Monocrystallized and coating modified lithium-rich manganese-based material as well as preparation method and application thereof

Abstract

The invention provides a single-crystallized and coating-modified lithium-rich manganese-based material as well as a preparation method and application thereof, the preparation method comprises the following steps: carrying out ball-milling mixing on a cobalt-free carbonate precursor material, a lithium source and molten salt, and then carrying out primary calcination, washing and repair heat treatment to obtain a single-crystal cobalt-free positive electrode material; the method comprises the following steps: mixing a single-crystal cobalt-free positive electrode material with a boron source solution, carrying out first coating to obtain a mixture, adding a magnesium source solution into the mixture, carrying out second coating, carrying out solid-liquid separation after coating, washing, drying and carrying out secondary calcination to obtain a gradient coating layer on the surface of the single-crystal cobalt-free positive electrode material, and one side far away from the single-crystal cobalt-free positive electrode material is rich in magnesium. The cobalt-free lithium-rich manganese-based positive electrode material is modified through single crystallization and coating synergistically, so that the problems of structural distortion, poor cycling stability, capacity fading and the like of the lithium-rich manganese-based material are solved, and the electrochemical performance of the lithium-rich manganese-based material is improved synergistically.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries, and relates to a single-crystalized and coated modified lithium-rich manganese-based material and a preparation method and application thereof. BACKGROUND

[0002] With the rapid growth of the electric vehicle industry, the demand for high-performance lithium-ion power batteries is increasingly urgent, and lithium iron phosphate positive electrode materials and ternary lithium battery positive electrode materials have become the most widely used materials in commercial power batteries. However, the current positive electrode materials are facing challenges in meeting the growing demand. In addition, in order to meet the demand of high energy density applications such as electric aviation, mobile robots, and long-range unmanned vehicles, it is urgent to develop a new generation of lithium-ion batteries with higher energy density. The positive electrode material is the main factor restricting the development of high-energy density lithium-ion batteries, and the lithium-rich manganese-based (LRM) positive electrode material with anion redox function can usually achieve higher discharge specific capacity (>250 mAh·g -1 ) and energy density (>1000 Wh·kg -1 ) than conventional layered positive electrode materials, and is the most promising candidate material for high-energy lithium-ion batteries.

[0003] The LRM positive electrode material can be considered as a composite material with a layered structure LiTMO2 and Li2MnO3 randomly arranged at an atomic scale, where TM is a transition metal. In order to activate the redox reaction in the Li2MnO3 phase, the charging voltage of the LRM positive electrode needs to exceed 4.5 V. However, the redox reaction is unstable, which leads to the irreversible release of oxygen active species into the electrolyte, intensifying the interface side reactions between the LRM electrode and the electrolyte, especially during long-term cycling. This can lead to the decomposition of the electrolyte, resulting in the dissolution of transition metals and the irreversible loss of active lithium, leading to morphological degradation and irreversible structural transformation, ultimately resulting in rapid degradation of electrochemical performance.

[0004] Specifically, due to the significant electrochemical characteristics of the LRM positive electrode material in two-stage voltage platforms during the first charging, the voltage platform of the second stage is between 3.8-4.4 V, and Li + is provided from the LiMO2 phase. The voltage platform of the second stage is about 4.5 V, and Li +Provided. The reason for the high capacity of LRM is the redox reaction of ions, which is much more complex than that of conventional cathode materials. By comparison with traditional cathode materials, the oxygen vacancies caused by the loss of oxygen will lead to irreversible structural transformation of LRM from outside to inside, which is manifested as low coulombic efficiency (ICE) in the first charge-discharge process and continuous decay of discharge voltage and capacity during the cycle process. In addition, the migration, segregation and continuous decrease of average valence state of transition metal elements in the cathode material are also the reasons for the loss of discharge capacity and voltage decay during the cycle process. Therefore, it is inevitable to modify the LRM.

[0005] To solve these problems, the prior art has adopted various methods to modify the LRM cathode material. These methods include element doping, surface coating, single crystal structure design, etc., among which element doping helps to improve the rate performance of LRM, surface coating can inhibit the capacity decay during the cycle process, and the design of single crystal structure can show unique advantages over polycrystalline particles, such as higher crystallinity, fewer grain boundaries, better particle integrity, higher mechanical stability, etc. In addition to the modification strategy, it is also feasible to use the preparation method based on the ternary cathode material to improve the inherent performance of the material. However, the modification of the prior art is generally only a single modification of the material, and there is no synergistic modification to effectively improve the electrochemical performance of the material. The comprehensive electrochemical performance of the lithium-rich manganese-based material needs to be improved.

