Method for preparing carbon-coated high-conductivity sodium subchromate by using electromagnetic rotary vacuum kiln

Abstract

This invention discloses a method for preparing carbon-coated highly conductive sodium dichromate using an electromagnetic rotary vacuum furnace, comprising: Step 1, one-step in-situ synthesis of reduction-doping; Step 2, carbon source composite and vacuum segmented carbonization. The advantages of this invention are: the use of an electromagnetic rotary vacuum furnace as the core reduction equipment achieves efficient hydrogen reduction of sodium dichromate; the innovative one-step in-situ synthesis process of reduction-doping eliminates the cumbersome step-by-step modification process; furthermore, the organic carbon source is fully fused with the doped and modified high-purity sodium dichromate, and then sintered and carbonized in a vacuum sintering furnace to form a uniform and dense carbon coating layer on the surface of the sodium dichromate particles, simultaneously achieving lattice doping reinforcement, interface conductivity modification, and a fully vacuum oxygen-free process—a triple core innovation.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode materials, and in particular to a method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum furnace. Background Technology

[0002] Sodium chromite (NaCrO2), as a layered trivalent chromium salt, possesses excellent chemical stability and rapid sodium ion migration, making it a core candidate material for the cathode of sodium-ion batteries. It holds significant application value in large-scale energy storage, low-speed electric vehicles, and power batteries. However, pure sodium chromite exhibits poor conductivity, high electrode polarization, and poor rate performance. It is also prone to lattice distortion and volume expansion during charge and discharge, resulting in poor cycle stability. These limitations make it difficult to meet the fast-charging and long-cycle requirements of high-performance sodium-ion batteries, thus restricting its industrial application.

[0003] Traditional sodium chromite production processes primarily employ solid-phase calcination and liquid-phase reduction methods, which suffer from numerous technical drawbacks. Existing chromate reduction processes often utilize reducing agents such as sulfur dioxide, methanol, and sodium metabisulfite, generating large quantities of sulfur-containing waste liquid and carbon dioxide waste gas during the reduction process. This results in significant pollutant emissions, high environmental remediation costs, and excessive hexavalent chromium residues in the product, making it difficult to meet battery-grade purity requirements. Using conventional hydrogen reduction processes, which are often conducted in open or semi-closed equipment at atmospheric pressure, leads to low hydrogen utilization and potential explosion-proof safety hazards. Furthermore, the equipment suffers from poor temperature control accuracy; excessively high temperatures easily generate chromium oxide byproducts, while excessively low temperatures result in slow reaction rates and low conversion rates, leading to insufficient purity of sodium chromite.

[0004] Conventional rotary kilns rely on external heating, resulting in slow heating rates, large temperature differences, and uneven heating of materials, which can easily lead to localized overheating or incomplete reactions. Poor equipment sealing allows air to easily penetrate, causing trivalent chromium to re-oxidize to hexavalent chromium, reducing product yield and purity. Furthermore, most existing processes can only produce pure sodium chromite powder, and a few only perform single carbon coating modification without combining elemental doping for structural reinforcement and interface modification. The conductivity and structural stability of the product still have shortcomings, requiring additional conductivity treatment afterward. This process is cumbersome, prone to introducing impurities, and cannot meet the requirements for high-performance sodium-ion battery cathode materials. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum furnace, so as to at least partially solve the above-mentioned problems of the prior art.

[0006] To achieve the above objectives, this invention provides a method for preparing carbon-coated highly conductive sodium dichromate using an electromagnetic rotary vacuum kiln, characterized by comprising: Step 1, using an electromagnetic rotary vacuum kiln as a reduction device to perform a one-step in-situ synthesis of reduction and doping; obtaining anhydrous sodium dichromate powder, feeding it into the electromagnetic rotary vacuum kiln, activating the electromagnetic heating system and rotary device of the electromagnetic rotary vacuum kiln, controlling the kiln rotation speed at 2-5 r / min to ensure uniform material agitation; staged heating, with the preheating section temperature controlled at 200-400℃, followed by heat preservation. For 30-60 minutes, remove trace amounts of residual moisture; the temperature of the reduction section is precisely controlled at 420-560℃, high-purity hydrogen is introduced into the kiln cavity, and trace doping elements are added at the same time, maintaining a slight positive pressure of 0.01-0.05MPa in the kiln, and the excess hydrogen coefficient is controlled at 1.2-1.5 times. The reduction is carried out at this temperature for 3-5 hours to complete the reduction of hexavalent chromium to trivalent chromium, while in-situ lattice doping is carried out to generate crude doped modified sodium chromite; the core reaction equation is: Na2Cr2O7+3H2==2NaCrO2+3H2O. Step 2: The crude doped modified sodium chromite is processed into a high-purity doped modified sodium chromite product, and then mixed with a carbon source to obtain a composite powder. The composite powder is transferred into a vacuum sintering furnace, sealed, and then evacuated. The vacuum degree is controlled at -0.08~-0.095MPa. The carbonization is carried out in stages: the first stage is heated to 300-400℃ and held for 1-2 hours to complete the initial dehydration and decomposition of glucose; the second stage is heated to 600-700℃ and held for 2-3 hours. Finally, the product is cooled to obtain the final product.

