Black phosphorus-based negative electrode material, preparation method thereof, negative electrode sheet and battery

Abstract

The application discloses a negative electrode material and a preparation method thereof. The negative electrode material is in a core-shell structure, the inner core comprises black phosphorus, carbon nanotubes and a transition metal phosphide, and the shell comprises amorphous carbon. The negative electrode material has good electronic conductivity and cycle stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of black phosphorus-based negative electrode materials, in particular to a transition metal phosphide modified black phosphorus-based negative electrode material, a preparation method thereof, a negative electrode sheet and a battery. BACKGROUND

[0002] Black phosphorus is a two-dimensional semiconductor material with high electrical conductivity (300 S / m), low lithium ion diffusion barrier and high theoretical specific capacity (2596 mAh / g), and also has a black metal luster appearance similar to that of graphite crystals. Black phosphorus has an orthorhombic crystal structure with a space group of Cmca. There are 8 phosphorus atoms in a single unit cell, forming a layered folded structure. The phosphorus atoms within each layer are connected in a zigzag chain, and the length of the P-P bond varies from to Due to the strong P-P bond energy in the structure of black phosphorus, black phosphorus has the highest density (2.70 g / cm 3 ) and the lowest chemical reactivity among all allotropes, and does not burn in air. Black phosphorus has a larger interlayer distance than graphite , making it easier to embed ions and having higher ionic conductivity. First-principles calculations show that the folded structure of black phosphorus can provide a super-fast diffusion channel for lithium ions, indicating that the dynamic performance of black phosphorus for storing lithium ions also has great potential. Although black phosphorus (BP) is considered a promising high-specific-capacity lithium-ion battery negative electrode material, its preparation difficulty hinders its application. To solve the application problem, high-energy ball milling is used to convert commercial red phosphorus into air-stable black phosphorus; and graphite oxide is introduced. The composite material can buffer the expansion of the electrode, and the in-situ formed covalent bond can tightly fix the black phosphorus on the graphite oxide. The composite material minimizes the deformation of the BP structure through interfacial compounding, thereby exhibiting excellent lithium storage reversibility. At the same time, carbon nanotubes (CNTs) can be used to form a three-dimensional conductive network, and BP is tightly combined with CNTs through chemical bonds to obtain a BP@CNTs composite material. The initial discharge specific capacity of BP@CNTs at 0.1C silver electrode is as high as 2229 mAh / g, and the coulombic efficiency of the first charge and discharge is 90%, which is much higher than that of the unmodified BP electrode.

[0003] Generally, there are two modes for the composite of black phosphorus-based materials. For example, CN112018363A discloses a black phosphorus-based material composite composed of black phosphorus, carbon material and conductive polymer. The carbon material is covalently connected to the black phosphorus by high-energy ball milling to form a black phosphorus-carbon modified composite, and the conductive polymer is continuously or discontinuously coated on the surface of the black phosphorus-carbon modified composite by liquid phase in-situ polymerization to obtain the required black phosphorus-based composite negative electrode material. Another mode is disclosed in CN102491294A, which uses a high-pressure synthesis device to synthesize pure-phase black phosphorus from white phosphorus and red phosphorus at different pressures and temperatures. It also discloses a modified black phosphorus synthesized from white phosphorus and red phosphorus doped with a modification material, with the proportion of raw materials being 10% to 90% and the proportion of modification material being 5% to 50%.

[0004] The above technical solutions improve the electronic conductivity of black phosphorus material by modifying and synthesizing black phosphorus material with functionalized carbon material or by modifying black phosphorus material with red phosphorus and white phosphorus composite. However, the polarization characteristics of the material are not improved, the charge transfer resistance of the material is not reduced, and the uniformity of the original phase and the composite phase of the material is poor. SUMMARY

[0005] The present application aims to at least partially solve one of the technical problems in the related art. To this end, the present application provides a transition metal phosphide modified black phosphorus-based negative electrode material, a preparation method thereof, a negative electrode sheet and a battery.

[0006] The first aspect of the present application provides a negative electrode material, which is a core-shell structure. The inner core includes black phosphorus (BP), carbon nanotubes (CNT) and transition metal phosphide (TMPs), and the shell includes amorphous carbon.

