Enhancement of Initial Coulombic Efficiency of Molybdenum Oxide Electrodes for Lithium-ion Batteries

Abstract

Although graphite, the negative electrode for commercialized lithium-ion batteries, has lots of advantages, it shows limited specific capacity to apply it to the high-capacity lithium-ion batteries such as electric vehicles and energy storage systems. Among alternative materials to overcome such drawback, transition metal oxides which react with lithium through a conversion reaction having high specific capacity have been exploited. During the lithiation, as metal ? oxygen bond in metal oxide is broken, metal ion is reduced elemental state by taking electrons and oxygen ion forms chemical bonds with lithium ion to be generated Li2O. In de-lithiation process, the reverse reaction occurs by the oxidation of metal component and formation of metal ? oxygen bond again. Molybdenum oxides react with lithium by an insertion or conversion reaction according to Mo valence, or bond strength between Mo and O. In the lithiation by conversion reaction, it delivers high specific capacity, especially very high in MoO3, 6 Li+/electrons per formula unit giving corresponding theoretical specific capacity of 1117 mAh g-1. However, due to the constantly cleavage and formation of metal ? oxygen bond and severe electrolyte decompositions at the surface of newly formed nano-sized metal, they show poor electrochemical performance including low initial Coulombic efficiency. Since the lithium sources in full-cell are limited, low ICE causes the decrease in cell capacity dramatically in the subsequent cycles. Thus, such drawback should be overcome for the use in practical LIBs. In this study, to enhance the electrochemical performance, especially the initial Coulombic efficiency, of molybdenum oxides, three strategies are performed. Firstly, by the change of pH in the preparation of amorphous molybdenum oxides, the Mo valence of them are changed, which results in the improvement of electrochemical performance. In the second strategy, the reaction mechanism of MoO3 electrode is examined, and its initial Coulombic efficiency and electrochemical performance is improved by just short-time ball-milling. The reason is likely due to the effect from the grinded surface of particles and tens of nanometric particles generated by ball-milling. Lastly, a new type of negative electrode, Li2MoO3, is introduced, which is designed to release larger amount of lithium ions and electrons in de-lithiation than amount of them taken in lithiation. Through such reaction, Li2MoO3 electrode shows the initial Coulombic efficiency higher than 100 % and good cycle performance as well. In addition, for in-depth analysis about what happens in these electrodes proposed in three strategies, several kinds of electrochemical and spectroscopic methods are used. By using these solutions, it is expected that the improvement of electrochemical performance can be achieved not only in molybdenum oxides but also other conversion reaction-type metal oxides. Furthermore, this work can help the use of conversion reaction-type transition metal oxides into the negative electrode for practical LIBs in the near future.