Positive electrode active material, preparation method therefor and use thereof

Abstract

The present application provides a positive electrode active material, a preparation method therefor and a use thereof. The positive electrode active material comprises a positive electrode base material, a transition layer, and a coating layer; the transition layer covers the surface of the positive electrode base material; the coating layer covers the surface of the transition layer; the surface smoothness of the positive electrode active material is greater than 95%; the positive electrode base material comprises a manganese element; the chemical formula of the coating layer is Lia1Alb1(PO4-δF2δ)c1Fd1; and the transition layer has a chemical composition of Lia2Nix1Coy1Mnz1MeFefAlb2Pc2Fd2Og. The positive electrode active material provided by the present application comprises a positive electrode base material, a transition layer, and a coating layer, and the surface smoothness of the positive electrode active material is defined. When the positive electrode active material is applied to a lithium-ion battery, the cycle performance of the lithium-ion battery can be improved.

Description

A positive electrode active material and a preparation method and application thereof

[0001] The present disclosure claims priority to the Chinese patent application No. 202411345274.4, filed on September 25, 2024, and entitled "A positive electrode active material and a preparation method and application thereof", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the technical field of lithium ion batteries, and particularly relates to a positive electrode active material and a preparation method and application thereof. BACKGROUND

[0003] With the rapid development of electric vehicles, portable electronic products, and artificial intelligence machines, the lithium battery market has higher requirements for the development of energy storage devices with high energy density. Generally, high specific capacity and high-voltage positive electrode active materials determine the energy density of lithium ion batteries, such as 5V-level spinel LiNi 0.5 Mn 1.5 O4(LNMO) can provide a specific capacity of 650Wh / Kg, thus improving the energy density of lithium ion batteries. However, during the cycle process, LNMO has poor stability and is prone to interface side reactions with electrolyte. In addition, the decomposition products of the electrolyte will cover the surface of the positive electrode to form a CEI film, increasing the impedance of the battery.

[0004] To solve the above problems, the surface coating method is usually used to modify the LNMO positive electrode active material. The conventional coating agents mainly include metal oxides (such as Al2O3, ZrO2, TiO2, etc.), fluorides (such as LiF, AlF3, NH4F, etc.), phosphates (such as FePO4, Li3PO4, etc.), polymers, fast ion conductors, etc. However, the traditional coating method mostly has the problems of uneven and non-dense coating, which cannot completely cover the surface of the positive electrode active material, resulting in that the coating layer is quickly damaged during the cycle process, further causing the transition metal Mn 2+ dissolution, causing continuous capacity decay, and leading to poor cycle stability of the battery. SUMMARY

[0005] The main purpose of the present application is to provide a positive electrode active material which is applied to a lithium ion battery and can improve the cycle performance of the lithium ion battery.

[0006] The present application also provides a preparation method of a positive electrode active material, which can prepare the above-mentioned positive electrode active material and has simple process and low cost.

[0007] The application further provides a positive electrode sheet, since the positive electrode sheet comprises the positive electrode active material, the positive electrode sheet is used in a lithium ion battery, and the cycle performance of the lithium ion battery can be improved.

[0008] The application further provides a lithium ion battery, since the lithium ion battery comprises the positive electrode sheet, the lithium ion battery has excellent cycle performance.

[0009] In a first aspect, the application provides a positive electrode active material, comprising a positive electrode base material, a transition layer and a coating layer; the transition layer is coated on the surface of the positive electrode base material, and the coating layer is coated on the surface of the transition layer.

[0010] The surface smoothness of the positive electrode active material is >95%;

[0011] The positive electrode base material comprises manganese elements;

[0012] The chemical formula of the coating layer is Li a1 Al b1 (PO 4-δ F 2δ ) c1 F d1 , wherein 0≤a1≤1.0, 0.001≤b1≤0.2, 0.01≤c1≤0.5, 0.01≤d1≤0.5, 0.01≤δ≤0.2, d1=a1+3b1-3c1;

[0013] The chemical formula of the transition layer is Li a2 Ni x1 Co y1 Mn z1 M e Fe f Al b2 P c2 F d2 O g , 0.95≤a2≤1.20, 0≤x1≤0.95, 0≤y1≤0.25, 0.05≤z1≤1.60, 0.001≤e≤0.02, 0≤f≤0.65, 1.5≤g≤4.5, 0.001≤b2≤0.1, 0.01≤c2≤1.2, 0.005≤d2≤0.1, M comprises at least one of Mg, Ca, Sc, Ti, V, Cr, Fe, Co, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Ta, W, B, Si, Ge, Sb, Te and S.

[0014] The positive electrode active material as described above, b2≤b1, d2≤d1;

[0015] c2>1, c2-1≤c1; c2<1, c2≤c1.

[0016] In the positive electrode active material described above, the coating layer accounts for 0.5-3.0% of the mass fraction of the positive electrode active material;

[0017] The transition layer accounts for 0.05-1.0% of the mass fraction of the positive electrode active material.

[0018] In the positive electrode active material described above, the thickness ratio of the coating layer to the transition layer is 1:0.01-0.5;

[0019] And / or, the thickness of the coating layer is 5-30 nm; the thickness of the transition layer is 0.5-10 nm.

[0020] The positive electrode active material as described above, wherein the positive electrode substrate material is a single crystal material, and the average particle size of the positive electrode substrate material is 0.2-20 μm;

[0021] Alternatively, the positive electrode substrate material is a polycrystalline material, and the average particle size of the positive electrode substrate material is 3-50 μm.

[0022] The positive electrode active material as described above, wherein the positive electrode matrix material includes LiNi 0.5 Mn 1.5-x M x O4, LiMn 2-x M x O4, LiMn 0.6-x Fe 0.4 M x PO4, Li(Ni) y Co z Mn 1-y-z ) 1-x M x At least one of O2; wherein 0.001≤x≤0.050, 0.50≤y≤0.095, 0.001≤z≤0.02, and M includes at least one of Mg, Ca, Sc, Ti, V, Cr, Fe, Co, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Ta, W, B, Si, Ge, Sb, Te, and S.

[0023] Secondly, this application provides a method for preparing the positive electrode active material as described above, comprising the following steps:

[0024] After nano-sizing of aluminum, phosphorus, and fluorine sources, they are mixed with the positive electrode matrix material to obtain a mixed system. The mixed system is then sintered at 700-1000℃ for 5-10 hours to obtain the positive electrode active material.

[0025] The preparation method described above, the mass ratio of the positive electrode base material to the aluminum source, the phosphorus source and the fluorine source is 1:0.01-0.5;

[0026] And / or, the mass ratio of the aluminum source, the phosphorus source and the fluorine source is 1:0.3-15:0.2-5.

[0027] In a third aspect, the present application provides a positive electrode sheet comprising the positive electrode active material described above or the positive electrode active material prepared by the preparation method described above.

[0028] In a fourth aspect, the present application provides a lithium ion battery comprising the positive electrode sheet described above.

[0029] The positive electrode active material provided by the present application comprises a positive electrode base material, a transition layer and a coating layer, and the surface smoothness of the positive electrode active material is limited, which realizes the complete and uniform coating of the coating layer on the positive electrode base material, inhibits the direct contact of the positive electrode base material with the electrolyte, reduces the occurrence of interface side reactions, enhances the structural stability and interface stability of the material, and when applied to a lithium ion battery, can improve the cycle performance of the lithium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the related art, the drawings needed in the description of the embodiments of the present application or the related art are briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0031] FIG. 1 is an SEM image of the positive electrode active material prepared in Example 1;

[0032] FIG. 2 is an SEM image of the positive electrode active material prepared in Example 3;

[0033] FIG. 3 is an SEM image of the positive electrode active material prepared in Example 9;

[0034] FIG. 4 is an SEM image of the positive electrode active material prepared in Example 11;

[0035] FIG. 5 is an SEM image of the positive electrode active material prepared in Comparative Example 1;

[0036] FIG. 6 is an SEM image of the positive electrode active material prepared in Comparative Example 2;

[0037] FIG. 7 is an SEM image of the positive electrode active material prepared in Comparative Example 3;

[0038] FIG. 8 is an SEM image of the positive electrode active material prepared in Comparative Example 4;

[0039] Figure 9 is an SEM image of the positive electrode active material prepared from Comparative Example 5;

[0040] Figure 10 is an SEM image of the positive electrode active material prepared from Comparative Example 11;

[0041] Figure 11 is an SEM image of the positive electrode active material prepared from Comparative Example 12. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without any creative work fall within the scope of protection of the present application.

[0043] In a first aspect, the present application provides a positive electrode active material, comprising a positive electrode base material, a transition layer and a coating layer; the transition layer is coated on the surface of the positive electrode base material, and the coating layer is coated on the surface of the transition layer;

[0044] The surface smoothness of the positive electrode active material is >95%;

[0045] The positive electrode base material comprises manganese element;

[0046] The chemical formula of the coating layer is Li a1 Al b1 (PO 4-δ F 2δ ) c1 F d1 , wherein 0≤a1≤1.0, 0.001≤b1≤0.2, 0.01≤c1≤0.5, 0.01≤d1≤0.5, 0.01≤δ≤0.2, d1=a1+3b1-3c1;

[0047] The chemical formula of the transition layer is Li a2 Ni x1 Co y1 Mn z1 M e Fe f Al b2 P c2 F d2 O g0.95 < a2 < 1.20, 0 < x1 < 0.95, 0 < y1 < 0.25, 0.05 < z1 < 1.60, 0.001 < e < 0.02, 0 < f < 0.65, 1.5 < g < 4.5, 0.001 < b2 < 0.1, 0.01 < c2 < 1.2, 0.005 < d2 < 0.1, M comprises at least one of Mg, Ca, Sc, Ti, V, Cr, Fe, Co, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Ta, W, B, Si, Ge, Sb, Te, S.

