Positive electrode composite material, preparation method thereof and battery

Abstract

The present invention relates to the field of new energy batteries, and in particular to a positive electrode composite material, a preparation method thereof and a battery, wherein the positive electrode composite material comprises positive electrode material particles, a first coating layer coated on the surface of the positive electrode material particles and a second coating layer coated on the surface of the first coating layer; wherein the positive electrode material particles comprise lithium iron phosphate particles, the first coating layer material comprises a high-nickel material; the second coating layer material comprises a fast ion conductor; the high-nickel material comprises LiNi x Co y Mn z O2, wherein x+y+z=1, 0.80≤x≤0.83, 0.05≤y≤0.11, 0.06≤z≤0.15; the fast ion conductor comprises Li3PO4 and / or Li4P2O7. The positive electrode composite material of the present invention can greatly improve the rate and cycle performance of lithium iron phosphate.

Description

Technical Field

[0001] The present invention relates to the field of new energy batteries, and in particular to a positive electrode composite material, a preparation method thereof, and a battery. Background Art

[0002] At present, most of the positive electrode materials using lithium iron phosphate involve coating a conductive material on the surface, and the conductive material is mainly a carbon-coated material. When the conductive material coating layer on the surface of the positive electrode material is too thick, it will hinder the deintercalation of lithium ions, thereby increasing polarization and reducing the capacity of the positive electrode material. Therefore, the main shortcoming of using lithium iron phosphate as a positive electrode material is still the low energy density. Therefore, this field needs to develop a positive electrode material that uses lithium iron phosphate as the main active material and, through the combined action of the coating layer material, has high conductivity and can avoid capacity loss caused by a decrease in the proportion of active substances. At the same time, it has high ionic conductivity and improved rate performance, and will not be affected by side reactions of the electrolyte due to high activity. Summary of the invention

[0003] In view of this, the present invention is committed to providing a positive electrode composite material and a preparation method and a battery, so as to solve the problems in the prior art that the energy density of lithium iron phosphate is relatively low and the conventional carbon coating layer is not conducive to improving the rate performance of lithium iron phosphate.

[0004] In order to solve the above technical problems, this application is implemented as follows:

[0005] The present invention provides a positive electrode composite material, the positive electrode composite material comprises positive electrode material particles, a first coating layer coated on the surface of the positive electrode material particles and a second coating layer coated on the surface of the first coating layer;

[0006] Wherein, the positive electrode material particles include lithium iron phosphate particles, the first coating layer material includes high nickel material; the second coating layer material includes a fast ion conductor;

[0007] The high nickel material includes LiNi x Co y Mn z O2, where x+y+z=1, 0.80≤x≤0.83, 0.05≤y≤0.11, 0.06≤z≤0.15;

[0008] The fast ion conductor includes Li3PO4 and / or Li4P2O7.

[0009] Optionally, the D50 of the lithium iron phosphate particles is 0.7-6 μm, preferably 1-4 μm.

[0010] Optionally, the thickness of the first coating layer is 0.5-3 μm, preferably 0.8-1.5 μm; the thickness of the second coating layer is 0.5-20 nm, preferably 2-10 nm.

[0011] Optionally, based on the total mass of the positive electrode composite material, the content of the lithium iron phosphate particles is 78.0~99.49wt%, the content of the high-nickel material is 0.5~20wt%, and the content of the fast ion conductor is 0.01~2.0wt%; preferably, the content of the lithium iron phosphate particles is 89~98.9wt%, the content of the high-nickel material is 1~10wt%, and the content of the fast ion conductor is 0.1~1wt%.

[0012] A second aspect of the present invention provides a method for preparing a positive electrode composite material, the preparation method comprising the following steps:

[0013] S1, dispersing a phosphorus-containing organic acid in an organic solvent to obtain a first solution; mixing lithium iron phosphate particles coated with a high-nickel material on the surface with the first solution to obtain a first material;

[0014] S2, filtering and drying the first material to obtain a precursor;

[0015] S3, calcining the precursor.

[0016] Optionally, in step S1, the molar ratio of the high-nickel material contained in the lithium iron phosphate particles coated with the high-nickel material to the phosphorus-containing organic acid is 1:(0.0001-0.1), preferably 1:(0.002-0.006).

[0017] Optionally, the phosphorus-containing organic acid is phytic acid; the organic solvent is selected from at least one of dimethyl sulfoxide solution, tetrahydrofuran, tetrachloroethane, isopropanol and methanol; preferably, the organic solvent is dimethyl sulfoxide solution.

[0018] Optionally, in step S1, the conditions for the mixing treatment include: a temperature of 10~80°C, a rotation speed of 400~1500rmp, and a time of 0.2~1h; in step S2, the conditions for the drying treatment include: a temperature of 60~100°C, and a time of 1~2h; in step S3, the calcination treatment is carried out in a first oxygen-containing atmosphere, and the oxygen concentration in the first oxygen-containing atmosphere is greater than 90vol%; the conditions for the calcination treatment include: a temperature of 400~800°C, and a time of 3~8h.

