Lithium iron phosphate cathode material, preparation method, cathode electrode sheet and lithium battery

Abstract

The present invention relates to the technical field of lithium batteries, and discloses a lithium iron phosphate cathode material, a preparation method thereof, a cathode pole piece and a lithium battery. By regulating the crystal structure of the lithium iron phosphate cathode material, a lithium iron phosphate cathode material with a crystal structure degree α satisfying 0.15 nm⁻¹ ≤ α ≤ 4.5 nm⁻¹ is obtained. The reaction activation energy for lithium ion extraction and insertion of this lithium iron phosphate cathode material is relatively low, so the lithium ion diffusion rate is high, and thus excellent electrode reaction kinetics is possessed; the corresponding lithium ion battery has excellent rate performance, low-temperature resistance and low polarization degree.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and more specifically, to a lithium iron phosphate cathode material, a preparation method thereof, a cathode electrode sheet, and a lithium battery. Background Art

[0002] Lithium iron phosphate (LiFePO4, abbreviated as LFP) is an important cathode material for lithium-ion batteries. Due to its excellent safety, long cycle life, and low-cost advantages, it stands out in many application fields. Lithium iron phosphate is formed by the edge-sharing connection of PO4 tetrahedrons, LiO6 octahedrons, and FeO6 octahedrons, which are alternately arranged to form a stable crystal structure to accommodate the repeated insertion and extraction of lithium ions over a long time. However, this stable crystal structure hinders the + transport of Li + ions, causing Li

[0003] ions to tend to insert / extract along a one-dimensional direction within the lithium iron phosphate crystal, and the iron-lithium antisite defects existing in this direction further hinder the transport of lithium ions, resulting in a low lithium ion diffusion rate. Therefore, LiFePO4 is difficult to exhibit excellent electrochemical performance at high current densities. Summary of the Invention

[0004] The purpose of the present application is to provide a lithium iron phosphate cathode material with a high lithium ion diffusion rate and electrode reaction kinetics, so that the battery prepared from this lithium iron phosphate cathode material has excellent rate performance, low-temperature resistance, and low polarization.

[0005] To achieve the above object, the present invention is implemented as follows:

[0006] In a first aspect, the present invention provides a lithium iron phosphate cathode material, including a lithium iron phosphate matrix and a carbon coating layer located on the surface of the lithium iron phosphate matrix.

[0007] The crystallinity of the lithium iron phosphate cathode material ×10000, with the unit of nm -1 , where α satisfies 0.15 nm -1 ≤α≤4.5 nm -1 ;

[0008] In the formula, represents the percentage of iron-lithium antisite defects in the lithium iron phosphate cathode material,

[0009] D （020） represents the grain size of the (020) crystal plane of the lithium iron phosphate cathode material, with the unit of nm,

[0010] I （200）Represents the peak intensity of the (200) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate cathode material.

[0011] I （020） Represents the peak intensity of the (020) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate cathode material.

[0012] In an alternative embodiment, the lithium iron phosphate cathode material satisfies at least one of the following characteristics (1)-(4):

[0013] Characteristic (1): The crystallinity α of the lithium iron phosphate cathode material satisfies 0.5 nm -1 ≤α≤1.1 nm -1 ;

[0014] Characteristic (2): The ≤5%;

[0015] Characteristic (3): The D of the lithium iron phosphate cathode material （020） has a value of 35 nm - 85 nm;

[0016] Characteristic (4): The has a value of 0.25 - 0.45.

[0017] In an alternative embodiment, the structural general formula of the lithium iron phosphate matrix is Li 1-x J x Fe 1-y M y (PO 4-z )Q z ;

[0018] In the formula, J is selected from at least one of Na and Mg;

[0019] 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;

[0020] Q is selected from at least one of F, S, N, and Cl;

[0021] 0≤x≤0.1, 0≤y≤0.1 and 0≤z≤0.1.

[0022] In a second aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material according to any one of the foregoing embodiments, comprising the following steps:

[0023] Mix a crystal form regulator and a metal salt in a macromolecular solution to form a first solution, adjust the pH of the first solution to obtain a gel; soak the gel in a solution of the metal salt to obtain an iron source carrier.

[0024] Mix the iron source carrier, lithium source and phosphorus source in water to obtain a mixture; perform a hydrothermal reaction on the mixture, followed by solid-liquid separation to obtain a lithium iron phosphate precursor;

[0025] Mix the lithium iron phosphate precursor and a carbon source and calcine them under an inert atmosphere to obtain a lithium iron phosphate cathode material.

[0026] In an alternative embodiment, the metal salt includes an iron salt and an M salt, and the molar ratio of iron element in the iron salt to M element in the M salt is 1:0 - 0.11.

[0027] The iron salt is a soluble salt of divalent iron, including at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate and ferrous acetate.

[0028] The M salt is a water-soluble salt of M element, and the M element includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y.

[0029] In an alternative embodiment, the process of preparing the iron source carrier satisfies at least one of the following characteristics (5)-(11):

[0030] Characteristic (5): The percentage content of the macromolecule in the first solution is 1 wt% - 30 wt%.

[0031] Characteristic (6): The mass ratio of the macromolecule, crystal form regulator and metal salt in the first solution is 1:0.05 - 0.3:0.05 - 0.5.

[0032] Characteristic (7): The macromolecule includes at least one of chitosan, carboxymethyl chitosan, tannic acid, gelatin and aminoethyl-β-cyclodextrin, and the crystal form regulator includes at least one of ethylene glycol, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone and diethylene glycol.

[0033] Characteristic (8): Adjusting the pH means adjusting the pH of the first solution to 7.0 - 7.5.

[0034] Characteristic (9): The concentration of divalent iron ions in the solution of the metal salt is 0.1 mol / L - 1.0 mol / L.

[0035] Characteristic (10): The solid-liquid ratio of the gel and the solution of the metal salt is 1 g:100 mL - 500 mL, and the soaking time is 10 min - 30 min.

[0036] Feature (11): The first solution and / or the solution of the metal salt further comprises an antioxidant, the antioxidant comprises ascorbic acid, and the molar ratio of the antioxidant to iron element in the first solution and / or the solution of the metal salt is 0.1 - 0.3:1.

[0037] In an alternative embodiment, the preparation method satisfies at least one of the following features (12)-(15):

[0038] Feature (12): The lithium source comprises at least one of lithium hydroxide, lithium chloride and lithium acetate, and the concentration of the lithium source in the mixture is 0.8 mol / L - 2.5 mol / L;

[0039] Feature (13): The mixture further comprises a doping source, the doping source comprises a metal doping source and / or a non-metal doping source; the metal doping source is a J source, and the J source is a water-soluble compound containing at least one of Na and Mg elements; the non-metal doping source is a Q source, and the Q source is a water-soluble compound containing at least one of F, S, N and Cl elements;

[0040] Feature (14): The pH of the mixture is 2.0 - 6.0;

[0041] Feature (15): The reaction temperature of the hydrothermal reaction is 140°C - 240°C, and the reaction time is 5 h - 20 h.

[0042] In an alternative embodiment, the preparation method satisfies at least one of the following features (16)-(18):

[0043] Feature (16): The carbon source comprises at least one of sucrose, glucose, starch, polyethylene glycol, phenolic resin, cellulose and citric acid, and the mass of the carbon source is 0.1% - 3% of the mass of the lithium iron phosphate precursor;

[0044] Feature (17): The inert atmosphere comprises at least one of nitrogen, helium and argon;

[0045] Feature (18): The calcination temperature is 500°C - 800°C, and the calcination duration is 4 h - 14 h.

