Cathode material, preparation method thereof, cathode sheet and secondary battery

Abstract

A positive electrode material, a preparation method thereof, a positive electrode sheet, and a secondary battery, the positive electrode material comprising a core, a buffer layer, and a coating layer; the core comprising a lithium-rich compound; the coating layer being coated on an outer surface of the core; the buffer layer being located between the core and the coating layer; the anti-deformation capacity of the coating layer being greater than that of the core, and the anti-deformation capacity of the buffer layer being between that of the coating layer and that of the core. The structural stability of the positive electrode material can be improved by using the combination of the buffer layer and the coating layer.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, and particularly relates to a positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] As a representative of efficient and clean energy technology, lithium ion batteries have the advantages of high voltage, large energy density, good cycle performance and no memory effect, and have been widely used in many fields of national economy such as portable electronic devices, electric vehicles and large-scale energy storage.

[0003] However, while lithium ion batteries are widely used, the safety of lithium ion batteries has also received more attention. In the process of manufacturing lithium ion batteries, the positive electrode sheet usually needs to be extruded so that the positive electrode sheet can have a higher energy density. However, this process can also cause the structure of the positive electrode material to be damaged, thereby affecting the performance of the lithium battery. Moreover, the positive electrode material with damaged structure will cause deformation and expansion of the atomic lattice during use, thereby causing the battery sheet to appear phenomena such as powdering or peeling, which not only reduces the electrochemical performance and service life of the lithium ion battery, but also affects the safety of the battery.

[0004] Therefore, how to avoid the damage of the structure of the positive electrode material during the manufacturing process of the lithium ion battery and ensure that the positive electrode material does not appear phenomena such as powdering or peeling during the charging and discharging process has become a key problem. SUMMARY

[0005] The present application aims to provide a positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery.

[0006] The present application provides the following technical solutions:

[0007] In a first aspect, the application provides a positive electrode material, comprising a core, a buffer layer and a coating layer; wherein the core comprises a lithium-rich compound; the coating layer is coated on the outer surface of the core; the buffer layer is located between the core and the coating layer; the deformation resistance of the coating layer is greater than that of the core, and the deformation resistance of the buffer layer is between that of the coating layer and the core. By coating the core with a material having a high Young's modulus, the present application improves the deformation resistance of the core under pressure, reduces the surface residual alkali content, reduces the erosion of harmful substances in the external environment on the positive electrode material, and further improves the structural stability of the positive electrode material. When the lithium battery is assembled by using the positive electrode material, the positive electrode material will not collapse under uneven stress. Under the premise that the positive electrode material has a complete structure, the positive electrode plate formed also has a more uniform pore structure, which is more conducive to the penetration of electrolyte and the transmission of active ions. In addition, during the charging and discharging process of the lithium battery, the buffer layer and the coating layer can prevent the volume expansion of the lithium-rich compound, reduce the powdering of the electrode plate, and improve the electrochemical performance of the lithium battery.

[0008] In a possible implementation, the Young's modulus G1 of the buffer layer and the Young's modulus G2 of the coating layer are both greater than the Young's modulus G0 of the core, and the Young's modulus G2 of the coating layer is greater than the Young's modulus G1 of the buffer layer. Specifically, based on the above-mentioned mode, when the Young's modulus of the coating layer is greater than that of the buffer layer, the coating layer has more excellent deformation resistance. When the coating layer is extruded by external force, its deformation amount is small, so that the pressure on the outer surface of the buffer layer is uniform. Moreover, after the buffer layer and the coating layer cooperate, the deformation degree of the positive electrode material can be gradually relieved, so that the outer surface of each layer (from the coating layer to the core) is uniformly stressed on the basis of small deformation. Therefore, the positive electrode material will not collapse under uneven stress. In the case that the positive electrode material has a uniform particle size, the positive electrode plate formed also has a more uniform pore structure, which is more conducive to the penetration of electrolyte and the transmission of active ions.

[0009] In a possible implementation, the difference between the Young's modulus G2 of the coating layer 30 and the Young's modulus G1 of the buffer layer 20 is 40Gpa-350Gpa. By controlling the difference between the Young's modulus G2 of the coating layer and the Young's modulus G1 of the buffer layer within the above range, it can be ensured that the buffer layer and the coating layer can control their own deformation layer by layer through a significant difference, thereby further achieving the purpose of protecting the core.

[0010] In a possible implementation, the positive electrode material satisfies a relationship: 0.1≤(G2-G1) / (G1-G0)≤9, where G0 is the Young's modulus of the core, G1 is the Young's modulus of the buffer layer, and G2 is the Young's modulus of the cladding layer. When the relationship is satisfied, the difference between the Young's moduli of the layers is in a similar range, so that the pressure on the positive electrode material is consistent. When the difference between the Young's moduli of the buffer layer and the cladding layer is smaller than the range, the anti-deformation abilities of the buffer layer and the cladding layer are similar, and the effect of the cladding layer is small. When the difference between the Young's moduli of the buffer layer and the core is smaller than the range, the anti-deformation abilities of the buffer layer and the core are similar, and the effect of the buffer layer is small.

[0011] In a possible implementation, the thickness H1 of the buffer layer is 1 nm to 200 nm. Controlling the thickness of the buffer layer in the range facilitates adjustment of the particle size of the positive electrode material, and also ensures the specific capacity and electronic conduction environment of the positive electrode material.

