Composite cathode active material, preparation method therefor and application thereof

Abstract

The present application relates to a composite cathode active material comprising an iron phosphide compound, and further discloses a preparation method thereof, as well as an application thereof for use in the preparation of battery positive electrode plates and secondary batteries. The composite cathode active material of the present application comprises lithium iron phosphate particles, lithium iron phosphate particles coated with carbon, lithium iron phosphate particles coated with an iron phosphide compound, lithium iron phosphate particles coated with an iron phosphide compound and carbon, and iron phosphide compound particles.

Description

[0001] COMPOSITE CATHODE ACTIVE MATERIAL, PREPARATION METHOD THEREFOR AND APPLICATION THEREOF

[0002] FIELD OF THE INVENTION

[0003] The present application belongs to the technical field of secondary batteries, specifically relates to a composite cathode active material, more specifically relates to a composite cathode active material comprising an iron phosphide compound, and further discloses a preparation method thereof, as well as an application thereof for use in the preparation of battery positive electrode plates and secondary batteries.

[0004] BACKGROUND OF THE INVENTION

[0005] Lithium-ion battery is a kind of energy storage device which is widely used in energy storage field, power battery and portable electronic equipment, and has the advantages of high working voltage, low self-discharge, good safety and the like. Lithium-ion battery is mainly composed of cathode active material, anode material, electrolyte solution, separator and shell, and the cathode active material system mainly comprises lithium cobaltate, lithium manganate, lithium -nickel-manganese oxide, lithium-nickel-cobalt-manganese oxide, lithium-nickel-cobalt-aluminum oxide, lithium iron phosphate (LFP), lithium iron manganese phosphate, etc. Among them, lithium iron phosphate LiFePCh material has the advantages of low cost, high safety, good cycling life and the like.

[0006] At present, the lithium iron phosphate cathode active material usually adopts processing of large-sized lithium iron phosphate particles to ensure compaction of the material, and the large-sized particles are mainly obtained by fusion of small particles under high-temperature calcination conditions, which results in relatively poor carbon coating of the large-particle material and directly causes relatively poor electron conduction among particles, which in turn leads to severe material polarization. In the prior art, although the technology of doping iron phosphide in the lithium iron phosphate material to improve the low-temperature cycling performance and the rate capability of the lithium iron phosphate cathode active material has been developed, the method of adding iron phosphide compounds is unable to effectively control the coating uniformity of the material, and the improvement of the performance of the lithium iron phosphate cathode active material still needs to be improved.

[0007] SUMMARY OF THE INVENTION

[0008] The present application provides a composite cathode active material, a positive electrode plate and a secondary battery, which effectively improves the polarization of lithium iron phosphate particles, and enhances the rate capability and cycling stability of the material.

[0009] In a first aspect, the present application provides a composite cathode active material, wherein the composite cathode active material comprises a mixture of lithium iron phosphate particles, lithium iron phosphate particles coated with carbon, lithium iron phosphate particles coated with an iron phosphide compound, lithium iron phosphate particles coated with an iron phosphide compound and carbon, and iron phosphide compound particles; and the content of the iron phosphide compound in the composite cathode active material is in a range from 0.01mol% to 0.22mol%.

[0010] In an optional embodiment, the iron phosphide compound comprises ferrous phosphide (Fe2P) and / or ferric phosphide (FeP).

[0011] In an optional embodiment, the intensity of the diffraction peaks ranging from 40° to 41° is 2% to 7% of the intensity of the strongest diffraction peak in the XRD spectrum of the composite cathode active material, wherein the diffraction peaks ranging from 40° to 41° are characteristic peaks of the iron phosphide compound, and the strongest diffraction peak is a crystal plane diffraction peak of LiFePOi phase (311).

[0012] In an optional embodiment, the composite cathode active material further comprises a doping element M; the doping element M comprises at least one of Ti, Zr, V, Nb and Mg; and the content of the doping element M is in a range from 1,000 ppm to 8,000 ppm based on the weight of the composite cathode active material. In an optional embodiment, the composite cathode active material has a particle size of 200 nm to 8,000 nm. It should be noted that the particle size described herein characterizes the particle diameter size shown in the SEM.

[0013] In a second aspect, the present application also provides a method for preparing the composite cathode active material, comprising the following steps:

[0014] SI: mixing an iron source material, a phosphorus source material, a lithium source material, a carbon source material and an oxygen-donating compound to obtain a precursor; and

[0015] S2: carrying out a high temperature sintering treatment to the precursor under an inert atmosphere to obtain the composite cathode active material.

[0016] In an optional embodiment, in SI, the oxygen-donating compound comprises an oxygen-containing salt which undergoes thermal decomposition at a temperature of 600°C to 800°C.

[0017] In an optional embodiment, in SI, the oxygen-donating compound comprises at least one of potassium perchlorate (KCIO4), potassium manganate (K^MnCh), manganese sulfate (MnSCh), and copper sulfate (CuSCh); and / or the iron source material comprises at least one of iron phosphate, iron(III) oxide, ferrous oxalate, ferrous phosphate, iron hydroxide oxide, and ferric hydroxide; and / or the phosphorus source material comprises at least one of iron phosphate, lithium phosphate, lithium dihydrogen phosphate, phosphoric acid, monoammonium phosphate, diammonium hydrogen phosphate, and triammonium phosphate; and / or the lithium source material comprises at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate and lithium chloride; and / or the carbon source material comprises at least one of sucrose, glucose, starch, ascorbic acid, citric acid, polyethylene glycol, cellulose and phenolic resin. In an optional embodiment, in SI, the oxygen-donating compound is added in an amount of 0.1-1 mol% based on the molar amount of iron element in the iron source material; and / or in the precursor, a molar ratio of iron element in the iron source material, phosphorus element in the phosphorus source material, and lithium element in the lithium source material is (0.95-1.00): 1 : (0.98-1.10); and / or the carbon source material is added in an amount of 10 wt% to 20 wt% based on the weight of the iron source material.

[0018] In an optional embodiment, SI further comprises a step of adding a dopant containing the doping element M; and based on the weight of the composite cathode active material, the doping element M in the dopant is added in an amount of 1,000 ppm to 8,000 ppm.

[0019] In an optional embodiment, SI further comprises a step of grinding and / or drying the precursor; and in the step of grinding, the precursor is controlled to be ground to a particle size of 100 nm to 700 nm.

[0020] In an optional embodiment, in S2, the high temperature sintering treatment is carried out at a temperature of 700°C to 900°C for a time period of 6h to 15h.

[0021] In an optional embodiment, S2 further comprises a step of pre-sintering treatment at a temperature of 350°C to 400°C for a time period of Ih to 3h.

[0022] In an optional embodiment, S2 comprises:

[0023] S2-a: heating up to 350-400°C at a heating rate of 2-4°C / min and performing the pre-sintering treatment;

[0024] S2-b: heating up to 550-600°C at a heating rate of 2-4°C / min; and

[0025] S2-c: heating up to 700-800°C at a heating rate of l-2°C / min and performing the high temperature sintering treatment.

[0026] In an optional embodiment, in S2, in S2-a and / or S2-b, a gas feeding rate of the inert atmosphere is controlled to be 50-100 L / min, and a pressure in a furnace for sintering is controlled to be 10-100 Pa; and / or in S2-c, a gas feeding rate of the inert atmosphere is controlled to be less than or equal to 10 L / min, and a pressure in a furnace for sintering is controlled to be more than or equal to 200 Pa.

