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

Abstract

The present invention discloses a lithium iron phosphate cathode material, a preparation method thereof, and a lithium ion battery, relating to the technical field of cathode materials. The lithium iron phosphate cathode material includes a matrix and a carbon coating layer coated on the surface of the matrix. The crystal structure factor A of the lithium iron phosphate cathode material satisfies 4.600<supgt;‑4< / supgt; ≤ A ≤ 9.500<supgt;‑4< / supgt>. The calculation formula of the crystal structure factor A is: A = ; where C refers to the crystallinity of the crystal; V refers to the unit cell volume; is the grain size of the crystal plane, and the value satisfies 4 ≤ ≤ 9. The lithium iron phosphate cathode material within the above range has excellent low-temperature electrochemical performance.

Description

Technical Field

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

[0002] Lithium iron phosphate is one of the most competitive cathode active materials for lithium ion batteries on the market currently. Compared with lithium cobalt oxide and ternary cathode materials, it has a long service life and good safety performance. In addition, lithium iron phosphate has a theoretical specific capacity of 170 mAh g -1 and a platform discharge voltage of 3.4 V, thus having a considerable energy density.

[0003] LiFePO with an olivine structure 4 , whose space group is of the Pbnm type. In the LiFePO 4 crystal, O atoms are arranged in a slightly distorted hexagonal close-packed manner, P atoms occupy the 4c position of the O atom tetrahedron to form a PO 4 tetrahedron, and Fe and Li are respectively located at the center positions of the oxygen atom octahedron to form FeO 6 and LiO 6 octahedrons. On the bc plane, adjacent FeO 6 octahedrons share one oxygen atom and thus are linked to each other to form a zigzag FeO 6 layer. Between the FeO 6 layers, adjacent LiO 6 octahedrons are linked through two oxygen atoms in the b direction to form a continuous straight chain of Li parallel to the c axis, which enables Li + to possibly form a two-dimensional diffusion motion. Structurally, the PO 4 tetrahedron is located between the FeO 6 layers, which to a certain extent hinders the diffusion motion of Li + . At the same time, adjacent FeO 6 octahedrons are connected by sharing vertices and do not form an octahedron structure sharing edges, resulting in a very low conductivity. Further, due to the above crystal configuration of LiFePO 4 , its capacitance, rate performance, etc. at low temperature are worse than those of ternary cathode material batteries because the chemical reaction of Li + insertion and extraction is more difficult to carry out in a low temperature environment.

[0004] In order to improve the lithium ion diffusion rate and conductivity of LiFePO 4 , a large amount of research has been carried out on the nanosizing and carbon coating of LiFePO 4 . In order to improve the low temperature performance of lithium iron phosphate batteries, the current commercial solution is to install a self-warming system on the power device to keep the battery warm or heated, but for LiFePO4 The LiFePO with its crystal structure adjusted and improved 4 There are few research reports on the low-temperature electrochemical performance of batteries.

[0005] In view of this, the present invention is specifically proposed. Summary of the Invention

[0006] The purpose of the present invention is to provide a lithium iron phosphate cathode material, its preparation method and a lithium ion battery.

[0007] The present invention is implemented as follows:

[0008] In the first aspect, the present invention provides a lithium iron phosphate cathode material, which includes a matrix and a carbon coating layer coated on the surface of the matrix. The crystal structure factor A of the lithium iron phosphate cathode material satisfies 4.600 -4 ≤A≤9.500 -4 The calculation formula of the crystal structure factor A is:

[0009] A = ;

[0010] Wherein, C refers to the crystallinity of the crystal of the lithium iron phosphate cathode material, which is measured by the ratio of the XRD diffraction peak intensity of the crystal plane (311) to the integral area of the diffraction peak;

[0011] V refers to the unit cell volume of the lithium iron phosphate cathode material, with the unit 3 ;

[0012] refers to the grain size of the crystal plane of the lithium iron phosphate cathode material, with the unit , and the value satisfies 4 ≤ ≤9 .

[0013] In an optional embodiment, the lithium iron phosphate cathode material satisfies at least one of the following characteristics (1) to (3):

[0014] Characteristic (1): The value satisfies 4 ≤ ≤6 ;

[0015] Characteristic (2): The value of C satisfies 0.0900≤C≤0.1200;

[0016] Characteristic (3): The value of V satisfies 290 ≤V≤292 .

[0017] In an alternative embodiment, the general formula of the matrix is as follows: Li 1-x A x Fe 1-y M y (PO 4-z )D z , where A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from at least one of F, S, N and Cl; 0 ≤ x ≤ 0.1; 0 ≤ y ≤ 0.1 and 0 ≤ z ≤ 0.1.

[0018] In an alternative embodiment, the mass of the carbon coating layer is 0.5% - 5% of the mass of the lithium iron phosphate cathode material.

[0019] In a second aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, which includes:

[0020] Stirring and reacting a mixture of lithium carbonate, divalent iron source, phosphorus source and ethylene glycol compound solution to obtain a precursor suspension;

[0021] Subjecting the precursor suspension to a hydrothermal reaction, followed by solid-liquid separation to obtain a solid precursor;

[0022] Mixing and drying the solid precursor and a carbon source, and then calcining in an inert atmosphere to obtain the lithium iron phosphate cathode material.

[0023] In an alternative embodiment, during the preparation of the precursor suspension, at least one of features (4) - (9) is included:

[0024] Feature (4): The divalent iron source is selected from at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate and ferrous acetate;

[0025] Feature (5): The phosphorus source is selected from at least one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate and monoammonium hydrogen phosphate;

[0026] Feature (6): The mass percentage content of the ethylene glycol compound in the ethylene glycol compound solution is 5% - 20%;

[0027] Feature (7): The ethylene glycol compound refers to a water-soluble compound containing an ethylene glycol structure, and the ethylene glycol compound is selected from at least one of 1,2-ethylene glycol, squaric acid, croconic acid, sodium crotonate, 2,3-dihydroxycyclopent-2-ene-1-one, 2,3-dihydroxy-4-(hydroxymethyl)cyclopent-2-ene-1-one, and 3,4-dihydroxy-5-methyl-2-furanone;

[0028] Feature (8): The pH of the solution for the stirring reaction is 1 to 2.5, the reaction temperature is 80 to 90 °C, the reaction time is 1 to 4 h, and the stirring speed is 300 to 800 rpm;

[0029] Feature (9): After the stirring reaction, it further includes cooling to 10 to 25 °C and stirring until no bubbles are generated.

[0030] In an alternative embodiment, an M source is further added during the preparation of the mixture, which includes at least one of Features (10) to (13):

[0031] Feature (10): Dissolve the divalent iron source and the M source in the ethylene glycol compound solution to form a metal salt solution, and dissolve the phosphorus source in water to form a phosphorus source solution; disperse the lithium carbonate in the metal salt solution to obtain a dispersion, and then add the phosphorus source solution to obtain the mixture, wherein the molar concentration of the metal element in the metal salt solution is 0.8 to 1.5 M, and the molar concentration of the phosphorus source in the phosphorus source solution is 0.85 to 3 M;

[0032] Feature (11): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of the phosphorus source is 1:1.05 to 1.5;

[0033] Feature (12): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of the lithium element of the lithium carbonate is 1:1.01 to 1.1;

[0034] Feature (13): The M source includes a water-soluble salt of at least one element selected from Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y.

[0035] In an alternative embodiment, the conditions for the hydrothermal reaction during the preparation of the solid precursor are: the temperature is 160 to 200 °C, the time is 1 to 3 h, and the stirring rate is 100 to 300 rpm.

