Next generation batteries may shift from lithium ion chemistries to those based on more earth-abundant cations such as sodium, potassium, or magnesium ions and employ ionic liquid or solid electrolytes with stability windows of over 6 volts. We have developed a new theoretical approaches to analyze lithium ion battery cathode materials to gain insight into the requisite properties for electrode materials in next-generation batteries, which we demonstrate using spinel lithium manganese oxide (LiMn₂O₄). Nanoscale spinel LiMn₂O₄ is a potential high-rate cathode material for advanced battery technologies and other electrochemical applications. Using a coupled electrochemical and band structure analysis of LiMn₂O₄ informed by screened hybrid density functional theory calculations we identify new features of the charge storage properties and stability of LiMn₂O₄. The accuracy and predictive power of this theoretical framework to determine the equilibrium potential(s) for charge-transfer in intercalated compounds is validated by comparison with experimental results. We first demonstrate the synthesis of ultrathin films of spinel LiMn₂O₄ between 20 and 200 nm in thickness by room-temperature electrochemical conversion of MnO grown by atomic layer deposition (ALD). Next, we investigate the charge storage properties of LiMn₂O₄ thin films in electrolytes containing Li+ with Na+, K+, and Mg2+. Our CV experiments demonstrate that for sweep rates between 1 and 400 mV/s, up to 30% of the capacity of LiMn₂O₄ arises from bulk electronic charge-switching which does not require compensating cation mass transport. The hybrid ALD-electrochemical synthesis, coupled electrochemical and electronic structure analysis, and unique charge storage mechanism we describe provide a fundamental framework to guide the development of future nanoscale electrode materials for ion-incorporation charge storage.