Lithium iron phosphate positive electrode material and preparation method therefor, and lithium-ion battery

Abstract

The present disclosure relates to the technical field of positive electrode materials, and provides a lithium iron phosphate positive electrode material and a preparation method therefor, and a lithium-ion battery. The lithium iron phosphate positive electrode material comprises a substrate and a carbon coating layer coated on a surface of the substrate. The crystal structure factor A of the lithium iron phosphate positive electrode material satisfies: 4.600 Å-4≤A≤9.500 Å-4, and is calculated by formula (I), wherein C represents the crystallinity of a crystal; V represents the unit cell volume; and D(010) represents the grain size of the (010) crystal plane, the value of which satisfies: 4 Å≤D(010)≤9 Å. The lithium iron phosphate positive electrode material falling within the above-mentioned ranges exhibits good low-temperature electrochemical performance.

Description

Lithium iron phosphate cathode material and its preparation method and lithium-ion battery

[0001] Cross-references to related applications

[0002] This disclosure claims priority to Chinese Patent Application No. 202411719686.X, filed on November 28, 2024, entitled "Lithium iron phosphate cathode material and preparation method thereof and lithium-ion battery", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of cathode material technology, and more specifically, to lithium iron phosphate cathode materials, their preparation methods, and lithium-ion batteries. Background Technology

[0004] Lithium iron phosphate (LFP) is one of the most competitive cathode active materials for lithium-ion batteries currently on the market. Compared with lithium cobalt oxide and ternary cathode materials, it has a longer lifespan and better safety performance. Furthermore, LFP has a 170mAh / g capacity. -1 With its theoretical specific capacity and a plateau discharge voltage of 3.4V, it possesses considerable energy density.

[0005] LiFePO4, with its olivine-type structure and space group Pbnm, exhibits a slightly distorted hexagonal close-packed arrangement of oxygen atoms. P atoms occupy the 4c ​​positions of the oxygen atom tetrahedra, forming PO4 tetrahedra. Fe and Li atoms occupy the centers of the oxygen atom octahedra, forming FeO6 and LiO6 octahedra, respectively. On the bc plane, adjacent FeO6 octahedra share a single oxygen atom, thus linking together to form a zigzag FeO6 layer. Between FeO6 layers, adjacent LiO6 octahedra are linked by two oxygen atoms along the b-axis, forming a continuous linear chain of Li parallel to the c-axis. This allows Li... + Two-dimensional diffusion may occur. Structurally, the PO4 tetrahedra are located between the FeO6 layers, which to some extent hinders the diffusion of Li. + The diffusion motion. Simultaneously, adjacent FeO6 octahedrons are connected by shared vertices, failing to form an octahedral structure with shared edges, resulting in very low electrical conductivity. Furthermore, due to the aforementioned crystal configuration of LiFePO4, its capacity and rate performance at low temperatures are worse than those of ternary cathode materials, because Li... + The chemical reaction of insertion and extraction is more difficult to carry out.

[0006] To improve the lithium-ion diffusion rate and conductivity of LiFePO4, traditional methods have been extensively studied for LiFePO4 nano-sizing and carbon coating. To improve the low-temperature usability of lithium iron phosphate batteries, current commercial solutions involve adding self-heating systems to the power unit to insulate or heat the battery. However, there are few reports on research into improving the low-temperature electrochemical performance of LiFePO4 batteries by adjusting the crystal structure of LiFePO4.

[0007] In view of this, this disclosure is hereby made.

[0008] Public content

[0009] The purpose of this disclosure is to provide a lithium iron phosphate cathode material, a method for preparing the same, and a lithium-ion battery.

[0010] This disclosure is implemented as follows:

[0011] In a first aspect, this disclosure provides a lithium iron phosphate cathode material, comprising a matrix and a carbon coating layer covering the surface of the matrix, wherein the crystal structure factor A of the lithium iron phosphate cathode material satisfies The formula for calculating the crystal structure factor A is as follows:

[0012] Wherein, C refers to the crystallinity of the lithium iron phosphate cathode material, which is measured by the ratio of the XRD diffraction peak intensity of the crystal plane (311) to the integral area of ​​the diffraction peak.

[0013] V refers to the unit volume of the lithium iron phosphate cathode material.

[0014] D(010) refers to the grain size of the (010) crystal plane of the lithium iron phosphate cathode material, in units of... The value satisfies

[0015] In an optional embodiment, the lithium iron phosphate cathode material satisfies at least one of the following characteristics (1) to (3):

[0016] Feature (1): The value of D(010) satisfies

[0017] Feature (2): The value of C satisfies 0.0900≤C≤0.1200;

[0018] Feature (3): The value of V satisfies

[0019] In an optional embodiment, the matrix has the following general formula: Li 1-x A x Fe1-y M y (PO 4-z )D z Wherein, A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from at least one of F, S, N and Cl; 0≤x≤0.1; 0≤y≤0.1 and 0≤z≤0.1.

[0020] In an optional embodiment, the mass of the carbon coating layer is 0.5% to 5% of the mass of the lithium iron phosphate cathode material.

[0021] Secondly, this disclosure provides a method for preparing a lithium iron phosphate cathode material, comprising:

[0022] A mixture of lithium carbonate, ferrous iron source, phosphorus source and ethylene glycol compound solution was stirred and reacted to obtain a precursor suspension.

[0023] The precursor suspension was subjected to a hydrothermal reaction, followed by solid-liquid separation to obtain a solid precursor.

[0024] The solid precursor and carbon source are mixed and dried, and then calcined under an inert atmosphere to obtain lithium iron phosphate cathode material.

[0025] In an optional embodiment, the preparation of the precursor suspension includes at least one of features (4) to (9):

[0026] Feature (4): The ferrous iron source is selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate and ferrous acetate;

[0027] Feature (5): The phosphorus source is selected from at least one of phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, ammonium dihydrogen phosphate and ammonium monohydrogen phosphate;

[0028] Feature (6): The mass percentage of ethylene glycol compound in the ethylene glycol compound solution is 5% to 20%;

[0029] Feature (7): The ethylene glycol compound refers to a water-soluble compound containing an ethylene glycol structure, wherein the ethylene glycol compound is selected from at least one of 1,2-ethylene glycol, squaric acid, ketone acid, sodium crotonate, 2,3-dihydroxycyclopent-2-en-1-one, 2,3-dihydroxy-4-(hydroxymethyl)cyclopent-2-en-1-one and 3,4-dihydroxy-5-methyl-2-furanone;

[0030] Feature (8): The pH of the solution in the stirring reaction is 1 to 2.5, the reaction temperature is 80 to 90°C, the reaction time is 1 to 4 hours, and the stirring speed is 300 to 800 rpm;

[0031] Feature (9): After the stirring reaction is completed, the temperature is further reduced to 10-25°C and stirred until no bubbles are generated.

[0032] In an optional embodiment, a source M, comprising at least one of features (10) to (13), is also added during the preparation of the mixture:

[0033] Feature (10): The divalent iron source and the M source are dissolved in the ethylene glycol compound solution to form a metal salt solution, and the phosphorus source is dissolved in water to form a phosphorus source solution; the lithium carbonate is dispersed in the metal salt solution to obtain a dispersion, and then the phosphorus source solution is added to obtain the mixture, wherein the molar concentration of the metal element in the metal salt solution is 0.8-1.5M, and the molar concentration of the phosphorus source in the phosphorus source solution is 0.85-3M;

[0034] Feature (11): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of the phosphorus source is 1:1.05 to 1.5;

[0035] Feature (12): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of lithium in the lithium carbonate is 1:1.01 to 1.1;

[0036] Feature (13): The M source comprises a water-soluble salt of at least one element selected from Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y.

