Lithium iron phosphate positive electrode material and preparation method thereof, positive plate and battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium iron phosphate positive electrode material, a preparation method thereof, a positive plate and a battery. The disclosed lithium iron phosphate positive electrode material comprises large lithium iron phosphate particles and small lithium iron phosphate particles, the particle size of the large lithium iron phosphate particles is micron-scale, and the particle size of the small lithium iron phosphate particles is nano-scale; the large lithium iron phosphate particles comprise at least two doping elements, and the doping elements comprise metal elements and non-metal elements; and / or, the lithium iron phosphate small particles comprise at least one doping element, and the doping element comprises a metal element; wherein the metal elements comprise one or more of alkali metal, alkaline earth metal or transition metal; the non-metallic element comprises one or more of halogen, S or N. According to the invention, the diffusion coefficient of lithium ions can be improved, and the discharge capacity and rate capability of the lithium iron phosphate positive electrode material are enhanced; and the problem of cycle attenuation acceleration caused by cracking and crushing of large lithium iron phosphate particles in the long-term cycle process can be avoided.

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and in particular to a lithium iron phosphate positive electrode material and a preparation method thereof, a positive electrode sheet and a battery. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in energy storage power supplies, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. The performance of the cathode material in lithium-ion batteries largely determines the overall performance of the entire lithium-ion battery. Lithium iron phosphate (LiFeO4), with its olivine structure, has outstanding advantages such as long cycle life, good safety, low price, and non-toxicity and environmental protection, and is therefore used in the production of cathode materials. With continuous improvements in material synthesis and battery manufacturing processes, the energy density of lithium iron phosphate has been significantly improved, and it also has good cycle stability and rate performance.

[0003] With the rapid increase in demand for electric vehicles and renewable energy storage, the market demand for high-performance lithium iron phosphate cathode materials has also increased. The theoretical density of lithium iron phosphate is 3.60g / cm 3 , its powder compaction density still has a lot of room for improvement. The commonly used improvement methods currently include: raw material process, adjustment of sintering method and size particle grading, etc. Among them, particle size grading and mixing of large and small particles are the main means to improve the compaction density of lithium iron phosphate. Due to the low diffusion coefficient of lithium iron phosphate positive electrode materials, increasing the primary particle size will make the lithium ion deintercalation path longer, significantly affecting the material's kinetic properties, and thus leading to a decrease in performance capacity. Since the large particles in high-density lithium iron phosphate are formed by the fusion of small particles, these large particles are prone to cracking during long-term cycles and react with the electrolyte, thereby exacerbating the dissolution of iron and accelerating the cycle decay of the battery.

[0004] In view of this, it becomes particularly important to improve the dynamic performance of large lithium iron phosphate particles and solve the problems of low specific capacity of high-density lithium iron phosphate particles and cracking during the cycle, which leads to accelerated cycle attenuation. Summary of the Invention

[0005] The present invention provides a lithium iron phosphate positive electrode material and a preparation method thereof, a positive electrode sheet and a battery, which are used to solve the technical problems existing in the prior art of lithium iron phosphate positive electrode materials, such as low specific capacity of large lithium iron phosphate particles and cracking during the cycle, which leads to accelerated cycle attenuation.

[0006] In a first aspect, the present invention provides a lithium iron phosphate positive electrode material, comprising: large lithium iron phosphate particles and small lithium iron phosphate particles; the particle size of the large lithium iron phosphate particles is micrometer-level, and the particle size of the small lithium iron phosphate particles is nanometer-level;

[0007] The large lithium iron phosphate particles include at least two doping elements, wherein the doping elements include metal elements and non-metal elements;

[0008] And / or, the lithium iron phosphate small particles include at least one doping element, and the doping element includes a metal element;

[0009] The metal elements include one or more of alkali metals, alkaline earth metals or transition metals; and the non-metal elements include one or more of halogens, S or N.

[0010] In an optional embodiment, the lithium iron phosphate positive electrode material satisfies at least one of the following characteristics (1) to (4):

[0011] (1) The metal element is selected from one or more of Na, Mg, Ti, V, Zr, Mn, Nb or Mo;

[0012] (2) The non-metallic element is selected from one or more of F, S, Cl or N;

[0013] (3) The mass content of the doping element in the large lithium iron phosphate particles is 0.02% to 1%;

[0014] (4) In the lithium iron phosphate small particles, the mass content of the doping element is 0.02% to 1%; preferably, the mass content of the metal element is 0.02% to 1%;

[0015] (5) When the lithium iron phosphate particles include at least two doping elements, one of the doping elements is a metal element.

[0016] In an optional embodiment, the chemical formula of the large lithium iron phosphate particles is LiFe 1-x M x PO y N z ; Among them, the value range of x is 0 to 0.05, the value range of y is 3.95 to 4.00, and the value range of z is 0 to 0.05, and the values ​​of x and z do not include 0;

[0017] And / or, the chemical formula of the lithium iron phosphate small particles is LiFe 1-x M x PO y N z ; Among them, the value range of x is 0 to 0.03, the value range of y is 3.97 to 4.00, and the value range of z is 0 to 0.03, and the value of x does not include 0;

[0018] Wherein, M is a metal element and N is a non-metal element.

[0019] In an optional embodiment, the lithium iron phosphate positive electrode material meets the following characteristics (1)~

[0020] At least one of (6):

[0021] (1) The average particle size of the large lithium iron phosphate particles is 0.5 μm to 2 μm;

[0022] (2) The average particle size of the lithium iron phosphate small particles is 150nm to 300nm;

[0023] (3) In the lithium iron phosphate positive electrode material, the mass proportion of the large lithium iron phosphate particles is 50% to 85%;

[0024] (4) In the lithium iron phosphate positive electrode material, the mass proportion of the lithium iron phosphate small particles is 15% to 50%;

[0025] (5) The large lithium iron phosphate particles are primary particles;

[0026] (6) The small lithium iron phosphate particles are primary particles.

[0027] In an optional embodiment, the large lithium iron phosphate particles include a doped lithium iron phosphate core and a carbon coating layer, wherein the carbon coating layer is coated on the surface of the doped lithium iron phosphate core, and the doped lithium iron phosphate core includes at least two doping elements;

[0028] Preferably, in the large lithium iron phosphate particles, the thickness of the carbon coating layer is 2 nm to 50 nm;

[0029] And / or, the lithium iron phosphate small particles include a doped lithium iron phosphate core and a carbon coating layer, the carbon coating layer is coated on the surface of the doped lithium iron phosphate core, and the doped lithium iron phosphate core includes at least one doping element;

[0030] Preferably, in the lithium iron phosphate small particles, the thickness of the carbon coating layer is 2 nm to 50 nm;

[0031] Preferably, in the lithium iron phosphate positive electrode material, the mass content of carbon element is 1.0% to 2.0%.

[0032] In an optional embodiment, the lithium iron phosphate positive electrode material satisfies at least one of the following characteristics (1) to (3):

[0033] (1) The average particle size D of the lithium iron phosphate positive electrode material v50 0.8μm~3.0μm;

[0034] (2) The powder compaction density of the lithium iron phosphate positive electrode material is 2.5 to 2.8 g / cm 3;

[0035] Preferably, the powder compaction density of the lithium iron phosphate positive electrode material is 2.6 to 2.8 g / cm 3 ;

[0036] (3) The powder resistivity of the lithium iron phosphate positive electrode material is 2 to 50 Ω*cm.

[0037] In a second aspect, the present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0038] Synthesizing large lithium iron phosphate particles: adding a lithium source, iron phosphate, a carbon source, and a first doping source into a solvent and grinding them, drying them to obtain a precursor A, and sintering the precursor A to obtain the large lithium iron phosphate particles;

[0039] Synthesizing small lithium iron phosphate particles: adding a lithium source, iron phosphate, a carbon source, and a second doping source into a solvent, grinding, and drying to obtain a precursor B, sintering the precursor B, and crushing to obtain the small lithium iron phosphate particles;

[0040] Evenly mixing the large lithium iron phosphate particles and the small lithium iron phosphate particles to obtain the lithium iron phosphate positive electrode material;

[0041] Wherein, the first doping source contains metal elements and non-metal elements; the second doping source contains metal elements;

[0042] The metal element includes one or more of alkali metals, alkaline earth metals or transition metals; the non-metal element includes one or more of halogens, S or N.

