Iron-based composite lithium supplementing material and preparation method thereof

Abstract

The invention discloses an iron-based composite lithium supplementing material which comprises a lithium-rich lithium ferrite matrix and a coating layer coating the lithium-rich lithium ferrite matrix, the coating layer sequentially comprises a first coating layer, a second coating layer and a third coating layer from inside to outside, the first coating layer is made of carbon, the second coating layer is made of carbon, and the third coating layer is made of carbon. The second coating layer comprises one or more of lithium borate, lithium tungstate and lithium silicate, and the third coating layer is carbon. The invention also provides a preparation method of the iron-based composite lithium supplementing material. According to the iron-based composite lithium supplementing material and the preparation method thereof, a fluorine-doped three-stage coating structure is adopted, the characteristic of high lithium supplementing capacity of lithium-rich lithium ferrite is fully played, the defects of poor intrinsic conductivity, high residual lithium, serious gas production, circulation deterioration and the like of the lithium-rich lithium ferrite are overcome, lower metal dissolution and lower gas production are realized, and the lithium supplementing capacity of the lithium-rich lithium ferrite is improved. And the capacity and the long-term cycle retention rate of the battery are further ensured.

Description

Technical Field

[0001] The invention belongs to the field of battery materials, and in particular relates to a lithium supplement and a preparation method thereof. Background Art

[0002] During the first charge and discharge process of lithium-ion batteries, the formation of SEI film on the surface of the negative electrode will permanently consume the active lithium in the positive electrode material, thereby reducing the battery's first coulombic efficiency. As the proportion of silicon in the negative electrode increases, problems such as low first efficiency, high expansion, and cycle attenuation are exacerbated, and the first irreversible capacity is as high as 30%. Existing studies have shown that positive electrode pre-lithiation technology can effectively improve the battery's first coulombic efficiency. Among them, lithium-rich lithium iron oxide is used as a positive electrode lithium supplement, with a theoretical capacity of up to 867mAh / g. It has the advantages of simple process, low price and high safety, and has a broad application prospect. However, lithium-rich lithium iron oxide prepared by the traditional solid phase method has low purity, low conductivity, and easy hygroscopicity. It is very easy to gel during the homogenization process, which ultimately makes its actual lithium supplement capacity low. It will also produce a lot of gas when used with the positive electrode material, resulting in cycle deterioration, thereby limiting its application.

[0003] In the patent application number CN117012929A, a variety of carbon sources were tried to be sintered and coated on lithium-rich ferrite, and the finished product with a 5% addition was mixed with lithium iron phosphate to test the electrical performance. Although the capacity retention rate after 500 cycles reached 97.2% (the reference was 83.5%), its first charge capacity was only increased by 8.8mAh / g relative to the reference. Although the effect of initially improving the conductivity and cycle retention rate can be achieved, the lithium replenishment capacity only plays 38% of the theoretical capacity, which shows that simple single-layer carbon coating cannot solve the problem of poor conductivity and high residual lithium of lithium-rich ferrite.

[0004] In the patent application number CN117577799A, an attempt was made to sinter and coat heteropolyacids and lithium-rich materials, which effectively reduced the residual alkali content on the surface of lithium-rich materials and reduced the gas production generated by the first charge and discharge decomposition. However, heteropolyacids are expensive and difficult to mass produce. Direct use will aggravate the corrosion of lithium-rich materials and reduce electrochemical performance.

[0005] Therefore, how to improve the shortcomings of lithium-rich lithium iron oxide, such as poor conductivity, high residual lithium, severe gas production and cycle deterioration, is an urgent problem to be solved. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the above background technology and provide an iron-based composite lithium-supplementing material and a preparation method thereof. The iron-based composite lithium-supplementing material can overcome the defects of lithium-rich lithium ferrite, such as poor intrinsic conductivity, high residual lithium, severe gas production and cycle deterioration, and can effectively improve the battery capacity and cycle performance.

[0007] In order to solve the above technical problems, the technical solution proposed by the present invention is: An iron-based composite lithium supplement material, comprising a lithium-rich lithium iron oxide matrix and a coating layer coated on the outside of the lithium-rich lithium iron oxide matrix. The coating layer sequentially comprises a first coating layer, a second coating layer and a third coating layer from inside to outside. The first coating layer is carbon, the second coating layer comprises one or more of lithium borate, lithium tungstate and lithium silicate, and the third coating layer is carbon.

[0008] In the above iron-based composite lithium supplement material, preferably, the chemical formula of the lithium-rich lithium iron oxide matrix is Li a FeF x O 4-x , where a is 4.7 - 5.3 and x is 0.01 - 0.1. F doping can inhibit the metal dissolution and oxygen evolution of the material itself during the cycling process. If the F content is too low, the inhibitory effect on metal dissolution and oxygen evolution is poor. If the F content is too high, it will lead to structural transformation.

