Hierarchically mesoporous carbon nanofiber/Mn3O4 coaxial nanocables as anodes in lithium ion batteries

doi:10.1016/j.jpowsour.2015.01.156

Journal of Power Sources

Volume 281, 1 May 2015, Pages 301-309

https://doi.org/10.1016/j.jpowsour.2015.01.156 Get rights and content

Highlights

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CNF/Mn₃O₄ was prepared via electrospinning and EPD.
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Coaxial CNF/Mn₃O₄ consisted of bark-like Mn₃O₄ shell and CNF core.
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CNF/Mn₃O₄ formed hierarchically mesoporous structure.
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CNF/Mn₃O₄ represented high reversible capacity due to elastic CNF core.

Abstract

Carbon nanofiber/Mn₃O₄ (CNF/Mn₃O₄) coaxial nanocables with a three-dimensional (3D) structure are prepared for lithium ion batteries by electrophoretic deposition on an electrospun CNF cathode followed by heat treatment in air. The bark-like Mn₃O₄ shell with a thickness of 30 nm surrounds the CNFs with a diameter of 200 nm; this hierarchically mesoporous Mn₃O₄ shell consisted of interconnected nanoparticles grows radially toward the CNF core when viewed from the cross-section of the coaxial cables. The charge transfer resistance of the CNF/Mn₃O₄ is much smaller than that of the Mn₃O₄ powder, because of (i) the abundant inner spaces provided via the formation of the 3D coaxial core/shell nanocables, (ii) the high electric pathway for the Mn₃O₄ nanoparticles attained with the 1D CNFs, and (iii) the structural stability obtained through the cushioning effect created by the CNF/Mn₃O₄ coaxial morphology. These unique characteristics contribute to achieving a high capacity, excellent cyclic stability, and good rate capability. The CNF/Mn₃O₄ nanocables deliver an initial capacity of 1690 mAh g⁻¹ at a current density of 100 mA g⁻¹ and maintain a high reversible capacity of 760 mAh g⁻¹ even after 50 charge–discharge cycles without showing any obvious decay.

Introduction

Lithium ion batteries (LIBs) are a promising candidate for use in plug-in electric vehicles, hybrid electric vehicles, and portable electronics [1]. Electrically active 3D transition metal oxides (M_xO_y, M = Ni, Co, Cu, Fe, Mn, etc.) have attracted much attention in the energy industry, because of their high theoretical capacity derived from the unique conversion mechanism to form lithium oxide and metal nanoparticles, which is $M O + 2 L i^{+} + 2 e^{-} = L i_{2} O + M$ , along with their long cycle life and high recharging rates [2], [3], [4], [5]. Among them, Mn₃O₄ is one of the most viable materials for use as an anode material for LIBs, due to its low electromotive force, abundance in natural resources, environmental benignity, and high theoretical specific capacity (937 mAh g⁻¹) compared to graphite (372 mAh g⁻¹) [6], [7], [8], [9], [10], [11]. However, pure Mn₃O₄ exhibits poor lithiation activity and a low electric conductivity of 10⁻⁷ to 10⁻⁸ S cm⁻¹ [10], [12]. Mn₃O₄ suffers from high voltage hysteresis, a large volume change and low conductivity during lithiation and de-lithiation, which results in a low charge/discharge efficiency, rapid capacity fading and poor rate performance. To overcome these problems, conductive carbonaceous material/manganese oxide composites have been used to improve the mechanical properties and electrical conductivity of manganese oxide when it is used as an electrode. In this respect, many studies of manganese oxide have been conducted in an attempt to increase its specific capacity, long-term cyclic performance, and reversible capacity by preparing coaxial manganese oxide/carbon nanotube arrays, manganese oxide-nanoparticle-loaded porous carbon nanofibers, manganese oxide/graphene composites, manganese oxide/MWCNT composites, and manganese oxide/carbon composite nanowires [10], [13], [14], [15], [16], [17].

