Composite material with two-dimensional sheet-shell structure and application of composite material

Abstract

The invention belongs to the technical field of ion battery materials, and discloses a composite material with a two-dimensional sheet-shell structure and an application of the composite material as a sodium ion battery negative electrode active material, a sheet-shaped Co and Ni double-metal hydroxide precursor (CoNi-LDH) is prepared, then the CoNi-LDH is vulcanized through a hydrothermal method to obtain a sheet-shaped double-metal sulfide (CoNi) S2-1, 2-propylene glycol, and the CoNi-S2-1, 2-propylene glycol is used as a negative electrode active material of a sodium ion battery. The preparation method comprises the following steps: coating an organic matter on a (CoNi) S2-1 sheet as a carbon source, and then transferring into an inert gas atmosphere for high-temperature carbonization to obtain a composite material with a two-dimensional sheet-shell structure. When the composite material with the sheet-shell structure is used as a sodium battery negative electrode material for manufacturing a sodium ion battery, the charge-discharge capacity and the cycling stability of the sodium ion battery can be remarkably improved.

Description

Technical Field

[0001] This invention belongs to the technical field of ion battery materials, specifically relating to a two-dimensional "sheet-shell" structured composite material and its application as a negative electrode active material for sodium-ion batteries. Background Technology

[0002] Against the backdrop of the rapid development of new energy storage and power battery technologies, sodium-ion batteries have shown significant application advantages in large-scale energy storage systems and low-speed transportation due to their low raw material costs, excellent safety performance, and wide temperature range operation characteristics, making them a candidate technology to replace traditional lead-acid batteries and some lithium battery applications.

[0003] As the core component of sodium-ion batteries, the performance of the anode material directly determines the battery's energy density and cycle life. Currently, carbon-based materials are widely regarded as the preferred anode solution due to their excellent sodium storage reversibility and low cost. However, their capacity bottleneck is significant. For example, the theoretical specific capacity of hard carbon is typically only 300-400 mAh / g, which is insufficient to meet the high energy density requirements. In contrast, transition metal sulfides, with their higher theoretical specific capacity (over 600 mAh / g), have become potential materials for improving battery performance. However, these materials suffer from inherently poor conductivity and electrode structure collapse due to volume expansion during charging and discharging, severely limiting their practical application.

[0004] Currently, the bottlenecks facing sodium-ion battery anode active materials are low energy density and rapid capacity decay during cycling. These two shortcomings severely limit their application in energy storage and power applications. Therefore, overcoming the performance bottlenecks of sodium battery anode materials is of paramount importance for promoting the commercialization of sodium-ion batteries. Summary of the Invention

[0005] To address the bottleneck problem of sodium battery anode materials, this invention provides a two-dimensional "sheet-shell" structured composite material and its application as an active material for sodium-ion battery anodes.

[0006] Firstly, the two-dimensional "sheet-shell" composite material is prepared by first coating a carbon source organic material with sheet-like Co and Ni bimetallic hydroxide, namely CoNi-LDH, after sulfidation, and then carbonizing it at high temperature to obtain the two-dimensional "sheet-shell" structure.

[0007] The CoNi-LDH is prepared by urea hydrolysis. Preferably, the urea hydrolysis method includes the following steps: dissolving cobalt and nickel metal salts and urea together in deionized water, stirring to form a homogeneous solution, heating the solution in an oil bath while maintaining stirring, and maintaining the liquid level by reflux. After the reaction is complete, the product is centrifuged, washed, and dried to obtain flake-shaped CoNi bimetallic hydroxide (CoNi-LDH). Further, the metal salts are water-soluble salts of nickel and cobalt, selected from any one of the sulfates, nitrates, chlorides, phosphates, and oxalates of nickel and cobalt. The molar ratio of nickel salt to cobalt salt is (2:1) to (1:2), and the molar ratio of metal salt to urea is 1:5. It should be noted that the metal salts here refer to nickel and cobalt salts. Therefore, the molar ratio of metal salt to urea refers to the total molar amount of nickel and cobalt salts and the molar ratio of urea. The oil bath temperature is 100°C, and the oil bath time is 12 hours. More preferably, when the nickel salt and cobalt salt are Ni(NO3)2•6H2O and Co(NO3)2•6H2O respectively, the molar ratio of nickel salt to cobalt salt is 1:2.

