Scalable synthesis of self-standing sulfur-doped flexible graphene films as recyclable anode materials for low-cost sodium-ion batteries Xiang Deng a , Kongyan Xie a , Li Li c , Wei Zhou a , Jaka Sunarso b, ** , Zongping Shao c, d, * a Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, PR China b Faculty of Engineering, Computing and Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350 Kuching, Sarawak, Malaysia c Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engineering, College of Energy, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, PR China d Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia article info Article history: Received 3 April 2016 Received in revised form 20 May 2016 Accepted 22 May 2016 Available online 24 May 2016 abstract The commercialization of anodes for sodium ion batteries (SIBs) requires multitude approaches such as the practicality of the synthesis process, battery performance and the recycling option which most studies overlooked. Herein, we showed the scalable synthesis of self-standing sulfur-doped flexible graphene films as the anode materials for SIBs, demonstrating up to 377 mA h g 1 capacity at 100 mA g 1 current density as well as an excellent rate capability and a moderate decay rate of 0.106% per cycle during long cycling test. The work also shows that sulfur doping creates additional redox sites for re- action with Na þ and induces larger interspacing layers, leading to an enhanced capacity compared to the non-doped graphene. The self-standing nature of the films also eliminates the need for other additional anode components, such as conductive carbon or binder, and more importantly, allows their recovery and subsequent reuse in new applications. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Sodium ion batteries (SIBs) constitute an attractive energy storage technology, which is highly suitable for power grid and other large scale applications. Recently, this technology is regarded as the ideal alternative to lithium ion batteries (LIBs), considering sodium’s natural abundance, environmental benignity and low cost [1,2]. Despite the common chemistry Na and Li shares, the funda- mental differences between the two actually exist, for example, Na has the 0.3 Å larger ionic radius and approximately three times larger mass than Li [3,4]. This may manifest into the incompatibility of commonly used LIB cell components for applications in SIB cells. Graphite, for example, which forms a commercial negative elec- trode (anode) component in current LIB cells given its high theoretical reversible capacity of 372 mA h g 1 and long cycle life, fails to provide identical performance in SIB cells [3,5]. The inter- layer distance of graphite (approximately 0.34 nm) is apparently too small to accommodate the insertion of the large Na ion [6,7]. In attempts to exploit and develop carbon-based anode mate- rials other than graphite, Dahn group has made early efforts to develop high capacity non-graphitized materials, where Na þ essentially enters into and exits from the random stacking layers of the non-graphitized carbon (also known as hard carbon) and the nanopores between the randomly stacked layers [8]. More recently, Yan et al. also demonstrated that another non-graphitized carbon material, i.e., porous carbon, can be combined with graphene via a facile ionothermal route to form the sandwich-like hierarchically porous carbon/graphene (G@HPC) composite. This anode achieved a high specific capacity of 400 mA h g 1 at 50 mA g 1 current density and excellent cycling stability, enabled by high Na þ diffu- sion on the porous carbon structure and fast electron transport on the graphene layers [9]. Reverting back to the graphite based car- bon, the concept of “expanded graphite” was recently reported by Wen et al. where they utilized oxidation and partial reduction processes to attain an enlarged graphite interlayer distance of 0.43 nm. Such distance indeed favors the electrochemical * Corresponding author. Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engineering, College of Energy, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, PR China. ** Corresponding author. E-mail addresses: barryjakasunarso@yahoo.com, jsunarso@swinburne.edu.my (J. Sunarso), shaozp@njtech.edu.cn (Z. Shao). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon http://dx.doi.org/10.1016/j.carbon.2016.05.052 0008-6223/© 2016 Elsevier Ltd. All rights reserved. Carbon 107 (2016) 67e73