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Article

Preparation of Li3PS4–Li3PO4 Solid Electrolytes by Liquid-Phase Shaking for All-Solid-State Batteries

by
Nguyen H. H. Phuc
*,
Takaki Maeda
,
Tokoharu Yamamoto
,
Hiroyuki Muto
and
Atsunori Matsuda
*
Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi, Aichi 441-8580, Japan
*
Authors to whom correspondence should be addressed.
Electron. Mater. 2021, 2(1), 39-48; https://doi.org/10.3390/electronicmat2010004
Submission received: 2 February 2021 / Revised: 4 March 2021 / Accepted: 6 March 2021 / Published: 12 March 2021
(This article belongs to the Special Issue Feature Papers of Electronic Materials)
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Abstract

:
A solid solution of a 100Li3PS4·xLi3PO4 solid electrolyte was easily prepared by liquid-phase synthesis. Instead of the conventional solid-state synthesis methods, ethyl propionate was used as the reaction medium. The initial stage of the reaction among Li2S, P2S5 and Li3PO4 was proved by ultraviolet-visible spectroscopy. The powder X-ray diffraction (XRD) results showed that the solid solution was formed up to x = 6. At x = 20, XRD peaks of Li3PO4 were detected in the prepared sample after heat treatment at 170 °C. However, the samples obtained at room temperature showed no evidence of Li3PO4 remaining for x = 20. Solid phosphorus-31 magic angle spinning nuclear magnetic resonance spectroscopy results proved the formation of a POS33− unit in the sample with x = 6. Improvements of ionic conductivity at room temperature and activation energy were obtained with the formation of the solid solution. The sample with x = 6 exhibited a better stability against Li metal than that with x = 0. The all-solid-state half-cell employing the sample with x = 6 at the positive electrode exhibited a better charge–discharge capacity than that employing the sample with x = 0.

