A lithium iron phosphate material and preparation method thereof, and a positive electrode material for lithium-ion batteries

Abstract

The present invention relates to a lithium iron phosphate material and a preparation method thereof, and a positive electrode material for lithium-ion batteries, and relates to the technical field of battery materials. The preparation method of the lithium iron phosphate material comprises the following steps: 1) a first solution containing a phosphorus source and polyethylene glycol is subjected to a co-precipitation reaction with a second solution containing an iron source to obtain a polyethylene glycol-coated ferrous phosphate precursor; 2) a third solution containing a lithium source and a polyethylene glycol-coated ferrous phosphate precursor are subjected to a hydrothermal crystallization reaction to obtain a reaction product, lithium iron phosphate; 3) the reaction product, lithium iron phosphate, is calcined under a reducing atmosphere to obtain a lithium iron phosphate material. The present invention first in-situ coats ferrous phosphate with polyethylene glycol, and uses a hydrothermal crystallization method to synthesize the reaction product, lithium iron phosphate crystals, with low antisite defects, and then calcines under a reducing atmosphere to reduce the Li + Diffusion impedance, improved electronic conductivity, and demonstrated excellent rate and capacity performance.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a lithium iron phosphate material and a preparation method thereof, and a lithium ion battery positive electrode material. Background Art

[0002] Environmental degradation and the depletion of non-renewable fossil fuels have driven the development of sustainable / renewable resources and energy storage devices. Iron-based electrode materials, known for their safety, low cost, abundant resources, and sustainable development, have attracted widespread attention. Among iron-containing cathodes, olivine-type lithium iron phosphate (LiFePO4) has emerged as a key player in lithium-ion batteries due to its high theoretical specific capacity and long cycle life.

[0003] The traditional carbon thermal reduction method for preparing lithium iron phosphate has achieved certain success in the market, but its disadvantage is that the particle size of the prepared product is large, which causes the electronic conductivity and lithium ion diffusion rate to decrease, resulting in poor electrochemical performance and limiting its further commercial application.

[0004] Therefore, in order to obtain high-performance (high energy density and high power density) lithium-ion cathode materials, researchers have adopted methods such as optimizing particle size, surface modification with highly conductive materials, and ion doping. The main related technologies are as follows:

[0005] The first related technology discloses a carbon-coated lithium iron phosphate composite material, its preparation method and use, which uses a hydrothermal method (240-280°C) to synthesize nano-lithium iron phosphate material, and then uses an organic carbon source and an inorganic carbon source to jointly coat the nano-lithium iron phosphate material to obtain a carbon-coated lithium iron phosphate composite material. Here, the temperature of the hydrothermal process is high and the energy consumption is large, which may cause certain safety hazards in the production process (the high hydrothermal temperature will generate a large pressure in the closed container, so there is a risk of explosion); secondly, the volume energy density of the lithium iron phosphate material is reduced by coating with organic and inorganic carbon, which is not conducive to the use of lithium-ion batteries (there is an indicator in the commercial evaluation of positive electrode materials that measures the capacity discharged per unit weight / volume (compacted density), and the carbon material has a low density and occupies a large volume. If more carbon material is added, this will reduce the compacted density of the lithium iron phosphate, so lithium iron phosphate with a higher compacted density is generally selected).

[0006] The second related art discloses a hydrothermal synthesis method for preparing lithium iron phosphate, a positive electrode material for lithium-ion batteries. The method involves reacting a lithium source, a phosphorus source, and a refined divalent iron salt in a sealed autoclave, wherein the molar ratio of each substance is: Li:Fe:P=(3.0-3.15):1:(1.0-1.15). However, lithium sources are currently expensive, and their effective utilization rate is only 1 / 3. Furthermore, the remaining lithium source needs to be recycled, increasing production costs. Therefore, reducing the cost of the hydrothermal method is particularly important.

[0007] In summary, the lithium iron phosphate preparation methods provided by the prior art have disadvantages such as low volumetric energy density and high production costs. However, the hydrothermal process reaction is very complex. The inventors of the present invention have discovered that by controlling the parameters of the hydrothermal process and the crystallization trend of the lithium iron phosphate, it can be induced to exhibit desired properties (such as high-rate charge and discharge, low-rate high capacity, low-temperature resistance, etc.). Therefore, the development of such high-performance lithium iron phosphate cathode materials has important practical significance and application value. Summary of the Invention

[0008] In view of this, the present invention provides a lithium iron phosphate material and a preparation method thereof, and a lithium ion battery positive electrode material, the main purpose of which is to improve the electrochemical properties of the lithium iron phosphate material.

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

[0010] In one aspect, an embodiment of the present invention provides a method for preparing a lithium iron phosphate material, wherein the method for preparing the lithium iron phosphate material comprises the following steps:

[0011] 1) co-precipitating a first solution containing a phosphorus source and polyethylene glycol with a second solution containing an iron source to obtain a polyethylene glycol-coated ferrous phosphate precursor;

[0012] 2) subjecting the third solution containing the lithium source to a hydrothermal crystallization reaction with the polyethylene glycol-coated ferrous phosphate precursor to obtain a reaction product, lithium iron phosphate;

[0013] 3) calcining the reaction product, lithium iron phosphate, under a reducing atmosphere to obtain a lithium iron phosphate material.

[0014] Preferably, the phosphorus source includes one or more of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and phytic acid.

[0015] Preferably, the iron source includes one or both of ferrous sulfate and ferrous chloride.

[0016] Preferably, the lithium source includes one or more of lithium hydroxide, lithium nitrate, and lithium sulfate.

[0017] Preferably, the molecular weight of the polyethylene glycol is 2-20K.

