First-Principles Investigation on Delithiation Mechanisms in a Li-Rich Monoclinic Li2MoO3 Cathode Material for Li-Ion Batteries.

Li2MoO3 is a promising cathode material for high-capacity Li-ion batteries. However, during cycling, migration of Mo to Li sites results in capacity fading. The present study analyzed structural, electronic, electrochemical, and mechanical characteristics of ordered monoclinic C2/m-Li2MoO3 and found that this phase has improved electrochemical properties compared to the rhombohedral R3̅m phase. Nudged elastic band calculations showed that Mo migration to the Li site is less probable in C2/m-Li2MoO3. The charge and chemical bonding analyses during delithiation showed Mo4+/Mo6+ oxidation and partial oxygen oxidation, but no spontaneous oxygen release occurred. The voltage profile calculated using the SCAN + U method exhibits high voltage, and partial W substitution at Mo sites suppresses intralayer Mo migration to the Li site and improves the voltage characteristics. These findings suggest that monoclinic Li2MoO3 is a potential cathode material for high-capacity Li-ion batteries with reduced Mo migration and maintained Mo4+/Mo6+ oxidation and oxygen stability. Moreover, partial W substitution at Mo sites further enhances the electrochemical properties of C2/m-Li2MoO3.

Role of W-site substitution on mechanical and electronic properties of cubic tungsten carbide.

In order to understand the role of W-site substitution on properties of cubic tungsten carbide ([Formula: see text]-WC), we have investigated the structural, mechanical, and electronic properties of WXC2 (X = Si, Sc, Ti, V, Cr, Ge, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Sn, Hf, Ta, Re, Os, Ir, Pt, Th, U) using first principles calculations based on density functional theory, within generalized gradient approximation. The structural optimization has carried out for all these compounds using force as well as stress minimization. The optimized structural parameters for experimentally known compounds are in good agreement with the available x-ray diffraction measurements and structural parameters for nineteen WXC2 compounds are newly predicted. The W-site substitution of the above-listed elements into [Formula: see text]-WC reduces the symmetry of the primitive lattice to tetragonal structure. The heat of formation ([Formula: see text]) and the mechanical stability studies are carried out to investigate the stability of these systems. The single-crystal elastic constants c ij , elastic moduli of the polycrystalline aggregates, anisotropy in elastic constants and related properties of the WXC2 materials have calculated and discussed in detail. The hardness of the above materials is predicted using two different criteria, based on the softest elastic mode as well as the Pugh's modulus ratio. There is a correlation in the hardness predicted from these two approaches except in the case of [Formula: see text]-WC. The chemical bonding interaction between the constituents is analysed using the density of states, crystal orbital Hamiltonian population, and charge density for selected systems. All these compounds are predicted to be metal and our calculations suggest that W-site substitutions do not improve the hardness of [Formula: see text]-WC. However, from the heat of formation studies, we have identified five new stable compounds such as CrWC2, NbWC2, ScWC2, YWC2, and UWC2 with reasonably good hardness and those need experimental verifications.