Lithium iron phosphate cathode material, its preparation method and application

Abstract

The present invention discloses a lithium iron phosphate cathode material, a preparation method thereof and an application thereof, belonging to the technical field of lithium iron phosphate cathode materials. The lithium iron phosphate cathode material includes a lithium iron phosphate matrix and a carbon coating layer coated on 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 cathode material is #imgabs0#, 0.05 nm ≤ σ(d) ≤ 0.35 nm; the average standard deviation of the carbon content in the lithium iron phosphate cathode material is σ ( C wt% ), 0.02 wt% ≤ σ (C wt% ) ≤ 0.35 wt%. The lithium iron phosphate cathode material has a uniform carbon coating layer, and 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 of the lithium iron phosphate cathode material and further improving the electrochemical performance of the battery prepared from the lithium iron phosphate cathode material.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron phosphate cathode materials, and in particular, to a lithium iron phosphate cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] As a new generation of lithium-ion battery cathode materials, LiFePO 4 materials have become a current research hotspot due to their low price, small environmental pollution, excellent thermal stability and structural stability, and relatively high theoretical specific capacity. However, due to the 4 structural defects of LiFePO itself, it has low electronic conductivity and lithium ion diffusion coefficient, resulting in insufficient utilization during high-current charge and discharge, and unsatisfactory rate performance.

[0003] Currently, carbon coating, ion doping, and particle nanosizing are conventional means to improve the rate performance of lithium iron phosphate materials. However, uneven carbon coating layers will reduce the carrier mobility on the material surface and deteriorate the conductivity; too thick carbon coating layers will hinder the diffusion of lithium ions at the electrode-electrolyte interface; too high carbon content will lead to too low tap density of the material and poor fracture strength of the electrode. All of the above factors will affect the electrochemical performance and cycle stability of the battery corresponding to the cathode material.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] The purpose of the present invention is to provide a lithium iron phosphate cathode material, a preparation method thereof, and an application thereof to solve or improve the above technical problems.

[0006] The present invention can be implemented as follows:

[0007] In a first aspect, the present invention provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix and a carbon coating layer coated on the surface of the lithium iron phosphate matrix;

[0008] The average standard deviation of the thickness of the carbon coating layer in the lithium iron phosphate cathode material is , , 0.05 nm ≤ σ(d) ≤ 0.35 nm;

[0009] The average standard deviation of the carbon content in the lithium iron phosphate cathode material is σ ( C wt% ), , 0.02 wt% ≤ σ(C wt% ) ≤ 0.35 wt%;

[0010] Wherein, N1 is the sample size of the lithium iron phosphate cathode material particles taken during the test of the lithium iron phosphate cathode material, N 1 ≥10;

[0011] 、 is the standard deviation of the carbon coating layer thickness and the standard deviation of the carbon content corresponding to the i th lithium iron phosphate cathode material particle taken during the test of the lithium iron phosphate cathode material;

[0012] 、 , n is the sample size of the local position taken during the test of the i th lithium iron phosphate cathode material particle, n ≥5; d m is the thickness of the carbon coating layer corresponding to the m th local position taken, is the n average value of the d m of the local position samples, in nm; C m is the carbon content corresponding to the m th local position taken, is the n average value of the C m of the local position samples, in wt%;

[0013] The carbon coating layer thickness of the lithium iron phosphate cathode material is d , ,1 nm ≤ d ≤ 10 nm; is the i average value of the n local position samples taken from the d m th lithium iron phosphate cathode material particle;

[0014] The carbon content of the lithium iron phosphate cathode material is , ,1 wt% ≤ C ≤ 10 wt%; is the i average value of the n local position samples taken from the C m th lithium iron phosphate cathode material particle.

[0015] In an alternative embodiment, the shape factor of the lithium iron phosphate cathode material is , , 0.5 ≤ ≤ 1;

[0016] Among them, N 2 is the sample size of the lithium iron phosphate cathode material particles taken during the test of the lithium iron phosphate cathode material, N 2 ≥ 10;

[0017] is the shape factor corresponding to the j th lithium iron phosphate cathode material particle taken during the test of the lithium iron phosphate cathode material;

[0018] , D s is the diameter of the standard circle with the same projected area as the projected area in the TEM two-dimensional projection diagram of the j th lithium iron phosphate cathode material particle, and D is the diameter of the minimum circumscribed circle of the particle projection in the TEM two-dimensional projection diagram of the j th lithium iron phosphate cathode material particle.

[0019] In an alternative embodiment, the parameter factor of the lithium iron phosphate cathode material Q = , 0.07 nm -1 ≤ Q ≤ 7 nm -1 .

[0020] In an alternative embodiment, the lithium iron phosphate cathode material further includes at least one of the following characteristics:

[0021] Characteristic 1: The of the lithium iron phosphate cathode material is 0.550~0.910;

[0022] Characteristic 2: The of the lithium iron phosphate cathode material is 1.5 nm~9.1 nm;

[0023] Characteristic 3: The of the lithium iron phosphate cathode material is 0.055 nm~0.343 nm;

[0024] Characteristic 4: The of the lithium iron phosphate cathode material is 1.94 wt%~9.68 wt%;

[0025] Characteristic 5: The σ ( C wt% ) of the lithium iron phosphate cathode material is 0.024 wt%~0.336 wt%;

[0026] Feature 6: of the lithium iron phosphate cathode material Q is 0.072 nm -1 ~6.596 nm -1 ;

[0027] Feature 7: The specific surface area of the lithium iron phosphate cathode material is 9.3 m 2 / g ~ 15.1 m 2 / g;

[0028] Feature 8: of the lithium iron phosphate cathode material D 50 is 0.85 μm ~ 1.50 μm;

[0029] Feature 9: The carrier mobility of the lithium iron phosphate cathode material is 20.03×10 -6 cm 2 / V·s ~ 21.71×10 - 6 cm 2 / V·s;

[0030] 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.

[0031] Second aspect, the present invention provides a preparation method of a lithium iron phosphate cathode material according to any one of the foregoing embodiments, comprising the following steps: ball-milling the lithium iron phosphate matrix and a composite carbon source to obtain a mixed precursor; drying and sintering the mixed precursor to obtain the lithium iron phosphate cathode material.

[0032] In an optional embodiment, the preparation of the lithium iron phosphate cathode material includes at least one of the following features:

[0033] Feature 11: The preparation of the lithium iron phosphate matrix includes: mixing a first solution containing a ferrous salt with a second solution containing a lithium source and a phosphate to obtain a mixed solution; adjusting the pH value of the mixed solution to 6 - 8 and then performing a hydrothermal reaction, and collecting the solid after the reaction ends;

[0034] Feature 12: The preparation of the composite carbon source includes: mixing polyethylene glycol gel with the melted carbon source; 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 polyethylene glycol is 1:(0.03 - 0.05):(0.02 - 0.04);

[0035] Feature 13: The ball milling time is 1h - 2h;

[0036] Feature 14: The drying temperature of the mixed precursor is 90°C - 110°C;

[0037] Feature 15: The drying time of the mixed precursor is 10h - 14h;

[0038] Feature 16: The sintering includes: first insulating for 2h - 5h under the condition of 400°C - 550°C, and then insulating for 6h - 8h under the condition of 650°C - 750°C.

[0039] In an alternative embodiment, the preparation of the lithium iron phosphate matrix includes at least one of the following conditions:

[0040] 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 further contains elements M, A, and D, the first solution further contains an M source, and the second solution further contains an A source and a D source;

[0041] Condition 2: Spraying the first solution into the second solution under stirring conditions by high-pressure atomization; or, spraying the second solution into the first solution under stirring conditions by high-pressure atomization;

[0042] Condition 3: The temperature of the hydrothermal reaction is 150°C - 180°C;

[0043] Condition 4: The time of the hydrothermal reaction is 8h - 10h.

[0044] In an alternative embodiment, the ferrous salt includes at least one of ferrous oxalate, ferrous chloride, and ferrous acetate;

[0045] Or, the lithium source includes at least one of lithium oxalate, lithium chloride, and lithium acetate;

[0046] Or, the phosphate includes at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate;

[0047] Or, the M source is a water-soluble salt of the M element;

[0048] Alternatively, the A source includes at least one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride, and sodium sulfide;

[0049] Alternatively, the D source includes at least one of ammonium fluoride, lithium fluoride, ammonium sulfide, lithium sulfide, ammonium bisulfite, and thiourea;

[0050] Alternatively, the molar ratio of the total moles of iron element in the ferrous salt to the M element in the M source, the total moles of lithium element in the lithium source to the A element in the A source, and the moles of P element in the phosphate is 1:1:(0.95 - 1.1); wherein, the molar ratio of lithium element to A element is (0.9 - 1):(0 - 0.1); the molar ratio of iron element to M element is (0.9 - 1):(0 - 0.1); the molar ratio of oxygen element in the phosphate to the D element in the D source is (3.9 - 4):(0 - 0.1);

[0051] Alternatively, the nozzle diameter of the high-pressure atomization spray gun used for high-pressure atomization is 0.3 mm to 0.6 mm, and the operating pressure is 0.2 MPa to 0.4 MPa;

[0052] Alternatively, the stirring rate corresponding to the stirring condition is 600 r / min to 1000 r / min.