[0048] The positive electrode active material provided by the application comprises a positive electrode base material, a transition layer and a coating layer. The transition layer is formed by mutual diffusion and penetration of the positive electrode base material and the coating layer. Therefore, the transition layer is coated on the surface of the positive electrode base material, and the coating layer is coated on the surface of the transition layer, so that a higher surface coating of the positive electrode base material can be achieved. The surface smoothness of the positive electrode active material in the application is >95%, which indicates that the surface interface bonding degree of the coating layer and the positive electrode base material is high, and the positive electrode base material surface can be completely wrapped, so as to achieve uniform and dense coating effect.

[0049] From the chemical formula of the coating layer and the transition layer of the positive electrode active material in the application, it can be seen that the coating layer and the transition layer simultaneously comprise aluminum, phosphorus and fluorine. Therefore, the positive electrode active material simultaneously has the protection ability of the aluminum oxide inert layer, the conductive ability of the phosphate and the HF corrosion resistance of the fluoride. Not only can the contact of the positive electrode base material with the electrolyte be inhibited, the formation of the surface Mn 3+ can be reduced, and the occurrence of the interface side reaction can be reduced, but also the diffusion of lithium ions can be promoted. In addition, the fluorine element can combine with oxygen in the phosphate at high temperature, and the capacity reduction caused by the consumption of oxygen in the positive electrode base material can be avoided. Therefore, the structural stability and the interface stability of the positive electrode active material can be enhanced, and the cycle performance of the positive electrode active material can be effectively improved.

[0050] The application does not limit the test method of the surface smoothness. For example, the scanning electron microscope can be used to test at an acceleration voltage of 5kV, a magnification of 50K and a working distance of 8.1mm. The surface smoothness is the area of the non-point coating on the surface of the positive electrode active material / the field area at the magnification.

[0051] The positive electrode active material provided by the application comprises a positive electrode base material, a transition layer and a coating layer, and the surface smoothness of the positive electrode active material is limited, so that the coating layer completely and uniformly coats the positive electrode base material, direct contact between the positive electrode base material and the electrolyte is inhibited, and the agglomeration and segregation of the coating layer can also be avoided, thereby reducing the specific surface area of the positive electrode active material, reducing the contact area between the positive electrode active material and the electrolyte, reducing the occurrence of interface side reactions, enhancing the structural stability and interface stability of the material, and when applied to a lithium ion battery, the cycle performance of the lithium ion battery can be improved.

[0052] In some embodiments of the application, b2≤b1, d2≤d1;

[0053] c2>1, c2-1≤c1; c2<1, c2≤c1.

[0054] c2>1 is the case where the positive electrode base material comprises P elements, for example, the positive electrode base material is lithium manganese iron phosphate, and after removing the P elements in the positive electrode base material from the transition layer, the remaining P element content is less than or equal to the P element content in the coating layer, i.e. c2-1≤c1. c2<1 is the case where the positive electrode base material does not comprise P elements, and the P element content in the transition layer is less than or equal to the P element content in the coating layer, i.e. c2≤c1.

[0055] As can be seen from the above limitations, the three elements of aluminum, phosphorus and fluorine are incorporated into the surface of the positive electrode base material in the order of the coating layer and the transition layer, and the high-concentration doped elements on the surface can form a dense and uniform coating layer, improve the interface stability of the positive electrode active material, effectively inhibit the corrosion of HF in the electrolyte, and reduce the oxidative decomposition of the electrolyte and the dissolution of transition metal Mn 2+ , thereby improving the cycle performance of the battery.

[0056] In some embodiments of the application, the mass fraction of the coating layer in the positive electrode active material is 0.5-3.0%;

[0057] The mass fraction of the transition layer in the positive electrode active material is 0.05-1.0%.

[0058] The mass fraction of the coating layer in the positive electrode active material and the mass fraction of the transition layer in the positive electrode active material in the application are in a suitable range, so that the transition layer and the coating layer can completely coat the positive electrode base material, inhibit the contact between the positive electrode base material and the electrolyte, reduce the occurrence of interface side reactions, improve the structural stability and interface stability of the positive electrode active material, and further improve the cycle stability of the lithium ion battery. In addition, the positive electrode active material can fully exert its capacity performance, and the energy density of the lithium ion battery can be improved.

[0059] In some embodiments of the present application, the thickness ratio of the coating layer and the transition layer is 1:0.01-0.5;

[0060] And / or, the thickness of the coating layer is 5-30 nm; the thickness of the transition layer is 0.5-10 nm.

[0061] It can be understood that the thickness ratio of the coating layer and the transition layer, and the thickness of the coating layer and the transition layer will affect the electronic conductivity of the positive electrode active material to some extent, thereby affecting the cycle performance of the lithium ion battery. The coating layer and the transition layer in the present application have a suitable thickness and thickness ratio, which can take into account the conduction of electrons and ions.

[0062] In an embodiment, the thickness ratio of the coating layer and the transition layer is controlled to be 1:0.01-0.5, the thickness of the coating layer is 5-30 nm, and the thickness of the transition layer is 0.5-10 nm, which are in a suitable range. The suitable thickness and thickness ratio can inhibit the growth of particles, reduce the diffusion distance of lithium ions, in addition, the suitable thickness and thickness ratio have excellent electrical conductivity, which is beneficial to the transmission of electrons, improves the electronic conductivity of the positive electrode active material, and thus can improve the cycle performance of the battery.

[0063] The present application does not limit the test method of the thickness of the coating layer and the transition layer, for example, the thickness of the coating layer and the transition layer of the positive electrode active material in the transmission electron microscope can be measured.

[0064] In some embodiments of the present application, the positive electrode base material is a single crystal material, and the average particle size of the positive electrode base material is 0.2-20 μm;

[0065] Or, the positive electrode base material is a polycrystalline material, and the average particle size of the positive electrode base material is 3-50 μm.

[0066] When the positive electrode base material in the present application is a single crystal material, the average particle size of the positive electrode base material is controlled to be 0.2-20 μm; when the positive electrode base material in the present application is a polycrystalline material, the average particle size of the positive electrode base material is controlled to be 3-50 μm, which can shorten the diffusion path of lithium ions inside the positive electrode active material, thereby improving the diffusion rate of lithium ions, which is beneficial to improve the rate performance of the battery; and the appropriate average particle size can reduce the volume change of the positive electrode active material during charging and discharging, thereby improving the cycle stability of the lithium ion battery.

[0067] In some embodiments of the present application, the positive electrode base material comprises LiNi 0.5 Mn 1.5-x M x O4, LiMn 2-x M x O4, LiMn 0.6-x Fe0.4 M x PO4, Li(Ni y Co z Mn 1-y-z ) 1-x M x O2; wherein 0.001≤x≤0.050, 0.50≤y≤0.095, 0.001≤z≤0.02, M comprises at least one of Mg, Ca, Sc, Ti, V, Cr, Fe, Co, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Ta, W, B, Si, Ge, Sb, Te, S.

[0068] The doping element M in the nickel-manganese lithium phosphate, lithium manganese phosphate, lithium manganese iron phosphate and nickel-cobalt-manganese lithium phosphate in the positive electrode matrix material of the present application comprises at least one of Mg, Ca, Sc, Ti, V, Cr, Fe, Co, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Ta, W, B, Si, Ge, Sb, Te, S, and these transition metal elements have a strong dispersion effect on the positive electrode matrix material, which can reduce the mutual adhesion of primary grain phases, is conducive to subsequent coating treatment of the dispersed material particles, so that the coated positive electrode active material is applied to a lithium ion battery, and the cycle performance of the lithium ion battery can be improved.

[0069] In a second aspect, the present application provides a preparation method of the positive electrode active material as described above, comprising the following steps:

[0070] After nanocrystallization treatment of the aluminum source, the phosphorus source and the fluorine source, the positive electrode matrix material is mixed to obtain a mixed system, and the mixed system is subjected to sintering treatment at 700-1000℃ for 5-10h to obtain the positive electrode active material.

[0071] In the preparation method of the positive electrode active material of the present application, the aluminum source, the phosphorus source and the fluorine source can be first nano-treated by airflow milling or sand milling, then mixed with the positive electrode matrix material to obtain a mixed system, and the mixed system is subjected to sintering treatment at a temperature of 700-1000°C, preferably 750-950°C, for 1-15h, preferably 5-10h. The aluminum source, the phosphorus source and the fluorine source are coated more uniformly and are less likely to form agglomeration or segregation. If sintering is performed at a low temperature, for example, at a temperature lower than 700°C, the aluminum source, the phosphorus source and the fluorine source will not be completely melted, and there will be many fine particles distributed on the surface of the positive electrode matrix material, resulting in low smoothness of the surface of the positive electrode active material, increased specific surface area of the positive electrode active material, increased contact area between the positive electrode active material and the electrolyte, and thus poor surface stability of the positive electrode active material. In the present application, high-temperature sintering is adopted, and the sintering temperature is 700-1000°C. The aluminum source, the phosphorus source and the fluorine source can be completely melted, and the smoothness of the surface of the positive electrode active material is greater than 95%, thereby reducing the specific surface area of the positive electrode active material.

[0072] Both aluminum and phosphorus are elements with low melting points, and can form low-melting substances with fluorine at high temperature to form a complete and uniform coating layer on the surface of the positive electrode matrix material. At high temperature, the aluminum source, the phosphorus source and the fluorine source are completely melted and spread on the entire surface of the positive electrode matrix material, and are not present in the form of agglomeration or dot-shaped coating on the positive electrode matrix material, but are doped into the surface interface of the positive electrode matrix material in the form of a concentration gradient, so that a dense and uniform coating effect can be achieved. It is possible that because the ionic radius of aluminum and fluorine is similar to that of nickel and manganese, and the phosphate radical in the phosphate can form a strong covalent bond with nickel and manganese ions, the aluminum, phosphorus and fluorine elements penetrate into the surface of the positive electrode matrix material in the form of a concentration gradient at high temperature, and the high-concentration doped elements on the surface are sufficient to form a dense and uniform coating layer, improve the interface stability of the positive electrode active material, effectively hinder the corrosion of HF in the electrolyte, reduce the oxidation decomposition of the electrolyte and the dissolution of the transition metal Mn 2+ , and thus improve the cycle performance of the battery.