[0018] Preferably, the amounts and concentrations of the first solution containing the phosphorus source and polyethylene glycol, the second solution containing the iron source, and the third solution containing the lithium source must ensure that the molar ratio of divalent ferrous ions, phosphate ions, and lithium ions is (0.95-1):1:(1-2).

[0019] Preferably, in the first solution containing the phosphorus source and polyethylene glycol, the concentration of the phosphorus source is 0.3-0.8 mol / L, and the mass concentration of the polyethylene glycol aqueous solution is 2-10 wt%.

[0020] Preferably, in the second solution containing the iron source, the concentration of the iron source is 0.3-0.8 mol / L.

[0021] Preferably, in the third solution containing the lithium source, the concentration of the lithium source is 0.3-0.8 mol / L.

[0022] Preferably, the second solution containing the iron source is prepared by the following steps: dissolving the iron source in oxygen-free water and continuously introducing a protective gas to prevent oxidation, thereby obtaining the second solution containing the iron source.

[0023] Preferably, the preparation steps of the first solution containing a phosphorus source and polyethylene glycol are as follows: dissolving the phosphorus source in an aqueous solution of polyethylene glycol to obtain a first solution containing a phosphorus source and polyethylene glycol; preferably, the mass concentration of the aqueous solution of polyethylene glycol is 2-10 wt%.

[0024] Preferably, the preparation steps of the third solution containing the lithium source are as follows: dissolving the lithium source in a mixture of solvent and water to obtain the third solution containing the lithium source; preferably, the solvent includes any one or both of ethylene glycol and diethylene glycol.

[0025] Preferably, the step 1) is specifically as follows: gradually adding a first solution containing a phosphorus source and polyethylene glycol to a second solution containing an iron source, stirring until uniformly dispersed, adding an auxiliary agent thereto, adjusting the pH value, heating under inert gas protection for a coprecipitation reaction, and reacting for a set time to obtain a polyethylene glycol-coated ferrous phosphate precursor; preferably, the auxiliary agent is triethylamine (here, the selection of triethylamine is, on the one hand, to adjust the pH and control the morphology, and on the other hand, to achieve doping nitrogen in the carbon layer to improve conductivity); preferably, the pH value is adjusted to 3.5-10.0; preferably, the stirring rate is 100-1500 rpm.

[0026] Preferably, in step 1), the temperature of the coprecipitation reaction is 55-120° C., and the reaction time is 0.5-2 h.

[0027] Preferably, the polyethylene glycol-coated ferrous phosphate precursor has a core-shell structure with ferrous phosphate as the core and polyethylene glycol as the shell; wherein the particle size of the polyethylene glycol-coated ferrous phosphate precursor is 0.1-1 μm.

[0028] Preferably, in the step 2), under a protective atmosphere (nitrogen as a protective atmosphere), the third solution containing a lithium source is gradually added to the polyethylene glycol-coated ferrous phosphate precursor prepared in the step 1), and the stirring is continued, and the stirring rate is gradually increased; then, an auxiliary agent is added thereto, and the pH value is adjusted. After stirring, the mixture is transferred to a hydrothermal kettle for a hydrothermal crystallization reaction to obtain a reaction product, lithium iron phosphate; preferably, the auxiliary agent is triethylamine; preferably, the pH value is adjusted to 3.5-10.0; preferably, the stirring rate is 100-1500 rpm.

[0029] Preferably, the temperature of the hydrothermal crystallization reaction is 160-200° C., and the time of the hydrothermal crystallization reaction is 1.5-12 hours.

[0030] Preferably, the particle size of the reaction product, lithium iron phosphate, is 0.1-1 μm.

[0031] Preferably, in step 3), the calcination temperature is 700-800°C and the time is 4-10 hours; and / or the reducing atmosphere comprises argon and hydrogen, wherein hydrogen accounts for 3-7% of the volume of argon; and / or the gas flow rate of the reducing atmosphere is 100-500 mL / min.

[0032] On the other hand, an embodiment of the present invention provides a lithium iron phosphate material, wherein the lithium iron phosphate material is prepared by the preparation method of the lithium iron phosphate material described in any one of the above items; preferably, the lithium iron phosphate material is used in the preparation of lithium ion battery positive electrode materials. Preferably, the particle size of the lithium iron phosphate material is 0.1-1 μm; the carbon content in the lithium iron phosphate material is 1.5-3%; preferably, the morphology of the lithium iron phosphate is rugby-shaped, spherical, or a shape varying between rugby and spherical; further preferably, the shape varying between rugby and spherical includes a rice grain shape. Preferably, as the hydrothermal crystallization reaction parameters are adjusted, the unit cell parameters of the lithium iron phosphate material are changed to regulate the lithium ion diffusion rate, thereby exhibiting excellent rate (low temperature) and capacity performance.

[0033] On the other hand, an embodiment of the present invention provides a positive electrode material for a lithium-ion battery, wherein the positive electrode material for a lithium-ion battery comprises the lithium iron phosphate material described in any one of the above items;

[0034] Preferably, when the preparation parameters of the lithium iron phosphate material are controlled as follows: the iron source concentration is less than or equal to 0.5 mol / L [the iron source concentration here refers to: (the amount of the iron source substance, mol) ÷ (the total volume of water and solvent used in the preparation of the lithium iron phosphate material, L], the hydrothermal crystallization reaction time is greater than or equal to 5 h, and the mass concentration of the polyethylene glycol aqueous solution is 2-5%; the lithium ion battery positive electrode material made of the lithium iron phosphate material is used in the lithium ion battery, so that the performance is as follows: at room temperature and 0.2C charge and discharge conditions, the discharge specific capacity reaches 150-164 mAh / g, which can reach 96.47% of the theoretical capacity (low rate high capacity performance).