[0036] In an alternative embodiment, during the calcination of the solid precursor and the carbon source mixture in an inert atmosphere, it includes at least one of Features (14) to (17):

[0037] Feature (14): The mass ratio of the solid precursor to the carbon source is 1:0.04-0.1;

[0038] Feature (15): The carbon source is selected from at least one of glucose, starch, phenolic resin, sucrose, cellulose, polyethylene glycol and citric acid;

[0039] Feature (16): The mixing process further includes adding a source A and / or a source D, wherein the source A is selected from at least one of magnesium fluoride, sodium fluoride, sodium chloride and sodium sulfide; and the source D is selected from at least one of potassium fluoride, magnesium fluoride, sodium fluoride, ammonium fluoride, lithium fluoride, melamine, amino acid, thiocyanate, thiourea, elemental sulfur, sodium sulfide, ammonium sulfide, sodium chloride, ammonium chloride, lithium chloride and urea;

[0040] Feature (17): The calcination includes calcination at 550-650°C for 4-8 hours.

[0041] In a third aspect, a lithium-ion battery comprises the lithium iron phosphate positive electrode material as described in any one of the aforementioned embodiments or the lithium iron phosphate positive electrode material prepared by the method for preparing the lithium iron phosphate positive electrode material as described in any one of the aforementioned embodiments.

[0042] The present invention has the following beneficial effects:

[0043] (1) The present invention provides a lithium iron phosphate positive electrode material, the crystal structure factor A of which satisfies 4.600 -4 ≤A≤9.500 -4 , while its grain size Meet 4 ≤ ≤9 The lithium iron phosphate cathode material that meets the above range has excellent low-temperature electrochemical performance. The crystal structure factor comprehensively evaluates the crystallinity C, unit cell volume V and crystal surface of the lithium iron phosphate cathode material. The grain size Three indicators. Among them, the higher the crystallinity C, the fewer internal defects in the material, and the higher the lithium ion diffusion rate; the smaller the unit cell volume, the shorter the lithium ion deintercalation channel, the easier it is to transition, which is beneficial to improving the cycle performance; the smaller D(010), the more conducive to shortening the lithium ion diffusion channel, improving the charge and discharge specific capacity and rate performance; crystallinity C, unit cell volume V, grain size Each of them has an impact on the performance of the lithium iron phosphate positive electrode material, but it is actually difficult to obtain a positive electrode material that is superior in all three aspects. Therefore, the present invention provides a crystal structure factor A that comprehensively evaluates the three aspects of the lithium iron phosphate positive electrode material, which has a crystal structure factor A that meets 4.600 -4 ≤A≤9.500 -4 and simultaneously satisfying 4 ≤ ≤ 9 the lithium iron phosphate cathode material has excellent low-temperature electrochemical performance.

[0044] (2) The preparation method of the lithium iron phosphate cathode material provided by the present invention can prepare a lithium iron phosphate cathode material with D(010) within the preferred range (4 ~6 ) and high crystallinity. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0046] Figure 1 is a schematic diagram of the comparison between the XRD of the lithium iron phosphate cathode materials prepared in Example 3, Example 5, Comparative Example 1 and Comparative Example 2 of the present invention and the standard card;

[0047] Figure 2 is the SEM diagram of the lithium iron phosphate cathode materials prepared in Example 3 (A) and Comparative Example 1 (B) of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0048] In order 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. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can be obtained as conventional products through commercial purchase.

[0049] The present invention provides a lithium iron phosphate cathode material, which includes a matrix and a carbon coating layer coated on the surface of the matrix. The crystal structure factor A of the lithium iron phosphate cathode material satisfies 4.600 -4 ≤ A ≤ 9.500 -4 , and the calculation formula of the crystal structure factor A is:

[0050] A = ;

[0051] wherein, C refers to the crystallinity of the lithium iron phosphate cathode material, which is measured by the ratio of the intensity of the XRD diffraction peak of the crystal plane (311) to the integral area of the diffraction peak;

[0052] V refers to the unit cell volume of the lithium iron phosphate cathode material, in units of 3 ;

[0053] refers to the crystal plane of the lithium iron phosphate cathode material of the grain size, in units of , and the value satisfies 4 ≤ D(010) ≤ 9 .

[0054] In the present invention, the lithium iron phosphate cathode material is used as a sample for X-ray diffraction (XRD) testing. The Cu K 2 background is subtracted using JADE software, and the XRD spectrum is obtained after full-spectrum fitting and refinement; the crystallinity C, unit cell volume V, and D(010) are obtained from the XRD spectrum, and the crystal structure factor A is calculated. The sample with the crystal structure factor A value satisfying 4.600 -4 ≤ A ≤ 9.500 -4 , and at the same time its grain size D(010) satisfies 4 ≤ D(010) ≤ 9 is used as the lithium iron phosphate cathode material of the present invention. The lithium iron phosphate cathode material satisfying the above range has excellent low-temperature electrochemical performance.

[0055] Among them, C refers to the crystallinity of the lithium iron phosphate cathode material. The crystallinity C of the lithium iron phosphate cathode material is measured by the ratio of the diffraction peak intensity of the diffraction peak with the largest peak intensity in the XRD spectrum ((311) crystal plane) to the integral area of the diffraction peak, corresponding to 2θ = 34.8~35.8°.

[0056] Further, the diffraction peak intensity and the integral area of the diffraction peak can reflect the sharpness of the diffraction peak. The smaller the integral area at the same intensity, the sharper the diffraction peak; similarly, the stronger the peak intensity at the same integral area, the sharper the diffraction peak; the sharper the diffraction peak, the higher the crystallinity; the higher the crystallinity, the higher the lithium ion diffusion rate and the better the conductivity. The value of C satisfies 0.0900 ≤ C ≤ 0.1200. Theoretically, the larger C is, the better, but 0.12 is the maximum value that can be obtained in the present invention.

[0057] V refers to the unit cell volume of the lithium iron phosphate cathode material. The unit cell is the basic unit of the crystal structure, which describes the arrangement of atoms or molecules in the crystal. The smaller the unit cell volume, the shorter the lithium ion insertion / extraction channel, and it is easier to transition, which is beneficial to improving the conductivity of the material and the cycle performance. The unit cell volume is generally related to the crystal form and chemical composition. The value of V satisfies 290 ≤ V ≤ 292 , too small unit cell volume will lead to poor structural stability of the cathode material and a decline in cycling performance.

[0058] D(010) refers to the grain size of the crystal plane (010) of the lithium iron phosphate cathode material, with the unit being in accordance with the diffraction angle of the crystal plane ( ) and the full width at half maximum of the diffraction peak ( ) and calculated according to the Scherrer formula. The Scherrer formula: ; k is a constant value, taking 0.9; refers to the wavelength of the incident ray; refers to the full width at half maximum of the diffraction peak, with the unit being rad; refers to the diffraction angle, with the unit being °. The grain size can reflect the macroscopic size of the crystal because D = nd, where d refers to the interplanar spacing of the crystal plane and n refers to the number of crystal layers; the smaller D(010) is, the fewer the number of crystal layers corresponding to the crystal plane or / and the smaller the interplanar spacing, and the crystal is plate-like; the smaller D(010) is, it indicates that the growth of the crystal along the b-axis is restricted, and the crystal grows along the ac crystal plane. Since lithium ions diffuse along the b-axis direction, it is beneficial to shorten the diffusion channel of lithium ions and improve the conductivity. In the present invention, the value of D(010) satisfies 4 ≤D(010)≤9 . Theoretically, the smaller D(010) is, the more beneficial it is to improve the electrochemical performance of the material. However, if D(010) is too small, it is not only difficult to achieve technically but also the plate-like crystals are prone to breakage during the preparation of the electrode sheet or in use, resulting in problems such as unstable performance and easy piercing of the separator. Preferably, D(010) takes a value satisfying 4 ≤D(010)≤6 .