[0009] In the above iron-based composite lithium supplement material, preferably, the lithium-rich lithium iron oxide matrix has a small single crystal morphology, the particle size of the lithium-rich lithium iron oxide matrix is 1.0 µm ≤ D50 ≤ 5.0 µm, and the particle size distribution is 0.1 ≤ (D90 - D10) / D50 ≤ 1.

[0010] In the above iron-based composite lithium supplement material, preferably, the first coating layer accounts for 0.5 - 10% of the mass of the iron-based composite lithium supplement material, the second coating layer accounts for 0.5 - 20% of the mass of the iron-based composite lithium supplement material, the third coating layer accounts for 0.5 - 30% of the mass of the iron-based composite lithium supplement material, and the mass ratio of the first coating layer, the second coating layer and the third coating layer is (0.1 - 0.5):(0.1 - 1):1.

[0011] As a general technical concept, the present invention also provides a preparation method of the above iron-based composite lithium supplement material, comprising the following steps: (1) Mix a lithium source, an iron source, a fluorine source and a first carbon source evenly and then sinter (under an inert gas), and obtain lithium-rich lithium iron oxide coated with the first coating layer after sieving; (2) Mix a weak acid for forming the second coating layer and a second carbon source for forming the third coating layer evenly, and then perform a first heat treatment under an inert gas to load the weak acid on the pitch to obtain a coated material; the weak acid includes one or more of boric acid, tungstic acid and silicic acid, and the second carbon source is pitch; (3) Mix the coated material obtained in step (2) and the lithium-rich lithium iron oxide coated with the first coating layer obtained in step (1), and then perform a second heat treatment under an inert gas to obtain the iron-based composite lithium supplement material.

[0012] In the present invention, the content of the coating layer formed in step (1) is low and uneven, and a large amount of the matrix surface is still exposed. In step (2), when the weak acid is loaded onto the asphalt, no chemical reaction occurs, nor is it a simple physical mixture. The first heat treatment slightly softens the asphalt, increasing the viscosity and adsorption energy of the asphalt, so that the weak acid is adsorbed on the asphalt particles. In the second heat treatment, as the temperature increases, the fluidity of the asphalt increases, and the weak acid is slowly separated and released, and preferentially reacts with the lithium-rich lithium iron ferrite matrix to form a coating layer. This coating process can inhibit the direct erosion of the weak acid on the lithium-rich lithium iron ferrite matrix.

[0013] In the above preparation method, preferably, the lithium source includes one or more of lithium carbonate, lithium oxide, lithium dihydrogen phosphate, lithium hydroxide, and lithium acetate, and the iron source includes one or more of iron oxalate, iron phosphate, ferrous phosphate, ammonium ferrous phosphate, and iron(III) oxide; the molar ratio of the lithium source to the iron source is (4.7 - 5.3):1.

[0014] In the above preparation method, preferably, the fluorine source and the first carbon source are PVDF; the dosage of PVDF is 0.5 - 20% of the total mass of the lithium source and the iron source. That is, the fluorine source and the first carbon source are the same substance, which can reduce the feeding steps. If the content of PVDF is too low, an effective carbon conductive network cannot be formed (there is a conductive network in the lithium-rich lithium iron ferrite matrix), and it is also difficult to inhibit grain growth. At the same time, if the F content is too low, the inhibitory effect on metal dissolution and oxygen evolution is poor; if the content of PVDF is too high, the carbon coating layer will be too thick, resulting in a serious decrease in the efficiency of the weak acid to reduce residual lithium. At the same time, if the F content is too high, it will lead to a structural transformation. More preferably, the dosage of PVDF is 3 - 5% of the total mass of the lithium source and the iron source.

[0015] In the above preparation method, preferably, the sintering temperature is 500 - 900 °C, and the isothermal time is 1 - 20 h. More preferably, the temperature is 600 - 800 °C, and the isothermal time is 5 - 15 h. If the sintering temperature and time are too high or too long, Fe impurities harmful to battery performance will be generated due to matrix phase transformation; if the sintering temperature and time are too low or too short, the reaction will be incomplete and the purity of the lithium-rich lithium iron ferrite will be reduced. At the same time, the sintering temperature and time will also affect the doping of F and the formation of the first coating layer. Controlling within the above range is beneficial to the performance of the product.

