<i>Operando</i> Study of Mass, Thickness and Resistance Variation of the Ti₃C₂T_x MXene in a Water-in-Salt Electrolyte

MXenes are a versatile family of 2D materials with promising energy storage capabilities arising from their high electronic and ionic conductivity, large surface area, and potential for reversible surface redox processes. 1,2 Ti 3 C 2 T x MXene (T being =O, –OH, –Cl or –F terminations) exhibits distinct electrochemical behavior depending on the nature of the electrolyte. On one hand, in acidic electrolyte, Ti 3 C 2 T x displays pseudocapacitive behavior based on the Ti as redox center, involving proton exchange between the electrolyte and its surface terminations. 3,4 In contrast, it shows purely capacitive behavior in neutral to basic solutions, as well as in organic or ionic liquid electrolytes. 5,6 Furthermore, an intriguing behavior has been reported in LiCl water-in-salt (WIS) concentrated electrolyte, with the apparition of reversible surface-controlled redox-like behavior during Li + intercalation/deintercalation. 7,8 Using the combination of different operando techniques, including Electrochemical Quartz Crystal Microbalance (EQCM), Electrochemical Dilatometry (ECD) and in-plane resistance measurement, 9,10 we tracked the evolution of mass, thickness and electronic resistance of the Ti 3 C 2 T x MXene in both LiCl WIS and salt-in-water (SIW) diluted LiCl electrolytes to elucidate the charge storage mechanisms. Results show that different behaviours are observed in both cases, highlighting the key role of the MXene surface chemistry / electrolyte interactions. The study highlights the power of advanced operando techniques to further understand the charge/discharge storage mechanism of energy storage materials. A. VahidMohammadi, J. Rosen, and Y. Gogotsi, Science , 372 , eabf1581 (2021). Y. Gogotsi, Chem. Mater. , 35 , 8767–8770 (2023). C. Zhan et al., J. Phys. Chem. Lett. , 9 , 1223–1228 (2018). X. Mu et al., Adv. Funct. Mater. , 29 , 1902953 (2019). M. R. Lukatskaya et al., Science , 341 , 1502–1505 (2013). Z. Lin et al., Electrochemistry Communications , 72 , 50–53 (2016). X. Wang et al., ACS Nano , 15 , 15274–15284 (2021). D. Zhang, R. Wang, X. Wang, and Y. Gogotsi, Nat Energy , 8 , 567–576 (2023). V. Maurel et al., J. Electrochem. Soc. , 169 , 120510 (2022). A. Perju et al., J. Electrochem. Soc. , 171 , 110511 (2024). Figure 1