[0019] Optionally, the preparation method further includes: mixing the lithium iron phosphate particles with the high-nickel material particles, performing grinding and calcining to obtain the lithium iron phosphate particles whose surface is coated with the high-nickel material; optionally, the D50 of the lithium iron phosphate particles is 0.7~6μm, preferably 1~4μm; optionally, the D50 of the high-nickel material particles is 50~4000nm, preferably 200~500nm; optionally, the mass ratio of the lithium iron phosphate to the high-nickel material is 1:(0.01~0.1); optionally, the grinding treatment is ball milling; optionally, the calcination treatment is carried out in a second oxygen-containing atmosphere, and the oxygen concentration in the second oxygen-containing atmosphere is greater than 90vol%; the conditions of the calcination treatment include: temperature of 700~1000℃, time of 6~12h.

[0020] A third aspect of the present invention provides a battery, comprising a positive electrode sheet, wherein the positive electrode sheet contains a positive electrode active material, wherein the positive electrode active material comprises the above-mentioned positive electrode composite material and / or the positive electrode composite material prepared according to the above-mentioned preparation method.

[0021] Through the above technical solution, the beneficial technical effects of the present invention are:

[0022] (1) The present invention uses high-nickel material to coat lithium iron phosphate, thereby improving the inherent conductivity of lithium iron phosphate. The high-nickel material coating layer has significantly improved electrochemical activity compared to the traditional carbon material coating layer, which can avoid the problem of reduced active material ratio due to the addition of the coating layer to a certain extent, thereby improving the rate and cycle performance of lithium iron phosphate.

[0023] (2) The present invention uses a fast ion conductor as the second coating layer material to enhance the surface Li + While diffusing, it can inhibit interface side reactions, thereby avoiding possible side reactions caused by high-nickel materials on the outer surface, and greatly improving the rate and cycle performance of lithium iron phosphate.

[0024] Other features and advantages of the present invention will be described in detail in the following detailed description. DETAILED DESCRIPTION

[0025] The present invention discloses a positive electrode composite material, a preparation method thereof and a battery. Those skilled in the art can refer to the content of this article and appropriately improve the process parameters to achieve it. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The method and application of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0026] In the description of the present invention, a list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A, B are listed, the phrase "at least one of A, B" means only A; only B; or A and B. In another example, if items A, B, C are listed, the phrase "at least one of A, B, C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0027] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range or the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0028] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0029] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0030] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0031] In order to solve the problem in the prior art that the energy density of lithium iron phosphate is low and the conventional carbon coating layer is not conducive to improving the rate performance of lithium iron phosphate, the present invention adopts the following technical solution:

[0032] The present invention provides a positive electrode composite material, the positive electrode composite material comprises positive electrode material particles, a first coating layer coated on the surface of the positive electrode material particles and a second coating layer coated on the surface of the first coating layer;

[0033] Wherein, the positive electrode material particles include lithium iron phosphate particles, the first coating layer material includes high nickel material; the second coating layer material includes a fast ion conductor;

[0034] The high nickel material includes LiNi x Co y Mn zO2, where x+y+z=1, 0.80≤x≤0.83, 0.05≤y≤0.11, 0.06≤z≤0.15;

[0035] The fast ion conductor includes Li3PO4 and / or Li4P2O7.

[0036] The present invention uses a high nickel material as the first coating layer material to coat the lithium iron phosphate particles, which improves the inherent conductivity of the lithium iron phosphate and can avoid the problem of a reduction in the proportion of active materials due to the addition of a coating layer to a certain extent. At the same time, the present invention uses a fast ion conductor as the second coating layer material to enhance the surface Li + While diffusing, it can inhibit side reactions on the interface, thereby avoiding possible side reactions caused by high-nickel materials on the outer surface.

[0037] According to the present invention, a suitable lithium iron phosphate particle size has a better gram capacity and a better coating effect. If the particle size of the lithium iron phosphate particles is too small, the coating effect of the second coating layer will be poor and the coating will be uneven; if the particle size of the lithium iron phosphate particles is too large, it will cause the internal Li to be difficult to escape and the gram capacity will be low. The D50 of the lithium iron phosphate particles in the present invention can be 0.7~6μm. Exemplarily, the D50 of the lithium iron phosphate particles can be any value among 0.7μm, 1μm, 2μm, 4μm and 6μm or any value within the range of any two of the above values. Preferably, the D50 of the lithium iron phosphate particles is 1~4μm.

