Preprint Article Version 1 This version is not peer-reviewed

Molecular Dynamics Study on the Li Diffusion Mechanism and Delithiation Process of Li2MnO3

Version 1 : Received: 10 September 2019 / Approved: 10 September 2019 / Online: 10 September 2019 (15:41:27 CEST)

How to cite: Huang, Y.; Liu, L.; Gao, M. Molecular Dynamics Study on the Li Diffusion Mechanism and Delithiation Process of Li2MnO3. Preprints 2019, 2019090111 (doi: 10.20944/preprints201909.0111.v1). Huang, Y.; Liu, L.; Gao, M. Molecular Dynamics Study on the Li Diffusion Mechanism and Delithiation Process of Li2MnO3. Preprints 2019, 2019090111 (doi: 10.20944/preprints201909.0111.v1).

Abstract

Li2MnO3 is critical component in the well-studied Li-excess cathode materials xLi2MnO3•(1- x)LiMO2 for achieving high lithium storage capacity. In this article, the diffusion of Li ions in Li2MnO3 is studied using molecular dynamics (MD) simulation with well-behaved empirical force fields obtained by fitting against the crystal structure from experiment and phonons calculated using Density Function Theory (DFT). We have found two possible tetrahedral hopping channels, 0-TM and 1-TM(Mn4+) channel, which are differentiated by the face sharing octahedral cations. Simulation results show that the 0-TM channel is active for Li hopping, while 1-TM(Mn4+) channel is inactive. During the delithiation process, the Li ions in the TM layered are firstly removed, then those in the Li layer. However, the Li ions will be trapped in the tetrahedral 0-TM channels as long as the four face sharing octahedral sites are cleared. Up to x=1.0 for Li2-xMnO3, almost all the Li ions are located at the tetrahedral sites, forming a regular array along a axis. The de-intercalation of tetrahedral Li ions requires a high voltage (>5.2 V vs. Li/Li+), limiting the practical capacities measured in lab. The diffusion of Mn ions into the Li layers is observed in a deeper delithiated structure (x=1.2 for Li2-xMnO3), indicating an initial phase transformation to a spinel-like structure. However, the Mn ions are mainly trapped in the tetrahedral sites in the Li layer, instead of the octahedral sites fin spinel-like structure. A few of Mn ions diffusing into the octahedral sites in Li layers have no face sharing tetrahedral Li ions, revealing a further Li de-intercalation is imperative for the complete phase transformation. Our model is not stable for x≥1.4 in Li2-xMnO3. Other charge compensation mechanism should be considered in this high delithiation stage, eg. oxygen release.

Subject Areas

Li2MnO3; Molecular dynamics; diffusion; delithiation

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