Graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material for sodium ion battery and preparation method of graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material
By doping iron into Na2CoP2O7, the cathode material for sodium-ion batteries, and combining it with graphene, a nanosheet composite structure was prepared using a sol-gel-hydrothermal-calcination process. This solved the problems of poor conductivity and insufficient cycle stability of sodium-ion battery cathode materials, and achieved efficient sodium ion diffusion and excellent electrochemical performance.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-30
- Publication Date
- 2026-04-10
AI Technical Summary
The existing sodium-ion battery cathode material Na2CoP2O7 has poor conductivity, and its electrochemical performance depends on the microstructure and crystal structure integrity of the material. Traditional synthesis methods make it difficult to accurately control the product morphology, resulting in insufficient rate performance and cycle stability of the material.
By doping Na2CoP2O7 with iron and combining it with graphene as a supporting matrix, a well-formed and uniformly dispersed nanosheet composite structure was prepared using a sol-gel-hydrothermal-calcination process. Graphene constructs a three-dimensional conductive network, and iron doping optimizes the sodium ion transport channels, resulting in an interfacial synergistic effect.
The composite material significantly improves sodium ion diffusion efficiency and electrochemical cycling stability. At a current density of 0.5 A/g, the specific capacity exceeds 120 mAh/g, the sodium ion diffusion coefficient is increased to the order of 10-10 cm2/s, and the capacity retention rate is as high as 92% after 10,000 cycles.
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Figure CN121839627A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application belongs to the technical field of sodium ion battery electrode material preparation, and particularly relates to a graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material for a sodium ion battery and a preparation method. BACKGROUND
[0002] With the transformation of global energy structure to clean and renewable direction, the large-scale utilization of intermittent energy sources such as solar energy and wind energy puts forward urgent demand for efficient energy storage systems. Sodium ion batteries are concerned due to their low cost and abundant resources. Among many positive electrode materials, sodium cobalt pyrophosphate (Na2CoP2O7) has become a potential polyanion positive electrode material due to its unique open tunnel structure (rose phase) and high working voltage.
[0003] However, the intrinsic electronic conductivity of Na2CoP2O7 is poor, and its electrochemical performance is seriously dependent on the micro-morphology and crystal structure integrity of the material. At present, the synthesis of the material is mostly by traditional solid phase method or solution combustion method combined with high temperature calcination process. These methods usually directly high-temperature treat the precursor, which easily leads to rapid grain growth, uneven particle size distribution (mostly micron level), thereby prolonging the sodium ion solid phase diffusion path and increasing the interface impedance. Even if the calcination temperature is controlled at 500℃, such "one-step" process is difficult to accurately control the product morphology, which restricts the improvement of the rate performance and cycle stability of the material.
[0004] Although there are schemes in the prior art to composite graphene with electrode materials to improve the conductivity, the effect is usually simply understood as providing a conductive network. How to improve the conductivity through material design and synthesis process innovation, realize accurate regulation of the nanostructure of the electrode material, and produce interface synergistic effect beyond simple superposition, is still a technical problem to be solved in the field. SUMMARY
[0005] The application aims to overcome the defects of poor conductivity, slow ion transport kinetics and insufficient cycle stability of the existing sodium ion battery positive electrode material, and provides a graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material for a sodium ion battery and a preparation method.
[0006] The application successfully obtains a nano sheet composite structure with regular morphology and uniform dispersion by doping iron elements into a Na2CoP2O7 crystal lattice in a specific ratio, using graphene as a support matrix, and combining an innovative "sol-gel-hydrothermal-calcination" preparation process. The iron doping effectively regulates the material crystal structure and optimizes the sodium ion transmission channel; the graphene constructs a three-dimensional conductive network and produces an interface synergistic effect with the iron-doped nano sheet to inhibit agglomeration and stabilize the structure. The synergistic effect is not a simple superposition of the functions of the two, but significantly improves the ion diffusion efficiency and electrochemical cycle stability.