[0037] In an optional embodiment, the hydrothermal reaction conditions during the preparation of the solid precursor are: temperature of 160–200°C, time of 1–3 h, and stirring rate of 100–300 rpm.

[0038] In an optional embodiment, the process of calcining the solid precursor and carbon source under an inert atmosphere after mixing and drying includes at least one of features (14) to (17):

[0039] Feature (14): The mass ratio of the solid precursor to the carbon source is 1:0.04 to 0.1;

[0040] Feature (15): The carbon source is selected from at least one of glucose, starch, phenolic resin, sucrose, cellulose, polyethylene glycol and citric acid;

[0041] Feature (16): The mixing process further includes the addition of source A and / or source D, wherein source A is selected from at least one of magnesium fluoride, sodium fluoride, sodium chloride and sodium sulfide; and source D is selected from at least one of potassium fluoride, magnesium fluoride, sodium fluoride, ammonium fluoride, lithium fluoride, melamine, amino acids, trithiocyanate, thiourea, elemental sulfur, sodium sulfide, ammonium sulfide, sodium chloride, ammonium chloride, lithium chloride and urea;

[0042] Feature (17): The calcination includes calcination at 550-650℃ for 4-8 hours.

[0043] Thirdly, a lithium-ion battery is provided, comprising a lithium iron phosphate cathode material as described in any of the foregoing embodiments or a lithium iron phosphate cathode material prepared by the preparation method of the lithium iron phosphate cathode material as described in any of the foregoing embodiments.

[0044] This disclosure has the following beneficial effects:

[0045] (1) This disclosure provides a lithium iron phosphate cathode material, wherein the crystal structure factor A satisfies the following value: Meanwhile, its grain size D(010) satisfies Lithium iron phosphate cathode materials meeting the above-mentioned ranges exhibit excellent low-temperature electrochemical performance. The crystal structure factor comprehensively evaluates three indicators of lithium iron phosphate cathode materials: crystallinity C, cell volume V, and grain size D(010) of the (010) crystal plane. Among these, higher crystallinity C results in fewer internal defects and a higher lithium-ion diffusion rate; a smaller cell volume shortens the lithium-ion insertion / extraction channel, facilitating transitions and improving cycle performance; a smaller D(010) further shortens the lithium-ion diffusion channel, improving charge / discharge specific capacity and rate performance. While crystallinity C, cell volume V, and grain size D(010) each influence the performance of lithium iron phosphate cathode materials, it is practically difficult to obtain cathode materials that simultaneously excel in all three aspects. Therefore, this disclosure provides a crystal structure factor A to comprehensively evaluate the three aspects of lithium iron phosphate cathode materials, possessing the ability to meet the requirements of crystal structure factor A. Simultaneously satisfy The lithium iron phosphate cathode material exhibits excellent low-temperature electrochemical performance.

[0046] (2) The method for preparing lithium iron phosphate cathode material provided in this disclosure can prepare D(010) in the range Lithium iron phosphate cathode material with high crystallinity. Attached Figure Description

[0047] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this disclosure and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 is a schematic diagram comparing the XRD patterns of the lithium iron phosphate cathode materials prepared in Examples 3, 5, Comparative Example 1, and Comparative Example 2 of this disclosure with those of the standard card.

[0049] Figure 2 is a SEM image of the lithium iron phosphate cathode materials prepared in Example 3(A) and Comparative Example 1(B) of this disclosure. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions in the embodiments of this disclosure will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0051] This disclosure provides a lithium iron phosphate cathode material, comprising a matrix and a carbon coating layer on the surface of the matrix, wherein the crystal structure factor A of the lithium iron phosphate cathode material satisfies The formula for calculating the crystal structure factor A is:

[0052] Wherein, C refers to the crystallinity of lithium iron phosphate cathode material, which is measured by the ratio of the XRD diffraction peak intensity of the crystal plane (311) to the integral area of ​​the diffraction peak.

[0053] V refers to the unit volume of the lithium iron phosphate cathode material.

[0054] D(010) refers to the grain size of the (010) crystal plane of the lithium iron phosphate cathode material, in units of... The value satisfies

[0055] This disclosure uses lithium iron phosphate cathode material as a sample for X-ray diffraction (XRD) testing. The Cu Kα2 background is subtracted using JADE software, and the XRD pattern is obtained after full-spectrum fitting and refinement. Crystallinity C, cell volume V, and D(010) are obtained from the XRD pattern, and the crystal structure factor A is calculated. Values ​​of crystal structure factor A that satisfy... Meanwhile, its grain size D(010) satisfies The sample used as the lithium iron phosphate cathode material of this disclosure, meeting the above-mentioned range, exhibits excellent low-temperature electrochemical performance. Among them, (Angstrom, As a commonly used unit of length in chemistry and crystallography, A meter = 0.1 nanometers, which is on the order of magnitude equal to the diameter of a hydrogen atom or the length of a typical chemical bond.

[0056] Wherein, C refers to the crystallinity of lithium iron phosphate cathode material. The crystallinity C of lithium iron phosphate cathode material is measured by the ratio of the intensity of the diffraction peak with the largest peak intensity ((311) crystal plane) in the XRD spectrum to the integrated area of ​​the diffraction peak, and the corresponding 2θ = 34.8~35.8°.

[0057] Furthermore, the intensity and integrated area of ​​the diffraction peaks reflect their sharpness. For the same intensity, a smaller integrated area indicates a sharper peak; similarly, for the same integrated area, a stronger peak intensity indicates a sharper peak. A sharper peak indicates higher crystallinity; higher crystallinity results in a higher lithium-ion diffusion rate and better conductivity. The value of C satisfies 0.0900 ≤ C ≤ 0.1200. Theoretically, a larger C is better, but 0.12 is the maximum value achievable in this disclosure.

[0058] V refers to the unit volume of lithium iron phosphate cathode material. The unit cell is the basic unit of crystal structure, describing the arrangement of atoms or molecules within the crystal. A smaller unit cell volume shortens the lithium-ion insertion / extraction channels, making lithium-ion transitions easier and improving the material's conductivity and cycle performance. The unit cell volume is generally related to the crystal form and chemical composition. The value of V satisfies... Too small a cell volume will lead to poor structural stability of the cathode material and a decrease in cycle performance.

[0059] D(010) refers to the grain size of the (010) crystal plane of the lithium iron phosphate cathode material, in units of... D(010) is calculated based on the diffraction angle (θ) and full width at half maximum (FWHM) of the diffraction peaks of the (010) crystal plane, according to the Scherrer formula: k is a constant value, taking the value 0.9; λ refers to the incident light wavelength; β refers to the full width at half maximum (FWHM) of the diffraction peak, in rad; θ refers to the diffraction angle, in °. Grain size reflects the macroscopic size of the crystal because D = nd, where d is the interplanar spacing of the crystal plane and n is the number of crystal layers. The smaller D(010), the fewer the number of crystal layers and / or the smaller the interplanar spacing corresponding to the crystal plane, resulting in a plate-like crystal. A smaller D(010) indicates that the crystal growth along the b-axis is restricted, and the crystal grows along the ac-plane. Since lithium ions diffuse along the b-axis, this helps to shorten the diffusion channel of lithium ions and improve conductivity. The D(010) value in this disclosure satisfies... Theoretically, a smaller D(010) is more beneficial to improving the electrochemical performance of the material. However, a D(010) that is too small is not only technically difficult to achieve, but also makes the plate-like crystals prone to breakage during electrode preparation or use, leading to problems such as performance instability and easy puncture of the separator. Optionally, the value of D(010) satisfies

[0060] This disclosure satisfies the crystal structure factor A of the lithium iron phosphate cathode material by limiting it. The lithium iron phosphate cathode material exhibits excellent low-temperature electrochemical performance, which is related to the lithium-ion diffusion rate of lithium iron phosphate at low temperatures.

