Secondary battery positive electrode material, secondary battery positive electrode sheet, and secondary battery

Abstract

A secondary battery positive electrode material, relating to the field of battery materials. The secondary battery positive electrode material comprises large particles with a particle size greater than or equal to 2 μm and small particles with a particle size smaller than or equal to 1 μm. The surfaces of some of the small particles are provided with a carbon coating layer; and the surfaces of some of the large particles are not provided with a carbon coating layer. According to the positive electrode material, by means of gradation design of the large and small particles, the effect that the small particles fill gaps left by accumulation of the large particles can be achieved, and then the compaction density is improved. Moreover, the surfaces of the small particles are coated with a carbon layer to provide sufficient electron transport paths, so that a stable electron pathway for the large particles wrapped by the small particles can be maintained in a charge and discharge cycle. Additionally, the surfaces of the large particles with the particle size larger than or equal to 2 μm are not hindered by a carbon coating layer, so that the wetting capacity of an electrolyte to the electrode sheet can be improved, and faster transmission of lithium ions at an interface can be realized, thereby reducing the impedance in a charge and discharge process.

Description

Secondary battery positive electrode material, secondary battery positive electrode sheet and secondary battery Technical Field

[0001] The present invention belongs to the field of battery materials, and in particular relates to a secondary battery positive electrode material, a secondary battery positive electrode sheet and a secondary battery. Background Art

[0002] In olivine-structured secondary battery cathode materials, such as lithium iron phosphate, oxygen atoms and phosphorus atoms form stable phosphate polyanions, resulting in significantly better structural and thermal stability than layered α-sodium ferrite cathode materials, such as ternary cathode materials. However, the intrinsic ionic and electronic conductivities of olivine-structured cathode materials are relatively low, resulting in inferior low-temperature and high-rate charge-discharge performance compared to layered cathode materials.

[0003] To address the above issues, the olivine-structured secondary battery cathode material needs to be ground and sintered to ensure that the particle size reaches the micrometer and nanometer levels, and the material surface needs to be carbon-coated. However, adjusting the size of the cathode material significantly affects the compaction density of the material after it is made into a pole piece, which in turn affects the energy density of the battery. Although carbon coating on the surface of the material can significantly improve the electronic conductivity between the materials, the carbon layer itself has weak lithium ion conductivity and needs to be generated by a reduction reaction of a carbon source under inert or reducing gas conditions. It is inherently difficult to complete the same process as oxide coating that can generate fast ion conductor species. This limits the possibility of using oxides to modify the material surface to improve ion interface transmission.

[0004] Therefore, preparing a secondary battery positive electrode material that can improve the material compaction density, simultaneously improve the electronic conductivity and ion conduction rate, and reduce the battery impedance is a technical problem that urgently needs to be solved.

[0005] Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems existing in the above-mentioned prior art and provide a secondary battery positive electrode material with high compaction density and low impedance.

[0007] The present invention is achieved through the following technical solutions:

[0008] The present invention provides a secondary battery positive electrode material, which comprises: large particles with a particle size of ≥2 μm and small particles with a particle size of ≤1 μm; at least some of the small particles have a carbon coating layer on their surfaces; and at least some of the large particles do not have a carbon coating layer on their surfaces.

[0009] The positive electrode material of the present invention improves its compaction density through the gradation of large and small particles. The present invention uses a combination of large particles with a particle size of ≥2μm and small particles with a particle size of ≤1μm, which can achieve the effect of small particles filling the gaps between large particles, thereby improving the compaction density of the positive electrode material in the positive electrode sheet. The surface of the small-particle positive electrode material with a particle size of ≤1μm is coated with a carbon layer, which can partially act as a conductive agent. In the electrode sheet, it surrounds the large particles with a particle size of ≥2μm, which can maintain a stable electron pathway for the large-particle positive electrode material during the charge and discharge cycle, providing a sufficient electron transmission path without affecting the electronic conductivity of the positive electrode sheet. The surface of the large particles with a particle size of ≥2μm is directly in contact with the electrolyte, without the obstruction of the carbon coating layer. This not only helps to improve the electrolyte's ability to penetrate the electrode sheet, but also enables faster transmission of lithium ions at the interface, thereby reducing the impedance of the charge and discharge process.

[0010] The particle size of the present invention is a size parameter of the particle, that is, the particle size is the maximum length of the positive electrode material particle in any shape direction.

[0011] As a preferred embodiment of the secondary battery positive electrode material of the present invention, at least part of the large particles have an oxide coating layer.

[0012] The present invention further performs oxide coating and doping on the surface of large-particle positive electrode materials with a particle size of ≥2μm. On the one hand, it can form a fast ion conductor or fine-tune the lattice of the material, thereby improving the interface and bulk ion transmission rate of the large-particle material and reducing the impedance during the charge and discharge process; on the other hand, it can reduce the damage of the electrolyte to the surface of the material particles and improve the cycle life of the material.

