Lithium Sulfide–Carbon Composites via Aerosol Spray Pyrolysis as Cathode Materials for Lithium–Sulfur Batteries

Introduction

Lithium-sulfur (Li-S) batteries are regarded as one of the most promising electrochemical energy storage technologies due to their low cost, environmental benignity, and outstanding theoretical capacity (Wang et al., 2013; Son et al., 2015). However, despite tremendous research and development efforts, there are still a number of challenges hindering their commercialization. Among these key challenges are the polysulfides shuttle effect and high electrolyte/sulfur ratio, which are significantly magnified by the instability of the Li metal anode (Chen J. et al., 2017; Chen S. et al., 2017; Pan et al., 2018; Wu et al., 2018). Therefore, high capacity non-Li anodes, particularly those comprised of silicon-based materials, have been proposed as replacements for Li metal in Li-S batteries (Yang et al., 2010). The use of silicon anode materials would require a pre-lithiated sulfur cathode, i.e., lithium sulfide (Li₂S). In recent years, various methods to synthesize Li₂S-carbon composite materials have been reported, including high-energy mixing Li₂S with carbon (Cai et al., 2012; Jha et al., 2015), chemical lithiation of S-C composites (Hwa et al., 2015), Li₂S-C composites synthesis via dissolving and precipitating Li₂S in ethanol (Wu et al., 2014a,b,c, 2015, 2016), embedding Li₂S in carbon matrix via Li-nitrogen interaction (Guo et al., 2013), reaction between Li metal and carbon disulfide (Tan et al., 2017), converting LiOH to Li₂S via sulfurization with H₂S (Dressel et al., 2016), and thermal reduction of Li₂SO₄ by carbon (Yang et al., 2013; Kohl et al., 2015; Li et al., 2015; Yu et al., 2017; Zhang et al., 2017; Ye et al., 2018). In addition, the mechanism studies on Li₂S activation and capacity degradation were also reported (Vizintin et al., 2017; Piwko et al., 2018). In this work, we report a new scalable method for synthesizing Li₂S-C composites via aerosol spray pyrolysis (ASP) followed by sulfurization.

Materials and Methods

Materials Synthesis

Three lithium salts including lithium nitrate (LiNO₃), lithium acetate (CH₃COOLi) and lithium carbonate (Li₂CO₃) were used as the precursors for Li₂S with sucrose as the precursor for carbon. Each Li salt was dissolved in deionized water with sucrose at different concentrations as listed in Table S1. The obtained solutions were used in the ASP process.

The ASP system in this study is illustrated in Figure S1. The commercial aerosol generator (TSI, Model 3076) consisting of a nebulizer and a solution reservoir is attached to a diffusion dryer followed by a tubular furnace and a filter collector. The diffusion dryer was composed by two concentric tubes: The outer tube is made of 3-inch inner diameter PVC tubing and the inner tube is made of 0.5-inch diameter steel mesh with the annular space filled with porous silica gel. The aerosol of the precursor solution was generated by the nebulizer and carried through the diffusion dryer by argon gas to desiccate the water content. The resultant dry particles were continuously carried into the tube furnace heated at 850°C to produce the Li₂CO₃-C nanoparticles, which are collected with a stainless-steel filter down stream outside the tube furnace.

The synthesized Li₂CO₃-C composite is placed in an alumina boat in a tubular furnace, followed by purging with argon for an hour. The furnace was then heated to 725°C and maintained at this temperature for 5 h under a flow of 5 vol.% H₂S and 95 vol.% argon. After 5 h the flow gas was switched to pure argon and the furnace was cooled naturally to room temperature. The product was collected in an argon-filled glovebox due to the sensitivity of Li₂S to moisture.

Materials Characterization

The nitrogen adsorption-desorption isotherms of the produced composite materials were obtained with a surface area and porosity analyzer (Micromeritics ASAP2020). For a particular analysis, approximately 200 mg sample was first degassed at 150°C for 3 h, then the nitrogen adsorption-desorption isotherms were measured from 0 to 1 relative pressure. The surface area was obtained with the Brunauer-Emmett-Teller (BET) method. The crystalline species in the composites were characterized by powder X-Ray diffraction (XRD, PANalytical) with a CuKα source and a scan rate of 0.11° s⁻¹. Kapton tape was used to seal the Li₂S-C composites to protect Li₂S from reacting with the moisture in ambient environment during measurement. The morphology and microstructure of the composites were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM, Tecnai T12). Carbon content in the Li₂CO₃-C composites was measured with thermogravimetric analysis (TGA, TA Instruments). The TGA samples were held at 120°C for 30 min to remove the moisture absorbed from environment, followed by heating to 600°C at a rate of 10°C min⁻¹ with an isothermal step in dry air. The carbon contents in Li₂CO₃-C_NitS, Li₂CO₃-C_AceS and Li₂CO₃-C_CarS (Figure S2) are very consistent at 20.7, 22.8, and 21.2 wt.%, respectively. Assuming complete conversion from Li₂CO₃ to Li₂S without carbon loss, the Li₂S content in Li₂S-C_NitS, Li₂S-C_AceS and Li₂S-C_CarS can be estimated as 70.4, 67.8, and 69.8 wt.%, respectively. The accurate Li₂S content in the Li₂S-C composites is determined as follows: 100 mg Li₂S-C was thoroughly washed 4 times using 15 mL ethanol each time in the glovebox to remove Li₂S. The obtained carbon was weighed after dried at 120°C for 8 h in the glovebox. The Li₂S content is 71.3 wt.% in Li₂S-C_NitS, 69.1 wt.% in Li₂S-C_AceS, and 71.6 wt.% in Li₂S-C_CarS, which all agree very well with the estimated values.