[0061] In other embodiments of this disclosure, the value of the crystal structure factor A can be, for example, [missing information]. or The crystallinity C can be any value or a range between any two of the values ​​in the range. For example, the crystallinity C can be any value or a range between any two of the values ​​in the range, such as 0.0900, 0.1000, 0.1100, or 0.1200. The unit cell volume V can be, for example, a value between any two of the values ​​in the range, such as 0.0900, 0.1000, 0.1100, or 0.1200. or The range of values ​​between any one or both of the given values. For example, the value of D(010) can be... or The range of values ​​between any one or any two of them.

[0062] Furthermore, the matrix has the following general formula: Li 1-x A x Fe 1-y M y (PO 4-z )D z Wherein, A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y; D is selected from at least one of F, S, N, and Cl; 0≤x≤0.1; 0≤y≤0.1 and 0≤z≤0.1. By doping with A source, D source, and / or M source, various properties of lithium iron phosphate can be improved, including electrochemical performance, hardness, morphology, particle size and its distribution, etc.

[0063] Further, the mass of the carbon coating layer is 0.5% to 5% of the mass of the lithium iron phosphate cathode material, for example, it can be any or a range between any two of 0.5%, 0.8%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, and 5%. Optionally, in some embodiments, the mass of the carbon coating layer is 1.1% to 1.5% of the mass of the lithium iron phosphate cathode material.

[0064] In addition, this disclosure also provides a typical but non-limiting method for preparing the above-mentioned lithium iron phosphate cathode material, which includes the following steps:

[0065] S1. A mixture of lithium carbonate, ferrous iron source, phosphorus source and ethylene glycol compound solution is stirred and reacted to obtain a precursor suspension.

[0066] The ferrous iron source includes, but is not limited to, at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate, and ferrous acetate; the phosphorus source includes, but is not limited to, at least one of phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, ammonium dihydrogen phosphate, and ammonium monohydrogen phosphate; ethylene glycol compounds refer to water-soluble compounds containing an ethylene glycol structure in their molecular structure, further specifying that the ethylene glycol structure is OH-C=C-OH; ethylene glycol compounds include, but are not limited to, at least one of 1,2-ethylene glycol, squaric acid, ketone acid, sodium crotonate, 2,3-dihydroxycyclopent-2-en-1-one, 2,3-dihydroxy-4-(hydroxymethyl)cyclopent-2-en-1-one, and 3,4-dihydroxy-5-methyl-2-furanone. The ethylene glycol compound solution contains 5% to 20% by mass, and the solvent is water.

[0067] In this disclosure, the reaction pH of the mixture during the stirring reaction is 1–2.5, the reaction temperature is 80–90°C, the reaction time is 1–4 h, and the stirring speed is 300–800 rpm. After the stirring reaction is completed, the temperature is lowered to 10–25°C, and stirring continues until no more bubbles are generated. In other embodiments of this disclosure, the pH of the stirring reaction can be, for example, any one or a range between 1, 1.5, 2, and 2.5; the reaction temperature can be, for example, any one or a range between 80°C, 82°C, 85°C, 88°C, and 90°C; the reaction time can be, for example, any one or a range between 1 h, 2 h, 3 h, and 4 h; and the stirring speed can be, for example, any one or a range between 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, and 800 rpm.

[0068] This disclosure uses lithium carbonate as a lithium source, mixed with a ferrous iron source, a phosphorus source, and an ethylene glycol compound solution. The mixture is stirred and reacted at a relatively low temperature (80–90°C) and a low pH (1–2.5). Phosphate ions in the phosphorus source react with ferrous ions to form an unstable ferrous phosphate precipitate (precipitation and dissolution occur simultaneously). Simultaneously, lithium carbonate reacts with the acid to release Li. + The carbon dioxide gas reacts to form tiny bubbles, and the bursting of these bubbles can lead to the precipitation of ions (Fe). 2+ Li + PO4 3- Local supersaturation promotes primary nucleation of lithium iron phosphate precipitation, thereby shortening the hydrothermal reaction time and lowering the hydrothermal reaction temperature. Ethylene glycol compounds act as both antioxidants and chelating agents for ferrous ions, preventing the oxidation of ferrous ions to ferric ions during stirred reactions. Compared to saturated polyols (such as ethylene glycol in traditional methods), ethylene glycol compounds have a stronger chelating effect on ferrous ions due to the bond between hydroxyl groups and unsaturated carbon atoms, thus having a more significant effect on slowing down crystal growth and greatly improving crystallinity. In addition, because ethylene glycol compounds have lower water solubility than saturated polyols such as ethylene glycol, they tend to chelate with ferrous ions and adsorb onto the crystal surface to regulate crystal morphology.

[0069] Furthermore, a dopant element source M may be added to the mixture. The M source includes a water-soluble salt of at least one element selected from Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y.

[0070] As an optional preparation method of this disclosure, firstly, a divalent iron source and a M source are dissolved in an ethylene glycol compound solution to form a metal salt solution; a phosphorus source is dissolved in water to form a phosphorus source solution; lithium carbonate is dispersed in the metal salt solution to obtain a dispersion, and then the phosphorus source solution is added to obtain a mixture.

[0071] Optionally, the molar concentration of the metal element in the metal salt solution is 0.8–1.5 M, and the ratio of the divalent iron source and the M source in the metal salt solution is according to the matrix general formula Li 1-x A x Fe 1-y M y (PO 4-z )D z The ratio of ferric and methyl elements is determined when feeding the materials.

[0072] Optionally, the molar concentration of phosphorus source in the phosphorus source solution is 0.85–3M, and the ratio of the total molar amount of ferrous iron source and M source to the molar amount of phosphorus source is 1:1.05–1.5.

[0073] Optionally, the ratio of the total molar amount of the divalent iron source and the M source to the molar amount of lithium in lithium carbonate is 1:1.01 to 1.1.

[0074] S2. The precursor suspension is subjected to a hydrothermal reaction, and solid-liquid separation is performed to obtain a solid precursor.

[0075] The hydrothermal reaction conditions for preparing the solid precursor are: temperature of 160–200°C, time of 1–3 h, and stirring rate of 100–300 rpm. In other embodiments of this disclosure, the hydrothermal reaction temperature may be, for example, any one or a range between 160°C, 170°C, 180°C, 190°C, and 200°C; the time may be any one or a range between 1 h, 2 h, and 3 h; and the stirring rate may be any one or a range between 100 rpm, 200 rpm, and 300 rpm.

[0076] During the hydrothermal reaction, the chelating effect of ethylene glycol compounds on ferrous ions can reduce the diffusion rate of ferrous ions, inhibit the orderly arrangement of ions, and thus slow down the crystal growth rate, resulting in lithium iron phosphate crystals with higher crystallinity. On the other hand, ethylene glycol compounds combine with ferrous ions arranged on the (010) crystal surface of lithium iron phosphate and are adsorbed on the crystal surface, inhibiting the growth of the crystal surface, thereby obtaining lithium iron phosphate crystals with (010) crystal surface orientation.

[0077] S3. After mixing and drying the solid precursor and carbon source, calcining them under an inert atmosphere yields the lithium iron phosphate cathode material.

