Nanometer lithium phosphide modified silicon-carbon negative electrode material and preparation method thereof, secondary battery

Abstract

This invention relates to the preparation and application of silicon-carbon anode materials, and discloses a silicon-carbon anode material modified with lithium phosphide nanoparticles, its preparation method, and a secondary battery. The material has a core-shell structure, with the core comprising porous carbon and lithium phosphide nanoparticles and silicon nanoparticles generated in situ within the micropores of the porous carbon; the shell comprises a composite coating layer composed of amorphous carbon and carbon nanotubes; silicon accounts for 45-65 wt% of the modified silicon-carbon anode material by mass, and the P / Si molar ratio of the lithium phosphide nanoparticles to the silicon nanoparticles is 2:98~30:70. A small amount of phosphorus source is first deposited in the porous carbon micropores, followed by lithiation treatment to convert the phosphorus in the porous carbon into lithium phosphide nanoparticles, then a second deposition of silicon nanoparticles, and finally overall carbon coating. This invention's modified silicon-carbon anode material improves the problem of rapid impedance increase in the low-charge range of silicon-carbon anode materials, significantly enhancing the user experience.

Description

Technical Field

[0001] This invention relates to the preparation and application of silicon-carbon anode materials, and particularly to a nano-lithium phosphide modified silicon-carbon anode material, its preparation method, and a secondary battery. Background Technology

[0002] With the widespread application of lithium-ion batteries in mobile phones, laptops, electric vehicles, and energy storage systems, the functions of terminal devices are increasing, and the requirements for battery energy density are also increasing. Silicon anode materials have great application potential in the field of lithium-ion batteries due to their high theoretical specific capacity (4200 mAh / g). With the mature application of porous carbon technology and CVD vapor deposition process (Xiao Mu, Xintong Xu, Haibang Xu, Tao Huang, Aishui Yu. Ultrastable Monodisperse Resin-Based Spherical Si-C Materials With Micropore Confined Growth of Silicon Nanoclusters for Lithium-Ion Battery Anodes. Advanced Functional Materials, 35(38), 202504545), the volume expansion of nano-silicon particles during the lithiation process is well suppressed under the confinement effect of porous carbon channels, which greatly promotes the commercial application of vapor-deposited silicon-carbon anode materials.

[0003] However, silicon-carbon anode materials have not been well resolved in practical applications due to the problem of high impedance or sudden impedance increase at low state of charge (SOC). (Li, Y., et al. SiliconNanoparticles Embedded in Porous Carbon Matrix via CVD for High-PerformanceLithium-Ion Batteries: The Role of Carbon Scaffold in Mitigating VolumeExpansion and Enhancing Conductivity, Journal of Power Sources, 2022, 539, 129987; Chen, J., et al. Influence of Temperature and Electrolyte Additives on Low SOC Performance of CVD Silicon-Carbon Anode in Full Cells. ACS Applied Energy Materials, 2023, 6, 12, 13842-13851). There are two main reasons: First, the nano-silicon in the porous carbon framework exhibits a tubular structure with "wall-attached growth," meaning lithium ions can only be transported along the pore walls, making three-dimensional diffusion difficult. At low SOC (low lithium content), the lithium ion concentration within the channels decreases, significantly increasing transport resistance. Second, silicon itself has poor electrical conductivity and relies on the carbon network for electron conduction. Although the electronic conductivity of the lithium-silicon alloy formed after lithiation is improved, at low SOC, the low lithium content in the silicon lattice obstructs the electron conduction pathway, leading to a rapid increase in contact resistance. Therefore, solving this problem would further promote the widespread application of vapor-deposited silicon-carbon materials.

[0004] Lithium phosphide (Li3P) is an excellent solid electrolyte with both high electronic and ionic conductivity, exhibiting composite conductivity. If it can be combined with CVD porous silicon-carbon materials, it is expected to significantly reduce the high impedance problem of silicon-carbon materials.

[0005] Chinese patent CN116895747A discloses a method for preparing phosphorus-doped modified silicon-carbon materials. The method employs vapor deposition to simultaneously deposit a phosphorus source and a silicon source into porous carbon channels in a single step. By adjusting the ratio of phosphorus and silicon sources, phosphorus atoms are doped into the silicon molecular lattice, replacing some silicon atoms. The resulting phosphorus-doped silicon achieves higher electronic conductivity than pure silicon, thereby reducing material impedance, lowering battery internal resistance, and improving battery cycle performance. However, the silicon anode material prepared by this method does not contain elemental phosphorus within the channels; it only addresses the electronic conductivity issue, and its properties differ from those of Li. + The main element in the lithiation / delithiation reaction is still silicon. In silicon-carbon materials at low state of charge (SOC), due to the low lithium content, Li... x Si y The problem of reduced lithium-ion concentration and high ion transport resistance within lithium-silicon grains has not yet been effectively solved.

[0006] Chinese patent CN120796938A discloses a method for preparing a phosphorus / silicon composite anode material. To avoid the problem of phosphorus vapor sublimation and loss during high-temperature pyrolysis deposition, the process adopts a two-step deposition method. First, nano-silicon particles are deposited in porous carbon channels, and then nano-phosphorus particles are deposited to finally obtain the phosphorus / silicon composite anode material. As an anode material, this material can form lithium phosphide (Li3P) with high electronic and ionic conductivity in actual full cells, which will simultaneously improve the electronic and ionic conductivity of silicon particles, reduce battery polarization resistance, and improve rate and cycle performance. However, due to the large difference in lithiation potential between phosphorus and silicon (lithium insertion potential of silicon is ~0.2V vs. Li, and delithiation potential is ~0.4V vs. Li), the lithiation potential of phosphorus is relatively higher than that of lithium metal (lithium insertion potential is ~0.7V vs. lithium metal, and delithiation potential is ~1.0V vs. lithium metal). (Unlocking theside reaction mechanism of phosphorus anode with binder and the development of a multifunctional binder for enhancing the performance. Xu Liang., et al. Journal of Power Sources, 541 (2022)) (231686) After being assembled into a full cell, the discharge voltage of the full cell will drop too much, which may lead to a decrease in the energy density of the full cell. If the lithium in the lithium phosphide is no longer extracted by controlling the delithiation potential, and only the silicon anode is lithiated / delithilated, the high conductivity of the lithium phosphide can be maintained to improve the impedance. However, in a full cell assembled in this way, the lithium source of the lithium phosphide needs to be obtained from the positive electrode material of the battery, which will reduce the initial coulombic efficiency of the battery to a certain extent, and also reduce the energy density of the battery.

