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

摘要

Phase stabilities of Ni-rich LiNi1-2xCoxMnxO2 (NCM) are investigated as a function of Ni concentration by first-principles calculation within DFT+U framework and a bond model based on the interaction of transition metal (TM) ions. The DFT-based bond model reveals the intrinsic relationship between phase stability and TM-TM bonds in NCM, which explains 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 predicted by bond model and used to understand the intricate LiTMO2 (TM = Ni, Co, Mn) phase triangle. The results also show that Co and Mn atomic segregation happened when x < 0.1 (Ni > 80 at.%) is responsible to the electrochemical degradation during cycling. The change from strong Ni2+Mn4+, Ni3+Mn4+ and Co3+Mn4+ bond to weak Ni-Ni bond is identified as the intrinsic mechanism for the Co and Mn atomic segregation. The calculated results agree well with experimental observations and could clarify the controversies of Ni-rich NCM in the literature. In addition, we have demonstrated that the possible way to improve the stability of Ni-rich NCM is through the methods which can replace the weak Ni-Ni bonds by another strong TM-TM bonds.