CN114614198A - Phase change diaphragm for lithium-sulfur battery and preparation method thereof - Google Patents

Abstract

The invention discloses a phase-change diaphragm for lithium-sulfur batteries and a preparation method thereof. A core-shell structure nanofiber membrane PPW was prepared from nitrile PAN solution and paraffin PW solution by coaxial electrospinning technology; the MOF/BP heterojunction powder was dispersed in deionized water to form a MOF/BP heterojunction dispersion, The MOF/BP heterojunction dispersion liquid is deposited on the nanofiber membrane PPW by vacuum filtration, and vacuum dried to obtain a PPW/MOF/BP phase change membrane. That is, a phase-change separator material for high-safety and high-performance lithium-sulfur batteries is obtained. The invention improves the safety performance of the lithium-sulfur battery through the phase-change nanofiber separator, suppresses the "shuttle effect" in the lithium-sulfur system through the MOF/BP heterojunction, and improves the cycle performance of the battery, thereby constructing a high-safety and high-performance battery. Lithium-sulfur battery.

Description

A kind of phase change separator for lithium-sulfur battery and preparation method thereof

technical field

The invention belongs to the technical field of material synthesis, and relates to a phase-change diaphragm for a lithium-sulfur battery and a preparation method thereof.

Background technique

With the demand for higher energy density secondary batteries in emerging market applications, especially in the fields of hybrid electric vehicles (PHEV) and pure electric vehicles (PEV), the energy density of secondary batteries needs to reach 300Wh kg ^-1 .

Lithium-sulfur batteries have been widely studied due to their theoretical energy density of 2600Wh kg ^-1 , closer to practicality, abundant resources, low price and environmental friendliness.

However, the main problem that hinders the development of current lithium-sulfur batteries is: the elemental sulfur cathode will form soluble polysulfide intermediates during the charge and discharge process, and the continuous dissolution and diffusion of polysulfides in the electrolyte will make it pass through the diaphragm. The continuous deposition on the surface of the lithium negative electrode is also known as the "shuttle effect", resulting in an increase in the internal impedance of the battery, the loss of active material sulfur and a sharp decline in the electrochemical performance. The key to overall performance. At the same time, the battery has a potential safety hazard of combustion and explosion during operation, which also makes it difficult for lithium-sulfur batteries to be applied in real life.

SUMMARY OF THE INVENTION

Purpose: In order to overcome the deficiencies in the prior art, solve the potential safety hazards of the existing commercial separators and the problem that the "shuttle effect" in lithium-sulfur batteries cannot be suppressed, the present invention provides a phase-change separator for lithium-sulfur batteries and the same. Preparation methods to improve the safety and performance of lithium-sulfur batteries.

Black phosphorus (BP) is the most thermodynamically stable allotrope of phosphorus, with a low resistivity between 0.48 and 0.77 Ω·cm and an ultra-high room temperature pore mobility ~1000 cm ² V ^-1 s ^-1 . BP also has a low density of 2.69 g cm ^-3 , a good volume conductivity of ~3S cm ^-1 , a high lithium ion diffusion constant, and a high binding energy with sulfur. These properties suggest that BP has the ability to chemically bind Li polysulfides, while its high electrical conductivity and high lithium ion diffusion constant can effectively promote the kinetics of the conversion of Li polysulfides to LiS. The literature also verifies the excellent performance of black phosphorus to efficiently adsorb and convert lithium polysulfides. However, black phosphorus is easily oxidized and decomposed in air, limiting its application in lithium-sulfur batteries. As a porous material, organometallic framework MOFs can provide fast channels for lithium ion transport and uniform deposition with a fixed pore size, and at the same time, they can block the shuttle of lithium polysulfides through the role of physical barriers, and have also been widely used in lithium-sulfur batteries. application. However, most of the MOFs have poor electrical conductivity and weak catalytic ability for the conversion kinetics of lithium polysulfides, which are not conducive to improving the electrochemical performance of batteries. In summary, the combination of black phosphorus and MOF is an effective way to make up for the deficiencies of the two. The MOF on the surface of black phosphorus can block oxygen, improve the air stability of black phosphorus, and the high conductivity of black phosphorus. The high rate and high catalytic ability can also enhance the application of MOFs in Li-S batteries.

