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Fast Na Ion Conduction in NASICON-Type Na Sc(SiO) (PO) Observed by Na NMR Relaxometry 3.4

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Maximilian Kaus, Marie Guin, Murat Yavuz, Michael Knapp, Frank Tietz, Olivier Guillon, Helmut Ehrenberg, and Sylvio Indris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10523 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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The Journal of Physical Chemistry

Fast Na+ Ion Conduction in NASICON-Type Na3.4Sc2(SiO4)0.4(PO4)2.6 Observed by 23Na NMR Relaxometry

Maximilian Kaus1,2, Marie Guin3, Murat Yavuz1,2, Michael Knapp1,2, Frank Tietz3,4, Olivier Guillon3,4,5, Helmut Ehrenberg1,2, Sylvio Indris*,1,2

1

Institute for Applied Materials – Energy Storage Systems (IAM-ESS), Karlsruhe Institute of

Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2

Helmholtz Institute Ulm for Electrochemical Energy Storage, P.O. Box 3640, 76021 Karlsruhe,

Germany 3

Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Materials

Synthesis and Processing (IEK-1), 52425 Jülich, Germany 4

Helmholtz Institute Münster, c/o Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany

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Jülich Aachen Research Alliance, JARA Energy, 52425 Jülich, Germany

Corresponding Author: Sylvio Indris Institute for Applied Materials – Energy Storage Systems (IAM-ESS) Karlsruhe Institute of Technology (KIT) Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany Tel.: +49-721-680-28508 Fax.: +49-721-608-28521 Email: [email protected] 1 ACS Paragon Plus Environment


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Abstract The bulk diffusion of Na in Na3.4Sc2(SiO4)0.4(PO4)2.6 was investigated by

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Na NMR relaxometry

in the temperature range from 250 K to 670 K. These measurements reveal fast Na diffusion with hopping rates of 3·108 s-1 for the Na+ ions at 350 K and activation barriers for single Na+ ion jumps of (0.20 ± 0.01) eV. From these values a diffusion coefficient of D = 6.4·10-12 m2/s and a Na ion conductivity of σNa = 4 mS/cm (both at 350 K) can be estimated. Measurements on two samples, one stored in air and one stored in Ar, do not show significant differences, which reveals that these NMR measurements are probing the bulk diffusion while conductivity measurements usually are also influenced by grain boundaries that can be affected by the moisture level during storage.

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Introduction Portable electronic devices such as mobile phones and notebooks are powered by Li-ion batteries which offer high energy and power densities.1–3 In recent years, research on Na-ion batteries has emerged in an attempt to find cost-effective solutions for larger energy storage systems.4–7 As for Li-ion batteries, the use of solid electrolytes instead of liquid ones might offer an enhanced thermal and chemical stability and thus an increased safety of such devices.8–12 Na3.4Sc2(SiO4)0.4(PO4)2.6 crystallizes with a so-called NASICON-type (Na Superionic conductor) structure. This rhombohedral structure (space group R3c) is depicted in Fig. 1 and can be derived from that of NaZr2(PO4)3.13 The Sc3+ ions are located in oxygen octahedra (12c site) while the Si and P ions share a common tetrahedral site (18e site). These octahedral and tetrahedral units are connected only via common corners and thus are forming an open network structure that enables fast diffusion of Na+ ions. Two octahedral units are connected via three tetrahedral units and thus form ‘lantern-like’ units14 that are stacked along the c axis. Between these lantern units there is one Na site (6b site) that is often fully occupied. Since the octahedral sites are occupied by trivalent Sc ions instead of tetravalent Zr as it is the case for NaZr2(PO4)3, a second 18e site is occupied by Na. The substitution of Si for P further enhances the amount of Na in the crystal structure in order to compensate the aliovalent substitution. The exact composition of this sample was chosen from a large number of NASICON-type compounds because it revealed the highest ionic conductivity.12



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Fig. 1: The rhombohedral crystal structure (space group R3c) of Na3.4Sc2(SiO4)0.4(PO4)2.6. Purple tetrahedra represent SiO4/PO4 units, green octahedra represent ScO6 units, and grey spheres depict Na+ ions.

