Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Cubic Sodium Cobalt Metaphosphate [NaCo(PO3)3] as a Cathode Material for Sodium Ion Batteries Ritambhara Gond,‡ Rayavapuru Prasada Rao,§ Valerie Pralong,† Oleg I. Lebedev,† Stefan Adams,§ and Prabeer Barpanda*,‡ ‡
Faraday Materials Laboratory, Materials Research Centre, Indian Institute of Science, C. V. Raman Avenue, Bangalore 560012, India Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117546, Singapore † Laboratoire de Cristallographie et Sciences des Matériaux, ENSICAEN, Université de Caen, CNRS, 6 bd Maréchal Juin, 14050 Caen, France §
ABSTRACT: Cubic-framework sodium cobalt-based metaphosphate NaCo(PO3)3 was recently demonstrated to be an attractive Na+ cationic conductor. It can be potentially used in the next-generation rechargeable Na ion batteries. The crystal structure and electrical conductivity were studied and found to have a three-dimensional framework with interconnecting tunnels for Na+ migration (J. Solid State Electrochem., 2016, 20, 1241). This inspired us to study the electrochemical (de)intercalation behavior of Na+ in the NaCo(PO3)3 assuming a cubic Pa3̅ framework. Herein, synergizing experimental and computational tools, we present the first report on the electrochemical activity and Na+ diffusion pathway analysis of cubic NaCo(PO3)3 prepared via conventional solid-state route. The electrochemical analyses reveal NaCo(PO3)3 to be an active sodium insertion material with well-defined reversible Co3+/Co2+ redox activity centered at 3.3 V (vs Na/ Na+). Involving a solid-solution redox mechanism, close to 0.7 Na+ per formula unit can be reversibly extracted. This experimental finding is augmented with bond valence site energy (BVSE) modeling to clarify Na+ migration in cubic NaCo(PO3)3. BVSE analyses suggest feasible Na+ migration with moderate energy barrier of 0.68 eV. Cubic NaCo(PO3)3 forms a 3.3 V sodium insertion material.
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INTRODUCTION There is considerable interest in the development of superior cathode materials for the next-generation sodium ion batteries (SIB), which have emerged as economic energy storage devices particularly for stationary grid storage. SIBs can complement Li ion batteries (LIB) in empowering large-scale energy storage applications that are not restricted by gravimetric and/or volumetric energy density.1 The low cost of SIB stems from the abundance of sodium-based precursors in earth crust with uniform geographical distribution.2 A variety of efficient cathode insertion materials can be designed using various chemistry such as transition-metal oxides, sulfides, fluorides, oxyanionic compounds, Prussian blue analogues, and polymers. One can use similar compounds for both SIB and LIB systems (e.g., NaxCoO2 and LixCoO2). As SIB has striking operational similarity with LIB along with similar (intercalation/conversion/alloying) redox mechanisms, it is possible to use similar compounds for both the systems. Apart from suites of O3- and P2-type transition-metal oxides, various polyanion insertion frameworks have gained scientific attention owing to their rich structural diversity, redox potential tunability, and chemical/thermal stability.3−5 Among them, phosphate PO43−containing compounds are widely explored due to their easy synthesis, good electrochemical performance, stability, and © XXXX American Chemical Society
operation safety. Several of these phosphates exhibit a favorable combination of ionic conductivity and electrochemical properties. In light of this, phosphates (maricite/triphylite polymorphs) [(PO4)3−], pyrophosphates [(P2O7)4−], hydrogenophosphates [(HPO4)2−] and mixed phosphates [(PO4)2(P2O7)10−] have been reported.6−11 Pursuing phosphate chemistry further, a new polyanionic system known as metaphosphate [(PO3)33−] has been recently reported containing either infinite chains of corner-sharing PO43− tetrahedra or finite cycles of 3, 4, 6, or 12 corner-sharing PO43−. Electrochemical activity involving Fe3+/2+ redox potential in 2.5−3.5 V window has been recently observed in polyanionic NaFe(PO3)3 system having an orthorhombic crystal structure with P212121 symmetry.12 Moving to the Co-based analogue, NaCo(PO3)3 has been reported with one-dimensional open channel and good electrical conductivity.13 It may be noted that, while orthorhombic NaFe(PO3)3 and NaMn(PO3)3 have one-dimensionally infinite interconnected phosphate chains,14 NaCo(PO3)3 as well as the isostructural cubic phases of NaZn(PO3)3 and NaMg(PO3)3 consist of 12-ring cyclic metaphosphates.15 It Received: February 1, 2018
