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Sep 14, 2018 - graphene oxide (RGO) with uniformly distributed CeO2 particles is reported. ... First, sulfur insulation reduces the use of cathode-active materials. Second, a large volume change (80%) is produced during charging/discharging, which leads ..... Liang, X.; Kwok, C.Y.; Lodi-Marzano, F.; Pang, Q.; Cuisinier, M.; ...
materials Article

Three-Dimensional S/CeO2/RGO Composites as Cathode Materials for Lithium–Sulfur Batteries Qiuyan Hao 1 , Guoliang Cui 1 , Yuan Tian 1 , Taizhe Tan 2 and Yongguang Zhang 1, * 1

2

*

School of Materials Science and Engineering, Research Institute for Energy Equipment Materials, Hebei University of Technology, Tianjin 300130, China; [email protected] (Q.H.); [email protected] (G.C.); [email protected] (Y.T.) Synergy Innovation Institute of GDUT, Heyuan 517000, China; [email protected] Correspondence: [email protected]; Tel.: +86-22-6020-1447

Received: 8 July 2018; Accepted: 9 September 2018; Published: 14 September 2018

 

Abstract: In this paper, the synthesis of the three-dimensional (3D) composite of spherical reduced graphene oxide (RGO) with uniformly distributed CeO2 particles is reported. This synthesis is done via a facile and large-scalable spray-drying process, and the CeO2 /RGO materials are hydrothermally compounded with sulfur. The morphology, composition, structure, and electrochemical properties of the 3D S/CeO2 /RGO composite are studied using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), Raman spectra and X-ray photoelectron spectroscopy (XPS), etc. The electrochemical performance of the composites as electrodes for lithium–sulfur batteries is evaluated. The S/CeO2 /RGO composites deliver a high initial capacity of 1054 mAh g−1 , and retain a reversible capacity of 792 mAh g−1 after 200 cycles at 0.1 C. Profiting from the combined effect of CeO2 and RGO, the CeO2 /RGO materials effectively inhibit the dissolution of polysulfides, and the coating of spherical RGO improves the structural stability as well as conductivity. Keywords: Lithium–sulfur battery; CeO2 /RGO composite; electrochemical performance

1. Introduction With the booming use of electric vehicles and portable electronic devices, the demand for rechargeable batteries that have higher power densities and long-term stability has increased substantially [1,2]. Lithium–sulfur batteries are secondary batteries that have high-energy current density (2600 Wh kg−1 ), as well as great potential for development and application prospects [3]. In addition, in terms of source, cost, and environmental impact, sulfur has also been shown to have unparalleled advantages for being used as a positive electrode [4,5]. However, lithium–sulfur batteries still have some shortcomings [6,7]. First, sulfur insulation reduces the use of cathode-active materials. Second, a large volume change (80%) is produced during charging/discharging, which leads to reduced mechanical properties. Third, the dissolution of polysulfides leads to a shuttle effect between the cathode and anode, and this results in the loss of active materials and poor coulomb efficiency, poor utilization, and obvious degradation [8]. Numerous design methods, including the combination of sulfur and carbon materials [9–11], metal oxides [12,13], and conductive polymers [14,15], have been explored to avoid these problems. Among these materials, reduced graphene oxide (RGO) (which is a carbon material) has high surface area, excellent intrinsic conductivity, excellent mechanical flexibility, and chemical stability. Due to these excellent properties, RGO has been widely used to prepare S/RGO composites to mitigate the dissolution of intermediate polysulfides [16,17]. However, the physical interactions between nonpolar RGO and polar polysulfides are weak, and they cannot ensure the long-term confinement Materials 2018, 11, 1720; doi:10.3390/ma11091720

