Lithium iron phosphate positive electrode material and preparation method therefor, positive electrode sheet, and lithium battery
Patent Information
- Application Number
- PCT/CN2025/133388
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-03-12
- Filing Date
- 2025-11-07
- Publication Date
- 2026-09-17
Smart Images
Figure CN2025133388_17092026_PF_FP_ABST
Abstract
Description
Lithium iron phosphate cathode material and preparation method, cathode sheet and lithium battery Technical Field
[0001] This disclosure relates to the field of lithium battery technology, and more specifically, to lithium iron phosphate cathode materials and preparation methods, cathode sheets, and lithium batteries. Background Technology
[0002] Lithium iron phosphate (LiFePO4, LFP) is an important cathode material for lithium-ion batteries, standing out in numerous application fields due to its excellent safety, long cycle life, and low cost. Lithium iron phosphate has a stable crystal structure formed by alternating edges of PO4 tetrahedra, LiO6 octahedra, and FeO6 octahedra, which can accommodate repeated lithium-ion insertion and extraction over long periods. However, this stable crystal structure hinders the absorption and extraction of lithium ions. + The transmission enables Li + In lithium iron phosphate crystals, insertion / extraction often occurs along a one-dimensional direction. The lithium iron phosphate antisite defects present 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.
[0003] In view of this, this disclosure is made. Summary of the Invention
[0004] The purpose of this disclosure 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 by the lithium iron phosphate cathode material has excellent rate performance, low temperature resistance and low polarization.
[0005] To achieve the above objectives, this disclosure is made in the following ways:
[0006] In a first aspect, this disclosure 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 crystal structure degree of the lithium iron phosphate cathode material is: The unit is nm -1 α satisfies 0.15nm - 1 ≤α≤4.5nm -1 ;
[0008] Among them, Fe Li This represents the percentage of lithium iron phosphate antisite defects in the lithium iron phosphate cathode material;
[0009] D (020) The grain size of the (020) crystal plane of the lithium iron phosphate cathode material is expressed in nm.
[0010] I (200) The peak intensity of the characteristic diffraction peak (200) in the X-ray diffraction pattern of the lithium iron phosphate cathode material is indicated;
[0011] I (020) The peak intensity represents the characteristic diffraction peak (020) in the X-ray diffraction pattern of the lithium iron phosphate cathode material.
[0012] In some implementations, the lithium iron phosphate cathode material satisfies at least one of the following characteristics (1)-(4):
[0013] Feature (1): The crystal structure α of the lithium iron phosphate cathode material satisfies 0.5 nm -1 ≤α≤1.1nm -1 ;
[0014] Feature (2): The Fe of the lithium iron phosphate cathode material Li ≤5%;
[0015] Feature (3): The D of the lithium iron phosphate cathode material (020) The value ranges from 35nm to 85nm;
[0016] Feature (4): The lithium iron phosphate cathode material The value ranges from 0.25 to 0.45.
[0017] In some implementations, the general structural formula of the lithium iron phosphate matrix is Li 1-x J x Fe 1-y M y (PO 4-z )Q z ;
[0018] Wherein, 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] Secondly, this disclosure provides a method for preparing any of the aforementioned embodiments of the lithium iron phosphate cathode material, comprising the following steps:
[0023] A crystal form regulator and a metal salt are mixed in a macromolecular solution to form a first solution. The pH of the first solution is adjusted to obtain a gel.
[0024] The gel was immersed in a solution of the metal salt to obtain an iron source carrier;
[0025] The iron source carrier, lithium source, and phosphorus source are mixed in water to obtain a mixture;
[0026] The mixture was subjected to a hydrothermal reaction, followed by solid-liquid separation, to obtain a lithium iron phosphate precursor.
[0027] The lithium iron phosphate precursor and carbon source are mixed and calcined under an inert atmosphere to obtain lithium iron phosphate cathode material.
[0028] In some embodiments, the metal salt includes an iron salt and an M salt, wherein the molar ratio of iron in the iron salt to M in the M salt is 1:0-0.11;
[0029] The iron salt is a soluble salt of ferrous iron, including at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate, and ferrous acetate.
[0030] The M salt is a water-soluble salt of element M, and element M includes at least one selected from Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y.
[0031] In some embodiments, the process for preparing the iron source support satisfies at least one of the following features (5)-(11):
[0032] Feature (5): The mass percentage of the macromolecule in the first solution is 1wt%-30wt%;
[0033] Feature (6): The mass ratio of macromolecules, crystal form regulators and metal salts in the first solution is 1:0.05-0.3:0.05-0.5;
[0034] Feature (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;
[0035] Feature (8): Adjusting the pH of the first solution means adjusting the pH of the first solution to 7.0-7.5;
[0036] Feature (9): The concentration of ferrous ions in the solution of the metal salt is 0.1 mol / L-1.0 mol / L;
[0037] Feature (10): The solid-liquid ratio of the gel and the metal salt solution is 1g:100mL-500mL, and the soaking time is 10min-30min;
[0038] Feature (11): The first solution and / or the solution of the metal salt further includes an antioxidant, the antioxidant including ascorbic acid, and 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.
[0039] In some embodiments, the preparation method satisfies at least one of the following features (12)-(15):
[0040] Feature (12): 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;
[0041] 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, which is a water-soluble compound containing at least one of Na and Mg elements; the non-metal doping source is a Q source, which is a water-soluble compound containing at least one of F, S, N and Cl elements;
[0042] Feature (14): The pH of the mixture is 2.0-6.0;
[0043] Feature (15): The hydrothermal reaction has a reaction temperature of 140℃-240℃ and a reaction time of 5h-20h.
[0044] In some embodiments, the preparation method satisfies at least one of the following features (16)-(18):
[0045] 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;
[0046] Feature (17): The inert atmosphere includes at least one of nitrogen, helium and argon;
[0047] Feature (18): The calcination temperature is 500℃-800℃ and the calcination time is 4h-14h.
[0048] Thirdly, this disclosure provides a positive electrode sheet, including any of the lithium iron phosphate positive electrode materials in the foregoing embodiments or lithium iron phosphate positive electrode materials prepared by any of the preparation methods in the foregoing embodiments.
[0049] Fourthly, this disclosure provides a lithium battery, including the positive electrode sheet of the aforementioned embodiment.
[0050] This disclosure has the following beneficial effects:
[0051] (1) This disclosure optimizes the crystal structure of the lithium iron phosphate cathode material, resulting in a lower activation energy for lithium-ion extraction and insertion, and a higher lithium diffusion rate, thus exhibiting excellent electrode reaction kinetics. Furthermore, the corresponding lithium-ion battery possesses excellent rate performance, low-temperature performance, and low polarization.
