Unraveling the Origin of Instability in Ni-Rich LiNi1-2xCoxMnxO2 (NCM) Cathode Materials

In this work, phase stability of Ni-rich LiNi1-2xCoxMnxO2 (NCM) (x < 0.20) is investigated by means of a bond model based on the effective interaction of transition metal (TM) ions (represented as TM-TM bond), fitted to results obtained within the DFT+U framework. The developed bond model reveals the intrinsic relationship between phase stability and TM-TM bonds, which explains the different roles of TM ions in the phase stability of Ni-rich NCM. A sequence of TM-TM bond strengths (Mn4+Mn4+ > Ni2+Mn4+ > Ni3+Mn4+ > Co3+Mn4+ > Co2+Mn4+ > Ni2+Ni4+) is then predicted by bond model and subsequently used to understand the intricate layered LiTMO2 (TM = Ni, Co, Mn) phase triangle. Our results also show that Co and Mn ions segregate to form clusters within the Ni environment when x < 0.1 (Ni >= 80 at. %), and such segregation is responsible for the electrochemical degradation during cycling. The obtained results agree excellently with the validation experiment in the present work and also other experiments in the literature, and could help to clarify the existing controversies about the origin of the instability of Ni-rich NCM compounds. Finally, we show that, by tailoring the TM-TM interaction, i.e., the atomic uniformity of the as-synthesized cathode material, the electrochemical stability of the Ni-rich NCM can be substantially improved.