Lithium iron phosphate positive electrode material, and preparation method therefor and use thereof

Abstract

The present invention belongs to the technical field of lithium iron phosphate positive electrode materials. Disclosed are a lithium iron phosphate positive electrode material, and a preparation method therefor and the use thereof. The lithium iron phosphate positive electrode material comprises a lithium iron phosphate matrix and a carbon coating layer coating the surface of the lithium iron phosphate matrix. The average standard deviation of the thickness of the carbon coating layer in the lithium iron phosphate positive electrode material is σ(d), where 0.05 nm≤σ(d)≤0.35 nm; and the average standard deviation of the carbon content in the lithium iron phosphate positive electrode material is σ(Cwt%), where 0.02 wt%≤σ(Cwt%)≤0.35 wt%. The lithium iron phosphate positive electrode material has a uniform carbon coating layer, and the thickness of the carbon coating layer and the carbon content at each position are both relatively consistent, which is conducive to improving the carrier mobility of the lithium iron phosphate positive electrode material and to further improving the electrochemical performance of a battery prepared from the lithium iron phosphate positive electrode material.

Description

Lithium iron phosphate cathode materials, their preparation methods and applications

[0001] Cross-references to related applications

[0002] This disclosure claims priority to Chinese Patent Application No. 202411719693X, filed on November 28, 2024, entitled "Lithium Iron Phosphate Cathode Material and Preparation Method Thereof and Application", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of lithium iron phosphate cathode material technology, and more specifically, to a lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0004] As a new generation of cathode material for lithium-ion batteries, LiFePO4 has become a research hotspot due to its low cost, low environmental pollution, excellent thermal and structural stability, and high theoretical specific capacity. However, due to structural defects in LiFePO4, it has low electronic conductivity and lithium-ion diffusion coefficient, which leads to its underutilization during high-current charge and discharge, resulting in unsatisfactory rate performance.

[0005] In view of this, this disclosure is hereby made.

[0006] Public content

[0007] The purpose of this disclosure is to provide a lithium iron phosphate cathode material, its preparation method, and its application, in order to solve or improve the above-mentioned technical problems.

[0008] This disclosure can be implemented as follows:

[0009] In a first aspect, this disclosure provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix and a carbon coating layer covering the surface of the lithium iron phosphate matrix;

[0010] The average standard deviation of the carbon coating thickness in lithium iron phosphate cathode materials is σ(d). 0.05nm≤σ(d)≤0.35nm;

[0011] The average standard deviation of carbon content in lithium iron phosphate cathode materials is σ(C wt％ ), 0.02wt%≤σ(C) wt％ ≤0.35wt%;

[0012] Wherein, N1 is the sample size of lithium iron phosphate cathode material particles taken during the lithium iron phosphate cathode material test, and N1≥10;

[0013] σ(d) iσ(C wt％ ) i These represent the standard deviation of the carbon coating thickness and the standard deviation of the carbon content corresponding to the i-th lithium iron phosphate cathode material particle taken during the testing of lithium iron phosphate cathode material;

[0014] n is the sample size at a local location taken during the testing of the i-th lithium iron phosphate cathode material particle, n≥5; d m Let the thickness of the carbon coating be the value at the m-th local location. For the n local location samples taken, d m The average value, in nm; C m Let m be the carbon content corresponding to the m-th local position. C is the value of the n local location samples taken. m The average value is expressed in wt%.

[0015] The thickness of the carbon coating layer in the lithium iron phosphate cathode material is d. 1nm≤d≤10nm; d is the number of local position samples taken from the i-th lithium iron phosphate cathode material particle. m The average value;

[0016] The carbon content of lithium iron phosphate cathode material is C. 1wt% ≤ C ≤ 10wt%; C is the value of n local location samples taken for the i-th lithium iron phosphate cathode material particle. m The average value.

[0017] In an optional embodiment, the shape factor of the lithium iron phosphate cathode material is:

[0018] Wherein, N2 is the sample size of lithium iron phosphate cathode material particles taken during the testing of lithium iron phosphate cathode material, and N2≥10;

[0019] The shape factor is the j-th lithium iron phosphate cathode material particle taken during the lithium iron phosphate cathode material test.

[0020] D s Let D be the diameter of the standard circle whose projected area is equal to that of the j-th lithium iron phosphate cathode material particle in the TEM two-dimensional projection diagram, and let D be the diameter of the smallest circumcircle of the particle projection in the TEM two-dimensional projection diagram of the j-th lithium iron phosphate cathode material particle.

[0021] In an optional implementation, the parameter factor of the lithium iron phosphate cathode material 0.07nm -1 ≤Q≤7nm -1 .

[0022] In optional embodiments, the lithium iron phosphate cathode material further includes at least one of the following features:

[0023] Feature 1: Lithium iron phosphate cathode material The range is 0.550 to 0.910;

[0024] Feature 2: The d of the lithium iron phosphate cathode material is 1.5nm to 9.1nm;

[0025] Feature 3: The σ(d) of the lithium iron phosphate cathode material is 0.055 nm to 0.343 nm;

[0026] Feature 4: The C content of lithium iron phosphate cathode material is 1.94 wt% to 9.68 wt%;

[0027] Feature 5: σ(C) of lithium iron phosphate cathode materials wt％ The content ranges from 0.024 wt% to 0.336 wt%.

[0028] Feature 6: The Q of the lithium iron phosphate cathode material is 0.072 nm. -1 ~6.596nm -1 ;

[0029] Feature 7: The specific surface area of ​​the lithium iron phosphate cathode material is 9.3 m². 2 / g~15.1m 2 / g;

[0030] Feature 8: D of lithium iron phosphate cathode material 50 Its thickness ranges from 0.85 μm to 1.50 μm;

[0031] Feature 9: The carrier mobility of the lithium iron phosphate cathode material is 20.03 × 10⁻⁶. -6 cm 2 / V·s~21.71×10 -6 cm 2 / V·s;

[0032] Feature 10: The general formula of lithium iron phosphate matrix is ​​Li 1-x A x Fe 1-y M y (PO 4-a )D aWherein, A includes at least one of Na and Mg; M includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D includes at least one of F and S; 0≤x≤0.1, 0≤y≤0.1 and 0≤a≤0.1.

[0033] Secondly, this disclosure provides a method for preparing a lithium iron phosphate cathode material as described in any of the foregoing embodiments, comprising the following steps: ball milling a lithium iron phosphate matrix with a composite carbon source to obtain a mixed precursor; drying the mixed precursor and sintering it to obtain a lithium iron phosphate cathode material.

[0034] In an optional embodiment, the preparation of the lithium iron phosphate cathode material includes at least one of the following features:

[0035] Feature 11: The preparation of the lithium iron phosphate matrix includes: mixing a first solution containing ferrous salt with a second solution containing lithium source and phosphate to obtain a mixture; adjusting the pH of the mixture to 6-8 and then carrying out a hydrothermal reaction; collecting the solid after the reaction is completed.

[0036] Feature 12: The preparation of the composite carbon source includes: mixing polyethylene glycol gel with molten carbon source; the carbon source includes at least one of sorbitol, erythritol, fructose, rhamnose and malic acid; the molar ratio of lithium element in lithium iron phosphate matrix to carbon source and polyethylene glycol is 1:(0.03~0.05):(0.02~0.04);

[0037] Feature 13: The ball milling time is 1 hour to 2 hours;

[0038] Feature 14: The drying temperature of the mixed precursor is 90℃~110℃;

[0039] Feature 15: The drying time of the mixed precursor is 10h to 14h;

[0040] Feature 16: Sintering includes: first holding at 400℃~550℃ for 2h~5h, and then holding at 650℃~750℃ for 6h~8h.

[0041] In an optional embodiment, the preparation of the lithium iron phosphate matrix includes at least one of the following conditions:

[0042] Condition 1: The general formula of lithium iron phosphate matrix is ​​Li 1-x A x Fe 1-y M y (PO 4-a )D a When the lithium iron phosphate matrix also contains M, A and D elements, the first solution also contains M source, and the second solution also contains A source and D source.

[0043] Condition 2: The first solution is sprayed into the second solution under stirring conditions by high-pressure atomization; or, the second solution is sprayed into the first solution under stirring conditions by high-pressure atomization.

[0044] Condition 3: The temperature of the hydrothermal reaction is 150℃~180℃;

[0045] Condition 4: The hydrothermal reaction time is 8 to 10 hours.

[0046] In an optional embodiment, the ferrous salt includes at least one of ferrous oxalate, ferrous chloride, and ferrous acetate;

[0047] Alternatively, the lithium source may include at least one of lithium oxalate, lithium chloride, and lithium acetate;

[0048] Alternatively, the phosphate includes at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate;

[0049] Alternatively, the source of M is a water-soluble salt of element M;

[0050] Alternatively, source A may include at least one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride, and sodium sulfide;

[0051] Alternatively, the D source may include at least one of ammonium fluoride, lithium fluoride, ammonium sulfide, lithium sulfide, ammonium bisulfite, and thiourea;

[0052] Alternatively, the ratio of the total moles of iron in the ferrous salt to the total moles of M in the M source, the total moles of lithium in the lithium source to the total moles of A in the A source, and the moles of P in the phosphate is 1:1:(0.95~1.1); wherein the molar ratio of lithium to A is (0.9~1):(0~0.1); the molar ratio of iron to M is (0.9~1):(0~0.1); and the molar ratio of oxygen in the phosphate to D in the D source is (3.9~4):(0~0.1).