[0052] (2) The iron source support prepared in this disclosure releases Fe slowly during the hydrothermal reaction. 2+ And crystal form regulators, on the one hand, reduce Fe 2+ The diffusion rate is reduced, and the antisite defects of lithium iron phosphate are reduced. On the other hand, the crystal form regulator is released slowly during the growth stage of lithium iron phosphate to induce the crystal to grow in a suitable preferred orientation, so that the obtained lithium iron phosphate cathode material has both excellent electrode reaction kinetics and high tap density. Attached Figure Description
[0053] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings involved 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 considered as limiting the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.
[0054] Figure 1 shows the X-ray diffraction pattern of the refined lithium iron phosphate cathode material provided in Example 6;
[0055] Figure 2 is a transmission electron microscope image of the lithium iron phosphate cathode material provided in Example 7;
[0056] Figures 3(a) and 3(b) are cyclic voltammetry curves (-15°C) of lithium iron phosphate cathode materials, where Figure 3(a) is the lithium iron phosphate cathode material provided in Example 6 and Figure 3(b) is the lithium iron phosphate cathode material provided in Comparative Example 2. Detailed Implementation
[0057] To more clearly illustrate the purpose, technical solutions, and advantages of the embodiments of this disclosure, the technical solutions of this disclosure will be clearly and completely described below in conjunction with the implementation methods. Where technical content known in the art and content with identical structures are repeated, they will not be elaborated upon to avoid unnecessary redundancy and to facilitate understanding and implementation by those skilled in the art.
[0058] Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this disclosure and are not intended to limit the scope of the claims. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.
[0059] In the description of the embodiments of this disclosure, the terms "first," "second," etc., are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. The term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone.
[0060] For the sake of brevity, this disclosure specifically discloses several numerical ranges, which can be combined to form corresponding implementation schemes. The endpoints and any values of the ranges disclosed in this disclosure are not limited to the precise ranges or values; these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of various ranges, the endpoint values of various ranges and individual point values, and individual point values can be combined to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.
[0061] Unless otherwise stated, the terms used in this disclosure have the common meanings as commonly understood by those skilled in the art.
[0062] Due to the olivine crystal structure of lithium iron phosphate (LFP) cathode materials, lithium ions can only diffuse along a one-dimensional channel. The presence of anti-site defects in this channel severely hinders lithium ion diffusion, resulting in a low diffusion rate and poor electrode reaction kinetics. Therefore, nano-sizing is commonly used to prepare small-sized LFP particles to shorten the lithium ion diffusion distance. However, nano-sizing is often accompanied by unstable reaction states, which can easily increase anti-site defects. Alternatively, the lithium ion diffusion channel can be shortened by inducing preferred orientation growth of LFP crystals, but this results in a plate-like or sheet-like structure, reducing tap density and consequently affecting discharge specific capacity and energy density.
[0063] Through extensive experiments and research, the inventors of this publication have discovered a technical solution that enables lithium iron phosphate cathode materials to possess both excellent electrode reaction kinetics and high tap density, thereby enabling lithium-ion batteries to have both excellent rate performance, low-temperature performance, and low polarization.
[0064] This disclosure provides a lithium iron phosphate cathode material, comprising a lithium iron phosphate matrix and a carbon coating layer on the surface of the lithium iron phosphate matrix, wherein the crystal structure of the lithium iron phosphate cathode material is [insert crystal structure here]. The unit is nm -1 α satisfies 0.15nm -1 ≤α≤4.5nm -1 ;
[0065] Among them, Fe Li The percentage (%) of lithium iron phosphate antisite defects in the lithium iron phosphate cathode material; D (020) This indicates the grain size of the (020) crystal plane of the lithium iron phosphate cathode material, in nm; (200) Indicates the peak intensity of the (200) characteristic diffraction peak in the X-ray diffraction pattern of lithium iron phosphate cathode material; I (020) The peak intensity represents the characteristic diffraction peak (020) in the X-ray diffraction pattern of lithium iron phosphate cathode material.
[0066] The crystal density α of the lithium iron phosphate cathode material disclosed herein is 0.15 nm. -1 -4.5nm -1 For example, it could be 0.15nm -1 0.5nm -1 1.0nm -1 2.0nm -1 3.0nm -1 4.0nm -1 4.5nm -1 The range of values between any one or any two of them.
[0067] It should be noted that the lithium iron phosphate inversion defect is an inherent lattice dislocation phenomenon, meaning that iron ions occupy positions that should be occupied by lithium ions. The presence of this defect reduces the lithium iron phosphate content. + The storage point severely hinders or even cuts off the diffusion of lithium ions in lithium iron phosphate cathode materials, increases the activation energy of lithium ion extraction and insertion, and causes a significant reduction in electrochemical performance.
[0068] D (020) This refers to the grain size of the lithium iron phosphate cathode material along the (020) crystal plane. In lithium iron phosphate cathode materials, due to the... + The activation energy required for lithium ion transport along the b-axis is the lowest, causing lithium ions in lithium iron phosphate cathode materials to diffuse along a one-dimensional channel in the b-axis direction (perpendicular to the (020) crystal plane). In this direction, the smaller the crystal size, the shorter the lithium ion diffusion channel distance, which is more conducive to achieving high-rate performance.
[0069] I (200) and I (020)The peak intensities are 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, respectively. The ratio reflects the orientation growth mode of lithium iron phosphate cathode materials. The smaller the ratio, the shorter the crystal length of the lithium iron phosphate cathode material is along the ac plane direction, i.e., along the b axis direction. The crystal orientation of lithium iron phosphate is divided into three directions: a, b, and c axes. Since the activation energy required for lithium ion diffusion along the b axis (perpendicular to the (020) crystal plane direction) is the lowest, lithium ions tend to diffuse along the b axis direction. In this transport mode, the b axis is the lithium ion diffusion channel and is perpendicular to the ac plane formed by the a and c axes. The larger the ac plane, the more channels are available for lithium ion diffusion. The ratio of the peak intensities of the characteristic diffraction peaks of the crystal plane can reflect the relative content of the crystal plane. (020) I represents the diffraction peak intensity of the ac crystal plane. (200) The intensity of the diffraction peak on the bc crystal plane. When the ratio is small, it indicates that the relative content of the ac crystal plane is high, which means that the crystal orientation of lithium iron phosphate is preferentially oriented along the ac plane. That is, the lithium ion diffusion channels along the b axis are relatively short and numerous, which can effectively shorten the lithium ion transport distance and improve the transport efficiency.
