Highly Stable Iron- and Manganese-Based Cathodes for Long-Lasting

Sep 30, 2016 - The development of long-lasting and low-cost rechargeable batteries lies at the heart of the success of large-scale energy storage syst...
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Highly Stable Iron- and Manganese-Based Cathodes for Long-Lasting Sodium Rechargeable Batteries Hyungsub Kim,†,‡,§ Gabin Yoon,†,‡ Inchul Park,†,‡ Jihyun Hong,† Kyu-Young Park,† Jongsoon Kim,§ Kug-Seung Lee,∥ Nark-Eon Sung,∥ Seongsu Lee,§ and Kisuk Kang*,†,‡ †

Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ‡ Center for Nanoparticle Research at Institute for Basic Science (IBS), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea § Korea Atomic Energy Research Institute (KAERI), 111 Daedeok-daero 989 Beon-Gil, Yuseong-gu, Daejeon 34057, Republic of Korea ∥ Beamline Department, Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea S Supporting Information *

ABSTRACT: The development of long-lasting and low-cost rechargeable batteries lies at the heart of the success of largescale energy storage systems for various applications. Here, we introduce Fe- and Mn-based Na rechargeable battery cathodes that can stably cycle more than 3000 times. The new cathode is based on the solid-solution phases of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) that we successfully synthesized for the first time. Electrochemical analysis and ex situ structural investigation reveal that the electrodes operate via a onephase reaction upon charging and discharging with a remarkably low volume change of 2.1% for Na4MnFe2(PO4)(P2O7), which is one of the lowest values among Na battery cathodes reported thus far. With merits including an open framework structure and a small volume change, a stable cycle performance up to 3000 cycles can be achieved at 1C and room temperature, and almost 70% of the capacity at C/20 can be obtained at 20C. We believe that these materials are strong competitors for large-scale Na-ion battery cathodes based on their low costs, long-term cycle stability, and high energy density.

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INTRODUCTION Developing cost-efficient large-scale energy storage systems (ESSs) that can connect to renewable energy resources (i.e., solar, wind, hydro, geothermal, and tidal energies) is one of the key issues to be addressed in an effort to satisfy ever-growing energy demands. Although recent advances in Li-ion battery (LIB) technology have commercialized electric vehicles (EVs) and increased the energy density to a level applicable to largescale ESSs, the cost issues arising from the use of costly transition metals in current LIB chemistry and concern over the cost and uneven global distribution of Li around the world remain great challenges.1 Among various candidates for postLIB technologies, Na-ion batteries (NIBs) are an attractive alternative to LIBs because of the nearly unlimited Na resources and because their electrochemistry is similar to that of LIBs. However, the large ionic size of Na (1.02 Å) requires an electrode material with an open framework structure, and the low standard electrochemical potential of Na (∼2.71 V vs Na+/ Na) compared with that of Li (∼3.04 V vs Li+/Li) resulted in a low energy density. Accordingly, intensive studies have focused on searching for new electrodes for NIBs with an open framework structure and high operating potential.2−6 Recent studies have introduced a © 2016 American Chemical Society

few important groups of potential electrode materials such as layered oxides and Prussian blue analogues based on earthabundant transition metals such as Fe and Mn as well as transition metal-free organic electrodes with promising electrochemical properties.2−4,6 Although these materials possess clear potential merits, the insufficient cycle performance (typically well below 1000 cycles) arising from the large volume change of most layered oxides or the dissolution issues of Prussian blue and organic cathodes in the electrolyte has yet to be addressed.7−11 In addition, the absence of Na ions in Prussian blue analogues and the organic cathodes requires the use of metallic Na as the counter electrode.8,12,13 The polyanion-based compounds, including phosphate, fluorophosphate, pyrophosphate, sulfate, and carbonophosphate materials, have also been extensively studied as an important group of potential electrode materials. 14−27 A few important V-based compounds, Na3(VOx)2(PO4)2F3−2x (x = 0 or 1), have been introduced as promising cathodes for NIBs.28,29 They exhibited high energy densities comparable to those of LIBs based on the high Received: May 1, 2016 Revised: September 30, 2016 Published: September 30, 2016 7241

