Understanding sodium storage properties of ultra-small FeS 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 Fe 3 S 4 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 Fe 3 S 4 into nanocrystalline intermediates occurs accompanied by reduction of Fe 3+ to Fe 2+ cations. Discharge to 0.1 V leads to formation of strongly disordered Fe 0 finely dispersed in a nanosized Na 2 S 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 Na 2 S 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, Fe 3 S 4 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 Fe 3 S 4 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. The Na storage mechanism of Fe 3 S 4 nanoparticles is studied via electrochemical techniques, synchrotron-based X-ray diffraction and absorption methods. The results explain the relation of the electrodes cycle life and cut-off potentials.