A sulfur-carbon compound composite cathode material and its preparation and application

Abstract

The present invention relates to a sulfur-carbon compound composite cathode material and a preparation method thereof. The electrochemically active structure of the sulfur-carbon compound composite cathode material is a chain structure of -S-S- (number of sulfur atoms ≤ 4) and C-S. The sulfur-carbon compound composite cathode material can be stably charged and discharged in both ether-based electrolytes and carbonate-based electrolytes, and both show a single plateau discharge curve. This discharge characteristic proves that the electrochemical reaction of the material is not the dissolution and deposition reaction process of the elemental sulfur cathode, and the dissolution and loss of polysulfides are avoided during the electrochemical process. The sulfur-carbon compound composite material developed by the present invention can be used as a novel cathode material for lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical power sources and electrode materials, and relates to a preparation method of a sulfur-carbon compound composite cathode material, in particular to a preparation method of a sulfur-carbon compound composite cathode material with high structural stability and a non-traditional dissolution and precipitation reaction mechanism. The prepared sulfur-carbon compound cathode material can be used as a new sulfur cathode for lithium-sulfur batteries with high cycle stability. Background Art

[0002] The demand for energy storage technologies in social development is increasing day by day. Lithium-sulfur batteries have a high theoretical specific capacity (1675 mAh / g) and theoretical energy density (2600 Wh / kg), and in the future, they may meet the application requirements of new battery energy storage systems (such as portable electronic devices, electric vehicles, and electrochemical energy storage stations), with broad application potential and becoming a research hotspot for high specific energy secondary batteries.

[0003] Conventional lithium-sulfur secondary batteries use elemental sulfur as the cathode active material and metallic lithium as the anode active material. For the elemental sulfur cathode, during the discharge process, the elemental sulfur ring opens and becomes long-chain S8 molecules, reacts with lithium to form soluble polysulfides, and is further reduced to insoluble Li2S2 and insoluble Li2S. The discharge platforms are two platforms at 2.3 V and 2.1 V. The dissolution of polysulfides exists in the first discharge platform, which will cause the loss of the sulfur cathode active material. The electrochemical conversion rate of the second discharge platform is slow, which limits the fast charge and discharge performance of the battery. Conventional technical means are to use carbon materials or oxides and sulfides with polar surface properties as carriers or catalytic reaction active sites to prepare a composite cathode material in which elemental sulfur is physically mixed with the carrier. Sulfur still exists in the form of micron-sized or larger particles in the carrier, and the dissolution and loss of polysulfides still occur during the discharge process of sulfur. The capture, physical confinement, and catalytic conversion effects of the carrier on polysulfides are limited, and it is difficult to achieve the effect of completely inhibiting the dissolution and loss of polysulfides. Therefore, it is necessary to develop a new reaction mechanism sulfur cathode material without the dissolution process of polysulfides and with faster electrochemistry reaction kinetics characteristics in the electrochemical reaction. The sulfur cathode of the new electrochemical reaction avoids the dissolution and deposition reaction of the conventional sulfur electrode and the dissolution and loss of active sulfur, and can significantly improve the cycle stability of lithium-sulfur batteries.

[0004] This invention patent regulates the synthesis method and preparation process mechanism of the active sulfur material, develops a sulfur-carbon compound-based cathode material with high structural stability without polysulfide intermediates in the electrochemical reaction process, and this sulfur-carbon compound composite cathode can carry out charge and discharge reactions in ether-based electrolytes and carbonate-based electrolyte systems. The sulfur cathode developed by the present invention avoids the dissolution and deposition reaction mechanism of traditional elemental sulfur and can effectively improve the cycle stability of lithium-sulfur batteries. Summary of the Invention

[0005] The object of the present invention is to provide a sulfur-carbon compound composite cathode material and a preparation method thereof. The sulfur-carbon compound composite cathode material prepared by this method can be stably charged and discharged in ether-based and carbonate-based electrolytes, and the discharge path is a single-platform conversion reaction, rather than the two-platform reaction of the dissolution and deposition reaction mechanism of elemental sulfur, avoiding the dissolution and loss of polysulfides, and thus having good cycle stability. The capacity hardly decays after 200 cycles.