[0007] In one aspect, the application provides a preparation method of a graphene / iron-doped sodium cobalt pyrophosphate nano sheet composite electrode material, comprising the following steps: S1, Na:Co:Fe:P = 2: (1-x):x:2 (wherein x is 0.05 to 0.20, and preferably 0.1) NaH2PO4·2H2O, Co(NO3)2·4H2O, Fe(NO3)3·9H2O, and citric acid are weighed according to the molar ratio; the raw materials are dissolved in deionized water to obtain a metal precursor solution; the molar ratio of the citric acid to the sum of the metal ions (Co 2+ and Fe 3+ ) is 1:1.
[0008] S2, graphene dispersion liquid is added dropwise to the above metal precursor solution, and stirred uniformly to obtain a mixed solution; Preferably, the graphene dispersion liquid in step S2 is prepared by adding single-layer graphene into ethylene glycol and ultrasonic treatment, wherein the mass-volume ratio (mg:mL) of graphene to ethylene glycol is 0.5:1 to 3:1. More preferably, the mass-volume ratio (mg:mL) of graphene to ethylene glycol is 1:1.
[0009] S3, the mixed solution is heated in an oil bath at 70-90°C for 1-4 hours to form a gel; preferably, the mixed solution is transferred to an 80°C oil bath and heated for 2 hours to form a gel.
[0010] S4, after the gel is cooled, self-propagating combustion is initiated at 180-250°C to obtain fluffy precursor powder; preferably, the reaction temperature is 200°C.
[0011] S5, the precursor powder is mixed with deionized water and placed in a reaction kettle, and hydrothermal reaction is carried out at 160-200°C for 8-16 hours; preferably, after reaction at 180°C for 12 hours, the reaction is cooled.
[0012] S6, after washing and drying the hydrothermal reaction product, heat to 450-550℃ at a rate of 2-5℃ / min under a protective atmosphere, and calcine for 2-5 hours, to obtain a graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material; preferably, heat to 500℃ at a rate of 5℃ / min under a nitrogen atmosphere, and calcine for 3 hours.
[0013] Preferably, the mass fraction of graphene in the final product of the composite material is 10 wt%.
[0014] In another aspect, the present application provides a graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material prepared by the above method, having the chemical formula Na2Co 1-x Fe x P2O7 / graphene, wherein 0.05 ≤ x ≤ 0.20.
[0015] Compared with the prior art, the present application has the following beneficial effects: (1) The present application creatively combines iron doping, graphene compounding, and a hydrothermal pre-forming process. Iron doping (x = 0.05-0.20) causes moderate lattice expansion, optimizing the sodium ion diffusion channel; the hydrothermal step pre-directs the formation of a nanosheet structure under relatively mild conditions, so that the subsequent calcination can be completed at a lower temperature (450-550℃), effectively avoiding abnormal grain growth and agglomeration.
[0016] (2) The addition of graphene not only provides a high-speed electron conduction path, but also produces a significant interface synergistic effect between the graphene and the iron-doped nanosheet. This effect can effectively inhibit the stacking and structural collapse of the nanosheet during long-term cycling, thereby achieving dual regulation of the material's crystal structure stability and microscopic morphology at the molecular scale.
[0017] (3) The synergistic effect enables the material to exhibit excellent comprehensive electrochemical performance. Experiments show that when the graphene addition amount is 5-15 wt%, the specific capacity of the composite material exceeds 120 mAh / g at a current density of 0.5 A / g, the sodium ion diffusion coefficient is increased to the order of 10 -10 cm 2 / s, and the capacity retention rate after 10,000 cycles is still as high as 92% or more, which is far superior to undoped Fe, non-composite graphene, or process-different comparative samples. BRIEF DESCRIPTION OF DRAWINGS
[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the following embodiment or prior art description will be briefly introduced. Obviously, for those skilled in the art, other drawings can also be obtained without creative labor.
