CN111477873A

CN111477873A - Lithium-sulfur battery conductive agent based on nano transition metal phosphide/carbon composite material and preparation method and application thereof

Info

Publication number: CN111477873A
Application number: CN202010289535.0A
Authority: CN
Inventors: 胡成; 陈传忠; 侠光
Original assignee: Shandong University
Current assignee: Shandong University
Priority date: 2020-04-14
Filing date: 2020-04-14
Publication date: 2020-07-31

Abstract

The invention relates to the technical field of lithium-sulfur battery conductive agents, in particular to a lithium-sulfur battery conductive agent based on a nanometer transition metal phosphide/carbon composite material and a preparation method and application thereof. The conductive agent comprises a conductive agent body and nano transition metal phosphide particles growing on the conductive agent body in situ, wherein the conductive agent body is a carbon material capable of conducting electricity. The conductive agent of the lithium-sulfur battery with the new structure is formed by compounding the conductive nano transition metal phosphide on the conductive carbon material, and the conductive agent not only has better conductivity than the traditional conductive agent, but also can effectively prevent the shuttle of polysulfide and generate certain catalytic action. Due to the catalytic action of the transition metal phosphide, the time of the soluble lithium polysulfide conversion liquid state reaction in the charging and discharging process can be effectively shortened, and the nucleation of the lithium polysulfide can be accelerated, so that the charging and discharging speed of the anode material is accelerated, the utilization rate of the active substance of the anode material is improved, and the specific capacity and the cycling stability of the lithium sulfur battery are improved.

Description

Lithium-sulfur battery conductive agent based on nano transition metal phosphide/carbon composite material and preparation method and application thereof

Technical Field

The invention relates to the technical field of lithium-sulfur battery conductive agents, in particular to a lithium-sulfur battery conductive agent based on a nanometer transition metal phosphide/carbon composite material and a preparation method and application thereof.

Background

The information in this background section is only for enhancement of understanding of the general background of the invention and is not necessarily to be construed as an admission or any form of suggestion that this information forms the prior art that is already known to a person of ordinary skill in the art.

The lithium-sulfur battery takes the elemental sulfur and the metallic lithium as the positive and negative active materials of the battery, the theoretical specific capacity is large (1675mAh/g), and the lithium-sulfur battery has the advantages of light weight, large capacity, environmental protection, easy obtainment of raw materials and the like, thereby being a secondary battery with great application prospect.

Although lithium sulfur batteries have great advantages in high energy density, lithium sulfur batteries also have some problems to be solved, among which a positive active material (S/L i)₂S) and the shuttling effect of soluble polysulfides are particularly prominent. At room temperature. The thermodynamically most stable sulfur molecule is S consisting of 8S atoms joined together₈Typical electron and ion insulators (5 × 10)^-30S cm^-1) Thus S₈In addition to the difficulty of activation when used as an electrode active material, the final discharge product L i₂S_x(x is 1-2) is electrically insulated and insoluble in electrolyte, the utilization rate of active substances is low, and the capacity is greatly attenuated, while the ether electrolyte commonly used in the lithium-sulfur battery at present is used for charge-discharge intermediate product polysulfide L i₂S_xThe (x is 4-8) has strong solubility, so that the charging and discharging process of the lithium-sulfur battery is a complex solid-liquid-solid process. This process is accompanied by dissociation and reconstruction of the positive electrode structure, resulting in poor stability of the electrode. Meanwhile, when the lithium-sulfur battery is charged, long-chain polysulfide ions generated by the positive electrode are driven by the concentration gradient to migrate to the negative electrode to react with metal lithium to generate short-chain polysulfide ions, and the short-chain polysulfide ions migrate back to the positive electrode under the action of an electric field and are further oxidized, and the process is continuously and circularly repeated to form a shuttle effect. The shuttle effect not only reduces the coulomb efficiency of the battery, but also causes the problems of corrosion of a metal lithium cathode, increase of the viscosity of the electrolyte and the like, so that the battery performance is improvedAnd is worsened.

In order to solve the above problems, carbon materials having excellent mechanical, electrical, and thermal conductive properties, adjustable pore structures, and abundant surface characteristics have received attention. Since 2009 researchers reported that mesoporous carbon CMK-3 and sulfur composite anodes, carbon materials such as porous activated carbon, graphene, carbon nanotubes, carbon nanofibers, carbon microspheres, and the like began to be applied to lithium sulfur batteries as support materials for the active substance sulfur. In the carbon-sulfur composite anode obtained by efficiently compounding the carbon material and sulfur, the nano carbon material can form an efficient anode conductive framework structure, so that the problem of low conductivity of sulfur and lithium sulfide is solved to a great extent; the unique pore structure of the nano carbonaceous material can also modulate the dissolution, migration and shuttling of polysulfide, and reduce the loss of active materials.

The method has been the research trend of lithium sulfur battery carbon anode material, but the method has the disadvantages of complicated preparation process, low yield, high cost and difficult mass production, and in addition, the discharge product L i is discharged in the long-cycle process₂Sx is deposited on the surface of the conductive framework, and part of Sx possibly breaks away from the conductive framework and cannot react to become sulfur or high-order polysulfide through a reversible charging process, so that the capacity is attenuated, and the service life of the battery is further shortened.

With the intensive research on sulfur positive electrodes, various forms of carbon-sulfur composite positive electrode materials have been developed. At present, most of sulfur composite positive electrodes are in the form of powder, and a conductive agent and a binding agent are used for further mixing with an active material, pulping and smearing. The commonly used conductive agents include conductive carbon black, carbon nanotubes, carbon nanofibers, graphene and the like, however, the inventors found that the conductive agents only provide a simple conductive effect and have a single function.

Disclosure of Invention

The invention aims to provide a conductive agent which can effectively prevent shuttle of polysulfide and generate certain catalysis besides conductive performance, thereby improving the electrochemical performance of a lithium-sulfur battery. Therefore, the invention discloses a lithium-sulfur battery conductive agent based on a nano transition metal phosphide/carbon composite material, and a preparation method and application thereof.

Specifically, to achieve the above object, the technical solution of the present invention is as follows:

in a first aspect of the present invention, a lithium-sulfur battery conductive agent based on a nano transition metal phosphide/carbon composite material is provided, which comprises a conductive agent body and nano transition metal phosphide particles grown in situ on the conductive agent body, wherein the conductive agent body is a carbon material capable of conducting electricity.

In some embodiments of the present invention, the carbon material comprises one or more of carbon black, carbon fiber, acetylene black, flake graphite, carbon nanotubes, graphene, porous activated carbon, and the like. The carbon material has excellent conductivity and mechanical property, and can be used as a carrier and an in-situ growth point of transition metal phosphide particles besides a conductive agent, so that the function of the conductive agent is enriched.

In some embodiments of the invention, the transition metal phosphide has a formula comprising MP, M₂P、M₃P、M₂P₃、M₅P₄、MNP_xEtc., wherein M, N represents different transition metal elements, and x represents the number of phosphorus atoms. For example FeP, CoP, Ni₂P、Mn₂P、Cu₃P、Zn₂P₃、Ni₄P₅. To say thatIt is clear that when the transition metal phosphide is MNP_xWhen x is the total number of phosphorus atoms collocated with metal M, N, e.g., the transition metal phosphide is Zn₂Mn₂P₄When it is made of Mn₂P and Zn₂P₃And (4) forming.