[0016] In the above preparation method, preferably, the temperature of the first heat treatment is 50-300 °C, the constant temperature time is 1-20 h, and the temperature of the first heat treatment is higher than the softening point of the asphalt (50-300 °C). More preferably, the temperature is 50-200 °C and the constant temperature time is 2-10 h. If the temperature and time of the first heat treatment are too high or too long, the asphalt will be overly softened and agglomerated, and even carbonized in advance, affecting the coating effect; if the temperature and time of the first heat treatment are too low or too short, the weak acid will not be completely adsorbed on the asphalt particles, and in the next heat treatment, the weak acid will still directly erode the lithium-rich lithium iron phosphate matrix, generating impurity phases.

[0017] In the above preparation method, preferably, the temperature of the second heat treatment is 300-800 °C, and the constant temperature time is 1-20 h. More preferably, the temperature is 500-700 °C and the constant temperature time is 2-10 h. If the temperature and time of the second heat treatment are too high or too long, Fe impurities harmful to the battery performance will be generated due to the phase change of the matrix; if the temperature and time of the second heat treatment are too low or too short, the asphalt will not be completely decomposed into carbon and a large amount of weak acid will remain, both of which will lead to a decrease in electrical performance.

[0018] In the above preparation method, the first coating layer is derived from the pyrolysis of PVDF; the second coating layer is obtained by the reaction of a weak acid with the residual lithium (surface residual Li2CO3 and LiOH) on the surface of the lithium-rich lithium iron phosphate matrix, and the weak acid includes one or more of boric acid, tungstic acid and silicic acid; the third coating layer is derived from the pyrolysis of asphalt; after the weak acid is loaded on the asphalt and then mixed and reacted with the lithium-rich lithium iron phosphate matrix, the second coating layer and the third coating layer are obtained. In the above iron-based composite lithium supplement material, the weak acid and the asphalt have a synergistic effect. When the amount of the weak acid is small, the reaction with the residual lithium on the surface of the lithium-rich lithium iron phosphate matrix is insufficient, which not only affects the processing and use performance of the product, but also reduces the cycle retention rate. If the amount of asphalt is too small, a uniform coating network cannot be formed, resulting in a decrease in the conductivity and charging capacity of the product. When the ratio of the weak acid to the asphalt is selected within the above range, the prepared product has better effects. The iron-based composite lithium supplement material prepared by the present invention has three-layer coating and F doping. This structure gives full play to the characteristics of the high lithium supplement capacity of the lithium-rich lithium iron phosphate and realizes lower metal dissolution and lower gas generation amount, thereby ensuring the long-term cycle retention rate of the battery. Before the three-layer coating, the lithium-rich lithium iron phosphate matrix has a small single crystal morphology and uniform particle distribution. After the three-layer coating, the surface of the finished product is smooth and there is no fine powder.

[0019] In the above preparation method, first, an iron source, a lithium source, and PVDF are subjected to high-temperature sintering to prepare a high-purity lithium-rich lithium ferrite matrix coated with a first coating layer. During this process, not only a carbon coating layer is formed, but also the carbon layer prevents the growth of lithium-rich lithium ferrite grains, effectively improving the electronic conductivity and ionic conductivity. At the same time, the doping of F inhibits the metal dissolution and oxygen evolution of the material itself during the cycling process, improving the cycling performance. Then, a weak acid and a low softening point asphalt are fully mixed in proportion. During the low-temperature heat treatment process, the asphalt is slightly softened, and the weak acid is evenly loaded on the asphalt to obtain a weak acid composite asphalt (i.e., the coating material). Subsequently, the weak acid composite asphalt and the lithium-rich lithium ferrite matrix are mixed and sintered to prepare the finished product. The weak acid effectively reduces the content of residual alkali on the surface of the matrix, thereby reducing the side reaction with the electrolyte and improving the cycling retention rate at room temperature and high temperature. In addition, the lithium-containing compound formed by the reaction of the weak acid and residual lithium improves the diffusion kinetics of lithium ions and inhibits the interfacial side reaction between the electrode and the electrolyte.

