Optimized Structure of Silicon/Carbon/Graphite Composites as an Anode Material for Li-Ion Batteries

Abstract

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...

Germanium Nanotubes Prepared by Using the Kirkendall Effect as Anodes for High‐Rate Lithium Batteries

“…The slurry was prepared as described elsewhere [28] by thoroughly mixing an N-methyl-2-pyrrolidone solution (NMP; Aldrich, 99.9 %) of PVDF, carbon black, and the active anode material. The coin-type half cells (2016R size), prepared in a heliumfilled glove box, contained Ge NTs, Li metal, a microporous polyethylene separator, and an electrolyte solution of 1.1m LiPF 6 in ethylene carbonate/ethyl methyl carbonate/dimethyl carbonate (EC/ EMC/DMC; 3:3:4 vol.…”

“…It has been reported that the irreversible capacity loss of the lithium reactive alloys is closely related to some side reactions between the active material and the electrolyte (especially LiPF 6 ). [28] We believe that the contribution of antimony to capacity was negligible owing to its low content.…”

Relationship between the Properties and Cycle Life of Si/C Composites as Performance-Enhancing Additives to Graphite Electrodes for Li-Ion Batteries

“…Various carbon precursors were reported in the literature, some of them more common * Electrochemical Society Member. [43][44][45] In some cases, carbon materials, such as graphite 27,28,34,37,41,44,45 or carbon nanotubes, 30 were added to the precursor mixture before the pyrolysis process to enhance carbon content. Reasonable cycling stability was achieved for some of these composites but, despite numerous studies, it is still unclear, which of the Si/C composite properties are indeed contributing to the stability enhancement of silicon-containing electrodes.…”

“…5 One of these, enabling a better contact between silicon and carbon, is mechanical mixing or ballmilling of silicon particles with an organic carbon precursor, which is then converted into carbon by pyrolysis in inert atmosphere. Various carbon precursors were reported in the literature, some of them more common than others, such as polyvinyl chloride (PVC), [27][28][29][30][31] poly(vinyl alcohol) (PVA), 32,33 sucrose, 26,33 poly(p-phenylene) (PPP), 31 polyacrylonitrile (PAN), 34 polystyrene (PS), 35 pitch 26,[36][37][38][39][40][41][42] and other resins. [43][44][45] In some cases, carbon materials, such as graphite 27,28,34,37,41,44,45 or carbon nanotubes, 30 were added to the precursor mixture before the pyrolysis process to enhance carbon content.…”

One‐to‐One Comparison of Graphite‐Blended Negative Electrodes Using Silicon Nanolayer‐Embedded Graphite versus Commercial Benchmarking Materials for High‐Energy Lithium‐Ion Batteries

“…[1,15,38,39] Moreover, the included oxygen in SiO x is known to create unfavorable irreversible products (Li 2 O and Li 4 SiO 4 ) and promote poor initial CEs. [22,40] For this reason, the large difference in volume variation between C-SiO x and conventional graphite, even with the irreversible products, forms localized cracks within the electrode and causes loss of electrical contact with relatively high capacity fading.…”