Insights into the application of cerium dioxide nanoparticle-modified cobalt phosphide as an efficient electrocatalyst for high-performance lithium–sulfur batteries

Abstract

Developing high-efficiency catalysts is an effective strategy to boost the hysteretic polysulfide conversion behavior of lithium–sulfur (Li–S) batteries. Cobalt phosphide (CoP) is a typical promising catalyst due to its inherent advantages such as good electron conductivity, facile synthesis route and moderate catalytic capability and binding energy to polysulfide. However, the design and fabrication of highly active CoP remains a challenge. Herein, aiming to optimize the catalytic activity of CoP, the regulation of the electronic structure of CoP by cerium dioxide (CeO₂) was explored, and the sulfur conversion capability and the electrochemical performance in Li–S batteries using the obtained CoP/CeO₂ nanocomposites were demonstrated. Microstructure analysis demonstrates that CeO₂ nanoparticles can embed in CoP and increase its exposed active sites, and the introduced CeO₂ can adjust the electronic structure and optimize the charge transfer and polysulfide conversion behavior of CoP. Although DFT indicates a moderate adsorption energy of CoP/CeO₂ towards Li₂S₆, practical catalytic activity depends strictly on the amount of CeO₂, and the optimum amount is ∼10 mol%. A Li–S battery with a CoP/CeO₂-10-modified separator exhibits a high specific capacity of 1400 mA h g⁻¹ at 0.1C, an excellent rate performance of 722 mA h g⁻¹ at 3C and a long-term cycling durability of 535 mA h g⁻¹ at 1C after 1000 cycles. This work expands the range of CoP-based catalysts based on CeO₂ in Li–S batteries.

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Introduction

In order to drive electric vehicles to a long distance, the development of high-energy-density electrochemical storage devices has become an imperative and essential approach.^1,2 Currently, over half of commercial energy storage systems employ Li-ion batteries; nevertheless, the practical specific capacity and energy density based on common anodes (such as graphite) and cathodes (such as NMC) almost reach the theoretical limit. In particular, energy density is still far from the targeted value (500 W h kg⁻¹, proposed by a battery consortium).³ In sharp contrast, lithium–sulfur (Li–S) batteries, based on a novel sulfur conversion reaction, have gained ever-increasing interest due to their potential high energy density of 2600 W h kg⁻¹.^4,5 Meanwhile, the practical energy density of the constructed pouch-type Li–S battery has exceeded that of existing commercial Li-ion batteries.⁶ Moreover, sulfur, a cathode active material, has a low cost compared with commercial Li-ion cathodes, and the above advantages make Li–S batteries a potential candidate for the next-generation battery system.

Although significant efforts have been made, the development of Li–S batteries is still in its early stage, and their practical application will face several scientific and technical bottlenecks: the insulating nature of sulfur, the dissolution and shuttling of lithium polysulfide (LiPS) intermediates, and the corrosion of Li metal.^7,8 These issues will inevitably cause sluggish redox kinetics, limit sulfur utilization, cause fast capacity fading, and finally hinder the application of Li–S batteries. Researchers have witnessed the promising prosperity of Li–S batteries by exploring various kinds of catalysts (including heterogeneous, homogeneous, and semi-immobilized materials) to improve the electrochemical conversion reaction kinetics of LiPSs;⁹ for example, transition-metal-containing oxides,^10,11 nitrides,¹² sulfides^13,14 and phosphides^15,16 have been theoretically explored; developing polar catalysts can effectively capture polar polysulfides and inhibit the shuttle effect; and the synergistic effect of catalysis and adsorption to polysulfides can at least ensure high kinetic behavior.

It is worth mentioning that too strong adsorption interaction of catalysts to polysulfide may lead to the irreversibility of polysulfide conversion, while too weak interaction may result in insufficient binding force to the suppression of shuttle effect.¹⁷ For example, the binding energy between polysulfide and Co₃O₄ was 5 eV, which changed to 4.13 eV for CoP, and the battery using Co₃O₄ exhibited a lower rate performance than that of CoP.¹⁸ Although some researchers applied CoS₂ as a catalyst, its binding energy to polysulfide was only 1.84 eV, hardly to completely prevent the loss of sulfur.¹⁹ On the premise of ensuring moderate binding energy to polysulfide, promoting the following conversion reaction of polysulfides is the focus. In this regard, the essential strategy is to regulate the binding energy of the catalyst towards catalytic intermediates; thus, electronic structure, an important factor in the decision of the concentration of active sites, attracts much attention and this usually has a close relationship with the Fermi level.^20,21 For CoP, the unoccupied d orbitals of Co and p originate from P usually labeled as active sites,¹⁴ and the number of the valence electrons in their orbitals is a key factor in bonding and breaking bonds with other ions and can influence the catalyst effect toward polysulfides. Till date, although many efforts have been made to fabricate novel CoP-based catalysts in Li–S batteries,^22–24 a single phase is hardly to regulate the valence electron and limits the number of active sites; therefore, the requirement for practical electric vehicles is still hard to achieve by applying single CoP due to its restricted electrocatalytic activity.