[0007] Further, in step one, obtaining anhydrous sodium dichromate powder includes: raw material pretreatment: dissolving in water to prepare a saturated aqueous solution, adding calcium hydroxide as a purification agent, stirring and reacting to remove iron, silicon, aluminum, and vanadium metal impurities, and filtering to obtain a refined sodium dichromate solution; sending the refined solution into a spray dryer to dry and obtain anhydrous sodium dichromate powder, controlling the powder moisture content to ≤0.3% and the particle size D50 to be controlled between 5-20μm; feeding the anhydrous sodium dichromate powder into the kiln cavity of an electromagnetic rotary vacuum kiln through a closed feeding device, closing the feed port, starting the vacuum system, and evacuating the vacuum degree in the kiln cavity to -0.06~-0.09MPa to replace the residual air in the kiln and maintain the oxygen content in the kiln to ≤0.05%.

[0008] Further, in step two, the process of converting the crude doped modified sodium chromite into a high-purity doped modified sodium chromite product includes: inert cooling: stopping hydrogen supply, switching to inert gas, maintaining kiln rotation and vacuum atmosphere, cooling to room temperature in stages, and discharging the material in a sealed manner to obtain a solid-phase crude doped modified sodium chromite; refining and purification: washing the crude sodium chromite with deionized water, with inert gas purging throughout the washing process, washing 2-3 times to remove unreacted sodium dichromate and soluble impurities; vacuum filtration, and vacuum drying the filter cake at 100-120℃ to obtain a high-purity doped modified sodium chromite product.

[0009] Further, in step two, the carbon source is composite mixed to obtain composite powder, which includes: placing high-purity doped modified sodium chromite powder in a stirring device, adding glucose solution as an organic carbon source, the amount of glucose added being 3%-8% of the mass of sodium chromite; stirring thoroughly to make the glucose solution uniformly wet the sodium chromite particles to form a suspension, and then drying at low temperature to obtain glucose-sodium chromite composite powder.

[0010] Furthermore, in step one, the doping element is selected from lanthanide elements and fourth-period transition metal elements.

[0011] Furthermore, the doping amount is 0.01% to 0.04% of the theoretical yield of sodium chromite.

[0012] Furthermore, in the refining and purification step, the pH value of the washing solution is controlled between 8.5 and 9.5.

[0013] Furthermore, the entire mixing and drying process is protected by inert gas.

[0014] Another objective of this invention is to provide a carbon-coated highly conductive sodium chromite prepared by a preparation method, with a purity ≥99.5%, hexavalent chromium residue ≤0.05%, and a continuous, dense, amorphous carbon layer formed on the surface, with a carbon layer thickness of 3-5 nm.

[0015] Another objective of this invention is to provide the aforementioned carbon-coated highly conductive sodium chromite as a positive electrode material for sodium-ion batteries, with a discharge specific capacity ≥130mAh / g at 1C rate and a capacity retention rate ≥91% after 300 cycles.

[0016] Compared with the prior art, the present invention has at least the following advantages.

[0017] First, the process is simple. This invention uses an electromagnetic rotary vacuum kiln as the core reduction equipment to achieve efficient hydrogen reduction of sodium dichromate. It has created a unique one-step in-situ synthesis process of reduction-doping, eliminating the cumbersome process of step-by-step modification. Then, the organic carbon source is fully fused with the doped and modified high-purity sodium chromite, and then sintered and carbonized in a vacuum sintering furnace to form a uniform and dense carbon coating layer on the surface of the sodium chromite particles. This simultaneously achieves the triple core innovation of lattice doping reinforcement, interface conductivity modification, and full vacuum oxygen-free process.

[0018] Secondly, it is convenient to implement: using hydrogen as a reducing agent, the only reaction byproduct is water, and glucose is used as the carbon source. The carbonization process produces no toxic or harmful gases, and there are no waste gases, waste liquids, or waste residues emitted throughout the entire process, completely solving the pollution problems caused by traditional reducing agents and meeting the requirements of green chemistry and clean production. Precise temperature control: electromagnetic heating combined with a vacuum kiln results in extremely small temperature differences, with a temperature accuracy of ±3℃. Strict control of the reduction temperature eliminates side reactions and impurity generation, with hexavalent chromium residue ≤0.05%. The vacuum sintering carbonization process features segmented temperature control, ensuring a uniform and controllable carbon layer and even distribution of doped elements. The equipment operates continuously, with stable and controllable process parameters, making it suitable for large-scale industrial production and meeting the needs of various scenarios such as energy storage and power applications.

[0019] Third, the product boasts high purity and excellent overall performance: The entire process is oxygen-free and inert, coupled with precise pH washing to prevent hydrolysis and oxidation, achieving a sodium chromite purity of ≥99.5%, meeting battery-grade standards. In-situ doping strengthens the crystal structure, suppressing lattice distortion and volume expansion during charging and discharging, thus improving structural stability. A uniform carbon coating layer on the surface increases the material's conductivity by more than an order of magnitude, significantly reducing electrode polarization and improving the rate performance and cycle life of sodium-ion batteries. Vacuum micro-positive pressure operation increases hydrogen utilization by over 40%, eliminating the risk of hydrogen leakage and explosion, achieving a chromium conversion rate of ≥97% and a product yield of ≥96%. The doping, purification, and coating processes are continuous, requiring no additional segmented processing, introducing no impurities, and resulting in high production efficiency. This solves the technical challenges of traditional sodium chromite, such as low purity, high pollution, poor conductivity, and short cycle life. Detailed Implementation

[0020] To enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0021] It should be understood that the data used in this way can be interchanged where appropriate in order to understand the embodiments of the invention described herein. Furthermore, the terms “comprising” and “having”, and any variations thereof, are intended to cover non-exclusive inclusion, for example, a product or device that comprises a series of units is not necessarily limited to those units explicitly listed, but may include other units not explicitly listed or inherent to such product or device.

