Secondary battery, preparation method thereof and electric device

Abstract

The invention provides a secondary battery, a preparation method thereof and an electric device. The secondary battery comprises a positive pole piece, a negative pole piece, an isolating membrane arranged between the positive pole piece and the negative pole piece, and an electrolyte comprising an organic solvent and a lithium salt, the positive pole piece comprises a positive current collector and a positive membrane layer arranged on at least one side of the positive current collector, the positive membrane layer comprises a positive active material, and the negative pole piece comprises a negative active material. The positive electrode active material comprises lithium-containing transition metal phosphate and a carbon coating layer coating the outer surface of the lithium-containing transition metal phosphate, the carbon coating layer comprises SP2-form carbon and SP3-form carbon, the molar ratio of the SP2-form carbon to the SP3-form carbon is 1.2-6.5, and based on the mass of the carbon coating layer, the mass content of an oxygen element in the carbon coating layer is less than or equal to 16%. The secondary battery has excellent cycle performance and long service life.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a secondary battery, a preparation method thereof, and an electric device. BACKGROUND

[0002] Secondary batteries have the advantages of high energy density, high working voltage, low self-discharge rate, small size, and light weight, and are widely used in the field of consumer electronics.

[0003] At present, with the rapid development of electric vehicles and mobile electronic devices, people have increasingly high requirements for the cycle performance of secondary batteries. How to improve the cycle performance of batteries is a scientific and technical problem to be solved in the current application field of secondary batteries. SUMMARY

[0004] The present application is made in view of the above-mentioned problems, and aims to provide a secondary battery and a preparation method thereof, which has excellent cycle capacity retention rate and long service life.

[0005] A first aspect of the present application provides a secondary battery, comprising:

[0006] a positive electrode sheet, a negative electrode sheet, a separator film arranged between the positive electrode sheet and the negative electrode sheet, and an electrolyte comprising an organic solvent and a lithium salt,

[0007] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises a lithium-containing transition metal phosphate and a carbon coating layer coated on the outer surface of the lithium-containing transition metal phosphate, the carbon coating layer contains SP2 form carbon and SP3 form carbon, the molar ratio of the SP2 form carbon to the SP3 form carbon is 1.2-6.5, and the mass content of oxygen in the carbon coating layer is less than or equal to 16% based on the mass of the carbon coating layer.

[0008] In one aspect, by selecting the carbon coating layer to contain both SP2 form carbon and SP3 form carbon, the SP2 form carbon has less structural defects and internal pores, which is conducive to improving the powder compaction density of the material, and a high-energy-density secondary battery can be obtained. Meanwhile, the less structural defects and internal pores of the SP2 form carbon can also improve the powder resistivity of the carbon coating layer and the material, and improve the conductivity of the material. The SP3 form carbon has relatively more internal defects and internal pores, which can provide a sufficient number of lithium ion channels, and can also improve the wettability of the electrolyte to the material, promote the transmission of lithium ions in the cycle process, and is conducive to the deintercalation of lithium ions. By controlling the SP2 form carbon and the SP3 form carbon in the carbon coating layer within a suitable range, both a high powder compaction density and a powder resistivity can be achieved, and the wettability of the electrolyte to the material and the transmission of lithium ions can also be promoted, thereby comprehensively improving the cycle performance of the battery. On the other hand, by controlling the mass content of oxygen elements in the carbon coating layer to be less than or equal to 16%, the possibility of side reactions between the oxygen elements in the carbon coating layer and the electrolyte can be reduced, the side reactions at the interface between the material and the electrolyte can be effectively reduced, and the cycle performance and safety performance of the battery can be improved.

[0009] In any embodiment, the mass content of oxygen elements in the carbon coating layer is 1%-5% based on the mass of the carbon coating layer.

[0010] Controlling the mass content of oxygen elements in the carbon coating layer within a suitable range can reduce the possibility of side reactions between the oxygen elements in the carbon coating layer and the electrolyte, and a certain content of oxygen elements is conducive to improving the compatibility of the carbon coating layer with the lithium-containing transition metal phosphate, conducive to forming a dense carbon coating layer, improving the capacity of the material, and improving the cycle performance and energy density of the battery.

[0011] In any embodiment, the molar ratio of the SP2 form carbon to the SP3 form carbon is 3.0-6.5.

[0012] Controlling the SP2 form carbon and the SP3 form carbon in the carbon coating layer within a suitable range can improve the powder compaction density and the powder resistivity of the material, promote the wettability of the electrolyte to the material and the transmission of lithium ions, improve the cycle performance of the battery, and prolong the service life of the battery.

[0013] In any embodiment, the lithium-containing transition metal phosphate includes lithium iron manganese phosphate and modified compounds thereof.

[0014] In any embodiment, the mass content of the carbon coating layer is 1.20%-2.0% based on the total mass of the positive electrode active material.

[0015] The mass content of the carbon coating layer is within a suitable range, which can form a complete coating on the surface of the positive electrode active material, achieve the purpose of improving the powder resistivity and conductivity of the material, is also conducive to the deintercalation of lithium ions in the carbon coating layer, and also enables the material to have a certain gram capacity, thereby comprehensively improving the cycle performance and energy density of the battery.

[0016] In any embodiment, the powder compaction density of the positive electrode active material is 2.15 g / cm3 under a pressure of 29400 N. 3 - 2.50 g / cm3. 3 .

[0017] The powder compaction density of the positive electrode active material is within a suitable range, the positive electrode film layer has a high compaction density, which can improve the energy density of the secondary battery; and is also conducive to the positive electrode film layer having a suitable pore structure, reducing the difficulty of ion liquid phase transmission, improving the ion and electron transmission performance, improving the impregnation and retention properties of the electrolyte of the positive electrode film layer, thereby further improving the rate performance and cycle performance of the secondary battery.