The construction of heterostructures provides a breakthrough point to enhance the catalytic activity, and the stitching of various phases can lead to the formation of heterostructured catalysts with rich heterointerfaces or heterojunctions, leading to the re-modulation of the electronic structure, and can finally regulate the catalytic activity, and thus, a serious of catalysts including CoP–Co₃S₄,²⁵ CoP–Co₂N,²⁶ Co₉S₈–MoS₂,²⁷ Co–CoOOH,²⁸ MoSe₂–MXene²⁹ and CoP–CeO₂³⁰ were designed, which exhibited excellent electrocatalytic activity. Take cerium dioxide (CeO₂), for example, in which Ce has a flexible valence shift between Ce³⁺ and Ce⁴⁺, and the valence-change via redox reaction provides an opportunity for the redistribution of electrons.³¹ Up to now, numerous transition metal phosphides have been coupled with CeO₂ to adjust the active sites and electronic structure, and exhibited excellent performance in various kinds of aspects such as water splitting,³² Zn–air batteries,³³ and supercapacitors.³⁴ For example, Wang et al. and co-workers applied CeO₂ to decorate CoP and successfully modulated the d-band center and achieved promising overall water splitting behavior.³⁵ In the field of Li–S batteries, the electronic structure of CoP can be adjusted and rearranged by introducing CeO₂, strong interactions at the interface of CoP and CeO₂ will optimize the electronic environment around the active sites, which can theoretically reduce the LiPS conversion reaction energy barrier and thus enhance the conversion behavior of LiPSs. Besides, polar CeO₂ exhibits strong capture ability to polysulfides, and even some researchers point out that CeO₂ can catalyze the following conversion reaction of LiPSs.³⁶ However, a deep understanding of the enhanced effect of CeO₂-decorated CoP, including the shuttle effect and the corresponding electrochemical performance in Li–S battery, is still not clear.

Currently, there are two strategies for the application of highly efficient catalysts to Li–S batteries: by designing sulfur hosts or functional separators, theoretically, sulfur will experience dissolution and diffusion stages and the dissolved LiPSs can easily be captured by functional separator; meanwhile, the fabrication of functional separators is simple and can easily be scaled up and applied to various sulfur cathodes. In this work, we designed and fabricated a CoP/CeO₂ heterostructure via a facile hydrothermal strategy, considering the inevitable dissolution of the polysulfides, and the fabricated CoP/CeO₂ was coated onto a PP separator to obtain a modified separator to capture and catalyze the polysulfides. The microstructure characterization reveals that CeO₂ can be embedded in CoP and increase the interface interaction. Meanwhile, the electronic structure can be regulated. Benefitting from sufficient interfaces and synergistic electron interaction between CoP and CeO₂, the Li–S battery with a CoP/CeO₂ heterostructure-modified separator exhibits dramatically enhanced sulfur redox kinetic behaviors compared with CoP. Meanwhile, the cycling stability can also be increased due to the enhanced adsorption capability to polysulfides, which can significantly inhibit the shuttling of polysulfides.

Experimental section

Chemicals

Cobaltous nitrate hexahydrate (Co(NO₃)₂·6H₂O), cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O), sodium hypophosphite (NaH₂PO₂) and hexamethylenetetramine (HMT) were obtained from Aladdin. All compounds were used without further purification.

Synthesis of CoP/CeO₂-x

The CoP/CeO₂ heterostructures were fabricated via a simple hydrothermal process followed by low-temperature phosphorization. In brief, 4 mmol mixture of metal salts (Ce(NO₃)₃·6H₂O and Co(NO₃)₂·6H₂O) and 12 mmol hexamethylenetetramine (HMT) were dissolved in 60 mL deionized water, and the molar ratios of Ce³⁺/Co²⁺ were adjusted to obtain optimized composites. The obtained clarified solution was then transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 100 °C for 8 h. The product was then washed three times with water and ethanol, and dried at 60 °C under vacuum overnight to obtain Co/Ce precursor. Then, 100 mg of the as-obtained precursor and 1 g NaH₂PO₂ were placed in two quartz vessels separately and NaH₂PO₂ was specifically placed upstream of the N₂ stream. After experiencing a heating process at 350 °C for 2 h, the targeted sample (CoP/CeO₂) was fabricated. To confirm the optimal amount of Ce(NO₃)₃·6H₂O, 0, 0.2, 0.4, and 0.6 mmol Ce(NO₃)₃·6H₂O were used and the corresponding Co(NO₃)₂·6H₂O were 4, 3.8, 3.6 and 3.4 mmol. The fabricated samples were labeled as CoP/CeO₂-0, CoP/CeO₂-5, CoP/CeO₂-10, and CoP/CeO₂-15, respectively.

Synthesis of CoP/CeO₂-x modified separators

First, 1 wt% Super P, 1 wt% poly(vinylidene fluoride) (PVDF) and 8 wt% CoP/CeO₂-x were thoroughly mixed in N-methyl-2-pyrrolidinone (NMP), and the obtained slurry was then coated onto a PP separator (Celgard 2500) dried at 60 °C for 12 h under vacuum to obtain the CoP/CeO₂-x modified separators. The areal S loading was ∼0.6 mg cm⁻² and the diameter of the modified separator was 18 mm.

