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 1^st 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.

References:

[1] C. Chen, N. Ding, L. Wang, Y. Yu, I. Lieberwirth, Journal of Power Sources. 189 (2009) 552–556

[2] X. Fang, X. Lu, X. Guo, Y. Mao, Y.-S. Hu, J. Wang, Z. Wang, F. Wu, H. Liu, L. Chen, Electrochemistry Communications. 12 (2010) 1520-1523

[3] M. Kundu, C.C. Albert Ng, D.Y. Petrovykh, L. Liu, Chem. Commun. 49 (2013) 8459-8461

[4] J. Chen, Y. Wang, X. He, S. Xu, M. Fang, X. Zhao, Y. Shang, Electrochimica Acta. 142 (2014) 152-156