[0018] In any embodiment, the thickness of the carbon coating layer is 2 nm-40 nm.

[0019] The thickness of the carbon coating layer is within a suitable range, which can reduce the dissolution of manganese ions, improve the conductivity of the material, is also conducive to the deintercalation of lithium ions in the carbon coating layer, and also enables the material to have a certain gram capacity, thereby comprehensively improving the cycle performance and energy density of the battery.

[0020] A second aspect of the present application provides a preparation method of a secondary battery, comprising the following steps:

[0021] Assembling the positive electrode sheet, the negative electrode sheet, the separator, and the electrolyte into a secondary battery.

[0022] The positive electrode sheet comprises a positive electrode current collector and a positive electrode film layer arranged on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, the positive electrode active material comprises a lithium-containing transition metal phosphate and a carbon coating layer coated on the outer surface of the lithium-containing transition metal phosphate, the carbon coating layer comprises SP2 form carbon and SP3 form carbon, the molar ratio of the SP2 form carbon to the SP3 form carbon is 1.2-6.5, and the mass content of oxygen in the carbon coating layer is less than or equal to 16% based on the mass of the carbon coating layer.

[0023] The positive electrode active material with the carbon coating layer containing both SP2 carbon and SP3 carbon can realize high powder compaction density and powder resistivity, promote the infiltration of electrolyte and the transmission of lithium ions, and improve the cycle performance of the battery. Controlling the content of oxygen element in the carbon coating layer can further reduce the side reaction between the active material and the electrolyte and improve the cycle performance of the battery.

[0024] In any embodiment, the preparation method of the positive electrode active material is specifically as follows:

[0025] Spray drying: spray drying the mixed slurry containing the lithium-containing transition metal phosphate raw material, the SP2 carbon source and the SP3 carbon source to obtain a precursor powder;

[0026] Sintering treatment: sintering the precursor powder to obtain the positive electrode active material.

[0027] The carbon source includes the SP2 carbon source and the SP3 carbon source, and the carbon coating layer containing both SP2 carbon and SP3 carbon can be obtained.

[0028] In any embodiment, the preparation method of the positive electrode active material further includes,

[0029] Pre-oxidation treatment: pre-oxidizing the SP2 carbon initial carbon source in an oxygen-containing atmosphere to obtain the SP2 carbon source.

[0030] The pre-oxidation treatment of the SP2 carbon initial carbon source can destroy the double bond in the SP2 carbon source, facilitate the decomposition of the carbon source in the subsequent sintering treatment, facilitate the release of oxygen element in the subsequent sintering treatment, and be conducive to reducing the mass content of oxygen element in the carbon coating layer, reducing the possibility of side reaction between the oxygen element in the carbon coating layer and the electrolyte, and improving the cycle performance of the battery.

[0031] In any embodiment, the treatment temperature of the pre-oxidation treatment is 150-300°C; and / or, the treatment time of the pre-oxidation treatment is 1-20h.

[0032] Controlling the temperature and time of the pre-oxidation treatment in a suitable range is conducive to obtaining the SP2 carbon source with the mass content of oxygen-containing functional groups in a suitable range, conducive to controlling the mass content of oxygen element in the carbon coating layer in a suitable range, and conducive to forming a dense carbon coating layer on the material surface while reducing the side reaction between the material and the electrolyte, thereby comprehensively improving the cycle performance of the battery.

[0033] In any embodiment, the mass ratio of the SP2 form carbon source and the SP3 form carbon source is 1:9-9:1.

[0034] Controlling the mass ratio of the SP2 form carbon source and the SP3 form carbon source within a suitable range is conducive to controlling the SP2 form carbon and the SP3 form carbon in the carbon coating layer within a suitable range, which can achieve high powder compaction density and powder resistivity of the material, promote the infiltration of electrolyte and the transmission of lithium ions, and comprehensively improve the cycle performance of the battery.

[0035] In any embodiment, the total mass content of the SP2 form carbon source and the SP3 form carbon source is 2%-8%, based on the mass of the lithium-containing transition metal phosphate raw material.

[0036] The total mass content of the SP2 form carbon source and the SP3 form carbon source within a suitable range is conducive to controlling the mass content of the carbon coating layer within a suitable range, which can form a complete coating on the surface of the positive active material and achieve the purpose of improving the powder resistivity and conductivity of the material.

[0037] In any embodiment, the SP2 form carbon source and the SP2 form carbon initial carbon source include one or more of phenolic resin, epoxy resin, melamine resin, polyfurfur alcohol, polyaniline, polyethylene oxide, polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, lignin, butylbenzene polymer, and polyethylene terephthalate.

[0038] In any embodiment, the SP3 form carbon source includes one or more of sucrose, glucose, dextrin, citric acid, fructose, and starch.

[0039] In any embodiment, the sintering temperature of the sintering treatment is 650°C-780°C; and / or, the sintering time of the sintering temperature is 8h-16h.

[0040] The sintering temperature and / or time within a suitable range is conducive to forming a complete and dense carbon coating layer on the surface of the lithium-containing transition metal phosphate by the SP2 form carbon source and the SP3 form carbon source, which achieves the purpose of improving the conductivity of the material and promoting the transmission of lithium ions, and improves the cycle performance of the battery.

[0041] The third aspect of the present application provides a power utilization device including the secondary battery of the first aspect or the secondary battery prepared by the preparation method of the second aspect. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0043] Figure 2 is Figure 1An exploded view of the secondary battery of one embodiment of the present application.

[0044] Figure 3 FIG. 1 is a schematic view of a battery module of one embodiment of the present application.

[0045] Figure 4 FIG. 2 is a schematic view of a battery pack of one embodiment of the present application.