[0073] The aluminum source is mainly an aluminum-containing compound, for example, at least one of aluminum oxide, aluminum hydroxide, aluminum chloride, aluminum sulfate, aluminum nitrate, aluminum silicate, aluminum sulfide, sodium aluminate, aluminum titanate, aluminum stearate and alum, preferably at least one of aluminum oxide and aluminum hydroxide.

[0074] The phosphorus source is mainly at least one of a phosphorus-containing compound, such as phosphoric acid, lithium phosphate, lithium dihydrogen phosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, magnesium phosphate, calcium phosphate, iron phosphate, copper phosphate, zinc phosphate, titanium phosphate, zirconium phosphate, nickel phosphate, cobalt phosphate, manganese phosphate, pyrophosphoric acid, lithium pyrophosphate, sodium pyrophosphate, magnesium pyrophosphate, calcium pyrophosphate, iron pyrophosphate, copper pyrophosphate, zinc pyrophosphate, titanium pyrophosphate, zirconium pyrophosphate, nickel pyrophosphate, cobalt pyrophosphate, manganese pyrophosphate, elemental phosphorus, diphosphorus pentoxide, lithium iron phosphate, and phosphate ester, preferably at least one of lithium phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium pyrophosphate, nickel phosphate, and manganese phosphate.

[0075] The fluorine source is mainly at least one of a fluorine-containing compound, such as lithium fluoride, hydrogen fluoride, ammonium fluoride, sodium fluoride, copper fluoride, cerium fluoride, lanthanum fluoride, cobalt fluoride, manganese fluoride, nickel fluoride, and polytetrafluoroethylene. Preferably, at least one of lithium fluoride and ammonium fluoride.

[0076] The aluminum source, the phosphorus source, and the fluorine source can also be at least one of some Al-P-F compounds, such as aluminum fluoride, sodium hexafluoroaluminate, fluoroaluminate, aluminum phosphate, monohydrogen aluminum phosphate, dihydrogen aluminum phosphate, aluminum tripolyphosphate, and lithium difluorophosphate. Preferably, at least one of aluminum fluoride, aluminum phosphate, aluminum tripolyphosphate, and lithium difluorophosphate.

[0077] It should be noted that when the aluminum source, the phosphorus source, or the fluorine source does not contain lithium, the capacity of the prepared positive electrode active material will decrease slightly, because part of the phosphate ions will combine with lithium in the positive electrode matrix material to form a stable lithium phosphate phase, causing part of the lithium in the positive electrode matrix material to be consumed, resulting in loss of lithium during the charging and discharging process, and causing capacity reduction.

[0078] The preparation method provided in the present application can prepare the positive electrode active material of the first aspect, and the positive electrode active material is applied to a lithium ion battery, which can improve the cycle performance of the lithium ion battery.

[0079] In some embodiments of the present application, the mass ratio of the positive electrode matrix material to the total mass of the aluminum source, the phosphorus source, and the fluorine source is 1:0.01-0.5;

[0080] And / or, the mass ratio of the aluminum source, the phosphorus source, and the fluorine source is 1:0.3-15:0.2-5.

[0081] The application controls the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source and the fluorine source, and the mass ratio of the aluminum source, the phosphorus source and the fluorine source is in a suitable range, so that the coating layer and the transition layer with a suitable mass ratio can be obtained, and the coating layer and the transition layer have a suitable thickness, so that the transition layer and the coating layer can completely cover the positive electrode base material, the contact between the positive electrode base material and the electrolyte is inhibited, the occurrence of the interface side reaction is reduced, the structural stability and the interface stability of the positive electrode active material are improved, and then the cycle stability of the lithium ion battery is improved.

[0082] In a third aspect, the application provides a positive electrode sheet, which comprises the positive electrode active material as described above or the positive electrode active material prepared by the preparation method as described above.

[0083] The positive electrode sheet of the application can be prepared by using conventional technical means in the art. Specifically, the positive electrode active material, the conductive agent and the binder can be uniformly dispersed in a solvent to obtain a positive electrode active layer slurry, and then the positive electrode active layer slurry can be coated on at least one functional surface of the positive electrode current collector, and after drying, the positive electrode sheet of the application can be obtained.

[0084] The application does not particularly limit the specific types of the conductive agent and the binder, and the components such as the conductive agent and the binder can be selected from conventional substances in the art. For example, the conductive agent can be selected from one or more of conductive carbon black, carbon nanotube, conductive graphite and graphene, and the binder can be selected from one or more of polyvinylidene fluoride (PVDF), acrylic modified PVDF, polyacrylate polymer, polyimide, butadiene styrene rubber and styrene butadiene rubber.

[0085] The application does not particularly limit the coating method, and any one of gravure coating, extrusion coating, spraying and screen printing can be used to coat the positive electrode active layer slurry.

[0086] Since the positive electrode sheet of the application comprises the positive electrode active material as described above, the positive electrode sheet used in the lithium ion battery can improve the rate performance and the cycle performance of the lithium ion battery.

[0087] In a fourth aspect, the application provides a lithium ion battery, which comprises the positive electrode sheet as described above.

[0088] The lithium ion battery of the application comprises the positive electrode sheet, a separator, a negative electrode sheet and an electrolyte. The composition of the negative electrode sheet can refer to the conventional negative electrode sheet in the art, and the separator can also use the separator commonly used in the art, such as a PP film and a PE film.

[0089] The lithium ion battery of the application can be prepared by using conventional methods in the art. Specifically, the positive electrode sheet, the separator and the negative electrode sheet can be sequentially stacked, and then the stacked sheets or the wound sheets can be obtained by a stacking or winding process, and then the above-mentioned lithium ion battery can be obtained by the processes of baking, liquid injection, formation and packaging.

[0090] Since the lithium-ion battery provided in this application includes the above-mentioned positive electrode, the lithium-ion battery has excellent rate performance and cycle performance.

[0091] The technical solution of this application will be further described below with reference to specific embodiments.

[0092] Example 1

[0093] The preparation method of the positive electrode active material in this embodiment includes the following steps:

[0094] 1000g LiNi 0.5 Mn 1.495 Nb 0.004 O4, 2.5g AlF3, and 27.8g Li3PO4 (AlF3 and Li3PO4 were treated in a sand mill for 8 hours, dried, and then passed through a 10,000-mesh sieve) were thoroughly mixed and sintered at 750℃ for 8 hours. The mass ratio of the positive electrode matrix material to the total mass of the aluminum, phosphorus, and fluorine sources was 1:0.0303, and the mass ratio of the aluminum, phosphorus, and fluorine sources was 1:9.17:2.11. After crushing and sieving, lithium nickel manganese oxide positive electrode active material was obtained. The coating layer contains Li... 0.6 Al 0.02 (PO 3.95 F 0.1 ) 0.2 F 0.06 The chemical composition of the transition layer is Li 1.046 Ni 0.48 Mn 1.44 Nb 0.004 Al 0.01 P 0.04 F 0.02 O 3.99 The chemical composition of the cathode active material is as follows: the molar amounts of aluminum, phosphorus, and fluorine in the transition layer are all less than those in the coating layer. The coating layer accounts for 2.51% of the mass fraction of the positive electrode active material, while the transition layer accounts for 0.22%. The coating layer has a thickness of 18 nm, the transition layer has a thickness of 2.5 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.14. The positive electrode substrate material is a single crystal material with a particle size of 8 μm.

[0095] Example 2

[0096] The preparation method of the positive electrode active material in this embodiment includes the following steps:

[0097] 1000g Li 1.03 Ni 0.5 Mn 1.48 Nb 0.01O4, 2.8g LiF, 10.4g Al2O3, 18g Li3PO4 (Al2O3, Li3PO4 are treated by jet mill for 3h and passed through 10000 mesh screen) are mixed thoroughly and sintered at 750℃ for 8h, wherein the mass ratio of the positive electrode matrix material to the total mass of aluminum source, phosphorus source and fluorine source is 1:0.0312, and the mass ratio of aluminum source, phosphorus source and fluorine source is 1:0.87:0.37, and the nickel-manganese acid lithium positive electrode active material is obtained after crushing and screening treatment. The coating layer has a chemical composition of Li 0.2 Al 0.12 (PO 3.98 F 0.04 ) 0.1 F 0.26 , and the transition layer has a chemical formula of Li 1.038 Ni 0.44 Mn 1.423 Nb 0.01 Al 0.08 P 0.02 F 0.02 O 3.99 , wherein the molar amount of aluminum, phosphorus and fluorine in the transition layer is less than that in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 1.90%, and the mass fraction of the transition layer in the positive electrode active material is 0.32%. The thickness of the coating layer is 20nm, and the thickness of the transition layer is 4nm. The thickness ratio of the coating layer to the transition layer is 1:0.2. The positive electrode matrix material is a single crystal material, and the particle size of the positive electrode matrix material is 6μm.

[0098] Example 3

[0099] The preparation method of the positive electrode active material of the present embodiment comprises the following steps:

[0100] 1000g Li 1.06 Ni 0.48 Mn 1.485 Nb 0.008 O4, 10.2g AlF3, 18.5g LiH2PO4 (AlF3, LiH2PO4 are treated by sand mill for 8h and then dried and passed through 10000 mesh screen) are mixed thoroughly and sintered at 750℃ for 8h, wherein the mass ratio of the positive electrode matrix material to the total mass of aluminum source, phosphorus source and fluorine source is 1:0.0287, and the mass ratio of aluminum source, phosphorus source and fluorine source is 1:1.68:2.11, and the nickel-manganese acid lithium positive electrode active material is obtained after crushing and screening treatment. The coating layer has a chemical composition of Li 0.1 Al 0.09 (PO 3.90 F 0.2 ) 0.08 F 0.13 , and the transition layer has a chemical formula of Li 1.07 Ni 0.425 Mn1.47 Nb 0.008 Al 0.03 P 0.05 F 0.08 O 3.96 , wherein the molar amount of aluminum, phosphorus, and fluorine in the transition layer is less than that in the coating layer. The coating layer accounts for 1.33% of the mass fraction of the positive electrode active material; the transition layer accounts for 0.39% of the mass fraction of the positive electrode active material. The thickness of the coating layer is 19.5 nm, the thickness of the transition layer is 4 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.205. The positive electrode base material is a polycrystalline material, and the particle size of the positive electrode base material is 12 μm.