[0020] In the present invention, the addition of PVDF inhibits the growth of lithium-rich lithium ferrite grains. Without the need for high-intensity crushing such as ball milling, a lithium-rich lithium ferrite matrix with a particle size of 1.0 µm ≤ D50 ≤ 5.0 µm and a particle size distribution of 0.1 ≤ (D90 - D10) / D50 ≤ 1 is directly obtained. In addition, this method avoids the crystal defects and a large amount of fine powder generated by crushing processes such as ball milling. Both crystal defects and a large amount of fine powder will lead to the deterioration of the battery during the cycling process. The small single crystal morphology improves the ionic conductivity of the matrix, and the high-temperature carbonization of PVDF improves the electronic conductivity. At the same time, the F element is uniformly doped inside the matrix. Since the bond energy of Li-F bond and Fe-F bond is stronger than that of Li-O bond and Fe-O bond, the doping of F inhibits the metal dissolution and oxygen evolution of the material itself during the cycling process, thereby improving the structural stability and enhancing the cycling performance. After the asphalt is heated to the softening point, it can be uniformly melted on the surface of the lithium-rich lithium ferrite matrix, and a highly uniform carbon layer is retained on the surface of the lithium-rich lithium ferrite matrix after high-temperature carbonization. Under the dual action of PVDF and asphalt, a high-conductivity network structure combining internal and external is achieved, further improving the electronic conductivity and ionic conductivity of the iron-based composite lithium supplement material, and thus significantly improving the lithium supplement capacity.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The iron-based composite lithium supplement material of the present invention adopts a three-layer coating structure doped with fluorine. The doping of F inhibits the metal dissolution and oxygen evolution of the material itself during the cycle, and improves the cycle performance. The first layer of carbon coating obtains a small single crystal morphology by inhibiting the growth of lithium-rich lithium ferrate grains, and at the same time improves the electronic conductivity and ionic conductivity of the material. The second layer of coating neutralizes the residual alkali on the surface of the matrix through a weak acid, and the lithium-containing compound formed by the neutralization reaction improves the diffusion kinetics of lithium ions and also inhibits the interfacial side reactions between the electrode and the electrolyte. The third layer of carbon coating provides a highly conductive network structure and improves the air stability. The three-layer coating structure doped with fluorine gives full play to the characteristics of high lithium supplement capacity of lithium-rich lithium ferrate, and overcomes the defects of poor intrinsic conductivity, high residual lithium, serious gas generation and cycle deterioration of lithium-rich lithium ferrate, realizing lower metal dissolution and lower gas generation, thereby ensuring the capacity and long-term cycle retention rate of the battery.

[0022] 2. In the preparation method of the iron-based composite lithium supplement material of the present invention, during the coating process, after the asphalt temperature rises above the softening point, it will melt on the surface of the lithium-rich lithium ferrate matrix to form a uniform third coating layer, which not only improves the conductivity of the iron-based composite lithium supplement material, but also effectively isolates the electrolyte, avoiding gas generation and cycle deterioration caused by side reactions with the electrolyte; by introducing a weak acid during the process of coating the lithium-rich lithium ferrate matrix with asphalt, the weak acid and the surface residual alkali undergo a neutralization reaction to form a second coating layer, effectively reducing the content and pH value of the residual alkali on the matrix surface, avoiding gas generation due to the decomposition of residual lithium and further cycle deterioration. At the same time, the lithium compound formed by the reaction of the weak acid and the residual lithium improves the diffusion kinetics of lithium ions and inhibits the interfacial side reactions between the electrode and the electrolyte. In addition, the reduction of the residual alkali effectively avoids the damage to the binder and the gel phenomenon during the homogenization process of the residual alkali, improving the slurry processability and stability of the homogenization.

[0023] 3. In the preparation method of the iron-based composite lithium supplement material of the present invention, the direct erosion of the weak acid on the lithium-rich lithium ferrate matrix is inhibited by first forming an asphalt protection layer. When the weak acid directly contacts the lithium-rich lithium ferrate matrix, it will react with the matrix to generate a heterophase LiFeO2. Asphalt with a low softening point is selected to melt first on the matrix surface to form a uniform protection layer, so that the weak acid can only contact the surface of the matrix, inhibiting the direct erosion of the weak acid on the lithium-rich lithium ferrate matrix and avoiding the formation of heterophases. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1It is the scanning electron microscope image of the lithium-rich lithium ironate matrix prepared in Example 1.

[0026] Figure 2 It is the scanning electron microscope image of the iron-based composite lithium supplement material prepared in Example 1.

[0027] Figure 3 It is the comparison chart of charge-discharge curves of the iron-based composite lithium supplement materials prepared in Example 1 and Comparative Example 5. Detailed implementation manners

[0028] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0029] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0030] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchases or can be prepared by existing methods.

[0031] Example 1: An iron-based composite lithium supplement material is composed of fluorine-doped lithium-rich lithium ironate, a carbon coating layer, a lithium borate coating layer, and a carbon coating layer from the inside to the outside in sequence. Its preparation method includes the following steps: (1) Fe2O3 and Li2O are fully mixed evenly according to the molar ratio of Li:Fe = 5.0:1, and at the same time, 3 wt% of PVDF by mass fraction is added as a carbon source. Under a nitrogen atmosphere, it is sintered at a high temperature of 700 °C for 15 h. After sieving, a lithium-rich lithium ironate matrix with uniformly dispersed particles and doped element F is obtained, and its surface is coated with a carbon coating layer.