[0013] As a preferred embodiment of the secondary battery positive electrode material of the present invention, the surfaces of the small particles are all coated with a carbon layer, and the surfaces of the large particles are all coated with an oxide layer.

[0014] In some embodiments, the oxide coating layer partially forms a fast ion conductor, with the remainder existing in the form of an oxide.

[0015] In a preferred embodiment of the secondary battery positive electrode material of the present invention, the oxide coating layer comprises at least one of aluminum oxide, zirconium oxide, cobalt oxide, boron oxide, nickel oxide, tungsten oxide, titanium oxide, and vanadium oxide. The present invention selects at least one of aluminum oxide, zirconium oxide, cobalt oxide, boron oxide, nickel oxide, tungsten oxide, titanium oxide, and vanadium oxide as the oxide coating layer to increase ionic conductivity, thereby reducing the impedance of the secondary battery.

[0016] The present invention selects at least one of aluminum oxide, zirconium oxide, cobalt oxide, boron oxide, nickel oxide, tungsten oxide, titanium oxide, and vanadium oxide and places it on the surface of a large-particle positive electrode material with a particle size of ≥2μm. Some of these oxides can generate fast ion conductors such as lithium metaaluminate, lithium borate, or lithium titanate, while the remaining oxides exist in the form of oxides, which can improve the ion conductivity of the material surface while reducing damage to the material surface byproducts such as HF in the electrolyte. In some embodiments, the oxide coating on the surface of the large-particle positive electrode material includes one or more selected from aluminum oxide, boron oxide, and titanium oxide, and optionally or preferably may also include one or more selected from zirconium oxide, cobalt oxide, nickel oxide, tungsten oxide, and vanadium oxide.

[0017] As a preferred embodiment of the secondary battery positive electrode material of the present invention, at least one of the following is satisfied:

[0018] (1) The positive electrode material has an olivine structure;

[0019] (2) The chemical formula of the positive electrode material is: Li x Mn y Fe 1-y PO4, where 0.75≤x≤1.05, 0≤y≤0.9.

[0020] Preferably, the positive electrode material is at least one of lithium iron phosphate, lithium iron manganese phosphate, and lithium manganese phosphate. In some embodiments, the positive electrode material is lithium iron phosphate and / or lithium iron manganese phosphate.

[0021] As a preferred embodiment of the secondary battery positive electrode material of the present invention, the positive electrode material further contains a metal doping element.

[0022] In the process of preparing positive electrode materials, metal doping elements can be introduced into the bulk phase of the positive electrode material to dope and fine-tune the material's lattice, thereby improving the interface and bulk ion transport rate of the positive electrode material.

[0023] As a preferred embodiment of the secondary battery positive electrode material of the present invention, the metal doping element is at least one of aluminum, zirconium, cobalt, tungsten, titanium, vanadium, nickel, and magnesium.

[0024] In the cathode material of the present invention, the large particles and the small particles can be the same cathode material or different cathode materials. Preferably, the large particles and the small particles in the cathode material have the same crystal structure.

[0025] As a preferred embodiment of the secondary battery positive electrode material of the present invention, the particle size distribution of the positive electrode material includes at least two distribution peaks after decomposition, wherein the maximum distribution peak area S1 for a median particle size ≥ 1.5 μm and the maximum distribution peak area S2 for a median particle size < 1.5 μm satisfy 1:4≤S1:S2≤4:1. The specific decomposition method can be a deconvolution method.

[0026] The present invention uses a particle size distribution to improve the performance of the positive electrode material. The particle size distribution diagram of the positive electrode material has at least two distribution peaks, namely, a distribution peak with a median particle size ≥1.5μm and a distribution peak with a median particle size <1.5μm. The inventors found through research that when the two distribution peaks meet 1:4≤S1:S2≤4:1 (S1 is the distribution peak area with a median particle size ≥1.5μm, and S2 is the distribution peak area with a median particle size <1.5μm), the positive electrode material has a higher compaction density and lower internal resistance, and better performance. In some embodiments, the two distribution peaks meet 1:3≤S1:S2≤3:1. In some embodiments, the two distribution peaks meet 2:3≤S1:S2≤3:1.

[0027] The present invention does not limit the preparation method of the positive electrode material. Preferably, the secondary battery positive electrode material is obtained by mixing positive electrode materials with different particle sizes.

[0028] Specifically, the present invention provides the following method for preparing a positive electrode material for a secondary battery as an example, which should not be construed as limiting.

