High-conductivity silicon-carbon negative electrode material for lithium ion batteries and preparation method thereof

Abstract

This invention discloses a highly conductive silicon-carbon anode material for lithium-ion batteries and its preparation method, belonging to the field of lithium-ion battery technology. SiO2 synthesized using the Stober method is combined with sodium hypophosphite (Na2H2PO4) as a phosphorus source. Defect-modified nano-silicon materials are synthesized through high-temperature phosphating and further magnesothermic reduction. These nano-silicon materials are then combined with graphene in a high-energy ball mill to obtain a silicon-carbon composite material. This invention effectively improves the conductivity of the silicon-carbon anode material by constructing internal defects in silicon and an external ultra-high conductivity graphene layer. Simultaneously, the synthesized silicon itself has a hollow and porous structure, effectively suppressing volume expansion during charge and discharge. When used as a lithium-ion battery anode material, this material exhibits high reversible specific capacity and good capacity retention.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and specifically relates to a high-conductivity silicon-carbon anode material for lithium-ion batteries and its preparation method. Specifically, it relates to a phosphorus-doped, graphene-coated hollow porous silicon-carbon anode material, which can be used as a lithium-ion battery anode material. Background Technology

[0002] Lithium-ion batteries possess characteristics such as high energy density, long lifespan, no pollution, and excellent storage performance, leading to their widespread application in 3C products, electric vehicles, and energy storage power stations. Currently, the capacity of commercial graphite anodes (372 mAh g⁻¹) is... -1 Gradually, these materials are no longer able to meet the electric vehicle market's demand for long driving range, necessitating the development of high-capacity lithium-ion battery anode materials.

[0003] Silicon (Si) has a high theoretical capacity (3579 mAh g⁻¹). -1 Silicon, with its abundant reserves, low cost, and environmental friendliness, holds promise as a potential replacement for graphite as the next-generation anode material for lithium-ion batteries. However, silicon as an anode material for lithium-ion batteries suffers from problems such as volume expansion and low intrinsic conductivity. Silicon prepared by the magnesiothermal reduction method has a hollow porous structure, which prevents silicon from expanding outwards during lithium intercalation, instead causing inward expansion and effectively alleviating the volume expansion problem during silicon intercalation. However, this does not solve the problem of poor conductivity, so researchers have further proposed strategies such as atomic doping and coating with highly conductive materials. However, the depth of atomic doping and the degree of coating still need to be addressed. Therefore, this work improves the atomic doping depth by doping during the precursor process and uses nano-silicon materials with greater activity to composite with graphene. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for preparing a phosphorus-doped, graphene-coated hollow porous silicon-carbon anode material. The silicon-carbon composite anode material, as a lithium-ion battery anode material, exhibits high conductivity and excellent initial reversible capacity and cycle performance.

[0005] This invention provides a highly conductive silicon-carbon anode material for lithium-ion batteries and its preparation method. The preparation method includes: calcining SiO2 prepared by the Stobol method with sodium hypophosphite at high temperature to obtain a precursor, followed by magnesothermic reduction and ball milling to obtain a silicon-carbon composite material for lithium-ion battery anodes. The specific preparation method includes the following steps:

[0006] (1) Hexadecyl ammonium bromide (CTAB) was added to a mixed solution containing ethanol, deionized water, and ammonia (25%). After the mixed solution became clear, tetraethyl orthosilicate (TEOS) was added. The mixture was stirred at 60°C for 3 h, and then centrifuged and dried to collect SiO2. Sodium hypophosphite was then added and calcined at high temperature to obtain the precursor.

[0007] The precursor was mixed with magnesium powder and reduced at high temperature. After the reaction was cooled, the product was immersed in hydrochloric acid and hydrofluoric acid solutions to remove byproducts. The product was collected by centrifugation and vacuum dried to obtain defect-modified nano-silicon materials.

[0008] The preferred mass ratio of silicon dioxide to sodium hypophosphite is 1:0.02-0.1. Too much sodium hypophosphite will introduce too many defects; too little sodium hypophosphite will not be enough to oxidize the surface phosphides into phosphates.

[0009] (2) The precursor was mixed with magnesium powder and reduced at high temperature. After the reaction was cooled, the product was immersed in hydrochloric acid and hydrofluoric acid solutions to remove byproducts. The product was collected by centrifugation and vacuum dried to obtain defect-modified nano-silicon material.

