Composite coated silicon-carbon negative electrode material as well as preparation method and application thereof

Abstract

The invention discloses a composite coated silicon-carbon negative electrode material as well as a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The method comprises the following steps of: oxidizing and etching graphite in mixed gas of air and nitrogen at the temperature of between 400 and 500 DEG C to form graphite oxide; the preparation method comprises the following steps: dispersing water-soluble asphalt, graphene, nano silicon, melamine and monoammonium phosphate in water to form a composite liquid; and then mixing the composite liquid with graphite oxide, and then carrying out carbonization treatment. The high-performance silicon-carbon composite negative electrode material is prepared through graphite oxidation etching and a composite coating process, nano silicon is uniformly dispersed in carbon pores which are stably combined on the surface of graphite and have rich mesoporous structures, graphene and a nitrogen-doped carbon layer form a three-dimensional conductive network, and the composite negative electrode material is applied to lithium ion batteries and can be applied to lithium ion batteries. And the lithium ion battery has good low-temperature charging proportion, first efficiency and cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a composite-coated silicon-carbon negative electrode 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 negative electrode materials. However, the application proportion of silicon negative electrodes in actual lithium-ion batteries is low. The main obstacles are the low intrinsic conductivity and the huge volume change of more than 400% of silicon particles during lithium insertion / extraction processes, which lead to the pulverization of silicon particles and permanent capacity attenuation.

[0003] Traditional silicon / carbon composite materials have limited achievements in being used as negative electrode 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-packaged silicon-carbon negative electrode batteries in the current market to reach more than 500 cycles, and the charging capacity at low temperature of -40 °C is very difficult 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 negative electrode material, a preparation method thereof, and an application thereof, so as to at least achieve the purpose of improving the cycle 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 negative electrode material provided by the present invention includes: 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; 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 oxidized graphite 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.

[0006] Further, in Step 1, the temperature of oxidative etching is 450 °C.

[0007] Further, in the mixed gas in Step 1, the volume ratio of air to nitrogen is 2:1 - 6:1.

[0008] Further, the mass of water-soluble asphalt is 3 - 7% of the mass of oxidized graphite.

[0009] Further, the mass of graphene is 0.5 - 2% of the mass of oxidized graphite.

[0010] Furthermore, the mass of the nano-silicon is 6% of the mass of the graphite oxide.

[0011] Furthermore, the mass of the melamine is 0.5 - 3% of the mass of the graphite oxide.

[0012] Furthermore, the mass of the monoammonium phosphate is 3 - 7% of the mass of the graphite oxide.

[0013] According to another aspect of the present invention, there is provided a composite-coated silicon-carbon negative electrode material obtained by the preparation method described above.

[0014] According to another aspect of the present invention, there is provided the application of the composite-coated silicon-carbon negative electrode material described above in the preparation of lithium-ion batteries.

[0015] The present invention prepares a high-performance silicon-carbon composite negative electrode material through oxidative etching of graphite and composite coating process. A flexible conductive network is constructed by using water-soluble asphalt and graphene together. The nitrogen doping of melamine improves the conductivity of carbon and silicon. The monoammonium 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, achieving the synergistic improvement of high capacity and long cycle life.

[0016] In the silicon-carbon negative electrode material provided by the present invention, the nano-silicon is uniformly dispersed in the carbon pores with a rich mesoporous structure that is stably bonded to the surface of the graphite. The 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

[0017] Figure 1 It is the first charge-discharge curve graph of the silicon-carbon negative electrode material in Example 1; Figure 2 It is the cycle capacity retention rate graph of the battery assembled with the silicon-carbon negative electrode in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] The basic concept of the present invention is to form active reaction sites through oxidative 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 negative electrode 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.

[0019] Based on this, a preparation method of a composite-coated silicon-carbon negative electrode material provided by a typical embodiment of the present invention includes the following steps 1 to 3, wherein steps 1 and 2 have no sequence.

[0020] Step 1: Graphite oxidation etching.

[0021] 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 with active functional groups on the surface.

[0022] Among them, the temperature of oxidation etching can be 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.

[0023] 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.

[0024] Step 2: Preparation of composite liquid.

[0025] Water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate are dispersed in water to form a composite liquid.

[0026] Preferably, the composite liquid is a homogeneous 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.

[0027] 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 oxidized graphite respectively.