[0078] The mass ratio of the solid precursor to the carbon source is 1:0.04 to 0.1; the carbon source is selected from at least one of glucose, starch, phenolic resin, sucrose, cellulose, polyethylene glycol, and citric acid; during the mixing process, dopant elements A source and / or D source may also be added, where A source is selected from at least one of magnesium fluoride, sodium fluoride, sodium chloride, and sodium sulfide; and D source is selected from at least one of potassium fluoride, magnesium fluoride, sodium fluoride, ammonium fluoride, lithium fluoride, melamine, amino acids, trithiocyanate, thiourea, elemental sulfur, sodium sulfide, ammonium sulfide, sodium chloride, ammonium chloride, lithium chloride, and urea.

[0079] The purpose of calcination is to further improve the crystallinity of lithium iron phosphate and introduce a carbon coating layer, thereby improving its electrical conductivity. The calcination temperature is 550–650℃, and the calcination time is 4–8 hours.

[0080] The method for preparing lithium iron phosphate cathode material disclosed herein can prepare D(010) in the range A lithium iron phosphate cathode material with high crystallinity. Where D(010) is not present. Within the range but satisfy The lithium iron phosphate cathode material also has good electrochemical performance, so it is not necessary to use the preparation method provided in this disclosure to prepare a product that can solve the technical problem.

[0081] The aforementioned lithium iron phosphate cathode material can be widely used in the preparation of batteries, and the resulting batteries exhibit excellent electrochemical performance. In this regard, this disclosure also provides a lithium-ion battery comprising the aforementioned lithium iron phosphate cathode material.

[0082] The features and performance of this disclosure will be further described in detail below with reference to embodiments.

[0083] This disclosure provides lithium iron phosphate cathode materials as shown in Tables 1 and 2, and performs X-ray diffraction (XRD) tests, chemical composition analysis, tap density, specific surface area, and particle size analysis on them.

[0084] The detection methods are as follows:

[0085] (1) XRD: The crystal phase and crystal structure of the material were studied using an X-ray powder diffractometer (Rigaku D / max-2600PC). Kα rays of Cu were used for the test, with a wavelength λ of 0.154056 nm, a voltage of 40 kV, a current of 40 mA, and a scanning range of 2θ of 10–80°. The XRD test results were analyzed using Jade 6 software, with full spectrum fitting and R-score refinement. wp ≤10% (R) wp (This is a weighted residual variance factor for the graph, which ensures the accuracy of the refinement).

[0086] (2) Chemical composition analysis was performed using a PE Avio200 inductively coupled plasma optical emission spectrometer (ICP-OES).

[0087] (3) Tap density TD: The tap density of powder products shall be determined in accordance with GB / T 21354-2008.

[0088] (4) Specific surface area BET determination: The determination was performed according to GB / T 21650.2—2008 "Determination of pore size distribution and porosity of solid materials by mercury intrusion porosimetry and gas adsorption method – Part 2: Analysis of mesopores and macropores by gas adsorption method". An Autosorb IQ2 fully automatic specific surface area and pore size analyzer was used to test the powdered samples and obtain nitrogen adsorption-desorption curves. The temperature conditions were constant at -196℃, and the pressure conditions ranged from 0.005 MPa to 0.100 MPa. The specific surface area was calculated using the BET formula based on the data from the low and medium pressure stages.

[0089] (5) Particle size Dv(50): Tested using an MS3000 laser particle size analyzer.

[0090] Please refer to Tables 1 and 2 for the test results.

[0091] Table 1. Statistical table of crystal structure test results for different materials

[0092] Figure 1 shows the XRD patterns of the lithium iron phosphate cathode materials prepared in Examples 3, 5, Comparative Example 1, and Comparative Example 2, as well as magnified images of the diffraction peaks of the (010) and (311) crystal planes. Comparison with the standard card shows that LiFePO4 was successfully synthesized in this disclosure. Furthermore, the crystal structure factor A was obtained by analyzing and calculating the diffraction peaks of the (010) and (311) crystal planes according to the method described in this disclosure.

[0093] Figure 2 shows scanning electron microscope (SEM) images of the lithium iron phosphate cathode materials prepared in Example 3 (A) and Comparative Example 1 (B). As can be seen from the figure, Example 3 has a sheet-like morphology, while Comparative Example 2 has an irregular particle morphology. The morphologies of both are consistent with the measured D(010), indicating that Example 3 has a (010) crystal plane orientation.

[0094] Table 2. Statistical table of composition and performance test results of different samples

[0095] Electrochemical tests were performed on the lithium iron phosphate cathode materials provided in the above embodiments and comparative examples.

[0096] Battery Assembly: A uniform slurry was prepared by mixing positive electrode material, acetylene black, and PVDF in a mass ratio of 75:15:10. This slurry was then evenly coated onto an aluminum foil substrate to serve as the positive electrode of the simulated battery. The negative electrode of the simulated battery used a lithium sheet, and the separator was a polypropylene porous membrane. The electrolyte was 1 mol LiPF6 dissolved in 1 L of a mixed solvent of EC and DMC (volume ratio 1:1). The positive electrode, negative electrode, electrolyte, and separator were assembled into a battery within an argon-protected glove box.

[0097] Electrochemical performance at room temperature: After charging and discharging at 0.1C rate for 1 week (i.e., 1 charge-discharge cycle) at 25℃, the 0.1C discharge capacity (25℃) is obtained; then the charge-discharge cycle performance is tested at 1C rate (200 cycles) to obtain the 1C discharge capacity (25℃) of the first week and the 1C discharge capacity (25℃) of the 200th week, and the capacity retention rate is calculated based on this.

[0098] Low-temperature electrochemical performance includes low-temperature discharge rate and low-temperature DCR growth rate.

[0099] Low-temperature DCR growth rate test: After one week of charge-discharge at 0.1C rate at 25℃, the charge-discharge cycle performance was tested at 1C rate at -10℃ (200 cycles). Wherein, DCR growth rate = (DCR value in week 200 - DCR value in week 1) ÷ DCR value in week 1 × 100%; DCR value = (static voltage before 1C discharge - voltage after 10s of 1C discharge) ÷ constant current at 1C.

[0100] Low-temperature discharge rate test: After charging and discharging at 0.1C rate for 1 cycle at 25℃, charge at 0.5C to 3.8V and record the charging capacity (i.e., 0.5C charging capacity); under -10℃ temperature conditions, discharge at 1C constant current to 2.0V and record the discharge capacity (i.e., 1C discharge capacity).

[0101] Low-temperature discharge rate = (1C discharge capacity ÷ 0.5C charging capacity) × 100%.

[0102] The charging and discharging voltage range is 2 to 3.8V.

[0103] Table 3. Statistical table of electrochemical performance tests for different samples

[0104] As can be seen from Tables 1, 2, and 3, the crystal structure factors A and D(010) of Examples 1-8 provided in this disclosure all meet the criteria defined in this disclosure. and However, the crystal structure factor A in Comparative Examples 1-3 does not meet the criteria defined in this disclosure.

