Multi-element carbon-coated lithium manganate, method for preparing the same, and secondary battery

Abstract

The application relates to the technical field of material preparation, and discloses multi-element carbon-coated lithium manganate, a preparation method thereof and a secondary battery. The multi-element carbon-coated lithium manganate comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer which are sequentially coated on the lithium manganate core. The chemical formula of the lithium manganate core is LiAl a X b Mn 2‑a‑b O4, wherein 0.01<=a<=0.10, 0.01<=b<=0.05, and X comprises at least one of Co, Cr, Ti, V and B. The preparation method of the multi-element carbon-coated lithium manganate comprises the following steps: (I) preparing doped lithium manganate, (II) preparing carbon nanotubes and (III) multi-element carbon coating. The multi-element carbon-coated lithium manganate has high conductivity, rate performance, cycle performance and safety performance, and can be used as an alternative positive electrode material with high cycle and excellent fast-charging performance.

Description

Technical Field

[0001] This invention relates to the field of materials preparation technology, particularly to lithium manganese oxide, and more particularly to multi-element carbon-coated lithium manganese oxide, its preparation method, and secondary batteries. Background Technology

[0002] Although spinel lithium manganese oxide (LiMn₂O₄, LMO) offers advantages such as high voltage, good stability, low toxicity, and good safety when used as a cathode material in lithium-ion batteries, it still suffers from capacity decay caused by a combination of structural transformation and transition metal dissolution. The Jan Taylor effect distortion during discharge causes LMO to shift from a cubic phase to a less symmetric tetragonal phase, resulting in large anisotropic volume changes and structural damage. In addition, the dissolution of manganese (Mn) in spinel lithium manganese oxide is also a significant problem. Erosion from residual water in the electrolyte and HF formed by LiPF₆ can cause manganese dissolution, leading to the dissolution of Mn. 3+ It will deposit on the surface of the negative electrode. During the overcharging process, Mn... 4+ In the enriched state, the increased solubility of Mn due to the instability of the charge-terminated delithiation structure leads to the loss of MnO and the formation of a more stable single-phase structure. The phase transition can cause manganese to dissolve from spinel at high potentials; after manganese dissolution, Mn... 2+ Dissolving in the electrolyte and migrating to the negative electrode under the drive of concentration gradient and / or electric field, zero-valent manganese gradually covers the negative electrode surface, interfering with lithium intercalation into the graphite and increasing the negative electrode impedance. Furthermore, the deposited Mn can react with carbonates and Li in the electrolyte. + The reaction forms inactive Li₂CO₃. This process consumes active lithium ions over a long period, leading to severe capacity decay.

[0003] To solve the above problems, researchers have done a lot of research. At present, the main method is to modify LMO, such as synthesizing nanomaterials, ion doping and surface coating. But the above methods still have various problems in practical application. For example, the doping and synthesis of nanomaterials lead to the decrease of the overall battery capacity due to the non-active cation dopant. In addition, they cannot prevent the side reaction of Mn dissolution at the interface between the electrode and the electrolyte during the charging / discharging process. Surface coating can maximize the reduction of direct contact between active positive electrode material and electrolyte, reduce Mn dissolution, and reduce the deposition of Mn on the negative electrode surface, thereby reducing impedance growth. However, surface coating usually requires complex high-cost synthesis processes such as wet chemical coating and atomic layer deposition, which is not suitable for commercial scale lithium ion batteries. Therefore, carbon materials with high electronic / ionic conductivity, good structure and thermal stability have good modification effect on LMO. Carbon surface modification can promote the continuous electron path of the coating shell, thereby improving the utilization rate of active materials. It not only avoids direct contact between LMO and electrolyte, prevents electrode dissolution and material side reactions, but also provides good conductive support for the electrode and inhibits nanoparticle agglomeration. With further research, it is found that the type, content and modification method of carbon material have a great influence on the electrochemical performance of the bulk LiMn2O4 material, so the method still needs to be improved to optimize the appropriate process route to meet the development of commercial carbon-coated LMO. SUMMARY

[0004] In view of this, the purpose of the present application is to provide a multi-element carbon-coated lithium manganate and a preparation method thereof, and a secondary battery. The multi-element carbon-coated lithium manganate has high conductivity, rate capability, cycle performance and safety performance, and can be used as a high-cycle alternative positive electrode material with excellent fast-charging performance.

[0005] To achieve the above purpose, the first aspect of the present application provides a multi-element carbon-coated lithium manganate. The multi-element carbon-coated lithium manganate comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core. The chemical formula of the lithium manganate core is LiAl a X b Mn 2-a-b O4, wherein 0.01≦a≦0.10, 0.01≦b≦0.05, and X comprises at least one of Co, Cr, Ti, V and B.

[0006] The poly-element carbon-coated lithium manganate of the present application is doped with at least two elements, and one of the elements is Al. The lithium manganate material doped with Al can improve the structural stability. The lithium manganate material prepared by doping with multiple elements has uniform particle size, and the particle size is large and the powder is less. The safety performance of the material is higher, and the electrochemical decay is reduced. The double-layer coating of the carbon nanotube layer and the hard carbon layer can avoid the side reaction of the lithium manganate core directly contacting with the electrolyte, thereby inhibiting the irreversible loss of manganese, avoiding the severe interface reaction under high pressure and the oxygen evolution problem of the material itself, so as to improve the cycle performance. At the same time, the double-layer coating of the carbon nanotube layer and the hard carbon layer can provide good conductive support for the material, improve the electron transport capacity of the electrode material, accelerate the deintercalation of lithium ions, improve the high-rate performance, and further improve the utilization rate of the active material. The design of the double-layer coating layer can not only inhibit the phase change and the agglomeration of the nanoparticles, but also maximize the reduction of the structural deformation caused by the volume change due to the intercalation and deintercalation of Li + , thereby improving the battery performance.

[0007] As a technical solution of the present application, the thickness of the carbon nanotube layer is 20-30 nm, and the thickness of the hard carbon layer is 0.5-1.5 μm.

[0008] The present application provides a preparation method of the poly-element carbon-coated lithium manganate.

[0009] (I) Preparation of doped lithium manganate

[0010] According to the formula, lithium source, aluminum source, X source and manganese source are taken into a ball mill, a solvent is added for ball milling, and then the filter cake is taken and sequentially dried, crushed and sintered;

[0011] (II) Preparation of carbon nanotubes

[0012] After mixing ferrocene and carbon source, heating to 700-900 DEG C under inert atmosphere, introducing water vapor into the system, and performing constant temperature reaction under reducing atmosphere and 700-900 DEG C, then cooling to room temperature under inert atmosphere and performing acid washing and post-treatment;

[0013] (III) Poly-element carbon coating

[0014] The doped lithium manganate, carbon nanotubes and soft carbon coating agent are added into the ball mill, a solvent is added for ball milling to obtain a slurry, the slurry and water are ultrasonically mixed to obtain a dispersion liquid, the dispersion liquid is subjected to hydrothermal reaction, then cooled and filtered to obtain a precursor, and the precursor is carbonized under inert atmosphere.

[0015] The preparation method of the present application can dope multiple elements by liquid-phase ball milling. When preparing carbon nanotubes from a carbon source by introducing water vapor using ferrocene as a catalyst, defects can be introduced at the edges of the prepared carbon nanotubes and hydroxyl groups, carboxyl groups and other oxygen-containing groups can be co-embedded, and at the same time, the ferrocene can not be passivated at the late stage of carbon nanotube growth, and can still maintain catalytic activity during the long growth process. When performing multi-element carbon coating, the dispersion liquid is subjected to hydrothermal reaction, which can uniformly coat the LMO and carbon nanotube surface with soft carbon coating agent, and under the action of oxygen-containing groups on the carbon nanotube, the soft carbon is carbonized to become hard carbon, thereby improving the performance of the LMO material. Carbonizing the carbon nanotubes and the soft carbon coating agent together for coating helps the soft carbon coating agent to uniformly coat the surface of the LMO and the carbon nanotubes, thereby reducing fine powder, and the prepared material has large particle size, better anisotropy than the material prepared by first forming a carbon nanotube layer and then forming a hard carbon layer, and is more conducive to the de-intercalation of lithium ions on the surface and inside of the material, thereby the material has better rate performance.

[0016] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium acetate.

[0017] In some embodiments, the aluminum source includes aluminum oxide and / or aluminum hydroxide.

[0018] In some embodiments, the manganese source includes manganese dioxide.

[0019] In some embodiments, the X source is an oxide of X, X including at least one of Co, Cr, Ti, V, and B.

[0020] In some embodiments, the X source includes at least one of Co3O4, Cr2O3, TiO2, V2O5, and H2BO3.

[0021] In some embodiments, the solvent in step (I) for preparing the manganese-doped lithium manganate includes at least one of ethanol, acetone, isopropanol, n-butanol, ethylene glycol, n-hexanol, and methanol.

[0022] In some embodiments, the ball milling in step (I) for preparing the manganese-doped lithium manganate and step (III) for multi-element carbon coating is performed in a protective atmosphere, the protective atmosphere including at least one of nitrogen, argon, and helium.

[0023] In some embodiments, the drying temperature is 60-100°C, and the time is 6-20h.

[0024] In some embodiments, the crushing is to a particle size of 300-500 mesh.

[0025] In some embodiments, the sintering device is a muffle furnace.

[0026] In some embodiments, the sintering temperature is 700-900 °C for 10-30 h.

[0027] In some embodiments, the carbon source is at least one of starch, coal tar pitch, resin, and melamine.

[0028] In some embodiments, the inert atmosphere in step (II) for preparing carbon nanotubes and step (III) for multi-element carbon coating is at least one of nitrogen, argon, and helium.

[0029] In some embodiments, the gas flow rate of the inert atmosphere in step (II) for preparing carbon nanotubes and step (III) for multi-element carbon coating is independently 0.3-10.0 L / min.

[0030] In some embodiments, the carbon nanotubes are prepared in a tube furnace.

[0031] In some embodiments, the reducing atmosphere comprises at least hydrogen.

[0032] In some embodiments, the gas flow rate of the reducing atmosphere is 0.3-10.0 L / min.

[0033] In some embodiments, the gas flow rate of the water vapor is 0.1-5.0 L / min.

[0034] In some embodiments, the water vapor is derived from a water vapor generator.

[0035] In some embodiments, the ball milling time in step (I) for preparing manganese-doped lithium oxide and step (III) for multi-element carbon coating is independently 8-20 h.

[0036] In some embodiments, the acid used in the acid washing is hydrochloric acid, nitric acid, or dilute sulfuric acid.

[0037] In some embodiments, the post-treatment comprises water washing, drying, and grinding in sequence after the acid washing.

[0038] In some embodiments, the mass ratio of the manganese-doped lithium oxide, the carbon nanotubes, and the soft carbon-based coating agent is 85-95:0.5-2:5-15.

[0039] In some embodiments, the soft carbon-based coating agent comprises solid pitch and / or liquid pitch.

[0040] In some embodiments, the coking value of the soft carbon-based coating agent is 10-80%.

[0041] In some embodiments, the solvent in step (II) for preparing carbon nanotubes comprises at least one of ethanol, acetone, isopropanol, n-butanol, ethylene glycol, n-hexanol, and methanol.

[0042] In some embodiments, the power of the ultrasonic is 600 W, and the ultrasonic time is 1-10 h.

[0043] In some embodiments, the temperature of the hydrothermal reaction is 150-250℃, and the time is 1-10 h.

[0044] In some embodiments, the equipment for carbonization is a roller kiln, a rotary furnace, a box furnace or a vertical tank.

[0045] In some embodiments, the heating in step (II) for preparing carbon nanotubes employs gradient heating.

[0046] In some embodiments, the heating in step (II) for preparing carbon nanotubes is first raised to 200-400℃ at a temperature raising rate of 3-10℃ / min, and then raised to 700-900℃ at a temperature raising rate of 1-5℃ / min.

[0047] In some embodiments, the temperature of carbonization is raised in a gradient, and the highest temperature is 800-900℃.

[0048] In some embodiments, the temperature raising rate of carbonization is 1-5℃ / min.

[0049] In some embodiments, the temperature of carbonization is first raised to 250-300℃, and then raised to 300-480℃, 480-850℃ in sequence, and then kept for 120-300 min.

[0050] To achieve the above-mentioned purpose, the third aspect of the present application provides a secondary battery, comprising a positive electrode material and a negative electrode material, wherein the positive electrode material is the aforementioned multi-element carbon-coated lithium manganate or the multi-element carbon-coated lithium manganate prepared by the aforementioned method for preparing multi-element carbon-coated lithium manganate. DETAILED DESCRIPTION

[0051] The multi-element carbon-coated lithium manganate of the present application refers to lithium manganate coated with at least two kinds of carbon materials. Specifically, the present application employs carbon nanotubes and hard carbon to coat lithium manganate. The multi-element carbon-coated lithium manganate of the present application can be used as a positive electrode active material in a secondary battery. The secondary battery comprises a positive electrode material and a negative electrode material. The positive electrode material can be further mixed with lithium cobaltate-based positive electrode materials, lithium iron phosphate-based positive electrode materials, nickel cobalt lithium manganate-based positive electrode materials or nickel cobalt aluminum lithium manganate-based positive electrode materials, etc. The negative electrode material can be but is not limited to carbon-based materials and silicon-based materials. The carbon-based materials can be but are not limited to graphite-based materials, soft carbon or hard carbon. The silicon-based materials can be but are not limited to SiO x or carbon-coated SiO x, 0≤x<2. The multi-element carbon-coated lithium manganate of the present application has better conductivity itself, so that less conductive agent can be added when preparing the positive electrode sheet according to actual use.

[0052] The multi-element carbon-coated lithium manganate of the present application comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core. The chemical formula of the lithium manganate core is LiAl a X b Mn 2-a-b O4, wherein 0.01≦a≦0.10, 0.01≦b≦0.05, and X comprises at least one of Co, Cr, Ti, V and B. As an example, a can be but is not limited to 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1. b can be but is not limited to 0.01, 0.02, 0.03, 0.04, 0.05. As an example, x is Ti.

[0053] As an embodiment of the present application, the thickness of the carbon nanotube layer is 20-30 nm. As an example, the thickness of the carbon nanotube layer can be but is not limited to 20-30 nm. The thickness of the hard carbon layer is 0.5-1.5 μm. As an example, the thickness of the hard carbon layer can be but is not limited to 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm.

[0054] The preparation method of the multi-element carbon-coated lithium manganate of the present application comprises steps (I) preparing doped lithium manganate, (II) preparing carbon nanotubes and (III) multi-element carbon coating.

[0055] In step (I) for preparing doped lithium manganate, lithium source, aluminum source, X source and manganese source are taken in a ball mill according to the formula ratio, a solvent is added for ball milling, the filter cake is taken after filtration, and drying, crushing and sintering are successively performed.

[0056] As an embodiment of the present application, the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium acetate, and as an example, the lithium source is lithium carbonate. The aluminum source comprises aluminum oxide and / or aluminum hydroxide, and as an example, the aluminum source is aluminum oxide. The manganese source comprises manganese dioxide.

[0057] As an embodiment of the present application, the X source is an oxide of X, and X comprises at least one of Co, Cr, Ti, V and B. In some embodiments, the X source comprises at least one of Co3O4, Cr2O3, TiO2, V2O5 and H2BO3. In other embodiments, the X source is TiO2.

[0058] As an embodiment of the present application, the solvent includes at least one of ethanol, acetone, isopropyl alcohol, n-butanol, ethylene glycol, n-hexanol, and methanol. As an example, the solvent is ethanol.

[0059] As an embodiment of the present application, the ball milling is performed under a protective atmosphere, which includes at least one of nitrogen, argon, and helium.

[0060] As an embodiment of the present application, the ball milling time is 8h to 20h, and as an example, the ball milling time can be, but is not limited to, 68h, 10h, 12h, 14h, 16h, 18h, 20h.

[0061] As an embodiment of the present application, the drying temperature is 60℃ to 100℃, and as an example, the drying temperature can be, but is not limited to, 60℃, 70℃, 80℃, 90℃, 100℃. The drying time is 6h to 20h, and as an example, the drying time can be, but is not limited to, 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h.

[0062] As an embodiment of the present application, the pulverization is performed to pass through a 300 mesh to 500 mesh screen. The sintering device is a muffle furnace.

[0063] As an embodiment of the present application, the sintering temperature is 700℃ to 900℃. As an example, the sintering temperature can be, but is not limited to, 700℃, 730℃, 750℃, 780℃, 800℃, 830℃, 850℃, 880℃, 900℃. The sintering time is 10h to 30h, and as an example, the sintering time can be, but is not limited to, 10h, 13h, 15h, 17h, 20h, 23h, 26h, 28h, 30h.

[0064] The step (II) for preparing the carbon nanotube includes mixing the ferrocene and the carbon source, heating to 700℃ to 900℃ under an inert atmosphere, introducing water vapor into the system, performing a constant temperature reaction under a reducing atmosphere and 700℃ to 900℃, and then reducing to room temperature under an inert atmosphere and performing acid washing and post-treatment.

[0065] As an embodiment of the present application, the carbon source is at least one of starch, coal tar pitch, resin, and melamine.

[0066] As an embodiment of the present application, the heating is performed by gradient heating to 700-900 °C, for example, but not limited to, 700 °C, 730 °C, 750 °C, 770 °C, 800 °C, 830 °C, 850 °C, 880 °C, 900 °C. The gradient heating is performed by preheating, which is helpful to form carbon nanotubes. In some embodiments, the heating is performed by first increasing the temperature to 200-400 °C at a rate of 3-10 °C / min, and then increasing the temperature to 700-900 °C at a rate of 1-5 °C / min.

[0067] As an embodiment of the present application, the inert atmosphere is at least one of nitrogen, argon and helium. The flow rate of the inert atmosphere is 0.3-10.0 L / min, for example, but not limited to, 0.3 L / min, 1.0 L / min, 2.0 L / min, 3.0 L / min, 4.0 L / min, 5.0 L / min, 6.0 L / min, 7.0 L / min, 8.0 L / min, 9.0 L / min, 10.0 L / min.

[0068] As an embodiment of the present application, the carbon nanotubes are prepared in a tube furnace. The reducing atmosphere comprises at least hydrogen, which is helpful to form carbon nanotubes. The flow rate of the reducing atmosphere is 0.3-10.0 L / min, for example, but not limited to, 0.3 L / min, 1.0 L / min, 2.0 L / min, 3.0 L / min, 4.0 L / min, 5.0 L / min, 6.0 L / min, 7.0 L / min, 8.0 L / min, 9.0 L / min, 10.0 L / min. The flow rate of the water vapor is 0.1-5.0 L / min, for example, but not limited to, 0.1 L / min, 1.0 L / min, 2.0 L / min, 3.0 L / min, 4.0 L / min, 5.0 L / min. As an embodiment of the present application, the water vapor is generated by a water vapor generator.

[0069] As an embodiment of the present application, the ball milling time is independently 8-20 h, for example, but not limited to, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h.

[0070] As an embodiment of the present application, the acid used in the acid washing is hydrochloric acid, nitric acid or dilute sulfuric acid.

[0071] As an embodiment of the present application, the post-treatment includes water washing, drying and grinding in sequence after the acid washing. The acid washing and water washing can be repeated for multiple times to remove the ferrocene catalyst. The temperature of the drying is 60-100°C, and as an example, the temperature of the drying can be but is not limited to 60°C, 70°C, 80°C, 90°C, 100°C. The time of the drying is 6-20h, and as an example, the time of the drying can be but is not limited to 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h. The grinding can be to a particle size of 300-500 mesh.

[0072] The step (III) of the multi-element carbon coating includes adding the doped lithium manganese oxide, the carbon nanotube and the soft carbon coating agent into a ball mill, adding a solvent to ball mill to obtain a slurry, ultrasonically mixing the slurry and water to obtain a dispersion, performing a hydrothermal reaction on the dispersion, cooling and filtering to obtain a precursor, and performing carbonization on the precursor under an inert atmosphere.

[0073] As an embodiment of the present application, the mass ratio of the doped lithium manganese oxide, the carbon nanotube and the soft carbon coating agent is 85-95:0.5-2:5-15. As an example, the mass ratio can be but is not limited to 85:1.2:6, 86:2:6, 88:1.0:7, 90:2.0:8, 93:2.0:10, 95:2.0:13, 86:1.5:7, 88:1.5:10, 89:1.5:10, 90:1:9, 91:1.5:10, 94:1.8:12, 94:1.0:12, 87:1.0:8, 88:1.7:8, 90:1.3:9, 93:1.8:10, 95:1.4:9, 93:2.0:11.

[0074] As an embodiment of the present application, the soft carbon coating agent is solid phase pitch and / or liquid phase pitch, and the coking value is 10%-80%, and as some embodiments, the coking value is 30%-80%, and as some embodiments, the coking value is 50%-80%. The coking value can be but is not limited to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% in particular.

[0075] As an embodiment of the present application, the ball milling is performed under a protective atmosphere, and the protective atmosphere includes at least one of nitrogen, argon and helium.

[0076] As an embodiment of the present application, the inert atmosphere in the carbonization is at least one of nitrogen, argon and helium, independently. The gas flow of the inert atmosphere is 0.3 L / min to 10.0 L / min, for example, the gas flow of the inert atmosphere can be, but is not limited to, 0.3 L / min, 1.0 L / min, 2.0 L / min, 3.0 L / min, 4.0 L / min, 5.0 L / min, 6.0 L / min, 7.0 L / min, 8.0 L / min, 9.0 L / min, 10.0 L / min.

[0077] As an embodiment of the present application, the ball milling time is 8 h to 20 h, for example, the ball milling time can be, but is not limited to, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h.

[0078] As an embodiment of the present application, the solvent includes at least one of ethanol, acetone, isopropanol, n-butanol, ethylene glycol, n-hexanol and methanol.

[0079] As an embodiment of the present application, the power of the ultrasonic is 600 W. The ultrasonic time is 1 h to 10 h, for example, the ultrasonic time can be, but is not limited to, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h.

[0080] As an embodiment of the present application, the temperature of the hydrothermal reaction is 150 °C to 250 °C, for example, the temperature of the hydrothermal reaction can be, but is not limited to, 150 °C, 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, 210 °C, 220 °C, 230 °C, 230 °C, 250 °C. The time of the hydrothermal reaction is 1 h to 10 h, for example, the time of the hydrothermal reaction can be, but is not limited to, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h.

[0081] As an embodiment of the present application, the equipment for carbonization is a roller kiln, a rotary furnace, a box furnace or a vertical tank.

[0082] As an embodiment of the present application, the temperature of carbonization is raised by gradient, and the highest temperature is raised to 800-900°C, and the highest temperature can be but is not limited to 800°C, 820°C, 840°C, 860°C, 880°C, 900°C. The gradient raising of temperature is advantageous to the conversion of the soft carbon coating agent from soft carbon to hard carbon. The temperature of carbonization is raised at a rate of 1-5°C / min, and the rate can be but is not limited to 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min. The temperature of carbonization can be raised by four gradients, as an example, the temperature of carbonization is first raised to 250-300°C, and then raised to 300-480°C, 480-850°C in turn, and then kept for 120-300 min, and the time can be but is not limited to 120 min, 130 min, 150 min, 170 min, 190 min, 210 min, 230 min, 250 min, 270 min, 290 min, 300 min.

[0083] For better illustrating the purpose, technical scheme and beneficial effects of the present application, the present application will be further explained in combination with specific examples. It should be noted that the following described method is a further explanation of the present application, and should not be regarded as a limitation of the present application.

[0084] Example 1

[0085] The present example is a multi-element carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0086] The preparation method of the multi-element carbon-coated lithium manganate of the present example comprises the following steps.

[0087] (I) Preparation of doped lithium manganate

[0088] According to the formula, lithium carbonate, nano-alumina, titanium dioxide and electrolytic manganese dioxide are taken in a ball mill, ethanol is added, and ball milling is carried out under nitrogen atmosphere for 10 h, then the filter cake is taken after suction filtration, dried at 80°C for 12 h, then crushed and passed through a 400 mesh sieve, and then put into a muffle furnace and sintered at 800°C under nitrogen atmosphere for 20 h.

[0089] (II) Preparation of carbon nanotubes

[0090] The ferrocene and coal tar pitch were mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace, the tube furnace temperature program was set, the furnace temperature was heated from room temperature to 300℃ at a heating rate of 5℃ / min under nitrogen atmosphere (gas flow rate was 5L / min), and kept at 300℃ for 1h, then the furnace temperature was heated from 300℃ to 800℃ at a heating rate of 3℃ / min, after the temperature reached stability, the water vapor generator was opened, water vapor (water vapor gas flow rate was 3.0L / min) was introduced into the reaction system, and immediately moved the tube furnace so that the inner container was placed in the middle of the heated area, kept at 800℃ for 30min under hydrogen atmosphere (gas flow rate was 2.0L / min), after the carbon nanotube growth was completed, the furnace temperature was turned off under hydrogen atmosphere and naturally cooled to room temperature under nitrogen atmosphere (gas flow rate was 3L / min). After 10 cycles of hydrochloric acid pickling-water washing, filtration, drying at 100℃ for 10h, and grinding to 500 mesh screen, the product was obtained.

