Two-Dimensional CeO2/RGO Composite-Modified Separator for Lithium/Sulfur Batteries
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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.
KeywordsCeO2/RGO composite Modified separator Lithium/sulfur batteries
High-resolution transmission electron microscope
Reduced graphene oxide
Selected area electron diffraction
Scanning electron microscope
Transmission electron microscope
X-ray photoelectron spectroscopy
High-performance rechargeable batteries are currently being developed to meet the urgent demands of high-specific 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 (S8) and its discharge products (Li2S2/Li2S) 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 Li2S2 or Li2S 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 , implementation of a functional interlayer between the cathode and separator as polysulfides host [8, 9], and optimization of organic electrolytes . 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 , graphene , active carbon , polypyrrole , and various metal oxides  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 (RCT) of sulfur cathodes while acting as an upper-current collector . The latter is related to the improved utilization of sulfur. On the other hand, metal oxides like Al2O3 , MgO , NiFe2O4 , and SiO2  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.
Materials and Reagents
Graphene oxide was purchased from The Sixth Element (Changzhou) Materials Technology Company; Ce(NO3)3·6H2O, acrylic acid, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China); polyvinylidene fluoride (PVDF) was obtained from Kynar, HSV900; N-methyl-2-pyrrolidone (NMP) and pyrrole (Py) were obtained from Tianjin Guangfu Chemical Reagent; nanosulfur aqueous suspension was purchased from Alfa Chemistry (US Nanomaterials 10 wt%), lithium trifluoromethanesulfony imide (LiTFSI), 1,3-dioxolane (DOL), and 1,2-dimethoxyethane (DME) were obtained from Sigma-Aldrich (Hong Kong, China); Super-P, normal separator (Celgard 2400), Al foil, and lithium mental anode foil was purchased from Li Zhi Yuan battery sales department. Unless otherwise stated, all regents were of analytical grade and used without further purification.
Preparation of CeO2/RGO Composite and Modified Separator
Nanosized CeO2 was synthesized using an adapted polymer pyrolysis technique . Firstly, Ce(NO3)3·6H2O and acrylic acid (C3H4O2) 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 CeO2 nanoparticles were finally formed by calcining the polyacrylate salt at 600 °C for 3 h.
A facile hydrothermal technique was used for preparation of CeO2/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 CeO2 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 CeO2/RGO composite was finally obtained.
The CeO2/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 CeO2/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 , 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, CeO2/RGO composite modified separator, Li metal foil anode, and electrolyte containing 1.0 M LiTFSI with 0.1 M LiNO3 in mixed dioxolane (DOL) and dimethoxyethane (DME) (1:1 by volume). The amount of electrolyte is around 30 μL.
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 CeO2/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, ESCALAB250Xi). 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.
Results and Discussion
Raman spectroscopy was conducted to detect the disorder degree of the carbon materials by calculating the intensity ratio of D band to G band (ID/IG). As shown in Fig. 2b, the values of ID/IG were estimated to 0.874 and 0.915 for RGO and CeO2/RGO composite, respectively. The increased values suggested the anchoring of CeO2 nanoparticles onto RGO sheets. The sharp peak at 455 cm−1 was associated with crystalline CeO2. Also, no distinct disturbance peak was observed, confirming the successful and efficient synthesis of high purity CeO2/RGO composite.
The results of investigation of specific surface area and pore size distribution of as-prepared CeO2/RGO composite are shown in Fig. 2c, d, respectively. The N2 adsorption/desorption isotherm of CeO2/RGO composite shows a large BET-specific surface area of 59.62 m2 g−1 with the pore volume of 0.1331 cm3 g−1 and the average pore size of 9.213 nm. Results indicate the porous CeO2/RGO composite would benefit the infiltration of electrolyte and transport of electrons.
High-resolution TEM image of selected area in Fig. 3d is illustrated in Fig. 3e. The lattice spacing of CeO2 particles was estimated to 0.2705 nm, well in accordance with the theoretical d-spacing of cubic CeO2 (200) plane. The SAED pattern (Fig. 3f) showed homogeneous diffraction rings, confirming the polycrystalline features of the as-prepared CeO2/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 CeO2/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 CeO2/RGO composite can serve as a barrier to block the migration of polysulfides in both physically and chemically.
Polymer pyrolysis and hydrothermal method were employed as facile and efficient ways to prepare CeO2/RGO composite with superior structure, where ultrafine CeO2 nanoparticles were anchored on RGO sheets. The chemical suppression of the shuttling effect of polysulfides for CeO2 was confirmed by XPS after electrochemical processes. The performance of Li/S battery was significantly enhanced due to the cooperation of RGO and CeO2. 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 CeO2/RGO composite-coated separator was also higher than values obtained by traditional commercial separators.
This work was supported by the National Natural Science Foundation of China [grant number 51505122] and Cultivation project of National Engineering Technology Center [grant number 2017B090903008].
Availability of Data and Materials
All data generated or analyzed during this study are included in this published article.
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.
The authors declare that they have no competing interests.
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