A separator modified by spray-dried hollow spherical cerium oxide and its application in lithium sulfur batteries

Abstract

Large-scale application of lithium sulfur batteries (LSBs) has been hindered by certain intrinsic obstacles, particularly the shuttle effect of lithium polysulfides (LiPSs) generated during the redox process. A separator modified with various materials was proposed and demonstrated as an effective method to handle these obstacles. In this study, hollow spherical cerium oxide (CeO₂) fabricated by a practical approach, spray granulation, served as the separator coating material. LSBs with this hollow spherical cerium oxide-coated separator presented excellent electrochemical performances compared with reference samples using a super P-coated separator or a routine separator. This CeO₂ layer can not only block and absorb lithium polysulfides (LiPSs), but also facilitate the transportation of electrons and ions as an upper current collector as well as a catalyst. The initial discharge capacity of this CeO₂-coated separator sample reaches 1004 mA h g⁻¹ at 1C (1675 mA h g⁻¹) and the reversible capacity is maintained at 625 mA h g⁻¹ after 500 cycles, implying an excellent capacity retention. The high rate discharge capacity at 2C is as high as 760 mA h g⁻¹, which demonstrates a promising rate performance. The elemental mappings of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as the testing results of cyclic voltammetry (CV), further confirm the function of this hollow spherical CeO₂-coated separator.

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Introduction

The increasing energy demand on powered electronic devices and electric vehicles poses great pressure on conventional lithium-ion batteries (LiBs), which are far from satisfying the requirement of high energy density, even if fully developed (387 W h kg⁻¹).¹ Accordingly, LSBs have emerged at the right moment, and are considered as one of the most promising next generation energy storage systems due to the high energy density (2600 W h kg⁻¹) and excellent theoretical capacity (1675 mA h g⁻¹). Moreover, as the cathode active material of LSBs, sulfur (S) has its own advantages, such as natural abundance, being environmental benign, economical, and having a low operating voltage (2.15 V vs. Li/Li⁺), which improve the safety.² Despite the advantages of the S cathode, its wide application is still hindered by some intrinsic obstacles. First, the insulating nature of sulfur and end discharge products (Li₂S₂/Li₂S) lead to the low utilization of the active material and poor specific capacity. Secondly, the large volume change of sulfur during the charge/discharge process reaches 80%, and gives rise to a pulverization of the cathode electrode, low coulombic efficiency as well as poor cyclability. Finally, the dissolution of the intermediate reaction products (Li₂S_X, 2 < X < 8) in liquid organic electrolyte, which induces shuttle effects, results in a great loss of the active material and inferior capacity retention during long cycles. Moreover, the lithium dendrites formed in the lithium anode cause a serious security problem.^3–6

In the context of the aforementioned electrochemical characteristics of LSBs, much effort has been contributed to modify LSBs in order to reach its high energy density and long life spans. The strategies mainly include decoration of the sulfur cathode, modification of the separator, protection of the lithium anode and adjustment of the electrolyte.^7–9 The most common approach is to impregnate sulfur into a carbon matrix, which can not only enhance the conductivity, but also ease the great volume change of sulfur and trap LiPSs by pores or tunnels.¹⁰ Various carbonaceous materials with different morphologies, such as porous carbon (PC),^11–13 carbon nanotubes (CNTs),^14–16 carbon nanofibers (CNFs),¹⁷ graphene (GN),¹⁸ and graphene oxide (GO) have been investigated.¹⁹ In addition, metallic oxides, for example MgO, Ti₄O₇, MnO₂, TiO₂, La₂O₃ and Mg_0.6Ni_0.4O, have been employed as additives in cathode and have been expected to have chemical interactions with LiPSs to catalyze sulfur reduction.^20–25 Furthermore, electrolyte and lithium anode optimization have also been investigated for the enhancement of LSB electrochemical performances.^26–28

