A NC@Li5FeO4 composite lithium supplement additive and its preparation and application in lithium-ion batteries

Abstract

The present invention belongs to the field of lithium-ion batteries, and specifically relates to an N-C@Li5FeO4 composite lithium supplement additive having a core-shell structure, wherein the core is Li5FeO4 and the shell is N-doped amorphous carbon. The present invention also relates to a method for preparing the lithium supplement agent and its application in lithium supplementation of the positive electrode of a lithium-ion battery. Coating Li5FeO4 with a nitrogen-doped carbon material can improve air stability and also help improve the lithium supplementation effect of the positive electrode. Studies have found that this material has excellent air stability, can preferentially participate in the construction of the negative electrode SEI film, can significantly improve the initial charge and discharge coulombic efficiency, and improve electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the field of energy storage devices, and in particular relates to a lithium supplement, a preparation method of a lithium ion battery, and a lithium ion battery. Background Art

[0002] Lithium ion battery (LIB) is currently the most promising and fastest-growing high-efficiency secondary battery, with many advantages such as high specific energy, low self-discharge, good cycle performance, and no memory effect.

[0003] Lithium in lithium-ion batteries + All of them come from the positive electrode material, and the negative electrode generally uses graphite material. When the battery is first charged, the negative electrode surface of the lithium-ion battery consumes Li + The formation of an SEI film causes initial charge and discharge capacity loss (ICL). The ICL of lithium-ion batteries with graphite anodes is approximately 7% to 10%. High-capacity silicon anode materials are gradually being used in lithium-ion batteries, but the ICL of silicon anodes can be as high as 50% to 70%. Therefore, the development of simple and efficient lithium replenishment technologies is extremely important.

[0004] The current lithium replenishment scheme is mainly negative electrode lithium replenishment, which can be further divided into primary battery lithium replenishment technology and auxiliary anode lithium replenishment technology. However, research results show that the existing lithium replenishment technology has the following problems: the lithium insertion current of the primary battery lithium replenishment is uncontrollable, the production process is extremely demanding and there are safety hazards; the auxiliary anode lithium replenishment is difficult to produce continuously and is prone to safety hazards. In recent years, domestic and foreign researchers have gradually turned their attention to positive electrode lithium replenishment. Positive electrode lithium replenishment is to select a positive electrode material with a high lithium content in addition to the traditional positive electrode material, mix it with the traditional positive electrode material in a certain proportion and use it as a new positive electrode material for battery assembly. During the first charge and discharge process, the excess Li released by the high lithium content positive electrode material as an additive + It will fill the irreversible Li + Loss, thereby reducing the ICL of the entire battery. Compared with negative electrode lithium replenishment technology, positive electrode lithium replenishment is safer and easier to industrialize. It does not require changes to existing factory equipment and processes and is a very promising lithium replenishment technology. However, the high-lithium cathode materials required for positive electrode lithium replenishment still require further research. In other words, the core of positive electrode lithium replenishment technology lies in finding a cathode material with a high lithium content that can maximize lithium release under the charge and discharge conditions of existing batteries, and is low-cost and simple to prepare.

[0005] Li5FeO4 is a lithium-rich transition metal oxide with an inverse fluorite structure. It boasts a very high specific capacity of up to 867 mAh / g. Its initial charge and discharge efficiency is very low, allowing for maximum lithium removal and replenishment of the ICL on the negative electrode. Therefore, Li5FeO4 has enormous potential for addressing the ICL issue in lithium-ion batteries. However, the conventional sintering process reported for preparing Li5FeO4 suffers from harsh conditions, poor air stability, and large particle size, resulting in low electronic conductivity, which compromises its electrochemical performance and applications. Summary of the Invention

[0006] In order to address the shortcomings and defects of the existing technology, the first purpose of the present invention is to provide a nitrogen-doped carbon-coated lithium ferrate (also known as NC@Li5FeO4 lithium supplement in the present invention), which aims to improve the air stability of the material and improve its electrochemical performance through the N-doped carbon coating.

[0007] The second object of the present invention is to provide a method for preparing NC@Li5FeO4, aiming to prepare a material with uniform and dense N-doped carbon coating, excellent air stability and lithium replenishment performance.

[0008] The third object of the present invention is to provide the use of the lithium supplement additive in lithium ion batteries and the obtained positive electrode and battery.

