One-step synthesis of NiCo 2 S 4 ultrathin nanosheets on conductive substrates as advanced electrodes for high-efficient energy storage Jian-Gan Wang a , Dandan Jin a , Rui Zhou a , Chao Shen a , Keyu Xie a , Bingqing Wei a, b, * a Center for Nano Energy Materials, State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, 710072, China b Department of Mechanical Engineering, University of Delaware, Newark, DE19716, USA highlights graphical abstract  One-step and low-temperature syn- thesis method is developed to pro- duce NiCo 2 S 4 .  Ultrathin NiCo 2 S 4 nanosheets are grown directly on Ni foams as binder- free anodes.  The NiCo 2 S 4 eNi anode delivers a high specific capacity of 1368 mAh g 1 .  The anode exhibits excellent cycling stability and high-rate capability. article info Article history: Received 5 November 2015 Received in revised form 4 December 2015 Accepted 7 December 2015 Available online xxx Keywords: Ultrathin nanosheet Nickel cobalt sulfide Li-ion battery Electrode material abstract A simple one-step and low-temperature synthesis approach has been developed to grow hierarchical NiCo 2 S 4 ultrathin nanosheets (2e3 nm in thickness) on Ni foam. Owing to the unique nanoarchitecture, the NiCo 2 S 4 nanosheets not only offer abundant electro-active sites for energy storage, but also have good electrical and mechanical connections to the conductive Ni foam for enhancing reaction kinetics and improving electrode integrity. When used as anodes for Li-ion batteries, the NiCo 2 S 4 nanosheets demonstrate exceptional energy storage performance in terms of high specific capacity, excellent rate capability, and good cycling stability. The mild-solution synthesis of NiCo 2 S 4 nanostructures and the outstanding electrochemical performance enable the novel electrodes to hold great potential for high- efficient energy storage systems. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The ever-increasing fossil depletion and the worsening envi- ronmental problems have stimulated a fast development of clean and sustainable energy storage systems, such as rechargeable Li-ion batteries [1,2] It is known that electrode materials play a dominant role in determining the energy storage performance. To explore a high-performance electrode material that can become an alterna- tive replacement to the commercial graphite anode, researchers in science and technology communities have carried out intensive studies on various transition metal oxides (or sulfides, nitrides, and phosphides) in the past decades [3]. Among these promising candidates, metal sulfides, such as co- balt sulfides, nickel sulfides, and molybdenum sulfides, have been considered to be highly applicable for Li-ion batteries, because they * Corresponding author. Center for Nano Energy Materials, State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, North- western Polytechnical University, Xi'an, 710072, China. E-mail address: weib@udel.edu (B. Wei). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2015.12.014 0378-7753/© 2015 Elsevier B.V. All rights reserved. Journal of Power Sources 306 (2016) 100e106