1 Introduction

With the soaring of electronic applications in various fields, there is tremendous need to develop new energy storage equipments with high energy density, high specific capacity and long service life [1, 2]. In recent years, Li–S batteries are deemed to be one of the most promising candidates owning to their high theoretical specific capacity (1675 mAh g−1) and energy density (2500 kW kg−1) [3, 4]. However, the commercialization of Li–S batteries is hindered by the insulating property of sulfur and lithium sulfides, the shuttle effect arising from the high solubility of polysulfides Li2Sx (2 < x ≤ 8) in electrolytes, and volumetric changes during charging/discharging [5, 6]. To solve these problems, tremendous effort has been done on the design of nanostructured materials to serve as sulfur host.

High conductivity, appropriate porosity and high mechanical strength are essential for nanostructured sulfur host materials [7]. Various carbon materials, for instance microporous carbon [8], mesoporous carbon [9], hierarchical porous materials [10], carbon nanotubes [11, 12] and carbon fibers [13], have been used as host materials of sulfur in Li–S batteries. However, the weak interaction between polar lithium polysulfides and non-polar porous carbon materials could still results in severe shuttle effect, e.g., the irreversible loss of lithium polysulfides from electrodes and therefore significant decay of Li–S battery over long-term cycling [14]. Typically, metal oxide has a very polar surface. Compared to porous carbon materials, metal oxide has the advantage of having more polar sites to adsorb lithium polysulfides. Cui et al. [18]combined theoretical calculations with adsorption experiments to demonstrate the existence of a single layer of chemisorption between metal oxide and lithium polysulfides. The adsorption strength of polysulfides by various metal oxides was discussed. The results showed that MgO, CeO2 and La2O3 exhibited enhanced electrochemical performance. Due to the stronger adsorption of metal oxide for polysulfides, loading metal oxide (such as Al2O3 [15], MnO2 [16], TiO2 [17]) onto nanostructured carbon host could not only mitigate the shuttle effect efficiently, but also offset the disadvantages of poor electrical conductivity of metal oxides.

Metal oxide loading onto nanoporous carbon materials can be conducted through hydrothermal/solvothermal process [19, 20], precipitation [21], pyrolysis method [22], and incipient wetness impregnation [23], etc. However, it is still challenging to achieve uniform loading of metal oxides within complicated nanoporous structure as well as more intimate interaction of metal oxide with carbon substrate [24]. Recently, as a unique solvent with hybrid properties of “gas-like” and “liquid-like”, supercritical carbon dioxide (scCO2) has received great attention [25] in materials synthesis. ScCO2 (Tc = 31.1 °C, Pc = 7.38 MPa) exhibits high diffusivity, low viscosity, zero surface tension and powerful wettability [24, 26], therefore allowing for highly dispersed deposition of metal oxides into nanoporous structure of carbon with more intimate interaction in between, favoring improved utilization of active material in Li–S batteries. The same advantages could also be achieved if scCO2 is used as solvent to impregnate sulfur into porous carbon host materials.

To alleviate the shuttle effect of Li–S batteries, herein a nanosheet-like porous carbon (NSPC) was synthesized with corn flour as carbon source, and CeO2 and sulfur were loaded on NSPC stepwisely using scCO2-assited deposition to obtain Li–S batteries cathode material, CeO2 decorated porous carbon/sulfur (C/CeO2/S) composites. The preparation of C/CeO2/S was shown in Scheme 1. CeO2 could provide a strong absorption to polysulfides as well as excellent electrocatalytic activity, facilitating the redox reactions of polysulfides during charging/discharging. NSPC would not only provide porous structure for sufficient accommodation and effective confinement of sulfur, ample channels for lithium ion transportation, but also improve the electrical conductivity of CeO2. Moreover, the scCO2-assisted deposition is beneficial for more highly dispersed loading of CeO2 and sulfur, which might improve the utilization efficiency of active materials. Compared with the traditional methods, the preparation of Li–S batteries cathode material by scCO2 deposition is beneficial for the sulfur nanoparticles to enter the porous channels with high aspect ratio and achieve high dispersion. ScCO2 deposition is also advantageous for improving the dispersibility of CeO2 on NSPC with complicate surface features and controlling the size of CeO2 nanoparticles, which is difficult to achieve by conventional methods. To the best of our knowledge, this is the first time that metal oxides deposited by scCO2-assisted process have been utilized in Li–S batteries, with the potential to enable improved cycling performance and rate performance.

