Lithium iron phosphate cathode active material, preparation method therefor and use thereof

Abstract

The present application discloses a lithium iron phosphate cathode active material and a preparation method and use thereof, and the lithium iron phosphate cathode active material of the present application comprises a core, an intermediate layer coating the core, and an outer layer coating the intermediate layer; the core has a chemical formula of LiyFeMxPO4, wherein 0.80 ≤ y ≤ 0.95, and 0 < x ≤ 0.02, and M is a doped metal element; the intermediate layer has a chemical formula of LiaFebPO4, wherein 1 < a ≤ 3 and 0 ≤ b < 1; and the outer layer is a carbon layer.

Description

[0001] LITHIUM IRON PHOSPHATE CATHODE ACTIVE MATERIAL, PREPARATION METHOD THEREFOR AND USE THEREOF

[0002] FIELD OF THE INVENTION

[0003] The present application relates to the technical field of lithium iron phosphate cathode active material, in particular to a lithium iron phosphate cathode active material and a preparation method therefor and use thereof.

[0004] BACKGROUND OF THE INVENTION

[0005] Lithium iron phosphate crystals are olivine structures, in which [FeOe] octahedrons and [PO4] tetrahedrons constitute the skeleton of the whole crystal cell, but the [FeOe] octahedrons are connected by concurrents, and their low overlap degree of electron cloud makes the electronic conductivity low; moreover, due to the limitation of the [PO4] tetrahedrons, in the process of charging and discharging, Li+can only be migrated along the one-dimensional channels, which makes its ionic conductivity also low; and the ionic conductivity and electronic conductivity together limit the rate capability of lithium iron phosphate.

[0006] In order to solve this problem, common methods are doping, coating, and nanocrystallization, but existing technology is still limited in terms of the enhancement on rate capability.

[0007] SUMMARY OF THE INVENTION

[0008] The present application provides a lithium iron phosphate cathode active material and a preparation method and use thereof to solve the problem of poor rate capability of lithium iron phosphate.

[0009] In a first aspect, the present application provides a lithium iron phosphate cathode active material, comprising a core, an intermediate layer coating the core, and an outer layer coating the intermediate layer; wherein the core has a chemical formula of LiyFeMxP04, wherein 0.80 < y < 0.95, 0 < x < 0.02, and M is a doping metal element; the intermediate layer has a chemical formula of LiaFebPCU, wherein l<a<3, 0<b<l; and the outer layer is a carbon layer.

[0010] In an optional embodiment, the core has a particle size ranging from 0.8pm to 3 pm, optionally from 1pm to 2 pm.

[0011] In an optional embodiment, the outer layer has a thickness of 5 nm or less, optionally the thickness is in a range from 2 nm to 3 nm.

[0012] In an optional embodiment, the lithium iron phosphate cathode active material satisfies at least one of the following conditions:

[0013] (1) M is one or more selected from the group consisting of V, Ti, Mg, Sn, Mn, and W;

[0014] (2) the intermediate layer is discontinuously coated in a shape of island;

[0015] (3) the content of carbon is in a range from 1 wt% to 1.5 wt% based on the total weight of the lithium iron phosphate cathode active material; and

[0016] (4) the lithium iron phosphate cathode active material has a specific surface area BET ranging from 8 m2 / g to 11 m2 / g.

[0017] In a second aspect, the present application also provides a method for preparing the lithium iron phosphate cathode active material, wherein the raw material for preparing the lithium iron phosphate cathode active material comprises a lithium source, an iron source, a phosphorus source, a dopant, a reducing agent, and a carbon source; and a molar ratio of lithium element in the lithium source, iron element in the iron source, and phosphorus element in the phosphorus source is (1-1.05): 1 : (1-1.05); and the method for preparing the lithium iron phosphate cathode active material comprises the following steps:

[0018] 51, mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent, and carrying out a first sintering to obtain intermediate product A; in SI, a molar ratio of lithium element in the lithium source, iron element in the iron source, phosphorus element in the phosphorus source and M element in the dopant is (0.8-0.95): 1 : (1-1.05): (0-0.02), and a molar amount of M is not 0;

[0019] 52, mixing intermediate product A with the remaining lithium source, and carrying out a second sintering to obtain intermediate product B; and

[0020] 53, mixing intermediate product B with the carbon source and carrying out a third sintering to obtain the lithium iron phosphate cathode active material.

[0021] In an optional embodiment, the first sintering is carried out at a temperature ranging from 700°C to 800°C for a time period ranging from Ih to 12h; optionally, the first sintering is carried out at a temperature ranging from 750°C to 800°C.

[0022] In an optional embodiment, the second sintering is carried out at a temperature ranging from 350°C to 500°C for a time period ranging from Ih to 6h.

[0023] In an optional embodiment, the third sintering is carried out at a temperature ranging from 650°C to 800°C for a time period ranging from Ih to 12h; optionally, the third sintering is carried out at a temperature ranging from 720°C to 750°C.

[0024] In an optional embodiment, SI satisfies at least one of the following conditions:

[0025] (1) a molar amount of carbon in the reducing agent is 10% to 20% of the molar amount of iron element in the iron source;

[0026] (2) the reducing agent comprises an organic carbon source; optionally, the organic carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, glucose, sucrose, starch, and water-soluble phenolic resin;

[0027] (3) the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate and lithium phosphate; (4) the iron source comprises at least one of ferric phosphate, diiron trioxide and ferrous oxalate;

[0028] (5) the phosphorus source comprises at least one of phosphoric acid, ammonium dihydrogen phosphate, ferric phosphate, lithium phosphate and lithium dihydrogen phosphate;

[0029] (6) the dopant comprises at least one of titanium dioxide, magnesium oxide, trimanganese tetraoxide, ammonium metavanadate, tungsten trioxide and tin dioxide;

[0030] (7) the first sintering is carried out under a protective atmosphere, optionally the protective atmosphere is N2 atmosphere or Ar atmosphere;

[0031] (8) the step of mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent is carried out in a mode of wet mixing; and

[0032] (9) intermediate product A has a particle size ranging from 0.8pm to 3 pm, optionally from 1pm to 2 pm.