[0043] In an optional embodiment, the preparation method satisfies at least one of the following features (1) to (11):

[0044] (1) In the synthesis of the large lithium iron phosphate particles, the mass ratio of the lithium source, iron phosphate, carbon source and first doping source is (0.2-0.35):1:(0.06-0.2):(0.001-0.1);

[0045] (2) In the synthesis of the lithium iron phosphate small particles, the mass ratio of the lithium source, iron phosphate, carbon source and second doping source is (0.2-0.35):1:(0.06-0.2):(0.001-0.1);

[0046] (3) The mass ratio of the large lithium iron phosphate particles to the small lithium iron phosphate particles is (50-90): (10-50); preferably, the mass ratio of the large lithium iron phosphate particles to the small lithium iron phosphate particles is (50-85): (15-50);

[0047] (4) In the process of synthesizing the large lithium iron phosphate particles, sand milling is used for grinding, and the particle size D of the sand milling is v50 0.6μm~1.3μm;

[0048] (5) In the process of synthesizing the lithium iron phosphate small particles, sand milling is used for grinding, and the particle size D of the sand milling is v50 0.2μm~0.4μm;

[0049] (6) The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium oxalate or lithium acetate;

[0050] (7) The carbon source is selected from at least one of acetylene black, glucose, sucrose, polyvinyl alcohol, polypropylene alcohol, polyethylene glycol black, carbon fiber tubes, or graphite;

[0051] (8) The first doping source includes at least one of NaO, NaNO3, MgO, Mg(NO3)2, TiO2, Li2TiO3, TiOSO4, Ti(OCH3)4, V2O5, NH4VO3, VOSO4, ZrO2, Zr(NO3)4, MnO2, Nb2O5, MoO3, or MoCl5; the first doping source also includes at least one of NaF, NH4F, HF, (NH4)2SO4, NH4Cl, or CO(NH2)2;

[0052] (9) The second doping source includes at least one of NaO, NaNO3, MgO, Mg(NO3)2, TiO2, Li2TiO3, TiOSO4, Ti(OCH3)4, V2O5, NH4VO3, VOSO4, ZrO2, Zr(NO3)4, MnO2, Nb2O5, MoO3 or MoCl5;

[0053] (10) During the synthesis of large lithium iron phosphate particles, the sintering conditions are as follows: heating to 300-550°C at a heating rate of 1-10°C / min, keeping warm for 1-5 hours, then heating to 720-820°C at a heating rate of 1-10°C / min, and keeping warm for 6-15 hours;

[0054] (11) The sintering conditions during the synthesis of the lithium iron phosphate small particles are as follows: heating to 200-350°C at a heating rate of 1-10°C / min, keeping warm for 1-5 hours, then heating to 680-780°C at a heating rate of 1-10°C / min, and keeping warm for 5-10 hours.

[0055] In a third aspect, the present invention provides a positive electrode sheet comprising: the lithium iron phosphate positive electrode material described in any embodiment of the first aspect of the present invention, and / or the lithium iron phosphate positive electrode material obtained by the preparation method described in any embodiment of the second aspect of the present invention.

[0056] In a fourth aspect, the present invention provides a battery, comprising: the positive electrode sheet described in any one embodiment of the third aspect of the present invention.

[0057] The lithium iron phosphate positive electrode material provided by the present invention has at least the following beneficial effects:

[0058] 1. The lithium iron phosphate (LFP) cathode material provided in the present invention is doped with doping elements in large and small lithium iron phosphate particles, respectively. The metal ions doped in the large and small lithium iron phosphate particles can cause lithium ions to diffuse along a one-dimensional path, thereby increasing the diffusion rate of lithium ions and the diffusion coefficient of lithium ions, thereby enhancing the discharge capacity and rate performance of the lithium iron phosphate cathode material. Furthermore, doping the large lithium iron phosphate particles with non-metallic elements not only improves the lithium ion diffusion coefficient but also ensures that the lithium ion transmission channel is not blocked, further ensuring the rapid deintercalation of lithium ions and preventing the large lithium iron phosphate particles from cracking and breaking during long-term cycling, which leads to accelerated cycle attenuation.

[0059] 2. In a preferred embodiment of the present invention, the metal element at the iron site and the non-metal element at the oxygen site are co-doped to improve the specific capacity of the high-density LFP large particles and the stability of long-term cycling. + diffusion along one-dimensional pathways, thereby improving the conductivity and Li + The diffusion coefficient of lithium iron ore is increased, thereby enhancing the discharge capacity and rate capability of lithium iron ore. Non-metallic element doping at the oxygen position can weaken the Li-O bond and shorten the distance between the P-O bonds without blocking the Li + The transmission channel improves the ion diffusion capacity of the iron-lithium material, giving the material better rate performance and structural stability.

[0060] 3. In a preferred embodiment of the present invention, the formation of large lithium iron phosphate particles in the provided lithium iron phosphate cathode material is not entirely achieved by fusion with small lithium iron phosphate particles during high-temperature sintering. Instead, it is primarily achieved through high-temperature sintering of a larger sand-milled particle size precursor, thereby avoiding accelerated attenuation caused by cracking of the fused grain boundaries during long-term cycling, leading to particle breakage. The primary particle size of the small lithium iron phosphate particles is relatively small, and through metal element doping alone, it is possible to achieve particle size grading while achieving good specific capacity, low temperature and rate performance. Moreover, during long-term cycling, the internal stress is low and particle cracking and breakage will not occur, thereby enabling the composite lithium iron phosphate to achieve high compaction, high specific capacity and long cycling. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0062] Figure 1 : is a SEM image of the lithium iron phosphate positive electrode material provided in Example 1 of the present invention;

[0063] Figure 2 : is a SEM image of the lithium iron phosphate positive electrode material provided in Comparative Example 1 of the present invention;

[0064] Figure 3 45° C. cycle capacity attenuation comparison diagram of the lithium iron phosphate positive electrode materials provided in Example 1 of the present invention and Comparative Example 1. DETAILED DESCRIPTION

[0065] The present invention will be further described below with reference to specific examples. It should be understood that these examples are only intended to illustrate the present invention and are not intended to limit the scope of the present invention.

[0066] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range or the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0067] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form a new technical solution. Unless otherwise specified, all technical features and optional technical features of the present invention can be combined with each other to form a new technical solution.

[0068] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0069] The compaction density of lithium iron phosphate positive electrode materials has a certain impact on the comprehensive performance of the positive electrode materials, such as the energy density. In order to improve the compaction density of lithium iron phosphate positive electrode materials, a particle size grading and mixing method is usually adopted. However, when using the graded mixing method, the large size of lithium iron phosphate particles is larger, which will make the path of lithium ion deintercalation and deintercalation longer, affecting the diffusion coefficient of the positive electrode material, and thus resulting in lower capacity. Moreover, in lithium iron phosphate materials with high compaction density, large lithium iron phosphate particles are formed by the fusion of small particles. During long-term cycling, these large particles are prone to cracking and react with the electrolyte, exacerbating the dissolution of iron and accelerating the cycle decay of the battery.

[0070] In view of this, the present invention provides a lithium iron phosphate positive electrode material and a preparation method thereof, a positive electrode sheet and a battery, and this application is described in detail below.

[0071] [Lithium iron phosphate cathode material]

[0072] In some embodiments, a lithium iron phosphate positive electrode material is provided, which includes: large lithium iron phosphate particles and small lithium iron phosphate particles; the particle size of the large lithium iron phosphate particles is micron-level, and the particle size of the small lithium iron phosphate particles is nanometer-level.

[0073] In this embodiment, the large lithium iron phosphate particles include at least two doping elements, including metal elements and non-metal elements; and / or the small lithium iron phosphate particles include at least one doping element, including metal elements.

[0074] The large lithium iron phosphate particles are doped with no less than two elements, and contain at least two types of element doping, metal elements and non-metallic elements. The small lithium iron phosphate particles are doped with at least one metal element.

[0075] In an embodiment of the present invention, the lithium iron phosphate positive electrode material improves the cracking and breakage of the large lithium iron phosphate particles during the cycle, thereby affecting the cycle performance of the battery. Among them, the large lithium iron phosphate particles use lithium iron phosphate particles with a particle size of micron-level, for example, the particle size of the large lithium iron phosphate particles can be 0.01μm, 0.02μm, 0.05μm, 0.1μm or greater than 0.1μm, etc.; the small lithium iron phosphate particles use lithium iron phosphate particles with a particle size of micron-level, for example, the particle size of the small lithium iron phosphate particles can be 800nm, 500nm, 300nm, 100nm or less than 100nm, etc. Of course, it can be understood by those skilled in the art that the particle sizes of the large lithium iron phosphate particles and the small lithium iron phosphate particles are relative, that is, in this embodiment, the particle size of the large lithium iron phosphate particles is larger than the particle size of the small lithium iron phosphate particles.

[0076] The large lithium iron phosphate particles also include doping elements, wherein the types of doping elements can be two, three, four, or more than four, and the doping elements can be metal elements or non-metal elements. Exemplarily, the large lithium iron phosphate particles contain two doping elements, namely doping element A and doping element B. Doping element A and doping element B can both be metal elements or non-metal elements, or doping element A can be a metal element and doping element B can be a non-metal element. Preferably, the doping elements in the large lithium iron phosphate particles include both metal elements and non-metal elements.

[0077] The lithium iron phosphate particles also include doping elements, where the doping elements may be one, two, three, or more than one, and are metallic elements. Preferably, the doping elements in the lithium iron phosphate particles comprise only metallic elements. Alternatively, in other embodiments, the doping elements in the lithium iron phosphate particles primarily comprise metallic elements and also contain a small amount of non-metallic elements.