[0046] Figure 5 FIG. 3 is a schematic view of a power storage device of one embodiment of the present application. Figure 4 FIG. 4 is an exploded view of the battery pack of one embodiment of the present application.

[0047] Figure 6 FIG. 5 is a schematic view of a power storage device of one embodiment of the present application.

[0048] REFERENCE NUMERALS:

[0049] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 case; 52 electrode assembly; 53 lid plate. DETAILED DESCRIPTION

[0050] Hereinafter, embodiments of the secondary battery of the present application and a method for manufacturing the same, and a power storage device will be specifically described with appropriate reference to the accompanying drawings. However, there will be cases of omission of unnecessary detailed description. For example, there will be cases of omission of detailed description of matters that are already well known, and repeated description of substantially identical structures. This is in order to avoid the following description from becoming unnecessarily lengthy, and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided in order for those skilled in the art to sufficiently understand the present application, and are not intended to limit the subject matter recited in the claims.

[0051] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0052] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0053] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0054] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0055] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0056] If not specifically stated, the term "or" is inclusive in this application, by way of example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following satisfy the condition "A or B": A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present).

[0057] Lithium-containing transition metal phosphate has the advantages of good safety performance, wide raw material sources, long cycle life, etc., and is a hot spot in the industry. However, this kind of material itself has the problem of poor conductivity, which restricts its further development. Carbon coating can improve the electrochemical performance of the material, but the carbon coating layer in the prior art is mostly in a loose and porous state. The loose and porous carbon coating layer structure will accelerate the side reaction between the positive active material and the electrolyte, which will significantly reduce the cycle performance of the battery.

[0058] [Secondary battery]

[0059] Based on this, the application provides a kind of secondary battery, comprising:

[0060] Positive pole piece, negative pole piece, separator film arranged between the positive pole piece and the negative pole piece, and electrolyte comprising organic solvent and lithium salt,

[0061] The positive pole piece comprises a positive pole current collector and a positive pole film layer arranged on at least one side of the positive pole current collector, the positive pole film layer comprises a positive pole active material, the positive pole active material comprises a lithium-containing transition metal phosphate and a carbon coating layer coated on the outer surface of the lithium-containing transition metal phosphate, the carbon coating layer contains SP2 form carbon and SP3 form carbon, the molar ratio of the SP2 form carbon to the SP3 form carbon is 1.2-6.5, and the mass content of oxygen elements in the carbon coating layer is less than or equal to 16% based on the mass of the carbon coating layer.

[0062] In some embodiments, the molar ratio of the SP2 form carbon to the SP3 form carbon can be selected as 1.2, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, 6.5 or any numerical range between any two of them.

[0063] The molar ratio of the SP2 form carbon to the SP3 form carbon can be tested by any known method in the art. As an example, first measure the Raman spectrum of the positive pole active material, and then obtain Ig / Id (where Id is the characteristic peak of SP3 form carbon and Ig is the characteristic peak of SP2 form carbon) by peak separation of the energy spectrum of Raman test, and then calculate the peak area ratio of the characteristic peak of SP3 form carbon to the characteristic peak of SP2 form carbon by integration, so as to obtain the molar ratio of the SP2 form carbon to the SP3 form carbon.

[0064] In some embodiments, based on the mass of the carbon coating, the mass content of oxygen in the carbon coating can be selected as 1%, 5%, 10%, 15%, 16%, or any value range between the two.

[0065] The mass content of oxygen in the carbon coating can be tested using any method known in the art. As an example, the positive electrode active material is placed in a reaction vessel for a high-temperature pyrolysis-reduction reaction to obtain a reaction gas. The reaction gas is then separated by chromatography under the impetus of a carrier gas, and carbon monoxide (the oxygen-containing functional groups in the carbon coating are converted into carbon monoxide through a high-temperature pyrolysis-reduction reaction) is detected by a thermal conductivity detector. The mass content of oxygen can then be calculated.

[0066] In this article, the term "lithium-containing transition metal phosphate" refers to LiFePO4, LiMnPO4, and LiMn 1-z Fe z PO4 and its modified compounds, where 0 < z < 1.

[0067] In this application, "modified compound" refers to a compound obtained by doping or coating a material.

[0068] In this paper, lithium transition metal phosphate-doped and modified compounds refer to those with the general formula Li 1+y1 Fe y2 Mn y3 M1 y4 P 1- y5 O 4-y6 M2 y7 Materials,

[0069] Wherein, -0.2≤y1≤0.2, 0≤y2≤1, 0≤y3≤1, 0≤y4≤0.1, 0≤y5≤0.1, 0≤y6≤0.4, 0≤y7≤0.1, M1 includes one or more of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge, and M2 includes one or more of B, S, Si, N, F, Cl, and Br. The value of y1 is, for example, but not limited to, -0.8, -0.7, -0.6, -0.5, -0.4, -0.3, -0.2, -0.1, 0, 0.1, and 0.2. The values ​​of y2 and y3 are, for example, but not limited to, 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1. The values ​​of y4, y5, and y7 are, for example, but not limited to, 0, 0.05, 0.1, etc. The values ​​of y6 are, for example, but not limited to, 0, 0.1, 0.2, 0.3, 0.4, etc.

[0070] In the present disclosure, the coated modified compound of lithium-containing transition metal phosphate includes a core of lithium-containing transition metal phosphate and its doped modified compound, and a polymer coating layer or a metal oxide layer coated on the surface of the lithium-containing transition metal phosphate and its doped modified compound, wherein the polymer coating layer includes one or more of polypyrrole, polyaniline, and polythiophene, and the metal oxide layer includes one or more of aluminum oxide, tungsten oxide, niobium oxide, zinc oxide, and zirconium oxide.