[0059] By limiting the crystal structure factor A of the lithium iron phosphate cathode material in the present invention, the lithium iron phosphate cathode material satisfying 4.600 -4 ≤A≤9.500 -4 has excellent low-temperature electrochemical performance. Among them, the low-temperature electrochemical performance is related to the lithium ion diffusion rate of lithium iron phosphate at low temperature.

[0060] In other embodiments of the present invention, the value of the crystal structure factor A can be, for example, 4.600 -4 , 5.200 -4 , 5.800 -4 , 6.800 -4 , 7.500 -4 , 8.500 -4 , 9.000 -4 , 9.500 -4 Any one of the values in the range between any two of them. For example, the value of crystallinity C can be any one of 0.0900, 0.1000, 0.1100, 0.1200 or the range value between any two of them. The value of unit cell volume V can be, for example, any one of 290 , 291 , 292 Any one of the values in the range between any two of them. The value of D(010) can be, for example, any one of 4 , 5 , 6 , 7 , 8 , Any one of the values in the range between any two of them.

[0061] Furthermore, the general formula of the matrix is as follows: Li 1-x A x Fe 1-y M y (PO 4-z )D z , where A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from at least one of F, S, N and Cl; 0 ≤ x ≤ 0.1; 0 ≤ y ≤ 0.1 and 0 ≤ z ≤ 0.1. By doping the A source, D source or / and M source, various properties of lithium iron phosphate can be improved, including electrochemical properties, hardness, morphology, particle size and its distribution, etc.

[0062] Furthermore, the mass of the carbon coating layer is 0.5% - 5% of the mass of the lithium iron phosphate cathode material, preferably 1.1% - 1.5%.

[0063] In addition, the present invention also provides a typical but non-limiting preparation method of the above lithium iron phosphate cathode material, which includes the following steps:

[0064] S1. Stir and react a mixture of lithium carbonate, divalent iron source, phosphorus source and ethylene glycol compound solution to obtain a precursor suspension.

[0065] Among them, the divalent iron source includes but is not limited to at least one of ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate, and ferrous acetate; the phosphorus source includes but is not limited to at least one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the ethylene glycol compound refers to a water-soluble compound containing an ethylene glycol structure in its molecular structure. Further, the ethylene glycol structure refers to OH-C=C-OH; the ethylene glycol compound includes but is not limited to at least one of 1,2-ethylene glycol, squaric acid, croconic acid, sodium crotonate, 2,3-dihydroxycyclopent-2-en-1-one, 2,3-dihydroxy-4-(hydroxymethyl)cyclopent-2-en-1-one, and 3,4-dihydroxy-5-methyl-2-furanone. The mass percentage content of the ethylene glycol compound in the ethylene glycol compound solution is 5% - 20%, and the solvent is water.

[0066] In the present invention, the reaction pH for the stirring reaction of the mixture is 1 - 2.5, the reaction temperature is 80 - 90 °C, the reaction time is 1 - 4 h, and the stirring speed is 300 - 800 rpm. After the stirring reaction is completed, the temperature is lowered to 10 - 25 °C and stirred until no bubbles are generated. In other embodiments of the present invention, the pH of the stirring reaction can be, for example, any one of 1, 1.5, 2, 2.5 or the range value between any two of them, the reaction temperature can be, for example, any one of 80 °C, 82 °C, 85 °C, 88 °C, 90 °C or the range value between any two of them, the reaction time can be, for example, any one of 1 h, 2 h, 3 h, 4 h or the range value between any two of them, and the stirring speed can be, for example, any one of 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm or the range value between any two of them.

[0067] The present invention uses lithium carbonate as the lithium source and mixes it with the divalent iron source, phosphorus source, and ethylene glycol compound solution, and conducts a stirring reaction at a relatively low temperature (80 - 90 °C) and a low pH value (1 - 2.5). The phosphate radical in the phosphorus source forms an unstable ferrous phosphate precipitate with divalent iron ions (precipitation and dissolution occur simultaneously), and at the same time, lithium carbonate reacts with the acid to release Li + and carbon dioxide gas, thereby forming tiny bubbles. The rupture of the bubbles can cause the precipitation ions (Fe 2+ 、Li + 、PO 4 3-Local supersaturation promotes the primary nucleation of lithium iron phosphate precipitation, thereby shortening the hydrothermal reaction time and reducing the hydrothermal reaction temperature; ethylene glycol compounds also act as antioxidants and chelating agents for divalent iron ions. Ethylene glycol compounds can prevent divalent iron ions from being oxidized to trivalent iron ions during the stirring reaction. Compared with saturated polyols (such as ethylene glycol in the prior art), since the hydroxyl group of ethylene glycol compounds is connected to unsaturated carbon atoms, the chelating effect on divalent iron ions is stronger. Therefore, it has a more obvious effect on slowing down the crystal growth rate, thus greatly improving the crystallinity. In addition, due to the lower water solubility of ethylene glycol compounds compared to saturated polyols such as ethylene glycol, they are more inclined to chelate with divalent iron ions and adsorb on the crystal surface to achieve the effect of regulating the crystal morphology.

[0068] Furthermore, a doping element M source can be added to the mixture. The M source includes water-soluble salts of at least one element selected from Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn, and Y.

[0069] As a preferred preparation method of the present invention, first, a divalent iron source and an M source are dissolved in an ethylene glycol compound solution to form a metal salt solution; a phosphorus source is dissolved in water to form a phosphorus source solution; lithium carbonate is dispersed in the metal salt solution to obtain a dispersion, and then the phosphorus source solution is added to obtain a mixture.

[0070] Preferably, the molar concentration of metal elements in the metal salt solution is 0.8 - 1.5 M, and the ratio of the divalent iron source to the M source in the metal salt solution is fed according to the ratio of iron element to M element in the matrix general formula Li 1-x A x Fe 1-y M y (PO 4-z )D z .

[0071] Preferably, the molar concentration of the phosphorus source in the phosphorus source solution is 0.85 - 3 M, and the ratio of the total molar amount of the divalent iron source and the M source to the molar amount of the phosphorus source is 1:1.05 - 1.5.

[0072] Preferably, the ratio of the total molar amount of the divalent iron source and the M source to the molar amount of lithium element of lithium carbonate is 1:1.01 - 1.1.

[0073] S2. Perform a hydrothermal reaction on the precursor suspension, followed by solid-liquid separation to obtain a solid precursor.

[0074] The conditions of the hydrothermal reaction in the process of preparing the solid precursor are as follows: the temperature is 160-200°C, the time is 1-3 h, and the stirring rate is 100-300 rpm. In other embodiments of the present invention, the temperature of the hydrothermal reaction can be, for example, any one of 160°C, 170°C, 180°C, 190°C, 200°C or the range value between any two of them, the time is any one of 1 h, 2 h, 3 h or the range value between any two of them, and the stirring rate is any one of 100 rpm, 200 rpm, 300 rpm or the range value between any two of them.

[0075] During the hydrothermal reaction, the chelating effect of the ethylene glycol compound on the divalent iron ions can, on the one hand, reduce the diffusion rate of the divalent iron ions, inhibit the orderly arrangement of the ions, thereby slowing down the crystal growth rate, and obtaining lithium iron phosphate crystals with higher crystallinity; on the other hand, the ethylene glycol compound combines with the divalent iron ions arranged on the (010) crystal plane of lithium iron phosphate and is adsorbed on this crystal plane, inhibiting the growth of this crystal plane, thereby obtaining lithium iron phosphate crystals with (010) crystal plane orientation.

