Modified positive electrode material and preparation method thereof, positive plate and lithium ion battery

Abstract

The invention relates to the field of lithium ion batteries, and discloses a modified positive electrode material and a preparation method thereof, a positive plate and a lithium ion battery. The modified positive electrode material comprises a positive electrode material matrix and a composite coating layer, wherein the composite coating layer contains lithium manganese iron phosphate and nano oxide; wherein the positive electrode material matrix and the lithium manganese iron phosphate are connected through an O1-Q-O2 bond, oxygen atoms O1 and O2 at two ends are respectively connected with transition metals in the positive electrode material matrix and the lithium manganese iron phosphate, and Q is selected from nitrogen N, sulfur S, phosphorus P and boron B. According to the modified positive electrode material provided by the invention, the surface of the positive electrode material substrate is uniformly coated with lithium manganese iron phosphate, and the binding force is enhanced, so that the cycle performance and the safety performance of the material are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a modified positive electrode material, a preparation method of the modified positive electrode material, a positive electrode sheet and a lithium ion battery. BACKGROUND

[0002] In recent years, lithium ion batteries are widely used in energy storage devices of electronic devices, electric vehicles and other devices due to their high specific energy, environmental protection and other advantages. The rapid development of electric vehicles increases the demand for high-performance rechargeable lithium ion batteries. In order to meet the endurance requirements of electric vehicles, lithium ion batteries must achieve higher energy density, which mainly depends on the specific capacity and working voltage of the positive electrode. Among them, ternary positive electrode materials (NCM, NCA) have been rapidly developed and widely used in new energy vehicles and other fields due to their better electrochemical performance. In order to pursue higher capacity, methods such as increasing Ni content or extending the material charge-discharge voltage window are usually adopted. However, the continuous increase of Ni content and the cycle of the material at a higher voltage will inevitably bring about a non-negligible safety risk to the material, which seriously hinders its large-scale commercial application. In addition, with the increase of Ni content, the cycle performance becomes poor, the surface side reaction increases and is unevenly distributed, the thermal stability decreases, and even under high temperature conditions, problems such as swelling, explosion and fire occur, which greatly limits the practical application of ternary materials in lithium ion batteries.

[0003] Although the lithium manganese iron phosphate material has high safety performance and low cost when used alone as a positive electrode, it has the disadvantages of low energy density, poor conductivity and poor rate performance. Therefore, high-energy-density ternary materials are often compounded with high-safety-performance LMFP. However, when LMFP is directly mixed with ternary materials, the homogenization process is difficult and the slurry dispersion is poor. Coating LMFP material on the surface of ternary materials can effectively solve this problem, and can also inhibit the chain reaction of ternary materials under thermal runaway conditions and improve the safety performance of the battery. However, in practical applications, the following defects exist: conventional LMFP coating only forms point / island-shaped coating with positive electrode material particles, which is prone to local agglomeration and has poor bonding ability. The positive electrode material surface is unevenly coated, and the performance improvement effect of the material is poor. SUMMARY

[0004] The purpose of the present application is to overcome the above technical problems, provide a modified positive electrode material, a preparation method of the modified positive electrode material, a positive electrode sheet and a lithium ion battery. The modified positive electrode material is provided with a coating layer, and the positive electrode material matrix and lithium manganese iron phosphate are connected by O1-Q-O2 bond. Not only is the lithium manganese iron phosphate uniformly coated on the surface of the positive electrode material matrix, but also the bonding force is enhanced, thereby improving the cycle performance and safety performance of the material.

[0005] In order to achieve the above object, the first aspect of the present application provides a modified positive electrode material, which comprises a positive electrode material matrix and a composite coating layer, and the composite coating layer contains lithium manganese iron phosphate and nano-oxide; wherein the positive electrode material matrix and lithium manganese iron phosphate are connected by O1-Q-O2 bond, and the two end oxygen atoms O1 and O2 are connected to transition metals in the positive electrode material matrix and lithium manganese iron phosphate respectively, and Q is selected from nitrogen N, sulfur S, phosphorus P and boron B.

[0006] In the present application, lithium manganese iron phosphate is referred to as LMFP without special circumstances.

[0007] Preferably, the composite coating layer is a coating layer of lithium manganese iron phosphate and nano-oxide.

[0008] The inventors of the present application have found that, compared with the prior art, by introducing a molecule self-assembly solvent containing a specific functional group, the specific functional group is tightly combined by forming a hydrogen bond or by condensation reaction between the positive electrode material matrix / LMFP particles, and then an O1-Q-O2 bond is formed between the two by sintering, thereby enhancing the bonding force between the positive electrode material matrix and the LMFP particles, and at the same time forming a LMFP-containing coating layer; in addition, the nano-oxide sol is used to coat the surface of the LMFP particles and the exposed part of the positive electrode material matrix after LMFP coating, forming a uniform composite coating layer containing LMFP and nano-oxide, which not only makes up for the uneven coating of LMFP, but also improves the coating degree, effectively inhibits the interface side reaction, and through the synergistic effect of LMFP and nano-oxide, the modified positive electrode material has high safety performance and excellent cycle performance.

[0009] The second aspect of the present application provides a preparation method of a modified positive electrode material, comprising the following steps:

[0010] S1, dispersing, drying lithium manganese iron phosphate, a molecule self-assembly solvent containing R1 group and R2 group, a positive electrode material matrix and a first organic solvent;

[0011] S2, secondly dispersing nano-oxide sol and a second organic solvent;

[0012] S3, thirdly dispersing the product of step S1 and the product of step S2, heating and stirring until all the liquid is evaporated, and performing solid phase sintering to coat lithium manganese iron phosphate and nano-oxide on the surface of the positive electrode material matrix to obtain a modified positive electrode material; wherein R1 is selected from hydroxyl and / or carboxyl; and R2 is selected from at least one of amino, boronic acid group, phosphoric acid group, sulfonic acid group and thiol group.

[0013] The third aspect of the present application provides a positive electrode sheet, which comprises the modified positive electrode material provided in the first aspect or the modified positive electrode material prepared by the preparation method provided in the second aspect.

[0014] The fourth aspect of the present application provides a lithium ion battery, which comprises the positive electrode sheet provided by the third aspect.