[0105] Correspondingly, data from Examples 1-8 and Comparative Examples 1-3 show that the electrochemical performance of cathode materials with similar TD, BET, and Dv(50) but different crystal structure factors A varies, with even greater differences in low-temperature electrochemical performance. Specifically, the larger the crystal structure factor, the better the low-temperature electrochemical performance of the cathode material. While the electrochemical performance at room temperature also improves, the change is less significant than that at low temperatures. The specific reason is that low temperatures are insufficient to provide Li. + The activation energy for insertion and extraction is relatively low, resulting in generally low low-temperature electrochemical performance of lithium iron phosphate. However, this disclosure modulates the crystal structure of the lithium iron phosphate cathode material to meet the standards defined herein. and The Li iron phosphate cathode material + Shorter diffusion paths are beneficial for improving Li + The insertion / extraction efficiency is improved, thus enhancing low-temperature performance. However, adjusting the crystal structure factor has little effect on room-temperature electrochemical performance because sufficient Li can be provided at room temperature. +The energy required for insertion / extraction. From the above electrochemical performance test results, it can be seen that when the crystal structure factors A and D(010) of the lithium iron phosphate cathode material meet the range defined in this disclosure ( and When the range of D(010) is met, good electrochemical performance can be obtained. Even if the range of D(010) in Examples 2, 5 and 8 is satisfied, However, it does not meet the range. In Example 8, the crystallinity C does not satisfy 0.0900≤C≤0.1200, and the unit cell volume V also does not satisfy... It also exhibits good electrochemical performance. However, Comparative Examples 1-3 do not meet the limitations of this disclosure, and their electrochemical performance is significantly worse than that of the above-described examples.

[0106] Specifically, the crystal structure factors of Comparative Examples 1 and 2 are lower than […]. The crystallinity of Comparative Example 1 was less than 0.09, and the D(010) of Comparative Example 2 was higher than... The low-temperature electrochemical performance of Comparative Examples 1-2 was significantly reduced, but the room-temperature electrochemical performance of Comparative Example 2 remained at an average level, indicating that crystallinity has a significant impact on the room-temperature electrochemical performance of the cathode material. However, the crystallinity, cell volume, and D(010) of Comparative Example 3 were all within the range of this disclosure, but its crystal structure factor was lower than [the specified value]. This proves that a cathode material does not necessarily have good electrochemical performance if its crystallinity, cell volume, and D(010) are within a certain range; the crystal structure factor A must also be within a certain range.

[0107] In addition, this disclosure also provides preparation methods for the above-mentioned product embodiments and product comparative examples.

[0108] Method Example 1

[0109] This embodiment provides a method for preparing lithium iron phosphate cathode material, and the product obtained corresponds to the product of Example 1 above (the matrix molecular formula is LiFe). 0.988 Ti 0.012 The preparation method of this embodiment includes the following steps: PO4

[0110] (1) Dissolve ferrous sulfate heptahydrate and titanium oxysulfate in an aqueous solution of 1,2-ethylenediol to form a metal salt solution; dissolve phosphoric acid in water to form a phosphorus source solution.

[0111] The molar concentration of the metal element in the metal salt solution is 1.2M, and the mass percentage concentration of 1,2-ethylenediol compound in the aqueous solution of 1,2-ethylenediol is 13wt%. The ratio of ferrous sulfate heptahydrate and titanium oxysulfate is n(Fe):n(Ti) = 0.988:0.012.

[0112] The concentration of the phosphorus source solution is 2M, and the ratio of the total molar amount of iron and Ti to the molar amount of phosphorus is 1:1.22.

[0113] (2) Lithium carbonate with a particle size of 300-500 mesh was ultrasonically dispersed in a metal salt solution to obtain a dispersion. Then, a phosphorus source solution was added and stirred to dissolve the dispersion. Ammonia water was added to adjust the pH of the solution to 1.8, and then the reaction was carried out with stirring at a temperature of 90°C for 2 hours and a stirring speed of 500 rpm. After the reaction was completed, the temperature was lowered to 20°C and ultrasonicated for 30 minutes to obtain a precursor suspension.

[0114] The amount of lithium carbonate fed is based on a ratio of 1:1.05 between the total molar amount of iron and Ti and the molar amount of lithium.

[0115] (3) The precursor suspension was subjected to a hydrothermal reaction at 200℃ for 3 hours with a stirring rate of 180 rpm. After the reaction was completed, the reaction solution was concentrated, cooled to room temperature, and then subjected to solid-liquid separation. The obtained solid was washed with deionized water and cleaned to obtain the solid precursor.

[0116] (4) The solid precursor and glucose were ball-milled with ethanol as a dispersant. The mass ratio of the solid precursor to the carbon source was 1:0.07. After being dried by heat, the mixture was calcined at 600°C for 6 hours under a nitrogen atmosphere. After crushing and sieving, the lithium iron phosphate cathode material was obtained.

[0117] Method Example 2

[0118] This embodiment provides a method for preparing lithium iron phosphate cathode material by carbon reduction, and the product obtained corresponds to the product in Example 2 above (the matrix molecular formula is LiFe). 0.988 Ti 0.012 PO4), which includes the following steps:

[0119] (1) According to the molar ratio of n(Fe):n(Ti)=0.988:0.012, a certain mass of ferrous sulfate heptahydrate and titanium oxysulfate were weighed and dissolved in deionized water to prepare a 2M mixed salt solution; a certain mass of phosphoric acid was weighed and mixed with water to obtain a phosphoric acid solution with a phosphate ion concentration of 2M.

[0120] (2) The mixed salt solution, phosphoric acid solution, 0.6M hydrogen peroxide and 2M ammonia solution were added to the reaction vessel in parallel using a constant flow pump. The flow rates of the mixed salt solution, phosphoric acid solution and hydrogen peroxide were all 120 mL / h. The flow rate of ammonia solution was controlled to maintain the pH of the reaction solution at 1.2 ± 0.1. The temperature was controlled to be maintained at 60℃ during the reaction. The reaction time was 2 h and the stirring speed was 600 rpm. After the reaction was completed, the mixture was filtered and washed with deionized water to obtain an amorphous precipitate (wet material, solid content of 85.22%).

[0121] (3) The amorphous precipitate is uniformly dispersed in the aging solution for aging and crystallization reaction. After the reaction is completed, the solution is filtered and washed with deionized water. The filter residue is spray-dried to obtain the crystalline precipitate.

[0122] The aging solution was an aqueous solution of phosphoric acid containing 1,2-ethylenediol, with a pH of 2 and a concentration of 15 wt% for 1,2-ethylenediol. The feeding ratio of amorphous precipitate to aging solution was 1 g: 200 mL. The aging and crystallization reaction conditions were: temperature 90℃, time 2 h, and stirring rate 200 rpm.

[0123] (4) The crystallized precipitate was calcined at a temperature of 700℃, a heating rate of 4℃ / min, and a calcination time of 2h to obtain a titanium-doped iron phosphate precursor.

[0124] (5) Titanium-doped iron phosphate precursor, lithium carbonate, and glucose were mixed with ethanol at a molar ratio of n(precursor):n(Li):n(glucose) = 1:1.04:0.07. The mixture was ground in a grinder for 6 hours to obtain a slurry. The slurry was then dried in a vacuum oven at 80°C for 4 hours to obtain a dry material. The dry material was calcined at 750°C for 9 hours under a nitrogen atmosphere. After crushing and sieving, lithium iron phosphate cathode material was obtained.

[0125] Method Examples 3-4 and 6-7

[0126] Method Examples 3-4 and 6-7 are prepared using the same methods as Example 1, except that the parameters used in the preparation process are different. The products obtained in Method Examples 3-4 and 6-7 correspond to the products in Examples 3-4 and 6-7, respectively. For specific preparation parameters, please refer to Table 4.