[0046] In a third aspect, the present invention provides a positive electrode sheet, comprising the lithium iron phosphate positive electrode material in any one of the foregoing embodiments or the lithium iron phosphate positive electrode material prepared by the preparation method in any one of the foregoing embodiments.

[0047] In a fourth aspect, the present invention provides a lithium battery, comprising the positive electrode sheet in the foregoing embodiment.

[0048] The present invention has the following beneficial effects:

[0049] (1) By optimizing the crystal structure of the lithium iron phosphate cathode material, the present invention enables the reaction activation energy for the extraction and insertion of lithium ions in the lithium iron phosphate cathode material to be relatively low and the lithium diffusion rate to be relatively high, thus having excellent electrode reaction kinetics. Further, the corresponding lithium ion battery has excellent rate performance, low-temperature performance, and low polarization degree.

[0050] (2) By preparing an iron source carrier that slowly releases Fe during the hydrothermal reaction 2+ and a crystal form regulator, on the one hand, the diffusion rate of Fe 2+ is reduced, and the iron-lithium anti-site defects are reduced; on the other hand, the crystal form regulator is slowly released during the growth stage of lithium iron phosphate, inducing the crystal to grow with appropriate preferred orientation, so that the obtained lithium iron phosphate cathode material has both excellent electrode reaction kinetics and a relatively high tap density. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0052] Figure 1 The X-ray diffraction pattern refined for the lithium iron phosphate cathode material provided in Example 6;

[0053] Figure 2 The transmission electron microscope image of the lithium iron phosphate cathode material provided in Example 7;

[0054] Figure 3 The cyclic voltammogram of the lithium iron phosphate cathode material (-15 °C), where (a) is the lithium iron phosphate cathode material provided in Example 6 and (b) is the lithium iron phosphate cathode material provided in Comparative Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0055] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. However, there will be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually identical structures are omitted, which is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art.

[0056] In addition, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present invention, and are not intended to limit the subject matter recited in the claims. For those conditions not specified in the examples, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments whose manufacturers are not indicated, they are all conventional products that can be obtained by commercial purchase.

[0057] In the description of the embodiments of the present application, the terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary and secondary relationship of the indicated technical features. The term "and / or" is only a description of the association relationship of associated objects, indicating that there can be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist simultaneously, and B exists alone these three situations.

[0058] For the sake of simplicity, the present invention specifically discloses some numerical ranges, and various numerical ranges can be combined with each other to form corresponding embodiments. In the present invention, the endpoints and any values of the disclosed ranges are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0059] Unless otherwise specified, the terms used in the present invention have the well-known meanings commonly understood by those skilled in the art.

[0060] Due to the olivine crystal structure of the lithium iron phosphate cathode material, lithium ions can only diffuse along a one-dimensional channel, and the iron-lithium anti-site defects existing on this channel seriously hinder the diffusion of lithium ions, resulting in a low lithium ion diffusion rate and poor electrode reaction kinetics. Therefore, in the art, nanosizing is often used to prepare small-sized lithium iron phosphate particles to shorten the lithium ion diffusion distance, but nanosizing is usually accompanied by an unstable reaction state, which easily increases iron-lithium anti-site defects. Or by inducing the preferred orientation growth of lithium iron phosphate crystals to shorten the lithium ion diffusion channel, but this will make the morphology of lithium iron phosphate present a plate-like or flaky structure, and the tap density is reduced, thereby affecting the discharge specific capacity and energy density.

[0061] Through a large number of experiments and studies, the inventors of the present application have found a technical solution that enables the lithium iron phosphate cathode material to have excellent electrode reaction kinetics and high tap density, making the lithium ion battery have excellent rate performance, low-temperature performance and low polarization degree.

[0062] The present invention provides a lithium iron phosphate cathode material, including a lithium iron phosphate matrix and a carbon coating layer on the surface of the lithium iron phosphate matrix. The crystal structure degree of this lithium iron phosphate cathode material × 10,000, with the unit of nm -1 , where α satisfies 0.15 nm -1 ≤ α ≤ 4.5 nm -1 ;

[0063] In the formula, represents the percentage (%) of the iron-lithium anti-site defects in the lithium iron phosphate cathode material; D （020） represents the grain size of the (020) crystal plane of the lithium iron phosphate cathode material, with the unit of nm; I （200） represents the peak intensity of the (200) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate cathode material; I （020） represents the peak intensity of the (020) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate cathode material.

[0064] The value of the crystal structure degree α of the lithium iron phosphate cathode material of the present invention ranges from 0.15 nm -1 to 4.5 nm -1 , for example, it can be 0.15 nm -1 , 0.5 nm -1 , 1.0 nm -1 , 2.0 nm -1 , 3.0 nm -1 , 4.0 nm -1 , 4.5 nm -1 or any value within the range between any two of them.

[0065] It should be noted that the iron-lithium anti-site defect is an inherent lattice dislocation phenomenon, which means that iron ions occupy the positions that should be occupied by lithium ions. The existence of this defect will reduce the storage points of Li + , seriously hinder or even truncate the diffusion of lithium ions in the lithium iron phosphate cathode material, increase the reaction activation energy for lithium ion deintercalation and intercalation, and cause a significant reduction in electrochemical performance.

[0066] D （020） is the grain size of the lithium iron phosphate cathode material in the (020) crystal plane direction. In the lithium iron phosphate cathode material, since the activation energy required for Li + to transport along the b-axis direction is the lowest, the lithium ions in the lithium iron phosphate cathode material tend to diffuse along the one-dimensional channel in the b-axis direction (perpendicular to the (020) crystal plane direction). The smaller the crystal size in this direction, the shorter the lithium ion diffusion channel distance, which is more conducive to the realization of high-rate performance.

[0067] I （200） and I （020） are respectively the peak intensities of the characteristic diffraction peaks of the (200) crystal plane and the (020) crystal plane in the X-ray diffraction pattern of the lithium iron phosphate cathode material. The ratio reflects the orientation growth mode of the lithium iron phosphate cathode material. The smaller it is, the more it indicates that the lithium iron phosphate cathode material grows along the ac plane direction, that is, the length along the b-axis is shorter. The crystal orientation of lithium iron phosphate is in three directions of the a, b, and c axes. Since Li + has the lowest activation energy required for transmission along the b-axis (perpendicular to the (020) crystal plane direction), the lithium ions in the lithium iron phosphate cathode material tend to diffuse more along the b-axis direction. In this transmission mode, that is, the b-axis is the lithium ion diffusion channel and is perpendicular to the ac plane formed by the a-axis and the c-axis. If the ac plane is larger, it indicates that there are more lithium ion diffusion channels. The peak intensity ratio of the characteristic diffraction peaks of the crystal plane can reflect the relative content of the crystal plane. I （020） is the diffraction peak intensity of the ac crystal plane, and I （200） is the diffraction peak intensity of the bc crystal plane. The smaller it is, the higher the relative content of the ac crystal plane indicates, that is, the crystal orientation of lithium iron phosphate grows preferentially along the ac plane, that is, the lithium ion diffusion channels along the b-axis direction are relatively short and numerous. Effectively shorten the transmission distance of Li + and improve the transmission efficiency.

[0068] In summary, the lithium iron anti-site defect, D （020） , I （200） and I （020） will all affect the lithium ion diffusion. By studying the above parameters, the crystal structure degree is proposed. This crystal structure degree ×10000 comprehensively reflects the quality of the lithium ion diffusion channels of the lithium iron phosphate cathode material. The better the quality, the faster the lithium ion diffusion and the better the electrode reaction kinetics. When the value range of the crystal structure degree is between 0.15 nm -1 -4.5 nm -1 , the lithium iron phosphate cathode material has high-quality lithium ion diffusion channels. Its lithium ion diffusion channels not only have less hindrance, but also have appropriate quantity and length, making the lithium iron phosphate cathode material have a high tap density and at the same time showing excellent electrode reaction kinetics. Further, the corresponding lithium ion battery has excellent rate performance, low-temperature resistance and low polarization degree.