Electrode Preparation and Cell Testing

The electrode is composed of 80 wt.% of Li₂S-C composite, 10 wt.% of carbon black additive, and 10 wt.% of polystyrene as the binder. Polystyrene was selected as the binder to avoid the use of polar solvents (both protic and aprotic), most of which dissolve Li₂S to some extent. Instead, mesitylene (Sigma-Aldrich) was used as the solvent for polystyrene in the electrode slurry. The electrodes were coated on carbon-coated aluminum current collector (MTI Corporation) in the argon-filled glovebox, with the average loading of Li₂S-C composite at 2 mg cm⁻². The electrodes were dried overnight in argon glovebox at room temperature, followed by drying at 120°C for 4 h. The dried electrodes are assembled into 2032-type coin cells with lithium foil anode (99.9%, Alfa Aesar) and Celgard^® 2,500 separator. The electrolyte used in this study is 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) solution in a mixture of 1,3-dioxolane (DOL), dimethoxyethane (DME) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) (1:3:1 by vol.) with 1.5 wt.% of LiNO₃. The electrolyte to Li₂S ratio (μL/mg) was kept at 10 in all coin cells testing. To activate the Li₂S-C electrode, the first anodic scan in the cyclic voltammetry (CV) was to 3.9 V vs. Li⁺/Li, and the anodic limit in the following scans was 2.6 V vs. Li⁺/Li. Similarly, the first charge was run at a rate of 50 mA g⁻¹ (with respect to Li₂S) to a charge cutoff of 3.5 V. Subsequent cycles are run at 117 mA g⁻¹ between 2.6 V and 1.8 V vs. Li⁺/Li.

Results and Discussion

During ASP synthesis, three aqueous solutions containing sucrose (as carbon precursor) and either lithium nitrate (LiNO₃), lithium acetate (CH₃COOLi), or lithium carbonate (Li₂CO₃), denoted as NitS, AceS, and CarS, respectively, were atomized into aerosols with a pressure-enabled atomizer. The aerosols were subsequently carried by argon gas through a diffusion dryer and a tubular furnace for pyrolysis within an inert environment. The powder X-ray diffraction (XRD) patterns in Figure 1A clearly indicate that the obtained composites from all three lithium salts are Li₂CO₃-C composite with comparable carbon content (20.7 wt.% in Li₂CO₃-C_NitS, 22.8 wt.% in Li₂CO₃-C_AceS and 21.2 wt.% in Li₂CO₃-C_CarS via thermalgravimetric analysis, Figure S2). It is worth noting that sucrose solution without the lithium salts (i.e., precursors of Li₂CO₃) completely decomposes during the same ASP without any carbon formation. This observation reveals that Li₂CO₃ serves as the nucleation sites for carbonization of sucrose in ASP (Skrabalak and Suslick, 2006). However, the formation mechanisms of Li₂CO₃ from these three Li salts are clearly different. For LiNO₃, its thermal decomposition is known to proceed according to Reaction 1: (Stern and Weise, 1969)

$\begin{array}{ll}2\text{L}\text{i}\text{N}{\text{O}}_3\to {\text{L}\text{i}}_2\text{O}+{2\text{N}\text{O}}_{\text{x}}+(2.5-\text{x}){\text{O}}_2& (1)\end{array}$

Based on the XRD evidence of Li₂CO₃ with the absence of crystalline Li₂O, it can be speculated that carbon dioxide (CO₂) released from pyrolysis of sucrose further reacts with Li₂O to generate Li₂CO₃ according to Reaction 2:

$\begin{array}{ll}{\text{L}\text{i}}_2\text{O}+\text{C}{\text{O}}_2\to {\text{L}\text{i}}_2\text{C}{\text{O}}_3& (2)\end{array}$

CH₃COOLi undergoes thermal decomposition to generate Li₂CO₃ and acetone according to Reaction 3: (Roe and Finlay, 1952)

$\begin{array}{ll}{2\text{C}\text{H}}_3\text{C}\text{O}\text{O}\text{L}\text{i}\to {\text{L}\text{i}}_2\text{C}{\text{O}}_3+{\text{C}}_3{\text{H}}_6\text{O}& (3)\end{array}$

For the CarS precursor, Li₂CO₃ undergoes precipitation during ASP without decomposition, thus becoming directly embedded into the carbon matrix formed by the carbonization of sucrose.

