Synthesis of sulfide solid electrolytes from Li₂S and P₂S₅ in anisole

Abstract

We report the liquid-phase synthesis of sulfide solid electrolytes from Li₂S and P₂S₅ using anisole at 200–300 °C under microwave irradiation, in which β-Li₃PS₄ and Li₇P₃S₁₁ were directly precipitated in anisole in 30 min. Anisole afforded reasonable reactivity toward Li₂S and P₂S₅ to form β-Li₃PS₄ and Li₇P₃S₁₁ powders at 200–300 °C. Moreover, a Li₆PS₅Cl precursor solution was synthesized from a Li₃PS₄–anisole suspension by adding ethanol, Li₂S, and LiCl. The proposed synthesis using anisole is advantageous as a simple, short-time process and would be applicable for the production of all-solid-state batteries.

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1. Introduction

The development of next-generation Li-ion secondary batteries is desired in various fields, including mobile phones, electric vehicles, and power storage systems.^1–4 One urgent target for next-generation of Li-ion secondary batteries is to improve their safety. Current Li-ion secondary batteries use organic electrolytes, which involve the risk of leakage and flammability. For this reason, all-solid-state lithium batteries, in which liquid electrolytes are replaced with flame-retardant inorganic solid electrolytes, have drawn considerable attention.

Sulfide solid electrolytes are attractive because of their high ionic conductivity, which in some cases is comparable to that in liquid electrolytes.^5–7 In addition, the good ductility of sulfide solid electrolytes is advantageous for producing all-solid-state batteries because the resistance between grain boundaries can be greatly reduced by only cold pressing.⁸ Among sulfide solid electrolytes, Li₂S–P₂S₅ systems with different structures and compositions have been reported with a conductivity ranging between 10⁻⁶ and 10⁻² S cm⁻¹.^9–11 For example, α-, β-, and γ-Li₃PS₄ electrolytes each comprise different arrangements of PS₄³⁻.¹² Li₇P₃S₁₁ is composed of Li⁺, PS₄³⁻ and P₂S₇⁴⁻, and shows a higher ionic conductivity (1.7 × 10⁻² S cm⁻¹) than β-Li₃PS₄ (3.3 × 10⁻⁴ S cm⁻¹).^6,13 In addition to a simple Li₂S–P₂S₅ system, Li₂S–P₂S₅–LiX (X: Cl, Br, and I) is a well-known system for producing sulfide electrolytes with a high ionic conductivity (∼10⁻³ S cm⁻¹), such as Li₆PS₅X.^14,15

Liquid-phase synthesis is a promising approach for producing sulfide electrolytes because it has the potential for large-scale synthesis and production of composite electrodes.^16–19 β-Li₃PS₄ and Li₇P₃S₁₁ are synthesized in two steps: formation of Li₃PS₄–solvent complexes from Li₂S and P₂S₅ precursors in a solvent; and their decomposition via post heat treatment in an inert atmosphere.^13,20–28 Synthesis of the complexes generally takes more than several hours, and thus a number of studies have focused on shortening the synthesis time of Li₃PS₄–solvent complexes by ultrasonic and microwave irradiation.^24,25,29 Li₆PS₅X halide-containing argyrodite can be synthesized by adding ethanol to dissolve all ion species, followed by heat treatment to remove the ethanol.²⁹

In liquid phase synthesis, selection of solvents is a key issue. Synthesis has been reported with solvents such as acetonitrile (ACN),^{22,24,25,27,28,30,31} THF^20,25,32,33 and ethyl propionate.^21,29 In the present study, we focused on anisole as a solvent for the synthesis of Li₂S–P₂S₅ based solid electrolytes. There are four reasons for our selection. First, anisole is an aprotic solvent. When protons are present in the solvent, there is a risk of toxic hydrogen sulfide generation through the exchange reaction between Li⁺ in Li₂S and H⁺ in the solvent. Second, P₂S₅ is easily dissolved in anisole.³⁴ It can be expected that the reaction will proceed uniformly and quickly in the liquid phase. Third, the electron-donating ability, which can be quantified using the donor number, is suitable for the synthesis reaction. While a donor number of 0, such as for toluene, does not advance the reaction for the synthesis of electrolytes, a number of 14 or above, such as for acetonitrile and ethanol, decomposes the solid electrolyte.³⁵ The donor number for anisole is 9,³⁶ which is suitable for the synthesis reaction without decomposing the synthesized electrolytes.³⁵ Fourth, anisole is a solvent that dissolves rubber-based binders.³⁵ A suspension of the electrode composite, in which the binder is dissolved, can be applied. A sheet-type all-solid-state battery with good uniformity can be constructed. Thus, the proposed liquid-phase synthesis will be easy to apply in the future fabrication of electrode composites.

