Article pubs.acs.org/cm
Structural Evolution and Li Dynamics in Nanophase Li3PS4 by SolidState and Pulsed-Field Gradient NMR Mallory Gobet,† Steve Greenbaum,*,† Gayatri Sahu,‡ and Chengdu Liang‡ †
Department of Physics & Astronomy, Hunter College of the City University of New York, New York, New York 10065, United States ‡ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States S Supporting Information *
ABSTRACT: The ceramic lithium ion conductor β-Li3PS4 has a disordered and nanoporous structure that leads to an enhancement in ionic conductivity by some 3 orders of magnitude compared to the crystalline γ phase. The β phase is prepared by thermal treatment of an inorganic−organic complex based on Li3PS4 and THF. Multinuclear (1H, 6,7Li, 31P) solid-state NMR spectroscopy is used to characterize the structural phase evolution of the starting material at various steps in the thermal treatment. The β phase formed after high temperature treatment is recognized as spectroscopically distinct from the bulk γ-Li3PS4 compound. Also formed is an amorphous lithium thiophosphate phase that is metastable as verified by annealing over an extended period. Lithium ion self-diffusion coefficients are measurable by standard pulsed-field gradient NMR methods at 100 °C and with values consistent with the high ionic conductivity previously reported for this material.
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INTRODUCTION
nanostructure stabilizes the high conduction beta phase and provides an additional conduction mechanism through surface conduction.4 The nanoporous form of Li3PS4, hereafter designated as β-Li3PS4, exhibits some 3 orders of magnitude enhancement in ionic conductivity compared to the bulk crystalline phase of the same composition, γ-Li3PS4. Given the same stoichiometric composition of Li3PS4, the essential difference in ionic conductivity of the nanoporous β-Li3PS4 and bulk γ-Li3PS4 arises from their synthesis procedures. The desolvation process of Li3PS4·3THF is a novel synthesis approach for sulfide-based solid electrolytes, which are conventionally synthesized through solid-state reactions. In order to understand this new synthesis approach, in this research, we used solid-state nuclear magnetic resonance (NMR) to provide insights on the structural evolution and the origin of the enhanced ionic conductivity of nanoporous βLi3PS4. As a distinct difference from powder X-ray diffraction, which is somewhat insensitive to short-range bonding arrangements in disordered or amorphous phases characteristic of heterogeneous structures, solid-state NMR provides shortrange structural details of β-Li3PS4 and thus can provide important details related to the anomalously high ionic conductivity of nanoporous β-Li3PS4.
The integration of intermittent renewable energy to the electricity grid and the electrification of transportation stimulate the research interest of large-scale electric energy storage.1 Lithium batteries that use metallic lithium as the anode are considered as one of the enabling technologies for such a paradigm shift of energy infrastructure. However, the formation of dendrites in combination of flammable organic electrolytes prohibits the use of metallic lithium as the anode because of safety concerns, particularly for large-scale and energy-dense storage setups.2 To alleviate the safety concerns of lithium batteries, nonflammable solid electrolytes have been intensively studied as a promising alternative to flammable liquid electrolytes. To date the most conductive solid electrolytes for lithium ions are sulfide-based materials, which achieved 12 mS/cm conductivity at ambient temperature.3 Among all sulfide-based solid electrolytes, the stoichiometric compound of lithium thiophosphate, Li3PS4, is considered as the most chemically stable compound against metallic lithium.4 In addition to its excellent compatibility with metallic lithium, Li3PS4 also has negligible interfacial resistance when used in lithium batteries.1 Nevertheless, at room temperature, Li3PS4 presents as the gamma phase, which has a very low ionic conductivity.5 In a recent report, Liang and co-workers discovered that stoichiometric Li3PS4 can be synthesized through the desolvation of Li3PS4·3THF, which resulted in a nanoporous structure. The © 2014 American Chemical Society
Received: April 4, 2014 Revised: May 6, 2014 Published: May 9, 2014 3558
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Table 1. δiso Is the Isotropic Chemical Shift, ηQ Is the Anisotropy of the Quadrupolar Tensor, CQ Is the Quadrupolar Coupling, and δCS and ηCS Are the Axiality and the Anisotropy of the CSA Tensor, Respectively 7
P
sample
δiso (ppm)
CQ (kHz)
ηQ
δiso (ppm)