[0027] In a third aspect, the present application also provides a positive electrode plate, comprising: a positive electrode current collector, and a cathode active material layer provided on at least one side of the positive electrode current collector; wherein the cathode active material layer comprises the composite cathode active material or a composite cathode active material prepared by the method.

[0028] In a fourth aspect, the present application also provides a secondary battery, comprising the positive electrode plate.

[0029] In a fifth aspect, the present application also provides a powered device, comprising the secondary battery.

[0030] The technical solution of the present application has the following advantages: for the composite cathode active material provided in the present application, an internal reduction method is used to form iron phosphide compound particles during the sintering process of lithium iron phosphate material, and then a controllable iron phosphide compound coating layer is introduced to coat the large-sized particles that are not coated with carbon, which effectively improves the polarization of lithium iron phosphate particles due to the better electronic conductivity of the iron phosphide compound material, enhances the rate capability and cycling stability of the material, and effectively improves the problem of non-uniformity of carbon coating in lithium iron phosphate materials; and for the method for preparing the composite cathode active material provided in the present application, by adding selected oxygen-donating compounds to the raw material system during the formation of lithium iron phosphate materials, and the oxygen-donating compounds decompose during high-temperature sintering to produce oxygen, which in turn reacts with the carbon source material that has not formed a coating to generate carbon monoxide, while the gas-phase CO can reduce the uncoated lithium iron phosphate (LFP) particles, thereby forming an iron phosphide compound-coated structure, i.e., an iron phosphide compound can be introduced to coat large-sized LFP particles that are not coated with carbon, which in turn effectively improves the polarization of the lithium iron phosphate particles by taking advantage of the electrical conductivity of the iron phosphide compound and enhances the rate capability and cycling stability of the material.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the specific embodiments of the present application or prior art, the following will briefly introduce the accompanying drawings that need to be used in the description of specific embodiments or prior art, and it is obvious that the accompanying drawings in the following description are some of the embodiments of the present application, and that for those skilled in the art, other accompanying drawings can be obtained based on these drawings without creative work.

[0033] FIG. 1 is an XRD spectrum of a portion of synthesized samples of the present application; and

[0034] FIG. 2 shows the comparison results of the discharge capacity at 1C of Example 1 and Comparative Example 1 of the present application.

[0035] DETAILED DESCRIPTION OF THE INVENTION

[0036] Reference will be made clearly and completely to the technical solutions in the embodiments of the present application with accompanying drawings. The embodiments described here are only part of the embodiments of the present application and are not all embodiments of the present application. Based on the embodiments of the present application, other embodiments obtained by those skilled in the art without creative work are within the scope of the present application.

[0037] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs. The terms used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. The terms “includes” and “comprises” and any variation thereof in the description and claims of the present application are intended to indicate a non-exclusive inclusion.

[0038] Reference to an “embodiment” herein means that a feature, structure or characteristic described in connection with the embodiment may be comprised in at least one embodiment of the present application. The “embodiment” in various places in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. One skilled in the art explicitly and implicitly understands that an embodiment described herein may be combined with other embodiments.

[0039] Term “range” disclosed in the present application is defined in the form of a lower limit and an upper limit, a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The range defined in this way can be inclusive or exclusive, 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 understood that ranges of 60-110 and 80-120 are also obtained. In addition, if the listed minimum values are 1 and 2, and if the listed maximum values are 3, 4 and 5, the ranges of 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5 may be obtained. In the present application, unless otherwise specified, the numerical range “a-b” means the abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range “0-5” means that all the real numbers between “0-5” have been listed, and “0-5” is only the abbreviated representation of these numerical combinations. In addition, when a parameter is an integer >2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0040] In the description of the present application, the term “and / or”, which describes an associated relationship of associated objects, means that there may be three relationships, for example, A and / or B, which may mean that A exists alone, A and B exist at the same time, and B exists alone. A character “ / ” generally indicates that contextual objects are in an “or” relationship.

[0041] In the description of the present application, the term “a plurality of’ refers to two or more (comprising two), and similarly, “a plurality of groups” refers to two or more (comprising two) groups, and “a plurality of pieces” refers to two or more (comprising two) pieces.

[0042] At present, the lithium iron phosphate cathode active material usually adopts processing of large-sized lithium iron phosphate particles to ensure compaction of the material, and the large-sized particles are mainly obtained by fusion of small particles under high-temperature calcination conditions, which results in relatively poor carbon coating of the large-particle material and directly causes relatively poor electron conduction among particles, which in turn leads to severe material polarization.

[0043] In a first aspect, the present application provides a composite cathode active material, comprising a mixture of lithium iron phosphate particles, lithium iron phosphate particles coated with carbon, lithium iron phosphate particles coated with an iron phosphide compound, lithium iron phosphate particles coated with an iron phosphide compound and carbon, and iron phosphide compound particles; and the content of the iron phosphide compound in the composite cathode active material is in a range from 0.01mol% to 0.22mol%. For example, the content of the iron phosphide compound in the composite cathode active material can be adjusted to be 0.01mol%, 0.03mol%, 0.05mol%, 0.08mol%, 0.10mol%, 0.15mol%, 0.20mol%, 0.22mol%, or in a range composed of the above any values.

[0044] For the composite cathode active material described in the present application, an iron phosphide compound is introduced to coat large-sized particles that are not coated with carbon by means of in-situ reduction to form an iron phosphide compound during sintering of the lithium iron phosphate material, which effectively improves the polarization of the lithium iron phosphate particles due to the better electronic conductivity of the iron phosphide compound material, and enhances the rate capability and cycling stability of the material.

[0045] In some embodiments, the iron phosphide compound comprises ferrous phosphide (Fe2P) and / or ferric phosphide (FeP). For example, the conductive properties of the iron phosphide compound are utilized for optimizing the properties of the material.

[0046] In some embodiments, the intensity of the diffraction peaks ranging from 40° to 41° is 2% to 7% of the intensity of the strongest diffraction peaks in the XRD spectrum of the composite cathode active material, wherein the diffraction peaks ranging from 40° to 41° are characteristic peaks of the iron phosphide compound, and the strongest diffraction peak is a crystal plane diffraction peak of LiFePCh phase (311). The lithium iron phosphate material obtained in the present application has higher compaction density and better rate capability at 1C.

[0047] In an optional embodiment, the composite cathode active material further comprises a doping element M; and the doping element M comprises at least one of Ti, Zr, V, Nb and Mg.

[0048] The composite cathode active material provided by the present application can improve the application performance of the material by doping metal elements in the lithium iron phosphate material.

[0049] In some embodiments, the composite cathode active material has a particle size of 200 nm to 8,000 nm. It should be noted that the particle size described herein characterizes the particle diameter size shown in the SEM. In the composite cathode active material provided in the present application, for example, the particle size of the composite cathode active material is controlled to be 200nm, 500nm, 800nm, l,000nm, l,500nm, 2,000nm, 2,500nm, 3,000nm, 3,500nm, 4,000nm, 4,500nm, 5,000nm, 5,500nm, 6,000nm, 6,500nm, 7,000nm, 7,500nm, 8,000nm, or in a range composed of the above any values.