[0071] By controlling the SP2 form carbon and SP3 form carbon to be contained in the carbon coating layer at the same time, on the one hand, the SP2 form carbon has less structural defects and internal pores, which is beneficial to improve the powder compaction density of the material, and a high-energy-density secondary battery can be obtained. Meanwhile, the SP2 form carbon also has less structural defects and internal pores, which can improve the powder resistivity of the carbon coating layer and the material, and improve the conductivity of the material. The SP3 form carbon has relatively more internal defects and internal pores, which can provide a sufficient number of lithium ion paths, and can also improve the wettability of the electrolyte to the material, promote the transmission of lithium ions in the cycle process, and facilitate the deintercalation of lithium ions. Meanwhile, the SP3 form carbon is easy to form a uniform carbon coating on the surface of the material. By controlling the SP2 form carbon and SP3 form carbon in the carbon coating layer within a suitable range, both high powder compaction density and powder resistivity can be achieved, and the wettability of the electrolyte to the material and the transmission of lithium ions can also be promoted, thereby comprehensively improving the cycle performance of the battery. Meanwhile, the mass content of oxygen elements in the carbon coating layer is less than or equal to 16%, which can reduce the possibility of side reactions between the oxygen elements in the carbon coating layer and the electrolyte, effectively reduce the side reactions at the interface between the material and the electrolyte, and improve the cycle performance and safety performance of the battery.

[0072] In some embodiments, the mass content of oxygen elements in the carbon coating layer is 1%-5% based on the mass of the carbon coating layer. In some embodiments, the mass content of oxygen elements in the carbon coating layer can be selected as 1%, 2%, 3%, 4%, 5%, or a numerical range between any two of them.

[0073] Controlling the mass content of oxygen elements in the carbon coating layer within a suitable range can reduce the possibility of side reactions between the oxygen elements in the carbon coating layer and the electrolyte, and a certain content of oxygen elements is beneficial to improve the compatibility of the carbon coating layer and the lithium-containing transition metal phosphate, facilitate the formation of a dense carbon coating layer, improve the capacity of the material, and improve the cycle performance and energy density of the battery.

[0074] In some embodiments, the molar ratio of the SP2 form carbon to the SP3 form carbon is 3.0-6.0. In some embodiments, the molar ratio of the SP2 form carbon to the SP3 form carbon can be selected as 3.0, 3.5, 4.0, 4.5, 5.0, 6.0, or a numerical range between any two of them.

[0075] By controlling the SP2 and SP3 carbon forms in the carbon coating layer within a suitable range, the powder compaction density and powder resistivity of the material can be improved, while also promoting the wetting of the material by the electrolyte and the transport of lithium ions, thus improving the cycle performance of the battery and extending its service life.

[0076] In some embodiments, the lithium-containing transition metal phosphate includes lithium manganese iron phosphate and its modified compounds.

[0077] In this article, the term "lithium manganese iron phosphate and its modified compounds" refers to LiMn 1-z Fe z PO4 and its modified compounds, where 0 < z < 1,

[0078] In this application, "modified compound" refers to a compound obtained by doping or coating a material.

[0079] In this article, the doped and modified compounds of lithium manganese iron phosphate refer to those with the general formula Li 1+y1 Fe y2 Mn y3 M1 y4 P 1-y5 O 4-y6 M2 y7 Materials,

[0080] Wherein, -0.2≤y1≤0.2, 0<y2<1, 0<y3<1, 0≤y4≤0.1, 0≤y5≤0.1, 0≤y6≤0.4, 0≤y7≤0.1, M1 includes one or more of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge, and M2 includes one or more of B, S, Si, N, F, Cl, and Br. The value of y1 is, for example, but not limited to, -0.2, -0.1, 0, 0.1, 0.2, etc. The values ​​of y2 and y3 are, for example, but not limited to, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc. The values ​​of y4, y5, and y7 are, for example, but not limited to, 0, 0.05, 0.1, etc. The values ​​of y6 can be, for example, but not limited to, 0, 0.1, 0.2, 0.3, 0.4, etc.

[0081] In this paper, the coating modification compound of lithium manganese iron phosphate includes a core of lithium manganese iron phosphate and its doped modification compound and a polymer coating layer or metal oxide layer coating the outer surface of lithium manganese iron phosphate and its doped modification compound. The polymer coating layer includes one or more of polypyrrole, polyaniline, and polythiophene, and the metal oxide layer includes one or more of alumina, tungsten oxide, niobium oxide, zinc oxide, and zirconium oxide.

[0082] The manganese site doping element M1 selected from the above elements helps to reduce the lattice change rate of lithium manganese phosphate during lithium deintercalation, improve the structural stability of the positive electrode active material, greatly reduce the dissolution of manganese and reduce the oxygen activity on the particle surface; the phosphorus site doping element M2 selected from the above elements also helps to change the difficulty of Mn-O bond length change, thereby improving the electronic conductivity and reducing the lithium ion migration barrier, promoting lithium ion migration and improving the rate performance of the secondary battery.

[0083] The lithium manganese iron phosphate and the modified compound thereof have high specific capacity and low raw material cost. Since the positive electrode active material will undergo an electrochemical reaction when applied in a battery, the participation of electrons is required. Therefore, in order to increase the electron transmission between particles, the SP2 form carbon and the SP3 form carbon are coated on the surface of the lithium manganese iron phosphate and the modified compound thereof, and the mass content of oxygen elements in the carbon coating layer is controlled to be less than or equal to 16%. In this way, the conductivity of the material is improved, the transmission speed of lithium ions is also improved, the possibility of side reaction between the carbon coating layer and the electrolyte is reduced, and a positive electrode active material with low cost, high specific capacity, high conductivity and high cycle stability is obtained, thereby improving the cycle performance of the battery.