[0033] In an optional embodiment, the reducing agent comprises at least one of polyvinyl alcohol, polyethylene glycol, and water-soluble phenolic resin; and the reducing agent further comprises at least one of glucose, sucrose, and starch.

[0034] In an optional embodiment, the step of mixing intermediate product A with the remaining lithium source is carried out in a mode of dry mixing.

[0035] In an optional embodiment, S3 satisfies at least one of the following conditions:

[0036] (1) the carbon source comprises an organic carbon source; optionally, the carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, glucose, sucrose, starch, and water-soluble phenolic resin; optionally, the carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, and water-soluble phenolic resin, and further comprises at least one of glucose, sucrose, and starch; (2) the content of the carbon source is in a range from 7.5 wt% to 11 wt% based on the weight of intermediate product B;

[0037] (3) the second sintering and / or the third sintering is carried out under a protective atmosphere; and

[0038] (4) the step of mixing intermediate product B with the carbon source is carried out in a mode of wet mixing.

[0039] In a third aspect, the present application 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 lithium iron phosphate cathode active material as described above or a lithium iron phosphate cathode active material prepared by the method as described above.

[0040] In a fourth aspect, the present application provides a secondary battery, comprising the positive electrode plate as described above.

[0041] In a fifth aspect, the present application provides a powered device, comprising the secondary battery as described above.

[0042] The technical solution of the present application has the following advantages.

[0043] The present application provides a lithium iron phosphate cathode active material, comprising a core, an intermediate layer coating the core, and an outer layer coating the intermediate layer; wherein the core has a chemical formula of LiyFeMxP04, wherein 0.80 < y < 0.95, 0 < x < 0.02, and M is a doping metal element; the intermediate layer has a chemical formula of LiaFebPC , wherein l<a<3, 0<b<l; and the outer layer is a carbon layer. The core of the lithium iron phosphate cathode active material of the present application is lithium-deficient type lithium iron phosphate, and the intermediate layer is lithium-rich type lithium iron phosphate, which partially enriches Li+inside the lithium iron phosphate cathode active material to the surface of the particles, which is able to reduce Li+migration paths, realize rapid deintercalation of Li+, and improve the Li+utilization rate, so as to improve the capacity and rate capability of the batteries without increasing the overall lithium amount.

[0044] BRIEF DESCRIPTION OF THE DRAWINGS

[0045] 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.

[0046] FIG. 1 is a SEM image of intermediate product A prepared in Example 1 of the present application.

[0047] FIG. 2 is a TEM image of the lithium iron phosphate cathode active material prepared in Example 1 of the present application.

[0048] FIG. 3 is a XRD spectrum of the lithium iron phosphate cathode active materials prepared in Example 1 and Example 8 of the present application.

[0049] FIG. 4 is a partially enlarged view of FIG. 3.

[0050] DETAILED DESCRIPTION OF THE INVENTION

[0051] 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.

[0052] 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.

[0053] In the description of the embodiments of the present application, the technical terms “first”, “second” and the like are only used for distinction between different objects and are not to be understood as indicating or implying relative importance or implicitly indicating a number, a particular order or a primary or secondary relationship of the technical features. In the description of the embodiments of the present application, “a plurality of’ means two or more, unless specified otherwise.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] In the description of the present application, the term “a plurality of’ refers to two or more (including two), and similarly, “a plurality of groups” refers to two or more (including two) groups, and “a plurality of pieces” refers to two or more (including two) pieces.

[0058] In the description of the present application, it is to be understood that, terms such as “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “over”, “below”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “in”, “out”, “clockwise”, “anti -clockwise”, “axial”, “radial” and “circumference” refer to the directions and location relations which are the directions and location relations shown in the drawings, and for describing the present application and for describing in simple, and which are not intended to indicate or imply that the device or the elements are disposed to locate at the specific directions or are structured and performed in the specific directions, which could not to be understood to the limitation of the present application.

[0059] In the present application, unless specified or limited otherwise, the terms “mounted,” “connected,” “coupled” and “fixed” are understood broadly, such as fixed, detachable mountings, connections and couplings or integrated, and can be mechanical or electrical mountings, connections and couplings, and also can be direct and via media indirect mountings, connections, and couplings, and further can be inner mountings, connections and couplings of two components or interaction relations between two components, which can be understood by those skilled in the art according to the detail embodiment of the present application.

[0060] When charging and discharging at large rates, a part of Li+inside the lithium iron phosphate cathode active material particles often cannot migrate in time, unable to play a role in the charging and discharging cycle process, forming a “dead zone”, and resulting in poor battery rate capability.

[0061] In order to solve the above problems in the related technology, according to a first aspect of the present application, there is provided a lithium iron phosphate cathode active material, comprising a core, an intermediate layer coating the core, and an outer layer coating the intermediate layer; wherein the core has a chemical formula of LiyFeMxP04, wherein 0.80 < y < 0.95, 0 < x < 0.02, and M is a doping metal element; the intermediate layer has a chemical formula of LiaFebPCU, wherein l<a<3, 0<b<l; and the outer layer is a carbon layer.

[0062] For example, x may be 0.005, 0.01, 0.015, or 0.02.

