Drexel researchers, along with colleagues at Aix-Marseille University in France, have synthesized two-dimensional carbon/sulfur (C/S) nanolaminate materials. Covalent bonding between C and S is observed in the nanolaminates, which along with and an extremely uniform distribution of sulfur between the atomically thin carbon layers make them promising electrode materials for Li-S batteries. A paper on their work is published in the journal Angewandte Chemie International Edition.
The international research collaboration led by Drexel’s Dr. Yury Gogotsi produced the nanolaminate by extracting the titanium from a three-dimensional material called a Ti2SC MAX phase. (Earlier post.) The resulting products are composed of multi-layers of C/S flakes, with predominantly amorphous and some graphene-like structures. The paper was selected as a VIP article and will be featured on the journal cover.
MAX Phases and MXene. The MAX Phases form a large family of ternary carbides with the general formula Mn+1AXn, where n = 1–3, M is an early transition metal, A is an A-group element (mostly IIIA and IVA), and X is C and/or N. Discovered two decades ago by Michel Barsoum, PhD, Distinguished professor in Drexel’s Department of Materials Science & Engineering, these layered ceramics have been used as the basis for much of Drexel’s materials research intended to find better materials for batteries.
Selective etching of the ‘A’ elements in MAX phases has yielded a new family of 2D early transition metal (e.g. titanium, vanadium, niobium, etc.) carbides and carbonitrides: “MXene”. These 2D materials offer a large variety of chemical compositions compared to graphene and they can exist as 3, 5, or 7 atomic layers.
MXenes may have a wide range of potential applications, including composite reinforcements, industrial catalysts, energy storage, transparent optical conductors, and electronic devices.
C/S nanolaminates. Sulfur-infiltrated carbon nanomaterials are currently considered promising cathode materials for Li-S batteries. In these materials, the uniform distribution of sulfur in carbon matrix and the strong interaction between carbon and sulfur are two important factors that affect the performance.
The carbon/sulfur nanolaminates synthesized by Gogotsi’s group demonstrate the same uniformity as the infiltrated carbon nanomaterials, but the sulfur in the nanolaminates is uniformly deposited in the carbon matrix as atomically thin layers and a strong covalent bonding between carbon and sulfur is observed. This may have a significant impact on increasing the life-span of next generation batteries.
To evaluate the electrochemical performance of the C/S nanolaminates as cathode materials for Li-S batteries, they constructed coin-type cells using a C/S nanolaminated cathode slurry prepared by mixing 80 wt.% of the latter with 10 wt.% acetylene carbon black and 10 wt.% polyvinylidene fluoride (PVDF) binder in a N-methylpyrrolidone (NMP) solvent dispersant.
Working electrodes were produced by coating the slurry onto Al foils, which were then dried at 60 °C for 24 h and cut into into circular pieces 12 mm in diameter. The areal density of the sulfur in the electrode was around 1.0 mg cm-2. A solution of 1.0 mol L-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in 1,3-dioxolane:1,2- dimethoxyethane (1:1 v/v) was employed as the electrolyte, and Li metal foil was used as the counter and reference electrodes. Polypropylene membranes (Celgard, Inc.) were used as separators.
|Cycling performance of the C/S nanolaminates showing the charge capacities at 0.1 C. Click to enlarge.|
The research team also showed that it is possible to extract Ti from other MAX phases, such as Ti3AlC2, Ti3SnC2 , and Ti2GeC, suggesting that electrochemical etching can be a powerful method to selectively extract the “M” elements from the MAX phases, to produce “AX” layered structures, that cannot be made otherwise. The latter hold promise for a variety of applications, such as energy storage, catalysis, etc.
This is a significant discovery, because there are more than 70 MAX phases in known existence.
It is not difficult to foresee that the ‘AX’ structures represent a new family of nanostructured materials, much of which will probably be 2D. The various ‘A’ and ‘X’ combinations already known make the ‘AX’ structures highly attractive for a number of potential applications, such as electrical energy storage and catalysis.—Yuri Gogotsi
This work was supported by the US Department of Energy, Office of Basic Energy Science.
Zhao, M.-Q., Sedran, M., Ling, Z., Lukatskaya, M. R., Mashtalir, O., Ghidiu, M., Dyatkin, B., Tallman, D. J., Djenizian, T., Barsoum, M. W. and Gogotsi, Y. (2015), “Synthesis of Carbon/Sulfur Nanolaminates by Electrochemical Extraction of Titanium from Ti2SC.” Angew. Chem. Int. Ed. doi: 10.1002/anie.201500110
Lukatskaya, M. R., Halim, J., Dyatkin, B., Naguib, M., Buranova, Y. S., Barsoum, M. W. and Gogotsi, Y. (2014), Inside Cover: ”Room-Temperature Carbide-Derived Carbon Synthesis by Electrochemical Etching of MAX Phases” (Angew. Chem. Int. Ed. 19/2014). Angew. Chem. Int. Ed., 53: 4728 doi: 10.1002/anie.201403559