[0012] In a possible implementation, the thickness H2 of the cladding layer is 1 nm to 200 nm. Controlling the thickness of the cladding layer in the range facilitates adjustment of the particle size of the positive electrode material, and also ensures the specific capacity and electronic conduction environment of the positive electrode material.

[0013] In a possible implementation, the thicknesses of the buffer layer and the cladding layer satisfy a relationship: 0.005≤H1 / H2≤200. When the relationship is satisfied, the thicknesses of the buffer layer and the cladding layer can be controlled in a suitable range, so that the buffer layer and the cladding layer can achieve the anti-deformation effect on the core through a suitable thickness ratio. When the thickness ratio is smaller than the range, the buffer layer is too thin relative to the cladding layer, so that the buffer layer cannot provide sufficient anti-deformation effect under the extrusion of the cladding layer, and the deformation degree of the buffer layer is large, which affects the core. When the thickness ratio is larger than the range, the cladding layer is too thin relative to the buffer layer, and a large deformation amount can be generated after the cladding layer is initially stressed, which affects the buffer layer.

[0014] In a possible implementation, the conductivity of the buffer layer is 1×10 -13 S / cm to 1.84 S / cm.

[0015] In a possible implementation, the conductivity of the cladding layer is 1×10 -13 S / cm to 1.84 S / cm.

[0016] In a possible implementation, the specific surface area of the positive electrode material is 0.1 m 2 / g to 35 m 2 / g.

[0017] In a possible implementation, the particle size D50 of the positive electrode material is 1 μm to 20 μm.

[0018] In a possible implementation, the mass of the buffer layer is 0.1% to 10% of the mass of the core. By controlling the mass ratio of the buffer layer within the above range, the cladding thickness of the buffer layer is more easily adjusted, the effective protection of the core by the buffer layer is achieved, and the buffer layer is prevented from being too thin or too thick, thereby affecting the structural stability or performance of the core.

[0019] In a possible implementation, the mass of the cladding layer is 0.1% to 10% of the mass of the core. By controlling the mass ratio of the cladding layer within the above range, the cladding thickness of the cladding layer is more easily adjusted, the effective protection of the core by the cladding layer is achieved, and the cladding layer is prevented from being too thin or too thick, thereby affecting the structural stability or performance of the core and the buffer layer.

[0020] In a possible implementation, the residual alkali degree of the positive electrode material is 0.1% to 3%.

[0021] In a possible implementation, the chemical formula of the lithium-rich compound is Li 1+x M y O z , wherein M is one or more elements selected from Fe, Ni, Mn, Cu, Zn, Co, Cr, Zr, Ni, Sb, Ti, V, Mo, Sn, and 0 < x ≤ 1, 0 < y, and 0 < z < 10. Specifically, the lithium-rich compound can be one or more of Li2NiO2, Li2CuO2, Li2CoO2, Li2MnO2, Li2Ni 0.5 Mn 1.5 O4.

[0022] In a second aspect, the present application also provides a preparation method of a positive electrode material, including: mixing a lithium-rich compound and a buffer material in a certain proportion, and sintering under an inert atmosphere to obtain a first positive electrode material; mixing the first positive electrode material and a cladding material in a certain proportion, and sintering under an inert atmosphere to obtain a second positive electrode material; the second positive electrode material includes a core, a buffer layer, and a cladding layer, the cladding layer is cladded on the outer surface of the core, and the buffer layer is located between the core and the cladding layer; the anti-deformation ability of the cladding layer is greater than that of the core, and the anti-deformation ability of the buffer layer is between that of the cladding layer and the core.

[0023] In a third aspect, the present application also provides a positive electrode tab, including a current collector and an active material layer arranged on the current collector, the active material layer including the positive electrode material according to any one of the above, or the active material layer including the positive electrode material obtained by the preparation method according to any one of the above.

[0024] In a fourth aspect, the present application provides a secondary battery comprising the positive electrode plate described in the above item, or the secondary battery comprises the positive electrode material described in any one of the above items, or the secondary battery comprises the positive electrode material prepared by the method described in any one of the above items. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced. Obviously, the drawings in the following description only represent some of the embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor on the basis of these drawings.

[0026] Figure 1 A cross-sectional view of a positive electrode material in an embodiment;

[0027] Figure 2 A flow chart of the preparation of a positive electrode material in an embodiment. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present application will be described in detail below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only represent some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0029] It should be noted that when a component is referred to as being "fixed" to another component, it can be directly on the other component or there can be an intervening component. When a component is referred to as being "connected" to another component, it can be directly connected to the other component or there can be an intervening component.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The use of the terms "and / or" includes a combination of one or more of the associated listed items.

[0031] Some embodiments of the present application will be described in detail below with reference to the accompanying drawings. The following embodiments and features in the embodiments can be combined with each other without conflict.

[0032] In a first aspect, the present application provides a positive electrode material, please refer to Figure 1, including a core 10, a buffer layer 20 and a cladding layer 30; wherein the core 10 comprises a lithium-rich compound; the cladding layer 30 is coated on the outer surface of the core 10; the buffer layer 20 is located between the core 10 and the cladding layer 30; the deformation resistance of the cladding layer 30 is greater than that of the core 10, and the deformation resistance of the buffer layer 20 is between that of the cladding layer 30 and the core 10.

[0033] Specifically, the positive electrode material is a core-shell structure, and the core 10 is a lithium-rich compound. The lithium-rich compound can be a positive electrode material particle, which is used to assemble a lithium battery with a negative electrode material and provide lithium ions to the negative electrode; the lithium-rich compound can also be a lithium supplement material particle, which is used as a "sacrificial agent" to supplement lithium source, thereby ensuring the first charge efficiency.

