A composite-coated silicon-carbon anode material, its preparation method and application

Abstract

The present invention discloses a composite-coated silicon-carbon anode material, a preparation method thereof and an application thereof, relating to the technical field of lithium-ion batteries. Graphite is oxidized and etched in a mixed gas of air and nitrogen at a temperature of 400°C - 500°C to form oxidized graphite; water-soluble asphalt, graphene, nano-silicon, melamine and monoammonium phosphate are dispersed in water to form a composite liquid; then the composite liquid is mixed with the oxidized graphite and subjected to carbonization treatment. The present invention prepares a high-performance silicon-carbon composite anode material through the oxidation etching of graphite and the composite coating process. Nano-silicon is uniformly dispersed and stably bonded in the carbon pores with a rich mesoporous structure on the surface of graphite, and graphene and the nitrogen-doped carbon layer form a three-dimensional conductive network. When applied to lithium-ion batteries, it has good low-temperature charging ratio, initial efficiency and cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a composite-coated silicon-carbon anode material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries have been widely used in portable electronic products, energy storage products, and electric vehicles. Due to the highest theoretical capacity (about 4200 mAh / g), silicon has become the most attractive choice among various lithium-ion battery anode materials. However, the application proportion of silicon anodes in actual lithium-ion batteries is low. The main obstacles are low intrinsic conductivity and a huge volume change of more than 400% in silicon particles during lithium insertion / extraction processes, which leads to pulverization of silicon particles and permanent capacity decay.

[0003] Traditional silicon / carbon composite materials have limited achievements in being used as anode materials because their internal volume is insufficient to accommodate severe volume changes, especially in low-temperature and high-rate scenarios, resulting in that it is very difficult for the soft-pack silicon-carbon anode batteries currently on the market to achieve more than 500 cycles, and it is very difficult for the charging capacity at -40°C to reach more than 60%. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a composite-coated silicon-carbon anode material, a preparation method thereof, and an application thereof, so as to at least achieve the purpose of improving the cycling performance and low-temperature performance of lithium-ion batteries.

[0005] To solve the above technical problems, according to one aspect of the present invention, a preparation method of a composite-coated silicon-carbon anode material provided by the present invention includes:

[0006] Step 1: Oxidatively etching graphite in a mixed gas of air and nitrogen at a temperature of 400°C - 500°C to form oxidized graphite;

[0007] Step 2: Disperse water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate in water to form a composite solution;

[0008] Step 3: Mix the composite solution obtained in Step 2 with the oxidized graphite obtained in Step 1 according to a mass ratio of 15:80 - 90, and then perform carbonization treatment at a carbonization temperature of 800 - 1050°C.

[0009] Further, in Step 1, the temperature of oxidative etching is 450°C.

[0010] Further, in Step 1, in the mixed gas, the volume ratio of air to nitrogen is 2:1 - 6:1.

[0011] Further, the mass of water-soluble asphalt is 3 - 7% of the mass of oxidized graphite.

[0012] Further, the mass of graphene is 0.5 - 2% of the mass of graphite oxide.

[0013] Further, the mass of nano - silicon is 6% of the mass of graphite oxide.

[0014] Further, the mass of melamine is 0.5 - 3% of the mass of graphite oxide.

[0015] Further, the mass of mono - ammonium phosphate is 3 - 7% of the mass of graphite oxide.

[0016] According to another aspect of the present invention, a composite - coated silicon - carbon anode material is provided, which is obtained by the preparation method described above.

[0017] According to another aspect of the present invention, the application of the above - mentioned composite - coated silicon - carbon anode material in the preparation of lithium - ion batteries is provided.

[0018] The present invention prepares a high - performance silicon - carbon composite anode material through oxidation etching of graphite and composite coating process. Water - soluble asphalt and graphene are used together to construct a flexible conductive network. The nitrogen doping of melamine improves the conductivity of carbon and silicon. Mono - ammonium phosphate forms a hierarchical pore structure through decomposition to accommodate the volume expansion of silicon. The combination of the above factors not only improves the conductivity and mechanical stability of the material, but also inhibits the volume effect of silicon through the dual mechanisms of chemical bonding and physical encapsulation, realizing the synergistic improvement of high capacity and long cycle life.

[0019] In the silicon - carbon anode material provided by the present invention, nano - silicon is uniformly dispersed in the carbon pores with a rich mesoporous structure that is stably bound to the surface of graphite. Graphene and the nitrogen - doped carbon layer form a three - dimensional conductive network. When this material is applied to lithium - ion batteries, it has good low - temperature charging ratio, initial efficiency and cycling performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is the first charge - discharge curve diagram of the silicon - carbon anode material in Example 1;

[0021] Figure 2 It is the cyclic capacity retention rate diagram of the battery assembled with the silicon - carbon anode in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0022] The basic concept of the present invention is to form active reaction points through oxidation etching on the surface of graphite, create appropriate pores in the carbon layer of the silicon / carbon composite material, encapsulate nano - silicon in the pores of carbon and graphite to form Si - doped modified porous carbon, and prepare a new type of doped - modified porous - carbon silicon - carbon anode material by means of nitrogen - atom doping and coating with lone - pair electrons and composite coating with graphene to construct a stable carbon - coating layer.

