English

• Over the past few years, the excessive consumption of fossil fuels has triggered the problems of the century such as air pollution, global warming, and the oil crisis, so it is particularly important to develop sustainable new energy sources to solve environmental problems [1–5]. Meanwhile, the continuously increasing requirement for electronic and portable devices have rapidly accelerated the progress of lithium-ion batteries (LIBs) and sodium/potassium ion batteries with high-energy-density [6–9]. However, the traditional LIBs have approached their theoretical energy density limits, typically falling below 400 Wh/kg, this limitation has hindered their ability to meet the increasing demands of practical applications. Accordingly, lithium-sulfur batteries (LSBs) have been considered as one of the promising high-energy storage systems for the next generation due to their high theoretical capacity (1675 mAh/g), specific energy density (2600 Wh/kg), plentiful natural reserves and cost-effectiveness of the sulfur element [10]. Nevertheless, despite the potential advantages of LSBs, there are still several obstacles that hinder their industrial applications, including inadequate energy density and subpar capacity retention, attributed to the following challenges: (1) The inadequate ionic/electronic conductivity with sulfur and Li₂S₂/Li₂S, causing sluggish reaction kinetics; (2) The formation of soluble lithium polysulfides (LiPSs) as intermediate reaction products in the electrolyte, causing a serious “shuttle effect” because of the slow redox kinetics of Li₂S in the multi-step sulfur redox process, which leads to reduced Coulombic efficiency, anode degradation, and rapid capacity decline; (3) The remarkable expansion in volume experienced by the sulfur cathode during charge and discharge cycles, causing structural instability. The aforementioned issues collectively present formidable challenges in achieving high-performance LSBs with excellent cyclic stability and a prolonged lifespan.

  Up to date, researchers have carried out significant efforts to alleviate the shuttling of polysulfides. Key strategies include employing host materials to encapsulate sulfur within the cathodes [11,12], preparing modified separators [13,14] and protecting lithium anode [15], with the ultimate goal of enhancing the cyclic stability of LSBs. Modified commercial separators have been more widely investigated due to their ease of operation and practicality than the search for novel separators [16,17]. Generally, modified separators bind to carbon materials because of their excellent electrical conductivity, while nanostructured carbon materials can act as physical barriers to hinder the movement of LiPSs and facilitate their utilization in the battery system [18–20]. However, the weak affinity between non-polar carbon materials and polar LiPSs is not sufficient to completely prevent the shuttling of LiPSs during the cycling process [21]. Therefore, introducing some polar oxides into the separator can effectively immobilize the LiPSs by physicochemical action [22]. However, most of the poorly conductive oxide LiPSs have slow reaction kinetics, a multitude of materials have been utilized to modify the separator with the aim of promoting the catalytic conversion and chemisorption of LiPSs, and inhibiting its shuttle effect in LSBs, such as metal oxides [23,24], metal sulfides [25], metal nitrides [26], COFs (covalent organic frameworks) [13], and MOFs (metal-organic frameworks) [27,28]. Among these, metal oxides are particularly favored due to their cost-effectiveness and straightforward preparation methods. In recent reports, it has been highlighted that rare-earth-based materials, including CeO₂, Sc₂O₃, CeF₃, Eu₂O₃, and Sm₂O₃, can effectively promote the catalytic conversion of LiPSs in LSBs [29–33]. CeO₂, a naturally abundant material, has been commonly utilized in the fields of energy storage and catalysis [34]. Rare-earth materials typically possess distinctive physical and chemical properties. Specifically, CeO₂ with a cubic fluorite structure, characterized by high ionic and electronic conductivity, has the capability to transition between Ce⁴⁺ and Ce³⁺ oxidation states. In addition to this, the oxygen defects of CeO₂ provide plentiful active sites that can enhance their interactions [35]. However, when used as a polysulfide catalyst in LSBs, metal oxides-based separators also have serious drawbacks, such as weak electron conductivity, limited ability to bind with LSBs, and sluggish catalytic kinetics [36,37]. Studies have shown that the incorporation of C, S, P, N and other heteroatoms in metal oxides will promote their polarity and conductivity [38–40], thereby improving the catalytic kinetics and binding capacity of LSBs. Particularly, in the 3p orbitals and unoccupied 3d orbitals of phosphorus (P) where possessing a lone pair of electrons, exhibits robust interactions with lithium ions, rendering it an exceptionally promising doping element for enhancing the redox kinetics in metal oxides, thereby contributing to the advancement of LSBs. However, there are few reports on the design of P-doped metal oxides for LSBs [41], the major challenge is that metal oxides usually undergo a transformation into metal phosphides during the high-temperature phosphating process, this conversion makes it hard to control and analyze the impact of surface phosphorus atoms on the properties of metal oxides.

