High area specific capacity iron negative electrode material and preparation method and application thereof

Abstract

This invention belongs to the field of electrochemical energy storage and relates to a method for synthesizing and applying a high areal capacity iron anode material. The method uses iron foam as a substrate, and forms an active layer containing iron sulfide on the iron foam substrate through a low-temperature hydrothermal reaction in an alkaline sulfur source system. The substrate is then air-annealed at 300°C for 2 hours to regulate the interface structure and conductivity. This results in a composite layer of iron sulfide and iron oxide grown in situ on the iron foam. The iron anode material exhibits high crystallinity and excellent interfacial stability, demonstrating high areal capacity, excellent rate performance, and stable cycle life. Applying this material to aqueous alkaline batteries can significantly improve energy density and cycle stability, making it particularly suitable for high power density energy storage devices.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage materials technology, and more specifically, to a high areal capacity iron anode material, its preparation method, and its application. Background Technology

[0002] Against the backdrop of increasing global demands for energy structure transformation and sustainable development, developing efficient, low-carbon, and clean energy systems has become a key focus of research and industrial development. As a typical electrochemical energy storage device, rechargeable batteries play a crucial role in reducing dependence on fossil fuels and lowering carbon emissions and environmental pollution. In recent years, lithium-ion batteries have achieved rapid development in the fields of electric vehicles and portable electronic devices due to their high energy density and long cycle life. However, with the market's continued pursuit of higher energy density and lower costs, the scarcity and high price of lithium resources, as well as potential safety hazards, have become increasingly prominent, limiting the further promotion and application of lithium-ion batteries. Therefore, exploring alternative energy storage systems with advantages such as abundant resources, safety, environmental friendliness, and ease of preparation has become an important research direction in the current energy storage field.

[0003] Iron-based aqueous alkaline batteries, as a long-established rechargeable battery system, have regained widespread attention in the "post-lithium era" due to their advantages such as inexpensive raw materials, system safety, and environmental friendliness. Compared with lithium-ion and lead-acid batteries, iron-based batteries have lower costs and better scalability, making them suitable for large-scale energy storage scenarios. Current research mainly focuses on cathode materials, and some progress has been made. However, anode performance remains a key factor limiting the development of iron-based batteries. Iron anodes rely on reversible redox reactions between iron and iron oxides for energy storage during charge and discharge, but their reaction kinetics are slow and charge transfer efficiency is low, resulting in low capacity and poor rate performance, thus severely affecting the overall battery performance. Existing iron anode modification strategies mainly include carbon material composites, morphology control, and defect engineering. For example, constructing S-doped Fe2O3 nanowire arrays on carbon cloth through hydrothermal and high-temperature sulfidation; preparing carbon-coated Fe3O4 composite materials through hydrothermal and carbonization reactions; or obtaining FeP-type anodes through phosphating reactions. While the aforementioned methods have improved the electrochemical performance of iron anodes to some extent, they generally suffer from low capacity and energy density. Therefore, there is an urgent need to propose a method for preparing high-capacity iron anode materials to enable the practical application of high-performance iron-based aqueous batteries. Summary of the Invention

[0004] The technical problem to be solved by this invention is to overcome the shortcomings of existing iron-based materials in terms of capacity and reaction kinetics, and to provide an iron anode material with high capacity, good rate capability and interfacial structural stability. Furthermore, a simple preparation method combining low-temperature hydrothermal and air annealing is proposed to achieve efficient and low-energy-consumption preparation of iron anode materials.

[0005] Another object of the present invention is to provide the application of iron anode materials in the preparation of aqueous alkaline batteries.

