(Digital Presentation) Ti3C2 Tx MXene As an Anode in Li- and Na-Ion Batteries: Where Does Electrochemical Capacity Stem from?

Abstract

The unique properties of the Ti3C2 Tx MXene continue to attract attention within the energy storage field. Many of the previous reports on the use of this material in Li- and Na-ion batteries mainly concentrate on modifying the surface of the MXene or the cycling protocol in attempts to achieve higher specific capacities. Very few studies have, on the other hand, addressed the nature of the electrochemical reactions taking place during the electrochemical cycling of the material.This study aims at gaining a deeper understanding of the electrochemical behavior of freestanding Ti3C2 Tx electrodes without any binder and conductive additive. The results obtained with these electrodes, which were manufactured by vacuum filtration of MXene suspension in deionized water, are used to identify the electrochemical reactions yielding the observed capacities obtained in Li-metal cells using 1 M LiPF6 in EC:DEC (50:50 vol%) as the electrolyte and in Na-metal cells containing 1 M NaFSI in TEG-DME. Constant current and cyclic voltammetry cycling as well as in-house ex-situ XPS and synchrotron-based ex-situ HAXPES and XAS were used to identify the redox processes taking place upon the charging and discharging of the MXene electrodes.The results, obtained when cycling the freestanding Ti3C2 Tx MXene electrodes in Li-metal cells, indicate that the reversible capacity can be explained by redox reactions involving Ti species present in the Ti3C2 Tx surface layer. The charge storage mechanism is hence faradic. The results of XAS measurements carried out in the transmission mode likewise indicate that the redox reactions are surface confined and do not involve the inner C-Ti-C layer of the Ti3C2 Tx MXene.When cycling material between 0 and 3 V vs. Na+/Na in Na-metal cells, the specific capacity of the Ti3C2 Tx electrodes increases progressively with the cycle number (from 5 mAh/g on the first cycles to 50 mAh/g on the 200th cycle using a cycling rate of 10 mA/g). This effect is suggested to be connected to the deposition of sodium at about 0 V vs. Na+/Na as this count yield an activation of the electrodes. Comparisons of the cycling behavior of freestanding Ti3C2 Tx electrodes with those of monolithic TiO2 (amorphous) nanotube electrodes with nanotubes lengths of about 4 μm suggest that the behavior of the Ti3C2 Tx electrodes cannot be explained by the expected spontaneous formation of TiO2 on the surface of Ti3C2 Tx electrodes. The capacity for the TiO2 nanotube anode was relatively high, i.e., 160 mAh/g, on the first cycle but was seen to decrease to about 90 mAh/g after 100 cycles when cycling at a rate of 10 mA/g. The reasons for the differences seen between the Ti3C2 Tx and TiO2 nanotube electrodes will be discussed based on the general electrochemical behavior of the Ti3C2 Tx electrodes.