Understanding the role of trace amount of Fe incorporated in Ni-rich Li[Ni1-x-yCoxMny]O2 cathode material

Abstract A trace amount of Fe (0.25%) is incorporated in Ni-rich Li[Ni1-x-yCoxMny]O2 cathode active material at an impurity level, Li[Ni0.78Co0.11Mn0.11]Fe0.0023O2 (NCMF), to investigate the effects of Fe on structural and electrochemical properties. The modified structure of NCMF shows a lithium-deficient composition with a reduced cation mixing ratio compared to the bare sample, Li[Ni0.78Co0.11Mn0.11]O2 (NCM), notwithstanding the tendency of Fe3+ ions that easily migrate into Li layers. NCMF electrochemically outperforms NCM including the increase in the initial charge/discharge capacity with higher Coulombic efficiency, cycleability at high charging cut-off voltage, and rate performance with relatively lowered overpotentials. These enhanced electrochemical properties of NCMF are analyzed by in-situ X-ray diffraction and post-mortem high angle annular dark field scanning transmission electron microscopy. Mitigated structural changes in lattice parameters and lattice volume during the second charging are observed in NCMF, and the cation-disordered region of NCMF becomes narrower than that of NCM after cycling. Incidentally, NCMF exhibits a smaller amount of residual lithium compounds than NCM, of which rate performance is inferior to NCMF. Although the improved cycleability of NCMF is consistent with the result of our previous report [Park et al., Electrochim. Acta, 296 (2019) 814–822] adopting NCM111, a different mechanism in Ni-rich NCM is herein proposed.