[0101] Example 4

[0102] The preparation method of the positive electrode active material of the present example comprises the following steps:

[0103] 1000 g of Li 1.07 Ni 0.465 Mn 1.48 Nb 0.016 O4, 10 g of LiPO2F2, and 10 g of Al2O3 (both LiPO2F2 and Al2O3 are treated by airflow milling for 3 h and then passed through a 10,000-mesh screen) are thoroughly mixed and then sintered at 750°C for 8 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source is 1:0.02, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source is 1:0.54:0.67. After crushing and sieving, the lithium nickel-manganese acid positive electrode active material is obtained. The coating layer has a chemical composition of Li 0.08 Al 0.11 (PO 3.93 F 0.14 ) 0.05 F 0.26 , and the transition layer has a chemical formula of Li 1.074 Ni 0.44 Mn 1.394 Nb 0.016 Al 0.08 P 0.03 F 0.06 O 3.97 , wherein the molar amount of aluminum, phosphorus, and fluorine in the transition layer is less than that in the coating layer. The coating layer accounts for 1.33% of the mass fraction of the positive electrode active material; the transition layer accounts for 0.39% of the mass fraction of the positive electrode active material. The thickness of the coating layer is 19.5 nm, the thickness of the transition layer is 4 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.205. The positive electrode base material is a polycrystalline material, and the particle size of the positive electrode base material is 12 μm.

[0104] Example 5

[0105] The preparation method of the positive electrode active material of the present example comprises the following steps:

[0106] 1000 g of LiNi 0.5 Mn 1.497 Nb 0.004 O4, 2.5 g of AlF3, and 27.8 g of Li3PO4 (AlF3and Li3PO4were dried after being treated by a sand mill for 8 h and then sieved through a 10,000-mesh screen) were thoroughly mixed and sintered at 850 °C for 8 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source was 1:0.0303, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source was 1:9.17:2.11, to obtain a lithium nickel manganese acid positive electrode active material after crushing and sieving. The coating layer has a chemical composition of Li 0.4 Al 0.018 (PO 3.95 F 0.1 ) 0.15 F 0.04 , the transition layer has a chemical composition of Li 1.048 Ni 0.441 Mn 1.435 Nb 0.004 Al 0.02 P 0.05 F 0.02 O 3.99 , and the molar amounts of aluminum, phosphorus, and fluorine in the transition layer are all less than those in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 1.84%, and the mass fraction of the transition layer in the positive electrode active material is 0.28%. The thickness of the coating layer is 17.8 nm, the thickness of the transition layer is 3.2 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.18. The positive electrode base material is a single crystal material, and the particle size of the positive electrode base material is 8 μm.

[0107] Example 6

[0108] The preparation method of the positive electrode active material of the present example includes the following steps:

[0109] 1000 g of LiNi 0.5 Mn 1.497 Nb 0.004 O4, 2.5 g of AlF3, and 27.8 g of Li3PO4 (AlF3and Li3PO4were dried after being treated by a sand mill for 8 h and then sieved through a 10,000-mesh screen) were thoroughly mixed and sintered at 950 °C for 8 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source was 1:0.0303, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source was 1:9.17:2.11, to obtain a lithium nickel manganese acid positive electrode active material after crushing and sieving. The coating layer has a chemical composition of Li 0.16 Al 0.008 (PO 3.90 F 0.2 )0.06 F 0.004， The chemical composition of the transition layer is Li. 1.2 Ni 0.38 Mn 1.34 Nb 0.004 Al 0.06 P 0.08 F 0.06 O 3.97 The molar amounts of aluminum, phosphorus, and fluorine in the transition layer are all less than those in the coating layer. The coating layer accounts for 0.72% of the mass fraction of the positive electrode active material, while the transition layer accounts for 0.95%. The coating layer has a thickness of 11.6 nm, and the transition layer has a thickness of 13.8 nm, with a thickness ratio of 1:1.19. The positive electrode substrate material is a single-crystal material with a particle size of 8 μm.

[0110] Example 7

[0111] The preparation method of the positive electrode active material in this embodiment includes the following steps:

[0112] 1000g LiNi 0.5 Mn 1.497 Nb 0.004 O4, 2.5g AlF3, and 27.8g Li3PO4 (AlF3 and Li3PO4 were treated in a sand mill for 8 hours, dried, and then passed through a 10,000-mesh sieve) were thoroughly mixed and sintered at 750℃ for 6 hours. The mass ratio of the positive electrode matrix material to the total mass of the aluminum, phosphorus, and fluorine sources was 1:0.0303, and the mass ratio of the aluminum, phosphorus, and fluorine sources was 1:9.17:2.11. After crushing and sieving, lithium nickel manganese oxide positive electrode active material was obtained. The coating layer contains Li... 0.62 Al 0.025 (PO 3.96 F 0.08 ) 0.22 F 0.03 The chemical composition of the transition layer is Li. 1.034 Ni 0.49 Mn 1.448 Nb 0.004 Al 0.008 P 0.03 F 0.01 O 3.995 In this process, the molar amounts of aluminum, phosphorus, and fluorine in the transition layer are all less than those in the coating layer. The coating layer accounts for 2.67% of the mass fraction of the positive electrode active material, while the transition layer accounts for 0.16%. The coating layer has a thickness of 18.6 nm, the transition layer has a thickness of 2.5 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.13. The positive electrode substrate material is a single-crystal material with a particle size of 8 μm.

[0113] Example 8

[0114] The preparation method of the positive electrode active material of the present example comprises the following steps:

[0115] 1000g of LiNi0.5Mn1.5O4, 2.5g of AlF3, 27.8g of Li3PO4(the AlF3and Li3PO4are treated by a sand mill for 8h, then dried and sieved through a 10000 mesh screen) are mixed thoroughly and sintered at 750℃ for 10h, wherein the mass ratio of the positive electrode matrix material to the total mass of the aluminum source, phosphorus source and fluorine source is 1:0.0303, and the mass ratio of the aluminum source, phosphorus source and fluorine source is 1:9.17:2.11, and the lithium nickel manganese acid positive electrode active material is obtained after crushing and sieving. 0.5 Mn 1.497 Nb 0.004 O4, 2.5g of AlF3, 27.8g of Li3PO4(the AlF3and Li3PO4are treated by a sand mill for 8h, then dried and sieved through a 10000 mesh screen) are mixed thoroughly and sintered at 750℃ for 10h, wherein the mass ratio of the positive electrode matrix material to the total mass of the aluminum source, phosphorus source and fluorine source is 1:0.0303, and the mass ratio of the aluminum source, phosphorus source and fluorine source is 1:9.17:2.11, and the lithium nickel manganese acid positive electrode active material is obtained after crushing and sieving. 0.56 Al 0.018 (PO 3.94 F 0.12 ) 0.2 F 0.014 of the chemical composition, and the chemical formula of the transition layer is LiNi0.5Mn1.5O2. The molar amount of aluminum, phosphorus and fluorine in the transition layer is less than that in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 2.39%, and the mass fraction of the transition layer in the positive electrode active material is 0.29%. The thickness of the coating layer is 17.5nm, and the thickness of the transition layer is 3.3nm, and the thickness ratio of the coating layer to the transition layer is 1:0.19. The positive electrode matrix material is a single crystal material, and the particle size of the positive electrode matrix material is 8μm. 1.05 Ni 0.476 Mn 1.4255 Nb 0.004 Al 0.012 P 0.048 F 0.04 O 3.98

[0116] Example 9

[0117] The preparation method of the positive electrode active material of the present example comprises the following steps:

[0118] 1000g of Li 1.01 Ni 0.49 Mn 1.495 Ta 0.006 ​O4, 5.3g AlF3, 30.6g Li3PO4 (AlF3, Li3PO4 is treated by sand mill for 8h and then dried and sieved through 10000 mesh screen) are mixed thoroughly and sintered at 750°C for 8h, wherein the mass ratio of the positive electrode matrix material to the total mass of the aluminum source, phosphorus source and fluorine source is 1:0.0359, and the mass ratio of the aluminum source, phosphorus source and fluorine source is 1:4.81:2.11, and the lithium nickel manganese acid positive electrode active material is obtained after crushing and sieving treatment. The coating layer has a chemical composition of Li 0.62 Al 0.05 (PO 3.96 F 0.08 ) 0.22 F 0.11 , and the transition layer has a chemical formula of Li 1.03 Ni 0.48 Mn 1.425 Ta 0.006 Al 0.01 P 0.05 F 0.06 O 3.97 , wherein the molar amount of aluminum, phosphorus and fluorine in the transition layer is less than that in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 2.88%, and the mass fraction of the transition layer in the positive electrode active material is 0.31%. The thickness of the coating layer is 20.5nm, and the thickness of the transition layer is 3.1nm, and the thickness ratio of the coating layer to the transition layer is 1:0.15. The positive electrode matrix material is a single crystal material, and the particle size of the positive electrode matrix material is 6μm.