[0007] Chinese patent CN110071264A proposes a method for improving the performance of silicon-carbon electrodes using lithium phosphide. A layer of lithium phosphide is deposited on the surface of the silicon-carbon electrode, utilizing the lithium in the lithium phosphide to compensate for the lithium required during the initial lithium intercalation process of the silicon-carbon anode, thereby improving the battery's initial coulombic efficiency. This method improves performance at the electrode level, with the core focus on enhancing the initial coulombic efficiency of the silicon-carbon material. However, the deposited lithium phosphide layer and silicon-carbon layer are physically stacked, failing to achieve close contact between nanoscale particles. Furthermore, due to the inherent properties of lithium phosphide, it undergoes a violent decomposition reaction upon contact with air or trace amounts of moisture, producing toxic phosphine gas. While the filtration process described in the patent is relatively simple to operate for large-scale commercial applications, the workshop environment required for mass production of battery electrodes must be a strictly water- and oxygen-free environment. This will significantly increase the investment in auxiliary engineering for the production workshop, making the application and promotion of this technology more difficult.

[0008] Chinese patent CN118833802A proposes a method for preparing a high-capacity silicon / phosphorus / carbon composite anode material. This method involves coating sulfur onto silicon particles; removing the sulfur template through annealing to create void structures; and then depositing elemental phosphorus within these void structures to ultimately obtain the silicon / phosphorus / carbon composite anode material. During the first charge, this material produces Li3P products with high electronic and ionic conductivity. The presence of Li3P can improve the overall rate performance and cycle performance of the material. However, in practical full-cell applications, the lithium source for Li3P in the anode material obtained by this method is lithium from the cathode. If Li3P participates in charge-discharge cycles, the high lithium phosphide delithiation potential of Li3P (1.0~1.1V vs. Li) will lead to a decrease in the average discharge voltage of the full cell, significantly reducing the battery's energy density. The energy of a real battery, E=V×Ah, depends not only on the specific capacity of the material but also on the lithium insertion / extraction potential (voltage).

[0009] Therefore, it is necessary to explore a better method for preparing vapor-deposited silicon-carbon anode materials. This method should be able to utilize the porous structure to suppress the volume expansion of silicon anode materials and obtain better cycle performance, while simultaneously combining nano-silicon particles with Li3P, which has high electronic and ionic conductivity, in porous carbon to improve the high impedance problem of silicon particles during charge and discharge. At the same time, it should ensure that the formation of Li3P does not consume the valuable lithium source in the cathode material and facilitate the battery production process in an air environment to reduce the difficulty of application. Summary of the Invention

[0010] To achieve the above objectives, this invention provides a nano-lithium phosphide modified silicon-carbon anode material, its preparation method, and a secondary battery. Through a process route of "first depositing phosphorus → lithium phosphide fixation to lithium phosphide (Li3P) → then depositing silicon," a nano-Li3P conductive network is formed in situ inside the porous carbon. This network does not participate in the lithium insertion / extraction reaction, thereby significantly improving the ionic and electronic conductivity of the silicon-carbon anode at low SOC without consuming positive electrode lithium or reducing the operating voltage. This fundamentally improves the high impedance problem and cycle stability of silicon-carbon anode batteries during charge and discharge.

[0011] The technical solution of this invention is: The first aspect of this invention provides a nano-lithium phosphide modified silicon-carbon anode material, wherein the material has a core-shell structure, the core of the core-shell structure comprising porous carbon and nano-lithium phosphide and nano-silicon generated in situ in the micropores of the porous carbon; the shell of the core-shell structure comprises a composite coating layer composed of amorphous carbon and carbon nanotubes; in the modified silicon-carbon anode material, silicon accounts for 45-65 wt% of the mass of the nano-lithium phosphide modified silicon-carbon anode material, and the P / Si molar ratio in the nano-lithium phosphide and nano-silicon particles is 2:98~30:70.

[0012] Preferably, in the composite coating layer, the mass ratio of amorphous carbon to carbon nanotubes is 1:1 to 1:0.01. To achieve better entanglement of porous carbon particles with carbon nanotubes, single-walled carbon nanotubes are preferred, with a tube length >3μm and a tube diameter of 0.5~2nm. Due to the very low density of carbon nanotubes, under the impact of the carbon source gas flow, the carbon nanotubes mix with the amorphous carbon particles generated by the high-temperature decomposition of the carbon source gas and deposit together on the porous carbon surface, forming a dense coating layer.

[0013] Preferably, the ratio of the particle size of the porous carbon to the thickness of the composite coating layer is 1000:2 to 1000:20.

[0014] Preferably, the porous carbon has an average pore size between 2 and 10 nm and a pore volume between 0.5 and 1.5 cm³. 3 / g, and the percentage of pore volume in the range of 0.5~2nm is ≥40% of the total pore volume, and the percentage of pore volume in the range of 0.5~5nm is ≥60% of the total pore volume; the median diameter D50 of the porous carbon particles is 1~30um.