[0015] Compared with the prior art, the present application has the following advantages:

[0016] (1) The modified positive electrode material provided by the present application, the LMFP and the positive electrode material matrix in the composite coating layer are connected by a specific O1-Q-O2 bond to enhance the binding force between them; at the same time, the composite coating layer also introduces nano-oxide to coat the surface of the LMFP particles and the exposed part of the positive electrode matrix after the LMFP coating, which not only makes up for the uneven coating of LMFP, but also further improves the coating degree and inhibits the interface side reaction, through the synergistic effect of the manganese iron lithium phosphate and the nano-oxide composite coating layer, the modified positive electrode material has high safety performance and cycle performance at the same time;

[0017] (2) The preparation method provided by the present application adopts liquid phase coating means, by introducing a molecule self-assembly solvent containing a specific functional group, the specific functional group is tightly combined through hydrogen bonding or condensation reaction between the positive electrode material matrix / lithium manganese iron phosphate particles, and then the O1-Q-O2 bond is formed between them through sintering, to enhance the binding force; at the same time, nano-oxide sol is used for wet coating to form a uniform and dense composite coating layer, and then the performance of the material is optimized. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is the SEM diagram of the modified positive electrode material P1 prepared in Example 1;

[0019] Figure 2 is the first cycle charge-discharge curve of the battery assembled by the modified positive electrode material P1 prepared in Example 1 and the modified positive electrode material DP1 prepared in Comparative Example 1;

[0020] Figure 3 is the DSC diagram of the battery assembled by the modified positive electrode material P1 prepared in Example 1 and the modified positive electrode material DP1 prepared in Comparative Example 1. DETAILED DESCRIPTION

[0021] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The endpoints of the ranges and any values are understood to be approximate values. The exact numerical values of the endpoints of the ranges and any values are approximations that allow for a reasonable range that will accommodate minor variations from the values that can result, for example, from round off error in reporting numeric values. The endpoints of the ranges and any values should be considered to be open-ended ranges, unless the context clearly indicates the closed-ended nature of the ranges. It is specifically contemplated that the endpoints of the ranges and any values are not to be understood to be limited to the precise values stated. The endpoints of the ranges and any values are understood to be approximate values that are understood to be open-ended ranges for purposes of the application.

[0022] In this invention, unless otherwise specified, "first" and "second" do not indicate a sequence or limit the various materials or steps, but are used only to distinguish or indicate that they are not the same material or step.

[0023] The first aspect of the present invention provides a modified cathode material, the modified cathode material comprising a cathode material matrix and a composite coating layer, wherein the composite coating layer contains lithium manganese iron phosphate and nano-oxide.

[0024] The cathode material matrix and lithium manganese iron phosphate are connected by O1-Q-O2 bonds, and the oxygen atoms at both ends are respectively connected to the transition metals in the cathode material matrix and lithium manganese iron phosphate, where Q is selected from nitrogen (N), sulfur (S), phosphorus (P), and boron (B).

[0025] In this invention, in the O1-Q-O2 bond, O1 and O2 respectively represent oxygen atoms O, and are only used to distinguish the two ends being connected to the positive electrode material matrix and the transition metal in lithium manganese iron phosphate, respectively.

[0026] In this invention, the composite coating layer is composed of LMFP and nano-oxide. The LMFP is wrapped around the surface of the cathode material matrix by the bonding effect of molecular self-assembly, forming O1-Q-O2 bonds between the cathode material matrix and the LMFP particles, which enhances the bonding force between the two and improves safety performance. At the same time, the nano-oxide is uniformly coated on the surface of the LMFP particles and the part of the cathode matrix exposed after LMFP coating, which improves the cycle performance of the material.

[0027] In this invention, preferably, the D50 particle size values ​​of the cathode material matrix, lithium manganese iron phosphate, and nano-oxide satisfy: 0.1≤a / [50(b+c) / 1000]≤7.7; where a is the D50 particle size of the cathode material matrix in micrometers; b and c are the D50 particle sizes of the lithium manganese iron phosphate and nano-oxide in nanometers, respectively.

[0028] In this invention, if a / [50(b+c) / 1000]<0.1, the larger particle size of LMFP and the coating of nano-oxides result in an excessively thick composite coating layer, leading to increased interfacial impedance and hindering Li + Transmission is not conducive to the utilization of material capacity; if a / [50(b+c) / 1000]>7.7, then coating with smaller particle size LMFP will not significantly improve the safety performance of the material.

[0029] In this invention, the D50 particle size parameter was measured using SEM.

[0030] In this invention, preferably, the D50 particle size a of the positive electrode material is 3-20 μm, for example, 3 μm, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, and any value in any range of any two values, preferably 3-15 μm.

[0031] In this invention, preferably, the D50 particle size b of the lithium manganese iron phosphate is 50-600nm, for example, 50nm, 60nm, 80nm, 100nm, 150nm, 200nm, 250nm, 300nm, 400nm, 500nm, 600nm, and any value within the range of any two values, preferably 50-300nm.

[0032] In this invention, preferably, the D50 particle size c of the nano-oxide is 2-100nm, for example, 2nm, 5nm, 8nm, 10nm, 12nm, 15nm, 20nm, 30nm, 40nm, 50nm, 70nm, 100nm, and any value in any range of any two values, preferably 5-50nm.

[0033] In this invention, preferably, the mass ratio of lithium manganese iron phosphate to the cathode material matrix is ​​0.1-30:100, for example, 0.1:100, 0.5:100, 1:100, 3:100, 5:100, 8:100, 10:100, 15:100, 20:100, 25:100, 30:100, and any value within any range of two such values, preferably 1-20:100. Satisfying the above mass ratio ensures that the modified cathode material exhibits high safety.

[0034] In this invention, preferably, the mass ratio of the nano-oxide to the cathode material matrix is ​​0.1-5:100, for example, 0.1:100, 0.5:100, 1:100, 2:100, 3:100, 5:100, and any value within any range of two such values, preferably 0.5-3:100. Satisfying the above mass ratio results in the modified cathode material exhibiting high cycle performance.

[0035] In this invention, preferably, the cathode material matrix is ​​selected from at least one of ternary lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt oxide, lithium nickel manganese oxide, lithium manganese oxide, lithium cobalt oxide, lithium-rich manganese-based cathode materials, and the above-mentioned doped and modified cathode materials.

[0036] In this invention, ternary lithium nickel cobalt manganese oxide includes, but is not limited to, LiNi. 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (abbreviated as NCM333), LiNi 0.5 Co0.2 Mn 0.3 O2 (abbreviated as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (abbreviated as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (abbreviated as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (abbreviated as NCM811), Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2 (abbreviated as LRM), etc.

[0037] In this invention, in the O1-Q-O2 bond, O1 is connected to the transition metal in the positive electrode material matrix, including but not limited to nickel (Ni), cobalt (Co), manganese (Mn), etc.

[0038] In this invention, preferably, the lithium manganese iron phosphate has an electronic conductivity ≥ 1 × 10⁻⁶. -9 S / cm.

[0039] In this invention, the electronic conductivity parameter is measured using a powder resistivity meter at room temperature, which is 25±2℃.

[0040] In this invention, preferably, the lithium manganese iron phosphate has the general formula shown in Formula I, Li α Mn x Fe 1-x-y M y PO4(I); wherein, 0.9≤α≤1.2, 0.2≤x<1, 0≤y≤0.1, and x+y<1, and M is selected from at least one of Mg, Al, Ti, Zr, Y, Sn, Nb and Zn.