The preparation method (1) of the present invention develops a method for preparing a MOF/BP heterojunction by self-assembly at an oil-water interface, thereby solving the problem that BP is easily oxidized in air. (2) A nanofiber phase change separator PPW with polyacrylonitrile PAN as the shell and paraffin PW as the core was developed to improve the safety performance of the battery operation. (3) The finally obtained PPW/MOF/BP phase change separator can effectively suppress the "shuttle effect" of lithium polysulfides, realize high safety and high performance lithium-sulfur batteries, and have broad application prospects.

Technical scheme: the preferred technical scheme adopted in the present invention is:

According to a first aspect of the present invention, there is provided a preparation method of a phase change diaphragm, comprising:

Step (a) preparation of MOF/BP heterojunction: forming metal-organic framework/black phosphorus (MOF/BP) heterojunction through interfacial assembly in an oil-water system, washing and drying to obtain MOF/BP heterojunction powder;

Step (b) preparation of phase change nanofiber membrane PPW: the core-shell structure nanofiber membrane PPW is prepared by coaxial electrospinning technology from polyacrylonitrile PAN solution and paraffin PW solution;

Step (c) Preparation of PPW/MOF/BP phase change diaphragm: the MOF/BP heterojunction powder is dispersed in deionized water to form a MOF/BP heterojunction dispersion liquid, and the MOF/BP heterojunction dispersion liquid is passed through The method of vacuum filtration is deposited on the nanofiber membrane PPW, and vacuum drying is performed to obtain a PPW/MOF/BP phase change membrane.

In some embodiments, in step (a), the oil-water system is a hexane/water system;

The metal organic framework MOF is UiO-66, and black phosphorus nanosheets are used.

In some embodiments, the preparation of step (a) MOF/BP heterojunction comprises: dispersing black phosphorus nanosheets in deionized water, adding aminoterephthalic acid and acetic acid, adding n-hexane after ultrasonic mixing, ultrasonic mixing, An aqueous solution of zirconium oxychloride is added, the reaction system is reacted in an oil bath, and after the reaction is completed, the MOF/BP heterojunction powder is obtained by centrifugation, washing and drying.

In some embodiments, in step (b), the solvent of the polyacrylonitrile PAN solution is N,N-dimethylformamide (DMF), and the preferred concentration of the polyacrylonitrile PAN solution is 10wt%; The solvent is kerosene; the preferred concentration of the paraffin solution is 40% (v/v).

In some embodiments, the step (b) of preparing the phase-change nanofiber membrane PPW includes: adding a polyacrylonitrile PAN solution and a paraffin PW solution into a syringe, respectively, and preparing a core-shell structure nanofiber by coaxial electrospinning technology. Fiber membrane PPW.

Further, in the coaxial electrospinning technology, the parameters of electrospinning are a voltage of 18.5kV, the distance between the receiver and the needle is 17cm, and the advancing rate of the polyacrylonitrile PAN solution and the paraffin PW solution is 0.5mL/h.

In some embodiments, in step (c), the concentration of the MOF/BP heterojunction dispersion liquid is 0.5-5 mg/mL, preferably 1-2 mg/mL.

In some embodiments, in step (c), the vacuum drying temperature is 60-80°C, preferably 70°C.

According to the second aspect of the present invention, there is provided a PPW/MOF/BP phase change membrane, which is prepared by the preparation method.

According to a third aspect of the present invention, the application of the PPW/MOF/BP phase change separator in a lithium-sulfur battery is provided.

Beneficial effects: The present invention is a phase change diaphragm for lithium-sulfur batteries and a preparation method thereof. The MOF/BP heterojunction is prepared by self-assembly at the oil-water interface, so as to solve the problem that black phosphorus is easy to oxidize and decompose in the air, and at the same time effectively Suppress the problem of "shuttle effect" in lithium-sulfur batteries and improve the cycle performance of batteries. And composite with phase change separator to obtain lithium-sulfur separator material with high safety and high performance. Has the following advantages:

(1) The method of preparing MOF/BP heterojunction by self-assembly at the oil-water interface, thus solving the problem that BP is easily oxidized in air.

