Solvation structure engineering with co-solvents enables tunable charge storage mechanisms in MXenes

• ACN co-solvents tune Na⁺ solvation, enabling new high-potential MXene intercalation. • High-potential intercalation raises MXene capacitance by ∼30 % in aqueous media. • Increased ACN ratio shifts MXene’s charge storage from non-Faradaic to redox-active. • At 25 % ACN, MXene shows higher rate retention from fast Na⁺-H 2 O intercalation. Modulating ion-solvent interactions offers a powerful approach to tune the desolvation process, which in turn influences both the capacity and kinetics of electrochemical charge storage. This influence is particularly complex in 2D MXenes due to their surface redox activity and flexible interlayer spacing and thus remains underexplored. In this study, we investigate how tuning the Na + solvation structure using acetonitrile (ACN) co-solvents affects charge storage mechanism of Ti 3 C 2 T x MXene. The addition of ACN enables a new intercalation process at relatively positive potential, which enhances the overall capacitance by ∼30 %. More interestingly, varying the ACN content leads to a transition in the charge storage mechanism of this additional process from non-Faradaic to redox-active. At lower ACN concentrations, strongly solvated Na + ions intercalate rapidly through a primarily non-Faradaic process, resulting in even better rate retention (72 % at 1 V s -1 ) than in the pure aqueous electrolyte. Meanwhile, higher ACN content (>50 %) promotes ion desolvation, enabling distinct redox activity (confirmed by in-situ UV–vis) but reduces rate capability. These findings demonstrate a clear correlation between solvation structure and charge storage mechanism in 2D materials, offering a rational strategy to optimize performance via co-solvent design.