Theoretical Study on the Correlation between the Nature of Atomic Li Intercalation and Electrochemical Reactivity in TiS₂ and TiO₂

Abstract

The electronic structures of LiTiS(2) and LiTiO(2) (having alpha-NaFeO(2) structure) have been investigated using discrete variational Xalpha molecular orbital methods. The alpha-NaFeO(2) structure is the equilibrium structure for LiCoO(2), which is widely used as a commercial cathode material for lithium secondary batteries. This study especially focused on the charge state of Li ions and the magnitude of covalency around Li ions. When the average voltage of lithium intercalation was calculated using pseudopotential methods, the average intercalation voltage of LiTiO(2) (2.076 V) was higher than that of LiTiS(2) (1.958 V). This can be explained by the differences in Mulliken charge of lithium and the bond overlap population between the intercalated Li ions and anion in LiTiO(2) as well as LiTiS(2). The Mulliken charge, which is the ionicity of Li atom, was approximately 0.12 in LiTiS(2), and the bond overlap population (BOP) indicating the covalency between Ti and S was about 0.339. When compared with the BOP (0.6) of C-H, which is one of the most famous example of covalent bonding, the intercalated Li ions in LiTiS(2) tend to form a quite strong covalent bond with the host material. In contrast, the Mulliken charge of lithium was about 0.79, which means that Li is fully ionized and the BOP, the covalency between Ti and O, was 0.181 in LiTiO(2). Because of the high ionicity of Li and the weak covalency between Ti and the nearest anion, LiTiO(2) has a higher intercalation voltage than LiTiS(2).

Iodine Vapor Transport-Triggered Preferential Growth of Chevrel Mo6S8 Nanosheets for Advanced Multivalent Batteries

“…Regarding the Mo 3d and S 2p core-level evolution upon galvanostatic discharge/charge, the magnesiation process occurs in two steps. In the first plateau, the charge transfer involves mainly sulfur atoms when the anion p-states arise to the top of the valence band and become dominant. , In the second plateau, the outer sites are filled, involving a reduction of Mo 3+ to Mo 2+ , which shifts the Fermi level of Mo 6 S 8 into its pseudogap, inducing metal to semiconductor transition . Accordingly, the different Mg 2+ mobilities in Mo 6 S 8 within the first and second plateau may originate from the polarization effects induced by sulfur anions.…”

Charge Transfer Mechanism into the Chevrel Phase Mo6S8 during Mg Intercalation

“…It was reported that the cation inside the cavity 1 creates strong bonds with 3-coordinated sulfurs whereas the interactions with 4-coordinated sulfurs are weak . The redox reaction may occur at the anions instead of the transition metals when the anion p-states arise to the top of the valence band and become dominant. , …”

“…12 The redox reaction may occur at the anions instead of the transition metals when the anion p-states arise to the top of the valence band and become dominant. 30,31 During the second insertion plateau (1 < Mg < 2), the cavities 2 are filled involving a reduction of Mo 3+ to Mo 2+ giving rise to symmetric Mo 6 octahedron rearrangement. The Mo 6 cluster can easily accommodate the four electrons required to compensate the two Mg 2+ by modifying their Mo−Mo interatomic distances.…”

Density Functional Theory Evaluated for Structural and Electronic Properties of 1T-Li x TiS2 and Lithium Ion Migration in 1T-Li0.94TiS2

“…The accurate description of structural properties of 1T-Li x TiS 2 in the whole range of 0 ≤ x ≤ 1 represents a challenge for available theoretical methods. With standard DFT methods, the calculated lattice parameters -in particular the c lattice parameter -for x→0 [27][28][29] strongly deviate from experimental values [4]. This is mainly due to the local description of electron correlation with generalized gradient approximation (GGA) DFT which does not account for nonlocal effects such as London dispersion.…”