Boosting the Rate Performance of Li–S Batteries via Highly Dispersed Cobalt Nanoparticles Embedded into Nitrogen-Doped Hierarchical Porous Carbon

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE5 Aug 2022Boosting the Rate Performance of Li–S Batteries via Highly Dispersed Cobalt Nanoparticles Embedded into Nitrogen-Doped Hierarchical Porous Carbon Cheng Yuan, Pan Zeng, Chen Cheng, Tianran Yan, Genlin Liu, Wenmin Wang, Jun Hu, Xin Li, Junfa Zhu and Liang Zhang Cheng Yuan Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Pan Zeng Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Chen Cheng Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Tianran Yan Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Genlin Liu Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Wenmin Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected]u.cn Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Jun Hu National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Xin Li Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Junfa Zhu National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author and Liang Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101214 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Lithium–sulfur (Li–S) batteries are one of the most promising alternatives to lithium–ion batteries because of the advantageous high energy density and low cost. However, the practical applications of Li–S batteries are hampered by a severe shuttle effect and sluggish polysulfide redox conversion. Herein, highly dispersed cobalt nanoparticles (∼0.8 wt %) embedded into nitrogen-doped hierarchical porous carbon ([email protected]) are designed as an effective electrocatalyst for Li–S batteries, which exhibit a synergistic effect of anchoring and dual-directional catalytic conversion of polysulfides. The experimental and theoretical studies reveal that [email protected] not only provides strong chemical affinity to polysulfides but also lowers the Li+ diffusion barrier and facilitates the precipitation and decomposition of Li2S, thus effectively inhibiting the shuttle effect and promoting the reaction kinetics of polysulfides. In addition, the well-dispersed Co nanoparticles in the three-dimensional carbon matrix guarantee the exposure of abundant polysulfide confining sites and catalytically active sites. Accordingly, the Li–S batteries assembled with [email protected] functional separators harvest a high rate performance (808.4 mAh g−1 at 10 C), a long-lasting cycle stability (0.055% decay per cycle over 1000 cycles at 4 C), and a superior areal capacity retention (5.78 mAh cm−2 after 100 cycles with a high sulfur loading of 7 mg cm−2). Download figure Download PowerPoint Introduction With the advancement of personal portable electronic devices and electric vehicles, there is an ever-increasing demand for high-power and fast-charging battery systems.1,2 Lithium–sulfur (Li–S) batteries have been proposed as a promising candidate for next-generation energy storage systems because of the high energy density, nature abundance, and environmental friendliness of sulfur. Nevertheless, the practical application of Li–S batteries faces the challenges of poor conductivity, shuttled polysulfides, and sluggish reaction kinetics.3 Multifarious strategies, including design of sulfur hosts, novel electrolytes and additives, and functionalized separators, have been adopted to circumvent the above-mentioned challenges.4–11 Among them, functionalizing separators with certain materials has proved effective at inhibiting the shuttle effect and improving the electrochemical performance.12,13 Initially, various carbonaceous materials, for example, one-dimensional carbon nanotubes, two-dimensional graphene, and three-dimensional porous carbon, have been utilized as functional materials to physically confine polysulfides.14–17 However, the weak interaction between nonpolar carbon materials and polar polysulfides cannot effectively suppress the polysulfide shuttle effect, resulting in a low sulfur utilization. In contrast, polar metallic compounds, such as metal oxides, metal sulfides, and metal–organic frameworks, have been introduced into carbon materials to improve the bonding with polysulfides through the Lewis acid–base interaction.18–21 However, the slow redox kinetics of polysulfides still leads to the accumulation of polysulfides and increases the electrolyte viscosity, especially under high sulfur loading and high-current rate conditions.22 Recently, catalysis has been introduced into Li–S batteries to simultaneously capture and convert polysulfides to accelerate the reaction kinetics.15,23–25 Noble metals, such as Pt and Ir, which possess excellent catalytic activity, have been utilized to improve the electrochemical performance of Li–S batteries; however, the