Iron-Based Mixed Phosphate Na4Fe3(PO4)2P2O7 Thin Films for Sodium-Ion Microbatteries

Baskar Senthilkumar,* Angalakurthi Rambabu, Chinnasamy Murugesan, Saluru Baba Krupanidhi, and Prabeer Barpanda*

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ABSTRACT: Iron-based polyanionic materials can be exploited to realize low cost, durable, and safe cathodes for both bulk and thinfilm sodium-ion batteries.

Herein, we report pulsed laser deposited mixed phosphate Na₄Fe₃(PO₄)₂P₂O₇as a positive electrode for thinfilm sodium-ion microbatteries. The bulk material and thinfilms of Na₄Fe₃(PO₄)₂P₂O₇ are employed by solution combustion synthesis (SCS) and the pulsed laser deposition (PLD) technique, respectively. Phase purity and the nature of the crystallinity of the thin films were confirmed by grazing incidence X-ray diffraction and transmission electron microscopy. Identification of surface roughness and morphology was obtained from atomic force microscopy and scanning electron microscopy, respectively. Emerging electrochemical properties were observed from charge−discharge profiles of the thinfilms, which are well comparable to bulk material properties. The Na₄Fe₃(PO₄)₂P₂O₇thinfilm electrodes delivered a highly reversible Na⁺storage capacity of∼120 mAh g⁻¹with an excellent stability of over 500 cycles. Electrochemical analysis results revealed that the thickness of thefilm affects the storage capacity.

1. INTRODUCTION

Lithium-ion batteries (LIBs) have realized unprecedented commercial success in myriads of portable electronic devices such as mobile phones, laptops, and digital generators.

Recently, the usage of LIBs has entered into large-scale applications such as (hybrid) electric vehicles and power grid applications. However, high cost and limited resources of lithium have raised concerns vis-à-vis the developments in large-scale applications. At this juncture, it has triggered an effort to develop alternative energy storage technology over LIBs employing earth-abundant economic elements. In this emerging era of post-Li-ion batteries, sodium-ion batteries (NIBs) have garnered much attention due to the abundance, ease of accessibility, low cost of sodium-based resources, and strikingly similar charge−discharge characteristics compared to LIBs.¹⁻⁵ It has triggered extensive research on sodium-ion batteries as an attractive alternative to LIBs, leading to numerous reports on sodium-based materials acting as cathodes⁶⁻⁸ and anodes.⁹⁻¹¹The ongoing quest is to design suitable electrodes with enhanced performance factors like energy density, power density, and cycling stability that are crucial for large-scale energy storage systems. In this pursuit, polyanionic-type insertion materials form a rich treasure house because of their robust three-dimensional crystal framework and tunable operating voltage. Over the years, a variety of polyanionic cathodes have been unraveled successfully. Some such candidates are the Prussian blue family,^12,13 phos-

phates,¹⁴⁻¹⁶fluorophosphates,¹⁷⁻¹⁹ and pyrophosphates.²⁰⁻²² Going a step further, mixed phosphate compounds form a niche class of polyanionic cathodes.²³ One such candidate is the mixed phosphates family with the general chemical formula of Na₄M₃(PO₄)₂P₂O₇(M = Co, Ni, Mn, Mg, or Fe), as first reported in 2001 by Sanz et al.²⁴⁻²⁶These mixed phosphates have an open framework with long-range interconnected channels (M₃P₂O₁₃) along the bdirection, favoring fast Na⁺- ion migration and enhanced electrochemical performance.

These long-range channels render appropriate pathways for Na⁺ migration with a low activation energy barrier.²⁷ Moreover, it has been reported that there are four crystallo- graphically distinct Na⁺ sites: even if some channels are blocked by defects, other channels are still accessible to facilitate Na⁺insertion/extraction into the electrode material.²⁸ In this mixed phosphate family, the iron-based Na₄Fe₃(PO₄)₂P₂O₇analogue forms the most attractive positive electrode material for NIBs owing to the low cost/toxicity, an abundance of iron-based resources, and suitable energy/power density. Besides, it can be easily prepared by conventional

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solid-state synthesis and works as a 3.1 V (vs Na⁺) sodium battery cathode involving multiple-electron redox reaction delivering high energy density with robust cycling stabil- ity.^25,27,29

On another note, in addition to bulk batteries, the development of thin film microbatteries has emerged as a key sector in this era of nanotechnology.^30,31 These micro- batteries have niche applications such as flexible/wearable devices, smart cards, remote sensors, metal oxide semi- conductor (CMOS) memory chips, implantable medical devices, and microelectromechanical systems (MEMS). An all-solid-state thin film microbattery is ideal in this context.

