Hanzhang
Fang
a,
Wenshuo
Hou
*a,
Chuanlong
Li
a,
Shuo
Li
a,
Fulu
Chu
a,
Xuting
Li
a,
Xianping
Zhang
*b,
Linrui
Hou
a,
Changzhou
Yuan
*a and
Yanwei
Ma
b
aSchool of Materials Science & Engineering, University of Jinan, Jinan, 250022, P. R. China. E-mail: mse_houws@ujn.edu.cn; mse_yuancz@ujn.edu.cn; ayuancz@163.com
bInstitute of Electrical Engineering and Advanced Electromagnetic Drive Technology, Qilu Zhongke, Jinan 250013, P. R. China. E-mail: zxp@mail.iee.ac.cn
First published on 4th April 2025
Sn-based halide perovskites are expected to solve the problems of the shuttle effect and sluggish redox kinetics of lithium polysulfides (LiPSs) in lithium–sulfur batteries (LSBs) due to their high conductivity and electrocatalytic activity, but their intrinsic catalytic mechanism for LiPSs remains to be explored. Herein, halide perovskites with varying halide anions, Cs2SnX6 (X = Cl, Br, I), are purposefully designed to unveil the halogen-induced regulatory mechanism. Theoretical calculations demonstrate that increasing the halogen atomic number induces the shift of the p-band center closer to the Fermi level, which results in the localized charge distribution around halide anions and rapid charge separation/transfer at Sn sites, enhancing the adsorptive and catalytic activity and redox kinetics of LiPSs. Experimental investigations exhibit that LSBs assembled with the Cs2SnI6 modified separator deliver a high initial capacity of 1000 mA h g−1 at 2C, with a minimum decay rate of 0.068% per cycle after 500 cycles. More impressively, the Cs2SnI6 battery with a high sulfur loading (6.1 mg cm−2) and a low electrolyte/sulfur ratio (5.5 μL mg−1) achieves a remarkable reversible capacity of 768.8 mA h g−1, along with robust wide-temperature-tolerant cycling performance from −20 to 50 °C. These findings underscore the critical role of p-band center regulation in rationally designing advanced LSBs.
In previous research studies, various polar and non-polar electrocatalytic materials have been explored to adsorb LiPSs and catalyze their redox reactions,12–21 including metal oxides,22 metal phosphides,23 and metal sulfides.24 Among them, oxide perovskites demonstrate good adsorptive and catalytic effects on LiPSs through their ferroelectric effect.25 For instance, oxide perovskites such as LaNiO3,26 SrTiO3,27 and the layered double perovskite PrBaCo2O628 have been utilized to fabricate separators for LSBs. These oxide perovskites inhibit the shuttle effect and promote the conversion of LiPSs. However, oxide perovskites still face challenges related to a lack of active sites and poor conductivity.29 In practical applications, the conductivity and electrocatalytic activity of electrocatalytic materials can be enhanced by adjusting their surface electronic state and the rate of charge transfer. Halide perovskites are known for their superior charge transport properties, tunable band structure, and high defect tolerance, which endow them with excellent electrocatalytic activity.30–34 Halide perovskites can also regulate structural characteristics by altering the material composition. However, the presence of toxic lead in the chemical composition of most halide perovskites and their structural instability remain the two main limiting factors hindering their application.35 Compared to other halide perovskites, halide double perovskites possess a greater number of halogen active sites and exhibit enhanced chemical stability in air and polar solutions. In contrast, halide perovskites are prone to oxidation in air and react with water and polar solutions.36 Traditional lead-based halide perovskites have restricted applications due to the toxicity of lead. Sn-based halide double perovskites are environmentally friendly and have a narrower band gap than other metal-based halide perovskites, which render better conductivity to them.37 Moreover, by regulating the p-band center, higher ion/electron conductivity can be achieved, which helps to improve the adsorption and catalytic performance toward LiPSs. To date, the exploration of p-band center regulation of Sn-based halide double perovskites and their application in LSBs has been limited, particularly lacking in-depth investigations into the intrinsic mechanism of halide anions catalyzing LiPS conversion at the atomic scale.