[0009] A NC@Li5FeO4 composite lithium supplement additive has a core-shell structure, wherein the core is Li5FeO4 and the shell is N-doped amorphous carbon.

[0010] The present invention has found that coating Li5FeO4 with a nitrogen-doped carbon material can improve air stability and also contribute to improved lithium replenishment at the positive electrode. The research found that this material has excellent air stability and can participate in the construction of the negative electrode SEI film before the active material, significantly improving the initial charge and discharge coulombic efficiency and electrochemical performance.

[0011] The N content is preferably 0.1 to 2 atm %, more preferably 0.5 to 2 atm %.

[0012] Preferably, the shell is also doped with silicon.

[0013] Preferably, the mass ratio of the core to the shell is 3-5:95-97.

[0014] Preferably, the particle size of the NC@Li5FeO4 composite lithium supplement additive is less than or equal to 5 μm; the specific surface area is 200 to 800 m 2 ·g -1 .

[0015] The present invention also provides a method for preparing the NC@Li5FeO4 composite lithium supplement additive, comprising the following steps:

[0016] Step (1):

[0017] Ball milling a mixed solution containing a carbon source, a nitrogen source, a silicate, a lithium source, an iron source, and a surfactant to obtain a slurry;

[0018] Step (2):

[0019] The slurry is sprayed to obtain a precursor; wherein the spraying temperature is 160 to 200° C.;

[0020] Step (3):

[0021] The precursor is sintered in a protective atmosphere at a temperature of 600-900° C. to obtain the NC@Li5FeO4 composite lithium supplement additive.

[0022] In the present invention, controlling the Li5FeO4 unit cell and crystal phase, and achieving uniform and dense coating, are the main difficulties in improving air stability and lithium replenishment activity. To address this preparation difficulty, the present invention has found that by using the synergistic materials of the carbon source, nitrogen source, silicate, lithium source, iron source, and surfactant, and further coordinating ball milling activation, spraying, and roasting treatment, a synergistic effect can be achieved, which helps to improve the crystal phase purity of Li5FeO4 and avoid grain agglomeration. In addition, it also helps to form a uniform and dense nitrogen-doped coating on the surface. Research has found that the combination of the synergistic raw materials and processes can effectively improve the air stability, lithium replenishment activity, and cycle stability of the resulting material.

[0023] In the present invention, the synergy of the material components (such as N source, silicate) and the ball milling activation-spraying-calcination process is the key to improving the uniform and dense coating of the material, improving its air stability, improving the electrochemical performance and storage electrochemical stability.

[0024] In the present invention, the carbon source is a carbon-containing organic matter, preferably at least one of citric acid, glucose, sucrose, acetic acid, and p123;

[0025] Preferably, the N source is a nitrogen-containing organic matter with an N:C ratio greater than or equal to 1, preferably at least one of urea, melamine, dicyandiamide, and corn steep liquor;

[0026] Preferably, the silicate is tetrasilicate, preferably at least one of tetramethyl silicate, tetraethyl silicate, tetrapropyl silicate and tetrabutyl silicate.

[0027] Preferably, the lithium source is at least one of Li2O, Li2CO3, LiF, Li3PO4, and Li2C2O4; more preferably, it is at least one of Li2CO3, LiF, and Li2C2O4.

[0028] Preferably, the Fe source is at least one of Fe2O3, Fe3O4, iron powder, and FeC2O4; more preferably, it is at least one of Fe2O3 and Fe3O4.

[0029] Preferably, the surfactant is at least one of dodecyltrimethylammonium bromide, stearic acid, sodium dodecylbenzenesulfonate and polyvinylpyrrolidone;

[0030] Preferably, the solvent in the mixed solution is at least one of water and alcohol;

[0031] Preferably, the alcohol is a C1-C4 mono- or polyol, preferably ethanol.

[0032] Preferably, the weight ratio of the carbon source to the iron source is 0.3 to 0.8:1; more preferably 0.4 to 0.6:1;

[0033] Preferably, the weight ratio of the N source to the iron source is 0.05 to 0.2:1; more preferably 0.1 to 0.15:1;

[0034] Preferably, the weight ratio of silicate to iron source is 0.01 to 0.1:1; more preferably 0.05 to 0.1:1;

[0035] Preferably, the element molar ratio of Li:Fe in the lithium source and the iron source is 4.5 to 9.5:1;

[0036] Preferably, the molar ratio of Li:Fe in the lithium source and the iron source is 4.5 to 9.5:1; more preferably 5 to 6:1;

[0037] Preferably, the added amount of the surfactant is 2 to 10 wt% of the iron source; more preferably 5 to 10 wt%.

