A Novel Synthesizing Strategy of 3D Cose 2 Porous Hollow Flowers for High Performance Lithium–Sulfur Batteries

: Redox kinetics of lithium polysulﬁdes (LiPSs) conversion and poor electrical conductivity of sulfur during the charge-discharge process greatly inhibit the commercialization of high-performance lithium–sulfur (Li–S) batteries. Herein, we synthesized CoSe 2 porous hollow ﬂowers (CoSe 2 -PHF) by etching and further selenizing layered double hydroxide, which combined the high catalytic activity of transition metal compound and high electrical conductivity of selenium. The obtained CoSe 2 -PHF can efﬁciently accelerate the catalytic conversion of LiPSs, expedite the electron transport, and improve utilization of active sulfur during the charge-discharge process. As a result, with CoSe 2 -PHF/S-based cathodes, the Li–S batteries exhibited a reversible speciﬁc capacity of 955.8 mAh g − 1 at 0.1 C and 766.0 mAh g − 1 at 0.5 C, along with a relatively small capacity decay rate of 0.070% per cycle within 400 cycles at 1 C. Even at the high rate of 3 C, the speciﬁc capacity of 542.9 mAh g − 1 can be maintained. This work enriches the way to prepare porous composites with high catalytic activity and electrical conductivity as sulfur hosts for high-rate, long-cycle rechargeable Li–S batteries.

Tremendous efforts have been invested in addressing the challenges mentioned above by developing novel strategies to fabricate state-of-the-art cathodes for Li-S batteries. Porous carbon materials such as mesoporous carbon, carbon nanotubes and graphene have been widely used to enhance the conductivity and mitigate the volume expansion of electrodes [13][14][15][16][17][18]. However, it is difficult to inhibit the dissolving of LiPSs into electrolyte due to the poor polarity of these carbon materials [19][20][21][22]. Research shows that a judicious method is to absorb LiPSs through chemical reactions [23]. Therefore, metallic compounds, such as metallic oxide, carbide, nitride, and sulfide have become a hotspot study in the past Figure 1 presents the synthesis process of CoSe 2 -PHF. Firstly, CoAl-LDH precursors were synthesized by the facile water bath method (The details can be seen in the experimental section). Secondly, Co-PHF were obtained by treating CoAl-LDH with 5mol L −1 NaOH aqueous solution for 4 h. Notably, Al 3+ can be etched by excess NaOH in this process so that generating many pores and exposing abundant active sites of catalyzing the conversion of LiPSs. Finally, Co-PHF were mixed with selenium powder, and then were heated at 500 • C for 3 h under N 2 atmosphere to obtain CoSe 2 -PHF. compounds, such as metallic oxide, carbide, nitride, and sulfide have become a hotspot study in the past several years [24][25][26][27][28]. Among these, transition metal oxides and sulfides exhibit superior electrochemical performance, which can promote the catalytic conversion of LiPSs [29][30][31][32][33]. The visualized adsorption test of polysulfides shows that titanium, vanadium and cobalt based oxides or sulfides have notable advantages of absorbing LiPSs. However, the poor conductivity of transition metal oxides and sulfides compromise their advantages of the superior cycle and rate performance for Li-S batteries.

