Characterization of electrolytic deposited α-Fe2O3 thin films on stainless steel as anodes for Li-ion batteries

Electrolytic hematite (α-Fe2O3) thin films on 304 stainless steel substrates are carried out in FeCl3 aqueous solution for anodes in thin film lithium ion batteries. Through cathodic polarization tests, three major regions are verified: (1) O2 + 4 H+ + 4e− → 2H2O (from 0.12 to − 0.4 V), (2) 2H+ + 2e− → H2 (from − 0.4 to − 0.88 V), and (3) 2H2O + 2e− → H2 + 2OH− and/or Fe(OH)2 + + H2O + 2e− → FeOOH + H2 (from − 0.88 to − 2.0 V). The as-deposited film carried out at region 3 is porous α-FeOOH, and condensed into uniform α-Fe2O3 at 250 °C revealing thickness 5 μm and particle size 10 nm. Cyclic voltammetry (CV) measurements show oxidation peaks at 1.0 and 1.8 V, and reduction peaks at 1.2 and 0.63 V (Li/Li+). At a discharge/charge current density of 20 μA cm− 2, the Fe2O3 film reveals 1162 for the 1st, 997 for the 2nd, and 482 mAh g− 1 for the 50th cycle. However, the capacity is lowered with increasing current density and increasing cycle number, accompanied with the coarsening of nano-sized particles. It is speculated that the diffusion flux of Li+ into and out from α-Fe2O3 dominates the electrochemical reaction rate. The coarsening of α-Fe2O3 particles elongates the diffusion length of Li+ during charge/discharge tests, resulting in the capacity decay.

► Hematite (α-Fe2O3) thin films are originally prepared and optimized by electrochemical method.
► The uniform α-Fe2O3 film shows a discharge capacity of 1162 mAhg− 1 for the 1st cycle.
► The coarsening of α-Fe2O3 particles during charge/discharge tests results in the capacity decay.