Li-ion batteries are promising power sources for portable electronics and electric vehicles due to their high energy density. However, its application in grid storage is limited due to high cost and low abundance of Li resources. Next to Li-ion, Na-ion batteries (NIBs) are getting prominent interest due to its low cost, intercalation properties similar to Li and electrochemical potential of Na/Na⁺ (-2.71 V vs SHE) [1]. Recent studies based on layered NaMnO₂ showed high specific capacity of 140 mA h g^-1 by six biphasic transitions. However, the layered materials deliver poor cycling performance due to multiple phase transitions in Na insertion/extraction reactions [2]. On the other hand, Polyanion-type compounds are the most promising electrode materials for NIBs due to their stability, safety, and suitable operating voltages. Recently, Na₄M₃(PO₄)₂P₂O₇ (M=Co, Mn, Ni and Fe) are evaluated as electrode materials for energy storage applications due to its Na-ion intercalating properties, low cost and environmental friendliness [3,4]. In this work, the bulk powder of carbon coated Na₄Fe₃(PO₄)₂P₂O₇ material was prepared by solution combustion synthesis technique using ascorbic acid. The crystal structure was identified to be orthorhombic with Pna21 symmetry from Rietveld refinement. It has a 3D intercalating structure for Na-ion with a theoretical capacity of 129 mAh g^-1. The Na-ion cell with the carbon coated nano-Na₄Fe₃(PO₄)₂P₂O₇ cathode delivered a discharge capacity of ~110 mAh g^-1 at a 0.1C rate (Fig. 1a). The Na₄Fe₃(PO₄)₂P₂O₇ showed excellent rate capability with good cycling stability 100 cycles. Overall the voltage profile shows two plateau regions with the average voltage of ~3.1 V (Fig. 1b). The electrochemical results motivated us to synthesize the Na₄Fe₃(PO₄)₂P₂O₇ thin films for all-solid-state Na-ion batteries, which can be useful to design thin film micro-batteries. Thin films of Na₄Fe₃(PO₄)₂P₂O₇ electrode was deposited on stainless steel substrates using pulsed laser deposition (PLD) using KrF Laser source. Various deposition parameters were optimized to grow uniform thin films on substrates. From X-ray diffraction it was observed that films were crystallized into orthorhombic structure with Pna21 symmetry. Nano size and thickness of Na₄Fe₃(PO₄)₂P₂O₇ thin film grains were identified by field emission electron microscopy (FE-SEM) and atomic force microscopy (AFM) (Fig. 1d). Here, Na-ion intercalating properties of iron-based mixed-polyanion Na₄Fe₃(PO₄)₂P₂O₇ thin films was investigated. The electrochemical properties of both bulk and thin films of Na₄Fe₃(PO₄)₂P₂O₇ were studied. The thin films with thickness of ~200 nm delivered a maximum capacity and similar voltage profiles to the bulk material (Fig. 1d). The work demonstrates the capability of the mixed polyanionic material as high performance thin film Na-ion battery cathode. The electrochemical performances of the bulk and thin films of Na₄Fe₃(PO₄)₂P₂O₇ will be presented. Finally, an all-solid-state thin film micro-battery will be demonstrated using the full cell configuration having Na₄Fe₃(PO₄)₂P₂O₇ as cathode, NASICON Na₃Zr₂Si₂O₁₂ as solid electrolyte and Na₃Ti₂(PO₄)₃ as anode [5].

Figure 1. Electrochemical performances of Na₄Fe₃(PO₄)₂P₂O₇ in sodium half-cell architecture. (a) Galvanostatic charge/discharge curves of Na₄Fe₃(PO₄)₂P₂O₇ at a C/10 current rate. (b) Differential capacity vs. voltage (dQ/dV) plots, (c) Rate capability at different C rates and (d) Galvanostatic charge/discharge curves of Na₄Fe₃(PO₄)₂P₂O₇ thin film (inset) AFM and SEM images of the thin films grown by PLD technique [5].

Acknowledgement

Author B.S. gratefully acknowledges the DST (SERB), New Delhi, India (PDF/2015/00217) for providing Fellowship.