“Motivated by the rapid development of portable electronic, electric vehicle, and renewable energy harvest, advanced energy storage devices such as lithium battery are highly required,” said Dr. Qiang Zhang, an associate professor at Department of Chemical Engineering, Tsinghua University, “since the traditional lithium-ion battery has met their theoretical limitation on energy density, our group found the tremendous potential of lithium-sulfur battery, a novel electrochemical energy storage system, and has carried out wide researched for about two years.”
Lithium-sulfur battery, employing sulfur as cathode and metallic lithium as anode, theoretically delivers 3-6 times higher energy density of 2600 Wh kg-1 than traditional lithium-ion batteries when sulfur and lithium are fully reacted. Besides, the cathode material sulfur is also naturally abundant, low cost, and environmental friendly. However, there are still several challenges preventing the lithium-sulfur battery far beyond practical application.
“On one hand, sulfur is highly electrical and ionic insulating. Its conductivity is even several-to-ten orders of magnitude lower than typical LiCoO2 or LiFePO4 cathode material for lithium-ion batteries, leading to 25-40 weight percent addition of conductive agents in the whole cathode, thus, hindering the full demonstration of the intrinsic high energy density,” Qiang told Nanowerk. “One the other hand, due to the multi-step and multi-phase reaction path, high soluble intermediate, always in form of chain-like polysulfide anions, generate at the cathode side, diffuse through the membrane, react with lithium anode, and shuttle back. During the whole process, polysulfides dissolve and irreversibly react with lithium and organic components, causing the destruction of cathode structure, depletion of lithium anode, and loss of active materials. Thus, very poor cycle life can be achieved.”
Actually, similar to advanced anode material such as silicon and tin, there is huge volume change (about 60-70 %) when sulfur is fully lithiated into lighter lithium sulfide, resulting in the failure of conductive scaffold and also the poor lifespan. To solve such multifaceted problems, multifunctional material with high electrical conductivity, interconnected ion pathway, and enough space for accommodating sulfur and retarding the diffusion of polysulfides need to be developed.
“Carbon material plays a vital role in advanced energy storage systems like lithium-sulfur battery due to their excellent conductivity, mechanical flexibility, and tailored morphology and surface chemistry” said Prof. Fei Wei, “Our group has investigated nanocarbon material for a long time and developed a series of methodologies for mass production of carbon nanotubes (CNTs) and graphene, as well as their application for energy storage. The sp2 nanocarbon possess extraordinary electron conductivity while limited specific surface area and confined space. Nanostructured porous carbon such as activated carbon and mesoporous carbon have high specific surface area and porosity but low conductivity due to the defective nature. Since both of the two cannot meet the requirement of lithium-sulfur batteries, the hybridization, or the ‘marriage’ of such two carbon will lead to a novel carbon nanoarchitecture inheriting the advantages and exhibiting superior functionality.”
Based on this concept, Hong-Jie Peng, a graduate student and the first author, developed an in-situ chemical vapor deposition strategy followed by hydrocarbon pyrolysis and chemical activation. A CNT/graphene/porous carbon nanoarchitecture with extraordinary electrical conductivity and hierarchical micro-/mesopores was fabricated for advanced carbon/sulfur composite cathode. Due to the rational marriage of the two carbon, the potential of carbon material as both electron/ion pathway and active mass reservoir was fully demonstrated. The as-obtained lithium-sulfur exhibited extending cycle life and superior power capability.
“We hope that the advanced carbon materials can help lithium-sulfur battery to be comparable to the engine-driven system for future electric transportation.” said Hong-Jie. The further study will focus on the increase of areal sulfur loading and actual content, as well as the innovation of membrane, anode, electrolyte, and the whole configuration of the cell. Besides, the hybridized carbon material can also find amazing application on supercapacitors, sodium-ion battery, and metal-air battery, et al.
“In fact, the marriage of sp2 nanocarbon and nanostructured porous carbon has not ended yet.” Qiang mentioned.