δCS (ppm)
ηCS
γ-Li3PS4 Li3PS4·3THF Li3PS4-70 °C
2.82 1.78 1.81 1.58 1.46 1.76 2.13
34 86 86 6 6
1 0.4 0.4
88.2 86.8; 87.7; 89.6 86.8; 87.7; 89.6 83.9 83.9 83.9 86.3
−19 28 28 25 25 25 −36.2
0.93 0.7;1;1 0.7;1;1 1 1 1 0.09
Li3PS4-100 °C Li3PS4-140 °C
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31
Li
area fraction (%) 30; 35; 35 20; 18; 17 45 64 36
Simulations of the solid-state NMR spectra were obtained using Dmfit software.6 The NMR parameters of the samples are summarized in Table 1. The CSA parameters are defined according to the following conventions:
EXPERIMENTAL SECTION
Synthesis of Materials. Li3PS4·3THF was synthesized by using a previously reported procedure.4 In brief, Li2S (Aldrich, 99%) and P2S5 (Sigma-Aldrich, 99%) with a stoichiometry of 3 to 1 were mixed in anhydrous THF (Sigma-Aldrich, >99.9%) at room temperature in an argon-filled glovebox. The mixture was then stirred overnight. The white precipitation was collected and washed with copious THF. The as-synthesized Li3PS4·3THF was dried on a Petri dish inside a glovebox at ambient temperature. The desolvation of Li3PS4·3THF was conducted in a homemade vacuum line without exposure to air and moisture. Three desolvated samples were prepared at 70, 100, and 140 °C. These samples are denoted as Li3PS4-70 °C, Li3PS4-100 °C, and Li3PS4-140 °C. Bulk Li3PS4 was synthesized through conventional solid-state synthesis. Li2S and P2S5 with a stoichiometry of 3 to 1 were mixed in a high-energy ball mill for 1 min. The powder was then dry-pressed into pellets (diameter 1.27 cm, thickness ∼1.5 cm) and calcined in a Pyrex glass ampule. The temperature was ramped to 550 °C in 4 h and then held for 48 h. The sample was cooled down to room temperature at a rate of 10 °C/h. The final product is denoted as γ-Li3PS4. All materials were handled in the glovebox. The crystal structures of materials were confirmed by powder X-ray diffraction. Powder XRD patterns, already published elsewhere,4 are included in the Supporting Information. Briefly, the XRD confirms the presence of the β phase as distinct from the bulk γ phase. Solid-State NMR Experiments. Samples were packed in 3.2 mm MAS rotors in an argon-filled glovebox. MAS NMR experiments were performed with a 7.05 T Varian-S Direct Drive Wide Bore spectrometer and a 3.2 mm MAS probe operating at 301.4 MHz, 122.0 MHz, 117.1 MHz, and 44.4 MHz to study 1H, 31P, 7Li, and 6Li, respectively. Single-pulse experiments (SPE) were run with MAS speeds ranging from 4 kHz to 20 kHz. For 1H SPE, a 2.8 μs π/2 pulse length and a 12 s recycle delay were used, and 128 transients were averaged. An empty rotor measurement was also run under the same conditions in order to check for background signals. 1H chemical shifts are referenced to TMS (δ = 0 ppm). 31P SPE were done using a 3.5 μs π/2 pulse length, a 60 to 600 s recycle delay, and 1024 transients. 31P chemical shifts are referenced to 85% H3PO4 aqueous solution (δ = 0 ppm). 7Li SPE were performed using a 3.1 μs π/2 pulse length, a 15 s recycle delay, and 64 transients. 6Li SPE were done using a 4.2 μs π/2 pulse length, a 30 s recycle delay, and 2048 transients. 7Li and 6Li chemical shifts are referenced to solid LiCl (δ = 0 ppm). Cross-polarization (CP) experiments were undertaken at 4 kHz spinning rate, and the experimental conditions were optimized directly on the samples. For 31P[1H] CP experiments, a 2.8 μs 1H π/2 pulse length and a 5 ms contact time were used. A dipolar decoupling of 35 kHz was applied during the 20 ms acquisition time. A 12 s recycle delay was used, and 1024 transients were acquired. For 7Li[1H] CP experiments, a 2.8 μs 1H π/2 pulse length and a 6 ms contact time were used, and a dipolar decoupling of 45 kHz was applied during the 16 ms acquisition time. 128 transients were acquired with a 12 s recycle delay. Static NMR 31P experiments were performed with a 5 mm broadband static probe using a 3 μs π/2 pulse length, 60 to 600 s recycle delays, and 1024 transients.
δiso = 1/3(δ11 + δ22 + δ33) δCS = δ33 − δiso ηCS = (δ22 − δ11)/(δ33 − δiso) with |δ33 − δiso| ≥ |δ11 − δiso| ≥ |δ22 − δiso| NMR Diffusion Experiments. The sample was packed in a 5 mm tube in an argon-filled glovebox. Experiments were performed with a DOTY Scientific, Inc. probe equipped with a Z-gradient coil. Diffusion measurements were done using a pulsed-field gradient stimulated echo pulse sequence with 11 μs π/2 pulse length and a 2 s recycle delay. Ten gradient values ranging from 2 to 1000 G cm−1 were used. The gradient pulse durations and the diffusion delay were 2.2 and 550 ms, respectively, with ring-down times of 1 ms, and a 70 G cm−1 spoiler gradient was set to 2 ms. 2500 transients were recorded.