[0078] In some embodiments, large lithium iron phosphate particles are doped with metal elements and non-metal elements, and small lithium iron phosphate particles are mainly doped with metal elements. This can improve the expansion coefficient of the lithium iron phosphate positive electrode material to a certain extent, reduce the lithium ion deintercalation path, and make the lithium iron phosphate positive electrode material have good gram capacity and rate performance.

[0079] In a preferred embodiment of the present invention, the metal element includes an iron-based metal element, and the non-metal element includes an oxygen-based non-metal element.

[0080] Thus, the present invention can effectively improve the electronic and ionic conductivity of large lithium iron phosphate particles while improving the structural stability of the material by co-doping with metal elements at the iron position and non-metallic elements at the oxygen position; in addition, the synthesis of large lithium iron phosphate particles is mainly achieved by sintering of larger sand-milled particle size precursors, rather than fusion of small particles, which can avoid the breakage of material particles during the cycle; thereby achieving better rate-specific capacity and cycle stability of high-density LFP large particles. Furthermore, based on the above, the present invention can preferably alleviate the problems of low specific capacity of high-density lithium iron phosphate large particles and accelerated cycle attenuation caused by cracking during the cycle.

[0081] In some embodiments, the metal element includes one or more of an alkali metal, an alkaline earth metal, or a transition metal; and the non-metal element includes one or more of a halogen, sulfur (S), or nitrogen (N).

[0082] In this embodiment, the metal element may be an alkali metal element, such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), or francium (Fr); or an alkaline earth metal element, such as beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), or radium (Ra); or a transition metal element, such as titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), or zinc (Zn). The non-metallic element may be a halogen, such as fluorine (F), chlorine (Cl), bromine (Br), or iodine (I); or one or more of S or N.

[0083] For example, the large lithium iron phosphate particles contain two doping elements, where doping element A is K and doping element B is Mg, or doping element B is Cl. The doping elements in the small lithium iron phosphate particles can be Mg or K, or Mg and K.

[0084] By doping large and small lithium iron phosphate particles with doping elements, the doped metal ions can cause lithium ions to diffuse along a one-dimensional path, increasing the lithium ion diffusion rate and the lithium ion diffusion coefficient, thereby enhancing the discharge capacity and rate performance of the lithium iron phosphate cathode material. The doped non-metallic elements, while improving the lithium ion diffusion coefficient, also ensure that they do not block the lithium ion transmission channel, further ensuring the rapid deintercalation of lithium ions and preventing the large lithium iron phosphate particles from cracking and breaking during long-term cycling, which can lead to accelerated cycle attenuation.

[0085] In some embodiments, the metal element is selected from one or more of Na, Mg, Ti, V, Zr, Mn, Nb, or Mo.

[0086] In some embodiments, the non-metallic element is selected from one or more of F, S, Cl, or N.

[0087] By further limiting the types of metal elements and non-metal elements, the above-mentioned metal ions are doped into the iron site of lithium iron phosphate. The doping of metal ions promotes the + Diffusion along a one-dimensional path reduces the diffusion path, thereby improving the conductivity and Li + The diffusion coefficient is increased to enhance the discharge capacity and rate capability of lithium iron phosphate. The above non-metallic elements are doped on the oxygen sites of lithium iron phosphate. The doping of non-metallic elements on the oxygen sites can weaken the Li-O bond and shorten the distance between the P-O bonds without blocking the Li + transmission channel, thereby further improving the ion diffusion capacity in the lithium iron phosphate material, making the material have better rate performance and structural stability.

[0088] In some embodiments, the mass content of the doping element in the large lithium iron phosphate particles is 0.02% to 1%.

[0089] Specifically, the mass content of the doping element in the large lithium iron phosphate particles can be any one of 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%, or a range of values ​​between any two of them. By limiting the above range, it is possible to ensure that the lithium iron phosphate has a higher diffusion coefficient, good gram capacity and cycle rate performance. If it is less than this range, the content of the doping element is too low and cannot effectively improve the diffusion coefficient of lithium ions; if it is greater than this range, it will affect the content of the active material part in the lithium iron phosphate positive electrode material, thereby reducing the gram capacity.

[0090] In some embodiments, the mass content of the doping element in the lithium iron phosphate small particles is 0.02% to 1%.

[0091] Specifically, the mass content of the doping element in the lithium iron phosphate particles can be any one of 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%, or any range therebetween. If the content is outside this range, the lithium ion diffusion and cycle performance will be affected.

[0092] In a preferred embodiment, the mass content of the metal element is 0.02% to 1%.

[0093] In the small lithium iron phosphate particles, the mass of the doped metal elements can be any one of 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%, or any range between two of them.

[0094] In some embodiments, when the lithium iron phosphate particles include at least two doping elements, one of the doping elements is a metal element.

[0095] In lithium iron phosphate cathode materials, large lithium iron phosphate particles and small lithium iron phosphate particles are fused together. Due to their large size, large lithium iron phosphate particles lengthen the lithium ion deintercalation path. By grading with small lithium iron phosphate particles, the deintercalation path can be shortened to a certain extent. To ensure that lithium iron phosphate has a high diffusion coefficient and that small lithium iron phosphate particles do not block the transmission channel, at least one metal element is doped into the small lithium iron phosphate particles. This allows lithium ions to diffuse along a one-dimensional path without blocking the ion transmission channel. This effectively prevents the large lithium iron phosphate particles from cracking and breaking during long-term cycling, which can affect the cycle rate performance.

[0096] In some embodiments, the general chemical formula of the large lithium iron phosphate particles is LiFe 1-x M x PO y N z ; Among them, the value range of x is 0~0.05, the value range of y is 3.95~4.00, the value range of z is 0~0.05, and the values ​​of x and z do not include 0; Among them, M is a metal element and N is a non-metal element.

[0097] The chemical formula of large lithium iron phosphate particles can be LiFe 0.95 Mg 0.05 PO 3.95 N 0.05 、LiFe 0.96 Na 0.04 PO 3.97 F 0.04 、LiFe 0.97 Ti 0.03 PO 3.98 S 0.03 or LiFe 0.99 Mn 0.01 PO 3.99 Cl 0.01 ; Of course, the chemical formula of large lithium iron phosphate particles includes but is not limited to the above-mentioned ones. As long as the value range of x is 0-0.05, the value range of y is 3.95-4.00, and the value range of z is 0-0.05 within the corresponding range, they are all within the protection scope of the present invention.

[0098] In some embodiments, the general chemical formula of the lithium iron phosphate particles is LiFe 1-x M x PO y N z ; Among them, the value range of x is 0~0.03, the value range of y is 3.97~4.00, the value range of z is 0~0.03, and the value of x does not include 0; Among them, M is a metal element and N is a non-metal element.

[0099] The general chemical formula of lithium iron phosphate particles is LiFe 0.97 Na 0.03 PO4、LiFe 0.97 Mg 0.03 PO4S 0.01 、LiFe 0.99 Ti 0.01 PO 3.97 、LiFe 0.98 Mn 0.02 PO 3.98 or LiFe 0.98 Mo 0.02 PO4N 0.03 ; Of course, the chemical formula of lithium iron phosphate small particles includes but is not limited to the above-mentioned ones. As long as the value range of x is 0 to 0.03, the value range of y is 3.97 to 4.00, and the value range of z is 0 to 0.03 within the corresponding range, they are all within the protection scope of the present invention.

[0100] In some embodiments, the average particle size of the large lithium iron phosphate particles in the lithium iron phosphate positive electrode material is 0.5 μm to 2 μm. Preferably, the average particle size of the large lithium iron phosphate particles in the lithium iron phosphate positive electrode material is 0.5 μm to 1.6 μm.

[0101] Specifically, the average particle size of the large lithium iron phosphate particles can be any one of 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, or 2 μm, or any value between any two of them. By limiting the average particle size of the large lithium iron phosphate particles, it is possible to avoid the problem of cracking and breaking of the large lithium iron phosphate particles during long-term cycling, which can cause side reactions with the electrolyte in the battery, exacerbate iron dissolution, and thus accelerate the cycle degradation of the battery.

[0102] In some embodiments, the lithium iron phosphate positive electrode material satisfies the requirement that the average particle size of the lithium iron phosphate small particles is 150 nm to 300 nm.

[0103] Specifically, the average particle size of the small lithium iron phosphate particles can be any one of 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, 210nm, 220nm, 230nm, 240nm, 250nm, 260nm, 270nm, 280nm, 290nm or 300nm, or any value between any two of them. By limiting the average particle size of the small lithium iron phosphate particles, the compaction density of the lithium iron phosphate positive electrode material can be increased, thereby better integrating the large lithium iron phosphate particles and the small lithium iron phosphate particles, thereby increasing the gram capacity.

[0104] In some embodiments, the lithium iron phosphate positive electrode material satisfies the requirement that the mass proportion of large lithium iron phosphate particles in the lithium iron phosphate positive electrode material is 50% to 85%.

[0105] Specifically, in the lithium iron phosphate cathode material, the mass percentage of large lithium iron phosphate particles can be any one of 50%, 55%, 60%, 65%, 70%, 75%, 80%, or 85%, or any value between any two. By limiting the range of the mass percentage of large lithium iron phosphate particles, the rate performance and low-temperature performance of the high-density lithium iron phosphate cathode material can be ensured.