[0070] In summary, the inversion defect of lithium iron phosphate, D (020) I (200) and I (020) All of these factors affect lithium-ion diffusion. Based on the study of the above parameters, this disclosure proposes a crystal structure degree α. The crystal structure degree α comprehensively reflects the quality of the lithium-ion diffusion channels in lithium iron phosphate cathode materials. Better quality indicates faster lithium-ion diffusion and superior electrode reaction kinetics. When the crystal structure degree α is within the range of 0.15 nm... -1 -4.5nm -1 In this process, lithium iron phosphate cathode materials possess high-quality lithium-ion diffusion channels. These channels not only have minimal obstruction but also possess an appropriate number and length, resulting in high tap density and excellent electrode reaction kinetics. Furthermore, the corresponding lithium-ion batteries exhibit excellent rate performance, low-temperature resistance, and low polarization.
[0071] In some implementations, the crystal structure density α satisfies 0.5 nm. -1 ≤α≤1.1nm -1 Lithium iron phosphate cathode materials with crystal structure degree α within this range have better electrochemical performance.
[0072] In some implementations, Fe Li ≤5%.
[0073] In some implementations, D (020) The value can be 35nm-85nm, for example, it can be any one of 35nm, 45nm, 55nm, 65nm, 75nm and 85nm or any two of them.
[0074] D (020) If the density is too small, it will reduce the tap density of the lithium iron phosphate cathode material, affecting the discharge specific capacity. It will also reduce the mechanical strength of the lithium iron phosphate cathode material, causing the crystal structure to collapse easily during repeated charge and discharge, resulting in reduced cycle performance.
[0075] In some implementations... The value can be between 0.25 and 0.45, for example, it can be any one of 0.25, 0.30, 0.35, 0.40 and 0.45 or a range between any two.
[0076] when If the length is too small, it indicates that the length of the lithium iron phosphate cathode material along the b-axis is too short, causing the morphology of the lithium iron phosphate cathode material to be plate-like or sheet-like, affecting the tap density and compaction density, and thus affecting the specific capacity and energy density. When the value is too large, it indicates that the length of the lithium iron phosphate cathode material along the b-axis is too long, that is, the lithium-ion diffusion channel is too long, which affects the rate of lithium-ion diffusion and thus affects the rate performance.
[0077] In some embodiments, the general structural formula of the lithium iron phosphate matrix is Li 1-x J x Fe 1-y M y (PO 4-z )Q z Wherein, J is selected from at least one of Na and Mg, and J can be any one or more 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 more of the above elements; Q is selected from at least one of F, S, N, and Cl, and Q can be any one or more of the above elements. The values of x, y, and z in the general formula are as follows: 0≤x≤0.1, 0≤y≤0.1, and 0≤z≤0.1. In some embodiments, x, y, and z can be equal or unequal, and can be independently 0.00, 0.03, 0.05, 0.08, 0.10, etc.
[0078] This disclosure provides a method for preparing the above-mentioned lithium iron phosphate cathode material, but the preparation method is not limited to this. The preparation method includes the following steps:
[0079] S1. Mix the crystal form regulator and the metal salt in a macromolecular solution to form a first solution. Adjust the pH of the first solution to obtain a gel. Immerse the gel in a solution of the metal salt to obtain an iron source carrier.
[0080] S2. Mix the iron source carrier, lithium source and phosphorus source in water to obtain a mixture; subject the mixture to a hydrothermal reaction and separate the solid and liquid phases to obtain the lithium iron phosphate precursor.
[0081] S3. Mix the lithium iron phosphate precursor and carbon source, and calcine them under an inert atmosphere to obtain the lithium iron phosphate cathode material.
[0082] This disclosure describes the preparation of a product containing a crystal form regulator and Fe via a gelation method. 2+ Iron source carrier. Utilizing the functional groups of macromolecules and crystal form regulators, and metal ions (mainly Fe) in metal salts... 2+ Coordination forms a complex, and the macromolecules and / or crystal form regulators interact through hydrogen bonds, resulting in a physically cross-linked gel composed of the macromolecules, crystal form regulators, and metal ions. The gel is then immersed in a solution of metal salts for reinforcement, and further loaded with metal ions to obtain a metal source support. Since the metal source in the metal source support is primarily Fe... 2+ For better representation, this disclosure uses "iron source carrier" instead of "metal source carrier" throughout. A portion of the iron source carrier is Fe. 2+ The functional groups of the Fe group coordinate with those of macromolecules and crystal form regulators to form the backbone structure of the gel, ensuring its structural stability; another part of the Fe group... 2+ When loaded into the gel during immersion reinforcement, its degree of freedom of movement is greater than that of Fe in the framework. 2+ This allows the carrier to have differentiated Fe 2+ Release rate, resulting in a sustained-release effect.
[0083] An iron source support, a lithium source, and a phosphorus source are mixed in water, and a hydrothermal reaction is performed to obtain a lithium iron phosphate precursor. In the initial stage of the reaction, the Fe supported in the iron source support... 2+ The iron source carrier preferentially releases Fe, which reacts with lithium and phosphorus sources under high temperature and pressure to nucleate and grow. 2+ This makes Fe 2+ The diffusion rate is higher than that of Li + Slow down, Li + It will be more than Fe 2+ First, it is introduced into the crystal structure to occupy lithium positions, avoiding the formation of lithium iron phosphate antisites. Later in the reaction, as the hydrothermal reaction proceeds, Fe in the iron source support framework... 2+As the release begins, the iron source carrier gradually collapses, and the macromolecules and crystal form regulator disperse into the solvent. The crystal form regulator forms hydrogen bonds with oxygen atoms on the (020) crystal plane of LiFePO4 along the b-axis, inhibiting the growth of this crystal plane along the b-axis and promoting the growth of the crystal along the uninhibited crystal plane. This induces lithium iron phosphate to grow into crystals with exposed (020) crystal planes, shortening the lithium-ion diffusion channel. Since the crystal form regulator is gradually released only in the later stage of the hydrothermal reaction, the lithium iron phosphate crystals already have a spherical morphology of a certain size. Therefore, crystal form regulation at this stage can avoid the appearance of rod-shaped or plate-shaped crystals with ultra-short b-axis, keeping the lithium iron phosphate crystals in a near-spherical shape to ensure good tap density and compaction density.
[0084] In some embodiments, the metal salt includes an iron salt and an M salt, wherein the molar ratio of iron in the iron salt to M in the M salt is 1:0-0.11. The iron salt is a soluble salt of ferrous iron, including at least one selected from ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate, and ferrous acetate. The M salt is a water-soluble salt of element M, which includes at least one selected from Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y. When the molar ratio of iron to M is 1:0, it indicates that the metal salt contains only iron.
[0085] In some embodiments, the process of preparing the iron source support includes being carried out under an inert atmosphere, which can effectively prevent ferrous iron from being oxidized to ferric iron.