With these comprehensive considerations in mind, in this contribution, we have purposefully constructed a series of Sn-based halide double perovskites with different halide anions, Cs2SnX6 (X = Cl, Br, I), to explore the regulatory mechanism of halide anions on the electronic structure. Both theoretical calculations and experimental results coherently confirmed that the p-band center of halogen anions shifted positively toward the Fermi level with the increase in the halogen atomic number. Such a unique electronic structural change endowed the material with remarkable electrocatalytic performance due to several advantages it possesses. Specifically, the I atoms in Cs2SnI6 first promoted the p-band center to be closer to the Fermi level, resulting in a higher charge transfer rate, which was more beneficial for catalyzing the conversion of LiPSs. Second, Cs2SnI6 achieved more localized charge distribution around the I atom, rendering a weaker charge distribution at the Sn center and promoting charge separation and transfer, which benefited the adsorptive and catalytic reactions of LiPSs on its surface. Third, the binding energy between Cs2SnI6 and LiPSs was higher than the other two counterparts, while Cs2SnI6 had the lowest energy barriers for LiPSs and Li2S decomposition reactions. As a result, Cs2SnI6 exhibited excellent adsorption and catalytic conversion capability for LiPSs. The initial specific capacity of LSBs assembled with Cs2SnI6 modified separators was as high as 1000 mA h g−1 at 2C, along with a decay rate of only 0.068% after 500 cycles. Notably, the Cs2SnI6 battery with a high-S loading (6.1 mg cm−2) and a low electrolyte/sulfur (E/S) ratio (5.5 μl mg−1) still exhibited an extremely high capacity of 768.8 mA h g−1. Additionally, the Cs2SnI6 battery exhibited initial capacities of 912.7 mA h g−1 at −20 °C at 0.1C and 1350.7 mA h g−1 at 50 °C at 0.5C, respectively. Encouragingly, the reversible capacities of 772.6 and 892.3 mA h g−1 could still be retained after 100 cycles at −20 at 50 °C, respectively. All in all, LSBs assembled with the Cs2SnI6 modified separator presented high reversible capacity and stable cycling performance, even under harsh operation conditions, including high sulfur loading, low E/S ratios, and extremely high/low working temperatures.
Eads = Etotal − Ead − Esub | (1) |
ΔG = ΔEads + ΔEzpe − TΔS | (2) |
During the charge–discharge cycles of LSBs, LiPSs are always adsorbed onto the surface of electrocatalysts Cs2SnX6. This process is accompanied by the redistribution of electrons between the electrocatalysts and LiPSs. The PDOS plot reveals that the p-band center of I in the Cs2SnI6–Li2S6 system is located at −0.727 eV (Fig. 1g), which is the closest to the Fermi level among the three systems. This electronic structural feature enhances the interaction between Cs2SnI6 and Li2S6, thereby facilitating the adsorption and activation of Li2S6. In the Cs2SnBr6–Li2S6 system, the p-band center of Br is located at −1.025 eV (Fig. 1h). In contrast, in the Cs2SnCl6–Li2S6 system, the p-band center of Cl is located at −1.197 eV (Fig. 1i), representing the most negative value among the three systems. The results here suggest that as the element changes from I to Cl, its p-band center gradually shifts upwards and away from the Fermi level, which will weaken the interaction strength between the electrocatalyst and Li2S6, thereby resulting in decreased electrocatalytic performance. By investigating the interaction between Li2S6 and Cs2SnX6, the underlying mechanisms through which these electronic structure changes influence the electrocatalytic performance have been elucidated further. The schematic diagram (Fig. 1j) explicitly illustrates the regulatory mechanism of the p-band center and electronic structure induced by the halogen change in Cs2SnX6, where Cs2SnCl6, Cs2SnBr6, and Cs2SnI6 all exhibit octahedral structures. The transition of halide anions from Cl to I first causes a shift in the position of the p-band center relative to the Fermi level. As the atomic number of the halogen increases, the p-band center of the halide anion shifts toward the Fermi level, indicating an enhanced electron transfer rate, and consequently improves the electrocatalytic performance. Besides, the band gap also changes accordingly. The band gaps of the conduction band (CB) and valence band (VB) of Cs2SnCl6, Cs2SnBr6, and Cs2SnI6 gradually decrease, which leads to an increase in their conductivity. Overall, a series of theoretical calculations for Cs2SnX6 (X = Cl, Br, I) demonstrate that by contrapuntally adjusting the halogen composition in halide perovskites, the p-band center and electronic structure of halide anions can be effectively regulated, thereby improving the conductivity and electrocatalytic activity of the materials.
Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) were conducted to elaborately investigate the morphological and structural evolution of the samples. FESEM observations (Fig. S3, ESI†) reveal that the as-prepared Cs2SnCl6, Cs2SnBr6, and Cs2SnI6 exhibit well-defined octahedral structures with particle size ranging approximately from 1 to 10 μm, and Cs2SnI6 presents the most uniform size distribution among them. Due to their micro-scale size, the subsequent ball-milling is highly necessary to obtain the nano-dimensional Cs2SnX6 samples before their application in LSBs. As noted, Cs2SnI6 forms uniform block shaped particles with sizes of approximately 200–500 nm after ball-milling (Fig. 2d). In contrast, the size of Cs2SnBr6 significantly decreases to ∼200 nm (Fig. S4a, ESI†), along with the smaller size of ∼100 nm for Cs2SnCl6 (Fig. S4b, ESI†), indicating that ball milling effectively reduces the particle size, which will contribute to its uniform distribution in the modified separator. The clear lattice fringes with a well-defined spacing of 0.32 nm, as presented in Fig. 2g, correspond to the (222) crystal plane of Cs2SnI6, indicating its good crystallinity. Similarly, the well-discerned lattice fringes with spacings of 0.31 and 0.30 nm are easily identified and ascribed to the (222) crystal planes of Cs2SnBr6 (Fig. S5a, ESI†) and Cs2SnCl6 (Fig. S5b, ESI†), respectively. The variation trend in spacing follows the order of Cs2SnI6 > Cs2SnBr6> Cs2SnCl6, which is in good agreement with the XRD refinement results presented above. Furthermore, the clear diffraction rings of the selected area electron diffraction (SAED) pattern (Fig. 2h) indicate the polycrystalline nature of Cs2SnI6, along with the well-indexed (111), (222), and (620) crystal planes. Similarly, both Cs2SnBr6 (Fig. S5c, ESI†) and Cs2SnCl6 (Fig. S5d, ESI†) exhibit polycrystalline features as well, owing to their multiple clear diffraction rings. Further elemental energy dispersive spectroscopy (EDS) mappings visualize the homogeneous distribution of Cs, Sn, and I throughout the Cs2SnI6 particles, as depicted in Fig. 2i. Correspondingly, EDS mappings (Fig. S6, ESI†) also authenticate the uniform distribution of Cs, Sn, and halogen elements (Br or Cl) in Cs2SnBr6 and Cs2SnCl6 particles. The uniform morphology, high crystallinity, and homogeneous elemental distribution of nanosized Cs2SnX6 particles jointly promote their excellent electrochemical performance in LSBs.
