티타늄 산화물과 유화물의 전지 전압을 결정하는 요소에 대한 제일원리계산
First Principles Study on Factors Determining Battery Voltages of TiS2 and TiO2
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Electronic structures and chemical bonding of Li-intercalated LiTiS2 and LiTiO2 were investigated by using discrete variational Xα method as a first-principles molecular-orbital method. α-NaFeO2 structure is the equilibrium structure for LiCoO2, which is widely used as a commercial cathode material for lithium secondary battery. The study especially focused on the charge state of Li ions and the magnitude of covalency around Li ions. The average voltage of lithium intercalation was calculated using pseudopotential method and the average intercalation voltage of LiTiO2 was higher than that of LiTiS2. It can be explained by the differences in Mulliken charge of lithium and the bond overlap population between the intercalated Li ions and anions in LiTiO2 as well as LiTiS2. The Mulliken charge, which means the ionicity of Li atom, was approximately 0.12 in LiTiS2 and the bond overlap population (BOP) indicating the covalency between Ti and S was about 0.339. One the other hands, the Mulliken charge of lithium was about 0.79, which means that Li is fully ionized. The BOP, the covalency between Ti and O, was 0.181 in LiTiO2. Because of high ionicity of Li and the weak covalency between Ti and the nearest anion, LiTiO2 has a higher intercalation voltage than that of LiTiS2.
Abstract
Electronic structures and chemical bonding of Li-intercalated LiTiS2 and LiTiO2 were investigated by using discrete variational Xα method as a first-principles molecular-orbital method. α-NaFeO2 structure is the equilibrium structure for LiCoO2, which is widely used as a commercial cathode material for lithium secondary battery. The study especially focused on the charge state of Li ions and the magnitude of covalency around Li ions. The average voltage of lithium intercalation was calculated using pseudopotential method and the average intercalation voltage of LiTiO2 was higher than that of LiTiS2. It can be explained by the differences in Mulliken charge of lithium and the bond overlap population between the intercalated Li ions and anions in LiTiO2 as well as LiTiS2. The Mulliken charge, which means the ionicity of Li atom, was approximately 0.12 in LiTiS2 and the bond overlap population (BOP) indicating the covalency between Ti and S was about 0.339. One the other hands, the Mulliken charge of lithium was about 0.79, which means that Li is fully ionized. The BOP, the covalency between Ti and O, was 0.181 in LiTiO2. Because of high ionicity of Li and the weak covalency between Ti and the nearest anion, LiTiO2 has a higher intercalation voltage than that of LiTiS2.
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