Composite positive electrode material and preparation method thereof, electrochemical device and electronic equipment

Abstract

The invention discloses a composite positive electrode material and a preparation method thereof, an electrochemical device and electronic equipment, the composite positive electrode material comprises an inner core and a coating layer coating the surface of the inner core, the inner core comprises a lithium transition metal oxide, and the coating layer is a rock salt phase structure layer; and the rock salt phase structure layer comprises doped transition metal with the valence state of + 4 to + 6. The electrochemical device prepared from the composite positive electrode material has excellent cycle performance and rate capability at the same time.

Description

Technical Field

[0001] The present invention relates to a composite cathode material, a preparation method thereof, an electrochemical device, and an electronic device. Background Art

[0002] Transition metal oxide cathode materials are widely used in high energy density power battery systems due to their high theoretical specific capacity. Especially for layered oxide cathode materials, such as LiNixCoyMn1-x-yO2 (0 < x < 1, 0 < y < 1, and x + y < 1), limited by the characteristics of layered oxides, the layered structure is prone to phase transformation and structural collapse during charge and discharge processes, resulting in significant impact on its long-term cycle stability and limiting the practical application performance of the battery.

[0003] Chinese and foreign scholars have considered the following strategies to solve the problems of unstable structure and poor long-term performance of layered oxides, such as doping modification, surface coating, optimizing synthesis process, structural design, heat treatment technology, etc. Among them, doping modification has a wide range of applications, and excellent cathode materials can be obtained through simple operations. There are various choices of doping elements, such as Zr 4+ 、Y 3 + 、Ti 4+ 、Nb 5+ and Ta 5+ etc. Among them, doping with high-valence elements can enhance the stability of the layered structure, inhibit phase transformation, and reduce structural collapse during charge and discharge; and is beneficial to improving the stability of the material at high temperatures and reducing the risk of thermal runaway. However, since element doping will occupy the transition metal sites in the layered structure, resulting in deformation of this part of the structure, thus affecting the insertion / extraction of Li+, reducing the Li+ migration rate, and ultimately affecting the capacity performance of the material. And the layered oxide cathode is prone to Li evaporation in the high-temperature zone during the sintering process, resulting in a change in the Li stoichiometry (LiaNixCoyMn1-x-yO2, where a < 1) and a decrease in capacity.

[0004] How to ensure that while exerting the high stability brought by high-valence element doping, the capacity of the material does not suffer a large loss is a problem that needs to be solved currently. Summary of the Invention

[0005] In order to solve the defect that the transition metal oxide cathode material in the prior art cannot simultaneously meet high stability and high capacity, a composite cathode material, a preparation method thereof, an electrochemical device, and an electronic device are provided. The electrochemical device prepared by using this composite cathode material can simultaneously have excellent cycle performance and rate performance.

[0006] The present invention solves the above technical problems through the following technical solutions:

[0007] In a first aspect, the present invention provides a composite cathode material comprising a core and a coating layer covering the surface of the core, wherein,

[0008] The core contains a lithium transition metal oxide;

[0009] The coating layer is a rock salt phase structure layer, which contains doped transition metals with valence states of +4 to +6.

[0010] Secondly, the present invention provides a method for preparing a composite cathode material, comprising the following steps:

[0011] The mixture is subjected to a first sintering;

[0012] The mixture comprises a transition metal precursor, a lithium-doped transition metal oxide, and a lithium salt compound; the transition metal in the lithium-doped transition metal oxide comprises a doped transition metal with a valence state of +4 to +6.

[0013] Thirdly, the present invention provides a composite cathode material prepared by the method described above.

[0014] Fourthly, the present invention provides an electrochemical device comprising the composite cathode material as described above.

[0015] Fifthly, the present invention provides an electronic device comprising the electrochemical device as described above.

[0016] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0017] The reagents and raw materials used in this invention are all commercially available.

[0018] The positive and progressive effects of this invention are as follows:

[0019] The composite cathode material provided by this invention has a core-shell structure. The core contains lithium transition metal oxide, and the outer shell is a rock salt phase structure layer containing doped transition metals with valence states of +4 to +6. This rock salt phase structure layer can effectively improve the overall structural stability of the composite cathode material without affecting ion / electron transport. When the composite cathode material of this invention is applied in electrochemical devices, it can simultaneously exhibit excellent cycle performance and rate performance. Attached Figure Description

[0020] Figure 1 The image shows the XRD pattern of the composite cathode material prepared in Example 1. Detailed Implementation

[0021] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0022] Composite cathode materials

[0023] In a first aspect of the present invention, a composite cathode material is provided, comprising a core and a coating layer covering the surface of the core, wherein,

[0024] The core contains a lithium transition metal oxide;

[0025] The coating layer is a rock salt phase structure layer, which contains doped transition metals with valence states of +4 to +6.

[0026] In this invention, the term "doped transition metal" refers to transition metals other than nickel, cobalt, and manganese that are conventionally added to cathode materials in the art.

[0027] In this invention, the core comprises a lithium transition metal oxide, which is mainly used for storing and de-intercalating lithium.

[0028] In this invention, the lithium transition metal oxide can be single-crystal or polycrystalline, preferably polycrystalline. Lithium transition metal oxides undergo volume changes during charging and discharging due to lithium ion insertion / extraction. Polycrystalline materials are more prone to cracking due to the presence of grain boundaries, and these grain boundaries are "defect-rich regions" (such as oxygen vacancies and dislocations), reducing the material's stability. This invention, by forming a rock-salt phase structure layer of specific composition and thickness, significantly improves upon these problems in polycrystalline materials.

[0029] In this invention, the lithium transition metal oxide may have a layered structure or a spinel structure.

[0030] In some embodiments, the lithium transition metal oxide includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide.

[0031] In some embodiments, the lithium nickelate has the chemical formula LiNiO2.

[0032] In some embodiments, the lithium nickel manganese oxide has the chemical formula LiNi. 0.5 Mn 1.5 O2.