[0035] Preferably, when the preparation parameters of the lithium iron phosphate material are controlled as follows: the iron source concentration is greater than or equal to 0.55 mol / L [the iron source concentration here refers to: (the amount of substance of the iron source, mol) ÷ (the total volume of water and solvent used in the preparation of the lithium iron phosphate material, L], the hydrothermal crystallization reaction time is less than 3 hours, the pH value during the hydrothermal crystallization reaction is adjusted to 5-10, the mass concentration of the aqueous solution of polyethylene glycol is 4-10%, and the mass ratio of the solvent in the lithium source to the water used in the preparation of the lithium iron phosphate material is greater than 1 / 3; the lithium ion battery positive electrode material made of the lithium iron phosphate material is used in the lithium ion battery, so that the performance is as follows: at room temperature and 10C charge and discharge, the discharge specific capacity reaches 120-140 mAh / g (high rate charge and discharge performance); in a low temperature environment of -20°C and 0.1C charge and discharge conditions, the discharge specific capacity reaches 120-131 mAh / g (low temperature performance).

[0036] In yet another aspect, an embodiment of the present invention provides a lithium-ion battery, wherein the lithium-ion battery includes the above-mentioned lithium-ion battery positive electrode material.

[0037] Compared with the prior art, the lithium iron phosphate material and preparation method thereof, and the lithium ion battery positive electrode material of the present invention have at least the following beneficial effects:

[0038] The present invention first prepares polyethylene glycol to in-situ coat ferrous phosphate. On the one hand, the grain size can be regulated (the higher the concentration of polyethylene glycol, the smaller the grain size). On the other hand, in the subsequent (reducing atmosphere) calcination process, polyethylene glycol is cracked and carbonized to form a conductive carbon chain skeleton, thereby improving the conductivity of lithium iron phosphate. Secondly, the ratio of Fe and Li is strictly controlled during the hydrothermal reaction, and the reaction kinetics (nucleation rate) is controlled by regulating the concentration of the reactants, thereby preparing lithium iron phosphate crystals with low (Fe / Li) antisite defects. Finally, the crystallinity of the lithium iron phosphate material is further improved by calcination treatment under a reducing atmosphere. In addition, triethylamine and polyethylene glycol form a conductive network of nitrogen-doped carbon after carbonization, which plays a positive role in both electronic conductivity and ion transfer rate. The lithium iron phosphate material obtained in the present application has a smaller grain size (shortened Li + transmission channel) and carbon-nitrogen conductive network structure (to improve electronic conductivity), which reduces the Li + Diffusion impedance, improve the electron transfer rate, and significantly improve the capacity and rate discharge performance of lithium iron phosphate batteries. In addition, the present invention uses a two-step liquid phase method to synthesize lithium iron phosphate particles (reaction product lithium iron phosphate), which has the advantages of adjustable grain crystallization trend, controllable performance and low raw material cost, and is easy to achieve large-scale production. In addition, the lithium iron phosphate material prepared by the embodiment of the present invention can effectively reduce (lithium iron) anti-site defects when used as a lithium battery positive electrode material, reducing Li + The resistance to movement greatly improves the material's electrical conductivity, which has important practical significance and application value for improving the poor endurance of electric vehicles at low temperatures and high speeds. Furthermore, the method of the present invention uses a low content of polyethylene glycol, resulting in a low carbon content during calcination, which has a minimal impact on the compaction density of lithium iron phosphate.

[0039] Furthermore, it should be noted that prior art typically uses polyethylene glycol as a carbon source, added before calcination, to coat lithium iron phosphate. However, this coating is less uniform and has little effect on the morphology of the lithium iron phosphate. The present invention, on the other hand, pre-coats ferrous phosphate with polyethylene glycol. This has the advantage of controlling the particle size and morphology of the precursor and forming a uniformly distributed conductive carbon layer during the subsequent calcination process.

[0040] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is the SEM image of the polyethylene glycol-coated ferrous phosphate precursor prepared in Example 1.

[0042] Figure 2 1 is the XRD and standard card diagram of the calcined lithium iron phosphate material prepared in Example 1.

[0043] Figure 3 This is an SEM image of the calcined lithium iron phosphate material prepared in Example 1.

[0044] Figure 4 This is an SEM image of the calcined lithium iron phosphate material prepared in Example 2.

[0045] Figure 5 This is an SEM image of the calcined lithium iron phosphate material prepared in Example 3.

[0046] Figure 6 This is an SEM image of the calcined lithium iron phosphate material prepared in Example 4. DETAILED DESCRIPTION

[0047] To further illustrate the technical means and effects employed by the present invention to achieve its intended objectives, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention is provided in conjunction with the accompanying drawings and preferred embodiments. In the following description, different references to "one embodiment" or "embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics of one or more embodiments may be combined in any suitable manner.

