Self-Formed Channel Boosts Ultrafast Lithium Ion Storage in Fe₃O₄@Nitrogen-Doped Carbon Nanocapsule

Investigations into conversion-type materials such as transition-metal oxides have dominated in energy-storage systems, especially for lithium ion batteries in recent years. A common understanding of taking account of high energy density and high power density allows us to design reasonable electrodes. In this study, the unique Fe₃O₄@nitrogen-doped carbon (denoted as Fe₃O₄@NC) nanocapsule with self-formed channels was synthesized based on a facile hydrothermal-coating-annealing route. With respect to the effect of this rational architecture on lithium-storage performance, excellent behavior (a high reversible capacity of 480 mAh g^-1) could be maintained at 20 A g^-1 during 1000 cycles, with an average Coulombic efficiency of 99.97%. It also means that such a Fe₃O₄@NC electrode can meet a fast-charge challenge (end-of-charge within ∼2 min). By a series of investigations, we certainly considered that uniform carbon coating improved electrical conductivity and acted as a buffer layer to accommodate volume variations of Fe₃O₄ nanoparticles during cycling. It is more interesting that self-formed channels can effectively shorten the ion diffusion path and provide a necessary space to buffer volume expansion as well. Benefiting from these synergetic advantages, this Fe₃O₄@NC nanocapsule also delivered outstanding electrochemical performances in full cells.