A positive electrode composite material and its preparation method and application

Abstract

The present application provides a positive electrode composite material, a preparation method thereof, and an application. The positive electrode composite material includes a core and a shell layer in-situ grown on the core. The material of the core includes Li x AO y , and the material of the shell layer includes a phosphate containing lithium and element M, where 1 < x ≤ 8, 0 < y ≤ 6, element A includes at least one of Co, Cu, Ni, Fe, Zr, Zn, and Mn, and element M includes a non-lithium metal element; there is a metallic bond at the interface between the core and the shell layer. The positive electrode composite material has a core-shell structure, the core material has a lithium supplement effect, and the positive electrode composite material can be converted into a positive electrode active material with a relatively high conductivity, a relatively high energy density, and good cycle stability. In addition, the positive electrode composite material has relatively high structural stability, good storage stability, and is not prone to side reactions with the electrolyte in the battery, which is conducive to the normal performance of the battery.

Description

Technical Field

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

[0002] With the development of the new energy industry, the market's requirements for the energy density, cycle performance and battery capacity of lithium batteries are also increasing. At present, phosphoric acid-based positive electrode active materials are popular in the market due to their high safety and stability, wide source of raw materials, high theoretical specific capacity, and stable discharge voltage. However, phosphoric acid-based positive electrode active materials have the pain point of low conductivity, and their cycle performance and capacity retention rate still need to be further improved. In addition, during the first charge and discharge process of all types of lithium batteries, the positive electrode active materials will undergo active Li + The irreversible loss affects the energy density, battery capacity and cycle performance of lithium batteries. Summary of the invention

[0003] In view of this, the present application provides a positive electrode composite material and a preparation method and application thereof, wherein the positive electrode composite material has a core-shell structure, the core material has a lithium supplement effect, and the positive electrode composite material can be converted into a positive electrode active material with low surface manganese content, high conductivity, high energy density and good cycle stability. In addition, the positive electrode composite material has high structural stability, good storage stability, and is not prone to side reactions with the electrolyte in the battery, which is conducive to the normal performance of the battery performance.

[0004] Specifically, the first aspect of the present application provides a positive electrode composite material, which includes a core and a shell layer in-situ grown on the core, wherein the material of the core includes Li x AO y , the material of the shell layer includes phosphate containing lithium and M element, wherein 1<x≤8, 0<y≤6, the A element includes at least one of Co, Cu, Ni, Fe, Zr, Zn and Mn, and the M element includes a non-lithium metal element; there is a metallic bond at the interface between the core and the shell layer.

[0005] The above core material can be used as a lithium supplement to provide active lithium ions during the charge and discharge process of the battery, and at the same time form a Li+ with higher conductivity and faster charge transfer with the shell material. + The positive electrode composite material has a high migration speed, a high energy density and good cycle stability. In addition, the positive electrode composite material is not easy to react with the electrolyte in the battery, and there is a metal bond at the interface between the core and the shell, and the shell has a high environmental stability, so that the positive electrode composite material has good structural stability and good storage stability. In summary, the above-mentioned positive electrode composite material can be used to provide a secondary battery with high energy density, good rate performance and long cycle life.

[0006] The second aspect of the present application provides a method for preparing a positive electrode composite material, comprising the following steps:

[0007] (1) mixing an A source, an M source, and a phosphoric acid source to obtain a positive electrode composite material precursor;

[0008] (2) mixing the positive electrode composite material precursor and a lithium source in a solvent so that lithium element is infiltrated into the positive electrode composite material precursor to obtain a composite material, filtering and drying the obtained composite material to obtain a solid, and sintering the solid under an inert atmosphere to obtain a positive electrode composite material; wherein the positive electrode composite material comprises a core and a shell layer in situ grown on the core, and the material of the core comprises Li x AO y The material of the shell layer includes phosphate containing lithium and M element, wherein 1<x≤8, 0<y≤6, the A element includes at least one of Co, Cu, Ni, Fe, Zr, Zn and Mn, and the M element includes a non-lithium metal element; and a metal bond exists at the interface between the core and the shell layer. The preparation method has simple steps, strong process controllability, high production efficiency, and is suitable for large-scale industrial production.

[0009] The third aspect of the present application provides a positive electrode plate, which contains the positive electrode composite material provided by the first aspect of the present application or the positive electrode composite material prepared by the preparation method provided by the second aspect of the present application.

[0010] The positive electrode plate has a self-replenishing lithium effect, and has a high energy density and a long cycle life.

[0011] The fourth aspect of the present application provides a secondary battery having the positive electrode plate provided in the third aspect of the present application.