[0006] The above-mentioned objective of this invention is achieved through the following technical solution: An iron anode material includes a foamed iron substrate, characterized in that an active layer containing iron sulfide (hereinafter referred to as FS) is generated in situ on the surface of the foamed iron substrate through a low-temperature hydrothermal reaction. After partial surface oxidation by heat treatment in air, the active layer forms a composite layer of iron sulfide and iron oxide with stable structure and good conductivity (hereinafter referred to as FSO). The oxide phase and the sulfide phase together constitute an oxidation-sulfidation synergistic interface structure. The low-temperature hydrothermal reaction solution includes water, an alkaline reagent, a sulfur source compound, and a complexing agent. The composite layer is composed of an interwoven nanocluster structure formed by stacked nanosheets, with a smooth and dense surface that is firmly bonded to the foamed iron substrate, which helps to improve electron transport channels and ion diffusion paths.

[0007] Furthermore, the iron oxide in the composite layer accounts for 20–30% of the composite layer mass.

[0008] Furthermore, the iron anode material as described in claim 1, 2, or 4 is characterized in that the heat treatment involves placing the material in an air atmosphere for programmed heating and holding at that temperature, followed by natural cooling to room temperature for annealing; the holding temperature is 280–320°C, and the holding time is 1.5–2.5 h; more preferably, the heat treatment involves heating to 300°C at a heating rate of 3–5°C·min⁻¹, holding at that temperature for 2 h, and then naturally cooling to room temperature. After annealing, the surface of the material is dark black. Cyclic test results show that the iron anode material has good cycle stability, and the coulombic efficiency remains at a high level.

[0009] Furthermore, the nanosheets have a thickness of 50–100 nm, and the nanoclusters have a size of 0.5–5.5 μm.

[0010] Furthermore, the alkaline reagent in the low-temperature hydrothermal reaction solution is selected from one or more of sodium hydroxide, potassium hydroxide, or lithium hydroxide, with a concentration of 0.8–1.2 mol / L; the sulfur source compound is selected from one or more of sodium sulfide, ammonium sulfide, or calcium sulfide, with a concentration of 0.15–0.30 mol / L; the complexing agent is selected from one or more of trisodium citric acid, disodium ethylenediaminetetraacetate, or sodium gluconate, with a concentration of 0.015–0.030 mol / L, and the pH of the solution is 9–11.

[0011] Furthermore, the low-temperature hydrothermal reaction solution is composed of sodium hydroxide, sodium sulfide, and trisodium citrate; wherein the concentration of sodium hydroxide is 0.95–1.05 mol / L, the concentration of sodium sulfide is 0.20–0.25 mol / L, and the concentration of trisodium citrate is 0.015–0.020 mol / L; the molar ratio of sodium hydroxide, sodium sulfide, and trisodium citrate is 1:0.20–0.25:0.015–0.020, preferably 1:0.25:0.02.

[0012] Furthermore, the low-temperature hydrothermal reaction temperature is 160–200℃, preferably 180℃; the reaction time is 18–25h, preferably 24h.

[0013] Furthermore, the preparation method of the iron anode material includes the following steps: S1. Iron substrate cleaning: The foamed iron material is cleaned in sequence with hydrochloric acid, acetone, deionized water and ethanol to remove surface impurities and oxide layer, and obtain a clean foamed iron substrate. S2. Low-temperature hydrothermal reaction: The foamed iron substrate obtained in S1 is placed in a solution containing an alkaline reagent, a sulfur source compound and a complexing agent, and reacted at 160–200℃ for 18–25 h to obtain a precursor for the in-situ formation of an active layer containing iron sulfide on the surface; the alkaline reagent is sodium hydroxide, the sulfur source compound is sodium sulfide and the complexing agent is trisodium citric acid.

[0014] S3. Air annealing: The precursor containing the iron sulfide active layer is subjected to programmed temperature annealing in an air atmosphere, with a heating rate of 3–5℃·min⁻¹ to 280–320℃, held at that temperature for 1.5–2.5 h, and then naturally cooled to room temperature to obtain a composite layer of iron sulfide and iron oxide; preferably, the annealing temperature is 300℃ and the holding time is 2 h. In the composite layer of iron sulfide and iron oxide, the oxides and sulfides synergistically construct a stable oxide-sulfide interface structure.