[0084] In some embodiments, the mass content of the carbon coating layer is 1.20%-2.0% based on the total mass of the positive electrode active material. In some embodiments, the mass content of the carbon coating layer can be selected as 1.2%, 1.4%, 1.6%, 1.8%, 2.0% or any numerical range between any two of them based on the total mass of the positive electrode active material.

[0085] The mass content of the carbon coating layer in the positive electrode active material can be tested by any method known in the art. As an example, turn on all power switches of the carbon sulfur analyzer, press the "zero" button, open the oxygen valve of the carbon sulfur analyzer, adjust the oxygen pressure to 0.02-0.04 MPa. Open the "front oxygen" "rear control", adjust the flowmeter to about 100 L / h. Add silicon molybdenum powder (0.3 g), weighed sample (250 mg), tin particles (0.3 g), pure iron (1 g) in the crucible in turn, close the crucible. Click the "test" button to start the test, and the test result is automatically displayed after the test is completed. Record this result as the mass content of the carbon coating layer.

[0086] The mass content of the carbon coating layer is within a suitable range, which can form a complete coating on the surface of the positive electrode active material, achieve the purpose of improving the powder resistivity and conductivity of the material, and also facilitate the deintercalation of lithium ions in the carbon coating layer, and make the material have a certain specific capacity, thereby comprehensively improving the cycle performance and energy density of the battery.

[0087] In some embodiments, the compacted density of the positive electrode active material at a pressure of 29400 N is 2.15 g / cm³. 3 -2.50g / cm 3 In some embodiments, the compacted density of the positive electrode active material at a pressure of 29400 N can be selected as 2.15 g / cm³. 3 2.2g / cm 3 2.30g / cm 3 2.35g / cm 3 2.40 g / cm 3 2.50g / cm 3 Or the range of values ​​between any two.

[0088] The compaction density of the positive electrode active material powder under 29400 N pressure can be tested using any method known in the art. As an example, referring to GB / T 24533-2009, 1 g of positive electrode active material powder is weighed and added to a container with a bottom area of ​​1.327 cm². 2 In the mold, pressure is applied to a specific pressure, such as 29400N, held for 30s, then depressurized and held for 10s. The compaction density of the positive electrode active material under the selected pressure is determined by an electronic pressure testing machine (such as a UTM7305 electronic pressure testing machine).

[0089] When the powder compaction density of the positive electrode active material is within a suitable range, the positive electrode film layer has a high compaction density, which can improve the energy density of the secondary battery. It is also beneficial for the positive electrode film layer to have a suitable pore structure, reduce the difficulty of ion liquid phase transport, improve ion and electron transport performance, and improve the wetting and retention characteristics of the positive electrode film layer to the electrolyte, thereby further improving the rate performance and cycle performance of the secondary battery.

[0090] In some embodiments, the thickness of the carbon coating layer is 2nm-40nm. In some embodiments, the thickness of the carbon coating layer can be selected from 2nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, or any value range between the two.

[0091] The thickness of the carbon coating can be measured using any method known in the art. As an example, a thin slice approximately 100 nm thick is cut from the middle of a single particle of the positive electrode active material using FIB (fiber optics), and then TEM (thermometry) is performed on the slice to obtain the raw TEM image, which is saved in the raw image format (xx.dm3). The raw TEM image is opened in DigitalMicrograph software, and the carbon coating is identified using lattice spacing and angle information. The thickness of the carbon coating is then measured. The thickness is measured at three locations on the selected particle, and the average value is taken.

[0092] Within the thickness range of the carbon coating layer, the dissolution of manganese ions can be reduced, the conductivity of the material can be improved, and the insertion and extraction of lithium ions in the carbon coating layer can be facilitated. It also enables the material to have a certain specific capacity, thus comprehensively improving the cycle performance and energy density of the battery.

[0093] This application also provides a method for preparing a secondary battery, comprising the following steps:

[0094] The positive electrode, negative electrode, separator, and electrolyte are assembled into a secondary battery;

[0095] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a lithium-containing transition metal phosphate and a carbon coating layer covering the outer surface of the lithium-containing transition metal phosphate. The carbon coating layer contains SP2 carbon and SP3 carbon, and the molar ratio of SP2 carbon to SP3 carbon is 1.2-6.5. Based on the mass of the carbon coating layer, the mass content of oxygen in the carbon coating layer is less than or equal to 16%.

[0096] By employing a positive electrode active material that contains both SP2 and SP3 carbon in its carbon coating layer, and controlling the SP2 and SP3 carbon forms within a suitable range, high powder compaction density and powder resistivity can be achieved. This also promotes electrolyte wetting of the material and lithium-ion transport, comprehensively improving the battery's cycle performance. Furthermore, controlling the oxygen content in the carbon coating layer can further reduce side reactions between the active material and the electrolyte, thus improving the battery's cycle performance.

[0097] In some embodiments, the preparation method of the positive electrode active material is as follows:

[0098] Spray drying: A mixed slurry containing lithium transition metal phosphate raw material, SP2 carbon source and SP3 carbon source is spray dried to obtain precursor powder.

[0099] Sintering treatment: The precursor powder is sintered to obtain the positive electrode active material.

[0100] Compared to the existing technology that controls the sintering time and / or sintering temperature to regulate the SP2 and SP3 carbon sources, this application selects the SP2 and SP3 carbon sources as carbon sources, making it easier to control the type of carbon in the coating layer by selecting the raw materials.

[0101] In some embodiments, the method for preparing the positive electrode active material further includes,

[0102] Pre-oxidation treatment: The initial carbon source in SP2 form is pre-oxidized in an oxygen-containing atmosphere to obtain the SP2 form carbon source.

