The charge-discharge behavior of a Si-C (Si:graphite = 20:80 wt%) mixed electrode was investigated by in-situ solid-state 7Li 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) at high SOC (State of Charge) values (above 80% SOC, low cell voltage region) and could be released from the same compound. The cell resistance measured by electrochemical impedance spectroscopy (EIS) increased steeply at high SOC values, revealing structural changes in the Li-Si alloy. The result is consistent with the gradual decrease of the Li15Si4 signal intensity in the NMR spectra observed immediately after a discharge process at 80% SOC.