Comparative Study of Lithium Storage Behavior in Mesoporous and Bulk Mn2o3 Anodes for Li Batteries
Authors: Yoon Jaesang , Choi Woosung , KIM Hyunwoo , Yoon Won-Sub 
Published: December 2020

Over the last two decades, lithium-ion batteries (LIBs) have been recognized as the most satisfactory energy storage technology for numerous electric devices. In recent years, as the application of LIBs extends to electric vehicles (EVs) and energy storage systems, the demand for LIBs with increased specifications continues to increase. To meet high-power and high-energy storage systems in the rapidly growing energy storage markets, in the anode, there have been many attempts to develop high capacity anode materials to replace the graphite [1,2]. In particular, transition metal oxides have attracted attention due to their high theoretical capacity based on the conversion reaction mechanism [3,4]. However, the conversion reaction has poor kinetics leading to large polarization and causes large volume changes during discharging and charging, resulting in poor capacity retention upon cycling [5,6]. To solve these problems, nanotechnology has been applied in this field [7,8]. Nanostructured materials can act as a buffer against volume expansion, and provide high surface area and short path lengths for Li+ transport, which results in high specific capacity, excellent rate capability and cycle life [9,10]. To effectively use the advantages of nanostructured materials in next-generation batteries, it is necessary to understand the advantage of nanostructure that positively affects the electrochemical performance.
In this work, using synchrotron-based X-ray techniques, we compare lithium storage of mesoporous and bulk Mn2O3 in terms of Mn central charge storage and non-Mn central charge to investigate why the mesoporous material outperforms the bulk material. As a consequence, the mesoporous structure with short path lengths for Li+ and electron transport and high surface area in contact with the electrolyte can enable larger lithium storage. X-ray absorption spectroscopy result indicates that both samples show a phase transition from Mn2O3 to Mn via MnO during the first discharge, followed by a reversible reaction between MnOx to Mn metal. However, since the mesoporous structure lowers the resistance for the conversion reaction, and accelerates the Li+ and electron transport, the mesoporous Mn2O3 has a larger capacity than bulk Mn2O3 through the further oxidation reaction from Mn2+ to a higher oxidation state during charging, and the additional reversible reaction during cycling. Furthermore, the combined results from X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy indicate that additional lithium storage reaction by reversible formation-decomposition of the electrolyte-derived surface layer occurs more in mesoporous Mn2O3 than bulk Mn2O3. Consequently, the improved oxidation reaction during charge and enhanced contribution of reversible formation and decomposition of the electrolyte-derived surface layer due to the increased specific surface area and reduced particle size as the cycle progresses also affect cycle performance of mesoporous Mn2O3. This report provides an understanding of the synergistic relationship between nanostructures and electrochemical performance and practical strategies for developing high energy density anode materials for next-generation lithium-ion batteries. More details will be discussed in the meeting.

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