A method for constructing a hypervalent ion doped lithium storage structure by high-temperature solid-phase carbon thermal reduction

Abstract

The application relates to a high-temperature solid-phase carbon thermal reduction hypervalent ion doping lithium storage structure construction method and belongs to the technical field of electrochemical energy storage materials. The method comprises the following steps: mixing a lithium source, a metal oxide precursor, a hypervalent ion doping source and light / heavy double carbon sources, ball-milling and then performing low-temperature pyrolysis to induce oxygen vacancy formation, and then performing high-temperature heat preservation to realize hypervalent ion gradient doping and graphite carbon coating. Through the cooperation of the two-stage heat treatment and the double carbon sources, the electronic conductivity and the lithium ion diffusion rate of the material are effectively improved, and the obtained material is suitable for high-rate and long-life lithium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage materials technology, specifically relating to a method for constructing a high-temperature solid-phase carbothermal reduction supervalent ion doped lithium storage structure. Background Technology

[0002] In the modification research of lithium-ion battery cathode materials (such as lithium iron phosphate, lithium cobalt oxide, and lithium nickel manganese oxide), hypervalent ion doping (i.e., introducing doping elements with a valence higher than that of the host cation) is widely used to improve the intrinsic electronic conductivity and lithium-ion diffusion capability of the materials. High-temperature solid-phase carbothermal reduction (HSP) has become a common method for preparing such doped materials due to its simple process, low cost, and ease of large-scale production. However, this method still faces certain challenges in achieving efficient and uniform hypervalent ion doping.

[0003] Patent CN111149241B discloses a silicon-based lithium storage material and its preparation method. This method improves the material's cycle performance and coulombic efficiency by constructing a core containing silicon elements with valences of 0 to 4 and coating it with a highly conductive carbon-containing material and a metal oxide shell with a specific dielectric constant. Although this approach optimizes interface stability through multilayer structure design, its core doping strategy mainly focuses on the valence state control of silicon and surface coating, without addressing the effective doping of supervalent ions through high-temperature solid-state reactions within the bulk lattice. Therefore, this approach has limitations for cathode material systems that require bulk doping to control the electronic structure.

[0004] Patent CN116264271B discloses a lithium-storage silicon carbide twin material and its preparation method. The method employs steps such as coating, mixing, heat treatment, and post-treatment to prepare a silicon carbide twin material with high lithium storage capacity using a thermochemical method. Although this scheme uses a high-temperature heat treatment process, its core lies in constructing a silicon carbide-lithium composite structure, rather than performing substitutional doping of supervalent ions in the lattice of traditional cathode materials. Its doping mechanism is fundamentally different from the atomic-level substitution within the lattice pursued in the high-temperature solid-state carbothermal reduction method, and it does not solve the problem of uneven doping or surface segregation caused by charge repulsion and slow diffusion kinetics of supervalent ions in high-temperature solid-state reactions.

[0005] In summary, existing technologies for preparing supervalent ion-doped lithium storage materials using high-temperature solid-state carbothermal reduction generally face challenges such as difficulty in effectively integrating dopant elements into the crystal lattice, insufficient uniformity of dopant distribution, and the inability of a single carbon source to synergistically optimize the reduction process and the final carbon coating quality. These problems limit the improvement in the intrinsic conductivity of the material and adversely affect rate performance and long-term cycling stability. Summary of the Invention

[0006] To overcome the problems in the prior art, this invention provides a method for constructing a high-temperature solid-phase carbothermal reduction supervalent ion doped lithium storage structure. The aim is to achieve deep and uniform doping of supervalent ions in the main lattice through a staged heat treatment process and the synergistic effect of dual carbon sources, and simultaneously construct a highly conductive graphitized carbon coating layer, thereby improving the electronic conductivity and lithium ion migration rate of the material.

[0007] In a first aspect, the present invention provides a method for constructing a high-temperature solid-phase carbothermal reduction supervalent ion-doped lithium storage structure, comprising the following steps: S10: The lithium source, metal oxide precursor, supervalent ion doped source, light carbon source and heavy carbon source are mixed according to the target stoichiometric ratio, and mechanical ball milling is performed after adding ball milling media to obtain a uniform mixed slurry; then the mixed slurry is vacuum dried at 60 to 90 degrees Celsius for 12 to 24 hours to obtain a dried powder. S20: The dried powder is placed in a tube furnace or box furnace and heated to 300 to 500 degrees Celsius at a heating rate of 3 to 5 degrees Celsius per minute under an inert atmosphere. The temperature is maintained for 2 to 4 hours to allow partial pyrolysis of the light carbon source and release of reducing gas. The weight loss of the light carbon source at 300 to 500 degrees Celsius is not less than 40% of its initial mass, so as to induce the formation of oxygen vacancy defects on the surface of the metal oxide precursor. S30: Under the condition of maintaining an inert atmosphere, continue to raise the furnace temperature to 700 to 900 degrees Celsius at a heating rate of 2 to 4 degrees Celsius per minute, and hold for 6 to 12 hours, so that the supervalent ions can enter the interior of the crystal lattice through the diffusion channels provided by the oxygen vacancies to complete the substitution. At the same time, the heavy carbon source is carbonized and graphitized in situ on the particle surface to form a continuous and dense conductive carbon layer. S40: After the heat preservation is completed, stop heating and allow the furnace body to cool naturally to room temperature to obtain a supervalent ion doped lithium storage material with gradient doping distribution and graphitized carbon coating structure.