[0053] Alternatively, the nozzle diameter of the high-pressure atomizing gun used for high-pressure atomization is 0.3mm to 0.6mm, and the operating pressure is 0.2MPa to 0.4MPa;

[0054] Alternatively, the stirring conditions correspond to a stirring rate of 600 r / min to 1000 r / min.

[0055] Thirdly, this disclosure provides a positive electrode sheet, wherein the active material in the positive electrode sheet includes the lithium iron phosphate positive electrode material of the aforementioned embodiments.

[0056] Fourthly, this disclosure provides a battery that includes the positive electrode sheet of the aforementioned embodiments.

[0057] The beneficial effects of this disclosure include:

[0058] The standard deviation σ(d) of the carbon coating thickness proposed in this disclosure i and the standard deviation of carbon content σ(C) wt％ ) i These figures reflect the dispersion of the carbon coating thickness and carbon content at different locations on the lithium iron phosphate cathode material particles relative to their respective average values, thus characterizing the uniformity of the carbon coating on the lithium iron phosphate substrate surface. Uniform carbon coating on the lithium iron phosphate substrate surface can inhibit grain fusion and particle growth, achieving the purpose of controlling particle size and curbing particle agglomeration. The average standard deviation σ(d) of the carbon coating thickness and the average standard deviation σ(C) of the carbon content are also shown. wt％ ) can measure the uniformity of the carbon layer distribution on the surface of lithium iron phosphate cathode materials, σ(d) and σ(C) wt％ The smaller the value of σ(d) is, the more uniform the carbon coating on the material surface, and the better the conductivity of the material; σ(d) and σ(C) wt％ The higher the value, the worse the uniformity of the carbon coating layer, which leads to a worse charge and discharge performance of the cathode material.

[0059] In addition, the thickness d of the carbon coating layer and the carbon content C of the lithium iron phosphate cathode material need to be within a suitable range; the higher the value of d, the less favorable it is for lithium ion deintercalation and diffusion at the electrode-electrolyte interface; the higher the value of C, the easier it is to inhibit the formation of crystalline LiFePO4 phase and reduce the compaction density of the material.

[0060] The lithium iron phosphate cathode material disclosed herein has σ(d) and σ(C) wt％ ), d and C are provided in this disclosure as σ(d), σ(C) wt％ The range of ), d and C has a uniform carbon coating layer. The thickness of the carbon coating layer and the carbon content at each position are relatively consistent, which is beneficial to improving the carrier mobility and conductivity of the lithium iron phosphate cathode material, and is also beneficial to further improving the electrochemical performance of the battery prepared from the lithium iron phosphate cathode material. Attached Figure Description

[0061] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this disclosure and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0062] Figure 1 is a TEM image of the lithium iron phosphate cathode material prepared in Example 6;

[0063] Figure 2 is a TEM image of the lithium iron phosphate cathode material prepared in Comparative Example 4. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions in the embodiments of this disclosure will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0065] The following provides a detailed description of the lithium iron phosphate cathode material, its preparation method, and its applications.

[0066] This disclosure provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix and a carbon coating layer covering the surface of the lithium iron phosphate matrix.

[0067] Among them, the average standard deviation of the carbon coating thickness in the lithium iron phosphate cathode material is σ(d, 0.05nm≤σ(d)≤0.35nm;

[0068] The average standard deviation of carbon content in lithium iron phosphate cathode materials is σ(C wt％ ), 0.02wt%≤σ(C) wt％ ≤0.35wt%;

[0069] Wherein, N1 is the sample size of lithium iron phosphate cathode material particles taken during the lithium iron phosphate cathode material test, and N1≥10;

[0070] σ(d) i σ(C wt％ ) i These represent the standard deviation of the carbon coating thickness and the standard deviation of the carbon content corresponding to the i-th lithium iron phosphate cathode material particle taken during the testing of lithium iron phosphate cathode material;

[0071] n is the sample size at a local location taken during the testing of the i-th lithium iron phosphate cathode material particle, n≥5; d m Let the thickness of the carbon coating be the value at the m-th local location. For the n local location samples taken, d m The average value, in nm; C m Let m be the carbon content corresponding to the m-th local position. C is the value of the n local location samples taken. m The average value is expressed in wt%.

[0072] The thickness of the carbon coating layer in the lithium iron phosphate cathode material is d. 1nm≤d≤10nm;

[0073] The carbon content of lithium iron phosphate cathode material is C. 1wt% ≤ C ≤ 10wt%;

[0074] d is the number of local position samples taken from the i-th lithium iron phosphate cathode material particle. m The average value;

[0075] C is the value of n local location samples taken for the i-th lithium iron phosphate cathode material particle. m The average value.

[0076] The selection of local locations of the lithium iron phosphate cathode material particles is random. In some preferred embodiments, random selection does not specifically define a certain range, and the selection is not concentrated in a small range or region of the particles. The sample selection range preferably covers the entire cathode material particle.

[0077] Standard deviation of carbon coating thickness σ(d) i and the standard deviation of carbon content σ(C) wt％ ) i These figures reflect the dispersion of the carbon coating thickness and carbon content at different locations on the lithium iron phosphate cathode material particles relative to their respective average values, thus characterizing the uniformity of the carbon coating on the surface of the lithium iron phosphate matrix particles. Typically, σ(d) i and σ(C wt％ ) i The smaller the value of σ(d), the smaller the dispersion of the carbon coating thickness and carbon content on the surface of the cathode material particles relative to their respective average values, the more uniform the carbon coating, and the better the conductivity of the material; i and σ(C wt％ ) i The higher the value, the greater the dispersion of the carbon coating thickness and carbon content on the surface of the cathode material particles relative to their respective average values, indicating a poorer uniformity of the carbon coating and a deterioration in the charge-discharge performance of the material. Furthermore, the average standard deviation σ(d) of the carbon coating thickness and the average standard deviation σ(C) of the carbon content... wt％ It also conforms to the above-mentioned variation pattern, and its value can measure the uniformity of the carbon layer distribution on the surface of lithium iron phosphate cathode material.

[0078] In addition, the thickness d of the carbon coating layer and the carbon content C of the lithium iron phosphate cathode material need to be within a suitable range. The higher the value of d, the less favorable it is for lithium ion deintercalation and diffusion at the electrode-electrolyte interface; the higher the value of C, the easier it is to inhibit the formation of crystalline LiFePO4 phase and reduce the compaction density of the material.

[0079] Continuing from the above, this disclosure provides conditions that satisfy σ(d) and σ(C). wt％Lithium iron phosphate cathode materials in the ranges of ), d, and C have a uniform carbon coating layer. The thickness of the carbon coating layer and the carbon content at each location are relatively consistent, which is beneficial to improving the carrier mobility and conductivity of lithium iron phosphate cathode materials, and further improving the electrochemical performance of batteries prepared from lithium iron phosphate cathode materials.

[0080] In some alternative embodiments, the d of the lithium iron phosphate cathode material can be 1.5nm to 9.1nm, such as 1.5nm, 1.6nm, 1.7nm, 2.7nm, 3.0nm, 3.8nm, 4.6nm, 5.5nm, 6.3nm, 8.7nm or 9.1nm, or other values ​​within the range of 1.5nm to 9.1nm.

[0081] In some alternative embodiments, the σ(d) of the lithium iron phosphate cathode material can be 0.055nm to 0.343nm, such as 0.055nm, 0.063nm, 0.101nm, 0.112nm, 0.116nm, 0.136nm, 0.210nm, 0.254nm, 0.304nm, 0.308nm, 0.310nm, 0.314nm, 0.338nm, or 0.343nm, or other values ​​within the range of 0.055nm to 0.343nm.

[0082] In some alternative embodiments, the C of the lithium iron phosphate cathode material can be 1.94wt% to 9.68wt%, such as 1.94wt%, 1.95wt%, 1.96wt%, 1.97wt%, 1.98wt%, 1.99wt%, 3.47wt%, 3.75wt%, 4.75wt%, 5.15wt%, 6.25wt%, 7.55wt%, 9.23wt%, or 9.68wt%, or other values ​​within the range of 1.94wt% to 9.68wt%.

[0083] In some alternative implementations, the σ(C) of the lithium iron phosphate cathode material wt％ The value can be 0.024wt% to 0.336wt%, such as 0.024wt%, 0.040wt%, 0.089wt%, 0.105wt%, 0.107wt%, 0.130wt%, 0.204wt%, 0.240wt%, 0.296wt%, 0.298wt%, 0.301wt%, 0.306wt%, 0.325wt%, or 0.336wt%, etc., or other values ​​within the range of 0.024wt% to 0.336wt%.

[0084] In some alternative implementations, the shape factor of the lithium iron phosphate cathode material is...

[0085] Wherein, N2 is the sample size of lithium iron phosphate cathode material particles taken during the testing of lithium iron phosphate cathode material, and N2≥10;

[0086] The shape factor is the j-th lithium iron phosphate cathode material particle taken during the lithium iron phosphate cathode material test.

[0087] D s Let D be the diameter of the standard circle whose projected area is equal to that of the j-th lithium iron phosphate cathode material particle in the TEM two-dimensional projection diagram, and let D be the diameter of the smallest circumcircle of the particle projection in the TEM two-dimensional projection diagram of the j-th lithium iron phosphate cathode material particle.

