Article pubs.acs.org/cm
Sodium Extraction from NASICON-Structured Na3MnTi(PO4)3 through Mn(III)/Mn(II) and Mn(IV)/Mn(III) Redox Couples Hongcai Gao, Yutao Li, Kyusung Park, and John B. Goodenough* Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Sodium extraction behavior in the rhombohedral Na3MnTi(PO4)3 with the NASICON structure was investigated and shown to have a 0.5 eV separation of the localized-electron Mn3+/Mn2+ and Mn4+/Mn3+ redox energies. Used as a cathode in a sodium-ion half-cell, the redox couples of Mn3+/Mn2+ and Mn4+/Mn3+ exhibit two voltage plateaus located at about 3.6 and 4.1 V vs Na+/Na, respectively. Na3MnTi(PO4)3 also exhibits a small volume change after the extraction of two sodium ions, and the NASICON structure is well-maintained after repeated extraction and insertion of sodium ions.
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phosphate framework.22−24 The redox couple of Fe3+/Fe2+ in Fe2(SO4)3 delivers an average voltage of ∼3.2 V vs Na+/Na.25 The redox couple of V4+/V3+ in Na3V2(PO4)3 is located at a voltage of ∼3.4 V vs Na+/Na.26,27 The redox couple of Ti4+/ Ti3+ in NaTi2(PO4)3 has also been exploited, but its voltage is too low (∼2.1 V vs Na+/Na) to be used as a cathode for sodium-ion batteries.28 Besides the well-documented NASICON compositions of ATi2(PO4)3 (A = Li+, Na+) and A3M2(PO4)3 (M = V3+, Fe3+, Cr3+; A = Li+, Na+), limited work has been conducted on mixed transition-metal phosphates with the NASICON structure, apart from the reports of Na2TiM(PO4)3 (M = Cr3+, Fe3+, Ga3+, and Rh3+).29−31 The preliminary investigation of NASICON-structured Li2TiFe(PO4)3 and Li2TiCr(PO4)3 in lithium-ion batteries revealed electrochemical insertion of lithium ions proceeds through the reduction of Fe3+ into Fe2+ (∼2.8 V vs Li+/Li) and Ti4+ into Ti3+ (∼2.5 V vs Li+/Li).22,32,33 The insertion of sodium ions into Na1.5Fe0.5Ti1.5(PO4)3 and Na1+xTi2−xFex(PO4)3 was reported recently; besides the plateau at about ∼2.1 V vs Na+/Na from the Ti4+/Ti3+ redox couple, a continuous voltage decrease is observed between 2.6 and 2.2 V vs Na+/Na presumably because of the reduction from Fe3+ to Fe2+.34,35 However, the Fe3+ and Ti4+ mixed phosphate cannot provide high voltages due to the difficulty of oxidation of Fe3+ to Fe4+ in the NASICON framework. The design of M2+ and Ti4+ mixed phosphate would open a way to access high voltage with the help of the M3+/M2+ and M4+/M3+ redox couples, especially with M2+ = Mn2+. Herein, we report our investigation of sodium-ion extraction/insertion behavior in NASICON-related rhombohedral Na3MnTi(PO4)3. The redox couples of Mn3+/
INTRODUCTION The successful integration of sustainable and renewable energy generated from solar and wind power plants into modern society requires the development of energy storage devices at a low cost to smooth the supply of electric power into the grid.1−4 The natural abundance of sodium makes rechargeable sodium-ion batteries preferable candidates to lithium-ion batteries for large-scale electric-energy storage provided costeffective sodium host anodes and cathodes can be found with the necessary capacity, voltage, and cycle life.5 A number of layered oxide materials have been investigated recently as cathodes for sodium-ion batteries with the expectation to achieve high capacity.6−8 However, the large volume change and the phase instability during cycling are among the largest obstacles for realization of layered cathodes with a long cycle life. Prussian blue analogues have also been intensively explored as cathodes for sodium-ion batteries because of their low cost, facile synthesis procedure at low temperature, and the low activation energies for Na+ insertion/extraction.9−12 Nevertheless, their inferior thermal stability, their low volume density, and the toxicity of cyanide are the intrinsic limitations toward practical applications