Material characterization

The crystallographic structures of the samples as produced were characterized using a powder XRD (Smart Lab 9 kW, Japan,) over a range of 10°–80°. The microstructure of the products was determined using a SEM (Regulus 8100) and a TEM (FEI Tecnai G2 F20 S-TWIN). The compositions of the elements and their valence states were determined using an XPS (Thermofisher Nexsa). The specific surface area of the samples was analyzed using a BET (Micromeritics ASAP 2460) model from the N₂ adsorption isotherm obtained at a temperature of 77 K. These methods were employed in conducting a thorough investigation of the prepared samples.

Electrochemical measurements

The CNT-S composite was constructed using a mass ratio of 7/3 of sublimed sulfur to carbon nanotubes (CNTs), which were heated at 155 °C for 12 h in an Ar atmosphere. Subsequently, a mixture of the obtained CNT-S composite, Super P and PVDF binder was prepared in NMP with a mass ratio of 8/1/1. This mixture was stirred for 4 h and then coated onto a carbon-coated Al foil, followed by vacuum-drying at 60 °C. The areal S loading was ∼2.0 mg cm⁻² and the diameter of cathode was 12 mm. In addition, a high S loading of ∼4.0 mg cm⁻² was prepared, and the difference was that the mass ratio of S to CNT was 75/25.

The battery was evaluated using a CR2032 coin cell system with a piece of sulfur cathode, a piece of lithium anode, and a piece of CoP/CeO₂-x-modified separator. The electrolyte was 1.0 M LiTFSI with 1 wt% LiNO₃ in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (v/v = 1 : 1). The electrolyte/sulfur (E/S) ratio of the battery using a low S loading cathode (2.0 mg cm⁻²) was 12 μL mg⁻¹, and the E/S ratio of the battery using a high sulfur loading cathode (4 mg cm⁻²) was 6 μL mg⁻¹. The electrochemical performance was evaluated within a voltage range of 2.8–1.7 V at different current densities (1C = 1675 mA h g⁻¹). Cyclic voltammetry (CV) and electrochemical impedance spectrometry (EIS) measurements were performed using a CHI660E electrochemical workstation.

Adsorption and catalytic conversion tests of Li₂S₆

Li₂S₆ solution was prepared by dissolving Li₂S and sulfur powder in DME/DOL (v/v = 1 : 1) using a molar ratio of 1/5 in an Ar-filled glove box. To assess the chemisorption capability, 20 mg CoP/CeO₂-x powders were separately put in 10 mL Li₂S₆ solution (5 mM) to observe the adsorption behavior.

To assemble a symmetric cell, 8 wt% CoP/CeO₂-x, 1 wt% Super P and 8 wt% PVDF were dissolved into an NMP solution to form a homogeneous slurry, which was then coated onto Al foil and dried at 60 °C for 12 h. The fabricated electrode could be used as the cathode and anode, the areal mass loading of CoP/CeO₂-x was ∼1.0 mg cm⁻² and the diameter of cathode was 12 mm. To evaluate the catalytic conversion performance of CoP/CeO₂-x to LiPSs, a symmetrical CR2032 coin cell was assembled using a piece of Celgard 2500 and 20 μL Li₂S₆ as the electrolyte, and CV tests were conducted at a sweep rate of 10 mV s⁻¹ from −1 to 1 V.

Nucleation and dissolution of Li₂S

A typical nucleation and dissolution test was also conducted using a CR2032 coin cell with a piece of PP separator, a piece of Li anode and a piece of CoP/CeO₂-x cathode. The electrolyte was a Li₂S₈ solution (0.25 mol L⁻¹), prepared by dissolving Li₂S and sulfur powder in DME/DOL (v/v = 1 : 1) using a molar ratio of 1/7; note that 10 μL Li₂S₈ was added to the cathode side, while 10 μL pristine DME/DOL (v/v = 1 : 1) was added to the anode side. The assembled cell was discharged to 2.06 V at 0.112 mA to consume most of the high-order polysulfides, and then the cell was discharged at 2.05 V to induce nucleation and growth of Li₂S until the current decreased to 10⁻⁵ A. For the dissolution of Li₂S, the cell was first discharged to 1.7 V at 0.112 mA to generate Li₂S, which was further dissolved into LiPSs by potentiostatically charging the cell at 2.4 V until the current decreased below 10⁻⁵ A.

Computational details

The density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP), the exchange–correlation interactions were described by using the van der Waals force corrected Perdew–Burke–Ernzerhof (PBE) functional DFT-D3 method, and the Generalized Gradient Approximation (GGA) was applied to implement DFT.³⁷ The CoP (011) and CeO₂ (111) models were employed to simulate the interaction surface with Li₂S₆. The atom position was optimized by using the conjugated gradient optimization method, and the geometric convergence tolerance at each atom was set to below 0.03 eV Å⁻¹, and the energy was converged within 10⁻⁵ eV. A vacuum layers were wet at least 15 Å along the z-direction to avoid interaction between planes. A cutoff energy of 400 eV was used. The binding strength of Li₂S₆ on the slab surface was calculated using the following expression and labeled as adsorption energy (ΔE_ads):³⁸

ΔE_ads = ΔE_ads/base − ΔE_ads − ΔE_base

where, ΔE_ads/base, ΔE_ads and ΔE_base are the total energy of the adsorbed systems, the Li₂S₆ and substrate, respectively.