The separator, as an essential part of LSBs, has recently been studied by several scholars intending to block the shuttle phenomenon of the dissolution of LiPSs in electrolyte and enhance the overall performances. For example, the modified cell configuration with a coating layer^29–38 on the common separator, or an interlayer^39–46 between the cathode electrode and the separator, was designed to immobilize the sulfur and polysulfides (PSs) in the cathode region and to block the PSs migration. Although the introduction of a coating layer or an interlayer can constrain the active material in the cathode region like a shield, trap PSs as a physical barrier and promote the transmission of electrons and ions as an upper current collector, it still has its own limitations because of the weak physical interaction between the carbon matrix on the membrane and the PSs.²⁹ Recently, nitrogen-doping conductive carbons have attracted much attention for their application as the coating materials for LSBs, which increase the electronic conductivity and form chemical interactions to adsorb PSs.^33,34 Furthermore, heteroatom-doped carbon coatings^50–52 and metal/metal oxide-containing coatings^53–55 have also been used as functional separators that show strong LiPSs adsorption abilities as well as electrocatalytic effects. Accordingly, we were inspired to propose hollow spherical CeO₂ as a coating material; it is considered to possess a catalytic effect for sulfur reduction, which is beneficial for a superior discharge capacity and high capacity retention even after long cycles. In this study, the hollow spherical CeO₂ was coated on the surface of a commercial separator using Ketjen Black (KB) as the conductive agent via an automatic coating machine to manufacture a multifunctional layer that could constrain the migration of PSs and serve as an upper current collector. This hybrid CeO₂/KB/PVDF coating layer can maintain a high conductivity because of the use of KB nanoparticles. In addition, STEM elemental mapping analysis demonstrated that the hollow spherical CeO₂ had a better lithium polysulfides adsorption ability than conductive carbon materials like super P and KB. Moreover, CV tests showed that hollow spherical CeO₂ also had a catalytic effect for the redox reactions. Thus, cells equipped with this modified separator showed better electrochemical performances compared with the cells equipped with a PVDF/carbon coating separator. The hollow spherical CeO₂-coated separator delivered a high initial discharge capacity of 1004 mA h g⁻¹ at 1C, excellent capacity retention of 625 mA h g⁻¹ at 1C after 500 cycles, outstanding average coulombic efficiency of up to 97% and a superior high rate discharge capacity of 760 mA h g⁻¹ at 2C.

Experimental details

Preparation and characterization of CeO₂ coated separator

The hollow spherical CeO₂ was prepared by a spray granulation method. In a typical fabrication process, 4.34 g Ce(NO₃)₃·6H₂O and 6.20 g citric acid (C₆H₈O₇) were dissolved in 150 mL deionized water. The solution was magnetically stirred until it was totally clear and then granulated by a spray dryer with a spray nozzle temperature of 250 °C. The spray dried precursor was calcined at 500 °C in air for 4 h with a heating rate of 5 °C min⁻¹ to acquire the end product. The obtained product was mixed with KB and polyvinylidene fluoride (PVDF) (Kynar, HSV900) in the weight ratio of 7 : 1 : 2 with N-methyl-pyrrolidinone (NMP) as the dispersant to obtain the coating slurry, which was then coated onto one side of the commercial separator Celgard 2400 by an automatic coating machine. The coated separator was then dried in a circulation oven at 50 °C for 12 h and in a vacuum oven at 40 °C for another 12 h. Finally, the CeO₂-coated separator was cut into discs with a diameter of 19 mm. For comparison, a super P-coated separator was synthesized with the method mentioned above, but with changing CeO₂ to super P. The coated separator and routine separator are shown in Fig. 1(a) and (b) and indicate that the thickness of the coating layer is approximately 10 μm. The cross sectional picture of the coated separator in Fig. 1(c) further proves the thicknesses. The mass of the CeO₂ coating layer was estimated to be 0.3 mg cm⁻² and the super P coating layer was controlled with the same mass loading.

Fig. 1 Thickness of (a) CeO₂ coated separator, (b) routine separator and (c) the SEM cross section of CeO₂ coated separator.

Cell assembly and characterization

KB and sublimed sulfur were mixed by high energy milling for 4 h at the weight ratio of 1 : 4, using ethyl alcohol as the lubricant, and dried at 60 °C for 12 h in a circulation oven. The uniformly dispersed mixture was heat treated at 155 °C for 12 h in a sealed reactor to obtain the KB/S composite with almost the same weight ratio before the heat treatment. The slurry of the working cathode electrode was fabricated by mixing 80 wt% KB/S mixture, 5 wt% conductive carbon (super P, Timcal) and 15 wt% PVDF in NMP solution. The cathode slurry was uniformly spread onto aluminum foil by an automatic coating machine that could control the layer thickness and coating speed accurately. The coated foil was placed in a vacuum oven at 60 °C for 15 h and then cut into discs with a diameter of 12 mm to serve as the cathode electrode. The thickness of the cathode electrode was approximately 40 μm and the total mass of the cathode material was 3 mg cm⁻²; therefore, the sulfur mass loading in the cathode electrode was estimated to be 1.92 mg cm⁻². As far as the mass loading of the coated layer is concerned, the percentage of the sulfur content in the cathode side was about 58%. Li–S coin cells (CR2025) were assembled in an argon-filled glove box using lithium foil as the anode electrode and routine/super P-coated/CeO₂-coated Celgard 2400 as the separator. The electrolyte was 1 M lithium bis(trifluoromethanesulfone) imide (LiTFSI) and 0.1 M LiNO₃ in a mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) at a volume ratio of 1 : 1. The amount of the electrolyte dropped in a single Li–S cell was approximately 70 μL. Therefore, the sulfur/electrolyte ratio was about 1.9 mg/70 μL. The schematic of the discharge process in LSBs with CeO₂ and routine separator is displayed in Fig. 2(a) and (b).

