Aluminum and lithium sulfur batteries: a review of recent progress and future directions.

Advanced materials with various micro-/nanostructures have attracted plenty of attention for decades in energy storage devices such as rechargeable batteries (ion- or sulfur based batteries) and supercapacitors. To improve the electrochemical performance of batteries, it is uttermost important to develop advanced electrode materials. Moreover, the cathode material is also important that it restricts the efficiency and practical application of aluminum-ion batteries. Among the potential cathode materials, sulfur has become an important candidate material for aluminum-ion batteries cause of its considerable specific capacity. Two-dimensional materials are currently potential candidates as electrodes from lab-scale experiments to possible pragmatic theoretical studies. In this review, the fundamental principles, historical progress, latest developments, and major problems in Li-S and Al-S batteries are reviewed. Finally, future directions in terms of the experimental and theoretical applications have prospected.

Monitoring the electronic, thermal and optical properties of two-dimensional MoO2 under strain via vibrational spectroscopies: a first-principles investigation.

This study presents the electronic, mechanical, thermal, vibrational and optical properties of the MoO2 monolayer under the effect of biaxial and uniaxial compressive/tensile strain, using first-principles calculations based on density functional theory. It has been found that the mechanical strength of MoO2 is higher than other MoX2 (X = S, Se, Te) monolayers. Dynamical stability analysis shows that MoO2 is stable up to 6% compressive and at least 8% tensile strain. Strain dependent Raman modes are investigated along the biaxial directions. It was obtained that phonon softening and hardening occurred under tensile and compressive strain owing to the increase and decrease of the bond lengths of the MoO2 structure. Our results also imply that the electronic band structure of the MoO2 monolayer can be tuned with strain and the energy bandgap decreases with increasing biaxial tensile strain up to 4%. For larger values of strain, a semiconductor to semimetal transition is observed; however, this kind of transition is not observed for uniaxial tensile strain. Besides, we report that MoO2 has a negative thermal expansion coefficient (TEC) in the range of extremely low-temperatures (0 K to 33 K) similar to other 2D MoX2 monolayers. For temperatures above 600 K, it possesses a positive TEC with an approximate maximum value of 12 × 10-6 K-1. We carried out optical property calculations by solving the Bethe-Salpeter equation and found that the MoO2 monolayer has two strongly bound excitons below the quasiparticle absorption edge. Overall, our results shed light on experimental studies and suggest that the MoO2 monolayer should be an excellent candidate for new design layered semiconductors, electronics, and optoelectronic devices.

The influence of surface functionalization on thermal transport and thermoelectric properties of MXene monolayers.

The newest members of a two-dimensional material family, involving transition metal carbides and nitrides (called MXenes), have garnered increasing attention due to their tunable electronic and thermal properties depending on the chemical composition and functionalization. This flexibility can be exploited to fabricate efficient electrochemical energy storage (batteries) and energy conversion (thermoelectric) devices. In this study, we calculated the Seebeck coefficients and lattice thermal conductivity values of oxygen terminated M2CO2 (where M = Ti, Zr, Hf, Sc) monolayer MXene crystals in two different functionalization configurations (model-II (MD-II) and model-III (MD-III)), using density functional theory and Boltzmann transport theory. We estimated the thermoelectric figure-of-merit, zT, of these materials by two different approaches, as well. First of all, we found that the structural model (i.e. adsorption site of oxygen atom on the surface of MXene) has a paramount impact on the electronic and thermoelectric properties of MXene crystals, which can be exploited to engineer the thermoelectric properties of these materials. The lattice thermal conductivity κl, Seebeck coefficient and zT values may vary by 40% depending on the structural model. The MD-III configuration always has the larger band gap, Seebeck coefficient and zT, and smaller κl as compared to the MD-II structure due to a larger band gap, highly flat valence band and reduced crystal symmetry in the former. The MD-III configuration of Ti2CO2 and Zr2CO2 has the lowest κl as compared to the same configuration of Hf2CO2 and Sc2CO2. Among all the considered structures, the MD-II configuration of Hf2CO2 has the highest κl, and Ti2CO2 and Zr2CO2 in the MD-III configuration have the lowest κl. For instance, while the band gap of the MD-II configuration of Ti2CO2 is 0.26 eV, it becomes 0.69 eV in MD-III. The zTmax value may reach up to 1.1 depending on the structural model of MXene.