Sulfur/graphitic hollow carbon sphere nano-composite as a cathode material for high-power lithium-sulfur battery
© Shin et al.; licensee Springer. 2013
Received: 26 June 2013
Accepted: 25 July 2013
Published: 3 August 2013
The intrinsic low conductivity of sulfur which leads to a low performance at a high current rate is one of the most limiting factors for the commercialization of lithium-sulfur battery. Here, we present an easy and convenient method to synthesize a mono-dispersed hollow carbon sphere with a thin graphitic wall which can be utilized as a support with a good electrical conductivity for the preparation of sulfur/carbon nano-composite cathode. The hollow carbon sphere was prepared from the pyrolysis of the homogenous mixture of the mono-dispersed spherical silica and Fe-phthalocyanine powder in elevated temperature. The composite cathode was manufactured by infiltrating sulfur melt into the inner side of the graphitic wall. The electrochemical cycling shows a capacity of 425 mAh g−1 at 3 C current rate which is more than five times larger than that for the sulfur/carbon black nano-composite prepared by simple ball milling.
KeywordsLithium-sulfur battery Hollow carbon sphere Graphitic carbon Nano-composite Cathode
The advent of new commercial markets for the hybrid electric vehicle and the large-scale energy storage system urges the development of novel battery systems with much higher energy density and lower price than the conventional Li-ion battery based on the transition metal oxide and graphite[1, 2]. For decades, lithium-sulfur battery has been investigated as a viable candidate to meet these requirements due to its high theoretical energy density of over 2,500 Wh/kg and the low material cost of sulfur[3, 4]. The lithium-sulfur battery utilizes a series of conversion reactions of elemental sulfur (S8) to lithium sulfide (Li2S) on the cathode, resulting in a high cathodic capacity of 1,678 mAh g−1. These reactions involve complex intermediate steps, where various lithium polysulfides (Li2S n , 3 < n < 8) participate as temporary soluble species[5, 6]. Since the solubilized lithium polysulfides can cause a significant shuttle reaction, and thus, an excessive overcharge behavior may occur during the charge process, the dissolution of polysulfide species needs to be suppressed as much as possible. So far, many attempts have been made to control this phenomenon, with a partial success including an addition of mesoporous metal oxide to cathode, an encapsulation of sulfur nanoparticles by hollow metal oxide, and an adoption of the highly concentrated electrolyte system.
The other fundamental challenge of Li-S battery is associated with the insulating low electrical conductivity of sulfur (approximately 5.0 × 10−14 S/cm) which leads to poor electrochemical performance even at moderate current rate. The formation of nano-composite cathode with conducting materials such as carbon and conducting polymer is a common tactic to tackle this issue. For example, the imbibition of sulfur melt into micro-/meso-/macro-porous carbon network such as CMK-3[5, 10, 11], the introduction of sulfur melt into hollow carbon sphere (HCS) or carbon nano-tubes, and the encapsulation of nano-scale sulfur with polythiophene or poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) have been tried to provide an electrical pathway to nano-scaled sulfur particles. In this study, we utilized and improved the idea of using a HCS by preparing HCS with a highly graphitic wall structure (GHCS) in order to promote its electrical conductivity[16, 17]. We developed a simple and convenient methodology to synthesize a mono-dispersed GHCS by simple pyrolysis of Fe-phthalocyanine (Fe-Pc) in elevated temperature. We utilized this GHCS to manufacture GHCS/sulfur nano-composite for the application to cathode under high current rate for lithium-sulfur battery.
For the preparation of GHCS, 1.0 g of commercially available mono-dispersed silica sphere of 500 nm (Fluka Analytical, St. Louis) was mixed homogenously with 2.0 g of Fe-Pc (Aldrich Chemistry, St. Louis) using mortar and pestle. The mixture was subjected to heat treatment at 900°C for 2 h under argon atmosphere to get silica/carbon composite. Then, GHCS was obtained by removing the silica template and iron particles by stirring the composite in a 10% hydrofluoric solution for 5 h.
Characterization of GHCS
The morphological feature was observed by field emission scanning electron microscopy (S-4200, Hitachi Ltd., Chiyoda, Tokyo) with energy dispersive X-ray spectroscope (EDX) attachment and high-resolution transmission electron microscopy (Tecnai G2, operating at 200 keV, FEI Co., Hillsboro). The crystallographic structure was measured by powder X-ray diffraction (XRD) using CuKα1 radiation (λ = 1.5406 Å, D/MAX-2500/PC, Rigaku Corporation, Tokyo). The surface area and pore size distribution were measured from the N2 adsorption isotherm (Belsorp mini 2, BEL Japan, Inc., Osaka). Raman spectrum was collected in a spectral range from 2,000 to 500 cm−1 (Nicolet™ Almega™ dispersive Raman spectrometer (Thermo Fisher Scientific Inc., Pittsburgh) with He-Ni 633-nm laser).
Preparation of sulfur/GHCS nano-composite cathode
Commercial sulfur powder (200 mg) and GHCS (100 mg) were ground thoroughly using mortar and pestle to make a homogenous mixture. Then, the mixture was put in a vacuum oven at 155°C for 6 h to let the sulfur melt smear into the inner part of the hollow carbon. After that, the composite was gently ground again using mortar and pestle. Thermogravimetric analysis (TGA) was carried out under nitrogen atmosphere up to 800°C at a rate of 10°C/min (TGA 2050, TA Instruments, New Castle, DE, USA).
In a typical procedure, sulfur/GHCS nano-composite (200 mg) was ball milled in N-methyl-2-pyrrolidone for 30 min together with polyvinylidene fluoride binder (25 mg) and casted on an aluminum foil with a loading around 2 mg cm−2 of sulfur. The electrochemical behavior of the composite electrodes was observed with 2032 coin cells using an electrolyte composed of 3 M lithium bis(trifluoromethanesulfonyl)imide in the cosolvent of 1,2-dimethoxyethane and 1,3-dioxolane 1:1 (v/v) solution. The electrochemical cycling was carried out between 1.5 and 3.0 V in C/10 rate for the initial three cycles and thereafter C/2 (1 C = 1,675 mA g−1 of sulfur).
Results and discussion
The intrinsic low conductivity of sulfur which leads to a low performance at high current rate is one of the most limiting factors for the commercialization of lithium-sulfur battery. In this work, we showed an easy and convenient method to synthesize a hollow carbon sphere with a thin graphitic wall which can provide a support with a good electrical conductivity for the preparation of sulfur/carbon composite cathode. The hollow carbon sphere was prepared by heating the homogenous mixture of mono-dispersed spherical silica and Fe-phthalocyanine powders in elevated temperature. The composite cathode was manufactured by infiltrating sulfur melt into the inner side of the graphitic wall at 155°C. The electrochemical cycling shows a capacity of 425 mAh g−1 at a 3 C current rate which is more than five times larger than that for the sulfur/carbon black nano-composite prepared by simple ball milling.
SHO is currently working as a senior researcher at the Korea Institute of Science and Technology and an active member of the Korean Electrochemical Society and the Korean Chemical Society.
This work was supported by the Energy Efficiency and Resources Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Knowledge Economy (20118510010030).
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