Solid-State NMR Investigations on the Structure ... - ACS Publications

Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES

Article

Solid-State NMR Investigations on the Structure and Dynamics of the Ionic Conductor Li1+xAlxTi2x(PO4)3 (0.0 # x # 1.0) C. Vinod Chandran, Sylke Pristat, Elena Witt, Frank Tietz, and Paul Heitjans J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00318 • Publication Date (Web): 01 Apr 2016 Downloaded from http://pubs.acs.org on April 7, 2016

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 24

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

Solid-State NMR Investigations on the Structure and Dynamics of the Ionic Conductor Li1+xAlxTi2−x(PO4)3 (0.0 ≤ x ≤ 1.0) C. Vinod Chandrana∗ , Sylke Pristatb , Elena Witta , Frank Tietzb,c , Paul Heitjansa∗ March 28, 2016

a

Institut f¨ ur Physikalische Chemie und Elektrochemie, Leibniz Universität

Hannover, Callinstr. 3-3a, 30167 Hannover, Germany b

Forschungszentrum J¨ ulich GmbH, Institute of Energy and Climate Re-

search (IEK-1), 52425 J¨ ulich, Germany c

Helmholtz-Institute M¨ unster, c/o Forschungszentrum J¨ ulich GmbH, 52425

J¨ ulich, Germany

∗

Corresponding authors.

E-mail address: [email protected] [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 The local structure and mobility of lithium ions of the NASICON-type ionic conductor Li1+x Alx Ti2−x (PO4 )3 (with x = 0.0, 0.1, 0.2, 0.35, 0.5, 0.7 and 1.0), synthesized using conventional solid-state reaction route, have been studied with solid-state nuclear magnetic resonance (NMR) techniques. 6 Li, 7 Li, 27 Al, 31 P

solid-state NMR experiments have been employed to trace

the structural changes with varying cation concentration. The structural evolution and the creation of new Al and P environments with changing cation contents were studied by magic-angle spinning (MAS) NMR measurements.

6 Li

MAS NMR and

27 Al

triple-quantum MAS (3QMAS) show

high-resolution spectra enabling site assignments and phase-purity inspections. The temperature dependences of 7 Li NMR spin-lattice relaxation (SLR) rates for different compositions yield important information on the lithium ion mobility in the systems. Li ion jump rates, the activation energies and the dimensionality of Li diffusion were deduced from the SLR experiments. A vacancy migration model has been proposed for the Li+ ionic diffusion process in pure-phase Li1+x Alx Ti2−x (PO4 )3 prepared by solid-state reaction. Above a certain threshold value of x (0.5) additional phosphate phases appear which slow down diffusion. This phenomenon can be observed from 6 Li exchange spectroscopy. The optimum cation concentration for maximum ionic mobility in the phase-pure Li1+x Alx Ti2−x (PO4 )3 system can be read directly from the solid-state NMR results.

Keywords Solid-state NMR; Li ion conductor; NASICON-type; Li1+x Alx Ti2−x (PO4 )3 ; LATP; Spin-lattice relaxation;

2


Page 2 of 24

Page 3 of 24

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

Introduction

Highly mobile lithium ion containing solids are excellent candidates for lithium ion battery (LIB) materials. While electrodes are usually made solely of solids, polymer electrolytes along with organic solvents are widely used as electrolytes in LIB’s. For all-solid-state batteries, the electrolyte has to be replaced with a fast Li+ ion conducting solid. LISICON (Li-superionic conductor)1 and NASICON (Na-superionic conductor)2 structured solids can be used as potential solid electrolytes.3 NASICON-type lithium titanium phosphate (LiTi2 (PO4 )3 ; abbreviated as LTP) is one of such systems. By substituting Al for Ti the system Li1+x Alx Ti2−x (PO4 )3 (abbreviated as LATP) with highly mobile Li+ ions can be achieved.4, 5 The LATP ceramics have very high bulk ionic conductivity of the order of 10−3 S cm−1 being three orders of magnitude higher than that of the unsubstituted LTP.6 LATP possesses three-dimensional channels suitable for fast Li ion diffusion. The factors affecting this diffusion process include the concentration (x) of Li ions, the porosity of the system, the grain boundary contribution, the formation of additional phases of oxides and phosphates, and their structural stability on sintering. Therefore the synthesis route has a very important influence on the ionic conductivity of LATP.1, 2, 4–7 LTP crystallizes in the rhombohedral structure (Figure 1) with R3c space group. The unit cell is composed of corner sharing TiO6 octahedra and PO4 tetrahedra. Li is located in oxygenated distorted octahedral sites (Li1) between the TiO6 planes. In LATP, some of the Ti4+ sites are occupied by Al3+ and the net charge is compensated by Li+ . The additional Li occupies irregular 8-O-coordinated sites (Li2). The Li+ ionic diffusion mechanism in LATP involves the Li1-Li2-Li1 pathway. The Al-substitution has been shown to enhance the three-dimensional Li diffusion.4 Similarly different trivalent cations can induce this effect to enhance the Li+ ionic conductivity.4 When the value of x is above 0.5, additional phases like TiO2 , Li4 P2 O7 , 3