of Prussian blue cathodes for sodium-ion batteries. Another important category of cathodes for sodiumion batteries is polyanionic compounds with open framework structures.13−18 The strong X−O (X = P, S, Si, and B) covalent bonding can offer a high structural stability with a relatively small volume change during insertion and extraction of sodium ions. Within the family of polyanionic frameworks, NASICON (Na+ super ionic conductor)-related compounds are of particular interest because of their high Na+ mobility and reasonable capacities.19,20 The most useful redox potentials of transition metals in the NASICON-related cathode materials have been identified as Fe3+/Fe2+ in the sulfate framework21 and V4+/V3+ in the © 2016 American Chemical Society
Received: May 24, 2016 Revised: August 25, 2016 Published: August 30, 2016 6553
DOI: 10.1021/acs.chemmater.6b02096 Chem. Mater. 2016, 28, 6553−6559
Article
Chemistry of Materials Mn2+ and Mn4+/Mn3+ were accessed in the Na3MnTi(PO4)3 cathode, exhibiting two voltage plateaus located at about 3.6 and 4.1 V vs Na+/Na, respectively.
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EXPERIMENTAL SECTION
The sol−gel method was applied to prepare Na3MnTi(PO4)3 by adding titanium isopropoxide into the aqueous solution of sodium acetate, manganese acetate, NH4H2PO4, and citric acid with stoichiometric ratio. After evaporation of water, the precursor was ground and sintered in a tube furnace under an argon atmosphere at 600 °C for 12 h.36 Scanning electron microscopy (SEM, FEI Quanta 650) and transmission electron microscopy (TEM, JEOL 2010F) were used to characterized the morphology of the material. The chemical composition of the material was analyzed by energy dispersive X-ray spectroscopy (EDS). Powder X-ray diffraction (XRD) data were collected on Rikagu MiniFlex 600 diffractometer (Cu Kα radiation). Synchrotron XRD data were collected at the 17BM beamline of Advanced Photon Source, Argonne National Laboratory, with an X-ray wavelength of 0.72768 Å. The XRD patterns were refined by the Rietveld refinement with the program of FullProf Suite. Thermogravimetric analysis (TGA) was performed in a Mettler Thermogravimetric Analyzer (Model TGA/DSC 1) under an air atmosphere. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos X-ray Photoelectron Spectrometer. After the electrochemical test, the coin cells were disassembled in an argon-filled glovebox and the electrodes were washed with dimethyl carbonate. After drying, the electrodes were transferred into an ultra-high-vacuum X-ray photoelectron spectrometer chamber directly from the glovebox without exposing them to air by employing a sample transfer interface. Na3MnTi(PO4)3 electrodes were prepared by rolling a mixture of Na3MnTi(PO4)3 (70%), carbon black (20%), and polytetrafluoroethylene (10%) into thin films with the loading of active materials about 2.0 mg cm−2. Before electrochemical tests, the electrodes were dried in a vacuum oven at 100 °C for 3 h. The Na3MnTi(PO4)3 electrodes were evaluated standard CR2032 coin cells with sodium metal as counter and reference electrodes and glass fiber as separators. The liquid electrolyte consisted of NaClO4 (1.0 M) in a solution of propylene carbonate and fluoroethylene carbonate with a volume ratio of 9:1. The fabrication of the half-cells was conducted in an argonfilled glovebox, and the cells were aged for 2 h before testing to ensure full absorption of the electrolyte into the electrodes. Cyclic voltammetry (CV) was conducted on an electrochemical workstation (Solartron Analitical Instrument, Model 1287A). The charge/ discharge tests were performed at room temperature in a battery testing system (Land Instruments). Galvanostatic intermittent titration technique (GITT) measurements of the electrode were conducted with 30 min charge or discharge with open-circuit periods of 5 h.