DOI: 10.1021/acs.chemmater.6b01766 Chem. Mater. 2016, 28, 7241−7249

Article

Chemistry of Materials operating voltage and multielectron redox reaction (V3+/V4+ and V4+/V5+); however, the use of toxic and costly V element should be addressed.28,29 Recent advances have also introduced various Fe- and Mn-based polyanionic compounds for NIBs.14−27 While they generally showed high structural stability and exhibited a relatively smaller volume change compared with that of common layered materials upon electrochemical cycling, a sufficiently long cycle stability with high energy density suitable for large-scale ESSs has not been achieved until now. Recently, we reported a new crystalline framework based on a mixed polyanion of (PO4)3− and (P2O7)4−: Na4Fe3(PO4)2(P2O7) and Na4Mn3(PO4)2(P2O7) for use in Na rechargeable batteries.30−32 The open framework structure of these compounds supports three-dimensional diffusion of Na ions in the crystal with low activation barriers for hopping, allowing fast Na de/insertion during electrochemical cycling in Na-ion cells. For Na4Fe3(PO4)2(P2O7), a capacity of 110 mAh g−1 with an average voltage of 3.2 V versus Na+/Na can be achieved via a one-phase reaction with a volume change of ∼4% during the charge/discharge processes.31 In addition, a high voltage of approximately 3.8 V versus Na+/Na and an energy density of 385 Wh kg −1 can be obtained for the Na 4 Mn 3 (PO 4 ) 2 (P 2 O 7 ) electrode. 3 2 In contrast to Na4Fe3(PO4)2(P2O7), the Mn-based electrode operated via a multiphase reaction with a volume change of approximately 7%. Although these electrode materials display promising electrochemical activities in Na-ion cells, the low energy density of the Na4Fe3(PO4)2(P2O7) electrode and the relatively faster capacity degradation of the Na4Mn3(PO4)2(P2O7) electrode due to a large volume change upon electrochemical cycling must be resolved to be suitable for NIBs in large-scale ESSs. To address this issue, herein we attempt to tune the electrochemical properties of this family of materials within the binary mixed-phosphate phase, Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2). Surprisingly, it is found that the binary system exhibits a higher practical power/energy density and a smaller volume change (∼2%) upon cycling compared with those of the end members. The electrochemical characterization illustrates that the high energy density is attributed to the combined (i) high potential of the Mn2+/Mn3+ redox couples as well as the upshifted Fe2+/Fe3+ redox potential and (ii) enhanced intercalation kinetics. Moreover, the Na4MnFe2(PO4)2(P2O7) electrode is capable of exhibiting a remarkable cycle stability up to 3000 cycles at a current rate of 1C at room temperature. Ex situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) analyses at an intermediate charge/discharge state of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) electrodes reveal that the stable electrochemical properties can be ascribed to the minimal structural change achieved (∼2.1%) in the binary mixedphosphate phase.