  Metal-organic frameworks (MOFs), which consist of organic ligands and inorganic metal ions, have recently been demonstrated as promising template/precursor to synthesize the metal compounds and widely used in sensors, separators, electrocatalysis and electrochemistry due to their homogeneous dispersed catalytic sites, large surface area, flexible pore size of MOFs, and excellent chemical properties [42,43]. In addition, it has previously been reported that the open metal sites of MOFs can serve as catalytic active sites for many reactions [44]. It has been reported that cerium oxide nanoparticles can adsorb polysulfides and promote their catalytic conversion [45]. In this manuscript, we have proposed a facile approach for fabrication of the phosphorus doped porous CeO₂ (P-CeO₂) separator materials in order to analyze the controllable influence of phosphorus atoms on the surface of metal oxides in LSBs, which is inspired by the remarkable chemical inertness [46], polyvalent states [47] and distinctive redox properties of CeO₂ [48]. When combined with oxygen to form nanoparticles, CeO₂ exhibits its fluorite crystal structure. Cerium atoms can easily transition quickly between Ce⁴⁺ and Ce³⁺ oxidation states, adjusting their electronic configuration to suit the chemical environment. In addition, due to its excellent chemical inertia, the problem of converting cerium oxide into cerium phosphide during phosphating can be solved. Firstly, the experimental analysis reveals that the phosphorus doped porous CeO₂ modified PP separator (P-CeO₂//PP) exhibits enhanced chemical interactions and superior catalytic activity compared with CeO₂//PP separator. In addition, the substantial specific surface area and plentiful oxygen defects of P-CeO₂ contribute to the manifestation of a highly exposed active interface, accelerate the rapid transformation of LiPSs, and regulate the nucleation of Li₂S deposition. Surface morphology and structural characterization analyses reveal that P-CeO₂ is primarily composed of numerous P-doped CeO₂ nanocrystals. Benefitting from these advantages, the P-CeO₂//PP separator exhibits exceptional electrochemical performance (a higher initial capacity of 1180 mAh/g at 0.5 C) and remarkable cycling stability with a low-capacity fading rate of only 0.048% per cycle over 1000 cycles at 2 C than that of bare CeO₂//PP. In addition, the experimental studies have confirmed that the batteries consist of sulfur anode, lithium cathode and P-CeO₂//PP separator exhibit superior adsorption capabilities for Li₂S₆, higher redox peak current, and facilitate the earlier precipitation of Li₂S compared to the CeO₂//PP batteries. To sum up, our research illustrates that the P-CeO₂ porous separator coating material has the dual function of catalyzing polysulfide transformation and obstructing the polysulfide transport pathway, which can successfully mitigate the shuttle effect in LSBs. This study presents a practical method for improving the cycle stability and electrochemical performance of LSBs.

