A MXene two-dimensional material, a graphite negative electrode material and a preparation method thereof

Abstract

The application belongs to the technical field of lithium ion battery negative electrode materials, and particularly relates to a MXene two-dimensional material, a graphite negative electrode material and a preparation method of the MXene two-dimensional material, the preparation method of the MXene two-dimensional material being as follows: lithium sulfate solution is added into a MXene dispersion liquid, stirred uniformly, vacuum dried, calcined at 300-500 DEG C, and the MXene two-dimensional material is obtained; and the application can effectively improve lithium ion conductivity and improve the performance of a battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery anode material technology, specifically relating to an MXene two-dimensional material, a graphite anode material and its preparation method. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density and long cycle life, are widely used in consumer electronics, electric vehicles, and energy storage systems, serving as a core support for the development of the new energy industry. However, with the increasing demands for fast charging, long cycle life, and extreme operating conditions, the performance bottlenecks of existing commercial anode materials have become apparent, necessitating breakthroughs through structural design and modification technologies.

[0003] Graphite, as a mainstream anode material, has advantages such as stable potential and controllable cost, but it also has inherent defects: low lithium-ion diffusion and electron conduction rates, resulting in poor high-rate performance; lithium-ion insertion / extraction causes volume expansion, leading to electrode pulverization, SEI film rupture, and shortened cycle life; SEI film formation is accompanied by side reactions, resulting in low initial coulombic efficiency; and interface reactions are aggravated under high-temperature conditions, leading to decreased performance and safety.

[0004] To address these issues, surface coating modification has become a research hotspot, but existing solutions each have their limitations: carbon-based coating only improves conductivity and cannot suppress volume expansion; metal oxides optimize interface stability but increase electrode impedance; lithium salt coating can construct a stable SEI film, but it has low ionic conductivity and weak interfacial bonding, making it difficult to meet multiple requirements.

[0005] Two-dimensional MXene materials possess excellent conductivity, a layered structure, and abundant functional groups, enabling simultaneous optimization of conductivity and buffering of volume expansion, providing a new direction for graphite modification. However, their susceptibility to aggregation, narrow interlayer spacing, and insufficient bonding with the graphite interface limit their applications. Existing MXene-graphite composite technologies are mostly simple mixing processes without controlling the layer structure; while intercalation modification can expand the interlayer spacing, it is limited by the performance of the intercalating agent. Therefore, developing a graphite modification technology with simple processes and controllable costs to achieve simultaneous improvement in comprehensive performance and prepare high-performance anode materials is key to breaking through the bottlenecks in high-end applications of lithium-ion batteries and is of great significance to promoting the upgrading of the new energy industry. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide an MXene two-dimensional material, a graphite anode material and a method for preparing the same, which can effectively improve the lithium-ion conductivity and improve the performance of the battery.

[0007] This invention provides a method for preparing MXene two-dimensional materials, wherein a lithium sulfate solution is added to an MXene dispersion, stirred evenly, vacuum dried, and calcined at 300-500°C to obtain the MXene two-dimensional material.

[0008] Preferably, the MXene dispersion is a mixture of multilayer MXene powder and water.

[0009] Preferably, the preparation method of the multilayer MXene powder is as follows: the MXene precursor MAX phase material is immersed in an etching solution, mixed evenly, kept warm, washed, and dried to obtain multilayer MXene powder.

[0010] Preferably, the general formula of the MXene precursor MAX phase material is M n+1 AX n M is Ti, V, Nb or Mo, A is Al, Si, Ga or Zn, X is C or N, n=1–3; the etching solution is a mixed HF / HCl etching solution or a mixed fluoride / HCl etching solution, and the fluoride is LiF, NaF, KF or CaF2; the holding temperature is 150-200℃.

[0011] Preferably, the solvent for the lithium sulfate solution is water.

[0012] Preferably, the concentration of the lithium sulfate solution is 0.02-0.08 M, more preferably 0.05 M (5.5 mg / mL), the concentration of the MXene dispersion is 2-8 mg / mL, more preferably 5 mg / mL, and the volume ratio of the lithium sulfate solution to the MXene dispersion is 1:3-5, more preferably 1:4.

[0013] Preferably, the calcination temperature is 350-450℃, and the calcination is carried out under a protective atmosphere.

[0014] This invention provides a two-dimensional MXene material, prepared using the aforementioned method.

