Lithium iron phosphate positive electrode material and preparation method and application thereof

Abstract

The invention relates to a lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material comprises a nitrogen and sulfur co-doped lithium iron phosphate core and a nitrogen and sulfur co-doped in-situ carbon coating layer. According to the technical scheme, in-situ carbon-coated NS-doped lithium iron phosphate is realized through a one-step method, most of NS is internally doped, a small part of NS is doped into a surface carbon source through a doping agent, an in-situ carbon coating layer is formed on the surface of lithium iron phosphate, multiple effects are realized by utilizing one substance in a simple manner, and the preparation method is simple and easy to implement. The problem of slow dynamics at low temperature can be improved, and the conductivity is improved, so that the problems of low low-temperature capacity retention ratio, poor rate capability and the like are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries, with their advantages of high energy density, high power density, and long life, play a vital role in portable electronics, transportation, and large-scale energy storage, particularly in electric vehicles. However, the application of lithium-ion batteries in transportation and large-scale energy storage presents both development opportunities and new challenges. In addition to energy density, power density, fast charging capability, lifespan, and safety, there are also urgent requirements for the operating temperature of lithium-ion batteries. In Northeast and North China, in particular, the industry's pain point of reduced winter range, fueled by anxiety about vehicle use, is a major factor influencing the purchase of new energy vehicles in northern China. LFP's poor low-temperature performance limits its application in low-temperature environments. Furthermore, the Ministry of Industry and Information Technology has established technical indicators for low-temperature range degradation, requiring a degradation rate of no more than 35%. Therefore, improving the low-temperature performance of lithium iron phosphate is of great significance.

[0003] Lithium-ion batteries inherently have low ionic and electronic conductivity, and their electrochemical performance deteriorates significantly at low temperatures, exhibiting significant energy and power loss, difficulty in charging, decreased lifespan, and safety issues. This has become one of the biggest challenges facing lithium-ion batteries. Specifically, this is manifested in the following aspects: (1) Capacity drop: At -20°C, conventional lithium batteries with LiFePO4 as cathodes only provide 40% to 60% of their room temperature capacity. (2) Capacity fading: Conventional lithium batteries with LiFePO4 as cathodes experience rapid capacity fading at low temperatures. (3) Safety issues: Low-temperature charging affects the diffusion kinetics of Li+, resulting in Li+ not being fully inserted into the graphite anode and forming dendrites. As these lithium dendrites grow, they can pierce the separator between the electrodes, causing an internal short circuit.

[0004] Regarding low-temperature improvement methods, the first option is to reduce the particle size to improve kinetics, but too small a particle size can easily lead to a large specific surface area, making processing difficult. The second option is to coat the LFP surface with a single or multiple layers of carbon to increase conductivity, thereby improving conductivity at low temperatures. The third method may be to dope some metal substances to improve the stability of the crystal lattice and thus improve conductivity. There are also methods that combine doping and coating for improvement, but the combination of two substances does not significantly improve processing and cost, including improving the overall performance of the low-temperature performance, and is not conducive to industrial production.

[0005] Other improvement options include placing a layer of insulation on the outside of the battery cell or using other structural methods to reduce heat loss. However, heating insulation takes too long, is generally ineffective, and is expensive. Alternatively, the electrolyte could be improved to develop one with high conductivity and low viscosity at low temperatures. However, the electrolyte composition is complex, and the results have been limited. Summary of the Invention

[0006] Problems to be solved by the invention

[0007] In view of the above problems existing in the prior art, the purpose of the present invention is to improve the problem of slow kinetics of lithium-ion batteries at low temperatures, improve the electrical conductivity, and thus improve the problems of low low-temperature capacity retention and poor rate performance.

[0008] Solutions for solving problems

[0009] The present invention provides a lithium iron phosphate positive electrode material, which comprises: a nitrogen-sulfur co-doped lithium iron phosphate core and a nitrogen-sulfur co-doped in-situ carbon coating layer;

[0010] The doping amount of nitrogen and sulfur in the lithium iron phosphate core is 1-5 wt %, and the doping amount of nitrogen and sulfur in the in-situ carbon coating layer is 2-3 wt %.

[0011] Preferably, the particle size of the lithium iron phosphate core is 100 to 500 nm;

[0012] Preferably, the thickness of the in-situ carbon coating layer is 10 to 30 nm.

[0013] The present invention also provides a method for preparing the lithium iron phosphate positive electrode material, the method comprising the following steps:

[0014] (1) dissolving a phosphorus source, an iron source, and a lithium source in an organic solvent, adding a carbon source and a sulfur source, mixing, and drying to obtain a precursor;

[0015] (2) grinding the precursor obtained in step (1) and sintering it under a protective atmosphere to obtain the lithium iron phosphate positive electrode material;

[0016] Wherein, the sintering includes a first sintering and a second sintering;

[0017] Preferably, the temperature of the first sintering is 500-600°C;

[0018] Preferably, the first sintering time is 8 to 12 hours;

[0019] Preferably, the temperature of the second sintering is 650-750°C;

[0020] Preferably, the second sintering time is 3 to 5 hours.

[0021] Preferably, the molar ratio of the phosphorus source, iron source and lithium source in step (1) is (0.9-1.1):(0.9-1.1):(0.9-1.1), preferably (0.9-1):(0.9-1):(0.9-1);

[0022] Preferably, the phosphorus source in step (1) is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, and ferric phosphate, preferably ammonium dihydrogen phosphate;

[0023] Preferably, the iron source in step (1) is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, ferric citrate, ferric oxide, and ferrous acetate, preferably ferrous oxalate;

[0024] Preferably, the lithium source in step (1) is selected from one or more of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, lithium nitrate, lithium ethoxide, and lithium fluoride, preferably lithium carbonate.

[0025] Preferably, the molar ratio of the carbon source to the phosphorus source in step (1) is (0.17-0.285):1, preferably (0.225-0.25):1;

[0026] Preferably, the carbon source in step (1) is N-acetyl-D-glucosamine.

[0027] Preferably, the molar ratio of the sulfur source to the phosphorus source in step (1) is (10-150):1, preferably (60-80):1;

[0028] Preferably, the sulfur source in step (1) is thiourea.

[0029] Preferably, the heating rate of the first sintering is 1-3°C / min;

[0030] Preferably, the heating rate of the second sintering is 4-6°C / min.

[0031] Preferably, the molar ratio of the organic solvent to the phosphorus source in step (1) is (20-40):1, preferably (33.8-35):1;

[0032] Preferably, the organic solvent in step (1) is acetone.

[0033] The present invention also provides a lithium-ion battery, which includes the lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared according to the preparation method.

[0034] The present invention also provides an electrical device, which includes the battery.

[0035] Effects of the Invention

[0036] The present invention utilizes an organic carbon source containing N and C, N-acetyl-D-glucosamine, and thiourea to achieve in-situ carbon coating and NS-doped lithium iron phosphate in a one-step process. Most of the NS is internally doped, and a small portion of the NS is doped into the surface carbon source with a dopant. The N-acetyl-D-glucosamine can form an in-situ carbon coating layer on the surface of the lithium iron phosphate. Using one substance to achieve multiple effects in a simple manner can improve the problem of slow kinetics at low temperatures, increase electrical conductivity, and thus improve problems such as low low-temperature capacity retention and poor rate performance.

[0037] (1) Using N-acetyl-D-glucosamine to achieve in-situ carbon coating can enhance electrical conductivity and improve the kinetic diffusion capacity at low temperatures. The N element has two functions. One part of the N is doped into the interior of the lithium iron phosphate. N doping can optimize the crystal structure of the material, making it have a smaller band gap and the lowest delithiation energy barrier, thereby accelerating the diffusion of lithium ions and reducing the lithium insertion and deinsertion energy barrier, thereby improving the electronic conductivity and electrochemical kinetics, thereby improving the low-temperature electronic conductivity problem. The other small amount of N is still attached to the carbon source matrix of the coating layer. The hybridization of the π electrons in the N-doped carbon and the lone pair electrons of N not only increases the specific surface area of ​​the material and broadens the diffusion path, but also produces a strong interaction between Li and N under the action of the electric field, which is beneficial to the active sites provided in the lithium insertion.

[0038] (2) Adding this substance to the precursor allows for doping and coating of different elements, simplifying the tedious process. During the high-temperature reaction, in-situ pyrolytic carbon is coated on the LiFePO4 particles. During the decomposition of the organic carbon source at high temperature, a large amount of CO2, CO, H2O and other gases escape from the precursor, forming pores and forming the LiFePO4 / C material. This makes the in-situ carbon coating have good uniformity. The uniform coating can effectively improve the electronic conductivity at low temperatures and reduce polarization.

[0039] (3) Sulfur doping at the O position also suppresses lithium antisite defects in the lithium ion diffusion channel. The increased lattice spacing allows Li to migrate easily in the diffusion channel without hindrance, thereby improving the kinetic performance.

[0040] (4) The gas generated by heating the sulfur-containing substance is used to reduce the surface heat of the (010) surface, so that the crystal plane growth obtains lower energy, thereby inhibiting its crystal plane growth and reducing its crystal plane thickness, thereby narrowing the lithium ion path and improving the kinetic performance of lithium ion transmission, thereby improving the effect of poor low-temperature kinetics.

[0041] (5) The use of N-acetyl-D-glucosamine can replace expensive metal dopants with low-cost materials, which has the characteristics of low material loss, short production cycle, simple process, and scalability. It has high industrial value, cost advantages and market potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Schematic diagram of the structure of the lithium iron phosphate positive electrode material of the present invention.

[0043] Figure 2 Schematic diagrams of crystal plane growth, (a) is a schematic diagram of crystal plane growth of comparative example 1, and (b) is a schematic diagram of crystal plane growth of embodiment 2.

[0044] Figure 3 This is the SEM image of the lithium iron phosphate positive electrode material of Example 1.

[0045] Figure 4 The XRD diagrams of Examples 1-3 show that the intensity of the (002) crystal plane of Example 2 is relatively strong, indicating that growth occurs along this crystal plane during the synthesis of the material, indicating that the (020) crystal plane is suppressed, representing a reduction in the thickness direction, thereby shortening the lithium ion diffusion path. DETAILED DESCRIPTION

[0046] In order to make the technical solutions and beneficial effects of the present invention more obvious and easy to understand, the following is described in detail by enumerating specific embodiments. Wherein, the accompanying drawings are not necessarily drawn to scale, and local features may be enlarged or reduced to more clearly show the details of local features. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods for which specific conditions are not specified in the following examples are generally based on conventional experimental conditions. Unless otherwise specified, the reagents and raw materials used in the present invention are all commercially available.

[0047] In the following description, numerous specific details are provided to provide a more thorough understanding of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced without one or more of these details. In other instances, certain technical features known in the art are not described to avoid confusion with the present application; that is, all features of actual embodiments are not described herein, nor are well-known functions and steps described in detail.

[0048] The purpose of the terms used herein is only to describe specific embodiments and is not intended to limit the present application. When used herein, the singular forms "a", "an", and "the" are intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "comprising" and / or "including", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0049] In order to fully understand the present application, detailed steps and detailed structures will be presented in the following description to illustrate the technical solution of the present application. The preferred embodiments of the present application are described in detail below. However, in addition to these detailed descriptions, the present application may also have other implementation methods.

[0050] Unless otherwise defined, technical and scientific terms used in this application have the same meanings as those in the technical field to which this application belongs.

[0051] In the following examples, if no specific techniques or conditions are specified, the methods are generally carried out according to conventional techniques or conditions described in the literature in the field, or according to the product instructions and the conditions recommended by the manufacturer. The numerical ranges in the following examples are all inclusive.

[0052] The present invention provides a lithium iron phosphate positive electrode material, which comprises: a nitrogen-sulfur co-doped lithium iron phosphate core and a nitrogen-sulfur co-doped in-situ carbon coating layer;

[0053] The doping amount of nitrogen and sulfur in the lithium iron phosphate core is 1-5 wt %, and the doping amount of nitrogen and sulfur in the in-situ carbon coating layer is 2-3 wt %.

[0054] In some embodiments, the amount of nitrogen and sulfur doped in the lithium iron phosphate core is 1 wt%, or 1.1 wt%, or 1.2 wt%, or 1.3 wt%, or 1.4 wt%, or 1.5 wt%, or 1.6 wt%, or 1.7 wt%, or 1.8 wt%, or 1.9 wt%, or 2 wt%, or 2.1 wt%, or 2.2 wt%, or 2.3 wt%, or 2.4 wt%, or 2.5 wt%, or 2.6 wt%, or 2.7 wt%, or 2.8 wt%. , or 2.9wt%, or 3wt%, or 3.1wt%, or 3.2wt%, or 3.3wt%, or 3.4wt%, or 3.5wt%, or 3.6wt%, or 3.7wt%, or 3.8wt%, or 3.9wt%, or 4wt%, or 4.1wt%, or 4.2wt%, or 4.3wt%, or 4.4wt%, or 4.5wt%, or 4.6wt%, or 4.7wt%, or 4.8wt%, or 4.9wt%, or 5wt%.

[0055] In certain embodiments, the doping amount of nitrogen and sulfur in the lithium iron phosphate core is 2-4 wt %.

[0056] In certain embodiments, the doping amount of nitrogen and sulfur in the in-situ carbon coating layer is 2 wt%, or 2.1 wt%, or 2.2 wt%, or 2.3 wt%, or 2.4 wt%, or 2.5 wt%, or 2.6 wt%, or 2.7 wt%, or 2.8 wt%, or 2.9 wt%, or 3 wt%.

[0057] In certain embodiments, the particle size of the lithium iron phosphate core is 100 to 500 nm.

[0058] In certain embodiments, the particle size of the lithium iron phosphate core is 100 nm, or 150 nm, or 200 nm, or 250 nm, or 300 nm, or 350 nm, or 400 nm, or 450 nm, or 500 nm.

[0059] In certain embodiments, the particle size of the lithium iron phosphate core is 200-300 nm.

[0060] In certain embodiments, the in-situ carbon coating layer has a thickness of 10 to 30 nm.

[0061] In certain embodiments, the thickness of the in-situ carbon coating layer is 10 nm, or 15 nm, or 20 nm, or 25 nm, or 30 nm.

[0062] In certain embodiments, N has a relatively low electronegativity relative to oxygen atoms and a relatively weak ability to attract electrons, so it has a relatively strong coordination ability and can replace the vacant oxygen in the phosphate group and form bonds with P and Fe, which is favorable in terms of formation energy. Since the length of the PN bond (195ppm) is slightly longer than the PO (165ppm) bond, and the length of the Fe(1)-N(1) bond is shorter than the Fe(1)-O(1) bond, the N atom tends to be attracted by the adjacent Fe atom, so that after N replaces the O position, the length of the Fe-N bond decreases after N doping, while the length of the Fe(1)-O(3) bond increases significantly. The bond energy of Li-S is 312.5kJ mol / 1, which is smaller than the 40.5kJ mol / 1 of Li-O3 of lithium oxygen. According to the doping substitution principle, the lowest energy reaction takes precedence, causing it to replace the O position.

[0063] In certain embodiments, the present invention adds N-acetyl-D-aminoglucose organic carbon source material containing N and C and thiourea to the precursor, so that part of NS and part of NS are doped into the interior of lithium iron phosphate during the sintering stage, and a small amount of NS forms an in-situ coating with N doped in the carbon coating.

[0064] The present invention also provides a method for preparing the lithium iron phosphate positive electrode material, the method comprising the following steps:

[0065] (1) dissolving a phosphorus source, an iron source, and a lithium source in an organic solvent, adding a carbon source and a sulfur source, mixing, and drying to obtain a precursor;

[0066] (2) grinding the precursor obtained in step (1) and sintering it under a protective atmosphere to obtain the lithium iron phosphate positive electrode material;

[0067] Wherein, the sintering includes a first sintering and a second sintering.

[0068] In certain embodiments, the temperature of the first sintering is 500-600°C.

[0069] In certain embodiments, the temperature of the first sintering is 500°C, or 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, or 600°C.

[0070] In certain embodiments, the first sintering time is 8 to 12 hours.

[0071] In certain embodiments, the first sintering time is 8 hours, or 8.5 hours, or 9 hours, or 9.5 hours, or 10 hours, or 10.5 hours, or 11 hours, or 11.5 hours, or 12 hours.

[0072] In certain embodiments, the longer holding time of the first sintering provides higher energy, which can ensure that the NS elements in the thiourea enter the LFP lattice during this process, wherein S replaces the oxygen site and N replaces the Fe site.

[0073] In certain embodiments, the second sintering temperature is 650-750°C.

[0074] In certain embodiments, the second sintering temperature is 650°C, or 660°C, or 670°C, or 680°C, or 690°C, or 700°C, or 710°C, or 720°C, or 730°C, or 740°C, or 750°C.

[0075] In certain embodiments, the second sintering time is 3 to 5 hours.

[0076] In certain embodiments, the second sintering time is 3 hours, or 3.5 hours, or 4 hours, or 4.5 hours, or 5 hours.

[0077] In certain embodiments, the second sintering with a shorter holding time and a higher temperature will cause excess thiourea to remain on the surface, gradually forming a coating layer, which together with the carbon coating forms a surface carbon layer containing NS coating.

[0078] In certain embodiments, the molar ratio of the phosphorus source, iron source, and lithium source in step (1) is (0.9-1.1):(0.9-1.1):(0.9-1.1).

[0079] In certain embodiments, the molar ratio of the phosphorus source, iron source, and lithium source in step (1) is 0.9:0.9:0.9, or 0.9:0.9:1, or 0.9:0.9:1.1, or 0.9:1:0.9, or 0.9:1:1, or 0.9:1:1.1, or 0.9:1.1:0.9, or 0.9:1.1:1, or 0.9:1.1:1.1, or 1:0.9:0.9, or 1:0.9:1, or 1:0.9:1. 1, or 1:1:0.9, or 1:1:1, or 1:1:1.1, or 1:1.1:0.9, or 1:1.1:1, or 1:1.1:1.1, or 1.1:0.9:0.9, or 1.1:0.9:1, or 1.1:0.9:1.1, or 1.1:1:0.9, or 1.1:1:1, or 1.1:1:1.1, or 1.1:1.1:0.9, or 1.1:1.1:1.1, or 1.1:1.1:1.1.

[0080] In certain embodiments, the molar ratio of the phosphorus source, iron source, and lithium source in step (1) is (0.9-1):(0.9-1):(0.9-1).

[0081] In certain embodiments, the phosphorus source in step (1) is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, and ferric phosphate.

[0082] In certain embodiments, the phosphorus source in step (1) is ammonium dihydrogen phosphate.

[0083] In certain embodiments, the iron source in step (1) is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, ferric citrate, ferric oxide, and ferrous acetate.

[0084] In certain embodiments, the iron source in step (1) is ferrous oxalate.

[0085] In certain embodiments, the lithium source in step (1) is selected from one or more of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, lithium nitrate, lithium ethoxide, and lithium fluoride.

[0086] In certain embodiments, the lithium source in step (1) is lithium carbonate.

[0087] In certain embodiments, the molar ratio of the carbon source to the phosphorus source in step (1) is (0.17-0.285):1.

[0088] In certain embodiments, the molar ratio of the carbon source to the phosphorus source in step (1) is 0.17:1, or 0.175:1, or 0.18:1, or 0.185:1, or 0.19:1, or 0.195:1, or 0.2:1, or 0.205:1, or 0.21:1, or 0.215:1, or 0.22:1, or 0.225:1, or 0.23:1, or 0.235:1, or 0.24:1, or 0.245:1, or 0.25:1, or 0.255:1, or 0.26:1, or 0.265:1, or 0.27:1, or 0.275:1, or 0.28:1, or 0.285:1.

[0089] In certain embodiments, the molar ratio of the carbon source to the phosphorus source in step (1) is (0.225-0.25):1.

[0090] In certain embodiments, the carbon source in step (1) is N-acetyl-D-glucosamine.

[0091] In certain embodiments, the molar ratio of the sulfur source to the phosphorus source in step (1) is (10-150):1.

[0092] In certain embodiments, the molar ratio of the sulfur source to the phosphorus source in step (1) is 10:1, or 20:1, or 30:1, or 40:1, or 50:1, or 60:1, or 70:1, or 80:1, or 90:1, or 100:1, or 110:1, or 120:1, or 130:1, or 140:1, or 150:1.

[0093] In certain embodiments, the molar ratio of the sulfur source to the phosphorus source in step (1) is (60-80):1.

[0094] In certain embodiments, the sulfur source in step (1) is thiourea.

[0095] In certain embodiments, the present invention utilizes thiourea to generate gas during the high-temperature synthesis process, and through thermal convection, the LFP precursor grains are continuously acted upon during the growth process, so that the deviation between the preferred orientation of the crystal and the direction of heat flow is reduced, thereby leading to a change in the effective orientation of the grains, thereby destroying the preferred orientation growth of the active crystal plane of the layer, limiting the growth of the crystal in the {010} direction, narrowing the ion path, and reducing the particle size.

[0096] In certain embodiments, the heating rate of the first sintering is 1-3° C. / min.

[0097] In certain embodiments, the heating rate of the first sintering is 1° C. / min, or 1.5° C. / min, or 2° C. / min, or 2.5° C. / min, or 3° C. / min.

[0098] In certain embodiments, the slow heating rate of the first sintering can ensure the removal of crystallization water and form a uniform and stable phase.

[0099] In certain embodiments, the heating rate of the second sintering is 4-6° C. / min.

[0100] In certain embodiments, the heating rate of the second sintering is 4° C. / min, or 4.5° C. / min, or 5° C. / min, or 5.5° C. / min, or 6° C. / min.

[0101] In certain embodiments, the molar ratio of the organic solvent to the phosphorus source in step (1) is (20-40):1.

[0102] In certain embodiments, the molar ratio of the organic solvent to the phosphorus source in step (1) is 20:1, or 21:1, or 22:1, or 23:1, or 24:1, or 25:1, or 26:1, or 27:1, or 28:1, or 29:1, or 30:1, or 31:1, or 32:1, or 33:1, or 33.1:1, or 33.2:1, or 33.3:1, or 33.4:1, or 33.5 :1, or 33.6:1, or 33.7:1, or 33.8:1, or 33.9:1, or 34:1, or 34.1:1, or 34.2:1, or 34.3:1, or 34.4:1, or 34.5:1, or 34.6:1, or 34.7:1, or 34.8:1, or 34.9:1, or 35:1, or 36:1, or 37:1, or 38:1, or 39:1, or 40:1.

[0103] In certain embodiments, the molar ratio of the organic solvent to the phosphorus source in step (1) is (33.8-35):1.

[0104] In certain embodiments, the organic solvent in step (1) is acetone.

[0105] The present invention also provides a lithium-ion battery, which includes the lithium iron phosphate positive electrode material or the lithium iron phosphate positive electrode material prepared according to the preparation method.

[0106] In certain embodiments, the lithium-ion battery further comprises a negative electrode sheet and a separator. Typically, a lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, with the separator being disposed between the positive and negative electrode sheets. During the battery's charge and discharge process, active ions are intercalated and released back and forth between the positive and negative electrode sheets. The electrolyte acts as an ion conductor between the positive and negative electrode sheets. The separator is disposed between the positive and negative electrode sheets, primarily to prevent a short circuit between the positive and negative electrodes while allowing ions to pass through.

[0107] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode current collector may be aluminum foil, copper foil, titanium foil, nickel foil, iron foil, zinc foil, etc. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, lithium metal alloy, a material capable of doping / dedoping lithium, or a transition metal oxide. Examples of negative electrode active materials include lithium metal, structured lithium metal, graphite (e.g., natural graphite, artificial graphite), mesophase carbon spheres, hard carbon, soft carbon, silicon, silicon-oxygen materials (e.g., silicon dioxide, silicon monoxide), silicon-carbon composites (Si / C composites), Li-Sn alloys, Li-Sn-O alloys, spinel-structured lithiated TiO2-Li4Ti5O12, Li-Al alloys, etc. In certain embodiments, the negative electrode active material comprises graphite.

