A functional separator for lithium-sulfur batteries based on oxygen vacancy content regulation, and its preparation method and application

Abstract

The present invention discloses a functional diaphragm for lithium-sulfur batteries based on oxygen vacancy content regulation, as well as its preparation method and application. CeO2 is an important rare earth catalytic material with excellent oxygen storage / release capacity. At high temperatures, CeO2 releases different amounts of oxygen in different gas atmospheres, thereby producing CeO2 with different oxygen vacancy contents. 2‑x The prepared CeO with different oxygen vacancy contents 2‑x / CNT composite materials are used as the separator functional layer of lithium / sodium-sulfur batteries. On the one hand, the interwoven network of CNTs can provide good electronic conductivity and have a physical barrier effect on polysulfides; on the other hand, CeO 2‑x It can efficiently adsorb polysulfides that migrate to the membrane surface and catalyze and accelerate their bidirectional reversible transformation, thereby inhibiting the "shuttle effect" of polysulfides. 2‑x The lithium-sulfur battery assembled with the CNT functional separator exhibits high specific capacity, excellent rate performance and cycle stability.

Description

Technical Field

[0001] The present invention belongs to the field of new energy materials and devices, and specifically relates to a lithium-sulfur battery functional diaphragm based on oxygen vacancy content regulation, and a preparation method and application thereof. Background Art

[0002] With the development of society, energy issues have become increasingly prominent. The increasing scarcity of traditional non-renewable energy and the serious pollution to the environment have forced people to look for new renewable energy sources. At present, solar energy, wind energy, tidal energy, etc. are considered to be important candidates for the next generation of energy. Therefore, finding and developing storage and conversion devices for clean energy has become an urgent problem to be solved. Lithium-ion batteries have been widely used in portable electronic products, electric vehicles and other fields due to their high energy density and good cycle stability. However, the energy density of lithium-ion batteries is currently close to its theoretical limit and cannot meet people's demand for high energy density energy. Therefore, the development of high energy density and low-cost energy storage systems has become an urgent matter.

[0003] Lithium-sulfur batteries are widely used due to their high specific capacity and energy density (1675 mAh g -1 and 2600 Wh kg -1 ), low potential and the fact that cathode sulfur is abundant in natural resources, inexpensive and environmentally friendly are advantages that make it a highly promising candidate for the next generation of energy storage and conversion devices; however, lithium-sulfur batteries still face many challenges in their practical applications, such as the non-conductivity of elemental sulfur and the discharge product lithium sulfide, the volume expansion of the electrode and the problem of lithium dendrites during charge and discharge reactions, the rapid capacity decay and low utilization rate of active materials caused by the "shuttle effect" of soluble polysulfides.

[0004] As a crucial component of lithium-sulfur batteries, the separator performs the crucial functions of separating the positive and negative electrodes, preventing short circuits, and facilitating ion diffusion. Its performance directly impacts the overall performance of the battery. Currently, commercial separators for lithium-sulfur batteries are typically non-polar films such as polypropylene / polyethylene (PP / PE). These membranes are unable to prevent polysulfides dissolved in the electrolyte from shuttling between the positive and negative electrodes. Therefore, various carbon materials, polar materials, and their composites are being used as functional layers on the separator surface to modify commercial separators. Carbon materials act as conductive networks to facilitate electron transport and physically block the "shuttling effect" of polysulfides. Polar materials, on the other hand, act as chemical scavengers to anchor polysulfides and catalyze their bidirectional, reversible transformation. However, solely utilizing physical barriers or chemical adsorption cannot completely suppress the shuttling effect, resulting in limited improvement in overall battery performance. Therefore, the rational design and development of a high-performance composite functional separator to effectively inhibit the polysulfide shuttling effect is essential for improving the electrochemical performance of lithium-sulfur batteries. Summary of the Invention

[0005] The purpose of the present invention is to provide a functional lithium-sulfur battery diaphragm based on oxygen vacancy content regulation, as well as its preparation method and application. The functional diaphragm can effectively suppress the "shuttle effect" of lithium-sulfur batteries, realize the efficient utilization of active material sulfur, and greatly improve the battery capacity, coulombic efficiency and cycle life.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A lithium-sulfur battery functional diaphragm based on oxygen vacancy content regulation, comprising a diaphragm substrate and a functional layer laid on one side of the diaphragm substrate, wherein the diaphragm substrate is a common commercial battery diaphragm substrate, and the functional layer is composed of CeO 2-x / CNT composite material, the CeO 2-x It contains abundant oxygen vacancies and is loaded on the surface of hydroxylated CNTs. The oxygen vacancy content can be regulated by high-temperature calcination in different atmospheres.