Here, we proposed a new approach to synthesize various sulfide solid electrolytes using anisole and microwave irradiation. By irradiating with microwaves, the suspension of anisole and sulfide electrolytes was directly obtained from Li₂S and P₂S₅ in 30 min.

2. Experimental

2.1 Synthesis of the solid electrolyte

Fig. 1 shows the overall synthesis scheme. For the synthesis of β-Li₃PS₄ and Li₇P₃S_11, the starting materials were Li₂S (Mitsuwa Chemical, 99.9%) and P₂S₅ (Aldrich, 99%), and anisole (Fujifilm Wako Pure Chemical) was used as a solvent. After heating by microwave irradiation, a white suspension was obtained. The suspension was centrifuged, and the obtained precipitates were evaluated. The solid electrolyte powders were obtained by drying under vacuum followed by heat-treatment. For Li₆PS₅Cl, Li₂S, LiCl (Aldrich, 99.9%) and ethanol (Fujifilm Wako Pure Chemical) were added to the suspension, producing a transparent solution. This solution was dried under vacuum and powders were sintered. Details of reagent and solvent amounts are summarized in the supplementary information (Table S1).†

Fig. 1 Synthesis process and obtained suspension, solution and powders. Precipitates from suspension, vacuumed powders and sintered Li₆PS₅Cl were characterized.

2.1.1. Synthesis of β-Li₃PS₄. Anisole was kept in bottles with molecular sieves to remove water. Pristine Li₂S and pulverized Li₂S by planetary ball milling in a zirconium jar at 500 rpm for 12 hours were used. This ball-milled Li₂S was used only for β-Li₃PS₄ composition after heat treatment in order to examine the effect of the size of Li₂S particles on residual Li₂S (shown in Fig. 5). These samples were treated under an Ar atmosphere to avoid moisture.

Powders of Li₂S (Mitsuwa Chemical, 99.9%) and P₂S₅ (Aldrich, 99%) at a molar ratio of Li₂S : P₂S₅ = 75 : 25 were mixed in a mortar and pestle. The powders and anisole were placed in a 20 mL borosilicate microwave vial (Anton Paar). Microwave irradiation (2.45 GHz) was applied to the mixture using a microwave reactor (Monowave 400, Anton Paar). The microwave output power was set to 100 W, and an infrared sensor was used to control the temperature. The microwave irradiation was applied until the temperature reached 180–300 °C in 2–7 min (stirring rate was 1200 rpm). Liquid-phase synthesis above the boiling point (anisole: 153.8 °C) was performed using a closed vial. After heating to the target temperature, the temperature was maintained for 20 min by adjusting the microwave output. A portion of the suspension was centrifuged, and the precipitates were evaluated. The obtained suspension was dried at 150 °C for 3 h under vacuum to remove the solvent, and the powder was subsequently heat-treated at 200 °C for 1 h.

2.1.2. Synthesis of Li₇P₃S₁₁. Powders of Li₂S without ball milling and P₂S₅ at a molar ratio of Li₂S : P₂S₅ = 70 : 30 were mixed. The powder and anhydrous anisole were heated to 260 °C by microwave irradiation and dried under vacuum under the same conditions as for β-Li₃PS₄. The powder was heat-treated at 300 °C for 1 h.

2.1.3. Synthesis of argyrodite, Li₆PS₅Cl. Powders of Li₂S without ball milling and P₂S₅ at a molar ratio of Li₂S : P₂S₅ = 75 : 25 were mixed. The powder and anhydrous anisole were heated to 220 °C by microwave irradiation under the same conditions as for β-Li₃PS₄. Then, Li₂S, LiCl, and super anhydrous ethanol were mixed in the obtained suspension (total molar ratio Li₂S : P₂S₅ : LiCl = 5 : 1 : 2). This addition changed the suspension to a transparent solution. The solution was dried at 180 °C for 4 h under vacuum to remove the solvent to produce the sulfide electrolyte powder. A portion of the electrolyte powder was pressed under 360 MPa and sintered at 550 °C for 7 h in a sealed glass tube.

