Ab initio study of LiMn2O4 cathode: electrochemical and optical properties for li-ion batteries and optoelectronic devices

Abstract

In this study, the structural, electrochemical and optical properties of Lithium manganese oxide (LiMn₂O₄) were studied through first-principles calculations based on density functional theory (DFT) using generalized gradient approximation (GGA). The LiMn₂O₄ compound is metallic and The MnO₂ has a direct band gap equal to 0.42 eV using the GGA-PBE (Perdew-Burke-Ernzerhof) approach, 0.21 eV using the GGA-mBJ (modified Becke-Johnson) approach and 2.21 eV using the DFT + U (Density Functional Theory for the Hubbard model) approach. The insertion and extraction of lithium ions induce slight changes in the crystal volume of MnO₂. In addition, the valence state of manganese shifts from + 4 to + 3.5 upon lithium insertion in MnO₂. The spin polarisation (SP%) of LiMn₂O₄ is 24% at the Fermi level (E_F). We found that The Curie temperature (T_c) of LiMn₂O₄ equals 924 K using the GGA-PBE method, 955 K using the GGA-mBJ method and 1290 K using the DFT + U method. The calculations show that the typical full-cycle LiMn₂O₄ battery balancing voltage (V_cell) is 3.4 V and the cell capacity is 148 mAh.g⁻¹. We also found that the energy density of the cell is 504 Wh.kg⁻¹. We determined the optical properties of the two materials in particular absorption and conductivity. The static dielectric constants e₁(0) of LiMn₂O₄ and MnO₂ compounds are 39.64 and 9.08 respectively. The LiMn₂O₄ compound shows an excellent absorption capacity in the UV region, indicating its potential application in optical memory devices. The high reflectivity in low energy ranges opens the possibility of using LiMn₂O₄ as a coating material to reduce solar heating. The calculated formation energies confirmed that the LiMn₂O₄ and MnO₂ compounds are thermodynamically stable. This means that LiMn₂O₄ is more suitable for use in batteries and optoelectronic applications.