CeO₂ nanodots decorated ketjen black for high performance lithium–sulfur batteries

Abstract

A CeO₂ nanodots decorated ketjen black composite was fabricated by a simple wet impregnation method and used as the host of sulfur for a lithium–sulfur battery. The microstructure and chemical components were evaluated by XRD, SEM, TEM, surface area analysis and thermogravimetric analysis. Electrochemical tests and microanalysis demonstrated that CeO₂ nanodots served as the sulfur fixation spots as well as the catalytic agent compared with the reference sample without CeO₂ nanodots. The CeO₂/KB–S cathode material with the CeO₂/KB mass ratio of approximately 15/85 shows a high initial discharge capacity of 905 mA h g⁻¹ at the current rate of 1C and remains at 710 mA h g⁻¹ after 300 cycles. Furthermore, the CeO₂/KB–S cathode shows a promising rate performance with the discharge capacity of 800 mA h g⁻¹ even at the current rate of 2C.

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Introduction

The lithium–sulfur battery is a potential candidate for the next generation of energy storage systems because of its large theoretical energy density and low cost. However, the low conductance of sulfur and volume expansion during the charging/discharging processes can induce the low utilization of active materials and fast capacity fading. Moreover, the shuttle effect induced by the soluble lithium polysulfides will lead to the loss of active material and low coulombic efficiency.^1–6

Different strategies have been attempted to overcome these problems, including the combination of sulfur with carbon materials,^7–9 metal oxides,^10,11 and conductive polymers etc.^12,13 Among these approaches, carbon materials are most studied because of their excellent conductivity, good mechanical properties and high adsorptive capability. Although the sulfur/carbon composites have been shown good physical confinement of sulfur and high conductivity, the suppress ability of lithium polysulfide still needs much improvement before they can put into practical use. In order to enhance the adsorption ability of sulfur/carbon cathode for soluble lithium polysulfide, metal oxide such as La₂O₃, Mg_0.6Ni_0.4O and Mg_0.8Cu_0.2O were used as the adsorbing additives.^14–16 Electrochemical measurements showed that these additives were good at absorbing lithium polysulfides, therefore the specific capacity of sulfur and cycling stability of the cathode electrode were significantly improved. However, the use of metal oxide additives is somehow disadvantageous for the increase of specific discharge capacity base on the total weight of sulfur cathode because the metal oxide additives will decrease the weight ratio of active sulfur in the cathode electrode. Inspired by this dilemma, metal oxide doped carbonaceous materials were investigated as the sulfur hosts instead of pure carbonaceous materials. It can not only avoid the use of metal oxide additives which will increase the total weight of sulfur cathode but can also exhibit a better polysulfide adsorption ability compared with the pure carbonaceous materials because of the embedded metal oxides. For example, sun and his coworkers reported a La₂O₃ nanodots decorated nitrogen-enriched mesoporous carbon and its application in Li–S battery. The results showed that La₂O₃ nanodots can be served as the polysulfide absorbing spots and an excellent redox reaction catalyst.¹⁷ Ding et al. fabricated a TiO₂ nanocrystal decorated graphene nanosheet and used it as the sulfur host. The TiO₂ nanocrystals can not only absorb the dissolved lithium polysulfides but also facilitate the charge transport.¹⁸

CeO₂ is an excellent kind of adsorbent and catalyst, but its application in Li–S battery has seldom been intensively studied. In this paper, we proposed a novel CeO₂ nanodots decorated ketjen black (KB) composite which was prepared by a simple wet impregnation method. Unlike the metal oxide introduced in ref. 14–16, the CeO₂ nanoparticles are embedded in the KB matrix instead of using as the additives which will decrease the energy density of cathode electrode. Electrochemical measurements and nano structure analysis indicate that CeO₂ nanodots decorated KB is more favorable for the immobilization of sulfur compared with the pure KB host. What is more, the CeO₂ nanodots also show a catalytic effect on the redox reaction of Li–S battery which result in an excellent cycling performance and rate capability. KB is a kind of commercial conductive carbon, the simple modification of CeO₂ decoration and the obvious improvement in electrochemical measurements proved its potential in practical application.

