Nitrogen-Enriched Shell Graphite-Core C–Si–N Composite for Reduced Swelling in Si/Graphite Negative Electrodes

This study reports a graphite-core, multiphase gradient C–Si–N composite architecture for Si-containing graphite-based negative electrodes in lithium-ion batteries. The increase in electrode thickness is used as a practical metric of expansion-driven degradation. The composite is prepared by the simultaneous nitridation and carbonization of a graphite core–Si precursor using polyvinylpyrrolidone (PVP) as the N source. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy indicates a quasi-continuous radial trend in the relative N signal toward the outer shell, consistent with preferential N enrichment near the particle exterior. This spatially distributed N arrangement may spatially separate the Si-rich expansion-prone region from the carbon-rich exterior containing nitrides and other N-bearing species, thereby enabling stress partitioning. The shell architecture is designed to disperse expansion-induced stress and stabilize the electrode–electrolyte interface. During electrochemical cycling, the C–Si–N electrode with 10% PVP preserves its core–shell morphology and exhibits the smallest average electrode thickness expansion (~58% after 40 cycles, based on four independent cells). The reduced thickness growth is discussed in relation to a mechanically robust Si–N matrix (Si3N4-like/SiNx-like), potential Li–N interphase species, and N-containing carbon, together with the post-mortem morphology and electrochemical impedance evolution. This study presents reduced swelling as an electrode-level trend versus nominal PVP addition, along with associated nitride-related signatures, thereby highlighting spatially graded stress buffering as an electrode-level design principle.