Room-Temperature PotassiumâSulfur Batteries Enabled by

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Room Temperature Potassium-Sulfur Batteries Enabled by Microporous Carbon Stabilized Small-Molecule Sulfur Cathodes Peixun Xiong, Xinpeng Han, Xinxin Zhao, Panxing Bai, Yang Liu, Jie Sun, and Yunhua Xu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09503 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Room

Temperature

Potassium-Sulfur

Batteries

Enabled by Microporous Carbon Stabilized SmallMolecule Sulfur Cathodes Peixun Xiong,† Xinpeng Han,§ Xinxin Zhao,† Panxing Bai,† Yang Liu,‡ Jie Sun,§ Yunhua Xu,†,,* †

School of Materials Science and Engineering, Key Laboratory of Advanced Ceramics and

Machining Technology (Ministry of Education), Tianjin Key Laboratory of Composite and Functional Materials and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, China; §

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China;

‡

Department of Materials Science and Engineering, North Carolina State University, Raleigh,

NC 27695, USA; 

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

300072, China.

ACS Paragon Plus Environment

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ABSTRACT: Potassium-sulfur (K-S) batteries are a promising alternative to lithium-ion batteries for large area energy storage applications, owing to their high capacity and inexpensiveness, but have been seldom investigated. Here we report room-temperature K-S batteries utilizing a microporous carbon-confined small-molecule sulfur composite cathode. The synergetic effects of the strong confinement of microporous carbon matrix and the smallmolecule sulfur structure can effectually eliminate the formation of soluble polysulfides and ensure a reversible capacity of 1198.3 mA h g−1 and retain 72.5% after 150 cycles with a Coulombic efficiency of ~97%. The potassium-storage mechanism was investigated by X-ray photoelectron spectroscopy analysis and theoretical calculations. The results suggest that K2S is the final potassiation product along with the reaction of 2K + S ↔ K2S, giving a theoretical capacity of 1675 mA h g−1. Our findings not only provide an effective strategy to fabricate high performance room-temperature K-S batteries but also offer a basic comprehension of the potassium storage mechanism of sulfur cathode materials. KEYWORDS: potassium-sulfur battery, microporous carbon, small-molecule sulfur cathode, potassiation mechanism, electrochemical energy storage As the dominant power technologies for portable electronic equipment, emerging electric vehicles, lithium-ion batteries (LIBs) have attracted growing concern in the past decades, which are expected to alleviate environmental pollution and energy crisis caused by utilization of fossil fuels.1-3 However, the limited Li resources and increasing demand for LIBs may significantly increase the cost of Li compounds, and further hinder LIBs from being deployed in large-scale applications. Therefore, alternative metal-ion batteries that use low-cost and environment-benign materials are highly desired.


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Recently, potassium-ion batteries (KIBs) have been attracted growing concern because of natural abundance and low redox potential (-2.93 V vs. K+/K) of potassium, which may provide cost advantages and high voltage batteries as reported in the literature.4-7 However, the realization of high performance KIBs is of great challenge due to large K ions.4,5 Particularly, the cathode materials reported in the literature show much lower capacities (~100 mA h g-1),8-11 compared with those anode materials (~250 mA h g-1 for carbon and ~400 mA h g-1 for Bi anodes).12-17 Sulfur, a low cost and abundant element, has been intensively explored as a promising cathode material in LIBs and sodium-ion batteries (NIBs) due to its high theoretical lithium/sodium storage capacity upon the formation of the final discharge product of Li2S or Na2S.18,19 These make it very appealing for KIB cathodes. Lu et. al reported a potassium-sulfur (K-S) battery that uses a solid-state electrolyte of β″-Al2O3 and operates at a high temperature of 150 °C, where encouraging performance was gained with a capacity of ~400 mA h g−1 and long cyclability of 1000 cycles without palpable capacity loss.20 However, the complicated fabrication process of the cells, the solid-state electrolytes, and the high operation temperature apparently diminish the resource merits of potassium and sulfur, and finally make it unsuitable for practical applications. In liquid electrolyte systems, sulfur cathodes have been severely plagued by shuttle effect of soluble polysulfide intermediate products, including inadequate use of sulfur, low Coulombic efficiency, severe capacity decay, and quick self-discharge.21-25 These have been widely experienced in lithium-sulfur (Li-S) batteries. Although the shuttle effect can be mitigated through embedding sulfur into matrixes of PAN and porous carbon in LIBs, the bigger volume changes caused by the larger K ions make the conditions worse, and thus quite lower capacity (0.4% capacity loss per cycle) in KIBs,26-28