Figure 1. (A) SEM image of the Na3MnTi(PO4)3 sample. (B) Area used for EDS mapping of the Na3MnTi(PO4)3 sample. (C) Elements distribution in the selected area.
titanium, and phosphorus are distributed uniformly in the particles (Figure 1C). The synchrotron X-ray diffraction (XRD) pattern of Na3MnTi(PO4)3 and its Rietveld analysis are given in Figure 2A. The diffraction pattern of Na3MnTi(PO4)3 can be indexed to a rhombohedral lattice in the space group R3̅c with the lattice parameters a = 8.825 Å and c = 21.708 Å.39,40 The detailed atomic positions of Na3MnTi(PO4)3 are listed in Table S1 (Supporting Information). Different from the calculated occupancy factors of 1 and 0.67 for Na(1) and Na(2) in NASICON-type compounds,41 the Rietveld refinement of Na3MnTi(PO4)3 indicates the occupancy factors were 0.87 and 0.66 for Na(1) and Na(2) sites, respectively, leading to 17.1 (6 × 0.87 + 18 × 0.66) sodium per unit cell (Z = 6) and in agreement with the expected chemical formula.40,42 Na3MnTi(PO4)3 prepared at a higher temperature of 800 °C has a XRD pattern similar to that of the material prepared at 600 °C, and there are no additional peaks contributing to the long-range ordering between the transition metals of Mn and Ti (Figure S2, Supporting Information). Therefore, the octahedral transition-metal sites of the NASICON framework are statistically equally occupied by Mn and Ti.33,35 The NASICON-type Na3MnTi(PO4)3 has a framework structure formed by corner-sharing MnO6 or TiO6 octahedral and PO4 tetrahedral units with large open channels between them (Figure 2B). The strong covalent PO4 units are responsible for an inductive effect on the redox energies of transition metals. The polyanionic phosphate framework can also provide structural stability and intrinsic safety even at high voltage states. In the framework structure, sodium ions occupy two different sites: M1 sites with sixfold coordination and M2 sites with eightfold coordination. During charge/discharge process, the sodium ions positioned at M1 sites are immobilized, and only sodium ions residing at M2 sites can be extracted and inserted for electrochemical activities.43,44 The reversible extraction/insertion of two sodium ions at M2 sites with the
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RESULTS AND DISCUSSION Na3MnTi(PO4)3 prepared by the sol−gel method has a wide particle size distribution (Figure 1A) that is composed of nanometer-sized primary particles embedded in a carbon matrix. The lattice fringes with d-spacing of 0.62 nm correspond to the (012) planes of the NASICON-structured Na3MnTi(PO4)3 (Figure S1, Supporting Information). While the transition-metal phosphates usually suffer from low electrical conductivity when used as the cathode for lithiumor sodium-ion batteries, our sol−gel method enables the direct synthesis of Na3MnTi(PO4)3 particles with a uniform carbon coating, which is expected to improve the electrical conductivity. TGA revealed a carbon content of about 7.8 wt % in Na3MnTi(PO4)3.36−38 The chemical composition was analyzed by energy-dispersive X-ray spectroscopy (EDS) of the Na3MnTi(PO4)3 particles in a selected area as shown in Figure 1B. It can be seen that the elements of sodium, manganese, 6554
DOI: 10.1021/acs.chemmater.6b02096 Chem. Mater. 2016, 28, 6553−6559
