CN113745510A

CN113745510A - Ordered porous hard carbon for sodium ion battery

Info

Publication number: CN113745510A
Application number: CN202111038617.9A
Authority: CN
Inventors: 白莹; 吴锋; 刘明权; 吴川
Original assignee: Beijing Institute of Technology BIT
Current assignee: Beijing Institute of Technology BIT
Priority date: 2021-09-06
Filing date: 2021-09-06
Publication date: 2021-12-03

Abstract

The preparation method of the ordered porous hard carbon for the sodium ion battery comprises the following steps: sequentially dissolving a polymer precursor and a phase separation promoter in an organic solvent to form a mixed solution; heating the mixed solution in an oil bath and stirring to form a sol solution; dripping the sol solution on the surface of a mould uniformly to form a gel film; immersing the mould with the gel film formed on the surface into deionized water for phase separation; separating and drying the obtained porous membrane from the mold; and carbonizing the dried porous membrane in a protective atmosphere to form hard carbon. According to the invention, a sol-gel film is formed by selecting a phase separation promoter with proper polarity and a specific precursor, ordered pore forming can be rapidly carried out on the film through simple water washing phase separation, and the carbonized material is similar to natural wood and has a large number of vertically ordered macroporous channels, so that the wettability and the rapid transmission of the electrolyte can be improved.

Description

Ordered porous hard carbon for sodium ion battery

Technical Field

The invention relates to an electrode material of a sodium ion battery.

Background

Battery technology has become an integral part of national life as an energy storage device, and batteries are widely used from small portable instruments to large electric vehicles, military energy devices, and the like. Lithium ion batteries are currently the most widely used secondary batteries, and play an important role in scientific research and industrialization. However, due to the global uneven distribution, limited reserves, high production cost and other factors, lithium resources are difficult to meet the requirements of sustainable development, and therefore, new electrochemical energy storage technologies are required to be developed to replace lithium ion batteries. In the rear lithium ion battery technology, the sodium ion battery has feasibility of replacing a lithium battery due to rich sodium resources and balanced global distribution. While graphite anodes that have been commercialized can be used to store lithium, graphite does not have sodium storage properties due to its inability to form a thermodynamically stable product with sodium, thereby limiting further applications of sodium-ion batteries.

The carbon-based negative electrode for the sodium ion battery mainly comprises hard carbon, soft carbon and an amorphous carbon material doped with heterogeneous elements. The soft carbon has high graphitization degree, small interlayer spacing and limited sodium storage capacity. The heterogeneous element doped amorphous carbon mainly adsorbs and stores sodium, although the capacity is high, the discharge voltage is high, the introduction of heteroatoms causes irreversible capacity and aggravation of electrolyte decomposition, and the first-week coulombic efficiency is low, so that the high-energy full-cell system is not favorable for assembly. In contrast, a hard carbon material with short-range order exhibits excellent sodium storage capacity, a lower discharge plateau, a higher first effect, and is most commercially feasible.

The lower rate capability is a great challenge for hard carbon. The hard carbon material is nano-sized and porous, so that the material transmission is improved, and a thin and stable SEI film is formed in the discharge process, which is a key factor for improving the rate capability. However, the nanocrystallization and porous structure inevitably introduces more defects, so that the electrolyte is continuously decomposed, and the first effect is reduced.

Disclosure of Invention

It is an object of the present invention to provide a porous carbon electrode material which avoids or ameliorates the disadvantages associated with the prior art.

According to a first aspect of the present invention, there is provided a carbon electrode material preparation method, comprising:

a method of making a carbon electrode material, comprising:

providing a polymer precursor, wherein the polymer precursor is selected from at least one of polyvinyl butyral, polyacrylonitrile, polyvinyl alcohol, polyvinylidene fluoride and polytetrafluoroethylene;

providing an organic solvent selected from at least one of N, N-dimethylformamide, dimethyl sulfoxide and acetone;

providing a phase separation promoter selected from at least one of polyvinylpyrrolidone, polyoxyethylene polyoxypropylene, and polyoxyethylene;

sequentially dissolving a polymer precursor and a phase separation promoter in an organic solvent to form a mixed solution, wherein the mass ratio of the polymer precursor to the phase separation promoter is 1: 0-1: 3;

heating the mixed solution in an oil bath, and stirring to form a sol solution, wherein the heating temperature of the oil bath is 75-85 ℃, and the time is 8-10 h;

dripping the sol solution on the surface of a mould uniformly to form a gel film;

immersing the mould with the gel film formed on the surface into deionized water for phase separation;

continuously replacing deionized water during phase separation until the deionized water is not turbid any more and a porous membrane is obtained on the mold;

separating and drying the porous membrane from the mold;

carbonizing the dried porous membrane in a protective atmosphere to form hard carbon; and then ball milling the hard carbon into particles to obtain the carbon electrode material.