Graphical Abstract

1. Introduction

Li3PS4 is one of the most important substances in the fields of sulfide-based solid electrolytes and all-solid-state batteries because of its ease of preparation and application. In general, Li3PS4 has three different crystal structures and an amorphous (glass) state [1,2]. The ionic conductivity and air stability of Li3PS4 can also be easily tuned by the addition of different materials, i.e., oxides, nitrides and halogens. The addition of an appropriate amount of LiI into the Li3PS4 structure is known to boost its ionic conductivity from approximately 5.0 × 10−4 S cm−1 to 6.5 × 10−4 S cm−1 [3,4]. Ambient air stability or suppression of H2S evolution is also improved with the existence of LiI, CuO or FeS in a Li3PS4 glassy electrolyte [5,6].
Among the three crystal structures of α, β and γ for Li3PS4, the β phase has been reported to be only stable at temperatures higher than 160 °C [1]. Owing to its high ionic conductivity, the stabilization of the β phase at room temperature has been attracting efforts from researchers for years. One of the first attempts was made by Takada et al., who employed a melt quenching method to freeze this high-temperature phase and made it stable at room temperature [7]. The β phase was found to be easily synthesized at room temperature by the reaction between Li2S and P2S5 in tetrahydrofuran (THF) [8]. The Li3PS4 glass-ceramic crystallized from 75Li2S·25P2S5 glass also showed the structure of the β phase [9,10].
The synthesis of Li3PS4 using liquid-phase synthesis with organic solvents as reaction media has been intensively investigated recently as an alternate chemical route for the preparation of sulfide-based solid electrolytes rather than the conventional solid state reaction [11,12,13,14,15,16]. In most of the reports, the effect of the solvents on the formation of Li3PS4 was of primary interest [17,18,19,20,21]. Many other types of organic solvents have been reported to be effective in promoting the reaction between Li2S and P2S5, including tetrahydrofuran (THF), acetonitrile, ethyl acetate and ethyl propionate (EP). Furthermore, Li3PS4, Li7P3S11 and argyrodite-type Li7−yPS6−yX (X = Cl, Br and I) have also been reported to be obtained by the liquid-phase route. The formation of Li7P3S11 was reported to be strongly dependent on the solvent structure. Dimethoxyethane and acetonitrile were the only two solvents that could promote the formation of Li7P3S11 [22,23]. The preparation of argyrodite-type solid electrolytes usually requires a two-step process: Li3PS4 is first prepared by either a solid-state or liquid reaction, which is followed by the use of ethanol to dissolve all the components of the desired solid electrolytes [24,25,26,27].
The number of methods to improve the ionic conductivity of solid electrolytes prepared by the liquid-phase synthesis route is very limited; these methods include the addition of LiX (X = Cl, Br and I) and a thermal treatment procedure in the case of argyrodite-type solid electrolytes [28,29,30,31,32]. Doping Li3PS4 with oxygen (75Li2S·23P2S5·2P2O5) resulted in the increase of ionic conductivity at room temperature (RT) from 1.83 × 10−4 Scm−1 to 2.53 × 10−4 Scm−1 [33]. However, Li3PO4 has been employed as a common additive to improve the ionic conductivity and electrochemical property of sulfide-based solid electrolytes [7,34,35,36]. In this study, 100Li3PS4·xLi3PO4 solid electrolytes were prepared by liquid-phase synthesis using ethyl propionate (EP). EP was employed in this study since it was suitable for Li3PS4 and Li3PS4—LiI solid electrolytes preparation [19,30]. Ultraviolet-visible (UV-Vis) spectroscopy evidenced the solvation of P2S5 but not of Li3PO4 in EP. The reaction between P2S5 and Li3PO4 in EP was also detected by UV-Vis spectroscopy. Solid phosphorus-31 magic angle spinning nuclear magnetic resonance (31P MAS NMR) also revealed the formation of POS33−, which confirmed the results from UV-Vis spectroscopy. An increase in ionic conductivity and decrease in activation energy were achieved by the addition of Li3PO4. An improvement of the stability against Li metal was also observed using the direct current polarization test. Better charge–discharge capacity of the all-solid-state (ASS) half-cell was also observed employing the sample with x = 6 compared with that employing the sample with x = 0.

