NOVELTY - The negative active material has silicon-based alloy that is made up of an alloy matrix made up of aluminum (Al)-copper (Cu) and Al2Cu, and silicon nanoparticles that is dispersed in the alloy matrix, in which a ratio of a sum of atomic fractions of Al and Cu contained in Al2Cu to a sum of atomic fractions of Al and Cu contained in AlCu ranges from 0-0.6 in the silicon-based alloy. The silicon-based alloy includes 20-70at.% silicon, 15-45at.% aluminum, and 10-40 at.% of copper, in which the sum of atomic fractions of silicon, aluminum and copper is 100at.%.    USE - Negative active material made up of silicon-based alloy having silicon, aluminum, and copper, for lithium battery (claimed), used in electric vehicles e.g., hybrid vehicles, and electronic devices e.g., mobile phones and portable computers.    ADVANTAGE - The inactive phases may efficiently inhibit volumetric expansion of the silicon-based alloy during charging and discharging, and provide excellent battery capacity properties. Since the binary alloy phase of AlCu and Al2Cu does not include inactive silicon, the content of Si that is consumed by formation of the inactive silicon may be reduced, a high battery capacity can be provided. The increase in the strength of the alloy matrix may inhibit destruction of the inactive phase caused by the volumetric expansion of the silicon nanoparticles during the repeated charging and discharging of a lithium battery and may also significantly increase the capacity retention rate during the repeated charging and discharging cycles, and thus, influence of the ratio between Al2Cu and AlCu on lifespan may be accurately tested by maintaining the discharge capacity in a similar level.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a manufacturing method of negative active material.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of the lithium battery.    Negative electrode (22)    Positive electrode (23)    Battery case (25)    Sealing portion (26)    Lithium ion battery (30)