[0063] For example, y may be 0.80, 0.85, 0.90, or 0.95.

[0064] For example, a may be 1.2, 1.5, 2, 2.5, or 3.

[0065] For example, b may be 0, 0.2, 0.5, 0.6, 0.8, or 0.9.

[0066] In the lithium iron phosphate cathode active material of present application, the core is lithium-deficient type lithium iron phosphate, and the intermediate layer is lithium-rich type lithium iron phosphate, which enriches the Li+inside the lithium iron phosphate cathode active material that is excess and can not enter the charging and discharging cycle to the surface of the particles, and reduces the migration path of Li+, so as to realize the rapid deintercalation of Li+, solve the problem of “dead zone”, improve the utilization rate of Li+, and increase the capacity and rate capability of the battery. Lithium-deficient type lithium iron phosphate refers to lithium iron phosphate in which the molar amount of Li element is less than the molar amount of Fe element, and lithium -rich type lithium iron phosphate refers to lithium iron phosphate in which the molar amount of Li element is greater than the molar amount of Fe element.

[0067] In an optional embodiment, the outer layer has a thickness of 5 nm or less, optionally the thickness is in a range from 2 nm to 3 nm. For example, the outer layer can have a thickness of Inm, 2nm, 3nm, 4nm or 5nm.

[0068] In an optional embodiment, at least one of the following conditions is satisfied:

[0069] (1) M is one or more selected from the group consisting of V, Ti, Mg, Sn, Mn, and W;

[0070] (2) the intermediate layer is discontinuously coated in a shape of island;

[0071] (3) the content of carbon is in a range from 1 wt% to 1.5 wt% based on the total weight of the lithium iron phosphate cathode active material; for example, the content of carbon can be 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, or 1.5 wt%; and

[0072] (4) the lithium iron phosphate cathode active material has a specific surface area BET ranging from 8 m2 / g to 11 m2 / g; for example, the specific surface area BET can be 8 m2 / g, 8.5 m2 / g, 9 m2 / g, 9.5 m2 / g, or 10 m2 / g.

[0073] In a second aspect, the present application provides a method for preparing the lithium iron phosphate cathode active material, wherein the raw material for preparing the lithium iron phosphate cathode active material comprises a lithium source, an iron source, a phosphorus source, a dopant, a reducing agent, and a carbon source; and a molar ratio of lithium element in the lithium source, iron element in the iron source, and phosphorus element in the phosphorus source is (1-1.05): 1 : (1-1.05); and the method for preparing the lithium iron phosphate cathode active material comprises the following steps:

[0074] SI, mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent, and carrying out a first sintering to obtain intermediate product A; in SI, a molar ratio of lithium element in the lithium source, iron element in the iron source, phosphorus element in the phosphorus source and M element in the dopant is (0.8-0.95): 1 : (1-1.05): (0-0.02), and a molar amount of M is not 0;

[0075] S2, mixing intermediate product A with the remaining lithium source, and carrying out a second sintering to obtain intermediate product B; and

[0076] S3, mixing intermediate product B with the carbon source and carrying out a third sintering to obtain the lithium iron phosphate cathode active material.

[0077] In the present application, by adjusting the amount of lithium distribution during the preparation of the lithium iron phosphate cathode active material, the excess lithium inside the particles is enriched to the surface of the particles under the condition that the total amount of the lithium source remains unchanged, so as to form a lithium iron phosphate cathode active material with an core of lithium-deficient type lithium iron phosphate, an intermediate layer of lithium-rich lithium iron phosphate, and an outer carbon layer. By enriching this part of Li+that is excess in the interior and cannot enter the charging and discharging cycles to the particle surface, the Li+migration path is reduced, thus realizing the rapid deintercalation of Li+and improving the Li+utilization rate, so as to improve the battery capacity and rate capability without increasing the lithium amount.

[0078] In the present application, intermediate product A is mixed with the remaining lithium source for a second sintering, and then the carbon source is added for a third sintering, which can avoid the lithium source from being coated by the carbon source before contacting intermediate product A to form lithium iron phosphate, thereby improving the utilization rate of lithium and avoiding the generation of heterogeneous phase lithium phosphate.

[0079] In an optional embodiment, the first sintering is carried out at a temperature ranging from 700°C to 800°C for a time period ranging from Ih to 12h; optionally, the first sintering is carried out at a temperature ranging from 750°C to 800°C. For example, the temperature of the first sintering may be 700°C, 720°C, 750°C, 770°C, or 800°C; and the time period of the first sintering may be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h or 12 h.

[0080] In an optional embodiment, the second sintering is carried out at a temperature ranging from 350°C to 500°C for a time period ranging from Ih to 6h. For example, the temperature of the second sintering may be 350°C, 400°C, 450°C or 500°C; and the time period of the second sintering may be 1 h, 2 h, 3 h, 4 h, 5 h, or 6 h. In an optional embodiment, the third sintering is carried out at a temperature ranging from 650°C to 800°C for a time period ranging from Ih to 12h; optionally, the third sintering is carried out at a temperature ranging from 720°C to 750°C. The third sintering at 720°C to 750°C results in a higher graphitization of the carbon layer and better electrical conductivity. For example, the temperature of the third sintering may be 650°C, 700°C, 750°C, or 800°C. The time period of the third sintering may be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h or 12 h.

