Understanding the Degradation Mechanisms of LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Material in Lithium Ion Batteries

LiNi x Co y Mn z O 2 (NCM, 0 ≤ x , y , z < 1) has become one of the most important cathode materials for next‐generation lithium (Li) ion batteries due to its high capacity and cost effectiveness compared with LiCoO 2 . However, the high‐voltage operation of NCM (>4.3 V) required for high capacity is inevitably accompanied by a more rapid capacity fade over numerous cycles. Here, the degradation mechanisms of LiNi 0.5 Co 0.2 Mn 0.3 O 2 are investigated during cycling under various cutoff voltage conditions. The surface lattice structures of LiNi 0.5 Co 0.2 Mn 0.3 O 2 are observed to suffer from an irreversible transformation; the type of transformation depends on the cutoff voltage conditions. The surface of the pristine rhombohedral phase tends to transform into a mixture of spinel and rock salt phases. Moreover, the formation of the rock salt phase is more dominant under a higher voltage operation (≈4.8 V), which is attributable to the highly oxidative environment that triggers the oxygen loss from the surface of the material. The presence of the ionically insulating rock salt phase may result in sluggish kinetics, thus deteriorating the capacity retention. This implies that the prevention of surface structural degradation can provide the means to produce and retain high capacity, as well as stabilize the cycle life of LiNi 0.5 Co 0.2 Mn 0.3 O 2 during high‐voltage operations.