[0029] The present invention provides a method for preparing large particles in a positive electrode material, comprising the following steps: mixing a precursor with a lithium source, calcining, and crushing to a particle size of D50 ≥ 2 μm, thereby obtaining a positive electrode material comprising large particles with a particle size ≥ 2 μm; the precursor comprises at least one of iron phosphate, manganese phosphate, and an iron-manganese phosphate solid solution. Furthermore, after mixing the precursor with the lithium source, the precursor is ball-milled, dried, and then calcined. The lithium source may be lithium carbonate and / or lithium hydroxide.

[0030] The method for coating the surface of large particles in the positive electrode material with an oxide comprises the following steps: mixing the large particles having a particle size ≥ 2 μm and an oxide additive, calcining the mixture, and cooling the mixture. Preferably, the mass ratio of the large-particle positive electrode material to the oxide additive is 100:(0.1-0.3). The oxide additive can be one or more selected from metal salts, metal oxides, and metal hydroxides, wherein the metal salts, metal oxides, and metal hydroxides contain one or more elements selected from aluminum, zirconium, cobalt, boron, nickel, tungsten, titanium, and vanadium.

[0031] The method for preparing small particles of a positive electrode material comprises the following steps: mixing a precursor with a lithium source, calcining, and crushing to a particle size of D50 ≤ 1 μm, thereby obtaining small particles of a positive electrode material having a particle size of ≤ 1 μm; the precursor comprises at least one of iron phosphate, manganese phosphate, and an iron-manganese phosphate solid solution. Furthermore, after mixing the precursor with the lithium source, the precursor is ball-milled, dried, and then calcined.

[0032] The method for coating the surface of small particles of the positive electrode material with carbon comprises the following steps: mixing the small particles having a particle size of ≤1 μm with a carbon source, calcining under an inert atmosphere, cooling, and crushing to a D50 of ≤1 μm. The carbon source can be a carbon-containing compound such as a sugar, for example, glucose, fructose, etc.

[0033] The positive electrode materials of different particle sizes obtained above are mixed to obtain the secondary battery positive electrode material. Furthermore, the positive electrode material with D50 ≥ 2 μm can be mixed with the positive electrode material with D50 ≤ 1 μm in a mass ratio of 1:4 to 4:1, for example, 1:3 to 3:1, 1:2 to 3:1, or 1:1.5 to 3:1. Furthermore, the positive electrode material with D50 ≥ 2 μm and the surface-coated oxide can be mixed with the surface-coated carbon positive electrode material with D50 ≤ 1 μm in a mass ratio of 1:4 to 4:1.

[0034] In some embodiments, the secondary battery positive electrode material of the present invention is a mixture of a positive electrode material having an oxide coating on the surface of some or all particles of D50 ≥ 2 μm and a positive electrode material having a carbon coating on the surface of some or all particles of D50 ≤ 1 μm, and the mass ratio of the positive electrode material having D50 ≥ 2 μm to the positive electrode material having D50 ≤ 1 μm is preferably 1:4 to 4:1, for example, 1:3 to 3:1, 1:2 to 3:1, or 1:1.5 to 3:1. In some embodiments, the secondary battery positive electrode material of the present invention is a mixture of a positive electrode material having an oxide coating on the surface of all particles of D50 ≥ 2 μm and a positive electrode material having a carbon coating on the surface of all particles of D50 ≤ 1 μm.

[0035] In the above preparation method example, the material is crushed to D50 ≥ 2μm, which can ensure that the surface of the positive electrode material with a particle size ≥ 2μm has an oxide coating layer. If D50 ≤ 1μm, it can ensure that the surface of the positive electrode material with a particle size ≤ 1μm has a carbon coating layer.

[0036] Furthermore, a method for introducing a metal doping element into a cathode material comprises the following steps: mixing a precursor, a lithium source, and a dopant containing the target metal doping element, calcining, and crushing to a target particle size. This method for introducing a metal doping element is applicable to the preparation of both large-particle and small-particle cathode materials. Furthermore, after mixing the precursor, the lithium source, and the dopant containing the target metal doping element, the mixture is ball-milled, dried, and then calcined.

[0037] Another object of the present invention is to provide a secondary battery positive electrode sheet, wherein the secondary battery positive electrode sheet comprises the secondary battery positive electrode material.

[0038] The positive electrode sheet made of the secondary battery positive electrode material of the present invention can take into account both the compaction density and the dynamic performance of the sheet.

[0039] Preferably, the secondary battery positive electrode plate comprises positive electrode materials with different crystal structures; more preferably, the secondary battery positive electrode plate comprises a positive electrode material with an olivine structure and a positive electrode material with an α-sodium ferrite structure.

[0040] Another object of the present invention is to provide a secondary battery, wherein the secondary battery comprises the secondary battery positive electrode sheet.

