All-solid-state batteries (ASSBs) offer a viable solution to mitigate the safety concerns of conventional lithium-ions batteries (LIBs), in addition to their potential for exploiting the Li-metal anode with a theoretical specific capacity of 3,860 mAh g⁻¹ and electrochemical potential of −3.04 V versus the standard hydrogen electrode, thereby enabling a significant enhancement of the energy-density of the batteries (Janek and Zeier, 2016). To make ASSBs practical, it is crucial to advance the development of solid-state electrolytes (SSEs) with exceptional performance (Xia et al., 2019; Abakumov et al., 2020; Zhao et al., 2020). Typical SSEs mainly include sulfide, halide, oxide, and other systems. Sulfide SSEs commonly exhibit high ionic conductivity and good processability, but the low intrinsic electrochemical stability windows (Zhu et al., 2015). Halide SSEs offer high ionic conductivity and compatibility with high voltage cathodes such as LiCoO₂, but are not stable with Li-metal anode (Kwak et al., 2022). Oxide SSEs exhibit wide electrochemical stability windows, but feature high interfacial and grain boundary resistance (van den Broek et al., 2016). Each SSE owns distinct properties in terms of ionic conductivity, electrochemical window, and stability in the air (Kamaya et al., 2011; Manthiram et al., 2017; Yao et al., 2019; Kim et al., 2020; Li et al., 2020; Xu et al., 2023). Notably, ionic conductivity is a vital performance indicator that impacts the application of SSEs (Kwak et al., 2022; Yang and Wu, 2022).

The ionic conductivity of SSEs can be optimized by manipulating lattice structure, element substitution, phase change, amorphization, etc (Asano et al., 2018; Wang et al., 2019; Luo et al., 2021; Kwak et al., 2022; Schweiger et al., 2022; Szczuka et al., 2022). Among these methods, amorphization has gained attention due to the emergence of mechanochemical synthesis methods, which is an effective approach to synthesizing SSEs with lower grain boundary resistance (Dalvi and Shahi, 2004; Morimoto et al., 2004; Kim and Martin, 2006; Enayati and Mohamed, 2014). Representatively, the amorphous Li₂S-P₂S₅ binary system SSEs can be prepared by mechanical milling and exhibit a high ionic conductivity (>10^–4 S/cm) (Hayashi et al., 2004). In addition, some SSEs such as Li₆PS₅I (Brinek et al., 2020), Li₂B₄O₇ (Wohlmuth et al., 2016), Li₂ZrCl₆ (Chen et al., 2021), and Li₃YCl₆ (Asano et al., 2018) show higher ionic conductivity after undergoing amorphization. However, the impact of amorphization on the ionic conductivity varies depending on the specific SSEs system, crystalline structures play a critical role in ionic conductivity for numerous SSEs. (Zhao et al., 2019; Schweiger et al., 2022). For instance, a recent study by Schweiger et al. revealed that Li₁₀GeP₂S₁₂ experienced an increase in grain boundary resistance and a decrease in ionic conductivity with increasing milling time against the behavior of other SSEs. The mechanism behind this phenomenon is that defects and site disorder caused by ball milling impede the migration of lithium ions within the lattice (Schweiger et al., 2022). Therefore, it is essential to investigate the impact of amorphization on the grain boundary resistance and ionic conductivity of SSEs, while also elucidating the underlying mechanism.

In this study, the amorphous SSEs of binary xLi₂S-(100-x)LiI (10 ≤ x ≤ 90) were synthesized by mechanical ball-milling method for the first time. PXRD analysis indicates that the amorphization degree of xLi₂S-(100-x)LiI system is significantly influenced by the stoichiometry of Li₂S and LiI. Furthermore, electrochemical impedance spectroscopy (EIS) analysis reveals a strong correlation between the amorphization degree of the xLi₂S-(100-x)LiI system and its ionic conductivity, with the effect of grain boundary resistance being the primary contributing factor. Additionally, the increase of Li₂S content in xLi₂S-(100-x)LiI may restrict the grain boundary impedance reduction caused by amorphization.