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of polysulfides during the charging/discharging process, during which the polysulfides remain vulnerable to slow dissolution in electrolytes, thus triggering the “shuttle effect” and resulting in an unsatisfactory calendar life [18]. Polar materials can be firmly combined with polysulfides via chemical adsorption, and thus polysulfides can be effectively captured at the cathode. Many polar host materials for sulfur, including SiO2 , TiO2 , Al2 O3 , La2 O3 and MnO2 , have thus far been introduced into the cathodes [19–21]. For example, Sun et al. reported a method of modifying nitrogen-rich mesoporous carbon using La2 O3 nanodots [22]. Their results show that the La2 O3 nanoparticles can be used as the adsorption point of polysulfides and oxidation-reduction catalyst. Ding et al. fabricated nanoscale graphene modified with TiO2 nanocrystals and used it as the sulfur host [23]. The TiO2 nanocrystals can adsorb dissolved polysulfides and also promote the transmission of charge. CeO2 , which is a polar substance, is also an excellent adsorbent and catalyst. CeO2 has been applied to the preparation of cathode materials for lithium sulfur batteries. In addition to effectively slowing down the dissolution of polysulfides in electrolytes, CeO2 also has a catalytic effect on the redox reaction. However, the conductivity of CeO2 is relatively low, which inevitably affects the electrochemical performance. Herein, a simple and large-scale spray-drying technique has been used to prepare RGO coated with CeO2 particles. The CeO2 /RGO composites have several apparent advantages. First, spherical RGO greatly improves the conductivity of the electron and ion transmission during the charging/discharging process. In addition, CeO2 particles provide several strong binding sites for polysulfide intermediates, and keep them bound to the cathode materials during the charging/discharging process, which results in a longer cycle life. Therefore, the S/CeO2 /RGO cathodes have the advantages of a high reversible capacity, good multiplying performance, and good circulation stability. 2. Materials and Methods 2.1. Materials All of the chemicals that were used were analytical grade and used without further purification. Cerium nitrate hexahydrate (Ce(NO3 )3 ·6H2 O), ammonia solution ((NH3 ·H2 O), graphene oxide solution (GO), polyvinylidene fluoride (PVDF), and N-methyl-pyrrolidinone (NMP) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). 2.2. Sample Preparation CeO2 was synthesized via a precipitation process. Ammonia solution (NH3 ·H2 O) was added dropwise to an aqueous solution of Ce(NO3 )3 ·6H2 O solution, which had a concentration 0.4 mol L−1 , until the pH of the mixture became 10. After stirring for 30 min, the mixture was then left standing for 12 h. The precipitate was filtered out of the solution using a filtration device, and then it was repeatedly washed with water. Afterward, the samples were desiccated at 60 ◦ C for 12 h in an electronic oven. The sample was then calcined at 300 ◦ C for 4 h in a muffle furnace to obtain the desired CeO2 . The second step was to composite CeO2 and RGO. A commercially available graphene oxide (GO) solution (2 mg mL−1 ) was mixed with CeO2 in ratio of 1:5. The mixture was sonicated for 2 h at 50 kHz using an ultrasonic cell crusher at room temperature to obtain a uniformly mixed suspension of CeO2 /GO. The spray-drying technique was then used to obtain CeO2 /GO powders. The spray-drying equipment that was used was a normal air pressurizer with an inlet air temperature of 200 ◦ C and a feed rate of 4 mL min−1 . The precursor was calcined in a tube furnace under an argon atmosphere at 900 ◦ C for 2 h to achieve the CeO2 /RGO composites (Figure 1). In the final step, sulfur was loaded into the CeO2 /RGO composite. The weight ratio of CeO2 /RGO to sulfur was set to 1:2. The mixture was heated at 155 ◦ C, and maintained at this temperature for 12 h to obtain the

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S/CeO2 /RGO composites. The chemical equations associated with the preparation of CeO2 are as follows: (a) Ce3+ + 3OH−→Ce(OH)3↓ (a) Ce3+ + 3OH− →Ce(OH)3 ↓ (b) Ce(OH)3 + 1/4O2 + 1/2H2O→Ce(OH)4

(b) Ce(OH)3 + 1/4O2 + 1/2H2 O→Ce(OH)4 (c) Ce(OH)4→CeO2 + 2H2O

(c) Ce(OH)4 →CeO2 + 2H2 O

Figure 1. Schematic diagram of the fabrication process of CeO2 /reduced graphene oxide Figure 1. Schematic diagram of the fabrication process of CeO2/reduced graphene oxide (RGO) (RGO) composites. composites.

