A positive electrode lithium supplementing agent coated with carbon and nitrogen layers, and a preparation method and application thereof

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to a carbon-nitrogen double-layer coated positive electrode lithium supplementing agent and a preparation method and application thereof. The preparation method of the carbon-nitrogen double-layer coated positive electrode lithium supplementing agent provided by the application is as follows: a carbon source such as sucrose, a nitrogen source such as melamine, and an iron source such as nano-iron oxide are uniformly mixed, and then one-step sintering is performed to make the mixture dehydrate; then in the second-step sintering, a lithium source such as pitch and lithium oxide (Li2O) is introduced to generate lithium ferrite (Li5FeO4), which effectively avoids water generated by dehydration of the carbon source such as sucrose and impurities generated by the reaction of lithium ferrite, improves the purity and performance of the positive electrode lithium supplementing agent, and meanwhile, the carbon-nitrogen double-layer coating can effectively coat the lithium ferrite and introduce nitrogen, thereby improving the stability, ion diffusivity and conductivity of the positive electrode lithium supplementing agent, so as to solve the technical problem of poor performance of the lithium manganate positive electrode lithium supplementing agent prepared by the existing preparation method.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a carbon-nitrogen double-layer coated positive electrode lithium supplementing agent and a preparation method and application thereof. BACKGROUND

[0002] With the development of technology, lithium ion batteries are used in digital products, new energy automobile batteries and energy storage of wind and solar power plants, and have broad application prospects and market demand. However, in the process of charging and discharging of lithium ion batteries, a negative electrode SEI film is formed on the surface of the electrode material of the lithium ion battery. When the negative electrode material used in the lithium ion battery is a high-capacity silicon-based negative electrode material, a large amount of active lithium is consumed, resulting in low first-week coulomb efficiency and battery capacity of the lithium ion battery. Research has found that the problem can be effectively alleviated by adding a lithium supplementing agent. At present, the irreversible active lithium loss caused by SEI growth can be alleviated by two ways, i.e. a negative electrode lithium supplementing agent and a positive electrode lithium supplementing agent, to improve the performance of the lithium ion battery.

[0003] Compared with the negative electrode lithium supplementing agent, the positive electrode lithium supplementing agent has the advantages of easy synthesis and low price, can be directly added in the homogenization process of the positive electrode slurry, does not need additional process improvement and has low cost, and is thus more suitable for the current lithium ion battery manufacturing process and has better application prospects. In the positive electrode lithium supplementing agent, lithium ferrite (Li5FeO4) is a mainstream positive electrode lithium supplementing agent due to its high specific capacity (theoretical 867 mAh / g) and suitable delithiation voltage (3.5-4.7 V). However, the lithium ferrite has poor conductivity and air stability. At room temperature, a small amount of water in the air will decompose to form lithium compound impurities, the purity of the lithium ferrite decreases, and the performance of the lithium ferrite decreases and the polarization increases. To improve the performance of the lithium ferrite, the main preparation method at present is to ball mill Li2O as a lithium source, iron oxide as an iron source and sucrose as a carbon source uniformly and then perform high-temperature calcination to obtain lithium ferrite positive electrode lithium supplementing agent with a core-shell structure. The carbon shell layer may also be doped with nitrogen, sulfur and silicon elements. However, high-temperature dehydration of the sucrose and other carbon sources will affect the performance of the lithium ferrite. In addition, the lithium ferrite positive electrode lithium supplementing agent prepared may have the problems of being unable to be effectively coated by the shell layer and uneven particle size distribution of the lithium ferrite positive electrode lithium supplementing agent. Therefore, it is necessary to further study the preparation method of the lithium ferrite to synthesize lithium ferrite positive electrode lithium supplementing agent with better performance. SUMMARY

[0004] Therefore, the application provides a carbon-nitrogen double-layer coated lithium ferrite positive electrode lithium supplementing agent and a preparation method and application thereof to solve the technical problem of poor performance of the lithium ferrite positive electrode lithium supplementing agent prepared by the existing preparation method.