[0006] The sulfur-carbon compound material obtained in the present invention is prepared by dehydrogenative sulfidation reaction between an organic oligomer containing R1 or R2 group and sulfur radicals to obtain a nano-sulfide with C-S and S-S structural units, and then dehydrogenative carbonization is carried out to prepare the sulfur-carbon compound material. At the same time, by using the hydrogen bond interaction between the surface functional groups of the high electron conductivity material and the surfactant, and the hydrogen bond interaction between the surfactant and the organic small molecule compound, the organic small molecule compound is induced to polymerize with the high electron conductivity material as the matrix to obtain a heterostructure composite with high structural stability; the sulfur-carbon compound composite cathode material is obtained by the dehydrogenative sulfidation reaction and dehydrogenative carbonization of the composite with sulfur radicals. The composite cathode material can be charged and discharged on a single platform (platform voltage is 1.5V - 2.1V) in both ether-based and carbonate-based electrolytes, and has good cycle stability.

[0007] The present invention provides a preparation method of a sulfur-carbon compound composite material with high structural stability, which is characterized in that, firstly, under the induction of a surfactant, by using the grafting and assembling action of a conductive material and an organic small molecule, a composite of an organic oligomer and a conductive material is prepared, and further, a sulfur-carbon compound composite material is prepared by the dehydrogenative sulfidation reaction of sulfur radicals and the organic oligomer. By regulating the mass ratio of the organic oligomer, the conductive material and the sulfur radicals in the synthesis, a nano-scale organic sulfur-carbon compound composite material can be prepared. The specific steps are as follows:

[0008] Step 1: Add a high electron conductivity nano material and a surfactant into a certain amount of solution to prepare a dispersion;

[0009] Step 2: Add an organic small molecule compound to the above dispersion, such as: furyl alcohol, polyhydroxy aldehyde, polyhydroxy alcohol, and one or more of the compounds that can be hydrolyzed to obtain the above substances;

[0010] Step 3: Add a certain amount of acid or base to the above mixture;

[0011] Step 4: Carry out a reaction in a closed system at a certain temperature for the above mixed solution to obtain a composite of an organic copolymer / conductive material;

[0012] Step 5: After the reaction, wash and dry the above composite;

[0013] Step 6: Grind the above-mentioned composite with sulfur according to a certain mass ratio, and carry out secondary polycondensation, dehydrogenation sulfidation and carbonization reactions under the protection of inert gas at a certain temperature, and finally obtain a sulfur-carbon compound composite cathode material with high structural stability.

[0014] Advantages of the present invention:

[0015] 1. A design and preparation method for a sulfur-carbon compound composite cathode material with high structural stability is proposed. The sulfur-carbon compound composite cathode material prepared by the present invention undergoes a conversion reaction on a single platform, rather than the dissolution and deposition reaction path of traditional elemental sulfur, avoiding the decline in the cycling performance of lithium-sulfur batteries caused by the loss of polysulfides.

[0016] 2. The sulfur-carbon compound prepared by the present invention is tightly combined with the conductive matrix and has high electronic conductivity. The conductivity of the composite sulfur cathode is as high as 3-5 S / cm, the electrochemical reaction voltage polarization is low, and the material has faster rate discharge characteristics.

[0017] 3. The cathode prepared with the sulfur-carbon compound composite material of the present invention can be charged and discharged cyclically in carbonate and ether electrolytes, which can effectively improve the use safety performance and cycling stability performance of lithium-sulfur batteries at different working environmental temperatures.

[0018] 4. The preparation method of the present invention is simple to operate, the reaction conditions are mild, the raw materials are widely sourced and low in cost, and it can realize the stable and reliable preparation of sulfur-carbon compound cathode materials at low cost and in large quantities, and has the characteristics of industrial practicality. Description of the Drawings

[0019] Figure 1 Infrared spectrum of the material prepared in Example 1;

[0020] Figure 2 First charge-discharge, 2nd and 29th discharge curves of the material prepared in Example 1 (in ether electrolyte);

[0021] Figure 3 Cycling stability curve of the material prepared in Example 1 (in ether electrolyte);

[0022] Figure 4 TEM image of the composite material prepared in Example 4. Detailed Embodiments

[0023] The following examples can enable those of ordinary skill in the art to more comprehensively understand the present invention, but do not limit the present invention in any form.

