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Identifying and understanding charge storage mechanisms is important for advancing energy storage. Well-separated peaks in cyclic voltammograms (CVs) are considered key indicators of diffusion-controlled electrochemical processes with distinct Faradaic charge transfer. Herein, we report on an electrochemical system with separated CV peaks, accompanied by surface-controlled partial charge transfer, in 2D Ti 3 C 2 T x MXene in water-in-salt electrolytes. The process involves the insertion/desertion of desolvation-free cations, leading to an abrupt change of the interlayer spacing between MXene sheets. This unusual behavior increases charge storage at positive potentials, thereby increasing the amount of energy stored. This also demonstrates opportunities for the development of high-rate aqueous energy storage devices and electrochemical actuators using safe and inexpensive aqueous electrolytes.

One of the primary factors limiting further research and commercial use of the two-dimensional (2D) titanium carbide MXene Ti 3 C 2 , as well as MXenes in general, is the rate at which freshly made samples oxidize and degrade when stored as aqueous suspensions. Here, we show that including excess aluminum during synthesis of the Ti 3 AlC 2 MAX phase precursor leads to Ti 3 AlC 2 grains with improved crystallinity and carbon stoichiometry (termed Al-Ti 3 AlC 2 ). MXene nanosheets (Al-Ti 3 C 2 ) produced from this precursor are of higher quality, as evidenced by their increased resistance to oxidation and an increase in their electronic conductivity up to 20 000 S/cm. Aqueous suspensions of stoichiometric single- to few-layer Al-Ti 3 C 2 flakes produced from the modified Al-Ti 3 AlC 2 have a shelf life of over ten months, compared to 1 to 2 weeks for previously published Ti 3 C 2 , even when stored in ambient conditions. Freestanding films made from Al-Ti 3 C 2 suspensions stored for ten months show minimal decreases in electrical conductivity and negligible oxidation. Furthermore, oxidation of the improved Al-Ti 3 C 2 in air initiates at temperatures that are 100-150 °C higher than that of conventional Ti 3 C 2 . The observed improvements in both the shelf life and properties of Al-Ti 3 C 2 will facilitate the widespread use of this material.

MXenes are a family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides with a general formula of M n +1 X n T x , in which two, three, or four atomic layers of a transition metal (M: Ti, Nb, V, Cr, Mo, Ta, etc.) are interleaved with layers of C and/or N (shown as X), and T x represents surface termination groups such as -OH, ═O, and -F. Here, we report the scalable synthesis and characterization of a MXene with five atomic layers of transition metals (Mo 4 VC 4 T x ), by synthesizing its Mo 4 VAlC 4 MAX phase precursor that contains no other MAX phase impurities. These phases display twinning at their central M layers which is not present in any other known MAX phases or MXenes. Transmission electron microscopy and X-ray diffraction were used to examine the structure of both phases. Energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and high-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy were used to study the composition of these materials. Density functional theory calculations indicate that other five transition metal-layer MAX phases (M' 4 M″AlC 4 ) may be possible, where M' and M″ are two different transition metals. The predicted existence of additional Al-containing MAX phases suggests that more M 5 C 4 T x MXenes can be synthesized. Additionally, we characterized the optical, electronic, and thermal properties of Mo 4 VC 4 T x . This study demonstrates the existence of an additional subfamily of M 5 X 4 T x MXenes as well as a twinned structure, allowing for a wider range of 2D structures and compositions for more control over properties, which could lead to many different applications.