[0038] According to the present invention, the thickness of the appropriate first coating layer has both high conductivity and can avoid capacity loss caused by a decrease in the proportion of active materials. At the same time, the ionic conductivity is high, which can improve the rate performance of the battery. If the thickness of the first coating layer is too small, it will affect the conductivity, increase the impedance, and reduce the gram capacity and cycle performance; if the thickness of the first coating layer is too large, the conductivity effect will not be improved much, and it will tend to be ternary as the Ni and Co metal content increases, and lose the cost advantage. The thickness of the first coating layer in the present invention can be 0.5~3μm. Exemplarily, the thickness of the first coating layer can be any value among 0.5μm, 1μm, 2μm and 3μm, or any value within the range of any two of the above values. Preferably, the thickness of the first coating layer can be 0.8~1.5μm.

[0039] According to the present invention, the thickness of the second coating layer is suitable for reducing residual alkali, enhancing surface Li +The technical effect of inhibiting interface side reactions and improving cycle performance while diffusing. If the thickness of the second coating layer is too small, it will cause serious phase change on the material surface, increase side reactions, increase surface impedance, and increase gas production, which will lead to rapid attenuation of life; if the thickness of the second coating layer is too large, it will exceed the amount of residual alkali on the surface that needs to be combined with the reaction, and the effect will not be improved much. While increasing the cost, it will reduce the conductive effect of the overall coating layer and improve the overall impedance. The thickness of the second coating layer in the present invention can be 0.5~20nm. Exemplarily, the thickness of the second coating layer can be any value among 0.5μm, 2μm, 5μm, 10μm, 15μm and 20μm, or any value within the range of any two of the above values. Preferably, the thickness of the second coating layer can be 2~10nm.

[0040] According to the present invention, based on the total mass of the positive electrode composite material, the content of the lithium iron phosphate particles can be 78.0-99.49wt%, the content of the high nickel material can be 0.5-20wt%, and the content of the fast ion conductor can be 0.01-2.0wt%. If the content of the lithium iron phosphate particles is too high, the thickness of the coating layer will be reduced, and it will be difficult to achieve the expected improvement in energy density and rate performance, and it will not be able to maintain excellent cycle performance; if the content of the lithium iron phosphate particles is too low, the cost advantage of the LFP matrix will be lost; if the content of the high nickel material is too high, it will tend to be ternary, and the cost and the cycle advantage brought by the stable olivine structure of LFP will be lost; if the content of the high nickel material is too low, the conductivity will be reduced, the impedance will increase, and the gram capacity and cycle performance will deteriorate; if the content of the fast ion conductor is too high, it will exceed the amount of residual alkali on the surface that needs to be combined with the reaction, and the cost will be increased while the effect is not improved; if the content of the fast ion conductor is too low, the serious side reactions of the phase change on the surface of the material will increase, the surface impedance will increase, and the gas production will increase, which will lead to the attenuation of life. Preferably, the content of the lithium iron phosphate particles is 89-98.9 wt %, the content of the high nickel material is 1-10 wt %, and the content of the fast ion conductor is 0.1-1 wt %.

[0041] A second aspect of the present invention provides a method for preparing a positive electrode composite material, the preparation method comprising the following steps:

[0042] S1, dispersing a phosphorus-containing organic acid in an organic solvent to obtain a first solution; mixing lithium iron phosphate particles coated with a high-nickel material on the surface with the first solution to obtain a first material;

[0043] S2, filtering and drying the first material to obtain a precursor;

[0044] S3, calcining the precursor.

[0045] According to the present invention, in step S1, the molar ratio of the high nickel material contained in the lithium iron phosphate particles with the high nickel material coated on the surface to the phosphorus-containing organic acid can be 1: (0.0001-0.1). Exemplarily, the molar ratio of the high nickel material contained in the lithium iron phosphate particles with the high nickel material coated on the surface to the phosphorus-containing organic acid can be any value among 1: 0.0001, 1: 0.001, 1: 0.005, 1: 0.01, 1: 0.05 and 1: 0.1 or any value within the range of values ​​composed of any two of the above values. Preferably, the molar ratio of the high nickel material contained in the lithium iron phosphate particles with the high nickel material coated on the surface to the phosphorus-containing organic acid can be 1: (0.002-0.006).

[0046] According to the present invention, the phosphorus-containing organic acid may be phytic acid; the organic solvent may be selected from at least one of dimethyl sulfoxide solution, tetrahydrofuran, tetrachloroethane, isopropanol and methanol; preferably, the organic solvent is dimethyl sulfoxide solution.

[0047] According to the present invention, in step S1, the conditions for the mixing treatment may include: a temperature of 10-80°C, a rotation speed of 400-1500 rpm, and a time of 0.2-1h; in step S2, the conditions for the drying treatment may include: a temperature of 60-100°C and a time of 1-2h; in step S3, the calcination treatment is carried out in a first oxygen-containing atmosphere, and the oxygen concentration in the first oxygen-containing atmosphere is greater than 90 vol%; the conditions for the calcination treatment may include: a temperature of 400-800°C and a time of 3-8h.