[0038] In the present invention, the raw materials are ball-milled and activated with the assistance of a nitrogen source, a silicate and a surfactant, which helps to further activate the materials and improve the properties of the prepared materials.

[0039] In the present invention, the carbon source, the N source and the silicate are pre-ball milled and activated, and then mixed with the lithium source and the iron source and ball milled and activated to prepare the slurry.

[0040] More preferably, the carbon source, N source, and silicate are pre-ball-milled in alcohol (such as C1-C4 alcohol), and then mixed with an aqueous solution dispersed with a lithium source and an iron source and ball-milled again to obtain the slurry.

[0041] Preferably, the ball milling speed is 200-400 r / min;

[0042] Preferably, the ball milling time is 1 to 20 hours, more preferably 3 to 15 hours.

[0043] In the present invention, the synergistically innovative raw materials and ball milling activation are further combined with spraying, roasting and synergistic conditions to further improve air stability, lithium replenishment activity and electrochemical performance.

[0044] Preferably, the temperature of the spray treatment is 160-180°C;

[0045] Preferably, the feed rate during the spray treatment process is 10 to 30 ml / min.

[0046] Preferably, the protective atmosphere is at least one of nitrogen and an inert gas;

[0047] Preferably, the calcination temperature is 600-850° C., and more preferably 750-800° C. At this preferred sintering temperature, the electrochemical properties of the obtained material can be further improved.

[0048] Preferably, the calcination time is 20 to 48 hours.

[0049] A more preferred preparation method of the present invention comprises the following steps:

[0050] Step (1): Weigh a certain proportion of glucose, urea and tetraethyl orthosilicate, dissolve them in an appropriate amount of anhydrous ethanol, and ball mill for 1 to 3 hours at a ball milling speed of 200 to 400 r / min;

[0051] Step (2): LiOH and Fe2O3 weighed in a stoichiometric ratio are then added and dispersed in deionized water containing a surfactant, and the mixture is ball-milled for 1 to 3 hours at a ball-milling speed of 200 to 400 r / min; the Li / Fe stoichiometric ratio is 4.5 to 9.5:1, and the surfactant comprises one or more of dodecyltrimethylammonium bromide, stearic acid, sodium dodecylbenzenesulfonate, and polyvinylpyrrolidone, and the amount of the surfactant added is 2 to 10 wt% of the Fe2O3.

[0052] Step (3): spraying the slurry obtained in step (2) at 160-200° C. to obtain a uniformly mixed precursor at a feed rate of 10-30 ml / min;

[0053] Step (4): The precursor obtained in step (3) is placed in an Ar atmosphere and sintered at 600-900° C. for 24-48 hours. After cooling, uniform NC@Li5FeO4 is obtained.

[0054] The present invention also includes NC@Li5FeO4 prepared by the preparation method. The material has the physical and chemical properties imparted by the preparation method, and has better air stability and electrochemical performance.

[0055] In the present invention, the first charge capacity of the prepared NC@Li5FeO4 is 450-750 mAh·g -1 The initial charge and discharge efficiency is 1-10%, the particle size is less than or equal to 5 μm, and the specific surface area is 200-800 m 2 ·g -1 .

[0056] The present invention also provides the application of the NC@Li5FeO4 composite lithium supplement additive as an additive for preparing lithium-ion batteries;

[0057] Preferably, it is used to prepare the positive electrode of a lithium ion battery;

[0058] Preferably, it is used to prepare positive electrode materials for lithium-ion batteries.

[0059] In the present invention, based on existing materials, equipment and principles, the NC@Li5FeO4 composite lithium supplement additive can be used to prepare lithium-ion batteries and their associated components and materials.

[0060] The present invention also provides a lithium-ion battery lithium-supplementing positive electrode material, comprising a positive electrode active material and the NC@Li5FeO4 composite lithium-supplementing additive.

[0061] The positive electrode active material can be an existing conventional positive electrode active material.