Results and discussion

The CoP/CeO₂ heterostructure's fabrication process is schematically illustrated in Fig. 1a; according to a previous report,³⁴ the Co(OH)₂/CeO₂ precursor can be first produced after a facile hydrothermal reaction for 8 hours at 100 °C, and the alkaline environment can satisfy the demand of the growth of Co/Ce precursor, which can be adjusted via HMT. In practical applications, temperature conduction in the hydrothermal process itself is more uniform than that of other methods such as solid-phase calcination, which can ensure the reproducibility of the fabrication of samples. Meanwhile, hydrothermal method can also ensure the consistency of the preparation process even if the volume of the container is increased multiple times, facilitating a large-scale production. As the morphology of the heterostructure is a vital parameter for the creation and exposure of active sites, we investigated the morphology. Under the condition without Ce(NO₃)₃·6H₂O, the precursor is composed of numerous nanosheets, which exhibit an average length of 2 μm, and these nanosheets are tightly interlaced and form clusters (Fig. S1a†). The size of the nanosheets decreases with the increase in the amount of Ce(NO₃)₃·6H₂O, the average length decreases to ∼0.5 μm and even some nanoparticles appear after adding 5 mmol Ce(NO₃)₃·6H₂O (Fig. S1b†). No significant nanosheets were found in the residue with the further increase in the amount of Ce(NO₃)₃·6H₂O to 10 and 15 mmol (Fig. S1c and d†). It should be noted that other fabrication parameters were not changed, indicating that the addition of Ce(NO₃)₃·6H₂O can regulate the morphology of Co(OH)₃/CeO₂ precursors. After experiencing a phosphorization treatment process, the morphology maintained its original shape (Fig. 1b–e), without any collapse or aggregation that usually confronted in most phosphating processes, indicating excellent microstructure stability of the precursor.³⁹

Fig. 1 (a) Schematic of the preparation of the CoP/CeO₂ material. SEM images of (b) CoP/CeO₂-0, (c) CoP/CeO₂-5, (d) CoP/CeO₂-10, and (e) CoP/CeO₂-15. (f) TEM image of CoP/CeO₂-10. (g) HRTEM image of CoP/CeO₂-10. (h) and (i) IFFT images and the corresponding line scan of the areas marked by squares in (g): (h and j) Region 1 and (i and k) Region 2. (l) Elemental mapping images of CoP/CeO₂-10.

The detailed morphology of CoP/CeO₂-0 is further explored using TEM (Fig. S2a†), and a relatively tight nanosheet can be observed, which is in sharp contrast to CoP/CeO₂-5 (Fig. S2b†), where abundant microspores highlighted in yellow labels can be observed among the nanosheet, and this is also true for CoP/CeO₂-10. As shown in Fig. 1f, numerous microspores accompany with many nanoparticles appear, and these characteristics are significantly different from that of CoP/CeO₂-0. We then conducted N₂ adsorption–desorption experiments to study the specific surface area and pore characteristic of various samples (Fig. S3†), and as expected, the specific surface area increases with the increase in the amount of CeO₂, and the corresponding values are 9.8, 12.3, 12.7 and 16.4 m² g⁻¹ for CoP, CoP/CeO₂-5, CoP/CeO₂-10, and CoP/CeO₂-15, respectively. Meanwhile, the pore-size characteristic appears with the introduction of CeO₂ into CoP, and the pore sizes of the samples mainly focus in the range of 30–35 nm. Theoretically, the high surface area and microporous structure facilitate the adsorption of LiPSs on the sample. The high-resolution TEM (HRTEM) image of the edge of CoP/CeO₂-10 shows that two types of lattice fringes appear (Fig. 1g); from the two-dimensional inverse fast Fourier transform (IFFT) pattern in Fig. 1h and 1i, it is clear that an interplanar distance of 0.283 nm corresponds to the (011) plane of the orthorhombic CoP, while an interplanar distance of 0.312 nm corresponds to the (111) plane of cubic CeO₂.³⁴ Furthermore, additional elemental mapping results of CoP/CeO₂-10 suggest that Co, P, Ce, and O elements can be uniformly distributed on the sample (Fig. 1l), revealing the homogeneous embedding of CeO₂.

The crystalline phase of the fabricated sample was then investigated by XRD, as shown in Fig. 2a; some characteristic peaks of CoP/CeO₂-0 appear at 31.6°, 36.3°, 46.2°, 48.2° and 56.7°, corresponding to the (011), (111), (112), (211) and (301) crystal planes of CoP (JCPDS no. 29-0497), respectively. After the addition of Ce(NO₃)₃·6H₂O, some new typical peaks at 28.5°, 33.1°, 47.5° and 56.3° appear and the corresponding peak intensity becomes stronger with the increase in the amount of Ce(NO₃)₃·6H₂O; these peaks can be assigned to the (111), (200), (220), and (311) planes of CeO₂ (JCPDS no. 34-0394), respectively.³⁴ Meanwhile, the peak intensity of CoP gradually becomes weak with the increase in the content of CeO₂. Combined with the TEM results, the above-mentioned results demonstrate the successful synthesis of CoP/CeO₂ heterostructures.