In the invention, the transition metal phosphide and the carbon material form a heterostructure after in-situ growth of the transition metal phosphide on the carbon material, so that the overall stability of the material is enhanced, and the conductivity of the material is obviously improved compared with that of a pure carbon material due to the conductivity of the transition metal phosphide, and sufficient electrons are provided for the absorbed lithium polysulfide while the liquid lithium polysulfide is restrained to prevent shuttling in the charging and discharging processes of the lithium-sulfur battery.

In some embodiments of the invention, the transition metal in the transition metal phosphide comprises one or more of iron, cobalt, nickel, copper, zinc and manganese, and the existing carbon material with only conductive function is modified to have catalytic capability, so as to shorten the conversion of soluble lithium polysulfide during charge and discharge (L i)₂S₂/Li₂S) time of liquid reaction, acceleration L i₂S₂/Li₂And S-shaped nucleation is performed, so that the charge and discharge rate of the positive electrode material is accelerated, the utilization rate of active substances of the positive electrode material is improved, and the specific capacity and the cycling stability of the lithium-sulfur battery are improved.

In some embodiments of the present invention, the mass percentage of the nano transition metal phosphide in the lithium-sulfur battery conductive agent is 10 to 50%, and the balance is the conductive agent body. Among them, the content of 25 to 45% is preferable, for example, 25%, 35%, 40%, 45%, etc., and particularly, 40% by weight is more preferable. At these contents, the lithium-sulfur battery conductive agent has a superior effect in catalysis. It is to be understood that the balance may also contain some inevitable impurities introduced during the preparation and the like, but it should be present in trace amounts.

In some embodiments of the present invention, the transition metal phosphide particles have a particle size of 10 to 20 nm. In the invention, the nano catalyst particles have strong dispersibility and larger surface area, can fully contact with lithium polysulfide, and improve the utilization rate of the catalyst; in addition, the small particle size also has a beneficial effect on the conductivity of the conductive agent itself.

In a second aspect of the present invention, there is provided a method for preparing the conductive agent for a lithium sulfur battery as described in the first aspect, comprising: and (2) reacting the oxidized conductive carbon material with a solution of transition metal cations, separating a solid product, then carrying out oxidation treatment on the solid product, and carrying out phosphating treatment on the obtained nano transition metal oxide-carbon composite material and hypophosphite in a protective atmosphere to obtain the nano transition metal phosphide/carbon composite material.

In some embodiments of the invention, the oxidized conductive carbon material is reacted with the solution containing the transition metal cation at a temperature of 20 to 100 ℃ for 6 to 14 hours, for example, at 60 ℃ for 14 hours and at 80 ℃ for 12 hours. The optimal reaction temperature and time are different for different solvents and different transition metal cations, and in the case of ferric nitrate solution with N, N-dimethylformamide as the solvent, the preferred reaction condition is to react for 14h at 80 ℃. The reaction time and temperature will affect the number and size of the conductive agent surface particles.

In some embodiments of the present invention, the conductive carbon material used in the preparation method includes one or more of conductive carbon black, carbon fiber, acetylene black, flake graphite, carbon nanotube, graphene, polypyrrole, polyaniline, polyacetylene, porous activated carbon, and the like. According to the preparation method, the carbon material can be used as a carrier and an in-situ growth point of transition metal phosphide particles besides a conductive agent, so that the function of the conductive agent is enriched.

It should be understood that polypyrrole, polyaniline and the like are carbon sources, and are not carbon materials which are already formed, but in the subsequent phosphating process, carbonization is realized under the action of protective atmosphere and high temperature to form carbon materials, and meanwhile, in-situ generation of transition metal phosphide on the carbon materials is realized.

In some embodiments of the invention, a method of preparing the oxidized conductive carbon material comprises: conducting hydrothermal reaction on a conductive carbon material and hydrogen peroxide at 100-200 ℃ for 2-5 h, or conducting annealing treatment on the conductive carbon material in an oxygen atmosphere at 100-300 ℃, or placing the conductive carbon material in an ozone atmosphere for oxidation treatment, or obtaining the conductive carbon material by adopting a Hummers method. The conductive carbon material is oxidized to load a great amount of oxygen-containing functional groups (such as hydroxyl, carboxyl and the like) on the surface, and the functional groups are electronegative in the solution and can attract the free electropositive transition metal cations in the solution.

In some embodiments of the invention, the method of separating the solid product comprises filtration, centrifugation, or the like; preferably, the method further comprises the steps of washing and drying the solid product obtained after filtration to remove residual liquid in the solid product. Optionally, the washing method comprises centrifugal washing, suction filtration washing and the like; the drying method includes any one of forced air drying, vacuum drying, freeze drying, supercritical drying, and the like.

In some embodiments of the present invention, the mass ratio of the nano transition metal oxide-carbon composite material to the hypophosphite powder is 1 (5-30), and may be, for example, 1:5, 1:10, 1:15, 1:20, or 1:30, wherein 1:20 is a more preferable addition ratio. The proportion of the two substances has a more remarkable influence on the nano transition metal phosphide/carbon composite material, because the PH released by the decomposition of hypophosphite in heating₃And the hypophosphite is a toxic and flammable gas, and when the addition amount of the hypophosphite is increased and exceeds the range of the proportion, the environment is polluted and the synthetic danger is increased.

Alternatively, the hypophosphite salt comprises any one of sodium hypophosphite, potassium hypophosphite, calcium hypophosphite, aluminum hypophosphite, and the like. Alternatively, pH may be used₃In place of hypophosphite, a mixed gas with nitrogen or an inert gas, preferably, the pH of the mixed gas₃The volume fraction of (A) is 5-100%. Considering the cost and safety issues, sodium hypophosphite is preferred, which has lower cost than potassium hypophosphite and other hypophosphites such as potassium hypophosphite, and the sodium hypophosphite gradually releases PH by means of thermal decomposition₃Has a pH higher than that of the direct use of₃And higher safety of its mixed gasAnd therefore, the method is more suitable for being used in some laboratories or enterprises. However, it should be noted that PH may also be used in enterprises with strict or regulated safety measures₃The mixed gas with nitrogen or inert gas is used as a raw material, which has the advantage of lower cost than sodium hypophosphite, but has higher requirements on production safety, so at least the tightness of a production device is required to be paid attention to during production, the concentration of oxygen is ensured to be controlled within a safe range, an oxygen concentration monitoring device is arranged at a feed inlet and the like, and the production tail gas is recycled.

In some embodiments of the present invention, the phosphating temperature is 300-600 ℃ and the time is 0.5-4 h, for example, phosphating at 450 ℃ for 3h, phosphating at 350 ℃ for 3h, phosphating at 600 ℃ for 0.5h, and phosphating at 300 ℃ for 4h may be performed, wherein phosphating at 450 ℃ for 3h is preferred. Transition metal oxide is converted into transition metal phosphide through phosphating treatment, the transition metal phosphide has strong polarity, polar polysulfide can be effectively adsorbed, and the dissolution of polysulfide is inhibited, so that the shuttle of polysulfide is limited, and the corrosion to negative lithium is reduced.

In some embodiments of the invention, the solution containing the oxidized conductive carbon material and the transition metal cations is prepared by: mixing the oxidized carbon-based conductive agent powder with a solvent, adding one or more of an iron source, a cobalt source, a nickel source, a copper source, a zinc source and a manganese source, and performing ultrasonic dispersion to obtain a suspension.