[0076] S3. After mixing and drying the solid precursor and the carbon source, calcine them under an inert atmosphere to obtain the lithium iron phosphate cathode material.

[0077] Among them, the mass ratio of the solid precursor to the carbon source is 1:0.04-0.1; the carbon source is selected from at least one of glucose, starch, phenolic resin, sucrose, cellulose, polyethylene glycol and citric acid; during the mixing process, a doping element A source and / or D source can also be added. The A source is selected from at least one of magnesium fluoride, sodium fluoride, sodium chloride and sodium sulfide; the D source is selected from at least one of potassium fluoride, magnesium fluoride, sodium fluoride, ammonium fluoride, lithium fluoride, melamine, amino acid, trithiocyanuric acid, thiourea, sulfur, sodium sulfide, ammonium sulfide, sodium chloride, ammonium chloride, lithium chloride and urea;

[0078] The purpose of calcination is to further improve the crystallinity of lithium iron phosphate and introduce a carbon coating layer, thereby improving the conductivity. The calcination temperature is 550-650°C, and the calcination time is 4-8 h.

[0079] The preparation method of the lithium iron phosphate cathode material provided by the present invention can prepare a lithium iron phosphate cathode material with D(010) in the preferred range (4 ~6 ) and high crystallinity. However, a lithium iron phosphate cathode material that does not have D(010) in the preferred range but satisfies 4.600 -4 ≤A≤9.500 -4 also has good electrochemical performance. Therefore, it is not necessary to use the preparation method provided by the present invention to prepare a product that can solve the technical problems.

[0080] The above lithium iron phosphate cathode material can be widely used in the preparation of batteries, and the batteries prepared therefrom have excellent electrochemical performance. In this regard, the present invention also provides a lithium ion battery, which includes the above lithium iron phosphate cathode material.

[0081] The features and properties of the present invention will be further described in detail below in conjunction with examples.

[0082] The present invention provides lithium iron phosphate cathode materials as shown in Table 1 and Table 2, and conducts tests such as X-ray diffraction (XRD) test, chemical composition analysis, tapped density, specific surface area, and particle size on them.

[0083] Among them, the detection methods are as follows:

[0084] (1) XRD: Use an X-ray powder diffractometer (XRD, Rigaku D / max-2600PC) to study the crystal phase and crystal structure of the material. When testing, use the Kα ray of Cu, the wavelength λ is 0.154056 nm, the voltage is 40 kV, the current is 40 mA, and the scanning range 2θ is 10~80°. The XRD test results are analyzed using Jade 6 software, and the whole spectrum is fitted and refined with R wp ≤10% (R wp is the weighted profile residual variance factor, which can ensure the refinement accuracy).

[0085] (2) Chemical composition analysis is carried out using a PE Avio200 type inductively coupled plasma optical emission spectrometer (ICP-OES).

[0086] (3) Tapped density TD: It is measured in accordance with the method for determining the tapped density of powder products in GB / T 21354-2008.

[0087] (4) BET specific surface area determination: It is carried out in accordance with GB / T 21650.2—2008 "Determination of pore size distribution and porosity of solid materials by mercury intrusion method and gas adsorption method - Part 2: Gas adsorption method for analysis of mesopores and macropores". An Autosorb IQ2 type fully automatic specific surface area and pore size analyzer is used to test the powder sample to obtain the nitrogen adsorption and desorption curve. The temperature condition is constant temperature at -196 °C, and the pressure condition ranges from 0.005 MPa to 0.100 MPa; the data in the medium and low pressure stages are processed and calculated using the BET formula to obtain the specific surface area.

[0088] (5) Particle size Dv(50): It is measured using an MS3000 type laser particle size analyzer.

[0089] Please refer to Table 1 and Table 2 for the detection results.

[0090] Table 1. Statistical table of crystal structure test results of different materials

[0091]

[0092] Figure 1 XRD of the lithium iron phosphate cathode materials prepared in Example 3, Example 5, Comparative Example 1 and Comparative Example 2, and enlarged views of the diffraction peaks of the (010) and (311) crystal planes. By comparison with the standard card, it can be seen that the present invention has successfully synthesized LiFePO 4 . In addition, the diffraction peaks of the (010) and (311) crystal planes were analyzed and calculated according to the method described in the present invention to obtain the crystal structure factor A.

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

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

[0095]

[0096] The lithium iron phosphate cathode materials provided in the above examples and comparative examples were electrochemically detected.

[0097] Battery assembly: A uniform slurry was prepared according to a mass ratio of cathode material∶acetylene black∶PVDF of 75∶15∶10, and evenly coated on an aluminum foil substrate as the cathode of the simulated battery. A lithium sheet was used as the negative electrode of the simulated battery, a polypropylene porous membrane was used as the separator, and the electrolyte was 1 mol LiPF 6 dissolved in a mixed solvent of 1 L EC and DMC (volume ratio 1∶1). The positive electrode, negative electrode, electrolyte, and separator were assembled into a battery in a glove box under argon protection.

[0098] Room temperature electrochemical performance: After charging and discharging at a rate of 0.1C for 1 week at 25°C, the 0.1C discharge capacity (25°C) was obtained; then, the charge and discharge cycle performance test was carried out at a rate of 1C (200 cycles), and the 1C discharge capacity (25°C) of the first week and the 1C discharge capacity (25°C) of the 200th week were obtained, and the capacity retention rate was calculated therefrom.

[0099] Low temperature electrochemical performance includes low temperature discharge rate and low temperature DCR growth rate.

[0100] Low-temperature DCR growth rate test: After charging and discharging at a rate of 0.1C for 1 week at 25°C, the charge-discharge cycle performance test (200 cycles) was carried out at a rate of 1C at -10°C. Among them, the DCR growth rate = (DCR value at the 200th cycle - DCR value at the 1st cycle) ÷ DCR value at the 1st cycle × 100%; the DCR value = (resting voltage before 1C discharge - voltage after 1C discharge for 10s) ÷ constant current of 1C.

[0101] Low-temperature discharge rate test: After charging and discharging at a rate of 0.1C for 1 cycle at 25°C, then charging to 3.8V at a rate of 0.5C, and recording the charging capacity (i.e., 0.5C charging capacity); under the temperature condition of -10°C, discharging at a constant current of 1C to 2.0V, and recording the discharge capacity (i.e., 1C discharge capacity).

[0102] Low-temperature discharge rate = (1C discharge capacity ÷ 0.5C charging capacity) × 100%.

[0103] The charge-discharge voltage range is 2 - 3.8 V.

[0104] Table 3. Statistical table of electrochemical performance tests of different samples

[0105]

[0106] As can be seen from Table 1, Table 2 and Table 3, the crystal structure factors A and D(010) of Examples 1 - 8 provided by the present invention all meet the standards defined by the present invention (4.600 -4 ≤A≤9.500 -4 and 4 ≤D(010)≤9 ), while the crystal structure factor A in Comparative Examples 1 - 3 does not meet the standards defined by the present invention.

