Two-Dimensional CeO2/RGO Composite-Modified Separator for Lithium/Sulfur Batteries

In this work, a modified separator coated with a functional layer of reduced graphene oxide (RGO) anchored by cerium oxide (CeO2) nanoparticles was developed. The superior conductivity of RGO and chemical immobilization of high-ordered sulfur-related species (mainly Li2Sn 4 ≤ n ≤ 8) of CeO2 yielded batteries with enhanced characteristics. A remarkable original capacity of 1136 mAh g−1 was obtained at 0.1 C with capacity retention ratio of 75.7% after 100 charge/discharge cycles. Overall, these data indicate that the separator with CeO2/RGO composite is promising to suppress the shuttling of polysulfides for better utilization of the active material.


Background
High-performance rechargeable batteries are currently being developed to meet the urgent demands of highspecific capacity and superior energy density application devices. Li/S batteries have widely been considered as promising energy storage for power grids and electric devices because of their outstanding theoretical capacity (1672 mAh g −1 ) and prominent energy density (2600 Wh kg −1 ) [1,2]. However, despite their numerous advantages, some major obstacles hindering their practical commercial usage of Li/S batteries are still to be solved. Firstly, the insulating nature of the active material (S 8 ) and its discharge products (Li 2 S 2 /Li 2 S) can cause poor electrochemical accessibility and decrease utilization of active materials [3,4]. Secondly, polysulfides tend to dissolve in organic electrolytes after numerous charge/discharge processes and readily diffuse across the separator to be finally reduced to Li 2 S 2 or Li 2 S solids at the surface of counter electrode. This results in low coulombic efficiency and poor cycling life of Li/S batteries [5,6].
Tremendous efforts have been made to improve the conductivity and deal with the shuttling of polysulfides. These include modification of sulfur cathodes by confining more sulfur into porous conductive frameworks [7], implementation of a functional interlayer between the cathode and separator as polysulfides host [8,9], and optimization of organic electrolytes [10]. However, recent studies have shown that the diffusion of polysulfides is difficult to fully overcome. Considering that, the newly developed methods used to improve the performance of Li/S batteries by modifying the separator have attracted increasing attention. For example, materials like functional carbon [11], graphene [12], active carbon [13], polypyrrole [14], and various metal oxides [15] have been used as coatings for the separator or as free-standing interlayers. These functional components would inhibit the migration of sulfur-related species to the anode and improve the electrical conductivity of cathodes. Studies have shown that reduced graphene oxide (RGO) interlayer might reduce the charge-transfer resistance (R CT ) of sulfur cathodes while acting as an upper-current collector [16]. The latter is related to the improved utilization of sulfur. On the other hand, metal oxides like Al 2 O 3 [17], MgO [18], NiFe 2 O 4 [19], and SiO 2 [20] can absorb polysulfides by introducing strong chemical bonds. However, the added interlayer may increase the total mass of the cell, resulting in declined energy density.
Considering the complex fabrication process of selfstanding interlayer, simple and lightweight coating methods were employed in this study. As shown in Fig. 1a, CeO 2 /RGO composite was prepared using a facile polymer pyrolysis followed by hydrothermal technique. The obtained material presented a unique twodimensional (2D) structure with uniform CeO 2 nanoparticles anchored on RGO sheets. The CeO 2 /RGO composite was then coated on the traditional commercial separator (Celgard 2400), and the Li/S battery with the modified separator was assembled. The schematic diagrams in Fig. 1b, c revealed that the 2D CeO 2 /RGO composite did not only efficiently inhibit the "shuttle effect" through strong interactions between CeO 2 and polysulfides, but also enhanced utilization of the active materials due to the fast electron transport of RGO.

Preparation of CeO 2 /RGO Composite and Modified Separator
Nanosized CeO 2 was synthesized using an adapted polymer pyrolysis technique [21]. Firstly, Ce(NO 3 ) 3 ·6H 2 O and acrylic acid (C 3 H 4 O 2 ) at stoichiometric amounts were dissolved in 50 ml deionized (DI) water under constant magnetic stirring at 40°C to facilitate polymerization. The mixture was then kept stirred until the precursor solution became dry. The obtained product was transferred into a furnace and calcined at 200°C for 2.5 h in an air atmosphere to yield a polyacrylate salt. The CeO 2 nanoparticles were finally formed by calcining the polyacrylate salt at 600°C for 3 h. A facile hydrothermal technique was used for preparation of CeO 2 /RGO composite. Typically, 4 g graphene oxide was dispersed in DI water to form 40 ml graphene oxide dispersion. After ultrasonication for 1 h, 0.1 g of the as-prepared CeO 2 nanoparticles were added to the suspension. Next, the mixture was stirred for 2 h to promote the self-assembly of functional groups. Subsequently, the mixture was transferred into an autoclave and heated to 140°C for 4 h. After drying overnight at 60°C, the CeO 2 /RGO composite was finally obtained.
The CeO 2 /RGO composite modified separator was prepared by coating the as-prepared composite material onto the surface of normal separator. Typically, 90 wt% of the as-prepared CeO 2 /RGO composite and 10 wt% PVDF in NMP were mixed to form a slurry. After grounding for 40 min, the slurry was coated onto a normal separator by using a spreader with height of 10 mm. Finally, the coated separator was dried at 60°C in an oven for 6 h.