[0069] In some embodiments, the crystal structure degree α satisfies 0.5 nm -1 ≤α≤1.1 nm -1 . For the lithium iron phosphate cathode material with the crystal structure degree α within this range, its electrochemical performance is better.

[0070] In some embodiments, ≤5%.

[0071] In some embodiments, D （020）The value of （020） is too small, which will reduce the tap density of the lithium iron phosphate cathode material, affect the discharge specific capacity, and at the same time reduce the mechanical strength of the lithium iron phosphate cathode material. During repeated charge and discharge processes, the crystal structure is prone to collapse and the cycle performance is reduced.

[0072] In some embodiments, the value of is 0.25 - 0.45. For example, it can be any one of 0.25, 0.30, 0.35, 0.40, and 0.45 or the range value between any two of them. When is too small, it means that the length of the lithium iron phosphate cathode material along the b-axis direction is too short, making the morphology of the lithium iron phosphate cathode material plate-like or flaky, affecting the tap density and compaction density, and thus affecting the specific capacity and energy density. When is too large, it means that the length of the lithium iron phosphate cathode material along the b-axis direction is too long, that is, the lithium ion diffusion channel is too long, affecting the speed of lithium ion diffusion, and thus affecting the rate performance.

[0073] In some embodiments, the structural general formula of the lithium iron phosphate matrix is Li 1-x J x Fe 1-y M y (PO 4-z )Q z ; in the formula, J is selected from at least one of Na and Mg, and J can be any one or several of the above elements; 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, and M can be any one or several of the above elements; Q is selected from at least one of F, S, N, and Cl, and Q can be any one or several of the above elements. The values of the subscripts x, y, and z in the general formula are as follows: 0 ≤ x ≤ 0.1, 0 ≤ y ≤ 0.1, and 0 ≤ z ≤ 0.1. x, y, and z can be equal or unequal and can independently be 0.00, 0.03, 0.05, 0.08, 0.10, etc.

[0074] The present invention provides a preparation method for the above lithium iron phosphate cathode material, but the preparation method is not limited thereto. The preparation method includes the following steps:

[0075] S1. Mix the crystal form regulator and metal salts in a macromolecular solution to form a first solution, adjust the pH of the first solution to obtain a gel; soak the gel in a solution of metal salts to obtain an iron source carrier.

[0076] S2. Mix an iron source carrier, a lithium source, and a phosphorus source in water to obtain a mixture; perform a hydrothermal reaction on the mixture and separate the solid from the liquid to obtain a lithium iron phosphate precursor.

[0077] S3. Mix the lithium iron phosphate precursor and a carbon source and calcine them under an inert atmosphere to obtain a lithium iron phosphate cathode material.

[0078] The present invention prepares an iron source carrier containing a crystal form regulator and Fe 2+ by a gel method. The functional groups of the macromolecule and the crystal form regulator coordinate with metal ions (mainly Fe 2+ ) in the metal salt to form a complex. The macromolecule and / or the crystal form regulator interact with each other through hydrogen bonds, so that the macromolecule, the crystal form regulator, and the metal ions form a gel crosslinked in a physical crosslinking manner. Immerse the gel in a solution of the metal salt to reinforce it and further load metal ions to obtain a metal source carrier. Since the metal source in the metal source carrier is mainly Fe 2+ source, for better expression, the metal source carrier is replaced by the iron source carrier throughout the text of the present invention. A part of Fe 2+ in the iron source carrier coordinates with the functional groups of the macromolecule and the crystal form regulator to become the backbone structure of the gel to ensure the structural stability of the gel; another part of Fe 2+ is loaded into the gel during the immersion reinforcement, and its degree of freedom of movement is greater than that of Fe 2+ in the backbone, forming an iron source slow-release carrier with different release rates of Fe 2+ .

[0079] Mix the iron source carrier, the lithium source, and the phosphorus source in water and perform a hydrothermal reaction to obtain a lithium iron phosphate precursor. In the initial stage of the reaction, the Fe 2+ loaded in the iron source carrier is preferentially released and reacts with the lithium source and the phosphorus source to nucleate and grow under high temperature and high pressure; since the iron source carrier slowly releases Fe 2+ , the diffusion rate of Fe 2+ is slower than that of Li + , and Li + will be introduced into the crystal structure earlier than Fe 2+ to occupy the lithium position, avoiding the generation of iron-lithium inversion. In the later stage of the reaction, as the hydrothermal reaction proceeds, Fe 2+At the beginning of the release, the iron source carrier gradually collapses, and the macromolecule and the crystal form regulator are dispersed into the solvent. The crystal form regulator forms hydrogen bond adsorption with the oxygen atoms on the (020) crystal plane of LiFePO4 in the b-axis direction, inhibiting the growth of this crystal plane along the b-axis direction, and promoting the crystal to grow along the non-inhibited crystal plane direction, thereby inducing the growth of lithium iron phosphate into a crystal with an exposed (020) crystal plane and shortening the lithium ion diffusion channel. Since the crystal form regulator is gradually released only in the later stage of the hydrothermal reaction, at this time the lithium iron phosphate crystal already has a spherical morphology of a certain size. At this stage, crystal form regulation can avoid the appearance of rod-shaped and sheet-shaped crystal morphologies with ultra-short b-axes, enabling the lithium iron phosphate crystal to maintain a spherical-like morphology to ensure good tapped density and compression density.

[0080] In some embodiments, the metal salt includes an iron salt and an M salt. The molar ratio of the iron element in the iron salt to the M element in the M salt is 1:0 - 0.11. The iron salt is a soluble salt of divalent iron, including at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate, and ferrous acetate; the M salt is a water-soluble salt of the M element, and the M element includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y. When the molar ratio of the iron element to the M element is 1:0, it means that the metal salt contains only the iron salt.

[0081] In some embodiments, the process of preparing the iron source carrier is carried out under an inert atmosphere. Preparing the iron source carrier under an inert atmosphere can effectively prevent divalent iron from being oxidized to trivalent iron.

[0082] In some embodiments, the percentage content of the macromolecule in the first solution is 1wt% - 30wt%, for example, it can be any one of 1wt%, 5wt%, 10wt%, 20wt%, and 30wt% or the range value between any two of them.

[0083] In some embodiments, the mass ratio of the macromolecule, the crystal form regulator, and the metal salt in the first solution is 1:0.05 - 0.3:0.05 - 0.5, for example, it can be any one of 1:0.05:0.05, 1:0.05:0.5, 1:0.3:0.05, 1:0.3:0.5, and 1:0.2:0.3 or the range value between any two of them. The mass ratio of the macromolecule, the crystal form regulator, and the metal salt can affect the crosslinking density of the gel, and thus affect the mechanical strength and decomposition rate of the gel.

[0084] In some embodiments, the macromolecule includes at least one of chitosan, carboxymethyl chitosan, tannic acid, gelatin, and aminoethyl-β-cyclodextrin, and the crystal form regulator includes at least one of ethylene glycol, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and diethylene glycol.

[0085] In some embodiments, the pH of the first solution is adjusted to 7.0 - 7.5 to obtain a gel. For example, the pH range can be any one of 7.0, 7.1, 7.2, 7.3, 7.4, and 7.5 or the range values between any two of them. The above macromolecule contains a large number of hydrogen-containing active groups. Adjusting the pH of the first solution to 7.0 - 7.5 is beneficial for the deprotonation of the hydrogen-containing active groups, making it easier to coordinate with metal ions and promoting the formation of a gel through physical cross-linking.