[0091] (III) Multi-element carbon-coated

[0092] The manganese-doped lithium manganate, carbon nanotubes and liquid pitch (the coking value of the liquid pitch was 60%, and the mass ratio of the three was 90:1:9) were added into a ball mill and ball milled for 14h, ethanol was added for ball milling to obtain a slurry, the slurry and water were mixed at 600W for 6h to obtain a dispersion liquid, the dispersion liquid was transferred into a hydrothermal reaction kettle, and hydrothermal reaction was carried out at 200℃ for 5h, after self-ignition cooling to room temperature, the product in the reaction kettle was suction filtered and dried to obtain a precursor, the precursor was loaded into a carbonization crucible and then placed into a roller hearth kiln for carbonization, and the carbonization product was obtained by reacting under the following temperature rising mode under the atmosphere of nitrogen gas with a flow rate of 5L / min.

[0093] Temperature rising mode: (1) first temperature rising step: room temperature to 300℃ at a heating rate of 3℃ / min; (2) second temperature rising step: temperature to 480℃ at a heating rate of 1.25℃ / min; (3) third temperature rising step: temperature to 850℃ at a heating rate of 2.06℃ / min; (4) fourth temperature rising step: 850℃ for 180min.

[0094] The obtained multi-element carbon-coated lithium manganate includes a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core. The thickness of the carbon nanotube layer is 20nm, and the thickness of the hard carbon layer is 0.5μm.

[0095] Example 2

[0096] The multi-element carbon-coated lithium manganate of the present example includes a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.07 Co 0.06 Mn 1.87 O4.

[0097] The preparation method of the multi-element carbon-coated lithium manganate of the present embodiment comprises the following steps.

[0098] (I) Preparation of doped lithium manganate

[0099] According to the formula, lithium carbonate, nano-aluminum oxide, chromium trioxide and electrolytic manganese dioxide were taken into a ball mill, ethanol was added, and ball milling was carried out under a nitrogen atmosphere for 10 h. After suction filtration, the filter cake was dried at 80°C for 12 h. After crushing and passing through a 400-mesh sieve, it was placed in a muffle furnace and sintered at 800°C under a nitrogen atmosphere for 20 h.

[0100] (II) Preparation of carbon nanotubes

[0101] Ferrocene and coal tar pitch were mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace. The tube furnace temperature program was set, and the furnace temperature was heated from room temperature to 300°C at a heating rate of 5°C / min under a nitrogen atmosphere (gas flow rate of 5 L / min). After 1 h at 300°C, the furnace temperature was increased from 300°C to 800°C at a heating rate of 3°C / min. After the temperature reached a stable state, the water vapor generator was turned on, and water vapor (water vapor gas flow rate of 3.0 L / min) was introduced into the reaction system. Immediately move the tube furnace so that the inner container is placed in the middle of the heated area. React at 800°C for 30 min under a hydrogen atmosphere (gas flow rate of 2.0 L / min). After the growth of carbon nanotubes was completed, the furnace temperature was turned off under a hydrogen atmosphere and naturally cooled to room temperature under a nitrogen atmosphere (gas flow rate of 3 L / min). After 10 cycles of hydrochloric acid pickling-water washing, filtration, drying at 100°C for 10 h, and grinding to pass through a 500-mesh screen.

[0102] (III) Multi-element carbon coating

[0103] Doped lithium manganate, carbon nanotubes, and liquid pitch (liquid pitch coking value of 60%, mass ratio of 90:1:9) were added to a ball mill and ball milled for 14 h. Ethanol was added to the slurry, and the slurry and water were mixed at 600 W for 6 h to obtain a dispersion liquid. The dispersion liquid was transferred to a hydrothermal reaction kettle and reacted at 200°C for 5 h. After spontaneous cooling to room temperature, the product in the reaction kettle was suction filtered and dried to obtain a precursor. The precursor was loaded into a carbonization crucible and placed in a roller hearth kiln. Under a nitrogen atmosphere of 5 L / min, the following temperature rising mode was used for reaction to obtain a carbonization product.

[0104] Temperature rising mode: (1) First temperature rising step: room temperature to 300°C, heating rate of 3°C / min; (2) Second temperature rising step: temperature to 480°C, heating rate of 1.25°C / min; (3) Third temperature rising step: temperature to 850°C, heating rate of 2.06°C / min; (4) Fourth temperature rising step: 850°C for 180 min.

[0105] Example 3

[0106] The present example is a multi-element carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0107] The preparation method of the multi-element carbon-coated lithium manganate of the present example comprises the following steps.

[0108] (I) Preparation of doped lithium manganate

[0109] According to the formula, lithium carbonate, nano-aluminum oxide, titanium dioxide and electrolytic manganese dioxide were taken into a ball mill, ethanol was added, and ball milling was carried out under a nitrogen atmosphere for 10 h. After suction filtration, the filter cake was dried at 80°C for 12 h. After crushing and passing through a 400 mesh sieve, it was placed in a muffle furnace and sintered at 800°C under a nitrogen atmosphere for 20 h.

[0110] (II) Preparation of carbon nanotubes

[0111] Ferrocene and coal tar pitch were mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace. The temperature rising program of the tube furnace was set, and the furnace temperature was heated from room temperature to 800°C at a heating rate of 5°C / min under a nitrogen atmosphere (gas flow rate was 5 L / min). After the temperature reached a stable state, the water vapor generator was turned on, and water vapor (water vapor gas flow rate was 3.0 L / min) was introduced into the reaction system, and immediately moved the tube furnace so that the inner container was placed in the middle of the heated area. Under a hydrogen atmosphere (gas flow rate was 2.0 L / min), the temperature was kept at 800°C for 30 min. After the growth of carbon nanotubes was completed, the furnace temperature was turned off under a hydrogen atmosphere and naturally cooled to room temperature under a nitrogen atmosphere (gas flow rate was 3 L / min). After 10 cycles of hydrochloric acid pickling-water washing, filtration, drying at 100°C for 10 h, and grinding to pass through a 500 mesh screen.

[0112] (III) Multi-element carbon coating

[0113] The doped lithium manganate, carbon nanotubes and liquid pitch (the coking value of the liquid pitch was 60%, and the mass ratio of the three was 90:1:9) were added into the ball mill and ball milled for 14 h. Ethanol was added for ball milling to obtain a slurry. The slurry and water were mixed at 600 W for 6 h to obtain a dispersion liquid. The dispersion liquid was transferred to a hydrothermal reaction kettle, and hydrothermal reaction was carried out at 200°C for 5 h. After spontaneous cooling to room temperature, the product in the reaction kettle was suction filtered and dried to obtain a precursor. The precursor was loaded into a carbonization crucible and then placed in a roller hearth kiln for carbonization. Under a nitrogen atmosphere with a flow rate of 5 L / min, the following temperature rising mode was used for reaction to obtain a carbonization product.

[0114] Ramp to temperature: (1) first temperature ramp: from room temperature to 300℃ at a rate of 3℃ / min; (2) second temperature ramp: from 300℃ to 480℃ at a rate of 1.25℃ / min; (3) third temperature ramp: from 480℃ to 850℃ at a rate of 2.06℃ / min; (4) fourth temperature ramp: 850℃ for 180min.

[0115] Example 4

[0116] The present example is a multi-element carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0117] The preparation method of the multi-element carbon-coated lithium manganate of the present example comprises the following steps.

[0118] (I) Preparation of manganese-doped lithium manganate

[0119] According to the formula, lithium carbonate, nano-aluminum oxide, titanium dioxide and electrolytic manganese dioxide were taken into a ball mill, ethanol was added, and ball milling was carried out under a nitrogen atmosphere for 10h. After suction filtration, the filter cake was dried at 80℃ for 12h. After crushing and passing through a 400 mesh sieve, it was placed in a muffle furnace and sintered at 800℃ for 20h under a nitrogen atmosphere.

[0120] (II) Preparation of carbon nanotubes

[0121] Ferrocene and coal tar pitch were mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace. The tube furnace temperature program was set, and the furnace temperature was heated from room temperature to 300℃ at a rate of 5℃ / min under a nitrogen atmosphere (gas flow rate was 5L / min), and then held at 300℃ for 1h. Then the furnace temperature was raised from 300℃ to 800℃ at a rate of 3℃ / min. After the temperature reached a stable state, the water vapor generator was turned on, and water vapor (water vapor gas flow rate was 3.0L / min) was introduced into the reaction system, and immediately the tube furnace was moved so that the inner container was placed in the center of the heated area. The reaction was carried out at 800℃ for 30min under a hydrogen atmosphere (gas flow rate was 2.0L / min). After the growth of carbon nanotubes was completed, the furnace temperature was turned off under a hydrogen atmosphere and naturally cooled to room temperature under a nitrogen atmosphere (gas flow rate was 3L / min). After 10 cycles of hydrochloric acid washing-water washing, filtration, drying at 100℃ for 10h, and grinding to pass through a 500 mesh screen.