2. Materials and Methods

Li2S (99.9%, Mitsuwa, Torrance, CA, USA), P2S5 (99%, Merck, Kenilworth, NJ, USA) and Li3PO4 (99.99%, Aldrich, St. Louis, MO, USA) were purchased and used without any further treatment process. Super dehydrated ethyl propionate (EP) (99.5%, Aldrich) was dried again using a 3-Å molecular sieve for at least 24 h prior to usage. The positive electrode material, LiNbO3-coated LiNi1/3Mn1/3Co1/3O2, was kindly donated by Toda Kogyo.
Li2S, P2S5, Li3PO4 and P2S5–Li3PO4 were put into a screw vial followed by the addition of EP. The three bins were sealed in an Ar-filled glovebox and then ultrasonically treated for 30 min. The as-prepared samples were then filtered for UV-Vis measurement.
100Li3PS4·xLi3PO4 solid electrolytes in this study were prepared from raw materials using the liquid-phase shaking method that has previously been reported [19]. Li2S, P2S5 and Li3PO4 to form 1 g of 100Li3PS4–xLi3PO4 were weighted and put into a plastic centrifuge tube together with zirconia balls (4 mm, 150 balls). The tube was filled with 20 mL of EP and then shaken at 1500 rpm for 6 h with amplitude of 1 cm. The received suspension was centrifuged and decanted to receive the precipitated pastes. The thus obtained samples were then dried under vacuum at room temperature and heat treated at 170 °C for 2 h.
The crystal structures of the samples were examined by X-ray diffraction (XRD; Ultima IV, Rigaku, Tokyo, Japan) using an air-tight holder to protect the samples from air humidity. Local structure of the prepared solid electrolytes was investigated by solid-state 31P MAS NMR spectroscopy (Avance III 400, Bruker, Tokyo, Japan) using the typical single pulse sequence with a spinning rate of 5 kHz. UV-Vis spectra were recorded using a V-670 spectrophotometer (Jasco, Tokyo, Japan).
Alternating-current impedance spectroscopy (SI 1260, Solartron, Tokyo, Japan) was employed to measure the temperature dependence of the total conductivity of the prepared samples. Powder samples were placed in an PEEK holder (10 mm in diameter) with two SUS electrodes (Misumi) then pressed at 550 MPa (RT). The cell was placed under an Ar stream in a glass tube to measure the temperature dependence of resistivity. The temperature was gradually increased from room temperature to 210 °C and held at each temperature for 1 h prior to the impedance measurement.
The pelletized sample (diameter of 10 mm) with lithium metal sheets attached on both sides to serve as nonblocking electrodes were made to study the stability of solid electrolytes against lithium metal by DC polarization examination. Current collectors were Au-sputtered SUS rods. The cells were then cycled at ±0.05 mA cm−2 under a dry Ar atmosphere using a charge-discharge device (NAGANO BST-2004H, Nagano, Japan).
An ASS half-cell was fabricated to investigate the electrochemical performance of the prepared solid electrolytes (SE). The prepared SE was employed as a separator and an In–Li alloy was used as the negative electrode. The positive electrode composites (PE) composed of LiNbO3-coated LiNi1/3Mn1/3Co1/3O2 (NMC), solid electrolytes and acetylene black (weight ratio of 70:30:3) were prepared using agate mortar and pestle. The bilayer pellets (ϕ10 mm), composed of PE and separator layers (80 mg), were prepared by uniaxial pressing at 550 MPa. Indium foil was then attached onto the pellets by pressing at 200 MPa. The loading of PE in each cell was about 15 mg, in according to loading of NMC of 11–13 mg. Two stainless steel rods served as current collectors. Cyclic voltammetry (at RT) at a scan rate of 0.1 mV s−1 (SI1287 potentiostat, Solartron, Tokyo, Japan) was employed to study the electrochemical compatibility of the obtained SEs. The cells were cycled using a charge–discharge device (NAGANO BST-2004H, Nagano, Japan) in a dry Ar atmosphere. The cutoff voltages were in the range of 3.7–2.0 V vs. Li–In at 0.1 C. All the cells were placed in an insulation box that was kept at 30 ± 2 °C for 4 h prior to being tested. Cell preparation was conducted inside an Ar-filled glove-box with O2 ≤ 1.0 ppm and H2O ≤ 0.1 ppm.