Experimental details

Fabrication of CeO₂/KB composite

CeO₂/KB composite was fabricated though a facile wet impregnation process. At first, 2 g KB was put into 200 mL ethanol. Then 0.8 g cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was dissolved in the solution and the mixture was ultrasonic stirred for 2 hours to ensure the cerium nitrate hexahydrate fully absorbed in the pores of KB. After a filtration procedure, the obtained Ce(NO₃)₃·6H₂O/KB precursor was dried in a circulation oven at 60 °C. In the end, the precursor was calcined in a tube furnace under argon atmosphere at 400 °C for 4 hours to achieve the CeO₂/KB composite (Fig. 1).

Fig. 1 Schematic diagram of the fabrication process of CeO₂/KB composite.

Assemble of Li–S battery

The as-prepared CeO₂/KB composite was served as the host of sulfur and the CeO₂/KB–S cathode materials was fabricated by a melt infiltration strategy. CeO₂/KB composite and S were mixed together at a weight ratio of 1 : 3, the mixture was firstly heat treated in a tube furnace under argon atmosphere at 155 °C for 8 hours and then increased to 200 °C for 2 hours to evaporate the sulfur covered on the surface of the CeO₂/KB composite. CeO₂/KB–S cathode electrode was fabricated by a slurry coating method using in previous literatures. It contains 80 wt% CeO₂/KB–S composite, 10 wt% super-P and 10 wt% polyvinylidene difluoride (PVDF). CR2025-type coin cells were assembled by stacking CeO₂/KB–S cathode electrode, Celgard 2400 separator and Li foil in an argon filled glove box. The electrolyte was 1 M lithium bis(trifluoromethanesulfone)imide (LIFSI) and 0.1 M LiNO₃ in mixed solvent of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) at the volume ratio of 1 : 1. KB–S cathode electrode was fabricated with the same conditions of CeO₂/KB–S cathode electrode in order to verify the function of CeO₂ nanodots embedded in KB.

Structural characterizations and electrochemical measurements

The micro structures of CeO₂/KB, CeO₂/KB–S and KB–S composites was determined by X-ray diffraction (XRD), thermal gravimetric analysis (TGA), specific surface area analyzer scanning electron microscope (SEM) and transmission electron microscopy (TEM). The cyclic voltammetry (CV) experiments and electrochemical impedance spectra (EIS) of CeO₂/KB–S and KB–S cathode electrodes were evaluated by the VMP2 electrochemical workstation. The galvanostatic charge–discharge test and rate capability were conducted by the CT2001A cell test instrument (LAND model, Wuhan RAMBO testing equipment, Co. Ltd). All the current densities and specific capacities in this paper are calculated based on the mass of sulfur.

Results and discussion

Fig. 2 illustrates the XRD patterns of S, KB, CeO₂/KB, KB–S and CeO₂/KB–S composites. The standard spectrum of sulfur (vertical blue lines, ICDD/JCPDS no. 08-0247) and CeO₂ (vertical green lines, ICDD/JCPDS no. 43-1002) are also given in the figure for comparison. The pure KB material shows two wide peaks at 26 and 44 degree which demonstrate its carbonaceous structure. The spectra of CeO₂/KB and CeO₂/KB–S composite exhibits several sharp diffraction peaks around 28.5, 33.1, 47.5, 56.3, 59.1, 69.4, 76.7, 79.1 and 88.4 degree which are well-indexed to (111), (200), (220), (311), (222), (400), (331), (420) and (420) planes of CeO₂.¹⁹ Interestingly, the spectra of KB–S shows some obvious diffraction peaks of sulfur, while the CeO₂/KB–S only display some weak peaks of sulfur. Therefore, XRD results suggest that sulfur was not fully absorbed by KB after the melt infiltration procedure. On the contrary, it was sufficiently infiltrated in to the pores of CeO₂/KB composite. This phenomenon indicates that the sulfur absorption ability of CeO₂/KB composite is better than the pure KB nanoparticles which might be attributed to the doping of CeO₂.

Fig. 2 XRD patterns of S, KB, KB–S, CeO₂/KB and CeO₂/KB–S composites.

Fig. 3(a) and (b) show the SEM images of KB and CeO₂/KB which display almost the same appearance. It indicates that CeO₂ particles was successively enwrapped by the KB particles. Fig. 3(c) and (d) are KB–S and CeO₂/KB–S composites displayed as monodispersed nanoparticles. Nevertheless, the KB–S composite appears more agglomerated than the CeO₂/KB–S composite. The relatively agglomerated image of KB–S composite may be induced by the small amount of sulfur covered on the KB particles. On the contrary, sulfur was supposed to be fully absorbed in the pores of CeO₂/KB on considering its well dispersed image of nanoparticles. Therefore, its superior absorbing ability might be related to the decoration of CeO₂ nanodots.

