Cr3+ Redox Couple in Polyanion Compounds

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A High-Voltage Cr /Cr Redox Couple in Polyanion Compounds Kosuke Kawai, Wenwen Zhao, Shin-ichi Nishimura, and Atsuo Yamada ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00105 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Energy Materials

A High-Voltage Cr4+/Cr3+ Redox Couple in Polyanion Compounds Kosuke Kawai,† Wenwen Zhao,† Shin-ichi Nishimura,†, ‡, and Atsuo Yamada†, ‡,* †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan

‡

Elemental Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan *[email protected]

ACS Paragon Plus Environment

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KEYWORDS Na-ion batteries; High-voltage; Cathode; Chromium; Phosphate; NASICON

ABSTRACT

Higher-voltage operation is crucial to increase energy density of rechargeable batteries. Here we explored Cr4+/Cr3+ redox couple in polyanion framework for high-voltage sodium battery cathode. An archetypal NASICON-phase Na3Cr2(PO4)3 exhibited a reversible redox activity at ca. 4.5 V vs. Na/Na+, offering the further scope for searching new high-voltage materials based on Cr4+/Cr3+ redox couple.


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For the past three decades, lithium-ion batteries (LIBs) have dominated the portable electronics market and nowadays its application is extending to larger-scale energy storage systems for electric vehicles and stationary use for leveling renewable energy. As the market expands, sodium-ion batteries (SIBs) can be a possible alternative due to the abundance of sodium in the earth crust and the sea.1,2 However, inherently higher redox potential of Na/Na+ than Li/Li+ by ca. 0.35 V brings about lower operating voltages of a cell. To overcome this disadvantage, high-potential cathode materials for SIBs are required. Use of inductive effect in polyanionic framework is a major strategy to raise up the redox potential of a transition metal while increasing structural integrity and safety.3,4 Sulfate ion (SO4)

2-

demonstrates the highest electronegativity and hence the strongest inductive effect among all polyanions, while compounds containing (SO4)2- are apt to decompose above ~ 400 ℃ (generating SO2 gas) and highly hygroscopic. Phosphate compounds, being chemically much more stable but less electronegative, forms practical candidates. Among them, NASICON-type compounds AxM2(PO4)3 (A = Li, Na; M = transition metal) have been widely studied because of their high sodium-ion conductivity and robust structures, incorporating various transition metals (M = Ti, V, Mn, Fe, Co, Ni) as redox centers.5-11 In addition to the selection of anionic species, transition metal has a critical influence on operating potential. So far, some cobalt- and nickel-based materials are known as 4 V class cathodes, for example Na2-2xCo1+xP2O7 (~ 4.0 V vs. Na/Na+) and Na4M3(PO4)2P2O7 (~ 4.4 V and ~ 4.8 V vs. Na/Na+ for M = Co and Ni, respectively).12-14 More abundant options containing vanadium or iron, such as Na3V2(PO4)2F3-xOx, Na7V3(P2O7)4 and Na2+2xFe2-x(SO4)3, have been identified while their average operating potentials lies around 4.0 V vs. Na/Na+.15-17 On the further development of high-voltage class cathode materials, utilizing other transition metal is of


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significant challenge. Theoretical calculation by Hautier et al. predicted that redox potential of a Cr4+/Cr3+ couple in a phosphate framework would be at ca. 4.9 V vs. Li/Li+,18 while the experimental verification has been very limited to date. The only example was reported by Herklotz et al., demonstrating electrochemical oxidation of monoclinic Li3Cr2(PO4)3, but with small reversible capacity of 32 mAh g–1, which was however turns out to occur only at the surface of particles.19 Therefore, demonstrating an active Cr4+/Cr3+ redox reaction in phosphate matrix is an urgent task before further exploration. Herein, we investigated the electrochemical properties of NASICON-type compound Na3Cr2(PO4)3 to demonstrate the Cr4+/Cr3+ redox couple operating around 4.5 V vs. Na/Na+ in a sodium cell. Na3Cr2(PO4)3 / acetylene black (AB) composite powder was synthesized by the solid state method. First, Cr2O3 (1st grade, Wako) and NaH2(PO4)3 (Wako, min. 99.0 %) in the stoichiometric ratio were ball-milled with 5 wt% excess of AB (Li-400, DENKA) on the basis of the product weight. Then, the pelletized mixture was heated in a tubular furnace under Ar flow for 22 hours at 1123 K and 12 hours at 1173 K with two intermediate grindings. The obtained powders were observed by field-emission scanning electron microscopy (FE-SEM) (S-4800, HITACHI) with an acceleration voltage of 1 kV. Synchrotron X-ray powder diffraction patterns was obtained at the beam line 8B of Photon Factory (PF), High Energy Accelerator Research Organization (KEK) Tsukuba, Japan. The wave length was calibrated to be 1.10901 Å using a diffraction pattern of standard reference material, (SRM640d, NIST). For all XRD measurements, borosilicate glass capillaries (Hilgenberg GmbH or Müller GmbH) were adopted as sample holders. The collected data was analyzed by Rietveld refinement program, TOPAS-Academic Ver. 6.