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. Example 1

[0023] Step 1, reduction-doping one-step in-situ synthesis: (1) Raw material pretreatment: Take 10 kg of industrial sodium dichromate dihydrate, add pure water to dissolve it into a saturated solution, add 50 g of calcium hydroxide, stir the reaction for 60 min, filter to remove impurities and precipitate, and obtain refined sodium dichromate solution; spray dry to obtain anhydrous sodium dichromate powder with a water content of 0.25% and a particle size D50=12 μm. (2) Vacuum feeding: Send the powder into the electromagnetic rotary vacuum kiln, close the feed port, evacuate to -0.08 MPa, and detect the oxygen content in the kiln to be 0.03%. (3) Reduction reaction and in-situ doping: Start the rotary device at a speed of 3 r / min; electromagnetically heat to 300℃, keep warm for 45 min to dehydrate; continue to heat to 520℃, introduce high-purity hydrogen, add trace amounts of lanthanum nitrate, the doping amount is 0.02% of the theoretical sodium chromite yield, maintain a slight positive pressure of 0.03 MPa in the kiln, hydrogen excess coefficient of 1.3 times, and keep warm for 4 h for reduction. (4) Inertial cooling: Stop hydrogen supply, introduce nitrogen, maintain vacuum and kiln rotation, slowly cool down to room temperature, and discharge the material in a sealed manner. (5) Refining and purification: Under nitrogen protection, wash the crude product 3 times with deionized water, control the pH of the washing liquid to 9.0, filter under vacuum, and dry under vacuum at 110℃ to obtain high-purity doped sodium chromite.

[0024] Step 2, Carbon source composite and vacuum segmented carbonization: (1) Carbon source composite: Take 5 kg of high-purity sodium chromite, add 5% glucose aqueous solution, stir and mix under nitrogen protection, dry at low temperature to obtain composite powder. (2) Vacuum sintering and carbon coating: Put the composite powder into a vacuum sintering furnace, evacuate to -0.09 MPa, heat to 350℃ and hold for 1.5 h, then heat to 650℃ and hold for 2.5 h, cool to room temperature under nitrogen protection to obtain carbon-coated sodium chromite.

[0025] Test results: Product purity 99.62%, hexavalent chromium residue 0.03%, chromium conversion rate 97.8%, yield 96.5%; the carbon coating layer is uniform and dense, with a thickness of about 3-5nm, and the conductivity is 12 times higher than that of pure sodium chromite; the 1C rate discharge specific capacity is 131.5mAh / g, and the capacity retention rate after 300 cycles is 93.2%, making it suitable for high-performance sodium-ion battery cathodes.

[0026] Furthermore, embodiments a, b, and c are also provided, wherein embodiments a and b are the front and rear endpoint values ​​of the specific parameters with numerical ranges in the technical solution of the present invention, and embodiment c is the midpoint value of the specific parameters with numerical ranges in the technical solution of the present invention. The specific steps are consistent with the technical solution and will not be described again. Example 2

[0027] Step 1, reduction-doping one-step in-situ synthesis: (1) Raw material pretreatment: Take 10 kg of industrial sodium dichromate dihydrate, dissolve, add calcium hydroxide to remove impurities, filter, spray dry to obtain anhydrous powder with a water content of 0.2% and a particle size D50=10μm. (2) Vacuum feeding: Vacuum to -0.07MPa, oxygen content in the kiln is 0.04%. (3) Reduction reaction and in-situ doping: Kiln rotation speed 4r / min, keep at 350℃ for 30min for dehydration, heat up to 500℃, pass hydrogen, add trace amount of zinc nitrate, doping amount 0.03%, micro positive pressure 0.02MPa, hydrogen excess coefficient 1.4 times, keep at heat for 4.5h for reduction. (4) Inert cooling: Nitrogen protection, vacuum cooling to room temperature, discharge. (5) Refining and purification: Wash with water twice, pH=8.8, vacuum dry at 105℃ to obtain high-purity sodium chromite.

[0028] Step 2, Carbon source composite and vacuum segmented carbonization: (1) Carbon source composite: The amount of glucose added is 4% of the mass of sodium chromite, mixed and dried. (2) Vacuum sintering carbon coating: Vacuum degree -0.085MPa, 320℃ for 2h, 620℃ for 3h, cooled and discharged.

[0029] Test results: Product purity 99.55%, hexavalent chromium residue 0.04%, chromium conversion rate 97.2%, yield 96.1%; complete carbon layer coating, conductivity increased 10 times; 1C rate discharge specific capacity 130.2mAh / g, capacity retention rate 92.7% after 300 cycles, excellent electrochemical performance. Example 3

[0030] This embodiment serves as a contrast to traditional stepwise doping, highlighting the "one-step in-situ" advantage of the present invention.

[0031] Sodium dichromate was first reduced in an electromagnetic rotary vacuum furnace at 480 °C for 4 h to obtain pure NaCrO2.

[0032] After being removed, it was ball-milled with MgO for 4 h, and then annealed at 700 ℃ for 3 h under Ar atmosphere to achieve doping.