2.3. Characterization

2.3. Characterization

Morphology and crystal structure information were acquired using scanning electron microscopy Morphology and crystal structure information were acquired using scanning electron (SEM, Rigaku S4800, Neu-Isenburg, Germany), transmission electron microscopy (TEM, TECNAI microscopy (SEM, Rigaku S4800, Neu-Isenburg, Germany), transmission electron microscopy (TEM, F-20, TECNAI Thermo F-20, Fisher Scientific, USA), and X-ray diffraction (XRD,(XRD, D/max-rB, Rigaku, Thermo FisherWaltham, Scientific, MA, Waltham, MA, USA), and X-ray diffraction D/max-rB, Toyko, Japan).Toyko, The surface functional groups in the S/CeO were determined Rigaku, Japan). The surface functional groups2 /RGO in the composites S/CeO2/RGO composites wereusing a Physical Electronics 5700 spectrometer (Chanhassen, MN, USA). The pyrolysis determined usingPHI a Physical Electronics PHI 5700 spectrometer (Chanhassen, MN, weight USA). analysis The weight analysis wasToledo-TGA/DSC performed using a Mettler (TGA)pyrolysis was performed using a(TGA) Mettler (HK). Toledo-TGA/DSC (HK). 2.4. Electrochemical Measurements 2.4. Electrochemical Measurements S/CeO 2/RGO, acetylene black, and PVDF were mixed using magnetic stirring in a weight ratio S/CeO 2 /RGO, acetylene black, and PVDF were mixed using magnetic stirring in a weight ratio of of 8:1:1 with N-methylpyrrolidone a solvent to prepare theN-methylpyrrolidone cathode slurry. 8:1:1 with N-methylpyrrolidone (NMP) as a(NMP) solventas to prepare the cathode slurry. N-methylpyrrolidone (NMP) was slowly added to the materials and ground until a similar viscous (NMP) was slowly added to the materials and ground until a similar viscous oil-like slurry was oil-like slurry was formed. The obtained slurry was then cast on aluminum foil and dried at 60 °C for formed. The obtained slurry was then cast on aluminum foil and dried at 60 ◦ C for 12 h in 12 h in vacuum, and the NMP evaporated completely during the drying process. Aluminum foil vacuum, thedisks, NMPeach evaporated completely during the foil was was and cut into with a diameter of 15 mm, for use asdrying current process. collectors. Aluminum The electrolyte was 1 cut into disks, each with a diameter of 15 mm, for use as current collectors. electrolyte was 1 M M of lithium bis (trifluoromethane)sulfonimide (LiTFSI) in a mixed solvent ofThe 1,2-dimethoxyethane of lithium bis (trifluoromethane)sulfonimide (LiTFSI) in a mixed solvent of 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1 v/v) containing 1 wt % of LiNO3. Cyclic voltammetry (CV) and (DME) and 1,3-dioxolane (DOL) (1:1 v/v) containing wt %conducted of LiNO3 . using Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) 1 were an electrochemical workstationimpedance (CHI660E, Austin, TX, USA) thatwere was operated in the frequency range of 10 kHz to 10 electrochemical spectroscopy (EIS) conducted using an electrochemical workstation mHz with an amplitude 10 mV. (CHI660E, Austin, TX, USA)ofthat was operated in the frequency range of 10 kHz to 10 mHz with an amplitude of 10 mV. 3. Results and Discussion

3. ResultsFigure and Discussion 2 shows the XRD patterns of CeO2 and S/CeO2/RGO composites. XRD peaks were recorded at 2θ = 28.5°, 33.1°, 47.4°, 56.3°, 69.4°, 76.6° and 79.0°, and could be well allocated to the

Figure 2 shows the XRD patterns of CeO2 and S/CeO2 /RGO composites. XRD peaks were (111), (200), (220),◦(311), (400), (331) and (420) planes, respectively, of CeO2 (JCPDS No. 34-0394) [24]. recorded at 2θ = 28.5 , 33.1◦ , 47.4◦ , 56.3◦ , 69.4◦ , 76.6◦ and 79.0◦ , and could be well allocated to the Two feeble peaks of 3D RGO are observed at 26.2° and 43.7° because of a fairly low diffraction (111),intensity (200), (220), (331) andpeaks (420)are planes, CeO 34-0394) 2 (JCPDS of 3D(311), RGO (400), [25]. The other sulfur respectively, peaks (JCPDSof No. 42-1278) [26].No. A few strong [24]. ◦ ◦ Two feeble of 3DinRGO are observed at 26.2 and 43.7 because of a fairly low diffraction intensity peaks peaks are marked the figure. of 3D RGO [25]. The other peaks are sulfur peaks (JCPDS No. 42-1278) [26]. A few strong peaks are marked in the figure.

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Figure patterns CeO 22 and S/CeO 22/RGO. Figure 2. XRD XRD patterns of CeO and S/CeO /RGO. Figure 2. 2. XRD patterns ofof CeO S/CeO 2 and 2 /RGO.

To the structural intricacies present inin the CeO To further confirm the structural intricacies present the CeO 22/RGO composites, we collected Tofurther furtherconfirm confirm the structural intricacies present in the CeO /RGOcomposites, composites,we wecollected collected 2 /RGO Raman spectra, and the results are shown in Figure All of the CeO /RGO composites exhibited Raman spectra, and the results are shown 3. All of the CeO 2 /RGO composites exhibited Raman spectra, and the results are shown in Figure 3. CeO22/RGO composites exhibited an an −1 and G-breathing zone at an inherent mode of graphite structure (D-breathing zone at ~1350 inherent and inherent mode mode of of graphite graphite structure structure (D-breathing (D-breathing zone zone at at ~1350 ~1350 cm cm−1−1cm and G-breathing G-breathing zone zone at at ~1580 ~1580 −1 ) and CeO structure (F mode at ~461 −1 [27]. The degree of graphitization in the ~1580 cmThe cm and (F cm−1−1))cm and CeO CeO22 structure structure (F2g2g mode mode2gat at ~461 ~461 cm cm−1−1)) [27]. [27]. The) degree degree of of graphitization graphitization in in the the CeO CeO22/RGO /RGO 2 CeO /RGO composites is low, because the addition of metal oxide leads to an increase in the ratio composites is low, because the addition of metal oxide leads to an increase in the ratio I D :I G , thereby composites is low, because the addition of metal oxide leads to an increase in the ratio ID:IG, thereby 2 IDincreasing :IG , thereby increasing theof defect level of graphene andthe increasing the conductivity graphene [28]. the defect graphene and increasing conductivity of increasing the defect level level of graphene and increasing the conductivity of graphene grapheneof[28]. [28].

