Room-temperature Synthesis of Nonstoichiometric Copper Sulfide (Cu2-Xs) for Sodium Ion Storage

Nonstoichiometric transition metal chalcogenides, characterized by intrinsic vacancy defects and high conductivity, have garnered significant interest for their diverse applications in catalysis, sensing, biomedicine, and energy conversion. Nevertheless, conventional synthesis strategies often necessitate harsh conditions or intricate procedures. It remains challenging to develop a rapid, facile, energy-efficient, and environmental-friendly strategy for the preparation of nonstoichiometric chalcogenides. Herein, we propose a surprisingly efficient yet simple method for the preparation of nonstoichiometric face-centered cubic (fcc) Cu2-xS (0 < x < 1) nanoparticles, which are p-type semiconducting and non-toxic, by simply mixing aqueous solutions of Cu2+ with excess S2-/HS- at room temperature. The Cu2-xS is resulted from the redox reaction between the Cu2+ and excess S2-/HS- with S-2(2-) as the side product, as has been demonstrated by the color change and the UV-Vis characterization of the supernatant. Moreover, the cyclic utilization of the excess S2-/HS- for repeatedly synthesizing Cu2-xS is demonstrated. In contrast, the mixing of similar amounts of Cu2+ and S2-/HS- produces hexagonal CuS through the well-known precipitation reaction. The cubic Cu2-xS exhibits outstanding rate capability and cycling stability as an anode material for sodium ion batteries, maintaining high specific capacities of 288 and 237 mA h g(-1) at rates of 2 and 5 A g(-1) respectively after 3000 cycles. Density functional theory (DFT) calculations unveil the exceptional Na+ storage properties of the as-prepared cubic Cu2-xS, attributing them to its elevated structural stability. Moreover, the substantiation of a reduced Na+-diffusion barrier energy provides theoretical reinforcement to these observations. The inorganic synthesis chemistry reported in this work paves a new pathway for the preparation of nonstoichiometric transition metal sulfides. In addition, the exceptional sodium-ion storage properties and the related understanding offer novel insights for optimizing the ion storage performances of transition metal chalcogenides.