The sluggish redox kinetics of polysulfides and difficult oxidation process of Li2S severely hinder practical application of Li–S batteries under high sulfur loading and a low electrolyte dosage. To address these issues, we develop a bifunctional catalyst by manipulation of anion N doping in CoSe2 (N-CoSe2). Theoretical simulation results uncover that an introduced N element into CoSe2 could form a shorter Co–N bond, create a higher charge number of the Co central atom, bring new defect levels, and induce the Co 3d band closer to Fermi level. Further atomic level analysis revealed that N-CoSe2 could form shorter Co–S bonds with sulfur species and simultaneously weaken the S–S bridged bond of Li2S4 and Li–S bond of Li2S, which eventually facilitate the polysulfide conversion reaction in the discharge process and the Li2S oxidation in the charge process. With N-CoSe2 as a bifunctional catalyst, the battery exhibited a high areal capacity of 9.26 mAh g–1 under the low E/S (electrolyte/sulfur) ratio of 4.4 μL mg–1. Understanding the design concept of a bifunctional catalyst with anion doping would provide a new vision for realizing a high-performance Li–S battery.

drastically superior specific energy density. [1][2][3][4][5][6][7] However, the successful implementation of Li-S batteries is still hindered by many challenges. One of the largest problems facing the current Li-S battery is the rapid capacity decay and serious selfdischarge caused by the dissolution and migration of intermediate lithium polysulfides (LiPSs). [8][9][10][11][12][13][14][15] Significant efforts have been dedicated to the research of suitable approach to address polysulfide shuttling. As one of the strategies, inserting an interlayer between the separator and sulfur electrode is widely applied due to the minimal changes to existing applications. [16][17][18][19] Various materials such as metal oxides, sulfides, and metal-organic frameworks have been proposed as functional interlayer to trap LiPSs through chemisorption, and this strategy has been demonstrated to be effective to block the migration of polysulfide species. [20][21][22][23][24] The interlayer serves as both adsorbent and current collector, where polysulfides can be reduced to insoluble products. [25,26] While this helps block LiPSs and improve the sulfur utilization, the produced Li 2 S is hard to oxidize back to soluble LiPSs due to the intrinsically poor electrical and ionic conductivity. [27] The accumulation of Li 2 S not only leads to the loss of active materials but also passivates the interlayer and impairs the electrochemical performance because of the insufficient transport of Li ions especially for cathodes with high sulfur loading. [1,16,[27][28][29] There is thereby an urgent need but it is still a significant challenge to develop interlayers that can not only block LiPSs but also accelerate Li 2 S oxidation on interlayers.To achieve this, we propose several key factors to consider when developing a practical bifunctional interlayer: 1) the adsorption energies between interlayer and LiPSs/Li 2 S that provides a strong anchoring; 2) the Li ion diffusion barrier that ensures good Li ion mobility; 3) the electrical conductivity of interlayer materials that facilitates electron transport and electrochemical conversions. In this work, we introduce MoN as a suitable interlayer material for Li-S batteries. On the one hand, the surface Mo atoms with unoccupied orbitals act as Lewis acid sites, and thus, MoN can serve as a good adsorbent for LiPSs. [30,31] On the other hand, our theoretical calculation reveals an extremely low Li ion diffusion barrier on MoN Rational design of effective polysulfide barriers is highly important for highperformance lithium-sulfur (Li-S) batteries. A variety of adsorbents have been applied as interlayers to alleviate the shuttle effect. Nevertheless, the unsuccessful oxidation of Li 2 S on interlayers leads to loss of active materials and blocks Li ion transport. In this work, a MoN-based interlayer sandwiched between the C-S cathode and the separator is developed. Such an interlayer not only strongly binds lithium polysulfides via Mo-S bonding but also efficiently accelerates the decomposition of Li 2...

Precisely tuning the coordination environment of the metal center and further maximizing the activity of transition metal–nitrogen carbon (M-NC) catalysts for high-performance lithium–sulfur batteries are greatly desired. Herein, we construct an Fe-NC material with uniform and stable Fe-N2 coordination structure. The theoretical and experimental results indicate that the unsaturated Fe-N2 center can act as a multifunctional site for anchoring lithium polysulfides (LiPSs), accelerating the redox conversion of LiPSs and reducing the reaction energy barrier of Li2S decomposition. Consequently, the batteries based on a porous carbon nitride supported Fe-N2 site (Fe-N2/CN) host exhibit excellent cycling performance with a capacity decay of 0.011% per cycle at 2 C after 2000 cycles. This work deepens the understanding of the relationship between electronic structure of M-NC sites and the catalysis effect for the conversion of LiPSs. This strategy also provides a potent guidance for the further application of M-NC materials in advanced lithium–sulfur batteries.

The serious shuttle effect of soluble polysulfides inevitably leads to low sulfur utilization and faster capacity decay, thus preventing the development of Li–S batteries. Array electrodes have attracted much attention owing to their binder-free and freestanding features. However, the insufficient surface area, lack of active sites with polysulfides, and poor conductive nature of the array electrode could not satisfy the need for high-rate and long-life Li–S batteries, especially for the high sulfur loading of Li–S batteries. Thus, in this work, we constructed the hierarchical C@SnO2/1T-MoS2 (C@SnO2@TMS) array electrode as the sulfur host. The hierarchical C@SnO2@TMS demonstrated strong adsorption with polysulfides, which could effectively facilitate polysulfide redox kinetics. With the C@SnO2@TMS/S as the electrode, the batteries achieved superb C-rate properties, high specific capacity, and ultralong lifespan. Even undergoing 4000 cycles at 5 C, the battery could retain a high specific capacity of 448 mAh g–1 with the capacity decay as low as 0.009% per cycle.

The lithium–sulfur battery system contains a complex reaction process of sulfur involving multielectron reactions and phase conversions. Moreover, the diffusion of intermediate polysulfides during reduction and sluggish kinetic conversion of polysulfides into insoluble Li2S still plague the use of Li–S batteries. Herein, BiOX was employed as sulfur host material in Li–S batteries, which could integrate suppression of the shuttle effect and promote kinetics redox reactions of lithium polysulfides. The polar BiOX displays a robust chemical adsorption ability with polysulfides, and the electrocatalytic activity of BiOX would accelerate the fragmentation of polysulfides into shorter chains. The results indicate that the good polysulfide reactivity not only ensures the effective reduction of polarization but also performs high discharge capacity and stable cycle performance. The battery with a BiOCl/G-S cathode reveals a high capacity of 1414 mA h/g at a current of 0.1 C and a low capacity decay rate of 0.007% during 2000 cycles at a current of 2 C. This work proposes the prospect of application of the BiOX materials in lithium sulfur batteries.