[0007] The inner core of the negative electrode material of the present application includes black phosphorus, carbon nanotubes and transition metal phosphide. The presence of CNT in the inner core is beneficial to the construction of a stable electronic conduction network, which improves the electronic conductivity of black phosphorus material. The TMPs intermediate optimizes the interface of BP-CNT, reduces the local stress of BP-TMPs-CNT composite material, and thus improves the electronic transfer rate and structural stability of the active material. The amorphous carbon on the outermost layer can form a stable interface with the inner core, and the electronic conductivity of the carbon layer is high, which can effectively transfer electrons and thus improve the conductivity of the negative electrode material. Specifically, the addition of transition metal phosphide optimizes the interface resistance of BP and CNT in direct contact, i.e. the polarization difference between BP and CNT is large, and the transition metal phosphide plays a role in the contact between the two substances, reducing the resistance of the interface contact, thereby accelerating the charge transfer characteristics between the two interfaces, and thus making the negative electrode material have good electronic transfer rate and structural stability.

[0008] According to an embodiment of the present application, the mass ratio of the black phosphorus, the carbon nanotube and the transition metal phosphide is (8-10):(8-10):1.

[0009] According to an embodiment of the present application, the transition metal phosphide comprises at least one of monophosphide, binary phosphide.

[0010] According to an embodiment of the present application, the monophosphide comprises at least one of nickel phosphide, molybdenum phosphide, tungsten phosphide, cobalt phosphide.

[0011] According to an embodiment of the present application, the binary phosphide comprises at least one of nickel molybdenum phosphide, cobalt molybdenum phosphide.

[0012] The second aspect of the present application provides a preparation method of a transition metal phosphide modified black phosphorus-based negative electrode material, comprising the following steps:

[0013] (1) mixing black phosphorus, carbon nanotube and transition metal phosphide as a precursor;

[0014] (2) high-energy ball milling the expanded graphite and the precursor under inert gas protection, so that the carbon nanotube and the transition metal phosphide are covalently connected to the black phosphorus to obtain a black phosphorus / transition metal phosphide / carbon nanotube composite;

[0015] (3) pyrolyzing a carbon source gas to deposit an amorphous carbon layer on the black phosphorus / transition metal phosphide / carbon nanotube composite.

[0016] The method of the present application, by high-energy ball milling, covalently connects the carbon nanotube and the transition metal phosphide to the black phosphorus and forms van der Waals force between the carbon nanotube, the black phosphorus and the transition metal phosphide to improve the stability between the material structures, thereby improving the capacity retention rate and cycle life during the recycling process of the negative electrode material; by using expanded graphite as a precursor to provide sufficient space to accommodate active substances such as black phosphorus, transition metal phosphide and carbon nanotube; then depositing an amorphous carbon layer on the black phosphorus / transition metal phosphide / carbon nanotube composite, because gas phase deposition is used, the amorphous carbon layer is deposited uniformly, and the present application solves the problem of poor uniformity of the original phase and the composite phase of the material.

[0017] According to an embodiment of the present application, in step (1), the mass ratio of the black phosphorus, the carbon nanotube and the transition metal phosphide in the precursor is (8-10):(8-10):1.

[0018] According to an embodiment of the present application, in step (2), the mass ratio of the precursor to the expanded graphite is 70:(20-50).

[0019] According to an embodiment of the present application, the rotation speed of the ball mill is 800 rpm to 1600 rpm, and the ball-to-powder ratio is (20-100):1.

[0020] According to an embodiment of the present application, the inert gas includes at least one of argon and nitrogen.

[0021] According to an embodiment of the present application, in step (3), a carbon source gas is introduced into the CVD rotary furnace to pyrolyze the coating and form an amorphous carbon coating layer on the black phosphorus / transition metal phosphide / carbon nanotube composite.

[0022] According to an embodiment of the present application, the carbon source gas includes a hydrocarbon gas.

[0023] Further, the hydrocarbon gas includes at least one of methane and acetylene.

[0024] According to an embodiment of the present application, in step (3), the flow rate of the carbon source gas in the CVD rotary furnace is 10 SLM to 30 SLM, and the reaction is operated for 4 h to 8 h.

[0025] A third aspect of the present application provides a negative electrode tab including the negative electrode material of the first aspect or the negative electrode material prepared by the method of the second aspect. Thus, the negative electrode tab has high rate capability and good cycle stability.

[0026] A fourth aspect of the present application provides a secondary battery including the negative electrode tab of the third aspect. Thus, the secondary battery has high rate capability and good cycle stability.

[0027] Additional aspects and advantages of the present application will be partially given in the following description, partially will become obvious from the following description, or will be learned by practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0028] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the following description, taken in conjunction with the accompanying drawings, in which:

[0029] Figure 1 is a structural schematic diagram of a negative electrode material according to some embodiments of the present application.