3. Results

Figure 1 shows the optical images of the Li2S, Li3PO4, P2S5 and Li3PO4–P2S5 solutions in EP solvent and UV-Vis spectra of the Li3PO4, P2S5 and Li3PO4–P2S5 solutions in EP solvent. The spectrum of the Li3PO4 solution exhibited no absorbance peak in the measurement range from 190 to 1200 nm. This result illustrated that Li3PO4 was unable to be dissolved in the EP solvent. The P2S5 solution had one sharp peak centered at approximately 264 nm, which indicated the dissolution of P2S5 in EP. The spectrum of Li3PO4–P2S5 in EP exhibited a large shoulder shape that was then deconvoluted into three other peaks centered at 264, 280 and 359 nm (small inset in Figure 1). The peak at 264 nm indicated the existence of P2S5 in the solution. The other two peaks originated from neither Li3PO4 nor P2S5, so they were proposed to arise from the products of the reaction between P2S5 and Li3PO4. The UV-Vis spectra proved that P2S5 dissolution in EP was the initial step for the reaction with Li3PO4 in EP solvent. To the best of our knowledge, this is the first time that such a reaction has been reported and evidenced using simple UV-Vis spectroscopy.
Figure 2 shows the XRD patterns of 100Li3PS4·xLi3PO4 obtained after solvent elimination at room temperature (Figure 2a) and at 170 °C for 2 h (Figure 2b). The powder samples obtained at room temperature exhibited similar XRD patterns without any peak from the starting materials with a value of x of up to 20. The pattern resembled that of a cocrystal of Li3PS4 and EP that has been reported previously [19]. After solvent elimination at 170 °C, the XRD patterns of the retrieved samples exhibited the feature of β-Li3PS4, and signals of Li3PO4 (LPO) were also detected for the sample with x = 20 [8,18]. The XRD results proved that Li3PO4 could be incorporated into Li3PS4 up to at least 6 mol% by the proposed liquid-phase synthesis. As mentioned above, a solid solution between Li3PS4 and Li3PO4 was successfully prepared by the melt quenching method with x up to 50 but the structure of Li3PS4 was changed upon interaction with Li3PO4 [7]. Li3PO4 was doped into 0.6Li2S·0.4SiS2 glass and glass-ceramic. It was observed that the 0.6Li2S·0.4SiS2 glass could dissolve 20 mol% of Li3PO4 but its crystal structure was detected by XRD in the 0.6Li2S·0.4SiS2 glass-ceramic [37]. Li3PO4 was also doped into the 70Li2S·30P2S5 glass-ceramic and was found to change the crystal structure from Li7P3S11 to thio-LISICON II with only 5 mol% of dopant [38].
Figure 3 shows the solid 31P MAS NMR results of Li3PO4 and the samples with x = 0 and 6 (Figure 3a) and their deconvolution results (Figure 3b,c). The sample with x = 6 had a small peak located at 8.0 ppm that indicated the existence of Li3PO4, which resembled that of pristine Li3PO4. The large shoulder observed for the sample with x = 0 was composed of three other peaks located at 81.5, 83.6 and 85.7 ppm (Figure 3b). The peak located at 81.5 ppm was assigned to amorphous PS43− [39,40]. The peaks at 83.6 and 85.7 were from PS43− in β- and γ-Li3PS4, respectively [40]. Surprisingly, the area fraction of the amorphous phase was the highest, approximately 78%, while that of the β phase was just 15%. The main peak in the NMR spectrum of the sample with x = 6 was also decomposed into three peaks located at 82.4, 83.5 and 85.8 ppm (Figure 3c). The peaks at 82.4 and 85.8 originated from amorphous and crystal PS43−. The peak at 83.5 ppm arose from both PS43− and POS33− ions [37]. In addition, the area fraction of the amorphous peak for the sample with x = 6 was 62% and that of the peak at 83.5 ppm was 32%. These values showed the reverse trend to that observed for the sample with x = 0 but the area fraction of PS43− from γ-Li3PS4 was almost unchanged. These results proved the formation of a POS33− ion and thus evidenced the reaction between P2S5 and Li3PO4 in an EP medium. The area fraction of the amorphous phase in the NMR spectra was surprisingly high in this study, which might be the reason for the low intensity of the XRD peak shown in Figure 2b.
Figure 4 shows the temperature dependence of the ionic conductivity (Figure 4a) and conductivity at room temperature and activation energy as a function of x (Figure 4b) for 100Li3PS4·xLi3PO4. The temperature dependence of the ionic conductivity showed that at a temperature higher than 60 °C, the sample with x = 0 exhibited the highest ionic conductivity but below this temperature, the sample with x = 6 had the highest ionic conductivity. Conductivity at room temperature and activation energy as a function of x showed that upon addition of Li3PO4, the ionic conductivity started to increase and reached the highest value at x = 6. The activation energy showed the reverse trend with the ionic conductivity and reached the lowest value at x = 6. For the samples with x = 0 and 6, conductivity at room temperature was 1.9 × 10−4 and 3.3 × 10−4 Scm−1, respectively, and activation energy was 42 and 27 kJ mol−1, respectively. At x values higher than 6, the ionic conductivity started to reduce together with the increase in activation energy due to the remaining of Li3PO4 in the samples. The ionic conductivity of the samples obtained in this study was higher than the reported values of Li3PS4–Li3PO4 prepared by melt-quenching method but lower than the glass-ceramic samples [7,41]. Li ion movement changing from 2D to 3D with oxygen doping was the main reason for improvement in both ionic conductivity and activation energy of β-Li3PS4 as shown by Wang et al. in their DFT calculation study [42]. Oxygen doping in both glass and glass-ceramic Li3PS4 solid electrolytes was found to be an effective method to boot their ionic conductivity by using either Li2O or P2O5 as oxygen sources [41,43].
Two symmetric cells, Li|Li3PS4|Li and Li|100Li3PS4·6Li3PO4|Li, were constructed to investigate the stability of the samples with x = 0 and 6 against Li metal using the direct current polarization test. The results are illustrated in Figure 5a. The Li|Li3PS4|Li cell had an initial voltage of approximately 32 mV while that of the Li|100Li3PS4·6Li3PO4|Li cell was approximately 18 mV because of the smaller resistivity. The Li|Li3PS4|Li cell exhibited a slight change of its voltage to approximately 35 mV after 100 h in the test condition. In contrast, the Li|100Li3PS4·6Li3PO4|Li cell exhibited a voltage profile that was nearly constant after 200 h in the working condition. These results proved that the addition of Li3PO4 was able to improve the stability of sulfide-based solid electrolytes against metallic Li. A similar enhancement of the stability against metallic Li was obtained when β-Li3PS4 was doped with Sb2O5 [44]. The improvement of the stability against metallic Li was explained by the formation of an interfacial buffer layer from the reaction between solid electrolytes and metallic Li [45].
LiNbO3-coated LiNi1/3Mn1/3Co1/3O2 instead of bare LiNi1/3Mn1/3Co1/3O2 was employed to reduce the side reaction occurring at the solid electrolyte and active material interface [45]. The initial charge capacities of both cells using the samples with x = 0 and x = 6 were 170 mAh g−1NMC, respectively, and the discharge capacities were 148 and 150 mAh g−1NMC, respectively. From the second cycle, the discharge capacity of the cell for the sample with x = 6 increased to 158 mAh g−1NMC whereas that of the sample with x = 0 reduced to 146 mAh g−1NMC. The superior discharge capacity of the cell employing the sample with x = 6, compared with that of the cell employing the sample with x = 0, remained for the investigated 20 cycles. These results proved that doping Li3PS4 with 6 mol% Li3PO4 improved the cell performance compared with the intrinsic electrolyte sample.