(2) The nanofiber phase change separator PPW with polyacrylonitrile PAN as the shell and paraffin PW as the core improves the safety performance of the battery operation.

(3) The finally obtained PPW/MOF/BP phase change separator can effectively suppress the "shuttle effect" of lithium polysulfides, realize high safety and high performance lithium-sulfur batteries, and have broad application prospects.

Description of drawings

Fig. 1 is the morphology electron microscope image of MOF/BP heterojunction prepared in Example 1;

Fig. 2 is the electron microscope image of the PPW/MOF/BP phase change diaphragm prepared in Example 1;

Fig. 3 is the temperature change diagram of the PPW/MOF/BP phase change diaphragm of the embodiment under heating;

Figure 4 shows the cycle performance of the PPW/MOF/BP phase change separator in a lithium-sulfur battery.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and embodiments.

The endpoints of ranges and any values disclosed herein are not limited to the precise ranges or values, which are to be understood to encompass values proximate to those ranges or values. For ranges of values, the endpoints of each range, the endpoints of each range and the individual point values, and the individual point values can be combined with each other to yield one or more new ranges of values that Ranges should be considered as specifically disclosed herein.

For the purposes of this specification and the appended claims, unless stated otherwise, all figures expressing quantities, percentages or ratios and other numerical values used in this specification and the appended claims are to be understood in all cases as Modified by the term "about". Furthermore, all ranges disclosed herein are inclusive of the endpoints and independently combinable.

Example 1

(1) Preparation of MOF/BP heterojunction

First, 20 mg of black phosphorus nanosheets were dispersed in 10 mL of deionized water in a 50 mL single-necked flask, then 50 mg of aminoterephthalic acid and 2 mL of acetic acid were added, 10 mL of n-hexane solution was added after sonicating for 5 minutes, and 1 mL was added after sonicating for 20 min. Zirconium oxychloride octahydrate (50 mg/mL) aqueous solution, and the reaction system was reacted in an oil bath at 70 °C for 24 h. After the reaction, the crude product was obtained by centrifugation, then washed with DMF and methanol to remove unreacted monomers, and vacuum dried at 50°C to obtain MOF/BP heterojunction powder.

Fig. 1 is the morphology electron microscope image of the prepared MOF/BP heterojunction; wherein: Fig. 1a is pure black phosphorus nanosheets, Fig. 1b is MOF UiO-66 prepared by conventional hydrothermal method, Fig. 1c is in the example MOF/BP heterojunction fabricated in oil-water system. It can be seen that the MOF/BP heterojunction was successfully self-assembled at the oil-water interface system, and the prepared MOF particles were smaller, which was beneficial to protect black phosphorus and improve the stability of black phosphorus in air.

(2) Preparation of phase change nanofiber membrane PPW

1 g of PAN powder was dissolved in 10 mL of DMF under heating at 50° C. to prepare a PAN solution with a concentration of 10 wt %. The solid paraffin was heated and melted, and then 4 mL of liquid paraffin was added to 6 mL of kerosene to prepare a 40% (v/v) paraffin solution. The PAN solution and the paraffin solution were added into the syringe respectively, and the phase-change nanofiber membrane PPW with core-shell structure was prepared by coaxial electrospinning technology. The electrospinning process was as follows: the voltage was 18.5 kV, the distance between the receiver and the needle was 17 cm, and the advancing rate of the PAN and PW solutions was 0.5 mL/h. After spinning, the film was detached from the receiver and vacuum-dried at 70°C for 24h to obtain a PPW film.

(3) Preparation of PPW/MOF/BP composite membrane

The PPW in (2) was cut into a square size of 4 × 4 cm ² , placed on a vacuum filtration device, then 10 mg of the MOF/BP heterojunction powder obtained from (1) was dispersed in 10 mL of water, and the method was vacuum filtered. It was deposited on the surface of the PPW film, and then vacuum-dried at 70 °C for 24 h to obtain the PPW/MOF/BP phase change separator, which was cut into 19 mm diameter discs for cycle testing of lithium-sulfur batteries.