[0106] In some embodiments, the lithium iron phosphate positive electrode material satisfies the requirement that the mass proportion of small lithium iron phosphate particles in the lithium iron phosphate positive electrode material is 15% to 50%.

[0107] Specifically, in the lithium iron phosphate cathode material, the mass percentage of the small lithium iron phosphate particles can be any one of 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%, or any value between any two. By limiting the mass percentage range of the small lithium iron phosphate particles, a higher compaction density can be achieved to ensure volumetric energy density, while ensuring that the material's rate performance and low-temperature performance meet requirements.

[0108] In some embodiments, the large lithium iron phosphate particles in the lithium iron phosphate positive electrode material are primary particles; and the small lithium iron phosphate particles in the lithium iron phosphate positive electrode material are primary particles.

[0109] The large lithium iron phosphate particles of the present application are primary particles, and the large lithium iron phosphate particles have good structural integrity, a good crystal structure, etc., and have better activity, so that lithium ions can diffuse faster and improve the cycle performance of lithium iron phosphate.

[0110] In some embodiments, the large lithium iron phosphate particles include a doped lithium iron phosphate core and a carbon coating layer, wherein the carbon coating layer is coated on the surface of the doped lithium iron phosphate core, and the doped lithium iron phosphate core includes at least two doping elements. The selection of the doping elements for the doped lithium iron phosphate core in the large lithium iron phosphate particles can be referred to above.

[0111] In some embodiments, the lithium iron phosphate particles include a doped lithium iron phosphate core and a carbon coating layer. The carbon coating layer is coated on the surface of the doped lithium iron phosphate core. The doped lithium iron phosphate core includes at least one doping element. The doping element of the doped lithium iron phosphate core in the lithium iron phosphate particles can be selected as described above.

[0112] By providing a protective carbon coating on the doped lithium iron phosphate core, electrical conductivity can be improved, making the lithium iron phosphate more stable during long-term cycling, ensuring stable cycling performance, and effectively mitigating volume changes in the lithium iron phosphate material. In this embodiment, the core surfaces of large and small lithium iron phosphate particles are each coated with a carbon coating. This improves the diffusion coefficient of lithium ions while also enhancing the stability of the lithium iron phosphate material, reducing cracking of the large lithium iron phosphate particles during long-term cycling and improving cycle rate performance.

[0113] In a preferred embodiment, in the large lithium iron phosphate particles, the thickness of the carbon coating layer is 2 nm to 50 nm.

[0114] Specifically, in the large lithium iron phosphate particles, the thickness of the carbon coating layer can be 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, 25nm, 26nm, 27nm, 28nm, 29nm, 30nm, 31nm, 32nm, 33nm, 34nm, 35nm, 36nm, 37nm, 38nm, 39nm, 40nm, 41nm, 42nm, 43nm, 44nm, 45nm, 46nm, 47nm, 48nm, 49nm or 50nm, or any value between any two of them.

[0115] In a preferred embodiment, in the lithium iron phosphate small particles, the thickness of the carbon coating layer is 2 nm to 50 nm.

[0116] Specifically, in the small lithium iron phosphate particles, the thickness of the carbon coating layer can be 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, nm, 16nm, 17nm, 18nm, 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, 25nm, 26nm, 27nm, 28nm, 29nm, 30nm, 31nm, 32nm, 33nm, 34nm, 35nm, 36nm, 37nm, 38nm, 39nm, 40nm, 41nm, 42nm, 43nm, 44nm, 45nm, 46nm, 47nm, 48nm, 49nm or 50nm, or any value between any two of them.

[0117] In a preferred embodiment, the mass content of carbon in the lithium iron phosphate positive electrode material is 1.0% to 2.0%. Preferably, the mass content of carbon in the lithium iron phosphate positive electrode material is 1.0% to 1.6%.

[0118] Specifically, the mass content of carbon element can be any one of 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9% or 2.0%, or any value between any two of them.

[0119] By controlling the carbon content in the lithium iron phosphate positive electrode material within the above-mentioned appropriate range, the electronic conductivity of the lithium iron phosphate positive electrode material can be improved, and the basic performance of the lithium iron phosphate positive electrode material, such as specific capacity and long-term cycle stability, can be ensured.

[0120] In some embodiments, the lithium iron phosphate positive electrode material satisfies the average particle size D of the lithium iron phosphate positive electrode material. v50 0.8μm~3.0μm.

[0121] Specifically, the average particle size D of the lithium iron phosphate positive electrode material v50 It can be any one of 0.8μm, 0.9μm, 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, 2.0μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm, 2.6μm, 2.7μm, 2.8μm, 2.9μm or 3.0μm or any value between any two of them.

[0122] In some embodiments, the lithium iron phosphate cathode material satisfies the powder compaction density of the lithium iron phosphate cathode material of 2.5 to 2.8 g / cm 3 .

[0123] Specifically, the powder compaction density of the lithium iron phosphate positive electrode material is 2.5 g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 or 2.8g / cm 3 Any one or any value between any two.

[0124] In a preferred embodiment, the powder compaction density of the lithium iron phosphate cathode material is 2.6 to 2.8 g / cm 3 .

[0125] By limiting the powder compaction density range of lithium iron phosphate positive electrode materials, we can better balance the energy density and requirements such as low-temperature fast charging, thereby optimizing the application effect.

[0126] In some embodiments, the powder resistivity of the lithium iron phosphate positive electrode material is 2 to 50 Ω*cm.

[0127] Specifically, the powder resistivity of the lithium iron phosphate positive electrode material can be 2Ω*cm, 3Ω*cm, 4Ω*cm, 5Ω*cm, 6Ω*cm, 7Ω*cm, 8Ω*cm, 9Ω*cm, 10Ω*cm, 11Ω*cm, 12Ω*cm, 13Ω*cm, 14Ω*cm, 15Ω*cm, 16Ω*cm, 17Ω*cm, 18Ω*cm, 19Ω*cm, 20Ω*cm, 21Ω*cm, 22Ω*cm, 23Ω*cm, 24Ω*cm, 25Ω*cm, 26Ω*cm , 27Ω*cm, 28Ω*cm, 29Ω*cm, 30Ω*cm, 31Ω*cm, 32Ω*cm, 33Ω*cm, 34Ω*cm, 35Ω*cm, 36Ω*cm, 37Ω*cm, 38Ω*cm, 39Ω*cm, 40Ω*cm, 41Ω*cm, 42Ω*cm, 43Ω*cm, 44Ω*cm, 45Ω*cm, 46Ω*cm, 47Ω*cm, 48Ω*cm, 49Ω*cm, or 50Ω*cm, or any value between any two of them. By limiting the powder resistivity of the lithium iron phosphate cathode material, the effect of the carbon coating and particle contact of the lithium iron phosphate material is ensured, and the electronic conductivity, rate performance, and interface reaction kinetics are improved, thereby achieving better power characteristics, energy density, and cycle stability.

[0128] Therefore, based on the above settings, compared with the existing lithium iron phosphate materials, the lithium iron phosphate cathode material provided by the present invention improves the high-density LFP large particle specific capacity and long-term cycle stability by co-doping with metal elements at the iron site and non-metallic elements at the oxygen site. Among them, the metal ion doping at the iron site promotes the Li + diffusion along one-dimensional pathways, thereby improving the conductivity and Li +Diffusion coefficient, which enhances the discharge capacity and rate capability of lithium iron. Doping with non-metallic elements at the oxygen position can weaken the Li-O bond, while shortening the distance between the P-O bonds, and will not block the Li+ transmission channel, thereby improving the ion diffusion capacity of the lithium iron material, and making the material have better rate performance and structural stability. At the same time, in this application, the formation of large lithium iron phosphate particles is not entirely formed by the melting of small particles during high-temperature sintering, but is mainly achieved by high-temperature sintering of a larger sand-milled particle size precursor, which can avoid the accelerated attenuation caused by the cracking of the fused grain boundaries during long-term cycles, leading to particle breakage. The primary particle size of small lithium iron phosphate particles is relatively small, and only by doping with metal elements can good specific capacity, low temperature and rate performance be achieved while achieving particle size grading, and the internal stress is small during long-term cycles, and particle cracking and breakage will not occur, thereby making the composite lithium iron phosphate take into account the characteristics of high compaction, high specific capacity and long cycle.

[0129] [Preparation method of lithium iron phosphate positive electrode material]

[0130] The present invention also provides a method for preparing a lithium iron phosphate cathode material, comprising the following steps:

[0131] S101. Synthesizing large lithium iron phosphate particles: adding a lithium source, iron phosphate, a carbon source, and a first doping source into a solvent and grinding them, and obtaining a precursor A after drying, and sintering the precursor A to obtain large lithium iron phosphate particles.

[0132] In the process of synthesizing large lithium iron phosphate particles, the specific material composition of the lithium source, iron phosphate, and carbon source is not specifically limited. Those skilled in the art can select them based on the actual situation based on the embodiments of the present invention. The first doping source can be specifically selected based on the above embodiments. After grinding, the ground product can also be dried, preferably by spray drying, to ensure a more uniform particle size of the product particles.

