same as Example 1, except that the reducing short-chain low molecular weight carbon source is different. Specifically, 65g of anhydrous glucose (2.2mol carbon) is replaced with 65g of fructose (2.2mol carbon).

[0068] Example 11

[0069] This embodiment is basically the same as Example 1, except that the doped metal is different. Specifically, 6.8g of tetrabutyl titanate (0.02mol titanium) is replaced with 0.8g of magnesium oxide (0.02mol magnesium).

[0070] Example 12

[0071] This embodiment is basically the same as embodiment 1, except that the sintering step in step S2 is different.

[0072] Specifically: S2. 750g of prepared lithium iron phosphate, 45g of mixed carbon source (wherein the mass ratio of glucose monohydrate, PVP, and SMA-g-MPEG is 7:2:1), and water are mixed until the solid content is 60%, resulting in a mixture. The mixture is ball-milled to obtain a nano-slurry with a particle size of 350nm; the nano-slurry is spray-dried to obtain dry powder particles with a particle size of 25μm; the dry powder particles are sintered under an inert atmosphere: the heating rate is controlled at 3℃ / min, the holding temperature is 785℃, and the holding time is 8h. Finally, it is naturally cooled to room temperature, and carbon-coated lithium iron phosphate is obtained after air jet milling.

[0073] Comparative Example 1

[0074] This comparative example is basically the same as Example 1, except that the mixed carbon source is different. Specifically, the mass ratio of glucose monohydrate to PEG in the mixed carbon source is 8:2.

[0075] TEM images of the particles were obtained as follows Figure 2 As shown.

[0076] Comparative Example 2

[0077] This comparative example is basically the same as Example 1, except that the mixed carbon source is different. Specifically, the mass ratio of glucose monohydrate, PEG, and SMA-g-MPEG in the mixed carbon source is 7:2:1.

[0078] Comparative Example 3

[0079] This comparative example is basically the same as Example 1, except that the mixed carbon source is different. Specifically, the mass ratio of anhydrous glucose, PVP, and SMA-g-MPEG in the mixed carbon source is 7:2:1.

[0080] Comparative Example 4

[0081] This comparative example is basically the same as Example 1, except that the mixed carbon source is different. Specifically, the mass ratio of glucose monohydrate, PVP, and sodium polystyrene sulfonate in the mixed carbon source is 7:2:1.

[0082] Comparative Example 5

[0083] This comparative example is basically the same as Example 1, except that the carbon source used in step S1 is different. Specifically, 65g of anhydrous glucose (2.2mol carbon) was replaced with 51g of polyethylene glycol 400 (2.2mol carbon).

[0084] Comparative Example 6

[0085] This comparative example is basically the same as Example 1, except that the carbon source used in step S1 is different. Specifically, 65g of anhydrous glucose (2.2mol carbon) is replaced with 99g of oxalic acid (2.2mol carbon).

[0086] Comparative Example 7

[0087] This comparative example is basically the same as Example 1, except that the amount of reducing short-chain low molecular weight carbon source used in step S1 is larger. Specifically, 65g of anhydrous glucose (2.2mol carbon) is replaced with 130g of anhydrous glucose (4.4mol carbon).

[0088] Performance testing

[0089] The carbon-coated lithium iron phosphate obtained in the examples and comparative examples had their compaction density measured according to GB / T 44330-2024 "Determination of compaction density of lithium-ion battery cathode material powder". The results are shown in Table 1 below.

[0090] The carbon-coated lithium iron phosphate obtained in the examples and comparative examples were mixed with SP and PVDF in a mass ratio of 90:5:5 to form an electrode. Then, coin cells were made and activated. The cells were charged and discharged at a rate of 0.1C in a constant current and constant voltage mode. The first discharge capacity was recorded and the specific capacity was calculated. The cells were then charged and discharged at a rate of 1C. The 1C discharge capacity of the fifth cycle was recorded and the specific capacity was calculated. The results are shown in Table 1 below.

[0091] Table 1. experimental group <![CDATA[Compaction density (g / cm 3 )]]> 0.1C gram capacity (mAh / g) 1C gram capacity (mAh / g) Example 1 2.64 160.3 147.5 Example 2 2.63 158.9 146.9 Example 3 2.64 159.7 147.1 Example 4 2.65 161.2 147.5 Example 5 2.64 161.3 148.3 Example 6 2.66 162.0 148.6 Example 7 2.66 161.5 148.7 Example 8 2.65 162.1 148.6 Example 9 2.61 157.5 143.2 Example 10 2.62 158.2 142.9 Example 11 2.63 158.4 143.8 Example 12 2.63 159.2 142.5 Comparative Example 1 2.62 157.3 144.6 Comparative Example 2 2.60 156.0 140.1 Comparative Example 3 2.59 156.9 141.2 Comparative Example 4 2.58 158.4 142.3 Comparative Example 5 2.60 157.8 141.5 Comparative Example 6 2.60 158.0 141.3 Comparative Example 7 2.60 158.4 143.0 .