[0081] In an optional embodiment, SI satisfies at least one of the following conditions:

[0082] (1) a molar amount of carbon in the reducing agent is 10% to 20% of the molar amount of iron element in the iron source; for example, 10%, 15% or 20%;

[0083] (2) the reducing agent comprises an organic carbon source; optionally, the organic carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, glucose, sucrose, starch, and water-soluble phenolic resin;

[0084] (3) the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate and lithium phosphate;

[0085] (4) the iron source comprises at least one of ferric phosphate, diiron trioxide and ferrous oxalate;

[0086] (5) the phosphorus source comprises at least one of phosphoric acid, ammonium dihydrogen phosphate, ferric phosphate, lithium phosphate and lithium dihydrogen phosphate;

[0087] (6) the dopant comprises at least one of titanium dioxide, magnesium oxide, trimanganese tetraoxide, ammonium metavanadate, tungsten trioxide and tin dioxide;

[0088] (7) the first sintering is carried out under a protective atmosphere, optionally the protective atmosphere is N2 atmosphere or Ar atmosphere; and

[0089] (8) the step of mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent is carried out in a mode of wet mixing.

[0090] In an optional embodiment, intermediate product A has a particle size ranging from 0.8pm to 3 pm, optionally from 1pm to 2 pm. For example, the particle size of intermediate product A may be 0.8 pm, 1 pm, 1.5 pm, 2 pm, 2.5 pm, or 3 pm. The particle size of intermediate product A within the scope of the present application avoids the reduction in capacity caused by too large a particle size.

[0091] In an optional embodiment, the step of mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent comprises adding pure water, the reducing agent, the lithium source, the phosphorus source, the dopant, and the iron source sequentially to the mixing tank of the sand mill, controlling the solid content to be in a range from 30% to 45wt%, and grinding to the suitable particle size (Dv50 of 0.8-1.5pm).

[0092] In SI, after mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent, it further comprises drying; optionally, the drying is carried out at mode of spray drying.

[0093] In an optional embodiment, the reducing agent comprises a polymer, such as at least one of polyvinyl alcohol, polyethylene glycol, and water-soluble phenolic resin; and the reducing agent further comprises a sugar, such as at least one of glucose, sucrose, and starch.

[0094] The reducing agent comprises both the polymer and the sugar as described above, the polymer having a relatively high viscosity in water to prevent particle agglomeration, and the sugar having a relatively high residual carbon rate to improve the reducing capacity and reduce the cost at the same time.

[0095] Optionally, the polymer has a molecular weight Mnranging from 1,500 to 20,000.

[0096] Optionally, in the reducing agent, the weight ratio of sugar to polymer is (4-7.5): (2.5-6).

[0097] When the amount of reducing agent is calculated according to “the molar amount of carbon in the reducing agent is 10% to 20% of the molar amount of Fe element in the iron source”, the molar amount of the polymer is defined as the weight of the polymer / molecular weight Mnof the polymer.

[0098] For example, polyethylene glycol has a repeat unit of CH2CH2O, and polyvinyl alcohol has a repeat unit of CH2CHOH.

[0099] In an optional embodiment, the step of mixing intermediate product A with the remaining lithium source is carried out in a mode of dry mixing.

[0100] In an optional embodiment, S3 satisfies at least one of the following conditions:

[0101] (1) the carbon source comprises an organic carbon source; optionally, the carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, glucose, sucrose, starch, and water-soluble phenolic resin; optionally, the carbon source comprises at least one polymer of polyvinyl alcohol, polyethylene glycol, and water-soluble phenolic resin, and further comprises at least one sugar of glucose, sucrose, and starch; the carbon source comprises both the polymer and the sugar, the polymer having a relatively high viscosity in water to prevent particle agglomeration, and the sugar having a relatively high residual carbon rate to improve the reducing capacity and reduce the cost at the same time; optionally, the polymer has a molecular weight Mnranging from 1,500 to 20,000; optionally, in the carbon source, the weight ratio of sugar to polymer is (4-7.5): (2.5-6);

[0102] (2) the content of the carbon source is in a range from 7.5 wt% to 11 wt% based on the weight of intermediate product B; for example, the content of the carbon source can be 7.5 wt%, 8 wt%, 9 wt%, 10 wt% or 11 wt%;

[0103] (3) the second sintering and / or the third sintering is carried out under a protective atmosphere; and

[0104] (4) the step of mixing intermediate product B with the carbon source is carried out in a mode of wet mixing. In an optional embodiment, the step of mixing intermediate product B with the carbon source comprises adding pure water, the carbon source, and intermediate product B sequentially in the mixing tank of the sand mill, controlling the solid content to be in a range from 30% to 45 wt%, and spray-drying after grinding to a suitable particle size (Dv50 of 1-2 pm).

[0105] In an optional embodiment, before mixing intermediate product A with the lithium source B, intermediate product A is mechanically pulverized or air-flow pulverized until the secondary balls are broken up (based on SEM test).

[0106] In a third aspect, the present application 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 lithium iron phosphate cathode active material as described above or a lithium iron phosphate cathode active material prepared by the method as described above.

[0107] In some embodiments, as the positive electrode current collector, a metal foil plate or a composite current collector may be adopted. For example, as the metal foil plate, an aluminum foil may be used. The composite current collector may comprise a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metallic material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a substrate of a polymer material such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0108] In some embodiments, the cathode active material layer can optionally 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 fluori de-hexafluor opropylene-tetrafluoroethylene terpolymer, tetrafluoroethyl ene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin.

[0109] In some embodiments, the cathode active material layer can optionally 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.

[0110] In some embodiments, the positive electrode plate may be prepared by: 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, coating the positive electrode slurry on the positive electrode current collector, and obtaining the positive electrode plate after drying, cold pressing and other processes.

[0111] In a fourth aspect, the present application also provides a secondary battery, comprising the positive electrode plate as described above.

[0112] The following illustrates the secondary battery of the present application.

[0113] [Negative electrode plate]

[0114] 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.

[0115] 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.