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 24

TiP2 O7 and/or AlPO4 (amorphous and crystalline) can be formed. Below x = 0.5, the lattice parameters decreased with increasing cation content, as shown in Figure 1. These small changes in the structure may lead to changes in ionic transport properties. However, the influencing factors such as the sintering procedure and the formation of the additional phases causing changes in intrinsic conductivity properties of LATP have rarely been studied. Nairn et al.8 studied Li1.3 Al0.3 Ti1.7 (PO4 )3 with 27 Al and 31 P MAS NMR and observed that Al3+ is not only substituting Ti4+ but occupies also a very small percentage of the P-sites. Hence, the authors contradicted a previous result4 where only Ti-substitution was observed.This suggests the important influence of sample preparation methods on the structure of LATP. Nairn et al.8 observed a single Li-site by a 7 Li NMR powder pattern being characterized by a quadrupole coupling constant (CQ ) of 45 kHz. The asymmetry of the

31 P

NMR signal was attributed to different P-sites which appear

upon Al-substitution.8 In a following paper Forsyth et al.,9 with the help of

27 Al

MAS NMR, observed a considerable amount of Al3+ reaching the

tetrahedral sites of LATP with x = 0.3, whereas there was no tetrahedrally coordinated Al visible for LATP with x = 0.1. The increasing asymmetry of the 31 P NMR signal with x suggested the formation of additional P-sites. This was accompanied with an increase of the 7 Li CQ indicating a reduction of structural symmetry. In addition, the work details the effects on replacing the PO4 tetrahedra with VO4 and/or NbO4 .9 With

27 Al

MAS NMR,

Best et al.10 observed five-fold coordinated Al in addition to tetrahedral and octahedral Al in Li1.3 Al0.3 Ti1.7 (PO4 )3 . With some stoichiometric changes (Li1.3+4y Al0.3 Ti1.7−y (PO4 )3 ) the authors managed to remove the tetrahedral AlPO4 phase, thereby increasing the grain-boundary conductivity. However, the bulk ionic conductivity was never seen to be higher than that in LATP. Arbi et al.11 studied LATP (0.0 ≤ x ≤ 0.7) using 7 Li,

4


27 Al

and

31 P

MAS

Page 5 of 24

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


NMR and suggested a few important structure models. They claimed the different P-sites are of the type P(OTi)4−n (OAl)n , where n varies from 0 to 4. These sites change their number density according to the extent of Al substitution, and their overlapped 31 P MAS NMR signals (in the range from -20 to -30 ppm) provide different asymmetries. The authors of Ref.11 observed a gradual increase of the tetrahedral

27 Al

signal (≈ 40 ppm) intensity with

the increase of x from 0.0 to 0.7. In addition, for LATP with x = 0.5 and 0.7, there was a new species of octahedral Al centered at around -17 ppm. From 7 Li NMR, they concluded that when the value of x is above 0.2, the Li4 P2 O7 phase appears and causes the decrease in ionic conductivity. From conductivity measurements, the authors estimated grain interior activation energies in the range between 0.27 eV and 0.35 eV.11 Key et al.12 studied variable-temperature MAS NMR of Li1.3 Al0.3 Ti1.7 (PO4 )3 and observed a room-temperature coalescence of two low-temperature 7 Li signals into one signal, indicating fast Li+ diffusion. Furthermore, they also reported several high-temperature structural changes in LATP, observed with NMR, above 770 K.12 In the present work, we carried out solid-state NMR characterization of different LATP compositions (with x = 0.0, 0.2, 0.35, 0.5, 0.7 and 1.0) synthesized by solid-state reaction. We have also done ion dynamics studies on all the compositions using 7 Li NMR spin-lattice relaxation (SLR) time (T1ρ ) measurements in the rotating frame as well as by 6 Li NMR exchange spectroscopy. Activation energies for the ionic jump process have been estimated along with the jump rates at various temperatures. The present work shows how the combined utilization of a variety of NMR techniques to study both structure and dynamics of ion conductors at different compositions can help to optimize the properties of an important potential solid electrolyte system.