[0041] In this invention, preferably, the surface of the lithium manganese iron phosphate further includes a carbon coating layer; the thickness of the carbon coating layer is 2-10 nm, preferably 2-5 nm.

[0042] In this invention, in the O1-Q-O2 bond, O2 connects to the transition metal in LMFP, including but not limited to manganese (Mn), iron (Fe), etc.

[0043] In this invention, preferably, the nano-oxide has the general formula shown in Formula II, A β B γ O δ(II); where 1≤β≤2, 0≤γ≤0.3, 1≤δ≤5, A is selected from at least one of Y, Al, Zr, Ti, Si, Mg and Sb, and B is selected from at least one of Zn, Sn, Mo, Nb and Ta.

[0044] In this invention, the nano-oxides include, but are not limited to, ZrO2, MgO, yttrium zirconium oxide, Al2O3, TiO2, Y2O3, etc.

[0045] In this invention, preferably, the O1-Q-O2 bond originates from a molecular self-assembly solvent containing R1 and R2 groups, wherein R1 is selected from hydroxyl and / or carboxyl groups; and R2 is selected from at least one of amino, borate, phosphate, sulfonic acid, and thiol groups.

[0046] In this invention, unless otherwise specified, in the O1-Q-O2 bond, the oxygen atom O1 originates from the R1 group, the oxygen atom O2 originates from the R2 group, and Q also originates from the R2 group.

[0047] In this invention, more preferably, the molecular self-assembly solvent is selected from at least one of aminoethanol, 5-aminosalicylic acid, 2-carboxyethyl phosphate, phenylboronic acid, mercaptoethanol, 4-carboxyphenylboronic acid, p-aminobenzenesulfonic acid, and 3-mercaptopropionic acid.

[0048] In this invention, the preparation method of the modified cathode material is widely selected, as long as it meets the above limitations. Preferably, the modified cathode material is prepared by solid-state sintering of a mixed slurry containing the cathode material matrix, lithium manganese iron phosphate, nano-oxide, and a molecular self-assembly solvent.

[0049] In this invention, preferably, the thickness of the composite coating layer is 50-800nm, for example, 50nm, 55nm, 60nm, 80nm, 100nm, 120nm, 150nm, 180nm, 200nm, 240nm, 300nm, 400nm, 500nm, 800nm, and any value within the range of any two values, preferably 80-400nm.

[0050] In this invention, when the thickness of the composite coating layer is less than 50 nm, the excessively thin composite coating layer cannot effectively block the direct contact between the material and the electrolyte and suppress interfacial side reactions, thus having a limited effect on improving the cycling performance of the material; when the thickness of the composite coating layer is greater than 800 nm, the excessively thick composite coating layer will form an "ion barrier," hindering Li + Migration between positive electrode particles leads to increased battery polarization and reduced discharge capacity.

[0051] In this invention, more preferably, the composite coating layer is a coating layer of lithium manganese iron phosphate and nano-oxide.

[0052] In this invention, preferably, the nano-oxide exists in the composite coating layer in the form of a flocculent structure. This configuration can, on the one hand, compensate for the problem of low coating degree after LMFP coating, and on the other hand, the flocculent structure has a porous network structure, which acts as an "elastic buffer layer" to relieve internal stress through pore deformation, reduce particle breakage, and thus maintain the integrity of the crystal structure. At the same time, it prevents the electrolyte from directly contacting the positive electrode material matrix, suppresses side reactions, and extends cycle life.

[0053] The modified cathode material provided by the present invention includes a cathode material matrix, a composite coating layer of lithium manganese iron phosphate and nano-oxide, wherein the cathode material matrix and lithium manganese iron phosphate are connected by O1-Q-O2 bonds.

[0054] The modified cathode material provided by this invention forms a continuous and uniform composite coating layer on the surface of the cathode material matrix, which can prevent the cathode material from directly contacting the electrolyte, inhibit the dissolution of transition metals, reduce the specific surface area of ​​the material, reduce side reactions, and give the material excellent cycle performance.

[0055] In this invention, preferably, the coating degree of the modified cathode material is ≥60%, for example, 60%, 65%, 70%, 75%, 80%, 82%, 85%, 88%, 90%, 91%, 95%, 100%, and any value within any range of any two values, preferably ≥80%.

[0056] In this invention, the method for testing the coating degree parameter is as follows: the cross-section of the positive electrode sheet prepared in each embodiment and comparative example is obtained by cutting it with an ion polishing machine, the cross-section is observed with a scanning electron microscope at an appropriate magnification, and the perimeter L1 of the matrix material and the total perimeter L2 of the uncoated area are identified using Image J software. The coating degree M = 1 - L2 / L1.

[0057] A second aspect of the present invention provides a method for preparing a modified cathode material, the method comprising:

[0058] S1. Lithium manganese iron phosphate, a molecular self-assembly solvent containing R1 and R2 groups, a cathode material matrix, and a first organic solvent are first dispersed and dried.

[0059] S2. The nano-oxide sol and the second organic solvent are then dispersed in a second dispersion.

[0060] S3. After the products of step S1 and step S2 are dispersed in the third step, they are heated and stirred until the liquid is completely evaporated, and solid-state sintering is performed to coat the surface of the cathode material matrix with lithium manganese iron phosphate and nano-oxide to obtain a modified cathode material; wherein, R1 is selected from hydroxyl and / or carboxyl groups; R2 is selected from at least one of amino, boric acid, phosphoric acid, sulfonic acid and thiol groups.

[0061] In this invention, unless otherwise specified, the types and physical properties of lithium manganese iron phosphate, molecular self-assembly solvent, and cathode material matrix are all as defined above, and will not be elaborated upon further in this invention.

[0062] The preparation method provided by this invention employs a liquid-phase coating technique, utilizing LMFP slurry and nano-oxide sol to coat the cathode material matrix. By introducing a molecular self-assembly solvent containing specific functional groups, these functional groups form hydrogen bonds or tightly bind with the cathode material matrix / lithium manganese iron phosphate particles through condensation reactions. Subsequently, sintering forms O1-Q-O2 bonds between the two, enhancing the bonding force between the cathode material matrix and LMFP particles. This effectively solves the problem of LMFP being introduced into the cathode material or slurry through blending in practical applications, where LMFP and LMFP... The poor bonding force between the cathode material matrix and its tendency to agglomerate lead to poor improvement in safety performance. At the same time, the use of nano-oxide sol to in-situ coat the surface of LMFP particles and the exposed parts of the cathode matrix after LMFP coating forms a uniform and dense coating layer. This not only makes up for the uneven coating of LMFP, but also enhances the bonding force between the coating layer and the cathode material matrix, improves the coating degree, and effectively suppresses interfacial side reactions. Through the synergistic effect of lithium manganese iron phosphate and nano-oxide coating layer, the modified cathode material has both high safety performance and excellent cycle performance.

