A layer-by-layer assembly route to [Mn1/3Co1/3Ni1/3]O2 hollow spheres with electrochemical activity

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Abstract

The monolayered nanosheets of layered [Mn1/3Co1/3Ni1/3]O2 are prepared by the exfoliation of protonated Li[Mn1/3Co1/3Ni1/3]O2 compound and applied as a precursor for synthesizing the hollow spheres of manganese cobalt nickel oxide. The hollow spheres of layered [Mn1/3Co1/3Ni1/3]O2 are synthesized via the layer-by-layer (LBL) coating of the nanosheets and polycations on the polystyrene (PS) beads and the following heat-treatment. According to powder X-ray diffraction and diffuse reflectance UV–vis spectroscopy, the layered crystal structure and optical property of [Mn1/3Co1/3Ni1/3]O2 nanosheets are well-maintained upon the coating on the PS beads. The high-resolution transmission electron microscopy clearly demonstrates the hollow sphere morphology of the obtained material. The electrode functionality of the [Mn1/3Co1/3Ni1/3]O2 hollow spheres is verified by cyclic voltammetry.

Highlights

► The hollow spheres of layered [Mn1/3Co1/3Ni1/3]O2 are for the first time synthesized. ► A coating of [Mn1/3Co1/3Ni1/3]O2 nanosheets on template PS bead produces hollow spheres. ► The electrode activity of the obtained hollow spheres is verified.

Introduction

Layered transition metal oxides receive intense research activity because of their promising functionality as electrode materials for lithium ion batteries [1], [2]. Judging from energy density, working potential, and electrochemical stability, oxides of 3d transition metal ions like cobalt, nickel, and manganese are promising candidates for cathodes for lithium rechargeable batteries [1], [2]. Currently layered LiCoO2 is successfully commercialized as a cathode material for Li ion batteries [3]. However, the high price and high toxicity of cobalt ions evoke a great deal of research efforts to replace LiCoO2 with new cathode materials having better performance and lower Co content [4], [5]. The layered Li[Mn1/3Co1/3Ni1/3]O2 attracts prime interest as an alternative cathode material because of its larger capacity, higher thermal stability, and lower cobalt content than the LiCoO2 [6], [7]. For many electrode materials, the formation of nanostructure is effective in improving their electrode performance, which is due to the shortening of Li ion diffusion path and the provision of more reaction sites [8]. Also attempts are made to synthesize nanostructured Li[Mn1/3Co1/3Ni1/3]O2 electrode materials such as 0D nanocrystals, 0D core–shell particles, and 3D mesoporous materials [9], [10], [11]. Recently our group reports the synthesis of the 2D nanosheets of layered [Mn1/3Co1/3Ni1/3]O2 via the exfoliation of the pristine layered metal oxide [12]. Since the exfoliated nanosheets of [Mn1/3Co1/3Ni1/3]O2 possess negative surface-charge and very thin layer thickness, they can be used as a building block to synthesize novel hybrid materials and can be transformed into other type of nanostructured materials. In one instance, the layer-by-layer (LBL) coating of [Mn1/3Co1/3Ni1/3]O2 nanosheets and positively charged species on the polymer beads is expected to yield porous materials with hollow sphere morphology. But at the time of the publication of the present study, we are aware of no other reports on the synthesis of the hollow sphere of [Mn1/3Co1/3Ni1/3]O2.

In the present study, the hollow spheres of layered [Mn1/3Co1/3Ni1/3]O2 materials are synthesized by the LBL coating of [Mn1/3Co1/3Ni1/3]O2 nanosheets and polyethyleneimine (PEI) polycations on the polystyrene (PS) beads, which is followed by heat-treatment at elevated temperature. The crystal structure, optical property, and crystal morphology of the PS–[Mn1/3Co1/3Ni1/3]O2–PEI core–shells and the [Mn1/3Co1/3Ni1/3]O2 hollow spheres are investigated with X-ray diffraction (XRD), diffuse reflectance UV–vis spectroscopy, and high resolution-transmission electron microscopy/selected area electron diffraction (HR-TEM/SAED), respectively. The electrochemical property of the obtained hollow sphere material is examined to probe its functionality as the electrode material for lithium secondary batteries.

Section snippets

Synthesis

The aqueous colloidal suspension of [Mn1/3Co1/3Ni1/3]O2 2D nanosheets was synthesized by the exfoliation of layered Li[Mn1/3Co1/3Ni1/3]O2 compound; the exfoliation of layered Li[Mn1/3Co1/3Ni1/3]O2 could be done by the intercalation of tetramethylammonium (TMA) cations into the protonated Li[Mn1/3Co1/3Ni1/3]O2 [12]. For the fabrication of hollow sphere of [Mn1/3Co1/3Ni1/3]O2, PS beads were used as template. As illustrated in Fig. 1, the PS beads were coated by immersing them in an aqueous

Powder XRD and diffuse reflectance UV–vis spectroscopic analyses

The crystal structures of template PS and PS–[Mn1/3Co1/3Ni1/3]O2–PEI core–shell are examined with powder XRD analysis. As presented in the top panel of Fig. 2, the core–shell particles show (00l) reflections at low angles, which are absent in the XRD pattern of bare PS beads. This observation indicates the formation of layered structure via the LBL stacking of negatively charged [Mn1/3Co1/3Ni1/3]O2 nanosheets and PEI polycations. As can be seen clearly from the inset of Fig. 2, the core–shell

Conclusions

In the present study, novel hollow spheres of [Mn1/3Co1/3Ni1/3]O2 are synthesized by the LBL assembly of exfoliated [Mn1/3Co1/3Ni1/3]O2 nanosheets and polyelectrolytes on PS bead templates and the subsequent thermal removal of the polymer cores. The formation of hollow sphere of layered [Mn1/3Co1/3Ni1/3]O2 is confirmed by powder XRD, diffuse reflectance UV–vis, and HR-TEM analyses. The obtained hollow sphere clearly possesses electrochemical activity, suggesting their functionality as electrode

Acknowledgments

This research was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST)(NRF-2010-C1AAA001-2010-0029065) and by the Ewha Global Top5 Grant 2011 of Ewha Womans University

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These two authors contributed equally to this work.

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