[0041] The secondary battery manufactured using the secondary battery positive electrode sheet of the present invention has higher energy density and lower impedance.

[0042] The present invention has the following beneficial effects: the present invention uses a combination of particles with a particle size of ≥2μm and particles with a particle size of ≤1μm to improve the compaction density of the positive electrode material, and further provides a carbon coating layer on the surface of the small particles with a particle size of ≤1μm, which can partially play the role of a conductive agent. In the electrode sheet, it surrounds the large particles with a particle size of ≥2μm, and can maintain a stable electron path for the large particles during the charge and discharge cycle. In addition, oxide coating is performed on the surface of large particles with a particle size of ≥2μm, which can partially generate fast ion conductors such as lithium aluminate, lithium borate or lithium titanate, and the remaining part exists in the form of oxide, overcoming the problem that large-particle positive electrode materials have small specific surface area, long ion transmission distance, and weak interface ion conduction ability. It can improve the surface ion conduction rate of the material while reducing the damage to it by byproducts such as HF in the electrolyte. The positive electrode material of the present invention is applied to the positive electrode sheet, which can take into account both the electrode sheet compaction density and kinetic performance. This further enables the secondary battery to have a higher energy density and lower impedance. DETAILED DESCRIPTION

[0043] To better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments. Those skilled in the art should understand that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0044] Unless otherwise specified, the experimental methods used in the examples are conventional methods; the materials, reagents, etc. used are all available from commercial sources unless otherwise specified.

[0045] In the present invention, the D50 particle size is the particle size corresponding to when the cumulative particle size distribution percentage of the particles reaches 50%, and is measured using a laser particle size analyzer.

[0046] Example 1

[0047] Positive electrode material 1: large particles with a D50 particle size of ≥2 μm and an oxide coating layer on the surface, the preparation method of which is as follows:

[0048] (1) Precursor iron phosphate and lithium source lithium carbonate were mixed in a molar ratio of 2:1.03 and added to water to form a slurry with a mass fraction of 20%. The slurry was ball-milled for 12 hours until the particle size D50 reached 1.7 μm as measured by a laser particle size analyzer; the slurry was dried to obtain a solid mixture 1;

[0049] (2) The solid mixture 1 was placed in a sintering furnace and nitrogen containing 5% hydrogen was introduced. After the air in the furnace was exhausted, the mixture was heated to 850°C at a heating rate of 5°C / min and maintained for 12 hours. After cooling, the mixture was subjected to air flow crushing to obtain an intermediate product 1. The D50 of the intermediate product 1 was 2.3 μm.

[0050] (3) Adding an oxide additive, Co(OH)2 powder, to the intermediate product 1 in a mass ratio of 0.20:100, the mixture was sintered at 850°C for 8 hours in a nitrogen atmosphere, and cooled to obtain a positive electrode material 1. The oxide coating layer was nanoscale and had little effect on the particle size. The D50 of the obtained positive electrode material 1 was 2.3 μm.

[0051] Positive electrode material 2: small particles with a D50 particle size of ≤1 μm and a carbon coating layer on the surface, the preparation method of which is as follows:

[0052] S1. The precursor iron phosphate and the lithium source lithium carbonate were mixed in a molar ratio of 2:1.03 and added to water to form a 20% mass fraction slurry. The slurry was ball milled for 12 hours until the particle size D50 reached 0.4μm as measured by a laser particle size analyzer;

[0053] S2. Add 1% of the total mass of glucose to the above slurry as a carbon source and stir to dissolve. Then, dry the above slurry to obtain a solid mixture 2. Place the solid mixture 2 in a sintering furnace and introduce nitrogen. After the air in the furnace is exhausted, heat it to 750°C at a heating rate of 5°C / min and maintain it for 12 hours. After cooling, the positive electrode material 2 is obtained by air flow crushing. The D50 of the positive electrode material 2 is 0.74μm.

[0054] The positive electrode sheet preparation method is as follows: Positive electrode material 1 and positive electrode material 2 are mixed in a mass ratio of 67:33, the mixed positive electrode material is mixed with conductive agent carbon black, carbon nanotubes (CNT), and binder polyvinylidene fluoride (PVDF) in a mass ratio of 97:1:0.5:1.5, and the mixture is added to the organic solvent N-methylpyrrolidone (NMP) and stirred at high speed to form a uniform dispersion. The dispersion is defoamed under negative pressure in a mixing tank to obtain a positive electrode material slurry suitable for coating. The resulting positive electrode slurry is applied to aluminum foil using a transfer coater, dried, cold pressed, and slit to produce positive electrode sheets of the desired shape. During the cold pressing process, the compaction density of the positive electrode material coating is controlled to the maximum compaction density required for winding the sheet.