Significant efforts have been devoted to searching advanced thin film electrode materials to enhance capacity and to improve stability for all-solid-state thinfilm batteries. To date, very few sodium-based insertion materials have been employed in thinfilm batteries.³² Here, with due material optimization (like nanosizing), we have implemented mixed phosphate Na₄Fe₃(PO₄)₂P₂O₇in thin film batteries for the first time by pulsed laser deposition. It delivers robust electrochemical performance for thinfilm Na-ion batteries.

2. EXPERIMENTAL SECTION

2.1. Synthesis of Iron-Based Mixed Phosphate Na4Fe3(PO4)2P2O7. The Na₄Fe₃(PO₄)₂P₂O₇ product was synthesized by the solution combustion synthesis (SCS) route. In a typical synthesis process, 1.19 g of NaH₂PO₄, 3.03 g of Fe(NO₃)₃·9H₂O (oxidizer), and 2.64 g of C₆H₈O₆ (fuel) were thoroughly dissolved in 50 mL of double-distilled water. In the initial step, the precursor mixture was heated at 120°C (for 2 h) with steady magnetic stirring to evaporate the excess water. Then, it was directly transferred to a hot plate maintained at 300 °C. It led to the ascorbic acid (fuel)- triggered exothermic ignition of reactants, resulting in the formation of an amorphous intermediate product. Finally, this product was ground to afine powder and was annealed at 600

°C for 12 h (under steady Ar flow) to obtain a phase-pure mixed phosphate Na₄Fe₃(PO₄)₂P₂O₇ (denoted as NFPPO) product.

2.2. Pulsed Laser Deposition of Na₄Fe₃(PO₄)₂P₂O₇ Thin Films.The NFPPO powder was uniaxially pressed and calcined to make a circular pellet (diameter, 13 mm; thickness, 4 mm) acting as a target. Employing the pulsed laser deposition (PLD) technique, NFPPO thin films were

uniformly deposited on circular stainless steel (SS) disks acting as substrates. A krypton fluoride excimer laser with a pulse energy of 150 mJ/pulse was used to ablate the Na₄Fe₃(PO₄)₂P₂O₇ target. The laser spot was aligned to focus on the target with a laser fluence of 2 J/cm². The substrate-to-target distance was adjusted to an optimized value of 4.5 cm to get efficient deposition. The thin films were deposited with a continuous flow of Ar gas, maintaining the substrates at 600°C. The PLD processing parameters such as vacuum level, laser energy, the distance between the substrate to target, gasflow, deposition time, and substrate temperature were optimized carefully to obtain homogeneous films of controlled thickness.

2.3. Materials Characterization.The crystal structure of the Na₄Fe₃(PO₄)₂P₂O₇ bulk powder was verified by a PANalytical X’Pert Pro diffractometer equipped with Cu Kα target (λ = 1.5404 Å). Rietveld refinement was performed using the GSAS program with the EXPGUI front end.

Microstructural features of the Na₄Fe₃(PO₄)₂P₂O₇ powder were captured by an FEI Inspect F50 scanning electron microscope. Next, (HR)TEM images and selected area diffraction patterns were collected with an FEI Tecnai T20 U-Twin TEM unit. For the electrochemical study of the NFPPO bulk sample, the cathode composite was prepared using an 80:10:10 (w/w) mixture of working electrode, carbon black, and polyvinylidene fluoride (PVDF) in a minimal amount of N-methyl-2-pyrrolidone (NMP) to form a thick slurry, which was coated on aluminum foil and was dried under vacuum. Initially, we studied the activity of the NFPPO system in CR2032-type coin cells by using 1 M NaClO₄dissolved in propylene carbonate acting as the electrolyte and sodium metal foil as the counter electrode.

