Low-temperature carbon-coating method and application

Abstract

This invention discloses a low-temperature carbon coating method and its application. The method involves first ball milling the solid material to be coated with a conductive or supporting material under an inert atmosphere, followed by switching the milling atmosphere to a carbon source atmosphere. Under continuous mechanical energy application, non-thermal decomposition of the gaseous carbon source is induced. The carbon species generated by the decomposition are deposited in situ on the surface of the composite precursor, thereby forming a composite material with a carbon coating layer. This method eliminates the need for external high-temperature treatment, effectively preventing structural damage to materials with low thermal stability or sensitive structures under high-temperature conditions. By controlling the type and pressure of the carbon source gas, the degree of carbon layer formation and the composition of the composite material can be adjusted. When applied to the anode of a lithium-ion battery, the composite material exhibits stable electrochemical performance under high-rate and long-cycle conditions. This invention features a simple process flow, adjustable parameters, good repeatability, and potential for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of materials preparation and surface engineering technology, specifically relating to a low-temperature carbon coating method and its application as a negative electrode in lithium-ion batteries. Background Technology

[0002] With the development of functional materials and energy storage materials, improving the structural integrity and interfacial stability of material systems that undergo significant volume changes during electrochemical reactions or ion insertion / deintercalation has become a key area of ​​continuous optimization in materials preparation and surface engineering. Taking high-specific-capacity materials such as black phosphorus (BP) as an example, they are prone to significant volume expansion during charging and discharging, leading to structural pulverization, repeated rupture of the interfacial film, and loss of active materials, ultimately resulting in performance degradation.

[0003] To mitigate structural and interfacial instability caused by volume changes, existing technologies often employ composite methods with conductive carbon materials such as graphite, graphene, or carbon nanotubes to improve overall conductivity and buffer mechanical stress to some extent. For example, patent CN117374263A discloses a method for ball milling composites of black phosphorus, carbon materials, and nitrogen-containing compounds to introduce phosphorus-carbon and phosphorus-nitrogen phase bond structures into the material system; patent CN116826005A discloses a method for ball milling composites of black phosphorus, Na-β(β”)-Al2O3, and conductive carbon materials to form PC, Al-P, and POC bond structures, thereby achieving composite structure construction. However, the above technical solutions mainly rely on interparticle composites or local interface bonds, making it difficult to form a continuous and dense overall coating layer on the surface of active materials. Their ability to constrain the volume evolution of materials during charge and discharge is limited, and problems such as local interface instability and structural disintegration may still occur during cycling. Therefore, it is difficult to achieve continuous and stable constraint on volume expansion at the structural level.

[0004] In contrast, constructing a continuous and dense carbon coating layer on the surface of the active material is considered a superior technical approach to achieve effective mechanical confinement and stabilize the electrode / electrolyte interface. A dense carbon layer can not only buffer volume changes and maintain electron transport channels to a certain extent, but also reduce the direct erosion of the active material by the electrolyte, thereby suppressing side reactions and improving interface stability. In existing carbon coating technologies, chemical vapor deposition (CVD) methods typically deposit a carbon layer on the material surface by thermally decomposing a gaseous carbon source at high temperatures. For example, patent CN116093275B discloses a method for CVD deposition on SiO₂ in a tube furnace at 800-1000℃. XA method for forming a carbon coating on the surface of particles; Patent CN120261549A discloses a method for achieving carbon coating on the surface of silicon-based materials by means of a mixed gas source under fluidized bed conditions of 450-700℃.

[0005] All of the above technical routes rely on high-temperature thermal energy to drive the decomposition and deposition process of gaseous carbon sources. For material systems with low thermal stability or sensitive structure, such as black phosphorus, high-temperature treatment may cause structural damage or performance degradation, making such high-temperature vapor deposition methods difficult to apply. This creates a significant technical contradiction between "the need for continuous and dense carbon coating" and "the inability to withstand high-temperature treatment conditions".

