High-energy-density carbon fluoride-sulfur carbon composite positive electrode material and preparation method thereof

Abstract

The invention discloses a high-energy-density carbon fluoride-sulfur-carbon composite positive electrode material and a preparation method thereof in the field of battery material preparation, the composite positive electrode material is mainly formed by compounding a carbon fluoride material and nano sulfur particles through a hydrothermal reaction, and the nano sulfur particles are uniformly distributed on the surface and between layers of carbon fluoride. In the carbon fluoride-sulfur-carbon composite positive electrode material, the open-circuit voltage of carbon fluoride is slightly higher than that of a sulfur-carbon material, the carbon fluoride material starts to discharge at the initial discharge stage of the composite material, and carbon fluoride discharge products mainly comprise lithium fluoride and conductive carbon and can provide a conductive agent effect for sulfur particles, so that the problem of low sulfur conductivity can be solved without adding the conductive agent. As an active substance, nano sulfur provides a large amount of capacity in the middle and later periods of discharge, so that the problem of material activity failure caused by lithium fluoride is solved, and the voltage platform and capacity in the later period of discharge are improved; and the actual energy density of the carbon fluoride-sulfur carbon composite positive electrode material is integrally improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of battery material preparation, and particularly relates to a high-energy-density fluorocarbon-sulfur-carbon composite positive electrode material and a preparation method thereof. BACKGROUND

[0002] The fluorocarbon material has important application potential in the field of batteries due to its high theoretical capacity and high safety and reliability. However, in practical application, it has the problems of poor conductivity and serious polarization, resulting in that the actual capacity is much lower than the theoretical capacity.

[0003] During the discharging process, the discharge product of fluorocarbon is conductive carbon and lithium fluoride, in which lithium fluoride is an inert and inactive substance and is irreversible. With the increase of the discharging depth, the deposition amount of lithium fluoride in the fluorocarbon body and interlayer increases, gradually isolating the fluorocarbon active substance from the electrolyte, resulting in the decrease of the fluorocarbon material activity and the decrease of the working voltage, which is manifested as serious polarization in the late stage of discharging, rapid decrease of the voltage platform and low actual capacity.

[0004] In the prior art, the ideas for improving the performance of fluorocarbon include: (1) improving the conductivity, reducing the electrode polarization and increasing the electron and ion transmission capacity of the fluorocarbon electrode, but at the same time, the compaction density and the proportion of active substance are affected, and the energy density is limited to improve; (2) processing the material, such as metal ion doping, oxide mixing or particle nanocrystallization, to improve the material performance, but this method can only improve the initial stage performance, and has limited effect on the middle and late stage voltage and capacity. Therefore, the current methods cannot solve the problem of activity failure in the middle and late stages of discharging caused by lithium fluoride. SUMMARY

[0005] In view of the problem that the fluorocarbon material in the prior art has the problems of activity decrease in the middle and late stages of discharging, increased contact resistance with the electrolyte and obvious voltage polarization, thereby affecting the voltage platform and energy density, due to the generation of lithium fluoride during the discharging process, the present application provides a high-energy-density fluorocarbon-sulfur-carbon composite positive electrode material and a preparation method thereof, so as to improve the performance in the late stage of discharging and the actual energy density.

[0006] In the first aspect, the present application provides a fluorocarbon-sulfur-carbon composite positive electrode material prepared by a hydrothermal method, which is mainly composed of fluorocarbon material and nano-sulfur particles which are compounded by a hydrothermal reaction, wherein the nano-sulfur particles are uniformly distributed on the surface and interlayer of the fluorocarbon.

[0007] Further, the particle size of the nano-sulfur particles is ≤20 nm.

[0008] Further, the fluorination degree of the fluorocarbon material is 0.8-1.

[0009] Further, the fluorocarbon material is selected from one of fluorocarbon nanotubes, fluorinated graphite, fluorocarbon nanofibers, fluorinated graphene and fluorocarbon nanospheres.