[0012] The secondary battery has high energy density, good rate performance and long cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A schematic diagram of the structure of a positive electrode composite material provided in one embodiment of the present application;

[0014] Figure 2 A scanning electron microscopy (SEM) photograph of the positive electrode composite material provided in Example 12 of the present application;

[0015] Figure 3 This is the X-ray diffraction (XRD) spectrum of the positive electrode composite material provided in Example 12 of the present application. DETAILED DESCRIPTION

[0016] The technical solution of the present application is described in detail below with reference to the accompanying drawings.

[0017] See also Figure 1 The present invention provides a positive electrode composite material 100, which includes a core 10 and a shell 20 in-situ grown on the core 10. The material of the core 10 includes Li x AO y The material of the shell 20 includes a phosphate containing lithium and an M element, wherein 1<x≤8, 0<y≤6, the A element includes at least one of Co, Cu, Ni, Fe, Zr, Zn and Mn, and the M element includes a non-lithium metal element; a metallic bond exists at the interface between the core 10 and the shell 20.

[0018] The core 10 can be used as a lithium supplement to provide active lithium ions during the charge and discharge process of the battery, making up for the active lithium ions lost during the initial charge and discharge process of the battery, thereby increasing the capacity and energy density of the battery. x AO y There is a metallic bond between the elements in the core and the shell material elements, so that the core 10 is activated to release Li + At the same time, the A element in the core 10 can form a high conductivity, Li + A positive electrode active material with fast migration speed, high energy density and good cycle stability. Furthermore, the shell 20 is in-situ grown on the surface of the core 10, and there is a metal bond at the shell-core interface, so that the interface bonding force between the shell and the core is good, the structural stability is high, and the environmental stability of the shell 20 is higher than that of the core 10, thereby protecting the core 10 from being easily reacted with moisture, CO2, etc. in the air, thereby improving the storage stability of the positive electrode composite material. In addition, the positive electrode composite material is not easy to react with the electrolyte in the battery. In summary, the above-mentioned positive electrode composite material can be used to provide a secondary battery with high energy density, good rate performance and long cycle life.

[0019] The above-mentioned positive electrode composite material can be used alone as a positive electrode active material, or can be used as a lithium supplement together with other positive electrode active materials known to those skilled in the art.

[0020] In the present application, the above-mentioned metal bond is formed after the materials in the core 10 and the shell 20 undergo thermal diffusion during the preparation process of the positive electrode composite material 100 .

[0021] In some embodiments of the present application, the above-mentioned phosphate containing lithium and M element includes but is not limited to LiM a PO b, where 0<a≤1, 0<b≤4.

[0022] In some embodiments of the present application, the M element includes but is not limited to at least one of Co, Cu, Ni, Ti, Al, Mg, Fe, V, Zr, Nd, Mo, Zn and Mn. The lithium phosphates containing the M element can form a phosphoric acid-based positive electrode active material with high energy density, high conductivity and good cycle stability together with the A element.

[0023] In some specific embodiments, the M element includes but is not limited to at least one of Co, Cu, Ni, Fe, Zr, Zn and Mn. In other specific embodiments, the M element includes at least one of Zn, Cu and Co. In other specific embodiments, the M element includes at least one of Zn, Cu and Co, and the A element is Mn. In this case, the shell can form LiMn 0.95 Zn 0.05 PO4、LiMn 0.2 Cu 0.8 PO4、LiCo 0.6 Mn 0.4 PO4 and other high-performance positive electrode active materials.

[0024] In some embodiments of the present application, the M element includes at least one of Mn and Fe. Further, the M element is Fe, or the M element is Mn and Fe. In some specific embodiments, the A element is Mn, and the M element is Fe, that is, the core 10 is Li2MnO2, and the shell 20 is LiFePO4. In other specific embodiments, the A element is Mn, and the M element is Mn and Fe, that is, the core 10 is Li2MnO2, and the shell 20 is LiMn x Fe 1-x PO4(0<x<1). In some embodiments of the present application, the above-mentioned M element is the same as the A element. At this time, if part of the M element in the shell is dissolved under the action of the battery electrolyte, part of the A element can be dissolved from the core material and enter the bulk phase of the shell material. Of course, part of the A element itself has a metallic bond with the shell material. At this time, the above-mentioned two parts of the A element can replace the dissolved M element to support the skeleton structure of the phosphate material containing lithium and M elements, thereby inhibiting the capacity decay of the material and improving the thermal stability, electronic conductivity and electrochemical properties of the shell material.

[0025] In some embodiments of the present application, the above-mentioned M element is different from the A element. In this case, the A element can be used as a doping element in the formed positive electrode active material to increase the diffusion coefficient of lithium ions, thereby significantly improving the cycle performance and rate performance of the battery.

[0026] In some embodiments of the present application, the M element is not Mn. In this case, the Mn content on the surface of the positive electrode composite material 100 itself and the positive electrode active material formed during charging and discharging is lower, and it is less likely to have side reactions with the electrolyte in the battery, which is more conducive to the performance of the battery.