[0033] In some embodiments, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. x Co y Mn 1-x-y O2, x is >0, y is >0.

[0034] In one specific embodiment, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. x Co y Mn 1-x-y O2, x is 0.8 or 0.9, y is 0.1 or 0.06.

[0035] In one specific embodiment, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. 0.9 Co 0.06 Mn 0.04 O2.

[0036] In one specific embodiment, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. 0.8 Co 0.1 Mn 0.1 O2.

[0037] In some embodiments, the lithium transition metal oxide further includes a doped transition metal.

[0038] The doped transition metal is, for example, one or more of zirconium, titanium, niobium and tantalum.

[0039] In one specific embodiment, the lithium transition metal oxide is, for example, LiNi. 0.9 Co 0.06 Mn 0.03 Nb 0.01 O2.

[0040] In some embodiments, the doped transition metal in the rock salt phase structure layer includes one or more of zirconium, titanium, niobium, and tantalum.

[0041] In this invention, the aforementioned composite cathode material is subjected to TEM testing. Fourier transform is performed on the TEM images, and the core and coating layer are distinguished by their crystal phase structure. The core is a lithium transition metal oxide, and the coating layer is a rock salt phase structure layer. The thickness of the rock salt phase structure layer is measured. Next, the composite cathode material is subjected to SEM & EDS testing. EDS scanning is performed on the rock salt phase structure layer region to obtain the types of transition metals within the rock salt phase structure layer. Finally, XPS is used to obtain the valence states of the transition metals within the rock salt phase structure layer. The specific operations and calculation methods described above are common knowledge in the art and will not be elaborated further.

[0042] In some implementations, the thickness of the rock salt phase structure layer is less than 10 nm.

[0043] In some implementations, the thickness of the rock salt phase structure layer is 3-10 nm.

[0044] In some embodiments, the thickness of the rock salt phase structure layer is 3 nm, 4 nm, 5 nm, 6 nm, 9 nm, or 10 nm.

[0045] In some embodiments, the Dv50 of the composite cathode material is 5-18 μm, for example, 10-15 μm. Dv50 is defined as the median of the particle size distribution, i.e., in a given sample, 50% of the particles are smaller than this value, while 50% of the particles are larger than this value.

[0046] In some embodiments, the Dv50 of the composite cathode material is 12 μm, 13 μm, or 14 μm.

[0047] In some embodiments, the doped transition metal in the rock salt phase structure layer accounts for more than 70% of the molar percentage of all doped transition metals in the composite cathode material, for example, 70%-90%.

[0048] In some embodiments, the molar percentage of the doped transition metal in the rock salt phase structure layer relative to all doped transition metals in the composite cathode material is 74%, 78%, 83%, 84%, 86%, or 87%.

[0049] In some embodiments, the rock salt phase structure layer is a NiO type rock salt layer.

[0050] Preparation method of composite cathode material

[0051] In a second aspect of the present invention, a method for preparing a composite cathode material is provided, comprising the following steps:

[0052] The mixture is subjected to a first sintering;

[0053] The mixture comprises a transition metal precursor, a lithium-doped transition metal oxide, and a lithium salt compound; the transition metal in the lithium-doped transition metal oxide comprises a doped transition metal with a valence state of +4 to +6.

[0054] In some embodiments, the transition metal precursor includes one or more of cobalt precursor, nickel precursor, nickel-manganese precursor, and nickel-cobalt-manganese precursor.

[0055] In some embodiments, the nickel precursor has the chemical formula Ni(OH)2.

[0056] In some embodiments, the chemical formula of the nickel-manganese precursor is Ni 0.25 Mn 0.75 (OH)2.

[0057] In some embodiments, the chemical formula of the nickel-cobalt-manganese precursor is Ni 0.9 Co 0.06 Mn 0.04 (OH)2.

[0058] In some embodiments, the chemical formula of the nickel-cobalt-manganese precursor is Ni 0.8 Co 0.1 Mn 0.1 (OH)2.

[0059] In some embodiments, the nickel-cobalt-manganese precursor further includes a transition metal dopant.

[0060] The doping element is, for example, one or more of zirconium, titanium, niobium and tantalum.

[0061] In one specific embodiment, the chemical formula of the nickel-cobalt-manganese precursor is Ni 0.9 Co 0.06 Mn 0.03 Nb 0.01 (OH)2.

[0062] In some embodiments, the lithium-doped transition metal oxide includes one or more of lithium titanate (LiTiO3), lithium zirconate (Li2ZrO3), and lithium niobate (LiNbO3).

[0063] In some embodiments, the lithium-doped transition metal oxide is lithium niobate (LiNbO3) and lithium titanate (LiTiO3).

[0064] The mass ratio of lithium niobate to lithium titanate is 1:1.

[0065] In this invention, the lithium salt compound is a lithium source conventionally used in the art for preparing lithium-containing cathode materials.

[0066] In some embodiments, the lithium compound includes one or more of lithium hydroxide, lithium oxalate, lithium carbonate, lithium acetate, and lithium nitrate.

[0067] In some embodiments, the ratio of the molar amount of lithium in the lithium-doped transition metal oxide to the molar amount of transition metal in the transition metal precursor is (0.05-0.15):1, preferably (0.08-0.12):1.

[0068] In some embodiments, the ratio of the molar amount of lithium in the lithium-doped transition metal oxide to the molar amount of transition metal in the transition metal precursor is 0.05:1, 0.07:1, 0.1:1, 0.13:1, or 0.15:1.

[0069] In some embodiments, the ratio of the sum of the molar amounts of lithium in the lithium salt compound and the lithium-doped transition metal oxide to the molar amount of the transition metal in the transition metal precursor is (1.02-1.05):1, preferably (1.03-1.04):1, for example 1.03:1.

[0070] In some embodiments, a pre-sintering process is included before the first sintering, wherein the temperature of the pre-sintering is lower than the temperature of the first sintering.