5



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

2 2.1

Page 6 of 24

Experimental Materials

The samples were synthesized by conventional solid state reaction using stoichiometric amounts of Li2 CO3 (VWR International, Belgium, 99%), TiO2 (VWR International, Belgium, 99%), (NH4 )H2 PO4 (Merck, 99%) and Al2 O3 (Alfa Aesar, 99.99%), plus an excess of 10% Li2 CO3 . Pellets with a diameter of 13 mm were pressed with 190 MPa and heated in a platinum crucible up to 1000 ◦ C for 6 hours in air. After calcination, the pellets were crushed, homogenized in a mortar and again pressed to pellets. During the second heat treatment the pellets were again sintered at 1000 ◦ C for 6 h. The composition measurements of the LATP samples prepared using the same synthesis route have been reported in a recent publication.13

2.2

Solid-state NMR

The solid-state NMR spectra were acquired on a Bruker Avance III 600 spectrometer, operating at a magnetic field strength of 14 T, corresponding to a 7 Li Larmor frequency of 233.30 MHz and a 6 Li frequency of 88.34 MHz. A 5 mm single-resonance probe has been employed for static NMR measurements and a 2.5 mm double-resonance probe for magic-angle spinning (MAS) experiments. For static 7 Li experiments a solid-echo sequence has been used. For high resolution spectra, all the samples were spun at 20 kHz MAS frequency. A very small tip angle of π/16 was chosen to obtain quantitative central transition spectra of the quadrupolar 7 Li and The

27 Al

and

31 P

27 Al

nuclei.

Larmor frequencies were 156.42 MHz and 243.01 MHz,

respectively. For each sample, the T1 time was determined using saturation recovery experiments and recycle delays of 5×T1 were used for all experiments. The recycle delays for 7 Li and

27 Al

NMR experiments were of the

order of a few seconds, for 6 Li NMR tens of seconds and for 31 P NMR thou-

6


Page 7 of 24

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


sands of seconds. The

27 Al

triple quantum MAS experiments were carried

out using the standard z-filter pulse sequence. The 7 Li T1ρ times were determined at a spin-lock frequency ν1 of 33.3 kHz using the static NMR probe in the temperature range 203 - 453 K. The 6 Li 2D exchange spectra were collected under MAS using a standard P90 -t1 -P90 -τmix -P90 pulse sequence. The

6/7 Li

and

27 Al

NMR spectra were referenced (0 ppm) against diluted

solutions of LiCl and Al(NO3 )3 , respectively. KH2 PO4 has been used as a secondary chemical shift standard (4.3 ppm) for

3 3.1

31 P

NMR.

Results and discussion Multinuclear NMR spectroscopy

Li-NMR is a very efficient characterization tool to simultaneously probe dynamics and structure of a solid system. NMR active Li nuclei have comparatively small quadrupole interactions, and usually do not have a secondorder quadrupole interaction. While 7 Li (spin I = 3/2) has a quadrupole moment of -40.1 mb that of 6 Li (spin I = 1) is only -0.808 mb. Therefore, 6 Li

NMR provides high-resolution spectra. Li-NMR of the LATP samples

revealed several interesting static and motional features of the system. Figure 2a shows static 7 Li solid-echo spectra of the LATP samples. When x = 0, it shows a well-defined single first-order quadrupolar line shape with a Oh of about 41 kHz for Li in the octahedral quadrupole coupling constant CQ

(Oh) site. As x increases, the line shape changes with the change in the intensities and separations in the powder pattern. Towards the end of the series a very broad feature can be seen in addition to the small Oh-Li signal. The quadrupole coupling constant increases from ∼41 kHz to ∼43.5 kHz and then stabilizes with increasing x. This indicates a structural symmetry loss during the process of the Al-substitution until x reaches 0.5. 7 Li MAS NMR spectra at 20 kHz spinning frequency are shown in Figure 2b. LTP shows

7



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

again a single Li contribution. But as soon as x increases to 0.10, there is another Li component under partial coalescence with the Li1 peak due to fast Li+ ion exchange. This process becomes faster when x = 0.20, and complete coalescence occurs for the cases with x = 0.35 and 0.50, leading to narrow single peaks. The 7 Li chemical shifts also move from -1.9 ppm to -1.1 ppm. When x increases above 0.50, additional broad and asymmetric Li peaks appear corresponding to Li4 P2 O7 . Similar observations were made in the case of 6 Li MAS (20 kHz) NMR spectra (Figure 2c), but with a better resolution. Two broad 6 Li peaks corresponding to two Li sites are observed for the samples with x = 0.10 and 0.20. Sample spinning at 20 kHz increases the temperature by 30 K at the NMR rotor, reaching a total sample temperature of 323 K during the MAS experiments. The complete coalescence of the Li-NMR signals at 323 K indicates very fast ionic diffusion for LATP with x = 0.35 and 0.50. The loss of the structural symmetry seen from the 7 Li

static spectra can now be associated with the Li2 site occupation due

to Al-substitution which enhances the Li-diffusion. With the appearance of the Li4 P2 O7 phase, the intensities of the LATP Li-signals are observed to be diminishing. However, a complete disintegration of the LATP phase has never been observed for this series. Figure 3 shows the temperature dependence of 7 Li central transition line widths (FWHM) for LATP with x = 0.00 and 0.35, under static conditions. The averaging of 7 Li-7 Li dipolar broadening due to Li motion (motional narrowing) can be observed in both cases. For the LATP system with x = 0.00, the motional narrowing (MN) starts at around 220 K and reaches the extreme line narrowing limit near 400 K. Comparable activation energies were obtained from the analysis of this MN data using the methods proposed by Waugh and Fedin14 (0.36 eV), Hendrickson and Bray15 (0.31 eV) and Abragam16 (0.27 eV). For the system with x = 0.35, the extreme line narrowing limit was already reached at around 220 K. This indicates a very fast

