A Bimetallic Organic Framework with Mn in MIL-101(Cr) for Lithium–Sulfur Batteries

Lithium–sulfur batteries (LSBs) show excellent performance in terms of specific capacity and energy density. However, the cyclic stability of LSBs is compromised due to the “shuttle effect”, which hinders the practical applications of LSBs. Herein, a metal–organic framework (MOF) based on Cr ions as the main body composition, commonly known as MIL-101(Cr), was utilized to minimize the shuttle effect and improve the cyclic performance of LSBs. To obtain MOFs with a certain adsorption capacity for lithium polysulfide and a certain catalytic capacity, we propose an effective strategy of incorporating sulfur-loving metal ions (Mn) into the skeleton to enhance the reaction kinetics at the electrode. Based on the oxidation doping method, Mn2+ was uniformly dispersed in MIL-101(Cr) to produce bimetallic Cr2O3/MnOx as a novel sulfur-carrying cathode material. Then, a sulfur injection process was carried out by melt diffusion to obtain the sulfur-containing Cr2O3/MnOx-S electrode. Moreover, an LSB assembled with Cr2O3/MnOx-S showed improved first-cycle discharge (1285 mAh·g−1 at 0.1 C) and cyclic performance (721 mAh·g−1 at 0.1 C after 100 cycles), and the overall performance was much better than that of monometallic MIL-101(Cr) as a sulfur carrier. These results revealed that the physical immobilization method of MIL-101(Cr) positively affected the adsorption of polysulfides, while the bimetallic composite Cr2O3/MnOx formed by the doping of sulfur-loving Mn2+ into the porous MOF produced a good catalytic effect during LSB charging. This research provides a novel approach for preparing efficient sulfur-containing materials for LSBs.


Introduction
The energy storage technology revolution is in full swing. Lithium-sulfur batteries (LSBs) have a high energy density (2600 W·h·kg −1 ) [1], which demonstrates their strong potential in new energy storage applications [2][3][4]. However, the inferior cyclic stability of LSBs has hindered their practical application and commercialization. The shuttle effect and high potential energy barrier of the reaction when lithium polysulfides (Li 2 S x , 4 ≤ x ≤ 8) are reconverted into sulfur monomers (S 8 ) significantly contribute to the poor cyclic stability of LSBs [5][6][7]. The shuttle effect occurs when the soluble lithium polysulfide generated at the positive electrode during the electrode reaction of LSBs is transferred with electrons to the negative electrode and directly reacts with the negative lithium electrode, resulting in an irreversible loss of active material in the battery. Hence, the cyclic capacity of LSBs quickly decays [8][9][10].
Sulfur/carbon (S/C) composites have been a popular cathode material for LSBs. The carbon material is mostly porous carbon, which can solve the problem of poor electrical conductivity in sulfur cathodes, and the porous carbon material usually has some pore structure to physically adsorb Li 2 S x , which can enhance the utilization of sulfur in LSBs. Although the addition of carbon materials allows for some improvement in the electrochemical properties of S/C composites, their cycling efficiency is not generally satisfactory, Materials 2023, 16, 3794 2 of 13 most notably due to the lack of active sites which do not adsorb Li 2 S x strongly enough and still result in the loss of large amounts of Li 2 S x , eventually leading to a rapid decrease in the specific capacity of LSBs.
Significant effort has been made in the past few years to reduce the shuttle effect. The incorporation of metal-organic frameworks (MOFs) is one of the more promising solutions for solving this challenge [11][12][13][14]. MOFs represent a kind of organic-inorganic porous material comprising metal ions and organic ligands with ligand bonding. MOFs possess an ultra-high specific surface area and void fraction, and they show relatively good physical adsorption capacities for lithium polysulfides [15,16]. MOFs have more metal nodes (Lewis acidic sites) and organic functional groups than conventional porous carbon materials, which can provide binding sites for Li 2 S x . Moreover, these binding sites can confine lithium polysulfides to the pore sites of MOFs by physical adsorption [17][18][19]. Among the various types of MOFs, Lavoisier skeleton materials possess a large pore size and excellent stability. MIL-101(Cr) is an example of a Lavoisier skeleton material. Furthermore, MIL-101(Cr) possesses certain catalytic properties due to the presence of Cr [20,21]. It should be noted that high catalytic activity can increase the activation energy of chemical reactions, thereby reducing the effect of potential energy barriers in the reaction process [9]. Both Mn and Cr have exhibited excellent catalytic performance in different cells [9,22] and synergistic interactions between Cr and Mn oxides due to electron migration can lead to enhanced catalytic activity [23]. However, although MIL-101(Cr) performs well when used as a sulfur-containing cathode, certain problems still need to be solved. MOFs, including MIL-101(Cr), have poor conductivity [24], which affects their electrochemical performance. For the issue of insufficient electrical conductivity of MOF materials, some researchers have used pyrolysis to increase the electrical conductivity of MOF materials and it has worked well [25].