[0063] In this invention, preferably, the lithium manganese iron phosphate is added in the form of a slurry with a solid content of 20-60 wt%, for example, 20 wt%, 30 wt%, 35 wt%, 40 wt%, 50 wt%, 60 wt%, and any value within any range of any two values, preferably 30-50 wt%.

[0064] In this invention, the solvent for the lithium manganese iron phosphate slurry is selected from at least one of carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), N-methylpyrrolidone (NMP), and polyvinylpyrrolidone (PVP).

[0065] In this invention, preferably, the mass ratio of lithium manganese iron phosphate to the molecular self-assembly solvent is 0.1-30:100, for example, 0.1:100, 0.5:100, 1:100, 2:100, 3:100, 4:100, 5:100, 8:100, 10:100, 20:100, 30:100, and any value within any range of any two values, preferably 0.1-5:100.

[0066] In this invention, when the mass of lithium manganese iron phosphate and the molecular self-assembly solvent is relatively low, an excessive amount of molecular self-assembly solvent may cause lithium manganese iron phosphate particles to stack and adsorb, forming a locally thicker coating layer. At the same time, the excessive molecular self-assembly solvent will leave residues, which will subsequently affect the performance of the material. When the mass of lithium manganese iron phosphate and the molecular self-assembly solvent is relatively high, the insufficient amount of molecular self-assembly solvent may cause the lithium manganese iron phosphate particles to lack anchoring points, making it impossible to ensure a strong binding force between the particles and the cathode material matrix. This makes them prone to falling off during stirring, thus failing to improve the safety performance of the material.

[0067] In this invention, preferably, the molecular self-assembly solvent is selected from at least one of aminoethanol, 5-aminosalicylic acid, 2-carboxyethyl phosphate, phenylboronic acid, mercaptoethanol, 4-carboxyphenylboronic acid, p-aminobenzenesulfonic acid, and 3-mercaptopropionic acid.

[0068] In this invention, the first organic solvent and the second organic solvent can be the same or different, but preferably the same.

[0069] In this invention, preferably, the first organic solvent and the second organic solvent are each independently selected from at least one of alcohol, acetone, diethyl ether, N-methylpyrrolidone, and N,N-dimethylformamide. In this invention, the alcohol is selected from C1-C5 alcohols, including but not limited to methanol, ethanol, isopropanol, etc.

[0070] In this invention, preferably, the conditions for the first dispersion include: a rotation speed of 100-1000 rpm and a time of 1-10 h.

[0071] In this invention, preferably, the first dispersion process includes: first dispersing lithium manganese iron phosphate, molecular self-assembly solvent and first organic solvent, and then adding positive electrode material matrix for a first dispersion.

[0072] In this invention, the drying is intended to remove the first organic solvent. Preferably, the drying temperature is 60-150°C.

[0073] In this invention, preferably, in step S2, the solid content of the nano-oxide sol is 3-20 wt%, for example, 3 wt%, 5 wt%, 8 wt%, 10 wt%, 15 wt%, 20 wt%, and any value within the range of any two values ​​combined, preferably 3-10 wt%.

[0074] In this invention, preferably, the pH of the nano-oxide sol is 4-7 at 25±2℃.

[0075] In this invention, unless otherwise specified, the particle size of the nano-oxide sol is equivalent to the D50 particle size c of the nano-oxide; the nano-oxide sol is a dispersion system of nano-oxide and dispersion medium.

[0076] In this invention, preferably, the dispersion medium of the nano-oxide sol is at least one selected from ethanol, isopropanol, ethylene glycol, polyvinylpyrrolidone (PVP), and polyacrylic acid (PAA); the nano-oxide has the general formula shown in II, A β B γ O δ (II); where 1≤β≤2, 0≤γ≤0.3, 1≤δ≤5, A is selected from at least one of Y, Al, Zr, Ti, Si, Mg and Sb, and B is selected from at least one of Zn, Sn, Mo, Nb and Ta.

[0077] In this invention, preferably, the mass ratio of the nano-oxide sol and the second organic solvent is 1:1-100, for example, 1:1, 1:5, 1:10, 1:18, 1:20, 1:30, 1:40, 1:45, 1:50, 1:55, 1:60, 1:80, 1:100, and any value within the range of any two values, preferably 1:20-60.

[0078] In this invention, preferably, the conditions for the second dispersion include: a rotation speed of 100-1000 rpm and a time of 0.5-5 h.

[0079] In this invention, preferably, in step S3, the conditions for the third dispersion include: a rotation speed of 100-1000 rpm and a time of 0.5-5 h.

[0080] In this invention, the heating is intended to remove the second organic solvent. Preferably, the heating temperature is 70-120°C, for example, 70°C, 80°C, 95°C, 100°C, 110°C, 120°C, or any value within a range of any two of these values.

[0081] In this invention, preferably, the solid-state sintering is carried out in an oxygen-containing atmosphere, wherein the oxygen content in the oxygen-containing atmosphere is ≥20 vol%, including but not limited to air, oxygen atmosphere, etc.

[0082] In this invention, preferably, the solid-state sintering temperature is 200-800℃, for example, 200℃, 300℃, 400℃, 500℃, 600℃, 700℃, 800℃, and any value within the range of any two values, preferably 300-800℃.

[0083] In this invention, preferably, the solid-state sintering time is 1-20h, for example, 1h, 2h, 6h, 8h, 10h, 12h, 15h, 20h, and any value within the range of any two values, preferably 6-12h.

[0084] In this invention, preferably, the heating rate of the solid-state sintering is 1-10℃ / min, for example, 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 8℃ / min, 10℃ / min, and any value within the range of any two values, preferably 3-8℃ / min.

[0085] A third aspect of the present invention provides a positive electrode sheet, the positive electrode sheet comprising: the modified positive electrode material provided in the first aspect, or the modified positive electrode material prepared by the preparation method provided in the second aspect.

[0086] In this invention, preferably, the content of the modified positive electrode material in the positive electrode sheet is ≥70wt%, for example, 70wt%, 75wt%, 80wt%, 85wt%, 88wt%, 90wt%, 92wt%, 95wt%, and any value within the range of any two values, preferably 85-95wt%.

[0087] In this invention, the positive electrode sheet, in addition to the modified positive electrode material, also includes: an electrolyte, a conductive agent, and a binder; wherein the total mass of the modified positive electrode material, the electrolyte, the conductive agent, and the binder is 100 wt%.

[0088] In this invention, the electrolyte includes, but is not limited to, sulfide electrolytes, oxide electrolytes, halide electrolytes, polymer electrolytes, borohydride electrolytes, etc.

[0089] In this invention, the conductive agent includes, but is not limited to, acetylene black, conductive carbon black, carbon fiber, carbon nanotubes, Ketjen black, etc.

[0090] In this invention, the adhesive includes, but is not limited to, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), carboxymethyl chitosan (CMCS), polyamide-imide (PAI), polyethyleneimine (PEI), polyimide (PI), tert-butyl polyacrylate-triethoxyvinylsilane (TBATEVS), etc.

