Accelerated Sulfur Evolution Reactions by TiS2/TiO2@MXene Host for High‐Volumetric‐Energy‐Density Lithium–Sulfur Batteries

The poor cycling stability and low volumetric energy density of lithium–sulfur batteries compared with lithium‐ion batteries are hindering their practical applications. Here, it is demonstrated that a dense sulfur electrode containing heavy TiS2/TiO2@MXene heterostructures can tackle these issues. It is observed that the TiO2 part functionally anchors the lithium polysulfides through the strong chemical affinity, and the TiS2 part serves as an efficient electrocatalyst to enhance the kinetics of sulfur evolution reactions. Benefitting from these synergistic effects, the TiS2/TiO2@MXene heterostructures effectively suppress the shuttle effects, leading to superior cyclability of the sulfur cathode with a low capacity decay of 0.038% per cycle for 500 cycles at a current rate of 1 C. More encouragingly, a highly dense S/TiS2/TiO2@MXene cathode exhibits a high volumetric energy density of 2476 Wh L−1 (based on the volume of the composite) at a high sulfur mass loading of 7.5 mg cm−2 and lean electrolyte of 5 µL mg−1. The electrochemical performance is comparable to or even superior to the lithium‐ion and lithium–sulfur batteries reported in the literature. This study provides an effective strategy to design stable and high‐volumetric‐energy‐density lithium–sulfur batteries for practical energy storage applications.


Introduction
The endless demand for a battery with high volumetric/gravimetric energy density likely will not be finished unless a new secondary battery to outperform the current batteries comes into the market for portable electronic devices and electric vehicles. [1][2][3] Among the various batteries, lithium-sulfur (Li-S) batteries look promising because Li-S batteries are known to have a high theoretical energy density (2600 Wh kg −1 or 2800 Wh L −1 ). [4,5] The practical gravimetric energy density of Li-S batteries has been demonstrated to be superior when compared with that of lithiumion (Li-ion) batteries. [6] However, the volumetric energy density of Li-S batteries (325-581 Wh L −1 ) is less competitive (≈670 Wh L −1 for Li-ion batteries). [7] The inferior volumetric energy density can be attributed to the low tap density of the sulfur cathode (less than 1.0 g cm −3 ). [8] In addition, the shuttle effect of soluble lithium polysulfides (LiPSs, Li 2 S n , n = 4-8), sluggish reaction kinetics, and the insulating nature of S and Li 2 S 2 /Li 2 S cause declined utilization efficiency of the active material and poor cycling stability. [9][10][11] These obstacles are degrading the performance of Li-S batteries and waiting for a proper technical solution.
Carbonaceous materials, such as carbon spheres, carbon nanotubes, graphene, and carbon cloth, have widely been used as sulfur hosts in cathodes. [12][13][14] Their high electrical conductivity can alleviate the insulating effect of S and Li 2 S 2 /Li 2 S. However, unfortunately, the seemingly advantageous properties of carbonaceous materials (such as the lightweight structure and the large pore volume) could cause a low tap density of the cathode and excessive usage of the electrolyte. [15,16] Moreover, under high sulfur mass loading conditions, the volume of the carbon-based cathode is drastically expanded. It inevitably leads to the declined volumetric energy density of Li-S batteries. [17] Therefore, developing novel sulfur host materials having high density is crucial for achieving the high volumetric energy density of Li-S batteries.
MXenes, the newest class of 2D transition metal carbides and nitrides, can act as a conductive framework in Li-S batteries and is a promising host to resolve the aforementioned hurdles in some way because it has high electronic conductivity to enhance charge transfer kinetics, allowing for significant improvements in sulfur utilization. [18][19][20][21][22] In addition, the high density of MXenes (3.8 g cm −3 for Ti 3 C 2 T x film [23] ) is highly advantageous to improve the volumetric energy density of the Li-S batteries. However, numerous F-terminal groups on the surface of MXenes generated during the preparation process have been reported to greatly restrict the activity for capturing and catalyzing the LiPSs. [24][25][26] Moreover, numerous studies have also demonstrated that modified MXenes showed a better performance compared to the sole MXenes. This indicated that even the nice-looking MXenes need to be further modified to realize the Li-S batteries with better cycling stability and higher energy density. To improve the efficiency of MXenes, several strategies of surface modification by decorating with adsorptive materials (TiO 2 [26][27][28] ) or catalysts (VS 2 , [25] CoS 2 , [29] single-atom Zn [30] ) have been proposed. Yet, the reported strategies seem to asymmetrically increase the adsorption capability or the catalytic activity. For instance, although introducing adsorptive TiO 2 on the surface of MXenes can improve the adsorption capability, the trivial catalytic activity remains nearly unchanged. Meanwhile, the presence of VS 2 or CoS 2 catalysts can enhance the catalytic activity of MXenes, whereas the change in the trivial adsorption capability is less noticeable. This is because a single additive always leads to a single side of functionality. [31] Moreover, it is well known that the performance of Li-S batteries is highly dependent on both the LiPSs adsorption capability and the catalytic activity of the sulfur host rather than the individual effect. [32] Therefore, simultaneously and equally enhancing the adsorption ability and catalytic property of MXenes could maximize their efficiency in regulating sulfur evolution reactions. Due to the lack of a proper modification strategy, a modified MXene with improved and balanced adsorption and catalytic properties has not been reported yet.
Recently, the construction of heterostructures to achieve cooperation between different components has been regarded as an effective strategy for assigning both adsorptive and catalytic properties to the selected hosts. [31,[33][34][35][36] For instance, Liu et al. demonstrated that the VO 2 -VN binary host generates synergistic effects of VO 2 and VN to promote the adsorption ability and catalytic activity toward LiPSs. [33] Yang et al. prepared a twinborn TiO 2 -TiN heterostructure combining the merits of highly adsorptive TiO 2 with conducting TiN to enable smooth trapping-diffusionconversion of LiPSs. [34] These previous studies gave us a hint that building a proper heterostructured material on the surface of MXenes could make MXenes simultaneously have high adsorption capability and good catalytic activity.
Here, we took advantage of the presence of titanium in the Ti 3 C 2 T x MXene to modify its surface. By a one-step partial sulfurization of few-layer Ti 3 C 2 T x MXene sheets, for the first time, a novel TiS 2 /TiO 2 @MXene heterostructure was elaborately designed and utilized as a stable and dense sulfur host to produce high-volumetric-energy-density Li-S batteries. Unlike other modified MXenes, the novel TiS 2 /TiO 2 @MXene heterostructure simultaneously and equally possesses a high LiPSs adsorption capability and excellent catalytic activity. The high LiPSs adsorption capability is donated by the TiO 2 part, while the excellent catalytic activity is endowed by the TiS 2 part. Thanks to the balanced adsorption-catalysis synergistic effect, the TiS 2 /TiO 2 @MXene heterostructure could effectively inhibit the shuttle effect and accelerate the conversion reactions of LiPSs. The resulting Li-S batteries using TiS 2 /TiO 2 @MXene heterostructure delivered a high-rate capability of 1227 mAh mg −1 (based on the sulfur mass) at 0.1 C and exhibited ultrastable cyclability with a low capacity decay of 0.038% per cycle for 500 cycles at a current of 1 C. In addition, a highly dense S/TiS 2 /TiO 2 @MXene cathode was observed to have a volumetric energy density of over 2476 Wh L −1 (based on the volume of the composite) at a high sulfur mass loading of 7.5 mg cm −2 and a low electrolyte-to-sulfur (E/S) ratio of 5 μL mg −1 . The electrochemical performance was found to be comparable to or even superior to the Li-ion and Li-S batteries reported in the literature.

Design of the TiS 2 /TiO 2 @MXene Heterostructure
Designing an ideal mediator for LiPSs regulation needs an indepth understanding of the intrinsic linkage between the theoretical prediction and experimental evidence. [37] Theoretically, it is believed that the higher the binding energy between the host material and LiPSs is, the better the adsorption capability of the host material is. [24,38] Moreover, a material can be considered to be a good catalyst if it can effectively promote the sulfur evolution reactions by lowering the activation energy of the redox reactions. [39] Therefore, to alleviate the LiPSs shuttle effect and the sluggish reaction kinetics, the sulfur host material should simultaneously have high binding energy and low barrier energy for LiPSs adsorption and conversion. With this in mind, we first carried out density functional theory (DFT) calculations to predict the interaction behavior of TiO 2 , TiS 2 , and Ti 3 C 2 T x MXene with Li 2 S 6 . As shown in Figure 1a-c, the binding energies of Li 2 S 6 on the surface of MXene with F-terminal groups, TiO 2 , and TiS 2 were calculated to be 1.05, 3.78, and 1.12 eV, respectively. The high binding energy of Li 2 S 6 on TiO 2 indicated that TiO 2 has an advantageous anchoring ability for LiPSs. However, the high barrier energy for the decomposition of Li 2 S molecule into LiS cluster and a single Li atom on the surfaces of MXene and TiO 2 (0.89 and 1.31 eV, respectively) revealed their limited catalytic activity (Figure 1d,e). In contrast, on the surface of TiS 2 , Li 2 S was predicted to be readily dissociated due to the fairly low barrier energy of 0.31 eV (Figure 1f). This prediction indicated that TiS 2 could act as a good catalyst to accelerate the conversion reactions of LiPSs.
The above theoretical calculation results reaffirmed that the sole MXene has weak adsorption and catalyzing capability, thereby limiting the performance of Li-S batteries ( Figure 1g). The decoration of MXene with TiO 2 was believed to likely improve the ability to trap of LiPSs significantly, although high catalytic activity was not able to be expected yet ( Figure 1h). Therefore, we expected that a host in the form of TiS 2 /TiO 2 decorated on the MXene (i.e., TiS 2 /TiO 2 @MXene) could exhibit both the adsorptive activity by TiO 2 and the catalytic property by TiS 2 (Figure 1i). It was naturally thought that the host could restrain the shuttle effect and improve the efficiency of sulfur utilization.

Fabrication and Characterization of the TiS 2 /TiO 2 @MXene Heterostructure
Based on the theoretical guide, we modified the Ti 3 C 2 T x MXene by constructing TiS 2 /TiO 2 heterostructure on its surface. The Figure 1. Design principle of the TiS 2 /TiO 2 @MXene heterostructure. a-c) DFT calculation of the adsorption energies of Li 2 S 6 on the MXene, TiO 2 , and TiS 2 surfaces, respectively. d-f) Summary of energy profiles for the decomposition of Li 2 S on the MXene, TiO 2 , and TiS 2 surface, respectively. IS, TS, and FS stand for the initial state, transition state, and final state, respectively. g-i) Schematic illustration of the conversion process of sulfur on the MXene, TiO 2 @MXene, and TiS 2 /TiO 2 @MXene, respectively. TiS 2 /TiO 2 heterostructure was synthesized via a facile one-pot synthesis method using partial sulfurization of few-layer Ti 3 C 2 T x MXene sheets (Figure 2a). At 600°C, the sulfur powder was turned into sulfur gas and reacted with MXene according to the reaction as follows: Ti 3 C 2 T x + S → Ti 3 C 2 T x ' + TiS 2 + CS 2 (g). [40] Moreover, the formation of TiS 2 on the surface of MXene made the surrounding areas prone to being oxidized by numerous O/OH-terminal groups, [41] thereby leading to the in situ formation of TiO 2 /TiS 2 @MXene heterostructure.
The morphology and microstructure of the TiO 2 /TiS 2 @MXene are characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). Figure S1 (Supporting Information) showed Ti 3 C 2 T x MXene nanosheets possessed a flat and clean surface. After sulfurization, the nanosheets became crumpled, and numerous TiS 2 /TiO 2 nanoparticles appeared (Figure 2b,c). These nanoparticles were uniformly decorated on the surface of MXene nanosheets and had an average size of 10-20 nm (Figure 2d). The high-resolution TEM further revealed that TiS 2 , TiO 2 , and MXene are coupled to each other, and the interface was sharp, demonstrating the in situ formation of TiS 2 /TiO 2 @MXene heterostructure (Figure 2e; Figure S2, Supporting Information). The numerous interfaces between TiS 2 , TiO 2 , and MXene are conducive to regulating sulfur evolution reactions. [42] The lattice spacing of 2.62, 3.5, and 2.08 Å could be ascribed to TiS 2 (011), [43] TiO 2 (101), [26,44] and MXene (0012) [45] planes, respectively. It was further confirmed by the corresponding inverse fast Fourier transform (FFT) lattice images and interplanar spacing profiles in Figure 2f. The selected area electron diffraction (SAED) pattern showed a set of hexagonal diffraction spots of TiS 2 and concentric diffraction rings of TiO 2 , further verifying the co-existence of TiS 2 /TiO 2 in the heterostructure (Figure 2g). The uniform distribution of TiS 2 /TiO 2 was confirmed by the homogeneous signals of Ti, O, and S atoms in the energy-dispersive x-ray spectroscopy (EDX) analyses (Figure 2h). The results obtained so far indicated that the surface of MXene is successfully modified by the in situ formation of the heterostructured TiS 2 /TiO 2 , which is beneficial for the smooth LiPS trapping, diffusion, and conversion process. For an upcoming comparative study, we also synthesized the TiO 2 /MXene with a similar procedure ( Figure S3, Supporting Information). X-ray diffraction (XRD) also supported that TiS 2 /TiO 2 @MXene heterostructure is well prepared. As shown in Figure 3a, a combination of TiS 2 (JCPDS No. 15-0853) and TiO 2 (JCPDS No. 21-1272) peaks appeared after partial sulfurization of the few-layer Ti 3 C 2 T x MXene. Moreover, the ratio between TiS 2 and TiO 2 was able to be tuned by changing the reactant composition ( Figure S4, Supporting Information). As a larger amount of sulfur was used, the intensity of TiS 2 peaks in the XRD patterns increased, indicating a larger proportion of TiS 2 . From Raman spectroscopy analysis, similar results were obtained (Figure 3b). The characteristic peaks could be assigned to E g (234 cm −1 ), A 1g (335 cm −1 ), A 2u (388 cm −1 ) vibration modes of TiS 2 and E g (151, 204 cm −1 ), B 1g (426 cm −1 ), A 1g (521 cm −1 ) vibration modes of TiO 2 . [46,47] Compared with the previous study, [46] the E g mode of TiS 2 was redshifted, while the E g mode of TiO 2 was slightly blue-shifted. The results implied that there is an interaction between TiS 2 and TiO 2 . X-ray photoelectron spectroscopy (XPS) spectra were employed to accurately analyze the composition and the chemical bonds in the TiS 2 /TiO 2 @MXene heterostructure. The characteristic bonds in Ti 3 C 2 T x MXene were well observed in Ti 2p and C 1s regions [48][49][50] (Figure 3c,d). The strong peaks located at 458.9 and 456.2 eV were able to be attributed to the Ti-O and Ti-S bonds in TiO 2 and TiS 2 . [27,51] Importantly, in the high-resolution O 1s spectrum (Figure 3e), the fitted peaks of O-Ti-S bonds (531 eV) and O-S (532 eV) indicated the formation of covalent bonds at the interface between TiS 2 and TiO 2 . [52] The S-Ti and S-O bonds were also detected at 160.8 and 168.3 eV in the S 2p spectrum [51,53] (Figure 3f). The covalent bonds formed at the interface were expected to significantly increase the interaction of components in the TiS 2 /TiO 2 @MXene heterostructure, leading to smooth trapping-diffusion-conversion of LiPSs.

Adsorption and Electrocatalytic Effects
To intuitively investigate the adsorption capability of the TiS 2 /TiO 2 @MXene and the reference materials, visualized adsorption tests were carried out by soaking host materials with the same weight into the Li 2 S 6 solution. As witnessed in Figure 4a, the solution with TiO 2 @MXene and TiS 2 /TiO 2 @MXene became nearly colorless after 5 h, while the one with the sole MXene still showed yellow color. This demonstrated that TiO 2 has a strong LiPSs adsorption ability. In addition, the numerous interfaces between TiS 2 , TiO 2 , and MXene also considerably contributed to LiPSs adsorption. [42] This endowed TiS 2 /TiO 2 @MXene with the highest LiPSs adsorption capability. It was consistent with the DFT calculation results in Figure 1. In addition, the solution containing the TiS 2 /TiO 2 @MXene showed the weakest signal of LiPSs in the UV-vis absorbance test, evincing the smallest concentration of LiPSs in the solution (Figure 4b). This indicated that the TiS 2 /TiO 2 @MXene could effectively suppress the diffusion of soluble LiPSs into the electrolyte. The interaction between the TiS 2 /TiO 2 @MXene and LiPSs was investigated by the XPS S 2p spectrum (Figure 4c). Compared to the S 2p spectrum of asprepared TiS 2 /TiO 2 @MXene, the appearance of terminal sulfur (S T −1 ) and bridging sulfur (S B 0 ) evinced that LiPSs are anchored on the TiS 2 /TiO 2 @MXene after adsorption tests. Moreover, the high intensity of thiosulfates and polythionate peaks indirectly suggested that the chemical reaction between TiO 2 and LiPSs has likely occurred.