Figure 3.3. Raman spectra obtained from CeO Figure Raman spectra obtained from CeO 22/RGO composites. Figure 3. Raman spectra obtained from CeO /RGOcomposites. composites. 2 /RGO

As inin thethe SEM andand TEM images of the /RGO sample (Figure 4a,b), 4a,b), RGO been made As seen SEM TEM images of the sample (Figure RGO has been Asseen seen in the SEM and TEM images ofCeO the2CeO CeO22/RGO /RGO sample (Figure 4a,b),has RGO has been into a three-dimensional (3D) spherical structure via spray-drying, and CeO was distributed uniformly 2 and made made into into aa three-dimensional three-dimensional (3D) (3D) spherical spherical structure structure via via spray-drying, spray-drying, and CeO CeO22 was was distributed distributed inuniformly the RGO.in Inthe theRGO. corresponding high-resolution TEM imageTEM shown in Figure 4c,in has4c, lattice In high-resolution image shown Figure RGO uniformly in the RGO. In the the corresponding corresponding high-resolution TEM image shown inRGO Figure 4c, RGO spacings of ca. 0.34 nm, which is indexed to the (200) planes, and ca. 0.312 nm, which corresponds has has lattice lattice spacings spacings of of ca. ca. 0.34 0.34 nm, nm, which which is is indexed indexed to to the the (200) (200) planes, planes, and and ca. ca. 0.312 0.312 nm, nm, which which tocorresponds the interspacing of interspacing the (111) planes of selective diffraction 2 [27]. to of (111) planes of cubic CeO [27]. selective electron corresponds to the the interspacing of the thecubic (111)CeO planes ofThe cubic CeO22 electron [27]. The The selective (SAED) electron pattern of the(SAED) composites materials the polycrystalline naturethe of the materials (Figure 4d)of diffraction pattern of composites materials polycrystalline nature the diffraction (SAED) pattern of the thereveals composites materials reveals reveals the polycrystalline nature of[29]. the The above results show that CeO and RGO are well combined to form composite materials. 2above results materials materials (Figure (Figure 4d) 4d) [29]. [29]. The The above results show show that that CeO CeO22 and and RGO RGO are are well well combined combined to to form form

composite composite materials. materials.

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Figure 4. (a) SEM image of CeO2/RGO. (b) TEM image of CeO2/RGO. (c) High-resolution Figure 4. (a) SEM image of CeO2 /RGO. (b) TEM image of CeO2 /RGO. (c) High-resolution transmission transmission electron(HRTEM) microscopy (HRTEM) of(d) CeO 2/RGO. (d) SAED pattern of CeO2/RGO. electron microscopy image of CeOimage /RGO. SAED pattern of CeO /RGO. 2

2

SEM and TEM TEMimages imagesofofthe the S/CeO 2/RGO sample are shown in Figure 5a,f, respectively. As SEM and S/CeO 2 /RGO sample are shown in Figure 5a,f, respectively. As seen seen in the figure, the resulting sphere has a diameter about µ m. Additionally, themapping element in the figure, the resulting sphere has a diameter of aboutof1–2 µm. 1–2 Additionally, the element mapping results (Figure 5b–e) that Ce,S are O, C, and S arethroughout distributedthe throughout structure, results (Figure 5b–e) reveal that reveal Ce, O, C, and distributed structure, the indicating the indicating the component uniformity of the S/CeO 2 /RGO composites. component uniformity of the S/CeO2 /RGO composites.

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Figure 5. (a) SEM image of S/CeO2/RGO. (b–e) Element mapping of S/CeO2/RGO. (f) TEM image of

Figure 5. (a) SEM image of S/CeO2 /RGO. (b–e) Element mapping of S/CeO2 /RGO. (f) TEM image of S/CeO 2/RGO. S/CeO2 /RGO.

The high-resolution XPS spectrum of 3D Ce is shown in Figure 6a, and demonstrates the presence of a mixed valence state. The O 1s XPS peak at 530.8 eV corresponds to the oxygen in CeO2, and further confirms the The high-resolution XPS spectrum ofO 3D Ceatis528.6 shown in Figure demonstrates the presence presence of CeO 2 (Figure 6b) [24]. The 1s peak eV indicates that there6a, are and residual oxygen groups associated withThe the C O atoms 3D RGO. The at C 1s530.8 XPS spectrum of S/CeO2/RGO is to shown Figure 6c. The of a mixed valence state. 1sinXPS peak eV corresponds theinoxygen inpeak CeO2 , and further observed at 283.34 eV is related to the graphitic carbon in the 3D RGO, and the peak at 286.48 eV is assigned to confirms the presence 6b)energies [24].of SThe O163.8 1sand peak at and 528.6 eV indicates that there 2 (Figure the C–O bondof [30].CeO In Figure 6d, the binding 2p3/2 are 164.3 eV, are attributed to the S–S and S–O species, respectively [30]. The additional small shoulder of 167.7 eV is attributed to the sulfate are residual oxygen groups associated with the C atoms in 3D RGO. The C 1s XPS spectrum of species, which is associated with sulfur oxidation [31].