[0050] In a second aspect, the present application also provides a method for preparing the composite cathode active material, comprising the following steps:

[0051] SI: mixing an iron source material, a phosphorus source material, a lithium source material, a carbon source material and an oxygen-donating compound to obtain a precursor; and

[0052] S2: carrying out a high temperature sintering treatment to the precursor under an inert atmosphere to obtain the composite cathode active material.

[0053] For the composite cathode active material provided in the present application, selected oxygen-donating compounds are added to the raw material system during the formation of lithium iron phosphate materials, and the oxygen-donating compounds decompose to produce oxygen at higher than 600°C, which in turn reacts with carbon to generate carbon monoxide, while the gas-phase CO can reduce the uncoated lithium iron phosphate (LFP) particles to generate an iron phosphide compound, which then introduces a controllable iron phosphide compound coating layer for the large-sized particles that are not coated with carbon, which effectively improves the polarization of the lithium iron phosphate particles by taking advantage of the electrical conductivity of the iron phosphide compound and enhances the rate capability and cycling stability of the material.

[0054] In some embodiments, in SI, the oxygen-donating compound comprises a compound which decomposes at high temperature to produce oxygen; for example, the oxygen-donating compound comprises an oxygen-containing salt which undergoes thermal decomposition at a temperature of 600°C to 800°C.

[0055] In some embodiments, in SI, the oxygen-donating compound comprises at least one of copper sulfate (CuSCh), manganese sulfate (MnSCh), potassium manganate (K2MnO4) and potassium perchlorate (KCIO4); for example, copper sulfate (CuSC ), manganese sulfate (MnSCh), potassium manganate (K2MnO4) or potassium perchlorate (KCIO4) alone, or a mixture of any combination of at least two of these, are chosen to be added to the reaction feedstock system. In some embodiments, in SI, the iron source material is a material conventionally selected in the art, and as an exemplary embodiment, the iron source material comprises at least one of iron phosphate, iron(III) oxide, ferrous oxalate, ferrous phosphate, iron hydroxide oxide, and ferric hydroxide.

[0056] In some embodiments, in SI, the phosphorus source material is a material conventionally selected in the art, and as an exemplary embodiment, the phosphorus source material comprises at least one of iron phosphate, lithium phosphate, lithium dihydrogen phosphate, phosphoric acid, monoammonium phosphate, diammonium hydrogen phosphate, and triammonium phosphate.

[0057] In some embodiments, in SI, the lithium source material is a material conventionally selected in the art, and as an exemplary embodiment, the lithium source material comprises at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate and lithium chloride.

[0058] In some embodiments, in SI, the carbon source material is a material conventionally selected in the art, and as an exemplary embodiment, organic carbon materials can be chosen, for example, the carbon source material comprises at least one of sucrose, glucose, starch, ascorbic acid, citric acid, polyethylene glycol, cellulose and phenolic resin.

[0059] In some embodiments, in SI, the oxygen-donating compound is added in an amount of 0.1-1 mol% based on the molar amount of iron element in the iron source material; for example, 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol%, 0.5 mol%, 0.6 mol%, 0.7 mol%, 0.8 mol%, 0.9 mol%, 1.0 mol%, or in a range composed of the above any values. The more oxygen-donating compounds that are added in a controlled manner during the reaction, the higher the content of the iron phosphide compound.

[0060] In some embodiments, in the precursor of SI, a molar ratio of iron element in the iron source material, phosphorus element in the phosphorus source material, and lithium element in the lithium source material is (0.95-1.00): 1 : (0.98-1.10); for example, the ratio of the three is controlled to be 0.95:1 :0.98, 0.95: 1 :1.10, 1.00: 1 :0.98, 1.00:1 : 1.10, or in a range composed of the above any values. In some embodiments, in the precursor of SI, the carbon source material is added in an amount of 10 wt% to 20 wt% based on the weight of the iron source material; for example, 10wt%, llwt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, or in a range composed of the above any values.

[0061] In some embodiments, SI further comprises a step of adding a dopant containing the doping element M.

[0062] In some embodiments, based on the weight of the composite cathode active material, the doping element M in the dopant is added in an amount of 1,000 ppm to 8,000 ppm. For example, by controlling the amount of dopant, the content of the doping element M is controlled to be 1,000 ppm, 3,000 ppm, 5,000 ppm, 8,000 ppm, or in a range composed of the above any values.

[0063] In some embodiments, the doping element M comprises at least one of Ti, Zr, V, Nb and Mg.

[0064] In some embodiments, the dopant comprises at least one of an oxide, a hydroxide, an oxalate, an acetate, a chloride, and a nitrate of the dopant element M.

[0065] In some embodiments, SI further comprises a step of grinding the precursor to a particle size of 100-700 nm, i.e. controlling the size characteristics of the particles by controlling the grinding granularity.

[0066] In some embodiments, in S2, the high temperature sintering treatment is carried out at a temperature of 700°C to 900°C. Under the high temperature sintering, the oxygen-donating compounds decompose to produce oxygen at higher than 600°C, which in turn reacts with carbon to generate carbon monoxide, while the gas-phase CO can reduce the uncoated lithium iron phosphate (LFP) particles to generate an iron phosphide compound, which then introduces a controllable iron phosphide compound coating layer for the large-sized particles that are not coated with carbon, which effectively improves the polarization of the lithium iron phosphate particles by taking advantage of the electrical conductivity of the iron phosphide compound and enhances the rate capability and cycling stability of the material. For example, the high temperature sintering treatment can be carried out at a temperature of 700°C, 720°C, 750°C, 780°C, 800°C, 820°C, 850°C, 870°C, 900°C, or in a range composed of the above any values.

[0067] In some embodiments, in S2, the high temperature sintering treatment is carried out for a time period of 6h to 15h, for example, 6h, 7h, 8h, 9h, lOh, llh, 12h, 13h, 14h, 15h, or in a range composed of the above any values.

[0068] In some embodiments, S2 further comprises a step of pre-sintering treatment at a temperature of 350°C to 400°C for a time period of Ih to 3h. For example, the pre-sintering treatment is carried out at 350°C, 360°C, 370°C, 380°C, 390°C, or 400°C, and the time period for the pre-sintering treatment can be controlled to be Ih, 2h, 3h, or in a range composed of the above any values.

[0069] In some embodiments, in S2, the whole step comprises pre-sintering treatment and sintering treatment as well as temperature controlling, specifically:

[0070] S2-a: heating up to 350-400°C at a heating rate of 2-4°C / min and performing the pre-sintering treatment;

[0071] S2-b: heating up to 550-600°C at a heating rate of 2-4°C / min; and

[0072] S2-c: heating up to 700-800°C at a heating rate of l-2°C / min and performing the high temperature sintering treatment.

[0073] In some embodiments, in S2, in S2-a and / or S2-b, a gas feeding rate of the inert atmosphere is controlled to be 50-100 L / min, and a pressure in a furnace for sintering is controlled to be 10-100 Pa; and / or in S2-c, a gas feeding rate of the inert atmosphere is controlled to be less than 10 L / min, and a pressure in a furnace for sintering is controlled to be more than or equal to 200 Pa.

[0074] In the method for preparing the composite positive electrode active material, in S2, S2-a belongs to the decomposition stage of the organic carbon source and lithium carbonate, while the process of formation and fusion of the LFP particles occurs within the sintering step of S2-b, and at the same time, the carbon source is coated on the surface of the particles, or is dislodged from the surface of the particles. These two stages mentioned above have a faster heating rate, which can favor the formation of fused particles and ensure the high compaction property of the material. The decomposition of oxygen -donating compounds occurs in the stage of S2-c, which generates trace oxygen inside the material. It is necessary to reduce the gas feeding rate and increase the pressure in the furnace to minimize the gas replacement rate in the furnace to avoid rapid loss of oxygen generated, and the carbon source on the surface of the stage is more active, which captures trace oxygen to generate CO, and then forms iron phosphide compounds with the uncoated LFP.

