Electrochemical behavior of layered Li-excess transition metal oxide in a Li rechargeable battery

Abstract

Recently layered Li-excess transition metal oxide($Li_{1+x}M_{1-x}O_2$), one of the most promising cathode candidates for next generation Li rechargeable batteries, has been consistently investigated especially because of its high lithium storage capacity, which exceeds beyond the theoretical capacity based on conventional chemical concepts. Yet the mechanism and the origin of the overcapacity have not been clearly understood. Previous reports on simultaneous oxygen evolution during the first delithiation may only explain the high capacity of the first charge process, and not of the subsequent cycles. In this work a clarified interpretation of the structural evolution of Li-excess transition metal oxide upon the electrochemical cycling, which is the key element in understanding its anomalously high capacity, was reported through careful study of electrochemical profiles, ex-situ X-ray diffraction pattern analysis, high resolution transmission electron microscopy, Raman spectroscopy, and first-principles calculations. Moreover, we successfully resolved the intermediate states of structural evolution upon electrochemical cycles by intentionally synthesizing sample with large particle size. All observations made through various tools lead to the result that spinel-like cation arrangement and lithium environment are gradually created and locally embedded in layered framework during repeated electrochemical cycling. Moreover, through analyzing the intermediate states of the structural transformation, this gradual structural evolution could explain the mechanism of the continuous development of the electrochemical activity below 3.5 V and over 4.25 V. Additionally, structural evolution of the Li-excess transition metal oxides with different nickel-manganese compositions was compared. The results of analysis indicate that the amount of manganese components in transition metal layer is the key factor of cation rearrangement.