[0015] A second aspect of this invention provides a method for preparing a nano-lithium phosphide-modified silicon-carbon anode material, comprising the following steps: (1) Vapor deposition and lithiation: Under an inert atmosphere, phosphorus source is vapor-deposited into the pores of porous carbon, and a small amount of nano-phosphorus is deposited in the nano-pores. The nano-phosphorus occupies only a small part of the pores, resulting in porous carbon with deposited nano-phosphorus. Then, the porous carbon with deposited nano-phosphorus is immersed in a lithiation organic solution for lithiation treatment. The nano-phosphorus is converted into nano-lithium phosphide in the porous carbon, resulting in porous carbon loaded with nano-lithium phosphide. (2) Vapor deposition and coating treatment: Under an inert atmosphere, silicon source is introduced into porous carbon loaded with nano-lithium phosphide to continue depositing nano-silicon particles. Finally, carbon source gas and carbon nanotube aerosol are introduced to coat the surface of porous carbon particles to obtain nano-lithium phosphide modified silicon-carbon anode material.

[0016] Preferably, the inert gas is argon or helium.

[0017] Preferably, the silicon source is one or more of silane, silane, and dichlorosilane.

[0018] Preferably, the phosphorus source is one or more of white phosphorus, red phosphorus, purple phosphorus, and phosphine (PH3).

[0019] Preferably, the lithium-ionizing agent in the lithium-ionizing organic solution is selected from one or more of alkyl lithium, lithium-aromatic complex, methylnaphthalene lithium, 9-phenanthyl lithium, 1-pyrene lithium, and methylated tertiary amine lithium. The alkyl lithium is one or more of methyl lithium, n-butyl lithium, and tert-butyl lithium, and the lithium-aromatic complex is one or more of lithium-biphenyl complex, lithium-anthracene complex, and lithium-toluene complex.

[0020] Preferably, the method for converting nano-elemental phosphorus in porous carbon into nano-lithium phosphide is by immersion in a lithium-ion organic solution, and the lithium-ionization depth is based on the lithium potential of the modified silicon-carbon anode material with deposited nano-phosphorus being ≤ 0.5V vs. Li.

[0021] Preferably, the carbon source gas includes one or more of gaseous alkanes, alkenes, or alkynes. The alkanes are one or more of methane, ethane, and propane; the alkenes are one or more of ethylene, propylene, and 1-butene; and the alkynes are one or more of acetylene, propyne, and 1-butyne.

[0022] Preferably, the coating treatment temperature is 550~750℃.

[0023] Preferably, the vapor deposition conditions in step (1) are as follows: the temperature is raised to 450-550℃, a phosphorus source is introduced, vapor deposition is carried out in porous carbon for 10-20 min, the temperature is lowered to 280-350℃ and maintained for 20-30 h.

[0024] Preferably, the vapor deposition conditions in step (2) are as follows: the temperature is raised to 450-550℃, a silicon source is introduced, vapor deposition is carried out in porous carbon for 5-8 hours, and the temperature is lowered to 280-350℃ and maintained for 20-30 hours.

[0025] A third aspect of the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the active material of the negative electrode comprises the aforementioned modified silicon-carbon negative electrode material. The active material of the positive electrode comprises at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, 5V spinel lithium nickel manganese oxide, lithium-rich manganese-based material, lithium iron phosphate, and lithium iron manganese phosphate.

[0026] The advantages and beneficial effects of this invention are: (1) A two-step deposition method is adopted. In the first step, a small amount of elemental phosphorus nanoparticles are deposited in porous carbon. Then, through a lithiation process, the phosphorus nanoparticles in the porous carbon are converted into lithium phosphide nanoparticles. Then, the second step is to deposit silicon nanoparticles. This design cleverly solves the technical contradictions in traditional co-deposition or stepwise deposition processes: since the process of silane cracking to deposit silicon particles is usually carried out at a high temperature of 500-700℃, if silicon is directly deposited on elemental phosphorus, the elemental phosphorus will sublimate and escape at this temperature, causing loss of active materials and environmental pollution. However, this invention pre-converts phosphorus into highly stable lithium phosphide, completely eliminating the problem of its sublimation in the subsequent high-temperature process, and ensuring the precise retention and uniform distribution of phosphorus in the material.

[0027] (2) After the second step of silicon nanoparticle deposition is completed, a porous carbon coating process is carried out. A composite component of amorphous carbon and carbon nanotubes is used for coating. Amorphous carbon is used to construct a dense coating layer to isolate oxygen and moisture in the air, avoid side reactions between oxygen and moisture and nano-silicon particles and nano-lithium phosphide in porous carbon, and reduce the stringent environmental requirements for actual mass production. At the same time, the wrapping effect of carbon nanotubes (single-walled carbon nanotubes) can further suppress the expansion effect of silicon nanoparticles in the battery. The high rigidity of carbon nanotubes can also be used to ensure that the silicon-carbon anode material particles will not be damaged under the strong friction during the actual battery slurry mixing. This avoids the side reactions of water molecules entering the porous carbon channels and reacting with nano-silicon and nano-lithium phosphide after damage.