Fig. 2 Schematic of the sample cells of (a) CeO₂-coated separator cell and (b) routine separator cell.

Characterization

X-ray diffraction (XRD, D/Mmax 2500PC) with CuKα radiation (λ = 0.154178 nm) was carried out for crystal structural analysis. The morphology of the hollow spherical CeO₂ was characterized by field emission scanning electron microscopy (FE-SEM, JSM-7001F) and transmission electron microscopy (TEM). Furthermore, SEM and STEM elemental mapping analysis were employed to characterize the elemental distribution of the CeO₂-coated layer before and after cycling, respectively.

Electrochemical measurement

The sample Li/S cells were galvanostatically charged/discharged at different current densities between 1.7 and 2.8 V (vs. Li/Li⁺) using a CT2001A cell test instrument (LAND model, Wuhan RAMBO testing equipment, Co. Ltd). The cyclic voltammetry (CV) measurements were carried out at a scanning rate of 0.2 mV s⁻¹ in the potential range of 1.7–2.8 V. All the measurements were executed at 25 °C. All the current densities and specific capacities were calculated based on the mass of sulfur on the cathode electrode.

Results and discussion

The as-prepared hollow spherical CeO₂ was characterized by XRD in a flat sample stage mode with a step width of 0.02°. As the sweeping speed was 2 s per step, the measurement time was approximately 100 min in the scan range 2θ = 20–80°. The diffraction peaks of Fig. 3 around 28.5°, 33.1°, 47.5°, 56.4°, 59.1°, 69.5°, 76.8° and 79.2° marked by red hearts are well-indexed to the (111), (200), (220), (311), (222), (400), (331) and (420) planes of CeO₂, which match well with ICDD-PDF2-2004 no. 43-1002 for CeO₂.⁴⁷

Fig. 3 XRD pattern of the hollow spherical CeO₂.

The morphologies of CeO₂ are presented in Fig. 4. As shown in Fig. 4(a) and (b), the as-prepared CeO₂ presents a uniform hollow sphere structure with particle sizes of about 1 μm. The PSs generated during the charge/discharge process is supposed to be adsorbed by these hollow structures. Fig. 4(c) shows the plane view of a routine separator, which exhibits evenly distributed submicron pore structures, guarantying fast ion transportation for redox reactions during the charge/discharge process.⁴⁸Fig. 4(d) presents the surface morphology of the super P-coated separator. Carbon nanoparticles are uniformly stacked together, forming a nano-carbon sponge on the routine separator that serves as an effective physical barrier to prevent the diffusion of LiPSs and promote the transportation of electrons and ions. Fig. 4(e) exhibits the hollow CeO₂-coated separator, conductive KB nanoparticles and CeO₂ hollow spheres stacked together homogeneously. The KB nanoparticles ensure the conductivity of the coated layer and CeO₂ hollow spheres are supposed to be more efficient at absorbing LiPSs. These conclusions are well supported by the SEM images and the corresponding elemental mapping of the CeO₂-modified separator before and after cycling.

Fig. 4 SEM photographs of (a) and (b) the hollow CeO₂; (c) routine separator; (d) super P-coated separator; (e) and CeO₂-coated separator.

Fig. 5(a) and (b) illustrate the elemental mapping of the CeO₂-coated separator before cycling and after 200 cycles at the current rate of 1C. The acceleration voltages used in the microscopic measurements and elemental mapping tests were 3 kV and 20 kV, respectively. Fig. 5(a) displays no elemental S before cycling while elemental Ce and oxygen (O) are uniformly arranged in the coating layer. In Fig. 5(b), elemental S is uniformly distributed in the CeO₂ coating layer after cycling even in the low carbon distributed zone (the relatively dark area in the C map), demonstrating that the effective LiPSs adsorption ability of this layer was not merely attributed to the KB nanoparticles, but the hollow spherical CeO₂ could also block the migration of LiPSs. According to the elemental map, CeO₂ was distributed homogeneously in all the areas of the coated layer and its content was more than the KB nanoparticles, as judged by the brightness of C and Ce. After cycling, the map of element sulfur was distributed in the way of cerium and was somehow much brighter than the C map, which demonstrated that the hollow spherical CeO₂ was considerably effective at absorbing the LiPSs.

Fig. 5 SEM photographs and elemental map of CeO₂-coated separator (a) before cycling and (b) after cycling.

STEM elemental mappings of the coated hollow CeO₂ spheres were also conducted in order to further demonstrate its excellent LiPSs absorption ability. The acceleration voltage used for TEM images and STEM elemental mapping were 200 kV. Fig. 6(a) shows the TEM image of a single CeO₂ sphere, which demonstrates a hollow structure. The image is a little dark because the CeO₂ sphere was in the micron size and the electrons were difficult to transmit through the sample, but the white area in the middle of the CeO₂ sphere proves the hollow structure. Fig. 6(b) is the HRTEM of the CeO₂ shell, the lattice fringe of approximately 0.3 nm is in good agreement with the (111) plane of CeO₂. Fig. 6(c) displays the STEM elemental mapping of a little part of a single CeO₂ sphere and some KB particles as the separator coating material after 200 cycles at a current rate of 1C. Interestingly, the map of elemental S was distributed more concentrated in the area of elemental Ce than that of elemental C. This indicates that the hollow CeO₂ spheres exert stronger LiPSs adsorption abilities than the carbon nanoparticles like KB and super P.⁴⁹

Fig. 6 (a) TEM image of a hollow CeO₂ sphere, (b) HRTEM of a CeO₂ sphere and (c) STEM elemental mapping of a CeO₂ sphere after cycling.

CV tests of the cells with a CeO₂-coated, super P-coated and routine separator were examined in the potential range of 1.7–2.8 V with a scanning rate of 0.2 mV s⁻¹. Fig. 7 displays the first CV cycle of the CeO₂-coated, super P-coated and routine separator samples, respectively. All samples presented two reduction peaks at about 2.0 V and 2.3 V in the cathodic scan, which corresponds to the conversion of elemental sulfur into soluble LiPSs and insoluble short polysulfide (Li₂S₂/Li₂S), respectively. In the anodic scan, one broad oxidation peak at about 2.4 V appears, corresponding to the oxidation of short polysulfide (Li₂S₂/Li₂S) to LiPSs and sulfur.³⁴ Clearly, the CeO₂-coated separator sample presents much sharper charge/discharge peaks, which prove the rapid electron/ion transfer process compared with the super P-coated and routine separator samples. The reduction and oxidation peak current densities and its CeO₂-covered separator cell are larger than the others, indicating a fast redox process, good stability and high specific capacity. In addition, the CeO₂-coated separator sample shows a higher reduction potential than the others, demonstrating the catalytic effect of CeO₂ on the redox process. This conclusion is similar to some recent studies on metal oxide nanoparticles showing a catalytic effect on the sulfur redox reaction; for example Ti₄O₇, La₂O₃ and Mg_0.6Ni_0.4O, etc.^21,24,25 Moreover, a smaller voltage gap (ΔE) between oxidation and reduction peaks indicates that the CeO₂-coated separator cell has better electrochemical reversibility considering that ΔE is determined by the polarization of the active material during the charge/discharge process. These observations allow us to suggest that the addition of CeO₂ leads to the enhanced electrochemical kinetics of the sulfur redox process, resulting in the reduced polarization of the electronic process on the cathode.

Fig. 7 CV curves of the cells with conventional separator, super P-coated separator and CeO₂-coated separator at a scan rate of 0.2 mV s⁻¹.

