A novel lithium/sulfur battery based on sulfur/graphene nanosheet composite cathode and gel polymer electrolyte
© Zhang et al.; licensee Springer. 2014
Received: 9 February 2014
Accepted: 14 March 2014
Published: 21 March 2014
A novel sulfur/graphene nanosheet (S/GNS) composite was prepared via a simple ball milling of sulfur with commercial multi-layer graphene nanosheet, followed by a heat treatment. High-resolution transmission and scanning electronic microscopy observations showed the formation of irregularly interlaced nanosheet-like structure consisting of graphene with uniform sulfur coating on its surface. The electrochemical properties of the resulting composite cathode were investigated in a lithium cell with a gel polymer electrolyte (GPE) prepared by trapping 1 mol dm−3 solution of lithium bistrifluoromethanesulfonamide in tetraethylene glycol dimethyl ether in a polymer matrix composed of poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methylmethacrylate)/silicon dioxide (PVDF-HFP/PMMA/SiO2). The GPE battery delivered reversible discharge capacities of 809 and 413 mAh g−1 at the 1st and 50th cycles at 0.2C, respectively, along with a high coulombic efficiency over 50 cycles. This performance enhancement of the cell was attributed to the suppression of the polysulfide shuttle effect by a collective effect of S/GNS composite cathode and GPE, providing a higher sulfur utilization.
Lithium-ion batteries are leading power sources for portable applications from small consumer electronics to electricity-powered transport. Despite this, their wider application is restricted due to the limited energy density of available cathode materials. Alternative cathode materials with high energy density and low cost are thus needed . Sulfur is very attractive as a cathode material for the next-generation high-energy rechargeable lithium batteries because of its advantages of a large theoretical capacity of 1,672 mAh g−1, which is the highest among all known cathode materials, low cost, and environmental friendliness [2–4]. Despite this, due to its insulating nature, large volume changes during electrochemical processes, and the solubility of the polysulfides formed during these processes, the practical application of sulfur as a cathode in lithium rechargeable batteries has not been successful yet [5, 6].
Therefore, intensive efforts have been devoted to overcome the mentioned problems. The preparation of sulfur/carbon and sulfur/conductive polymer composites has received considerable attention, and recent results show that the sulfur/carbon composites benefit from their hierarchical design resulting in the performance improvement [7–21]. Microporous and mesoporous carbon nanoparticles [10, 11], carbon nanotubes , and graphene sheets [14–16] have been employed to encapsulate sulfur. However, the preparation techniques used to obtain these materials have the disadvantages of side products and prolonged and complicated processing, increasing the final product cost .
In this work, we report on the preparation of a novel sulfur/graphene nanosheet (S/GNS) composite via a simple ball milling of sulfur and commercial multi-layer graphene nanosheets, followed by a heat treatment, and investigation of its physical and electrochemical properties as a cathode for Li|S batteries.
Diffusion of lithium polysulfides is largely determined by the electrolyte components; adopting an appropriate electrolyte is critical to promote the performance of Li|S batteries . In previous studies [9, 10], it was shown that a gel polymer membrane can act as a physical barrier, controlling the cathode reaction product dissolution, restricting their diffusion from the cathode, and thus preventing their reaction at the anode side. Herein, in the present work, to further enhance the battery performance, a common liquid organic electrolyte was replaced with an original gel polymer electrolyte, formed by trapping a liquid electrolyte in tetraethylene glycol dimethyl ether electrolyte in a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/poly(methylmethacrylate) (PMMA) polymer matrix doped with silicon dioxide (SiO2) nanoparticles. The electrochemical and structural properties of this GPE and the electrochemical performance of Li|S/GNS batteries with this GPE were also investigated.
The preparation of the GPE is schematically represented in Figure 1b. Among other polymer pore-making technologies, we adopted the phase inversion method to obtain a porous structured system through a solvent exchange route [23, 24]. The membrane is formed by polymer precipitation, which occurs as a consequence of concentration variations following diffusive interchange between the solvent (acetone) and the non-solvent (water). PVDF-HFP (KynarFlex 2801, Arkema Inc., Philadelphia, PA, USA), PMMA (average molecular weight 350,000 g mol−1, Sigma-Aldrich, St. Louis, MO, USA), and SiO2 nanopowder (US Research Nanomaterials, Inc.) were added to acetone in a weight ratio of 3:2:0.25 under stirring followed by sonication. Deionized water was then added dropwise and the mixture was continuously stirred for 3 h. The resulting slurry was cast on an aluminum plate and the solvent was evaporated overnight at ambient temperature. The resulting membrane was dried under vacuum at 50°C for 5 h. The resulting mechanically stable membranes, approximately 80 μm thick, were activated inside an argon-filled glove box (As One Co., Osaka, Japan) by immersion in a 1 mol dm−3 solution of lithium bistrifluoromethanesulfonamide (LiTFSI) in tetraethylene glycol dimethyl ether (99.95% purity, Sigma-Aldrich). The liquid uptake (%) was determined using the relation (W2 − W1) × 100/W1, where W1 and W2 denote the respective weights of the polymer electrolyte before and after absorbing the lithium salt solution .
