Local Structures and Li Ion Dynamics in a Li10SnP2S12-Based

Subscriber access provided by University of Michigan-Flint

Article 10

2

12

Local Structures and Li Ion Dynamics in a Li SnPS -Based Composite Observed by Multinuclear Solid-State NMR Spectroscopy Maximilian Kaus, Heike Stoeffler, Murat Yavuz, Tatiana Zinkevich, Michael Knapp, Helmut Ehrenberg, and Sylvio Indris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08350 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Local Structures and Li Ion Dynamics in a Li10SnP2S12-Based Composite

Observed

by

Multinuclear

Solid-State

NMR

Spectroscopy

Maximilian Kaus1,2, Heike Stöffler1, Murat Yavuz1, Tatiana Zinkevich1,2, Michael Knapp1,2, Helmut Ehrenberg1,2, Sylvio Indris1,2,*

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, Helmholtzstraße 11, 89081 Ulm, 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


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Multinuclear solid-state nuclear magnetic resonance spectroscopy was used in combination with Mössbauer spectroscopy and synchrotron diffraction in order to investigate the local and longrange 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 about 2 mS/cm (both at 303 K), and a small activation barrier of 0.13 eV for single Li+ ion jumps.


Page 2 of 29

Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Portable electronic devices such as mobile phones and notebooks, but recently also electric cars, are powered by Li-ion batteries which 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 (Fig. 1). Sn and P are four-fold coordinated by S, i.e. 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. Since 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 non-correlated motion of the Li+ ions, i.e. the directions of subsequent Li ion jumps are independent from each other. This corresponds to fast long-range transport of the ions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

Fig. 1: The 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 grey spheres show the four different Li positions.

In this paper, we investigate the local and long-range structures of a commercially available Li10SnP2S12

material

by

multinuclear

solid-state

spectroscopy on the different probe nuclei

6

Li,

7

nuclear

Li,

31

magnetic

P, and

resonance

(NMR)

119

Sn, in combination with

synchrotron diffraction (SRD) and Sn Mössbauer spectroscopy. The mobility of the Li ions is investigated by temperature dependent 7Li NMR spectroscopy, relaxometry, and pulsed fieldgradient NMR (PFG NMR), and then compared to 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. While 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 4 ACS Paragon Plus Environment

Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ions. These changes are described by the autocorrelation function g(t) of the local environment, represented, e.g., 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 towards 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 2ωL 13–17: ∝ + 4 2

(1)

While 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 three-dimensional jumps of the Li ions, the autocorrelation function can be described by a simple exponential function, as described first by Bloembergen, Purcell, and Pound16:

= exp −

(2)

c

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

∝

(3)

In general, 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 energy, kB the Boltzmann constant, and T the temperature. Eqs. (1), (3), and (4) can be used to describe the temperature dependence of the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

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. While 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-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, 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 ' = ' ∙ exp(−)* + + , + ∆ − ,/30

(5)

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

Experimental 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 Perkin-Elmer area detector. 6

Li,

31

P, and

119

Sn 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 MHz, 202.5 MHz, 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 6 ACS Paragon Plus Environment

Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 µs for 6Li, 2.8 µs for

31

P, and 1.2 µs for

119

Sn. Temperature-dependent 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

6/7

Li, 31P, and 119Sn were referenced to 1M 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 µs 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 base 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 ∆ were found to be 3 ms and 100 ms, respectively. Sn Mössbauer spectroscopy was performed at room temperature with a constant acceleration spectrometer and a Ba119SnO3 source.

Results and Discussion



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

The synchrotron diffraction pattern of Li10SnP2S12 is shown in Fig. 2 together with the Rietveld refinement. The different 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


Page 8 of 29

Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 3:

31

P,

119

6

Sn, and Li MAS NMR spectra of the Li10SnP2S12-based composite.