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coated C/Na4MnxFe3−x(PO4)(P2O7) (x = 1 or 2) samples were prepared via a postannealing method with PA carbon sources. PA (5 wt %) was mixed again with Na4MnxFe3−x(PO4)(P2O7) (x = 1 or 2) samples using dry ball-milling at 80 rpm, and the mixture was annealed at 600 °C for 2 h under an Ar atmosphere. The carbon contents in carbon-coated samples were confirmed to be 6 wt %. C/ Na4Mn3(PO4)2(P2O7) and Na4Fe3(PO4)2(P2O7) powder samples were prepared via the same method with previous reports.31,32 Characterization Methods. The crystal structure of as-prepared Na4MnxFe3−x(PO4)(P2O7) (x = 0, 1, 2, or 3) powder and ex situ electrode samples of Na4−yMnxFe3−x(PO4)(P2O7) (x = 1 or 2, and 0 ≤ y ≤ 3) were analyzed by using an X-ray diffractometer (Bruker, D2 PHASER) equipped with Cu Kα radiation (λ = 1.5406 Å). The data were recorded over a 2θ range of 8−60° with a step size of 0.02 and a step time of 1 s. High-resolution powder diffraction (HRPD) analyses using synchrotron X-ray and neutron sources were performed to determine the atomic configuration of Fe and Mn in the structure. Synchrotron XRD data were obtained from beamline 9B at Pohang Accelerator Laboratory (PAL). The data were collected over a 2θ range of 8−148.5° with a step size of 0.02 and a step time of 7 s using a constant wavelength (λ) of 1.4647 Å. ND data were obtained from the HANARO facility in the Korea Atomic Energy Research Institute (KAERI). The measurement was conducted at a 2θ range of 0−180° with a step size of 0.05° using a wavelength λ of 1.834333 Å. The particle size of the samples was evaluated using field emission scanning electron microscopy (FESEM) (SUPRA 55VP/Carl Zeiss). Atomic ratios between Na and Fe and Mn were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Thermo Jarrel Ash, Polyscan 60E). The carbon contents in the C/ Na4MnxFe3−x(PO4)(P2O7) (x = 1 or 2) samples were analyzed by using an elemental analyzer (EA 1108 CHNS-O, FISONS Instruments). The valence states of Mn and Fe in the structure of Na4−yMnxFe3−x(PO4)2(P2O7) (x = 1 or 2, and 0 ≤ y ≤ 3) were characterized using X-ray absorption near-edge structure (XANES) analysis. The XANES spectra were recorded on beamline 8C at the PAL. The Mn and Fe k-edge spectra were recorded in transmission and fluorescence mode with an electron energy of 3 GeV and a current of 320 mA. Fe and Mn reference spectra were recorded simultaneously from each metal foil. Electrochemical Tests. The electrodes were prepared using Na4MnxFe3−x(PO4)(P2O7) (x = 0, 1, 2, or 3), super P, and a polyvinylidene fluoride (PVDF) binder in a 7:2:1 mass ratio in Nmethyl-2-pyrrolidone (NMP) (99.5%, Aldrich) solvent. The slurry was pasted onto Al foil with a 250 μm thickness. After NMP was evaporated at 120 °C for 1 h, the electrode was pressed using a roll press machine. The loading of Na4MnxFe3−x(PO4)(P2O7) (x = 0, 1, 2, or 3) was approximately 2 mg cm−2. The electrochemical test was conducted with a Na half-cell that was assembled into a CR2032 coin cell using metallic Na (sodium cube, 99%, Aldrich) as a counter electrode. A glass microfiber filter (grade GF/F, Whatman) and a 1 M solution of NaBF4 in an ethyl carbonated/prophylene carbonate electrolyte (EC/PC, 1:1, anhydrous, Aldrich) were used as a separator and an electrolyte, respectively. Galvanostatic measurement of Na4MnxFe3−x(PO4)(P2O7) (x = 0, 1, 2, or 3) electrode samples was performed with various C rates (C/20, C/10, C/5, 1C, 5C, 10C, and 20C) at room temperature and 60 °C with cutoff voltages of 1.7 and 4.5 V (vs Na+/Na). Galvanostatic intermittent titration technique (GITT) measurement of the electrodes was performed with a 2 h constant current time at a current rate of C/50 and a 4 h relaxation time. Cyclic voltammetry (CV) measurement of the electrodes was conducted over the voltage range of 1.7−4.5 V at a scan rate of 0.1 mV s−1.

EXPERIMENTAL SECTION

Synthesis of Na4MnxFe3−x(PO4)(P2O7) (x = 0, 1, 2, or 3). Na4MnxFe3−x(PO4)(P2O7) (x = 1 or 2) powder samples were synthesized via the solid-state method. A stoichiometric amount of Na4P2O7 (95%, Aldrich), xMnC2O4·2H2O (99%, Alfa Aesar), (1− x)FeC2O4·2H2O (99%, Aldrich), and 5 wt % pyromellitic acid (PA) (C10H6O2, 96%, Alfa Aesar) was mixed using planetary ball-milling at 400 rpm for 12 h. The precursors were sealed in an Ar glovebox to avoid the oxidation of Fe during the ball-milling process. The wellground mixture was calcined at 300 °C for 6 h under an Ar flow. The resulting powder was ground and pelletized under a pressure of 250 kg cm−2 and sintered again at 600 °C for 12 h under an Ar flow. Carbon-

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RESULTS AND DISCUSSION Synthesis and Structural Characterization of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3). Binary transition metal (TM) mixed phosphates, Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2), were successfully synthesized via conventional solid-state synthesis involving a 7242

