High-rate lithium and sodium storage devices that can deliver energy densities considerably higher than those of conventional aqueous or non-aqueous electrochemical capacitors have recently received considerable attention [1]. Li-ion capacitors (LICs) and sodium-ion capacitors (NACs) can combine the high-energy density of batteries and with a high-power density of supercapacitor by utilizing electrode materials that are capable of fast intercalation of Li⁺ and Na⁺ [1-4]. 2D transition metal carbides and/or carbonitrides called MXenes are a new family of 2D materials that are produced by selective etching and liquid exfoliation of MAX phases, a large group of ternary carbides and/or nitrides, where M is an early transition metal, A is a group A element and X is carbon and/or nitrogen [5]. Two-dimensional (2D) Ti₃C₂, which is produced by selective removal of Al atoms from Ti₃AlC₂ MAX phase, is the most studied MXene material which presents an excellent volumetric capacitance of around 900 F cm^-1 in some aqueous electrolytes [5]. However, like other 2D materials, Ti₃C₂ shows much lower capacitance in Li and Na ion containing organic electrolytes mainly due to the restacking of Ti₃C₂ sheets that decrease the accessibility of Li⁺ and Na⁺ ions to the electrode surface, thus hindering the redox reactions at the surface of the electrodes. Herein, we address this issue by reporting freestanding hybrid electrodes of Ti₃C₂ and polyaniline (PANI) synthesized by oxidant-free polymerization of aniline monomers on the surface of individual MXene sheets. The results show that a few-nanometer-thick PANI coating on the surface of single layer MXene sheets increases the layer spacing of the freestanding electrodes. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman analysis confirmed the formation of PANI thin films on the surface of Ti₃C₂ nanosheets. The conducting and electrochemically active PANI also contribute to the overall capacity of the electrodes, resulting in specific capacities as high as 200 Fg^-1 when tested as an LIC electrode. The PANI/Ti₃C₂ electrodes were also investigated as anode material for both Li-ion and Na-ion batteries and showed highly improved capacities compared to bare Ti₃C₂ electrodes.

 

Keywords: Ti₃C₂, MXene, 2D materials, Lithium capacitor, Batteries

 

References

1. A. Byeon et al., J. Power Sources. 326, 686-694 (2016).

2. C. E. Ren et al., ChemElectroChem. 3, 689-693 (2016).

3. X. Xie et al., Nano Energy26, 513-523 (2016).

4. J. Luo et al., ACS Nano11 (3), 2459-2460 (2017).

5. B. Anasori, M. R. Lukatskaya, Y. Gogotsi, Nat. Rev. Mater. 2, 16098 (2017).