[0028] The functions and synergistic effects of each component in the composite liquid are as follows: 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.

[0029] Graphene: Composites with the carbon matrix to form a three-dimensional conductive network, enhancing the overall conductivity of the material and simultaneously improving the stability of the carbon structure.

[0030] Nano-silicon: As a high-capacity active material, its theoretical capacity is 4200 mAh / g. Through encapsulation by a porous carbon skeleton, it alleviates the pulverization problem caused by volume expansion.

[0031] Melamine: After pyrolysis, it generates a nitrogen-doped carbon layer, and its lone pair electrons form strong chemical bonds with the surfaces of silicon and oxidized graphite, stabilizing the interface and enhancing the lithium-ion diffusion rate.

[0032] Monoammonium phosphate: Decomposes at high temperature to generate phosphate and ammonia, forming abundant hierarchical pores in the carbon layer to accommodate the volume expansion of silicon.

[0033] Step 3, Composite Coating and Carbonization.

[0034] Mix the composite liquid obtained in Step 2 and the graphene oxide obtained in Step 1 in a mass ratio of 15:80 - 90, and then perform carbonization treatment at a carbonization temperature of 800 - 1050 °C.

[0035] Among them, the mass ratio of the composite liquid to graphene 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 graphene oxide are mixed in a mass ratio of 15:85 and then carbonized.

[0036] After the composite liquid and graphene oxide are mixed, heat them to the carbonization temperature at a heating rate of 3 - 5 °C / min, and keep the temperature for 3 - 5 hours.

[0037] In the above embodiment, through the oxidation etching and composite coating processes, a three-dimensional conductive network formed by the composite of nitrogen-doped carbon layer and graphene is constructed on the surface of graphite. The structural characteristics are as follows: 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 and the carbon layer with graphite, and forming a more stable structure.

[0038] Nitrogen-doped carbon coating: The nitrogen-doped carbon generated by the pyrolysis of melamine binds to silicon and the surface of graphene oxide through lone pair electrons, enhancing the interface stability and promoting lithium ion transport.

[0039] Multi-level pores formed by monoammonium phosphate: During carbonization, monoammonium phosphate generates phosphate and gas to form multi-level 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.

[0040] 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.

[0041] In this embodiment, the combination of oxidation etching and composite coating is not a simple process superposition, but achieves synergistic effects through the following mechanisms: 1. Active site provision: Oxidation etching introduces oxygen-containing functional groups (such as hydroxyl, carbonyl, etc.) on the surface of graphite, providing active anchor sites for the uniform loading of the subsequent composite liquid and enhancing the interfacial binding force between each component and graphite.

[0042] 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 monoammonium phosphate, multi-level pore structures are formed through the formation of gas and phosphate groups, which not only accommodate the volume change of nanosilicon, but also form a "graphite - silicon - carbon" multi-level buffer structure.

[0043] 3. Chemical bond synergy: The active groups on the surface of graphite oxide react chemically with melamine, and a nitrogen-doped stable carbon layer is formed after carbonization, significantly improving the electrochemical stability of the material.

[0044] The following further illustrates the technical solutions claimed in the present invention through some examples. However, the examples 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 examples.

[0045] Unless otherwise specified, the materials and reagents used in the present invention can be obtained from commercial products in this field. The water-soluble asphalt used in the examples and comparative examples was purchased from Liaocheng Hongxin Chemical Co., Ltd. (the residue carbon value is 50%); the particle size of the nano-silicon is 70 nm; the graphite is needle coke artificial graphite with a D50 of 10 μm (the specific capacity is 360 mAh / g).

[0046] Example 1 Graphite was placed in a tube furnace, and a mixed gas with a volume ratio of air: nitrogen = 3:1 was introduced, and it was oxidized and etched at a constant temperature of 450 °C for 2 hours to form graphite oxide.

[0047] The water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate were respectively taken at 5%, 1%, 6%, 3%, and 7% of the mass of the graphite oxide and dispersed into an appropriate amount of deionized water, and a composite liquid was formed by ultrasonic dispersion at 40 °C.

[0048] The composite liquid and the graphite oxide were mixed uniformly at a mass ratio of 15:85, and 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.

[0049] Examples 2 - 13 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 the water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate to the mass of the graphite oxide were adjusted. The specific parameter differences are shown in Table 1.

