2D niobium and vanadium carbides as promising materials for high-power Li-ion batteries; extending MXenes
Researchers at Drexel University report in a paper in the Journal of the American Chemical Society on the potential for 2D niobium and titanium carbide materials as high-power electrode materials for Li-ion batteries (LIBs). Earlier this year in a paper in Science, the team had reported on the high volumetric capacitance and intercalation of other “MXene” structures. (Earlier post.)
Testing of the two new MXenes—Nb2CTx and V2CTx—as electrodes materials in LIBs showed that each has its own voltage profile. Nb2CTx showed good reversible capacity (170 mAh·g-1 at 1C) at lower lithiation voltages; V2CTx showed higher capacities (210 mAh·g−1 and 260 mAh·g−1) at higher lithiation voltages. Both Nb2CTx and V2CTx showed excellent capability to handle high cycling rates (10C), suggesting fast Li diffusion between MXene layers and potential use in high power applications, the team found.
Dr. Michel W. Barsoum and Dr. Yury Gogotsi, professors in Drexel’s College of Engineering, discovered several years ago that atomically thin, two-dimensional materials—similar to graphene—that have good electrical conductivity and a surface that is hydrophilic, or can hold liquids. They named these new materials “MXenes,” which hearkens to their genesis through the process of etching and exfoliating atomically thin layers of aluminum from layered carbide “MAX phases.” (The latter also discovered at Drexel about 15 years ago by Barsoum.)
Recently, our discovery of 2D early transition metal carbides and carbonitrides that we labeled MXenes further extended the family of 2D inorganic materials to include: Ti3C2, Ti2C, Ta4C3, (Ti0.5,Nb0.5)2C, (V0.5,Cr0.5)3C2, and Ti3CN...Potential applications for MXenes range from conductive reinforcement fillers for polymers, to catalysts and sensors, transparent conductors, and many others. Of special interest to this work is the use of MXenes as electrode materials in electrical energy storage such as supercapacitors, lithium ion batteries (LIBs), and lithium-ion capacitors.
The lithiation and delithiation mechanisms were found to be Li intercalation and deintercalation between the MXene layers. In general, MXenes with n = 1, viz., M2X ...should have higher gravimetric capacities compared to their higher order counterparts such as M3X2 or M4X3, because the former have less atomic layers compared to the latter (3 atomic layers vs 5 and 7, respectively). Furthermore, M2X-based MXenes should possess higher specific surface areas as compared to their higher order counterparts. Herein we report, for the first time, on the synthesis of two new 2D phases, Nb2CTx and V2CTx, and their Li uptake and cyclability at high rates.—Naguib et al.
In the analysis of the electrochemical testing results, the team noted that although the reversible capacity of MXenes at high cycling rates (i.e., 10 C) is comparable to titania-based anodes, the latter have maximum theoretical capacities of the order of 170 mAh·g−1 even at slow scan rates, while V2CTx displayed a reversible capacity of up to 260 mAh·g−1 at 1 C.
Further, the results were obtained on just synthesized and not well-purified compounds; refinement could deliver some upside.
The higher rate performances, however, are encouraging and suggest that Nb2CTx and V2CTx can be used as promising electrode materials in LIBs, especially for high power applications. For example, the Li-capacities of additives-free fully delaminated Ti3C2Tx electrodes were roughly 4 times those of non-delaminated Ti3C2Tx.—Naguib et al.
Michael Naguib, Joseph Halim, Jun Lu, Kevin M. Cook, Lars Hultman, Yury Gogotsi, and Michel W. Barsoum (2013) “New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries”, Journal of the American Chemical Society doi: 10.1021/ja405735d