[0006] In recent years, mechanochemistry, as a method for material preparation and transformation driven by mechanical energy, has gradually attracted attention. Related studies have shown that mechanical stress, defect activation, and localized high-energy states can promote the breaking of some chemical bonds and non-equilibrium reaction processes, providing new possibilities for material modification under non-high-temperature conditions. However, existing mechanochemistry research mostly focuses on solid-state reactions or solid-state modification processes, and its effects are usually limited to particle refinement, component composite formation, or localized interfacial bonding, making it difficult to further form a continuous and dense carbon coating layer on the material surface. Summary of the Invention

[0007] To address the problem of long-term structural stability in material systems that are prone to significant volume changes during electrochemical reactions or ion insertion / deintercalation, especially considering the difficulty in forming continuous and effective external constraints through conductive carbon material composites, and the shortcomings of existing thermally driven gas-phase carbon coating processes that rely on high-temperature conditions and are unsuitable for material systems with low thermal stability, this invention provides a low-temperature carbon coating method.

[0008] The present invention provides a low-temperature carbon coating method based on mechanical energy-driven gas-phase pyrolysis as follows: (1) Under an inert atmosphere, the solid material to be coated and the conductive or supporting material are mixed, and grinding balls are added. The mixture is ball-milled at a speed of 800-2100 rpm for 6-24 hours to obtain a uniformly dispersed composite precursor. The solid material to be coated is a material that undergoes significant volume change during electrochemical reaction or ion insertion / extraction, including elemental materials, alloy materials and their oxides or composites, including one or more of red phosphorus, black phosphorus, silicon, silicon oxide, and tin; the conductive or supporting material is a conductive carbon material, including one or more of graphite, graphene, carbon nanotubes, hard carbon, soft carbon, carbon black, and porous carbon; the mass ratio of the coating solid material to the conductive or supporting material is (1-9):1; (2) Switch the inert atmosphere of the ball milling system in step (1) to a carbon source atmosphere, mix and ball mill at 800-2100 rpm for 6-24 hours, and under the condition of continuous application of mechanical energy, make the carbon source gas undergo non-thermal decomposition driven by mechanical energy. The carbon species generated by the decomposition are deposited in situ on the surface of the composite precursor, thereby obtaining a composite material with a carbon coating on the surface. Non-thermal pyrolysis refers to a pyrolysis or activation process that does not require external high temperature and is mainly induced by mechanical energy.

[0009] The carbon source gas is a gas that can be cracked to produce carbon, selected from one or more of methane, ethylene, acetylene, and propane; the pressure of the carbon source gas in the ball mill jar is 0.01-0.15 MPa.

[0010] The ball milling method described above is a high-energy planetary ball milling method. The grinding jar is a stainless steel grinding jar or an agate grinding jar. The grinding balls are selected from zirconia balls, agate balls or stainless steel balls, and the diameter of the grinding balls is 3-20mm. In step (1) or step (2), the mass ratio of the grinding balls to the mixture is (20-100):1.

[0011] The present invention uses the composite material with a carbon coating obtained by the above method as the negative electrode of a lithium-ion battery. In the electrochemical performance test, it can still maintain stable performance under high rate conditions and exhibit excellent capacity retention and rate performance under long cycle and fast charging conditions.

[0012] Advantages or technical effects of the present invention: 1. In the ball milling system, the inert atmosphere is switched to a carbon source atmosphere, and the gas phase carbon source is induced to decompose / activate under the action of continuous mechanical energy, so that carbon species can be deposited in situ on the surface of the composite precursor to form a carbon layer. From the process path, the traditional CVD relies on high temperature thermal decomposition, thereby reducing the risk of structural damage or performance degradation of materials with low thermal stability or structural sensitivity under high temperature treatment conditions. 2. Since the carbon species generated by pyrolysis are directly generated and deposited on the particle surface in situ, this invention helps to form a continuous carbon coating layer on the surface of active materials, thereby providing an overall external constraint on the particles and improving interface stability, thereby alleviating failure problems such as structural pulverization and repeated rupture of the interface film caused by volume evolution, and improving the cycle stability and reliability of the material. 3. The present invention adopts an integrated process flow of "ball milling-atmosphere switching-continued ball milling". The whole process does not require solvents and complex post-treatment, the process is relatively simplified, and key parameters such as carbon source gas pressure, ball milling time and grinding media have adjustable windows, which facilitates process control and batch consistency. 4. This invention takes ball milling equipment as its core and is compatible with existing industrial ball milling / mixing equipment systems. The supply and pressure control of carbon source gas can be achieved through conventional gas pipelines and valve control systems, which has the engineering basis for continuous or scale-up production, thus making it suitable for industrial applications of preparing surface carbon-coated composite materials. Attached Figure Description