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RESULTS AND DISCUSSION Overview of 1H, 31P, and 7Li SPE-MAS Spectra. The 1H SPE MAS spectra of the samples (Li3PS4·3THF, Li3PS4-70 °C, Li3PS4-100 °C, and Li3PS4-140 °C) are shown in Figure 1. Even though the experiments were run with the same acquisition parameters, no precise quantitative comparison as to proton concentration can be made, mainly because sample densities are unknown. The liquid-state NMR spectrum of THF (not shown) is composed of two equal intensity peaks resonating at 3.75 ppm (O−CH2−) and 1.85 ppm (−CH2−). The 1H spectrum of Li3PS4·3THF here consists of a broad peak appearing around 3 ppm and experiencing important anisotropic interactions evidenced by the presence of spinning sidebands for this 1/2 spin nucleus. This feature confirms the presence of cocrystallized THF in this phase. The spectrum of Li3PS4-70 °C is very similar although less intense (about 25% decrease in signal integral), implying that, despite thermal treatment at 70 °C, the sample still contains a significant amount of entrapped THF molecules. In contrast, the very weak 1H signals of Li3PS4-100 °C and Li3PS4-140 °C indicate that the thermal treatment at 100 °C was sufficient to remove almost all the THF molecules. 31 P NMR SPE MAS spectra of the 4 samples and the standard γ-Li3PS4 are presented in Figure 2. The spectrum of γLi3PS4 is mainly composed of one narrow peak, which reflects the presence of only one P site (isolated PS43− tetrahedra) in this phase. The measured chemical shift δiso = 88.2 ppm is very close to the expected value (88.4 ppm).7 The presence of weak spinning sidebands is due to CSA that was estimated as δCSA = 3559
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spectrum of Li3PS4-100 °C. As already indicated by the 1H spectrum of the latter sample (Figure 1), there is no more evidence of Li3PS4·3THF presence. The broad signal is also present in the Li3PS4-140 °C spectrum. In addition, there is the emergence of a single peak, slightly resonating upfield to γLi3PS4. This peak is attributed to isolated PS43− in a new chemical environment: β-Li3PS4. 7 Li NMR SPE MAS spectra of the four samples and the standard γ-Li3PS4 are presented in Figure 3. The spectrum of γ-
Figure 1. 1H MAS NMR single-pulse spectra (after subtraction of rotor and probe background signal) of Li3PS4·3THF crystalline phase and the samples obtained after 70 °C, 100 °C, and 140 °C heat treatments. The spinning rate is 17 kHz. Spinning sidebands are designated by asterisks.
Figure 3. 7Li MAS NMR single-pulse spectra acquired at ambient temperature for the four samples at the spinning rate of 4 kHz.
Li3PS4 displays the typical spinning sideband pattern of the 3/2 → 1/2 and −1/2 → −3/2 transitions under first-order quadrupolar perturbations. The wider span of sideband pattern of Li3PS4·3THF spectrum indicates a larger quadrupolar coupling constant, a consequence of the greater electric field gradient surrounding the 7Li nuclei attributed to the presence of cocrystallized THF molecules. Li3PS4-70 °C, Li3PS4-100 °C, and Li3PS4-140 °C spectra are characterized by less intense side bands. This can be due to lithium ions being either in a more symmetric environment or experiencing rapid motion at room temperature so that the quadrupolar interaction is averaged. This point will be discussed further. Structural Aspect and Composition of Each Phase. Li3PS4·3THF. The central band area of the 31P SPEMAS spectrum of Li3PS4·3THF is displayed in Figure 4. It is composed of three different 31P peaks resonating at 86.8, 87.7, and 89.6 ppm of comparable integrated intensity (30, 35, and 35% of the SPE MAS spectrum). The presence of these distinct peaks originates from molecular-scale variations in the vicinity of the PS43− tetrahedra and demonstrates how solid-state NMR is sensitive to local order, unlike long-range structural characterization techniques such as XRD. Generally used to improve the detection of nuclei of lowabundance or low sensitivity, the transfer of polarization from abundant nuclei to others via cross-polarization (CP) can also be used for spectral editing. This experiment is particularly useful for detecting nuclei spatially close to the excited nuclei, as the transfer is mediated by the nuclear dipolar interaction. In this work, the presence of entrapped proton-containing
Figure 2. 31P MAS NMR single-pulse spectra acquired for the five samples at the spinning rate of 4 kHz. Spinning sidebands are designated by asterisks.
