CN114068932B - A flexible self-doping material for lithium-sulfur batteries and its preparation method and application - Google Patents

Abstract

The invention discloses a flexible self-doping material for lithium-sulfur batteries and its preparation method and application. The flexible self-doping material is prepared from fibrillated PBO fibers, has a three-dimensional structure, and has a relatively high specific surface area and Rich pore structure can better adsorb polysulfides and inhibit their shuttling. It can maintain a high proportion of N atoms after carbonization treatment. The distribution of N elements is uniform and the structure is very stable. It has a good anchor for polysulfides. It can effectively improve the shuttle problem of polysulfides, realize the physical barrier to the transmission of polysulfides, and strengthen its chemical adsorption to improve the cycle capacity and long-term cycle stability of lithium-sulfur batteries. The provided flexible self-doping material can be widely used as a lithium-sulfur battery interlayer material to improve battery performance.

Description

A flexible self-doping material for lithium-sulfur batteries and its preparation method and application

technical field

The invention relates to the technical field of new energy batteries, and more specifically, to a flexible self-doping material for lithium-sulfur batteries and its preparation method and application.

Background technique

Lithium-sulfur batteries usually have the shuttle effect of polysulfides, which leads to a decrease in the capacity retention and Coulombic efficiency of the battery, and a decrease in battery performance. As a good electronic conductor, carbon is beneficial to current distribution. In addition, porous carbon materials with high specific surface area can provide a large reaction area, reduce the electrochemical polarization of the battery, and hinder the clustering of sulfur. Carbon materials with high pore volume can accommodate A large amount of sulfur, and produce a certain physical adsorption to inhibit the dissolution of lithium polysulfide Li ₂ S _n in the electrolyte. Carbon materials have a good affinity for the electrolyte of ether lithium-sulfur batteries, which can speed up the charging and discharging process. transport of lithium ions. However, due to the non-polar nature of the CC bond, pure carbon materials have poor adsorption capacity for polar lithium polysulfides, and lithium polysulfides are easily lost from the positive carbon network. In order to increase the polarity of carbon materials and make them bind lithium polysulfide, researchers often do heteroatom doping treatment on carbon materials. After doping heteroatoms, the surface of carbon materials is rich in polar groups, so that it can achieve To fix the effect of lithium polysulfide, the main means of doping N atoms is to use nitrogen-containing substances (urea, ammonia, ammonium chloride, etc.) to post-treat carbon materials, or to place the prepared samples directly in a tube furnace , high-temperature treatment in NH ₃ atmosphere, or soak the prepared sample with urea, ammonium chloride and other solutions for a period of time, and then use a tube furnace for high-temperature treatment. Chinese patent CN106684389A discloses a sulfur-nitrogen double-doped graphene nanomaterial and its preparation method and application. After carbonization of graphene oxide and thiourea, a sandwich structure material for lithium-sulfur batteries is prepared. However, due to the complicated doping process, N The uneven distribution of elements will be removed during the cycle, so that the shuttle effect of lithium-sulfur batteries cannot be avoided, resulting in reduced battery performance.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the defects and deficiencies that the existing lithium-sulfur battery interlayer cannot avoid the shuttle effect, which leads to the decrease of battery performance, and provide a flexible self-doping material for lithium-sulfur batteries, using fibrillated PBO fibers It has a high specific surface area and rich pore structure, good adsorption effect, and can effectively inhibit the shuttle of polysulfides. As a lithium-sulfur battery interlayer, it is beneficial to electron and ion transmission, improves electrolyte wettability, and improves battery life. Ionic conductivity and Coulombic efficiency, thus ensuring the performance of the battery.

Another object of the present invention is to provide a method for preparing a flexible self-doping material for lithium-sulfur batteries.

Another object of the present invention is to provide an application of a flexible self-doping material for lithium-sulfur batteries.