[0086] In some embodiments, the macromolecule in the first solution is 1 wt% to 30 wt%, for example, it can be any one of 1 wt%, 5 wt%, 10 wt%, 20 wt%, and 30 wt%, or a range between any two.
[0087] In some embodiments, the mass ratio of macromolecules, crystal form regulators, and metal salts 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 a range between any two. The mass ratio of macromolecules, crystal form regulators, and metal salts can affect the crosslinking density of the gel, and thus affect the mechanical strength and decomposition rate of the gel.
[0088] In some embodiments, the macromolecules include at least one of chitosan, carboxymethyl chitosan, tannic acid, gelatin, and aminoethyl-β-cyclodextrin; the crystal form regulators include at least one of ethylene glycol, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and diethylene glycol.
[0089] In some embodiments, the pH of the first solution is adjusted to 7.0-7.5 to obtain a gel; for example, the pH of the first solution can be any one of 7.0, 7.1, 7.2, 7.3, 7.4, and 7.5, or a range between any two. The aforementioned macromolecules contain a large number of hydrogen-containing active groups. Adjusting the pH of the first solution to 7.0-7.5 facilitates the deprotonation of these hydrogen-containing active groups, making them more readily coordinate with metal ions and promoting physical cross-linking to form a gel.
[0090] In some embodiments, the metal salt solution is a mixed aqueous solution of metal salts, and the concentration of ferrous ions in the metal salt solution is 0.1 mol / L to 1.0 mol / L, for example, it can be any one or any two of 0.1 mol / L, 0.3 mol / L, 0.5 mol / L, 0.7 mol / L and 1.0 mol / L.
[0091] In some embodiments, the solid-liquid ratio of the gel and the metal salt solution is 1g:100mL-500mL, for example, it can be any one of 1g:100mL, 1g:300mL and 1g:500mL or any range between two of them; the soaking time is 10min-30min, for example, it can be any one of 10min, 20min and 30min or any range between two of them.
[0092] In some embodiments, the first solution and / or the metal salt solution further include an antioxidant, including ascorbic acid, wherein the molar ratio of the antioxidant to iron in the first solution and / or the metal salt solution is 0.1-0.3:1, for example, any one of 0.1:1, 0.2:1, and 0.3:1, or a range between any two. Adding an antioxidant prevents ferrous iron from being oxidized to ferric iron.
[0093] In some embodiments, 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 0.8 mol / L to 2.5 mol / L, for example, it can be any one or a range between any two of 0.8 mol / L, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L, and 2.5 mol / L.
[0094] In some embodiments, the mixture further includes a dopant source, which includes a metal dopant source and / or a non-metal dopant source; the metal dopant source is a J source, which is a water-soluble compound containing at least one of Na and Mg elements; the non-metal dopant source is a Q source, which is a water-soluble compound containing at least one of F, S, N and Cl elements.
[0095] In some embodiments, the pH of the mixture is 2.0-6.0, for example, it can be any or any combination of 2.0, 3.0, 4.0, 5.0, and 6.0, or any combination thereof. The pH of the mixture affects the slow release rate of the iron source support; the lower the pH of the mixture, the faster the iron source support releases Fe. 2+ The faster the reaction rate, the higher the saturation of the hydrothermal reaction system, and the faster the nucleation and growth of lithium iron phosphate crystals. In some embodiments, the pH value of the hydrothermal reaction system can be adjusted by adding common acidic regulators such as hydrochloric acid to ensure that it is within a set range.
[0096] In some embodiments, the hydrothermal reaction temperature is 140°C-240°C, for example, it can be any one or any two of 140°C, 160°C, 180°C, 200°C and 240°C; the reaction time is 5h-20h, for example, it can be any one or any two of 5h, 10h, 15h and 20h.
[0097] In some embodiments, the carbon source includes at least one selected from 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 or a range between any two of 0.1%, 0.5%, 1.0%, 1.5%, 2%, and 3%. Adding a carbon source during calcination can form a carbon coating layer, improving the conductivity of the lithium iron phosphate cathode material.
[0098] In some embodiments, the inert atmosphere includes at least one of nitrogen, helium, and argon.
[0099] In some embodiments, the calcination temperature is 500°C-800°C, for example, it can be any one or any two of 500°C, 600°C, 700°C and 800°C; the calcination time is 4h-14h, for example, it can be any one or any two of 4h, 8h, 10h, 12h and 14h.
[0100] This disclosure also provides a positive electrode sheet comprising the above-described lithium iron phosphate positive electrode material. In some embodiments, the positive electrode sheet may further comprise a positive current collector, on which a positive active coating is formed at least one surface, wherein the lithium iron phosphate positive electrode material exists as a positive active material in the positive active coating.
[0101] This disclosure also provides a lithium battery, including the aforementioned positive electrode. In some embodiments, the lithium battery may further include a negative electrode, an electrolyte, a separator, etc., to form a complete battery structure with good electrochemical performance. The specific types of the negative electrode, electrolyte, and separator are not limited.
[0102] In some embodiments, the lithium battery can be in the form of a rechargeable battery. During the charging and discharging process of a rechargeable battery, active ions are inserted and removed back and forth between the positive and negative electrode plates, and the electrolyte plays a role in conducting ions between the positive and negative electrode plates. In some embodiments, the lithium battery may not be in the form of a rechargeable battery, but may be in the form of a battery module, battery pack, etc.
[0103] This disclosure provides an apparatus including the aforementioned secondary battery, battery module, or battery pack. The secondary battery, battery module, or battery pack can serve as a power source for the apparatus or as an energy storage unit. The apparatus includes, but is not limited to, mobile devices (e.g., mobile phones, laptops), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, and energy storage systems.
[0104] The features and performance of this disclosure will be further described in detail below with reference to embodiments.
[0105] Example 1
[0106] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0107] 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 the solution is allowed to stand to obtain a gel. The gel is immersed 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.
[0108] The acetic acid solution contains 1.5 wt% acetic acid, and the chitosan solution contains 2 wt% chitosan. The mass ratio of chitosan, polyethylene glycol, and ferrous sulfate in the first solution is 1:0.2:0.3. Both the first solution and the ferrous sulfate solution contain ascorbic acid, and the molar ratio of ascorbic acid to iron in both the first solution and the ferrous sulfate solution is 0.2:1.
[0109] S2. Mix the iron source carrier, lithium hydroxide and phosphoric acid in water, adjust the pH to 3.5 to obtain a mixture; react the mixture in a reactor at 170℃ for 15h, filter, wash and dry to obtain the lithium iron phosphate precursor.
[0110] 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.