[0008] The sulfidation step involves using thioacetamide as a sulfur source and sulfiding CoNi-LDH via a hydrothermal method to obtain sheet-like (CoNi)S2-1. Preferably, the sulfidation step specifically includes: dispersing the CoNi-LDH in ethanol, sonicating and stirring to ensure full dispersion, adding thioacetamide as a sulfur source, stirring vigorously to ensure full dissolution, transferring the mixture to a hydrothermal reactor, sulfiding the CoNi-LDH via hydrothermal treatment, centrifuging, washing, and drying the product to obtain CoNi bimetallic sulfide nanosheets (CoNi)S2-1. Further, the mass ratio of CoNi-LDH to thioacetamide is (1:2) to (1:3), the hydrothermal treatment temperature is 120℃, and the time is 12 hours. More preferably, the mass ratio of CoNi-LDH to thioacetamide is 1:3.

[0009] The carbon source organic compound is a water-soluble organic compound, such as dopamine hydrochloride, glucose, polyvinylpyrrolidone, tannic acid, or polyvinyl alcohol. Preferably, the specific steps for coating the carbon source organic compound include: dispersing (CoNi)S2-1 in deionized water, sonicating and stirring to ensure full dispersion, adding the carbon source organic compound and stirring vigorously to ensure full dissolution, forming a thin film of organic compound on the surface of (CoNi)S2-1 nanosheets by stirring at room temperature or hydrothermal treatment, centrifuging, washing, and drying the product to obtain carbon source organic compound-coated (CoNi)S2-1. Further, the mass ratio of (CoNi)S2-1 to the carbon source organic compound is (1:0.5) to (1:5). More preferably, the carbon source organic compound used is dopamine hydrochloride, the mass ratio of (CoNi)S2-1 to dopamine hydrochloride is 1:1, the pH of the system is adjusted to 8.5 using Tris-Cl solution, and stirring is maintained for 24 hours.

[0010] The high-temperature carbonization step involves transferring the material to an inert gas atmosphere for high-temperature carbonization to obtain a composite material with a "plate-shell" structure. Preferably, the high-temperature carbonization step specifically includes: placing the sulfurized product (CoNi)S2-1 or (CoNi)S2-1 coated with carbon source organic matter in a tube furnace, introducing an inert gas as a protective gas, and heating to carbonize the carbon source organic matter while simultaneously increasing the crystallinity of (CoNi)S2-1. Then, the material is naturally cooled to room temperature, and the resulting product is the two-dimensional "plate-shell" composite material. Further, the inert gas used is argon or nitrogen, the tube furnace heating rate is 1~5℃ / min, the carbonization temperature is 300~600℃, and the holding time is 1~4 hours. More preferably, the inert gas used is argon, the heating rate is 2℃ / min, the carbonization temperature is 600℃, and the holding time is 4 hours.

[0011] Secondly, the present invention provides the application of the above-mentioned two-dimensional "sheet-shell" structured composite material as a negative electrode active material for sodium-ion batteries.

[0012] Thirdly, the present invention also provides a sodium-ion battery negative electrode, wherein the sodium-ion battery negative electrode comprises the two-dimensional "sheet-shell" structured composite material; for example, as one embodiment of the sodium-ion battery negative electrode of the present invention, the sodium-ion battery negative electrode is made by mixing the two-dimensional "sheet-shell" structured composite material, conductive carbon black SuperP, and polyvinylidene fluoride in a mass ratio of 7:2:1, using N-methylpyrrolidone as a solvent, mixing and grinding to obtain a slurry, coating the slurry onto copper foil, and after the solvent evaporates, placing it in a vacuum oven at 100°C for vacuum drying for 24 hours, and then punching it into an electrode sheet to obtain the sodium-ion battery negative electrode.

