As the most promising energy storage system, rechargeable lithium ion batteries (LIBs) greatly extend their market from portable electronics to smart grid, hybrid electric vehicles and electric vehicles, owing to their high energy density, long cycle life and environmental friendliness.^[1] Nevertheless, LIBs using graphite as the anode material cannot meet the stringent requirements for these applications because of the limited theoretical capacity of graphitic carbon.^[2] Among various novel anode materials studied to date, nickel sulfides have attracted particular attention due to their attractive high specific capacity and low cost. However, the low electrical conductivity and large volume change during lithium uptake/removal process hinder their practical applications. The common efforts to address the problems mainly focus on the acquisition of nanoscaled materials and embedding them in buffering matrix. Nonetheless, these electrode materials need to be mixed with conductive and binder additives for battery tests, which is time-consuming. Besides, the involved binder may impede the electrolyte penetration and lithium ion transport.

In this work, a binder- and conductive-agent-free electrode with nest-like interwoven Ni₃S₂ nanoplates directly grown on Ni foam (Fig. 1a) was prepared by a facile one-step hydrothermal method, which can be directly used as electrode without any further processing. This design has many advantages required for high performance electrodes. The open space between the interconnected Ni₃S₂ nanoplates can buffer the volume changes caused by electrochemical reaction, enabling the electrode to maintain its integrity on cycling. Additionally, the unique structure that the Ni₃S₂ nanoplates are grown directly on the Ni foam ensures the strong adhesion of active component Ni₃S₂ to the current collector Ni substrate, offering an improved cyclic stability. The porous network structure can permit facile penetration of the liquid electrolyte and thus promote lithium ion diffusion through the electrode film. Meanwhile, the highly conductive Ni matrix makes Ni₃S₂/Ni electrode electrochemically active because the electron can be effectively and rapidly conducted back and forth between the active material and the current collector during the delithiation/lithiation process, efficiently reducing ohmic polarization. As a result, the Ni₃S₂/Ni electrode maintains a high reversible capacity of 623 mAh g^-1 after 150 cycles at a current density of 0.1 A g^-1 and delivers a high reversible capacity of 370 mAh g^-1 at a high current density of 1.5 A g^-1 (Fig. 1b).

Fig.1 FE-SEM image (a) and electrochemical performance (b) of the prepared Ni₃S₂/Ni electrode.

Acknowledgements

This work was financially supported by National Basic Research Program of China (2013CB934003), “863” program (2013AA050902), National Natural Science Foundation of China (21273019), Guangdong Industry-Academy-Research Alliance (2013C2FC0015) and Program of Introducing Talents of Discipline to Universities (B14003).

Reference

[1] Armand M, Tarascon J M. Building Better Batteries. Nature, 2008, 451: 652-657.

[2] Buqa H, Goers D, Holzapfel M, Spahr M E, Novák P. High Rate Capability of Graphite Negative Electrodes for Lithium-Ion Batteries. J. Electrochem. Soc. 2005, 152, A474-A481.