[0116] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may comprise a polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector may be formed by forming a metallic material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a substrate of a polymeric material such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0117] 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).

[0118] 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).

[0119] 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.

[0120] In some embodiments, the anode active material layer optionally comprises other adjuvants, such as thickeners (e.g. sodium carboxymethylcellulose (CMC -Na)).

[0121] 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.

[0122] [Electrolyte]

[0123] 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.

[0124] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution comprises an electrolyte salt and a solvent.

[0125] In some embodiments, the electrolyte salt may comprise at least one selected from lithium hexafluorophosphate, lithium tetrafluorob orate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonimide, lithium bi s-trifluorom ethane sulfonimide, lithium triflate, lithium difluorophosphate, lithium difluorooxalato borate, lithium dioxalato borate, lithium difluorooxalato phosphate, and lithium tetrafluorooxalato phosphate.

[0126] 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.

[0127] 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. In some embodiments, the solid electrolyte base material is a lithium ion solid electrolyte.

[0128] In some embodiments, the lithium ion solid electrolyte may be a variety of lithium ion solid electrolytes commonly used in the art.

[0129] Examples of the lithium ion solid electrolyte are described herein, including but not limited to:

[0130] LISICON type materials, for example, y-LiaPCU, etc.;

[0131] NASICON type materials, for example, Li(i+xi)QxiM(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;

[0132] 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;

[0133] LIPON type materials, for example, LixaPOyiNzi; where 0<x3<l, 0<y 1<1, and 0<zl<l;

[0134] 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;

[0135] Anti-Perovskite type materials, for example, LLOCl;

[0136] 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, LialnCL; Hydride type materials, for example, at least one selected from O.7Li(CB9Hio) - 0.3Li(CBnHi2); for example, a type 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, and H comprises at least one selected from F, Cl, Br and I; a type of (100-x7)Li2S*x7M»y4Q, where 20 < x7 < 30 and 0 < y4 < 50, M comprises at least one selected from B2S3, AI2S3, Ir S3, SiS2, GeS2, SnS2, P2S5, AS2S3, Sb2Ss, Bi2S3, WS2 and M0S2, Q comprises at least one selected from B2O3, AI2O3, I Ch, SiCh, GeCb, SnC>2, 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, LisMJ 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.

[0137] The above 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.

[0138] Specifically, the above sulfide electrolyte is selected from at least one of Li2S-P2Ss, Li2S-SiS2, Li2S-GeS, Li2S-B2S3 and Li2S-Me-P2S5.

[0139] [Separator]

[0140] In some embodiments, the secondary battery also comprises a separator. 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.

[0141] 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.

[0142] In some embodiments, the positive electrode plate, the negative electrode plate, and the separator of a secondary battery may be prepared into an electrode assembly by a winding process or a lamination process.

[0143] 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.

[0144] 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.

[0145] The shape of the secondary battery may be cylindrical, square or any other shape, which is not limited in the present application.

[0146] In some embodiments, the secondary batteries may be assembled into a battery module, and the number of secondary batteries contained in the battery module may be one or more, the specific number being selectable by a person skilled in the art based on the application and capacity of the battery module.

[0147] In some embodiments, the above battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by the person skilled in the art according to the application and capacity of the battery pack.

[0148] In a fifth aspect, the present application provides a powered device, comprising the secondary battery described in the above.

[0149] In some embodiments, the above powered device may also comprise a battery module or a battery pack obtained by assembling the above 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.

[0150] 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.

[0151] As another example, the powered 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.

[0152] 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 by the present application.

[0153] Example 1

[0154] The present example provided a method for preparing a lithium iron phosphate cathode active material, comprising the following steps:

[0155] SI, adding lithium carbonate as a lithium source, iron phosphate as a phosphorus source and an iron source, titanium dioxide as a dopant, glucose and polyethylene glycol (Mn=1500) as a reducing agent into pure water, mixing homogeneously, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, and then carrying out a first sintering to the dried granules under N2 atmosphere to obtain intermediate product A; in SI, a molar ratio of lithium element, iron element, phosphorus element, and titanium element was 0.9: 1 : 1 : 0.01; the first sintering was carried out at a temperature of 750 °C for a time period of 6 h; a molar amount of carbon in the reducing agent was 20% of the molar amount of iron element in the iron source; and a weight ratio of glucose to polyethylene glycol was 2: 3; 52, carrying out airflow pulverizing on intermediate product A, and then mixing intermediate product A with lithium carbonate as a lithium source and carrying out a second sintering in N2 atmosphere to obtain intermediate product B; in S2, a molar ratio of lithium element in the lithium source to iron element in intermediate product A was 0.1: 1; and the second sintering was carried out at a temperature of 400°C for a time period of 3h;

[0156] 53, adding intermediate product B, as well as glucose and polyethylene glycol as a carbon source into pure water, mixing uniformly, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, carrying out a third sintering to the spray-dried granules in N2 atmosphere to obtain a lithium iron phosphate cathode active material after airflow pulverizing (gas pressure in pulverization chamber was in a range from 0.4 MPa to 0.5 MPa, and the frequency of the classification wheel was 140 Hz); wherein the content of the carbon source was 10 wt% based on the weight of intermediate product B; a weight ratio of glucose to polyethylene glycol was 2: 3; and the third sintering was carried out at a temperature of 725°C for a time period of 6h.

[0157] Example 2

[0158] The present example provided a method for preparing a lithium iron phosphate cathode active material, which was essentially the same as Example 1, with the difference that in SI, the molar ratio of Li element in the lithium source, Fe element the iron source, P element in the phosphorus source and Ti element in the dopant was 0.95: 1: 1 : 0.01; and in S2, the molar ratio of Li element in the lithium source to Fe element in intermediate product A was 0.05: 1.

