Positive electrode active material and preparation method therefor, and electrochemical device

Abstract

Provided in the present application are a positive electrode active material and a preparation method therefor, and an electrochemical device. The positive electrode active material comprises a lithium transition metal phosphate substrate having an olivine structure and a carbon coating layer on a surface of the substrate. The positive electrode active material is subjected to XPS measurement using Al / Kα rays, and among peaks in obtained C1s spectra, the area of the peak corresponding to a C-C bond is a, the area of the peak corresponding to a C-O bond is b, and the area of the peak corresponding to an O-C=O bond is c, wherein X=a / (a+b+c), and 60%≤X≤80%. By controlling the microstructure of a carbon material in the carbon coating layer, the rate capability, the low-temperature performance and the cycle performance thereof are effectively improved.

Description

Positive electrode active material, preparation method thereof, and electrochemical device

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application CN202311693860.3, filed on December 7, 2023, entitled “A positive electrode active material, a preparation method thereof, and an electrochemical device”. The entire contents of the above Chinese patent application are incorporated into this application by reference. Technical Field

[0003] The present application relates to a positive electrode active material and a preparation method thereof, as well as an electrochemical device comprising the positive electrode active material. Background Art

[0004] Olivine-type transition metal phosphate materials, such as lithium iron phosphate (LFP), have become one of the mainstream cathode materials for power batteries due to their abundant raw material resources, low cost, environmental friendliness, excellent safety performance, and long cycle life. In recent years, their market share has surpassed that of ternary materials, and their share has gradually increased. However, the intrinsically low electronic conductivity and lithium ion diffusion coefficient of LFP materials result in rapid capacity decay under high-rate charge and discharge conditions and low low-temperature capacity retention, thus reducing the competitiveness of LFP cathode materials. To improve the performance of LFP cathode materials, researchers primarily use surface coating, nanomaterialization, and metal doping to manipulate their electrochemical properties. Carbon materials, due to their excellent electronic conductivity, have become the preferred material for improving the performance of LFP materials. Improving the rate capability, low-temperature performance, and cycle performance of LFP cathode materials through appropriate carbon coating is a key issue.

[0005] Summary of the Invention

[0006] In view of the above-mentioned problems existing in the prior art, the present application provides a positive electrode active material, which includes a lithium transition metal phosphate substrate with an olivine structure and a carbon coating layer arranged on the surface of the substrate. By controlling the microstructure of the carbon material in the carbon coating layer, the rate performance, low temperature performance and cycle performance are effectively improved.

[0007] A first aspect of the present application provides a positive electrode active material, which includes a lithium transition metal phosphate substrate having an olivine structure and a carbon coating layer arranged on the surface of the substrate. When the positive electrode active material is measured by XPS using Al Kα rays, the peak area from the CC bond in the C1s spectrum peak is a, the peak area from the CO bond is b, and the peak area from the OC=O bond is c, wherein X=a / (a+b+c), 60%≤X≤80%, preferably, 65%≤X≤80%.

[0008] A second aspect of the present application provides a method for preparing a positive electrode active material, the method comprising the following steps:

[0009] S1. providing a slurry comprising a transition metal phosphate precursor, a lithium source, a carbon source and deionized water,

[0010] S2, heating the slurry obtained in step S1 at a preset temperature and for a preset time to obtain a positive electrode active material;

[0011] Wherein, the preparation method satisfies at least one of the following conditions:

[0012] (1) The carbon source includes one or more of glucose, sucrose, fructose, maltose, starch, polyvinyl alcohol and polyethylene glycol;

[0013] (2) The preset temperature is 600° C. to 800° C.;

[0014] (3) The preset time is 4 hours to 20 hours.

[0015] A third aspect of the present application provides an electrochemical device comprising the above-mentioned positive electrode active material.

