Preparation of Li₂S-AlI₃-LiI Composite Solid Electrolyte and Its Application in All-Solid-State Li-S Battery

Abstract

Novel (80Li₂S − 20AlI₃)·yLiI composite solid electrolytes (y = 5, 10, 15) were prepared by mechannochemical synthesis. XRD results showed that the pattern of 80Li₂S − 20AlI₃ was similar to that of AlI₃, which means that Li₂S was dissolved in AlI₃ matrix during preparation. This structure was still maintained after LiI addition. The current measured at constant applied DC voltage indicated that (80Li₂S − 20AlI₃)·yLiI composites are intrinsically pure Li-ion conductors. The ionic conductivity at 25 °C of y = 10 was about 2.3 × 10⁻⁴ Scm⁻¹, which was about three times higher than that of y = 0. The conductivity of y = 10 increased 20 times to 2.2 × 10⁻³ Scm⁻¹ at 70 °C. These values were highest among those observed from Li₂S-based materials. It was revealed that Li-ion moves in 80Li₂S − 20AlI₃ by a hoping mechanism, while the lattice dipoles are the origin of Li-ion movement in (80Li₂S − 20AlI₃)·yLiI. The polarization measurements using Liǀ90 (80Li₂S − 20AlI₃)·10LiI ǀLi and LiǀLi₆PS₅Clǀ90 (80Li₂S − 20AlI₃)·10LiIǀLi₆PS₅ClǀLi cells proved that 90 (80Li₂S − 20AlI₃)·10LiI reacts with Li metal, but it is relatively stable at a low voltage. Sample y = 10 was also employed as a solid electrolyte in the positive electrode of a solid-state Li-S battery to study its stability in the voltage range of the positive electrode. CuS and Li_4.4Si were the electrode-active materials. The cell was cycled in CC-CV mode at 1.0 mA cm⁻² (CC) with a cut-off voltage of 1.0–2.3 V. The cell delivered a stable capacity of about 400 mAh g⁻¹_CuS after 40 cycles.

1. Introduction

Sulfide-based solid electrolytes (SEs) are one of the candidates for all-solid-state (ASS) Li-ion batteries due to their high ionic conductivity at room temperature and suitable mechanical properties [1,2,3]. Li₇P₃S₁₁, Li₁₀GeP₂S₁₂, and Li_5.5PS_4.5Cl_1.5 exhibited ionic conductivity higher than 10⁻² Scm⁻¹ at 25 °C [4,5,6]. Sulfide-based solid electrolytes can be prepared using different methods: solid-state reaction at high temperature, mechano-synthesis, and liquid phase synthesis [7,8,9]. Until now, most of the reported sulfide-based solid electrolytes have belonged to Li₂S-P₂S₅-LiX (X = halogen) groups; therefore, the search for sulfide-based electrolytes that do not contain P₂S₅ is an interesting research direction.

The glass and glass-ceramic electrolytes of the (100 − x)Li₂S-xAl₂S₃ group were synthesized using planetary ball milling and thoroughly studied [10]. Amorphous 60Li₂S·40AlS_1.5 was obtained with a milling speed of 230 rpm, while glass-ceramic containing Li₅AlS₄ crystal was directly obtained with a milling speed of 510 rpm for 2 h. The highest conductivity of 3.4 × 10⁻⁵ Scm⁻¹ at 25 °C was obtained for the 60Li₂S·40AlS_1.5 amorphous electrolytes. Li₂S-SiS₂ and Li₃PO₄-Li₂S-SiS₂ glassy SEs were also prepared by the melt-quenching method, and their ionic conductivities and electrochemical properties were investigated [11,12]. The highest conductivity, of about 7.6 × 10⁻⁴ Scm⁻¹ at 25 °C, was obtained in those systems. The solid-state cell using graphite as active material and 0.03Li₃PO₄-0.58Li₂S-0.39SiS₂ as solid electrolyte showed good cycle performance. On the other hand, Li₂S-SiS₂ amorphous materials were also prepared by mechanochemical synthesis and showed the highest conductivity of about 3.4 × 10⁻⁴ Scm⁻¹ at 25 °C [13].