Scheme 1
scheme 1

Schematic illustration of the synthesis process for C/CeO2/S

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2 Experiment

2.1 Preparation of NSPC

NSPC was synthesized using corn flour and KOH. Typically, corn flour was pre-carbonized by heating in N2 atmosphere at 450 °C for 3 h. Afterwards 2 g of pre-carbonized solid was mixed with 40 ml of 5 mol/L KOH aqueous solution, and the mixture was dried at 110 °C for 8 h to evaporate water. The resultant solid was chemically activated in a tubular furnace under nitrogen atmosphere at 800 °C for 2 h. The obtained powder was washed with dilute HCl solution and then deionized water until the eluate reached a pH value of 7. The powder was finally dried at 100 °C for 12 h in a vacuum oven.

2.2 Preparation of C/CeO2

CeO2 was loaded onto NSPC through scCO2-assisted deposition. In a 50 ml high-pressure reactor, 0.1 g of Ce(NO3)3·6H2O, 4 ml of ethanol and 0.2 g of NSPC was mixed to form a slurry and sealed. The reactor was then charged with 6.0 MPa of CO2 and heated at 150 °C for 2 h. Subsequently the reactor was cooled down to ambient temperature and depressurized slowly. The powder retained in the reactor was collected and calcinatedin flowing N2 at 600 °C for 2 h. The solid after calcination was labeled as C/CeO2. For comparison, CeO2 was loaded onto NSPC using pyrolysis method,named C/CeO2-p.

2.3 Preparation of C/CeO2/S

C/CeO2/S was prepared according to a similar process of preparing C/CeO2. 0.2 g of C/CeO2 and 0.4 g of sulfur were milled for 1.5 h and then transferred to a 50 ml high-pressure reactor. The reactor was then filled with 6.0 MPa of CO2, heated at 100 °C for 3 h with agitation applied, and discharged at room temperature to harvest black powder, which was further treated at 250 °C for 1 h under N2 atmosphere to remove sulfur on the external surface. The product was denoted as C/CeO2/S. For comparison, sulfur was also loaded onto NSPC directly using the identical process, giving rise to composite material C/S.

2.4 Electrochemical measurements

To compare electrochemical performance of C/S and C/CeO2/S as cathode materials of Li–S battery, CR2025 type cells were fabricated and tested. The cathode slurry was constituted by dispersing 10 wt% of acetylene black, 80 wt% of active materials and 10 wt% of PVDF in NMP solvent. The mixed slurry was pasted on aluminum foil and then dried in vacuum at 90 °C for overnight. Sulfur loading is approximately 2.8 mg cm−2. Lithium metal and Celgard 2400 polyethylene were served as counter electrode and separator, respectively. The electrolyte consisted of 1.0 M lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) dissolved in 1,3-dioxolane and 1,2-dimethoxyethane (1:1 by volume) solution with 1 wt% LiNO3 as additive. The amount of electrolyte used to assemble a cell is 20 μL. Fresh coin cells were assembled in glove box under argon atmosphere. The galvanostatic discharge/charge tests were carried out with Battery Test System 7.5.× (NEWARE, China) between 1.7 and 2.8 V versus Li+/Li0. Cyclic Voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements was performed on CHI660E (Shanghai Chenhua, China). EIS were measured in the frequency range of 100 m HZ–100 k HZ.

3 Results and discussion

The morphology and structural feature of NSPC were imaged by SEM and TEM. As shown in Fig. 1a and S1a, NSPC displays an irregular nanosheet-like structure which facilitates rapid ion diffusion and electron transfer [27, 28]. High-resolution TEM image (HRTEM) in Fig. 1b reveals exhibits the existence of numerous micopores in NSPC.

