Hybrid Electrodes of Conducting Polymers and Two-Dimensional Ti3C2 (MXene) for Li and Na Ion Storage

Abstract

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