[0006] Based on the above research, it is necessary to provide a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, which synergistically modifies the single-crystallization and coating, and the synergistic modification effect effectively improves the electrochemical performance of the material. SUMMARY

[0007] The purpose of the present application is to provide a single-crystallized and coated modified lithium-rich manganese-based material and a preparation method and application thereof. The preparation method synergistically modifies the cobalt-free lithium-rich manganese-based cathode material through single-crystallization and coating, so as to improve the problems of structural distortion, poor cycle stability and capacity decay of the lithium-rich manganese-based material, and synergistically improve the electrochemical performance of the lithium-rich manganese-based material.

[0008] To achieve the purpose of the present application, the following technical solutions are adopted:

[0009] In a first aspect, the present application provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, which comprises the following steps:

[0010] (1) mixing a cobalt-free carbonate precursor material, a lithium source and a molten salt by ball milling, and then performing a first calcination, washing and repairing heat treatment to obtain a single-crystalline cobalt-free cathode material;

[0011] (2) mixing the single-crystal cobalt-free cathode material in step (1) with a boron source solution to perform first coating, obtaining a mixture, then adding a magnesium source solution to the mixture to perform second coating, after the coating is completed, performing solid-liquid separation, washing, drying and secondary calcination, and obtaining a gradient coating layer on the surface of the single-crystal cobalt-free cathode material, wherein the gradient coating layer is rich in boron on the side close to the single-crystal cobalt-free cathode material and is rich in magnesium on the side away from the single-crystal cobalt-free cathode material, thereby obtaining the single-crystal and coating modified lithium-rich manganese-based material.

[0012] The present application obtains complete crystal grains by high-temperature crystallization assisted by molten salt, and obtains a single-crystal cobalt-free cathode material, and the design of the single-crystal structure can improve the structural and interface stability of the material, thereby improving the cycle stability of the material. Then the single-crystal cobalt-free cathode material in step (1) is mixed with a boron source solution to perform first coating, and then a magnesium source solution is added to perform second coating, that is, the surface of the single-crystal cobalt-free cathode material is coated with boron first, and then coated with magnesium, and after secondary calcination, the gradient coating layer obtained is rich in boron inside and rich in magnesium outside, and the magnesium enriched on the surface also reacts with the boron near the surface, and finally a gradient coating layer of boron-containing magnesium is obtained, which is rich in boron inside and rich in magnesium outside, wherein the inner layer is rich in B, so that the boron near the surface may form a Li-B-O or M-B-O (M=transition metal) interface layer or realize trace phase doping, boron is a grain boundary strengthening agent and an oxygen stabilizer, which can significantly inhibit the release of lattice oxygen, the migration of transition metals and the formation of rock salt phase, thereby stabilizing the body phase structure from the source, and is particularly helpful in inhibiting voltage drop. The outer layer is rich in Mg, so that the magnesium-rich magnesium borate provides an excellent physical barrier to directly block the corrosion of the electrolyte, reduce side reactions, and protect the inner layer of the boron-rich stable layer, and the magnesium borate compound itself has good chemical stability and high mechanical strength, therefore, the double protection mechanism of the inner layer rich in B+the outer layer rich in Mg can more synergistically inhibit phase transformation and side reactions, and can more improve the capacity and cycle performance of the material.

[0013] Preferably, the temperature of the second coating in step (2) is lower than that of the first coating.

[0014] Preferably, the time of the second coating in step (2) is lower than that of the first coating.

[0015] The present application coats magnesium at a lower temperature and time than coating boron, and since the reaction rate of Mg 2+ with H3BO3 / B4O7 2- is slow at a lower temperature, mainly forming an enrichment layer of Mg 2+ on the surface, and forming a preliminary concentration gradient with the boron penetration zone in the inner layer, if the temperature of the second coating is too high or the time is too long, the formation of the magnesium-rich region will be affected, and it is difficult to form a gradient, but if the temperature of the second coating is too low or the time is too short, the enrichment of Mg 2+ will also be affected.