[0027] In some embodiments of the present application, the core material Li x AO y 2≤x≤6, 2≤y≤4. Controlling the molar ratio of the above elements within the above range, on the one hand, enables the core material to stably release lithium ions and contain a sufficient amount of A element to form a high-performance positive electrode active material with the shell material, and on the other hand, it is also beneficial to ensure that the core material can maintain the original lattice structure after releasing a large amount of lithium ions, which is more conducive to the performance of the battery.

[0028] In some specific embodiments, x=2, y=2, and element A is Mn. That is, the core material is Li2MnO2. In this case, the core 10 can be used as a lithium supplement and a positive electrode active material at the same time. The shell 20 can not only form a high-performance positive electrode active material with the core material, but also effectively inhibit the volume expansion of Li2MnO2 during the charge and discharge process, thereby optimizing the expansion characteristics of the battery.

[0029] In some other specific embodiments, x=6, y=4, and the element A is Mn, that is, the core material is Li6MnO4. In this case, the lithium replenishing efficiency of the core 10 is higher.

[0030] In some embodiments of the present application, the mass percentage of the material of the core 10 to the material of the shell 20 is in the range of 1%-10%. Exemplarily, the mass percentage of the material of the core 10 to the material of the shell 20 can be 1%, 1.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc. In some specific embodiments, the mass percentage of the material of the core 10 to the material of the shell 20 is in the range of 1%-5%. Controlling the content ratio of the lithium element between the core and the shell within the above range is conducive to ensuring a high lithium replenishment efficiency of the core 10, and is also conducive to improving the energy density, conductivity and cycle stability of the finally formed positive electrode active material.

[0031] In some embodiments of the present application, the above-mentioned metal bond includes but is not limited to at least one of a Li-M bond, an AM bond and a Li-A bond. Among them, the Li element in the above-mentioned Li-M bond specifically refers to the Li element in the core 10, and the Li element in the above-mentioned Li-A bond specifically refers to the Li element in the shell 20. At this time, the A element in the core 10 and the M element or Li element in the shell have a metal bond, which is conducive to the A element and the shell material to form a high-performance, high-stability positive electrode active material. In some specific embodiments, the A element is the Mn element. At this time, the above-mentioned metal bond includes at least one of a Li-M bond, a Mn-M bond and a Li-Mn bond. Correspondingly, the Li element in the above-mentioned Li-Mn bond specifically refers to the Li element in the shell 20, and the Mn in the Li-Mn bond and Mn-M specifically refers to the Mn element in the core 10.

[0032] In some embodiments of the present application, the diameter of the core 10 is in the range of 0.01 μm-15 μm. For example, the diameter of the core 10 can be 0.01 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 15 μm, etc. In some cases, the core 10 is made of Li x AO y In some cases, the core 10 is formed directly from primary particles. In this case, the diameter of the core 10 can be controlled within the range of 0.01 μm to 6 μm, and further controlled within the range of 0.01 to 5 μm. x AO y In this case, the diameter of the core 10 can be controlled within the range of 0.1 μm-15 μm, and further within the range of 0.1 μm-10 μm. Among them, secondary particles refer to agglomerated particles formed by the aggregation of multiple primary particles. Controlling the diameter of the core 10 within the above range means controlling the content of Li element and A element in the core within a suitable range, which is conducive to ensuring that the lithium replenishment efficiency of the core 10 is high, and at the same time, it can stably form a high-performance positive electrode active material with the shell material.

[0033] In some embodiments of the present application, the thickness of the shell 20 is in the range of 0.1 μm-10 μm. In some specific embodiments, the thickness of the shell 20 is in the range of 2 μm-5 μm. Exemplarily, the thickness of the shell 20 can be 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc. Controlling the thickness of the shell 20 within the above range means controlling the content of the Li element in the shell within a suitable range, which is conducive to ensuring that the performance of the positive electrode active material formed after the first charge and discharge is good.

[0034] In some embodiments of the present application, the diameter of the positive electrode composite material 100 is in the range of 0.1 μm-20 μm. In some specific embodiments, the diameter of the positive electrode composite material 100 is in the range of 0.1 μm-15 μm. Exemplarily, the diameter of the positive electrode composite material 100 can be 0.1 μm, 0.5 μm, 1 μm, 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, etc. By controlling the diameter of the positive electrode composite material 100 within the above range, the migration path of the active lithium ions can be controlled to be shorter, which is conducive to further improving the lithium replenishment efficiency of the positive electrode composite material 100.

[0035] In some embodiments of the present application, the BET specific surface area of ​​the positive electrode composite material 100 is 0.1 m 2 / g-30m 2 In some specific embodiments, the BET specific surface area of ​​the positive electrode composite material 100 is within the range of 0.3 m 2 / g-20m 2 When the positive electrode composite material 100 is made into a positive electrode sheet and assembled into a battery, controlling the BET specific surface area of ​​the positive electrode composite material within the above range is conducive to ensuring that the positive electrode composite material or the positive electrode active material converted therefrom is in full contact with the electrolyte, which is more conducive to the transmission of active lithium ions.