[0013] Fourthly, the present invention also provides a sodium-ion battery, wherein the sodium-ion battery comprises a composite material with the two-dimensional "sheet-shell" structure; for example, as an embodiment of the sodium-ion battery of the present invention, the sodium-ion battery is manufactured by stacking, pressing and sealing the positive electrode shell, spring sheet, gasket, sodium-ion battery negative electrode as described in the third aspect above, glass fiber separator, electrolyte, sodium sheet and negative electrode shell in that order.

[0014] Beneficial effects

[0015] In the two-dimensional "plate-shell" composite material provided in this application, the "plate" refers to a bimetallic sulfide (CoNi)S 2， Bimetallic sulfide (CoNi)S2 can improve the inherent defects of metal sulfides, such as poor conductivity and volume expansion. The bimetallic component helps to form heterojunctions rich in crystal defects, creating more separated electrons and holes, increasing active sites, and improving the electrochemical activity of the composite material. The high aspect ratio two-dimensional sheet structure can alleviate the volume change of the active material during charge and discharge. Compared with ordinary monometallic sulfides, (CoNi)S2 exhibits superior charge and discharge capacity and cycle stability in sodium-ion batteries. Ordinary monometallic sulfide (NiS2) has a specific capacity of only about 100 mAh / g after 400 cycles in the battery, while (CoNi)S2-1 nanosheets can maintain a specific capacity of 338 mAh / g.

[0016] The "shell" in the two-dimensional "sheet-shell" composite material provided in this application refers to a highly conductive carbon outer shell layer based on the (CoNi)S2-1 "sheet" structure. The carbon layer can improve the conductivity of the bimetallic sulfide and provide a large number of sodium ion adsorption sites, further mitigating the negative impact of volume changes on the material. Compared with (CoNi)S2-1, the cycle capacity of sodium-ion batteries based on (CoNi)S2-1@C is further improved, and a specific capacity of 645 mAh / g can be maintained after 400 cycles. Furthermore, a nitrogen-doped carbon layer, namely (CoNi)S2-1@CN, is introduced. The doping of nitrogen further improves the conductivity of the carbon layer and provides more active sites. Compared with (CoNi)S2-1@C, the sodium-ion battery made of (CoNi)S2-1@CN has significantly improved cycle performance, maintaining a discharge specific capacity of up to 1000 mAh / g after 400 cycles. Currently, among the publicly disclosed sodium-ion battery anode active materials, such as CN115321503A and CN115000361A, there is no technology that can achieve the same level of performance.

[0017] Compared to similar materials, the two-dimensional "sheet-shell" structure of (CoNi)S2-1@CN exhibits superior electrochemical performance. Sodium-ion batteries based on this structure demonstrate better charge-discharge capacity and cycle stability than those made from (CoNi)S2-2 nanorods, specifically the (CoNi)S2-2@CN "rod-shell" structure. Its preparation process is simple, environmentally friendly, and low-cost. As a negative electrode active material for sodium-ion batteries, it exhibits excellent electrochemical performance and shows great potential for large-scale application. Attached Figure Description

[0018] Figure 1 Scanning electron microscope image of the CoNi bimetallic hydroxide (CoNi-LDH) prepared in Example 1;

[0019] Figure 2 The X-ray diffraction (XRD) pattern of the CoNi bimetallic hydroxide (CoNi-LDH) prepared in Example 1;

[0020] Figure 3 X-ray diffraction (XRD) spectra of (CoNi)S2-1 nanosheets prepared in Example 2 and (CoNi)S2-2 nanorods prepared in Comparative Example 1;

[0021] Figure 4 Scanning electron microscope image of the (CoNi)S2-1@CN "sheet-shell" structured composite material prepared in Example 6;

[0022] Figure 5 Transmission electron microscopy image of the (CoNi)S2-1@CN "sheet-shell" structured composite material prepared in Example 6;

