N-Containing Carbon-Coated β-Si₃N₄ Enhances Si Anodes for High-Performance Li-Ion Batteries

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 LiNi_0.8Co_0.1Mn_0.1O₂ cathode is constructed and its properties are evaluated. The potential of the proposed composite anodes for Li-ion battery applications is demonstrated.