8


Page 8 of 24

Page 9 of 24

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


ionic motion in the latter case even at low temperatures. 27 Al

(spin I = 5/2) NMR provides chemical shift fingerprints for Al

coordination, especially in tetrahedral (Td), five-coordinated and octahedral (Oh) environments in the solid state. In the case of the LATP samples, as shown in Figure 4a, the main Al signal centered at about -15 ppm belongs to the Oh-Al which substitutes for the Ti in TiO6 . A very small amount of Td-Al centered at about 40 ppm is present for LATP with x = 0.10 and 0.20. But for the samples with x = 0.35 and 0.50, only the Oh contribution was visible. The Td-Al signal is due to small impurities of AlPO4 and grows significantly when x increases above 0.5. From x = 0.50 onwards, another Oh-Al signal (centered at about -17 ppm) starts to intensify as more Al replaces Ti. In the Figures 4b and 4c, the 27 Al triple quantum MAS (3QMAS) NMR spectra of the samples with x = 0.70 and 1.00 are shown. The 3QMAS experiments helped to achieve high-resolution and apparently three Al signals were identified, two Oh and one Td. None of the sites were seen to have a crystalline second-order quadrupolar line shape. The second Oh-Al signal (at -17 ppm) seems to be associated with some distribution of quadrupolar interaction, indicating the presence of an amorphous LATP species. This is the most intense signal when x = 1.00, although a fair amount of the crystalline LATP phase co-exists. It is possible that the presence of these additional phases involving Al has some influence in the Li+ ionic mobility. In addition, we have carried out

31 P

(spin I = 1/2) MAS NMR experi-

ments, which has shown the expected behavior of increasing peak asymmetry with increasing Al substitution (Figures 5). While LTP has shown a very narrow

31 P-signal

(centered at -27 ppm), on substitution of Al, the inten-

sities of the peaks corresponding to P(OTi)4 (-26.8 ppm), P(OTi)3 (OAl) (-25.5 ppm), P(OTi)2 (OAl)2 (-24.7 ppm), P(OTi)(OAl)3 (-23.6 ppm) and P(OAl)4 (-21.6 ppm) increase, adding to the total width and asymmetry of

9



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 signal. When x increases above 0.5, Li4 P2 O7 and AlPO4 phases contribute to the signal, at around -20 ppm. We did not observe the presence of the phase TiP2 O7 , which shows positive chemical shifts. The very small peaks between -5 and -10 ppm were due to unresolved impurities. Comparison of the MAS NMR results with those in the literature11 mentioned in the Introduction section indicates the strong influence by the synthesis route and sintering procedure on the structure of LATP. Since many of the amorphous phases are hard to detect with X-ray diffraction methods, MAS NMR can be used for the quantitative analysis during fine tuning of sample properties using various preparation methods.

3.2 7 Li

7

Li spin-lattice relaxation

relaxometry is a well-known method to investigate ion dynamics in Li-

containing solids.17–20 Spin-lattice relaxation times are sensitive to the motion of the probed nucleus with respect to the surroundings. The resulting fluctuating internal dipolar or quadrupolar fields can cause a diffusion induced SLR rate peak with a characteristic maximum and flanks at the high-temperature (HT) and low-temperature (LT) sides. The maximum SLR rate in the laboratory frame of reference (T1−1 ) is observed when the angular resonance frequency (ω0 ) equals the jump rate (τ −1 ) of the mobile species in a diffusion process. Similarly at the maximum SLR rate in the −1 rotating frame (T1ρ ) the angular spin-lock frequency (ω1 ) and the jump rate

fullfil the condition, ω1 ≈ 0.5 × τ −1 . The slopes of the LT and HT flanks show Arrhenius behavior and yield the activation energy (EA ) for the jump LT ) refers to barprocess. However, the value taken from the LT flank (EA HT ) riers experienced in short-range motion and that from the HT flank (EA

to barriers seen in the long-range diffusion process. For a three dimensional (3D) diffusion process with non-correlated motion both the flanks should have equivalent slopes. The spectral density function J(ω) in the case of

10


Page 10 of 24

Page 11 of 24

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


such a 3D-jump relaxation process can be expressed as, τc J(ω) ∝ 1 + (ωτc )β

(1)

;where τc is the Li+ ionic jump correlation time in our case, and β is 2 for all the fits in the present work. A 3D-diffusion model21 is employed for fitting the SLR rate peaks in all cases. In the case of LATP samples, we have −1 employed 7 Li T1ρ measurements in a temperature range between 203 K and