The above object of the present invention is achieved through the following technical solutions:

A flexible self-doping material for lithium-sulfur batteries, comprising fibrillated PBO fibers, the specific surface area of the flexible self-doping material is 100-2600g/m ² , the basis weight is 30-100g/m ² , and the thickness is 40-160um.

The present invention utilizes fibrillated PBO (poly-p-phenylenebenzobisoxazole) fibers to prepare flexible self-doping materials. Since the PBO fibers themselves contain N and can still maintain a relatively high N content after high-temperature activated carbonization, self-doping The N atoms of N atoms can be evenly distributed in the carbon skeleton, and the fibrillated PBO fibers have a high specific surface area and rich pore structure, which can enhance the charge density of carbon materials, improve the adsorption performance of polysulfides, and after high temperature treatment Still maintaining a high ratio, it can be used as an interlayer material in lithium-sulfur batteries to inhibit the shuttle of polysulfides, which can significantly improve the capacity retention and Coulombic efficiency of the battery, and improve battery performance.

Preferably, the fibrillated PBO fibers have a diameter of 100-1000 nm.

Preferably, the basis weight is 30-50g/m ² , and the thickness is 40-70um.

The invention protects the above-mentioned preparation method of flexible self-doping materials for lithium-sulfur batteries, comprising the following steps:

S1. After fully disentangling the fibrillated PBO fibers, add water to dilute to a sheet concentration of 0.02wt%-0.05wt% to obtain a slurry;

S2. forming the slurry in step S1 to obtain a wet paper web;

S3. Drying the wet paper web in step S2 at a temperature of 100-150° C. for 5-10 minutes to obtain a dry paper web;

S4. Calendering the dry paper web in step S3 at a temperature of 180-200° C. for 5-10 minutes;

S5. Carbonizing the calendered material in step S4 to prepare a flexible self-doping material for lithium-sulfur batteries.

Preferably, the carbonization in step S5 is one of direct carbonization, physical activation carbonization and chemical activation carbonization.

Preferably, the direct carbonization is to heat up the calendered material in step S4 to 500-600°C under the protection of nitrogen, and keep it warm for 1-2h; The temperature is kept at 1000° C. for 50-60 minutes, and then cooled to normal temperature to obtain a flexible self-doping material.

Preferably, the physical activation carbonization is to heat up the calendered material in step S4 to 500-600°C under the protection of nitrogen, keep it warm for 1-2h, then pass carbon dioxide, heat up to 850-1000°C, and The temperature is kept at -1000° C. for 50-60 minutes, and then cooled to normal temperature to obtain a flexible self-doping material.

Preferably, the chemically activated carbonization is to impregnate the calendered material in step S4 in KOH solution, then raise the temperature to 500-600°C under the protection of nitrogen, keep it warm for 1-2h, then continue to pass nitrogen, and raise the temperature to 850°C -1000°C, and heat preservation at 850-1000°C for 50-60min, then cooling to normal temperature to obtain a flexible self-doping material.

Preferably, the beating degree of the PBO fibrillated fiber diluted with water in step S1 is 75-92°SR.

Preferably, the sheet forming in step S2 is performed in a former.

Preferably, step S2 also includes sieving and rectifying the slurry before papermaking, so that the slurry exhibits a high-strength micro-turbulent flow state.

Preferably, the drying treatment in step S3 is carried out in a drum dryer.

Preferably, the calendering treatment in step S4 is performed using metal rolls and soft rolls.

Preferably, the preparation method of the fibrillated PBO fiber is as follows: soak the PBO chopped fiber at a concentration of 2wt% for 6 hours, then use a tank beater to perform fibrillation treatment, and the PBO fiber undergoes skin removal, fiber separation Fibrillation is complete after cleavage. The trough beater cuts, crushes, kneads, splits, moistens and refines the PBO fiber slurry through the mechanical action of the flying knife roller and the bottom knife. By controlling the flow velocity of 0.03m/s-0.15m/s and the beating time of 1h-1000h, fibrillated fibers with a beating degree of 40°SR-69°SR can be obtained. The fibrillation process of PBO ensures its proper length, diameter and fibrillation degree through process control, and ensures the uniform dispersion and formation of fibrillated ultrafine fibers with 0.05% sizing and 0.1% forming concentration in the forming process , in the process of high-speed turbulence, the control of the structure and uniformity of the paper base material is enhanced, the fibrillated ultrafine fibers are evenly distributed and tightly combined, and the dimensional stability of the base material is improved. The passing flow rate is 100-160m ³ /h and The concentration of 0.01wt%-0.1wt% is controlled to realize quantitative control of the self-supporting material substrate in a large ratio range, and to adjust the structure of the substrate efficiently and flexibly.

