Synergistic Catalysis on Dual‐Atom Sites for High‐Performance Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries promise ultrahigh theoretical energy density and attract great attention as next‐generation energy storage devices. However, the sluggish sulfur redox kinetics severely restricts the practical performances of Li–S batteries. Introducing electrocatalysts can accelerate the sulfur redox kinetics and enhance the discharge capacity and rate performances, where advanced electrocatalysts are required for better performance promotion. Herein, a Fe–Co‐based dual‐atom catalyst (DAC) is adopted to accelerate the sulfur redox kinetics and construct high‐performance Li–S batteries. The unique structure of the dual‐atom site allows synergistic effect between the adjacent metal atoms, thus enhancing the interactions with lithium polysulfides and promoting the sulfur redox kinetics over the single‐atom counterparts. As a result, Li–S batteries with DAC afford a high discharge capacity of 1034.6 mAh g−1 at 0.1 C and excellent rate performances of 728.0 mAh g−1 at 4.0 C. The introduction of DAC demonstrates the promising potential of applying advanced materials for constructing high‐performance Li–S batteries.


Introduction
The booming development of electric vehicles and mobile electronic devices has placed increasing demands on high-energy-density secondary batteries. [1] Li-S batteries afford a theoretical energy density of 2600 Wh kg À1 and are promising candidates as next-generation high-energy-density energy storage devices. [2] Typical Li-S batteries employ a sulfur cathode, a Li metal anode, and ether-based electrolyte. Multiphase and multielectron sulfur redox reactions take place at the sulfur cathode during charge and discharge. [3] Specifically, during the discharge process, solid sulfur is first reduced to lithium polysulfides (LiPSs) through solid-liquid conversion, then LiPSs undergo liquid-liquid reduction reactions in the electrolyte, and eventually liquid-solid conversion occurs to form Li 2 S 2 /Li 2 S precipitates from LiPS reduction. [4] The cathode sulfur redox reactions provide a high specific capacity of 1672 mAh g À1 that greatly exceeds the intercalation cathodes of Li-ion batteries and promises a high energy density of Li-S batteries. However, the sluggish kinetics of sulfur redox reactions leads to low utilization of active materials and severe polarization, which consequently result in low discharge capacity and poor rate performance of Li-S batteries and severely limit the practical implementation of Li-S batteries. [5] Therefore, facilitating the sulfur redox kinetics is essential for the construction of highperformance Li-S batteries. Introducing electrocatalysts is regarded as a promising solution to promote the sluggish sulfur redox kinetics. [6] Previous works have established that electrocatalysts such as metal oxides, [7] nitrides, [8] sulfides, [9] and carbides [10] can accelerate the sulfur redox kinetics and improve the performance of Li-S batteries. Among the various electrocatalysts, single-atom catalysts (SACs) are able to make efficient use of almost every transition metal atom as active site and the unsaturated coordinated transition metal atoms are more likely to interact with the active species over traditional catalyst particles. [11] As a result, Cobased, [12] Fe-based, [13] Ni-based, [14] and Mn-based [15] SACs have been massively reported as electrocatalysts for Li-S batteries to accelerate the sulfur redox kinetics. For example, Du et al. reported that atomic Co embedded in nitrogen-doped graphene could trigger the surface-mediated reaction and reduce the activation energy for the conversion from Li 2 S 4 to Li 2 S 2 /Li 2 S. [16] Ye et al. reported Fe-N and Co-N co-doped SACs that could catalyze the conversion reactions for both long-chain and short-chain LiPSs. [17] Accordingly, the Li-S batteries with the DOI: 10.1002/sstr.202200205 Lithium-sulfur (Li-S) batteries promise ultrahigh theoretical energy density and attract great attention as next-generation energy storage devices. However, the sluggish sulfur redox kinetics severely restricts the practical performances of Li-S batteries. Introducing electrocatalysts can accelerate the sulfur redox kinetics and enhance the discharge capacity and rate performances, where advanced electrocatalysts are required for better performance promotion. Herein, a Fe-Cobased dual-atom catalyst (DAC) is adopted to accelerate the sulfur redox kinetics and construct high-performance Li-S batteries. The unique structure of the dualatom site allows synergistic effect between the adjacent metal atoms, thus enhancing the interactions with lithium polysulfides and promoting the sulfur redox kinetics over the single-atom counterparts. As a result, Li-S batteries with DAC afford a high discharge capacity of 1034.6 mAh g À1 at 0.1 C and excellent rate performances of 728.0 mAh g À1 at 4.0 C. The introduction of DAC demonstrates the promising potential of applying advanced materials for constructing high-performance Li-S batteries.
above SACs exhibited improved specific capacity and rate performances.