453 K with a spin-lock frequency ν1 of 33 kHz. The corresponding angular frequency is ω1 = 207 × 103 s−1 . Therefore, the estimated Li+ ionic jump rate is approximately 4 × 105 s−1 where the diffusion induced SLR rate maximum occurs at temperature TDmax . In the case of LTP (x = 0), TDmax was observed at around 380 K with an EA of 0.41 eV (Figure 6a). For x = 0.10, two superimposed SLR rate peaks were observed with maxima at 280 and 245 K, respectively. The activation energies for these diffusion processes were reduced to 0.37 and 0.29 eV, respectively. Since both the rate peaks overlap, it is hard to understand the nature of the diffusion process. But since for x = 0.10 still only a few Li+ ions are present in the Li2 sites, it is possible that they are subjected to a different jump mechanism compared to those in the Li1 sites. Moreover, the additional Li+ ions are displaced towards Al. These could be the reasons for the appearance of these two SLR rate peaks. For LATP with x = 0.20, a slightly broader SLR rate peak HT = 0.30 eV. From the figure, it was observed at TDmax of 245 K with EA

can be imagined that the second diffusion process may occur at a slightly lower temperature (Fig 6a, black curve). In the case of x = 0.35 and 0.50, the maxima were observed at the same low temperature of 213 K (-60 ◦ C) HT of 0.29 eV. For with the same Li+ ionic jump rate of ∼105 s−1 and EA

LATP with x = 0.70 and 1.00, the ionic mobility seems to be the slowest in this series. As shown in Figure 6b, the SLR rates increased reaching the diffusion-induced maxima while the HT flanks of the rate peaks were not observed. The peaks for x = 0.70 and 1.00 show TDmax close to 420 K and 11



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

again larger activation energies of 0.40 and 0.33 eV, respectively. In short, at ambient conditions the fastest ionic motion in LATP can be expected when 0.35 ≤ x ≤ 0.5. This is in good agreement with the reported values of x for excellent ionic conductivities in LATP.4, 11, 13, 22, 23 In the case of LATP prepared by solid-state reaction, the bulk ionic conductivity was observed to be increased from x = 0.1 (1.73 × 10−3 S cm−1 ) to x = 0.4 (5.63 × 10−3 S cm−1 ) and then decreased slightly (4.70 × 10−3 S cm−1 for x = 0.5).13 Figure 6c shows the expected ionic jump rates for all the LATP composiHT = E LT ) and tions estimated using the activation energies (assuming EA A

TDmax values at 300 and 400 K. A recent SLR NMR study24 on a sol-gel −1 maxsynthesized LATP with x = 0.5 also reported a diffusion induced T1ρ

imum at around 213 K with ω1 /2π = 30 kHz. However, a lower activation energy (0.16 eV) was estimated from the HT flanks of the SLR rate peaks. The authors also observed two additional diffusion processes at lower and higher temperatures. These processes are activated outside the temperature range of the measurements (for x = 0.50) reported in the present work. Nevertheless, in the present work the two main contributions to the Li diffusion in LATP were observed fully for LATP with x = 0.10 and partially for that with x = 0.20. This would indicate the possibilities of similar dynamic trajectories for all the pure-phase materails (LATP from x = 0.10 to x = 0.50) after Al3+ substitution. But the processes are activated at higher temperatures for lower Al3+ /Li+ concentration (x). In Ref,24 the dynamic processes were attributed to distinct Li+ elementary hopping processes including jumps between the two crystallographic Li sites and/or involving interstitials. However, the preparation method seems to have an influence on the ionic motion and that is reflected in the slightly lower activation energy for the sol-gel synthesized material. In a recent report,25 the calulated activation energies for Li1.2 Al0.2 Ti1.8 (PO4 )3 varied between 0.35 eV and 0.47 eV for a vacancy migration model while the activation barrier for

12


Page 12 of 24

Page 13 of 24

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


interstitial migration was 0.19 eV. Therefore, the vacancy migration model can be used to explain the dynamic processes in LATP synthesized using solid-state reactions in the present work. In short the sol-gel synthesis and the solid-state reactions introduce different levels of vacancies and/or defects to the LATP system to have different ion diffusion barriers. Therefore the significant conclusion of the present work is the fact that, for an ionic conductor, the difference in synthesis routes, ionic compositions and the structural properties immensely influence its ionic mobility.