[0071] In some embodiments, the pre-sintering temperature is 400-700°C, preferably 450-600°C, and more preferably 480-550°C.

[0072] In some specific embodiments, the pre-sintering temperature is 400°C, 450°C, 500°C, 600°C, or 700°C.

[0073] In some implementations, the pre-sintering time is 4-6 hours, for example, 4 hours.

[0074] In some implementations, the temperature of the first sintering is 720-950°C.

[0075] In some specific embodiments, the temperature of the first sintering is 720°C, 750°C, or 880°C.

[0076] In some implementations, the first sintering time is 8-15 hours, for example, 10 hours.

[0077] In some embodiments, the first sintering also includes additives, said additives including one or more of B2O3, Al2O3, MgO and TiO2.

[0078] In some embodiments, the additive accounts for 0.05%-1.5% of the mass of the precursor, preferably 0.05%-0.25%, and more preferably 0.05%-0.2%.

[0079] In some specific embodiments, the additives are B2O3 and Al2O3.

[0080] The mass ratio of B2O3 to Al2O3 is, for example, 1:1.

[0081] In some specific embodiments, the additives are B2O3 and MgO.

[0082] The mass ratio of B2O3 to MgO is, for example, 1:1.

[0083] In some specific embodiments, the additives are B2O3 and TiO2.

[0084] The mass ratio of B2O3 to TiO2 is, for example, 1:1.5.

[0085] In some specific embodiments, the additives are Al2O3 and MgO.

[0086] The mass ratio of Al2O3 to MgO is, for example, 1:1.

[0087] In some specific embodiments, the additives are MgO and TiO2.

[0088] The mass ratio of MgO to TiO2 is, for example, 1:1.5.

[0089] In some specific embodiments, the additives are Al2O3 and TiO2.

[0090] The mass ratio of Al2O3 to TiO2 is, for example, 1:1.5.

[0091] In some specific embodiments, the additive may be added by: putting the pre-sintered material and the additive into a high-speed mixer and mixing them according to the program of 200r / min-3min, 500r / min-5min, and 800r / min-15min.

[0092] In some implementations, a second sintering is included after the first sintering.

[0093] In some embodiments, the temperature of the second sintering is 300-600°C, for example, 400°C.

[0094] In some implementations, the second sintering time is 4-8 hours, for example, 6 hours.

[0095] In some implementations, a water washing step is also included after the first sintering.

[0096] In some specific implementations, the water washing can be carried out by the following steps: add the above-mentioned first sintered material to pure water at 8-10℃ with a solid content of 65%, stir at a speed of 2300r / min for 1min, filter the first sintered material to remove excess pure water, and then dry it in a vacuum oven at 150℃ for 6h.

[0097] In this invention, lithium-doped transition metal oxides are used to replace lithium salt compounds. During the pre-sintering stage, the temperature is raised from room temperature to the decomposition temperature of the lithium salt compound, but below the decomposition temperature of the lithium-doped transition metal oxide. The lithium salt compound decomposes, but the lithium-doped transition metal oxide does not. Due to insufficient lithium source, the material is in a lithium-poor state, generating a partial lithium-poor phase (i.e., rock salt phase). In the first sintering stage, the temperature is raised above the decomposition temperature of the lithium-doped transition metal oxide. The lithium-doped transition metal oxide decomposes into Li₂O and the doped transition metal oxide. Due to the high ionic conductivity and strong ionic bonding of the rock salt phase, it tends to combine with the doped transition metal oxide to form a uniform solid solution, resulting in a rock salt phase structure layer with high ionic conductivity and high thermal stability. Li₂O converts the remaining lithium-poor phase into lithium transition metal oxide in the composite cathode material. The second sintering stage serves as low-temperature annealing, repairing surface dislocations and vacancies, further improving long-term stability.

[0098] Composite cathode materials

[0099] In a third aspect of the present invention, a composite cathode material prepared by the method described above is provided.

[0100] In some embodiments, the composite cathode material includes a core and a coating layer covering the surface of the core, wherein,

[0101] The core contains a lithium transition metal oxide;

[0102] The coating layer is a rock salt phase structure layer, which contains doped transition metals with valence states of +4 to +6.

[0103] In some embodiments, the lithium transition metal oxide includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide.

[0104] In some embodiments, the lithium nickelate has the chemical formula LiNiO2.

[0105] In some embodiments, the lithium nickel manganese oxide has the chemical formula LiNi. 0.5 Mn 1.5 O2.

[0106] In some embodiments, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. x Co y Mn 1-x-y O2, x is >0, y is >0.

[0107] In one specific embodiment, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. x Co y Mn 1-x-yO2, x is 0.8 or 0.9, y is 0.1 or 0.06.

[0108] In one specific embodiment, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. 0.9 Co 0.06 Mn 0.04 O2.

[0109] In one specific embodiment, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. 0.8 Co 0.1 Mn 0.1 O2.

[0110] In some embodiments, the lithium transition metal oxide further includes a doped transition metal.

[0111] The doped transition metal is, for example, one or more of zirconium, titanium, niobium and tantalum.

[0112] In one specific embodiment, the transition metal oxide is, for example, LiNi. 0.9 Co 0.06 Mn 0.03 Nb 0.01 O2.

[0113] In some embodiments, the doped transition metal in the rock salt phase structure layer includes one or more of zirconium, titanium, niobium, and tantalum.

[0114] In some implementations, the thickness of the rock salt phase structure layer is less than 10 nm.

[0115] In some implementations, the thickness of the rock salt phase structure layer is 3-10 nm.

[0116] In some embodiments, the thickness of the rock salt phase structure layer is 3 nm, 4 nm, 5 nm, 6 nm, 9 nm, or 10 nm.

[0117] In some embodiments, the Dv50 of the composite cathode material is 5-18 μm, for example, 10-15 μm.