Despite the success of SACs in promoting the sulfur redox kinetics, developing electrocatalyst with higher intrinsic activity is still in great need to construct high-performance Li-S batteries especially under practical working conditions such as employing high-sulfur-loading cathodes, lean electrolyte, and high cycling rates. To this end, dual-atom catalysts (DACs), a novel class of catalysts with contacted metal atom pairs dispersed on carbon substrate as the electrocatalytic active sites, have attracted widespread attention as advanced electrocatalysts beyond SACs. [18] DACs inherit the advantages of high atomic efficiency as SACs, while the unique structure of the dual-atom sites allows synergistic effect between the two transition metal atoms. [19] These merits contribute to the excellent performance of DACs in many important electrocatalytic processes. For example, Fe-Mn-based DAC could lower the reaction energy barrier of oxygen reduction reaction [20] and Zn-Co-based DAC could facilitate CO 2 reduction reaction due to the electronic interaction between Zn and Co atoms. [21] Accordingly, these advantages also afford promising potential for DACs to achieve superior intrinsic activity on accelerating sulfur redox kinetics. [22] Employing advanced DACs in Li-S batteries is expected to realize high discharge capacity and high working rates under practical working conditions.
Herein, a Fe-Co-based DAC is innovatively employed as a high-active electrocatalyst in Li-S batteries to promote the sulfur redox kinetics. Considering the excellent performances of Fe-based and Co-based SACs in Li-S batteries, Fe and Co atoms are selected as active sites of DAC. The DAC (named Fe, Co-NC) is obtained by uniformly dispersing Co atoms onto a Fe-based SAC substrate. The synergistic effect between the Fe and Co atoms significantly enhances the interaction between LiPSs and the active sites. Accordingly, the liquid-liquid and liquid-solid reduction kinetics of LiPSs are greatly promoted. Li-S batteries with the Fe, Co-NC electrocatalyst exhibit a high initial discharge capacity of 1268.0 mAh g À1 at 0.1 C (1 C ¼ 1672 mA g À1 ) and maintain 728.0 mAh g À1 at a high current of 4.0 C. Even under the demanding conditions of using ultrahigh-sulfur-loading cathodes of 8.0 mg S cm À2 and an ultralow electrolyte/sulfur (E/S) ratio of 5.2 μL mg À1 , Li-S batteries with the Fe, Co-NC electrocatalyst present a high initial discharge capacity of 950.4 mAh g À1 at 0.1 C. The utilization of Fe-Co DAC provides a viable strategy for the construction of high-performance Li-S batteries and inspires rational design of electrocatalytic materials for analogous multiphase and multielectron energy-related electrochemical reactions.