[0091] A fourth aspect of the present invention provides a lithium-ion battery, comprising the positive electrode sheet provided in the third aspect.

[0092] The present invention will be described in detail below through embodiments.

[0093] Example 1

[0094] S1. Prepare a lithium manganese iron phosphate slurry with a solid content of 35wt% (solvent is NMP, and LiMn has a D50 particle size b of 80nm). 0.6 Fe 0.4 PO4 was dispersed in 300 mL of NMP, and 4-carboxyphenylboronic acid was added as a self-assembly solvent. The mixture was stirred at 300 rpm for 2 h, and then LiNi with a D50 particle size of 10 μm was added. 0.9 Co 0.05 Mn 0.05 O2 was used as the positive electrode material matrix. After stirring at 300 rpm for 2 hours, the mixture was filtered and dried at 100°C.

[0095] The mass ratio of lithium manganese iron phosphate to the cathode material matrix is ​​10:100; the mass ratio of lithium manganese iron phosphate to the molecular self-assembly solvent is 5:100.

[0096] S2. Disperse a 5 wt% nano-oxide sol (pH 5 at 25°C, isopropanol as the dispersion medium, and ZrO2 with a D50 particle size c of 10 nm) in NMP and stir at 300 rpm for 1 h.

[0097] The mass ratio of the above-mentioned nano-oxide sol to NMP is 1:60; the mass ratio of the above-mentioned nano-oxide sol to the cathode material matrix, based on solid content, is 1:100.

[0098] S3. Stir the products of S1 and S2 at 450 rpm for 2 hours, heat to 100°C and stir until the liquid is completely evaporated. Place the resulting powder in a tube furnace and heat to 450°C at a heating rate of 5°C / min in an O2 atmosphere. Sinter the solid phase for 8 hours to obtain the modified cathode material P1.

[0099] The SEM image of the modified cathode material P1 is shown below. Figure 1As shown, a uniform and dense composite coating layer is formed on the surface of the cathode material matrix, and the nano-oxide ZrO2 exists in the form of a flocculent structure.

[0100] Example 2

[0101] The method is the same as in Example 1, except that...

[0102] In step S1, the mass ratio of lithium manganese iron phosphate to the cathode material matrix is ​​adjusted to 1:100.

[0103] Under the same conditions, the modified cathode material P2 was obtained.

[0104] Example 3

[0105] The method is the same as in Example 1, except that...

[0106] In step S1, the mass ratio of lithium manganese iron phosphate and the cathode material matrix is ​​adjusted to 30:100.

[0107] Under the same conditions, the modified cathode material P3 was obtained.

[0108] Example 4

[0109] The method is the same as in Example 1, except that...

[0110] In step S1, the D50 particle size b of the above-mentioned lithium manganese iron phosphate is adjusted to 300 nm;

[0111] Under the same conditions, the modified cathode material P4 was obtained.

[0112] Example 5

[0113] The method is the same as in Example 1, except that...

[0114] In step S1, the D50 particle size b of the above-mentioned lithium manganese iron phosphate is adjusted to 600 nm;

[0115] Under the same conditions, the modified cathode material P5 was obtained.

[0116] Example 6

[0117] The method is the same as in Example 1, except that...

[0118] In step S1, the mass adjustment ratio of the above-mentioned lithium manganese iron phosphate and molecular self-assembly solvent is 0.1:100;

[0119] Under the same conditions, the modified cathode material P6 was obtained.

[0120] Example 7

[0121] The method is the same as in Example 1, except that...

[0122] In step S1, the mass adjustment ratio of the above-mentioned lithium manganese iron phosphate and molecular self-assembly solvent is 30:100;

[0123] Under the same conditions, the modified cathode material P7 was obtained.

[0124] Example 8

[0125] The method is the same as in Example 1, except that...

[0126] In step S2, the mass ratio of the above-mentioned nano-oxide sol and the cathode material matrix, based on solid content, is adjusted to 0.1:100;

[0127] Under the same conditions, the modified cathode material P8 was obtained.

[0128] Example 9

[0129] The method is the same as in Example 1, except that...

[0130] In step S2, the mass ratio of the above-mentioned nano-oxide sol and the cathode material matrix, based on solid content, is adjusted to 5:100;

[0131] Under the same conditions, the modified cathode material P9 was obtained.

[0132] Example 10

[0133] The method is the same as in Example 1, except that...

[0134] In step S2, the D50 particle size c of ZrO2 is adjusted to 2nm;

[0135] Under the same conditions, the modified cathode material P10 was obtained.

[0136] Example 11

[0137] The method is the same as in Example 1, except that...

[0138] In step S2, the D50 particle size c of ZrO2 is adjusted to 100 nm;

[0139] Under the same conditions, the modified cathode material P11 was obtained.

[0140] Example 12

[0141] The method is the same as in Example 1, except that...

[0142] In step S1, LiMn 0.6 Fe 0.4 The D50 particle size b of PO4 was adjusted to 60 nm;

[0143] LiNi 0.9 Co0.05 Mn 0.05 The D50 particle size a of O2 was adjusted to 20 μm;

[0144] In step S2, the D50 particle size c of ZrO2 is adjusted to 5 nm;

[0145] Under the same conditions, the modified cathode material P12 was obtained.

[0146] Example 13

[0147] The method is the same as in Example 1, except that...

[0148] In step S1, LiMn 0.6 Fe 0.4 The D50 particle size b of PO4 was adjusted to 200 nm;

[0149] LiNi 0.9 Co 0.05 Mn 0.05 The D50 particle size a of O2 was adjusted to 6μm;

[0150] Under the same conditions, the modified cathode material P13 was obtained.

[0151] Example 14

[0152] The method is the same as in Example 1, except that...

[0153] In step S1, LiMn 0.6 Fe 0.4 The D50 particle size b of PO4 was adjusted to 450 nm;

[0154] LiNi 0.9 Co 0.05 Mn 0.05 The D50 particle size a of O2 was adjusted to 5 μm;

[0155] In step S2, the D50 particle size c of ZrO2 is adjusted to 100 nm;

[0156] Under the same conditions, the modified cathode material P14 was obtained.

[0157] Example 15

[0158] S1. Carbon-coated lithium manganese iron phosphate slurry with a solid content of 20wt% (solvent is NMP, D50 particle size b is 50nm) is prepared. 0.7 Fe 0.3 PO4@C (with a C coating thickness of 3 nm) was dispersed in 300 mL of ethanol, and aminoethanol was added as a self-assembly solvent. The mixture was stirred at 100 rpm for 1 h, and then NCA (LiNi) with a D50 particle size of 3 μm was added. 0.9 Co0.05 Al 0.05 O2) was used as the positive electrode material matrix. After stirring at 100 rpm for 1 hour, the mixture was filtered and dried at 80°C.

