A Rational Design of a CoS2-CoSe2 Heterostructure for the Catalytic Conversion of Polysulfides in Lithium-Sulfur Batteries

Lithium-sulfur batteries are anticipated to be the next generation of energy storage devices because of their high theoretical specific capacity. However, the polysulfide shuttle effect of lithium-sulfur batteries restricts their commercial application. The fundamental reason for this is the sluggish reaction kinetics between polysulfide and lithium sulfide, which causes soluble polysulfide to dissolve into the electrolyte, leading to a shuttle effect and a difficult conversion reaction. Catalytic conversion is considered to be a promising strategy to alleviate the shuttle effect. In this paper, a CoS2-CoSe2 heterostructure with high conductivity and catalytic performance was prepared by in situ sulfurization of CoSe2 nanoribbon. By optimizing the coordination environment and electronic structure of Co, a highly efficient CoS2-CoSe2 catalyst was obtained, to promote the conversion of lithium polysulfides to lithium sulfide. By using the modified separator with CoS2-CoSe2 and graphene, the battery exhibited excellent rate and cycle performance. The capacity remained at 721 mAh g−1 after 350 cycles, at a current density of 0.5 C. This work provides an effective strategy to enhance the catalytic performance of two-dimensional transition-metal selenides by heterostructure engineering.

Duan's group analyzed the activation energy for sulfur reduction reactions (SRRs) and revealed the difficulty of different reaction steps [30]. The results show that the activation energy in the initial stage of a SRR is relatively low, indicating that the conversion reaction between solid S 8 and liquid Li 2 S x is easier. However, the activation energy for the reduction reaction of Li 2 S x to Li 2 S 2 /Li 2 S is large, which means the transformation of liquid Li 2 S x to solid Li 2 S 2 /Li 2 S is difficult. Therefore, the conversion of liquid Li 2 S x to solid Li 2 S is the

Materials and Methods
Synthesis of CoSe 2 NBs. CoSe 2 NBs were prepared by the solvothermal method [41]. The specific steps are as follows. (1) First, 1 mmol of cobalt acetate (Co(AC) 2 H 2 O, 0.249 g) was added to 5 mL of deionized water and stirred until Co(AC) 2 H 2 O was completely dissolved, followed by adding 8 mL of graphene oxide dispersion (0.3 mg mL −1 ). A small amount of graphene oxide was added to prevent agglomeration of CoSe 2 NBs. The dispersion solution was treated by strong ultrasonic for 2 h. (2) Then, 26 mL diethylenetriamine and 1 mmol sodium selenite (Na 2 SeO 3 , 0.173 g) were added to the dispersion solution and stirred for 3 h until Na 2 SeO 3 was completely dissolved. (3) The completely dissolved solution was transferred to a 50 mL reactor and reacted at 180 • C for 16 h, after which the product was cleaned with deionized water. Finally, black powder was obtained by freeze drying. The as-prepared sample was labeled as CoSe 2 .
Synthesis of cathode and modified separators. Synthesis of CNT/S cathode: The sulfur/carbon composite was prepared by heat treatment at 155 • C for 12 h after mixing the nano-sulfur powder and carbon nanotubes at a mass ratio of 7.5:2.5. Then, the sul-fur/carbon composite, Super P, and PVDF were mixed at a mass ratio of 8:1:1, and NMP solvent was added and stirred for 6 h. The stirred slurry was coated on aluminum foil, dried at 60 • C for 12 h and cut into a disk with a diameter of 12 mm. The loading mass of sulfur was 1 mg cm −2 . Synthesis of modified separators: Firstly, a dispersion liquid containing 3 mg of CoS 2 -CoSe 2 powder, 14 mg graphene (GN), and 100 mL of ethanol was obtained via strong ultrasonic for 2 h. The dispersion was vacuum filtered on the blank separator and dried naturally for 12 h. Then, the separator loaded with CoS 2 -CoSe 2 /GN and was cut into discs with a diameter of 19 mm, and the mass loading of CoS 2 -CoSe 2 /GN on the disc was about 0.17 mg cm −2 . The CoSe 2 /GN composite separator was prepared by the same method.