[0118] In some embodiments, the Dv50 of the composite cathode material is 12 μm, 13 μm, or 14 μm.

[0119] In some embodiments, the doped transition metal in the rock salt phase structure layer accounts for more than 70% of the molar percentage of all doped transition metals in the composite cathode material, for example, 70%-90%.

[0120] In some embodiments, the molar percentage of the doped transition metal in the rock salt phase structure layer relative to all doped transition metals in the composite cathode material is 74%, 78%, 83%, 84%, 86%, or 87%.

[0121] In some embodiments, the rock salt phase structure layer is a NiO type rock salt layer.

[0122] Electrochemical device

[0123] In a fourth aspect of the invention, an electrochemical device is provided, comprising the composite cathode material as described above.

[0124] In this invention, the electrochemical device is preferably a lithium-ion battery. The lithium-ion battery can be a liquid battery, a solid-state battery, or a semi-solid-state battery. For example, a liquid lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; a solid lithium-ion battery includes a positive electrode, a negative electrode, and a solid electrolyte. The type of battery does not limit the scope of protection of this invention.

[0125] The following uses a liquid battery as a specific embodiment to illustrate the technical content of the present invention.

[0126] Positive electrode film

[0127] In this invention, the positive electrode sheet may include a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, the positive electrode material layer including the composite positive electrode material as described above.

[0128] In some embodiments, the positive electrode material layer further includes a binder. The binder may be a component that facilitates the bonding between the active material and the conductive agent and facilitates the bonding between the positive electrode material and the positive electrode current collector. It can typically be selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0129] In some embodiments, the positive electrode material layer further includes a conductive agent. The conductive agent is a reagent used to ensure that the electrode has good charge-discharge performance. It can be selected from graphite materials such as natural graphite and artificial graphite; carbon black materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers such as carbon fiber and metal fiber; metal powders such as fluorinated carbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; and conductive metal oxides or polyphenylene derivatives such as titanium dioxide.

[0130] In some embodiments, the positive electrode material layer includes a composite positive electrode material, polyvinylidene fluoride, and conductive carbon black.

[0131] In one specific implementation, the mass ratio of the composite cathode material, conductive carbon black, and polyvinylidene fluoride is 90:5:5.

[0132] In this invention, the positive electrode current collector can be a conventional positive electrode current collector in the art. For the positive electrode current collector, materials that do not cause chemical changes and have high conductivity can be used without limitation. For example, commonly used materials include stainless steel, aluminum, nickel, titanium, or calcined carbon, or aluminum or stainless steel materials surface-treated with carbon, nickel, titanium, silver, etc. To enhance adhesion, micro-embossing can be formed on the surface of the positive electrode current collector. The positive electrode current collector can be used in various forms, such as films, sheets, foils, meshes, or porous bodies.

[0133] In some alternative implementations, the positive current collector is aluminum foil.

[0134] In some alternative implementations, the thickness of the positive current collector can be 8-16 μm, for example 10 μm.

[0135] In this invention, the positive electrode sheet can be prepared using methods conventional in the art.

[0136] In some embodiments, the positive electrode sheet is prepared by the following method: a composite positive electrode material, a conductive agent and a binder are mixed in a certain mass ratio, and a solvent is added and mixed evenly to obtain a positive electrode slurry; then the positive electrode slurry is uniformly coated on at least one surface of the positive electrode current collector; and then the positive electrode sheet is prepared by processes such as drying, rolling and cutting.

[0137] negative electrode sheet

[0138] In some implementations, the negative electrode is a lithium electrode.

[0139] In other embodiments, the negative electrode sheet may include a negative electrode current collector and a negative electrode material layer located on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a negative electrode material.

[0140] In this invention, the negative electrode material in the negative electrode material layer can be a negative electrode material conventionally used in the art, preferably including one or more of artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microspheres, silicon suboxide and silicon carbide materials, such as artificial graphite.

[0141] In some implementations, the negative electrode material layer further includes a conductive agent.

[0142] The conductive agent is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. For example, specific materials that can be used include: graphite, such as natural or artificial graphite; carbon-based materials, such as conductive carbon black (Super P), carbon nanotubes (CNTs), acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium dioxide; or conductive polymers, such as polyphenylene derivatives.

[0143] In some implementations, the negative electrode material layer further includes a binder.

[0144] The type of adhesive is not particularly limited and can be selected from polyvinylidene fluoride, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid (PAA), ethylene-propylene-diene terpolymer and its sulfonates, styrene-butadiene rubber (SBR), fluororubber and various copolymers, such as SBR.

[0145] In some implementations, the negative electrode material layer also includes a thickener.

[0146] The addition of the thickener can increase the viscosity of the system of each component in the negative electrode slurry. It can be a thickener commonly used in the art to prepare negative electrode sheets, such as sodium carboxymethyl cellulose (CMC).

[0147] In this invention, the negative electrode current collector can be a conventional negative electrode current collector in the art. As a substrate supporting the negative electrode material layer, the negative electrode current collector is typically a metal foil with a thickness of 3-500 micrometers. There are no particular restrictions on the material, as long as it has high conductivity and does not produce a chemical reaction in the secondary battery system. For example, it can be a foil formed by surface treatment of nickel, titanium, aluminum, silver, stainless steel, carbon, etc. The negative electrode current collector usually has a smooth surface, but fine textures can also be formed on its surface to improve the adhesion between the negative electrode material layer and the current collector. Besides foil, the negative electrode current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam, or non-woven fabric. Generally, the negative electrode current collector is copper foil.

[0148] In some embodiments, the method for preparing the negative electrode sheet includes the following steps: coating the negative electrode slurry obtained by thoroughly mixing the components of the negative electrode material layer in a solvent onto at least one surface of the negative electrode current collector, drying, cold pressing, and slitting to obtain the final product.

[0149] diaphragm

[0150] In some alternative embodiments, the diaphragm may be a polypropylene film or a polyethylene film.