The current work focuses on the preparation of bimetallic MOFs using Mn-doped MIL-101(Cr) as a sulfur-containing cathode. The combination of Mn with Cr in the MIL-101(Cr) material provides a synergistic effect that improves its catalytic properties and maintains the MOF framework structure after pyrolysis. Therefore, the loading of S in the final electrode material does not decrease. MIL-101(Cr) was prepared by a hydrothermal method. This was followed by the preparation of Cr-Mn complexes, which were synthesized by combining manganese oxide with MIL-101(Cr), which has good catalytic oxidation activity and a high specific surface area. The content of Mn in the Cr-Mn complexes was the only factor that was changed, and the pyrolysis temperature was fixed. Finally, an electrode material was prepared by the melt-diffusion method. The current study investigated the structure of the composite Cr 2 O 3 /MnO x material and its electrochemical performance as a cathode in LSBs.

Prepare of MIL-101(Cr) Precursor
The MIL-101(Cr) precursor was prepared using a hydrothermal method. Typically, 4.8 g of Cr(NO 3 ) 3 ·9H 2 O and 1.996 g of terephthalic acid (TPA) were mixed and ground. Then, the mixture was added to deionized water and stirred for 25 min. Next, 225 µL HF was added, and stirring was continued for a few minutes. This mixture was ultrasonically dispersed for 10 min, then placed in a polytetrafluoroethylene-lined reactor. The reactor was held at 220 • C for 15 h. A light green solid was obtained after the reaction. This solid was filtered and allowed to stand at 120 • C for 12 h to precipitate out the terephthalic acid vacancies. Finally, the powder was alternately washed 3 times with N,N-dimethylformamide (DMF), ethanol, and ultrapure water, then dried in a vacuum oven at 60 • C for 12 h. The light green powder obtained was the precursor of this research. A total of 500 mg precursor was added to 80 mL ultrapure water and thoroughly stirred. Then, a certain mass of KMnO 4 (53 mg, 1334 mg, or 2556 mg) was added into the stirred suspension, which was stirred for a further 2 h to prepare Cr-Mn composite materials with Mn loadings of 5 wt%, 10 wt% and 20 wt%. These were labeled as MIL-101-5Mn, MIL-101-10Mn and MIL-101-20Mn, respectively. Next, 4 mL of H 2 O 2 (volume fraction of 30%) was diluted in 50 mL ultrapure water and stirred for 2 h. The stirred solution was washed three times with deionized water to obtain a dark green solid, which was denoted as MIL-101-Mn. The solid was heated at 900 • C at a heating rate of 5 • C/s in a protective atmosphere and pyrolysed for 2 h. The resulting dark green solid was denoted Cr 2 O 3 /MnO x .

Preparation of MIL-101-S and Cr 2 O 3 /MnO x -S Electrode Materials
MIL-101(Cr) and Cr 2 O 3 /MnO x were mixed with sulfur powder in a mass ratio of 3:7 using a grinding tool. Then, the mixed powders were homogeneously mixed by grinding for 30 min. After grinding, each powder was placed in an inert-atmosphere reactor and heated at 155 • C for 18 h. The dark green solid was obtained after cooling and denoted as MIL-101-S or Cr 2 O 3 /MnO x -S.

Preparation of Batteries
To prepare the LSBs, super carbon black (super P) was applied as the conductive agent, polyvinylidene fluoride (PVDF) was applied as the binder, N-methyl-2-pyrrolidone (NMP) was applied as the solvent, and aluminum foil was used as the collector. The positive electrode of the battery was prepared using the active material after the sulfur loading with an active material: super P:PVDF ratio of 8:1:1. These components were added to a small bottle with NMP and stirred for 12 h. The mixed liquid was then evenly applied to the aluminum foil, which was dried in a vacuum oven at 70 • C for 12 h. After drying, the positive electrode was obtained. CR2025 button cells were used, and the electrolyte was 1 M LiTFSI (1,3-dioxolane: 1,2-dimethoxyethane = 1:1 v/v) + 1% LiNO 3 . The anode was lithium metal and the separator system was Polyethylene (Cyber Electrochemistry Materials, thickness: 25 µm, porosity: 45%). The corresponding positive electrode pieces were separately assembled into cells; the amount of electrolyte used in each cell was 20 µL.