[0088] Shape factor This indicates that the shape of the lithium iron phosphate cathode material particles is similar to that of a standard sphere. The degree of deviation, The closer the value is to 1, the closer the particle morphology is to spherical shape, and the more regular the particle morphology. In this disclosure, The closer the value is to 1, the more regular the shape of the lithium iron phosphate cathode material particles, the more uniform the particle size, the better the particle dispersion, and the more conducive it is to the diffusion and transport of lithium ions and electrons. The smaller the value, the worse the morphological regularity of the lithium iron phosphate cathode material particles, the more severe the particle aggregation, which affects the electrochemical performance of the battery made from it.

[0089] Continuing from the above, this disclosure provides for satisfying The lithium iron phosphate cathode material has a regular and uniform particle morphology. This uniform particle morphology is not only conducive to the diffusion of lithium ions, but also helps to improve the yield strength of the electrode prepared from the lithium iron phosphate cathode material, as well as the electrochemical performance of the battery prepared from the electrode.

[0090] In some alternative implementations, lithium iron phosphate cathode materials It can be 0.550 to 0.910, such as 0.550, 0.670, 0.673, 0.677, 0.695, 0.702, 0.712, 0.761, 0.858, 0.880, 0.893, 0.907 or 0.910, or other values ​​within the range of 0.550 to 0.910.

[0091] In some alternative implementations, the carrier mobility of the lithium iron phosphate cathode material can be 20.03 × 10⁻⁶. -6 cm 2 / V·s~21.71×10 -6cm 2 / V·s, such as 20.03×10 -6 cm 2 / V·s、21.05×10 -6 cm 2 / V·s、20.20×10 -6 cm 2 / V·s, 20.26×10 -6 cm 2 / V·s、20.29×10 -6 cm 2 / V·s、20.30×10 -6 cm 2 / V·s、20.33×10 -6 cm 2 / V·s, 20.42×10 -6 cm 2 / V·s、20.50×10 -6 cm 2 / V·s、20.60×10 -6 cm 2 / V·s, 20.62×10 -6 cm 2 / V·s, 20.86×10 -6 cm 2 / V·s、21.38×10 -6 cm 2 / V·s or 21.71×10 -6 cm 2 / V·s, etc., can also be 20.03×10 -6 cm 2 / V·s~21.71×10 -6 cm 2 Other values ​​within the range / V·s.

[0092] In some alternative implementations, the parameter factors of the lithium iron phosphate cathode material 0.07nm -1 ≤Q≤7nm -1 .

[0093] The above parameter factors and σ(d) and σ(C) wt％ The correspondence between these parameters can comprehensively reflect the electrochemical performance of batteries using lithium iron phosphate cathode materials. When... The closer to 1 (i.e., the more regular the particle morphology) and the better σ(d) and σ(C) wt％ The smaller the value of Q (i.e., the more uniform the carbon coating on the material surface), the larger the parameter factor Q value, and the better the electrochemical performance of the battery corresponding to the lithium iron phosphate cathode material.

[0094] In some alternative implementations, the Q of the lithium iron phosphate cathode material can be 0.072 nm. -1 ~6.596nm -1 For example, 0.072nm -1 0.073nm -1 0.074nm -1 0.076nm -1 0.079nm -1 0.111nm -1 0.205nm -1 0.505nm -1 0.540nm -1 0.648nm -1 1.012nm -1 3.405nm -1 Or 6.596nm -1 etc., can also be 0.072nm -1 ~6.596nm -1 Other values ​​within the range.

[0095] Continuing from the above, this disclosure provides simultaneous satisfaction of σ(d) and σ(C) wt％ ), Furthermore, lithium iron phosphate cathode materials with Q values ​​within a certain range have regular particle morphology and uniform carbon coating, which enables batteries using them as cathode materials to have better electrochemical performance (such as specific capacity, rate capability, and cycle performance).

[0096] In some optional embodiments, the specific surface area of ​​the above-mentioned lithium iron phosphate cathode material can be 9.3 m². 2 / g~15.1m 2 / g, such as 9.3m 2 / g, 10.2m 2 / g, 11.7m 2 / g, 11.9m 2 / g, 12.0m 2 / g, 12.1m 2 / g, 12.3m 2 / g, 12.4m 2 / g, 12.6m 2 / g, 12.8m 2 / g, 13.2m 2 / g, 13.7m 2 / g, 14.3m 2 / g or 15.1m 2 / g, etc., can also be 9.3m 2 / g~15.1m2 Other values ​​within the / g range.

[0097] In some alternative embodiments, the D of the above-mentioned lithium iron phosphate cathode material 50 The value can be 0.85μm to 1.50μm, such as 0.85μm, 0.90μm, 0.96μm, 1.00μm, 1.04μm, 1.07μm, 1.06μm, 1.08μm, 1.10μm, 1.12μm, 1.14μm, 1.16μm, 1.35μm or 1.50μm, or other values ​​within the range of 0.85μm to 1.50μm.

[0098] In some alternative embodiments, the general formula of the lithium iron phosphate matrix can be Li 1-x A x Fe 1-y M y (PO 4-a )D a Wherein, A may include at least one of Na and Mg; M may include at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D may include at least one of F and S; 0≤x≤0.1, 0≤y≤0.1 and 0≤a≤0.1.

[0099] Accordingly, this disclosure also provides a method for preparing the above-mentioned lithium iron phosphate cathode material, which may include the following steps: ball milling a lithium iron phosphate matrix and a composite carbon source to obtain a mixed precursor; drying the mixed precursor and sintering it to obtain the lithium iron phosphate cathode material.

[0100] In some optional embodiments, the preparation of the lithium iron phosphate matrix may include: mixing a first solution containing ferrous salt with a second solution containing lithium source and phosphate to obtain a mixture; adjusting the pH of the mixture to 6-8 and then carrying out a hydrothermal reaction, and collecting the solid after the reaction is completed.

[0101] The ferrous salt may, by way of example but not by way of limitation, include at least one of ferrous oxalate, ferrous chloride and ferrous acetate.

[0102] Lithium sources may include, by way of example but not by way of limitation, at least one of lithium oxalate, lithium chloride and lithium acetate.

[0103] Phosphates may, by way of example but not by way of limitation, include at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate.

[0104] In this disclosure, when the lithium iron phosphate matrix also contains element M, the source of M is present in the first solution; when the lithium iron phosphate matrix also contains at least one of element A and element D, the corresponding source of A and source of D are present in the second solution.

[0105] For example, the lithium iron phosphate matrix also contains elements M, A, and D. The first solution containing ferrous salt also contains an M source, and the second solution containing lithium source and phosphate also contains an A source and a D source. Under these conditions, the ferrous salt and M source can be dispersed in deionized water, and dissolved oxygen can be removed by purging with nitrogen gas for a period of time (e.g., 1 hour) under stirring to obtain the first solution. Correspondingly, the lithium source, phosphate, A source, and D source can be dispersed in deionized water and stirred at high speed under a nitrogen atmosphere to obtain the second solution.

[0106] The aforementioned source M can be a water-soluble salt of element M.

[0107] Source A may, by way of example but not limitation, include at least one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride, and sodium sulfide.

[0108] D source may, by way of example but not limitation, include at least one of ammonium fluoride, lithium fluoride, ammonium sulfide, lithium sulfide, ammonium bisulfite, and thiourea.

[0109] In some optional embodiments, the ratio of the total number of moles of iron in the ferrous salt to the total number of moles of M in the M source, the total number of moles of lithium in the lithium source to the total number of moles of A in the A source, and the ratio of the number of moles of P in the phosphate is 1:1:(0.95 to 1.1), such as 1:1:0.95, 1:1:0.96, 1:1:0.97, 1:1:0.98, 1:1:0.99, 1:1:1, 1:1:1.05, or 1:1:1.1, etc., or other values ​​within the range of 1:1:(0.95 to 1.1).

[0110] In some alternative implementations, the molar ratio of lithium to alumina can be (0.9-1):(0-0.1), such as 0.9:0.1, 0.95:0.05, or 1:0, or other values ​​within the range of (0.9-1):(0-0.1).

[0111] In some alternative implementations, the molar ratio of iron to M can be (0.9-1):(0-0.1), such as 0.9:0.1, 0.95:0.05 or 1:0, or other values ​​within the range of (0.9-1):(0-0.1).

[0112] In some alternative embodiments, the molar ratio of oxygen in the phosphate to D in the D source can be (3.9–4):(0–0.1), such as 3.9:0.1, 3.95:0.05, or 4:0, or other values ​​within the range of (3.9–4):(0–0.1).

[0113] In some alternative embodiments, the first solution can be sprayed into the second solution under stirring conditions via high-pressure atomization. In other alternative embodiments, the second solution can also be sprayed into the first solution under stirring conditions via high-pressure atomization.

[0114] The nozzle diameter of the high-pressure atomizing spray gun used for high-pressure atomization can be 0.3mm to 0.6mm, such as 0.3mm, 0.35mm, 0.4mm, 0.45mm, 0.5mm, 0.55mm or 0.6mm, or other values ​​within the range of 0.3mm to 0.6mm.

[0115] The operating pressure for high-pressure atomization can be 0.2MPa to 0.4MPa, such as 0.2MPa, 0.25MPa, 0.3MPa, 0.35MPa or 0.4MPa, or other values ​​within the range of 0.2MPa to 0.4MPa.

[0116] In some optional embodiments, the stirring rate corresponding to the stirring conditions can be 600 r / min to 1000 r / min, such as 600 r / min, 650 r / min, 700 r / min, 750 r / min, 800 r / min, 850 r / min, 900 r / min, 950 r / min or 1000 r / min, or other values ​​within the range of 600 r / min to 1000 r / min.