[0133] S102. Synthesizing small lithium iron phosphate particles: adding a lithium source, iron phosphate, a carbon source, and a second doping source into a solvent, grinding, and drying to obtain a precursor B, sintering the precursor B, and crushing it to obtain small lithium iron phosphate particles.

[0134] In steps S101 and S102, sintering can be performed under an inert gas atmosphere, which can better avoid side reactions of the precursor during the sintering process, thereby affecting the comprehensive performance of subsequent products.

[0135] S103, mixing the large lithium iron phosphate particles and the small lithium iron phosphate particles evenly to obtain a lithium iron phosphate positive electrode material.

[0136] The method for preparing the lithium iron phosphate positive electrode material provided by the present invention can first prepare large lithium iron phosphate particles and small lithium iron phosphate particles separately, and then mix the prepared large lithium iron phosphate particles and small lithium iron phosphate particles to obtain the lithium iron phosphate positive electrode material.

[0137] The preparation method provided by the embodiment of the present invention has simple process, convenient operation, strong feasibility, and is easy to industrialize; the lithium iron phosphate positive electrode material that can be prepared by this method can have the characteristics of high compaction, high gram capacity and long cycle performance.

[0138] It should be understood that all the features and advantages described above for the “lithium iron phosphate positive electrode material” are also applicable to the “method for preparing the lithium iron phosphate positive electrode material” and will not be described in detail here.

[0139] In some embodiments, the first doping source contains metal elements and non-metal elements; the second doping source contains metal elements; the metal elements include one or more of alkali metals, alkaline earth metals or transition metals; the non-metal elements include one or more of halogens, S or N.

[0140] The metal element may be an alkali metal element such as Li, Na, K, Rb, Cs, or Fr; an alkaline earth metal element such as Be, Mg, Ca, Sr, Ba, or Ra; or a transition metal element such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn. The non-metal element may be a halogen such as F, Cl, Br, or I; or one or more of S and N.

[0141] In some embodiments, in the synthesis of large lithium iron phosphate particles, the mass ratio of the lithium source, iron phosphate, carbon source and first doping source is (0.2-0.35):1:(0.06-0.2):(0.001-0.1).

[0142] Specifically, the mass ratio of the lithium source, iron phosphate, carbon source, and first doping source can be any one of 0.2:1:0.06:0.001, 0.25:1:0.1:0.05, 0.3:1:0.12:0.065, or 0.35:1:0.2:0.1, or any value within any range. Preferably, the first doping source includes a metal element doping source and a non-metal element doping source, and the mass ratio of the metal element doping source to the non-metal element doping source can be (0.001-0.1): (0.001-0.1), specifically any one of 0.1:0.1, 0.01:0.05, or 0.07:0.09, or any value within any range. That is, in the synthesis of large lithium iron phosphate particles, the mass ratio of lithium source, iron phosphate, carbon source, metal element doping source and non-metal element doping source is (0.2~0.35):1:(0.06~0.2):(0.001~0.1):(0.001~0.1).

[0143] By limiting the proportion of raw materials in the synthesis of large lithium iron phosphate particles, the proportion of various elements in the final product, lithium iron phosphate positive electrode material, can be better controlled, thereby making the lithium iron phosphate positive electrode material have excellent comprehensive performance.

[0144] In some embodiments, in the synthesis of small lithium iron phosphate particles, the mass ratio of the lithium source, iron phosphate, carbon source, and the second doping source is (0.2-0.35):1:(0.06-0.2):(0.001-0.1).

[0145] Specifically, the mass ratio of the lithium source, iron phosphate, carbon source, and the second doping source can be any one of 0.2:1:0.06:0.001, 0.24:1:0.1:0.05, 0.31:1:0.12:0.065, or 0.35:1:0.2:0.1, or any value within any range. Preferably, the second doping source includes a metal element doping source and a non-metal element doping source, and the mass ratio of the metal element doping source and the non-metal element doping source can be (0.001-0.1):(0-0.05), specifically, any one of 0.001:0, 0.02:0.01, or 0.1:0.05, or any value within any range. In the synthesis of lithium iron phosphate small particles, the mass ratio of lithium source, iron phosphate, carbon source, metal element doping source and non-metal element doping source is (0.2-0.35):1:(0.06-0.2):(0.001-0.1):(0-0.05).

[0146] In some embodiments, the mass ratio of large lithium iron phosphate particles to small lithium iron phosphate particles is (50-90):(10-50). Preferably, the mass ratio of large lithium iron phosphate particles to small lithium iron phosphate particles is (50-85):(15-50).

[0147] Specifically, the mixing mass ratio of the large lithium iron phosphate particles to the small lithium iron phosphate particles can be 50:50, 55:45, 60:40, 70:30, 80:20 or 90:10, etc.

[0148] In some embodiments, during the synthesis of large lithium iron phosphate particles, sand milling is used for grinding, and the particle size D of the sand milling is v50 0.6~1.3μm; in the process of synthesizing lithium iron phosphate small particles, sand milling is used for grinding, and the particle size D v50 0.2~0.4μm.

[0149] Specifically, in the process of synthesizing large lithium iron phosphate particles, the particle size D of the sand milling is v50It can be any one of 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm or 1.3 μm or any value between any two. In the process of synthesizing lithium iron phosphate small particles, the particle size D of the sand milling v50 It can be any one of 0.2 μm, 0.22 μm, 0.24 μm, 0.26 μm, 0.28 μm, 0.30 μm, 0.32 μm, 0.34 μm, 0.36 μm, 0.38 μm or 0.4 μm, or any value between any two of them.

[0150] By limiting the sanding particle size of the synthesized large lithium iron phosphate particles and small lithium iron phosphate particles during the sand grinding process, the particle size of the final product large lithium iron phosphate particles and small lithium iron phosphate particles can be effectively controlled; if it is larger than the above range, the particle size of the final product lithium iron phosphate particles will become larger, and the particles will also grow during the sintering process, thereby affecting the gram capacity, low temperature performance and cycle performance of the positive electrode material.

[0151] In some embodiments, the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium oxalate, or lithium acetate.

[0152] In some embodiments, the carbon source is selected from one or more of acetylene black, glucose, sucrose, polyvinyl alcohol, polypropylene alcohol, polyethylene glycol black, carbon fiber tubes, or graphite.

[0153] In some embodiments, the first doping source includes at least one of NaO, NaNO3, MgO, Mg(NO3)2, TiO2, Li2TiO3, TiOSO4, Ti(OCH3)4, V2O5, NH4VO3, VOSO4, ZrO2, Zr(NO3)4, MnO2, Nb2O5, MoO3, and MoCl5; the first doping source also includes one or more of NaF, NH4F, HF, (NH4)2SO4, NH4Cl, and CO(NH2)2.

[0154] In some embodiments, the second doping source includes one or more of NaO, NaNO3, MgO, Mg(NO3)2, TiO2, Li2TiO3, TiOSO4, Ti(OCH3)4, V2O5, NH4VO3, VOSO4, ZrO2, Zr(NO3)4, MnO2, Nb2O5, MoO3, and MoCl5.

[0155] In some embodiments, during the synthesis of large lithium iron phosphate particles, the sintering conditions are: heating to 300-550°C at a heating rate of 1-10°C / min, keeping warm for 1-5 hours, then heating to 720-820°C at a heating rate of 1-10°C / min, and keeping warm for 6-15 hours.

[0156] The sintering conditions during the synthesis of lithium iron phosphate small particles are as follows: heating to 200-350°C at a heating rate of 1-10°C / min, keeping warm for 1-5 hours, then heating to 680-780°C at a heating rate of 1-10°C / min, and keeping warm for 5-10 hours.

[0157] During the synthesis of large and small lithium iron phosphate particles, the sintering conditions have a great influence on the final product. For example, if the sintering temperature is too high, the crystal structure of the lithium iron phosphate will be destroyed, and side reactions will increase, making the uniformity of the particles worse. By separately limiting the sintering conditions of large and small lithium iron phosphate particles, the particle size of large and small lithium iron phosphate particles can be well controlled. The grading of large and small particles can be achieved, further promoting the fusion of large and small lithium iron phosphate particles, and avoiding the accelerated attenuation caused by particle breakage due to the cracking of the fusion grain boundaries during long-term circulation.

[0158] [Positive electrode]

[0159] An embodiment of the present invention further provides a positive electrode sheet, which includes: the lithium iron phosphate positive electrode material in any of the above embodiments of the present invention, and / or the lithium iron phosphate positive electrode material obtained by the preparation method in any of the above embodiments of the present invention.

[0160] The positive electrode sheet includes the above-mentioned lithium iron phosphate positive electrode material provided in the embodiment of the present application. Therefore, the positive electrode sheet also has the characteristic of excellent electrochemical performance.

[0161] In some embodiments, the positive electrode sheet includes a current collector and a positive electrode active material layer disposed on the surface of the current collector; the positive electrode active material layer includes the aforementioned lithium iron phosphate positive electrode material. Furthermore, the positive electrode active material layer may also include a conductive agent and a binder.