[0086] In some embodiments, the solution of the metal salt is an aqueous mixed solution of metal salts. The concentration of divalent iron ions in the solution of the metal salt is 0.1 mol / L - 1.0 mol / L. For example, it can be any one of 0.1 mol / L, 0.3 mol / L, 0.5 mol / L, 0.7 mol / L, and 1.0 mol / L or the range values between any two of them.

[0087] In some embodiments, the solid-liquid ratio of the gel to the solution of the metal salt is 1 g : 100 mL - 500 mL. For example, it can be any one of 1 g : 100 mL, 1 g : 300 mL, and 1 g : 500 mL or the range values between any two of them; the soaking duration is 10 min - 30 min. For example, it can be any one of 10 min, 20 min, and 30 min or the range values between any two of them.

[0088] In some embodiments, the first solution and / or the solution of the metal salt further includes an antioxidant. The antioxidant includes ascorbic acid. The molar ratio of the antioxidant to the iron element in the first solution and / or the solution of the metal salt is 0.1 - 0.3 : 1. For example, it can be any one of 0.1 : 1, 0.2 : 1, and 0.3 : 1 or the range values between any two of them. Adding an antioxidant can prevent divalent iron from being oxidized to trivalent iron.

[0089] In some embodiments, the lithium source includes at least one of lithium hydroxide, lithium chloride, and lithium acetate. The concentration of the lithium source in the mixture is 0.8 mol / L - 2.5 mol / L. For example, it can be any one of 0.8 mol / L, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, and 2.5 mol / L or the range values between any two of them.

[0090] In some embodiments, the mixture further includes a doping source. The doping source includes a metal doping source and / or a non-metal doping source; the metal doping source is a J source, and the J source is a water-soluble compound containing at least one of Na and Mg elements; the non-metal doping source is a Q source, and the Q source is a water-soluble compound containing at least one of F, S, N, and Cl elements.

[0091] In some embodiments, the pH of the mixture is 2.0 - 6.0, for example, it can be any one of 2.0, 3.0, 4.0, 5.0, and 6.0 or a range value between any two of them. The pH of the mixture affects the slow-release rate of the iron source carrier. The lower the pH of the mixture, the faster the Fe 2+ release rate, the higher the saturation of the hydrothermal reaction system, and the faster the nucleation and growth of lithium iron phosphate crystals. In some cases, the pH value of the hydrothermal reaction system can be regulated by adding common acid regulators such as hydrochloric acid to ensure that it is within the set range.

[0092] In some embodiments, the reaction temperature of the hydrothermal reaction is 140°C - 240°C, for example, it can be any one of 140°C, 160°C, 180°C, 200°C, and 240°C or a range value between any two of them; the reaction time is 5h - 20h, for example, it can be any one of 5h, 10h, 15h, and 20h or a range value between any two of them.

[0093] In some embodiments, the carbon source includes at least one of sucrose, glucose, starch, polyethylene glycol, phenolic resin, cellulose, and citric acid. The mass of the carbon source is 0.1% - 3% of the mass of the lithium iron phosphate precursor, for example, it can be any one of 0.1%, 0.5%, 1.0%, 1.5%, 2%, and 3% or a range value between any two of them. Adding a carbon source during the calcination process can form a carbon coating layer and improve the conductivity of the lithium iron phosphate cathode material.

[0094] In some embodiments, the inert atmosphere includes at least one of nitrogen, helium, and argon.

[0095] In some embodiments, the calcination temperature is 500°C - 800°C, for example, it can be any one of 500°C, 600°C, 700°C, and 800°C or a range value between any two of them; the calcination duration is 4h - 14h, for example, it can be any one of 4h, 8h, 10h, 12h, and 14h or a range value between any two of them.

[0096] The embodiment of the present invention also provides a positive electrode sheet, which includes the above lithium iron phosphate cathode material, and may further include a positive electrode current collector. A positive electrode active coating is formed on at least one surface of the positive electrode current collector, and the lithium iron phosphate cathode material exists as a positive electrode active material in the positive electrode active coating.

[0097] The embodiment of the present invention also provides a lithium battery, which includes the above positive electrode sheet, and may further include a negative electrode sheet, an electrolyte, a separator, etc. to form a complete battery structure, having good electrochemical performance.

[0098] Specifically, there are no specific limitations on the types of the negative electrode sheet, electrolyte, and separator. The lithium battery can be in the form of a secondary battery. During the charge and discharge process of the secondary battery, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet.

[0099] In other embodiments, it may not be in the form of a secondary battery, and forms such as a battery module or a battery pack can be adopted.

[0100] An embodiment of the present invention provides a device, including the above-mentioned secondary battery, battery module, or battery pack. The secondary battery, battery module, or battery pack can be used as the power source of the device or as the energy storage unit of the device. The device can be, but is not limited to, mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc.

[0101] The features and performance of the present invention will be further described in detail below in conjunction with embodiments.

[0102] Embodiment 1

[0103] This embodiment provides a lithium iron phosphate positive electrode material, and the preparation method includes the following steps: [[ID=;16]]

[0104] S1. Under a nitrogen atmosphere, polyethylene glycol and ferrous sulfate are heated and mixed in an acetic acid aqueous solution of chitosan at 45 °C to form a first solution. The pH of the first solution is adjusted to 7.5, and then left standing to obtain a gel; the gel is soaked in a 0.3 mol / L ferrous sulfate solution at a solid-liquid ratio of 1 g: 200 mL for 10 min to obtain an iron source carrier.

[0105] Among them, the concentration of acetic acid in the acetic acid aqueous solution is 1.5 wt%, and the concentration of chitosan in the first solution is 2 wt%; the mass ratio of chitosan, polyethylene glycol, and ferrous sulfate in the first solution is 1: 0.2: 0.3; the first solution and the ferrous sulfate solution contain ascorbic acid, and the molar ratio of ascorbic acid to iron elements in the first solution and the ferrous sulfate solution is both 0.2: 1.

[0106] S2. The iron source carrier, lithium hydroxide, and phosphoric acid are mixed in water, the pH is adjusted to 3.5 to obtain a mixture, and the mixture is reacted in a reaction kettle at 170 °C for 15 h. After filtration, washing, and drying, a lithium iron phosphate precursor is obtained.

[0107] Among them, the molar ratio of P: Li: Fe in the mixture is 1: 1.3: 1, and the concentration of lithium hydroxide in the mixture is 1.0 mol / L.

[0108] S3. After ball-milling and mixing the lithium iron phosphate precursor and glucose and spray-drying them, calcine at 680 °C for 9 h under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.

[0109] Among them, the mass of glucose is 2.2% of the mass of the lithium iron phosphate precursor.

[0110] Example 2

[0111] This example provides a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0112] S1. Under a nitrogen atmosphere, mix polyvinyl alcohol and metal salts in an aqueous solution of gelatin to form a first solution, adjust the pH of the first solution to 7.2, and let it stand to obtain a gel; soak the gel in a metal salt solution at a solid-liquid ratio of 1 g: 500 mL for 30 min to obtain an iron source carrier.

[0113] Among them, the metal salt is a mixture of ferrous chloride and cobalt acetate, and the molar ratio of iron element to cobalt element in the mixture is 1: 0.08; the concentration of gelatin in the first solution is 20 wt%, and the mass ratio of gelatin, polyvinyl alcohol and metal salt in the first solution is 1: 0.1: 0.5; the concentration of divalent iron ions in the metal salt solution is 0.1 mol / L; both the first solution and the metal salt solution contain ascorbic acid, and the molar ratio of ascorbic acid to iron element in the first solution and the metal salt solution is 0.1: 1.