[0159] The mass ratio of the carbon-coated lithium manganese iron phosphate to the cathode material matrix is ​​20:100; the mass ratio of the carbon-coated lithium manganese iron phosphate to the molecular self-assembly solvent is 3:100.

[0160] S2. Disperse 3wt% nano-oxide sol (pH 5 at 25℃, ethylene glycol as the dispersion medium, and MgO with a D50 particle size c of 2nm) in ethanol and stir at 100rpm for 0.5h.

[0161] The mass ratio of the above-mentioned nano-oxide sol to ethanol is 1:5; the mass ratio of the above-mentioned nano-oxide sol to the cathode material matrix, based on solid content, is 4:100.

[0162] S3. Stir the products of S1 and S2 at 270 rpm for 1.5 h, heat to 70 °C and stir until the liquid is completely evaporated. Place the resulting powder in a tube furnace and heat to 280 °C in air at a heating rate of 3 °C / min. Sinter the solid phase for 5 h to obtain the modified cathode material P15.

[0163] Example 16

[0164] S1. Carbon-coated lithium manganese iron phosphate slurry with a solid content of 30wt% (solvent is PVP, D50 particle size b is 60nm) is used. 0.7 Fe 0.3 PO4@C (with a C coating thickness of 4 nm) was dispersed in 300 mL of acetone, 5-aminosalicylic acid was added as a molecular self-assembly solvent, and the mixture was stirred at 280 rpm for 3 h. LiCoO2 with a D50 particle size a of 8 μm was added as the positive electrode material matrix, and the mixture was stirred at 280 rpm for 3 h before being filtered and dried at 95 °C.

[0165] The mass ratio of the carbon-coated lithium manganese iron phosphate to the cathode material matrix is ​​13:100; the mass ratio of the carbon-coated lithium manganese iron phosphate to the molecular self-assembly solvent is 7:100.

[0166] S2. Disperse yttrium zirconium oxide (pH 4 at 25°C, isopropanol as the dispersion medium, and D50 particle size c of 10 nm) with a solid content of 5 wt% in acetone and stir at 280 rpm for 2.5 h.

[0167] The mass ratio of the above-mentioned nano-oxide sol to acetone is 1:18; the mass ratio of the above-mentioned nano-oxide sol to the cathode material matrix, based on solid content, is 1:100.

[0168] S3. Stir the products of S1 and S2 at 420 rpm for 3 hours, heat to 80°C and stir until the liquid is completely evaporated. Place the resulting powder in a tube furnace and heat to 450°C at a heating rate of 5°C / min in an O2 atmosphere. Sinter the solid phase for 8 hours to obtain the modified cathode material P16.

[0169] Example 17

[0170] S1. Prepare a slurry of lithium manganese iron phosphate with a solid content of 40 wt% (solvent is NMP, and LiMn has a D50 particle size b of 120 nm). 0.65 Fe 0.35 PO4) was dispersed in 300 mL of diethyl ether, and 2-carboxyethyl phosphoric acid was added as a self-assembly solvent. The mixture was stirred at 520 rpm for 5 h, and then LRM (Li) with a D50 particle size of 10 μm was added. 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2) was used as the positive electrode material matrix. After stirring at 520 rpm for 5 h, the mixture was filtered and dried at 100 °C.

[0171] The mass ratio of lithium manganese iron phosphate to the cathode material matrix is ​​15:100; the mass ratio of lithium manganese iron phosphate to the molecular self-assembly solvent is 15:100.

[0172] S2. Disperse 10wt% nano-oxide sol (pH 4.5 at 25℃, dispersion medium is isopropanol, D50 particle size c of Al2O3) in diethyl ether and stir at 450 rpm for 3 h.

[0173] The mass ratio of the above-mentioned nano-oxide sol to diethyl ether is 1:45; the mass ratio of the above-mentioned nano-oxide sol to the cathode material matrix, based on solid content, is 2:100.

[0174] S3. Stir the products of S1 and S2 at 600 rpm for 2 hours, heat to 95°C and stir until the liquid is completely evaporated. Place the resulting powder in a tube furnace and heat to 500°C in air at a heating rate of 6°C / min. Sinter the powder in solid state for 10 hours to obtain the modified cathode material P17.

[0175] Example 18

[0176] S1. Prepare a slurry of lithium manganese iron phosphate with a solid content of 50 wt% (solvent is NMP, and LiMn has a D50 particle size b of 180 nm). 0.65 Fe 0.35 PO4) was dispersed in 300 mL of isopropanol, and p-aminobenzenesulfonic acid was added as a self-assembly solvent. The mixture was stirred at 800 rpm for 10 h, and then LNMO (LiNi) with a D50 particle size of 15 μm was added. 0.9 Mn0.1 O2) was used as the positive electrode material matrix. After stirring at 800 rpm for 10 h, the mixture was filtered and dried at 110 °C.

[0177] The mass ratio of lithium manganese iron phosphate to the cathode material matrix is ​​17:100; the mass ratio of lithium manganese iron phosphate to the molecular self-assembly solvent is 25:100.

[0178] S2. Disperse a 15wt% nano-oxide sol (pH 5 at 25℃, dispersion medium PAA, TiO2 with D50 particle size c of 70nm) in isopropanol and stir at 820rpm for 4h.

[0179] The mass ratio of the above-mentioned nano-oxide sol to isopropanol is 1:80; the mass ratio of the above-mentioned nano-oxide sol to the cathode material matrix, based on solid content, is 3:100.

[0180] S3. Stir the products of S1 and S2 at 820 rpm for 4.5 h, heat to 110 °C and stir until the liquid is completely evaporated. Place the resulting powder in a tube furnace and heat to 700 °C at a heating rate of 8 °C / min in an O2 atmosphere. Sinter the solid phase for 15 h to obtain the modified cathode material P18.

[0181] Example 19

[0182] S1. Prepare a slurry of lithium manganese iron phosphate with a solid content of 60wt% (solvent is PVP, and LiMn has a D50 particle size b of 240nm). 0.8 Fe 0.2 PO4 was dispersed in 300 mL of N,N-dimethylformamide, and p-mercaptoethanol was added as a self-assembly solvent. The mixture was stirred at 1000 rpm for 8 h. NC(LiNi) with a D50 particle size of 20 μm was then added. 0.9 Co 0.1 O2) was used as the positive electrode material matrix. After stirring at 1000 rpm for 8 hours, the mixture was filtered and dried at 130℃.

[0183] The mass ratio of lithium manganese iron phosphate to the cathode material matrix is ​​16:100; the mass ratio of lithium manganese iron phosphate to the molecular self-assembly solvent is 20:100.

[0184] S2. Disperse 18wt% nano-oxide sol (pH 5 at 25℃, dispersion medium is PVP, D50 particle size c of Y2O3) in isopropanol and stir at 1000rpm for 5h.

