COMPUTER SIMULATION STUDIES OF SPINEL LiMn2O4 AND SPINEL LiNiXMn2-XO4 (0≤x≤2) By KEMERIDGE TUMELO MALALTJI Thesis Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy In Physics in the FACULTY OF SCIENCES AND AGRICULTURE (School of Physical and Mineral Sciences) at the UNIVERSITY OF LIMPOPO SUPERVISOR: Prof. R.R. Maphanga CO- SUPERVISORS: Prof. P.E. Ngoepe (Dated: 29 October 2019) Declaration I, Kemeridge Tumelo Malatji confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. I agree that the Library may lend or copy this dissertation on request. Signature: Date: 29/10/2019 Name: Mr K.T. Malatji ii Abstract LiMn2O4 spinel (LMO) is a promising cathode material for secondary lithium-ion batteries which, despite its high average voltage of lithium intercalation, suffers crystal symmetry lowering due to the Jahn-Teller active six-fold Mn3+ cations. Although Ni has been proposed as a suitable substitutional dopant to improve the energy density of LiMn2O4 and enhance the average lithium intercalation voltage, the thermodynamics of Ni incorporation and its effect on the electrochemical properties of this spinel are not fully understood. Firstly, structural, electronic and mechanical properties of spinel LiMn2O4 and LiNixMn2-xO4 have been calculated out using density functional theory employing the pseudo-potential plane-wave approach within the generalised gradient approximation, together with Virtual Cluster Approximation. The structural properties included equilibrium lattice parameters; electronic properties cover both total and partial density of states and mechanical properties investigated elastic properties of all systems. Secondly, the pressure variation of several properties was investigated, from 0 GPa to 50 GPa. Nickel concentration was changed and the systems LiNi0.25Mn1.75O4, LiNi0.5Mn1.5O4 LiNi0.75Mn1.25O4 and LiNi0.875Mn1.125O4 were studied. Calculated lattice parameters for LiMn2O4 and LiNi0.5Mn1.5O4 systems are consistent with the available experimental and literature results. The average Mn(Ni)-O bond length for all systems was found to be 1.9 Å. The bond lengths decreased with an increase in nickel content, except for LiNi0.75Mn1.25O4, which gave the same results as LiNi0.25Mn1.75O4. Generally, analysis of electronic properties predicted the nature of bonding for both pure and doped systems with partial density of states showing the contribution of each metal in our systems. All systems are shown to be metallic as it has been previously observed for pure spinel LiMn2O4, and mechanical properties, as deduced from elastic properties, depicted their stabilities. Furthermore, the cluster expansion formalism was used to investigate the nickel doped LiMn2O4 phase stabilities. The method determines stable multi-component crystal structures and ranks metastable structures by the enthalpy of formation while iii maintaining the predictive power and accuracy of first-principles density functional methods. The ground-state phase diagram with occupancy of Mn 0.81 and Ni 0.31 generated various structures with different concentrations and symmetries. The findings predict that all nickel doped LMO structures on the ground state line are most likely stable. Relevant structures (Li4Ni8O16, Li12MnNi17O48, Li4Mn6Ni2O16, Li4Mn7NiO16 and Li4Mn8O16) were selected on the basis of how well they weighed the cross-validation (CV) score of 1.1 meV, which is a statistical way of describing how good the cluster expansion is at predicting the energy of each stable structure. Although the structures have different symmetries and space groups they were further investigated by calculating the mechanical and vibrational properties, where the elastic constants and phonon vibrations indicated that the structures are stable in accordance with stability conditions of mechanical properties and phonon dispersions. Lastly, a computer program that identifies different site occupancy configurations for any structure with arbitrary supercell size, space group or composition was employed to investigate voltage profiles for LiNixMn2-xO4. The density functional theory calculations, with a Hubbard Hamiltonian (DFT+U), was used to study the thermodynamics of mixing for Li(Mn1-xNix)2O4 solid solution. The results suggested that LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at least 1000K, which is in excellent agreement with experiments. It was also found that the configurational entropy is much lower than the maximum entropy at 1000K, indicating that higher temperatures are required to reach a fully disordered solid solution. The maximum average lithium intercalation voltage of 4.8 eV was calculated for the LiMn1.5Ni0.5O4 composition which correlates very well with the experimental value. The temperature has a negligible effect on the Li intercalation voltage of the most stable composition. The approach presented here shows that moderate Ni doping of the LiMn2O4 leads to a substantial change in the average voltage of lithium intercalation, suggesting an attractive route for tuning the cathode properties of this spinel. iv ACKNOWLEDGEMENTS I would like to thank my supervisor Prof. R.R. Maphanga for all the unconditional encouragement, guidance, support, mentoring. Thanks for making the journey in the research field an interesting one. My gratitude also goes to my co-supervisor Prof. P.E. Ngoepe for this opportunity, contribution throughout this study and award of the project. I am forever humbled by what you saw in me and for your mentorship. Above all, I would like to thank the almighty God for all the strength, patience and perseverance He has installed in me during all the years of my studies, without Him this work wouldn’t have been started. I fully acknowledge the financial support from the National Research Foundation. This research wouldn’t have been a success if it was not of their financial backing and it encouraged my work ethic. Ample thanks to all the compatriots of the Material Modeling Center whose friendship, guidance and support had made the center more than an area to study at; “It is and will always be a jam to work with you fellas”. Finally, abundant thanks to my parents Mosadi Sophy Malatji and Johannes Anaph Malatji for their warm support, constant believe, encouragement and the love they showed me during this journey and life. They have contributed to me being the man I am today. My brothers (Kholofelo Brian Malatji and Tshegofatjo Anaph Malatji) thanks a lot your brotherhood, which reminded me that my efforts would make a difference in your lives. To my grandmothers (Shibe and Boreadi), aunts and uncle, your contribution will always be of paramount significance in my heart. To my beautiful partner your support from reading my work and pushing me to finish meant a lot. I would also like to give thanks to my many friends and extended family members who were always positively inquisitive about my progress. I wish to thank everybody with whom I have shared experiences in life. Most importantly, ample thanks to my late grandfather Samson Galiade Gozo ‘aka Gita’ for always making me believe on how important education is. Always full of laughter. v Dedication This work is dedicated to my lovely parents Johannes Anaph Malatji and Mosadi Sophy Malatji, my brothers (Kholofelo Brian Malatji and Tshegofatjo Anaph Malatji). To my late grandfather wherever you are, this is for you, may your soul rest in peace “Galiade”. “It’s better to die for an idea that will live than to live for an idea that will die” –Onkgopotse Abram Tiro vi Contents Declaration .............................................................................................................. ii Abstract .................................................................................................................. iii Dedication .............................................................................................................. vi List of Figures .......................................................................................................... x List of Tables ......................................................................................................... xv ............................................................................................................... xv Introduction ......................................................................................................... 1 General Introduction ..................................................................................... 1 Lithium-ion Batteries ..................................................................................... 2 Doping in LiMn2O4 ........................................................................................ 7 Structural Properties of LiMn2O4 ................................................................. 10 The rationale of the Study ........................................................................... 12 Outline of the Study .................................................................................... 14 .............................................................................................................. 15 Methodology ..................................................................................................... 15 Introduction ................................................................................................. 15 Density Functional Theory