Lanthanum silicide/silicon composite electrodes have shown remarkable lithiation-delithiation properties, whereas their reaction behaviors have not yet been clarified. Herein, we investigated changes in the volume of the composite layer and the metallographic structure after cycling. The electrode has approximately the same relative thickness regardless of the electrolyte, whereas the thickness of a Si electrode is greater in the organic liquid electrolyte. Scanning transmission electron microscopy revealed that the Si phase, with a diameter of hundreds of nanometers, was highly dispersed in the silicide matrix phase before cycling. Enabled by this metallographic structure, the silicide phase can relieve the stress generated from Si. In contrast, the positional relationship of Si and silicide was inverted after cycling. Due to this inversion, the silicide phase could not relieve the Si-generated stress and the reversible capacity decayed. Because of the difference in surface film properties, the metallographic structure changed later in the ionic liquid electrolyte. Obtained insights into the reaction behaviors of silicide/Si composite electrodes would contribute to improve the performance of lithium-ion batteries.