[0159] Example 3

[0160] The present example provided a method for preparing a lithium iron phosphate cathode active material, which was essentially the same as Example 1, with the difference that in SI, the molar ratio of Li element in the lithium source, Fe element the iron source, P element in the phosphorus source and Ti element in the dopant was 0.8: 1 : 1 : 0.01; and in S2, the molar ratio of Li element in the lithium source to Fe element in intermediate product A was 0.2: 1.

[0161] Example 4

[0162] The present example provided a method for preparing a lithium iron phosphate cathode active material, which was essentially the same as Example 1, with the difference that the doped metal element was V element.

[0163] Example 5

[0164] The present example provided a method for preparing a lithium iron phosphate cathode active material, which was essentially the same as Example 1, with the difference that the content of carbon source in S3 was 7.5 wt% based on the weight of intermediate product B.

[0165] Example 6

[0166] The present example provided a method for preparing a lithium iron phosphate cathode active material, comprising the following steps:

[0167] 51, adding lithium carbonate as a lithium source, iron phosphate as a phosphorus source and an iron source, titanium dioxide as a dopant, as well as glucose and polyethylene glycol as a reducing agent into pure water, mixing homogeneously, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, and then carrying out a first sintering to the dried granules under N2 atmosphere to obtain intermediate product A; in SI of this example, a molar ratio of lithium element, iron element, phosphorus element, and titanium element was 0.9: 1 : 1 : 0.02; the first sintering was carried out at a temperature of 700 °C for a time period of 12 h; a molar amount of carbon in the reducing agent was 10% of the molar amount of iron element in the iron source; and a weight ratio of glucose to polyethylene glycol was 2: 3;

[0168] 52, carrying out airflow pulverizing on intermediate product A, and then mixing it with lithium carbonate as a lithium source and carrying out a second sintering in N2 atmosphere to obtain intermediate product B; in S2, a molar ratio of lithium element in the lithium source to iron element in intermediate product A was 0.1 : 1; and the second sintering was carried out at a temperature of 350°C for a time period of 6h;

[0169] S3, adding intermediate product B, as well as glucose and polyethylene glycol as a carbon source into pure water, mixing uniformly, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, carrying out a third sintering to the spray-dried granules in N2 atmosphere to obtain a lithium iron phosphate cathode active material after airflow pulverizing (gas pressure in pulverization chamber was in a range from 0.4 MPa to 0.5 MPa, and the frequency of the classification wheel was 140 Hz); wherein the content of the carbon source was 11 wt% based on the weight of intermediate product B; a weight ratio of glucose to polyethylene glycol was 2: 3; and the third sintering was carried out at a temperature of 650°C for a time period of 12h.

[0170] Example 7

[0171] The present example provided a method for preparing a lithium iron phosphate cathode active material, comprising the following steps:

[0172] SI, adding lithium carbonate as a lithium source, iron phosphate as a phosphorus source and an iron source, titanium dioxide as a dopant, as well as glucose and polyethylene glycol as a reducing agent into pure water, mixing homogeneously, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, and then carrying out a first sintering to the spray-dried granules under N2 atmosphere to obtain intermediate product A; in SI of this example, a molar ratio of lithium element, iron element, phosphorus element, and titanium element was 0.9: 1 : 1 : 0.02; the first sintering was carried out at a temperature of 800 °C for a time period of 1 h; a molar amount of carbon in the reducing agent was 20% of the molar amount of iron element in the iron source; and a weight ratio of glucose to polyethylene glycol was 2: 3;

[0173] 52, carrying out airflow pulverizing on intermediate product A, and then mixing it with lithium carbonate and carrying out a second sintering in N2 atmosphere to obtain intermediate product B; in S2, a molar ratio of lithium element in the lithium source to iron element in intermediate product A was 0.1 : 1; and the second sintering was carried out at a temperature of 500°C for a time period of Ih;

[0174] 53, adding intermediate product B, as well as glucose and polyethylene glycol as a carbon source into pure water, mixing uniformly, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, carrying out a third sintering to the spray-dried granules in N2 atmosphere to obtain a lithium iron phosphate cathode active material after airflow pulverizing (gas pressure in pulverization chamber was in a range from 0.4 MPa to 0.5 MPa, and the frequency of the classification wheel was 140 Hz); wherein the content of the carbon source was 10 wt% based on the weight of intermediate product B; a weight ratio of glucose to polyethylene glycol was 2: 3; and the third sintering was carried out at a temperature of 800°C for a time period of Ih.

[0175] Example 8

[0176] The present example provided a method for preparing a lithium iron phosphate cathode active material, which was essentially the same as Example 1, with the difference that in S2, after mixing intermediate product A with the remaining lithium source, a second sintering was not carried out and S3 was carried out directly.

[0177] Comparative Example 1

[0178] This comparative example provided a method for preparing a lithium iron phosphate cathode active material, comprising the following steps: 51, adding lithium carbonate as a lithium source, iron phosphate as a phosphorus source and an iron source, titanium dioxide as a dopant, as well as glucose and polyethylene glycol as a reducing agent into pure water, mixing homogeneously, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, and then carrying out a first sintering to the spray-dried granules under N2 atmosphere to obtain intermediate product A; in SI, a molar ratio of lithium element, iron element, phosphorus element, and titanium element was 1 : 1 : 1 : 0.01; the first sintering was carried out at a temperature of 750 °C for a time period of 6 h; a molar amount of carbon in the reducing agent was 20% of the molar amount of iron element in the iron source; and a weight ratio of glucose to polyethylene glycol was 2:3;

[0179] 52, adding intermediate product A as well as glucose and polyethylene glycol as the carbon source into pure water, mixing homogeneously, grinding to Dv50 of 1.0 pm using a sand mill to obtain a slurry with a solid content of 40 wt%, spray drying, then carrying out a second sintering to the spray-dried granules under N2 atmosphere, and carrying out airflow pulverizing (gas pressure in pulverization chamber was in a range from 0.4 MPa to 0.5 MPa, and the frequency of the classification wheel was 140 Hz) to obtain the lithium iron phosphate cathode active material; wherein the content of the carbon source was 10 wt% based on the weight of intermediate product A; a weight ratio of glucose to polyethylene glycol was 2: 3; and the second sintering was carried out at a temperature of 725°C for a time period of 6h.