high cost of noble metals restricts their practical applications.25–27 In contrast, 3d transition metals have also attracted extensive attention because of their low cost and decent catalytic activity.28–31 For example, Co and Ni nanoparticles combined with carbon materials have been synthesized using zeolitic imidazolate frameworks to improve the polysulfide redox kinetics.32–35 Nevertheless, the content of metals in the synthesized materials is usually high, and the rate performance and cycling stability are still unsatisfactory.36 Additionally, most of these reports only focused on one direction sulfur reaction (mainly reduction), and the fast cycling of sulfur species has not yet been finally realized. In principle, to realize the rapid polysulfide redox conversion, the designed catalyst should simultaneously possess a strong affinity for polysulfides, fast Li+/electron diffusion rate, and high catalytic activity toward polysulfide redox conversion. Here, we endow these properties to one dual-directional catalyst, namely well dispersed Co nanoparticles embedded into nitrogen-doped hierarchical porous carbon ([email protected]) with a low Co content (∼0.8 wt %), which is synthesized using a simple one-step pyrolysis strategy.37 The Li–S batteries assembled with [email protected] functional separators show a high rate performance of 808 mAh g−1 at 10 C, a low capacity fading rate of 0.055% per cycle over 1000 cycles at 4 C, as well as a high areal capacity of 5.78 mAh cm−2 after 100 cycles with a sulfur loading of 7 mg cm−2. Combining experiments and density functional theory (DFT) calculations, we demonstrate that the multiple effects of [email protected] are responsible for the improved electrochemical performance of Li–S batteries, which can not only strongly anchor polysulfides and facilitate the Li+/electron transport but also accelerate the polysulfide redox kinetics by reducing the energy barriers for Li2S deposition and decomposition. This work presents a facile strategy to develop cost-effective catalysts toward dual-directional polysulfide redox conversion for practical Li–S batteries. Experimental Methods Preparation of [email protected] First, 1.441 g urea and 0.198 g Co(NO3)2·6H2O were completely dissolved in 7.5 mL deionized water (DIW) and stirred for 0.5 h to form solution A. Then, 0.75 g polyethylene oxide-polypropylene oxide-polyethylene (P123) was completely dissolved in 7.5 mL DIW, 1 mL commercial acidic silica was added, and then the solution was stirred for 0.5 h to form solution B. After that, solution B was added to solution A and stirred for 3 h at room temperature. The mixture was gradually evaporated at 80 °C. The evaporated precursor was fully ground and moved into a porcelain boat under high-temperature pyrolysis. The precursor was heated to 500 °C for 2 h, and then heated to 900 °C for 2 h under N2 atmosphere. After cooling to room temperature, the obtained black powder was etched with 5% hydrofluoric acid (HF) (v/v in water) solution. The [email protected] was obtained by vacuum filtration and dried at 60 °C in a vacuum oven. As contrast samples, N-HPC was prepared without Co source by the same procedure, and N-C was obtained without Co source and silica sol. Polysulfide absorption experiments Sulfur and lithium sulfide (Li2S) in a 5:1 molar ratio were dissolved in 1,2-dimethoxyethane (DME) solvent with stirring at 60 °C for 48 h to produce Li2S6 solution. The concentration of Li2S6 solution was diluted to 2 mmol L−1 (mM). Then 10 mg of [email protected], N-HPC, and N-C powders were placed in 2 mL Li2S6 solution, respectively. After the solution was left standing for a designated time, the supernatant was absorbed for UV–vis spectroscopy, and the adsorbed powder was dried for X-ray photoelectron spectroscopy (XPS). Evaluation of polysulfide conversion kinetics [email protected], N-HPC, and N-C were mixed with polyvinylidene fluoride (PVDF) in N-methylpyrrolidinone (NMP) solvent with a mass ratio of 4:1, respectively. The mixed material was coated on the carbon coated aluminum foil, and then dried under vacuum at 60 °C for 12 h. The obtained electrodes were cut into disks with a diameter of 12 mm. Li2S6 (0.2 M) solution containing 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 2.0 wt % LiNO3 in DME and 1,3-dioxolane (DOL) (1:1, v/v) solvent (conversional electrolyte) was used as electrolyte. Two identical electrodes were used to assemble a symmetrical battery with 30 μL electrolyte. Cyclic voltammetry (CV) tests were performed with a scan rate of 3 mV s−1 and a voltage range of 1 to −1 V. The nucleation and dissolution experiments of Li2S A 0.5 M Li2S8 solution was prepared by mixing a 7:1 molar ratio of sulfur and Li2S in tetraglyme solvent. The [email protected] and N-C powders were ultrasonically dispersed in