Nitrogen / sulfur co-doped porous carbon-iron-based composite sodium battery negative electrode material as well as preparation method and application thereof

Abstract

The invention discloses a nitrogen / sulfur co-doped porous carbon iron-based composite sodium ion battery negative electrode material and a preparation method thereof. According to the material, chitosan is taken as a carbon source, and the honeycomb-shaped composite carbon material with a hierarchical mesoporous structure is prepared by combining a sodium chloride template method and a two-step calcination process through a double-doping synergistic effect of cyanamine and ferric sulfate. FeS and Fe4 (Fe (CN) 6) 3 active phases are uniformly distributed in the material, FeS and Fe4 (Fe (CN) 6) 3 exist in a heterojunction form, nitrogen is doped in forms of pyridine nitrogen (more than or equal to 50%) and pyrrole nitrogen, and sulfur exists in forms of C-S-C bonds and sulfur oxides. The first efficiency of the material is greater than 90%, the reversible capacity under the current density of 1A / g reaches more than 450mAh / g, the capacity retention ratio after 1000 cycles is greater than or equal to 90%, and the reversible specific capacity under the high rate of 20A / g is still kept more than 150mAh / g. The preparation method comprises the steps of spray drying, two-step calcination and pickling pore-forming. The invention solves the problems of low capacity and poor cycling stability of the existing sodium-ion battery negative electrode material, and is suitable for high-power energy storage equipment.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery anode materials, specifically to a method for preparing a nitrogen / sulfur co-doped porous carbon-iron-based composite material and its application in sodium-ion batteries. This material utilizes a template method combined with a dual-doping synergistic effect to construct a carbon-based composite anode with a hierarchical mesoporous structure and high conductivity. This significantly improves the capacity, rate capability, and cycle stability of sodium-ion batteries, making it suitable for high-power energy storage devices. Background Art

[0002] As the global energy structure transitions to renewable energy, electrochemical energy storage technology has become the key to balancing energy supply and demand. Lithium-ion batteries dominate due to their high energy density and mature technology, but lithium resources are scarce and unevenly distributed, resulting in high costs. Sodium-ion batteries have become the most promising alternative due to their abundant sodium resources (2.3% content in the earth's crust), low cost, and similar electrochemical mechanisms to lithium-ion batteries. However, the larger ionic radius of sodium ions ( vs. Lithium-ion ) makes it difficult to embed / eject in traditional graphite negative electrodes. The theoretical sodium storage capacity of graphite is only 35mAh / g, and repeated charging and discharging will cause the interlayer structure to expand and collapse, which cannot meet actual needs.

[0003] To overcome the limitations of graphite materials, researchers have developed new negative electrode systems such as hard carbon, alloys, and organic composite materials. Hard carbon materials have become the mainstream choice due to their large interlayer spacing (0.37-0.42 nm) and rich microporous structure. For example, Li et al. prepared hard carbon microspheres by solution method, with an initial capacity of 313 mAh / g at 50 mA / g, but it can only be maintained for about 100 cycles (ACS Appl. Mater. Interfaces, 16, 2024, 28461-28472); Sun et al. prepared hard carbon with pitch as the carbon source, with a reversible capacity of about 250 mAh / g, but the capacity decayed significantly at high currents (Journal of Alloys and Compounds, 786, 2019, 468-474). Although alloy materials (such as Sn and Sb-based materials) have high theoretical capacities (e.g., Sb: 660 mAh / g), volume expansion rates of up to 300%–400% during charge and discharge lead to electrode pulverization and short cycle life (Journal of Alloys and Compounds, 938, 2023, 168472). Organic composite materials (such as chitosan-based hard carbon) use cellulose nanocrystals as a carbon source to improve stability, but the first-cycle coulombic efficiency is only 79–82%, and high-rate performance is insufficient (Journal of Energy Storage, 89, 2024, 111491).