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compared with ~1000 mA h g−1 and >1000 cycles in LIBs.21 Although the utilization of sulfur is improved by modified separators, the cycle life is still poor.29 In addition, discordant potassium storage mechanisms in sulfur are reported in the literature.20,26-33 These clearly show that more barriers are facing room-temperature K-S batteries. Among the reported strategies, the most successful way is to embed fragmented sulfur in microporous carbon at a high temperature.34-37 The strong chemical interaction between the ending sulfur atoms of the fragmented sulfur molecules and carbon atoms can effectively sequester the sulfur within the microporous carbon matrix and essentially eliminate the formation of soluble polysulfide intermediates. An ultra-long cycle life of 4000 cycles with high Coulombic efficiency has been realized in a Li-S battery, showing that the microporous carbon/sulfur composite is promising.36 In this study, room-temperature K-S batteries are fabricated using microporous carbon stabilized small-molecule sulfur composite cathode materials. The strong confinement of the microporous carbon to sulfur was validated by the high resistance to heat treatment and irradiation of electrons in a transmission electron microscope (TEM). The existing form of smallmolecule sulfur was revealed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurement. As a cathode in KIBs, the microporous C/S composite delivered a reversible capacity of 1198.3 mA h g−1 and retained 72.5% after 150 cycles, which is attributed to the sulfur stabilization effect of microporous carbon, and the performance is much better than those previous reports. The potassium storage mechanism of sulfur was explored by a combined method of theoretical calculation and experimental analysis. RESULTS AND DISCUSSION


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Generally, sulfur has a cyclic structure (cyclo-S8). As temperature increases, cyclo-S8 will be decomposed into small sulfur molecules (S2 to S7).38 The content of S8 species decreases along with the increase of smaller molecules. In particular, when the temperature is above 600 °C and the pressure is below 1 torr, S2 is the main species, up to 99% of all species.38 Upon cooling down to room temperature, cyclo-S8 will reform if no further treatment. It is reported that small sulfur molecules could be confined in appropriate pores of carbon matrix.34-36 This motivates us to construct small sulfur molecules at a proper temperature and pressure. In our work, microporous carbon was regarded as the matrix material to sequester and confine the small sulfur molecules through strong interaction between fragmented sulfur and carbon atoms (Figure 1). This method has been demonstrated as an available way to make stable microporous C/S composite electrode materials for LIBs.36 In the scanning electron microscopy (SEM) and TEM images, there is no difference in morphology between the microporous carbon and the microporous C/S composite. They both exhibit a spherical morphology with smooth surface and have a similar particle size of ~500 nm (Figure 2a-b and 2d-2e), indicative of a structural stability of the microporous carbon as a sulfur supporting matrix. The structure stability was also validated by the similarity in the microstructures of the microporous carbon and the microporous C/S composite (Figure 2c and 2f). The energy-dispersive X-ray spectroscopy (EDS) mappings in Figure 2h-i show a uniform distribution of sulfur in the microporous carbon, suggesting a good sulfur permeation into the microporous carbon. Only diffuse diffraction rings for carbon are observed in the selected area electron diffraction (SAED) pattern, demonstrating amorphous phase of sulfur in the composite (Figure 2j). The porous structures of the carbon host and C/S composite were also analyzed by CO2 adsorption/desorption measurement (Figure S1a). The carbon host has a specific surface


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area of 308 m2 g−1, a pore volume of 0.11 cm3 g−1 and the pore sizes are mainly below 1 nm (Figure S1b). After sulfur infusion, however, the specific surface area of the C/S composite dramatically reduced to 8.5 m2 g−1, and the micropores thoroughly vanished, showing that the micropores in the porous carbon matrix are successfully filled with sulfur. The composite shows no distinct diffraction peaks as the cyclo-S8 displays, suggesting uniform distribution of sulfur in the microporous carbon (Figure 3a). The nature of small sulfur molecules in the microporous carbon were further investigated using Raman spectroscopy with pristine sulfur and microporous carbon as references (Figure 3b). The peak at 1350 cm−1 for disordered (D) carbon and the peak at 1580 cm−1 for graphitic (G) carbon were observed in the spectrum of microporous carbon, indicating partial graphitization of the microporous carbon. Typical Raman spectrum is presented for the pristine sulfur with strong stretching bands below 500 cm−1 (Figure 3b).39 Whereas, the peaks for sulfur are absent for the composite, and an alike shape to the microporous carbon was displayed (Figure 3b). These clearly suggest that the microporous carbon-confined sulfur exists in a different form from cyclo-S8, which may be resulted from the fragmented sulfur molecules produced during the high temperature process and the confinement of the microporous carbon. The existing state of the microporous carbon-confined sulfur was evidenced by time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis, which is one forceful technique to probe molecular information.40 Both of the negative and positive ion polarity data were collected in TOF-SIMS analysis (Figure S2 and S3). Figure 3c and Table S1 show the relative mass intensities. Obviously, the signal of single-atom sulfur is the main species for sulfur, following by S2 and S3 with reduced intensity. It is worth noting that trace amounts of S4~S7 molecules (0.14% for total of S4~S7, Figure S4) were also observed in the beginning of beam sputtering but