[0036] In some embodiments of the present application, the surface of the positive electrode composite material 100 also has an encapsulation layer; the material of the encapsulation layer includes at least one of carbon material, polyaniline, polypyrrole, polyethylene oxide and poly-3,4-ethyldioxythiophene. The good conductivity of the encapsulation layer is conducive to further reducing the internal resistance of the final positive electrode active material, thereby improving the lithium replenishment effect and lithium replenishment stability of the positive electrode composite material 100 and the electrochemical properties of the final positive electrode active material. The encapsulation layer can also further isolate water vapor, oxygen and CO2 in the air, further improving the storage stability of the positive electrode composite material 100.

[0037] In some embodiments of the present application, the thickness of the encapsulation layer may be in the range of 2nm-200nm. In some specific embodiments, the thickness of the encapsulation layer may be in the range of 2nm-50nm.

[0038] In some embodiments of the present application, the material of the encapsulation layer includes but is not limited to at least one of a carbon material, a conductive oxide, and a conductive organic matter. In some specific embodiments, the material of the encapsulation layer is a carbon material, and the above-mentioned carbon material includes but is not limited to at least one of amorphous carbon, carbon nanotubes, graphite, carbon black, and graphene. In other specific embodiments, the material of the encapsulation layer is a conductive oxide, and the above-mentioned conductive oxide includes but is not limited to at least one of In2O3, ZnO, and SnO2. In some other specific embodiments, the material of the encapsulation layer is a conductive organic matter, and the above-mentioned conductive organic matter can be a conductive polymer, etc.

[0039] Accordingly, the embodiment of the present application provides a method for preparing a positive electrode composite material, which can be used to prepare a positive electrode composite material 100. Specifically, the preparation method includes the following steps:

[0040] (1) mixing an A source, an M source, and a phosphoric acid source to obtain a positive electrode composite material precursor;

[0041] (2) mixing the positive electrode composite material precursor and a lithium source in a solvent so that lithium element is infiltrated into the positive electrode composite material precursor to obtain a composite material, filtering and drying the obtained composite material to obtain a solid, and sintering the solid under an inert atmosphere to obtain a positive electrode composite material; wherein the positive electrode composite material comprises a core and a shell layer in situ grown on the core, and the material of the core comprises Li x AO y , the material of the shell layer includes phosphate containing lithium and M element, wherein 1<x≤8, 0<y≤6, the A element includes at least one of Co, Cu, Ni, Fe, Zr, Zn and Mn, and the M element includes a non-lithium metal element; there is a metallic bond at the interface between the core and the shell layer.

[0042] In the above step (1), the M source and the phosphate source can be deposited on the surface of the A source to form a positive electrode composite material precursor having a core-shell structure. In step (2), the lithium element gradually penetrates into the shell and the core of the above positive electrode composite material precursor to form a solid with lithium element in both the shell and the core. After sintering the above solid, the above positive electrode composite material can be obtained.

[0043] The preparation method has simple steps, strong process controllability, high production efficiency, and is suitable for large-scale industrial production. In addition, the positive electrode composite material prepared by the above preparation method has less residual lithium on its outer surface, which can improve the storage stability of the positive electrode composite material, and can further improve the cycle stability of the positive electrode active material formed in the battery, and can also further improve the safety performance of the battery.

[0044] In the present application, in step (1), the above-mentioned solvent includes water and / or ethanol.

[0045] In the present application, in step (1), the positive electrode composite material precursor can be prepared by solid phase reaction or liquid phase reaction. Those skilled in the art can choose according to actual production conditions.

[0046] In the present application, in step (2), the relative excess of the lithium source can be controlled.

[0047] In the present application, the inert gas in step (2) can be argon or nitrogen.

[0048] In the present application, in step (2), under the action of high temperature, thermal diffusion occurs between the shell and core materials, and metal bonds can be formed between the metal elements at the interface, thereby improving the bonding strength between the shell and the core, and further improving the structural stability of the positive electrode composite material.

[0049] The sintering process is a multi-stage sintering. Specifically, in some embodiments of the present application, the sintering process includes the following steps: reacting the solid at 250°C-400°C for 5h-10h, and then heating to 500°C-900°C for 10h-24h.

[0050] In some embodiments of the present application, the above-mentioned M element includes but is not limited to at least one of Co, Cu, Ni, Ti, Al, Mg, Fe, V, Zr, Nd, Mo, Zn and Mn.

[0051] In some embodiments of the present application, the above-mentioned A source includes but is not limited to at least one of sulfate, carbonate, acetate and oxide of element A. The above-mentioned phosphate source includes but is not limited to at least one of potassium phosphate, sodium phosphate, ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate and phosphoric acid.