[0013] Figure 1 The image shows a scanning electron microscope (SEM) image of the BP / G(BM-Ac)-0.02 composite material prepared in Example 1 of this invention. Figure 2 The images show the dark-field scanning transmission electron microscope (STEM) images of the BP / G(BM-Ac)-0.02 composite material prepared in Example 1 of this invention, as well as the overlapping distribution diagrams of P, O and C elements and the corresponding elemental distribution (Mapping) diagrams. Figure 3 The image shows the P 2p level X-ray photoelectron spectroscopy (XPS) spectrum of the BP / G(BM-Ac)-0.02 composite material prepared in Example 1 of this invention. Figure 4 The analytical results of the P content in Example 1, Comparative Example 1 and Comparative Example 2 of the present invention, determined by inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0014] Figure 5 The image shows a scanning electron microscope (SEM) image of the BP / G(BM-Ar) composite material prepared in Comparative Example 1 of this invention. Figure 6 The image shows a scanning electron microscope (SEM) image of the BP / G composite material prepared in Comparative Example 2 of this invention. Figure 7 The BP / G(BM-Ac)-0.02 composite material in Example 1 and the BP / G(BM-Ar) composite material in Comparative Example 1 were compared at 5 A·g. -1 Comparison of long-cycle performance at current density; Figure 8 The BP / G(BM-Ac)-0.02 composite material in Example 1 and the BP / G composite material in Comparative Example 2 are compared at 10 A·g. -1 Comparison of long-cycle performance at current density; Figure 9 The graph shows a comparison of the long-cycle performance of the BP / G(BM-Ac)-0.02 composite material in Example 1 and the BP / G(CVD-Ac) composite material in Comparative Example 3 at a current density of 5 A·g-1. Figure 10The BP / G(BM-Ac)-0.02 composite material in Example 1 and the BP / G(BM-Ac)-0.04 composite material in Example 2 of this invention are compared at 5 A·g. -1 Comparison of long-cycle performance under current density. Detailed Implementation

[0015] The present invention will be further described below with reference to specific embodiments. It should be understood that the embodiments are only used to illustrate the technical solutions and implementation methods of the present invention and do not constitute a limitation on the scope of protection of the present invention. Any equivalent substitutions, equivalent improvements, or modifications made to the present invention by those skilled in the art without departing from the concept of the present invention should fall within the scope of protection of the present invention. Unless otherwise expressly stated, the ball milling operation, atmosphere replacement / switching, gas pressure control, product collection, and conventional material characterization and testing methods involved in the embodiments all employ conventional technical means well known to those skilled in the art; the raw materials, reagents, and gases (including inert gases and carbon source gases) used, unless otherwise specified, are all commercially available or commonly obtained industrial products; the pressure ranges, unless otherwise specified, are all gauge pressures.

[0016] Example 1: Preparation of low-temperature carbon-coated black phosphorus-graphite materials based on mechanical energy-driven gas-phase pyrolysis 1. In an argon-filled glove box (O2 and H2O content not exceeding 0.1ppm), weigh 0.7g of black phosphorus and 0.3g of graphite and place them in a 50mL stainless steel ball mill jar and mix well. Add 77g of zirconia grinding balls, which are composed of balls of different sizes, including 5 balls with a diameter of 15mm and 40 balls with a diameter of 5mm. Use a high-energy planetary ball mill, set the ball milling speed to 1700rpm, and continuously ball mill for 12h in forward / reverse rotation cycle mode to obtain black phosphorus-graphite (BP / G) composite precursor powder. 2. While maintaining the ball milling system in a sealed state, the atmosphere inside the ball mill jar was switched from argon to acetylene gas, and the residual inert gas inside the jar was replaced with acetylene gas at a gauge pressure of 0.02 MPa. After the atmosphere replacement was completed, continuous ball milling was continued for 12 hours at 1700 rpm in a forward / reverse rotation cycle under an acetylene atmosphere. Under continuous application of mechanical energy, the acetylene gas underwent mechanically driven non-thermal decomposition. The carbon species generated by the decomposition were deposited in situ on the surface of the composite precursor to obtain a black phosphorus-graphite (BP / G(BM-Ac)-0.02) composite material with a carbon-coated surface.