A
DOI: 10.1021/acs.inorgchem.8b00291 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
acquired in a 2θ range of 10−90° with a step scanning increment of 0.026 26° in Bragg−Brentano geometry. Rietveld refinement was performed on diffraction intensity data of NaCo(PO3)3 using GSAS program with the EXPGUI front-end.19−21 VESTA software was employed for the crystal structure visualization.22 Morphology of the sample was observed by field emission scanning electron microscopy (FE-SEM; FEI Inspect F 50 at 10 kV). Further transmission electron microscopy (TEM) including high-resolution TEM (HRTEM) and electron diffraction (ED) studies were applied to study crystallinity of sample using an FEI Tecnai G2 30 UT microscope operated at 300 kV and having 0.17 nm resolution. For TEM study, powder sample dispersed in methanol was drop-casted onto a holey carbon film fixed on a 3 mm Cu-grid. Vibrational State Analysis. Fourier transform infrared (FTIR) absorption spectra of powder sample were acquired by PerkinElmer Spectrum100 Series FTIR spectrometer in the wavenumber range of 400−4000 cm−1 (cycle number = 4). The sample was diluted with crystalline KBr and pressed into pellets. Raman spectra of the NaCo(PO3)3 powder were recorded on a Horiba Jobin Yvon HRRaman-123 MicroPL spectrometer with a green laser source of excitation wavelength of 519 nm. Chemical State Analysis. The chemical states of all elements in as-synthesized NaCo(PO3)3 powder as well as its desodiated (charged) state were analyzed by a Kratos Axis Ultra DLD with an incident monochromated X-ray beam of Kα radiation (ℏν = 1486.6 eV) from the Al target. X-ray photoelectron spectroscopy (XPS) data were collected at accelerating voltage of 13 kV and an emission current of 9 mA. The main carbon signal at a binding energy of 284.6 eV was considered as reference for calibration of shift corrections. These spectra were fitted after Shirley background subtraction. Electrochemical Analysis. To gauge the electrochemical properties of NaCo(PO3)3 as cathode, it was used as active material to prepare composite electrode slurry. An intimate slurry of active materials along with acetylene black and poly(vinylidiene fluoride) (PVDF) binder in 80:10:10 (w/w/w) ratio was prepared in minimal amount of N-methyl-pyrrolidone (NMP). This cathode slurry was coated on aluminum (Al) foil by using a doctor blade (cutting edge gap of 100 μm). These cathode-coated Al foils were kept inside a vacuum oven for overnight to remove excess NMP. CR2032-type coin cells were assembled using circular disks of cathode-coated Al foils as positive electrode, Na metal foil serving as counter electrode and a sheet of glass fiber as separator soaked in 1 mol/L NaPF6 in EC/PC (1:1 v/v) acting as electrolyte. These Na half-cell architectured coin cells were assembled inside an MBraun Labstar glovebox maintaining an inert argon ambience (H2O/O2 level < 0.5 ppm). These cells were tested at 25 °C using a Bio-Logic BCS 805/810 automatic battery tester. Galvanostatic charge−discharge cycling was performed in the voltage range of 2.0−4.7 V at a rate of C/10 and 1C for 10 cycles each. Ex situ XRD patterns were recorded at the end of multiple charge− discharge cycles (at a rate of 1C). Post the electrochemical analyses, the CR2032 coin cells were decrimped inside the glovebox, and the electrodes were recuperated and were thoroughly washed with EC/PC (1:1 v/v) solvent to remove any surface residues. These electrodes were dried inside a vacuum oven at 80 °C for overnight. These dried electrodes were employed for acquiring ex situ XRD patterns and ex situ XPS spectra. Diffusion Pathway Analysis. Diffusion pathways and the corresponding