[0127] Table 4. Statistical Table of Preparation Parameters

[0128] Method Example 5

[0129] This embodiment provides a method for preparing lithium iron phosphate cathode material using carbon reduction, and the product obtained corresponds to the product of Example 5 above (matrix molecular formula is LiFePO4). The method includes the following steps:

[0130] (1) Weigh out ferrous sulfate heptahydrate and dissolve it in water to prepare a 2M salt solution; weigh out a certain mass of phosphoric acid and mix it with water to obtain a phosphoric acid solution with a phosphate ion concentration of 2M.

[0131] (2) Salt solution, phosphoric acid solution, 0.6M hydrogen peroxide and 2M ammonia solution were added to the reaction vessel in parallel using a constant flow pump. The flow rates of salt solution, phosphoric acid solution and hydrogen peroxide were all 120 mL / h. The flow rate of ammonia solution was controlled to maintain the pH of the reaction solution at 1.2±0.1. The temperature was controlled to be maintained at 60℃ during the reaction. The reaction time was 2 h and the stirring speed was 600 rpm. After the reaction was completed, the mixture was filtered and washed with deionized water to obtain an amorphous precipitate (wet material, solid content of 89.57%).

[0132] (3) The amorphous precipitate is uniformly dispersed in the aging solution for aging and crystallization reaction. After the reaction is completed, the solution is filtered and washed with deionized water. The filter residue is spray-dried to obtain the crystalline precipitate.

[0133] The aging solution was an aqueous solution of phosphoric acid containing 1,2-ethylenediol, with a pH of 2 and a concentration of 20 wt% for 1,2-ethylenediol. The feeding ratio of the amorphous precipitate to the aging solution was 1 g: 200 mL. The aging and crystallization reaction conditions were: temperature 90℃, time 2 h, and stirring rate 200 rpm.

[0134] (4) The crystallized precipitate was calcined at a temperature of 700℃, a heating rate of 4℃ / min, and a calcination time of 2h to obtain the iron phosphate precursor.

[0135] (5) The iron phosphate precursor, lithium carbonate, and glucose were mixed with ethanol at a molar ratio of n(precursor):n(Li):n(glucose) = 1:1.04:0.06. The mixture was ground in a grinder for 6 hours to obtain a slurry. The slurry was then dried in a vacuum oven at 80°C for 4 hours to obtain a dry material. The dry material was calcined at 750°C for 9 hours under a nitrogen atmosphere. After crushing and sieving, the lithium iron phosphate cathode material was obtained.

[0136] Method Example 8

[0137] This embodiment provides a method for preparing lithium iron phosphate cathode material using carbon reduction, and the product obtained corresponds to the product of Example 8 above (matrix molecular formula is LiFePO4), which includes the following steps:

[0138] The difference from Example 5 is that squaric acid is used instead of 1,2-ethylenediol in step (3); and the calcination temperature in step (4) is 600°C and the heating rate is 2°C / min.

[0139] Method Comparison Example 1

[0140] This comparative example provides a method for preparing lithium iron phosphate cathode material by solvothermal reaction. The product obtained corresponds to the product of Comparative Example 1 above (matrix molecular formula is LiFePO4), and includes the following steps:

[0141] (1) Weigh ferrous sulfate heptahydrate and dissolve it in ethylene glycol to prepare an iron salt solution of 0.1 g / mL; weigh lithium hydroxide monohydrate and dissolve it in ethylene glycol to prepare a lithium source solution of 0.04 g / mL.

[0142] (2) Add 2.8 mL of 85 wt% phosphoric acid to 90 mL of lithium source solution, stir for 30 min and then add iron salt solution, continue stirring for 30 min to obtain precursor suspension.

[0143] (3) The precursor suspension was placed in an oven at 180°C for a solvothermal reaction for 10 h. After solid-liquid separation, the obtained solid was washed and dried to obtain lithium iron phosphate (LiFePO4).

[0144] (4) Lithium iron phosphate (LiFePO4) and glucose were mixed at a mass ratio of 1:0.07, ball milled with ethanol as a dispersant, then dried by heat and calcined at 600°C for 6 hours under a nitrogen atmosphere. After crushing and sieving, lithium iron phosphate cathode material was obtained.

[0145] Method Comparison Example 2

[0146] This comparative example provides a method for preparing lithium iron phosphate cathode material by solid-state synthesis. The product obtained corresponds to the product of Comparative Example 2 above (the matrix molecular formula is LiFePO4), and the method includes the following steps:

[0147] (1) Weigh lithium phosphate, ferrous oxalate dihydrate and ammonium phosphate in a stoichiometric ratio of 1:3:3. Ball mill with ethanol as a dispersant for 6 hours and dry to obtain a precursor. Calcine the precursor at 500°C for 12 hours, cool, crush and sieve to obtain lithium iron phosphate (LiFePO4).

[0148] (2) Lithium iron phosphate (LiFePO4) and glucose were mixed at a mass ratio of 1:0.07, ball milled with ethanol as a dispersant, and then dried by heat drying. After being placed in a nitrogen atmosphere and calcined at 600°C for 2 hours, the lithium iron phosphate cathode material was obtained after crushing and sieving.

[0149] Method Comparison Example 3

[0150] This comparative example provides a method for preparing lithium iron phosphate cathode material by solvothermal reaction. The product obtained corresponds to the product of Comparative Example 3 above (matrix molecular formula is LiFePO4), and includes the following steps:

[0151] (1) Dissolve 1,2-ethylenediol in ethylene glycol to prepare a mixed solvent with a mass percentage concentration of 10% for 1,2-ethylenediol; weigh ferrous sulfate heptahydrate and dissolve it in the above mixed solvent to prepare an iron salt solution with a mass concentration of 0.1 g / mL; weigh lithium hydroxide monohydrate and dissolve it in the above mixed solvent to prepare a lithium source solution with a mass concentration of 0.04 g / mL.

[0152] (2) Add 2.8 mL of 85 wt% phosphoric acid to 90 mL of lithium source solution, stir for 30 min and then add iron salt solution, continue stirring for 30 min to obtain precursor suspension.

[0153] (3) The precursor suspension was placed in a 200℃ oven for a solvothermal reaction for 10 h. After solid-liquid separation, the obtained solid was washed and dried to obtain lithium iron phosphate (LiFePO4).

[0154] (4) Lithium iron phosphate (LiFePO4) and glucose were mixed at a mass ratio of 1:0.07, ball milled with ethanol as a dispersant, then dried by heat and calcined at 600°C for 6 hours under a nitrogen atmosphere. After crushing and sieving, lithium iron phosphate cathode material was obtained.

[0155] Based on the products and preparation methods disclosed herein, and the performance of the products listed in Tables 1-3 above, it can be seen that Examples 1 and 2 have the same chemical composition. However, due to the different preparation methods, the D(010) of Example 1 differs. Within the scope, while Example 2 is not. Within this range, the crystal structure factor of Example 2 is lower than that of Example 1. Similarly, in Examples 3-5, the crystal structure factor values ​​of Examples 3-4 prepared using the method of this disclosure are greater than those of Example 5. This fully demonstrates that the preparation method provided by this disclosure can prepare D(010) in... A lithium iron phosphate cathode material with a wide range and high crystallinity. Among them, D(010) is not... Range but satisfy The lithium iron phosphate cathode material also has good electrochemical performance, so it is not necessary to use the preparation method provided in this disclosure to prepare a product that can solve the technical problem.

[0156] In addition, this disclosure also provides method embodiments 9-16 and method comparative examples 4-5, which perform single-factor variable analysis on a certain parameter of method embodiment 1.