[0019] Figure 1 XRD pattern of Fe-doped Na2CoP2O7 material prepared for Example 1; Figure 2 Scanning electron microscope (SEM) image of Fe-doped Na2CoP2O7 material prepared for Example 4. DETAILED DESCRIPTION
[0020] In order to make the objects, features, and advantages of the present application more apparent and easy to understand, the technical solutions in the present application will be described clearly and completely below in connection with specific embodiments. Obviously, the embodiments described below are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present patent, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present patent.
[0021] The raw materials in the embodiments can be obtained from the market, and specific details are shown in Table 1. Table 1. Raw material table Material name Chemical formula Purity Supplier Single-layer graphene C >99% Alfa-Aesar Ethylene glycol [C2H6O2] >99.5% Merck Sodium dihydrogen phosphate NaH2PO4.2H2O >99.9 % Merck Cobalt nitrate Co(NO3)2.4H2O >99.9 % Himedia Iron nitrate Fe(NO3) 3· 9H2O]]> >99.9 % Sigma-Aldrich Citric acid [C6H8O7] >98% Merck Deionized water H2O - - Example 1: Graphene / iron-doped sodium cobalt pyrophosphate nanosheet (x = 0.10, graphene 10 wt%) Preparation steps: (1) 373 mg of single-layer graphene was added to 373 mL of ethylene glycol, and ultrasonic treatment was performed at 300 W for 30 minutes to form a dispersion liquid. (2) According to a molar ratio of Na:Co:Fe:P = 2:0.9:0.1:2, 3.41 g of NaH2PO4·2H2O, 2.46 g of Co(NO3)2·4H2O, 0.404 g of Fe(NO3)3·9H2O, and 2.29 g of citric acid (molar ratio 1:1 with Co 2+ + Fe 3+ ) were weighed and dissolved in 30 mL of deionized water. (3) The graphene dispersion liquid was added dropwise to the above solution, and stirred for 1 hour. (4) The gel was formed by heating in an 80°C oil bath for 2 hours. (5) After cooling, self-spread combustion was initiated at 200°C to obtain a precursor powder. (6) The precursor was mixed with 40 mL of deionized water, and hydrothermal reaction was performed at 180°C for 12 hours. (7) After washing the product, calcination was performed at 500°C for 3 hours under a N2 atmosphere at a rate of 5°C / min to obtain the final composite electrode material.
[0022] Example 2: Iron doping amount optimization (x = 0.15, graphene 10 wt%) Preparation procedure: (1) 371 mg of single-layer graphene was weighed into 371 mL of ethylene glycol and sonicated for 30 minutes at 300 W to form a dispersion. (2) 3.41 g of NaH2PO4·2H2O, 2.17 g of Co(NO3)2·4H2O, 0.606 g of Fe(NO3)3·9H2O, and 2.29 g of citric acid (in a 1:1 molar ratio with Co 2+ + Fe 3+ ) were dissolved in 30 mL of deionized water. (3) Subsequent steps were performed according to steps (3)-(7) of Example 1.
[0023] Example 3: Optimization of graphene addition amount (x = 0.10, graphene 5 wt%) Preparation procedure: (1) 168 mg of single-layer graphene was weighed into 168 mL of ethylene glycol and sonicated for 30 minutes at 300 W to form a dispersion. (2) 3.41 g of NaH2PO4·2H2O, 2.46 g of Co(NO3)2·4H2O, 0.404 g of Fe(NO3)3·9H2O, and 2.29 g of citric acid (in a 1:1 molar ratio with Co 2+ + Fe 3+ ) were dissolved in 30 mL of deionized water. (3) Subsequent steps were performed according to steps (3)-(7) of Example 1.