Fig. 3 SEM images of (a) KB, (b) CeO₂/KB, (c) KB–S and (d) CeO₂/KB–S composite.

Fig. 4(a) displays the TEM picture of CeO₂/KB–S composite which demonstrates that CeO₂ nanodots are homogeneously distributed in the pores of KB. Moreover, no obvious sulfur particles are observed in the CeO₂/KB–S composite which is similar to the SEM results. The HRTEM image of CeO₂/KB composite shown in Fig. 3(b) further proves that CeO₂ nanodots are approximately 5 nm, and the lattice fringe of about 0.3 nm is in good agreement with the (111) plane of CeO₂.¹⁹ Fig. 3(c) illustrates the elemental map of CeO₂/KB–S composite. As been discovered, the yellow dots in the sulfur map are much brighter in the area where Ce distributed which indicates more sulfur are distributed on the spots of CeO₂ nanodots. Therefore, the sulfur absorption ability of KB is increased by the decoration of CeO₂ nanodots. Consequently, more sulfur will be confined in the CeO₂/KB–S composite and the agglomerated appearance of KB–S composite will be alleviated.

Fig. 4 (a) TEM image of CeO₂/KB–S composite; (b) HRTEM image of CeO₂/KB composite and (c) STEM elemental mapping of C, Ce and S.

Thermal gravimetric analysis was carried out to investigate the composition of CeO₂/KB, KB–S and CeO₂/KB–S composites. The TG curve of CeO₂/KB shown in Fig. 5(a) was conducted in air atmosphere from 25 to 900 °C. According to the figure, the mass loss reaches 85% when the temperature increased to 900 °C which indicates the mass percentage of CeO₂ in the CeO₂/KB composite is approximately 15%. The TG curve of KB–S and CeO₂/KB–S composites shown in Fig. 5(b) were conducted in argon atmosphere from 25 to 600 °C. As illustrated in the figure, both the KB–S and CeO₂/KB–S composites show a fast mass loss at around 300 °C, the mass loss of the two samples are all about 75% when the temperature increased to 600 °C which coincide well with the preparation condition.

Fig. 5 TGA curves of (a) CeO₂/KB composite, (b) KB–S and CeO₂/KB–S composites.

Fig. 6(a) and (b) are nitrogen adsorption–desorption isotherms of KB, KB–S, CeO₂/KB and CeO₂/KB–S composites. The specific surface areas of KB and CeO₂/KB composite are estimated to be 1020 and 1180 m² g⁻¹ respectively, using the Brunauer–Emmett–Teller (BET) method. After the infiltration of sulfur, the BET specific surface areas of KB–S and CeO₂/KB–S composites decreased to 57 and 73 m² g⁻¹ correspondingly. The DFT pore size distribution curves of KB, KB–S, CeO₂/KB and CeO₂/KB–S composites shown in Fig. 6(c) and (d) indicate that after the melt diffusion treatment, the pores in the range of 2–20 nm dramatically disappeared, demonstrating that the sulfur were infiltrated into the pores of KB and CeO₂/KB composite. Additionally, we noticed that the BET specific surface area of CeO₂/KB is a little larger than that of KB which means that the absorbing ability of KB will be increased by the decoration of CeO₂ nanodots.

Fig. 6 Nitrogen adsorption–desorption isotherms of (a) KB, KB–S and (b) CeO₂/KB, CeO₂/KB–S composite; DFT pore size distribution curves of (c) KB, KB–S and (d) CeO₂/KB, CeO₂/KB–S composite.

The charging/discharging curves of KB–S and CeO₂/KB–S cathodes were conducted at different current rates from 0.1 to 2C in Fig. 7(a) and (b), respectively. As shown in the figure, both of the samples present two apparent discharging plateaus at around 2.3 V and 2.0 V, corresponding to the reduction of S₈ to long chain (Li₂S_n, 4 ≤ n ≤ 8) and short chain (Li₂S_n, 1 ≤ n ≤ 2) lithium polysulfides, respectively.^20–22 Compared with the KB–S cathode, the CeO₂/KB–S cathode delivers larger discharge capacities at each current rate, which indicates that the structure of CeO₂ nanodots decorated KB is more favourable for the utilization of active materials.

