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Hierarchical clews of carbon nanobelts (CsCNBs) with ultrahigh specific surface area (2300 m g) and large pore volume (up to 1.29 cm g) has been successfully fabricated through carbonization and KOH activation of phenolic resin based nanobelts. The product possesses hierarchically porous structure, three-dimensional conductive network framework, and polar oxygen-rich groups, which are very befitting to load sulfur leading to excellent cycling stability of lithium-sulfur batteries. The composites of CsCNBs/sulfur exhibit an ultrahigh initial discharge capacity of 1245 mA h g and ultralow capacity decay rate as low as 0.162% per cycle after 200 cycles at 0.1 C. Even at high current rate of 4 C, the cells still display a high initial discharge capacity (621 mA h g) and ultralow capacity decay rate (only 0.039% per cycle) after 1000 cycles. These encouraging results indicate that polar oxygen-containing functional groups are important for improving the electrochemical performance of carbons. The oxygen-doped carbon nanobelts have excellent energy storage potential in the field of energy storage. Introduction Lithium batteries are widely used in energy storage power systems such as hydropower, firepower, wind power and solar power plants, electric bicycles, aerospace and many other fields. [1-2]. However, the traditional Lithium batteries cannot satisfy the high requirement of more integrated devices with high energy-consumption as well as the environmental protection concept [3-5]. Lithium-sulfur batteries (LiSBs) are notable for their ultrahigh theoretical energy density (2600 W h kg) that doubles the energy density of lithium-ion batteries [6-8], cheap raw materials and little pollution to the environment [9-10]. Therefore, LiSBs are very promising batteries for energy storage applications. However, there are three major drawbacks for LiSBs which hamper their development in energy storage fields. First, elemental sulfur has poor electrical conductivity, and final reaction products (Li2S2 and Li2S) are also electronic insulators, which are not beneficial to the high-rate performance of batteries [11-14]. Second, lithium polysulfide compounds (Li2S8, Li2S6 and Li2S4) are easily soluble in the electrolyte, resulting in a shuttle effect which easily destroys the electrolyte interface film (SEI film) [12, 15]. Third, sulfur will undergo a volumetric expansion of ~80% during charging and discharging process, which causes the pulverization of the electrode materials and thus damages the batteries [16-17]. In recent ten years, in order to solve the problems in LiSBs, researchers have conducted a lot of research on the modification of composite cathode materials. One of the most important, porous carbons have received great attention and research for its high electrical conductivity, high specific surface area (SSA) [18], high porosity, high chemical and mechanical stability, and ease of preparation. According to its structure, porous carbon is divided into one-dimensional materials (such as carbon nanotubes), two-dimensional materials (such as graphene) and three-dimensional materials (such as hollow carbon spheres). In a variety of porous carbon, hierarchical porous carbons (HPCs) have been more widely researched due to the co-existence of micropore (< 2 nm), mesopore (2-50 nm), and macropore (> 50 nm) [19-22]. Hierarchical pores can make a synergistic effect of in LiSBs. Micropores provide anchoring sites for binding lithium polysulfide. Mesopores are commonly used to enhance the physical properties of microporous or macroporous networks. The smaller mesopores along with the micropores store sulfur and adsorb polysulfides, further slowing down the decay of capacity. The larger mesopores along with macropores promote charge transport and ensure electrolyte infiltration, which thereby improves sulfur utilization of active materials. The macroporous network serves to contain and transport the electrolyte to ensure good electrolyte impregnation. The physisorption characteristic of the pores can effectively improve the cycling stability of LiSBs. In addition, Cui groups [15, 23] studied the interaction and binding energies of various functional groups (such as carbon groups and oxygen-rich groups) and lithium sulfide (LiS) species using density functional theory based on the vinyl polymer. As a result, carbon-based groups, such as alkanes, demonstrate lower binding energy with LiS species (0.30 eV) and oxygen-rich groups containing esters, amides, ketones and ethers, demonstrate higher binding energies between 1.01 and 1.26 eV. Oxygen-containing functional groups have been found to improve the combination with lithium polysulfides. In short, the chemisorption between LiS species and oxygen-rich groups has a potential improvement effect on the cycle-stability of LiSBs. Cheng and colleagues [24] found that the strong chemical combination between sulfur and oxygen-containing functional groups (mainly -OH and C=O) can significantly