[0108] In certain embodiments, the negative electrode active material layer may further include one or both selected from a conductive agent and a binder. The conductive agent is used to improve the conductivity of the electrode. Examples of negative electrode conductive agents include conductive carbon black, conductive graphite, vapor deposited carbon fiber (VGCF), carbon nanotubes, graphene, and the like. The binder of the negative electrode improves the bonding performance between the negative electrode active material particles and between the negative electrode active material particles and the current collector. Examples of negative electrode binders include polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC-Na), cyclodextrin, gelatin, polyvinyl alcohol, polyacrylate, acrylonitrile multipolymer, and the like.

[0109] In certain embodiments, the conductive agent in the negative electrode active material layer is acetylene black, and the binder is styrene-butadiene rubber and sodium carboxymethyl cellulose. The mass ratio of each component in the negative electrode active material layer can be conventional.

[0110] In some embodiments, the negative electrode active material layer is obtained by coating a negative electrode slurry containing the components of the negative electrode active material layer and a solvent onto a negative electrode current collector, and then rolling and slitting. The solvent of the negative electrode slurry can be a conventional solvent in the art, such as deionized water.

[0111] In certain embodiments, the separator can be a polymer porous separator, an inorganic porous separator, or a polymer-inorganic composite porous separator. The polymer porous separator includes a single-layer polymer porous separator and a multi-layer polymer porous separator.

[0112] In certain embodiments, the lithium-ion battery of the present application further includes a packaging case for housing the positive electrode sheet, separator, negative electrode sheet, and electrolyte, as well as other components known in the art for lithium-ion batteries. This application does not limit these other components. This application does not particularly limit the packaging case and may be any packaging case known in the art, as long as it can achieve the objectives of this application.

[0113] The present invention has no particular limitation on the preparation method of the lithium-ion battery, and any technical solution for preparing a negative electrode material into a lithium-ion battery such as a secondary battery, which is well known to those skilled in the art, can be used.

[0114] It should be understood that, since the lithium-ion battery provided in the present application includes the positive electrode material described in the present invention, the beneficial effects of the positive electrode material described in any of the above embodiments are applicable to the lithium-ion battery.

[0115] The present invention also provides an electrical device, which includes the battery.

[0116] Example 1: Preparation of lithium iron phosphate positive electrode material using a small amount of N-acetyl-D-glucosamine and a small amount of thiourea

[0117] (1) 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) were dissolved in 50 mL of acetone until the solution turned yellow. Furthermore, N-acetyl-D-glucosamine (0.75 g) was added as a carbon source, and thiourea (0.2 mmol) was added to the reaction solution. The above materials were stirred thoroughly for 10 h, drained, and dried in a vacuum oven at 80°C for 10 h. The precursor sample was then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0118] (2) Under an inert argon atmosphere, the material was first annealed in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h, and then annealed in a tube furnace at 700°C at a heating rate of 5°C / min for 3-5 h to obtain a nitrogen-sulfur co-doped lithium iron phosphate positive electrode material.

[0119] The prepared lithium iron phosphate positive electrode material was subjected to SEM testing (the powder sample was dispersed on a conductive tape, sprayed with gold / carbon coating to enhance conductivity (to avoid charge accumulation), and tested using a scanning electron microscope (SEM)). The particle size of the core was measured to be 200 nm and the thickness of the in-situ coating layer was 10 nm.

[0120] The prepared lithium iron phosphate positive electrode material was ground, and the sintered block powder was subjected to liquid nitrogen brittle fracture. The middle and edge parts of the cross-sectional area of ​​the material were selected for powder scraping and ICP testing. The measured nitrogen and sulfur doping amounts in the core were 1w%, and the nitrogen and sulfur doping amounts in the in-situ carbon coating layer were 1wt%.

[0121] ICP testing: Powdered samples are dissolved in a strong acid and digested at high temperature to convert them into a solution. Standard solutions of known concentrations of Li, Fe, P, N, and S are prepared, and a concentration-signal intensity curve is established. The sample solution is atomized and fed into a plasma torch for ionization, where it is analyzed using a gas phase liquid chromatography-mass spectrometer (GC-MS). The concentrations of each element are calculated based on the mass spectrometric signal and converted to the actual sample content based on the dilution factor. The test results are shown in Table 1.

[0122] Table 1: ICP test results

[0123]

[0124] The results show that, according to Examples 1-3, the doping levels of the inner and outer layers of the NS increase proportionally with the addition of N-acetyl-D-glucosamine, which is consistent with the designed values. When this substance is not added, the measured contents of the inner and outer layers of the comparative example are absent. We found that the N content is higher than the S content in the same layer. This is because S is volatile at high temperatures, resulting in a non-1:1 ratio.

[0125] The diffraction analysis of the prepared lithium iron phosphate cathode material was carried out, and the results were as follows Figure 4 As shown in the figure, some particles are shorter in thickness due to the small amount of airflow, making them longer in length, appearing as long strips rather than spheres. However, it was found that a small amount of thiourea can cause a small number of particles to grow preferentially.

[0126] Example 2: Preparation of lithium iron phosphate positive electrode material using a moderate amount of N-acetyl-D-glucosamine and a moderate amount of thiourea

[0127] (1) 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) were dissolved in 50 mL of acetone until the solution turned yellow. Furthermore, N-acetyl-D-glucosamine (1 g) was added as a carbon source, and thiourea (1.2 mmol) was added to the reaction solution. The above materials were stirred thoroughly for 10 h, drained, and dried in a vacuum oven at 80°C for 10 h. The precursor sample was then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0128] (2) Under an inert argon atmosphere, the material was first annealed in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h, and then annealed in a tube furnace at 700°C at a heating rate of 5°C / min for 3-5 h to obtain a nitrogen-sulfur co-doped lithium iron phosphate positive electrode material.

[0129] The prepared lithium iron phosphate cathode material was subjected to SEM testing (the method was the same as in Example 1), and the particle size of the core was measured to be 300 nm, and the thickness of the in-situ coating layer was 20 nm.

[0130] The prepared lithium iron phosphate positive electrode material was not ground, and the sintered block powder was subjected to liquid nitrogen brittle fracture. The middle and edge parts of the cross-sectional area of ​​the material were selected for powder scraping and ICP testing (the method was the same as in Example 1, and the results are shown in Table 1). The measured nitrogen and sulfur doping amounts in the core were 3w%, and the nitrogen and sulfur doping amounts in the in-situ carbon coating layer were 2wt%.

[0131] Example 3: Preparation of lithium iron phosphate positive electrode material using a large amount of N-acetyl-D-glucosamine and a large amount of thiourea

[0132] (1) 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) were dissolved in 50 mL of acetone until the solution turned yellow. Furthermore, N-acetyl-D-glucosamine (1.25 g) was added as a carbon source, and thiourea (3 mmol) was added to the reaction solution. The above materials were stirred thoroughly for 10 h, drained, and dried in a vacuum oven at 80°C for 10 h. The precursor sample was then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0133] (2) Under an inert argon atmosphere, the material was first annealed in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h, and then annealed in a tube furnace at 700°C at a heating rate of 5°C / min for 3-5 h to obtain a nitrogen-sulfur co-doped lithium iron phosphate positive electrode material.

[0134] The prepared lithium iron phosphate positive electrode material was subjected to SEM testing (the method was the same as that in Example 1), and the particle size of the core was measured to be 500 nm, and the thickness of the in-situ coating layer was 30 nm.

[0135] The prepared lithium iron phosphate positive electrode material was not ground, and the sintered block powder was subjected to liquid nitrogen brittle fracture. The middle and edge parts of the cross-sectional area of ​​the material were selected for powder scraping and ICP testing (the method was the same as that in Example 1, and the results are shown in Table 1). The nitrogen and sulfur doping amounts in the core were measured to be 5w%, and the nitrogen and sulfur doping amounts in the in-situ carbon coating layer were 3wt%.

[0136] Comparative Example 1: Preparation of lithium iron phosphate positive electrode material without using N-acetyl-D-glucosamine and thiourea

[0137] (1) 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) were dissolved in 50 mL of acetone until the solution turned yellow. The mixture was stirred thoroughly for 10 h, drained, and dried in a vacuum oven at 80°C for 10 h. The precursor sample was then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0138] (2) Under an inert argon atmosphere, the lithium iron phosphate positive electrode material was obtained by annealing in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h and then annealing in a tube furnace at 700°C at a heating rate of 5°C / min for 3-5 h.

[0139] The prepared lithium iron phosphate cathode material was subjected to SEM testing (using the same method as in Example 1), and the particle size of the core was measured to be 500 nm. No modification was performed, and no doping or coating material was detected.

[0140] Comparative Example 2: Preparation of lithium iron phosphate positive electrode material by adding only thiourea without using N-acetyl-D-glucosamine

[0141] (1) 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) were dissolved in 50 mL of acetone until the solution turned yellow. Furthermore, glucose (1 g) was added as a carbon source, and thiourea (1.2 mmol) was added to the reaction solution. The above materials were stirred thoroughly for 10 h, drained, and dried in a vacuum oven at 80°C for 10 h. The precursor sample was then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0142] (2) Under an inert argon atmosphere, the material was first annealed in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h, and then annealed in a tube furnace at 700°C at a heating rate of 5°C / min for 3-5 h to obtain a nitrogen-free lithium iron phosphate positive electrode material.

[0143] The prepared lithium iron phosphate cathode material was subjected to SEM testing (the method was the same as in Example 1), and the particle size of the core was measured to be 400 nm, and the thickness of the in-situ coating layer was 20 nm.

[0144] The prepared lithium iron phosphate positive electrode material was not ground, and the sintered block powder was subjected to liquid nitrogen brittle fracture. The middle and edge parts of the cross-sectional area of ​​the material were selected for powder scraping and ICP testing (the method was the same as that in Example 1, and the results are shown in Table 1). The nitrogen and sulfur doping amounts in the core were measured to be 3w%, and the nitrogen and sulfur doping amounts in the in-situ carbon coating layer were 1wt%.

[0145] Comparative Example 3: Preparation of lithium iron phosphate cathode material by adding N-acetyl-D-glucosamine after synthesizing LFP

[0146] (1) Dissolve 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) in 50 mL of acetone until the solution turns yellow. Add 1.2 mmol of thiourea to the reaction solution. Stir the mixture thoroughly for 10 h, drain, and dry in a vacuum oven at 80°C for 10 h. The precursor sample is then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0147] (2) Under an inert argon atmosphere, the lithium iron phosphate positive electrode material was first annealed in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h.

[0148] (3) In addition, N-acetyl-D-glucosamine (1 g) was added as a carbon source and ball-milled with the above-mentioned lithium iron phosphate positive electrode material for 5 h. After being uniformly mixed, the mixture was first annealed in an inert argon atmosphere in a tube furnace at 600 °C at a heating rate of 2 °C / min for 10 h, and then annealed in a tube furnace at 700 °C at a heating rate of 5 °C / min for 3-5 h to obtain a nitrogen-doped lithium iron phosphate positive electrode material.

[0149] The prepared lithium iron phosphate cathode material was subjected to SEM testing (the method was the same as in Example 1), and the particle size of the core was measured to be 400 nm, and the thickness of the in-situ carbon coating layer was 30 nm.

[0150] The prepared lithium iron phosphate positive electrode material was not ground, and the sintered block powder was subjected to liquid nitrogen brittle fracture. The middle and edge parts of the cross-sectional area of ​​the material were selected for powder scraping and ICP testing (the method was the same as that in Example 1, and the results are shown in Table 1). The nitrogen and sulfur doping amounts in the core were measured to be 3w%, and the nitrogen and sulfur doping amounts in the in-situ carbon coating layer were 1wt%.

[0151] Comparative Example 4: Preparation of lithium iron phosphate positive electrode material by adding only N-acetyl-D-glucosamine without using thiourea

[0152] (1) 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) were dissolved in 50 mL of acetone until the solution turned yellow. Furthermore, N-acetyl-D-glucosamine (1 g) was added as a carbon source. The above materials were stirred thoroughly for 10 h, drained, and dried in a vacuum oven at 80°C for 10 h. The precursor sample was then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0153] (2) Under an inert argon atmosphere, the material was first annealed in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h, and then annealed in a tube furnace at 700°C at a heating rate of 5°C / min for 3-5 h to obtain a nitrogen-doped lithium iron phosphate positive electrode material.

[0154] The prepared lithium iron phosphate cathode material was subjected to SEM testing (the method was the same as in Example 1), and the particle size of the core was measured to be 400 nm, and the thickness of the in-situ coating layer was 20 nm.

[0155] The prepared lithium iron phosphate positive electrode material was not ground, and the sintered block powder was subjected to liquid nitrogen brittle fracture. The middle and edge parts of the cross-sectional area of ​​the material were selected for powder scraping and ICP testing (the method was the same as that in Example 1, and the results are shown in Table 1). The nitrogen and sulfur doping amounts in the core were measured to be 3w%, and the nitrogen and sulfur doping amounts in the in-situ carbon coating layer were 1wt%.

[0156] Comparative Example 5: Preparation of lithium iron phosphate positive electrode material by adding only thiourea without using N-acetyl-D-glucosamine

[0157] (1) 0.02 mol of ammonium dihydrogen phosphate (NH4H2PO4), 0.02 mol of lithium carbonate (Li2CO3), and 0.02 mol of ferrous oxalate (FeC2O4·2H2O) were dissolved in 50 mL of acetone until the solution turned yellow. Furthermore, thiourea (1.2 mmol) was added to the reaction solution. The above materials were stirred thoroughly for 10 h, drained, and dried in a vacuum oven at 80°C for 10 h. The precursor sample was then dried in a vacuum oven at 80°C for 10 h and ground with a mortar to reduce its size.

[0158] (2) Under an inert argon atmosphere, the material was first annealed in a tube furnace at 600°C at a heating rate of 2°C / min for 10 h, and then annealed in a tube furnace at 700°C at a heating rate of 5°C / min for 3-5 h to obtain a nitrogen-doped lithium iron phosphate positive electrode material.

[0159] The prepared lithium iron phosphate positive electrode material was subjected to SEM testing (the method was the same as that in Example 1), and the particle size of the core was measured to be 400 nm.

[0160] The prepared lithium iron phosphate positive electrode material was not ground, and the sintered block powder was subjected to liquid nitrogen brittle fracture. The middle and edge parts of the cross-sectional area of ​​the material were selected for powder scraping and ICP testing (the method was the same as that in Example 1, and the results are shown in Table 1). The nitrogen and sulfur doping amounts in the core were measured to be 3w%, and the nitrogen and sulfur doping amounts in the in-situ carbon coating layer were 1wt%.

[0161] Example 4: Electrode fabrication and electrochemical performance testing

[0162] The synthetic materials in Examples 1-3 and Comparative Examples 1-3 were used as positive electrodes. To test the electrochemical properties of the lithium-ion positive electrode materials obtained in the Examples and Comparative Examples, the positive electrode materials obtained in the Examples or Comparative Examples, the binder polyvinylidene fluoride (PVDF), and the conductive agent SP were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 8:1:1 and stirred to form a uniform and stable slurry. The slurry was then coated on aluminum foil and dried to obtain a positive electrode sheet. The sheet was cut, weighed, and placed in a glove box. In the glove box, a sodium metal sheet was used as the negative electrode, LiPF6 was dissolved in diethylene glycol dimethyl ether as the electrolyte, and pp was used as the separator. CR2032 button batteries were assembled in an argon glove box.

[0163] The method for testing the initial discharge capacity at different temperatures is as follows: 1. In an environment of n±2°C, n=25°C; 0°C, -20°C; 2. Allow to stand for 2 hours; 3. Charge at 0.1C to the upper limit voltage, then charge at constant voltage, with a cutoff current of 0.05C and a cutoff voltage of 3.65V; 4. Allow to stand for 30 minutes; 5. Discharge at 0.1C to the lower limit voltage of 2.5V; 6. Repeat steps 1-5 at different temperatures. Record the initial discharge capacity at different temperatures.

[0164] -20℃ 100-cycle capacity retention rate: 1. Stand at -20℃±2℃ for 2h; 2. Charge at 0.1C to the upper limit voltage, then charge at constant voltage with a cut-off voltage of 3.65V and a cut-off current of 0.05C; 3. Stand for 30min; 4. Discharge at 0.1C to the lower limit voltage of 2.5V; 5. Repeat steps 1 to 4 and record the capacity retention rate after 100 cycles. Capacity retention rate after 100 cycles = discharge capacity at 100 cycles / discharge capacity at 1st cycle.

[0165] The test conditions and results are shown in Table 2:

[0166] Table 2: Electrochemical performance test results

[0167]

[0168] The results show that by comparing Example 2 with Examples 1, 4, and 5, the analysis of D50 shows that nitrogen-sulfur organic matter generates gas during synthesis, which inhibits the growth of preferential {010} in thickness at high temperatures, thereby shortening the particle size of lithium ions during the extraction and embedding process. When too much sulfur-containing raw material is added, excessive gas is generated, which also inhibits the growth of lithium ions. If there is no NS-containing organic matter, the D50 of the particles is relatively large. Compared with others, especially the capacity at low temperatures, the capacity retention is poor.

[0169] Comparing the results of Examples 1-3 and Comparative Examples 1-3, it can be seen that the more the coating amount, the smaller the initial discharge capacity. Since the in-situ coating is relatively uniform, the coating is not significantly affected by the loss content. Comparing all the data below, the decreasing trends at 0°C and 25°C are similar, and it is known that Example 2 has the best performance. Due to incomplete coating, Example 1 has a lower low-temperature decay than Example 3 and Example 2. Combining Example 2 and Comparative Example 1, although the initial capacity of the comparative example is relatively high, since the comparative example is not modified, it decays too quickly at low temperatures, which shows that the modification method of Example 2 has a more obvious performance improvement, especially the 100-cycle capacity retention rate. Combining Example 2 and Comparative Example 2, it is found that although the capacity decay is not particularly fast at low temperatures, N doping optimizes the crystal structure of the material, giving it a smaller band gap and the lowest delithiation energy barrier, which speeds up the diffusion of lithium ions and reduces the lithium insertion and deintercalation energy barrier, thereby improving the electronic conductivity and electrochemical kinetics at low temperatures. It can be seen from the comparison of the relevant capacity retention rates that single coating has limited effect on improving conductivity, and the most important thing is the crystal doping modification of internal diffusion. Combining Example 2 and Comparative Example 3, it can be found that the in-situ coating is not only more obvious for improving the low-temperature conductivity, but the non-in-situ coating is prone to produce uneven coatings, and the transmission capacity is limited. Combined with the capacity retention rate, it can be found that the advantage of Example 2 is also due to the addition of a carbon source to the precursor to achieve a uniform coating. Reasonable design of electrode materials has good conductivity. Through reasonable process adjustment, not only the characteristics of stable material structure are brought into play, but also the characteristics of high conductivity are achieved, thereby improving the overall electrochemical low-temperature performance of the electrode.

