## 1 Introduction  
With the sustained growth of energy density requirements in advanced portable electronic devices and electric vehicles, the development of a new generation of secondary batteries with high energy density is extremely demanded [1; 2; 3]. As a strong competitor, lithium-sulfur (Li-S) batteries have attracted tremendous attention due to their ultrahigh energy density of \(\sim\)2600 Wh kg\({}^{-1}\), and elemental sulfur also possesses multiple advantages of low cost, rich availability, and environmental friendliness [4]. Nevertheless, the practical applications of Li-S cells currently face several intrinsic challenges, such as the notorious shuttle effect and sluggish reaction kinetics of intermediate polysulfides. The shuttle effect is caused by the diffusion of polysulfides dissolved in electrolytes through the separator, which usually leads to irreversible capacity attenuation and poor Coulombic efficiency [5; 6]. Even worse, the sluggish redox kinetics originated from the inherent multi-step solid-liquid-solid heterogeneous reaction mechanism inevitably begets the accumulation of various sulfur species instead of immediate conversion and then causes more diffusion behavior of polysulfides, ultimately resulting in low sulfur use and a short cycling lifespan [7]. To address the aforementioned issues, extensive efforts have been devoted to mitigate the intrinsic drawbacks. One of the most effective strategies is to design functional conductive carbonaceous materials for capturing sulfur/ polysulfides, such as tuning the pore size and/or porosity (micropore, mesopore, etc.) [7; 8; 9], doping various heteroatoms (N, O, S, P, etc.) [10; 11; 12; 13], and tailoring the morphology (CNTs, graphene, etc.) [14; 15], which indeed achieves a positive effect on boosting the performance of Li-S cells, owing to the physical confinement or chemical adsorption of carbonaceous substances toward polysulfides. Nonetheless, the shuttle behavior of intrinsic soluble polysulfides in electrolytes can be only partially restricted because of the limited absorbability originated from the functional carbons for dissolved polysulfides, leading to unsatisfied cyclic stability over long-term cycling [16; 17]. In addition, it can cause the accumulation of sulfur species due to their sluggish redox conversion kinetics on the carbonaceous matrixes, which thus easily results in the diffusion of intercepted polysulfides along cycling [16; 18; 19]. Impressively, it has been indicated that some metal-based additives introduced into carbon substrates play an important role in the chemical adsorption and/or catalytic conversion of intermediate polysulfides, such as metal oxides (Ti\({}_{4}\)O\({}_{7}\), Nb\({}_{2}\)O\({}_{5}\), etc.) [20; 21], metal sulfides (CoS\({}_{2}\), MoS\({}_{2}\), etc.) [22; 23], metal nitrides (MoN, VN, etc. [18; 24]), and metal nanoparticles (Fe, Co, Ni, etc.) [16; 25; 26], synergistically expediting the reaction kinetics of polysulfides in the redox process. Unfortunately, the chemical interaction between sulfur species and some inorganics usually can be influenced significantly by the dimension and other physical nature of the inorganic additives, thus leading to the formation of a thick passivation layer on the surface of inorganics, especially under high sulfur areal loading and long-term cycling [27; 28], which finally has an adverse impact on the performance of Li-S cells under long-term cycling conditions. Therefore, it is of great importance to elaborate suitable metal-based additives with chemisorption polysulfides and timely accelerating their conversion to Li\({}_{2}\)S during the redox reaction. Specifically, recent studies have demonstrated that tailoring metal compounds to an ultra-small size, such as nanocrystals, nanodots, and clusters, and even to the single-atomic scale could generate strong adsorption and excellent catalytic activity for polysulfides, owing to enhanced surface energy and more active terminations [16; 29; 30; 31].