FIGURE 1

Figure 1. XRD patterns of (A) the Li₂CO₃-C composites obtained from ASP and (B) the Li₂S-C composites after sulfurization.

Although the obtained Li₂CO₃-C composites have consistent composition and carbon content, they have distinctively different microstructures as displayed by the transmission electron microscopy (TEM) images in Figure 2 (scanning electron microscopy images in Figure S3). The Li₂CO₃-C_NitS nanoparticles in Figure 2A have a hollow-shell structure with irregular-shaped interior voids due to the release of NO_x and O₂ gases during pyrolysis. The high solubility of LiNO₃ in water also contributes to the formation of this hollow structure. When water evaporates during ASP, LiNO₃ precipitates at the outer surface of the aerosol droplets following the surface precipitation mechanism (Messing et al., 1993). The microstructure of the Li₂CO₃-C_NitS nanoparticles is further revealed by the TEM image in Figure 2D, after the removal of Li₂CO₃ using diluted hydrochloric acid (HCl). The carbon matrix of Li₂CO₃-C_NitS has a highly porous structure after Li₂CO₃ removal, indicating that Li₂CO₃ occupies the majority of the volume in the Li₂CO₃-C_NitS nanoparticles. The specific surface area of Li₂CO₃-C_NitS before and after Li₂CO₃ removal obtained from the nitrogen adsorption-desorption isotherms (Figure 3 and Table S2) is consistent with this observation: the specific surface area of Li₂CO₃-C_NitS is significantly increased from 26.8 to 608.2 m² g⁻¹ after Li₂CO₃ removal.

FIGURE 2

Figure 2. TEM images of (a) Li₂CO₃-C_NitS, (b) Li₂CO₃-C_AceS, (c) Li₂CO₃-C_CarS; TEM images of the carbon matrix of (d) Li₂CO₃-C_NitS, (e) Li₂CO₃-C_AceS, (f) Li₂CO₃-C_CarS after Li₂CO₃ removed; and TEM images of (g) Li₂S-C_NitS, (h) Li₂S-C_AceS, (i) Li₂S-C_CarS.

FIGURE 3

Figure 3. N₂ adsorption-desorption isotherms of (A) Li₂CO₃-C nanoparticles and (B) the carbon matrix after Li₂CO₃ removal.

On the other hand, Li₂CO₃-C_AceS nanoparticles show a denser spherical structure in Figure 2B. It is worth noting that the AceS precursor solution has a significantly lower sucrose/lithium salt molar ratio at 1:15 compared to 1:1.5 in NitS and 1:1.18 in CarS. Given the 22.8 wt.% carbon content in Li₂CO₃-C_AceS, it is believed the generated acetone during the pyrolysis of CH₃COOLi must function as the major source for carbon formation. The TEM image of the carbon matrix after Li₂CO₃ removal in Figure 2E reveals the distribution of Li₂CO₃ in the Li₂CO₃-C_AceS nanoparticles is not as uniform as in Li₂CO₃-C_NitS. The carbon matrix has a golf ball-like structure with relatively large pores, previously occupied by Li₂CO₃, distributed within. The specific surface area of Li₂CO₃-C_AceS is 76.3 m² g⁻¹, which increases to 184.9 m² g⁻¹ after Li₂CO₃ removal. This modest increase of surface area also indicates the relatively larger size of Li₂CO₃ compared to that of Li₂CO₃-C_NitS.

As shown in Figure 2C, the Li₂CO₃-C_CarS nanoparticles clearly have a different structure resembling crumpled spheres, which is due to the much lower solubility of Li₂CO₃ in water than those of LiNO₃ and CH₃COOLi. The concentration of Li₂CO₃ in the CarS precursor solution is 0.1 M, which is close to saturation (Zou et al., 2013). Therefore, Li₂CO₃ undergoes fast and uniform precipitation from the aerosol droplets' evaporation in ASP according to the volume precipitation mechanism (Messing et al., 1993). In addition, the ASP of CarS precursor also releases fewer gaseous species without decomposition of Li₂CO₃. Both factors contribute to better confinement and more uniform distribution of Li₂CO₃. After Li₂CO₃ removal, the carbon matrix retains its original structure with apparently higher porosity as shown in Figure 2F. The specific surface area of Li₂CO₃-C_CarS nanoparticles is 43.7 m² g⁻¹, which increases to 443.6 m² g⁻¹ after Li₂CO₃ removal.