[0010] (3) Defect-modified nano-silicon materials and graphene are processed in a high-energy ball mill to obtain silicon-carbon anode materials with high conductivity.

[0011] The mass ratio of nano-silicon material to graphene is 2-0.5:1; the ratio of the total mass of nano-silicon material and graphene to the zirconia balls is 1:20; the power of the high-energy ball mill is 800-1000 r / min, and the time is 2 h, so that the nano-silicon material and graphene can be fully composited.

[0012] The prepared silicon-carbon anode material was coated onto copper foil to form the anode of a lithium-ion battery. Silicon-carbon, conductive agent (superconducting carbon (Super P)), and binder (sodium carboxymethyl cellulose (CMC)) were dispersed in an aqueous solvent at a mass ratio of 8:1:1, and then uniformly coated onto copper foil. After drying, a circular electrode sheet with a diameter of 12 mm was prepared.

[0013] In this invention, the electrochemical performance of the silicon-carbon anode was tested using a lithium-ion battery system consisting of a dual electrode. The silicon-carbon anode material was used as the working electrode, while a high-purity lithium sheet served as both the counter electrode and the reference electrode. The electrolyte was 1M LiPF6 dissolved in ethylene carbonate (EC) / dimethyl carbonate (DMC) (1:1 v / v) with 10 wt% fluoroethylene carbonate (FEC) added. Battery assembly was conducted in an argon-filled glove box. Charge-discharge experiments of the lithium-ion battery were performed using a Newway battery testing system.

[0014] In Example 4 of the present invention, the lithium-ion battery containing the silicon-carbon anode of the present invention was prepared at 0.2 A g.-1 At the current density, the reversible specific capacity after 100 cycles is 1762 mAh g. -1 The capacity retention rate is as high as 98% or more.

[0015] In this invention, nano-silicon is synthesized via a magnesothermic reduction method, which offers advantages such as readily available raw materials, simple processes, and low energy consumption compared to current mainstream nano-silicon preparation methods. This invention employs a solid-phase mixture of SiO2 and a phosphorus source, followed by high-temperature calcination. The phosphorus element occupies oxygen positions, creating defects. Further thermal reduction, as seen in the XRD pattern, reveals that the phosphorus element is located within the silicon crystal structure as bulk doping, thus forming phosphorus-doped porous silicon nanomaterials. Furthermore, this invention synthesizes silicon dioxide using stobol before bulk doping, without affecting the particle size. SEM images show that the carbon-silicon material prepared in this invention has a porous hollow spherical structure with a particle size of 200-250 nm, ensuring better composite with graphene and maintaining high activity.

[0016] The nano-silicon prepared by this invention has a particle size of approximately 200-250 nm, and its hollow, porous structure effectively reduces the volume expansion effect of silicon during charge and discharge. Phosphorus doping creates defects within the silicon, which, when used as a negative electrode material for lithium-ion batteries, effectively improves the electron transport speed of silicon and ensures structural stability during cycling. Furthermore, the nano-silicon adheres to highly conductive graphene during ball milling, further addressing the issue of silicon's low intrinsic conductivity. In summary, this invention offers the following advantages: simple process, low cost, and suitability for industrial production; better cycle stability, superior rate performance, and lower internal resistance when used as a negative electrode material for lithium-ion batteries. Attached Figure Description

[0017] Figure 1 XRD patterns of the nano-silicon materials prepared in Example 2 and Comparative Example 1;

[0018] Figure 2 SEM image of the nano-silicon material prepared in Example 2;

[0019] Figure 3 SEM image of the silicon-carbon material prepared in Example 4;

[0020] Figure 4 Example 4, Comparative Example 1, and Comparative Example 2 were prepared at 0.2 A g. -1 Cyclic curves at current density;

[0021] Figure 5 The rate performance of Example 4, Comparative Example 1, and Comparative Example 2 at different current densities is shown. Detailed Implementation

[0022] The high-stability and high-conductivity silicon-based anode material for lithium-ion batteries and its preparation method proposed in this invention can be implemented through the following method. Example 1

[0023] (1) 0.3 g of cetylammonium bromide (CTAB) was added to an ethanol aqueous solution containing 2 ml of ammonia (25%) (60 ml ethanol, 100 ml water). After the mixed solution became clear, 2 ml of tetraethyl orthosilicate (TEOS) was added. The mixture was stirred at 60 °C for 3 h, and then centrifuged and dried to collect SiO2. Subsequently, sodium hypophosphite (0.02 g) and SiO2 (1 g) were added to a mortar and ground and mixed evenly. The mixture was then placed in a muffle furnace and calcined at 500 °C to obtain the precursor. The heating rate was 5 °C / min.

