Si/carbon nanocomposites with different Si distribution were prepared to the anode of Li-ion batteries for better cyclic properties. Si particles were milled by a high-energy multi-ring-type mill to decrease the crystallite size to 15 nm. Nanocomposite particles with controlled inhomogeneous spatial Si distribution were obtained by co-grinding with Cu powder. Heterogeneity of the Si distribution exhibited better cyclability than those with homogeneous Si distribution prepared from well-dispersed Si nanoparticles. We attribute the better cyclability to two factors, reduction of mechanical stress developed by electrochemical volume change of Si and networking of Si particles for maintaining conductive paths in the electrode.

Si/carbon/graphite composites were prepared by mixing submicrometer Si, pitch cokes, and graphite powders followed by heattreatment. Effects of particle sizes and the mixing method on the cycle performance were examined and the optimized composite structure was investigated by transmission electron microscopy and scanning electron microscopy of the composite electrodes. A "surface-coat-type" composite prepared by mixing Si with pitch cokes before mixing with graphite powders showed better cycle performances. The particle size ͑30 m͒ of graphite was also a very important factor in keeping the composite's surface area low, resulting in low irreversible capacity. The downsizing of Si particles was also very effective to have good cycle performances.Lithium-silicon alloys are attractive candidates for anode materials in rechargeable Li-ion batteries due to their high specific capacity or energy density. 1,2 However, they usually exhibit rapid capacity decay during charge-discharge cycling. 1 This is mainly attributed to the large volume change up to 323% between the delithiated ͑discharged͒ and fully lithiated ͑charged͒ states. 3 The internal stress accumulated during the repetition of dilatation and shrinkage leads to the generation of microcracks, and hence, the degradation of active materials and the breakage of electronic conduction paths. 4 Better cycleability is expected when the mechanical stress induced by the large volume change was eased and the surplus reactions between Si and electrolyte was suppressed by the appropriate composite structure.To date, numerous methods have been developed to fabricate Si composites with inactive or active materials more appropriate for Li-intercalation. 2,5-7 Recently, we have also reported that a Si-Cu/carbon/graphite composite showed excellent cycle characteristics. 8 To improve the cycle characteristics of Si/carbon/ graphite composites, the following three points were considered: maintain the electronic conductive paths, avoid the direct contact between Si and electrolyte to form a solid electrolyte interphase ͑SEI͒ on the surface of Si, 9 and relieve severe Si volume change.The mixing methods of Si, carbon-precursor, and graphite powder, and the sizes of graphite and Si particles are very important factors. 10,11 However, the interrelation between composite structure and cycle performance have not been studied in detail yet. Recently, Lee and Lee reported on a good cycle performance of carbon-coated nano-Si dispersed oxides/graphite composite. 12 We report here the configuration effects of Si/carbon/graphite composites by controlling the dispersion of Si and graphite particles. Moreover, we propose the optimized structure of the Si/carbon/graphite composites to improve the cycle performance, based on the analysis of deteriorated electrodes after cycling. ExperimentalComposite preparation.-Si, pitch cokes, and graphite powders were used to prepare the Si/carbon/graphite composites. Submicrometer Si powders ͑mean particle size 0.8 m͒ were prepared from micrometer Si powde...

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By the combination of two successive dry milling processes (i.e., by using a multiring mill and subsequently a pin-type disintegrator), downsizing of Si powders was achieved to 30 nm crystallites and their aggregated mean volume diameter about 100 nm, with the top size less than 400 nm. Combination of the two dry milling processes proved to be effective to reduce the particle size and sharpen the size distribution of the aggregates without using any additives, tedious wet processes, or subsequent classification. Increase in the impurity due to milling was <0.1% for Fe and <0.4% for Zr, being well within a tolerable limit for most of the technological purposes.

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