CEJ: Two-dimensional sandwich composites of covalently grafted MXenes with conjugated microporous polymers as sulfur carriers for lithium-sulfur batteries

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Two-dimensional sandwich composites of covalently grafted MXenes with conjugated microporous polymers as sulfur carriers for lithium-sulfur batteries

First author: Cao Yawen

Corresponding author: Geng Jianxin

Unit: Beijing University of Chemical Technology, Tianjin University of Technology



Research Background

The development of portable electronic devices, electric vehicles and smart grids has put forward higher requirements for the energy density of electrochemical energy storage systems. Lithium-sulfur batteries have high theoretical energy density (2600 W h kg-1) and low cost of cathode sulfur. , environment-friendly and has received widespread attention. However, the slow cathode reaction kinetics and the shuttle effect of polysulfides hinder the commercialization of lithium-sulfur batteries. At present, researchers have developed materials with different functions as sulfur carriers to solve the above problems.

Conjugated microporous polymers (CMPs) have the advantages of p-conjugated framework, tunable chemical composition and pore structure, and their applications in lithium-sulfur batteries have attracted much attention in recent years. However, the electrical conductivity of most CMPs is very poor, limiting their application as sulfur carriers. MXene is a class of two-dimensional sheet materials with good electrical conductivity, which can not only realize the chemical capture of polysulfides through Lewis acid-base interactions, but also promote the conversion of polysulfides and accelerate the reaction kinetics of sulfur cathodes. However, MXene nanosheets are prone to restack due to interlayer van der Waals forces, resulting in the ineffective utilization of their active sites.



Introduction to the article

Professor Geng Jianxin from Tianjin University of Technology and his graduate student Cao Yawen from Beijing University of Chemical Technology, etc., published a paper entitled "Covalently grafting conjugated porous polymers to MXene offers a two-dimensional sandwich-structured electrocatalytic sulfur host for lithium" in Chemical Engineering Journal, an internationally renowned journal -sulfur batteries" research article.

This work resulted in a CMP with larger pore size and molecular polarity by using two monomers with longer molecular backbones and heteroatoms. Using an in situ polymerization strategy, CMP was grown in situ on the surface of functionalized MXene, and a two-dimensional sandwich-like composite (CMP-M) was obtained by covalently grafting CMP to MXene. Comparative study of materials.

A series of electrochemical tests showed that the lithium-sulfur batteries prepared with CMP-M as the sulfur support material exhibited superior rate performance and cycle stability compared with pure CMP. Through cyclic voltammetry and lithium sulfide deposition experiments at different scan rates, the contribution of MXene components to improving the reaction kinetics of the sulfur cathode was found. The two-dimensional sandwich structure formed by the grafting improves the mechanical properties of the cathode material and avoids the pulverization of the electrode during the charging and discharging process. This work provides a new idea for realizing the design of novel high-performance electrocatalysts that can be used in lithium-sulfur batteries and other energy storage devices.



Key points of this article

Point 1: Preparation and characterization of two-dimensional sandwich composite CMP-M

First, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) containing triazine group and 2,7-triazine containing benzothiophene unit were prepared by molecular design Dibromo[1]benzothieno[3,2-b][1]benzothiophene (DiBr-BTBT) monomer; p-bromophenyl modified MXene (denoted as BrPh-MXene). Secondly, in the presence of BrPh-MXene, TAPT and DiBr-BTBT were polymerized through Buchwald-Hartwig coupling reaction to obtain a two-dimensional sandwich composite CMP-M with CMP covalently grafted to MXene.

SEM and TEM characterizations showed that the CMP was composed of a fiber network with a large number of mesopores, presenting an agglomerated state. On the other hand, CMP-M presents a sheet state, CMP grows uniformly on the surface of the MXene sheet, and maintains the fiber network characteristics. There are also a large number of mesopores in CMP-M. Therefore, CMP-M can achieve fast electron transfer in electrochemical reactions through the MXene layer while retaining the advantages of CMP. The elemental distribution map demonstrates that CMP is uniformly grafted on MXene. The N2 adsorption and desorption isotherms and pore size distribution indicated that the introduction of MXene template did not significantly affect the CMP pore structure.

Figure 1. Synthetic routes of (a) CMP and (b) CMP-M and (c) schematic diagram of the transformation of sulfur species in the two cathode materials. Due to the catalysis of MXene, the conversion rate of sulfur species in the CMP-M support is higher than that in the CMP support, and therefore, the tendency of polysulfides in CMP to diffuse into the electrolyte is more pronounced.



Figure 2. Morphology and structural characterization of CMP-M. SEM images of (a) CMP and (b) CMP-M at different magnifications. (c) TEM images of CMP and (d) CMP-M. (e) SEM image of CMP-M and (f) corresponding element distribution map of C, N, S, and Ti. (g) N2 adsorption and desorption isotherms and pore size distributions of CMP and CMP-M.



Point 2: Electrochemical performance evaluation

Cyclic voltammetry (CV) tests at different scan rates show that the S@CMP-M cathode has faster reaction kinetics compared to the S@CMP cathode. Lithium sulfide deposition experiments show that CMP-M can promote the conversion of polysulfides to lithium sulfide. CV and LiS deposition tests demonstrate that the introduction of MXene can enhance the electrocatalytic performance of the composites.

A series of electrochemical characterizations (CV curve, Tafel curve, charge-discharge curve, rate performance test, impedance test, cycle stability test) proved that the batteries prepared by CMP-M as sulfur carrier had higher performance than those prepared by CMP. Sulfur utilization, better rate performance (discharge capacity of 610 mA h g-1 at 4 C), and more stable cycling performance (1000 cycles at 0.5 C, with an average capacity decay rate of 0.044% per cycle; 2 C The next cycle is 1000 laps, and the average capacity decay per lap is 0.025%). The excellent electrochemical performance can be attributed to the synergistic effect of the CMP layer and the MXene layer. While the CMP layer captures polysulfides, the MXene layer promotes the conversion of polysulfides.