[0010] In a second aspect, the present application also provides a method for preparing the carbon fluoride-sulfur carbon composite cathode material by using a hydrothermal method, comprising the following steps: (1) dispersing an emulsifier, 10-50 wt.% carbon fluoride material into deionized water to obtain a carbon fluoride solution; (2) dissolving 1-10 wt.% elemental sulfur in an organic solvent to obtain a sulfur solution; (3) adding the sulfur solution into the carbon fluoride solution, stirring for 20-60 min, and then transferring into a reaction kettle for hydrothermal reaction at 120-150℃ for 2-5h; (4) obtaining the carbon fluoride-sulfur carbon composite cathode material by filtering and drying the reaction product.

[0011] In step (1), the solubility of elemental sulfur is increased by using an organic solvent to form a liquid sulfur solution, which can realize the penetration and diffusion of elemental sulfur on the surface and interlayer of carbon fluoride, and is conducive to the preparation of a bulk homogeneous carbon fluoride / sulfur carbon composite cathode material.

[0012] In step (2), the lipophilic group of the emulsifier can improve the hydrophilicity of carbon fluoride, realize uniform dispersion in the aqueous solvent, and form micelles with elemental sulfur in the sulfur solution through the lipophilic group, which is uniformly adsorbed on the surface of carbon fluoride particles through van der Waals force and forms uniform reaction sites. After hydrothermal reaction, the micelles form a uniform elemental sulfur coating layer on the surface of carbon fluoride.

[0013] In step (3), the stable pressure and temperature reaction conditions provided by the hydrothermal reaction can control the reaction depth of elemental sulfur micelles and carbon fluoride material on the surface and interlayer, regulate the fluorine content of carbon fluoride material, and uniformly control the particle size and growth rate of elemental sulfur on the surface of carbon fluoride material, realizing the uniform and controllable distribution of elemental sulfur on the surface and interlayer of carbon fluoride.

[0014] Further, the emulsifier is one of polydimethylsiloxane, polyglycerol-isostearate, polysorbate, sorbitan fatty acid ester, or sodium dodecyl sulfate.

[0015] Further, the amount of the emulsifier is 0.1-1 wt.%.

[0016] Further, the organic solvent is one of carbon disulfide, carbon tetrachloride, isopropyl alcohol, or N-methylpyrrolidone.

[0017] Further, the amount of the organic solvent is 10-100 mL.

[0018] Further, the ultrasonic dispersion time is 20-40 min.

[0019] The working principle of the present application is: The present application forms a uniform precursor mixed solution by sulfur solution and carbon fluoride solution under the action of emulsifier. Through hydrothermal reaction, the uniform growth of nano sulfur particles in the interlayer and surface of carbon fluoride is realized, and finally the carbon fluoride / sulfur carbon composite positive electrode material is formed.

[0020] The problem of difficult solubility of sulfur molecules in water is solved by using organic solvent, and the emulsifier is used to change the hydrophobicity of carbon fluoride material, and the lipophilic group and the sulfur solution with large polarity form micelles and are adsorbed on the surface of carbon fluoride material particles. The stable pressure and temperature conditions provided by the hydrothermal reaction not only promote the interlayer penetration of sulfur solution in carbon fluoride material, realize the uniform growth of elemental sulfur in the interlayer and surface of carbon fluoride, but also effectively control the size and growth rate of sulfur particles.

[0021] In the carbon fluoride / sulfur carbon composite positive electrode material, the discharge platform of carbon fluoride and sulfur carbon material is close, so that the discharge curve of the composite material is smooth. The open circuit voltage of carbon fluoride is slightly higher than that of sulfur carbon material, and the discharge of carbon fluoride material starts first at the initial stage of discharge of the composite material. The discharge product of carbon fluoride is mainly lithium fluoride and conductive carbon, which can provide conductive agent for sulfur particles, and the problem of low sulfur conductivity can be solved without adding conductive agent.

[0022] The beneficial technical effects of the present application are: (1) The nano sulfur particles are uniformly distributed on the surface and interlayer of carbon fluoride, the carbon layer structure of carbon fluoride is regulated, and the reaction active site is increased; (2) The discharge product of carbon fluoride, conductive carbon, can be used as the conductive agent of sulfur to solve the problem of low sulfur conductivity without adding additional conductive agent; (3) The nano sulfur as active material provides a large capacity in the late stage of discharge, solves the problem of material activity failure caused by lithium fluoride, and improves the voltage platform and capacity in the late stage of discharge; (4) The actual energy density of the carbon fluoride-sulfur carbon composite positive electrode material is improved as a whole. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The flow chart of the present application for preparing high energy density carbon fluoride-sulfur carbon composite positive electrode material by hydrothermal method.

