2.1 The construction and electrochemical performance of quasi-solid-state Al−S batteries
The design principle of quasi-solid-state aluminum-sulfur (Al−S) batteries and its working mechanism are illustrated in Figure 1a. The cobalt-nitrogen co-doped graphene (CoNG) is elected as the sulfur host for positive electrode ([email protected]), and the zirconium-based metal-organic frameworks (Uio-66, MOF) immersed in ionic liquid ([email protected]) serve as the quasi-solid-state electrolyte for Al−S batteries. The CoNG can catalyze both the dissociation of the Al3+ and reactions of sulfur conversion. Meanwhile, the [email protected] quasi-solid-state electrolyte plays a role in delivering active ions (AlCl4− and Al2Cl7−) and inhibiting the shuttle effect of polysulfide due to its narrow channels. The characterization and performance of CoNG electrocatalyst and [email protected] quasi-solid-state electrolyte will be further discussed.
The quasi-solid-state Al−S batteries was assembled with composite positive electrode, in which the [email protected] was mixed with [email protected] and acetylene black (AB) and pressed into a circular pellet. The optimal composition of [email protected]: [email protected]: AB was first affirmed by galvanostatic charge and discharge processes (Figure S1). Apparently, the samples with [email protected]: [email protected]: AB ratios of 7: 6: 1 exhibit the highest discharge capacity (~820 mAh g−1) and the smallest charge-discharge voltage gap (~0.43 V), and this positive electrode composition is used in the following tests. Figure 1b shows the cyclic voltammetry (CV) curves of the Al−S battery within the voltage range of 0.1−2.0 V. One cathodic peak at 0.81 V and two anodic peaks at 1.43 V and 1.59 V are observed in the first scan, which corresponds to the reduction of sulfur and the oxidation of sulfide, respectively.9,10 Simultaneously, the CV curves at subsequent scanning cycles show similar shape and size, representing a superior reversibility. Figure 1c demonstrates the initial three charge/discharge curves of the battery at a current density of 50 mA g−1. One discharge voltage plateau appears at 0.83 V, and two charge voltage plateaus appear at 1.42 and 1.55 V in 1st cycle, in good agreement with the CV curves. Subsequently, a higher discharge voltage plateau at 0.90 V and only one charge voltage plateau at 1.33 V in subsequent two cycles are observed. The lower voltage hysteresis and reduced amount of charge voltage plateau may be attributed to improved electrolyte infiltration, and the sluggish ion transmission results in the two charge voltage plateaus in the first cycle. The electrochemical impedance spectroscopy (EIS) and corresponding distribution of relaxation times (DRT) analysis are employed to further investigate the reaction dynamics during the first few cycles. Before cycling, the Nyquist plots were composed of two depressed semicircles at high frequencies corresponding to the interfacial charge transfer resistance (Rct). Whereas, after the initial cycle, only one semicircle remains and its radius decreases, meaning a reduced Rct (Figure S2a). The DRT analysis confirms that the response time of ion diffusion has moved forward after cycling, thus resulting in sharp decrease of electrochemical reaction impedance (Figure S2b).30 Therefore, the decreased Rct may be due to the formation of a stable interface between the sulfur and [email protected] electrolyte in composite positive electrode, which can also explain the change in voltage plateaus in the first two cycles.
