Effect of Annealing on the First-Cycle Performance and Reversible Capabilities of Lithium-Rich Layered Oxide Cathodes

The cathode materials of lithium-rich 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 have been synthesized via coprecipitation method. To propose a better understanding of the effects of annealing on the cathode material, the bulk and surface structures of the pristine and cycled samples of 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 are characterized by X-ray diffraction (XRD) together with transmission electron microscopy (TEM). According to XRD and TEM observations, the surface structure of the sample annealed at 800 °C (LMCNO-800) tends to transform from layer to spinel after the first cycle, whereas the sample annealed at 950 °C (LMCNO-950) inclines to turn amorphous. Combined with the electrochemical characterizations, it was found that the LMCNO-800 cathode has a high activation degree, which enables a high discharge capacity of 274 mAh g–1. However, the spinel surface structure caused by activation is apt to suffer from corrosion by the electrolyte during cycling, which leads to rapid capacity decay. On the contrary, the surface amorphous structure of the first cycled LMCNO-950 sample may serve as a barrier layer between the bulk structure and the electrolyte, thereby guaranteeing extremely high capacity retention of 98.5% after 100 cycles. Further, the effects of different annealing temperatures on electrochemical performance of the Li-rich compounds are investigated. It is thus concluded that the structure stability and electrochemical performance of the Li-rich cathode is strongly dependent on the annealing temperature, and the results suggest that the sample annealed at 900 °C exhibits the best electrochemical performance.