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The working electrode was fabricated by a wet slurry process: mixing 80 wt% Na3Cr2(PO4)3 / AB, 10 wt% AB and poly(vinylidene difluoride) (KUREHA) in N-methyl pyrrolidone (Kanto Chemical Co., Inc., min. 99.0 %) to form a homogeneous slurry, spreading the slurry on an aluminum current collector and then drying the sheet at 338 K in a vacuum condition overnight. The counter electrode was Na metal (Wako). The separator was a glass filter (GB-100R, Advantec). The electrolyte was 1 M NaPF6 ethylenecarbonate:diethylcarbonate (1:1 vol.) (Battery Grade, Kishida Chemical). Electrochemical tests were conducted by using 2032-type coin cells assembled in an Ar-filled grove box. Cyclic voltammetry was measured at a constant rate of 1.0 mV s–1 in a potential region between the rest potential and 5.2 V vs. Na/Na+. For all galvanostatic charge/discharge measurement, the C-rates were calculated by defining 1 C as the current density of 117 mA g–1 and the potential region was between 2.5 V and 4.7 V vs. Na/Na+. Ex situ synchrotron XRD patterns were collected at the beam line described above. Coin cells at the various charging and discharging states were disassembled in a grove box filled with Ar, then the working electrodes were washed with dimethyl ether and dried at 353 K under a vacuum condition. Powder peeled from the working electrodes were enclosed in the same glass capillaries mentioned above. The oxidation state of chromium during charging and discharging process was measured by Xray absorption near edge structure (XANES) spectroscopy at room temperature in a transmission mode at the BL-9A of PF, KEK, Japan. Data analyses was performed with Athena software.20 The synchrotron XRD pattern and its Rietveld refinement for as-prepared Na3Cr2(PO4)3 / acetylene black (AB) powder are shown in Figure 1. Most of reflections could be indexed to a rhombohedral lattice with a space group R3തc, which accords to a typical high-temperature form of NASICON structure.21-23 Lattice constants were a = b = 8.661(3) Å and c = 21.736(7) Å.


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Small additional diffraction peaks were observed in a low two theta region below 20 °, which reflects superstructure formed by Na+ ordering. Similar observation was also reported in Na3M2(PO4)3 (M = Ti, V, Fe).24,25 The detailed crystallographic data are listed in Table S1 and S2. A SEM image in the inset of Figure 1 shows a wide range of particle diameters from 2 µm to 10 µm embedded in nanometer particles of AB.

Figure 1. Rietveld refinement of the as-prepared Na3Cr2(PO4)3 / AB electrode. Black arrows indicate peaks ascribed to superstructure. A SEM image of synthesized Na3Cr2(PO4)3 / AB particles is shown in the insert. A cyclic voltammogram of Na3Cr2(PO4)3 / AB electrode is shown in Figure 2a. Only one redox couple at ca. 4.45 V vs. Na/Na+ was identified. The Cr4+/Cr3+ redox potential is positioned at 4.45 V vs. Na/Na+, which is competitive to Co3+/Co2+ and Ni3+/Ni2+ in polyanion matrix.12-14 Galvanostatic charge/discharge curves in Figure 2b indicated that an initial charge capacity at a current rate of 0.5 C was ~ 98 mAh g–1 being equal to 84 % of the theoretical capacity (117 mAh g–1 for 2 Na per formula unit) with single voltage plateau, and the reversible discharge capacity was ~ 79 mAh g–1 exhibiting a small potential difference of ca. 0.2 V between charging and discharging curves. Figure 2c shows galvanostatic discharge curves after maintaining the cell at 4.70 V (vs. Na/Na+) for the various time durations. Continuous increase in overvoltage with


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evident initial voltage vumps indicates the time-dependent degradation in this system could be initiated at the partial surface prohibiting the smooth nucleation of the discharged phase.