[0075] In a third aspect, the present application also provides a positive electrode plate, comprising: a positive electrode current collector, and a cathode active material layer provided on at least one side of the positive electrode current collector; wherein the cathode active material layer comprises the composite cathode active material or a composite cathode active material prepared by the method.

[0076] In a fourth aspect, the present application also provides a secondary battery, comprising the positive electrode plate.

[0077] Typically, a secondary battery comprises a positive electrode plate, a negative electrode plate, an electrolyte, and a separator. During the charging and discharging process of the battery, active ions are intercalated and deintercalated back and forth between the positive electrode plate and the negative electrode plate. The electrolyte serves to transmit ions between the positive electrode plate and the negative electrode plate. The separator is set between the positive electrode plate and the negative electrode plate, mainly plays the role of preventing the positive and negative electrodes from short-circuiting, and at the same time allows the ions to pass through.

[0078] The following uses a lithium ion battery as an example to illustrate the secondary battery of the present application.

[0079] [Positive electrode plate] The positive electrode plate comprises a positive electrode current collector and a cathode active material layer disposed on at least one surface of the positive electrode current collector. The cathode active material layer comprises the cathode active material.

[0080] As an example, the positive electrode current collector has two surfaces facing in opposite directions along the thickness direction itself, and the cathode active material layer is provided on either or both of the two surfaces of the positive electrode current collector facing in opposite directions.

[0081] The cathode active material layer comprises the cathode active material. The cathode active material may be selected from materials capable of absorbing and releasing lithium. The specific types of the cathode active material are not particularly limited and may be selected according to requirements. The cathode active material described in the present application is selected from the lithium iron phosphate (LiFePCh) type. As an example, in addition thereto, the cathode active material suitable for the battery system may also comprise at least one of the following materials: lithium manganese phosphate (LiMnPCh), lithium cobalt phosphate (LiCoPCh), iron pyrophosphate (Li2FeP2O?), lithium cobaltate (LiCoCh), spinel-type lithium manganate (LiMmC ), spinel-type lithium nickel manganate (LiNio.5Mn1.5O4), layered lithium manganate (LiMnO2), lithium nickelate (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium magnesiate (LiMgO2), lithium calciate (LiCaO2), lithium cuprate (LiCuO2), lithium zincate (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiW02), lithium nickel cobalt aluminum oxides (LiNixCoyAli.x.yO2, 0<x<l, 0<y<l, 0 <x + y<l, e.g. LiNi0.sCo0.15Al0.05O2), lithium nickel cobalt manganese oxides (LiNixCoyMni-x-yO2, 0<x<l, 0<y<l, 0<x + y<l, e.g., LiNii / 3Coi / 3Mm / 3O2, LiNio.5Coo.2Mno.3O2, LiNio.6Coo.2Mno.2O2, LiNio.sCoo.1Mno.1O2, etc.), lithium-rich materials (e.g. lithium-rich nickel cobalt manganese oxides), manganese oxides (Mn02), vanadium oxides, sulfur oxides, silicate oxides, and at least one of their respective modified compounds. These materials may be used separately or in combination, for example two or more kinds of materials are used together.

[0082] The secondary battery provided in the present application is a secondary battery in which lithium iron phosphate (LiFePCh) cathode active material is selected. The modification of each of the above cathode active materials may comprise a doping modification, a surface coating modification, or a doping-coating simultaneous modification of the cathode active material, and the like.

[0083] In some embodiments, the cathode active material layer can also comprise a binder. As an example, the binder may comprise at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0084] In some embodiments, the cathode active material layer can also comprise a conductive agent. As an example, the conductive agent may comprise at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0085] In some embodiments, the positive electrode plate may be prepared by the following method: dispersing the above-mentioned components for preparing the positive electrode plate, such as the cathode active material, the conductive agent, the binder and any other components, in a solvent (such as N-methyl pyrrolidone) to form a positive electrode slurry; and coating the positive electrode slurry on the positive electrode current collector, and obtaining the positive electrode plate after drying, cold pressing and other processes.

[0086] [Negative electrode plate]

[0087] The negative electrode plate comprises a negative electrode current collector and an anode active material layer disposed on at least one surface of the negative electrode current collector. The anode active material layer comprises an anode active material.

[0088] As an example, the negative electrode current collector has two surfaces facing in opposite directions along the thickness direction itself, and the anode active material layer is provided on either or both of the two surfaces facing in opposite directions.

[0089] In some embodiments, the anode active material may be an anode active material known in the art. As an example, the anode active material may comprise at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate and the like. The silicon-based material may be at least one selected from elemental silicon, silicon-oxygen compounds, silicon-carbon complexes, silicon-nitrogen complexes, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. The present application is not limited to these materials, and other traditional materials that may be used as an anode active material for a battery may be used. These anode active materials may be used separately or in combination (for example two or more kinds of materials are used).

[0090] In some embodiments, the anode active material layer optionally comprises a binder. The binder may be at least one selected from 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).

[0091] In some embodiments, the anode active material layer optionally comprises a conductive agent. The conductive agent may be at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0092] In some embodiments, the anode active material layer optionally comprises other adjuvants, such as thickeners (e.g. sodium carboxymethylcellulose (CMC -Na)).

[0093] In some embodiments, the negative electrode plate may be prepared by: dispersing the above-mentioned components for preparing the negative electrode plate, such as the anode active material, the conductive agent, the binder and any other components in a solvent (such as deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and obtaining the negative electrode plate after drying, cold pressing and other processes.

[0094] [Electrolyte]

[0095] The electrolyte serves to conduct ions between the positive electrode plate and the negative electrode plate. The kind of the electrolyte is not particularly limited in the present application, and may be selected according to requirements. For example, the electrolyte may be liquid, gel, or solid.

[0096] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution comprises an electrolyte salt and a solvent.

[0097] In some embodiments, the electrolyte salt may comprise at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonimide, lithium bis-trifluoromethane sulfonimide, lithium triflate, lithium difluorophosphate, lithium difluorooxalato borate, lithium dioxalato borate, lithium difluorooxalato phosphate, and lithium tetrafluorooxalato phosphate.

[0098] In some embodiments, the solvent may comprise at least one selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene 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.

[0099] In some embodiments, the electrolyte solution optionally comprises an additive. For example, the additive may comprise a negative electrode film-forming additive, a positive electrode film-forming additive, and may further comprise an additive capable of improving properties of the battery, such as an additive for improving overcharge properties of the battery, and an additive for improving high-temperature or low-temperature properties of the battery.