Electrode Preparation and Battery Assembly
The fabrication process of the sulfur composite was reported in our previous work [22], which fabricated well-dispersed sulfur anchored on interconnected polypyrrole nanofiber network by mixing PPy and nano-sulfur aqueous suspension via a simple ball-milling followed by a low-temperature heat treatment. The sulfur cathode was prepared by mixing 80 wt% sulfur composite, 10 wt% conductive Super-P, and 10 wt% PVDF binder in NMP then laminated on an aluminum foil at sulfur composite loading around 2.0 mg cm −2 . Subsequently, the coated foil was dried in vacuum at 60°C for 6 h. The CR 2032 coin-type cells were assembled using the following components: sulfur cathode, CeO 2 /RGO composite modified separator, Li metal foil anode, and electrolyte containing 1.0 M LiTFSI with 0.1 M LiNO 3 in mixed dioxolane (DOL) and dimethoxyethane (DME) (1:1 by volume). The amount of electrolyte is around 30 μL.

Characterization
The morphologies and structures of the samples were observed by scanning electron microscopy (SEM, NovaNano SEM450, FEI) and transmission electron microscopy (TEM, JEM2010F), respectively. The phase composition of CeO 2 /RGO composite was obtained by X-ray diffraction (XRD, Vinci, AXS) with Cu Kα-radiation. The surface functional groups present on the samples were identified by X-ray photoelectron spectroscopy (XPS, ESCA-LAB250Xi). The Raman spectra were measured using Raman spectroscopy (LabRAM HR Evolution, HORIBA). The specific surface area was examined by the Brunaner-Emmet-Teller (BET) and Barret-Joyner-Halenda (BJH) methods at 77 K (Autosorb iQ, Quantachrome Corporation). The batteries were discharged and charged on a battery test system (BTS-5 V 20 mA, Shenzhen Neware) from 1.5 to 3.0 V at 0.1 C. The electrochemical impedance spectra were collected on an electrochemical workstation (CH1600E) over the frequency range from 0.01-1 MHz.
Raman spectroscopy was conducted to detect the disorder degree of the carbon materials by calculating the intensity ratio of D band to G band (I D /I G ). As shown in Fig. 2b, the values of I D /I G were estimated to 0.874 and 0.915 for RGO and CeO 2 /RGO composite, respectively. The increased values suggested the anchoring of CeO 2 nanoparticles onto RGO sheets. The sharp peak at 455 cm −1 was associated with crystalline CeO 2 . Also, no distinct disturbance peak was observed, confirming the successful and efficient synthesis of high purity CeO 2 / RGO composite.
The results of investigation of specific surface area and pore size distribution of as-prepared CeO 2 /RGO composite are shown in Fig. 2c, d, respectively. The N 2 adsorption/desorption isotherm of CeO 2 /RGO composite shows a large BET-specific surface area of 59.62 m 2 g −1 with the pore volume of 0.1331 cm 3 g −1 and the average pore size of 9.213 nm. Results indicate the porous CeO 2 / RGO composite would benefit the infiltration of electrolyte and transport of electrons.
Representative micro-morphologies of RGO, CeO 2 , and CeO 2 /RGO composite are depicted in Fig. 3a-d. Pure RGO sheets showed restacked structures, suffering from reduction in specific surface area. Pure CeO 2 particles possessed uniform nano sizes but with tendency to agglomerate. Fortunately, the recombination of CeO 2 and RGO by polymer pyrolysis and hydrothermal methods resulted in unique 2D structure with CeO 2 nanoparticles well dispersed on RGO sheets. The High-resolution TEM image of selected area in Fig. 3d is illustrated in Fig. 3e. The lattice spacing of CeO 2 particles was estimated to 0.2705 nm, well in accordance with the theoretical d-spacing of cubic CeO 2 (200) plane. The SAED pattern (Fig. 3f ) showed homogeneous diffraction rings, confirming the polycrystalline features of the as-prepared CeO 2 /RGO composite. The commercial separator (Celgard 2400) revealed a smooth surface with numerous pores of several micrometers in size (Fig. 3g), whereas sizes of polysulfides (1-1.8 nm) were too small to be hindered by the membrane. By contrast, the holes of pristine separator were completely covered by CeO 2 / RGO composite with a thickness of about 15 μm (Fig. 3h), as well as its inset whose rough surface would benefit the infiltration of electrolyte and transport of electrons. Moreover, the coated CeO 2 /RGO composite can serve as a barrier to block the migration of polysulfides in both physically and chemically.
Charge/discharge voltage-capacity profiles of the cell assembled with CeO 2 /RGO composite modified separator at 0.1 C after different cycling processes are displayed in Fig. 4a. The first discharge plateau was associated with the oxidation processes of S 8 to Li 2 S n (4 ≤ n ≤ 8), and the lower plateau corresponded to reduction processes of high-order polysulfides to Li 2 S/Li 2 S 2 [23]. Stable electrochemical performances were evidently confirmed by the close coincidence curves. The cells assembled with CeO 2 /RGO composite coated separator exhibited overlapping upper discharge plateaus even after 100 cycles, revealing that the modified cell was extremely beneficial for polysulfide inhibition and electrochemical stability. By comparison, the cells with normal separator exhibited shorter upper discharge plateaus accompanied by increased cycling processes. Moreover, the polarization (ΔE) of the cells with CeO 2 /RGO composite coated separator (0.224) looked smaller than those assembled with normal separator (0.238). The latter would suggest fast redox reaction kinetics and high system reversibility [24,25].
