FeOx@carbon yolk/shell nanowires with tailored void spaces as stable and high-capacity anodes for lithium ion batteries

Abstract

Transition metal oxides as anodes of lithium ion batteries (LIBs) promise high theoretical capacities. However, the significant volume change upon lithium insertion/extraction cycles causes pulverization of the materials, leading to a quick loss of electrical conductivity and a rapid decrease in capacity. The present work provides a possible solution to this problem by using a yolk/shell nanostructure featuring porous FeO_x nanowires encapsulated with carbon shells as anodes. It was found that the FeO_x@carbon yolk–shell nanowires with a volumetric ratio (void/FeO_x) of ∼1.04 present high gravimetric and volumetric capacities of 1045.4 mA h g⁻¹ and 380.5 mA h cm⁻³ at 0.2 A g⁻¹, respectively, a good rate capability at various current densities and a robust cycling stability. The appropriate void space plays a critical role in accommodating volume swing, preserving the structural integrity of porous FeO_x nanowires, confining electroactive materials inside of carbon shells and maintaining good electrical contact between FeO_x and carbon shells during the lithiation and delithiation cycling. However, if the void space is too small or too large, the structural damage of FeO_x@C yolk/shell nanowires would occur attributed to a large volume expansion (∼90%) during lithiation/delithiation processes or the divulgation of the pulverized FeO_x from carbon shells, respectively, leading to their poor performance as anodes of LIBs.