[0023] Figure 6 X-ray photoelectron spectroscopy (XPS) of the (CoNi)S2-1@CN "sheet-shell" composite material prepared in Example 6;

[0024] Figure 7 Scanning electron microscope image of (CoNi)S2-2 nanorods prepared in Comparative Example 1;

[0025] Figure 8 Scanning electron microscope image of the (CoNi)S2-2@CN "rod-shell" structured composite material prepared in Comparative Example 1;

[0026] Figure 9 The X-ray diffraction (XRD) pattern of NiS2 prepared in Comparative Example 2;

[0027] Figure 10 The discharge specific capacity of the batteries assembled in Examples 2, 3, 4 and Comparative Examples 1 and 2 during charge-discharge cycle testing. Detailed Implementation

[0028] Example 1: Preparation of CoNi-LDH nanosheets

[0029] 8.7 g of Ni(NO3)2•6H2O, 17.4 g of Co(NO3)2•6H2O, 27 g of urea, and 300 mL of deionized water were added to a three-necked flask and stirred for 10 minutes to homogenize the solution. The flask was then placed in an oil bath with a condenser attached to the neck. The oil bath temperature was set to 100 °C, and stirring was maintained for 12 hours. The dark green precipitate was separated by centrifugation, washed three times each with deionized water and ethanol, and dried in an oven at 60 °C to obtain CoNi-LDH nanosheets. The scanning electron microscope image of the product prepared in Example 1 is shown below. Figure 1 As shown, the morphology is nanosheets, and its XRD pattern is as follows. Figure 2 As shown, the characteristic peaks are related to CO3. 2- The intercalated CoNi bimetallic hydroxide (PDF#33-0429) is consistent, demonstrating that the method provided in Example 1 can prepare two-dimensional sheet-like CoNi bimetallic hydroxide (CoNi-LDH).

[0030] Example 2: Preparation of (CoNi)S2-1 nanosheets

[0031] Take the CoNi-LDH nanosheets prepared in Example 1, add 1g of CoNi-LDH powder to 400mL of ethanol, and ultrasonically stir for 10min to ensure thorough dispersion; add 3g of thioacetamide and stir vigorously for 10min, then transfer the mixture to a hydrothermal reactor for hydrothermal treatment at 120℃ for 12 hours; centrifuge to separate the precipitate, wash three times each with deionized water and ethanol, and dry in an oven at 60℃ to obtain (CoNi)S2-1 nanosheets. The X-ray diffraction pattern of the product prepared in Example 2 is shown below. Figure 3 As shown, the characteristic peaks are consistent with those of CoNi bimetallic sulfide (PDF#62-4479), proving that the method provided in Example 2 can sulfide CoNi-LDH nanosheets into CoNi bimetallic sulfide (CoNi)S2-1 nanosheets.

[0032] Example 3: Fabrication of (CoNi)S2-1 electrode and battery

[0033] The (CoNi)S2-1, conductive carbon black SuperP, and polyvinylidene fluoride prepared in Example 2 were mixed in a mass ratio of 7:2:1. N-methylpyrrolidone was used as a solvent to mix and grind the mixture to obtain a slurry. The slurry was coated onto a copper foil. After the solvent evaporated, the foil was placed in a vacuum oven and dried at 100°C for 24 hours. After removal, the electrode was punched into a round disc with a diameter of 12 mm to obtain the (CoNi)S2-1 electrode.

[0034] Using a 2032 type battery case, the positive electrode shell, spring, gasket, (CoNi)S2-1 electrode, glass fiber separator, 80μL electrolyte (1.0M NaCF3SO3 / Digylme), sodium sheet, and negative electrode shell are stacked in that order, pressed and sealed to obtain the (CoNi)S2-1 battery. After standing for 12 hours, the battery is tested.