[0015] Furthermore, in the preparation method, in step S1, the foamed iron is first soaked in 2 mol / L dilute hydrochloric acid for 1 min to remove the surface oxide layer, and then washed sequentially with acetone, deionized water and ethanol for 10 min each, and then dried in an oven at 60 ℃ for later use; the pH value of the low-temperature reaction system in step S2 is 9–11, preferably 9.5–10.5; after the reaction, the sample is washed with deionized water and ethanol and dried at 60 ℃; the air annealing temperature in step S3 is 280–320 ℃, and the holding time is 1.5–2.5 h.

[0016] Furthermore, the iron anode material prepared by this invention forms a uniform and dense FSO composite layer on its surface, which is firmly bonded to the foamed iron substrate and has good conductivity continuity and structural integrity, and can be used as an anode material for aqueous alkaline batteries.

[0017] In this invention, sodium sulfide ionizes under hydrothermal conditions in an alkaline sulfur source system to release S²⁻, which then reacts in situ with Fe foam to form a FeS layer. The chemical reaction is as follows: In this invention, during the annealing process in an air atmosphere, a controllable partial oxidation reaction occurs on the surface of the iron sulfide layer, forming a composite interface layer where iron sulfide and iron oxide phases coexist. Moderate oxidation is beneficial for improving interface stability and charge transport behavior, thus balancing the structural stability and conductivity of the material. The chemical reaction is as follows: or Compared with the prior art, the present invention has the following advantages and beneficial effects: The iron anode material preparation method proposed in this invention adopts a strategy combining low-temperature hydrothermal treatment and air annealing. The entire process requires no inert protective atmosphere or complex equipment, the operation steps are simple, the reaction conditions are mild, energy consumption is low, and the raw materials are widely available and inexpensive, making it suitable for large-scale preparation and widespread application. This method uses foamed iron itself as the iron source and substrate, generating an FeS layer in situ on the surface through a low-temperature hydrothermal reaction, avoiding the cumbersome steps of adding binders, conductive agents, and metal current collectors required in traditional slurry-coated electrodes. The resulting FeS layer is firmly bonded to the iron substrate, forming a continuous and dense conductive network, which is conducive to rapid electron transport. After air annealing, the surface of the FeS layer undergoes partial oxidation, generating an iron oxide phase that coexists with the sulfide phase, forming an FSO composite interface structure. This oxidation-sulfide interface can significantly improve the charge transfer behavior and interfacial stability of the material, promoting the reversibility of the electrode reaction. The resulting iron anode material has a lamellar surface structure with a high specific surface area, which can fully expose active sites, shorten the ion diffusion path, and thus improve the reaction kinetics of the electrode. The material obtained by this invention can be directly applied to the negative electrode of aqueous alkaline batteries such as Ni / / Fe, exhibiting good interfacial adaptability and structural stability, and has the potential to be further optimized into a high-performance energy storage device. The reaction system used in the preparation method is environmentally friendly and the operating conditions are easy to control. The morphology and interfacial structure of the material can be flexibly adjusted by regulating parameters such as sulfur source concentration and annealing temperature, demonstrating good repeatability and industrial scale-up prospects. Attached Figure Description

[0018] Figure 1 The image in the middle is a high-magnification scanning electron microscope (SEM) image of the iron anode material FSO in Example 1.

[0019] Figure 2 The image in the middle is a high-magnification scanning electron microscope (SEM) image of the FS precursor in Example 1.

[0020] Figure 3 The cyclic voltammetry (CV) curves of the iron anode material in Example 1 at different scan rates in 6 M KOH solution are shown.

[0021] Figure 4 The figures show the charge-discharge curves and rate performance of the iron anode material in Example 1 at different current densities.

[0022] Figure 5 The iron anode material of Example 1 was tested at 150 mA cm⁻¹. -2 Comparison of long-cycle stability under current density. Detailed Implementation

[0023] The present invention will be further described below with reference to specific embodiments and accompanying drawings, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in this technical field.