[0052] In some embodiments of the present application, the lithium source includes but is not limited to at least one of lithium hydroxide, lithium carbonate, lithium oxide, lithium acetate and lithium oxalate.

[0053] In the present application, step (1) can be to slowly add source A to a mixed solution of the shell metal source and the phosphate, or to directly mix the three. A person skilled in the art can choose according to the physical state of the raw materials and the activity of the reactants.

[0054] The embodiment of the present application also provides a positive electrode plate, which contains the positive electrode composite material 100 provided in the embodiment of the present application or contains the positive electrode composite material 100 prepared by the preparation method provided in the embodiment of the present application.

[0055] The positive electrode plate has a self-replenishing lithium effect, and has a high energy density and a long cycle life.

[0056] In some embodiments of the present application, the positive electrode sheet includes a current collector and a positive electrode active material layer disposed on at least one side of the current collector, and the positive electrode active material layer includes the positive electrode composite material, a binder, and an optional conductive agent. In some specific embodiments, the positive electrode active material layer contains only the positive electrode composite material, a binder, and an optional conductive agent. In other specific embodiments, the positive electrode active material layer also contains other positive electrode active materials, and the positive electrode composite material is mainly used as a lithium supplement additive. At this time, the positive electrode active material can be a material well known to those skilled in the art.

[0057] In the present application, the preparation process of the positive electrode sheet may include the following steps:

[0058] (1) mixing a positive electrode composite material, a binder, an optional conductive agent and a solvent to obtain a positive electrode slurry; or mixing a positive electrode active material, a positive electrode composite material, a binder, an optional conductive agent and a solvent to obtain a positive electrode slurry;

[0059] (2) The positive electrode slurry is coated on the current collector, and the positive electrode sheet is prepared through steps such as drying, rolling, and die cutting.

[0060] The embodiment of the present application also provides a secondary battery, which has the positive electrode plate provided in the embodiment of the present application.

[0061] The secondary battery has high energy density, good rate performance and long cycle life.

[0062] The technical solution of the present application is described in detail below in conjunction with specific embodiments.

[0063] Example 1

[0064] (1) Preparation of positive electrode composite material precursor: Weigh a certain mass of CoC2O4, NH4H2PO4, and MnO respectively, so that the molar ratio of Co element, phosphate, and manganese ion is 1:1:1, and disperse the above three substances in ethanol respectively. Then slowly add MnO solution to the mixed solution of CoC2O4 and NH4H2PO4 to obtain a positive electrode composite material precursor containing manganese oxide coated with a phosphate metal compound.

[0065] (2) Preparation of positive electrode composite materials: Slowly add a sufficient amount of lithium source - lithium oxide (ethanol dispersion of lithium oxide) into the mixed solution containing the positive electrode composite material precursor. After the reaction is complete, the above reaction solution is vacuum filtered, and the obtained solid is dried at 100°C and transferred to a tubular furnace for high-temperature sintering: under nitrogen protection, first sinter at 250°C for 5h, and then heat to 750°C and sinter for 10h, finally obtaining a positive electrode composite material with Li2MnO2 as the core and LiCoPO4 as the shell.

[0066] The core material mass is 1wt.% of the shell material, the shell thickness is 1.9μm, and the BET specific surface area is 0.55m 2 / g.

[0067] Example 2

[0068] (1) Preparation of positive electrode composite material precursor: Weigh a certain mass of ZnC2O4, NH4H2PO4, and MnO respectively, so that the molar ratio of Zn element, phosphate, and manganese ion is 1:1:1, and disperse the above three substances in water respectively. Then slowly add MnO solution to the mixed solution of ZnC2O4 and NH4H2PO4 to obtain a positive electrode composite material precursor containing manganese oxide coated with a phosphate metal compound.

[0069] (2) Preparation of positive electrode composite materials: Slowly add a sufficient amount of lithium source - lithium oxide to the mixed solution containing the positive electrode composite material precursor. After the reaction is complete, the above reaction solution is vacuum filtered, and the obtained solid is dried at 100°C and transferred to a tubular furnace for high-temperature sintering: under nitrogen protection, first sinter at 250°C for 5h, then heat to 750°C and sinter for 10h, finally obtaining a positive electrode composite material with Li2MnO2 as the core and LiZnPO4 as the shell.

[0070] The core material mass is 1wt.% of the shell material, the shell thickness is 2μm, and the BET specific surface area is 0.57m 2 / g.

[0071] Example 3

[0072] The difference from Example 2 is that the mass ratio of the raw materials in Example 2 is slightly adjusted, and the core material of the prepared positive electrode composite material is Li6MnO4.

[0073] Example 4

[0074] The difference from Example 2 is that the mass ratio of the raw materials in Example 2 is slightly adjusted, and the core material of the prepared positive electrode composite material is Li2MnO2, and the shell material is LiZnPO4.