[0151] In one specific embodiment, the diaphragm is a polypropylene film; the thickness of the diaphragm is 12 μm.

[0152] electrolyte

[0153] In some embodiments, the electrolyte may be a conventional electrolyte used in batteries, typically including non-aqueous solvents and lithium salts.

[0154] In this invention, the non-aqueous solvent can be a conventional non-aqueous solvent in the art.

[0155] In some embodiments, the non-aqueous solvent preferably includes ester solvents, more preferably carbonate solvents. The carbonate solvent may optionally be one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC). The non-aqueous solvent may also include ethyl acetate.

[0156] In this invention, the lithium salt can be a conventional lithium salt in the art, such as LiPF6.

[0157] In some embodiments, the electrolyte includes LiPF6, ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl acetate.

[0158] The mass ratio of ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl acetate is, for example, 1:1:2:6. The concentration of the lithium salt is, for example, 1 mol / L.

[0159] In some embodiments, the electrolyte can be prepared by conventional methods in the art. Optionally, it can be prepared by the following method: mixing the various non-aqueous solvents in proportion in an argon atmosphere glove box with a water content of <10 ppm, and then adding a fully dried lithium salt and mixing evenly to obtain the electrolyte.

[0160] In this invention, the method for preparing the lithium-ion battery can be a conventional method in the art, which can be to wind the positive electrode, the separator and the negative electrode in sequence to obtain the cell, then package it in a packaging shell and inject the electrolyte; or it can be to stack the negative electrode, the separator and the positive electrode in sequence to obtain the cell, then package it in a packaging shell and inject the electrolyte.

[0161] electronic devices

[0162] The electronic device provided in the fifth aspect of the present invention includes the electrochemical device as described above.

[0163] For example, the electronic devices described in this invention may be, but are not limited to, mobile devices (such as mobile phones, tablets, laptops, video recorders, portable printers / copiers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems and backup power supplies, etc.

[0164] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention. The present invention is further illustrated below by way of embodiments, but this does not limit the present invention to the scope of the described embodiments. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or according to the product instructions. Unless otherwise specified, all reagents and raw materials used in the present invention are commercially available.

[0165] Example 1

[0166] The preparation method of composite cathode materials includes the following steps:

[0167] For those containing precursors (Ni 0.9 Co 0.06 Mn 0.04 A mixture of (OH)2, lithium-doped transition metal oxide (LiNbO3), and lithium salt compound (LiOH·H2O) is subjected to pre-sintering, first sintering, water washing, and second sintering in an oxygen atmosphere (oxygen volume percentage of more than 95%) to obtain a composite cathode material.

[0168] The ratio of the molar amount of lithium in the lithium-doped transition metal oxide to the molar amount of transition metal in the precursor is 0.1:1, and the ratio of the sum of the molar amounts of lithium in the lithium compound and the lithium-doped transition metal oxide to the molar amount of transition metal in the precursor is 1.03:1.

[0169] The pre-sintering temperature is 500℃ and the time is 4 hours.

[0170] The first sintering process also includes additives (B2O3 and Al2O3, with a mass ratio of B2O3 to Al2O3 of 1:1), with the additives accounting for 0.2% of the precursor's mass (B2O3 accounts for 0.1% of the precursor's mass, and Al2O3 accounts for 0.1% of the precursor's mass). The pre-sintered material and additives are then fed into a high-speed mixer and mixed according to a program of 200 r / min-3 min, 500 r / min-5 min, and 800 r / min-15 min. The mixed material is then subjected to a first sintering at a temperature of 750°C for 10 h.

[0171] The water washing process involves the following steps: adding the first sintered material to pure water at 8-10℃ with a solid content of 65%, stirring at 2300 r / min for 1 min, filtering the first sintered material to remove excess pure water, and then drying it in a vacuum oven at 150℃ for 6 h.

[0172] The second sintering temperature is 400℃ and the time is 6 hours.

[0173] The parameters used in the above preparation process are listed in Tables 1 and 2.

[0174] The composite cathode material was prepared using the above method.

[0175] Example 2-19

[0176] The parameters in the preparation process of Examples 2-19 are listed in Tables 1 and 2. The other unlisted parameters are the same as in Example 1, and the preparation methods are the same as in Example 1.

[0177] Comparative Examples 1-3

[0178] The parameters in the preparation process of Comparative Examples 1-3 are listed in Tables 1 and 2. The other unlisted parameters are the same as in Example 1, and the preparation methods are the same as in Example 1.

[0179] Among them, Comparative Example 2 is a composite cathode material with a core-shell structure that does not fully form a lithium transition metal oxide core and a NiO-type rock salt phase structure layer coated on the surface of the transition metal oxide. It is only a mixture of precursor, lithium-doped transition metal oxide and lithium compound.

[0180] Table 1

[0181]

[0182] Note: " / " in the table indicates that the substance is not present.

[0183] Table 2

[0184]

[0185] Note: " / " in the table indicates that the step is not included.

[0186] Example 1

[0187] The composite cathode materials prepared in the above embodiments and comparative examples were subjected to the following tests:

[0188] 1. XRD test: The testing instrument was a Bruker D8 Advance, and the testing method was as follows:

[0189] (1) Sample preparation

[0190] Grinding: Grind the composite cathode material into a uniform powder (particle size ≤ 5μm) to avoid the particles being too large and affecting the diffraction intensity.

[0191] Compressing: Compress the powder into a flat sheet (or load it into a sample holder) to ensure a smooth surface.

[0192] (2) Instrument calibration

[0193] Use standard samples (such as silicon powder) to calibrate the instrument's angle and intensity to ensure data accuracy.

[0194] (3) Parameter settings

[0195] Scan range: The 2θ angle is usually set to 10-80° (to cover the main diffraction peaks);

[0196] Step size: 0.02°;

[0197] Scanning speed: 2° / min (adjustable according to accuracy requirements).