DOI: 10.1021/acs.chemmater.6b01766 Chem. Mater. 2016, 28, 7241−7249

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Chemistry of Materials relatively low-temperature heat treatment (∼600 °C) under an Ar atmosphere. The crystal structures of these compounds were characterized using XRD analyses, which confirmed that Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) in orthorhombic space group Pn21a were obtained without traceable impurities in Figure 1a.31,32 Figure 1b shows the variation of the cell

length can be attributed to the distortion induced by the neighboring larger Mn ion sharing the polyhedron edges, whose detailed structure and effect on the electrochemistry will be discussed later. A combined XRD and neutron diffraction (ND) study was performed to determine the atomic position as well as the configuration of Fe and Mn in Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) in panels a and b of Figure 2 and panels c and d of Figure 2, respectively. The atomic positions of heavy elements such as Fe and Mn were investigated using XRD analysis, whereas the light elements (i.e., O, P, and Na) were primarily analyzed by ND. A possible atomic ordering or preferential site occupancy of Fe and Mn in the structure was examined using ND analyses due to a significant contrast in the neutron scattering length between Mn and Fe ions (i.e., Fe = 9.45 fm, and Mn = −3.73 fm). Note that the ND pattern of Na4MnFe2(PO4)2(P2O7) in Figure 2c shows significantly different peak intensity ratios over the entire 2θ range compared with that of Na4Mn2Fe(PO4)2(P2O7), even though the overall crystal structure remains the same. This finding is ascribed to distinct Fe and Mn occupancy ratios at the three symmetrically inequivalent TM sites shown in the insets of panels c and d of Figure 2. To quantify the Fe and Mn site occupancies, we carefully simulated the ND patterns by varying the Fe and Mn ratios at three different TM sites based on the atomic position derived from synchrotron XRD results in Figure S3. Table 1 presents the Fe and Mn occupancies from Rietveld refinement. The results indicate that Mn has a clear preference in TM1 sites both in Na4MnFe2(PO4)2(P2O7) and in Na4Mn2Fe(PO4)2(P2O7), while a relatively higher Fe occupancy was found in other TM sites. Nevertheless, we could not detect additional new peaks from a specific ordering of Mn and Fe in the structure. The detailed structural information along with the measurement conditions is presented in Table S1. The particle morphology of carboncoated Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3) is investigated in Figure S4, which reveals that all the samples have similar particle sizes of 200 nm to 1 μm. Electrochemical Investigation of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3) Electrodes. Galvanostatic charge/ discharge measurements of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3) were conducted using Na metal as a counter electrode. Panels a and b of Figure 3 present the typical charge/ discharge profiles of the four different electrodes (x = 0, 1, 2, or 3) at a current rate of C/20 at room temperature. All the electrodes delivered similar capacities of approximately 110 mAh g−1, which correspond to approximately 85% of the theoretical capacity (i.e., one electron per TM, three-Na extraction/insertion, yielding ∼129 mAh g−1). The substitution of Mn into the structure induces the electrochemical activity at the redox potential of ∼3.8 V involving a Mn2+/Mn3+ redox couple from both charge and discharge profiles. The activation of the Mn redox couple resulted in a higher energy density for Mn-rich samples, as shown in Figure S5. Note that Na4Mn2Fe(PO4)2(P2O7) can display an energy density of approximately 380 Wh kg−1 at room temperature, which is comparable to that of the pure Mn phase; this behavior is most likely a result of the enhanced kinetics with the addition of Fe in the structure. Notably, the increased energy density can be attributed to not only the Mn2+/Mn3+ redox activity but also the upshifted Fe2+/ Fe3+ redox potentials. The magnified discharge curves around the Fe2+/Fe3+ redox potentials in Figure 3c and cyclic voltammetry (CV) plots in Figure S6a−c indicate that the

Figure 1. (a) XRD patterns and (b) cell parameters of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3).

parameters as a function of Mn content (x) in Na4MnxFe3−x(PO4)2(P2O7). The lattice parameters and volume linearly change with the Mn substitution, indicating a complete solid solut ion between the two end ph ases of Na4Fe3(PO4)2(P2O7) and Na4Mn3(PO4)2(P2O7). The increase in the cell volume is attributed to the larger ionic radius of Mn2+ (0.83 Å) compared with that of Fe2+ (0.61 Å) in the structure. The cell parameters and R-factors from Rietveld refinement are listed in Figure S1. We performed XANES analyses using a synchrotron radiation source to probe the valence states of Fe and Mn in the binary compounds. The XANES spectra at the Fe k-edge in Figure S2a reveal that the oxidation state of Fe in Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) remains 2+, which is comparable to the reference state in Na4Fe3(PO4)2(P2O7).31 In addition, the valence state of Mn was determined to be 2+ using a Na4Mn3(PO4)2(P2O7) reference, as observed in Figure S2b.32 The local environment around Fe and Mn determined by the extended X-ray absorption fine structure (EXAFS) analyses indicates that the bonding nature around Fe and Mn in Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) is almost the same as that of the single-metal mixed phosphates (see the inset of Figure S2a,b).31,32 However, it was noted that the Fe−O length slightly increases as x increases, whereas the Mn−O bond length remains constant. The increase in the Fe−O bond 7243

