• Umesh V Waghmare

      Articles written in Bulletin of Materials Science

    • Effect of curvature on structures and vibrations of zigzag carbon nanotubes: A first-principles study

      Mousumi Upadhyay Kahaly Umesh V Waghmare

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      First-principles pseudopotential-based density functional theory calculations of atomic and electronic structures, full phonon dispersions and thermal properties of zigzag single wall carbon nanotubes (SWCNTs) are presented. By determining the correlation between vibrational modes of a graphene sheet and of the nanotube, we understand how rolling of the sheet results in mixing between modes and changes in vibrational spectrum of graphene. We find that the radial breathing mode softens with decreasing curvature. We estimate thermal expansion coefficient of nanotubes within a quasiharmonic approximation and identify the modes that dominate thermal expansion of some of these SWCNTs both at low and high temperatures.

    • Electrochemical potentials of layered oxide and olivine phosphate with aluminum substitution: A first principles study

      Arun Kumar Varanasi Phani Kanth Sanagavarapu Arghya Bhowmik Mridula Dixit Bharadwaj Balasubramanian Narayana Umesh V Waghmare Dipti Deodhare Alind Sharma

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      First-principles prediction of enhancement in the electrochemical potential of LiCoO2 with aluminum substitution has been realized through earlier experiments. For safer and less expensive Li-ion batteries, it is desirable to have a similar enhancement for alternative cathode materials, LiFePO4 and LiCoPO4. Here, we present first-principles density functional theory based analysis of the effects of aluminum substitution on electrochemical potential of LiCoO2, LiFePO4 and LiCoPO4. While Al substitution for transition metal results in increase in electrochemical potential of LiCoO2, it leads to reduction in LiFePO4 and LiCoPO4. Through comparative topological analysis of charge density of these materials, we identify a ratio of Bader charges that correlates with electrochemical potential and determine the chemical origin of these contrasting effects: while electronic charge from lithium is transferred largely to oxygen in LiCoO2, it gets shared by the oxygen and Co/Fe in olivine phosphates due to strong covalency between O and Co/Fe. Our work shows that covalency of transition metal–oxygen bond plays a key role in determining battery potential.

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