Neutron diffraction and magnetic susceptibility studies on a high-voltage Li_1.2Mn_0.55Ni_0.15Co_0.10O ₂ lithium ion battery cathode: Insight into the crystal structure

Lithium- and manganese-rich oxides undergo structural transformation and/or atomic rearrangements during the delithiation/lithiation process and ultimately suffer from several issues such as first cycle irreversible capacity and voltage fade. In order to understand the mechanism of these issues, perception of a detailed crystal structure of pristine material is obviously demanding. In this study, combined powder neutron diffraction (ND) and temperature-dependent magnetic susceptibility techniques were employed to investigate the structure of a pristine lithium- and manganese-rich Li_1.2Mn_0.55Ni _0.15Co_0.10O₂ cathode oxide. Rietveld refinement on the experimental ND pattern yields good fits by considering either Li ₂MO₃ (M = Co, Mn, Ni) type monoclinic (C2/m space group) phase with 1% of Ni residing in the 4h lithium site or a composite structure consisting of 50% of Li₂MnO₃ type monoclinic (C2/m space group) and 50% LiMO₂ (M = Co, Mn, Ni) type trigonal (R3m space group) structure. In the composite structure, 3% Li/Ni site exchange in the trigonal phase is also proposed. Further, temperature-dependent dc magnetic susceptibility shows Curie-Weiss paramagnetic behavior at T ≥ 100 K, and no ordering/deviation of the field cooling (FC) curve in the temperature range 2-320 K indicates the random distribution of metal ions in the transition metal (TM) layer in the trigonal phase. Bifurcation of the zero-field cooling (ZFC) curve from the FC curve showing a magnetic ordering at T_N ∼ 50 K reveals the presence of cation ordering in the TM layers arising from a distinct Li₂MnO₃-like phase. These results suggest that the lithium- and manganese-rich oxide with a composition Li_1.2Mn _0.55Ni_0.15Co_0.10O₂ is more likely a composite of monoclinic and trigonal phases. The report also highlights the unique materials diagnostic capability of combined ND and magnetic susceptibility techniques to obtain detailed structural information of complex oxide systems.