[0053] In a third aspect, the present invention provides a positive electrode sheet, and the active material in the positive electrode sheet includes the lithium iron phosphate positive electrode material of the foregoing embodiment.

[0054] In a fourth aspect, the present invention provides a battery, and the battery contains the positive electrode sheet of the foregoing embodiment.

[0055] The beneficial effects of the present invention include:

[0056] The standard deviation of the carbon coating layer thickness σ(d) i and the standard deviation of the carbon content σ ( C wt% ) i respectively reflect the dispersion of the carbon coating layer thickness and carbon content at different positions of the lithium iron phosphate positive electrode material particles relative to their respective average values, so as to characterize the coating uniformity of the carbon coating layer on the surface of the lithium iron phosphate matrix. The uniform coating of the carbon coating layer on the surface of the lithium iron phosphate matrix can inhibit the fusion of crystal grains and the growth of particles, achieving the purpose of controlling the particle size and curbing particle agglomeration. The average standard deviation of the carbon coating layer thickness σ(d) and the average standard deviation of the carbon content σ ( C wt% ) can measure the uniformity of the carbon layer distribution on the surface of the lithium iron phosphate positive electrode material, σ(d) and σ ( C wt%The smaller the value of , the more uniform the carbon coating layer on the material surface is, and the better the conductivity of the material is; σ(d) and σ ( C wt% ) The higher the value, the worse the uniformity of the carbon coating layer, resulting in poor charge-discharge performance of the cathode material.

[0057] In addition, the thickness of the carbon coating layer of the lithium iron phosphate cathode material and the carbon content should be within an appropriate range; The higher the value, the more unfavorable the deintercalation of lithium ions and their diffusion between the electrode and the electrolyte interface; The higher , the more likely it is to inhibit the formation of the crystalline LiFePO 4 phase and reduce the tap density of the material.

[0058] The lithium iron phosphate cathode material provided by the present invention, its σ(d), σ ( C wt% ), and within the range provided by the present invention σ(d), σ ( C wt% ), and have a uniform carbon coating layer, and 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, etc., and is beneficial to further improving the electrochemical performance of the battery prepared from the lithium iron phosphate cathode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the embodiments will be briefly introduced below. 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, other related drawings can be obtained based on these drawings without creative efforts.

[0060] Figure 1 FIG. is a TEM image of the lithium iron phosphate cathode material prepared in Example 6;

[0061] Figure 2 FIG. is a TEM image of the lithium iron phosphate cathode material prepared in Comparative Example 4. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0062] 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 described clearly and completely below. For those not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments not indicated by the manufacturer can all be obtained as conventional products through commercial purchase.

[0063] The lithium iron phosphate cathode material provided by the present invention, its preparation method, and application will be specifically described below.

[0064] The present invention provides a lithium iron phosphate cathode material, which includes a lithium iron phosphate matrix and a carbon coating layer coated on the surface of the lithium iron phosphate matrix.

[0065] Among them, the average standard deviation of the thickness of the carbon coating layer in the lithium iron phosphate cathode material is , , 0.05 nm ≤ σ(d) ≤ 0.35 nm;

[0066] The average standard deviation of the carbon content in the lithium iron phosphate cathode material is σ ( C wt% ), , 0.02 wt% ≤ σ(C wt% ) ≤ 0.35 wt%;

[0067] Among them, N 1 is the sample size of the lithium iron phosphate cathode material particles taken during the test of the lithium iron phosphate cathode material, N 1 ≥ 10;

[0068] 、 is the standard deviation of the carbon coating layer thickness and the standard deviation of the carbon content corresponding to the i th lithium iron phosphate cathode material particle taken during the test of the lithium iron phosphate cathode material;

[0069] 、 , n is the sample size of the local position taken during the test of the i th lithium iron phosphate cathode material particle, n ≥ 5; d m is the thickness of the carbon coating layer corresponding to the m th local position taken, is the n th local position sample's d mThe average value, in nm; C m is the carbon content corresponding to the m th local position selected, is the n average value of the C m for the

[0070] The carbon coating thickness of the lithium iron phosphate cathode material is d , , 1 nm ≤ d ≤ 10 nm;

[0071] The carbon content of the lithium iron phosphate cathode material is , , 1 wt% ≤ C ≤ 10 wt%;

[0072] is the i average value of the n local position samples taken from the d m th lithium iron phosphate cathode material particle;

[0073] is the i average value of the n local position samples taken from the C m th lithium iron phosphate cathode material particle.

[0074] The selection of the local positions of the above-mentioned lithium iron phosphate cathode material particles is randomly selected. In some preferred embodiments, the random selection does not specifically delimit a certain range, and the selection is not concentrated in a certain small range or area of the particle. The sample selection range preferably covers the entire cathode material particle.

[0075] The standard deviation of the carbon coating thickness σ(d) i and the standard deviation of the carbon content σ ( C wt% ) i respectively reflect the dispersion of the carbon coating thickness and carbon content at different positions of the lithium iron phosphate cathode material particles relative to their respective average values, so as to characterize the coating uniformity of the carbon coating on the surface of the lithium iron phosphate matrix particles. Generally, σ(d) i and σ ( C wt% ) iThe smaller the value, the smaller the dispersion of the carbon coating layer thickness and carbon content on the surface of the positive electrode material particles relative to their respective average values, the more uniform the carbon coating layer, and the better the conductivity of the material; σ(d) i and σ ( C wt% ) i The higher the value, the greater the dispersion of the carbon coating layer thickness and carbon content on the surface of the positive electrode material particles relative to their respective average values, indicating that the uniformity of the carbon coating layer is worse, and the charge-discharge performance of the material also deteriorates. Further, the average standard deviation of the carbon coating layer thickness σ(d) and the average standard deviation of the carbon content σ ( C wt% ) also conform to the above variation law and their numerical values can measure the uniformity of the carbon layer distribution on the surface of the lithium iron phosphate positive electrode material.

[0076] In addition, the carbon coating layer thickness and the carbon content of the lithium iron phosphate positive electrode material need to be within an appropriate range. The higher the value, the more unfavorable it is for the deintercalation of lithium ions and their diffusion between the electrode and the electrolyte interface; The higher it is, the more likely it is to inhibit the formation of the crystalline LiFePO 4 phase and reduce the tap density of the material.

[0077] Continuing from the above, the lithium iron phosphate positive electrode material provided by the present invention that satisfies σ(d), σ ( C wt% )、 and has a uniform carbon coating layer within the range. The thickness of this 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 positive electrode material, etc., and is also beneficial to further improving the electrochemical performance of the battery prepared from the lithium iron phosphate positive electrode material.

[0078] In some alternative embodiments, the of the lithium iron phosphate positive electrode material can be 1.5 nm to 9.1 nm, such as 1.5 nm, 1.6 nm, 1.7 nm, 2.7 nm, 3.0 nm, 3.8 nm, 4.6 nm, 5.5 nm, 6.3 nm, 8.7 nm or 9.1 nm, etc., or other values within the range of 1.5 nm to 9.1 nm.

[0079] In some alternative embodiments, the It can be 0.055 nm to 0.343 nm, such as 0.055 nm, 0.063 nm, 0.101 nm, 0.112 nm, 0.116 nm, 0.136 nm, 0.210 nm, 0.254 nm, 0.304 nm, 0.308 nm, 0.310 nm, 0.314 nm, 0.338 nm or 0.343 nm, etc., or it can also be other values within the range of 0.055 nm to 0.343 nm.

[0080] In some alternative embodiments, the of the lithium iron phosphate cathode material can be 1.94 wt% to 9.68 wt%, such as 1.94 wt%, 1.95 wt%, 1.96 wt%, 1.97 wt%, 1.98 wt%, 1.99 wt%, 3.47 wt%, 3.75 wt%, 4.75 wt%, 5.15 wt%, 6.25 wt%, 7.55 wt%, 9.23 wt% or 9.68 wt%, etc., or it can also be other values within the range of 1.94 wt% to 9.68 wt%.