[0100] In some embodiments, the electrolyte is a solid electrolyte, which may be a variety of lithium ion solid electrolytes commonly used in the art. Examples of the lithium ion solid electrolyte comprise, but are not limited to:

[0101] LISICON type materials, for example, y-LisPCh, etc.;

[0102] NASICON type materials, for example, Li(i+xi)QxM(2-xi)(PO4)3, where 0 < xl <1, and Q comprises at least one selected from Al, Cr, Ba, Fe, Sc, In, Lu, Y and La; Garnet type materials, for example, Li(7-x2)La3Zr(2-x2)Mx2Oi2, where 0 < x2 <1, and M comprises at least one selected from Sb, Nb, Ta, Te and W;

[0103] LIPON type materials, for example, LixsPOyiNzi; where 0<x3<l, 0<y 1<1, and 0<zl<l;

[0104] Perovskite type materials, for example, LiX4Q(2 / 3-x4)MO3, where 0.04 <x4 <0.17, Q comprises at least one selected from La, Sr, Ba and Nd, M comprises at least one selected from Al, Ti and Ge;

[0105] Anti-Perovskite type materials, for example, LisOCl;

[0106] Thio-LiSICON type materials, for example, Li(3+x5)My2A(i-y2)Q(4-z2)TZ2, where -1 <x5 <2, 0 < y2 < 1, and 0 < z2 < 2, M comprises at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A comprises at least one selected from P, As, Sb and Bi, Q comprises at least one selected from S or Se, and T comprises at least one selected from F, Cl, Br and I; sulfide solid electrolytes, comprising: Thiophosphate type materials, for example, Li3PS4, Argyrodite type materials, for example, LiePSsCl, Halide type materials, for example, LisInCL, Hydride type materials, for example at least one selected from O.7Li(CB9Hio) - 0.3Li(CBuHi2); for example, a material of Li(io+x6)M(i+y3)A(2-y3)Q(i2-z3)HZ3, where -2 < x6 <2, 0 < y3 < 2, and 0 < z3 < 2, M comprises at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A comprises at least one selected from P, As, Sb and Bi, Q comprises at least one selected from S and Se, H comprises at least one selected from F, Cl, Br and I, for another example, a material of (100-x7)Li2S*x7M»y4Q, where 20 < x7 < 30 and 0 < y4 < 50, M comprises at least one selected from B2S3, AI2S3, ImSs, SiS2, GeS2, SnS2, P2S5, AS2S3, Sb2Ss, Bi2S3, WS2 and M0S2, Q comprises at least one selected from B2O3, AI2O3, ImCh, SiCh, GeCh, SnCh, P2O5, Sb20s, Bi2C>3, WO2, WO3, MoO2, MoO3, Fe2C>3, ZnO, MgO, CuO, CaO, LiN, Li2O, LiF, LiCl, LiBr and Lil; Argyrodite type materials, for example, Li(6+x8)My5A(i-y5)Q(5-z5)T(i+z5), where -I < x8 < 1, 0 < y5 < 1 and -1 <z5 < 1, M comprises at least one selected from B, Al, In, Si, Ge, Sn, Ti, W and Mo, A comprises at least one selected from P, As, Sb and Bi, Q comprises at least one selected from S and Se, T comprises at least one selected from F, Cl, Br and I; Halide type materials, for example, LLMJ or Li2Sc2 / 3J, where M comprises at least one selected from Y, Er, In, Sc and Ga, and J comprises at least one selected from F, Cl, Br and I.

[0107] The sulfide solid electrolyte comprises, but is not limited to, sulfur silver germanium mineral electrolytes; binary sulfide solid materials such as Li2S-P2Ss, Li2S-SiS2, Li2S-GeS and Li2S-B2S3, and ternary materials such as Li2S-Me-P2S5, where Me is selected from Si, Ge, Sn and Al.

[0108] Specifically, the sulfide electrolyte is selected from at least one of Li2S-P2S5, Li2S-SiS2, Li2S-GeS, Li2S-B2S3and Li2S-Me-P2S5.

[0109] [Separator]

[0110] In some embodiments, the separator is further comprised in the secondary battery. The type of the separator is not particularly limited in the present application, and any known separator having a porous structure and good chemical and mechanical stability may be used.

[0111] In some embodiments, the material of the separator may be at least one selected from glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, which is not limited in the present application. In a case where the separator is a multilayer composite film, the materials of individual layers may be the same or different.

[0112] In some embodiments, the positive electrode plate, the negative electrode plate, and the separator may be prepared into an electrode assembly by a winding process or a lamination process.

[0113] In some embodiments, a secondary battery may comprise an outer package. The outer package may be used to encapsulate the electrode assembly and the electrolyte.

[0114] In some embodiments, the outer package of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell and the like. Alternatively, the outer package of the secondary battery may be a soft package, such as a soft bag. The soft bag may be made of a polymer material such as plastics, polypropylene, polybutylene terephthalate and polybutylene succinate. The shape of the secondary battery may be cylindrical, square or any other shape, which is not limited in the present application.

[0115] In a fifth aspect, the present application also provides a powered device, comprising the secondary battery described in the fourth aspect.

[0116] In some embodiments, the above powered device may also comprise a battery module or a battery pack obtained by assembling the secondary battery. The secondary battery, battery module, or battery pack may be used as a power source for the powered device or as an energy storage unit for the powered device. The powered device may comprise, but is not limited to, mobile devices (e.g., cell phones, laptop computers, 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, and the like.

[0117] As the powered device, a secondary battery, a battery module or a battery pack can be selected according to the needs of its use. As an example, for a powered device that is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc., a battery pack or a battery module may be used to meet the demand for high power and high energy density of the secondary battery of the powered device.

[0118] As another example, the device may be a cell phone, a tablet computer, a laptop computer, and the like. This device usually requires thinness and lightness, and a secondary battery may be used as the power source.

[0119] The present application is described in further detail below in connection with specific examples, which are not to be construed as limiting the scope of the protection claimed herein.

[0120] The following examples of the present application involve the following special terms and related definitions: wt%: weight percentage; mol%: molar percentage; Mu: relative atomic mass of Li;

[0121] MP: relative atomic mass of P;

[0122] M FeP04: relative molecular mass of LiFePCh;

[0123] Nu: the mass fraction of element Li tested by ICP; and

[0124] NP: the mass fraction of element P tested by ICP.

[0125] Example 1

[0126] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0127] SI : weighing iron phosphate and lithium carbonate as raw materials in accordance with a molar ratio of Fe: Li of 0.97: 1, mixing, adding a carbon source material (sucrose mixed with polyethylene glycol at a weight ratio of 1 : 1) accounting for 12 wt% of the weight of the iron phosphate, adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), and adding 500 ppm of copper sulfate as an oxy gen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, controlling the sanding granularity to be 300 nm, spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0128] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: controlling N2 gas feeding rate to be lOOL / min and a pressure in a furnace for sintering to be 30Pa; under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 2h; then continuing to heat up to 600°C at a heating rate of 3°C / min while controlling N2 gas feeding rate to be 5L / min and a pressure in the furnace for sintering to be 200Pa; then continuing to heat up to 800°C at a heating rate of l°C / min, holding at this temperature and sintering for 12h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0129] Example 2

[0130] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0131] SI: weighing ferrous oxalate, monoammonium phosphate and lithium oxalate as raw materials in accordance with a molar ratio of Fe: P: Li of 0.95: 1 :0.98, mixing, adding a carbon source material (glucose) accounting for 10 wt% of the weight of the above raw materials, adding MgO as a dopant (with an addition amount of 3,000 ppm), and adding 1,000 ppm of potassium manganate as an oxygen-donating compound, transferring a mixture of the above mixed materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, controlling the sanding granularity to be 300 nm, spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0132] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: controlling N2 gas feeding rate to be lOOL / min and a pressure in a furnace for sintering to be 30Pa; under N2 atmosphere, heating up to 350°C at a heating rate of 2°C / min, and holding at 350°C for 3h; then continuing to heat up to 550°C at a heating rate of 2°C / min while controlling N2 gas feeding rate to be 5L / min and a pressure in the furnace for sintering to be 200Pa; then continuing to heat up to 700°C at a heating rate of l°C / min, holding at this temperature and sintering for lOh; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0133] Example 3