[0198] (4) Data collection

[0199] Place the sample on the sample stage and turn on the X-ray source (usually a Cu target, wavelength λ=1.5406 Å).

[0200] The detector collects diffraction signals and generates raw data curves.

[0201] (5) Data processing

[0202] Smoothing: Eliminate noise interference;

[0203] Background subtraction: Removes amorphous scattering background;

[0204] Peak finding: Identify the positions of diffraction peaks (such as the (003) and (104) characteristic peaks of NCM);

[0205] Phase analysis: Match crystal phases (e.g., α-NaFeO2 type layered structure) using databases (e.g., ICDD PDF).

[0206] Cell parameter calculation: Lattice constants (a and c axes) are calculated using the Bragg equation and Rietveld refinement. 6. Result output

[0207] Generate XRD patterns and analysis reports, including:

[0208] Phase composition (e.g., whether it contains the impurity phase Li2CO3);

[0209] Crystallinity and grain size (estimated using the Scherrer formula);

[0210] The layered structure order (the (003) / (104) peak intensity ratio reflects the degree of cation mixing).

[0211] The XRD pattern of the composite cathode material prepared in Example 1 is as follows: Figure 1 As shown. Figure 1 In the curve, the dots marked below the curve represent characteristic peaks of LiNiO2 (layered phase), and the triangles marked below the curve represent characteristic peaks of NiO (rock salt phase). This curve shows that in the composite cathode material, characteristic peaks of the rock salt phase appear around 37° and 43°. Since the rock salt phase accounts for a relatively small proportion of the entire composite cathode material, its characteristic peaks are also relatively small. However, 37° and 43° are the strongest and second strongest peaks of the NiO rock salt phase, respectively, hence their appearance in the curve. This indicates that the composite cathode material prepared in Example 1 includes a rock salt phase structural layer.

[0212] The XRD test results of Examples 2-19 are similar to those of Example 1. In different examples, due to the different thicknesses of the rock salt phase structure layer, the proportion of the rock salt phase structure layer in the composite cathode material is different, so there may be slight differences in peak intensity. However, since the amount is relatively small, the strongest and second strongest peaks of the NiO rock salt phase will basically only appear in the layered phase XRD, as shown in the figure.

[0213] 2. Thickness test of rock salt phase structure layer: TEM test was performed on the composite cathode materials prepared in the examples and comparative examples (using a transmission electron microscope, TEM, manufacturer: Hitachi, model: HT7800). By performing Fourier transform on the TEM images, it can be proved that the composite cathode material includes a lithium transition metal oxide core and a NiO type rock salt phase structure layer coated on the surface of the lithium transition metal oxide core; and the thickness of the rock salt phase structure layer was obtained.

[0214] The thickness was obtained as follows: At a magnification of 200,000x, 10 sites were randomly selected in the TEM image (20nm*20nm) of the same composite cathode material, and Fourier transforms were performed. Different crystal phase structures yielded different spectra through Fourier transforms, and the lithium transition metal oxide core and the rock salt phase structure layer were distinguished by the crystal phase structure. The thickness of the rock salt phase structure layer at each site was measured using Digital Micrograph. The average thickness of the rock salt phase structure layer was obtained by averaging the thicknesses of the 10 sites, and this average thickness was taken as the thickness of the rock salt phase structure layer.

[0215] 3. Testing of the type and valence state of transition metals doped in the rock salt phase structure layer: SEM & EDS testing was performed on the composite cathode material. EDS scanning was performed on the rock salt phase structure layer region to determine the type of transition metals doped in the rock salt phase structure layer. The valence state was further determined using XPS. The XPS (Thermo Fisher ESCALAB 250Xi) testing procedure is as follows:

[0216] (1) Sample preparation: Cut the composite cathode material sample into appropriate size, clean the surface, and may need to grind or polish it before placing it in the sample chamber;

[0217] (2) Vacuuming: Evacuate the sample chamber to an ultra-high vacuum (usually <10). -8 mbar);

[0218] (3) X-ray irradiation: The sample surface is irradiated with X-rays to excite photoelectrons;

[0219] (4) Energy analysis: The analyzer separates photoelectrons based on their kinetic energy;

[0220] (5) Detection: The detector records the number of photoelectrons with different kinetic energies, forming an energy spectrum;

[0221] (6) Data processing: Data is processed using Avantage software to obtain the types, chemical states and concentrations of elements.

[0222] Taking the determination of the +5 valence state of Nb in an XPS image as an example, the specific determination process is as follows:

[0223] (1) Obtaining XPS spectra of Nb: Measuring the 3d orbital spectra of niobium (Nb) (usually including 3d orbitals). 5 / 2 and 3D 3 / 2 (Two peaks)

[0224] (2) Binding energy calibration: The charge effect is calibrated with C 1s (284.8 eV) as a reference to ensure the accuracy of the binding energy data;

[0225] (3) Analysis of binding energy location: Nb5+ Characteristic binding energy of Nb 5+ 3D 5 / 2 The peaks are typically located in the range of 207.0–208.0 eV (e.g., Nb in Nb2O5).

[0226] 4. Test of the molar percentage of doped transition metals in the rock salt phase structure layer relative to all doped transition metals in the composite cathode material: SEM & EDS tests were performed on the composite cathode material. The rock salt phase structure layer region was selected for EDS scanning to obtain the molar percentage of doped transition metals in the rock salt phase structure layer relative to all doped transition metals in the composite cathode material.

[0227] 5. Testing of Dv50 of composite cathode materials: Particle size was measured using laser diffraction technology with a Malvern Mastersizer 3000 particle size analyzer. The particle size was measured by measuring the intensity of the scattered light as the laser beam passed through the dispersed composite cathode material particles. The data was then used to analyze and calculate the particle size distribution curve of the cathode material forming the scattered spectrum. The specific steps are as follows:

[0228] S1. Weigh 0.1g of composite cathode material;

[0229] S2. After turning on the particle size analyzer, clean it to ensure that there is no remaining sample in the internal pipes.