[0062] Preferably, the positive electrode active material is at least one of LiCoO2, LiFePO4, and NCM ternary materials;

[0063] Preferably, the weight percentage of the NC@Li5FeO4 composite lithium-supplementing additive is 2-15%, preferably 5-10wt%. Studies have found that controlling the NC@Li5FeO4 lithium-supplementing material within the above range can further enhance the performance of the additive, further improve the electrical performance, and improve the initial charge and discharge coulombic efficiency.

[0064] In the present invention, the lithium-supplementing positive electrode material, in addition to the lithium-supplementing additive and the positive electrode active material, also contains other components allowed to be added to the positive electrode material, such as a conductive agent, a binder, etc.

[0065] The conductive agent can be a conductive material known in the industry that can be used for the positive electrode; for example, at least one of acetylene black and Ketjen black. The weight percentage of the conductive agent is 1-15%, preferably 5-10%.

[0066] The binder can be a material known in the industry that can be used to bond the positive electrode components together; for example, it can be at least one of PVDF and PTFE. The weight percentage of the binder is 1-15%, preferably 5-10%.

[0067] In the present invention, the balance is the positive electrode active material.

[0068] In the present invention, the preparation method of the lithium-supplementing positive electrode material comprises slurrying the lithium-supplementing additive, positive electrode active material, conductive agent, binder and solvent to prepare positive electrode slurry, coating the positive electrode slurry on the surface of the positive electrode collector, solidifying, and compounding on the surface of the positive electrode collector to obtain the lithium-supplementing positive electrode material.

[0069] The present invention also provides a lithium-ion battery, which includes the NC@Li5FeO4 composite lithium-supplementing additive; preferably, includes the lithium-supplementing positive electrode material.

[0070] In the present invention, the lithium-ion battery, except for the positive electrode containing the NC@Li5FeO4 composite lithium supplement additive of the present invention, can have other components and materials that are known in the art. For example, the negative electrode active material can be one or more of graphite, hard carbon, and silicon-carbon materials.

[0071] The lithium-replenishing positive electrode material is used to replenish lithium in the assembled lithium-ion battery. Preferably, the replenishment process involves a single charge-discharge cycle, wherein the first charge is performed using a constant current or constant voltage charge of 0.02 to 0.1 C with a cutoff voltage of 4.0 to 4.5 V, and the first discharge is performed using a constant current discharge of 0.02 to 0.1 C with a cutoff voltage of 1.5 to 2.0 V. A low current is used during charging to completely remove lithium from the material, while a high current is used during discharge to destroy the material structure and prevent the lithium from returning.

[0072] As a general technical concept, the present invention also provides a lithium-ion battery assembled from the lithium-supplementing positive electrode material, wherein the lithium-ion battery has an initial charge and discharge coulombic efficiency of 90-99%.

[0073] Compared with the prior art, the advantages of the present invention are:

[0074] 1. This invention provides a new material, NC@Li5FeO4, and discovers that thanks to the nitrogen-doped carbon coating, it has a synergistic effect with the Li5FeO4 core, which helps further improve air stability and lithium replenishment. Using it as a lithium battery negative electrode for lithium replenishment can effectively reduce the battery's capacity loss during the initial charge and discharge process, and improve the overall battery's energy density and cycle performance.

[0075] 2. The existing Li5FeO4 sintering process has stringent conditions and also presents issues such as grain and crystalline purity, uneven coating, air stability, and suboptimal electrochemical performance. To address these technical issues, the present invention innovatively utilizes ball milling activation in the presence of a nitrogen source and a silicate, combined with subsequent spraying and calcination. This creates a synergistic effect, improving the grain and crystalline structure and facilitating the formation of a uniform, dense coating. This effectively improves air stability, reduces ICL, and enhances battery electrochemical performance. The NC@Li5FeO4 synthesized in this invention has relaxed environmental requirements and can be co-coated with existing cathode materials. The process is simple, easy to control, and low-cost.

[0076] 3. In the present invention, N-doped C coating has a synergistic effect on the air stability of Li5FeO4, which can effectively improve the stability of the material in air.