[0035] Example 4: Preparation of (CoNi)S2-1@C using glucose as a carbon source

[0036] Take 1g of (CoNi)S2-1 powder prepared in Example 2, add it to 200mL of deionized water, and ultrasonically stir for 10min to ensure thorough dispersion. Add 1g of glucose, stir for 10min, and then transfer the mixture to a hydrothermal reactor for hydrothermal treatment at 150℃ for 12 hours. Centrifuge to separate the precipitate, wash three times each with deionized water and ethanol, and dry in an oven at 60℃ to obtain glucose-coated (CoNi)S2-1 nanosheets. Place the glucose-coated (CoNi)S2-1 nanosheets in a tube furnace, purge with argon gas to protect the product, set the heating rate of the tube furnace to 2℃ / min, heat to 600℃, and hold for 4 hours. Allow to cool naturally to room temperature to obtain the (CoNi)S2-1@C composite material.

[0037] Example 5: Preparation of (CoNi)S2-1@C electrode and battery based on the composite material of Example 4

[0038] The (CoNi)S2-1@C composite material prepared in Example 4, conductive carbon black superP, and polyvinylidene fluoride were mixed in a mass ratio of 7:2:1. N-methylpyrrolidone was used as a solvent to mix and grind the mixture to obtain a slurry. The slurry was coated onto a copper foil. After the solvent evaporated, the foil was placed in a vacuum oven and dried at 100°C for 24 hours. After removal, the electrode was punched into a round disc with a diameter of 12 mm to obtain the (CoNi)S2-1@C electrode.

[0039] Using a 2032 type battery case, the positive electrode shell, spring, gasket, (CoNi)S2-1@C electrode, glass fiber separator, 80μL electrolyte (1.0M NaCF3SO3 / Digylme), sodium sheet, and negative electrode shell are stacked in that order, pressed and sealed to obtain the (CoNi)S2-1@C battery. After standing for 12 hours, the battery is tested.

[0040] Example 6: Preparation of (CoNi)S2-1@CN using dopamine as a carbon source

[0041] Take 1g of (CoNi)S2-1 powder prepared in Example 2 and add it to 400mL of deionized water. Stir ultrasonically for 10min to ensure thorough dispersion. Add 1g of dopamine hydrochloride and stir vigorously for 1 hour. Adjust the pH of the system to 8.5 using 50mM Tris-Cl solution and stir continuously for 24 hours. After standing, centrifuge to separate the precipitate. Wash three times each with deionized water and ethanol, and dry in an oven at 60℃ to obtain dopamine-coated (CoNi)S2-1 nanosheets. Place the dopamine-coated (CoNi)S2-1 nanosheets in a tube furnace, purge with argon gas to protect the product, set the heating rate of the tube furnace to 2℃ / min, heat to 600℃, and hold for 4 hours. Allow to cool naturally to room temperature to obtain the (CoNi)S2-1@CN "sheet-shell" composite material. Its scanning electron microscope image is shown below. Figure 4 As shown, (CoNi)S2-1@CN is a nanosheet that retains the sheet-like structure of the precursor CoNi-LDH, with a rough, amorphous carbon shell on the surface; the transmission electron microscope image of (CoNi)S2-1@CN is shown below. Figure 5 As shown, the dark inner portion is the bimetallic sulfide (CoNi)S2-1, and the light outer portion is an amorphous carbon shell doped with nitrogen, indicating that the structure of (CoNi)S2-1@CN is a "sheet-shell" structure formed by a carbon shell encapsulating bimetallic sulfide nanosheets; the X-ray electron spectrum of (CoNi)S2-1@CN is as follows. Figure 6 As shown, the binding energy characteristic peaks of Co, Ni, S and C, N elements in the carbon shell of (CoNi)S2-1 can be clearly distinguished.

[0042] Example 7: (CoNi)S2-1@CN electrode and battery based on the composite material of Example 6

[0043] (CoNi)S2-1@CN, conductive carbon black superP, and polyvinylidene fluoride were mixed in a mass ratio of 7:2:1, with N-methylpyrrolidone as the solvent. The mixture was ground to obtain a slurry, which was then coated onto copper foil. After the solvent evaporated, the foil was placed in a vacuum oven and dried at 100°C for 24 hours. After removal, the electrode was punched into a round disc with a diameter of 12 mm to obtain the (CoNi)S2-1@CN electrode.

