Silicon-core–carbon-shell nanoparticles have been widely studied as promising candidates for the replacement of graphite in commercial lithium-ion batteries. Over more than 10 years of R&D, the many groups actively working in this field have proposed a profusion of distinctive nanomaterial designs. This broad variety makes it extremely challenging to establish mechanistic insight into how fundamental material structure and properties affect battery performance. In particular, the interplay between the character of the carbon encapsulation layer and the electrochemical performance of the composite is still poorly understood. In this work, we aim to address this lack of knowledge through the development of a modified chemical vapor deposition approach that enables precise control of the degree of graphitization of the carbon coating. We provide a comparison between core–shell structures maintaining identical silicon cores with different types of carbon shells, that is, graphitic carbon and amorphous carbon. A highly graphitic carbon layer is not only characterized by higher electrical conductivity but markedly favors the transport of lithium ions into the silicon core with respect to an amorphous one. This advantageous property confers better cycling stability to the composite material. We also demonstrate that the graphitic-carbon-coated particles display excellent electrochemical performance even when used as a simple “drop-in” additive in graphite-dominant anodes for current generation Li-ion batteries. Replacement of 10% by weight of graphite in the electrode composition results in an increase of 60% in the storage capacity with a first cycle Coulombic efficiency of 91% and capacity retention over 100 cycles of 86%.

By combining experimental measurements with ab initio molecular dynamics simulations, we provide the first microscopic description of the interaction between metal surfaces and a low-temperature nitrogen-hydrogen plasma. Our study focuses on the dissociation of hydrogen and nitrogen as the main activation route. We find that ammonia forms via an Eley-Rideal mechanism where atomic nitrogen abstracts hydrogen from the catalyst surface to form ammonia on an extremely short timescale (a few picoseconds). On copper, ammonia formation occurs via the interaction between plasma-produced atomic nitrogen and the H-terminated surface. On platinum, however, we find that surface saturation with NH groups is necessary for ammonia production to occur. Regardless of the metal surface, the reaction is limited by the mass transport of atomic nitrogen, consistent with the weak dependence on catalyst material that we observe and has been reported by several other groups. This study represents a significant step towards achieving a mechanistic, microscopic-scale understanding of catalytic processes activated in lowtemperature plasma environments.

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