[0034] Optionally, the material of the buffer layer 20 can be organic or inorganic, and the buffer layer 20 can be an aerogel structure. When the material of the buffer layer 20 is organic, the material includes but is not limited to one or more of polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, polyamide, polyimide, polyvinylpyrrolidone, polyethylene oxide, polypyrrole, polytetrafluoroethylene, polyurethane, polyethylene dioxythiophene, pitch-based fibers such as pitch-based fibers. When the material of the buffer layer 20 is inorganic, the material includes but is not limited to one or more of artificial graphite, aluminum oxide, silicon dioxide, tungsten carbide, silicon carbide, silicon nitride, Li 1.4 Al 0.4 Ti 1.6 (PO4)3(LATP), Li7La3Zr2O 12 (LLZO), Li 6.4 La3Zr 1.4 Ta 0.6 O 12 and the like.

[0035] Optionally, the material of the cladding layer 30 can be organic or inorganic, and the material of the cladding layer 30 can be the same as or different from the material of the buffer layer 20, and the material of the cladding layer 30 can refer to the material of the buffer layer 20. It can be understood that Young's modulus is related to the structural properties of the material itself and the processing technology, such as single crystal, polycrystal, amorphous state, structure, dimension, processing technology. Therefore, when the materials of the buffer layer 20 and the cladding layer 30 are the same, the Young's modulus of the buffer layer 20 and the cladding layer 30 can be further adjusted by controlling the above parameters.

[0036] Further, the deformation resistance of the buffer layer 20 and the coating layer 30 is greater than that of the core 10. It can be understood that the deformation resistance can be reflected by the Young's modulus. In the case of the same stress degree, the greater the Young's modulus of a substance, the stronger the deformation resistance and the smaller the deformation degree. Of course, in other embodiments, the deformation resistance can also be embodied by other mechanical performance parameters, such as hardness, strength, toughness, etc. During the assembly of the battery, the pole piece will be extruded, and the positive electrode material will be extruded from the outside to the inside. The conduction process occurs from the coating layer to the core 10. In the case of the same compaction density, since the deformation degree of the coating layer itself is small, the pressure received by the coating layer can be uniformly transmitted to the core 10. In this way, the pressure received by the outer surface of the core 10 is also uniform, thereby avoiding the structural deformation or collapse of the core 10 due to uneven stress. At the same time, during the use of the positive electrode material, the core 10 can expand in lattice volume. Therefore, by using the buffer layer 20 and the coating layer 30 with stronger deformation resistance to coat the core 10, the expansion of the lattice volume of the positive electrode material can also be inhibited, and the phenomenon of powdering and falling of the pole piece assembled therefrom can be prevented.

[0037] The present application coats the core with a material with a high Young's modulus, thereby improving the pressure resistance of the core and reducing the surface residual alkali degree of the core, reducing the erosion of harmful substances in the external environment on the positive electrode material, and further improving the structural stability of the positive electrode material. When the positive electrode material is assembled into a lithium battery, the positive electrode material will not collapse under uneven stress. Under the premise that the positive electrode material has a complete structure, the positive electrode pole piece formed thereby also has a more uniform pore structure, which is more conducive to the penetration of the electrolyte and the transmission of active ions. In addition, during the charging and discharging process of the lithium battery, the buffer layer and the coating layer can also prevent the volume expansion of the lithium-rich compound, reduce the powdering of the pole piece, and improve the electrochemical performance of the lithium battery.

[0038] In a possible implementation, the chemical formula of the lithium-rich compound is Li 1+x M y O z , wherein M is one or more elements selected from Fe, Ni, Mn, Cu, Zn, Co, Cr, Zr, Ni, Sb, Ti, V, Mo, Sn, and 0 0.5 Mn 1.5 04.

[0039] In a possible implementation, the buffer layer can be one or more layers; and / or, the coating layer can be one or more layers.

[0040] In a possible implementation, the Young's modulus G1 of the buffer layer and the Young's modulus G2 of the cladding layer are both greater than the Young's modulus G0 of the core, and the Young's modulus G2 of the cladding layer is greater than the Young's modulus G1 of the buffer layer. Specifically, on the basis of the above manner, when the Young's modulus of the cladding layer 30 is greater than the Young's modulus of the buffer layer 20, the cladding layer 30 can have more excellent deformation resistance. When the cladding layer 30 is extruded by external force, the deformation amount thereof is small, so that the pressure on the outer surface of the buffer layer 20 is uniform. Moreover, after the buffer layer 20 and the cladding layer 30 cooperate, the deformation degree of the positive electrode material can be gradually relieved, so that the outer surface of each layer (to the core 10) is uniformly stressed on the basis of small deformation. Therefore, the positive electrode material will not collapse under uneven stress, and in the case that the positive electrode material has a uniform particle size, the positive electrode plate formed can also have a more uniform pore structure, which is more conducive to the penetration of electrolyte and the transmission of active ions.

[0041] Moreover, in order to obtain higher energy density when assembling a battery, the electrode plate is usually extruded, so that the positive electrode material inside the electrode plate is more dense. Based on the above process, the present application uses two materials with different Young's moduli to clad the positive electrode material, that is, the buffer layer of the positive electrode material uses an active substance with a low Young's modulus, and the cladding layer uses an active substance with a high Young's modulus. In the case of the same compaction density, the material with a high Young's modulus has a small deformation under pressure, and therefore will not damage the pore structure of the surface layer region of the electrode. In addition, because the deformation of the positive electrode material particles in the outer layer is small, more force can be transmitted to the positive electrode material particles located in the interior of the electrode plate, thereby ensuring that the upper and lower layer particles have consistent pressure, and the pore structure of the entire electrode is more conducive to the penetration of electrolyte and the transmission of active ions. On the other hand, the better pore structure is conducive to the retention of electrolyte in the electrode during the cycle process, thereby improving the cycle performance of the battery.