[0081] In some alternative embodiments, the σ ( C wt% ) can be 0.024 wt% to 0.336 wt%, such as 0.024 wt%, 0.040 wt%, 0.089 wt%, 0.105 wt%, 0.107 wt%, 0.130 wt%, 0.204 wt%, 0.240 wt%, 0.296 wt%, 0.298 wt%, 0.301 wt%, 0.306 wt%, 0.325 wt% or 0.336 wt%, etc., or it can also be other values within the range of 0.024 wt% to 0.336 wt%.

[0082] In some alternative embodiments, the shape factor of the lithium iron phosphate cathode material is , , 0.5 ≤ ≤ 1;

[0083] Among them, N 2 is the sample size of the lithium iron phosphate cathode material particles taken during the test of the lithium iron phosphate cathode material, N 2 ≥ 10;

[0084] is the shape factor corresponding to the j th lithium iron phosphate cathode material particle taken during the test of the lithium iron phosphate cathode material;

[0085] , Ds For the diameter of the standard circle with an area equal to the projected area in the TEM two-dimensional projection diagram of the j th lithium iron phosphate cathode material particle, and D is the diameter of the minimum circumscribed circle of the particle projection in the TEM two-dimensional projection diagram of the j th lithium iron phosphate cathode material particle.

[0086] Shape factor , indicating the degree of deviation of the shape of the lithium iron phosphate cathode material particle from the standard sphere ( = 1). The closer the value of is to 1, the closer the particle morphology is to a spherical shape and the more regular the particle morphology is. In the present invention, the closer the value of is to 1, the more regular the morphology of the lithium iron phosphate cathode material particle is, the relatively more uniform the particle size of the lithium iron phosphate cathode material particle is, the better the particle dispersion is, and the more conducive it is to the diffusion and transmission of lithium ions and electrons; The smaller the value of

[0087] Continuing from the above, the lithium iron phosphate cathode material particles provided by the present invention that satisfy the range of have regular and uniform particle morphology, which is not only conducive to the diffusion of lithium ions, but also conducive to improving the yield strength of the electrode prepared from the lithium iron phosphate cathode material and the electrochemical performance of the battery prepared from the above electrode.

[0088] In some alternative embodiments, the of the lithium iron phosphate cathode material can be 0.550 - 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, etc., or other values within the range of 0.550 - 0.910.

[0089] In some alternative embodiments, the carrier mobility of the lithium iron phosphate cathode material can be 20.03×10 - 6 cm 2 / V·s - 21.71×10 -6 cm 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 / V·s within the range of other values.

[0090] In some alternative embodiments, the parameter factor of the lithium iron phosphate cathode material Q = , 0.07 nm -1 ≤ Q ≤ 7 nm -1 .

[0091] The above parameter factor and , and σ ( C wt% ) The corresponding relationship can comprehensively reflect the electrochemical performance of the battery corresponding to the lithium iron phosphate cathode material. When is closer to 1 (i.e., the particle morphology is more regular) and and σ ( C wt% ) are smaller (i.e., the carbon coating layer on the material surface is more uniform), the parameter factor Q value is larger, and the electrochemical performance of the battery corresponding to the lithium iron phosphate cathode material is also more excellent.

[0092] In some alternative embodiments, the Q can be 0.072 nm -1 ~6.596 nm -1 , such as 0.072 nm -1 , 0.073 nm -1 , 0.074 nm -1 , 0.076 nm -1 , 0.079 nm -1 , 0.111 nm -1 , 0.205 nm -1 , 0.505 nm -1 , 0.540 nm -1 , 0.648 nm -1 , 1.012 nm -1 , 3.405 nm -1 or 6.596 nm -1 etc., and can also be other values within the range of 0.072 nm -1 ~6.596 nm -1 .

[0093] Continuing from the above, the lithium iron phosphate cathode material provided by the present invention that simultaneously satisfies , σ ( C wt% ), and Q has a regular particle morphology and a uniform carbon coating layer, and can enable a battery using it as a cathode material to have better electrochemical performance (such as specific capacity, rate performance, and cycle performance, etc.).

[0094] In some alternative embodiments, the specific surface area of the above lithium iron phosphate cathode material can be 9.3 m 2 / g ~ 15.1 m 2 / g, such as 9.3 m 2 / g, 10.2 m 2 / g, 11.7 m 2 / g, 11.9 m 2 / g, 12.0 m 2 / g, 12.1 m 2 / g, 12.3 m 2 / g, 12.4 m 2 / g, 12.6 m 2 / g, 12.8 m 2 / g, 13.2 m 2 / g, 13.7 m 2 / g, 14.3 m 2 / g or 15.1 m 2 / g, etc., and can also be 9.3m 2 / g to 15.1m 2 Other values within the range of / g.

[0095] In some alternative embodiments, the above-mentioned lithium iron phosphate cathode material D 50 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, etc., and can also be other values within the range of 0.85μm to 1.50μm.

[0096] 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 . Among them, A can include, for example, at least one of Na and Mg; M can include, for example, at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Ga, V, Nb, Zr, In, Zn and Y; D can include, for example, at least one of F and S; 0≤x≤0.1, 0≤y≤0.1 and 0≤a≤0.1.

[0097] Correspondingly, the present invention also provides a preparation method of the above-mentioned lithium iron phosphate cathode material, which may include the following steps: ball-milling the lithium iron phosphate matrix and the composite carbon source to obtain a mixed precursor; drying and sintering the mixed precursor to obtain the lithium iron phosphate cathode material.

[0098] In some alternative embodiments, the preparation of the lithium iron phosphate matrix may include: mixing a first solution containing a ferrous salt with a second solution containing a lithium source and a phosphate to obtain a mixed solution; adjusting the pH value of the mixed solution to 6-8 and then performing a hydrothermal reaction, and collecting the solid after the reaction ends.

[0099] Among them, the ferrous salt can include, by way of example but not limitation, at least one of ferrous oxalate, ferrous chloride and ferrous acetate.

[0100] The lithium source can include, by way of example but not limitation, at least one of lithium oxalate, lithium chloride and lithium acetate.

[0101] The phosphate can include, by way of example but not limitation, at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate.

[0102] In the present invention, when the lithium iron phosphate matrix further contains element M, the M source is present in the first solution; when the lithium iron phosphate matrix further contains at least one of element A and element D, the corresponding A source and D source are present in the second solution.

[0103] By way of example, the lithium iron phosphate matrix further contains elements M, A, and D at the same time. The first solution containing the ferrous salt further contains the M source, and the second solution containing the lithium source and phosphate further contains the A source and the D source. Under this condition, the ferrous salt and the M source can be dispersed in deionized water, and nitrogen is blown for a period of time (such as 1 h) under stirring to remove the dissolved oxygen in the solution, thereby obtaining the first solution. Correspondingly, the lithium source, phosphate, A source, and D source can be dispersed in deionized water, and high-speed stirring is carried out under the protection of a nitrogen atmosphere to obtain the second solution.

[0104] The above M source can be a water-soluble salt of element M.

[0105] The A source can exemplarily but non-limitingly include at least one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride, and sodium sulfide.

[0106] The D source can exemplarily but non-limitingly include at least one of ammonium fluoride, lithium fluoride, ammonium sulfide, lithium sulfide, ammonium bisulfite, and thiourea.

[0107] In some alternative embodiments, the total molar ratio of the iron element in the ferrous salt to the M element in the M source, the total molar ratio of the lithium element in the lithium source to the A element in the A source, and the molar ratio of the P element in the phosphate is 1:1:(0.95 - 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., and can also be other values within the range of 1:1:(0.95 - 1.1).

[0108] In some alternative embodiments, the molar ratio of the lithium element to the A element can be (0.9 - 1):(0 - 0.1), such as 0.9:0.1, 0.95:0.05, or 1:0, etc., and can also be other values within the range of (0.9 - 1):(0 - 0.1).

[0109] In some alternative embodiments, the molar ratio of the iron element to the M element can be (0.9 - 1):(0 - 0.1), such as 0.9:0.1, 0.95:0.05, or 1:0, etc., and can also be other values within the range of (0.9 - 1):(0 - 0.1).

[0110] In some alternative embodiments, the molar ratio of the oxygen element in the phosphate to the D element in the D source can be (3.9 to 4):(0 to 0.1), such as 3.9:0.1, 3.95:0.05, or 4:0, etc., or other values within the range of (3.9 to 4):(0 to 0.1).

