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The aligned freestanding nanorods (NR) of Co3O4 and nanoporous hollow spheres (NHS) of SnO2 and Mn2O3 were investigated as the anodes for Li-ion rechargeable batteries. The Co3O4 nanorods demonstrated 1433 mAh/g of reversible capacity initially and then decreased gradually. The NHS of SnO2 and Mn2O3 delivered energy densities as 400 and 250 mAh/g, respectively, in multiple galvonastatic discharge–charge cycles. The morphologic changes of the nanostructure anodes were investigated. It was found that Co3O4 NR broke down during cycles, but SnO2 NHS still maintained their structural integrity in multiple cycles resulting in sustainable high capacity. The nanostructured metal oxides exhibit great potential as the new anode materials for Li-ion rechargeable batteries with high energy density, low cost, and inherent safety.

I. INTRODUCTION

Recently, the plug-in hybrid or electric vehicles were proposed as one of the solutions for energy independence and clean transportation. The lighter, cheaper, and safer Li-ion batteries are the key for migration from petroleum propulsion to electrification. The theoretic capacity of carbon intercalation anodes currently used in Li-ion rechargeable batteries is limited to 372 mAh/g.1 The new anode materials that can meet major criteria such as high energy density, long cycling life, low cost, and safety will change the whole landscape of Li-ion rechargeable batteries. A number of metals have been investigated for the next generation of high capacity anodes (Table I).2–10 However, the large volume expansion (300–400%) during metal lithiation causes material pulverization resulting in a rapid decrease of initial high capacity. On the other hand, the volume of the most metal oxides shrinks slightly during lithium conversion (Table I). For example, the volumes of Co3O4 and Mn2O3 are reduced 87% and 94% after conversions of Co3O4 þ 8Li ! 3Co þ 4Li2O and Mn2O3 þ 6Li ! 2Mn þ 3Li2O. Although the theoretic energy densities of the metal oxide anodes are generally not as high as metals, they have the inherent advantage over the metallic anodes regarding sustainable high capacity. The aligned nanorods and hollow microspheres of metal oxides were synthesized for various applications such as gas absorption,11 magnetic materials,12 gas sensors,13 photocatalysis,14 capacitors,15 and batteries.16,17 To assess the effect of the different nanoscale geometries on their electrochemical properties in a Li-ion battery a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0212

setting, the Co3O4 freestanding nanorods (NR) and hollow nanoporous spheres (HNS) of SnO2 and Mn2O3 with porous wall were selected for investigation of the sustainability of initial high capacity. II. EXPERIMENTAL

A thin film of Co3O4 NR were grown on a titanium substrate (10 mm  40 mm  1 mm) using the ammoniaevaporation-induced-deposition process reported by the literature17 and the HNS of SnO2 and Mn2O3 were synthesized by hydrothermal processes reported by the literature.15,18 The morp