Understanding the Structural Evolution of Multiple Cationic Centres during Electrochemical Cycling of Lithium Rich Layered Li2Ru1-XFexO3
Authors: Satish Rohit , Vanchiappan Aravindan , Bucher Nicolas , Hartung Steffen , Franklin Joseph , Wong Chui Ling , Srinivasan Madhavi 
Published: February 2020

Lithium rich layered cathode materials have recently garnered a lot of attention as high capacity cathodes in Li-ion batteries, and their possible use for vehicular applications. However, they have been shown to be poor in terms of high power performance and long-term stability[1], these issues are mainly attributed to the structural changes occurring in the material during electrochemical cycling[2]. The understanding of these changes are still at an infant stage. In this work, we concentrate on the formation and study of the effect of partial substitution of ruthenium with iron in a Lithium rich ruthenium oxide (Li2RuO3). The purpose of substituting ruthenium with iron is to be able to understand the effect of multiple redox centers on the electrochemical performance of Lithium rich layered cathodes. Apart from having an active redox couple (Fe3+/Fe4+) in our interest region (3 - 4.6V vs. Li), iron is cost effective and environmentally benign.

Li2Ru1-xFexO3 (0<x<1)was synthesised using a high temperature ceramic route [3]. Rietveld refinement of the X-ray diffraction pattern was carried out to confirm the formation of a pure phase. Initial electrochemical measurements conducted at 20 mA g-1 show a high reversible capacity of ≈240 mAh g-1 in an organic electrolyte using Li as the counter electrode. This was accomplished  by the reversible removal/insertion of ≈1.4 moles of Lithium from the host structure. This is beyond the capacity that can be expected due to the ruthenium and iron redox couples alone (≈ 174 mAh g-1). Apart from the high capacity, it was observed that the material had a much-reduced voltage decay during cycling, when compared to other ruthenium based Lithium rich layered cathodes[1, 4-6]. In order to understand the structural changes, responsible for the extra capacity, in-operando X-ray diffraction measurements were made using a specially designed electrochemical cell [7]. The results obtained contribute to a unique insight into the charge storage mechanism of Lithium rich layered cathode materials and will help in the future development of new chemistries involving Lithium rich layered materials for energy storage applications

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