Figure 1d shows the rate performance of the quasi-solid-state Al−S battery. It can deliver an overall capacity of 820 mA h g−1 at 50 mA g−1, and the capacity of the sulfur positive electrode (not including doped graphene host) can reach as high as 1335 mA h g−1 calculated using the known sulfur weight ratio (61.4%, Figure S3). It is clear that the Al−S battery shows stable reversible capacities of 580, 450, and 202 mAh g−1 at the current densities of 100, 200, and 300 mA g−1, respectively. Furthermore, the rate capability curves are highly reversible (no hysteresis) when cycled at the same rate (Figure S4), attributing to the efficient ion transport in the interface between the sulfur and solid-state electrolyte. In addition, the cycle performances of liquid-state and quasi-solid-state Al−S batteries are evaluated and compared (Figure 1e). For the liquid-state Al−S battery, although it can deliver a reversible capacity of 945 mAh g−1 in the first cycle (at a current density of 50 mA g−1), the capacity remains at 175 mAh g−1 after only 100 cycles. The poor capacity retention of liquid-state battery is ascribed to the shuttle effect of polysulfides, as described in the previous reports.8,11,15 In comparison, the initial capacity of quasi-solid-state Al−S battery is slightly lower than that of liquid-state one, whereas it could still deliver a high capacity of 640 mAh g−1 after 300 cycles with an excellent capacity retention of 78%, revealing a stable cycle stability. The superior cycle stability of quasi-solid-state Al-S batteries is attributed to the successful suppression of shuttle effect of polysulfide by [email protected] quasi-solid-state electrolyte. Most importantly, compared to the documented RABs, the relation of specific capacity versus discharge potential of such quasi-solid-state Al−S battery implies that the performance is competitive with the record-setting values in terms of energy density (Figure 1f and Table S1).9,14,15,31−41
2.2 The structure characterization and evolution of sulfur positive electrode
The morphology and structure of CoNG and [email protected] are characterized by scanning electron microscopy (SEM) and transmission electron microcopy (TEM). The CoNG consists of a typical wrinkled sheet-like structure (Figure 2a), which is beneficial to sulfur loading. No obvious nanoparticles are found on the surface of the graphene sheet (Figure S5a), indicating the absence of residual Co particles in CoNG. Meanwhile, the elemental analysis reveals that Co, N and C elements are distributed in CoNG and these composition distributions are in accordance with the observed morphology (Figure S6). The SEM and TEM results indicate that the morphology of the CoNG composite remains intact after sulfur loading (Figure 2b), and the sulfur element is clearly detected in [email protected] sample (Figure S5b and S7), thus confirming the successful loading of sulfur on [email protected] The [email protected] powder was mixed with [email protected] quasi-solid-state electrolyte and acetylene black and pressed to form the positive electrode (Figure 2c), and the three components in mixture are uniform dispersion and seamless contact to afford a favorable electron and ion transport network, guarantying a high-performance sulfur positive electrode.
X-ray photoelectron spectroscopy (XPS) is then used to analyze the valence state and chemical composition of C, N, and Co in CoNG. The C 1s XPS spectra can be fitted to three peaks centered at 284.8, 285.4, and 286.8 eV, corresponding to sp2-hybridized C−C, C−N, and C−O, respectively (Figure S8).42 Meanwhile, the N 1s spectra exhibit two major components (Figure 2d), which could be ascribed to pyridinic-N (398.5 eV) and graphitic-N (401.5 eV). Notably, a distinct peak located at 399.3 eV is indexed to the Co-N bond, suggesting that some of the nitrogen atoms are integrated into the graphene lattice along with cobalt atoms.43,44 Furthermore, in the Co 2p3/2 signal (Figure 2e), the peaks located at 780.5 eV and 782.5 eV are ascribed to Co−N4 and Co−N, respectively, without any metallic Co signal (778.5 eV),45,46 indicating that cobalt enters the carbon skeleton mainly in the form of Co−Nx structures.
X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were further conducted to analyze the coordination mode of Co atoms in CoNG. The results for standard samples including Co foil and Co2O3 were also probed as references. The XANES profile at the Co K-edge of CoNG is significantly different from those of the reference samples, and the pre-edge peak at 7705−7715 eV of CoNG situates between Co foil and Co2O3 (marked by the dotted box in Figure 2f), indicating that Co atoms in CoNG are positively charged. As shown in Figure 2g, the Fourier-transformed EXAFS (FT-EXAFS) curve for CoNG exhibits one strong peak at about 1.45 Å, which is slightly shorter than Co−O (1.52 Å) distance in the standard Co2O3 sample, probably due to the dominated bonding between Co and N.47 Meanwhile, a Co−Co peak (2.18 Å) is noticed for the standard Co foil, which is almost undetectable in CoNG, suggesting the absence of metallic Co in CoNG. Based on this observation, we performed the tetrahedral geometry (Co−N4) structure model (inset in Figure 2h) to fit the FT-EXAFFS curve of CoNG. The fitted coordination number of Co (3.8) in Figure 2h is in good agreement with tetrahedral Co−N4 structure, and the calculated Co−N bond length is 1.82 Å (Table S2), indicating that Co atom coordinates with four N atoms with a scatting distance of 1.82 Å in CoNG.42,47 The k-spaces results confirm that the fitting result is in good agreement with the experimental data (Figure S9). To more clearly understand atomic dispersion of Co atom, the wavelet transform (WT) was performed to analyze the k2-weighted EXAFS signal in view of the powerful resolution in radial distance and k-space.48 The Co foil shows one WT maximum intensity at 6.9 Å−1 corresponding to Co−Co coordination, and Co2O3 has a WT maximum at 6.3 Å−1 due to Co−Co and another WT maximum at 4.8 Å−1 assigned to Co−O (Figure S10). For CoNG, the WT contour plot appears one intensity maximum at 4.7 Å−1, corresponding to Co−N coordination (Figure 2i). Based on the above analysis, it can be concluded that Co atoms are atomically dispersed and coordinated with four N atoms to form Co−N4 centers in CoNG.