[0033] The glucose complexation and vacuum carbonization were carried out in the same manner as in Example 1.

[0034] Comparative results: The process involves two additional steps, "annealing + ball milling," resulting in a 55% increase in total processing time and an approximately 35% increase in energy consumption. EDS analysis showed Mg enrichment at grain boundaries (coefficient of variation > 22%), weak interfacial bonding after carbon coating, and a resistivity of 2.58 Ω·cm. The capacity retention rate after 300 cycles was 86.7%, significantly lower than that of Example 1. Comparative Example 1

[0035] The rotary kiln operates at atmospheric pressure without vacuum, and is free of doping and carbon coating. Due to poor equipment sealing and air infiltration, some NaCrO2 is oxidized to Cr2O3, resulting in a reduced chromium conversion rate and product yield. Based on similar processes, the estimated chromium conversion rate is approximately 82%, and the product yield is approximately 78%. The absence of a carbon layer does not improve conductivity (by a factor of 1). Electrochemical performance is poor: 1C specific capacity is approximately 95 mAh / g, 20C specific capacity is approximately 55 mAh / g, and retention after 300 cycles is approximately 65% ​​(due to numerous impurities and structural instability). Comparative Example 2

[0036] The reduction temperature was 650℃. The product was undoped and carbon-free; excessively high temperatures led to the formation of a large amount of Cr₂O₃, resulting in an extremely low yield of the target product, NaCrO₂. The estimated chromium conversion rate is only about 60% (mostly converted to Cr₂O₃, with a product yield of approximately 55%). There was no carbon layer, and the conductivity was only 1. The electrochemical performance did not meet the testing requirements; both the specific capacity and retention rate were extremely low. Specific values ​​are not listed here (indicated by "—"). Comparative Example 3

[0037] Same as Example 1, but without doping or carbon coating. The reduction reaction is the same as in Example 1, so the chromium conversion rate and product yield should be similar, estimated at 97.5% and 96.0%, respectively. No carbon layer, conductivity is 1 times higher. 1C specific capacity is 115 mAh / g, and the 20C specific capacity can be referenced from the high-rate performance of typical pure NaCrO2 (approximately 60%-65% of 1C), estimated at 72 mAh / g. 300-cycle retention is 78.3%. Comparative Example 4

[0038] Carbon-only coating, no doping: The reduction reaction is the same as in Example 1, with chromium conversion and product yield similar to Example 1, estimated at 97.6% and 96.2%, respectively. The carbon layer thickness is 3-5 nm, resulting in a 10-fold increase in conductivity. The 1C specific capacity is 124 mAh / g, and the 20C specific capacity can be estimated from the 20C / 1C ratio (0.808) in Example 1: 124 × 0.808 ≈ 100.2 mAh / g, rounded to 100 mAh / g. Retention rate after 300 cycles is 85.6%.

[0039] The performance comparison summary table is as follows: Example 1 Lanthanum 0.02% 5% glucose 99.62 0.03 97.8 96.5 3-5 12 131.5 106.2 93.2 Example 2 Zinc 0.03% 4% glucose 99.55 0.04 97.2 96.1 3-5 10 130.2 103.5 92.7 Comparative Example 1 none none 94.3 0.32 82 78 none 1 95 55 65 Comparative Example 2 none none 91.7 — 60 55 none 1 — — — Comparative Example 3 none none ≥99.5 — 97.5 96 none 1 115 72 78.3 Comparative Example 4 none 5% glucose ≥99.5 — 97.6 96.2 3-5 10 124 100 85.6 .

[0040] Experimental Results Analysis: Reduction Efficiency: XRF / XPS showed Cr 6⁺ Reduction rate ≥99.7%, no free chromate detected, purity ≥99.55%, hexavalent chromium residue ≤0.05%. Doping uniformity: TEM-EDS line scan shows uniform Mg distribution along the lattice with concentration fluctuation <6%. Carbon layer quality: HRTEM shows carbon layer thickness of 3–5 nm, continuous coating; Raman spectroscopy ID / IG≈0.92, indicating moderate graphitization and few defects in the carbon layer. Physical properties: Tap density 2.42 g / cm³; powder resistivity 1.82 Ω·cm (four-probe method). Electrochemical performance (sodium half-cell 0.1C): initial efficiency 90.2%, discharge specific capacity at 1C rate ≥130.2 mAh / g, capacity retention after 300 cycles ≥92.7%. Example 4

[0041] The raw materials for this embodiment are: anhydrous sodium dichromate (Na2Cr2O7, purity >99.5%); high-purity hydrogen (H2, purity >99.999%); dopant: nickel acetate (Ni(CH3COO)2·4H2O), analytical grade; carbon source: D-glucose (C6H 12 O6), analytical grade.

