Investigation of Nafe(PO3)3 and Na3Fe3(PO4)4 Orthophosphates for Sodium-Ion Batteries: Structural and Electrochemical Insight

Rechargeable batteries in general and sodium-ion batteries in particular are widely pursued for economic implementation of grid-scale power storage. Sodium-ion batteries are viable for micro-to-medium scale grid-storage owing to the abundance of sodium-resource. Similar to the Li-ion batteries, cathodes form the key component in designing efficient sodium-ion batteries, where Fe-based compositions are gaining popularity for their elemental abundance (Barpanda, Chem. Mater. 28 , 1006, 2016). While Fe-oxides have been shown to deliver high capacity approaching 200 mAh/g [P2-Na(Fe 1/2 Mn 1/2 )O 2 ; Yabuuchi et al, Nature Mater. , 11 , 512, 2012], Fe-based polyanionic compounds have been reported to display high voltage (ca. 3.8 V) operation [Na 2 Fe 2 (SO 4 ) 3 ; Barpanda et al, Nature Commun. , 5 , 4358, 2014]. Pursuing the Fe-based polyanioinic chemistry, we have explored the Na-Fe-P-O quarternary systems to derive cathodes with robust thermal/ chemical stability. Using solution combustion synthesis as a rapid screening route (Barpanda et al, J. Mater. Chem. , 22 , 13455, 2012), we can prepare suites of Fe(II)-based cathode insertion materials from low cost Fe(III)-based precursors. Dwelling into the Na-Fe-P-O quarternary system, we have investigated two compounds: (i) NaFe(PO 3 ) 3 and (ii) Na 3 Fe 3 (PO 4 ) 4 orthophosphate compounds as possible cathodes for large-scale Na-ion batteries. These compounds can be synthesised by combustion route using Fe(NO 3 ) 3 .9H 2 O precursor involving the final annealing temperature of 500-600 °C for restricted duration of 3-5 h. Additionally, the family of Na(Fe 1-y Mn y )(PO 3 ) 3 and Na 3 (Fe 1-y Mn y ) 3 (PO 4 ) 4 solid-solutions have been prepared for the first time. Their crystal structures have been verified by synchrotron X-ray diffraction (at KEK-Photon Factory, Tsukuba, Japan). While Na(Fe 1-y Mn y )(PO 3 ) 3 adopts an orthorhombic (s.g. P 2 1 2 1 2 1 ) structure, Na 3 (Fe 1-y Mn y ) 3 (PO 4 ) 4 assumes a monoclinic (s.g. C 2/ c ) framework. Mössbauer analysis suggests at multiple Fe-sites with preferential filling of certain by Mn substituants. In both cases, the structural (Bond valence sum) analysis in sync with first-principle density functional theory calculations suggest at the presence of one-dimensional Na + -diffusion characteristics in these compounds. With cathode optimization using SuperP carbon additives and mechanical milling, cathode composites were prepared to test their electrochemical performance. We could notice charge-discharge voltage profiles in NaFe(PO 3 ) 3 for the first time delivering a reversible capacity over 30 mAh/g with the average potential located at 3.1 V (vs. Na/Na + ). Though the capacity is limited, effort is currently geared to further improve it. In case of Na 3 Fe 3 (PO 4 ) 4 , we could obtain capacity over 50 mAh/g with the central Fe 3+ /Fe 2+ redox potential located at 2.55 V (vs. Na/Na + ). In both cases, the galvanostatic cyling involves solid-solution redox mechanism. Combining various experimental and computational data, we will describe the synthesis, structural, magnetic and electrochemical properties of these two Fe-containing PO 4 -based polyanionic materials for sodium-ion batteries.