The crystal structure of the PLD deposited NFPPO thin films was characterized by grazing incidence X-ray diffraction (GIXRD; SmartL, Bruker) using Cu Kαradiation (λ= 1.5406 Å). The surface morphology and roughness were estimated by atomic force microscopy (AFM; Veeco dII system). Micro- structural studies and average thickness of the film were estimated by FE-SEM (Zeiss Ultra) analysis. Transmission electron microscopy (TEM) was used to confirm the crystalline phase of NFPPO thin films. The electrochemical properties of NFPPO thin films were evaluated in CR2032 coin cells, as mentioned in a previous report.³² These cells were galvanostatically cycled at different rates in the potential Figure 1.(a) Rietveld refined XRD pattern of the Na4Fe₃(PO₄)₂P₂O₇sample prepared at 600°C. The experimental data (red stars), simulated pattern (black line), their difference (blue line), and Bragg positions (pink ticks) are shown. The goodness offit values obtained from the refinement were Rp =1.30, Rwp = 2.28, andχ²= 1.61. The inset illustrates the crystal structure with alternate arrays consisting of PO₄and P₂O₇ units. (b) GIXRD pattern of the PLD deposited Na4Fe3(PO4)2P2O7thinfilm. The peaks with asterisk marks the stem from the stainless steel (SS) substrate.

range of 1.8−3.8 V using a Bio-Logic (Claix, France) BCS-810 battery cycler.

3. RESULTS AND DISCUSSION

The Rietveld refined XRD pattern and corresponding structural model of the Na₄Fe₃(PO₄)₂P₂O₇ end-member are depicted in Figure 1a. The resulting lattice parameters were determined to bea = 18.0165(16) Å, b= 6.5510(6) Å, c = 10.6972(9) Å, and unit cell volume = 1262.54(24) ųfor the Na₄Fe₃(PO₄)₂P₂O₇ end-member assuming an orthorhombic structure with the Pn2₁a space group. The structure can be described as a 3D network of corner-sharing (Fe₃P₂O₁₃)_∞ infinite layers parallel to the b−c plane comprising three crystallographically independent iron atoms in octahedra abridged by PO₄ tetrahedra. These layers are interconnected along theaaxis by (P₂O₇) pyrophosphate groups (Figure 1a, inset).^24,25 Na₄Fe₃(PO₄)₂P₂O₇ is stable until 600 °C, above which it undergoes phase decomposition to form a mixture of NaFePO₄ (maricite) and Na₂FeP₂O₇.²⁹ Therefore, the Na₄Fe₃(PO₄)₂P₂O₇ mixed phosphate framework was found to form and remain stable at temperatures ranging from 400 to 600°C.²⁷

After the PLD deposition of Na₄Fe₃(PO₄)₂P₂O₇ thinfilms on stainless steel substrates (deposited at 600°C), the phase purity of the deposited thinfilm was confirmed by GIXRD, as presented inFigure 1b. The observed XRD pattern of the thin film was in good agreement with the corresponding bulk powder with no evidence of any secondary phases. Three characteristic peaks for Na₄Fe₃(PO₄)₂P₂O₇ in the 2θ range between 35 and 40°were obtained along with two sharp peaks at 45 and 51° arising from the SS substrate. By carefully optimizing all thin film deposition parameters such as laser energy, partial pressure (vacuum level), and deposition temperature during the PLD process, we could obtain phase- pure thinfilm target products.

Figure 2a displays the scanning electron microscopy (SEM) image of bulk Na₄Fe₃(PO₄)₂P₂O₇ particles prepared by the SCS route. It confirms the formation of highly agglomerated micrometric particles. The dehydration reaction involving direct condensation polymerization of phosphate groups leads to the formation of the micrometric particles. A closer look by transmission electron microscopy (TEM) revealed secondary

near-spherical nanometric Na₄Fe₃(PO₄)₂P₂O₇ particles with thin layers of carbon coating arising due to the decomposition o f c a r b o n a c e o u s p r e c u r s o r (F i g u r e 2b , c ) . T h e Na₄Fe₃(PO₄)₂P₂O₇ particles are uniformly coated by in situ deposited 3D interconnected networks of carbon. The uniform carbon coating enhances the electronic conductivity of Na₄Fe₃(PO₄)₂P₂O₇particle network, which is beneficial during the Na⁺ (de)intercalation process. SEM elemental mapping and energy-dispersive X-ray spectroscopy (EDS) were carried out for Na₄Fe₃(PO₄)₂P₂O₇ materials to confirm the con- stituent elements and the homogeneous distribution of all the elements in these materials given inFigure S1 and Table S1.

The high-resolution TEM image of Na₄Fe₃(PO₄)₂P₂O₇ in Figure 2c reveals the crystallinity with lattice fringes having an interplanar spacing of 0.907 nm, corresponding to the (200) lattice planes of the Na₄Fe₃(PO₄)₂P₂O₇ mixed phosphate product.