Li₂S has been intensively used as positive electrode material in the Li-S battery because its usage enables the employment of graphite or silicon in the negative electrode. However, Li₂S is a natural insulator to both electrons and ions; much effort has been made to increase the electron and ionic conductivity of Li₂S. Fabricating composites with carbonaceous materials, such as graphene, carbon nanotubes, or carbon nanofibers, will increase the electronic conductivity of Li₂S [14,15,16,17]. On the other hand, many other methods for increasing the ionic conductivity of Li₂S have been reported so far. Composite nanoparticles with Li₂S as the core and Li₃PS₄ as the shell exhibit ionic conductivity of about 10⁻⁷ Scm⁻¹ at 25 °C, which is 10⁴ times higher than that of Li₂S nanoparticles [18]. The ionic conductivity of Li₂S- Li₃PS₄ reaches 10⁻⁴ Scm⁻¹ at 25 °C in another report [19]. The ionic conductivity of Li₂S-LiX solid solution (X = halogen) is about 10⁻⁶ Scm⁻¹, which is about 10² times higher than that of Li₂S [20]. Solid solutions of Li₂S and multivalence cation sulfides, Li_2-2xMg_xS (0.05 ≤ x ≤ 0.2) and Li_2-3xAl_xS (0 ≤ x ≤ 0.1667), also show improved ionic conductivity compared with Li₂S [21,22]. The ionic conductivity of 80Li₂S·20AlI₃ even reaches 6.0 × 10⁻⁵ Scm⁻¹ at 25 °C [23]. In the same study, 99.5Li₂S·0.5AlI₃ acted as both SE and active material in an all-solid-state battery. In addition, antiperovskite-like type Li₃SI had an ionic conductivity of 2.2 × 10⁻⁵ Scm⁻¹ at 25 °C and was employed as SE in the ASS Li-ion battery [24]. It should be noted that the application of Li₂S-based as SE in the ASS battery is very rare due to its low ionic conductivity.

In this study, a series of (100 − x)Li₂S-xAlI₃ and (80Li₂S − 20AlI₃)·yLiI composite solid electrolytes (y = 5, 10, 15) were prepared by mechanochemical synthesis and investigated using XRD and AC impedance spectroscopy. The ionic conductivity at 25 °C (80Li₂S − 20AlI₃)·yLiI (y = 5, 10, 15) is higher than that of 80Li₂S·20AlI₃ and Li₃SI because LiI addition reduces the resistance at the grain boundary and interface; to the best of our knowledge, such effects of LiI have not been reported before. Among them, sample (80Li₂S − 20AlI₃)·10LiI has the highest ionic conductivity, which is about 2.3 × 10⁻⁴ Scm⁻¹ at 25 °C. In addition, the complex conductivity suggested the formation of a composite consisting of 80Li₂S − 20AlI₃ and LiI rather than a solid solution due to the existence of two slopes in the low-frequency region. The use of complex conductivity to differentiate solid solutions from composites has also not been noted before. The electronic conductivity of (80Li₂S − 20AlI₃)·10LiI is about 1.1 × 10⁻⁸ Scm⁻¹; therefore, (80Li₂S − 20AlI₃)·10LiI is a pure Li-ion conductor. The solid-state cell, which employs CuS as active material and (80Li₂S − 20AlI₃)·10LiI as solid electrolyte, delivers a stable capacity after 40 cycles.

2. Experimental

Li₂S (99.9%), LiCl (99.9%), AlI₃ (99.9%), Si (99.99%), Li metal (99.99%), CuS (99.9%), and LiI (99.9%) were purchased from Sigma. P₂S₅ (99%) was purchased from Merck Group. All chemicals were used without purification.