[0042] In a possible implementation, the difference between the Young's modulus G2 of the cladding layer 30 and the Young's modulus G1 of the buffer layer 20 is 40 GPa to 350 GPa. Specifically, the difference between the Young's modulus G2 of the cladding layer and the Young's modulus G1 of the buffer layer can be, but is not limited to, 40 GPa, 50 GPa, 60 GPa, 80 GPa, 100 GPa, 150 GPa, 200 GPa, 250 GPa, or 350 GPa. By controlling the difference between the Young's modulus G2 of the cladding layer 30 and the Young's modulus G1 of the buffer layer 20 within the above range, it can be ensured that the buffer layer and the cladding layer can control their own deformation through a significant difference layer by layer, thereby further achieving the purpose of protecting the core.

[0043] In a possible implementation, the positive electrode material satisfies a relationship: 0.1≤(G2-G1) / (G1-G0)≤9, where G0 is the Young's modulus of the core 10, G1 is the Young's modulus of the buffer layer, and G2 is the Young's modulus of the coating layer. Specifically, (G2-G1) / (G1-G0) can be, but is not limited to, 0.1, 0.2, 0.3, 0.5, 0.7, 1, 1.2, 1.5, 2, 3, 5, 7, or 9. When the relationship is satisfied, the difference between the Young's moduli of the layers is in a similar range, so that the pressure on the positive electrode material inside and outside can be consistent. When the difference between the Young's moduli of the buffer layer 20 and the coating layer 30 is smaller than the above range, the anti-deformation abilities of the coating layer 30 and the buffer layer 20 are similar, and the coating layer 30 is less likely to play a role. When the difference between the Young's moduli of the buffer layer 20 and the core 10 is smaller than the above range, the anti-deformation abilities of the buffer layer 20 and the core 10 are similar, and the buffer layer 20 is less likely to play a role.

[0044] In a possible implementation, the thickness H1 of the buffer layer 20 is 1 nm to 200 nm. Specifically, the thickness of the buffer layer 20 can be, but is not limited to, 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 150 nm, or 200 nm. Controlling the thickness of the buffer layer 20 in the above range is beneficial to adjusting the particle size of the positive electrode material, and can also ensure the specific capacity and electronic conductivity environment of the positive electrode material. When the thickness of the coating layer is less than the above range, the buffer layer 20 does not completely coat the core 10, and the anti-deformation effect of the core 10 cannot be achieved through the buffer layer 20. When the thickness of the buffer layer 20 is greater than the above range, the particle size of the positive electrode material is too large, and the overall mass capacity of the positive electrode material is reduced because the buffer layer 20 does not contribute lithium ions.

[0045] In a possible implementation, the thickness H2 of the coating layer 30 is 1 nm to 200 nm. Specifically, the thickness of the coating layer 30 can be, but is not limited to, 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 100 nm, 150 nm, or 200 nm. Controlling the thickness of the coating layer 30 in the above range is beneficial to adjusting the particle size of the positive electrode material, and can also ensure the specific capacity and electronic conductivity environment of the positive electrode material. When the thickness of the coating layer is less than the above range, the coating layer 30 does not completely coat the core 10, and the anti-deformation effect of the buffer layer 20 and the core 10 cannot be achieved through the coating layer 30. When the thickness of the coating layer 30 is greater than the above range, the particle size of the positive electrode material is too large, and the overall mass capacity of the positive electrode material is reduced because the buffer layer 20 does not contribute lithium ions.

[0046] In a possible implementation, the thicknesses of the buffer layer 20 and the cladding layer 30 satisfy the relationship: 0.005≤H1 / H2≤200. Specifically, the thickness ratio of the buffer layer 20 and the cladding layer 30 can be, but is not limited to, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 5, 10, 20, 50, 100, 200. When the above relationship is satisfied, the thicknesses of the buffer layer 20 and the cladding layer 30 can be controlled within a proper range, so that the buffer layer 20 and the cladding layer 30 can achieve the deformation prevention effect on the core 10 through a proper thickness ratio. When the thickness ratio is lower than the above range, the buffer layer 20 is too thin relative to the cladding layer 30, so that the buffer layer 20 cannot provide sufficient deformation resistance under the extrusion of the cladding layer 30, thereby causing a large deformation of the buffer layer 20 and affecting the core 10. When the thickness ratio is higher than the above range, the cladding layer 30 is too thin relative to the buffer layer 20, and a large deformation may occur in the cladding layer 30 after the cladding layer 30 is initially stressed, thereby affecting the buffer layer 20.

[0047] In a possible implementation, the conductivity of the buffer layer 20 is 1×10 -13 S / cm to 1.84 S / cm. Specifically, the conductivity of the buffer layer 20 can be, but is not limited to, 1×10 -13 S / cm, 0.67 S / cm, 0.85 S / cm, 1.02 S / cm, 1.28 S / cm, 1.49 S / cm, 1.62 S / cm, 1.84 S / cm.