[0230] S3. Create the Standard Operating Procedure (SOP) for the sample to be tested in the program. After clicking OK, the measurement display interface will be entered. After clicking Start, the instrument will initialize according to the set SOP. After completion, it will automatically enter the "Measurement Background" stage, at which time the red and blue light values ​​of the background will be measured.

[0231] S4. When the "measuring background" stage ends, add positive electrode material until the shading bar indicates a range of 10-20%;

[0232] S5. Click "Start" again to begin the measurement of the cathode material; after completion, record the relevant data to obtain the particle size distribution curve and the Dv50 of the cathode material.

[0233] Dv50 is defined as the median of the particle size distribution, meaning that in a given sample, 50% of the particles are smaller than this value, while 50% of the particles are larger than this value.

[0234] 6. Residual Alkali Test: The residual alkali in the material is titrated using a potentiometric titrator to obtain the LiOH and Li2CO3 values. (Model: 905 Ai Intelligent Fully Automatic Potentiometric Titrator)

[0235] 7. Free Li +Test: The values ​​of LiOH and Li₂CO₃ in the material are obtained using a potentiometric titrator, and the Free Li can be calculated from this value. + The amount is calculated as follows: Free Li + =LiOH*0.2898+ Li2CO3*0.1879;

[0236] Where 0.2898 = M Li / M LiOH =6.9410 / 28.9483; 0.1879=2*M Li / M Li2CO3 .

[0237] 8. Li element content: The Li element content of the composite cathode material was tested by ICP using a Shimadzu ICPE-9800 series plasma atomic emission spectrometer.

[0238] The test results are listed in Table 3.

[0239] Table 3

[0240]

[0241] In Table 3, Comparative Examples 1 and 3 do not have a rock salt phase structure layer, while Comparative Example 2 does not have a lithium transition metal oxide core and a rock salt phase structure layer.

[0242] Example 2

[0243] The composite cathode materials prepared in the above examples and comparative examples were subjected to the following electrochemical performance tests:

[0244] Positive electrode: First, PVDF and NMP are mixed, then Super P is added and stirred at high speed to prepare a conductive slurry. The aforementioned composite positive electrode material is then added to prepare a positive electrode slurry with a certain viscosity. The prepared slurry is then uniformly coated onto aluminum foil and dried in a forced-air drying oven at 100°C for 120 minutes. Finally, the dried electrode sheet is rolled, cut, and formed into a positive electrode sheet, which is then placed in a vacuum drying oven and dried at 105°C for 240 minutes to obtain the positive electrode. The mass ratio of the composite positive electrode material, conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) is 90:5:5; the mass of NMP is adjusted appropriately according to the slurry's fluidity.

[0245] Negative electrode: Lithium foil is used.

[0246] The negative and positive electrodes were assembled in a glove box to prepare C2032 button cells. The C2032 button cells were then activated, and their capacity after activation was tested using a Blue Electric M340A precision battery tester on an electrochemical workstation. The following tests were also performed:

[0247] The charging specific capacity obtained by constant current and constant voltage charging (cutoff current of 0.05C) and constant current discharging (cutoff voltage of 3.0V) within a voltage range of 3-4.3V is CC, and the discharging specific capacity is DC. ICE is obtained from DC / CC.

[0248] Constant current and constant voltage charging (cutoff current of 0.05C) and constant current discharging (cutoff voltage of 3.0V) were performed within a voltage range of 3-4.3V at 0.33C. After 100 cycles under these conditions, the capacity was tested using a Blue Electric M340A precision battery tester on an electrochemical workstation, and the capacity retention rate after 100 cycles was calculated.

[0249] The test results are listed in Table 4:

[0250] Table 4

[0251]

[0252] As shown in the table above, when the composite cathode material of the embodiment is applied in the battery, the activated capacity is 128.1 mAh / g or more, the capacity after 100 cycles is 117.9 mAh / g or more, and the capacity retention rate after 100 cycles is 82% or more.

[0253] For composite cathode materials with nickel-cobalt-manganese precursors, when applied in batteries, the activated capacity is above 199.4 mAh / g, the capacity after 100 cycles is above 166.5 mAh / g, and the capacity retention rate after 100 cycles is above 82%.

[0254] For composite cathode materials with nickel precursors, when applied in batteries, the activated capacity is above 217.7 mAh / g, the capacity after 100 cycles is above 193.5 mAh / g, and the capacity retention rate after 100 cycles is above 88.9%.

[0255] For composite cathode materials with Nb-doped nickel-cobalt-manganese precursors, when applied in batteries, the activated capacity is above 207.1 mAh / g, the capacity after 100 cycles is above 185.2 mAh / g, and the capacity retention rate after 100 cycles is above 89.4%.

[0256] The composite cathode material of the present invention has a core-shell structure. The core contains lithium transition metal oxide, and the outer shell is a rock salt phase structure layer covering the surface of the lithium transition metal oxide core. This may be because the embodiments of the present invention use partially lithium-doped transition metal oxide to replace lithium compounds. Therefore, in the stage from room temperature to the decomposition temperature of lithium-doped transition metal oxide, due to insufficient lithium source, the material is in a lithium-poor state and a partial lithium-poor phase (i.e., rock salt phase) will be generated. In the sintering stage above the decomposition temperature of lithium-doped transition metal oxide, lithium-doped transition metal oxide will decompose into Li2O and doped transition metal oxide. Due to the high ionic conductivity and strong ionic bonding of the rock salt phase, it tends to combine with the doped transition metal oxide to form a uniform solid solution (the final rock salt phase structure layer), which has high ionic conductivity and high thermal stability. Li2O converts the remaining lithium-poor phase into the lithium transition metal oxide core in the composite cathode material, which is mainly used for lithium storage and lithium insertion / extraction.

