Layered-to-Spinel Phase Transformation of Li2MnO3 during the Initial Charge

Abstract

Lithium- and Manganese-Rich layered oxides (LMRs) are one of the promising candidates of positive electrode materials for the advanced LIB due to their high reversible capacities. The chemical formula is often described as xLi2MnO3•(1–x)LiMO2 that consists of Li2MnO3- and LiMO2-rich nano-domains. The LMR-based electrodes show a voltage plateau at ca. 4.5 V in the initial charge process, which leads to an irreversible capacity. This plateau is understood to appear for the activation of Li2MnO3.[1,2] This activation process is not thoroughly understood yet and the elucidation of this reaction would lead to the practical use of this material. Some groups have reported that the charge compensation on the delithiation from Li2MnO3 is associated with the oxidation of oxide ion including O2release from the structure.[3,4] The present work focuses on clarifying the structural changes taking place inside LMR bulk at the initial delithiation (charge) using SR-X ray diffraction, STEM, and solid-state NMR.[5] SR-XRD profiles showed that all the diffraction intensities were gradually decreased and broadened during the initial charge up to 4.8 V, which indicated that the initial delithiation caused the change to highly disordered material. The high-resolution 6Li MAS NMR spectra showed that Li+ was monotonously extracted from both the Li and TM layers. EELS mapping by STEM suggested that the Li+extraction from the sample of 50% SOC occurred in a biphasic manner inside a single particle. The lattice structures of the active material after charge-discharge cycle was also examined on samples at the 20th discharged and 21stcharged states. This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) and the Research and Development Initiative for Scientific Innovation of New Generation Batteries II (RISING II) projects from the New Energy and Industrial Technology Development Organization (NEDO), Japan. [1] Thackeray et al., J. Mater. Chem., 2007, 17, 3112. [2] Yu & Zhou, J. Phys. Chem. Lett., 2013, 4, 1268. [3] Yu et al., J. Electrochem. Soc., 2009, 156, A417. [4] Oishi et al., J. Mater. Chem. A, 2016, 4, 9293. [5] Shimoda et al., J. Mater. Chem. A, 2017, 5, 6695.