ethanol, and then the dispersed solution was dropped on a carbon cloth with a diameter of 12 mm and an areal mass loading of 0.2 mg cm−2. The loaded carbon cloth was used as the cathode, and lithium metal was used as the anode; 15 μL of 0.5 M Li2S8 solution was placed on the cathode side, and 15 μL of conversional electrolyte was placed on the anode side. The assembled batteries were galvanostatically discharged to 2.06 V at a current of 0.112 mA and then potentiostatically discharged to 2.05 V until the discharged current was below 0.01 mA. Similarly, the batteries were assembled using the same electrode material and electrolyte. The batteries were first galvanostatically discharged to 1.7 V at a current of 0.1 mA and then allowed to stand for 300 s. During the standing process, the voltage moved toward the equilibrium potential higher than 1.8 V, and therefore, the batteries then were galvanostatically discharged to 1.8 V at 0.1 mA to ensure that polysulfides were fully converted into solid Li2S. Finally, the nucleation and dissolution capacity of Li2S were evaluated by calculating the integral area of the curve drawn according to Faraday’s law. Preparation of [email protected] functional separations [email protected] functional separations were prepared by a slurry casting method. [email protected], acetylene black, and PVDF were mixed in NMP solvent with a mass ratio of 7:2:1, and the mixed precursor was coated on the polypropylene separator (PP, Celgard-2500). After drying, functional separators were cut into disks with a diameter of 19 mm. The areal loading of the material was controlled at 0.4 mg cm−2. Fabrication of Li–S batteries and electrochemical tests The Li–S batteries were assembled under Ar in a glove box with the oxygen and water content below 0.1 ppm. The amount of electrolyte was maintained at an electrolyte to sulfur (E/S) ratio of 15 μL mg−1. The sulfur cathode consists of 70% sulfur powder, 20% acetylene black, and 10% PVDF. Sulfur loading was approximately 1 mg cm−2 for routine testing (E/S = 15 μL mg−1) and 7 mg cm−2 for high loading testing (E/S = 15 and 10 μL mg−1). Note that the high sulfur loading cathode was obtained by mixing 90% sulfur and 10% conductive carbon with carbon cloth as the current collector, and the weight of carbon cloth was not taken into consideration in general. The batteries were assembled with lithium metal as the anode and sulfur as the cathode with modified separators to form the standard 2025 coin-type batteries. The galvanostatic charge–discharge test was measured by the Landt CT2001A battery test system. A CHI660E electrochemical workstation was used for CV tests, and the scan rate was from 0.1 to 0.5 mV s−1 in the voltage range of 1.7 to 2.8 V. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 10 mHz to 100 kHz. Computational Methods All the DFT simulations in this work were performed using the Vienna ab initio Simulation Package (VASP).38 The all-electron projector augmented wave (PAW) model was used for describing the electron-ion interaction.39 The Perdew–Burke–Ernzerhof (PBE) exchange and correlation functional was employed for the structural optimization and the energy calculation.40 A kinetic energy cutoff of 400 eV was used for the plane-wave expansion of the electronic wave function. The Hellmann–Feynman force and energy convergence criteria were set to 0.02 eV/Å and 10–4 eV in structural optimization, respectively. The four layers of (3 × 3) supercell of Co (111) containing 144 atoms with the bottom two layers fixed were used as the substrates for redox reaction of Li–S molecules, with periodic vacuum layers of 15 Å and Г-centered k-point sampling grid of 1 × 1 × 1.The climbing-image nudged-elastic-band (CI-NEB) approach was applied to calculate the energy barrier of single Li diffusion and Li decomposition from Li2S molecule on the monolayer nitrogen-doped graphene (N-G) and Co (111) surfaces.41 To further study the absorption effect between Li2Sx (x = 1, 6) and samples, the first-principle simulations were performed. As shown in Supporting Information Figure S9, two atom monolayers (AMs) [N-G, Co (111) surfaces] were selected as the representative to calculate the binding energy with Li2Sx (x = 1, 6), respectively. From the top view of adsorption conformations of Li2Sx (x = 1, 6) on the AMs, we can conclude that the chemical interaction is the most important interaction between Li2Sx (x = 1, 6) and AMs. The binding strengthens between Li2Sx (x = 1, 6) and AMs, and the Co (111) surface has the highest binding energy for Li2Sx (x = 1, 6) as compared to N-G surface. The Co (111) surface has the highest binding energy for Li2Sx (x = 1, 6) as compared with N-G surface. The binding energy, Eb, is computed to measure the binding strength between Li2Sx (x = 1, 6) and AMs. It is defined as the energy