[0134] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0135] SI : weighing ferrous phosphate, diammonium hydrogen phosphate and lithium acetate as raw materials in accordance with a molar ratio of Fe: P: Li of 0.95: 1 :1.0, mixing, adding a carbon source material (citric acid) accounting for 10 wt% of the weight of the above raw materials, adding Nb2Os as a dopant (with an addition amount of 2,000 ppm), and adding 100 ppm of manganese sulfate as an oxygen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, controlling the sanding granularity to be 300 nm, spraying to obtain a dry sprayed material, and obtaining a precursor; and S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: controlling N2 gas feeding rate to be lOOL / min and a pressure in a furnace for sintering to be 30Pa; under N2 atmosphere, heating up to 400°C at a heating rate of 4°C / min, and holding at 400°C for 2h; then continuing to heat up to 600°C at a heating rate of 4°C / min while controlling a N2 gas feeding rate to be 5L / min and a pressure in the furnace for sintering to be 200Pa; then continuing to heat up to 750°C at a heating rate of 2°C / min, holding at this temperature and sintering for 8h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0136] Example 4

[0137] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0138] SI : weighing iron(III) oxide, lithium dihydrogen phosphate and lithium hydroxide as raw materials in accordance with a molar ratio of Fe: P: Li of 1.00:1 :0.98, mixing, adding a carbon source material (glucose) accounting for 12 wt% of the weight of the above raw materials, adding ZrCh as a dopant (with an addition amount of 4,000 ppm), and adding 300 ppm of potassium perchlorate as an oxygen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, controlling the sanding granularity to be 300 nm, spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0139] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: controlling a N2 gas feeding rate to be lOOL / min and a pressure in a furnace for sintering to be 30Pa; under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 2h; then continuing to heat up to 600°C at a heating rate of 2°C / min while controlling a N2 gas feeding rate to be 5L / min and a pressure in the furnace for sintering to be 200Pa; then continuing to heat up to 800°C at a heating rate of l°C / min, holding at this temperature and sintering for 5h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0140] Example 5

[0141] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0142] SI: weighing ferric hydroxide, lithium dihydrogen phosphate and lithium chloride as raw materials in accordance with a molar ratio of Fe: P: Li of 1.00: 1.00: 1.00, mixing, adding a carbon source material (PEG) accounting for 15 wt% of the weight of the above raw materials, adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), and adding 500 ppm of copper sulfate as an oxygen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, subjecting sand milling (controlling the sanding granularity to be 500 nm), spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0143] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 3h; then continuing to heat up to 580°C at a heating rate of 3°C / min while controlling a N2 gas feeding rate to be 5L / min and a pressure in the furnace for sintering to be 200Pa; then continuing to heat up to 780°C at a heating rate of l°C / min, holding at this temperature and sintering for 8h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0144] Example 6

[0145] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0146] SI : weighing iron phosphate and lithium carbonate as raw materials in accordance with a molar ratio of Fe: Li of 0.97: 1, mixing, adding a carbon source material (sucrose mixed with polyethylene glycol at a weight ratio of 1 : 1) accounting for 12 wt% of the weight of the iron phosphate, adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), and adding 1,500 ppm of copper sulfate as an oxy gen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, subjecting sand milling (controlling the sanding granularity to be 600 nm), spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0147] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: controlling a N2 gas feeding rate to be lOOL / min and a pressure in a furnace for sintering to be 40Pa; under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 2h; then continuing to heat up to 600°C at a heating rate of 3°C / min while controlling a N2 gas feeding rate to be 5L / min and a pressure in the furnace for sintering to be 200Pa; then continuing to heat up to 800°C at a heating rate of l°C / min, holding at this temperature and sintering for 12h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0148] Example 7

[0149] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0150] SI : weighing iron phosphate and lithium carbonate as raw materials in accordance with a molar ratio of Fe: Li of 0.97: 1, mixing, adding a carbon source material (sucrose mixed with polyethylene glycol at a weight ratio of 1 : 1) accounting for 12 wt% of the weight of the iron phosphate, adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), and adding 50 ppm of copper sulfate as an oxygen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, subjecting sand milling (controlling the sanding granularity to be 300 nm), spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0151] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 2h; then continuing to heat up to 600°C at a heating rate of 3°C / min while controlling a N2 gas feeding rate to be 8L / min and a pressure in the furnace for sintering to be 300Pa; then continuing to heat up to 800°C at a heating rate of l°C / min, holding at this temperature and sintering for 12h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0152] Example 8

[0153] The method for preparing the composite cathode active material described in this example comprised the following steps:

[0154] SI: weighing iron phosphate and lithium carbonate as raw materials in accordance with a molar ratio of Fe: Li of 0.97: 1, mixing, adding a carbon source material (sucrose mixed with polyethylene glycol at a weight ratio of 1 :1) accounting for 2.5 wt% of the weight of the iron phosphate, adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), and adding 500 ppm of copper sulfate as an oxygen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, subjecting sand milling (controlling the sanding granularity to be 300 nm), spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0155] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 2h; then continuing to heat up to 600°C at a heating rate of 3°C / min while controlling a N2 gas feeding rate to be lOL / min and a pressure in the furnace for sintering to be 300Pa; then continuing to heat up to 800°C at a heating rate of l°C / min, holding at this temperature and sintering for 12h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0156] Example 9

[0157] The method for preparing the composite cathode active material described in this example comprised the following steps: SI: weighing iron phosphate and lithium carbonate as raw materials in accordance with a molar ratio of Fe: Li of 0.97: 1, mixing, adding a carbon source material (sucrose mixed with polyethylene glycol at a weight ratio of 1 : 1) accounting for 12 wt% of the weight of the iron phosphate, adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), and adding 500 ppm of potassium perchlorate as an oxygen-donating compound, transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, subjecting sand milling (controlling the sanding granularity to be 400 nm), spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0158] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: under N2 atmosphere, heating up to 450°C at a heating rate of 5°C / min, and holding at 450°C for 2h; then continuing to heat up to 650°C at a heating rate of 5°C / min while controlling a N2 gas feeding rate to be lOL / min and a pressure in the furnace for sintering to be 280Pa; then continuing to heat up to 800°C at a heating rate of l°C / min, holding at this temperature and sintering for 12h; and airflow pulverizing the sintered materials to obtain the composite cathode active material of the present application.

[0159] Comparative Example 1

[0160] The method for preparing the composite cathode active material described in this comparative example comprised the following steps:

[0161] SI : weighing iron phosphate and lithium carbonate as raw materials in accordance with a molar ratio of Fe: Li of 0.97: 1, mixing, adding a carbon source material (sucrose mixed with polyethylene glycol at a weight ratio of 1 : 1) accounting for 12 wt% of the weight of the iron phosphate, and adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, subjecting sand milling (controlling the sanding granularity to be 600 nm), spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0162] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 2h; then continuing to heat up to 800°C at a heating rate of 3°C / min, holding at this temperature and sintering for 12h; during the whole process of sintering, controlling a N2 gas feeding rate to be lOOL / min and a pressure in the furnace for sintering to be 30Pa; and airflow pulverizing the sintered materials to obtain a lithium iron phosphate finished material.