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rapidly disappeared after a few seconds, which may come from the residue sulfur molecules on the surface. Considering the high energy barrier to produce single-atom sulfur molecules, it can be deduced that the confined sulfur only exists in forms of S2~S3 and there is no larger sulfur species within the microporous carbon matrix, which can completely eliminate the formation of soluble polysulfides and thus avoiding the shuttle effect (Figure 3d). It would be specially mentioned that the existing state of small sulfur molecule in the microporous C/S composites has not been validated previously. It is well known that the pristine cyclo-sulfur suffers serious sublimation under the condition of ultra-high vacuum as verified by X-ray photoelectron spectroscopy (XPS) measurements. Before collecting data, the samples were firstly put into a test chamber, and then evacuated to a pressure of ~1×10−8 torr and left over 12 h. Figure S5a shows the XPS spectra of the composite and bulk cyclo-S8. After normalization based on the carbon peaks, strong sulfur peaks were observed in the spectrum of the composite with a sulfur content of 20 wt%, while the bulk cycloS8 shows weak signal with a low sulfur content of only 2 wt% due to sublimation. These strongly suggest that the microporous C/S composite shows superior stability compared with the severe sublimation of pristine cyclo-S8 under UHV conditions. To clearly show the comparison of the XPS data between the two samples, the zoomed-in sulfur peaks are shown in Figure S5b, where the composite presents much stronger sulfur peaks than those of pristine sulfur sample. The stability of the composite was also examined using Thermogravimetric (TGA) measurement. As given in Figure S6, the microporous C/S composite mainly underwent a weight loss from 400 °C to 600 °C. The temperature is much higher than the evaporation temperature of the pristine cyclo-S8 (~300 °C). Similar to the pristine sulfur, the residue sulfur on the surface shows an easy evaporation below 300 °C as presented by the TGA curve of the


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microporous C/S composite without removing the extra superficial sulfur (Figure S7).37,41 The enhanced thermal stability of the confined sulfur should be attributed to the strong confinement of the microporous carbon host and the interaction between the fragmented sulfur species and carbon atoms. Under a 300 keV electron beam irradiation inside a TEM, the composite also shows strong stability. The pure sulfur and the composite without removing superficial sulfur were first dispersed on Cu micro-grid, and then put into TEM chamber and further placed into electron beam. Under different electron beam doses, the elemental composition data was collected and analyzed before and after irradiation. Sulfur contents in different irradiation conditions were displayed in Table 1. Upon the exposure to electron beam, the pure sulfur quickly disappeared even at a low dose rate of 7.43 A m−2 (Video S1 and Figure S8a). After a durative electron irradiation for 20 minutes, because of the sublimation of sulfur, only 1.9 wt% sulfur was remained. The instable behavior is in accordance with the XPS measurements (Figure S5). Whereas, the composite showed a super stability. After 30 minutes irradiation with a moderate dose rate of 74.6 A m−2 (Video S2 and Figure S8b), which is over 10 times greater than that used for pure sulfur, or 20 minutes irradiation with a stronger dose rate of 796 A m−2, 32.3 wt% and 31.3 wt% sulfur can be retained, respectively, indicating that little sulfur loss occurred in the microporous C/S composite. Amazingly, even under exceedingly strong electron beam exposure with a much higher dose rate of 35000 A m−2 over 20 minutes, 24.3 wt% sulfur still retains, and the weight loss is possibly due to the evaporation of superficial cyclo-S8. This is consistent with the TGA analysis (shown in Figure S7). These clearly indicate that in the composite, sulfur was confined and bonded in the carbon host matrix, which could stand up to the electron beam irradiation. This presents sulfur from sublimating and decomposing upon high vacuum and


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electron irradiation. Therefore, an improved cycling performance can be anticipated for the microporous C/S composite cathode material. Electrochemical performance of the composite was evaluated using coin cells. The cyclic voltammograms (CV) test of the composite electrode was operated from 0.01 to 3.0 V with a scan rate of 0.01 mV s−1 (Figure 4a). A major peak at 0.51 V was observed in the initial cathodic scan, which is replaced by two peaks at 1.5 V and 0.85 V in the subsequent cycles, corresponding to the potassiation process of the confined sulfur. In the anodic scans, two depotassiation peaks were observable at 1.65 V and 1.95 V, showing a reversible electrochemical behavior. The reaction voltages are lower than those cyclo-S8 electrodes where soluble potassium polysulfides are formed, suggesting a different charge/discharge processes in the composite. This is consistent with the different existing form of the confined sulfur from the bulk cyclo-S8. Figure 4b displays selected charge/discharge profiles of the composite electrodes at 20 mA g−1. A high reversible capacity of 1198.3 mA h g−1 was achieved, and the initial Coulombic efficiency is 61.9%. Two voltage slopes are displayed below 1.6 V, which are lower than the voltage of cyclo-S8 electrodes (~2.4 V and ~1.8 V).30 The lower voltage of the small-molecule sulfur cathodes may be related to the different structure and reaction mechanism. The fragmented small sulfur molecules may produce strong chemical interaction between the ending sulfur and carbon atoms and as a result charge delocalization. This should be responsible for the lower voltage of the small-molecule sulfur cathodes. Afterwards, no obvious change in the CV and voltage profiles was observed, showing the high reversibility and stability of the composite. Good cycling stability was further displayed in Figure 4c. A high reversible capacity of 869.9 mA h g−1 was obtained after 150 cycles with a capacity retention of 72.5% and a low capacity loss of 0.18% each cycle (Figure 4c, Figure S9-S10). In order to compare the electrochemical