The S/GNS composite surface morphology was examined by field emission scanning electron microscopy (SEM; JSM-6490, JEOL, Akishima, Tokyo, Japan). The interior structure of the composite was observed by transmission electron microscopy (TEM; High Voltage LIBRA 120, Сarl Zeiss, Oberkochen, Germany) with energy-dispersive X-ray spectroscopy (EDX). The ionic conductivity of the GPE was determined at 25°C by electrochemical impedance spectroscopy (EIS) over the frequency range from 0.1 Hz to 1 MHz using potentiostat/galvanostat VMP3 (Biologic, Claix, France), sandwiching the GPE membranes between two blocking stainless steel electrodes. The electrochemical stability window of GPE was determined by cyclic voltammetry (CV) conducted with VMP3 in coin-type cells where GPE was interleaved between lithium metal and stainless steel electrodes.
The electrochemical performance of the S/GNS composite cathode was investigated in coin-type cells (CR2032) with PVDF-HFP/PMMA/SiO2 GPE. The cell was composed of a lithium metal anode and the S/GNS composite cathode separated by the GPE film. The cathode is comprised of 80 wt% S/GNS composite, 10 wt% acetylene black (AB; 99.5% purity, MTI, Richmond, CA, USA) as a conductive agent, and 10 wt% polyvinylidene fluoride (PVDF; 99.5% purity, MTI) as a binder. These materials were dispersed in 1-methyl-2-pyrrolidinone (NMP; ≥99% purity, Sigma-Aldrich). The resultant slurry was spread onto aluminum foil using a doctor blade and dried at 50°C for 12 h. The resulting cathode film was used to prepare the cathodes by punching circular disks of 1 cm in diameter. The coin cells were assembled in high-purity argon (99.9995%) atmosphere. The cells were tested galvanostatically on multi-channel battery tester (BT-2000, Arbin Instruments, College Station, TX, USA) between 1 and 3 V vs. Li+/Li. The applied currents and specific capacities were calculated on the basis of the weight of S in the cathode.
Results and discussion
A novel S/GNS composite with irregular interlaced nanosheet-like structure and homogeneous distribution of the components was successfully prepared via a simple ball milling of sulfur with commercial multi-layer graphene nanosheets, followed by a heat treatment. This composite was studied in a lithium cell with an original gel polymer electrolyte, 1 mol dm−3 of LiTFSI in PVDF-HFP/PMMA/SiO2 polymer electrolyte, prepared by phase separation. The GPE exhibited a pore-rich structure, a high ability to absorb liquid electrolyte exceeding 71 wt%, and a high ionic conductivity at ambient temperature. The Li|GPE|S cells exhibited a high initial specific discharge capacity and maintained a reversible discharge capacity of 413 mAh g−1 after 50 cycles at 0.2C, along with a high coulombic efficiency. Due to a combined positive effect of the nanosheet-like structure of conductive S/GPE composite cathode, retaining the S cathode reaction products-polysulfides, and a highly conductive GPE as a physical barrier for these products’ shuttle, the system could deliver reversible capacity of 316 mAh g−1 even at 1C. The results of this work show that the S/GNS composite cathode prepared in this work via a simple preparation technique, in combination with the original GPE, provides a promising way to develop the Li|S battery with very attractive overall performances and, due to its simplicity, could be a good choice for the scale-up technology for Li/S batteries.
energy-dispersive X-ray spectroscopy
electrochemical impedance spectroscopy
gel polymer electrolyte
poly(vinylidene fluoride-co-hexafluoropropylene)/poly(methylmethacrylate)/silicon dioxide
scanning electron microscopy
transmission electron microscopy.
This research was supported by the Research Grant from the Ministry of Education and Science of the Republic of Kazakhstan and partially by the World Bank - Ministry of Education and Science of the Republic of Kazakhstan grant. The authors acknowledge the Nazarbayev University Research and Innovation System (the General Director Dr. Baigarin) for the overall support to the work. Nazarbayev University (President Mr. S. Katsu, Vice-President Mr. M. Mamashev) assisted in meeting the publication costs of this article.
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