[0028] (3) The modified silicon-carbon anode material of the present invention contains a small amount of nano-lithium phosphide at the molecular level interface. During the charge and discharge process, the lithium phosphide has a very high delithiation potential (~1V vs. Li). Within the normal operating voltage range of a real full cell, lithium phosphide basically does not participate in the charge and discharge reaction process and only acts as a composite conductive agent. Utilizing the high electronic conductivity / high ionic conductivity of lithium phosphide, the high impedance problem of silicon particles during the charge and discharge process is improved at the nanoscale, especially solving the problem of Li in the low charge range of the silicon-carbon anode. + The problem of a sharp decrease in ion transport capacity leading to a rapid increase in impedance; moreover, since the lithium in lithium phosphide is pre-lithiated during the material production process, it does not consume the valuable lithium resources of the cathode material in the actual production of lithium-ion full batteries. This characteristic avoids the problem of full battery voltage window shift or energy density decrease caused by additional lithium replenishment, and achieves the improvement of anode performance without sacrificing the overall battery performance. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the preparation process of the modified silicon-carbon anode material of the present invention; Figure 2 The image shows the XRD pattern of the modified silicon-carbon anode material in Example 1. Figure 3 The first charge-discharge curve of the half-cell assembled with the negative electrode material of Example 1; Figure 4 The first charge-discharge curves of the half-cells assembled with the negative electrode materials of Example 2 and Comparative Example 1 are shown. Figure 5 The dQ / dV curves for the first cycle of the charge and discharge process of the half-cell assembled with the negative electrode materials of Example 2 and Comparative Example 1 are shown. Figure 6 AC impedance spectral curves of half-cells assembled with the negative electrode materials of Example 3, Comparative Example 1 and Comparative Example 2. Figure 7 The first charge-discharge curves of the full cells prepared with the negative electrode materials of Example 2 and Comparative Example 1 are shown. Figure 8 The rate discharge temperature rise curve of the full cell prepared with the negative electrode material of Example 4 and Comparative Example 1 is shown in the figure. Figure 9 The DC impedance curves of the full cells made from the negative electrode materials of Example 5 and Comparative Example 1 at different depths of discharge (DOD) are shown. Detailed Implementation

[0030] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0031] Example 1 A method for preparing a nano-lithium phosphide-modified silicon-carbon anode material, comprising the following steps: (1) 100g of porous carbon material (D50 is 10μm, pore volume is 1.4 cm³) 3 / g, with an average pore size of 1.5nm) is placed in a vapor deposition furnace. The reaction chamber is evacuated and then argon is passed through until the oxygen content is below 100ppm. Heating is turned on and the temperature of the reaction chamber is raised to 500℃. At this time, phosphorus vapor is introduced and vapor deposition is carried out in the porous carbon for 15min. The temperature is then lowered to 300℃ and maintained for 24h. During this process, the elemental phosphorus in the porous carbon is converted into red phosphorus. After cooling to room temperature, porous carbon powder with deposited nano-phosphorus is obtained. (2) Under an argon atmosphere, the above-deposited nano-phosphorus porous carbon powder was immersed in a 0.75 mol / L methylnaphthalene lithium solution and stirred for 8 hours. After filtration, it was vacuum dried. At this time, the nano-phosphorus nanoparticles in the porous carbon were converted into nano-lithium phosphide Li3P particles, and porous carbon powder loaded with nano-lithium phosphide Li3P particles was obtained. (3) The porous carbon powder loaded with the above-mentioned lithium phosphide (Li3P) nanoparticles was put back into the vapor deposition furnace. The reaction chamber was evacuated and then argon gas was introduced until the oxygen content was less than 60 ppm. Heating was turned on and the temperature of the reaction chamber was raised to 510°C. At this time, silane gas was introduced and vapor deposition was carried out in the remaining pores of the porous carbon for 6 hours. The temperature of the reaction chamber was then raised to 650°C and an aerosol composed of acetylene and single-walled carbon nanotube powder (the mass concentration of single-walled carbon nanotubes was 10 μg / L, the tube length was ~4 μm, and the tube diameter was 1 nm) was introduced. After reacting for 2 hours, the aerosol was turned off. After cooling, the powder was collected to obtain the lithium phosphide nanoparticle-modified silicon-carbon anode material.

[0032] The modified silicon-carbon anode material prepared in Example 1 has a core-shell structure. The core of the core-shell structure includes porous carbon and nano-lithium phosphide and nano-silicon generated in situ within the micropores of the porous carbon. The shell of the core-shell structure includes a composite coating layer composed of amorphous carbon and carbon nanotubes. The mass ratio of amorphous carbon to carbon nanotubes in the composite coating layer is 95:5. The ratio of the particle size of the porous carbon to the thickness of the composite coating layer is 1000:5.

[0033] The molar ratio of phosphorus to silicon in the nano-lithium phosphide modified silicon-carbon anode material of Example 1 was tested by ICP. The P / Si molar ratio was 6:94, and the silicon element accounted for 51% of the mass of the nano-lithium phosphide modified silicon-carbon anode material.

[0034] The crystal structure of the lithium phosphide-modified silicon-carbon anode material obtained in Example 1 was tested by XRD, and the results are as follows: Figure 2 As shown. By Figure 2 It can be seen that in 24 o and 26.5 o Characteristic diffraction peaks of Li3P appear on both sides.

[0035] Example 2 A method for preparing a nano-lithium phosphide-modified silicon-carbon anode material, comprising the following steps: (1) In a vapor deposition furnace, 150g of porous carbon material (D50 of 6.5μm and pore volume of 1.1 cm³) was placed. 3 / g, with an average pore size of 2.3nm), first evacuate to a vacuum level below 50Pa, then turn on the heating and raise the temperature of the reaction chamber to 540℃, then pass phosphorus vapor through for 12min deposition, then cool down to 340℃ and maintain for 22h, and then cool to room temperature to obtain porous carbon powder with deposited nano-phosphorus.

[0036] (2) Under an argon atmosphere, the above-deposited nano-phosphorus porous carbon powder was immersed in a 0.8 mol / L 1-pyrene lithium solution and stirred thoroughly for 10 h. After filtration, it was vacuum dried to obtain porous carbon powder loaded with nano-lithium phosphide Li3P particles. (3) The powder was put back into the vapor deposition furnace, the reaction chamber was evacuated and then argon gas was introduced until the oxygen content was less than 50 ppm. The heating was turned on and the temperature of the reaction chamber was raised to 550°C. At this time, silane gas was introduced and vapor deposition was carried out in the remaining pores of the porous carbon for 8 hours. The temperature of the reaction chamber was then raised to 680°C and an aerosol composed of acetylene and single-walled carbon nanotube powder (the mass concentration of single-walled carbon nanotubes was 15 μg / L, the tube length was ~4.5 μm, and the tube diameter was 1.2 nm) was introduced. After reacting for 3 hours, the introduction of the mixed aerosol was turned off. After cooling, the powder was collected to obtain nano-lithium phosphide modified silicon-carbon anode material.

