ϵ‑FeOOH: A Novel Negative Electrode Material for Li- and Na-Ion Batteries

The demand for eco-friendly materials for secondary batteries has stimulated the exploration of a wide variety of Fe oxides, but their potential as electrode materials remains unknown. In this contribution, ϵ-FeOOH was synthesized using a high-pressure/high-temperature method and examined for the first time in nonaqueous Li and Na cells. Under a pressure of 8 GPa, α-FeOOH transformed into ϵ-FeOOH at 400 °C and then decomposed into α-Fe2O3 and H2O above 500 °C. Here, FeO6 octahedra form [2 × 1] tunnels in α-FeOOH or [1 × 1] tunnels in ϵ-FeOOH. The ϵ-FeOOH/Li cell exhibited a rechargeable capacity (Q recha) of ∼700 mA h·g–1 at 0.02–3.0 V, whereas the ϵ-FeOOH/Na cell indicated a Q recha of less than 30 mA h·g–1 at 0.02–2.7 V. The discharge and charge profiles of ϵ-FeOOH and α-FeOOH were similar, but the rate capability of ϵ-FeOOH was superior to that of α-FeOOH.