The present invention protects the application of the above-mentioned flexible self-doping material for lithium-sulfur batteries in the preparation of lithium-sulfur batteries.

A lithium-sulfur battery interlayer material is prepared from the above-mentioned flexible self-doping material for lithium-sulfur batteries.

Compared with prior art, the beneficial effect of the present invention is:

The flexible self-doping material provided by the invention is made of fibrillated PBO fibers, has a three-dimensional structure, has a high specific surface area and a rich pore structure, and can better absorb polysulfides and inhibit their shuttling. It can maintain a high proportion of N atoms after carbonization, the distribution of N elements is uniform, the structure is very stable, and it has a good anchoring effect on polysulfides, which can effectively improve the shuttle problem of polysulfides and realize the transmission of polysulfides On the basis of the physical barrier, strengthen its chemical adsorption to improve the cycle capacity and long-term cycle stability of the lithium-sulfur battery. The flexible self-doping material provided by the present invention can be widely used as a lithium-sulfur battery interlayer material to improve battery life. performance.

Description of drawings

FIG. 1 is a schematic diagram of a lithium-sulfur battery prepared by using the flexible self-doping material in Example 1 of the present invention.

Fig. 2 is an electron microscope image of the flexible self-doping material prepared in Example 1 of the present invention.

Fig. 3 is a physical diagram of the flexible self-doping material prepared in Example 1 of the present invention.

Fig. 4 is a 2000 times magnified electron microscope image of the flexible self-doping material prepared by the direct carbonization method in Example 1 of the present invention.

Fig. 5 is an electron microscope image magnified 10,000 times of the flexible self-doping material prepared by the direct carbonization method in Example 1 of the present invention.

Fig. 6 is a bending picture of the flexible self-doping material obtained by the direct carbonization method in Example 1 of the present invention.

detailed description

The present invention will be further described below in combination with specific embodiments, but the examples do not limit the present invention in any form. Unless otherwise specified, the raw material reagents used in the examples of the present invention are conventionally purchased raw material reagents.

The raw materials used in each of the following examples and comparative examples:

PBO chopped fiber, Toray Corporation, Japan, AS-6mm.

PBO fibrillation fiber: soak PBO chopped fiber at a concentration of 2wt% for 6 hours, and then use a tank beater for fibrillation treatment. The trough beater cuts, crushes, kneads, splits, moistens and refines the PBO fiber slurry through the mechanical action of the flying knife roller and the bottom knife. The flow velocity is 0.15m/s, beating for 1000h, and PBO fibrillated fibers with a beating degree of 69°SR can be obtained.

Example 1

A flexible self-doping material for lithium-sulfur batteries, including fibrillated PBO fibers, the specific surface area, quantitative, and thickness of the flexible self-doping material are shown in Table 1 below.

The above-mentioned preparation method of the flexible self-doping material for lithium-sulfur battery comprises the following steps:

S1. Stock preparation and delivery

After the PBO chopped fibers are fibrillated, the fibrillated PBO fibers are mixed, dispersed and diluted with water in a fiber disintegrator, and the solid weight percentage concentration is 0.02% of the sheet concentration to obtain a slurry;

S2. Copy forming

The slurry is sent to the vacuum sheet machine for copying and forming to obtain a wet paper web;

S3. After dehydration treatment, the wet paper sheet is obtained, and the wet paper sheet is dried at a Yankee temperature of 150°C for 10 minutes to obtain a dry paper web;

S4. Then go through the heat calendering treatment of metal rollers and elastic rollers with a temperature of 200 ° C and a time of 10 minutes;

S5. Carbonization

Put the calendered material in step S4 into a tube furnace, heat it up to 600°C at a heating rate of 10°C/min under the protection of nitrogen, keep it for 1 hour, and then heat it from 600°C at a heating rate of 8°C/min to 850°C, and kept at 850°C for 60 minutes, and then cooled to room temperature to obtain a self-supporting material.

