ZHU ET AL. VOL. 9 ’ NO. 3 ’ 3254–3264 ’ 2015 www.acsnano.org 3254 March 04, 2015 C 2015 American Chemical Society Red Phosphorus ÀSingle-Walled Carbon Nanotube Composite as a Superior Anode for Sodium Ion Batteries Yujie Zhu, † Yang Wen, † Xiulin Fan, † Tao Gao, † Fudong Han, † Chao Luo, † Sz-Chian Liou, ‡ and Chunsheng Wang * ,† † Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States and ‡ Maryland Nanocenter, University of Maryland, College Park, Maryland 20742, United States L ithium ion batteries (LIBs) currently dominate the power supply for porta- ble electronic devices and have been integrated into (plug-in hybrid) electric vehicles to reduce the energy dependence on fossil fuels. 1 Considering the massive consumption of LIBs in the future, cost will become one of the major issues for LIBs due to the very limited and unevenly distributed lithium resources. 2 Thus, alternative energy storage systems with low cost will be urgently needed for future applications. Recently, room temperature sodium ion batteries (SIBs) have attracted an intensive attention and been considered as a poten- tial alternative to LIBs because of the earth abundance and low cost of sodium com- pared with lithium. 3À5 It is reported that the relative earth abundance of sodium in crust is more than 1000 times higher than that of lithium (23600 ppm for sodium vs 20 ppm for lithium). 6 Given the similarity between lithium and sodium chemistry, an intuitive idea is to directly transfer well-developed electrode materials in LIBs to SIBs with the replacement of lithium by sodium. Unfortu- nately, the most common and commercially successful graphite anode for LIBs was pro- ven to only store a very little amount of sodium due to the insufficient interlayer distance and stretched CÀC bond induced by sodiation, 7À10 which makes the Na- graphite intercalation compound thermo- dynamically unstable. Therefore, a diversity of alternative materials, 11 including nongra- phitized carbon, 12À15 metal oxides, 16À18 and alloy-based materials, 19À24 have been explored as potential anodes for SIBs. Among all proposed anode materials, red phosphorus (P) is a very promising candi- date because it has the highest theoretical sodium storage capacity (∼2600 mAh/g, based on the assumption that P is trans- formed to Na 3 P) and a relatively safer oper- ating potential (∼0.45 V vs Na/Na þ ) than other materials (for example, hard carbon which has a sodiation potential close to 0V vs Na/Na þ ). 25 However, the application of red P in SIBs is handicapped by two factors. First, pristine amorphous red P has * Address correspondence to cswang@umd.edu. Received for review January 17, 2015 and accepted March 4, 2015. Published online 10.1021/acsnano.5b00376 ABSTRACT Sodium ion batteries (SIBs) have been considered as a top alternative to lithium ion batteries due to the earth abundance and low cost of sodium compared with lithium. Among all proposed anode materials for SIBs, red phosphorus (P) is a very promising candidate because it has the highest theoretical capacity (∼2600 mAh/g). In this study, a red PÀsingle-walled carbon nanotube (denoted as red PÀSWCNT) composite, in which red P is uniformly distributed between tangled SWCNTs bundles, is fabricated by a modified vaporization-condensation method. Benefiting from the nondestructive preparation process, the highly conductive and mechanically strong SWCNT network is preserved, which enhances the conductivity of the composite and stabilizes the solid electrolyte interphase. As a result, the red PÀSWCNT composite presents a high overall sodium storage capacity (∼700 mAh/g composite at 50 mA/g composite ), fast rate capability (∼300 mAh/g composite at 2000 mA/g composite ), and stable long-term cycling performance with 80% capacity retention after 2000 sodiationÀdesodiation cycles. The red PÀSWCNT composite fabricated by the vaporizationÀcondensation method significantly extends the cycling stability of P/carbon composite from current ∼100 cycles to ∼2000 cycles. KEYWORDS: sodium ion batteries . anode . phosphorus . carbon nanotubes ARTICLE