The high electrocatalytic activities of host materials are of importance in promoting conversion reactions in Li-S batteries. To evaluate the kinetics of sulfur evolution reactions on the prepared materials, cyclic voltammetry (CV) analyses of both symmetric and asymmetric cells were carried out. In the symmetric cells, the Li 2 S 6 electrolyte was added between two same electrodes. As shown in Figure 4d, although the cell with the TiO 2 @MXene electrode delivered higher currents than that with the MXene electrode, the CV curves of these cells were similar. In contrast, the cell with the TiS 2 /TiO 2 @MXene electrode showed a different CV profile with strong redox peaks. Even at high scan rates, the redox peaks were still observed ( Figure S5, Supporting Informa-tion). These results revealed that TiS 2 in the TiS 2 /TiO 2 @MXene plays a key role in the electrocatalytic activities, as already predicted from the theoretical calculation. In asymmetric cells, the superior electrocatalytic properties of TiS 2 /TiO 2 @MXene were displayed through the negative shift in the anodic peaks and the positive shift in the cathodic peaks ( Figure 4e; Figure S6, Supporting Information). These peak shifts proved the remarkably reduced polarization of liquid-solid conversion and the accelerated kinetics of sulfur evolution reactions in the TiS 2 /TiO 2 @MXene cell. [54] In addition, the sharpest and strongest peaks further verified the high efficiency of LiPSs conversion and fast current exchange in the TiS 2 /TiO 2 @MXene heterostructure. To further analyze the charge transfer kinetics, Tafel slopes were taken from the linear regions in the redox peaks (Figure 4f). Compared with other counterparts, the TiS 2 /TiO 2 @MXene cell delivered the lowest fitted Tafel slopes of 38 and 73 mV dec −1 toward cathode and anode peaks, respectively. This demonstrated that the TiS 2 /TiO 2 @MXene heterostructure could effectively promote charge transfer to catalyze the conversion of LiPSs. [36] Theoretically, 75% of the capacity of Li-S batteries arises from the conversion of the soluble intermediate product Li 2 S 4 to the insoluble discharge product Li 2 S. [55] However, this liquidsolid conversion reaction requires high overpotential to overcome the large energy barrier. The host materials should possess a catalytic ability to lower the energy barrier and facilitate Li 2 S formation. Hence, we investigated the catalytic effects of asprepared materials on the Li 2 S nucleation using potentiostatic discharge measurements. Specifically, the assessed capacity of the Li 2 S precipitation on the TiS 2 /TiO 2 @MXene heterostructure (165 mAh g s −1 ) was markedly higher than those on the MXene (95 mAh g s −1 ) and TiO 2 @MXene (106 mAh g s −1 ) (Figure 4gi). Besides, the TiS 2 /TiO 2 @MXene electrode showed a faster response to Li 2 S precipitation than others ( Figure S7, Supporting Information), indicating its outstanding catalytic effect. The larger amount of Li 2 S deposited in the TiS 2 /TiO 2 @MXene electrode was visualized in SEM images. The same phenomena were observed in the Li 2 S dissolution test with the highest capacity and fastest response of the TiS 2 /TiO 2 @MXene electrode ( Figure  S8, Supporting Information). All of these results substantiated that TiS 2 /TiO 2 @MXene heterostructure could act as a bidirec-tional catalyst to accelerate sulfur evolution reactions in Li-S batteries. [25]

Electrochemical Performance of Li-S Batteries
