Local Interactions Governing the Performances of Lithium and Manganese-Richcathodes

Abstract

Li-rich layered transition metal oxides Li[Li0.2Ni0.16Mn0.56Co0.08]O2 are attractive electrode materials providing high energy densities, relatively good cycling stability with reduced environmental costs [1]. However, their utility in many applications suffers from both voltage and capacity fade, most likely related to structural transformations and not yet fully understood [2].We investigated the manganese oxidation and spin states which are linked to the Mn size and magnetic interactions, resulting key parameters in defining the possible phases in which this cation may be involved [1, 3], and then in controlling the reversibility of the reactions along cycling. The Mn redox-state, constantly in opposition to the expected charge compensation, and spin-state resulted correlated with Ni oxidation/reduction, also spatially, suggesting that strain induced on the Mn−O sublattice by Ni oxidation triggers Mn reduction [3].To elaborate design strategies that can be used to control the structure, for instance, hindering the spinel formation at the benefit of the electrode cycle life, it results then fundamental to address quantitatively how the strain controls the Mn oxidation and spin state.Here we report about the local structural and electronical transformations during electrochemical cycling of a Li[Li0.2Ni0.16Mn0.56Co0.08]O2 cathode, combining several complementary techniques: X-ray absorption (Mn, Ni, and O K-edge) and X-ray emission spectroscopy (Mn Kβ emission line).We focus mainly on the first charge and discharge cycle, identifying few key states of charge: the pristine state, the beginning and the end of the high voltage plateau, and the fully charged and following discharged states. The partially irreversible spinel formation, occurring at the expenses of the cycling layered phase, is quantified. Moreover, the local strains induced by the Ni oxidation have been characterized. Finally, the quantification of the different Mn electronic phases present at the different charge states along the first cycle permits to determine the charge redistribution among all elements of the cathode material.The reported results unravel the role of strains in controlling the electrochemistry of Li-rich cathodes.[1] Rozier, P.; Tarascon, J. M. Li-Rich Layered Oxide Cathodes forNext-Generation Li-Ion Batteries: Chances and Challenges. J. Electrochem.Soc. 2015, 162 (14), A2490.[2] D. Mohanty et al. J. of Power Sources 229 (2013), 239[3] L. Simonelli et al. J. Phys. Chem. Lett. 10, (2019), 3359Corresponding author: lsimonelli@cells.es