difference between the summation of pure Li2Sx and pure AMs and Li2Sx − AM adsorbed system (ELi2Sx + AM) and can be expressed as Eb = ELi2Sx + AM − (ELi2Sx + EAM). With this definition, a negative binding energy indicates that the binding interaction is favored. All the Eb magnitudes and atomic configurations showed below are those of the most stable adsorption cases. Results and Discussion Synthesis and characterization of [email protected] We employed a one-step pyrolysis to synthesize [email protected] (Figure 1a). First, a mixture of cobalt nitrate [Co(NO3)2], urea, P123 (surfactant), and SiO2 sol were dissolved in DIW at room temperature and evaporated at 80 °C to evenly distribute Co and SiO2. Then the mixed precursors were pyrolyzed at 900 °C and etched by HF solution to obtain the functional composite of [email protected] For comparison, two other samples, that is, N-HPC without Co and N-C without Co or SiO2 sol, were also prepared by the same method. As shown in the scanning electron microscopy (SEM) images, the as-synthesized [email protected] exhibits a hierarchical porous structure with a three-dimensional carbon skeleton (Figures 1b and 1c and Supporting Information Figures S1a and S1b). N-HPC shows a similar structure to that of [email protected], whereas N-C displays unevenly distributed carbon blocks ( Supporting Information Figures S1c–S1f), implying that the formation of a hierarchical porous structure is related to the synergistic effect of the silica template and carbon pyrolysis. Figure 1 | (a) Illustration of the [email protected] synthesis. (b and c) SEM images, (d and e) TEM images, (f and g) high-resolution HADDF-STEM images, (h) HADDF-STEM image and the corresponding element mapping of [email protected] Download figure Download PowerPoint The transmission electron microscopy (TEM) images of [email protected] (Figures 1d and 1e) further reveal the three-dimensional porous structure and uniformly distributed mesopores with an average size of 10–20 nm. Furthermore, the high-resolution high-angle annular dark-field scanning TEM (HADDF-STEM) images (Figures 1f and 1g and Supporting Information Figure S2) show well-dispersed Co nanoparticles that are surrounded by a graphitic carbon matrix. In addition, the distance between adjacent lattice fringes is 0.21 nm, corresponding to the (111) crystal plane of metallic Co. The Co content of [email protected] determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) is ∼0.8 wt % ( Supporting Information Table S1). Energy-dispersive X-ray spectroscopy (EDS) mappings also demonstrate that Co, N, and C elements are homogeneously distributed and lack aggregated Co nanoparticles in [email protected] (Figure 1h and Supporting Information Figure S3). The formation of highly dispersed Co nanoparticles is related to the etching of unstable and agglomerated Co nanoparticles by HF solution. The X-ray diffraction (XRD) pattern of [email protected] (Figure 2a) clearly demonstrates the characteristic graphitic carbon peak at 26.2°, which is consistent with the TEM results. However, no such peak is observed for N-HPC and N-C, suggesting that the Co source facilitates the formation of graphitic carbon. Note that the diffraction peak of metallic Co is absent because of the low Co content in [email protected]42 Moreover, the Raman results (Figure 2b) suggest that the intensity ratio of G band (1590 cm−1) to D band (1330 cm−1) (IG/ID) is higher for [email protected] (1.02) compared with that of N-HPC (0.92) and N-C (0.87),43 implying a higher degree of graphite and therefore promoted electrical conductivity for [email protected] The N2 adsorption and desorption isotherms show the characteristics of typical IV isotherms with the pore size of 5–20 nm, indicating that [email protected] is mainly dominated by mesoporous structure (Figures 2c and 2d). [email protected] also exhibits the largest specific surface area (1248.32 m2 g−1) among the samples ( Supporting Information Table S2), which should be beneficial for the physical constraint of polysulfides and nucleation of Li2S.44 Figure 2 | (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption/desorption isotherms, and (d) the corresponding pore size distributions of [email protected], N-HPC, and N-C. (e) N 1s XPS spectrum of [email protected] Co and of [email protected], Co foil, and Download figure Download PowerPoint XPS was to the chemical of Co and N in [email protected] As shown in Figure the N 1s XPS spectrum displays four N species ( Supporting Information Table that is, N N graphitic N and N Furthermore, the N of [email protected], N-HPC, and N-C samples were ( Supporting Information Table indicating the of N in the carbon matrix. It has been that an carbon surface facilitate polysulfide redox kinetics by improving the electron resulting in sulfur