[0016] The technical solution of this application can achieve the following beneficial effects:

[0017] By controlling the ratio of the peak area of ​​CC bond to the sum of the peak area of ​​CC bond, the peak area of ​​CO bond and the peak area of ​​OC=O bond in the C1s spectrum peak of the carbon coating layer on the surface of the positive electrode active material of the present application based on the XPS test, the carbon coating layer has a higher degree of graphitization, the conductivity of the active material is improved, and good low temperature performance, fast charging performance and cycle performance are guaranteed. If the ratio of the peak area of ​​CC bond is too low, it means that the degree of defect of the carbon coating layer is large and the conductivity is poor; if the ratio of the peak area of ​​CC bond is too high, the sintering temperature needs to be increased or the sintering time needs to be extended. If the sintering temperature is too high (>800℃), the active material is prone to produce mixed phase byproducts, and extending the sintering time too much will increase the production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present application is further explained and illustrated below in conjunction with the accompanying drawings. It should be understood that the following drawings are only for illustrative purposes and are not intended to limit the technical solutions of the present application.

[0019] FIG1 is a schematic diagram of the peak separation of the C1s spectrum peak obtained by XPS testing of the active material of Example 1;

[0020] FIG2 is a TEM image of the active material of Example 1. DETAILED DESCRIPTION

[0021] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of this application, rather than all the embodiments. The relevant embodiments described herein are illustrative and are used to provide a basic understanding of this application. The embodiments of this application should not be interpreted as limiting this application.

[0022] For the sake of clarity, only some numerical ranges are specifically disclosed herein. However, any lower limit may be combined with any upper limit to form an unspecified range; and any lower limit may be combined with other lower limits to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, each individually disclosed point or single value may itself serve as a lower limit or upper limit and be combined with any other point or single value, or with other lower limits or upper limits, to form an unspecified range.

[0023] In the description herein, unless otherwise specified, “above” and “below” include the number itself.

[0024] Unless otherwise specified, the terms used in this application have the commonly understood meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0025] 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 component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0026] The term "silicon-based material" is not particularly limited, as long as the material contains silicon. In this application, "silicon-based material" can be silicon, silicon alloys, silicon oxides, silicon carbon compounds, and any mixtures of the above silicon-based materials.

[0027] The term "carbon-based material" is not particularly limited and may include graphite, soft carbon, hard carbon, carbon nanotubes, graphene, and any mixture thereof. The term "graphite" is not particularly limited and may include artificial graphite and / or natural graphite.

[0028] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0029] 1. Cathode Active Materials

[0030] One or more embodiments of the present application provide a positive electrode active material, comprising a lithium transition metal phosphate substrate having an olivine structure and a carbon coating layer disposed on the surface of the substrate. When the positive electrode active material is subjected to XPS measurement using Al Kα radiation, the peak area of ​​the C1s spectrum peak derived from the C—C bond is a, the peak area derived from the C—O bond is b, and the peak area derived from the O—C═O bond is c, where X=a / (a+b+c), and 60%≤X≤80%.

[0031] Surface carbon coating not only enhances the conductivity of lithium transition metal phosphate cathode active materials but also inhibits grain growth during high-temperature processing, thereby improving their rate capability and low-temperature performance. Carbon coating also reduces the contact surface between the active material and the electrolyte, thereby mitigating side reactions with the electrolyte and improving high-temperature and cycling performance. The performance of lithium transition metal phosphate cathode active materials is closely related to the microstructure of the carbon coating.

[0032] X-ray photoelectron spectroscopy (XPS) analyzes the constituent elements and interatomic bonding patterns of a sample surface by irradiating it with X-rays and measuring the energy of the emitted photoelectrons. XPS spectra reveal the substance's inherent microstructure and peak areas proportional to the amount of substance, enabling both qualitative and quantitative analysis. Peak areas can be calculated using, for example, the "MultiPak" software from ULVAC PHI.