[0048] The purpose of the present invention is to provide a lithium iron phosphate material and a preparation method thereof; wherein, the lithium iron phosphate material is mainly used as a positive electrode material for lithium-ion batteries, which can achieve rate and capacity performance conversion. Specifically, the present invention first uses phosphate and ferrous salt to carry out a coprecipitation reaction in an aqueous solution of polyethylene glycol under an inert atmosphere, and regulates the concentration of the solution reactants, pH value, etc. to achieve the purpose of coating and crystallization of ferrous phosphate. Here, the benefits of coating are: on the one hand, it can limit the growth and agglomeration of the grain size of ferrous phosphate, and then indirectly limit the growth of lithium iron phosphate crystals during the hydrothermal process; on the other hand, in addition to regulating the pH value of the reaction solution, triethylamine also incorporates a nitrogen source into the coating layer of polyethylene glycol, laying the foundation for nitrogen-doped carbon in the subsequent calcination process. In addition, under a high temperature and high pressure environment, ferrous phosphate is recrystallized and combined with lithium ions by a hydrothermal method, and its hydrothermal parameters, especially the ratio of iron to lithium, are controlled to reduce the number of iron-lithium antisite defects, thereby greatly improving the electrochemical properties of lithium iron phosphate. Finally, the lithium iron phosphate is calcined at high temperature under a reducing atmosphere to further improve the crystallinity of the lithium iron phosphate. At the same time, the polyethylene glycol coating layer and triethylamine are cracked to form a lithium iron phosphate positive electrode material with a porous nitrogen-doped carbon conductive network structure. The benefit of this lithium iron phosphate material is that the electrolyte can easily penetrate the porous gaps, which can shorten the Li + transmission path, improving Li +The diffusion rate is significantly reduced, effectively reducing diffusion impedance, improving electrochemical kinetics, and significantly improving the electrochemical performance of lithium iron phosphate batteries. In addition, by regulating reaction parameters, especially the concentration of reactants, reaction time, and pH, the oriented crystallization of lithium iron phosphate can be controlled, specifically by changing the crystal parameters, particle size, and morphology, thereby converting between high current charge and discharge and low current high capacity performance.

[0049] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0050] An embodiment of the present invention provides a method for preparing a lithium iron phosphate material, wherein the method for preparing the lithium iron phosphate material comprises the following steps:

[0051] 1) A first solution containing a phosphorus source and polyethylene glycol is subjected to a coprecipitation reaction with a second solution containing an iron source to obtain a polyethylene glycol-coated ferrous phosphate precursor.

[0052] In this step, the second solution containing the iron source is prepared by dissolving the iron source in oxygen-free water and continuously introducing a protective gas (nitrogen) to prevent oxidation, thereby obtaining the second solution containing the iron source. The first solution containing the phosphorus source and polyethylene glycol is prepared by dissolving the phosphorus source in an aqueous solution of polyethylene glycol to obtain the first solution containing the phosphorus source and polyethylene glycol. Preferably, the mass concentration of the aqueous solution of polyethylene glycol is 2-10 wt%.

[0053] This step specifically comprises: gradually adding a first solution containing a phosphorus source and polyethylene glycol to a second solution containing an iron source, stirring until uniform dispersion is achieved, adding an auxiliary agent thereto, adjusting the pH value, heating under inert gas protection for a coprecipitation reaction, and reacting for a set time to obtain a polyethylene glycol-coated ferrous phosphate precursor; preferably, the auxiliary agent is triethylamine; preferably, the pH value is adjusted to 3.5-10.0; preferably, the stirring rate is 100-1500 rpm.

[0054] Preferably, the coprecipitation reaction temperature is 55-120° C., and the reaction time is 0.5-2 h.

[0055] The polyethylene glycol-coated ferrous phosphate precursor has a core-shell structure with ferrous phosphate as the core and polyethylene glycol as the shell; wherein the particle size of the polyethylene glycol-coated ferrous phosphate precursor is 0.1-1 μm.

[0056] 2) subjecting the third solution containing the lithium source and the polyethylene glycol-coated ferrous phosphate precursor to a hydrothermal crystallization reaction (under high temperature and high pressure conditions) to obtain lithium iron phosphate as a reaction product.

[0057] In this step: the preparation steps of the third solution containing the lithium source are as follows: the lithium source is dissolved in a mixture of solvent and water to obtain the third solution containing the lithium source; preferably, the solvent includes any one or both of ethylene glycol and diethylene glycol.

[0058] This step specifically comprises: under a protective atmosphere (nitrogen may be used), gradually adding the third solution containing a lithium source to the polyethylene glycol-coated ferrous phosphate precursor prepared in step 1), while continuously stirring at a gradually increasing stirring rate; then, adding an auxiliary agent, adjusting the pH, and transferring the mixture to a hydrothermal reactor after stirring for hydrothermal crystallization to obtain the reaction product, lithium iron phosphate. Preferably, the auxiliary agent is triethylamine; the pH is adjusted to 3.5-10.0; and the stirring rate is 100-1500 rpm.

[0059] The temperature of the hydrothermal crystallization reaction is 160-200° C., and the time of the hydrothermal crystallization reaction is 1.5-12 hours.

[0060] The particle size of the reaction product, lithium iron phosphate, is 0.1-1 μm.

[0061] 3) calcining the reaction product, lithium iron phosphate, under a reducing atmosphere to obtain a lithium iron phosphate material.

[0062] In this step, the calcination temperature is 700-800°C for 4-10 hours. The reducing atmosphere comprises argon and hydrogen, wherein the volume ratio of argon to hydrogen is 3-7%. The gas flow rate of the reducing atmosphere is 100-500 mL / min.

[0063] Preferably, in the above steps: the phosphorus source includes one or more of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and phytic acid. The iron source includes one or two of ferrous sulfate and ferrous chloride. The lithium source includes one or more of lithium hydroxide, lithium nitrate, and lithium sulfate. The molecular weight of polyethylene glycol is 8-20K. The dosage and concentration of the first solution containing the phosphorus source and polyethylene glycol, the second solution containing the iron source, and the third solution containing the lithium source must ensure that the molar ratio of iron ions, phosphorus ions, and lithium ions is (0.95-1):1:(1-2). In the first solution containing the phosphorus source and polyethylene glycol, the concentration of the phosphorus source is 0.3-0.8 mol / L. In the second solution containing the iron source, the concentration of the iron source is 0.3-0.8 mol / L. In the third solution containing the lithium source, the concentration of the lithium source is 0.3-0.8 mol / L.