Results and Discussion
The Fe, Co-NC DAC was prepared by sequentially loading Fe atoms and Co atoms at the atomic scale on a nitrogen-doped carbon precursor (NC). Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of Fe, Co-NC demonstrate that the catalyst exhibits a rhombic dodecahedral morphology with a diameter ranging from 300 to 500 nm ( Figure S1 and S2, Supporting Information). The element contents of Fe and Co were quantified using X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectrometer (ICP-OES) measurements ( Figure 1a). The results display that the Fe and Co contents in Fe, Co-NC are 3.13 and 0.093 wt% by XPS and 2.12 and 0.062 wt% by ICP-OES, respectively, demonstrating that the Fe and Co atoms were successfully introduced. X-Ray diffraction (XRD) measurements of Fe, Co-NC ( Figure 1b) were further conducted to detect the existence of undesired metal nanoparticles. Apart from the peaks of (002) and (100) planes from the carbon substrate, [23] no other diffraction peaks  were observed (PDF#50-1275: Fe and PDF#05-0727: Co as references), indicating that there are no undesired aggregated metal or metal compound nanoparticles. The XRD results suggest that the Fe and Co atoms are dispersed at the atomic scale and potentially form the structure of dual-atom sites. [24] To further visualize the distribution of the isolated transition metal atoms, high-angle annular dark-field scanning TEM (HAADF-STEM) technique was used to observe the Fe, Co-NC DAC. Stemming from the difference in Z-contrast, the transition metal atoms loaded on NC show up as bright spots in the HAADF-STEM image (Figure 1c). Uniformly distributed Fe and Co atoms are observed over the electrocatalyst particle. It is noteworthy that many atom pairs are formed as highlighted by the red circles, indicating the successful construction of dual-atom sites. Quantitative analysis by line scan on the contrast of the HAADF-STEM image across the atom pair (yellow arrow, inset in Figure 1c) reveals that the distance between the two peaks is 0.21 nm, illustrating that the adjacent metal atoms form dualatom sites instead of discrete single-atom sites. Energydispersive spectrometer (EDS) elemental mapping was further carried out under the HAADF-STEM mode (Figure 1d), showing a high degree of dispersion of the Fe and Co atoms with no formation of atomic clusters, in correspondence with the results of XRD and validating the successful synthesis of DAC. In addition, XPS measurement was carried out to analyze the surface structures of the Fe, Co-NC DAC. The XPS survey spectra of Fe, Co-NC show the existence of Fe, Co, C, N, and O elements ( Figure S3, Supporting Information). In the deconvoluted N 1s XPS spectrum (Figure 1e), the existence of significant M-N (M refers to metal) interaction directly demonstrates that the Fe and Co atoms are atomically dispersed and coordinated by N atoms rather than aggregated. The above characterization results demonstrate the successful introduction of atomic-level dispersed active sites and verify the construction of Fe-Co-based DAC. Notably, the Fe content is higher than the Co content in Fe, Co-NC. The extra Fe atoms in addition to those forming dualatom sites with Co are suggested to remain in single-atom sites considering the synthesis procedure and absence of Fe-Fe interactions in Fe, Co-NC.
Meanwhile, counterpart SACs were synthesized by loading Fe atoms and Co atoms, respectively, on NC as a comparison (named Fe-NC and Co-NC, respectively). Fe-NC and Co-NC show similar dodecahedral morphology as Fe, Co-NC ( Figure S4 and S5, Supporting Information). XRD patterns of Fe-NC and Co-NC convince the absence of agglomerated particles ( Figure S6, Supporting Information). The XPS survey spectra exhibit reasonable contents of Fe and Co ( Figure S7, Supporting Information, 2.99 wt% of Fe in Fe-NC and 2.17 wt% of Co in Co-NC), and similar M-N interaction can be identified to indicate that the Fe and Co atoms are atomically dispersed and coordinated with N ( Figure S8, Supporting Information). HAADF-STEM characterization and corresponding EDS element mapping of Fe-NC and Co-NC exhibit that the transition metal atoms are atomically distributed on the substrate as highlighted by the red circles ( Figure S9-S12, Supporting Information). The adjacent Fe or Co atoms in Fe-NC or Co-NC are farther apart with a distance ranging from 0.8 to 2.8 nm, indicating that the active sites in Fe-NC or Co-NC are isolated metal atoms instead of transition metal pairs formed in the Fe, Co-NC DAC.
