Graphene is one type of promising functional material for a variety of applications because of its extraordinary electronic and mechanical properties. However, the huge surface area and strong interactions between graphene layers causes the facile stacking with an interlayer distance of ca. 0.334 nm; this stacking results in a much smaller surface area of the as-obtained graphene and poor energy-storage performance. To amplify the intrinsic properties of graphene, its stacking needs to be effectively prevented.
Researchers have explored numerous novel approaches to inhibit the stacking of graphene. Most of them based on the introduction of spacers such as metal oxides, conducting polymers, carbon black, or carbon nanotubes into the interlayer spaces. However, such hybridization processes always result in changes in the intrinsic properties of graphene and/or induce poor interfaces.
New work by Prof. Qiang Zhang and Fei Wei’s research group in Department of Chemical Engineering at Tsinghua Univ. (China) provides not only new insights into the unstacked graphene structure, but also a general route to obtain unstacked graphene via facile templated catalytic growth and their applications for high-rate lithium-sulfur battery.
By introducing a large number of protuberances on graphene layers during the CVD synthesis, scientists have obtained intrinsically unstacked double-layer graphene that are with high specific surface area, excellent electrical conductivity, and mesoporous structure. The unstacked double layer graphene, described in the journal of Nature Communications in the March 3, 2014 could serve as excellent cathode materials for high-power lithium-sulfur batteries.
“We explored the idea of using mesoporous nanoflakes as the template,” says Qiang Zhang, an Associate Professor in Tsinghua Univ. “The graphene layers are deposited onto the mesoporous template and cast into its mesoporous structure, where the carbon atoms deposited in the mesopores form the graphene protuberances and act as spacers to prevent the stacking of the graphene layers deposited on both sides of the mesoporous flakes.”
Consequently, unstacked double-layer template graphene composed of two graphene layers with a large quantity of protuberances can be recovered after the removal of the mesoporous flakes.
“The presence of a large quantity of mesopores in the nanoflake template gives rise to protuberances with a high density of ca. 5.8 × 1014 m-2 and a size between 2 and 7 nm,” first-author Meng-Qiang Zhao explained to Nanowerk, “The protuberances play an important role in preventing the stacking of graphene layers. Besides, the presence of such protuberances on the surface of graphene can weaken the π-π interactions between graphene layers and thus prevent the stacking of neighboring double-layer templated graphene to a certain extent.” As a result, the as-obtained double-layer graphene shows a high specific surface area of 1628 m2 g-1, a pore size ranging from 2 to 7 nm, and a total pore volume of 2.0 cm3 g-1.
Lithium-sulfur batteries are one of the most promising energy storage devices due to their high energy density. However, their power density and poor cycling stability have always been a key issue for their practical application. ‘We try to fabricate lithium-sulfur batteries with excellent high-power performance with unstacked double-layer graphene as the cathode materials’ explains Fei Wei, a professor at the Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology. High reversible capacities of 1034 and 734 mA h g-1 were achieved at high discharge rates of 5 and 10 C, respectively. Even after 1000 cycles, high reversible capacities of ca. 530 and 380 mA h g-1 were retained at 5 and 10 C, with coulombic efficiency constants at ca. 96 and 98 %, respectively.
“The excellent high-power performance is attributed to the extraordinary electrical conductivity and unique mesoporous structure of the unstacked double-layer graphene,” Prof. Zhang explained. A high electrical conductivity of 438 S cm-1 for the unstacked double-layer graphene was determined. Even after the infiltration of sulfur, a high electrical conductivity of 107 S cm-1 can still be retained. The unstacked double-layer graphene’s unique porous structure allows the effective storage of sulfur in the meso-sized lamellar interlayer space, which gives rise to an efficient connection between the sulfur and graphene and prevents the diffusion of polysulfides into the electrolyte. Consequently, an excellent high-power performance of the lithium-sulfur cells with a high capacity and good stability can be achieved.
In the future, the researchers hope to precisely control the fine structure of unstacked double-layer graphene materials, and further explore their applications in the areas of environmental protection, nanocomposites, electronic devices, and healthcare because of their intrinsic large surface area, extraordinary thermal and electric conductivity, robust 3D scaffold, tunable surface chemistry, and biocompatible interface. ‘There is enough space to be explored’ says Prof. Zhang, “The concept of unstacked layered nanostructures are not limited to graphene, a new branch of chemistry evolving in the stabilization of nanostructures through 3D topological porous systems is foreseen.”
Source: Tsinghua Univ.