The majority of etching methods for synthesizing MXenes use water as the main solvent, which in turn limits direct use of MXenes in water-sensitive applications. In this work, we show that it is possible to etch, and delaminate, MXenes in the absence of water by using organic polar solvents and ammonium dihydrogen fluoride. We also demonstrate that electrodes made from Ti 3 C 2 T z etched in propylene carbonate, resulted in Na-ion battery anodes with double the capacity to those etched in water.

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The first MXene discovered, Ti3C2T z , was synthesized by etching aluminum, Al, from the nanolaminated MAX phase, Ti3AlC2, using hydrofluoric acid, HF. To delaminate the resulting MXene multilayers, MLs, it was necessary to increase the interlayer spacing, by first treating them with relatively large organic cations such as tetrabutylammonium hydroxide, TBAOH, dimethyl sulfoxide, DMSO, etc. When etched with a combination of LiF and HCl on the other hand, the Li cations spontaneously intercalated and no extra delamination step was needed. Herein, we attempt to understand why some molecules intercalate into the HF-etched MXene, while others do not. We find that treating HF-etched Ti3C2T z MLs with a base, like NaOH, renders them ion exchangeable. This base treatment was found to reduce the −F terminations on the MXene surfaces, which most likely weakens the interlayer hydrogen bonding and therefore allows for ion exchange and concomitant hydration. We exploit this nucleophilic dehalogenation to functionalize the Ti3C2T z surfaces using several different nucleophiles like sodium stearate, lithium ethoxide, and diisopropyl xanthogen polysulfide. We also demonstrate the effect of interlayer ions and other functional terminations on the electrochemical performance of Ti3C2T z in sodium ion and lithium sulfur batteries. Finally, we find that the interlayer spacing between MXene sheets derived using LiF + HCl increases dramatically when exposed to low-concentration salt solutions; this was attributed to osmotic swelling. This phenomenon was earlier observed in clays but is shown for the first time in the case of MXenes.

This past decade has seen extensive research in lithium-sulfur batteries with exemplary works mitigating the notorious polysulfide shuttling. However, these works utilize ether electrolytes that are highly volatile severely hindering their practicality. Here, we stabilize a rare monoclinic γ-sulfur phase within carbon nanofibers that enables successful operation of Lithium-Sulfur (Li-S) batteries in carbonate electrolyte for 4000 cycles. Carbonates are known to adversely react with the intermediate polysulfides and shut down Li-S batteries in first discharge. Through electrochemical characterization and post-mortem spectroscopy/ microscopy studies on cycled cells, we demonstrate an altered redox mechanism in our cells that reversibly converts monoclinic sulfur to Li2S without the formation of intermediate polysulfides for the entire range of 4000 cycles. To the best of our knowledge, this is the first study to report the synthesis of stable γ-sulfur and its application in Li-S batteries. We hope that this striking discovery of solid-to-solid reaction will trigger new fundamental and applied research in carbonate electrolyte Li-S batteries.