[0111] In some alternative embodiments, the first solution can be sprayed into the second solution under stirring conditions by means of high-pressure atomization. In some other alternative embodiments, the second solution can also be sprayed into the first solution under stirring conditions by means of high-pressure atomization.

[0112] Among them, the nozzle diameter of the high-pressure atomization spray gun used for high-pressure atomization can be 0.3 mm to 0.6 mm, such as 0.3 mm, 0.35 mm, 0.4 mm, 0.45 mm, 0.5 mm, 0.55 mm, or 0.6 mm, etc., or other values within the range of 0.3 mm to 0.6 mm.

[0113] The operating pressure of high-pressure atomization can be 0.2 MPa to 0.4 MPa, such as 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.35 MPa, or 0.4 MPa, etc., or other values within the range of 0.2 MPa to 0.4 MPa.

[0114] In some alternative 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, etc., or other values within the range of 600 r / min to 1000 r / min.

[0115] In some alternative embodiments, ammonia water can be used to adjust the pH value of the mixed solution. The pH value can be adjusted to 6, 6.5, 7, 7.5, or 8, etc., or other values within the range of 6 to 8.

[0116] After adjusting the pH value, it can be continuously stirred for a period of time under the protection of room temperature and an inert atmosphere (such as a nitrogen atmosphere, etc.), and then transferred to a stainless-steel autoclave with a polytetrafluoroethylene lining for hydrothermal reaction. After the reaction is completed, the product is cooled to room temperature and filtered, and then the solid phase obtained by filtration is centrifugally washed with deionized water and absolute ethanol, and finally dried to obtain the lithium iron phosphate matrix.

[0117] Among them, the temperature of the hydrothermal reaction can be 150 °C to 180 °C, such as 150 °C, 155 °C, 160 °C, 165 °C, 170 °C, 175 °C, or 180 °C, etc., or other values within the range of 150 °C to 180 °C.

[0118] The hydrothermal reaction time can be 8 h to 10 h, such as 8 h, 8.5 h, 9 h, 9.5 h or 10 h, etc., and can also be other values within the range of 8 h to 10 h.

[0119] The drying temperature can be 60 °C to 80 °C, such as 60 °C, 65 °C, 70 °C, 75 °C or 80 °C, etc., and can also be other values within the range of 60 °C to 80 °C.

[0120] The drying time can be 10 h to 14 h, such as 10 h, 10.5 h, 11 h, 11.5 h or 12 h, etc., and can also be other values within the range of 10 h to 14 h.

[0121] Continuing from the above, the present invention pours the first solution (or the second solution) into a high-pressure atomizing spray gun, and after atomization by a high-pressure nozzle, sprays it into the second solution (or the first solution) under high-speed stirring in an atomized state to obtain a uniform seed precursor solution. Then, the pH value is adjusted with ammonia water, and it is placed in a high-pressure reaction kettle for hydrothermal reaction to obtain lithium iron phosphate matrix particles. The high-pressure atomization-high-speed stirring method can enhance the initial microscopic mixing and uniform dispersion among ferrous ions, lithium ions and phosphate ions, effectively alleviate the rate difference of lithium iron phosphate crystal nucleus formation caused by too high local concentration of reaction ions, and provide a uniform microscopic nucleation environment for the generation of seed precursors. Continuing the hydrothermal reaction of this seed can form lithium iron phosphate matrix with concentrated particle size distribution, good dispersion and uniform morphology.

[0122] In some alternative embodiments, the preparation of the composite carbon source may include: mixing polyethylene glycol gel with the melted carbon source.

[0123] Among them, the carbon source can exemplarily but non-limitingly include at least one of sorbitol, erythritol, fructose, rhamnose and malic acid.

[0124] The molar ratio of lithium element, 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., and can also be other values within the range of 1:(0.03 - 0.05):(0.02 - 0.04).

[0125] The above carbon source can be melted in a high-temperature electric furnace by heating to 130 °C to 150 °C. Specifically, the melting temperature can be set according to the selected carbon source to achieve the melting of the carbon source.

[0126] The polyethylene glycol gel can be prepared by dissolving polyethylene glycol powder in dimethyl sulfoxide, stirring evenly, and then heating it in a water bath at 50°C to 60°C. In addition, it is not excluded to prepare the polyethylene glycol gel by other methods and conditions.

[0127] In some alternative embodiments, the ball milling time of the lithium iron phosphate matrix and the composite carbon source can be 1 h to 2 h, such as 1 h, 1.5 h, or 2 h, etc.

[0128] 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, etc., or other values within the range of 90°C to 110°C.

[0129] The drying time of the mixed precursor can be 10 h to 14 h, such as 10 h, 11 h, 12 h, 13 h, or 14 h, etc., or other values within the range of 10 h to 14 h.

[0130] In some alternative embodiments, the dried mixed precursor is first ground and pulverized, and then the well-ground precursor is placed in a muffle furnace filled with high-purity argon for sintering.

[0131] The sintering process includes a first stage and a second stage. Among them, the first stage can be carried out under the condition of 400°C to 550°C (such as 400°C, 420°C, 450°C, 480°C, 500°C, 520°C, or 550°C, etc.) for heat preservation for 2 h to 5 h (such as 2 h, 3 h, 4 h, or 5 h, etc.), and the second stage can be carried out under the condition of 650°C to 750°C (such as 650°C, 680°C, 700°C, 720°C, or 750°C, etc.) for heat preservation for 6 h to 8 h (such as 6 h, 7 h, or 8 h, etc.).

[0132] To further improve the conductivity and crystallinity of lithium iron phosphate, in the present invention, the molten carbon source is first mixed with the polyethylene glycol gel to obtain a composite carbon source, and then it is ball milled and mixed with the lithium iron phosphate matrix, dried, and placed in a muffle furnace for low-temperature and high-temperature roasting to obtain the lithium iron phosphate cathode material. In this process, the molten carbon source and the polyethylene glycol gel act as a composite carbon source to perform carbon coating on the lithium iron phosphate matrix. Compared with the traditional solid-phase method, the molten carbon source can easily wrap and coat on the surface of the lithium iron phosphate matrix. The carbon network structure formed by the polyethylene glycol gel during the drying and calcination processes can keep the carbon coating layer coated on the particle surface stable to form a uniform carbon coating layer. In addition, compared with the water solvent, dimethyl sulfoxide has lower viscosity and surface tension. Using it 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 dimethyl sulfoxide can coat the molten carbon source more evenly on the surface of the lithium iron phosphate matrix and penetrate into its micropores.

[0133] In summary, the preparation method provided by the present invention can be used to prepare a lithium iron phosphate cathode material with regular morphology and uniform carbon coating.

[0134] In addition, the present invention also provides a positive electrode sheet, and the active material in the positive electrode sheet includes the above-mentioned lithium iron phosphate cathode material.

[0135] The present invention also provides a battery cell, and the battery cell includes the above-mentioned positive electrode sheet.

[0136] By way of example, the above-mentioned battery cell can be used in power-consuming devices such as vehicles, ships or aircraft, etc.

[0137] The present invention also provides a battery including the above-mentioned battery cell.

[0138] The present invention also provides a power-consuming device, and the power-consuming device includes the above-mentioned battery cell and / or battery. By way of example, the power-consuming device can be, but is not limited to, mobile phones, tablets, laptop computers, electric toys, electric tools, battery cars, electric vehicles, ships, spacecraft, etc. Among them, the electric toys can include stationary or mobile electric toys, for example, game consoles, electric vehicle toys, electric ship toys and electric aircraft toys, etc., and the spacecraft can include airplanes, rockets, space shuttles and spaceships, etc.

[0139] The features and performance of the present invention will be further described in detail below in conjunction with embodiments.

[0140] Example 1

[0141] The difference between this example and Example 3 below is as follows:

[0142] 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, and the operating pressure is 0.1 MPa; the stirring speed of the high-speed stirring is 550 rpm; the pH value is adjusted to 6; the heat preservation time in the electrothermal blast drying oven is 9 h;

[0143] In S2, the melting temperature of rhamnose is 120 °C;

[0144] In S3, the molar ratio of Li element in the lithium iron phosphate matrix to rhamnose and polyethylene glycol is 1:0.04:0.02; the temperature in the first stage of sintering is 450 °C.

[0145] Example 2

[0146] The difference between this example and Example 3 below is as follows:

[0147] 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, and the operating pressure is 0.2 Mpa; the stirring speed of the high-speed stirring is 650 rpm; the pH value is adjusted to 6; the heat preservation time in the electrothermal blast drying oven is 9 h;

[0148] In S2, the melting temperature of rhamnose is 130 °C;

[0149] In S3, the molar ratio of Li element, rhamnose and polyethylene glycol in the lithium iron phosphate matrix is 1:0.04:0.02; the temperature in the first stage of sintering is 450 °C.

