Marcasite-FeS₂@carbon nanodots anchored on 3D cell-like graphenic matrix for high-rate and ultrastable potassium ion storage

Marcasite-FeS₂ has significant potential as an anode material for potassium-ion batteries (KIBs) for its lower band gap, high theoretical capacity, low cost and environmental friendliness but suffers poor cycle stability and rate capability. Here, we design a marcasite-FeS₂@carbon nanodots anchored on 3D cell-like S, N co-doped graphenic matrix (m-FeS₂@C-SSNFG). The 3D graphenic matrix is constructed by graphenic cages and sustained by conductive graphitic struts that can curb the agglomeration of graphenic sheets during cycling. Remarkably, the yolk-shell m-FeS₂@graphenic cage structure not only ensures uniform dispersion of m-FeS₂ on the cell wall, but also offers enough void room to accommodate the volume expansion of m-FeS₂. Meanwhile the graphitic carbon shell wrapped around m-FeS₂ also prevents the active m-FeS₂ nanodots from agglomeration and dissolving into electrolyte. It is worth underlining that the phase transition from semiconductor (m-FeS₂) to conductor (K_xFeS₂) is firstly validated through first-principles calculations, beneficial to the electron transfer and K-ions diffusion in subsequent cycles. Besides, optimizing electrolyte and cut-off voltage can further boost its long-term cycle stability. Thus, it delivers high capacity of 419 mA h g⁻¹ after 100 cycles at 0.05 A g⁻¹ and ultralong cycle stability of 140 mA h g⁻¹ at 5 A g⁻¹ over 4000 cycles.