The dual-atom structure illustrated above makes the Fe, Co-NC DAC as a potential kinetic accelerator for Li-S batteries. Kinetic characterization was performed to evaluate the intrinsic electrocatalytic activity of Fe, Co-NC for the sulfur redox kinetics. Cyclic voltammetry (CV) characterization of Li-S cells was first carried out. The Li-S cell with the Fe, Co-NC electrocatalyst shows early voltage shifts ranging from 10 to 30 mV in the reduction and oxidation peaks according to the CV profiles (marked as peaks a, b, c, and d) and higher corresponding peak currents, demonstrating enhanced sulfur redox kinetics and outstanding electrocatalytic activity of Fe, Co-NC ( Figure S13, Supporting Information). Li 2 S 6 symmetric cells were then assembled to analyze the activity of Fe, Co-NC in electrocatalyzing the liquid-liquid conversion reactions of LiPSs. The redox kinetics of LiPSs was characterized using CV test at a large scanning rate of 800 mV s À1 (Figure 2a). The Li 2 S 6 symmetric cell with Fe, Co-NC displays a maximum current of 37.8 mA, much higher than that of 25.0 mA for Fe-NC and 16.2 mA for Co-NC, indicating the superior electrocatalytic activity of Fe, Co-NC for LiPSs liquid-liquid conversion.
Electrochemical impedance spectroscopy (EIS) spectra of the Li 2 S 6 symmetric cells with different catalysts were further measured ( Figure 2b). [25] In order to quantitatively analyze the promotion of the LiPSs redox kinetics, an equivalent circuit was employed to fit the experimental EIS data and decouple the charge transfer resistance (R ct ) as the key indicator ( Figure S14, Supporting Information). [26] The symmetric cell with Fe, Co-NC exhibits the smallest overall impedance and has the smallest R ct of 5.4 Ω. The R ct of Fe, Co-NC is only 50% of Fe-NC and 10% of Co-NC, indicating that LiPSs are most likely to gain or lose electrons on the Fe, Co-NC electrocatalyst and undergo fast redox reactions than the SAC counterparts (Figure 2c). In order to further investigate the origin of the kinetic advantage, the activation energy of the liquid-liquid conversion reaction of LiPSs was obtained from EIS data measured at different temperatures. R ct was first determined and then fitted with an Arrhenius relationship to temperature to obtain the activation energy (Figure 2d and S15, Supporting Information). The activation energy of the LiPSs liquid-liquid conversion reaction on Fe-NC and Co-NC is both around 40 kJ mol À1 , while that of Fe, Co-NC is only 31.8 kJ mol À1 . Therefore, the Fe, Co-NC DAC can efficiently reduce the reaction energy barrier of liquid-liquid conversion, and the accelerated conversion kinetics of LiPSs on Fe, Co-NC is attributed to the intrinsic activity advantage of the dual-atom sites for sulfur redox electrocatalysis.
Meanwhile, the catalytic activity of DAC on the reduction of dissolved LiPSs to solid Li 2 S 2 /Li 2 S was evaluated using a potentiostatic nucleation experiment (Figure 2d). The Li 2 S 6 cell with Fe, Co-NC displayed the earliest and the highest discharge current peak (peak time of 2.8 h and maximum peak current of 0.057 mA) under the potentiostatic discharge of 2.12 V, while those for Fe-NC and Co-NC were inferior (peak time for Fe-NC and Co-NC was 3.5 and 2.8 h and maximum peak current for Fe-NC and Co-NC was 0.031 and 0.045 mA, respectively). As a result, Fe, Co-NC is able to substantially promote the liquid-solid conversion kinetics of LiPSs. The deposition capacity was further analyzed. Fe, Co-NC afforded the highest Li 2 S 2 /Li 2 S electrodeposition capacity of 338.1 mAh g S À1 , while that for Fe-NC and Co-NC was 272.9 and 317.5 mAh g S À1 , respectively, further demonstrating the efficient kinetic enhancement afforded by the Fe-Co DAC. Based on the promotion on the sulfur