Multiple unfavorable features, such as poor electronic conductivity of sulfur cathodes, the dissolution and shuttling of sodium polysulfides (Na 2 S n ) in electrolytes, and the slower kinetics for the decomposition of solid Na 2 S, make sodium−sulfur batteries (NaSBs) impractical. To overcome these obstacles, novel double-transition metal (DTM) MXenes, Mo 2 TiC 2 T 2 , (T = O and S) are studied as an anchoring material (AM) to immobilize higher-order polysulfides and to expedite the otherwise slower kinetics of insoluble short-chain polysulfides. Density functional theory (DFT) calculations are carried out to justify and compare the effectiveness of Mo 2 TiC 2 S 2 and Mo 2 TiC 2 O 2 as AMs by analyzing their interactions with S 8 /Na 2 S n (n = 1, 2, 4, 6, and 8). Mo 2 TiC 2 S 2 provides moderate adsorption strength compared to Mo 2 TiC 2 O 2 , therefore, it is expected to effectively inhibit Na 2 S n dissolution and shuttling without causing decomposition of Na 2 S n . The calculated Gibbs free energies of the rate-determining step for sulfur reduction reactions (SRR) are found to be significantly lower (0.791 eV for S and 0.628 eV for O functionalization) than that in vacuum (1.442 eV), suggesting that the SRR is more thermodynamically favorable on Mo 2 TiC 2 T 2 during discharge. Additionally, both Mo 2 TiC 2 S 2 and Mo 2 TiC 2 O 2 demonstrated effective electrocatalytic activity for the decomposition of Na 2 S, with a substantial reduction in the energy barrier to 1.59 eV for Mo 2 TiC 2 S 2 and 1.67 eV for Mo 2 TiC 2 O 2 . While Mo 2 TiC 2 O 2 had superior binding properties, structural distortion is observed in Na 2 S n , which may adversely affect cyclability. On the other hand, because of its moderate binding energy, enhanced electronic conductivity, and significantly faster oxidative decomposition kinetics of polysulfides, Mo 2 TiC 2 S 2 can be considered as an effective AM for suppressing the shuttle effect and improving the performance of NaSBs.

The Na−S and Li−S batteries are in the forefront to supplant ubiquitously used lithium-ion batteries. The understanding of mechanistic differences between Na−S and Li−S is critical to enable the inter-transfer of developed technologies toward designing high-performance cathode materials. The anchoring materials (AMs) are required to overcome the performance-limiting factors such as sluggish kinetics of metal polysulfides' (M 2 S n , M = Na and Li, n = 1−8) conversion reactions and their dissolution into electrolytes. This study undertakes the challenges to critically understand the role of AMs on the polysulfide chemistry in both the batteries. We employ firstprinciples density functional theory simulations to comprehensively examine the adsorption mechanisms of M 2 S n and the kinetics of sulfur reduction reactions (SRRs) and the catalytic decomposition of short-chain polysulfides across Na−S and Li−S batteries on pristine and vanadium (V) single-atom catalyst embedded WSe 2 (V@WSe 2 ) substrates. We found that pristine WSe 2 cannot immobilize the higher-order M 2 S n ; however, V@WSe 2 endows adequate binding energies to trap the higher-order M 2 S n . The degree of M 2 S n adsorption strengths and the effectiveness of the V@WSe 2 varies between Na−S and Li−S systems. We elucidate the underlying mechanistic details with the aid of charge transfer, bond strength, and density of state analysis. Importantly, our simulations reveal that, in V@WSe 2 , the rate-limiting step of the SRR is kinetically faster in Li−S, whereas the oxidative decomposition of the discharge end product M 2 S exhibits accelerated kinetics in Na−S batteries. These findings are pivotal to understand the role of AMs in the design of cathode materials for addressing the performance-limiting factors in Na−S and Li−S batteries, in particular, and metal−sulfur batteries, in general.

The dissolution of polysulfides into electrolytes and sluggish electrochemical conversion kinetics primarily impede the practical realization of Li–S batteries. Homogeneous catalysis is an effective strategy to overcome the challenges involved under lean electrolyte conditions. Metallocenes, a class of organometallic compounds, hold promise to anchor and catalyze polysulfides. In this study, we used first-principles density functional theory (DFT) simulations to understand the role of metallocenes (using titanocene (TiCp2) as an archetypical example) as homogeneous catalysts in the electrolyte medium to suppress the shuttle effect and promote the reaction kinetics. The calculated electrochemical stability window of TiCp2 reveals that the composition is electrochemically inactive in the operating potential range of Li–S batteries and can thus be leveraged as an additive to expedite the reduction kinetics of soluble lithium polysulfides (LiPSs). We studied the detailed characteristic behavior of LiPS interactions with TiCp2 in both gas and solvent phases and the kinetics of elementary sulfur reduction reactions (SRRs). We found that TiCp2 provides adequate binding toward various LiPSs to mitigate the shuttle effect, and the structural integrity of LiPSs is well retained without any chemical decomposition. The catalyzing effect of TiCp2 is evident from the observed significant reduction in the SRR barriers, particularly for the rate-determining step, which is expected to favorably promote the deposition of Li2S on the cathode surface. We further propose a mechanistic scheme of TiCp2 homogeneous catalyst-boosted sulfur redox cycles. Overall, our simulations predicted moderate binding and improved kinetics of polysulfide chemistry with the metallocene-based homogeneous catalyst and are expected to lead to a paradigm shift in the design of organometallic additives for achieving high-performance metal–sulfur batteries.

The use of trinitrotoluene (TNT) in industrial processes or military operations presents a significant threat to both the environment and human health due to its toxicity. Recently, it has been discovered that bis(1,2,4-oxadiazole)bis(methylene) dinitrate (BOM) can be an appropriate substitute of TNT due to its low sensitivity, high detonation velocity, and nearly insignificant impact on the surrounding environment. In this study, we utilize molecular dynamics (MD) simulations with a ReaxFF force field to investigate the thermomechanical and chemical response of BOM to shock loading. We simulate shocks using the Hugoniostat technique and observe shock-induced, volume-expanding exothermic reactions following a short induction time for strong enough insults. We analyze the shock behavior at various pressures to determine the conditions necessary to initiate detonation and evaluate the consequent events of detonation. A transition between unreacted and reacted materials has been observed and several detonation properties, such as detonation pressure and velocity, have been calculated at the Chapman–Jouguet state. We elucidate the reaction initiation pathways by predicting the intermediates and final products of the exothermic reaction. The quantity of intermediates and products has been studied for different applied shock loadings to understand the effect of loadings on chemical reactions. This study illustrates how reactive MD simulations can be used to characterize the physics and chemistry of high-energy materials subjected to shock loading, and we believe that our research can assist to shed light on numerous features of BOM that may establish it as a viable alternative to TNT.