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.