Multiple Edge in Situ X-Ray Absorption Spectroscopy (XAS) Investigations of Fe Substituted Li-Rich Cathode Materials for Libs
Authors: Dixon Ditty , Bhaskar Aiswarya , Mangold Stefan , Ehrenberg Helmut 
Published: February 2020

Layered Li-rich materials with a general formula xLi2MnO3·(1-x)LiMO2 (M = Co, Ni, Fe etc.) can offer specific capacities >200 mAh/g and hence are highly promising candidates as high-capacity cathode materials for lithium-ion batteries[1]. Out of these, the Co containing materials are extensively investigated with respect to their electrochemical performance and cycling mechanism[2]
[3]. Discrepant results are available in literature regarding the electrochemical mechanism especially in the 1st charge-discharge cycle[4]
[5]. Some literature reports claim a loss of Li+ and corresponding amount of oxygen (net loss “Li2O”) from the Li2MnO3 component of the material in the 1st charge at ~4.6 V[6]
[7]
[8]. As a result, the Li2MnO3 component transforms to MnO2-like species which then intercalates Li+ in the subsequent discharge to form LiMnO2-like structure.  A conversion of the MnO2-like species to Li2MnO3 –type structure at the end of discharge was also reported in few works[5]
[4]. It was also observed that during the lithium and oxygen losses, the transition metal reordering occurs[9]. A recent X-ray diffraction study proposes an oxygen loss at the surface and reversible oxygen oxidation at the bulk of the Li-rich material[10].

The layered Li-rich materials containing Fe are attractive candidates as high capacity cathode materials for lithium-ion batteries as they are environmentally friendly and cost effective in comparison with the current commercial Co-containing cathode materials[11]
[12]. There are some literature reports dealing with the electrochemical performance of these materials[12]. However, the information on electrochemical mechanism especially in the wide voltage range 2.00-4.95, where the maximum capacity can be obtained is missing in literature.

Electrochemical mechanism of Fe substituted Li rich cathode material with composition xLi2MnO3. (1-x) LiNi0.4Mn0.4Fe0.2O2 was investigated using multiple edge in situ XAS. In situ XAS experiment was conducted at Mn, Fe and Ni K edges in one go with specially modified in situ cell and optimized XAS beamline optics and ionization chambers[13]. All the in situ XAS measurements were carried out at XAS beamline ANKA, Karlsruhe, in transmission mode with a scan rate of 5 min per each spectrum.  The oxidation state changes and the local coordination changes around each transition metal will be discussed at different potentials. Local coordination changes can also give information about the transition metal reordering which is expected during the activation process by the loss of oxygen and Li+. The mechanism of this activation process during the first cycle will be discussed. Further, a comparison between the first and second charging processes will be made.

Acknowledgement:

This research has benefitted from beamtime allocation at the XAS Beam Line at ANKA (Karlsruhe, Germany).

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