As presented in Figure 1, the amorphous degree of xLi₂S-(100-x)LiI (x = 10, 30, 50, 70 and 90) system significantly depends on the stoichiometry of Li₂S and LiI. Before ball-milling, all PXRD patterns of xLi₂S-(100-x)LiI exhibit sharp-peak feature, which indicates their good crystallinity (Figure 1A). In contrast, the PXRD patterns of xLi₂S-(100-x)LiI after ball-milling exhibit different degrees of broadening (Figure 1B). Representatively, FWHM of the PXRD peaks in the range of 40°–50° is used here to quantitatively analyze the amorphization degree of xLi₂S-(100-x)LiI binary system (Indris et al., 2000; Sasano et al., 2011; Holder and Schaak, 2019; Londono-Restrepo et al., 2019; Schweiger et al., 2022; Sun et al., 2022). It should be emphasized that the peak positions and FWHM of Li₂S or LiI at x = 10 or 90 are not discernible from the PXRD pattern due to the low content. Surprisingly, different stoichiometric ratios of Li₂S and LiI in xLi₂S-(100-x)LiI lead to obviously different amorphization degrees, even under the same ball-milling conditions. As shown in Figures 1C, D, the FWHM of LiI presents an increasing trend with the increase of Li₂S and changes from 0.239 (x = 10) to 1.124 (x = 70), which demonstrates that the presence of Li₂S can promote the amorphization of LiI. In contrast, the FWHM of Li₂S seems to tend to remain constant as x increases in xLi₂S-(100-x)LiI (x ≥ 50). Interestingly, the amorphization degree of the xLi₂S-(100-x)P₂S₅ binary system is also dependent on the stoichiometric ratios of Li₂S and P₂S₅ (Minami et al., 2006; Tatsumisago and Hayashi, 2012; Kudu et al., 2018). However, the amorphization degree of xLi₂S-(100-x)P₂S₅ diminishes as Li₂S increases, accompanied by the appearance of sharp peaks of Li₂S in the PXRD patterns (Hayashi et al., 2004). Therefore, the difference between the xLi₂S-(100-x)LiI and xLi₂S-(100-x)P₂S₅ suggests that the amorphization degree depends not only on the stoichiometric ratios but also on the composition of the compound in the binary system.

The stoichiometric ratios of Li₂S and LiI determine the amorphization degree of the xLi₂S-(100-x)LiI binary system, which significantly affects its ionic conductivity. Figure 2A shows the Nyquist plots of amorphous xLi₂S-(100-x)LiI binary system at room temperature (RT), and each curve exhibits a typical semicircle at high frequency representing the resistance and the linear part at low frequency representing ion blocking electrode. The EIS data were processed based on the formula: Z = (Z ₀ × S)/l to eliminate the effect of SSE pellet thickness and area on the impedance, in which Z ₀ is the raw data of the measured EIS, l is the thickness, and S is the area of SSE pellet. Fitting the plot by the equivalent circuit leads to the resistance R, which corresponds to the value of the real part of the Nyquist curve, and the ionic conductivity is calculated according to the formula of σ = l/(R × S). As presented in Figure 2B, the ionic conductivities of xLi₂S-(100-x)LiI show a non-monotonic variation with the increase of x. As x increased from 10 to 70, the ionic conductivity of xLi₂S-(100-x)LiI increased from 1.03 × 10⁻⁶ S/cm to 8.43 × 10⁻⁶ S/cm. Subsequently, after x continued to increase to 90, the ionic conductivity appeared to drop significantly to 1.78 × 10⁻⁷ S/cm. The above non-monotonic ionic conductivity changes may be attributed to both the amorphization degree of LiI and the content of Li₂S in xLi₂S-(100-x)LiI. In the first stage (x from 10 to 70), the amorphization of LiI is the dominant factor in influencing the ionic conductivity of xLi₂S-(100-x)LiI (Figure 1C). However, in the next stage (x from 70 to 90), the adverse effect of Li₂S content on ionic conductivity may play a major role.