In some embodiments of the present invention, the mass-to-volume ratio (w/v) of the conductive carbon material to the solvent is 1g (100-3500) m L, the concentration of the transition metal cation in the suspension is 0.02-3 mol/L, for example, any one of the concentrations of 0.02 mol/L, 0.1 mol/L, 0.5 mol/L, 1 mol/L, 2.2 mol/L, 3 mol/L, etc. when the above ranges, the metal cation is prone to agglomeration, coagulation, etc. during the reaction.

For example, assuming that the mass of the conductive carbon material is 1g and the volume of the solution containing the transition metal cations is 1000m L, in which the concentration of the transition metal cations is 0.5 mol/L (excluding the volume change due to the addition of the conductive carbon material), when a solution containing the transition metal cations is prepared using cobalt acetate and water as raw materials, 0.5mol of cobalt acetate is required and 88.54g of cobalt acetate is required in terms of mass, but since cobalt acetate is often present in a form containing four crystal waters, 124.54g of cobalt acetate tetrahydrate is required.

In the invention, the conductive carbon material with the functional group after oxidation can attract transition metal cations to form nuclei and grow on the surface of the conductive carbon material in a solution to form oxide particles.

In some embodiments of the invention, the solvent comprises: deionized water, ethanol, N-dimethylformamide, ammonia water, etc. The solvent functions as a dispersion of oxidized carbon-based conductive agent powder and provides a site for the reaction of the carbon-based conductive agent powder and transition metal ions.

According to the production method of the present invention, preferably, a transition metal salt of a monovalent acid group is used, which has better solubility in a solvent. Thus, the iron source includes at least one of ferric nitrate, ferric chloride, ferric acetate, and the like. The cobalt source includes at least one of cobalt nitrate, cobalt chloride, cobalt acetate, and the like. The copper source includes at least one of copper nitrate, copper chloride, copper acetate, and the like. The zinc source includes at least one of zinc nitrate, zinc chloride, zinc acetate, and the like. The manganese source includes at least one of manganese nitrate, manganese chloride, manganese acetate, and the like. The nickel source includes at least one of nickel nitrate, nickel chloride, nickel acetate, and the like.

In some embodiments of the invention, the protective atmosphere comprises nitrogen or an inert gas. The main function of the protective atmosphere is to prevent explosions during the phosphating process.

In a third aspect of the invention, the invention also provides the application of the lithium-sulfur battery conductive agent based on the nano transition metal phosphide/carbon composite material in the field of energy storage; in particular to the application of the conductive agent of the lithium-sulfur battery in the lithium-sulfur battery.

Compared with the prior art, the invention has the following beneficial effects:

(1) the invention is based on the use of electrically conductive carbon materialsThe conductive agent of the lithium-sulfur battery with a new structure is formed by compounding the conductive nano transition metal phosphide, the conductive agent is superior to the conductive property of the traditional conductive agent, and the shuttle of polysulfide can be effectively prevented and certain catalytic action can be generated₂S₂/Li₂S) time of liquid-state-transition reaction, acceleration L i₂S₂/Li₂And S-shaped nucleation is performed, so that the charge and discharge rate of the positive electrode material is accelerated, the utilization rate of active substances of the positive electrode material is improved, and the specific capacity and the cycling stability of the lithium-sulfur battery are improved.

(2) The transition metal phosphide formed on the surface of the conductive agent prepared by the invention has the characteristic of strong polarity, so that a polar intermediate product polysulfide (such as lithium polysulfide) generated in the charging and discharging process can be effectively adsorbed, the dissolution of the polysulfide is inhibited, the shuttle of the polysulfide is limited, and the corrosion to negative lithium is reduced.

(3) According to the invention, the conductive nano transition metal phosphide is compounded on the conductive carbon material, so that the conductive carbon material and the conductive nano transition metal phosphide form a stable heterostructure with excellent conductivity, and sufficient electrons can be provided for the adsorbed lithium polysulfide while liquid lithium polysulfide is bound in the charging and discharging processes of the lithium-sulfur battery. In addition, the d electron orbit which is not completely occupied by the transition metal can obviously improve the electron exchange between the lithium polysulfide and the conductive carbon material, effectively accelerate the phase change from the lithium polysulfide to the solid lithium sulfide, and ensure the continuous activity of the transition metal phosphide catalyst in the reaction process.

(4) Furthermore, the transition metal phosphide can reduce the decomposition barrier of insulating lithium sulfide, lithium atoms migrate under the traction of phosphorus atoms to form lithium ions, and the decomposition of lithium sulfide is promoted, so that the phenomenon that lithium sulfide pinning on a positive electrode carrier does not participate in further reaction is avoided, and the utilization rate of active substances, the battery coulombic efficiency and the cycling stability are improved.

(5) The invention prepares the lithium sulfur conductive agent with a new structure by modifying the traditional carbon material lithium sulfur battery conductive agent for the first time, the conductive agent can be prepared into carbon sulfur composite anode materials in any form by continuing the existing processes of lithium battery slurry mixing, coating, baking and the like, the preparation process of the lithium sulfur battery anode material is simplified, the preparation method is simple, inert gas is not needed, high temperature and high pressure are not needed, the cost is low, the energy consumption is low, and the large-scale industrial production of the lithium sulfur battery is facilitated.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention. Embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein:

FIG. 1 is a Scanning Electron Microscope (SEM) image of iron phosphide/Super P in a first embodiment of the present invention.

FIG. 2 is a Transmission Electron Microscope (TEM) image of iron phosphide/Super P in the first embodiment of the present invention.

FIG. 3 is an X-ray diffraction (XRD) pattern of iron phosphide/Super P in the first example of the present invention.

FIG. 4 shows a first embodiment of the invention of an iron phosphide/Supe P pair discharge intermediate L i₂S₆And (4) adsorbing the model.

FIG. 5 shows the iron phosphide/Super P surface charging reactant L i in the first embodiment of the present invention₂And S, decomposing the model.

Fig. 6 is a graph showing the rate cycle of a lithium-sulfur battery using iron phosphide/Super P as a conductive agent according to the first embodiment of the present invention.

Fig. 7 is a 0.5C cycle chart of a lithium sulfur battery using iron phosphide/Super P as a conductive agent according to the first embodiment of the present invention.

Fig. 8 is a graph showing the rate cycle of a lithium-sulfur battery using iron phosphide/Super P as a conductive agent in accordance with a second embodiment of the present invention.

Fig. 9 is a 0.5C cycle chart of a lithium sulfur battery using iron phosphide/Super P as a conductive agent according to a second embodiment of the present invention.

FIG. 10 is an X-ray diffraction (XRD) pattern of cobalt phosphide/Super P in a third example of the present invention.

Fig. 11 is a graph showing the rate cycle of a lithium-sulfur battery using cobalt phosphide/Super P as a conductive agent in a third embodiment of the present invention.

Fig. 12 is a 0.5C cycle chart of a lithium sulfur battery using cobalt phosphide/Super P as a conductive agent according to a third embodiment of the present invention.

FIG. 13 is a Scanning Electron Microscope (SEM) image of an iron phosphide/MWCNT in the fourth example of the present invention.

FIG. 14 is a Transmission Electron Microscope (TEM) image of an iron phosphide/MWCNT in the fourth example of the present invention.

Fig. 15 is a graph of the rate cycle of a lithium sulfur battery using an iron phosphide/MWCNT as a conductive agent in a fourth example of the present invention.