[0150] Example 3

[0151] This example provides a lithium iron phosphate cathode material, and its preparation method includes:

[0152] S1: Prepare the lithium iron phosphate matrix.

[0153] Weigh ferrous acetate and disperse it in deionized water, and blow nitrogen for 1 h under stirring conditions to remove the dissolved oxygen in the solution to obtain the first solution. Weigh lithium oxalate and ammonium hydrogen phosphate and disperse them in deionized water, and perform high-speed stirring under the protection of a nitrogen atmosphere, with a stirring rate of 600 r / min, to obtain the second solution. The molar ratio of Li element in lithium oxalate, Fe element in ferrous acetate and P element in ammonium hydrogen phosphate is 1:1:0.95.

[0154] Pour the first solution into a high-pressure atomizing spray gun with a nozzle diameter of 0.5 mm and an operating pressure of 0.3 MPa, and after atomization by the high-pressure nozzle, spray it into the second solution under high-speed stirring in an atomized state to obtain a mixed solution. Adjust the pH value of the obtained mixed solution to 7 with ammonia water, and continue to stir for 30 min at room temperature under the protection of a nitrogen atmosphere, and then transfer it to a stainless steel high-pressure reaction kettle with a polytetrafluoroethylene lining and place it in an electrothermal blast drying oven at 150 °C for heat preservation for 8 h. After the reaction is completed, cool the product to room temperature and filter it, and then successively perform centrifugal washing on the filtered solid phase with deionized water and absolute ethanol, and finally dry it in a vacuum drying oven at 70 °C for 12 h to obtain the lithium iron phosphate matrix.

[0155] S2: Prepare the composite carbon source.

[0156] Weigh rhamnose and place it in a high-temperature electric furnace, and raise the temperature to 135 °C for melting; weigh polyethylene glycol powder and dissolve it in dimethyl sulfoxide, and after stirring evenly, raise the temperature of the mixed solution to 50 °C for water bath heating to obtain a polyethylene glycol gel; mix the melted rhamnose with the polyethylene glycol gel to obtain a composite carbon source.

[0157] S3: Prepare the lithium iron phosphate cathode material.

[0158] Mix the lithium iron phosphate matrix in S1 with the composite carbon source in S2 by ball milling for 1.5 h to obtain a mixed precursor; place the mixed precursor in a vacuum oven at 100 °C and dry for 12 h, then grind and crush it; subsequently, place the well-ground mixed precursor in a muffle furnace filled with high-purity argon for sintering to obtain the lithium iron phosphate cathode material.

[0159] Among them, the molar ratio of Li element in the lithium iron phosphate matrix, rhamnose, and polyethylene glycol is 1:0.035:0.02.

[0160] The sintering process includes: heating up to 400 °C at a rate of 5 °C / min and holding for 2 h; then continuing to heat up to 650 °C, and the roasting time is 6 h.

[0161] Example 4

[0162] The difference between this example and Example 3 is that:

[0163] In S1, the molar ratio of Li element in lithium oxalate, Fe element in ferrous acetate, and P element 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 stirring is 700 rpm;

[0164] In S2, the carbon source is sorbitol, and the melting temperature of sorbitol is 145 °C; the water bath heating temperature of polyethylene glycol is 60 °C;

[0165] In S3, the molar ratio of Li element in the lithium iron phosphate matrix, sorbitol, and polyethylene glycol is 1:0.05:0.04; the temperature in the first stage of sintering is 550 °C and the time is 5 h; the temperature in the second stage of sintering is 750 °C and the time is 8 h.

[0166] Example 5

[0167] The difference between this example and Example 3 is that:

[0168] 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 element in lithium acetate, Fe element in ferrous chloride, and P element 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 stirring is 800 rpm; the holding temperature in the electrothermal blast drying oven is 155 °C and the holding time is 9 h;

[0169] In S2, the melting temperature of rhamnose is 150 °C; the water bath heating temperature of polyethylene glycol is 55 °C;

[0170] In S3, the molar ratio of Li element in the lithium iron phosphate matrix, rhamnose and polyethylene glycol is 1:0.05:0.03; the temperature in the first stage of sintering is 500 °C and the time is 3 h; the temperature in the second stage of sintering is 700 °C and the time is 7 h.

[0171] Example 6

[0172] The difference between this example and Example 3 is as follows:

[0173] 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 element in lithium acetate, Fe element in ferrous chloride and P element 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 high-speed stirring is 900 rpm; the pH value is adjusted to 8; the holding temperature in the electrothermal blast drying oven is 160 °C and the holding time is 9 h;

[0174] In S2, the melting temperature of rhamnose is 150 °C; the water bath heating temperature of polyethylene glycol is 60 °C;

[0175] In S3, the molar ratio of Li element in the lithium iron phosphate matrix, rhamnose and polyethylene glycol is 1:0.05:0.025; the temperature in the first stage of sintering is 500 °C and the time is 3 h; the time in the second stage of sintering is 7 h.

[0176] Example 7

[0177] The difference between this example and Example 3 is as follows:

[0178] 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 molar number of Li element in lithium acetate, the total molar number of Fe element in ferrous chloride and Ni element in nickel chloride to the molar number of P element in ammonium dihydrogen phosphate is 1:1:0.95, wherein the molar ratio of Fe element in ferrous chloride to Ni element in nickel 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 0.35 MPa; the stirring speed of high-speed stirring is 950 rpm; the pH value is adjusted to 8; the holding temperature in the electrothermal blast drying oven is 170 °C and the holding time is 9 h;

[0179] In S2, the carbon source is sorbitol and the melting temperature of sorbitol is 140 °C;

[0180] In S3, the molar ratio of Li element in the lithium iron phosphate matrix, sorbitol and polyethylene glycol is 1:0.045:0.02; the temperature in the first stage of sintering is 450 °C.

[0181] Example 8

[0182] The difference between this embodiment and Embodiment 3 is as follows:

[0183] 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 element in lithium acetate, Fe element in ferrous chloride, and P element in ammonium dihydrogen phosphate is 1:1:1; the molar ratio of O element in ammonium dihydrogen phosphate to F element 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 high-speed stirring is 950 rpm; the holding temperature in the electrothermal blast drying oven is 175 °C, and the holding time is 10 h;

[0184] In S2, the carbon source is sorbitol; the melting temperature of sorbitol is 145 °C; the water bath heating temperature of polyethylene glycol is 55 °C;

[0185] 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 in the first stage of sintering is 480 °C, and the time is 3 h; the temperature in the second stage of sintering is 680 °C.

[0186] Embodiment 9

[0187] The difference between this embodiment and Embodiment 3 is as follows:

[0188] 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 number of Li element in lithium acetate and Na element in sodium chloride, the molar number of Fe element in ferrous chloride, and the molar number of P element in ammonium dihydrogen phosphate is 1:1:0.95; the molar ratio of Li element in lithium acetate to Na element 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 high-speed stirring is 1000 rpm; the holding temperature in the electrothermal blast drying oven is 180 °C, and the holding time is 10 h;

[0189] In S2, the carbon source is fructose; the melting temperature of fructose is 145 °C; the water bath heating temperature of polyethylene glycol is 60 °C;

[0190] In S3, the molar ratio of Li element in the lithium iron phosphate matrix to fructose and polyethylene glycol is 1:0.05:0.035; the temperature in the first stage of sintering is 500 °C, and the time is 5 h; the temperature in the second stage of sintering is 750 °C, and the time is 8 h.

[0191] Embodiment 10

[0192] The difference between this example and Example 3 lies in that: the nozzle diameter in the high-pressure atomization mixing step is 0.4 mm, and the operating pressure is 0.4 Mpa; the high-speed stirring rate is 650 r / min; the remaining steps and conditions remain unchanged.

[0193] Example 11

[0194] The difference between this example and Example 3 lies in that: the holding temperature in the electrothermal blast drying oven is 170 °C, and the holding time is 9 h; the remaining steps and conditions remain unchanged.

[0195] Example 12

[0196] The difference between this example and Example 3 lies in that: the temperature for melting the carbon source is 150 °C, and the temperature of the gel water bath is 60 °C; the remaining steps and conditions remain unchanged.

[0197] Example 13

[0198] The difference between this example and Example 3 lies in that: the temperature and time in the first stage of sintering are 450 °C and 3 h respectively, and the temperature and time in the second stage are 700 °C and 7 h respectively; the remaining steps and conditions remain unchanged.