[0048] In a possible implementation, the conductivity of the cladding layer 30 is 1×10 -13 S / cm to 1.84 S / cm. Specifically, the conductivity of the cladding layer 30 can be, but is not limited to, 1×10 -13 S / cm, 0.67 S / cm, 0.85 S / cm, 1.02 S / cm, 1.28 S / cm, 1.49 S / cm, 1.62 S / cm, 1.84 S / cm.

[0049] In a possible implementation, the specific surface area of the positive electrode material is 0.1 m 2 / g to 35 m 2 / g. Specifically, the specific surface area of the positive electrode material can be, but is not limited to, 0.1 m 2 / g, 1 m 2 / g, 2 m 2 / g, 5 m 2 / g, 10 m 2 / g, 20 m 2 / g, 35 m 2 / g.

[0050] In a possible implementation, the particle size D50 of the positive electrode material is 1 μm to 20 μm. Specifically, the particle size D50 of the positive electrode material can be, but is not limited to, 1 μm, 2 μm, 3 μm, 5 μm, 8 μm, 10 μm, 12 μm, 13 μm, 15 μm, or 20 μm.

[0051] In a possible implementation, the mass of the buffer layer 20 accounts for 0.1% to 10% of the mass of the core 10. Specifically, the mass of the buffer layer 20 can be, but is not limited to, 0.1%, 0.3%, 0.5%, 1%, 2%, 5%, 8%, or 10% of the mass of the core 10. By controlling the mass of the buffer layer 20 within the above range, the coating thickness of the buffer layer 20 can be more effectively adjusted, the effective protection of the core 10 by the buffer layer 20 can be achieved, and the structural stability or performance of the core 10 can be affected by the buffer layer 20 being too thin or too thick. When the mass of the buffer layer 20 is less than the above range, the buffer layer 20 can be too thin to resist deformation. When the mass of the buffer layer 20 is greater than the above range, the buffer layer 20 can be too thick, but the overall mass capacity of the positive electrode material can be reduced because the buffer layer 20 does not contribute lithium ions.

[0052] In a possible implementation, the mass of the buffer layer 20 accounts for 0.1% to 10% of the mass of the core 10. Specifically, the mass of the buffer layer 20 can be, but is not limited to, 0.1%, 0.3%, 0.5%, 1%, 2%, 5%, 8%, or 10% of the mass of the core 10. By controlling the mass of the buffer layer 20 within the above range, the coating thickness of the buffer layer 20 can be more effectively adjusted, the effective protection of the core 10 by the buffer layer 20 can be achieved, and the structural stability or performance of the core 10 can be affected by the buffer layer 20 being too thin or too thick. When the mass of the buffer layer 20 is less than the above range, the buffer layer 20 can be too thin to resist deformation. When the mass of the buffer layer 20 is greater than the above range, the buffer layer 20 can be too thick, but the overall mass capacity of the positive electrode material can be reduced because the buffer layer 20 does not contribute lithium ions.

[0053] In a possible implementation, the residual alkalinity of the positive electrode material is 0.1% to 3%. Specifically, the residual alkalinity of the positive electrode material can be, but is not limited to, 0.1%, 0.5%, 1%, 1.5%, 2.5%, or 3%. It can be understood that the above residual alkalinity is the residual alkalinity of the positive electrode material after the removal of residual alkalinity by the organic acid. The residual alkalinity within the above range has little effect on the performance of the positive electrode material, and the content of residual alkalinity in the positive electrode material after coating by the organic acid is significantly reduced.

[0054] In a second aspect, the present application also provides a preparation method of the positive electrode material, which is specifically used for preparing the positive electrode material in the first aspect. The preparation method includes the following steps: Figure 2

[0055] ​Step S10, mixing the lithium-rich compound and the buffer material in a certain proportion, and sintering under an inert atmosphere to obtain a first positive electrode material;

[0056] Step S20, mixing the first positive electrode material and the coating material in a certain proportion, and sintering under an inert atmosphere to obtain a second positive electrode material.

[0057] Specifically, the second positive electrode material includes a core, a buffer layer, and a coating layer, the coating layer is coated on the outer surface of the core, and the buffer layer is located between the core and the coating layer; the deformation resistance of the coating layer is greater than that of the core, and the deformation resistance of the buffer layer is between that of the coating layer and the core.

[0058] In one possible implementation, the preparation method of the lithium-rich compound in step S10 is not specifically limited, as long as the lithium-rich compound can be prepared. For example, the method for preparing the lithium-rich compound can adopt solid-phase sintering. In one possible implementation, the step for preparing the lithium-rich compound includes:

[0059] Step S001, uniformly mixing an M source and a lithium source in a certain molar ratio to obtain a mixture; and step S002, sintering the mixture under an inert atmosphere, and breaking after cooling to obtain a lithium-rich compound.

[0060] In step S001, the M source is a compound composed of the M element in the chemical formula of the lithium-rich compound provided in the first aspect, including at least one of an oxide, a hydroxide, a carbonate, a sulfate, and a chloride of M. The lithium source is at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium sulfate, and lithium oxalate.

[0061] In one possible implementation, in step S002, the inert atmosphere can be an atmosphere formed by any one of nitrogen, argon, and nitrogen-argon mixed gas.

[0062] In one possible implementation, in step S002, the sintering temperature of the mixture can be 650-900°C, and the sintering time can be 2-10 h. The temperature rising speed can be 100-500°C / h.

[0063] In one possible implementation, in step S10, the buffer material can be one or more of the raw materials of the buffer layer provided in the first aspect.

