Carbon-coated Si-Cu3Si-Al2O3 nanocomposites have been synthesized via a facile mechanochemical reaction and employed as anode materials for lithium-ion batteries. Combined X-ray and microscopic studies show that the nanocomposites are composed of agglomerated nanostructured particles with uniform distribution of crystalline silicon, Cu3Si, and amorphous Al2O3. Electrochemical characterization reveals that the in situ incorporation of both the conductive Cu3Si and electrochemically stable Al2O3 phases results in a dramatic improvement of cyclability and rate capability, while the specific capacity decreases with increasing amount of Cu3Si. By controlling the Cu3Si content, the composite with a high tap density of ∼1.2 g cm−3 delivers a high reversible capacity of 841 mA h g−1, excellent cyclability, and good rate performance up to 3.2 A g−1 in half cells. Full-cell test coupled with a commercial spinel cathode also displays a high average operating voltage of >3.5 V, a relatively good capacity retention of ∼77.2% after 50 cycles with a high initial efficiency of ∼86.3%. The enhanced electrochemical performance is mainly attributed to the presence of the conductive Cu3Si buffer phase that mitigates structural degradation and offers high conductivity.