Conductive and Catalytic Triple-Phase Interfaces Enabling Uniform Nucleation in High-Rate Lithium Sulfur Batteries

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1 FULL PAPER Lithium Sulfur Batteries Conductive and Catalytic Triple-Phase Interfaces Enabling Uniform Nucleation in High-Rate Lithium Sulfur Batteries Hong Yuan, Hong-Jie Peng, Bo-Quan Li, Jin Xie, Long Kong, Meng Zhao, Xiao Chen, Jia-Qi Huang,* and Qiang Zhang* Rechargeable lithium sulfur batteries have attracted tremendous scientific attention owing to their superior energy density. However, the sulfur electrochemistry involves multielectron redox reactions and complicated phase transformations, while the final morphology of solid-phase Li 2 S precipitates largely dominate the battery s performance. Herein, a triplephase interface among electrolyte/cose 2 /G is proposed to afford strong chemisorption, high electrical conductivity, and superb electrocatalysis of polysulfide redox reactions in a working lithium sulfur battery. The triplephase interface effectively enhances the kinetic behaviors of soluble lithium polysulfides and regulates the uniform nucleation and controllable growth of solid Li 2 S precipitates at large current density. Therefore, the cell with the CoSe 2 /G functional separator delivers an ultrahigh rate cycle at 6.0 C with an initial capacity of 916 mah g 1 and a capacity retention of 459 mah g 1 after 500 cycles, and a stable operation of high sulfur loading electrode ( mg cm 2 ). This work opens up a new insight into the energy chemistry at interfaces to rationally regulate the electrochemical redox reactions, and also inspires the exploration of related energy storage and conversion systems based on multielectron redox reactions. 1. Introduction The constant consuming energy sources have promoted the continuous innovation of energy storage technologies and devices, especially rechargeable battery systems. [1] However, current cutting-edge Li-ion batteries are unable to meet the ever-increasing demands of personal electronics and electric vehicles (EVs) for high energy density. [2] Rechargeable lithium Dr. H. Yuan, Dr. H.-J. Peng, B.-Q. Li, J. Xie, Dr. L. Kong, Dr. X. Chen, Prof. Q. Zhang Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology Department of Chemical Engineering Tsinghua University Beijing , China zhang-qiang@mails.tsinghua.edu.cn M. Zhao, Prof. J.-Q. Huang Advanced Research Institute of Multidisciplinary Science Beijing Institute of Technology Beijing , China jqhuang@bit.edu.cn The ORCID identification number(s) for the author(s) of this article can be found under DOI: /aenm sulfur (Li S) battery has been regarded as one of the most promising candidates for the next-generation battery technology because of its high theoretical energy density of 2600 Wh kg 1, low cost, and environmental benignity. [3] Despite great advantages, the practical application of Li S batteries is hindered by the complex energy chemistry in a working Li S cell. Generally, the sulfur redox electrochemistry involves a series of sophisticated phase transformation and phase migration in a cell. [4] The insulation and insolubility of charged and discharged products (S and Li 2 S) result in sluggish redox kinetics and a low sulfur utilization. The shuttle of soluble polysulfide intermediates (Li 2 S x, 4 x 8) leads to low Coulombic efficiency and rapid capacity degradation. [5] Moreover, the diffusion of polysulfides induces the redistribution of solid products on electrode/electrolyte interfaces, causing the passivation of active surfaces, aggregation of Li 2 S, and large overpotential for sulfur redox conversion. [6] With the continuous redox consumption of surface products, the inner core of aggregates could be loss of electronic contact with a 3D conductive matrix and then readily form dead sulfur, leading to low sulfur utilization. [7] The final morphology of Li 2 S precipitates is determined by the nucleation density and relative kinetics of nucleation versus growth. This strongly depends on the surface chemistry where chemical/electrochemical reactions take place. Generally, the deposition of Li 2 S precipitates depends on the binding strength between sulfur species and substrates, electron transfer capability, number density of active centers, as well as their reactivity at the reactive interfaces. [8,9] The properties of the interfaces can be effectively regulated by doping heteroatoms (such as oxygen, [10] nitrogen, [11] ) or decorating diverse additives (such as metals, [12] metal oxides, [13] sulfides, [14,15] carbides, [16] nitrides, [17] phosphides, [18] organic mediates, [19] and so on). The enhanced catalytic capability and a large number of reactive centers can improve the kinetic behaviors of sulfur intermediates. However, it is difficult to achieve an effective collaborative interface with strong adsorption, high electrical conductivity, and high reactive sites simultaneously, which hence induces a feeble regulation for Li 2 S nucleation and growth. Therefore, the imperative for controllable precipitation of solid Li 2 S is to (1 of 8)

