Consumers use batteries daily, and the lithium-ion battery has undergone a lot of engineering advances in the last few decades. There is a need to understand and improve the cathode chemistry to adapt to the rapidly growing electronics and electric vehicle market that is continually demanding more energy from batteries. Nickel-rich layered LiNi_1-x-yMn_xCo_yO₂ (1-x-y ≥ 0.6, NMC) cathodes could potentially provide the necessary energy to meet the demand of the high energy applications. Overcoming the stability issues from oxygen activation in nickel-rich materials is one of the largest challenges facing the commercial incorporation of NMCs. This thesis focuses on, LiNi_0.8Mn_0.1Co_0.1 (NMC811). Using surface sensitive techniques, such as Xray Absorption (XAS), our research reveals that degradation of NMC811 occurs during cycling, regardless of temperature, and that oxygen activation plays a role in the overall surface changes and degradation observed in NMC811. The thesis then explores the role of substituting a transition metal in the NMC811. Then we used a gradient addition of titanium to the NMC811 material to stabilize the battery performance. Theoretical techniques, such as Finite Difference Method Near Edge Structure, and experimental techniques, such as XAS, revealed how transition metal substitution, specifically titanium, stabilized the lattice. The results indicated that titanium deactivates oxygen by limiting the nickel and oxygen covalency that typically leads to oxygen activation upon charging. We observed that the titanium substitution increases cycling reversibility after hundreds of cycles. Overall, the work indicates that a more stable nickel-rich material is possible. It identifies the reasons why substitution can work in cathode materials. Additionally, the methods described can provide a guideline to further studies of stabilization of the cathode. / Master of Science / Consumers across the world use lithium-ion batteries in some fashion in their everyday life. The growing demand for energy has led to batteries dying quicker than consumers want. Thus, there are calls for researchers to develop batteries that are longer lasting. However, the initial increase in battery life over the years has been from better engineering and not necessarily from making a better material for a battery. This thesis focuses on the understanding of the chemistry of the materials of a battery. Throughout the chapters, the research delves into the how and why materials with extra nickel degrade quickly. Then, it investigates a method of making these nickel-rich materials last longer and how the chemistry within these materials are affected by the addition of a different metal. Overall, the findings indicate that the addition of titanium creates a more stable material because it mitigates the release of oxygen and prevents irreversible changes within the structure of the material. It determines that the chemistry behind the failings of nickel-rich lithium-ion batteries and a potential method for allowing the batteries to last longer. It also provides insight and guidance for potential future research of stabilization of lithium-ion materials.

Lithium-rich layered transition metal oxide cathode, represented as the chemical formula of xLi₂MnO₃ · (1 - x)LiMO₂(M = Mn, Ni, Co) , retains immense interest as one of the most promising candidates for energy storage system ranging from mobile devices to electric vehicle applications (EV/HEV/PHEV). This battery type benefits from superior theoretical capacity (>250 mAhg^-1), high chemical potential (>4.6 V vs Li⁰), good thermal stability, high discharge capacity and lower cost compared with conventional cathodes (e.g. LiCoO₂, Li(Ni_1/3Mn_1/3Co_1/3)O₂ cathodes). However, there remain major barriers which still need to be improved in order to achieve a successful commercialization for large-scale devices or electric vehicle applications. The irreversible capacity loss of 40-100 mAhg^-1 during the initial electrochemical cycle and the battery fading phenomena (capacity fading/voltage decay) on further cycles are the major problems which have emerged. The Li⁺ ion extraction accompanied by oxygen release from the active material in the form of oxide known as lithia (Li₂O) along with the transition metal migration has been suggested as the dominant processes underlying the capacity fading mechanism. Those processes, in turn, cause a phase transition from a layered structure into a spinel within the electrode material. The interplay of the local atomic environments between Li₂MnO₃ (monoclinic, C2/m) and LiMO₂ (trigonal/hexagonal, R3m) holds the key to developing better cathodes with enhanced stability. In the present thesis, an in operando XAS study using a specially-designed cell of the graphene- coated Li(Li_0.2Mn_0.54Ni_0.13Co_0.13)O₂ cathode is employed to examine the chemical, electronic, and structural states of the transition metals (Mn, Co, and Ni) during electrochemical cycle(s). Precise oxidation states for the transition metals is evaluated by the combined analyses from the XANES and SQUID measurements. The K-edge XANES spectral shift is quantified to investigate the contribution to the charge compensation mechanism by the oxidation change. Absorption features in K-edge XANES are identified. These features describe the electronic state of the individual atoms in the cathode composite, as well as the local distortion from the octahedral structure of MO₆. The Fourier transform of EXAFS offers a satisfactory description of the local structure changes with the connection to the cation arrangement. The description is generally involved with the peak amplitude, position, shape changes (trend), and coordination numbers in the real space. Hence, similarities or discrepancies in the local atomic environments could be compared at different state of charge. Major structural parameters are deduced from the EXAFS fitting process. These parameters can be used to distinguish different atomic environments upon voltage bias levels or investigate the appearance of the Jahn-Teller effect. A new approach to understand the atomic environment upon charge-discharge is demonstrated, namely, a Continuous Cauchy Wavelet Transform (CCWT) which enables the visualization of the EXAFS spectra in three dimensions by decomposing the k-space and R-space (uncorrected for phase shift) signals. The wavelet transform analysis provides possible evidence of the precursor that leads to the spinel phase transition in this battery system.