Subsequently, the phase transition and structural evolution of [email protected] positive electrode during charge and discharge processes were investigated. Before cycling, the X-ray diffraction (XRD) pattern of [email protected] shows a series of characteristic diffraction peaks of sulfur and a weak carbon peak (Figure S11), representing a high sulfur loading. For the discharged sample, the peaks of sulfur disappear and are replaced by Al2S3, a discharging product in this system. When charging to 1.8 V, the characteristic peaks of sulfur reappear again, indicating a highly reversible electrochemical process. The evolution in the chemical state of sulfur during the charge and discharge processes is further investigated by ex-situ XPS and Raman spectrum (Figure 2j-l). During the discharge process, the signal of S shifts significantly toward a smaller binding energy, and the distinct peak of Sx− (163 eV, Al2Sx) appears and gradually increases with discharge depth (Figure 2k), corresponding to the formation of polysulfides.49,50 During the subsequent charging process, the signal of Sx− gradually disappears until it returns to the original state, suggesting that the Sx− are continuously oxidized.51 Meanwhile, the reduction of sulfur undergoes several stages in the discharge process, in while the S8 (468 cm−1 and 512 cm−1) molecules are first reduced to S3− (418 cm−1) and then successively reduced to S2− (135 cm−1), S− (161 cm−1), and the end product S2− (190 cm−1)52,53, as shown in Figure 2l. Likewise, the reduction products S2− are also progressively oxidized to form S−, S2−, S3−, and S8 molecules in the charging process, revealing highly reversible redox process. The above results demonstrate that, for the sulfur positive electrode, sulfur is reduced to form aluminum sulfide in the discharge process, and then oxidized to sulfur in the subsequent charge process. The electrochemical reaction mechanism can be described by the following equations:
Positive electrode: 8Al2Cl7− + 6e− + 3S ⇌ Al2S3 + 14AlCl4− (1)
Negative electrode: 2Al + 14AlCl4− ⇌ 8Al2Cl7− + 6e− (2)
Overall reaction: 3S + 2Al ⇌ Al2S3 (3)
2.3 The electrocatalytic effect of CoNG on the sulfur conversion
Based on the above reaction mechanism and previous works on Al−S batteries, the electrochemical conversion of sulfur and aluminum sulfide would undergo high kinetics energy barrier, ascribing to the insulating nature of sulfur, the tough release of Al3+ from Al2Cl7−, and the difficulties in breaking three robust Al−S bonds (332 kJ mol−1 for Al–S single bond).44 The sluggish kinetics leads to a high charge–discharge voltage gap, which is usually higher than 0.6 V in the previous reports.9,14,15 Apparently, the voltage gap is as low as 0.43 V in our quasi-solid-state Al−S battery, which could be related to the CoNG host. To figure out the functional mechanism of CoNG host, a series of contrast samples are synthesized for comparison, including graphene (G), nitrogen doped graphene (NG) and cobalt decorated graphene (CoG), which are further used as sulfur host to form [email protected], [email protected] and [email protected], respectively.
Similar to CoNG, these contrast samples also present a sheet-like structure (Figure S12a-c). Particularly, many small particles are found on the graphene sheet in CoG sample (Figure S12c). After loading sulfur, these materials remain intact sheet structure and sulfur is distributed on their graphene surfaces (Figure S12d-f). The XRD results further confirm that sulfur is successfully loaded onto these carbon host (Figure S13a). Note that the Co diffraction peak exists in CoG and [email protected] samples, indicating that these small particles distributed on the graphene are elemental cobalt. To remove redundant Co particles, the CoNG is treated by acid soaking during the synthesis process. Obviously, after acid treatment, no elemental Co remains in the CoNG sample (Figure S13b), which is consistent with the XPS results (Figure 2e). All CoNG samples used in this work are obtained with the above method.
Figure 3a shows the charge/discharge profiles of liquid-state Al−S batteries assembled by [email protected], [email protected], [email protected] and [email protected] positive electrodes, respectively. It is clear that the discharge capacities are in the order of [email protected] > [email protected] > [email protected] > [email protected] Moreover, the discharge voltage and voltage hysteresis (∆E) of these batteries are compared (Figure S14), among which [email protected] exhibits the highest discharge voltage of 0.98 V. Meanwhile, the largest voltage hysteresis of 1.2 V is obtained for [email protected], whereas a significant reduction of the voltage hysteresis is observed in the nitrogen (and cobalt)-doped or cobalt-decorated graphene samples. In particular, the [email protected] Al−S battery possesses the smallest voltage hysteresis of 0.32 V, which is superior to that of all previous reports.8–11, 14,15 Obviously, the improved electrochemical performance of [email protected] was derived from the CoNG host, which may be beneficial to accelerate the entire reaction kinetics. To further evaluate the effects of CoNG on the electrochemical conversion of sulfur, half cells and symmetric cells are assembled with these four types of hosts as electrode, respectively. CV curves are conducted on half cells with host materials as working electrodes and Al foil as counter electrodes in the IL electrolyte containing sulfur (S+IL), as shown in Figure 3b. Each CV curve exhibits a pair of redox peaks, which is agreement with the CV results of the Al−S batteries with four sulfur-based positive electrodes (Figure S15), indicating that the pair of redox peaks correspond to the reduction of sulfur and the oxidation of aluminum sulfide. Note that the reduction peak for CoNG shows the most positive potential and the largest peak current among these four host materials, further confirming that CoNG could promote the reduction kinetics of sulfur. Figure 3c presents the CV curves of symmetric cells with identical working and counter electrodes in the IL electrolyte containing sulfur and aluminum sulfide (S+Al2S3+IL). The CV curve of CoNG exhibits a pair of reduction/oxidation peaks located at −0.44 V (Peak A) and 0.45 V (Peak B). Peak A in the cathodic scan arises from the reduction of sulfur on the working electrode and oxidation of aluminum sulfide on the counter electrode, and peak B is due to the oxidation of aluminum sulfide to generate sulfur on the working electrode.14,47 The CV curves of NG and CoG also show a pair of redox peaks, but the potential difference between the cathodic and anodic peaks is much higher than that of GoNG, and the CV peaks are barely visible for G electrodes. Furthermore, Tafel plots have been derived to analyze the effect of four graphene-based materials on redox reaction kinetics of sulfur (Figure 3d). Obviously, GoNG shows the lowest Tafel slope of 219 mV dec−1 for the sulfur reduction and 173 mV dec−1 for sulfide oxidation among the four host materials, indicating the accelerated sulfur conversion. Meanwhile, exchange current densities (j0) were calculated from the Tafel plots, which reflect the intrinsic electron transfer rate of reaction (Table S3). The CoNG electrode shows the largest exchange current densities for both the cathodic process and the anodic process, which are 4.12 and 0.19 mA cm−2, compared to those of the G (1.39 and 0.01 mA cm−2), NG (1.98 and 0.02 mA cm−2) and CoG (2.63 and 0.09 cm−2). Thus, the increase of exchange current density values clearly validates more faster charge transfer induced by CoNG in both charge and discharge processes. Note that these four host materials possess similar graphene-like morphology except different heteroatoms doping, and the reaction kinetics of sulfur conversion on CoNG is the fastest. Therefore, the Co−N center in CoNG should be the major reason responsible for the improved electrochemical performance.
To verify the effect of Co−N structure on sulfur conversion, the evolution of chemical states for Co and N in [email protected] Al−S batteries are studied by ex-situ XPS. In the N signal of pristine sample, there are three peaks assigned to pyridinic-N (Py N), Co-N and graphitic-N (Gr N), respectively (Figure 3e). In the case of the charging/discharging intermediate products (discharge to 0.8 V and charge to 1.3 V, as abbreviated to D-0.8V and C-1.3V, respectively), a new peak (400.5 eV) ascribed to the Co−N−Al signal is found, indicating that the Co−N structures interact with the aluminum-based cluster (AlxCly and AlxSy) during cycling.54 Likewise, compared with the Co signal of the pristine sample, the intermediate product also presents a new peak (778.8 eV) allocated to S−Co−N (Figure 3f), suggesting the interaction between the Co−N structure and sulfur-based compounds (AlxSy) in the charge and discharge processes.14 Based on the above XPS results, we speculate that the Co−N center could catalyze the breaking of the Al−Cl and S−S bonds during the discharge process and the breaking of the Al−S bond during the charge process.
To further understand the catalytic mechanism of Co−N structure on the electrochemical reaction, the first-principles calculations are performed to investigate the possible reactions from sulfur to aluminum sulfide in the four host materials. Based on above Raman results (Figure 2l), the models of four graphene-based hosts and various reaction primitives are considered (Figure S16), and the electrochemical reduction reaction of sulfur is illustrated in Figure 3g, including dissociation of Al3+ from Al2Cl7− and transformation from S to Al2S3. Firstly, the Al3+ release pathways are investigated, and the first step involves decomposition of Al2Cl7− to form AlCl4−, which is stepwise dissociated into AlCl2+, AlCl2+, and end product Al3+ (Figure S17). The Gibbs free energies are calculated for the above reactions on the four graphene-based substrates (Figure 3h and Table S4). It is clear that all reactions are unspontaneous, and the formation of AlCl2+ from AlCl4− has the largest positive Gibbs free energy (∆G2), indicating that this is the rate-limiting step in the whole process. This value is 1.22 eV for CoNG, which is lower than that of G (1.69 eV), NG (1.55 eV) and CoG (1.41 eV), manifesting that the formation of Al3+ is thermodynamically most favorable on CoNG substrate. During the subsequent S reduction process, the S8 couples with Al3+ ions and undergoes further reduction with the stepwise formation of Al2S18, Al2S12, Al2S6, and end product Al2S3 (Figure S18 and Table S5).55 Note that after the spontaneous exothermic conversion from S8 to Al2S18, the subsequent three steps to form Al2S12, Al2S6, and Al2S3 are either endothermic or nearly thermoneutral (Figure 3i). Obviously, the formation of Al2S3 from Al2S6 has the largest positive Gibbs free energy (∆G4), corresponding to the rate-limiting step in this process. Likewise, the energy barrier of this process on CoNG (2.25 eV) is the lowest among the four host materials (3.62 eV on G, 3.03 eV on NG and 2.70 eV on CoG), clearly confirming that Co−N structure in CoNG serves as catalytic sites to accelerate Al2S3 formation. To further reveal how the CoNG accelerate the two rate-limiting steps, the electron distribution is simulated to evaluate the charge transfer process. During the dissociation of AlCl4− into AlCl2+ on CoNG, the Bader charge of Co atoms and N atoms significantly increases from 8.0 to 8.6 and from 6 to 6.5 respectively, along with an unchanged state for Al and Cl atoms (Figure 3j, k), suggesting that the Co−N structure loots the charge from Al atoms and eventually breaks the Al−Cl bonds. For the formation of Al2S3 from Al2S6 on CoNG, the Bader charge of S increases from 6.5 to 7.2 and the charge of Co decreases from 8.0 to 7.1 (Figure 3l, m), indicating that Co atoms donate electrons to S atoms and cause the breaking of S−S and S−Al bonds. The theoretical calculation results confirm that the Co-N structure acts as the active center to induce the breaking of Al−Cl and S−S bonds, thus accelerating the kinetics of sulfur reduction reaction.
2.4 The electrochemical performance and restriction on polysulfide of [email protected] quasi-solid-state electrolyte
The porous zirconium-based MOF was selected as the host for impregnation of IL to construct [email protected] quasi-solid-state electrolyte. The BET surface area of the MOF is 1611 m2 g−1 and the dominating pore sizes are 0.98 nm and 1.25 nm according to the N2 adsorption/desorption isothermal tests (Figure S19a, b). The size of IL ions (AlCl4−, Al2Cl7−, and EMI+) have been calculated to be less than 0.8 nm in the longest dimension, so the porous MOF can load a large amount of IL and allow IL ions to pass through.56 After IL impregnating, the BET surface area sharply declines to only 18.6 m2 g−1 and the pore size distribution is almost nonexistent for [email protected] composite (Figure S19c, d), indicating that the pores of the MOF have been almost filled up with IL ions. Meanwhile, for [email protected], the characteristic XRD peaks of MOF become weak and most of them disappear as reported in previous works (Figure S20),57 further confirming the successful impregnation of IL. The SEM images show that the synthesized [email protected] crystals are regular polyhedron with a particle size of about 200~300 nm (Figure 4a), and plentiful Al elements are detected in [email protected] (Figure S21), indicating the uniform distribution of chloroaluminate-based ions in MOF. The [email protected] particles are pressed into circular pellet (inset in Figure 4a) for further testing.
High ionic conductivities of 4.2 × 10−4 and 2.5 × 10−3 S cm−1 for [email protected] are observed at 20 and 100°C (Figure 4b), respectively, indicating a fast ion migration under a wide temperature range. Meanwhile, the activation energy (Ea) of [email protected] is evaluated to be 233 meV (Figure S22), manifesting an efficient ion transport. Apparently, the high ionic mobility of [email protected] is derived from IL ions, since the MOF is an ionic insulator and merely serves as a porous framework to enable rapid IL ions movement.58 Furthermore, the [email protected] electrolyte can achieve reversible deposition and stripping of aluminum with a small polarization potential (Figure S23), demonstrating free movement of chloroaluminate-based ions (Al2Cl7− and AlCl4−). The interface features between [email protected] electrolyte and Al metal are evaluated by the direct current Al plating/stripping cycles, as shown in Figure 4c. A large polarization voltage of ±98 mV is observed in the initial cycle at 0.05 mA cm−2, indicating a high interfacial impedance between [email protected] and Al metal. During subsequent cycles, the polarization voltage decreases and stabilizes at 71 mV, and it remains at 83 mV over 200 h at a higher current density of 0.1 mA cm−2 (Figure S24), representing the formation of a stable [email protected]/Al interface layer with high ion transport. The above results demonstrate that the [email protected] electrolyte possesses high active ion transport and can establish a stable interface layer with Al metal for long-time cycling, meeting the requirements for high-performance Al−S batteries.
Sulfur, polysulfide and aluminum sulfide could dissolve in IL electrolyte to some extent 10,15. In the liquid-state Al−S battery after 20 cycles, black substance can be clearly seen on both the conventional separator (GF/A) and Al foil (Figure S25). The black substance is detected by XPS and proved to be sulfur and aluminum compounds, which must have originated from the sulfur positive electrode. This result indicates that sulfur and intermedium will pass through the separator and deposit on the Al negative electrode during the cycling, which is responsible for the poor cycle stability of liquid-state Al−S battery (Figure 1e). The [email protected] electrolyte is used in this work to inhibit the shuttle effect of these active substances. To verify the suppression of sulfur-based compound diffusion, permeation experiments are conducted using H-type cell for visualization, in which the IL electrolyte with dissolved S and Al2S3 (left chamber) and blank IL electrolyte (right chamber) are separated by the GF/A separator and [email protected] pellet, respectively. As illustrated in Figure 4d, the solution in right chamber gradually darkens when routine GF/A separator is used. For the case of [email protected], no visible change in color of blank electrolyte is found even after 48 hours, indicating the effective inhibition of the shuttle effect. After permeation experiment, the solution in right chamber is further monitored with Fourier transform infrared spectroscopy (FTIR). The sulfur signal is clearly observed in the solution when GF/A separator is used, but is not found when using [email protected] (Figure 4e), suggesting that S and Al2S3 could not travel through the [email protected] Apparently, polysulfides with larger sizes are also unable to pass through the [email protected] in quasi-solid-state Al−S batteries, fundamentally addressing the issue of shuttle effect. To further evaluate the inhibition ability of [email protected] toward these sulfur-based compounds, element information at three different points inside cycled quasi-solid-state Al−S battery is detected, as shown in Figure 4f and 4g. The S and Sx− (Al2Sx and Al2S3) signal are obviously detected in point 1 (in the composite positive electrode) and point 2 (the interface between positive electrode and [email protected] electrolyte), but there is no sulfur signal at point 3 (in the middle of the [email protected] electrolyte), further confirming the effective suppression of polysulfide diffusion. For [email protected], the narrow pore of MOF (size: ~1.2 nm) is filled up with the chloroaluminate-based IL ions, which effectively inhibits the shuttle effect of soluble sulfur compounds, achieving an excellent cycle stability of quasi-solid-state Al−S batteries.