Investigation of the Reaction Mechanism in Mesoporous MnO2 As Anode Material for Lithium Ion Batteries
Authors: Yoon Jaesang , Choi Woo Sung , Kim Hyunwoo , Yoon Won-Sub 
Published: February 2020

Lithium ion batteries (LIBs) are the mostly used energy storage systems because of their high energy density and excellent cycle life. Recently, as the more expanding application fields of LIBs from portable devices to electric vehicle (EV) and energy storage system (ESS), the demands for better LIBs increase. Energy density is one of the most important required performances of batteries. A high energy density in batteries can be achieved by increasing the capacity of the electrodes or by increasing the working potential of the cathode materials. In the field of field of anode materials, research is underway to increase capacity, because the capacity of graphite widely used as anode in commercial LIBs is limited (~372mAh/g). Transition metal oxides such as CoO, Co3O4, NiO, FeO and Fe2O3 have attracted great attention due to their high theoretical capacity (700~1500 mAh/g) based on the conversion reaction mechanism. Mn-based oxides are the promising anode materials for lithium ion battery due to low cost, lower operating voltage compared to the Co, Ni, Fe and Cu based oxides and their high theoretical capacity (MnO : 756 mAh/g, Mn3O4 : 934 mAh/g, Mn2O3 : 1018 mAh/g, MnO2 : 1232 mAh/g). Many groups reported that nanostructured manganese oxides show higher capacity than theoretical capacity based on the conversion reaction mechanism. However, their exact reaction mechanisms are not yet clear and information about the reaction mechanism of MnO2 is more insufficient compared to other manganese oxide materials, MnO, Mn3O4 and Mn2O3 because they have focused on the synthesis method of materials to improve their electrochemical performances. Chen et al. reported that electron energy loss spectra (EELS) and selected area electron diffraction (SAED) results propose that MnO2 is only partially reduced to LiMn3O4, instead of being reduced to Mn [1]. Furthermore, Fang et al. reported that the LixMnO2 (X=0.96) and Li2MnO2 are observed by ex situ X-ray diffraction (XRD) but MnO is not observed until Li2MnO2 is fully reduced to metallic Mn and Li2O at 0V. From high resolution transmission electron microscopy (HRTEM), Mn is one of the end products of discharge to 0V, while MnO is the end product of recharge for MnO2 at 3.0V [2]. Recently, by cyclic voltammetry, MnO2 is reduced to Mn via MnO intermediate and the oxidation of Mn0 to Mn2+ and Mn2+ to Mn4+ occur during charge [3,4]. But cyclic voltammetry cannot provide obvious evidence of phase formation.
In our report, to propose the detailed reaction mechanism of MnO2, we synthesize ordered mesoporous MnO2 and use synchrotron-based X-ray techniques, high resolution transmission electron microscopy (HRTEM) and electrochemical techniques. Our new proposed reaction mechanism suggest experimentally that, for the first time, the formation of the MnO intermediate during 1st discharge and the further oxidation of Mn2+ to higher oxidation state during charge are involved in the reaction mechanism of MnO2. Therefore, we provide a better understanding of the reaction mechanism of MnO2 anode for Li ion batteries. More details will be discussed in the meeting.

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