The need for high performance electrochemical energy storage devices has increased due to the fast development of portable electronics and electric vehicles. For decades, lithium ion batteries (LIBs) have been considered as one of the most attractive examples in many electric applications, because of their high energy density and good power performance. However, its limited resource restricts the application of LIBs for large-scale energy storage systems. Among many other candidates, sodium ion batteries (SIBs) have been demonstrated as a promising alternative for its high abundance in earth crust and sea water. In recent years, there were numerous attempts to find new anode materials for high performance SIBs, including carbonaceous materials, metal oxides, phosphides, and alloy-based materials.

Transition metal sulfides have received much interests as promising SIB anode materials because of their high theoretical capacity and good electrical conductivity. Numerous researches have been reported about iron sulfide, cobalt sulfide, and many others with good electrochemical properties. Among them, nickel sulfides have great advantages such as high theoretical capacity, low cost, and environmental friendliness. Especially, nickel disulfide (NiS₂) delivers a theoretical capacity of 873 mAh/g, according to conversion reaction (NiS₂ + 4Na = Ni + 2Na₂S).

However, like other conversion-type transition metal sulfides, NiS₂ suffers from large volume changes which cause structural pulverization problem. To overcome this hindrance, synthesizing the composite with carbon or forming 3D network has been generally adopted. Such modifications help the active material to buffer volume changes and ensure its structural integrity.

Herein, we report the electrochemical properties of NiS₂ nanospheres synthesized by hydrothermal process as an anode for SIB. Compared to micro sized particles, nanospheres have superior rate capability due to shorter Na ion and electron diffusion path during the electrochemical process by offering large contact areas between electrode and electrolyte. Also, it is well known that nanoscale particles prevent the accumulation of internal stresses during volume changes. Therefore, they show the improved cyclability by additional inhibition of crack formation within the particles.

NiS₂ powders were synthesized via a simple hydrothermal method. Ni nitrate hexahydrate (Ni(NO)₃6H₂O) and L-cysteine (C₃H₇NO₂S) were used for nickel and sulfur source respectively. First, they were dispersed in DI water and hexadecyltrimethylammonium bromide (CTAB, C₁₉H₄₂BrN) was added as surfactant. Then the mixture solution was transferred to stainless steel autoclaves and maintained at 160 °C for 12 h. After they were cooled to room temperature, the mixture was filtrated and the separated black precipitate was dried at 50 °C under vacuum. The resulting powder was confirmed to be NiS₂ by XRD with particle size of ~120 nm.

The first discharge and charge curves are shown in Fig. 1. The first discharge capacity achieved was 837 mAh/g, which is 96 % of the theoretical value. The first charge capacity was 696 mAh/g, corresponding to the coulombic efficiency of 83%.

Based on these results, we tried various methods such as carbon coating, to improve electrochemical properties of NiS₂ nanospheres.