Layered Li2MnO3·3LiNi0.5−xMn0.5−xCo2xO2 microspheres with Mn-rich cores as high performance cathode materials for lithium ion batteries

Abstract

Layered Li₂MnO₃·3LiNi_0.5−xMn_0.5−xCo_2xO₂ (x = 0, 0.05, 0.1, 0.165) microspheres with Mn-rich core were successfully synthesized by a simple two-step precipitation calcination method and intensively evaluated as cathode materials for lithium ion batteries. The X-ray powder diffractometry (XRD) results indicate that the growth of Li₂MnO₃-like regions is impeded due to the presence of cobalt (Co) in the material. The field-emission scanning electron microscopy (FESEM) data reveal the core–shell-like structure with a Mn-rich core in the as-prepared particles. The charge–discharge testing reveals that the capacity is markedly improved by adding Co. The activation of the cathode after Co doping becomes easier and can be accomplished completely when charged to 4.6 V at the C/40 rate in the initial cycle. Superior electrochemical performances are obtained for samples with x = 0.05 and 0.1. The corresponding initial discharge capacities are separately 281 and 285 mA h g⁻¹ at C/40 between 2 and 4.6 V at room temperature. After 250 cycles at C/2, the respective capacity retentions are 71.2% and 70.4%, which are better compared to the normal Li-excess Li₂MnO₃·3LiNi_0.4Mn_0.4Co_0.2O₂ sample with a uniform distribution of Mn element in the particles. The initial discharge capacities of both samples are approximately 250 mA h g⁻¹ at a rate of C/2 between 2 and 4.6 V at 55 °C after activation. Furthermore, the samples are investigated by electrochemical impedance spectroscopy (EIS) at room and elevated temperature, revealing that the key factor affecting electrochemical performance is the charge transfer resistance in the particles.