[0077] 4. The carbon source and nitrogen source selected in the present invention are cheap and easily available, the process is simple, and it is easy to industrialize. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 This is the SEM image of NC@Li5FeO4;

[0079] Figure 2 This is the cycle diagram of the material prepared in Example 1 at 0.1C;

[0080] Figure 3 This is the cycle diagram of the material prepared in Example 2 at 0.1C;

[0081] Figure 4 This is the cycle diagram of the material prepared in Example 3 at 0.1C;

[0082] Figure 5 This is the cycle diagram of the material prepared in Example 4 at 0.1C;

[0083] Figure 6 This is the cycle diagram of the material prepared in Example 5 at 0.1C;

[0084] Figure 7 This is the cycle diagram of the material prepared in Example 6 at 0.1C;

[0085] Figure 8 This is the cycle diagram of the material prepared in Example 7 at 0.1C;

[0086] Figure 9 This is the cycle diagram of the material prepared in Example 8 at 0.1C. DETAILED DESCRIPTION

[0087] In order to facilitate understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0088] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0089] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0090] Example 1

[0091] A method for preparing a lithium-ion battery using an NC@Li5FeO4 lithium supplement comprises the following steps:

[0092] 1: Preparation of lithium supplement materials

[0093] (1) Glucose, urea, and tetraethyl orthosilicate (TES) were weighed in a mass ratio of 10:2:1 and dissolved in anhydrous ethanol (the weight ratio of ethanol to glucose was 40:1) under magnetic stirring. The mixture was then ball-milled for 3 h at a speed of 400 r / min.

[0094] (2) adding the solution of step (1) into an aqueous solution in which Li2O, Fe2O3 and a surfactant are dispersed (the molar ratio of Li / Fe is 5.5:1; the surfactant is dodecyltrimethylammonium bromide, and the concentration of the surfactant in the aqueous solution is 5 wt%; the weight ratio of the surfactant to Fe2O3 is 0.05:1; and the weight ratio of glucose to Fe2O3 is 0.5:1), and continuing ball milling at a speed of 400 r / min for 3 h to obtain a slurry;

[0095] (3) spraying the slurry obtained in step (2) at 160° C. to obtain a uniformly mixed precursor at a feed rate of 30 ml / min;

[0096] (4) The precursor obtained in step (3) was placed in an Ar atmosphere and sintered at 800°C for 24 hours. After cooling, a uniform NC@Li5FeO4 was obtained. SEM showed Figure 1 Its particle size is 2μm; its specific surface area is 600m 2 ·g -1. ;N content is 1atm%;C content is 5atm%;

[0097] 2. Cathode materials

[0098] NCM811 (75wt%), NC@Li5FeO4 (5wt%), SuperP (10wt%) and PVDF (10wt%) were mixed evenly, and then the positive electrode sheet was obtained by slurry preparation, coating, drying and rolling.

[0099] 3. Anode material:

[0100] After graphite (90 wt%), SuperP (5 wt%) and PVDF (5 wt%) are uniformly mixed, the negative electrode sheet is obtained by slurry preparation, coating, drying and rolling.

[0101] 4. After assembling the positive and negative electrode sheets, a lithium-ion battery is obtained, and lithium replenishment of the negative electrode material is achieved during the first charge and discharge cycle.

[0102] Example 2

[0103] Compared with Example 1, the only difference is that the N source is melamine, and other conditions are the same as Example 1.

[0104] Example 3

[0105] Compared with Example 1, the only difference is that the Fe source is Fe3O4, and other conditions are the same as Example 1.

[0106] Example 4

[0107] Compared with Example 1, the only difference is that the Li source is LiOH, and other conditions are the same as Example 1.

[0108] Example 5

[0109] Compared with Example 1, the only difference is that the Li / Fe stoichiometric ratio is 5:1, and other conditions are the same as Example 1.

[0110] Example 6

[0111] Compared with Example 1, the only difference is that the Li / Fe stoichiometric ratio is 6:1, and other conditions are the same as Example 1.

[0112] Example 7

[0113] Compared with Example 1, the only difference is that the ball milling time in (2) is 12 h, and the other conditions are the same as Example 1.

[0114] Example 8

[0115] Compared with Example 1, the only difference is that the temperature of the spray treatment is 200° C., and other conditions are the same as Example 1.

[0116] Example 9

[0117] Compared with Example 1, the only difference is that the sintering temperature is 600° C., and other conditions are the same as Example 1.

[0118] Example 10

[0119] Compared with Example 1, the only difference is that the matched positive electrode material is LiFePO4, and other conditions are the same as Example 1.