[0170] First charge and discharge capacity and cycle test of sodium ion battery: first charge at 0.01C constant current for 4h at 45℃, then charge at 0.05C constant current for 4h, then charge to 3.75V at 0.1C constant current and constant voltage at 25 / 0 / -20℃ respectively. After standing for 2min, discharge at 0.1C constant current to 2.0V, and record the discharge capacity at this time as the first discharge capacity. Record the discharge capacity of the first week and 100 cycles (1000-week capacity retention rate = 100th week cycle discharge capacity / 1st week cycle).

[0171] Simulation calculations: The calculations in this work are performed on a plane wave basis set using the projected augmented wave (PAW) method in the generalized gradient approximation (GGA) in the simulation package (VASP) code, which usually calculates the lattice parameters and ground state energies through LDA / GGA (local density approximation).

[0172] E form =[E N-LFP(010)- E LFP(010) +N N (E O -E N )] / N N

[0173] where EN-LFP(010), ELFP(010), EO, and EN represent the DFT total energies of N-LiFePO4(010), LiFePO4(010), and isolated O and N atoms, respectively; NN is the number of N atoms in the supercell.

[0174] The results are shown in Table 3:

[0175] Table 3: Simulation results

[0176]

[0177] The result shows: by embodiment 2 in table 3 compared to unmodified comparative example 1, it is found that the spacing after abc axis doping increases, and this is mainly because the atomic radius of N is greater than the atomic radius of O, which also shows that the interlayer spacing is increased, so as to promote dynamics, improve low temperature dynamics. It is found by calculation that the formation energy of N doping is smaller, which also shows that it is conducive to doping to replace the position of O. According to the d band center theory, the movement of d band close to (away from) the Fermi level causes more (less) occupation of anti-bonding state with the surrounding atoms, corresponding to weaker (stronger) bonding. The d band center of Fe shifts upward after the introduction of N atoms, so the electrons of O2p orbital tend to fill in Fe-O bonding orbital, strengthen the bonding between Fe-O, and Fe-O bond length is also due to shortening after N doping, meaning that greater energy fracture is required. Secondly, the change of ab axis is greater than c axis, and this is mainly due to S being doped to O atomic vacancy. If it is doped to Fe position, it should be that abc axis is all changed relatively greatly.

[0178] It should be understood that the above embodiments are exemplary and are not intended to encompass all possible implementations of the claims. Various modifications and variations may be made to the above embodiments without departing from the scope of this disclosure. Similarly, the various technical features of the above embodiments may be arbitrarily combined to form additional embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments merely illustrate several implementations of the present invention and do not limit the scope of protection of the patent of this invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material comprises: a nitrogen-sulfur co-doped lithium iron phosphate core and a nitrogen-sulfur co-doped in-situ carbon coating layer; The doping amount of nitrogen and sulfur in the lithium iron phosphate core is 1-5 wt %, and the doping amount of nitrogen and sulfur in the in-situ carbon coating layer is 2-3 wt %.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The particle size of the lithium iron phosphate core is 100 to 500 nm; Preferably, the thickness of the in-situ carbon coating layer is 10 to 30 nm.

3. A method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1-2, characterized in that: The method comprises the following steps: (1) dissolving a phosphorus source, an iron source, and a lithium source in an organic solvent, adding a carbon source and a sulfur source, mixing, and drying to obtain a precursor; (2) grinding the precursor obtained in step (1) and sintering it under a protective atmosphere to obtain the lithium iron phosphate positive electrode material; Wherein, the sintering includes a first sintering and a second sintering; Preferably, the temperature of the first sintering is 500-600°C; Preferably, the first sintering time is 8 to 12 hours; Preferably, the temperature of the second sintering is 650-750°C; Preferably, the second sintering time is 3 to 5 hours.

4. The preparation method according to claim 3, characterized in that The molar ratio of the phosphorus source, iron source and lithium source in step (1) is (0.9-1.1):(0.9-1.1):(0.9-1.1), preferably (0.9-1):(0.9-1):(0.9-1); Preferably, the phosphorus source in step (1) is selected from one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, and ferric phosphate, preferably ammonium dihydrogen phosphate; Preferably, the iron source in step (1) is selected from one or more of ferric phosphate, ferrous oxalate, ferric nitrate, ferric citrate, ferric oxide, and ferrous acetate, preferably ferrous oxalate; Preferably, the lithium source in step (1) is selected from one or more of lithium carbonate, lithium hydroxide, lithium oxalate, lithium acetate, lithium nitrate, lithium ethoxide, and lithium fluoride, preferably lithium carbonate.

5. The preparation method according to claim 3, characterized in that The molar ratio of the carbon source to the phosphorus source in step (1) is (0.17-0.285):1, preferably (0.225-0.25):1; Preferably, the carbon source in step (1) is N-acetyl-D-glucosamine.

6. The preparation method according to claim 3, characterized in that The molar ratio of the sulfur source to the phosphorus source in step (1) is (10-150):1, preferably (60-80):1; Preferably, the sulfur source in step (1) is thiourea.

7. The preparation method according to claim 3, characterized in that The heating rate of the first sintering is 1-3°C / min; Preferably, the heating rate of the second sintering is 4-6°C / min.

8. The preparation method according to claim 3, characterized in that The molar ratio of the organic solvent to the phosphorus source in step (1) is (20-40):1, preferably (33.8-35):1; Preferably, the organic solvent in step (1) is acetone.

9. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the lithium iron phosphate positive electrode material according to any one of claims 1-2, or the lithium iron phosphate positive electrode material prepared by the preparation method according to any one of claims 3-8.

10. An electrical device, characterized in that: The electric device comprises the battery according to claim 9.

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