Advanced multiphase silicon-based anodes for high-energy-density Li-ion batteries

We report on the formation of composite silicon-based multiphase nanopowders as promising active-anode materials for highenergy- density, high-capacity-retention lithium-ion batteries. Simple synthetic routes have been developed for the preparation of core-shell Si and p-doped SiNi composite particles attached to carbon nanotubes. The particles were characterized by a variety of analytical and electrochemical methods. Environmental scanning-electron-microscopy and transmission-electron-microscopy images and electron-energy-loss-spectroscopy maps indicated formation of nanoparticles wrapped by carbon nanotubes and coated by nanometer-thick amorphous carbon. It was found that nickel diffuses to the bulk of silicon and forms either nickel-rich or nickel-depleted composite entities. The presence of boron in the synthesized composite powder has been detected by time-of-flight secondary-ion mass spectroscopy (TOFSIMS). Li/LiPF₆ EC:DEC/Si-C-MWCNT cells with anodes composed of about 80% coreshell Si-C composite (36% Si in the anode) ran for more than 1000 cycles with a degradation rate of 0.07%/cycle (beginning from cycle 25). The SiNi/MWCNT composite anode revealed a remarkably higher capacity-retention rate at initial cycles and higher C-rate capability. Li/SiNi/MWCNT cells ran for about 250 cycles demonstrating a reversible capacity of about 620 mAh/gSi at 120 μA/cm₂ and cycle 210, and 800 mAh/gSi at 50 μA/cm₂ at cycle 240.