[0004] In recent years, element doping and structural manipulation have been widely used to optimize the performance of carbon-based materials. Nitrogen doping can improve the electronic conductivity of the carbon layer, but the capacity retention rate of single N-doped hard carbon materials after 100 cycles is only 94%, and the capacity is less than 100 mAh / g at 5 A / g. Although sulfur doping can promote sodium ion diffusion by expanding the interlayer spacing, its insulating properties can easily lead to polarization effects. Porous structure design (such as the template method) can improve ion transport efficiency, but the initial coulombic efficiency of the porous Sb / C material prepared by Duan et al. is only 69.4%.

[0005] Despite this, existing technologies still face bottlenecks such as the contradiction between capacity and cycle performance, insufficient conductivity and ion diffusion rate, and complex preparation process. For example, high-capacity alloy materials have short cycle life due to volume expansion, while stable hard carbon materials have limited capacity; single doping or unoptimized pore structure leads to low electron / ion transmission efficiency; traditional preparation methods (such as vapor deposition, solution method) are costly and difficult to scale up. In response to the above problems, the present invention synergistically regulates the electronic structure of the carbon layer through nitrogen / sulfur dual doping, combines the redox activity of iron-based compounds (FeS, Fe4(Fe(CN)6)3) and template method to construct a hierarchical mesoporous structure, achieves improved conductivity, expanded interlayer spacing (0.52nm) and optimized ion diffusion path, and balances material performance and industrial feasibility through spray drying and two-step calcination process, providing a new solution for the commercialization of sodium ion batteries. Summary of the Invention

[0006] Technical Solution

[0007] The present invention provides a nitrogen / sulfur co-doped porous carbon-iron composite sodium ion battery anode material and its preparation method. The material uses chitosan as a carbon source, cyanamide as a nitrogen source, ferric sulfate as an iron / sulfur source, and sodium chloride as a template. It is prepared through a spray drying combined with a two-step calcination process. Specifically, it includes the following features:

[0008] Material composition and structure:

[0009] Nitrogen (N) and sulfur (S) co-doped porous carbon matrix, carbon layer spacing ≥ 0.38nm, pore size distribution range of 2-80nm, specific surface area greater than 100m 2 / g; FeS and Fe4(Fe(CN)6)3 nanoparticles are uniformly dispersed in the carbon matrix, and the two exist in the form of heterojunctions; iron-based compounds account for 10% to 30%; nitrogen exists in the form of pyridinic nitrogen (accounting for ≥50%) and pyrrolic nitrogen, and sulfur exists in the form of CSC bonds and sulfur oxides (SO, SO4 2- ) form of doping.

[0010] Preparation method:

[0011] (1) Chitosan was prepared into a gel with a mass ratio of 3.5%. Chitosan (net weight), cyanamide, ferric sulfate, and sodium chloride were dissolved in an ethanol / water (1:9) mixed solvent at a mass ratio of 1:1-6:0.9-1.8:1-6, and stirred to form a homogeneous solution;

[0012] (2) spray drying (inlet air temperature 150-200°C, outlet air temperature 80-120°C) to obtain precursor powder;

[0013] (3) Under inert atmosphere, pre-carbonize at 600 °C for 5 h, then final carbonize at 900 °C for 5 h, with a heating rate of ≤5 °C / min;

[0014] (4) Acid pickling is performed to remove the template and impurities, and the porous composite material is obtained after washing and drying.

[0015] application:

[0016] As the negative electrode of sodium ion batteries, within the voltage window of 0.01-3V, at a current density of 1A / g, the reversible capacity is ≥450mAh / g, and the capacity retention rate is ≥90% after 1000 cycles; the reversible specific capacity is still maintained at more than 150mAh / g at a high rate of 20A / g.

[0017] Innovation

[0018] Double doping synergistic effect: N / S co-doping is used to regulate the electronic structure of the carbon layer, pyridinic nitrogen improves electronic conductivity, and sulfur doping expands the interlayer spacing and introduces sulfur active sites, synergistically improving the sodium ion adsorption and diffusion rate.

