Li-Alloying and Conversion Reactions in Silver Ferrite Nanoparticles for Li-Ion Battery Negative Electrodes
Authors: Berastegui Pedro , Tai Cheuk-Wai , Valvo Mario 
Published: February 2020

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 polytypes1,2 (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 AgxFeOy compositions.5-7

However, AgFeO2 and AgxFeOy 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 change3-7 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 resistivity6,7 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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Figure 1