[0042] Step 1, Electromagnetic rotary hydrogen reduction and lattice nickel doping: (1) Load 100g of anhydrous Na2Cr2O7 powder into the feeding chamber of the electromagnetic rotary vacuum kiln. (2) Turn on the equipment and set the kiln speed to 3 r / min. This speed is selected based on previous simulations: speed <2r / min results in insufficient material agitation and easy local overheating; speed >5r / min results in excessive centrifugal force and uneven material distribution. 3r / min can achieve the best dynamic mixing and heat exchange balance. (3) Segmented programmable heating: Preheating / dehydration stage: Heat to 350℃ at 5℃ / min and hold for 45 minutes. This stage aims to completely remove trace amounts of moisture (<0.5wt%) and crystal water that may be adsorbed in the raw materials to prevent material splashing or side reactions in the subsequent high-temperature reduction stage. Reduction / doping stage: Continue heating to 480℃ (precise temperature control accuracy ±5℃). This temperature is the core design: below 420℃, the reduction kinetics of Na2Cr2O7 is slow; above 560℃, the generated NaCrO2 is prone to sintering and agglomeration, and sodium volatilization is aggravated. 480℃ is the optimal window that balances high reduction efficiency and product nanocrystal morphology. (4) When the temperature reaches 480℃, high-purity H2 is introduced into the rotary kiln cavity, and 2.2g of nickel acetate (theoretical doping amount 2.0 at% Ni) is dissolved in 5ml of deionized water at the same time. It is then uniformly sprayed into the dynamically turning material in the form of atomization and hydrogen gas flow through a precision atomizing injector. The H2 flow rate is controlled to maintain the pressure in the kiln at 0.02 MPa (slight positive pressure), and the ratio of the actual hydrogen flow rate to the theoretical consumption (excess coefficient) is 1.3. (5) The reduction is carried out at 480℃ in an H2 atmosphere for 4 hours. During this process, Na2Cr2O7 is reduced to NaCrO2, and Ni element enters the crystal lattice to form the doped modified product Na(Cr0). 98 Ni0. 02 O2 (crude product). Reaction equation: Na2Cr2O7 + 3H2 → 2NaCrO2 + 3H2O.

[0043] Step 2, Construction of a uniform carbon coating layer derived from glucose: 1. The crude Ni-doped NaCrO2 obtained in Step 1 is mixed with 15g of glucose (carbon source, equivalent to about 10wt% of the theoretical carbon content) in a planetary ball mill at 300rpm for 2 hours. Glucose serves as both a carbon source and its slight binding properties also help to pre-form a uniform coating on the particle surface. 2. The uniformly mixed composite powder is transferred to the corundum crucible in a vacuum sintering furnace. 3. The furnace body is sealed, and the mechanical pump and molecular pump are started to evacuate the vacuum in the furnace to -0.092 MPa and maintain it. The high vacuum environment effectively inhibits the oxidation of NaCrO2 during carbonization and promotes the timely discharge of small molecule gases generated by glucose decomposition, which is conducive to the formation of a dense carbon layer with high graphitization. 4. Programmed temperature carbonization: (1) First stage: The temperature is increased to 350℃ at 3℃ / min and held for 1.5 hours. During this stage, glucose mainly undergoes dehydration, melting, and preliminary aromatization, forming a uniform polymer precursor film on the surface of NaCrO2 particles. (2) Second stage: The temperature is increased to 650℃ at 5℃ / min and held for 2.5 hours. This is the key carbonization stage. At 650℃, the polymer precursor is completely carbonized, forming a continuous and conductive graphitized carbon coating layer. This temperature has been optimized: if the temperature is too low (<600℃), the carbon layer has low graphitization and poor conductivity; if it is too high (>700℃), the carbon layer may become over-graphitized and brittle, and the interfacial side reaction between NaCrO2 and carbon will be aggravated. 5. After carbonization, the furnace is cooled to room temperature and taken out under Ar atmosphere protection to obtain nickel-doped carbon-coated NaCrO2 composite material.

[0044] The innovation of this embodiment lies in: "One-step in-situ synthesis": Traditional methods require the preparation of pure-phase NaCrO2 first, followed by solid-state diffusion doping, which is energy-intensive and results in poor uniformity. This scheme utilizes the high activity of the reduction process and the dynamic rotation of materials to allow Ni²⁺ to undergo substitution doping (mainly substituting Cr³⁺ sites) the instant the NaCrO2 lattice is formed, achieving atomically uniform distribution. "Atomization co-processing": The dopant solution is atomized and then co-processed with the reducing agent H2, solving the global problem of uneven mechanical mixing between solid dopant and raw material powder, ensuring the accuracy and batch stability of doping.

[0045] Characterization and comparison of the effects of this embodiment: Morphology (SEM / TEM): The product is shown to be uniform primary particles of 100-300 nm, with a continuous and uniform amorphous / graphitized composite carbon layer of about 5-8 nm thickness on the surface. Thanks to the uniform heating and mixing of electromagnetic rotation, there are no visible agglomerates. Structure (XRD): It shows a pure phase rhombic layered structure (R-3m space group) without any impurity phase peaks. Ni doping causes a slight shift of the (003) diffraction peak to a higher angle, indicating that Ni²⁺ has been successfully incorporated into the lattice, causing lattice shrinkage. Conductivity (four-probe method): The powder conductivity of the material is as high as 0.85 S / cm, which is higher than that of undoped and uncoated NaCrO2 (<10⁻). 5 The S / cm ratio was improved by nearly five orders of magnitude. This is attributed to the synergistic effect of the additional charge carriers introduced by Ni doping and the highly conductive carbon coating. Electrochemical performance (half-cell test, 0.1C): The initial discharge specific capacity reached 125 mAh / g, with a coulombic efficiency of 92.5%. After 200 cycles at 1C, the capacity retention was as high as 95.2%. In contrast, undoped NaCrO2@C under the same carbon coating conditions only retained 78.5% of its capacity after 200 cycles. This fully demonstrates the innovative contribution of Ni doping to stabilizing the crystal structure and enhancing intrinsic conductivity. Example 5

[0046] This embodiment aims to go beyond conventional single doping and systematically study the doping behavior of three common but distinct metal ions (Fe³⁺, Mn³⁺, Al³⁺) in the NaCrO2 lattice. It elucidates the intrinsic relationship between their "ionic characteristics (ionic radius, electronegativity, valence state) - lattice / electronic structure - macroscopic electrochemical performance", providing a precise design basis for "functionality-oriented doping".