[0015] This invention provides a method for preparing a graphite anode material. The method involves mixing the MXene two-dimensional material, water, and dispersant evenly, centrifuging to obtain a supernatant; adding modified graphite to the supernatant, stirring evenly, drying, and calcining to obtain the graphite anode material.

[0016] Preferably, the modified graphite is prepared by mixing graphite and nitric acid solution evenly, then washing, drying, grinding and sieving to obtain modified graphite.

[0017] This invention provides a graphite anode material prepared using the aforementioned preparation method.

[0018] The beneficial effects of this invention are that it uses lithium salt to intercalate and modify MXene, combining ion exchange and high-temperature calcination processes, utilizing Li +By increasing the interlayer spacing of MXene, sulfate anions in the lithium salt form stable coordination with functional groups on the MXene surface, and then uniform coating of graphite is achieved through electrostatic self-assembly. This invention increases the interlayer spacing of MXene, accelerates lithium-ion transport, and the coordination effect enhances the dispersion and interfacial bonding of MXene, preventing the coating layer from falling off.

[0019] The lithium salt used in this invention is lithium sulfate. The inventors discovered that, compared with other anions, sulfate ions can better maintain the interlayer spacing of MXene after intercalation at a suitable distance, and can significantly improve the conductivity of lithium ions. As a result, the batteries prepared in this way have significantly higher specific capacity and energy retention.

[0020] This invention locks the MXene interlayer structure through the steric hindrance effect of anions during lithium salt intercalation, and combines a two-step heat treatment process (calcination after intercalation + curing after coating) to enhance interlayer stability and interface bonding, thereby constructing a "rigid graphite + flexible composite coating" structure; effectively improving the cycle performance and rate performance of the battery. Detailed Implementation

[0021] Example 1 A method using Li2SO4 intercalated MXene (Li2SO4-Ti3C2T) x The preparation method of two-dimensional material-coated artificial graphite anode material includes the following steps: 1) Immerse 5g of Ti3AlC2 powder in 100mL of HF / HCl mixed etching solution (HF / HCl mixed etching solution includes HF aqueous solution and HCl aqueous solution, HF aqueous solution concentration is 30% wt%, HCl aqueous solution concentration is 35 wt%, HF aqueous solution and HCl aqueous solution volume ratio is 1:3), transfer to a reaction vessel and disperse evenly, then react at 160℃ for 6 hours, centrifuge, wash until pH=6, and vacuum dry at 60℃ for 12 hours to obtain multilayer Ti3C2T x powder; 2) Take 0.5 g of multilayer Ti3C2T x The powder was added to 100 mL of deionized water and sonicated at 300 W for 30 min to obtain a monolayer Ti3C2T with a concentration of ≈5 mg / mL. x Dispersion; 3) Weigh 0.275 g Li2SO4, dissolve it in 50 mL of deionized water, and stir for 30 min to obtain a transparent 0.05 M Li2SO4 solution; 4) Slowly add 20 mL of Li₂SO₄ solution dropwise to 80 mL of Ti₃C₂T₄ solution. x The dispersion was sonicated for 30 minutes and then evaporated at 80°C with magnetic stirring until it reached a viscous state, ensuring sufficient ion exchange to obtain a viscous substance. 5) The viscous material was transferred to a vacuum drying oven and dried at 80℃ for 12 h. After grinding, it was placed in a tube furnace and calcined at 400℃ for 2 h under an Ar atmosphere (heating rate 5℃ / min). After natural cooling, Li2SO4 intercalated Ti3C2T was obtained. x Powder (Li2SO4-Ti3C2T) x ); 6) Weigh 0.2 g of Li2SO4 intercalated Ti3C2T x The powder was mixed with 100 mL of deionized water, 0.02 g of PVP dispersant was added, and the mixture was sonicated for 60 min. Then, it was centrifuged at 3000 rpm for 10 min, and the supernatant uniform dispersion was collected to obtain Li₂SO₄-Ti₃C₂T₄. x Dispersion; 7) Weigh 10g of artificial graphite into a 500mL beaker, add 100mL of 1M HNO3, and stir magnetically at room temperature for 30min. Then wash with deionized water until neutral, dry under vacuum at 60℃ for 12h, grind and sieve to obtain modified graphite powder; 8) Add 5g of modified graphite powder to the above 100 mL Li2SO4-Ti3C2T x The dispersion was stirred at room temperature for 4 hours, followed by vacuum drying at 80°C for 12 hours. 9) The dried coated graphite powder was placed in a tube furnace and heated to 350℃ (heating rate 3℃ / min) under Ar atmosphere protection, held at that temperature for 2 h, and then naturally cooled to room temperature to obtain graphite@Li2SO4-Ti3C2T x That is, graphite anode material.