Example 2

A flexible self-doping material for lithium-sulfur batteries, including fibrillated PBO fibers, the specific surface area, quantitative, and thickness of the flexible self-doping material are shown in Table 1 below.

The difference between the preparation method of the above-mentioned flexible self-doping material for lithium-sulfur batteries and Example 1 is that S5: put the material after the calendering treatment in step S4 into a tube furnace, and heat it at 10°C/min under the protection of nitrogen. The heating rate was heated to 600°C, and then heated from 600°C to 1000°C at a heating rate of 8°C/min, kept at 1000°C for 60 minutes, and then cooled to room temperature to obtain activated carbon fibers.

Example 3

A flexible self-doping material for lithium-sulfur batteries, including fibrillated PBO fibers, the specific surface area, quantitative, and thickness of the flexible self-doping material are shown in Table 1 below.

The difference between the preparation method of the above-mentioned flexible self-doping material for lithium-sulfur batteries and Example 1 is that S5: the physical activation carbonization method is used for activation. In the first step, the temperature is raised at a heating rate of 10°C/min to 600°C, keep warm for one hour; in the second stage, pass carbon dioxide, heat up to 850°C at a heating rate of 8°C/min, keep at 850°C for 60 minutes, and then cool to room temperature.

Example 4

A flexible self-doping material for lithium-sulfur batteries, including fibrillated PBO fibers, the specific surface area, quantitative, and thickness of the flexible self-doping material are shown in Table 1 below.

The difference between the preparation method of the above-mentioned flexible self-doping material for lithium-sulfur batteries and Example 1 is that S5: the physical activation carbonization method is used for activation. In the first step, the temperature is raised at a heating rate of 10°C/min to 600°C, keep warm for one hour; in the second stage, pass carbon dioxide, heat up to 1000°C at a heating rate of 8°C/min, keep at 1000°C for 60 minutes, and then cool to room temperature.

Example 5

A flexible self-doping material for lithium-sulfur batteries, including fibrillated PBO fibers, the specific surface area, quantitative, and thickness of the flexible self-doping material are shown in Table 1 below.

The difference between the preparation method of the above-mentioned flexible self-doping material for lithium-sulfur batteries and Example 1 is that S5: use chemical activation carbonization method for activation, after impregnating with KOH solution, carry out carbonization treatment in two steps, the first step, in Under the protection of nitrogen, the temperature was raised to 600°C at a heating rate of 10°C/min, and kept for one hour; in the second stage, the temperature was raised to 850°C at a heating rate of 8°C/min, and kept at 850°C for 60 minutes, and then cooled to room temperature.

Example 6

A flexible self-doping material for lithium-sulfur batteries, including fibrillated PBO fibers, the specific surface area, quantitative, and thickness of the flexible self-doping material are shown in Table 1 below.

The difference between the preparation method of the above-mentioned flexible self-doping material for lithium-sulfur batteries and Example 1 is that S5: use chemical activation carbonization method for activation, after impregnating with KOH solution, carry out carbonization treatment in two steps, the first step, in Under the protection of nitrogen, the temperature was raised to 600°C at a heating rate of 10°C/min, and kept for one hour; in the second stage, the temperature was raised to 1000°C at a heating rate of 8°C/min, and kept at 1000°C for 60 minutes, and then cooled to room temperature.

