The development of cheap, high energy density electrode materials for Li ion batteries (LIBs) is critical to reaching net zero emissions targets and mitigating climate change. To this end, Ni- and Mn-rich transition metal oxide cathodes simultaneously offer high energy densities as well as cost effective solutions for LIBs. Despite the immense potential of these materials, both Ni- and Mn-rich cathodes suffer from severe interfacial instabilities that lead to crystallographic rearrangement of the active material surface, transition metal dissolution, and the formation of a cathode electrolyte interphase (CEI) layer on the composite surface during electrochemical cycling. While changes in the crystallographic structure of these materials are readily detected with diffraction-based methods, probing the chemistry associated with transition metal dissolution and characterizing the disordered, heterogeneous CEI layer have proven to be substantially more challenging. In this talk, I will discuss our efforts to use a combination of solid-state nuclear magnetic resonance (SSNMR) spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide information on the CEI deposited on composite cathode films with high chemical and spatial resolution (on the order of nanometers). In situ and operando magnetic resonance spectroscopies are used to correlate specific cathode degradation modes with electrolyte oxidation events. The final properties of the resulting CEI architectures are evaluated in the context of battery performance, where poor capacity retention and high interfacial resistance in LIBs is related to specific surface moieties on the cathode composite.