[0048] According to the present invention, the preparation method may further include: mixing lithium iron phosphate particles with high-nickel material particles, grinding and calcining the mixture to obtain lithium iron phosphate particles with the high-nickel material coated on the surface.

[0049] The present invention uses a simple high-temperature solid phase method to coat a layer of high-nickel material on the surface of lithium iron phosphate to obtain lithium iron phosphate particles with the surface coated with high-nickel material. The high-nickel material coated on the surface significantly improves the inherent conductivity of lithium iron phosphate. Compared with the traditional carbon material coating, the high-nickel material coating has electrochemical activity, so it can avoid the problem of reducing the proportion of active materials due to the addition of the coating to a certain extent, and at the same time can greatly improve the rate and cycle performance of the battery.

[0050] In the present invention, the D50 of the lithium iron phosphate particles may be 0.7-6 μm. For example, the D50 of the lithium iron phosphate particles may be any value among 0.7 μm, 1 μm, 2 μm, 4 μm and 6 μm, or any value within the range of any two of the above values. Preferably, the D50 of the lithium iron phosphate particles may be 1-4 μm.

[0051] In the present invention, the D50 of the high-nickel material particles may be 50-4000 nm. Exemplarily, the D50 of the high-nickel material particles may be any value among 50 nm, 100 nm, 400 nm, 800 nm, 1000 nm, 2000 nm, 3000 nm and 400 nm or any value within the range of any two of the above values. Preferably, the D50 of the high-nickel material particles may be 200-500 nm.

[0052] In the present invention, the mass ratio of the lithium iron phosphate to the high nickel material may be 1:(0.01-0.1). Exemplarily, the mass ratio of the lithium iron phosphate to the high nickel material may be any value among 1:0.01, 1:0.02, 1:0.04, 1:0.06, 1:0.08 and 1:0.1 or any value within the range of any two of the above values.

[0053] Exemplarily, the grinding process may be ball milling; optionally, the calcination process is performed in a second oxygen-containing atmosphere, wherein the oxygen concentration in the second oxygen-containing atmosphere is greater than 90 vol%; the conditions of the calcination process may include: a temperature of 700-1000°C and a time of 6-12 hours.

[0054] The preparation method of the high-nickel material in the present invention can be: use nickel salt, manganese salt and cobalt salt to mix and prepare a ternary mixed solution; under a nitrogen protective atmosphere, add a precipitant to the ternary mixed solution and react at 75-85° C. for 7-9 hours to obtain a ternary precursor; mix the lithium salt and the ternary precursor and then ball mill them, and calcine them at 700-1000° C. for 14-16 hours to obtain a high-nickel material.

[0055] Exemplarily, the nickel salt includes Ni(CH3COO)2·4H2O and / or NiSO4·6H2O; the manganese salt includes Mn(CH3COO)2·4H2O and / or MnSO4·H2O; the cobalt salt includes Co(CH3COO)2·4H2O and / or CoSO4·7H2O; the precipitant includes at least one of NaOH, KOH and Ba(OH)2; the mass ratio of the lithium salt and the ternary precursor is 1:(1~2); the lithium salt includes at least one of lithium carbonate, lithium sulfate and lithium nitrate; the ball milling conditions include: ball milling temperature of 10~60°C and ball milling time of 9~11 hours.

[0056] A third aspect of the present invention provides a battery, comprising a positive electrode sheet, wherein the positive electrode sheet contains a positive electrode active material, wherein the positive electrode active material comprises the above-mentioned positive electrode composite material and / or the positive electrode composite material prepared according to the above-mentioned preparation method.

[0057] The present invention is further described in detail by way of examples. The raw materials used in the examples can be obtained through commercial sources.

[0058] Example 1

[0059] The positive electrode material of this embodiment includes a core and a coating layer arranged on the surface of the core. The material constituting the core is lithium iron phosphate, and the D50 of the lithium iron phosphate particles is 2 μm. The material constituting the first coating layer is a high-nickel material LiNi 0.83 Co 0.11 Mn 0.06 O2, the material constituting the second coating layer is Li3PO4 / Li4P2O7, the first coating layer accounts for 5wt% of the positive electrode material, the thickness of the first coating layer is 1μm, the second coating layer accounts for 0.2wt% of the positive electrode material, and the thickness of the second coating layer is 4nm.

[0060] The positive electrode material provided in this embodiment is prepared by the following steps:

[0061] (1) Ni(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O and Co(CH3COO)2·4H2O were mixed in a molar ratio of 0.83:0.11:0.06 to prepare a ternary mixed solution.

[0062] (2) Under a nitrogen atmosphere, NaOH was added to the ternary mixed solution until a colloidal precipitate was completely formed and heated in a water bath at 80°C for 8 hours. The product was then precipitated for 24 hours to obtain a ternary precursor.

