Operando X‐Ray Diffraction Study of MXene Electrode Structure in Supercapacitors with Alkali Metal Electrolytes

Ti-MXene is a promising electrode material for supercapacitors. The layered structure of MXene expands due to swelling in electrolytes allowing the penetration of ions into the interlayers. A study of effects related to the match between the size of cations in hydrated or dehydrated state and the interlayer distance of MXene is performed here using operando X-ray diffraction (XRD) in capillary-size supercapacitors with alkali metal chloride electrolytes. The supercapacitors are studied during charging and discharging over several cycles revealing structural changes at both MXene electrodes. Experiments reveal an expansion of the MXene c-lattice in LiCl, NaCl, and KCl electrolytes (compared to the expansion in pure water) under an increase of applied voltage from 0 to 1 V and structural oscillations related to a change of polarity. The interlayer spacing of MXene remains close to the water-swollen state in RbCl, CsCl, and NH₄Cl electrolytes showing no further expansion as a function of applied voltage. Only rather small variations of interlayer spacing are found in H₂SO₄ electrolyte during tens of charge-discharge cycles. Analysis of the match between the sizes of ions and the width of MXene interlayers demonstrates that some cations and anions could be inserted into MXene interlayers only in dehydrated state.