The morphology of PLD deposited Na₄Fe₃(PO₄)₂P₂O₇thin films were obtained from SEM and atomic force microscopy (AFM). It confirmed the uniform deposition of nanostructured target thin films (Figure 2d). The PLD-grown thin films exhibited crystalline grains with a homogeneous distribution of well-dispersed particles. The particle agglomeration is almost negligible as compared to bulk samples owing to the formation of uniform nanoparticles obtained by laser deposition. The surface morphology of thinfilms exhibited a smooth surface and small crystalline grains in a size range of 65−75 nm. The average thickness of the thinfilm was∼220 nm, as measured from cross-sectional SEM (Figure 2d, inset). The three- dimensional AFM image depicted uniform surface morphology with a near homogeneous distribution of well-crystallized grains with less degree of agglomeration (Figure 2e). The surface roughness of thefilm was found to be∼11 nm.Figure 2f represents the high-resolution TEM image of the Na₄Fe₃(PO₄)₂P₂O₇thinfilm, confirming the highly crystalline nature of the PLD-coated thin films. The corresponding interplanar spacing was determined to be 0.450 nm, related to the (202) lattice plane of the Na₄Fe₃(PO₄)₂P₂O₇ mixed phosphate.

First, galvanostatic (dis)charge analysis of the bulk Na₄Fe₃(PO₄)₂P₂O₇sample was performed in Na-ion half-cell architecture in the voltage range of 1.8−3.8 V. Representative Figure 2.Morphology of the bulk Na₄Fe₃(PO₄)₂P₂O₇powder: (a) SEM, (b) TEM, and (c) high-resolution TEM images showing lattice fringes.

Thinfilm surface morphology: (d) SEM, (inset: cross-sectional image), (e) three-dimensional AFM, and (f) high-resolution TEM images (inset showing the SAED pattern).

voltage profiles registered at a current rate of 0.1 C are presented in Figure 3a. The voltage profiles show a stable voltage profile with three plateau regions around 3.2, 2.9, and 2.5 V (during discharge). These reversible multiple plateau regions stem from (de)insertion of Na⁺ions from different Na sites in the parent structure. There are four Na sites present in the crystal framework where three Na sites favor efficient Na⁺- ion (de)insertion, while the fourth Na site works as spectator ions enhancing the structural stability during the Na⁺-ion (de)insertion process.

A slight downshift was observed between the first and subsequent cycles due to an irreversible structural rearrange- ment during thefirst charge process.²⁹The following charge− discharge cycles (10 cycles) were very stable, exhibiting a high degree of reversibility. This carbon-coated Na₄Fe₃(PO₄)₂P₂O₇ cathode delivered an initial discharge capacity of 126 mAh g⁻¹ (i.e., 97% of its theoretical capacity). Though the capacity dropped to 121 mAh g⁻¹in the second cycle, it was very steady in subsequent cycles. The observed discharge capacity of∼121 mAh g⁻¹ corresponds to 2.8 Na⁺ intercalations per formula unit.

I n s p i r e d b y t h e r o b u s t p e r f o r m a n c e o f t h e Na₄Fe₃(PO₄)₂P₂O₇ cathode in bulk form, its electrochemical performance of thin film form was tested in a CR2032 coin- type half-cell fabricated with the Na₄Fe₃(PO₄)₂P₂O₇thinfilm as a cathode, sodium metal as a counter electrode, and 1 M NaClO₄ dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 EC:DMC) acting as an electrolyte.

Upon cycling in a potential window of 1.8−3.8 V (vs Na), the thinfilms with a thickness of∼220 nm exhibited galvanostatic charge−discharge curves very similar to the bulk Na₄Fe₃(PO₄)₂P₂O₇ cathode (Figure 3b). It was very robust with no capacity fading during thefirst 100 cycles. At ambient temperature, this ∼220 nm thinfilm delivered a capacity of

∼118 mAh g⁻¹, very similar to the capacity of the bulk sample,

retaining most of the characteristic features like the occurrence of three distinct plateaus with an average working potential of