FIG. 16 is a 0.5C cycle chart of a lithium sulfur battery using an iron phosphide/MWCNT as a conductive agent according to a fourth embodiment of the present invention.

Fig. 17 is a graph showing the rate cycle of a lithium-sulfur battery using cobalt phosphide/CNF as a conductive agent in a fifth example of the present invention.

Fig. 18 is a 0.5C cycle chart of a lithium-sulfur battery using cobalt phosphide/CNF as a conductive agent in the fifth example of the present invention.

FIG. 19 is a graph showing the rate cycle of a lithium-sulfur battery using a nickel phosphide/MWCNT as a conductive agent in accordance with a sixth embodiment of the present invention.

FIG. 20 is a 0.5C cycle chart of a lithium sulfur battery using a nickel phosphide/MWCNT as a conductive agent according to a sixth embodiment of the present invention.

Fig. 21 is a rate cycle diagram of a lithium sulfur battery according to a first comparative example of the present invention.

Fig. 22 is a 0.5C cycle chart of a lithium sulfur battery in the first comparative example of the present invention.

Fig. 23 is a rate cycle plot for a lithium sulfur battery according to a second comparative example of the present invention.

Detailed Description

The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The experimental procedures, in which specific conditions are not noted in the following examples, are generally carried out according to conventional conditions or according to conditions recommended by the manufacturers.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The reagents or starting materials used in the present invention can be purchased from conventional sources, and unless otherwise specified, the reagents or starting materials used in the present invention can be used in a conventional manner in the art or in accordance with the product specifications. In addition, any methods and materials similar or equivalent to those described herein can be used in the methods of the present invention. The preferred methods and materials described in this invention are exemplary only.

As previously described, discharge product L i is discharged during long cycles₂Sx is deposited on the surface of the conductive framework, and part of Sx possibly breaks away from the conductive framework and cannot react to become sulfur or high-order polysulfide through a reversible charging process, so that the capacity is attenuated, and the service life of the battery is further shortened. Therefore, the invention provides a lithium-sulfur battery conductive agent based on a nano transition metal phosphide/carbon composite material and a preparation method thereof. The invention will now be further described with reference to the drawings and detailed description of the specification.

First embodiment

1. The preparation process of nanometer transition metal phosphide/carbon composite material includes the following steps:

(1) weighing 1g of Super P (conductive carbon black), weighing 10m L30 wt.% of hydrogen peroxide, putting the two into a stainless steel reaction kettle with the capacity of 100m L and the inner lining of Teflon, carrying out hydrothermal reaction for 4h at 150 ℃, centrifugally cleaning black powder obtained after the reaction for 3 times by using deionized water, and then carrying out forced air drying for 3h at 120 ℃ to obtain the Super P with oxygen-containing groups.

(2)80mg of the Super P with the oxygen-containing group and 3.2mmol of ferric nitrate are added into 160m L N, N-dimethylformamide and placed in a sealed glass bottle for ultrasonic dispersion for 1 hour to form 0.2 mol/L Fe³⁺And (4) suspending the solution. The mixed solution is transferred to an oil bath environment at 60 ℃, and the reaction is stirred for 14 hours. And centrifugally cleaning and separating the reacted mixed solution, and freeze-drying to obtain the iron oxide/Super P composite material.

(3) The iron oxide/Super P composite material and sodium hypophosphite are placed into two ends of a corundum boat (the front end of the sodium hypophosphite in the airflow direction) according to the mass ratio of 1:20, and the corundum boat is transferred to a tubular furnace. And reacting for 3 hours at 450 ℃ under the protection of argon in a tubular furnace with the flow rate of 60sccm, cooling, and collecting black powder, namely the iron phosphide (FeP)/Super P conductive agent, wherein the content of FeP is 40 wt% (mass percentage).

2. The preparation of a lithium sulfur battery based on the iron phosphide/Super P conductive agent of this example included the steps of:

(4) uniformly mixing acidified carbon nanotubes and sublimed sulfur according to the mass ratio of 2:8 to obtain a main material of the anode material, sequentially preparing the main material, the ferric phosphate/Super P conductive agent and the PVDF binder into slurry according to the mass ratio of 8:1:1, coating the slurry on hydrophobic carbon paper, drying, rolling and cutting into pole pieces with the diameter of 12mm to obtain the sulfur anode.

(5) The electrolyte adopts dioxolane (DO L) and ethylene glycol dimethyl ether (DME) as 1:1 electrolyte, 0.2M anhydrous lithium nitrate and 1.0M bis (trifluorosulfonyl) imide lithium (L iTFSI) are added, a metal lithium sheet with the thickness of 0.5mm is adopted as a negative electrode, a commercial polypropylene diaphragm (Celgard 2400) is adopted as a battery diaphragm, and the electrochemical performance of the CR2025 button cell is tested after the CR2025 button cell is assembled in a glove box according to the sequence of a sulfur positive electrode, the diaphragm and the metal lithium.

Fig. 1 and fig. 2 are a scanning electron microscope image and a transmission electron microscope image of the iron phosphide/Super P conductive agent prepared in the embodiment, respectively, and it can be clearly seen that: the shape of the iron phosphide is nanoparticles growing on the surface of the Super P, the distribution is uniform, and the particle size is about 10-20 nm. Meanwhile, the iron phosphide/Super P conductive agent presents a porous structure on the whole, and the iron phosphide/Super P conductive agent has cooperative work and can strengthen the adsorption effect on dissolved lithium polysulfide.

FIG. 3 is an XRD pattern of the iron phosphide/Super P conductive agent prepared in the present example, and it can be seen that: the two phases contained in the conductive agent are carbon and iron phosphide.

FIG. 4 is a graph of iron phosphide-adsorbed lithium sulfur battery reaction intermediate L i calculated using density functional theory₂S₆Iron atom in iron phosphide L i₂S₆Two sulfur atoms in the middle part form a bond, electrons are transferred to the S atom from the iron atom so as to strip the two sulfur atoms at the tail end under the action of coulomb force, and a next product L i is formed₂S₄。

FIG. 5 is a calculated phosphorization using density functional theoryIron apparent charging reactant L i₂Decomposition diagram of S L i₂Under the traction of phosphorus atoms, lithium atoms in S gradually move away from S atoms around the phosphorus atoms, so that bonds are broken to form lithium ions.

Further, the battery obtained in this example was tested for charge/discharge rate performance at 0.2C, 0.5C, 1C, 0.5C, and 0.2C rates, and the relationship between the gram capacity of sulfur and the charge/discharge rate was shown in fig. 6. It can be seen that: the discharging specific capacities of the battery under 0.2C, 0.5C and 1C are 1516mAh g respectively^-1、1340mAh g^-1、1122mAh g^-1And after the battery is cycled for 5 times at the rate of 1C, the specific discharge capacity of the battery can still be recovered to 1316mAh g at the rate of 0.5C^-1。

Further, the battery obtained in this example was subjected to a cycle stability test at a rate of 0.5C, and the results are shown in fig. 7. It can be seen that: the highest discharge specific capacity of the battery can reach 1344mAh g^-1And after 300 cycles of charging and discharging, the specific capacity of the battery can still be kept at 1232mAh g^-1The capacity retention rate was 94.57%, the average attenuation rate per cycle was 0.018%, and the coulombic efficiency was close to 100%.

