The realization of the broad range of application of MXenes relies on the successful and reproducible synthesis of quality materials of tailored properties. To date, most MXenes have been produced making use of acid-based etching methods, yet an in-depth understanding of etching processes is lacking. Herein, we have engaged in a comprehensive study of the multiple variables involved in the synthesis of V₂CT_x with focus on the properties of etched materials. Two main sets of experiments were considered, each using a different V₂AlC precursor and a range of synthesis variables including reaction time and temperature, mixing rate, and type of acid. Correlations of synthesis conditions-materials properties were investigated using a broad range of characterization techniques including analytical methods, scanning and transmission electron microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Findings indicated the crucial relevance of properties of the MAX precursor such as elemental composition, particle size, and crystal structure on etching processes and properties of etched materials. Particularly, depending on the MAX precursor, two etching patterns were identified, core-shell and plate-by-plate, the latter describing a more efficient etching. Combined studies of elemental composition, crystal structure, and yield quantification allowed us to evaluate the effectiveness of etching processes. XRD studies revealed key crystal-structure-type of acid correlations showing advantages of using a HF/HCl mix over only HF. Analytical methods XRD and XPS delivered insights into undergoing chemical processes and their influence on bulk and surface chemistry of etched materials. The relevance for reaction kinetics of highly correlated variables such as reaction vessel dimensions, mixing efficiency, and reaction temperature was recognized. For the first time, a MXene synthesis has been investigated comprehensively highlighting its multivariable nature and the high variable intercorrelation, opening up venues for further investigation on MAX and MXene synthesis.

Monolayer V${}_4$C${}_3$T${}_z$ is synthesized and its atomic structure is studied using high-end transmission electron microscopy. X-Ray diffraction reveals key 3D to 2D crystal transformations as a V${}_4$AlC${}_3$ crystal is transformed into V${}_4$C${}_3$T${}_z$ in synthesis processes. The charge storage properties of V${}_4$C${}_3$T${}_z$ film electrodes are investigated for supercapacitor applications in 3 M H${}_2$SO${}_4$. V${}_4$C${}_3$T${}_z$ film electrodes shows an excellent capacitance of up to 469.6 F g${}^{-1}$ and 845.7 F cm${}^{-3}$, rate performance up to 30 A g${}^{-1}$ and cycling stability up to 10,000 cycles. A combination of electrochemical kinetics/mass transport models, staircase potentio-electrochemical impedance spectroscopy and in situ X-Ray absorption spectroscopy reveals, for the first time for this MXene, the underlying charge storage mechanisms, consisting of double layer capacitance, pseudocapacitance and a minor contribution from mass transport-controlled processes. The latter two implying a outstanding redox activity superior to Ti-based MXenes. The stability in standard environments, mechanical flexibility and the demonstrated excellent charge storage performance of V${}_4$C${}_3$T${}_z$ makes it one of the best candidates for supercapacitor applications, especially in miniaturized devices.

The use of highly concentrated electrolytes to enlarge the operational electrochemical window of MXenes is a strategy to enhance their energy density. Here, we demonstrate that V$$ _4 $$ C$$ _3 $$ T$$ _z $$ can operate in a -0.7 to 0.8 V vs . Ag electrochemical window in a 17.5 m LiBr/H$$ _2 $$ O electrolyte achieving a high capacity/capacitance of 237.1 C g-1/745.5 C cm-3/158 F g-1, electrode energy density of 49.4 Wh kg-1/155.3 mWh cm-3, and a high cycling stability up to 10, 000 cycles. This performance is superior to previously reported MXenes, including Ti$$ _3 $$ C$$ _2 $$ T$$ _z $$ and Ti$$ _2 $$ CT$$ _z $$ tested in water-in-salt electrolytes and hydrate melts. We demonstrate the key role of electrolyte concentration in maximizing the electrochemically stable window. Electrolyte formulations in the low-concentration (5 m, 7.5 m and 10 m) and high-concentration (12.5 m, 15 m, 17.5 m, and 19 m) regimes were investigated. The best performance balance of capacity, capacity retention, Coulombic efficiency and cycling stability was achieved in the 17.5 m electrolyte. Electrochemical methods showed that this electrolyte formulation enabled the stabilization of the electrode against the hydrogen evolution reaction and oxidation processes at negative and positive potentials vs . Ag, respectively, where an interfacial film at the electrode-electrolyte interface, confirmed by electrochemical impedance spectroscopy, played a key role. Physical properties of the electrolyte were correlated to electrode performance. Importantly, this optimum performance was achieved without reaching the electrolyte concentration at the LiBr solubility limit at room temperature (19 m), which undermines rate performance and brings other operational issues.

The practical implementation of P2-Na0.85Li0.17Ni0.21Mn0.64O2 as sodium-ion battery cathode has been hampered by problems of self-discharge and poor cycling stability caused by electrolyte oxidation. Here, we developed an electrolyte formulation that makes use of succinic anhydride as a key additive to alleviate these drawbacks. The composition of the electrolyte was optimized and the so-optimized electrolyte enables a safe performance of the cathode in a 2-4.2 V vs. Na+/Na electrochemical window. Electrochemical studies including electrochemical impedance spectroscopy and microscopy studies showed that succinic anhydride favours the formation of a cathode electrolyte interface that protects the cathode against electrolyte oxidation. As compared to the electrodes tested in the succinic anhydride-free electrolyte, self-discharge processes were substantially reduced and cycling stability (up to 100 cycles, 2-fold improvement) and Coulombic efficiency were improved. More remarkable was the improvement on power density performance. The cathode tested in the optimized electrolyte performed up to the high current density of 5.78C delivering a high power of 1.88 kW kg−1, whereas the cathode tested in the SA-free electrolyte faded readily at 1.77C. Equally, the cathode showed a much more stable average voltage of 3.17-2.89 V vs. Na+/Na, over such a wider range of operating current rates. The cathode delivered a maximum energy density of 377 Wh kg−1. Importantly, SA also aided in preventing corrosion of aluminum current collectors in the electrochemical window of interest.