[0005] The first aspect of the application provides a preparation method of a carbon-nitrogen double-layer coated lithium ferrite positive electrode lithium supplementing agent, which comprises the following steps:

[0006] Step S1, mix the carbon source, nitrogen source and iron source uniformly to obtain a mixture;

[0007] Step S2, sinter the mixture to obtain carbon-nitrogen coated iron oxide;

[0008] Step S3, mix the carbon-nitrogen coated iron oxide, lithium source and pitch uniformly to obtain a positive electrode lithium supplement precursor;

[0009] Step S4, two-step sintering of the positive electrode lithium supplement precursor to obtain a carbon-nitrogen double-layer coated positive electrode lithium supplement.

[0010] Preferably, in step S1, the step of mixing uniformly comprises:

[0011] Step S11, ball milling the carbon source, nitrogen source and iron source with anhydrous ethanol as a dispersant to obtain a ball-milled material;

[0012] Step S12, evaporating the dispersant during stirring to obtain a dry ball-milled material;

[0013] Step S13, grinding and mixing the dry ball-milled material uniformly to obtain a mixture.

[0014] Preferably, in step S11, the ball-to-material ratio of the ball milling is 20-60:1, the rotation speed is 200-1000 r / min, and the ball milling time is 1-5 h;

[0015] In step S12, the temperature for evaporating the dispersant is 60-80°C.

[0016] Preferably, in step S2, the temperature for one-step sintering is 300-500°C, and the time is 4-8 h.

[0017] Preferably, in step S3, the step of mixing uniformly comprises grinding and mixing the carbon-nitrogen coated iron oxide, lithium source and pitch uniformly.

[0018] Preferably, in step S4, the temperature for two-step sintering is 400-700°C, and the time is 10-30 h.

[0019] Preferably, in step S1, the carbon source is selected from at least one of sucrose, glucose and citric acid;

[0020] The nitrogen source is selected from at least one of melamine, dicyandiamide and urea;

[0021] The iron source is selected from at least one of nano-iron oxide, nano-magnetite and nano-ferrous oxide;

[0022] Preferably, in step S3, the lithium source is selected from at least one of lithium oxide, lithium hydroxide and lithium carbonate.

[0023] Preferably, the mass ratio of the carbon source, the nitrogen source, the iron source, the lithium source, and the pitch is 0.1-0.2:0.1-0.2:0.5:0.78:0.1-0.2 in mass parts.

[0024] The second aspect of the present application provides a positive electrode lithium supplement of lithium iron oxide coated with carbon and nitrogen, which is prepared by the preparation method of the first aspect.

[0025] The third aspect of the present application provides a positive electrode material of a lithium ion battery, which comprises the positive electrode lithium supplement of lithium iron oxide coated with carbon and nitrogen of the second aspect, a positive electrode material, a conductive agent, and a binder.

[0026] Preferably, the positive electrode material is at least one selected from lithium nickel cobalt manganese oxide, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, and lithium iron phosphate.

[0027] The conductive agent is at least one selected from carbon black, conductive graphite, and carbon nanotubes.

[0028] The binder is selected from polyvinylidene fluoride.

[0029] The fourth aspect of the present application provides a positive electrode of a lithium ion battery, which comprises the positive electrode material of a lithium ion battery of the third aspect and a current collector.

[0030] The positive electrode material of a lithium ion battery is coated on the surface of the current collector.

[0031] The fifth aspect of the present application provides a lithium ion battery, which comprises the positive electrode of a lithium ion battery of the fourth aspect, a negative electrode, a separator, and an electrolyte.

[0032] The sixth aspect of the present application provides the application of the lithium ion battery of the fifth aspect in digital products, new energy vehicles, and wind and solar power plants.