[0016] Preferably, the temperature of the first coating in step (2) is 40-60℃, for example, it can be 40℃, 45℃, 50℃, 55℃ or 60℃, and the time is 4-8h, for example, it can be 4h, 5h, 6h, 7h or 8h, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0017] Preferably, the temperature of the second coating in step (2) is 30-50℃, for example, it can be 30℃, 35℃, 40℃, 45℃ or 50℃, and the time is 1-2h, for example, it can be 1h, 1.2h, 1.4h, 1.6h, 1.8h or 2h, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0018] Preferably, the rotation speed of the first coating and the second coating in step (2) is independently 500-700r / min, for example, it can be 500r / min, 550r / min, 600r / min, 650r / min or 700r / min, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0019] Preferably, the boron source solution in step (2) comprises a boric acid solution and / or a borax solution.

[0020] The present application uses a boric acid solution and / or a borax solution for coating. At the temperature of the first coating system, boron is easily controlled to penetrate to the near surface layer, laying the foundation for the inner layer to be rich in boron. If NaBH4 is used, the coating reaction is violent and it is difficult to form a gradient.

[0021] Preferably, the concentration of the boron source in the mixture in step (2) is 0.05-0.15mol / L, for example, it can be 0.05mol / L, 0.07mol / L, 0.09mol / L, 0.11mol / L, 0.13mol / L or 0.15mol / L, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0022] Preferably, the magnesium source solution in step (2) comprises a magnesium nitrate ethanol solution.

[0023] Preferably, after the magnesium source solution is added in step (2), the concentration of the magnesium source in the system is 0.1-0.3mol / L, for example, it can be 0.1mol / L, 0.15mol / L, 0.2mol / L, 0.25mol / L or 0.3mol / L, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0024] Preferably, the way of adding the magnesium source solution in step (2) is dropwise.

[0025] The magnesium source is added in a dropwise manner to promote the formation of a surface magnesium-rich layer.

[0026] Preferably, the temperature of the secondary calcination in step (2) is 450-550℃, for example, it can be 450℃, 470℃, 490℃, 510℃, 530℃ or 500℃, and the time is 1-3h, for example, it can be 1h, 1.5h, 2h, 2.5h or 3h, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0027] Preferably, the content of the gradient coating layer in the single-crystalized and coated modified lithium-rich manganese-based material in step (2) is 1-3wt%, for example, it can be 1wt%, 1.5wt%, 2wt%, 2.5wt% or 3wt%, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0028] Preferably, the primary calcination in step (1) includes first holding at 800-900℃, for example, it can be 800℃, 825℃, 850℃, 875℃ or 900℃, for 7-9h, for example, it can be 7h, 7.5h, 8h, 8.5h or 9h, and then heating to 950-1050℃, for example, it can be 950℃, 970℃, 990℃, 1010℃, 1030℃ or 1050℃, and holding for 0.5-1.5h, for example, it can be 0.5h, 0.75h, 1.0h, 1.25h or 1.5h, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0029] The present application uses a temperature as high as 950-1050℃ for high-temperature crystallization, and cooperates with molten salt assistance to obtain complete single-crystal grains.

[0030] Preferably, the temperature of the repair heat treatment in step (1) is 250-350℃, and the time is 4-6h.

[0031] After the primary calcination, the obtained powder is washed with deionized water to remove excess molten salt, and then the single-crystal positive electrode material is subjected to single-crystal particle surface repair by again repairing heat treatment.

[0032] Preferably, the total mole ratio of the cobalt-free carbonate precursor material and the lithium source to the molten salt in step (1) is 1:(3-5), for example, it can be 1:3, 1:4 or 1:5, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0033] Preferably, the molten salt in step (1) includes potassium chloride.

[0034] Preferably, the rotation speed of the ball milling mixing in step (1) is 250-350 r / min, for example, it can be 250 r / min, 275 r / min, 300 r / min, 325 r / min or 350 r / min, and the time is 3-4 h, for example, it can be 3 h, 3.25 h, 3.5 h, 3.75 h or 4 h, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0035] Preferably, the method for preparing the cobalt-free carbonate precursor material in step (1) comprises the following steps:

[0036] The mixed metal salt solution and the carbonate solution are passed into the bottom liquid to carry out a co-precipitation reaction to obtain the cobalt-free carbonate precursor material.

[0037] Preferably, the mixed metal salt solution comprises nickel ions and manganese ions.