[0199] Example 14

[0200] The difference between this example and Example 3 lies in that: manganese chloride and iron chloride are dispersed together in deionized water at a molar ratio of Fe to Mn of 0.985:0.015 to obtain a first solution; the remaining steps and conditions remain unchanged.

[0201] Comparative Example 1

[0202] The difference between this comparative example and Example 3 lies in that:

[0203] In S1, the molar ratio of Li element in lithium oxalate, Fe element in ferrous acetate to P element in ammonium hydrogen phosphate is 1:1:1.1; the nozzle diameter of the high-pressure atomization spray gun is 0.6 mm, and the operating pressure is 0.2 MPa; the pH value is adjusted to 6; the holding time in the electrothermal blast drying oven is 9 h;

[0204] In S2, the melting temperature of rhamnose is 100 °C;

[0205] In S3, the molar ratio of Li element in the lithium iron phosphate matrix to rhamnose and polyethylene glycol is 1:0.03:0.02; the temperature in the first stage of sintering is 350 °C; the temperature in the second stage is 650 °C.

[0206] Comparative Example 2

[0207] The difference between this comparative example and Example 3 is that the preparation of the lithium iron phosphate matrix does not use the method of high-pressure atomization mixing, but directly mixes the materials and then reacts.

[0208] Specifically:

[0209] S1: Directly disperse ferrous acetate, lithium oxalate, and ammonium hydrogen phosphate in deionized water at a molar ratio of Li:Fe:P = 1:1:0.95. Under stirring conditions, use nitrogen to blow for 30 min to remove the dissolved oxygen in the solution, then transfer it to a stainless-steel high-pressure reaction kettle with a polytetrafluoroethylene liner, and place it in an electric blast drying oven at 150 °C for heat preservation for 8 h. After the reaction is completed, cool the product to room temperature and then filter it. Centrifuge and wash the obtained solid phase successively with deionized water and absolute ethanol, and finally dry it in a vacuum drying oven at 70 °C for 12 h to obtain the lithium iron phosphate precursor.

[0210] The steps and conditions of S2 and S3 are the same as those in Example 3.

[0211] Comparative Example 3

[0212] 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.

[0213] Specifically:

[0214] The steps and conditions of S1 are the same as those in Example 3.

[0215] S2: Disperse the lithium iron phosphate matrix in S1 and rhamnose (the molar ratio of lithium element in the lithium iron phosphate matrix particles to rhamnose is 1:0.035) in absolute ethanol, and ball-mill for 1.5 h to mix evenly to obtain a mixed precursor; place the mixed precursor in a vacuum drying oven at 100 °C for 12 h, grind and crush it; then, place the well-ground precursor in a muffle furnace filled with high-purity argon for sintering, heat it to 400 °C at a rate of 5 °C / min, and keep the temperature for 2 h; then continue to heat it to 650 °C, and roast for 6 h to obtain the lithium iron phosphate cathode material.

[0216] Comparative Example 4

[0217] The difference between this comparative example and Example 3 is that the hydrothermal-high temperature calcination method is directly used to prepare lithium iron phosphate.

[0218] 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, and the difference is only that: the molar ratio of lithium element in the lithium iron phosphate matrix to rhamnose is 1:0.05.

[0219] Test Example

[0220] (1) Perform performance tests on the lithium iron phosphate cathode materials prepared in the above Examples 1-14 and Comparative Examples 1-4.

[0221] The performance test method is as follows:

[0222] ① Use a combined device of FEI Tecnai F20 transmission electron microscope (TEM) - energy dispersive spectrometer (EDX) to test the carbon coating thickness, surface carbon element content and distribution of the lithium iron phosphate cathode material. Use the TEM imaging system to select a microarea (local position) of a single lithium iron phosphate cathode material particle, and operate the electron microscope at a high magnification to obtain a high-resolution image of the particle microarea using the objective aperture. Then, use the image acquisition system to measure and analyze the morphological image data of the particle, and measure the coating thickness of the selected microarea of the particle; at the same time, qualitatively and quantitatively analyze the carbon element distribution on the surface of the selected microarea of the particle in combination with the EDX detector. The sample collection amount of the cathode material particles in the lithium iron phosphate cathode material sample is 10, randomly selected.

[0223] Among them, and σ ( C wt% ) is calculated as follows:

[0224] 、 , N 1 is the sample size of the lithium iron phosphate cathode material particles taken for testing, with a value of 10;

[0225] 、 , n is the sample size of the local position taken during the test of the i th lithium iron phosphate cathode material particle, with a value of 5; d m is the carbon coating thickness corresponding to the m th local position taken for the lithium iron phosphate cathode material particle, is the d m average value of the 5 local position samples taken for the lithium iron phosphate cathode material particle, in nm; C m is the carbon content corresponding to the m th local position taken for the lithium iron phosphate cathode material particle, is the 5 samples of the local positions taken for the lithium iron phosphate cathode material particle C m average value, in wt%.

[0226] The carbon coating thickness of the lithium iron phosphate cathode material is , the carbon content is .

[0227] ②. Use a FEI Tecnai F20 transmission electron microscope to observe the microstructure of the lithium iron phosphate cathode material particles and obtain its transmission electron microscope image (TEM two-dimensional projection image); use ImageJ to perform shape analysis on the TEM two-dimensional projection image corresponding to the lithium iron phosphate cathode material particles, and collect 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; the sample collection amount of the cathode material particles in the lithium iron phosphate cathode material sample is 10, and they are randomly selected.

[0228] Among them, The calculation method is as follows: , is defined as the shape factor corresponding to the j th lithium iron phosphate cathode material particle taken during the test. Specifically, is the diameter of the standard circle ( D s , μm) in the TEM two-dimensional projection image corresponding to the lithium iron phosphate cathode material particle, which is equal to the two-dimensional projection area of the lithium iron phosphate cathode material particle, and the diameter of the minimum circumscribed circle of the two-dimensional projection of the lithium iron phosphate cathode material particle ( D , μm); N 2 is the sample size of the lithium iron phosphate cathode material particles tested, and the value is 10.

[0229] ③. Specific surface area (BET) test: Use a Micromeritics ASAP 2460 fully automatic specific surface area analyzer to perform isothermal adsorption and desorption analysis by the static volumetric method, and measure the specific surface area of the lithium iron phosphate cathode material by measuring the adsorption and desorption behavior of nitrogen on the lithium iron phosphate cathode material, with the unit of m 2 / g.

[0230] ④. Particle size: Use a GSL-101BI laser particle size analyzer to test the particle size of the lithium iron phosphate cathode material.

[0231] ⑤. Carrier mobility test: Use a HMS-7000 Hall effect tester to test the carrier mobility of the lithium iron phosphate cathode material. The carrier mobility is an important parameter to measure the conductivity of the material, and is used to characterize the speed of movement of carriers (electrons, holes) inside and on the surface of the lithium iron phosphate cathode material under the action of a unit electric field, with the unit of cm 2 / V·s.

[0232] ⑥. Electrochemical performance test

[0233] A. Electrode Preparation and Button Cell Assembly: The lithium iron phosphate cathode material, conductive agent acetylene black, and adhesive polyvinylidene fluoride are uniformly mixed in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry. Then, the slurry is coated on aluminum foil and dried in a vacuum drying oven. Subsequently, it is pressed into a positive electrode plate using a tablet press. The negative electrode plate is a lithium metal sheet, the electrolyte is 1 mol / L lithium hexafluorophosphate-ethylene carbonate:dimethyl carbonate (by volume, V(EC):V(DMC)=1:1), and a polypropylene porous membrane is used as the separator. The assembly of the button cell is carried out in an argon glove box.

[0234] B. Electrochemical Performance Test: The assembled button cell is subjected to an electrochemical performance test within a charge-discharge voltage range of 2.5 V to 4.5 V. First, it is charged at a constant current to 4.5 V, then discharged at a rate current to 2.5 V, and the discharged capacity is the discharge capacity at that rate. After the discharge is completed, it is further discharged at a constant current to 2.5 V; then, the test for the next rate is carried out.

[0235] ⑦. Electrode Yield Strength Test: The yield strength (unit: MPa, i.e., the stress value that generates a 0.2% residual deformation) of the lithium iron phosphate electrode plate is tested using a universal testing machine in combination with a video extensometer and a special pneumatic fixture for foil materials to evaluate the influence of the microscopic morphology of lithium iron phosphate particles and the surface carbon coating on the mechanical strength of the electrode. The prepared electrode is clamped using the special pneumatic fixture for foil materials, and the fixture air pressure is set between 0.4 MPa and 0.6 MPa for a tensile test. The deformation of the electrode plate during the tensile process is measured through the video extensometer.