Results and Discussion
Recently, transition metal selenides gain their popularity as potential candidates for cathode materials of high-performance Li-S batteries. It is because selenium, as the chalcogen, shares relatively similar electronegativity and ionic radius with sulfur, while has higher electrical conductivity (1 × 10 −3 S/cm) [34,35]. Hence, we combined the high catalytic activity of transition metal compound and high electrical conductivity of selenium to fabricate the CoSe2-PHF through etching and further selenizing layered double hydroxide. The etching process created many pores and exposed abundant active sites that could efficiently accelerate the catalytic conversion of LiPSs. The further selenizing process enhanced the electrical conductivity and improved the utilization of the active sulfur. Thanks to these merits, the batteries assembled with CoSe2-PHF/S-based cathodes exhibited a reversible specific capacity of 959.2 mAh g −1 at 0.1 C and 766.0 mAh g −1 at 0.5 C, along with a relatively small capacity decay rate of 0.070% per cycle within 400 cycles at 1 C. Even at the high rate of 3 C, the specific capacity of 542.9 mAh g −1 can still be maintained. Figure 1 presents the synthesis process of CoSe2-PHF. Firstly, CoAl-LDH precursors were synthesized by the facile water bath method (The details can be seen in the experimental section). Secondly, Co-PHF were obtained by treating CoAl-LDH with 5mol L −1 NaOH aqueous solution for 4 h. Notably, Al 3+ can be etched by excess NaOH in this process so that generating many pores and exposing abundant active sites of catalyzing the conversion of LiPSs. Finally, Co-PHF were mixed with selenium powder, and then were heated at 500 °C for 3 h under N2 atmosphere to obtain CoSe2-PHF. As shown in Figure 2a, Co-PHF exhibited flower-like sphere with a diameter of around 10 μm. Additionally, Figure 2b clearly showed the hollow structure of Co-PHF, which would provide a large space to accommodate the volume fluctuation of active sulfur in cathodes during the cycling process. After selenylation, the obtained CoSe2-PHF (Figure 2c) maintained the flower-like morphology of Co-PHF parent. The transmission electron microscope (TEM) image ( Figure 2d) showed that CoSe2-PHF were composed of hexagonal sheets, and the insert figure exhibited that CoSe2-PHF were porous, which can expose abundant active sites towards catalyzing the conversion of LiPSs. Figure 2e showed the high-resolution TEM image of CoSe2-PHF, the d-spacing lattice of CoSe2-PHF was 0.237 nm, corresponding to the (211) plane of CoSe2. As illustrated in Figure 2f, the SAED pattern can be well indexed according to the crystallographic data, indicating the high crystallinity of CoSe2-PHF. The element distribution of CoSe2-PHF, as shown in Figure 2g, indicated As shown in Figure 2a, Co-PHF exhibited flower-like sphere with a diameter of around 10 µm. Additionally, Figure 2b clearly showed the hollow structure of Co-PHF, which would provide a large space to accommodate the volume fluctuation of active sulfur in cathodes during the cycling process. After selenylation, the obtained CoSe 2 -PHF ( Figure 2c) maintained the flower-like morphology of Co-PHF parent. The transmission electron microscope (TEM) image ( Figure 2d) showed that CoSe 2 -PHF were composed of hexagonal sheets, and the insert figure exhibited that CoSe 2 -PHF were porous, which can expose abundant active sites towards catalyzing the conversion of LiPSs. Figure 2e showed the high-resolution TEM image of CoSe 2 -PHF, the d-spacing lattice of CoSe 2 -PHF was 0.237 nm, corresponding to the (211) plane of CoSe 2 . As illustrated in Figure 2f, the SAED pattern can be well indexed according to the crystallographic data, indicating the high crystallinity of CoSe 2 -PHF. The element distribution of CoSe 2 -PHF, as shown in Figure 2g, indicated that Co, Se, and C were evenly distributed on the flower-like sphere, further suggesting the successful preparation of CoSe 2 -PHF. that Co, Se, and C were evenly distributed on the flower-like sphere, further suggesting the successful preparation of CoSe2-PHF. Thermogravimetric analysis (TGA) was conducted to determine the sulfur loading in CoSe2-PHF/S. As shown in Figure 3a, the sulfur loading in CoSe2-PHF/S composite was about 74 wt%. This high sulfur loading can be ascribed to the hollow structure of CoSe2-PHF. The specific surface area and pore structure of CoAl-LDH, Co-PHF and CoSe2-PHF were determined by using N2 adsorption/desorption measurement. All of the three samples showed the type-IV curves (Figure 3b), which implied the mesoporous structure of these samples. The BET test results for different samples were summarized in Table 1. The specific surface area of Co-PHF was measured as 40.177 m 2 •g −1 , which was approximately twice to that of CoAl-LDH (19.515 m 2 •g −1 ). This result may be caused by the etching of NaOH in CoAl-LDH leading to the formation of porous structure in Co-PHF. After selenizing treatment, the specific surface area was decreased to 31.986 m 2 •g −1 due to the formation of CoSe2-PHF. These three samples' results of N2 adsorption−desorption measurements were in line with their morphological features observed by SEM and TEM measurements. Figure 3c demonstrated the pore size distribution plots of the three samples tested by the Barrett−Joyner−Halenda (BJH) method. The average pore diameter of CoAl-LDH was 5.012 nm, and this value was increased to 17.296 nm after etching, and the average pore diameter was decreased to 15.310 nm with the selenizing treatment. The large specific surface and the abundant mesoporous textures of CoSe2-PHF are ideal for enhancing the performance of sulfur cathodes. It is because they can provide sufficient adsorptive and catalytic sites for LiPSs, and efficiently mitigate the volume changes of the cathodes during the charge-discharge process. Thermogravimetric analysis (TGA) was conducted to determine the sulfur loading in CoSe 2 -PHF/S. As shown in Figure 3a, the sulfur loading in CoSe 2 -PHF/S composite was about 74 wt%. This high sulfur loading can be ascribed to the hollow structure of CoSe 2 -PHF. The specific surface area and pore structure of CoAl-LDH, Co-PHF and CoSe 2 -PHF were determined by using N 2 adsorption/desorption measurement. All of the three samples showed the type-IV curves (Figure 3b), which implied the mesoporous structure of these samples. The BET test results for different samples were summarized in Table 1. The specific surface area of Co-PHF was measured as 40.177 m 2 ·g −1 , which was approximately twice to that of CoAl-LDH (19.515 m 2 ·g −1 ). This result may be caused by the etching of NaOH in CoAl-LDH leading to the formation of porous structure in Co-PHF. After selenizing treatment, the specific surface area was decreased to 31.986 m 2 ·g −1 due to the formation of CoSe 2 -PHF. These three samples' results of N 2 adsorption−desorption measurements were in line with their morphological features observed by SEM and TEM measurements. Figure 3c demonstrated the pore size distribution plots of the three samples tested by the Barrett−Joyner−Halenda (BJH) method. The average pore diameter of CoAl-LDH was 5.012 nm, and this value was increased to 17.296 nm after etching, and the average pore diameter was decreased to 15.310 nm with the selenizing treatment. The large specific surface and the abundant mesoporous textures of CoSe 2 -PHF are ideal for enhancing the performance of sulfur cathodes. It is because they can provide sufficient adsorptive and catalytic sites for LiPSs, and efficiently mitigate the volume changes of the cathodes during the charge-discharge process.   X-ray photoelectron spectroscopy (XPS) tests were then conducted to explore the chemical composition and valence states of CoSe2-PHF. As observed from the survey spectrum of CoSe2-PHF (Figure 3d), the element of Co, Se, C and O were all existed. According to the literature, O 1s can be ascribed to the unavoidable surface adsorption of the sample  X-ray photoelectron spectroscopy (XPS) tests were then conducted to explore the chemical composition and valence states of CoSe 2 -PHF. As observed from the survey spectrum of CoSe 2 -PHF (Figure 3d), the element of Co, Se, C and O were all existed.

Results and Discussion
According to the literature, O 1s can be ascribed to the unavoidable surface adsorption of the sample due to the exposure to the air. [36]. Peak fitting analysis of Co in 2p region showed Co 2p 1/2 (797.1 eV), Co 2p 3/2 (781.1 eV) and two satellite peaks (Figure 3e), which were consistent with Co 2+ in CoSe 2 [36,37]. In addition, two obvious satellites at the higher energy side of the Co 2p indicated the antibonding orbital between the atom of Co and Se [38]. In Figure 3f, the peaks located at 54.5 eV and 55.8 eV were attributed to Co-Se bond, while the peak located at around 61.2 eV was assigned to SeO x , which was formed by partial oxidation of CoSe 2 [36,37,39,40]. Figure 4a,b displayed the x-ray diffraction (XRD) patterns of Co-PHF, CoSe 2 -PHF, and CoSe 2 -PHF/S. Although most of the aluminum in the precursor (CoAl-LDH) was etched by NaOH, the diffraction peaks of Co-PHF were still matched well with CoAl-LDH (PDF#51-0045) [41][42][43]. After selenylation, since that the characteristic diffraction peaks of CoSe 2 (PDF#09-0234) appear in the obtained CoSe 2 -PHF sample. In addition, the XRD pattern of CoSe 2 -PHF/S composite is actually the overlap of XRD peaks of CoSe 2 -PHF and S 8 , indicating that sulfur is successfully loaded into CoSe 2 -PHF.
