Solving the Capacitive Paradox of 2D MXene using Electrochemical Quartz‐Crystal Admittance and In Situ Electronic Conductance Measurements

Abstract

Fast ion adsorption processes in supercapacitors enable quick storage/ delivery of signifi cant amounts of energy, while ion intercalation in battery materials leads to even larger amounts of energy stored, but at substantially lower rates due to diffusional limitations. Intercalation of ions into the recently discovered 2D Ti 3 C 2 T x (MXene) occurs with a very high rate and leads to high capacitance, posing a paradox. Herein, by characterizing the mechanical deformations of MXene electrode materials at various states-ofcharge with a variety of cations (Li, Na, K, Cs, Mg, Ca, Ba, and three tetraalkylammonium cations) during cycling by electrochemical quartz-crystal admittance (EQCA, quartz-crystal microbalance with dissipation monitoring) combined with in situ electronic conductance and electrochemical impedance, light is shone on this paradox. Based on this work, it appears that the capacitive paradox stems from cationic insertion, accompanied by signifi cant deformation of the MXene particles, that occurs so rapidly so as to resemble 2D ion adsorption at solid-liquid interfaces. The latter is greatly facilitated by the presence of water molecules between the MXene sheets.

High-Voltage Symmetric Supercapacitor Based on 2D Titanium Carbide (MXene, Ti2CTx)/Carbon Nanosphere Composites in a Neutral Aqueous Electrolyte

“…This distortion may be because of greater difficulty for Li + ions to diffuse into or out of the materials' structure as the rate increases, therefore limiting effective interaction of the cations with the electrode surface to shallower adsorption sites. 19,[69][70][71] This behavior can also explain the line becoming rounded during charging at the higher voltages for the lower current density of 1 A·g −1 . Both the CV scans and the GCPL curves show no peaks.…”

“…The unstable cycling data may be related to the use of the high voltage and/or oxidation of the MXene as was also observed by Levi at al. 69 who noted that in long-time CV cycling, at potential sweeps beyond 1.1 V, there is deterioration due to changes in the mechanical properties of the electrodes related to electrode oxidation and associated with hydrodynamic dissipation phenomena occurring at the solid-liquid interface.…”

Electrochemical and in-situ X-ray diffraction studies of Ti 3 C 2 T x MXene in ionic liquid electrolyte

“…Then the peaks reversibly shift back to their original 2θ position when discharged from 1.5 V back to 0 V, indicating the increase of interlayer spacing. As the change of interlayer spacing can be induced by both electrostatic or steric effects during ion intercalation [6,[25][26][27][28], the reduction of interlayer spacing with increasing applied potential within the positive potential range can be explained in two ways: 1) electrostatic attraction between intercalated TFSI − anions and positively charged Ti 3 C 2 T x layers; 2) steric effects caused by the de-intercalation of EMI + cations, which has been reported in both aqueous and organic electrolytes elsewhere [14,29]. The exact identification of origins of the observed dimensional change can possibly be done by using electrochemical quartz crystal microbalance (EQCM) [29] or in-situ nuclear magnetic resonance (NMR).…”

“…As the change of interlayer spacing can be induced by both electrostatic or steric effects during ion intercalation [6,[25][26][27][28], the reduction of interlayer spacing with increasing applied potential within the positive potential range can be explained in two ways: 1) electrostatic attraction between intercalated TFSI − anions and positively charged Ti 3 C 2 T x layers; 2) steric effects caused by the de-intercalation of EMI + cations, which has been reported in both aqueous and organic electrolytes elsewhere [14,29]. The exact identification of origins of the observed dimensional change can possibly be done by using electrochemical quartz crystal microbalance (EQCM) [29] or in-situ nuclear magnetic resonance (NMR). In the negative potential range, the appearance of the (002) peak at a lower 2θ degree position can be ascribed to the pillared structure of Ti 3 C 2 T x flakes when intercalated by EMI + cations, which has already been observed for MXene [9,14] and other 2D materials [30,31].…”

EQCM Analysis of Redox Behavior of CuFe Prussian Blue Analog in Mg Battery Electrolytes

“…Conversely, as the diameter of the hydrated Mg 2+ ion is reported to be larger than 6 Å, 50 partial or full dehydration is necessary for the insertion of Mg 2+ ions; the partial dehydrations of Mg 2+ and other cosmotropic ions have been also confirmed by EQCM during adsorption into carbon materials 51 and a state-of-the-art multilayered 2D Ti 3 C 2 T x MXene, where T x stands for a general surface termination. 52 For a more quantitative discussion, the fraction of Mg 2+ ions as carrier ions was calculated from the mass change during the 2nd cycle by the following equation under the assumption that completely dehydrated Mg 2+ ions and SO 2− 4 ions are the carrier ions:…”