[0033] In summary, the application provides a positive electrode lithium supplementing agent coated with carbon-nitrogen double layers of lithium iron oxide and a preparation method and application thereof. The preparation method comprises the following steps: firstly, uniformly mixing a carbon source such as sucrose, a nitrogen source such as melamine, and an iron source such as nano-iron oxide, and then sintering to obtain iron oxide coated with carbon-nitrogen, wherein the carbon-nitrogen coating is incomplete; and then, uniformly mixing a lithium source such as lithium oxide and a carbon-nitrogen source such as pitch, and then sintering again to obtain the positive electrode lithium supplementing agent coated with carbon-nitrogen double layers of lithium iron oxide. In the preparation process of the positive electrode lithium supplementing agent, the carbon source such as sucrose is sintered, carbonized and dehydrated in advance, and then the lithium source such as lithium oxide and the raw material such as pitch are added for sintering, thereby avoiding the water generated by dehydration of the carbon source such as sucrose and impurities generated by reaction of lithium iron oxide, and improving the purity and performance of the positive electrode lithium supplementing agent. Meanwhile, the pitch added in the sintering can control the particle size of the generated positive electrode lithium supplementing agent, promote the growth of lithium iron oxide within the coating framework of CN, and is conducive to the formation of a dense coating layer, thereby improving the stability and other performances of the positive electrode lithium supplementing agent. In addition, the nitrogen source introduced in the sintering can also improve the conductivity and other performances of the positive electrode lithium supplementing agent. Thus, the technical problem of poor performance of the lithium manganese oxide positive electrode lithium supplementing agent prepared by the existing preparation method is solved. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the specific embodiments or prior art of the application, the drawings needed in the description of the specific embodiments or prior art will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0035] Figure 1 XRD diagram of the carbon-nitrogen double-layer coated positive electrode lithium supplementing agent (Li5FeO4@CN composite material) prepared by the preparation method provided in Embodiments 1-2 of the application;

[0036] Figure 2 Stability test result diagram of the carbon-nitrogen double-layer coated positive electrode lithium supplementing agent prepared by the preparation method provided in Embodiment 1 of the application, the positive electrode lithium supplementing agent without carbon-nitrogen coating prepared by the preparation method provided in Comparative Example 1, and the carbon-nitrogen single-layer coated positive electrode lithium supplementing agent prepared by the preparation method provided in Comparative Example 2;

[0037] Figure 3 Coating performance test result diagram of the carbon-nitrogen double-layer coated positive electrode lithium supplementing agent prepared by the preparation method provided in Embodiment 1 of the application, the positive electrode lithium supplementing agent without carbon-nitrogen coating prepared by the preparation method provided in Comparative Example 1, and the carbon-nitrogen single-layer coated positive electrode lithium supplementing agent prepared by the preparation method provided in Comparative Example 2;

[0038] Figure 4The ion diffusion performance test results of the carbon-nitrogen double-layer coated positive electrode lithium supplement agent prepared by the preparation method provided in Embodiment 1 of the present application, the positive electrode lithium supplement agent without carbon-nitrogen coating prepared by the preparation method provided in Comparative Example 1, and the carbon-nitrogen single-layer coated positive electrode lithium supplement agent prepared by the preparation method provided in Comparative Example 2 are shown in the following figure. DETAILED DESCRIPTION

[0039] The present application provides a carbon-nitrogen double-layer coated lithium iron oxide positive electrode lithium supplement agent, a preparation method and an application thereof, which are used to solve the technical problem of poor performance of lithium iron oxide positive electrode lithium supplement agents prepared by existing preparation methods.

[0040] The technical solutions of the present application will be described clearly and completely below with reference to the accompanying drawings. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0041] In view of the defect that the lithium iron oxide positive electrode lithium supplement agents prepared by existing preparation methods have poor performance, the present application provides a preparation method of a carbon-nitrogen double-layer coated lithium iron oxide (Li5FeO4) positive electrode lithium supplement agent. The raw materials used in the preparation method of the positive electrode lithium supplement agent include common carbon sources such as sucrose, nitrogen sources such as melamine, iron sources such as nano-iron oxide, lithium sources such as lithium oxide (Li2O), and pitch as a carbon-nitrogen source. The preparation process of the positive electrode lithium supplement agent is to mix the carbon sources such as sucrose, the nitrogen sources such as melamine, and the iron sources such as nano-iron oxide uniformly and then sinter in one step, so that the carbon sources such as sucrose and the nitrogen sources such as melamine are carbonized and dehydrated to obtain Fe2O3@CN composite material, i.e., carbon-nitrogen coated iron oxide. Then, the lithium sources such as lithium oxide (Li2O), pitch, and the carbon-nitrogen coated iron oxide are mixed uniformly and then sintered in two steps to obtain the carbon-nitrogen double-layer coated positive electrode lithium supplement agent (Li5FeO4@CN composite material), in which carbon-nitrogen is the shell layer and Li5FeO4 is the core layer. In the preparation process, the carbon sources such as sucrose have been dehydrated in the one-step sintering process, and lithium iron oxide (Li5FeO4) is generated in the two-step sintering process, thereby avoiding the water generated by dehydration and impurities generated by the reaction of lithium iron oxide, and improving the purity and performance of the positive electrode lithium supplement agent.