DOI: 10.1021/acs.chemmater.6b01766 Chem. Mater. 2016, 28, 7241−7249

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Chemistry of Materials

Figure 2. Rietveld refinement of XRD patterns of Na4MnxFe3−x(PO4)2(P2O7) for (a) x = 1 and (b) x = 2. Structure representations of each structure are shown in the insets of the XRD patterns. Rietveld refinement of ND patterns of Na4MnxFe3−x(PO4)2(P2O7) for (c) x = 1 and (d) x = 2. Occupied Fe and Mn ions at three different transition metal sites with their fraction in color are described in the insets of the ND patterns.

state of intermediate compounds.37 Similarly, we speculate that unfavorable Na + −Fe 3 + and/or V N a −Mn 2 + pairs in Na4−yMnxFe3−x(PO4)2(P2O7) (0 < y < 3) may also increase the Gibbs free energy of intermediate states, resulting in a higher Fe2+/Fe3+ redox potential compared with that of Na4−yFe3(PO4)2(P2O7). In addition, the variation in Fe−O bond lengths in Na4MnxFe3−x(PO4)2(P2O7), as observed above, would affect the potential of the Fe2+/Fe3+ redox couple. Considering that the longer TM−O bond length generally induces a higher ionic character of the TM in the structure based on the inductive effect descriptor, an increased average bond length between Fe and O in Na4MnxFe3−x(PO4)2(P2O7) would result in a higher Fe redox potential (see Figure S2 and Table S2).33,38 Electrochemical Mechanism of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) Electrodes upon Charge/Discharge Processes. To understand the structural evolution of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) during electrochemical reaction, we performed ex situ XRD analyses at various states of charge. Figure 4a presents ex situ XRD patterns of Na4MnFe2(PO4)2(P2O7) at intermediate states of charge and discharge processes. A continuous peak shift of the XRD pattern is observed for the entire process, which indicates the one-phase reaction of Na4−xMnFe2(PO4)2(P2O7). The (200), (011), and (210) peaks shifted to higher angles during charging and returned to the initial 2θ during the subsequent discharge reaction. The Na4Mn2Fe(PO4)2(P2O7) electrode also operates via the one-phase reaction during electrochemical cycling with a continuous shift of XRD peaks in Figure 4b. The entire

Table 1. Fe and Mn Occupancies at Three Different Transition Metal Sites (TM1−TM3 sites) phase

site

atom

occupancy

x=1

TM1

Fe Mn Fe Mn Fe Mn Fe Mn Fe Mn Fe Mn

0.482 0.518 0.828 0.172 0.725 0.275 0.171 0.829 0.406 0.594 0.434 0.536

TM2 TM3 x=2

TM1 TM2 TM3

(7) (7) (5) (5) (6) (6) (7) (7) (7) (7) (8) (8)

potential of the Fe2+/Fe3+ redox reaction increases in both the anodic and cathodic scans. The onset potential of Fe2+/Fe3+ increases from 2.89 to 3.06 V versus Na+/Na in the anodic scan and from 2.78 to 2.89 V versus Na+/Na in the cathodic scan as x increases from 0 to 2 in Na4MnxFe3−x(PO4)2(P2O7). An upshift of the Fe2+/Fe3+ redox potential (∼0.2 V vs Na+/Na) was also observed in galvanostatic intermittent titration technique (GITT) profiles in Figure S7. The upshift of the Fe2+/Fe3+ redox couple has been previously reported for Li(Fe1−yMny)PO4, Li(Mn1/3Fe1/3Co1/3)PO4, and Li2(Fe1−yMny)P2O7 with higher values of x.33−37 Malik et al. claimed that it arises from the unfavorable interaction between the oxidized Fe3+ and remaining Li+, which increases the energy 7244