[0063] (3) Lithium hydroxide monohydrate and the ternary precursor were mixed in a molar ratio of 1.03:1 and then ball-milled to a material particle size D50 of 0.4 μm. The mixture was calcined at 650°C for 15 hours to obtain a high-nickel material with a particle size D50 of 300 nm. The ball-milling operation was performed at a ball-milling temperature of 35°C and a ball-milling time of 10 hours.

[0064] (4) Lithium iron phosphate particles and high nickel material were mixed in a mass ratio of 95:5 and ball milled, and calcined at 800°C for 9 hours in a pure oxygen atmosphere to obtain LFP@NCM material.

[0065] (5) The LFP material coated with high nickel (denoted as LFP@NCM) was put into a transparent dimethyl sulfoxide (DMSO) solution dispersed with phytic acid (PA), and the high nickel material and phytic acid were mixed in a molar ratio of 1:0.005 and stirred for 4 hours. The resulting mixture was filtered and dried at 80°C. The filtrate was collected and calcined at 600°C for 6 hours in a pure oxygen atmosphere to obtain a positive electrode material. In the positive electrode active material, the phytic acid in the second coating layer was converted into Li3PO4 / Li4P2O7 by calcination and combined with the residual alkali on the high nickel surface. The positive electrode material of this embodiment is LFP@NCM@Li3PO4 / Li4P2O7.

[0066] Example 2

[0067] The positive electrode material of this embodiment includes a core and a coating layer arranged on the surface of the core. The material constituting the core is lithium iron phosphate, and the D50 of the lithium iron phosphate particles is 2 μm. The material constituting the first coating layer is a high-nickel material LiNi 0.97 Co 0.02 Mn 0.01 O2, the material constituting the second coating layer is Li3PO4 / Li4P2O7, the first coating layer accounts for 5wt% of the positive electrode material, the thickness of the first coating layer is 1μm, the second coating layer accounts for 0.2wt% of the positive electrode material, and the thickness of the second coating layer is 4nm.

[0068] The preparation method of the positive electrode material in this embodiment is generally the same as that in Embodiment 1, except that in step (1), Ni(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O and Co(CH3COO)2·4H2O are mixed in a molar ratio of 97:2:1 to prepare a ternary mixed solution.

[0069] Example 3

[0070] The positive electrode material of this embodiment includes a core and a coating layer arranged on the surface of the core. The material constituting the core is lithium iron phosphate, and the D50 of the lithium iron phosphate particles is 2 μm. The material constituting the first coating layer is a high-nickel material LiNi 0.7 Co 0.1 Mn 0.2 O2, the material constituting the second coating layer is Li3PO4 / Li4P2O7, the first coating layer accounts for 5wt% of the positive electrode material, the thickness of the first coating layer is 1μm, the second coating layer accounts for 0.2wt% of the positive electrode material, and the thickness of the second coating layer is 4nm.

[0071] The preparation method of the positive electrode material in this embodiment is generally the same as that in Embodiment 1, except that in step (1), Ni(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O and Co(CH3COO)2·4H2O are mixed in a molar ratio of 70:10:20 to prepare a ternary mixed solution.

[0072] Example 4

[0073] The positive electrode material of this embodiment includes a core and a coating layer arranged on the surface of the core. The material constituting the core is lithium iron phosphate, and the D50 of the lithium iron phosphate particles is 2 μm. The material constituting the first coating layer is a high-nickel material LiNi 0.83 Co 0.11 Mn 0.06 O2, the material constituting the second coating layer is Li3PO4 / Li4P2O7, the first coating layer accounts for 0.5wt% in the positive electrode material, the thickness of the first coating layer is 0.2μm, the second coating layer accounts for 0.2wt% in the positive electrode material, and the thickness of the second coating layer is 4nm.

[0074] The preparation method of the positive electrode material in this embodiment is generally the same as that in Embodiment 1, except that in step (4), the lithium iron phosphate particles and the high nickel material are mixed in a mass ratio of 99.5:0.5 and then ball milled, and calcined at 800° C. for 9 hours in a pure oxygen atmosphere to obtain the LFP@NCM material.

[0075] Example 5

[0076] The positive electrode material of this embodiment includes a core and a coating layer arranged on the surface of the core. The material constituting the core is lithium iron phosphate, and the D50 of the lithium iron phosphate particles is 2 μm. The material constituting the first coating layer is a high-nickel material LiNi 0.83 Co 0.11 Mn 0.06 O2, the material constituting the second coating layer is Li3PO4 / Li4P2O7, the first coating layer accounts for 15wt% of the positive electrode material, the thickness of the first coating layer is 2μm, the second coating layer accounts for 0.2wt% of the positive electrode material, and the thickness of the second coating layer is 4nm.