(100 − x)Li₂S-xAlI₃ and (80Li₂S − 20AlI₃)·yLiI composite solid electrolytes (y = 5, 10, 15) were prepared via planetary ball milling using the same conditions as reported previously [23]. For a typical batch, Li₂S, AlI₃, and LiI were manually mixed for 10 min prior to introduction into 45 mL zirconia pots with zirconia balls (10 mm, 15 balls). The pots were then rotated at 500 rpm for 12 h using a Pulverisette 7 (Fritsch). The obtained samples were recovered and used without any further heat treatment. Li_4.4Si was prepared using the procedure reported by Tamori et al. [25]. A total of 0.7814 g Li and 0.7186 g Si (Li:Si = 4.4:1.0 mole ratio) was manually mixed using an agate mortar for about 15 min; the obtained mixture was then introduced into 45 mL zirconia pots with zirconia balls (4 mm, 10 balls). The pots were then rotated at 200 rpm. For one cycle, 10 min milling time and 5 min resting time was employed to avoid the sample from temperature increasing. Li_4.4Si was recovered after 50 h of milling time. Li₆PS₅Cl was prepared using the same method. In a typical batch, 2 g of Li₂S, P₂S₅, and LiCl with a stoichiometric ratio of 5:1:2 was employed. The raw materials were manually mixed for 10 min prior to introduction into 45 mL zirconia pots with zirconia balls (10 mm, 10 balls). A rotation speed of 600 rpm and a rotation time of 20 h were employed.

XRD patterns were recorded using Bruker X8 with a CuK_α radiation. The samples were covered with amorphous Kapton film because of their hygroscopic properties.

The total resistivity of the prepared samples was investigated using AC impedance spectroscopy, as reported previously [26]. DC conduction was recorded using stainless steel (SUS) rods as blocking electrodes and lithium metal sheets as non-blocking electrodes. Prior to the measurements, the sample (80Li₂S − 20AlI₃)·10LiI was made into pellets of about 10 mm in diameter (550 MPa, at RT). Since the sample (80Li₂S − 20AlI₃)·10LiI continuously reacts with Li metal, a thin layer of Li₆PS₅Cl solid electrolyte was introduced to the cell to prevent (80Li₂S − 20AlI₃)·10LiI from direct contact with Li metal. Two SUS rods were employed as current collectors. The currents were measured using a potentiostat (PGSTAT302N, Autolab, Herisau, Switzerland) with applied voltages of 0.2–0.7 V (DC) for 60 min.

DC polarization curves were recorded using the cell LiǀLi₆PS₅Clǀ(80Li₂S − 20AlI₃)·10LiIǀLi₆PS₅ClǀLi cycling at ±0.05, ±0.1, and ±0.2 mA cm⁻¹ [27].

Two-step planetary ball milling was used to prepare the composite cathode. In total, 0.3 g of CuS and Ketjen Black (weight ratio of 4:1) were mixed in zirconia pots using 30 g of zirconia balls (4 mm) at 410 rpm for 10 h. Then, 0.2 g of (80Li₂S − 20AlI₃)·10LiI was added, and the positive electrode composites were obtained after 2 h mixing at 300 rpm. The ASS Li-S cell was constructed as previously reported using the prepared electrode composite, Li₆PS₅Cl, and Li_4.4Si [26]. The cells were cycles using either CC or CC-CV mode to study the stability of (80Li₂S − 20AlI₃)·10LiI at a high voltage range. The current density was 0.25 mA cm⁻².

All the experiments were carried out in a glove box (Ar-filled, [H₂O] < 0.1 ppm) or an airtight sample holder.