2 Figure 1. Schematic illustration of the Li 2 S nucleation and growth on routine conductive surface (left) and on conductive and catalytic nanotriple-phase interface with uniformly distributed nucleation sites (right). rationally regulate the interfaces that are capable of manipulating not only conversion process of lithium polysulfides but also their following nucleation and growth. In this contribution, a unique triple-phase interface (denoted as the reactive interface among catalyst, conductive support, and electrolyte) is proposed to afford synergistic properties of strong chemisorption, large electrical conductivity, and highly active electrocatalysis that can regulate the kinetic behaviors of soluble lithium polysulfides and enable the uniform nucleation and controllable growth of solid Li 2 S in a working Li S battery (Figure 1). In details, the trogtalite CoSe 2 nanodots provide abundant exposed sites for intimate adsorption of liquid-phase lithium polysulfides and regulate the nucleation density, while the intrinsic metallic attribute of CoSe 2 facilitates the rapid electron transfer. More importantly, the uniformly dispersed CoSe 2 nanodots with an average size of 5 nm lead to densely and uniformly distributed sulfiphilic active sites. It guides effective equilibrium to nucleation and growth of Li 2 S. Therefore, the well-designed triple-phase interface realizes the uniform precipitation of Li 2 S at nanoscale and inhibits their blocky growth and aggregation. By virtue of the cooperative triple-phase interfaces, unprecedented high rate cycle at 6 C ( 10 ma cm 2 ) is demonstrated with an initial capacity (916 mah g 1 ) and an ultralong life (459 mah g 1 after 500 cycles) in a working Li S battery. 2. Results and Discussion The unique triple-phase interface was constructed through in situ growing trogtalite CoSe 2 nanodots on reduced graphene oxide nanosheets (CoSe 2 /G). Abundant oxygen-containing groups on graphene oxide served as absorption sites for metal cations to promote the local growth of CoSe 2 nanocrystalline during the hydrothermal synthesis of CoSe 2. [20] The heat treatment was conducted to further reduce GO and therefore improved the overall conductivity of CoSe 2 /G hybrids (Figure S1, Supporting Information). The as-obtained CoSe 2 /G hybrids exhibited an interconnected and self-supported porous scaffold, in which graphene sheets displayed a lateral size of 10 µm without large CoSe 2 nanoparticle (Figure 2a). The CoSe 2 nanodots with an average diameter of 5 nm were uniformly distributed on the graphene sheets (Figure 2b and Figure S2, Supporting Information). Such a small diameter endowed CoSe 2 nanodots with abundant exposed active sites that afford strong interactions with lithium polysulfides. The content of CoSe 2 in CoSe 2 /G hybrids was around 30% (Figure S1d, Supporting Information). The high-resolution transmission electron microscopy (TEM) image of an individual CoSe 2 nanodot revealed clear lattice fringe with a d-spacing of 0.26 nm, corresponding to the (210) plane of trogtalite CoSe 2 (Figure 2c). The X-ray powder diffraction (XRD) pattern (Figure 2d) exhibited the well-defined characteristic peaks at 30.5 (200), 34.2 (210), 37.6 (211), and 51.7 (311), further proving the crystal structure of trogtalite CoSe 2 (PDF# ). [21] In the CoSe 2 /G hybrid, conductive G sheets provided long-range electron transfer pathway, while metallic CoSe 2 nanodots facilitated short-range electron transportation to active sites. The high specific surface area of m 2 g 1 and abundant mesoporous structure for CoSe 2 /G hybrids afford sufficient triple-phase interfaces as well as their direct accessibility for the redox of sulfur species (Figure S3, Supporting Information). The chemical adsorption of polysulfides is the precondition for their rapid redox reactions on the CoSe 2 electrocatalyst. Therefore, a visualized adsorption test was carried out by adding CoSe 2 or G with the same surface area into the Li 2 S 4 solution to validate the strong interaction between CoSe 2 and typical polysulfides. As shown in Figure 3a, the solution with CoSe 2 was entirely decolored after static adsorption for 12 h, while the control solution