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to the theoretical capacity of Na3V2(PO4)3 with the V4+/V3+ redox couple. The electrochemical properties of Na3MnTi(PO4)3 as a cathode for sodium-ion batteries were evaluated by CV and galvanostatic charge/discharge tests in half-cells with sodium metal as the reference and counter electrodes at room temperature. The CV plots of the Na3MnTi(PO4)3 electrodes were tested in the voltage range of 2.5−4.2 V vs Na+/Na at a scanning rate of 0.05 mV s−1 (Figure 3A). Two pairs of redox peaks were observed, corresponding to the oxidation (Na+ extraction) and reduction (Na+ insertion) of Na3MnTi(PO4)3. The redox peaks centered at about 3.6 V vs Na+/Na is close to the equilibrium voltage of the Mn3+/Mn2+ redox couple in Na3MnTi(PO4)3. The peaks centered at about 4.1 V vs Na+/Na correspond to the Mn4+/Mn3+ redox couple. Theoretical calculation indicates that the average voltage gap between Mn4+/Mn3+ and Mn3+/Mn2+ redox couples in phosphates is about 0.5 V,45 which is consistent with the observed voltages of Mn4+/Mn3+ and Mn3+/Mn2+ in Na3MnTi(PO4)3. The nearly overlapped CV curves during repeated scans indicates an excellent reversibility of extraction/insertion of sodium ions from/into Na3MnTi(PO4)3. In accordance with the CV curves, two obvious voltage plateaus are located at about 3.6 and 4.1 V vs Na+/Na in the galvanostatic charge/discharge plots of the Na3MnTi(PO4)3 electrode (Figure 3B). The average voltage of Na3MnTi(PO4)3 is apparently higher than other NASICON-structured materials for sodium-ion batteries, such as Na3V2(PO4)3 (∼3.4 V vs Na+/ Na)27 and NaTi2(PO4)3 (∼2.1 V vs Na+/Na).46 The redox couple of Mn3+/Mn2+ is generally responsible for the electrochemical activity in lithium or sodium manganese phos ph at es, incl udin g LiM n PO 4 , 4 7 Li 2 MnP 2 O 7 , 4 8 Na2MnP2O7,49 and Na4Mn3(PO4)2P2O7.50 Herein, the redox couple of Mn4+/Mn3+ is accessed in Na3MnTi(PO4)3 with a high voltage located at about 4.1 V vs Na+/Na. The Na3MnTi(PO4)3 electrode delivers an initial charge capacity
Figure 2. (A) Synchrotron X-ray diffraction data and Rietveld refinement of Na3MnTi(PO4)3. (B) Structural illustration of the NASICON-structured Na3MnTi(PO4)3. The MnO6 or TiO6 octahedron is presented in blue, the PO4 octahedron in green, and the sodium ions in yellow.
Mn3+/Mn2+ and Mn4+/Mn3+ redox couples could yield a theoretical capacity of 117 mA h g−1, which is almost identical
Figure 3. (A) Cyclic voltammograms of the Na3MnTi(PO4)3 electrode. (B) Galvanostatic charge/discharge curves of the Na3MnTi(PO4)3 electrode at a rate of 0.1 C. (C) Galvanostatic charge/discharge curves of the Na3MnTi(PO4)3 electrode at different current rates from 0.1 to 2 C. (D) The capacity retention and Coulombic efficiency of the Na3MnTi(PO4)3 electrode at a constant charge/discharge rate of 0.1 C. 6555
DOI: 10.1021/acs.chemmater.6b02096 Chem. Mater. 2016, 28, 6553−6559
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Chemistry of Materials of 114 mA h g−1 at a current rate of 0.1 C, corresponding to the extraction of about two sodium ions. The Coulombic efficiency of the Na3MnTi(PO4)3 electrode is about 70% in the first charge/discharge cycle, with a discharge capacity of 80 mA h g−1. The low initial Coulombic efficiency of manganese phosphate cathodes may be caused by instability of the cathode surface induced by the reactivity of the cathode with the electrolytes.49−53 The rate performance of the Na3MnTi(PO4)3 electrode was tested at different current rates of 0.1, 0.2, 0.5, 1, and 2 C, with discharge capacities of 80, 64, 56, 49, and 40 mA h g−1, respectively (Figure 3C). After the current density came back to the rate of 0.1 C, the discharge capacity was recovered to 70 mA h g−1, demonstrating an excellent structural stability of the Na3MnTi(PO4)3 cathode (Figure S3, Supporting Information). The cycling stability of the Na3MnTi(PO4)3 electrode was tested at a constant current rate of 0.1 C (Figure 3D). The discharge capacity dropped from 80 to 66 mA h g−1 in the first 20 cycles, but remained at 62 mA h g−1 after 100 cycles with Coulombic efficiency approaching 99%. The capacity fade of Na3MnTi(PO4)3 is mainly from the Mn4+/Mn3+ redox couple (Figure S4, Supporting Information). After the cycling test, the characteristic peaks in the XRD pattern of the Na3MnTi(PO4)3 electrode remained almost the same compared with that of the pristine electrode (Figure 4). Thus, the repeated extraction and