[0114] S2. Mix the iron source carrier, lithium acetate and phosphoric acid in water, adjust the pH to 2.2 to obtain a mixture, place the mixture in a reaction kettle, react at 200 °C for 12 h, filter, wash and dry to obtain the lithium iron phosphate precursor.

[0115] Among them, the molar ratio of P: Li: Fe + Co in the mixture is 1: 1.3: 1, and the concentration of lithium acetate in the mixture is 2.3 mol / L.

[0116] S3. After ball-milling and mixing the lithium iron phosphate precursor and citric acid and spray-drying them, calcine at 770 °C for 5 h under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.

[0117] Among them, the mass of citric acid is 1.5% of the mass of the lithium iron phosphate precursor.

[0118] Example 3

[0119] This example provides a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0120] S1. Mix polyethylene glycol and ferrous chloride in an aqueous solution of tannic acid under a nitrogen atmosphere to form a first solution. Adjust the pH of the first solution to 7.0 and let it stand to obtain a gel. Immerse the gel in a 1 mol / L ferrous chloride solution at a solid-liquid ratio of 1 g: 350 mL for 15 min to obtain an iron source carrier.

[0121] Among them, the concentration of tannic acid in the first solution is 28 wt%, and the mass ratio of tannic acid, polyethylene glycol, and ferrous chloride in the first solution is 1: 0.26: 0.1; both the first solution and the ferrous chloride solution contain ascorbic acid, and the molar ratio of ascorbic acid to iron in the first solution and the ferrous chloride solution is 0.3: 1.

[0122] S2. Mix the iron source carrier, lithium chloride, and phosphoric acid in water, adjust the pH to 5.8 to obtain a mixture, react the mixture in a reaction kettle at 230 °C for 7 h, and after filtration, washing, and drying, obtain a lithium iron phosphate precursor.

[0123] Among them, the molar ratio of P: Li: Fe in the mixture is 1: 1.3: 1, and the concentration of lithium chloride in the mixture is 0.8 mol / L.

[0124] S3. After milling and mixing the lithium iron phosphate precursor and sucrose and spray-drying, calcine at 530 °C for 13 h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material.

[0125] Among them, the mass of sucrose is 0.3% of the mass of the lithium iron phosphate precursor.

[0126] Example 4

[0127] This example provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the concentration of chitosan in the first solution is 1.5 wt%, and other conditions are exactly the same as those in Example 1.

[0128] Example 5

[0129] This example provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the concentration of chitosan in the first solution is 2.8 wt%, and other conditions are exactly the same as those in Example 1.

[0130] Example 6

[0131] This example provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the mass ratio of chitosan, polyethylene glycol, and ferrous sulfate in the first solution is 1: 0.09: 0.3, and other conditions are exactly the same as those in Example 1.

[0132] Example 7

[0133] This embodiment provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the mass ratio of chitosan, polyethylene glycol, and ferrous sulfate in the first solution is 1:0.25:0.3, and other conditions are exactly the same as those in Embodiment 1.

[0134] Example 8

[0135] This embodiment provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the mass ratio of chitosan, polyethylene glycol, and ferrous sulfate in the first solution is 1:0.2:0.15, and other conditions are exactly the same as those in Embodiment 1.

[0136] Example 9

[0137] This embodiment provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the mass ratio of chitosan, polyethylene glycol, and ferrous sulfate in the first solution is 1:0.2:0.45, and other conditions are exactly the same as those in Embodiment 1.

[0138] Example 10

[0139] This embodiment provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the concentration of the ferrous sulfate solution is 0.5 mol / L, and other conditions are exactly the same as those in Embodiment 1.

[0140] Example 11

[0141] This embodiment provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the concentration of the ferrous sulfate solution is 0.8 mol / L, and other conditions are exactly the same as those in Embodiment 1.

[0142] Example 12

[0143] This embodiment provides a lithium iron phosphate cathode material. In step S2 of the preparation method, an iron source carrier, lithium hydroxide, and phosphoric acid are mixed in water, and the pH is adjusted to 2.8 to obtain a mixture, and other conditions are exactly the same as those in Embodiment 1.

[0144] Example 13

[0145] This embodiment provides a lithium iron phosphate cathode material. In step S2 of the preparation method, an iron source carrier, lithium hydroxide, and phosphoric acid are mixed in water, and the pH is adjusted to 4.5 to obtain a mixture, and other conditions are exactly the same as those in Embodiment 1.

[0146] Example 14

[0147] This embodiment provides a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0148] S1. Under a nitrogen atmosphere, polyethylene glycol and ferrous sulfate are heated and mixed in an aqueous acetic acid solution of chitosan at 45 °C to form a first solution. The pH of the first solution is adjusted to 7.5, and it is left standing to obtain an iron source carrier.

[0149] Among them, the concentration of acetic acid in the aqueous acetic acid solution is 1.5 wt%, and the concentration of chitosan in the first solution is 2 wt%; the mass ratio of chitosan, polyethylene glycol, and ferrous sulfate in the first solution is 1:0.2:0.3; the first solution contains ascorbic acid, and the molar ratio of ascorbic acid to iron element in the first solution is 0.2:1.

[0150] S2. The iron source carrier, lithium hydroxide, lithium fluoride, and phosphoric acid are mixed in water, and the pH is adjusted to 4.5 to obtain a mixture. The mixture is reacted in a reaction kettle at 180 °C for 16 h, and after filtration, washing, and drying, a lithium iron phosphate precursor is obtained.

[0151] Among them, the molar ratio of P:Li:Fe:F in the mixture is 1:1.3:1:0.01, and the concentration of lithium hydroxide in the mixture is 1.0 mol / L.

[0152] S3. After the lithium iron phosphate precursor and glucose are ground and mixed and spray-dried, they are calcined at 650 °C for 7 h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material.

[0153] Among them, the mass of glucose is 2.2% of the mass of the lithium iron phosphate precursor.

[0154] Comparative Example 1

[0155] This comparative example provides a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0156] S1. Ferrous sulfate, lithium hydroxide, and phosphoric acid are dissolved in water to obtain a mixed solution. The mixed solution is reacted in a reaction kettle at 170 °C for 15 h, and after filtration, washing, and drying, a lithium iron phosphate precursor is obtained.

[0157] Among them, the molar ratio of P:Li:Fe in the mixed solution is 1:1.3:1; the concentration of lithium hydroxide in the mixed solution is 1.0 mol / L; the mixed solution also includes ascorbic acid, and the molar ratio of ascorbic acid to iron element in the mixed solution is 0.2:1.

[0158] S2. After the lithium iron phosphate precursor and glucose are ground and mixed and spray-dried, they are calcined at 680 °C for 9 h under a nitrogen atmosphere to obtain a lithium iron phosphate cathode material.

[0159] Among them, the mass of glucose is 2.2% of the mass of the lithium iron phosphate precursor.

[0160] Comparative Example 2

[0161] This comparative example provides a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0162] S1. Dissolve ferrous sulfate, lithium hydroxide, polyethylene glycol and phosphoric acid in water to obtain a mixed solution. React the mixed solution in a reaction kettle at 170 °C for 15 h, and then filter, wash and dry to obtain a lithium iron phosphate precursor.

[0163] Among them, the molar ratio of P:Li:Fe in the mixed solution is 1:1.3:1; the concentration of lithium hydroxide in the mixed solution is 1.0 mol / L; the mass ratio of polyethylene glycol to ferrous sulfate is 0.2:0.5. The mixed solution also includes ascorbic acid, and the molar ratio of ascorbic acid to iron element in the mixed solution is 0.2:1.