Fig. 3 shows the

31

P (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 ppm and 77.7 ppm with very similar integral intensity. The chemical shift range of

31

P between 120

ppm and 50 ppm is characteristic of sulfidic environments around P, while oxidic environments result in chemical shifts between about 10 ppm 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 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

presence of either β-Li3PS4

27

Page 10 of 29

or Li7PS6.28 As described above in the SRD measurements, this

peak has to be ascribed to the single Li site in Li7PS6. The 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 that might hint at the presence of a small amount of a disordered phosphorous 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 smaller than 0.5 % with respect to the overall spectral area. The 119Sn NMR spectrum is dominated by a strong peak at 86 ppm, which is thus assigned to Sn on the 4d site of Li10SnP2S12. Additionally, a weak shoulder is observed at 77 ppm which might hint at some cation disorder in the main phase, e.g. a small amount of Sn on the 2a site. The third, broad peak at about 51 ppm is consistent with the presence of a small amount of Li4SnS4.25 The relative areas of the peaks assigned to Li10SnP2S12 and Li4SnS4 (1.7 : 1) are in good agreement with the relative amounts of Sn in the phases Li10SnP2S12 (41.3 mol%) and Li4SnS4 (24.8 mol%) as determined by SRD. The 6Li NMR spectrum reveals a strong peak at 1.0 ppm and smaller contributions at 1.8 ppm and 2.2 ppm. These peaks reveal the presence of multiple Li environments. These occur in the main phase Li10SnP2S12 but they can also be caused by the presence of additional phases.


Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4: The Sn Mössbauer spectrum of the Li10SnP2S12-based composite with a Lorentzian fit and the difference profile.

Fig. 4 shows the Sn Mössbauer spectrum of Li10SnP2S12. It reveals a quite simple shape with only a Lorentzian singlet at an isomer shift of IS = (1.18 ± 0.01) mm/s and a width of Γ = (1.19 ± 0.02) mm/s. The spectrum suggests that just a single Sn environment is present although different phases containing Sn (Li10SnP2S12 and Li4SnS4) could be detected from SRD and NMR. This fact reveals that the local environments around the Sn sites in the different phases are too similar to be discriminated by Mössbauer spectroscopy. The isomer shift is characteristic of tetravalent Sn4+ in a sulfidic environment. Sn Mössbauer spectroscopy makes use of the transition from the nuclear ground state with I = 1/2 to an excited state with spin I = 3/2. Therefore, in contrast to

119

Sn NMR spectroscopy, Sn Mössbauer spectroscopy is sensitive to

electric field gradients being present at the site of the Sn nuclei interacting with the nuclear quadrupolar moment of

119

Sn in the excited state.29 The fact that a singlet is visible reveals that

the electric field gradient at the Sn site is close to zero, i.e. the environment around Sn is highly symmetric. Fig. 5a shows the temperature dependence of the static 7Li NMR spectra (I = 3/2, 92.6 %) of Li10SnP2S12 in the temperature range from 250 K 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 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 29

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 (Figs. 5b and 5c) 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 (Fig. 5c). This clearly shows that the Li+ ions are highly mobile in this material with average hopping rates well above the low-temperature linewidth, i.e. 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 (Fig. 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 Fig. 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, i.e. some milliseconds. This is consistent with the 6Li MAS NMR spectrum described above that was dominated by a strong peak at 1.0 ppm.


Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7

Fig. 5: Static Li NMR spectra of the Li10SnP2S12-based composite for temperatures between 250 K and 525 K. a) full spectrum, b) magnified view of the central transition, c) temperature dependence of the position of the central transition, of the width of the central transition, and of the quadrupole coupling constant Cq, d) the spectrum at 310 K (solid line) together with the simulated pattern (dashed line).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7

Page 14 of 29

-1

Fig. 6: Li NMR relaxation rates T1 vs inverse temperature, for temperatures between 250 K and 525 K.

The 7Li spin-lattice relaxation rate T1-1 of Li10SnP2S12 is shown in Fig. 6 vs inverse temperature in the temperature range from 250 K to 525 K. At low temperatures, at 250 K, the relaxation rate T1-1 is about 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 about 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 about 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 NernstEinstein 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 (about 4 mS at 300 K).6 When the temperature is further increased, the rate T1-1 decreases again because the 14 ACS Paragon Plus Environment

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hopping rate of the Li+ ions more and more exceeds the Larmor frequency (ωL⋅τ

Local Structures and Li Ion Dynamics in a Li10SnP2S12-Based

Recommend Documents