[0236] The above performance test results are shown in Table 1, Table 2, Figure 1 and Figure 2 as shown below.

[0237] Table 1 Test Results

[0238]

[0239] Table 2 Test Results

[0240]

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

[0242] It can be seen from the data in Table 1 that , and will all affect the parameter factor of the lithium iron phosphate materialQ has an impact on the value.

[0243] As can be seen from Examples 1 to 9, when the of the lithium iron phosphate cathode material particles is larger, that is, closer to 1, the regularity of the morphology of the lithium iron phosphate cathode material particles is better; and is smaller, the carbon coating layer in the lithium iron phosphate cathode material particles is more uniform. At this time, the obtained parameter factor Q has a larger value, and the size of the parameter factor Q will have a certain impact on the comprehensive electrochemical performance of the lithium battery prepared from the lithium iron phosphate cathode material. Generally, the lithium iron phosphate cathode material with regular morphology has good dispersibility, can improve the tap density of the lithium iron phosphate cathode material and reduce the internal resistance of the battery; the uniform conductive carbon coating layer can effectively reduce the occurrence of side reactions between the lithium iron phosphate cathode material and the electrolyte, and improve the diffusion and conduction of electrons and lithium ions, which will improve the electrical performance of the battery corresponding to the lithium iron phosphate cathode material.

[0244] From the comprehensive data in Table 1 and Table 2, it can be seen that even if the lithium iron phosphate cathode material has the same chemical composition, there will be corresponding different , and , and thus there will be corresponding parameter factors Q of different sizes. When the value of the parameter factor Q is in the range of 0.07 to 7, and , and are also within the preferred ranges ( is 0.5 to 1, is 0.05 nm to 0.35 nm, is 0.02 wt% to 0.35 wt%), 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.

[0245] Among them, in Example 1, only and are within the preferred ranges, and the electrochemical performance and electrode yield strength value of the battery prepared from the corresponding lithium iron phosphate cathode material are not as good as those of other examples; for Example 2, , and are all within the preferred ranges, but the parameter factor Q is not within the preferred range. The electrochemical performance and electrode yield strength value of the battery prepared from the corresponding lithium iron phosphate cathode material are improved compared with Example 1, but not as good as those of Examples 3 to 14; for Examples 3 to 9, , , andQ All are within the preferred range. Compared with Examples 1 to 2, the electrochemical performance and electrode yield strength performance of the batteries prepared from the corresponding lithium iron phosphate cathode materials are significantly improved.

[0246] Moreover, when the parameter factors Q of the lithium iron phosphate cathode materials in Examples 3 to 9 are higher, the electrical performance of the corresponding batteries is better. At this time, the lithium iron phosphate cathode material particles have a relatively regular morphology and a relatively uniform carbon coating layer, which is beneficial to accelerating the deintercalation / insertion rate of lithium ions, shortening the transmission and diffusion path of lithium ions, increasing the conductivity of the lithium iron phosphate cathode material, and avoiding the occurrence of side reactions between the lithium iron phosphate cathode material and the electrolyte, thereby improving the charge-discharge performance of the lithium-ion battery.

[0247] In addition, the electrode yield strength corresponding to the lithium iron phosphate cathode material is jointly determined by the microscopic morphology of the material particles and the carbon coating. Appropriate carbon content, regular particle morphology, and uniform carbon coating layer are beneficial to increasing the tap density of the lithium iron phosphate cathode material particles, thereby improving the electrode yield strength prepared therefrom and enhancing the mechanical properties of the electrode. From the data, it can be seen that except for Examples 4 and 9, the electrode yield strength values corresponding to the lithium iron phosphate cathode materials in the remaining examples increase with the increase of the parameter factor Q value; relatively speaking, although the parameter factors Q of Examples 4 and 9 are within the preferred range, their surface carbon content is slightly higher, and more amorphous carbon is generated after high-temperature carbonization, which will reduce the tap density of the lithium iron phosphate cathode material to a certain extent, thereby affecting its electrode yield strength.

[0248] In addition, the lithium iron phosphate cathode material prepared in the examples has a relatively excellent carrier mobility, indicating that the electrons inside and on the surface of the lithium iron phosphate cathode material have a relatively fast moving speed in the electric field per unit area, and it also further reflects that the carbon coating layer has good uniformity on the matrix surface. It can be seen from Examples 3 to 4, Examples 8 to 9, and Examples 6 to 7 that when the and values of the lithium iron phosphate cathode material are smaller and the carbon content ( , wt%) is higher, the carrier mobility of the lithium iron phosphate cathode material is higher, which means that its conductivity is also better, which is beneficial to improving the charge-discharge performance of the lithium battery. Moreover, when the carbon content and and values are not very 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 element doping.

[0249] In addition, from the comparison between Examples 1 to 9 and Comparative Example 1, the and The value is not within the range provided by the present invention, although its The value is within the range provided by the present invention, and the apparent carrier mobility, electrochemical performance, and electrode yield strength of the corresponding lithium iron phosphate cathode material are all poor. Therefore, when , , and Q are all within the range provided by the present invention, the performance indicators of the obtained lithium iron phosphate cathode material will be more excellent.

[0250] It can be seen from Example 3, Comparative Example 2, and Comparative Example 4 that if the high-pressure atomization mixing method is not adopted during the preparation process, the values of Comparative Example 2 and Comparative Example 4 are small and differ greatly from those of Example 3, indicating that the shape regularity of the lithium iron phosphate cathode materials obtained in Comparative Example 2 and Comparative Example 4 is poor. Moreover, the parameter factor Q value in Comparative Example 2 and Comparative Example 4 is small, and the comprehensive electrochemical performance of the lithium iron phosphate cathode material is also low; the shape factor value of the lithium iron phosphate cathode material prepared in Example 3 is large (less than 1), indicating that its particle morphology is more regular. This is due to the fact that the high-pressure atomization-high-speed stirring mixing operation mode provides seeds with uniform particle size and uniform morphology for the formation of the lithium iron phosphate cathode material. After the hydrothermal reaction process at an appropriate temperature, lithium iron phosphate precursor particles with good crystallinity and regular and uniform morphology are obtained, and the electrochemical performance of the battery prepared from the corresponding lithium iron phosphate cathode material is also better.

[0251] Compared with Example 3, in Example 10, the nozzle diameter in the high-pressure atomization mixing step is smaller, the operating pressure is larger, and the mixing liquid stirring rate is faster. This is conducive to obtaining seed precursors with smaller particle sizes. The specific surface area of the lithium iron phosphate precursor particles prepared by the subsequent hydrothermal reaction is larger, and the morphological regularity is also better. This is beneficial for shortening the diffusion path of lithium ions and increasing the lithium ion deintercalation / insertion rate, thereby enhancing the charge and discharge performance of the battery prepared from the lithium iron phosphate cathode material.

[0252] Compared with Example 3, the particle morphology regularity of the lithium iron phosphate cathode material in Example 11 is improved, and the particle size is slightly reduced. This is because within the appropriate reaction temperature range, the increase in the synthesis temperature makes the interaction between reactants stronger, which will accelerate the crystal growth process. At the same time, the higher temperature can provide sufficient energy to overcome the energy barrier required to form smaller particles, resulting in smaller crystal particle sizes and more regular morphologies, which is beneficial for improving the electrochemical performance of the battery prepared from the lithium iron phosphate cathode material.

[0253] Compared with Example 3, the shape factor The value increases because an appropriate amount of element 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 directions or crystal planes, further 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 prepared from the lithium iron phosphate cathode material.

[0254] It can be seen from Example 3, Comparative Example 3 and Comparative Example 4 that if the carbon source is not melt-treated and not compounded with polyethylene glycol gel during the high-temperature calcination of lithium iron phosphate in the preparation process, the and values of the lithium iron phosphate cathode materials prepared in Comparative Example 3 and 4 are relatively large (greater than 0.9), indicating that the coating uniformity of the carbon coating layer in the cathode material is poor. Therefore, the apparent carrier mobility value of the material is low, and the obtained parameter factor Q value is also small, and the comprehensive performance of the cathode material is also low; the and values of the lithium iron phosphate cathode material prepared in Example 3 are smaller (less than 0.5), indicating that the coating uniformity of the carbon coating layer in the cathode material is better. This benefits from the co-infiltration coating of the composite carbon source composed of the molten carbon source and polyethylene glycol gel during the high-temperature calcination process, which is conducive to further improving the electrochemical performance of the lithium iron phosphate cathode material.