[0064] In one possible implementation, in step S10, the sintering temperature of the mixture of the lithium-rich compound and the buffer material can be 450-700°C, and the sintering time can be 1-4 h.

[0065] In a possible implementation, in step S20, the coating material can be one or more of the raw materials of the coating layer provided in the first aspect. Moreover, the buffer material and the coating material can be different.

[0066] In a possible implementation, in step S20, the sintering temperature of the mixture of the first positive electrode material and the coating material can be 450-700°C, and the sintering time can be 1-4 hours.

[0067] In the third aspect, the application further provides a positive electrode tab, which includes a current collector and an active material layer arranged on the current collector, and the active material layer includes the positive electrode material of any one of the second aspect. The positive electrode material of the application can not only supplement the active lithium consumed in the formation of the SEI film during the first charging of the battery as a lithium supplement additive, but also participate in the cycle as a positive electrode active material, and has a good application prospect. In some embodiments of the application, the current collector includes any one of a copper foil and an aluminum foil. In some embodiments, the active material layer includes an electrode active material, a lithium-rich material, a binder, and a conductive agent. In some embodiments, the active material layer includes a lithium-rich material, a binder, and a conductive agent, i.e., the lithium-rich material directly serves as an active material. In the embodiments of the application, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene butadiene rubber, hydroxypropyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives. In the embodiments of the application, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotube. In the embodiments of the application, the electrode active material includes one or more of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxyphosphate, fluorinated lithium vanadium phosphate, lithium titanate, lithium nickel cobalt manganate, and lithium nickel cobalt aluminum.

[0068] In the fourth aspect, the application further provides a secondary battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode includes the positive electrode tab provided by the application. The secondary battery provided by the application has good cycle performance and safety performance due to the use of the positive electrode material of the application, and is conducive to the application of the secondary battery in various fields.

[0069] The technical solutions of the application are described in detail below through specific embodiments.

[0070] Embodiment 1

[0071] The embodiment provides a positive electrode material and a preparation method thereof. The positive electrode material comprises an inner core with a Li2NiO2 composition, and a buffer layer and a coating layer are arranged outside the inner core. The material of the buffer layer and the coating layer is artificial graphite. The Young's modulus of the inner core is 8 Gpa, the Young's modulus of the buffer layer is 50 Gpa, the Young's modulus of the coating layer is 100 Gpa, the thickness of the buffer layer is 3.22 nm, and the thickness of the coating layer is 4.15 nm.

[0072] The preparation method of the positive electrode material comprises the following steps.

[0073] (1) Nickel oxide and lithium hydroxide are uniformly mixed according to a molar ratio of 1:2, and then sintered under a nitrogen atmosphere for 10 hours. After the tube furnace is cooled, the material is taken out and crushed to obtain a lithium-rich compound. The sintering temperature is 770 DEG C.

[0074] (2) Artificial graphite is added to the lithium-rich compound, and then uniformly mixed and sintered under an inert atmosphere for 1 hour to obtain a positive electrode material containing a buffer layer. The artificial graphite is weighed according to 0.5% of the mass of the lithium-rich compound, and the Young's modulus of the artificial graphite in this step is 50 Gpa. The sintering temperature is 450 DEG C.

[0075] (3) Artificial graphite is added to the positive electrode material containing the buffer layer, and then uniformly mixed and sintered under an inert atmosphere for 2 hours to obtain a positive electrode material containing a coating layer. The artificial graphite is weighed according to 0.5% of the mass of the lithium-rich compound, and the Young's modulus of the tungsten carbide in this step is 100 Gpa. The sintering temperature is 450 DEG C.

[0076] Embodiment 2

[0077] The embodiment provides a positive electrode material and a preparation method thereof. The positive electrode material comprises an inner core with a Li2NiO2 composition, and a buffer layer and a coating layer are arranged outside the inner core. The material of the buffer layer and the coating layer is artificial graphite. The Young's modulus of the inner core is 8 Gpa, the Young's modulus of the buffer layer is 50 Gpa, the Young's modulus of the coating layer is 100 Gpa, the thickness of the buffer layer is 3.22 nm, and the thickness of the coating layer is 4.15 nm.

[0078] The preparation method of the positive electrode material comprises the following steps.

[0079] (1) The same as in embodiment 1.

[0080] (2) The same as in embodiment 1, except that the buffer layer material is pitch-based fiber.

[0081] (3) The same as in embodiment 1, except that the coating layer material is pitch-based fiber.

[0082] Embodiment 3

[0083] The embodiment provides a positive electrode material and a preparation method thereof. The positive electrode material comprises an inner core with a Li2NiO2 composition, a buffer layer and a coating layer outside the inner core, and the buffer layer is made of 3,4-ethylenedioxythiophene (PEDOT) aerogel, and the coating layer is made of silicon carbide. The Young's modulus of the inner core is 8 Gpa, the Young's modulus of the buffer layer is 164 Gpa, the Young's modulus of the coating layer is 450 Gpa, the thickness of the buffer layer is 3.51 nm, and the thickness of the coating layer is 5.10 nm.

[0084] The preparation method of the positive electrode material comprises the following steps.

[0085] (1) The same as in example 1.

[0086] (2) The same as in example 1, except that the buffer layer is made of 3,4-ethylenedioxythiophene (PEDOT) aerogel.

[0087] (3) The same as in example 1, except that the coating layer is made of silicon carbide.