[0008] As a preferred technical solution of the present invention, the functional layer is composed of CeO 2-x The dispersion of CNT composite materials was filtered on one side of the common commercial battery separator substrate. The CeO 2-x The area loading of the CNT functional layer is 0.1~0.3 mg / cm 2 .

[0009] A method for preparing a functional separator for lithium-sulfur batteries based on the regulation of oxygen vacancy content is firstly performed by refluxing and purifying ordinary commercial multi-walled carbon nanotubes (CNTs) to obtain hydroxylated CNTs; then, the hydroxylated CNTs are used as carriers and CeCl3 is used as a Ce source, and CeO is obtained by high-temperature calcination in different gas atmospheres. 2-x / CNT composite material, and finally filter its dispersion onto one side of a common commercial battery separator matrix; the specific steps are as follows:

[0010] (1) First, ordinary commercial multi-walled carbon nanotubes (CNTs) were added to HNO3 with a concentration of 65% to 70%, refluxed at 90 to 140 °C for 2 to 6 h, washed, and vacuum-dried to obtain hydroxylated CNTs;

[0011] (2) Ultrasonic dispersion of the hydroxylated CNT obtained in step (1) in deionized water to obtain dispersion A; CeCl3 is fully dissolved in deionized water to obtain solution B; solution B is slowly added to dispersion A to obtain mixed solution C, and the pH value of the reaction system of solution C is adjusted to 8-12 with 0.1-0.5 mol / L NaOH solution. The reaction is stirred at room temperature for 2-6 h, and the product is centrifuged, washed, and dried; finally, the obtained product is calcined at high temperature in argon hydrogen (Ar / H2), argon (Ar), nitrogen (N2), and air (Air) atmosphere to obtain CeO2-x / CNT-Ar / H2、CeO 2-x / CNT-Ar、CeO 2-x / CNT-N2、CeO 2-x / CNT-Air composite materials; calcination temperature is 380~450℃, heating rate is 2~5℃ / min, holding time is 0.5~3h, and the volume percentage of argon and hydrogen in argon hydrogen (Ar / H2) atmosphere is 19:1;

[0012] (3) The calcined product obtained in step (2) is ultrasonically dispersed in anhydrous ethanol to obtain a dispersion, and then the dispersion is filtered onto the surface of one side of a common commercial battery separator substrate, and finally a functional separator for lithium-sulfur batteries is obtained after vacuum drying.

[0013] As a preferred technical solution of the present invention, the common commercial battery separator substrate is one of polypropylene porous membrane, polyethylene porous membrane, polyethylene / polypropylene composite porous membrane, or one of polyimide separator and polyethylene terephthalate-based non-woven fabric separator.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] The present invention utilizes the fact that CeO2 at high temperature releases different amounts of oxygen in different gas atmospheres, thus realizing CeO2 with adjustable oxygen vacancy content. 2-x Preparation of CeO / CNT composite materials; 2-x Fine CeO in CNT composites 2-x Nanocrystals are uniformly loaded on the surface of CNTs. The interwoven network structure of CNTs has the advantages of light weight, high conductivity and excellent mechanical properties. It not only provides good electronic conductivity, but also has a physical barrier effect on polysulfides. Polar CeO 2-x Nanocrystals contain abundant oxygen vacancy defects, which can efficiently adsorb polysulfides that migrate to the membrane surface and catalyze and accelerate their bidirectionalization, thereby inhibiting the "shuttle effect" of polysulfides. 2-x The lithium-sulfur battery assembled with the CNT / CNT composite material as a lithium-sulfur battery separator has high specific capacity, excellent rate performance and cycle stability.

[0016] In addition, the CeO 2-x The preparation method of the CNT / Li-S composite material is simple, non-toxic, reproducible, high-yield, and environmentally friendly, which is conducive to industrial promotion and has important significance for promoting the practical application of lithium-sulfur batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The CeO prepared in Example 1 2-xSEM and TEM images of / CNT-Ar / H2 composite materials.

[0018] Figure 2 The CeO prepared in Examples 1, 2, 3, and 4 2-x XRD patterns of / CNT composites.

[0019] Figure 3 The CeO prepared in Examples 1, 2, 3, and 4 2-x Raman graph of / CNT composite material.

[0020] Figure 4 The CeO prepared in Examples 1, 2, 3, and 4 2-x X-ray photoelectron spectroscopy (XPS) of / CNT composites.

[0021] Figure 5 The constant current charge-discharge cycle test results of the lithium-sulfur batteries prepared in Example 1 and Comparative Example 1 at a rate of 1 C are shown.