[0122] (III) Multi-element carbon coating

[0123] The manganese-doped lithium manganate, carbon nanotubes and liquid pitch (the coking value of the liquid pitch is 60%, and the mass ratio of the three is 90:1:9) were added into a ball mill and ball-milled for 14 h, ethanol was added to ball-mill to obtain a slurry, the slurry and water were mixed at 600 W for 6 h to obtain a dispersion liquid, the dispersion liquid was transferred into a hydrothermal reaction kettle, and hydrothermal reaction was carried out at 200 ℃ for 5 h, then the product in the reaction kettle was dried after spontaneous cooling to room temperature, the precursor was obtained, the precursor was loaded into a carbonization crucible, and then was placed into a roller hearth kiln for carbonization, and the temperature was increased to 850 ℃ at a heating rate of 10 ℃ / min under the atmosphere of nitrogen gas with a flow rate of 5 L / min, and the temperature was kept for 180 min.

[0124] Example 5

[0125] The manganese-doped lithium manganate, carbon nanotubes and liquid pitch (the coking value of the liquid pitch is 60%, and the mass ratio of the three is 90:1:9) were added into a ball mill and ball-milled for 14 h, ethanol was added to ball-mill to obtain a slurry, the slurry and water were mixed at 600 W for 6 h to obtain a dispersion liquid, the dispersion liquid was transferred into a hydrothermal reaction kettle, and hydrothermal reaction was carried out at 200 ℃ for 5 h, then the product in the reaction kettle was dried after spontaneous cooling to room temperature, the precursor was obtained, the precursor was loaded into a carbonization crucible, and then was placed into a roller hearth kiln for carbonization, and the temperature was increased to 850 ℃ at a heating rate of 10 ℃ / min under the atmosphere of nitrogen gas with a flow rate of 5 L / min, and the temperature was kept for 180 min. 0.05 Ti 0.05 Mn 1.9 O4.

[0126] The preparation method of the manganese-doped lithium manganate of the embodiment comprises the following steps.

[0127] (I) Preparation of manganese-doped lithium manganate

[0128] According to the formula, lithium carbonate, nano-aluminum oxide, titanium dioxide and electrolytic manganese dioxide were taken into a ball mill, ethanol was added and ball-milled under a nitrogen atmosphere for 10 h, then the filter cake was dried at 80 ℃ for 12 h after filtration, then it was crushed and passed through a 400-mesh sieve, and then was placed into a muffle furnace and sintered at 800 ℃ for 20 h under a nitrogen atmosphere.

[0129] (II) Preparation of carbon nanotubes

[0130] The ferrocene and coal tar pitch were mixed and placed in the inner bag of the tube furnace and placed in the unheated area of the tube furnace, the temperature rising program of the tube furnace was set, the furnace temperature was heated from room temperature to 300 ℃ at a heating rate of 5 ℃ / min under a nitrogen atmosphere (the gas flow rate was 5 L / min), and kept at 300 ℃ for 1 h, then the furnace temperature was increased from 300 ℃ to 800 ℃ at a heating rate of 3 ℃ / min, after the temperature reached a stable state, the water vapor generator was opened, water vapor (the gas flow rate of the water vapor was 3.0 L / min) was introduced into the reaction system, and immediately the tube furnace was moved so that the inner bag was placed in the middle of the heated area, and kept at 800 ℃ for 30 min, after the growth of the carbon nanotubes was completed, it was naturally cooled to room temperature. After 10 cycles of hydrochloric acid acid washing-water washing, filtration, drying at 100 ℃ for 10 h, and grinding to pass through a 500-mesh screen.

[0131] (III) Multi-element carbon coating

[0132] The doped lithium manganate, carbon nanotubes and liquid pitch (the coking value of the liquid pitch is 60%, and the mass ratio of the three is 90:1:9) are added into a ball mill for ball milling for 14 h, ethanol is added for ball milling to obtain a slurry, the slurry and water are mixed at 600 W for 6 h to obtain a dispersion liquid, the dispersion liquid is transferred into a hydrothermal reaction kettle, and hydrothermal reaction is carried out at 200 ℃ for 5 h. After spontaneous cooling to room temperature, the product in the reaction kettle is extracted and dried to obtain a precursor. The precursor is loaded into a carbonization crucible and then placed into a roller hearth kiln for carbonization. In the nitrogen atmosphere with a flow rate of 5 L / min, the reaction is carried out according to the following heating mode to obtain a carbonized product.

[0133] Heating mode: (1) first heating step: room temperature to 300 ℃, heating rate is 3 ℃ / min; (2) second heating step: temperature to 480 ℃, heating rate is 1.25 ℃ / min; (3) third heating step: temperature to 850 ℃, heating rate is 2.06 ℃ / min; (4) fourth heating step: 850 ℃ for 180 min.

[0134] Example 6

[0135] The present embodiment is a multi-element carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core. The chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0136] The preparation method of the multi-element carbon-coated lithium manganate of the present embodiment comprises the following steps.

[0137] (I) Preparation of doped lithium manganate

[0138] Lithium acetate, aluminum hydroxide, titanium dioxide and electrolytic manganese dioxide are taken according to the formula ratio, isopropanol is added, ball milling is carried out in a nitrogen atmosphere for 10 h, the filter cake is extracted and dried at 80 ℃ for 12 h, then it is crushed and passed through a 400 mesh sieve, and then it is placed into a muffle furnace for sintering at 800 ℃ in an argon atmosphere for 20 h.

[0139] (II) Preparation of carbon nanotubes

[0140] The ferrocene and melamine were mixed and placed in the inner container of the tube furnace in the unheated area of the tube furnace, the temperature rising program of the tube furnace was set, the furnace temperature was heated from room temperature to 300℃ at a heating rate of 5℃ / min under nitrogen atmosphere (gas flow rate was 5L / min), and the furnace temperature was kept at 300℃ for 1h, then the furnace temperature was raised from 300℃ to 800℃ at a heating rate of 3℃ / min, after the temperature reached stability, the water vapor generator was opened, water vapor (water vapor gas flow rate was 3.0L / min) was introduced into the reaction system, and immediately the tube furnace was moved so that the inner container was placed in the middle of the heated area, and the reaction was kept at 800℃ for 30min under hydrogen atmosphere (gas flow rate was 4.0L / min), after the growth of carbon nanotubes was completed, the furnace temperature was turned off under hydrogen atmosphere and naturally cooled to room temperature under nitrogen atmosphere (gas flow rate was 3L / min). After 7 cycles of dilute sulfuric acid washing-water washing, filtration, drying at 100℃ for 10h, and grinding to 500 mesh screen, the product was obtained.

[0141] (III) Multi-element carbon-coated

[0142] The manganese-doped lithium manganate, carbon nanotubes and liquid phase pitch (the coking value of the liquid phase pitch was 70%, and the mass ratio of the three was 93:1.5:6) were added into a ball mill for ball milling for 14h, ethylene glycol was added for ball milling to obtain a slurry, the slurry and water were mixed at 600W for 6h to obtain a dispersion liquid, the dispersion liquid was transferred into a hydrothermal reaction kettle, and hydrothermal reaction was carried out at 200℃ for 5h, after self-ignition cooling to room temperature, the product in the reaction kettle was suction filtered and dried to obtain a precursor, the precursor was loaded into a carbonization crucible, and then was placed into a roller hearth kiln for carbonization, under the atmosphere of 5L / min of argon, the reaction was carried out according to the following temperature rising mode to obtain a carbonization product.

[0143] Temperature rising mode: (1) first temperature rising step: room temperature to 300℃ at a heating rate of 3℃ / min; (2) second temperature rising step: temperature to 480℃ at a heating rate of 1.25℃ / min; (3) third temperature rising step: temperature to 850℃ at a heating rate of 2.06℃ / min; (4) fourth temperature rising step: 850℃ for 180min.

[0144] Example 7

[0145] This example is a multi-element carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0146] The preparation method of the multi-element carbon-coated lithium manganate of this example comprises the following steps.

[0147] (I) Preparation of manganese-doped lithium manganate

[0148] According to the formula, lithium carbonate, nano-alumina, titanium dioxide and electrolytic manganese dioxide were taken into a ball mill, ethanol was added, and ball milling was carried out under a nitrogen atmosphere for 8 h. After pressure filtration, the filter cake was dried at 100°C for 8 h. After crushing and passing through a 350 mesh sieve, it was placed in a muffle furnace and sintered at 900°C for 15 h under a nitrogen atmosphere.