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behavior of small sulfur and cyclo-S8, the S8/C cathode was produced by mixing bulk sulfur and microporous carbon followed by heat treatment at 155 ℃. Only one voltage plateau at 2.38 V is displayed in the charge/discharge profiles, and no reversible capacity was delivered, showing that an irreversible reaction occurred between the soluble potassium polysulfides and the carbonate electrolyte during the discharge process (Figure S11). The same phenomenon has been commonly observed for sulfur cathodes in LIBs.34 The rate performance was also evaluated using current rates from 20 to 2000 mA g−1 (Figure 4d-4e). A capacity of 741.2 mA h g−1 was achieved at 2000 mA g−1, indicating a good rate performance. There is no doubt that the excellent performance is associated with the confinement effect of microporous carbon matrix and the stabilized small-molecule sulfur structure, which effectively prevents and eliminates the shuttling behavior of soluble polysulfides. The electrochemical properties of the composite outperform those reported sulfur-based cathodes for KIBs (Table S2). The morphology, microstructure and chemical property of the cycled microporous C/S composite electrodes were examined by electrochemical impedance spectroscopy (EIS), SEM, TEM, and XPS. The EIS data show that the solid-electrolyte-interphase (SEI) layer gets stable after 50 cycles (Figure 4f). At discharge states (0.5 V), a thicker layer of SEI was formed with a rougher surface morphology as shown by the SEM and TEM images (Figure 5a-5b). At charge states (3 V), no obvious morphology change was observed apart from a thin SEI layer coated on the surface compared with the pristine electrodes (Figure 5d-5e and Figure S12). The EDS mapping images reveal that sulfur is still uniformly distributed within the microporous carbon matrix after cycling, validating a strong confinement of small-molecule sulfur by the microporous carbon (Figure S13).


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To thoroughly understand the reaction mechanism between S and K, ex-situ XPS analysis was conducted on the cycled electrodes. At discharge state, the high-resolution S 2p spectrum shows strong peaks at 165.98, 162.05, and 159.46 eV (Figure 5c), assigned to RSOx species, S22-(K2S2), and S2-(K2S), respectively.42 The RSOx species decrease significantly after sputtering and are assigned to the SEI films raised from the irreversible reaction between electrolytes and sulfur (Figure S15a). It’s worth noting that the binding energy of 159.46 eV for K2S is much lower than that of bulk K2S (~161.2 eV)42, which may arise from the sectional charge-transfer effect between the confined sulfide anion and the carbon matrix.36 The sulfide species disappeared upon charging (Figure 5f). They are oxidized to S-S, indicating a reversible reaction of the microporous C/S cathodes with potassium (Figure 5f).42 The peak at 161.1 eV could be assigned to the residue sulfide species (S2- + S22-), which may arise from incomplete oxidation reaction. The remaining K 2p peaks at charged state in Figure S14-S15 may come from the SEI layer and the intensity decreased dramatically when charged to 3.0 V. These results show that, similar to the sulfur cathodes in LIBs and NIBs, sulfur can also be fully potassiated along the reaction of 2K + S ↔ K2S in KIBs, giving a theoretical capacity of 1675 mA h g-1. This is in accordance with previous results on K-S batteries.29,33 To further get insight into the reaction mechanism between potassiation and sulfur, the calculations based on density functional theory (DFT) were performed (Figure 6). According to the K-S binary phase diagram, five phases, K2S6, K2S5, K2S3, K2S2, and K2S, were considered in our calculation study (Figure S16).43 Compared with K2S6 and K2S5, the short chain potassium sulfides, K2S3, K2S2 and K2S, show lower formation energies, -168.2, -170.1 and -168.8 eV, respectively, meaning that they are stable forms. K2S phase exhibited the lowest formation energy, suggesting it is the most stable form on thermodynamics. It was noted that K2S2 shows a


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higher formation energy than K2S3 and K2S, indicating that it is a less thermodynamically stable phase. If K2S2 is formed in an incomplete potassiation reaction, a disproportionation reaction may occur along the reaction of 2K2S2  K2S3 + K2S.44 This may explain why K2S3 was observed in previous reports.26,27 Therefore, the theoretical calculation results suggest that K2S is the final potassiation product. CONCLUSIONS In summary, room temperature K-S batteries are realized using microporous carbon-confined small-molecule sulfur cathode materials. The small sulfur molecules (Sn, n≤3) were validated by TOF-SIMS analysis, which completely eliminate the formation of soluble polysulfides. The strong confinement of the microporous carbon matrix can stabilize the sulfur cathodes as evidenced by TGA, high vacuum and TEM beam irradiation measurements. The synergetic effects of the microporous carbon confinement and the small-molecule sulfur structure promise a good performance of composite as a KIB cathode with a reversible capacity of 1198.3 mA h g−1, moderate Coulombic efficiency of 97%, and good cycling stability retaining 869.9 mA h g−1 after 150 cycles. This property is much better than those previous reports on K-S batteries. The XPS analysis and the theoretical calculations suggest that K2S is the final potassiation reaction product of sulfur in the reaction of 2K + S ↔ K2S, giving a theoretical capacity of 1675 mA h g−1. Our findings not only provide an effective strategy to fabricate high performance roomtemperature K-S batteries but also offer a basic comprehension of the potassium storage mechanism of sulfur cathode materials. EXPERIMENTAL SECTION Preparation of Microporous Carbon. The microporous carbon was prepared using previously reported route.36 In a typical synthesis procedure, sucrose was firstly dissolved in sulfuric acid