[0022] Figure 6 These are the performance test results of the lithium-sulfur batteries prepared in Example 1 and Comparative Example 1 at different rates.

[0023] Figure 7 CeO prepared in Example 2 2-x SEM image of / CNT-Ar composite material.

[0024] Figure 8 The constant current charge-discharge cycle test results of the lithium-sulfur batteries prepared in Example 2 and Example 1 at a rate of 0.5 C are shown. DETAILED DESCRIPTION

[0025] The embodiments of the present technical solution will be further described in detail below with reference to the accompanying drawings and specific embodiments. Example 1

[0026] This embodiment provides a CeO 2-x Preparation of CNT / CNT composite materials and using them as lithium-sulfur battery separators to prepare lithium-sulfur batteries. The specific steps are as follows:

[0027] (1) Preparation of hydroxylated CNTs: First, ordinary commercial multi-walled carbon nanotubes were added to 150 mL of 68% HNO3 and refluxed at 120 °C for 3 h to obtain hydroxylated CNTs.

[0028] (2) CeO 2-xPreparation of CNT / CNT composites: 20 mg of hydroxylated CNTs were ultrasonically dispersed in 40 mL of deionized water to obtain dispersion A; 20 mg of CeCl3 was fully dissolved in 60 mL of deionized water to obtain solution B; solution B was slowly added to dispersion A to obtain mixed solution C, and the pH value of solution C was adjusted to 10 with 0.1 mol / L NaOH solution. The mixture was stirred for 4 h, and the product was centrifuged, washed, and dried; finally, CeO was calcined at high temperature in an argon / hydrogen (Ar / H2) atmosphere. 2-x / CNT-Ar / H2 composite material. The calcination temperature was 400 °C, the heating rate was 2 °C / min, the holding time was 0.5 h, and the volume percentage of argon and hydrogen in the argon-hydrogen (Ar / H2) atmosphere was 19:1.

[0029] (3) Preparation of lithium-sulfur battery separator: The calcined product obtained in step (2) was ultrasonically dispersed in anhydrous ethanol to obtain a dispersion, and then the dispersion was filtered on the surface of one side of the substrate of a common commercial battery PP separator (Celgard 2500). After vacuum drying, a functional separator for lithium-sulfur batteries was finally obtained. The unit area loading of the functional layer was 0.18 mg / cm 2 .

[0030] See also Figure 1 , this figure shows the CeO prepared in this example 2-x SEM images of / CNT-Ar / H2 composites Figure 1 a and TEM Figure 1 b. As can be seen from the figure, CNTs are interwoven network structures, which not only have a physical barrier effect on polysulfides, but also provide good electronic conductivity; small polar CeO 2-x The nanoparticles are evenly loaded on the CNT surface, which can efficiently adsorb polysulfides and catalytically accelerate their bidirectional reversible transformation, thereby inhibiting the "shuttle effect" of polysulfides and improving the electrochemical performance of lithium-sulfur batteries.

[0031] See also Figure 2 , this figure shows CeO prepared in Examples 1, 2, 3, and 4 2-x XRD patterns of CeO / CNT composite materials. 2-x The XRD spectra of the CeO / CNT composites are consistent with the XRD characteristic peak positions in the standard card JCPDS No.34-0394, indicating that CeO 2-x The oxygen vacancies on the surface did not change its phase structure, and no other impurity peaks were found in the XRD spectrum, proving that CeO was successfully synthesized. 2-x / CNT composite materials.

[0032] See also Figure 3, this figure shows CeO prepared in Examples 1, 2, 3, and 4 2-x / CNT composite material Raman spectrum, as shown in the figure, CeO prepared in Examples 1, 2, 3, and 4 2-x / CNT composites at 466 cm -1 CeO2 characteristic peaks appeared nearby. With the increase of oxygen vacancy content, the characteristic peaks gradually shifted to lower wavenumbers, proving that the oxygen vacancy content can be controlled by calcining in different gas atmospheres.