[0030] REFERENCE NUMERALS

[0031] 1: carbon nanotube; 2: transition metal phosphide; 3: black phosphorus; 4: amorphous carbon. DETAILED DESCRIPTION

[0032] Embodiments of the present application are described below in detail with reference to the accompanying drawings, wherein the same or similar components or components having the same or similar functions are denoted by the same or similar reference numerals throughout. The embodiments described below by reference to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be construed as limiting the present application.

[0033] The first aspect of the present application provides a negative electrode material, which is a core-shell structure, the inner core comprises black phosphorus (BP), carbon nanotubes (CNT) and transition metal phosphide (TMPs), and the shell comprises amorphous carbon.

[0034] The inner core of the negative electrode material of the present application comprises black phosphorus, carbon nanotubes and transition metal phosphide. The presence of CNT in the inner core is conducive to the construction of a stable electron conductive network, improving the electron conductivity of the black phosphorus material. The TMPs intermediate optimizes the interface of BP-CNT, reduces the local stress of the BP-TMPs-CNT composite material, thereby improving the electron transfer rate and structural stability of the active material. The amorphous carbon on the outermost layer can form a stable interface with the inner core, and the electron conductivity of the carbon layer is high, which can effectively transfer electrons, thereby improving the conductivity of the negative electrode material. Specifically, the addition of transition metal phosphide optimizes the interface resistance of BP and CNT in direct contact, that is, the polarization difference between BP and CNT is large, and the transition metal phosphide plays a role in the contact between the two substances, reducing the resistance of the interface contact, thereby accelerating the transfer characteristics of the electric charge between the two interfaces, and thereby making the negative electrode material have good electron transfer rate and structural stability.

[0035] According to the present application, with reference to Figure 1 , the inner core comprises a composite of carbon nanotubes 1, transition metal phosphide 2 and black phosphorus 3, and the shell comprises amorphous carbon 4.

[0036] According to the present application, van der Waals forces are formed between the carbon nanotubes, transition metal phosphide and black phosphorus in the inner core to improve the stability between the material structures, thereby improving the capacity retention rate and cycle life of the negative electrode material during the recycling process, and thereby improving the structural stability of the negative electrode material.

[0037] In some embodiments, the mass ratio of the black phosphorus, carbon nanotubes and the transition metal phosphide is (8-10):(8-10):1. Excessive black phosphorus leads to a decrease in the transition phase, an increase in the contact resistance and a decrease in the rate performance of the material. Too little black phosphorus reduces the lithium storage space of the material and decreases the capacity. Excessive carbon nanotubes can lead to a decrease in the stability of the material and the structure is easily damaged during the recycling process. Therefore, by limiting the mass ratio of the black phosphorus, carbon nanotubes and the transition metal phosphide within the above range, the structural stability of the negative electrode material can be improved.

[0038] In some embodiments, the transition metal phosphide comprises at least one of monophosphide, binary phosphide.

[0039] Further, the monophosphide comprises at least one of nickel phosphide, molybdenum phosphide, tungsten phosphide, cobalt phosphide, titanium phosphide.

[0040] Further, the binary phosphide comprises at least one of nickel molybdenum phosphide, cobalt molybdenum phosphide.

[0041] The second aspect of the present application provides a method for preparing a negative electrode material, comprising the following steps:

[0042] (1) mixing black phosphorus, carbon nanotubes and transition metal phosphide as a precursor;

[0043] (2) high-energy ball milling the expanded graphite and the precursor under inert gas protection, so that the carbon nanotubes and the transition metal phosphide are covalently connected to the black phosphorus, to obtain a black phosphorus / transition metal phosphide / carbon nanotube composite;

[0044] (3) pyrolyzing a carbon source gas to deposit an amorphous carbon layer on the black phosphorus / transition metal phosphide / carbon nanotube composite.

[0045] In the second aspect of the present application, the carbon nanotubes and the transition metal phosphide are covalently connected to the black phosphorus by high-energy ball milling, and the van der Waals force is formed between the carbon nanotubes, the black phosphorus and the transition metal phosphide, so as to improve the stability between the material structures, thereby improving the capacity retention rate and the cycle life during the recycling of the negative electrode material; the expanded graphite is used as a precursor to provide sufficient space for accommodating the active substances such as the black phosphorus, the transition metal phosphide and the carbon nanotubes; and then the amorphous carbon layer is deposited on the black phosphorus / transition metal phosphide / carbon nanotube composite, and the amorphous carbon layer is deposited uniformly due to the use of gas phase deposition, so that the problem of poor uniformity of the original phase and the composite phase of the material is solved.