[0103] In this paper, the term "oxygen-containing atmosphere" refers to an atmosphere containing oxygen, including air or an atmosphere with an oxygen volume content greater than 20.95%.

[0104] Pre-oxidation of the initial carbon source in SP2 form can break the double bonds in the SP2 form carbon source, which is beneficial for the decomposition of the carbon source in the subsequent sintering process. This facilitates the release of oxygen elements during the subsequent sintering process, which helps to reduce the mass content of oxygen elements in the carbon coating layer. This reduces the possibility of side reactions between the oxygen elements in the carbon coating layer and the electrolyte, thereby improving the cycle performance of the battery.

[0105] In some embodiments, the pre-oxidation treatment is carried out at a temperature of 150°C-300°C.

[0106] In some embodiments, the processing temperature of the pre-oxidation treatment can be selected as 150°C, 200°C, 250°C, 300°C, or any value range between the two.

[0107] In some embodiments, the pre-oxidation treatment takes 1-20 hours.

[0108] In some embodiments, the processing time of the pre-oxidation treatment can be selected as 1h, 5h, 10h, 15h, 20h or any value range between the two.

[0109] Controlling the temperature and time of the pre-oxidation treatment within a suitable range is beneficial for obtaining SP2-form carbon sources with an appropriate mass content of oxygen-containing functional groups. It is also beneficial for controlling the mass content of oxygen in the carbon coating layer within a suitable range. This can reduce side reactions between the material and the electrolyte, and also facilitate the formation of a dense carbon coating layer on the material surface by the SP2-form carbon source, thereby comprehensively improving the cycle performance of the battery.

[0110] In some embodiments, the mass ratio of the SP2 carbon source to the SP3 carbon source is 1:9 to 9:1. In some embodiments, the mass ratio of the SP2 carbon source to the SP3 carbon source can be selected as 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, or any range between the two.

[0111] Controlling the mass ratio of SP2 and SP3 carbon sources within a suitable range is beneficial for maintaining the appropriate balance of SP2 and SP3 carbon in the carbon coating layer. This achieves high powder compaction density and resistivity, promotes electrolyte wetting of the material and lithium-ion transport, and comprehensively improves battery cycle performance. Furthermore, controlling the mass ratio of SP2 and SP3 carbon sources within a suitable range also regulates the oxygen content in the carbon coating layer, reducing side reactions between the carbon coating layer and the electrolyte, and further improving battery cycle performance.

[0112] In some embodiments, the total mass content of the SP2 and SP3 carbon sources is 2%-8%, based on the mass of the lithium transition metal phosphate feedstock.

[0113] In some embodiments, based on the mass of the lithium transition metal phosphate raw material, the total mass content of the SP2 and SP3 carbon sources can be selected as 2%, 4%, 6%, 8%, or any value range between the two.

[0114] When the total mass content of SP2 and SP3 carbon sources is within a suitable range, it is beneficial to control the mass content of the carbon coating layer within a suitable range, so as to form a complete coating on the surface of the positive electrode active material, thereby improving the powder resistivity and conductivity of the material.

[0115] In some embodiments, the SP2 form carbon source and the SP2 form carbon include one or more of phenolic resin, epoxy resin, melamine resin, polyfurfuryl alcohol, polyaniline, polyethylene oxide, polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, lignin, styrene-butadiene polymer, and polyethylene terephthalate.

[0116] In some embodiments, the SP3 form of carbon source includes one or more of sucrose, glucose, dextrin, citric acid, fructose, and starch.

[0117] In some embodiments, the sintering temperature of the sintering treatment is 650℃-780℃, and can be selected from 650℃, 670℃, 700℃, 720℃, 740℃, 760℃, 780℃ or any value range between the two.

[0118] In some embodiments, the sintering time at the sintering temperature is 8h-16h, and can be selected as 8h, 10h, 12h, 14h, 16h or any range between the two.

[0119] When the sintering temperature and / or time are within a suitable range, it is beneficial for SP2 and SP3 carbon sources to form a complete and dense carbon coating layer on the surface of lithium-containing transition metal phosphates, thereby improving the conductivity of the material, promoting lithium-ion transport, and improving the cycle performance of the battery.

[0120] In some embodiments, the lithium-containing transition metal phosphate raw material includes a phosphorus source, an iron source, a manganese source, and a lithium source.

[0121] In some embodiments, the lithium-containing transition metal phosphate feedstock also includes an M1 source or an M2 source.

[0122] In some implementations, the M1 source includes one or more of the following: Al source, Cu source, Mg source, Zn source, Ni source, Ti source, V source, Zr source, Ni source, Co source, Ga source, Sn source, Sb source, Nb source, and Ge source.

[0123] In some implementations, the M2 source includes one or more of the following: B source, S source, Si source, N source, F source, Cl source, and Br source.

[0124] [Positive electrode plate]

[0125] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector, the positive electrode film layer including a positive active material.

[0126] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0127] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0128] In some embodiments, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0129] In some embodiments, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0130] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0131] [Negative electrode plate]

[0132] The negative electrode includes a negative current collector and a negative electrode film layer optionally disposed on at least one surface of the negative current collector. The negative electrode film layer includes a negative electrode active material.

[0133] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0134] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0135] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0136] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0137] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0138] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0139] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0140] Electrolyte

[0141] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements.

[0142] In some embodiments, the electrolyte comprises a lithium salt and an organic solvent.

[0143] In some embodiments, the lithium salt may be selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bistrifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0144] In some embodiments, the organic solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0145] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0146] [Isolation membrane]

[0147] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0148] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation. In some embodiments, the separator may also have one or more coatings.

