Band-Diagram Framework for Materials Development in Cation Intercalation Charge Storage

Abstract

The next generation of batteries may employ ionic liquid or solid electrolytes with stability windows of more than 6 volts, and migrate from lithium ion chemistries to more earth-abundant cations such as sodium, potassium, or magnesium ions. In this work, we apply new theoretical and experimental approaches to lithium ion battery cathode materials to gain insight into the requisite properties for electrode materials in next-generation batteries, with particular emphasis on spinel lithium manganese oxide (LiMn2O4). Nanoscale spinel LiMn2O4 is of interest as a high-rate cathode material for advanced battery technologies among other electrochemical applications. 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+. 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. Furthermore, 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. Figure 1