Elsevier

Electrochimica Acta

Volume 92, 1 March 2013, Pages 188-196
Electrochimica Acta

Remarkable enhancement of the electrode performance of nanocrystalline LiMn2O4 via solvothermally-assisted immobilization on reduced graphene oxide nanosheets

https://doi.org/10.1016/j.electacta.2013.01.022Get rights and content

Abstract

A facile solvothermal way to immobilize nanocrystalline LiMn2O4 on the surface of graphene nanosheets is developed to improve the functionality of lithium manganate as lithium intercalation electrode. A solvothermal treatment for the colloidal mixture of graphene oxide (GO) nanosheets and LiMn2O4 nanocrystals gives rise not only to the reduction of GO to reduced graphene oxide (RGO) but also to the immobilization of lithium manganate nanoparticles on the surface of RGO nanosheets. According to powder X-ray diffraction and electron microscopic analyses, the crystal structure and morphology of spinel lithium manganate remain intact upon the composite formation with the RGO nanosheets. The application of larger aldehyde molecule as a reductant leads to the increase of crystallinity and the lowering of Mn oxidation state for the pristine LiMn2O4 and its nanocomposite with the RGO nanosheets. The present LiMn2O4–RGO nanocomposites display promising cathode performances for lithium rechargeable batteries, which are much superior to those of the pristine LiMn2O4 nanocrystals. The observed enhancement of electrode performance upon the composite formation with the RGO nanosheets is attributable both to the improvement of the surface ion transport of nanocrystalline lithium manganate and to the increase of electrical conductivity. The present experimental findings demonstrate that the solvothermal treatment with RGO nanosheets provides an effective way of improving the electrochemical activity of nanocrystalline lithium metal oxides.

Introduction

Many economic and ecological merits of manganese element evoke intense research interest on lithium manganese oxides as alternative electrode materials for lithium secondary batteries [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Among various polymorphs of lithium manganese oxides, spinel-structured LiMn2O4 phase boasts many advantages such as a fast Li+ diffusion in 3D connected diffusion paths, a high tunability of chemical composition, and facile synthesis [2], [11], [12], [13], [14]. For the optimization of the electrode performance of this spinel phase, a great deal of researches are carried out with the control of Li/Mn ratio, chemical substitution, surface coating, morphology control, nanostructure formation, composite formation, and so on [2], [13], [14], [15], [16], [17], [18], [19]. The composite formation with highly conductive carbon species can provide a powerful way of improving the electrochemical performance of electrode materials especially under high current density condition [20], [21], [22]. As an effective support for immobilizing electrode material, graphene, an exfoliated 2D sheet of graphite, attracts prime attention because of its high electrical conductivity and its unique 2D morphology providing many surface sites for the anchoring of metal oxide crystals [23], [24], [25], [26]. Since the graphene is synthesized in the form of the colloidal suspension of reduced graphene oxide (RGO) nanosheets via the chemical reduction of precursor graphene oxide (GO) [27], [28], [29], most of metal oxide–graphene nanocomposites are prepared by the crystal growth of metal oxide crystals on the surface of RGO nanosheets [30], [31]. Such a direct crystal growth method is not readily applicable for multicomponent metal oxides like LiMn2O4. Alternatively, the presence of many hydrophilic functional groups on the surface of GO nanosheets would render an anchoring of presynthesized lithium metal oxide on the surface of GO nanosheet a useful method to prepare lithium metal oxide–graphene nanocomposites.

In the present study, the LiMn2O4–RGO nanocomposites are synthesized by the solvothermal treatment of the precursor LiMn2O4 spinel oxides with aqueous/ethanolic GO suspension. The structural, morphological, and bonding characteristics of the resulting nanocomposites are investigated with the combination of diffraction, microscopic, and spectroscopic tools. The evolution of the electrode performance of lithium manganate nanocrystals upon the composition formation with RGO is also examined.

Section snippets

Synthesis

The cubic spinel LiMn2O4 nanocrystals were prepared by hydrothermal reaction of KMnO4 and LiOH in the presence of reductant organic molecule, i.e. formaldehyde or isobutyraldehyde, at 200 °C for 3 h in Teflon-lined 100 mL hydrothermal vessel [32], [33]. The resulting powdery materials were thoroughly washed with distilled water. The molar ratio of KMnO4/organic reductant was adjusted to unity commonly for both the aldehydes. The obtained LiMn2O4 materials prepared with formaldehyde and

Powder XRD analysis

The effect of the molecular weight of reductant aldehyde molecules on the crystal structures of the pristine LiMn2O4 nanocrystals is examined with powder XRD analysis, together with the evolution of their crystal structures upon the composite formation with RGO nanosheets. Fig. 1 represents the powder XRD patterns of the pristine LMOF and LMOB nanocrystals and their nanocomposites with RGO nanosheets (LMOFG and LMOBG). Both the pristine lithium manganese oxides display typical XRD patterns of

Conclusions

The nanocomposites of LiMn2O4–RGO are synthesized by the solvothermal treatment for the mixture colloidal suspension of GO nanosheets and LiMn2O4 nanocrystals. The formation of RGO as well as the immobilization of LiMn2O4 nanoparticles on the surface of graphene nanosheets occurs during the solvothermal treatment. The crystal structure and morphology of spinel lithium manganate are well-maintained before and after the coupling with the RGO nanosheets. The application of larger isobutyraldehyde

Supporting information

Mn dissolution behavior of the pristine LiMn2O4 nanocrystals and their nanocomposites with RGO nanosheets during the storage in electrolyte solution.

Acknowledgments

This work was supported by the Core Technology of Materials Research and Development Program of the Korea Ministry of Intelligence and Economy (grant no. 10041232), the National Research Foundation of Korea Grant funded by the Korean Government (MEST)” (NRF-2010-C1AAA001-2010-0029065), and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2010-0027517). The experiments at PAL were supported in part by MOST and POSTECH.

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