[0022] Example 2 Compared with Example 1, the difference is that step 2) is: take 0.25g of multilayer Ti3C2T x Add 100 mL of deionized water and sonicate at 300 W for 30 min to obtain a monolayer Ti3C2T with a concentration of ≈2.5 mg / mL. x Dispersion. Everything else is the same as in Example 1.

[0023] Example 3 Compared with Example 1, the difference is that step 2) is: take 0.75g of multilayer Ti3C2T x Add 100 mL of deionized water and sonicate at 300 W for 30 min to obtain a monolayer Ti3C2T with a concentration of ≈7.5 mg / mL. x Dispersion. Everything else is the same as in Example 1.

[0024] Example 4 Compared with Example 1, the difference is that step 3) is: weigh 0.412 g of Li2SO4, dissolve it in 50 mL of deionized water, and stir for 30 min to obtain a transparent 0.075 M Li2SO4 solution. Everything else is the same as in Example 1.

[0025] Example 5 Compared with Example 1, the difference is that step 3) is: weigh 0.137 g Li2SO4, dissolve it in 50 mL of deionized water, and stir for 30 min to obtain a transparent 0.025 M Li2SO4 solution. The rest is the same as in Example 1.

[0026] Example 6 Compared with Example 1, the difference is that step 5) is as follows: the viscous material is transferred to a vacuum drying oven, dried at 80°C for 12 h, ground, and then placed in a tube furnace and calcined at 300°C for 2 h under an Ar atmosphere (heating rate 5°C / min). After natural cooling, Li2SO4 intercalated Ti3C2T is obtained. x Powder (Li2SO4-Ti3C2T) x The rest is the same as in Example 1.

[0027] Example 7 Compared with Example 1, the difference is that step 5) is as follows: the viscous material is transferred to a vacuum drying oven, dried at 80°C for 12 h, ground, and then placed in a tube furnace and calcined at 500°C for 2 h under an Ar atmosphere (heating rate 5°C / min). After natural cooling, Li2SO4 intercalated Ti3C2T is obtained. x Powder (Li2SO4-Ti3C2T) x The rest is the same as in Example 1.

[0028] Comparative Example 1 Compared with Example 1, the difference is that 0.275g Li2SO4 in step 3) is replaced with 0.29g Li3PO4, and a 0.05 M Li3PO4 solution is prepared using a dispersion of 50mL deionized water and 0.15g citric acid. Everything else is the same as in Example 1.

[0029] Comparative Example 2 Compared with Example 1, the difference is that 0.275g Li2SO4 in step 3) is replaced with 0.185g Li2CO3, and 45mL deionized water + 5mL 1M dilute nitric acid is used as the solvent to aid dissolution. Everything else is the same as in Example 1.

[0030] Comparative Example 3 Compared with Example 1, the difference is that 0.275g Li2SO4 in step 3) is replaced with 0.106g LiCl, and the rest is the same as in Example 1.

[0031] Comparative Example 4 Compared with Example 1, the difference is that 0.275g Li2SO4 in step 3) is replaced with 0.72g LiTFSI, and the solvent is replaced with DMC (dimethyl carbonate). Otherwise, it is the same as Example 1.

[0032] Example 8 For MXene materials with different lithium salt intercalations (i.e., the intercalated Ti3C2T obtained in Example 1 and Comparative Examples 1-4), x The interlayer spacing variation of the powder was characterized by high-resolution scanning electron microscopy (HRTEM) and X-ray diffraction (XRD).

[0033] To verify the impact of different graphite anodes on battery performance, 8.8Ah pouch cells were assembled for electrical performance testing. The anode material used in the various examples and comparative examples was graphite@Li-MXene, the cathode was lithium iron phosphate, and the electrolyte was 1 mol / L LiPF6 EC and DMC (volume ratio 1.1-3). Pouch cells were assembled for lithium-ion conductivity measurement. Lithium-ion conductivity was obtained using electrochemical impedance spectroscopy (EIS), and the results are shown in the table below.