Solid-state nuclear magnetic resonance (NMR) relaxometry is a powerful tool to observe microscopic diffusion parameters such as hopping rates of mobile ions and activation barriers for single ion jumps.15–18 These parameters are directly related to the performance of these materials in a Na-ion battery, especially at high charging and discharging rates. While NMR spectroscopy yields information about the local environment of specific elements (and even isotopes),19,20 NMR relaxometry is able to probe changes in these local environments that occur by movement of the ions. These changes can be described by the auto-correlation function g(t) of the local environment, represented, e.g., by the local electric and magnetic fields. The Fourier transform of this auto-correlation function is the spectral density J(ω) of local field fluctuations. The spin-lattice relaxation time T1 describes the reorientation of a macroscopic nuclear spin polarization towards its equilibrium state after distortion by a radiofrequency pulse. Its inverse, the relaxation rate T1-1, is proportional to the components of this spectral density at the Larmor frequency ωL and twice the Larmor frequency 2ωL 21: ∝ 4 2

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Bloembergen, Purcell, and Pound22 proposed a simple exponential function for the autocorrelation function c

= exp −

(2)

with τc being the correlation time. Apart from a factor of the order of unity, the correlation time τc can be identified with the average residence time τ of the hopping ions, and its inverse is the average jump rate τ-1. Such an auto-correlation function corresponds to a spectral density

∝

(3)

The temperature dependence of the jump rate can be described by an Arrhenius-type behavior $A B (

! = !" ∙ exp − &

(4)

with τ0-1 being a pre-factor, EA the activation barrier, kB the Boltzmann constant, and T the temperature. Eqs. (1), (3), and (4) can be used to describe the temperature dependence of the relaxation rate T1-1 and thus to extract microscopic diffusion parameters such as the hopping rate τ-1 of the Na+ ions and the activation barrier EA for single Na ion jumps.

Experimental Na3.4Sc2(SiO4)0.4(PO4)2.6 was synthesized by conventional solid state reaction. A stoichiometric homogenized mixture of NH4H2PO4 (Merck KGaA, 99 %), Sc2O3 (Projector GmbH, 99.5 %), Na2CO3 (Alfa Aesar GmbH & Co KG, 99.5 %), and SiO2 (Alfa Aesar GmbH & Co KG, 99.8 %) was heated with 300 K h-1 to 1173 K for 4 h. After grinding, the powder was again annealed at 1573 K for 20 h. The obtained powder was milled and pressed into pellets (13 mm in diameter, approximately 2 - 5 mm height) and sintered at 1553 K for 10 h. Afterwards samples were either stored in ambient air or in dry argon to investigate the impact of possible uptake of water on the transport properties in the material.



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X-ray diffraction (XRD) was performed on a STOE-Stadi-P diffractometer with Mo-Kα1 radiation, a focusing Ge 111 monochromator, Si-strip detector with 50µm pitch (Dectris ©) and equipped with an capillary oven.

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Na solid-state nuclear magnetic resonance (NMR) spectroscopy was performed on a Bruker

Avance spectrometer at a magnetic field of 4.7 T, corresponding to a Larmor frequency of 52.9 MHz, with a π/2 pulse length of 2 µs. A quadrupolar-echo sequence ((π/2)x-τ-(π/2)y-τ-acq.) was used to acquire the spectra. 256 transients were measured with a recycle delay of 2 s. This value is well above 5 T1 for all temperatures. The spectra were referenced to an aqueous solution of 1M NaCl at 0 ppm. 23Na relaxation times T1 were acquired with an inversion-recovery pulse sequence23.

Results and Discussion The XRD patterns of the sample stored in air are shown in Fig. 2 for temperatures between 298 K and 873 K. These measurements were carried out on an open glass capillary. The overall patterns can be well described with the structure depicted in Fig. 1 (red lines in Fig. 2). During heating to 873 K, no new reflections occur, and some of the reflections show a clear shift to smaller diffraction angles while others do not change their position. The reflections that show large shifts belong to lattice plane orientations involving large l values in the Miller indices, i.e. stacking along the c axis, while the peaks that do not shift have l values that are very small or equal to zero.