[0162] This application does not particularly limit the material of the current collector in the positive electrode sheet, as long as it can achieve the purpose of this application, and can be selected according to actual needs. For example, in some embodiments, the positive electrode current collector can include aluminum foil, aluminum alloy foil, etc. Of course, in other embodiments, composite current collectors (such as aluminum-carbon composite current collectors) can also be used.

[0163] The present application embodiment has no particular restrictions on the types of conductive agents and binders in the positive electrode active material layer, as long as the purpose of the application can be achieved. For example, in some embodiments, the binder may include but is not limited to one or more of polyacrylate, polyimide, polyvinyl alcohol, polyamide, polyamide-imide, polyvinylidene fluoride, polystyrene butadiene copolymer (styrene-butadiene rubber), polytetrafluoroethylene, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose or sodium hydroxymethyl cellulose. The conductive agent may include but is not limited to at least one of conductive carbon black (such as acetylene black, Ketjen black), carbon nanotubes (CNTs), carbon fiber, graphene, etc. The above-mentioned carbon nanotubes can be single-walled carbon nanotubes and / or multi-walled carbon nanotubes.

[0164] [Battery]

[0165] An embodiment of the present invention further provides a battery, comprising: a positive electrode sheet according to any implementation manner of any of the above embodiments of the present invention.

[0166] In the embodiment of the present invention, the battery further includes a negative electrode sheet, a separator and an electrolyte; and the specific type of the battery is not limited. As an example, the battery can be any one of a cylindrical lithium battery, a soft-pack lithium battery or an aluminum shell lithium battery.

[0167] The battery includes the above-mentioned lithium iron phosphate positive electrode material provided in the embodiment of the present application. Therefore, the battery, such as a lithium-ion secondary battery, also has excellent electrochemical performance, such as the battery has a long cycle life, a high compaction density and a high gram capacity.

[0168] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents, materials, or instruments used that do not specify the manufacturer are all conventional products that can be purchased commercially.

[0169] Since the battery provided by the embodiment of the present invention adopts all the technical solutions of the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be described one by one here.

[0170] The present application is described in detail below with reference to the accompanying drawings and examples, but the implementation and protection of the present invention are not limited thereto. The following embodiments are only some embodiments of the present application and are not limitations on the present application.

[0171] Preparation Example

[0172] 1. Preparation of large lithium iron phosphate particles:

[0173] 1. Preparation of large lithium iron phosphate particles (LFP-A1): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.032 kg of ammonium chloride and 0.044 kg of titanium dioxide) were weighed and mixed and ground to a frosted particle size of 0.6 μm. The resulting precursor was spray-dried and sintered at 790°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.515 μm. The resulting large lithium iron phosphate particles were designated LFP-A1.

[0174] 2. Preparation of Large Lithium Iron Phosphate Particles (LFP-A2): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.032 kg of ammonium chloride and 0.044 kg of titanium dioxide) were weighed and mixed and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 790°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.752 μm. The resulting large lithium iron phosphate particles were designated LFP-A2.

[0175] 3. Preparation of Large Lithium Iron Phosphate Particles (LFP-A3): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.032 kg of ammonium chloride and 0.044 kg of titanium dioxide) were weighed and mixed and ground to a frosted particle size of 1.2 μm. The resulting precursor was spray-dried and sintered at 810°C to obtain large lithium iron phosphate particles with an average primary particle size of 1.212 μm. The resulting large lithium iron phosphate particles were designated LFP-A3.

[0176] 4. Preparation of Large Lithium Iron Phosphate Particles (LFP-A4): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.032 kg of ammonium chloride and 0.044 kg of titanium dioxide) were weighed and mixed and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 805°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.794 μm. The resulting large lithium iron phosphate particles were designated LFP-A4.

[0177] 5. Preparation of Large Lithium Iron Phosphate Particles (LFP-A5): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.032 kg of ammonium chloride and 0.044 kg of titanium dioxide) were weighed and mixed and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 820°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.941 μm. The resulting large lithium iron phosphate particles were designated LFP-A5.

[0178] 6. Preparation of Large Lithium Iron Phosphate Particles (LFP-A6): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.016 kg of ammonium chloride and 0.044 kg of titanium dioxide) were weighed and mixed and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 805°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.786 μm. The resulting large lithium iron phosphate particles were designated LFP-A6.

[0179] 7. Preparation of large lithium iron phosphate particles (LFP-A7): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.021 kg of ammonium chloride and 0.044 kg of titanium dioxide) were weighed and mixed and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 805°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.791 μm. The resulting large lithium iron phosphate particles were designated LFP-A7.

[0180] 8. Preparation of large lithium iron phosphate particles (LFP-A8): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.032 kg of ammonium chloride, 0.031 kg of titanium dioxide, and 0.014 kg of vanadium pentoxide) were weighed and mixed and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 805°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.781 μm. The resulting large lithium iron phosphate particles were designated LFP-A8.

[0181] 9. Preparation of large lithium iron phosphate particles (LFP-A9): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and 0.044 kg of a first doping source (titanium dioxide) were weighed, mixed, and ground to a frosted particle size of 0.6 μm. The resulting precursor was spray-dried and sintered at 790°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.534 μm. The resulting large lithium iron phosphate particles were designated LFP-A9.

[0182] 10. Preparation of large lithium iron phosphate particles (LFP-A10): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and 0.044 kg of a first doping source (titanium dioxide) were weighed, mixed, and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 805°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.834 μm. The resulting large lithium iron phosphate particles were designated LFP-A10.

[0183] 11. Preparation of large lithium iron phosphate particles (LFP-A11): 1.25 kg of lithium carbonate, 5 kg of iron phosphate, 0.35 kg of glucose, 0.2 kg of polyethylene glycol, and a first doping source (0.014 kg of vanadium pentoxide and 0.031 kg of titanium dioxide) were weighed, mixed, and ground to a frosted particle size of 0.8 μm. The resulting precursor was spray-dried and sintered at 805°C to obtain large lithium iron phosphate particles with an average primary particle size of 0.867 μm. The resulting large lithium iron phosphate particles were designated LFP-A11.

[0184] 2. Preparation of small lithium iron phosphate particles:

[0185] 1. Preparation of lithium iron phosphate small particles LFP-B1: 1.26 kg of lithium carbonate, 5.0 kg of iron phosphate, 0.45 kg of glucose, a second doping source (0.033 kg of titanium dioxide), and 0.25 kg of polyethylene glycol were mixed and ground to a frosted particle size of 0.20 μm. After drying, a precursor was obtained. The precursor was sintered at 730°C and crushed to obtain lithium iron phosphate small particles with an average primary particle size of 0.143 nm. The obtained lithium iron phosphate small particles were designated LFP-B1.

[0186] 2. Preparation of lithium iron phosphate small particles LFP-B2: 1.26 kg of lithium carbonate, 5.0 kg of iron phosphate, 0.45 kg of glucose, a second doping source (0.033 kg of titanium dioxide), and 0.25 kg of polyethylene glycol were mixed and ground to a frosted particle size of 0.25 μm. After drying, a precursor was obtained. The precursor was sintered at 730°C and crushed to obtain lithium iron phosphate small particles with an average primary particle size of 0.169 nm. The obtained lithium iron phosphate small particles were designated LFP-B2.

[0187] 3. Preparation of lithium iron phosphate small particles LFP-B3: 1.26 kg of lithium carbonate, 5.0 kg of iron phosphate, 0.45 kg of glucose, a second doping source (0.033 kg of titanium dioxide), and 0.25 kg of polyethylene glycol were mixed and ground to a frosted particle size of 0.30 μm. After drying, a precursor was obtained. The precursor was sintered at 730°C and crushed to obtain lithium iron phosphate small particles with an average primary particle size of 0.195 nm. The obtained lithium iron phosphate small particles were designated LFP-B3.

[0188] Example 1

[0189] In Example 1, 70% of LFP-A1 and 30% of LFP-B2 are used, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.23μm, powder compaction density 2.61g / cm 3 .

[0190] Example 2

[0191] In Example 2, 70% LFP-A2 and 30% LFP-B2 are used, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.34μm, powder compaction density 2.68g / cm 3 .

[0192] Example 3

[0193] In Example 3, 70% LFP-A3 and 30% LFP-B2 are used, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.52μm, powder compaction density 2.77g / cm 3 .

[0194] Example 4

[0195] In Example 4, 70% LFP-A4 and 30% LFP-B2 are used, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.36μm, powder compaction density 2.71g / cm 3 .

[0196] Example 5

[0197] In Example 5, 70% LFP-A5 and 30% LFP-B2 are used, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.48μm, powder compaction density 2.73g / cm 3 .

[0198] Example 6

[0199] In Example 6, 70% LFP-A6 and 30% LFP-B2 are used, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.34μm, powder compaction density 2.67g / cm 3 .

[0200] Example 7

[0201] In Example 7, 70% LFP-A7 and 30% LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.34μm, powder compaction density 2.68g / cm 3 .

[0202] Example 8

[0203] In Example 8, 70% LFP-A8 and 30% LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.33μm, powder compaction density 2.66g / cm 3 .

