Scalable synthesis of sub-100 nm hollow carbon nanospheres for energy storage applications

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1 Electronic Supplementary Material Scalable synthesis of sub-100 nm hollow carbon nanospheres for energy storage applications Hongyu Zhao 1,2,, Fan Zhang 2,, Shumeng Zhang 1, Shengnan He 1, Fei Shen 2, Xiaogang Han 2 ( ), Yadong Yin 3, and Chuanbo Gao 1 ( ) 1 Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi an Jiaotong University, Xi an , China 2 Center of Nanomaterials for Renewable Energy, Key Lab of Smart Grid of Shaanxi Province, State Key Laboratory of Electrical Insulation and Power Equipment, Xi an Jiaotong University, Xi an , China 3 Department of Chemistry, University of California, Riverside, CA 92521, USA Hongyu Zhao and Fan Zhang contributed equally to this work. Supporting information to Figure S1 Effect of polymerization time of RF on the morphology of the hollow carbon nanospheres. (a c) TEM images of the hollow carbon nanospheres with RF polymerized after different lengths of time: (a) 2 h, (b) 12 h, and (c) 24 h. It is inferred that 24 h of polymerization is required for a uniform coating of RF on the silica nanospheres. Address correspondence to Chuanbo Gao, gaochuanbo@mail.xjtu.edu.cn; Xiaogang Han, xiaogang.han@xjtu.edu.cn

2 Figure S2 Effect of amino group functionalization on the coating of RF on the silica nanospheres. (a) TEM image of the product after coating of RF on the silica nanospheres without amino group functionalization. The coating of RF failed under this condition. (b d) TEM images of the hollow carbon nanospheres by templating of amino group functionalized silica nanospheres. The volumes of APS were 20 (b), 50 (c) and 150 µl (d) for the synthesis of the respective silica nanospheres. Figure S3 FTIR of the hollow carbon nanospheres obtained by carbonization at 600 C, showing the retention of phenolic groups.

3 Nano Res. Figure S4 TEM image of the hollow carbon nanospheres synthesized without silica protection before the thermal carbonization process at 600 C. Aggregates of the hollow carbon nanospheres can be observed, instead of well dispersed ones, which can be attributed to the sintering of RF/carbon during the thermal treatment. Figure S5 Control of the thickness of the hollow carbon nanospheres by tuning the amount of resorcinol in a typical synthesis. The amounts of the resorcinol were 2.27 (a), 4.5 (b) and 6.8 mmol (c), respectively. Nano Research

4 Figure S6 Raman spectra of the hollow carbon nanospheres obtained under different carbonization conditions. After carbonization at 1000 C for 3 h, the hollow carbon nanospheres showed increased intensity ratio of the G/D bands, relative to that of the nanospheres carbonized at 600 C, confirming an increased graphitization degree of the carbon at an elevated temperature. When the carbonization process (1000 C) was prolonged from 3 h to 12 h, no significant change can be observed in the G/D ratio, indicating that the graphitization of carbon cannot be effectively improved by solely relying on the prolonged carbonization time. Figure S7 TEM images of the sub-100 nm silica nanospheres synthesized in reverse micelles, when the volume of the TEOS was increased to 5, 6 and 10 ml in a typical experiment (see Experimental Section). Inset: Size distributions of the silica nanospheres; unit: nm. The average sizes were denoted as mean ± standard deviation. With increasing volume of TEOS, the size of the silica nanospheres becomes larger. However, significant aggregation of the silica nanospheres was observed, which can be attributed to incomplete hydrolysis of silicate in the presence of insufficient amount of water (628 μl in a typical synthesis), and thus increased hydrophobicity of the silica nanospheres in a polar aqueous environment. It is worth noting that the volume of water cannot be significantly increased to maintain the spherical micelles in the reverse micelle system. Therefore, it is difficult to synthesize uniform silica nanospheres of large sizes without sacrificing their dispersity by the reported method.

5 Nano Res. Figure S8 Chemical stability of the sub-100 nm hollow carbon nanospheres (carbonized at 600 C) in acid and base. (a) TEM image of the hollow carbon nanospheres after treatment in NaOH (2 M) for 48 h at 50 C. (b) TEM image of the hollow carbon nanospheres after treatment in HCl (1 M) for 48 h at 50 C. Figure S9 Thermal stability of the sub-100 nm hollow carbon nanospheres (carbonized at 600 C). (a) Thermal gravimetric analysis (TGA) of the hollow carbon nanospheres in air. (b, c) TEM images of the hollow carbon nanospheres after calcination in air at 300 and 400 C, respectively, for 12 h. These results indicate that the hollow carbon nanospheres stayed stable at temperatures below ~ 400 C in ambient air, thus are applicable in many aerobic applications at relatively high temperatures. Nano Research

6 Nano Res. Figure S10 TEM image of the hollow carbon nanospheres (carbonized at 600 C) after boiling in water (100 C) for 12 h. The hollow nanostructure has been well retained, confirming their high hydrothermal stability. Figure S11 Voltage profiles for the 1st cycle of the electrode fabricated with hollow carbon nanospheres (carbonized at 1000 C) cycled between 0.01 and 2.0 V vs Li/Li+ at a current density of 500 ma g 1. Figure S12 TEM images of the hollow carbon nanospheres in LIB after 1000 cycles (a) and SIB after 200 cycles (b), respectively. The hollow structure of the carbon nanospheres was successfully retained.

7 Figure S13 Cycle performance of the commercial graphite for the LIB (a) and SIB (b). The capacities of commercial graphite were tested under the same conditions as Fig. 5. At a current density of 500 ma g 1, commercial graphite delivers a low capacity of mah g 1 for LIB at the first cycle (a). The capacity gradually increases upon cycling, which suggests an obvious delay of electrochemical reaction due to slow kinetics. The maximal capacity of commercial graphite reaches 300 mah g 1 after 100 cycles, which is still much lower than that of the hollow carbon nanospheres (375 mah g 1 after 100 cycles). In addition, the commercial graphite exhibits a very limited capacity for SIB, less than 15 mah g 1 (b), which is significantly lower than that of the hollow carbon nanospheres (166 mah g 1 after 100 cycles). Figure S14 Rate performance of the hollow carbon nanospheres (carbonized at 1000 C) in LIB. The hollow carbon nanospheres show excellent kinetics and rate performance, which deliver 470 mah g 1, 375 mah g 1, 315 mah g 1 and 255 mah g 1 at 200 ma g 1, 500 ma g 1, 1000 ma g 1 and 2000 ma g 1, respectively. It indicates great electrochemical kinetics of the carbon electrode arising from the short diffusion distance and excellent electronic conductivity of the sub-100 nm hollow carbon nanospheres. Figure S15 Voltage profiles for the 1st cycle of the electrode fabricated with hollow carbon nanospheres (carbonized at 1000 C) cycled between 0.01 and 2.0 V vs Na/Na + at a current density of 100 ma g Nano Research