Cobalt-doped Hollow Carbon Framework as Sulfur Host for the Cathode of Lithium Sulfur Battery-Part 1

Cobalt-doped Hollow Carbon Framework as Sulfur Host for the Cathode of Lithium Sulfur Battery - Part 1

JIN Gaoyao, HE Haichuan, WU Jie, ZHANG Mengyuan, LI Yajuan, LIU Younian

Hunan Provincial Key Laboratory of Micro & Nano Materials Interface Science, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

Lithium-sulfur batteries are deemed to be the next generation of cost-effective and high energy density systems for energy storage. However, low conductivity of active materials, shuttle effect and sluggish kinetics of redox reaction lead to serious capacity fading and poor rate performance. Herein, a sodium citrate derived three-dimensional hollow carbon framework embedded with cobalt nanoparticles is designed as the host for sulfur cathode. The introduced cobalt nanoparticles can effectively adsorb the polysulfides, enhance the kinetics of conversion reaction and further improve the cyclic and rate performance. The obtained cathode delivered a high initial discharge capacity of 1280 mAh·g-1 at 0.5C, excellent high-rate performance up to 10C and stable cyclic capacity of 770 mAh·g-1 at 1C for 200 cycles with high Columbic efficiency.
Keywords： lithium sulfur battery ; cobalt nanoparticle ; conversion reaction ; sulfur cathode

The lithium-sulfur (Li-S) batteries contain elemental sulfur, which possesses the superiorities of natural abundance, low cost, and high specific capacity (1672 mAh∙g-1). However, the poor performance due to the low electrical conductivity of elemental sulfur (5×10-30 S∙cm-1), “shuttle effect” caused by dissolution of polysulfides and large volume expansion (~80%) during cycling seriously hinders the development of Li-S batteries. Vigorous studies have been devoted to the aforementioned issues, while cathode designing forms the largest class to date. Previous work focused on encapsulating sulfur cathode into light host with excellent electronic conductivity, robust framework structure and enough pore volume. Though carbonaceous materials can satisfy the criteria of cathode substrates, the forces between the nonpolar host and polar lithium polysulfides species (hereafter denoted as LiPSs) can be too weak. The polar LiPSs species gradually diffuse during long- term cycling due to the single physical confinement. To increase the polarity of barrier skeletons, heteroatoms were introduced into the carbon host to produce stronger interaction with the LiPSs. These dopants can effectively capture the soluble polysulfide and restrain the shuttling effect.
Although the cathode performance can be improved to some extent with the synergy of heteroatoms and carbon framework, it is still significantly limited by the sluggish kinetics of polysulfide conversion reaction, which causes the excessive accumulation of LiPSs and inevitable diffusion. Transition metal compounds have been widely introduced into the sulfur host to accelerate the kinetics of conversion reaction. In recent years, specific metal nanoparticles, such as Co, Fe and Pt, showed similar accelerating effect. Among these metals, cobalt metal has attracted the attention of researchers for its excellent conductivity and strong interaction with polysulfides. During the charging and discharging process, it can effectively capture the polysulfides and promote the conversion reaction. Li, et al. obtained the Co- and N-doped carbon as the sulfur host by the calcination of ZIF-67 precursor. The uniformly dispersed Co nanoparticles distinctly accelerated the redox reaction with the synergic effect of N-doped groups. Furthermore, Du, et al. presented the monodisperse cobalt atoms embedded nitrogen-doped graphene cathode, and Wu, et al. fabricated Co nanodots/N-doped mesoporous carbon with the in-situ calcination of adenine and CoCl2. In all of these reports, the Co-contained systems gained excellent cycling performances.
In this work, to improve the cyclic and rate performance of Li-S batteries, a 3D hollow carbon framework decorated with cobalt nanoparticles was designed as the host of sulfur cathode. Sodium citrate, a cheap and plentiful additive, is employed as the carbon source for its unique character during direct calcination. And the electrochemical performance of the cobalt-containing system (Co/C-700) and carbon framework (HEC-700) was systematically evaluated to ensure the effect of doped cobalt nanoparticles for the sulfur cathode.

Experimental

Synthesis of materials

All chemical reagents used in this work were of analytical grade without further purification. Briefly, 0.25 g Co(NO3)2·6H2O and 5.0 g sodium citrate were dissolved in 20 mL deionized water under magnetic stirring to form a homogeneous solution. Then, the solution was freeze-dried, ground into fine powder and calcined at 700 ℃ under N2 for 1 h with a heating rate of 5 ℃∙min-1. The obtained composites (named as UWC- 700) were washed with deionized water for 3 times in order to remove the by-products. After being dried at 60 ℃ overnight, the final product was collected and denoted as Co/C-700. To further confirm the effect of Co, hydrochloric acid etched carbon (HEC-700) was obtained by etching Co/C-700 in 2 mol/L HCl for 12 h, washing until neutral and drying at 80 ℃ for 12 h.
The cathode composites were prepared via a conventional melting-diffusion method. In brief, a mixture of sulfur (70wt%) and Co/C-700 (or HEC-700) composites were milled for 20 min, transferred into a 20 mL Teflon container autoclave and heated at 155 ℃ for 12 h. The obtained powder was collected as S@Co/C-700 and S@HEC-700.
The materials characterization and static adsorption of polysulfides are shown in supporting materials.

Electrochemical characterization
The electrochemical performance of the S@Co/C-700 and S@HEC-700 cathodes were tested by CR2025 type coin cells, fabricated in an argon-filled glove box (MBraun, Germany). The sulfur cathode slurry was prepared by mixing S@Co/C-700 (or S@HEC-700), acetylene black and polyvinylidene difluoride (PVDF) binder with a weight ratio of 7 : 2 : 1 in N-methyl-2- pyrrolidinone (NMP). Then the obtained slurry was uniformly casted onto an Al foil. Furthermore, the membrane was dried at 50 ℃ under vacuum overnight and cut into discs (1 cm in diameter) with a sulfur loading of 1.1-1.7 mg∙cm-2. The routine polypropylene membrane (Celgard 2400) was used to separate the cathode and lithium anode. The electrolyte used in each cell was 50 μL 1mol/L LiN(CF3SO2)2 and 1wt% LiNO3 solution in DOL/DME (1:1 in volume). Galvanostatic charge- discharge tests were performed by a LAND CT 2001A battery test system (Jinnuo Electronic Co, Wuhan, China) within the voltage window of 1.7-2.8 V. The cyclic voltammetry (CV) measurement was performed from 1.5 to 3.0 V at a scan rate of 0.1 mV∙s-1. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range from 0.1 MHz to 10 mHz with a voltage amplitude of 5 mV at open-circuit. The CV and EIS measurements were carried out on a CHI 660E electrochemical Workstation (Chenhua Instruments Co, Shanghai, China). The symmetrical cells were assembled with Co/C-700 or HEC-700 (8:2 with PVDF in weight ratio) as identical cathode and anode, and 50 μL electrolyte of 1 mol/L LiN(CF3SO2)2, 1wt% LiNO3 and 0.2 mol/L Li2S6 in DOL/DME(1:1 in volume) solution.

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