Local Structures and Li Ion Dynamics in a Li10SnP2S12-Based

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2017, 121, 23370-23376

Local Structures and Li Ion Dynamics in a Li10SnP2S12-Based Composite Observed by Multinuclear Solid-State NMR Spectroscopy Maximilian Kaus,†,‡ Heike Stöffler,† Murat Yavuz,† Tatiana Zinkevich,†,‡ Michael Knapp,†,‡ Helmut Ehrenberg,†,‡ and Sylvio Indris*,†,‡ †

J. Phys. Chem. C 2017.121:23370-23376. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 11/25/18. For personal use only.

Institute for Applied Materials − Energy Storage Systems (IAM-ESS), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Helmholtz Institute Ulm, Helmholtzstraβe 11, 89081 Ulm, Germany ABSTRACT: Multinuclear solid-state nuclear magnetic resonance spectroscopy was used in combination with Mössbauer spectroscopy and synchrotron diffraction to investigate the local and long-range structure as well as the Li-ion dynamics in a Li10SnP2S12-based composite. Although two additional phases could be detected (Li7PS6 and Li4SnS4), the Li ion dynamics turn out to be very fast with a Li diffusion coefficient of 1.6 × 10−12 m2/s, a Li+ ion conductivity of ∼2 mS/cm (both at 303 K), and a small activation barrier of 0.13 eV for single Li+ ion jumps.

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INTRODUCTION

Portable electronic devices such as mobile phones and notebooks, but recently also electric cars, are powered by Liion batteries that offer high energy and power densities.1−3 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.4−6 Li10GeP2S12 was reported to be a superionic conductor with a high Li+ ion conductivity of 12 mS/cm at 300 K.5 The corresponding Sn compound (Li10SnP2S12) has an only slightly lower ionic conductivity (4 mS/cm) but offers the advantage of being a less expensive material.6 It crystallizes with a tetragonal structure, space group P42/nmc (Figure 1). Sn and P are fourfold coordinated by S, that is, located in sulfur tetrahedra, on two different sites. The 4d site is occupied by Sn and P in a ratio of 1:1, and the 2a site is occupied exclusively by P. Because the 4d site is present twice as often as the 2a site, P is located on both sites with the same probability. In between the PS4 and SnS4 tetrahedra, 4 different Li sites are present in the crystal structure that are partially occupied.6 Therefore, multiple accessible sites are present for the Li+ ions, which enables fast hopping/transport. This occupational disorder should also result in a noncorrelated motion of the Li+ ions; that is, the directions of subsequent Li ion jumps are independent of each other. This corresponds to fast longrange transport of the ions. We investigate the local and long-range structures of a commercially available Li10SnP2S12 material by multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy on the different probe nuclei 6Li, 7Li, 31P, and 119Sn, in combination with synchrotron diffraction (SRD) and Sn © 2017 American Chemical Society

Figure 1. Tetragonal crystal structure of Li10SnP2S12 (space group P42/nmc). Purple tetrahedra depict PS4 units (with P on the 2a site), blue tetrahedra depict sulfur tetrahedra with P or Sn in the center (in a ratio of 1:1, on the 4d site), and gray spheres show the four different Li positions.

Received: August 21, 2017 Revised: September 29, 2017 Published: October 2, 2017 23370

DOI: 10.1021/acs.jpcc.7b08350 J. Phys. Chem. C 2017, 121, 23370−23376

Article

The Journal of Physical Chemistry C

dependent and the position of the Li+ ions can be labeled via this resonance frequency. If the Li+ ions are moving within the duration of this echo experiment, then their resonance frequency will change and they will not contribute to the echo intensity, resulting in an echo damping as a function of the magnetic field gradient strength g. The diffusion coefficient D can be extracted from the echo decay according to the equation by Stejskal and Tanner20

Mössbauer spectroscopy. The mobility of the Li ions is investigated by temperature dependent 7Li NMR spectroscopy, relaxometry, and pulsed field-gradient NMR (PFG NMR) and then compared with the formerly reported conductivity measurements. NMR relaxometry is a powerful tool to observe microscopic diffusion parameters such as hopping rates of Li+ ions and activation energies for single Li ion jumps.7−10 These are important parameters for understanding the transport mechanisms in solid electrolytes. Whereas NMR spectroscopy yields information about the local environments of specific elements,11,12 NMR relaxometry is able to probe changes in these local environments that occur by movement of the ions. These changes are described by the autocorrelation function g(t) of the local environment, represented, for example, by the local electric and magnetic fields. The Fourier transform of this autocorrelation function is the spectral density J(ω) of local field fluctuations. The spin−lattice relaxation time T1 describes the relaxation of a macroscopic nuclear magnetization toward its equilibrium state after distortion by a radiofrequency pulse. Its inverse, the relaxation rate T1−1, is proportional to the components of the spectral density at the Larmor frequency, ωL, and twice the Larmor frequency, 2ωL13−17 T1−1 ∝ [J(ωL) + 4J(2ωL)]

I = I0·exp(−Dγ 2g 2δ 2(Δ − δ /3))

where I0 and I are the echo intensities without and with gradient, respectively, γ is the magnetogyric ratio, and δ and Δ are gradient duration and diffusion time, respectively.