Material Characterization
XRD analysis was performed with a Bruker D8 Advance (Billerica, MA, USA). Nitrogen adsorption-desorption analysis was concluded with a Micromeritics ASAP 2460 (Norcross, GA, USA) to determine BET surface areas and pore sizes. SEM and EDS analyses were performed with a ZEISS Gemini SEM 300 (Oberkochen, Germany), and TEM analysis was performed with a JEOL F200 (Tokyo, Japan). XPS was conducted with a Thermo Scientific K-Alpha spectrometer (Waltham, MA, USA) to investigate the elemental composition and valence states. The wavelengths of X-ray sources: XRD (Cu Kα1 = 0.15406 nm) and XPS (Al Kα1 = 0.8339 nm).

Electrochemical Characterization
The batteries were tested using a battery test system (SenveBTS, 5 V, 20 mA). The main current rate was 0.1 C (1 C = 1675 mA·g −1 ) and testing was carried out in the voltage range of 1.7 to 2.8 V. Cyclic voltammetry was carried out using a CHI760E workstation (Shanghai, China) in the voltage range of 1.6 to 3.0 V at a 1 mV·s −1 scanning rate.

Material Characterization
Characterization of each material was carried out. MIL-101-Mn is a composite made of Cr-MOF loaded with different proportions of Mn, and Cr 2 O 3 /MnO x is the MIL-101-Mn material after pyrolysis. Figure 1a shows the XRD patterns before pyrolysis. The XRD pat- tern of MIL-101(Cr) was almost identical to that of a previously reported work [26,27]. The peaks of the MIL-101(Cr) spectrum remained almost unchanged after Mn loading; meanwhile, the diffraction peaks at 19.3 • and 37.6 • were weakened due to the better dispersion of Mn. This demonstrates that the crystal structure of MIL-101(Cr) was not damaged during the loading process. However, the crystallinity was slightly lower, as demonstrated by the SEM images (Figure 2b-d). Figure 1b shows the XRD profiles of the Cr 2 O 3 /MnO x samples after heat treatment, which are in agreement with the standard Cr 2 O 3 diffraction peaks (JCPDS no. 00-038-1479). This demonstrates that the main form of Cr in Cr 2 O 3 /MnO x after heat treatment was its oxide. Moreover, these diffraction patterns show that the dispersion of Mn was good for all loading ratios of MIL-101(Cr). No significant peaks related to Mn were observed in the XRD patterns because of the low Mn content and its existence in the amorphous state [28]. Table 1 shows the lattice parameters of each sample after heat treatment, and there was no significant change in the lattice parameters of the material as the Mn loading increased.

