MOFs Reinforce CNTs Sponge Derived Functional Carbon Network As Sulfur Host for Lithium Sulfur Battery
Authors: Nguyen Hung Quoc , Luu Tung Van , Pham Kiet Tuan , Karima Neema Cyril , Nguyen Long Hoang , Ahn Wook 
Published: December 2020

Lithium sulfur battery is one of the most prospective alternatives for lithium ion battery. It possesses high theoretical capacity of 1672 mAh/g and high theoretical gravimetric energy density of a Li-S cell is approximately 2,510 Wh/kg, which is about eight folds compare to transition metal oxide [1]. The mechanism of lithium sulfur battery is based on the conversion reaction of sulfur to lithium sulfide (Li2S) which create high capacity. The transfer of two electrons per sulfur atom is more preferred than one or less than one electron per transition-metal ion [2]. However, tremendous challenges such as natural insulator characteristic of sulfur and lithium sulfide, dissolution of mediators into electrolyte, significant volume change during cycling and lithium metal anode corrosion have hindered the commercialization in industry [3].
To overcome these drawbacks, the design of the sulfur cathode material plays an important role in boosting sulfur electrochemistry and accelerating the battery performance. In the past few years, metal–organic frameworks (MOFs) which consist of metal ions and organic moiety as linker have been used as novel sulfur host materials in lithium sulfur batteries [4]. Particularly, MOFs are prospective as both a precursor and derived materials. The porous carbon materials created from MOFs shows a uniform porosity due to its highly ordered crystalline structures [5]. Besides the confining sulfur within the porous carbon materials, the functional materials have been recently employed to supply both a strong interaction surface with lithium polysulfides and to provide electron transport for enhancing the redox reaction [6]. Moreover, the polysulfide shuttle effect could be manipulated by electrocatalysis [7]. Recent researches found that the atomic cobalt dopants can serve as active sites to improve the kinetics of the sulfur redox reactions. It is therefore pivotal to design an effective electrode that not only improve the electrochemical processes of sulfur but also could efficiently suppress the intermediate polysulfide shuttle effect [8].
In this study, we designed a novel sulfur film coated on the three-dimensional nitrogen and cobalt co-doped carbon polyhedral wrapped on multi-walled carbon nanotubes sponge composite as a high-performance cathode material for rechargeable lithium-sulfur batteries. CNTs is employed to supply an interconnected conductive carbon network. Along with the abundant uniform micropores that provided by carbon derived from ZIF 67, sponge-like structure provides abundant mesopores that create favorable pathway for the electrolyte permeability and lithium ion diffusion. The doping of nitrogen and cobalt atom leads to improve electrical conductivity and enhance the surface polarity of the composite. Moreover, the strong interaction between cobalt and lithium polysulfides leads to efficiently suppress the shuttle effect. Figure 1 presents the photo of sponge-like composite and SEM image of composite after sulfur infiltration. We will report in detail about the latest results achieved on as cathode material for lithium-sulfur batteries.

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