redox kinetics, the Fe, Co-NC DAC is expected to enhance the specific capacity and rate performances in working Li-S batteries. Concretely, Fe, Co-NC was adopted as a functional interlayer in Li-S batteries and Fe-NC and Co-NC underwent the same treatment to obtain interlayers with similar thickness for comparison ( Figure S16, Supporting Information). The rate performance was first investigated using low-sulfur-loading cathodes of 1.2 mg S cm À2 (Figure 3a). The Li-S cell with Fe, Co-NC displays a high initial discharge capacity of 1268.0 mAh g À1 at 0.1 C. At higher cycling rates of 0.5, 1.0, 2.0, 3.0, and 4.0 C, the cell with Fe, Co-NC still maintains high specific discharge capacities of 1025.6, 924.6, 849.2, 796.0, and 728.0 mAh g À1 , respectively. In contrast, the initial discharge capacity of the Li-S cell with Fe-NC or  Co-NC is only about 1000 mAh g À1 at 0.1 C and the discharge capacity decreases rapidly with increasing rates. The charge-discharge profiles also demonstrate the superior activity of Fe, Co-NC over its SAC counterparts ( Figure S17, Supporting Information). The Li-S cell with Fe, Co-NC exhibits the smallest polarization as evidenced by the smallest difference of the median charge and discharge voltage (ΔE Fe, Co-NC ¼ 0.144 V) in comparison with Fe-NC (ΔE Fe-NC ¼ 0.174 V) and Co-NC (ΔE Co-NC ¼ 0.178 V) at 0.1 C. Moreover, the polarization advantage of Fe, Co-NC over Fe-NC and Co-NC is maintained with increased cycling rate. In addition, the cycling performance at 0.5 C was evaluated. The Li-S cell with Fe, Co-NC demonstrates a higher specific capacity of about 100 mAh g À1 than the cell with Fe-NC or Co-NC with little degradation throughout the 150 cycles ( Figure S18, Supporting Information). Therefore, the advantages of the Fe, Co-NC DAC exist during the prolonged cycles and ensure long-term and efficient electrocatalysis for working Li-S batteries.
To demonstrate the electrocatalytic ability of the Fe, Co-NC DAC under more demanding conditions, a sulfur cathode with high sulfur loading of 4.2 mg S cm À2 was used for evaluation. The Li-S cell with Fe, Co-NC maintains its superiority under high-sulfur-loading working conditions (Figure 3b). The initial discharge capacity is 1184.4 mAh g À1 at 0.05 C and sustains 749.6 mAh g À1 at 0.5 C, much higher than both Fe-NC and Co-NC (1132.5 and 1098.2 mAh g À1 at 0.05 C, while 675.6 and 643.1 mAh g À1 at 0.5 C for Fe-NC and Co-NC, respectively), reflecting the high electrocatalytic activity of the Fe, Co-NC DAC in accelerating the sulfur redox kinetics. The cycling performance of Fe, Co-NC with high-sulfur-loading cathodes was further investigated. The initial discharge capacity with the Fe, Co-NC DAC is up to 1034.6 mAh g À1 at 0.1 C (Figure 3c), and the discharge capacity remains 981.7 mAh g À1 after 60 cycles (capacity decay rate of 0.087% per cycle). In comparison, the initial discharge capacity of the Li-S cell with Fe-NC or Co-NC is much lower. The high discharge capacity and low capacity decay rendered by the Fe, Co-NC DAC demonstrate its excellent electrocatalytic performances in practical Li-S batteries.
Considering the high-energy-density requirement for practical applications, Li-S cells with an ultrahigh areal sulfur loading of 8.0 mg S cm À2 and an ultralow E/S ratio of 5.2 μL mg À1 were assembled and evaluated. The initial discharge capacity of the Li-S cell with Fe, Co-NC is 950.4 mAh g À1 at 0.1 C (Figure 3d), while that of the cell with Fe-NC or Co-NC is only around 800 mAh g À1 . Under such a harsh condition, Fe, Co-NC maintains its significant electrocatalytic activity, evidently indicating efficient acceleration of the sulfur redox kinetics on the Fe, Co-NC DAC and promising potential of using advanced DACs in practical Li-S batteries.