2.2 Characterization

X-ray diffraction (XRD) patterns were measured using an X-ray diffractometer (Miniflex 600, Rigaku). Diffraction data were collected in the 2θ range between 10° and 40° at a step size of 0.02°.

Raman spectra were measured using a Raman spectrometer (XploRA PLUS, Horiba Scientific) to identify structural units of the solid electrolyte samples. The excitation and intensity of the laser beam were 532 nm and 17 mW, respectively. The ionic conductivity of the pelletized samples was evaluated by electrochemical impedance spectroscopy (EIS). The solid electrolyte powders (30 or 80 mg) were pressed under 360 MPa (at room temperature) and two stainless steel (SS) disks were used as the current collectors. EIS was performed using an impedance analyzer (SI 1260, Solartron) in the frequency range of 0.1 Hz to 1 MHz at an amplitude of 10–30 mV.

3. Results and discussion

3.1 Reaction requirements under microwave irradiation

To examine which chemical species absorb microwaves, we investigated the change in the temperature of the starting materials under microwave irradiation. Fig. 2a shows the temperature change during microwave irradiation. The temperature of all the samples, including anisole without Li₂S and P₂S₅, increased under microwave irradiation, indicating that anisole absorbs microwaves. While the temperature increase in anisole with Li₂S was comparable to that of anisole alone, the temperature increase in anisole with P₂S₅ was enhanced. Furthermore, the sample with both Li₂S and P₂S₅ in anisole showed the highest temperature. The temperature increase due to microwave irradiation was also confirmed in ACN and THF (Fig. S1a and b†), which have been reported as useful solvents in the liquid-phase synthesis of Li₂S–P₂S₅ electrolytes. In contrast, no temperature difference was observed in toluene (Fig. 2b). The number of donors in toluene is 0, and the synthesis reaction cannot proceed in toluene.³⁵ Even though microwave absorption is also affected by the dielectric constant of each solvent, the product of the chemical reaction between Li₂S and P₂S₅ would further absorb microwaves.

Fig. 2 (a) The temperature change in anisole during microwave irradiation and the Raman spectrum after microwave irradiation (anisole + Li₂S + P₂S₅). (b) Temperature change in toluene during microwave irradiation. The microwave output power was 100 W.

The inset of Fig. 2a shows the Raman spectra of the precipitate after microwave irradiation of Li₂S and P₂S₅ in the anisole suspension and subsequent centrifugation at 8000 rpm for 10 min. Two Raman bands were centered at 419 and 405 cm⁻¹, attributed to PS₄³⁻ and P₂S₇⁴⁻ units,³⁷ and the precipitates were β-Li₃PS₄ or Li₇P₃S₁₁, as described in the next section. These ion species can enhance the temperature by Joule heating under microwave irradiation, as suggested earlier. Furthermore, we examined the reactivity between Li₂S and Lawesson's reagents, which have similar components to P₂S₄–anisole (Scheme 1) and are soluble in anisole. In a similar experiment heated at 300 °C, the only product was Li₂S (Fig. S2†).

Scheme 1 Structural formulae of anisole, Lawesson's reagent and toluene.

To investigate how microwave irradiation affects the products, the same synthesis was performed using a SiC ampule instead of a glass ampule. Because the SiC vial absorbs almost all the microwaves to generate Joule heat, the microwave cannot reach inside the sample. Thus, the reactions proceeded only by the thermal effect. Li₂S and P₂S₅ prepared at a molar ratio of 75 : 25 and anisole as a solvent were added to a 6 mL SiC vial, and microwave irradiation was performed at an output of 100 W for 4 min. No significant difference was observed in the Raman spectra of the products synthesized with glass and SiC (Fig. S3†). Given that the P–S unit was confirmed, as shown in Fig. 2a, it can be seen that Li₂S and P₂S₅ reacted even when a SiC vial was used. Therefore, the reaction of Li₂S and P₂S₅ would dominantly proceed by the thermal effect, and microwave irradiation was effective for quick heating.