[0033] The positive electrode active material of the present application can measure the peak of the C1s spectrum (typically, a peak in the binding energy range of 279eV to 298eV) by the above-mentioned XPS. Among them, the peak of the C1s spectrum is the peak of the energy from the 1s orbital of the carbon atom. Since the peak of the C1s spectrum can be composed of a plurality of overlapping peaks close to each other, it can be separated into, for example, a peak at a position with a binding energy of 284.8eV, a peak at a position of 286eV, and a peak at a position of 288.7eV by performing waveform separation based on curve fitting (typically, fitting based on the nonlinear least squares method). It should be noted that waveform separation can be implemented, for example, by using the software "MultiPak" manufactured by ULVACPHI. Typically, the peak at the position of 284.8eV is a peak from a C-C bond, the peak at the position of 286.0eV is a peak from a C-O bond, and the peak at the position of 288.7eV represents a peak from a C-O bond. The peak area of ​​the "peak at 288.7 eV" includes the sum of the peak areas of both C-O-C and C-OH bonds. Note that, as used herein, the term "peak at 284.8 eV" includes peak top positional deviations that may occur due to measurement conditions, and may include peaks near 284.8 eV. Specifically, the peak may typically be at 284.8 eV ± 0.2 eV, for example, at 284.8 eV ± 0.2 eV or at 284.8 eV ± 0.1 eV. The same applies to the "peak at the position of 286 eV" and the "peak at the position of 288.7 eV", which can be the peak at the position of 286.0 eV±0.2 eV (for example, 286.0 eV±0.2 eV, 286.0 eV±0.1 eV) and the peak at the position of 288.7 eV±0.2 eV (for example, 288.7 eV±0.2 eV, 288.7 eV±0.1 eV), respectively.

[0034] When the ratio of the peak area of ​​the C-C bond peak to the C1s peak area (i.e., the sum of the peak area of ​​the C-C bond, the peak area of ​​the C-O bond, and the peak area of ​​the O-C=O bond, the same below), i.e., X, is within the range of 60% ≤ X ≤ 80%, the degree of graphitization is high and the defects are relatively few, indicating that the active material has better conductivity, thereby improving the low-temperature performance and cycle performance of the lithium-ion battery. Generally speaking, a higher value of X is more beneficial for improving the performance of lithium-ion batteries. However, a higher value of X means that a longer sintering time or sintering temperature is required during the preparation of the positive electrode active material, which increases the corresponding manufacturing cost. In addition, a sintering temperature that is too high can easily lead to the formation of impurity phases (e.g., iron phosphide phase) in the positive electrode active material, resulting in performance degradation.

[0035] In some embodiments, the ratio of the peak area of ​​the peak derived from the C-C bond in the positive electrode active material to the total C1s peak area, i.e., X, is in the range of 65% ≤ X ≤ 80%. When within this range, the lithium-ion battery containing the positive electrode active material has both performance and cost advantages.

[0036] In some embodiments, the carbon coating layer has a thickness of 2 nm to 15 nm. For example, the carbon coating layer has a thickness of 2 nm, 5 nm, 8 nm, 10 nm, 15 nm, or a range consisting of any two of the above values.

[0037] In some embodiments, the carbon coating layer has a content of 1% to 3% based on the total mass of the positive electrode active material.

[0038] Appropriate carbon coating thickness and amount can improve the coating uniformity of the carbon coating, enhance the conductivity of lithium transition metal phosphates, reduce Fe dissolution, and enhance the low-temperature performance, cycle performance, and rate performance of the active material. On the contrary, too little carbon coating or too thin a carbon coating will have limited effect on improving the conductivity of the material. If the carbon coating is too thick or the amount is too much, it will affect the lithium ion intercalation and deintercalation process, thereby affecting the cycle and rate performance of the lithium-ion battery.

[0039] In some embodiments, the specific surface area of ​​the positive electrode active material is 8 m 2 / g~20m 2 / g.

[0040] For positive electrode active materials containing a carbon coating layer, their specific surface area is not only related to the particle size of the particles, but also to the uniformity of the carbon coating. If the specific surface area is within the above range, it means that the carbon coating layer of the positive electrode active material has high coating uniformity, the powder internal resistance of the material is low, and the transmission performance of lithium ions and electrons is improved.