[0044] Using a 2032 type battery case, the positive electrode case, spring sheet, gasket, (CoNi)S2-1@CN electrode, glass fiber separator, 80μL electrolyte (1.0M NaCF3SO3 / Digylme), sodium sheet, and negative electrode case are stacked in that order, pressed and sealed to obtain the (CoNi)S2-1@CN battery. After standing for 12 hours, the battery is tested.

[0045] Comparative Example 1: Preparation of (CoNi)S2-2 nanorods, (CoNi)S2-2@CN, (CoNi)S2-2@CN electrodes and batteries

[0046] A mixture of Co(CH3COO)2•4H2O (0.6 g), Ni(CH3COO)2•4H2O (0.3 g), thiourea (0.689 g), and water (65 mL) was stirred for 30 minutes. The resulting mixture was then transferred to a polytetrafluoroethylene-lined hydrothermal reactor and maintained at 160 °C for 12 hours. After natural cooling to room temperature, a light black powder was obtained by centrifugation, washed three times each with deionized water and ethanol, and dried in an oven at 60 °C to obtain (CoNi)S2-2 nanorods. The scanning electron microscope image is shown below. Figure 7 As shown, the X-ray diffraction pattern of the (CoNi)S2-2 nanorod product prepared in Comparative Example 1 is also... Figure 3 As shown, the characteristic peaks are also consistent with those of CoNi bimetallic sulfide (PDF#62-4479), and its scanning electron microscope image is shown below. Figure 7 As shown, the morphology is rod-shaped nanoparticles, proving that the method provided in Comparative Example 1 can prepare rod-shaped metal sulfide (CoNi)S2-2;

[0047] 1 g of (CoNi)S2-2 powder was added to 400 mL of deionized water and ultrasonically stirred for 10 min to ensure complete dispersion. 1 g of dopamine hydrochloride was added and stirred vigorously for 1 hour. The pH of the system was adjusted to 8.5 using 50 mM Tris-Cl solution, and the mixture was stirred continuously for 24 hours. After standing, the precipitate was separated by centrifugation, washed three times each with deionized water and ethanol, and dried in an oven at 60 °C to obtain dopamine-coated (CoNi)S2-1 nanosheets.

[0048] Dopamine-coated (CoNi)S2-2 nanorods were placed in a tube furnace, and argon gas was introduced to protect the product. The furnace was heated at a rate of 2 °C / min to 600 °C and held for 4 hours. After natural cooling to room temperature, a rod-shell structured (CoNi)S2-2@CN composite material was obtained. A scanning electron microscope image of the (CoNi)S2-2@CN product prepared in Comparative Example 1 is shown below. Figure 8 As shown, with Figure 7 Compared to the smooth surface of the (CoNi)S2-2 nanorods, its surface is clearly covered with a rough carbon shell, indicating that the structure of (CoNi)S2-2@CN is a "rod-shell" structure formed by the carbon shell covering the bimetallic sulfide nanorods.

[0049] (CoNi)S2-2@CN, conductive carbon black superP, and polyvinylidene fluoride were mixed in a mass ratio of 7:2:1, with N-methylpyrrolidone as the solvent. The mixture was ground to obtain a slurry, which was then coated onto copper foil. After the solvent evaporated, the foil was placed in a vacuum oven and dried at 100°C for 24 hours. The electrode was then punched into a 12mm diameter disc to obtain the (CoNi)S2-2@CN electrode.

[0050] Using a 2032 type battery casing, the positive electrode casing, spring sheet, gasket, (CoNi)S2-2@CN electrode sheet, glass fiber separator, 80μL electrolyte (1.0M NaCF3SO3 / Digylme), sodium sheet, and negative electrode casing are stacked in that order, pressed and sealed to obtain the (CoNi)S2-2@CN battery. After standing for 12 hours, the battery is tested.

