118 Conference of the South African Advanced Materials Initiative 2021 Available online at https://doi.org/10.36303/SATNT.2021cosaami.23 Conference of the South African Advanced Materials Initiative 1. Introduction The Li-air battery has taken a lot of attention to researchers because of its ability to deliver ultra-high energy density and also gained its advantage because its lower weight due to the absence of metallic cathode.1 Metal oxides (MO 2 ) have attracted extensive attention because they possess various applications such as lithium- ion and Li-air batteries (LIBs), dye sensitized solar cells (DSSCs), fuel cells, catalysis, gas sensors, water splitting and super capacitors due to low cost and high compatibility with the environment. 2 However, the fundamental challenges that limits the use of metal air battery technology is the ability to fnd a catalyst that will catalyse the formation and decomposition of Li 2 O 2 during charging and discharging cycle i.e. oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Catalytic materials have been proposed and synthesized to promote the oxygen reduction reactions and oxygen evolution reactions ORR/OER process, which can be mainly classifed into three groups: carbon-based materials, 3,4 noble metal/metal oxides 5,6 and transition metal oxides. 7,8 These catalytic materials show notable strengths and weaknesses, e.g. although noble metals/metal oxides, such as Ru/RuO 2 , 9,10 could signifcantly decrease the ORR/ OER over-potentials, their high cost greatly hinders the practical application. In addition, some metal oxides have appeared to cause the decomposition of electrolytes. 11 In this paper we investigate the electronic properties of the metal oxides which will give a better insight in approaching further calculations which will be applicable in the performance of the metal-air battery. The instability of the material does not rule out the ability of the material to be a better catalyst. 2. Methodology The calculations were performed using ab initio density functional theory (DFT) formalism as implemented in the VASP total energy package 12,13 with the projector augmented wave. 14 The number of planewaves was determined by a cutof kinetic energy of 500 eV and the Brillouin zone sampling scheme of Monkhorst-Pack with 6x6x9 k-points mesh for rutile MnO 2 bulk structures. The phonon dispersion spectra were evaluated using PHONON code 15 as implemented in the Materials Design within MedeA software of VASP code. 16,17 The 2x2x3 supercell was used for the tetragonal whereas the supercell of 2x2x2 was used for monoclinic MnO 2 . Calculations carried out within the full Brillouin zone, parameters with interaction range of 7.0 Ǻ, displacement of atoms was set at 0.02 Ǻ which gave a supercell which extends equally in all directions and does not reduce the symmetry of the system. The above parameters gave new lattice vectors of a’, b’ and c’, 1x2x1, 24 number of atoms in the supercell was 24 and resulted in 73 supercells to be calculated. Convergence was assumed when the maximum component of the residual forces on the ions was less than 0.01 eV/Å. 3. Results and discussion 3.1 Lattice parameters The lattice parameters allude to the physical dimension of unit cells in a crystal lattice. Lattices in three dimensions predominantly have three lattice constants, referred to as a, b, and c. However, the full set of lattice parameters consist of the three lattice constants and the three angles between them. Calculated lattice parameters are presents and the cell volume of the bulk structure of the metal oxide are shown in Table 1 and they are compared with the lattice parameters of the experimental lattice parameters calculated previously. 18-20 Table 1: Lattice parameters and cell volume of tetragonal (rutile) MnO 2 , TiO 2 and VO 2 bulk structure Structure a (Ǻ) c (Ǻ) V (Ǻ 3 ) β-MnO 2 4.366 4.410 [18] 2.961 2.887 [19] 56.44 β-TiO 2 4.827 4.954 [20] 3.008 2.959 [18] 64.40 β-VO 2 4.617 4.554 [18] 2.774 2.857 [19] 59.13 The lattice parameters were in good agreement with the experimental with deviations of approximately +0.8% and -3.1% for a and c, respectively, and of 1.6 % in the cell volume for the MO 2 shown in the above table. Abstract We investigate the structural stability of metal oxides β-MnO 2 , TiO 2 and VO 2 (MO 2 ) which are used as catalyst in metal air batteries, using the density functional theory (DFT) within the generalized gradient approximation (GGA). Their mechanical property was determined to show the stability trend of the metal oxides catalyst. Cell parameters of the bulk structures of the MO 2 are in reasonable agreement with the experimental values (deviations of approximately 0.8% and -3.1% for a and c, respectively, and of 1.6 % in the cell volume). Phonon dispersion curves show that rutile (R) TiO 2 is the most stable structure since it does not have vibrations in the negative frequencies. Density functional theory study of MnO 2 , TiO 2 and VO 2 KP Maenetja, HR Chauke, PE Ngoepe Materials Modelling Centre, University of Limpopo Private Bag X1106, Sovenga, 0727, South Africa Email: khomotso.maenetja@gmail.com