Abstract:

Li-S batteries are considered promising next-generation energy storage systems because they offer high theoretical specific capacity (1675 mAh∙g⁻¹), high energy density (2600 Wh∙kg⁻¹), environmental friendliness, and low cost. However, large-scale commercial applications are hindered by the low electrical conductivity of S, high volume expansion ratio, and high solubility of intermediate polysulfides in organic electrolytes. Li-Se batteries using Se as the cathode material have high discharge rates, good cyclic performance, high electrical conductivities, high output voltages, and high volumetric capacity densities, and therefore, they are potential alternatives to Li-S systems. Recently, covalent organic frameworks (COFs) have emerged as new porous crystalline materials with large specific surface areas, high porosities, low densities, good thermal stabilities, and controllable structures. Therefore, COFs have wide potential applicability in the fields of gas adsorption, heterogeneous catalysis, energy storage, and drug delivery. Based on the above analysis, a simple core-shell multiwalled carbon nanotube (MWCNT)/1, 3, 5-triformylphloroglucinol (Tp)-phenylenediamine (Pa) COF nanocomposite (MWCNT@TpPa-COF) was prepared by growing a TpPa-COF on MWCNTs through a simple solvothermal reaction. The MWCNT@TpPa-COF high-performance cathode material realizes the first application of a COF in Li-Se batteries. The MWCNTs can encapsulate Se, limit the diffusion of polyselenides (Li₂Se_n, 3 ≤ n ≤ 8), and provide rapid electron conduction and ion transmission. In addition, the π-π interaction between MWCNTs and COFs promotes COF growth and distribution on the MWCNTs, thereby forming core-shell MWCNT@TpPa-COF nanocomposites, which can further increase the loading of Se. Measurements via field-emission scanning electron microscopy, transmission electron microscopy, and Fourier-transform infrared spectroscopy confirmed the successful combination of MWCNTs and COFs. The rich micro- and mesoporous hierarchical structure provides the MWCNT@TpPa-COF nanocomposites with initial specific discharge capacities reaching 463.5 mAh∙g⁻¹ at the current density of 3C (1C = 675 mA∙g⁻¹). Cells utilizing the nanocomposite electrodes maintained 99% Coulombic efficiency, with the average cyclic capacitive loss of 0.14% after 500 cycles. In addition, electrochemical impedance spectroscopy, cyclic voltammetry, and multiple-rate cycling analyses support the excellent electrochemical performance of the proposed cathode material. This work provides a promising new prospect for the future development of rechargeable Li-Se batteries utilizing new COF-based cathode materials.

Key words: Lithium-selenium battery, Multi-walled carbon nanotube, Covalent organic frameworks, Nanocomposites, Core-shell structure