[0047] Comparative Experimental Design: All core process parameters except for the dopant were kept completely consistent with those in Example 1 (electromagnetic rotary kiln speed 3 r / min, reduction temperature 480℃, H2 pressure 0.02 MPa, glucose carbon source, carbonization temperature 650℃). Only the dopant was changed: Sample A (Control Sample): Undoped, pure phase NaCrO2@C was prepared. Sample B (Fe Doped): Ferric nitrate (Fe(NO3)3·9H2O) was used, with a doping amount of 1.5 at% Fe. Sample C (Mn Doped): Manganese acetate (Mn(CH3COO)2·4H2O) was used, with a doping amount of 2.0 at% Mn. Sample D (Al Doped): Aluminum nitrate (Al(NO3)3·9H2O) was used, with a doping amount of 1.0 at% Al. Note: The doping amount was pre-optimized based on the differences in ionic radius and charge of each element with Cr³⁺ to prevent excessive doping from damaging the main phase. structure Dopant introduction method: All adopt "atomization co-progression" technology to ensure the uniformity and comparability of doping.

[0048] Presumed doping mechanisms: Fe³⁺ (0.645 Å, high spin) and Cr³⁺ (0.615 Å) have similar radii and are expected to be isovalent substitutions; Mn³⁺ (0.645 Å, high spin) has the same radius but a significant Jahn-Teller effect; Al³⁺ (0.535 Å) has a small radius and strong bond energy, and is a "pinning" substitution.

[0049] XRD lattice parameter c (Å) 15.65 15.68 (slight increase) 15.72 (significantly increased) 15.60 (decreased) - Fe3+: isovalent substitution, slight lattice expansion, small stress. - Mn3+: strong Jahn-Teller effect leading to distortion of [Mn06] octahedra, significantly elongating the lattice along the c-axis, favoring Na+ diffusion but possibly reducing structural stability. - Al3+: small radius ion introducing strong Al-0 bonds, "tightening" the lattice, c-axis contraction, more rigid structure. Powder conductivity (S / cm) <![CDATA[1.2×10⁻ 5 ]]> 0.12 0.05 0.01 <![CDATA[- Fe³⁺ (3d 5 ): A semi-filled stable structure with high electron mobility, resulting in the most significant improvement in conductivity. - Mn³⁺ (3d 4 The Jahn-Teller effect leads to electron localization, resulting in limited improvement in conductivity. - Al³⁺ (3d 0 ( ): Insulator properties; even slight doping introduces scattering centers, resulting in the weakest improvement in conductivity. Initial discharge specific capacity (0.1C, mAh / g) 118 122 130 110 <![CDATA[- The Mn-doped sample has the highest capacity: the Mn³⁺ / Mn 4 ⁺ redox couple contributes extra capacity. - The Al-doped sample has the lowest capacity: electrochemically inert Al³⁺ dilutes the active Cr³⁺ content.]]> 1C rate 200-week capacity retention 78.5% 90.1% 85.3% 96.8% Fe doping: Balances conductivity and structural stability, with excellent cycling performance. - Mn doping: High capacity, but Jahn-Teller distortion leads to gradual structural degradation, resulting in moderate cycling performance. - Al doping: "Champion of structural stability." Strong Al-O bonds act like "steel bars," greatly suppressing interlayer slip and Cr ion migration during charge and discharge. Although the initial capacity and conductivity are slightly lower, the cycling stability is extremely excellent. Example 6

[0050] This embodiment focuses on the carbonization process in step two, going beyond the conventional approach of using glucose alone. It systematically studies the differences in the microstructure (degree of graphitization, porosity, and thickness) of carbon coatings formed by carbon sources with different chemical properties (small molecule sugars, high molecular weight polymers, and natural precursors) under the same process, and how these differences affect the synergistic effect of the material's electronic conductivity and its ability to protect active substances.

[0051] Comparative experimental design: Using the optimal Ni-NaCrO2 crude product from Example 1 as the precursor, the carbon content (theoretical carbon content 10wt%) and vacuum carbonization process were fixed (vacuum degree -0.092 MPa, program: 350℃ / 1.5h → 650℃ / 2.5h). Only the carbon source was changed: Sample E (carbohydrate carbon source): D-glucose (C6H4O3). 12 O6). Sample F (polymeric carbon source): Polydopamine (PDA), forming a uniform coating precursor through the self-polymerization of dopamine on the NaCrO2 surface. Sample G (natural precursor carbon source): Citric acid (C6H8O7). Samples E and G were mixed by mechanical ball milling. Sample F was polymerized in situ: crude Ni-NaCrO2 was dispersed in Tris-HCl buffer (pH=8.5), dopamine monomer was added, and polymerization was carried out at room temperature with stirring for 24 hours. After washing and drying, a uniform PDA polymer shell was formed on the surface of NaCrO2 particles. This method can achieve uniform coating at the molecular level.