[0117] In some alternative embodiments, ammonia can be used to adjust the pH of the mixture. The pH can be adjusted to 6, 6.5, 7, 7.5 or 8, or other values ​​within the range of 6 to 8.

[0118] After adjusting the pH value, the mixture can be stirred for a period of time under the protection of room temperature and an inert atmosphere (such as nitrogen atmosphere). Then, it is transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene for hydrothermal reaction. After the reaction is completed, the product is cooled to room temperature and filtered. The filtered solid phase is then washed by centrifugation with deionized water and anhydrous ethanol, and finally dried to obtain the lithium iron phosphate matrix.

[0119] The temperature of the hydrothermal reaction can be between 150℃ and 180℃, such as 150℃, 155℃, 160℃, 165℃, 170℃, 175℃ or 180℃, or other values ​​within the range of 150℃ to 180℃.

[0120] The hydrothermal reaction time can be 8h to 10h, such as 8h, 8.5h, 9h, 9.5h or 10h, or other values ​​within the range of 8h to 10h.

[0121] The drying temperature can be between 60℃ and 80℃, such as 60℃, 65℃, 70℃, 75℃ or 80℃, or other values ​​within the range of 60℃ to 80℃.

[0122] The drying time can be 10h to 14h, such as 10h, 10.5h, 11h, 11.5h or 12h, or other values ​​within the range of 10h to 14h.

[0123] Continuing from the above, this disclosure involves pouring the first solution (or the second solution) into a high-pressure atomizing spray gun, atomizing it through a high-pressure nozzle, and then spraying it into a high-speed stirred second solution (or the first solution) to obtain a uniform seed precursor solution. The pH value is then adjusted with ammonia, and the solution is placed in a high-pressure reactor for hydrothermal reaction to obtain lithium iron phosphate matrix particles. The high-pressure atomization-high-speed stirring method enhances the initial micro-mixing and uniform dispersion among ferrous ions, lithium ions, and phosphate ions, effectively mitigating the rate differences in lithium iron phosphate crystal nucleus formation caused by excessively high local concentrations of reacting ions. It also provides a uniform micro-nucleation environment for the formation of the seed precursor. Further hydrothermal reaction of this seed precursor can form a lithium iron phosphate matrix with concentrated particle size distribution, good dispersibility, and uniform morphology.

[0124] In some alternative embodiments, the preparation of the composite carbon source may include mixing polyethylene glycol gel with the molten carbon source.

[0125] The carbon source may, by way of example but not by way of limitation, include at least one of sorbitol, erythritol, fructose, rhamnose and malic acid.

[0126] The molar ratio of lithium to carbon source and polyethylene glycol in the lithium iron phosphate matrix can be 1:(0.03~0.05):(0.02~0.04), such as 1:0.03:0.02, 1:0.03:0.03, 1:0.03:0.04, 1:0.04:0.02, 1:0.04:0.03, 1:0.04:0.04, 1:0.05:0.02, 1:0.05:0.03 or 1:0.05:0.04, etc., or other values ​​within the range of 1:(0.03~0.05):(0.02~0.04).

[0127] The aforementioned carbon source can be melted in a high-temperature electric furnace at a temperature of 130℃ to 150℃. Specifically, the melting temperature can be set according to the selected carbon source to achieve the melting of the carbon source.

[0128] Polyethylene glycol gel can be prepared by dissolving polyethylene glycol powder in dimethyl sulfoxide, stirring until homogeneous, and then heating in a water bath at 50°C–60°C. Alternatively, other methods and conditions can also be used to prepare polyethylene glycol gel.

[0129] In some optional embodiments, the ball milling time between the lithium iron phosphate matrix and the composite carbon source can be 1h to 2h, such as 1h, 1.5h or 2h.

[0130] In some alternative embodiments, the drying temperature of the mixed precursor can be 90°C to 110°C, such as 90°C, 95°C, 100°C, 105°C or 110°C, or other values ​​within the range of 90°C to 110°C.

[0131] The drying time for the mixed precursors can be 10h to 14h, such as 10h, 11h, 12h, 13h or 14h, or other values ​​within the range of 10h to 14h.

[0132] In some alternative embodiments, the dried mixed precursor is first ground and pulverized, and then the fully ground precursor is placed in a muffle furnace filled with high-purity argon gas for sintering.

[0133] The sintering process includes a first stage and a second stage. The first stage can be held at 400℃~550℃ (e.g., 400℃, 420℃, 450℃, 480℃, 500℃, 520℃ or 550℃, etc.) for 2h~5h (e.g., 2h, 3h, 4h or 5h, etc.). The second stage can be held at 650℃~750℃ (e.g., 650℃, 680℃, 700℃, 720℃ or 750℃, etc.) for 6h~8h (e.g., 6h, 7h or 8h, etc.).

[0134] To further improve the conductivity and crystallinity of lithium iron phosphate, this disclosure first mixes molten carbon source with polyethylene glycol gel to obtain a composite carbon source, then ball-mills and mixes it with a lithium iron phosphate matrix, dries it, and calcines it in a muffle furnace at low and high temperatures to obtain lithium iron phosphate cathode material. In this process, the molten carbon source and polyethylene glycol gel, as a composite carbon source, carbon-coat the lithium iron phosphate matrix. Compared to the traditional solid-phase method, the molten carbon source can easily coat the surface of the lithium iron phosphate matrix. The carbon network structure formed by the polyethylene glycol gel during drying and calcination can stabilize the carbon coating layer on the particle surface, thus forming a uniform carbon coating layer. Furthermore, compared to water solvents, dimethyl sulfoxide (DMSO) has lower viscosity and surface tension. Using DMSO as a solvent to prepare the polyethylene glycol gel, after mixing the lithium iron phosphate matrix with the composite carbon source, the good wettability of DMSO allows the molten carbon source to be more uniformly coated onto the surface of the lithium iron phosphate matrix and penetrate into its micropores.

[0135] In summary, the preparation method provided in this disclosure can be used to prepare lithium iron phosphate cathode materials with regular morphology and uniform carbon coating.

[0136] In addition, this disclosure also provides a positive electrode sheet in which the active material includes the above-mentioned lithium iron phosphate positive electrode material.

[0137] This disclosure also provides a battery cell including the aforementioned positive electrode plate.

[0138] For example, the aforementioned battery cells can be used, but are not limited to, in electrical devices such as vehicles, ships, or aircraft.

[0139] This disclosure also provides a battery comprising the aforementioned battery cells.

[0140] This disclosure also provides an electrical device comprising the aforementioned battery cell and / or battery. As examples, the electrical device may include, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys may include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys; spacecraft may include airplanes, rockets, space shuttles, and spacecraft.

[0141] The features and performance of this disclosure will be further described in detail below with reference to embodiments.

[0142] Example 1

[0143] The difference between this embodiment and Embodiment 3 below is that:

[0144] In S1, the ferrous salt is ferrous chloride, the lithium source is lithium acetate, and the phosphate is ammonium dihydrogen phosphate; the nozzle diameter of the high-pressure atomizing spray gun is 0.7 mm, the operating pressure is 0.1 MPa; the stirring speed of the high-speed stirrer is 550 rpm; the pH value is adjusted to 6; and the heat preservation time in the electric heating blast oven is 9 hours.

[0145] In S2, the melting temperature of rhamnose is 120℃;

[0146] In S3, the molar ratio of Li to rhamnose and polyethylene glycol in the lithium iron phosphate matrix is ​​1:0.04:0.02; the temperature of the first stage of sintering is 450℃.

[0147] Example 2

[0148] The difference between this embodiment and Embodiment 3 below is that:

[0149] In S1, the ferrous salt is ferrous chloride, the lithium source is lithium acetate, and the phosphate is ammonium dihydrogen phosphate; the nozzle diameter of the high-pressure atomizing spray gun is 0.6 mm, the operating pressure is 0.2 MPa; the stirring speed of the high-speed stirrer is 650 rpm; the pH value is adjusted to 6; and the heat preservation time in the electric heating blast oven is 9 hours.

[0150] In S2, the melting temperature of rhamnose is 130℃;

[0151] In S3, the molar ratio of Li to rhamnose and polyethylene glycol in the lithium iron phosphate matrix is ​​1:0.04:0.02; the temperature of the first stage of sintering is 450℃.

[0152] Example 3

[0153] This embodiment provides a lithium iron phosphate cathode material, the preparation method of which includes:

[0154] S1: Preparation of lithium iron phosphate matrix.

[0155] Ferrous acetate was weighed and dispersed in deionized water. Under stirring, nitrogen was applied for 1 hour to remove dissolved oxygen, yielding the first solution. Lithium oxalate and ammonium hydrogen phosphate were weighed and dispersed in deionized water. The mixture was stirred at high speed (600 r / min) under a nitrogen atmosphere, yielding the second solution. The molar ratio of Li in lithium oxalate, Fe in ferrous acetate, and P in ammonium hydrogen phosphate was 1:1:0.95.

[0156] The first solution was poured into a high-pressure atomizing spray gun with a nozzle diameter of 0.5 mm and an operating pressure of 0.3 MPa. After atomization by the high-pressure nozzle, it was sprayed into the second solution under high-speed stirring to obtain a mixture. The pH of the obtained mixture was adjusted to 7 with ammonia water, and the mixture was stirred for 30 min at room temperature under a nitrogen atmosphere. Then, it was transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene and placed in an electric heating oven at 150°C for 8 h. After the reaction was completed, the product was cooled to room temperature and filtered. The filtered solid phase was then washed by centrifugation with deionized water and anhydrous ethanol, and finally dried in a vacuum oven at 70°C for 12 h to obtain the lithium iron phosphate matrix.