[0055] Example 2

[0056] The difference between this embodiment and embodiment 1 is that: in step (2), the air flow crushing is performed to the intermediate product 1 having a D50 of 2.5 μm; in step (3), the oxide additive is NiO, and the mass ratio of the oxide additive to the intermediate product 1 is 0.17:100, and the D50 of the obtained positive electrode material 1 is 2.5 μm; the rest is the same as in embodiment 1, and a positive electrode sheet is prepared.

[0057] Example 3

[0058] The difference between this embodiment and embodiment 1 is that: in step (2), the air flow crushing is performed to the intermediate product 1 with a D50 of 2.1 μm; in step (3), the oxide additive is Y2O3, the mass ratio of the oxide additive to the intermediate product 1 is 0.15:100, the sintering temperature is 800°C, and the D50 of the obtained positive electrode material 1 is 2.1 μm; the rest is the same as in embodiment 1, and a positive electrode sheet is obtained.

[0059] Example 4

[0060] The difference between this embodiment and embodiment 1 is that in step (3), the oxide additive is TiO2, and the mass ratio of the oxide additive to the intermediate product 1 is 0.25:100; the rest is the same as embodiment 1, and a positive electrode sheet is prepared.

[0061] Example 5

[0062] The difference between this embodiment and embodiment 1 is that: in step (2), the air flow crushing is performed to the intermediate product 1 having a D50 of 2.7 μm; in step (3), the oxide additive is B2O3, the sintering temperature is 750°C, and the D50 of the obtained positive electrode material 1 is 2.7 μm; the rest is the same as in embodiment 1, and a positive electrode sheet is obtained.

[0063] Example 6

[0064] The difference between this embodiment and embodiment 1 is that in the preparation method of the positive electrode sheet, the positive electrode material 1 and the positive electrode material 2 are mixed in a mass ratio of 40:60; the rest is the same as in embodiment 1 to prepare the positive electrode sheet.

[0065] Example 7

[0066] The difference between this embodiment and embodiment 1 is that in the preparation method of the positive electrode sheet, the positive electrode material 1 and the positive electrode material 2 are mixed in a mass ratio of 75:25; the rest is the same as in embodiment 1 to prepare the positive electrode sheet.

[0067] Example 8

[0068] The difference between this embodiment and embodiment 1 is that: in step S1, the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 30:70; in step S2, the air flow is crushed to a D50 of 0.67 μm for the positive electrode material 2; in the preparation method of the positive electrode sheet, the positive electrode material 1 and the positive electrode material 2 are mixed in a mass ratio of 40:60; the rest is the same as in embodiment 1, and a positive electrode sheet is prepared.

[0069] Example 9

[0070] The difference between this embodiment and embodiment 1 is that: in step S1, the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 30:70; in step S2, the air flow is crushed to a D50 of 0.67 μm for the positive electrode material 2; in the preparation method of the positive electrode sheet, the positive electrode material 1 and the positive electrode material 2 are mixed in a mass ratio of 60:40; the rest is the same as in embodiment 1, and a positive electrode sheet is prepared.

[0071] Example 10

[0072] The difference between this embodiment and embodiment 1 is that: in step (1), the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 30:70; in step (2), the air flow is crushed to the D50 of the intermediate product 1 being 2.4 μm; in step (3), the sintering temperature is 900°C; in step S1, the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 30:70; in step S2, the air flow is crushed to the D50 of the positive electrode material 2 being 0.67 μm; the rest is the same as in embodiment 1, and a positive electrode sheet is obtained.

[0073] Example 11

[0074] The difference between this embodiment and embodiment 1 is that: in step (1), the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 40:60; in step (2), the air flow is crushed to the D50 of the intermediate product 1 being 2.7 μm; in step (3), the sintering temperature is 950°C; in step S1, the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 40:60; in step S2, the air flow is crushed to the D50 of the positive electrode material 2 being 0.61 μm; the rest is the same as in embodiment 1, and a positive electrode sheet is obtained.

[0075] Example 12

[0076] The difference between this embodiment and embodiment 1 is that: in step (1), the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 30:70; in step (2), the air flow crushing is performed to the intermediate product 1 having a D50 of 2.4 μm; in step (3), the sintering temperature is 900°C; and the rest is the same as in embodiment 1, to obtain a positive electrode sheet.

[0077] Example 13

[0078] The difference between this embodiment and embodiment 1 is that, in the preparation method of the positive electrode material 1, the intermediate product 1 is not coated with an oxide, that is, step (3) is not performed, and the intermediate product 1 is the positive electrode material 1 of this embodiment; the rest is the same as in embodiment 1, and a positive electrode sheet is prepared.