Fig. 1
figure 1

Morphological and structural characterizations of NSPC and C/CeO2/S. a SEM images of NSPC. b–d TEM images of NSPC and C/CeO2/S. e–g EDS ma**s (C, S and Ce) of C/CeO2/S

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The TEM images of C/CeO2 are shown in Fig. 1c, d. CeO2 has been impregnated into NSPC as highly dispersed nanoparticles with approximate size of 3 nm. But for C/CeO2-p, on the external surface of NSPC, there exist some obviously agglomerated CeO2 nanoparticles with approximate size of 5 nm as shown in Fig. S1c and S1d. This indicates that supercritical CO2 facilitate highly dispersed loading of CeO2 on NSPC with less agglomeration than pyrolysis method. HRTEM images (inset in Fig. 1c and S1d) of the nanoparticles reveals a main interplanar lattice distance of 0.31 nm, which is match well with the (111) plane of CeO2, implying the highly crystalline nature of the loaded CeO2. As Fig.S1b indicates, the TEM image of C/S make clear that sulfur exists in a highly dispersed amorphous state after supercritical CO2 treatment. Furthermore, as shown in Fig. 2e–g, the EDS map** images of C/CeO2/S demonstrate the uniform dispersion of C, S and Ce in the material.

Fig. 2
figure 2

a N2 adsorption–desorption isotherms, and b pore size distributions of NSPC and C/CeO2/S, respectively. c XRD patterns of S, NSPC, C/CeO2, C/S and C/CeO2/S. d Raman spectra of NSPC and C/CeO2/S

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The specific surface area and porous characteristics of NSPC and C/CeO2/S were illustrated by nitrogen adsorption/desorption isotherms (Fig. 2a) and pore size distribution curves (Fig. 2b). NSPC displays a hybrid isotherm of type-I and type-IV according to the IUPAC, and causes steep uptakes at low relative pressure range (P/P0 < 0.1),indicating a huge amount of micropores [29] which are beneficial to confine polysulfide dissolution and relocation [30]. In addition, the appearance of very narrow hysteresis loop within the P/P0 range of 0.5 and 0.9 (type H4) reveals the existence of mesopores [31] which are facilitate the access of electrolyte [32]. BET specific surface area, which is as high as 2329.4 m2 g−1 for NSPC, dramatically decreases to 18.5 m2 g−1 after CeO2 and sulfur loading, implying that CeO2 and sulfur have been successfully incorporated into NSPC, which has been further demonstrated by the significant difference of pore size distribution profiles between NSPC and C/CeO2/S. In addition, the sulfur content in C/CeO2/S and C/S was measured through TGA method. As shown in Fig. S2, the TG curves indicate that the sulfur content in C/CeO2/S and C/S is 54 wt% and 59 wt%, respectively.

XRD patterns of S, NSPC, C/CeO2, C/S and C/CeO2/S materials are shown in Fig. 2c. For NSPC and C/S, a broad diffraction peak can be observed in the 20°–25° range, which is caused by the (002) plane of graphite [33]. However, this peak is covered by strong diffraction of CeO2 in C/CeO2 and C/CeO2/S. Furthermore, different from the crystalline state of sulfur at normal temperature, no diffraction peak of S could be found in both C/S and C/CeO2/S samples, indicating that sulfur exists in a highly dispersed amorphous state, which could improve the utilization of sulfur in the process of charging/discharging [34]. In addition, the XRD pattern also reveals that the sharp diffraction peaks of C/CeO2 and C/CeO2/S are in coincide with CeO2 compound (JCPDS no. 34-0394).

The carbonization degree and structural features of NSPC and C/CeO2/S were investigated by Raman spectroscopy. As shown in Fig. 2d, the D peak at 1347 cm−1 and G peak at 1595 cm−1 could be related to the disordered structures and graphite in-plane vibrations of carbon materials [27]. The ID/IG ratio of NSPC is 0.95, indicating the partially graphitic nature with a relatively good electronic conductivity [35]. In addition, the Raman spectrum of C/CeO2/S also shows a peak at ∼ 461 cm−1, which is caused by the F2g vibration of cerium oxide [36].

The surface chemical state of C/CeO2/S was further confirmed by XPS analysis. In the full scan spectrum of Fig. 3a, the peaks in 228.4, 163.9, 284.6 and 531.9 eV can be assigned to S2s, S2p, C1s and O1s, respectively. From the high-resolution XPS spectrum of Ce3d (Fig. 3b), six peaks collected from CeO2 are visible [37]. Furthermore, as displayed in Fig. 3c, four different peaks centering at 284.6, 285.1, 286 and 286.7 eV are consistent with C–C/C=C, C=O, C–O/C–S and O–C=O, respectively. In addition, the state of sulfur could be identified through peak fitting of S2p spectrum in Fig. 3d. The S2p3/2 (163.9 and 164.3 eV) and S2p1/2 (165.1 and 165.5 eV) peaks could be attributed to S–S and S–O bonds [38].