3.3 6 Li

6

Li exchange spectroscopy

2D EXSY experiment is a very convenient NMR method to determine

Li+ ionic exchange processes between Li crystallographic sites. The experiment works better for the rare 6 Li than for the abundant 7 Li, since spin diffusion effects can be avoided. Therefore, this experiment can also be used to probe very slow 6 Li exchange processes in the range of several minutes.26 In the LATP with x = 0.70, the 6 Li MAS NMR showed two broad and slightly asymmetric signals belonging to the lithium phosphate phase in addition to the LATP phase. We have done 6 Li 2D EXSY experiments on this system with variable mixing times (5, 10, 100, 250, 500, 750 and 1000 ms) under 20 kHz MAS. Figure 7 shows the 2D spectra obtained with mixing times (tm ) of (a) 5 ms, (b) 100 ms and (c) 1000 ms. Apparently the two broad signals, which show cross peak intensities, correspond to Li sites with Li+ exchange. But the exchange process is a moderately slow one here. The ratio of the intensities of the cross peaks (Ic ) to those of the diagonal peaks (Id ) follows a stretched exponential function, (Ic /Id ) = 1−exp(−(tm /τex )γ ). The exchange correlation time τex in this case is estimated as 0.249 s from the fit, indicating approximately 4 jumps per second at 323 K. The stretching exponent γ gives a value of 0.5 which might indicate a small distribution in the ionic exchange processes, confirmed by the asymmetric line shape

13



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

belonging to a multi-component system (amorphous). However, the Li signal from the LATP (at -0.85 ppm) is not getting involved in the exchange process. Therefore the generation of the phosphate phase at the expense of LATP phase seems to be associated with the suppression of ionic mobility. The amorphous nature of the remaining LATP phase observed from the 27 Al

3QMAS experiments can now be attributed to the random degradation

process due to the formation of the phosphate phase.

4

Conclusions

Solid-state NMR methods have been used to characterize structure and dynamics in an ionic conductor, Li1+x Alx Ti2−x (PO4 )3 (0.0 ≤ x ≤ 1.0). MAS NMR provided high-resolution 6 Li, 7 Li,

27 Al, 31 P

spectra of the samples

differing in cation concentrations. The site assignments were done using the multinuclear NMR data. The coalescence of the 7 Li indicated fast ionic motion even at ambient conditions. The faster ionic motion in the case of LATP with x = 0.35 when compared to that for x = 0.0 can be seen from the 7 Li motional narrowing data. This was also proven by 7 Li relaxometry experiments. The SLR rates showed diffusion-induced maxima at very low temperatures when 0.35 ≤ x ≤ 0.5, indicating a faster Li ionic motion for them in the LATP series. The activation barriers for the pure-phase LATP systems were in the range between 0.29 and 0.37 eV. With the help of different solid-state NMR methods, the ionic mobility landscape of LATP as a function of ionic composition and temperature can be visualized. A vacancy migration model for the ionic transport is proposed in the LATP system prepared using solid-state reaction. 6 Li exchange spectroscopy provided hints of slowly moving Li ions in additional phosphate phases formed at the expence of LATP when x is above 0.5. In short, a variety of solidstate NMR techniques were exploited to analyze a series of ion conductors to explore the most favorable mobility characteristics. However, the structural 14


Page 14 of 24

Page 15 of 24

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


features and hence the ion dynamics of the LATP system show a strong dependence on the preparation method, as understood from the comparison with previous reports.

Acknowledgements The authors are grateful for the financial support by ‘Graduiertenkolleg Energiespeicher und Elektromobilität Niedersachsen (GEENI)’ and DFG Research Unit 1277 ‘Mobilit¨ at von Lithium-Ionen in Festkörpern (molife)’.

References [1] Goodenough, J. B.; Hong, H. Y. P.; Kafalas, J. A. Fast Na+ -ion Transport in Skeleton Structures. Mater. Res. Bull. 1976, 11, 203-220. [2] Hong, H. Y. -P. Crystal Structure and Ionic Conductivity of Li14 Zn(GeO4 )4 and Other New Li+ Superionic Conductors. Mater. Res. Bull. 1978, 13, 117-124. [3] Knauth, P. Inorganic Solid Li Ion Conductors: An Overview. Solid State Ionics 2009, 180, 911-916. [4] Aono, H.; Sugimoto, E.; Sadaoka, Y.; Imanaka, N.; Adachi, G.-Y. Ionic Conductivity of Solid Electrolytes Based on Lithium Titanium Phosphate. J. Electrochem. Soc. 1990, 137, 1023-1027. [5] Aono, H.; Sugimoto, E.; Sadaoka, Y.; Imanaka, N.; Adachi, G.-Y. The Electrical Properties of Ceramic Electrolytes for LiMx Ti2−x (PO4 )3 + yLi2 O, M = Ge, Sn, Hf, and Zr Systems. J. Electrochem. Soc. 1993, 140, 1827-1833.