[0024] (2) The precursor was mixed with magnesium powder at a ratio of 1:1 and reduced at 650℃ (heating rate 1℃ / min). After the reaction was cooled, the product was immersed in 1M hydrochloric acid and hydrofluoric acid solution to remove byproducts. The product was collected by centrifugation and dried under vacuum at 80℃ to obtain defect-modified nano-silicon material.

[0025] This embodiment also provides a lithium battery, including a negative electrode material, wherein the negative electrode material includes the silicon-carbon composite material described above. The preparation method and type of the lithium battery are carried out using methods known in the art, and are not specifically limited in this application.

[0026] The following is an example to illustrate this:

[0027] (1) A method for preparing a negative electrode material includes the following steps: dispersing the silicon-carbon composite material, SuperP and CMC obtained above in an aqueous solvent at a mass ratio of 8:1:1 to obtain a mixed dispersion, coating the mixed dispersion on a copper foil, and drying to obtain an electrode sheet, i.e., a negative electrode material;

[0028] (2) The above negative electrode material was used as the working electrode, Celgard 2500 was used as the separator, high-purity lithium sheet was used as the counter electrode and reference electrode, and 1M LiPF6 was dissolved in ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) (1:1:1 vol) and 10 wt% fluoroethylene carbonate (FEC) was added. The cells were assembled into a 2032 type button cell, i.e., a lithium battery, in a glove box (H2O<0.01ppm, O2<0.01ppm) filled with high-purity argon (99.999%).

[0029] The charge / discharge experiments of the lithium battery in this application were conducted on the Xinwei Battery Testing System. Test conditions: 50mA g -1 Activation was performed for three cycles at a current density, followed by activation at 200 mA g. -1cycle. Example 2

[0030] The difference between this embodiment and Embodiment 1 is that the mass of sodium hypophosphite, the phosphorus source, added is 0.05g, while the other methods are the same as in Embodiment 1. Example 3

[0031] The difference between this embodiment and Embodiment 1 is that the mass of sodium hypophosphite, the phosphorus source, added is 0.1g; the other methods are the same as in Embodiment 1. Example 4

[0032] The defect-modified nano-silicon material (0.6 g), graphene (0.3 g), and zirconia balls (18 g) prepared in Example 2 were placed in a ball mill jar and milled in a high-energy ball mill at 800 r / min. -1 The material was ball-milled for 5 hours (with a 3-minute rest after every 10 minutes of ball milling) to obtain a silicon-carbon anode material with high conductivity. Example 5

[0033] The difference between this embodiment and Embodiment 4 is that the high-energy ball mill operates at 500 r / min. -1 The material was ball-milled for 5 hours (with a 3-minute rest after every 10 minutes of ball milling) to obtain a silicon-carbon anode material with high conductivity. Example 6

[0034] The difference between this embodiment and Embodiment 4 is that the high-energy ball mill operates at 1000 r / min. -1 The material was ball-milled for 5 hours (with a 3-minute rest after every 10 minutes of ball milling) to obtain a silicon-carbon anode material with high conductivity. Example 7

[0035] The defect-modified nano-silicon material (0.5 g), graphene (0.5 g), and zirconia balls (20 g) prepared in Example 2 were placed in a ball mill jar and milled in a high-energy ball mill at 800 r / min. -1 The material was ball-milled for 5 hours (with a 3-minute rest after every 10 minutes of ball milling) to obtain a silicon-carbon anode material with high conductivity. Example 8

[0036] The defect-modified nano-silicon material (0.3 g), graphene (0.6 g), and zirconia balls (18 g) prepared by the method in Example 2 were placed in a ball mill jar and milled in a high-energy ball mill at 800 r / min. -1 The material was ball-milled for 5 hours (with a 3-minute rest after every 10 minutes of ball milling) to obtain a silicon-carbon anode material with high conductivity. Comparative Example 1

[0037] The difference between this comparative example and Example 1 is that no phosphorus source was added; the other methods are the same as in Example 1. Comparative Example 2

[0038] In contrast, the basic sample of commercial silicon nanoparticles was assembled into a Si / Li half cell according to the method of Example 1, forming Comparative Example 2.

[0039] The lithium batteries prepared in Examples 1-8 and Comparative Examples 1-2 were subjected to charge-discharge experiments on the Newway Battery Testing System using the constant current charge-discharge test standard. The results are as follows:

[0040] .