[0008] According to the present invention, the method introduces a reducing atmosphere generated by the pyrolysis of a light carbon source at a low temperature stage, thereby controllably generating oxygen vacancy defects on the surface of the host material, providing a low-barrier path for the intra-lattice diffusion of supervalent ions at a subsequent high temperature stage. At the same time, a combination strategy of light and heavy carbon sources is adopted, with the former used for defect regulation and reduction kinetic control at low temperature stage, and the latter used for carbon coating quality optimization at high temperature stage. The two play their respective functions in different temperature ranges, avoiding the mutual constraints between reduction intensity and coating performance of a single carbon source.

[0009] In some embodiments, in step S10, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium nitrate; the metal oxide precursor is selected from at least one of iron phosphate, titanium dioxide, cobalt trioxide, manganese tetroxide, and nickel cobalt manganese oxide; the supervalent ion doping source is selected from at least one of niobium pentoxide, tantalum pentoxide, tungsten trioxide, molybdenum dioxide, rhenium trioxide, tin tetrachloride, and zirconium fluoride; the light carbon source is selected from at least one of glucose, sucrose, citric acid, ascorbic acid, and polyethylene glycol; and the heavy carbon source is selected from at least one of phenolic resin, asphalt, polyacrylonitrile, polyvinylidene fluoride, and polystyrene.

[0010] In some embodiments, in step S10, the molar ratio of the lithium source, the metal oxide precursor, and the hypervalent ion dopant source satisfies the stoichiometric relationship of the target product, wherein the amount of hypervalent ion dopant source added is 0.1% to 5.0% of the molar amount of the main metal element in the metal oxide precursor; the mass of the light carbon source is 2% to 8% of the total solid material mass; and the mass of the heavy carbon source is 3% to 10% of the total solid material mass.

[0011] In some embodiments, in step S10, the mechanical ball milling uses a planetary ball mill or a stirred ball mill, with a ball-to-material mass ratio of 10:1 to 30:1, the milling media being zirconia balls or agate balls, the milling speed being 200 to 600 revolutions per minute, and the milling time being 2 to 8 hours; anhydrous ethanol or deionized water is added as a dispersion medium during the milling process, with a liquid-to-solid mass ratio of 1:1 to 3:1.

[0012] In some embodiments, in step S20, the inert atmosphere is high-purity nitrogen, argon, or a mixture thereof, and the gas flow rate is 50 to 200 standard milliliters per minute; the holding temperature is preferably 350 to 450 degrees Celsius, and the holding time is preferably 2.5 to 3.5 hours; within this temperature range, light carbon sources such as glucose undergo dehydration condensation and partial carbonization, releasing carbon monoxide, carbon dioxide, and small molecule hydrocarbon gases, wherein carbon monoxide, as the main reducing component, reacts with lattice oxygen on the surface of the metal oxide precursor to generate oxygen vacancies.

[0013] In some embodiments, in step S30, the high-temperature holding temperature is determined according to the target material system: for lithium iron phosphate system, the holding temperature is 650 to 800 degrees Celsius; for lithium titanate system, the holding temperature is 750 to 900 degrees Celsius; for lithium cobalt oxide or lithium nickel manganese oxide system, the holding temperature is 800 to 900 degrees Celsius; the holding time is adjusted according to the diffusion coefficient of the doped ions, ranging from 6 to 12 hours.

[0014] In some embodiments, in step S30, the heavy carbon source undergoes pyrolysis, crosslinking, aromatization and graphitization processes at 700 to 900 degrees Celsius, with a residual carbon rate of not less than 50%, and the thickness of the carbon layer formed is 2 to 10 nanometers. The degree of graphitization is characterized by the intensity ratio of the D peak to the G peak (ID / IG) in Raman spectroscopy, and this ratio is not greater than 1.2.

[0015] In some embodiments, the dopant element in the supervalent ion doping source exists as a cation at high temperatures, with an ionic radius differing from the host cation by no more than 30% and a valence higher than that of the host cation by 1 to 3; for example, in the lithium iron phosphate system, Fe²⁺ is the main cation, and Nb is doped. 5 ⁺、Ta 5 ⁺ or W 6 ⁺; In the lithium titanate system, Ti 4 ⁺ is the main cation, doped with Nb 5 ⁺、Mo 6 ⁺ or Re 7 ⁺.