with pristine G exhibited negligible discoloration. The CoSe 2 exhibited a superior binding capability of liquid-phase polysulfides than G. To further reveal the intrinsic chemical interaction between CoSe 2 and lithium polysulfides, X-ray photoelectron spectroscopy (XPS) of CoSe 2 was implemented after polysulfide adsorption (denoted as CoSe 2 Li 2 S 4 ) (Figure 3b, and Figures S4 and S5, Supporting Information). The high-resolution Co 2p XPS spectrum exhibited typical characteristic resonances of 2p 3/2 and 2p 1/2. The Co 2p spectrum could be further deconvoluted into four peaks, in which and ev were corresponding to Co 3+, while and ev were attributed to the Co 2+. [22] However, in comparison to original CoSe 2, these four characteristic peaks of Co 2p 3/2 and 2p 1/2 overall upshifted to higher binding energy, indicating the intense interaction of exposed Co site with surrounding strong electronegative sulfur ligand. In Se 3d XPS spectrum, the upshifts of characteristic peaks of 3d 5 / 2 and 3d 3 / 2, which located at 54.8 and 55.6 ev, respectively, were observed as well. This indicated the electron transfer from (2 of 8)

3 Figure 2. Morphological characterization of CoSe 2 /G hybrids. a) Scanning electron microscopy (SEM), b) transmission electron microscopy (TEM), and c) high resolution TEM images of CoSe 2 /G hybrids. d) XRD pattern of CoSe 2 /G hybrids and the JCPDS card of trogtalite CoSe 2. surface exposed Se atoms to electron-rich S atoms in Li 2 S 4. [23] The full sulfiphilic surface could significantly enhance the retention of sulfur species in cathode region and inhibit their outward diffusion to Li metal anode region. More importantly, metallic CoSe 2 possesses high electrical conductivity to facilitate charge transport. [24] The electrons can be rapidly transferred Figure 3. Kinetic behaviors for sulfur redox reactions. a) Visualized adsorption of Li 2 S 4 by pristine CoSe 2 and G. b) High-resolution XPS spectra of Co 2p and Se 3d of CoSe 2 before and after adsorption of Li 2 S 4. c) CV curve of symmetric dummy cells employing CoSe 2 /G and pristine G electrodes at a rapid scan rate of 2000 mv s 1. d) Potentiostatic discharge profile at 2.05 V on different electrodes. e) Potentiostatic charge profile at 2.40 V for evaluating dissolution kinetics of Li 2 S (3 of 8)

4 across the CoSe 2 grains to surface exposed sites and thereby to the S S bridge of polysulfides, further propelling the electrochemical redox conversion of polysulfides. The sulfur electrochemistry involves a serial of complicated liquid liquid, liquid solid, and solid liquid conversions, each of which is critical for realizing reliable Li S batteries. To gain insight into the effectiveness of elaborated interfaces in improving the liquid liquid transformation, the kinetics of redox reactions was systemically probed by cyclic voltammetry (CV) measurement using symmetric dummy cells with Li 2 S 6 electrolyte (Figure 3c). [14] The CoSe 2 /G exhibited a higher current density than that of pristine G, implying the significantly enhanced redox kinetics between liquid-phase polysulfides. The CV curves at different scan rates were also carried out to further illuminate the catalytic capability of the unique interface (Figure S6, Supporting Information). The redox current density increased with the rise of scan rates. Moreover, it was linear functions of the square root of the scan rate, suggesting a strong diffusion-controlled feature of the electrochemical conversion. Generally, the diffusion of active phases lags far behind their redox exhaustion at a rapid scan rate. In this case, the CV profile of CoSe 2 /G at a fast scan rate of 1000 mv s 1 still remained clearly reduction and oxidation peaks, indicating the ultrafast catalytic redox reactions of soluble lithium polysulfides. These were mainly attributed to the decoration of CoSe 2 nanodots on G, which afforded abundant triple-phase interfaces with the synergy of chemisorption, conductivity, and electrocatalysis to trigger the rapid polysulfide conversion (Figure S3, Supporting Information). A low electron transport resistance was further confirmed by electrochemical impedance spectroscopy (EIS), manifesting superior electrocatalytic performance of CoSe 2 /G interface (Figure S7, Supporting Information). In general, the nucleation of Li 2 S and its growth, i.e., the liquid solid conversion, strongly rely on the reactive interfaces. To demonstrate the regulation of Li 2 S precipitation, simple potentiostatic discharge experiments were monitored (Figure 3d). [8,25] The responsivity of Li 2 S nucleation was earlier on CoSe 2 /G over that on G. Moreover, the