[0051] Comparative Example 2: Preparation of NiS2, NiS2 Electrodes and Batteries

[0052] 1 g of Ni(OH)₂ powder was added to 400 mL of ethanol and ultrasonically stirred for 10 min to ensure thorough dispersion. 3 g of thioacetamide was added and stirred vigorously for 10 min. The mixture was then transferred to a hydrothermal reactor and hydrothermally treated at 120 °C for 12 hours. The precipitate was separated by centrifugation, washed three times each with deionized water and ethanol, and dried in an oven at 60 °C to obtain NiS₂. Its X-ray diffraction pattern is shown in the attached figure. Figure 9 As shown, the characteristic peaks match those of NiS2 (PDF#89-7142), proving that the method provided in Comparative Example 2 can prepare the monometallic sulfide NiS2.

[0053] NiS2, conductive carbon black SuperP, and polyvinylidene fluoride are mixed in a mass ratio of 7:2:1, with N-methylpyrrolidone as the solvent. The mixture is ground to obtain a slurry, which is then coated onto copper foil. After the solvent evaporates, the foil is placed in a vacuum oven and dried at 100°C for 24 hours. The electrode is then punched into a 12mm diameter disc to obtain the NiS2 electrode.

[0054] Using a 2032 battery case, stack the following components in the order of positive electrode case, spring sheet, gasket, NiS2 electrode, glass fiber separator, 80μL electrolyte (1.0M NaCF3SO3 / Digylme), sodium sheet, and negative electrode case, press and seal them to obtain a NiS2 battery. Test the battery after standing for 12 hours.

[0055] Test Example: Comparison of Electrochemical Performance Tests

[0056] The discharge specific capacity of the batteries assembled in Examples 3, 5, 7 and Comparative Examples 1, 2 during charge-discharge cycle testing is as follows: Figure 10 As shown.

[0057] Due to the poor conductivity and severe volume expansion of metal sulfides, the sodium-ion battery with NiS2 as the negative electrode in Comparative Example 2 exhibits extremely rapid capacity decay during charge-discharge cycle testing, with the specific capacity decreasing to approximately 150 mAh / g after only 100 discharge cycles.

[0058] In Example 3, the battery assembled based on two-dimensional bimetallic sulfide (CoNi)S2-1 nanosheets can form a heterojunction with high electrochemical activity, and the two-dimensional sheet morphology can alleviate the volume change during charge and discharge. Therefore, the capacity retention rate is higher than that of single metal sulfides, and it can still maintain a specific capacity of 338 mAh / g after 400 cycles.

[0059] Example 5: The (CoNi)S2-1@C "sheet-shell" structure formed by coating a carbon layer on (CoNi)S2-1 nanosheets exhibits improved conductivity of sulfides and further mitigated volume change shocks. As a result, the (CoNi)S2-1@C battery maintains a specific capacity of 645 mAh / g after 400 cycles.

[0060] In Example 7, the nitrogen-doped carbon layer in the (CoNi)S2-1@CN "sheet-shell" structure further improves the conductivity of the carbon layer and provides more active sites. Therefore, the cycle performance of the battery assembled based on (CoNi)S2-1@CN is greatly improved, and it can maintain a discharge specific capacity of up to 1000 mAh / g after 400 cycles.

[0061] In Comparative Example 1, (CoNi)S2-2@CN is also a "bimetallic sulfide + nitrogen-doped carbon layer" composite material. The difference lies in its structure, which is a "rod-shell" structure formed by nanorods coated with a carbon layer. The battery assembled using this structure achieved a discharge specific capacity of approximately 1000 mAh / g after 230 charge-discharge cycles. However, its capacity subsequently entered a relatively rapid decay range, with the specific capacity remaining at only 513 mAh / g after 400 cycles. This indicates that the capacity retention capability of the (CoNi)S2-2@CN "rod-shell" structure as a negative electrode material in sodium-ion batteries is weaker than that of the similar (CoNi)S2-1@C "sheet-shell" structure. Therefore, the battery performance comparison test shows that the (CoNi)S2-1@C "sheet-shell" structure has unique advantages in terms of specific capacity and cycle stability.

