Binder-free transition metal sulfide composite nitrogen-doped graphene negative electrode material and preparation method thereof
By designing a core-shell structure for a binder-free transition metal sulfide composite nitrogen-doped graphene anode material, the conductivity and stability issues of lithium-ion battery anode materials were solved, achieving high-efficiency electrochemical energy storage performance and long-life battery performance.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- ZIBO TORCH ENERGY
- Filing Date
- 2026-01-23
- Publication Date
- 2026-05-26
AI Technical Summary
Existing lithium-ion battery anode materials, such as graphite, have low Na+/Li+ storage capacity, while transition metal sulfides have low conductivity and significant volume effects, leading to electrode instability. Therefore, it is necessary to improve the overall performance and stability of secondary batteries.
A binderless transition metal sulfide composite nitrogen-doped graphene anode material is adopted. Through core-shell structure design, nickel foam is used as the substrate, nitrogen-doped graphene is used as the intermediate layer, and ternary transition metal sulfide is used as the shell to form a composite material, which provides high conductivity and catalytic activity. Combined with the high specific surface area and flexible structure of nitrogen-doped graphene, the lithium-ion transport efficiency and mechanical stability are improved.
It improves the battery's charge and discharge efficiency, lithium storage capacity, mechanical strength, and cycle life, reduces interface impedance, enhances electrode stability and cost-effectiveness, and is suitable for large-scale production.
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of secondary battery anode material technology, specifically relating to a binder-free transition metal sulfide composite nitrogen-doped graphene anode material and its preparation method. Background Technology
[0002] With the depletion of fossil fuels and increasingly serious environmental problems, improving the storage and utilization of renewable energy has become a key issue restricting human development. As a highly efficient electro-chemical energy conversion device, secondary batteries have become an indispensable energy storage and supply device for human society. Efforts are being made to develop secondary batteries based on lithium-ion batteries to meet human society's requirements for secondary batteries in terms of energy density, rate performance, cycle life, safety, and cost.
[0003] Graphite is a commercially available anode material for lithium-ion batteries, but its sodium storage... + / Li + Low capacity is a significant drawback. Modifying carbon-based materials with heteroatoms is crucial for achieving high capacity and high reversibility in energy storage. Furthermore, transition metal sulfides are considered potential electrode materials due to their high specific capacity. However, sulfides exhibit low conductivity and significant volume effects, leading to electrode instability. Therefore, structural modification and alteration are necessary. Consequently, a composite anode material for secondary batteries needs to be developed to improve the overall performance and stability of secondary batteries. Summary of the Invention
[0004] This invention provides a binder-free transition metal sulfide composite nitrogen-doped graphene anode material and its preparation method, which can improve the overall performance and stability of secondary batteries.
[0005] The technical solution of this invention is as follows: In the first aspect, a method for preparing binder-free transition metal sulfide composite nitrogen-doped graphene anode materials is disclosed, including the following steps: 1) Dissolve zinc nitrate, cobalt nitrate and nitrogen-doped graphene in a mixed solution of ethanol and water, mix evenly, then add ammonium fluoride and urea and continue stirring until evenly mixed; 2) Add the nickel foam and the above-mentioned uniformly mixed solution into a high-pressure reactor for hydrothermal treatment, cooling, washing, and drying to obtain the Zn-Co precursor; 3) The Zn-Co precursor was then calcined under an inert atmosphere to obtain ZnCo2O4@NGr / NF; 4) The prepared ZnCo2O4@NGr / NF is added to a high-pressure reactor containing a sulfur source for hydrothermal sulfidation treatment, washed with isopropanol and deionized water, and dried in a vacuum oven to obtain the binder-free transition metal sulfide composite nitrogen-doped graphene anode material ZnCo2S4@NGr / NF.
[0006] Preferably, the mass ratio of zinc nitrate, cobalt nitrate, and nitrogen-doped graphene is 1:(1-2):(3-6).
[0007] Preferably, the volume ratio of ethanol to water is (1-2):(1-2).
[0008] Preferably, the mass ratio of ammonium fluoride to urea is (0.4-1):1; and the mass ratio of zinc nitrate to ammonium fluoride is 1:(2-5).
[0009] Preferably, the nitrogen-doped graphene is prepared by: dry ball milling graphene with melamine, calcining the milled graphene under an argon atmosphere, and cooling to obtain nitrogen-doped graphene.
[0010] Preferably, the mass ratio of graphene to melamine is 1:(3-5), and the calcination is carried out at 300-600℃ for 3-6 hours under an argon atmosphere.
[0011] Preferably, in step 2), the hydrothermal reaction is carried out in a high-pressure reactor with a polytetrafluoroethylene liner at 130-150°C for 4-6 hours.