[0016] In some embodiments, the concentration of the oxygen vacancy is quantified by peak fitting of the O 1s orbital in X-ray photoelectron spectroscopy (XPS), and its relative content is 5% to 15% of the total oxygen signal; the occupancy state of the supervalent ion in the crystal lattice is confirmed by X-ray diffraction (XRD) refinement combined with extended X-ray absorption fine structure (EXAFS) analysis, and its substitution ratio is not less than 85% of the theoretical doping amount.

[0017] In some embodiments, in step S40, the cooling rate of the furnace is controlled at 1 to 3 degrees Celsius per minute to avoid lattice stress concentration or carbon layer cracking due to rapid cooling; inert gas is continuously introduced during the cooling process to prevent the material from being re-oxidized by contact with oxygen in the high-temperature section.

[0018] In a second aspect, the present invention provides a supervalent ion-doped lithium storage material, prepared by the method described in any embodiment of the first aspect, having the general chemical formula LiM1₋. x D x PO4 / C, Li4Ti5₋ x D x O 12 / C or LiCo1₋ x D x O2 / C, where M represents at least one of Fe, Mn, Ni, and Co, D represents at least one of Nb, Ta, W, Mo, Re, Sn, and Zr, x is from 0.001 to 0.05, and C represents the graphitized carbon coating.

[0019] According to the present invention, the supervalent ion-doped lithium storage material has the following microstructural features: supervalent ions are distributed in a gradient within the crystal lattice, with a surface concentration slightly lower than that in the central region of the bulk phase, indicating that the doping process is controlled by diffusion rather than surface adsorption; a carbon coating layer continuously covers the surface of the primary particles without obvious pores or fractures, and is tightly bonded to the matrix interface; the primary particle size of the material is 100 to 500 nanometers, and the secondary agglomerate particle size is 1 to 5 micrometers.

[0020] In some embodiments, the electronic conductivity of the supervalent ion-doped lithium storage material is determined by the four-probe method and is not less than 10⁻³ Siemens per centimeter; the lithium-ion diffusion coefficient is determined by the galvanostatic intermittent titration technique (GITT) and is not less than 10⁻¹² square centimeters per second.

[0021] In some embodiments, when the supervalent ion-doped lithium storage material is used as the positive electrode of a lithium-ion battery, it is mixed with conductive carbon black and polyvinylidene fluoride binder in a mass ratio of 80:10:10 using N-methylpyrrolidone as a solvent to form a slurry, which is then coated onto an aluminum foil current collector. After being vacuum dried at 120 degrees Celsius for 12 hours, the slurry is pressed, cut, and assembled into a coin cell. The initial discharge specific capacity is not less than 150 mAh / g at a voltage window of 2.5 to 4.2 volts and a 1C rate, and the capacity retention rate at a 10C rate is not less than 85% relative to 0.1C.

[0022] In some embodiments, the method is applicable to a variety of lithium storage material systems, including but not limited to olivine phosphates (such as LiFePO4, LiMnPO4) and spinel oxides (such as Li4Ti5O4). 12 By adjusting the type of doping element, doping concentration, and heat treatment parameters, the electronic structure and ion transport performance of different systems can be directionally controlled.

[0023] In some embodiments, the pyrolysis behavior of the light carbon source and the heavy carbon source is distinguished by thermogravimetric-differential scanning calorimetry (TG-DSC) analysis: the light carbon source shows a significant weight loss peak in the range of 200 to 500 degrees Celsius, corresponding to the release of volatile products and preliminary carbonization; the heavy carbon source exhibits slow weight loss in the range of 500 to 900 degrees Celsius, corresponding to the formation of cross-linked networks and graphitization process.

[0024] In some embodiments, the formation mechanism of the oxygen vacancy is as follows: carbon monoxide generated by the pyrolysis of a light carbon source reacts with lattice oxygen on the surface of a metal oxide precursor as follows: MO + CO → M + CO2, where M represents a transition metal. This reaction is thermodynamically feasible at 300 to 500 degrees Celsius, and its kinetics are jointly controlled by the carbon source decomposition rate and the gas diffusion rate.

[0025] In some embodiments, the diffusion mechanism of the hypervalent ion is a vacancy-assisted hopping mechanism, i.e., Nb 5 High-valence ions such as ⁺ migrate along channels formed by oxygen vacancies in the crystal lattice. Their migration activation energy is obtained by fitting the conductivity-temperature curve using the Arrhenius equation. The value is 30% to 50% lower than that of the control sample without oxygen vacancies.

[0026] In some embodiments, the formation of the graphitized carbon layer depends on the orderly stacking of aromatic structural units of heavy carbon source at high temperature. Phenolic resin, due to its rich benzene ring structure, can form local graphite microcrystals at temperatures above 800 degrees Celsius. The (002) interplanar spacing is determined by X-ray diffraction and the value is between 0.34 and 0.36 nanometers.

[0027] In summary, this invention solves the technical challenges of high diffusion barriers, uneven doping, and uncontrollable reduction of supervalent ions in solid-state reactions by using a two-stage heat treatment process of low-temperature defect induction and high-temperature diffusion doping, combined with the functional division of light and heavy carbon sources. The prepared material has high intrinsic conductivity, fast ion transport capability, and excellent interface stability, making it suitable for high-rate and long-life lithium-ion battery applications.