[0092] As shown in Table 1, the carbon-coated lithium iron phosphate prepared by the synthesis method of the present invention has high compaction density and excellent electrochemical performance, and the overall volumetric capacity density is improved. Specifically: Comparing Example 1 and Comparative Examples 1-7, in Comparative Examples 1 and 2, polyethylene glycol was used as an auxiliary carbon source. It is possible that because polyethylene glycol melts and depolymerizes rather than crosslinks during sintering, a suitable carbon network cannot be formed regardless of whether a dispersant is used. In Comparative Example 3, anhydrous glucose was used. It is possible that due to the relatively rapid and violent reaction, the concentrated release of gas during the reaction caused cracks in the carbon layer. In Comparative Example 4, there was a lack of suitable polyether chains to provide steric hindrance to ensure the uniform dispersion of lithium iron phosphate particles and carbon source components. All of these directly affected the compaction density and electrochemical performance of the final material. The carbon source used in Comparative Example 5 to prepare lithium iron phosphate has a relatively high molecular weight, the carbon source used in Comparative Example 6 to prepare lithium iron phosphate has a relatively low molecular weight, and the amount of carbon source used in Comparative Example 7 to prepare lithium iron phosphate is relatively high. This affects the carbon residue rate on the surface of lithium iron phosphate, which in turn affects the uniformity of subsequent carbon coating, and also affects the compaction density and electrochemical performance of the final material.

[0093] Furthermore, comparisons within the examples show that the selection and amount of cross-linking reactive carbon sources, the selection of reducing short-chain low-molecular-weight carbon sources, the selection of doped metals, and adjustments to the sintering steps can further improve the compaction density and electrochemical performance of the materials. Comparisons of Examples 1-8 show that a mixture of PVA and PAA or PVP as a cross-linking reactive carbon source can achieve functional complementarity, resulting in a uniform, dense composite carbon shell with ideal ion channels. Comparisons of Examples 1 and 10 show that glucose has a more suitable reducing power than fructose, which is beneficial for the formation of ferrous iron. Comparisons of Examples 1 and 11 show that titanium doping, compared to magnesium doping, can further suppress FeLi defects and enhance the strength of the particle skeleton. Comparisons of Examples 1 and 12 show that gradient sintering, compared to one-step calcination, is more conducive to the orderly reaction of the carbon source to form a high-quality carbon layer.

[0094] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for synthesizing coated and modified lithium iron phosphate, characterized in that: Includes the following steps: S1. Mix iron phosphate, lithium source, reducing short-chain low molecular weight carbon source and doped metal source, and sinter under an inert atmosphere to obtain lithium iron phosphate; The molar ratio of lithium to carbon is 1:(0.30~0.35); The molecular weight of the reducing short-chain low molecular weight carbon source is 150-200 g / mol; S2. The lithium iron phosphate is mixed with a mixed carbon source and sintered under an inert atmosphere to obtain carbon-coated lithium iron phosphate; The mixed carbon source includes glucose monohydrate, a cross-linking reactive carbon source, and a modified dispersant; The main chain of the modified dispersant molecule includes aromatic hydrocarbons, and the side chains include ether groups.

2. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S1, the reducing short-chain low molecular weight carbon source is selected from anhydrous glucose.

3. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S1, the doped metal is selected from Ti.

4. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S1, during sintering, the heating rate is 2-5℃ / min, the holding temperature is 550-650℃, and the holding time is 2-4h.

5. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S2, the reaction crosslinking carbon source is selected from at least one of polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylic acid.

6. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S2, the main chain of the modified dispersant molecule is selected from styrene-maleic anhydride copolymer, and the side chain is selected from at least one of polyetheramine and polyether alcohol.

7. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S2, the mass ratio of the mixed carbon source to the lithium iron phosphate is 1:(12-20).

8. A method for synthesizing coated and modified lithium iron phosphate according to claim 1 or 7, characterized in that: In step S2, the glucose monohydrate accounts for 50wt% to 70wt% of the mass of the mixed carbon source, the cross-linking reactive carbon source accounts for 20wt% to 30wt% of the mass of the mixed carbon source, and the modified dispersant accounts for 10wt% to 20wt% of the mass of the mixed carbon source.

9. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S2, during sintering, the initial holding temperature is 250–350℃ for 0.5–1.5 h; then the holding temperature is 550–650℃ for 0.5–1.5 h; and finally the holding temperature is 780–790℃ for 5–7 h; the heating rate is 2–5℃ / min.

10. The method for synthesizing coated and modified lithium iron phosphate according to claim 1, characterized in that: In step S1 or / and step S2, after mixing, the mixture is sequentially ball-milled, spray-dried, sintered under an inert atmosphere, and then air-jet pulverized to obtain the lithium iron phosphate or carbon-coated lithium iron phosphate.

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