[0024] Figure 2 The micro-morphology of the high energy density carbon fluoride-sulfur carbon composite positive electrode material obtained in Example 1; (a) is the morphology of the whole; (b) is the enlarged morphology of the local part.

[0025] Figure 3 The discharge curve of the high energy density carbon fluoride-sulfur carbon composite positive electrode material obtained in Example 1 at 1C rate. DETAILED DESCRIPTION

[0026] The following is further described in detail by specific embodiments, the method flow is as shown in Figure 1 : Embodiment 1: A preparation method of a high-energy-density fluorocarbon-sulfur-carbon composite positive electrode material, comprising the following steps: (1) Take 0.5 wt.% of polysorbate (emulsifier) and 30 wt.% of fluorinated graphite (fluorocarbon material) with a fluorination degree of 0.85, and add them into deionized water in turn, and ultrasonic dispersion for 30 min to obtain a fluorocarbon solution; (2) Dissolve 5 wt.% of elemental sulfur in 50 mL of carbon disulfide (organic solvent) to obtain a sulfur solution; (3) Add the sulfur solution into the fluorocarbon solution, stir for 40 min, and then transfer to a reaction kettle for hydrothermal reaction at 130°C for 3 h; (4) After the reaction product is naturally cooled to room temperature, it is suction filtered and dried to obtain a fluorocarbon-sulfur-carbon composite positive electrode material.

[0027] Embodiment 2: A preparation method of a high-energy-density fluorocarbon-sulfur-carbon composite positive electrode material, comprising the following steps: (1) Take 0.1 wt.% of sodium dodecyl sulfate (emulsifier) and 10 wt.% of fluorinated carbon nanotubes (fluorocarbon material) with a fluorination degree of 0.8, and add them into deionized water in turn, and ultrasonic dispersion for 20 min to obtain a fluorocarbon solution; (2) Dissolve 1 wt.% of elemental sulfur in 10 mL of carbon tetrachloride (organic solvent) to obtain a sulfur solution; (3) Add the sulfur solution into the fluorocarbon solution, stir for 20 min, and then transfer to a reaction kettle for hydrothermal reaction at 120°C for 2 h; (4) After the reaction product is naturally cooled to room temperature, it is suction filtered and dried to obtain a fluorocarbon-sulfur-carbon composite positive electrode material.

[0028] Embodiment 3: A preparation method of a high-energy-density fluorocarbon-sulfur-carbon composite positive electrode material, comprising the following steps: (1) Take 1 wt.% of polydimethylsiloxane (emulsifier) and 50 wt.% of fluorinated graphene (fluorocarbon material) with a fluorination degree of 1, and add them into deionized water in turn, and ultrasonic dispersion for 40 min to obtain a fluorocarbon solution; (2) Dissolve 10 wt.% of elemental sulfur in 100 mL of N-methylpyrrolidone (organic solvent) to obtain a sulfur solution; (3) The sulfur solution was added to the carbon fluoride solution, stirred for 60 min, and then transferred to a reaction kettle for hydrothermal reaction at 150°C for 5 h; (4) After the reaction product was naturally cooled to room temperature, it was filtered and dried to obtain a carbon fluoride-sulfur carbon composite positive electrode material.

[0029] The micro-morphology of the carbon fluoride-sulfur carbon composite positive electrode material prepared in Example 1 is shown in FIG. 1. Figure 2