According to the invention, a sol-gel film is formed by selecting a phase separation promoter with proper polarity and a specific precursor, ordered pore forming can be rapidly carried out on the film through simple water washing phase separation, and the carbonized material is similar to natural wood and has a large number of vertically ordered macroporous channels, so that the wettability of an electrolyte (which is beneficial to forming a stable SEI film) and the rapid transmission (which improves the electrode reaction kinetics and improves the electrode performance in a coordinated manner) can be improved.

According to the present invention, a gel film having a thickness of 0.2 to 0.8mm, more preferably about 0.5mm, is preferably formed on the surface of the mold. The die is preferably a Polytetrafluoroethylene (PTFE) mesh die to improve water wash phase separation efficiency.

According to the invention, the phase separation promoter is preferably polyvinylpyrrolidone (PVP).

According to the invention, the organic solvent is preferably N, N-Dimethylformamide (DMF).

According to the invention, the polymer precursor is preferably Polyacrylonitrile (PAN).

According to the invention, the temperature of the deionized water is preferably 60-80 ℃, and more effectively 70 ℃.

According to the invention, it is preferred that in the carbonization step: heating to 1000-1300 ℃ at a speed of about 3 ℃/min and preserving heat for 1.5-2.5 h. The carbonization is preferably carried out under a protective atmosphere.

According to the invention, the oil bath heating temperature is preferably 80 ℃ and the time is preferably 9 h.

According to the present invention, the drying temperature of the porous membrane is preferably 60 to 80 ℃, more preferably 70 ℃; the drying time is preferably 12-16 h, and more preferably 14 h.

According to a second aspect of the present invention, there is also provided a sodium ion battery anode formed from the carbon electrode material prepared according to the above method.

According to other aspects of the invention, there is also provided a capacitor or battery having the above electrode.

The invention has the following advantages:

1. the invention forms the porous membrane by one-step washing phase separation, and is simple and environment-friendly.

2. The present invention can adjust the porous membrane to a desired pore state by selecting the phase separation promoter and/or changing the ratio thereof.

3. The hard carbon material obtained by the carbonization process of the porous film has an ordered and cross-linked through hole structure similar to wood, so that the diffusion and transmission of electrolyte are facilitated, the utilization rate of the interior of an electrode material is improved, and particularly, the prepared cathode has excellent sodium storage capacity and rate capability.

4. The selected polymerization precursor has good film forming property and openness: the method can be expected to be used for preparing multi-element doped or composite hard carbon negative electrodes, such as nitrogen, phosphorus, sulfur, oxygen and other doped hard carbon materials, or composite materials coupled with metal sulfides, phosphides, selenides and the like.

Drawings

Fig. 1 and 2 are respectively SEM images of the micro-topography of the hard carbon material prepared according to the example. Fig. 3 shows cycle performance of the hard carbon anode obtained according to example 1.

Detailed Description

The present invention will be described in detail below by way of specific examples with reference to the accompanying drawings.

Example 1

Step (1): 4g of Polyacrylonitrile (PAN) was weighed out and dissolved in 36g N, N-Dimethylformamide (DMF) at room temperature, and stirring was continued for 8h until complete dissolution.

Step (2): to the above liquid was added 8g of polyvinylpyrrolidone (PVP) and stirring was continued for 4h until complete dissolution.

And (3): the liquid is put into an oil bath pot and stirred for reaction for 9 hours at 80 ℃ until the sol gel is complete.

And (4): the glue solution is evenly dripped on a PTFE mesh die with the size of 40mm multiplied by 1.5 mm.

And (5): the PTFE mold with the gel film is immersed into deionized water at 70 ℃ for phase separation, and the step is repeated until the deionized water is not turbid.

And (6): carbonizing the porous polymer film removed from the die in the step (5) in a tube furnace, and controlling the temperature rise rate to be 3 ℃/min until the carbonization temperature is 1200 ℃ and preserving the heat for 2 h.

And (7): the carbon material obtained in (6) was further pulverized by a ball mill at a rotation speed of 500 r/min.

