The lithiation/delithiation properties of α-Si3N4 and β-Si3N4 are compared and the carbon coating effects are examined. Then, β-Si3N4 at various fractions is used as the secondary phase in a Si anode to modify the electrode properties. The incorporated β-Si3N4 decreases the crystal size of Si and introduces a new N-Si-O species at the β-Si3N4/Si interface. The nitrogen from the milled β-Si3N4 diffuses into the surface carbon coating during the carbonization heat treatment, forming pyrrolic nitrogen and C-N-O species. The synergistic effects of combining β-Si3N4 and Si phases on the specific capacity are confirmed. The operando X-ray diffraction and X-ray photoelectron spectroscopy data indicate that β-Si3N4 is partially consumed during lithiation to form a favorable Li3N species at the electrode. However, the crystalline structure of the hexagonal β-Si3N4 is preserved after prolonged cycling, which prevents electrode agglomeration and performance deterioration. The carbon-coated β-Si3N4/Si composite anode shows specific capacities of 1068 and 480 mAh g−1 at 0.2 and 5 A g−1, respectively. A full cell consisting of the carbon-coated β-Si3N4/Si anode and a LiNi0.8Co0.1Mn0.1O2 cathode is constructed and its properties are evaluated. The potential of the proposed composite anodes for Li-ion battery applications is demonstrated.