[0037] In Example 2, the mass ratio of amorphous carbon to carbon nanotubes in the modified silicon-carbon anode composite coating was 90:10. The ratio of porous carbon particle size to composite coating thickness was 1000:8.

[0038] The molar ratio of phosphorus to silicon in the modified silicon-carbon anode material of Example 2 was tested by ICP. The P / Si molar ratio was 5:95, and the silicon element accounted for 58% of the mass of the nano-lithium phosphide modified silicon-carbon anode material.

[0039] Example 3 A method for preparing a nano-lithium phosphide-modified silicon-carbon anode material, comprising the following steps: (1) In a vapor deposition furnace, 150g of porous carbon material (D50 of 6.2μm and pore volume of 1.15 cm³) was placed. 3 / g (with an average pore size of 2.4nm), first, a vacuum was drawn to a vacuum level below 50Pa, then heating was turned on to raise the temperature of the reaction chamber to 460℃, then phosphorus vapor was introduced for deposition for 35min, then the temperature was lowered to 290℃ and held for 23h, and then cooled to room temperature to obtain porous carbon powder with deposited nano-phosphorus.

[0040] (2) Under an argon atmosphere, the above-deposited nano-phosphorus porous carbon powder was immersed in a 0.9 mol / L methylated tertiary amine lithium solution and stirred thoroughly for 8 hours. After filtration, it was vacuum dried to obtain porous carbon powder loaded with nano-lithium phosphide Li3P particles. (3) The powder was put back into the vapor deposition furnace, the reaction chamber was evacuated and then argon gas was introduced until the oxygen content was less than 50 ppm. Heating was turned on and the temperature of the reaction chamber was raised to 580°C. At this time, silane gas was introduced and vapor deposition was carried out in the remaining pores of the porous carbon for 8 hours. The temperature of the reaction chamber was then raised to 630°C and an aerosol composed of acetylene and single-walled carbon nanotube powder (the mass concentration of single-walled carbon nanotubes was 12 μg / L, the tube length was ~3.5 μm, and the tube diameter was 1.1 nm) was introduced. After reacting for 4 hours, the introduction of the mixed aerosol was turned off. After cooling, the powder was collected to obtain nano-lithium phosphide modified silicon-carbon anode material.

[0041] In Example 3, the mass ratio of amorphous carbon to carbon nanotubes in the modified silicon-carbon anode composite coating was 92:8. The ratio of porous carbon particle size to composite coating thickness was 1000:12.

[0042] The molar ratio of phosphorus to silicon in the modified silicon-carbon anode material of Example 3 was tested by ICP. The P / Si molar ratio was 15:85, and the silicon element accounted for 54% of the mass of the nano-lithium phosphide modified silicon-carbon anode material.

[0043] Example 4 A method for preparing a nano-lithium phosphide-modified silicon-carbon anode material, comprising the following steps: (1) In a vapor deposition furnace, place 200g of porous carbon material (D50 of 5.8μm, pore volume of 1.04 cm³) 3 / g, with an average pore size of 1.9nm), first evacuate to a vacuum level below 50Pa, then turn on the heating and raise the temperature of the reaction chamber to 460℃, then pass phosphorus vapor through for 18min deposition, then cool down to 310℃ and hold for 30h, and then cool to room temperature to obtain porous carbon powder with deposited nano-phosphorus.

[0044] (2) Under an argon atmosphere, the above-deposited nano-phosphorus porous carbon powder was immersed in an alkyl lithium solution with a concentration of 1.0 mol / L and stirred thoroughly for 9 h. After filtration, it was vacuum dried to obtain porous carbon powder loaded with nano-lithium phosphide Li3P particles. (3) The powder was put back into the vapor deposition furnace. The reaction chamber was evacuated and then argon gas was introduced until the oxygen content was below 50 ppm. The heating was turned on and the temperature of the reaction chamber was raised to 520°C. At this time, silane gas was introduced and vapor deposition was carried out in the remaining pores of the porous carbon for 6 hours. The temperature of the reaction chamber was then raised to 610°C and an aerosol composed of acetylene and single-walled carbon nanotube powder (the mass concentration of single-walled carbon nanotubes was 11 μg / L, the tube length was ~3.8 μm, and the tube diameter was 1.1 nm) was introduced. After reacting for 4.5 hours, the introduction of the mixed aerosol was turned off. After cooling, the powder was collected to obtain the nano-lithium phosphide modified silicon-carbon anode material.

[0045] In Example 4, the mass ratio of amorphous carbon to carbon nanotubes in the modified silicon-carbon anode composite coating was 93:7. The ratio of porous carbon particle size to composite coating thickness was 1000:15.

[0046] The molar ratio of phosphorus to silicon in the nano-lithium phosphide modified silicon-carbon anode material of Example 4 was tested by ICP. The P / Si molar ratio was 7:93, and the silicon element accounted for 57% of the mass of the nano-lithium phosphide modified silicon-carbon anode material.

[0047] Example 5 A method for preparing a nano-lithium phosphide-modified silicon-carbon anode material, comprising the following steps: (1) In a vapor deposition furnace, 175g of porous carbon material (D50 of 7.5μm, pore volume of 0.94 cm³) was placed. 3 / g, with an average pore size of 1.8 nm), first evacuate to a vacuum level below 50 Pa, then turn on the heating and raise the temperature of the reaction chamber to 505℃, then pass phosphorus vapor through for 20 min for deposition, then cool down to 330℃ and maintain for 20 h, and then cool to room temperature to obtain porous carbon powder with deposited nano-phosphorus.