[0257] Comparative Example 1 does not include lithium-doped transition metal oxides, which results in the absence of a rock salt phase structure layer, making it impossible to achieve high ionic conductivity and high thermal stability. Consequently, its capacity and capacity retention rate deteriorate after 100 cycles.

[0258] Comparative Example 2 did not have a first sintering and did not fully form a composite cathode material with a transition metal oxide and a NiO-type rock salt phase structure layer coated on the surface of the transition metal oxide. It was only a mixture of precursor, lithium-doped transition metal oxide and lithium compound. Its capacity after activation, capacity after 100 cycles and capacity retention rate after 100 cycles all deteriorated.

[0259] Comparative Example 3 uses a doped transition metal oxide that does not contain lithium, resulting in the absence of doped transition metals with valence states of +4 to +6 in its rock salt phase structure layer, which leads to a decrease in its capacity and capacity retention after 100 cycles.

[0260] Based on the results of Examples 1, 12, 13, 16, and 17, it can be seen that setting the pre-sintering temperature to 450-600℃ can further improve the electrochemical performance in the first cycle, the activated capacity, the capacity after 100 cycles, and the capacity retention rate after 100 cycles; setting the pre-sintering temperature to 480-550℃ can further achieve an activated capacity of more than 207.8 mAh / g, a capacity after 100 cycles of more than 187.2 mAh / g, and a capacity retention rate of more than 90.1% after 100 cycles.

[0261] According to the results of Examples 1 and 18, in Example 1, after the first sintering in the preparation process of the composite cathode material, a second sintering is also set. The second sintering can play the role of low-temperature annealing, repairing surface dislocations and vacancies, further improving long-term stability, further improving the capacity after activation, the capacity after 100 cycles, and the capacity retention rate after 100 cycles, and can also further improve the electrochemical performance of the first cycle.

[0262] According to the results of Examples 1 and 19, the ratio of the sum of the molar amounts of lithium in the lithium compound and the lithium-doped transition metal oxide to the molar amount of the transition metal in the precursor, controlled by Example 1, is (0.05-0.15):1, which can further improve the capacity after activation, the capacity after 100 cycles, and the capacity retention rate after 100 cycles.

[0263] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and the scope of protection of the present invention is defined by the appended claims. Those skilled in the art can make various changes or modifications to these embodiments without departing from the principles and essence of the present invention, but all such changes and modifications fall within the scope of protection of the present invention.

Claims

1. A composite cathode material, characterized in that, It includes a core and a covering layer covering the surface of the core, wherein, The core contains a lithium transition metal oxide; The coating layer is a rock salt phase structure layer, which contains doped transition metals with valence states of +4 to +6.

2. The composite cathode material as described in claim 1, characterized in that, It meets one or more of the following conditions: (a) The doped transition metal includes one or more of zirconium, titanium, niobium and tantalum; (b) The thickness of the rock salt phase structure layer is less than 10 nm; (c) The Dv50 of the composite cathode material is 5-18 μm; (d) The doped transition metal in the rock salt phase structure layer accounts for more than 70% of the molar percentage of all doped transition metals in the composite cathode material; (e) The rock salt phase structure layer is a NiO type rock salt layer; (f) The lithium transition metal oxide includes one or more of lithium cobalt oxide, lithium nickel oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide.

3. A method for preparing a composite cathode material, characterized in that, It includes the following steps: The mixture is subjected to a first sintering; The mixture comprises a transition metal precursor, a lithium-doped transition metal oxide, and a lithium salt compound; the transition metal in the lithium-doped transition metal oxide comprises a doped transition metal with a valence state of +4 to +6.

4. The method for preparing the composite cathode material as described in claim 3, characterized in that, It meets one or more of the following conditions: (a) The transition metal precursor includes one or more of cobalt precursor, nickel precursor, nickel-manganese precursor and nickel-cobalt-manganese precursor; (b) The lithium-doped transition metal oxide includes one or more of lithium titanate, lithium zirconate, and lithium niobate; (c) The lithium salt compound includes one or more of lithium hydroxide, lithium oxalate, lithium carbonate, lithium acetate and lithium nitrate.

5. The method for preparing the composite cathode material as described in claim 3, characterized in that, It meets one or more of the following conditions: (a) The ratio of the molar amount of lithium in the lithium-doped transition metal oxide to the molar amount of transition metal in the transition metal precursor is (0.08-0.12):1; (b) The ratio of the sum of the molar amounts of lithium in the lithium salt compound and the lithium-doped transition metal oxide to the molar amount of the transition metal in the transition metal precursor is (1.02-1.05):1; (c) The ratio of the molar amount of lithium in the lithium-doped transition metal oxide to the molar amount of transition metal in the transition metal precursor is (0.05-0.15):

1.

6. The method for preparing the composite cathode material as described in claim 3, characterized in that, It meets one or more of the following conditions: (a) The first sintering is preceded by a pre-sintering, wherein the temperature of the pre-sintering is lower than the temperature of the first sintering; (b) The temperature of the first sintering is 720-950℃; (c) The first sintering time is 8-15 hours; (e) The first sintering also includes additives, said additives including one or more of B2O3, Al2O3, MgO and TiO2; (f) The first sintering is followed by a second sintering, wherein the temperature of the first sintering is higher than the temperature of the second sintering; (g) The process after the first sintering also includes a water washing step.

7. The method for preparing the composite cathode material as described in claim 6, characterized in that, It meets one or more of the following conditions: (1) The pre-sintering temperature is 450-600℃; (2) The pre-sintering time is 4-6 hours; (3) The mass percentage of the additive relative to the mass of the precursor is 0.05%-1.5%; (4) The second sintering temperature is 300-600℃; (5) The second sintering time is 4-8 hours.

8. A composite cathode material prepared by a method according to any one of claims 3-7.

9. An electrochemical device, characterized in that, It includes the composite cathode material as described in any one of claims 1, 2 and 8.

10. An electronic device, characterized in that, It includes the electrochemical device as described in claim 9.

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