[0023] Based on this, a preparation method of a composite-coated silicon-carbon anode material provided by a typical embodiment of the present invention includes the following steps 1 to 3, where steps 1 and 2 have no sequential order.

[0024] Step 1, graphite oxidation etching.

[0025] Graphite is oxidized and etched in a mixed gas of air and nitrogen at a temperature of 400°C - 500°C to form graphite oxide with active functional groups on the surface.

[0026] Among them, the temperature of the oxidation etching is arbitrarily selected within the range of 400°C - 500°C, such as 400°C, 410°C, 420°C, 450°C, 480°C, 500°C, and preferably 450°C.

[0027] In the mixed gas, air is used as the oxidant, and the volume ratio of air to nitrogen is 2:1 - 6:1, such as 2:1, 3:1, 4:1, 5:1, 6:1.

[0028] Step 2, preparation of the composite liquid.

[0029] Water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate are dispersed in water to form a composite liquid.

[0030] Preferably, the composite liquid is a uniform suspension formed by dispersing water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate in water and ultrasonically dispersing for 1 - 2 hours at 40°C.

[0031] The masses of water-soluble asphalt, melamine, nano-silicon, monoammonium phosphate, and graphene are 3 - 7%, 0.5 - 3%, 6%, 3 - 7%, and 0.5 - 2% of the mass of graphite oxide respectively.

[0032] The functions and synergistic effects of each component in the composite liquid are as follows:

[0033] Water-soluble asphalt: Its solution state can fully react with melamine to achieve a uniform nitrogen doping effect; water-soluble asphalt serves as a carbon source and forms a continuous carbon matrix during the carbonization process, wrapping nano-silicon and graphite particles, providing mechanical support and an electron conduction path.

[0034] Graphene: It forms a three-dimensional conductive network in combination with the carbon matrix, enhancing the overall conductivity of the material and simultaneously improving the stability of the carbon structure.

[0035] Nano-silicon: As a high-capacity active material, its theoretical capacity is 4200 mAh / g. Through encapsulation by a porous carbon skeleton, the pulverization problem caused by volume expansion is alleviated.

[0036] Melamine: After pyrolysis, it generates a nitrogen-doped carbon layer, whose lone pair electrons form strong chemical bonds with the surfaces of silicon and graphite oxide, stabilizing the interface and enhancing the lithium-ion diffusion rate.

[0037] Ammonium dihydrogen phosphate: It decomposes at high temperature to produce phosphate and ammonia, forming rich hierarchical pores in the carbon layer to accommodate the volume expansion of silicon.

[0038] Step 3, composite coating and carbonization.

[0039] Mix the composite liquid obtained in Step 2 and the graphite oxide obtained in Step 1 in a mass ratio of 15:80 - 90 and then perform carbonization treatment. The carbonization temperature is 800 - 1050 °C.

[0040] Among them, the mass ratio of the composite liquid to graphite oxide can be arbitrarily selected within the range of 15:80 - 90, such as 15:80, 15:82, 15:85, 15:87, 15:90. Preferably, the composite liquid and graphite oxide are mixed in a mass ratio of 15:85 and then carbonization treatment is carried out.

[0041] After the composite liquid and graphite oxide are mixed, heat up to the carbonization temperature at a heating rate of 3 - 5 °C / min, and keep the temperature for 3 - 5 hours.

[0042] In the above embodiment, through the oxidation etching and composite coating processes, a three-dimensional conductive network formed by the composite of a nitrogen-doped carbon layer and graphene is constructed on the surface of graphite. The structural features are as follows:

[0043] A stable coating carbon layer penetrating into the graphite body: After oxidation etching, a rich pore structure is formed on the surface of graphite, providing more sites for the combination of melamine, the carbon layer and graphite, and forming a more stable structure.

[0044] Nitrogen-doped carbon coating: The nitrogen-doped carbon generated by the pyrolysis of melamine binds to the surfaces of silicon and graphite oxide through lone pair electrons, enhancing the interface stability and promoting lithium-ion transport.

[0045] Hierarchical pores formed by ammonium dihydrogen phosphate: Ammonium dihydrogen phosphate generates phosphate and gas during carbonization to form hierarchical pores. On the one hand, it can accommodate the volume expansion of silicon, and on the other hand, it is beneficial to the transmission of the electrolyte.

[0046] Graphene reinforcement layer: Graphene and the carbon skeleton form a three-dimensional conductive network, improving the overall conductivity of the material and reducing the polarization effect.

