A Co9S8 microsphere and N-doped carbon nanotube composite host material for lithium-sulfur batteries

Abstract

Lithium-sulfur batteries have emerged as extraordinarily favorable energy storage devices due to their high specific capacity and energy density, safety and low cost. Unfortunately, the wide applications of lithium-sulfur batteries are hampered by several issues, such as the low electronic conductivity and slow redox kinetics, serious volumetric expansion and polysulfide "shuttle effect". To overcome these issues, in our work, we design and synthesize a composite sulfur host material of Co9S8 microspheres and N-doped carbon nanotubes, where the metallic sulfide Co9S8 with a good conductivity enables the immobilization of the polar lithium polysulfides owing to the strong polar chemisorptive capability, and the one dimensional N-doped carbon nanotubes can provide channels for fast electron and lithium-ion transport. As the lithium polysulfides are well confined, and the redox conversions are promoted, the Co9S8@N-CNTs/S-based lithium-sulfur battery possesses a superior energy storage performance, exhibiting a large specific capacity of 1233 mAh g -1 at 0.1 C and an outstanding cyclic performance, with a low decay of 0.045% per cycle and a Coulombic efficiency of more than 99% after 1000 cycles. Keywords: Metal sulfides; N-doped carbon nanotube; Cathode material; Lithium-sulfur battery. tightly attached to the N-CNTs is formed. In this composite, the polar metal sulfide Co9S8 enables a superior lithium polysulfide absorptivity via the polar chemical bond, which decreases polysulfide dissolution and the shuttle effect and increases the specific capacity and cyclic performance, while the N-doped carbon nanotubes can assist to increase the electronic conductivity and enlarge the interface between the electrode and electrolyte to further enhance the rate performance. Meanwhile, the N-CNTs/S composite and the Co9S8@N-CNTs/S composites with different contents of N-CNTs have been studied and compared. The unique Co9S8@N-CNTs composite-based sulfur cathode displays an outstanding overall electrochemical performance, making it promising for applications in 6 lithium-sulfur batteries. Experimental Preparation of Co9S8 and Co9S8@N-CNTsThe Co9S8@N-CNTs composite host was prepared using a solvothermal process. First, 27 mg of the N-CNTs was ultrasonically dispersed into 30 mL of ethylene glycol and 10 mL of distilled water to form a suspension. CoCl2· 6H2O (0.7138 g) and thiourea (0.4567 g) were added with stirring, and the ratio of CoCl2:thiourea was fixed as 60:40. The obtained solution was then subjected to a hydrothermal reaction for 12 h at 180 C. The product was filtered, adequately washed, and vacuum-dried for 24 h at 80 C. The obtained precursor was annealed for 3 h at 500 °C in H2/Ar (10%:90%). The obtained sample is denoted as Co9S8@N-CNTs. The pristine Co9S8 material was also prepared using the same processes but without adding the N-CNTs. The other Co9S8@N-CNTs composites with different contents of N-CNTs (13.5 mg and 40.5 mg) were prepared by a similar procedure. Hereafter, these obtained samples with different N...

Microflower-like Co9S8@MoS2 heterostructure as an efficient bifunctional catalyst for overall water splitting

“…[13][14][15][16] However, MoS 2 also has drawbacks such as covering of active sites, poor conductivity, and low OER performance. [17][18][19] In contrast, transition metal sulphides and oxides of Co such as Co 3 S 4 , 20,21 Co 9 S 8 , [22][23][24] Co 4 O 4 , 25 and LaCoO 3 26,27 exhibit higher conductivity and efficient electrocatalytic performance for the OER, but their HER performance is unsatisfactory. Some studies have reported that the electronic structure of MoS 2 can be modulated and optimised by doping with transition metals (e.g.…”

Rational Construction of Sulfur-Deficient NiCo2S4–x Hollow Microspheres as an Effective Polysulfide Immobilizer toward High-Performance Lithium/Sulfur Batteries

“…23 While polar materials such as Co 3 O 4 and TiO 2 act as chemical trappers toward LiPSs, they have intrinsically low electrical conductivity, which makes them unable to diffuse/ transform polysulfides in time, leading to a slow redox reaction. 24,25 Therefore, an ideal sulfur carrier should simultaneously realize LiPS adsorption/trapping, which is very critical to achieve an outstanding cycling stability. Currently, transition-metal sulfides (TMSs) have aroused great research interest due to their unique metallic conductivity compared with corresponding metal oxides and appropriate adsorption capacity toward LiPSs.…”

“…Various strategies have been developed to tackle the issues mentioned above. − One of the most important strategies focuses on constructing various composites as sulfur host materials, such as carbon materials, , metal oxides, − and metal sulfides. − Yet, nonpolar carbon exhibits inferior physical adsorption to the polar intermediate polysulfides, which cannot effectively prevent the dissolution of polysulfides . While polar materials such as Co 3 O 4 and TiO 2 act as chemical trappers toward LiPSs, they have intrinsically low electrical conductivity, which makes them unable to diffuse/transform polysulfides in time, leading to a slow redox reaction. , Therefore, an ideal sulfur carrier should simultaneously realize LiPS adsorption/trapping, which is very critical to achieve an outstanding cycling stability. Currently, transition-metal sulfides (TMSs) have aroused great research interest due to their unique metallic conductivity compared with corresponding metal oxides and appropriate adsorption capacity toward LiPSs. , For example, Zhang et al introduced NiS 2 nanoparticles into graphene used as the sulfur host material, which exhibited a high specific capacity of 400 mAh g –1 over 800 cycles at 1 C with almost 98% Coulombic efficiency, benefiting from the chemical adsorption of LiPSs to NiS 2 nanoparticles .…”

Confine sulfur in double-hollow carbon sphere integrated with carbon nanotubes for advanced lithium–sulfur batteries

“…However, Li-S battery has been firstly hampered by the low electrical conductivity of both sulfur (5 × 10 −30 S cm −1 ) and the discharge product (Li 2 S), which lead to restricting the electron transport and the utilization of the active sulfur [6]. Second, the dissolution of polysulfides in the conventional organic electrolyte (Li 2 S x , 4 ≤ x ≤ 8) provides low coulombic efficiency and short cycle life [7,8]. Third, the volume change of sulfur (about 80%) during the charge-discharge process leads to the pulverization of sulfur cathode and gives up a rapid capacity decay [9,10].…”