[0024] Example 1

[0025] Take 0.3 g of PVP (polyvinylpyrrolidone), dissolve it in 60 mL of deionized water, add 0.5 g of expanded graphite to the above solution, and perform ultrasonic dispersion; weigh 10 g of starch, dissolve it in the above dispersion, and stir; add 4 mL of furan-5-ol to the above mixture, then add 0.2 g of boric acid to it, and stir until evenly mixed. React the above mixture under a closed system at a temperature of 180 °C for 4 h, then filter, wash with water, and dry to prepare an oligomer. Grind the obtained oligomer and sublimed sulfur in a mass ratio of 1:5, and then place it in a tube furnace for heat treatment under nitrogen protection. The heat treatment temperature is 250 °C, and the heat treatment time is 15 h to obtain a composite cathode material of sulfur carbide (C6H 10 O5)-(S-S)2-(C8H7O2) and graphite.

[0026] Prepare a cathode slurry using the prepared sulfur carbide composite cathode material according to a mass ratio of active material: conductive agent: binder = 8:1:1, using NMP as the dispersion liquid (using acetylene black as the conductive agent and PVDF as the binder). Coat the slurry on carbon-coated aluminum foil and dry to prepare a cathode electrode sheet. Use metallic lithium as the anode, and inject an electrolyte of 1 M LiPF6 in EC:MC:EMC = 1:1:1 (volume ratio) to assemble a lithium-sulfur battery. Set the discharge voltage range to 1 - 3 V, and test the electrochemical performance at a current density of 100 mA / g. The sulfur carbide composite cathode material exhibits a single-platform discharge curve at about 2.0 V, the initial discharge specific capacity reaches 780 mAh / g, and the specific capacity after 200 cycles is 550 mAh / g. Also test the electrochemical performance of the prepared electrode in an electrolyte of 1 M LiTFSI in DOL:DME = 1:1 (volume ratio). The sulfur carbide composite cathode material also has a single-platform discharge curve at about 2.0 V, and the discharge specific capacity is 650 mAh / g. After 100 cycles, the specific capacity is 540 mAh / g.

[0027] Example 2

[0028] Take 0.3 g of CTAB, dissolve it in 70 mL of deionized water, add 0.4 g of carbon fiber to the above solution, and perform ultrasonic dispersion; weigh 12 g of glucose, dissolve it in the above dispersion, and stir; add 2 mL of furfuryl alcohol to the above mixture, then add 0.2 g of phosphoric acid to it, and stir until evenly mixed. Polymerize the above mixture under a closed system at a reaction temperature of 200 °C for 4 h, then filter, wash with water, and dry to prepare an oligomer. Grind the obtained oligomer and sublimed sulfur in a mass ratio of 1:5, and then place it in a tube furnace for heat treatment under nitrogen protection. The heat treatment temperature is 220 °C, and the heat treatment time is 15 h to obtain sulfur carbide C6H 11Composite cathode material of O6-(S-S)-(C8H7O2) and carbon nanotubes.

[0029] Example 3

[0030] Take 0.3 g of F127 and dissolve it in a mixed solution of 60 mL of deionized water and ethanol (the volume ratio of water to ethanol is 1:1). Add 0.4 g of polyaniline to the above solution and perform ultrasonic dispersion; weigh 5 g of water-soluble starch, dissolve it in the above dispersion, and stir; add 2 mL of furfuryl alcohol to the above mixture, then add 0.2 g of sodium carbonate, and stir until evenly mixed. Polymerize the above mixture under a closed system, with a reaction temperature of 150 °C and a time of 24 h, then filter, wash with water, and dry to prepare an oligomer. Grind the obtained oligomer and sodium polysulfide in a mass ratio of 1:10, and then place it in a tube furnace for heat treatment under nitrogen protection conditions, at 150 °C for 10 h and 320 °C for 5 h, to obtain the sulfur-carbon compound composite cathode material PANI-(S-S)2-(C8H7O2).

