Nano-/Micro-structured Li₂MnO₃ And Its Surface Modification Composites As The Cathode Materials For Lithium Ion Batteries

Now lithium ion batteries are dominating the market of portable electronic devices. To expand the use of lithium ion batteries, for instance as the onboard energy storage for electric vehicles (EVs), the specific energy density has to be increased. In order to arrive at this goal, it is necessary to increase the voltage and the capacity of their cathode materials. However, most commercial lithium ion batteries use layered LiCoO2 or LiNi1/3Co1/3Mn1/3O2 as the cathode material and have a limited practical capacity (below 160 mAh g-1). The theoretical capacities of alternate cathode materials like spinel LiMn2O4 (148 mAh g-1) and olivine LiFePO4 (170 mAh g-1) are also low. The energy density of the current lithium ion batteries is mainly limited by the cathode materials and it is urgent to develop new cathodes with higher capacity and higher operating voltages. In this regard, solid solutions between layered Li[Li1/3Mn2/3]O2 (i.e. Li2Mn03) and LiMO2 (M=Mn, Ni, Cr, Co and Fe) are becoming appealing as they exhibit high capacities. However, these materials undergo an irreversible loss of oxygen from the lattice and suffer from a huge irreversible capacity loss of 30-120 mAh g-1 in the first cycle, which results from part of Li+ions that could not be accommodated into the layered lattice after the loss of oxygen. So the application of Li-rich layered materials with high capacities is significantly limited due to the huge irreversible capacity loss and oxygen emission from the lattice above 4.5 V.Therefore, inmydissertation, the work is focused on the preparation of nano-/micro-structured Li2MnO3 materials. Then we present a strategy to eliminate the large initial irreversible capacity and oxygen emission.1 Coaxial LiCoO2@Li2MnO3 nanoribbons was prepared by a simple method and characterized with X-ray diffraction, field emission scanning electron and transmission electron microscopy. As cathode material for lithium ion batteries they deliver a high reversible capacity of 180 mAh g-1, which is very stable without capacity fading during cycling in the range of 2.0-4.8 V.2. The Li2MnO3 nanoplate was synthesized by a facile method. After coating FePO4 nanoparticles on the surface of Li2MnO3 nanoplate, a method to eliminate the large irreversible capacity for the Li-rich materials is found. The FePO4 on the surface of Li2MnO3 can serve as a host for Li ions that deintercalate from Li2MnO3 during the initial charge. The composite as the cathode material for lithium ion batteries exhibits an attractive discharge capacity of 180 mAh g-1. Its cycling performance is also stable without any evident capacity fading, even when cycling in the high voltage range of 2.0-4.8 V. It is free of Co- and Ni, providing great attraction for electric vehicles of low cost.3. Li2Mn-3 microsphere was prepared successfully by a simple reaction of MnC03 microsphere with LiOH. Then we demonstrate a novel approach to solve the irreversible capacity loss and release O2 of Li-rich materials. Also, the Co3O4 coated on the surface of Li2MnO3 microsphere is porous and would not hinder the Li ion diffusion to the surface of Li2MnO3. The cycling performance of such core-shell structure of porous Co3O4 grown on Li2MnO3 microsphere is excellent. The attraction of our porous Co3O4 coating approach is that it can be extended to other coating to eliminate the irreversible capacities loss of Li2MnO3 and its solid solutions with layered LiMO2.