Structure, redox states, and magnetic properties of Li1-xNi1-zCoz/2Mnz/2 O2 cathode materials with z=1/5, 2/5, or 2/3 at different states of charge x

Abstract

Cathode materials Li1-xNi1-zCoz/2Mnz/2O2 with z=1/5 (NCM811), z=2/5 (NCM622), or z=2/3 (NCM111) in (i) the pristine state, (ii) charged to x=0.5 or x=0.65, and (iii) discharged after having been charged previously were characterized by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), direct current (DC) and alternating current (AC) magnetometry, heat capacity, and nuclear magnetic resonance (NMR) spectroscopy. The magnetic properties of these layered oxides are decisively determined by Ni2+ ions that have migrated from the transition metal (TM) layer to the Li layer and that are magnetically coupled to the Ni, Co, or Mn cations occupying neighboring sites within the adjacent TM layers. For the pristine and discharged states, this cluster formation causes a bifurcation between the zero-field-cooled (ZFC) and the field-cooled (FC) susceptibility vs temperature curve (which sets in continuously below about 100 K) and is responsible for the appearance of a broad maximum of the imaginary χ′′ susceptibility vs temperature curve that is centered around 14, 23, and 25 K for pristine NCM111, NCM622, and NCM811, respectively. The charged states with reduced Ni2+ amount do no longer show these features pointing to a strongly reduced cluster formation ability. Further, three-dimensional (frustrated) magnetic long-range ordering that leads to a broad local maximum in the heat capacity curve and to another abrupt bifurcation of the ZFC/FC susceptibility vs temperature curve (both observable slightly below 10 K) are additional features that all compositions (and states of charge) have in common. With increasing Ni to Co/Mn ratio, this low-temperature magnetic transition exhibits a more pronounced "partial"spin-glass-like characteristic as inferred from the frequency shift of the local AC susceptibility maximum. The results from AC/DC magnetometry and heat capacity measurements return complementary information to what can be inferred from XRD, XAS, and NMR investigations about the Ni to Co/Mn ratio, the Li/Ni disorder, and the TMs' oxidation states. The systematic investigation of samples with different Ni to Co/Mn ratios (going from NCM811 via NCM622 to NCM111) and with different oxidation states of the TMs (at different states of charge) allows for a better understanding of how the magnetic phenomena are correlated with the (real) structure and electronic properties in these layered oxide cathode materials.