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Controlled phase and crystallinity of FeNCN/NC dominating sodium storage performance.

FeNCN is a potentially fast-charging sodium ion anode due to the presence of lots of broad tunnels and its high electronic conductivity. However, FeNCN has been rarely investigated due to its complicated synthetic process and unclear synthetic mechanism, which affect the precise control of its phase and crystallinity. In this work, phase- and crystallinity-controlled FeNCN polyhedrons grown on nitrogen-doped carbon (FeNCN/NC) are successfully fabricated by adjusting the growing time and temperature. Moreover, the synthesis mechanism is disclosed in this paper. High-crystallinity FeNCN grows along the [001] direction, which exposes sufficient broad channels on the {010} planes and significantly improves the diffusion rate of sodium ions. Moreover, high-crystallinity FeNCN exhibits higher mechanical strength, which reduces its pulverization rate and endows it with durable cycling stability. When applied as an anode in a sodium-ion battery, high-crystallinity FeNCN/NC exhibits a high rate capability of 332 mA h g-1 at 5.0 A g-1 and a stable cycling performance of 368 mA h g-1 after 300 cycles at a high current density of 1.0 A g-1. This work confirms that the sodium-ion storage performance of FeNCN can be further improved by tuning its crystallinity.