  Fig. S1 (Supporting information) schematically depicts the fabrication process of P-CeO₂ modified separators and their subsequent integration into LSBs. Comprising three fundamental stages, the procedure includes: (1) Preparation of Ce-MOFs; (2) preparation of CeO₂ by calcination of Ce-MOFs in air; and (3) preparation of P-CeO₂ by phosphating CeO₂ in argon atmosphere. Firstly, Ce-MOFs were synthesized through a straightforward hydrothermal method, wherein Ce³⁺ ions coordinate with trimeric acid molecules [46], and then Ce-MOFs were calcined in air to obtain CeO₂. Afterwards, P was introduced by PH₃ gas in argon atmosphere at high temperature, and P-CeO₂ with a large amount of P doped was obtained after phosphorization process. After that, the modified separators were then assembled into button batteries by catalyzing the conversion of polysulfide intermediates and mitigating the shuttling effect of polysulfides to improve the electrochemical performance of LSBs. The morphologies of Ce-MOFs, CeO₂ and P-CeO₂ were observed by scanning electron microscopy (SEM) at different sizes (Figs. S2 and S3 in Supporting information, Figs. 1a and b). As depicted in Figs. S2a and b, the Ce-MOFs precursors exhibit a well-defined nanorod morphology with an average size of approximately 300 nm, which are densely arranged in a well-organized manner, and have a very smooth surface. After heat treatment at 500 ℃ for 4 h in air, CeO₂ was obtained (Figs. S3a and b). During this process, the nanorod-like morphology and crystallinity of Ce-MOFs were intact even after heating at high temperatures for 4 h. After the phosphorization of CeO₂ at high temperatures, P-CeO₂ retains its nanorod-like structure (Figs. 1a and b), which is attributed to the complete coverage of the inert gas argon in a sealed tube furnace. It can be suggested that Ce-MOFs and CeO₂ have excellent thermal stability during the heat treatment process.

  Figure 1

  

  Figure 1.  (a, b) SEM images of P-CeO₂. (c) HAADF-STEM image of P-CeO₂. (d) SAED pattern. (e, f) High-resolution TEM images and the corresponding EDX elemental mapping of (g) Ce, (h) O and (i) P.

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  Analysis of transmission electron microscopy (TEM) images provided a detailed exploration of the morphology of P-CeO₂. Fig. S5 (Supporting information) illustrates the uniform dispersion of P-CeO₂ nanocrystals with the average size of approximately 10 nm, within the nanowires in TEM images. The images of high-resolution TEM (HRTEM) in Figs. 1e and f reveal distinct interplanar spacings of 0.191, 0.270, and 0.312 nm, corresponding to the (220), (200), and (111) lattice fringes of the CeO₂ crystallites. Notably, these measurements are consistent with values derived from X-ray diffraction (XRD) data [47,48]. Fig. 1d showcases patterns of selected area electron diffraction (SAED), which clearly shows the presence of multiple rings, corresponding to the poly-crystalline property of CeO₂. For additional verification of the elemental distribution of phosphorus within the composite, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Fig. 1c), along with the corresponding element mapping of Ce, O, and P, are presented in Fig. S4 (Supporting information) and Figs. 1g–i. The mapping images further illustrate the uniform dispersion of phosphorus atoms in P-CeO₂, implying that the crystal structure of CeO₂ does not change after the doping of phosphorus. Obviously, the above properties demonstrated that the nanorod-like structure of P-CeO₂, which provides fast electron transport channels and abundant catalytic reaction sites.

  X-ray diffraction (XRD) was employed to analyze the crystal structures and compositions of all samples. In Fig. 2a, P-CeO₂ and CeO₂ present closely matching XRD patterns, in which the prominent peaks at 28.5°, 33.1°, 47.4°, and 56.3° correspond precisely to the (111), (200), (220), and (311) faces of the CeO₂ cubic phase (JCPDS No. 34-0394) with space group Fm-3m (225), respectively. The comparison of XRD peaks between CeO₂ and P-CeO₂ shows that P-CeO₂ has no obvious peak-to-peak shift of XRD characteristics, which indicates that P doping does not change the crystal structure of CeO₂. Afterwards, the decomposition temperature in nitrogen and air of Ce-MOFs was examined by the thermos gravimetric analysis (TGA). As shown in Fig. S6 (Supporting information), at 150 ℃, the mass loss of Ce-MOFs in air and nitrogen atmosphere are both 21.93%, when the temperature rises to 400 ℃, the mass loss of Ce-MOFs in air and nitrogen atmosphere are 64.23% and 25.41%, while when the temperature rises to 670 ℃, the mass loss are 64.99% and 52.36%, respectively. Exploration of the electronic states and detailed chemical composition of CeO₂ and P-CeO₂ were achieved through X-ray photoelectron spectra (XPS) analyses. Firstly, we calculated the contents of each element by XPS region scanning. The result shows that the atomic contents of O, Ce and P in CeO₂ are ~75.7%, ~19.45% and ~0%, while that of O, Ce and P in P-CeO₂ are ~76.9%, ~16.9% and ~10.2%, which means that we successfully doped P into CeO₂ (Fig. 2b). The XPS spectra of P-CeO₂ are showcased in Fig. S7 (Supporting information), distinctly illustrating the presence of C, O, P, and Ce elements in the samples. Additionally, the XPS spectra of P 2p show that P 2p was carefully deconvolved into P 2p_1/2 and P 2p_3/2, elucidating the presence of POx species (Fig. 2e). It is revealed that existing two characteristic spin-orbit states of Ce 3d_5/2 and Ce 3d_3/2 from the XPS of Ce 3d, as depicted in Figs. 2c and f. Notably, the Ce 3d_3/2 peaks exhibit a subdivision into five components, of which three peaks attributed to Ce⁴⁺ (899.5, 905.8, and 915.6 eV) and two peaks attributed to Ce³⁺ (897.5 and 904.3 eV). High-resolution O 1s spectra provide insights into different oxygen species in P-CeO₂. The peak at 531.0 eV belongs to adsorbed oxygen species and the peak at 529.2 eV is associated with metal-oxygen (Fig. 2d). Notably, the reversible conversion from Ce³⁺ to Ce⁴⁺ enhances the redox kinetics. The surface of P-CeO₂ is rich in defective Ce³⁺/Ce, which indicates that the high concentration of oxygen vacancy is conducive to enhancing the chemisorption and catalytic conversion of LiPSs to accelerate the redox kinetics. These aspects will be investigated in subsequent electrochemical experiments.

  Figure 2

  

  Figure 2.  (a) XRD patterns of two samples. (b) The atomic element contents of CeO₂ and P-CeO₂, (c) XPS survey spectrum of (c) Ce 3d, (d) O 1s, (e) P 2p of P-CeO₂ and (f) Ce 3d of CeO₂. (g) SEM image of P-CeO₂//PP. (h) Cross-sectional SEM image of P-CeO₂//PP. (i) Contact angle of PP and P-CeO₂//PP.

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  The difference between commercial separators (PP) and P-CeO₂ modified separators (P-CeO₂//PP) in terms of physical properties is shown in Fig. 2. It can be observed that the original PP has a uniformly interwoven fishnet-like structure (Fig. S8 in Supporting information), which has a significant effect on the Li⁺ shuttling and inhibits the electron transport. Then, the uniform distribution of P-CeO₂ nanorods on PP surface was observed through SEM image (Fig. 2g), and the diameter of P-CeO₂//PP was about 19 mm (Fig. S9a in Supporting information). In addition, the thickness of the P-CeO₂ modified layer was about 25 µm, which was confirmed by the SEM image of the membrane cross section (Fig. 2h). In addition, the mechanical toughness of the P-CeO₂//PP separator is excellent and shows no delamination of P-CeO₂ after various bending tests (Fig. S9b in Supporting information). Notably, we also performed electrolyte wettability tests by dropping electrolyte on the surfaces of both the P-CeO₂//PP separator and the pristine PP separator and comparing the diffusion area of both (Fig. 2i). The contact angle of the P-CeO₂//PP separator (7.01°) was found to be lower than that of the PP separator (33.34°), indicating that P-CeO₂ has superior electrolyte wettability compared to PP. The presented findings unequivocally establish the superior qualities and promising potential of P-CeO₂ as a modified diaphragm material for LSBs.