[0038] Preferably, the molar ratio of nickel ions to manganese ions in the mixed metal salt solution is 1:(2-4), for example, it can be 1:2, 1:2.5, 1:3, 1:3.5 or 1:4, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0039] Preferably, the pH of the co-precipitation reaction is 8.5-9, for example, it can be 8.5, 8.6, 8.7, 8.8, 8.9 or 9, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0040] Preferably, the temperature of the co-precipitation reaction is 55-65℃, for example, it can be 55℃, 57℃, 59℃, 61℃, 63℃ or 65℃, and the time is 15-25 h, for example, it can be 15 h, 17 h, 19 h, 21 h, 23 h or 25 h, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0041] Preferably, the carbonate solution comprises a sodium carbonate solution.

[0042] Preferably, the bottom liquid is obtained by mixing deionized water and a complexing agent solution.

[0043] Preferably, the complexing agent solution comprises ammonia water.

[0044] In a second aspect, the present application provides a single-crystallized and coated modified lithium-rich manganese-based material, which is prepared by the preparation method as described in the first aspect.

[0045] In a third aspect, the present application provides a lithium ion battery comprising the single-crystallized and coated modified lithium-rich manganese-based material as described in the second aspect.

[0046] Compared with the prior art, the present application has the following beneficial effects:

[0047] The present application obtains complete crystal grains by high-temperature crystallization assisted by molten salt, and obtains single-crystal cobalt-free positive electrode material. The design of the single-crystal structure can improve the structural and interface stability of the material, thereby improving the cycle stability of the material. Then, the single-crystal cobalt-free positive electrode material in step (1) is mixed with a boron source solution to perform first coating, and a magnesium source solution is added to perform second coating. That is, the present application first coats boron on the surface of the single-crystal cobalt-free positive electrode material, and then coats magnesium. After secondary calcination, a gradient coating layer is obtained, which is rich in boron inside and rich in magnesium outside. The magnesium enriched on the surface also reacts with the boron near the surface. Finally, a boron-magnesium-containing gradient coating layer is obtained, which is rich in boron inside and rich in magnesium outside. The inner layer is rich in B, which makes the boron near the surface possibly form a Li-B-O or M-B-O (M=transition metal) interface layer or achieve trace phase doping. Boron is a strong crystal boundary strengthening agent and an oxygen stabilizer, which can significantly inhibit the release of lattice oxygen, the migration of transition metals, and the formation of rock salt phase, thereby stabilizing the body phase structure from the source and particularly helping to inhibit voltage drop. The outer layer is rich in Mg, which makes the magnesium-rich borate provide an excellent physical barrier to directly block the corrosion of the electrolyte, reduce side reactions, and protect the inner layer of the boron-rich stable layer. In addition, the magnesium-rich borate compound itself has good chemical stability and high mechanical strength. Therefore, the dual protection mechanism of the inner layer rich in B and the outer layer rich in Mg can more effectively inhibit phase transformation and side reactions than uniform borate coating, and can more effectively improve the capacity retention rate and cycle performance of the material. DETAILED DESCRIPTION

[0048] The technical solutions of the present application will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations on the present application.

[0049] Embodiment 1

[0050] The present embodiment provides a preparation method of a single-crystal and coating-modified lithium-rich manganese-based material, which comprises the following steps:

[0051] (1) Dissolve nickel sulfate (Ni2SO4·6H2O) and manganese sulfate (MnSO4·H2O) in deionized water to obtain a transition metal salt mixed solution (wherein the molar ratio of nickel ions to manganese ions = 1:3). In addition, dissolve sodium carbonate (Na2CO3) in deionized water in another container to prepare a Na2CO3 aqueous solution, which is used as a precipitating agent;

[0052] First, 1 L of deionized water and 100 mL of ammonia water were sequentially added into a continuously stirred reactor, stirring was started with a stirring speed of 500 r / min, and then nitrogen was introduced for 30 min to remove oxygen in the reactor. Subsequently, 500 mL of a transition metal salt mixed solution and 500 mL of a sodium carbonate aqueous solution were simultaneously pumped in, and then continuous stirring was performed at 58℃ for 20 h with pH controlled at 8.8±0.2. After the reaction was completed, the prepared carbonate precursor was washed multiple times (water washing) and dried (dried at 70℃ in a vacuum drying oven for 12 h) and then collected after being ground to obtain a cobalt-free carbonate precursor material;