[0164] S2. Grind and mix the lithium iron phosphate precursor and glucose, spray dry, and then calcine at 680 °C for 9 h in a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.

[0165] Among them, the mass of glucose is 2.2% of the mass of the lithium iron phosphate precursor.

[0166] Comparative Example 3

[0167] This comparative example provides a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0168] S1. Under a nitrogen atmosphere, heat and mix ferrous sulfate in an acetic acid aqueous solution of chitosan at 45 °C to form a first solution. Adjust the pH of the first solution to 7.5 and let it stand to obtain a gel. Immerse the gel in a 0.3 mol / L ferrous sulfate solution at a solid-liquid ratio of 1 g:200 mL for 10 min to obtain an iron source carrier.

[0169] Among them, the concentration of acetic acid in the acetic acid aqueous solution is 1.5 wt%, and the concentration of chitosan in the first solution is 2 wt%; the mass ratio of chitosan to ferrous sulfate in the first solution is 1:0.3; the first solution and the ferrous sulfate solution contain ascorbic acid, and the molar ratio of ascorbic acid to iron element in the first solution and the ferrous sulfate solution is 0.2:1.

[0170] S2. Mix the iron source carrier, lithium hydroxide, phosphoric acid and polyethylene glycol in water, adjust the pH to 4.5 to obtain a mixture. React the mixture in a reaction kettle at 170 °C for 15 h, and then filter, wash and dry to obtain a lithium iron phosphate precursor.

[0171] Among them, the molar ratio of P:Li:Fe in the mixture is 1:1.3:1, the concentration of lithium hydroxide in the mixture is 1.0 mol / L; the mass ratio of polyethylene glycol to ferrous sulfate is 0.2:0.5.

[0172] S3. After ball-milling and mixing the lithium iron phosphate precursor and glucose and spray-drying, it is calcined at 680 °C for 9 h in a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.

[0173] Among them, the mass of glucose is 2.2% of the mass of the lithium iron phosphate precursor.

[0174] Comparative Example 4

[0175] This comparative example provides a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0176] S1. Under a nitrogen atmosphere, chitosan and polyethylene glycol are mixed and dissolved in water to obtain a first solution, and the pH of the first solution is adjusted to 7.5 to obtain a gel.

[0177] Among them, the concentration of chitosan in the first solution is 2 wt%, and the mass ratio of chitosan to polyethylene glycol is 1:0.2.

[0178] S2. The gel, ferrous sulfate, lithium hydroxide, and phosphoric acid are mixed in water, the pH is adjusted to 4.5 to obtain a mixture, and the mixture is reacted at 160 °C for 15 h in a reaction kettle, and after filtration, washing, and drying, a lithium iron phosphate precursor is obtained.

[0179] Among them, the molar ratio of P:Li:Fe in the mixture is 1:1.3:1, the concentration of lithium hydroxide in the mixture is 1.0 mol / L, and the mass ratio of polyethylene glycol to ferrous sulfate in the mixture is 0.2:0.5.

[0180] S3. After ball-milling and mixing the lithium iron phosphate precursor and glucose and spray-drying, it is calcined at 680 °C for 9 h in a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.

[0181] Among them, the mass of glucose is 2.2% of the mass of the lithium iron phosphate precursor.

[0182] The present invention tests the crystal structure degree and related parameters of the lithium iron phosphate cathode materials provided in Examples 1-14 and Comparative Examples 1-4 (as shown in Table 1), and simultaneously measures their physical properties and the electrochemical properties of the corresponding lithium batteries (as shown in Tables 1 and 2). The specific steps are as follows:

[0183] (1) X-ray diffraction (XRD) test

[0184] Use an X-ray powder diffractometer (Rigku Ultima IV) to test the crystal phase and crystal structure of the lithium iron phosphate cathode material to obtain an XRD pattern. The diffraction source used for the test is Cu-Ka, the wavelength λ = 0.15406 nm, the step size is 0.02°, and the scanning speed is 2° / min. Use GSAS-II to perform crystal structure refinement on the obtained XRD pattern to obtain an X-ray diffraction pattern, and the R of the refinement result wp(Weighted graph variance factor) ≤ 5%.

[0185] The percentage of iron-lithium anti-site defects was obtained by analysis and calculation using GSAS-II software ( ), the diffraction angles ( ), full width at half maximum ( ), and peak intensities of the (200) and (020) characteristic diffraction peaks. After subtracting the peak intensity of the background diffraction peak from the peak intensity of the (200) and (020) characteristic diffraction peaks, the peak intensities of the actual characteristic diffraction peaks were I （200） and I （020） . The Scherrer formula was used to calculate the grain size in the crystal plane direction of the (020) characteristic diffraction peak. The Scherrer formula ; k is a constant value, taking 0.9; refers to the wavelength of the incident ray, λ = 0.15406 nm; is the full width at half maximum of the characteristic diffraction peak, with the unit of rad; is the diffraction angle, with the unit of °. The test results of Example 6 are as Figure 1 shown.

[0186] (2) Transmission electron microscope (TEM) test

[0187] The morphology of the lithium iron phosphate cathode material was analyzed using a JEM-2000EX TEM. The test results of Example 7 are as Figure 2 shown. It can be seen from Figure 2 that the lithium iron phosphate cathode material prepared in Example 7 has a spherical-like morphology and is coated with a carbon layer on the surface. The interplanar spacing at a higher resolution indicates that the lithium iron phosphate crystal grows along the ac plane direction.

[0188] (3) Tap density test

[0189] The tap density of the lithium iron phosphate cathode material was tested according to the test standard GB / T 21354-2008.

[0190] (4) Electrochemical performance test

[0191] The lithium iron phosphate cathode material obtained above was formulated into a button cell for electrochemical performance testing. The specific steps are as follows: The lithium iron phosphate cathode material, acetylene black, and polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 90:5:5, and then coated on an aluminum foil and placed in a vacuum drying oven for drying. After drying, it was pressed into a positive electrode sheet with a diameter of 12 mm using a tablet press in an argon glove box. In the glove box, a button cell was composed of the positive electrode sheet, a polypropylene porous separator, a negative lithium sheet, and an electrolyte. The electrolyte was 1 M LiPF6-EC:DMC (volume ratio 1:1), and the electrochemical performance was tested.

[0192] (4-1)Cyclic Voltammetry Test

[0193] The CR2032 coin cell was tested by cyclic voltammetry using a CHI660E electrochemical workstation to investigate the reaction kinetics of the lithium iron phosphate electrode material. The test voltage window was 2.4V - 4.2V, and the test temperature was -15°C.

[0194] Within the tested voltage range, lithium ions in the electrode material began to intercalate and deintercalate due to redox reactions, manifested as a pair of redox peaks, and a cyclic voltammetry (CV) curve was obtained. The peak positions and peak potential values of the redox peaks in the cyclic voltammetry (CV) curve can reflect the phase change and ion diffusion processes in the lithium iron phosphate electrode material. The potential difference between the oxidation peak and the reduction peak can reflect the polarization degree of the electrode and the lithium ion diffusion degree. The smaller the potential difference between the oxidation peak and the reduction peak, the lower the polarization degree of the electrode and the higher the lithium ion diffusion ability, and the better the electrode reaction kinetics. Further, calculating the growth rate of the potential difference between the oxidation peak and the reduction peak at different scanning rates can reflect whether the diffusion and transfer of lithium ions are rapid and whether the intercalation and deintercalation are sufficient during high-rate charge and discharge processes. The smaller the growth rate of the potential difference, the more stable the lithium ion diffusion ability during charge and discharge at different rates, without significant attenuation; further, the polarization degree will not increase significantly, which directly affects the low-temperature performance and rate performance of lithium-ion batteries.