[0088] Example 4

[0089] The embodiment provides a positive electrode material and a preparation method thereof. The positive electrode material comprises an inner core with a Li2NiO2 composition, a buffer layer and a coating layer outside the inner core, and the buffer layer is made of 3,4-ethylenedioxythiophene (PEDOT) aerogel, and the coating layer is made of silicon carbide. The Young's modulus of the inner core is 8 Gpa, the Young's modulus of the buffer layer is 164 Gpa, the Young's modulus of the coating layer is 450 Gpa, the thickness of the buffer layer is 3.51 nm, and the thickness of the coating layer is 5.10 nm.

[0090] The preparation method of the positive electrode material comprises the following steps.

[0091] (1) The same as in example 3.

[0092] (2) The same as in example 3, except that 3,4-ethylenedioxythiophene (PEDOT) aerogel accounts for 2wt% of the mass fraction of the inner core material.

[0093] (3) The same as in example 3, except that silicon carbide accounts for 3wt% of the mass fraction of the inner core material.

[0094] Example 5

[0095] The embodiment provides a positive electrode material and a preparation method thereof. The positive electrode material comprises an inner core with a Li2NiO2 composition, a buffer layer and a coating layer outside the inner core, the material of the buffer layer is 3,4-ethylenedioxythiophene (PEDOT) aerogel, and the material of the coating layer is silicon carbide. The Young's modulus of the inner core is 8 Gpa, the Young's modulus of the buffer layer is 164 Gpa, the Young's modulus of the coating layer is 450 Gpa, the thickness of the buffer layer is 24.10 nm, and the thickness of the coating layer is 32.5 nm.

[0096] The preparation method of the positive electrode material comprises the following steps.

[0097] (1) The same as in example 3.

[0098] (2) The same as in example 3, except that the 3,4-ethylenedioxythiophene (PEDOT) aerogel accounts for 8wt% of the mass fraction of the inner core material.

[0099] (3) The same as in example 3, except that the silicon carbide accounts for 9wt% of the mass fraction of the inner core material.

[0100] Comparative example 1

[0101] The comparative example provides a positive electrode material and a preparation method thereof, and the positive electrode material is not provided with a buffer layer and a coating layer.

[0102] The preparation method of the positive electrode material comprises the following steps.

[0103] (1) The same as in example 1.

[0104] Comparative example 2

[0105] The comparative example provides a positive electrode material and a preparation method thereof. The positive electrode material comprises an inner core with a Li2NiO2 composition, and an outer coating layer of the inner core, the Young's modulus of the coating layer is greater than that of the inner core, the material of the coating layer is graphene, the Young's modulus of the coating layer is 15 Gpa, and the thickness of the coating layer is 2.51 nm.

[0106] The preparation method of the positive electrode material comprises the following steps.

[0107] (1) The same as in example 1.

[0108] (2) The same as in example 1.

[0109] The positive electrode materials provided in examples 1-5 and the positive electrode materials provided in comparative examples 1-2 are assembled into positive electrode electrodes and lithium ion batteries according to the following methods respectively.

[0110] Positive electrode: the positive electrode material was mixed with lithium iron phosphate at a mass ratio of 4:96 to obtain a mixture, the mixture was mixed with polyvinylidene fluoride and SP-Li at a mass ratio of 93:3:4 to obtain a positive electrode slurry, the positive electrode slurry was coated on the surface of an aluminum foil, vacuum dried at 110°C overnight, and rolled to obtain a positive electrode sheet;

[0111] Negative electrode: graphite was mixed with carboxymethyl cellulose (CMC), SBR and SP at a mass ratio of 95.8:1.2:2:1 to obtain a negative electrode slurry, the negative electrode slurry was coated on the surface of a copper foil, vacuum dried at 110°C overnight to obtain a negative electrode sheet;

[0112] Electrolyte: ethylene carbonate and methyl ethyl carbonate were mixed at a volume ratio of 3:7, and LiPF6 was added to form an electrolyte, the concentration of LiPF6 was 1 mol / L;

[0113] Separator: polypropylene microporous separator;

[0114] Lithium ion battery assembly: the lithium ion batteries were assembled in the order of graphite negative electrode sheet-separator-electrolyte-positive electrode sheet in an inert atmosphere glove box.

[0115] The electrochemical performance of each lithium ion battery assembled in the above lithium ion battery examples was tested as shown in Table 1, and the test conditions were as follows:

[0116] Constant current constant voltage charging, first cycle charging and discharging voltage 2.5-4.3V, current 0.1C, cutoff current 0.01C, 500 cycles with current 2C, cutoff current 0.01C.

[0117] The test results are shown in Table 1 below:

[0118] Table 1

[0119]

[0120] From the test results of Examples 1-5 and Comparative Examples 1-2 in Table 1, it can be seen that the lithium batteries prepared by using the positive electrode material provided by the present application all have high electrical performance. This shows that using a material with stronger deformation resistance as a buffer layer and coating layer to coat the lithium-rich compound can improve the electrochemical performance of the positive electrode material.

[0121] It can be seen from the comparison of Example 1 and Comparative Examples 1-2 that the positive electrode material with one layer of coating (coating layer) is superior to the positive electrode material without coating, and the positive electrode material with two layers of coating (buffer layer and coating layer) is superior to the positive electrode material with one layer of coating. Specifically, from the data of the cycle capacity retention rate at 500 cycles, it can be seen that the capacity retention rate in Examples 1-5 is greater than 87%, while the capacity retention rate in Comparative Examples 1-2 is less than 80%, which shows that the positive electrode material prepared in the present application with a double-layer coating structure has stable mechanical properties, and in the process of high-rate charging and discharging of lithium batteries, the coating layer can reduce the volume expansion of the positive electrode material, reduce the powdering of the electrode sheet, and improve the electrochemical performance of the lithium battery. This further shows that using a material with a higher Young's modulus than the positive electrode material to coat the positive electrode material can improve the stability of the surface of the positive electrode material and thus improve the electrochemical performance of the lithium battery.