[0107] Correspondingly, as can be seen from the data of Examples 1 - 8 and Comparative Examples 1 - 3, when the cathode materials have similar TD, BET, and Dv(50), but different crystal structure factors A, the electrochemical performance of the materials is different, and the difference in low-temperature electrochemical performance is greater. Specifically, the larger the value of the crystal structure factor, the better the low-temperature electrochemical performance of the cathode material. Although the electrochemical performance at room temperature also improves, the change amount is less than that of the low-temperature electrochemical performance. The specific reason is that at low temperatures, there is not enough activation energy for Li + intercalation and deintercalation. Therefore, the low-temperature electrochemical performance of lithium iron phosphate is generally low. However, the present invention regulates the crystal structure of the lithium iron phosphate cathode material to make it meet the standards defined by the present invention (4.600 -4 ≤A≤9.500 -4 and 4 ≤D(010)≤9 ), the Li of the lithium iron phosphate cathode material + diffusion path is shortened, which is beneficial to improving the Li + intercalation / deintercalation efficiency, thereby improving the low-temperature performance. However, the influence on the room-temperature electrochemical performance by regulating the crystal structure factor is not obvious. The reason is that enough energy for Li + intercalation / deintercalation can be provided at room temperature. From the test results of the above electrochemical performance, it can be seen that when the crystal structure factors A and D(010) of the lithium iron phosphate cathode material satisfy the ranges defined in the present invention (4.600 -4 ≤A≤9.500 -4 and 4 ≤D(010)≤9 ), good electrochemical performance can be obtained. Even though the ranges of D(010) in Examples 2, 5 and 8 satisfy 4 ≤D(010)≤9 , but they do not satisfy the preferred range 4 ≤D(010)≤6 , and the crystallinity C in Example 8 does not satisfy 0.0900≤C≤0.1200, and the unit cell volume V also does not satisfy 290 ≤V≤292 , they still have good electrochemical performance. However, Comparative Examples 1-3 do not satisfy the ranges defined in the present invention. At this time, their electrochemical performance is significantly worse than that of the above examples.

[0108] Specifically, the crystal structure factors of Comparative Examples 1 and 2 are lower than 4.6 -4 , the crystallinity of Comparative Example 1 is lower than 0.09, and D(010) of Comparative Example 2 is higher than 9 , which greatly reduces the low-temperature electrochemical performance of Comparative Examples 1-2. However, the room-temperature electrochemical performance of Comparative Example 2 is still at an average level, indicating that the crystallinity has a greater impact on the room-temperature electrochemical performance of the cathode material. However, the crystallinity, unit cell volume and D(010) of Comparative Example 3 are all within the ranges of the present invention, but the crystal structure factor is lower than 4.6 -4 , which proves that it is not that as long as the crystallinity, unit cell volume and D(010) are within the defined ranges, the cathode material will have good electrochemical performance. It is necessary to satisfy that the crystal structure factor A is within the defined range.

[0109] In addition, the present invention also correspondingly provides preparation methods for the above product examples and product comparative examples.

[0110] Method Example 1

[0111] This embodiment provides a method for preparing a lithium iron phosphate cathode material, and the product obtained by the preparation corresponds to the product of the above-mentioned Embodiment 1 (the matrix molecular formula is Li[Fe 0.988 Ti 0.012 PO 4 ). The preparation method of this embodiment includes the following steps:

[0112] (1) Dissolve ferrous sulfate heptahydrate and titanium oxysulfate in an aqueous solution of 1,2-ethylene glycol to form a metal salt solution; dissolve phosphoric acid in water to form a phosphorus source solution.

[0113] The molar concentration of metal elements in the metal salt solution is 1.2 M, and the mass percentage concentration of 1,2-ethylene glycol compound in the aqueous solution of 1,2-ethylene glycol is 13 wt%; the ratio of ferrous sulfate heptahydrate to titanium oxysulfate is fed according to n(Fe):n(Ti)=0.988:0.012.

[0114] The concentration of the phosphorus source solution is 2 M, and the ratio of the total molar amount of iron element and Ti element to the molar amount of phosphorus element is 1:1.22.

[0115] (2) Ultrasonically disperse lithium carbonate with a particle size of 300-500 mesh in the metal salt solution to obtain a dispersion liquid, then add the phosphorus source solution and stir to dissolve it, add ammonia water to adjust the pH of the solution to 1.8, then carry out stirring reaction, the reaction temperature is 90 °C, the reaction time is 2 h, and the stirring speed is 500 rpm. After the reaction is completed, cool down to 20 °C and ultrasonically treat for 30 min to obtain a precursor suspension.

[0116] The feeding amount of lithium carbonate is based on the ratio of the total molar amount of iron element and Ti element to the molar amount of lithium element of 1:1.05.

[0117] (3) Carry out hydrothermal reaction on the precursor suspension, the temperature is 200 °C, the time is 3 h, and the stirring rate is 180 rpm. After the reaction is completed, concentrate the reaction solution, cool it to room temperature and then carry out solid-liquid separation, and wash the obtained solid with deionized water and clean it to obtain a solid precursor.

[0118] (4) Ball-mill the solid precursor and glucose with ethanol as a dispersant, the mass ratio of the solid precursor to the carbon source is 1:0.07, then carry out heat drying and calcine at 600 °C in a nitrogen atmosphere for 6 h, and after crushing and screening, the lithium iron phosphate cathode material is obtained.

[0119] Method Example 2

[0120] This embodiment provides a method for preparing a lithium iron phosphate cathode material by a carbon reduction method, and the product obtained by the preparation corresponds to the product of the above-mentioned Embodiment 2 (the matrix molecular formula is Li[Fe 0.988 Ti 0.012 PO4 ), which comprises the following steps:

[0121] (1) Weigh a certain mass of ferrous sulfate heptahydrate and titanyl sulfate according to the molar ratio of n(Fe):n(Ti) = 0.988:0.012, and dissolve them in deionized water to prepare a 2M mixed salt solution; weigh a certain mass of phosphoric acid and mix it with water to obtain a phosphoric acid solution with an ionic concentration of phosphate radical of 2M.

[0122] (2) Use a constant flow pump to add the mixed salt solution, phosphoric acid solution, 0.6M hydrogen peroxide solution, and 2M ammonia water into the reaction kettle in parallel. The flow rates of the mixed salt solution, phosphoric acid solution, and hydrogen peroxide solution are all 120 mL / h. The flow rate of ammonia water is controlled to maintain the pH of the reaction solution at 1.2 ± 0.1, and the temperature during the reaction is maintained at 60 °C. The reaction time is 2 h, and the stirring speed is 600 rpm; after the reaction, filter and wash with deionized water to obtain an amorphous precipitate (wet material, solid content is 85.22%);

[0123] (3) Uniformly disperse the amorphous precipitate in the aging solution for aging and crystal transformation reaction. After the reaction, filter and wash with deionized water. Take the filter residue and perform spray drying to obtain a crystalline precipitate.

[0124] Among them, the aging solution is an aqueous phosphoric acid solution dissolved with 1,2-ethylene glycol, with a pH of 2 and a concentration of 1,2-ethylene glycol of 15 wt%. The feeding ratio of the amorphous precipitate to the aging solution is 1 g:200 mL; the conditions for the aging and crystal transformation reaction: the temperature is 90 °C, the time is 2 h, and the stirring rate is 200 rpm.

[0125] (4) The crystalline precipitate is calcined at a calcination temperature of 700 °C, a heating rate of 4 °C / min, and a calcination time of 2 h to obtain a titanium-doped iron phosphate precursor.

[0126] (5) Mix the titanium-doped iron phosphate precursor, lithium carbonate, and glucose according to the molar ratio of n(precursor):n(Li):n(glucose) = 1:1.04:0.07, and mix them with ethanol. Put the mixed material into a grinder and grind for 6 h. Put the obtained slurry into a vacuum oven at 80 °C and dry for 4 h to obtain a dry material; calcine the dry material in a nitrogen atmosphere at 750 °C for 9 h, crush and screen to obtain a lithium iron phosphate cathode material.

