Enhanced Electrochemical Lithium-Ion Charge Storage of Iron Oxide Nanosheets

Iron oxides are appealing cathode materials for low-cost electrochemical energy storage, but iron oxide nanoparticles (NPs) exhibit very low capacities, particularly at fast charging and discharging times, which are increasingly important for numerous applications. We report that synthesis and stabilization of iron oxide in nanosheets results in significantly improved lithium-ion charge storage capacities compared to those of iron oxide NPs at both slow and fast charging/discharging times. The iron oxide nanosheets have lateral dimensions of ∼50 nm and thicknesses of ∼1 nm and are composed of smaller crystallites. The structure of the nanosheets is consistent with maghemite, γ-Fe 2 O 3, which contains cation defects. The γ-Fe 2 O 3 phase is not typically observed within a nanosheet form, and γ-Fe 2 O 3 nanosheets transform to NPs at a relatively low temperature of 200 °C. The transformation of γ-Fe 2 O 3 from a nanosheet to an NP occurs in conjunction with removal of structural H 2 O. The γ-Fe 2 O 3 nanosheets exhibited lithium-ion charge storage capacities of up to 148 mA h g –1, which is significantly greater than that of commercial γ-Fe 2 O 3 NPs (32 mA h g –1 ). γ-Fe 2 O 3 nanosheets showed the ability to be rapidly charged and discharged (93.2 mA h g –1 at a 9 min discharge time) with significantly higher capacities than γ-Fe 2 O 3 NPs. The electronic conductivity of the nanosheets was 3 times higher than that of NPs, which is attributed to facilitated electron conduction within the nanosheets. Kinetic analysis of the charge storage mechanism suggests the nanosheets store charge predominantly via a capacitive charge storage process rather than conventional intercalation. The understanding of how to synthesize and stabilize iron oxide nanosheets, their unique electrochemical properties, and their distinct charge storage mechanism furthers the design of charge storage materials with improved capacities, enhanced rate capabilities, and lower cost.