Fig. 2 (a) XRD patterns of CoP/CeO₂-0, CoP/CeO₂-5, CoP/CeO₂-10 and CoP/CeO₂-15. High-resolution XPS spectra of (b) Co 2p and (c) P 2p of CoP/CeO₂-10 and CoP. High-resolution XPS spectra of (d) Ce 3d, (e) Co 2p, (f) P 2p and (g) S 2p of CoP/CeO₂-10 before and after adsorbing the Li₂S₆ solution. (h) CV curves of symmetric cells at a scan rate of 10 mV s⁻¹. (i) Digital image of Li₂S₆ solutions before and after adding CoP/CeO₂-0, CoP/CeO₂-5, CoP/CeO₂-10 and CoP/CeO₂-15.

The surface composition and chemical state of a catalyst have a close relationship with the catalytic activity, because some critical parameters such as electronic structure and oxygen vacancy will affect the adsorption/redox processes of an intermediate reaction product.⁴⁰ Therefore, XPS is applied to synthesize the surface characteristic and the electronic interaction of the samples, involving the effect of CeO₂ decorating on the elements of Co 2p and P 2p and the effect of Li₂S₆ adsorption via CoP and CoP/CeO₂ on the elements of Ce 3d, Co 2p, P 2p and S 2p. From the fitted high-resolution XPS spectrum of Co 2p shown in Fig. 2b, it can be easily found that two pairs of peaks that belong to Co 2p_3/2 (778.61 and 782.29 eV) and Co 2p_1/2 (793.58 and 798.26 eV) appear, along with two shakeup satellites (corresponding to 786.50 and 802.78 eV, respectively). The peaks at 782.29 and 798.26 eV can be assigned to the Co species for Co–O, and the peaks at 778.61 and 793.58 eV correspond to the Co–P compound.⁴¹ After the introduction of CeO₂, the shape of the fitted XPS spectrum of CoP/CeO₂ exhibits significant differences from that of CoP, and the peak intensity around 778.61 and 793.58 eV becomes stronger than that around 782.29 and 798.26 eV. Meanwhile, the peak of Co 2p is notably shifted to a lower value than that of CoP, the fitted values of Co 2p_3/2 located at 778.40 and 781.98 eV, and the Co 2p_1/2 located at 793.39 and 798.12 eV, along with the shakeup satellites located at 785.51 and 802.78 eV, respectively.⁴² This phenomenon indicates the charge redistribution and partial electron loss of Co, referring to a previous report,³⁵ and we believe the electronic concentration in the Fermi level of adsorbed LiPSs can be increased, which facilitates boosting the interfacial electronic transfer dynamics and promote the electrocatalytic reaction of LiPSs. Meanwhile, the electron transfer phenomenon (Co to CeO₂) is also beneficial for the conductivity of CoP.⁴³ The P 2p XPS spectra of both CoP and CoP/CeO₂ are presented in Fig. 2c, and the broad peak around 134.13 eV can be assigned to oxidized P species,³⁷ as expected. Although both types of samples exhibit a similar shape around 129.24 eV, the peak intensity at 130.08 and 129.24 eV that correspond to P 2p_1/2 and P 2p_3/2 is significantly different, and these peaks indicate the existence of Co–P, further confirming the fabrication of CoP and a strong interface interaction between CoP and CeO₂.

After experiencing the Li₂S₆ adsorption, the high-resolution XPS of Co 2p exhibits a similar shape compared with that of pristine CoP/CeO₂ except for a slight shift to a high binding energy of 0.4–0.6 eV, as shown in Fig. 2d, indicating that electrons can transfer from Li₂S₆ to Co, and this situation facilitates the decomposition of Li₂S₆. A similar condition can also be found from the high-resolution XPS of Ce 3d, as shown in Fig. 2e, where the peaks around 886.1 and 905.2 eV attribute to Ce 2d_5/2 and Ce 2d_3/2, respectively, and both can be fitted into three parts and belong to Ce³⁺, Ce³⁺ and Ce⁴⁺, respectively.⁴⁴ These peaks exhibit a shift of 0.7–1.0 eV to a high binding energy due to the adsorption of Li₂S₆, suggesting that particle Ce³⁺ can be oxidized into Ce⁴⁺ and this can realize an electron transfer from Ce to Li₂S₆. For the P 2p XPS spectra of both CoP/CeO₂ and CoP/CeO₂–Li₂S₆, no significant shift can be detected and the shape of the fitted curves is nearly maintained, which may indicate that P does not play a critical role in the catalytic activity of CoP/CeO₂ (Fig. 2f). Although the S 2p spectrum can be found in pristine CoP/CeO₂, two peaks appear after the adsorption of Li₂S₆, which can be assigned to S 2p_3/2 and S 2p_1/2 (Fig. 2g), indirectly reflecting the existence of certain interaction between CoP/CeO₂ and Li₂S₆.²⁰ Therefore, the above-mentioned XPS results indicate a strong interaction between CoP/CeO₂ and Li₂S₆, which facilitates the electron transfer from CoP/CeO₂ to Li₂S₆ and can accelerate the electrochemical conversion reaction between Li₂S₆ and Li₂S/Li₂S₂.⁴⁵

