Recently, a family of layered ternary transition-metal carbides and/or nitrides, known as the MAX phases, received increasing attention as precursors for two-dimensional (2D) transition-metal carbides and/or nitrides known as MXenes. The MAX phases have the general formula M_n+1AX_n, where n= 1 to 3 and M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen.^1-2 By selectively etching the A-element layers from MAX phases, MXenes have been synthesized. Given that the MXene surfaces are terminated with OH, and F groups resulting from the etching process, it is more accurate to refer to them as M_n+1X_nT_x, where T represents the surface terminating groups, such as O, F and OH. There are more than 70 compounds known in the MAX family to date (plus dozens of solid solutions),³ most of which offer good electronic conductivity, lamellar structure, and temperature and environmental stability. As-prepared MXenes exist in a multilayered structure, which appears as stacked multilayer flakes. The multilayered structures can be delaminated into single-layer flakes by intercalation and/or sonication.^4-5 Due to their high specific surface areas, metallic conductivity, and hydrophilic surfaces, MXenes have shown promising performance as electrodes for supercapacitors. MXenes are also promising anode materials for LIBs due to their excellent conductivity and cation intercalation capability.^6-7 Nb4C3 MXene shows stable capacity, also at high rates (Figure 1). A capacity of 410 mAh g⁻¹ for a Ti₃C₂ MXene “paper” anode at 1C rate without adding any binder has been reported. This Ti₃C₂ MXene “paper” was fabricated by intercalating Ti₃C₂ with dimethyl sulfoxide, bath sonication for 6 h, and filtering the Ti₃C₂ colloidal solution.^8-9 The experimental procedure is very simple. MXene-based LIB anodes exhibited Li-ion capacity up to 800 mAh g⁻¹, good rate performance, and excellent cycling stability.^9-10 These results suggest a potential for MXene-supported hybrid electrodes for energy storage applications.

By decreasing the particle size, we successfully demonstrated that not only MXenes but also their precursors - MAX phases, such as Ti₂SC and Ti₃SiC₂, exhibit Li-ion storage capacity and show promise as LIB anode materials. Capacities of 180 and 150 mAh g⁻¹ were achieved by submicrometer size Ti₂SC and Ti₃SiC₂, respectively, accompanied by good cycle life and excellent rate performance.¹¹ It opens the door to exploring a large family of potential anode materials.

1. Wang, X., et al. Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors. Nature communications 2015, 6.

2. Mashtalir, O.; Lukatskaya, M. R.; Zhao, M. Q.; Barsoum, M. W.; Gogotsi, Y., Amine-Assisted Delamination of Nb2C MXene for Li-Ion Energy Storage Devices. Advanced Materials 2015, 27 (23), 3501-3506.

3. Hu, C.; Zhang, H.; Li, F.; Huang, Q.; Bao, Y., New phases’ discovery in MAX family. International Journal of Refractory Metals & Hard Materials 2013, 36, 300-312.

4. Kim, S. J.; Naguib, M.; Zhao, M.; Zhang, C.; Jung, H. T.; Barsoum, M. W.; Gogotsi, Y., High mass loading, binder-free MXene anodes for high areal capacity Li-ion batteries. Electrochimica Acta 2015, 163, 246-251.

5. Ling, Z.; Ren, C. E.; Zhao, M. Q.; Yang, J.; Giammarco, J. M.; Qiu, J.; Barsoum, M. W.; Gogotsi, Y., Flexible and conductive MXene films and nanocomposites with high capacitance. Proceedings of the National Academy of Sciences 2014, 111 (47), 16676-81.

6. Naguib, M.; Come, J.; Dyatkin, B.; Presser, V.; Taberna, P. L.; Simon, P.; Barsoum, M. W.; Gogotsi, Y., MXene: a promising transition metal carbide anode for lithium-ion batteries. Electrochemistry Communications 2012, 16 (1), 61-64.

7. Sun, D.; Wang, M.; Li, Z.; Fan, G.; Fan, L. Z.; Zhou, A., Two-dimensional Ti₃C₂ as anode material for Li-ion batteries. Electrochemistry Communications 2014, 47 (10), 80-83.

8. Mashtalir, O.; Naguib, M.; Mochalin, V. N.; Dall’Agnese, Y.; Min, H.; Barsoum, M. W.; Gogotsi, Y., Intercalation and delamination of layered carbides and carbonitrides. Nature communications 2013, 4 (2), 216-219.

9. Luo, J.; Tao, X.; Zhang, J.; Xia, Y.; Huang, H.; Zhang, L.; Gan, Y.; Liang, C.; Zhang, W., Sn⁴⁺ Ions Decorated Highly Conductive Ti₃C₂ MXene: Promising Lithium-Ion Anodes with Enhanced Volumetric Capacity and Cyclic Performance. ACS Nano 2016, 10 (2).

10. Ren, C. E.; Zhao, M. Q.; Makaryan, T.; Halim, J.; Boota, M.; Kota, S.; Anasori, B.; Barsoum, M. W.; Gogotsi, Y., Porous Two-Dimensional Transition Metal Carbide (MXene) Flakes for High-Performance Li-Ion Storage. ChemElectroChem 2016, 3 (5), 689-693.

11. J. Xu, M.-Q. Zhao, Y. Wang, W. Yao, C. Chen, B. Anasori, A. Sarycheva, C. E. Ren, T. Mathis, L. Gomes, Z. Liang, Y. Gogotsi, Demonstration of Li-ion capacity of MAX phases, ACS Energy Letters, 2016, 1, 1094−1099.