[0041] Figure 1 The XRD patterns of Example 2 and Comparative Example 1 of this application show that elemental silicon was successfully synthesized based on its characteristic diffraction peaks, and the shift of the characteristic peaks proves that the doping of P element was successful.

[0042] Figure 2 The hollow nanoporous silicon material prepared in Example 2 of this application clearly shows that its surface is rough and porous, and the particle size is 248.3 nm.

[0043] Figure 3 The image shows a SEM image of the silicon-carbon material prepared in Example 4 of this application. The image shows that the particle size of the nano-silicon is about 200 nm, the structure is rough and porous, and it is attached to the graphene.

[0044] Figure 4 For Example 4, Comparative Example 1, and Comparative Example 2 of this application, the concentration was 0.2 A g. -1 Cycling curves at current density. The cycling stability of nano-hollow porous silicon is significantly improved, thanks to its unique structure which greatly avoids particle breakage and shedding caused by volume expansion; and the improved conductivity after being combined with graphene, which further improves its reversible capacity and capacity retention.

[0045] Figure 5 For the rate performance of Examples 4, Comparative Example 1, and Comparative Example 2 at different current densities, it can be noted that Examples 4 and Comparative Example 1 still exhibit a rate performance of 500 mAh g at a current density of 5 A / g. -1 The specific capacitance is around 0.1 A g, and when the current density returns to 0.1 A g -1 The specific capacity can return to its initial state, which proves that the hollow porous structure can effectively suppress the expansion effect and ensure that the material maintains structural stability under high current.

[0046] The above embodiments are only for illustrating the technical concept and features of the present invention. Their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a silicon-carbon anode material for lithium-ion batteries, characterized in that, Specifically, the following steps are included: (1) SiO2 and phosphorus source are mixed evenly and calcined at high temperature to obtain phosphorus-doped silicon precursor; the mass ratio of SiO2 to phosphorus source is 1:0.02-0.1; (2) The phosphorus-doped silicon precursor obtained in step (1) is mixed with magnesium powder, reduced at high temperature, and after the reaction is cooled, the product is immersed in hydrochloric acid and hydrofluoric acid solutions to remove byproducts, collected by centrifugation, and vacuum dried to obtain defect-modified nano-silicon material. (3) The defect-modified nano-silicon material prepared in step (2) and graphene are treated in a high-energy ball mill to obtain a silicon-carbon anode material with high conductivity; the mass ratio of the defect-modified nano-silicon material to graphene is 1:0.5-1; the rotation speed of the high-energy ball mill is 800-1000 r / min and the ball milling time is 5h.

2. The method for preparing the silicon-carbon anode material for lithium-ion batteries as described in claim 1, characterized in that, The method for preparing SiO2 is as follows: hexadecyl ammonium bromide is added to a mixed solution containing ethanol, deionized water, and saturated ammonia. After the mixed solution becomes clear, tetraethyl orthosilicate is added. The mixture is stirred at 60°C for 3 h, and then centrifuged and dried to collect SiO2. The ratio of the amounts of hexadecyl ammonium bromide, anhydrous ethanol, deionized water, saturated ammonia, and tetraethyl orthosilicate is 300 g: 2 L: 60 L: 100 L: 2 L.

3. The method for preparing the silicon-carbon anode material for lithium-ion batteries as described in claim 1, characterized in that, The phosphorus source in step (1) includes one or more of sodium hypophosphite, phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

4. The method for preparing the silicon-carbon anode material for lithium-ion batteries as described in claim 1, characterized in that, The high-temperature calcination in step (1) is carried out in an air atmosphere.

5. The method for preparing the silicon-carbon anode material for lithium-ion batteries as described in claim 1, characterized in that, In step (2), the mass ratio of the phosphorus-doped silicon precursor to magnesium powder is 1:1; the high-temperature reduction temperature is 650-700℃, and the reaction time is 5h.

6. The method for preparing the silicon-carbon anode material for lithium-ion batteries as described in claim 1, characterized in that, The total mass ratio of the nano-silicon material and graphene to the mass ratio of the zirconium oxide spheres is 1:

20.

7. A silicon-carbon anode material for lithium-ion batteries prepared by the method according to any one of claims 1-6, characterized in that, The lithium-ion battery anode material is a phosphorus-doped, graphene-coated hollow porous silicon-carbon anode material with a particle size of 200-250 nm.

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