Red Phosphorus Nano-Dots on Reduced Graphene Oxide as Flexible High-Performance Anode for Sodium-Ion Batteries Yihang Liu 1, Anyi Zhang 2, Chenfei Shen 2, Qingzhou Liu 2, Xuan Cao 2, Yuqiang Ma 2, Liang Chen 1, Christian Lau 1, Tian-Chi Chen 3, Fei Wei 3 and Chongwu Zhou 1,* 1 Department of Electrical Engineering, 2 Department of Materials Science and Engineering, University of Southern California, Los Angeles, CA 90089, USA 3 Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P. R. China
Reduced graphene oxide preparation: Graphene oxide water suspension was purchased from Graphene Supermarket (Graphene Laboratories Inc.), then was further treated according to modified Hummers method. 1 H 2 SO 4 and H 3 PO 4 were mixed with a volume ratio of 9:1, then 1% wt. graphite flakes (Sigma Aldrich) was added into the mixed acid solution. The mixture was heated to 50 o C and stirred for 10 hours, and then cooled with ice with appropriate amount of H 2 O 2 added. After a one-hour standing, the supernatant liquid was removed and the rest of the mixture was diluted with water, repeating the procedure several times. The remaining solid material was then washed with water, HCl and ethanol several times, and then filtered and dried at 60 o C in air overnight. 100 mg of graphene oxide was loaded in a ceramic boat in a tube furnace followed by a heat treatment at 600 o C for 15 minutes in a gas mixture of H 2 and Ar (5:95 in vol.) under a total flow rate of 300 ml/min for reduction. Free-standing P@RGO film preparation: the free-standing electrodes were prepared through vacuum filtration. A small amount of P@RGO powder was first added to the filtration system to obtain a thin layer of RGO network at the bottom. Then the P@RGO composite was mixed with ethanol, and the mixture was added to form the main part of the film. In order to obtain a smooth film with excellent mechanical properties, the power of the vacuum pump and the material loading rate during the filtration process were carefully adjusted. The obtained film was pressed to increase
the mechanical stability finally. The mass of the film electrodes for the CR 2032 coin cells is around 2.6 mg. Phosphorus-doped RGO preparation: Phosphorus-doped RGO was obtained by a 30-minute post heat-treatment of the P@RGO composite at 600 o C in argon gas flow to remove the phosphorus nano-dots on the RGO sheets. The free-standing phosphorus-doped RGO film preparation is the same as the P@RGO film described above. Bending test: A film with a surface area of 4 cm 3 cm with a thickness of 110 µm was used in tests. The film was first assembled with a piece polyethylene terephthalate (PET) film with a thickness of 1.4 µm as substrate by using small amount of glue. Then the two edges of the film was pasted with silver paste and bonded with indium wire for the testing. Bending radius from flat to 12.3, 8.4, 5.8, 3.4 and 2 mm were used in the bending test. Commercial red phosphorus control electrode preparation: 80 wt. % commercial red phosphorus (Spectrum Chemical Mfg. Corp.), 10 wt. % carbon black and 10 wt. % polyvinylidene fluoride (PVDF) binder was ball milled into slurry with appropriate amount of N-methyl pyrrolidone (NMP) added. The resulted slurry was casted onto an Al foil and dried at 90 o C in air overnight, and then punched into electrodes with the size of CR2032 type cell. The active material mass loading on the Al foil current collector was ~ 1.2 mg/cm 2.
Figure S1. (a,b,c) SEM image of the P@RGO composite at different locations. (d) The enlarged SEM image of the area marked with red dashed rectangle in (c).
Figure S2. (a) The XPS spectrum of P@RGO sample. The high-resolution XPS spectrum of phosphorus 2p (b) and carbon 1s (c). Figure S3. (a) The current-voltage (I-V) profile of pristine RGO and phosphorus-doped RGO samples with a voltage sweeping range from 0 to 1 V. (b) The cycling performance of the pristine RGO and the phosphorus-doped RGO films at a charge/discharge current density of 100 ma/g. (c) the 1 st, 2 nd and 50 th
charge/discharge profiles of phosphorus-doped RGO film electrode. Figure S4. Cycling Performance of the commercial red phosphorus anode at 0.1 C charge/discharge rate (259.6 ma/g).
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