To demonstrate the advantage of the TiS 2 /TiO 2 @MXene heterostructure with both the adsorptive property of TiO 2 and the catalytic property of TiS 2 toward LiPSs, electrochemical performance evaluations were conducted by coin-type batteries using the Li foil as the anode and S/TiS 2 /TiO 2 @MXene as the cathode (sulfur loading of 1.5 mg cm −2 and sulfur content of ≈79 wt % ( Figure S9, Supporting Information)). The galvanostatic chargedischarge (GCD) profiles showed that the S/TiS 2 /TiO 2 @MXene cell delivers a high capacity of 1227 mAh g −1 at 0.1 C, which is obviously higher than that of S/TiO 2 @MXene and S/MXene cells (1110 and 1031 mAh g −1 , respectively) (Figure 5a). Additionally, the GCD profile of the S/TiS 2 /TiO 2 @MXene cell displayed the smallest polarization accompanied by reduced overpotentials of Li 2 S reduction/oxidation ( Figure S10, Supporting Information), demonstrating the electrocatalytic effects of TiS 2 /TiO 2 @MXene on increasing the reaction kinetics. The S/TiS 2 /TiO 2 @MXene cell also revealed the lowest charge-transfer resistance, which was testified by the smallest diameter of the semicircle at high frequencies in the electrochemical impedance spectroscopy (EIS) analysis [56,57] (Figure S11, Supporting Information). Moreover, the high adsorption ability of the TiS 2 /TiO 2 @MXene heterostructure toward LiPSs resulted in a low shuttle current in the cell ( Figure S12, Supporting Information). Due to the high adsorptive and catalytic properties, the S/TiS 2 /TiO 2 @MXene cell delivered the best rate capability (Figure 5b). The discharge capacities at the current densities of 0.1, 0.2, 0.5, 1, 2, and 4 C were recorded to be 1227, 1099, 997, 918, 803, and 690 mAh g −1 , respectively. When the current rate was back to 0.1 C, the reversible capacity of 1192 mAh g −1 was able to be retained. This indicated superb reversibility and durable sulfur electrochemistry. As shown in Figure 5c and Figure S13 (Supporting Information), the GCD curves of the S/TiS 2 /TiO 2 @MXene cell at high current rates still well maintained the featured shape with the typical charge-discharge plateaus, implying the rapid LiPSs conversion reactions. More importantly, the synergistic effect of catalytic TiS 2 and adsorptive TiO 2 enabled superior cycling stability at various current densities. At 0.1 C, the S/TiS 2 /TiO 2 @MXene cell showed an initial capacity as high as 1232 mAh g −1 and excellent capacity retention of 86.7% after 100 cycles (Figure 5d). In comparison, the S/TiO 2 @MXene and S/MXene cells exhibited a gradual fading process with capacity retention of only 79.4% and 65.2%, respectively. Moreover, at the high current density of 1 C, the S/TiS 2 /TiO 2 @MXene cell presented superb stability with a low capacity decay rate of 0.038% per cycle. Even after 500 cycles, the cell still delivered a high reversible capacity of 804 mAh g −1 (Figure 5e). The average capacity fading rate of the S/TiS 2 /TiO 2 @MXene cell was much smaller than that of the S/TiO 2 @MXene and the S/MXene cells (0.046% and 0.053%, respectively). It should be pointed out that the electrochemical performance of our batteries is highly competitive among the recently reported Li-S cells using heterostructured materials as sulfur hosts (Table S1, Supporting Information). This result was the key evidence that the TiS 2 /TiO 2 @MXene heterostructure could effectively suppress the shuttle effect and enhance the kinetics of sulfur evolution reactions during long cycling life.
After the long-term cycling tests, the coin cells were disassembled for further analysis. As displayed in Figure 6a, the separator in the S/TiS 2 /TiO 2 @MXene cell showed the lightest yellow color, testifying to the good LiPSs trapping capability of the TiS 2 /TiO 2 @MXene heterostructure. It was proved that the LiPSs shuttle effects cause not only loss of the active material but also corrosion of the Li anode. [58,59] As shown in Figure 6bd and Figures S14 and S15 (Supporting Information), the cycled Li anode paired with S/TiS 2 /TiO 2 @MXene had the thinnest corroded layer (28 μm) and the smoothest surface, while the others were very porous. The S content on the surface of the Li anode paired with S/TiS 2 /TiO 2 @MXene was the lowest, evidently indicating that the shuttle effect in the cell using the S/TiS 2 /TiO 2 @MXene cathode is effectively suppressed. Moreover, the high catalytic activities of the TiS 2 /TiO 2 @MXene heterostructure led to a smooth surface of the corresponding cathode. In contrast, Li 2 S was severely agglomerated on the surface of the S/MXene and S/TiO 2 @MXene cells (Figure 6e-g).