[0012] Preferably, in step 3), the calcination is carried out in a muffle furnace under an argon or nitrogen atmosphere at 350-400℃ for 2-4 hours, with a heating rate of 2-5℃·min. -1 .
[0013] Preferably, the sulfur source in step 4) is sodium sulfide, and the hydrothermal sulfidation treatment in step 4) is carried out at 130-160℃ for 8-12 hours.
[0014] Secondly, the preparation method described above discloses a binder-free transition metal sulfide composite nitrogen-doped graphene anode material.
[0015] Compared with the prior art, the present invention has the following advantages: 1. The transition metal sulfide composite nitrogen-doped graphene anode material of this application is a "core-shell structure" composite material. Nickel foam serves as the substrate material, providing a large specific surface area and good conductivity; nitrogen-doped graphene acts as the intermediate layer, connecting and stabilizing the catalyst; and ternary transition metal sulfides serve as the outer shell, providing catalytic activity. This structure combines the high conductivity and stability of nitrogen-doped graphene with the excellent catalytic activity provided by ternary transition metal sulfides. This core-shell structure design can effectively improve the overall performance of the material, especially showing excellent application potential in the field of electrochemical energy storage.
[0016] 2. The nitrogen-doped graphene and ternary transition metal sulfides of the present invention have good conductivity, which can promote the rapid transport of lithium ions in the electrode, reduce polarization, and improve the charging and discharging efficiency of the battery.
[0017] 3. The nitrogen-doped graphene of the present invention has a high specific surface area, which can provide more lithium storage active sites, thereby improving the lithium storage capacity of the battery; the nitrogen-doped graphene has ultra-high conductivity, which can effectively reduce the interface impedance; the hydrophobic surface of NGr can reduce electrolyte decomposition, and the generated SEI film is mainly composed of LiF to stabilize the formation of the SEI film.
[0018] 4. The three-dimensional conductive network of NGr in this invention, synergistically with the porous structure of nickel foam, enables dual continuous electron / ion transport. The three-dimensional porous structure of the nickel foam substrate provides a high specific surface area, promoting electrolyte wetting and active material loading. Direct growth of active materials avoids the use of binders, reducing interfacial contact resistance. The three-dimensional network structure of nickel foam provides excellent mechanical support for the composite electrode, enhancing its overall mechanical strength and stability. By providing a stable support structure, it prevents volume expansion or pulverization of the electrode material during charge and discharge, thereby improving the battery's cycle life. This highly conductive three-dimensional network structure significantly improves electron transfer kinetics. This structure not only increases the loading of active materials but also effectively prevents pulverization and agglomeration of active materials, thus improving the overall performance of the battery.
[0019] 5. The Zn of the present invention 2+ and Co 3+ Through Zn 2+ / Zn 3+ and Co 3+ / Co 4+ The multi-electron redox reaction has a theoretical capacity greater than 1000 mAh / g, and the Zn / Co bimetal exhibits a synergistic effect.
[0020] 6. The ZnCo2S4 of the present invention is prone to volume expansion during charging and discharging, while the flexible coating layer of NGr buffers stress through elastic deformation, prevents the active material from cracking and falling off, and significantly extends the cycle life.
[0021] 7. The raw materials used in this invention, such as graphene and transition metal sulfides, are relatively low-cost materials. Combining them with nickel foam can further improve the cost-effectiveness of the material, making it suitable for large-scale production and application. Detailed Implementation
[0022] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions will be clearly and completely described below in conjunction with the embodiments of this invention.
[0023] Example 1 Preparation of nitrogen-doped graphene (NGr): First, 10g of graphene and 50g of melamine were placed in a ball mill jar and dry-milled under an argon atmosphere. Stainless steel balls were used as grinding balls, with a ball-to-material mass ratio of 5:1. The milling speed was 450 rpm, and the milling time was 10 h. The milled dry mixture was then removed and placed in a muffle furnace. Under an inert argon atmosphere, it was calcined at 600℃ for 3 h at a heating rate of 2℃·min. -1 Then cool to 25°C at a cooling rate of 2°C·min. -1 NGr powder was obtained; Pretreatment of nickel foam: First, the cut nickel foam (85mm×98mm) was ultrasonically treated with 3mol / L HCl for 20min. Then, it was taken out, centrifuged and washed with ethanol and deionized water respectively, and dried at 60℃ for later use.
[0024] The method for preparing the binder-free transition metal sulfide composite nitrogen-doped graphene anode material includes the following steps: 1) Add 0.6g zinc nitrate, 1.2g cobalt nitrate, and 1.8g nitrogen-doped graphene to 80mL ethanol and 80mL water. Stir the solution magnetically for 30min, then sonicate for 30min. Then add 3g urea and 1.2g ammonium fluoride to the clear solution and continue stirring for 20min.
