N-Containing Carbon-Coated β-Si₃ N₄ Enhances Si Anodes for High-Performance Li-Ion Batteries

The lithiation/delithiation properties of α-Si₃ N₄ and β-Si₃ N₄ are compared and the carbon coating effects are examined. Then, β-Si₃ N₄ at various fractions is used as the secondary phase in a Si anode to modify the electrode properties. The incorporated β-Si₃ N₄ decreases the crystal size of Si and introduces a new NSiO species at the β-Si₃ N₄ /Si interface. The nitrogen from the milled β-Si₃ N₄ 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-Si₃ N₄ and Si phases on the specific capacity are confirmed. The operando X-ray diffraction and X-ray photoelectron spectroscopy data indicate that β-Si₃ N₄ is partially consumed during lithiation to form a favorable Li₃ N species at the electrode. However, the crystalline structure of the hexagonal β-Si₃ N₄ is preserved after prolonged cycling, which prevents electrode agglomeration and performance deterioration. The carbon-coated β-Si₃ N₄ /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-Si₃ N₄ /Si anode and a LiNi_0.8 Co_0.1 Mn_0.1 O₂ cathode is constructed and its properties are evaluated. The potential of the proposed composite anodes for Li-ion battery applications is demonstrated.