Researchers from Hunan University and Changsha University in China have designed 3D hierarchical porous nitrogen-doped aligned carbon nanotubes (HPNACNTs) with well-directed 1D conductive electron paths as scaffold to load sulfur for use as a high-performance cathode in Li-S batteries. A paper on their work is published in the Journal of Power Sources.
The HPNACNTs have abundant micropores, mesopores and macropores with a relatively high specific surface area and a large total pore volume. The sulfur-HPNACNTs (with 68.8 wt% sulfur) composite exhibits a high initial discharge capacity of 1340 mAh g−1 at 0.1 C and retains as high as 979 mAh g−1 at 0.2 C after 200 cycles. It also shows high reversible capacity at high rates (817 mAh g−1 at 5 C).
As often stated, Li-sulfur batteries are extremely attractive as high-capacity next-generation energy storage systems due to their high theoretical specific capacity (S: 1675 mAh g-1) and excellent theoretical energy density (2600 Wh kg-1), low cost, environment friendliness and abundance.
Equally well-known are the basic obstacles to commercialization: the intrinsic insulative properties of sulfur; the dissolution of intermediate polysulfides; the shuttle effect in the electrolyte; and large volume expansion of the sulfur (~80%) cathode, which lead to its poor rate performances and severe capacity fade.
Numerous approaches have been taken to addresses these problems, including the design of conductive polymer/sulfur composites and the construct of carbon/sulfur composites such as carbon spheres/sulfur; hollow carbon spheres/sulfur; carbon nanotubes/sulfur; carbon nanofiber/sulfur; graphenes/sulfur; and porous carbon/sulfur.
Although porous carbon materials improve sulfur utilization and can alleviate the dissolution of lithium polysulfides, the C/S composites with ordered pore structures usually suffer from kinetic inhibition to Li+ diffusion within the framework due to carbon pores fully or partially filled with sulfur, causing inferior rate capability.
To address this, researchers developed hierarchical porous carbon structures—i.e., a combination of macro-, meso- and micro-pores—to enhance electrolyte permeability and electrolyte accessibility.
However, the authors of the new paper note, generally, the hierarchical porous carbon prepared by common methods shows a low graphitization degree and often exhibits poor electrical conductivity. This limits the electron transfer and thus restricts the rate performance.
As a way to enhance the electrical conductivity, researchers turned to carbon nanotubes (CNTs) to act as the scaffold for sulfur due to their excellent conductivity, porosity and mechanical properties. In particular, nitrogen-doped aligned CNTs (NACNTs) have been shown to improve electrolyte accessibility and charge transport capability significantly because of its well defined regular pore structure and the well 1D directed conductive electron paths, as well as the additional electron transport pathways.
These, in turn, have their own issues. The NACNTs/S composite exhibits low sulfur loading and relatively high decay rates, resulting from relatively low specific surface areas (SBET); specific total pore volume (VT) of NACNTs; and the weak interface bonding between S and the NACNTs.
According to the advantages of hierarchical porous structure in Li-S batteries, therefore, it is necessary to create hierarchical pores in NACNTs to increase SBET and VT and achieve high-electrochemical performance for sulfur-CNT electrode.
Based on this promising strategy, we report a rational 3D hierarchically porous nitrogen-doped aligned carbon nanotubes (HPNACNTs) array with 1D conductive electron paths to encapsulate sulfur.—Deng et al.
|Schematic illustration of the procedure for preparing HPNACNTs-S.|
The researchers purified NACNTs with nitric acid and then activated them with KOH. Hierarchical pores are introduced into nanotube walls, opening the inner cavities of the bamboo-like NACNTs an achieving interconnected macro/mesopores. They used a melt-diffusion method to load the scaffold with sulfur.
The resulting composite has a number of advantages, including:
the space between the nanotubes and the interconnected mesopores not only contributed to loading the sulfur but also facilitated Li+ transport at high rates so as to ensure high rate capability;
the 1D directed conductive electron path and the graphite layers inside the cavities provided rapid pathways for electronic transport to enhance the electrical conductivity;
the opened bamboo-like NACNTs provided high pore container and large pore volume for sulfur encapsulation; and
the unique hierarchical micro-meso-macro architectures are beneficial for high sulfur loading and for ion-transport kinetics enhancing.
Weina Deng, Aiping Hu, Xiaohua Chen, Shiying Zhang, Qunli Tang, Zheng Liu, Binbin Fan, Kuikui Xiao (2016) “Sulfur-impregnated 3D hierarchical porous nitrogen-doped aligned carbon nanotubes as high-performance cathode for lithium-sulfur batteries,” , Volume 322, Pages 138-146 doi: