Probing microstructure of solid-state synthesized LiCoO2 with MAS NMR spectroscopy

LiCoO2 is an important category of active cathode materials in lithium-ion batteries due to its high compacted electrode density, good thermal stability, and stable voltage platform. Recent works on LiCoO2 have focused on the realization of higher charging voltages to fully utilize its high theoretical capacity. However, an unambiguous atomic-level local probe is essential for the understanding of structure-function correlation. Here we employ high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy to study the local atomic environments in LiCoO2 synthesized with three common sintering methods. While one-dimensional 7Li NMR shows distinct linewidth and subtle dependence on lithium over-stoichiometry, both 7Li and 59Co relaxation times are highly dependent on the sintering method. We prove that the two-step sintering method favors the elimination of unreacted Co3O4, thereby enabling the best discharge capacity in all-solid-state lithium batteries assembled with LiCoO2/LGPS/LiIn, which is in accordance with its narrowest 7Li linewidth and the longest 7Li/59Co T1.