Figure 2. Electrochemical properties of Na3Cr2(PO4)3/AB electrode in Na half-cell; (a) A cyclic voltammogram of 1st cycle at a scan rate of 1.0 mV s-1 (b) 1st, 2nd, 3rd and 20th galvanostatic charge/discharge curves between 2.5 V and 4.7 V (vs. Na/Na+) at the rate of 0.5 C for charging and 1 C for discharging (c) 1st galvanostatic discharge curves after maintaining the cell at 4.70 V


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(vs. Na/Na+) for the various time durations. C-rates were 0.5 C for charging and 1 C for discharging. Structural evolution during charge/discharge process was investigated using ex situ synchrotron XRD measurement. The depths of charge or discharge at which the XRD patterns were collected are marked as A to E in Figure 3a. XRD patterns are shown in Figure 3b. Upon charging, new peaks grew up with expense to the peak intensity for the original phase. All of original peaks recovered in a reversible manner upon discharging. These typical features for "nucleation-growth-type" mechanism is consistent with the flat charge/discharge profile at the specific generating voltage at 4.5 V vs. Na/Na+.

Figure 3. (a) The 1st charge/discharge curve at which each XRD pattern was collected; 0.5 C for charging and 1 C for discharging, (b) Ex situ XRD patterns at various charge/discharge states during 1st cycle. Enlarged view in angular region [17.0 - 18.0 °] are shown on the right. Each alphabet from A to E corresponds the state at which XRD pattern was collected indicated in (a).


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Cr K-edge XANES spectra was measured to check the valence state of chromium in Na3xCr2(PO4)3.

A series of spectra obtained at various states of charge or discharge is shown in

Figure 4. Upon charging, intensity for the pre-edge peak (5988 - 5994 eV) increased, while the main edge shifted to higher energy by ca. 1 eV. The pre-edge peak is composed of the 1s-3d quadrupole transition and the oxygen mediated 1s-4p dipole transition. Their contributions become evident at high valence states due to the less 3d electron and the local structural change around Cr. The reversible shift of main edge also indicates the redox activity of the Cr4+/Cr3+ couple.

Figure 4. Cr K-edge XANES spectrums of Na3Cr2(PO4)3 / AB electrode in (a) 1st charging process and (b) 1st discharging process. The spectrum of Cr2O3 is a reference as a compound containing Cr(III).


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The Cr4+/Cr3+ redox couple in polyanion framework was demonstrated to generate extremely high-potential in sodium cell. As an initiating example, NASICON-type Na3Cr2(PO4)3 showed reversible electrochemical activity at ca. 4.5 V vs. Na/Na+ with an initial charging capacity of 98 mAh g–1 comparable to 84 % of theoretical capacity (117 mAh g–1) for 2 Na per formula unit. In addition to the well known high-potential with Ni3+/Ni2+ and Co3+/Co2+ redox couples shown in Figure 5, Cr4+/Cr3+ redox couple in several polyanion framework systems are now warrant exploration toward new high-voltage cathode materials. However, excessive oxidation or spontaneous disproportionation could produce highly toxic Cr6+ and should be recognized as a possible important risk.

(n+1)+

Figure 5. Positions of the M

/Mn+ redox couples with various polyanions (M = transition

metal). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:


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Details of the Rietveld refinement results of synchrotron X-ray powder diffraction, electrochemical performance and synchrotron X-ray powder diffraction pattern of full charged phase with miller indices and lattice constants (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Kosuke Kawai: 0000-0003-3840-2198 Shin-ichi Nishimura: 0000-0001-7464-8692 Atsuo Yamada: 0000-0002-7880-5701 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The present work was financially supported from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the “Element Strategy Initiative for Catalysts & Batteries” (ESICB) project. The synchrotron XRD experiments was performed under KEK-PF User Program (No. 2015G684). K.K. acknowledges financial support from “Materials Education Program for the Future Leaders in Research, Industry, and Technology” (MERIT) project. REFERENCES (1) Kubota, K.; Komaba, S. Review - Practical Issues and Future Perspective for Na-Ion Batteries. J. Electrochem. Soc. 2015, 162, A2538-A2550.