Electrophoretic deposition (EPD) is used in this study as a facile synthetic technique to coat Mn(NO₃)₂, the precursor of Mn(OH)₂ nanoparticles, on the surface of a CNF cathode under an applied electric field [18], [19], [20]. To the best of our knowledge, this is the first time that this convenient electrochemical technique has been used for this purpose. The EPD process is divided into two steps. Firstly, electrophoresis allows the charged ions in solution to move toward the oppositely charged electrode when an electric field is applied. Then, once the charged ions accumulate at the electrode, their deposition with proper structures can be achieved by controlling the rate of mass transfer. The deposited electrode undergoes crystallization or densification upon the application of a heat treatment process. The 3D hierarchically mesoporous CNF/Mn₃O₄ (coaxial core/shell) nanocables provide both high capacity and electrochemical stability with excellent retention. In general, Mn₃O₄ experiences inelastic deformation due to its low elasticity, whereas the CNFs undergo elastic deformation due to their high elastic modulus [21], [22]. The lithiated Mn₃O₄ shell compresses the CNF core as its volume expands radially via inelastic flow during lithiation; however, the 3D coaxial morphology, which is created by achieving good bonding between the Li/Mn₃O₄ shell and the conductive CNF, prohibits the volume change that causes capacity fading/battery failure. Meanwhile, the 1D CNFs in a well-woven network and porous structure offer a highly conductive pathway for electrons and enable the electrolyte to easily access the Mn₃O₄ anode material. Thus, the 3D structured coaxial core–shell morphology in CNF/Mn₃O₄ nanocables plays an important role in enhancing the electrochemical performance and stability.

The objective in this study is to design novel 3D coaxial CNF/Mn₃O₄ nanocables to obtain high capacity and good electrochemical retention without any obvious decay. The preparation is done by directly coating the Mn(OH)₂ nanoparticles on the surface of the CNFs through electrophoretic deposition (EPD) followed by heat treatment. The mesoporous CNF/Mn₃O₄ nanocables provide a high pore volume due to their 3D woven network morphology, which is essential to improve the electrochemical performance.

Section snippets

Preparation of carbon nanofibers

The polymer solution for electrospinning was prepared by dissolving 10 wt.% polyacrylonitrile (PAN, M_w = 150,000, Aldrich Chemical Co) in N, N-dimethylformamide (DMF) and was stirred gently for 24 h at 60 °C to obtain a homogeneous solution. The electrospinning process was conducted using the system described in a previous work, which is installed with a power supply (NT-PS-35K, NTSEE, Korea) with a variable high voltage [23], [24], [25]. The polymer solution was placed in a 30 ml syringe with

Results and discussion

The process of Mn₃O₄ deposition on the CNFs using a facile electrophoretic deposition (EPD) process is illustrated in Fig. 1. In the EPD process, the CNFs and Pt wire were used as a cathode and anode, respectively. The manganese salt, Mn(NO₃)₂·6H₂O, serves two purposes simultaneously. When an electric field is applied, the Mn²⁺ ions in the Mn(NO₃)₂·6H₂O ethanol solution move toward the surface of the 1D CNFs acting as a cathode; thus, the 1D CNFs become positively charged by forming CNF–Mn²⁺,

Conclusions

We designed and fabricated a novel material consisting of 3D CNF/Mn₃O₄ coaxial nanocables as an anode material for lithium ion batteries using electrophoretic deposition (EPD) on the surface of the CNFs followed by subsequent heat treatment. The bark-like mesoporous Mn₃O₄ shells are deposited onto the CNF surface in a 3D coaxial structure. The interconnected woven network formed by the hierarchically mesoporous Mn₃O₄ shells provides abundant inner spaces, which consequently facilitate the

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A2A01007540).

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Hierarchically mesoporous carbon nanofiber/Mn3O4 coaxial nanocables as anodes in lithium ion batteries

Highlights

Abstract

Introduction

Section snippets

Preparation of carbon nanofibers

Results and discussion

Conclusions

Acknowledgments

J. Power Sources

Electrochem. Commun.

J. Power Sources

J. Power Sources

Electrochim. Acta

J. Eur. Ceram. Soc.

J. Photochem. Photobiol. A

Electrochim. Acta

J. Power Sources

J. Power Sources

J. Power Sources

Angew. Chem. Int. Ed.

Small

Nano Lett.

Hierarchically mesoporous carbon nanofiber/Mn₃O₄ coaxial nanocables as anodes in lithium ion batteries