Slow stabilization of Si-Li alloys formed during charge & discharge of a Si-C mixed electrode studied by in situ solid-state ⁷Li nuclear magnetic resonance spectroscopy

The charge-discharge behavior of a Si-C mixed electrode (Si:graphite = 20:80 or 30:70 (wt%)) was investigated using in situ solidstate 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 or Li_15+δSi₄) 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 Li_15+δSi₄ shifted and decreased gradually, indicating that the Li-Si alloys are not sufficiently stable in the cell.