[0041] In some embodiments, based on the total volume of the positive electrode active material, the content of large particle material is m, 5%≤m≤40%, and the large particle material is a particle with a particle size greater than 2μm. Due to the unevenness of the lithium ion deintercalation reaction, large particles accumulate stress, and large particles are more likely to crack, which affects the cycle performance. Therefore, it is hoped that the volume proportion of large particle material in the positive electrode active material is as low as possible. However, because too low a content of large particle material will affect the compaction density of the active material, resulting in too low an energy density of the active material, controlling the volume proportion of large particle material within the above range can simultaneously achieve good cycle performance and high energy density.

[0042] In some embodiments, the positive electrode active material satisfies: Preferably,

[0043] when When the above numerical range is met, the positive electrode active material has good cycle performance, low temperature performance and high energy density.

[0044] In some embodiments, the lithium transition metal phosphate substrate comprises a a Fe (1-x) M x A compound represented by PO4, wherein M comprises at least one of Ti, V, Cr, Mn, Co, Ni, Cu, Zn, and Mg, 0.8≤a≤1.2, and 0≤x≤0.1. Exemplarily, the lithium transition metal phosphate substrate is lithium iron phosphate, lithium iron manganese phosphate, or lithium iron vanadium phosphate. Exemplarily, the doping element M is Ti or V.

[0045] 2. Preparation Method of Positive Electrode Active Materials

[0046] One or more embodiments of the present application further provide a method for preparing a positive electrode active material, comprising the following steps:

[0047] S1. providing a slurry comprising a transition metal phosphate precursor, a lithium source, a carbon source and deionized water,

[0048] S2, heating the slurry obtained in step S1 at a preset temperature and for a preset time to obtain the positive electrode active material;

[0049] In some embodiments, the slurry is further ball-milled and spray-dried before step S2.

[0050] In some embodiments, the carbon source comprises one or more of glucose, sucrose, fructose, maltose, starch, polyvinyl alcohol, and polyethylene glycol.

[0051] In some embodiments, the preset temperature is 600°C to 800°C.

[0052] In some embodiments, the preset time is 4 hours to 20 hours.

[0053] The positive electrode active material prepared by heating at the above preset temperature and preset time has a better degree of graphitization and conductivity, and does not increase the preparation cost due to too high a temperature or too long a time, while avoiding the presence of impurities (such as iron phosphide phase) caused by too high a temperature.

[0054] In some embodiments, the transition metal phosphate precursor is iron phosphate.

[0055] In some embodiments, the lithium source is lithium carbonate.

[0056] In some embodiments, the carbon source includes at least glucose and may also include another carbon source, such as one or more of sucrose, fructose, maltose, starch, polyvinyl alcohol, and polyethylene glycol, wherein the mass percentage of glucose in the carbon source is 50% or more. The carbon source also includes a monosaccharide, such as glucose, and a polysaccharide (such as sucrose, fructose, maltose, starch) or a polymer (such as polyethylene glycol), and the mass percentage of glucose is 50% or more, and the resulting positive electrode active material has a lower powder resistance.

[0057] In some embodiments, the preset temperature is 650°C to 750°C.

[0058] In some embodiments, the preset time is 6 hours to 12 hours.

[0059] 3. Electrochemical Device

[0060] One or more embodiments of the present application further provide an electrochemical device, including a positive electrode sheet, wherein the positive electrode sheet includes the aforementioned positive electrode active material.

[0061] In some embodiments, the positive electrode plate also includes a binder and a conductive agent, and the mass ratio of the positive electrode active material, the binder and the conductive agent satisfies: (90~98):(1~5):(1~5). The ratio of the binder and the conductive agent can be adjusted, and the embodiments of the present application do not impose specific restrictions on this.

[0062] In some embodiments, the binder includes, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0063] In some embodiments, the conductive material includes, but is not limited to, at least one of a carbon-based material, a metal-based material, a conductive polymer, and mixtures thereof. In some embodiments, the carbon-based material includes natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material includes metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0064] The electrochemical device of the present application further includes a negative electrode sheet and an electrolyte.

