Graphene-induced confined crystal growth of octahedral Zn 2 SnO 4 and its improved Li-storage properties
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A Zn2SnO4/graphene (Zn2SnO4/G) hybrid was prepared by a facile one-pot hydrothermal route using SnCl45H2O, ZnSO47H2O, and graphite oxide as the precursors and NaOH as the mineralizer. Microsized Zn2SnO4 crystals with an octahedral shape are firmly confined by the graphene sheets, forming a unique hybrid structure. The confining effect of graphene leads to a more homogeneous size distribution of Zn2SnO4 crystals in Zn2SnO4/G than in bare Zn2SnO4. The introduction of graphene also brings an improved Li-storage performance for Zn2SnO4 due to the combined buffering, conducting, and confining effects of graphene. After being cycled at 200 mA/g for 50 times, Zn2SnO4/G can still keep a charge capacity of 326 mAh/g, while for bare Zn2SnO4, its charge capacity drops to only 100 mAh/g after the same cycles.
I. INTRODUCTION
Recently, a considerable attention has been focused on the lithium metal alloys to meet ever increasing high energy density requirement for Li ion batteries. A special interest has been paid to Sn-based oxides since first reported by Idota et al.1 that tin-based amorphous oxide (Sn1.0B0.56P0.40Al0.42O3.6) could yield a stable capacity over 600 mAh/g. A typical tin oxide, SnO2, can yield a theoretical capacity as high as 781 mAh/g with the reactions: SnO2 1 4Li ! Sn 1 2Li2O, Sn 1 xLi 4 LixSn (x # 4.4).2 Similar to Sn, Zn also showed a Li-storage ability by forming various Li–Zn alloys with a maximum LiZn composition (410 mAh/g).3 A typical zinc-based material, ZnO, also exhibited reversible Li-storage ability.4,5 Besides simple tin or zinc oxides, some mixed oxides such as ZnCo2O4,6 ZnV2O4,7 Co2SnO4,8 and ZnFe2O49,10 with the AB2O4 (or A2BO4) structure also show promising applications as anodes for Li ion batteries. The energy density and working voltage can be tuned by varying the A and B elements.11 These simple or mixed oxides, however, generally exhibit poor electrochemical performance due to the large volume changes upon Li uptake/release reactions. For example, the volume variation of Sn will reach as high as 358% during the Li uptake/release processes. Forming a composite with a matrix of Li inert or with less volume changes has been proved to be a practical strategy to alleviate the volume changes. Previous work showed that the mixed oxides could deliver a stable electrochemical cycling by loading them onto carbon matrices.12–14 The Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.369 J. Mater. Res., Vol. 27, No. 24, Dec 28, 2012
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II. EXPERIMENTAL A. Preparation of Zn2SnO4/G hybrid
GO (60 mg), prepared by a modified Hummer’s method,22 was added into deionized (DI) water under sonication for 3 h to form a uniform solution. Then, 0.5 mmol of SnCl45H2O and 1 mmol of ZnSO47H2O were mixed in
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carbon materials are preferred as the matrices because they not only contribute to the overall capacity but also supply the electronically conducting channels, in addition to the strain buffering effect. Zinc stannate (ZnSn2O4) is
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