Catalysts 2021, 11, x FOR PEER REVIEW 5 of 13 due to the exposure to the air. [36]. Peak fitting analysis of Co in 2p region showed Co 2p1/2 (797.1 eV), Co 2p3/2 (781.1 eV) and two satellite peaks (Figure 3e), which were consistent with Co 2+ in CoSe2 [36,37]. In addition, two obvious satellites at the higher energy side of the Co 2p indicated the antibonding orbital between the atom of Co and Se [38]. In Figure 3f, the peaks located at 54.5 eV and 55.8 eV were attributed to Co-Se bond, while the peak located at around 61.2 eV was assigned to SeOx, which was formed by partial oxidation of CoSe2 [36,37,39,40]. Figure 4a,b displayed the x-ray diffraction (XRD) patterns of Co-PHF, CoSe2-PHF, and CoSe2-PHF/S. Although most of the aluminum in the precursor (CoAl-LDH) was etched by NaOH, the diffraction peaks of Co-PHF were still matched well with CoAl-LDH (PDF#51-0045) [41][42][43]. After selenylation, since that the characteristic diffraction peaks of CoSe2 (PDF#09-0234) appear in the obtained CoSe2-PHF sample. In addition, the XRD pattern of CoSe2-PHF/S composite is actually the overlap of XRD peaks of CoSe2-PHF and S8, indicating that sulfur is successfully loaded into CoSe2-PHF. Cyclic voltammetry (CV) was conducted to investigate the catalytic activity of CoSe2-PHF toward the conversion of LiPSs in the charge-discharge process. As shown in Figure 5a, two reduction peaks (R1 and R2) and one oxidation peak (O) were observed in both CoSe2-PHF/S and CoSe2/S electrodes. For CoSe2/S-based electrode, the first reduction peak (R1) corresponded to the conversion of sulfur (S8) to long-chain Li2Sn (4 ≤ n ≤ 8), and the second reduction peak (R2) was related to the further reduction of long-chain LiPSs to short-chain Li2S2 and LiS2, as well as CoSe2-PHF/S based electrode [44][45][46]. The broad oxidation peak centered at about 2.455 V represented the two continuous oxidation processes. As shown in Figure 5b and Figure S2 (see Supplementary Materials), both reduction and oxidation peaks were highly dependent on the potential scan rate. As the potential scan rate increased, two reduction peaks shifted to lower potential while the oxidation peak shifted to higher potential. Significantly, the area ratio of reduction peak to oxidation peak for CoSe2-PHF/S electrode was close to 1, indicating its highly reversible reaction of sulfur redox. By contrast, the two reduction peaks of CoSe2/S-based electrode centered at lower potential (2.238 V, and 2.027 V, respectively), and the oxidation peak centered at a higher potential (2.487 V), which suggested that CoSe2-PHF possessed higher catalytic activity than CoSe2. And CV tests on the symmetrical cells of CoSe2-PHF electrode and CoSe2 electrode (without loading sulfur) showed the same conclusion. As shown in Figure 5c, the redox current in CoSe2-PHF cell increased faster than that of CoSe2-based cell with the increasing of potential, implying that CoSe2-PHF electrode can really accelerate the conversion of LiPSs and enhance the kinetics of electrode reaction. Cyclic voltammetry (CV) was conducted to investigate the catalytic activity of CoSe 2 -PHF toward the conversion of LiPSs in the charge-discharge process. As shown in Figure 5a, two reduction peaks (R 1 and R 2 ) and one oxidation peak (O) were observed in both CoSe 2 -PHF/S and CoSe 2 /S electrodes. For CoSe 2 /S-based electrode, the first reduction peak (R 1 ) corresponded to the conversion of sulfur (S 8 ) to long-chain Li 2 S n (4 ≤ n ≤ 8), and the second reduction peak (R 2 ) was related to the further reduction of long-chain LiPSs to short-chain Li 2 S 2 and LiS 2 , as well as CoSe 2 -PHF/S based electrode [44][45][46]. The broad oxidation peak centered at