[0064] Regarding the lithium iron phosphate material prepared by the above-mentioned preparation scheme of the present invention, when used as a positive electrode material for lithium-ion batteries, it has the characteristics of rate and capacity performance conversion, which specifically refers to: by adjusting the concentration of raw materials, the time of hydrothermal crystallization, the coating carbon content and other parameters during preparation. For example, to develop a low-current, high-capacity lithium iron phosphate battery, the reaction kinetics process can be reduced by reducing the concentration of raw materials, extending the time of hydrothermal crystallization, reducing the coating carbon content and other control measures, so that a lithium iron phosphate material with a high degree of crystallization and few iron-lithium anti-site defects can be obtained, so that its discharge capacity is close to the theoretical capacity. For example, if a lithium iron phosphate battery with high current and fast charge and discharge is required, the hydrothermal reaction kinetics can be accelerated by increasing the reactant concentration, shortening the reaction time, increasing the coating carbon and other technical means, thereby greatly reducing the primary grain size, and obtaining a lithium iron phosphate material with good rate and capacity performance.

[0065] In addition, the above-mentioned preparation scheme of the present invention uses a two-step synthesis process to prepare lithium iron phosphate materials. Water and organic solvents are used as carriers, and ferrous phosphate crystals are obtained by coprecipitation. The reaction product, lithium iron phosphate, is then subjected to high-temperature and high-pressure hydrothermal treatment to obtain the reaction product, which is finally calcined under a high-temperature reducing atmosphere to obtain a high-performance lithium iron phosphate material. Unlike the traditional hydrothermal method with an iron-lithium ratio of 3:1, the present invention adopts a ratio of 1:(1-2), thereby reducing the cost of raw materials (lithium source cost is the highest). In addition, a reasonable iron-lithium ratio is an effective way to reduce iron-lithium antisite defects, thereby obtaining a higher capacity, and adjusting the reaction parameters can obtain lithium iron phosphate with controllable performance.

[0066] The present invention is further described below by means of specific experimental examples:

[0067] Example 1

[0068] Example 1: Preparation of a lithium iron phosphate material, which mainly includes the following steps:

[0069] 1) 5.722 g of ferrous sulfate heptahydrate was dissolved in 20 mL of oxygen-free water and nitrogen was continuously introduced to prevent oxidation, to obtain a second solution containing an iron source.

[0070] 0.882 g of lithium hydroxide monohydrate was weighed and dissolved in a mixture of 20 mL of ethylene glycol solvent and 15 mL of water to obtain a third solution containing a lithium source.

[0071] 2.773 g of diammonium hydrogen phosphate was dissolved in 15 mL of a 3% by mass polyethylene glycol (PEG-800) aqueous solution to obtain a first solution containing a phosphorus source and polyethylene glycol.

[0072] 2) gradually adding the first solution containing the phosphorus source and polyethylene glycol to the second solution containing the iron source and stirring at 500 rpm until uniformly dispersed, then adding triethylamine and adjusting the pH to 4.5. The mixture was heated to 60° C. under nitrogen protection for coprecipitation reaction. After reacting for 30 minutes, a polyethylene glycol-coated ferrous phosphate precursor was obtained, the particle size of which was 0.1-0.3 μm.

[0073] 3) Slowly add the third solution containing a lithium source to the sample (polyethylene glycol-coated ferrous phosphate precursor) obtained in step 2) with continuous stirring, still under nitrogen. The stirring rate is gradually increased from 500 rpm to 800 rpm. Triethylamine is then added, and the pH is adjusted to 7.0. After stirring for 10 minutes, the solution is transferred to a hydrothermal reaction vessel at 180°C for 6 hours for hydrothermal crystallization. The reaction product, lithium iron phosphate, is then centrifuged and dried under vacuum to obtain the reaction product.

[0074] 4) The reaction product lithium iron phosphate obtained in step 3) is calcined at 750° C. in a reducing atmosphere of argon and hydrogen (hydrogen accounts for 5% of the volume of argon) for 6 hours to finally obtain a high-performance lithium iron phosphate material.

[0075] in, Figure 1 This is an SEM image of the polyethylene glycol-coated ferrous phosphate precursor prepared in Example 1. Figure 2 1 is the XRD and standard card diagram of the calcined lithium iron phosphate material prepared in Example 1. Figure 3 This is an SEM image of the calcined lithium iron phosphate material prepared in Example 1.

[0076] Among them, from Figure 1 It can be seen that the polyethylene glycol-coated ferrous phosphate precursor prepared in this example has a spherical morphology and a particle size of 0.1-0.3 μm.

[0077] from Figure 2 It can be seen that the diffraction peaks of the lithium iron phosphate material prepared in this embodiment are basically coincident with those of the standard card (PDF#81-1173) after X-ray diffraction, which proves that the lithium iron phosphate material prepared in this embodiment is a typical Pnma space group, and no impurity peaks appear, that is, pure phase lithium iron phosphate; after XRD refinement, it is found that the value of the unit cell parameter b is smaller than the standard value (that is, the (010) crystal plane is smaller, which is easier for Li + De-embedding).

[0078] pass Figure 3It can be seen that the particle size of the calcined lithium iron phosphate material is 0.1-0.5 μm, with a rice grain shape, good dispersion, and a carbon content of (1.8±0.5)%. Due to its good crystal structure, it exhibits performance close to the theoretical capacity under low current charge and discharge conditions. The specific electrochemical performance test results are listed in Table 1.

[0079] Example 2

[0080] Example 2: Preparation of a lithium iron phosphate material, which mainly includes the following steps:

[0081] 1) 11.444 g of ferrous sulfate heptahydrate was dissolved in 20 mL of oxygen-free water and nitrogen was continuously introduced to prevent oxidation, to obtain a second solution containing an iron source.

[0082] 1.748 g of lithium hydroxide monohydrate was weighed and dissolved in a mixture of 20 mL of ethylene glycol solvent and 15 mL of water to obtain a third solution containing a lithium source.