[0119] Example 10

[0120] The preparation method of the positive electrode active material of the present embodiment comprises the following steps:

[0121] 1000g Li 1.03 Ni 0.486 Mn 1.477 Ta 0.018 O4, 2.8g LiF, 15g AlH2P3O 10 (LiF, AlH2P3O 10 are treated by air flow mill for 3h and then sieved through 10000 mesh screen) are mixed thoroughly and sintered at 750°C for 8h, wherein the mass ratio of the positive electrode matrix material to the total mass of the aluminum source, phosphorus source and fluorine source is 1:0.0178, and the mass ratio of the aluminum source, phosphorus source and fluorine source is 1:3.44:1.43, and the lithium nickel manganese acid positive electrode active material is obtained after crushing and sieving treatment. The coating layer has a chemical composition of Li 0.15 Al 0.04 (PO 3.97 F 0.06 ) 0.08 F 0.03 , and the transition layer has a chemical formula of Li 1.038Ni 0.48 Mn 1.402 Ta 0.018 Al 0.018 P 0.05 F 0.02 O 3.99 , wherein the molar amount of aluminum, phosphorus, and fluorine in the transition layer is less than that in the coating layer. The coating layer accounts for 1.03% of the mass fraction of the positive electrode active material; the transition layer accounts for 0.25% of the mass fraction of the positive electrode active material. The thickness of the coating layer is 10.2 nm, the thickness of the transition layer is 0.92 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.088. The positive electrode base material is a polycrystalline material, and the particle size of the positive electrode base material is 26 μm.

[0122] Example 11

[0123] The preparation method of the positive electrode active material of the present example comprises the following steps:

[0124] 1000 g of Li 1.02 Ni 0.48 Mn 1.49 Mo 0.01 W 0.01 O4, 6.8 g of AlF3, and 22.5 g of Li3PO4 (after sand mill treatment for 8 h, the AlF3 and Li3PO4 are dried and then sieved through a 10,000 mesh screen) are thoroughly mixed and then sintered at 750°C for 8 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source is 1:0.0293, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source is 1:2.76:2.12. After crushing and sieving, a lithium nickel-manganese acid positive electrode active material is obtained. The coating layer has a chemical composition of Li 0.5 Al 0.05 (PO 3.95 F 0.1 ) 0.15 F 0.2 , and the transition layer has a chemical formula of Li 1.05 Ni 0.465 Mn 1.4 Mo 0.01 W 0.01 Al 0.02 P 0.06 F 0.04 O 3.98 , wherein the molar amount of aluminum, phosphorus, and fluorine in the transition layer is less than that in the coating layer. The coating layer accounts for 1.03% of the mass fraction of the positive electrode active material; the transition layer accounts for 0.25% of the mass fraction of the positive electrode active material. The thickness of the coating layer is 10.2 nm, the thickness of the transition layer is 0.92 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.088. The positive electrode base material is a polycrystalline material, and the particle size of the positive electrode base material is 26 μm.

[0125] Example 12

[0126] The preparation method of the positive electrode active material of the embodiment comprises the following steps:

[0127] 1000g of LiNi0.5Mn1.5O4, 4.2g of NH4F, and 20.8g of AlPO4 are mixed thoroughly and sintered at 750°C for 8h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source is 1:0.025, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source is 1:1.15:0.58, and the lithium nickel manganese acid positive electrode active material is obtained after crushing and sieving treatment. 0.98 Ni 0.49 Mn 1.507 Mo 0.003 W 0.001 O4, 4.2g of NH4F, and 20.8g of AlPO4 are mixed thoroughly and sintered at 750°C for 8h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source is 1:0.025, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source is 1:1.15:0.58, and the lithium nickel manganese acid positive electrode active material is obtained after crushing and sieving treatment. 0.12 (PO 3.96 F 0.08 ) 0.1 F 0.06 The chemical composition of the coating layer is Li 0.98 Ni 0.486 Mn 1.4265 Mo 0.003 W 0.001 Al 0.05 P 0.06 F 0.03 O 3.985 , and the chemical formula of the transition layer is Li 0.5 Mn 1.4925 Mo 0.002 W 0.008 The molar amount of aluminum, phosphorus, and fluorine in the transition layer is less than that in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 1.4%, and the mass fraction of the transition layer in the positive electrode active material is 0.38%. The thickness of the coating layer is 16.2nm, and the thickness of the transition layer is 1.9nm. The thickness ratio of the coating layer to the transition layer is 1:0.12. The positive electrode base material is a single crystal material, and the particle size of the positive electrode base material is 8μm.

[0128] Example 13

[0129] The preparation method of the positive electrode active material of the embodiment comprises the following steps:

[0130] 1000g of LiNi0.5Mn1.5O4, 4.2g of NH4F, and 20.8g of AlPO4 are mixed thoroughly and sintered at 750°C for 8h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source is 1:0.025, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source is 1:1.15:0.58, and the lithium nickel manganese acid positive electrode active material is obtained after crushing and sieving treatment. 0.5 Mn 1.4925 Mo 0.002 W 0.008O4, 6.5g AlF3, 22.6g NH4H2PO4 (AlF3, NH4H2PO4 were treated by sand mill for 8h, then dried and sieved by 10000 mesh screen) were mixed thoroughly and sintered at 750℃ for 8h, wherein the mass ratio of the positive electrode matrix material to the total mass of the aluminum source, phosphorus source and fluorine source was 1:0.0291, and the mass ratio of the aluminum source, phosphorus source and fluorine source was 1:2.91:2.11, and the lithium nickel manganese acid positive electrode active material was obtained after crushing and sieving treatment. The coating layer has the chemical composition of Li 0.08 (PO 3.98 F 0.04 ) 0.06 F 0.06 , and the transition layer has the chemical formula of LiNi 0.492 Mn 1.419 Mo 0.002 W 0.008 Al 0.02 P 0.05 F 0.04 O 3.98 , wherein the molar amount of aluminum, phosphorus and fluorine in the transition layer is less than that in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 0.90%, and the mass fraction of the transition layer in the positive electrode active material is 0.28%. The thickness of the coating layer is 18.5nm, and the thickness of the transition layer is 3.3nm. The thickness ratio of the coating layer to the transition layer is 1:0.18. The positive electrode matrix material is a polycrystalline material, and the particle size of the positive electrode matrix material is 8μm.

[0131] Example 14

[0132] The preparation method of the positive electrode active material of the present embodiment comprises the following steps:

[0133] 1000g LiNi 0.8 Co 0.1 Mn 0.1 O2, 2.5g AlF3, 27.8g Li3PO4 (AlF3, Li3PO4 were treated by sand mill for 8h, then dried and sieved by 10000 mesh screen) were mixed thoroughly and sintered at 750℃ for 8h, wherein the mass ratio of the positive electrode matrix material to the total mass of the aluminum source, phosphorus source and fluorine source was 1:0.0303, and the mass ratio of the aluminum source, phosphorus source and fluorine source was 1:9.17:2.11, and the lithium nickel manganese acid positive electrode active material was obtained after crushing and sieving treatment. The coating layer has the chemical formula of Li 0.6 Al 0.02 (PO 3.95 F 0.1 ) 0.2 F 0.06 , and the transition layer has the chemical formula of Li 1.02 Ni 0.8 Co 0.1 Mn 0.06Al 0.01 P 0.022 F 0.04 O 1.98 , wherein the molar amount of aluminum, phosphorus, and fluorine in the transition layer is less than that in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 2.51%; the mass fraction of the transition layer in the positive electrode active material is 0.18%. The thickness of the coating layer is 16.5 nm, the thickness of the transition layer is 2.5 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.15. The positive electrode base material is a polycrystalline material, and the particle size of the positive electrode base material is 15 μm.

[0134] Example 15

[0135] The preparation method of the positive electrode active material of the present example includes the following steps:

[0136] 1000 g of LiFe 0.4 Mn 0.6 PO4, 2.5 g of AlF3, and 27.8 g of Li3PO4 (AlF3 and Li3PO4 are dried after being treated by a sand mill for 8 h and then sieved through a 10,000-mesh screen) are thoroughly mixed and sintered at 750°C for 8 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source is 1:0.0303, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source is 1:9.17:2.11, and the lithium nickel-manganese acid positive electrode active material is obtained after crushing and sieving. The chemical formula of the coating layer is Li 0.6 Al 0.02 (PO 3.95 F 0.1 ) 0.2 F 0.06 , and the chemical formula of the transition layer is Li 1.03 Fe 0.4 Mn 0.42 Al 0.01 F 0.06 P 1.06 O 3.97 , wherein the molar amount of aluminum and fluorine in the transition layer is less than that in the coating layer, and the molar amount of phosphorus in the transition layer minus the molar amount of phosphorus in the coating layer is less than 1. The mass fraction of the coating layer in the positive electrode active material is 2.51%; the mass fraction of the transition layer in the positive electrode active material is 0.35%. The thickness of the coating layer is 18.8 nm, the thickness of the transition layer is 4 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.21. The positive electrode base material is a single-crystal material, and the particle size of the positive electrode base material is 6 μm.

[0137] Example 16

[0138] The preparation method of the positive electrode active material of the present example includes the following steps:

[0139] 1000 g of LiMn2O4, 2.5 g of AlF3, 27.8 g of Li3PO4 (AlF3, Li3PO4 are dried after sand mill treatment for 8 h and then passed through a 10000 mesh screen) are mixed thoroughly and sintered at 750°C for 8 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, phosphorus source and fluorine source is 1:0.0303, and the mass ratio of the aluminum source, phosphorus source and fluorine source is 1:9.17:2.11, and the lithium nickel manganese oxide positive electrode active material is obtained after crushing and sieving treatment. The chemical formula of the coating layer is Li 0.6 Al 0.02 (PO 3.95 F 0.1 ) 0.2 F 0.06 , and the chemical formula of the transition layer is Li 1.05 Mn 1.863 Al 0.01 P 0.08 F 0.04 O 3.98 , wherein the molar amount of aluminum, phosphorus and fluorine in the transition layer is less than that in the coating layer. The mass fraction of the coating layer in the positive electrode active material is 2.51%; the mass fraction of the transition layer in the positive electrode active material is 0.39%. The thickness of the coating layer is 26.3 nm, the thickness of the transition layer is 8.5 nm, and the thickness ratio of the coating layer to the transition layer is 1:0.32. The positive electrode base material is a polycrystalline material, and the particle size of the positive electrode base material is 3 μm.