[0025] 2) The above reaction solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor, and a piece of nickel foam pretreated with HCl was placed inside. The high-pressure reactor was hydrothermally heated at 140°C for 6 hours. After cooling, the nickel foam was washed three times with deionized water and finally dried in a vacuum oven at 80°C for 12 hours to obtain the Zn-Co precursor.
[0026] 3) Then, the Zn-Co precursor is placed in a muffle furnace under an Ar atmosphere at 2℃·min. -1 The ZnCo2O4 nanowire structure (ZnCo2O4@NGr / NF) with composite nitrogen-doped graphene loaded on nickel foam was obtained by annealing at 350℃ for 4 h.
[0027] 4) Dissolve 0.48 g of sodium sulfide in 160 mL of deionized water under magnetic stirring to obtain a clear solution. Then transfer the clear solution to a high-pressure reactor and immerse a ZnCo2O4@NGr / NF anode material in it. React at 130 °C for 12 h, then wash the product three times with isopropanol and deionized water, and finally dry it in a vacuum oven at 110 °C for 10 h to obtain the target product, ZnCo2S4@NGr / NF anode material.
[0028] Example 2 Preparation of nitrogen-doped graphene (NGr): First, 10g of graphene and 40g of melamine were placed in a ball mill jar and dry-milled under an argon atmosphere. Stainless steel balls were used as grinding balls, with a ball-to-material mass ratio of 5:1. The milling speed was 450 rpm, and the milling time was 10 h. The milled dry mixture was then removed and placed in a muffle furnace. Under an inert argon atmosphere, it was calcined at 500℃ for 4 h, with a heating rate of 2℃·min. -1 Then cool to 25°C at a cooling rate of 2°C·min. -1 NGr powder was obtained; Pretreatment of nickel foam: First, the cut nickel foam (85mm×98mm) was ultrasonically treated with 3mol / L HCl for 20min. Then, it was taken out, centrifuged and washed with ethanol and deionized water respectively, and dried at 60℃ for later use.
[0029] The method for preparing the binder-free transition metal sulfide composite nitrogen-doped graphene anode material includes the following steps: 1) Dissolve 0.6g zinc nitrate, 0.6g cobalt nitrate, and 1.8g nitrogen-doped graphene in 80mL ethanol and 160mL water. Stir the solution magnetically for 30min, then sonicate for 30min. Add 3g urea and 1.8g ammonium fluoride to the clear solution and continue stirring for 20min.
[0030] 2) The above reaction solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor, and a piece of nickel foam pretreated with HCl was placed inside. The high-pressure reactor was hydrothermally heated at 140°C for 4 hours. After cooling, the nickel foam was washed three times with deionized water and finally dried in a vacuum oven at 80°C for 12 hours to obtain the Zn-Co precursor.
[0031] 3) Then, the Zn-Co precursor is placed in a muffle furnace under a N2 atmosphere at 3℃·min. -1 The ZnCo2O4 nanowire structure (ZnCo2O4@NGr / NF) with composite nitrogen-doped graphene loaded on nickel foam was obtained by annealing at 380℃ for 3 h.
[0032] 4) Dissolve 0.48 g of sodium sulfide in 160 mL of deionized water under magnetic stirring to obtain a clear solution. Then transfer the clear solution to a high-pressure reactor and immerse a ZnCo2O4@NGr / NF anode material in it. React at 140 °C for 10 h, then wash the product three times with isopropanol and deionized water, and finally dry it in a vacuum oven at 110 °C for 10 h to obtain the target product, ZnCo2S4@NGR / NF anode material.
[0033] Example 3 Preparation of nitrogen-doped graphene (NGr): First, 10g of graphene and 40g of melamine were placed in a ball mill jar and dry-milled under an argon atmosphere. Stainless steel balls were used as grinding balls, with a ball-to-material mass ratio of 5:1. The milling speed was 450 rpm, and the milling time was 10 h. The milled dry mixture was then removed and placed in a muffle furnace. Under an inert argon atmosphere, it was calcined at 400℃ for 5 h, with a heating rate of 2℃·min. -1 Then cool to 25°C at a cooling rate of 2°C·min. -1 NGr powder was obtained; Pretreatment of nickel foam: First, the cut nickel foam (85mm×98mm) was ultrasonically treated with 3mol / L HCl for 20min. Then, it was taken out, centrifuged and washed with ethanol and deionized water respectively, and dried at 60℃ for later use.