capacity of Li 2 S precipitation on CoSe 2 /G (91.7 mah g 1 ) was larger than those on G (75.5 mah g 1 ) even at a shorter nucleation and growth time. These results clearly confirmed the unique triple-phase interfaces possessed significantly reduced overpotential for the initial nucleation of Li 2 S and the rapid kinetics of promoting Li 2 S precipitation. To further validate the superiority of CoSe 2 /G in regulating the deposition morphology of Li 2 S, the cells were disassembled after s potentiostatic discharge. Interestingly, the surface of CoSe 2 /G was uniformly covered by a layer of Li 2 S nanoparticles with a diameter of <50 nm (Figure S8a,b, Supporting Information). The uniform deposition of Li 2 S nanoparticles was mainly attributed to the dense and well-distributed nucleation sites of CoSe 2 nanodots on the surface of conductive G matrix. The CoSe 2 nanodots guided uniform Li 2 S nuclei at the initial nucleation stage and thereby induced even precipitation of Li 2 S nanoparticles during the following growth stage. In contrast, Li 2 S solid exhibited an arbitrary deposition with bulky and agglomerate morphology on the surface of G electrode owing to lack of the induction of sulfiphilic nucleation sites (Figure S8c,d, Supporting Information). The sluggish kinetics of the oxidation of solid Li 2 S during the charge process is the main reason for oxidation overpotential in a working Li S battery. [26] To prove the positivity of promoting Li 2 S dissolution, similar kinetic investigations were conducted by employing a potentiostatic charge process after two stage galvanostatic discharge processes, that is to enable active sulfurs fully converted into Li 2 S (Figure 3e). An increased oxidation current density was detected on CoSe 2 /G, suggesting a low oxidation overpotential for Li 2 S dissolution. Furthermore, the dissolution capacity estimated by calculating the quantity of electric charge was also much higher than that on pristine G. These demonstrated excellent electrocatalysis of CoSe 2 /G in promoting Li 2 S dissolution. In addition, the uniform distribution and small Li 2 S precipitation on the surface of CoSe 2 /G formed at the previous potentiostatic discharge process could also reduce the deactivation of active surface and increase the utilization of sulfur. These results verified the superior electrocatalytic reactivity of triple-phase interfaces among electrolyte/ CoSe 2 /G hybrids in regulating kinetics of redox reactions of polysulfides. To investigate the actual superiority of improving electrochemical kinetics in a working Li S cell, CoSe 2 /G, and G coated on polypropylene (PP) membranes with an areal loading of 0.16 mg cm 2 and a thickness of 20 µm were employed as functional separator (Figure S9, Supporting Information). The accelerated polysulfide redox reaction was firstly examined by CV measurements of Li S batteries using different functional separators. As shown in Figure S10a (Supporting Information), CV profiles both exhibited obvious two pair of redox peaks (cathodic reduction peaks at and V, while anodic oxidation peaks at and V), indicating a reversible redox conversion. [27] However, an enhanced onset potential and an increased peak potential, as well as elevated peak current of both two cathodic peaks were clearly observed on the cell with CoSe 2 /G electrocatalysts. It was originated from the excellent kinetic promotion of sulfur species in liquid liquid and liquid solid transformations. Additionally, the improvement of Li 2 S dissolution was also demonstrated by the downshift of the first anode peak potential and its enhanced peak current value. EIS spectra further proved the improved sulfur reactive kinetics under CoSe 2 /G electrocatalysis (Figure S10b, Supporting Information). To confirm the role of CoSe 2 /G on facilitating sulfur redox chemistry, the rate performances of Li S batteries were evaluated (Figure 4a). With 63% sulfur content and an average sulfur loading of 1.0 mg cm 2, Li S cell assembled with CoSe 2 /G functional separator delivered an initial discharge capacity of 1331 mah g 1 at 0.2 C (1 C = 1672 ma g 1 ). When increasing current density to 0.5, 1.0, 2.0, and 4.0 C, a high reversible capacity of 1105, 1030, 972, and 931 mah g 1 was maintained, respectively. Even increasing to 6.0 C, an unprecedented reversible capacity of 902 mah g 1 remained, corresponding to 68% of initial capacity. After reversing current density back to 4.0, 2.0, 1.0, and 0.5 C in turn, reversible discharge capacities of 929, 980, 1031, and 1061 mah g 1 recovered after ultrahighrate test, corresponding to 100% of capacity restoration. In contrast, the cell with G separator exhibited a rapid capacity degradation with increasing rate current, displaying a reduced discharge capacity of 560 mah g 1 at 6.0 C. Moreover, the G cell (4 of 8)