15



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

[6] Aono, H.; Sugimoto, E. Ionic Conductivity and Sinterability of NASICON-type Ceramics: The Systems NaM2 (PO4 )3 + yNa2 O (M = Ge, Ti, Hf, and Zr). J. Am. Ceram. Soc. 1996, 79, 2786-2788. [7] Adachi, G.-Y.; Imanaka, N.; Aono, H. Fast Li+ Conducting Ceramic Electrolytes. Adv. Mater. 1996, 8, 127-135. [8] Nairn, K. M.; Forsyth, M.; Greville, M.; MacFarlane, D. R.; Smith, M. E. Solid State NMR Characterization of Lithium Conducting Ceramics. Solid State Ionics 1996, 86-88, 1397-1402. [9] Forsyth, M.; Wong, S.; Nairn, K. M.; Best, A. S.; Newman, P. J.; MacFarlane, D. R. NMR Studies of Modified Nasicon-like, Lithium Conducting Solid Electrolytes. Solid State Ionics 1999, 124, 213-219. [10] Best, A. S.; Forsyth, M.; MacFarlane, D. R. Stoichiometric Changes in Lithium Conducting Materials Based on Li1+x Alx Ti2−x (PO4 )3 : Impedance, X-ray and NMR Studies. Solid State Ionics 2000, 136-137, 339-344. [11] Arbi, K.; Mandal, S.; Rojo, J. M.; Sanz, J. Dependence of Ionic Conductivity on Composition of Fast Ionic Conductors Li1+x Alx Ti2−x (PO4 )3 , 0 ≤ x ≤ 0.7. A Parallel NMR and Electric Impedance Study. Chem. Mater. 2000,14, 1091-1097. [12] Key, B.; Schroeder, D. J.; Ingram, B. J.; Vaughey, J. T. Solution-based Synthesis and Characterization of Lithium-ion Conducting Phosphate Ceramics for Lithium Metal Batteries. Chem. Mater. 2012, 24, 287-293. [13] D. Rettenwander, A. Welzl, S. Pristat, F. Tietz, S. Taibl, G. J. Redhammer, J. Fleig, A Microcontact Impedance Study on NASICON-type Li1+x Alx Ti2−x (PO4 )3 (0 ≤ x ≤ 0.5) Single Crystals. J. Mater. Chem. A 2016, 4, 1506-1513.

16


Page 16 of 24

Page 17 of 24

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


[14] Waugh, J.; Fedin, E. Determination of Hindered-rotation Barriers in Solids. Sov. Phys. Solid State 1963, 4, 1633-1636. [15] Hendrickson, J.R.; Bray, P. J. A Phenomenological Equation for NMR Motional Narrowing in Solids. J. Magn. Reson. 1973, 9, 341-357. [16] Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, 1994. [17] P. Heitjans, J. K¨ arger (Eds.), Diffusion in Condensed Matter: Methods Materials Models; Springer: Berlin, 2005. [18] Kuhn, A.; Sreeraj, P.; Pöttgen, R.; Wiemhöfer, H.-D.; Wilkening, M.; Heitjans, P. Li Ion Diffusion in the Anode Material Li12 Si7 : Ultrafast Quasi-1D Diffusion and Two Distinct Fast 3D Jump Processes Separately Revealed by 7 Li NMR Relaxometry. J. Am. Chem. Soc. 2011, 133, 11018-11021. [19] Kuhn, A.; Kunze, M.; Sreeraj, P.; Wiemhöfer, H. -D.; Thangadurai, V.; Wilkening, M.; Heitjans, P. NMR Relaxometry as a Versatile Tool to Study Li Ion Dynamics in Potential Battery Materials. Solid State Nucl. Magn. Reson. 2012, 42, 2-8. [20] Kuhn, A.; Dupke, S.; Kunze, M.; Puravankara, S.; Langer, T.; Pöttgen, R.; Winter, M.; Wiemh¨ ofer, H.-D.; Eckert, H.; Heitjans, P. Insight into the Li Ion Dynamics in Li12 Si7 : Combining Field Gradient Nuclear Magnetic Resonance, One- and Two-Dimensional Magic-angle Spinning Nuclear Magnetic Resonance, and Nuclear Magnetic Resonance Relaxometry. J. Phys. Chem. C 2014, 118, 28350-28360. [21] Bloembergen, N.; Purcell, E. M.; Pound, R. V. Relaxation Effects in Nuclear Magnetic Resonance Absorytion. Phys. Rev. 1948, 73, 679-712.

17



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

[22] Morimoto, H.; Hirukawa, M.; Matsumoto, A.; Kurahayashi, T.; Ito, N.; Tobishima, S. Lithium Ion Conductivities of NASICON-type Li1+x Alx Ti2−x (PO4 )3 Solid Electrolytes Prepared from Amorphous Powder Using a Mechanochemical Method. Electrochemistry 2014, 82, 870-874. [23] Breuer, S.; Prutsch, D.; Ma, Q.; Epp, V.; Preishuber-Pfl¨ ugl, F.; Tietz, F.; Wilkening, M. Separating Bulk from Grain Boundary Li Ion Conductivity in the Sol-gel Prepared Solid Electrolyte Li1.5 Al0.5 Ti1.5 (PO4 )3 . J. Mater. Chem. A 2015, 3, 21343-21350. [24] Epp, V.; Ma, Q.; Hammer, E.; Tietz, F.; Wilkening, M. Very Fast Bulk Li Ion Diffusivity in Crystalline Li1.5 Al0.5 Ti1.5 (PO4 )3 as Seen Using NMR Relaxometry. Phys. Chem. Chem. Phys. 2015, 17, 32115-32121. [25] Lang, B.; Ziebarth, B.; Elsässer, C. Lithium Ion Conduction in LiTi2 (PO4 )3 and Related Compounds Based on the NASICON Structure: A First-Principles Study. Chem. Mater. 2015, 27, 5040-5048. [26] Wilkening, M.; Romanova, E. E.; Nakhal, S.; Weber, D.; Lerch, M.; Heitjans, P. Time-resolved and Site-specific Insights into Migration Pathways of Li+ in α-Li3 VF6 by 6 Li 2D Exchange MAS NMR. J. Phys. Chem. C 2010, 114, 19083-19088.

