The charge-discharge behavior of a Si-C mixed electrode (Si: graphite = 20: 80 or 30: 70 (wt%)) was investigated using in situ solid-state Li-7 nuclear magnetic resonance (NMR) spectroscopy. The spectra revealed the formation of Li-Si alloys and the intercalation of Li into graphite during the charge process and the corresponding reverse process (Li extraction) during discharge. Li was mainly stored as a Li-Si alloy (Li15Si4 or Li15+delta Si-4) at high SOC (state of charge) values (above 80% SOC, low cell voltage region) and could be released from the alloy. The cell resistance measured by electrochemical impedance spectroscopy (EIS) increased steeply at high SOC values, revealing that structural changes in the Li-Si alloy may influence the electrode resistance. The NMR peaks observed upon formation of Li15Si4 or Li15+delta Si-4 shifted and decreased gradually, indicating that the Li-Si alloys are not sufficiently stable in the cell. (C) The Author(s) 2017. All rights reserved.