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EXPERIMENTAL SECTION Nominally phase-pure Li10SnP2S12 powder was purchased from NEI company and used without further purification. Synchrotron diffraction (SRD) was performed at the synchrotron PETRA-III in Hamburg/Germany at beamline P02.121 at a photon energy of 60 keV, corresponding to a wavelength of 0.2070 Å. The sample was measured in a sealed glass capillary with a diameter of 0.5 mm and a PerkinElmer area detector. 6 Li, 31P, and 119Sn magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy was done with a Bruker Avance spectrometer at a field of 11.7 T, corresponding to resonance frequencies of 73.6, 202.5, and 186.5 MHz, respectively. Spinning was performed in 2.5 mm rotors at 30 kHz. The recycle delay was 60 s and the π/2 pulse length was 1.5 μs for 6Li, 2.8 μs for 31P, and 1.2 μs for 119Sn. Temperaturedependent static 7Li NMR line shape and relaxation time measurements were acquired with an inversion−recovery pulse sequence at a magnetic field of 4.7 T (77.8 MHz) on samples sealed in glass vials. The chemical shifts of 6/7Li, 31P, and 119Sn were referenced to 1 M LiCl (0 ppm), H3PO4 (85%, 0 ppm), and well-crystalline SnO2 (−604.3 ppm22), respectively. Simulation of the spectra was done with the DMFIT program.23 7 Li PFG NMR experiments were performed on a Bruker Avance 300 MHz spectrometer operated at a 7Li frequency of 116.6 MHz. The spectrometer was equipped with a Diff50 probe, which produces pulsed field gradients of up to 30 T/m. A stimulated-echo pulse sequence in combination with bipolar gradients18 was used to observe the echo damping as a function of gradient strength. The duration of the π/2 and π pulses varied slightly with temperature and was about 12 and 24 μs, respectively. 4096 scans were used to sample one echo for each of the 16 gradients per temperature. Recycle delays were chosen on the basis of the T1 measurement results and were in the range of 1 to 1.5 s. All delay times during each PFG-NMR experiment were kept constant, whereas the gradient amplitude was varied to cause the signal decay. By this, the influence of relaxation on the echo decay can be eliminated. The optimum values for the gradient duration δ and the diffusion time Δ were found to be 3 and 100 ms, respectively. Sn Mö ssbauer spectroscopy was performed at room temperature with a constant acceleration spectrometer and a Ba119SnO3 source.

(1)

Whereas the first term on the right side of eq 1 describes changes of the overall nuclear magnetization by a single spin flip, the second term describes changes by double spin flips of interacting spins. In the simplest case, assuming uncorrelated 3D jumps of the Li ions, the autocorrelation function can be described by a simple exponential function, as described first by Bloembergen, Purcell, and, Pound16

⎛ t⎞ g (t ) = exp⎜ − ⎟ ⎝ τc ⎠

(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 autocorrelation function corresponds to a spectral density τ J(ω) ∝ (3) 1 + ω 2τ 2 In general, the temperature dependence of the jump rate can be described by an Arrhenius-type behavior ⎛ E ⎞ τ −1 = τ0−1·exp⎜ − A ⎟ ⎝ kBT ⎠

(5)

(4)

τ0−1

with being a prefactor, EA the activation energy, kB the Boltzmann constant, and T the temperature. Equations 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 Li+ ions and the activation energy EA for single Li ion jumps. Whereas NMR relaxometry is able to probe the local hopping of the Li+ ions, 7Li PFG NMR can be used to investigate the long-range transport of these ions, on length scales of several micrometers.18,19 This is achieved by performing an echo experiment with magnetic field gradients. By this, the resonance frequency of each Li+ ion becomes space-

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RESULTS AND DISCUSSION The synchrotron diffraction pattern of Li10SnP2S12 is shown in Figure 2 together with the Rietveld refinement. The different 23371

DOI: 10.1021/acs.jpcc.7b08350 J. Phys. Chem. C 2017, 121, 23370−23376

Article

The Journal of Physical Chemistry C

Figure 2. Synchrotron diffraction pattern of the Li10SnP2S12-based composite with Rietveld refinement (λ = 0.2070 Å).

Figure 3. (a) 31P, (b)

119

Sn, and (c) 6Li MAS NMR spectra of the Li10SnP2S12-based composite.

characteristic of sulfidic environments around P, whereas oxidic environments result in chemical shifts between about 10 and −50 ppm.26 The two strong peaks can be assigned to P located on the 2a and 4d sites of Li10SnP2O12, where it is present with the same probability. In between these two peaks, a third, weaker peak is observed at 86.3 ppm. It cannot be unambiguously assigned to another phase from just the NMR data. It is consistent with the presence of either β-Li3PS427 or Li7PS6.28 As described above in the SRD measurements, this peak has to be ascribed to the single Li site in Li7PS6. The

reflections can be assigned to the main phase Li10SnP2S12 (62.1 wt % = 41.3 mol %) and two additional phases Li7PS6 (21.8 wt % = 33.9 mol %) and Li4SnS4 (16.1 wt % = 24.8 mol %). It is interesting to note that all three phases have been reported to be fast Li+ ion conductors.6,24,25 Figure 3 shows the 31P (nuclear spin I = 1/2, 100% isotope), 119 Sn (I = 1/2, 8.6%), and 6Li (I = 1, 7.4%) MAS NMR spectra of Li10SnP2S12. The 31P NMR spectrum exhibits two peaks at 92.3 and 77.7 ppm with very similar integral intensity. The chemical shift range of 31P between 120 and 50 ppm is 23372