[0185] The mass ratio of the above-mentioned nano-oxide sol to isopropanol is 1:100; the mass ratio of the above-mentioned nano-oxide sol to the cathode material matrix, based on solid content, is 2:100.

[0186] S3. Stir the products of S1 and S2 at 900 rpm for 1 h, heat to 120°C and stir until the liquid is completely evaporated. Place the resulting powder in a tube furnace and heat to 750°C in air at a heating rate of 10°C / min. Sinter the powder in solid state for 20 h to obtain the modified cathode material P19.

[0187] Comparative Example 1

[0188] Directly using LiNi with a D50 particle size of 10 μm 0.9 Co 0.05 Mn 0.05 O2 was sintered at 450℃ for 8 hours to obtain the modified cathode material DP1.

[0189] Comparative Example 2

[0190] The method is the same as in Example 1, except that...

[0191] Step S2 is skipped; the product of step S1 is directly processed into step S3.

[0192] Under the same conditions, the modified cathode material DP2 was obtained.

[0193] Comparative Example 3

[0194] The method is the same as in Example 1, except that...

[0195] Step S1 is skipped; LiNi with a D50 particle size of 10 μm is directly processed. 0.9 Co 0.05 Mn 0.05 The O2 cathode material matrix and the product of step S2 are then subjected to step S3;

[0196] Under the same conditions, the modified cathode material DP3 was obtained.

[0197] Comparative Example 4

[0198] The method is the same as in Example 1, except that...

[0199] In step S1, no molecular self-assembly solvent is added;

[0200] Under the same conditions, the modified cathode material DP4 was obtained.

[0201] Comparative Example 5

[0202] The method is the same as in Example 1, except that...

[0203] In step S1, the solvent for molecular self-assembly is adjusted to KH550 (i.e., γ-aminopropyltriethoxysilane);

[0204] Under the same conditions, the modified cathode material DP5 was obtained.

[0205] Comparative Example 6

[0206] The method is the same as in Example 1, except that...

[0207] In step S1, the nano-oxide sol is first coated;

[0208] In step S3, a lithium manganese iron phosphate slurry with a solid content of 35 wt% is coated and a molecular self-assembly solvent is added.

[0209] Under the same conditions, the modified cathode material DP6 was obtained.

[0210] Table 1

[0211]

[0212] As can be seen from the data in Table 1, compared with Comparative Examples 1-6, Examples 1-19 use the modified cathode material provided by the present invention. In the composite coating layer, LMFP and the cathode material matrix are connected by specific O1-Q-O2 bonds to enhance the bonding force between them, resulting in a higher coating degree and thus optimizing the performance of the material.

[0213] Test case

[0214] The modified cathode materials obtained in Examples 1-19 and Comparative Examples 1-6 were prepared into electrode sheets and assembled into coin cells for testing. The preparation steps were as follows: the modified cathode materials (P1-P19 and DP1-DP6), binder (PVDF), and conductive agent (SP) were dissolved in NMP, wherein the mass ratio of modified cathode materials, PVDF, and SP was 90:5:5. After uniform mixing, the mixture was coated onto an aluminum foil with a thickness of 12 μm. The compaction of the electrode sheet was 3.2 g / cm³. 3 ;

[0215] Assembly of button cells: The modified positive electrode material was used as the positive electrode, lithium metal sheet was used as the negative electrode, 1 mol / L LiPF6 was dissolved in a mixed solution of EC / DMC / EMC with a volume ratio of 1:1:1 as the electrolyte, and Celgard 2400 single-layer microporous PP membrane was used as the separator. The assembly was completed in a high-purity argon glove box with strictly controlled water and oxygen content. The assembled batteries were then subjected to charge and discharge tests.

[0216] Test method: Charge and discharge test was conducted at a rate of 0.1C, with a test voltage range of 2.8-4.25V. The nominal specific capacity was 200mAh / g. The test results are shown in Table 2.

[0217] The first-week charge-discharge curves of the modified cathode materials assembled in Example 1 and Comparative Example 1 are shown below. Figure 2As shown, compared to Comparative Example 1, the battery assembled with the modified cathode material P1 in Example 1 has a higher first-cycle charge specific capacity and first-cycle discharge specific capacity.

[0218] The DSC diagrams of the modified cathode material assembled batteries provided in Example 1 and Comparative Example 1 are shown below. Figure 3 As shown, compared with Comparative Example 1, the modified cathode material P1 assembled battery in Example 1 has a higher peak exothermic temperature and better safety performance.

[0219] Table 2

[0220]

[0221]

[0222] As can be seen from the data in Tables 1-2, compared with Comparative Examples 1-6, Examples 1-19 using the modified cathode material provided by the present invention have higher discharge specific capacity and better cycle performance, while also improving the peak exothermic temperature of DSC test and having better safety performance.

[0223] (1) Synergistic effect of lithium iron manganese phosphate and nano-oxide composite coating layer on the performance of modified positive electrode material

[0224] As can be seen from the data of Example 1 and Comparative Examples 1-3, compared with Comparative Example 1 which is uncoated, Comparative Example 2 which is only coated with LMFP and Comparative Example 3 which is only coated with nano-oxide, Example 1 uses a composite coating of LMFP and nano-oxide. The modified cathode material P1 has a higher DSC exothermic peak temperature and a higher cycle capacity retention rate. On the one hand, the LFMP coating layer can improve the safety performance of the material and increase the DSC exothermic temperature. On the other hand, the nano-oxide coating layer can make up for the uneven coating of LMFP particles and improve the coating degree of the material. Through the synergistic effect of lithium manganese iron phosphate and nano-oxide composite coating layer, the material has higher safety performance and excellent cycle performance.

[0225] (2) Effect of coating sequence of lithium iron manganese phosphate and nano-oxide on the performance of modified positive electrode material

[0226] As can be seen from the data of Example 1 and Comparative Example 6, when the cathode material matrix is ​​first coated with nano-oxide and then coated with LMFP particles, the improvement effect on the safety performance and cycle performance of the material is poor. On the one hand, there is no bonding between the LMFP particles and the cathode material matrix, the bonding force is poor, and it cannot play a role, resulting in a poor improvement effect on the safety performance of the material. On the other hand, when the nano-oxide is coated first and then the LMFP particles are coated, the coating degree of the material is low, and the cycle performance of the material is poor.

[0227] (3) Effect of molecular self-assembly solution on the performance of modified positive electrode material

[0228] As can be seen from the data of Example 1 and Comparative Examples 4-5, the safety and cycle performance of the material are poor when LMFP particles are coated without adding a molecular self-assembly solution or adding a silane coupling agent. Since LMFP particles cannot form O1-Q-O2 bonds with the cathode material matrix, the bonding force between LMFP and the cathode material matrix is ​​poor, resulting in limited improvement on the safety and cycle performance of the material.