[0140] Comparative Example 1

[0141] Comparative Example 1 is a lithium nickel manganese oxide positive electrode active material with a chemical composition of LiNi 0.5 Mn 1.5 O4, without metal ion doping and surface coating.

[0142] Comparative Example 2

[0143] Comparative Example 2 is a lithium nickel manganese oxide positive electrode active material with a chemical composition of LiNi 0.5 Mn 1.947 Nb 0.004 O4, without surface coating.

[0144] Comparative Example 3

[0145] Comparative Example 3 is a lithium nickel manganese oxide positive electrode active material with a chemical composition of Li 1.01 Ni 0.49 Mn 1.495 Ta 0.006 O4, without surface coating.

[0146] Comparative Example 4

[0147] Comparative Example 4 is a lithium nickel manganese oxide positive electrode active material with a chemical composition of Li 1.02 Ni0.48 Mn 1.49 Mo 0.01 W 0.01 LiNi0.5Mn1.5O4 without surface coating.

[0148] Comparative Example 5

[0149] The preparation method of the positive electrode active material of Comparative Example 5 includes the following steps:

[0150] 1000 g of LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving. 0.5 Mn 1.947 Nb 0.004 LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving.

[0151] Comparative Example 6

[0152] The preparation method of the positive electrode active material of Comparative Example 6 includes the following steps:

[0153] 1000 g of LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving. 0.5 Mn 1.947 Nb 0.004 LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving.

[0154] Comparative Example 7

[0155] The preparation method of the positive electrode active material of Comparative Example 7 includes the following steps:

[0156] 1000 g of LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving. 0.5 Mn 1.947 Nb 0.004 LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving.

[0157] Comparative Example 8

[0158] The preparation method of the positive electrode active material of Comparative Example 8 includes the following steps:

[0159] 1000 g of LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving. 1.01 Ni 0.49 Mn 1.495 Ta 0.006 LiNi0.5Mn1.5O4, 10.4 g of Al2O3, and 18 g of Li3PO4 were sufficiently mixed and sintered at 750°C for 8 h, and a nickel manganese acid lithium positive electrode active material was obtained after crushing and sieving.

[0160] Comparative Example 9

[0161] The preparation method of the positive electrode active material of Comparative Example 9 includes the following steps:

[0162] 1000 g of Li 1.01 Ni 0.49 Mn 1.495 Ta 0.006 O4, 2.8 g of LiF, and 10.4 g of Al2O3 were mixed well and sintered at 750°C for 8 h, and a lithium nickel manganese acid positive electrode active material was obtained after crushing and sieving.

[0163] Comparative Example 10

[0164] The preparation method of the positive electrode active material of Comparative Example 10 includes the following steps:

[0165] 1000 g of Li 1.01 Ni 0.49 Mn 1.495 Ta 0.006 O4, and 6 g of LiPO2F2 were mixed well and sintered at 750°C for 8 h, and a lithium nickel manganese acid positive electrode active material was obtained after crushing and sieving.

[0166] Comparative Example 11

[0167] The preparation method of the positive electrode active material of Comparative Example 11 includes the following steps:

[0168] 1000 g of Li 1.02 Ni 0.48 Mn 1.49 Mo 0.01 W 0.01 O4, 2.5 g of AlF3, and 27.8 g of Li3PO4 were mixed well and sintered at 500°C for 10 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source, and the fluorine source was 1:0.0303, and the mass ratio of the aluminum source, the phosphorus source, and the fluorine source was 1:0.87:0.37, and a lithium nickel manganese acid positive electrode active material was obtained after crushing and sieving.

[0169] Comparative Example 12

[0170] The preparation method of the positive electrode active material of Comparative Example 12 includes the following steps:

[0171] 1000 g of Li 1.06 Ni 0.48 Mn 1.485 Nb 0.008O4, 10.2 g of AlF3, and 18.5 g of LiH2PO4 were mixed thoroughly and sintered at 500°C for 10 h, wherein the mass ratio of the positive electrode base material to the total mass of the aluminum source, phosphorus source, and fluorine source was 1:0.287, and the mass ratio of the aluminum source, phosphorus source, and fluorine source was 1:1.68:2.11, and the lithium nickel manganese oxide positive electrode active material was obtained after crushing and sieving.

[0172] Comparative Example 13

[0173] The preparation method of the positive electrode active material of Comparative Example 13 comprises the following steps:

[0174] 1.39 g of Al (NO3) 3·9H2O was weighed and dissolved in 100 mL of deionized water to prepare an aluminum nitrate solution, and 10.0 g of LiNi 0.8 Co 0.1 Mn 0.1 O2 was added to the aluminum nitrate solution and stirred and dispersed to prepare a primary mixed solution; 0.31 g of NH4F and 0.12 g of (NH4) 2HPO4 were weighed and dissolved in 20 mL of deionized water to prepare a mixed solution of fluoride and phosphate; the prepared fluoride solution and phosphate solution were added dropwise to the primary mixed solution under continuous stirring, and after reaction for 2 h, standing, washing, filtering, and drying, sintering was performed at 400°C for 5 h under an inert atmosphere to obtain a composite positive electrode active material coated with an AlF3 / AlPO4 composite coating layer.

[0175] Comparative Example 14

[0176] The preparation method of the positive electrode active material of Comparative Example 14 comprises the following steps:

[0177] 1) 1 kg of LiNi 0.8 Co 0.1 Mn 0.1 O2 was placed in a high-speed mixer, and nitrogen protection (pressure 0.2 MPa) was started, and the stirring speed was 1200 rpm; 80 g of (Al2O3) 0.6 (TiO2) 0.4 The alcohol phase sol (solid content 10 wt%) was added in the form of a spray, and uniform stirring was performed at a low speed; heating was performed to 80°C, and drying was performed for 2 h;

[0178] 2) low-speed stirring was performed under an oxygen atmosphere (oxygen concentration 20%), and the temperature was gradually increased to 350°C, and the temperature was maintained for 3 h, and then the temperature was decreased;

[0179] 3) when the temperature was decreased to 30°C, high-speed stirring was performed under a nitrogen atmosphere, 27.38 g of lithium difluorophosphate was added, and stirring was continuously performed for 90 min to obtain a positive electrode active material. The stoichiometric formula of the positive electrode active material was LiNi 0.8 Co 0.1 Mn0.1 O2·0.015((Al2O3) 0.6 (TiO2) 0.4 )·0.02LiPO2F2.

[0180] Comparative Example 15

[0181] The preparation method of the positive electrode active material of Comparative Example 15 includes the following steps:

[0182] 1) Lithium difluorophosphate (average particle size: 1100 nm), lithium phosphate (average particle size: 130 nm), LiNi0.5Mn1.5O2, LiOH, NH4H2PO4, NH4F, and Al2O3 were weighed in a molar ratio of 1.0:1.1:1.0:1.0:0.5, and ball-milled for 2 h at a speed of 200 rpm to fully ball-mix them uniformly; then the finely mixed powder was placed in a tube furnace and sintered at 600°C for 3 h under a high-purity oxygen gas stream of 99.999%, with the heating rate set at 5°C / min, and naturally cooled; a lithium aluminum phosphate fluoride-coated ternary NCM811 positive electrode active material LAPF@NCM811 was obtained. 0.8 Co 0.1 Mn 0.1 O2(D50 is about 12 pm) were mixed uniformly in a mixer in a mass ratio of 0.5:0.2:100, and then heat-treated at 350°C under an oxygen atmosphere for 12 h to obtain a positive electrode active material.

[0183] Comparative Example 16

[0184] The preparation method of the positive electrode active material of Comparative Example 16 includes the following steps:

[0185] 1) LiNi0.5Mn1.5O2, LiOH, NH4H2PO4, NH4F, and Al2O3 were weighed in a molar ratio of 1.0:1.1:1.0:1.0:0.5, and ball-milled for 2 h at a speed of 200 rpm to fully ball-mix them uniformly; then the finely mixed powder was placed in a tube furnace and sintered at 600°C for 3 h under a high-purity oxygen gas stream of 99.999%, with the heating rate set at 5°C / min, and naturally cooled; a lithium aluminum phosphate fluoride-coated ternary NCM811 positive electrode active material LAPF@NCM811 was obtained. 0.8 Co 0.1 Mn 0.1 O2, LiOH, NH4H2PO4, NH4F, and Al2O3 were weighed in a molar ratio of 1.0:1.1:1.0:1.0:0.5, and ball-milled for 2 h at a speed of 200 rpm to fully ball-mix them uniformly; then the finely mixed powder was placed in a tube furnace and sintered at 600°C for 3 h under a high-purity oxygen gas stream of 99.999%, with the heating rate set at 5°C / min, and naturally cooled; a lithium aluminum phosphate fluoride-coated ternary NCM811 positive electrode active material LAPF@NCM811 was obtained.

