Elsevier

Electrochimica Acta

Volume 55, Issue 28, 1 December 2010, Pages 8521-8526
Electrochimica Acta

Submicron-sized cube-like α-Fe2O3 agglomerates as an anode material for Li-ion batteries

https://doi.org/10.1016/j.electacta.2010.07.078Get rights and content

Abstract

Submicron-sized cube-like α-Fe2O3 agglomerates were successfully fabricated via hydrothermal technique. The material showed a high reversible capacity of 900.2 mAh g−1 and excellent capacity retention of 88.9% after 35 cycles at a current density of 40 mA g−1. The initial columbic efficiencies of the as-prepared powder were 82.65 and 80.57% at current densities of 40 and 80 mA g−1, respectively, which is higher than that of other α-Fe2O3 electrodes reported so far. We believe that the small crystal size and the high structure stability are responsible for the drastic improvement in initial coulombic efficiency and reversibility.

Introduction

Lithium-ion batteries with high energy density have long been used as power sources for portable electronic devices, such as cellular phones, laptop computers, and cameras. With the improvement of their safety and high rate discharge capability, they are also becoming common power sources for electric vehicles (EV), hybrid electric vehicles (HEV), and power tools. Carbonaceous substances such as graphite are widely used in commercial lithium-ion batteries as anode materials. However, because of their relatively low capacity (e.g. 372 mAh g−1 for graphite), a worldwide effort has been made to search for alternative anode materials for lithium batteries during the past decade [1], [2], [3], [4], [5], [6], [7], [8]. It has been reported that nanoparticles of 3d transition-metal oxides (MO, where M is Co, Ni, Cu, or Fe) exhibit reversible capacities about three times larger than those of graphite at a relatively low potential, which has greatly prompted intensive research in this field [9], [10]. Among those transition-metal oxides, iron oxide is a relatively inexpensive material with low environmental impact that can be used as a promising anode material for lithium-ion batteries. Previous studies by Larcher et al. [11], [12], Morales et al. [13], Chen et al. [14], and our group [15], [16] have clearly indicated that the morphology of the nanostructured Fe2O3 plays a vital role in determining the discharge characteristics. Larcher et al. [11] investigated the effect of particle size on lithium intercalation into α-Fe2O3 and concluded that nanosized α-Fe2O3 had better electrochemical performance than micron-sized samples. They found that 0.5 moles of Li could be reversibly intercalated into nanoparticles (20 nm) of α-Fe2O3 in the potential range of 1.5–4.0 V (vs. Li). When discharged to 0.9 V (vs. Li), up to 2 moles of Li can be intercalated, which cannot be electrochemically extracted during the charging process without destroying the crystal structure. Further deep discharge down to 0.005 V can result in the reaction of 8.5 moles of Li with 1 mole of Fe2O3 [11]. However, deep discharge induced destruction of the crystal structure and formation of nanosized Fe0 particles, Li2O, and a polymer layer on the Fe0, as a result of the decomposition of the solvents in the electrolyte. The reduction of the Fe2O3 was found to be reversible due to the huge internal contact surface between Li2O and Fe0. According to this previous work, the reaction mechanism of Li with nanostructured Fe2O3 was proposed as follows [11], [12], [13], [14], [17]:Fe2O3 + 2Li+ + 2e  Li2(Fe2O3)Li2(Fe2O3) + 4Li+ + 4e  2Fe0 + 3Li2O2Fe0 + 2xLi2O  2FeOx + 4xLi+ + 4xeSeveral other works also confirmed that the particle size and morphology of hematite nanostructures exert a key influence on their electrochemical performance for lithium storage, therefore, hematite nanostructures with different morphologies and particle sizes have been synthesized in order to enhance the electrochemical performance [11], [12], [13], [14].

Based on the experience we obtained during our previous work on the synthesis of hematite nanostructures with controllable size and shape [15], [16], we herein report a simple hydrothermal procedure to produce submicron-sized cube-like agglomerates composed of 10–20 nm well-crystallized particles of α-Fe2O3. The electrochemical properties of these materials as anode materials for lithium-ion batteries were investigated by using electrochemical impedance and galvanostatic methods. The as-prepared submicron-sized cube-like agglomerates showed high initial coulombic efficiency and good reversibility when they were charged/discharged at a current density of 40 mA/g in the voltage range of 0.01–3.0 V.

Section snippets

Preparation of samples

All the chemical reagents were analytically pure and used without further purification. Poly(ethylene glycol) with an average molecular weight of 600 was employed as surfactant. It is a typical non-toxic, non-immunogenic, non-antigenic, and protein resistant polymer reagent with long polymer chains [18]. In the experiment, a 5 ml of 4 M aqueous solution of FeCl3 (Aldrich) was added dropwise in equivalent molar number to a 10 ml of 2 M PEG-600 (Aldrich) methanol solution under stirring at room

Results and discussions

Hydrothermal synthesis is affected by many factors, such as concentration of the precursor, heating temperature and heating duration. Compared to our previous experiments [15], the lower reaction temperature (120 °C) used in this experiment may be the main reason for the formation of the nano-ellipses. From our own results and the previous reports [15], [16], it is found that nanowires or nanorods are normally formed at higher temperature even at same concentration. The mechanism for the

Conclusions

The drawbacks for transition-metal oxides are their low initial columbic efficiency and poor capacity retention. In this work, the submicron-sized agglomerates, consisting of nanosized primary particles were fabricated via a simple hydrothermal technique. The material exhibited excellent electrochemical performance, with a high reversible capacity of 900.2 mAh g−1, excellent capacity retention of 88.9% after 35 cycles at a current density of 40 mA g−1, and high initial columbic efficiencies of

Acknowledgement

Financial support from the Australian Research Council through an ARC Linkage project (LP0775456) is gratefully acknowledged. Moreover, the authors would like to thank Prof. Paul Munroe for his help with the FESEM and TEM measurements. Finally, the authors also thank Dr. Tania Silver at the University of Wollongong for critical reading of the manuscript.

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