Cobalt Sulfide Confined in N-Doped Porous Branched Carbon Nanotubes for Lithium-Ion Batteries
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A novel hierarchical structure constructed by encapsulating cobalt sulfide nanowires within nitrogen-doped porous branched carbon nanotubes (NBNTs) is designed for lithium-ion batteries.
The unique hierarchical Co9S8@NBNT electrode displayed a reversible specific capacity of 1310 mAh g−1 at a current density of 0.1 A g−1.
KeywordsLithium-ion batteries Nitrogen doping Cobalt sulfide Branched carbon nanotubes
Recently, rechargeable energy-storage systems have attracted a great deal of interest because of the escalating requirements of various applications such as hybrid electric vehicles and electronic devices [1, 2, 3, 4]. Among various alternatives, lithium-ion batteries (LIBs) have attracted unprecedented attention owing to increasing market demand [5, 6, 7, 8]. However, LIBs suffer from a lack of high-performance anode materials, which hinders their practical application [7, 8, 9, 10, 11]. Extensive studies have been conducted to solve these problems by structure design to achieve different charge-storage mechanisms [10, 11]. Recently, various transition metal sulfides have been proposed because of their high theoretical capacities [12, 13, 14, 15, 16, 17]. However, they are limited by their poor rate performance and sharp capacity fading caused by their low electronic conductivity and large volume changes during the charging/discharging process [9, 10, 18]. In recent years, nanostructured carbonaceous materials have been widely investigated to overcome these low capacity and kinetic limitations . Carbon nanotubes (CNTs), one of the promising nanostructured carbonaceous materials with excellent electrical conductivity, large specific surface area, electrochemical and thermal stabilities, and easy ion accessibility, are expected to be an important option for LIBs [19, 20, 21]. In particular, Li+ ions can be intercalated not only into the intertube channel, but also into the inner space of the tube cavity, leading to excellent rate performance . However, using CNTs as active materials is difficult because of their limited capacity [23, 24, 25, 26].
Therefore, much work has been carried out to design new nanostructured hybrids for the construction of transition metal sulfide/carbon-based material composites as next-generation anodes [27, 28, 29, 30, 31, 32]. One-dimensional (1D) nanomaterials, which have a large accessible area, fast ion diffusion, and percolated electron transport, are considered to be ideal nanoscale building blocks for construction of multidimensional and multifunctional electrode configurations for advanced electrochemical energy storage [27, 33, 34, 35]. Hence, construction of integrated 1D nanostructured transition metal sulfides with CNTs can also be regarded as an attractive strategy for developing high-performance anode materials for LIBs. Although high-capacity transition metal sulfides have been incorporated onto the surface of hierarchical CNT networks to improve the rate and cycling performances, these active materials are still unstable because of surface exposure and interparticle aggregation .
2.1 Materials Synthesis
The Co9S8@NBNT was synthesized by a catalytic decomposition reaction of polyacrylonitrile (PAN) on a Co/MgO catalyst in the presence of sodium polysulfide. The Co/MgO catalyst was prepared as follows. Co(NO3)2·6H2O and Mg(NO3)2·6H2O were mechanically mixed and ground, and then calcined at 600 °C for 1 h in air to decompose the precursor and yield a cluster made of Co and Mg oxides. The formed powder was then reduced in H2 (100 sccm) and Ar (200 sccm) for 30 min at 600 °C to form Co nanoclusters supported on MgO particles, which were collected and used as the catalyst.
A sodium polysulfide (Na–poly-S) solution was prepared from sulfur (6 g, Puriss, precipitated, 99.5–100.5%) and sodium disulfide (nonahydrate, 18 g, ACS reagent 98%) in water (240 g, distilled) by sonication and stirring overnight with mild heating (40 °C). l-Ascorbic acid (80 mg, 99%) was dissolved in water (26 g, distilled) in a 40 mL glass vial with a PET cap. Subsequently, 4 g of the Na–poly-S solution was added and hand-shaken. Hydrochloric acid (0.1 mL, Puranal 37%, diluted to 5 M) was then added, and the vial was hand-shaken. The vial was closed, hand-shaken and sonicated at 40 °C for 30 min. The Co/MgO catalyst particles and PAN powder were placed in a graphite crucible enclosed within a graphite susceptor, and heated up to the reaction temperature using an induction furnace with a flow of Ar (2300 sccm) and H2 (100 sccm). H2 was allowed to bubble through the vial. The temperature of the susceptor was controlled to ensure that Co/MgO catalyst particles were heated to 1000 °C. After growth for 15 min, the H2 flow was stopped and the chamber was cooled down to room temperature. During the cooling process, the system was purged with Ar to prevent a backflow of air from the exhaust line.
