Efficient Synthesis of O2 Layered Structure for High Performance Li-Rich Cathode
Authors: Zhu Aipeng , Zhang Yun 
Published: February 2020

Nowadays, the growing grievous environment problems together with the exhaustion of energy resources put urgent demands for developing high energy density. Considering the factors including capacity, resource and environment, Manganese-based lithium-rich layer-structured cathode materials xLi2MnO3⋅(1-x)LiMO2 (M = Ni, Co, Mn, and other metals) are drawing increasing attention due to their high reversible capacities, high discharge potentials, and low cost. They are expected to be one type of the most promising cathode materials for the next-generation Li-ion batteries (LIBs) with higher energy densities. Unfortunately, their commercial applications are hindered with crucial drawbacks such as poor rate performance, limited cycle life and continuous falling of the discharge potential. With decades of extensive studies, significant achievements have been obtained in improving their cyclability and rate performances, but the potential decay remains a severe challenge. Fundamental investigations have clarified that the continuous migration of the transition metal (TM) ions into the lithium layer and the resultant irreversible structural transformation are responsible for the potential decay.

Recent study reports that the P2-NaxMO2 (M = Ni, Com Mn) shows a phase transformation from P2 to O2 with the disodium while the formation energy of O2 phase is around 25meV f.u.-1 lower than P2 structure. Such an energy difference can significantly affect the migration of the transition metal ions. Our work aims to improve the properties of Li-rich cathode by preparation of Mn based hybrid Li-Na materials and O2-typed layered structure materials for Li-rich cathode.

The precursor was prepared by a facile coprecipitation method. The as-prepared precursor was then thoroughly mixed with Na2CO3 and Li2CO3 and calcined in air at 500℃ for 5h and 900℃ for 12h to get NaxLi1.2-x[Ni0.2Mn0.6]O2. The lithiation was carried out in three different methods, that is, the solid phase calcination (LRLO-C) and the electrochemical reaction (LRLO-E). The formula of the as-prepared material was determined to be Li1.2Ni0.2Mn0.6O2. The cathode electrodes were prepared by mixing 80wt% LNMO, 15wt% acetylene black and 5% Polyvinylidene fluoride onto an Al foil.

The SEM of the as-prepared samples through different lithiation methods are shown in Figure 1. Obviously, we can see that the samples maintain spherical topography, but the details are not the same. The secondary particles of LRLO-C clustered and formed a solid sphere (Figure 1d), while the sample of LRLO-E keeps many hollows (Figure 2b). The result of electrochemical measurement of LRLO-E (Figure 1a) indicates such a structure has an enhanced cycle performance but relatively low capacity, while LRLO-C (Figure 1c) indicates that the O2 layered structure electrode from solid calcination delivered a discharge capacity of 238 mAh g-1 at 30mA g-1 and the retention after 50 cycles was 93%, but a long activation process.

In summary, the work described here shows a potential of O2 layered structure of Li-rich materials as promising materials to deal with voltage decay and long cycle life for Li-rich materials making it possible for commercial application.

Figure 1