Effect of Mg doping at the M site of layered AMO2 (A = Li/Na and M = Fe/Mn/Ni) for enhancing its electrochemical properties were studied using density functional theory. Structurally, Mg doping was found to suppress Jahn-Teller (JT) distortion in both AMnO2 and ANiO2 while it acts negatively by bringing JT distortion in AFeO2. The simulation of density of states indicate that after Mg doping electronic conduction increase in all AM(1-x)MgxO2 apart from LiMn0.5Mg0.5O2. The diffusion barrier was found to be ≤ 1.21 eV for all the Mg undoped and doped materials. Study of open circuit voltage (OCV) indicate that with Mg doping at M site, OCV increases for all the materials due to extraction of more electrons from M during charging. For LiNiO2 and LiNi0.5Mg0.5O2 the OCV are found to be respectively 4.022 V and 4.557 V, with moderate diffusion barriers of 1.057 eV and 0.841 eV respectively. Again with diffusion barrier of ∼0.47 eV to 0.64 eV and large OCV of 3.83 and 3.72 V, respectively NaMn0.5Mg0.5O2 and NaNi0.5Mg0.5O2 are found to be the best alternative to the commercial cathode materials.
The structural, mechanical, electronic, optical and thermoelectric properties of the earth-abundant quaternary chalcogenides A2VAgS4 (A = K, Rb, Cs) are investigated using first-principles density functional theory combined with semiclassical Boltzmann transport calculations. The optimized lattice parameters reproduce the reported experimental values to within 4 %, and the elastic constants satisfy the Born mechanical stability criteria. Pugh's ratios of 1.33–1.47 and Poisson's ratios of 0.19–0.22 identify all three compounds as brittle and elastically anisotropic solids. The TB-mBJ potential yields indirect band gaps of 1.75, 1.77 and 1.71 eV for the K, Rb and Cs compounds, respectively; the value obtained for Rb2VAgS4 agrees with the measured optical gap of 1.83 eV to within 3 %. Static dielectric constants of 3.89–5.10 accompany strong absorption throughout the visible and ultraviolet ranges. At 300 K the compounds exhibit Seebeck coefficients of 2711–2868 μV/K and σ/τ values of (3.04–3.43) × 105 (Ω m s)−1, which translate into figures of merit of 1.12, 1.07 and 1.08 for K2VAgS4, Rb2VAgS4 and Cs2VAgS4. Exceeding ZT > 1 at ambient temperature, these compounds emerge as promising earth-abundant absorbers for near-room-temperature waste-heat recovery and solid-state thermoelectric cooling.