Battery assembly and electrochemical measurements. This was performed using CNT/S as the cathode, a catalyst/GN modified separator, lithium foil as the anode, and 1 M LiTFSI dissolved in a DME/DOL (volume ratio is 1:1) with 2% LiNO 3 (mass ratio) as the electrolyte to assemble Li-S coin cells. The electrolyte/sufur (E/S) ratio is was 15 µL mg −2 . Galvanostatic charge-discharge curves were measured with battery test system (Land CT2001A). CV profiles were performed on VMP3 electrochemical workstation.
Materials Characterization. The morphologies of the samples were examined by scanning electron microscopy (SEM SU8010) and transmission electron microscopy (FEI Tecnai G2 F30). X-ray diffraction (XRD) patterns of the samples were carried out on a Bruker D8 Advance diffractometer using Cu Ka radiation. The N 2 adsorption-desorption isotherm of the samples was measured using a Belsorp Max II analyzer. X-ray photoelectron spectroscopy (XPS) analyses were carried on a PHI 5000 VersaProbe II spectrometer using monochromatic Al K(alpha) X-ray source.

Results and Discussion
The CoSe 2 NBs were prepared by solvothermal method; some protonated ammonia molecules were intercalated within the CoSe 2 NB [41]. The in situ sulfurization of CoSe 2 into CoS 2 -CoSe 2 heterostructure can be easily realized by heat treatment. In a typical preparation, thiourea and CoSe 2 NBs were first placed in two separate crucibles with different mass ratios, followed by heat treatment. Then, the H 2 S vapor was formed to react with CoSe 2 to realize the in situ sulfurization (Figure 1a). Note that the content of the CoS 2 was related to the thiourea content and gradually increased with higher thiourea contents ( Figure S1). More synthesis details can be found in the Experimental section. The morphologies of CoSe 2 and CoS 2 -CoSe 2 were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Figure 1b,e, The two-dimensional nanoribbon is flexible and transparent, indicating that ultra-thin nanoribbons exhibit good electrical conductivity. The nanoribbon was then sulfurized in situ through heat treatment using thiourea as sulfur source. As shown in Figure 1c,d, the smooth nanoribbon surface becomes rough after sulfurization, and a large number of nanoparticles were grown in situ on the nanoribbon of CoSe 2 . The nanoparticles were uniformly dispersed on the surface of the nanoribbon and were less than 20 nm in size ( Figure 1f). The uniform dispersion of nanoparticles may be ascribed to the following two reasons. Firstly, uniform selenium vacancies were formed on the surface of CoSe 2 during heat treatment, and the selenium vacancies were rapidly replaced by sulfur to generate CoS 2 nanoparticles. Secondly, the surface of CoSe 2 , prepared by the solvothermal method, contained a large number of amino functional groups, which could act as nucleation site, to promote the uniform growth of CoS 2 on the surface of CoSe 2 . The nanoparticles were still firmly anchored on the CoSe 2 nanoribbon after strong ultrasonic dispersion during the preparation process of the TEM sample, indicating that the strong bonding force existed between the CoS 2 and CoSe 2 , which was due to the fact that CoS 2 nanoparticles were attached in situ on selenium vacancies or amino functional groups. As shown in Figure 1g, high-resolution TEM confirms that the nanoparticles on the nanoribbon were CoS 2 , and a lattice spacing of 0.25 nm corresponded to the (210) plane of CoS 2 .