[0149] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0150] [Outer Packaging]

[0151] In some embodiments, the secondary battery may include an outer packaging for encapsulating the positive electrode, negative electrode, and electrolyte. As an example, the positive electrode, negative electrode, and separator may be stacked or wound to form a stacked or wound battery cell, with the cell encapsulated within the outer packaging; the number of cells in the secondary battery may be one or more, adjustable as needed.

[0152] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0153] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 5.

[0154] In some implementations, refer to Figure 2The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0155] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0156] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0157] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0158] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0159] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0160] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0161] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0162] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0163] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0164] Example

[0165] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0166] I. Preparation Method

[0167] Example 1

[0168] 1) Preparation of positive electrode active materials

[0169] Lithium carbonate and phosphoric acid were weighed according to a molar ratio of Li:P = 1.02:1 and slowly added to a stirred tank containing deionized water, and stirred until dissolved. Ferrous oxalate and manganese oxalate were weighed according to a molar ratio of Fe:Mn:P = 0.58:0.38:1 and added to the stirred tank, and stirred until dissolved to obtain lithium manganese iron phosphate raw material. Phenolic resin and glucose were added to the stirred tank and stirred to obtain a mixed slurry, wherein the mass ratio of phenolic resin to glucose was 1:9 and the mass percentage of phenolic resin and glucose was 4%, based on the total mass of lithium carbonate, phosphoric acid, ferrous oxalate and manganese oxalate. The above mixed slurry was milled until the volume distribution particle size Dv50 was 0.3 μm to obtain the precursor slurry.

[0170] After the precursor slurry was spray-dried, it was sintered under nitrogen protection for 12 hours at a temperature of 720°C to obtain carbon-coated lithium manganese iron phosphate cathode active material.

[0171] 2) Preparation of positive electrode sheet

[0172] Carbon-coated lithium manganese iron phosphate positive electrode active material, conductive carbon black, and PVDF were mixed in a weight ratio of 96:2.5:1.5. An appropriate amount of N-methylpyrrolidone (NMP) solvent was added, and the mixture was stirred evenly to obtain a positive electrode slurry. The positive electrode slurry was coated onto aluminum foil and dried to obtain a positive electrode sheet.

[0173] 3) Preparation of negative electrode sheet

[0174] A negative electrode slurry was prepared by dissolving graphite (anode active material), styrene-butadiene rubber (SBR) (binder), sodium carboxymethyl cellulose (CMC-Na) (thickener), and carbon black (SuperP) (conductive agent) in deionized water at a weight ratio of 96.2:1.8:1.2:0.8 and mixing thoroughly. The negative electrode slurry was then uniformly coated onto both sides of the copper foil used as the negative electrode current collector, with a coating density of 8.6 mg / cm³. 2 The negative electrode sheet is obtained by drying, cold pressing, and slitting.

[0175] 4) Separating membrane

[0176] Polyethylene film is used as the separation membrane.

[0177] 5) Preparation of electrolyte

[0178] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate (EC) and diethyl carbonate (DMC) are mixed at a volume ratio of 1:1 to obtain an electrolyte solvent. Then, lithium salt is mixed with the mixed solvent to prepare an electrolyte with a lithium salt concentration of 1 mol / L.

[0179] 6) Battery manufacturing

[0180] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a wound battery cell. The wound battery cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0181] Implementation 2-5

[0182] Compared with Example 1, Examples 2-5 have adjusted the mass ratio of phenolic resin to glucose. Specific parameters are shown in Table 1.

[0183] Example 6

[0184] Compared to Example 1, Example 6 requires a pre-oxidation treatment of the phenolic resin, as detailed below:

[0185] Pre-oxidation treatment of phenolic resin: Place phenolic resin powder in a 150℃ forced-air oven and heat-treat at a constant temperature for 2 hours to obtain pre-oxidized phenolic resin.

[0186] Examples 7-8

[0187] Compared with Example 6, Examples 7-8 replaced phenolic resin with lignin or polyethylene terephthalate, and the specific parameters are shown in Table 1.

[0188] Examples 9-10

[0189] Compared with Example 6, Examples 9-10 adjusted the time or temperature of the pre-oxidation treatment. See Table 1 for specific parameters.

[0190] Comparative Example 1

[0191] Compared with Example 1, the difference is that the total mass content of phenolic resin and glucose was adjusted to 4% glucose by mass, and the specific parameters are shown in Table 1.

[0192] Comparative Examples 2-3

[0193] Compared with Example 1, Comparative Examples 2-3 differed in that the phenolic resin was replaced with polyethylene glycol and polyvinyl alcohol, respectively.

[0194] Comparative Example 4

[0195] Compared with Example 6, the difference is that the total mass content of phenolic resin and glucose was adjusted to 4% by mass of phenolic resin, and the treatment temperature and treatment time of the pre-oxidation treatment were adjusted. For specific parameters, please refer to Table 1.

[0196] Comparative Example 5

[0197] Compared with Example 1, the difference is that the total mass content of phenolic resin and glucose was adjusted to 4% by mass of phenolic resin. For specific parameters, please refer to Table 1.

[0198] II. Testing Methods

[0199] 1. High-temperature cycle capacity retention of secondary batteries

[0200] At 65°C, the secondary batteries prepared in each embodiment and comparative example were charged at a constant current rate of 1C to the charging cutoff voltage of 4.2V, then charged at a constant voltage rate to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current rate of 1C to the discharge cutoff voltage of 2.0V, allowed to stand for 5 minutes. This constitutes one charge-discharge cycle. The batteries were subjected to cyclic charge-discharge tests according to this method. Using the capacity of the first discharge as 100%, the capacity retention rate after 1000 cycles was calculated. The capacity retention rate (%) after 1000 cycles = (Discharge capacity of the 1000th cycle / Capacity of the first discharge) × 100%.