migration barriers for the mobile Na+ in NaCo(PO3)3 were analyzed by BVSE calculations from the Rietveld refined crystal structure after geometry optimization. It may be noted that geometry optimizations of the structure model from this work and from Ben Smida13 yield to undistinguishable structure models. The methodology of BVSE analysis has been discussed in detail elsewhere.23−25 In brief, pathways for mobile Na+ are identified with regions of low bond valence site energy EBVSE (Na+). The approach is considered a simple and reliable tool to predict migration pathways and the energy barriers therein from local structure model.25,26 In the BVSE calculations, bond valences sNa‑X = exp[(R0,Na‑X − RNa‑X)/bNa‑X] are linked to an absolute energy scale by expressing the squared bond valence as a Morse-type interaction energy between Na+ cation and O2− anions.27−30
can be a potential high-voltage sodium battery cathode considering the fact that various Co-based phosphates are known to be electrochemically active involving Co3+/2+ redox potential in the window of 3−4.5 V (vs Na). For example, the red-phase polymorph of NaCoPO4 (NCP) has electrochemical activity at ∼4.1−4.4 V.16 Similarly, Na2CoP2O7 pyrophosphate and Na4Co3(PO4)2(P2O7) mixed phosphates deliver redox activity around 3 and 4.5 V, respectively.17,18 Battery sector has long been associated with Co-based cathode materials like LixCoO2 and NaxCoO2.1 While offering high energy density, recently there has been growing concern over the high cost and toxicity of Co-based resources giving impetus to design lowcost and environmentally benign Fe/Mn-based electrodes. The cost/safety issues associated with Co-based precursors can be neutralized by (i) achieving high energy density in Co-based cathode thereby reducing the overall energy cost ($/W h) and (ii) safe handling, packaging and recycling of Co-resources to reduce possible safety hazards. Focusing on the metaphosphate (PO3)33− chemistry, the present work reports systematic study of the electrochemical properties as well as Na+ diffusional pathways in NaCo(PO3)3 system. Unlike the orthorhombic NaFe(PO3)3 and NaMn(PO3)3 end-members, the Co-analogue assumes a cubic crystal structure with Pa3̅ space group. Our principal aim was to synthesize phase-pure NaCo(PO3)3 and to investigate the Na+ (de)intercalation mechanism and the Co3+/Co2+ redox activity (at room temperature). The underlying structural transformation during (de)sodiation was probed using ex situ Xray diffraction analysis. The degree and efficiency of Na+ intercalation was further examined using bond valence site energy (BVSE) modeling to decipher feasible Na+ migration pathways to establish first demonstration of cubic NaCo(PO3)3 as a cathode material for sodium ion batteries.
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EXPERIMENTAL SECTION
Material Synthesis. Sodium cobalt metaphosphate NaCo(PO3)3 was synthesized by taking stoichiometric amounts of NaH2PO4·H2O (Merck, ≥98%), Co(CH 3 COO) 2 ·4H2 O (Merck, ≥99%), and NH4H2PO4 (Merck, ≥99%) precursors. These precursors were mixed intimately by (dry) planetary milling for 1 h at 350 rpm using Cr-hardened stainless steel milling media and container. The resulting mixture (raw powder) was pelletized, and these pellets were calcined at 600 °C for 6 h at a heating rate of 5 °C/min in a tube furnace in air atmosphere. The desired phase has Co2+ state, which is royal blue in color and air-stable. The overall reaction can be expressed as Scheme 1
NaH 2PO4 · H 2O + Co(CH3COO)2 · 4H 2O + 2NH4H 2PO4 + 4O2 → NaCoII(PO3)3 + 2NH3 ↑ +4CO2 ↑ +12H2O↑
Scheme 1
Material Characterization. The powder X-ray diffraction (XRD) patterns were first collected for as-synthesized NaCo(PO3)3 product using PANalytical X′Pert Pro diffractometer equipped with a Cu Kα target (operating at 40 kV/30 mA) and Ni filter to obtain monochromatic wavelength (λ = 1.5404 Å). Typical scans were B
DOI: 10.1021/acs.inorgchem.8b00291 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Comparison of Known NaTM(PO3)3 Metaphosphate Structures ionic radius,a Å29,33
crystal system
space group
meta-phosphate ion type
ICSD No.