[0053] (2) The cobalt-free carbonate precursor material, lithium hydroxide and potassium chloride were mixed in a zirconia ball mill at a speed of 300 r / min for 3.5 h to achieve the crushing of the polycrystalline precursor, wherein the molar ratio of (precursor+LiOH) / KCl was 1:4. Subsequently, the mixture obtained above was heated to 850℃ at a heating rate of 5℃ / min in an air atmosphere, and calcined at this temperature for 8 h. Then, the temperature was continued to be raised to 980℃ at a heating rate of 5℃ / min, and calcined at this temperature for 1 h. After the calcination was completed, it was naturally cooled to room temperature. The obtained powder was washed with deionized water for 5 times to remove the excess molten salt. Then, the single-crystal positive electrode material was continued to be calcined in an air atmosphere at 300℃ for 5 h, and the surface of the single-crystal particles was repaired to obtain a single-crystal cobalt-free positive electrode material;

[0054] (3) The single-crystal cobalt-free positive electrode material of step (2) was mixed with a boric acid solution, and the first coating was performed at a speed of 600 r / min and a temperature of 50℃ for 6 h to obtain a mixture, wherein the concentration of boric acid in the mixture was 0.1 mol / L; then a magnesium nitrate ethanol solution was added dropwise into the mixture, and after the addition of the magnesium nitrate ethanol solution, the concentration of magnesium nitrate in the system was 0.2 mol / L. The second coating was performed at a speed of 600 r / min and a temperature of 40℃ for 1.5 h, and then the coating was completed. After the coating was completed, filtration, washing and drying at 100℃ for 24 h were performed. Then, the dried powder material was heated to 500℃ at a heating rate of 5℃ / min in a tube furnace and calcined for 2 h, and a gradient coating layer was obtained on the surface of the single-crystal cobalt-free positive electrode material, wherein the side of the gradient coating layer close to the single-crystal cobalt-free positive electrode material was rich in boron, and the side far from the single-crystal cobalt-free positive electrode material was rich in magnesium, thereby obtaining the single-crystallized and coated modified lithium-rich manganese-based material, and the content of the gradient coating layer in the single-crystallized and coated modified lithium-rich manganese-based material was 2 wt%.

[0055] Example 2

[0056] The present embodiment provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, which comprises the following steps:

[0057] (1) Nickel sulfate (Ni2SO4·6H2O) and manganese sulfate (MnSO4·H2O) were dissolved in deionized water to obtain a mixed solution of transition metal salts (wherein the molar ratio of nickel ions to manganese ions = 1:3). In addition, sodium carbonate (Na2CO3) was dissolved in deionized water in another container to prepare a Na2CO3 aqueous solution, which was used as a precipitant.

[0058] First, 1 L of deionized water and 100 mL of ammonia water were sequentially added to a continuously stirred reactor, and stirring was started at a stirring speed of 500 r / min. Subsequently, nitrogen gas was introduced for 30 min to remove oxygen from the reactor. Next, 500 mL of the mixed solution of transition metal salts and 500 mL of the sodium carbonate aqueous solution were simultaneously pumped in, and then continuous stirring was performed at 55°C for 25 h, with the pH being controlled at 8.8±0.2. After the reaction was completed, the prepared carbonate precursor was washed multiple times (water washing) and dried (drying at 70°C in a vacuum drying oven for 12 h) and then collected after being ground to obtain a cobalt-free carbonate precursor material;

[0059] (2) The cobalt-free carbonate precursor material, lithium hydroxide, and potassium chloride were mixed in a zirconia ball mill at a speed of 250 r / min for 4 h to break the polycrystalline precursor, wherein the molar ratio of (precursor + LiOH) / KCl was 1:5. Subsequently, the mixture obtained above was heated to 900°C at a heating rate of 5°C / min in an air atmosphere, and calcination was performed at this temperature for 7 h. Next, heating was continued at a heating rate of 5°C / min to 1050°C, and calcination was performed at this temperature for 0.5 h. After the calcination was completed, the sample was naturally cooled to room temperature. The obtained powder was washed with deionized water 5 times to remove excess molten salt. Then, calcination was continued at 250°C in an air atmosphere for 6 h to repair the surface of the single-crystal particles of the obtained single-crystal positive electrode material, thereby obtaining a single-crystal cobalt-free positive electrode material;

