High energy density batteries are essential for compact and lightweight power storage, enabling longer lasting and more efficient portable electronic devices, electric vehicles, and grid storage solutions. Advancements in battery technology to enhance energy storage capacity and extend range are crucial for the complete transition to renewable energy. Currently, the capacity of state-of-the-art batteries is limited by the cathode and improvements in their design are contingent on better understanding of their working mechanisms. This dissertation investigates the layered lithium transition metal oxide class of cathodes, with a focus on Co-free compositions (LiNi0.5Mn0.5O2 and LiNiO2), and explores the relationship between cathode composition, structure, and performance. We first develop advanced characterization techniques that probe the electron spins in cathodes to investigate reaction mechanisms occurring during battery operation. Operando cells for magnetometry and electron paramagnetic resonance (EPR) were designed and tested. These electron spin probes provide complementary insights, which aid in interpretation of ambiguous data. We demonstrate the tandem use of these cells on LiNi0.5Mn0.5O2 and show that the magnetism and redox reactions in the charge cycle are largely influenced by the Ni/Li antisite defects in the material. Similarly, in LiNiO2 the defects in the as-synthesized material are found to have a significant contribution to its irreversible capacity. Twin boundaries and Ni over-stoichiometry (y in Li1-yNi1+yO2) are quantified through X-ray diffraction, magnetometry, and solid-state nuclear magnetic resonance (NMR) assisted with first-principles calculation of ssNMR parameters. These planar and point defects prevent lithium reinsertion at low voltages in the initial cycle due to impeded lithium diffusion. Finally, the electrochemical aging mechanism in LiNiO2 is investigated. Structural changes in LiNiO2 induced by extended high voltage cycling are correlated with diminishing capacity retention. The capacity decay is attributed to Li inventory loss and interlayer Ni-migration. The methodology and defect-property relationships established here will aid in future design of improved batteries.