[0047] In this embodiment, the combination of oxidation etching and composite coating is not a simple process superposition, but realizes synergistic effects through the following mechanism:

[0048] 1. Active site provision: Oxidative etching introduces oxygen-containing functional groups (such as hydroxyl groups, carbonyl groups, etc.) on the graphite surface, providing active anchoring sites for the uniform loading of the subsequent composite solution and enhancing the interfacial binding force between each component and graphite.

[0049] 2. Pore structure optimization: The pore structure formed by etching strengthens the structural stability of the carbon layer, silicon, and graphite. During the carbonization of ammonium dihydrogen phosphate, a hierarchical pore structure is formed through pore formation by gases and phosphate groups, which not only accommodates the volume change of nanosilicon but also forms a "graphite-silicon-carbon" hierarchical buffer structure.

[0050] 3. Chemical bond synergy: The active groups on the surface of graphite oxide react with melamine, and after carbonization, a nitrogen-doped stable carbon layer is formed, significantly improving the electrochemical stability of the material.

[0051] The following further illustrates the technical solutions claimed in the present invention through some embodiments. However, the embodiments and comparative examples are used to explain the implementation solutions of the present invention and do not exceed the scope of the present invention's theme. The protection scope of the present invention is not limited by the described embodiments.

[0052] Unless otherwise specified, the materials and reagents used in the present invention can be obtained from commercial products in the art. The water-soluble asphalt used in the examples and comparative examples was purchased from Liaocheng Hongxin Chemical Co., Ltd. (the residual carbon value is 50%); the particle size of nanosilicon is 70 nm; the graphite is needle coke artificial graphite with a D50 of 10 μm (the specific capacity is 360 mAh / g).

[0053] Example 1

[0054] Graphite was placed in a tubular furnace, and a mixed gas with a volume ratio of air: nitrogen = 3:1 was introduced. Oxidative etching was carried out at a constant temperature of 450 °C for 2 hours to form graphite oxide.

[0055] Water-soluble asphalt, graphene, nanosilicon, melamine, and ammonium dihydrogen phosphate were respectively taken at 5%, 1%, 6%, 3%, and 7% of the mass of graphite oxide and dispersed into an appropriate amount of deionized water. A composite solution was formed by ultrasonic dispersion at 40 °C.

[0056] The composite solution and graphite oxide were mixed uniformly at a mass ratio of 15:85 at high speed. Then, under nitrogen protection, it was heated to 950 °C at a rate of 5 °C / min and carbonized for 3 hours to obtain a silicon-carbon negative electrode material.

[0057] Examples 2 - 13

[0058] On the basis of the preparation method provided in Example 1, the ratio of air to nitrogen in the mixed gas, the carbonization temperature, and the ratios of water-soluble asphalt, graphene, nanosilicon, melamine, and ammonium dihydrogen phosphate relative to the mass of graphite oxide were adjusted. The specific parameter differences are shown in Table 1.

[0059] Table 1 Main preparation conditions of each example

[0060]

[0061] In Table 1, the ratio of air to nitrogen is by volume; the ratios of water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate to graphite oxide are by mass.

[0062] Comparative Example 1

[0063] Unoxidized graphite, nano-silicon, and asphalt were mixed at a mass ratio of 89:6:5 at high speed, and then under nitrogen protection, the temperature was raised to 950 °C at a rate of 5 °C / min and carbonized for 3 hours to obtain a silicon-carbon composite material.

[0064] Comparative Example 2

[0065] Unoxidized graphite and nano-silicon were uniformly mixed at a mass ratio of 94:6 to obtain a silicon-carbon and graphite mixed material.

[0066] The negative electrodes in the above examples and comparative examples were made into lithium-ion batteries and their performance was tested. A 5 Ah soft-pack battery was made to test the cycle performance and low-temperature performance.

[0067] Test conditions:

[0068] The silicon-carbon materials prepared in the comparative examples and examples were used as negative electrode materials and mixed with the binder polyvinylidene fluoride (PVDF) and the conductive agent (Super-P) at a mass ratio of 80:10:10. An appropriate amount of N-methylpyrrolidone (NMP) was added as a solvent to make a slurry, which was coated on a copper foil, and then vacuum dried and roll-pressed to prepare a negative electrode sheet; a metal lithium sheet was used as the counter electrode, and an electrolyte composed of a 1 mol / L LiPF6 ternary mixed solvent with EC:DMC:EMC = 1:1:1 (v / v) was used. A polypropylene microporous membrane was used as the separator, and a CR2032 type button cell was assembled in a glove box filled with inert gas.

[0069] The charge and discharge tests of the button cell were carried out on the battery test system of Wuhan Landian Electronic Co., Ltd. Under normal temperature conditions, a constant current charge and discharge at 0.1C was performed, and the charge and discharge voltage was limited to 0.005 - 1.5V to test the first discharge capacity and the first discharge efficiency of the silicon-carbon negative electrode.

