Cyclic Properties and Structural Studies for Ruthenium-Substituted Li2MnO3

Lithium batteries have attracted much attention as the key issue for hybrid electric vehicle (HEV) and electric vehicle (EV) because of an interest in reducing emission of carbon dioxide and use of alternative energy sources.  Layered manganese oxide, Li₂MnO₃, with similar layered structure to the practical cathode materials represented as LiMO₂ (M = transition metal) is a promising candidate as an alternative cathode material having higher voltage and larger capacity.  Although Li₂MnO₃ synthesized at low temperature shows capacity of above 150 mAh g^-1, its cyclic capability is very poor.  Recently, we demonstrated that ruthenium substitution to Li₂MnO₃ is quite effective to improve electrical and electrochemical properties [1].  For instance, Li₂Mn_0.4Ru_0.6O₃ exhibited the electrical resistivity of 2.4 x 10³ Ω cm, which is six orders lower than that of Li₂MnO₃, and showed high capacity of about 200 mAh g^-1 and good cyclic capability.  And then, Sathiya et al. reported that its high capacity is involved in reversible anionic redox process between O^2- and O^- in addition to well known redox process of transition metals [2].  The better understanding of the charge-discharge mechanism of high capacity cathode materials such as LiMO₂-Li₂MO₃ system and development of new class of cathode materials utilizing anionic redox process require the detailed study of electrochemical properties and charge-discharge mechanism for ruthenium substituted Li₂MnO₃.  In this study, we investigated structural change during electrochemical cycling and cyclic properties for ruthenium substituted Li₂MnO₃.

   Li₂Mn_1-xRu_xO₃ was synthesized by a conventional solid-state reaction. Li₂CO₃, MnO₂ and RuO₂were used as starting materials.  The mixing powder was pelletized and then fired at 1200 ˚C for 6-9 h in an oxygen gas flow with intermediate grinding.  The powdered samples obtained were identified by X-ray diffraction method.  The electrochemical measurement was carried out using 2032 type coin-cell with a constant current of 1/10C rate between 2.0 and 4.8 V.  The positive electrodes after electrochemical (de)-intercalation reaction to various voltages were characterized by synchrotron XRD measurement in the beam line BL02B2 at SPring-8.

   Li₂Mn_1-xRu_xO₃ has different layer structure depending on ruthenium content which are the Li₂MnO₃and Li₂RuO₃ type structure at 0 ≤ x < 0.8 and 0.8 ≤ x ≤ 1.0, respectively.  In the XRD patterns for Li₂Mn_1-xRu_xO₃ at x = 0.4 and 0.6 the reflections corresponding to cation ordering in transition metal layer were observed after charging to 4.8 V.  It indicates that the ruthenium substitution stabilized layer structure of Li₂MnO₃.  Li₂Mn_1-xRu_xO₃with both type structures was also found to show reversible structural change during charge-discharge cycling. 

   Li₂Mn_1-xRu_xO₃ with 0.6 ≤ x ≤ 1.0 exhibited good cyclic performance between 2.0 and 4.8 V.  However, the cyclic capability of Li₂Mn_0.4Ru_0.6O₃ with Li₂MnO₃ type structure depended on cut-off voltage of charging.  During charge-discharge cycling between 2.0 and 4.8 V Li₂RuO₃ showed prominent variation of charge- discharge profiles and lowering discharge voltage with increasing cycle.  While, no significant changes in the charge-discharge profiles and lowering discharge voltage were observed upon cycling for Li₂Mn_1-xRu_xO₃ at x = 0.6 and 0.8.  We will discuss the relationship between composition, structure and electrochemical properties for ruthenium substituted Li₂MnO₃.

References

[1] D. Mori, H. Sakaebe, M. Shikano, H. Kojitani, K. Tatsumi, Y. Inaguma, J. Power Sources, 196(2011) 6934.

[2] M. Sathiya, K. Ramesha, G. Rousse, D. Foix, D. Gonbeau, A.S. Prakash, M. L. Doublet, K. Hemalatha, J. –M. Tarascon, Chem. Mater., 25 (2013) 1121.