[0204] Example 9

[0205] In Example 9, 70% of LFP-A4 and 30% of LFP-B1 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.35μm, powder compaction density 2.68g / cm 3 .

[0206] Example 10

[0207] In Example 10, 70% of LFP-A4 and 30% of LFP-B3 are used, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.38μm, powder compaction density 2.72g / cm 3 .

[0208] Example 11

[0209] In Example 11, 85% of LFP-A4 and 15% of LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.51μm, powder compaction density 2.68g / cm 3 .

[0210] Example 12

[0211] In Example 12, 80% of LFP-A4 and 20% of LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.46μm, powder compaction density 2.70g / cm 3 .

[0212] Example 13

[0213] In Example 13, 60% of LFP-A4 and 40% of LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.31μm, powder compaction density 2.61g / cm 3 .

[0214] Example 14

[0215] In Example 14, 50% LFP-A4 and 50% LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.23μm, powder compaction density 2.53g / cm 3 .

[0216] Comparative Example 1

[0217] In Comparative Example 1, 70% LFP-A9 and 30% LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.21μm, powder compaction density 2.58g / cm 3 .

[0218] Comparative Example 2

[0219] In Comparative Example 2, 70% LFP-A10 and 30% LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.37μm, powder compaction density 2.68g / cm 3 .

[0220] Comparative Example 3

[0221] In Comparative Example 3, 70% LFP-A11 and 30% LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50 =1.38μm, powder compaction density 2.71g / cm 3 .

[0222] Comparative Example 4

[0223] Comparative Example 4 uses 90% LFP-A4 and 10% LFP-B2, wherein the D of the lithium iron phosphate positive electrode material is 50 =1.16μm, powder compaction density 2.67g / cm 3 .

[0224] Comparative Example 5

[0225] In Comparative Example 5, 30% of LFP-A4 and 70% of LFP-B2 were used, wherein the D of the lithium iron phosphate positive electrode material was 50=0.94μm, powder compaction density 2.40g / cm 3 .

[0226] Performance Testing

[0227] 1. Test of primary particle size:

[0228] A ZEISS sigma300 scanning electron microscope was used to take 10K-fold electron microscope images, and the long sides of all primary particles within the field of view were measured using Nano Measurer 1.2 measurement software to obtain the average value of the primary particle size.

[0229] 2. Carbon content test:

[0230] A certain amount of lithium iron phosphate cathode material was weighed and placed in a crucible, and a solvent was added; the above experimental sample was burned, and then the carbon content of the gas generated after combustion was analyzed; the specific reference standard GB / T 20123-2006 was used.

[0231] 3. Compaction density test:

[0232] The compaction density test is carried out in accordance with GB / T24533-2009. A certain amount of lithium iron phosphate cathode material powder is placed on a special compaction mold of known diameter. A metal sheet is placed on the upper and lower parts of the mold, and the lithium iron phosphate cathode material powder is placed in the middle. The corresponding powder thickness is tested while applying a 3T pressure. The formula ρ = m / v is used, where ρ is the compaction density g / cm 3 , m is the mass g of the weighed sample, v is the sample volume, and it is obtained by multiplying the test thickness by the area of ​​the mold; calculate the compacted density.

[0233] 4. Preparation of batteries:

[0234] The prepared lithium iron phosphate positive electrode material, binder polyvinylidene fluoride (PVDF) and conductive agent Super-P were mixed in a mass ratio of 90:5:5, added to N-methylpyrrolidone solvent (NMP), and stirred in a drying room to form a slurry to obtain positive electrode slurry; the positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and after drying and cold pressing, the positive electrode sheet was obtained. The coating amount was 0.3g / cm 2 The lithium sheet is used as the negative electrode, and the electrolyte is a solution of 1 mol / L LiPF6 in EC+DEC+DMC (ethylene carbonate+diethyl carbonate+methyl carbonate) with a volume ratio of 1:1:1. Together with the above-mentioned positive electrode sheet, it is assembled into a CR3032 button battery in a button box.

[0235] (1) 0.1C discharge capacity in grams: At room temperature (25°C), charge the button cell at a constant current of 0.1C to 3.75V, then charge at a constant voltage of 3.75V to a current of 0.05C, and leave it for 5 minutes. The discharge capacity in grams obtained by discharging at 0.1C to 2.0V is recorded as the 0.1C discharge capacity in grams.

[0236] (2) 1C discharge capacity in grams test: At room temperature (25°C), charge the button cell at a constant current of 1C to 3.75V, then charge at a constant voltage of 3.75V to a current of 0.05C, and leave it for 5 minutes; the discharge capacity in grams obtained by discharging at 1C to 2.0V is recorded as the 1C discharge capacity in grams.

[0237] (3) 5C discharge capacity in grams test: At room temperature (25°C), charge the button cell at a constant current of 1C to 3.75V, then charge at a constant voltage of 3.75V to a current of 0.05C, and leave it for 5 minutes; the discharge capacity in grams obtained by discharging at 5C to 2.0V is recorded as the 5C discharge capacity in grams.

[0238] 5. Cycle times at 45℃

[0239] The battery was prepared as follows: the positive electrode materials prepared in the above embodiments and comparative examples were mixed evenly with polyvinylidene fluoride (PVDF) in NMP at a weight ratio of 98:2.0 to obtain a positive electrode slurry, which was coated on aluminum foil and then dried, cold pressed and cut to obtain a positive electrode sheet.

[0240] The negative electrode active material (artificial graphite), the conductive agent (acetylene black), the binder (styrene-butadiene rubber (SBR), and the thickener (sodium carboxymethyl cellulose (CMC)) were mixed uniformly in deionized water at a weight ratio of 96.5:0.7:1.8:1. The mixture was then coated on both sides of copper foil, dried, and cold-pressed to produce the negative electrode sheet. The coating weight on one side was sufficient to maintain a negative-to-positive electrode capacity ratio of 1.12. A polyethylene (PE) porous polymer film was used as the separator. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were dissolved in 1M LiPF6 at a volume ratio of 1:1:1 and stirred to produce a 1 mol / L LiPF6 electrolyte. The positive electrode sheet, separator, and negative electrode sheet were stacked in order, with the separator positioned between the positive and negative electrodes to provide separation. The resulting bare cell was then wound and placed in an outer packaging. The electrolyte was then added. The battery then underwent formation and resting processes to produce a lithium-ion battery.

[0241] Full battery cycle performance test:

[0242] At 45°C, the battery was charged at a constant current of 1C to 3.65V, then charged at a constant voltage of 3.65V until the current dropped to 0.05C. It was then discharged at a constant current of 1C to 2.0V. The first cycle discharge capacity (denoted as C1) was obtained. This cycle was repeated until the capacity decayed to 80% of C1. The number of cycles required for the capacity to decay to 80% of C1 was recorded. The test results are shown in Table 1.

[0243] 6. Powder compression * 1C gram capacity test: Powder compression * 1C gram capacity = powder compaction density * discharge 1C gram capacity; wherein, powder compaction density is the compaction density measured by the above test 3, compaction density test, and discharge 1C gram capacity is the gram capacity measured by the above test 4(2) 1C discharge gram capacity test.

[0244] Table 1. Test results:

[0245]

[0246]

[0247] Table 2. Raw material ratio and preparation conditions of large lithium iron phosphate particles:

[0248]

[0249]

[0250] Table 3. Raw material ratio and preparation conditions of lithium iron phosphate small particles:

[0251]

[0252] As can be seen from Tables 1 to 3, the large lithium iron phosphate particles A1-5 are comparative examples of different sanding particle sizes and sintering temperatures, A6 anion doping amounts, A7 different anion types, A8 dual cation doping, and A9-11 no anion doping. Examples 1 to 8 are different types of A1-8 lithium iron phosphate large particles mixed with A2 lithium iron phosphate small particles (7:3) through powder pressing * 1C gram capacity reaction positive electrode volume energy density. As the sand-milled particle size of the lithium iron phosphate large particles increases, the volume energy density first increases and then decreases at the same lithium iron phosphate small particle mixing ratio; the effect of the sintering temperature of the lithium iron phosphate large particles on the volume energy density also first increases and then decreases. Compared with the comparative examples 1 to 3 without anion doping, the volume energy density and cycle are significantly improved; the mixing ratio of lithium iron phosphate large particles and lithium iron phosphate small particles must also be controlled within a certain range. Compared with comparative example 4, the proportion of lithium iron phosphate large particles is too large (>85%), and the gram capacity, volume energy density, and rate performance are all reduced; and compared with comparative example 5, the lithium iron phosphate small particles are over-compressed and the volume energy density and cycle performance are reduced.