and cycling The Co XPS spectrum ( Supporting Information Figure that in to metallic Co, and are also which be to the of certain Co atoms the pyrolysis To further the electronic structure and of Co nanoparticles in [email protected], Co X-ray adsorption structure and X-ray absorption structure were performed. The of [email protected] is that of Co (Figure the metallic of Co in [email protected]42 Moreover, in contrast to only one peak at Å is observed for the spectrum of [email protected] (Figure corresponding to the first of the Note that the of [email protected] is than that of Co foil, which be related to the size effect of Co and [email The of [email protected] displays one intensity at that is with the (Figures further the of metallic Co in [email protected] Li + diffusion kinetics We utilized [email protected] as a functional material to the commercial separators of Li–S batteries ( Supporting Information Figures The functional separators exhibit an excellent electrolyte and a strong ( Supporting Information Figures For comparison, N-HPC and N-C functional separators were also and ( Supporting Information Figures and Figure demonstrates the voltage of Li–S batteries using [email protected], N-HPC, and N-C functional separators at 0.2 C C = mA and the from to and then to the process, Li–S batteries with [email protected] functional separators exhibit and (Figure and Supporting Information Figure suggesting that sulfur is in the electrochemical by to polysulfides and then to Li2S. Figure 3 | (a) voltage and (b) of Li–S batteries assembled with [email protected], N-HPC, and N-C functional CV of Li–S batteries with separators at scan from 0.1 to 0.5 mV s−1 and the corresponding of redox peak with and and energy of Li diffusion the energy on the of Co (111) and monolayer The and to Li, C, N, and Co respectively. Download figure Download PowerPoint The diffusion kinetics is a polysulfide redox which we by and CV The results reveal that [email protected] functional separators show the (Figure of the fast at the N-HPC also has an excellent electrical conductivity that the Figures show the CV of Li–S batteries assembled with separators at various scan from 0.1 to 0.5 mV The and to the of to Li2Sx and Li2Sx to the peak the from to with N-C and N-HPC, the Li–S batteries with [email protected] functional separators show the highest peak with voltage ( Supporting Information Figure suggesting the of Co nanoparticles the polysulfide redox to the the Li+ diffusion are to the of (Figures is the peak current and is the scan The and Li+ diffusion are in Supporting Information Table It that the Li–S batteries with [email protected] functional separators demonstrate the largest Li+ diffusion implying that the well dispersed Co nanoparticles in [email protected] the Li+ diffusion rate, which should be beneficial for the redox kinetics of polysulfides. In addition, the redox in the CV of Li–S batteries with [email protected] functional separators well in the cycles ( Supporting Information Figure suggesting the excellent cycling stability and high of sulfur conversion. To well distributed Co nanoparticles facilitate the Li+ diffusion we have the Li diffusion barriers on N-G and Co with the Li diffusion ( Supporting Information Figure and Figures and The diffusion barriers are and eV for N-G and Co, indicating the improved Li diffusion on the Co surface. the diffusion rate on [email protected] surface the polysulfide redox kinetics the electrochemical process, which suppress the shuttle effect and the sulfur utilization. Polysulfide adsorption and catalytic conversion The adsorption and conversion of polysulfides a in improving the electrochemical performance of Li–S batteries. To the bonding strength with polysulfides, adsorption experiments were performed by [email protected], N-HPC, N-C powders into Li2S6 solution using DME as solvent. As shown in Figure after 30 of the [email protected] and N-HPC whereas the N-C solution The UV–vis (Figure further demonstrate that [email protected] the with polysulfides. To further the strong affinity between [email protected] and polysulfides, spectroscopy are performed on Li2S6 and after with [email protected] (Figure For the spectrum of there are two which are to sulfur and sulfur After with [email protected], species to binding because of from Co to the strong chemical interaction between [email protected] and This is also by the first-principle Figures and show the adsorption conformations of Li2S6 on Co and N-G respectively. The binding energy between Li2S6 and Co is compared with that of N-G to the of Li2S ( Supporting Information Figure The results clearly that the well dispersed Co nanoparticles in [email protected] can effectively confine polysulfides, which is the first

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