[0032] (2) Boric acid and asphalt with a softening point of 70 °C are fully mixed according to the mass ratio of 1:2. Under a nitrogen atmosphere, it is heat-treated at 100 °C for 1 h to load boric acid on the asphalt, obtaining a weak acid composite carbon source.

[0033] (3) Then, the weak acid composite carbon source in step (2) and the lithium-rich lithium ironate matrix in step (1) are fully mixed. The carbon contained in the weak acid composite carbon source accounts for 2% of the total mass of the composite lithium supplement material. Under a nitrogen atmosphere, it is sintered at a high temperature of 600 °C for 5 h. After sieving, an iron-based composite lithium supplement material with uniformly dispersed particles and good fluidity is obtained.

[0034] Example 2: A kind of iron-based composite lithium supplement material is composed of lithium-rich iron ferrite doped with fluorine, a carbon coating layer, a lithium tungstate coating layer, and a carbon coating layer from the inside to the outside. Its preparation method includes the following steps: (1) Ferric oxalate and LiOH are fully mixed evenly according to the molar ratio of Li:Fe = 4.9:1. At the same time, 5wt% of PVDF by mass fraction is added as a carbon source. Under a nitrogen atmosphere, it is sintered at a high temperature of 800°C for 12h. After sieving, a lithium-rich iron ferrite matrix with uniformly dispersed particles and doped element F is obtained, and its surface is coated with a carbon coating layer.

[0035] (2) Tungstic acid and asphalt with a softening point of 70°C are fully mixed according to the mass ratio of 1:4. Under a nitrogen atmosphere, it is heat-treated at 90°C for 2h to load tungstic acid on the asphalt, obtaining a weak acid composite carbon source.

[0036] (3) Then the weak acid composite carbon source in step (2) and the lithium-rich iron ferrite matrix in step (1) are fully mixed. The carbon contained in the weak acid composite carbon source accounts for 2% of the total mass of the composite lithium supplement material. Under a nitrogen atmosphere, it is sintered at a high temperature of 500°C for 6h. After sieving, an iron-based composite lithium supplement material with uniformly dispersed particles and good fluidity is obtained.

[0037] Example 3: A kind of iron-based composite lithium supplement material is composed of lithium-rich iron ferrite doped with fluorine, a carbon coating layer, a lithium silicate coating layer, and a carbon coating layer from the inside to the outside. Its preparation method includes the following steps: (1) Ferric phosphate and Li2CO3 are fully mixed evenly according to the molar ratio of Li:Fe = 5.1:1. At the same time, 3wt% of PVDF by mass fraction is added as a carbon source. Under a nitrogen atmosphere, it is sintered at a high temperature of 750°C for 12h. After sieving, a lithium-rich iron ferrite matrix with uniformly dispersed particles and doped element F is obtained, and its surface is coated with a carbon coating layer.

[0038] (2) Silicic acid and asphalt with a softening point of 80°C are fully mixed according to the mass ratio of 1:3. Under a nitrogen atmosphere, it is heat-treated at 120°C for 1h to load the weak acid on the carbon source, obtaining a weak acid composite carbon source.

[0039] (3) Then the weak acid composite carbon source in step (2) and the lithium-rich iron ferrite matrix in step (1) are fully mixed. The carbon contained in the weak acid composite carbon source accounts for 2% of the total mass of the composite lithium supplement material. Under a nitrogen atmosphere, it is sintered at a high temperature of 650°C for 5h. After sieving, an iron-based composite lithium supplement material with uniformly dispersed particles and good fluidity is obtained.

[0040] Example 4: A kind of iron-based composite lithium supplement material is composed of lithium-rich iron ferrite doped with fluorine, a carbon coating layer, a lithium borate coating layer, and a carbon coating layer from the inside to the outside. Its preparation method includes the following steps: (1) Mix Fe2O3 and Li2O thoroughly in a molar ratio of Li:Fe = 5.0:1. Meanwhile, add 0.5 wt% of PVDF by mass as a carbon source. Under a nitrogen atmosphere, sinter at 600 °C for 5 h. After sieving, a lithium-rich lithium ferrate matrix with uniformly dispersed particles and doped element F is obtained, and its surface is coated with a carbon coating layer.

[0041] (2) Mix boric acid and asphalt with a softening point of 50 °C in a mass ratio of 1:10 thoroughly. Under a nitrogen atmosphere, heat-treat at 50 °C for 1 h to load boric acid onto the asphalt, obtaining a weak acid composite carbon source.