DOI: 10.1021/acs.jpcc.7b08350 J. Phys. Chem. C 2017, 121, 23370−23376

Article

The Journal of Physical Chemistry C

Figure 5a shows the temperature dependence of the static 7Li NMR spectra (I = 3/2, 92.6%) of Li10SnP2S12 in the temperature range from 250 to 525 K. The spectra reveal line shapes characteristic of a nucleus with I = 3/2. At about 0 ppm a strong peak is visible representing the central transition |−1/2⟩ → |+1/2⟩. Additionally, two maxima at ±85 ppm and two corresponding shoulders at ±160 ppm represent the transitions |−3/2⟩ → |−1/2⟩ and |+1/2⟩ → |+3/2⟩ that give resonance frequencies being dependent on the orientations of the different crystallites with respect to the external magnetic field and thus result in characteristic broad distributions of these resonance frequencies. The central transition (Figures 5b,c) shows a small shift with increasing temperature, from about 0 ppm at 250 K to −3 ppm at 525 K. Additionally, the full width at half-maximum (fwhm) of this central transition shows a clear narrowing from 470 Hz at 250 K to 240 Hz at 525 K (Figure 5c). This clearly shows that the Li+ ions are highly mobile in this material with average hopping rates well above the low-temperature line width, that is, 5 × 102 s−1. Furthermore, the splitting of the quadrupolar satellites decreases from about 27 kHz at 250 K to 21 kHz at 525 K (Figure 5c). This fact shows that the local environment around the Li+ ions gets more symmetric (on a temporal average) due to the motion of the Li+ ions. The overall line shape consists of only a single contribution, as shown exemplarily in Figure 5d for the spectrum at 310 K. The spectrum can be well described with a so-called quadrupolar line shape with asymmetry parameter ηQ = 0 and quadrupole coupling constant Cq = 25.5 kHz. Such a line shape is characteristic of a local Li environment with at least a four-fold rotational symmetry axis through the site of the Li nuclei. In view of the fact that three phases containing Li are present and that the main phase has four different Li sites, this line shape suggests that the Li+ ions are already highly mobile at 310 K and that even the Li exchange between these phases is fast enough to result in a time-averaging of the local environments on the time scale of these NMR experiments, that is, some milliseconds. This is consistent with the 6Li MAS NMR spectrum described above that was dominated by a strong peak at 1.0 ppm. The 7Li spin−lattice relaxation rate T1−1 of Li10SnP2S12 is shown in Figure 6 versus inverse temperature in the temperature range from 250 to 525 K. At low temperatures, at 250 K, the relaxation rate T1−1 is ∼5 s−1. With increasing temperature, the Li+ ions become more and more mobile and the transitions between the Zeeman levels of the Li nuclei are induced more and more effectively. When the jump rate of the Li+ ions τ−1 reaches the Larmor frequency ωL (ωL·τ ≈ 1), these transitions are induced most effectively and T1−1 passes through a clear maximum.15 This happens at 336 K, where T1−1 is ∼13 s−1. At this temperature, a jump rate of the Li+ ions of 4.9 × 108 s−1 can be extracted. Assuming a jump length of ∼2 Å, which is the shortest Li−Li distance in the crystal lattice of Li10SnP2S12, we can estimate the diffusion coefficient of the Li ions via the Einstein−Smoluchowski equation30,31 to be (3.3 ± 1.3)·10−12 m2/s. Using the Nernst−Einstein equation together with the Li+ ion concentration (2.1 × 1028 m−3), we can estimate the Li+ ion conductivity to be (3.8 ± 1.5) mS/cm (at 336 K). This is in good agreement with the value determined directly by impedance spectroscopy for phase-pure Li10SnP2S12 (∼4 mS at 300 K).6 When the temperature is further increased, the rate T1−1 decreases again because the hopping rate of the Li+ ions more and more exceeds the Larmor frequency (ωL·τ ≪ 1) and

overall intensity of the two peaks assigned to the main phase Li10SnP2S12 with respect to the third peak (2.66:1) is in good agreement with the amounts of P in the phases Li10SnP2S12 (2 × 41.3 mol %) and Li7PS6 (33.9 mol %), as determined by SRD. Furthermore, a broad and much weaker peak is observed at 68 ppm, which might hint at the presence of a small amount of a disordered phosphorus sulfide (without Li). A fifth, very small peak at 10 ppm reveals the presence of a small amount of Li3PO4 and demonstrates the high sensitivity of NMR to detect minor phases as its area fraction is