Electronic properties and structural properties of Li (1 - chi) FePO 4 (X = 0 , 0.5 ,1)

Lithium iron phosphate (LiFePO4) is currently applied as a cathode material for lithium ion batteries. However, its electronic properties and the mechanism by which lithium ions are extracted (delithiation) or inserted (lithiation) into the lattice are still not completely understood. In this thesis the electronic and structural properties of Li(1-x)FePO4 (LiFePO4 x = 0, FePO4 x = 1) have been investigated using valence and core loss electron energy loss spectroscopy (EELS) and high resolution transmission electron microscopy (HRTEM). In the first part of this thesis we present the study of the electronic structure before and after delithiation, LiFePO4 and FePO4 respectively. This is accomplished using valence EELS (VEELS), core-loss EELS and bandstructure calculations. We show that the changes in the electronic structure between FePO4 and LiFePO4 are quite significant such that FePO4 can be considered to be a charge transfer insulator while the LiFePO4 can be considered to be a Mott-Hubbard insulator. In LiFePO4 the energy states at the top of the valence band and bottom of the conduction band are dominated by Fe 3d states. These states form the lower Hubbard band (LHB) and the upper Hubbard band (UHB) respectively. Delithiation is characterized by shifting of the iron (Fe) 3d bands to lower energies and increased hybridization between the Fe-3d and oxygen (O) 2p states. The second part of this thesis is concerned with structural studies on partially delithiated (Li(0.5)FePO4). Examining the lattice parameter distribution in partially delithiated LiFePO4 grains we find lattice parameters for FePO4 and LiFePO4 phases. This shows that at a partially delithiated state both phase do co-exist in the same grain. These results support the one dimensional model (1D) of delithiation which states that extraction of Li ions proceeds through the ion channels parallel to the [010] direction resulting in the formation of lithiated and delithiated domains in the same grain

Electron-Beam- and Thermal-Annealing-Induced Structural Transformations in Few-Layer MnPS3

Funding Information: We want to thank all cooperation associates in the framework of this manuscript. Further, we especially thank the Institut Laue-Langevin for the synthesis of the TMPT materials and Gabriele Es-Samlaoui for the preparation of TEM samples used in this work. This work was supported by the German Science Foundation (DFG), project CRC 1279 (project number 316249678), projects KR 4866/8-1, and the collaborative research center “Chemistry of Synthetic 2D Materials” SFB-1415-417590517. This work was supported by a project funded by the Carl Zeiss foundation. The computational support from the Technical University of Dresden computing cluster (TAURUS) and High-Performance Computing Center (HLRS) in Stuttgart is gratefully appreciated. M.K.K. acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG) within the project DFG: KI 2546/1-1. Publisher Copyright: © 2023 The Authors. Published by American Chemical Society.Quasi-two-dimensional (2D) manganese phosphorus trisulfide, MnPS3, which exhibits antiferromagnetic ordering, is a particularly interesting material in the context of magnetism in a system with reduced dimensionality and its potential technological applications. Here, we present an experimental and theoretical study on modifying the properties of freestanding MnPS3 by local structural transformations via electron irradiation in a transmission electron microscope and by thermal annealing under vacuum. In both cases we find that MnS1-xPx phases (0 ≤ x < 1) form in a crystal structure different from that of the host material, namely that of the α- or γ-MnS type. These phase transformations can both be locally controlled by the size of the electron beam as well as by the total applied electron dose and simultaneously imaged at the atomic scale. For the MnS structures generated in this process, our ab initio calculations indicate that their electronic and magnetic properties strongly depend on both in-plane crystallite orientation and thickness. Moreover, the electronic properties of the MnS phases can be further tuned by alloying with phosphorus.Peer reviewe