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 Ti3C2T 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.

Two-dimensional (2D) materials offer distinct advantages for electrochemical energy storage (EES) compared to bulk materials, including a high surface-to-volume ratio, tunable interlayer spacing, and excellent in-plane conductivity, making them highly attractive for applications in batteries and supercapacitors. Gaining a fundamental understanding of the energy storage processes in 2D material-based EES devices is essential for optimizing their chemical composition, surface chemistry, morphology, and interlayer structure to enhance ion transport, promote redox reactions, suppress side reactions, and ultimately improve overall performance. This review provides a comprehensive overview of the characterization techniques employed to probe charge storage mechanisms in 2D and thin-layered material-based EES systems, covering optical spectroscopy, imaging techniques, X-ray and neutron-based methods, mechanical probing, and nuclear magnetic resonance spectroscopy. We specifically highlight the application of these techniques in elucidating ion transport dynamics, tracking redox processes, identifying degradation pathways, and detecting interphase formation. Furthermore, we discuss the limitations, challenges, and potential pitfalls associated with each method, as well as future directions for advancing characterization techniques to better understand and optimize 2D material-based electrodes.

MXenes, a thriving class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, demonstrate considerable potential in diverse electrochemical energy storage applications. To leverage MXenes for high-performance sulfur-based batteries, researchers have employed various strategies to modify their properties, aiming to tackle challenges such as the notorious shuttle effect induced by soluble polysulfides, sluggish redox reaction kinetics, and substantial volume expansion during the lithiation process. This review article offers an overview of MXene modification strategies, emphasizing their significant potential in adjusting the composition, surface chemistry, and morphology to address one or more challenges specially in sulfur cathodes. We first discuss internal regulation methods of MXene, including surface group engineering, heteroatom doping, and high-entropy MXene synthesis, which have been demonstrated to enhance MXene-polysulfide interactions and facilitate polysulfide conversion. Subsequently, we provide a summary of the recent design methods and advancements made in MXene-derived and MXene-based composites, with a particular emphasis on electronic structure reconstruction at the heterointerface and their synergistic roles in Li-S batteries. Following this, we outline the utilization of MXenes to address the challenges encountered in metal-sulfur batteries beyond Li-S batteries. Concluding the review, we offer prospects for the future development of utilizing MXenes in practical sulfur-based batteries.