Comparative example 1-2

Comparative example 1 is a material obtained by carbonization of coniferous wood paper-based materials doped with urea. The urea purity is 95%, which comes from Guangzhou Chemical Reagent Factory; Brand coniferous wood pulp board, obtained by carbonizing in a tube furnace at 500°C after sheeting.

The difference between Comparative Example 2 and Example 1 is that it is prepared by using unfibrillated PBO fiber material.

Performance Testing

1. Test method

(1) quantitative

Measured by TAPPI standard.

(2) average fiber pore size

Measured using a fiber analyzer.

(3) Appearance of flexible self-doping materials

Scanning electron microscopy was used to test and analyze the appearance and pore structure of the separator.

(4) Elemental analysis

Use an elemental analyzer to test the content of C, N, and O.

(5) Constant current charge and discharge test

The flexible self-doping materials obtained in the above Examples 1-6 were used as PBO flexible interlayers, and the materials prepared in Comparative Examples 1-2 were used as flexible interlayers, according to the negative electrode shell→lithium sheet→diaphragm→electrolyte (40μL)→ Lithium-sulfur batteries are assembled in the order of PBO flexible interlayer→positive electrode sheet→steel sheet→shrapnel→positive electrode shell, and finally the battery packaging machine seals the battery, as shown in Figure 1. In addition, a lithium-sulfur battery without an interlayer was assembled as a comparison, and a battery test cabinet was used to test the specific capacity, Coulombic efficiency, cycle life, and rapid charge-discharge capability of the battery. All the assembled batteries were put on hold for 6 hours before the test, and charged and discharged with a constant current, and the charge and discharge range was 1.7-2.6V.

2. Test results

Table 1 Elemental analysis data of self-supporting materials with different carbonization activation processes

sample N (wt%) C (wt%) H(wt%) S (wt%) Example 1 7.11 80.64 1.3 0 Example 2 6.62 78.53 1.22 0 Example 3 6.58 77.92 1.16 0 Example 4 6.01 75.33 1.03 0 Example 5 4.64 62.84 0.58 0 Example 6 3.52 60.1 0.36 0

Table 1 shows that by comparing the contents of C, N, H and S elements in the materials after PBO fibrillation, direct carbonization, physical activation carbonization and chemical activation carbonization, the content of N is still high after carbonization, and the direct carbonization The N content after carbonization treatment was higher than _CO2 activated carbonization treatment as well as KOH activated carbonization treatment. The doping of N elements can make the surrounding carbon atoms be electropositive, and introduce more defects and active sites in the carbon skeleton, thereby improving the interface adsorption capacity. Therefore, the present invention adopts the fibrillation of self-doped N PBO fibers can make flexible self-doping materials have specific adsorption and removal properties.

Table 2 BET data of flexible self-doping materials with different carbonization activation processes

It can be seen from Table 2 that the flexible self-doping material prepared in Examples 1-6 of the present invention has a higher specific surface area, and the specific surface area after KOH activated carbonization is higher than that of _CO2 activated carbonization and direct carbonization, and the polysulfide The adsorption effect is better, but the strength of the flexible material directly carbonized is better. It can be seen from Table 1 that its N retention rate is the highest at the same temperature.

Table 3 Coulombic efficiency comparison of lithium-sulfur batteries prepared using flexible self-doping materials

It can be seen from Table 3 that in Examples 1-6 of the present invention, PBO fibrillated fibers are used to prepare lithium-sulfur battery sandwich structures. Due to its self-supporting structure, high specific surface area and high N content, strong adsorption of self-supporting materials is ensured. Therefore, the flexible self-doping material prepared by the present invention is used as an interlayer to prepare a lithium-sulfur battery, which can significantly improve the capacity retention and Coulombic capacity of the battery. Efficiency, the specific capacity after 100 cycles at a rate of 0.2C remains above 80% of the initial specific capacity, both exceeding 920mAh/g, and the Coulombic efficiency is above 99%.