[0079] Example 14

[0080] The difference between this embodiment and embodiment 1 is that in the preparation method of positive electrode material 1, step (1) is changed to mixing the precursor iron phosphate, lithium source lithium carbonate, and nano-titanium oxide at a molar ratio of 2:1.03:0.004 and adding them to water; in the preparation method of positive electrode material 2, step (1) is changed to mixing the precursor iron phosphate, lithium source lithium carbonate, and nano-titanium oxide at a molar ratio of 2:1.03:0.004 and adding them to water. The rest of the steps are the same as in embodiment 1, and a positive electrode sheet is prepared.

[0081] Example 15

[0082] The difference between this embodiment and embodiment 8 is that in the preparation method of the positive electrode material 1, the precursor used is an iron-manganese phosphate solid solution with an iron-manganese element ratio of 3:7. The rest is the same as in embodiment 8 to prepare a positive electrode sheet.

[0083] Comparative Example 1

[0084] Positive electrode material 2: small particles with a particle size of ≤1μm and a carbon coating layer on the surface, the preparation method of which is as follows:

[0085] S1. The precursor iron phosphate and the lithium source lithium carbonate were mixed in a molar ratio of 2:1.03 and added to water to form a 20% mass fraction slurry. The slurry was ball milled for 12 hours until the particle size D50 measured by a laser particle size analyzer reached 0.3 to 0.7 μm;

[0086] S2. Add 1% of the total mass of glucose to the above slurry as a carbon source and stir to dissolve. Then, dry the above slurry to obtain a solid mixture 2. Place the solid mixture 2 in a sintering furnace and introduce nitrogen. After the air in the furnace is exhausted, heat it to 750°C at a heating rate of 5°C / min and maintain it for 12 hours. After cooling, the positive electrode material 2 is obtained by air flow crushing. The D50 of the positive electrode material 2 is 0.74μm.

[0087] Positive electrode material 3: large particles with a particle size of ≥2 μm and a carbon coating layer on the surface, the preparation method of which is as follows:

[0088] (a) Precursor iron phosphate and lithium source lithium carbonate were mixed in a molar ratio of 2:1.03 and added to water to form a 20% by mass slurry. The slurry was ball milled for 12 hours until the particle size (D50) reached 1.5 to 2 μm as measured by a laser particle size analyzer.

[0089] (b) 1% of the total mass of glucose was added to the above slurry as a carbon source and stirred to dissolve. The slurry was then dried to obtain a solid mixture 3. The solid mixture 3 was placed in a sintering furnace and nitrogen was introduced. After the air in the furnace was exhausted, it was heated to 850°C at a heating rate of 5°C / min and maintained for 12 hours. After cooling, it was subjected to air flow crushing to obtain the positive electrode material 3. The D50 of the positive electrode material 3 was 2.3μm.

[0090] The positive electrode sheet preparation method is as follows: Positive electrode material 2 and positive electrode material 3 are mixed in a mass ratio of 33:67, the mixed positive electrode material is mixed with conductive agent carbon black, carbon nanotubes (CNT), and binder polyvinylidene fluoride (PVDF) in a mass ratio of 97:1:0.5:1.5, and the mixture is added to the organic solvent N-methylpyrrolidone (NMP) and stirred at high speed to form a uniform dispersion. The dispersion is defoamed under negative pressure in a mixing tank to obtain a positive electrode material slurry suitable for coating. The resulting positive electrode slurry is applied to aluminum foil using a transfer coater, dried, cold pressed, and slit to produce positive electrode sheets of the desired shape. During the cold pressing process, the compaction density of the positive electrode material coating is controlled to the maximum compaction density required for winding the sheet.

[0091] Comparative Example 2

[0092] The difference between this comparative example and comparative example 1 is that: in step S1, the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 30:70; in step S2, the D50 of the positive electrode material 2 is 0.67 μm; in the preparation method of the positive electrode sheet, the positive electrode material 2 and the positive electrode material 3 are matched in a mass ratio of 60:40; the rest are the same as in comparative example 1, and a positive electrode sheet is prepared.

[0093] Comparative Example 3

[0094] The difference between this comparative example and comparative example 1 is that: in step S1, the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 40:60; in step S2, the D50 of the positive electrode material 2 is 0.61 μm; in step (a), the precursor is a mixture of iron phosphate and manganese phosphate, and the mass ratio of iron phosphate to manganese phosphate is 40:60; in step (b), the D50 of the positive electrode material 3 is 2.7 μm; the rest is the same as in comparative example 1, and a positive electrode sheet is prepared.

[0095] Comparative Example 4

[0096] The difference between this comparative example and Example 1 is that, in the preparation method of the positive electrode sheet, only positive electrode material 1 is used, and positive electrode material 2 is not used; the rest is the same as in Example 1, and a positive electrode sheet is prepared.