[0083] 4.831 g of ammonium dihydrogen phosphate was dissolved in 15 mL of a 7.5% by mass polyethylene glycol (PEG-1200) aqueous solution to obtain a first solution containing a phosphorus source and polyethylene glycol.

[0084] 2) gradually adding the first solution containing the phosphorus source and polyethylene glycol to the second solution containing the iron source at a speed of 500 rpm until uniformly dispersed, then adding triethylamine and adjusting the pH to 4.0. The mixture was heated to 80° C. under nitrogen protection for coprecipitation reaction. After reacting for 30 minutes, the polyethylene glycol-coated ferrous phosphate precursor was obtained, and the particle size was 0.1-0.2 μm.

[0085] 3) Slowly add the third solution containing a lithium source to the sample (polyethylene glycol-coated ferrous phosphate precursor) obtained in step 2) with continuous stirring, still under nitrogen. The stirring rate is gradually increased from 500 rpm to 1000 rpm. Triethylamine is then added, and the pH is adjusted to 7.0. After stirring for 10 minutes, the solution is transferred to a hydrothermal reaction vessel at 180°C for 2 hours for hydrothermal crystallization. The reaction product, lithium iron phosphate, is then centrifuged and dried under vacuum to obtain the reaction product.

[0086] 4) The reaction product lithium iron phosphate obtained in step 3) is calcined at 750° C. in a reducing atmosphere of argon and hydrogen (hydrogen accounts for 5% of the volume of argon) for 6 hours to finally obtain a high-performance lithium iron phosphate material.

[0087] in, Figure 4 This is the SEM image of the calcined lithium iron phosphate material prepared in Example 2. Figure 4It can be seen that the particle size of the lithium iron phosphate material prepared in this embodiment is about 0.1-0.3 μm, and the carbon content is (2.0±0.5)%. Due to the reduction of its particle size (the unit cell parameters a and b are reduced) and the appropriate increase of carbon content, its electrochemical performance shows excellent high current charge and discharge (rate performance) and low temperature resistance. The specific electrochemical performance test results are listed in Table 1.

[0088] Example 3

[0089] Example 3: Preparation of a lithium iron phosphate material, which mainly includes the following steps:

[0090] 1) Take 8.346 ml of ferrous chloride tetrahydrate and dissolve it in 15 mL of oxygen-free water while continuously introducing nitrogen gas to prevent oxidation, to obtain a second solution containing an iron source.

[0091] 2.895 g of anhydrous lithium nitrate was weighed and dissolved in a mixture of 20 mL of diethylene glycol solvent and 25 mL of water to obtain a third solution containing a lithium source.

[0092] 6.6 g of phytic acid with a mass fraction of 70% was dissolved in 15 mL of a polyethylene glycol (PEG-800:PEG-1200=1:1) aqueous solution with a mass concentration of 6% to obtain a first solution containing phosphoric acid and polyethylene glycol.

[0093] 2) gradually adding the first solution containing phosphoric acid and polyethylene glycol to the second solution containing the iron source at a speed of 500 rpm until uniformly dispersed, adding triethylamine and adjusting the pH to 4.0, heating to 85° C. under nitrogen protection for coprecipitation reaction, and reacting for 40 minutes to obtain a polyethylene glycol-coated ferrous phosphate precursor with a particle size of 0.1-0.2 μm.

[0094] 3) Slowly add the third solution containing a lithium source to the sample (polyethylene glycol-coated ferrous phosphate precursor) obtained in step 2) with continuous stirring, still under nitrogen. The stirring rate is gradually increased from 500 rpm to 1200 rpm. Triethylamine is then added, and the pH is adjusted to 6.5. After stirring for 10 minutes, the solution is transferred to a hydrothermal reaction vessel at 180°C for 3 hours for hydrothermal crystallization. The reaction product, lithium iron phosphate, is then centrifuged and vacuum dried.

[0095] 4) The obtained reaction product, lithium iron phosphate, was calcined at 750° C. in a reducing atmosphere of argon and hydrogen (hydrogen accounts for 5% of the volume of argon) for 8 h to finally obtain a high-performance lithium iron phosphate material.

[0096] in, Figure 5 This is the SEM image of the calcined lithium iron phosphate material prepared in Example 3. Figure 5It can be seen that the particle size of the lithium iron phosphate material prepared in this embodiment is about 0.1-0.2μm, the carbon content is (2.1±0.5)%, and phytic acid is used as phosphoric acid. On the one hand, it can provide the phosphate radical required for the reaction, and on the other hand, its organic part reacts more easily with polyethylene glycol to form a network structure. During the calcination process, a conductive network is formed, which is conducive to the migration of electrons and exhibits excellent high current charge and discharge (rate performance) and low temperature resistance. The specific electrochemical performance test results are listed in Table 1.

[0097] Example 4

[0098] Example 4: Preparation of a lithium iron phosphate material, which mainly includes the following steps:

[0099] 1) 5.722 g of ferrous sulfate heptahydrate was dissolved in 20 mL of oxygen-free water and nitrogen was continuously introduced to prevent oxidation, to obtain a second solution containing an iron source.

[0100] 1.762 g of lithium hydroxide monohydrate was dissolved in a mixture of 15 mL of ethylene glycol and 20 mL of water to obtain a third solution containing a lithium source.

[0101] 2.41563 g of ammonium dihydrogen phosphate was dissolved in 15 mL of a 4% polyethylene glycol (PEG-800:PEG-1200=1:1) aqueous solution to obtain a first solution containing a phosphorus source and polyethylene glycol.

