NANO LETTERS
Superior Lithium Electroactive Mesoporous Si@Carbon Core-Shell Nanowires for Lithium Battery Anode Material
2008 Vol. 8, No. 11 3688-3691
Hyesun Kim and Jaephil Cho* Department of Applied Chemistry, Hanyang UniVersity, Ansan, 426-791 Korea Received June 27, 2008; Revised Manuscript Received September 10, 2008
ABSTRACT Mesoporous Si@carbon core-shell nanowires with a diameter of ∼6.5 nm were prepared for a lithium battery anode material using a SBA-15 template. As-synthesized nanowires demonstrated excellent first charge capacity of 3163 mA h/g with a Coulombic efficiency of 86% at a rate of 0.2 C (600 mA/g) between 1.5 and 0 V in coin-type half-cells. Moreover, the capacity retention after 80 cycles was 87% and the rate capability at 2 C (6000 mA/g) was 78% the capacity at 0.2 C.
Si metal has been known to reach a highest lithium capacity of ∼4200 mA h/g corresponding to Li4.4Si;1 however a large volume change (>300%) during lithium alloying and dealloying can result in pulverization of the particles and electrical disconnection from the current collector.2 The electrical disconnection leads to a rapid capacity fade of the cell. Many reports have focused on reducing such volume change via composites with carbon and Si nanoparticles.2,9 These methods showed some improvement of the capacity retention because the carbon acts as an electron conductor between the pulverized particles. However, in order to achieve relatively good capacity retention, the carbon content in the composite should be greater than 50 wt %, and the capacity retention after 50 cycles less than 1500 mA h/g. Alternatively, reports on one-dimensional (1D) silicon nanowires for anode materials are very rare. Si nanowires prepared by the laser ablation system described by Lieber et al.10 had a wire diameter of ∼10-30 nm,11 and the first discharge and charge capacities of the nanowires were ∼1300 and ∼900 mA h/g, respectively, even though the reason for the low specific capacities was not reported. Si nanowires with a diameter