[0031] Example 4

[0032] Take 0.4 g of PVA and dissolve it in 50 mL of deionized water. Add 0.2 g of carbon nanotubes to the above solution and perform ultrasonic dispersion; weigh 0.2 g of sucrose, dissolve it in the above dispersion, and stir; add 2 mL of furan-5-ol and 0.1 g of boric acid to the above mixture, and stir until evenly mixed. Polymerize the above mixture under a closed system, with a reaction temperature of 180 °C and a time of 4 h, then filter, wash with water, and dry to prepare an oligomer. Grind the obtained oligomer and precipitated sulfur in a mass ratio of 1:4, and then place it in a tube furnace for heat treatment under nitrogen protection conditions, with a heat treatment temperature of 180 °C and a heat treatment time of 10 h, to obtain the boron-doped sulfur-carbon compound composite material B-C 12 H 20 O 11 Composite cathode of -(S-S)-(C8H5O2) and carbon nanotubes. The transmission electron micrograph of the prepared composite material is shown in the appendix Figure 4 .

[0033] Example 5

[0034] Take 0.3 g of CTAB and dissolve it in 50 mL of deionized water. Weigh 12 g of glucose, dissolve it in the above dispersion, and stir. Add 5 mL of furfuryl alcohol to the above mixture and stir until evenly mixed. Then add 0.1 g of phosphoric acid to it. Polymerize the above mixture under a closed system at a reaction temperature of 150 °C for 6 h, then filter, wash with water, and dry to prepare an oligomer. Grind the obtained oligomer and precipitated sulfur in a mass ratio of 1:5, and then place it in a tube furnace for heat treatment under nitrogen protection. The heat treatment temperature is 220 °C and the heat treatment time is 6 h to obtain a phosphorus-doped sulfur-carbon compound composite cathode material P-C6H 12 O6-(S-S)2-(C8H7O2).

[0035] Example 6

[0036] Weigh 12 g of glucose, dissolve it in 60 ml of deionized water, and stir. Add 5 mL of furfuryl alcohol to the above mixture and stir until evenly mixed. Then add 0.1 g of phosphoric acid to it. Polymerize the above mixture under a closed system at a reaction temperature of 150 °C for 6 h, then filter, wash with water, and dry to prepare an oligomer. Grind the obtained oligomer and precipitated sulfur in a mass ratio of 1:10, and then place it in a tube furnace for heat treatment under nitrogen protection. The heat treatment temperature is 300 °C and the heat treatment time is 6 h to obtain a phosphorus-doped sulfur-carbon compound composite cathode material P-C6H 12 O6-(S-S)2-(C8H7O2).

[0037] Example 7

[0038] Weigh 50 ml of deionized water, add 3 mL of furan-5-ol and 0.1 g of phosphoric acid to it, and stir until evenly mixed. Polymerize the above mixture under a closed system at a reaction temperature of 180 °C for 4 h, then filter, wash with water, and dry to prepare an oligomer. Grind the obtained oligomer and precipitated sulfur in a mass ratio of 1:10, and then place it in a tube furnace for heat treatment under nitrogen protection. The heat treatment temperature is 250 °C and the heat treatment time is 10 h to obtain a phosphorus-doped sulfur-carbon compound composite cathode material P-C6H 12 O6-(S-S)-(C8H7O2).

Claims

1. A sulfur-carbon composite cathode material, characterized in that: It is a composite cathode material with a sulfur-carbon compound as the electrochemically active component. The electrochemically active structural composition of the sulfur-carbon compound is: R1-(S-S) n -R2, where 1≤n≤2, and R1 or R2 is composed of one or more of oligomeric organic molecular chain segments containing a -C=C-C=C- conjugated structure and a furan ring structure or chain segment units of the above structure doped with non-carbon elements. The non-carbon elements include one or more of N, P, and B, and the mass content is 1% - 5%.

2. A sulfur-carbon composite cathode material, characterized in that: The composite material is composed of the sulfur-carbon compound material in Claim 1 and a conductive matrix material with high electron conductivity, and the conductive matrix material accounts for 3% - 15% of the total mass; The conductive matrix material with high electron conductivity can be one or more of carbon-based materials, conductive polymers, nano metal wires, etc.; The carbon-based material is one or more mixtures of carbon nanotubes, carbon fibers, acetylene black, graphene, graphite, expanded graphite, and the above carbon materials modified by hydroxylation; The conductive polymer is one or more of polyaniline, polythiophene, polyphenylene; the nano metal wire is one or more of nano copper, nano silver, nano aluminum, the diameter of the metal wire is 10nm - 100nm, and the length is 1μm - 100μm.

3. The sulfur-carbon composite cathode material according to claim 2, wherein: The conductive matrix material accounts for 5% - 10% of the total mass.

