Silicon based nanocomposites as lithium-ion battery anodes.

摘要

In this research, different silicon based nanocomposites have been developed and studied as potential anode materials for high-energy density lithium-ion batteries. The main challenge to use silicon as lithium-ion anode material is the pulverization caused by the tremendous volume changes associated with the phase transitions during electrochemical cycling, which eventually leads to the failure of electrode due to loss of electronic contact between active material particles.;Si/C/Al composite powders have been synthesized by thermal treatment of high-energy mechanically milled composite precursors comprising graphite, silicon, aluminium and polymethacrylonitrile. The polymer has been used to suppress the interfacial diffusion reactions between graphite, silicon and aluminium, which otherwise lead to the formation of electrochemically inactive SiC and Al4C3 intermetallics during high energy mechanical milling. The resultant Si/C/Al composite of nominal composition 75wt.% C-20wt.% Si-5wt.% Al exhibits a reversible capacity of ∼650mAh/g up to 30 cycles at a charge/discharge rate of ∼340mA/g. Scanning electron microscopy analysis of electrochemically cycled electrodes indicates that the microstructural stability and the structural integrity of the Si/C/Al composite is retained during electrochemical cycling contributing to the good cyclability demonstrated by the composites.;Nanocomposite comprising silicon (Si), graphite (C) and SWNTs, denoted as Si/C/SWNTs, has been synthesized by dispersing SWNTs via high-power ultrasonication into a pre-milled Si/C composite mixture, followed by subsequent thermal treatment. The Si/C powder was prepared by high-energy mechanical milling (HEMM) in which polymethacrylonitrile (PMAN) was used to act as a diffusion barrier to suppress the mechanochemical reaction between silicon and graphite to form undesired electrochemically inactive SiC and further prevent the amorphization of graphite during extended milling. A nanocomposite with nominal composition of Si-35 wt.% SWNTs-37 wt.% exhibits a reversible discharge capacity of ∼900mA/g with an excellent capacity retention of capacity loss of 0.3% per cycle up to 30 cycles. Functionalization of the SWNTs with LiOH significantly improves the cyclability of the nanocomposite containing Si-45 wt.% SWNTs-28 wt.% exhibiting a reversible capacity of 1066 mAh/g displaying almost no fade in capacity up to 30 cycles. The improved electrochemical performance is hypothesized to be attributed to the formation of a nanoscale conductive network by the dispersed SWNTs which successfully results in maintaining electrical contact between the electrochemically active particles during cycling. (Abstract shortened by UMI.);To tackle this problem, the research approach proposed in this study is to directly form active-inactive nanocomposites ex-situ prior to any electrochemical reactions aiming to capitalize on the advantages provided by nanoparticle, nanotubes, and the active-inactive composite interfaces. The feasibility of using Si/C/Al, Si/C/SWNTs (single-walled carbon nanotubes) nanocomposites, Si/MWNTs (multi-walled carbon nanotubes) hybrid nanocomposite, and Si/MWNTs hierarchical electrode as potential lithium-ion battery anodes is studied.