[0195] The cyclic voltammetry curves of Example 6 and Comparative Example 2 are as Figure 3 shown. As the scanning rate continuously increases, the cyclic voltammetry curve of Comparative Example 2 gradually deforms, and the potential difference between the oxidation peak and the reduction peak increases significantly, while the shape of the cyclic voltammetry curve of Example 6 remains stable.

[0196] Test method for the growth rate T of the potential difference between the oxidation and reduction peaks: The potential differences between the oxidation and reduction peaks of the lithium iron phosphate electrode material were measured successively at different scanning rates L (0.2 mV / s, 0.6 mV / s, 1.0 mV / s), and the growth rate T of the potential difference between the oxidation and reduction peaks was calculated using the following formula.

[0197] T1 and T2 . Among them, L 0.2 represents the potential difference between the oxidation and reduction peaks of the lithium iron phosphate electrode material at a scanning rate of 0.2 mV / s, and L 0.6 represents the potential difference between the oxidation and reduction peaks of the lithium iron phosphate electrode material at a scanning rate of 0.6 mV / s, and L 1.0 represents the potential difference between the oxidation and reduction peaks of the lithium iron phosphate electrode material at a scanning rate of 1.0 mV / s.

[0198] (4-2)Constant Current Charge and Discharge Test

[0199] The coin-type battery was tested for constant current charge and discharge using a LAND battery test system to investigate the rate performance and capacity retention rate of the lithium iron phosphate cathode material and other electrochemical properties. The test voltage was 2.0V - 3.9V. For 1 gram of lithium iron phosphate cathode material, the 1C charge and discharge rate was 170 mA.

[0200] Among them, the discharge specific capacity at different rates was tested at room temperature of 25°C; the low-temperature capacity retention rate was tested at 1C, and the calculation formula was ×100%.

[0201] Taking Example 6 as an example to illustrate the calculation process of the crystal structure degree and related parameters, as Figure 1 shown, Y obs is the observed data of XRD, Y calc is the calculated data after refining the observed data, Y diff is the difference curve between the observed data and the calculated data, GOF is the goodness-of-fit factor, and R wp is the refined weighted profile variance factor. Based on the calculated data after refinement, the present invention calculates that Fe Li is 1.53%, I （200） is 7323.83 and I （020） is 18579.56. The diffraction angle of the (020) characteristic diffraction peak is 14.8642°, the full width at half maximum is 0.00227 rad, and according to the Scherrer formula, D （020） is 62.9 nm.

[0202] Table 1 Crystal structure degree α and related parameters,

[0203] Test results of tap density

[0204]

[0205] Table 2 Test results of electrochemical performance of different examples and comparative examples

[0206] Combining Table 1 and Table 2, when the value of the crystal structure degree α is in the range of 0.15 nm -1 - 4.5 nm -1 the lithium iron phosphate cathode material has good electrode reaction kinetics, that is, the reaction activation energy required for lithium ion extraction and insertion is low, and the lithium ion diffusion rate is fast. Therefore, at low temperature (-15°C) and different scanning rates, it has a low growth rate of the redox peak potential difference, and the corresponding lithium ion battery has excellent rate performance and low temperature resistance. Furthermore, when the value of the crystal structure degree α is in the range of 0.5 nm -1 - 1.1 nm -1When the temperature is low (-15°C), the growth rate of the potential difference between the oxidation and reduction peaks is smaller, and the rate performance and low-temperature performance of the corresponding lithium-ion battery are better.

[0207] Comparing Examples 2, 4, and 10, D （020） and are not very different, but the Fe in Example 10 Li (1.95%) is the highest, and those in Example 2 and Example 4 are 1.63% and 1.1% respectively. When D （020） is approximate, the lower the Fe Li means the smaller the hindrance to the lithium-ion diffusion channels, and vice versa. Among Examples 2, 4, and 10, the Fe in Example 4 Li is the smallest, with α being 0.6933 nm -1 , indicating better lithium-ion diffusion. At the 5C rate and low temperature, lithium ions can also be fully and rapidly transported, thus having excellent rate performance (5C / 1C: 86.22%), low-temperature capacity retention rate, and a lower growth rate of the potential difference between the oxidation and reduction peaks (low polarization degree). It can be seen from the experimental data that as Fe Li increases, α increases accordingly, and both the rate performance and low-temperature performance of the lithium-ion battery decrease, and the polarization degree increases.

[0208] Comparing Examples 3, 5, and 11, Fe Li and are not very different. The D in Example 11 （020） is the smallest, being 47.1 nm, and those in Example 3 and Example 5 are 70.4 nm and 56.5 nm respectively. When Fe Li and are approximate, as D （020） increases, α decreases, and the discharge specific capacity of the lithium iron phosphate cathode material at 0.1C increases in turn. This is because D （020） affects the grain size of the material. The larger D （020） is, the larger the grain size of the material, thus improving the tap density of the material and being beneficial to increasing the specific capacity. However, with the increase of D （020） , the lithium-ion diffusion channels become longer, resulting in the rate performance and low-temperature capacity retention rate of Example 3 being less than those of Example 5, and the growth rate of the potential difference between the oxidation and reduction peaks being higher than that of Example 5. Although the D in Example 11 （020） is the shortest, but it has a similar Fe Li , making the hindrance degree per unit D （020） size larger, resulting in Example 11 and Example 5 having similar rate performance, low-temperature performance, and polarization degree.

[0209] Comparing Examples 6, 12, and 14, Fe Li and D （020）Not much difference indicates that their lithium-ion diffusion channels not only have similar lengths but also the same degree of obstruction. They are significantly different. The smaller it is, the higher the degree of preferred orientation. The morphology of the lithium iron phosphate cathode material tends to be plate-like or even flaky, resulting in a decrease in its tap density. Therefore, the discharge specific capacity decreases, but the low-temperature resistance, rate performance, and polarization degree have little difference.

[0210] For Comparative Example 2's and D （020） are both within the range set by the present invention. However, for Comparative Example 2, the Fe Li is relatively large, making the crystal structure degree α value too large at 5.2367 nm -1 . Although Comparative Example 2 has a shorter lithium-ion diffusion channel (42.1 nm), the large Fe Li increases the degree of obstruction of the lithium-ion diffusion channel, thereby affecting the electrochemical performance of the lithium-ion battery.

[0211] The crystal structure degree α value of Comparative Example 3 is 0.1460 nm -1 is not within the range set by the present invention. It can be seen from the table that Comparative Example 3 has a lower Fe Li (0.62%) and a short lithium-ion diffusion channel (64.8 nm), but the too low makes the lithium iron phosphate cathode material present a flaky morphology, resulting in the lowest tap density of the material and thus the lowest discharge specific capacity at 0.1C.

[0212] Although the Fe Li , D （020） and of Comparative Example 4 are all within the range set by the present invention, but the crystal structure degree α value is 4.6139 nm -1 does not meet the range set by the invention and cannot achieve excellent rate performance, low-temperature resistance, and polarization degree either.