[0097] Comparative Example 5

[0098] The difference between this comparative example and Example 1 is that, in the preparation method of the positive electrode sheet, only positive electrode material 2 is used, and positive electrode material 1 is not used; the rest is the same as in Example 1, and a positive electrode sheet is prepared.

[0099] Comparative Example 6

[0100] The difference between this comparative example and Example 1 is that in step (3), the oxide additive is calcium oxide; the rest is the same as Example 1, and a positive electrode sheet is prepared.

[0101] Comparative Example 7

[0102] The difference between this comparative example and Example 1 is that in step S2, glucose is not added to the slurry, that is, the positive electrode material 2 is not carbon-coated; the rest is the same as in Example 1, and a positive electrode sheet is prepared.

[0103] Comparative Example 8

[0104] The difference between this comparative example and Example 1 is that in the preparation method of the positive electrode sheet, the positive electrode material 1 and the positive electrode material 2 are mixed in a mass ratio of 10:90; the rest is the same as Example 1 to prepare the positive electrode sheet.

[0105] Comparative Example 9

[0106] The difference between this comparative example and Example 1 is that, in the preparation method of the positive electrode sheet, the positive electrode material 1 and the positive electrode material 2 are mixed in a mass ratio of 90:10; the rest is the same as in Example 1 to prepare the positive electrode sheet.

[0107] The positive electrode sheets obtained in the examples and comparative examples were subjected to a limit compaction test.

[0108] The ultimate compaction density test method involves rolling the positive electrode sheet to a set compaction density on a roller press, then manually folding it in half. After bending the sheet until the two folded sides are completely aligned, the crease is inspected. The highest compaction density at which the crease remains opaque is the ultimate compaction density. The test results are shown in Table 1.

[0109] The positive electrode sheets obtained in the examples and comparative examples were used as positive electrodes of secondary batteries to conduct a 50% SOC DC internal resistance test.

[0110] The preparation method of the negative electrode sheet is as follows:

[0111] The negative electrode active material artificial graphite is mixed with the conductive agent carbon black, the binder styrene-butadiene rubber, and the thickener sodium carboxymethyl cellulose (CMC-Na) in a mass ratio of 96.8:1.2:1.2:0.8 and added to deionized water and stirred at high speed to form a uniform dispersion; the dispersion is subjected to negative pressure defoaming in a stirring tank to obtain a negative electrode slurry suitable for coating, and the obtained negative electrode slurry is coated on a copper foil by a transfer coater, and then dried, cold pressed, and slit to form a negative electrode sheet of a desired shape; during the cold pressing process, the compaction density of the negative electrode active material coating is controlled at 1.65 g / cm 3 .

[0112] The preparation method of the secondary battery is as follows:

[0113] The positive and negative electrode sheets are placed on both sides of the separator (the separator is uniformly made of 9μm thick PE separator, with 1μm thick PVDF dotted on both sides) and wound to form a roll core. The exposed parts of the positive and negative electrode sheets are welded to the tabs, and the whole is placed in an aluminum-plastic film and pre-packaged, keeping the tabs exposed outside the aluminum-plastic film while leaving an opening on one side for electrolyte injection;

[0114] An electrolyte is prepared by adding 10 wt% of LiPF6, 1 wt% of vinylene carbonate, 2 wt% of vinyl sulfate (DTD), and 1 wt% of vinylene carbonate (VC) as a lithium salt and an additive to a mixed solvent of ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) = 3:5:2 (volume ratio). The mass percentages of the lithium salt and the additives are calculated based on the total mass of the electrolyte. The electrolyte is injected into the aforementioned aluminum-plastic film with a core, and then a secondary battery is obtained through packaging, standing, and formation processes.

[0115] The 50% SOC DC internal resistance test method involves using a charge-discharge device to adjust the battery's SOC to 50% (i.e., charging from a fully discharged state to 50% of its rated capacity) at a rate of 0.33C (i.e., 0.33 times the battery's rated capacity ampere-hours is used as the current). After 30 minutes of quiescence, the battery is discharged at a constant current of 3C. The voltage change during discharge, ΔV, is recorded. The 50% SOC DC internal resistance = ΔV / A. The test results are shown in Table 1.

[0116] The particle size distribution diagram of the positive electrode materials after matching in each embodiment and comparative example (positive electrode material 1 in comparative example 4 and positive electrode material 2 in comparative example 5) was measured using a laser particle size analyzer, and the particle size distribution diagram was decomposed by deconvolution to obtain each distribution peak, and the median particle size and maximum distribution peak area of ​​each distribution peak were obtained. The positive electrode materials of comparative examples 4 and 5 have only one distribution peak, and the positive electrode materials of the other embodiments and comparative examples all have two larger distribution peaks, namely, the distribution peak with a median particle size ≥1.5μm and the distribution peak with a median particle size <1.5μm. The maximum distribution peak areas of the distribution peak with a median particle size ≥1.5μm and the distribution peak with a median particle size <1.5μm are recorded as S1 and S2, respectively. The test results are shown in Table 1.

