Few-Layer Antimonene: Anisotropic Expansion and Reversible Crystalline-Phase Evolution Enable Large-Capacity and Long-Life Na-Ion Batteries Weifeng Tian, †,# Shengli Zhang, §,# Chengxue Huo, §,# Daming Zhu, † Qingwei Li, † Lei Wang, † Xiaochuan Ren, † Lei Xie, † Shiying Guo, § Paul K. Chu, || Haibo Zeng,* ,§ and Kaifu Huo* ,† † Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China § Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China || Department of Physics and Department of Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China * S Supporting Information ABSTRACT: Two-dimensional (2D) antimonene is a promising anode material in sodium-ion batteries (SIBs) because of its high theoretical capacity of 660 mAh g -1 and enlarged surface active sites. However, its Na storage properties and sodiation/desodiation mechanism have not been fully explored. Herein, we propose the sodiation/ desodiation reaction mechanism of 2D few-layer antimo- nene (FLA) based on results acquired by in situ synchrotron X-ray diffraction, ex situ selected-area electron diffraction, and theoretical simulations. Our study shows that the FLA undergoes anisotropic volume expansion along the a/b plane and exhibits reversible crystalline phase evolution (Sb ⇋ NaSb ⇋ Na 3 Sb) during cycling. Density-functional theory calculations demonstrate that the FLA has a small Na-ion diffusion barrier of 0.14 eV. The FLA delivers a larger capacity of 642 mAh g -1 at 0.1 C (1 C = 660 mA g -1 ) and a high rate capability of 429 mAh g -1 at 5 C and maintains a stable capacity of 620 mA g -1 at 0.5 C with 99.7% capacity retention from the 10th to the 150th cycle. Considering the 660 mAh g -1 theoretical capacity of Sb, the electrochemical utilization of Sb atoms of FLA is as high as 93.9% at a rate of 0.5 C for over 150 cycles, which is the highest capacity and Sb utilization ratio reported so far. Our study discloses the Na storage mechanism of 2D FLA, boosting promising applications of 2D materials for advanced SIBs. KEYWORDS: few-layer antimonene, 2D materials, anisotropic expansion, reversible crystalline-phase evolution, sodium-ion batteries, large capacity T he large demand for portable electronics, electronic vehicles, and large-scale power grid storage has spurred the development of advanced energy storage tech- nologies and systems. 1,2 Among the various energy storage devices, room-temperature sodium-ion batteries (SIBs) have attracted increasing attention as alternatives to current lithium- ion batteries (LIBs) because sodium (Na) is abundant, economical, and readily available worldwide. 3 Since the performance of SIBs significantly depends on the Na storage capability of electrode materials, developing promising electrode materials with rational structures and surface properties is of great importance to promote the performance of SIBs. 4 Inspired by the large-capacity alloy anodes in LIBs, alloy-based anode materials for Na storage have attracted much attention lately. 5,6 In particular, antimony (Sb) is an appealing anode material for SIBs because of its large theoretical capacity of 660 mAh g -1 and low discharge potential of about 0.5 V (vs Na + /Na). 7 Unfortunately, bulk Sb generally shows a quick capacity decay due to the severe pulverization as a result of the Received: December 9, 2017 Accepted: January 25, 2018 Published: January 25, 2018 Article www.acsnano.org Cite This: ACS Nano 2018, 12, 1887-1893 © 2018 American Chemical Society 1887 DOI: 10.1021/acsnano.7b08714 ACS Nano 2018, 12, 1887-1893