[0033] See also Figure 4 , this figure shows CeO prepared in Examples 1, 2, 3, and 4 2-x High-resolution spectra of (a) Ce3d and (b) O1s of / CNT composite materials. Figure 4 It can be seen from the high-resolution spectrum of Ce3d that the characteristic peaks at 882.8 eV, 889.4 eV, 898.7 eV, 901.3 eV, 907.7 eV and 917.2 eV correspond to CeO 2-x Central 4+ The characteristic peaks at 885.9 eV and 903.8 eV correspond to CeO 2-x Central 3+ state peak, proving that CeO 2-x The presence of oxygen vacancies in CeO prepared in Examples 1, 2, 3, and 4 2-x Ce in CNT composites 3+ / Ce 4+ The proportions are 25%, 19%, 18% and 11% respectively; Figure 4 As shown in b, in the high-resolution spectrum of O1s, the characteristic peaks at 529.6 eV, 531.6 eV and 532.6 eV correspond to lattice oxygen, oxygen vacancies and adsorbed oxygen, respectively. 2-x The oxygen vacancy ratios in the CeO / CNT composites were 31%, 23%, 21% and 13%, respectively (the oxygen content in CeO2 without oxygen vacancies was taken as 100%), which proved that the oxygen vacancy content could be regulated by calcining in different gas atmospheres.

[0034] The CeO prepared in this example 2-x / CNT-Ar / H2 functional separator and carbon / sulfur composite positive electrode, lithium metal negative electrode, 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME) based bis(trifluoromethanesulfonic acid) imide lithium (LiTFSI) electrolyte, and 2032 button cells were assembled in the order of negative electrode shell-spring sheet-gasket-lithium sheet-electrolyte-separator-electrolyte-positive electrode sheet-positive electrode shell and electrochemical performance tests were carried out. Among them, the area mass of active sulfur in the positive electrode sheet is 1.50 mg / cm 2 about.

[0035] Comparative Example 1

[0036] In order to compare and illustrate the lithium sulfur battery provided by the present invention with CeO 2-x Effect of / CNT-Ar / H2 functional membrane on the electrochemical performance of lithium-sulfur battery. The preparation of the lithium-sulfur battery in this comparative example 1 is basically the same as that in Example 1, except that the battery is assembled with ordinary Celgard 2500 membrane and the charge and discharge test is carried out.

[0037] See also Figure 5 , CeO prepared in Example 1 2-x The lithium-sulfur battery assembled with the / CNT-Ar / H2 functional membrane was subjected to constant current charge and discharge at 1 C, with an initial discharge capacity of up to 1132 mAh / g. After 500 cycles of charge and discharge, the capacity dropped to 599 mAh / g, with a capacity retention rate of 53% and an average capacity decay rate of about 0.09% per cycle. In contrast, the battery assembled with the ordinary commercial Celgard 2500 membrane in Comparative Example 1 had an initial discharge capacity of 838 mAh / g at 1 C, and after 500 cycles of charge and discharge, the capacity dropped to 374 mAh / g. The capacity retention rate was only 45% and the average capacity decay rate was only 0.11%. Compared with Comparative Example 1, the battery assembled with CeO in Example 1 had an initial discharge capacity of 838 mAh / g at 1 C, and after 500 cycles of charge and discharge, the capacity dropped to 374 mAh / g. The capacity retention rate was only 45% and the average capacity decay rate was only 0.11%. 2-x The specific capacity and cycle stability of lithium-sulfur batteries with / CNT-Ar / H2 functional membrane are significantly improved.

[0038] See also Figure 6 , This figure shows the rate performance test results of the lithium-sulfur battery in Example 1 and Comparative Example 1, which were charged and discharged at 0.2 C, 0.5 C, 1 C, 2 C, 3 C and 5 C respectively. The lithium-sulfur battery prepared in Example 1 has a discharge capacity of 1139 mAh / g at a low rate of 0.2 C, and a discharge capacity of 538 mAh / g at a high rate of 5 C. In contrast, the lithium-sulfur battery assembled with ordinary commercial Celgard 2500 diaphragm in Comparative Example 1 has a discharge capacity of 741 mAh / g at a low rate of 0.2 C, and a discharge capacity of only 341 mAh / g at a high rate of 5 C, indicating that the rate performance of the lithium-sulfur battery in Comparative Example 1 is significantly lower than that of the lithium-sulfur battery assembled with CeO 2-x Lithium-sulfur battery assembled with / CNT-Ar / H2 functional membrane.

[0039] The above data show that the use of CeO in Example 1 2-x The electrochemical properties of the lithium-sulfur battery with the / CNT-Ar / H2 functional membrane, such as capacity, cycle performance, rate performance, and coulombic efficiency, are significantly better than those of the battery assembled using the commercial Celgard 2500 battery membrane in Comparative Example 1. Example 2

[0040] The preparation method of this embodiment is the same as that of embodiment 1, except that the calcination process in step (2) is carried out in an argon (Ar) atmosphere, the calcination temperature is 400 °C, the heating rate is 2 °C / min, and the holding time is 0.5 h to prepare CeO 2-x / CNT-Ar composite material, wherein the CeO prepared in this embodiment 2-x The oxygen vacancy content in the / CNT-Ar composite material is 23% ( Figure 4 shown).