[0077] The preparation method of the positive electrode material in this embodiment is generally the same as that in Embodiment 1, except that in step (4), the lithium iron phosphate particles and the high nickel material are mixed in a mass ratio of 85:15 and then ball-milled, and calcined at 800° C. for 9 hours in a pure oxygen atmosphere to obtain the LFP@NCM material.

[0078] Example 6

[0079] The positive electrode material of this embodiment includes a core and a coating layer arranged on the surface of the core. The material constituting the core is lithium iron phosphate, and the D50 of the lithium iron phosphate particles is 2 μm. The material constituting the first coating layer is a high-nickel material LiNi 0.83 Co 0.11 Mn 0.06 O2, the material constituting the second coating layer is Li3PO4 / Li4P2O7, the first coating layer accounts for 5wt% of the positive electrode material, the thickness of the first coating layer is 1μm, the second coating layer accounts for 1wt% of the positive electrode material, and the thickness of the second coating layer is 20nm.

[0080] The preparation method of the positive electrode material in this embodiment is generally the same as that in Embodiment 1, except that: in step (3), lithium hydroxide monohydrate and the ternary precursor are mixed at a molar ratio of 1.06:1. In step (5), the high nickel material and phytic acid are mixed at a molar ratio of 1:0.15 and stirred for 4 hours.

[0081] Comparative Example 1

[0082] This comparative example provides a positive electrode material, which includes a core and a coating layer arranged on the surface of the core. The material constituting the core is lithium iron phosphate, the material constituting the first coating layer is a high-nickel material, the thickness of the first coating layer is 1 μm, and there is no second coating layer.

[0083] The positive electrode material provided in this comparative example is prepared by the following steps:

[0084] (1) A ternary mixed solution was prepared using Ni(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O, and Co(CH3COO)2·4H2O in a molar ratio of 0.83:0.11:0.06.

[0085] (2) Under a nitrogen atmosphere, NaOH was added to the ternary mixed solution until a colloidal precipitate was completely formed and heated in a water bath at 80°C for 8 hours. The product was then precipitated for 24 hours to obtain a ternary precursor.

[0086] (3) Lithium hydroxide monohydrate and the ternary precursor are mixed and then ball-milled to a material particle size D50 of 0.4 μm, and then calcined at 650°C for 15 hours to obtain a ternary material with a particle size D50 of 300 nm; wherein the ball milling operation meets the requirements, the ball milling temperature is 35°C, and the ball milling time is 10 hours.

[0087] (4) The lithium iron phosphate particles and high nickel were mixed according to the mass ratio of LFP:NCM=95%:5%, and then ball-milled. The mixture was calcined at 800°C for 9 hours in a pure oxygen atmosphere to obtain LFP@NCM material.

[0088] Comparative Example 2

[0089] 10% glucose was added to the lithium iron phosphate particles, and the two were ground in a ball mill for 3 hours until they were completely mixed to obtain a mixed material. The mixed material was transferred to a vacuum tube furnace and treated at 750°C for 8 hours. The heating rate of the vacuum tube furnace was 3°C / min to obtain a carbon-coated sample with a carbon coating layer thickness of 1μm, named LFP@C.

[0090] Test Example 1

[0091] The positive electrode materials in Examples 1-6 and Comparative Examples 1-2 were prepared into batteries, and the performance of the prepared batteries was tested. The test results are shown in Table 1.

[0092] The preparation method of the battery includes: mixing the positive electrode material powder, conductive carbon black, carbon nanotubes and polyvinylidene fluoride (PVDF) in Examples 1 to 6 and Comparative Examples 1 to 2 at a mass ratio of 95.2:1:0.8:3, and then adding N-methylpyrrolidone (NMP) in a high-speed mixer and uniformly mixing to form a positive electrode slurry with a solid content of 74%. The slurry is coated on a single side of an aluminum foil with a thickness of 12 microns using a transfer coater and dried to maintain a coating weight per unit area of ​​22.5 mg / cm after drying. 2 . Then, the same process is used to coat and dry the other side of the aluminum foil to obtain a semi-finished positive electrode sheet. Artificial graphite powder, conductive carbon black, CMC and SBR are mixed in a mass ratio of 94:1:2:2, and then deionized water is added in a high-speed mixer and evenly mixed into a negative electrode slurry with a solid content of 48%. The slurry is applied to a single side of a copper foil with a thickness of 8 microns using a transfer coater and dried to maintain a coating weight of 10.4 mg / cm per unit area after drying. 2 . Then, the other side of the copper foil is coated and dried using the same process to obtain a negative electrode semi-finished product. The exposed metal foil portion of the above-mentioned electrode is processed and welded into a pole ear, and then wound with a separator to form a winding core. The winding core is wrapped with an aluminum-plastic film to form a semi-finished battery cell, which is dried and then injected with electrolyte. After vacuum packaging, standing, formation, shaping and other processes, a lithium-ion battery is obtained.

