Nickel Regulating Endows Fe₇Se₈ With Stable Potassium-Ion Storage

Iron-based selenides are considered as potential electrode materials in potassium-ion batteries (PIBs) owing to the merits of high capacity, intrinsic safety, and cost-effectiveness. However, sluggish electronic/ionic transport kinetics and large volume variations result in suboptimal electrochemical performance. Herein, we report a nickel-doped Fe₇Se₈ with double-shell N-doped carbon (Ni-Fe₇Se₈@DNC) as anode for robust potassium ion storage. Notably, the introduction of Ni induces the lattice distortion and leads to a rearrangement of charge, thereby creating numerous active sites and optimizing the band structure to enhance charge transport. Additionally, the elastic carbon shell can synergistically mitigate the volume expansion upon cycling and maintain the structural stability. Thus, the Ni-Fe₇Se₈@DNC presented excellent cycling stability of more than 1 year (464.8 mAh g⁻¹after 1000 cycles at 0.1 A g⁻¹, the best stability among all iron-based selenides) and satisfactory rate capability. The potassium-ion hybrid capacitors (PIHCs) have also demonstrated a remarkable energy density of 186.5 Wh kg⁻¹ at 0.2 A g⁻¹. Density functional theory calculations, in conjunction with a range of characterization methods, validate the rapid pseudocapacitive effect and lower ion diffusion energy barriers, resulting from Ni doping, improve reaction kinetics. This study paves the avenue for novel anode material designs for PIBs.