[0028] The beneficial effects of this invention are: 1. Overcoming the diffusion energy barrier to achieve efficient and uniform doping of hypervalent ions. This invention employs a staged heat treatment process. In the low-temperature stage (300-500℃), a reducing atmosphere (such as CO) generated by the pyrolysis of a light carbon source induces oxygen vacancy defects in situ on the surface of the metal oxide precursor. These oxygen vacancies significantly reduce the number of hypervalent ions (such as Nb) in the subsequent high-temperature stage. 5 ⁺、W 6 The diffusion barrier of ions (such as ⁺ ions) entering the crystal lattice provides a low-barrier migration channel. Compared with traditional direct high-temperature solid-state reactions, this method achieves deep substitution and gradient distribution of hypervalent ions, effectively avoiding surface segregation problems caused by charge repulsion, and significantly improving the intrinsic electronic conductivity of the material.

[0029] 2. Synergistic effect of dual carbon sources, balancing reduction kinetics and conductive coating quality. This invention employs a combined strategy of lightweight carbon sources (such as glucose) and heavy carbon sources (such as phenolic resin) to achieve functional division of labor. The lightweight carbon source rapidly decomposes in the low-temperature region, primarily responsible for controlling the reducing atmosphere and creating lattice defects; the heavy carbon source undergoes aromatization and graphitization in the high-temperature region (700-900℃), primarily responsible for constructing a continuous and dense graphitized conductive carbon layer. This synergistic mechanism solves the problem that a single carbon source cannot simultaneously meet the requirements of low-temperature reduction and high-temperature, high-quality coating, enabling the prepared material to possess both excellent lattice stability and surface electron transport capabilities.

[0030] 3. Significantly improves the rate performance and cycle stability of materials. Benefiting from the enhanced lithium-ion diffusion coefficient (reaching the order of 10⁻¹² cm² / s) due to the doping of supervalent ions within the crystal lattice and the electron transport network provided by the high-quality graphitized carbon layer on the surface, the lithium storage material prepared in this invention exhibits superior electrochemical performance. The capacity retention at high rates (10C) is significantly improved, and the gradient doping structure alleviates lattice stress during charge and discharge, greatly enhancing the long-term cycling stability of the material. Detailed Implementation

[0031] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0032] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0033] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0034] As described above, current methods for preparing doped lithium storage materials using high-temperature solid-state methods often encounter problems such as high diffusion barriers for supervalent ions, uneven doping distribution, uncontrollable reduction levels, and poor carbon coating quality. Especially when using a single carbon source, it is difficult to simultaneously control the reduction kinetics at low temperatures and achieve graphitized carbon layer formation at high temperatures, resulting in limited improvements in the material's electronic conductivity and lithium-ion migration rate. Therefore, this application provides a method for constructing a high-temperature solid-state carbothermal reduction supervalent ion doped lithium storage structure. Through a staged heat treatment process and the synergistic effect of light and heavy dual carbon sources, deep and uniform doping of supervalent ions is achieved in the main crystal lattice, while simultaneously constructing a highly conductive graphitized carbon coating layer.

[0035] In a first aspect, this application provides a method for constructing a high-temperature solid-phase carbothermal reduction supervalent ion-doped lithium storage structure, comprising the following steps: S10: The lithium source, metal oxide precursor, supervalent ion doped source, light carbon source and heavy carbon source are mixed according to the target stoichiometric ratio, and mechanical ball milling is performed after adding ball milling media to obtain a uniform mixed slurry; then the mixed slurry is vacuum dried at 60 to 90 degrees Celsius for 12 to 24 hours to obtain a dried powder. S20: The dried powder is placed in a tube furnace or box furnace and heated to 300 to 500 degrees Celsius at a heating rate of 3 to 5 degrees Celsius per minute under an inert atmosphere. The temperature is maintained for 2 to 4 hours to allow partial pyrolysis of the light carbon source and release of reducing gas. The weight loss of the light carbon source at 300 to 500 degrees Celsius is not less than 40% of its initial mass, so as to induce the formation of oxygen vacancy defects on the surface of the metal oxide precursor. S30: Under the condition of maintaining an inert atmosphere, continue to raise the furnace temperature to 700 to 900 degrees Celsius at a heating rate of 2 to 4 degrees Celsius per minute, and hold for 6 to 12 hours, so that the supervalent ions can enter the interior of the crystal lattice through the diffusion channels provided by the oxygen vacancies to complete the substitution. At the same time, the heavy carbon source is carbonized and graphitized in situ on the particle surface to form a continuous and dense conductive carbon layer. S40: After the heat preservation is completed, stop heating and allow the furnace body to cool naturally to room temperature to obtain a supervalent ion doped lithium storage material with gradient doping distribution and graphitized carbon coating structure.