  In Fig. 3a, cyclic voltammetry (CV) curves of P-CeO₂//PP, CeO₂//PP, and PP batteries are presented at a uniform scan rate of 0.1 mV/s. The battery utilizing a P-CeO₂//PP displays two prominent cathodic peaks at approximately 2.3 V compared to 2.0 V and a noticeable anodic peak at approximately 2.4 V. During the process of cathodic scanning, both high and low potential regions display two reduction peaks that correspond to the conversion of S₈ to LiPSs and its therewith transformation into Li₂S/Li₂S₂, respectively, and the two reduction peaks correspond to the conversion of Li₂S₂ to LiPSs and ultimately to S₈ during the anodic scanning. Notably, the oxidation and reduction peaks of cells utilizing a P-CeO₂//PP exhibit higher negative and positive shifts, respectively. This suggests faster redox kinetics and efficient utilization of sulfur. Figs. 3b and d and Fig. S11a (Supporting information) show that the CV curves of P-CeO₂//PP, CeO₂//PP and PP batteries at different scan rates from 0.1 mV/s to 0.5 mV/s, CV curves have no obvious deformation at different scanning rates, which proves its excellent stability. To research the ion transport velocity about different separator, the peak current curves are shown in Figs. 3c and e and Fig. S11b (Supporting information), the comparison results of linear relationships between three redox current peaks for P-CeO₂, CeO₂ and PP batteries are shown in Fig. S12 (Supporting information). Significantly, P-CeO₂ demonstrated enhanced velocity in comparison to CeO₂, emphasizing the exceptional electrical conductivity inherent in the P-CeO₂ bimetallic composition as opposed to that of CeO₂. The CV curves of symmetric batteries were depicted in Fig. 3f. Concurrently, in comparison to CeO₂//PP and PP, P-CeO₂//PP demonstrated a notable peak current density. The EIS plot for symmetric batteries, utilizing CeO₂ and P-CeO₂ as identical electrodes, is presented in Fig. 3g. It is an evident that the charge transfer resistance of the P-CeO₂//PP symmetric batterie is lower than that of the CeO₂ sample, suggesting enhanced charge-transfer capability and heightened electrocatalytic characteristics. Simultaneously, experiments involving cells with Li₂S₈ electrolytes were conducted to achieve nucleation experiments, thus corroborating the catalytic effectiveness of P-CeO₂ towards LiPSs. The nucleation experiments for both P-CeO₂ and CeO₂ are graphically depicted in Figs. 3h and i. From the experimental arrangement, the P-CeO₂ material played the role of the cathode, lithium served as the anode and Li₂S₈ was chosen as electrolyte, and a polypropylene separator was integrated into the battery assembly. In the initial phase of the experiment, in order to achieve the full conversion of Li₂S₈ to Li₂S₄, all batteries were continuously discharged to 2.05 V at a current of 0.112 mA. Following this, to foster the nucleation of Li₂S, an electrostatic discharge was performed until the current dropped below 10^–5 at 2.05 V. This procedure aligns with the methodologies outlined in the previous works by Yang et al., along with the equation correlating time and peak current [49]:

  $ t_m=\left(\frac{2}{\pi A k^2}\right)^{\frac{1}{3}} $

  (1)

  Figure 3

  

  Figure 3.  (a) CV curves of P-CeO₂//PP, CeO₂//PP, and PP batteries at a scan rate of 0.1 mV/s. (b, d) CV curves of P-CeO₂//PP and CeO₂//PP batteries at different scan rates. (c, e) Comparison of linear relationships between current peak (I_p) and the square root of the scan rates for P-CeO₂//PP and CeO₂//PP batteries. (f) CV curves of symmetric batteries at 10 mV/s under the Li₂S₆ electrolyte. (g) EIS spectra of symmetric batteries. (h, i) Li₂S nucleation tests for evaluating the nucleation kinetics based on different electrodes.

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  In the above formula, the peak current time is expressed in seconds, where A denotes the nucleation rate constant (cm^–2 s^–1), and k signifies the growth rate (cm/s). Manifestly, the results show that after data fitting, P-CeO₂ manifests higher peak current, larger deposition peak area, and shorter nucleation time than CeO₂, which further proves the effectiveness of phosphorus-doped metal oxides in promoting the catalytic conversion of polysulfides.