4. Conclusions

100Li3PS4·xLi3PO4 solid electrolytes were successfully prepared by the liquid-phase shaking method using EP as the reaction medium. The reaction between Li3PO4 and P2S5 was evidenced by UV-Vis spectroscopy, where the dissolution of P2S5 in EP was the initial step of the activation of Li3PO4. The formation of the POS33− ion was further proved by solid 31P MAS NMR. It was observed that the incorporation of Li3PO4 into Li3PS4 could improve not only the ionic conductivity and activation energy but also stability against metallic Li. The charge–discharge capacities of the all-solid-state cell employing the 100Li3PS4·6Li3PO4 solid electrolyte at the positive electrode was slightly higher than those of the cell using Li3PS4 in the positive electrode.

Author Contributions

Conceptualization, methodology, software, validation, formal analysis, N.H.H.P.; investigation, N.H.H.P., T.M. and T.Y.; writing—original draft preparation, N.H.H.P.; writing—review and editing, N.H.H.P. and A.M.; supervision, A.M. and H.M.; project administration, A.M. and H.M.; funding acquisition, A.M. and H.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Advanced Low Carbon Technology Specially Promoted Research for Innovative Next Generation Batteries (JST-ALCA-SPRING, Grant No. JPMJAL1301) program of the Japan Science and Technology Agency.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Acknowledgments