[0036] According to this application, in step S10, the mixing ratio of the lithium source, metal oxide precursor, hypervalent ion dopant source, light carbon source, and heavy carbon source must be strictly controlled. Specifically, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium nitrate; the metal oxide precursor is selected from at least one of iron phosphate, titanium dioxide, cobalt trioxide, manganese tetroxide, and nickel-cobalt-manganese oxide; the hypervalent ion dopant source is selected from at least one of niobium pentoxide, tantalum pentoxide, tungsten trioxide, molybdenum dioxide, rhenium trioxide, tin tetrachloride, and zirconium fluoride; the light carbon source is selected from at least one of glucose, sucrose, citric acid, ascorbic acid, and polyethylene glycol; and the heavy carbon source is selected from at least one of phenolic resin, asphalt, polyacrylonitrile, polyvinylidene fluoride, and polystyrene.

[0037] In some embodiments, in step S10, the molar ratio of lithium source, metal oxide precursor, and hypervalent ion dopant source satisfies the stoichiometric relationship of the target product, wherein the amount of hypervalent ion dopant source added is 0.1% to 5.0% of the molar amount of the main metal element in the metal oxide precursor; the mass of light carbon source is 2% to 8% of the total solid material mass; and the mass of heavy carbon source is 3% to 10% of the total solid material mass.

[0038] In some embodiments, in step S10, the mechanical ball milling uses a planetary ball mill or a stirred ball mill, with a ball-to-material mass ratio of 10:1 to 30:1, and the milling media being zirconia balls or agate balls. The milling speed is 200 to 600 rpm, and the milling time is 2 to 8 hours. During the milling process, anhydrous ethanol or deionized water is added as a dispersion medium, with a liquid-to-solid mass ratio of 1:1 to 3:1. After milling, the resulting slurry is transferred to a vacuum drying oven and dried under vacuum at 70 degrees Celsius for 18 hours to obtain a dry powder with good flowability.

[0039] In some embodiments, in step S20, the inert atmosphere is high-purity nitrogen, argon, or a mixture thereof, with a gas flow rate of 50 to 200 standard milliliters per minute; the holding temperature is preferably 350 to 450 degrees Celsius, and the holding time is preferably 2.5 to 3.5 hours. Within this temperature range, light carbon sources such as glucose undergo dehydration condensation and partial carbonization, releasing carbon monoxide, carbon dioxide, and small molecule hydrocarbon gases. Carbon monoxide, as the main reducing component, reacts with lattice oxygen on the surface of the metal oxide precursor to generate oxygen vacancies. This reaction can be represented as: MO + CO → M + CO2, where M represents a transition metal.

[0040] In some embodiments, in step S30, the high-temperature holding temperature is determined according to the target material system: for lithium iron phosphate systems, the holding temperature is 650 to 800 degrees Celsius; for lithium titanate systems, the holding temperature is 750 to 900 degrees Celsius; for lithium cobalt oxide or lithium nickel manganese oxide systems, the holding temperature is 800 to 900 degrees Celsius; the holding time is adjusted according to the diffusion coefficient of the doped ions, ranging from 6 to 12 hours. During this stage, the supervalent ions exist as cations, with an ionic radius not exceeding 30% different from the host cation, and a valence 1 to 3 higher than the host cation. For example, in a lithium iron phosphate system, Fe²⁺ is the main cation, doped with Nb. 5 ⁺、Ta 5 ⁺ or W 6 ⁺; In the lithium titanate system, Ti 4 ⁺ is the main cation, doped with Nb 5 ⁺、Mo 6 ⁺ or Re 7 ⁺.

[0041] In some embodiments, in step S30, the heavy carbon source undergoes pyrolysis, crosslinking, aromatization, and graphitization processes at 700 to 900 degrees Celsius, with a residual carbon content of not less than 50%, and the resulting carbon layer thickness is 2 to 10 nanometers. The degree of graphitization is characterized by the intensity ratio of the D peak to the G peak (ID / IG) in Raman spectroscopy, and this ratio is not greater than 1.2. For example, phenolic resin, due to its rich benzene ring structure, can form local graphite microcrystals at temperatures above 800 degrees Celsius, and its (002) interplanar spacing is determined by X-ray diffraction, with values ​​ranging from 0.34 to 0.36 nanometers.

[0042] In some implementations, the concentration of oxygen vacancies is quantified by peak fitting of the O 1s orbital in X-ray photoelectron spectroscopy (XPS), with a relative content of 5% to 15% of the total oxygen signal; the occupancy state of supervalent ions in the crystal lattice is confirmed by X-ray diffraction (XRD) refinement combined with extended X-ray absorption fine structure (EXAFS) analysis, with a substitution ratio of not less than 85% of the theoretical doping amount.

[0043] In some embodiments, in step S40, the cooling rate of the furnace cooling is controlled at 1 to 3 degrees Celsius per minute to avoid lattice stress concentration or carbon layer cracking due to rapid cooling; inert gas is continuously introduced during the cooling process to prevent the material from being re-oxidized by contact with oxygen in the high-temperature section.