[0213] Adjusting the reaction parameters of steps S1 and S2 can regulate the crystal structure of the lithium iron phosphate cathode material, and thus regulate the crystal structure degree , so as to affect the physical and electrochemical properties of the lithium iron phosphate cathode material. Specifically, preparing iron source carriers with different slow-release properties in step S1 can affect the crystal structure of the lithium iron phosphate cathode material. The content of any one of the macromolecules, crystal form regulators, and metal salts in the first solution will affect the cross-linking state of the gel. The more the content, the greater the cross-linking density in the gel, and the network structure of the gel will be more compact and firm, and the mechanical strength will increase. The mechanical strength will affect the slow-release effect of the iron source carrier. The higher the mechanical strength, the slower the release of Fe 2+ and the crystal form regulator during the hydrothermal reaction, the less, and at the same time The higher; the lower the mechanical strength, Fe 2+ and the crystal form regulator are released faster in the hydrothermal reaction, the more, and at the same time the lower.

[0214] The content of the crystal form regulator in the iron source carrier will also affect the morphology of the lithium iron phosphate cathode material. The higher the content of the crystal form regulator, the easier it is for the lithium iron phosphate cathode material to preferentially orient, and the morphology tends to be plate-like or even flaky, resulting in a lower tap density of the lithium iron phosphate cathode material, thereby reducing the discharge specific capacity of the corresponding lithium-ion battery. In addition, the solution concentration of the metal salt will also affect the release effect of the iron source carrier. The gel is immersed in the solution of the metal salt for loading. The greater the solution concentration of the metal salt, the more Fe 2+ is loaded per unit volume of the gel, and the more Fe 2+ is released per unit time in the hydrothermal reaction, the higher the local ion concentration, the faster the reaction rate, the crystal nucleation rate is greater than the growth rate, D （020） the grain size becomes smaller, increases.

[0215] In step S2, the pH of the hydrothermal reaction system will affect the release of the iron source carrier, and thus affect the crystal structure of the lithium iron phosphate cathode material. When the pH of the hydrothermal reaction system is relatively low, the iron source carrier dissolves faster, making the relatively high. When the pH of the hydrothermal reaction system is relatively high, the iron source carrier dissolves slower, making the relatively low; at the same time, the ion reaction rate is slower, and the crystal is more inclined to grow, D （020） the grain size becomes larger. Due to the slower dissolution of the iron source carrier, there is a large speed difference in the formation of lithium iron phosphate particles, and a natural grading effect is generated, making the lithium iron phosphate cathode material have a high tap density and a high discharge specific capacity.

[0216] It should be noted that this application is not limited to the above embodiments. The above embodiments are only illustrative, and embodiments with the same composition and the same function and effect as the technical idea within the technical solution scope of this application are all included in the technical scope of this application. In addition, within the scope of not departing from the gist of this application, various deformations that can be thought of by those skilled in the art are applied to the embodiments, and other ways constructed by combining some constituent elements in the embodiments are also included in the scope of this application.

Claims

1. A lithium iron phosphate cathode material, characterized in that, It includes a lithium iron phosphate matrix and a carbon coating layer located on the surface of the lithium iron phosphate matrix; the crystallinity of the lithium iron phosphate cathode material × 10000, with the unit of nm -1 , α satisfies 0.15 nm -1 ≤ α ≤ 4.5 nm -1 , In the formula, represents the percentage of iron-lithium anti-site defects in the lithium iron phosphate cathode material, D （020） represents the grain size of the (020) crystal plane of the lithium iron phosphate cathode material, with the unit of nm I （200） represents the peak intensity of the (200) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate cathode material I （020） represents the peak intensity of the (020) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate cathode material; The ≤ 5%, The D of the lithium iron phosphate cathode material （020） has a value of 35 nm to 85 nm, The value ranges from 0.25 to 0.45; The structural general formula of the lithium iron phosphate matrix is Li 1-x J x Fe 1-y M y (PO 4-z )Q z ; In the formula, J 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; Q 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.

2. The lithium iron phosphate cathode material according to claim 1, wherein The lithium iron phosphate cathode material satisfies the following characteristics: Feature (1): The crystallinity α of the lithium iron phosphate cathode material satisfies 0.5 nm -1 ≤ α ≤ 1.1 nm -1 .

3. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1-2, characterized in that, It includes the following steps: Mix a crystal form regulator and metal salts in a macromolecular solution to form a first solution, adjust the pH of the first solution to obtain a gel; soak the gel in a solution of the metal salts to obtain an iron source carrier; the macromolecule includes at least one of chitosan, carboxymethyl chitosan, tannic acid, gelatin and aminoethyl-β-cyclodextrin; the metal salts include iron salts and M salts; Mix the iron source carrier, a lithium source and a phosphorus source in water to obtain a mixture; carry out a hydrothermal reaction on the mixture, and perform solid-liquid separation to obtain a lithium iron phosphate precursor; Mix the lithium iron phosphate precursor and a carbon source and calcine them under an inert atmosphere to obtain the lithium iron phosphate cathode material.

4. The preparation method according to claim 3, wherein The molar ratio of iron element in the iron salt to M element in the M salt is 1:0 - 0.11, The iron salt is a soluble salt of divalent iron, including at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate and ferrous acetate, The M salt is a water-soluble salt of M element, and the M element includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y.

5. The preparation method according to any one of claims 3-4, characterized in that, The process of preparing the iron source carrier satisfies at least one of the following characteristics (5)-(11): Characteristic (5): The percentage content of the macromolecule in the first solution is 1wt% - 30wt%; Characteristic (6): The mass ratio of the macromolecule, the crystal form regulator and the metal salts in the first solution is 1:0.05 - 0.3:0.05 - 0.5; Characteristic (7): The crystal form regulator includes at least one of ethylene glycol, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone and diethylene glycol; Characteristic (8): Adjusting the pH means adjusting the pH of the first solution to 7.0 - 7.5; Characteristic (9): The concentration of divalent iron ions in the solution of the metal salts is 0.1mol / L - 1.0mol / L; Characteristic (10): The solid-liquid ratio of the gel and the solution of the metal salts is 1g:100mL - 500mL, and the soaking time is 10min - 30min; Characteristic (11): The first solution and / or the solution of the metal salts further includes an antioxidant, the antioxidant includes ascorbic acid, and the molar ratio of the antioxidant to iron element in the first solution and / or the solution of the metal salts is 0.1 - 0.3:

1.

6. The preparation method according to any one of claims 3-4, characterized in that, The preparation method satisfies at least one of the following characteristics (12)-(15): Characteristic (12): The lithium source includes at least one of lithium hydroxide, lithium chloride and lithium acetate, and the concentration of the lithium source in the mixture is 0.8mol / L - 2.5mol / L; Feature (13): The mixture further includes a doping source, which includes a metal doping source and / or a non-metal doping source; the metal doping source is a J source, and the J source is a water-soluble compound containing at least one of Na and Mg elements; the non-metal doping source is a Q source, and the Q source is a water-soluble compound containing at least one of F, S, N, and Cl elements. Feature (14): The pH of the mixture is 2.0 - 6.

0. Feature (15): The reaction temperature of the hydrothermal reaction is 140°C - 240°C, and the reaction time is 5h - 20h.

7. The preparation method according to any one of claims 3-4, characterized in that, The preparation method satisfies at least one of the following features (16) - (18): Feature (16): The carbon source includes at least one of sucrose, glucose, starch, polyethylene glycol, phenolic resin, cellulose, and citric acid, and the mass of the carbon source is 0.1% - 3% of the mass of the lithium iron phosphate precursor. Feature (17): The inert atmosphere includes at least one of nitrogen, helium, and argon. Feature (18): The temperature of the calcination is 500°C - 800°C, and the duration of the calcination is 4h - 14h.

8. A positive electrode plate, characterized in that, It includes the lithium iron phosphate cathode material according to any one of claims 1 - 2 or the lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 3 - 7.

9. A lithium battery, characterized in that, It includes the positive electrode plate according to claim 8.

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