[0065] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode active material. The negative electrode active material includes at least one of a silicon-based material, a carbon-based material, a tin-based material, a phosphorus-based material, and metallic lithium. The silicon-based material includes at least one of silicon, a silicon alloy, a silicon oxide, and a silicon-carbon compound. The carbon-based material includes at least one of graphite, soft carbon, hard carbon, carbon nanotubes, and graphene. The tin-based material includes at least one of tin, tin oxide, and a tin alloy. The negative electrode current collector includes at least one of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0066] In some embodiments, the negative electrode plate further includes a binder and a conductive agent. The binder includes, but is not limited to: at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon. The conductive agent includes, but is not limited to: at least one of carbon-based materials, metal-based materials, conductive polymers and mixtures thereof. In some embodiments, the carbon-based material includes natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based material includes metal powder, metal fiber, copper, nickel, aluminum or silver. In some embodiments, the conductive polymer includes a polyphenylene derivative.

[0067] The electrochemical device of the present application further includes an electrolyte, which includes a lithium salt and a non-aqueous solvent.

[0068] In some embodiments of the present application, the lithium salt includes but is not limited to one or more of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, LiBOB or lithium difluoroborate.

[0069] Exemplarily, the lithium salt may be LiPF6.

[0070] The non-aqueous solvent includes, but is not limited to, one or more of a carbonate compound, a carboxylate compound, or an ether compound.

[0071] Exemplary, carbonate compounds include, but are not limited to, one or more of linear carbonate compounds or cyclic carbonate compounds. Specifically, linear carbonate compounds include, but are not limited to, one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), or methylethyl carbonate (MEC); cyclic carbonate compounds include, but are not limited to, one or more of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinylethylene carbonate (VEC).

[0072] Exemplary carboxylic acid ester compounds include, but are not limited to, one or more of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerolactone, or caprolactone.

[0073] Exemplarily, the ether compound includes, but is not limited to, one or more of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran.

[0074] In some embodiments, a separator is provided between the positive and negative electrode sheets to prevent short circuits. The material and shape of the separator used in the embodiments of the present application are not particularly limited and may be any known prior art material. In some embodiments, the separator comprises a polymer or inorganic material, for example, formed from a material that is stable to the electrolyte of the present application.

[0075] The electrochemical devices of the present application include, but are not limited to, all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors. In some embodiments, the electrochemical device is a lithium secondary battery. In some embodiments, the lithium secondary battery includes, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0076] In a specific example of the present invention, the electrochemical device is a lithium-ion battery. The present application does not impose any specific restrictions on the type of lithium-ion battery, and it can be any type of lithium-ion battery, such as button-type, cylindrical, soft-pack lithium-ion battery, etc.

[0077] Hereinafter, the positive electrode active material and electrochemical device of the present application will be further described in conjunction with specific examples and comparative examples.

[0078] Examples and Comparative Examples

[0079] Example 1

[0080] Preparation of positive electrode active materials:

[0081] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 400 g of glucose, and 15 L of deionized water were mixed to obtain a slurry;

[0082] (2) taking the slurry and ball milling it;

[0083] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0084] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 770°C for 12 hours, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0085] Preparation of electrochemical device:

[0086] Preparation of the positive electrode sheet: The positive electrode active material LiFePO4, conductive agent carbon black (Super P), and binder PVDF prepared in Example 1 were fully mixed in an N-methylpyrrolidone solvent system in a weight ratio of 97:1:2 to form a positive electrode slurry, and the positive electrode slurry was coated on a 12 μm thick positive electrode current collector aluminum foil to form a positive electrode active material layer. The thickness of the positive electrode active material layer was 150 μm; after drying and cold pressing, the positive electrode sheet was obtained.