[0024] Unless otherwise specified, all reagents and materials used in this invention are commercially available.

[0025] Example: The iron anode material was prepared by a combination of low-temperature hydrothermal and air annealing methods. The synthesis steps are as follows: (a) Foamed iron (2 cm × 3 cm) was washed in hydrochloric acid, acetone, deionized water and ethanol for 10 minutes each, and then dried in an oven at 60°C for later use.

[0026] (b) Prepare a mixed reaction solution containing sodium hydroxide, sodium sulfide, and trisodium citrate. In this embodiment, the concentration of sodium hydroxide is 1.0 mol / L, the concentration of sodium sulfide is 0.25 mol / L, and the concentration of trisodium citrate is 0.020 mol / L, with a molar ratio of 1:0.25:0.020. After thoroughly stirring the above solution until completely dissolved, transfer it to a polytetrafluoroethylene-lined reactor for later use.

[0027] (c) Immerse the clean foamed iron substrate obtained in step (a) into the mixed solution prepared in step (b) and carry out a low-temperature hydrothermal reaction under set conditions, so that the Fe substrate reacts with the S in the solution. 2-The ions react to form an in-situ FeS layer on its surface. After the reaction is complete, the foamed iron is removed and thoroughly rinsed with deionized water and ethanol in sequence to remove unreacted substances and residual electrolytes from the surface. It is then dried at 60°C to obtain the FS precursor.

[0028] 1. (d) In this embodiment, the FS precursor obtained in step (c) is placed in an air atmosphere and heated to 300°C at a heating rate of 5°C·min⁻¹, and held at that temperature for 2 h. During the annealing process, the surface of the iron sulfide active layer undergoes moderate oxidation, forming a composite interface structure in which the iron sulfide phase and the iron oxide phase coexist. After the holding period, it is naturally cooled to room temperature to obtain the FSO iron anode material of this embodiment. In this embodiment, by controlling the annealing temperature and time, the iron oxide content of the prepared FSO composite layer is 20–30% of the composite layer mass. This mass ratio allows the oxidation-sulfidation interface of the material to significantly improve the charge transfer behavior and interface stability of the material, promote the reversibility of the electrode reaction, and enable the electrode material to exhibit excellent performance.

[0029] To investigate the effect of annealing temperature on the material interface structure and electrochemical performance, annealing treatments at 200℃ and 400℃ were set as comparative examples under the same preparation conditions.

[0030] The conditions and results in the examples are shown in Table 1: Table 1: Morphological characteristics and properties of iron anode materials obtained under different preparation conditions in the examples and comparative examples As a comparative example, FS precursors were prepared under the same low-temperature hydrothermal conditions, but without subsequent air annealing, to investigate the effect of air annealing on the material's interfacial structure and electrochemical performance. Electrochemical tests showed that the FS electrode without air annealing exhibited poor energy storage performance, with a significantly lower areal capacity than the annealed FSO electrode, indicating that the single FS precursor is difficult to use directly as a high-performance iron anode material. In contrast, air annealing promoted the formation of the iron oxide phase and facilitated the construction of a composite interface between iron sulfide and iron oxide, thereby improving electrode reaction kinetics and enhancing electrochemical activity.

[0031] The test results of the iron anode material prepared in Example 1 are as follows: Figures 1 to 5 As shown. Figure 2 As shown, the FS precursor before air annealing forms uniform iron sulfide nanosheets on the surface of the foamed iron substrate, and the nanosheets stack together to form a nanocluster structure. After air annealing, as... Figure 1As shown, the obtained FSO composite iron anode material still maintains a similar nanosheet / nanocluster morphology, indicating that the annealing treatment did not significantly damage the original micro / nano structure, but rather promoted moderate oxidation of the iron sulfide active layer surface, forming a composite structure of iron sulfide and iron oxide. This structure is beneficial for increasing the contact area between the electrode and the electrolyte, exposing more active sites, and shortening the ion diffusion path, thereby improving electrode reaction kinetics and cycle stability.