[0017] Scanning electron microscope (SEM) image of BP / G(BM-Ac)-0.02 composite material as shown below. Figure 1 As shown, after ball milling in an acetylene atmosphere, the composite material exhibits irregular particles with particle sizes mainly distributed in the submicron scale range, and no obvious delamination or peeling phenomena were observed.

[0018] The elemental distribution of the BP / G(BM-Ac)-0.02 composite material was obtained using scanning transmission electron microscopy (STEM-Mapping). Figure 2 As shown in the diagram, the overlapping distribution of P, O, and C elements reveals that the outer layer of the composite material primarily exhibits the distribution characteristics of C and O elements, while P is mainly distributed in the inner region, indicating that the black phosphorus phase is effectively coated by the carbon layer. Simultaneously, the relatively uniform distribution of P, O, and C elements within the composite particles demonstrates that during the mechanically driven gas-phase processing, the components can achieve full composite formation and stable coexistence, thus forming a black phosphorus-graphite composite material with a carbon-coated surface.

[0019] The fine P 2p XPS spectrum of the BP / G(BM-Ac)-0.02 composite material is shown below. Figure 3 As shown, the spectrum yielded four peaks through deconvolution, located at 130.10 eV, 131.00 eV, 132.61 eV, and 133.77 eV, corresponding to P 2p3 / 2, P2p1 / 2, PC, and PO / P=O, respectively. These results indicate that black phosphorus successfully formed PC bonds with graphite, and the surface carbon layer effectively suppressed the structural instability caused by volume expansion of black phosphorus during charging and discharging.

[0020] Simultaneously set up Comparative Example 1, Comparative Example 2 and Comparative Example 3; Comparative Example 1: In an argon-filled glove box (O2 and H2O contents not exceeding 0.1 ppm), 0.7 g of black phosphorus and 0.3 g of graphite were weighed and mixed in a 50 mL stainless steel ball mill jar. 77 g of zirconia grinding balls were added. The grinding balls consisted of 5 balls with a diameter of 15 mm and 40 balls with a diameter of 5 mm. A high-energy planetary ball mill was used, with a milling speed of 1700 rpm, and continuous milling for 12 hours in a forward / reverse rotation cycle mode to obtain BP / G composite precursor powder. While maintaining the ball milling system in a sealed state, the gas inside the jar was replaced according to the atmosphere replacement procedure in Example 1, and argon gas was introduced into the ball mill jar at a gauge pressure of 0.02 MPa. Subsequently, continuous milling was continued for 12 hours in an argon atmosphere at 1700 rpm in a forward / reverse rotation cycle mode to obtain the BP / G (BM-Ar) composite material of Comparative Example 1.

[0021] Comparative Example 2: In an argon-filled glove box (with O2 and H2O contents not exceeding 0.1 ppm), 0.7 g of black phosphorus and 0.3 g of graphite were weighed and placed in a 50 mL stainless steel ball mill jar and mixed. 77 g of zirconia grinding balls were added. The grinding balls consisted of balls of different sizes, including 5 balls with a diameter of 15 mm and 40 balls with a diameter of 5 mm. A high-energy planetary ball mill was used, with the milling speed set at 1700 rpm. After continuous milling for 12 hours in a forward / reverse rotation cycle mode, the ball mill jar was opened and the product was collected in an argon-filled glove box to obtain the BP / G composite material of Comparative Example 2.

[0022] Comparative Example 3 involved placing the BP / G composite material obtained in Comparative Example 2 in an open quartz boat, which was then placed inside a tube furnace. Under an argon protective atmosphere, the argon flow rate was 50 sccm, and the furnace temperature was 10℃·min. -1 The heating rate was increased from room temperature to 580℃; after the temperature reached 580℃, an acetylene / argon mixture was introduced, with an acetylene flow rate of 30 sccm and an argon flow rate of 20 sccm, and the temperature was maintained at 580℃ for 30 min; then the acetylene gas was turned off, and the mixture was naturally cooled to room temperature under an argon protective atmosphere to obtain the BP / G(CVD-Ac) composite material of Comparative Example 3.