[0163] Comparative Example 2

[0164] The method for preparing the composite cathode active material described in this comparative example comprised the following steps:

[0165] SI : weighing iron phosphate and lithium carbonate as raw materials in accordance with a molar ratio of Fe: Li of 0.97: 1, mixing, adding a carbon source material (sucrose mixed with polyethylene glycol at a weight ratio of 1 : 1) accounting for 12 wt% of the weight of the iron phosphate, and adding titanium dioxide as a dopant (with an addition amount of 3,000 ppm), transferring a mixture of the above materials to a mixing tank, mixing the mixture in water using a homogenizer, controlling the solid content to be 30%, subjecting sand milling (controlling the sanding granularity to be 300 nm), spraying to obtain a dry sprayed material, and obtaining a precursor; and

[0166] S2: sintering the precursor according to the following sintering curve, wherein the sintering curve was set as follows: controlling a N2 gas feeding rate to be 80L / min and a pressure in a furnace for sintering to be 80Pa; under N2 atmosphere, heating up to 400°C at a heating rate of 3°C / min, and holding at 400°C for 2h; then continuing to heat up to 600°C at a heating rate of 3°C / min while controlling a N2 gas feeding rate to be lOL / min and a pressure in the furnace for sintering to be 200Pa; then continuing to heat up to 800°C at a heating rate of l°C / min, holding at this temperature and sintering for 12h; and airflow pulverizing the sintered materials to obtain a lithium iron phosphate finished material.

[0167] Comparative Example 3

[0168] The method for preparing the composite cathode active material described in this comparative example was the same as that of Example 1, which differed only in that, in S I, instead of adding the oxygen-donating compound, reductive sintering was carried out by passing H2 during the sintering process in S2.

[0169] Comparative Example 4

[0170] The method for preparing the composite cathode active material described in this comparative example was the same as that of Example 1, which differed only in that, in S I, instead of adding the oxygen-donating compound, reductive sintering was carried out by passing CO during the sintering process in S2.

[0171] Comparative Example 5

[0172] The method for preparing the composite cathode active material described in this comparative example was the same as that of Example 1, which differed only in that, in S I, instead of adding the oxygen-donating compound, an equal amount of iron phosphide compound was added to mix.

[0173] Test Examples

[0174] The test parameters and methods involved in the following test examples of the present application were as follows:

[0175] SEM: Zeiss Sigma 500 type field emission scanning electron microscope (SEM) was used to characterize the material;

[0176] Compaction density test: compaction density of powder was tested using a powder compaction tester (pressure 3t) according to standard GB / T 30835-2014, “Lithium iron phosphate - carbon composite cathode material for lithium-ion battery”;

[0177] Electrochemical performance test: the capacity of button cell was tested using a button cell tester at 2.0-3.75V according to standard 20202915-T-610, “Electrochemical performance test of lithium iron phosphate - Test method for specific capacity and charge-discharge efficiency of the first cycle”;

[0178] Button cell battery assembly and electrochemical performance test: 1) the cathode active material, acetylene black and polyvinylidene fluoride in a weight ratio of 80:10: 10 were dissolved in N-methyl-pyrrolidone, stirred uniformly, and then coated on an aluminum foil, and then dried at 100°C in a blasting blowing drying oven to obtain a positive electrode plate precursor; 2) the dried positive electrode plate precursor was punched and sliced into small round pieces with a diameter of 12 mm as the positive electrode plate; 3) lithium metal piece were used as negative electrode plate, polypropylene microporous membrane was used as a separator, and a mixture of 1 mol / L of LiPFe in EC and DMC (volume ratio of 1 : 1 :1) was used as the electrolyte solution to assemble the CR2025-type button cell in a glove box filled with argon gas. NEWARE 5V50mA battery tester (available from NEWARE TECHNOLOGY LIMITED) was used to test the charging and discharging performance of the battery with a voltage of 3.75-2.0 V. The cycling test method was charging and discharging at 0.1 C for two cycles, followed by cycling with a charging at 1C and discharging at 1C program, and the cycling retention rate after 100 cycles was calculated as a ratio of discharging capacity at 1C after 100 cycles to discharging capacity at 1C after the first cycle.

[0179] Quantitative analysis of iron phosphide compounds: 200g of composite cathode active material sample was weighed and dispersed in 300g of water, and a clean magnetic bar of 12,000 GS was placed therein for rolling stirring for 30 min. After removing the magnetic bar, the material was gently rinsed with clean water and cleaned with ultrasonic waves for plurality of times to wash away the excess lithium iron phosphate, and the remaining magnetic material was dissolved with aqua regia; and the mass fractions of Li and P were tested by ICP, labelled as Nu and Np, respectively, of which Li element was supplied by LiFePCU, and P element was supplied by LiFePCh and iron phosphide compound, so that the molar fraction of the iron phosphide compound (N) in the sample was: N=(Np / Mp-NLi / MLi) / (200 / MLiFePO4), wherein Np represents the mass fraction of element P tested by ICP; Mp represents relative atomic mass of P; Nu represents the mass fraction of element Li tested by ICP; Mu represents relative atomic mass of Li; and MuFePO4 represents relative molecular mass of LiFePCh.

[0180] 1. Quantitative analysis of the iron phosphide compound in the materials

[0181] The lithium iron phosphate materials prepared in Examples 1-9 and Comparative Examples 1-5 were quantitatively analyzed according to the aforementioned methods, respectively, and the results are shown in Table 1 below.

[0182] Table 1. Quantitative analysis results of materials

[0183] It can be seen that the lithium iron phosphate material prepared by the method of the present application has relatively ideal iron phosphide compound properties and compaction density.

[0184] 2. XRD diagram of the material

[0185] The samples of lithium iron phosphate materials synthesized in Examples 1, 5, 6, and Comparative Example 1 were taken for XRD spectra test, and the results are shown in FIG. 1, where the standard card number of lithium iron phosphate, JCPDS: 83-2092, is shown.

[0186] It can be seen that the diffraction peak of the material phase of iron phosphide compound appeared at the position of 29=40.2°, which proved the production of the iron phosphide compound in the finished material. Meanwhile, the comparison of different examples reveals that the diffraction peak intensity of the iron phosphide compounds rises with the increase of the mass ratio of the added oxygen-donating compounds, which means the content of the iron phosphide compounds is elevated.

[0187] 3. Electrochemical performance test

[0188] The lithium iron phosphate materials prepared in Examples 1-9 and Comparative Examples 1-5 were tested for electrochemical performance according to the aforementioned methods, respectively, and the results are shown in Table 2 below. The comparison results of the discharge capacity at 1C of Example 1 and Comparative Example 1 are shown in FIG. 2.

[0189] Table 2. Electrochemical performance test results

[0190] As shown in the results in the above table, the composite cathode active materials prepared in the technical solution of the present application has an ideal content and uniform distribution of the iron phosphide compounds, which effectively improves the polarization of the lithium iron phosphate particles, enhances the rate capability and compaction density of the material, and contributes to the improvement of the electrical conductivity and the cycling stability performance of the lithium iron phosphate material, which effectively improves its electrochemical application performance.