5 Figure 4. Electrochemical performance of Li S batteries with a CoSe 2 /G functional separator. a) The rate capability of Li S batteries and b) corresponding galvanostatic discharge charge profiles at 0.5 and 6.0 C (1 C = 1672 ma g 1 ). c) Long-term cycling performances at 4.0 and 6.0 C. d) Cycling stability of Li S batteries with high sulfur loadings of 2.69, 3.55, and 4.35 mg cm 2, respectively. exhibited inferior capacity restoration after high-rate test. The superb rate capability and their capacity recovery for CoSe 2 /G cell were ascribed to the outstanding kinetic promotion of sulfur redox chemistry in Li S cell. Galvanostatic charge discharge profiles at different current densities revealed significant differences in median voltage at every charge and discharge plateaus (Figure 4b). Compared with G cell, the CoSe 2 /G cell exhibited a higher discharge voltage at first and second discharge plateaus and a lower charge voltage at 6.0 C. This implied a rapid polysulfide conversion kinetics, a low Li 2 S nucleation overpotential, and a weak Li 2 S dissolution resistance, respectively, which considerably promoted the sulfur utilization in a working battery. Consequently, the extraordinary rate performance was ascribed to fully accelerated sulfur redox kinetics at triple-phase interfaces among electrolyte/cose 2 /G hybrids. The well-designed triple-phase interfaces also regulated the long-term cycling stability of Li S batteries at high current rate (Figure 4c). The CoSe 2 /G delivered an ultrahigh discharge capacity of 984 mah g 1 at 4.0 C after initial activation at a low current density, which even increased to 1002 mah g 1 after the next 8 cycles. After continuous 500 cycles, a reversible capacity of 503 mah g 1 was remained. Surprisingly, an unparalleled initial discharge capacity of 916 mah g 1 with increasing current density to 6.0 C and a capacity retention of 459 mah g 1 after 500 cycles was demonstrated. Over the whole cycles at 6.0 C, the charge/discharge curves exhibited two obvious discharge plateaus. Moreover, the charge/discharge overpotential reduced with elevating the number of cycles, indicating the longterm efficiency in the effective electrocatalysis of sulfur redox reaction at CoSe 2 /G interfaces (Figure S11, Supporting Information). More importantly, the Coulombic efficiency (CE) for both cells at 4.0 and 6.0 C had no significant decline, indicating a minor Li anode corrosion. In comparison to the cell with G separator, a low initial discharge capacity of 756 mah g 1 with a CE of 95% was displayed at 4.0 C. Unfortunately, a rapid fade to 71% in CE was observed after only 17 cycles, followed by fluctuant CE, indicating the irreversible cycling. It was ascribed to the invalidation in electrocatalysis for polysulfides and thus, leading to the repeated corrosion toward Li metal owing to the shuttling, which were further confirmed by the SEM images of Li metal anode through disassembling Li S cells after 50 cycles at 4.0 C (Figure S12, Supporting Information). The dissolution and diffusion of soluble polysulfides is more serious in a slow charge/discharge process owing to their long relaxation time in electrolyte. Nevertheless, for the cell with CoSe 2 /G separator, a discharge capacity of 886 mah g 1 was delivered after 200 cycles at 0.5 C, which is 74% of initial capacity. A discharge capacity of 805 mah g 1 with 79% of retention after 200 cycles at 2.0 C was also displayed, indicating identical regulation of polysulfides at (5 of 8)

6 Figure 5. Morphological evolution of active sulfur species on different reactive interfaces during cycling. a) TEM image of CoSe 2 /G after first discharge at 0.5 C and high-resolution TEM image of individual CoSe 2 nanodot (the inset), and b) corresponding STEM elemental mapping images. c) TEM image of pristine G after first discharge at 0.5 C. d) TEM image of CoSe 2 /G at charged state after 200 cycles at 4.0 C and e) its high-resolution TEM image. f) Schematic illustration of polysulfide redox reaction and Li 2 S nucleation. low current density (Figure S13, Supporting Information). The comparison of electrochemical performance of present work with the state-of-the-art studies on functional interlayers was summarized in Table S1 (Supporting Information), indicating the unique superiority in cycle performance and rate capability. In view of the demand of high-energy density in practical applications, the cycling performances of sulfur cathodes with a high