[0157] Specifically, the specific steps of method examples 9-16 and method comparative examples 4-5 are as follows:

[0158] Method Example 9

[0159] This embodiment is basically the same as Method Embodiment 1, except that the mass percentage concentration of 1,2-ethylenediol compound in the aqueous solution of 1,2-ethylenediol in this embodiment is 5 wt%.

[0160] Method Example 10

[0161] This embodiment is basically the same as Method Embodiment 1, except that the mass percentage concentration of 1,2-ethylenediol compound in the aqueous solution of 1,2-ethylenediol in this embodiment is 20 wt%.

[0162] Method Example 11

[0163] This embodiment is basically the same as Method Embodiment 1, except that the ethylene glycol compound in this embodiment is squaric acid.

[0164] Method Example 12

[0165] This embodiment is basically the same as Method Embodiment 1, except that the ethylene glycol compound in this embodiment is sodium crotonate.

[0166] Method Example 13

[0167] This embodiment is basically the same as Method Embodiment 1, except that the pH of the stirring reaction in this embodiment is 1.

[0168] Method Example 14

[0169] This embodiment is basically the same as Method Embodiment 1, except that the pH of the stirring reaction in this embodiment is 2.5.

[0170] Method Example 15

[0171] This embodiment is basically the same as Method Embodiment 1, except that the stirring speed in this embodiment is 300 rpm.

[0172] Method Example 16

[0173] This embodiment is basically the same as Method Embodiment 1, except that the stirring speed in this embodiment is 800 rpm.

[0174] Method Comparison Example 4

[0175] This comparative example is basically the same as Example 1, except that in this comparative example, 1,2-ethylene glycol in Example 1 is replaced with ethylene glycol.

[0176] Method Comparison Example 5

[0177] This comparative example is basically the same as Example 1, except that in this comparative example, lithium carbonate in Example 1 is replaced with lithium hydroxide.

[0178] The performance of the lithium iron phosphate cathode materials prepared by the above methods in Examples 9-16 and Comparative Examples 4-5 was tested. The test results are shown in Tables 5 and 6.

[0179] Table 5. Statistical table of crystal structure test results for different samples

[0180] Table 6. Statistical table of composition and performance test results of different samples

[0181] As can be seen from the table above, Method Examples 9-16 and Method Comparative Examples 4 and 5 are single-factor variable analyses of Method Example 1. The results of Method Examples 9-16 and Method Comparative Examples 4-5 show that the crystal structure of cathode materials with the same chemical composition can be controlled by the preparation method.

[0182] Specifically, in Method Examples 9-12, the amount and type of ethylene glycol compound added were varied. It can be seen that the amount and type of ethylene glycol compound added affect the crystal structure factor of lithium iron phosphate. Specifically, the crystal structure factor initially increases and then decreases with increasing amount of ethylene glycol compound added. Excessive addition leads to an increase in free Fe in the reaction solution. 2+ Reducing the amount of lithium iron phosphate (LFP) reduces its saturation, which restricts its nucleation and thus decreases its crystallinity. If the amount added is too small, it is difficult to achieve crystal face adjustment, which increases the crystal structure factor.

[0183] In Method Examples 13-16, the pH and stirring speed were varied during the stirring reaction. It can be seen that pH and stirring speed affect the crystal structure factor of lithium iron phosphate. Specifically, the crystal structure factor first increases and then decreases with increasing pH, and first increases and then decreases with increasing stirring speed. The pH and stirring speed are closely related to the dissolution rate of lithium carbonate (i.e., the production of carbon dioxide gas). If the carbon dioxide production rate is too fast, it leads to uneven distribution of elements, making nucleation defects more likely and affecting crystallinity. If the carbon dioxide production rate is too slow, it results in lower local supersaturation, inhibiting nucleation.

[0184] In Comparative Example 4, ethylene glycol was used instead of ethylene glycol. The results show that ethylene glycol effectively reduces the D(010) of lithium iron phosphate compared to ethylene glycol, and also improves the crystallinity of lithium iron phosphate to some extent. Therefore, the crystal structure factor of lithium iron phosphate prepared in this example is larger than that of the product obtained in Comparative Example 4. This is because the hydroxyl groups in the ethylene glycol compound are linked through unsaturated carbons, which interact with Fe... 2+ The chelating ability is greatly improved, thus greatly enhancing the control effect on the crystal plane, and slowing down the crystal growth rate is beneficial to improving crystallinity.

[0185] In Comparative Example 5, lithium hydroxide was used as the lithium source, compared to lithium carbonate in Example 1. The carbon dioxide bubbles generated by lithium carbonate in an acidic environment significantly promoted the crystallinity of lithium iron phosphate, while also reducing the D(010) of lithium iron phosphate to some extent. Therefore, the crystal structure factor of lithium iron phosphate prepared in Example 1 was larger than that of the product prepared in Comparative Example 5. This is because the rupture of microbubbles can lead to local supersaturation, promoting primary nucleation of lithium iron phosphate precipitation. Therefore, under the same hydrothermal reaction conditions, the resulting lithium iron phosphate exhibited better crystallinity.

[0186] In summary, this disclosure provides a lithium iron phosphate cathode material whose crystal structure factor A satisfies the following values: Meanwhile, its grain size D(010) satisfies Lithium iron phosphate cathode materials meeting the above-mentioned range exhibit excellent low-temperature electrochemical performance. The crystal structure factor comprehensively evaluates three indicators of lithium iron phosphate cathode materials: crystallinity C, cell volume V, and grain size D(010) of the (010) crystal plane. Among them, higher crystallinity leads to higher lithium-ion diffusion rate and better conductivity; smaller cell volume shortens the lithium-ion insertion / extraction channel, making it easier for lithium ions to jump, which is beneficial to improving the conductivity and cycle performance of the material; smaller D(010) is more conducive to shortening the lithium-ion diffusion channel and improving conductivity. Furthermore, the preparation method of lithium iron phosphate cathode material provided in this disclosure is merely an exemplary method. In this method, lithium carbonate is used as the lithium source, mixed with a ferrous iron source, a phosphorus source, and an ethylene glycol compound, and stirred to obtain a precursor suspension. The stirring reaction is carried out at a relatively low temperature (80–90°C) and a low pH (1–2.5). The phosphorus source reacts with ferrous ions to form an unstable ferrous phosphate precipitate (precipitation and dissolution occur simultaneously), while lithium carbonate reacts with acid to release Li. + The presence of carbon dioxide gas forms microbubbles, and the bursting of these bubbles leads to localized supersaturation, promoting primary nucleation of lithium iron phosphate precipitation, thereby shortening the hydrothermal reaction time and lowering the hydrothermal reaction temperature. Ethylene glycol compounds also act as Fe... 2+As an antioxidant and chelating agent, ethylene glycol compounds can prevent Fe from forming during stirred reactions. 2+ Oxidized to Fe 3+ During the hydrothermal reaction, ethylene glycol compounds affect Fe... 2+ The chelation effect can reduce Fe 2+ The diffusion rate is reduced, inhibiting the orderly arrangement of ions and thus slowing down the crystal growth rate, resulting in lithium iron phosphate crystals with higher crystallinity; on the other hand, the ethylene glycol compound reacts with the Fe arranged on the (010) crystal surface of lithium iron phosphate. 2+ The compounds are adsorbed onto the crystal plane, inhibiting its growth and thus yielding lithium iron phosphate crystals with a (010) crystal plane orientation. Compared to saturated polyols (such as ethylene glycol in traditional methods), ethylene glycol compounds, due to the hydroxyl groups being bonded to unsaturated carbon atoms, are more effective at adsorbing onto Fe. 2+ The chelating effect is stronger, thus having a more significant effect on slowing down crystal growth and greatly improving crystallinity; in addition, since ethylene glycol compounds have lower water solubility than saturated polyols such as ethylene glycol, they tend to chelate with Fe. 2+ Chelation allows for adsorption onto the crystal surface, thereby controlling the crystal morphology. This method enables the preparation of D(010) crystals with... A lithium iron phosphate cathode material with a wide range and high crystallinity. Among them, D(010) is not... Range but satisfy The lithium iron phosphate cathode material also has good electrochemical performance, so it is not necessary to use the preparation method provided in this disclosure to prepare a product that can solve the technical problem.