To understand the ionic transport mechanism of the amorphous xLi₂S-(100-x)LiI in depth, the Nyquist plots were fitted with the equivalent circuit consisting of bulk resistance (R _b), grain boundary resistance (R _gb) and constant phase element (CPE). As illustrated in Figure 3A, lithium ions transport in the bulk phase and grain boundary of SSEs, which determines the overall ionic conductivity of xLi₂S-(100-x)LiI (Gao et al., 2016; Goswami and Kant, 2019; Vadhva et al., 2021). Obviously, the hindrance of lithium ions transport at the grain boundaries is stronger than that of the bulk phase according to Figures 3B–F. For 10Li₂S-90LiI, for example, its R _gb is 947.7 kΩ cm, which is much higher than that of R _b (8,403 Ω cm). Besides, the variation of R _gb is significantly higher than that of R _b. The R _b and R _gb of 70Li₂S-30LiI with the highest ionic conductivity are 3,632 Ω cm and 116.1 kΩ cm respectively. In contrast, the R _b and R _gb of 90Li₂S-10LiI with the lowest ionic conductivity are 9,925 Ω cm and 5,566.4 kΩ cm respectively.

Furthermore, to present the dependence of R _b and R _gb on x in xLi₂S-(100-x)LiI, the differences between R _b and R _gb on logarithmic scale are presented in Figure 4A. While the ionic conductivity of xLi₂S-(100-x)LiI undergoes the significant change with x from 10 to 90 (Figure 2), R _b does not undergo a distinct fluctuation, as well as the bulk phase conductivity σ _b. In contrast, R _gb and the grain boundary conductivity σ _gb show the significant changes and are in agreement with the trend of the ionic conductivity (Figure 4B). Also, the conductivity isotherms extracted from EIS can reflect the dependence of the grain boundary conductivity on x in xLi₂S-(100-x)LiI, which is consistent with the results of the Nyquist curves fitted with the equivalent circuit. As shown in Figure 5A, conductivity isotherms are plotted from the real part (σ′) of the complex ionic conductivity as a function of frequency. Typically, the frequency independent plateaus (marked by arrow) correspond to the ionic conductivities at the grain boundary of SSEs (Schweiger et al., 2022). As x increases, the plateau of σ′ gradually reaches a maximum of 8.50 × 10⁻⁶ S/cm at x = 70, then dropping to a minimum of 1.79 × 10⁻⁶ S/cm at x = 90. It is worth emphasizing that the feature of conductivity isotherms not only agrees with the analysis of the Nyquist curve, but also the values corresponding to the plateau of σ′ are very close to the grain boundary conductivity σ _gb in Figure 4B, which confirms the above analysis of ionic conductivity of amorphous xLi₂S-(100-x)LiI. In addition, the imaginary part (Z″) of the complex impedance as a function of frequency is plotted in Figure 5B, and the Z″ peak height is usually considered to be equal to half of the most resistive elements (here, i.e., the grain boundary resistance) in SSEs (Irvine et al., 1990). Consistently, the dependence of Z″ peak height on x can also corroborate the results of Nyquist curves fitted with the equivalent circuit.

Obviously, the above results indicate that the ionic conductivity change of amorphous xLi₂S-(100-x)LiI depends directly on the grain boundary conductivity σ _gb and is almost unaffected by the bulk phase conductivity σ _b. On the other hand, in combination with the PXRD data of xLi₂S-(100-x)LiI (Figure 1), it can be concluded that the increase in grain boundary conductivity σ _gb may depend on the enhanced amorphization of LiI as x increases from 10 to 70, while the decrease in grain boundary conductivity σ _gb may be mainly affected by the increase in Li₂S content as x increases from 70 to 90. In other words, there is a competitive relationship between the amorphization of LiI and the content of Li₂S in affecting the grain boundary conductivity of amorphous xLi₂S-(100-x)LiI.

In conclusion, the amorphous xLi₂S-(100-x)LiI (10 ≤ x ≤ 90) binary system was synthesized by mechanical ball-milling method. The PXRD analysis significantly demonstrated that the increase of Li₂S content can promote the amorphization of LiI, and the amorphous degree of Li₂S tend to remain constant as x increases in xLi₂S-(100-x)LiI (x ≥ 50). The EIS analysis revealed that the change in ionic conductivity of amorphous xLi₂S-(100-x)LiI depends on the grain boundary conductivity and is almost unaffected by the bulk phase conductivity. In addition, the competitive mechanism between the amorphization of LiI and the content of Li₂S in affecting the grain boundary conductivity was found. The findings of xLi₂S-(100-x)LiI binary system provide insights into the future design of new amorphous SSEs.