vs Na+/Na. Upon further charging and after the voltage plateau of 3.6 V vs Na+/Na, the potential jumped to another plateau of 4.1 V vs Na+/Na. Both the low voltage (3.6 V vs Na+/Na) and high voltage (4.1 V vs Na+/Na) plateaus are quite flat, suggesting these plateaus are associated with a two-phase reaction mechanism during the extraction of sodium ions. In the charge curve, the capacity of the low-potential plateau is almost equal to the capacity of the high-potential plateau, suggesting the existence of Na2MnTi(PO4)3 as an intermediate phase between the two terminal phases of Na3MnTi(PO4)3 and NaMnTi(PO4)3. To reveal the structural evolution of the Na3MnTi(PO4)3 cathode during the extraction and insertion of sodium ions, ex situ XRD analysis of the electrodes was performed at different states of charge and discharge (Figure 5A). At the beginning of charging the Na3MnTi(PO4)3 electrode, all peaks were indexed to the rhombohedral NASICON structure in the space group R3c̅ (Figure 5B), except for a peak from the polymer binder of PTFE (Figure S6, Supporting Information). The first plateau at around 3.6 V vs Na+/Na is the signature of a two-phase process for the sodium-ion extraction from Na3MnTi(PO4)3 to Na2MnTi(PO4)3. Upon charging of the electrode to 3.9 V vs Na+/Na, a new phase is observed corresponding to the rhombohedral phase of Na2MnTi(PO4)3. The second plateau at around 4.1 V vs Na+/Na corresponds to another two-phase domain between Na2MnTi(PO4)3 and NaMnTi(PO4)3. When the electrode is charged to 4.2 V vs Na+/Na, the rhombohedral phase of NaMnTi(PO4)3 within space group R3̅c was obtained with the diffraction peaks of (012), (104), (113), (204), and (300) moving to higher angles.41,54 After complete charging, the volume change is about 4.8% from Na3MnTi(PO4)3 (a = 8.825 Å, c = 21.708 Å) to NaMnTi(PO4)3 (a = 8.564 Å, c = 21.886 Å), as a result of full oxidation of Mn2+ to Mn4+ and the extraction of two sodium ions, but it does not generate a change of space group (Table S2, Supporting Information). The decrease of a and the slight increase of c implies the progressive emptying of the M2 sites while the occupancy of M1 sites remains unchanged from Na3MnTi(PO4)3 to NaMnTi(PO4)3 with the increase of c/a ratio.33,55 The volume change of Na3MnTi(PO4)3 is smaller than that of Na3V2(PO4)3 (∼8.3%) after extraction of two sodium ions.54 After discharge to 2.5 V vs Na+/Na, the structure of the material is completely transformed back to the original rhombohedral NASICON phase of Na3MnTi(PO4)3. The 0.5 V voltage step between the Mn4+/Mn3+ and the Mn3+/Mn2+ redox couples signals a localized-electron Mn(III):t3e1 configuration in Na2MnTi(PO4)3. Nevertheless, stabilization of this intermediate phase apparently indicates a Na+ ordering rather than a cooperative Jahn−Teller orbital ordering on the Mn(III), which is commonly observed with a greater concentration of Mn(III).56 These results invite exploration of other transition-metal cations than Ti(IV) in the NASICON-structured material of Na3+xMnM(PO4)3. XPS was used to probe the oxidation states of manganese and titanium in the Na3MnTi(PO4)3 electrode at different charging states. The transition from Na3MnTi(PO4)3 to NaMnTi(PO4)3 during charging is expected to be accompanied by the oxidation of manganese from Mn2+ to Mn3+ and then to Mn4+. The peak of Mn 2p3/2 in the Na3MnTi(PO4)3 electrode before charging was located at a binding energy of 640.8 eV, corresponding to Mn2+ (Figure 5C). After the electrode was charged to 3.9 V vs Na+/Na, the binding energy of Mn 2p3/2 increased to about 641.6 eV, indicating the formation of Mn3+.