S/CeO2 /RGO is shown in Figure 6c. The peak observed at 283.34 eV is related to the graphitic carbon in the 3D RGO, and the peak at 286.48 eV is assigned to the C–O bond [30]. In Figure 6d, the binding energies of S 2p3/2 are 163.8 and 164.3 eV, and are attributed to the S–S and S–O species, respectively [30]. The additional small shoulder of 167.7 eV is attributed to the sulfate species, which is 2018, 11, x FOR PEER REVIEW 7 of 13 associated Materials with sulfur oxidation [31].

Figure 6. X-ray photoelectron spectroscopy (XPS) binding energy spectra of the core level of (a) Ce, Figure 6. X-ray photoelectron spectroscopy (XPS) binding energy spectra of the core level of (a) Ce, (b) O, (c) C and (d) S in the resulting samples. (b) O, (c) C and (d) S in the resulting samples.

It is apparent from the TGA curves shown in Figure 7 that the weight drops rapidly when the temperature increases from 200 °C to 293 °C. Since the sulfur is completely evaporated [32], the rapid weight loss is about 64 wt %. Therefore, the overall sulfur content can be estimated to be about 64 wt %.

Figure 6. X-ray photoelectron spectroscopy (XPS) binding energy spectra of the core level of (a) Ce, (b) O, (c) C and (d) S in the resulting samples.

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It is apparent from the TGA curves shown in Figure 7 that the weight drops rapidly when the temperature increases from 200curves °C toshown 293 °C.inSince the sulfur completely [32], It is apparent from the TGA Figure 7 that theisweight dropsevaporated rapidly when thethe ◦ ◦ rapid weight loss is from about200 64 wt Therefore, thethe overall content can be estimated to be about temperature increases C %. to 293 C. Since sulfursulfur is completely evaporated [32], the rapid 64 wt %. weight loss is about 64 wt %. Therefore, the overall sulfur content can be estimated to be about 64 w %.

. Materials 2018, 11, x FOR PEER REVIEW Figure 7. TGA curve of S/CeO2 /RGO at a heating rate of 10 ◦ C /min.

Figure 7. TGA curve of S/CeO2/RGO at a heating rate of 10 °C /min.

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The S/CeO 2/RGO andand S/RGO cathodes were were testedtested after 200 cycles, the Sand 2p XPS of S/CeO S/RGO cathodes after 200 and cycles, the Sspectra 2p XPS 2 /RGO the twoofsamples shown Figurein8.Figure There8. are fourareapparent peaks peaks for each sample. For spectra the twoare samples areinshown There four apparent for each sample. S/CeO 2/RGO, these are at 156.2 157.3 163.1 and eV 164.3 eV.164.3 For S/RGO, are at 155.7 For S/CeO these are ateV, 156.2 eV,eV, 157.3 eV,eV 163.1 and eV. For they S/RGO, they areeV, at 2 /RGO, 156.8 eV, eV, 156.8 162.1 eV, eV,162.1 and 164 samples, the peaks eV correspond to lithium 155.7 eV, eV. and For 164 both eV. For both samples, thearound peaks 156 around 156 eV correspond to polysufides, and theand peaks 163 eV163 correspond to elemental sulfur. The The S 2pS XPS spectra of lithium polysufides, the around peaks around eV correspond to elemental sulfur. 2p XPS spectra the S/CeO 2/RGO cathode after after cycling obviously showshow higher binding energies compared with those of the S/CeO cathode cycling obviously higher binding energies compared with 2 /RGO of the of S/RGO cathode. Therefore, the CeO 2 particles embedded in spherical RGO can serve strong those the S/RGO cathode. Therefore, the CeO2 particles embedded in spherical RGO canasserve as adsorbents of lithium polysulfides, which in turnin improve the electrochemical characteristics. strong adsorbents of lithium polysulfides, which turn improve the electrochemical characteristics.

Figure8.8.SS2p 2pXPS XPSspectra spectraofofthe the(a) (a)S/CeO S/CeO22/RGO Figure /RGOand and(b) (b)S/RGO S/RGOcathodes cathodesafter after200 200cycles. cycles.

Figure 99 shows and Figure shows the the charge/discharge charge/discharge curves curves for for lithium–sulfur lithium–sulfur with with the theS/CeO S/CeO22/RGO /RGO and S/RGO cathodes at a scan rate of 0.1 C. In the discharge process with the S/CeO /RGO cathode, 2 S/RGO cathodes at a scan rate of 0.1 C. In the discharge process with the S/CeO2/RGO cathode, two two major stages appear in the potential distribution,which whichare areattributed attributed toto the the two-step two-step major stages appear in the potential distribution, electrochemical reaction between lithium and sulfur. A short discharge platform of about 2.3 V V electrochemical reaction between lithium and sulfur. A short discharge platform of about 2.3 indicates the first electrochemical reaction, and is related to the reduction of the S form of elemental indicates the first electrochemical reaction, and is related to the reduction of the S88 form of elemental sulfur [33]. [33]. The The lower lower extended extended plateau plateau around around 2.1 2.1 V V in in the the discharge discharge curve curve reflects reflects the sulfur the subsequent subsequent

reduction of higher polysulfides to lower polysulfides, and eventually to lithium sulfide Li2S [32]. The S/CeO2/RGO electrode presents a higher initial discharge capacity than the S/RGO electrode during discharge at 0.1 C. Meanwhile, the S/CeO2/RGO electrode shows two higher discharge potential plateaus than the S/RGO electrode. These are all because CeO2 decoration enhances catalytic activity.

Figure 9 shows the charge/discharge curves for lithium–sulfur with the S/CeO2/RGO and S/RGO cathodes at a scan rate of 0.1 C. In the discharge process with the S/CeO2/RGO cathode, two major stages appear in the potential distribution, which are attributed to the two-step electrochemical reaction between lithium and sulfur. A short discharge platform of about 2.3 V Materials 2018, 11, 1720 8 of 12 indicates the first electrochemical reaction, and is related to the reduction of the S8 form of elemental sulfur [33]. The lower extended plateau around 2.1 V in the discharge curve reflects the subsequent reductionof ofhigher higherpolysulfides polysulfides to tolower lowerpolysulfides, polysulfides, and and eventually eventually to to lithium lithium sulfide sulfide Li Li22SS [32]. [32]. reduction TheS/CeO S/CeO22/RGO higher initial initial discharge discharge capacity capacitythan thanthe theS/RGO S/RGOelectrode electrode The /RGO electrode electrode presents presents a higher during discharge discharge atat0.1 0.1C.C.Meanwhile, Meanwhile,the theS/CeO S/CeO2 /RGO 2/RGO electrode shows shows two two higher higher discharge discharge during potential plateaus plateaus than thanthe theS/RGO S/RGO electrode. electrode. These are are all all because because CeO CeO22 decoration decoration enhances enhances potential catalyticactivity. activity. catalytic

Figure9.9.Charging/discharging Charging/dischargingcurves curvesofofthe thelithium–sulfur lithium–sulfurbatteries batterieswith withthe the S/CeO 2/RGOand and Figure (a)(a) S/CeO 2 /RGO (b)S/RGO S/RGO cathodes cathodes at at 0.1 0.1 C. C. (b)

As with the S/CeO 2 /RGO Asseen seenininFigure Figure10a, 10a,the thecycle cycleperformances performancesofofbatteries batteries with the S/CeO 2/RGO cathode cathodewere were Materials 2018, 11, x FOR PEER REVIEW 9 of 13 −1−1, corresponding to a sulfur measured under 0.1 C. The initial discharge capacity was 1054 mAh g measured under 0.1 C. The initial discharge capacity was 1054 mAh g , corresponding to a sulfur utilization of 65%. Furthermore, the S/CeO2 /RGO cathodes enhanced the cyclability of the batteries, utilization of 65%. Furthermore, the S/CeO2/RGO cathodes enhanced the cyclability of the batteries, 1 even retaining a discharge capacity of 792 mAh g−−1 after 200 cycles. On the contrary, the S/RGO retaining a discharge capacity of 792 mAh g even after 200 cycles. On the contrary, the S/RGO cathode (Figure 10b) delivered a lower discharge capacity of approximately 965 mAh g−1−1 at the same cathode (Figure 10b) delivered a lower discharge capacity of approximately 965 mAh g at the same current rate. After 200 cycles, the discharge capacity quickly decreased to 623 mAh g−1 . The coulombic current rate. After 200 cycles, the discharge capacity quickly decreased to 623 mAh g−1. The efficiency of the batteries with the S/CeO2 /RGO cathode was close to 100%, whereas the coulombic coulombic efficiency of the batteries with the S/CeO2/RGO cathode was close to 100%, whereas the efficiency of the S/RGO cathode was lower than 98%, indicating that the soluble polysulfides from the coulombic efficiency of the S/RGO cathode was lower than 98%, indicating that the soluble cathodes were largely adsorbed by the S/CeO2 /RGO materials. polysulfides from the cathodes were largely adsorbed by the S/CeO2/RGO materials.