[0157] S2: Preparation of composite carbon source.

[0158] Rhamnose was weighed and placed in a high-temperature electric furnace and heated to 135°C to melt. Polyethylene glycol powder was weighed and dissolved in dimethyl sulfoxide. After stirring evenly, the mixture was heated to 50°C in a water bath to obtain polyethylene glycol gel. The molten rhamnose and polyethylene glycol gel were mixed to obtain a composite carbon source.

[0159] S3: Preparation of lithium iron phosphate cathode material.

[0160] The lithium iron phosphate matrix in S1 and the composite carbon source in S2 were ball-milled and mixed for 1.5 h to obtain a mixed precursor. The mixed precursor was dried in a vacuum oven at 100°C for 12 h and then ground. Subsequently, the fully ground mixed precursor was sintered in a muffle furnace with high-purity argon gas to obtain the lithium iron phosphate cathode material.

[0161] The molar ratio of Li to rhamnose and polyethylene glycol in the lithium iron phosphate matrix is ​​1:0.035:0.02.

[0162] The sintering process includes: heating to 400℃ at a rate of 5℃ / min and holding for 2 hours; then heating to 650℃ and calcining for 6 hours.

[0163] Example 4

[0164] The difference between this embodiment and embodiment 3 is as follows:

[0165] In S1, the molar ratio of Li in lithium oxalate, Fe in ferrous acetate and P in ammonium hydrogen phosphate is 1:1:1; the nozzle diameter of the high-pressure atomizing spray gun is 0.45 mm and the operating pressure is 0.3 MPa; the stirring speed of the high-speed stirrer is 700 rpm.

[0166] In S2, the carbon source is sorbitol, and the melting temperature of sorbitol is 145℃; the water bath heating temperature of polyethylene glycol is 60℃.

[0167] In S3, the molar ratio of Li element in the lithium iron phosphate matrix to sorbitol and polyethylene glycol is 1:0.05:0.04; the temperature of the first sintering stage is 550℃ and the time is 5h; the temperature of the second sintering stage is 750℃ and the time is 8h.

[0168] Example 5

[0169] The difference between this embodiment and embodiment 3 is as follows:

[0170] In S1, the ferrous salt is ferrous chloride, the lithium source is lithium acetate, and the phosphate is ammonium dihydrogen phosphate; the molar ratio of Li in lithium acetate, Fe in ferrous chloride, and P in ammonium dihydrogen phosphate is 1:1:1; the nozzle diameter of the high-pressure atomizing spray gun is 0.4 mm, and the operating pressure is 0.35 MPa; the stirring speed of the high-speed stirrer is 800 rpm; the heat preservation temperature in the electric heating blast oven is 155℃, and the heat preservation time is 9 hours.

[0171] In S2, the melting temperature of rhamnose is 150℃; the water bath heating temperature of polyethylene glycol is 55℃.

[0172] In S3, the molar ratio of Li to rhamnose and polyethylene glycol in the lithium iron phosphate matrix is ​​1:0.05:0.03; the temperature of the first sintering stage is 500℃ and the time is 3h; the temperature of the second sintering stage is 700℃ and the time is 7h.

[0173] Example 6

[0174] The difference between this embodiment and embodiment 3 is as follows:

[0175] In S1, the ferrous salt is ferrous chloride, the lithium source is lithium acetate, and the phosphate is ammonium dihydrogen phosphate; the molar ratio of Li in lithium acetate, Fe in ferrous chloride, and P in ammonium dihydrogen phosphate is 1:1:1.1; the nozzle diameter of the high-pressure atomizing spray gun is 0.4 mm, and the operating pressure is 0.35 MPa; the stirring speed of the high-speed stirrer is 900 rpm; the pH value is adjusted to 8; the heat preservation temperature in the electric heating blast oven is 160℃, and the heat preservation time is 9 hours.

[0176] In S2, the melting temperature of rhamnose is 150℃; the water bath heating temperature of polyethylene glycol is 60℃.

[0177] In S3, the molar ratio of Li to rhamnose and polyethylene glycol in the lithium iron phosphate matrix is ​​1:0.05:0.025; the temperature of the first sintering stage is 500℃ and the time is 3h; the time of the second sintering stage is 7h.

[0178] Example 7

[0179] The difference between this embodiment and embodiment 3 is as follows:

[0180] In S1, the ferrous salt is ferrous chloride, the lithium source is lithium acetate, and the phosphate is ammonium dihydrogen phosphate; the first solution also contains nickel chloride; the ratio of the total moles of Li in lithium acetate, Fe in ferrous chloride, and Ni in nickel chloride to the moles of P in ammonium dihydrogen phosphate is 1:1:0.95, wherein the molar ratio of Fe in ferrous chloride to Ni in nickel chloride is 0.985:0.015; the nozzle diameter of the high-pressure atomizing spray gun is 0.3 mm, the operating pressure is 0.35 MPa; the stirring speed of the high-speed stirrer is 950 rpm; the pH value is adjusted to 8; the temperature in the electric heating blast oven is 170℃, and the holding time is 9 hours;

[0181] In S2, the carbon source is sorbitol, and the melting temperature of sorbitol is 140℃;

[0182] In S3, the molar ratio of Li in the lithium iron phosphate matrix to sorbitol and polyethylene glycol is 1:0.045:0.02; the temperature of the first stage of sintering is 450℃.

[0183] Example 8

[0184] The difference between this embodiment and embodiment 3 is as follows:

[0185] In S1, the ferrous salt is ferrous chloride, the lithium source is lithium acetate, and the phosphate is ammonium dihydrogen phosphate; the second solution also contains ammonium fluoride; the molar ratio of Li in lithium acetate, Fe in ferrous chloride, and P in ammonium dihydrogen phosphate is 1:1:1; the molar ratio of oxygen in ammonium dihydrogen phosphate to F in ammonium fluoride is 3.998:0.002; the nozzle diameter of the high-pressure atomizing spray gun is 0.3 mm, and the operating pressure is 0.4 MPa; the stirring speed of the high-speed stirrer is 950 rpm; the heat preservation temperature in the electric heating blast oven is 175℃, and the heat preservation time is 10 h;

[0186] In S2, the carbon source is sorbitol; the melting temperature of sorbitol is 145℃; the water bath heating temperature of polyethylene glycol is 55℃.

[0187] In S3, the molar ratio of Li element in the lithium iron phosphate matrix to sorbitol and polyethylene glycol is 1:0.05:0.02; the temperature of the first sintering stage is 480℃ and the time is 3h; the temperature of the second sintering stage is 680℃.

[0188] Example 9

[0189] The difference between this embodiment and embodiment 3 is as follows:

[0190] In S1, the ferrous salt is ferrous chloride, the lithium source is lithium acetate, and the phosphate is ammonium dihydrogen phosphate; the second solution also contains sodium chloride; the ratio of the total molar amount of Li in lithium acetate to Na in sodium chloride, and the ratio of the molar amount of Fe in ferrous chloride to P in ammonium dihydrogen phosphate is 1:1:0.95; the molar ratio of Li in lithium acetate to Na in sodium chloride is 0.985:0.015; the nozzle diameter of the high-pressure atomizing spray gun is 0.3 mm, and the operating pressure is 0.4 MPa; the stirring speed of the high-speed stirrer is 1000 rpm; the heat preservation temperature in the electric heating blast oven is 180℃, and the heat preservation time is 10 h;

[0191] In S2, the carbon source is fructose; the melting temperature of fructose is 145℃; the water bath heating temperature of polyethylene glycol is 60℃.

[0192] In S3, the molar ratio of Li to fructose and polyethylene glycol in the lithium iron phosphate matrix is ​​1:0.05:0.035; the temperature of the first sintering stage is 500℃ and the time is 5h; the temperature of the second sintering stage is 750℃ and the time is 8h.

[0193] Example 10

[0194] The difference between this embodiment and Embodiment 3 is that the nozzle diameter in the high-pressure atomization mixing step is 0.4 mm, the operating pressure is 0.4 MPa, the high-speed stirring rate is 650 r / min, and the other steps and conditions remain unchanged.

[0195] Example 11

[0196] The difference between this embodiment and embodiment 3 is that the heat preservation temperature in the electric heating oven is 170℃ and the heat preservation time is 9h; the other steps and conditions remain unchanged.

[0197] Example 12

[0198] The difference between this embodiment and Embodiment 3 is that the melting temperature of the carbon source is 150°C and the temperature of the gel water bath is 60°C; the remaining steps and conditions remain unchanged.

[0199] Example 13

[0200] The difference between this embodiment and Embodiment 3 is that the temperature and time of the first stage of sintering are 450℃ and 3h, respectively, and the temperature and time of the second stage are 700℃ and 7h, respectively; the remaining steps and conditions remain unchanged.

[0201] Example 14

[0202] The difference between this embodiment and Embodiment 3 is that manganese chloride and ferric chloride are dispersed together in deionized water at a molar ratio of Fe to Mn of 0.985:0.015 to obtain the first solution; the remaining steps and conditions remain unchanged.

[0203] Comparative Example 1

[0204] The difference between this comparative example and Example 3 is as follows:

[0205] In S1, the molar ratio of Li in lithium oxalate, Fe in ferrous acetate, and P in ammonium hydrogen phosphate is 1:1:1.1; the nozzle diameter of the high-pressure atomizing spray gun is 0.6 mm, the operating pressure is 0.2 MPa; the pH value is adjusted to 6; and the heat preservation time in the electric heating oven is 9 hours.

