Significantly Improved Cyclability of Conversion‐Type Transition Metal Oxyfluoride Cathodes by Homologous Passivation Layer Reconstruction

Electrode stabilization by surface passivation has been explored as the most crucial step to develop long‐cycle lithium‐ion batteries (LIBs). In this work, functionally graded materials consisting of “conversion‐type” iron‐doped nickel oxyfluoride (NiFeOF) cathode covered with a homologous passivation layer (HPL) are rationally designed for long‐cycle LIBs. The compact and fluorine‐rich HPL plays dual roles in suppressing the volume change of NiFeOF porous cathode and minimizing the dissolution of transition metals during LIBs cycling by forming a structure/composition gradient. The structure and composition of HPL reconstructs during lithiation/delithiation, buffering the volume change and trapping the dissolved transition metals. As a result, a high capacity of 175 mAh g−1 (equal to an outstanding volumetric capacity of 936 Ah L−1) with a greatly reduced capacity decay rate of 0.012% per cycle for 1000 cycles is achieved, which is superior to the NiFeOF porous film without HPL and commercially available NiF2‐FeF3 powders. The proposed chemical and structure reconstruction mechanism of HPL opens a new avenue for the novel materials development for long‐cycle LIBs. Functionally graded materials are designed to solve transition metal dissolution and volume expansion issues of “conversion‐type” cathodes for long‐cycle lithium‐ion batteries. In such materials, a homologous passivation layer (HPL) with a structure/composition gradient is rationally designed to passivate “conversion‐type” cathodes. The structure and composition reconstruction of HPL help to buffer the volume change and trap the dissolved transition metals.