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(2) Kim, H.; Kim, H.; Ding, Z.; Lee, M. H.; Lim, K.; Yoon, G.; Kang, K. Recent Progress in Electrode Materials for Sodium-Ion Batteries. Adv. Energy Mater. 2016, 6, 1-38. (3) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Goodenough, J. B. Mapping of Transition Metal Redox Energies in Phosphates with NASICON Structure by Lithium Intercalation. J. Electrochem. Soc. 1997, 144, 2581-2586. (4) Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113, 6552–6591. (5) Jian, Z.; Hu, Y.-S.; Ji, X.; Chen, W. NASICON-Structured Materials for Energy Storage. Adv. Mater. 2017, 1601925, 1-16. (6) Delmas, C.; Cherkaoui, F.; Nadiri, A.; Hagenmuller, P. A Nasicon-Type Phase as Intercalation Electrode: NaTi2(PO4)3. Mater. Res. Bull. 1987, 22, 631-639. (7) Senguttuvan, P.; Rousse, G.; Arroyo Y De Dompablo, M. E.; Vezin, H.; Tarascon, J. M.; Palacín, M. R. Low-Potential Sodium Insertion in a Nasicon-Type Structure through the Ti(III)/Ti(II) Redox Couple. J. Am. Chem. Soc. 2013, 135, 3897-3903. (8) Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. The First Report on Excellent Cycling Stability and Superior Rate Capability of Na3V2(PO4)3 for Sodium Ion Batteries. Adv. Energy Mater. 2013, 3, 444-450. (9) Gao, H.; Li, Y.; Park, K.; Goodenough, J. B. Sodium Extraction from NASICONStructured Na3MnTi(PO4)3 through Mn(III)/Mn(II) and Mn(IV)/Mn(III) Redox Couples. Chem. Mater. 2016, 28, 6553-6559.


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(17) Barpanda, P.; Oyama, G.; Nishimura, S.; Chung, S.-C.; Yamada, A. A 3.8-V EarthAbundant Sodium Battery Electrode. Nat. Commun. 2014, 5, 4358. (18) Hautier, G.; Jain, A.; Ong, S. P.; Kang, B.; Moore, C.; Doe, R.; Ceder, G. Phosphates as Lithium-Ion Battery Cathodes: An Evaluation Based on High-Throughput Ab Initio Calculations. Chem. Mater. 2011, 23, 3495-3508. (19) Herklotz, M.; Scheiba, F.; Glaum, R.; Mosymow, E.; Oswald, S.; Eckert, J.; Ehrenberg, H. Electrochemical Oxidation of Trivalent Chromium in a Phosphate Matrix: Li3Cr2(PO4)3 as Cathode Material for Lithium Ion Batteries. Electrochim. Acta 2014, 139, 356-364. (20) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for XRay Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537541. (21) D’Yvoire, F.; Pintard-Scrépel, M.; Bretey, E.; de la Rochère, M. Phase Transitions and Ionic Conduction in 3D Skeleton Phosphates A3M2(PO4)3: A = Li, Na, Ag, K ; M = Cr, Fe. Solid State Ionics 1983, 9-10 (PART 2), 851-857. (22) de la Rochère, M.; d’Yvoire, F.; Collin, G.; Comès, R.; Boilot, J. P. NASICON Type Materials - Na3M2(PO4)3 (M = Sc, Cr, Fe): Na+-Na+ Correlations and Phase Transitions. Solid State Ionics 1983, 9-10 (PART 2), 825-828. (23) Lucazeau, G.; Barj, M.; Soubeyroux, J. L.; Dianoux, A. J.; Delmas, C. Neutron Scattering and Diffractuion Study of Na3Cr2(PO4)3, NaZr2(PO4)3 and Na3ZrMg(PO4)3. Solid State Ionics 1986, 18-19 (PART 2), 959-963.


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(24) Kabbour, H.; Coillot, D.; Colmont, M.; Masquelier, C.; Mentr, O. α-Na3M2(PO4)3 (M = Ti, Fe): Absolute Cationic Ordering in NASICON-Type Phases. J. Am. Chem. Soc. 2011, 2, 11900-11903. (25) Chotard, J.-N.; Rousse, G.; David, R.; Mentré, O.; Courty, M.; Masquelier, C. Discovery of a Sodium-Ordered Form of Na3V2(PO4)3 below Ambient Temperature. Chem. Mater. 2015, 27, 5982-5987. Table of Contents


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Cr3+ Redox Couple in Polyanion Compounds

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