3. Results and Discussion

Figure 1 shows the XRD patterns of (100 − x)Li₂S-xAlI₃ (Figure 1a) and (80Li₂S − 20AlI₃)·yLiI (Figure 1b) composite solid electrolytes (y = 5, 10, 15). Sample Li₂S (x = 0) was characterized by four intense peaks located at 2θ ≈ 27.03, 31.22, 44.77, and 53.15°. The pattern of AlI₃ (x = 100) has two intense peaks at 2θ ≈ 25.24 and 29.06°. Samples x = 7 and x = 10 exhibited characteristic peaks of both Li₂S and AlI₃; however, the features of AlI₃ were more clearly observed in sample x = 10 than in sample x = 7. The features of Li₂S disappeared in the samples x = 20, 25, and 30, indicating that Li₂S became amorphous or a new crystal structure was formed because the XRD pattern of those samples did not completely cover AlI₃ peaks. It was also proved that samples 5 ≥ x ≥ 0 exhibited the crystal structure of Li₂S [23]. Therefore, (100 − x)Li₂S-xAlI₃ exhibits the structure of Li₂S when 5 ≥ x ≥ 0 and the structure of AlI₃ when x ≥ 20; those results also show that the (100 − x)Li₂S-xAlI₃ solid solution can be prepared by mechanochemical synthesis, and its structure depends on the Li₂S mass fraction. XRD patterns of (80Li₂S − 20AlI₃)·yLiI composites and raw materials are illustrated in Figure 1b. The (80Li₂S − 20AlI₃)·yLiI composites exhibited the characteristic peaks of AlI₃ and no trace of LiI (y = 100) was detected. However, fitting the pattern of sample y = 15 disclosed that the peak at 25.4° is composed of two peaks at 25.4 and 25.9°, and the peak at 29.4° is composed of two peaks at 29.4 and 30.2° (Figure 1c,d). Those fitting results proved that LiI crystal still remained in sample y = 15, while its existence in y = 5 and 10 was not detected. Therefore, LiI might form a solid solution with 80Li₂S − 20AlI₃ or it exists in the amorphous form in samples y = 5 and 10.

Figure 2 shows the temperature dependence of the ionic conductivity of (100 − x)Li₂S-xAlI₃ (Figure 2a) and (80Li₂S − 20AlI₃)·yLiI (Figure 2b) composite solid electrolytes (y = 5, 10, 15). The small insets show the activation energy of the samples. The ionic conductivity of Li₂S at 25 °C was about 10⁻¹² Scm⁻¹ [18,23]. Sample x = 7 exhibited ionic conductivity at 25 °C of about 2.8 × 10⁻⁵ Scm⁻¹. Samples x = 20, 25, and 20 had ionic conductivity at 25 °C of about 5.8 × 10⁻⁵ Scm⁻¹, 3.8 × 10⁻⁵ Scm⁻¹, and 3.1 × 10⁻⁵ Scm⁻¹, respectively. The activation energy of sample x = 5, 7, 20, 25, and 30 was about 56, 44, 38, 37, and 60 kJ mol⁻¹. The activation energy of the samples decreased when x increased and reached the minima values of about 38 kJ mol⁻¹ in samples x = 20, 25. Thus, forming a solid solution with AlI₃ could enhance the ionic conductivity of Li₂S, but the ionic conductivity of (100 − x)Li₂S-xAlI₃ at 25 °C was still lower than 10⁻⁴ Scm⁻¹. It was reported that the ionic conductivity of Li₃PS₄ improved with LiI addition, so LiI was also added to 80Li₂S − 20AlI₃ to investigate the ionic conductivity improvement [28,29]. The temperature dependence of the ionic conductivity of (80Li₂S − 20AlI₃)·yLiI composite solid electrolytes (y = 0, 5, 10, 15) are shown in Figure 2b. The conductivity of 80Li₂S − 20AlI₃ (x = 20, y = 0) at 25 °C was about 5.8 × 10⁻⁵ Scm⁻¹; this value increased to 1.1 × 10⁻⁴, 2.3 × 10⁻⁴, and 1.0 × 10⁻⁴ Scm⁻¹. The activation energy for the Li-ion movement was about 38, 40, 42, and 42 in samples y = 0, 5, 10, and 15, respectively. The existence of LiI crystals, as shown in Figure 1c,d, is responsible for the decrease in conductivity of sample y = 15 compared with sample y = 10. Thus, LiI addition could enhance the ionic conductivity of 80Li₂S − 20AlI₃ and (80Li₂S − 20AlI₃)·10LiI had the highest Li-ion conductivity among the prepared sample at 25 °C.