[0060] (3) mixing the single-crystal cobalt-free cathode material of step (2) with a boric acid solution, carrying out first coating at a rotation speed of 700 r / min and a temperature of 40 °C for 8 h to obtain a mixture, in the mixture, the concentration of boric acid is 0.05 mol / L; then adding a magnesium nitrate ethanol solution dropwise into the mixture, after the addition of the magnesium nitrate ethanol solution, the concentration of magnesium nitrate in the system is 0.1 mol / L, carrying out second coating at a rotation speed of 700 r / min and a temperature of 30 °C for 1 h, and then coating is completed, after the completion of coating, carrying out filtration, washing and drying at 100 °C for 24 h, then heating the dried powder material to 450 °C at a heating rate of 5 °C / min in a tube furnace and calcining for 3 h, and obtaining a gradient coating layer on the surface of the single-crystal cobalt-free cathode material, wherein the side of the gradient coating layer close to the single-crystal cobalt-free cathode material is rich in boron, and the side of the gradient coating layer away from the single-crystal cobalt-free cathode material is rich in magnesium, and the single-crystal and coating-modified lithium-rich manganese-based material is obtained, and in the single-crystal and coating-modified lithium-rich manganese-based material, the content of the gradient coating layer is 1 wt%.

[0061] Example 3

[0062] The present embodiment provides a preparation method of a single-crystal and coating-modified lithium-rich manganese-based material, and the preparation method comprises the following steps:

[0063] (1) dissolving nickel sulfate (Ni2SO4·6H2O) and manganese sulfate (MnSO4·H2O) in deionized water to obtain a transition metal salt mixed solution (wherein the molar ratio of nickel ions to manganese ions = 1:5). In addition, sodium carbonate (Na2CO3) is dissolved in deionized water in another container to prepare a Na2CO3 aqueous solution, and the Na2CO3 aqueous solution is used as a precipitant.

[0064] First, 1 L of deionized water and 100 mL of ammonia water are sequentially added to a continuously stirred reactor, stirring is started, the stirring speed is 500 r / min, and then nitrogen is introduced for 30 min to remove oxygen in the reactor. Then, 500 mL of the transition metal salt mixed solution and 500 mL of the sodium carbonate aqueous solution are pumped in at the same time, and then continuously stirred at 65 °C for 15 h, with the pH controlled at 8.8±0.2, after the reaction is completed, the prepared carbonate precursor is washed (washed with water) and dried (dried at 70 °C in a vacuum drying oven for 12 h) and ground, and then collected to obtain a cobalt-free carbonate precursor material;

[0065] (2) The cobalt-free carbonate precursor material, lithium hydroxide and potassium chloride are mixed in a zirconium oxide ball mill at a speed of 350 r / min for 3 h to achieve the crushing of the polycrystalline precursor, wherein the molar ratio of (precursor + LiOH) / KCl is 1:3. Subsequently, the mixture obtained above is heated to 800℃ at a heating rate of 5℃ / min in an air atmosphere, and calcined at this temperature for 9 h. Then, the heating is continued at a heating rate of 5℃ / min to 950℃, and calcined at this temperature for 1.5 h, and after the calcination is completed, it is naturally cooled to room temperature. The obtained powder is washed with deionized water for 5 times to remove the excess molten salt. Then, the single-crystal positive electrode material obtained after the single-crystal particle surface repair is calcined at 350℃ in an air atmosphere for 4 h, to obtain a single-crystal cobalt-free positive electrode material;

[0066] (3) The single-crystal cobalt-free positive electrode material in step (2) is mixed with a boric acid solution, and the first coating is carried out at a speed of 500 r / min and a temperature of 60℃ for 4 h to obtain a mixture, wherein the concentration of boric acid in the mixture is 0.15 mol / L; then a magnesium nitrate ethanol solution is added dropwise to the mixture, and after the addition of the magnesium nitrate ethanol solution, the concentration of magnesium nitrate in the system is 0.3 mol / L. The second coating is carried out at a speed of 500 r / min and a temperature of 50℃ for 2 h, and then the coating is completed. After the coating is completed, filtration, washing and drying at 100℃ for 24 h are carried out, and then the dried powder material is heated to 550℃ at a heating rate of 5℃ / min in a tube furnace and calcined for 1 h, to obtain a gradient coating layer on the surface of the single-crystal cobalt-free positive electrode material, wherein the side of the gradient coating layer close to the single-crystal cobalt-free positive electrode material is rich in boron, and the side far from the single-crystal cobalt-free positive electrode material is rich in magnesium, to obtain the single-crystallized and coated modified lithium-rich manganese-based material, wherein the content of the gradient coating layer in the single-crystallized and coated modified lithium-rich manganese-based material is 3 wt%.