[0042] Meanwhile, the coating frame of CN formed by one-step sintering coats Fe2O3 and other iron oxides, so that the pitch added in the two-step sintering can promote the continuous growth of lithium ferrite (Li5FeO4) within the coating frame of CN, control the particle size of the positive electrode lithium supplement agent, and form a dense coating layer; the one-step sintering and the two-step sintering can introduce nitrogen sources through melamine and other nitrogen sources and pitch, so that the shell of the positive electrode lithium supplement agent contains nitrogen, the nitrogen atom has high rich electricity, the introduction of nitrogen into the carbon framework not only increases the smoothness of electron transfer, but also provides a large number of defects and active sites, reduces the delithiation energy barrier of the material, promotes the rapid migration of lithium ions on the interface between the coating surface and the active material, can effectively improve the interface charge transfer, and improves the diffusion ability of lithium ions in the interface, so as to obtain excellent ion diffusion performance, and improve the conductivity of the positive electrode lithium supplement agent and the lithium ion battery.

[0043] As preferred, for the reaction conditions in the preparation method of the positive electrode lithium supplement agent provided in the application, the temperature of one-step sintering is controlled at 300-500℃, and the time is controlled at 4-8h; the temperature of two-step sintering is controlled at 400-700℃, and the time is controlled at 10-30h, and the heating rate in the sintering process is controlled at 2-6℃ / min.

[0044] As preferred, for the amount of the reaction raw materials in the preparation method of the positive electrode lithium supplement agent provided in the application, the mass ratio of the carbon source such as sucrose, the nitrogen source such as melamine, the iron source such as nano iron oxide, the lithium source such as lithium oxide (Li2O), and the pitch is controlled at 0.1-0.2:0.1-0.2:0.5:0.78:0.1-0.2, and the addition amount of the lithium source such as lithium oxide (Li2O) is excessive by 5%-10% to compensate for the volatilization of lithium during sintering.

[0045] As preferred, for the application of the positive electrode lithium supplement agent prepared by the preparation method provided in the application, the positive electrode lithium supplement agent can be used to configure a positive electrode material of a lithium ion battery with better performance, or further used to prepare a lithium ion battery positive electrode and a lithium ion battery; and the lithium ion battery with better performance can also be used in a wind-solar power plant as an energy storage device.

[0046] Example 1

[0047] The application example 1 provides a preparation method of a carbon-nitrogen double-layer coated lithium ferrite positive electrode lithium supplement agent, which comprises raw material preparation, mixture preparation, one-step sintering to prepare carbon-nitrogen coated iron oxide, preparation of a positive electrode lithium supplement agent precursor, and two-step sintering to prepare a carbon-nitrogen double-layer coated positive electrode lithium supplement agent.

[0048] The raw material preparation step includes: weighing sucrose, melamine, nano-sized iron oxide, lithium oxide (Li2O) and pitch at a mass ratio of 0.15:0.15:0.5:0.78:0.2, respectively, 1.5g, 1.5g, 5.0g, 7.8g, 2.0g.

[0049] The step of configuring the mixed material includes: mixing the weighed sucrose, melamine and nano-sized iron oxide, ball milling with 20ml of anhydrous ethanol as a dispersant, a ball-to-material ratio of 20:1, a rotation speed of 400r / min, and a ball milling time of 4h; then heating the mixed material in an oil bath at 65℃ to evaporate the dispersant, continuously stirring during the evaporation, mixing the mixed material uniformly by evaporation, and then grinding the mixed material into powder to obtain the mixed material uniformly mixed.