As mentioned, the low density of the sulfur host causes the poor volumetric energy density of Li-S batteries, which is eagerly awaiting the appearance of a practical solution. In this research, due to the high density of the MXene substrate, the S/TiS 2 /TiO 2 @MXene cathode was expected to deliver a high volumetric energy density under practical conditions, including a high sulfur mass loading and low electrolyte-to-sulfur (E/S) ratio. As shown in Figure S16 (Supporting Information), the TiS 2 /TiO 2 @MXene possessed a low specific surface area (56 m 2 g −1 ) and low porosity, which were crucial for realizing a high-tap-density sulfur cathode and avoiding electrolyte consumption. [60] Especially, at an identical mass, the TiS 2 /TiO 2 @MXene material exhibited a significantly smaller volume compared with carbon nanotubes (CNTs), which have been used as a common sulfur host (Figure 7a). As shown in Figure 7b, with the ultrahigh sulfur loading of 7.5 mg cm −2 , the thickness of the sulfur cathode was measured to be 59 μm. The cathode owned a highly compact structure ( Figure S17, Supporting Information) with a high tap density of 1.76 g cm −3 . At 0.1 C, the dense S/TiS 2 /TiO 2 @MXene cathode initially exhibited a large volumetric capacity of 1315 Ah L −1 (based on the total volume of the whole sulfur cathode) under the sulfur loading of 5.2 mg cm −2 and low E/S ratio of 6 μL mg −1 . Impressively, even when the sulfur loading increased to 7.5 mg cm −2 and the E/S ratio reduced to 5 μL mg −1 , the dense S/TiS 2 /TiO 2 @MXene cathode still delivered a volumetric capacity as high as 1145 Ah L −1 (Figure 7c). The corresponding gravimetric capacities of these batteries were measured to be 1038 and 904 mAh g −1 (based on the sulfur mass), respectively ( Figure S18, Supporting Information). The corresponding volumetric capacities were estimated to be 1041 and 818 Ah L −1 even after 100 cycles, respectively (Figure 7d; Figure  S19, Supporting Information). Thanks to its high tap density, the volumetric energy density of the dense S/TiS 2 /TiO 2 @MXene cathode with a sulfur loading of 7.5 mg cm −2 was calculated to be 2476 Wh L −1 (based on the total volume of the whole sulfur cathode). Moreover, the practical cell with a high sulfur loading of 7.5 mg cm −2 , a low electrolyte-to-sulfur (E/S) ratio of 3 μL mg −1 , and a low negative-to-positive capacity (N/P) ratio of 2 was assembled and tested. The volumetric and gravimetric energy densities of the practical cell were estimated to be 910 Wh L −1 and 329 Wh kg −1 , respectively ( Figure S20 and Table  S2, Supporting Information). These values exceeded the volumetric energy densities of Li-ion batteries using the Ni-rich metal oxide cathode and other reported Li-S batteries using carbon, oxide, or sulfide as host materials [7,8,15,25,[61][62][63][64][65] (Figure 7e; Tables S3 and S4, Supporting Information). To demonstrate the applicability of the TiS 2 /TiO 2 @MXene heterostructure, a pouch cell (3 × 3 cm 2 in size) using the total sulfur mass of 38 mg and lean electrolyte of 6 μL mg −1 was assembled. The as-prepared pouch cell possessed an open circuit potential of 2.79 V ( Figure  S21, Supporting Information). Encouragingly, the pouch cell presented a high capacity of 35.4 mAh and capacity retention of 87% after 50 cycles (Figure 7f,g). It was able to light up 30 LEDs in parallel for 5 h (Figure 7h; Video S1, Supporting Information). These results convincingly demonstrated that the heavy TiS 2 /TiO 2 @MXene heterostructure possessing excellent adsorp-www.advancedsciencenews.com www.afm-journal.de tive and catalytic properties is a promising sulfur host material for Li-S batteries with high volumetric energy density.

Conclusion
We proposed a heavy TiS 2 /TiO 2 @MXene heterostructure as an efficient multifunctional sulfur host material for durable and high-volumetric-energy-density Li-S batteries. Theoretically, we predicted that the TiS 2 /TiO 2 @MXene heterostructure simultaneously could possess a strong chemical affinity (TiO 2 ) and bidirectional catalytic activity (TiS 2 ). By performing in-depth experiments, we demonstrated that the TiS 2 /TiO 2 @MXene heterostructure could effectively suppress the shuttle effects of LiPSs and accelerate the kinetics of sulfur evolution reactions in Li-S batteries. Impressively, the sulfur cathode containing the heavy TiS 2 /TiO 2 @MXene heterostructure delivered a superior volumetric energy density of 2476 Wh L −1 cathode with a high sulfur loading of 7.5 mg cm −2 and a low E/S ratio of 5 μL mg −1 . We expect that this work can promote the development of highly adsorptive, catalytic, and heavy sulfur host material to realize Li-S batteries with high stability and high volumetric energy density.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.