Negative electrode active material

A negative electrode active material which is represented by the following Forumla 1, and which comprises

• a phase A including active silicon;
• and
• a phase B having a metal alloy matrix including silicon and a transition metal,
• wherein the active silicon of the phase A forms a core and the alloy matrix of the phase B surrounds the core of the active silicon of the phase A,
• wherein peaks other than a peak derived from the active silicon of the phase A observed by an ex situ XRD analysis before a charge/discharge cycle are also observed by an ex situ XRD analysis after the charge/discharge cycle,
• the Formula 1 being aSi-bTi-cFe
• wherein the sum of a, b, and c is 100 atom %, a is 67 atom % to 74 atom %, b is from 13 atom % to 16.5 atom %, and c is 13 atom % to 16.5 atom %.