[0050] The step of one-step sintering to prepare the carbon-nitrogen coated iron oxide includes: feeding the mixed material into a tube furnace, calcining in an argon atmosphere, a calcining temperature of 450℃, a calcining time of 5h, and a temperature rising rate of 5℃ / min, to obtain the Fe2O3@CN composite material.

[0051] The step of configuring the positive electrode lithium supplement agent precursor includes: mixing and grinding the Fe2O3@CN composite material, lithium oxide (Li2O) and pitch to obtain the positive electrode lithium supplement agent precursor.

[0052] The step of two-step sintering to prepare the carbon-nitrogen double-layer coated positive electrode lithium supplement agent includes: feeding into a tube furnace, calcining in an argon atmosphere, a calcining temperature of 650℃, a temperature rising rate of 5℃ / min, and a calcining time of 15h, and finally cooling in the furnace to obtain the carbon-nitrogen double-layer coated positive electrode lithium supplement agent (Li5FeO4@CN composite material).

[0053] Example 2

[0054] The example 2 of the present application provides a preparation method of the carbon-nitrogen double-layer coated lithium iron oxide positive electrode lithium supplement agent, and the preparation method includes the steps of raw material preparation, mixed material configuration, one-step sintering to prepare carbon-nitrogen coated iron oxide, configuration of the positive electrode lithium supplement agent precursor, and two-step sintering to prepare the carbon-nitrogen double-layer coated positive electrode lithium supplement agent.

[0055] The raw material preparation step includes: weighing sucrose, melamine, nano-sized iron oxide, lithium oxide (Li2O) and pitch at a mass ratio of 0.2:0.1:0.5:0.78:0.15, respectively, 2.0g, 1.0g, 5.0g, 7.8g, 1.5g.

[0056] The step of configuring the mixed material includes: mixing the weighed sucrose, melamine and nanoscale iron oxide, ball milling with 20ml of anhydrous ethanol as a dispersant, a ball-to-material ratio of 40:1, a rotating speed of 500r / min, and a ball milling time of 3h; then, the mixed material after ball milling is heated in an oil bath at 70℃ to evaporate the dispersant, the mixture is stirred constantly during evaporation, the mixture is mixed uniformly through evaporation, and then the mixture is ground into powder to obtain the mixed material uniformly mixed.

[0057] The step of preparing the carbon-nitrogen coated iron oxide through one-step sintering includes: feeding the mixed material into a tube furnace, calcining in an argon atmosphere, a calcining temperature of 400℃, a calcining time of 6h, and a temperature rising rate of 5℃ / min to obtain the Fe2O3@CN composite material.

[0058] The step of configuring the positive electrode lithium supplement agent precursor includes: mixing and grinding the Fe2O3@CN composite material, lithium oxide (Li2O) and pitch to obtain the positive electrode lithium supplement agent precursor.

[0059] The step of preparing the carbon-nitrogen double-layer coated positive electrode lithium supplement agent through two-step sintering includes: feeding into a tube furnace, calcining in an argon atmosphere, a calcining temperature of 450℃, a temperature rising rate of 5℃ / min, a calcining time of 20h, and finally cooling in the furnace to obtain the carbon-nitrogen double-layer coated positive electrode lithium supplement agent (Li5FeO4@CN composite material).

[0060] Embodiment 3

[0061] The embodiment 3 of the present application provides a preparation method of the carbon-nitrogen double-layer coated lithium iron oxide positive electrode lithium supplement agent, and the preparation method includes raw material preparation, mixed material configuration, one-step sintering to prepare carbon-nitrogen coated iron oxide, positive electrode lithium supplement agent precursor configuration, and two-step sintering to prepare carbon-nitrogen double-layer coated positive electrode lithium supplement agent.

[0062] The step of raw material preparation includes: weighing sucrose, melamine, nanoscale iron oxide, lithium oxide (Li2O) and pitch according to a mass ratio of 0.10:0.2:0.5:0.78:0.1, 1.0g, 2.0g, 5.0g, 7.8g and 1.0g respectively.

