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

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 Li_xSi 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.