3.2 Synthesis of solid electrolytes: suspension and solution

Fig. 3a shows the XRD patterns of the precipitates obtained by microwave irradiation at different temperatures. The precipitates were collected by centrifugation; thus, no further heat treatment was performed after microwave irradiation. The XRD pattern of the sample heated at 100 °C exhibited peaks assigned to Li₂S as well as additional unknown XRD peaks. The XRD pattern of the sample heated at 180 °C showed Li₂S and β-Li₃PS₄ peaks. The temperature range between 200 and 300 °C showed β-Li₃PS₄ as the main phase, while Li₇P₃S₁₁ was detected as a minor phase between 240 and 280 °C. The increase in the ratio of starting materials, Li₂S and P₂S₅, to anisole heated at 220 °C increased the amount of Li₇P₃S₁₁ and Li₂S impurities (Fig. S4†).

Fig. 3 (a) XRD patterns and (b) Raman spectra of the precipitate (Li₂S : P₂S₅ = 75 : 25) after microwave irradiation.

Fig. 3b shows the Raman spectra of the obtained precipitates after centrifugation, corresponding to the conditions shown in Fig. 3a. The Raman spectrum of the sample heated at 100 °C exhibited a Raman band centered at 393 cm⁻¹. This band is not attributed to PS₄³⁻ (420 cm⁻¹) or P₂S₇⁴⁻ (405 cm⁻¹) units.³⁷ The Raman spectra of all samples heated above 100 °C exhibited a Raman band at approximately 420 cm⁻¹, attributed to PS₄³⁻ units, and an additional band at approximately 405 cm⁻¹, attributed to P₂S₇⁴⁻ units in the heating temperature range of 240–280 °C.

Unknown peaks in the XRD pattern and the Raman shift to 393 cm⁻¹ of the sample heated at 100 °C can be attributed to the complex of Li₃PS₄ and anisole. The formation of β-Li₃PS₄ in the samples heated at temperatures above 100 °C was confirmed by X-ray diffraction and Raman spectroscopy. The intensity of the Raman band attributed to P₂S₇⁴⁻ units increased in the heating temperature range of 240–280 °C, which explains the additional formation of Li₇P₃S₁₁, as observed by X-ray diffraction.

The Li₇P₃S₁₁ suspension was synthesized in the same way as the β-Li₃PS₄ suspension (Fig. 3) using a heating temperature of 260 °C and a stoichiometric molar ratio of Li₂S : P₂S₅ = 70 : 30. Fig. 4 shows the XRD pattern and Raman spectrum of the Li₇P₃S₁₁ suspension. The XRD pattern (Fig. 4a) exhibited the formation of the Li₇P₃S₁₁ crystal phase; only minor additional XRD peaks corresponding to Li₂S were observed. The Raman spectrum (Fig. 4b) exhibited two main bands at 420 and 405 cm⁻¹, assigned to PS₄³⁻ and P₂S₇⁴⁻ units.

Fig. 4 (a) XRD pattern and (b) Raman spectra of the solid electrolyte (Li₂S : P₂S₅ = 70 : 30). The suspension was heated at 260 °C by microwave irradiation.

Fig. 5 shows the XRD patterns of the solid electrolytes synthesized from anisole in the present study. After microwave irradiation, the solid electrolytes were dried under vacuum at 150 or 180 °C and subsequently heat-treated at a specific temperature, as described in Fig. 1. The synthesis conditions are summarized in Table S1.† β-Li₃PS₄ was synthesized from Li₂S with and without ball milling treatment. While ball-milled Li₂S produced single-phase β-Li₃PS₄, Li₂S without ball milling treatment brought about β-Li₃PS₄ with Li₂S impurity. Because ball milling treatment would decrease the particle size of Li₂S, smaller particles with a larger surface area enhance the reaction between Li₂S and P₂S₅ in anisole. Li₇P₃S₁₁ was precipitated as a major phase with a smaller amount of Li₂S. Li₆PS₅Cl was obtained as a single phase and sintering at 550 °C enhanced the crystallinity. Various sulfide solid electrolytes can be synthesized by a simple process using anisole for liquid-phase synthesis.