about 2.455 V represented the two continuous oxidation processes. As shown in Figure 5b and Figure S2 (see Supplementary Materials), both reduction and oxidation peaks were highly dependent on the potential scan rate. As the potential scan rate increased, two reduction peaks shifted to lower potential while the oxidation peak shifted to higher potential. Significantly, the area ratio of reduction peak to oxidation peak for CoSe 2 -PHF/S electrode was close to 1, indicating its highly reversible reaction of sulfur redox. By contrast, the two reduction peaks of CoSe 2 /S-based electrode centered at lower potential (2.238 V, and 2.027 V, respectively), and the oxidation peak centered at a higher potential (2.487 V), which suggested that CoSe 2 -PHF possessed higher catalytic activity than CoSe 2 . And CV tests on the symmetrical cells of CoSe 2 -PHF electrode and CoSe 2 electrode (without loading sulfur) showed the same conclusion. As shown in Figure 5c, the redox current in CoSe 2 -PHF cell increased faster than that of CoSe 2 -based cell with the increasing of potential, implying that CoSe 2 -PHF electrode can really accelerate the conversion of LiPSs and enhance the kinetics of electrode reaction. Theoretically, CoSe2-PHF could inhibit the shuttle effect of polysulfides via the enhanced chemical absorption and catalytic conversion between CoSe2-PHF and LiPSs. As shown in Figure 5d, LiPSs solution (Li2S6, 0.5 mol•L −1 ) was soaked with the CoSe2-PHF for 2 h and its color was changed from brown to almost water white, while the other two solutions (added with Super P, and CoSe2, respectively) were changed slightly, which corresponded with the results of UV−vis absorption spectra, indicating the much stronger adsorption and catalytic conversion capability for LiPSs of CoSe2-PHF.
Electrical conductivity measurements revealed the conductivity of CoSe2-PHF. As shown in Table 2, the electrical conductivity for CoSe2-PHF was estimated to be 1.49 × 10 −2 S cm −1 , which is an increase of nearly 3 orders of magnitude compared with Co3O4 (1.90 × 10 −5 S cm −1 ) and CoS2 (2.27 × 10 −5 S cm −1 ). The high electrical conductivity of CoSe2-PHF is beneficial for the electron transport and utilization of active sulfur during the charge-discharge process. Furthermore, electrochemical impedance spectra (EIS) of CoSe2-PHF/S cathodes before charge-discharge process were conducted to determine the internal resistance and charge transfer kinetics (Co3O4/S, CoS2/S and CoSe2/S cathodes as contrast samples). As shown in Figure 6, the Nyquist plots of CoSe2-PHF/S and other three contrast electrodes showed semicircles in the high frequency region (charge-transfer resistance, Rct) [35,47]. The Rct of electrode−electrolyte interface for CoSe2-PHF/S, CoSe2/S, CoS2/S and Co3O4/S cathodes was 38.509 Ω, 55.526 Ω, 74.221 Ω, and 84.337 Ω, respectively, indicating the lower resistance for charge transfer and faster kinetics for LiPSs conversion in CoSe2-PHF/S [48]. Therefore, CoSe2-PHF can significantly reduce charge-transfer resistance and promote the electrode reaction kinetics of LiPSs. Theoretically, CoSe 2 -PHF could inhibit the shuttle effect of polysulfides via the enhanced chemical absorption and catalytic conversion between CoSe 2 -PHF and LiPSs. As shown in Figure 5d, LiPSs solution (Li 2 S 6 , 0.5 mol·L −1 ) was soaked with the CoSe 2 -PHF for 2 h and its color was changed from brown to almost water white, while the other two solutions (added with Super P, and CoSe 2 , respectively) were changed slightly, which corresponded with the results of UV−vis absorption spectra, indicating the much stronger adsorption and catalytic conversion capability for LiPSs of CoSe 2 -PHF.
Electrical conductivity measurements revealed the conductivity of CoSe 2 -PHF. As shown in Table 2, the electrical conductivity for CoSe 2 -PHF was estimated to be 1.49 × 10 −2 S cm −1 , which is an increase of nearly 3 orders of magnitude compared with Co 3 O 4 (1.90 × 10 −5 S cm −1 ) and CoS 2 (2.27 × 10 −5 S cm −1 ). The high electrical conductivity of CoSe 2 -PHF is beneficial for the electron transport and utilization of active sulfur during the charge-discharge process. Furthermore, electrochemical impedance spectra (EIS) of CoSe 2 -PHF/S cathodes before charge-discharge process were conducted to determine the internal resistance and charge transfer kinetics (Co 3 O 4 /S, CoS 2 /S and CoSe 2 /S cathodes as contrast samples). As shown in Figure 6, the Nyquist plots of CoSe 2 -PHF/S and other three contrast electrodes showed semicircles in the high frequency region (chargetransfer resistance, R ct ) [35,47]. The R ct of electrode−electrolyte interface for CoSe 2 -PHF/S, CoSe 2 /S, CoS 2 /S and Co 3 O 4 /S cathodes was 38.509 Ω, 55.526 Ω, 74.221 Ω, and 84.337 Ω, respectively, indicating the lower resistance for charge transfer and faster kinetics for LiPSs conversion in CoSe 2 -PHF/S [48]. Therefore, CoSe 2 -PHF can significantly reduce charge-transfer resistance and promote the electrode reaction kinetics of LiPSs.   To further study the electrochemical performance of CoSe2-PHF/S electr of charge-discharge cycling experiments were tested. As shown in Figure  PHF/S electrodes showed much higher discharge capacity than CoSe2/S elec discharge voltage. In Figure 7c, the ratio of Q2 (Corresponding to Li2S4 + 2e − + Li2S2 + 2e − + 2Li + → 2Li2S) to Q1 (Corresponding to S8 + 4e − + 4Li + → 2Li2S4) for and CoSe2/S electrodes were 2.64, and 1.63, respectively. The charge-discharg (ΔE) of CoSe2-PHF/S electrodes were much smaller than that of CoSe2/S (Figure suggesting the faster ions transport and lower electrochemical polarizati PHF/S electrodes. To further study the electrochemical performance of CoSe 2 -PHF/S electrodes, a series of charge-discharge cycling experiments were tested. As shown in Figure 7a,b, CoSe 2 -PHF/S electrodes showed much higher discharge capacity than CoSe 2 /S electrodes at any discharge voltage. In Figure 7c, the ratio of Q2 (Corresponding to Li 2 S 4 + 2e − + 2Li + → 2Li 2 S 2 ; Li 2 S 2 + 2e − + 2Li + → 2Li 2 S) to Q1 (Corresponding to S 8 + 4e − + 4Li + → 2Li 2 S 4 ) for CoSe 2 -PHF/S and CoSe 2 /S electrodes were 2.64, and 1.63, respectively. The charge-discharge voltage gap (∆E) of CoSe 2 -PHF/S electrodes were much smaller than that of CoSe 2 /S (Figure 7c,d) [49][50][51], suggesting the faster ions transport and lower electrochemical polarization of CoSe 2 -PHF/S electrodes.
The corresponding voltage profiles based on different current rates were shown in Figure 8a. The discharge platform at a relatively high current rate of 3 C maintained well in CoSe 2 -PHF/S electrodes, suggesting a high operation stability of CoSe 2 -PHF/S electrodes. The value of rate capacity for CoSe 2 -PHF/S electrodes were 955.8 mAh g −1 at 0.1 C, 769.6 mAh g −1 at 0.5 C, 680.7 mAh g −1 at 1 C, 618.0 mAh g −1 at 2 C, and 542.9 mAh g −1 at 3 C (Figure 8b), which were much higher than these of CoSe 2 /S, CoS 2 /S and Co 3 O 4 /S. At the current rate of 0.1 C, CoSe 2 -PHF/S electrodes with sulfur loading at 1.3 mg cm −2 showed a much higher specific capacity other electrodes in each cycle (Figure 8c). Besides, with a high sulfur loading at 3.4 mg cm −2 (Figure 8d), initial specific capacity of CoSe 2 -PHF/S electrodes were 578.8 mAh g −1 , which could retain 86.3% of its initial capacity after 150 cycles at 0.5 C. The decrease of specific capacity with the increasing of sulfur loading may be caused by the quick dissolution of LiPSs intermediates into electrolyte. To further verify the electrochemical performance of CoSe 2 -PHF/S electrodes, long-term cycling was also conducted at 1 C. As shown in Figure 8e, in contrast to CoSe 2 /S, CoSe 2 /S, CoS 2 /S and Co 3 O 4 /S