[0120] Example 11

[0121] Compared with Example 1, the only difference is that in step (2), the content of NC@Li5FeO4 is 10wt%, the contents of the conductive agent and the binder remain unchanged, and the balance is NCM811 (70wt%);

[0122] Example 12

[0123] Compared with Example 1, the only difference is that in step (2), the content of NC@Li5FeO4 is 2wt%, and the balance is NCM811 (78wt%);

[0124] Example 13

[0125] Compared with Example 1, the only difference is that the weight ratio of glucose to Fe2O3 is 0.3:1;

[0126] The weight ratio of urea to Fe2O3 is 0.2:1;

[0127] The weight ratio of tetraethyl orthosilicate to Fe2O3 is 0.1:1; the amount of surfactant added is 10wt% of Fe2O3;

[0128] Example 14

[0129] Compared with Example 1, the only difference is that the weight ratio of glucose to Fe2O3 is 0.8:1;

[0130] The weight ratio of urea to Fe2O3 is 0.05:1;

[0131] The weight ratio of tetraethyl orthosilicate to Fe2O3 is 0.02:1; the amount of surfactant added is 2wt% of the iron source;

[0132] Comparative Example 1

[0133] Compared with Example 1, the only difference is that step (1) does not contain urea.

[0134] Comparative Example 2

[0135] Compared with Example 1, the only difference is that step (1) does not contain tetraethyl orthosilicate.

[0136] Comparative Example 3

[0137] Compared with Example 1, the only difference is that in step (1), a magnetic stirring step is used instead of ball milling.

[0138] Comparative Example 4

[0139] Compared with Example 1, the only difference is that no spraying treatment is performed, but the slurry is placed in an oven for treatment for 12 hours.

[0140] Comparative Example 5

[0141] Compared with Example 1, the only difference is that the outlet temperature of the spray treatment in step (1) is 120°C.

[0142] Comparative Example 6

[0143] Compared with Example 1, the only difference is that the sintering temperature is 400°C.

[0144] Comparative Example 7

[0145] Compared with Example 1, the only difference is that the sintering atmosphere in step (4) is O2.

[0146] Performance data:

[0147] 1. Air stability test

[0148] The cyclic stability data of the lithium supplement additives prepared in each case after being stored in an air atmosphere at a temperature of 30°C and a humidity of 30% for different periods of time are shown in Table 1:

[0149] Table 1

[0150]

[0151]

[0152] Note: The 0h mentioned above refers to materials not exposed to air, for example, materials stored under Ar atmosphere.

[0153] 2. Full battery cycle data:

[0154] The test results are shown in Table 2:

[0155] Table 2

[0156]

[0157] It can be seen from the embodiments and comparative examples that the technical solution of the present invention can effectively improve stability and improve its cyclic stability under air resistance.

Claims

1. A NC@Li5FeO4 composite lithium supplement additive, characterized in that: It has a core-shell structure, wherein the core is Li5FeO4 and the shell is N-doped amorphous carbon, and the shell is also doped with silicon; In the shell, the content of N is 0.1-2 atm%; The preparation method of the NC@Li5FeO4 composite lithium supplement additive comprises the following steps: Step (1): Ball milling and activating a mixed solution comprising a carbon source, a nitrogen source, a silicate, a lithium source, an iron source, and a surfactant to obtain a slurry; wherein the weight ratio of the carbon source to the iron source is 0.3-0.8:1; The weight ratio of the nitrogen source to the iron source is 0.05~0.2:1; The weight ratio of silicate to iron source is 0.01~0.1:1; The molar ratio of Li:Fe in the lithium source and the iron source is 4.5~9.5:1; Step (2): The slurry is sprayed to obtain a precursor; wherein the spraying temperature is 160-200°C; Step (3): The precursor is sintered in a protective atmosphere at a temperature of 600-900° C. to obtain the NC@Li5FeO4 composite lithium supplement additive.

2. The NC@Li5FeO4 composite lithium supplement additive according to claim 1, characterized in that The mass ratio of core to shell is 3~5:95~97.

