Improving the Specific Capacity and Cyclability of Sodium-Ion Batteries by Engineering a Dual-Carbon Phase-Modified Amorphous and Mesoporous Iron Phosphide

Based on the concept of the nanoconfinement reaction, a synthetic strategy is developed to construct carbon-coated iron phosphide (FeP) with an amorphous and mesoporous framework anchored on carbon nanotubes (CNTs). The synthesis involves direct growth of FeOOH on the CNTs followed by silica coating, carbon coating, and subsequent treatments of low-temperature phosphidation and silica removal procedures. During the synthesis, the silica layer is adopted to not only serve as a sacrificial internal spacer to increase the mesoporosity (311m(2)g(-1) and 0.36cm(3)g(-1)) of the FeP framework, but also to provide a confined environment in guiding the structural evolution of the homogenous/conformal framework and topologically amorphous nature of FeP. When used as an anode material in sodium-ion batteries (SIBs), the FeP-based electrode shows a utilization rate of 78% for the active material and a reversible capacity of 415 mAhg(-1). Even at a higher current density of 500 mAg(-1), a capacity retention rate of 90% over 500 cycles is obtained with a capacity-decay rate of only 0.02% per cycle. The much improved performance of the FeP-based electrode in SIBs clearly demonstrates the potential of FeP to be used as the anode material in SIBs when it is engineered in a nanoconfinement environment to overcome its structural constraints. In principle, this strategy can be adapted to engineer other transition-metal phosphide-based materials for energy-storage applications.