[0063] The step of configuring the mixed material includes: mixing the weighed sucrose, melamine and nanoscale iron oxide, ball milling with 20ml of anhydrous ethanol as a dispersant, a ball-to-material ratio of 40:1, a rotating speed of 500r / min, and a ball milling time of 3h; then, the mixed material after ball milling is heated in an oil bath at 70℃ to evaporate the dispersant, the mixture is stirred constantly during evaporation, the mixture is mixed uniformly through evaporation, and then the mixture is ground into powder to obtain the mixed material uniformly mixed.

[0064] The step of preparing the carbon-nitrogen coated iron oxide by one-step sintering comprises: feeding the mixed material into a tube furnace, calcining in an argon atmosphere, the calcining temperature is 450 DEG C, the calcining time is 5h, the heating rate is 5 DEG C / min, and Fe2O3@CN composite material is obtained.

[0065] The step of configuring the positive electrode lithium supplement agent precursor comprises: mixing and grinding Fe2O3@CN composite material, lithium oxide (Li2O) and pitch to obtain a positive electrode lithium supplement agent precursor.

[0066] The step of preparing the carbon-nitrogen double-layer coated positive electrode lithium supplement agent by two-step sintering comprises: feeding into a tube furnace, calcining in an argon atmosphere, the calcining temperature is 650 DEG C, the heating rate is 5 DEG C / min, the calcining time is 18h, and finally the furnace is cooled to obtain the carbon-nitrogen double-layer coated positive electrode lithium supplement agent (Li5FeO4@CN composite material).

[0067] Comparative Example 1

[0068] The present application provides a preparation method of a carbon-nitrogen uncoated positive electrode lithium supplement agent, the preparation method comprising: uniformly grinding 7.8g of nanoscale iron oxide and 1.0g of lithium oxide (Li2O), feeding into a tube furnace, calcining in an argon atmosphere, the calcining temperature is 450 DEG C, the calcining time is 5h, and the carbon-nitrogen uncoated positive electrode lithium supplement agent Li5FeO4 is obtained.

[0069] Comparative Example 2

[0070] The present application provides a preparation method of a carbon-nitrogen single-layer coated positive electrode lithium supplement agent, the preparation method comprising raw material preparation, configuration of mixed material, and sintering preparation of carbon-nitrogen coated iron oxide.

[0071] The step of raw material preparation comprises: weighing sucrose, melamine, nanoscale iron oxide and lithium oxide (Li2O) according to the mass ratio of 0.10:0.2:0.5:0.78, 1.0g, 2.0g, 5.0g and 7.8g respectively.

[0072] The step of configuring the mixed material comprises: mixing the weighed sucrose, melamine, nanoscale iron oxide and lithium oxide (Li2O), ball milling with 20ml of anhydrous ethanol as dispersant, the ball-to-material ratio is 30:1, the rotation speed is 450r / min, and the ball milling time is 5h; then the mixed material after ball milling is heated in an oil bath at 80 DEG C to evaporate the dispersant, the mixed material is mixed uniformly by evaporation, and then it is ground into powder to obtain the mixed material uniformly mixed.

[0073] The steps for preparing carbon-nitrogen coated iron oxide by sintering include: feeding the mixture into a tube furnace and calcining it in an argon atmosphere at a calcination temperature of 450℃ for 5 hours at a heating rate of 5℃ / min to obtain a carbon-nitrogen monolayer coated positive electrode lithium supplement Li5FeO4@CN composite material.

[0074] Test Example 1

[0075] This application tests the carbon-nitrogen bilayer coated positive electrode lithium replenisher, the carbon-nitrogen monolayer coated positive electrode lithium replenisher, and the positive electrode lithium replenisher without carbon-nitrogen coating obtained in the examples and comparative examples.

[0076] Among them, the carbon-nitrogen bilayer coated positive electrode lithium supplementer obtained in Examples 1-2 was tested using X-ray diffraction, and the test results are as follows: Figure 1 As shown, from Figure 1 As can be seen from the XRD pattern shown, the preparation method provided in this application can prepare a carbon-nitrogen bilayer coated positive electrode lithium replenishing agent (Li5FeO4@CN composite material).