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Figure 3. Galvanostatic (a) charge and (b) discharge curves of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3) at a C rate of C/20. (c) Magnified discharge curves around Fe2+/Fe3+ redox potentials. For a clear comparison of the electrochemical profiles, we used the data from the second charge/discharge reaction. Figure 4. Ex situ XRD patterns of Na4MnxFe3−x(PO4)2(P2O7) for (a) x = 1 and (b) x = 2 during electrochemical charge/discharge reactions in Na-ion cells. (c) Lattice parameter and cell volume changes upon charge/discharge reactions in Na-ion cells.

evolution of XRD patterns in the 2θ range of 8−60° can be found in Figure S8. Note that the one-phase-based Na de/ intercalation behavior is similar to that of Na4Fe3(PO4)2(P2O7) but is in marked contrast to that of the Na4Mn3(PO4)2(P2O7) electrode, where a multiphase reaction occurs during electrochemical reaction with two intermediate phases (β- and δphases).32 Figure 4c shows the lattice parameter change of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) electrodes with cycling. Upon extraction of Na from the Na4MnFe2(PO4)2(P2O7), the a lattice parameter linearly decreases whereas the b lattice

parameter exhibits a negligible change (95%) with energy densities of 376 and 418 Wh kg−1, respectively. The high power density is believed primarily to originate from the open framework structure of Na4MnxFe3−x(PO4)2(P2O7) and the de/sodiation involving a small volume change (∼2% for x = 1 and ∼4% for x = 2) during electrochemical cycling. Remarkable cycle stability was achieved for both Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) electrodes in Na cells. Figure 6d shows that approximately 96 and 95% of the initial capacities were retained from Na4MnFe2(PO4)2(P2O7) and Na4Mn2Fe(PO4)2(P2O7) electrodes, respectively, after 50 cycles at C/20. Although the overall discharge profile was relatively well preserved after 20 and 50 cycles without a significant voltage decay, as observed in the inset of Figure 6d, a capacity decay was observed especially in the high-voltage region (Mn2+/Mn3+) upon electrochemical cycling as shown Figure S12, which may originate from the large lattice contraction and Mn dissolution. It was noted that the Na4MnFe2(PO4)2(P2O7) electrode demonstrates an unprecedentedly stable cycle property at a current rate of 1C and room temperature; approximately 83% of the initial capacity was retained after 3000 cycles with a Coulombic efficiency of 99.8%, as observed in Figure 6e. Also, the discharge profiles upon electrochemical cycling display low voltage and capacity decays as shown in Figure S13. To the best of our knowledge, this demonstration is the first of such superior cycle stability for Feor Mn-based polyanion cathodes for NIBs. Note that Na4MnFe2(PO4)2(P2O7) exhibits the highest capacity retention of 95.8% after 100 cycles compared with those of other Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 2, or 3) electrodes (90.7% for x = 0, 84.8% for x = 2, and 81.3% for x = 3), as shown in Figure S14. We believe that the small volume change of the Na4MnFe2(PO4)2(P2O7) electrode upon electrochemical cycling (∼2.1%) may affect the cycle stability. Electrode operation of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) at 60 °C also revealed stable cycle retention up to 200 cycles in Figure S15. Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) electrodes exhibited retention of 93% (x = 1) and 75% (x = 2) after 200 cycles. Considering that the long-term cycle stability is one of the key factors in the success of large-scale ESSs, we believe that Na4MnFe2(PO4)2(P2O7) is a strong competitor among various cost-efficient cathode materials.