[0075] The core material mass is 10wt.% of the shell material, the shell thickness is 1.8μm, and the BET specific surface area is 0.57m 2 / g.

[0076] Example 5

[0077] The difference from Example 2 is that the mass ratio of the raw materials in Example 2 is slightly adjusted, and the core material of the prepared positive electrode composite material is Li2MnO2, and the shell material is LiZnPO4.

[0078] The core material mass is 5wt.% of the shell material, the shell thickness is 2.1μm, and the BET specific surface area is 0.58m 2 / g.

[0079] Example 6

[0080] The difference from Example 2 is that the mass ratio of the raw materials in Example 2 is slightly adjusted, and the core material of the prepared positive electrode composite material is Li2MnO2, and the shell material is LiZnPO4.

[0081] The mass of the core material is 1wt.% of the shell material, and the thickness of the shell is 1.1 μm.

[0082] Example 7

[0083] The difference from Example 2 is that the mass ratio of the raw materials in Example 2 is slightly adjusted, and the core material of the prepared positive electrode composite material is Li2MnO2, and the shell material is LiZnPO4.

[0084] The mass of the core material is 10wt.% of the shell material, and the thickness of the shell is 10μm.

[0085] Example 8

[0086] The difference from Example 2 is that the mass ratio of the raw materials in Example 2 is slightly adjusted, and the core material of the prepared positive electrode composite material is Li2MnO2, and the shell material is LiZnPO4.

[0087] The mass of the core material is 0.09wt.% of the shell material, and the thickness of the shell is 0.09μm.

[0088] Example 9

[0089] The difference from Example 2 is that the surface of the positive electrode composite material further has an encapsulation layer, which is a carbon layer with a thickness of 10 nm.

[0090] Example 10

[0091] The difference from Example 2 is that the mass ratio of the raw materials in Example 2 is slightly adjusted, and the core material of the prepared positive electrode composite material is Li2MnO2, and the shell material is LiZnPO4.

[0092] The mass of the core material is 12wt.% of the shell material, and the thickness of the shell is 9μm.

[0093] Embodiment 11

[0094] (1) Preparation of positive electrode composite material precursor: Weigh a certain mass of FeC2O4, NH4H2PO4, and MnO respectively, so that the molar ratio of Fe element, phosphate, and manganese ion is 1:1:1, and disperse the above three substances in ethanol respectively. Then slowly add MnO solution to the mixed solution of FeC2O4 and NH4H2PO4 to obtain a positive electrode composite material precursor containing manganese oxide coated with a phosphate metal compound.

[0095] (2) Preparation of positive electrode composite materials: Slowly add a sufficient amount of lithium source - lithium oxide (ethanol dispersion of lithium oxide) into the mixed solution containing the positive electrode composite material precursor. After the reaction is complete, the above reaction solution is vacuum filtered, and the obtained solid is dried at 100°C and transferred to a tubular furnace for high-temperature sintering: under nitrogen protection, first sinter at 350°C for 5h, and then heat to 850°C and sinter for 10h, finally obtaining a positive electrode composite material with Li2MnO2 as the core and LiFePO4 as the shell.

[0096] The core material mass is 5wt.% of the shell material, and the BET specific surface area is 0.56m 2 / g.

[0097] Example 12

[0098] (1) Preparation of positive electrode composite material precursor: Weigh a certain mass of FeC2O4, NH4H2PO4, and MnO respectively, so that the molar ratio of Fe element, phosphate, and manganese ion is 0.7:1:1.3, and disperse the above three substances in ethanol. Then slowly add MnO solution to the mixed solution of FeC2O4 and NH4H2PO4 to obtain a phosphate-containing metal compound Fe 0.7 Mn 0.3 Positive electrode composite precursor of PO4-coated manganese oxide.

[0099] (2) Preparation of positive electrode composite material: Slowly add a sufficient amount of lithium source - lithium oxide (ethanol dispersion of lithium oxide) to the mixed solution containing the positive electrode composite material precursor. After the reaction is complete, the reaction solution is vacuum filtered, and the obtained solid is dried at 100°C and transferred to a tube furnace for high-temperature sintering: under nitrogen protection, first sinter at 350°C for 5h, then heat to 850°C for 10h, and finally obtain a Li2MnO2 core and LiFe 0.7 Mn 0.3 PO4 is the positive electrode composite material of the shell.

[0100] The core material mass is 5wt.% of the shell material, and the BET specific surface area is 0.55m 2 / g.

[0101] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are provided.

[0102] Comparative Example 1

[0103] The commercially available lithium cobalt phosphate positive electrode material and Li2MnO2 were simply mixed according to the proportions of Example 1 to form a positive electrode composite material. That is, compared with Example 1, the composite positive electrode material prepared in this comparative example is not a core-shell structure, and there is no chemical bond between the two.