[0019] Iron-based active phase heterojunction composite: FeS provides high capacity redox reaction (Fe 2+ / Fe 3+ ), Fe4(Fe(CN)6)3 enhances structural stability and inhibits volume expansion. The cyano ligands of Fe4(Fe(CN)6)3 can preferentially react with the electrolyte to form a stable SEI layer, reducing the incidence of irreversible side reactions on the FeS surface.

[0020] Hierarchical porous structure design: NaCl template combined with acid washing is used to form a mesoporous (2-5nm) and microporous composite structure (greater than 50nm), shortening the ion diffusion path and improving the electrode-electrolyte contact efficiency.

[0021] Process optimization: Spray drying is combined with a two-step calcination process to achieve controllable material morphology, uniform element doping, and industrial feasibility.

[0022] Beneficial effects

[0023] High capacity: The reversible specific capacity at 1A / g is over 450mAh / g, far exceeding the theoretical specific capacity of hard carbon materials (<300mAh / g).

[0024] Long cycle life: reversible specific capacity retention rate ≥90% after 1000 cycles at 5A / g.

[0025] Excellent rate performance: The reversible specific capacity remains above 150mAh / g at a high current density of 20A / g.

[0026] High initial efficiency and stability: initial coulombic efficiency ≥ 90%, the electrode structure is intact after cycling, and there is no significant volume expansion.

[0027] Process universality: Raw materials are cheap and easily available, the process is simple and controllable, and suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 : The XRD patterns of the samples of Example 1, Comparative Example 1 and Comparative Example 2 show characteristic peaks of FeS (PDF#03-065-1894) and Fe4(Fe(CN)6)3 (PDF#00-001-0239), verifying the formation of iron-based compounds and the amorphous nature of the carbon matrix.

[0029] Figure 2 : Raman spectrum, D band (1340cm -1 ) and G belt (1580cm -1 )Intensity ratio (I D / I G ), indicating the defect structure introduced by N / S co-doping.

[0030] Figure 3 : The SEM and TEM images of Example 1 show the porous morphology and the interlayer spacing of the crystal planes of FeS and Fe4(Fe(CN)6)3 materials, and the EDS spectrum confirms the uniform distribution of C, N, Fe, and S elements.

[0031] Figure 4 and Figure 5 : N2 adsorption-desorption isotherm and pore size distribution curve of Example 1.

[0032] Figure 6 and Figure 7 : Cyclic performance test curve of Example 1 at different charge and discharge current densities in the range of 0.01-3.0V.

[0033] Figure 8 : Example 1 rate performance test in the range of 0.01-3.0V.

[0034] Figure 9 : Cyclic performance test curves of the active material in Example 1 at charge and discharge current densities of 5 A / g and 20 A / g in the range of 0.01-3.0 V. DETAILED DESCRIPTION

[0035] The following examples and comparative examples illustrate the implementation process and effects of the present invention in detail. In all examples, the purity of chitosan, cyanamide, ferric sulfate, and sodium chloride is ≥99%, the spray drying equipment is BA-PWGZ1000, and the calcination equipment is a tubular furnace (SGL-1200).

[0036] Dissolve chitosan, cyanamide, ferric sulfate, and sodium chloride in an ethanol / water mixed solvent at a mass ratio of 1:1-6:0.9-1.8:1-6, and stir to obtain a homogeneous solution;

[0037] Example 1 (Benchmark Example)

[0038] Raw material ratio: chitosan 0.83g, cyanamide 3g, FeSO4.7H2O 1.2g, NaCl 3g.

[0039] Preparation steps: First, dissolve 0.83g of chitosan in 23g of a water / acetic acid mixture to form a uniform gel; then mix all the above raw materials and dissolve them in 100mL of a 1:9 ethanol / water mixture and stir for 12h;

[0040] Spray drying (inlet air 180°C, outlet air 100°C) to obtain the precursor powder;

[0041] Under Ar atmosphere, pre-carbonization was performed at 600 °C for 5 h and final carbonization was performed at 800 °C for 5 h (heating rate 2 °C / min);

[0042] Soak in 1mol / L hydrochloric acid for 12h, wash and dry to obtain C-Fe-1.2 material.

[0043] The iron-based compound accounts for 10% to 30% of the total material mass; the resulting porous carbon matrix has a pore size distribution of mesopores (3 to 50 nm) accounting for ≥80%. The prepared negative electrode has an initial Coulombic efficiency of ≥90%. At a current density of 0.5 to 20 A / g, the reversible capacity is ≥350 mAh / g, and the capacity retention rate is ≥90% after 1,000 cycles. Even at a high rate of 20 A / g, the capacity still maintains a reversible specific capacity of 188 mAh / g.

[0044] Example 2

[0045] Example 2 (middle ratio raw material regulation)

[0046] Raw material ratio: chitosan: cyanamide: ferric sulfate: sodium chloride = 1:3:1.2:3; ethanol / water volume ratio 1:9, the mass of the solution is 12.5 times the total mass of the solid

[0047] Preparation steps: After chitosan is dissolved in water / acetic acid gel, the mixed raw materials are stirred and homogenized in ethanol / water solvent; spray drying (inlet air 180℃ / outlet air 100℃) is used to prepare precursor powder; pre-carbonization at 600℃ for 5h → final carbonization at 800℃ for 5h under inert atmosphere (heating rate 2℃ / min); soaking in 2mol / L hydrochloric acid for 12h to remove iron oxide impurities and retain the FeS / Fe4(Fe(CN)6)3 heterojunction.

[0048] Technical Effects

[0049] Material composition: iron-based compounds account for 22wt%; pore size distribution: mesopores (3-50nm) account for 83%; doping characteristics: pyridinic nitrogen accounts for 55% (total nitrogen 6.3at%), and sulfur exists in the form of CSC (70%) and SO (30%) (Claim 1); battery performance: 1A / g capacity 465mAh / g, capacity retention rate after 1000 cycles 91%; 20A / g capacity 168mAh / g; initial coulombic efficiency 92%.

[0050] Example 3 (High Ratio Raw Material Strengthening)

[0051] The raw material ratio is: chitosan: cyanamide: ferric sulfate: sodium chloride = 1:6:1.8:6; the solvent ratio is consistent with that in Example 2.

[0052] Process adjustment: The pre-carbonization heating rate was reduced to 1°C / min, and the reduction reaction time was extended to generate more FeS. After the final carbonization, a secondary pickling with 3 mol / L hydrochloric acid was added (the time was shortened to 6 h) to precisely control the heterojunction ratio.

[0053] Technical Effects

[0054] Material composition: Iron-based compounds account for 28wt%;

[0055] Pore size distribution: mesopores account for 88% (pore size concentrated in 5-40nm);

[0056] Heterojunction regulation: Fe4(Fe(CN)6)3 accounts for 35% of the total heterojunction;

[0057] Battery performance: 1A / g capacity 478mAh / g, capacity retention rate 93% after 1000 cycles; 20A / g capacity 195mAh / g.

[0058] Example 4 (low ratio raw material verification)

[0059] The raw material ratio is: chitosan: cyanamide: ferric sulfate: sodium chloride = 1:1:0.9:1; the solvent ratio is consistent with that in Example 2.

[0060] Process adjustment: the pre-carbonization temperature was adjusted to 550 ° C (maintained for 5 h) to suppress the excessive formation of FeS; after the final carbonization, a mild pickling with 1 mol / L hydrochloric acid was used (the time was extended to 24 h).

