Reduced graphene oxide wrapped MOFs-derived cobalt-doped porous carbon polyhedrons as sulfur immobilizers as cathodes for high performance lithium sulfur batteries

Keywords

• Metal organic frameworks;
• Cobalt nanocrystals;
• Reduced graphene oxide;
• Chemical immobilization;
• Lithium sulfur batteries

1. Introduction

Lithium sulfur (Li–S) batteries have recently drawn much attention because sulfur cathode can exhibit a theoretical specific capacity of 1672 mAh g⁻¹, much higher than that of the commercial cathode materials of lithium ion batteries (LIBs), making it a competitive candidate for the next generation secondary batteries along with the natural abundance, low cost and nontoxicity of elemental sulfur [1]; [2] ;  [3]. However, several issues needs to be solved before its wide industrial applications for Li–S batteries [4]; [5]; [6] ;  [7]. Firstly, the low electrical conductivity (5×10⁻²⁸ S m⁻¹) of sulfur results in limited utilization of active material and poor electrochemical activity. Secondly, the intermediates of lithium polysulfides generated during the electrochemical reaction are soluble in electrolyte, thus leading to a continuous loss of active materials and the subsequent shuttle effect. Furthermore, the dissolved polysulfides may react with lithium metal anode to form insoluble Li₂S/Li₂S₂, deposit on the Li anode surface, thus causing the deactivation of lithium anode.

Tremendous efforts have been made to solve these problems in recent years. One of the most popular methods is to build nanohybrids with various carbon materials [8]; [9]; [10]; [11]; [12]; [13] ;  [14]. Carbon materials with a better electrical conductivity not only facilitates charge transfer during electrochemical reactions, but also helps to trap sulfur species through physical adsorption. Various porous carbon materials with large surface area have been used as sulfur hosts to fabricate C-S nanocomposites and exhibit good electrochemical performance when used as cathode materials for Li–S batteries. Small sulfur molecules (S_2–4) stored in 0.5 nm micropores can avoid the unfavorable transition between S₈ and S₄²⁻, thus effectively alleviating the shuttle effect [15]; [16] ;  [17]. But the sulfur loading content is usually too low for practical use. Mesopores with larger pore volume can store more sulfur. However, owing to the weak interaction of the physical adsorption and the open pore structure, the active materials still show gradually loss during repeated cycles. So it’s urgent to take measures to further trap and immobilize sulfur and polysulfides.

Recently, chemisorption effect between sulfur species and some specific hosts have been proved to efficiently trap and immobilize sulfur species. Several metal oxides, such as Mg_0.6Ni_0.4O [18], TiO₂[19] ;  [20], MnO₂[21] ;  [22], MoO₂[23] and Ti₄O₇[24] ;  [25] have been used as sulfur hosts, and the possible chemical interactions between sulfur species and metal oxides hosts are responsible for the enhanced electrochemical performance when used as cathode materials for Li–S batteries. For example, Ti₄O₇ host with a high surface area fabricated by Nazar and coworkers shows strong ability to retain sulfur species. The demonstrated strong metal oxide-polysulfide interactions indicate that O–Ti–O units can chemically bind LiPSs [25]. Zheng et al. utilizes an electrically conductive MoO₂ as the matrix to encapsulate sulfur, and the strong O–S bonding interactions enable firmly confinement of sulfur [23]. Interestingly, transitional metals (such as Cu, Co, Ni, etc.) also show strong chemical interactions between sulfur species and metals. Wang et al. uses Cu-doped microporous carbon to confine S/polysulfides, and Cu nanoparticles anchored in the microporous carbon chemically interact with S/polysulfides through chemically bonding between Cu and S/polysulfides, thus leading to enhanced electrochemical performance when used as cathode for Li–S batteries [26]. Compared with physical absorption ability of traditional porous hosts to retain sulfur, the stronger chemical interactions between sulfur species and hosts may be more efficient to trap S/polysulfides.