[0044] Secondly, this application provides a supervalent ion-doped lithium storage material, prepared by the method described in any embodiment of the first aspect, with the general chemical formula LiM1₋. x D x PO4 / C, Li4Ti5₋ x D x O 12 / C or LiCo1₋ x D x O2 / C, where M represents at least one of Fe, Mn, Ni, and Co, D represents at least one of Nb, Ta, W, Mo, Re, Sn, and Zr, x is from 0.001 to 0.05, and C represents the graphitized carbon coating.

[0045] According to this application, the supervalent ion-doped lithium storage material has the following microstructural characteristics: supervalent ions are distributed in a gradient within the crystal lattice, with a surface concentration slightly lower than that in the bulk central region, indicating that the doping process is diffusion-controlled rather than surface adsorption; a carbon coating layer continuously covers the surface of the primary particles without obvious pores or fractures, and is tightly bonded to the matrix interface; the primary particle size of the material is 100 to 500 nanometers, and the secondary agglomerate particle size is 1 to 5 micrometers. The gradient doping distribution is achieved by controlling the heating rate and holding time in the S30 stage: when the heating rate is ≤3°C / min and the holding time is ≥8 hours, supervalent ions have sufficient time to diffuse from the particle surface to the interior, forming a gradient distribution with a bulk concentration higher than that on the surface; preferably, the S30 heating rate is 2–3°C / min, and the holding time is 8–12 hours, to ensure that the diffusion depth of the doped ions is greater than 50% of the particle radius.

[0046] In some embodiments, the electronic conductivity of the supervalent ion-doped lithium storage material is determined by the four-probe method and is not less than 10⁻³ Siemens per centimeter; the lithium-ion diffusion coefficient is determined by the galvanostatic intermittent titration technique (GITT) and is not less than 10⁻¹² square centimeters per second.

[0047] In some embodiments, when the supervalent ion-doped lithium storage material is used as the positive electrode of a lithium-ion battery, it is mixed with conductive carbon black and polyvinylidene fluoride binder in a mass ratio of 80:10:10 using N-methylpyrrolidone as a solvent to form a slurry, which is then coated onto an aluminum foil current collector. After being vacuum dried at 120 degrees Celsius for 12 hours, the slurry is pressed, cut, and assembled into a coin cell. The initial discharge specific capacity is not less than 150 mAh / g at a voltage window of 2.5 to 4.2 volts and a 1C rate, and the capacity retention rate at a 10C rate is not less than 85% relative to 0.1C.

[0048] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0049] Example 1 S10: Weigh 2.18 g of lithium carbonate, 7.98 g of iron phosphate, 0.048 g of niobium pentoxide, 0.42 g of glucose, and 0.65 g of phenolic resin, place them in a planetary ball mill jar, add zirconia balls (5 mm in diameter), the ball-to-material mass ratio is 20:1, add 20 ml of anhydrous ethanol, the liquid-to-solid mass ratio is 2:1, and ball mill at 400 rpm for 4 hours to obtain a uniform slurry; vacuum dry the slurry at 80 degrees Celsius for 18 hours to obtain a dry powder; S20: Load the dry powder into a quartz boat, place it in a tube furnace, introduce high-purity argon gas (flow rate 100 standard milliliters per minute), heat to 400 degrees Celsius at 4 degrees Celsius per minute, and hold for 3 hours; S30: Continue to heat up to 750 degrees Celsius at a rate of 3 degrees Celsius per minute and keep warm for 8 hours; S40: Stop heating and cool the furnace to room temperature at a rate of about 2 degrees Celsius per minute, with argon gas protection throughout, to obtain Nb-doped lithium iron phosphate / carbon composite material.

[0050] Example 2 S10: Weigh 1.68 g of lithium hydroxide, 4.76 g of titanium dioxide, 0.056 g of tungsten trioxide, 0.38 g of citric acid, and 0.72 g of pitch, place them in a stirred ball mill, add agate balls (ball-to-material mass ratio of 25:1), add 18 ml of deionized water (liquid-to-solid mass ratio of 1.8:1), and ball mill at 500 rpm for 6 hours to obtain a slurry; vacuum dry at 80 degrees Celsius for 20 hours to obtain a dry powder; S20: The powder is placed in a box furnace, and high-purity nitrogen gas (flow rate 150 standard milliliters per minute) is introduced. The temperature is increased to 380 degrees Celsius at a rate of 3.5 degrees Celsius per minute and held at that temperature for 3.5 hours. S30: Heats up to 850 degrees Celsius at a rate of 2.5 degrees Celsius per minute and holds for 10 hours; S40: Cool to room temperature in the furnace at a rate of 1.5 degrees Celsius per minute to obtain W-doped lithium titanate / carbon composite material.