Example 2

(1) Preparation of MOF/BP heterojunction

The preparation of MOF/BP heterojunctions was carried out according to Example 1.

(2) Preparation of phase change nanofiber membrane PPW

The preparation of phase change nanofiber membrane PPW was carried out according to Example 1.

(3) Preparation of PPW/MOF/BP composite membrane

Cut the PPW in (2) into a square size of 4 × 4 cm ² and place it on a vacuum filtration device, then take (1) to obtain 15 mg of the MOF/BP heterojunction powder and disperse it in 10 mL of water, and filter it by vacuum. It was deposited on the surface of the PPW film, and then vacuum-dried at 70 °C for 24 h to obtain the PPW/MOF/BP phase change separator, which was cut into 19 mm diameter discs for cycle testing of lithium-sulfur batteries.

Fig. 2 is the electron microscope picture of the PPW/MOF/BP phase change diaphragm obtained in Example 2;

As can be seen from Figure 2a, we successfully prepared nanofiber membranes by electrospinning technology. The interwoven nanofiber skeleton can construct a separator with rich pores, which is conducive to the infiltration of electrolyte and the transport of lithium ions. The difference in contrast between the core and the shell of the TEM image in Figure 2b shows that a nanofiber phase change separator with a core-shell structure was successfully prepared, in which the shell is polyacrylonitrile PAN, the core is paraffin PW, and the paraffin is encapsulated in PAN. Electrospinning The nanofiber membrane formed by silk is denoted as PPW. Figure 2c shows the PPW/MOF/BP membrane, indicating that the MOF/BP heterojunction was successfully deposited on the surface of the membrane by vacuum filtration, with a thickness of about 16 μm.

Figure 3 shows the temperature change diagram of the PPW/MOF/BP phase change diaphragm under heating;

Commercial separators, PPW and PPW/MOF/BP were placed on a heating plate, and the thermal runaway management of phase change separators was investigated by heating up. It can be seen from Figure 3 that the surface temperature of PPW/MOF/BP is much lower than that of the commercial separator after heating for 10 min. This is because the paraffin will absorb heat during the melting process, thereby reducing the system temperature, which appears during battery operation. In the process of instantaneous thermal runaway behavior, the rapid melting of paraffin can reduce the temperature of the entire battery system, thereby making the battery operation safer.

Figure 4 shows the cycle performance of the PPW/MOF/BP phase change separator in a lithium-sulfur battery;

It can be seen from Figure 4 that the PPW/MOF/BP separator prepared in Example 2 still has a capacity retention rate of 97.5% after 100 cycles of operation at 0.2C, while the capacity of the commercial separator rapidly decays to 476.8mAh/g after 100 cycles of operation , the capacity retention rate is only 51.7%. This is caused by the shuttle effect of lithium polysulfide, and the cycle performance also shows that PPW/MOF/BP can effectively block the shuttle of lithium polysulfide, thereby improving the cycle performance of the battery.

Example 3

(1) Preparation of MOF/BP heterojunction

The preparation of MOF/BP heterojunctions was carried out according to Example 1.

(2) Preparation of phase change nanofiber membrane PPW

The preparation of phase change nanofiber membrane PPW was carried out according to Example 1.

(3) Preparation of PPW/MOF/BP composite membrane

The PPW in (2) was cut into a square size of 4×4 cm ² , placed on a vacuum filtration device, and then 20 mg of the MOF/BP heterojunction powder obtained from (1) was dispersed in 10 mL of water, and the method was vacuum filtered. It was deposited on the surface of the PPW film, and then vacuum-dried at 70 °C for 24 h to obtain the PPW/MOF/BP phase change separator, which was cut into 19 mm diameter discs for cycle testing of lithium-sulfur batteries.

The cycle performance of the PPW/MOF/BP phase change diaphragm materials prepared in the above Examples 1-3 is shown in Table 1, and it can be seen that the cycle performance of the phase change diaphragm is greatly improved compared with the commercial diaphragm.