[0253] From the attached Figure 1 and Figure 2 It can be seen that the morphology of the lithium iron phosphate positive electrode material in Example 1 is good, and the large lithium iron phosphate particles and the small lithium iron phosphate particles are evenly and closely distributed. The large lithium iron phosphate particles in the lithium iron phosphate positive electrode material in Comparative Example 1 have cracking phenomena; Figure 3 It can be seen that the lithium iron phosphate cathode material in Comparative Example 1 accelerates the cycle attenuation faster than the lithium iron phosphate cathode material in Example 1 during the cycle. The longer the Li+ diffusion path in the large particles of Comparative Example 1, the more obvious the Li+ concentration gradient between the interior and the surface during the cycle, resulting in uneven volume changes and stress accumulation leading to cracking. After the particles crack, some fragments are separated from the conductive network or the current collector, becoming "dead zones" and losing electrochemical activity. At the same time, the exposed new interface reacts with the electrolyte, causing iron to dissolve and deposit on the negative electrode to break up the SEI, further exacerbating the capacity decay. Example 1 can effectively suppress the occurrence of particle cracking and improve the cycle capacity decay rate through anion doping.

[0254] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0255] The basic principles of the present invention have been described above in conjunction with specific embodiments. However, it should be noted that the advantages, strengths, and effects mentioned in the present invention are merely illustrative and non-limiting, and should not be construed as necessarily possessed by each embodiment of the present invention. Furthermore, the specific details disclosed above are provided for illustrative purposes and to facilitate understanding, and are not intended to be limiting. These details do not necessarily limit the present invention to being implemented using these specific details.

[0256] It should be noted that the terms "and / or" or " / " used herein are merely a description of an association relationship between associated objects, indicating that three relationships may exist. For example, A and / or B may represent three situations: A exists alone, A and B exist at the same time, and B exists alone. The singular forms "a," "the," and "the" used in the embodiments of the present invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0257] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0258] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: include: Large lithium iron phosphate particles and small lithium iron phosphate particles; the particle size of the large lithium iron phosphate particles is micrometer-level, and the particle size of the small lithium iron phosphate particles is nanometer-level; The large lithium iron phosphate particles include at least two doping elements, wherein the doping elements include metal elements and non-metal elements; And / or, the lithium iron phosphate small particles include at least one doping element, and the doping element includes a metal element; The metal elements include one or more of alkali metals, alkaline earth metals or transition metals; and the non-metal elements include one or more of halogens, S or N.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The lithium iron phosphate positive electrode material satisfies at least one of the following characteristics (1) to (4): (1) The metal element is selected from one or more of Na, Mg, Ti, V, Zr, Mn, Nb or Mo; (2) The non-metallic element is selected from one or more of F, S, Cl or N; (3) The mass content of the doping element in the large lithium iron phosphate particles is 0.02% to 1%; (4) In the lithium iron phosphate small particles, the mass content of the doping element is 0.02% to 1%; preferably, the mass content of the metal element is 0.02% to 1%; (5) When the lithium iron phosphate particles include at least two doping elements, one of the doping elements is a metal element.

3. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The chemical formula of the large lithium iron phosphate particles is LiFe 1-x M x PO y N z ; Among them, the value range of x is 0 to 0.05, the value range of y is 3.95 to 4.00, and the value range of z is 0 to 0.05, and the values ​​of x and z do not include 0; And / or, the chemical formula of the lithium iron phosphate small particles is LiFe 1-x M x PO y N z ; Among them, the value range of x is 0 to 0.03, the value range of y is 3.97 to 4.00, and the value range of z is 0 to 0.03, and the value of x does not include 0; Wherein, M is a metal element and N is a non-metal element.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that The lithium iron phosphate positive electrode material satisfies at least one of the following characteristics (1) to (6): (1) The average particle size of the large lithium iron phosphate particles is 0.5 μm to 2 μm; (2) The average particle size of the lithium iron phosphate small particles is 150nm to 300nm; (3) In the lithium iron phosphate positive electrode material, the mass proportion of the large lithium iron phosphate particles is 50% to 85%; (4) In the lithium iron phosphate positive electrode material, the mass proportion of the lithium iron phosphate small particles is 15% to 50%; (5) The large lithium iron phosphate particles are primary particles; (6) The small lithium iron phosphate particles are primary particles.

5. The lithium iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that The large lithium iron phosphate particles include a doped lithium iron phosphate core and a carbon coating layer, wherein the carbon coating layer is coated on the surface of the doped lithium iron phosphate core, and the doped lithium iron phosphate core includes at least two doping elements; Preferably, in the large lithium iron phosphate particles, the thickness of the carbon coating layer is 2 nm to 50 nm; And / or, the lithium iron phosphate small particles include a doped lithium iron phosphate core and a carbon coating layer, the carbon coating layer is coated on the surface of the doped lithium iron phosphate core, and the doped lithium iron phosphate core includes at least one doping element; Preferably, in the lithium iron phosphate small particles, the thickness of the carbon coating layer is 2 nm to 50 nm; Preferably, in the lithium iron phosphate positive electrode material, the mass content of carbon element is 1.0% to 2.0%.

6. The lithium iron phosphate cathode material according to any one of claims 1 to 4, characterized in that The lithium iron phosphate positive electrode material satisfies at least one of the following characteristics (1) to (3): (1) The average particle size D of the lithium iron phosphate positive electrode material v50 0.8μm~3.0μm; (2) The powder compaction density of the lithium iron phosphate positive electrode material is 2.5 to 2.8 g / cm 3 ; Preferably, the powder compaction density of the lithium iron phosphate positive electrode material is 2.6 to 2.8 g / cm 3 ; (3) The powder resistivity of the lithium iron phosphate positive electrode material is 2 to 50 Ω*cm.

7. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: Synthesizing large lithium iron phosphate particles: adding a lithium source, iron phosphate, a carbon source, and a first doping source into a solvent and grinding them, drying them to obtain a precursor A, and sintering the precursor A to obtain the large lithium iron phosphate particles; Synthesizing small lithium iron phosphate particles: adding a lithium source, iron phosphate, a carbon source, and a second doping source into a solvent, grinding, and drying to obtain a precursor B, sintering the precursor B, and crushing to obtain the small lithium iron phosphate particles; Evenly mixing the large lithium iron phosphate particles and the small lithium iron phosphate particles to obtain the lithium iron phosphate positive electrode material; Wherein, the first doping source contains metal elements and non-metal elements; the second doping source contains metal elements; The metal element includes one or more of alkali metals, alkaline earth metals or transition metals; the non-metal element includes one or more of halogens, S or N.

8. The method for preparing the lithium iron phosphate positive electrode material according to claim 7, characterized in that: The preparation method satisfies at least one of the following characteristics (1) to (11): (1) In the synthesis of the large lithium iron phosphate particles, the mass ratio of the lithium source, iron phosphate, carbon source and first doping source is (0.2-0.35):1:(0.06-0.2):(0.001-0.1); (2) In the synthesis of the lithium iron phosphate small particles, the mass ratio of the lithium source, iron phosphate, carbon source and second doping source is (0.2-0.35):1:(0.06-0.2):(0.001-0.1); (3) The mass ratio of the large lithium iron phosphate particles to the small lithium iron phosphate particles is (50-90): (10-50); preferably, the mass ratio of the large lithium iron phosphate particles to the small lithium iron phosphate particles is (50-85): (15-50); (4) In the process of synthesizing the large lithium iron phosphate particles, sand milling is used for grinding, and the particle size D of the sand milling is v50 0.6μm~1.3μm; (5) In the process of synthesizing the lithium iron phosphate small particles, sand milling is used for grinding, and the particle size D of the sand milling is v50 0.2μm~0.4μm; (6) The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium oxalate or lithium acetate; (7) The carbon source is selected from at least one of acetylene black, glucose, sucrose, polyvinyl alcohol, polypropylene alcohol, polyethylene glycol carbon black, carbon fiber tubes, or graphite; (8) The first doping source includes at least one of NaO, NaNO3, MgO, Mg(NO3)2, TiO2, Li2TiO3, TiOSO4, Ti(OCH3)4, V2O5, NH4VO3, VOSO4, ZrO2, Zr(NO3)4, MnO2, Nb2O5, MoO3, or MoCl5; the first doping source also includes at least one of NaF, NH4F, HF, (NH4)2SO4, NH4Cl, or CO(NH2)2; (9) The second doping source includes at least one of NaO, NaNO3, MgO, Mg(NO3)2, TiO2, Li2TiO3, TiOSO4, Ti(OCH3)4, V2O5, NH4VO3, VOSO4, ZrO2, Zr(NO3)4, MnO2, Nb2O5, MoO3 or MoCl5; (10) During the synthesis of large lithium iron phosphate particles, the sintering conditions are as follows: heating to 300-550°C at a heating rate of 1-10°C / min, keeping warm for 1-5 hours, then heating to 720-820°C at a heating rate of 1-10°C / min, and keeping warm for 6-15 hours; (11) The sintering conditions during the synthesis of the lithium iron phosphate small particles are as follows: heating to 200-350°C at a heating rate of 1-10°C / min, keeping warm for 1-5 hours, then heating to 680-780°C at a heating rate of 1-10°C / min, and keeping warm for 5-10 hours.

9. A positive electrode sheet, characterized in that: include: The lithium iron phosphate positive electrode material according to any one of claims 1 to 6, and / or the lithium iron phosphate positive electrode material obtained by the preparation method according to claim 7 or 8.

10. A battery, characterized in that: include: The positive electrode sheet according to claim 9.

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