[0201] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0202] Secondary batteries for each embodiment and comparative example were prepared according to the above method, and various parameters were measured. The results are shown in the table below.

[0203] Table 1

[0204]

[0205] Table 2

[0206]

[0207] As can be seen from the table, Examples 1-10 provide a positive electrode active material, including lithium manganese iron phosphate and a carbon coating layer on the outer surface of lithium manganese iron phosphate. The carbon coating layer includes SP2 carbon and SP3 carbon, and the molar ratio of SP2 carbon to SP3 carbon is 1.2-6.5. Based on the mass of the carbon coating layer, the mass content of oxygen in the carbon coating layer is less than or equal to 16%.

[0208] As can be seen from the comparison between Examples 1-10 and Comparative Examples 1-4, controlling the molar ratio of SP2 carbon to SP3 carbon to be 1.2-6.5 can improve the high-temperature cycle capacity retention rate of the battery and improve the cycle performance of the battery.

[0209] As can be seen from the comparison between Examples 1-10 and Comparative Example 5, based on the mass of the carbon coating layer, the mass content of oxygen in the carbon coating layer is less than or equal to 16%, which can improve the high-temperature cycle capacity retention rate of the battery and improve the cycle performance of the battery.

[0210] As can be seen from the comparison between Examples 3-10 and Examples 1-2, controlling the molar ratio of SP2 carbon to SP3 carbon to be 3.0-6.5 can further improve the high-temperature cycle capacity retention rate of the battery and improve the cycle performance of the battery.

[0211] As can be seen from Examples 6-10 and Examples 1-5, based on the mass of the carbon coating layer, the oxygen content in the carbon coating layer is 1%-5%, which can further improve the high-temperature cycle capacity retention rate of the battery and improve the cycle performance of the battery.

[0212] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A secondary battery, characterized in that, include: A positive electrode, a negative electrode, a separator disposed between the positive and negative electrode, and an electrolyte comprising an organic solvent and a lithium salt. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector, the positive electrode film layer including a positive electrode active material. The positive electrode active material includes a lithium-containing transition metal phosphate and a carbon coating layer covering the outer surface of the lithium-containing transition metal phosphate. The carbon coating layer contains SP2 carbon and SP3 carbon, and the molar ratio of SP2 carbon to SP3 carbon is 1.2-6.

5. Based on the mass of the carbon coating layer, the mass content of oxygen in the carbon coating layer is less than or equal to 16%.

2. The secondary battery according to claim 1, characterized in that, Based on the mass of the carbon coating layer, the oxygen content in the carbon coating layer is 1%-5% by mass.

3. The secondary battery according to claim 1 or the above, characterized in that, The molar ratio of SP2 carbon to SP3 carbon is 3.0-6.

5.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The lithium-containing transition metal phosphates include lithium manganese iron phosphate and its modified compounds.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, Based on the total mass of the positive electrode active material, the mass content of the carbon coating layer is 1.20%-2.0%.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, The compacted density of the positive electrode active material at a pressure of 29400N is 2.15 g / cm³. 3 -2.50g / cm 3 .

7. The secondary battery according to any one of claims 1 to 6, characterized in that, The thickness of the carbon coating layer is 2nm-40nm.

8. A method for preparing a secondary battery, characterized in that, Includes the following steps: The positive electrode, negative electrode, separator, and electrolyte are assembled into a secondary battery; The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a lithium-containing transition metal phosphate and a carbon coating layer covering the outer surface of the lithium-containing transition metal phosphate. The carbon coating layer contains SP2 carbon and SP3 carbon, and the molar ratio of SP2 carbon to SP3 carbon is 1.2-6.

5. Based on the mass of the carbon coating layer, the mass content of oxygen in the carbon coating layer is less than or equal to 16%.

9. The preparation method according to claim 8, characterized in that, The specific preparation method of the positive electrode active material is as follows: Spray drying: A mixed slurry containing lithium transition metal phosphate raw material, SP2 carbon source and SP3 carbon source is spray dried to obtain precursor powder. Sintering treatment: The precursor powder is sintered to obtain the positive electrode active material.

10. The preparation method according to claim 9, characterized in that, The method for preparing the positive electrode active material also includes... Pre-oxidation treatment: The initial carbon source in SP2 form is pre-oxidized in an oxygen-containing atmosphere to obtain the SP2 form carbon source.

11. The preparation method according to claim 10, characterized in that, The pre-oxidation treatment temperature is 150℃-300℃; and / or the pre-oxidation treatment time is 1h-20h.

12. The preparation method according to any one of claims 9 to 11, characterized in that, The mass ratio of the SP2 carbon source to the SP3 carbon source is 1:9-9:

1.

13. The preparation method according to any one of claims 9 to 12, characterized in that, The total mass content of the SP2 and SP3 carbon sources is 2%-8%, based on the mass of lithium transition metal phosphate raw materials.

14. The preparation method according to any one of claims 10 to 13, characterized in that, The SP2 form carbon source and the initial SP2 form carbon source include one or more of the following: phenolic resin, epoxy resin, melamine resin, polyfurfuryl alcohol, polyaniline, polyethylene oxide, polyvinylidene fluoride, polyacrylic acid, polyacrylonitrile, lignin, styrene-butadiene polymer, and polyethylene terephthalate.

15. The preparation method according to any one of claims 9 to 14, characterized in that, The SP3 form of carbon source includes one or more of sucrose, glucose, dextrin, citric acid, fructose, and starch.

16. The preparation method according to any one of claims 9 to 15, characterized in that, The sintering temperature of the sintering treatment is 650℃-780℃; and / or, the sintering time at the sintering temperature is 8h-16h.

17. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1 to 7 or a secondary battery prepared by any one of claims 8 to 16.

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