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