[0127] Method Examples 3 - 4 and 6 - 7

[0128] The preparation methods of Method Examples 3 - 4 and 6 - 7 are the same as that of Example 1, except that the parameters in the preparation process are different. The products obtained by Method Examples 3 - 4 and 6 - 7 correspond to the products of the above Examples 3 - 4 and 6 - 7 respectively. Specifically, for the preparation parameters, please refer to Table 4:

[0129] Table 4. Statistical Table of Preparation Parameters

[0130]

[0131] Method Example 5

[0132] This example provides a method for preparing lithium iron phosphate cathode material by carbon reduction method, and the product obtained by the preparation corresponds to the product of the above Example 5 (the matrix molecular formula is LiFePO 4 ,), which includes the following steps:

[0133] (1) Weigh ferrous sulfate heptahydrate and dissolve it in water to prepare a 2M salt solution; weigh a certain mass of phosphoric acid and mix it with water to obtain a phosphoric acid solution with an ionic concentration of phosphoric acid root of 2M.

[0134] (2) Use a constant current pump to add the salt solution, phosphoric acid solution, 0.6M hydrogen peroxide solution, and 2M ammonia water into the reaction kettle in parallel. The flow rates of the salt solution, phosphoric acid solution, and hydrogen peroxide solution are all 120 mL / h. The flow rate of ammonia water is controlled to maintain the pH of the reaction solution at 1.2 ± 0.1, and the temperature during the reaction is maintained at 60 °C. The reaction time is 2 h, and the stirring speed is 600 rpm; after the reaction is completed, filter, and wash with deionized water to obtain an amorphous precipitate (wet material, solid content is 89.57%);

[0135] (3) Uniformly disperse the amorphous precipitate in the aging solution for aging and crystal transformation reaction. After the reaction is completed, filter and wash with deionized water. Take the filter residue and perform spray drying to obtain a crystalline precipitate.

[0136] Among them, the aging solution is an aqueous phosphoric acid solution dissolved with 1,2-ethylene glycol, the pH is 2, and the concentration of 1,2-ethylene glycol is 20 wt%. The feeding ratio of the amorphous precipitate to the aging solution is 1 g: 200 mL; the aging and crystal transformation reaction conditions: the temperature is 90 °C, the time is 2 h, and the stirring rate is 200 rpm.

[0137] (4) The crystalline precipitate is calcined, the calcination temperature is 700 °C, the heating rate is 4 °C / min, and the calcination time is 2 h to obtain a lithium iron phosphate precursor.

[0138] (5) Mix the lithium iron phosphate precursor, lithium carbonate, and glucose according to the molar ratio of n(precursor): n(Li): n(glucose) = 1: 1.04: 0.06 and mix with ethanol. Put the mixed material into a grinder and grind for 6 h. Put the obtained slurry into a vacuum oven at 80 °C and dry for 4 h to obtain a dry material; calcine the dry material in a nitrogen atmosphere at 750 °C for 9 h, crush and screen to obtain the lithium iron phosphate cathode material.

[0139] Method Example 8

[0140] This embodiment provides a method for preparing lithium iron phosphate cathode material by carbon reduction method, and the obtained product corresponds to the product of the above embodiment 8 (the matrix molecular formula is LiFePO 4 ), and it includes the following steps:

[0141] The difference from Example 5 is that in step (3), squaric acid is used instead of 1,2-ethylene glycol; the calcination temperature in step (4) is 600 °C, and the heating rate is 2 °C / min.

[0142] Method Comparative Example 1

[0143] This comparative example provides a method for preparing lithium iron phosphate cathode material by solvothermal reaction, and the obtained product corresponds to the product of the above comparative example 1 (the matrix molecular formula is LiFePO 4 ), and it includes the following steps:

[0144] (1) Weigh ferrous sulfate heptahydrate and dissolve it in ethylene glycol to prepare an iron salt solution with a concentration of 0.1 g / mL; weigh lithium hydroxide monohydrate and dissolve it in ethylene glycol to prepare a lithium source solution with a concentration of 0.04 g / mL.

[0145] (2) Add 2.8 mL of phosphoric acid with a concentration of 85 wt% to 90 mL of the lithium source solution, stir and react for 30 min, then continue to add the iron salt solution and stir for another 30 min to obtain a precursor suspension.

[0146] (3) Place the precursor suspension in an oven at 180 °C for solvothermal reaction for 10 h. After solid-liquid separation, wash and dry the obtained solid to obtain lithium iron phosphate LiFePO 4 .

[0147] (4) Mix lithium iron phosphate LiFePO 4 with glucose at a mass ratio of 1:0.07, perform ball milling using ethanol as a dispersant, then perform hot drying and calcine at 600 °C for 6 h under a nitrogen atmosphere. After crushing and screening, the lithium iron phosphate cathode material is obtained.

[0148] Method Comparative Example 2

[0149] This comparative example provides a method for preparing lithium iron phosphate cathode material by solid-phase synthesis method, and the obtained product corresponds to the product of the above comparative example 2 (the matrix molecular formula is LiFePO 4 ), and it includes the following steps:

[0150] (1) Weigh lithium phosphate, ferrous oxalate dihydrate, and ammonium monohydrogen phosphate according to a stoichiometric ratio of 1:3:3. Use ethanol as a dispersant and ball-mill for 6 h, then dry to obtain a precursor. Place the precursor in a furnace at 500 °C and calcine for 12 h. After cooling, crush and screen to obtain lithium iron phosphate LiFePO 4 .

[0151] (2) Mix lithium iron phosphate LiFePO 4 with glucose at a mass ratio of 1:0.07. Use ethanol as a dispersant for ball-milling, then perform hot drying and place in a nitrogen atmosphere and calcine at 600 °C for 2 h. After crushing and screening, the lithium iron phosphate cathode material is obtained.

[0152] Method Comparative Example 3

[0153] This comparative example provides a method for preparing a lithium iron phosphate cathode material by a solvothermal reaction. The product obtained corresponds to the product of the above Comparative Example 3 (the matrix molecular formula is LiFePO 4 ), and it includes the following steps:

[0154] (1) Dissolve 1,2-ethylene glycol in ethylene glycol to prepare a mixed solvent with a mass percentage concentration of 10% of 1,2-ethylene glycol. Weigh ferrous sulfate heptahydrate and dissolve it in the above mixed solvent to prepare an iron salt solution with a concentration of 0.1 g / mL. Weigh lithium hydroxide monohydrate and dissolve it in the above mixed solvent to prepare a lithium source solution with a concentration of 0.04 g / mL.

[0155] (2) Add 2.8 mL of phosphoric acid with a concentration of 85 wt% to 90 mL of the lithium source solution, stir and react for 30 min, then continue to add the iron salt solution and stir for another 30 min to obtain a precursor suspension.

[0156] (3) Place the precursor suspension in an oven at 200 °C for a solvothermal reaction for 10 h. After solid-liquid separation, wash and dry the obtained solid to obtain lithium iron phosphate LiFePO 4 .

[0157] (4) Mix lithium iron phosphate LiFePO 4 with glucose at a mass ratio of 1:0.07. Use ethanol as a dispersant for ball-milling, then perform hot drying and place in a nitrogen atmosphere and calcine at 600 °C for 6 h. After crushing and screening, the lithium iron phosphate cathode material is obtained.