In order to verify the electrocatalytic activity of the samples in Li–S batteries, symmetric cells are assembled with identical CoP/CeO₂-x cathodes and anodes, as shown in Fig. 2h, and no significant CV peak can be observed in a Li₂S₆-free electrolyte. After adding Li₂S₆ to the electrolyte, the CV curve of the CoP/CeO₂-x electrode shows one pair of sharp redox peaks, and the peak intensity becomes stronger with the increase in the amount of CeO₂, indicating that the introduction of CeO₂ can significantly enhance the current response and obtain a large peak area at 10 mV s⁻¹. Generally speaking, the area enclosed by CV is proportional to the conversion ratio of polysulfides, which can also be used to analyze the catalytic activity, especially to study and compare the performance at the same CV scan rate. Apparently, these results point out the strong catalytic enhancement behavior of CeO₂ to CoP, and this can boost the conversion kinetic of LiPSs in Li–S batteries. Furthermore, Fig. 2i shows the optimal configuration with the corresponding energetic information of Li₂S₆ on CoP/CeO₂, and ΔE_ads was calculated to be −2.29 eV, indicating that a strong interaction exists between LiPSs and CoP/CeO₂. Meanwhile, we also noted that the adsorption behavior is greatly affected by the content of CeO₂. From the digital image of Li₂S₆ solutions before and after adding CoP/CeO₂-x in Fig. S4,† it can be found that the initial Li₂S₆ solution exhibits a brown color, and the addition of 20 mg CoP/CeO₂-x (x = 5, 10, 15) will lighten the color, especially CoP/CeO₂-10, after equal mass of CoP/CeO₂-x for 12 h, the color using CoP/CeO₂-10 is almost transparent, while the color containing other amount of CeO₂ exhibits certain degree of yellow, demonstrating the importance of CeO₂ in anchoring polysulfides.

To investigate the interfacial charge transfer dynamics of the CoP/CeO₂-x modified separator in the Li–S battery, EIS was used and the results are shown in Fig. 3a. All Nyquist diagrams contain a semicircle in the high-frequency region, which corresponds to charge transfer resistance at the interface of the electrolyte with the cathode, and a diagonal line appears in the low-frequency region corresponding to the diffusion behavior of ions.⁴⁶ It is inspiring to find that the diameter of the semicircle exhibits the following order: CoP/CeO₂-0 > CoP/CeO₂-15 > CoP/CeO₂-5 > CoP/CeO₂-10, and the smallest value of the semicircle implies that the battery with the CoP/CeO₂-10-modified separator can achieve the fastest reaction dynamics. CV was performed to investigate the electrochemical property of Li–S batteries, in the voltage range from 1.7 to 2.8 V at 0.1 mV s⁻¹, as shown in Fig. 3b, in which two pairs of redox peaks appear for all kinds of batteries, which can be assigned to the multi-step reaction mechanism of LiPSs. The reduction peak around 2.31 V is ascribed to the reduction of solid S₈ into soluble long-chain Li₂S_x (x = 4, 6, and 8), and another peak around 2.02 V is assigned to the conversion of Li₂S_x into solid short-chain sulfides (Li₂S₂/Li₂S).⁴⁷ Two anodic peaks appear around 2.32 and 2.4 V, which are ascribed to the reverse conversion of Li₂S into Li₂S_x and S₈, respectively. The difference mainly focuses on the values of peak voltage and current. The battery using a CoP/CeO₂-10-modified separator shows the highest current among all the batteries. Meanwhile, the peak current of the battery with the addition of CeO₂ is always higher than that of pure CoP, and the corresponding voltage polarization (the difference between the oxidation peak voltage and the reduction peak voltage) of CeO₂-added is always smaller than that of pure CoP. These results suggest that the introduction of CeO₂ can greatly enhance the electrochemical kinetic of the Li–S battery, while this has a close relationship with the amount of CeO₂, and here, the optimum value is CoP/CeO₂-10.

Fig. 3 (a) EIS curves and (b) CV curves of the Li–S battery with various kinds of separators. CV curves of Li–S batteries with a CoP/CeO₂-10-modified separator (c) at 0.1 mV s⁻¹ and (d) at different scan rates. (e) The corresponding linear fitting of the peak currents with the square root of the scan rate of Li–S batteries with CoP/CeO₂-10-modified separators. (f) Corresponding Li-ion diffusion coefficients calculated from the CV redox peaks. (g and h) Li₂S nucleation and dissolution curves of the battery using CoP/CeO₂-10. (i) Values of Li₂S deposition capacity and Li₂S oxidation capacity.

We then investigated the first three CV curves of the Li–S battery with four kinds of modified separators (Fig. 3c and Fig. S5a–c†), and it can be found that the battery with CoP/CeO₂-10 still presents the highest current density and all the curves nearly perfect overlap, suggesting excellent electrochemical reversibility of CoP/CeO₂-10 in accelerating the redox reaction of LiPSs. The diffusion coefficient of Li⁺ is another major parameter to evaluate whether the separator is suitable to accelerate the sulfur dynamics of L–S batteries. The CV tests of the battery using various kinds of separators were carried out at different scanning rates (0.1–0.5 mV s⁻¹) within the voltage range of 1.7–2.8 V, from the CV curves of the battery with CoP/CeO₂-10, as shown in Fig. 3d, from which it can be found that all CV curves show two reduction peaks at 2.3 and 1.9 V during the cathodic scan, corresponding to two oxidation peaks at 2.3 and 2.4 V during the anodic scan. The peak current of the CV curve increases with the increase in the scan rates, the nearly unchanged curve shape indicates excellent electrochemical stability. The value of the peak current has a linear relation with the square root of the scanning rates, according to the Randles–Sevcik equation,⁴⁸ and the diffusion coefficient can be obtained according to the following formula:

I_p = (2.69 × 10⁵)n^1.5AD_Li^0.5ν_Li^0.5C_Li

where I_p is the peak current; n is 2; A is the active area of the cathode; C_Li presents the Li⁺ concentration in the electrolyte; ν presents the scan rate of the CV curve. The value of D_Li of the battery using CoP/CeO₂-10 is in the range of 0.56 × 10⁻⁵ to 2.5 × 10⁻⁵ cm² s⁻¹, and these values are always higher than that of other kinds of samples (Fig. S6a–f†). The above-mentioned results confirm the best conversion kinetic of LiPSs via the assistance of CoP/CeO₂-10, which can suppress the shuttle effect of LiPSs and improve the utilization ratio of S.

The conversion process of polysulfides can be further evaluated by monitoring the nucleation and decomposition processes of Li₂S on CoP/CeO₂-x. The nucleation of Li₂S can be studied via a discharge curve with Li₂S₈ as the active material; during this process, the battery is set to galvanostatically discharge to 2.06 V and then potentiostatically discharge at 2.05 V, which can realize the conversion of long-chain LiPSs to Li₂S.⁴⁹ Generally, the integral parameters including current and time and the measured curve can be fitted mathematically to get the integral area, and the precipitation capacity of Li₂S can be calculated according to Faraday's law. The nucleation curve of Li₂S on CoP/CeO₂-10 is shown in Fig. 3g, and for comparison, the nucleation of other kinds of CoP/CeO₂-x is shown in Fig. S7a–c.† The nucleation capacities of Li₂S on CoP/CeO₂-0, CoP/CeO₂-5, CoP/CeO₂-10 and CoP/CeO₂-15 are 157.18, 303.13, 418.32 and 397.41 mA h g⁻¹, respectively; the high nucleation capacity suggests that CoP/CeO₂-10 can capture maximum LiPSs and achieve an effective deposition of Li₂S and high discharge capacity. The dissolution process of Li₂S on CoP/CeO₂-10 was further evaluated by a potentiostatic charging test, as shown in Fig. 3h, and the dissolution capacity contribution can be calculated to be 768.24 mA h g⁻¹, which is still higher than that of CoP/CeO₂-0 (343.81 mA h g⁻¹), CoP/CeO₂-5 (689.81 mA h g⁻¹) and CoP/CeO₂-15 (720.58 mA h g⁻¹), as shown in Fig. S7d–f.† From the intuitive diagram in Fig. 3i, it is easy to find that the nucleation and dissolution capacities of Li₂S exhibit the following order: CoP/CeO₂-10 > CoP/CeO₂-15 > CoP/CeO₂-5 > CoP; clearly, this result further indicates the importance of CeO₂ and the key role of the amount of CeO₂ in CoP/CeO₂-x in regulating the catalytic activity of the composite.

We then systematically studies the electrochemical performance of CoP/CeO₂-x in a practical Li–S battery. From the rate capabilities shown in Fig. 4(a), it can be found that although the capacity of all the batteries decreases with the increase in current density, the battery with a CoP/CeO₂-10-modified separator shows the smallest decrease in capacity; its specific capacity can achieve 1400, 1114, 992, 910, 806 and 722 mA h g⁻¹ at 0.1, 0.2, 0.5, 1, 2 and 3C, respectively, which are higher than those of CoP/CeO₂-5 (1300, 1028, 884, 786, 702 and 641 mA h g⁻¹), CoP/CeO₂-15 (1257, 948, 827, 747, 663 and 605 mA h g⁻¹) and CoP/CeO₂-0 (1211, 898, 771, 688, 573 and 131 mA h g⁻¹). Meanwhile, the capacities can still reach 867 and 1077 mA h g⁻¹ when the current density returns to 1 and 0.1C, respectively, verifying that CoP/CeO₂-10 can maintain superior rate property. This excellent rate performance of the CoP/CeO₂-10-modified separator can also be verified from the charge/discharge curves, as shown in Fig. 4b, and all the curves have two stable and typical voltage plateaus; however, the curves of other types of batteries nearly deform, especially at 3C (Fig. S8a–c†). We then compared the first galvanostatic charge/discharge curves at 0.2C, as shown in Fig. 4c, and the battery with a CoP/CeO₂-10-modified separator exhibits the smallest polarization overpotential, and the value is about 140 mV, while the values of other types of batteries are as follows: CoP/CeO₂-0 (270 mV), CoP/CeO₂-5 (200 mV) and CoP/CeO₂-15 (240 mV). The lowest polarization overpotential suggests that the battery using CoP/CeO₂-10 can ensure the highest energy efficiency. Similar to the CV curve, the discharge process can also be divided into two parts: the first high voltage plateau can be assigned to the conversion of S₈ to LiPSs, which can be denoted as ΔQ1, and the second low voltage plateau can be assigned to the conversion of LiPSs to Li₂S, which can be denotes as ΔQ1. The battery with a CoP/CeO₂-10-modified separator exhibits the largest values from either ΔQ1 or ΔQ2's respect, indicating that CoP/CeO₂-10 can ensure the conversion of more S₈ into LiPSs, allowing more LiPSs to take part in the following conversion process.

