Iron Selenide‐Based Heterojunction Construction and Defect Engineering for Fast Potassium/Sodium‐Ion Storage

Suitable anode materials with high capacity and long cycling stability, especially capability at high current densities, are urgently needed to advance the development of potassium ion batteries (PIBs) and sodium ion batteries (SIBs). Herein, a porous Ni‐doped FeSe2/Fe3Se4 heterojunction encapsulated in Se‐doped carbon (NF11S/C) is designed through selenization of MOFs precursor. The porous composite possesses enriched active sites and facilitates transport for both ion and electron. Ni‐doping is adopted to enrich the lattice defects and active sites. The Se–C bond and carbon framework endow integrity of the composite and hamper aggregation of selenide nano‐particles during potassiation/de‐potassiation. The NF11S/C exhibits exceptional rate performance and ultra‐long cycling stability (177.3 mA h g−1 after 3050 cycles at 2 A g−1 for PIBs and 208.8 mA h g−1 after 2000 cycles at 8 A g−1 for SIBs). The potassiation/de‐potassiation mechanism is investigated via ex‐situ X‐ray powder diffraction, high‐resolution transmission electron microscopy, X‐ray photoelectron spectrocopy and Raman analysis. PTCDA//NF11S/C full cell stably cycles for 1200 cycles at 200 mA g−1 with a capacity of 103.7 mA h g−1, indicating the high application potential of the electrode for highly stable rechargeable batteries. Defect engineering expands the lattice spacing of traditional selenide and boosts the ion/electron diffusion. In addition, heterojunction composite material with nanoparticles encapsulated in porous carbon framework and abundant interior space effectively alleviate the volume effect and enable the fluent transport of Na+/K+ ions and electrons, which enables a Ni‐doped FeSe2/Fe3Se4‐based electrode with highly stable cycling performance.