Fig. 7 Charging/discharging curves of (a) KB–S and (b) CeO₂/KB–S cathodes at different current densities.

CV measurement was conducted to learn the redox reaction of KB–S and CeO₂/KB–S cathodes at a scan rate of 0.2 mV s⁻¹ with the potential range from 1.7–2.8 V. As presented in the Fig. 8, both of the KB–S and CeO₂/KB–S cathodes display two obvious cathodic peaks and one anodic peak which matched well with the charging/discharging profiles. However, the CeO₂/KB–S sample presents sharper and higher charge/discharge peaks verifying a rapid electron/ion transfer process compared with the KB–S sample. The current densities of cathodic and anodic peaks of CeO₂/KB–S sample are apparently larger than the KB–S sample, demonstrating higher conductivity and faster reaction kinetics which in turn improve the specific capacities and rate capabilities. Besides, the cathodic peak potential of CeO₂/KB–S cathode is approximately 2.05 V, a little larger than that of the KB–S cathode which is about 2.0 V. The relatively larger cathodic peak potential indicates the sulfur in the cathode electrode can react with Li ions more easily caused by the decoration of CeO₂ nanodots which demonstrates the catalytic effect of CeO₂. This conclusion is similar to some recent studies on Ti₄O₇, MnO₂ and Mg_0.6Ni_0.4O nanoparticles which are found to have a catalytic effect on the sulfur redox reaction.^23–25

Fig. 8 CV curves of KB–S and CeO₂/KB–S cathodes at the scan rate of 0.2 mV s⁻¹.

In order to further study the superiorities of CeO₂/KB–S cathode, galvanostatic charge/discharge measurements of the KB–S and CeO₂/KB–S cathode at various current rates were executed. Fig. 9(a) describes the cycling stability of the two samples at the current rate of 1C. The CeO₂/KB–S cathode obviously displays a better cycling performance than the KB–S cathode. It owns an initial discharge capacity of approximately 905 mA h g⁻¹ at the current rate of 1C, after 300 cycles, the specific capacity gradually decreased to 710 mA h g⁻¹ which means a degradation rate of only 0.07% per cycle. On the contrary, the KB–S cathode delivers a lower discharge capacity of approximately 770 mA h g⁻¹ at the same current rate. After 300 cycles, the discharge capacity quickly decreased to 420 mA h g⁻¹ with a degradation rate of 0.15% per cycle. Apart from the superior cycling stability, the CeO₂/KB–S cathode also shows a better coulombic efficiency. After 300 cycles, the coulombic efficiency of CeO₂/KB–S cathode is still as high as 97%, while the coulombic efficiency of KB–S cathode is lower than 93%. We suppose the great enhanced cycling performance as well as the coulombic efficiency is bound up with the doping of CeO₂ nanodots. The CeO₂ nanodots played the role as an absorber and catalyzer of the lithium polysulfides which in turn alleviates the shuttle effect and optimizes the reaction kinetics. Thus, the cycling stability and coulombic efficiency will be improved as a result.

Fig. 9 (a) Cycling stability and (b) rate capability of KB–S and CeO₂/KB–S cathodes, (c) S 2p XPS spectra of KB–S and CeO₂/KB–S cathode after 200 cycles.