sulfur loading of mg cm 2 (63% sulfur content) were probed by adopting CoSe 2 /G functional separator (Figure 4d, and Figures S14 and Figure S15, Supporting Information). The cells with sulfur loadings of 2.69 and 3.55 mg cm 2 delivered initial capacities of 1151 and 1132 mah g 1 at 0.2 C, respectively. Even with a high sulfur loading of 4.35 mg cm 2, a high capacity of 1098 mah g 1 was also obtained, corresponding to an areal capacity of 4.78 mah cm 2 (at 1.45 ma cm 2 ). An approximate capacity of 900 mah g 1 for all cells was retained after 100 cycles. Furthermore, steady cycles with a high discharge capacity of 878, 876, and 832 mah g 1 (corresponding to areal capacity of 2.36, 3.11, and 3.62 mah cm 2, respectively) for the cells with sulfur loading of 2.69, 3.55, and 4.35 mg cm 2, respectively, were also maintained after 100 cycles at 0.5 C (corresponding to the areal current density of 2.25, 2.97, and 3.64 ma cm 2, respectively), corresponding to a capacity retention of 89%, 90% and 85%, respectively. Notably, there was almost no clear difference in discharge capacity for the cells with different sulfur loadings at the same C-rate current density, which can be attributed to the unparalleled electrocatalysis of conductive and catalytic triplephase interface for realizing high-efficiency manipulation of sulfur conversion chemistry even at a practical sulfur loading amount in a working battery. The surface chemistry of conductive matrix can determine the nucleation and growth of Li 2 S, while their precipitated morphology, especially the initial state, is of paramount significance to influence the electrochemical performance of the cell during long-term cycling. [8] To further validate the actual role of the unique interface on the control of Li 2 S deposition in a practical Li S cell, we disassembled the cell after first discharge at 0.5 C and investigated the initial morphology of Li 2 S precipitate on CoSe 2 /G surface. Commendably, the first electrochemical deposition of Li 2 S onto the CoSe 2 /G surface exhibited a uniform particle morphology with a size of a few nanometers surrounding the CoSe 2, which is in consistence with the results obtained in the Li 2 S nucleation test (Figure 5a). The high number density of precipitates on CoSe 2 /G surface was in good accordance with high nucleation sites. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding elemental mapping images further illustrated a uniform distribution of Li 2 S precipitate with no obvious large particles or aggregations (Figure 5b and Figure S16, Supporting Information). In addition, CoSe 2 nanodots remained their original crystal structure and particle size, indicating excellent chemical and electrochemical stability in discharge (inset of Figure 5a). In contrast, the completely different Li 2 S morphology appeared, and large agglomerations with sparse number density and a size of hundreds of nanometers displayed on pristine G, which was consistent with a low nucleation density (Figure 5c and Figure S17, Supporting Information). The discharged cells were also disassembled after 50 cycles at 4.0 C. Similar effects of CoSe 2 /G interfaces on the regulation of Li 2 S deposition were observed (Figure S18, Supporting Information). The morphological difference was in good agreement with high sulfur utilization and a small overpotential for CoSe 2 /G interface (Figure 4b). TEM measurement was also conducted to examine the morphology of elemental sulfur at charging state after 200 cycles at 4.0 C (Figure 5d,e). Owing to the positivity of accelerating the dissolution of Li 2 S and the (6 of 8)

7 subsequent conversion short-chain polysulfides to long-chain polysulfides, Li 2 S was fully oxidized and transformed into element sulfur particles with slightly enhanced size in comparison to initial Li 2 S diameter at first discharging. Meanwhile, the CoSe 2 still maintained the initial crystal structure and was surrounded by the sulfur particles, suggesting the sulfiphilic surface likewise induced uniform deposition of oxidized sulfur. Both the nucleation and growth of active sulfur species is very complicated in a practical Li S battery. There is plenty free room to precisely regulate Li 2 S deposition. Based on abovementioned excellent electrochemical data and postmortem analyses, it could be inferred that the surface chemistry of reactive interfaces, especially binding capability for sulfur species, electric conductivity, electrocatalysis, as well as their distribution, are the key to effectively regulate the deposition process of sulfur species (Figure 5f). 