[0229] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A modified cathode material, characterized in that, The modified cathode material includes a cathode material matrix and a composite coating layer, and the composite coating layer contains lithium manganese iron phosphate and nano-oxide. The cathode material matrix and lithium manganese iron phosphate are connected by O1-Q-O2 bonds, and the oxygen atoms O1 and O2 at both ends are respectively connected to the transition metals in the cathode material matrix and lithium manganese iron phosphate, where Q is selected from nitrogen (N), sulfur (S), phosphorus (P), and boron (B).

2. The modified cathode material according to claim 1, wherein, The D50 particle size values ​​of the cathode material matrix, lithium manganese iron phosphate, and nano-oxide satisfy: 0.1≤a / [50(b+c) / 1000]≤7.7; where a is the D50 particle size of the cathode material matrix in micrometers; b and c are the D50 particle sizes of the lithium manganese iron phosphate and nano-oxide in nanometers, respectively. And / or, the D50 particle size a of the positive electrode material is 3-20 μm, preferably 3-15 μm; And / or, the D50 particle size b of the lithium manganese iron phosphate is 50-600 nm, preferably 50-300 nm; And / or, the D50 particle size c of the nano-oxide is 2-100 nm, preferably 5-50 nm.

3. The modified cathode material according to claim 1 or 2, wherein, The mass ratio of lithium manganese iron phosphate to the cathode material matrix is ​​0.1-30:100, preferably 1-20:100; And / or, the mass ratio of the nano-oxide to the cathode material matrix is ​​0.1-5:100, preferably 0.5-3:100; And / or, the cathode material matrix is ​​selected from at least one of ternary lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt oxide, lithium nickel manganese oxide, lithium manganese oxide, lithium cobalt oxide, lithium-rich manganese-based cathode materials, and the above-mentioned doped and modified cathode materials; And / or, the electronic conductivity of the lithium manganese iron phosphate is ≥1×10⁻⁶. -9 S / cm; And / or, the lithium manganese iron phosphate has the general formula shown in Formula I, Li α Mn x Fe 1-x-y M y PO4(I); wherein, 0.9≤α≤1.2, 0.2≤x<1, 0≤y≤0.1, and x+y<1, and M is selected from at least one of Mg, Al, Ti, Zr, Y, Sn, Nb and Zn; And / or, the surface of the lithium manganese iron phosphate also includes a carbon coating layer; And / or, the nano-oxide has the general formula shown in Formula II, A β B γ O δ (II); where 1≤β≤2, 0≤γ≤0.3, 1≤δ≤5, A is selected from at least one of Y, Al, Zr, Ti, Si, Mg and Sb, and B is selected from at least one of Zn, Sn, Mo, Nb and Ta.

4. The modified cathode material according to any one of claims 1-3, wherein, The O1-Q-O2 bond originates from a molecular self-assembly solvent containing R1 and R2 groups, wherein R1 is selected from hydroxyl and / or carboxyl groups; and R2 is selected from at least one of amino, boric acid, phosphoric acid, sulfonic acid, and thiol groups. Preferably, the molecular self-assembly solvent is selected from at least one of aminoethanol, 5-aminosalicylic acid, 2-carboxyethyl phosphate, phenylboronic acid, mercaptoethanol, 4-carboxyphenylboronic acid, p-aminobenzenesulfonic acid, and 3-mercaptopropionic acid. Preferably, the modified cathode material is prepared by solid-state sintering of a mixed slurry containing the cathode material matrix, lithium manganese iron phosphate, nano-oxide and molecular self-assembly solvent.

5. The modified cathode material according to any one of claims 1-4, wherein, The thickness of the composite coating layer is 50-800 nm, preferably 80-400 nm; And / or, the composite coating layer is a coating layer of lithium manganese iron phosphate and nano-oxide; And / or, in the composite coating layer, the nano-oxide exists in the form of a flocculent structure; And / or, the coating degree of the modified cathode material is ≥60%, preferably ≥80%.

6. A method for preparing a modified cathode material, characterized in that, The preparation method includes: S1. Lithium manganese iron phosphate, a molecular self-assembly solvent containing R1 and R2 groups, a cathode material matrix, and a first organic solvent are first dispersed and dried. S2. The nano-oxide sol and the second organic solvent are then dispersed in a second dispersion. S3. After the products of step S1 and step S2 are dispersed in the third step, they are heated and stirred until the liquid is completely evaporated, and solid-state sintering is performed to coat the surface of the cathode material matrix with lithium manganese iron phosphate and nano-oxide to obtain a modified cathode material; wherein, R1 is selected from hydroxyl and / or carboxyl groups; R2 is selected from at least one of amino, boric acid, phosphoric acid, sulfonic acid and thiol groups.

7. The preparation method according to claim 6, wherein, In step S1, The lithium manganese iron phosphate is added in the form of a slurry, with a solid content of 20-60 wt%, preferably 30-50 wt%. And / or, the mass ratio of lithium manganese iron phosphate to molecular self-assembly solvent is 0.1-30:100, preferably 0.1-5:100; And / or, the molecular self-assembly solvent is selected from at least one of aminoethanol, 5-aminosalicylic acid, 2-carboxyethyl phosphate, phenylboronic acid, mercaptoethanol, 4-carboxyphenylboronic acid, p-aminobenzenesulfonic acid, and 3-mercaptopropionic acid.

8. The preparation method according to claim 6 or 7, wherein, In step S2, The solid content of the nano-oxide sol is 3-20 wt%, preferably 3-10 wt%. And / or, the pH of the nano-oxide sol is 4-7 at 25±2℃; And / or, the nano-oxide sol is dispersed in at least one of ethanol, isopropanol, ethylene glycol, polyvinylpyrrolidone, and polyacrylic acid; the nano-oxide has the general formula shown in II, A β B γ O δ (II); where 1≤β≤2, 0≤γ≤0.3, 1≤δ≤5, A is selected from at least one of Y, Al, Zr, Ti, Si, Mg and Sb, and B is selected from at least one of Zn, Sn, Mo, Nb and Ta; And / or, the mass ratio of the nano-oxide sol and the second organic solvent is 1:1-100, preferably 1:20-60.

9. The preparation method according to any one of claims 6-8, wherein, In step S3, The heating temperature is 70-120℃; And / or, the solid-phase sintering is carried out in an oxygen-containing atmosphere; And / or, the solid-state sintering conditions include: a temperature of 200-800℃, preferably 300-800℃; a time of 1-20h, preferably 6-12h; and a heating rate of 1-10℃ / min, preferably 3-8℃ / min.

10. A positive electrode plate, characterized in that, The positive electrode sheet comprises: the modified positive electrode material according to any one of claims 1-5, or the modified positive electrode material prepared by the preparation method according to any one of claims 6-9; Preferably, the content of the modified positive electrode material in the positive electrode sheet is ≥70wt%, and more preferably 85-95wt%.

11. A lithium-ion battery, characterized in that, The lithium-ion battery includes: the positive electrode sheet as described in claim 10.

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