[0077] To investigate the technical effects of no coating layer, carbon-nitrogen monolayer coating, and carbon-nitrogen double-layer coating, this application conducted performance tests. The performance test process was as follows: the carbon-nitrogen double-layer coated positive electrode lithium replenishing agents, carbon-nitrogen monolayer coated positive electrode lithium replenishing agents, and the carbon-nitrogen uncoated positive electrode lithium replenishing agents provided in Example 1 and Comparative Examples 1-2 were directly assembled into lithium-ion batteries. The carbon-nitrogen double-layer coated positive electrode lithium replenishing agents, carbon-nitrogen monolayer coated positive electrode lithium replenishing agents, and the carbon-nitrogen uncoated positive electrode lithium replenishing agents were exposed to air with 40% humidity for 24 hours before being assembled into lithium-ion batteries for charge-discharge tests. The performance test results are as follows: Figure 2 As shown, from Figure 2 It is evident that after 24 hours of exposure to air with 40% humidity, the lithium-ion battery assembled with the uncoated positive electrode lithium replenisher exhibits significant capacity decay, while the capacity decay of the single-layer carbon-nitrogen coated positive electrode lithium replenisher is moderate, and the capacity decay of the double-layer carbon-nitrogen coated positive electrode lithium replenisher is relatively small. This indicates that the preparation method provided in this application, by first using carbon sources such as sucrose and nitrogen sources such as melamine to prepare carbon-nitrogen coating, and then introducing pitch to prepare a double-layer carbon-nitrogen coated positive electrode lithium replenisher, can achieve effective coating of lithium ferrite positive electrode lithium replenisher. This improves the stability of the positive electrode lithium replenisher exposed to humid air, and makes it difficult for the Li5FeO4 coated by the double carbon-nitrogen layer to decompose and form lithium compound impurities, which is beneficial for maintaining the performance of the positive electrode lithium replenisher during long-term storage.

[0078] Meanwhile, in order to explore the technical effects of the carbon source such as sucrose in the preparation method provided in the present application dehydrating in the one-step sintering process and generating lithium ferrite (Li5FeO4) in the two-step sintering process, the present application performs performance testing, and the performance testing process is as follows: the carbon-nitrogen double-layer coated positive lithium supplement agent provided in Example 1, the carbon-nitrogen single-layer coated positive lithium supplement agent provided in Comparative Example 1-2 and the positive lithium supplement agent without carbon-nitrogen coating are directly assembled into lithium ion batteries for charge-discharge testing, and the addition amount of the positive lithium supplement agent is 10% of the ternary 811NCM positive material; and the performance testing results are as shown in Table 1. Figure 3 Figure 3 It can be seen that the lithium ion battery assembled by the carbon-nitrogen single-layer coated positive lithium supplement agent provided in Comparative Example 2 has a lower capacity, and the lithium ion battery assembled by the carbon-nitrogen double-layer coated positive lithium supplement agent provided in Example 1 has a higher capacity, which indicates that the water molecules generated by the high-temperature dehydration of the carbon source such as sucrose react with lithium ferrite to generate impurities, thereby reducing the capacity of the lithium ion battery assembled by the positive lithium supplement agent.

[0079] Further, in order to explore the influence of nitrogen doping on ion diffusion and conductivity, the present application performs performance testing, and the performance testing process is as follows: the carbon-nitrogen double-layer coated positive lithium supplement agent provided in Example 1, the carbon-nitrogen single-layer coated positive lithium supplement agent provided in Comparative Example 1-2 and the positive lithium supplement agent without carbon-nitrogen coating are assembled into lithium ion button cells for alternating current impedance testing, which is an important means for studying the process dynamics and surface phenomena of electrodes at present, the data obtained by alternating current impedance testing is calculated according to the formula, and the lithium ion diffusion coefficient D is as shown in Table 1:

[0080]

[0081] In the formula, R is the gas constant value, which is 8.314 J / (mol·K);