[0191] As for the materials prepared in the technical solutions in Comparative Example 3 and Comparative Example 4, although the iron phosphide compound can also be produced by reducing through the passage of a reducing atmosphere, the resulting iron phosphide compounds are almost only able to form a coating on the surface layer of the lithium iron phosphate material and are not homogeneous for the material, and also produce inactive extremely large particle (>10 pm) materials, thereby affecting the compaction density and capacity of the materials. In contrast, the technical solution in Comparative Example 5 adopted the method of directly blending iron phosphate particles, for lithium iron phosphate finished products, the newly blended particles will affect the original particle gradation resulting in lower compaction density; moreover, iron phosphide particles can only exist among the lithium iron phosphate particles, and did not improve the original lithium iron phosphate particles without carbon coating, resulting in a larger polarization, lower rate capability, which affects the application performance of the materials.

[0192] In summary, the composite cathode active material prepared in the present application utilizes an oxygen-donating compound to form a high conductivity iron phosphide compound in situ on the surface of the uncoated lithium iron phosphate material, and then introduces a controllable iron phosphide compound coating layer to coat the large-sized particles that are not coated with carbon, which effectively improves the polarization of the lithium iron phosphate particles, enhances the rate capability and cycling stability of the material, and effectively improves the problem of non-uniformity of carbon coating in lithium iron phosphate materials, and effectively enhances the activity and the application performance of the cathode active materials. Obviously, the above examples are merely examples for the purpose of clear illustration, and are not a limitation of the embodiments. For those skilled ordinary in the art, other variations or changes in different forms can be made on the basis of the above description. It is neither necessary nor possible to exhaust all of the embodiments herein. The obvious variations or changes derived therefrom are still within the scope of protection of the present application.

Claims

CLAIMS1. A composite cathode active material, wherein the composite cathode active material comprises a mixture of lithium iron phosphate particles, lithium iron phosphate particles coated with carbon, lithium iron phosphate particles coated with an iron phosphide compound, lithium iron phosphate particles coated with an iron phosphide compound and carbon, and iron phosphide compound particles; and the content of the iron phosphide compound in the composite cathode active material is in a range from 0.01mol% to 0.22mol%.

2. The composite cathode active material of claim 1, wherein the iron phosphide compound comprises ferrous phosphide (Fe2P) and / or ferric phosphide (FeP).

3. The composite cathode active material of claim 1 or 2, wherein the intensity of the diffraction peaks ranging from 40° to 41° is 2% to 7% of the intensity of the strongest diffraction peak in the XRD spectrum of the composite cathode active material, wherein the diffraction peaks ranging from 40° to 41° are characteristic peaks of the iron phosphide compound, and the strongest diffraction peak is a crystal plane diffraction peak of LiFePCh phase (311).

4. The composite cathode active material of any one of claims 1 to 3, wherein the composite cathode active material further comprises a doping element M; the doping element M comprises at least one of Ti, Zr, V, Nb and Mg; and the content of doping element M is in a range from 1,000 ppm to 8,000 ppm based on the weight of the composite cathode active material.

5. The composite cathode active material of any one of claims 1 to 4, wherein the composite cathode active material has a particle size of 200 nm to 8,000 nm.

6. A method for preparing the composite cathode active material of any one of claims 1 to 5, comprising the following steps:SI: mixing an iron source material, a phosphorus source material, a lithium source material, a carbon source material and an oxygen-donating compound to obtain a precursor; andS2: carrying out a high temperature sintering treatment to the precursor under an inert atmosphere to obtain the composite cathode active material.

7. The method for preparing the composite cathode active material of claim 6, wherein in SI, the oxygen-donating compound comprises an oxygen-containing salt which undergoes thermal decomposition at a temperature of 600°C to 800°C.

8. The method for preparing the composite cathode active material of claim 6 or 7, wherein in SI, the oxygen-donating compound comprises at least one of potassium perchlorate (KCIO4), potassium manganate (K^MnCh), manganese sulfate (MnSC ), and copper sulfate (CuSCh); and / or the iron source material comprises at least one of iron phosphate, iron(III) oxide, ferrous oxalate, ferrous phosphate, iron hydroxide oxide, and ferric hydroxide; and / or the phosphorus source material comprises at least one of iron phosphate, lithium phosphate, lithium dihydrogen phosphate, phosphoric acid, monoammonium phosphate, diammonium hydrogen phosphate, and triammonium phosphate; and / or the lithium source material comprises at least one of lithium carbonate, lithium hydroxide, lithium phosphate, lithium dihydrogen phosphate, lithium oxalate, lithium acetate and lithium chloride; and / or the carbon source material comprises at least one of sucrose, glucose, starch, ascorbic acid, citric acid, polyethylene glycol, cellulose and phenolic resin.

9. The method for preparing the composite cathode active material of any one of claims 6 to 8, wherein in SI, the oxygen-donating compound is added in an amount of 0.1 mol% to 1 mol% based on the molar amount of iron element in the iron source material; and / or in the precursor, a molar ratio of iron element in the iron source material, phosphorus element in the phosphorus source material, and lithium element in the lithium source material is (0.95-1.00): 1 : (0.98-1.10); and / or the carbon source material is added in an amount of 10 wt% to 20 wt% based on the weight of the iron source material.

10. The method for preparing the composite cathode active material of any one of claims 6 to 9, wherein,SI further comprises a step of adding a dopant containing the doping element M; and based on the weight of the composite cathode active material, the doping element M in the dopant is added in an amount of 1,000 ppm to 8,000 ppm.

11. The method for preparing the composite cathode active material of any one of claims 6 to 10, wherein,SI further comprises a step of grinding and / or drying the precursor; and in the step of grinding, the precursor is controlled to be ground to a particle size of 100 nm to 700 nm.

12. The method for preparing the composite cathode active material of any one of claims 6 to 11, wherein in S2, the high temperature sintering treatment is carried out at a temperature of 700°C to 900°C for a time period of 6h to 15h.

13. The method for preparing the composite cathode active material of any one of claims 6 to 12, wherein S2 further comprises a step of pre-sintering treatment at a temperature of 350°C to 400°C for a time period of Ih to 3h.

14. The method for preparing the composite cathode active material of claim 12 or 13, wherein S2 comprises:S2-a: heating up to 350-400°C at a heating rate of 2-4°C / min and performing the pre-sintering treatment;S2-b: heating up to 550-600°C at a heating rate of 2-4°C / min; andS2-c: heating up to 700-800°C at a heating rate of l-2°C / min and performing the high temperature sintering treatment.

15. The method for preparing the composite cathode active material of claim 14, wherein in S2, in S2-a and / or S2-b, a gas feeding rate of the inert atmosphere is controlled to be in a range from 50 L / min to 100 L / min, and a pressure in a furnace for sintering is controlled to be in a range from 10 Pa to 100 Pa; and / or in S2-c, a gas feeding rate of the inert atmosphere is controlled to be less than or equal to 10 L / min, and a pressure in a furnace for sintering is controlled to be more than or equal to 200 Pa.

16. A positive electrode plate, comprising: a positive electrode current collector, and a cathode active material layer provided on at least one side of the positive electrode current collector; wherein the cathode active material layer comprises the composite cathode active material of any one of claims 1 to 5 or a composite cathode active material prepared by the method of any one of claims 6 to 15.

17. A secondary battery, comprising the positive electrode plate of claim 16.

18. A powered device, comprising the secondary battery of claim 17.

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