[0034] The method for preparing the binder-free transition metal sulfide composite nitrogen-doped graphene anode material includes the following steps: 1) Dissolve 0.6g zinc nitrate, 0.9g cobalt nitrate, and 3g nitrogen-doped graphene in 160mL ethanol and 80mL water. Stir the solution magnetically for 30min, then sonicate for 30min. Add 3g urea and 2.4g ammonium fluoride to the clear solution and continue stirring for 20min.
[0035] 2) The above reaction solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor, and a piece of nickel foam pretreated with HCl was placed inside. The high-pressure reactor was hydrothermally heated at 140°C for 5 hours. After cooling, the nickel foam was washed three times with deionized water and finally dried in a vacuum oven at 80°C for 12 hours to obtain the Zn-Co precursor.
[0036] 3) Then, the Zn-Co precursor is placed in a muffle furnace under an Ar atmosphere at 4℃·min. -1 The ZnCo2O4 nanowire structure (ZnCo2O4@NGr / NF) with composite nitrogen-doped graphene loaded on nickel foam was obtained by annealing at 400℃ for 2 h.
[0037] 4) Dissolve 0.48 g of sodium sulfide in 160 mL of deionized water under magnetic stirring to obtain a clear solution. Then transfer the clear solution to a high-pressure reactor and immerse a ZnCo2O4@NGr / NF anode material in it. React at 150 °C for 9 h, then wash the product three times with isopropanol and deionized water, and finally dry it in a vacuum oven at 110 °C for 10 h to obtain the target product ZnCo2S4@NGr / NF anode material.
[0038] Example 4 Preparation of nitrogen-doped graphene (NGr): First, 10g of graphene and 30g of melamine were placed in a ball mill jar and dry-milled under an argon atmosphere. Stainless steel balls were used as grinding balls, with a ball-to-material mass ratio of 5:1. The milling speed was 450 rpm, and the milling time was 10 h. The milled dry mixture was then removed and placed in a muffle furnace. Under an inert argon atmosphere, it was calcined at 300℃ for 6 h, with a heating rate of 2℃·min. -1 Then cool to 25°C at a cooling rate of 2°C·min. -1 NGr powder was obtained; Pretreatment of nickel foam: First, the cut nickel foam (85mm×98mm) was ultrasonically treated with 3mol / L HCl for 20min. Then, it was taken out, centrifuged and washed with ethanol and deionized water respectively, and dried at 60℃ for later use.
[0039] The method for preparing the binder-free transition metal sulfide composite nitrogen-doped graphene anode material includes the following steps: 1) Dissolve 0.6g zinc nitrate, 1.2g cobalt nitrate, and 3.6g nitrogen-doped graphene in 160mL ethanol and 160mL water. Stir the solution magnetically for 30min, then sonicate for 30min. Add 3g urea and 3g ammonium fluoride to the clear solution and continue stirring for 20min.
[0040] 2) The above reaction solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor, and a piece of nickel foam pretreated with HCl was placed inside. The high-pressure reactor was hydrothermally heated at 140°C for 6 hours. After cooling, the nickel foam was washed three times with deionized water and finally dried in a vacuum oven at 80°C for 12 hours to obtain the Zn-Co precursor.
[0041] 3) Then, the Zn-Co precursor is placed in a muffle furnace under an Ar atmosphere at 5°C·min. -1 The ZnCo2O4 nanowire structure (ZnCo2O4@NGr / NF) with composite nitrogen-doped graphene loaded on nickel foam was obtained by annealing at 350℃ for 4 h.
[0042] 4) Dissolve 0.48 g of sodium sulfide in 160 mL of deionized water under magnetic stirring to obtain a clear solution. Then transfer the clear solution to an autoclave and immerse a ZnCo2O4@NGr / NF anode material in it. React at 160 °C for 8 h, then wash the product three times with isopropanol and deionized water, and finally dry it in a vacuum oven at 110 °C for 10 h to obtain the target product ZnCo2S4@NGr / NF anode material.
[0043] Comparative Example 1 Unlike Example 1, in this comparative example, "1.8g nitrogen-doped graphene" was replaced with "1.8g nitrogen-doped graphene oxide", and the resulting battery anode material was denoted as ZnCo2S4@NGO / NF.
[0044] The preparation method of nitrogen-doped graphene oxide is as follows: 5g of graphene and 10g of urea are dissolved in 250mL of water / ethanol (volume ratio 1:1) and sonicated for 30min. The solution is then transferred to a 100mL high-pressure reactor and hydrothermally reacted at 180℃ for 10h. After cooling, the solution is centrifuged, washed, and vacuum dried at 60℃ for 12h to obtain NGO.
[0045] Comparative Example 2 Unlike Example 1, in this comparative example, "1.8g nitrogen-doped graphene" was replaced with "1.8g commercially available graphene". The graphene was purchased from Qingdao Huagao Graphene Technology Co., Ltd. and was of analytical grade. The resulting battery anode material was denoted as ZnCo2S4@Gr / NF.
[0046] Comparative Example 3 Unlike Example 1, uncomposite nitrogen-doped graphene, i.e., ZnCo2S4@NF, is directly used as the negative electrode material of the battery. Its preparation method is as follows: 1) Add 0.6g of zinc nitrate and 1.2g of cobalt nitrate to 80mL of ethanol and 80mL of water. Stir the solution magnetically for 30min, then sonicate for 30min. Then add 3g of urea and 1.2g of ammonium fluoride to the clear solution and continue stirring for 20min.
[0047] 2) The above reaction solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor, and a piece of nickel foam pretreated with HCl was placed inside. The high-pressure reactor was hydrothermally heated at 140°C for 6 hours. After cooling, the nickel foam was washed three times with deionized water and finally dried in a vacuum oven at 80°C for 12 hours to obtain the Zn-Co precursor.
[0048] 3) Then, the Zn-Co precursor is placed in a muffle furnace under an Ar atmosphere at 2℃·min. -1The ZnCo2O4 nanowire structure (ZnCo2O4@NF) was obtained by annealing at 350℃ for 4 h at a rate of 0.
[0049] 4) Dissolve 0.48 g of sodium sulfide in 160 mL of deionized water under magnetic stirring to obtain a clear solution. Then transfer the clear solution to a high-pressure reactor and immerse a ZnCo2O4@NF anode material in it. React at 130 °C for 12 h, then wash the product three times with isopropanol and deionized water, and finally dry it in a vacuum oven at 110 °C for 10 h to obtain the target product, ZnCo2S4@NF anode material.
[0050] Comparative Example 4 Unlike Example 1, urea and ammonium fluoride were not added during the preparation process.
[0051] The preparation method includes the following steps: 1) Take 0.6g zinc nitrate, 1.2g cobalt nitrate and 1.8g nitrogen-doped graphene and add them to 80mL ethanol and 80mL water. Stir the solution under magnetic stirring for 30min.
[0052] 2) The above reaction solution was then transferred to a polytetrafluoroethylene-lined high-pressure reactor, and a piece of nickel foam pretreated with HCl was placed inside. The high-pressure reactor was hydrothermally heated at 140°C for 6 hours. After cooling, the nickel foam was washed three times with deionized water and finally dried in a vacuum oven at 80°C for 12 hours to obtain the Zn-Co precursor.
[0053] 3) Then, the Zn-Co precursor is placed in a muffle furnace under an Ar atmosphere at 2℃·min. -1 The ZnCo2O4 nanowire structure (ZnCo2O4@NGr) with composite nitrogen-doped graphene loaded on nickel foam was obtained by annealing at 350℃ for 4 h.
[0054] 4) Dissolve 0.48 g of sodium sulfide in 160 mL of deionized water under magnetic stirring to obtain a clear solution. Then transfer the clear solution to a high-pressure reactor and immerse a ZnCo2O4@NGr anode material in it. React at 130 °C for 12 h, then wash the product three times with isopropanol and deionized water, and finally dry it in a vacuum oven at 110 °C for 10 h to obtain the target product ZnCo2S4@NGr anode material.
[0055] Comparative Example 5 Unlike Example 1, zinc nitrate was not added during the preparation process; instead, an equal amount of nickel nitrate was added to obtain the NiCo2S4@NGr / NF anode material.
[0056] Comparative Example 6 Unlike Example 1, the preparation process does not include step 4) and directly uses a composite nitrogen-doped graphene ZnCo2O4 nanowire structure (ZnCo2O4@NGr / NF) loaded on nickel foam.
[0057] Comparative Example 7 Unlike Example 1, the amount of nitrogen-doped graphene added in this comparative example is 7.3g.
[0058] Comparative Example 8 Unlike Example 1, in step 4) of this comparative example, sodium sulfide is replaced with an equal mass of (NH4)2S.
[0059] Full cells were assembled using the materials prepared in the examples and comparative examples. The specific battery assembly process is as follows: lithium iron phosphate was used as the positive electrode, the prepared negative electrode material was used as the negative electrode, and 1 mol / L LiPF6 was used as the electrolyte.
[0060] The examples and comparative examples were tested by charging and discharging at a current of 0.1C in full cells.
[0061] The positive electrode sheets in the examples and comparative examples were subjected to AC impedance testing. A coin cell case was used, with lithium iron phosphate as the positive electrode and the prepared material as the negative electrode to assemble the coin cell. The results are shown in Table 1.
[0062] Table 1. Conductivity in Examples and Comparative Examples
[0063] As shown in Table 1, ZnCo2S4@NGr / NF has the highest conductivity, while ZnCo2S4 / NF has the lowest. The conductivity of ZnCo2S4@NGr / NF and ZnCo2S4@Gr / NF falls between these two. This is because nitrogen-doped graphene (NGr) exhibits the best conductivity, providing a three-dimensional conductive network, and nitrogen atoms enhance the interfacial coupling with ZnCo2S4. Undoped graphene has good conductivity but lacks the charge transport active sites associated with nitrogen doping. The oxygen-containing functional groups (such as -COOH and -OH) in nitrogen-doped graphene oxide (NGO) disrupt the sp² carbon network, thus reducing conductivity. ZnCo2S4 / NF, lacking graphene as an intermediate layer and without carbon material modification, has the worst conductivity and cannot function as a connecting and stabilizing catalyst, resulting in the worst performance among the four.
[0064] The batteries of the examples and comparative examples were charged and discharged, and the discharge capacity and capacity retention after 100 cycles were measured as shown in Table 2.
[0065] Table 2. Test results of discharge specific capacity and capacity retention rate for the examples and comparative examples.
[0066] As shown in Table 2, the discharge capacity and capacity retention rate of Comparative Example 1 after 100 cycles are both lower than those of Example 1. This is because when ZnCo2S4@NGr / NF is used as the anode material, the nitrogen-doped graphene retains complete sp[s]. 2 Carbon-based graphene networks exhibit significantly higher conductivity than NGO, and nitrogen doping further introduces electron donors, enhancing electron transfer capabilities. Nitrogen-doped graphene reduces electrode internal resistance and improves rate performance. Nitrogen-doped graphene oxide has defects; even after reduction, residual oxygen-containing groups such as carboxyl and epoxy groups remain on its surface, potentially leading to side reactions with the electrolyte (e.g., forming a thicker SEI film), increasing interfacial impedance, and reducing initial coulombic efficiency. NGr's inert surface lacks active oxygen-containing groups, reducing side reactions, promoting stable SEI formation, and improving cycle reversibility. NGr's flexible structure, with its complete graphene sheets, possesses higher mechanical strength, effectively buffering the volume changes of ZnCo2S4 during charge and discharge, preventing active material breakage. In contrast, the structural defects in NGO due to oxidation treatment may cause localized stress concentration, accelerating electrode pulverization. ZnCo2S4@NGr / NF exhibits higher cycle stability than NGO-based graphene. While the oxygen-containing groups in NGO can adsorb polysulfides (Li2S...),... x While physical adsorption is dominant, desorption can still occur during long-term cycling. NGr, on the other hand, has superior polysulfide confinement capabilities. The pyridine nitrogen and graphitic nitrogen in NGr can anchor polysulfides through stronger chemical bonds (such as Li–N), suppressing the "shuttle effect" and improving sulfur utilization.
[0067] Comparative Example 2 showed lower discharge capacity and capacity retention after 100 cycles compared to Example 1. This is because nitrogen atoms in NGr act as electron donors, increasing the carrier concentration in graphene and significantly improving conductivity. It also reduces electrode interface resistance, promotes the redox reaction kinetics of ZnCo2S4, and results in better rate performance. Nitrogen doping can introduce defect sites; nitrogen atoms and structural defects in NGr can serve as additional lithium storage active sites, further improving specific capacity. Furthermore, it exhibits stronger polysulfide confinement capability; nitrogen atom polysulfides (Li2S4) in NGr... x NGr forms strong chemical bonds (Li-N), suppressing the "shuttle effect" and reducing the loss of active material. Undoped Gr, on the other hand, relies solely on physical adsorption, and polysulfides are easily desorbed during long-term cycling. NGr's lithiophilic surface promotes the formation of a uniform and stable SEI film, reducing side reactions (such as electrolyte decomposition) and resulting in a more stable electrode / electrolyte interface. The lithiophores of undoped Gr, however, can lead to localized SEI inhomogeneities and increased interfacial impedance. NGr-based materials typically have higher initial coulombic efficiency than Gr-based materials due to reduced irreversible lithium consumption. NGr's doping defects enhance flexibility, mitigating the volume expansion stress of ZnCo2S4 and preventing graphene layer cracking. In contrast, the perfect lattice of undoped Gr is more prone to cracking during repeated expansion / contraction, leading to the shedding of active material.
[0068] After 100 cycles, the discharge capacity and capacity retention of Comparative Example 3 were lower than those of Example 1. This is because pure ZnCo2S4 has poor intrinsic conductivity (semiconductor properties), resulting in high internal resistance and slow charge transport, affecting rate performance. Polarization easily occurs during charge and discharge, reducing energy efficiency. Nitrogen-doped graphene, on the other hand, provides a highly conductive three-dimensional network (conductivity >1000 S / m), significantly improving electron transport rate, reducing charge transfer impedance, and enhancing reaction kinetics, allowing the material to maintain high capacity even at high rates. ZnCo2S4 undergoes drastic volume changes during lithiation / delithiation, leading to electrode structure damage, active material shedding, and short cycle life. After long-term cycling, capacity decays rapidly. ZnCo2S4@NGr / NF exhibits enhanced structural stability, suppressing volume expansion and active material pulverization. In particular, the flexible graphene network of NGr encapsulates ZnCo2S4, buffering volume expansion and preventing particle breakage. The 3D structure of nickel foam provides mechanical support, while NGr further reinforces the overall electrode structure. During the charge and discharge process, ZnCo2S4 / NF generates soluble polysulfides (Li2S). x This leads to a "shuttle effect," resulting in the loss of active material and capacity decay. In ZnCo2S4@NGr / NF, NGr exhibits chemisorption; nitrogen can form Li–N bonds with polysulfides, fixing Li2S... x It suppresses the shuttle effect. Compared with pure ZnCo2S4 / NF, the dissolution loss of polysulfides is reduced by more than 50% after NGr modification.
[0069] After 100 cycles, the discharge capacity and capacity retention of Comparative Example 4 were both lower than those of Example 2. This is because the presence of urea can adjust the pH of the solution for the formation of transition metals in the hydrothermal process. Many transition metal ions will undergo precipitation reactions within a specific pH range. The ammonia gas produced by urea decomposition will increase the pH of the solution, promoting the formation of hydrogen precipitates from transition metal ions. Urea molecules are relatively large and may affect the diffusion and aggregation processes of substances in the hydrothermal reaction system. It helps guide the uniform deposition of transition metals and nitrogen-doped graphene on the surface of nickel foam, forming an ordered structure, thereby improving the uniformity and stability of the loading. The fluoride ions (F) in ammonium fluoride -Ammonium fluoride exhibits strong chemical reactivity. In the reaction system, fluoride ions can react with the surface of nickel foam, etching it. This etching increases the surface roughness of the nickel foam, exposing more active sites. Simultaneously, the etched nickel foam surface becomes more activated, facilitating the adsorption and deposition of transition metals and nitrogen-doped graphene. During the formation of transition metals, fluoride ions can influence the crystal growth direction and morphology. They can form complexes with transition metal ions, altering the form and reactivity of the metal ions, thereby controlling the crystal structure and particle size. After participating in the reaction, ammonium fluoride may form chemical bonds or physical adsorption between the transition metals, nitrogen-doped graphene, and nickel foam. This interaction enhances the bonding force between the components of the composite material, preventing interlayer delamination or detachment during subsequent use, and improving the stability and durability of the composite material.
[0070] After 100 cycles, the discharge capacity and capacity retention of Comparative Example 5 were both lower than those of Example 3. This is because Zn is more reactive than Ni, and the theoretical capacity of ZnCo2S4 (~1200-1500 mAh / g) is higher than that of NiCo2S4 (~800-1000 mAh / g) because Zn... 2+ It can provide 2 electrons for the reaction (Zn) Zn 2+ +2e - ), and Ni 2+ Typically, only 1 electron is contributed (Ni) 2+ Ni 3+ +e - Zn has a lower redox potential (~0.5V vs. Li). + / Li), enabling it to store more lithium ions during charging and discharging. Due to the buffering effect of Zn, volume expansion is more controllable, Ni 2+ / Ni 3+ During redox processes, NiCo2S4 undergoes significant lattice distortion, leading to structural instability and limited cycle life. Meanwhile, the Zn in ZnCo2S4... 2+ During discharge, metallic Zn nanoparticles are formed. These particles act as a conductive buffer layer, mitigating volume expansion. The Ni-S bonds in NiCo2S4 are prone to breakage during cycling, leading to the formation of polysulfides (Li2S). x Dissolution of ZnCo2S4 leads to a "shuttle effect," reducing coulombic efficiency. In contrast, the Zn-S bonds in ZnCo2S4 are more stable, exhibiting stronger polysulfide confinement, and Zn... 2+ It can form a ZnS protective layer with polysulfides, reducing the loss of active materials. ZnCo2S4 exhibits faster Li... + Diffusion rate (thanks to Zn) 2+Its smaller ionic radius allows it to maintain a high capacity even at high current densities.
[0071] After 100 cycles, the discharge capacity and capacity retention of Comparative Example 6 were both lower than those of Example 1. This is because although NF and NGr can provide support, ZnCo2O4 still undergoes drastic volume changes during lithiation / delithiation. Long-term cycling may lead to the shedding of active material from NF and the collapse of the electrode structure, accelerating capacity decay. A thick SEI film forms on the ZnCo2O4 surface during the first discharge, which is an unstable solid electrolyte interface film that consumes a large number of lithium ions, resulting in low initial coulombic efficiency. Although NGr can improve conductivity, the intrinsic reaction kinetics of ZnCo2O4 are slow, and the capacity decreases significantly at high current densities.
[0072] After 100 cycles, the discharge capacity and capacity retention of Comparative Example 7 were lower than those of Example 1. This is because excess NGr may encapsulate the active material (ZnCo2S4), hindering its contact with the hydrothermal reaction solution, resulting in incomplete ZnCo2S4 crystallization and uneven size. Excess NGr may also cause graphene sheets to stack, clogging pores, reducing the effective specific surface area, leading to over-coverage of the NF substrate, and reducing the exposure of active sites. Excess NGr also leads to a relative decrease in the loading of the active material (ZnCo2S4), resulting in a decrease in specific capacity and an increase in interfacial resistance. Excess NGr may cause poor contact between the active material and the current collector (NF), leading to detachment during cycling. The NGr surface may adsorb sulfur sources, reducing the interaction with Zn. 2+ / Co 2+ The reaction efficiency is affected, leading to a decrease in the yield of ZnCo2S4. The nitrogen-containing groups in NGr may locally alter the pH, affecting the nucleation rate of ZnCo2S4.
[0073] After 100 cycles, the discharge capacity and capacity retention of Comparative Example 8 were both lower than those of Example 1. This is because if the reaction temperature is >120°C, ammonium sulfide will decompose rapidly, leading to insufficient sulfur source and the formation of amorphous or defect-rich phases. The ZnCo2S4 generated by the ammonium sulfide system may contain more sulfur vacancies, enhancing intrinsic conductivity, but excess NH3 will adsorb onto NGr, increasing interfacial resistance. Insufficient crystallinity of ZnCo2S4 will lead to a decrease in capacity. Materials synthesized with ammonium sulfide may experience accelerated structural degradation during cycling due to a higher number of defects.
Claims
1. A method for preparing binder-free transition metal sulfide composite nitrogen-doped graphene anode material, characterized in that, Includes the following steps: 1) Dissolve zinc nitrate, cobalt nitrate and nitrogen-doped graphene in a mixed solution of ethanol and water, mix evenly, then add ammonium fluoride and urea and continue stirring until evenly mixed; 2) Add the nickel foam and the above-mentioned uniformly mixed solution into a high-pressure reactor for hydrothermal treatment, cooling, washing, and drying to obtain the Zn-Co precursor; 3) The Zn-Co precursor was then calcined under an inert atmosphere to obtain ZnCo2O4@NGr / NF; 4) The prepared ZnCo2O4@NGr / NF is added to a high-pressure reactor containing a sulfur source for hydrothermal sulfidation treatment, followed by washing and drying to obtain the ZnCo2S4@NGr / NF anode material.
2. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 1, characterized in that, The mass ratio of zinc nitrate, cobalt nitrate, and nitrogen-doped graphene is 1:(1-2):(3-6).
3. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 1, characterized in that, The volume ratio of ethanol to water is (1-2):(1-2).
4. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 1, characterized in that, The mass ratio of ammonium fluoride to urea is (0.4-1):1; the mass ratio of zinc nitrate to ammonium fluoride is 1:(2-5).
5. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 1, characterized in that, The method for preparing the nitrogen-doped graphene is as follows: graphene and melamine are dry ball-milled, and after ball milling, the graphene is calcined under an argon atmosphere and cooled to obtain nitrogen-doped graphene.
6. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 5, characterized in that, The mass ratio of graphene to melamine is 1:(3-5), and the calcination is carried out at 300-600℃ for 3-6 hours under an argon atmosphere.
7. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 1, characterized in that, In step 2), the hydrothermal reaction is carried out in a high-pressure reactor with a polytetrafluoroethylene liner at 130-150°C for 4-6 hours.
8. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 1, characterized in that, In step 3), the calcination is carried out in a muffle furnace under an argon or nitrogen atmosphere at 350-400℃ for 2-4 hours, with a heating rate of 2-5℃·min. -1 .
9. The method for preparing the binderless transition metal sulfide composite nitrogen-doped graphene anode material as described in claim 1, characterized in that, In step 4), the sulfur source is sodium sulfide, and the hydrothermal sulfidation treatment in step 4) is carried out at 130-160℃ for 8-12 hours.
10. A binderless transition metal sulfide composite nitrogen-doped graphene anode material prepared by the preparation method according to any one of claims 1-9.