[0093] Battery performance test items and methods include:

[0094] 1. Discharge specific capacity test: At 25°C, let the lithium-ion battery stand for 30 minutes, then discharge it at a constant current of 0.5C to a voltage of 2.5V. Charge the lithium-ion battery at a constant current of 0.33C to a voltage of 4.2V, then charge it at a constant voltage to a current ≤0.05C, record the first cycle charge and discharge capacity, and calculate the discharge specific capacity based on the weight of the positive electrode.

[0095] 2. DCR test:

[0096] (1) 25℃ 50%SOC DCR test: 2C rate discharge for 30 seconds to measure DCR (2C-30s 50%SOC DCR@25℃): First test the 1C capacity, place the lithium-ion battery in a constant temperature environment of 25℃ for 1h, use the nominal capacity for 1C / 1C charging and discharging, charge at a constant current of 1C to a voltage of 4.2V, then charge at a constant voltage of 4.2V with a cut-off current of 0.05C, and leave it for 15min before discharging at a constant current of 1C to 2.5V. The discharge capacity is the 1C capacity C0; then adjust the SOC to 50%, charge at a constant current of 1C0 to a voltage of 4.2V, then charge at a constant voltage of 4.2V with a cut-off current of 0.05C0, and leave it for 15min before discharging at a constant current of 1C0 for 30min; place the lithium-ion battery in a constant temperature environment of 25℃ for 2h, discharge at 1C0 for 30s, record the voltage in the last second before discharge as V1, and the voltage for 30s of discharge as V2. The discharge DCR of 50% SOC at 25℃ is (V1-V2) / (2C0).

[0097] (2) -20℃ 50%SOC DCR test: DCR is measured by discharging at a rate of 0.36C for 30 seconds (0.36C-30s 50%SOC DCR@-20℃): After charging at a constant current of 1C0 to a voltage of 4.2V, the charging cut-off current is 0.05C0 at a constant voltage of 4.2V. After standing for 15 minutes, the battery is discharged at a constant current of 1C0 for 30 minutes. The lithium-ion battery is placed in a constant temperature environment of -20℃ for 2 hours, and then discharged at 1C for 30 seconds. The voltage before discharge is recorded as V1 in the last second, and the voltage after discharge for 30 seconds is recorded as V2. The discharge DCR of 50%SOC at -20℃ is (V1-V2) / (0.36C0).

[0098] 3. Cycle performance test:

[0099] (1) 25℃ cycle performance test: First, in an environment of 25℃, the first charge and discharge are performed. After constant current charging at a charging current of 1C to a voltage of 4.2V, the charging cut-off current is 0.05C at a constant voltage of 4.2V. After standing for 15 minutes, constant current discharge is performed at a discharge current of 1C to 2.5V. The discharge capacity of the first cycle is recorded as C1. Then, the charge and discharge cycle is performed until the capacity decays to 80% C1, and the number of cycles corresponding to the 80% discharge capacity retention rate of 80% is recorded.

[0100] (2) 45℃ cycle performance test: First, in an environment of 45℃, the first charge and discharge were performed. After constant current charging at a charging current of 1C to a voltage of 4.2V, the charging cut-off current was 0.05C at a constant voltage of 4.2V. After standing for 15 minutes, constant current discharge was performed at a discharge current of 1C to 2.5V. The discharge capacity of the first cycle was recorded as C1. Then, the charge and discharge cycle was performed until the capacity decayed to 80% C1, and the cycle number corresponding to the 80% discharge capacity retention rate of 80% was recorded.

[0101] Table 1

[0102]

[0103] It can be seen from Table 1 that Examples 1 to 3 compare the effects of different Ni, Co, and Mn contents in the high-nickel material in the first coating layer on the overall battery performance. 4+ Too high content, Ni 4+ The activity is strong and the structure is unstable. It is easy to deoxidize and react with the electrolyte, and the layered structure is converted into a rock salt phase structure, thereby increasing the impedance and causing the cycle effect to deteriorate. The Ni and Co content in the first coating layer of Example 3 is low, the conductivity is relatively poor, and the impedance is high. It cannot achieve the conductivity improvement of LFP materials brought by C coating, resulting in relatively poor overall electrical performance.

[0104] Examples 1, 4, and 5 compare the effects of different coating thicknesses of the first coating layer on the overall battery performance. The coating thickness of the first coating layer in Example 1 is 1 μm. The thickness of the first coating layer in Example 4 is 0.2 μm, which is too low and has a poor coating effect, and cannot improve the conductivity of lithium iron phosphate. At the same time, the overall impedance is high, and the gram capacity, rate, and cycle equalization performance will deteriorate. In Example 5, the thickness of the first coating layer is 2 μm, which causes the material to tend to be ternary, losing the cost advantage and the cycle advantage brought by the stable olivine structure of lithium iron phosphate.