[0111] S3. After mixing the lithium iron phosphate precursor and glucose by sand milling and spray drying, the mixture is calcined at 680°C for 9 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0112] The glucose content is 2.2% of the mass of the lithium iron phosphate precursor.
[0113] Example 2
[0114] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0115] S1. Under a nitrogen atmosphere, polyvinyl alcohol and metal salt are mixed in an aqueous solution of gelatin to form a first solution. The pH of the first solution is adjusted to 7.2, and the solution is allowed to stand to obtain a gel. The gel is immersed in a solution of metal salt at a solid-liquid ratio of 1g:500mL for 30 minutes to obtain an iron source carrier.
[0116] The metal salt is a mixture of ferrous chloride and cobalt acetate, with a molar ratio of iron to cobalt of 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 ferrous 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 in both the first solution and the metal salt solution is 0.1:1.
[0117] S2. Mix the iron source carrier, lithium acetate and phosphoric acid in water, adjust the pH to 2.2 to obtain a mixture; react the mixture in a reactor at 200℃ for 12h, filter, wash and dry to obtain the lithium iron phosphate precursor.
[0118] 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.
[0119] S3. The lithium iron phosphate precursor and citric acid are mixed by sand milling, spray dried, and then calcined at 770°C for 5 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0120] The citric acid content is 1.5% of the mass of the lithium iron phosphate precursor.
[0121] Example 3
[0122] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0123] S1. Under a nitrogen atmosphere, polyethylene glycol and ferrous chloride are mixed in an aqueous solution of tannic acid to form a first solution. The pH of the first solution is adjusted to 7.0, and the solution is allowed to stand to obtain a gel. The gel is immersed 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.
[0124] 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 both the first solution and the ferrous chloride solution is 0.3:1.
[0125] S2. Iron source carrier, lithium chloride and phosphoric acid are mixed in water and the pH is adjusted to 5.8 to obtain a mixture. The mixture is reacted in a reactor at 230°C for 7 hours. After filtration, washing and drying, lithium iron phosphate precursor is obtained.
[0126] 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.
[0127] S3. The lithium iron phosphate precursor and sucrose are mixed by sand milling, spray dried, and then calcined at 530°C for 13 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0128] The sucrose content is 0.3% of the mass of the lithium iron phosphate precursor.
[0129] Example 4
[0130] This embodiment 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 in Example 1.
[0131] Example 5
[0132] This embodiment 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 in Example 1.
[0133] Example 6
[0134] 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 1:0.09:0.3, and other conditions are exactly the same as in Example 1.
[0135] Example 7
[0136] 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 in Example 1.
[0137] Example 8
[0138] 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 in Example 1.
[0139] Example 9
[0140] 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 in Example 1.
[0141] Example 10
[0142] This embodiment provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the concentration of ferrous sulfate solution is 0.5 mol / L, and other conditions are exactly the same as in Example 1.
[0143] Example 11
[0144] This embodiment provides a lithium iron phosphate cathode material. In step S1 of the preparation method, the concentration of ferrous sulfate solution is 0.8 mol / L, and other conditions are exactly the same as in Example 1.
[0145] Example 12
[0146] This embodiment provides a lithium iron phosphate cathode material. In step S2 of the preparation method, the iron source support, lithium hydroxide and phosphoric acid are mixed in water and the pH is adjusted to 2.8 to obtain a mixture. Other conditions are exactly the same as in Example 1.
[0147] Example 13
[0148] This embodiment provides a lithium iron phosphate cathode material. In step S2 of the preparation method, the iron source support, lithium hydroxide and phosphoric acid are mixed in water and the pH is adjusted to 4.5 to obtain a mixture. Other conditions are exactly the same as in Example 1.
[0149] Example 14
[0150] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0151] 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 the solution is allowed to stand to obtain an iron source carrier.
[0152] The acetic acid solution contains 1.5 wt% acetic acid, and the chitosan solution contains 2 wt% chitosan. 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 in the first solution is 0.2:1.
[0153] S2. 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 reactor at 180°C for 16 hours. After filtration, washing and drying, lithium iron phosphate precursor is obtained.
[0154] 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.
[0155] S3. After mixing the lithium iron phosphate precursor and glucose by sand milling and spray drying, the mixture is calcined at 650°C for 7 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0156] The glucose content is 2.2% of the mass of the lithium iron phosphate precursor.
[0157] Comparative Example 1
[0158] This comparative example provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0159] S1. Ferrous sulfate, lithium hydroxide and phosphoric acid are dissolved in water to obtain a mixed solution. The mixed solution is reacted in a reactor at 170°C for 15 hours. After filtration, washing and drying, lithium iron phosphate precursor is obtained.
[0160] The mixed solution contains P:Li:Fe in a molar ratio of 1:1.3:1; lithium hydroxide in a mixed solution has a concentration of 1.0 mol / L; and ascorbic acid is also included in the mixed solution, with a molar ratio of ascorbic acid to iron in the mixed solution of 0.2:1.
[0161] S2. After mixing the lithium iron phosphate precursor and glucose by milling and spray drying, the mixture is calcined at 680°C for 9 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0162] The glucose content is 2.2% of the mass of the lithium iron phosphate precursor.
[0163] Comparative Example 2
[0164] This comparative example provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0165] S1. Ferrous sulfate, lithium hydroxide, polyethylene glycol and phosphoric acid are dissolved in water to obtain a mixed solution. The mixed solution is reacted in a reactor at 170°C for 15 hours. After filtration, washing and drying, lithium iron phosphate precursor is obtained.
[0166] The mixed solution contains P:Li:Fe in a molar ratio of 1:1.3:1; lithium hydroxide in a mixed solution has a concentration of 1.0 mol / L; and polyethylene glycol to ferrous sulfate in a mass ratio of 0.2:0.5. The mixed solution also includes ascorbic acid, with a molar ratio of ascorbic acid to iron in the mixed solution of 0.2:1.
[0167] S2. After mixing the lithium iron phosphate precursor and glucose by milling and spray drying, the mixture is calcined at 680°C for 9 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0168] The glucose content is 2.2% of the mass of the lithium iron phosphate precursor.
[0169] Comparative Example 3
[0170] This comparative example provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0171] S1. Under a nitrogen atmosphere, ferrous sulfate is heated and mixed with 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 the solution is allowed to stand to obtain a gel. The gel is immersed 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.
[0172] The acetic acid solution contains 1.5 wt% acetic acid, and the chitosan solution contains 2 wt% chitosan. The mass ratio of chitosan to ferrous sulfate in the first solution is 1:0.3. Both the first solution and the ferrous sulfate solution contain ascorbic acid, and the molar ratio of ascorbic acid to iron in both the first solution and the ferrous sulfate solution is 0.2:1.