Second embodiment

(2)80mg of the Super P with the oxygen-containing group and 0.16mol of ferric nitrate are added into 80m L deionized water and placed in a sealed glass bottle for ultrasonic dispersion for 1 hour to form 2 mol/L Fe³⁺And (4) suspending the solution. The reaction was stirred for 8h at 20 ℃. And centrifugally cleaning and separating the reacted mixed solution, and then blowing and drying at 60 ℃ to obtain the iron oxide/Super P composite material.

(4) uniformly mixing acidified carbon fibers and sublimed sulfur according to the mass ratio of 2:8 to obtain a main material of the anode material, sequentially preparing the main material, the ferric phosphate/Super P conductive agent and the PVDF binder into slurry according to the mass ratio of 8:1:1, coating the slurry on hydrophobic carbon paper, drying, rolling and cutting into pole pieces with the diameter of 12mm to obtain the sulfur anode.

The battery prepared in this example was tested for charge and discharge rate performance at 0.2C, 0.5C, 1C, 0.5C, and 0.2C rates, and the relationship between the gram capacity of sulfur and the charge and discharge rate was shown in fig. 8. It can be seen that: the discharging specific capacity of the battery at 0.2C, 0.5C and 1C is 1339mAh g^-1、1049 mAh g^-1、827mAh g^-1And the specific discharge capacity of the battery is increased after the battery is cycled for many times, because the catalyst is completely activated in the charging and discharging processes.

The battery prepared in this example was subjected to a cycle stability test at 0.5C rate, and the results are shown in fig. 9, where it can be seen that: the highest discharge specific capacity of the battery can reach 1144mAh g^-1And after 300 cycles of charging and discharging, the specific capacity of the battery can still be kept at 976mAh g^-1The capacity retention rate was 93.76%, the average attenuation rate per cycle was 0.020%, and the coulombic efficiency was close to 100%.

In addition, combining the first and second examples, it can be demonstrated that the prepared nano transition metal phosphide/carbon composite material has wide applicability as a conductive agent of a positive electrode material of a lithium-sulfur battery.

Third embodiment

(1) weighing 1g of Super P (conductive carbon black), weighing 10m L30 wt.% of hydrogen peroxide, putting the two into a stainless steel reaction kettle with the capacity of 100m L and the inner liner of Teflon, reacting for 4h at 150 ℃, centrifugally cleaning black powder obtained after reaction for 3 times by deionized water, and then blowing and drying for 3h at 120 ℃ to obtain the Super P with oxygen-containing groups.

(2) 60mg of the Super P with oxygen-containing groups, 2.8mol of cobalt acetate and 10.8m L25 wt.% of ammonia water are added into 140m L of absolute ethyl alcohol, and then placed into a flask with a plug for ultrasonic dispersion for 15min to form 0.2 mol/L of Co²⁺And (4) suspending the solution. And transferring the mixed solution to an oil bath environment at the temperature of 80 ℃, and stirring for reaction for 12 hours. And centrifugally cleaning and separating the reacted mixed solution, and freeze-drying to obtain the cobalt oxide/Super P composite material.

(3) The cobalt oxide/Super P composite material and sodium hypophosphite are placed into two ends of a corundum boat (the front end of the sodium hypophosphite in the airflow direction) according to the mass ratio of 1:20, and are transferred to a tube furnace. And reacting for 3 hours at 350 ℃ under the protection of argon in a tubular furnace with the flow rate of 60sccm, cooling, and collecting black powder, namely the cobalt phosphide (CoP)/Super P conductive agent, wherein the CoP content is 40 wt% (mass percentage).

2. The preparation of a lithium sulfur battery based on the cobalt phosphide/Super P conductive agent of this example included the steps of:

(4) uniformly mixing the acidified carbon nano tube and sublimed sulfur in a mass ratio of 2:8 to obtain a main material of the anode material, sequentially preparing the main material, the cobalt phosphide/Super P conductive agent and the PVDF binder into slurry in a mass ratio of 8:1:1, coating the slurry on hydrophobic carbon paper, drying, rolling and cutting into pole pieces with the diameter of 12mm to obtain the sulfur anode.

(5) The electrolyte adopts dioxolane (DO L) and ethylene glycol dimethyl ether (DME) as 1:1 electrolyte, 0.2M anhydrous lithium nitrate and 1.0M bis (trifluorosulfonyl) imide lithium (L iTFSI) are added, a metal lithium sheet with the thickness of 0.5mm is adopted as a negative electrode, a commercial polypropylene diaphragm (Celgard 2400) is adopted as a battery diaphragm, and the electrochemical performance is tested after the CR2025 button cell is assembled in a glove box according to the sequence of a sulfur positive electrode, the diaphragm and the metal lithium.

FIG. 10 is the XRD pattern of the cobalt phosphide/Super P conductive agent prepared in the present example, and it can be seen that: the two phases contained in the conductive agent are carbon and cobalt phosphide.

Further, the charge/discharge rate performance of the battery obtained in this example was tested at 0.2C, 0.5C, 1C, 0.5C, and 0.2C rates, and the relationship between the gram sulfur capacity and the charge/discharge rate was shown in fig. 11. It can be seen that: the battery is discharged for the first time under the multiplying power of 0.2C, and the specific capacity of the battery can reach 1412 mAh g^-1Discharge specific capacities at 0.2C, 0.5C and 1C were 1275mAh g, respectively^-1、1118mAh g^-1、928mAh g^-1. After the battery is cycled for 5 times under the multiplying power of 1C, the discharging specific capacity of the battery can still be recovered to 1103mAh g under the condition of 0.5C^-1。

Further, the battery obtained in this example was subjected to a cycle stability test at a rate of 0.5C, and the results are shown in fig. 12, where it can be seen that: the highest discharge specific capacity of the battery can reach 1117 mAh g^-1And after 300 cycles of charging and discharging, the specific capacity of the battery can still be maintained at 956mAh g^-1The capacity retention rate is 89.10%, the average attenuation rate per cycle is 0.036%, and the coulombic efficiency is close to 100%.

Fourth embodiment

(1) 80mg of purchased Arlatin MWCNT (multi-walled carbon nanotube) with carboxylation and 3.2mmol of ferric nitrate are added into 160m L N, N-dimethylformamide and placed in a sealed glass bottle for ultrasonic dispersion for 1h to form 0.2 mol/L Fe³⁺And (4) suspending the solution. And the mixed solution is transferred to an oil bath environment at 60 ℃, and the reaction is stirred for 14 hours. And centrifugally cleaning and separating the reacted mixed solution, and freeze-drying to obtain the iron oxide/MWCNT composite material.

(2) The iron oxide/MWCNT composite material and sodium hypophosphite are placed into two ends of a corundum boat (the front end of the sodium hypophosphite in the airflow direction) according to the mass ratio of 1:20, and the corundum boat is transferred to a tube furnace. Reacting for 3h at 450 ℃ under the protection of argon in a tubular furnace with the flow rate of 60sccm, cooling, and collecting black powder, namely the iron phosphide (FeP)/MWCNT conductive agent, wherein the content of FeP is 40 wt% (mass percentage).

2. A lithium-sulfur battery based on the iron phosphide/MWCNT conductive agent of this example was prepared, comprising the steps of:

(3) uniformly mixing the acidified carbon nano tube and sublimed sulfur according to the mass ratio of 2:8 to obtain a main material of the anode material, sequentially preparing the main material, the ferric phosphate/MWCNT conductive agent and the PVDF binder into slurry according to the mass ratio of 8:1:1, coating the slurry on hydrophobic carbon paper, drying, rolling and cutting into pole pieces with the diameter of 12mm to obtain the sulfur anode.