  To comprehensively investigate the performance of P-CeO₂//PP LSBs, a lot of electrochemical tests were executed. Fig. 4a portrays that the P-CeO₂//PP battery (with a sulfur load of 1.28 mg) obtains excellent performance of long cycle at 0.5 C. Significantly, it is obvious that P-CeO₂//PP demonstrates a substantial initial discharge-specific capacity of 1180 mAh/g at 0.5 C, accompanied by an outstanding decay rate of 0.08% per cycle over 500 cycles. Conversely, cells assembled with other separators displayed lower decay rates of 0.147% and 0.154%, which highlights the exceptional cycle stability of the P-CeO₂//PP separator. Additionally, charge/discharge curves of the first cycle at 0.5 C were depicted in Fig. 4b and Fig. S8 (Supporting information), revealing that P-CeO₂ maintains a small polarization voltage when compared to CeO₂ and the PP separator. This serves as robust evidence that the effectiveness of phosphorus-doped metal oxides in promoting catalytic conversion and reaction kinetics. Furthermore, P-CeO₂//PP batteries sustained a good long cycle performance with an attenuation rate of 0.08% even at 1 C after 800 cycles, highlighting the advantages associated with ultrafast reaction kinetics (Fig. 4c). Next, in order to explore the ionic conductivity of the three batteries directly, EIS tests were performed in Fig. 4d, which indicate that the P-CeO₂//PP cell displays the lowest resistance, signifying its superior conductivity and reinforcing the earlier point. Moreover, in order to explore the cyclic stability of P-CeO₂//PP batteries under high sulfur loading, P-CeO₂//PP batteries under sulfur loads of 2.0 and 3.0 mg at 0.5 C were depicted in Fig. 4e. It is worth noting that as the sulfur load increases, the battery consistently maintains a high specific capacity, and after 300 cycles, the coulomb efficiency is still approach 100%. Fig. 4f clearly shown the superior rate capability of P-CeO₂//PP batteries, arising from the electrochemical catalysis. When the current density varies from 0.2, 0.5, 1, 2 C to 3 C, the discharge capacity shows a steady decay of 1210, 1080, 890, 788 mAh/g to 648 mAh/g, which illustrates that the P-CeO₂ modified separator expedites the reaction kinetics and impedes the shuttle effect of polysulfide efficiently. Furthermore, in comparison to previous works, the P-CeO₂//PP configuration also demonstrates exceptional performance, offering a valuable strategy for enhancing the performance of LSBs.

  Figure 4

  

  Figure 4.  (a) Cycling performances of LSBs with various separators at 0.5 C. (b) Charge-discharge profiles at 0.5 C. (c) Cycling performances of LSBs with various separators at 1 C. (d) The EIS spectra of LSBs with various separators. (e) Cycle performance with the high sulfur loading of 2.0 and 3.0 mg/cm² of P-CeO₂//PP batteries at 0.5 C. (f) Rate capacities of LSBs with various separators.

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  Adsorption experiments were conducted to investigate the adsorption mechanism (Figs. 5a and b). Based on the observed color change, successful adsorption of Li₂S₆ was achieved. Furthermore, as shown in Fig. 5c, XPS analysis was performed on P-CeO₂ powder after 24 h adsorption. Two adsorption peaks for Ce³⁺ and three adsorption peaks for Ce⁴⁺ shift from the initial 898.3, 903.5, 900.6, 906.9 and 916.4 eV to 896.5, 901.4, 898.6, 904.7 and 915.1 eV. Thus, the material exhibits both chemical and physical interactions with Li₂S₆, providing compelling evidence that P-CeO₂ can promote the chemisorption and catalytic conversion of polysulfide. Simultaneously, the visual representation in Fig. 5d illustrates the impact of different separators on polysulfide through shuttle experiments. The PP and CeO₂//PP separators exhibit a noticeable shuttle effect of polysulfide, while the P-CeO₂//PP separator shows superior performance to mitigate the effect. From the experiment, we can see that the shuttle effect of polysulfide is most significant in PP separator, followed by CeO₂//PP separator, and P-CeO₂//PP separator has the best effect to outcome the shuttle of polysulfide. This serves as direct evidence confirming the effective inhibition of polysulfide shuttling by the modified separator. Besides, the calculation results presented the adsorption energy of the (100) crystal plane for P-CeO₂ on Li₂S₆ can reach –10.44 eV (Fig. S13 in Supporting information), which is higher than that of CeO₂, presenting an outstanding adsorption effect on Li₂S₆ and suggesting that the doping of P can promote the electron transfer and can effectively slow down the shuttle effect during the charge/discharge processes. Additionally, Raman shifts were utilized to identify surface defects in P-CeO₂ and CeO₂. Notably, the peak observed at 463.7 cm^–1 represents the symmetric stretching vibration of the CeO₂ lattice, while the faint peak approximately 600 cm^–1 is due to the presence of Ce³⁺ (Fig. S14 in Supporting information).