1) incorporating sulfiphilic CoSe 2 nanodots adsorbs soluble lithium polysulfides and thereby enriches them on CoSe 2 /G interface, in favor of increasing the Li 2 S nuclei density. The strong affinity with soluble polysulfides can mitigate the shuttle effect and thus improve the capacity retention. 2) The uniformly distributed CoSe 2 nanodots with a high number density and a large electrical conductivity considerably extend reactive boundary from the junction points between electrocatalysts and conductive matrices to entire catalyst surface, which reduced local current density surrounding with surface catalytic sites. A uniform electrical field is therefore built on the high surface area host. 3) The intrinsic electrocatalysis of CoSe 2 reduced reaction barrier and overpotential of sulfur chemistry, accelerating the conversion of active sulfur. The uniform electrical fields guide the consistent deposition of Li 2 S deposition on triple-phase CoSe 2 interfaces. Therefore, the unique triple-phase interfaces resulted in moderate Li 2 S nucleation and rendered their subsequent controllable growth in a working battery. 3. Conclusions We proposed a unique strategy by precisely constructing multifunctional triple-phase interfaces to regulate electrochemical redox reaction of polysulfide intermediates. The sulfiphilic surface of CoSe 2 was demonstrated to mitigate the shuttle and enhance the retention of sulfur, while their outstanding electrocatalysis was confirmed to propel sulfur species conversion kinetics. More importantly, well-distributed CoSe 2 nanodots on graphene matrix effectively manipulated Li 2 S nucleation and growth, realizing the uniform Li 2 S precipitates. Therefore, the cell with CoSe 2 /G functional separator delivered an ultrahigh rate cycle at 6.0 C (corresponding to a current density of 10 ma cm 2 ) with an initial capacity of 916 mah g 1, a capacity retention of 459 mah g 1 after 500 cycles, and a stable operation of high sulfur loading electrode ( mg cm 2 ). This sheds an in-depth understanding on Li 2 S precipitate process. Meanwhile, this work opens up a fresh insight from energy chemistry at the triple phase interfaces to propel sulfur transformation kinetics and realize the regulation of Li 2 S nucleation and growth. This strategy also inspires the scientific exploration of related energy storage and conversion systems based on multi electron redox reactions. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was supported by National Key Research and Development Program (2016YFA , 2015CB932500, and 2016YFA ), National Natural Scientific Foundation of China ( , , , , and ), Beijing Key Research and Development Plan (Z ), China Postdoctoral Science Foundation (2017M and 2018M630165), and Tsinghua University Initiative Scientific Research Program. The authors appreciate helpful discussion from Ze-Wen Zhang and Ge Zhang. Conflict of Interest The authors declare no conflict of interest. Keywords electrocatalysis, Li 2 S precipitate, lithium sulfur batteries, polysulfide redox reaction, triple-phase interface Received: September 5, 2018 Revised: October 4, 2018 Published online: October 30, 2018 [1] a) G. Li, S. Wang, Y. Zhang, M. Li, Z. Chen, J. Lu, Adv. Mater. 2018, 30, ; b) H.-J. Peng, J.-Q. Huang, X.-B. Cheng, Q. Zhang, Adv. Energy Mater. 2017, 7, ; c) X.-B. Cheng, R. Zhang, C.-Z. Zhao, Q. Zhang, Chem. Rev. 2017, 117, 10403; d) A. Manthiram, S.-H. Chung, C. Zu, Adv. Mater. 2015, 27, [2] a) R. Fang, S. Zhao, Z. Sun, D.-W. Wang, H.-M. Cheng, F. Li, Adv. Mater. 2017, 29, ; b) L. Peng, Y. Zhu, D. Chen, R. S. Ruoff, G. Yu, Adv. Energy Mater. 2016, 6, ; c) X. Q. Zhang, X. B. Cheng, Q. Zhang, J. Energy Chem. 2016, 25, 967. [3] a) H.-J. Peng, J.-Q. Huang, Q. Zhang, Chem. Soc. Rev. 2017, 46, 5237; b) M. Liu, X. Qin, Y.-B. He, B. Li, F. Kang, J. Mater. Chem. A 2017, 5, 5222; c) T. Tao, L. Shengguo, F. Ye, L. Weiwei, H. Shaoming, C. Ying, Adv. Mater. 2017, 29, ; d) G. Zhang, Z.-W. Zhang, H.-J. Peng, J.-Q. Huang, Q. Zhang, Small Methods 2017, 1, ; e) G.-P. Hao, C. Tang, E. Zhang, P. Zhai, J. Yin, W. Zhu, Q. Zhang, S. Kaskel, Adv. Mater. 2017, 29, [4] a) J. Schuster, G. He, B. Mandlmeier, T. Yim, K. T. Lee, T. Bein, L. F. Nazar, Angew. Chem., Int. Ed. 2012, 51, 3591; b) H. Chen, Y. Shen, C. Wang, C. Hu, W. Lu, X. Wu, L. Chen, J. Electrochem. Soc. 2018, 165, A6034; c) S.-H. Chung, C.-H. Chang, A. Manthiram, Adv. Funct. Mater. 2018, 28, [5] a) H. Pan, J. Chen, R. Cao, V. Murugesan, N. N. Rajput, K. S. Han, K. Persson, L. Estevez, M. H. Engelhard, J.-G. Zhang, K. T. Mueller, Y. Cui, Y. Shao, J. Liu, Nat. Energy 2017, 2, 813; b) D. Liu, C. Zhang, G. Zhou, W. Lv, G. Ling, L. Zhi, Q.-H. Yang, Adv. Sci. 2018, 5, ; c) T. Wang, K. Kretschmer, S. Choi, H. Pang, H. Xue, G. Wang, Small Methods 2017, 1, ; d) G. Li, W. Lei, D. Luo, Y.-P. Deng, D. Wang, Z. Chen, Adv. Energy Mater. 2018, 8, ; e) Y. Zhou, G. Hu, W. Zhang, Q. Li, Z. Zhao, Y. Zhao, F. Li, F. Geng, Energy Storage Mater. 2017, 9, 39; f) W. Liu, J. Jiang, K. R. Yang, Y. Mi, P. Kumaravadivel, Y. Zhong, Q. Fan, Z. Weng, Z. Wu, J. J. Cha, (7 of 8)