[0105] Examples 1, 6 and Comparative Example 1 compare the effects of different thicknesses of the second coating layer on the overall performance of the battery. The coating thickness of the second coating layer in Example 1 is 4nm. The thickness of the second coating layer in Example 6 is 20nm, and the coating thickness is increased, which increases the cost; and the conductivity of the second coating layer is relatively poor compared to the first coating layer, and the overall impedance will increase with a relatively appropriate coating amount, and the corresponding electrical performance will deteriorate. Comparative Example 1 has no second coating layer at all, only the first coating layer, and the residual alkali on the surface reacts with the electrolyte to produce gas, and the reactive Ni 4+ During the charge and discharge process, high nickel on the surface easily releases lattice oxygen, which leads to intensified side reactions, thereby increasing the surface impedance and deteriorating the overall cycle performance.

[0106] Comparative Example 2 is conventional carbon coating, and the thickness of the carbon coating is the same as the thickness of the first coating layer. Comparing Example 1 with Comparative Example 2, Example 1 has improved gram capacity, rate, and cycle. This is mainly because the first coating layer in Example 1 is a high-nickel material with activity, while carbon is inactive; and Example 1 has a second coating layer, which can isolate the first coating layer from side reactions with the electrolyte, and the second coating layer material is a fast ion conductor, which does not affect the overall impedance and can further improve the electrical performance.

[0107] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A positive electrode composite material, characterized in that: The positive electrode composite material comprises positive electrode material particles, a first coating layer coated on the surface of the positive electrode material particles, and a second coating layer coated on the surface of the first coating layer; The positive electrode material particles include lithium iron phosphate particles, the first coating layer material includes a high nickel material, the second coating layer material includes a fast ion conductor, the first coating layer has a thickness of 0.5 to 3 μm, and the second coating layer has a thickness of 0.5 to 20 nm. The high nickel material includes LiNi x Co y Mn z O2, where x+y+z=1, 0.80≤x≤0.83, 0.05≤y≤0.11, 0.06≤z≤0.15; The fast ion conductor includes Li3PO4 and / or Li4P2O 7； The D50 of the lithium iron phosphate particles is 0.7 to 6 μm; Based on the total mass of the positive electrode composite material, the content of the lithium iron phosphate particles is 78.0-99.49wt%, the content of the high nickel material is 0.5-20wt%, and the content of the fast ion conductor is 0.01-2.0wt%.

2. A method for preparing the positive electrode composite material according to claim 1, characterized in that: The preparation method comprises the following steps: S1, dispersing a phosphorus-containing organic acid in an organic solvent to obtain a first solution; mixing lithium iron phosphate particles coated with a high-nickel material on the surface with the first solution to obtain a first material; S2, filtering and drying the first material to obtain a precursor; S3, calcining the precursor.

3. The preparation method according to claim 2, characterized in that: In step S1, the molar ratio of the high nickel material contained in the lithium iron phosphate particles coated with the high nickel material to the phosphorus-containing organic acid is 1:(0.0001-0.1).

4. The preparation method according to claim 2, characterized in that: The phosphorus-containing organic acid is phytic acid; The organic solvent is selected from at least one of dimethyl sulfoxide solution, tetrahydrofuran, tetrachloroethane, isopropanol and methanol.

5. The preparation method according to claim 2, characterized in that: In step S1, the mixing treatment conditions include: temperature of 10 to 80°C, rotation speed of 400 to 1500 rpm, and time of 0.2 to 1 h; In step S2, the drying conditions include: temperature of 60-100°C and time of 1-2h; In step S3, the calcination treatment is carried out in a first oxygen-containing atmosphere, wherein the oxygen concentration in the first oxygen-containing atmosphere is greater than 90 vol%; the conditions of the calcination treatment include: a temperature of 400 to 800° C. and a time of 3 to 8 hours.

6. The preparation method according to claim 2, characterized in that: The preparation method further comprises: mixing lithium iron phosphate particles with high-nickel material particles, grinding them and calcining them to obtain lithium iron phosphate particles with the high-nickel material coated on the surface; The D50 of the lithium iron phosphate particles is 0.7 to 6 μm; The D50 of the high nickel material particles is 50 to 4000 nm; The mass ratio of the lithium iron phosphate to the high nickel material is 1:(0.01-0.1); The grinding process is ball milling; The calcination treatment is carried out in a second oxygen-containing atmosphere, wherein the oxygen concentration in the second oxygen-containing atmosphere is greater than 90 vol %; the calcination treatment conditions include: a temperature of 700 to 1000° C. and a time of 6 to 12 hours.

7. A battery, characterized in that: The battery comprises a positive electrode sheet, and the positive electrode sheet contains a positive electrode active material, wherein the positive electrode active material comprises the positive electrode composite material according to claim 1 and / or the positive electrode composite material prepared according to the preparation method according to any one of claims 2 to 6.

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