The adsorption capability of the four materials for LiPSs was compared through adsorption experiments in the Li2S6 solution (Fig. 3a). After 6 h, the yellow Li2S6 solution with Cs2SnI6 shows a nearly colorless appearance, highlighting its strongest absorbability to LiPSs among the three samples. The underlying reasons for this lie in the fact that Cs2SnI6 provides a certain adsorption capacity for Li2S6, as evidenced by the yellowish color of the Li2S6 solution containing Cs2SnBr6 and Cs2SnCl6. These intuitive results were further verified by UV-vis spectra, as the solution containing Cs2SnI6 exhibits the weakest signal of S62− in the range of 400–500 nm.45 The interactions between Cs2SnX6 and Li2S6 were further explored by high-resolution XPS before and after adsorption. Among them, the peaks at 486.8 eV and 495.2 eV are attributed to Sn4+. After adsorption, the Sn 3d peaks shift to higher binding energies, from 486.8/495.2 eV to 486.9/495.4 eV, respectively. And a new pair of signals attributed to Sn–S bonds at 487.5 eV emerge, suggesting the facile anchoring of Li2S6 on Cs2SnI6.46 The peak of the Sn–S bond appears in the Sn 3d XPS spectra of both Cs2SnBr6 (Fig. S7a, ESI†) and Cs2SnCl6 (Fig. S7b, ESI†). Notably, the Sn–S bond in Cs2SnI6 is the strongest among the three, indicating its strongest adsorption ability for Li2S6. Besides this, as shown in Fig. 3c, the peaks at 620.3 eV and 631.8 eV are attributed to I−. After adsorbing Li2S6, the I 3d peaks shift toward lower binding energy, from 620.3/631.8 eV to 619.0/630.5 eV, revealing an interaction between I and Li2S6. The noticeable interactions between Li2S6 and Br/Cl are also detected, as such shifts are still found both in Br 3d (Fig. S7c, ESI†) and Cl 2p spectra (Fig. S7d, ESI†). It is especially mentioned that the shift in I 3d is the most pronounced, which means that the interaction between I and Li2S6 is the strongest among the halogens and explicitly confirms the superior adsorption capability of Cs2SnI6 toward LiPSs. Such characteristic interactions between the materials and LiPSs are further embodied in S 2p spectra of Cs2SnX6 absorbed with Li2S6 (Fig. 3c and S8, ESI†). Specifically, the two pairs of peaks in the high-binding-energy region (167.3–169.5 eV) correspond to sulfates and thiosulfates formed on the material surfaces through chemical interactions with LiPSs, and the other two pairs of peaks in the low-binding-energy region (161.8–163.7 eV) are related to the bridging sulfur (S0B) and terminal sulfur (ST−).47 Compared with Cs2SnBr6 and Cs2SnCl6 (Fig. S8, ESI†), LiPSs captured on Cs2SnI6 (Fig. 3d) exhibit even higher peak intensities in the high-binding-energy region, further confirming the stronger chemical adsorption of Cs2SnI6 toward LiPSs from another point of view.48
To further analyze the regulatory mechanism of Cs2SnX6 (X = Cl, Br, I) on the adsorption of Li2S6, adsorptive models were constructed through DFT calculations (Fig. S9, ESI†). According to the DFT results, I and Sn atoms preferentially bond to Li and S atoms on LiPSs surface, which is consistent with the XPS analysis results presented above. Fig. 3e provides the charge density difference diagrams of Li2S6 adsorbed on different catalysts, where cyan and yellow correspond to charge depletion and accumulation, respectively. Apparently, the electron cloud density of the I center in Cs2SnI6 is higher than those of the Br center and Cl center, indicating that the contribution of halide anions to local charge distribution increases with the increase of atomic number. Consequently, I anions in Cs2SnX6 act as a more active site. Cs2SnI6 achieves more local charge distribution at the I center, resulting in weaker charge distribution at the Sn center, thereby promoting charge separation and transfer and improving the conversion reaction kinetics of LiPSs. While the local charge distributions at the Br center and Cl center in Cs2SnBr6 and Cs2SnCl6 become weak, leading to the relatively slower conversion reaction kinetics of LiPSs. Additionally, Li2S6 has the adsorption energy of −2.60 eV on the Cs2SnI6 surface, which is higher than those of Cs2SnBr6 (−2.04 eV) and Cs2SnCl6 (−1.03 eV), as manifested in Fig. 3f. The results above indicate that the regulation of the halogen anion p-band center indeed enhances the electron transfer ability of Cs2SnI6 and strengthens its binding energy with LiPSs. Therefore, Cs2SnI6 can better capture LiPSs and suppress the shuttle effect more effectively.