[0149] (II) Preparation of carbon nanotubes

[0150] Ferrocene and coal tar pitch were mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace. The tube furnace temperature program was set, and the furnace temperature was heated from room temperature to 400°C at a heating rate of 4°C / min under a nitrogen atmosphere (gas flow rate was 8 L / min), and then held at 400°C for 1 h. Then, the furnace temperature was increased from 400°C to 800°C at a heating rate of 3°C / min. After the temperature reached a stable state, the water vapor generator was turned on, and water vapor (water vapor gas flow rate was 5.0 L / min) was introduced into the reaction system, and immediately the tube furnace was moved so that the inner container was placed in the middle of the heated area. The reaction was carried out at 850°C for 20 min under a hydrogen atmosphere (gas flow rate was 5.0 L / min). After the growth of carbon nanotubes was completed, the furnace temperature was turned off under a hydrogen atmosphere and naturally cooled to room temperature under a nitrogen atmosphere (gas flow rate was 4 L / min). After 10 cycles of hydrochloric acid pickling-water washing, filtration, drying at 90°C for 9 h, and grinding to pass through a 500 mesh screen.

[0151] (III) Multi-element carbon coating

[0152] Manganese-doped lithium carbonate, carbon nanotubes, and liquid pitch (liquid pitch coking value was 60%, mass ratio of the three was 90:1:9) were added to a ball mill and ball milled for 12 h. Ethanol was added to the slurry and mixed with water at 600 W for 5 h to obtain a dispersion. The dispersion was transferred to a hydrothermal reaction kettle and hydrothermally reacted at 230°C for 4 h. After self-cooling to room temperature, the product in the reaction kettle was vacuum filtered and dried to obtain a precursor. The precursor was loaded into a carbonization crucible and placed in a roller hearth kiln for carbonization. Under a nitrogen atmosphere of 3.5 L / min, the reaction was carried out according to the following temperature rising mode to obtain a carbonized product.

[0153] Temperature rising mode: (1) First temperature rising step: room temperature to 300°C, heating rate was 4°C / min; (2) Second temperature rising step: temperature increased to 500°C, heating rate was 1.5°C / min; (3) Third temperature rising step: temperature increased to 850°C, heating rate was 2.5°C / min; (4) Fourth temperature rising step: 850°C for 180 min.

[0154] Comparative Example 1

[0155] The embodiment is carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0156] The preparation method of the carbon-coated lithium manganate of the embodiment comprises the following steps.

[0157] (I) Preparation of doped lithium manganate

[0158] According to the formula, lithium carbonate, nano-aluminum oxide, titanium dioxide and electrolytic manganese dioxide are taken into a ball mill, ethanol is added, and ball milling is carried out under a nitrogen atmosphere for 10 h. After suction filtration, the filter cake is dried at 80°C for 12 h. After crushing and passing through a 400-mesh sieve, it is placed in a muffle furnace and sintered at 800°C under a nitrogen atmosphere for 20 h.

[0159] (II) Preparation of carbon nanotubes

[0160] Ferrocene and coal tar pitch are mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace. The tube furnace temperature program is set, and the furnace temperature is heated from room temperature to 300°C at a heating rate of 5°C / min under a nitrogen atmosphere (gas flow rate is 5 L / min), and kept at 300°C for 1 h. Then the furnace temperature is raised from 300°C to 800°C at a heating rate of 3°C / min. After the temperature reaches a stable state, the water vapor generator is turned on, and water vapor (water vapor gas flow rate is 3.0 L / min) is introduced into the reaction system, and immediately move the tube furnace so that the inner container is placed in the middle of the heated area. The reaction is carried out at 800°C under a hydrogen atmosphere (gas flow rate is 2.0 L / min) for 30 min. After the growth of carbon nanotubes is completed, the furnace temperature is turned off under a hydrogen atmosphere and naturally cooled to room temperature under a nitrogen atmosphere (gas flow rate is 3 L / min). After 10 cycles of hydrochloric acid pickling-water washing, filtration, drying at 100°C for 10 h, and grinding to pass through a 500-mesh screen.

[0161] (III) Coating

[0162] The doped lithium manganate and carbon nanotubes (mass ratio of 90:10) are added to the ball mill and ball milled for 14 h. Ethanol is added for ball milling to obtain a slurry. The slurry and water are mixed at 600 W for 6 h to obtain a dispersion liquid. The dispersion liquid is transferred to a hydrothermal reaction kettle, and hydrothermal reaction is carried out at 200°C for 5 h. After spontaneous cooling to room temperature, the product in the reaction kettle is suction filtered and dried to obtain a precursor. The precursor is loaded into a carbonization crucible and placed in a roller hearth kiln for carbonization. Under a nitrogen atmosphere of 5 L / min, the reaction is carried out according to the following temperature rising mode to obtain a carbonization product.

[0163] Ramp-up mode: (1) the first temperature-rising step: from room temperature to 300℃ at a rate of 3℃ / min; (2) the second temperature-rising step: from 300℃ to 480℃ at a rate of 1.25℃ / min; (3) the third temperature-rising step: from 480℃ to 850℃ at a rate of 2.06℃ / min; (4) the fourth temperature-rising step: keeping 850℃ for 180min.

[0164] Comparative Example 2

[0165] The carbon-coated lithium manganate of the present example comprises a lithium manganate core and a soft carbon layer coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0166] The preparation method of the carbon-coated lithium manganate of the present example comprises the following steps.

[0167] (I) Preparation of doped lithium manganate

[0168] According to the formula, lithium carbonate, nano-aluminum oxide, titanium dioxide and electrolytic manganese dioxide were taken into a ball mill, ethanol was added, and ball milling was carried out under a nitrogen atmosphere for 10h. After suction filtration, the filter cake was dried at 80℃ for 12h. After crushing and passing through a 400 mesh sieve, it was placed in a muffle furnace and sintered at 800℃ under a nitrogen atmosphere for 20h.

[0169] (II) Coating

[0170] The doped lithium manganate and liquid pitch (the coking value of the liquid pitch is 60%, and the mass ratio of the two is 90:10) were added into a ball mill and ball milled for 14h. Ethanol was added for ball milling to obtain a slurry. The slurry and water were mixed at 600W for 6h to obtain a dispersion liquid. The dispersion liquid was transferred to a hydrothermal reaction kettle, and hydrothermal reaction was carried out at 200℃ for 5h. After spontaneous cooling to room temperature, the product in the reaction kettle was suction filtered and dried to obtain a precursor. The precursor was loaded into a carbonization crucible and placed in a roller hearth kiln for carbonization. Under the atmosphere of nitrogen gas with a flow rate of 5L / min, the following temperature-rising mode was used for reaction to obtain a carbonization product.

[0171] Ramp-up mode: (1) the first temperature-rising step: from room temperature to 300℃ at a rate of 3℃ / min; (2) the second temperature-rising step: from 300℃ to 480℃ at a rate of 1.25℃ / min; (3) the third temperature-rising step: from 480℃ to 850℃ at a rate of 2.06℃ / min; (4) the fourth temperature-rising step: keeping 850℃ for 180min.

[0172] Comparative Example 3

[0173] The carbon-coated lithium manganate of the present example comprises a lithium manganate core and a soft carbon layer coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl0.05 Ti 0.05 Mn 1.9 O4.

[0174] The preparation method of the multi-carbon coated lithium manganate of the present embodiment comprises the following steps.

[0175] (I) Preparation of doped lithium manganate

[0176] According to the formula, lithium carbonate, nano-alumina, titanium dioxide and electrolytic manganese dioxide were taken into a ball mill, ethanol was added, and ball milling was carried out under a nitrogen atmosphere for 10 h. After suction filtration, the filter cake was dried at 80°C for 12 h. After crushing and passing through a 400 mesh sieve, it was placed in a muffle furnace and sintered at 800°C under a nitrogen atmosphere for 20 h.

[0177] (II) Preparation of carbon nanotubes

[0178] Ferrocene and coal tar pitch were mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace. The tube furnace temperature program was set, and the furnace temperature was heated from room temperature to 300°C at a heating rate of 5°C / min under a nitrogen atmosphere (gas flow rate was 5 L / min). After 1 h at 300°C, the furnace temperature was increased from 300°C to 800°C at a heating rate of 3°C / min. After the temperature reached a stable state, the water vapor generator was turned on, and water vapor (water vapor gas flow rate was 3.0 L / min) was introduced into the reaction system, and immediately moved the tube furnace so that the inner container was placed in the middle of the heated area. The reaction was carried out at 800°C under a hydrogen atmosphere (gas flow rate was 2.0 L / min) for 30 min. After the growth of carbon nanotubes was completed, the furnace temperature was turned off under a hydrogen atmosphere and naturally cooled to room temperature under a nitrogen atmosphere (gas flow rate was 3 L / min). After 10 cycles of hydrochloric acid pickling-water washing, filtration, drying at 100°C for 10 h, and grinding to pass through a 500 mesh screen.