The Li₂CO₃-C nanoparticles obtained via ASP were subsequently reacted with mixed hydrogen sulfide and argon gas (H₂S/Ar at 5/95 vol.%) at 725°C to yield the Li₂S-C composites according to Reaction 4, confirmed by the XRD patterns shown in Figure 1B.

$\begin{array}{ll}{\text{L}\text{i}}_2\text{C}{\text{O}}_3+{\text{H}}_2\text{S}\to {\text{L}\text{i}}_2\text{S}+{\text{H}}_2\text{O}+{\text{C}\text{O}}_2& (4)\end{array}$

The TEM images of the Li₂S-C composites in Figures 2G–I (scanning electron microscopy images in Figure S4) demonstrate that these nanoparticles sustain their original structures after the conversion to Li₂S from Li₂CO₃.

Figure 4 shows the first three CV cycles of the Li₂S-C vs. Li counter/reference electrode in two-electrode cells. The cathodic peak in the first delithiation scan of Li₂S-C_NitS is centered at 3.5 V with a small shoulder at 3.4 V. The Li₂S-C_AceS composite demonstrates a broader delithiation peak at the same potential. In contrast, Li₂S-C_CarS shows two distinct cathodic peaks at 2.75 and 3.4 V vs. Li⁺/Li. The lower cathodic peak of the Li₂S-C_CarS composite at 2.75 V indicates a lower energy barrier for the delithiation reaction (Zhou et al., 2017). The Li₂S-C_CarS composite also demonstrates the highest peak current in the consecutive lithiation-delithiation scans. The superior performance of Li₂S-C_CarS may be reflective of the intimate contact of Li₂S and the carbon matrix. Figure 5 displays the representative charge-discharge curves and the cycle stability of the Li₂S-C composites. The electrolyte/Li₂S ratio is 10:1 (μL/mg), and all Li₂S-C composites are first charged to 3.5 V (activation) vs. Li⁺/Li with a current density of 50 mA g⁻¹. The charge-discharge curves demonstrate similar cycling behavior of these three Li₂S-C composites. However, Li₂S-C_AceS shows the highest charge-discharge hysteresis, which is consistent with the lowest surface area of its carbon matrix. On the other hand, although Li₂S-C_NitS shows the lowest voltage hysteresis due to the highest surface area of its carbon matrix, its capacity rapidly fades. As a composite with the balanced microstructure, Li₂S-C_CarS demonstrates the best overall performance: After 200 cycles, Li₂S-C_CarS can retain a capacity of 540 mAh g⁻¹, superior to 385 mAh g⁻¹ of Li₂S-C_NitS and 460 mAh g⁻¹ of Li₂S-C_AceS, indicating the effectiveness of the Li₂S-C_CarS composite architecture in sequestrating polysulfides. The overall performance demonstrated by Li₂S-C_CarS, in terms of areal loading, E/Li₂S ratio, overall capacity, and cycle stability, is on par with the best performance reported to date (Table S3).

FIGURE 4

Figure 4. CV scans of (A) Li₂S-C_NitS, (B) Li₂S-C_AceS, (C) Li₂S-C_CarS using 0.2 mV s^-1 scan rate.

FIGURE 5

Figure 5. Representative charge-discharge curves of (A) Li₂S-C_NitS, (B) Li₂S-C_AceS, (C) Li₂S-C_CarS at the 1st, 2nd, 10th, 50th, and 100th cycle, and (D) the cycle stability of these composites at 117 mA g^-1.

In summary, we examined a new synthetic route for the production of Li₂S-C composite materials for Li-S batteries. The combination of aerosol spray pyrolysis and sulfurization has been shown to be a robust method for the conversion of various lithium salts including nitrate, acetate, and carbonate to Li₂S-C nanocomposites using sucrose as the carbon precursor. Furthermore, the cycling performance of the Li₂S-C composite has been found to be closely correlated to its precursor-derived microstructure. The combination of Li₂CO₃ and sucrose results in the Li₂S-C composite with the best electrochemical performance, which has a non-hollow composite structure with Li₂S uniformly embedded in the carbon matrix. The detailed mechanism of aerosol spray pyrolysis and the optimization of the composite's structure and electrochemical performance will be further investigated in our future studies.

Author Contributions

NH completed most of the experiments. JS, JZ, and CF helped with the experiments and data analysis. JG designed the experiments. All authors co-wrote the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We gratefully acknowledge the financial support from Power Energy Solutions Inc.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00476/full#supplementary-material

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