Fabrication Of PAMAM Modified CoFe₂O₄@N-CNFs Composite Fibers Membrane And Its Application In Lithium-Sulfur Batteries

In recent years,lithium-sulfur batteries（LSB）have become one of the hot spots in the research field of high specific energy storage devices because of the abundant and cheap elemental sulfur in nature,which can provide a theoretical specific capacity of1675 m Ah g^-1 and a high theoretical specific energy of 2600 Wh kg^-1.At present,the key problem that LSB can not be industrialized is the shuttle effect of electrochemical reaction products-intermediate polysulfdies,which leads to a series of problems such as low utilization rate of active material and corrosion of lithium metal anode,thus affecting the electrochemical performance.Based on the above problems,this paper mainly focuses on the preparation process of electrode with high sulfur loading.Nitrogen-doped carbon nanofibers（N-CNFs）membrane as the matrix material which were prepared by electrospinning and heat treatment,and Li₂S₆ solution as the catholyte,then N-CNFs/Li₂S₆ self-supporting electrode was obtained.The above strategy avoids the cumbersome of the traditional coating process,improves the loading of the active material.Meanwhile,the three-dimensional networks structure of N-CNFs improves the polarity and electrical conductivity of the matrix material.Based on N-CNFs membrane,the dendritic polymer polyamide-amine（PAMAM）was used for in situ its surface functionalization by solution impregnation process.The electrochemical performance of PAMAM@N-CNFs/Li₂S₆ self-supporting electrode in LSB was studied.Research results show that when the sulfur loading is 4.74 mg,composite electrode under 0.2 C presents a first discharge specific capacity of 998 m Ah g^-1and remains 783 m Ah g^-1 after 300 cycles.When the sulfur loading is 7.11 mg,the first discharge capacity under 0.2 C can reach 5.77 m Ah with the capacity retention rate of 71.2%after 400 cycles.The excellent cycling performance is attributed to the abundant groups on the surface of PAMAM can effectively adsorb soluble polysulfides,thus inhibiting the shuttling effect.To regulate the kinetics of electrode electrochemical reaction and to construct catalyst for electrode is a hot research topic.N-CNFs membrane prepared based on the above optimized process were modified by in-situ loading of spinel oxide CoFe₂O₄（CFO）nanoparticles in hydrothermal process,and the electrochemical performance of CFONC/Li₂S₆ composite electrode was investigated.The results show that when the sulfur loading is 4.74 mg,the initial specific discharge capacity of the composite electrode at 0.2 C reaches 1096 m Ah g^-1,and the capacity decay rate of each cycle is only 0.076%in 500 cycles.In the higher sulfur loading（7.11 mg）,the composite electrode can provide a first high capacity of 6.1 m Ah at 0.2 C,and can maintain 4.8m Ah after 300 cycles.The excellent electrochemical performance of this electrode is attributed to the chemisorption and electrocatalytic activity of CFO to polysulfide,enhancing the deposition capacity of lithium sulfide and improving the electrochemical reaction kinetics of the electrode,and effectively inhibiting the transit of polysulfide.Based on the above results,PAMAM modified CFONC（organic-inorganic）functionalized composite fiber membrane（PCFONC）was constructed to investigate the electrochemical performance of PCFONC/Li₂S₆ high sulfur loading electrode.The results show the first specific discharge capacity of composite electrode at 0.2 C is 940m Ah g^-1 when the sulfur loading is 7.11 mg,and the capacity remains 690 m Ah g^-1 after450 cycles.When the sulfur loading is 14.22 mg and 22.65 mg:the first discharge capacity of 0.1 C is 11 m Ah and 12 m Ah,the capacity retention after 150 cycles is 9.7m Ah and 10.8 m Ah with capacity retention rates of 88.2%and 90%,respectively.This work lays a foundation for the study of the electrochemical performance of organic-inorganic functionalized composite fibers membrane at high sulfur loading electrode.