[0023] The relative phosphorus content results for BP / G(BM-Ac)-0.02, BP / G(BM-Ar), and BP / G composites are as follows: Figure 4 As shown, compared with BP / G(BM-Ar) and BP / G, the relative phosphorus content of the BP / G(BM-Ac)-0.02 composite decreased to 49.93-51.45%, while the relative phosphorus contents of BP / G(BM-Ar) and BP / G remained in the ranges of 63.17-63.98% and 62.04-63.30%, respectively. These results indicate that ball milling under an acetylene atmosphere introduces additional carbon components, leading to an increase in the relative carbon content of the composite. On the other hand, the relative phosphorus content of BP / G(BM-Ar) and BP / G was essentially the same, indicating that adding one ball milling process under an inert atmosphere did not cause a significant change in the relative carbon content of the composite.

[0024] The scanning electron microscope (SEM) images of BP / G (BM-Ar) and BP / G composite materials are shown in Figure 5 and 5, respectively. Figure 6 As shown in Figure 5, the BP / G (BM-Ar) sample after inert atmosphere ball milling mainly consists of irregular blocky and granular structures. Its overall particle size is significantly smaller than that of BP / G, indicating that the ball milling process has a certain physical crushing and particle refinement effect on the material. In contrast, as... Figure 6As shown, the BP / G sample is mainly composed of large elliptical particles, which exhibit more obvious agglomeration characteristics, and its overall particle size is significantly larger than that of BP / G (BM-Ar).

[0025] Example 2: Preparation of BP / G(BM-Ac)-0.04 composite material The method is the same as in Example 1, except that the acetylene gas pressure is 0.04 MPa (gauge pressure).

[0026] Example 3: Battery Assembly and Performance Testing The composite materials obtained in Examples 1-2, Comparative Examples 1, 2 and 3 were used as negative electrode active materials for lithium-ion batteries. The composite material, conductive carbon black (conductive agent) and sodium alginate (binder) were mixed in a mass ratio of 7:1.5:1.5 to prepare a slurry, which was uniformly coated on copper foil with a coating thickness of 200 μm. After being vacuum dried overnight, the slurry was cut into electrode sheets. CR2032 button cells were assembled in an argon-filled glove box environment (O2 and H2O content both not exceeding 0.1 ppm). The dried electrode sheets were transferred to the glove box and paired with lithium metal sheets (counter and reference electrodes), separated by a polypropylene separator. A few drops of 1M LiPF6 dissolved in an electrolyte solution of EC:DEC (volume ratio 1:1) containing 5wt% FEC additive were added. After encapsulation, the cells were allowed to stand for 24 hours to allow for full electrolyte wetting. Subsequently, constant current charge-discharge tests were performed on the Xinwei Battery Testing System, with the voltage range set to 0.01-3.0V and the current density at 5A·g. -1 and 10A·g -1 5A·g -1 Cyclic performance such as Figure 7 , 9 As shown, 10A·g -1 Cyclic performance such as Figure 8 As shown; Depend on Figure 7 It can be seen that the BP / G(BM-Ac)-0.02 composite material of Example 1 has a yield of 5A g. -1 After 300 cycles at a current density, it still maintains 1058.34 mAh·g. -1 The reversible specific capacity of the electrode, with a capacity retention of 105.04%, indicates that the electrode did not exhibit significant capacity decay during long-term cycling. In contrast, under the same test conditions, the reversible specific capacity of the BP / G(BM-Ar) composite material in Comparative Example 1 after 300 cycles was only 905.30 mAh·g. -1Its capacity retention rate was 69.38%, showing significantly lower cycling stability. These results indicate that, compared to BP / G(BM-Ar) obtained by ball milling under an inert atmosphere, the BP / G(BM-Ac)-0.02 composite exhibits superior cycling stability and capacity retention at high current densities.

[0027] Depend on Figure 8 It can be seen that at 10A·g -1 After 300 cycles at a current density, the BP / G(BM-Ac)-0.02 composite material of Example 1 still maintained 626.92 mAh·g. -1 The reversible specific capacity of the electrode exhibited a capacity retention of 105.51%, indicating that the electrode did not show significant capacity decay even under high current density conditions. In contrast, under the same test conditions, the reversible specific capacity of the BP / G electrode in Comparative Example 2 after 300 cycles was only 211.39 mAh·g. -1 The corresponding capacity retention rate was 14.79%, indicating significantly poor cycling stability. These results further demonstrate that the BP / G(BM-Ac)-0.02 composite material maintains stable electrochemical performance even under higher current density conditions, significantly outperforming the BP / G electrode that has not undergone ball milling in a carbon source gas atmosphere.