[0179] (III) Multi-carbon coating

[0180] The doped lithium manganate, carbon nanotubes and liquid pitch (the coking value of the liquid pitch was 60%, and the mass ratio of the three was 90:1:9) were added to the ball mill and ball milled for 14 h to obtain a precursor. The precursor was loaded into a carbonization crucible and placed in a roller kiln for carbonization. Under a nitrogen atmosphere of 5 L / min, the reaction was carried out according to the following temperature rising mode to obtain a carbonization product.

[0181] Temperature rising mode: (1) First temperature rising step: room temperature to 300°C, heating rate 3°C / min; (2) Second temperature rising step: temperature to 480°C, heating rate 1.25°C / min; (3) Third temperature rising step: temperature to 850°C, heating rate 2.06°C / min; (4) Fourth temperature rising step: 850°C for 180 min.

[0182] Comparative Example 4

[0183] The embodiment is multi-element carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core, and the chemical formula of the lithium manganate core is LiAl 0.05 Ti 0.05 Mn 1.9 O4.

[0184] The preparation method of the multi-element carbon-coated lithium manganate of the embodiment comprises the following steps.

[0185] (I) Preparation of doped lithium manganate

[0186] According to the formula, lithium carbonate, nano-aluminum oxide, titanium dioxide and electrolytic manganese dioxide are taken into a ball mill, ethanol is added, and ball milling is carried out under a nitrogen atmosphere for 10 h. After suction filtration, the filter cake is dried at 80°C for 12 h. After crushing and passing through a 400-mesh sieve, it is placed in a muffle furnace and sintered at 800°C under a nitrogen atmosphere for 20 h.

[0187] (II) Carbon nanotube coating

[0188] Ferrocene and coal tar pitch are mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace. The tube furnace temperature program is set, and the furnace temperature is heated from room temperature to 300°C at a heating rate of 5°C / min under a nitrogen atmosphere (gas flow rate is 5 L / min), and then held at 300°C for 1 h. The furnace temperature is then raised from 300°C to 800°C at a heating rate of 3°C / min. After the temperature stabilizes, the water vapor generator is turned on, and water vapor (water vapor gas flow rate is 3.0 L / min) is introduced into the reaction system, and the tube furnace is immediately moved so that the inner container is placed in the center of the heated area. The reaction is carried out at 800°C under a hydrogen atmosphere (gas flow rate is 2.0 L / min) for 30 min. After the carbon nanotube growth is completed, the furnace temperature is turned off under a hydrogen atmosphere and naturally cooled to room temperature under a nitrogen atmosphere (gas flow rate is 3 L / min). After 10 cycles of hydrochloric acid pickling-water washing, filtration, drying at 100°C for 10 h, and grinding to pass through a 500-mesh screen, the doped lithium manganate and carbon nanotubes (mass ratio of 90:1:9) are spray granulated, placed in a carbonization crucible, and placed in a roller kiln for carbonization. Under a nitrogen atmosphere of 5 L / min, the reaction is carried out according to the following temperature rising mode to obtain the precursor.

[0189] (III) Hard carbon coating

[0190] The precursor and liquid pitch (coking value of the liquid pitch is 60%, 9% of the mass of the precursor) are added into a ball mill for ball milling for 14 h, ethanol is added for ball milling to obtain a slurry, the slurry and water are mixed at 600 W for 6 h to obtain a dispersion liquid, the dispersion liquid is transferred into a hydrothermal reaction kettle, and hydrothermal reaction is carried out at 200 ℃ for 5 h. After spontaneous cooling to room temperature, the product in the reaction kettle is suction filtered and dried to obtain a precursor. The precursor is loaded into a carbonization crucible and then placed into a roller hearth kiln for carbonization. In the process of carbonization, 5 L / min of nitrogen gas is introduced, and the following temperature rising mode is used to obtain a carbonization product.

[0191] The temperature rising mode of step (II) carbon nanotube coating and step (III) hard carbon coating: (1) the first temperature rising step: the temperature is raised from room temperature to 300 ℃ at a rate of 3 ℃ / min; (2) the second temperature rising step: the temperature is raised to 480 ℃ at a rate of 1.25 ℃ / min; (3) the third temperature rising step: the temperature is raised to 850 ℃ at a rate of 2.06 ℃ / min; (4) the fourth temperature rising step: 850 ℃ is kept for 180 min.

[0192] Comparative Example 5

[0193] The present embodiment is a multi-element carbon-coated lithium manganate, which comprises a lithium manganate core and a carbon nanotube layer and a hard carbon layer successively coated on the lithium manganate core. The chemical formula of the lithium manganate core is LiAl 0.1 Mn 1.9 O4.

[0194] The preparation method of the multi-element carbon-coated lithium manganate of the present embodiment comprises the following steps.

[0195] (I) Preparation of doped lithium manganate

[0196] According to the formula, lithium carbonate, nano-aluminum oxide and electrolytic manganese dioxide are taken into a ball mill, ethanol is added, and ball milling is carried out under a nitrogen atmosphere for 10 h. After suction filtration, the filter cake is dried at 80 ℃ for 12 h, then crushed and passed through a 400 mesh sieve, and then placed into a muffle furnace for sintering at 800 ℃ under a nitrogen atmosphere for 20 h.

[0197] (II) Preparation of carbon nanotubes

[0198] The ferrocene and coal tar pitch were mixed and placed in the inner container of the tube furnace and placed in the unheated area of the tube furnace, the temperature rising program of the tube furnace was set, the furnace temperature was heated from room temperature to 300°C at a heating rate of 5°C / min under nitrogen atmosphere (gas flow rate was 5 L / min), and kept at 300°C for 1 h, then the furnace temperature was raised from 300°C to 800°C at a heating rate of 3°C / min, after the temperature reached stability, the water vapor generator was opened, water vapor (water vapor gas flow rate was 3.0 L / min) was introduced into the reaction system, and immediately the tube furnace was moved so that the inner container was placed in the middle of the heated area, and kept at 800°C for 30 min under hydrogen atmosphere (gas flow rate was 2.0 L / min), after the carbon nanotube growth was completed, the furnace temperature was turned off under hydrogen atmosphere and naturally cooled to room temperature under nitrogen atmosphere (gas flow rate was 3 L / min). After 10 cycles of hydrochloric acid washing-water washing, filtration, drying at 100°C for 10 h, and grinding to 500 mesh screen, the product was obtained.

[0199] (III) Multi-element carbon coating

[0200] The manganese-doped lithium manganate, carbon nanotubes and liquid pitch (the coking value of the liquid pitch was 60%, and the mass ratio of the three was 90:1:9) were added to a ball mill and ball milled for 14 h, ethanol was added for ball milling to obtain a slurry, the slurry and water were mixed at 600 W for 6 h to obtain a dispersion liquid, the dispersion liquid was transferred to a hydrothermal reaction kettle, and hydrothermal reaction was carried out at 200°C for 5 h, after spontaneous cooling to room temperature, the product in the reaction kettle was suction filtered and dried to obtain a precursor, the precursor was loaded into a carbonization crucible and then placed in a roller hearth kiln for carbonization, and the carbonization product was obtained by reacting under the following temperature rising mode under the atmosphere of 5 L / min of nitrogen.

[0201] Temperature rising mode: (1) first temperature rising step: room temperature to 300°C, heating rate was 3°C / min; (2) second temperature rising step: temperature rose to 480°C, heating rate was 1.25°C / min; (3) third temperature rising step: temperature rose to 850°C, heating rate was 2.06°C / min; (4) fourth temperature rising step: 850°C for 180 min.

[0202] The average particle size Dv50 of the lithium manganate materials obtained in Examples 1 to 7 and Comparative Examples 1 to 5 was measured by a particle size analyzer, and the specific surface area was tested by a McBain specific surface area analyzer 3020, and the results are shown in Table 1.