(4) The electrolyte adopts dioxolane (DO L) and ethylene glycol dimethyl ether (DME) as 1:1 electrolyte, 0.2M anhydrous lithium nitrate and 1.0M bis (trifluorosulfonyl) imide lithium (L iTFSI) are added, a metal lithium sheet with the thickness of 0.5mm is adopted as a negative electrode, a commercial polypropylene diaphragm (Celgard 2400) is adopted as a battery diaphragm, and the electrochemical performance is tested after the CR2025 button cell is assembled in a glove box according to the sequence of a sulfur positive electrode, the diaphragm and the metal lithium.

Fig. 13 and 14 are a scanning electron microscope image and a transmission electron microscope image of the iron phosphide/MWCNT conductive agent prepared in this example, respectively, and it can be clearly seen that the morphology of the iron phosphide is nanoparticles growing on the MWCNT tube wall, the distribution is uniform, and the particle size is about 10 to 20 nm.

Further, the charge/discharge rate performance of the battery obtained in this example was measured at 0.2C, 0.5C, 1C, 0.5C, and 0.2C rates, and the relationship between the sulfur gram capacity and the charge/discharge rate of the sample obtained by the measurement was shown in fig. 15. It can be seen that: the discharging specific capacities of the battery samples at 0.2C, 0.5C, 1C and the discharging specific capacities of 1413mAh g^-1、1206mAh g^-1、1040mAh g^-1And after the battery is cycled for 5 times under the multiplying power of 1C, the discharge specific capacity of the battery can still be recovered to 1200mAh g under the specific capacity of 0.5C^-1。

Further, at 0.5C magnificationThe following cycle stability test was performed on the battery obtained in this example, and the results are shown in fig. 16, where it can be seen that: the initial value of the discharge specific capacity of the battery can reach 1250 mAh g^-1After 300 cycles of charge and discharge, the specific capacity of the battery can still be kept at 1113mAh g^-1The capacity retention rate was 89.11%, the average attenuation rate per cycle was 0.036%, and the coulombic efficiency was close to 100%.

Fifth embodiment

(1) weighing 1g of CNF (carbon nanofiber), weighing 10m L30 wt.% of hydrogen peroxide, putting the CNF and the hydrogen peroxide into a stainless steel reaction kettle with the capacity of 100m L and the lining of Teflon, reacting for 4h at 150 ℃, centrifugally cleaning black powder obtained after reaction for 3 times by using deionized water, and then blowing and drying for 3h at 120 ℃ to obtain Super P with oxygen-containing groups.

(2) 60mg of CNF with oxygen-containing groups and 140m L anhydrous ethanol 60mg of Super P with oxygen-containing groups, 2.8mol of cobalt acetate and 10.8m L25 wt.% of ammonia water are added into 140m L anhydrous ethanol, and the mixture is placed in a reaction kettle and placed in a Teflon lining for ultrasonic dispersion for 15min to form 0.2 mol/L Co of 0.2 mol/L²⁺And (4) suspending the solution. And screwing the reaction kettle, transferring the reaction kettle to an air-blast drying oven at 80 ℃, and preserving heat for 12 hours. And centrifugally cleaning and separating the reacted mixed solution, and freeze-drying to obtain the cobalt oxide/CNF composite material.

(3) The cobalt oxide/CNF composite material and sodium hypophosphite are placed into two ends of a corundum boat (the front end of the sodium hypophosphite in the airflow direction) according to the mass ratio of 1:20, and the corundum boat is transferred to a tubular furnace. Reacting for 3 hours at 350 ℃ under the protection of argon in a tubular furnace with the flow rate of 60sccm, cooling, and collecting black powder, namely the cobalt phosphide (CoP)/CNF conductive agent, wherein the CoP content is 45 wt% (mass percentage).

2. The preparation of a lithium-sulfur battery based on the cobalt phosphide/CNF conductive agent of the present example comprises the steps of:

(4) uniformly mixing acidified carbon nanofibers and sublimed sulfur in a mass ratio of 2:8 to obtain a main material of the anode material, sequentially preparing the main material, a cobalt phosphide/CNF conductive agent and a PVDF binder into slurry in a mass ratio of 8:1:1, coating the slurry on hydrophobic carbon paper, drying, rolling and cutting into pole pieces with the diameter of 12mm to obtain the sulfur anode.

The charge and discharge rate performance of the battery obtained in this example was tested at 0.2C, 0.5C, 1C, 0.5C, and 0.2C rates, and the relationship between the obtained sulfur gram capacity and the charge and discharge rate was shown in fig. 17. It can be seen that: the discharging specific capacities of the battery at 0.2C, 0.5C and 1C are 1230 mAh g, 1067 mAh g and 860 mAh g respectively^-1And after the battery is cycled for 5 times at the multiplying power of 1C, the specific discharge capacity of the battery can still be recovered to 1042mAh g at the rate of 0.5C^-1。

The battery obtained in this example was subjected to the cycle stability test at 0.5C rate, and the results are shown in fig. 18, where it can be seen that: the initial value of the discharge specific capacity of the battery can reach 1078mAh g^-1After 300 cycles of charging and discharging, the specific capacity of the battery can still be kept at 890mAh g^-1The capacity retention rate is 82.56%, the average attenuation rate per cycle is 0.058%, and the coulombic efficiency is close to 100%.

Sixth embodiment

(1) synthesizing oxidized multi-walled carbon nanotubes (MWCNTs) by a modified Hummers method, assembling a 250m L reaction bottle in an ice-water bath, adding a proper amount of concentrated sulfuric acid, adding a solid mixture of 2g of MWCNTs and 1g of sodium nitrate under magnetic stirring, slowly adding 6g of potassium permanganate, controlling the reaction temperature to be less than or equal to 10 ℃, stirring for 2 hours under an ice bath condition, taking out, stirring for reaction at room temperature for 5 days, and reacting the sample with 5 wt.% of H₂SO₄The solution was diluted and stirred for 2H, then 6m L H was added₂O₂The solution turns into bright yellow and is stirred to react for 2 hoursAnd (4) centrifuging. Then using 0.2M H₂SO₄、H₂O₂And repeatedly washing the mixed solution and HCl, finally washing the mixed solution by using distilled water to ensure that the pH value of the mixed solution is 7, and finally fully drying the sample in a freeze dryer to obtain black powder, namely the oxidized MWCNT.

(2)80mg of oxidized MWCNT prepared in this example and 0.3mol of nickel chloride were added to 100m L deionized water and placed in a sealed glass bottle for ultrasonic dispersion for 1h to form 3 mol/L of Ni²⁺And (4) suspending the solution. And the mixed solution is transferred to an oil bath environment at 60 ℃, and the reaction is stirred for 14 hours. And centrifugally cleaning and separating the reacted mixed solution, and freeze-drying to obtain the nickel oxide/MWCNT composite material.

(3) The nickel oxide/MWCNT composite material and sodium hypophosphite are placed into two ends of a corundum boat (the front end of the sodium hypophosphite in the airflow direction) according to the mass ratio of 1:20, and the corundum boat is transferred to a tube furnace. Reacting at 450 deg.C for 3h under the protection of argon gas in a tubular furnace at a flow rate of 60sccm, cooling, and collecting black powder to obtain nickel phosphide (Ni)₅P₄) /MWCNT conductive agent, wherein, Ni₅P₄The content was 35 wt% (mass%).