[0034]

[0035] Typically, the interlayer spacing of MXene is 0.9-1.0 nm. The lithium-ion conductivity of Example 1 of this invention is higher. The possible reason is that the interlayer spacing of Example 1 is moderately increased due to its strong anion coordination and strong steric hindrance. The interlayer spacing expansion is balanced and stable, thus effectively protecting the stability of the MXene sheet structure while enhancing the ion transport channel.

[0036] The comparative example showed a lower effect than the embodiment, which may be due to the following reasons: Comparative Example 1 shows a moderate increase in interlayer spacing, but PO4... 3- Strong coordination ability, binding Li + migrate.

[0037] Comparative Example 2 has a moderate interlayer spacing expansion, but its anion coordination effect is relatively strong, and some Li... + Anchored, the increase in conductivity is limited.

[0038] The comparative example has a moderate interlayer spacing expansion, but the anions and Li... + The interaction is weak, and the conductivity is low.

[0039] In the comparative example, the interlayer spacing of the four layers was too large, resulting in an unstable MXene sheet structure and a large-volume anion TFSI. - Increasing the interface impedance results in higher conductivity but poor stability.

[0040] To verify the impact of different graphite anodes on battery performance, 8.8Ah pouch cells were assembled for electrical performance testing. The anode material used in the various examples and comparative examples was graphite@Li-MXene, the cathode was lithium iron phosphate, and the electrolyte was 1 mol / L LiPF6 EC and DMC (volume ratio 1.1-3). Pouch cells were assembled and subjected to rate performance and cycle testing. The test results are as follows:

[0041] The rate performance test conditions were as follows: At an ambient temperature of 25℃, the battery was fixed in place using a clamp, and subjected to two cycles of 0.5P charge / discharge and two cycles of 1P charge / discharge. The charge energy retention rate P at each rate was then calculated. rc and discharge energy retention rate P rd .

[0042] The cycle performance test conditions are as follows: at an ambient temperature of 25°C, the battery is fixed with a clamp and charged and discharged at 0.5P. The energy retention rate after 1000 cycles is calculated.

[0043] Compared to Example 1, Example 2 reduced the amount of Ti3C2T. x The content of [certain substances] leads to a decrease in the electronic and ionic conductivity of the material itself, which ultimately affects its electrical performance to some extent.

[0044] Compared to Example 1, Example 3 adds Ti3C2T. x The battery performance actually decreased due to the high content of MXene. This may be because the MXene surface itself contains a large number of groups (-OH, -F), which are electrophilic. Excessive MXene will introduce a large number of active sites, causing the electrolyte to decompose continuously on the MXene surface and generate a thick and loose SEI film.

[0045] Compared to Example 1, Example 4 increased the content of Li2SO4. The reason for the decrease in its electrical properties may be that an appropriate amount of Li2SO4 can pass through SO42-. 2 The coordination and steric hindrance effects stabilize the MXene interlayer spacing at 1.3-1.6 nm (suitable for Li). + (Optimal range for transport). When the Li2SO4 concentration is too high, a large amount of SO4... 2- Aggregation can occur between MXene layers, and even crystals may precipitate, resulting in excessively widened interlayer spacing (more than 1.8 nm) or local collapse.

[0046] Compared to Example 1, Example 5 reduced the Li2SO4 concentration. The reason for the decrease in battery performance may be that the MXene interlayer spacing control failed, the ion transport channels were narrow, and the coordination and anchoring effect with MXene was weak when the Li2SO4 concentration was too low. The MXene interlayer structure was easily destroyed under the volume expansion stress of graphite lithium insertion / delithiation. The ion channels gradually became blocked with the increase of cycle number, resulting in an increase in the battery cycle energy decay rate and a significant decrease in the energy retention rate after 1000 cycles.

[0047] Compared to Example 1, Example 6 reduced the calcination temperature in step 5). If the calcination temperature is too low, the heat energy will be insufficient to overcome the SO42-. 2 Due to the energy barrier between the MXene layers, a large amount of Li₂SO₄ is only adsorbed on the MXene surface and cannot achieve effective intercalation. Ultimately, the interlayer spacing of the MXene layers cannot be expanded to the optimal range of 1.3-1.6 nm and remains in a narrow state of 1.0-1.2 nm. The lithium-ion transport channel is blocked, and the rate performance of the battery is significantly reduced.

[0048] Compared to Example 1, Example 7 increases the calcination temperature in step 5). When the calcination temperature reaches 500°C, Li2SO4 will undergo a thermal decomposition reaction: Li2SO4→Li2O+SO2+O2, which leads to intercalation failure and thus affects the battery's electrical performance.