[0048] (2) Under an argon atmosphere, the above-deposited nano-phosphorus porous carbon powder was immersed in a 0.75 mol / L methylnaphthalene lithium solution and stirred thoroughly for 15 h. After filtration, it was vacuum dried to obtain porous carbon powder loaded with nano-lithium phosphide Li3P particles. (3) The powder was put back into the vapor deposition furnace, the reaction chamber was evacuated and then argon gas was introduced until the oxygen content was less than 50 ppm. Heating was turned on and the temperature of the reaction chamber was raised to 540°C. At this time, silane gas was introduced and vapor deposition was carried out in the remaining pores of the porous carbon for 5.5 h. The temperature of the reaction chamber was then raised to 625°C and an aerosol composed of acetylene and single-walled carbon nanotube powder (the mass concentration of single-walled carbon nanotubes was 14 μg / L, the tube length was ~4.6 μm, and the tube diameter was 1.3 nm) was introduced. After reacting for 5 h, the introduction of the mixed aerosol was turned off. After cooling, the powder was collected to obtain nano-lithium phosphide modified silicon-carbon anode material.

[0049] In Example 5, the mass ratio of amorphous carbon to carbon nanotubes in the modified silicon-carbon anode composite coating was 92:8. The ratio of porous carbon particle size to composite coating thickness was 1000:18.

[0050] The molar ratio of phosphorus to silicon in the nano-lithium phosphide modified silicon-carbon anode material of Example 5 was tested by ICP. The P / Si molar ratio was 11:89, and the silicon element accounted for 52% of the mass of the nano-lithium phosphide modified silicon-carbon anode material.

[0051] Comparative Example 1 150g of porous carbon material (D50 of 6.5μm and pore volume of 1.1 cm³) was placed in a vapor deposition furnace. 3 The mixture (with an average pore size of 2.3 nm) was first evacuated to a vacuum level below 50 Pa, then silane gas was introduced for 6 hours of deposition. Subsequently, the reaction chamber temperature was raised to 680°C, acetylene gas was introduced, and the reaction was continued for 3 hours. Afterward, the gas flow was stopped, and the powder was collected after cooling to obtain the fumed silicon-carbon anode material. ICP testing showed that silicon comprised 54% of the material by mass.

[0052] Comparative Example 2 The difference from Example 3 is that the lithium phosphide powder and the fumed silicon-carbon anode material are mixed by mechanical ball milling, and the steps are as follows: A small amount of lithium phosphide powder was prepared according to the method in the literature (Park, J., Kim, S., Lee, Y., Lee, J., Ahn, H., & Kim, J. (2023). Mixed ion-electron conducting Li3P for efficient cathode prelithiation of all-solid-state Li-ion batteries. Small Methods, 7(8), 2300120). It was then mechanically ball-milled with the fumed silicon-carbon anode material (without lithium phosphide) of Comparative Example 1 at a P:Si molar ratio of 15:85 in the lithium phosphide powder and the fumed silicon-carbon anode material to obtain a physically mixed lithium phosphide silicon-carbon anode material.

[0053] Electrochemical testing and analysis Electrochemical tests were conducted using the nano-lithium phosphide-modified silicon-carbon anode materials obtained in Examples 1, 2, and 3, the fumed silicon-carbon anode material (without lithium phosphide) of Comparative Example 1, and the silicon-carbon anode material physically mixed with lithium phosphide of Comparative Example 2 as the anode active materials for half-cells. (1) Preparation of anode sheet: The above negative electrode material is mixed with carbon black conductive agent and binder PVDF in a mass ratio of 80:10:10 in N-methylpyrrolidone (NMP) solvent and stirred evenly to obtain anode slurry; the anode slurry is evenly coated on copper foil, vacuum dried at 100℃ for 8h, and rolled to obtain anode sheet; (2) Using a lithium metal sheet as the counter electrode, a porous PP / PE / PP composite membrane as the separator, and a 1 mol / L lithium hexafluorophosphate (LiPF6) as the solute in the electrolyte, the solvent is composed of EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DMC (dimethyl carbonate) in a volume ratio of 1:1:1, and assembled into a CR2032 button half cell in an argon-protected glove box.

[0054] The test conditions were as follows: the battery test system was a Shenzhen Xinweier charge-discharge tester, the test temperature was 25℃±2℃, and the test procedure was: lithium insertion at a rate of 0.1C to 0.01V vs. Li, and then lithium extraction at a rate of 0.1C to 0.8V vs. Li. The initial coulombic efficiency was calculated based on the capacity of the negative electrode material when lithium was extracted to 0.8V vs. Li.

[0055] Example 1: The first charge-discharge curve of the half-cell assembled with the negative electrode material is shown below. Figure 3 As shown, the initial coulomb efficiency reaches 84.5%.

[0056] The first charge-discharge curves of the half-cells assembled with the negative electrode materials of Example 2 and Comparative Example 1 are as follows: Figure 4 As shown, from Figure 4 It can be seen that the first-cycle delithiation process curves of the two are almost identical, but the coulombic efficiency of the two differs significantly; in Example 2, the nano-phosphorus particles in the nano-lithium phosphide modified silicon-carbon anode material have already completed pre-lithiation, therefore there is no lithium phosphide formation process during the lithium intercalation process (3Li). + +P + 3e - →Li3P), when delithiated to 0.8V vs. Li, the first coulombic efficiency of the half-cell assembled with the anode material of Example 2 reached 84%; while the first coulombic efficiency of the unlithiated fumed silicon-carbon anode material of Comparative Example 1 when delithiated to 0.8V vs. Li was only 73.5%. Figure 5 The dQ / dV curves for the first cycle of charge-discharge of the half-cell using the negative electrode materials of Example 2 and Comparative Example 1 clearly show the characteristic peak of lithium phosphide formation at 0.65V (vs. Li) during the first lithium intercalation of the half-cell using the negative electrode material of Comparative Example 1. This irreversible process requires additional lithium source from the positive electrode. When assembling an actual full cell using the unlithiated fumed silicon-carbon material of Comparative Example 1, the lithium source for this lithiation process needs to be obtained from the positive electrode, consuming valuable lithium resources in the positive electrode and reducing the initial efficiency of the positive electrode material.