[0180] Test Example

[0181] (1) The carbon content was tested using HF-2000B high-frequency infrared carbon and sulfur analyzer according to GB / T 30835-2014 (Lithium iron phosphate-carbon composite cathode materials for lithium ion battery).

[0182] (2) The test method of the particle size of intermediate product A comprises: characterizing the morphology of intermediate product A using Zeiss Sigma 500 type field emission scanning electron microscopy (SEM), and measuring the particle size of intermediate product A using image J software.

[0183] (3) The test method for the thickness of the outer layer comprised: obtaining the TEM image of the lithium iron phosphate cathode material, and using TEM Digital Micrograph to measure the thickness of the outer layer in the TEM image. (4) The semi -quantitative test method for the elements of the intermediate layer comprised: performing XPS test, and using Avantage software to calculate the components of the intermediate layer of LiaFebPCU.

[0184] The test results were shown in FIG. 1, FIG. 2, and Table 1.

[0185] Table 1. Parameters of lithium iron phosphate cathode active materials produced by the examples and the comparative examples

[0186] Test of Electrochemical performance

[0187] Button cell assembly: 1) 1.6 g of lithium iron phosphate cathode active material produced by each example and comparative example and 0.2 g of acetylene black were ground and mixed, respectively; 2) 0.2 g of poly(vinylidene fluoride) was dissolved in 4 ml of N-m ethyl pyrrolidone, and then they were slowly added into the mixture of the above lithium iron phosphate cathode active material and acetylene black and stirred uniformly; and the above material was coated on an aluminum foil with a thickness of 20 pm, and then dried under 80 °C for 12 h in the air-blast drying oven; the dried electrode plate was punched and cut into a small disc with a size of 12 mm (containing about 6 mg of active material), which was used as the positive electrode plate; 3) the CR2016 type button cell was assembled with the lithium metal plate as the counter electrode, the ND525 separator as the separator, and 1 mol / L LiPFe in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1, v / v) as the electrolyte solution in an argon gas-filled glove box.

[0188] The charging and discharging performance of the above button cell was tested with a BTS-5 V / 5 mA battery testing system at a voltage of 3.65 V to 2.0 V. The button cell was firstly charged at 0.1C to 3.65 V, discharged at 0.1C to 2.0 V, and then charged at 0.1C to 3.65 V, discharged at 1C to 2.0 V, and then cycled at 1C / 1C.

[0189] The test results were shown in Table 2.

[0190] Table 2. Electrochemical performance

[0191] Capacity retention rate = (100th cycle discharge capacity of 1C / 1C cycle) / (first cycle discharge capacity of 1C / 1C cycle) X 100%

[0192] As can be seen from the comparison of Examples 1 to 8 and Comparative Example 1 in Table 2, the design of interface lithium-rich type lithium iron phosphate cathode active material can significantly improve the rate capability. In Comparative Example 1, there is no intermediate layer, and all of the lithium source with all the iron source, all the phosphorus source, all the dopant and all the reducing agent were directly mixed to prepare intermediate product A. The molar ratio of Li element to Fe element of the produced intermediate product A was set traditionally as 1 : 1, and a part of Li+inside the particles often cannot migrate in time, and it cannot play a role in the charging and discharging cycling process, forming a “dead zone”. The core of Examples 1-8 of the present application was lithium-deficient type lithium iron phosphate, and the intermediate layer was lithium-rich type lithium iron phosphate, under the condition that the total amount of lithium source remains unchanged, the Li+inside the lithium iron phosphate cathode active material that was excess and cannot enter the charging / discharging cycle was enriched to the surface of the particles, and the migration path of Li+was reduced, so as to realize the rapid deintercalation of Li+, solve the “dead zone” problem, and improve the Li+utilization rate, thereby improving the battery capacity and rate capability without increasing the lithium amount.

[0193] As can be seen from Examples 1 to 3, y in the range of 0.9 to 0.95 can further improve the rate capability and the first cycle discharge capacity of the battery.

[0194] As can be seen from Example 1 and Example 5, the content of carbon source in S3 was in the range from 9.5 wt% to 10.5 wt% based on the weight of intermediate product B, which can further improve the specific surface area of the lithium iron phosphate cathode active material, and rate capability and the first cycle discharge capacity of the battery.

[0195] FIG. 3 shows the XRD spectra of the lithium iron phosphate cathode active materials produced in Example 1 and Example 8, and FIG. 4 shows the XRD enlarged view, and it can be seen from FIGS. 3 and 4 that the heterogeneous phase lithium phosphate existed in Example 8, whereas this heterogeneous phase was not detected in Example 1. It demonstrated that mixing intermediate product A with the remaining lithium source for the second sintering, and then adding the carbon source for the third sintering can avoid the generation of the heterogeneous phase lithium phosphate in the lithium iron phosphate cathode active material, and further improve the cycling performance, the rate capability, and the first cycle discharge capacity of the battery.

