Modeling of the Electronic Properties of M-Doped Supercells Li4Ti5O12–M (М = Zr, Nb) with a Monoclinic Structure for Lithium-Ion Batteries

Abstract

The T–x phase diagram of the quasi-binary system \({\text{L}}{{{\text{i}}}_{2}}{\text{O}}- {\text{Ti}}{{{\text{O}}}_{2}}\) was refined and the isothermal cross section of the ternary \({\text{Li}}- {\text{Ti}}- {\text{O}}\) system at 298 K was constructed. The equilibrium phase regions of \({\text{Li}}- {\text{Ti}}- {\text{O}}\) in the solid state are determined with the participation of boundary binary oxides and four intermediate ternary compounds \({\text{L}}{{{\text{i}}}_{4}}{\text{Ti}}{{{\text{O}}}_{4}}\), \({\text{L}}{{{\text{i}}}_{2}}{\text{Ti}}{{{\text{O}}}_{3}}\), \({\text{L}}{{{\text{i}}}_{4}}{\text{T}}{{{\text{i}}}_{5}}{{{\text{O}}}_{{12}}}\) and \({\text{L}}{{{\text{i}}}_{2}}{\text{T}}{{{\text{i}}}_{3}}{{{\text{O}}}_{7}}\). Using the density functional theory (DFT LSDA) method, the formation energies \(({{\Delta }_{f}}E)\) of the indicated ternary compounds of the \({\text{L}}{{{\text{i}}}_{2}}{\text{O}}- {\text{Ti}}{{{\text{O}}}_{2}}\) system were calculated and the dependence of \({{\Delta }_{f}}E\) on the composition was plotted. Ab initio modeling of supercells based on M-doped \(\left( {{\text{M }} = {\text{ Zr}},{\text{ Nb}}} \right)\) anode material based on the \({\text{L}}{{{\text{i}}}_{4}}{\text{T}}{{{\text{i}}}_{5}}{{{\text{O}}}_{{12}}}\) (\({\text{LTO}}\)) compound with a monoclinic structure (m) was carried out. It has been shown that partial substitution of cations and oxygen in the \({\text{m}}- {\text{LTO}}- {\text{M}}\) structure increases the efficiency of a lithium-ion battery (\({\text{LIB}}\)) both by stabilizing the structure and by increasing the diffusion rate of \({\text{L}}{{{\text{i}}}^{ + }}\). Due to the contribution of d-orbitals (\({\text{Z}}{{{\text{r}}}^{{4 + }}}\,\,4{\text{d}},\) \({\text{N}}{{{\text{b}}}^{{3 + }}}\) 4d orbitals) to the exchange energy, partial polarization of electronic states occurs and the electronic conductivity of \({\text{m}}- {\text{LTO}}- {\text{M}}\) increases. The formation of oxygen vacancies in the \({\text{m}}- {\text{LTO}}- {\text{M}}\) crystal lattice, as in binary oxides, can create donor levels and improve the transport of \({\text{L}}{{{\text{i}}}^{ + }}\) and electrons. M-doping of the \({\text{m}}- {\text{LTO}}\) structure by replacing cations, in particular lithium, with Zr or Nb atoms noticeably reduces the band gap (\({{E}_{{\text{g}}}}\)) of \({\text{m}}- {\text{LTO}}- {\text{M}}\) supercells. In this case, in the \({\text{m}}- {\text{LTO}}- {\text{M}}\) band structure, the Fermi level shifts to the conduction band and the band gap narrows. Decreasing the \({{E}_{{\text{g}}}}\) value increases the electronic and lithium-ion conductivity of \({\text{m}}- {\text{LTO}}- {\text{M}}\) supercells.