[0255] Compared with Example 3, the and values of the lithium iron phosphate cathode material in Example 12 are significantly reduced, indicating that the uniformity of the carbon coating layer on the surface of the lithium iron phosphate matrix is improved. This is because within the appropriate temperature range, a higher water bath temperature is conducive to increasing the strength and swelling characteristics of the polyethylene glycol gel, and a higher melting temperature is conducive to increasing the fluidity of the liquid carbon source. After mixing the lithium iron phosphate matrix with the composite carbon source, the molten carbon source with large fluidity is conducive to being uniformly and firmly coated on the surface of the lithium iron phosphate matrix by the high-strength polyethylene glycol, thereby increasing the uniformity of the carbon coating layer.

[0256] Compared with Example 3, the and values of the lithium iron phosphate cathode material in Example 13 are reduced, and the thickness of the carbon coating layer increases slightly. This is because within a certain temperature range, a higher calcination temperature helps to form a more stable, continuous and dense carbon coating layer structure, thereby improving its coating uniformity and promoting the diffusion of lithium ions and the conduction of electrons; moreover, a higher sintering temperature will promote the rapid sintering of the carbon coating layer to a certain extent, thereby forming a thicker carbon layer. The lower and values indicate that the carbon coating layer is more uniformly coated on the surface of the lithium iron phosphate matrix, which is beneficial to improving the electrochemical performance of the corresponding battery.

[0257] In summary, the lithium iron phosphate cathode material provided by the present invention 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, etc., and is also beneficial to further improving the electrochemical performance (such as specific capacity, rate performance, and cycling performance, etc.) of the battery prepared from the lithium iron phosphate cathode material.

[0258] 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, various modifications and changes can be made to the present invention. Any modification, equivalent replacement, improvement, 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 method for preparing a lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: ball milling the lithium iron phosphate matrix and the composite carbon source to obtain a mixed precursor; The mixed precursor is dried and then sintered to obtain a lithium iron phosphate positive electrode material; The preparation of the composite carbon source comprises: mixing polyethylene glycol gel with a melted carbon source; wherein the carbon source comprises at least one of sorbitol, erythritol, fructose, rhamnose and malic acid; the molar ratio of the 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); The preparation of the lithium iron phosphate matrix includes: mixing a first solution containing ferrous salt with a second solution containing a lithium source and phosphate to obtain a mixed solution; adjusting the pH value of the mixed solution to 6-8 and then performing a hydrothermal reaction, and collecting solids after the reaction; wherein 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; The operating pressure of the high-pressure atomization is 0.2MPa~0.4MPa.

2. The preparation method according to claim 1, characterized in that: The preparation of the lithium iron phosphate positive electrode material includes at least one of the following features: Feature 1: The ball milling time is 1h~2h; Feature 2: The drying temperature of the mixed precursor is 90°C to 110°C; Feature 3: The drying time of the mixed precursor is 10h~14h; Feature 4: The sintering includes: first keeping the temperature at 400°C to 550°C for 2h to 5h, and then keeping the temperature at 650°C to 750°C for 6h to 8h.

3. The preparation method according to claim 2, 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 further contains an M source, and the second solution further contains an A source and a D source; Condition 2: The temperature of the hydrothermal reaction is 150°C to 180°C; Condition 3: The hydrothermal reaction time is 8h~10h.

4. The preparation method according to claim 3, characterized in that: The ferrous salt includes at least one of ferrous oxalate, ferrous chloride and ferrous acetate; Or, the lithium source includes at least one of lithium oxalate, lithium chloride and lithium acetate; Or, the phosphate includes at least one of diammonium phosphate and ammonium hydrogen phosphate; Or, the M source is a water-soluble salt of the M element; Or, the A source includes at least one of sodium fluoride, magnesium fluoride, sodium chloride, magnesium chloride and sodium sulfide; Or, the D source includes at least one of ammonium fluoride, lithium fluoride, ammonium sulfide, lithium sulfide, ammonium bisulfite and thiourea; Or, the ratio of the total molar number of the iron element in the ferrous salt to the M element in the M source, the total molar number of the lithium element in the lithium source to the A element in the A source, and the molar number of the P element in the phosphate is 1:1:(0.95-1.1); wherein the molar ratio of the lithium element to the A element is (0.9-1):(0-0.1); the molar ratio of the iron element to the M element is (0.9-1):(0-0.1); the molar ratio of the oxygen element in the phosphate to the D element in the D source is (3.9-4):(0-0.1); Or, the nozzle diameter of the high-pressure atomization spray gun used for the high-pressure atomization is 0.3mm~0.6mm; Alternatively, the stirring condition corresponds to a stirring rate of 600 r / min to 1000 r / min.

5. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by the preparation method according to any one of claims 1 to 4.

6. The lithium iron phosphate positive electrode material according to claim 5, characterized in that: The lithium iron phosphate positive electrode material comprises a lithium iron phosphate matrix and a carbon coating layer coated on 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 , , 0.05nm≤ σ(d) ≤0.35nm; The average standard deviation of the carbon content in the lithium iron phosphate positive electrode material is σ ( C wt% ), , 0.02wt%≤ σ(C wt% ) ≤0.35wt%; in, N 1 is the sample amount of lithium iron phosphate positive electrode material particles taken during the test of the lithium iron phosphate positive electrode material, N 1 ≥ 10; , The first value taken when testing the lithium iron phosphate positive electrode material i The standard deviation of carbon coating thickness and carbon content corresponding to each lithium iron phosphate cathode material particle; , , n For the said i The sample size of a local location taken when testing a lithium iron phosphate positive electrode material particle, n ≥5; d m For the m The thickness of the carbon coating layer corresponding to a local position, For what is taken n Local location samples d m The average value of , in nm; C m For the m The carbon content corresponding to each local position is For what is taken n Local location samples C m The average value, in wt%; The thickness of the carbon coating layer of the lithium iron phosphate positive electrode material is d , , 1nm≤ d ≤10nm; For the i The lithium iron phosphate positive electrode material particles are taken n Local location samples d m The average value of The carbon content of the lithium iron phosphate positive electrode material is , , 1wt%≤ C ≤10wt%; For the i The lithium iron phosphate positive electrode material particles are taken n Local location samples C m The average value of .

7. The lithium iron phosphate positive electrode material according to claim 6, characterized in that: The shape factor of the lithium iron phosphate positive electrode material is , , 0.5≤ ≤1; in, N 2 is the sample amount of lithium iron phosphate positive electrode material particles taken during the test of the lithium iron phosphate positive electrode material, N 2 ≥ 10; The first value taken when testing the lithium iron phosphate positive electrode material j The shape factor corresponding to each lithium iron phosphate cathode material particle; , D s For the j The diameter of the standard circle with the same projection area in the TEM two-dimensional projection image of each lithium iron phosphate positive electrode material particle, D For the j The diameter of the minimum circumscribed circle of the particle projection in the TEM two-dimensional projection image of a lithium iron phosphate positive electrode material particle.

8. The lithium iron phosphate positive electrode material according to claim 7, characterized in that: The parameter factors of the lithium iron phosphate positive electrode material , 0.07nm -1 ≤ Q ≤7nm -1 .

9. The lithium iron phosphate positive electrode material according to claim 8, characterized in that: The lithium iron phosphate positive electrode material also includes at least one of the following features: Feature 5: The lithium iron phosphate positive electrode material 0.550~0.910; Feature 6: The lithium iron phosphate positive electrode material 1.5nm~9.1nm; Feature 7: The lithium iron phosphate positive electrode material 0.055nm~0.343nm; Feature 8: The lithium iron phosphate positive electrode material 1.94wt%~9.68wt%; Feature 9: The lithium iron phosphate positive electrode material σ ( C wt% ) is 0.024wt%~0.336wt%; feature 10: The lithium iron phosphate positive electrode material Q 0.072nm -1 ~6.596nm -1 ; Feature 11: The specific surface area of ​​the lithium iron phosphate positive electrode material is 9.3m 2 / g~15.1m 2 / g; Feature 12: The lithium iron phosphate positive electrode material D 50 0.85μm~1.50μm; feature 13: The carrier mobility of the lithium iron phosphate positive electrode material is 20.03×10 -6 cm 2 / V·s~21.71×10 - 6 cm 2 / V·s; Feature 14: 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.

10. A positive electrode sheet, characterized in that: The active material in the positive electrode sheet includes the lithium iron phosphate positive electrode material according to any one of claims 5 to 9.

11. A battery, characterized in that: The battery comprises the positive electrode sheet according to claim 10.

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