[0187] The above description is merely an optional embodiment of this disclosure and is not intended to limit this disclosure. Various modifications and variations can be made to this disclosure by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the scope of protection of this disclosure. Industrial applicability

[0188] The lithium iron phosphate cathode material disclosed herein has a crystal structure factor A value that satisfies... Meanwhile, its grain size D(010) satisfies Lithium iron phosphate cathode materials meeting the above-mentioned ranges exhibit excellent low-temperature electrochemical performance. The crystal structure factor comprehensively evaluates three indicators of lithium iron phosphate cathode materials: crystallinity C, cell volume V, and grain size D(010) of the (010) crystal plane. Among these, higher crystallinity C results in fewer internal defects and a higher lithium-ion diffusion rate; a smaller cell volume shortens the lithium-ion insertion / extraction channel, facilitating transitions and improving cycle performance; a smaller D(010) further shortens the lithium-ion diffusion channel, improving charge / discharge specific capacity and rate performance. While crystallinity C, cell volume V, and grain size D(010) each influence the performance of lithium iron phosphate cathode materials, it is practically difficult to obtain cathode materials that simultaneously excel in all three aspects. Therefore, this disclosure provides a crystal structure factor A to comprehensively evaluate the three aspects of lithium iron phosphate cathode materials, possessing the ability to meet the requirements of crystal structure factor A. Simultaneously satisfy Lithium iron phosphate cathode materials exhibit excellent low-temperature electrochemical performance. The method for preparing lithium iron phosphate cathode materials disclosed herein can produce D(010) values ​​within the range... Lithium iron phosphate cathode material with high crystallinity.

Claims

1. A lithium iron phosphate cathode material, characterized in that, It includes a matrix and a carbon coating layer on the surface of the matrix, wherein the crystal structure factor A of the lithium iron phosphate cathode material satisfies The formula for calculating the crystal structure factor A is as follows: Wherein, C refers to the crystallinity of the lithium iron phosphate cathode material, which is measured by the ratio of the XRD diffraction peak intensity of the crystal plane (311) to the integral area of ​​the diffraction peak. V refers to the unit volume of the lithium iron phosphate cathode material. D(010) refers to the grain size of the crystal plane D(010) of the lithium iron phosphate cathode material, in units of... The value satisfies 2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium iron phosphate cathode material satisfies at least one of the following characteristics (1) to (3): Feature (1): The value of D(010) satisfies Feature (2): The value of C satisfies 0.0900≤C≤0.1200; Feature (3): The value of V satisfies 3. The lithium iron phosphate cathode material according to any one of claims 1 to 2, characterized in that, The general formula of the matrix is ​​as follows: Li 1-x A x Fe 1-y M y (PO 4-z )D z Wherein, A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from at least one of F, S, N and Cl; 0≤x≤0.1; 0≤y≤0.1 and 0≤z≤0.

1.

4. The lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, The mass of the carbon coating layer is 0.5% to 5% of the mass of the lithium iron phosphate cathode material.

5. A method for preparing a lithium iron phosphate cathode material as described in any one of claims 1 to 4, characterized in that, It includes: A mixture of lithium carbonate, ferrous iron source, phosphorus source and ethylene glycol compound solution was stirred and reacted to obtain a precursor suspension. The precursor suspension was subjected to a hydrothermal reaction, followed by solid-liquid separation to obtain a solid precursor. The solid precursor and carbon source are mixed and dried, and then calcined under an inert atmosphere to obtain lithium iron phosphate cathode material.

6. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, The preparation of the precursor suspension includes at least one of features (4) to (9): Feature (4): The ferrous iron source is selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate and ferrous acetate; Feature (5): The phosphorus source is selected from at least one of phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, ammonium dihydrogen phosphate and ammonium monohydrogen phosphate; Feature (6): The mass percentage of ethylene glycol compound in the ethylene glycol compound solution is 5% to 20%; Feature (7): The ethylene glycol compound refers to a water-soluble compound containing an ethylene glycol structure, wherein the ethylene glycol compound is selected from at least one of 1,2-ethylene glycol, squaric acid, ketone acid, sodium crotonate, 2,3-dihydroxycyclopent-2-en-1-one, 2,3-dihydroxy-4-(hydroxymethyl)cyclopent-2-en-1-one and 3,4-dihydroxy-5-methyl-2-furanone; Feature (8): The pH of the solution in the stirring reaction is 1 to 2.5, the reaction temperature is 80 to 90°C, the reaction time is 1 to 4 hours, and the stirring speed is 300 to 800 rpm; Feature (9): After the stirring reaction is completed, the temperature is further reduced to 10-25°C and stirred until no bubbles are generated.

7. The method for preparing the lithium iron phosphate cathode material according to any one of claims 5 to 6, characterized in that, In the process of preparing the mixture, a source M is also added, which includes at least one of the features (10) to (13): Feature (10): The divalent iron source and the M source are dissolved in the ethylene glycol compound solution to form a metal salt solution, and the phosphorus source is dissolved in water to form a phosphorus source solution; The lithium carbonate is dispersed in the metal salt solution to obtain a dispersion, and then the phosphorus source solution is added to obtain the mixture, wherein the molar concentration of the metal element in the metal salt solution is 0.8-1.5M, and the molar concentration of the phosphorus source in the phosphorus source solution is 0.85-3M; Feature (11): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of the phosphorus source is 1:1.05 to 1.5; Feature (12): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of lithium in the lithium carbonate is 1:1.01 to 1.1; Feature (13): The M source comprises a water-soluble salt of at least one element selected from Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y.

8. The method for preparing the lithium iron phosphate cathode material according to any one of claims 5 to 7, characterized in that, The hydrothermal reaction conditions during the preparation of the solid precursor are: temperature of 160–200°C, time of 1–3 h, and stirring rate of 100–300 rpm.

9. The method for preparing the lithium iron phosphate cathode material according to any one of claims 5 to 8, characterized in that, The process of calcining the solid precursor and carbon source under an inert atmosphere after mixing and drying includes at least one of features (14) to (17): Feature (14): The mass ratio of the solid precursor to the carbon source is 1:0.04 to 0.1; Feature (15): The carbon source is selected from at least one of glucose, starch, phenolic resin, sucrose, cellulose, polyethylene glycol and citric acid; Feature (16): The mixing process further includes the addition of source A and / or source D, wherein source A is selected from at least one of magnesium fluoride, sodium fluoride, sodium chloride and sodium sulfide; and source D is selected from at least one of potassium fluoride, magnesium fluoride, sodium fluoride, ammonium fluoride, lithium fluoride, melamine, amino acids, trithiocyanate, thiourea, elemental sulfur, sodium sulfide, ammonium sulfide, sodium chloride, ammonium chloride, lithium chloride and urea; Feature (17): The calcination includes calcination at 550-650℃ for 4-8 hours.

10. A lithium-ion battery, characterized in that, It includes lithium iron phosphate cathode materials prepared by any one of claims 1 to 4 or by any one of claims 5 to 9.

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