Porous CuO nanowires as the anode of rechargeable Na-ion batteries

We report the preparation of porous CuO nanowires that are composed of nanoparticles (~50 nm) via a simple decomposition of a Cu(OH)₂ precursor and their application as the anode materials of rechargeable Na-ion batteries. The as-prepared porous CuO nanowires exhibit a Brunauer–Emmett–Teller (BET) surface area of 13.05 m²·g⁻¹, which is six times larger than that of bulk CuO (2.16 m²·g^–1). The anode of porous CuO nanowires showed discharge capacities of 640 mA·h·g^–1 in the first cycle and 303 mA·h·g^–1 after 50 cycles at 50 mA·g^–1. The high capacity is attributed to porous nanostructure which facilitates fast Na-intercalation kinetics. The mechanism of electrochemical Na-storage based on conversion reactions has been studied through cyclic voltammetry, X-ray diffraction (XRD), Raman spectroscopy, and high resolution transmission electron microscopy (HRTEM). It is demonstrated that in the discharge process, Na⁺ ions first insert into CuO to form a ${\mathrm{C}\mathrm{u}}_{1-x}^{Ⅱ}{\mathrm{C}\mathrm{u}}_x^{Ⅰ}{\mathrm{O}}_{1-x/2}$ solid and a Na₂O matrix then ${\mathrm{C}\mathrm{u}}_{1-x}^{Ⅱ}{\mathrm{C}\mathrm{u}}_x^{Ⅰ}{\mathrm{O}}_{1-x/2}$ reacts with Na⁺ to produce Cu₂O, and finally Cu₂O decompose into Cu nanoparticles enclosed in a Na₂O matrix. During the charge process, Cu nanoparticles are first oxidized to generate Cu₂O and then converted back to CuO. This result contributes to the design and mechanistic analysis of high-performance anodes for rechargeable Na-ion batteries.