Material Characterization
Characterization of each material was carried out. MIL-101-Mn is a composite made of Cr-MOF loaded with different proportions of Mn, and Cr2O3/MnOx is the MIL-101-Mn material after pyrolysis. Figure 1a shows the XRD patterns before pyrolysis. The XRD pattern of MIL-101(Cr) was almost identical to that of a previously reported work [26,27]. The peaks of the MIL-101(Cr) spectrum remained almost unchanged after Mn loading; meanwhile, the diffraction peaks at 19.3°and 37.6° were weakened due to the better dispersion of Mn. This demonstrates that the crystal structure of MIL-101(Cr) was not damaged during the loading process. However, the crystallinity was slightly lower, as demonstrated by the SEM images (Figure 2b-d). Figure 1b shows the XRD profiles of the Cr2O3/MnOx samples after heat treatment, which are in agreement with the standard Cr2O3 diffraction peaks (JCPDS no. 00-038-1479). This demonstrates that the main form of Cr in Cr2O3/MnOx after heat treatment was its oxide. Moreover, these diffraction patterns show that the dispersion of Mn was good for all loading ratios of MIL-101(Cr). No significant peaks related to Mn were observed in the XRD patterns because of the low Mn content and its existence in the amorphous state [28]. Table 1 shows the lattice parameters of each sample after heat treatment, and there was no significant change in the lattice parameters of the material as the Mn loading increased.    Figure 2e shows that the particle size of Cr 2 O 3 /5MnO x became smaller after pyrolysis. This is because MnO x entered the apertures of MIL-101(Cr) after loading, and a tight bond was created after heat treatment. Moreover, the particles became slightly rougher after heat treatment, indicating a minimal amount of structural collapse. Figure 2f,g show that the surface of the particles became rougher with increased MnO x loading, demonstrating that the MOF skeleton collapsed to a certain extent after heat treatment and that the Cr 2 O 3 /20MnO x particles were deformed. Meanwhile, as MIL-101(Cr) started to decompose at 550 • C [29], the 900 • C pyrolysis used in this research caused the composite to decompose and the decomposed material was nucleated on top of the original material, which also caused the Cr 2 O 3 /MnO x material to show a change in crystal particle size. Figure 2h shows the surface morphology of Cr 2 O 3 /10MnO x -S. The overall morphology of this material was still ortho-octahedral after 70 wt% sulfur loading.    Figure 2e shows that the particle size of Cr2O3/5MnOx became smaller after pyrolysis. This is because MnOx entered the apertures of MIL-101(Cr) after loading, and a tight bond was created after heat treatment. Moreover, the particles became slightly rougher after heat treatment, indicating a minimal amount of structural collapse. Figure 2f,g show that the surface of the particles became rougher with increased MnOx loading, demonstrating that the MOF skeleton collapsed to a certain extent after heat treatment and that the Cr2O3/20MnOx particles were deformed. Meanwhile, as MIL-101(Cr) started to decompose at 550 °C [29], the 900 °C pyrolysis used in this research caused the composite to decompose and the decomposed material   Figure 3 shows the distribution of elements and EDS mapping of Cr 2 O 3 /10MnO x -S. As displayed in Figure 3a,b, S and Mn were very uniformly distributed, without any obvious sulfur agglomeration. The approximate content of each element in Cr 2 O 3 /10MnO x -S is shown in the energy spectrum displayed in Figure 3c. The elemental S content was approximately 67%, demonstrating the successful loading of S on Cr 2 O 3 /10MnO x . 70 wt% sulfur loading. Figure 3 shows the distribution of elements and EDS mapping of Cr2O3/10MnOx-S. As displayed in Figure 3a,b, S and Mn were very uniformly distributed, without any obvious sulfur agglomeration. The approximate content of each element in Cr2O3/10MnOx-S is shown in the energy spectrum displayed in Figure 3c. The elemental S content was approximately 67%, demonstrating the successful loading of S on Cr2O3/10MnOx.   [30], which is in good accordance with the small surface particles observed in the SEM images of Cr2O3/MnOx. This also indicates that, when MnOx loading was increased to >10%, some of the MnOx was encapsulated on the MOF surface in the form of α-MnO2. Moreover, some short and intricate lattices were visible in the TEM images. These lattices exposed some of the high-energy crystal surfaces of Cr2O3. This phenomenon reduces the activation energy required for the conversion of lithium polysulfide to sulfur monomers.    [30], which is in good accordance with the small surface particles observed in the SEM images of Cr 2 O 3 /MnO x . This also indicates that, when MnO x loading was increased to >10%, some of the MnO x was encapsulated on the MOF surface in the form of α-MnO 2 . Moreover, some short and intricate lattices were visible in the TEM images. These lattices exposed some of the high-energy crystal surfaces of Cr 2 O 3 . This phenomenon reduces the activation energy required for the conversion of lithium polysulfide to sulfur monomers.
to show a change in crystal particle size. Figure 2h shows the surface morphology of Cr2O3/10MnOx-S. The overall morphology of this material was still ortho-octahedral after 70 wt% sulfur loading. Figure 3 shows the distribution of elements and EDS mapping of Cr2O3/10MnOx-S. As displayed in Figure 3a,b, S and Mn were very uniformly distributed, without any obvious sulfur agglomeration. The approximate content of each element in Cr2O3/10MnOx-S is shown in the energy spectrum displayed in Figure 3c. The elemental S content was approximately 67%, demonstrating the successful loading of S on Cr2O3/10MnOx.   [30], which is in good accordance with the small surface particles observed in the SEM images of Cr2O3/MnOx. This also indicates that, when MnOx loading was increased to >10%, some of the MnOx was encapsulated on the MOF surface in the form of α-MnO2. Moreover, some short and intricate lattices were visible in the TEM images. These lattices exposed some of the high-energy crystal surfaces of Cr2O3. This phenomenon reduces the activation energy required for the conversion of lithium polysulfide to sulfur monomers.    Figure 5b which demonstrated the existence of a great number of microporous and mesoporous constructions that can provide binding sites for the sulfur in the cell and for the polysulfides during the reaction. Figure 4c shows the specific surface areas (SSAs) and mean pore sizes of these materials. Because the Cr 2 O 3 /MnO x composite is similar to Cr 2 O 3 , commercially available Cr 2 O 3 (Shanghai Macklin Biochemical Co., Ltd.) was used for comparison. The SSAs and pore sizes of the as-fabricated composites were significantly higher than those of ordinary Cr 2 O 3 . The BET SSA of Cr 2 O 3 /5MnO x was 65.34 m 2 ·g −1 with a Barrett-Joyner-Halenda (BJH) average pore diameter of 28.51 nm; the BET SSA of Cr 2 O 3 /10MnO x was 88.41 m 2 ·g −1 with a BJH average pore diameter of 32.5 nm; and the BET SSA of Cr 2 O 3 /20MnO x was 87.95 m 2 ·g −1 with a BJH average pore diameter of 28.13 nm. Cr 2 O 3 /20MnO x exhibited the highest SSA and Cr 2 O 3 /10MnO x had the largest average pore diameter. The large SSA and small pore diameter of Cr 2 O 3 /20MnO x was attributed to a large amount of Mn entering the apertures of MIL-101(Cr) to support the skeleton structure during pyrolysis and prevent excessive shrinkage; however, the excessive loading of MnO x decreased the average pore diameter. distribution of Cr2O3/10MnOx is shown in Figure 5b which demonstrated the existence of a great number of microporous and mesoporous constructions that can provide binding sites for the sulfur in the cell and for the polysulfides during the reaction. Figure 4c shows the specific surface areas (SSAs) and mean pore sizes of these materials. Because the Cr2O3/MnOx composite is similar to Cr2O3, commercially available Cr2O3 (Shanghai Macklin Biochemical Co., Ltd.) was used for comparison. The SSAs and pore sizes of the as-fabricated composites were significantly higher than those of ordinary Cr2O3. The BET SSA of Cr2O3/5MnOx was 65.34 m 2 ·g −1 with a Barrett-Joyner-Halenda (BJH) average pore diameter of 28.51 nm; the BET SSA of Cr2O3/10MnOx was 88.41 m 2 ·g −1 with a BJH average pore diameter of 32.5 nm; and the BET SSA of Cr2O3/20MnOx was 87.95 m 2 ·g −1 with a BJH average pore diameter of 28.13 nm. Cr2O3/20MnOx exhibited the highest SSA and Cr2O3/10MnOx had the largest average pore diameter. The large SSA and small pore diameter of Cr2O3/20MnOx was attributed to a large amount of Mn entering the apertures of MIL-101(Cr) to support the skeleton structure during pyrolysis and prevent excessive shrinkage; however, the excessive loading of MnOx decreased the average pore diameter.    [32,33]. Based on the data in Table 1, the Mn 3+ fraction of Cr 2 O 3 /5MnO x was lower than that of the other samples. The Cr 2p spectra of the samples are shown in Figure 6b. The main signals of Cr 2p 3/2 were 575.42 eV(Cr 2+ ), 576.57 eV(Cr 3+ ) and 578.34 eV(Cr 4+ ), while for Cr 2p 1/2 the main signals were 585.21 eV(Cr 2+ ), 586.47 eV(Cr 3+ ) and 588.10 eV(Cr 4+ ) [34][35][36][37]. A significant decline Materials 2023, 16, 3794 8 of 13 in Cr 5+ content occurred after Mn loading, which, in combination with the presence and change in the valence state of Mn, can be attributed to electron migration between Mn and Cr: 2Mn 2+ + Cr 5+ 2Mn 3+ + Cr 3+ . Electronic migration between different metallic ions facilitates faster reaction rates and improves the catalytic ability of materials [38]. The O 1s spectra are shown in Figure 6c. The peak at 530.05 eV corresponds to lattice oxygen (O Latt ), the peak at 531.70 eV corresponds to adsorbed oxygen (O Ads ), and the peak at 533.22 eV relates to surface hydroxyl oxygen (O OH ) [39,40]. The lattice oxygen content of Cr 2 O 3 /MnO x was higher than that of MIL-101 (Cr). This promoted the oxidation process and enabled a certain oxygen spillover effect between MnO x and Cr 2 O 3 after pyrolysis. Hence, lattice oxygen can readily migrate to the composite surface, thereby improving the catalytic efficiency of the Cr 2 O 3 /MnO x composite. elemental composition data are summarized in Table 2. The high-resolution Mn 2p spectra of each sample are shown in Figure 6a. The main signals of Mn 2p3/2 were 640.01 eV(Mn 2+ ), 641.38 eV(Mn 3+ ) and 642.48 eV(Mn 4+ ), while for Mn 2p1/2 the main signals were 652.22 eV(Mn 2+ ), 653.67 eV(Mn 3+ ) and 654.47 eV(Mn 4+ ) [32,33]. Based on the data in Table  1, the Mn 3+ fraction of Cr2O3/5MnOx was lower than that of the other samples. The Cr 2p spectra of the samples are shown in Figure 6b. The main signals of Cr 2p3/2 were 575.42 eV(Cr 2+ ), 576.57 eV(Cr 3+ ) and 578.34 eV(Cr 4+ ), while for Cr 2p1/2 the main signals were 585.21 eV(Cr 2+ ), 586.47 eV(Cr 3+ ) and 588.10 eV(Cr 4+ ) [34][35][36][37]. A significant decline in Cr 5+ content occurred after Mn loading, which, in combination with the presence and change in the valence state of Mn, can be attributed to electron migration between Mn and Cr: 2Mn 2+ + Cr 5+ ⇄ 2Mn 3+ + Cr 3+ . Electronic migration between different metallic ions facilitates faster reaction rates and improves the catalytic ability of materials [38]. The O 1s spectra are shown in Figure 6c. The peak at 530.05 eV corresponds to lattice oxygen (OLatt), the peak at 531.70 eV corresponds to adsorbed oxygen (OAds), and the peak at 533.22 eV relates to surface hydroxyl oxygen (OOH) [39,40]. The lattice oxygen content of Cr2O3/MnOx was higher than that of MIL-101 (Cr). This promoted the oxidation process and enabled a certain oxygen spillover effect between MnOx and Cr2O3 after pyrolysis. Hence, lattice oxygen can readily migrate to the composite surface, thereby improving the catalytic efficiency of the Cr2O3/MnOx composite.

Electrochemical Characterization
The electrochemical properties of MIL-101-S and Cr 2 O 3 /MnO x -S were analyzed to evaluate the immobilization of sulfur in the MOF and the effect of adding Mn to MIL-101(Cr) on the specific capacity and cycling stability. The mass loading of sulfur for all electrode materials was 1.6 mg·cm −2 . Figure 7 displays the cyclic voltammetry (CV) profiles of Cr 2 O 3 /10MnO x -S, which displayed clear reduction peaks at 2.23 and 2.02 V due to the conversion of S 8 molecules to higher-order Li 2 S x and their further conversion to Li 2 S 2 and Li 2 S during the discharge process. The oxidation peak at 2.45 V corresponds to the conversion of polysulfides into S 8 molecules (Equations (1) and (2)), as follows:

Electrochemical Characterization
The electrochemical properties of MIL-101-S and Cr2O3/MnOx-S were analyzed to evaluate the immobilization of sulfur in the MOF and the effect of adding Mn to MIL-101(Cr) on the specific capacity and cycling stability. The mass loading of sulfur for all electrode materials was 1.6 mg·cm −2 . Figure 7 displays the cyclic voltammetry (CV) profiles of Cr2O3/10MnOx-S, which displayed clear reduction peaks at 2.23 and 2.02 V due to the conversion of S8 molecules to higher-order Li2Sx and their further conversion to Li2S2 and Li2S during the discharge process. The oxidation peak at 2.45 V corresponds to the conversion of polysulfides into S8 molecules (Equations (1) and (2)), as follows: The CV peaks further indicate that the chemical reactions are reversible [17,41]. It should be noted that the Cr2O3/10MnOx-S electrode had a higher reduction peak and a lower oxidation peak than the MIL-101-S electrode. In the corresponding charge/discharge profiles, the charge/discharge potential difference of the Cr2O3/10MnOx-S electrode was also smaller. Thus, the CV analysis shows that Mn loading caused the chemical reaction to proceed more smoothly, leading to higher catalytic activity and higher utilization of the active material. The electrochemical stability was further assessed by carrying out