electrodes, which displayed much lower specific capacities and coulombic efficiencies, CoSe 2 -PHF/S electrodes can retain a high capacity of 522.1 mAh g −1 and a nearly 100% coulombic efficiency after 400 cycles, corresponding to a slow capacity decay rate of 0.070% per cycle. Therefore, CoSe 2 -PHF/S electrodes owned much better electrochemical performance, and can be considered as a promising sulfur host for Li−S batteries. The corresponding voltage profiles based on different current rates were shown in Figure 8a. The discharge platform at a relatively high current rate of 3 C maintained well in CoSe2-PHF/S electrodes, suggesting a high operation stability of CoSe2-PHF/S electrodes. The value of rate capacity for CoSe2-PHF/S electrodes were 955.8 mAh g −1 at 0.1 C, 769.6 mAh g −1 at 0.5 C, 680.7 mAh g −1 at 1 C, 618.0 mAh g −1 at 2 C, and 542.9 mAh g −1 at 3 C (Figure 8b), which were much higher than these of CoSe2/S, CoS2/S and Co3O4/S. At the current rate of 0.1 C, CoSe2-PHF/S electrodes with sulfur loading at 1.3 mg cm −2 showed a much higher specific capacity other electrodes in each cycle (Figure 8c). Besides, with a high sulfur loading at 3.4 mg cm −2 (Figure 8d), initial specific capacity of CoSe2-PHF/S electrodes were 578.8 mAh g −1 , which could retain 86.3% of its initial capacity after 150 cycles at 0.5 C. The decrease of specific capacity with the increasing of sulfur loading may be caused by the quick dissolution of LiPSs intermediates into electrolyte. To further verify the electrochemical performance of CoSe2-PHF/S electrodes, long-term cycling was also conducted at 1 C. As shown in Figure 8e, in contrast to CoSe2/S, CoSe2/S, CoS2/S and Co3O4/S electrodes, which displayed much lower specific capacities and coulombic efficiencies, CoSe2-PHF/S electrodes can retain a high capacity of 522.1 mAh g −1 and a nearly 100% coulombic efficiency after 400 cycles, corresponding to a slow capacity decay rate of 0.070% per cycle. Therefore, CoSe2-PHF/S electrodes owned much better electrochemical

Synthesis of CoAl-LDH Precursors
CoAl-LDH precursors were prepared by a typical hydrothermal process. Firstly, Co (NO3)2 6H2O (2.183 g), Al (NO3)3 9H2O (0.938 g), NH4F (0.371 g) and urea (1.501 g) were dissolved in 50 mL deionized water (DIW) and stirred at room temperature for 1 h. Secondly, the solution was added into a 100 mL Teflon-lined stainless-steel autoclave and maintained at 110 °C for 8 h. After cooling to room temperature, the precipitates were obtained by centrifuging and washing with DIW for several times. Finally, CoAl-LDH precursors were obtained by drying at 60 °C for 12 h.

Synthesis of CoAl-LDH Precursors
CoAl-LDH precursors were prepared by a typical hydrothermal process. Firstly, Co (NO 3 ) 2 6H 2 O (2.183 g), Al (NO 3 ) 3 9H 2 O (0.938 g), NH 4 F (0.371 g) and urea (1.501 g) were dissolved in 50 mL deionized water (DIW) and stirred at room temperature for 1 h. Secondly, the solution was added into a 100 mL Teflon-lined stainless-steel autoclave and maintained at 110 • C for 8 h. After cooling to room temperature, the precipitates were obtained by centrifuging and washing with DIW for several times. Finally, CoAl-LDH precursors were obtained by drying at 60 • C for 12 h.

Synthesis of CoSe 2 -PHF, CoSe 2 , CoS 2 and Co 3 O 4
Firstly, 20 g of NaOH and 1 g CoAl-LDH precursors were dispersed in 200 mL DIW and stirred for 4 h. Secondly, the brown precipitates (denoted as Co-PHF) were collected by centrifuging and washing with DIW for several times, followed by drying at 60 • C overnight. Finally, 200 mg of Co-PHF were mixed with 800 mg of selenium powder, and then the mixture was heated at 500 • C for 3 h under N 2 atmosphere to obtain CoSe 2 -PHF. For comparison, Co 3 O 4 was prepared by heating Co-PHF in the air at 500 • C for 3 h; CoS 2 was prepared by heating the mixture of Co-PHF and sulfur at 500 • C for 3 h under N 2 atmosphere, and CoSe 2 was prepared by heating the mixture of CoAl-LDH precursors and selenium powder at 500 • C for 3 h under N 2 atmosphere.