4. A method for preparing the sulfur-carbon compound composite cathode material according to claim 1, characterized in that, It includes the following steps: Step 1: Disperse the organic small molecule compound in an aqueous solution or a mixed solution of water and ethanol. The organic small molecule compound is one or more of polyhydroxy aldehydes, polyhydroxy alcohols, furyl alcohols, and the volume ratio of water to ethanol in the water and ethanol mixture is 1:0.5 - 1; Step 2: Add an acid or a base to the solution in Step 1; Step 3: React the solution in Step 2 in a closed system to obtain an organic oligomer; Step 4: Wash and dry the organic oligomer in Step 3 after the reaction; Step 5: Mix the organic oligomer in Step 4 evenly with sulfur, and under an inert gas, carry out polycondensation, dehydrogenation sulfidation, and carbonization reactions to finally obtain a sulfur-carbon compound cathode material with high structural stability.

5. A method for preparing the sulfur-carbon composite cathode material according to claim 2, characterized in that: It includes the following steps: S1: Disperse the organic small molecule compound, the material with high electron conductivity, and the surfactant in an aqueous solution or a mixed solution of water and ethanol. The organic small molecule compound is one or more of polyhydroxy aldehydes, polyhydroxy alcohols, furyl alcohols, and the volume ratio of water to ethanol in the water and ethanol mixture is 1:0.5 - 1; S2: Add an acid or a base to the solution in S1; S3: React the solution in S2 in a closed system to obtain an organic oligomer; S4: Wash and dry the organic oligomer in S3 after the reaction; S5: Mix the organic oligomer in S4 evenly with sulfur, and under an inert gas, carry out polycondensation, dehydrogenation sulfidation, and carbonization reactions to finally obtain a sulfur-carbon compound composite cathode material with high conductivity and high structural stability.

6. According to the preparation method described in Claim 4 or 5, it is characterized in that: The furyl alcohol in Step 1 includes one or more of furfuryl alcohol, furfuryl ethanol, furan dimethanol, furfuryl propanol, furan-5-ol; The polyhydroxy aldehyde in Step 1 includes one or more of dextran, glucose, deoxyascorbic acid, sucrose; The polyhydroxy alcohols in Step 1 include one or more of xylitol, sorbitol, maltol, water-soluble small molecule starch; The final concentration of one or more of the furyl alcohol, polyhydroxy aldehyde, and polyhydroxy alcohol is 0.001 - 1g / mL.

7. The preparation method according to claim 4 or 5, characterized in that: The acid in Step 2 includes one or more of formic acid, acetic acid, oxalic acid, boric acid, phosphoric acid, quinolinic acid, citric acid, malic acid, tartaric acid; In step 2, the base includes one or more of sodium bicarbonate, sodium carbonate, sodium hydroxide, methylamine, ethylamine, and ephedrine; the final concentration of the acid or base in step 2 is 0.001 - 0.01 g / mL.

8. The preparation method according to claim 4 or 5, characterized in that: In step 3, the reaction temperature is 90°C - 200°C; In step 5, the sulfur is one or more of elemental sulfur, sublimed sulfur, precipitated sulfur, insoluble sulfur, and sodium polysulfide; the mass ratio of sulfur to the organic oligomer in step 5 is 10 - 1:1; the dehydrogenative sulfidation and carbonization reaction temperature in step 5 is 150°C - 400°C, and the time is 10 - 20 hours.

9. The preparation method according to claim 8, wherein: In step 3, the reaction temperature is 120°C - 180°C, and the reaction time is 4 - 12 hours.

10. The preparation method according to claim 5, characterized in that: The molar ratio of the organic small molecule compound to the high electron conductivity material is 100 - 0.1:

1.

11. The preparation method according to claim 10, characterized in that: The molar ratio of the organic small molecule compound to the high electron conductivity material is 10 - 0.1:

1.

12. The preparation method according to claim 5, characterized in that: In step S1, the surfactant is one or more of F127, polyvinyl alcohol (PVA), cetyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), and sodium dodecylbenzenesulfonate (SDS); the final concentration of the surfactant is 0.001 - 0.01 g / mL.

13. Application of the sulfur-carbon compound composite cathode material according to claim 1 or 2 in a lithium-sulfur battery.

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