[0024] Example 4: Optimization of graphene addition amount (x = 0.10, graphene 15 wt%) Preparation procedure: (1) 592 mg of single-layer graphene was weighed into 592 mL of ethylene glycol and sonicated for 30 minutes at 300 W to form a dispersion. (2) 3.41 g of NaH2PO4·2H2O, 2.46 g of Co(NO3)2·4H2O, 0.404 g of Fe(NO3)3·9H2O, and 2.29 g of citric acid (in a 1:1 molar ratio with Co 2+ + Fe 3+ ) were dissolved in 30 mL of deionized water. (3) Subsequent steps were performed according to steps (3)-(7) of Example 1.
[0025] Comparative Example 1: Pure-phase Na2CoP2O7 (undoped, uncomplexed) Preparation procedure: (1) No graphene and graphene dispersion were added. (2) 3.41 g NaH2PO4·2H2O, 2.73 g Co(N03)2·4H2O, and 2.29 g citric acid (with Co 2+ molar ratio 1:1) were dissolved in 30 mL deionized water. (3) The subsequent steps were performed according to the procedure of Example 1, steps (3)-(7).
[0026] Comparative Example 2: Graphene only (5 wt% graphene, no iron doping) Preparation procedure: (1) 168 mg single-layer graphene was weighed into 168 mL ethylene glycol and ultrasonicated for 30 minutes at 300 W to form a dispersion. (2) 3.41 g NaH2PO4·2H2O, 2.73 g Co(N03)2·4H2O, and 2.29 g citric acid (with Co 2+ molar ratio 1:1) were dissolved in 30 mL deionized water. (3) The subsequent steps were performed according to the procedure of Example 1, steps (3)-(7).
[0027] Comparative Example 3: Iron only (x = 0.10, no graphene composite) Preparation procedure: (1) No graphene and graphene dispersion were added. (2) 3.41 g NaH2PO4·2H2O, 2.46 g Co(N03)2·4H2O, 0.404 g Fe(N03)3·9H2O, and 2.29 g citric acid (with Co 2+ + Fe 3+ molar ratio 1:1) were dissolved in 30 mL deionized water. (3) The subsequent steps were performed according to the procedure of Example 1, steps (3)-(7).
[0028] Comparative Example 4: Too low Fe doping amount (x = 0.02, 5 wt% graphene) Preparation procedure: (1) 159 mg single-layer graphene was weighed into 159 mL ethylene glycol and ultrasonicated for 30 minutes at 300 W to form a dispersion. (2) 3.41 g NaH2PO4·2H2O, 2.68 g Co(N03)2·4H2O, 0.081 g Fe(N03)3·9H2O, and 2.29 g citric acid (with Co 2+ + Fe 3+(3) The molar ratio is 1:1), and it is dissolved in 30 mL of deionized water. (4) Subsequent steps are carried out in accordance with steps (3)-(7) of Example 1.
[0029] Comparative Example 5: Excessive Fe doping (x=0.25, graphene 5 wt%) Preparation steps: (1) Weigh 148 mg of monolayer graphene and add it to 148 mL of ethylene glycol. Sonicate at 300 W for 30 minutes to form a dispersion. (2) Weigh 3.41 g of NaH2PO4·2H2O, 2.05 g of Co(NO3)2·4H2O, 1.010 g of Fe(NO3)3·9H2O, and 2.29 g of citric acid (with Co) according to the molar ratio of Na:Co:Fe:P = 2:0.75:0.25:2. 2+ +Fe 3+ (3) The molar ratio is 1:1), and it is dissolved in 30 mL of deionized water. (4) Subsequent steps are carried out in accordance with steps (3)-(7) of Example 1.