[0158] Combined with the products and preparation methods of the present invention, and the performance of the products in Tables 1-4 above, it can be seen that Examples 1 and 2 have the same chemical composition, but due to their different preparation methods, the Within the preferred range, Example 2 is not within the preferred range. At this time, the value of the crystal structure factor of Example 2 is lower than that of Example 1. Similarly, among Examples 3-5, the values of the crystal structure factors of Examples 3-4 prepared by the preparation method of the present invention are greater than those of Example 5. It is fully proved that the preparation method provided by the present invention can prepare a lithium iron phosphate cathode material with D(010) within the preferred range and high crystallinity. However, it does not have D(010) within the preferred range but satisfies 4.600 -4 ≤A≤9.500 -4 The lithium iron phosphate cathode material also has good electrochemical performance. Therefore, it is not necessary to use the preparation method provided by the present invention to prepare a product that can solve the technical problem.

[0159] In addition, the present invention also provides Method Examples 9-16 and Method Comparative Examples 4-5, and Method Examples 9-16 and Method Comparative Examples 4-5 perform a single-factor variable on a certain parameter of Method Example 1.

[0160] Specifically, the specific steps of Method Examples 9-16 and Method Comparative Examples 4-5 are as follows:

[0161] Method Example 9

[0162] This example is basically the same as Method Example 1, except that in this example, the mass percentage concentration of the 1,2-ethylene glycol compound in the aqueous solution of 1,2-ethylene glycol is 5 wt%.

[0163] Method Example 10

[0164] This example is basically the same as Method Example 1, except that in this example, the mass percentage concentration of the 1,2-ethylene glycol compound in the aqueous solution of 1,2-ethylene glycol is 20 wt%.

[0165] Method Example 11

[0166] This example is basically the same as Method Example 1, except that in this example, the ethylene glycol compound is squaric acid.

[0167] Method Example 12

[0168] This example is basically the same as Method Example 1, except that in this example, the ethylene glycol compound is sodium crotonate.

[0169] Method Example 13

[0170] This example is basically the same as Method Example 1, except that in this example, the pH of the stirring reaction is 1.

[0171] Method Example 14

[0172] This example is basically the same as Method Example 1, except that in this example, the pH of the stirring reaction is 2.5.

[0173] Method Example 15

[0174] This example is basically the same as Method Example 1, except that in this example, the rotation speed of the stirring reaction is 300 rpm.

[0175] Method Example 16

[0176] This example is basically the same as Method Example 1, except that in this example, the rotation speed of the stirring reaction is 800 rpm.

[0177] Method Comparative Example 4

[0178] This comparative example is basically the same as Example 1, except that in this comparative example, 1,2-ethylene glycol in Example 1 is replaced by ethylene glycol.

[0179] Method Comparative Example 5

[0180] This comparative example is basically the same as Example 1, except that in this comparative example, lithium carbonate in Example 1 is replaced by lithium hydroxide.

[0181] Perform performance tests on the lithium iron phosphate cathode materials prepared in the above Method Examples 9-16 and Method Comparative Examples 4-5. For the test results, please refer to Table 5 and Table 6.

[0182] Table 5. Statistical Table of Crystal Structure Test Results of Different Samples

[0183]

[0184] Table 6. Statistical Table of Composition and Performance Test Results of Different Samples

[0185]

[0186] As can be seen from the above table, Method Examples 9-16 and Method Comparative Examples 4 and 5 perform single-factor variable analysis on Method Example 1. From the results of Method Examples 9-16 and Method Comparative Examples 4-5, it can be seen that for cathode materials with the same chemical composition, their crystal structures can be regulated through the preparation method.

[0187] Specifically, in Method Examples 9 to 12, the addition amount and type of ethylene glycol compounds were changed. It can be seen that the addition amount and type of ethylene glycol compounds have an impact on the crystal structure factor of lithium iron phosphate. Among them, the crystal structure factor first increases and then decreases with the increase in the addition amount of ethylene glycol compounds. Due to excessive addition amount, the free Fe in the reaction solution 2+ decreases, reducing its saturation degree, resulting in limited nucleation of lithium iron phosphate, and thus the crystallinity decreases; if the addition amount is too small, it is difficult to play a role in crystal plane regulation, making the crystal structure factor larger.

[0188] In Method Examples 13 to 16, the pH and stirring speed during the stirring reaction were changed. It can be seen that the pH and stirring speed during the stirring reaction have an impact on the crystal structure factor of lithium iron phosphate. Among them, the crystal structure factor first increases and then decreases with the increase in pH, and also first increases and then decreases with the increase in stirring speed; the magnitude of pH and the stirring speed are closely related to the dissolution rate of lithium carbonate (i.e., the production rate of carbon dioxide gas). If the production rate of carbon dioxide is too fast, it will lead to uneven distribution of various elements, and defects are likely to occur during nucleation, thus affecting the crystallinity; if the production rate of carbon dioxide is too slow, it will lead to a relatively low local supersaturation degree, inhibiting nucleation.

[0189] In Method Comparative Example 4, ethylene glycol was used instead of ethylene glycol compounds. From the results, it can be seen that ethylene glycol compounds can effectively reduce the D(010) of lithium iron phosphate compared with ethylene glycol, and also improve the crystallinity of lithium iron phosphate to a certain extent. Therefore, the crystal structure factor of the lithium iron phosphate prepared in the example is larger than that in Comparative Example 4. The reason is that the hydroxyl groups in ethylene glycol compounds are connected through unsaturated carbons, and their chelating ability with Fe 2+ is greatly improved, so the regulation effect on the crystal plane is greatly improved, and slowing down the crystal growth rate is beneficial to improving the crystallinity.

[0190] In Method Comparative Example 5, lithium hydroxide was used as the lithium source. Compared with using lithium carbonate as the lithium source in Example 1, lithium carbonate produces carbon dioxide bubbles in an acidic environment, which has a significant promoting effect on the crystallinity of lithium iron phosphate, and also reduces the D(010) of lithium iron phosphate to a certain extent. Therefore, the crystal structure factor of the lithium iron phosphate prepared in Method Example 1 is larger than that in Comparative Example 5. The reason is that the rupture of tiny bubbles can lead to local supersaturation and promote the primary nucleation of lithium iron phosphate precipitation. Therefore, under the same hydrothermal reaction conditions, the crystallization performance of the obtained lithium iron phosphate is better.

