Stoichiometrically driven disorder and local diffusion in NMC cathodes

Abstract

Major structural differences in lithium nickel manganese cobalt oxides (NMC) prepared under identical conditions have been uncovered using neutron powder diffraction. Sample NMC-622 was obtained as a single Rm crystal structure with little defects, whereas NMC-811 showed significant Li deficiency and NMC-433 formed three distinct phases; ordered Rm, disordered Rm and a C2/m phase. Local diffusion behaviour was also studied by muon spin relaxation (μSR). It was observed that single phase Rm NMC-622 showed a higher lithium diffusion coefficient (4.4 × 10⁻¹¹ cm² s⁻¹) compared to lithium deficient NMC-811 (2.9 × 10⁻¹¹ cm² s⁻¹), or the highly disordered NMC-433 (3.4 × 10⁻¹¹ cm² s⁻¹). Furthermore, activation energies for the Li diffusion process were estimated to be 58 meV, 61 meV and 28 meV for NMC-811, NMC-622 and NMC-433, respectively.