[0030] Comparative Example 6: The amount of graphene added was too low (x=0.10, 2 wt% graphene). Preparation steps: (1) Weigh 65 mg of monolayer graphene and add it to 65 mL of ethylene glycol. Sonicate at 300 W for 30 minutes to form a dispersion. (2) Weigh 3.41 g of NaH2PO4·2H2O, 2.46 g of Co(NO3)2·4H2O, 0.404 g of Fe(NO3)3·9H2O, and 2.29 g of citric acid (with Co) according to the molar ratio of Na:Co:Fe:P = 2:0.9:0.1:2. 2+ +Fe 3+ (3) The molar ratio is 1:1), and it is dissolved in 30 mL of deionized water. (4) Subsequent steps are carried out in accordance with steps (3)-(7) of Example 1.
[0031] Comparative Example 7: Graphene addition was too high (x=0.10, graphene 20 wt%) Preparation steps: (1) Weigh 894 mg of monolayer graphene and add it to 894 mL of ethylene glycol. Sonicate at 300 W for 30 minutes to form a dispersion. (2) Weigh 3.41 g of NaH2PO4·2H2O, 2.46 g of Co(NO3)2·4H2O, 0.404 g of Fe(NO3)3·9H2O, and 2.29 g of citric acid (with Co) according to the molar ratio of Na:Co:Fe:P = 2:0.9:0.1:2. 2+ +Fe 3+ (3) The molar ratio is 1:1), and it is dissolved in 30 mL of deionized water. (4) Subsequent steps are carried out in accordance with steps (3)-(7) of Example 1.
[0032] Example 5: Performance Testing and Comparative Analysis All samples were tested using a uniform standard: Phase analysis: D8 Advance X-ray diffractometer (Cu-Kα radiation, λ=0.1542 nm, scanning range 10) 0 -80 0 Microstructure: SU8010 field emission scanning electron microscope (SEM, accelerating voltage 5 kV). Graphene content determination: NETZSCH STA 449F3 simultaneous thermal analyzer. Approximately 10 mg of dry sample was taken and heated from room temperature to 800 °C at a rate of 10 °C / min in air. The TGA curve was recorded, and the mass fraction (wt%) of graphene in the composite material was determined by calculating the percentage of mass loss in the 600 °C to 800 °C range. The calculation formula is as follows: Graphene content (wt%) = Each sample was tested in parallel three times, and the average value was taken.
[0033] Electrochemical performance: Working electrode preparation: Active material:PVDF:Super P = 8:1:1 (mass ratio), coated on copper foil, and vacuum dried for 12 hours. Battery assembly: CR2032 button cell, sodium metal sheet as counter electrode, 1 M NaOH aqueous solution as electrolyte, Celgard 2400 as separator. Testing equipment: Chenhua CHI660E electrochemical workstation (cyclic voltammetry, electrochemical impedance spectroscopy), LAND CT2001A battery testing system (constant current charge / discharge, current density 0.5 A / g, voltage range 1.5-4.2 V).
[0034] For detailed test results, please refer to [link / reference]. Figure 1-2 And Table 2. Figure 1 The image shows the XRD pattern of the Fe-doped Na2CoP2O7 material prepared in Example 1, which shows that the product has a triclinic Na2CoP2O7 structure and is a perfect match for the standard card JCPDS No. 80-2409.