The modified separators for LSBs were prepared by simply coating polypropylene (PP) with the nanosized Cs2SnX6 (X = Cl, Br, I). The Cs2SnI6, Cs2SnBr6, and Cs2SnCl6 modified separators were attached closely to PP with the thicknesses of 16.0, 14.3, and 15.0 μm, respectively (Fig. S10, ESI†).49 Evidently, Cs2SnX6 (X = Cl, Br, I) and the conductive agent carbon nanotubes (CNTs) were observed to be uniformly distributed across the surface of the separator (Fig. S11, ESI†). The visible permeation experiment was carried out with H-type electrolyzers (Fig. 3g). Visually, LiPSs partially pass through the separator modified with Cs2SnCl6 just after 6 h, as evidenced by the initially colorless right chamber turning pale yellow. While the colorless transparent solutions are still observed in the right chambers for the cases of Cs2SnBr6 and Cs2SnI6 after 6 h, indicating that the inhibiting ability of the Cs2SnBr6 and Cs2SnI6 separators is indeed better than that of the Cs2SnCl6 separator. After 12 h, the electrolyte color turns out to be deepened in the right chamber of the Cs2SnCl6 separator, and a light yellow solution appears for the case of Cs2SnBr6. In sharp contrast, the electrolyte in the right chamber of the Cs2SnI6 separator always remains almost colorless even after the uninterrupted penetration/diffusion for 12 h, convincingly corroborating that the migration of LiPSs is effectively inhibited even at high LiPS concentrations, which is undoubtedly ascribed to the robust chemical adsorption capability of Cs2SnI6.50
The electrochemical process in the discharge process involves both liquid–liquid and liquid–solid conversion reactions. To figure out the catalytic ability of Cs2SnI6 in the liquid–solid conversion process, the nucleation and decomposition behaviors of Li2S on different Cs2SnX6 substrates were investigated by chronoamperometry. The response time of Li2S nucleation (6000 s) in Cs2SnI6 (Fig. 4d) is significantly shorter than those of Cs2SnBr6 (13100 s, Fig. 4e) and Cs2SnCl6 (13
500 s, Fig. 4f), revealing its faster nucleation rate. Moreover, Cs2SnI6 exhibits a higher nucleation capacity of 154.0 mA h g−1, surpassing those of Cs2SnBr6 (137.5 mA h g−1) and Cs2SnCl6 (117.3 mA h g−1). The high nucleation ability of Li2S authenticates that Cs2SnI6 effectively catalyzes the liquid–solid conversion kinetics from Li2S4 to Li2S. For the decomposition curve of Li2S, Cs2SnI6 has the highest specific capacity of 493.4 mA h g−1 (Fig. 4g). By comparison, the lower Li2S dissolution specific capacities are delivered by Cs2SnBr6 (454.1 mA h g−1, Fig. 4h) and Cs2SnCl6 (437.7 mA h g−1, Fig. 4i). These observations corroborate the superb ability of Cs2SnI6 to decompose Li2S during the charging process, which is reasonably attributed to the more charge distribution at the I site in Cs2SnI6 after adsorbing LiPSs, thus exhibiting better catalytic activity.
The rate performance of the assembled LSBs was tested within the current rate range of 0.2–5C (1C = 1675 mA h g−1). The capacities of LSBs using Cs2SnI6 modified separators at rates of 0.2, 0.5, 1, 2, 3, 4, and 5C were estimated as 1271, 1106, 997, 906, 876, 830, and 771 mA h g−1, respectively (Fig. 5a). Upon switching back to 0.2C, a capacity as large as 1138 mA h g−1 can still be recovered, manifesting its admirable stability and high reversibility. The charge–discharge plots of the Cs2SnI6 battery present two clear plateaus evident even at a higher current rate of 5C, with a high-capacity response and a low polarization potential (Fig. 5b). Nevertheless, LSBs using Cs2SnBr6 and Cs2SnCl6 modified separators exhibited low capacities at the same rates (Fig. S15, ESI†). The excellent rate performance of the Cs2SnI6 battery is attributed to its outstanding adsorption and catalytic performance, which verifies the crucial role of halogen-induced p-band center regulation in Cs2SnI6. The cycling properties of LSBs with different separators at 1C are comparatively collected in Fig. 5c. An initial discharge capacity of 1097 mA h g−1 is achieved by LSBs using the Cs2SnI6 modified separator at 1C, and a retention capacity of 713 mA h g−1 is retained after 500 cycles, higher than those of LSBs with Cs2SnBr6 (1027 and 554 mA h g−1) and Cs2SnCl6 separators (1021 and 543 mA h g−1) under the same conditions. The fast capacity degradation of Cs2SnBr6 and Cs2SnCl6 batteries is ascribed to their poor adsorption and catalytic abilities toward LiPSs.