Fig. 5 XRD patterns of the solid electrolytes after heat treatment in Ar or a vacuum. BM: ball-milled Li₂S.

Fig. 6 shows the Nyquist plots and Table 1 shows the ionic conductivity at room temperature of the prepared solid electrolytes (β-Li₃PS₄, Li₇P₃S₁₁, and Li₆PS₅Cl). The ionic conductivities at 180–300 °C were in the range of 0.05–0.13 mS cm⁻¹. Even though further heat-treatment was necessary, the Li₆PS₅Cl pellet heated at 550 °C reached a higher conductivity of 2.1 mS cm⁻¹.¹⁸ Furthermore, we proved the lithium-ion conducting nature of the synthesized Li₇P₃S₁₁ electrolyte using the discharge–charge properties of the all-solid-state battery with the cathode composite composed of this synthesized Li₇P₃S₁₁ (Fig. S5†).

Fig. 6 Room-temperature Nyquist plot of the prepared solid electrolyte after heat treatment in Ar or a vacuum.

Table 1 Ionic conductivity of synthesized solid electrolyte at room temperature

Product	Microwave/°C	Heat treatment/°C	σ/mS cm^−1
β-Li3PS4	220	200	0.087
β-Li3PS4 (BM)	220	200	0.051
Li7P3S11	260	300	0.13
Li6PS5Cl	220	180	0.070
Li6PS5Cl	220	550	2.1

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Compared with those of previous reports,^20,24,33 the conductivities shown in Table 1 tend to be slightly lower. The conductivity may be improved by completely removing any residual water, anisole, and hydrogen in the final product considering that Li₆PS₅Cl electrolyte heated at 550 °C showed one-order higher conductivity than others. A recent study suggests that residual Li₂S which cannot be detected by XRD decreases the ionic conductivity of synthesized electrolytes.³⁸ Thus, further improvement of the conductivity may be achieved by completing the synthesis reaction.

As P₂S₅ dissolves in anisole at high temperatures (Fig. S6†), it is considered that dissolved P₂S₅ is involved in the reaction. In contrast, β-Li₃PS₄ was not obtained from Li₂S and Lawesson's reagent (which can be synthesized from P₂S₅ and anisole) instead of P₂S₅, indicating that the reaction intermediate should not be Lawesson's reagent. Because unreacted Li₂S remained after the reaction to produce β-Li₃PS₄ or Li₇P₃S₁₁ under microwave irradiation (Fig. 3 and 4), the reaction proceeded from the interface between the dissolved P₂S₅ species (but not Lawesson's reagent) and Li₂S particles. This is supported by an enhanced reaction by using ball-milled Li₂S (Fig. 5).

In the present study, we synthesized sulfide-based solid electrolytes by taking advantage of anisole with moderate nucleophilic aggression, having a donor number of 9. Because a reaction did not occur in toluene, having a donor number of 0, as a solvent,³⁵ it is clear that the solvent is involved in the reaction of Li₂S and P₂S₅. The synthesis reaction proceeds in ACN (donor number is 14) to form β-Li₃PS₄ at 200 °C, but further heat treatment above 220 °C resulted in the decomposition of β-Li₃PS₄ electrolytes (Fig. S7†). Therefore, anisole has the advantage of high-temperature and short-time synthesis, where the reaction kinetics increase without an unfavorable decomposition reaction of β-Li₃PS₄.

4. Conclusions

We succeeded in synthesizing sulfide solid electrolytes, Li₃PS₄, Li₇P₃S₁₁, and Li₆PS₅Cl by rapid heating under microwave irradiation using anisole as a solvent. The advantage of this process is the direct precipitation of solid electrolytes in a solvent, in contrast to other techniques that require a post-heating process to decompose complexes composed of Li₃PS₄ and solvents under an inert atmosphere. Moderate reactivity in anisole allows the synthesis of solid electrolytes at 200–300 °C in a short time. This proposed approach can simplify and shorten the synthesis process, and is promising for producing the components of all-solid-state batteries. These features are advantageous not only for industrial scale-up but also for producing composite materials with electrode particles and binders. Research on the production of sheet-type batteries using this approach is currently ongoing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was partially supported by KAKENHI Grant Number JP19H04682. This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ta08658d

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