Fig. 9(b) displays the rate capability of the KB–S and CeO₂/KB–S cathode. After 5 cycles at the current rate of 0.1C, the KB–S and CeO₂/KB–S cathode deliver the specific discharge capacity of 1030 and 1200 mA h g⁻¹. After the cycles at the following current rates of 0.2, 0.5, 1 and 2C. The corresponding average reversible discharge capacities of the two samples are 910 and 1080 mA h g⁻¹, 830 and 990 mA h g⁻¹, 710 and 900 mA h g⁻¹, 430 and 800 mA h g⁻¹, respectively. When the current rate returns to 0.5C, the discharge capacities of the two samples after 45th cycles are 800 and 1000 mA h g⁻¹, respectively. Apparently, the discharge capacities of the CeO₂/KB–S cathode at each current rate are larger than that of the KB–S cathode. Moreover, when the current rate returns to 0.5C, it remains almost the same value as the 0.5C before 2C cycles. It contributes to the absorbing and catalyzing effects of CeO₂ nanodots to the lithium polysulfides during the redox procedures. X-ray photoelectron spectroscopy (XPS) analysis was used to confirm the absorbing ability of CeO₂ nanodots to lithium polysulfides. All peak positions for analysis were calibrated using C 1s peak at 284.6 eV. KB–S and CeO₂/KB–S cathode electrodes were tested after 200 cycles and the S 2p XPS spectra of the two samples are shown in Fig. 9(c). A Gaussian–Lorentzian fit to the S 2p spectrum was performed. Three apparent peaks at around 159.6/160.3, 161.5/162.3 and 164.7/165.6 eV are discovered, corresponding to lithium polysulfides and elemental sulfur respectively.^23,26 The S 2p XPS spectra of CeO₂/KB–S cathode after cycling obviously show higher binding energies compared with that of the KB–S cathode. The binding energies shift of approximately 0.9 eV is supposed to be induced by the strong interactions between the electronegative lithium polysulfides and electropositive cerium and/or oxygen vacancies.²⁷ Therefore, the CeO₂ nanodots embedded in the KB nanoparticles can be served as strong adsorbents of lithium polysulfides which in turn improve the electrochemical characteristics.

Electrochemical impedance spectroscopy (EIS) analysis was carried out for further understanding the improved electrochemical characteristics of CeO₂/KB–S hybrids. Fig. 10 shows the Nyquist plots of KB–S and CeO₂/KB–S cathodes before cycling and after 300 cycles. The scatters are raw data and the lines are fitting curves using the inset as the equivalent circuit of Li–S cells. All Nyquist plots consist of an intercept at high frequency region on the real axis representing the resistance of the electrolyte (R_s), a depressed semicircle at medium to high frequency region indicating the charge transfer resistance (R_ct) and an inclined line at low frequency implying the Li ion diffusion into the active mass, respectively. As listed in Table 1, the R_ct values of the CeO₂/KB–S cathode before and after cycling are 20.1 and 9.0 Ω respectively, lower than that of the KB–S cathode. This phenomenon indicates that the doping of CeO₂ nanodots in the pores of KB can dramatically promote the charge transportation during the redox reactions. Therefore, the specific discharge capacity and the rate performance of the CeO₂/KB–S cathode will be considerably enhanced.

Fig. 10 EIS curves of KB–S and CeO₂/KB–S cathodes before and after cycling, the inset is the equivalent circuit of Li–S battery.

Table 1 Impedance parameters of different samples

Sample	Before cycling		After cycling
	Rs (Ω)	Rct (Ω)	Rs (Ω)	Rct (Ω)
KB–S	3.0	34.8	3.6	24.3
CeO2/KB–S	1.9	20.1	2.5	9.0

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Conclusions

In summary, CeO₂/KB composite was fabricated via a simple wet impregnation method. TEM images demonstrate that the CeO₂ nanodots which are less than 10 nm are homogeneously distributed in the pores of KB. Electrochemical measurements and microanalysis demonstrated that CeO₂ nanodots can not only act as the absorber of lithium polysulfides but also enhance its reaction kinetics in Li–S battery. Consequently, the incorporator of CeO₂/KB–S hybrid shows better cycling and rate performances than that of the KB–S hybrid. It shows an initial discharge capacity of approximately 905 mA h g⁻¹ at the current rate of 1C and remains a reversible capacity of about 710 mA h g⁻¹ after 300 cycles with the coulombic efficiency maintains over 97%. Moreover, it displays a promising rate capability and delivers a high discharge capacity of over 800 mA h g⁻¹ at a high current density of 2C. However, other details which can influence the electrochemical performances of Li–S batteries such as the mass content of CeO₂ nanodots in the CeO₂/KB composite, size of the CeO₂ particles still need to be investigated.

Acknowledgements

This work was financially supported by the Start-up Fund of Jiangsu University (Grant No. 14JDG060, 14JDG058), open fund of the Laboratory of Solid State Microstructures, Nanjing University (M28035), the Natural Science Foundation of Jiangsu Provincial Higher Education of China (Grant No. 16KJB430007), the National Natural Science Foundation of China (Grant No. 21401081, 51274106, 51474113, 51474037), China Postdoctoral Science Foundation (No. 2014M560397), the Natural Science Foundation of Jiangsu Province (BK20130511) and Jiangsu Postdoctoral Science Foundation (No. 1401051C and 1402196C).

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