Figure 4. X-ray diffraction patterns of the Na3MnTi(PO4)3 electrode before and after 100 charge/discharge cycles at the rate of 0.1 C.
insertion of sodium ions did not cause any significant change in the crystal structure of Na3MnTi(PO4)3. The manganese-based polyanionic compounds typically suffer from significant structural distortion because of the appearance of Jahn−Teller active Mn3+ during charge/discharge cycles. The presence of Ti4+ in Na3MnTi(PO4)3 ensures the fraction of Mn3+ does not exceed 50% of the total sites of transition metals, which is beneficial to maintain the structural stability of the NASICON framework of Na3MnTi(PO4)3 during repeated extraction and insertion of sodium ions. The phase behavior of the Na3MnTi(PO4)3 electrode during electrochemical cycling was first probed through GITT measurements (Figure S5, Supporting Information). Starting from open-circuit voltage of 2.7 V vs Na+/Na, the potential of the electrode was rapidly increased to a plateau of about 3.6 V 6556
DOI: 10.1021/acs.chemmater.6b02096 Chem. Mater. 2016, 28, 6553−6559
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Figure 5. (A) Galvanostatic charge/discharge profiles of the Na3MnTi(PO4)3 electrode and the marked points at which the samples were taken for ex situ XRD analysis. (B) XRD patterns of the Na3MnTi(PO4)3 electrode at various charge/discharge states (* indicates the peaks from the polymer binder of PTFE). XPS spectra of (C) Mn 2p and (D) Ti 2p of the Na3MnTi(PO4)3 electrode at different charging states.
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After further charging of the electrode to 4.2 V vs Na+/Na, the binding energy of Mn 2p3/2 increased to about 642.1 eV, indicating the appearance of Mn4+.57,58 The binding energies of titanium remain unchanged during the charge process (Figure 5D); thus, the oxidation and reduction of manganese accompanies the extraction and insertion of sodium ions. The above results showed that the extraction and insertion of two sodium ions from/into Na3MnTi(PO4)3 is possible through the utilization of Mn3+/Mn2+ and Mn4+/Mn3+ redox couples.
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Corresponding Author
*E-mail:
[email protected] (J.B.G.). Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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CONCLUSIONS
ACKNOWLEDGMENTS The synthesis, analysis, and electrochemical characterization of the electrode material were supported by the U.S. Department of Energy, ARPA-E Project (UTA13-000404), and by the U.S. Department of Energy, Office of Basic Energy Sciences (Grant No. DE-SC0005397). We acknowledge the technical support by Dr. Xiqian Yu and Dr. Enyuan Hu from Beamline 17BM of Advanced Photon Source at Argonne National Laboratory. J.B.G. also acknowledges support from the Robert A. Welch Foundation (Grant F-1066).
A NASICON-structured material of rhombohedral Na3MnTi(PO4)3 was prepared, and the sodium extraction/insertion behavior was investigated. The access to two redox couples, Mn3+/Mn2+ and Mn4+/Mn3+, with a voltage gap of 0.5 V between them, provides a high-voltage cathode for a sodiumion battery, but the poor electronic conductivity will require small particles with a carbon coating, which would limit the volumetric energy density of the cathode. Nevertheless, the small volume change and the robust NASICON framework may point a direction toward further exploration of NASICONbased chemistry to realize novel cathode materials for sodiumion batteries at a low cost with environmentally benign elements toward large-scale stationary applications.
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AUTHOR INFORMATION
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REFERENCES
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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.6b02096. Crystallographic data, TEM images, and electrochemical tests (PDF) 6557
DOI: 10.1021/acs.chemmater.6b02096 Chem. Mater. 2016, 28, 6553−6559
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DOI: 10.1021/acs.chemmater.6b02096 Chem. Mater. 2016, 28, 6553−6559
