Negative electrode active material

A negative electrode active material, comprising an alloy phase that undergoes thermoelastic diffusionless transformation either when releasing metal ions, or when occluding the metal ion, wherein

• the alloy phase undergoes the thermoelastic diffusionless transformation when occluding the metal ions, and undergoes reverse transformation when releasing the metal ions, wherein
• the alloy phase after the thermoelastic diffusionless transformation contains a crystal structure which is 2H in Ramsdell notation, and
• the alloy phase after the reverse transformation contains a crystal structure which is DO3 in Strukturbericht notation, wherein
• the negative electrode active material contains 21 to 27 at % of Sn, with the balance being Cu and impurities, or
• the negative electrode active material contains:
• Sn:
• 10 to 35 at %, and
• one or more selected from the group consisting of Ti:
• 9.0 at % or less, V:
• 49.0 at % or less, Cr:
• 49.0 at % or less, Mn:
• 9.0 at % or less, Fe:
• 49.0 at % or less, Co:
• 49.0 at % or less, Ni:
• 9.0 at % or less, Zn:
• 29.0 at % or less, Al:
• 49.0 at % or less, Si:
• 49.0 at % or less, B:
• 5.0 at % or less, and C:
• 5.0 at % or less, with the balance being Cu and impurities.