the charge/discharge process at 0.1 C. The CV peaks further indicate that the chemical reactions are reversible [17,41]. It should be noted that the Cr 2 O 3 /10MnO x -S electrode had a higher reduction peak and a lower oxidation peak than the MIL-101-S electrode. In the corresponding charge/discharge profiles, the charge/discharge potential difference of the Cr 2 O 3 /10MnO x -S electrode was also smaller. Thus, the CV analysis shows that Mn loading caused the chemical reaction to proceed more smoothly, leading to higher catalytic activity and higher utilization of the active material. The electrochemical stability was further assessed by carrying out the charge/discharge process at 0.1 C. Figure 8a shows that the initial discharge capacity of the MIL-101-based LSB was 901.8 mAh·g −1 , and this capacity decreased to 261.3 mAh·g −1 after 100 cycles. Furthermore, similar to the CV curves, the galvanostatic charge/discharge curves of Cr 2 O 3 /MnO x -S exhibited two distinct plateaus at about 2.23 V and 2.02 V. The materials prepared with different Mn contents showed significant differences in terms of initial discharge performance. The curves in the figure show that the Cr 2 O 3 /MnO x -S electrodes with different Mn loadings all performed better than the original MOF material in terms of first discharge specific capacity, with Cr 2 O 3 /10MnO x -S having the highest specific capacity (1286 mAh·g −1 ). The specific capacity of Cr 2 O 3 /20MnO x -S became lower compared to Cr 2 O 3 /10MnO x -S, probably because the Mn loading had exceeded the upper limit, the material pores were over-occupied and the sulfur in the electrode material formed aggregates, leading to a lower utilization of the active material during the electrode reaction. MIL-101-S and Cr 2 O 3 /20MnO x -S exhibited different degrees of overcharging during the charging process. It should be noted that MIL-101-S does not contain any Mn. Hence, MnO x crystals were not formed, resulting in a significant collapse of the MIL-101(Cr) channels during pyrolysis. Therefore, S molecules were not able to enter this MOF, leading to S agglomeration and reducing the utilization of sulfur during the initial discharge. The overcharging of Cr 2 O 3 /20MnO x -S was attributed to the excessive addition of Mn, which filled the MOF channels with MnO x . This reduced the ability of S to enter the structure and lowered the sulfur utilization. To show the difference in the performance of the Cr 2 O 3 /MnO x -S and MIL-101-S electrodes, Figure 8b shows the initial specific capacity of these materials and their specific capacity after 100 cycles at 0.1 C. The initial specific capacity of the Cr 2 O 3 /MnO x -based LSB and its specific capacity after cycling were better than those of MIL-101(Cr). discharge performance. The curves in the figure show that the Cr2O3/MnOx-S electrodes with different Mn loadings all performed better than the original MOF material in terms of first discharge specific capacity, with Cr2O3/10MnOx-S having the highest specific capacity (1286 mAh·g −1 ). The specific capacity of Cr2O3/20MnOx-S became lower compared to Cr2O3/10MnOx-S, probably because the Mn loading had exceeded the upper limit, the material pores were over-occupied and the sulfur in the electrode material formed aggregates, leading to a lower utilization of the active material during the electrode reaction. MIL-101-S and Cr2O3/20MnOx-S exhibited different degrees of overcharging during the charging process. It should be noted that MIL-101-S does not contain any Mn. Hence, MnOx crystals were not formed, resulting in a significant collapse of the MIL-101(Cr) channels during pyrolysis. Therefore, S molecules were not able to enter this MOF, leading to S agglomeration and reducing the utilization of sulfur during the initial discharge. The overcharging of Cr2O3/20MnOx-S was attributed to the excessive addition of Mn, which filled the MOF channels with MnOx. This reduced the ability of S to enter the structure and lowered the sulfur utilization. To show the difference in the performance of the Cr2O3/MnOx-S and MIL-101-S electrodes, Figure 8b shows the initial specific capacity of these materials and their specific capacity after 100 cycles at 0.1 C. The initial specific capacity of the Cr2O3/MnOx-based LSB and its specific capacity after cycling were better than those of MIL-101(Cr).  