Robust Design of Silicon/Graphite Composite Via Atomic-Scale Rearrangement for High Performance Lithium Ion Batteries

Abstract

State-of-the-art lithium-ion battery (LIB) technology are still challenged with many other requirements, including high-energy density, long cycle life, safety and cost by the increasing demand of the electric vehicle energy-storage market. Moreover, key to success for commercialization of EVs is reducing the charging time of the batteries. In general, the conventional graphite has shown not only low electrochemical performance but also safety problems arisen from the metallic lithium plating at the surface of the anode electrode at the high current density (4mA/cm2) due to a sluggish kinetics during intercalation reaction and working voltage potential neighboring almost 0V (versus Li/Li+). Alternatively, among the alloy-type anodes, silicon is considered as promising anode material due to the high gravimetric capacity of 3572 mAh g-1 and proper lithium-alloying potential (0.22 V vs. Li/Li+) to prevent unwanted lithium plating. However, to the best of our knowledge, the majority of previously reported Si anodes do not demonstrate the rate capabilities at high electrode density (g/cc) due to the large volume changes (>300%) and low electrical conductivity of Si anodes. Herein, we have proposed a composite consisting of a nanoscale silicon coating layer on graphite/carbon nanofibers through nickel-catalyzed-CNF growth on the graphite by chemical vapor decomposition (CVD) using both ethylene (C2H4) and silane (SiH4) gas. The chemical Si-C bonding by atomic rearrangement of CNF and a-Si layer prevents the exfoliation of the active Si layer from the surface of CNF during cycles. For practical viability, all electrochemical tests were performed in the full-cell as well as in the half-cell configurations with high energy density (> 1.6g cc-1), high areal capacity (3.3 mAh cm2) and limited binder composition (< 4wt %). As a result, by increasing electrical conductivity of silicon/graphite composite and existing high stability of ultrathin Si layer, our composite shows remarkable improvement in electrochemical performances even at high current density with those of the conventional graphite.