[0186] Comparative Example 17

[0187] The preparation method of the positive electrode active material of Comparative Example 17 includes the following steps:

[0188] 1) Preparation of LiAlPO4F material: 100 g of lithium carbonate, aluminum hydroxide, ammonium dihydrogen phosphate, and ammonium fluoride were weighed according to a ratio of 0.5:1:1:1 (molar ratio), and a planetary ball mill was used to disperse the above-mentioned powder in a solvent, i.e., deionized water, for 3 h, wherein the ball-to-material ratio was 5:1, the ratio of grinding solvent to powder was 2:1, the grinding ball material was zirconia ball, and the diameter was 5 mm. After grinding, the above-mentioned slurry was dried at a temperature of 100°C. The powder obtained after drying was placed in an alumina crucible, heated to 500°C at a rate of 2°C / min, and held for 6 h, and then naturally cooled to obtain LiAlPO4F powder material. The LiAlPO4F powder was sand milled for 3 h, and the particle size range was finally controlled to be 80 nm;

[0189] 2) Preparation of LiAlPO4F@NCM811 powder: the positive active material NCM (D50 = 8 μm) was coated using a coating machine (the mass of LiAlPO4F powder was 1% of the mass of the positive active material), and after coating, the LiAlPO4F@NCM811 powder was obtained by calcining at 600°C for 4 h;

[0190] 3) Preparation of PAA@LiAlPO4F@NCM811 powder dispersion liquid: 1 g of PAA was weighed in 1000 mL of deionized water (1 g / L), 101 g of the above-prepared LiAlPO4F@NCM811 powder was added, then 5 g of dilute sulfuric acid (8M) and 0.51 g of ethanol were added, the PAA part carboxyl group was esterified to generate ester group, and the PAA was adsorbed on the surface of LiAlPO4F by magnetic stirring for 1 h to obtain the PAA@LiAlPO4F@NCM811 powder dispersion liquid;

[0191] 4) Preparation of PAA-diamino ferrocene@LiAlPO4F@NCM811 powder dispersion liquid: the above-prepared dispersion liquid was evaporated to dryness and redissolved in 500 mL of acetone, 0.7 g of formic acid was added, and the mixture was stirred for 1 h under magnetic stirring to convert the carboxylate group to carboxyl group; 0.6 g of phosphorus trichloride was added, and 0.1 g of aluminum trichloride was added as a catalyst, and the mixture was stirred at 80°C for 3 h to convert the carboxylic acid group to acyl chloride group; 0.8 g of diamino ferrocene and 0.1 g of 4-dimethylamino pyridine were added, and the mixture was stirred at -10°C for 30 min to form a PAA-diamino ferrocene gel network coated on the LiAlPO4F@NCM811 particles to obtain the PAA-diamino ferrocene@LiAlPO4F@NCM811 powder dispersion liquid;

[0192] 5) Preparation of PAA-diaminoferrocene@LiAlPO4F@NCM811 powder: The PAA-diaminoferrocene@LiAlPO4F@NCM811 powder dispersion prepared above is continuously stirred and 0.7 g of PEG is added. The PEG introduces ether chains into the PAA-diaminoferrocene gel network to obtain a PAA-diaminoferrocene@LiAlPO4F@NCM811 powder dispersion;

[0193] The powder dispersion is washed with water and then spray dried at an inlet temperature of 200°C and an outlet temperature of 70°C to obtain PAA-diaminoferrocene@LiAlPO4F@NCM811 powder with a particle size of about 8 μm.

[0194] Test Example:

[0195] 1. Surface smoothness: A scanning electron microscope (Hitachi Regulus 8100 / SU 8010) is used to test at an acceleration voltage of 5 kV, a magnification of 50K and a working distance of 8.1 mm. The area of the positive electrode active material surface non-point coating / field of view at this magnification.

[0196] 2. Coating layer or transition layer thickness: A field emission transmission electron microscope (Talos F200X, Thermo, USA) is used to characterize the thickness of the coating layer and the transition layer. Anhydrous ethanol is usually used to disperse the powder sample, and the test acceleration voltage is 60 kV.

[0197] 3. Chemical formula of the coating layer and the transition layer: The coating sample is etched with a 1 mol / L nitric acid solution for 20-60 min, and the etching depth is the thickness of the coating layer measured by transmission electron microscopy. The etched coating layer is characterized by inductively coupled plasma emission spectrometer (ICP-OES Optima 7000 / Avio500) to quantitatively measure the content of each element in the coating layer, and then the chemical formula of the coating layer is calculated according to the content of each element (Li a1 Al b1 (PO 4-δ F 2δ ) c1 F d1 ); the chemical formula of the manganese-containing matrix material is determined by X-ray diffractometer (Bruker D8 A A25) and ICP test results; the contents of lithium, aluminum, phosphorus and fluorine in the transition layer are the contents of each element in the additive minus the contents in the coating layer, and the chemical formula of the transition layer is calculated according to the doping of lithium, aluminum, phosphorus and fluorine in the positive electrode active material (Li a2 Ni x1 Co y1 Mn z1 M e Fe f Alb2 P c2 F d2 O g )。

[0198] 4、Coating layer and transition layer mass fraction: according to the chemical formula and the content of each element of the coating layer and the transition layer in the test example 3, the mass m1 of the coating layer and the transition layer is calculated as m1=M1*a1+M2*b1+M3*c1+M4*(2-d1)C1+d1*C1, M1, M2, M3, M4, M5 are the relative atomic mass of lithium, aluminum, phosphorus, fluorine, and oxygen, respectively, and the ratio of the mass of the positive electrode active material is the mass fraction of the coating layer, and the mass fraction of the transition layer can be calculated in the same way.

[0199] 5、Specific capacity: at 25°C, under normal pressure (0.1 MPa), the positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) of each example and the comparative example were mixed uniformly in N-methylpyrrolidone solvent at a mass percentage ratio of 95:3:2 to obtain a positive electrode slurry. The positive electrode slurry was coated on an aluminum foil, and after drying and cold pressing, a positive electrode sheet containing a positive electrode active layer with a thickness of 100 pm was obtained. Then, the positive electrode sheet was treated by drying and weighing with a film-type punch with a small round sheet of 12 mm in diameter, and in an Ar-protected glove box, a 2025 button cell shell was used, Li metal round sheet was used as the negative electrode, and a conventional high-voltage lithium cobalt oxide electrolyte was used to assemble a button cell. The button cell was charged at a rate of 0.1C / 0.33C to the upper limit voltage, and then charged at a constant voltage at the upper limit voltage until the current was equal to 0.05C. At this time, the charging capacity was recorded as the first cycle charging specific capacity. After 5 min, the button cell was discharged at a rate of 0.1C / 0.33C to the lower limit voltage. The discharge capacity at this time was recorded as the first cycle discharge specific capacity of the button cell. The voltage window of lithium nickel manganese oxide is 3.5-4.95V, the voltage window of nickel cobalt manganese ternary material is 2.8-4.3V, the voltage window of lithium manganese iron phosphate is 2.5-4.3V, and the voltage window of lithium manganate is 3-4.35V.

[0200] 6、First coulombic efficiency: according to the method in the button capacity test, a button cell was assembled, and then charged at a rate of 0.1C to the upper limit voltage, and then charged at a constant voltage at the upper limit voltage until the current was equal to 0.05C. At this time, the charging capacity was recorded as the first cycle charging specific capacity, and then the button cell was discharged at a rate of 0.1C to the lower limit voltage. The discharge capacity at this time was recorded as the first cycle discharge specific capacity of the button cell. The obtained discharge specific capacity / charging specific capacity was the first coulombic efficiency.

[0201] 7. Capacity retention after 100 cycles at 1C: Following the method described in the coin cell capacity test, the positive electrode active material was coated to form a positive electrode sheet, and graphite was used as the negative electrode. A full cell was constructed with a polyethylene separator and an electrolyte (LiPF6 electrolyte, EC / DMC solvent). At 25°C, the cell was charged at a constant current of 1C to the upper limit voltage, then charged at a constant voltage of 0.5C to the lower limit voltage, and finally discharged at a constant voltage of 1C to the lower limit voltage. This charge-discharge cycle was repeated 100 times. The discharge capacity Q1 at the first cycle and Q at the 100th cycle were measured. 100 The capacity retention rate after 100 cycles is Q = Q 100 / Q1*100%.

[0202] 8. Number of cycles at 80% capacity retention: For the full cells prepared above, charge them at a constant current rate of 0.1C to the upper limit voltage at 25℃ / 45℃, then charge them at a constant voltage rate of 0.05C at the upper limit voltage, let them stand for 5 minutes, and then discharge them at a constant current rate of 0.1C to the lower limit voltage. Repeat this cycle test until the capacity decays to 80%, and record the number of cycles.

[0203] 9. 28-day capacity retention rate: After the soft-pack full battery is formed, the air bag is removed and it is sealed. The initial capacity after formation is tested. After charging to the upper limit voltage and storing for 28 days, the remaining capacity of the battery is tested. The ratio of the remaining battery capacity to the initial capacity is the 28-day storage capacity retention rate.

[0204] 10. High-temperature storage gas generation performance: The positive active material was coated into a positive electrode sheet according to the method in the coin cell capacity test to prepare a positive soft pack battery. The soft pack battery before storage was placed in water and the initial volume V0 of the soft pack battery was tested by the water displacement method. The temperature was controlled at 70℃. After storage for 28 days, the volume V1 of the soft pack battery was tested again by the water displacement method. The volume change (V1-V0) / V0 was calculated to obtain the gas generation performance during high-temperature storage.

[0205] 11. BET change rate σ: σ=(BET1-BET2) / BET1, where BET1 is the specific surface area of ​​the positive electrode matrix material and BET2 is the specific surface area of ​​the positive electrode active material.

[0206] Figure 1 is a SEM image of the positive electrode active material prepared in Example 1;

[0207] Figure 2 is a SEM image of the positive electrode active material prepared in Example 3;

[0208] Figure 3 is a SEM image of the positive electrode active material prepared in Example 9;

[0209] Figure 4 is a SEM image of the positive electrode active material prepared in Example 11;

[0210] As can be seen from FIGS. 1-4, the positive electrode active materials of Example 1, Example 9, and Example 11 have a relatively round particle shape, no adhesion, a smooth surface, and no small particles. The surface of the positive electrode active material of Example 3 is also very smooth, indicating that the aluminum source, phosphorus source, and fluorine source have entered the surface interface of the positive electrode base material.

[0211] FIG. 5 is an SEM image of the positive electrode active material prepared in Comparative Example 1;

[0212] As can be seen from FIG. 5, the undoped single crystal particles prepared in Comparative Example 1 have poor dispersibility.

