Unraveling the Underlying Redox Mechanism in Na2FeP2O7 Pyrophosphate Cathode for Sodium-Ion Batteries
Authors: Gond Ritambhara , Barpanda Prabeer 
Published: February 2020

Polyanionic materials have contributed plethora of electrodes (anode and cathode) in the field of Li-ion and Na-ion battery. Li-ion batteries (LIB) and Na-ion batteries (SIB) are building up in our landfills thus researchers need to develop more efficient, less hazardous and safer electrode materials for these class of rechargeable batteries. In the race of developing novel and safer cathode “pyrophosphate-based” polyanionic compounds has rekindled scientific attention.[1] Recently analysis of phase stabilities reveals that LiFeP2O7 (P21/c) is intrinsically more stable than FePO4 (olivine) against reduction (high temperature). Similar high thermal stability is also observed for Li1.4MnP2O7 which decomposes to Li2MnP2O7, Mn2P2O7, LiPO3, and O2 at 450 °C, much higher than the olivine counterpart MnPO4.[2] considering this fact and probing Li/Na-M-P-O quaternary system, Li2FeP2O7 and Na2FeP2O7 has been reported as a 3.5 V and 3 V cathode material for LIB and SIB (Figure A, B) .[3,4] These materials offer a robust three-dimensional (P2O7)-4-framework with multiple sites for alkali ions and robust chemical/ thermal stability. Based on earth-abundant elements, Na2FeP2O7 can form an economic cathode for grid storage. Thus in this work we focus our interest on Na2FeP2O7.
Na2FeP2O7 has triclinic (space group: P-1) structure, where each corner of FeO6 octahedra is bridged to PO4 tetrahedra. Such arrangements of FeO6 octahedra where no edge-sharing can be seen will delimit large channels for migration of charge carrier. During the galvanostatic charge-discharge reversible desodiation reaction occur between pristine Na2Fe2+P2O7 and desodiated NaFe3+P2O7 end-members involving an Fe3+/Fe2+ redox reaction. During this redox process, it goes through various mixed-valence intermediate compositions [Na2-xFe2+/3+P2O7, x = 0-1] with steady structural ordering/ reorientation. These intermediate stages can be traced with a combination of electrochemical techniques and in-situ X-ray diffraction technology.[5]
Here, in the present work we have studied the underlying redox mechanism by preparing a custom-made in-situ cell, able to perform both in reflection and transmission mode. Pristine Na2FeP2O7 was synthesized by combustion method using Ferric nitrate (III) precursor. Carbothermal reduction was done using Ascorbic acid (for reduction of Fe+3 into Fe+2) and Urea fuel. Electrodes were prepared by mixing 80 wt% of Na2FeP2O7 with 20 wt% of Carbon super P. 1 M NaClO4 dissolved in propylene carbonate (PC) was used as an electrolyte to form Na half-cell. In-situ X-ray diffraction patterns were recorded (Figure C) at C/25 C-rate during electrochemical testing. XRD pattern were recorded for 3624 seconds at an interval of 2 seconds in the angular domain of 9-60°. Observation of such measurement clearly tells the composition variation of Na into Na2FeP2O7 host during the Galvanostatic charge-discharge process. The exploration of structure-property relationship like de-insertion and re-insertion of Na from the host material is very clear from the phases recorded which actually tells what are the crystal planes involved for charge carrier migration. A close look in the 2θ range of 15-18°, 20-25° and 30-35° reveals the structural changes occurring due to Na de-insertion from the host Na2FeP2O7 giving rise to Na2-xFeP2O7 (at complete charge) shown in red color in Figure C. After complete charge the diffraction peaks are transforming and acquiring the pristine phase, shown in blue color which strongly suggest the re-insertion of Na into Na2-xFeP2O7 giving rise Na2FeP2O7 after the first electrochemical cycle. Volume strain of the crystal during cathode operation can be verified in the 2θ range 30-35° which shows widening of the peak position i.e. some peaks are shifting towards low 2θ, and some are towards high 2θ, possibly coming due to lattice expansion due to Na removal. Removal of Na has resulted stronger repulsive interactions between electronegative oxygens in Na2-xFeP2O7 during charge. No matter how strong is this interaction structure is not collapsing which is due to strong framework built by pyrophosphate units. Thus in-situ operando diffraction based experiments are very beneficial for the researchers as at the same time it gives both electrochemical data and structure insight which will reveal the stability of future cathode materials.

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Figure1. Galvanostatic charge-discharge profile of Li2FeP2O7 (A.), Na2FeP2O7 (B.) and In-situ XRD pattern (C.) of Na2FeP2O7 at C/25 rate from 2 to 3.7 V.

Figure 1