In order to understand the chemistry of the electrocatalytic activity origin of the Fe, Co-NC DAC, the interaction between LiPSs and different types of active sites is further explored. XPS measurements were carried out to characterize the chemical Figure 4. Mechanism investigation of the dual-atom sites. a) Deconvoluted S 2p XPS spectrum of Fe-NC, Co-NC, and Fe, Co-NC after the addition of Li 2 S 6 . b) 7 Li NMR spectra of Li 2 S 6 solution without and with FeTMPP, CoTMPP, and FeTMPP þ CoTMPP. c) Schematic illustration of the synergistic electrocatalysis mechanism on the dual-atom sites of the Fe, Co-NC DAC.
www.advancedsciencenews.com www.small-structures.com structure of Li 2 S 6 after being adsorbed on Fe-NC, Co-NC, or Fe, Co-NC ( Figure 4a). S 2p spectra on Fe-NC or Co-NC reveal typical chemical structure of Li 2 S 6 including bridged and terminal sulfur in polysulfide chain. However, for Fe, Co-NC, a new sulfur-metal interaction can be identified at 164.3 and 165.5 eV, indicating stronger interaction between LiPSs and the dual-atom sites. The sulfur-metal interaction between the dual-atom sites and LiPSs can be attributed to the electron modulation effect between the Fe and Co atoms that regulates the adsorption configuration of LiPSs on Fe, Co-NC to allow stronger metal-sulfur interaction. As a result, subsequent dissociation of the sulfur chain is facilitated to render a superior electrocatalytic activity. [27] The chemical interaction between the dual-atom sites and LiPSs was investigated by 7 Li nuclear magnetic resonance (NMR) measurements. Iron(III) tetramethoxyphenylporphyrin (FeTMPP) and cobalt(II) tetramethoxyphenylporphyrin (CoTMPP) were both added to the Li 2 S 6 solution for simulating the chemical environment of LiPSs on Fe, Co-NC. Similarly, only FeTMPP or CoTMPP was individually added to simulate the adsorption of LiPSs on Fe-NC or Co-NC, respectively. As shown in Figure 4b, the 7 Li chemical shift of Li 2 S 6 with FeTMPP moved to the low field, while that with CoTMPP moved to the high field in comparison with pristine Li 2 S 6 , indicating the different electronic effects of Fe and Co. The 7 Li chemical shift of Li 2 S 6 with both FeTMPP and CoTMPP moved to the lower field, illustrating that the Fe-Co dual-atom site exhibits the most powerful electron withdraw capability toward Li atoms. This can be attributed to the electronic inducible effect of the strong metal-sulfur interaction, thus confirming the synergistic effect between the transition metal atoms that leads to the unique adsorption configuration of LiPSs. As a result, the intrinsic high electrocatalytic activity of Fe, Co-NC on the sulfur redox kinetics can be achieved. [28] According to the above discussion, the electrocatalytic mechanism of the dual-atom site is inferred in Figure 4c. Due to the unique configuration of the dual-atom sites with metal-metal modulation effect, the Fe and Co atoms cooperate to afford strong sulfur-metal interactions between the dual-atom sites and LiPSs, in which the electron-withdrew effect by the dualatom sites contributes to the strong intermolecular interactions and the electrocatalytic activity of the Fe, Co-NC DAC. As a result, the liquid-liquid conversion and liquid-solid conversion kinetics are both accelerated to render improved performances in working Li-S batteries. Accordingly, Li-S batteries with the Fe, Co-NC DAC exhibit superior discharge capacity and rate performances than the batteries with SAC counterparts.

Conclusion
In conclusion, a novel Fe-Co-based DAC is developed to achieve efficiently synergistic electrocatalysis on accelerating the sulfur redox kinetics in working Li-S batteries. Owing to the synergistic effect of the dual-atom sites, strong sulfur-metal interactions are established between the LiPSs and the active sites to enable the DAC with high intrinsic electrocatalytic activity on both liquidliquid and liquid-solid conversion reactions. Consequently, Li-S batteries with the DAC exhibit a high initial discharge capacity of 1034.6 mAh g À1 at 0.1 C. Even with an ultrahigh areal sulfur loading of 8.0 mg S cm À2 and an ultralow E/S ratio of 5.2 μL mg À1 , the Li-S batteries with the DAC display an initial specific capacity of 950.4 mAh g À1 at 0.1 C. This work proposes a novel strategy of using DACs to synergistically accelerate the sulfur redox kinetics and broadens the design of advanced electrocatalysts for constructing high-performance Li-S batteries and other analogous energy storage devices based on multiphase and multielectron reactions.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.