Challenges and recent progress in LiNi x Co y Mn 1− x − y O 2 (NCM) cathodes for lithium ion batteries

High energy density lithium ion batteries (LIBs) are in urgent demand for portable electronic devices and electrical vehicles. As the energy density heavily relies on the cathode materials, extensive researches have been undertaken to develop the cathode materials with a high degree of lithium utilization. In this respect, the layered lithium transition metal oxides with the composition of LiNi x Co y Mn 1− x − y O 2 (NCM) have received much attention due to their low cost and high practical capacity. Unfortunately, the viability of NCM for the market is hindered by the challenges resulting from the deteriorations occurring on the length scale from nanometer to bulk. These degradation mechanisms interact each other during battery cycling and simultaneously deteriorate the NCM lifespan. Therefore, this review article focuses on the insightful understanding of the capacity/voltage fading mechanisms of NCM with an aim to provide important clues for the future design of layered lithium transition metal oxides. First, we discuss the crystal and electronic structure of the layered oxide as they are related to the NCM mechanochemical breakdown. Then, on overviewing each degradation mechanism, we provide an insightful opinion with regard to finding the correlation between the mechanisms. Finally, a variety of novel approaches that have been proposed with an attempt to prolong the cycle life of NCM cathodes are presented.