|Cycling performance of the GS-S/CZIF8-D composite and the unwrapped S/CZIF8-D composite. Chen et al. Click to enlarge.|
A team of researchers led by Dr. Vasant Kumar at the University of Cambridge and Professor Renjie Chen at the Beijing Institute of Technology has devised a three-dimensional hierarchical sandwich-type graphene sheet-sulfur/carbon (GS-S/CZIF8-D) composite to address performance-related issues in Lithium-sulfur batteries such as low efficiency and capacity degradation.
The thin graphene sheet, wrapped around the sulfur/zeolitic imidazolate framework-8 derived carbon (S/CZIF8-D) composite, has excellent electrical conductivity and mechanical flexibility. This facilitates rapid electron transport and accommodates the changes in volume of the sulfur electrode. Compared with an unwrapped S/CZIF8-D sample, Li-S batteries with the GS-S/CZIF8-D composite cathode showed enhanced capacity, improved electrochemical stability up to 120 cycles, and relatively high Coulombic efficiency. An open access paper on the work is published in the journal APL Materials.
|Schematic of the preparation of a 3-D hierarchically structured graphene-sulfur/carbonZIF8-D composite. Click to enlarge.|
Lithium-sulfur batteries offer a theoretical specific energy densities approaching 2600 Wh kg−1 (by cell weight), compared to current Li-ion batteries at 130–220 Wh kg−1. In the near future, the researchers note, Li-S batteries are poised to attain both high gravimetric and volumetric energy densities beyond the values of 500 Wh kg−1 and 500 Wh l−1, respectively. Capable of achieving high power densities, a Li-S battery system is also highly tolerant against overcharging and does not suffer from memory effect.
For Li-S batteries, the foremost technical issue to overcome is poor S utilization and capacity to fade with cycling. Several reasons have been discussed in the literature for relatively poor active material utilization and cycle life. The dominant view involves parasitic loss of active S owing to the high solubility of intermediate long-chain polysulfide anions in the organic solvent and the resulting shuttle effect. Specifically, S is lost by dissolving in the solvent and further by shuttling away from the cathode and even more by reacting with the Li anode. The soluble components can move in both directions in different parts of cycling. The shuttle mechanism has been directly implicated as the cause for low S utilization following the initial discharge, which is exacerbated in subsequent charge-discharge cycle.
Other factors associated with S chemistry, such as its poor conductivity, incomplete reduction in some solvent systems, degradation of the S cathode by random precipitation of the discharge and charge products, and volume variations arising from significant density differences between S and the solid discharge products are also the barriers to superior electrochemical performance. Theoretically, the capacity of S is 1675 mAh g−1 as derived from the reduction of S8 to the most reduced state in Li2S. It is, therefore, not surprising that any approach which can minimize these problems can produce improved S utilization and cycle life.—Chen et al.
One approach being explored by a number of research teams is utilizing porous carbon in the cathode with the intention of:
trapping polysulfide intermediate products by adsorption in the porous structure to prevent loss of active material and maintain cycling stability;
ensuring sufficient space for accommodating volume expansion during reactions; and
providing a large conductive surface area for electrochemical reactions and also allowing for deposition of insoluble products.
One potential choice of matrix for housing sulfur is micro/mesoporous carbons derived from carbonizing metal-organic frameworks (MOFs)—crystalline structures comprising metal clusters or ions coordinated to organic ligands in the form of highly porous spatial networks. However, the authors note, the capacity of MOF-derived microporous carbons is relatively low due to the sulfur/carbon composites having several interfaces and grain boundaries, leading to several amorphous and irregularly connected carbon networks. As a result, the internal resistance of the sulfur cathode is high as these interfaces act as scattering centers for electron transport.
Graphene, already shown to be effective when applied in other sulfur cathodes, can act as a “bridge” to form inter-connected networks, thereby reducing the internal resistances from each component. Therefore, hybridization of graphene and MOF-derived microporous carbons could be a promising method to further improve electrochemical performance of Li-S batteries, the team proposed.
To explore this possibility, the team prepared the microporous carbon host by a one-step pyrolysis of Zeolitic Imidazolate Framework-8 (ZIF-8), a typical zinc-containing metal organic framework (MOF), which offers a tunable porous structure into which electro-active sulfur was diffused. This was then wrapped with graphene.
The good cycle stability can be attributed to the synergistic effect of microporous carbon from ZIF-8 and the highly conductive graphene sheet. The improved capacity is attributed to the fast charge-transfer kinetics enabled by an inter-connected graphene network with its high electrical conductivity. The results show that the composite structure of porous scaffold with conductive connection can be a promising electrode structure design for rechargeable batteries.—Chen et al.
In terms of applications, the novel battery design’s integration of energy storage with an ion/electron framework has opened the door for fabrication of high-performance non-topotactic (not involving a structural change to a crystalline solid) reactions-based energy storage systems.
The team will next focus on fabricating hybrid free-standing sulfur cathode systems to achieve high-energy density batteries, which will involve tailoring novel electrolyte components and building lithium protection layers to enhance the electrochemical performance of batteries, said Kai Xi, one of the Cambridge researchers.
This work was funded by the National Science Foundation of China (21373028), National 863 Program (2011AA11A256), and the EPSRC IAA Partnership Development Award (RG/75759).
Renjie Chen, Teng Zhao, Tian Tian, Shuai Cao, Paul R. Coxon, Kai Xi, David Fairen-Jimenez, R. Vasant Kumar and Anthony K. Cheetham (2014) “Graphene-wrapped sulfur/metal organic framework (MOF)-derived microporous carbon composite for lithium sulfur batteries,” APL Materials 2, 124109 doi: 10.1063/1.4901751