[0061] Technical Effects

[0062] Material composition: Iron-based compounds account for 11wt%;

[0063] Doping characteristics: pyridinic nitrogen accounts for 50% (total nitrogen 5.8at%), sulfur doping amount is 4.2at%;

[0064] Battery performance: 1A / g capacity 432mAh / g, capacity retention rate 90% after 1000 cycles; 20A / g capacity 152mAh / g.

[0065] Comparative Example 1 (without cyanamide)

[0066] Raw material adjustment: no cyanamide is added, and the rest is the same as Example 1.

[0067] Comparative Example 2 (7g cyanoamine)

[0068] Raw material adjustment: 7 g of cyanoamine, and the rest are the same as in Example 1.

[0069] Comparative Example 3 (without ferrous sulfate)

[0070] Raw material adjustment: FeSO4.7H2O is not added, and the rest is the same as Example 1.

[0071] Comparative Example 4 (2g ferric sulfate)

[0072] Raw material adjustment: FeSO4.7H2O is 2g, and the rest is the same as Example 1.

[0073] Results: Examples 1-4 demonstrate that by regulating the nitrogen / sulfur doping levels, the iron-based compound content, and the amount of template, the material capacity, rate capability, and cycling stability can be optimized. Comparative examples confirm the necessity of dual doping and an iron-based active phase. The flexible process of this invention is suitable for the industrial production of high-performance sodium-ion battery anodes.

[0074] Material characterization and electrochemical performance testing

[0075] Next, the morphology and structure of the composite material are tested and characterized by phase testing, and the electrochemical performance of the composite material prepared by the present invention is tested and characterized by cycle performance testing.

[0076] 1.XRD analysis

[0077] Figure 1The XRD patterns of Example 1, Comparative Example 1, and Comparative Example 2 each clearly display significant peak intensity, indicating robust crystallinity. After the addition of ferric sulfate, cyanamide, and NaCl, Example 1 essentially matches the standard charts for Fe4(Fe(CN)6)3 (PDF#00-001-0239) and FeS (PDF#03-065-1894). The XRD pattern shows the presence of Fe4(Fe(CN)6)3 and FeS after the simultaneous addition of ferric sulfate and cyanamide, while only peaks of iron oxide are observed without cyanamide. This indicates that the addition of cyanamide creates a strong reducing environment that inhibits Fe oxidation and promotes sulfide formation. The carbon peaks significantly disappear upon addition of either a nitrogen source or ferric sulfate, indicating that amorphous carbon cannot be detected in the XRD data.

[0078] 2. Raman spectroscopy analysis

[0079] Figure 2 The Raman spectrum is at 1340 cm -1 and 1580cm -1 The presence of carbon atoms in the vicinity is attributed to disordered carbon (D band) and crystalline graphitic carbon (G band). D / I G The ion diffusion rate and rate performance of the material can be improved. Characterization of the material Example 1, Comparative Example 1 and Comparative Example 2, Example 1 and Example 1 D / I G The values are significantly higher than those in Comparative Example 1, which indicates that there are more defects caused by N and S doping in Example 1. D / I G It can be seen from the value that the doping of N alone will greatly increase I D / I G The larger interlayer spacing is beneficial to the storage of sodium. Example 1 shows high electrochemical performance when used as a sodium ion negative electrode. -1 The peak appearing nearby may be due to the presence of iron oxide, which corresponds to the XRD data of Comparative Example 1 above.

[0080] 3. SEM and TEM analysis

[0081] The figure shows that Example 1 was washed with dilute hydrochloric acid to remove surface metal ions, and then washed with deionized water to remove sodium chloride, forming a porous structure. The lattice spacing of Example 1 is 0.52 nm, corresponding to the (100) peak of Fe4(Fe(CN)6)3, and 0.20 nm, corresponding to the (102) peak of FeS. This is consistent with the XRD data results mentioned above. Elemental mapping shows the distribution of C, N, Fe, O, and S in the Example 1 sample. C, N, Fe, O, and S are evenly distributed throughout the entire area of the Example 1 sample, indicating the uniform presence of nitrogen, sulfur, and iron in the carbon structure.

