Environmentally friendly synthesis of crystalline NiS2 and NiS2/S with high utilization efficiency of precursor sulfur atoms enabled by polysulfide chemistry for electrochemical ion storage applications

Nickel disulfide (NiS₂) is a promising electrochemical storage material for varied charge carrier ions. However, previous preparation approaches for NiS₂-based materials have been haunted by the low utilization efficiency of the sulfur atoms in the precursor and the detrimental byproduct of SO₂. Inspired by the unique crystal structure of NiS₂, composed of Ni²⁺ cations and S₂²⁻ anions, herein, we develop an environmentally friendly synthesis method for crystalline NiS₂ with 100% utilization efficiency of precursor sulfur atoms through the precipitation reaction between Ni²⁺ and S₂²⁻, which can happen at varied temperatures and in different solvents. Furthermore, a NiS₂/S composite is in situ formed by the co-precipitation reaction between Ni²⁺ and S₈²⁻, which can also occur at varied temperatures and in different solvents. As demonstration, both the NiS₂ and NiS₂/S have been applied for the electrochemical storage of nonaqueous Na⁺ ions and aqueous Cu²⁺ ions. Owing to the in situ formed elemental sulfur with higher theoretical capacity and the considerable conductivity provided by the framework of NiS₂ in the NiS₂/S composite, it exhibits superior electrochemical performance compared to NiS₂ in terms of capacity, rate, and cycling stability. The polysulfide anions produced in the process of nonaqueous sodium ion storage enhance the electrochemical kinetics, and the sulfur can be converted into Cu₂S through the intermediate of CuS in the process of aqueous copper ion storage. The sodium ion storage capacity of NiS₂/S is retained at 578.5 mAh g⁻¹ after 375 cycles at 2.0 A g⁻¹, and its reversible copper ion storage can sustain a long life of 1500 cycles at 1.0 A g⁻¹. Our work paves the way towards liquid-state inorganic synthesis of metal sulfides composed of metal cations and disulfide anions.