To investigate the influence of the mechanism of Cs2SnX6 with different anions on the battery performance, the charge–discharge plots of Cs2SnI6, Cs2SnBr6, and Cs2SnCl6 batteries at 1C rate were analyzed in detail. All the LSBs with different modified separators at 1C illustrate two distinct discharge platforms at 2.30 V and 2.05 V (Fig. 5d), which correspond to the electrochemical conversion of elemental sulfur to soluble polysulfides (Li2Sn, 4 ≤ n ≤ 8) based on the cathodic peaks observed in the CV curve (Fig. 4b), followed by reduction to insoluble Li2S2/Li2S. Among them, the almost overlapping voltage platforms at different cycles are observed for the Cs2SnI6 battery (Fig. S16, ESI†), suggesting that the Cs2SnI6 modified separator has high sulfur redox chemical reversibility and cycling stability. The polarization overpotential of the Cs2SnI6 battery is 274 mV (Fig. 5e), even lower than those of Cs2SnBr6 (304 mV) and Cs2SnCl6 (348 mV). The observed smallest charge–discharge voltage hysteresis further suggests that Cs2SnI6 significantly enhances the catalytic activity for the conversion reaction of LiPSs, thereby improving the utilization of sulfur species.54 As the atomic number of halogen increases, the adsorption and catalytic ability of Cs2SnX6 (X = Cl, Br, I) for LiPSs in the cathode region also increases. Consequently, the Cs2SnI6 modified separator achieves high-performance LSBs.
The resistance characteristics of Cs2SnX6 were further explored through electrochemical impedance spectroscopy (EIS) analysis. The Nyquist curves of batteries equipped with different separators were obtained before (Fig. 5f) and after cycling (Fig. 5g). All the EIS plots of LSBs consist of the intersections with the Z′ axis, a semicircle in the middle frequency region and a tail line in the low-frequency region, corresponding to the solution resistance (Rs), charge-transfer resistance (Rct) and Warburg impedance, respectively. Using the equivalent circuit diagrams (the insets), the fitted plots are found to be in good agreement with the experimental data, along with the fitting results (Table S1, ESI†). Clearly, all the Cs2SnX6 batteries display low Rs values ranging from 1.6 to 2.9 Ω. These low Rs values are attributed to the excellent conductivity of the modified separator, which effectively promotes electron and ion transport, thereby enhancing the performance of LSBs. Additionally, the Rct of Cs2SnI6 battery is estimated as ∼62.0 Ω, significantly lower than those of Cs2SnBr6 (76.0 Ω) and Cs2SnCl6 (78.6 Ω), indicating its superior charge transfer kinetics. As a result, the Cs2SnI6 modified separator will render the faster redox reactions of LiPSs and reduce the electrochemical reaction resistance.55 More particularly, one additional semicircle appears in the high-medium frequency region of EIS plots for all batteries after cycling (Fig. 5g), which probably stems from the extra surface film resistance (Rsf) caused by the formation of an insoluble Li2S2/Li2S passivation layer on the electrode surface.56,57 Despite this, the Cs2SnI6 battery still consistently exhibits the lowest Rsf and Rct after cycling, suggesting the uniform deposition of Li2S2/Li2S induced by Cs2SnI6, thanks to the superb adsorption and catalytic ability of Cs2SnI6 towards LiPSs.58,59
The shuttle effect in LSBs inevitably leads to adverse side reactions between lithium metal anodes and migrated LiPSs. To investigate the inhibitory effects of Cs2SnI6, Cs2SnBr6, and Cs2SnCl6 on LiPSs shuttle, LSBs were disassembled after 500 cycles at 1C. Noticeably, the distribution of sulfur species on the cathode surface of the Cs2SnI6 battery is relatively uniform, whereas the cathode surfaces of Cs2SnBr6 and Cs2SnCl6 batteries exhibited varying degrees of uneven sulfur deposition after cycling (Fig. S17, ESI†). Moreover, FESEM observations and the corresponding digital photographs indicate that the lithium anode in the Cs2SnI6 battery exhibits a smooth and dense surface after cycling (Fig. S18, ESI†).60 In contrast, the lithium anode surfaces of Cs2SnBr6 and Cs2SnCl6 batteries