[0122] It can be seen from Examples 1-3 that the scheme provided in the present application is not limited to using inorganic materials for coating, but can also use organic materials or a combination of organic and inorganic materials. The advantage is that the buffer layer and the coating layer can be more flexibly constructed according to the type of the core or the type of the electrode sheet required.

[0123] It can be seen from the comparison of Examples 3-5 that controlling the ratio and thickness of the coating layer and the buffer layer within a suitable range can improve the lithium ion transmission rate and reduce the heterojunction resistance, so the first charge capacity of the lithium battery prepared using the positive electrode material of Example 4 is significantly higher than that of the other examples.

[0124] In the description of the embodiments of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like refer to the orientation or positional relationship based on the drawings described, and are only intended to facilitate the description of the present application and simplify the description, and therefore cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0125] The above disclosure is only one preferred embodiment of the present application, and of course cannot limit the scope of the present application. Those skilled in the art can understand that the entire or partial processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present application still fall within the scope of the present application.

Claims

1. A positive electrode material, characterized by, The positive electrode material comprises: a core comprising a lithium-rich compound; a coating layer coated on an outer surface of the core; and a buffer layer between the core and the coating layer; the coating layer has a deformation resistance greater than that of the core, and the buffer layer has a deformation resistance between that of the coating layer and the core; the Young's modulus G1 of the buffer layer and the Young's modulus G2 of the coating layer are both greater than the Young's modulus G0 of the core, and the Young's modulus G2 of the coating layer is greater than the Young's modulus G1 of the buffer layer; the positive electrode material satisfies the relationship: 0.1≤(G2-G1) / (G1-G0)≤9, wherein G0 is the Young's modulus of the core, G1 is the Young's modulus of the buffer layer, and G2 is the Young's modulus of the coating layer; the difference between the Young's modulus G2 of the coating layer and the Young's modulus G1 of the buffer layer is 40Gpa-350Gpa. The lithium-rich compound has a chemical formula of Li 1+x M y O z wherein M is one or more elements selected from Fe, Ni, Mn, Cu, Zn, Co, Cr, Zr, Ni, Sb, Ti, V, Mo, Sn, and 0 2. The positive electrode material of claim 1, wherein, the thickness H1 of the buffer layer is 1nm-200nm; and / or, the thickness H2 of the coating layer is 1nm-200nm; and / or, the thicknesses of the buffer layer and the coating layer satisfy the relationship: 0.005≤H1 / H2≤200.

3. The cathode material of claim 1, wherein, the mass of the buffer layer is 0.1%-10% of the mass of the core; and / or, the mass of the coating layer is 0.1%-10% of the mass of the core.

4. The positive electrode material of claim 1, wherein, 6. The positive electrode material according to any one of claims 1-5, wherein:

5. The cathode material of claim 1, wherein, The conductivity of the buffer layer is 1 x 10 -13 S / cm ~ 1.84 S / cm; and / or, the conductivity of the cladding layer is 1 x 10 -13 S / cm ~ 1.84 S / cm. the particle size D50 of the positive electrode material is 1um-20um; and / or The specific surface area of the positive electrode material is 0.1 m 2 / g~35 m 2 / g, and / or, the residual alkalinity of the positive electrode material is 0.1%-3%. The positive electrode material comprises:

7. A method for producing a positive electrode material, characterized by, a lithium-rich compound and a buffer material are mixed in a certain proportion, and after sintering in an inert atmosphere, a first positive electrode material is obtained; the first positive electrode material and a coating material are mixed in a certain proportion, and after sintering in an inert atmosphere, a second positive electrode material is obtained; the second positive electrode material comprises a core, a buffer layer and a coating layer, the coating layer is coated on the outer surface of the core, and the buffer layer is between the core and the coating layer; the deformation resistance of the coating layer is greater than that of the core, and the deformation resistance of the buffer layer is between that of the coating layer and the core; the Young's modulus G1 of the buffer layer and the Young's modulus G2 of the coating layer are both greater than the Young's modulus G0 of the core, and the Young's modulus G2 of the coating layer is greater than the Young's modulus G1 of the buffer layer; the positive electrode material satisfies the relationship: 0.1≤(G2-G1) / (G1-G0)≤9, wherein G0 is the Young's modulus of the core, G1 is the Young's modulus of the buffer layer, and G2 is the Young's modulus of the coating layer; the positive electrode tab comprises a current collector and an active material layer arranged on the current collector, and the active material layer comprises the positive electrode material according to any one of claims 1-6, or the active material layer comprises the positive electrode material obtained by the preparation method of the positive electrode material according to claim 7. The lithium-rich compound has a chemical formula of Li 1+x M y O z wherein M is one or more elements selected from Fe, Ni, Mn, Cu, Zn, Co, Cr, Zr, Ni, Sb, Ti, V, Mo, Sn, and 0 8. A positive electrode sheet characterized by comprising: ​ 9. A secondary battery characterized by comprising: The secondary battery includes the positive electrode material as claimed in any one of claims 1 to 6, or the positive electrode material obtained by the preparation method as claimed in claim 7, or the positive electrode tab as claimed in claim 8.

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