To further illustrate the enhanced electrochemical properties of the CeO₂ separator sample, cycle and rate performance tests were executed at the current density of 1675 mA h g⁻¹ (1C) between 1.7 and 2.8 V. Fig. 8(a) shows the rate capabilities of the three samples. In each cycle of the rate capability tests, the Li–S cell stood for 5 min in an Open Circuit Potential (OCP) state. This demonstrates that the CeO₂-coated separator leads to significant enhancements in the electrochemical stability because of the high discharge capacity and stable cycle stability, reaching discharge capacities of 1300, 1125, 970, 860 and 760 mA h g⁻¹ at 0.1, 0.2, 0.5, 1, and 2C current densities, correspondingly. Even when the current density returns to 0.5C after 20 cycles at 2C, the reversible capacity is still maintained at 960 mA h g⁻¹, almost the same capacity (970 mA h g⁻¹) of the 0.5C before 2C cycling, implying a superior rate capability due to the LiPSs blocking effect, promotion of electrons and ions transportation as well as the catalytic effect of the hollow CeO₂ coating barrier during the charge/discharge process. The highly reversible discharge capacity over a wide range of cycling rates from 0.1C to 2C indicates the stable cyclability and excellent rate capability of the CeO₂-coated separator cell to a certain extent.²⁹ In comparison, the cells with super P-coated and routine separator have relatively low reversible capacities of 650 and 410 mA h g⁻¹ at 2C rate. In addition, when the current rate came back to 0.5C, the discharge capacities of the contrast sample could not return to a similar value like the CeO₂-coated separator sample.

Fig. 8 (a) Rate capability and (b) cyclability at 1C of routine, super P = coated and CeO₂-coated separator samples.

Long-term cycling was also tested to prove the high electrochemical reversibility of the cell with the CeO₂-coated separator. As shown in Fig. 8(b), the capacities of all cells decrease gradually with increasing cycle number. However, the CeO₂ separator sample shows evident advantages over that of the super P-coated and routine separator cells in the initial capacity, coulombic efficiency (calculated by the formula: discharge capacity/charge capacity × 100%) and capacity retention in general. The CeO₂-coated separator sample delivers an excellent initial discharge capacity of 1004 mA h g⁻¹ at 1C and maintains at 625 mA h g⁻¹ after 500 cycles with 0.075% capacity degradation per cycle. In contrast, the initial discharge capacity of the super P and pristine separator sample are approximately 910 and 710 mA h g⁻¹ at 1C and drop to 470 and 260 mA h g⁻¹ after 500 cycles, correspondingly. Apparently, the super P-coated and pristine separator cells show much more serious capacity decay, with 0.10% and 0.13% capacity decreases per cycle. Consequently, we suggest that this CeO₂-coated layer can effectively suppress the shuttle effect of LiPSs, which can lead to great capacity degradation and low coulombic efficiency during long term cycling; this is further evidence to the advantages of the CeO₂-coated separator. In order to better understand the superiority of the CeO₂-coated separator sample, the details of the three separator samples are listed in Table 1.

Table 1 Key data comparison of the three samples

Sample	CeO2 separator	Super P separator	Routine separator
Coating layer mass (mg cm^−2)	0.3	0.3	0
Coating layer thickness (μm)	10	15	0
Sulfur loading (mg cm^−2)	1.92	1.92	1.92
Sulfur content	58%	58%	64%
Initial discharge capacity/1C (mA h g^−1)	1004	910	710
Cycle number	500	500	500
Reversible discharge capacity (mA h g^−1)	625	470	260
Degradation rate per cycle (%)	0.075	0.1	0.13

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Conclusions

In conclusion, the CeO₂-coated separator, which combines hollow spherical CeO₂ easily fabricated through spray granulation, with conductive KB nanoparticles, is an accessible and effective way to modify the separator for high performance LSBs. The light weight and thin CeO₂-coated separator shows great superiority in both long term cycling stability and rate capability due to the CeO₂ functioning as a physical barrier and a chemical catalyst for the sulfur redox reaction. The small amount of conductive KB nanoparticles can also facilitate the electron and ion transportation, working as an upper current collector and constrain LiPSs to a certain extent. The initial discharge capacity reached 1004 mA h g⁻¹ at 1C and the reversible capacity was maintained at 625 mA h g⁻¹ after 500 cycles, implying superb capacity retention. In addition, the CV results are effective evidence for demonstrating the dynamic and static stability of this CeO₂-coated separator cell. More efforts are needed for the investigation of separator modification and rare-earth oxides to promote the development of LSB commercialization.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Grant No. 21401081, 51274106, 51474113, 51474037), the Natural Science Foundation of Jiangsu Provincial Higher Education of China (Grant No. 16KJB430007), open fund of the Laboratory of Solid State Microstructures, Nanjing University (M28035), China Postdoctoral Science Foundation (No. 2014M560397), Jiangsu Postdoctoral Science Foundation (No. 1401051C and 1402196C) and the Start-up Fund of Jiangsu University (Grant No. 14JDG060, 14JDG058).

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