Fig. 4 (a) First charge/discharge curves of batteries with different materials at 0.2C. (b) Rate performance of the batteries with different types of separators. (c) Charge–discharge profiles of CoP/CeO₂-10 batteries at different current rates. (d) Long-term cycling performance of batteries with different separators at 0.2C. (e) Cycling performance of batteries with CoP/CeO₂-10 separators with a high sulfur loading at 0.2C. (f) Long-term cycling performance of batteries with a moderate sulfur loading at 1C; inset: digital photo of LEDs lightened by a coin cell made using CoP/CeO₂-10 separators.

The Li–S battery with CoP/CeO₂-10 further presents an excellent cycling stability at 0.2C. As shown in Fig. 4d, although the specific capacities of all the batteries decrease with the increase in the cycling number, it is also interesting to find that the battery with CoP/CeO₂-10 always presents the largest value among them, and it can deliver a high initial discharge capacity of 1214.5 mA h g⁻¹, and maintain 640.7 mA h g⁻¹ after 500 cycles, corresponding to a capacity retention ratio of 51.4%; the coulombic efficiency during the whole cycling stage can be over 99%. While the performance based only on CoP exhibits the lowest electrochemical performance, it only exhibits the lowest capacity of 377.7 mA h g⁻¹ after 500 cycles. We also note that the battery with a high sulfur loading of 4.0 mg cm⁻² exhibits superior electrochemical performance to that of a low sulfur loading of 2.0 mg cm⁻², at the same current density of 0.2C, the battery with CoP/CeO₂-10 presents an initial capacity of 1041.6 mA h g⁻¹ and the specific capacity can still reach 932.2 mA h g⁻¹ after 100 cycles, with a capacity retention rate of 89.5% and a corresponding capacity decay rate of 0.010% per cycle, as shown in Fig. 4e. The main reason is that the mass ratio of S to CNT applied in the high-sulfur-loaded cathode is 75/25, while the low-sulfur-loaded S/C cathode uses a ratio of 7/3, and the thickness of the coated layer of the slurry is higher than that of the low sulfur loading. A little difference in the quality of the cathode may lead to this result, we believe that the role of the functional separator cannot be affected by the cathode, on the contrary, this result can further confirm that the CoP/CeO₂-10-modified separator can play a very active role in Li–S batteries. After confirming the excellent electrochemical performance of CoP/CeO₂-10 in Li–S batteries, we then investigated the stability of various kinds of CoP/CeO₂-x during a long cycle number of 1000, as shown in Fig. 4f, and the initial capacity of battery with CoP/CeO₂-10 reaches 1027.7 mA h g⁻¹. It is worth noting that the current density is 1C, which is higher than that shown in Fig. 4d, and the capacity can be maintained at 535.0 mA h g⁻¹ after 1000 cycles. About 52.1% of the capacity is maintained with a capacity decay rate of 0.047% per cycle, while the corresponding data of other kinds of batteries are as follows: CoP/CeO₂-0 (717.9 mA h g⁻¹, 218.8 mA h g⁻¹, 30.5%, 0.070%), CoP/CeO₂-5 (852.6 mA h g⁻¹, 375.0 mA h g⁻¹, 44.0%, 0.056%) and CoP/CeO₂-15 (8817.4 mA h g⁻¹, 326.8 mA h g⁻¹, 40.0%, 0.060%). We further compare the electrochemical performance of Li–S battery using CoP or CeO₂-based catalyst, as summarized in Table S1,† it is clear that the performance has surpassed most reported batteries. In addition, we provide a photograph of an LED sign lightened by a coin cell based on the Li–S battery with the CoP/CeO₂-10-modified separator (inset in Fig. 4f), which highlights the promising practical application.

Conclusions

In summary, based on the unique electron transfer and ion exchange advantages of CeO₂, we introduced CeO₂ to regulate the catalytic activity of CoP, hoping to integrate the superiority of CeO₂ and CoP to boost the conversion behavior of LiPSs in Li–S batteries. The results indicate that the introduction of CeO₂ can not only induce electron redistribution but also accelerate the electron transfer between CoP/CeO₂ and LiPSs. We also find that the introduction of CeO₂ can create a porous structure in CoP, and the amount of CeO₂ is a significant factor in determining the final catalytic activity of CoP/CeO₂. As a result, the battery with CoP/CeO₂-10 exhibits excellent adsorbability and catalytic activity to LiPSs, and the Li–S battery using a CoP/CeO₂-10-modified separator exhibits high energy output, enhanced rate and excellent cycling performances. We hope this work can provide a reference for designing highly efficient catalysts in Li–S batteries based on the regulation effect of CeO₂ on metal phosphides.

Author contributions

Xiaofei Wang: methodology, data curation, writing – review & editing. Ganfan Zhang: resources, software. Yue Li: validation, conceptualization. Yuanting Wu: validation, funding acquisition. Wei Luo: investigation, supervision.

Data availability

Data will be made available on request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51608412), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2021JM-423), the Youth Innovation Team of Shaanxi Universities (No. 2022-70).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01349b

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