[0206] In S2, the melting temperature of rhamnose is 100℃;

[0207] In S3, the molar ratio of Li to rhamnose and polyethylene glycol in the lithium iron phosphate matrix is ​​1:0.03:0.02; the temperature of the first sintering stage is 350℃; and the temperature of the second stage is 650℃.

[0208] Comparative Example 2

[0209] The difference between this comparative example and Example 3 is that the preparation of the lithium iron phosphate matrix did not use high-pressure atomization mixing, but instead the materials were directly mixed and reacted.

[0210] Specifically:

[0211] S1: Ferrous acetate, lithium oxalate, and ammonium hydrogen phosphate were directly dispersed in deionized water at a molar ratio of Li:Fe:P = 1:1:0.95. After removing dissolved oxygen from the solution by blowing nitrogen gas for 30 min under stirring, the mixture was transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene and placed in an electric heating oven at 150℃ for 8 h. After the reaction was completed, the product was cooled to room temperature and filtered. The filtered solid phase was then washed by centrifugation with deionized water and anhydrous ethanol, and finally dried in a vacuum oven at 70℃ for 12 h to obtain the lithium iron phosphate precursor.

[0212] The steps and conditions for S2 and S3 are the same as those in Example 3.

[0213] Comparative Example 3

[0214] The difference between this comparative example and Example 3 is that the carbon source is not melt-treated and is not compounded with polyethylene glycol gel.

[0215] Specifically:

[0216] The steps and conditions in S1 are the same as in Example 3.

[0217] S2: The lithium iron phosphate matrix from S1 and rhamnose (the molar ratio of lithium to rhamnose in the lithium iron phosphate matrix particles is 1:0.035) are dispersed in anhydrous ethanol and ball-milled for 1.5 h to obtain a mixed precursor. The mixed precursor is dried in a vacuum oven at 100°C for 12 h and then ground. Subsequently, the fully ground precursor is placed in a muffle furnace with high-purity argon gas for sintering. The temperature is increased to 400°C at a rate of 5°C / min and held for 2 h. The temperature is then increased to 650°C and calcined for 6 h to obtain the lithium iron phosphate cathode material.

[0218] Comparative Example 4

[0219] The difference between this comparative example and Example 3 is that lithium iron phosphate was prepared directly using the hydrothermal-high temperature calcination method.

[0220] Specifically, S1 in this comparative example is the same as that in comparative example 2; S2 in this comparative example is basically the same as that in comparative example 3, the only difference being that the molar ratio of lithium to rhamnose in the lithium iron phosphate matrix is ​​1:0.05.

[0221] Test case

[0222] (1) The performance of the lithium iron phosphate cathode materials prepared in Examples 1 to 14 and Comparative Examples 1 to 4 was tested.

[0223] The performance testing method is as follows:

[0224] ① The thickness of the carbon coating layer and the content and distribution of surface carbon elements in lithium iron phosphate cathode materials were tested using a FEI Tecnai F20 transmission electron microscope (TEM) and energy dispersive spectroscopy (EDX) system. Micro-regions (local locations) of individual lithium iron phosphate cathode material particles were selected using the TEM imaging system. High-resolution images of these micro-regions were acquired at high magnification using the objective aperture of the electron microscope. The morphological image data of the particles were then measured and analyzed using an image acquisition system to determine the coating layer thickness of the selected micro-regions. Simultaneously, the carbon element distribution on the surface of the selected micro-regions was qualitatively and quantitatively analyzed using the EDX detector. Ten cathode material particles were randomly selected from each sample.

[0225] Where, σ(d) and σ(C) wt％ The calculation method for ) is as follows:

[0226] N1 is the sample size of lithium iron phosphate cathode material particles taken for testing, and its value is 10.

[0227] n is the sample size at a local location taken during the testing of the i-th lithium iron phosphate cathode material particle, and its value is 5; d m Let be the thickness of the carbon coating layer corresponding to the m-th local position of the lithium iron phosphate cathode material particle. d of five local location samples taken from lithium iron phosphate cathode material particles m The average value, in nm; C m This represents the carbon content at the m-th local position of the lithium iron phosphate cathode material particle. C values ​​of five local samples taken from lithium iron phosphate cathode material particles m The average value, in wt%.

[0228] The carbon coating thickness of the lithium iron phosphate cathode material is Carbon content is

[0229] ② The microstructure of lithium iron phosphate cathode material particles was observed using a FEI Tecnai F20 transmission electron microscope, and their transmission electron microscope images (TEM two-dimensional projection images) were obtained. ImageJ was used to perform shape analysis on the TEM two-dimensional projection images of the lithium iron phosphate cathode material particles, and parameters such as the two-dimensional projection area and the area of ​​the minimum circumscribed circle of the projection of the lithium iron phosphate cathode material particles were collected. Ten cathode material particles were randomly selected from the lithium iron phosphate cathode material samples.

[0230] in, The calculation method is as follows: Defined as the shape factor corresponding to the j-th lithium iron phosphate cathode material particle taken during testing; specifically... In the TEM two-dimensional projection image corresponding to the lithium iron phosphate cathode material particle, the diameter (D) of the standard circle with the same two-dimensional projected area as the lithium iron phosphate cathode material particle is shown. s The ratio of (μm) to the diameter (D, μm) of the smallest circumscribed circle of the two-dimensional projection of the lithium iron phosphate cathode material particle; N2 is the sample size of the tested lithium iron phosphate cathode material particles, with a value of 10.

[0231] ③ Specific Surface Area (BET) Test: Isothermal adsorption-desorption analysis was performed using a Micro ASAP2460 fully automated specific surface area analyzer with the static volumetric method. The specific surface area was determined by measuring the adsorption-desorption behavior of nitrogen on the lithium iron phosphate cathode material, with units of m². 2 / g.

[0232] ④ Particle size: The particle size of the lithium iron phosphate cathode material was tested using a GSL-101BI laser particle size analyzer.

[0233] ⑤ Carrier mobility test: The carrier mobility of the lithium iron phosphate cathode material was tested using an HMS-7000 Hall effect meter. Carrier mobility is an important parameter for measuring the conductivity of a material, used to characterize the speed at which charge carriers (electrons and holes) move inside and on the surface of the lithium iron phosphate cathode material under the action of a unit electric field. The unit is cm. 2 / V·s.

[0234] ⑥ Electrochemical performance testing

[0235] A. Electrode preparation and coin cell assembly: Lithium iron phosphate positive electrode material, conductive agent acetylene black and adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry, which was then coated onto aluminum foil and dried in a vacuum drying oven. The positive electrode sheet was then pressed into a sheet using a tablet press, and the negative electrode sheet was a lithium metal sheet. The electrolyte was 1 mol / L lithium hexafluorophosphate-ethylene carbonate:dimethyl carbonate (by volume, V(EC):V(DMC)=1:1). A polypropylene porous membrane was used as the separator. The coin cell assembly was carried out in an argon glove box.

[0236] B. Electrochemical performance test: The assembled coin cells are subjected to electrochemical performance tests in the charge and discharge voltage range of 2.5V to 4.5V. First, they are charged to 4.5V with constant current, and then discharged to 2.5V with a higher rate current. The capacity discharged is the discharge capacity at that rate. After the discharge is completed, they are discharged to 2.5V with constant current again. Then the next rate test is carried out.

[0237] ⑦ Electrode Yield Strength Test: The yield strength (unit: MPa, i.e., the stress value that produces 0.2% residual deformation) of the lithium iron phosphate electrode sheet was tested using a universal testing machine with a video extensometer and a pneumatic foil fixture to evaluate the influence of the microstructure of lithium iron phosphate particles and surface carbon coating on the mechanical strength of the electrode. The sample electrode was clamped using a pneumatic foil fixture with the air pressure set between 0.4 MPa and 0.6 MPa for tensile testing. The deformation of the electrode sheet during the tensile process was measured using a video extensometer.

[0238] The performance test results are shown in Table 1, Table 2, Figure 1, and Figure 2.

[0239] Table 1 Test Results

[0240] Table 2 Test Results

[0241] As can be seen from Figure 1, the lithium iron phosphate cathode material of Example 6 of this disclosure has a regular morphology and a uniform thin carbon coating layer; while Figure 2 shows that the lithium iron phosphate material of Comparative Example 4 has an irregular morphology and an uneven surface carbon coating layer, which is not conducive to improving the electrochemical performance of the cathode material.

[0242] As shown in Tables 1 and 2, the lithium iron phosphate cathode material provided in the examples meets the requirements of 0.05nm ≤ σ(d) ≤ 0.35nm and 0.02wt% ≤ σ(C) as disclosed in this disclosure. wt％ With a content of ≤0.35wt%, lithium iron phosphate cathode materials have relatively high carrier mobility and conductivity, which is beneficial to further improve the electrochemical performance (such as specific capacity, rate capability and cycle performance) of batteries prepared from lithium iron phosphate cathode materials.

[0243] As can be seen from the data in Table 1, σ(d) and σ(C) wt％ The magnitude of ) will affect the parameter factor Q value of lithium iron phosphate materials.

