Designing new materials with high efficiencies based on earth abundant nontoxic materials for renewable energy technologies is key to mitigate the climate change and develop prosperous sustainable society. With this idea in mind we are designing new materials useful for hydrogen economy, Li and Na ion battery cathodes, photovoltaic and thermoelectric applications using state-of-the-art density functional methods. Li-rich materials with simultaneous anionic and cationic reduction is one among the most promising class of cathode materials for high‐capacity Li-ion batteries. The recent studies on the Li-rich Li5FeO4 with defect antifluorite-type structure show that Li5FeO4 can enable high capacity by simultaneous reduction of Fe and oxygen atom without any obvious release of oxygen gas. In the present study we have substituted Fe site with Ti in different concentrations (Li5TixFe1-xO4 with x = 0.125, 0.250, 0.375, 0.500, 0.625, 0.750, 1.00) and investigated the structural, electronic and Li-diffusion properties using density functional theory calculations. The polyanionic compound Na2MnSiO4 is regarded as one of the promising cathode materials for Na-ion batteries due to good specific capacity along with its attractive prospect of utilization of two electrons in the redox processes. So, in this study we have performed the thermodynnamic and electronic structure analysis of Na2MnSiO4 using first principles density functional theory calculations. The intermediate ground state configurations for Na2MnSiO4 during Na de-intercalation were found using the cluster expansion method and are used to obtain the 0 K voltage profile as a function of Na concentration. This material shows an average voltage of 4.2 V and the finite temperature analysis at 300 K using Monte Carlo simulations indicates that this material undergo two phase mixing when desodiate beyond 1.5 Na/f.u. The involvement of oxygen in the redox reaction apart from the transition metal is identified using the Bader charge analysis. Relevant Na diffusion pathways and their corresponding calculated energy barriers are compared with the partially Fe substituted Na2MnSiO4 to understand the effect of Mn-site substitution on the process of Na migration through this material Compared with the remarkable achievement of using perovskites in photovoltaic applications, the role of antiperovskites in solar cells has not been adequately identified and reported. So we have predicted the crystal structures of the antiperovskites Be3PN, Mg3PN, Ca3PN, Sr3PN, Ba3PN and Zn3PN using structural optimization with stress as well as force minimization by considering 32 potential structural variants into the calculation. We found that all these compounds are having direct bandgap behavior with low carrier effective mass, high optical absorption, well separated electron-hole pair etc. Due to the iso-structural as well as isoelectronic nature of these materials with tunable bandgap value apart from the above mentioned other advantageous optoelectronic properties suggest that these materials are suitable for higher efficiency tandem solar cell applications. Multinary aliovalent substituted semiconducting half Heusler alloys are expected to have low thermal conductivity due to increase in the phonon scattering centers and thus it is expected to increase the thermoelectric figure of merit. So we have studied electronic structure, lattice dynamics, and thermoelectric (TE) transport properties of a new family of pentanary substituted TiNiSn systems using the 18 valence electron count (VEC) rule. The substitution of atoms with different mass creates more phonon scattering centers and hence lower the lattice thermal conductivity. The calculated lattice thermal conductivity for Hf containing systems La0.25Hf0.5V0.25NiSn and non Hf containing system La0.25Zr0.5V0.25NiSn are found to be 0.2 and 0.36 W/m-K, respectively and they can attain maximum ZT value of 0.78 at 500 K and 0.76 at 450 K, respectively. Based on the calculated results we conclude that one can design high efficiency thermoelectric materials by considering 18 VEC rule with aliovalent substitution. In order to identify potential materials for hydrogen storage application worldwide attention has been focused on hydrides with high gravimetric and volumetric capacity to have sustainable energy system. If one can find hydrogen storage materials where hydrogen is present in both negative and positive oxidation state within the same structural frame work then one can accommodate hydrogen with high volume density because of the attractive interaction between oppositely charged hydrogen. So, it is fundamentally as well as technologically important to identify compounds in which hydrogen is in amphoteric nature and understand the necessary criteria for its origin. We have identified several hydrogen storage materials with hydrogen in amphoteric state using van der Waals interactions corrected density functional calculations and explain such behaviour.
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