The authors appreciate R. Kato of the Cooperative Research Facility Center, Toyohashi University of Technology for his assistance in NMR measurement. We thank Edanz Group (https://en-author-services.edanzgroup.com/ac) for editing a draft of this manuscript.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. Optical photographs and UV-Vis spectra of Li3PO4, P2S5 and Li3PO4–P2S5 in EP solutions.
Figure 1. Optical photographs and UV-Vis spectra of Li3PO4, P2S5 and Li3PO4–P2S5 in EP solutions.
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Figure 2. XRD patterns of 100Li3PS4–xLi3PO4 samples dried at room temperature (RT) (a) and 170 °C (b).
Figure 2. XRD patterns of 100Li3PS4–xLi3PO4 samples dried at room temperature (RT) (a) and 170 °C (b).
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Figure 3. 31P Solid MAS NMR spectra of Li3PO4, Li3PS4 and 100Li3PS4·6Li3PO4 (a), the asterik * indicated the spinning side bands; deconvolution of main peak of Li3PS4 (b); deconvolution of main peak of 100Li3PS4·6Li3PO4 (c).
Figure 3. 31P Solid MAS NMR spectra of Li3PO4, Li3PS4 and 100Li3PS4·6Li3PO4 (a), the asterik * indicated the spinning side bands; deconvolution of main peak of Li3PS4 (b); deconvolution of main peak of 100Li3PS4·6Li3PO4 (c).
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Figure 4. Temperature dependence of conductivity of 100Li3PS4·xLi3PO4 samples dried at 170 °C; (a) room temperature ionic conductivity and (b) activation energy as a function of composition.
Figure 4. Temperature dependence of conductivity of 100Li3PS4·xLi3PO4 samples dried at 170 °C; (a) room temperature ionic conductivity and (b) activation energy as a function of composition.
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Figure 5. DC polarization of Li|LPSO|Li cell at 0.05 mA cm−2 (a); first charge–discharge curves and cyclic properties of all-solid-state cells employing either sample with x = 0 or 6 as solid electrolytes at the positive electrodes (b); cyclic performance of all-solid-state half-cells employing samples with x = 0 and x = 6 at the positive electrodes (c).
Figure 5. DC polarization of Li|LPSO|Li cell at 0.05 mA cm−2 (a); first charge–discharge curves and cyclic properties of all-solid-state cells employing either sample with x = 0 or 6 as solid electrolytes at the positive electrodes (b); cyclic performance of all-solid-state half-cells employing samples with x = 0 and x = 6 at the positive electrodes (c).
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Phuc, N.H.H.; Maeda, T.; Yamamoto, T.; Muto, H.; Matsuda, A. Preparation of Li3PS4–Li3PO4 Solid Electrolytes by Liquid-Phase Shaking for All-Solid-State Batteries. Electron. Mater. 2021, 2, 39-48. https://doi.org/10.3390/electronicmat2010004

AMA Style

Phuc NHH, Maeda T, Yamamoto T, Muto H, Matsuda A. Preparation of Li3PS4–Li3PO4 Solid Electrolytes by Liquid-Phase Shaking for All-Solid-State Batteries. Electronic Materials. 2021; 2(1):39-48. https://doi.org/10.3390/electronicmat2010004

Chicago/Turabian Style

Phuc, Nguyen H. H., Takaki Maeda, Tokoharu Yamamoto, Hiroyuki Muto, and Atsunori Matsuda. 2021. "Preparation of Li3PS4–Li3PO4 Solid Electrolytes by Liquid-Phase Shaking for All-Solid-State Batteries" Electronic Materials 2, no. 1: 39-48. https://doi.org/10.3390/electronicmat2010004

APA Style

Phuc, N. H. H., Maeda, T., Yamamoto, T., Muto, H., & Matsuda, A. (2021). Preparation of Li3PS4–Li3PO4 Solid Electrolytes by Liquid-Phase Shaking for All-Solid-State Batteries. Electronic Materials, 2(1), 39-48. https://doi.org/10.3390/electronicmat2010004

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