confirms that the nanoparticles on the nanoribbon were CoS2, and a lattice spacing of 0.25 nm corresponded to the (210) plane of CoS2. X-ray diffraction (XRD) was used to study the phases of CoSe2 and CoS2-CoSe2. As shown in Figure 2a, the CoSe2 nanoribbons prepared by solvothermal reaction are cubic phase (JCPDS No.09-0234). The characteristic peak of CoS2 appears at 32.3° (JCPDS No.65-3322), after in situ sulfurization, which further confirms the formation of CoS2-CoSe2 [39]. Meanwhile, the stable cubic CoSe2 was transformed into the metastable orthorhombic CoSe2 (JCPDS No.53-0449). The formation of orthorhombic CoSe2 was accompanied by the rotation of the Se-Se bond, which alters the coordination environment of Co sites. This metastable orthorhombic CoSe2 showed higher electrocatalytic activity than the stable cubic phase [42]. Nitrogen adsorption desorption tests were conducted on CoSe2 and CoS2-CoSe2, and the results are shown in Figure 2b,c. There was no difference in adsorptiondesorption curve types between CoSe2 and CoS2-CoSe2, and both of them had a small number of mesoporous pores. According to a Brunauer-Emmett-Teller theoretical calculation, the specific surface areas of CoSe2 and CoS2-CoSe2 are 74 and 45 m 2 g −1 , respectively. The formation of the CoS2-CoSe2 heterostructure was accompanied by the decrease of a specific surface area, which was caused by the growth of granular CoS2 on the surface of ultra-thin nanoribbons. X-ray diffraction (XRD) was used to study the phases of CoSe 2 and CoS 2 -CoSe 2 . As shown in Figure 2a, the CoSe 2 nanoribbons prepared by solvothermal reaction are cubic phase (JCPDS No.09-0234). The characteristic peak of CoS 2 appears at 32.3 • (JCPDS No.65-3322), after in situ sulfurization, which further confirms the formation of CoS 2 -CoSe 2 [39]. Meanwhile, the stable cubic CoSe 2 was transformed into the metastable orthorhombic CoSe 2 (JCPDS No.53-0449). The formation of orthorhombic CoSe 2 was accompanied by the rotation of the Se-Se bond, which alters the coordination environment of Co sites. This metastable orthorhombic CoSe 2 showed higher electrocatalytic activity than the stable cubic phase [42]. Nitrogen adsorption desorption tests were conducted on CoSe 2 and CoS 2 -CoSe 2 , and the results are shown in Figure 2b,c. There was no difference in adsorption-desorption curve types between CoSe 2 and CoS 2 -CoSe 2 , and both of them had a small number of mesoporous pores. According to a Brunauer-Emmett-Teller theoretical calculation, the specific surface areas of CoSe 2 and CoS 2 -CoSe 2 are 74 and 45 m 2 g −1 , respectively. The formation of the CoS 2 -CoSe 2 heterostructure was accompanied by the decrease of a specific surface area, which was caused by the growth of granular CoS 2 on the surface of ultrathin nanoribbons. The surface chemistries of CoS2-CoSe2 and CoSe2 were tested and compared by X-ray photoelectron spectroscopy. Figure 2d shows the Co 2p spectra of CoS2-CoSe2 and CoSe2, in which two peaks at 778.46 and 793.36 eV are attributed to Co 2p3/2 and Co 2p1/2 of CoSe2, after the sulfurization process, and the two Co-Se bonds move to higher binding energy positions, which are 779.19 and 794.09 eV, respectively. During the in situ sulfurization process, some Se atoms in CoSe2 are replaced with S atoms, which is accompanied by the formation of a CoS2-CoSe2 heterostructure. The Co 2p3/2 spectrum moves to the higher binding energy position with 0.73 eV, indicating a Co element under oxidized change. Moreover, electron transfer occurs between the heterostructure, and a strong interaction exists between CoS2 and CoSe2. The analysis of the Se 3d fine spectra of the two materials shows that 54.7 and 59.6 eV correspond to Se2 2− and SeOx, respectively; after the conversion of CoSe2 to CoS2-CoSe2, Se 3d moves to higher binding energy position (Figure 2e). The fine spectrum of S 2p confirms the existence of CoS2 and thiosulfate/sulfate (Figure 2f). Above all, the electronic structure and coordination environment of Co were changed after the in situ sulfurization.
To investigate the polysulfide adsorption capability of CoS2-CoSe2, and CoSe2 toward Li2Sx, visual static Li2S6 adsorption tests were performed on CoS2-CoSe2 and CoSe2. As shown in Figure 3a-c, the yellow Li2S6 solution of CoS2-CoSe2 begins to fade after 0.5 h and becomes clear after 3 h; however, the color of CoSe2 began to fade after 5 h, proving that the CoS2-CoSe2 heterostructure has a strong adsorption ability with Li2S6. The UV-vis technique was used to investigate the polysulfide adsorption capability of different materials. The peak of the UV-vis spectrum in the solution with CoS2-CoSe2 exhibited the weakest intensity, indicating Li2S6 immobilization of CoS2-CoSe2 (Figure 3d). This was because the electronic structure of Co in the CoS2-CoSe2 heterostructure was optimized and the Co coordination environment of metal was changed, increasing the adsorption capability of Li2S6. Furthermore, the formation of CoS2 also increased the adsorption ability. The surface chemistries of CoS 2 -CoSe 2 and CoSe 2 were tested and compared by Xray photoelectron spectroscopy. Figure 2d shows the Co 2p spectra of CoS 2 -CoSe 2 and CoSe 2 , in which two peaks at 778.46 and 793.36 eV are attributed to Co 2p 3/2 and Co 2p 1/2 of CoSe 2 , after the sulfurization process, and the two Co-Se bonds move to higher binding energy positions, which are 779.19 and 794.09 eV, respectively. During the in situ sulfurization process, some Se atoms in CoSe 2 are replaced with S atoms, which is accompanied by the formation of a CoS 2 -CoSe 2 heterostructure. The Co 2p 3/2 spectrum moves to the higher binding energy position with 0.73 eV, indicating a Co element under oxidized change. Moreover, electron transfer occurs between the heterostructure, and a strong interaction exists between CoS 2 and CoSe 2 . The analysis of the Se 3d fine spectra of the two materials shows that 54.7 and 59.6 eV correspond to Se 2 2− and SeO x , respectively; after the conversion of CoSe 2 to CoS 2 -CoSe 2 , Se 3d moves to higher binding energy position (Figure 2e). The fine spectrum of S 2p confirms the existence of CoS 2 and thiosulfate/sulfate (Figure 2f). Above all, the electronic structure and coordination environment of Co were changed after the in situ sulfurization.
To investigate the polysulfide adsorption capability of CoS 2 -CoSe 2 , and CoSe 2 toward Li 2 S x , visual static Li 2 S 6 adsorption tests were performed on CoS 2 -CoSe 2 and CoSe 2 . As shown in Figure 3a-c, the yellow Li 2 S 6 solution of CoS 2 -CoSe 2 begins to fade after 0.5 h and becomes clear after 3 h; however, the color of CoSe 2 began to fade after 5 h, proving that the CoS 2 -CoSe 2 heterostructure has a strong adsorption ability with Li 2 S 6 . The UVvis technique was used to investigate the polysulfide adsorption capability of different materials. The peak of the UV-vis spectrum in the solution with CoS 2 -CoSe 2 exhibited the weakest intensity, indicating Li 2 S 6 immobilization of CoS 2 -CoSe 2 (Figure 3d). This was because the electronic structure of Co in the CoS 2 -CoSe 2 heterostructure was optimized and the Co coordination environment of metal was changed, increasing the adsorption capability of Li 2 S 6 . Furthermore, the formation of CoS 2 also increased the adsorption ability. Materials 2023, 16, x FOR PEER REVIEW 6 of 10 The catalytic activity of the materials was evaluated through cyclic voltammogram (CV) curves of symmetrical cells, using an Li2S6 electrode. As shown in Figure 3e, compared with CoSe2, the CoS2-CoSe2 symmetrical cell has a higher current density, which indicates that CoS2-CoSe2 accelerates the catalytic conversion of Li2Sx and improves the utilization of sulfur. Moreover, the Li2S deposition experiment was conducted for CoS2-CoSe2 and CoSe2, to illustrate the catalytic activity. Catalysts were loaded on carbon fiber paper, and an Li2S8 solution was added on the catalysts. The coin cell was assembled using carbon fiber paper loaded with a catalyst, Li2S8 as the cathode, and lithium foil as the anode. The coin cell was galvanostatically discharged to 2.07 V and then potentiostatically discharged at 2.06 V. The potentiostatic discharge curves of cells with CoSe2 and CoS2-CoSe2 are shown in Figure 3f. The peak currents of CoSe2 and CoS2-CoSe2 were 0.21 and 0.3 mA, respectively. Moreover, compared with CoSe2, the deposition time of Li2Sx for CoS2-CoSe2 was shorter, demonstrating that the CoS2-CoSe2 delivered better catalytic performance toward Li2Sx. The morphology of the electrode was observed at the same deposition time point, as shown in Figure 3g,h. No obvious Li2S was present on the surface of CoSe2; in comparation, a large area of Li2S had already emerged on the surface of CoS2-CoSe2, confirming that the CoS2-CoSe2 heterostructure improved the conversion efficiency of Li2Sx to Li2S2/Li2S. The morphologies of the CoSe2 and CoS2-CoSe2 electrodes after Li2S deposition were also investigated. As shown in Figure S2, a large amount of Li2S could be observed on the surface of the CoS2-CoSe2 electrode, while only a few deposits were detected on the on the surface of the CoSe2 electrode, further confirming that CoS2-CoSe2 was beneficial for the deposition of Li2S.
Above all, compared with CoSe2, the adsorption and catalytic conversion ability of CoS2-CoSe2 have been largely improved, due to the presence of CoS2, which may be attributed to two reasons. Firstly, the heterostructure activated the Co catalytic activity on the surface of the CoSe2, and the formed hetero-interface ensured the rapid electron transfer of and Li-ion diffusion between CoS2 and CoSe2. Secondly, the orthorhombic CoSe2 changed the coordination environment of the Co, increasing the adsorption sites and catalytic activity.
To investigate the electrochemical performance of Li-S batteries with CoSe2 and CoS2-CoSe2, the CoS2-CoSe2/GN or CoSe2/GN modified separator was applied to the Li-S batteries. The modified separator had a loading of 0.17 mg cm −2 and a thickness of ~11 µm ( Figure S3). The rate performance of Li-S batteries with CoS2-CoSe2 and CoSe2 were tested under different current densities. As shown in Figure 4a, the battery with CoS2-CoSe2 had discharge capacities of 1133, 905, 765, and 658 mAh g −1 at 0.2, 0.5, 1, and 2 C current densities, respectively. The discharge capacities of the battery with the CoSe2 cell are 1031, The catalytic activity of the materials was evaluated through cyclic voltammogram (CV) curves of symmetrical cells, using an Li 2 S 6 electrode. As shown in Figure 3e, compared with CoSe 2 , the CoS 2 -CoSe 2 symmetrical cell has a higher current density, which indicates that CoS 2 -CoSe 2 accelerates the catalytic conversion of Li 2 S x and improves the utilization of sulfur. Moreover, the Li 2 S deposition experiment was conducted for CoS 2 -CoSe 2 and CoSe 2 , to illustrate the catalytic activity. Catalysts were loaded on carbon fiber paper, and an Li 2 S 8 solution was added on the catalysts. The coin cell was assembled using carbon fiber paper loaded with a catalyst, Li 2 S 8 as the cathode, and lithium foil as the anode. The coin cell was galvanostatically discharged to 2.07 V and then potentiostatically discharged at 2.06 V. The potentiostatic discharge curves of cells with CoSe 2 and CoS 2 -CoSe 2 are shown in Figure 3f. The peak currents of CoSe 2 and CoS 2 -CoSe 2 were 0.21 and 0.3 mA, respectively. Moreover, compared with CoSe 2 , the deposition time of Li 2 S x for CoS 2 -CoSe 2 was shorter, demonstrating that the CoS 2 -CoSe 2 delivered better catalytic performance toward Li 2 S x . The morphology of the electrode was observed at the same deposition time point, as shown in Figure 3g,h. No obvious Li 2 S was present on the surface of CoSe 2 ; in comparation, a large area of Li 2 S had already emerged on the surface of CoS 2 -CoSe 2 , confirming that the CoS 2 -CoSe 2 heterostructure improved the conversion efficiency of Li 2 S x to Li 2 S 2 /Li 2 S. The morphologies of the CoSe 2 and CoS 2 -CoSe 2 electrodes after Li 2 S deposition were also investigated. As shown in Figure S2, a large amount of Li 2 S could be observed on the surface of the CoS 2 -CoSe 2 electrode, while only a few deposits were detected on the on the surface of the CoSe 2 electrode, further confirming that CoS 2 -CoSe 2 was beneficial for the deposition of Li 2 S. Above all, compared with CoSe 2 , the adsorption and catalytic conversion ability of CoS 2 -CoSe 2 have been largely improved, due to the presence of CoS 2 , which may be attributed to two reasons. Firstly, the heterostructure activated the Co catalytic activity on the surface of the CoSe 2 , and the formed hetero-interface ensured the rapid electron transfer of and Li-ion diffusion between CoS 2 and CoSe 2 . Secondly, the orthorhombic CoSe 2 changed the coordination environment of the Co, increasing the adsorption sites and catalytic activity.
To investigate the electrochemical performance of Li-S batteries with CoSe 2 and CoS 2 -CoSe 2 , the CoS 2 -CoSe 2 /GN or CoSe 2 /GN modified separator was applied to the Li-S batteries. The modified separator had a loading of 0.17 mg cm −2 and a thickness of~11 µm ( Figure S3). The rate performance of Li-S batteries with CoS 2 -CoSe 2 and CoSe 2 were tested under different current densities. As shown in Figure 4a, the battery with CoS 2 -CoSe 2 had discharge capacities of 1133, 905, 765, and 658 mAh g −1 at 0.2, 0.5, 1, and 2 C current densities, respectively. The discharge capacities of the battery with the CoSe 2 cell are 1031, 757, 639, and 524 mAh g −1 , indicating that the CoS 2 -CoSe 2 improved the sulfur utilization. Figure 4b displays the charge-discharge curves of the two batteries. When the current density was 0.5 • C, the battery with CoS 2 -CoSe 2 had a longer discharge platform, which proves that CoS 2 -CoSe 2 promotes the conversion of Li 2 S x and improves the utilization of sulfur. By analyzing the charging curves of the two batteries, it can be seen that the battery with CoS 2 -CoSe 2 can reduce the energy barrier of conversion from Li 2 S 2 /Li 2 S to Li 2 S x , indicating that CoS 2 -CoSe 2 can promote its conversion reaction. Figure 4c shows the charge-discharge curves of the battery using CoS 2 -CoSe 2 at different current densities. The discharge curves at different current densities contained two discharge platforms, corresponding to the reduction reaction from solid S 8 to liquid Li 2 S x , and Li 2 S x to Li 2 S 2 /Li 2 S. CV tests were conducted at different scanning speeds for the battery with CoS 2 -CoSe 2 . It can be seen that, even at the scanning speed of 0.5 mV s −1 , there were still two obvious reduction peaks and oxidation peaks, indicating that the battery had excellent rate performance (Figure 4d). 757, 639, and 524 mAh g −1 , indicating that the CoS2-CoSe2 improved the sulfur utilization. Figure 4b displays the charge-discharge curves of the two batteries. When the current density was 0.5 °C, the battery with CoS2-CoSe2 had a longer discharge platform, which proves that CoS2-CoSe2 promotes the conversion of Li2Sx and improves the utilization of sulfur. By analyzing the charging curves of the two batteries, it can be seen that the battery with CoS2-CoSe2 can reduce the energy barrier of conversion from Li2S2/Li2S to Li2Sx, indicating that CoS2-CoSe2 can promote its conversion reaction. Figure 4c shows the chargedischarge curves of the battery using CoS2-CoSe2 at different current densities. The discharge curves at different current densities contained two discharge platforms, corresponding to the reduction reaction from solid S8 to liquid Li2Sx, and Li2Sx to Li2S2/Li2S. CV tests were conducted at different scanning speeds for the battery with CoS2-CoSe2. It can be seen that, even at the scanning speed of 0.5 mV s −1 , there were still two obvious reduction peaks and oxidation peaks, indicating that the battery had excellent rate performance (Figure 4d).  Figure 4e shows the long cycle performance of the two batteries at a current density of 0.5 C. The battery using the CoS2-CoSe2 had an initial capacity of 921 mAh g −1 and a capacity of 722 mAh g −1 after 350 cycles, corresponding to a capacity decay rate of 0.062% per cycle. The cell using the CoSe2 had an initial capacity of 740 mAh g −1 and a capacity of 352 mAh g −1 after 350 cycles, corresponding to a capacity decay rate of 0.15% per cycle. Although CoSe2 has a high specific surface area, its poor Li2Sx adsorption and catalytic ability resulted in poor cycling stability. CoS2-CoSe2 has good adsorption and catalytic conversion ability toward Li2Sx, which improved the capacity and cycle stability of the  Figure 4e shows the long cycle performance of the two batteries at a current density of 0.5 C. The battery using the CoS 2 -CoSe 2 had an initial capacity of 921 mAh g −1 and a capacity of 722 mAh g −1 after 350 cycles, corresponding to a capacity decay rate of 0.062% per cycle. The cell using the CoSe 2 had an initial capacity of 740 mAh g −1 and a capacity of 352 mAh g −1 after 350 cycles, corresponding to a capacity decay rate of 0.15% per cycle. Although CoSe 2 has a high specific surface area, its poor Li 2 S x adsorption and catalytic ability resulted in poor cycling stability. CoS 2 -CoSe 2 has good adsorption and catalytic conversion ability toward Li 2 S x , which improved the capacity and cycle stability of the battery. Figure 4f shows the long cycle performance of the two batteries at 1 C. After 400 cycles, the battery with CoS 2 -CoSe 2 had a capacity of 612 mAh g −1 , while the cell with CoSe 2 only had a capacity of 353 mAh g −1 , which demonstrates that CoS 2 -CoSe 2 can improve the utilization of sulfur and alleviate the shuttle effect of Li 2 S x . We also compared the cycle performance of batteries with other Co-based composite electrodes, measured at 0.5/1 C (Table S1), indicating that the battery with CoS 2 -CoSe 2 had a longer cycling life.

Conclusions
In conclusion, we prepared CoSe 2 nanoribbons with a high specific surface area and high conductivity, by the solvothermal method, and then prepared a CoS 2 -CoSe 2 heterostructure by in situ sulfurization. The strong interaction between the two components ensures rapid electron transfer and Li-ion diffusion. The formation of a CoS 2 -CoSe 2 heterostructure optimizes the electronic structure of Co, and simultaneously converts its phase from a stable cubic phase into the metastable orthorhombic phase, which accompanies the change of the coordination environment of Co. As a result, CoS 2 -CoSe 2 significantly increases adsorption and catalytic ability, thereby improving the electrochemical performance of the Li-S battery. This work provides an easy way to construct highly efficient heterostructure catalysts for Li-S batteries by the in situ sulfurization of a metal sulfide heterostructure.