[0041] See also Figure 7 , this figure shows the CeO prepared in this example 2-x SEM image of / CNT-Ar composite material. As can be seen from the figure, CNT presents an interwoven network structure, CeO 2-x The nanoparticles are uniformly loaded on the CNT surface.

[0042] See also Figure 8 , This figure shows the constant current charge and discharge cycle test results of the lithium-sulfur batteries prepared in this embodiment and Example 1 at a rate of 0.5 C. 2-x The electrochemical performance of the lithium-sulfur battery assembled with the CNT-Ar functional separator, including capacity, rate capability, and cycling stability, was slightly lower than that of Example 1: the initial discharge capacity at 0.5 C was 1253 mAh / g, and after 500 cycles, the capacity remained at 769 mAh / g, with a capacity retention rate of 61%. Furthermore, the rate capability was lower than that of Example 1. Example 3

[0043] The preparation method of this embodiment is the same as that of embodiment 1, except that the calcination process in step (2) is carried out in a nitrogen (N2) atmosphere, the calcination temperature is 400°C, the heating rate is 2°C / min, and the holding time is 0.5 h to prepare CeO 2-x / CNT-N2 composite material, wherein the CeO prepared in this embodiment 2-x The oxygen vacancy content in the / CNT-N2 composite material is 21% ( Figure 4 shown).

[0044] Compared with Example 1, the CeO prepared in this example 2-x The electrochemical performance of the lithium-sulfur battery assembled with the CNT-N2 functional separator, including capacity, rate capability, and cycling stability, was lower than that of Example 1: the initial discharge capacity at 1 C was 970 mAh / g, and after 500 cycles, the capacity remained at 490 mAh / g, with a capacity retention rate of 51%. Furthermore, the rate capability was also reduced. Example 4

[0045] The preparation method of this embodiment is the same as that of embodiment 1, except that the calcination process in step (2) is carried out in an air atmosphere, the calcination temperature is 400 °C, the heating rate is 2 °C / min, and the holding time is 0.5 h. CeO is prepared. 2-x / CNT-Air composite material, wherein the CeO prepared in this embodiment 2-x The oxygen vacancy content in the CNT-Air composite material is 13% ( Figure 4 shown).

[0046] Compared with Example 1, the CeO prepared in this example 2-x The electrochemical properties of the lithium-sulfur battery assembled with the / CNT-Air functional membrane, such as capacity, rate, and cycle stability, were significantly reduced compared with those in Example 1: the initial discharge capacity at 1 C was 914 mAh / g, and after 500 cycles, the capacity remained at 421 mAh / g, with a capacity retention rate of 46%.

[0047] The above content is merely an example and explanation of the concept of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in a similar manner. As long as they do not deviate from the concept of the invention or exceed the scope defined by the claims, they should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a functional separator for a lithium-sulfur battery based on regulation of oxygen vacancy content, characterized in that: The specific steps are as follows: (1) Preparation of hydroxylated CNTs: First, ordinary commercial multi-walled carbon nanotubes were added to 150 mL of 68% HNO3 and refluxed at 120 °C for 3 h to obtain hydroxylated CNTs. (2) CeO 2-x Preparation of CNT / CNT composites: 20 mg of hydroxylated CNTs were ultrasonically dispersed in 40 mL of deionized water to obtain dispersion A; 20 mg of CeCl3 was fully dissolved in 60 mL of deionized water to obtain solution B; solution B was slowly added to dispersion A to obtain mixed solution C, and the pH value of solution C was adjusted to 10 with 0.1 mol / L NaOH solution. The mixture was stirred for 4 h, and the product was centrifuged, washed, and dried; finally, CeO was calcined at high temperature in an argon-hydrogen atmosphere. 2-x / CNT-Ar / H2 composite material; wherein, the calcination temperature is 400 ℃, the heating rate is 2 ℃ / min, the holding time is 0.5 h, wherein, the volume percentage of argon and hydrogen in the argon-hydrogen atmosphere is 19:1; the prepared CeO 2-x The proportion of oxygen vacancies in the / CNT composite is 31%; (3) Preparation of lithium-sulfur battery separator: The calcined product obtained in step (2) was ultrasonically dispersed in anhydrous ethanol to obtain a dispersion, and then the dispersion was filtered on the surface of one side of a common commercial battery PP separator substrate. After vacuum drying, a functional separator for lithium-sulfur batteries was finally obtained; wherein the unit area loading of the functional layer was 0.18 mg / cm 2 .

2. Application of the functional diaphragm prepared by the method according to claim 1 in lithium-sulfur batteries.

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