[0117] Table 1 Performance test results of positive electrode sheets and secondary batteries obtained in Examples and Comparative Examples

[0118] It can be seen from the test results in Table 1 that the positive electrode sheets obtained in Examples 1-15 of the present invention have a higher ultimate compaction density, which can improve the energy density of the secondary battery. Moreover, when the positive electrode sheets of Examples 1-15 are used in the secondary battery, the secondary battery obtained has a smaller 50% SOC DC internal resistance, indicating that the positive electrode material provided by the present invention can improve the energy density of the secondary battery and has good electronic conductivity and ion conduction, thereby reducing the impedance during the battery charge and discharge process.

[0119] Compared with Example 1, the carbon coating of large particles with a particle size ≥ 2 μm in Comparative Example 1 resulted in a weakened ion conduction of the secondary battery and an increase in the 50% SOC DC internal resistance. Compared with Example 8, the carbon coating of large particles with a particle size ≥ 2 μm in Comparative Example 2 resulted in a weakened ion conduction of the secondary battery and an increase in the 50% SOC DC internal resistance. Compared with Example 11, the carbon coating of large particles with a particle size ≥ 2 μm in Comparative Example 3 resulted in a weakened ion conduction of the secondary battery and an increase in the 50% SOC DC internal resistance. Comparative Example 4 used a single positive electrode material 1, and Comparative Example 5 used a single positive electrode material 2. The positive electrode materials were not graded with different particle sizes, resulting in a lower ultimate compaction density of the positive electrode sheet and a reduced energy density of the secondary battery. Comparative Example 6 differs from Example 1 in that calcium oxide was used for coating. Due to its inertness, calcium oxide cannot increase ionic conductivity, resulting in an increase in the 50% SOC DC internal resistance of the secondary battery. The difference between Comparative Example 7 and Example 1 is that the small-particle positive electrode material is not carbon-coated, resulting in a weakening of both the electronic and ionic conduction of the secondary battery and a significant increase in the internal resistance. In Comparative Example 8, the gradation of large and small particles is unbalanced, and the proportion of small particles is too large, resulting in a decrease in the compaction density of the positive electrode sheet, a decrease in the energy density of the secondary battery, and a weakening of ionic conduction and an increase in impedance. In Comparative Example 9, the proportion of large particles in the positive electrode material is too high, the gradation is unbalanced, the compaction density of the positive electrode sheet is reduced, and there are too few small particles with carbon coating, insufficient electronic conductivity, and an increase in DC internal resistance.

[0120] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery positive electrode material, characterized in that: include: Large particles with a particle size of ≥2μm and small particles with a particle size of ≤1μm; At least part of the small particles have a carbon coating layer on their surfaces; At least part of the large particles have no carbon coating layer on their surfaces.

2. The secondary battery positive electrode material according to claim 1, characterized in that: At least some of the large particles have an oxide coating layer.

3. The secondary battery positive electrode material according to claim 2, characterized in that: The oxide coating layer is at least one of aluminum oxide, zirconium oxide, cobalt oxide, boron oxide, nickel oxide, tungsten oxide, titanium oxide, and vanadium oxide.

4. The secondary battery positive electrode material according to claim 1, characterized in that: The positive electrode material satisfies at least one of the following: (1) The positive electrode material has an olivine structure; (2) The chemical formula of the positive electrode material is: Li x Mn y Fe 1-y PO4, where 0.75≤x≤1.05, 0≤y≤0.

9.

5. The secondary battery positive electrode material according to claim 4, characterized in that: The positive electrode material also contains metal doping elements.

6. The secondary battery positive electrode material according to claim 5, characterized in that: The metal doping element is at least one of aluminum, zirconium, cobalt, tungsten, titanium, vanadium, nickel and magnesium.

7. The secondary battery positive electrode material according to claim 1, characterized in that: The large particles and the small particles have the same crystal structure.

8. The secondary battery positive electrode material according to claim 1, characterized in that: The particle size distribution diagram of the positive electrode material includes at least two distribution peaks after decomposition, wherein the maximum distribution peak area S1 for a median particle size ≥1.5 μm and the maximum distribution peak area S2 for a median particle size <1.5 μm satisfy: 1:4≤S1:S2≤4:

1.

9. A secondary battery positive electrode plate, characterized in that: Contains the secondary battery positive electrode material according to any one of claims 1 to 8.

10. A secondary battery, characterized in that: A secondary battery positive electrode sheet comprising the positive electrode sheet of claim 9.

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