Fig. 3
figure 3

a XPS spectra of C/CeO2/S composites. b Ce3d, c C1s and d S2p XPS spectra

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The charge/discharge mechanism of prepared C/CeO2/S and C/S materials in Li–S battery could be identified through cyclic voltammetry (CV). The CV curves of C/CeO2/S and C/S cathodes with a potential window of 1.5–3.0 V at a scan rate of 0.1 mV/s are shown in Fig. S3a and S3b, respectively. The two cathodic peaks at 2.31 V and 2.08 V are respectively correspond to the conversion from S8 to soluble long-chainpolysulfides (Li2Sx, 4 ≤ x < 8) and further reduction to low order Li2S and Li2S2 [9]. One anodic peak near 2.46 V which is associated with the oxidation from Li2S and Li2S2 to Li2S8 [

Fig. 4
figure 4

a The cycling performance of C/S and C/CeO2/S at 0.1 C rate for 100 cycles. b The cycling performance of C/CeO2/S at 0.5 C rate for 300 cycles

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To further explore the cycling stability of the C/CeO2/S, the long-term cycling was conducted at 0.5 C (Fig. 4b). After 300 cycles, the specific capacity gradually decreased to 729 mAh g−1, corresponding to high capacity retention of approximately 70%. Upon further increasing of cycling rate to 2.0 C and 5.0 C, the C/CeO2/S still exhibits stable capacity and long cycle life (Fig. S4). The cycling performance of C/CeO2/S at 2.0 C is presented in Fig. S4a. The initial discharge capacity reaches 730 mAh g−1, and decreases to 561 mAh g−1 after 400 cycles, accompanied with 0.058% capacity decay per cycle. The C/CeO2/S cathode delivers stable cycling performance with the lower discharge capacities of 407 mAh g−1 at 5.0 C after 300 cycles with 74% capacity retention (Fig. S4b). This suggests that C/CeO2/S as the cathode of Li–S battery could effectively mitigate shuttle effect. In addition, unlike most studies that conduct electrochemical tests in calorstats, this study was conducted in a real-world environment. The effects of day and night temperature differences lead to instability of Coulomb efficiency were inevitable.

Rate performances of the C/CeO2/S and the C/S cathodes were compared under various current densities (Fig. 5a). The C/CeO2/S cathode delivers a specific discharge capacity of 1404.6, 1076.1, 932.2 and 782.4 mAh g−1 at 0.1, 0.2, 0.5 and 1 C, respectively, higher than that of C/S cathode. When the current density returns to 0.1 C, the discharge capacities of the C/CeO2/S can be recovered well to 1092.4 mAh g−1. However, the C/S only retains 706.9 mAh g−1of discharge capacity after multi-rate tests. Such remarkable improvement in the rate capability of the C/CeO2/S indicates the importance of CeO2 in adsorbing polysulfide and catalyzing redox reaction.

Fig. 5
figure 5

a The rate performances of two electrodes at different current densities. b Nyquist plots of C/S and C/CeO2/S cathodes before cycling

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EIS measurement was employed to further reveal the improved electrochemical performance of C/CeO2/S. Figure 5b exhibits the Nyquist plots of C/CeO2/S and C/S cathodes before cycling and after 100 cycles. The Nyquist plots consist of one depressed semicircle in the high-frequency region and almost vertical-line in the low-frequency region. The depressed semicircle represents the charge transfer resistance (Rct) [9]. The line in the low-frequency range correlates to the Warburg impedance, associated with Li ion diffusion into the active mass. For the C/CeO2/S and C/S composites, the charge transfer resistances before cycling are 40.5 and 60.5 Ω, respectively, indicating a higher electrochemical activity of C/CeO2/S composite material, which could be ascribed to that the CeO2 can promote the charge transportation during the redox reactions [39]. As Fig. S5 indicates, after 100 cycles, the Rct of C/CeO2/S and C/S are 213.3 and 578.6 Ω, respectively, suggesting that the C/CeO2/S delivers a better electrochemical contact and much lower polarization.