[0196] 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 lithium iron phosphate cathode active material, comprising a core, an intermediate layer coating the core, and an outer layer coating the intermediate layer; wherein the core has a chemical formula of LiyFeMxP04, wherein 0.80 < y < 0.95, 0 < x < 0.02, and M is a doping metal element; the intermediate layer has a chemical formula of LiaFebPCU, wherein l<a<3, 0<b<l; and the outer layer is a carbon layer.

2. The lithium iron phosphate cathode active material of claim 1, wherein the outer layer has a thickness of 5 nm or less, optionally the outer layer has a thickness ranging from 2 nm to 3 nm.

3. The lithium iron phosphate cathode active material of claim 1 or 2, wherein at least one of the following conditions is satisfied:(1) M is one or more selected from the group consisting of V, Ti, Mg, Sn, Mn, and W;(2) the intermediate layer is discontinuously coated in a shape of island;(3) the content of carbon is in a range from 1 wt% to 1.5 wt% based on the total weight of the lithium iron phosphate cathode active material; and(4) the lithium iron phosphate cathode active material has a specific surface area BET ranging from 8 m2 / g to 11 m2 / g.

4. A method for preparing the lithium iron phosphate cathode active material of any one of claims 1 to 3, wherein the raw material for preparing the lithium iron phosphate cathode active material comprises a lithium source, an iron source, a phosphorus source, a dopant, a reducing agent, and a carbon source; and a molar ratio of lithium element in the lithium source, iron element in the iron source, and phosphorus element in the phosphorus source is (1-1.05): 1 : (1-1.05); and the method for preparing the lithium iron phosphate cathode active material comprises the following steps:SI, mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent, and carrying out a first sintering to obtain intermediate product A; in SI, a molar ratio of lithium element in the lithium source,iron element in the iron source, phosphorus element in the phosphorus source and M element in the dopant is (0.8-0.95): 1 : (1-1.05): (0-0.02), and a molar amount of M is not 0;52, mixing intermediate product A with the remaining lithium source, and carrying out a second sintering to obtain intermediate product B; and53, mixing intermediate product B with the carbon source and carrying out a third sintering to obtain the lithium iron phosphate cathode active material.

5. The method for preparing the lithium iron phosphate cathode active material of claim 4, wherein the first sintering is carried out at a temperature ranging from 700°C to 800°C for a time period ranging from Ih to 12h; optionally, the first sintering is carried out at a temperature ranging from 750°C to 800°C.

6. The method for preparing the lithium iron phosphate cathode active material of claim 4, wherein the second sintering is carried out at a temperature ranging from 350°C to 500°C for a time period ranging from Ih to 6h.

7. The method for preparing the lithium iron phosphate cathode active material of claim 4, wherein the third sintering is carried out at a temperature ranging from 650°C to 800°C for a time period ranging from Ih to 12h; optionally, the third sintering is carried out at a temperature ranging from 720°C to 750°C.

8. The method for preparing the lithium iron phosphate cathode active material of any one of claims 4 to 7, wherein SI satisfies at least one of the following conditions:(1) a molar amount of carbon in the reducing agent is 10% to 20% of the molar amount of iron element in the iron source;(2) the reducing agent comprises an organic carbon source; optionally, the organic carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, glucose, sucrose, starch, and water-soluble phenolic resin;(3) the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate and lithium phosphate;(4) the iron source comprises at least one of ferric phosphate, diiron trioxide and ferrous oxalate;(5) the phosphorus source comprises at least one of phosphoric acid, ammoniumdihydrogen phosphate, ferric phosphate, lithium phosphate and lithium dihydrogen phosphate;(6) the dopant comprises at least one of titanium dioxide, magnesium oxide, trimanganese tetraoxide, ammonium metavanadate, tungsten trioxide and tin dioxide;(7) the first sintering is carried out under a protective atmosphere, optionally the protective atmosphere is N2 atmosphere or Ar atmosphere;(8) the step of mixing a part of the lithium source with all of the iron source, all of the phosphorus source, all of the dopant, and all of the reducing agent is carried out in a mode of wet mixing; and(9) intermediate product A has a particle size ranging from 0.8pm to 3 pm, optionally intermediate product A has a particle size ranging from 1pm to 2 pm.

9. The method for preparing the lithium iron phosphate cathode active material of claim 8, wherein the reducing agent comprises at least one of polyvinyl alcohol, polyethylene glycol, and water-soluble phenolic resin; and the reducing agent further comprises at least one of glucose, sucrose, and starch.

10. The method for preparing the lithium iron phosphate cathode active material of any one of claims 4 to 7, wherein the step of mixing intermediate product A with the remaining lithium source is carried out in a mode of dry mixing.

11. The method for preparing the lithium iron phosphate cathode active material of any one of claims 4 to 7, wherein S3 satisfies at least one of the following conditions:(1) the carbon source comprises an organic carbon source; optionally, the carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, glucose, sucrose, starch, and water-soluble phenolic resin; optionally, the carbon source comprises at least one of polyvinyl alcohol, polyethylene glycol, and water-soluble phenolic resin, and further comprises at least one of glucose, sucrose, and starch;(2) the content of the carbon source is in a range from 7.5 wt% to 11 wt% based on the weight of intermediate product B;(3) the second sintering and / or the third sintering is carried out under a protectiveatmosphere; and(4) the step of mixing intermediate product B with the carbon source is carried out in a mode of wet mixing.

12. 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 lithium iron phosphate material of any one of claims 1 to 3 or a lithium iron phosphate material prepared by the method for preparing the lithium iron phosphate material of any one of claims 4 to 11.

13. A secondary battery, comprising the positive electrode plate of claim 12.

14. A powered device, comprising the secondary battery of claim 13.

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