RESUMO
Room-temperature sodium sulfur batteries (RT-NSBs) are among the promising candidates for large-scale energy storage applications because of the natural abundance of the electrode materials and impressive energy density. However, one of the main technical challenges of RT-NSBs is the shuttle effect by which active redox intermediates (i.e., sodium polysulfides Na2Sn, n = 1-8) are dissolved in electrolytes, which hamper the battery reversibility. The interfacial interplays between Na2Sn and the electrodes (or electrolytes) at the atomic level thus play an intrinsic role in elucidating the shuttle effect. This work reports the ab initio calculations to unravel the suppression of the shuttle effect using titanium carbide MXenes (Ti3C2Tx, Tx = F, O) as the cathode additives. The findings reveal that the shuttle phenomenon is efficiently resolved because the immense chemical bonding of Na2Sn-Ti3C2Tx interfaces competitively surpasses the binding magnitudes of Na2Sn-electrolyte interaction. The analysis of the electronic density of states and charge density further manifests that there is charge donation from the Na-3s orbital of Na2Sn to the unfilled F(O)-2p orbitals of metallic Ti3C2Tx. The metallicity of the Ti3C2Tx remains preserved during the entire course of the redox process, ensuring the rapid electrochemical kinetics. Furthermore, the presence of Ti3C2Tx additives drastically reduces the dissociation barrier of the final redox product Na2S, yielding the efficient utilization of sulfur during the discharge process. This work has proposed the unexplored functionality of Ti3C2Tx as the anchoring materials for RT-NSBs.
RESUMO
Green phosphorus and its monolayer variant, green phosphorene (GreenP), are the recent members of two-dimensional (2D) phosphorus polymorphs. The new polymorph possesses the high stability, tunable direct bandgap, exceptional electronic transport, and directionally anisotropic properties. All these unique features could reinforce it the new contender in a variety of electronic, optical, and sensing devices. Herein, we present gas-sensing characteristics of pristine and defected GreenP towards major environmental gases (i. e., NH3, NO, NO2, CO, CO2, and H2O) using combination of the density functional theory, statistical thermodynamic modeling, and the non-equilibrium Green's function approach (NEGF). The calculated adsorption energies, density of states (DOS), charge transfer, and Crystal Orbital Hamiltonian Population (COHP) reveal that NO, NO2, CO, CO2 are adsorbed on GreenP, stronger than both NH3 and H2O, which are weakly physisorbed via van der Waals interactions. Furthermore, substitutional doping by sulfur can selectively intensify the adsorption towards crucial NO2 gas because of the enhanced charge transfer between p orbitals of the dopant and the analyte. The statistical estimation of macroscopic measurable adsorption densities manifests that the significant amount of NO2 molecules can be practically adsorbed at ambient temperature even at the ultra-low concentration of part per billion (ppb). In addition, the current-voltage (I-V) characteristics of S-doped GreenP exhibit a variation upon NO2 exposure, indicating the superior sensitivity in sensing devices. Our work sheds light on the promising application of the novel GreenP as promising chemical gas sensors.