  Figure 5

  

  Figure 5.  (a, b) Adsorption test of P-CeO₂ in Li₂S₆ electrolyte. (c) XPS spectrum of P-CeO₂ after adsorption of Li₂S₆. (d) Shuttling effect measurements of three separators at the Li₂S₆ electrolyte.

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  In summary, the phosphorus doped porous CeO₂ (P-CeO₂) were successfully synthesized by solvothermal method, calcination and phosphorization, then the commercial PP separator is modified by coating process. The P-CeO₂ modified separator surpasses the commercial PP separator due to the synergistic effects in P-CeO₂, which combines the catalytic conversion and chemical absorption for polysulfides. Turn out, when the sulfur load is 1.28 mg/cm², the battery capacity is 1180 mA/h, and the decay rate per cycle is 0.10% at 0.5 C. When the sulfur load is increased to 2.0 mg/cm², the decay rate per cycle is 0.048% at 0.5 C. These performances are superior to previously reported separators modified by cerium-based compounds [50]. Furthermore, the P-CeO₂//PP battery maintained a high capacity of 658 mAh/g under a high sulfur loading of 3.0 mg/cm² after 300 cycles. In this study, a strategy of phosphorus-doped porous CeO₂ is introduced, which shows that P-CeO₂ separator can promote the chemical absorption and catalytic conversion of polysulfides in advanced LSBs, thus achieving high performance and long cycle life.

  Declaration of competing interests

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Xinyun Liu: Writing – original draft, Investigation, Conceptualization. Long Yuan: Data curation. Xiaoli Peng: Formal analysis. Shilan Li: Methodology. Shengdong Jing: Software. Shengjun Lu: Conceptualization. Hua Lei: Writing – original draft, Supervision. Yufei Zhang: Writing – review & editing, Supervision. Haosen Fan: Writing – review & editing, Supervision.

  Acknowledgement

  This work was supported by National Natural Science Foundation of China (Nos. 52472194, 52101243), Natural Science Foundation of Guangdong Province, China (No. 2023A1515012619) and the Science and Technology Planning Project of Guangzhou (No. 202201010565). We would like to thank Analysis and Test Center of Guangzhou University for their technical support.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110369.

  
  
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  Figure 1  (a, b) SEM images of P-CeO₂. (c) HAADF-STEM image of P-CeO₂. (d) SAED pattern. (e, f) High-resolution TEM images and the corresponding EDX elemental mapping of (g) Ce, (h) O and (i) P.

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  Figure 2  (a) XRD patterns of two samples. (b) The atomic element contents of CeO₂ and P-CeO₂, (c) XPS survey spectrum of (c) Ce 3d, (d) O 1s, (e) P 2p of P-CeO₂ and (f) Ce 3d of CeO₂. (g) SEM image of P-CeO₂//PP. (h) Cross-sectional SEM image of P-CeO₂//PP. (i) Contact angle of PP and P-CeO₂//PP.

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  Figure 3  (a) CV curves of P-CeO₂//PP, CeO₂//PP, and PP batteries at a scan rate of 0.1 mV/s. (b, d) CV curves of P-CeO₂//PP and CeO₂//PP batteries at different scan rates. (c, e) Comparison of linear relationships between current peak (I_p) and the square root of the scan rates for P-CeO₂//PP and CeO₂//PP batteries. (f) CV curves of symmetric batteries at 10 mV/s under the Li₂S₆ electrolyte. (g) EIS spectra of symmetric batteries. (h, i) Li₂S nucleation tests for evaluating the nucleation kinetics based on different electrodes.

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  Figure 4  (a) Cycling performances of LSBs with various separators at 0.5 C. (b) Charge-discharge profiles at 0.5 C. (c) Cycling performances of LSBs with various separators at 1 C. (d) The EIS spectra of LSBs with various separators. (e) Cycle performance with the high sulfur loading of 2.0 and 3.0 mg/cm² of P-CeO₂//PP batteries at 0.5 C. (f) Rate capacities of LSBs with various separators.

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  Figure 5  (a, b) Adsorption test of P-CeO₂ in Li₂S₆ electrolyte. (c) XPS spectrum of P-CeO₂ after adsorption of Li₂S₆. (d) Shuttling effect measurements of three separators at the Li₂S₆ electrolyte.

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