[0244] As can be seen from Examples 1-9, when the lithium iron phosphate cathode material particles... The larger the value, the closer it is to 1, the better the particle morphology regularity of lithium iron phosphate cathode material; σ(d) and σ(C) wt％The smaller the value of Q, the more uniform the carbon coating layer in the lithium iron phosphate cathode material particles, resulting in a larger parameter factor Q. The magnitude of the parameter factor Q has a certain impact on the overall electrochemical performance of lithium batteries prepared from lithium iron phosphate cathode materials. Generally, lithium iron phosphate cathode materials with regular morphology have good dispersibility, which can improve the compaction density of lithium iron phosphate cathode materials and reduce the internal resistance of the battery. A uniform conductive carbon coating layer can effectively reduce the occurrence of side reactions between lithium iron phosphate cathode materials and electrolytes, and improve the diffusion and conduction of electrons and lithium ions. All of these factors contribute to improving the electrical performance of batteries made from lithium iron phosphate cathode materials.

[0245] The data from Tables 1 and 2 show that even lithium iron phosphate cathode materials with the same chemical composition will have different properties. σ(d) and σ(C) wt％ This corresponds to different parameter factors Q. When the parameter factor Q is 0.07 nm... -1 ~7nm -1 When within the range, and σ(d) and σ(C) wt％ All of these are within the scope of this disclosure. The value is 0.5–1, σ(d) is 0.05 nm–0.35 nm, and σ(C) is 0.5–1. wt％ (The content of lithium iron phosphate cathode material is 0.02wt% to 0.35wt%). At this time, the battery corresponding to the lithium iron phosphate cathode material has good electrode yield strength, high specific capacity, and excellent rate performance and cycle performance.

[0246] In Example 1, only σ(d) is in the range of 0.05 nm to 0.35 nm, and σ(C) is... wt％ Within the range of 0.02wt% to 0.35wt%, the electrochemical performance and electrode yield strength of the battery made from the corresponding lithium iron phosphate cathode material are inferior to those in other examples; Example 2 In the range of 0.5 to 1, σ(d) is in the range of 0.05 nm to 0.35 nm, and σ(C) is in the range of 0.05 nm to 0.35 nm. wt％ The concentration is in the range of 0.02wt% to 0.35wt%, but the parameter factor Q is not above 0.07nm. -1 ~7nm -1 Within the specified range, the electrochemical performance and electrode yield strength of the battery made from the corresponding lithium iron phosphate cathode material are improved compared to Example 1, but not as good as Examples 3-14; Examples 3-9. In the range of 0.5 to 1, σ(d) is in the range of 0.05 nm to 0.35 nm, and σ(C) is in the range of 0.05 nm to 0.35 nm. wt％ The content is in the range of 0.02wt% to 0.35wt% and Q is in the range of 0.07nm. -1 ~7nm -1Compared with Examples 1-2, the electrochemical performance and electrode yield strength of the batteries made from the corresponding lithium iron phosphate cathode materials are significantly improved.

[0247] Furthermore, when the parameter factor Q value of the lithium iron phosphate cathode material in Examples 3 to 9 is high, the corresponding battery has better electrical performance. At this time, the lithium iron phosphate cathode material particles have a more regular morphology and a more uniform carbon coating layer, which is beneficial to accelerate the lithium ion deintercalation / intercalation rate, shorten the lithium ion transport and diffusion path, increase the conductivity of the lithium iron phosphate cathode material, and avoid the occurrence of side reactions between the lithium iron phosphate cathode material and the electrolyte, thereby improving the charge and discharge performance of the lithium-ion battery.

[0248] Furthermore, the yield strength of the lithium iron phosphate cathode material is determined by the microstructure of the material particles and the carbon coating. Suitable carbon content, regular particle morphology, and a uniform carbon coating layer are beneficial for increasing the compaction density of the lithium iron phosphate cathode material particles, thereby improving the yield strength of the electrode prepared from it and enhancing the mechanical properties of the electrode. Data shows that, except for Examples 4 and 9, the yield strength values ​​of the lithium iron phosphate cathode materials in the other examples increase with the increase of the parameter factor Q value; relatively speaking, although the parameter factor Q value of Examples 4 and 9 is around 0.07 nm... -1 ~7nm -1 Within the range, but its surface carbon content is slightly higher, and more amorphous carbon is produced after high-temperature carbonization, which to some extent reduces the compaction density of lithium iron phosphate cathode material, thereby affecting its electrode yield strength.

[0249] Furthermore, the lithium iron phosphate cathode material prepared in the examples exhibits superior carrier mobility, indicating that electrons inside and on the surface of the lithium iron phosphate cathode material move rapidly within a unit area electric field. This also further reflects the good uniformity of the carbon coating layer on the substrate surface. As can be seen from Examples 3-4, 8-9, and 6-7, when the σ(d) and σ(C) of the lithium iron phosphate cathode material... wt％ The lower the σ(d) value and the higher the carbon content (C, wt%), the higher the carrier mobility of the lithium iron phosphate cathode material, which means better conductivity and improved charge-discharge performance of lithium batteries. Furthermore, the lower the carbon content and the higher the σ(d) and σ(C) values, the better the carrier mobility of the lithium iron phosphate cathode material. wt％ When the values ​​are not significantly different, the apparent carrier mobility of Example 9 is slightly higher than that of Example 4, which is related to the lattice defects of lithium iron phosphate caused by elemental doping.

[0250] Furthermore, a comparison between Examples 1-9 and Comparative Example 1 shows that σ(d) and σ(C) of Comparative Example 1 are different. wt％ The value is not within the scope of this disclosure, although its The values ​​are within the range provided in this disclosure, but the corresponding apparent carrier mobility, electrochemical performance, and electrode yield strength of the obtained lithium iron phosphate cathode material are all poor. Therefore, when σ(d), σ(C) wt％ When both Q and Q are within the range provided in this disclosure, the performance indicators of the obtained lithium iron phosphate cathode material will be more superior.

[0251] As can be seen from Example 3, Comparative Example 2, and Comparative Example 4, if high-pressure atomization mixing is not used during the preparation process, the results of Comparative Example 2 and Comparative Example 4 are significantly different. The smaller value and the significant difference from Example 3 indicate that the lithium iron phosphate cathode materials obtained in Comparative Examples 2 and 4 have poor shape regularity. Furthermore, the smaller parameter factor Q values ​​in Comparative Examples 2 and 4 result in lower overall electrochemical performance of the lithium iron phosphate cathode materials. The shape factor of the lithium iron phosphate cathode material prepared in Example 3 is also lower. A larger value (less than 1) indicates that the particles have a more regular morphology. This is because the high-pressure atomization-high-speed stirring and mixing operation provides seed crystals with uniform particle size and morphology for the generation of lithium iron phosphate cathode materials. After a hydrothermal reaction process at a suitable temperature, lithium iron phosphate precursor particles with good crystallinity and regular and uniform morphology are obtained. The corresponding lithium iron phosphate cathode materials also have better electrochemical performance.

[0252] Compared with Example 3, the nozzle diameter is smaller, the operating pressure is higher, and the stirring rate of the mixture is faster in the high-pressure atomization mixing step of Example 10. This is beneficial to obtain seed precursors with smaller particle size. The lithium iron phosphate precursor particles prepared by hydrothermal reaction have a larger specific surface area and better morphological regularity. This is beneficial to shorten the diffusion path of lithium ions and improve the lithium ion deintercalation / intercalation rate, thereby enhancing the charge and discharge performance of the battery made from the lithium iron phosphate cathode material.

[0253] Compared with Example 3, the lithium iron phosphate cathode material of Example 11 has improved particle morphology regularity and slightly reduced particle size. This is because, within a suitable reaction temperature range, the increase in synthesis temperature makes the interaction between reactants stronger, which accelerates the crystal growth process. At the same time, the higher temperature can provide enough energy to overcome the energy barrier required to form smaller particles, thereby making the formed crystal particles smaller and more regular in shape, which in turn helps to improve the electrochemical performance of the battery made from the lithium iron phosphate cathode material.

[0254] Compared with Example 3, the shape factor of the lithium iron phosphate cathode material prepared in Example 14 is [not specified]. The value increases because appropriate elemental doping can not only form crystal defects such as holes in the lattice of lithium iron phosphate to reduce the resistance of lithium ion insertion / extraction, but also inhibit the growth of crystals in certain crystal orientations or crystal faces, thus playing a role in regulating the particle size and morphology of lithium iron phosphate particles. Therefore, lithium iron phosphate with better morphological regularity and larger specific surface area is obtained, which is beneficial to improving the electrochemical performance of the battery made from lithium iron phosphate cathode material.

[0255] As can be seen from Example 3 and Comparative Examples 3 and 4: if the carbon source is not melted or combined with polyethylene glycol gel during the high-temperature calcination of lithium iron phosphate in the preparation process, the σ(d) and σ(C) of the lithium iron phosphate cathode materials prepared in Comparative Examples 3 and 4 are relatively low. wt％ A large σ(d) value (greater than 0.9) indicates poor uniformity of the carbon coating layer in the cathode material, resulting in a low apparent carrier mobility, a small parameter factor Q value, and low overall performance of the cathode material. The σ(d) and σ(C) values ​​of the lithium iron phosphate cathode material prepared in Example 3... wt％ A smaller value (less than 0.35) indicates that the carbon coating layer in the cathode material has better uniformity. This is due to the co-wetting and coating of the composite carbon source composed of molten carbon source and polyethylene glycol gel during high-temperature calcination, which is beneficial to further improve the electrochemical performance of lithium iron phosphate cathode material.