[0104] Comparative Example 2

[0105] The commercially available lithium iron phosphate positive electrode material and Li2MnO2 were simply mixed in the proportions of Example 11 to form a positive electrode composite material. That is, compared with Example 11, the composite positive electrode material prepared in this comparative example is not a core-shell structure, and there is no chemical bond between the two.

[0106] Comparative Example 3

[0107] The commercially available lithium manganese iron phosphate positive electrode material and Li2MnO2 were simply mixed according to the proportions of Example 12 to form a positive electrode composite material. That is, compared with Example 12, the composite positive electrode material prepared in this comparative example is not a core-shell structure, and there is no chemical bond between the two.

[0108] The positive electrode composite materials prepared in the above-mentioned embodiments and comparative examples were directly used as positive electrode active materials to prepare positive electrode sheets and secondary batteries, respectively.

[0109] (1) Preparation of positive electrode sheet: N-methylpyrrolidone (NMP): positive electrode composite material: conductive agent-SuperP: binder-polyvinylidene fluoride PVDF are mixed in a mass ratio of 100:95:2:3 to obtain a positive electrode slurry; the above positive electrode slurry is coated on the positive electrode collector-aluminum foil, and a positive electrode sheet is prepared through the steps of drying, rolling, die cutting, etc.

[0110] (2) Preparation of negative electrode sheet: The negative electrode active material graphite, the conductive agent Super P, the thickener carboxymethyl cellulose (CMC), and the binder styrene butadiene rubber (SBR) are mixed evenly in deionized water to obtain a negative electrode slurry; the above negative electrode slurry is coated on the negative electrode current collector-copper foil, and the negative electrode sheet is prepared through the steps of drying, rolling, die cutting, etc.

[0111] (3) The positive electrode sheets and negative electrode sheets obtained in the above steps are alternately stacked together with the separator to prepare a battery by lamination, wherein the positive and negative electrode sheets are alternately arranged and the adjacent positive and negative electrode sheets are separated by the separator to obtain a dry battery cell. The dry battery cell is placed in an aluminum-plastic film outer packaging, injected with electrolyte, and then vacuum-sealed, and left at 60°C for 48 hours, and then pressurized at 60°C, secondary packaged, exhausted, and capacity divided to obtain a full battery.

[0112] Among them, the batteries with positive electrode sheets of the positive electrode composite materials provided in Examples 1-12 are respectively recorded as S1-S12, and the batteries with positive electrode composite materials provided in Comparative Examples 1-3 are respectively recorded as DS1-DS3.

[0113] (1) Characterization of the morphology and structure of the positive electrode composite materials prepared in some embodiments:

[0114] a) performing SEM test on the positive electrode composite material obtained in Example 12;

[0115] b) The positive electrode composite material prepared in Example 12 was subjected to XRD characterization test.

[0116] (2) The electrochemical performance tests were performed on the above batteries: 1) Charging: 0.05C constant current and constant voltage charging to 4.0V / Cell, cut-off current 0.02C, and standing for 10 min; 2) Discharging: 0.05C constant current and constant voltage charging to 2.5V / Cell, and standing for 10 min; 3) Cycling 3 times, recording the third discharge capacity as the actual capacity C0 of the battery; 4) Charging: With the actual capacity C0 of the battery as 1C, 1C constant current and constant voltage charging to 4.0V, cut-off current 0.02C; 5) Standing for 10 min; 6) Discharging: With the actual capacity C0 of the battery as 1C, 1C constant current and constant voltage charging to 2.5V; 7) Standing for 10 min; 8) Cycling 4) to 7) for a total of 200 times.

[0117] The first charge gram capacity of the battery and the cycle capacity retention rate after 200 cycles of the battery were tested, and the gram capacity improvement of each material was calculated. The results are summarized in Table 1. Among them, the gram capacity improvement of each positive electrode composite material refers to the difference between the first charge gram capacity of the above positive electrode composite material and the first charge gram capacity of the single shell material as the positive electrode active material. Generally, the first charge gram capacity of LiZnPO4 is 145mAh / g, the first charge gram capacity of LiCoPO4 is 130mAh / g, the first charge gram capacity of LiFePO4 is 155mAh / g, and the first charge gram capacity of LiFePO4 is 160mAh / g. 0.7 Mn 0.3 The first charge capacity of PO4 is 163 mAh / g.

[0118] Table 1 Electrochemical performance test results of batteries in various embodiments and comparative examples

[0119]

[0120]

[0121] From the SEM photo of the positive electrode composite material of Example 12 (see Figure 2 ) It can be seen that the positive electrode composite material is granular and nano-scale. In addition, from its XRD spectrum (see Figure 3 ) It can be seen that LiFe 0.7 Mn 0.3 The diffraction peak of PO4 is strong, while that of Li2MnO2 is weak. This is mainly because LiFe 0.7 Mn 0.3 PO4 is coated on the surface of Li2MnO2, and the shell has a certain thickness, but the penetration ability of X-rays is limited, which ultimately leads to a very weak signal of Li2MnO2.