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(H2SO4), and refluxed at 120 °C for 12 h. The resulting brown product was collected and washed several times with distilled water and then dried at 100 °C overnight. The sample was further annealed at 1000 °C for 3 h under 95% Ar/5% H2 atmosphere. Synthesis of Microporous C/S Composite. The resulting carbon was mixed with bulk sulfur (1:1 in a weight ratio) and sealed in a glass tube under vacuum. The glass tube was heated in a furnace at 600 °C for 5 h. After natural cooling to 25 °C, the resulting sample was further heated at 200 °C for 5 h under Ar atmosphere to eliminate superficial sulfur. The S8/C composite was prepared by mixing bulk sulfur and microporous carbon followed by heat treatment at 155 ℃. Material Characterization. XRD patterns were recorded on a Bruker D8 Advance X-ray diffractometer with CuKα radiation. TGA measurement was collected on a thermogravimetric analyzer (TA Instruments, USA) with a heating rate of 10 °C min−1 in argon. XPS spectra were carried out on Kratos AXIS 165 spectrometer with monochromatic Al Kα radiation (280 W) for microporous C/S composite, and charge neutralization was required to minimize sample charging. Raman spectra were recorded on a Horiba Jobin Yvon Labram Aramis with a 532 nm excitation source. CO2 adsorption/desorption was performed on a Micromeritics ASAP 2420–4 volumetric adsorption analyzer (USA). The pore size distribution and pore volume were calculated by NLDFT and Horvath–Kawazoe method, respectively. SEM and TEM measurement were performed on Hitachi S-4800 and FEI F20 S-TWIN instrument, respectively. TEM Electron Beam Irradiation. The pure sulfur and microporous C/S composites were dispersed on TEM grids and loaded inside the TEM (FEI Tecnai F30). After the column vacuum was pumped down to about 2×10−5 Pascal in several minutes, the samples were irradiated using 300 keV electron beam with various doses. Energy-dispersive X-ray (EDX) spectra were carried


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out after exposure to electron beam for a period of time. Sulfur retention was quantitatively analyzed based on the EDX spectra. The electron beam dose rates as well as the irradiation time were listed in Table 1. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). The TOF-SIMS analysis was conducted on a TOF.SIMS 5 analyzer (ION-TOF GmbH, Germany). The pulsed primary ion beam bombards the sample surface and generates neutrals and secondary ions, which were counted by a time-of-flight mass detector. TOF-SIMS data were recorded on a TOF.SIMS 5 analyzer with a liquid metal Bi1+ ion gun at an operation energy of 60 KeV. The collection time is 600 s at a sample current of 1.9 pA and analysis area of 200 × 200 µm2 and a 1.5 pA 10 keV Bi1+ beam at 45° rasterized over 50 μm × 50 μm for analysis. The sputtering speed is 0.214nm s−1 for SiO2 as the reference. Both of the negative and positive ion polarity data were collected. Electrochemical Measurement. The microporous C/S composite, carbon black and sodium carboxymethyl cellulose (CMC) were uniformly mixed to form a slurry (8:1:1 by weight). The slurry was casted onto Al foil and dried at 50 °C for 24 h in a vacuum oven. The mass loading is 0.5~1.0 mg cm−2 for the active material (sulfur). The coin cells were assembled in a glove box (argon atmosphere, O2 and H2O levels < 0.1 ppm). Potassium foil was used as counter electrodes. The separator was Celgard®2400 polypropylene membrane. The electrolyte was 0.8 M KPF6 in EC/DEC (1:1 by volume). The usage amount of electrolyte is about 10 μL mg−1. The charge/discharge measurements were performed in the voltage window from 0.5 to 3.0 V (vs. K+/K) on Land automatic battery testers (Land CT 2001A, Wuhan, China). Cyclic voltammetry (CV) test was conducted on an electrochemistry workstation (CHI660E). Capacity calculation was based on the mass of sulfur after eliminating the contribution of microporous carbon.


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Theoretical Calculations: The calculations based on DFT were performed to investigate the thermodynamic properties of K-S binary compounds. The CASTEP code was used, in which the correlation interaction and electron exchange are evaluated with generalized gradient approximation (GGA) method. Two valence electron configurations (K (4s1) and S (3s23p4)) were used. After checking for convergence, we used a plane-wave cutoff energy of 450 eV. The lattice parameters and atomic arrangements were fully relaxed. Atomic relaxation was performed until the tolerances of energy and gradient convergence were less than 1×10-5 eV and 0.01 eV Å−1, respectively. ASSOCIATED CONTENT Supporting Information Available: CO2 adsorption/desorption data of porous carbon and C/S composite, positive and negative TOF-SIMS mass spectrum of the microporous C/S composite, XPS spectra of the microporous C/S composite and pure sulfur, TGA curve of the microporous C/S composite, in situ TEM images of pure S and the microporous C/S composite, electrochemical performance of S8/C, the pristine microporous C/S composite electrode, elemental mappings and XPS spectrum of the discharged and charged microporous C/S composite electrodes, K-S phase diagram. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT


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This work was financially supported by the National Natural Science Foundation of China (Grand No. 51672188), and Natural Science Foundation of Tianjin City (Grand No.: 16JCYBJC40900). REFERENCES (1) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577-3613. (2) Hwang, J.-Y.; Myung, S.-T.; Sun, Y.-K. Sodium-Ion Batteries: Present and Future. Chem. Soc. Rev. 2017, 46, 3529-3614. (3) Li, M.; Lu, J.; Chen, Z.; Amine, K. 30 Years of Lithium-Ion Batteries. Adv. Mater. 2018, 30, 1800561. (4) Jian, Z.; Luo, W.; Ji, X. Carbon Electrodes for K-Ion Batteries, J. Am. Chem. Soc. 2015, 137, 11566-11569. (5) Zhao, J.; Zou, X.; Zhu, Y.; Xu, Y.; Wang, C. Electrochemical Intercalation of Potassium into Graphite. Adv. Funct. Mater. 2016, 26, 8103-8110. (6) Zou, X.; Xiong, P.; Zhao, J.; Hu, J.; Liu, Z.; Xu, Y. Recent Research Progress in Nonaqueous Potassium-Ion Batteries. Phys. Chem. Chem. Phys., 2017, 19, 26495-26506. (7) Kim, H.; Kim, J. C.; Bianchini, M.; Seo, D. H.; Rodriguez-Garcia. J.; Ceder. G. Recent Progress and Perspective in Electrode Materials for K-Ion Batteries. Adv. Energy Mater. 2018, 8, 1702384. (8) Chong. S.; Chen. Y.; Zheng. Y.; Tan. Q.; Shu. C.; Liu. Y.; Guo. Z. Potassium Ferrous Ferricyanide Nanoparticles as A High Capacity and Ultralong Life Cathode Material for Nonaqueous Potassium-Ion Batteries. J. Mater. Chem. A 2017, 5, 22465-22471.


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(9) Gao. H.; Xue. L.; Xin. S.; Goodenough. J. B. A High‐Energy‐Density Potassium Battery with a Polymer‐Gel Electrolyte and a Polyaniline Cathode. Angew. Chem. 2018, 130, 5547-5551. (10) Fan, Li.; Ma, R.; Wang, J.; Yang, H.; Lu, B. An Ultrafast and Highly Stable PotassiumOrganic Battery. Adv. Mater. 2018, 30, 1805486. (11) Zhang, Q.; Wang, Z.; Zhang, S.; Zhou, T.; Mao, J.; Guo, Z. Cathode Materials for Potassium-Ion Batteries: Current Status and Perspective. Electrochem. Energ. Rev. 2018, 1, 625. (12) Xiong, P.; Zhao, X.; Xu, Y. Nitrogen-Doped Carbon Nanotubes Derived from MetalOrganic Frameworks for Potassium-Ion Battery Anodes. ChemSusChem 2018, 11, 202-208. (13) Zhao, X.; Xiong, P.; Meng, J.; Liang, Y.; Wang. J.; Xu, Y. High Rate and Long Cycle Life Porous Carbon Nanofiber Paper Anodes for Potassium-Ion Batteries. J. Mater. Chem. A, 2017, 5, 19237-19244. (14) Wang, G.; Xiong X.; Xie, D.; Lin Z.; Zheng J.; Zheng, F.; Li, Y.; Liu, Y.; Yang, C.; Liu, M. Chemically Activated Hollow Carbon Nanospheres as A High-Performance Anode Material for Potassium Ion Batteries. J. Mater. Chem. A, 2018, 6, 24317-24323. (15) Xu, Y.; Zhang, C.; Zhou, M.; Fu, Q.; Zhao, C.; Wu, M.; Lei, Y. Highly Nitrogen Doped Carbon Nanofibers with Superior Rate Capability and Cyclability for Potassium Ion Batteries. Nat. Commun. 2018, 9, 1720. (16) Lei K.; Wang, C.; Liu, L.; Luo Y.; Mu. C.; Li, F.; Chen, J. A Porous Network of Bismuth Used as the Anode Material for High-Energy-Density Potassium-Ion Batteries. Angew. Chem. 2018, 130, 4777-4781. (17) Zhang, Q.; Mao, J.; Pang, W.; Zheng, T.; Sencadas, V.; Chen, Y.; Liu, Y.; Guo, Z. Boosting the Potassium Storage Performance of Alloy-Based Anode Materials via Electrolyte Salt Chemistry. Adv. Energy Mater. 2018, 8, 1703288.