[0051] Example 3 S10: Weigh 2.34 g of lithium acetate, 3.21 g of cobalt trioxide, 0.062 g of tantalum pentoxide, 0.45 g of sucrose, and 0.80 g of polyacrylonitrile. Use a planetary ball mill with a ball-to-material ratio of 30:1. Add 22 ml of anhydrous ethanol and mill at 600 rpm for 5 hours. Then vacuum dry at 70 degrees Celsius for 24 hours. S20: In a tube furnace, argon flow rate is 120 standard milliliters per minute, temperature is increased from 4 degrees Celsius to 420 degrees Celsius per minute, and held for 2.5 hours; S30: Temperature rises to 880 degrees Celsius in 3 degrees Celsius per minute and remains warm for 12 hours; S40: Cooled in the furnace to obtain Ta-doped lithium cobalt oxide / carbon composite material.

[0052] Comparative Example 1 (using only a light carbon source) S10: Weigh 2.18 g of lithium carbonate, 7.98 g of iron phosphate, 0.048 g of niobium pentoxide, and 1.07 g of glucose (total carbon content is equivalent to that in Example 1), and the rest are the same as in Example 1; S20-S40: The process is the same as in Example 1.

[0053] Comparative Example 2 (using only heavy carbon source) S10: Weigh 2.18 g of lithium carbonate, 7.98 g of iron phosphate, 0.048 g of niobium pentoxide, and 1.07 g of phenolic resin, and the rest are the same as in Example 1; S20-S40: The process is the same as in Example 1.

[0054] Comparative Example 3 (no staged heat treatment, direct high-temperature sintering) S10: Same as Example 1; S20: Omitted, directly raises the temperature from room temperature to 750 degrees Celsius at a rate of 3 degrees Celsius per minute and holds it for 8 hours; S40: Same as Example 1.

[0055] The materials obtained in the above embodiments and comparative examples were subjected to performance tests, and the results are shown in the table below: sample Specific surface area (m² / g) Pore ​​volume (cm^3 / g) Electron conductivity (S / cm) Lithium-ion diffusion coefficient (cm² / s) 1C initial discharge specific capacity (mAh / g) 10C / 0.1C capacity retention (%) Example 1 18.5 0.082 1.2×10⁻³ 2.1×10⁻¹² 158 87.3 Example 2 15.2 0.075 <![CDATA[8.5×10⁻ 4 ]]> 1.8×10⁻¹² 162 85.6 Example 3 12.8 0.068 <![CDATA[9.3×10⁻ 4 ]]> 1.9×10⁻¹² 155 86.1 Comparative Example 1 16.7 0.078 <![CDATA[3.5×10⁻ 4 ]]> 8.7×10⁻¹³ 132 62.4 Comparative Example 2 14.3 0.065 <![CDATA[4.1×10⁻ 4 ]]> 9.2×10⁻¹³ 128 58.9 Comparative Example 3 17.1 0.079 <![CDATA[5.2×10⁻ 4 ]]> 1.1×10⁻¹² 140 70.2 To verify the above-mentioned beneficial effects, the test data of Examples 1-3 and Comparative Examples 1-3 provided in the specification were compared and analyzed.

[0056] 1. Necessity analysis of dual carbon source synergy (Example 1 and Comparative Examples 1 and 2): Example 1 uses a combination of glucose (light) and phenolic resin (heavy), which has an electronic conductivity of up to 1.2×10⁻³ S / cm, a 1C discharge capacity of 158 mAh / g, and a 10C rate retention rate of 87.3%.

[0057] Comparative Example 1 used only a light carbon source. Although it had good reducibility, it had low residual carbon and low graphitization at high temperatures, resulting in a sharp drop in electronic conductivity to 3.5 × 10⁻⁻⁻⁶. 4 The S / cm ratio and rate retention rate are only 62.4%. This indicates that the lack of a heavy carbon source will lead to the failure of surface conductive network construction.

[0058] Comparative Example 2 used only a heavy carbon source. Due to the lack of sufficient volatile reducing gas to create oxygen vacancies at low temperatures, overvalent ions had difficulty entering the crystal lattice, resulting in a lithium-ion diffusion coefficient of only 9.2 × 10⁻¹³ cm² / s (lower than 2.1 × 10⁻¹² in Example 1), ultimately leading to the worst capacity and rate performance. This verifies the crucial role of a light carbon source in inducing defect doping.

[0059] 2. Key aspects of the staged heat treatment process (Example 1 and Comparative Example 3): Although Comparative Example 3 used a dual carbon source, it omitted the low-temperature holding stage (S20) and directly heated to a high temperature. Its test results showed that the lithium-ion diffusion coefficient (1.1×10⁻¹² cm² / s) and rate retention rate (70.2%) were significantly lower than those of Example 1.

