Electrochemical Li-ion storage in defect spinel iron oxides: the critical role of cation vacancies

Rechargeable lithium-ion batteries are the preferred power source for consumer electronic devices, but the cost and toxicity of many cathode materials limit their scale-up. Worldwide research efforts are addressing this concern by transitioning from conventional Co- and Ni-based intercalation hosts towards Fe- and Mn-based alternatives. The unfavorable energetics of the Fe2+/3+redox couple and limited Li-insertion capacities render the use of iron oxides impractical. We address this limitation with the defect spinel ferrite γ-Fe2O3 as a model structure for Li-ion insertion by replacing a fraction of the Fe3+ sites with highly oxidized Mo6+ to generate cation vacancies that shift the onset of Li-ion insertion to more positive potentials as well as increase capacity. In the present study, native and Mo-substituted iron oxides are synthesized via base-catalyzed precipitation in aqueous media, yielding nanocrystalline spinel materials that also exhibit short-range disorder characteristic of a proton-stabilized structure. The Mo-substituted ferrite reported herein is estimated to have ∼3× as many cation vacancies as γ-Fe2O3 with a corresponding increase in the Li-ion capacity to >100 mA h g−1 between 4.1 and 2.0 V vs.Li/Li+. This dual enhancement in capacity and insertion potential will enable these and related defect spinel ferrites to be explored as positive electrode materials for lithium batteries, while retaining the cost advantages of a material whose metal composition is still predominately iron based.