NEGATIVE ELECTRODE ACTIVE MATERIAL FOR LITHIUM ION BATTERY, AND NEGATIVE ELECTRODE FOR LITHIUM ION BATTERY USING THE SAME

PROBLEM TO BE SOLVED: To provide a negative electrode active material for a lithium ion battery capable of improving cycle characteristic.SOLUTION: The negative electrode active material contains Si-Sn-Fe-Cu base alloy, and has a system structure in which an area ratio of a Si phase against the entire negative electrode active material is within the range of 35-80%, with the Si phase dispersed in matrix phase. As the matrix phase, Si-Fe compound phase is crystallized around Si phase and further Sn-Cu compound phase is crystallized so as to surround Si phase and Si-Fe compound phase. The Si-Fe compound phase is crystallized at 35-90% of area ratio in the matrix phase. A Sn phase which is inevitably crystallized in the matrix phase is 15% or less by area ratio.