show an undesirable loose structure, uneven lithium deposition, and numerous cracks, primarily caused by the migration of soluble LiPSs to the anode side.61 In addition, the morphology of the cycled Cs2SnI6 modified separator (cathode side) remains largely unchanged, strongly confirming the good stability of the Cs2SnI6 separator during cycling (Fig. S19, ESI†). In summary, the Cs2SnI6 modified separator effectively suppresses the shuttle effect, which is mainly related to its excellent adsorption and catalytic properties for LiPSs. Subsequently, a long-cycle test was conducted at a high current rate to evaluate the long-cycle stability of the battery. At 2C, the initial capacity of the Cs2SnI6 battery remains as high as 1000 mA h g−1 (Fig. 5h), which is still kept at 660 mA h g−1 after 500 cycles, corresponding to a small capacity decay rate of only 0.068% per cycle. In sharp contrast, the Cs2SnBr6 and Cs2SnCl6 batteries display significant capacity degradation just after about 300 cycles. The higher cycling stability of the Cs2SnI6 battery is attributed to the modulation of the p-band center induced by halogen ions, which enhances LiPSs adsorption and catalytic ability of Cs2SnI6, thus effectively suppressing the shuttle effect of LiPSs.
The performance of LSBs assembled with the Cs2SnI6 modified separator under high sulfur load was further evaluated to check their potential application in high-energy-density devices. The gained initial capacities of Cs2SnI6 batteries with sulfur loadings of 3.0, 5.4, and 6.1 mg cm−2 at 0.2C are 928.6, 838.5, and 768.8 mA h g−1, respectively. Typically, the charge–discharge curve of the Cs2SnI6 battery reveals two distinct plateaus even at a high sulfur loading of 6.1 mg cm−2 (Fig. S20a, ESI†). Under the conditions of sulfur loading of 3.0, 5.4, and 6.1 mg cm−2 and corresponding E/S ratio of 8.0, 6.0, and 5.5 μL mg−1, the recorded charge–discharge plots of the Cs2SnI6 battery show almost overlapping voltage plateaus (Fig. S20b–d, ESI†), identifying that the separator modified by Cs2SnI6 has high sulfur redox chemical reversibility. Considering the multi-scenario application of LSBs, their battery performance in extremely high and low temperature environments is also of great significance. To evaluate this, cyclic performance tests were conducted on the Cs2SnI6 battery over a wide temperature range of −20 to 50 °C. The initial capacities of the Cs2SnI6 battery at −20 °C (0.1C) and 50 °C (0.5C) are 912.7 and 1350.7 mA h g−1, respectively, coupled with the reversible capacities of 772.6 and 892.3 mA h g−1 after 100 cycles, respectively (Fig. 5j). Although the polarization overpotential of the Cs2SnI6 battery over the wide temperature range increases with decreasing temperatures (Fig. S21, ESI†), it still maintains a modest overpotential, which proves that the Cs2SnI6 battery can undergo rapid LiPS conversion over such a wide temperature range, guaranteeing its high initial specific capacity and excellent cycling stability even at extreme low and high temperatures. Compared with other electrocatalytic materials under the same testing conditions, the Cs2SnI6 battery exhibits excellent performance under long cycling (Table S2, ESI†), high sulfur loading, low E/S ratio (Table S3, ESI†), and extreme temperature conditions (Table S4, ESI†). The above electrochemical performance tests established the regulatory relationship between different halide anions of Cs2SnX6 (X = Cl, Br, I) and electrochemical properties of LSBs. As the atomic number of the halogen increases, the rate performance, high-loading discharge capability, and cycling stability of LSBs are progressively enhanced. Among them, the Cs2SnI6 battery emanates superior electrochemical performance. Such distinctive improvement is primarily attributed to the effective adsorption of LiPSs by Cs2SnI6, which significantly suppresses the shuttle effect of LiPSs. Additionally, the inherent catalytic ability of Cs2SnI6 substantially accelerates the conversion reactions of LiPSs, further enhancing the overall electrochemical performance.