2. A lithium-sulfur battery based on the nickel phosphide/MWCNT conductive agent of this example was prepared, comprising the steps of:

(4) mixing the acidified carbon nano tube and the sublimed sulfur according to the mass ratio of 2:8, uniformly mixing the mixture and the main material for the anode material, sequentially preparing the main material, the nickel phosphide/MWCNT conductive agent and the PVDF binder into slurry according to the mass ratio of 8:1:1, coating the slurry on hydrophobic carbon paper, drying and rolling the hydrophobic carbon paper, and cutting the hydrophobic carbon paper into pole pieces with the diameter of 12mm to obtain the sulfur anode.

The batteries obtained in the present example were charged and discharged at 0.2C, 0.5C, 1C, 0.5C, and 0.2C magnificationsThe rate performance was tested and the relationship between the sulfur gram capacity and the charge-discharge rate was shown in fig. 19. It can be seen that: the discharging specific capacities of the battery at 0.2C, 0.5C, 1C and 915mAh g are 1246, 1073 and 915mAh g respectively^-1And after the battery is cycled for 5 times under the multiplying power of 1C, the specific discharge capacity of the battery can still be recovered to 1071mAh g under the condition of 0.5C^-1。

The battery obtained in this example was subjected to the cycle stability test at 0.5C rate, and the results are shown in fig. 20, where it can be seen that: the highest discharge specific capacity of the battery can reach 1080mAh g^-1After 300 cycles of charging and discharging, the specific capacity of the battery can still be kept at 900mAh g^-1The capacity retention rate is 85.30%, the average attenuation rate per cycle is 0.049%, and the coulombic efficiency is close to 100%.

Seventh embodiment

(1) weighing 1g of acetylene black, weighing 10m L30 wt.% of hydrogen peroxide, putting the acetylene black and the hydrogen peroxide into a stainless steel reaction kettle with the capacity of 100m L and the inner lining of teflon, reacting for 2 hours at 200 ℃, centrifugally cleaning black powder obtained after the reaction for 3 times by using deionized water, and then blowing and drying for 3 hours at 120 ℃ to obtain the acetylene black with oxygen-containing groups.

(2)80mg of acetylene black with oxygen-containing groups and 30mmol of copper acetate are added into 300m L deionized water and placed in a reaction kettle teflon lining for ultrasonic dispersion for 15min to form 0.1 mol/L Cu²⁺And (4) suspending the solution. And screwing the reaction kettle, transferring the reaction kettle to a 100 ℃ air blast drying oven, and preserving heat for 6 hours. And centrifugally cleaning and separating the reacted mixed solution, and freeze-drying to obtain the copper oxide/acetylene black composite material.

(3) The copper oxide/acetylene black composite material and potassium hypophosphite are placed into two ends of a corundum boat (the front end of the sodium hypophosphite in the airflow direction) according to the mass ratio of 1:30, and the corundum boat is transferred to a tubular furnace. Reacting at 600 deg.C for 0.5h under protection of nitrogen gas in a tube furnace at a flow rate of 60sccm, cooling, and collecting black powder as copper phosphide (Cu)₃P)/acetylene black conductive agent, wherein Cu₃The P content was 25 wt% (mass%).

Eighth implementationExample (b)

(1) weighing 1g of porous activated carbon, weighing 10m L30 wt.% of hydrogen peroxide, putting the two into a stainless steel reaction kettle with the capacity of 100m L and the inner lining of Teflon, reacting for 5 hours at 100 ℃, centrifugally cleaning black powder obtained after reaction for 3 times by using deionized water, and then blowing and drying for 3 hours at 120 ℃ to obtain the porous activated carbon with oxygen-containing groups.

(2)80mg of the porous activated carbon with oxygen-containing groups, 0.4mmol of zinc acetate and 20mmol of manganese acetate are added into 40m L deionized water and placed in a reaction kettle teflon lining for ultrasonic dispersion for 15min to form 0.5 mol/L Mn²⁺And 0.01 mol/L Zn²⁺Adding 20m L0.02 mol/L mol zinc acetate aqueous solution and 10m L2 mol/L mol manganese acetate aqueous solution into the suspension, screwing the reaction kettle, transferring the reaction kettle to an air-blast drying box at 80 ℃, preserving heat for 12 hours, centrifugally cleaning and separating the reacted mixed solution, and freeze-drying to obtain the zinc oxide/manganese oxide/porous activated carbon composite material.

(3) And (3) placing the zinc oxide/manganese oxide/porous activated carbon composite material and sodium hypophosphite at a mass ratio of 1:5 into two ends of a corundum boat (the front end of the sodium hypophosphite in the airflow direction) and transferring the corundum boat into a tubular furnace. Reacting at 300 deg.C for 4 hr under the protection of nitrogen gas at flow rate of 60sccm in a tubular furnace, cooling, and collecting black powder to obtain zinc phosphide (Zn)₂P₃) Manganese phosphide (Mn)₂P)/porous activated carbon as a conductive agent, wherein Zn₂P₃35 wt% of Mn₂The P content was 10 wt% (mass%).

First comparative example

A lithium sulfur battery is prepared comprising the steps of:

(1) uniformly mixing an acidified carbon nano tube and sublimed sulfur in a mass ratio of 2:8 to obtain a main material of a positive electrode material, sequentially preparing the main material, a commercial conductive agent (Super P, which is the same material as the material obtained in the previous embodiment of the invention and is purchased from Super-dense high graphite and carbon company) and a PVDF (polyvinylidene fluoride) binder into slurry in a mass ratio of 8:1:1, coating the slurry on an aluminum foil, drying and rolling the aluminum foil, and cutting the aluminum foil into pole pieces with the diameter of 12mm to obtain the sulfur positive electrode.

(2) The electrolyte adopts dioxolane (DO L) and ethylene glycol dimethyl ether (DME) as 1:1 electrolyte, 0.2M anhydrous lithium nitrate and 1.0M bis (trifluorosulfonyl) imide lithium (L iTFSI) are added, a metal lithium sheet with the thickness of 0.5mm is adopted as a negative electrode, a commercial polypropylene diaphragm (Celgard 2400) is adopted as a battery diaphragm, and the electrochemical performance is tested after the CR2025 button cell is assembled in a glove box according to the sequence of a sulfur positive electrode, the diaphragm and the metal lithium.

The charge and discharge rate performance of the battery obtained in the present comparative example was tested at 0.2C, 0.5C, 1C, 0.5C, and 0.2C rates, and the relationship between the obtained sulfur gram capacity and the charge and discharge rate was shown in fig. 21. It can be seen that: the discharging specific capacity of the battery is only 780mAh g at 0.2C, 0.5C, 1C and respectively^-1、 610mAh g^-1、540mAh g^-1。

The battery obtained in the present comparative example was subjected to the cycle stability test at 0.5C rate, and the result is shown in fig. 22, where it can be seen that: the capacity of the battery after 300 circles is 470mAh g^-1The capacity retention was only 75.82%, and the coulombic efficiency drop was significant, below 90%.