[0035] Table 2 Performance Test Data Sample Material composition (x is the amount of Fe doping) Graphene addition amount Graphene content (TGA) (wt%) Key structural characterization Specific capacity (0.5 A / g) Cycle retention rate (10000 times) Sodium ion diffusion coefficient (cm 2 / s) Comparative Example 1 pure Na2CoP207 (x = 0) 0 wt% 0.0 ± 0.1 Pure phase triclinic system 104.2 ± 8 88% 4.0 x 10 -11 ]]> Comparative Example 2 Na2CoP2O7 (x=0) 5 wt% 4.5 ± 0.3 Structurally complete, slightly lower crystallinity 108.5 ± 6 89% 6.5 × 10 -11 ]]> Comparative Example 3 Na2Co 0.9 Fe 0.1 P2O7]]> 0 wt% 0.2 ± 0.2 Peak broadening, indicating agglomeration 112.3 ± 7 87% 5.8 x 10 -11 ]]> Example 1 Na2Co 0.9 Fe 0.1 P2O7]]> 10 wt% 9.1 ± 0.4 Pure phase triclinic crystal system Figure 1 ) 123.7 ± 4 93% 1.3 x 10 -10 ]]> Example 2 Na2Co 0.85 Fe 0.15 P2O7]]> 10 wt% 9.0 ± 0.5 Good triclinic system structure 119.2 ± 6 91% 1.1 x 10 -10 ]]> Example 3 Na2Co 0.9 Fe 0.1 P2O7]]> 5 wt% 4.4 ± 0.3 Structurally complete, uniformly dispersed 121.4 ± 5 92% 1.2 x 10 -10 ]]> Example 4 Na2Co 0.9 Fe 0.1 P2O7]]> 15 wt% 13.2 ± 0.6 The structure is complete, SEM shows slight agglomeration Figure 2 ) 116.7 ± 5 90% 9.8 x 10 -11 ]] Comparative Example 4 Na2Co 0.98 Fe 0.02 P2O7]]> 5 wt% 4.3 ± 0.4 Poor crystallinity 105.3 ± 7 86% 4.8 x 10 -11 ]] Comparative Example 5 Na2Co 0.75 Fe 0.25 P2O7]]> 5 wt% 4.6 ± 0.5 XRD shows impurity peaks 98.6 ± 9 80% 5.2 x 10 -11 ]]> Comparative Example 6 Na2Co 0.9 Fe 0.1 P2O7]]> 2 wt% 1.7 ± 0.2 Poor dispersibility, agglomeration 110.2 ± 6 87% 6.0 x 10 -11 ]] Comparative Example 7 Na2Co 0.9 Fe 0.1 P2O7]]> 20 wt% 17.8 ± 0.8 Severe agglomeration 108.5 ± 8 85% 5.5 x 10 -11 ]]> Based on the above test data, we can find that: (1) The necessity and synergistic effect of Fe doping combined with graphene: Limitations of individual effects: Doping with only iron (Comparative Example 3) or adding only graphene (Comparative Example 2), although slightly improving the specific capacity or diffusion coefficient compared to the pure phase material (Comparative Example 1), the improvement is limited, and the improvement in cycle stability is not significant. Significant synergistic effect: When Fe doping (x=0.10) is combined with an appropriate amount of graphene (e.g., 5-15 wt%) (Examples 1, 3, 4), the overall performance of the material achieves a leap forward. In particular, the sodium ion diffusion coefficient jumps to 10. -10 cm 2 The efficiency is on the order of / s, more than twice that of a single improvement scheme. Simultaneously, both specific capacity and cycle retention are optimal. This demonstrates that Fe doping (optimizing crystal channels) and graphene (constructing a conductive network) produce a synergistic effect of "1+1>2".
[0036] (2) Optimization range of key parameters Fe doping amount (x): The optimal range is 0.05 ≤ x ≤ 0.20. When x is too low (0.02, Comparative Example 4), the performance improvement is negligible; when x is too high (0.25, Comparative Example 5), it will destroy the crystal structure and cause the performance to deteriorate sharply.
[0037] Graphene addition amount: The optimal range is 5 wt% ≤ addition amount ≤ 15 wt%. When the addition amount is too low (2 wt%, comparative example 6), an effective conductive network cannot be formed; when the addition amount is too high, it will cause agglomeration. Figure 2 The image shown is a scanning electron microscope (SEM) image of the Fe-doped Na2CoP2O7 material prepared in Example 4. The structure is intact, but the SEM shows slight agglomeration, which has affected some performance. When the amount added is too high (20 wt%, Comparative Example 7), it will cause severe agglomeration, shielding the active sites and reducing performance.
[0038] (3) The relationship between structure and performance Structure determines performance: The broadening and agglomeration of the XRD peaks in Comparative Example 3 directly resulted in a low cycle retention rate; Example 3, due to its uniform dispersion, achieved a higher specific capacity and cycle retention rate.