3. A method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 1 or 2, characterized in that: The following steps are involved: Step (1): Ball milling and activating a mixed solution comprising a carbon source, a nitrogen source, a silicate, a lithium source, an iron source, and a surfactant to obtain a slurry; wherein the weight ratio of the carbon source to the iron source is 0.3-0.8:1; The weight ratio of the nitrogen source to the iron source is 0.05~0.2:1; The weight ratio of silicate to iron source is 0.01~0.1:1; The molar ratio of Li:Fe in the lithium source and the iron source is 4.5~9.5:1; Step (2): The slurry is sprayed to obtain a precursor; wherein the spraying temperature is 160-200°C; Step (3): The precursor is sintered in a protective atmosphere at a temperature of 600-900° C. to obtain the NC@Li5FeO4 composite lithium supplement additive.

4. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The carbon source is carbon-containing organic matter.

5. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 4, wherein: The carbon source is at least one of citric acid, glucose, sucrose, acetic acid and p123.

6. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The N source is a nitrogen-containing organic matter with an N:C ratio greater than or equal to 1.

7. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The nitrogen source is at least one of urea, melamine, dicyandiamide and corn steep liquor.

8. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The silicate is at least one of tetramethyl silicate, tetraethyl silicate, tetrapropyl silicate and tetrabutyl silicate.

9. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The lithium source is at least one of Li2O, Li2CO3, LiF, Li3PO4, and Li2C2O4.

10. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The Fe source is at least one of Fe2O3, Fe3O4, iron powder and FeC2O4.

11. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The surfactant is at least one of dodecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, stearic acid, sodium dodecylbenzenesulfonate and polyvinylpyrrolidone.

12. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The solvent in the mixed solution is at least one of water and alcohol.

13. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The added amount of surfactant is 2-10 wt% of the iron source.

14. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The carbon source, the nitrogen source and the silicate are pre-ball milled and activated, and then mixed with the lithium source and the iron source and ball milled and activated to prepare the slurry.

15. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The ball mill speed is 200~400r / min; The ball milling time is 1~20h.

16. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The feed rate during the spray treatment process is 10~30ml / min.

17. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The protective atmosphere is at least one of nitrogen and inert gas.

18. The method for preparing the NC@Li5FeO4 composite lithium supplement additive according to claim 3, wherein: The sintering time is 20~48h.

19. Use of the NC@Li5FeO4 composite lithium supplement additive according to any one of claims 1 to 2 or the NC@Li5FeO4 composite lithium supplement additive prepared by the preparation method according to any one of claims 3 to 18, characterized in that: It is used to prepare lithium-ion batteries.

20. The use according to claim 19, characterized in that It is used to prepare the positive electrode of lithium-ion batteries.

21. The use according to claim 20, characterized in that It is used to prepare positive electrode materials for lithium-ion batteries.

22. A lithium-ion battery lithium-supplementing positive electrode material, characterized in that: The invention comprises a positive electrode active material and the NC@Li5FeO4 composite lithium supplement additive according to any one of claims 1 to 2 or the NC@Li5FeO4 composite lithium supplement additive prepared by the preparation method according to any one of claims 3 to 18.

23. The lithium-ion battery lithium-supplementing positive electrode material according to claim 22, wherein: The positive electrode active material is at least one of LiCoO2, LiFePO4 and NCM ternary materials.

24. The lithium-ion battery lithium-supplementing positive electrode material according to claim 22, wherein: Also contains a conductive agent and a binder.

25. The lithium-ion battery lithium-supplementing positive electrode material according to claim 24, characterized in that The weight percentage of the NC@Li5FeO4 composite lithium supplement additive is 2-15%; The weight percentage of the conductive agent is 1~15%; The weight percentage of the binder is 1~15%; The balance is positive electrode active material.

26. The lithium-ion battery lithium-supplementing positive electrode material according to claim 25, characterized in that The weight percentage of the NC@Li5FeO4 composite lithium supplement additive is 2-15%; The weight percentage of the conductive agent is 5~10%; The weight percentage of the binder is 5~10%; The balance is positive electrode active material.

27. A lithium ion battery, characterized in that: The invention comprises the NC@Li5FeO4 composite lithium supplement additive according to any one of claims 1 to 2 or the NC@Li5FeO4 composite lithium supplement additive prepared by the preparation method according to any one of claims 3 to 18.

28. The lithium ion battery according to claim 27, wherein Comprising the lithium-supplementing positive electrode material according to any one of claims 22 to 26.

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