synthesis temperature, °C
Ba2+ Sr2+ Ca2+ Cd2+ Mn2+ Fe2+ Co2+ Co2+
1.35 1.18 1.00 0.95 0.83 0.78 0.745 0.745
orthorhombic triclinic triclinic orthorhombic orthorhombic orthorhombic orthorhombic cubic
P212121 P1̅ P1̅ P212121 P212121 P212121 P212121 Pa3̅
chains chains chains chains chains chains chains 12-ring
2807 418 633 418 632 85 094 85 095 291 414 (missing) 291 572
RT 700 700 350 350 600 600 650
Ni2+
0.69
orthorhombic
Pcca
chains
700
Zn2+ Zn2+ Zn2+ (Ag doped) Mg2+ Mg2+
0.74 0.74 0.74 0.72 0.72
cubic cubic orthorhombic cubic orthorhombic
I43̅ d Pa3̅ Pbca Pa3̅ Pbca
4-ring 12-ring chains 12-ring chains
91 526 59 357 35 651 90 484 91 780 90 483 20 642
TM
a
350 650 350 650
ref Martin37 Abrahams15 Abrahams15 Murashova14 Murashova14 Lin31 Lin31 Ben Smida13 (and this work) Kapshuk38 Erragh39 Averbuch-Pouchot40 Abrahams15 Aouad41 Abrahams15 Shepelev42
For octahedral ions, where applicable high-spin values are given for TM cation radii. 2 ⎡ N ⎛⎛ ⎞⎤ s Na − O ⎞ s Na − O ⎟⎥ ⎢ ⎜ ⎜ ⎟ E BVSE(Na − O) = ∑ D⎢∑ ⎜ ⎟ −2 ⎜ s smin,Na − O ⎟⎠⎥ x ⎣ i = 1 ⎝⎝ min,Na − O ⎠ ⎦
+ Erepulsion
considering same synthetic conditions and types of precursors. Both in the orthorhombic (or lower) space groups adopted by the polyanion chain metaphosphates and in the cubic phases with cyclic metaphosphate anions, the TM occupies sixfold coordination sites. Structures realized for a variety of divalent transition-metal cations as well as for compounds where alkaline-earth cations replace the TM cations are listed in Table 1 along with the references, divalent cation radius, and type of metaphosphate anions.33,36 Among the listed divalent cations (Ba2+−Fe2+), for larger cations only the low-symmetry chaintype metaphosphates are observed. In this case the parallel orientation of the helical metaphosphate chains restricts the crystal symmetry to the orthorhombic system (or lower). For the smaller ions (Mg2+, Zn2+, Co2+) cubic structures containing cyclic metaphosphates are preferentially observed, but orthorhombic polymorphs remain also accessible (in the case of NaZn(PO3)3, e.g., by Ag-doping), so that, for the case of Co2+, both options must be considered. The known phases of the analogue AgTM(PO3)3 (where TM = Ba, Mn, Mg, Zn) crystallize all in orthorhombic phases with chain-type polyanion metaphosphates. With careful diffraction and Rietveld analysis, we found that our solid-state synthesized NaCo(PO3)3 phase crystallizes in the cubic crystal system in space group Pa3̅, isostructural to NaZn(PO3)3. Structure refinements were performed on powder X-ray data by the Rietveld method using the GSAS software package. During refinement, a shifted Chebyshev-type function was employed for the background. The resulting Rietveld refined plot of as-synthesized NaCo(PO3)3 is shown in Figure 1, and the corresponding details of structural parameters, final agreement factors, and crystallographic data are listed in Table 2. A careful analysis of Rietveld refinement (shown in Figure 1) revealed weak peaks of a secondary phase near 28°, which was identified as Co2P2O7 (∼6 wt %) having space group P21/c (No. 14), a thermodynamically stable phase in the Co− P−O ternary system. The atomic coordinates, occupancies, thermal parameters, and multiplicities are listed in Table 3. The structure of cubic NaCo(PO3)3 consists of 13 crystallographic independent atoms in the asymmetric unit. There are two phosphorus (P), two cobalt (Co), three sodium (Na), and six oxygen (O) atoms involved in the formation of the repeating
(1)
Here, smin,Na‑X corresponds to the expected BV of a Na−O bond at the equilibrium distance Rmin,Na−O. Similarly, D, which is the bond breaking energy, can be derived as a function of Rmin, the oxidation states VNa = 1 and |VO| = 2, the bond softness parameter b, and the period number n of Na and O as D0 = ε0
b2 VNa|VO| 2R min nNanO
(2)
All required bond valence parameters were taken from the softBV database.28,29 Regions of low bond valence site energy EBVSE(Na) in grids spanning the structure model with a resolution of (0.1) Å3 were analyzed yielding the Na+ migration pathways.