[0173] S2. Iron source carrier, lithium hydroxide, phosphoric acid and polyethylene glycol are mixed in water and the pH is adjusted to 4.5 to obtain a mixture. The mixture is reacted in a reactor at 170°C for 15 hours. After filtration, washing and drying, lithium iron phosphate precursor is obtained.
[0174] The mixture contains P:Li:Fe in a molar ratio of 1:1.3:1, lithium hydroxide in a concentration of 1.0 mol / L, and polyethylene glycol in a mass ratio of 0.2:0.5.
[0175] S3. After mixing the lithium iron phosphate precursor and glucose by sand milling and spray drying, the mixture is calcined at 680°C for 9 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0176] The glucose content is 2.2% of the mass of the lithium iron phosphate precursor.
[0177] Comparative Example 4
[0178] This comparative example provides a lithium iron phosphate cathode material, the preparation method of which includes the following steps:
[0179] S1. Under a nitrogen atmosphere, chitosan and polyethylene glycol are mixed and dissolved in water to obtain a first solution. The pH of the first solution is adjusted to 7.5 to obtain a gel.
[0180] The concentration of chitosan in the first solution is 2 wt%, and the mass ratio of chitosan to polyethylene glycol is 1:0.2.
[0181] S2. Mix the gel, ferrous sulfate, lithium hydroxide and phosphoric acid in water, adjust the pH to 4.5 to obtain a mixture, react it in a reactor at 160℃ for 15 h, filter, wash and dry to obtain the lithium iron phosphate precursor.
[0182] The mixture contains P:Li:Fe in a molar ratio of 1:1.3:1, lithium hydroxide in a concentration of 1.0 mol / L, and polyethylene glycol in a mass ratio of ferrous sulfate in a mass ratio of 0.2:0.5.
[0183] S3. After mixing the lithium iron phosphate precursor and glucose by sand milling and spray drying, the mixture is calcined at 680°C for 9 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.
[0184] The glucose content is 2.2% of the mass of the lithium iron phosphate precursor.
[0185] This disclosure tests the crystal structure 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 corresponding electrochemical performance of lithium batteries (as shown in Tables 1 and 2). The specific steps are as follows:
[0186] (1) X-ray diffraction (XRD) test
[0187] The crystal phase and structure of the lithium iron phosphate cathode material were analyzed using an X-ray powder diffractometer (Rigku Ultima IV), and XRD patterns were obtained. The diffraction source used was Cu-Ka, the wavelength was λ = 0.15406 nm, the step size was 0.02°, and the scan rate was 2° / min. The obtained XRD patterns were then refined using GSAS-II to obtain the final X-ray diffraction pattern. The refined R... wp (Weighted graphical variance factor) ≤ 5%.
[0188] The percentage of lithium iron phosphate inversion defects (Fe) was obtained through analysis and calculation using GSAS-II software. Li The diffraction angle (θ), full width at half maximum (β), and peak intensity of the characteristic diffraction peaks (200) and (020) are obtained. The peak intensities of the actual characteristic diffraction peaks are obtained by subtracting the peak intensity of the background diffraction peak from the peak intensities of the characteristic diffraction peaks (200) and (020), which are respectively I... (200) and I (020) The grain size along the crystal plane of the (020) characteristic diffraction peak is calculated using the Scherrer formula. Where k is a constant value, taking the value 0.9; λ refers to the incident light wavelength, λ = 0.15406 nm; β is the full width at half maximum (FWHM) of the characteristic diffraction peak, in rad; and θ refers to the diffraction angle, in °. The test results of Example 6 are shown in Figure 1.
[0189] (2) Transmission Electron Microscope (TEM) testing
[0190] The morphology of the lithium iron phosphate cathode material was analyzed using a JEM-2000EX TEM. The test results of Example 7 are shown in Figure 2. As can be seen from Figure 2, the lithium iron phosphate cathode material prepared in Example 7 exhibits a spherical morphology with a carbon layer covering the surface. The interplanar spacing at higher resolution indicates that the lithium iron phosphate crystals grow along the ac plane direction.
[0191] (3) Tap density test
[0192] The tap density of lithium iron phosphate cathode material was tested according to the testing standard GB / T 21354-2008.
[0193] (4) Electrochemical performance testing
[0194] The lithium iron phosphate cathode material obtained above was used to formulate a coin 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. This mixture was then coated onto aluminum foil and dried in a vacuum drying oven. After drying, the mixture was pressed into a 12mm diameter cathode sheet using a press in an argon-filled glove box. In the glove box, the cathode sheet, a polypropylene porous separator, a lithium anode sheet, and an electrolyte were assembled into a coin cell. The electrolyte was 1M LiPF6-EC:DMC (volume ratio 1:1), and the electrochemical performance was tested.
[0195] (4-1) Cyclic Voltmeter-Ammeter Test
[0196] Cyclic voltammetry tests were performed on coin cells using a CHI660E electrochemical workstation to investigate the reaction kinetics of lithium iron phosphate electrode materials. The test voltage window was 2.4V-4.2V, and the test temperature was -15℃.
[0197] Within the tested voltage range, lithium ions in the electrode material begin to intercalate and deintercalate due to redox reactions, manifesting as a pair of redox peaks, resulting in a cyclic voltammetry (CV) curve. The peak positions and potential values of the redox peaks in the CV curve reflect the phase transition and ion diffusion processes in the lithium iron phosphate electrode material. The potential difference between the oxidation and reduction peaks reflects the degree of electrode polarization and lithium ion diffusion. A smaller potential difference between the oxidation and reduction peaks indicates lower polarization and higher lithium ion diffusion capability, indicating better electrode reaction kinetics. Furthermore, calculating the growth rate of the potential difference between the oxidation and reduction peaks at different scan rates reflects the speed of lithium ion diffusion and transfer and the adequacy of intercalation and deintercalation during high-rate charge and discharge. A smaller growth rate of the potential difference means that the lithium ion diffusion capability remains relatively stable at different charge and discharge rates, without significant attenuation; furthermore, the degree of polarization does not increase significantly. These factors directly affect the low-temperature performance and rate performance of lithium-ion batteries.
[0198] The cyclic voltammetry curves of Example 6 and Comparative Example 2 are shown in Figures 3(a)-3(b). As the scan rate 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.
[0199] Test method for the growth rate T of the redox peak potential difference: The potential difference of the redox peak of the lithium iron phosphate electrode material at different scan rates L (0.2mV / s, 0.6mV / s, 1.0mV / s) is tested sequentially, and the growth rate T of the redox peak potential difference is calculated using the formula below.