[0203] The lithium manganate material obtained in Examples 1 to 7 and Comparative Examples 1 to 5 is mixed with a binder PVDF and a conductive agent (Super-P) in a mass ratio of 95:1.5:3.5, and an appropriate amount of N-vinylpyrrolidone is added as a solvent to prepare a slurry, which is coated on an aluminum foil and dried in vacuum and rolled to prepare a negative electrode sheet. A CR2032 button cell is assembled in an inert gas glove box using lithium metal as a counter electrode, a 1 mol / L LiPF6 electrolyte mixed with a three-component mixed solvent of EC:DMC:EMC = 1:1:1 (v / v), and a polypropylene microporous membrane as a separator. The charge-discharge test of the button cell is performed on a battery test system of Wuhan Blue Electronic Co., Ltd.

[0204] At 25°C, 0.1C constant current charging and discharging to 0.01V, then 0.02C constant current discharging to 0.005V, and finally 0.1C constant current charging to 4.0V, the capacity charged to 4.0V is the initial specific discharge capacity, the ratio of the discharge capacity to the charge capacity is the initial charge-discharge efficiency, and the corresponding 200th cycle discharge specific capacity is obtained after 200 cycles, and the 200th cycle charge-discharge efficiency is calculated.

[0205] After the first charge-discharge test of the material, constant current constant voltage discharging (0.2C, cutoff condition 0.01C) to 0.01V, standing for 5 min, and constant current charging 0.2C to 4.0V, the ratio of the charge capacity at 0.5C rate to the charge capacity at 0.2C rate is calculated.

[0206] Table 1: Physical properties and electrochemical properties of the lithium manganate materials obtained in the examples

[0207]

[0208] As can be seen from the results in Table 1, compared with Comparative Examples 1 to 5, the multi-element carbon-coated lithium manganate obtained in Examples 1 to 7 has a large particle size and good cycle and rate performance, which is mainly due to the multi-metal doping and double-layer coating of carbon nanotube layer and hard carbon layer of the obtained lithium manganate material.

[0209] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application, although the present application has been described in detail with reference to the preferred embodiments, but it is not limited to the examples listed in the embodiments, and those skilled in the art should understand that the technical solutions of the present application can be modified or replaced equivalently without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A method for preparing a multi-element carbon-coated lithium manganate, characterized by, The method comprises the following steps: (I) preparing manganese-doped lithium manganate According to the formula, lithium source, aluminum source, X source and manganese source are taken into a ball mill, a solvent is added for ball milling, a filter cake is taken after filtration, and drying, crushing and sintering are sequentially performed; (II) preparing carbon nanotubes After mixing ferrocene and carbon source, heating is performed to 700-900℃ under inert atmosphere, water vapor is introduced into the system, constant temperature reaction is performed under reducing atmosphere and 700-900℃, and then cooling is performed to room temperature under inert atmosphere, acid washing and post-treatment are performed; (III) multi-element carbon coating The manganese-doped lithium manganate, the carbon nanotubes and a soft carbon coating agent are added into a ball mill, a solvent is added for ball milling to obtain a slurry, the slurry and water are ultrasonically mixed to obtain a dispersion liquid, the dispersion liquid is subjected to hydrothermal reaction, and then cooling and filtration are performed to obtain a precursor, and the precursor is subjected to carbonization under inert atmosphere.

2. The method of claim 1, wherein the lithium manganate coated with multi-element carbon is prepared by the steps of: mixing lithium manganate and a carbon source to form a mixture; and heating the mixture at a temperature of 300 to 800°C for 1 to 10 hours in an inert gas atmosphere. The method comprises at least one of the following features (1) to (29): (1) the lithium source comprises at least one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium acetate; (2) the aluminum source comprises aluminum oxide and / or aluminum hydroxide; (3) the manganese source comprises manganese dioxide; (4) the X source is an oxide of X, and X comprises at least one of Co, Cr, Ti, V and B; (5) the X source comprises at least one of Co3O4, Cr2O3, TiO2, V2O5 and H2BO3; (6) in step (I), the solvent comprises at least one of ethanol, acetone, isopropanol, n-butanol, ethylene glycol, n-hexanol and methanol; (7) in steps (I) and (III), the ball milling is performed under a protective atmosphere, and the protective atmosphere comprises at least one of nitrogen, argon and helium; (8) the drying is performed at a temperature of 60-100℃ for 6-20h; (9) the crushing is performed to a particle size of 300-500 mesh; (10) the sintering is performed in a muffle furnace; (11) the sintering is performed at a temperature of 700-900℃ for 10-30h; (12) the carbon source is at least one of starch, coal tar pitch, resin and melamine; (13) in steps (II) and (III), the inert atmosphere is independently at least one of nitrogen, argon and helium; (14) in steps (II) and (III), the gas flow of the inert atmosphere is independently 0.3-10.0L / min; (15) the carbon nanotubes are prepared in a tube furnace; (16) the reducing atmosphere comprises at least hydrogen; (17) the gas flow of the reducing atmosphere is 0.3-10.0L / min; (18) the gas flow of the water vapor is 0.1-5.0L / min; (19) the water vapor is derived from a water vapor generator; (20) in steps (I) and (III), the ball milling is independently performed for 8-20h; (21) the acid used in the acid washing is hydrochloric acid, nitric acid or dilute sulfuric acid. (22) the post-treatment comprises water washing, drying and grinding in sequence after pickling; (23) the mass ratio of the doped lithium manganate, the carbon nanotube and the soft carbon-based coating agent is 85-95:0.5-2:5-15; (24) the soft carbon-based coating agent comprises solid-phase pitch and / or liquid-phase pitch; (25) the coking value of the soft carbon-based coating agent is 10% to 80%; (26) the solvent in the preparation of the carbon nanotube in step (II) comprises at least one of ethanol, acetone, isopropyl alcohol, n-butyl alcohol, ethylene glycol, n-hexyl alcohol and methanol; (27) the power of the ultrasonic is 600W, and the ultrasonic time is 1h to 10h; (28) the temperature of the hydrothermal reaction is 150℃ to 250℃, and the time is 1h to 10h; (29) the equipment for the carbonization is a roller kiln, a rotary furnace, a box furnace or a vertical kettle.

3. The method of claim 1, wherein the lithium manganate coated with multi-element carbon is prepared by the steps of: mixing lithium manganate and a carbon source to form a mixture; and heating the mixture at a temperature of 300 to 800°C for 1 to 10 hours in an inert gas atmosphere. The heating in the preparation of the carbon nanotube in step (II) adopts gradient heating.

4. The method of claim 1, wherein the multi-element carbon-coated lithium manganate is prepared by the steps of: mixing lithium manganate, carbon, and a carbon source; and heating the mixture at a temperature of 600 to 1,000°C for 1 to 10 hours in an inert gas atmosphere. The heating in the preparation of the carbon nanotube in step (II) is first raised to 200℃ to 400℃ at a temperature rising rate of 3℃ / min to 10℃ / min and kept constant for 0.5h to 3h, and then raised to 700℃ to 900℃ at a temperature rising rate of 1℃ / min to 5℃ / min.

5. The method of claim 1, wherein the multi-element carbon-coated lithium manganate is prepared by the steps of: mixing lithium manganate, carbon, and a carbon source; and heating the mixture at a temperature of 600 to 1,000°C for 1 to 10 hours in an inert gas atmosphere. The temperature of the carbonization adopts gradient heating, and the highest temperature is 800℃ to 900℃.

6. The method of claim 1, wherein the multi-element carbon-coated lithium manganate is prepared by the steps of: mixing lithium manganate, carbon, and a carbon source; and heating the mixture at a temperature of 600 to 1,000°C for 1 to 10 hours in an inert gas atmosphere. The temperature rising rate of the carbonization is 1℃ / min to 5℃ / min.

7. The method of claim 1, wherein the lithium manganate coated with multi-element carbon is prepared by the steps of: mixing lithium manganate and a carbon source to form a mixture; and heating the mixture at a temperature of 300 to 800°C for 1 to 10 hours in an inert gas atmosphere. The temperature of the carbonization is first raised to 250℃ to 300℃, and then raised to 300℃ to 480℃, 480℃ to 850℃ in sequence, and then kept constant for 120min to 300min. 8.The multi-element carbon-coated lithium manganate of any one of claims 1-7, wherein, The lithium manganate core has a chemical formula of LiAl a X b Mn 2-a-b O4, wherein 0.01≦a≦0.10, 0.01≦b≦0.05, and X includes at least one of Co, Cr, Ti, V, and B. 9.The multi-element carbon-coated lithium manganate of claim 8, wherein, The thickness of the carbon nanotube layer is 20nm to 30nm, and the thickness of the hard carbon layer is 0.5µm to 1.5µm.

10. A secondary battery comprising a positive electrode material and a negative electrode material, characterized by, The positive electrode material is the multi-element carbon-coated lithium manganate according to any one of claims 8 to 9.

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