High-rate and ultra-stable Na-ion storage for Ni 3 S 2 nanoarrays via self-adaptive pseudocapacitance Jun Tang a , Shibing Ni a, b, c, * , Dongliang Chao b , Jilei Liu b , Xuelin Yang a, ** , Jinbao Zhao c a College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, 443002, PR China b School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore c State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, PR China article info Article history: Received 3 November 2017 Received in revised form 27 December 2017 Accepted 29 January 2018 Available online 31 January 2018 Keywords: Self-adaptive Pseudocapacitance Nickel sulphide Nanoarray Sodium ion battery abstract Ni 3 S 2 nanoarrays directly growing on Ni foam are fabricated via an electrochemical corrosion method and used as freestanding electrode for sodium ion batteries. Based on high electronic conductivity facilitated by the 3D Ni backbone and fast surface redox reactions rendered by the ultrathin thickness of the Ni 3 S 2 nanoarrays, high pseudocapacitive contribution for the charge storage is induced in the Ni 3 S 2 - Ni electrode. Remarkably, the capacitive contribution is self-adaptively enhanced in cycling owing to the gradually reduced and stabilized charge transfer resistance, triggering exceptional electrochemical per- formance. The Ni 3 S 2 -Ni electrode delivers ultra-stable cycling with charge/discharge capacities of 344.2/ 350.6 mAh g 1 after 200 cycles at 150 mA g 1 as well as high capacity recovery of 427 mAh g 1 after 70 cycles from 150 to 3000 mA g 1 . Meanwhile, practical application for the Ni 3 S 2 -Ni electrode is also preliminarily assessed. It exhibits promising fast discharge/slow charge (750/150 mA g 1 ) performance with initial discharge/charge capacities of 285.4/275.7 mAh g 1 and 244.8/242.2 mAh g 1 after 300 cy- cles. When matching with Na 3 V 2 (PO 4 ) 3 cathode, it delivers discharge capacity of 347.8 mAh g 1 after 180 cycles at 200 mA g 1 . © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Increasing demand for sustainable and clean energy sources has been intensively stimulated owing to the exhaustion of traditional nonrenewable fossil fuel and serious environment pollution during the past few decades. Both energy harvesting from nature, and storing via batteries and supercapacitor technologies are the center of current research worldwide. Li-ion batteries (LIBs) have become a favorable power source for portable electronics and electric/ hybrid electric vehicle (EV/HEV) because of their long cycle life, low cost and high safety [1e3]. While the present LIBs technologies are quite mature, sodium-ion batteries (SIBs) caught up from behind are now receiving increasing attention partly due to the wide availability of Na sources and thus potentially low cost [4e10]. Three categories of anode materials for SIBs can also be distin- guished as insertion, alloying and conversion type, respectively, according to the charge/discharge mechanisms which is similar to that for LIBs. While insertion type anode materials perform poorly because of bigger size and sluggish kinetics of Na ions, and alloying type anode materials suffer from severe volume expansion [11e 14], conversion anode materials have come into the spotlight for research owing to the moderate reaction kinetics and volume variation [15e19]. Typically, Ni 3 S 2 has got attention owing to its low room temperature resistivity of ~1.2  10 4 U cm [20,21], much lower than its oxide counterparts, demonstrating great potential as anode for SIBs [21e24]. For instance, Qin et al. fabricated layered Ni 3 S 2 -RGO via microwave assisted route and subsequent sintering, which shows reversible capacity of 391.6 mAh g 1 after 50 cycles at 100 mA g 1 [22]. Shang et al. prepared PEDOT decorated Ni 3 S 2 , which shows reversible capacity of 280 mAh g 1 after 30 cycles [23]. Despite high reversible capacity achieved up to now, chal- lenges for Ni 3 S 2 SIB electrode are still there, such as i) poor rate capability and unsatisfactory cycling performance, and ii) the practical feasibility of NiS in full cell is unexplored. Capacitive charge storage has been demonstrated effective to * Corresponding author. College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key Labora- tory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, 443002, PR China. ** Corresponding author. E-mail addresses: shibingni07@126.com (S. Ni), xlyang@ctgu.edu.cn (X. Yang). Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2018.01.199 0013-4686/© 2018 Elsevier Ltd. All rights reserved. Electrochimica Acta 265 (2018) 709e716