Figure10. 10.Cycling Cycling performances and coulombic efficiencies (blue) of the lithium–sulfur Figure performances (red)(red) and coulombic efficiencies (blue) of the lithium–sulfur batteries batteries with the (a) S/CeO 2/RGO and (b) S/RGO cathodes under 0.1 C. with the (a) S/CeO /RGO and (b) S/RGO cathodes under 0.1 C. 2

Figure densities of of thethe S/CeO 2 /RGO Figure11 11shows showsthe therate ratecapability capabilityatatdifferent differentcurrent current densities S/CeO 2/RGOand andS/RGO S/RGO cathodes. As the current density increased from 0.1 C to 2 C, the discharge capacity changed steadily; cathodes. As the current density increased from 0.1 C to 2 C, the discharge capacity changed under 0.1under C, 0.50.1 C, C, 1 C, C,and for S/CeO the reversible capacities were were 1054 mAh g−1 , steadily; 0.5and C, 12 C, 2 C, for2 /RGO, S/CeO2/RGO, the reversible capacities 1054 mAh − 1 − 1 − 1 807 g , 674 , and mAh , respectively, and for S/RGO, reversible g−1, mAh 807 mAh g−1mAh , 674 gmAh g−1552 , and 552g mAh g−1, respectively, and forthe S/RGO, the capacities reversible −1 , 680 mAh −1 , 512 −1mAh g−1 and −1 ,−1respectively. Apparently, −1 −1 were 948 mAh g g 394 mAh g capacities were 948 mAh g , 680 mAh g , 512 mAh g and 394 mAh g , respectively. Apparently, the /RGOcathode cathodeatateach eachcurrent current rate were larger than those the discharge discharge capacities capacities of of the S/CeO S/CeO22/RGO rate were larger than those of

the S/RGO cathode. Moreover, when the current rate returned to 0.1 C, S/CeO2/RGO remains almost at capacity. This is ascribed to the absorbing and catalyzing effects of CeO2 particles on lithium polysulfides during the redox procedures [26].

Figure 11 shows the rate capability at different current densities of the S/CeO2/RGO and S/RGO cathodes. As the current density increased from 0.1 C to 2 C, the discharge capacity changed steadily; under 0.1 C, 0.5 C, 1 C, and 2 C, for S/CeO2/RGO, the reversible capacities were 1054 mAh g−1, 807 mAh g−1, 674 mAh g−1, and 552 mAh g−1, respectively, and for S/RGO, the reversible Materials 2018, 11, 1720 9 of 12 capacities were 948 mAh g−1, 680 mAh g−1, 512 mAh g−1 and 394 mAh g−1, respectively. Apparently, the discharge capacities of the S/CeO2/RGO cathode at each current rate were larger than those of thethe S/RGO cathode. Moreover, when the current rate returned to 0.1toC,0.1 S/CeO 2/RGO2 remains almost of S/RGO cathode. Moreover, when the current rate returned C, S/CeO /RGO remains at capacity. This isThis ascribed to the andand catalyzing effects 2 2particles almost at capacity. is ascribed to absorbing the absorbing catalyzing effectsofofCeO CeO particleson on lithium during the the redox redox procedures procedures [26]. [26]. polysulfides during

Figure 11. 11. Rate Rateperformances performancesof oflithium–sulfur lithium–sulfurbatteries batterieswith withthe the(a) (a)S/CeO S/CeO2 2/RGO /RGO and (b) (b) S/RGO S/RGO current densities. densities. cathodes at different current

As presented and S/RGO S/RGOcathodes cathodes display display two two obvious presented in in the the Figure Figure12, 12,both boththe theS/CeO S/CeO22/RGO /RGO and cathodic peaks and one anodic peak during the cathodic sweep; the peaks at 2.3 and 2.1 V are attributed to the change change of of elemental elemental sulfur sulfur into into soluble soluble lithium lithium polysulfide. polysulfide. In the subsequent anodic scan, the obvious peak Compared with the S/RGO sample, peak at at 2.4 2.4 V V corresponds correspondsto toLi Li22SS88 [34]. Compared S/RGO sample, the S/CeO /RGOsample samplehas hasaa higher higher charge/discharge charge/dischargepeak, peak, which which verifies the rapid S/CeO22/RGO rapid electron/ion electron/ion transfer and redox redox process process [35]. [35]. The Thecathode cathodepeak peakpotential potentialofofthe theS/CeO S/CeO cathode is about 2/RGO cathode is about 2.1 2 /RGO 2.1 V; this is11, slightly larger than the cathodepeak peakpotential potentialofofthe theS/RGO S/RGOcathode, cathode,which whichisisabout about 1.9 V. V; this is slightly larger than the cathode V. Materials 2018, x FOR PEER REVIEW 101.9 of 13 The relatively larger cathodic peak potential indicates that the sulfur in the cathode electrode can react relatively larger cathodic peak potential indicates that the sulfur in the cathode electrode can react with Limore ions easily more easily because of the decoration CeO2 particles, which demonstrates the with Li ions because of the decoration of CeO2ofparticles, which demonstrates the catalytic catalytic effect of effect CeO2 .of CeO2.