[0087] Preparation of negative electrode sheet: Mix the negative electrode active material artificial graphite, conductive agent acetylene black, binder styrene butadiene rubber, thickener sodium carboxymethyl cellulose, and polyacrylic acid in a weight ratio of 95:2:1.5:1:0.5, add deionized water, and stir to obtain a negative electrode slurry. The negative electrode slurry is evenly coated on an 8μm thick negative electrode current collector copper foil to form a negative electrode active material layer. After drying and rolling, the negative electrode sheet is obtained.

[0088] Preparation of electrolyte: In a dry argon atmosphere, LiFP6 was added to EC / DEC / EMC (volume ratio of 1:1:1) and mixed evenly to obtain an electrolyte.

[0089] Preparation of the diaphragm: A 9 μm thick polyethylene (PE) was used as the base membrane, and an aluminum oxide ceramic layer and an adhesive layer were coated on the surface of the base membrane.

[0090] Assembly of lithium-ion batteries: The positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed between the positive and negative electrode sheets to obtain a battery cell. Then, after winding and welding the tabs, the battery cell is placed in an outer packaging aluminum-plastic film and injected with electrolyte. After packaging and formation, a soft-pack lithium-ion battery is obtained.

[0091] Example 2

[0092] Preparation of positive electrode active materials:

[0093] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 300 g of glucose, 100 g of polyethylene glycol, and 15 L of deionized water were mixed to obtain a slurry;

[0094] (2) taking the slurry and ball milling it;

[0095] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0096] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 750°C for 10 hours, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0097] Example 3

[0098] Preparation of positive electrode active materials:

[0099] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 267 g of glucose, 133 g of polyethylene glycol, and 15 L of deionized water were mixed to obtain a slurry;

[0100] (2) taking the slurry and ball milling it;

[0101] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0102] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 700°C for 8 hours, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0103] Example 4

[0104] Preparation of positive electrode active materials:

[0105] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 300 g of glucose, 100 g of ascorbic acid and 15 L of deionized water were mixed to obtain a slurry;

[0106] (2) taking the slurry and ball milling it;

[0107] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0108] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 680°C for 8 hours, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0109] Example 5

[0110] Preparation of positive electrode active materials:

[0111] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 400 g of sucrose, and 15 L of deionized water were mixed to obtain a slurry;

[0112] (2) taking the slurry and ball milling it;

[0113] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0114] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 650°C for 6 hours, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0115] Example 6

[0116] Preparation of positive electrode active materials:

[0117] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 600 g of glucose, and 15 L of deionized water were mixed to obtain a slurry;

[0118] (2) taking the slurry and ball milling it;

[0119] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0120] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas at a gas flow rate of 5 L / min, sintered at a temperature of 700°C for 6 hours, and naturally cooled to room temperature to obtain lithium iron phosphate material.

[0121] Example 7

[0122] Preparation of positive electrode active materials:

[0123] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 600 g of glucose, and 15 L of deionized water were mixed to obtain a slurry;

[0124] (2) taking the slurry and ball milling it;

[0125] (3) spray drying the slurry obtained in step (2), with the inlet temperature of the spray dryer being 230°C, the outlet temperature being 80°C, and the gas flow rate being 15 m3 / h;

[0126] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 620° C. for 6 h, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0127] Example 8

[0128] Preparation of positive electrode active materials:

[0129] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 300 g of glucose, and 15 L of deionized water were mixed to obtain a slurry;

[0130] (2) taking the slurry and ball milling it;

[0131] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0132] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 600°C for 4 hours, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0133] Example 9

[0134] Preparation of positive electrode active materials:

[0135] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 300 g of sucrose, and 15 L of deionized water were mixed to obtain a slurry;

[0136] (2) taking the slurry and ball milling it;

[0137] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0138] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 600°C for 4 hours, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0139] Comparative Example 1

[0140] Preparation of positive electrode active materials:

[0141] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 400 g of glucose, and 15 L of deionized water were mixed to obtain a slurry;

[0142] (2) taking the slurry and ball milling it;

[0143] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0144] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 560° C. for 4 h, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0145] Comparative Example 2

[0146] Preparation of positive electrode active materials:

[0147] (1) 4000 g of ferric phosphate, 1000 g of lithium carbonate, 400 g of glucose, and 15 L of deionized water were mixed to obtain a slurry;

[0148] (2) taking the slurry and ball milling it;

[0149] (3) The slurry obtained in step (2) was spray dried. The inlet temperature of the spray dryer was 230°C, the outlet temperature was 80°C, and the gas flow rate was 15m 3 / h;

[0150] (4) The spray-dried material was placed in an atmosphere furnace with nitrogen as the gas and a gas flow rate of 5 L / min, and sintered at a temperature of 550° C. for 3 h, and then naturally cooled to room temperature to obtain lithium iron phosphate material.

[0151] The preparation methods of the electrochemical devices corresponding to the active materials prepared in Examples 2 to 9 and Comparative Examples 1 to 2 are the same as those of the electrochemical device in Example 1 and are omitted here.

[0152] Test Method

[0153] 1. XPS test

[0154] The test was performed using an X-ray photoelectron spectroscopy device with the following parameters: Device: PHI Model VP4; X-ray source: AlKα; X-ray output: 50W; Emission current: 3.3mA; Acceleration voltage: 15kV

[0155] For the C1s energy spectrum, the C-C bond, the C-O bond, and the O-C=O bond were subjected to peak separation. The C1s energy spectrum and peak separation results of Example 1 are shown in FIG1 as an example. In FIG1 , three peaks were observed in the C1s energy spectrum. This is the result of peak separation in which the peak near 284.8 eV was regarded as a signal from the C-C bond, the peak near 286.0 eV was regarded as a signal from the C-O bond, and the peak near 288.7 eV was regarded as a signal from the O-C=O bond. The peak area a of the C-C bond, the peak area b of the C-O bond, and the peak area c of the O-C=O bond were calculated, and X was calculated. The active materials obtained in each example and comparative example were processed using the same method, and the results are shown in Table 1.

[0156] 2. Volume ratio test of large particle materials

[0157] Place an appropriate amount of the sample to be tested and a pre-dispersant (5wt% sodium pyrophosphate, 4-5mL) in a beaker. Add ultrapure water to a volume of 20mL and ultrasonicate to evenly disperse the sample to obtain a dispersion. Particle size analysis of the dispersion was performed using a Malvern Master Size 3000 instrument. Follow the instrument's operating procedures and read the volume percentage of large particles from the cumulative volume curve of the particle size distribution.

[0158] 3. Carbon coating thickness test

[0159] The sample powder was added to anhydrous ethanol and dispersed evenly by ultrasonication. The sample was then dropped onto a microgrid with a dropper and dried in vacuum before being observed using a JEM-2100F high-resolution transmission electron microscope from JEOL, Japan.

[0160] 4. Specific surface area test

[0161] Test method: GB / T 19587-2017, "Determination of Specific Surface Area of ​​Solids by Gas Adsorption - BET Method." Weigh 3g of sample and completely degas at 300°C for 1 hour in a vacuum oven to remove surface adsorbed substances. Then, calculate the specific surface area of ​​the particles using the nitrogen adsorption method based on the amount of nitrogen adsorbed.

[0162] 5. 1 / 3C discharge specific capacity and 2C capacity retention rate test:

[0163] 1) Allow the lithium-ion battery to rest for 3 hours; 2) Discharge it at 1 / 3C to 2.3V; 3) After standing for 30 minutes, charge it at 1 / 3C CC-CV to 3.80V with a cutoff current of 0.05C; 4) After standing for 30 minutes, discharge it at 1 / 3C to 2.3V; 5) Repeat steps 3-4 three times, and take the third discharge capacity as the 1 / 3C discharge capacity. 6) After standing for 30 minutes, charge it at 1 / 3C CC-CV to 3.80V with a cutoff current of 0.05C; 7) After standing for 30 minutes, discharge it at 2C to 2.3V; 8) Repeat steps 6-7 three times, and take the third discharge capacity as the 2C discharge capacity. Calculate the ratio of the 2C discharge capacity to the 1 / 3C discharge capacity as the 2C capacity retention rate.