[0200] Among them, L0.2 L represents the potential difference between the redox peaks of the lithium iron phosphate electrode material at a scan rate of 0.2 mV / s. 0.6 The potential difference representing the redox peak of the lithium iron phosphate electrode material at a scan rate of 0.6 mV / s, L 1.0 This represents the potential difference between the redox peaks of the lithium iron phosphate electrode material at a scan rate of 1.0 mV / s.
[0201] (4-2) Constant current charge and discharge test
[0202] Constant current charge-discharge tests were performed on coin cells using the LAND battery testing system to examine the electrochemical performance of the lithium iron phosphate cathode material, including rate performance and capacity retention. The test voltage ranged from 2.0V to 3.9V. For 1 gram of lithium iron phosphate cathode material, the 1C charge-discharge rate was 170mA.
[0203] The discharge specific capacity at different rates was tested at room temperature (25℃); the low-temperature capacity retention rate was tested at 1C, and the calculation formula is as follows:
[0204] Taking Example 6 as an example, the calculation process of crystal structure degree and related parameters is explained, as shown in Figure 1, Y obs For XRD observation data, Y calc To refine the calculated data from the observation data, Y diff This is the difference curve between observed and calculated data, GOF is the goodness-of-fit factor, and R0 is the difference curve between observed and calculated data. wp To refine the weighted graph variance factor, this disclosure calculates Fe based on the refined computational data. Li It is 1.53%, I (200) For 7323.83 and I (020) The value is 18579.56. (020) The diffraction angle θ of the characteristic diffraction peak is 14.8642°, and the full width at half maximum (FWHM) is 0.00227 rad. According to the Scherrer formula, D is calculated to be... (020) It is 62.9nm.
[0205] Table 1. Test results of crystal structure density α and related parameters, and tap density of different embodiments and comparative examples.
[0206] Table 2. Electrochemical performance test results of different embodiments and comparative examples.
[0207] Combining Tables 1 and 2, when the crystal structure density α is 0.15 nm -1 -4.5nm -1When the lithium iron phosphate cathode material exhibits good electrode reaction kinetics, meaning it has a low activation energy required for lithium-ion extraction and insertion, and a fast lithium-ion diffusion rate. Therefore, it possesses a low redox peak potential difference growth rate at different scan rates at low temperatures (-15℃), resulting in lithium-ion batteries with both excellent rate performance and low-temperature resistance. Furthermore, when the crystal structure density α is at 0.5 nm... -1 -1.1nm -1 When the redox peak potential difference increases at low temperatures (-15℃), the rate performance and low-temperature performance of the lithium-ion battery are better.
[0208] By comparing embodiments 2, 4, and 10, it can be seen that D in these three embodiments... (020) and The values are similar, but the Fe in Example 10 is higher. Li The highest was 1.95%, with Examples 2 and 4 at 1.63% and 1.1%, respectively. In D (020) In approximate cases, Fe Li A lower value indicates less obstruction to the lithium-ion diffusion channel, and vice versa. In Examples 2, 4, and 10, the Fe in Example 4... Li Minimum, α is 0.6933 nm -1 This indicates superior lithium-ion diffusion, with lithium-ions being transported rapidly and sufficiently even at 5C rate and low temperatures, resulting in excellent rate performance (5C / 1C: 86.22%), low-temperature capacity retention, and a low redox potential difference growth rate (low polarization). Experimental data show that with Fe... Li As the value increases, α also increases, leading to a decrease in both the rate performance and low-temperature performance of lithium-ion batteries, while increasing the degree of polarization.
[0209] By comparing Examples 3, 5, and 11, it can be seen that Fe in these three examples... Li and The values are similar, but D(020) shows a significant difference: D in Example 11 (020) The smallest was 47.1 nm, while Examples 3 and 5 had 70.4 nm and 56.5 nm, respectively. In Fe... Li and When the values are similar, as D... (020) As α increases, the discharge specific capacity of lithium iron phosphate cathode material increases sequentially at 0.1C. This is due to D... (020) It will affect the grain size of the material, D (020) The larger the value, the larger the grain size of the material, thereby increasing the tap density and thus improving the specific capacity. However, D... (020)The increase in D also lengthens the lithium-ion diffusion channel, thus adversely affecting rate performance and low-temperature performance. Therefore, the rate performance and low-temperature capacity retention of Example 3 are both lower than those of Example 5, while the growth rate of the redox potential difference is higher than that of Example 5. In contrast, although the D of Example 11... (020) The smallest, but due to its Fe Li Similar to Example 5, its unit D (020) The greater size constraint results in Example 11 having similar rate performance, low-temperature performance and polarization to Example 5.
[0210] By comparing Examples 6, 12, and 14, it can be seen that Fe in these three examples... Li and D (020) The values are similar, indicating that their lithium-ion diffusion channels are not only similar in length but also similar in degree of obstruction. However, the three... The values show significant differences. The smaller the value, the higher the degree of preferred crystal orientation of the lithium iron phosphate cathode material. The morphology of the lithium iron phosphate cathode material tends to be plate-like or even sheet-like, which leads to a decrease in its tap density and thus a decrease in discharge specific capacity. However, it has little effect on low temperature resistance, rate performance and polarization degree.
[0211] Comparative Example 2 and D (020) All are within the scope set forth in this disclosure, but the Fe in Comparative Example 2 is different. Li The larger size leads to an increase in crystal density α to 5.2367 nm. -1 Although Comparative Example 2 has a shorter lithium-ion diffusion channel (42.1 nm), due to Fe... Li Excessive density increases the obstruction of lithium-ion diffusion channels, thereby affecting the electrochemical performance of lithium-ion batteries.
[0212] The crystal structure density α of Comparative Example 3 is 0.1460 nm. -1 This exceeds the reasonable range set forth in this disclosure. As can be seen from the table, Comparative Example 3 has a lower Fe content. Li The value (0.62%) and short lithium-ion diffusion channel (64.8 nm) are not ideal, but the low efficiency is a significant drawback. This causes the lithium iron phosphate cathode material to have a sheet-like morphology, resulting in the lowest tap density of the material, which in turn results in the lowest discharge specific capacity at 0.1C.
[0213] Despite the Fe in Comparative Example 4 Li D (020) and All are within the scope of this disclosure, but due to the crystal structure density α being 4.6139 nm -1If it exceeds the reasonable range set by this disclosure, it will also fail to achieve excellent rate performance, low temperature resistance and polarization.