8 H. Zhou, V. S. Batista, G. W. Brudvig, H. Wang, Proc. Natl. Acad. Sci. USA 2017, 114, [6] a) J. H. Yan, X. B. Liu, B. Y. Li, Adv. Sci. 2016, 3, ; b) C.-F. Chen, A. Mistry, P. P. Mukherjee, J. Phys. Chem. C 2017, 121, 21206; c) J. Xie, H.-J. Peng, J.-Q. Huang, W.-T. Xu, X. Chen, Q. Zhang, Angew. Chem. Int. Ed. 2017, 56, 16223; d) S. Chen, D. Wang, Y. Zhao, D. Wang, Small Methods 2018, 2, [7] a) Y. Z. Sun, J. Q. Huang, C. Z. Zhao, Q. Zhang, Sci. China Chem. 2017, 60, 1508; b) Y. Z. Wang, X. X. Huang, S. Q. Zhang, Y. L. Hou, Small Methods 2018, 2, [8] F. Y. Fan, W. C. Carter, Y.-M. Chiang, Adv. Mater. 2015, 27, [9] H.-J. Peng, Z.-W. Zhang, J.-Q. Huang, G. Zhang, J. Xie, W.-T. Xu, J.-L. Shi, X. Chen, X.-B. Cheng, Q. Zhang, Adv. Mater. 2016, 28, [10] L. Zhang, F. Wan, X. Wang, H. Cao, X. Dai, Z. Niu, Y. Wang, J. Chen, ACS Appl. Mater. Interfaces 2018, 10, [11] a) H. Zhang, Z. Zhao, Y. Liu, J. Liang, Y. Hou, Z. Zhang, X. Wang, J. Qiu, J. Energy Chem. 2017, 26, 1282; b) H. Yuan, W. Zhang, J.-g. Wang, G. Zhou, Z. Zhuang, J. Luo, H. Huang, Y. Gan, C. Liang, Y. Xia, J. Zhang, X. Tao, Energy Storage Mater. 2018, 10, 1; c) J. Song, Z. Yu, M. L. Gordin, D. Wang, Nano Lett. 2016, 16, 864. [12] H. Al Salem, G. Babu, C. V. Rao, L. M. R. Arava, J. Am. Chem. Soc. 2015, 137, [13] a) X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Adv. Mater. 2017, 29, ; b) Z. Li, B. Y. Guan, J. Zhang, X. W. Lou, Joule 2017, 1, 576; c) X. Tao, J. Wang, C. Liu, H. Wang, H. Yao, G. Zheng, Z. W. Seh, Q. Cai, W. Li, G. Zhou, C. Zu, Y. Cui, Nat. Commun. 2016, 7, 11203; d) Q. Sun, B. J. Xi, J. Y. Li, H. Z. Mao, X. J. Ma, J. W. Liang, J. K. Feng, S. L. Xiong, Adv. Energy Mater. 2018, 8, [14] Z. Yuan, H.-J. Peng, T.-Z. Hou, J.-Q. Huang, C.-M. Chen, D.-W. Wang, X.-B. Cheng, F. Wei, Q. Zhang, Nano Lett. 2016, 16, 519. [15] L. B. Ma, W. J. Zhang, L. Wang, Y. Hu, G. Y. Zhu, Y. R. Wang, R. P. Chen, T. Chen, Z. X. Tie, J. Liu, Z. Jin, ACS Nano 2018, 12, [16] a) R. Fang, S. Zhao, Z. Sun, D.-W. Wang, R. Amal, S. Wang, H.-M. Cheng, F. Li, Energy Storage Mater. 2018, 10, 56; b) W. Bao, D. Su, W. Zhang, X. Guo, G. Wang, Adv. Funct. Mater. 2016, 26, [17] a) T. Zhou, W. Lv, J. Li, G. Zhou, Y. Zhao, S. Fan, B. Liu, B. Li, F. Kang, Q.-H. Yang, Energy Environ. Sci. 2017, 10, 1694; b) X. Li, K. Ding, B. Gao, Q. Li, Y. Li, J. Fu, X. Zhang, P. K. Chu, K. Huo, Nano Energy 2017, 40, 655; c) D. R. Deng, F. Xue, Y. J. Jia, J. C. Ye, C. D. Bai, M. S. Zheng, Q. F. Dong, ACS Nano 2017, 11, [18] Y. Zhong, L. Yin, P. He, W. Liu, Z. Wu, H. Wang, J. Am. Chem. Soc. 2018, 140, [19] a) C.-Y. Chen, H.-J. Peng, T.-Z. Hou, P.-Y. Zhai, B.-Q. Li, C. Tang, W. Zhu, J.-Q. Huang, Q. Zhang, Adv. Mater. 2017, 29, ; b) J. Q. Zhou, T. Qian, N. Xu, M. F. Wang, X. Y. Ni, X. J. Liu, X. W. Shen, C. L. Yan, Adv. Mater. 2017, 29, [20] H. Yuan, J. Liu, Q. Jiao, Y. Li, X. Liu, D. Shi, Q. Wu, Y. Zhao, H. Li, Carbon 2017, 119, 225. [21] a) X. Zhao, H. Zhang, Y. Yan, J. Cao, X. Li, S. Zhou, Z. Peng, J. Zeng, Angew. Chem., Int. Ed. 2017, 56, 328; b) M.-R. Gao, X. Cao, Q. Gao, Y.-F. Xu, Y.-R. Zheng, J. Jiang, S.-H. Yu, ACS Nano 2014, 8, [22] a) V. Murugadoss, N. Wang, S. Tadakamalla, B. Wang, Z. Guo, S. Angaiah, J. Mater. Chem. A 2017, 5, 14583; b) J. Yang, Y. Yuan, W. Wang, H. Tang, Z. Ye, J. Lu, J. Power Sources 2017, 340, 6. [23] a) L. Petit, N. Carlie, A. Humeau, G. Boudebs, H. Jain, A. C. Miller, K. Richardson, Mater. Res. Bull. 2007, 42, 2107; b) Y. Wei, Y. Tao, Z. Kong, L. Liu, J. Wang, W. Qiao, L. Ling, D. Long, Energy Storage Mater. 2016, 5, 171. [24] a) Z. Hu, Q. Liu, S.-L. Chou, S.-X. Dou, Adv. Mater. 2017, 29, ; b) J.-F. Chang, Y. Xiao, Z.-Y. Luo, J.-J. GE, C.-P. Liu, W. Xing, Acta Phys.-Chim. Sin. 2016, 32, 1556; c) D. Kong, J. J. Cha, H. Wang, H. R. Lee, Y. Cui, Energy Environ. Sci. 2013, 6, [25] F. Y. Fan, Y.-M. Chiang, J. Electrochem. Soc. 2017, 164, A917. [26] a) A. Berger, A. T. S. Freiberg, A. Siebel, R. Thomas, M. U. M. Patel, M. Tromp, H. A. Gasteiger, Y. Gorlin, J. Electrochem. Soc. 2018, 165, A1288; b) G. Zhou, H. Tian, Y. Jin, X. Tao, B. Liu, R. Zhang, Z. W. Seh, D. Zhuo, Y. Liu, J. Sun, J. Zhao, C. Zu, D. S. Wu, Q. Zhang, Y. Cui, Proc. Natl. Acad. Sci. USA 2017, 114, 840. [27] Y. Ye, F. Wu, Y. Liu, T. Zhao, J. Qian, Y. Xing, W. Li, J. Huang, L. Li, Q. Huang, X. Bai, R. Chen, Adv. Mater. 2017, 29, (8 of 8)