[0082] T is the thermodynamic temperature, T=t+273.15℃, and the normal temperature 25℃ is 298.15K;

[0083] F is the Faraday constant, which is 96485.33289±0.00059 C / mol;

[0084] A is the electrode surface area, and the button cell CR2032 is used, and the electrode area is 1.76 cm 2 ;

[0085] C is the molar concentration of Li + , which is 2.33*10 -2 mol / cm 3 ;

[0086] ω=2Πf (Π is approximately 3.14, and f is the frequency of impedance);

[0087] ​ is the slope of the Warburg impedance coefficient (i.e. Figure 4

[0088]

[0089] Table 1

[0090] As can be seen from Table 1, the lithium ion diffusion coefficient of the lithium ion battery assembled by the carbon-nitrogen uncoated positive electrode lithium supplement agent, the lithium ion diffusion coefficient of the carbon-nitrogen single-layer coated positive electrode lithium supplement agent can be improved, and the lithium ion diffusion coefficient of the carbon-nitrogen double-layer coated positive electrode lithium supplement agent is improved, which shows that the preparation method provided by the present application can realize effective coating of the lithium supplement agent of the lithium iron oxide positive electrode by first preparing the carbon-nitrogen coating by using sucrose as a carbon source and melamine as a nitrogen source, and then introducing pitch to prepare the carbon-nitrogen double-layer coated positive electrode lithium supplement agent, and more doped nitrogen is introduced, the lithium ion diffusion coefficient of the positive electrode lithium supplement agent is improved, which is conducive to improving the conductivity of the positive electrode lithium supplement agent.

[0091] The above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the above embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.​

Claims

1. A preparation method of a positive electrode lithium supplement agent of lithium iron phosphate coated with carbon and nitrogen double layers, characterized in that, The method comprises the steps of: S1, mixing a carbon source, a nitrogen source and an iron source to obtain a mixture, wherein the carbon source is at least one selected from sucrose, glucose and citric acid; S2, sintering the mixture to obtain carbon-nitrogen coated iron oxide; S3, mixing the carbon-nitrogen coated iron oxide, a lithium source and pitch to obtain a positive electrode lithium supplement precursor; S4, two-step sintering the positive electrode lithium supplement precursor to obtain a carbon-nitrogen double-layer coated positive electrode lithium supplement; In S2, the one-step sintering is performed at a temperature of 300-500°C for 4-8h; In S4, the two-step sintering is performed at a temperature of 400-700°C for 10-30h.

2. The preparation method of the positive electrode lithium supplementing agent with carbon-nitrogen double-layer coated lithium iron phosphate according to claim 1, characterized in that; The nitrogen source is at least one selected from melamine, dicyandiamide and urea; The iron source is at least one selected from nano-iron oxide, nano-magnetite and nano-ferrous oxide; The lithium source is at least one selected from lithium oxide, lithium hydroxide and lithium carbonate.

3. The preparation method of the positive electrode lithium supplementing agent of carbon-nitrogen double-layer coated lithium iron phosphate according to claim 2, characterized in that, The mass ratio of the carbon source, the nitrogen source, the iron source, the lithium source and the pitch is 0.1-0.2:0.1-0.2:0.5:0.78:0.1-0.

2.

4. A positive electrode lithium supplementing agent in which lithium iron phosphate is coated with a carbon-nitrogen double layer, characterized by The positive electrode lithium supplement is prepared by the method of any one of claims 1-3.

5. A lithium-ion battery cathode material, characterized in that, The positive electrode lithium supplement, a positive electrode material, a conductive agent and a binder are mixed uniformly. The positive electrode lithium supplement, a positive electrode material, a conductive agent and a binder are mixed uniformly.

6. A lithium-ion battery cathode, characterized by, The positive electrode material and the current collector are mixed uniformly. The positive electrode material is coated on the surface of the current collector.

7. A lithium-ion battery, characterized by The positive electrode of the lithium ion battery is the positive electrode material of claim 6.

8. The lithium ion battery of claim 7 is applied in digital products, new energy vehicles and wind-solar power plants.

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