The nature of Li–Si alloy phases that are generated in electrochemical lithiation is examined as a function of temperature. The electrochemical lithiation is performed at 0.0V (vs. Li/Li+) by short-circuiting an amorphous Si thin-film electrode with a Li metal counter electrode. At 25–85°C, the well-known Li15Si4 phase (theoretical specific capacity=3580mAhg−1) forms. At 100–120°C, however, Li21Si5 (4008mAhg−1) that is known to be the most Li-rich phase in Li–Si system is generated. The crystallization into Li21Si5 is, however, so kinetically slow that it does not appear in the transient cycling experiment. The Li21Si5 phase is converted to amorphous Si upon de-lithiation, but the restoration back to the initial phase is only observed at 100–120°C after a prolonged lithiation at 0.0V. The cycleability of this phase is poor due to a successive Li trapping inside the Si matrix, which is caused by the formation of electrically isolated Si islands.