The method of doping N element in Comparative Example 1 has a relatively complicated process, uneven distribution of N element, and a low doping ratio, which is easy to introduce other impurities, which are removed during the cycle and affect the performance of the battery. Comparative example 2, the PBO unfibrillated paper-based material becomes fragmented after carbonization, cannot be bent at will, and the thickness is as high as 300um, which affects the cycle performance of lithium-sulfur batteries, and the steel rod structure on the surface is not rich The pores can adsorb polysulfides. Lithium-sulfur batteries without an interlayer cannot effectively adsorb polysulfides, and the cycle performance of lithium-sulfur batteries is relatively poor and is easy to decay.

Figure 2 shows the PBO fibrillated fibers after beating for 1000 hours, and the fibers are intertwined to form a compact paper-based structure. Figure 3 shows that the surface of PBO fibrillated fibers after beating for 1000 hours is very smooth and the fibers are evenly distributed.

It can be seen from Figure 4 and Figure 5 that the PBO fibrillated fibers still maintain a complete micro-nano structure after direct carbonization at 900 °C. It can be seen from Figure 6 that after the PBO fibrillated fiber is directly carbonized, it can be bent to a large extent under the action of an external force, and the surface is smooth.

The invention adopts PBO fibrillation fiber to prepare flexible self-supporting material, which has high specific surface area, multi-level pore structure, and good flexibility. It can be used as a lithium-sulfur battery interlayer and can absorb a large amount of lithium polysulfide to ensure polysulfide The stability of lithium clusters and avoiding their dissolution in the electrolyte can inhibit the "shuttle effect" of polysulfides and ensure battery performance.

Apparently, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, rather than limiting the implementation of the present invention. For those of ordinary skill in the art, on the basis of the above description, other changes or changes in different forms can also be made. It is not necessary and impossible to exhaustively list all the implementation manners here. All modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (3)

1. A lithium-sulfur battery interlayer material, characterized in that the specific surface area of the lithium-sulfur battery interlayer material is 100-2600g/m ² , the weight is 30-100g/m ² , and the thickness is 40-160um; The preparation method of the lithium-sulfur battery interlayer material comprises the following steps: S1. After fully disentangling the fibrillated PBO fibers, add water to dilute to a sheet concentration of 0.02wt%-0.05wt% to obtain a slurry; S2. forming the slurry in step S1 to obtain a wet paper web; S3. Drying the wet paper web in step S2 at a temperature of 100-150° C. for 5-10 minutes to obtain a dry paper web; S4. Calendering the dry paper web in step S3 at a temperature of 180-200° C. for 5-10 minutes; S5. Carbonizing the material after the calendering treatment in step S4 to obtain a lithium-sulfur battery interlayer material; The carbonization in step S5 is one of direct carbonization, physical activation carbonization and chemical activation carbonization; The direct carbonization is to heat up the material after the calendering treatment in step S4 to 500-600°C under the protection of nitrogen, and keep it warm for 1-2h; Keep it warm for 50-60min, then cool down to normal temperature to obtain a lithium-sulfur battery interlayer material; The physical activation carbonization is to heat up the calendered material in step S4 to 500-600°C under the protection of nitrogen, keep it warm for 1-2h, then pass carbon dioxide, heat up to 850-1000°C, and heat it at 850-1000°C Insulate at high temperature for 50-60min, then cool to normal temperature to obtain lithium-sulfur battery interlayer material; The chemical activated carbonization is to impregnate the calendered material in step S4 in KOH solution, then raise the temperature to 500-600°C under the protection of nitrogen, keep it warm for 1-2h, and then continue to pass nitrogen to raise the temperature to 850-1000°C , and kept at a temperature of 850-1000° C. for 50-60 minutes, and then cooled to normal temperature to obtain a lithium-sulfur battery interlayer material. 2 . The lithium-sulfur battery interlayer material according to claim 1 , wherein the diameter of the fibrillated PBO fibers is 100-1000 nm. 3. The lithium-sulfur battery interlayer material according to claim 1, wherein the weight is 30-50g/m ² and the thickness is 40-70um.

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