[0102] 2) The first solution containing the phosphorus source and polyethylene glycol was gradually added to the second solution containing the iron source at a speed of 500 rpm until uniformly dispersed. Triethylamine was added and the pH was adjusted to 4.5. The mixture was heated to 85° C. under nitrogen protection for coprecipitation. After reacting for 20 minutes, a polyethylene glycol-coated ferrous phosphate precursor was obtained with a particle size of 0.1-0.3 μm.

[0103] 3) Slowly add the third solution containing a lithium source to the sample (polyethylene glycol-coated ferrous phosphate precursor) obtained in step 2) with continuous stirring, still under nitrogen. The stirring rate is gradually increased from 500 rpm to 1200 rpm. Triethylamine is then added to adjust the pH to 6.4. After stirring for 10 minutes, the solution is transferred to a hydrothermal reaction vessel at 180°C for 5 hours for hydrothermal crystallization. The reaction product, lithium iron phosphate, is then centrifuged and dried under vacuum to obtain the reaction product.

[0104] 4) The reaction product lithium iron phosphate obtained in step 3) is calcined at 750° C. in a reducing atmosphere of argon and hydrogen (hydrogen accounts for 5% of the volume of argon) for 5 hours to finally obtain a high-performance lithium iron phosphate material.

[0105] in, Figure 6This is the SEM image of the calcined lithium iron phosphate material prepared in Example 4. Figure 6 It can be seen that the particle size of the lithium iron phosphate prepared in this embodiment is about 0.1-0.5μm, and the carbon content is (1.9±0.5)%. Here, by reducing the concentration of the reactants and prolonging the hydrothermal crystallization reaction time, the grain size of the prepared lithium iron phosphate material increases (the unit cell parameter a increases and b decreases). Under low current charge and discharge conditions, lithium ions are more likely to migrate along the b-axis, thereby exhibiting excellent capacity performance. The specific electrochemical performance test results are listed in Table 1.

[0106] Comparative Example 1

[0107] The only difference between this comparative example and Example 1 is that the first solution containing a phosphorus source and polyethylene glycol is replaced with a first solution containing only a phosphorus source (ie, without polyethylene glycol). Other conditions and parameters are exactly the same as in Example 1.

[0108] Comparative Example 2

[0109] This comparative example differs from Example 1 only in that: (1) the first solution containing a phosphorus source and polyethylene glycol is replaced with a first solution containing only a phosphorus source (i.e., no polyethylene glycol is added). (2) prior to high-temperature calcination, the reaction product, lithium iron phosphate, and glucose are ground and blended (wherein the mass of glucose is 12% of the mass of the reaction product, lithium iron phosphate), and calcined in an argon atmosphere. Other conditions and parameters are identical to those of Example 1.

[0110] Comparative Example 3

[0111] The only difference between this comparative example and Example 1 is that ammonia water is used instead of triethylamine when adjusting the pH. Other conditions and parameters are exactly the same as those in Example 1.

[0112] The lithium iron phosphate materials prepared in Examples 1-4 and Comparative Examples 1-3 were used as positive electrode materials for lithium batteries and applied to CR2032 button batteries. The active material (lithium iron phosphate), binder (PVDF), and conductive agent (Ketjen Black) were used in a ratio of 9:0.5:0.5. Charge and discharge performance tests were conducted. The test results are shown in Table 1.

[0113] Table 1 shows the discharge capacity and cycle performance test data of the lithium iron phosphate materials prepared in Examples 1-4 and Comparative Examples 1-3.

[0114] Table 1

[0115]

[0116] See Table 1, the experimental results show that:

[0117] When the lithium iron phosphate material prepared in this embodiment is used in a lithium battery, a high specific capacity is obtained under low current charge and discharge conditions. For example, the lithium iron phosphate materials prepared in Examples 1 and 4 have a specific capacity of 163 mAh / g and 164 mAh / g under 0.2C charge and discharge conditions (theoretical capacity 170 mAh / g). In addition, by changing the preparation parameters, a high specific capacity can be obtained under high current charge and discharge conditions. For example, the lithium iron phosphate materials prepared in Examples 2 and 3 have a specific capacity of 138 mAh / g and 137 mAh / g under 10C charge and discharge. Here, the lithium iron phosphate materials prepared in Examples 1 and 4 are used in lithium batteries to achieve low-rate high-capacity performance, and the lithium iron phosphate materials prepared in Examples 2 and 3 are used in lithium batteries to achieve high-rate charge and discharge performance and low-temperature performance.

[0118] Since polyethylene glycol is not added to the lithium iron phosphate material prepared in Comparative Example 1 during preparation, the discharge specific capacity of the lithium iron phosphate material is relatively low when used in button batteries.

[0119] The lithium iron phosphate material prepared in Comparative Example 3 has a relatively low discharge specific capacity when used in a lithium battery because ammonia water is used instead of triethylamine during preparation. The reason may be that ammonia water has a strong complexing ability, which causes the generated lithium iron phosphate to agglomerate more seriously and discharge insufficiently. In addition to the function of regulating pH value, triethylamine also has a certain ability to adjust morphology. Therefore, the use of triethylamine to regulate pH value can produce lithium iron phosphate with better electrochemical performance.

[0120] The lithium iron phosphate material prepared in Comparative Example 2 lacks polyethylene glycol during preparation, and the reaction product, lithium iron phosphate, is poorly mixed with glucose before calcination. This results in an uneven distribution of the carbon layer after calcination. This not only reduces the electrochemical performance of the lithium iron phosphate, but also reduces the material's tap density due to excessive carbon.

