In Situ Growth Of FeS Nanostructured Films For Lithium-Ion Batteries

A facile solution-based approach has been developed for the in-situ growth of mackinawite FeS microsheet arrays and FeS microspheres directly on Fe foil. The influences of the concentration of Fe3+ in the initial solution, the sulfur sources and solvent on the shape of the crystallites were investigated systematically. Besides, the formation mechanism of the FeS nanostructured films was proposed according to time-dependent experiments and adjusted synthesis conditions. Furthermore, the electrochemical properties of FeS microsheet arrays and microspheres as anode electrodes in lithium-ion storage were tested based on two-electrode cointype cells with Li metal as the counter-electrode. Finally, we in-situ investigated the volume change and the phase transformation mechanism during electrochemical cycles inside transmission electron microscopy (TEM).It is found that the concentration of Fe+ influences the dense degree of microsheets, while sulfur sources significantly impact the uniformity and purity of the products. Furthermore, ethylenediamine (EDA) as a strong donor ligand plays an important role in the formation of FeS microsheet arrays. The FeS-EDA molecular precursors may serve as molecular templates in the control of the crystal growth, which is lead to the microsheet networks obtained in the presence of EDA.The electrochemical properties of FeS microsheet arrays and microspheres as anode electrodes in lithium-ion storage were tested. We found that the FeS microsheet networks deliver promising Li storage capacity (772 mAh g-1 at the 1st cycle and 697 mAh g-1 at the 20th cycle), much higher than that of the FeS microspheres. The enhanced electrochemical performance of the FeS microsheet arrays can be attributed to following advantages:the microsheet structure greatly shortens the ionic diffusion length and provides sufficient electrode-electrolyte contact area for lithium-storage reactions; the FeS microsheets are interconnected with each other to form macropores, which can effectively accommodate large volume change induced by the Li+ intercalation/extercalation; in comparation with the fabrication of traditional battery electrode, in-situ growth of microsheet networks onto the metal current collector results in good electrical contact of every microsheet with the metal current collector, and no additive is needed for the preparation of electrode.We used FeS nanosheet as electrode to investigate their electrochemical behavior and mechanism during the charge and discharge process using in-situ TEM. FeS nanosheet shows a small size expansion (129% at 1st and 113% at 3rd cycles) and few fractures, which attributes to the excellent Li+ conductivity of FeS nanosheet and accounts for the improved cyclability. Furthermore, FeS nanosheet converts to Fe nanocrystals of 2-3 nm embedded within Li2S matrix after the first lithiation. The subsequent electrochemical reaction is a reversible phase conversion between Fe/Li2S and Li1.13FeS2 nanocrystals. Our direct observation provided important mechanistic insight for developing high-performance conversion electrodes for LIBs.