FULL PAPER 1800898 (1 of 9) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com Tailoring NiO Nanostructured Arrays by Sulfate Anions for Sodium-Ion Batteries Yingmeng Zhang, Yew Von Lim, Shaozhuan Huang, Mei Er Pam, Ye Wang, Lay Kee Ang, Yumeng Shi,* and Hui Ying Yang* Dr. Y. M. Zhang, Prof. Y. M. Shi International Collaborative Laboratory of 2D Materials for Optoelectronic Science and Technology of Ministry of Education Engineering Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province College of Optoelectronic Engineering Shenzhen University Shenzhen 518060, China E-mail: yumeng.shi@szu.edu.cn Dr. Y. M. Zhang, Y. V. Lim, Dr. S. Z. Huang, M. E. Pam, Dr. Y. Wang, Prof. L. K. Ang, Prof. Y. M. Shi, Prof. H. Y. Yang Pillar of Engineering Product Development Singapore University of Technology and Design 8 Somapah Road, Singapore 487372, Singapore E-mail: yanghuiying@sutd.edu.sg DOI: 10.1002/smll.201800898 structure, nanostructure engineering and carbon hybridization of anode materials are highly desirable for SIBs, [2b,3] and the approaches to control the morphology and structure are fundamentally crucial. Up to now, tremendous efforts have been invested in well-controlled synthesis of functional nanomaterials. [4] Two syn- thetic strategies are commonly used to control the dimensions and morphologies of nanostructured materials. One approach is by applying the templates (hard or soft template) [5] to physically confine the size, dimension, and mor- phology of the crystal grain growth, while the other is by applying appropriate additives (capping agent, surfactant, block copolymer, etc.) [6] to kinetically control the growth rate of spe- cific crystal facets. However, the removal process of templates or organic additives is usually time and energy consuming, accompanied with the risk of introducing heterogeneous impurities. As a result, alternative approaches have been devel- oped to control the morphology and structure with the help of inorganic species, [7] avoiding the complicated and consuming purification process. Hence, developing inorganic salts as the morphology-controlled agents is cost-effective, energy-saving, and environmental-friendly. Additionally, inorganic species usually perform as capping agents that selectively adsorb on specific crystal facets, resulting in anisotropic crystal growth and controlled morphologies. [7b,c,8] Recently, it has been reported that various anions are able to insert into the interlayers of the layered hydroxides, leading to the novel hydroxides with different morphologies. The relative binding energies toward these interlayer anions in hydroxides follow the order: CO 3 2- > SO 4 2- > OH - > F - > Cl - > Br - > NO 3 - . [9] As a divalent anion, sulfate ions (SO 4 2- ) have higher affinity than the monovalent anions, which is more probable creating new complex compounds. [9] Moreover, SO 4 2- ions can influ- ence the crystal growth remarkably when it is being adsorbed on the growing facets, owing to the steric hindrance effect [8b] and strong bridging-bidentate adsorption. [10] As a result, various metal hydroxysulfate (e.g., Ni(SO 4 ) 0.3 (OH) 1.4 , [11] Cu 4 (OH) 6 SO 4 , [12] Co 5 (OH) 6 (SO 4 ) 2 (H 2 O) 4 , [13] etc.) nanomaterials have been pre- pared as a family of layered hydroxides. Therefore, a systematic study on the effects of the SO 4 2- ions on the crystal growth habit and the nanomaterial morphology is necessary. Herein, a sulfate-ion-controlled method is successfully developed to tailor the morphologies and structures of the In this contribution, a novel sulfate-ion-controlled synthesis is developed to fabricate freestanding nickel hydroxide nanoarrays on Ni substrate. As an inorganic morphology-controlled agent, SO 4 2- ions play a critical role in controlling the crystal growth and the nanoarray morphologies, by modu- lating the growth rate of adsorbed crystal facets or inserting into the metal hydroxide interlayers. By controlling the SO 4 2- concentration, the nanostruc- tured arrays are tailored from one-dimensional (1D) Ni(SO 4 ) 0.3 (OH) 1.4 nano- belt arrays to hierarchical β-Ni(OH) 2 nanosheet arrays. With further graphene oxide modification and postheat treatment, the obtained NiO/graphene hybrid nanoarrays show great potential for high-performance sodium-ion bat- teries, which exhibit a cyclability of 380 mAh g -1 after undergoing 100 cycles at 0.5 C and reach a rate capability of 335 mA h g -1 at 10 C. Sodium-Ion Batteries 1. Introduction Sodium-ion batteries (SIBs) are promising low-cost alternative to current lithium-ion batteries (LIBs), considering the suit- able redox potential (-2.71 V vs standard hydrogen electrode) and the sustainability advantages. [1] But the commonly utilized graphite anode for LIBs cannot function well in SIBs, due to the extremely low capacity and unfavorable thermodynamics. [1b] Therefore, numerous attempts have been made to explore high-performance anodes for SIBs, such as hard carbon, alloy-based, metal oxide, sulfide, and organic-based anodes. [1,2] However, the radius of sodium ions (0.102 nm) is relatively larger than that of lithium ions (0.076 nm), resulting in severe volume expansion and low reaction kinetics of the anode materials. [2b,c] Fortunately, since the mechanical stability and sodium storage kinetics are related to the morphology and Small 2018, 14, 1800898