Structural and Electrochemical Characterization of Thin Film Li₂MoO₃ Cathodes

Development of low-cost cathodes with high energy density and long cycle life is critical to enable advanced Li-ion batteries for transportation applications. When charged beyond 4.3 V vs. Li/Li+, most lithium-transition metal oxides (e.g., LiMO2, M = Ni, Mn, Co) undergo irreversible structural changes with concomitant oxygen gas evolution, resulting in irreversible capacity loss and voltage fade during cycling. One strategy to improve cathode stability at high states of charge is to design composite structures containing LiMO2 as the primary Li storage site and Li2MoO3 as a structural stabilizing unit (i.e., cathodes with the formula xLi2MoO3•(1-x)LiMO2). In this regard, Li2MoO3 possesses several attractive properties including a stable oxygen framework (up to 4.8 V vs. Li/Li+) and the ability to reversibly cycle 1 Li per formula unit. To aid the development of Mo-based composite cathodes, the phenomena occurring at the cathode/electrolyte interface (e.g., Li transport, electrolyte decomposition, charge transfer, etc.) during battery cycling must be well-understood. However, interpreting in-situ and ex-situ data collected with traditional slurry cast electrodes is oftentimes complicated by the presence of conductive carbon and polymer binder which may participate in side reactions with the electrolyte and/or produce signals that overlap with the desired signal from the active material. To resolve this issue, radio frequency (RF) sputtered thin film cathodes composed entirely of active material represent a model system to study the electrode/electrolyte interface. In addition to eliminating unwanted signal arising from inactive components, sputtered thin films have a planar surface which greatly simplifies interpretation of AC impedance spectroscopy data. The present work describes the preparation and characterization of Li2MoO3 thin film cathodes shown in Figure 1a. When cycled in a conventional liquid carbonate electrolyte (Figures 1b and 1c), the cathodes exhibited an initial reversible capacity of 166 mAh/g which rapidly faded to 99 mAh/g after 20 cycles. In comparison, all-solid-state Li2MoO3/Lipon/Li thin film batteries showed excellent stability with negligible capacity fade during cycling. As will be discussed, these differences are related to the nature of the cathode/electrolyte interface. This presentation will provide additional findings from X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), vibrational spectroscopy, and AC impedance spectroscopy. Overall, these results provide important insights on the fundamental interfacial phenomena that govern cathode performance. Acknowledgements This research has been supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, of the U.S. Department of Energy (DOE) through the Advanced Battery Materials Research (BMR) Program. Figure 1