Structural and Electrochemical Characterization of Thin Film Li2MoO3 Electrodes

Abstract

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.