[0256] Compared with Example 3, the σ(d) and σ(C) of the lithium iron phosphate cathode material in Example 12 are significantly higher. wt％ The significant decrease in the carbon coating value indicates an improvement in the uniformity of the carbon coating layer on the lithium iron phosphate substrate surface. This is because, within a suitable temperature range, a higher water bath temperature is beneficial for increasing the strength and swelling characteristics of the polyethylene glycol gel, while a higher melting temperature is beneficial for increasing the fluidity of the liquid carbon source. After mixing the lithium iron phosphate substrate with the composite carbon source, the highly fluid molten carbon source is better able to be uniformly and stably coated onto the lithium iron phosphate substrate surface by the high-strength polyethylene glycol, thereby increasing the uniformity of the carbon coating layer.

[0257] Compared with Example 3, the σ(d) and σ(C) of the lithium iron phosphate cathode material in Example 13 are significantly higher. wt％ The decrease in σ(d) and slight increase in carbon coating thickness are due to the fact that, within a certain temperature range, higher calcination temperatures help form a more stable, continuous, and compact carbon coating structure, thereby improving its coating uniformity and promoting lithium-ion diffusion and electron conduction. Furthermore, higher sintering temperatures can, to some extent, promote the rapid sintering of the carbon coating, resulting in a thicker carbon layer. Lower σ(d) and σ(C) values... wt％ The value indicates that the carbon coating layer is relatively uniformly coated on the surface of the lithium iron phosphate substrate, which is beneficial to improving the electrochemical performance of the corresponding battery.

[0258] In summary, the lithium iron phosphate cathode material provided in this disclosure has a regular particle morphology and a uniform carbon coating layer, which is beneficial to improving the carrier mobility and conductivity of the lithium iron phosphate cathode material, and further improving the electrochemical performance (such as specific capacity, rate capability, and cycle performance) of the battery prepared from the lithium iron phosphate cathode material. Industrial applicability

[0259] The lithium iron phosphate cathode material disclosed herein has a regular particle morphology and a uniform carbon coating layer, which is beneficial to improving the carrier mobility and conductivity of the lithium iron phosphate cathode material, and is also beneficial to further improving the electrochemical performance (such as specific capacity, rate capability and cycle performance) of the battery prepared from the lithium iron phosphate cathode material.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material includes a lithium iron phosphate matrix and a carbon coating layer covering the surface of the lithium iron phosphate matrix; The average standard deviation of the carbon coating thickness in the lithium iron phosphate cathode material is σ(d). 0.05nm≤σ(d)≤0.35nm; The average standard deviation of the carbon content in the lithium iron phosphate cathode material is σ(C). wt％ ), 0.02wt%≤σ(C wt％ )≤0.35wt%; Wherein, N1 is the sample size of lithium iron phosphate cathode material particles taken during the testing of the lithium iron phosphate cathode material, and N1≥10; σ(d) i σ(C wt％ ) i These are the standard deviations of the carbon coating thickness and carbon content corresponding to the i-th lithium iron phosphate cathode material particle taken during the testing of the lithium iron phosphate cathode material. n is the sample size at a local location taken during the testing of the i-th lithium iron phosphate cathode material particle, n≥5; d m Let the thickness of the carbon coating be the value at the m-th local location. For the n local location samples taken, d m The average value, in nm; C m Let m be the carbon content corresponding to the m-th local position. C is the value of the n local location samples taken. m The average value is expressed in wt%. The thickness of the carbon coating layer of the lithium iron phosphate cathode material is d. 1nm≤d≤10nm; d is the number of local position samples taken from the i-th lithium iron phosphate cathode material particle. m The average value; The lithium iron phosphate cathode material has a carbon content of C. 1wt%≤C≤10wt%; C is the value of n local location samples taken for the i-th lithium iron phosphate cathode material particle. m The average value.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The shape factor of the lithium iron phosphate cathode material is: Wherein, N2 is the sample size of lithium iron phosphate cathode material particles taken during the testing of the lithium iron phosphate cathode material, and N2≥10; The shape factor is the shape factor corresponding to the j-th lithium iron phosphate cathode material particle taken during the testing of the lithium iron phosphate cathode material. D s D is the diameter of the standard circle whose projected area is equal to that of the j-th lithium iron phosphate cathode material particle in the TEM two-dimensional projection diagram, and D is the diameter of the smallest circumcircle of the particle projection in the TEM two-dimensional projection diagram of the j-th lithium iron phosphate cathode material particle.

3. The lithium iron phosphate cathode material according to claim 2, characterized in that, The parameter factors of the lithium iron phosphate cathode material 0.07nm -1 ≤Q≤7nm -1 。 4. The lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, The lithium iron phosphate cathode material also includes at least one of the following characteristics: Feature 1: The lithium iron phosphate cathode material The range is 0.550 to 0.910; Feature 2: The d of the lithium iron phosphate cathode material is 1.5 nm to 9.1 nm; Feature 3: The σ(d) of the lithium iron phosphate cathode material is 0.055 nm to 0.343 nm; Feature 4: The C content of the lithium iron phosphate cathode material is 1.94 wt% to 9.68 wt%; Feature 5: The σ(C) of the lithium iron phosphate cathode material wt％ The content ranges from 0.024 wt% to 0.336 wt%. Feature 6: The Q of the lithium iron phosphate cathode material is 0.072 nm. -1 ~6.596nm -1 ; Feature 7: The specific surface area of ​​the lithium iron phosphate cathode material is 9.3 m². 2 / g~15.1m 2 / g; Feature 8: The D of the lithium iron phosphate cathode material 50 Its thickness ranges from 0.85 μm to 1.50 μm; Feature 9: The carrier mobility of the lithium iron phosphate cathode material is 20.03 × 10⁻⁶. -6 cm 2 / V·s~21.71×10 -6 cm 2 / V·s; feature 10: The general formula of the lithium iron phosphate matrix is ​​Li 1-x A x Fe 1-y M y (PO 4-a )D a Wherein, A includes at least one of Na and Mg; M includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D includes at least one of F and S; 0≤x≤0.1, 0≤y≤0.1 and 0≤a≤0.

1.

5. A method for preparing the lithium iron phosphate cathode material according to any one of claims 1 to 4, characterized in that, The process includes the following steps: ball milling a lithium iron phosphate matrix with a composite carbon source to obtain a mixed precursor; drying the mixed precursor and sintering it to obtain a lithium iron phosphate cathode material.

6. The preparation method according to claim 5, characterized in that, The preparation of the lithium iron phosphate cathode material includes at least one of the following characteristics: Feature 11: The preparation of the lithium iron phosphate matrix includes: mixing a first solution containing ferrous salt with a second solution containing lithium source and phosphate to obtain a mixture; adjusting the pH of the mixture to 6-8 and then carrying out a hydrothermal reaction; and collecting the solid after the reaction is completed. Feature 12: The preparation of the composite carbon source includes: mixing polyethylene glycol gel with molten carbon source; wherein the carbon source includes at least one of sorbitol, erythritol, fructose, rhamnose and malic acid; the molar ratio of lithium element in the lithium iron phosphate matrix to the carbon source and the polyethylene glycol is 1:(0.03~0.05):(0.02~0.04); Feature 13: The ball milling time is 1 hour to 2 hours; Feature 14: The drying temperature of the mixed precursor is 90℃~110℃; Feature 15: The drying time of the mixed precursor is 10h to 14h; Feature 16: The sintering includes: first holding at 400℃~550℃ for 2h~5h, and then holding at 650℃~750℃ for 6h~8h.

7. The preparation method according to claim 6, characterized in that, The preparation of the lithium iron phosphate matrix includes at least one of the following conditions: Condition 1: The general formula of the lithium iron phosphate matrix is ​​Li 1-x A x Fe 1-y M y (PO 4-a )D a When the lithium iron phosphate matrix contains M, A and D elements, the first solution also contains an M source, and the second solution also contains an A source and a D source. Condition 2: The first solution is sprayed into the second solution under stirring conditions by high-pressure atomization; or, the second solution is sprayed into the first solution under stirring conditions by high-pressure atomization. Condition 3: The temperature of the hydrothermal reaction is 150℃~180℃; Condition 4: The hydrothermal reaction time is 8h to 10h.

8. The preparation method according to claim 7, characterized in that, The ferrous salt includes at least one of ferrous oxalate, ferrous chloride and ferrous acetate; Alternatively, the lithium source may include at least one of lithium oxalate, lithium chloride, and lithium acetate; Alternatively, the phosphate may include at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate; Alternatively, the M source is a water-soluble salt of element M; Alternatively, the A source may include at least one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride, and sodium sulfide; Alternatively, the D source may include at least one of ammonium fluoride, lithium fluoride, ammonium sulfide, lithium sulfide, ammonium bisulfite, and thiourea. Alternatively, the ratio of the total molar number of iron in the ferrous salt to the total molar number of M in the M source, the total molar number of lithium in the lithium source to the total molar number of A in the A source, and the molar number of P in the phosphate is 1:1:(0.95~1.1); wherein the molar ratio of lithium to A is (0.9~1):(0~0.1); the molar ratio of iron to M is (0.9~1):(0~0.1); and the molar ratio of oxygen in the phosphate to D in the D source is (3.9~4):(0~0.1). Alternatively, the nozzle diameter of the high-pressure atomizing gun used for high-pressure atomization is 0.3mm to 0.6mm, and the operating pressure is 0.2MPa to 0.4MPa; Alternatively, the stirring conditions correspond to a stirring rate of 600 r / min to 1000 r / min.

9. A positive electrode sheet, characterized in that, The active material in the positive electrode includes the lithium iron phosphate positive electrode material according to any one of claims 1 to 4.

10. A battery, characterized in that, The battery contains the positive electrode sheet as described in claim 9.

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