[0122] It can be seen from the data in Table 1 that the core material Li x AO y As a lithium supplement, it releases Li upon charging activation. + After that, the first charge capacity of the positive electrode active material can be significantly improved. This is because the addition of the lithium supplement itself can increase the first charge capacity of the material, and the A element can form a high-performance positive electrode active material with the shell material. In particular, due to LiZnPO4, LiCoPO4, LiFePO4, LiFe 0.7 Mn 0.3 The first charge gram capacity of the four materials PO4 is different, so when the shell material is the only variable, the electrochemical performance of the corresponding battery will also be different. In addition, the core of the positive electrode composite material in Example 3 is Li6MnO4, which has a better lithium supplement effect, so the electrochemical performance of S3 is slightly higher than that of S2. It can be understood that when the content of the core material in the positive electrode composite material is within the preferred range (Example 4), the electrochemical performance of the battery can be further improved.

[0123] In addition, by comparing the electrochemical properties of the example battery and the comparative example battery, it can be found that even if the positive electrode composite material contains the same material composition, the different microscopic structures lead to differences in the microscopic reactions of the materials during charging and discharging, making the positive electrode composite material prepared in the example of the present application more excellent in various performances than the comparative example.

[0124] The above is an exemplary embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made thereto without departing from the principles of the present application. These improvements and modifications are also considered to be within the scope of protection of the present application.

Claims

1. A method for preparing a positive electrode composite material, characterized in that: The following steps are involved: (1) Mixing the A source, the M source, and the phosphoric acid source to obtain a cathode composite material precursor; (2) Mixing the positive electrode composite material precursor and a lithium source in a solvent so that lithium element is infiltrated into the positive electrode composite material precursor to obtain a composite material, filtering and drying the obtained composite material to obtain a solid, and sintering the solid under an inert atmosphere to obtain a positive electrode composite material; wherein the sintering treatment includes: sintering the solid at 250°C-400°C for 5h-10h, and then heating to 500°C-900°C for 10h-24h; Among them, the positive electrode composite material includes a core and a shell layer in-situ grown on the core, and the material of the core includes Li x AO y , the material of the shell layer includes a phosphate containing lithium and M element, where 1 < x ≤ 8, 0 < y ≤ 6, the A element includes at least one of Co, Cu, Ni, Fe, Zr, Zn, and Mn, and the M element includes a non-lithium metal element; the Li x AO y There is a metal bond between the elements in and the material of the shell layer, and the metal bond includes Li-M bond and / or A-M bond; the core is a lithium supplement agent; The phosphate containing lithium and M element includes LiM a PO b , wherein 0<a≤1, 0<b≤4; the M element includes at least one of Co, Cu, Ni, Ti, Al, Mg, Fe, V, Zr, Nd, Mo, Zn and Mn; The mass percentage of the core material to the shell material is in the range of 1%-5%.

2. The preparation method according to claim 1, characterized in that: The A source includes at least one of sulfate, carbonate, acetate and oxide of element A; the phosphate source includes at least one of potassium phosphate, sodium phosphate, ammonium phosphate, ammonium hydrogen phosphate, diammonium phosphate and phosphoric acid; the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium oxide, lithium acetate and lithium oxalate.

3. A positive electrode composite material, characterized in that: The positive electrode composite material is prepared according to the preparation method according to claim 1 or 2.

4. The positive electrode composite material according to claim 3, characterized in that: The metal bonds also include Li-A bonds.

5. The positive electrode composite material according to claim 3, characterized in that: The M element includes at least one of Mn and Fe.

6. The positive electrode composite material according to claim 3, characterized in that: The thickness of the shell layer is in the range of 0.1 μm-10 μm; the diameter of the core is in the range of 0.01 μm-15 μm; and the diameter of the positive electrode composite material is in the range of 0.1 μm-20 μm.

7. The positive electrode composite material according to claim 3, characterized in that: The BET specific surface area of ​​the positive electrode composite material is 0.1 m 2 / g-30m 2 / g range.

8. The positive electrode composite material according to any one of claims 3 to 7, characterized in that: The surface of the positive electrode composite material also has an encapsulation layer; the material of the encapsulation layer includes at least one of carbon material, polyaniline, polypyrrole, polyethylene oxide and poly-3,4-ethyldioxythiophene.

9. A positive electrode sheet, characterized in that: The positive electrode sheet contains the positive electrode composite material as described in any one of claims 3 to 8.

10. A secondary battery, characterized in that: The secondary battery has the positive electrode sheet as claimed in claim 9.

Images