[0070] The following methods were used to test and calculate the cycling performance and low-temperature performance of the materials: The negative electrode materials in the examples and comparative examples were applied to a soft-packaged laminated battery with a nominal capacity of 5 Ah using lithium iron phosphate as the positive electrode. After assembling the battery, formation and grading were carried out according to the normal system, and then charge-discharge cycling was performed at a current of 5 A until the discharge capacity decreased to 80% of the first discharge capacity. The cycling performance was expressed by the number of charge-discharge cycles; The formed battery was subjected to one charge-discharge cycle at a current of 5 Ah at room temperature, and the charge capacity was marked as C 室温 , and then the discharged battery was placed in a low-temperature cabinet and kept at -40 °C for 24 h, and then charged at a current of 2.5 A. The charge capacity was marked as C 低温 , and P = C 低温 / C 室温 *100% was used to express the low-temperature performance.

[0071] Figure 1 is the first charge-discharge curve of the silicon-carbon negative electrode material in Example 1, Figure 2 is the cycling capacity retention rate diagram of the battery assembled with the silicon-carbon negative electrode in Example 1. The physical and electrochemical performance characterizations of the materials in each example and comparative example are shown in Table 2.

[0072] Table 2 Physical and Electrochemical Performance Characterizations of the Materials in Each Example and Comparative Example

[0073]

[0074] Compared with Example 1, in Comparative Example 1, unoxidized graphite was used and graphene, melamine, and monoammonium phosphate were not added. Its cycling performance was 343 times and the low-temperature charging ratio was 63.2%, both significantly lower than the cycling performance of 820 times and the low-temperature charging ratio of 93.7% in Example 1. This indicates that the absence of the oxidation etching and composite coating processes leads to the inability to relieve the volume expansion of silicon particles and insufficient interface stability.

[0075] Compared with Example 1, in Comparative Example 2, only unoxidized graphite and nano-silicon were mixed, and no carbon coating and carbonization treatment were carried out. Its first efficiency was 85.7% and the cycling performance was 298 times, both significantly lower than the first efficiency of 91.7% and the cycling performance of 820 times in Example 1, indicating the necessity of the composite coating process for constructing the conductive network and interface stability. The low-temperature charging ratio of Comparative Example 2 was 58.5%, significantly lower than the low-temperature charging ratio of 93.7% in Example 1, proving the contribution of the composite coating to structural stability and rate performance improvement.

[0076] According to the above comparison, it can be proved that the synergistic effect of oxidation etching and composite coating significantly improves the cycling stability, low-temperature performance, and interface kinetics of the materials.

Claims

1. A preparation method of a composite-coated silicon-carbon anode material, characterized in that, Comprising: Step 1: Oxidatively etching graphite in a mixed gas of air and nitrogen at a temperature of 400°C - 500°C to form graphite oxide; Step 2: Dispersing water-soluble asphalt, graphene, nano-silicon, melamine and monoammonium phosphate in water to form a composite liquid; Step 3: Mixing the composite liquid obtained in Step 2 with the graphite oxide obtained in Step 1 according to a mass ratio of 15:80 - 90, and then performing carbonization treatment at a carbonization temperature of 800 - 1050°C.

2. The preparation method of the composite-coated silicon-carbon anode material according to claim 1, characterized in that: In Step 1, the temperature of the oxidative etching is 450°C.

3. The preparation method of the composite-coated silicon-carbon anode material according to claim 2, characterized in that: In Step 1, in the mixed gas, the volume ratio of air to nitrogen is 2:1 - 6:

1.

4. The preparation method of the composite-coated silicon-carbon anode material according to claim 1, 2 or 3, characterized in that: The mass of the water-soluble asphalt is 3 - 7% of the mass of the graphite oxide.

5. The preparation method of the composite-coated silicon-carbon anode material according to claim 4, characterized in that: The mass of the graphene is 0.5 - 2% of the mass of the graphite oxide.

6. The preparation method of the composite-coated silicon-carbon anode material according to claim 5, characterized in that: The mass of the nano-silicon is 6% of the mass of the graphite oxide.

7. The preparation method of the composite-coated silicon-carbon anode material according to claim 6, characterized in that: The mass of the melamine is 0.5 - 3% of the mass of the graphite oxide.

8. The preparation method of the composite-coated silicon-carbon anode material according to claim 7, characterized in that: The mass of the monoammonium phosphate is 3 - 7% of the mass of the graphite oxide.

9. A composite-coated silicon-carbon anode material, characterized in that: Obtained by the preparation method according to any one of claims 1 - 8.

10. Use of the silicon-carbon negative electrode material with composite coating according to claim 9 in the preparation of a lithium-ion battery.

Images