Enhanced electroactivity with Li in Fe3O4/MWCNT nanocomposite electrodes

https://doi.org/10.1016/j.jallcom.2014.01.077Get rights and content

Highlights

  • Facile one-pot colloidal synthesis of Fe3O4/MWCNT composites.

  • Favorable utilization of the high surface area by the nanoparticles and in the Li electrochemical reactions.

  • Enhanced electrochemical performance (large reversible capacity, cycle life, and rate capability) of Fe3O4/MWCNT composites.

Abstract

We demonstrate a facile synthetic procedure to obtain Fe3O4/MWCNT (MWCNT = multiwalled carbon nanotube) composite electrode via the one-pot colloidal preparation method at 180 °C with a surface functionalized MWCNT. Fe3O4 nanoparticles were successfully precipitated on the surface of MWCNT with uniform size of approximately 10 nm. Fe3O4/MWCNT composite electrode exhibits a better reversible capacity, cycle life, and rate capability compared with the Fe3O4 powder electrode by higher electronic conductivity and stable structural properties with the support of MWCNT.

Introduction

In recent years, the emergence of various kinds of high-performance portable electronics requires more energy; further, the shortened working-time of devices is exposed by the insufficiency of the batteries’ essentially restrictive energy density. Moreover, some types of the electronic vehicle and energy storage system market, which is represented by a smart grid, were also rapidly increased in order keep up with the recent worldwide eco-friendly policy. Thus, the development necessity of high-energy and high-power electrode material is considered indispensable. There have been various candidates as substitutes for conventional graphitic anode materials of lithium ion batteries due to its low gravimetric energy density (6C + Li+ + e  LiC6, 372 mA h g−1) [1].

The conversion anode material, represented by the transition metal oxide, is a very suitable substitution for replacing the graphitic material. Ever since Poizot et al. reported the conversion reaction in the 3d transition metal oxide (MxOy, M = Co, Ni, Fe, Cu) matrix, where they meet the lithium ion at a specific voltage range [2], the material was improved by the efforts of many researchers. The conversion reaction allows the anode materials to accept a larger amount of lithium ion compared with the insertion/desertion reaction of graphitic materials. The reaction arises through the destruction and reconstruction of an initial metal-oxide structure to the metal particles, dispersed in the amorphous Li2O matrix after a complete reaction with lithium at low voltage. Typically, transition metal oxides are able to react with various amounts of lithium depending on their oxidation number, from 2 of CuO and NiO [3], [4] to 8 of Fe3O4 and Co3O4 [5], [6]. It has an advantage of accessibility for large amounts of lithium.

Fe3O4, magnetite is a typical transition metal oxide anode, which also has a conversion reaction with 8 lithium ions (Fe3O4 + 8Li+ + 8e → 3Fe + 4Li2O, 926 mA h g1) [7]. It is abundant in nature, environmental-friendly, and its nanoparticles can be applied as a contrast agent of magnetic resonance imaging and a carrier of a drug delivery system [7], [8]. However, such high-capacity anode materials have more complex reactions than insertion materials, causing sluggish reaction kinetics [9]. To overcome these disadvantages, several ways have been reported such as formation of various types of nanostructure [10], [11], [12], and composite with another material [13], [14], [15] for increasing reaction efficiency and improvement of cycle life.

Herein, we synthesized Fe3O4/MWCNT nanocomposite using a facile one-pot colloidal process with surface-functionalized MWCNT. Furthermore, we performed heat treatment for the decomposition of surface residual organics in order to form a carbon layer of particles surfaces. The prepared Fe3O4/MWCNT nanocomposite electrode presented high capacity, good rate capability, and long cycle life by enhanced electric conductivity and stress relaxation properties.

Section snippets

Experimental details

The colloidal crystallization process was conducted for the synthesis of Fe3O4/MWCNT nanocomposite. First, pristine MWCNT (CM 100, Hanwha Nanotech) was functionalized by a typical method using a concentrated acidic medium in order to attach the carboxylic functional group unto their surface [16]. Thereafter, 0.1 g of functionalized MWCNT and 0.006 mmol of Fe(acac)3 (99.9%, Sigma–Aldrich) were thoroughly dispersed and dissolved in 100 mL of a mixture of oleylamine (C18H37N, 70%, Sigma–Aldrich) and

Results and discussion

Fig. 1a shows the powder XRD pattern of Fe3O4/MWCNT samples before and after heat-treatment under an inert atmosphere for the decomposition and carbonization of organic residue. As a result of diffraction patterns, every peak well corresponds with the reference powder X-ray diffraction of magnetite (PDF # 01-71-4918), and the calculated size of Fe3O4 nanoparticles (NP) are about 10 nm by the Debye–Scherrer equation. In particular, there is a peak at around 25°, which indicates the presence of

Conclusions

In this work, we prepared the Fe3O4/MWCNT nanocomposites via a one-pot colloidal synthetic route in oleylamine as a solvent and a stabilizer. The Fe3O4 NP were well precipitated on each MWCNT. The composite electrode was electrochemically analyzed and compared with the Fe3O4 NP sample. The Fe3O4/MWCNT composite electrode shows a superior reversible capacity of approximately 1000 mA h g−1 at 0.2C after 30 cycles, and shows a good rate capability of approximately 600 mA h g−1 at 1C. It shows better

Acknowledgement

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2010-0029027 and 2012R1A2A2A01045382).

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