Structural and electrochemical characterization of thin film Li₂MoO₃ electrodes

Li₂MoO₃ is a promising structural stabilizing unit for use in composite layered-layered cathodes for Li-ion batteries. To enable the rational design of such cathodes, studies on fundamental phenomena related to the active material structure and electrode/electrolyte interface are needed. The present work details the fabrication and characterization of thin film Li₂MoO₃ cathodes and shows that their electrochemical performance greatly depends on the nature of the cathode/electrolyte interface. The Li₂MoO₃ thin films exhibit poor cyclability in a liquid carbonate electrolyte (e.g., initial capacity of 166 mAh/g with 40% capacity fade over 20 cycles) whereas all-solid-state Li₂MoO₃/Lipon/Li batteries show negligible fade during cycling. A suite of characterization methods including Raman spectroscopy and X-ray photoelectron spectroscopy are used to study the evolution of the cathode structure and cathode electrolyte interphase (CEI) layer during charge/discharge cycling. Li transport rates are another important factor which affect cathode performance. AC impedance spectroscopy studies reveal that the Li diffusion coefficient (DLi) in Li₂MoO₃ decreases from 4.36 × 10−¹¹ cm²/s in the fully discharged state to 4.51 × 10−¹³ cm²/s when charged to 3.6 V vs. Li/Li⁺. Overall, the results presented herein provide insight on the fundamental phenomena which govern Li₂MoO₃ cathode performance.