[0067] Example 4

[0068] The present embodiment provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, and the temperature of the second coating in step (3) is 20℃, and the rest are the same as in example 1.

[0069] Example 5

[0070] The present embodiment provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, and the temperature of the second coating in step (3) is 60℃, and the rest are the same as in example 1.

[0071] Example 6

[0072] The present embodiment provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, and the time of the second coating in step (3) is 0.5 h, and the rest are the same as in example 1.

[0073] Example 7

[0074] This example provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, and steps (3) is different from example 1 in that the second coating time is 3h, and the rest are the same as example 1.

[0075] Example 8

[0076] This example provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, and step (2) is different from example 1 in that the temperature is continuously increased to 900℃ at a rate of 5℃ / min, and the rest are the same as example 1.

[0077] Example 9

[0078] This example provides a preparation method of a single-crystallized and coated modified lithium-rich manganese-based material, and step (2) is different from example 1 in that the temperature is continuously increased to 1150℃ at a rate of 5℃ / min, and the rest are the same as example 1.

[0079] Comparative Example 1

[0080] This comparative example provides a preparation method of a polycrystalline cobalt-free positive electrode material, which is different from example 1 in that the cobalt-free carbonate precursor material in step (1) is mixed with 5wt% excess LiOH powder in a polyurethane ball mill (to maintain the polycrystalline precursor morphology undamaged), and then calcined at 450℃ for 5h in a tube furnace under an oxygen atmosphere, and then the temperature is increased to 850℃ and calcined for 12h, and the temperature increasing rate is 5℃ / min, and finally a polycrystalline cobalt-free positive electrode material is obtained, and step (3) is not performed, and the rest are the same as example 1.

[0081] Comparative Example 2

[0082] This comparative example provides a preparation method of a single-crystallized lithium-rich manganese-based material, which is different from example 1 in that step (3) is not performed, and the rest are the same as example 1.

[0083] Comparative Example 3

[0084] This comparative example provides a preparation method of a single-crystallized lithium-rich manganese-based material, which is different from example 1 in that step (3) is different from example 1 in that the single-crystalline cobalt-free positive electrode material in step (2), boric acid solution and magnesium nitrate ethanol solution are mixed, and the coating is completed at a speed of 600r / min and a temperature of 50℃ for 6h, and the rest are the same as example 1.

[0085] The material obtained in the above examples and comparative examples is prepared into a positive electrode sheet, and a polypropylene separator (Celgrad 2400), a lithium sheet and an electrolyte (1 mol / L of LiPF6 is dissolved in a mixed solvent of EC, DEC and EMC in a volume ratio of 1:1:1, and assembled in an argon-filled glove box to obtain a lithium ion battery. Then the capacity and cycle performance of the lithium ion battery are tested, and the capacity test conditions are as follows: at room temperature 25℃, the battery charge and discharge voltage range is 2.5-4.6V (vs. Li / Li + ), and the first charge and discharge test is carried out at 0.1C rate. The cycle performance test conditions are as follows: at room temperature 25℃, the voltage range is 2.5-4.6V, and the charge and discharge cycle test is carried out at 1C rate, and the cycle is 200 times.

[0086] The test results are shown in Table 1:

[0087] Table 1

[0088]

[0089] From the above Table 1, it can be seen that:

[0090] From Example 1 and Comparative Example 1, it can be seen that the Comparative Example 1 prepared is a polycrystalline cobalt-free positive electrode material, and compared with the single-crystal and coated synergistically modified material of the application, the capacity and cycle performance are greatly reduced; from Example 1 and Comparative Example 2, it can be seen that if no coating treatment is performed, only single-crystal modification is performed, although the electrochemical performance can be improved compared with polycrystalline material, the electrochemical performance is still inferior to the single-crystal and coated synergistically modified material of the application; from Example 1 and Examples 4-7, it can be seen that the temperature and time of the second coating of the application will affect the gradient distribution of the coating layer, and affect the effect of the gradient coating layer; from Example 1 and Examples 8-9, it can be seen that the temperature during the preparation of the single-crystal material will affect the quality of the single crystal, thereby affecting the performance of the material.