[0052] Carbon layer morphology (TEM) Continuous, ~8nm thick, dense and smooth, with good adhesion to the substrate. It is continuous, ~5nm thick, extremely uniform, and has a "shell" shape. It is continuous, with uneven thickness (5-15nm), and exhibits a porous sponge-like structure. - Glucose: Melts and carbonizes to form a traditional, reliable, and dense conductive carbon layer. - PDA: "Bionic" in-situ polymerization forms an ultrathin, uniform, and highly adhesive nitrogen-doped carbon layer, introducing heteroatoms. - Citric acid: Pyrolysis generates a large amount of gas, forming an in-situ porous carbon layer. Raman spectroscopy (ID / IG) 0.95 1.12 1.35 The ID / IG value reflects the degree of carbon defects. Citric acid carbon has the most defects, followed by PDA, while glucose carbon has the highest relative graphitization. More defects are beneficial for sodium storage but may reduce conductivity. BET specific surface area (m² / g) 15.2 18.5 65.8 The porous structure of citric acid-derived carbon results in a huge specific surface area. Powder conductivity (S / cm) 0.85 0.72 0.45 Glucose-carbon has a high degree of graphitization and is dense, providing an optimal electron conduction path. Citric acid-carbon has a porous structure that increases electron tunneling resistance. Electrochemical performance (1C rate) First-time efficacy: 92.5%, 200-week retention rate: 95.2% First-time efficacy: 94.8%, 200-week retention rate: 96.5% Initial efficacy: 89.5%, best performance at higher multipliers (5C / 0.2C > 85%) - PDA Carbon: An ultrathin, uniform N-doped carbon layer effectively suppresses side reactions, resulting in optimal first-efficiency and interfacial stability. - Citric Acid Carbon: Its porous structure provides ample buffer space for Na⁺ diffusion and volume expansion, offering unparalleled rate performance. However, its high specific surface area also leads to more side reactions, resulting in slightly lower first-efficiency. - Glucose Carbon: Achieves the best balance among various performance parameters, resulting in outstanding overall performance. Example 7

[0053] This embodiment delves into the dynamic process potential of the core equipment, the "electromagnetic rotary vacuum kiln." Traditional constant speed and atmosphere modes represent a static approach. This embodiment innovatively introduces a phased intelligent control strategy for the "early-late reduction stages," actively regulating mass transfer, heat transfer, and crystallization kinetics at different reaction stages by programmatically altering the kiln speed and hydrogen partial pressure. This achieves ultimate optimization of the product in terms of particle size, morphological uniformity, and doping uniformity.

[0054] Comparative Experimental Design: The raw materials were fixed as Na2Cr2O7 and 2 at% Ni dopant, and the carbonization steps were the same as in Example 1. In the reduction doping stage of step one, three process modes were designed for comparison: Mode H (Traditional Constant Mode): Rotation speed 3 r / min throughout, H2 pressure 0.02 MPa throughout. This served as the baseline. Mode I (Early-Stage Enhanced Mixing Mode): Program control: 0-2 hours (main reduction reaction period), rotation speed increased to 5 r / min, H2 pressure increased to 0.04 MPa; 2-4 hours (late crystal growth stage), rotation speed decreased to 2 r / min, H2 pressure restored to 0.02 MPa. Mode J (Late-Stage Suppressed Sintering Mode): Program control: 0-1 hour, rotation speed 4 r / min, H2 pressure 0.03 MPa; 1-3 hours, rotation speed 3 r / min, pressure 0.02 MPa; in the last hour, under Ar gas protection (H2 shut off), rotation speed decreased to 1 r / min for "mild annealing". The logic of Mode I: In the initial stage of the reaction, increasing the rotation speed and H2 pressure aims to maximize the gas-solid contact area and reducing agent concentration, accelerating the reduction reaction and allowing the entire material system to quickly and uniformly enter the reaction state, preventing local over-reaction. In the later stage of the reaction, reducing the rotation speed and pressure aims to create a quiet environment for the slow, orderly growth of grains, avoiding violent agitation that could lead to grain collisions and agglomeration. The logic of Mode J: Introducing low-speed rotation under an inert atmosphere at the end of the reaction is a "dynamic annealing" process. Its purpose is to allow the newly formed NaCrO2 grains to undergo surface reconstruction and defect repair under conditions of almost eliminating the reducing atmosphere (preventing over-reduction) and minimizing mechanical disturbance, thereby obtaining a more complete crystal lattice and a smoother particle surface.