[0042] (3) Then mix the weak acid composite carbon source in step (2) and the lithium-rich lithium ferrate matrix in step (1) thoroughly. The carbon contained in the weak acid composite carbon source accounts for 0.5% of the total mass of the composite lithium supplement material. Under a nitrogen atmosphere, sinter at 500 °C for 1 h. After sieving, an iron-based composite lithium supplement material with uniformly dispersed particles and good fluidity is obtained.

[0043] Example 5: An iron-based composite lithium supplement material is composed of fluorine-doped lithium-rich lithium ferrate, a carbon coating layer, a lithium borate coating layer, and a carbon coating layer from the inside out. Its preparation method includes the following steps: (1) Mix Fe2O3 and Li2O thoroughly in a molar ratio of Li:Fe = 5.0:1. Meanwhile, add 20 wt% of PVDF by mass as a carbon source. Under a nitrogen atmosphere, sinter at 900 °C for 20 h. After sieving, a lithium-rich lithium ferrate matrix with uniformly dispersed particles and doped element F is obtained, and its surface is coated with a carbon coating layer.

[0044] (2) Mix boric acid and asphalt with a softening point of 300 °C in a mass ratio of 1:1 thoroughly. Under a nitrogen atmosphere, heat-treat at 300 °C for 10 h to load boric acid onto the asphalt, obtaining a weak acid composite carbon source.

[0045] (3) Then mix the weak acid composite carbon source in step (2) and the lithium-rich lithium ferrate matrix in step (1) thoroughly. The carbon contained in the weak acid composite carbon source accounts for 30% of the total mass of the composite lithium supplement material. Under a nitrogen atmosphere, sinter at 700 °C for 10 h. After sieving, an iron-based composite lithium supplement material with uniformly dispersed particles and good fluidity is obtained.

[0046] Comparative Example 1: An iron-based composite lithium supplement material is composed of fluorine-doped lithium-rich lithium ferrate, a lithium borate coating layer, and a carbon coating layer from the inside out. Its preparation method includes the following steps: (1) Fe2O3 and Li2O were fully and evenly mixed according to the molar ratio of Li:Fe = 5.0:1. Meanwhile, 3 wt% of LiF by mass fraction was added as a fluorine source. Under a nitrogen atmosphere, it was sintered at a high temperature of 700 °C for 15 h. After crushing and sieving, a lithium-rich lithium ferrate matrix doped with element F, with large particle size and relatively wide distribution, was obtained.

[0047] (2) Boric acid and asphalt with a softening point of 70 °C were fully mixed according to the mass ratio of 1:2. Under a nitrogen atmosphere, it was heat-treated at 100 °C for 1 h to load the weak acid on the carbon source, obtaining a weak acid composite carbon source.

[0048] (3) Then, the weak acid composite carbon source in step (2) and the lithium-rich lithium ferrate matrix in step (1) were fully mixed. The carbon contained in the weak acid composite carbon source accounted for 2% of the total mass of the composite lithium supplementing material. Under a nitrogen atmosphere, it was sintered at a high temperature of 600 °C for 5 h. After sieving, an iron-based composite lithium supplementing material was obtained.

[0049] Comparative Example 2: An iron-based composite lithium supplementing material is composed of lithium-rich lithium ferrate doped with fluorine, a carbon coating layer, a lithium borate coating layer, and a carbon coating layer from the inside out. Its preparation method includes the following steps: (1) Fe2O3 and Li2O were fully and evenly mixed according to the molar ratio of Li:Fe = 5.0:1. Meanwhile, 3 wt% of PVDF by mass fraction was added as a carbon source. Under a nitrogen atmosphere, it was sintered at a high temperature of 700 °C for 15 h. After sieving, a lithium-rich lithium ferrate matrix with uniformly dispersed particles and doped with element F was obtained, and its surface was coated with a carbon coating layer.

[0050] (2) Boric acid and the lithium-rich lithium ferrate matrix in step (1) were fully mixed. Under a nitrogen atmosphere, it was heat-treated at 100 °C for 1 h to obtain lithium-rich lithium ferrate coated with lithium borate.

[0051] (3) Then, asphalt with a softening point of 70 °C and the lithium-rich lithium ferrate in step (2) were fully mixed. The carbon contained in the asphalt accounted for 2% of the total mass of the composite lithium supplementing material. Under a nitrogen atmosphere, it was sintered at a high temperature of 600 °C for 5 h. After sieving, an iron-based composite lithium supplementing material with uniformly dispersed particles and good fluidity was obtained.

[0052] Comparative Example 3: An iron-based composite lithium supplementing material is composed of lithium-rich lithium ferrate, a lithium borate coating layer, and a carbon coating layer from the inside out. Its preparation method includes the following steps: (1) Fe2O3 and Li2O were fully and evenly mixed according to the molar ratio of Li:Fe = 5.0:1. Under a nitrogen atmosphere, it was sintered at a high temperature of 700 °C for 15 h. After crushing and sieving, a lithium-rich lithium ferrate matrix with large particle size and relatively wide distribution was obtained.