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(18) Ji, X.; Lee, K.; Nazar, L. A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium–Sulphur Batteries. Nat. Mater. 2009, 8, 500. (19) Xin, S.; Yin, Y.; Guo, Y.; Wan, L. A High-Energy Room-Temperature Sodium-Sulfur Battery. Adv. Mater. 2014, 26, 1261. (20) Lu, X.; Bowden, M.; Sprenkle, V.; Liu, J. A Low Cost, High Energy Density, and Long Cycle Life Potassium-Sulfur Battery for Grid-Scale Energy Storage. Adv. Mater. 2015, 27, 59155922. (21) Peng, H.; Huang, J.; Zhang, Q. A Review of Flexible Lithium-Sulfur and Analogous Alkali Metal-Chalcogen Rechargeable Batteries. Chem. Soc. Rev. 2017, 46, 5237-5288. (22) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Li-O2 and Li-S batteries with High Energy Storage. Nat. Mater. 2012, 11, 19. (23) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S. Rechargeable Lithium–Sulfur Batteries. Chem. Rev. 2014, 114, 11751-11787. (24) Fu, C.; Oviedo, M.; Zhu, Y.; von Wald Cresce, A.; Xu, K.; Li, G.; E. Itkis, M.; C. Haddon, R.; Chi, M.; Han, Y.; M. Wong, B.; Guo, J. Diameter Carbon Nanotubes Reveal Enhanced Electrochemical Reactivity. ACS Nano 2018, 12, 9775-9784. (25) Fu, C.; Xu, L.; W. Aquino, F.; v. Cresce, A.; Gobet, M.; G. Greenbaum, S.; Xu K.; M. Wong, B.; Guo, J. Correlating Li+‑Solvation Structure and its Electrochemical Reaction Kinetics with Sulfur in Subnano Confinement. J. Phys. Chem. Lett. 2018, 9, 1739-1745. (26) Zhao, Q.; Hu, Y.; Zhang, K.; Chen, J. Potassium-Sulfur Batteries: A New Member of Room-Temperature Rechargeable Metal-Sulfur Batteries. Inorg. Chem. 2014, 53, 9000-9005.


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(27) Liu, Y.; Wang, W.; Wang, J.; Zhang, Y.; Zhu, Y.; Chen, Y.; Fu, L.; Wu, Y. Sulfur Nanocomposite as a Positive Electrode Material for Rechargeable Potassium-Sulfur Batteries. Chem. Commun. 2018, 54, 2288-2291. (28) Wei, S.; Ma, L.; Hendrickson, K. E.; Tu, Z.; Archer, L. A.; Hwang, J.-Y.; Kim, H. M.; Yoon, C. S.; Sun, Y.-K. Toward High-Safety Potassium-Sulfur Batteries Using a Potassium Polysulfide Catholyte and Metal-Free Anode. ACS Energy Lett. 2018, 3, 540-541. (29) Yu, X. W.; Manthiram, A. A Reversible Nonaqueous Room-temperature Potassium-Sulfur Chemistry for Electrochemical Energy Storage. Energy Storage Materials, 2018, 15, 368-373. (30) Gu, S.; Xiao, N.; Wu, F.; Bai, Y.; Wu, C.; Wu, Y. Chemical Synthesis of K2S2 and K2S3 for Probing Electrochemical Mechanisms in K-S Batteries. ACS Energy Lett. 2018, 3, 2858-2864. (31) Ma, R.; Fan, L.; Wang, J.; Lu, B. Confined and Covalent Sulfur for Stable Room Temperature Potassium-Sulfur Battery. Electrochim. Acta, 2019, 293, 191-198. (32) Wang, L.; Bao, J.; Liu, Q.; Sun, C. Concentrated Electrolytes Unlock the Full Energy Potential of Potassium-Sulfur Battery Chemistry. Energy Storage Materials, 2018, https://doi.org/10.1016/j.ensm.2018.10.004. (33) Hwang, J.-Y.; Kim, H. M.; Sun, Y.-K. High Performance Potassium-Sulfur Batteries Based on a Sulfurized Polyacrylonitrile Cathode and Polyacrylic Acid Binder. J. Mater. Chem. A, 2018, 6, 14587-14593. (34) Xin, S.; Gu, L.; Zhao, N.; Yin, Y.; Zhou, L.; Guo, Y.; Wan, L. Smaller Sulfur Molecules Promise Better Lithium-Sulfur Batteries. J. Am. Chem. Soc. 2012, 134, 18510-18513. (35) Guo, J.; Xu, Y.; Wang, C. Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium-Sulfur Batteries. Nano Lett. 2011, 11, 4288-4294.


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(36) Xu, Y.; Wen, Y.; Zhu, Y.; Gaskell, K.; Cychosz, K. A.; Eichhorn, B.; Xu, K.; Wang, C. Confined Sulfur in Microporous Carbon Renders Superior Cycling Stability in Li/S Batteries. Adv. Funct. Mater. 2015, 25, 4312-4320. (37) Fu, C.; Wong, B. M.; Bozhilov, K. N.; Guo, J. Solid State Lithiation-Delithiation of Sulphur in Sub-nano Confinement: a New Concept for Designing Lithium-Sulphur Batteries. Chem. Sci. 2016, 7, 1224-1232. (38) Steudel, R.; Elemental Sulfur, Sulfur-Rich Compounds II, Springer-Verlag Berlin Heidelberg NY, USA 2003. (39) Ozin, G. A. The Single-Crystal Raman Spectrum of Rhombic Sulphur. J. Chem. Soc. A, 1969, 116-118. (40) Beninghven, A. Chemical Analysis of Inorganic and Organic Surfaces and Thin Films by Static Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Angew. Chem. Engl. 1994, 33, 1023-1043. (41) Liang, C.; Dundney, N. J.; Howe, J. Y. Hierarchically Structured Sulfur/Carbon Nanocomposite Material for High-Energy Lithium Battery. Chem. Mater. 2009, 21, 4724-4730. (42) Tauson, V. L.; Sapozhnikov, A. N.; Shinkareva, S. N.; Lustenberg, E. E. The Nature of the Stability of an Incommensurate 3D Structural Modulation in Baikal lazurite: Experimental Data at 550 °C. Geochem. Int. 2009, 47, 815-830. (43) Sangster, J.; Pelton, A. D. The KS (Potassium-Sulfur) System. J. Phase Equilib. 1997, 18, 82. (44) Jaroudi, O. E.; Picquenard, E.; Demortier, A.; Lelieur, J.; Corset, J. Polysulfide Anions. 1. Structure and Vibrational Spectra of the S22- and S32- Anions. Influence of the Cations on Bond Length and Angle. Inorg. Chem., 1999, 38, 2394-2401.


