Computational modelling study on the stability of Li1.2Mn0.8O2 cathode material

Abstract

Lithium-ion batteries are significant rechargeable power sources for use in various electronic devices and electric vehicles due to their high specific energy and high energy density. The cathode material of lithium-ion batteries greatly influences specific energy, lifespan, and safety since they possess a minimised specific capacity and a more unstable structure than anode materials. As a result, it is crucial to develop high performance cathode materials. Lithium-rich Mn-based layered oxides have been extensively investigated as possible cathodes with high capacity of over 250mAh/g, low cost, safety, and long lifetime for lithium-ion batteries, however, commercial applications of these cathodes are mainly hindered by voltage fade, irreversible oxygen loss, structural transition, and low coulombic efficiency which results in poor cycling performance. To solve these challenges, many approaches have been used, including elemental doping, surface modification, anionic doping, and coating. Nevertheless, finding suitable cathode materials with appropriate characteristics and stable structural compositions is still difficult, and as such, materials such as Li2MnO3, LiMnO2, LiNiO2, and LiCoO2 are being studied for a satisfactory outcome. This study merges computational and experimental methods to study the stability of the Li1.2Mn0.8O2 cathode material which is derived from the promising cathode material Li2MnO3. Firstly, structural, electronic, and mechanical properties of Li1.2Mn0.8O2 were calculated using the density functional theory within the Vienna ab initio simulation package implementing the generalized gradient approximation functionals. It was found that the Li1.2Mn0.8O2 material is thermodynamically stable with a negative heat of formation, it was also found to be a semi-conductor with a band gap of 0.269eV at the Fermi level. Moreover, the structure was found to be mechanically stable under a strain of 0.001 and mechanically unstable under larger strains 0.005 and 0.025. In addition, the Pugh ratio predicted the material to be brittle. Secondly, the material was doped with Nickel on the manganese sites and also doped with fluorine on the oxygen sites to study the effects of these elements on the material Li1.2Mn0.8O2. First-principles calculations coupled with cluster expansion simulations were performed on the doped structures to generate new phases of the doped material. The cluster expansion generated 12, 78, and 165 new phases for the Ni-doped Li1.2Mn0.8O2, F-doped Li1.2Mn0.8O2, and F-doped Li1.2Mn0.6Ni0.2O2 structures respectively. It was observed that doping Li1.2Mn0.8O2 with Ni enhances the thermodynamic stability of the material and improves the material’s conductivity as it led to a magnetic metal with no bandgap. We further observed that the presence of Ni in the structure does not compromise the mechanical stability of the material as the generated phases were found to be mechanically stable. However, the Ni-doped phases are more brittle than the pristine structure suggesting that introducing Ni into the material reduces the ductility of the material. From the F-doped materials it was found that the presence of fluorine in the material increases the thermodynamic stability of the material. Consequently, the generated materials retain the pristine structure’s electronic conductivity as they show metallic behaviour. Furthermore, it was found that only a minimum concentration of fluorine is required for the material to be mechanically stable, in addition, the presence of fluorine has the potential to improve the ductility of the material. Experimentally a co-precipitation method was used to generate precursors for the Li1.2Mn0.8O2, Li1.2Mn0.6Ni0.2O2, and Li1.2Mn0.4Ni0.2Co0.2O2 structures using the semi-batch technique. Morphology of the samples was studied with the Scanning Electron Microscopy (SEM). The SEM analysis of the material Li1.2Mn0.8O2 showed spherical shaped particles. With Li1.2Mn0.6Ni0.2O2 the morphology analysis revealed primary particles with irregular shapes, and which agglomerate into secondary particles with spherical shapes. The Li1.2Mn0.4Ni0.2Co0.2O2 particles also showed spherical particles and empty spaces in between the agglomerated particles. The measured tapped density (1.43g/cm3) of the Li1.2Mn0.6Ni0.2O2 material was consistent with the tapped density value reported in literature, and this suggests the accuracy of the synthesis method. Furthermore, particle size distribution analysis was performed on the Li1.2Mn0.6Ni0.2O2 material and it was found that D10=14.29μm, D50=22.95μm, and D90=33.16μm.