[0053] (2) Boric acid and asphalt with a softening point of 70°C are fully mixed in a mass ratio of 1:2, and heat treated at 100°C for 1 h in a nitrogen atmosphere to load the weak acid on the carbon source to obtain a weak acid composite carbon source.

[0054] (3) The weak acid composite carbon source in step (2) and the lithium-rich lithium ferrite matrix in step (1) are fully mixed, wherein the carbon contained in the weak acid composite carbon source accounts for 2% of the total mass of the composite lithium supplement material, and the mixture is sintered at 600° C. for 5 h in a nitrogen atmosphere, and then sieved to obtain an iron-based composite lithium supplement material.

[0055] Comparative Example 4: An iron-based composite lithium supplement material is composed of lithium-rich lithium ferrite and a carbon coating layer. The preparation method thereof comprises the following steps: (1) Fe2O3 and Li2O were fully mixed at a molar ratio of Li:Fe=5.0:1, sintered at 700°C for 15 h in a nitrogen atmosphere, and crushed and sieved to obtain a lithium iron oxide matrix with large particle size and wide distribution.

[0056] (2) Asphalt with a softening point of 70°C and a lithium-rich lithium ferrite matrix are fully mixed, wherein the carbon content of the asphalt accounts for 2% of the total mass of the composite lithium supplement material. The mixture is sintered at 600°C for 5 hours under a nitrogen atmosphere, and the iron-based composite lithium supplement material is obtained after screening.

[0057] Comparative Example 5: An iron-based lithium supplement material, composed of lithium-rich lithium ferrite, is not doped or coated. The preparation method thereof comprises the following steps: Fe2O3 and Li2O were fully and evenly mixed according to the molar ratio of Li:Fe=5.0:1, sintered at 700°C for 15h in a nitrogen atmosphere, and crushed and sieved to obtain an iron-based lithium supplement material with large particle size and wide distribution.

[0058] Figure 1 This is a scanning electron microscope image of the lithium-rich lithium ferrite matrix prepared in Example 1. Figure 2 This is a scanning electron microscope image of the iron-based composite lithium supplement material prepared in Example 1. Figure 3 The following is a comparison chart of the charge-discharge curves of the iron-based composite lithium-supplementing material prepared in Example 1 and Comparative Example 5. Figure 1 It can be seen that the lithium-rich lithium ferrite matrix prepared in Example 1 is dispersed small single crystal particles, which proves that PVDF effectively inhibits the growth of lithium-rich lithium ferrite grains during the sintering process. Figure 2 It can be seen that the iron-based composite lithium supplement material prepared in Example 1 is a large single crystal particle with a smooth surface, indicating that a continuous carbon coating network has been formed. Figure 3 It can be seen that compared with the undoped and uncoated lithium-rich lithium ferrite, the iron-based composite lithium-supplementing material prepared in Example 1 has a significantly lower voltage platform, and a significantly improved charging capacity and lithium-supplementing capacity.

[0059] The lithium-rich lithium ferrate matrix and the iron-based composite lithium supplement material prepared in the above embodiments and comparative examples were subjected to physical and chemical performance test comparison and power-off test comparison, and the results are shown in Table 1 and Table 2. The iron-based composite lithium supplement material prepared in the above embodiments and comparative examples was respectively matched with the LMO positive electrode material for full power test comparison, and the results are shown in Table 3, and the addition amount of the lithium supplement material was 3%.

[0060] Residual alkali test: Dissolve the above products in ethanol at a ratio of solid: liquid = 1:20, stir for 10 minutes and filter out the solid material, transfer the filtrate to a titration cup, and perform electrochemical titration with 0.1M hydrochloric acid standard solution. The sum of the two breakpoints is taken as the residual alkali. Other performance test results refer to the national standard test.

[0061] Table 1: Performance test results of lithium-rich lithium ferrite matrix of Examples 1-5 and Comparative Examples 1-5

[0062] Table 2: Performance test results of the products of Examples 1-5 and Comparative Examples 1-5

[0063] Table 3: Full electrical test results of products of Examples 1-5 and Comparative Examples 1-5