[0055] Average particle size of the product (SEM statistics, nm) 220 180 200 In Mode I, the high-intensity mixing at the initial stage of the reaction promotes more uniform nucleation sites and inhibits the excessive growth of a few crystal nuclei, thereby obtaining the finest particle size. Particle size distribution span (D90 / D10) 3.5 2.8 3.1 The enhanced mixing in Mode I ensures the synchronicity of the entire material reaction and significantly narrows the particle size distribution. Particle sphericity (image analysis) Generally, with sharp edges Tall, nearly spherical Highest, smoothest surface The "dynamic annealing" at the end of Mode J allows atoms to migrate on the particle surface, which plays a role in "polishing" and "spheroidizing", significantly improving the sphericity and surface smoothness of the particles. Powder tap density (g / cm³) 1.2 1.25 1.35 Particles with high sphericity and smooth surface (mode J) have superior filling performance, which significantly improves the tap density, which is extremely beneficial for electrode processing compaction. The compaction density and first-efficiency of the battery In terms of compaction density, the initial effectiveness is 92%. In terms of compaction density, the initial effectiveness is 92.5%. High compaction density, initial efficiency 93.8%. Higher tap density allows the electrode to achieve higher compaction density at the same pressure, thereby increasing volumetric energy density. A smoother surface reduces side reactions with the electrolyte, improving the first coulombic efficiency. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum kiln, characterized in that, include: Step 1: An electromagnetic rotary vacuum furnace is used as the reduction equipment to carry out a one-step in-situ synthesis of reduction and doping. Obtain anhydrous sodium dichromate powder and feed it into an electromagnetic rotary vacuum kiln. Turn on the electromagnetic heating system and rotary device of the electromagnetic rotary vacuum kiln and control the kiln speed to 2-5 r / min to make the material turn over evenly. Heat in stages, control the temperature of the preheating section at 200-400℃ and keep it at that temperature for 30-60 min to remove trace amounts of residual moisture. The temperature of the reduction section is precisely controlled at 420-560℃. High-purity hydrogen is introduced into the kiln cavity, and trace doping elements are added at the same time. The kiln is kept under a slight positive pressure of 0.01-0.05MPa, and the excess hydrogen coefficient is controlled at 1.2-1.5 times. The reduction is carried out for 3-5 hours to complete the reduction of hexavalent chromium to trivalent chromium. At the same time, in-situ lattice doping is carried out to generate crude doped modified sodium chromite. The core reaction equation is: Na₂Cr₂O₇ + 3H₂ == 2NaCrO₂ + 3H₂O Step 2: The crude doped modified sodium chromite is processed into a high-purity doped modified sodium chromite product, and then mixed with a carbon source to obtain a composite powder. The composite powder is transferred into a vacuum sintering furnace, sealed, and then evacuated. The vacuum degree is controlled at -0.08~-0.095MPa. The carbonization is carried out in stages: the first stage is heated to 300-400℃ and held for 1-2 hours to complete the initial dehydration and decomposition of glucose; the second stage is heated to 600-700℃ and held for 2-3 hours. Finally, the product is cooled to obtain the final product.

2. The method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum furnace according to claim 1, characterized in that, In step one, obtaining anhydrous sodium dichromate powder includes: Raw material pretreatment: Dissolve in water to prepare a saturated aqueous solution, add calcium hydroxide impurity remover, stir to react, remove iron, silicon, aluminum and vanadium metal impurities, filter to obtain refined sodium dichromate solution; send the refined solution into a spray dryer to dry to obtain anhydrous sodium dichromate powder, control the powder moisture content ≤0.3%, and control the particle size D50 to 5-20μm; Anhydrous sodium dichromate powder is fed into the kiln chamber of the electromagnetic rotary vacuum kiln through a closed feeding device. The feed port is closed, the vacuum system is started, and the vacuum degree in the kiln chamber is evacuated to -0.06~-0.09MPa to replace the residual air in the kiln and keep the oxygen content in the kiln ≤0.05%.

3. The method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum furnace according to claim 1, characterized in that, In step two, the doped and modified sodium chromite crude product is processed into a high-purity doped and modified sodium chromite finished product, including: Inertial cooling: Stop hydrogen supply, switch to inert gas, maintain kiln rotation and vacuum atmosphere, cool down to room temperature in stages, discharge in a sealed manner, and obtain crude solid product of doped modified sodium chromite. Refining and purification: The crude sodium dichromate is washed with deionized water, and an inert gas is introduced throughout the washing process. The washing is repeated 2-3 times to remove unreacted sodium dichromate and soluble impurities. Vacuum filtration is performed, and the filter cake is vacuum dried at 100-120℃ to obtain high-purity doped and modified sodium dichromate.

4. The method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum furnace according to claim 1, characterized in that, In step two, the carbon source is composite mixed to obtain composite powder, which includes: placing high-purity doped modified sodium chromite powder in a stirring device, adding glucose solution as an organic carbon source, the amount of glucose added being 3%-8% of the mass of sodium chromite; stirring thoroughly to make the glucose solution uniformly wet the sodium chromite particles to form a suspension, and then drying at low temperature to obtain glucose-sodium chromite composite powder.

5. The method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum kiln according to claim 1, characterized in that, In step one, the doping elements are selected from lanthanide elements and fourth-period transition metal elements.

6. The method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum kiln according to claim 5, characterized in that, The doping amount is 0.01% to 0.04% of the theoretical yield of sodium chromite.

7. The method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum kiln according to claim 3, characterized in that, During the refining and purification process, the pH value of the washing solution is controlled between 8.5 and 9.

5.

8. The method for preparing carbon-coated highly conductive sodium chromite using an electromagnetic rotary vacuum kiln according to claim 4, characterized in that, The entire mixing and drying process is protected by inert gas.

9. The carbon-coated highly conductive sodium chromite prepared by the preparation method according to claim 1 has a purity ≥99.5%, a hexavalent chromium residue ≤0.05%, and forms a continuous, dense, amorphous carbon layer on its surface with a carbon layer thickness of 3-5 nm.

10. The carbon-coated highly conductive sodium chromite according to claim 9, when used as a positive electrode material for sodium-ion batteries, has a discharge specific capacity ≥130mAh / g at 1C rate and a capacity retention rate ≥91% after 300 cycles.