Understanding sodium storage properties of ultra-small Fe ₃ S ₄ nanoparticles – a combined XRD, PDF, XAS and electrokinetic study

Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, we present a rigorous study about Na storage properties of ultra-small Fe₃S₄ nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe₃S₄ into nanocrystalline intermediates occurs accompanied by reduction of Fe³⁺ to Fe²⁺ cations. Discharge to 0.1 V leads to formation of strongly disordered Fe⁰ finely dispersed in a nanosized Na₂S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na₂S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention, e.g., 486 mAh g^-1 after 300 cycles applying 0.5 A g^-1 and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe₃S₄ with smaller domains are reversibly generated in the 1^st cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe₃S₄ in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.