[0064] As can be seen from Tables 1 to 3, compared with Comparative Examples 1 to 5, the iron-based composite lithium supplement materials prepared in Examples 1, 2, and 3 all exhibited a high lithium supplement capacity of more than 800mAh / g and extremely low Fe dissolution under the conditions of 3.0-4.5V@0.05C buckle test. After the lithium supplement material is matched with the LMO positive electrode material, it exhibits a higher cycle retention rate and a lower gas production under the conditions of 2.8-4.2V@0.1C full-electric test. The 500-cycle capacity retention rate is higher than 92%, and the gas production is lower than 0.8ml / Ah. The performance advantages of Examples 1, 2, and 3 depend on the technical solutions of three-layer coating, F doping, and premixing and pre-burning of the second and third layers of coatings. In Comparative Example 1, LiF replaces PVDF as the F source, so that the lithium supplement material lacks the first layer of carbon coating, and the D50 particle size is greatly increased, which leads to crystal surface defects and micropowders after crushing. Finally, the capacity retention rate is reduced and the gas production is increased under the full-electric test. In Comparative Example 2, the premixing and pre-burning of the second and third layers of coatings were cancelled, so that the weak acid corroded the lithium iron oxide matrix, producing impurities, which led to a decrease in the capacity retention rate of the full-electric test and an increase in gas production. In Comparative Examples 3-5, due to the lack of the required coating layer, the lithium replenishment capacity of the buckle test was lower, the Fe dissolution was higher, the capacity retention rate of the full-electric test was lower, and the gas production was higher.

Claims

1. A lithium - supplementing iron - based composite material, comprising a lithium - rich lithium ferrate matrix and a coating layer coated on the outside of the lithium - rich lithium ferrate matrix, characterized in that, The coating layer sequentially includes a first coating layer, a second coating layer, and a third coating layer from inside to outside. The first coating layer is carbon, the second coating layer includes one or more of lithium borate, lithium tungstate, and lithium silicate, and the third coating layer is carbon.

2. The iron-based composite lithium supplement material according to claim 1, wherein The chemical formula of the lithium-rich lithium iron ferrite matrix is Li a FeF x O 4-x , where a is 4.7 - 5.3 and x is 0.01 - 0.

1.

3. The iron-based composite lithium supplement material according to claim 1, wherein The lithium-rich lithium ferrate matrix is in the shape of small single crystals, and the particle size of the lithium-rich lithium ferrate matrix is 1.0 µm ≤ D50 ≤ 5.0 µm, and the particle size distribution is 0.1 ≤ (D90 - D10) / D50 ≤ 1.

4. The iron-based composite lithium supplement material according to any one of claims 1-3, characterized in that, The first coating layer accounts for 0.5 - 10% of the mass of the iron-based composite lithium supplement material, the second coating layer accounts for 0.5 - 20% of the mass of the iron-based composite lithium supplement material, the third coating layer accounts for 0.5 - 30% of the mass of the iron-based composite lithium supplement material, and the mass ratio of the first coating layer, the second coating layer, and the third coating layer is (0.1 - 0.5):(0.1 - 1):

1.

5. A preparation method of the iron-based composite lithium supplement material according to any one of claims 1-4, characterized in that, It includes the following steps: (1) Mix a lithium source, an iron source, a fluorine source, and a first carbon source evenly and then sinter them. After sieving, lithium-rich lithium ferrate coated with the first coating layer is obtained. (2) Mix a weak acid for forming the second coating layer and a second carbon source for forming the third coating layer evenly, and then perform a first heat treatment to load the weak acid on the asphalt to obtain a coating material. The weak acid includes one or more of boric acid, tungstic acid, and silicic acid, and the second carbon source is asphalt. (3) Mix the coating material obtained in step (2) and the lithium-rich lithium ferrate coated with the first coating layer obtained in step (1), and then perform a second heat treatment to obtain the iron-based composite lithium supplement material.

6. The preparation method according to claim 5, wherein The lithium source includes one or more of lithium carbonate, lithium oxide, lithium dihydrogen phosphate, lithium hydroxide, and lithium acetate, and the iron source includes one or more of iron oxalate, iron phosphate, ferrous phosphate, ammonium ferrous phosphate, and iron(III) oxide. The molar ratio of the lithium source to the iron source is (4.7 - 5.3):

1.

7. The preparation method according to claim 5, characterized in that, The fluorine source and the first carbon source are PVDF. The dosage of PVDF is 0.5 - 20% of the total mass of the lithium source and the iron source.

8. The preparation method according to claim 5, characterized in that, The temperature of the sintering is 500 - 900 °C, and the constant temperature time is 1 - 20 h.

9. The preparation method according to any one of claims 5-8, characterized in that, The temperature of the first heat treatment is 50 - 300 °C, and the constant temperature time is 1 - 20 h, and the temperature of the first heat treatment is higher than the softening point of the asphalt.

10. The preparation method according to any one of claims 5-8, characterized in that, The temperature of the second heat treatment is 300 - 800 °C, and the constant temperature time is 1 - 20 h.

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