The feasibility of MXenes as energy dense cathodes for rechargeable Mg batteries is assessed, revealing major challenges but also some promise.

A versatile group of 2D carbide materials from the past decade, MXenes, have attracted attention for their rich chemistry and wide range of properties. The perhaps best known MXene, namely, Ti3C2T x , has been observed to stack in two distinct ways, and simulations show that this influences interlayer bonding energy and Li diffusion. In this DFT study, six types of Ti3C2T2 interlayer bonds resulting from O, F, and OH termination groups are assessed with respect to stability. It is shown that OH termination groups are highly stable up to 50% coverage, but unstable for higher coverage. A model to predict stacking type based on termination group chemistry shows that the degree of hydrogen bonding is the deciding factor. The model is also tested on V2CT2 and Zr3C2T2, giving similar results to those of Ti3C2T2. By calculating migration barriers for Ti3C2O2, we show that Li, Na, and Mg have orders of magnitude faster diffusion in the stacking favored by hydrogen bonds. XRD patterns calculated for both stackings show they are close to indistinguishable, highlighting the need for caution when classifying stacking.

The realization of sustainable and cheap Mg‐S batteries depends on significant improvements in cycling stability. Building on the immense research on cathode optimization from Li‐S batteries, for the first time a beneficial role of MXenes for Mg‐S batteries is reported. Through a facile, low‐temperature vacuum‐filtration technique, several novel current collector‐ and binder‐free cathode films were developed, with either dipenthamethylene thiuram tetrasulfide (PMTT) or S8 nanoparticles as the source of redox‐active sulfur. The importance of combining MXene with a high surface area co‐host material, such as carbon nanotubes, was demonstrated. A positive effect of MXenes on the average voltage and reduced self‐discharge was also discovered. Ascribed to the rich polar surface chemistry of Ti3C2Tx MXene, an almost doubling of the discharge capacity (530 vs. 290 mA h g−1) was achieved by using MXene as a polysulfide‐confining interlayer, obtaining a capacity retention of 83 % after 25 cycles.

Rechargeable Mg batteries (RMBs) represent a possible route for low-cost energy storage applications, but they are lacking a satisfactory cathode material. Conventional sulfur (S 8 ) cathodes have shown promise, yet they suffer from poor cycling stability and low reversibility. Here, we investigate the organosulfur compound dipentamethylene thiuram tetrasulfide (PMTT) as the source of redox active sulfur. In its pristine form with carbon black as a conductive additive, an initial discharge capacity of 295 mA h g −1 is reported, which is one of the highest capacities reported for an organosulfur compound for RMBs. A reaction mechanism is proposed, supported by density functional theory calculations. Through a mild heat treatment, a PMTT-derived sulfur/mesoporous carbon composite is investigated. PMTT's unique chemistry and the resulting molecular mixture of active and inactive components enable a high cycling stability (76% capacity retention in the 100th cycle after one formation cycle) and excellent rate performance (185 mA h g −1 at 500 mA g −1 ) for an RMB. The PMTTderived sulfur/mesoporous carbon composite outperforms reference cells with a conventional S 8 composite and, combining with further electrolyte development, may open up for cost-competitive RMBs.