[0032] Electrochemical tests were all performed in 6 M KOH electrolyte using a three-electrode system, with FSO composite iron anode material as the working electrode, carbon rod as the counter electrode, and Hg / HgO electrode as the reference electrode. Figure 3 Cyclic voltammetry curves at different scan rates are shown, with a test potential window of -1.4 to -0.4 V. The results indicate that the electrode exhibits a significant redox response and good reversibility. Figure 4 The results are for constant current charge / discharge and rate performance tests, with current densities ranging from 100 to 260 mA cm⁻¹. -2 ; at 100mA cm -2 Below, the area capacity can reach 70 mAh cm⁻¹ -2 At 260 mA cm -2 It can still maintain 27 mAh cm -2 This indicates that the material has high areal capacity and good rate performance. Figure 5 The results are for cycle stability testing, with a test current density of 150 mA cm⁻¹. -2 The coulombic efficiency can be maintained at about 99.6%, indicating that the material has good cycle stability.

[0033] To further illustrate the performance advantages of the material of this invention, see Reference 1 (Reference 1: Lixia Li, Tianyi Gao, Yunshuang Ge, Qing Zhang, Jingyu Wang, Zhipeng Ma, Wenfeng Guo, Shengxue Yu, Yuqian Fan, “Ultra-long KFeS2 nanowires grown on Fe foam as a high-performance anode for aqueous solid-state energy storage”). J. Mater. Chem. A (2021, 9, 27727–27735, DOI: 10.1039 / D1TA08310D.) reported an iron-based anode material with in-situ growth of KFeS2 nanowires on a foamed iron substrate, and its performance in 6 M KOH electrolyte at 50 mA cm⁻¹. -2 The surface area capacity is 21.1 mAh cm⁻¹.-2 At 250 mA cm -2 It still maintains 18.8 mAh cm⁻¹ -2 The document also points out that the areal capacity of traditional iron-based thin-film anodes is typically below 2 mAh cm⁻¹. -2 In contrast, the FSO composite iron anode material obtained in this invention still exhibits a high areal capacity at higher current densities, indicating that the composite structure of iron sulfide and iron oxide is beneficial to improving the utilization rate of active materials and charge transport capability of iron anode materials.

[0034] The iron anode material preparation method provided by this invention has a simple process flow, is easy to operate, uses widely available and inexpensive raw materials, and has low overall energy consumption, making it suitable for large-scale preparation and widespread application. This method uses foamed iron as a substrate and iron source, generating an iron sulfide active layer in situ on its surface through a low-temperature hydrothermal reaction, followed by air annealing to form an FSO composite interface structure. The entire process requires no binder, conductive agent, or additional metal current collector; the material directly adheres to the conductive substrate, resulting in a strong bond and continuous conductivity. The obtained iron anode material exhibits a micro / nano sheet-like structure with a high specific surface area and abundant electrochemical active sites, facilitating rapid electron and ion transport. Through reasonable structural design and interface control, the material exhibits excellent conductivity and good reaction kinetics. The method of this invention has advantages such as strong scalability and good repeatability, enabling large-area continuous preparation of the material. The obtained FSO composite iron anode material exhibits good structural stability and application potential in aqueous alkaline batteries, providing a new technical approach and broad development prospects for developing high-performance, low-cost, and environmentally friendly energy storage devices.

Claims

1. A high areal capacity iron anode material, comprising a foamed iron substrate, characterized in that, The surface of the foamed iron substrate is generated in situ with an active layer containing iron sulfide through a low-temperature hydrothermal reaction. After the active layer is partially oxidized by heat treatment in air, it forms a composite layer of iron sulfide and iron oxide with stable structure and good conductivity. The low-temperature hydrothermal reaction solution includes water, alkaline reagent, sulfur source compound and complexing agent. The composite layer is composed of interwoven nanoclusters formed by stacking nanosheets.

2. The high areal capacity iron negative material of claim 1, wherein, The iron oxide content of the composite layer accounts for 20–30% of the composite layer mass.

3. The high areal capacity iron negative material of claim 1, wherein, The heat treatment involves placing the material in an air atmosphere for programmed heating and holding at that temperature, followed by natural cooling to room temperature for annealing. The holding temperature is 280–320°C, and the holding time is 1.5–2.5 h. More preferably, the heat treatment involves heating the material to 300°C at a heating rate of 3–5°C·min⁻¹, holding at that temperature for 2 h, and then naturally cooling to room temperature.

4. The high areal capacity iron negative material of claim 1, wherein, The nanosheets have a thickness of 50–100 nm, and the nanoclusters have a size of 0.5–5.5 μm.

5. The high areal capacity iron anode material of claims 1-4, wherein, The alkaline reagent in the low-temperature hydrothermal reaction solution is selected from one or more of sodium hydroxide, potassium hydroxide, or lithium hydroxide, with a concentration of 0.8–1.2 mol / L; the sulfur source compound is selected from one or more of sodium sulfide, ammonium sulfide, or calcium sulfide, with a concentration of 0.15–0.30 mol / L; the complexing agent is selected from one or more of trisodium citric acid, disodium ethylenediaminetetraacetate, or sodium gluconate, with a concentration of 0.015–0.030 mol / L, and the pH of the solution is 9–11.

6. The high areal capacity iron negative material of claim 5, wherein, The low-temperature hydrothermal reaction solution is composed of sodium hydroxide, sodium sulfide, and trisodium citrate; wherein the concentration of sodium hydroxide is 0.95–1.05 mol / L, the concentration of sodium sulfide is 0.20–0.25 mol / L, and the concentration of trisodium citrate is 0.015–0.020 mol / L; the molar ratio of sodium hydroxide, sodium sulfide, and trisodium citrate is 1:0.20–0.25:0.015–0.020, preferably 1:0.25:0.

02.

7. The high areal capacity iron anode material of claim 1, wherein, The low-temperature hydrothermal reaction temperature is 160–200℃, preferably 180℃; the reaction time is 18–25h, preferably 24h.

8. The method of making high areal capacity iron negative materials of claim 1, wherein, Includes the following steps: S1. Iron substrate cleaning: The foamed iron material is cleaned in sequence with hydrochloric acid, acetone, deionized water and ethanol to remove surface impurities and oxide layer, and obtain a clean foamed iron substrate. S2. Low-temperature hydrothermal reaction: The foamed iron substrate obtained in S1 is placed in a solution containing an alkaline reagent, a sulfur source compound and a complexing agent, and reacted at 160–200℃ for 18–25 h to obtain a precursor for the in-situ formation of an active layer containing iron sulfide on the surface; the alkaline reagent is sodium hydroxide, the sulfur source compound is sodium sulfide and the complexing agent is trisodium citric acid. S3. Air annealing treatment: The precursor containing the iron sulfide active layer is placed in an air atmosphere for programmed temperature annealing treatment, heated to 280–320°C at a heating rate of 3–5°C·min⁻¹, held at that temperature for 1.5–2.5 h, and then naturally cooled to room temperature to obtain a composite layer of iron sulfide and iron oxide; preferably, the annealing temperature is 300°C and the holding time is 2 h.

9. The method for preparing the high areal specific capacity iron anode material as described in claim 8, characterized in that, In step S1, the foamed iron is first soaked in 2 mol / L dilute hydrochloric acid for 1 min to remove the surface oxide layer, and then washed sequentially with acetone, deionized water and ethanol for 10 min each, and then dried in an oven at 60 ℃ for later use; the pH value of the low-temperature reaction system in step S2 is 9–11, preferably 9.5–10.5; after the reaction, the sample is washed with deionized water and ethanol and dried at 60 ℃; the air annealing temperature in step S3 is 280–320 ℃ and the holding time is 1.5–2.5 h.

10. The application of the iron anode material according to any one of claims 1-4, 6, or 7 in an aqueous alkaline battery.

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