The cycling performances of cells assembled with and without CeO 2 /RGO composite modified separator at 0.1 C and 1 C are gathered Fig. 5. At the current rate of 0.1 C, the modified battery achieved a high capacity of 1136 mAh g −1 after the 1st cycle and retained a capacity of 886 mAh g −1 after 100 cycles with high coulombic efficiency throughout the processes. These values were superior to that of the cell assembled with normal separator (992 mAh g −1 and 501 mAh g −1 , respectively), suggesting the key role played by the functional separator. In addition, when the current rate increased to 1 C, the modified cells can also delivered an outstanding initial capacity of 917 mAh g −1 and maintained 72.9% of its initial capacity as well as high coulombic efficiency throughout the processes. The well-designed structure would not only allow better transport of electrons by contribute to superior electrical conductivity of RGO. Also, the shuttling of polysulfides could efficiently be impeded by the strong chemical bond between CeO 2 and sulfur-related species.
The Nyquist plots of the cells assembled with and without CeO 2 /RGO composite-modified separator were first obtained then fitted with an equivalent circuit model. As shown in Fig. 6, both cells exhibited depressed semicircle in high-frequency region and inclined line at low frequencies. These would correspond to charge-transfer resistance (R CT ) for sulfur cathode and Li-ion diffusion or so-called Warburg impedance, respectively [26,27]. The smaller semicircle represented moderate R CT value of the modified cell, which mainly attributed to the efficiently suppressed shuttling of polysulfides by CeO 2 nanoparticles and superior electron transport of RGO. Moreover, the CeO 2 /RGO composite would improve the electrochemical contact and maximize the utilization of active materials. The larger slope of To gain a better understanding about contributions of CeO 2 /RGO composite-coated separator in impeding the shuttle of sulfur-related species, H-type glass cells were introduced and tested. As displayed in Fig. 7, the dark brown solution in the left side was composed of DOL/ DME with 0.05 M Li 2 S 6 as an additive. The right side solution contained pure DOL/DME. Li 2 S 6 would spontaneously diffuse through the membrane from high to low concentration, which can be reflected by changes in color [28,29]. In cells with normal separator (Fig. 7a), the color of the right cell changed evidently over time to become dark brown after 16 h, confirming that traditional commercial separator was unable to hinder the diffusion of polysulfide. By comparison, in cells with CeO 2 /RGO composite coated separator (Fig. 7b), no distinct color change took place over time, suggesting the shuttling of polysulfide was inhibited by CeO 2 /RGO composite modified separator.
XPS was used to confirm the existence of interactions between CeO 2 and sulfur-related species. The elemental composition and valence states of CeO 2 /RGO composite after cycling are displayed in Fig. 8a. Four elements (C, O, Ce, and S) were detected. The peak in S 2p spectrum of CeO 2 /RGO composite after cycling can be fitted by   (Fig. 8c), corresponding to CeO 2 3d 5/2. The peak at 885.3 eV can be attributed to CeO 2 3d 5/2. The peaks located at 901.2, 907.7, and 917.1 eV were associated with CeO 2 3d 3/2. The peaks of CeO 2 /RGO composite-coated separator after cycling appeared slightly shifted to negative values (Fig. 8d). This indicated absorption of sulfur-related species by Ce-S bonding [30], corresponding to S 2p spectrum of CeO 2 /RGO composite after cycling.

Conclusions
Polymer pyrolysis and hydrothermal method were employed as facile and efficient ways to prepare CeO 2 / RGO composite with superior structure, where ultrafine CeO 2 nanoparticles were anchored on RGO sheets. The chemical suppression of the shuttling effect of polysulfides for CeO 2 was confirmed by XPS after electrochemical processes. The performance of Li/S battery was significantly enhanced due to the cooperation of RGO and CeO 2 . A high initial capacity of 1136 mAh g −1 was obtained at 0.1 C with about 75.7% capacity retention after 100 cycles. The coulombic efficiency of the cell with CeO 2 /RGO composite-coated separator was also higher than values obtained by traditional commercial separators.

Availability of Data and Materials
All data generated or analyzed during this study are included in this published article.

Authors' Contributions
SW and FG carried out the experiments. SW, FG, and TT analyzed the data. SW, YZ, NL, TT, and XW contributed in the drafting and revision of the manuscript. YZ, XW, and NL supervised the work and finalized the manuscript. All authors read and approved the final manuscript.

Competing Interests
The authors declare that they have no competing interests.

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