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RESULTS AND DISCUSSION Crystal Structure and Morphology. Crystal structure of NaTM(PO3)3 metaphosphate family is largely found to adopt either orthorhombic or cubic frameworks, based on the constituent transition-metal (TM) species. On the one hand, in NaTM(PO3)3, if TM is Mn, Cd, and Fe (or if TM by a large alkaline-earth cation), the system crystallizes into orthorhombic structure with space group of P212121 (No. 19) containing onedimensionally oriented polyanionic metaphosphate chains.12,14,31 On the other hand, when TM is Co and Zn (or when TM is replaced by Mg), the cyclic metaphosphates are arranged to form a cubic crystal structure in space group of Pa3̅ (No. 205).13,32,33 With (PO3)−, TM bonded with O2− generally forms high-spin complexes due to the weak crystal field energy of (PO3)− ligand. To adopt cubic crystal system, TM ionic radii play a major role for the arrangement of atoms to build crystal in any system along with the synthetic conditions.34 Still, there have been conflicting reports about the space group of NaCo(PO3)3. Lin et al. have described an orthorhombic NaCo(PO3)3 phase with P212121 (No. 19) symmetry;35 the cubic polymorph having space group Pa3̅ (No. 205) has been previously reported by Smida et al.13 To compare the preferences for the formation of the different possible phases, one can compare and analyze the atomic radii of T M C
DOI: 10.1021/acs.inorgchem.8b00291 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
octahedra leaves three types of octahedral sites for Na+ that control the electrochemical performance of the material as further explained using BVSE modeling in the later section. Main metal−oxygen bond distances are listed in Table 4. The P−O bond distances range from 1.301 to 1.726 Å. The three types of sixfold coordinated Na atoms are located in hexagonal tunnels, similar along the 3̅ axes due to the cubic symmetry. This unique property may serve as a possible path for the Na+ migration in NaCo(PO3)3 metaphosphate framework. Figure 2a shows the scanning electron microscopy (SEM) micrographs of solid-state synthesized NaCo(PO3)3 powder. Agglomeration of small particles so as to form large micrometric particles is clearly visible as expected due to annealing at 600 °C for 6 h. The primary particles have micrometric size in the range of 1−5 μm, which are basically agglomerates of 500−600 nm nanometric particles as depicted from TEM image shown in Figure 2b. ED patterns obtained from main zone axis (Figure 2c) confirm NaCo(PO3)3 crystal structure determined by XRD and can be completely indexed based on cubic Pa3̅ structure (a = 1.4278 nm). No extra spots due to any possible superstructure or secondary phase were observed in the ED patterns. HRTEM images of NaCo(PO3)3 along [011] and [111], shown in Figure 2d, confirm good crystallinity of the as-synthesized powder. Vibrational State Analysis. The purity of as-prepared sample was further confirmed by probing the vibrational state of constituent polyanionic units in the structure using FTIR and Raman spectroscopy. In metaphosphate composition, the assignment of FTIR spectra is based on the characteristic vibrations of PO2 (terminal) and POP (bridging) bands. Figure 3a shows FTIR spectra plotted within the 400−1300 cm−1 region with specific vibration bands characteristic of linear polyphosphate groups.18,31 There are no bands lying around 1700 cm−1 that could be assigned for CO stretching bonds, confirming the complete conversion of Co(CH3COO)2·4H2O precursor into the final product. The vibration bands in the range of 1290−1220 and 1180−1030 cm−1 can be assigned to asymmetric and symmetric PO2 bands [νas(PO2) and νs(PO2)], respectively. Similarly, νas(POP) and νs(POP) stretching vibrations lead to bands around 1020−900 and 830−600 cm−1, respectively. Other than the PO4 bands, there are some external bonds like Co−O or Na−O bonds giving rise to additional bands in FTIR as well as Raman spectra at low wavenumber range (600 and