Claims

1. A composite material of two-dimensional "sheet-shell" structure, characterized in that, The two-dimensional "sheet-shell" structure composite material is prepared by coating a carbon source organic matter on a sheet-shaped CoNi-LDH after sulfidation, and then carbonizing at high temperature; the carbon source organic matter is a water-soluble organic matter; the sulfidation step is to sulfidize the CoNi-LDH by a hydrothermal method with thioacetamide as a sulfur source to obtain sheet-shaped (CoNi) S2-1; the high-temperature carbonization step is to carbonize the product in an inert gas atmosphere to obtain the two-dimensional "sheet-shell" structure composite material; and the CoNi-LDH is prepared by a urea hydrolysis method.

2. A composite material of two-dimensional "sheet-shell" structure according to claim 1, characterized in that, The urea hydrolysis method comprises the following steps: dissolving metal salts of cobalt and nickel and urea in deionized water together, stirring to form a uniform solution, heating the solution in an oil bath, keeping stirring, and maintaining the liquid level by condensation reflux; after the reaction is completed, centrifuging, washing, and drying the product to obtain sheet-shaped CoNi double metal hydroxide CoNi-LDH.

3. A two-dimensional "plate-shell" structural composite material as claimed in claim 2, wherein, The metal salt is a water-soluble salt of nickel and cobalt, and the water-soluble salt is selected from any one of sulfates, nitrates, chlorides, phosphates, and oxalates of the metals nickel and cobalt, the molar ratio of the nickel salt to the cobalt salt is (2:1)~(1:2), the molar ratio of the metal salt to urea is 1:5, the metal salt includes the nickel salt and the cobalt salt; the oil bath temperature is 100℃, and the oil bath time is 12 hours.

4. A two-dimensional "plate-shell" structural composite material as claimed in claim 1, wherein, The sulfidation step specifically comprises the following steps: dispersing the CoNi-LDH in ethanol, ultrasonicating and stirring to fully disperse, adding thioacetamide as a sulfur source, and stirring vigorously to fully dissolve, transferring the mixed system into a hydrothermal kettle, sulfidizing the CoNi-LDH by hydrothermal treatment, and centrifuging, washing, and drying the product to obtain CoNi double metal sulfide nanosheet (CoNi) S2-1.

5. A two-dimensional "plate-shell" structural composite material as claimed in claim 1, wherein, The carbon source organic matter is any one of hydrochloric acid dopamine, glucose, polyvinylpyrrolidone, tannic acid, and polyvinyl alcohol.

6. A two-dimensional "plate-shell" structural composite material as claimed in claim 1, wherein, The specific operation steps of the first coating of the carbon source organic matter include the following steps: dispersing (CoNi) S2-1 in deionized water, ultrasonicating and stirring to fully disperse, adding the carbon source organic matter and stirring vigorously to fully dissolve, forming a thin film of the organic matter on the surface of the (CoNi) S2-1 nanosheet by normal-temperature stirring or hydrothermal treatment, and centrifuging, washing, and drying the product to obtain the carbon source organic matter coated (CoNi) S2-1.

7. A two-dimensional "plate-shell" structural composite material as claimed in claim 1, wherein, The high-temperature carbonization step specifically comprises the following steps: placing the sulfidized product (CoNi) S2-1 or the carbon source organic matter coated (CoNi) S2-1 in a tube furnace, introducing inert gas as a protective gas, carbonizing the carbon source organic matter and improving the crystallinity of (CoNi) S2-1 by heating, and then naturally cooling to room temperature, and the obtained product is the two-dimensional "sheet-shell" structure composite material.

8. Application of the two-dimensional "sheet-shell" structure composite material of claim 1 as a negative active material of a sodium ion battery.

9. A sodium ion battery negative electrode comprising the two-dimensional "sheet-shell" structure composite material of claim 1.

10. A sodium-ion battery comprising the composite material of claim 1 in the form of two-dimensional "sheet-shell" structures.

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