the recently reported Na2Fe2(SO4)3 electrode (∼1.6%) (see Figure S9).15,17,22,23,31,32,39,40 The origin of the small volume change with a particular Fe and Mn ratio is unclear; however, we speculate that it is related to the preferential occupancies of Fe and Mn in the symmetrically distinguishable TM sites and corresponding sequential distortion of TM along with the desodiation procedure.31,32 The b and c lattice constants from the Mn-rich electrode, Na4Mn2Fe(PO4)2(P2O7), exhibit tendencies similar to those of Na4MnFe2(PO4)2(P2O7) during charge and discharge reactions. However, the change in the a lattice parameter is much larger with a collapse at a certain composition of more than one Na extraction, which may originate from the Jahn−Teller distortion of Mn3+, as previously observed for Na4Mn3(PO4)2(P2O7).32 Thus, the lattice volume contraction is slightly larger by approximately 4.1% after a full charging. The lattice parameter change upon desodiation was further examined by using density functional theory (DFT) calculations. However, the lattice parameter changes from DFT calculation exhibited a trend different from those from experimental results, which may originate from Fe/ Mn mixing in TM sites in the structures of Na4MnxFe1−x(PO4)2(P2O7) (x = 1 or 2) (see Table S3). The valence state of TMs in Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) during electrochemical reaction was evaluated using XANES analyses, as shown in Figure 5a−d. These results clearly demonstrate that the Fe k-edge spectrum of Na4MnFe2(PO4)2(P2O7) shifts to a higher energy during the initial charging up to 3.5 V versus Na+/Na, indicating the occurrence of Fe2+/Fe3+ redox reactions. The negligible shift of the Fe k-edge spectrum is observed in the following charging up to 4.5 V. Instead, the Mn2+/Mn3+ redox reaction occurs in this region. Although the shift in the XANES spectra was not clear from the Mn k-edge, the pre-edge shifts to the right in the voltage range of 3.5−4.5 V, as observed in the inset of the Mn k-edge spectra. The reversible Fe2+/Fe3+ and Mn2+/Mn3+ redox reactions occur during subsequent discharge processes (Figure S10). Similarly, the Fe k-edge spectra of the Na4Mn2Fe(PO4)2(P2O7) electrode primarily shift to the right until charging up to 3.85 V, which is followed by the main shift of the Mn k-edge until the end of the charging reaction. On the basis of the XANES analyses, we can confirm that the capacities at low potential below 3.5 V and high potential above 3.8 V were delivered from Fe2+/Fe3+ and Mn2+/Mn3+ redox reactions, respectively. Electrochemical pProperties of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) Electrodes under Various Conditions. Galvanostatic charge/discharge experiments at various C rates were performed at room temperature using coin-type Na cells. Figure 6a displays the discharge profiles of Na4MnFe2(PO4)2(P2O7) at various C rates. Approximately 85% (110 mAh g−1) of the theoretical capacity was delivered at a C/20 rate, and a capacity of approximately 101 mAh g−1 was retained at a current rate of 2C. Furthermore, we can obtain more than 60% of the theoretical capacity even for a 3 min discharging procedure (20C). The high rate performance is also demonstrated for the Na4Mn2Fe(PO4)2(P2O7) electrode up to 20C in Figure 6b. The high energy density of approximately 380 Wh kg−1 was obtained at C/20, and this value is comparable to those of polyanion cathodes such as Na4Mn3(PO4)2(P2O7) and Na2Fe2(SO4)3.23,32 The Ragone plot of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) compared with those of other representative Fe- and Mn-based polyanion compounds [i.e., NaFePO4, Na2FePO4F, Na2Fe2(SO4)3,

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CONCLUSION We successfully synthesized solid-solution phases of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) for the first time and characterized the structure using ND, XRD, and XANES analyses. A high energy density (376 Wh kg−1 for x = 1 and 418 Wh kg−1 for x = 2 at 60 °C) was obtained from both electrodes by utilizing the Mn2+/Mn3+ and Fe2+/Fe3+ redox couple in Naion cells. Both electrodes operate via a one-phase reaction with a remarkably small volume change (∼2% for x = 1 and ∼4% for x = 2). The Na4MnFe2(PO4)2(P2O7) electrode exhibited outstanding rate and cycle performances at both room temperature and 60 °C, which are attributed to its open framework structure and small volume change (∼2%). The 7247

DOI: 10.1021/acs.chemmater.6b01766 Chem. Mater. 2016, 28, 7241−7249

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electrode operation at 1C at room temperature revealed unprecedentedly stable cycle retention of 83% over 3000 charge/discharge cycles. We believe that the combination of high energy density, long-term cycle stability, and low element cost of Fe and Mn makes the Na4MnFe2(PO4)2(P2O7) electrode a strong competitor for large-scale Na-ion battery cathodes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01766. Detailed structural information from Rietveld refinement of XRD and ND patterns of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2), XANES spectra at Fe and Mn k-edges of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3), SEM image of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3), CV and GITT profiles of Na4MnxFe3−x(PO4)2(P2O7) (x = 0, 1, 2, or 3), ex situ XRD and XANES profiles of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) upon charge and discharge processes, and galvanostatic charge/ discharge profiles of Na4MnxFe3−x(PO4)2(P2O7) (x = 1 or 2) at 60 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (20158510050040). This work was also supported by the National Research Foundation of Korea (NRF) funded by the Korean Government (MSIP) (2014M2B2A4031968 and 2012M2A2A6002461).

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