−19 ppm (ηCS = 0.93). The 31P spectrum of Li3PS4·3THF exhibits 3 different P sites. The new structure induced by the cocrystallization of THF provides three distinct chemical environments around the PS43− tetrahedra. The 31P spectrum of Li3PS4-70 °C also highlights the presence of a Li3PS4·3THF phase. This is consistent with the strong signal of THF in the corresponding 1H spectrum (Figure 1). Up-field (i.e., lower chemical shift) to these three sharp peaks, a broad shoulder arises. This broad component is the only signal detected in the 3560
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Figure 5. 31P MAS NMR (4 kHz) of the 70 °C-heated sample: singlepulse (experimental and simulated) and 31P[1H] CP-MAS spectra in the centerband region.
Figure 4. 31P MAS NMR (4 kHz) of Li3PS4·3THF crystalline phase: single-pulse (experimental and simulated) and 31P[1H] CP-MAS spectra in the centerband region.
molecules (THF) allows probing the distance between PS43− tetrahedra and the cocrystallized molecules of solvent by means of 31P[1H] CP-MAS. As shown in Figure 4, the 31P[1H] CPMAS and 31P SPE spectra are almost identical, meaning that all the thiophosphate units are closely surrounded by THF molecules. Since the spatial proximity of nuclei can be deduced through the effectiveness of the CP signal enhancement, the CP contact time was arrayed, and the relative areas of each peak of the CP signal were determined. The CP build-up curves (not shown) of the three P peaks were very similar, suggesting that the differences between the three sites are not due to variations in distance from THF molecules but rather to the small differences in geometric arrangement of these organic molecules. Similarly, the proximity between lithium ions and THF molecules can be investigated with 7Li[1H] CP-MAS. As found for 31P, 7Li[1H] CP-MAS and 7Li SPE spectra are almost identical (CP-MAS spectrum shown in the SI), confirming the homogeneous distribution of THF molecules within the Li3PS4· 3THF structure. Li3PS4-70 °C. As shown in Figure 5, the 31P[1H] CP-MAS allows isolation of the signal from the remaining Li3PS4·3THF phase in the 70 °C-treated sample. The decomposition of the SPE-MAS spectrum is then possible and it highlights a new P site not coupled to the remaining THF protons, representing about 45% of the total P content of the sample and of which the nature (amorphous phase) will be discussed in the section describing the Li3PS4-100 °C sample. Similar spectral editing can be done by 7Li[1H] CP-MAS, the resulting spectrum reproducing the exact signature of Li3PS4· 3THF phase (Figure 6). The spinning sideband pattern of Li3PS4·3THF is present in the 7Li SPE but with a smaller intensity because of the contribution of the amorphous phase which, as expected, does not exhibit a resolved quadrupolar interaction pattern but results, instead, in broadening observed at the base of the central transition.
Figure 6. 7Li MAS NMR (4 kHz) of the 70 °C-heated sample: complete single-pulse and 7Li[1H] CP-MAS spectra.
Li3PS4-100 °C. The central band area of the 31P SPEMAS spectrum of Li3PS4-100 °C is displayed in Figure 7. It consists of a broad Gaussian signal, indicating a distribution of environments as in an amorphous phase. The xLi2S-(1-x)P2S5 system is well-known to yield glasses over a wide range of composition that have been widely studied and characterized by NMR.7 Assuming the Li:P ratio had not changed during the heating process, the phase was expected to consist of monomeric PS43− units, as in γ-Li3PS4, and thus the chemical shift was expected in the vicinity of 88.2 ppm. Surprisingly, the measured chemical shift of the glassy phase is actually much lower (δiso = 83.9 ppm). In previous studies of lithium oxysulfide-containing glasses and ceramics,8 31P NMR peaks resonating at 84 ppm were assigned to a combination of monomeric PS43− and PS3O3− units. If such an assignment is 3561
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Figure 9. Single-pulse 6Li MAS NMR (20 kHz) spectrum of the 140 °C-treated sample.
Figure 7. P MAS NMR (4 kHz) of the 100 °C-heated sample: single-pulse spectra and its simulation in the centerband region. 31
glassy phase and β-Li3PS4. Through in situ high-temperature XRD structural characterization of β-Li3PS4, three crystallographically distinct Li sites were identified: 2 tetrahedral and 1 octahedral.5 However, due to fast ionic motion, intersite exchange occurs at room temperature, and the NMR experiment detects only an average signal. Metastable Nature of Amorphous Phase. Figure 10 shows the 31P spectra of the 100 °C- and 140 °C samples
made in the present study, it would imply a sulfur−oxygen exchange during the heating process and further imply that some THF molecules had decomposed during the solvent removal process. This point was confirmed when the Li3PS4140 °C sample was extracted with toluene. A trace amount of organic extraction (