reduce the dissolution/diffusion of lithium polysulfides on reduced graphene oxide. Further, the cycle stability of the lithium-sulfur battery is improved. KOH activation is a very promising method for the preparation of HPCs, because of its low activation temperature, high carbon yield, well-defined pore size distribution and ultrahigh specific surface area [18]. Using biomass waste (soybean residue) as precursor, Zhou et al. [25] successfully prepared a new type of honeycomb nitrogen-oxygen co-doped porous carbon using KOH activation. The derived porous carbon has a special multi-stage pore structure, a large specific surface area (SSA) (2690.3 m g), a high pore volume (1.34 cm g), and a suitable N, O co-doping. The carbon/sulfur composite [26-31] prepared by the porous carbon (sulfur content of 64.5%) exhibits an excellent cycle stability in LiSBs, and the capacity decay rate after cycling 600 times at a current rate of 1 C is as low as 0.063% per cycle. Therefore, we prepared the hierarchical clews of carbon nanobelts (CsCNBs) by KOH activation for LiSBs (Scheme 1) in this work. KOH activation is employed to increase the SSA, pore volume, and number of polar oxygen-containing functional groups. By controlling the mass ratio of KOH/C, the CsCNBs-800*4 has obtained the highest SSA of 2300 m g and the biggest pore volume of 1.29 cm g. Fundamentally, the CsCNBs with hierarchically porous structure, three-dimensional conductive network framework, and polar oxygen-rich groups are very befitting to load sulfur for the improvement of cycle-stability of LiSBs. Hence, CsCNBs/S composites were prepared. CsCNBs-800*4/S cells have an ultrahigh capacity at low current rates. The initial discharge/charge capacity is 1245/1187 mA h g at 0.1 C. The capacity decay rate is as low as 0.162% per cycle for 200 cycles (voltage window is 1.8-2.8 V). At high current rates (>xxxC), the cells still exhibit an excellent cycle stability. At 4 C, the initial discharge/charge capacity is 621/518 mA h g. After 1000 cycles, the capacity decay rate (only 0.039% per cycle) is extremely low. Scheme 1. A schematic illustration of the preparation of carbon nanobelts. Experimental section Materials Hydroquinone [Analytical reagent (AR), 99%], formaldehyde (AR, 37 wt%), hydrochloric acid (AR, 37wt%), sulfur (AR, 99.9%), CS2 (AR), KOH (GR, 95%) and tetrahydrofuran (THF, AR) were purchased from Shanghai Macklin Company (P. R. China). Synthesis of CsPNBs CsPNBs were synthesized according to ref. 22. In general, 1.65 g of hydroquinone, 2.5 mL of 37 wt% formaldehyde and 115 mL of 10 wt% hydrochloric acid are thoroughly mixed in a 200 mL autoclave with a polytetrafluoroethylene liner. Then, the autoclave was sealed and heated in an oven at 180 oC for 12 hours. The resulting black spongy product was filtered and washed with deionized water. The filter cake was dried under vacuum at 60 oC for 6 h. Finally, 2.0 g of light and dark brown powder was collected. The yield is close to the quantitative value (based on hydroquinone). Fabrication of CsCNBs CsPNBs (1.80 g) was heated to 800 °C (with a heating rate of 5 °C min) under a flow of argon (50 mL min) and then cooled down. This sample was called CsCNBs-800. CsCNBs-800 was chemically activated with KOH. The mixture, consisting of CsCNBs-800 and a certain amount of KOH, was activated under a flow of argon (50 mL min) at 400 °C for 0.5 h and then at 800 °C for 1 h at a heating rate of 5 °C min. The weight ratios of KOH/C were 3, 4 and 5. Therefore, the samples were denoted according to the mass ratio as CsCNBs-800*3, CsCNBs-800*4 and CsCNBs-800*5, respectively. CsCNBs-950-40 was physically activated with CO2 according to ref. 22. The activation time was 40 min. Synthesis of CsCNBs/S composites CsCNBs (0.08 g) and sulfur (0.12 g) were dispersed in CS2 (1 ml) and stirred till totally dry. To ensure a complete infiltration of sulfur into CsCNBs, the mixture was heated at 155 °C for 12 h. Therefore, all composites contain CsCNBs and sulfur are denoted as CsCNBs-800/S, CsCNBs-800*3/S, CsCNBs-800*4/S, CsCNBs-800*5/S and CsCNBs-950-40/S. Meanwhile, the mass fraction of sulfur in each composite was 60%. Similarly, composite (90 wt% of sulfur) was also prepared and named CsCNBs-800*4/S90. Characterization Field emission scanning electron microscopy (FESEM) images were collected via a field emission SU-70 microscope. High-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM2010 electron microscope. High-resolution X-ray diffraction (HR-XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu-Kα radiation (40 kV and 200 mA). Raman spectra were measured using a laser confocal Raman spectrometer quipped with an argon-ion laser (514.5 nm). Measurement of the Brunauer–Emmett–Teller (BET) specific surface areas was performed at 77 K using a low-temperature nitrogen adsorption surface area analyser (ASAP 2020, Micromeritics Ins, USA). The total pore volume an

Optimized Synthesis of Ultrahigh-Surface-Area and Oxygen-Doped Carbon Nanobelts for High Cycle-Stability Lithium-Sulfur Batteries