[0028] Figure 9 It can be seen that in 5A·g -1 At the specified current density, the BP / G(BM-Ac)-0.02 composite material of Example 1 exhibited excellent long-term cycling stability. In contrast, under the same test conditions, the BP / G(CVD-Ac) composite material of Comparative Example 3 only showed 151.97 mAh·g⁻¹ in the initial stage. -1 The reversible specific capacity remained at a low level throughout the entire cycle, with a negligible capacity contribution. These results indicate that the conventional CVD method for carbon coating BP / G composites at a processing temperature of 580℃ is unsuitable for thermally unstable materials like black phosphorus. The high-temperature process easily leads to the volatilization and loss of black phosphorus components, significantly weakening the reversible lithium storage activity of the composite. Therefore, traditional high-temperature CVD carbon coating methods are not suitable for thermally unstable material systems. In contrast, the mechanical energy-driven low-temperature vapor-phase pyrolysis carbon coating method described in this invention does not require external high temperatures, making it more conducive to maintaining the integrity of the active components and achieving stable electrochemical performance.

[0029] Figure 10 It can be seen that in 5A·g 1At the specified current density, the BP / G(BM-Ac)-0.02 composite material of Example 1 exhibited a higher overall specific capacity than the BP / G(BM-Ac)-0.04 composite material of Example 2 throughout the entire cycling process. Specifically, the BP / G(BM-Ac)-0.02 composite material maintained a specific capacity of 1058.34 mAh·g after 300 cycles. -1 The reversible specific capacity of the BP / G(BM-Ac)-0.04 composite material is 865.21 mAh·g⁻¹, while the reversible specific capacity of the BP / G(BM-Ac)-0.04 composite material is 865.21 mAh·g⁻¹. -1 The capacity retention rates of the two materials are basically similar, at 105.04% and 107.67%, respectively. These results indicate that under the conditions described in this invention, as the acetylene pressure increases, the carbon phase content formed in the material increases accordingly. Although cycle stability is maintained, excessively high carbon content is not conducive to further improvement of specific capacity.

Claims

1. A low-temperature carbon coating method, characterized in that: In an inert atmosphere, the solid material to be coated and the conductive or supporting material are mixed, and grinding balls are added. The mixture is ball-milled at 800-2100 rpm for 6-24 hours to obtain a uniformly dispersed composite precursor. The inert atmosphere is then switched to a carbon source atmosphere, and the mixture is ball-milled at 800-2100 rpm for another 6-24 hours. Under continuous application of mechanical energy, the carbon source gas undergoes non-thermal decomposition driven by mechanical energy. The carbon species generated by the decomposition are deposited in situ on the surface of the composite precursor, thereby obtaining a composite material with a carbon-coated surface.

2. The low-temperature carbon coating method according to claim 1, characterized in that: The solid material to be coated is selected from one or more of red phosphorus, black phosphorus, silicon, silicon oxide, and tin.

3. The low-temperature carbon coating method according to claim 1, characterized in that: The conductive or supporting material is a conductive carbon material, selected from one or more of graphite, graphene, carbon nanotubes, hard carbon, soft carbon, carbon black, and porous carbon.

4. The low-temperature carbon coating method according to claim 3, characterized in that: The mass ratio of the solid material to be coated to the conductive or supporting material is (1-9):

1.

5. The low-temperature carbon coating method according to claim 1, characterized in that: The grinding balls are made of zirconia, agate, or stainless steel, and their diameter ranges from 3 to 20 mm.

6. The low-temperature carbon coating method according to claim 5, characterized in that: The mass ratio of grinding balls to the mixture is (20-100):

1.

7. The low-temperature carbon coating method according to claim 1, characterized in that: The carbon source gas is selected from one or more of methane, ethylene, acetylene, and propane.

8. The application of the composite material with a surface-coated carbon layer obtained by the low-temperature carbon coating method according to any one of claims 1-7 as the negative electrode of a lithium-ion battery.

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