[0060] This strongly demonstrates that inducing oxygen vacancies during the low-temperature pretreatment stage is a prerequisite for achieving efficient doping of hypervalent ions. Without a low-temperature pore / defect creation process, hypervalent ions mainly accumulate on the surface at high temperatures, failing to effectively enhance the ion transport capacity of the bulk phase.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for constructing a high-temperature solid-phase carbothermal reduction supervalent ion-doped lithium storage structure, characterized in that... Includes the following steps: S10: The lithium source, metal oxide precursor, supervalent ion doped source, light carbon source and heavy carbon source are mixed according to the target stoichiometric ratio, and mechanical ball milling is performed after adding ball milling media to obtain a uniform mixed slurry; then the mixed slurry is vacuum dried at 60 to 90 degrees Celsius for 12 to 24 hours to obtain a dried powder. S20: The dried powder is placed in a tube furnace or box furnace and heated to 300 to 500 degrees Celsius at a heating rate of 3 to 5 degrees Celsius per minute under an inert atmosphere, and held for 2 to 4 hours. The weight loss ratio of the light carbon source at 300 to 500 degrees Celsius is not less than 40% of its initial mass, so that the light carbon source undergoes partial pyrolysis and releases reducing gas, inducing the formation of oxygen vacancy defects on the surface of the metal oxide precursor. S30: Under the condition of maintaining an inert atmosphere, continue to raise the furnace temperature to 700 to 900 degrees Celsius at a heating rate of 2 to 4 degrees Celsius per minute, and hold for 6 to 12 hours, so that the supervalent ions can enter the interior of the crystal lattice through the diffusion channels provided by the oxygen vacancies to complete the substitution. At the same time, the heavy carbon source is carbonized and graphitized in situ on the particle surface to form a continuous and dense conductive carbon layer. S40: After the heat preservation is completed, stop heating and allow the furnace body to cool naturally to room temperature to obtain a supervalent ion doped lithium storage material with gradient doping distribution and graphitized carbon coating structure.

2. The method according to claim 1, characterized in that, In step S10, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium nitrate; the metal oxide precursor is selected from at least one of iron phosphate, titanium dioxide, cobalt trioxide, manganese tetroxide, and nickel cobalt manganese oxide; the supervalent ion doping source is selected from at least one of niobium pentoxide, tantalum pentoxide, tungsten trioxide, molybdenum dioxide, rhenium trioxide, tin tetrachloride, and zirconium fluoride; the light carbon source is selected from at least one of glucose, sucrose, citric acid, ascorbic acid, and polyethylene glycol; and the heavy carbon source is selected from at least one of phenolic resin, asphalt, polyacrylonitrile, polyvinylidene fluoride, and polystyrene.

3. The method according to claim 1, characterized in that, In step S10, the molar ratios of the lithium source, metal oxide precursor, and hypervalent ion dopant source satisfy the stoichiometric relationship of the target product, wherein the amount of hypervalent ion dopant source added is 0.1% to 5.0% of the molar amount of the main metal element in the metal oxide precursor; the mass of the light carbon source is 2% to 8% of the total solid material mass; and the mass of the heavy carbon source is 3% to 10% of the total solid material mass.

4. The method according to claim 1, characterized in that, In step S10, the mechanical ball milling adopts a planetary ball mill or a stirred ball mill, the ball-to-material mass ratio is 10:1 to 30:1, the ball milling media is zirconia balls or agate balls, the ball milling speed is 200 to 600 revolutions per minute, and the ball milling time is 2 to 8 hours; anhydrous ethanol or deionized water is added as a dispersion medium during the ball milling process, and the liquid-to-solid mass ratio is 1:1 to 3:

1.

5. The method according to claim 1, characterized in that, In step S20, the inert atmosphere is high-purity nitrogen, argon, or a mixture thereof, and the gas flow rate is 50 to 200 standard milliliters per minute; the heat preservation temperature is 350 to 450 degrees Celsius, and the heat preservation time is 2.5 to 3.5 hours.

6. The method according to claim 1, characterized in that, In step S30, the holding temperature is 650 to 800 degrees Celsius for the lithium iron phosphate system; 750 to 900 degrees Celsius for the lithium titanate system; and 800 to 900 degrees Celsius for the lithium cobalt oxide or lithium nickel manganese oxide system.

7. The method according to claim 1, characterized in that, In step S30, the carbon layer formed by the pyrolysis of the heavy carbon source at 700 to 900 degrees Celsius has a thickness of 2 to 10 nanometers, and its graphitization degree is characterized by the intensity ratio of the D peak to the G peak in the Raman spectrum, which is not greater than 1.

2.

8. The method according to claim 1, characterized in that, The dopant element in the supervalent ion doping source exists in the form of a cation at high temperature, and its ionic radius differs from that of the host cation by no more than 30%, and its valence is 1 to 3 higher than that of the host cation.

9. The method according to claim 1, characterized in that, In step S40, the cooling rate of the furnace is controlled at 1 to 3 degrees Celsius per minute, and inert gas is continuously introduced during the cooling process.

10. A lithium storage material doped with supervalent ions, characterized in that, Prepared by the method according to any one of claims 1 to 9, having the general chemical formula LiM1₋ x D x PO4 / C, Li4Ti5₋ x D x O 12 / C or LiCo1₋ x D x O2 / C, where M represents at least one of Fe, Mn, Ni, and Co, D represents at least one of Nb, Ta, W, Mo, Re, Sn, and Zr, x is from 0.001 to 0.05, and C represents the graphitized carbon coating.