Transition metal oxides (TMOs) represent a crucial source for realizing electrode materials for Li-ion batteries (LIBs) and, as such, have been investigated intensively for this purpose. Among several transition metal oxide compounds, a class of ternary oxides known as “Delafossites” is worthy of note due to a characteristic layered structure and the presence of two different polytypes^1,2 (i.e. rhombohedral and hexagonal) depending on the particular orientation of the layers. AgFeO₂ is a typical Delafossite prototype and has attracted attention in LIB studies, as it can be synthesized easily at low temperatures,^3,4 while a good control of Ag/Fe ratios can also be achieved to produce several Ag_xFeO_y compositions.^5-7

However, AgFeO₂ and Ag_xFeO_y have mainly been considered for possible application in positive LIB electrodes so far, despite the fact that this Delafossite structure is not entirely stable towards insertion of Li⁺ ions. Indeed, moderate Li insertion into AgFeO₂ (e.g. one mole per formula unit) causes an irreversible structural change^3-7 at 1.7 V vs. Li⁺/Li with a concomitant reduction of Ag⁺ to Ag⁰, which is progressively extruded from its original lattice in the form of nanoparticles. The latter cannot be re-oxidized to Ag⁺ upon Li⁺ removal, thereby causing an irreversible charge loss. Conversely, the formation of Ag⁰ in these compounds has been reported to have a very positive impact on the overall resistance of the resulting electrodes, in which a massive decrease of the resistivity^6,7 was noticed upon additional Li⁺ incorporation. Therefore, extensive lithiation of AgFeO₂ at low potentials vs. Li⁺/Li is clearly attractive for possible use of this compound as alternative negative electrode. Further Li storage can be achieved in this active material via combined conversion reaction⁸ of its FeO₆ units and Li-alloying⁹ of these in situ-formed Ag⁰ nanoparticles. With these ideas in mind, we have synthesized AgFeO₂ nanoparticles via a simple precipitation route at room temperature and included them in composite electrodes with Na-alginate binder and carbon black conductive additive.¹⁰

A synergy between Li-Ag alloying and a characteristic pseudocapacitive behavior,¹¹ due to the formation of Fe⁰/Li₂O boundaries, is observed at low voltages vs. Li⁺/Li, promoting convenient charge storage. This combined mechanism is also employed in full cells having deeply lithiated AgFeO₂ as negative electrode and LiFePO₄ as positive counterpart to realize an entirely Fe-based rechargeable charge storage device.^10,12 Results from the cycling of both Li half-cells (Figure 1) and full cells will be presented, highlighting current challenges and opportunities for this intriguing Fe-based compound.

Figure 1. (a) Characteristic voltage profiles of a composite electrode containing AgFeO₂ nanoparticles, Na-alginate and carbon black cycled in a Li half-cell at a current density of 0.05 mAcm^-2 between 0.05 and 2.80 V vs. Li⁺/Li with a standard LP40 electrolyte. (b) Discharge and charge capacities for the same cell in correspondence of increasing cycle numbers and associated Coulombic efficiencies.

Acknowledgements

The funding by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS) via the personal grant no. 245-2014-668 is acknowledged together with the Knut and Alice Wallenberg (KAW) Foundation for providing the electron microscopy facilities at Stockholm University. StandUp for Energy is acknowledged as well.

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