A Li-Rich Layered Cathode Material with Enhanced Structural Stability and Rate Capability for Li-on Batteries

Abstract

A new lithium rich composite positive electrode material of the composition 0.3Li2MnO3.0.7LiNi0.5Co0.5O2 (LLNC) was synthesized using the conventional co-precipitation method. Its crystal structure and electrochemistry in Li cells have been compared to that of the previously known material, 0.3Li2MnO3.0.7LiMn0.33Ni0.33Co0.33O2 (LLNMC). The removal of Mn from the LiMO2 (M = transition metal) segment of the composite cathode material allowed us to determine the location of the manganese oxide moiety in its structure that triggers the layered to spinel conversion during cycling. The new material resists the layered to spinel structural transformation under conditions in which LLNMC does. X-ray diffraction patterns revealed that both compounds, synthesized as approximately 300 nm crystals, have identical super lattice ordering attributed to Li2MnO3 existence. Using X-ray absorption spectroscopy we elucidated the oxidation states of the K edges of Ni and Mn in the two materials with respect to different charge and discharge states. The XAS data along with electrochemical results revealed that Mn atoms are not present in the LiMO2 structural segment of LLNC. Electrochemical cycling data from Li cells further revealed that the absence of Mn in the LiMO2 segment significantly improves the rate capabilities of LLNC with good capacity maintenance during long term cycling. Removing the Mn from the LiMO2 segment of lithium rich layered metal oxides appears to be a good strategy for improving the structural robustness and rate capabilities of these high capacity cathode materials for Li-ion batteries.