NMR investigations on the lithiation and delithiation of nanosilicon-based anodes for Li-ion batteries

Abstract

Lithiation and delithiation of nanosilicon anodes of 100–200 nm diameter have been probed by ex situ solid-state high-resolution 7Li nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM) methods. Samples were charged within pouch cells up to capacities of 1,500 mAh/g at 0.1 C, and subsequently discharged at the same rate. The NMR spectra reveal important quantitative information on the local lithium environments during the various stages of the charging/discharging process. The TEM experiments show that the electrochemical lithiation of nanosilicon particles results in core-shell materials, consisting of LixSi shells surrounding a core of residual silicon. The NMR spectra yield approximate Li/Si ratios of the lithium silicides present in the shells, based on the distinct local environments of the various types of 7Li nuclei present. The combination of NMR with TEM gives important quantitative conclusions about the nature of the electrochemical lithiation process: Following the initial formation of the solid electrolyte interphase layer, which accounts for an irreversible capacity of 240 mAh/g, lithium silicide environments with intermediate Li concentrations (Li12Si7, Li7Si3, and Li13Si4) are formed at the 500 to 1,000 mAh/g range during the charging process. At a certain penetration depth, further lithiation does not progress any further toward the interior of the silicon particles but rather leads to the formation of increasing amounts of the lithium-richest silicide, Li15Si4-type environments. Delithiation does not result in the reappearance of the intermediate-stage phases but rather only changes the amount of Li15Si4 present, indicating no microscopic reversibility. Based on these results, a detailed quantitative model of nanophase composition versus penetration depth has been developed. The results indicate the power and potential of solid-state NMR spectroscopy for elucidating the charging/discharging mechanism of nano-Si anodes.