[0091] The above is only a specific embodiment of the application, but the protection scope of the application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed by the application can be easily thought of by those skilled in the art, and all fall within the protection scope and disclosure scope of the application.

Claims

1. A method for preparing a single-crystallized and coating-modified lithium-rich manganese-based material, characterized in that, The preparation method comprises the following steps: (1) mixing a cobalt-free carbonate precursor material, a lithium source and a molten salt by ball milling, then performing a first calcination, washing and repairing heat treatment to obtain a single-crystal cobalt-free positive electrode material; (2) mixing the single-crystal cobalt-free positive electrode material in step (1) with a boron source solution to perform a first coating, obtaining a mixture, then adding a magnesium source solution to the mixture to perform a second coating, after the coating is completed, performing solid-liquid separation, washing, drying and secondary calcination to obtain a gradient coating layer on the surface of the single-crystal cobalt-free positive electrode material, wherein the side of the gradient coating layer close to the single-crystal cobalt-free positive electrode material is rich in boron, and the side far from the single-crystal cobalt-free positive electrode material is rich in magnesium, thereby obtaining the single-crystal and coating-modified lithium-rich manganese-based material.

2. The production method according to claim 1, characterized by, The temperature of the second coating in step (2) is lower than that of the first coating; Preferably, the time of the second coating in step (2) is lower than that of the first coating; Preferably, the temperature of the first coating in step (2) is 40-60℃, and the time is 4-8h; Preferably, the temperature of the second coating in step (2) is 30-50℃, and the time is 1-2h; Preferably, the rotation speed of the first coating and the second coating in step (2) is independently 500-700r / min.

3. The production method according to claim 1 or 2, characterized by, The boron source solution in step (2) comprises a boric acid solution and / or a borax solution; Preferably, in the mixture in step (2), the concentration of the boron source is 0.05-0.15mol / L; Preferably, the magnesium source solution in step (2) comprises a magnesium nitrate ethanol solution; Preferably, after the magnesium source solution is added in step (2), the concentration of the magnesium source in the system is 0.1-0.3mol / L.

4. The production method according to any one of claims 1 to 3, characterized by, The method for adding the magnesium source solution in step (2) is dropwise addition; Preferably, the temperature of the secondary calcination in step (2) is 450-550℃, and the time is 1-3h; Preferably, in the single-crystal and coating-modified lithium-rich manganese-based material in step (2), the content of the gradient coating layer is 1-3wt%.

5. The production method according to any one of claims 1 to 4, characterized by, The first calcination in step (1) comprises first holding at 800-900℃ for 7-9h, then increasing the temperature to 950-1050℃ and holding for 0.5-1.5h; Preferably, the temperature of the repairing heat treatment in step (1) is 250-350℃, and the time is 4-6h.

6. The method of any one of claims 1-5, wherein, The total molar amount of the cobalt-free carbonate precursor material and the lithium source to the molar amount of the molten salt in step (1) is 1:(3-5); Preferably, the molten salt in step (1) comprises potassium chloride; Preferably, the rotation speed of the ball milling in step (1) is 250-350r / min, and the time is 3-4h.

7. The method of any one of claims 1-6, wherein, The method for preparing the cobalt-free carbonate precursor material in step (1) comprises the following steps: passing a mixed metal salt solution and a carbonate solution into a bottom liquid to perform a coprecipitation reaction to obtain the cobalt-free carbonate precursor material; Preferably, the mixed metal salt solution comprises nickel ions and manganese ions; Preferably, in the mixed metal salt solution, the molar ratio of nickel ions to manganese ions is 1:(2-4).

8. The production method according to claim 7, characterized by, The pH of the coprecipitation reaction is 8.5-9; Preferably, the temperature of the coprecipitation reaction is 55-65℃, and the time is 15-25h; Preferably, the carbonate solution comprises a sodium carbonate solution; Preferably, the base solution is obtained by mixing deionized water and a complexing agent solution; Preferably, the complexing agent solution comprises ammonia.

9. A single-crystallized and coating-modified lithium-rich manganese-based material, characterized in that, The single-crystallized and coated modified lithium-rich manganese-based material is prepared by the preparation method of any one of claims 1-8.

10. A lithium-ion battery, characterized by, The lithium ion battery comprises the single-crystallized and coated modified lithium-rich manganese-based material of claim 9.

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