Figure 3a shows the time dependence of DC current when a voltage of 0.2, 0.5, and 0.7 V (DC) was applied to sample (80Li₂S − 20AlI₃)·10LiI using a blocking electrode. The polarization was initially observed, and then the current became nearly constant. The electronic conductivity was about 1.1 × 10⁻⁸ Scm⁻¹. Since (80Li₂S − 20AlI₃)·10LiI reacts with Li metal, a thin layer of Li₆PS₅Cl was employed to prevent (80Li₂S − 20AlI₃)·10LiI from direct contact with Li metal. A constant current was obtained when 0.5 V (DC) was applied to the cell LiǀLi₆PS₅Clǀ(80Li₂S − 20AlI₃)·10LiIǀ Li₆PS₅ClǀLi (Figure 3b). The constant current obtained using the non-blocking electrode was about 10⁴ times higher than that obtained using the blocking electrode; thus, (80Li₂S − 20AlI₃)·10LiI is considered a pure Li-ion conductor with a Li-ion transport number higher than 0.999. In addition, the ionic conductivity calculated from the current values was about 2.1 × 10⁻⁴ Scm⁻¹ at 25 °C, which resembled the value 2.3 × 10⁻⁴ Scm⁻¹ obtained from AC impedance spectroscopy.

Figure 4a,b shows the Nyquist plots of 80Li₂S − 20AlI₃ and (80Li₂S − 20AlI₃)·10LiI measured at 28–90 °C, respectively. All the plots consist of a semicircle and a low-frequency tail, which is consistent with Li⁺ blocking at the SUS electrodes, expressing ionic conductive behavior. The diameter of the semicircle gradually decreased with increasing temperature and was almost negligible at 90 °C. The total impedance value of the electrolyte pellet is the value at the intersection point between the semicircle and the x-axis in the intermediate frequency region [30]. The impedance spectra recorded at room temperature of 80Li₂S − 20AlI₃ and (80Li₂S − 20AlI₃)·10LiI are plotted in Figure 4c,d, respectively. The equivalence circuit of the impedance spectrum is shown in Figure 4c. The impedance resistance of 80Li₂S − 20AlI₃ is composed of bulk, interface, and grain boundary resistances (the color-filled regions) [31,32]. The bulk resistance of the SE (R_bulk) is obtained from the intersection between the semicircle and the x-axis in the high-frequency region. The semicircle in the intermediate region is composed of interfacial resistance (R_Interfacial) and grain boundary resistance (R_{Grain boundary}). The R_bulk value of 80Li₂S − 20AlI₃ is close to that of (80Li₂S − 20AlI₃)·10LiI. While the R_Interfacial + R_{Grain boundary} value of (80Li₂S − 20AlI₃)·10LiI is smaller than that of 80Li₂S − 20AlI₃. These results indicate that LiI addition effectively reduces the value of R_Interfacial + R_{Grain boundary}.

The conductivity isotherms of 80Li₂S − 20AlI₃ and (80Li₂S − 20AlI₃)·10LiI recorded from 28 to 90 °C, the real part σ″ of the complex conductivity plotted against frequency F, are depicted in Figure 4e,f, respectively. The isotherms display a frequency independence plateau in high and intermediate frequency regions, which is the so-called direct current (DC) region [33,34]. This region indicates long-range ion transport. Frequency dependence of conductivity is clearly observed in the low-frequency region, which exhibits the polarization effect. Jonscher’s power law σ″ ∝ ωⁿ was observed in the low-frequency region, where ω is the angular frequency and n represents the interaction between the mobile ion and its surrounding environment [35,36]. Temperature dependence of n is clearly observed in sample 80Li₂S − 20AlI₃ and the n values are higher than 0.6, indicating the dipole behaviors of charge carriers in this sample [36]. Two distinct slopes appear in the low-frequency region of the isotherms of sample (80Li₂S − 20AlI₃)·10LiI, with n values close to 0.7 and 0.5. The n values also indicated the dipoles behaviors of charge carriers, but the presence of two slopes suggested the existence of two types of materials in the sample (80Li₂S − 20AlI₃)·10LiI.