[0030] In comparison with the existing commercial carbon fluoride (fluorine-carbon ratio 0.85) and sulfur carbon material (sulfur-carbon ratio 0.8), the specific experimental process is as follows: the carbon fluoride-sulfur carbon composite positive electrode prepared in Example 1 was used as the positive electrode material, SP and CNTS were used as the conductive agent, and CMC+SBR was used as the binder, and the positive electrode slurry was prepared by uniformly mixing the positive electrode material, the conductive agent and the binder in a mass ratio of 80:10:10, coated on an aluminum foil, dried at 100°C, and then assembled into a lithium-carbon fluoride battery with metal lithium as the negative electrode in a 1% dry room. The commercial carbon fluoride (fluorine-carbon ratio 0.85) and sulfur carbon material (sulfur-carbon ratio 0.75) were used as the positive electrode material, and the rest was the same as in Example 1 to assemble lithium-carbon fluoride and lithium-sulfur batteries. The discharge test curves of the three groups of batteries under the same conditions of room temperature 25°C and 1C rate are shown in FIG. 2. Figure 3 Figure 3 It can be observed that the battery prepared from the sulfur carbon material (sulfur-carbon ratio 0.75) has two low-voltage platforms at 2.32 and 2.07 V under the 1C rate, and the discharge capacity is 837.48 mAh / g (cut-off discharge voltage is 1.70 V). The battery prepared from the commercial carbon fluoride (fluorine-carbon ratio 0.85) has an obvious voltage lag peak under the 1C rate, and the low-wave voltage is as low as 2.57 V, the discharge platform voltage is 2.63 V, and the specific capacity of the material is 753.46 mAh / g when the cut-off discharge voltage is 1.70 V. In comparison, the battery prepared from the carbon fluoride-sulfur carbon composite positive electrode material in Example 1 has a discharge low-wave voltage of 2.76 V, a discharge early-stage platform voltage of 2.71 V, and a new voltage platform at 2.40, so that the specific capacity of the composite material is increased to 844.46 mAh / g. Therefore, under the same battery preparation conditions, the battery prepared from the carbon fluoride-sulfur carbon composite positive electrode material has a voltage platform increased from 2.63 V to 2.76 V under the same rate of 1C, and the specific energy is increased by 17.61%, which shows that the carbon fluoride-sulfur carbon composite positive electrode material has a higher working voltage and energy density.​​

Claims

1. A hydrothermally prepared carbon fluoride-carbon sulfur composite cathode material, characterized in that: It is mainly composed of fluorinated carbon material and nano-sulfur particles through hydrothermal reaction, wherein the nano-sulfur particles are evenly distributed on the surface and interlayer of fluorinated carbon.

2. The hydrothermally prepared carbon fluoride-carbon sulfur composite cathode material according to claim 1, characterized in that: The particle size of nanosulfur particles is ≤20 nm.

3. The hydrothermally prepared carbon fluoride-carbon sulfur composite cathode material according to claim 1, characterized in that: The degree of fluorination of the carbon fluoride material is 0.8-1.

4. The hydrothermally prepared carbon fluoride-carbon sulfur composite cathode material according to claim 1, characterized in that: The fluorinated carbon material is selected from one of fluorinated carbon nanotubes, fluorinated graphite, fluorinated carbon nanofibers, fluorinated graphene, and fluorinated carbon nanospheres.

5. A method for preparing the fluorinated carbon-sulfur-carbon composite cathode material according to any one of claims 1 to 4 by a hydrothermal method, characterized in that: The following steps are involved: (1) adding an emulsifier and 10-50 wt.% of a carbon fluoride material into deionized water and dispersing the mixture to obtain a carbon fluoride solution; (2) dissolving 1-10 wt.% elemental sulfur in an organic solvent to obtain a sulfur solution; (3) Add the sulfur solution to the carbon fluoride solution, stir for 20-60 min, transfer to the reactor, and hydrothermally react at 120-150°C for 2-5 h; (4) The reaction product is filtered and dried to obtain a fluorinated carbon-sulfur carbon composite positive electrode material.

6. The method according to claim 5, wherein: The emulsifier is one of polydimethylsiloxane, polyglycerol isostearate, polysorbate, sorbitan fatty acid ester or sodium lauryl sulfate.

7. The method according to claim 6, wherein: The amount of the emulsifier is 0.1-1 wt.%.

8. The method according to claim 7, wherein: The organic solvent is one of carbon disulfide, carbon tetrachloride, isopropyl alcohol or N-methylpyrrolidone.

9. The method according to claim 8, wherein: The amount of the organic solvent used is 10-100 mL.

10. The method according to claim 9, wherein: In the step (1), ultrasonic dispersion is used for 10 to 60 minutes.

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