[0121] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiment based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a lithium iron phosphate material, characterized in that: The preparation method of the lithium iron phosphate material comprises the following steps: 1) co-precipitating a first solution containing a phosphorus source and polyethylene glycol with a second solution containing an iron source to obtain a polyethylene glycol-coated ferrous phosphate precursor; wherein, step 1) is specifically as follows: gradually adding the first solution containing the phosphorus source and polyethylene glycol to the second solution containing the iron source, stirring until uniformly dispersed, adding an auxiliary agent thereto, adjusting the pH value, heating under inert gas protection to perform a co-precipitation reaction, and reacting for a set time to obtain a polyethylene glycol-coated ferrous phosphate precursor; wherein the auxiliary agent is triethylamine; wherein the pH value is adjusted to 3.5-10.0; wherein the co-precipitation reaction temperature is 55-120° C., and the reaction time is 0.5-2 hours; wherein the polyethylene glycol-coated ferrous phosphate precursor has a core-shell structure with ferrous phosphate as the core and polyethylene glycol as the shell; wherein the particle size of the polyethylene glycol-coated ferrous phosphate precursor is 0.1-1 μm; 2) subjecting the third solution containing the lithium source to a hydrothermal crystallization reaction with the polyethylene glycol-coated ferrous phosphate precursor to obtain a reaction product, lithium iron phosphate; 3) calcining the reaction product, lithium iron phosphate, under a reducing atmosphere to obtain a lithium iron phosphate material; wherein the calcination temperature is 700-800° C.; The amounts and concentrations of the first solution containing a phosphorus source and polyethylene glycol, the second solution containing an iron source, and the third solution containing a lithium source must ensure that the molar ratio of divalent ferrous ions, phosphate ions, and lithium ions is (0.95-1):1:(1-2).

2. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The phosphorus source includes one or more of phosphoric acid, ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, and phytic acid; and / or The iron source includes one or both of ferrous sulfate and ferrous chloride; and / or The lithium source includes one or more of lithium hydroxide, lithium nitrate, and lithium sulfate; and / or The molecular weight of the polyethylene glycol is 2-20K; and / or In the first solution containing a phosphorus source and polyethylene glycol, the concentration of the phosphorus source is 0.3-0.8 mol / L, and the concentration of the polyethylene glycol is 2-10 wt %; and / or In the second solution containing the iron source, the concentration of the iron source is 0.3-0.8 mol / L; and / or In the third solution containing the lithium source, the concentration of the lithium source is 0.3-0.8 mol / L; and / or The preparation steps of the second solution containing the iron source are as follows: dissolving the iron source in oxygen-free water and continuously introducing a protective gas to prevent oxidation, thereby obtaining the second solution containing the iron source.

3. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The preparation steps of the first solution containing a phosphorus source and polyethylene glycol are as follows: dissolving the phosphorus source in an aqueous solution of polyethylene glycol to obtain the first solution containing a phosphorus source and polyethylene glycol.

4. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The preparation steps of the third solution containing the lithium source are as follows: dissolving the lithium source in a mixture of a solvent and water to obtain the third solution containing the lithium source.

5. The method for preparing the lithium iron phosphate material according to claim 4, wherein: The solvent includes any one or both of ethylene glycol and diethylene glycol.

6. The method for preparing the lithium iron phosphate material according to claim 1 or 2, characterized in that: In step 1), the stirring rate is 100-1500 rpm.

7. The method for preparing lithium iron phosphate material according to claim 1, characterized in that: In step 2): Under a protective atmosphere, gradually adding the third solution containing the lithium source to the polyethylene glycol-coated ferrous phosphate precursor prepared in step 1), and continuously stirring at a gradually increasing stirring rate; Then, an auxiliary agent is added thereto, and the pH value is adjusted. After stirring, the mixture is transferred to a hydrothermal kettle to perform a hydrothermal crystallization reaction to obtain a reaction product, lithium iron phosphate.

8. The method for preparing the lithium iron phosphate material according to claim 7, characterized in that: In step 2): The auxiliary agent is triethylamine; and / or Adjust the pH to 3.5-10.0; and / or The stirring rate is 100-1500 rpm.

9. The method for preparing lithium iron phosphate material according to claim 1, characterized in that: The temperature of the hydrothermal crystallization reaction is 160-200° C., and the time of the hydrothermal crystallization reaction is 1.5-12 hours; and / or The particle size of the reaction product, lithium iron phosphate, is 0.1-1 μm.

10. The method for preparing lithium iron phosphate material according to claim 1, characterized in that: In step 3): The calcination treatment time is 4-10h; and / or The reducing atmosphere comprises argon and hydrogen; wherein the hydrogen accounts for 3-7% of the volume of the argon; and / or The gas flow rate of the reducing atmosphere is 100-500 mL / min.

11. A lithium iron phosphate material, characterized in that: The lithium iron phosphate material is prepared by the preparation method of the lithium iron phosphate material according to any one of claims 1-10.

12. The lithium iron phosphate material according to claim 11, characterized in that The particle size of the lithium iron phosphate material is 0.1-1 μm; the carbon content in the lithium iron phosphate material is 1.5-3%.

13. The lithium iron phosphate material according to claim 11, characterized in that: The lithium iron phosphate has a rugby-shaped, spherical-shaped, or a shape varying between the rugby-shaped and spherical-shaped shapes.

14. The lithium iron phosphate material according to claim 13, characterized in that: The shape that changes between the rugby ball and the ball-like shape is a rice grain shape.

15. Use of the lithium iron phosphate material according to any one of claims 11 to 14 in preparing a positive electrode material for lithium-ion batteries.

16. A positive electrode material for a lithium ion battery, characterized in that: The lithium-ion battery positive electrode material includes a lithium iron phosphate material; wherein the lithium iron phosphate material is prepared by the preparation method of the lithium iron phosphate material according to any one of claims 1 to 10.

17. A lithium ion battery, characterized in that: The lithium-ion battery comprises the lithium-ion battery positive electrode material according to claim 16.

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