Liǀ(80Li₂S − 20AlI₃)·10LiIǀLi and LiǀLi₆PS₅Clǀ(80Li₂S − 20AlI₃)·10LiIǀLi₆PS₅ClǀLi symmetric cells were employed to investigate the stability of (80Li₂S − 20AlI₃)·10LiI at a low voltage region. The small inset in Figure 5a proved that the reaction between (80Li₂S − 20AlI₃)·10LiI and Li metal is irreversible. However, the LiǀLi₆PS₅Clǀ(80Li₂S − 20AlI₃)·10LiIǀLi₆PS₅ClǀLi symmetric cell exhibited a relatively steady charge–discharge voltage for more than 200 h. These results indicated that (80Li₂S − 20AlI₃)·10LiI irreversibly reacts with Li metal, but it is quite stable in the low-voltage region. All-solid-state cells, which employed (80Li₂S − 20AlI₃)·10LiI, as solid electrolyte in the positive electrode, were fabricated to study their stability at high voltage regions. CuS and Li_4.4Si were used as active materials. Figure 5b–e illustrates the electrochemical performances of the prepared cells. The 1st and 10th charge—discharge curves of the cells cycling using CC and CC-CV modes are depicted in Figure 5b,d, respectively. Two distinct plateaus are observed in the discharge curves, which arise from the following reactions [37]:

• The first plateau: CuS + 1.96Li⁺ + 1.96e⁻ → Li₂S + Cu_1.96S.
• The second plateau: Cu_1.96S + 2Li⁺ → Li₂S + 1.96Cu.

Accordingly, two distinct plateaus also appeared in the charge curves. The charge curve in Figure 5b was smooth at low voltage and bumpy at voltage higher than 2.3 V, which illustrated the decomposition of (80Li₂S − 20AlI₃)·10LiI composite solid electrolyte. The discharge and charge capacity were about 400 and 600 mAh g⁻¹_CuS, respectively (Figure 5c), resulting in the capacity loss of about 200 mAh g⁻¹_CuS and unstable Coulombic efficiency (Figure 5c). Figure 5c indicates that the discharge capacity was stable at about 400 mAh g⁻¹_CuS but the charge capacity was unsteady, showing the continuous decomposition of solid electrolyte. The cyclic voltammogram of Li₃SI solid electrolyte also indicated the oxidation of S²⁻ at about 2 V, which was one of the reasons for low capacity retention of the all-solid-state cell cycling in the voltage range of 2.0–3.6 V [24]. Thus, (80Li₂S − 20AlI₃)·10LiI composite solid electrolyte was irreversibly decomposed at voltage higher than 2.3 V. The cell cycling in CC-CV mode (CC: 0.25 mA cm⁻², CV: 2.30 V) exhibits smooth charge–discharge curves (Figure 5d), confirming that (80Li₂S − 20AlI₃)·10LiI was stable in this voltage region. The charge–discharge capacity was stable at about 400 mAh g⁻¹_CuS, resulting in a Coulombic efficiency higher than 99.5%.

4. Conclusions

(100 − x)Li₂S-xAlI₃ and (80Li₂S − 20AlI₃)·yLiI (y = 0, 5, 10, 15) composite solid electrolytes were successfully prepared by mechanochemical synthesis. XRD results revealed that Li₂S and AlI₃ formed a solid solution, while LiI crystals remained in sample y = 15. It was also proved that (80Li₂S − 20AlI₃)·10LiI is a pure Li-ion conductor with the ionic conductivity of about 2.3 × 10⁻⁴ Scm⁻¹ at 25 °C, about three times higher than that of the 80Li₂S − 20AlI₃ sample. The impedance spectroscopy results revealed that LiI addition enhanced the grain boundary and interfacial resistance of the prepared samples. The results from XRD and AC impedance suggested that LiI might be present in the amorphous form in samples y = 5 and 10. The symmetric and all-solid-state cell performance illustrated that (80Li₂S − 20AlI₃)·10LiI was relatively stable at voltage lower than 2.3 V.