The obtained product was washed three times by replacing the liquid products with distilled water and HCl, allowing soluble components to diffuse out of the product for at least 2 h. The washed product was freeze-dried to remove the liquid component. The resulting product was cut into disks with a razor blade. Prior to use as an anode, the disks were dried in a vacuum oven at 90 °C for 1 h and directly transferred to an argon-filled glove box.
Co9S8@CNT was obtained by a catalytic decomposition reaction of dimethyl sulfide (C2H6S) on the Co/MgO catalyst, which was reported in our previous work .
Commercial multiwalled CNTs were obtained from Shenzhen Nano Co. Ltd.
The products were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM, JEM-2010). N2 sorption analysis was performed on an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics) equipped with an automated surface area analyzer at 77 K, using Barrett–Emmett–Teller (BET) calculations for the surface area. The pore-size distribution (PSD) plot was prepared from the adsorption branch of the isotherm based on a density functional theory (DFT) model. X-ray powder diffraction (XRD) patterns of the sample were recorded using a D/Max-3C diffractometer equipped with a Cu-Kα X-ray source. Raman spectra were measured using a Renishaw inVia Raman spectrometer system (Gloucestershire, UK) equipped with a Leica DMLB microscope (Wetzlar, Germany) and a 17 mW at 633 nm Renishaw helium–neon laser source. X-ray photoelectron spectroscopy (XPS) measurements were taken on a Kratos XSAM 800 spectrometer with a Mg–Kα radiation source. TGA was carried out using a DuPont 2200 thermal analysis station.
2.3 Electrochemical Measurements
The electrochemical tests were conducted by cycling two-electrode 2032 coin cells with Li disks as both the counter and reference electrode, a Celgard 2400 film as the separator, and a mixed slurry consisting of the prepared Co9S8@NBNT structure (80 wt%) with poly(vinylidene fluoride) (PVDF, 20 wt%) in N-methyl-2-pyrrolidone (NMP) without conducting agents. The Co9S8@NBNT composite electrodes were pressed before being assembled into the coin cells. The loading density, diameter, and thickness of the prepared electrodes were ~ 1 mg cm−2, ~ 12 mm, and ~ 65–85 μm, respectively. The electrolyte was 1 M LiPF6 in a 50:50 (w/w) mixture of ethylene carbonate and diethyl carbonate. Cyclic voltammetry and electrochemical impedance spectroscopy were conducted with a CHI 660C electrochemical workstation.
3 Results and Discussion
The outstanding cycling capability of Co9S8@NBNT at various current densities shown in Fig. 3c further proves its remarkable reversibility and stability. The capacity of the Co9S8@NBNT electrode reached 945 mAh g−1 after the first 2500 cycles at 1000 mA g−1, 889 mAh g−1 at 2000 mA g−1 after 1500 cycles, and 779 mAh g−1 after another 1000 cycles at 5000 mA g−1, respectively. The coulombic efficiency of the Co9S8@NBNT electrode was nearly 100% after the first few cycles. The irreversible capacity loss in the first cycle can mainly be attributed to the incomplete extraction of Li ions as a result of lithium being trapped in the porous electrodes and unable to be released in the first cycle, the irreversible formation of a solid-electrolyte interface layer, and the formation of insulating Li2S, which is generally observed for nanostructured conversion-based anode materials [35, 36]. This result is consistent with the CV results in which the cathodic peaks only exist in the first cycle and are absent in subsequent cycles.
In summary, we have designed and prepared a novel hierarchical structure constructed by encapsulating cobalt sulfide nanowires within nitrogen-doped porous branched carbon nanotubes. The Co9S8@NBNT nanohybrid exhibited remarkable electrochemical performance as an anode material in LIBs with a very high specific capacity of up to 1310 mAh g−1 at a current density of 0.1 A g−1, outstanding rate capability, and long cycle life. The Co9S8@NBNT with hierarchical porosity, the incorporation of nitrogen doping, and interconnected NBNT networks played a role in improving the rate capability by allowing rapid Li+ diffusion and facilitating electronic transport. The CNT-confinement of the active cobalt sulfide nanowires offered a proximate electron pathway for the isolated nanoparticles and shielding of the cobalt sulfide nanowires from pulverization for long cycling time periods. Such a strategy could be readily extended to other materials for energy-storage and conversion applications.
This work was financially supported by the Natural Science Foundation of Anhui Province (KJ2018A0534), the research fund of Anhui Science and Technology University (ZRC2014402), and Materials Science and Engineering Key Discipline Foundation (AKZDXK2015A01). The authors would like to express their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project No. Prolific Research Group No. 1436-011.
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