[0191] In summary, the present invention provides a lithium iron phosphate cathode material, and the value of its crystal structure factor A satisfies 4.600 -4 ≤A≤9.500 -4, and at the same time, its grain size D(010) satisfies 4 ≤D(010)≤9 , the lithium iron phosphate cathode material within the above range has excellent low-temperature electrochemical performance. The crystal structure factor comprehensively evaluates three indicators of the crystallinity C, unit cell volume V, and grain size D(010) of the (010) crystal plane of the lithium iron phosphate cathode material. Among them, the higher the crystallinity, the higher the lithium ion diffusion rate and the better the electrical conductivity; the smaller the unit cell volume, the shorter the intercalation / deintercalation channels of lithium ions, making it easier to transition, which is beneficial to improving the conductivity of the material and the cycle performance; the smaller D(010), the more conducive to shortening the diffusion channels of lithium ions and improving the conductivity. In addition, the preparation method of the lithium iron phosphate cathode material provided by the present invention only provides an exemplary preparation method. In the preparation method of the present invention, lithium carbonate is used as the lithium source and mixed with a divalent iron source, a phosphorus source, and an ethylene glycol compound for stirring reaction to obtain a precursor suspension. The stirring reaction is carried out at a lower temperature (80~90 °C) and a low pH value (1~2.5). The phosphorus source forms an unstable iron phosphate precipitate with divalent iron ions (precipitation and dissolution occur simultaneously). At the same time, lithium carbonate reacts with an acid to release Li + and carbon dioxide gas, thereby forming tiny bubbles. The rupture of the bubbles can cause local supersaturation, promoting the primary nucleation of lithium iron phosphate precipitation, thereby shortening the hydrothermal reaction time and reducing the hydrothermal reaction temperature; the ethylene glycol compound also acts as an antioxidant and chelating agent for Fe 2+ . The ethylene glycol compound can prevent Fe 2+ from being oxidized to Fe 3+ during the stirring reaction; during the hydrothermal reaction process, the chelating effect of the ethylene glycol compound on Fe 2+ can, on the one hand, reduce the diffusion rate of Fe 2+ , inhibit the orderly arrangement of ions, thereby slowing down the crystal growth rate, and obtaining lithium iron phosphate crystals with higher crystallinity; on the other hand, the ethylene glycol compound binds to the Fe 2+ arranged on the (010) crystal plane of lithium iron phosphate and is adsorbed on this crystal plane, inhibiting the growth of this crystal plane, thereby obtaining lithium iron phosphate crystals with a (010) crystal plane orientation; compared with saturated polyols (such as ethylene glycol in the prior art), since the hydroxyl group of the ethylene glycol compound is connected to an unsaturated carbon atom, the chelating effect on Fe 2+ is stronger, and thus has a more obvious effect on slowing down the crystal growth rate, thereby greatly improving the crystallinity; in addition, due to the lower water solubility of the ethylene glycol compound compared to saturated polyols such as ethylene glycol, it is more inclined to chelate with Fe 2+ and adsorb on the crystal plane to achieve the effect of regulating the crystal morphology. Through this method, a lithium iron phosphate cathode material with D(010) in the preferred range and high crystallinity can be prepared. However, it does not have D(010) in the preferred range but satisfies 4.600 -4 ≤A≤9.500 -4 The lithium iron phosphate cathode material also has good electrochemical performance. Therefore, it is not necessary to adopt the preparation method provided by the present invention to prepare a product that can solve the technical problems.

[0192] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: It includes a substrate and a carbon coating layer coated on the surface of the substrate, and the crystal structure factor A of the lithium iron phosphate positive electrode material satisfies 4.

600. -4 ≤A≤9.500 -4 , the calculation formula of the crystal structure factor A is: A= ， Wherein, C refers to the crystallinity of the lithium iron phosphate positive electrode material, which is measured by the ratio of the diffraction peak intensity of the (311) crystal plane with the largest peak intensity in the XRD spectrum to the integrated area of ​​the diffraction peak, corresponding to 2θ=34.8~35.8°; V refers to the unit cell volume of the lithium iron phosphate positive electrode material, unit 3 ; Refers to the crystal surface of the lithium iron phosphate positive electrode material The grain size, unit , the value satisfies 4 ≤ ≤9 .

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The lithium iron phosphate positive electrode material satisfies at least one of the following characteristics (1) to (3): Feature (1): The value satisfies 4 ≤ ≤6 ; Feature (2): The value of C satisfies 0.0900≤C≤0.1200; Feature (3): The value of V satisfies 290 ≤V≤292 .

3. The lithium iron phosphate positive electrode material according to any one of claims 1 to 2, characterized in that The general formula of the matrix is ​​as follows: Li 1-x A x Fe 1-y M y (PO 4-z )D z , wherein A is selected from at least one of Na and Mg; M is selected from at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D is selected from at least one of F, S, N and Cl; 0≤x≤0.1; 0≤y≤0.1 and 0≤z≤0.

1.

4. The lithium iron phosphate positive electrode material according to any one of claims 1 to 2, characterized in that The mass of the carbon coating layer is 0.5% to 5% of the mass of the lithium iron phosphate positive electrode material.

5. A method for preparing a lithium iron phosphate positive electrode material as claimed in any one of claims 1 to 4, characterized in that: It includes: A mixture of lithium carbonate, a divalent iron source, a phosphorus source and an ethylene glycol compound solution is stirred for reaction to obtain a precursor suspension, wherein the mass percentage of the ethylene glycol compound in the ethylene glycol compound solution is 5% to 20%; the ethylene glycol compound refers to a water-soluble compound containing an ethylene glycol structure, and the ethylene glycol compound is selected from at least one of 1,2-ethylene glycol, squaric acid, crotonic acid, sodium crotonate, 2,3-dihydroxycyclopent-2-ene-1-one, 2,3-dihydroxy-4-(hydroxymethyl)cyclopent-2-ene-1-one and 3,4-dihydroxy-5-methyl-2-furanone; the pH of the solution for the stirring reaction is 1 to 2.5, the reaction temperature is 80 to 90° C., the reaction time is 1 to 4 hours, and the stirring speed is 300 to 800 rpm; The precursor suspension is subjected to a hydrothermal reaction at a temperature of 160-200° C., a time of 1-3 h, a stirring rate of 100-300 rpm, and solid-liquid separation to obtain a solid precursor; The solid precursor and the carbon source are mixed and dried, and then calcined under an inert atmosphere to obtain a lithium iron phosphate positive electrode material.

6. The method for preparing the lithium iron phosphate positive electrode material according to claim 5, characterized in that: The process of preparing the precursor suspension includes at least one of the characteristics (4) to (6): Feature (4): The divalent iron source is at least one selected from ferrous sulfate, ferrous chloride, ferrous nitrate, ferrous oxalate and ferrous acetate; Feature (5): The phosphorus source is at least one selected from phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, ammonium dihydrogen phosphate and ammonium monohydrogen phosphate; Feature (6): After the stirring reaction is completed, the temperature is lowered to 10-25°C and stirred until no bubbles are generated.

7. The method for preparing a lithium iron phosphate positive electrode material according to claim 6, characterized in that: During the preparation of the mixture, an M source is also added, which includes at least one of the characteristics (7) to (10): Feature (7): dissolving the divalent iron source and the M source in the ethylene glycol compound solution to form a metal salt solution, dissolving the phosphorus source in water to form a phosphorus source solution; dispersing the lithium carbonate in the metal salt solution to obtain a dispersion, and then adding the phosphorus source solution to obtain the mixture, wherein the molar concentration of the metal element in the metal salt solution is 0.8~1.5M, and the molar concentration of the phosphorus source in the phosphorus source solution is 0.85~3M; Feature (8): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of the phosphorus source is 1:1.05-1.5; Feature (9): The ratio of the total molar amount of the divalent iron source and the M source to the molar amount of the lithium element in the lithium carbonate is 1:1.01-1.1; Feature (10): The M source includes a water-soluble salt of at least one element selected from the group consisting of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y.

8. The method for preparing a lithium iron phosphate positive electrode material according to claim 5, characterized in that: After the solid precursor and the carbon source are mixed and dried, the process of calcining under an inert atmosphere includes at least one of the following features (11) to (14): Feature (11): The mass ratio of the solid precursor to the carbon source is 1:0.04-0.1; Feature (12): The carbon source is selected from at least one of glucose, starch, phenolic resin, sucrose, cellulose, polyethylene glycol and citric acid; Feature (13): The mixing process further includes adding a source A and / or a source D, wherein the source A is selected from at least one of magnesium fluoride, sodium fluoride, sodium chloride and sodium sulfide; and the source D is selected from at least one of potassium fluoride, magnesium fluoride, sodium fluoride, ammonium fluoride, lithium fluoride, melamine, amino acid, thiocyanate, thiourea, elemental sulfur, sodium sulfide, ammonium sulfide, sodium chloride, ammonium chloride, lithium chloride and urea; Feature (14): The calcination includes calcination at 550-650°C for 4-8 hours.

9. A lithium ion battery, characterized in that: It includes the lithium iron phosphate positive electrode material according to any one of claims 1 to 4 or the lithium iron phosphate positive electrode material prepared by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 5 to 8.

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