Table 1 Cycling performance of PPW/MOF/BP phase change separator materials

The present invention successfully synthesizes the heterojunction of MOF/BP in the oil-water system, and improves the stability of black phosphorus in air. At the same time, a phase change nanofiber membrane PPW with a core-shell structure is prepared by the coaxial electrospinning process, which can effectively solve the problem of thermal runaway of the battery during operation. Combining MOF/BP and PPW through a simple suction filtration process, MOF/BP can effectively inhibit the "shuttle effect" of lithium polysulfides in lithium-sulfur batteries, and significantly improve the cycle performance of lithium-sulfur batteries, while PPW phase change separator, Then the thermal runaway problem of the battery can be regulated and the safety of the battery operation can be improved. The prepared PPW/MOF/BP phase change separator significantly improves the cycling performance of lithium-sulfur batteries and is expected to be applied in real life in the future.

The above are only the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the technical principle of the present invention, several improvements and modifications can also be made. These improvements and modifications It should also be regarded as the protection scope of the present invention.

Claims (10)

1. A method for producing a phase change separator, comprising:

step (a) preparation of MOF/BP heterojunction: forming a metal organic framework/black phosphorus (MOF/BP) heterojunction through interface assembly in an oil-water system, and washing and drying to obtain MOF/BP heterojunction powder;

preparing a phase-change nanofiber membrane (PPW) in the step (b): preparing a Polyacrylonitrile (PAN) solution and a paraffin PW solution into a nanofiber membrane (PPW) with a core-shell structure by a coaxial electrostatic spinning technology;

step (c) preparation of a PPW/MOF/BP phase-change membrane: and dispersing the MOF/BP heterojunction powder in deionized water to form MOF/BP heterojunction dispersion liquid, depositing the MOF/BP heterojunction dispersion liquid on the nanofiber membrane PPW by a vacuum filtration method, and carrying out vacuum drying to obtain the PPW/MOF/BP phase change membrane.

2. The method according to claim 1, wherein in the step (a), the oil-water system is a hexane/water system;

and/or the metal organic framework MOF is UiO-66, and black phosphorus is black phosphorus nanosheet.

3. A method according to claim 1 or 2, wherein the step (a) of preparing the MOF/BP heterojunction comprises: dispersing the black phosphorus nanosheets in deionized water, adding amino terephthalic acid and acetic acid, adding n-hexane after ultrasonic mixing, ultrasonically mixing, adding a zirconium oxychloride aqueous solution, reacting a reaction system in an oil bath, and centrifuging, washing and drying after the reaction is finished to obtain the MOF/BP heterojunction powder.

4. The method according to claim 1, wherein in step (b), the solvent of polyacrylonitrile PAN solution is N, N-Dimethylformamide (DMF), preferably the concentration of polyacrylonitrile PAN solution is 10 wt%;

and/or, the solvent of the paraffin PW solution adopts kerosene; the preferred concentration of the paraffin solution is 40% (v/v).

5. The preparation method according to claim 1 or 3, wherein the step (b) of preparing the phase-change nanofiber membrane PPW comprises: adding Polyacrylonitrile (PAN) solution and Paraffin (PW) solution into an injector respectively, and preparing the nanofiber membrane (PPW) with the core-shell structure by a coaxial electrostatic spinning technology.

6. The method according to claim 1, 3 or 5, wherein in the coaxial electrospinning technique, the electrospinning parameters are voltage of 18.5kV, the distance between the receiver and the needle is 17cm, and the advancing rates of the polyacrylonitrile PAN solution and the paraffin PW solution are 0.5 mL/h.

7. The preparation method according to claim 1, wherein in the step (c), the concentration of the MOF/BP heterojunction dispersion is 0.5-5 mg/mL, preferably 1-2 mg/mL.

8. The preparation method according to claim 1, wherein in the step (C), the vacuum drying temperature is 60-80 ℃, preferably 70 ℃.

9. A PPW/MOF/BP phase change membrane prepared by the preparation method of any one of claims 1 to 8.

10. Use of the PPW/MOF/BP phase change membrane of claim 9 in a lithium sulfur battery.

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