[0046] In some embodiments, in step (1), the mass ratio of the black phosphorus, the carbon nanotubes and the transition metal phosphide in the precursor is (8-10):(8-10):1.

[0047] In some embodiments, in step (1), the mixing is carried out under vacuum conditions at a temperature of 100-150°C.

[0048] In some embodiments, in step (2), the mass ratio of the precursor to the expanded graphite is 70:(20-50).

[0049] In some embodiments, the rotation speed of the ball milling is 800rpm-1600rpm, and the ball powder ratio is (20-100):1.

[0050] In some embodiments, the inert gas comprises at least one of argon, nitrogen.

[0051] In some embodiments, in step (3), a carbon source gas is introduced into the CVD rotary furnace to pyrolyze the coating, so as to form an amorphous carbon coating layer on the black phosphorus / transition metal phosphide / carbon nanotube composite. In this way, the conductivity of the negative electrode material can be enhanced.

[0052] In some embodiments, the carbon source gas comprises a hydrocarbon gas.

[0053] Further, the hydrocarbon gas comprises at least one of methane, acetylene.

[0054] In some embodiments, in step (3), the flow rate of the carbon source gas in the CVD rotary furnace is 10 SLM-30 SLM, and the reaction is operated for 4 h-8 h.

[0055] In some embodiments, in step (3), the coating treatment temperature is 500-700 degrees Celsius, and the coating treatment pressure is 0.2-1 MPa.

[0056] The third aspect of the present application provides a negative electrode tab comprising the negative electrode material of the first aspect or the negative electrode material prepared by the method of the second aspect. In this way, the negative electrode tab has high rate capability and good cycle stability.

[0057] The fourth aspect of the present application provides a secondary battery comprising the negative electrode tab of the third aspect. In this way, the secondary battery has high rate capability and good cycle stability.

[0058] The present application will be described below with reference to specific examples, and it should be noted that these examples are merely descriptive and do not limit the present application in any way.

[0059] Example 1

[0060] (1) BP, titanium phosphide (TiP), and single-walled carbon nanotubes (SWCNT) were mixed uniformly in a mass ratio of 47.5:47.5:5, and dried in a vacuum drying box, with a temperature setting of 120°C and a vacuum degree of 100 Pa. After vacuum drying, the mixture was taken out as a precursor;

[0061] (2) 70wt% of the precursor and 30wt% of expanded graphite were placed in a ball milling device (QM-3A), with a ball milling speed of 1200 rpm, a ball powder ratio of 50:1, and a ball milling time of 3 h. The ball milling was carried out in an Ar buffer atmosphere to obtain an intermediate product;

[0062] (3) The intermediate product is placed in a CVD rotating furnace for coating treatment. The coating gas is acetylene, the gas flow is 20 SLM, the temperature of the rotating furnace is 580°C, the pressure is 0.5 MPa, the coating time is 6 h, and the product is collected after the reaction, to obtain the transition metal phosphide modified black phosphorus-based composite material.

[0063] Example 2

[0064] The other operations are the same as in Example 1, except that the titanium phosphide in step (1) is replaced by nickel phosphide (Ni2P).

[0065] Comparative Example 1

[0066] The other operations are the same as in Example 1, except that the precursor in step (1) is 95wt% BP and 5wt% SWCNT.

[0067] Comparative Example 2

[0068] The other operations are the same as in Example 1, except that no SWCNT is added in step (1).

[0069] Test Example

[0070] The black phosphorus-based negative electrode materials provided by Examples 1-2 and Comparative Examples 1 and 2 are used to prepare batteries. The specific operations for preparing the batteries are as follows:

[0071] The black phosphorus-based negative electrode material, super P and carboxymethyl cellulose (CMC) are uniformly mixed in deionized water at a mass ratio of 70:15:15, coated on a copper foil and dried at 100°C under vacuum overnight. The obtained film is cut into a circular piece with a diameter of 12 mm, and the loading amount of active material is 250.7-0.8 mg cm-2. A lithium foil is used as the counter electrode, 1M LiPF6, ethyl carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) (volume ratio 1:1:1) and 10wt% fluoroethylene carbonate (FEC) are used as the electrolyte, and a test battery is assembled in an argon-filled glove box. The constant current charge-discharge cycle test is carried out using a battery test system (LAND CT2001A) between 0.01V and 2.0V (vs. Lit / Li). The results are shown in Table 1.