Second comparative example

A lithium sulfur battery is prepared comprising the steps of:

(1) uniformly mixing acidified carbon nanofibers and sublimed sulfur according to the mass ratio of 2:8 to obtain a main material of the anode material, sequentially preparing the main material, a commercial conductive agent (Super P) and a PVDF binder into slurry according to the mass ratio of 8:1:1, coating the slurry on hydrophobic carbon paper, drying, rolling and cutting into pole pieces with the diameter of 12mm to obtain the sulfur anode.

At 0.2C, 0.5C, 1C, 0.5C, 0.2C magnificationThe charge-discharge rate performance of the battery obtained in this comparative example was tested, and the relationship between the obtained sulfur gram capacity and the charge-discharge rate was shown in fig. 23. It can be seen that: the discharge specific capacities of the battery samples at 0.2C, 0.5C and 1C are only 826mAh g respectively^-1、 610mAh g^-1、507mAh g^-1。

The battery obtained in the comparative example was subjected to a cycle stability test at a rate of 0.5C, and the capacity after 300 cycles of the battery was 423mAh g^-1The capacity retention was only 69.34%, and the coulombic efficiency drop was significant, below 90%.

In addition, by comparing the first, third, fourth, and sixth embodiments with the first comparative example, and the second embodiment with the second comparative example, respectively, it can be seen that: after the conductive agent prepared in the embodiment of the invention is adopted to replace a commercial conductive agent, the electrochemical performance is obviously improved, and the long cycle is more stable, because the transition metal phosphide composite conductive agent realizes the additional functions of lithium polysulfide adsorption and catalytic conversion while maintaining the original excellent conductive performance of the conductive agent. The d electron orbit which is not completely occupied by the transition metal can obviously improve the electron exchange between the lithium polysulfide and the conductive carbon material, effectively accelerate the phase change from the lithium polysulfide to the solid lithium sulfide, and ensure the continuous activity of the catalyst in the reaction process. Meanwhile, the decomposition barrier of insulating lithium sulfide can be reduced, lithium atoms are migrated under the traction of phosphorus atoms to form lithium ions, and the decomposition of the lithium sulfide is promoted, so that the phenomenon that lithium sulfide pinning on a positive electrode carrier does not participate in further reaction is avoided, the redox reaction process of a sulfur-containing active substance is promoted, and the utilization rate of the active substance, the battery coulombic efficiency and the cycling stability are improved.

Although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A lithium sulfur battery conductive agent based on a nano transition metal phosphide/carbon composite, comprising:

a conductive agent body, a carbon material capable of conducting electricity, and

nano transition metal phosphide particles grown in situ on the conductive agent body.

2. The lithium sulfur battery conductive agent of claim 1, wherein the transition metal phosphide has a formula comprising MP, M₂P、M₃P、M₂P₃、M₅P₄、MNP_xWherein M, N represents different transition metal elements, and x represents the number of phosphorus atoms.

3. The lithium-sulfur battery conductive agent according to claim 1, wherein the mass percent of the nano transition metal phosphide in the lithium-sulfur battery conductive agent is 10-50%, and the balance is the conductive agent; preferably, the content of the transition metal phosphide particles is 25-45%; more preferably, the transition metal phosphide particles content is 40%.

4. The lithium-sulfur battery conductive agent according to claim 1, wherein the transition metal phosphide particles have a particle size of 10 to 20 nm.

5. The lithium sulfur battery conductive agent according to any one of claims 1 to 4, wherein the carbon material comprises one or more of carbon black, carbon fiber, acetylene black, flake graphite, carbon nanotubes, graphene, porous activated carbon;

or the transition metal in the transition metal phosphide comprises one or more of iron, cobalt, nickel, copper, zinc and manganese.

6. A preparation method of a lithium-sulfur battery conductive agent based on a nano transition metal phosphide/carbon composite material is characterized by comprising the following steps: and (2) reacting the oxidized conductive carbon material with a solution of transition metal cations, separating a solid product, then carrying out oxidation treatment on the solid product, and carrying out phosphating treatment on the obtained nano transition metal oxide-carbon composite material and hypophosphite in a protective atmosphere to obtain the nano transition metal oxide-carbon composite material.

7. The method for preparing the conductive agent for the lithium-sulfur battery according to claim 6, wherein the oxidized conductive carbon material is reacted with the solution containing the transition metal cations at a temperature of 20 to 100 ℃ for 6 to 14 hours;

preferably, the method for preparing the oxidized conductive carbon material comprises: carrying out hydrothermal reaction on a conductive carbon material and hydrogen peroxide at 100-200 ℃ for 2-5 h, or carrying out annealing treatment on the conductive carbon material in an oxygen atmosphere at 100-300 ℃, or placing the conductive carbon material in an ozone atmosphere for oxidation treatment, or obtaining the conductive carbon material by adopting a Hummers method;

preferably, the conductive carbon material adopted in the preparation method comprises one or more of conductive carbon black, carbon fiber, acetylene black, crystalline flake graphite, carbon nano tube, graphene, polypyrrole, polyaniline, polyacetylene and porous activated carbon;

alternatively, the method of separating the solid product comprises filtration or centrifugation; preferably, the method further comprises the steps of washing and drying the solid product obtained after filtration; preferably, the washing method comprises centrifugal washing or suction filtration washing; or the drying method comprises any one of air-blast drying, vacuum drying, freeze drying and supercritical drying.

8. The preparation method of the lithium-sulfur battery conductive agent according to claim 6, wherein the mass ratio of the nano transition metal oxide-carbon composite material to the hypophosphite powder is 1 (5-30), preferably 1: 20;

or the hypophosphite comprises any one of sodium hypophosphite, potassium hypophosphite, calcium hypophosphite and aluminum hypophosphite;

alternatively, pH is used₃In place of the hypophosphite, a mixed gas with nitrogen or an inert gas, preferably, the pH of the mixed gas₃The volume fraction of (A) is 5-100%;

or, the phosphating temperature is 300-600 ℃, and the time is 0.5-4 h;

alternatively, the protective atmosphere comprises nitrogen or an inert gas.

9. The method for preparing a conductive agent for a lithium-sulfur battery according to any one of claims 6 to 8, wherein the solution containing the oxidized conductive carbon material and the transition metal cation is prepared by: mixing oxidized carbon-based conductive agent powder with a solvent, adding one or more of an iron source, a cobalt source, a nickel source, a copper source, a zinc source and a manganese source, and ultrasonically dispersing into a suspension to obtain the carbon-based conductive agent;

preferably, the mass volume ratio of the conductive carbon material to the solvent is 1g (100-3500) m L, and the concentration of transition metal cations in the suspension is 0.02-3 mol/L;

preferably, the solvent comprises: any one of deionized water, ethanol, N-dimethylformamide and ammonia water;

preferably, the iron source comprises at least one of ferric nitrate, ferric chloride, and ferric acetate;

preferably, the cobalt source comprises at least one of cobalt nitrate, cobalt chloride, cobalt acetate;

preferably, the copper source comprises at least one of copper nitrate, copper chloride, copper acetate;

preferably, the zinc source comprises at least one of zinc nitrate, zinc chloride, zinc acetate;

preferably, the manganese source comprises at least one of manganese nitrate, manganese chloride, manganese acetate;

preferably, the nickel source comprises at least one of nickel nitrate, nickel chloride, and nickel acetate.

10. Use of a lithium sulphur battery conductive agent based on a nano transition metal phosphide/carbon composite according to any one of claims 1 to 5 or prepared by the method of any one of claims 6 to 9 in the field of energy storage, preferably in a lithium sulphur battery.