Rice University team develops new nanocomposite material for Li-sulfur battery cathode with high cycling stability
|Schematic illustration of the synthesis of SPGs. Credit: ACS, Li et al. Click to enlarge.|
Researchers at Rice University led by Dr. James Tour have developed a hierarchical nanocomposite material of graphene nanoribbons combined with polyaniline and sulfur (Sulfur-PANI-GNRs, SPG) using an inexpensive, simple method. The composite shows good rate performance and excellent cycling stability for use as a cathode material in Lithium-sulfur batteries.
As reported in an an open access paper in the journal ACS Applied Materials & Interfaces, the stable reversible specific discharge capacity was 567 mAh/g at the 26th with only a 9% decay in the following 374 cycles, at the rate of 0.4 C.
Practical application of lithium-sulfur battery chemistry, despite its high theoretical energy density, has been limited by a number of challenges, among them the being that (1) sulfur is elecrtically insulating, and (2) the severe degradation of the lithium-sulfur battery capacity with cycling due to volume change and the high solubility of the polysulfide products.
A number of approaches have been and are being developed to try to address these issues. For example:
The poor electrical conductivity of sulfur can be improved by the introduction of conducting materials that form composites, such as graphene, carbon nanotubes, conducting polymers, and other carbon matrixes.
To reduce capacity decay by enhancing the stability of polysulfides in the electrolyte, additives such as LiNO3 have been used.
Another approach is to protect the anode to reduce the sulfur shuttle effect, and thereby extend cycling life.
Various matrices have been developed to trap the soluble polysulfides, cluding mesoporous carbon, amorphous carbon, carbon nanotubes, graphene, hollow carbon spheres, metal oxides, and conducting polymers.
Among all these matrixes, conducting polymers open new possibilities for the cycling life improvement in the LSBs due to their easy preparation and scale-up, mechanical structure, self- healing, and good electrical conductivity. … Polyaniline is an interesting conducting polymer because it works as a substrate to load sulfur and can be used as a cathode in lithium sulfur batteries. However, polyaniline suffers from two major problems which hinder its application in lithium sulfur batteries. The first problem is the limited electrical conductivity, and the other is the mechanical degradation caused by its large volumetric change, leading to its poor cycling stability in energy storage devices. Therefore, to mitigate these negative effects, it is important to improve polyaniline related materials for lithium sulfur batteries.
In this study, a unique structure where sulfur was loaded on polyaniline-graphene nanoribbons (PANI-GNRs) was designed to reduce the capacity decay in lithium sulfur batteries. The PANI- GNR composite was prepared by the in situ polymerization of aniline in the presence of GNRs. GNRs serve as the substrate for polyaniline growth, and increase the electronic conductivity and effective utilization of PANI in the composite. The GNRs also improved the mechanical properties of the composite, resulting in an enhancement of its ability to recover from the volume expansion. Therefore, PANI-GNRs effectively overcome the negative deficiencies of PANI alone.—Tour et al.
The Rice team prepared the SPGs by heat treatment of a mixture of elemental sulfur and PANI-GNRs. The PANI-GNRs work as an electronic conductivity framework for sulfur and they enhance the mechanical properties of SPGs. A fraction of sulfur reacts with polyaniline to form a cross-linked network with the inter-chain or intra-chain disulfide bond interconnectivity during the vulcanization process.
The rest of the sulfur diffuses into the hierarchical network of PANI-GNRs and newly formed polymer networks. PANI traps the soluble intermediate lithium polysulfide through strong physical and chemical absorption effects. The GNR reinforcement reduces the damage that normally occurs from volume change during the electrochemical reaction.
|(a) Rate performance of SPGs at various rates from 0.1 C to 1.0 C with respect to the cycle numbers. (b) Cycling performance of SPGs at a rate of 0.4 C. Credit: ACS, Li et al. Click to enlarge.|
Electrochemical experiments demonstrate that the SPGs exhibit good rate performance and high cycling stability as cathode materials, compared to pure elemental sulfur and sulfur-polyaniline, due to the synergic effect between the PANI, GNRs and sulfur. The synthesis of the SPGs composite has been shown to produce an effective component to improve the electrochemical stability of the electrode materials for LSBs.—Li et. al
Lei Li, Gedeng Ruan, Zhiwei Peng, Yang Yang, Huilong Fei, Abdul-Rahman O. Raji, Errol L. G. Samuel, and James M. Tour (2014) “Enhanced Cycling Stability of Lithium Sulfur Batteries Using Sulfur-Polyaniline-Graphene Nanoribbon Composite Cathodes” ACS Applied Materials & Interfaces doi: 10.1021/am5030116