[0164] 6. Cycle capacity retention test:

[0165] 1) Allow the lithium-ion battery to stand at 45°C for 3 hours to reach temperature equilibrium; 2) Discharge at 1 / 3C to 2.3V; 3) After standing for 30 minutes, charge the battery at 1C CC-CV to 3.80V with a cutoff current of 0.05C; 4) After standing for 30 minutes, discharge the battery at 1C to 2.3V; 5) Repeat steps 3-4 400 times. The ratio of the 400th discharge capacity to the first discharge capacity is used as the high-temperature cycle capacity retention rate.

[0166] 7. Capacity retention at -20℃

[0167] The capacity released by placing the battery cell at room temperature, fully charging it at a rate of 1 / 3C, and discharging it to the cut-off voltage at a rate of 1 / 3C is recorded as C0; the capacity released by placing the battery cell at room temperature, fully charging it at a rate of 1 / 3C, placing it in a constant temperature environment of -20℃ for 12 hours, and discharging it to the cut-off voltage at a rate of 0.1C is recorded as C1; the capacity retention rate at -20℃ is C1 / C0.

[0168] Table 1

[0169] Compared with Comparative Examples 1 and 2, the lithium-ion battery containing the positive electrode active material prepared in Examples 1 to 9 exhibits better cycle performance, rate performance and low-temperature performance when the CC peak area ratio X in its C1s spectrum peak is in the range of 60% to 80%.

[0170] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A positive electrode active material, characterized in that: The positive electrode active material includes a lithium transition metal phosphate substrate with an olivine structure and a carbon coating layer arranged on the surface of the substrate. When the positive electrode active material is measured by XPS using Al Kα rays, the peak area from the CC bond in the spectrum peak of C1s is a, the peak area from the CO bond is b, the peak area from the OC=O bond is c, X=a / (a+b+c), 60%≤X≤80%.

2. The positive electrode active material according to claim 1, characterized in that 65％≤X≤80％。 3. The positive electrode active material according to claim 1, characterized in that Based on the total mass of the positive electrode active material, the content of the carbon coating layer is 1% to 3%, and the thickness of the carbon coating layer is 2nm to 15nm.

4. The positive electrode active material according to claims 1 to 3, characterized in that: The specific surface area of ​​the positive electrode active material is 8 m 2 / g~20m 2 / g.

5. The positive electrode active material according to claim 1, characterized in that Based on the total volume of the positive electrode active material, the content of the large particle material is m, 5%≤m≤40%, and the large particle material is particles with a particle size greater than 2 μm.

6. The positive electrode active material according to claim 5, characterized in that satisfy:

7. The positive electrode active material according to claim 6, characterized in that satisfy:

8. A method for preparing a positive electrode active material as claimed in claim 1, characterized in that: The preparation method comprises the following steps: S1. providing a slurry including a transition metal phosphate precursor, a lithium source, a carbon source and deionized water, S2, heating the slurry obtained in step S1 at a preset temperature and for a preset time to obtain the positive electrode active material; Wherein, the preparation method satisfies at least one of the following conditions: (1) The carbon source includes one or more of glucose, sucrose, fructose, maltose, starch, polyvinyl alcohol and polyethylene glycol; (2) The preset temperature is 600° C. to 800° C.; (3) The preset time is 4 hours to 20 hours.

9. The preparation method according to claim 8, characterized in that: The preparation method satisfies at least one of the following conditions: (1) before the step S2, the slurry is ball-milled and spray-dried; (2) the carbon source comprises at least glucose, and the mass percentage of glucose in the carbon source is greater than 50%; (3) The preset temperature is 650° C. to 750° C.; (4) The preset time is 6 hours to 12 hours.

10. An electrochemical device, characterized in that: The electrochemical device comprises the positive electrode active material according to claims 1 to 7 or the positive electrode active material prepared by the preparation method according to claims 8 to 9.

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