Abstract
We demonstrate significant improvement of CuO nanowire arrays as anode materials for lithium ion batteries by coating with thin NiO nanosheets conformally. The NiO nanosheets were designed two kinds of morphologies, which are porous and non-porous. By the NiO nanosheets coating, the major active CuO nanowires were protected from direct contact with the electrolyte to improve the surface chemical stability. Simultaneously, through the observation and comparison of TEM results of crystalline non-porous NiO nanosheets, before and after lithiation process, we clearly prove the effect of expected protection of CuO, and clarify the differences of phase transition, crystallinity change, ionic conduction and the mechanisms of the capacity decay further. Subsequently, the electrochemical performances exhibit lithiation and delithiation differences of the porous and non-porous NiO nanosheets, and confirm that the presence of the non-porous NiO coating can still effectively assist the diffusion of Li+ ions into the CuO nanowires, maintaining the advantage of high surface area, and improves the cycle performance of CuO nanowires, leading to enhanced battery capacity. Optimally, the best structure is validated to be non-porous NiO nanosheets, in contrary to the anticipated porous NiO nanosheets. In addition, considering the low cost and facile fabrication process can be realized further for practical applications.
Highlights
CuO nanowires have been predominantly synthesized by a hydrothermal method at low temperatures, which offers the advantages of low cost but at the expense of crystallinity and adhesion[5,8,16]
We first develop a facile process for synthesizing patterned CuO nanowires, with the aim of reducing the adverse effect of volume expansion; this is followed by coating the CuO nanowires with NiO nanosheets
Single crystalline CuO nanowires emerged from the top of the stacked film via a stress-assisted growth mechanism
Summary
The Cu film sequentially turned into Cu2O and CuO, and formed a CuO/Cu2O/Cu stacked film structure. CuO nanowires exhibit single and twin crystal structures, as shown in TEM images and diffraction patterns of Fig. S2 in the supporting information. Growth of the NiO phase in both the CuO/NiO(NiSO4) and CuO/NiO(Ni(NO3)[2] nanosheets is clearly observed in the XRD patterns in Fig. S4 of the supporting information. In comparing the attainable maximum capacity after the first cycle, CuO/NiO(Ni(NO3)2) is higher at 821 mAh g−1, followed by 617 mAh g−1 for the CuO/NiO(NiSO4) nanowires, both are closer to their theoretical capacities and outperform the 400 mAh g−1 of pure CuO This indicates that two types of NiO nanosheets contribute to part of the capacity, and protect the core CuO nanowires from degradation by boosting the chemical stability of the surfaces.
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