[0039] In summary, the combination of Fe doping amount x=0.10 and graphene addition amount of 10 wt% (Example 1) showed the most balanced and excellent performance in this test, achieving a specific capacity (123.7 mAh / g), cycle life (93% retention), and ion diffusion rate (1.3×10⁻⁶). -10 cm 2 / s).
[0040] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. A method for preparing a graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material for sodium-ion batteries, characterized in that, Includes the following steps: S1. Take NaH2PO4·2H2O, Co(NO3)2·4H2O, Fe(NO3)3·9H2O, and citric acid; dissolve them in deionized water to obtain a metal precursor solution; The molar ratio of Na:Co:Fe:P in the raw materials is 2:(1-x):x:2, where x is 0.05 to 0.20; the molar ratio of citric acid to the total metal ions is 1:
1. S2. The graphene dispersion is added dropwise to the above metal precursor solution to obtain a mixed solution; S3. Heat the mixed solution to form a gel; S4. After cooling the formed gel, heat it up to initiate self-propagating combustion to obtain the precursor powder. S5. Mix the precursor powder with deionized water, and cool after the reaction. S6. After cooling and washing the product, it is calcined under a protective atmosphere to finally obtain the graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material.
2. The method according to claim 1, characterized in that, The amount of graphene added in the composite electrode material is 5 wt% to 15 wt%.
3. The method according to claim 1, characterized in that, In S2, the graphene dispersion is prepared as follows: monolayer graphene is added to ethylene glycol and ultrasonically treated to form a uniform and stable graphene dispersion. The mass-to-volume ratio of graphene to ethylene glycol (mg:ml) is 0.5:1 to 3:
1.
4. The method according to claim 1, characterized in that: In S3, the mixed solution is transferred to an oil bath at 70-90℃ and heated for 1-4 hours to form a gel.
5. The method according to claim 1, characterized in that: In S4, after the formed gel is cooled, the temperature is raised to 180-250℃ to initiate self-propagating combustion, thus obtaining the precursor powder.
6. The method according to claim 1, characterized in that: In S5, the precursor powder is mixed with deionized water and reacted at 160-200℃ for 8-16 hours, then cooled.
7. The method according to claim 1, characterized in that: In S6, the temperature is increased to 450-550℃ and calcined for 2-5 hours under a nitrogen atmosphere at a heating rate of 2-5℃ / min.
8. The method according to claim 1, characterized in that: Includes the following steps: S1. Take NaH2PO4·2H2O, Co(NO3)2·4H2O, Fe(NO3)3·9H2O, and citric acid; dissolve them in deionized water to obtain a metal precursor solution; The molar ratio of Na:Co:Fe:P in the raw materials is 2:(1-x):x:2, where x is 0.1; the molar ratio of citric acid to the total metal ions is 1:
1. S2. The graphene dispersion is added dropwise to the above metal precursor solution to obtain a mixed solution. The mass-to-volume ratio of graphene to ethylene glycol in the graphene dispersion is 1:1 (mg:mL). S3. Transfer the mixed solution to an 80°C oil bath and heat for 2 hours to form a gel; S4. After cooling the formed gel, raise the temperature to 200°C to initiate self-propagating combustion and obtain the precursor powder. S5. Mix the precursor powder with deionized water, react at 180°C for 12 hours, and then cool. S6. After cooling and washing the product, it is heated to 500℃ and calcined for 3 hours under a nitrogen atmosphere at a heating rate of 5℃ / min to finally obtain graphene-supported Fe-doped Na2CoP2O7 nanosheets.
9. The graphene / iron-doped sodium cobalt pyrophosphate nanosheet composite electrode material obtained by the method according to any one of claims 1-8, wherein the chemical formula is Na₂Co 1-x Fe x P2O7 / graphene, where 0.05 ≤ x ≤ 0.20.