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Abstract

The sodium ion battery is a promising successor for the lithium-ion battery. Its energy density is limited by the anode, where sodium ideally is stored at low potentials vs. Na/Na⁺. The understanding of the fundamental relationships between material properties and sodium storage is often lagging behind materials development. There is a discord regarding the involvement of so-called “closed pores” in carbons in sodium storage. To investigate their influence, a chemical vapour deposition (CVD) process to deposit polymeric carbon nitride (p-C₃N₄) on hard carbon fibres of both, open and closed microporosity is developed. High storage capacity at a low potential is only possible, when suitable, sealed pores are present. In fibers without notable gas-accessible surface, p-C₃N₄ is deposited on the external area, whereas in open-microporous samples the p-C₃N₄ phase grows in micropores. Consequently, except for the untreated fibres with closed pores, the composite with this pore gradient is the only one in the study that is able to accommodate sodium at low potentials. Neither the remaining graphitic domains, nor the introduced p-C₃N₄ are able to accommodate sodium in a quasimetallic state. Finally, not only the sodium storage but also the solid-electrolyte interphase (SEI) build-up is influenced by the additional p-C₃N₄ layer.