[0072] Table 1

[0073]

[0074] As can be seen from Table 1, the capacity and capacity retention at different stages of Example 1 and Example 2 are higher than those of Comparative Example 1 and Comparative Example 2. There is also a slight difference between Example 1 and Example 2, specifically, the capacity of Example 2 is higher than that of Example 1 at the beginning, and after cycling, the capacity decreases significantly and is lower than that of Example 1; this may be due to the replacement of TiP with Ni2P, the intermediate phase Ti structure changes to Ni structure, and Ni has better performance in energy density, but Ti phase has a more stable structure in structural stability, and also maintains better performance in the recycling process. Example 1 vs. Comparative Example 1, the initial capacities of the two are close, while the subsequent cycling performance of Comparative Example 2 is lower than that of Example 1 after the first cycle, which may be due to the lack of transition metal phosphide in Comparative Example 1, which lacks a transition phase between BP and carbon materials when forming a composite conductive structure. Due to the large volume expansion of the BP material, the formation of the intermediate phase will cause cracks and breakage of the BP-CNT material during charging and discharging, reducing the service life of the battery. Example 1 vs. Comparative Example 2, the capacity of Comparative Example 2 is lower than that of Example 1 after the first cycle, and the subsequent cycling performance is also insufficient, which may be due to the lack of CNT, which reduces the stability of the structure. In the composite structure, CNT is beneficial to the construction of a stable electronic conductive network, improving the electronic conductivity of black phosphorus material, which is reflected in the rate performance of the electrode material, which helps to improve the electrochemical performance of the material.

[0075] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, different embodiments or examples described in the present specification and the features of different embodiments or examples can be combined and combined by those skilled in the art without contradiction.

[0076] Although the embodiments of the present application have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application.

Claims

1. A method for preparing a negative electrode material, characterized in that, Includes the following steps: (1) Black phosphorus, carbon nanotubes and transition metal phosphides are mixed as a precursor; (2) The expanded graphite and the precursor were subjected to high-energy ball milling under inert gas protection so that carbon nanotubes and transition metal phosphides were covalently linked to the black phosphorus to obtain a composite of black phosphorus / transition metal phosphide / carbon nanotubes. (3) Pyrolysis of carbon source gas to deposit an amorphous carbon layer on the composite of black phosphorus / transition metal phosphide / carbon nanotube; The negative electrode material has a core-shell structure, with the core consisting of black phosphorus, carbon nanotubes, and transition metal phosphides, and the shell consisting of amorphous carbon.

2. The method according to claim 1, characterized in that, In the negative electrode material, the mass ratio of the black phosphorus, carbon nanotubes and the transition metal phosphide is (8~10):(8~10):

1.

3. The method according to claim 1, characterized in that, The transition metal phosphides include at least one of monophosphides and binary phosphides.

4. The method according to claim 3, characterized in that, The monophosphide includes at least one of titanium phosphide, nickel phosphide, molybdenum phosphide, tungsten phosphide, and cobalt phosphide.

5. The method according to claim 3, characterized in that, The binary phosphide includes at least one of nickel-molybdenum-phosphide and cobalt-molybdenum-phosphide.

6. The method according to claim 1, characterized in that, In step (1), the mass ratio of black phosphorus, carbon nanotubes and transition metal phosphides in the precursor is (8~10):(8~10):

1.

7. The method according to claim 1, characterized in that, In step (2), the mass ratio of the precursor to the expanded graphite is 70:(20~50).

8. The method according to claim 1, characterized in that, The ball mill operates at a speed of 800 rpm to 1600 rpm, with a ball-to-powder ratio of (20 to 100):

1.

9. The method according to claim 1, characterized in that, The inert gas includes at least one of argon and nitrogen.

10. The method according to claim 1, characterized in that, In step (3), carbon source gas is introduced into the CVD rotary furnace for pyrolysis coating to form an amorphous carbon coating layer on the composite of black phosphorus / transition metal phosphide / carbon nanotubes.

11. The method according to claim 1 or 10, characterized in that, The carbon source gas includes hydrocarbon gases.

12. The method according to claim 11, characterized in that, The hydrocarbon gas includes at least one of methane and acetylene.

13. The method according to claim 10, characterized in that, In step (3), the flow rate of carbon source gas in the CVD rotary kiln is 10 SLM to 30 SLM, and the reaction runs for 4 h to 8 h.

14. A negative electrode sheet, characterized in that, Includes the negative electrode material prepared by the method according to any one of claims 1-13.

15. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 14.

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