ZnFe$_2$O$_4$ and ZnO/Fe$_2$O$_3$ nanoparticles as anode materials for lithium-ion batteries: understanding of the lithium storage mechanism

Abstract : Lithium-ion batteries have been widely used in portable electronic devices for years. With the depletion of fossil fuels and the growing environmental concerns, lithium-ion batteries received considerable attention to contribute to the development of electric vehicles and storage devices for renewable energies. However, existing lithium-ion batteries cannot reach sufficient energy density to address the needs for such applications. One of the issues limiting the energy density is the low specific capacity of the graphite anode (372 mAh/g). Mixed-transition metal oxides (also called ternary oxides) with a spinel structure (AB$_2$O$_4$ with A and B transition metals) appear as a promising solution to replace graphite with a higher theoretical capacity (between 750 and 1000 mAh/g depending on the oxide). Among various oxides, ZnFe$_2$O$_4$ is an interesting substitute for graphite, as an abundant, cheap, non-toxic and environmentally friendly material with a high theoretical capacity of 1000 mAh/g. This capacity corresponds to 9 Li$^+$ ions stored per formula unit as shown in the widely accepted storage mechanism described below.