Synthesis of CoSe 2 -PHF/S, CoS 2 /S and Co 3 O 4 /S and CoSe 2 /S Composite
Sulfur was thoroughly mixed with CoSe 2 -PHF at a mass ratio of 3:1. Subsequently, the mixture was heated under Ar atmosphere at 155 • C for 12 h and then further treated at 200 • C for 1 h in a quartz tubular furnace. The resultant samples were defined as CoSe 2 -PHCS/S composite. CoS 2 /S and Co 3 O 4 /S and CoSe 2 /S were also prepared in the same method.

Visualized Adsorption Test of Polysulfides
Li 2 S 6 (0.5 mol L −1 ) solution was prepared by dissolving sulfur and Li 2 S at a molar ratio of 5:1 in a mixture of dimethoxyethane (DME) and 1,3-dioxolane (DOL) (v/v, 1:1), and vigorously stirring at 65 • C in an Ar-filled glovebox. Subsequently, 20.0 mg of samples (i.e., CoSe 2 -PHF and CoSe 2 ) were added to 5.0 mL of Li 2 S 6 solution for a 2 h static adsorption to evaluate their LiPSs adsorption ability.

Electrochemical Measurement
The cathodes were prepared by mixing CoSe 2 -PHF/S (or Co 3 O 4 /S, CoS 2 /S, and CoSe 2 /S) composite, Super P, and PVDF in NMP (7:2:1) and stirring the mixtures for 6 h, then coating the obtained slurry on Al foil and drying in an oven at 60 • C for 24 h. CR2016 coin cells were assembled in the glovebox and the sulfur areal mass loading of the batteries is 1.1-1.5 mg cm −2 . Celgard 2400 film was placed between cathode and lithium foil. The electrolyte was consisted of 1.0 M LiTFSI, a mixture of DOL and DME (1:1, v/v), and 2.0 wt% LiNO 3 additive. The cycling and rate performance were tested on the LAND battery test instrument (LAND CT2001A, Wuhan, China) between 1.6 V and 2.8 V. Cyclic voltammetry curves and electrochemical impedance spectra were obtained with the electro-chemical workstation (CHI660D, Shanghai, China).

Characterization
The morphologies of the materials were investigated by using a Hitachi SU8010 field-emission SEM (Hitachi, Tokyo, Japan). The TEM images were investigated by a JEOL JEM-2100F microscope. The crystal structures were obtained by using a Bruker D8 Advance powder X-ray diffract meter at the 2θ range of 5-90 • . TGA curves were obtained with a METTLER instrument. The samples were tested under a N 2 atmosphere at a heating rate of 10 • C min −1 . XPS measurements were conducted with a Phi X-tool XPS instrument. BET specific surface area and porous structure were determined by a Micromeritics ASAP 2020 analyzer. Electrical conductivity was determined by using a four-point probe resistivity measurement system (RTS-9).

Conclusions
In conclusion, CoSe 2 -PHF were prepared by a novel synthesizing strategy of etching and further selenizing CoAl-LDH. Due to the porous transition metal compound's superior adsorption and catalytic conversion capabilities towards LiPSs and the high electrical conductivity of metal sulfides, CoSe 2 -PHF can not only efficiently accelerate the catalytic conversion of LiPSs during the charge-discharge process, but also expedite the electron transport and improve the utilization of the active sulfur. As a result, CoSe 2 -PHF/S cathode materials in Li−S batteries exhibit a reversible specific capacity of 955.8 mAh g −1 at 0.1 C and 766.0 mAh g −1 at 0.5 C, along with a small capacity decay rate of 0.070% per cycle within 400 cycles at 1 C. Even at the high rate of 3 C, the specific capacity of 542.9 mAh g −1 can be maintained. This work offers a new way to prepare porous composites with high catalytic activity and electrical conductivity as efficient sulfur hosts for high-rate, long-cycle rechargeable Li−S batteries.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.