[0049] Compared to Example 1, the battery performance decreased when Li2SO4 was replaced with Li3PO4 in Comparative Example 1. This is because the PO4 in Li3PO4... 3- The coordination with the -OH and -F functional groups on the MXene surface is too strong; excessively strong coordination bonds will cause PO4 to... 3- Firmly anchored to the MXene surface, it is difficult to penetrate the interlayer to exert an expansion effect; at the same time, the high-charge anions easily cause electrostatic aggregation of MXene sheets, ultimately leading to a decrease in rate performance and cycling performance.

[0050] Comparative Example 2, compared to Example 1, replaced Li2SO4 with Li2CO. 3， The poor battery performance may be due to the fact that the solubility of Li2CO3 in the water / isopropanol mixture is much lower than that of Li2SO4, making the prepared intercalation solution prone to solute precipitation, and CO3 during the intercalation process. 2- Aggregates on the MXene surface, making uniform coating impossible, while CO3... 2- Side reactions easily occur at the electrode / electrolyte interface, forming a thick and loose Li2CO3-based SEI film, which severely reduces ion conductivity.

[0051] Compared to Example 1, Comparative Example 3, by replacing Li2SO4 with LiCl, showed a significant decrease in rate performance and long-cycle performance. This may be due to the fact that LiCl... - Its coordination effect is weaker than that of SO4. 2- The inability to effectively guide the orderly transport of lithium ions leads to irregular diffusion of lithium ions between layers, increasing transport resistance. At the same time, LiCl is an electronic insulator, and incompletely intercalated free LiCl will cover the highly conductive surface of MXene, destroying the continuous electronic conduction network inside the electrode and increasing internal resistance.

[0052] The poor battery performance of Comparative Example 4 compared to Example 1, where Li2SO4 was replaced with LiTFSI, may be due to the large volume of anion TFSI. - Increased interfacial transport resistance and the disruption of the conductive network by free LiTFSI impede electron conduction, resulting in a decrease in both ionic and electronic conductivity.

[0053] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; within the framework of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of one or more embodiments of this application as described above, which are not provided in detail for the sake of brevity.

[0054] One or more embodiments in this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments in this application should be included within the protection scope of this application.

Claims

1. A method for preparing MXene two-dimensional materials, characterized in that, Lithium sulfate solution was added to MXene dispersion, stirred until homogeneous, vacuum dried, and calcined at 300-500℃ to obtain MXene two-dimensional material.

2. The preparation method according to claim 1, characterized in that, The MXene dispersion is a mixture of multilayer MXene powder and water.

3. The preparation method according to claim 2, characterized in that, The preparation method of the multilayer MXene powder is as follows: the MXene precursor MAX phase material is immersed in an etching solution, mixed evenly, kept warm, washed, and dried to obtain multilayer MXene powder.

4. The preparation method according to claim 3, characterized in that, The general formula of the MXene precursor MAX phase material is M n+ 1AX n M is Ti, V, Nb or Mo, A is Al, Si, Ga or Zn, X is C or N, n=1–3; the etching solution is a mixed HF / HCl etching solution or a mixed fluoride / HCl etching solution, and the fluoride is LiF, NaF, KF or CaF2; the holding temperature is 150-200℃.

5. The preparation method according to any one of claims 1-4, characterized in that, The solvent for the lithium sulfate solution is water.

6. The preparation method according to any one of claims 1-4, characterized in that, The lithium sulfate solution has a concentration of 0.02-0.08 M, the MXene dispersion has a concentration of 2-8 mg / mL, the volume ratio of the lithium sulfate solution to the MXene dispersion is 1:3-5, the calcination temperature is 350-450℃, and the calcination is carried out under a protective atmosphere.

7. A two-dimensional MXene material, characterized in that, It is prepared by the preparation method described in any one of claims 1-6.

8. A method for preparing a graphite anode material, characterized in that, The MXene two-dimensional material as described in claim 7, water, and dispersant are mixed evenly and centrifuged to obtain a supernatant. Modified graphite is added to the supernatant, stirred evenly, dried, and calcined to obtain a graphite anode material.

9. The preparation method according to claim 8, characterized in that, The modified graphite is prepared by mixing graphite and nitric acid solution evenly, then washing, drying, grinding and sieving to obtain modified graphite.

10. A graphite anode material, characterized in that, It is prepared using the preparation method described in claim 8 or 9.

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