To thoroughly elucidate the specific catalytic mechanisms of Cs2SnX6 for the redox reactions of LiPSs, an atomic-level analysis was further carried out. Accordingly, the adsorption models of Cs2SnX6 with different LiPSs generated at various charge–discharge stages were constructed through DFT calculations. The Gibbs free energy distribution of Cs2SnX6 (X = Cl, Br, I) surfaces from S8 to Li2S is shown in Fig. 6a. The thermodynamic analysis of the continuous reduction pathway reveals that all transformation steps from Li2S8 to Li2S exhibit positive Gibbs free energy changes, indicating the thermodynamically nonspontaneous characteristics, which necessitate the use of catalysts to facilitate LiPS conversion. Overall, the ΔG value of electrochemical reactions from Li2S8 to Li2S in the Cs2SnI6 system is smaller than those in Cs2SnBr6 and Cs2SnCl6 systems, indicating that the electrochemical conversions of LiPSs are more favorable on the surface of Cs2SnI6. In particular, in the rate-limiting processes of liquid–solid conversion from Li2S4 to Li2S2 and solid–solid conversion from Li2S2 to Li2S, the ΔG values of Cs2SnI6 are 0.31 and 0.42 eV, respectively, which are lower than those of Cs2SnBr6 (0.39 and 0.70 eV) and Cs2SnCl6 (0.45 and 0.84 eV), suggesting that the sulfur reduction reaction of Cs2SnI6 is thermodynamically more favorable. The decomposition of Li2S is the first step in the charging process of LSBs and involves a solid–liquid phase transition, which results in relatively slow reaction kinetics. Consequently, the decomposition rate of Li2S is critical to the charging process of LSBs. The calculated energy barriers for the decomposition of Li2S on the surfaces of Cs2SnX6 (X = Cl, Br, I) reveal that Cs2SnI6 has a decomposition barrier of 0.62 eV (Fig. 6b), which is lower than those of Cs2SnBr6 (0.92 eV) (Fig. 6c) and Cs2SnCl6 (1.32 eV) (Fig. 6d). Such a finding fully substantiates that, during the charging process of LSBs, Li2S undergoes accelerated dissociation on the surface of Cs2SnI6, which is in good agreement with the aforementioned electrochemical results. The calculation results show that the thermodynamic and kinetic processes can be improved by changing the halogen in the Sn-based halide double perovskite, which can simultaneously catalyze both the reduction of LiPSs and the oxidation of Li2S. Specifically, as the atomic number of the halogen increases, the p-band center gradually approaches the Fermi level and the band gap decreases. These changes promote electron transfer, which is beneficial for the transformation of S species, thereby weakening the energy barriers in LiPSs reduction process and Li2S oxidation process.
The action mechanism of the Cs2SnI6 modified separator in LSBs is schematically illustrated in Fig. 6e. Clearly, during the discharge process, a large amount of soluble LiPSs is present in the cathode region, while the anode region only contains Li+. This demonstrates the effective inhibition of LiPSs shuttle by the Cs2SnI6 modified separator. The Cs2SnI6 modified separator effectively inhibits LiPSs shuttle due to its capability to chemically adsorb soluble LiPSs and the catalytic activity of surface I sites in promoting LiPSs conversion. The generated Li2S can be uniformly distributed around the active sites. Additionally, the I sites on the surface of Cs2SnI6 also catalyze the oxidative reaction and decomposition of Li2S, re-exposing the active sites for subsequent adsorption and catalysis. These structural characteristics of Cs2SnI6 modified separators ensure excellent catalytic effects on both oxidation and reduction reactions in LSBs, thereby effectively inhibiting LiPSs shuttle and enhancing the electrochemical performance of LSBs.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sc01266j |
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