∼3 V. The nanosized grains with well-dispersed surface morphology are key to get excellent electrochemical perform- ance in thin film batteries. The crystallinity, grain size, and surface morphology affect thefinal electrochemical properties of Na₄Fe₃(PO₄)₂P₂O₇thinfilms. Further, the open and stable polyanion crystal structure is beneficial for fast ion transport and plays a vital role in practical applications.³³ Overall, the robust electrochemical performance of Na₄Fe₃(PO₄)₂P₂O₇ thinfilms can be useful for all-solid-state batteries. However, the electrochemical properties were affected as a function of film thickness. Thicker (∼300 nm) films led to reduced Na⁺ diffusion, thereby merging three distinct plateaus to a single sloping plateau with a lower average working potential of∼2.7 V (vs Na) (Figure 3c). The optimized electrochemical activity was obtained for afilm thickness of∼220 nm, in which case, three distinct oxidation−reduction peaks involving Fe²⁺/Fe³⁺

oxidation−reduction centered at ∼3 V were observed in the differential capacity (dQ/dV) plot (Figure 3d). They are related to the different sodium sites present in the Na₄Fe₃(PO₄)₂P₂O₇thinfilm.

The rate capability of the Na₄Fe₃(PO₄)₂P₂O₇ thin film cathode tested at various current rates is shown inFigure 3e.

Without any optimization, it provides a capacity of∼125 mAh g⁻¹at a low current rate of 2μA cm⁻². Even at faster cycling (∼10 μA cm⁻²), it delivered a capacity of ∼110 mAh g⁻¹, indicating excellent rate capability stemming from large Na⁺ diffusion channels in this material. Cycling stability and Coulombic efficiency of the ∼220 nm thin film sample was tested up to 500 cycles, as shown inFigure 3f. The enlarged region of the Coulombic efficiency plot is given in the Supporting Information (Figure S2). It exhibited no significant capacity fading and a high Coulombic efficiency of ∼100%, c o nfir m i n g t he h i g hl y r e v e r s i b l e n a t u r e o f t h e Figure 3.Electrochemical performance of the Na4Fe3(PO4)2P2O7mixed phosphate cathode in sodium half-cell architecture. (a) Charge−discharge voltage profiles of bulk Na4Fe3(PO4)2P2O7powder. Charge−discharge voltage profiles of thinfilm electrodes of the thicknesses of (b) 220 and (c) 300 nm. (d) Corresponding differential capacity vs voltage (dQ/dV) plots for 10 cycles. (e) Comparative voltage profiles of the thin film (thickness, 220 nm) electrode cycled at different rates and (f) capacity retention at 1C rate and Couloumbic efficiency for 500 cycles.

implemented in Na-ion half-cells, these thin films exhibited excellent electrochemical performance with robust cycling stability, similar to the bulk batteries. To the best of our knowledge, Na₄Fe₃(PO₄)₂P₂O₇thin films (∼220 nm) deliver reasonable electrochemical performance with good reversibility among all sodium insertion materials. The mixed phosphate Na₄Fe₃(PO₄)₂P₂O₇forms an inexpensive and safe cathode for designing all-solid-state sodium-ion microbatteries.

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*^sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03835.

EDS spectrum, SEM image, elemental mapping images of the Na₄Fe₃(PO₄)₂P₂O₇powder sample, EDS analysis results table, and the Coulombic efficiency plot (PDF)

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Baskar Senthilkumar− Faraday Materials Laboratory (FaMaL), Materials Research Centre, Indian Institute of Science, Bangalore 560012, India; Email:senphysics@

gmail.com

Prabeer Barpanda−Faraday Materials Laboratory (FaMaL), Materials Research Centre, Indian Institute of Science, Bangalore 560012, India; orcid.org/0000-0003-0902-3690;

Phone: +91-80 2293 2783; Email:prabeer@iisc.ac.in;

Fax: +91-80 2360 7316 Authors

Angalakurthi Rambabu−Faraday Materials Laboratory (FaMaL), Materials Research Centre and Quantum Structures and Device Laboratory, Materials Research Centre, Indian Institute of Science, Bangalore 560012, India; Department of Basic Sciences and Humanities, GMR Institute of Technology, Rajam, Andhra Pradesh 532127, India

Chinnasamy Murugesan− Faraday Materials Laboratory (FaMaL), Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

Saluru Baba Krupanidhi−Quantum Structures and Device Laboratory, Materials Research Centre, Indian Institute of Science, Bangalore 560012, India; orcid.org/0000-0001- 6393-0908

Complete contact information is available at:

https://pubs.acs.org/10.1021/acsomega.9b03835 Author Contributions

∥B.S. and A.R. contributed equally.

Notes

The authors declare no competingfinancial interest.

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