18


Page 18 of 24

Page 19 of 24

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


Table 1. The activation energies estimated from the high-temperature HT ) and low-temperature (E LT ) flanks of the diffusion-induced 7 Li SLR (EA A

rate peak and the temperatures (TDmax ) at which the maximum relaxation rate was measured for Li1+x Alx Ti2−x (PO4 )3 . The asterisks indicate the two SLR rate peaks observed when x = 0.10 (see Figure 6). The Li+ ionic jump rate corresponding to the SLR rate maxima is 4 × 105 s−1 . The standard error in the estimation of EA is approximately ±0.01 eV. x

0.00

0.10

0.20

0.35

0.50

0.70

1.00

HT / eV EA

0.41

0.37*

0.30

0.29

0.29

–

–

LT / eV EA

0.41

0.29**

–

–

–

0.40

0.33

TDmax / K

380

280*, 245**

245

213

213

423

413

19



Figures

o

c/A

21.0

20.8

8.6 o

a/A

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

8.4 c

a

b

0.0

0.1

0.2

x

0.3

0.4

0.5

Figure 1: Crystal structure (left) of rhombohedral LATP showing Li1-O6 octahedra in light blue, oxygen in red, Ti/Al-O6 octahedra in orange, PO4 tetrahedra in dark green and 8-coordinated lithium (Li2) in grey polyhedra. The lattice parameters a and c with their dependence on the Al content are shown on the right side.

Figure 2: Li NMR spectra of the LATP samples with different cation concentrations at ambient conditions: (a) 7 Li static solid-echo spectra showing Oh ) for lithium in octahedral position, (b) different quadrupole couplings (CQ 7 Li

MAS (at 20 kHz) NMR and (c) 6 Li MAS (at 20 kHz) NMR spectra. 64

transients were collected for each spectrum at an external magnetic field of 14 T.

20


Page 20 of 24

Page 21 of 24

2.0

1.8

1.6

1.4

7

FWHM ( Li) / kHz

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


1.2

x = 0.00 1.0

x = 0.35

0.8

0.6 200

250

300

350

400

450

T/K

Figure 3:

7 Li

NMR linewidths (FWHM in kHz) of the central transition

signals of LATP with x = 0.00 and 0.35 plotted against temperature (T ).

Figure 4: (a)

27 Al

frequency and

NMR spectra of LATP under MAS of 20 kHz spinning

27 Al

3QMAS NMR of LATP with (b) x = 0.70 and (c)

x = 1.00. 1024 transients were collected for each 1D spectrum at an external magnetic field of 14 T.

21



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

x = 0.00 x = 0.10 x = 0.20 x = 0.35 x = 0.50 x = 0.70 x = 1.00 40

20

0

-20

-40

-60

-80

-100

31

(

Figure 5:

31 P

P) / ppm

MAS NMR spectra of LATP samples at a spinning frequency

of 20 kHz. Four transients were collected for each spectrum at an external magnetic field of 14 T.

Figure 6: The 7 Li SLR rates in the rotating frame against the measurement temperature (203 - 453 K) for LATP samples with (a) x = 0.00, 0.10, 0.20, 0.35 and 0.5 and (b) 0.70 and 1.00. The spin-lock frequency employed was ν1 = 33 kHz corresponding to ω1 = 207 × 103 s−1 . Therefore, at the diffusioninduced SLR rate maxima, the ionic jump rate (τ −1 ) is approximately 4 × 105 s−1 . A 3D-diffusion model21 is used to fit the SLR rate peaks in all cases. The expected ionic jump rates (c) for all compositions of LATP (solid symbols), estimated from the activation energies (open symbols; assuming HT = E LT from Table 1) and T EA Dmax are shown for 300 and 400 K. The A

grey symbols indicate the jump rates and the activation energy of the second dynamic process observed for LATP with x = 0.1.

22


Page 22 of 24

Page 23 of 24

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


Figure 7: 2D 6 Li EXSY NMR spectra of LATP with x = 0.70, under 20 kHz MAS frequency. The mixing times used are (a) 5 ms, (b) 100 ms and (c) 1000 ms.

23



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

146x102mm (300 x 300 DPI)


Page 24 of 24

Solid-State NMR Investigations on the Structure ... - ACS Publications

Recommend Documents