Enhancing Li-ion capacity and rate capability in cation-defective vanadium ferrite aerogels via aluminum substitution

Cation-defective iron oxides have proven to be effective Li-ion charge-storage hosts in nonaqueous electrolytes, particularly when expressed in disordered, nanoscale forms such as aerogels. Replacing a fraction of Fe sites in ferrites with high-valent cations such as V⁵⁺ introduces cation-vacancy defects that increase Li-ion capacity. Herein, we show that compositional substitution with electroinactive Al³⁺ further increases Li-ion capacity by 30% when incorporated within a disordered VFe₂Ox aerogel, as verified by electrochemical tests in a two-terminal Li half-cell. We use electroanalytical techniques to show that both Al-VFe₂Ox and VFe₂Ox aerogels exhibit many of the hallmarks of pseudocapacitive materials, including fast charge-discharge and surface-controlled charge-storage kinetics. These disordered, substituted ferrites also provide the high specific capacity expected from battery-type electrode materials, up to 130 mA h g^-1 for Al-VFe₂Ox. Our findings are discussed in the context of related Li-insertion hosts that blur the distinctions between battery-like and capacitor-like behavior.