Figure 12. 12. Cyclic Cyclic voltammetry (CV) curves /RGOand and(b) (b) S/RGO S/RGOcathodes cathodesat ataascan scan Figure curves of of the the (a) (a) S/CeO S/CeO22/RGO rateof of 0.1 0.1 mV mV ss−1−.1 . rate

To gain gain further further insight insight into into the the reaction reaction kinetics, kinetics, the the charge charge transfer transfer resistance resistance (Rct) (Rct) of of the the To S/CeO2/RGO and S/RGO examined withwith EIS data The EIS demonstrate 2 /RGOand S/CeO S/RGO cathodes cathodeswas was examined EIS(Figure data 13). (Figure 13).data The EIS data a semicircle ina the mediumin frequency regionfrequency and a tail region with a slope the with lowerafrequency [36]. demonstrate semicircle the medium and aintail slope in region the lower As presented in Figure 13, the RCT value of the S/CeO /RGO cathode before cycling is 90, which is 2 frequency region [36]. As presented in Figure 13, the RCT value of the S/CeO2/RGO cathode before lower than of the S/RGO cathode This phenomenon indicates that the CeO in 2 particles cycling is 90,that which is lower than that of(120). the S/RGO cathode (120). This phenomenon indicates that spherical RGO can dramatically promote charge transportation during the redox reactions. Therefore, the CeO2 particles in spherical RGO can dramatically promote charge transportation during the the specific discharge capacity the rate performance of and the S/CeO cathode will be 2 /RGO redox reactions. Therefore, the and specific discharge capacity the rate performance of the considerably enhanced. S/CeO 2/RGO cathode will be considerably enhanced.

demonstrate a semicircle in the medium frequency region and a tail with a slope in the lower frequency region [36]. As presented in Figure 13, the RCT value of the S/CeO2/RGO cathode before cycling is 90, which is lower than that of the S/RGO cathode (120). This phenomenon indicates that the CeO2 particles in spherical RGO can dramatically promote charge transportation during the redox Materialsreactions. 2018, 11, 1720Therefore, the specific discharge capacity and the rate performance of 10 ofthe 12 S/CeO2/RGO cathode will be considerably enhanced.

Figure (EIS) Nyquist Nyquist plots plots of ofthe the(a) (a)S/CeO S/CeO22/RGO /RGO and Figure 13. 13. Electrochemical Electrochemical impedance spectroscopy (EIS) and (b) S/RGO electrodes. (b) S/RGO electrodes.

As shown shown in Table cathode isis compared compared with with other other As Table 1, 1, the the performance performance of of 3D 3DS/CeO S/CeO22/RGO /RGO cathode reported results. results. The The results results show show that the prepared /RGOcathode cathode has has good good cycling cycling reported prepared 3D 3D S/CeO S/CeO2/RGO performance. The discharge specific the decay rate remains at 0.25% for performance. specificcapacity capacityisisstable stableatat0.1 0.1C,C,and and the decay rate remains at 0.25% 200 cycles. for 200 cycles. Table 1. 1. Comparison Comparisonof ofthe theelectrochemical electrochemical performances performances from from previous previous reports reports and and from from our our work. work. Table Current Density Initial Initial Discharge Density Discharge (discharge) Capacity (mAh/g) (discharge)0.1 C Capacity859 (mAh/g) SnO2 @rGO/S 988 SnO2@rGO/S ZnO@S/CNT 0.1 C 0.16 A/g 859 MnO2 @HCF/S 0.5 C 890 ZnO@S/CNT MgO@S 0.16 A/g 0.2 C 988 940 710 MnO2@HCF/SFibrous rGO/S 0.5 C 0.75 A/g 890

Current Cathodes

Cathodes

3D S/CeO2 /RGO

0.1 C

1054

Discharge Capacity Capacity Discharge Reference (after n th) (mAh/g)

(after n th) (mAh/g) 671 (50) [37] 942 (70) 671 (50) [38] 662 (300) [39] 942 (70) 620 (100) [40] 541 (100) 662 (300) [41] 792 (200)

This work

Reference [37] [38] [39]

4. Conclusions The 3D S/CeO2 /RGO composite materials were successfully synthesized via spray drying. Since this is a very simple synthesis route, high-throughput commercial manufacturing can easily be achieved. When S/CeO2 /RGO composites are used for cathodes, they retain a capacity of 792 mAh g−1 , even after 200 cycles of operation, under a current density of 0.1 C. Such excellent performance makes the S/CeO2 /RGO composite a promising candidate for a low-cost, high-performance material for use in lithium–sulfur batteries. Author Contributions: Formal Analysis, G.C., Y.T. and T.T.; Investigation, G.C. and Y.T.; Writing-Original Draft Preparation, G.C. and Q.H.; Writing-Review & Editing, Y.Z., T.T. and Q.H.; Supervision, Y.Z. and Q.H.; Project Administration, Y.Z. Funding: This work was supported by the Scientific Research Foundation for Selected Overseas Chinese Scholars, Ministry of Human Resources and Social Security of China [Grant No. CG2015003002]; Program for the Outstanding Young Talents of Hebei Province. Conflicts of Interest: The authors declare no conflict of interest.

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