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The mechanical alloying technique was applied to the Si-Ag system. SiAg powders formed by the mechanical alloying process appear to contain a uniform dispersion of Si in ductile Ag matrix. When the SiAg powders were tested as an anode material in a lithium cell, the cycling stability was significantly improved by limiting the cutoff potential. The SiAg electrode prepared by milling for 50 h showed good cyclability with little fade over the first 50 cycles. It exhibited a stable capacity of ϳ280 mAh/g or 1150 mAh/cm 3 , suggesting that the material system is promising as an anode material for lithium-ion batteries.Graphite has been commonly used as anode material for lithiumion batteries. However, the theoretical capacity is limited to 372 mAh/g and the volumetric capacity to 830 Ah/L. 1,2 To increase the specific energy of lithium-ion batteries, new anode materials which have higher energy density and specific capacity compared to graphite are needed. Several materials, such as tin oxide-based composites and nanocomposites of intermetallics, have recently been investigated as possible anode materials to replace graphite. [3][4][5][6][7][8] Whereas most of these materials, so far, have utilized Sn as the active element, another active element, Si, can also react with lithium to form Li x Si to maximum uptake of Li 21 Si 5 with a theoretical capacity of 4000 mAh/g. 9,10 But the expansion and contraction of lithium silicide during charge-discharge processes leads to a pulverization of the silicon powder, which results in a fast fading of the capacity during cycling. A nano-Si composite electrode prepared by mixing nanometer-scale Si powder and carbon black showed better cycling performance and higher reversible capacity over 1700 mAh/g, but the high weight ratio of carbon black in the composite electrode reduced the volume energy density to 425 mAh/cm 3 . 11 It is expected that a nano-Si composite material, in which nanometer-scale Si particles are finely dispersed within a solid, mixed-conducting, ductile metallic matrix, would demonstrate a good electrochemical performance. A similar approach has been previously suggested. 12,13 Mechanical alloying ͑MA͒ is a powder processing technique allowing production of microscopically homogeneous materials starting from blended elemental powder mixture. In this study, we applied MA to the immiscible Si-Ag system to synthesize a homogeneous mixture of Si-50 atom % Ag for use as anode material in lithium-ion batteries. ExperimentalA high-energy ball mill ͑SPEX-8000͒ was used for the mechanical alloying. The elemental powders of Si ͑Aldrich, 99%͒ and Ag ͑Aldrich, 99.9%͒ were mixed with an atomic ratio of 1:1, and then loaded into a stainless steel vial together with stainless steel balls measuring 7.9 mm in diam. Note that Si powders were ground for 10 h prior to mechanical alloying. All processes prior to milling were conducted inside an argon-filled glove box and the vial was sealed with an elastomer O-ring seal. The ratio of ball weight to powder weight was 4:1. Milling was ...

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