[0213] FIG. 6 is an SEM image of the positive electrode active material prepared in Comparative Example 2;

[0214] FIG. 7 is an SEM image of the positive electrode active material prepared in Comparative Example 3;

[0215] FIG. 8 is an SEM image of the positive electrode active material prepared in Comparative Example 4;

[0216] As can be seen from FIGS. 6-8, the single crystal particles in Comparative Examples 2-4 have good dispersibility, indicating that the transition metal elements Nb, Ta, Mo, and W have a significant dispersing effect on the single crystal material.

[0217] FIG. 9 is an SEM image of the positive electrode active material prepared in Comparative Example 5;

[0218] As can be seen from FIG. 9, the positive electrode active material prepared at low temperature in Comparative Example 5 has many small particles on the surface of the aluminum-coated material, in the form of point-like coating, and the coating is uneven.

[0219] FIG. 10 is an SEM image of the positive electrode active material prepared in Comparative Example 11.

[0220] FIG. 11 is an SEM image of the positive electrode active material prepared in Comparative Example 12.

[0221] As can be seen from FIGS. 10 and 11, the positive electrode active materials prepared at low temperature in Comparative Example 11 (single crystal) and Comparative Example 12 (polycrystal) have aluminum, phosphorus, and fluorine elements in the form of point-like coating on the surface of the lithium nickel manganese oxide material, and the coating is uneven.

[0222] Table 1

[0223] Compared with the comparative examples, the positive electrode active material provided by the application includes a positive electrode matrix material, a transition layer and a coating layer, and the surface smoothness of the positive electrode active material is limited, the coating layer is uniformly coated on the positive electrode matrix material, direct contact between the positive electrode matrix material and the electrolyte is inhibited, the occurrence of interface side reactions is reduced, the structural stability and interface stability of the material are enhanced, and when applied to a lithium ion battery, the cycle performance of the lithium ion battery can be improved.

[0224] As can be seen from the comparative example 1, the specific capacity, cycle performance and high-temperature storage performance of the positive electrode active material without transition metal element doping and surface coating are very poor.

[0225] As can be seen from the comparative examples 2-4, the positive electrode active material without surface coating has poor high-temperature cycle performance and high-temperature storage performance, because the surface stability of the positive electrode matrix material is poor, interface side reactions easily occur at high temperatures, the electrolyte is oxidized and decomposed, transition metal Mn 2+ is dissolved, and the high-temperature cycle performance and high-temperature storage performance are poor.

[0226] As can be seen from the comparative examples 5-7, the specific surface area of the positive electrode active material obtained by low-temperature sintering is obviously increased, and the surface smoothness is less than 30%, which shows that the coating material is mainly distributed in the form of dots on the surface of the positive electrode active material at low temperature (as shown in FIG. 8), this coating method has poor uniformity, the thin coating layer is easily eroded by hydrofluoric acid in the electrolyte, and the surface of the positive electrode active material cannot be completely coated, the coating layer and the material surface are not tightly adhered, and the coating layer is easily detached under long cycle, thereby causing rapid capacity decay, poor cycle performance and poor high-temperature storage performance.

[0227] As can be seen from the comparative example 8, when no fluorine is added, the cycle performance needs to be improved, and the high-temperature storage performance is also poor, because the fluoride ion has an important acid-proof effect, the presence of the fluoride ion can effectively inhibit the interface reaction between the hydrofluoric acid in the electrolyte and the positive electrode active material, and the gas production is reduced.

[0228] As can be seen from the comparative example 9, when no phosphorus is added, the capacity decreases, the cycle performance is poor, and the high-temperature storage performance is also poor, because the phosphate has good electrical conductivity, which is conducive to the transmission of lithium ions and will not significantly reduce the capacity in the cycle.

[0229] As can be seen from the comparative example 10, when no aluminum is added, the cycle performance and high-temperature storage performance need to be improved, because the aluminum oxide coating has low electrochemical activity, can directly block the contact between the electrolyte and the surface of the positive electrode active material, and inhibit the dissolution of transition metal Mn 2+ . In addition, aluminum and phosphorus are both low-melting-point substances, and when they are coated together, they can have a co-melting effect, which is helpful for surface doping.

[0230] From the comparative example 11, it can be seen that the specific surface area of the positive active material will become larger by using low-temperature sintering, which indicates that the transition layer and the coating layer of surface gradient doping are not formed. It can be known from the morphology of Figure 9 that at low temperature, the coating layer still exists in the form of point-like coating, and the cycle performance is poor due to the unevenness of the coating layer.

[0231] From the comparative examples 12-17, it can be seen that the specific surface area of the lithium nickel cobalt manganese phosphate coated with aluminum, phosphorus and fluorine at low temperature is obviously larger than that coated at high temperature (Example 14), and the surface smoothness is much lower, the coating effect is poor, resulting in poor overall capacity and cycle of the battery.

[0232] Finally, it should be pointed out that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A positive electrode active material, characterized by, The positive electrode active material comprises a positive electrode base material, a transition layer and a coating layer; the transition layer is coated on the surface of the positive electrode base material, and the coating layer is coated on the surface of the transition layer; The surface smoothness of the positive electrode active material is > 95%; The positive electrode base material comprises manganese element; The chemical formula of the coating layer is Li a1 Al b1 (PO 4-δ F 2δ ) c1 F d1 , wherein 0≤a1≤1.0, 0.001≤b1≤0.2, 0.01≤c1≤0.5, 0.01≤d1≤0.5, 0.01≤δ≤0.2, d1=a1+3b1-3c1. The transition layer has a chemical formula of Li a2 Ni x1 Co y1 Mn z1 M e Fe f Al b2 P c2 F d2 O g , 0.95≤a2≤1.20, 0≤x1≤0.95, 0≤y1≤0.25, 0.05≤z1≤1.60, 0.001≤e≤0.02, 0≤f≤0.65, 1.5≤g≤4.5, 0.001≤b2≤0.1, 0.01≤c2≤1.2, 0.005≤d2≤0.1, M includes at least one of Mg, Ca, Sc, Ti, V, Cr, Fe, Co, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Ta, W, B, Si, Ge, Sb, Te, and S.

2. The positive electrode active material according to claim 1, characterized by b2≤b1, d2≤d1; c2>1, c2-1≤c1; c2<1, c2≤c1.

3. The positive electrode active material according to claim 1 or 2, characterized by The mass fraction of the coating layer in the positive electrode active material is 0.5-3.0%; The mass fraction of the transition layer in the positive electrode active material is 0.05-1.0%.

4. The positive electrode active material according to any one of claims 1 to 3, characterized by, The thickness ratio of the coating layer to the transition layer is 1:0.01-0.5; And / or, the thickness of the coating layer is 5-30 nm; the thickness of the transition layer is 0.5-10 nm.

5. The positive electrode active material according to any one of claims 1 to 4, characterized by, The positive electrode base material is a single crystal material, and the average particle size of the positive electrode base material is 0.2-20 μm; Or, the positive electrode base material is a polycrystalline material, and the average particle size of the positive electrode base material is 3-50 μm.

6. The positive electrode active material according to any one of claims 1 to 5, characterized by, The positive electrode base material includes LiNi 0.5 Mn 1.5-x M x O4, LiMn 2-x M x O4, LiMn 0.6-x Fe 0.4 M x PO4, Li(Ni y Co z Mn 1-y-z ) 1-x M x O2; wherein 0.001≤x≤0.050, 0.50≤y≤0.095, 0.001≤z≤0.02, M includes at least one of Mg, Ca, Sc, Ti, V, Cr, Fe, Co, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Ta, W, B, Si, Ge, Sb, Te, S.

7. A method for producing the positive electrode active material as claimed in any one of claims 1 to 6, characterized by, The method comprises the following steps: After nano-treatment of an aluminum source, a phosphorus source and a fluorine source, the sources are mixed with a positive electrode base material to obtain a mixed system, and the mixed system is sintered at 700-1000 ℃ for 5-10 h to obtain the positive electrode active material.

8. The preparation method according to claim 7, characterized in that, The mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source and the fluorine source is 1:0.01-0.5; And / or, the mass ratio of the aluminum source, the phosphorus source and the fluorine source is 1:0.3-15:0.2-5.

9. A positive electrode sheet characterized by comprising: The positive electrode active material comprises a positive electrode base material, a transition layer and a coating layer; the transition layer is coated on the surface of the positive electrode base material, and the coating layer is coated on the surface of the transition layer; 10. A lithium-ion battery, characterized by, The surface smoothness of the positive electrode active material is > 95%; The positive electrode base material comprises manganese element; b2≤b1, d2≤d1; c2>1, c2-1≤c1; c2<1, c2≤c1. The mass fraction of the coating layer in the positive electrode active material is 0.5-3.0%; The mass fraction of the transition layer in the positive electrode active material is 0.05-1.0%. The thickness ratio of the coating layer to the transition layer is 1:0.01-0.5; And / or, the thickness of the coating layer is 5-30 nm; the thickness of the transition layer is 0.5-10 nm. The positive electrode base material is a single crystal material, and the average particle size of the positive electrode base material is 0.2-20 μm; Or, the positive electrode base material is a polycrystalline material, and the average particle size of the positive electrode base material is 3-50 μm. The method comprises the following steps: After nano-treatment of an aluminum source, a phosphorus source and a fluorine source, the sources are mixed with a positive electrode base material to obtain a mixed system, and the mixed system is sintered at 700-1000 ℃ for 5-10 h to obtain the positive electrode active material. The mass ratio of the positive electrode base material to the total mass of the aluminum source, the phosphorus source and the fluorine source is 1:0.01-0.5; And / or, the mass ratio of the aluminum source, the phosphorus source and the fluorine source is 1:0.3-15:0.2-5. The positive electrode active material comprises a positive electrode base material, a transition layer and a coating layer; the transition layer is coated on the surface of the positive electrode base material, and the coating layer is coated on the surface of the transition layer; The surface smoothness of the positive electrode active material is > 95%; The positive electrode base material comprises manganese element; b2≤b1, d2≤d1; c2>1, c2-1≤c1; c2<1, c2≤c1. The mass fraction of the coating layer in the

Images