[0214] Adjusting the reaction parameters in steps S1 and S2 can regulate the crystal structure of the lithium iron phosphate cathode material, thereby controlling the crystal density α and influencing its physical and electrochemical properties. Specifically, in step S1, preparing iron source supports with different sustained-release properties 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 affects the crosslinking state of the gel. A higher content indicates a higher crosslinking density in the gel, resulting in a denser and stronger network structure and increased mechanical strength. Mechanical strength affects the sustained-release effect of the iron source support; higher mechanical strength leads to increased Fe... 2+ The slower the crystal form regulator is released during the hydrothermal reaction, the better the Fe... Li The less, at the same time The higher the value, the lower the mechanical strength; Fe 2+ The faster the crystal form regulator is released during the hydrothermal reaction, the better the Fe... Li The more, at the same time The lower.
[0215] The content of crystal form regulators in the iron source carrier also affects the morphology of lithium iron phosphate cathode materials. Higher crystal form regulator content makes it easier for the lithium iron phosphate cathode material to achieve preferred orientation, resulting in a plate-like or even sheet-like morphology. This leads to a decrease in the tap density of the lithium iron phosphate cathode material, thereby reducing the discharge specific capacity of the corresponding lithium-ion battery. Furthermore, the concentration of the metal salt solution also affects the release effect of the iron source carrier. When the gel is immersed and loaded in a metal salt solution, the higher the concentration of the metal salt solution, the higher the Fe content per unit volume of gel. 2+ The more Fe released per unit time in the hydrothermal reaction, the higher the yield. 2+ The more ions there are, the higher the local ion concentration and the faster the reaction rate, resulting in a crystal nucleation rate that exceeds the growth rate, thus causing D... (020) Grain size decreases, Fe Li The value increased.
[0216] In step S2, the pH of the hydrothermal reaction system also affects the release of the iron source support, thereby influencing the crystal structure of the lithium iron phosphate cathode material. When the pH of the hydrothermal reaction system is low, the iron source support dissolves more quickly, leading to a decrease in the Fe content of the lithium iron phosphate cathode material. Li The value is too high. When the pH of the hydrothermal reaction system is high, the iron source carrier dissolves more slowly, leading to a higher Fe content in the lithium iron phosphate cathode material. Li The value is low. At the same time, the ionic reaction rate is slower at higher pH environments, and crystals tend to grow more, leading to a lower D value. (020)The grain size increases. In addition, due to the slow dissolution rate of the iron source carrier, large velocity differences are easily generated during the formation of lithium iron phosphate particles, resulting in a natural gradation effect, which gives the lithium iron phosphate cathode material a high tap density and a high discharge specific capacity.
Claims
1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate-based positive electrode material comprises a lithium iron phosphate matrix and a carbon coating layer on the surface of the lithium iron phosphate matrix; the crystal structure degree of the lithium iron phosphate positive electrode material is Unit: nm -1 , and alpha satisfies 0.15nm -1 ≤ alpha ≤ 4.5nm - 1 ; wherein Fe Li represents the percentage of anti-site defects of iron in the lithium iron phosphate positive electrode material; D (020) D50 represents the grain size of the crystal plane (020) of the lithium iron phosphate positive electrode material (020), in nm; I (200) represents the peak intensity of the (200) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate positive electrode material; I (020) represents the peak intensity of the (020) characteristic diffraction peak in the X-ray diffraction pattern of the lithium iron phosphate positive electrode material.
2. The lithium iron phosphate cathode material of claim 1, wherein, The lithium iron phosphate positive electrode material satisfies at least one of the following characteristics (1)-(4): Feature (1): the crystalline structure degree a of the lithium iron phosphate positive electrode material satisfies 0.5nm -1 ≤ a ≤ 1.1nm -1 ; Feature (2): Fe of the lithium iron phosphate cathode material Li ≤ 5%; Feature (3): the D50 of the lithium iron phosphate positive electrode material is 35 nm-85 nm. (020) of 35 nm-85 nm; Feature (4): the lithium iron phosphate cathode material of The value of a is 0.25-0.
45.
3. The lithium iron phosphate cathode material according to claim 1 or 2, characterized in that 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 ; 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.
4. A method of producing the lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: Mixing the crystal form regulator and the metal salt in the macromolecule solution to form a first solution, adjusting the pH of the first solution to obtain a gel; Soaking the gel in a solution of the metal salt to obtain an iron source carrier; Mixing the iron source carrier, a lithium source and a phosphorus source in water to obtain a mixture; Performing a hydrothermal reaction on the mixture, and performing solid-liquid separation to obtain a lithium iron phosphate precursor; Mixing the lithium iron phosphate precursor and a carbon source, and calcining under an inert atmosphere to obtain the lithium iron phosphate positive electrode material.
5. The preparation method according to claim 4, characterized in that, The metal salt comprises an iron salt and an M salt, and the molar ratio of iron in the iron salt to M in the M salt is 1:0-0.11; The iron salt is a soluble salt of divalent iron, and comprises 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, and M comprises at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y.
6. The production method according to claim 4 or 5, characterized by, The process for preparing the iron source carrier satisfies at least one of the following characteristics (5)-(11): Characteristic (5): The mass percentage 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 salt in the first solution is 1:0.05-0.3:0.05-0.5; Characteristic (7): The macromolecule comprises at least one of chitosan, carboxymethyl chitosan, tannic acid, gelatin and aminoethyl-β-cyclodextrin, and the crystal form regulator comprises at least one of ethylene glycol, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone and diethylene glycol; Characteristic (8): The adjustment of the pH of the first solution refers to 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 salt is 0.1mol / L-1.0mol / L; Characteristic (10): The solid-liquid ratio of the gel and the solution of the metal salt is 1g:(100mL-500mL), and the soaking time is 10min-30min; Characteristic (11): The first solution and / or the solution of the metal salt further comprises an antioxidant, and the antioxidant comprises ascorbic acid, and the molar ratio of the antioxidant to iron in the first solution and / or the solution of the metal salt is 0.1-0.3:
1.
7. The production method according to claim 4 or 5, characterized by, The preparation method satisfies at least one of the following characteristics (12)-(15): Characteristics (12): 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 0.8-2.5 mol / L; Characteristics (13): 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, 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, the Q source is a water-soluble compound containing at least one of F, S, N and Cl elements; Characteristics (14): the pH of the mixture is 2.0-6.0; Characteristics (15): the reaction temperature of the hydrothermal reaction is 140-240℃, and the reaction time is 5-20h.
8. The production method according to claim 4 or 5, characterized by, The preparation method satisfies at least one of the following characteristics (16)-(18): Characteristics (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; Characteristics (17): the inert atmosphere includes at least one of nitrogen, helium and argon; Characteristics (18): the calcination temperature is 500-800℃, and the calcination time is 4-14h.
9. A positive electrode sheet characterized by comprising: The lithium iron phosphate positive electrode material prepared by the preparation method of any one of claims 4-8.
10. A lithium battery, characterized by, The positive electrode sheet of claim 9.