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FIGURES AND CAPTIONS

+ Sucrose

Refluxing

Dehydration

120 ℃, 12h Dehydration Product

6 M H2SO4

Ar/H2 (95%/5%) 1000 ℃, 3h

+ Sulfur

Ar

600 ℃, 5 h

200 ℃, 5 h C/S composite

Sx (x≤3)

S8

Microporous Carbon

Figure 1. Schematic illustration for the formation of microporous C/S composite.


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Figure 2. (a) SEM and (b-c) TEM images of the microporous carbon. (d) SEM and (e-f) TEM images of the microporous C/S composite. (g-i) Elemental mapping images of C and S for the marked region in (g). (j) SAED image of the microporous C/S composite.


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Figure 3. XRD patterns (a) and Raman spectra (b) of pure sulfur, porous carbon and the microporous C/S composite. (c) The TOF-SIMS data of the microporous C/S composite. (d) Schematic illustration of the existing forms of sulfur in the porous carbon matrix: S2 (purple) and S3 (green).


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2400

0.0

0.5 1.0 1.5 2.0 2.5 + Voltage (V vs. K /K)

-1

1.5 1.0

3.0 2.5

+

-1

mA g 20 50 100 200 500 800 1000

1200

2.0

200 2000

discharge charge

600

0

3

6

9 12 15 18 21 24 27 Cycle number

1.5 1.0 0.5 0

500 1000 1500 2000 -1 Specific Capacity (mA h g )

40

0 0

20 -1

20 mA g -1 50 mA g -1 100 mA g -1 200 mA g -1 500 mA g -1 800 mA g -1 1000 mA g -1 2000 mA g

2000

60

discharge charge

600

2500

(e)

500 1000 1500 -1 Capacity (mAh g )

80 -1

20 mA g

1200

1st 2nd 5th

0.5 0

3.0

(d)

1800

0

2.0

1800

30

60 90 Cycle number

20 120

Coulombic efficiency/%

-150 -200

Capacity (mAh g )

1st 2nd 5th

(c) 100

2.5

+

-50

-100

-1

Voltage (V vs. K /K)

0

2400

(b)

0 150

(f)

16 12

-Z'' (kΩ)

3.0

(a)

Voltage (V vs K /K)

Current (μA)

50

Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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fresh 1st 5th 50th 100th

8 4 0 0

4

8 12 Z' (kΩ)

16

20

Figure 4. Electrochemical performance of the microporous C/S composite: (a) cyclic voltammograms at a scan rate of 0.01 mV s−1 between 0.01 and 3.0 V, (b) galvanostatic charhedischarge profiles at 20 mA g−1, (c) cycling performance and Coulombic efficiency at 20 mA g−1, (d) rate performance, (e) the corresponding charge-discharge profiles at different current density, and (f) electrochemical impedance spectra collected at 3.0 V at different cycles.


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(c)

S22-(K2S2)

S 2p

S2-(K2S)

Intensity (a.u.)

RSOx

(f)

S-S

S 2p S2-+S22-

RSOx

176 172 168 164 160 156 Binding Energy (eV)

Figure 5. Morphologies and structure of the microporous C/S composite electrodes disassembled at different conditions. The tests state of electrode in (a-c) is discharged to 0.5 V in the first cycle: (a) SEM, (b) TEM images and (c) the high-resolution S 2p XPS spectrum at fully discharged state. The tests state of electrode in (d-f) is charged to 3 V in the first cycle: (d) SEM, (e) TEM images and (f) the high-resolution S 2p XPS spectrum at fully charged state. 50 E per S atom(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 -50 K2S6

-100 -150

K2S5

-200 0.0

0.5

K2S3

K2S2

1.0 1.5 K content (x) in KxS

K2S

2.0

Figure 6. Formation energies of potassiation sulfides by theoretical calculations.


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Table 1. Sulfur retention of the microporous carbon/sulfur composite under the electron beam irradiation in TEM. Samples

Electron dose rate (A m−2)

Irradiation time (min)

Sulfur retention (wt%)

Pure S

7.43

20

1.9

C/S composite

74.6

0

31.7

C/S composite

74.6

30

32.3

C/S composite

796

20

31.3

C/S composite

35000

20

24.3

*S content retention after irradiation of electron beam at different electron dose rates and different irradiation time.


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