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Fig. 2: X-ray diffraction patterns of Na3.4Sc2(SiO4)0.4(PO4)2.6 acquired at temperatures between 298 K and 873 K.

This behavior reveals a strongly anisotropic thermal expansion. Very similar results were obtained earlier for systems with the same crystal structure (containing tetravalent Ti4+/Zr4+ instead of Sc3+ and no silicate units, and thus smaller alkali contents).14,24–26 A Rietveld refinement of the structure model to all data sets confirmed this anisotropic behavior. The results for the lattice constants a and c are shown in Fig. 3. The error bars are determined from the Rietveld refinement, including the Berar factor.27 While the lattice constant a shows no significant changes (the changes are smaller than 0.1%), the c constant shows a strong increase from 22.2 Å to 22.7 Å, i.e. an increase by 2.1%, in this temperature regime. The observed changes in the lattice parameters are highly reversible, and when the sample is cooled down to 298 K after heating to 873 K, the original XRD pattern and the original lattice parameters are obtained again.



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Fig. 3: Changes of lattice parameters a and c of Na3.4Sc2(SiO4)0.4(PO4)2.6 at temperatures between 298 K and 873 K.

The static

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Na NMR lineshapes of Na3.4Sc2(SiO4)0.4(PO4)2.6 at temperatures between 219 K and

550 K are displayed in Fig. 4. These measurements were performed on the sample stored in Ar. Measurements on the sample stored in air showed identical results. At the lowest temperature, at 219 K, the spectrum reveals a broad lineshape with a full width at half maximum (FWHM) of 18 kHz. A slight asymmetry is visible and at temperatures between 250 K and 275 K a shoulder on the right side is formed. This might be caused by the anisotropic quadrupolar interaction of the 23Na nucleus (nuclear spin I = 3/2, nuclear quadrupole moment Q = 104 mb28–30) with electric field gradients at the site of this nucleus. The

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Na magic-angle spinning (MAS) NMR spectrum

of the same sample shows only a single narrow Lorentzian contribution to the lineshape (Fig. S2)31. At temperatures above 300 K a clear so-called motional narrowing of the lineshape, i.e. an averaging of local environments around the Na nuclei caused by movement of the Na+ ions and thus a decrease of the full width at half maximum (FWHM) of the

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Na NMR lineshapes, can be

observed with a final line width of 2.5 kHz at 550 K (Fig. 5). This is caused by movement of the 8 ACS Paragon Plus Environment

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Na+ ions with average jump rates well above the line width of 18 kHz observed at low temperatures.

Fig. 4: Static

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Na NMR spectra of Na3.4Sc2(SiO4)0.4(PO4)2.6 stored in Ar, at temperatures

between 219 K and 550 K.



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Fig. 5: Linewidth of the static

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Na NMR spectra of Na3.4Sc2(SiO4)0.4(PO4)2.6 for temperatures

between 300 K and 550 K.

Na NMR spin-lattice relaxation rates T1-1 have been measured on Na3.4Sc2(SiO4)0.4(PO4)2.6

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stored in air and in Ar in the temperature range from 260 K to 650 K. For these measurements the samples were sealed in evacuated glass containers. At all temperatures, the magnetization transients could be well described with a single-exponential function. The relaxation rates are plotted in Fig. 6 as a function of inverse temperature. Both samples show a very similar behavior. At low temperatures (right side of Fig. 6) the relaxation rates are in the range of half a millisecond. In this regime, the hopping rate τ-1 is much smaller than the Larmor frequency ωL (ωL·τ >> 1). When the temperature is increased, the motions of the Na+ ions become faster, the value of the spectral density at the Larmor frequency J(ωL) increases and the transitions between the nuclear Zeeman levels are induced more and more effectively, i.e. the relaxation rate T1-1 increases. At a temperature of 350 K, the relaxation rate reaches a clear maximum with values of the relaxation time T1 of about 70 µs. Here the hopping rate equals the Larmor frequency (ωL·τ ≈ 1). After further increasing the temperature, the hopping rate becomes much larger than the Larmor frequency (ωL·τ

Fast Na+ Ion Conduction in NASICON-Type Na3 ... - ACS Publications

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