[0082] 4. Specific surface area and pore size test

[0083] Figure 4 and Figure 5 The N2 adsorption-desorption isotherm and pore size distribution curve of Example 1 show that the sample shows a type IV isotherm, indicating that the sample is mainly mesoporous.

[0084] 5. Cycle performance test

[0085] Figure 6 and 7 The graphs show the cycling performance test curves of the active material in Example 1 of the present invention at different charge and discharge current densities in the range of 0.01-3.0 V. As can be seen from the graph, Example 1 exhibits ultra-high cycling performance after being doped with N and S.

[0086] Figure 8 This is a rate performance test in the range of 0.01-3.0V in Example 1 of the present invention, and it can be seen that the rate performance is excellent.

[0087] Figure 9 The cycling performance test curves of the active material in Example 1 of the present invention at a charge and discharge current density of 5 A / g and 20 A / g in the range of 0.01-3.0 V are shown. It can be seen that the capacity remains at 394 and 188 mAh / g after 1000 cycles.

[0088] The protection content of the present invention is not limited to the above embodiments. Without departing from the spirit and scope of the inventive concept, changes and advantages that can be thought of by those skilled in the art are included in the present invention and are protected by the appended claims.

Claims

1. A nitrogen / sulfur co-doped porous carbon-iron-based composite sodium ion battery negative electrode material, characterized in that include: Nitrogen (N) and sulfur (S) co-doped honeycomb porous carbon matrix with a pore size distribution range of 2-80nm and a specific surface area greater than 100m 2 / g; the iron-based compounds uniformly embedded in the carbon matrix include FeS and Fe4(Fe(CN)6)3, which exist in the form of heterojunctions; the nitrogen exists in the form of pyridinic nitrogen (accounting for ≥50%) and pyrrolic nitrogen, and the sulfur exists in the form of CSC bonds and sulfur oxides (SO, SO4 2- ) form.

2. The material according to claim 1, characterized in that: The iron-based compound accounts for 10% to 30% of the total mass of the material.

3. The material according to claim 1, wherein: The pore size distribution of the porous carbon matrix is such that mesopores (3 to 50 nm) account for ≥80%.

4. A method for preparing the material according to any one of claims 1 to 3, comprising the following steps: a. Prepare chitosan into a 3.5% gel by weight. Dissolve chitosan (net weight), cyanamide, ferric sulfate, and sodium chloride in an ethanol / water mixture at a mass ratio of 1:1-6:0.9-1.8:1-6, and stir to obtain a homogeneous solution. b. preparing precursor powder by spray drying; c. Under inert atmosphere, pre-carbonize at 600℃ for 5h, then final carbonize at 800℃ for 5h, with a heating rate of ≤5℃ / min; d. Soaking with 1-3 mol / L hydrochloric acid to remove metal impurities (iron oxide), washing and drying to obtain a composite material.

5. The method according to claim 4, wherein: The air inlet temperature of the spray drying is 150-200°C, and the air outlet temperature is 80-120°C.

6. The method according to claim 4, wherein: In the pre-carbonization step, Fe 3+ Reduced to Fe by cyanamide 2+ And combine with sulfur to form FeS.

7. The method according to claim 4, wherein: The volume ratio of the ethanol / water mixed solvent is 1:9, and the mass of the solution is 12.5 times the total mass of the solid.

8. A sodium ion battery, characterized in that: The material according to any one of claims 1 to 3 is used as the negative electrode.

9. The sodium ion battery according to claim 8, wherein: At a current density of 1A / g, the reversible capacity is ≥450mAh / g, and the capacity retention rate after 1000 cycles is ≥90%; at a high rate of 20A / g, the reversible specific capacity is still maintained at above 150mAh / g.

10. The sodium ion battery according to claim 8, wherein: The initial coulombic efficiency of the negative electrode is ≥90%.

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