The formation and mechanism of nano-monocrystalline γ-Fe2O3 with graphene-shell for high-performance lithium ion batteries

Using a sintering process with Prussian Blue (PB) and 20 wt% glucose at high temperature (950 °C for 6 hours in Ar/H2) with oxidation in the air at room temperature, we synthesized a nano-monocrystalline γ-phase iron oxide (γ-Fe2O3) compound coated with carbon comprising a number of graphene layers, which was named as core–shell nano-monocrystalline γ-Fe2O3@graphene. It can be noted that the formation of nano-monocrystal is different from forming core–shell nano-polycrystalline hollow γ-Fe2O3@graphene sintered at lower temperature (650 °C 6 hours in Ar) via a simple Kirkendall process with oxidation at room temperature as reported in our previous study. We further investigate how nano-monocrystalline γ-Fe2O3 is formed by controlling the synthesis process and testing with TEM and SEM. We confirmed that the nano-monocrystalline γ-Fe2O3 is grown from nano-monocrystalline Fe with interface catalysis of O2 and the related mechanism is discussed through comparing the structures of γ-Fe2O3 and the Fe crystals. The core–shell nano-monocrystalline γ-Fe2O3@graphene shows high performance as an anode material in Li-ions batteries (much better than nano-polycrystalline hollow γ-Fe2O3@graphene reported in previous study). For example, the cycling stability and rate performance are remarkable as an anode material for lithium ion batteries with a high reversible capacity of 848.08 and 782.54 mA h g−1 at 1C and 5C for 600 cycles, respectively, and a high rate performance (284.42 mA h g−1 at 20C). Another interesting performance is that during the first 80 cycles, the specific capacity increases, which may result from more interface area being generated by the γ-Fe2O3 nano-monocrystal crushing with protection of the graphene-shell during the initial charging/discharging cycles. This synthesis method and mechanism can be used as a guide to produce γ-Fe2O3 as an anode material for lithium ion batteries with high performance on a large scale.