Finally, uniformly mixing and stirring the obtained multi-carbon hard carbon material serving as an active substance, carbon black serving as a conductive agent and PVDF serving as a binder at a ratio of 8:1:1 overnight to form slurry, then blade-coating the slurry on a copper foil current collector, drying the copper foil current collector in vacuum at 80 ℃ for 24 hours, cutting the copper foil current collector into a negative electrode wafer with a certain size, taking sodium as a counter electrode and 1M NaPF₆The ethers are used as electrolyte to assemble the button cell.

FIG. 1 is a low power electron micrograph of a PAN4-PVP8 hard carbon sample made according to this example; FIG. 2 is a high magnification electron microscope image thereof; fig. 3 is a graph of the long cycle performance at 1A/g current density for a PAN4-PVP 8-based hard carbon anode of a battery assembled according to this example.

Examples 2 to 4

Examples 2 to 4 were sequentially carried out by changing the amount of PVP added in step (2) of example 1 to 0g, 4g and 12g, respectively, all other things being unchanged.

Table 1 shows the specific surface area and sodium ion battery performance of the hard carbon materials prepared in examples 1 to 4.

TABLE 1

Examples 5 to 9

The PAN in step (1) of example 1 was changed to polyvinyl butyral (PVB), polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE) in this order, and the others were not changed.

Table 2 shows the specific surface areas and sodium ion battery performance of the hard carbon materials prepared in examples 5 to 9.

TABLE 2

Examples 10 to 17

The PVP in the step (2) of example 1 was sequentially replaced with FeCl₃、NaCl、Na₂SO₄、KCl、ZnCl₂LiCl, polyoxyethylene polyoxypropylene, polyoxyethylene, others.

Table 3 shows the specific surface areas and sodium ion battery properties of the hard carbon materials prepared in examples 10 to 17.

TABLE 3

Comparative examples 1 to 3

The same as example 1 except that the temperature in step (6) was changed to 1000, 1100 and 1300 ℃.

Table 4 shows the sodium ion battery performance of the hard carbon materials prepared in comparative examples 1 to 3.

TABLE 4

Comparative examples 4 to 6

Otherwise, the same as example 1, except that the rotation speed in step (7) was changed to 300, 400 and 600r/min, respectively.

Table 5 shows the sodium ion battery performance of the hard carbon materials prepared in comparative examples 4 to 6.

TABLE 5

And (4) analyzing results:

electron micrographs 1 and 2 show that the hard carbon material prepared in example 1 has rich and interconnected open-pore structures, and the channels are favorable for the rapid diffusion of electrolyte, so that the sodium storage performance of the material is improved. FIG. 3 shows the cycling performance of the corresponding negative electrode, which can maintain a capacity of 160mAh/g at a current density of 1A/g, indicating that the cycling stability is good. The coulomb efficiency of the first circle under low current density reaches more than 70 percent.

The results of examples 2 to 4 show that the specific surface area gradually increases with the gradual increase of PVP, and the optimal value is PAN4-PVP 8. The sodium storage performance is improved along with the increase of open pores, including first effect and reversible capacity, which shows that the open pore structure is very important for improving the sodium storage performance.

Examples 5-9 compare the sodium storage performance of different polymer precursors and show that hard carbon materials made from PAN have higher first-pass and reversible capacity.

Examples 10-17 compare the effect of different phase separation promoters on material performance, which on the one hand demonstrates the general applicability of the pore structure modulation method of the invention and on the other hand demonstrates the optimal performance of the choice of PVP.

Comparative examples 1-3 show that as the temperature increases, the degree of graphitization of the carbon material increases, providing more active sites for the deintercalation of sodium ions, and the capacity gradually increases. However, beyond 1200 ℃, there is a possibility of decreased interlayer spacing, surface defects and reduced functional group content, resulting in a slight decrease in capacity.

Comparative examples 4-6 show that higher rotation speed is favorable to improving carbon material performance, the particles are finer and more uniform at high rotation speed, and are favorable to wettability of electrolyte, and meanwhile, partial defect concentration can be improved at high rotation speed, so that adsorption capacity of a slope area is improved.

Claims

1. A method of making a carbon electrode material, comprising:

separating and drying the porous membrane from the mold;

carbonizing the dried porous membrane in a protective atmosphere to form hard carbon; and

and then ball milling the hard carbon into particles to obtain the carbon electrode material.

2. The production method according to claim 1, wherein a gel film having a thickness of 0.2 to 0.8mm is formed on the surface of the mold.

3. The preparation method according to claim 1, wherein the temperature of the deionized water is 60 to 80 ℃.

4. The production method according to claim 1, wherein the carbonization is performed under a protective atmosphere.

5. A sodium ion battery negative electrode formed from the carbon electrode material prepared according to the method of any one of claims 1 to 4.