Air-Stable Porous Fe₂N Encapsulated in Carbon Microboxes with High Volumetric Lithium Storage Capacity and a Long...

The development of inexpensive electrode materials with a high volumetric capacity and long cycle-life is a central issue for large-scale lithium-ion batteries. Here, we report a nanostructured porous Fe₂N anode fully encapsulated in carbon microboxes (Fe₂N@C) prepared through a facile confined anion conversion from polymer coated Fe₂O₃ microcubes. The resulting carbon microboxes could not only protect the air-sensitive Fe₂N from oxidation but also retain thin and stable SEI layer. The appropriate internal voids in the Fe₂N cubes help to release the volume expansion during lithiation/delithiation processes, and Fe₂N is kept inside the carbon microboxes without breaking the shell, resulting in a very low electrode volume expansion (the electrode thickness variation upon lithiation is ∼9%). Therefore, the Fe₂N@C electrodes maintain high volumetric capacity (1030 mA h cm^-3 based on the lithiation-state electrode volume) comparable to silicon anodes, stable cycling performance (a capacity retention of over 91% for 2500 cycles), and excellent rate performance. Kinetic analysis reveals that the Fe₂N@C shows an enhanced contribution of capacitive charge mechanism and displays typical pseudocapacitive behavior. This work provides a new direction on designing and constructing nanostructured electrodes and protective layer for air unstable conversion materials for potential applications as a lithium-ion battery/capacitor electrode.