[0050] Table 1 Main preparation conditions of each example

[0051] In Table 1, the ratio of air to nitrogen is a volume ratio; the ratios of the water-soluble asphalt, graphene, nano-silicon, melamine, and monoammonium phosphate to the graphite oxide are mass ratios.

[0052] Comparative Example 1 Mix unoxidized graphite, nano-silicon, and asphalt at a mass ratio of 89:6:5 at high speed, and then under nitrogen protection, heat it to 950 °C at a rate of 5 °C / min and carbonize for 3 hours to obtain a silicon-carbon composite material.

[0053] Comparative Example 2 Mix unoxidized graphite and nano-silicon evenly at a mass ratio of 94:6 to obtain a silicon-carbon and graphite mixed material.

[0054] Prepare the negative electrodes in the above examples and comparative examples into lithium-ion batteries and test their performance. Prepare a 5 Ah soft-pack battery to test the cycle performance and low-temperature performance.

[0055] Test conditions: Use the silicon-carbon materials prepared in the comparative examples and examples as negative electrode materials, mix them with the binder polyvinylidene fluoride (PVDF) and the conductive agent (Super-P) at a mass ratio of 80:10:10, add an appropriate amount of N-methylpyrrolidone (NMP) as a solvent to make a slurry, coat it on a copper foil, and after vacuum drying and rolling, prepare a negative electrode sheet; use a lithium metal sheet as the counter electrode, use an electrolyte of 1 mol / L LiPF6 three-component mixed solvent mixed according to EC:DMC:EMC = 1:1:1 (v / v), use a polypropylene microporous membrane as the separator, and assemble it into a CR2032 type button battery in a glove box filled with inert gas.

[0056] The charge-discharge test of the button battery is carried out on the battery test system of Wuhan Landian Electronic Co., Ltd. Under normal temperature conditions, charge and discharge at a constant current of 0.1C, and the charge-discharge voltage is limited to 0.005 - 1.5V to test the first discharge capacity and first discharge efficiency of the silicon-carbon negative electrode.

[0057] Use the following method to test and calculate the cycle performance and low-temperature performance of the materials: Apply the negative electrode materials in the examples and comparative examples to a soft-pack laminated battery with a nominal capacity of 5 Ah using lithium iron phosphate as the positive electrode. After assembling the battery, carry out formation and grading according to the normal system, and then charge and discharge at a current of 5 A until the discharge capacity drops to 80% of the first discharge capacity. Express the cycle performance in terms of the number of charge-discharge cycles; Carry out one charge-discharge cycle of the formed battery at a current of 5 Ah at room temperature, and mark the charging capacity as C 室温 , then place the discharged battery in a low-temperature cabinet and keep it at -40 °C for 24 h, and then charge it with a current of 2.5 A, and mark the charging capacity as C 低温 , and use P = C 低温 / C 室温 *100% to express the low-temperature performance.

[0058] Figure 1 It is the first charge-discharge curve diagram of the silicon-carbon negative electrode material in Example 1. Figure 2It is the graph of the cyclic capacity retention rate of the battery assembled with the silicon-carbon negative electrode in Example 1. The physical and electrochemical property characterizations of the materials in each example and comparative example are shown in Table 2.

[0059] Table 2 Physical and Electrochemical Property Characterizations of the Materials in Each Example and Comparative Example

[0060] Compared with Example 1, in Comparative Example 1, unoxidized graphite was used and graphene, melamine, and monoammonium phosphate were not added. Its cyclic performance was 343 times and the low-temperature charging ratio was 63.2%, both of which were significantly lower than the cyclic 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 interfacial stability.

[0061] 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 cyclic performance was 298 times, both of which were significantly lower than the first efficiency of 91.7% and the cyclic performance of 820 times in Example 1, indicating the necessity of the composite coating process for constructing the conductive network and interfacial stability. The low-temperature charging ratio of Comparative Example 2 was 58.5%, which was 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.

[0062] According to the above comparison, it can be proved that the synergistic effect of oxidation etching and composite coating significantly improves the cyclic stability, low-temperature performance, and interfacial kinetics of the material.

Claims

1. A preparation method of a composite-coated silicon-carbon anode material, characterized in that, Including: 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; 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 oxidized graphite 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 oxidative etching is 450°C.

3. The preparation method of the composite-coated silicon-carbon negative electrode 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 water-soluble asphalt is 3 - 7% of the mass of oxidized graphite.

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

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

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

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

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.

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