To assess their rate performance, the Cr 2 O 3 /10MnO x -S and MIL-101-S electrodes were cycled at different charge and discharge rates (0.1, 0.2, 0.3, 0.5, 0.8 and 1 C) [17]. After cycling, the electrodes were tested again at low current rates (0.1 C and 0.2 C) to assess their capacity recovery. The results are presented in Figure 8c. In general, a lower discharge rate corresponds to higher capacity and vice versa [42,43]. It should be noted that capacity declines at high discharge rates due to the limited free space for Li + ions [17,44]. There is a relatively high potential energy barrier for the conversion of polysulfides into sulfur molecules during the charge/discharge reaction of LSBs, leading to inferior cyclic performance [45]. However, the addition of a catalyst can reduce the activation energy of these reactions and improve the cycling efficiency of LSBs [46]. The specific capacity of Cr 2 O 3 /10MnO x -S exceeded that of MIL-101-S at all discharge rates. In terms of overall cyclic efficiency, the addition of Mn improved the specific capacity and cyclic efficiency of the battery at low discharge rates, while only a limited improvement was observed at high discharge rates. This was because the addition of Mn led to the Mn occupying some of the pore volume of the MOF material, limiting the space for Li + ions. Thus, the effect of the catalyst on the cyclic efficiency of the battery became insignificant. Meanwhile, Figure 8c shows that the Cr 2 O 3 /10MnO x -S and MIL-101-S electrodes behaved differently in terms of charging efficiency. The Cr 2 O 3 /10MnO x -S electrode maintained a charging efficiency of about 98%, but the MIL-101-S electrode exhibited a lower charging efficiency than Cr 2 O 3 /10MnO x -S, indicating a certain degree of electrochemical polarization. The poor electrical conductivity of the MIL-101-S electrode resulted in a low rate of electron conduction during the cell reaction. The Cr 2 O 3 /10MnO x -S material had enhanced electrical conductivity after pyrolysis and achieved higher catalytic activity after loading with Mn. Hence, at the same discharge rate, the electrode reaction of Cr 2 O 3 /MnO x -S was enhanced compared to that of the MIL-101-S electrode, resulting in a higher charging efficiency.
To fully demonstrate the overall improvement of the cyclic efficiency of the MOF after adding Mn addition, the MIL-101-S and Cr 2 O 3 /MnO x -S electrodes were tested for 100 cycles at 0.1 C, and the results are shown in Figure 8d. The performance of the MIL-101-S electrode was inferior to that of the Cr 2 O 3 /MnO x -S electrodes in terms of both initial discharge capacity and cyclic stability. The Cr 2 O 3 /5MnO x -S electrode had a slightly improved performance compared to the MIL-101-S electrode, while the optimal performance was achieved at a Mn loading of 10%. The Cr 2 O 3 /10MnO x -S electrode had an initial specific capacity of 1286 mAh·g −1 and a 100th cycle capacity of 721 mAh·g −1 . At a Mn loading of 20%, the initial specific capacity decreased but the cyclic performance was slightly improved because the excessive loading of Mn lowered the average pore size (Figure 5b), resulting in sulfur aggregation and reduced sulfur utilization. Hence, the overall specific capacity of this electrode was lower.

Conclusions
In summary, MIL-101(Cr) was synthesized by a hydrothermal method. Cr 2 O 3 /MnO x was then prepared by pre-impregnation, reduction and pyrolysis. The resulting Cr 2 O 3 /MnO x -S material was utilized as a cathode for lithium-sulfur batteries. The effects of Mn content on the morphology, structure, specific surface area, catalytic performance and electrochemical properties of this electrode were systematically investigated. The results revealed that, even with a small amount of Mn, the performance of the battery cathode was significantly improved. The morphology of the Cr 2 O 3 /MnO x composite prepared after Mn loading was ortho-octahedral, where Cr 2 O 3 /10MnO x possessed a BET SSA of 88.41 m 2 ·g −1 and a BJH average pore diameter of 32.5 nm. The positive material Cr 2 O 3 /10MnO x -S possessed the best electrochemical performance with a sulfur mass loading of 1.6 mg·cm −2 . Cr 2 O 3 /10MnO x -S achieved an initial discharge capacity of 1286 mAh·g −1 (0.1 C) and retained a capacity of 721 mAh·g −1 after 100 charge/discharge cycles.

Data Availability Statement:
The data used to support the findings of this study are available from the corresponding author upon request.