[0057] After lithium insertion at a rate of 0.1C to 0.1V (vs. Li), AC impedance spectroscopy (EIS) tests were performed on CR2032 coin cells assembled from the negative electrode materials obtained in Example 3, Comparative Example 1, and Comparative Example 2, with an amplitude of 10mV and a frequency range of 0.1~106 Hz. Figure 6 As shown, by Figure 6 It can be seen that the half-cell with the modified silicon-carbon anode material of Example 3 has the lowest impedance, followed by the half-cell with the silicon-carbon anode material of Comparative Example 2 which uses physically mixed lithium phosphide, and the half-cell with the lithium phosphide-free gas-phase silicon-carbon anode material of Comparative Example 1 has the highest impedance. This indicates that the modified silicon-carbon material using in-situ lithiation to generate lithium phosphide has superior performance.

[0058] Full cells were assembled using the negative electrode materials from Examples 2, 4, 5, and Comparative Example 1. The preparation steps for the full cells are as follows: (1) Selecting lithium nickel cobalt manganese oxide material (NCM811) LiNi 0.8 Co 0.1 Mn 0.1 Using O2 as the positive electrode material, a positive electrode slurry was prepared by uniformly mixing NCM811, PVDF (polyvinylidene fluoride), conductive carbon black, and multi-walled carbon nanotubes in a mass ratio of 96:2:1.5:0.5. The solid content of the positive electrode slurry was 65%. After coating, rolling, and cutting, the positive electrode slurry was used to obtain a single-sided loading of 200 g / m². 2The compacted density is 3.5 g / cm³. 3 The positive electrode sheet.

[0059] (2) The negative electrode material was mixed with graphite, conductive carbon black, single-walled carbon nanotubes, sodium hydroxymethyl cellulose (CMC), and lithium polyacrylate (PAALi) in a mass ratio of 15:78:1.9:0.1:0.5:4.5 to obtain a negative electrode slurry. The solid content of the negative electrode slurry was 35%, and the viscosity was 3000-5000 Pa·s. After coating, rolling, and cutting, the negative electrode slurry was obtained with a single-sided loading of 85 g / m². 2 The compacted density is 1.5 g / cm³. 3 The negative electrode sheet.

[0060] (3) A ceramic-coated polyethylene membrane is used as the battery separator (12μm polyethylene membrane + 2μm Al2O3 ceramic layer). The electrolyte is 1mol / L lithium hexafluorophosphate (LiPF6) and the solvent is composed of EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DMC (dimethyl carbonate) in a volume ratio of 1:1:1. In an argon-protected glove box, the above positive electrode, negative electrode and separator are stacked and sealed. The aluminum-plastic film is then used for sealing. After injecting the electrolyte in an inert atmosphere glove box, the battery is pre-charged for 3 cycles at a rate of 0.1C within a voltage range of 2.5~4.25V to obtain a soft-pack battery with a capacity of about 8Ah. At this point, the full battery is obtained.

[0061] Figure 7 The first charge-discharge curve tests were conducted on the full cells prepared using the negative electrode materials of Example 2 and Comparative Example 1. The test methods and conditions were as follows: the first charge-discharge cycle was performed at 25℃±2℃ at a 0.1C rate and a voltage range of 2.5~4.25V. Figure 7 As can be seen, the full cell with nano-lithium phosphide-modified silicon-carbon material as the negative electrode in Example 2 exhibits an initial coulombic efficiency of 89.5% and an initial reversible specific capacity of 214.8 mAh / g, both significantly higher than Comparative Example 1. The full cell with fumed silicon-carbon negative electrode material in Comparative Example 1 has an initial coulombic efficiency of 83% and an initial reversible specific capacity of 199.4 mAh / g. This demonstrates that the present invention effectively avoids the consumption of the positive electrode lithium source during the lithium phosphide formation process by generating nano-lithium phosphide in situ within porous carbon channels and completing pre-lithiation, thereby simultaneously improving both the initial coulombic efficiency of the full cell and the reversible capacity of the positive electrode material.

[0062] Figure 8The table shows the rate discharge temperature rise curves of the full cells prepared using the negative electrode materials of Example 4 and Comparative Example 1. The test method and conditions were as follows: the batteries were charged to a full charge state of 4.25V at a constant current and constant voltage of 0.5C in a 25°C constant temperature chamber, and then discharged to 2.5V at a constant current of 3C. Temperature data of the battery's center surface was continuously collected during the discharge process at a sampling frequency of 0.1Hz. Figure 8 As can be seen, under the same 3C discharge rate conditions, the full battery prepared using the modified silicon-carbon anode material of Example 4 had a 7°C lower temperature at the end of discharge compared to Comparative Example 1, with a temperature rise rate reduced by more than 20%. Furthermore, the temperature rise rate of the full battery prepared using the nano-lithium phosphide modified silicon-carbon anode material of Example 4 was significantly lower than that of Comparative Example 1 throughout the entire discharge process. This indicates that the present invention effectively reduces heat generation during high-rate discharge by generating nano-lithium phosphide in situ within silicon-carbon materials, suppressing temperature rise and thus improving battery safety and user experience.

[0063] Figure 9 To obtain the DC impedance curves of the full cells prepared using the negative electrode materials of Example 5 and Comparative Example 1, the test method and conditions were as follows: The batteries were charged to a full state of 4.25V at a constant current and constant voltage rate of 0.5C in a 25°C constant temperature chamber, discharged at a constant current rate of 1C for 6 minutes to 10% DOD, allowed to stand for 4 hours, and then pulsed discharged at a rate of 3C for 10 seconds. The voltage difference ΔV before and after the 10-second discharge was divided by the 3C current value to obtain the DC impedance at this DOD, i.e., DCR = ΔV / I. Then, the battery was adjusted to 20% DOD, allowed to stand for 4 hours, and then pulsed discharged at a rate of 3C again until 90% DOD was reached. In the range of 10-90% DOD, the DC impedance (DCR) of the full cell prepared using the negative electrode material of Example 5 was significantly lower; especially in the range of 70-90% DOD at the end of the discharge, due to the decrease in lithium content in the negative electrode, the DC impedance of both increased significantly, but the impedance increase of the full cell of Example 5 was significantly smaller than that of Comparative Example 1. This is because lithium phosphide (Li3P) has a high delithiation potential (vs. Li) and does not undergo a delithiation reaction. It always exists in a stable form in the negative electrode channel. By utilizing the high ionic conductivity of Li3P, the excessively rapid rise of DCR in the low charge range (70~90% DOD) of the battery is greatly suppressed, which greatly improves the user experience of electronic devices.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any modifications or equivalent changes made based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

Claims

1. A nano-lithium phosphide-modified silicon-carbon anode material, characterized in that, The material has a core-shell structure. The core comprises porous carbon and nano-lithium phosphide and nano-silicon formed in situ within the micropores of the porous carbon. The shell comprises a composite coating layer composed of amorphous carbon and carbon nanotubes in a mass ratio of 1:1 to 1:0.

01. In the material, silicon accounts for 45-65 wt% of the mass of the nano-lithium phosphide-modified silicon-carbon anode material, and the P / Si molar ratio of the nano-lithium phosphide to the nano-silicon particles is 2:98 to 30:

70. The composite coating layer composed of amorphous carbon and carbon nanotubes is formed by co-deposition of carbon source gas and carbon nanotube aerosol. The nano-lithium phosphide does not participate in the lithium insertion / extraction reaction during charge and discharge, but only acts as a composite conductive agent. The lithium in the nano-lithium phosphide originates from the pre-lithiation treatment during the preparation process and does not consume lithium resources in the cathode material. The average pore size of the porous carbon is 2-10 nm, and the pore volume is 0.5-1.5 cm³. 3 / g, wherein the median diameter D50 of the porous carbon is 1~30μm; The preparation method of the nano-lithium phosphide modified silicon-carbon anode material includes: (1) Vapor deposition and lithiation: Under an inert atmosphere, phosphorus source is vapor-deposited into the pores of porous carbon to obtain porous carbon with deposited nano-phosphorus. Then, the porous carbon with deposited nano-phosphorus is immersed in a lithiation organic solution for lithiation treatment. The nano-phosphorus is converted into nano-lithium phosphide in the porous carbon to obtain porous carbon loaded with nano-lithium phosphide. (2) Vapor deposition and coating treatment: Under an inert atmosphere, silicon source is introduced into porous carbon loaded with nano-lithium phosphide to continue depositing nano-silicon particles. Finally, carbon source gas and carbon nanotube aerosol are introduced to coat the surface of porous carbon particles to obtain nano-lithium phosphide modified silicon-carbon anode material.

2. A method for preparing the nano-lithium phosphide-modified silicon-carbon anode material as described in claim 1, characterized in that, include: (1) Vapor deposition and lithiation: Under an inert atmosphere, phosphorus source is vapor-deposited into the pores of porous carbon to obtain porous carbon with deposited nano-phosphorus. Then, the porous carbon with deposited nano-phosphorus is immersed in a lithiation organic solution for lithiation treatment. The nano-phosphorus is converted into nano-lithium phosphide in the porous carbon to obtain porous carbon loaded with nano-lithium phosphide. (2) Vapor deposition and coating treatment: Under an inert atmosphere, silicon source is introduced into porous carbon loaded with nano-lithium phosphide to continue depositing nano-silicon particles. Finally, carbon source gas and carbon nanotube aerosol are introduced to coat the surface of porous carbon particles to obtain nano-lithium phosphide modified silicon-carbon anode material.

3. The preparation method according to claim 2, characterized in that, The phosphorus source includes one or more of white phosphorus, red phosphorus, purple phosphorus, and phosphine; the lithiation reagent of the lithiation organic solution is selected from one or more of alkyl lithium, methylnaphthalene lithium, 9-phenanthyl lithium, and 1-pyrene lithium; the lithiation depth of the lithiation treatment is based on the lithium metal potential of the porous carbon material with deposited nano-phosphorus ≤ 0.5V vs. Li.

4. The preparation method according to claim 2, characterized in that, Step (1) Vapor deposition conditions are as follows: raise the temperature to 450-550℃, introduce a phosphorus source, perform vapor deposition in porous carbon for 10-20 min, cool down to 280-350℃ and maintain for 20-30 h.

5. The preparation method according to claim 2, characterized in that, Step (2) Vapor deposition conditions are as follows: raise the temperature to 450-550℃, introduce a silicon source, perform vapor deposition in porous carbon for 5-8 hours, cool down to 280-350℃ and maintain for 20-30 hours.

6. The preparation method according to claim 2, characterized in that, The carbon source gas includes one or more of alkanes, alkenes, or alkynes.

7. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The active material of the negative electrode comprises the nano-lithium phosphide modified silicon-carbon negative electrode material as described in claim 1.

8. The secondary battery according to claim 7, characterized in that, The active material of the positive electrode includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium iron manganese phosphate.

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